CA1303814C - Heat recovery from concentrated sulfuric acid - Google Patents

Abstract

HEAT RECOVERY FROM CONCENTRATED SULFURIC ACID
ABSTRACT

A method and apparatus for the recovery of heat from a sulfuric acid process are provided.
Sulfur trioxide is absorbed into hot concentrated sulfuric acid, acid having a concentration greater than 98% and less than 101% and a temperature greater than 120°C., in a heat recovery tower and the heat created by the exothermic reaction is recovered in a useful form in a heat exchanger.

Description

17-21(~083)A

~3~)~B191 HEAT RECOVERY FROM CONCENTRATED SULFURIC ACID

Back~round of the Invention This invention relates to a process for the recovery of heat from a sulfuric acid plant. More particularly, this invention relates to a process ~or the recoverY oE the exo-thermic heat resulting from the absorption of sulfur trioxideinto concen~rated sulfuric acid. ~nis invention also relates to a heat recovery tower which is used to recover the heat energy from csncentrated sulfuric acid.

DESCRIPTION OF THE PRIOR ART
.
The process for the manufacture of sulfuric acid starts with a gas stream which contains sulfur dioxide. The sulfur dioxide is catalytically oxidized in a converter to sulfur trioxide whi~h is removed from the gas stream in one or more absorption stages to form sulfuric acid. The oxidation of sulfur dioxide to sulfur trioxide is an exothermic reaction.
To prevent the loss of this heat, steam has been generated in boiler~, and low level process heat has been recovered by heating boil~r feed water in economizers.
~ After oxidation the gas stream containi~g ~he sulfur trioxide passes through absorption towers in which the gaseous suIfur trioxide is absorbed into concentrated sulfuric acid, .

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13~3~3~4 having a typical concentration of 98%. In a modern s~lfuric acid plant there are typically two absorption towers, designat-ed as the interpass absorp~ion tower and the f inal a~sorption ~ower, which are respectively located in the process upstream from and downstream from the final catalyst stage in the con-verter. In ~he sulfuric acid plants of today, the gas stream is cooled prior to entry into the absorption towers to maximize the recovery o~ energy f rom the gas stream. The absorption to~er is operated at a temperature which is selected to facili-tate the absorption of sulfur trioxide into ~he sulfuric acid,to minimize the corrosion of piplng and heat exchangers that occurs at higher te~peratures, and to minimize the ~ormation of acid`mist. The absorption of sul~ur trioxide into sulfuric acid is a highly exoth~rmic reactionO and large amounts of heat are lost to cooling water ~hile maintaining the low tempera-tures in the typical absorption towers.
The absorption tower is typically constructed such that the sulfuric acid flows downward through the tower and the enclosed packing material while the gas stream which contains the sulfur trioxide passes through the tower. The packing promotes contact between the sulfuric acid and the gas stream such that the sulfur trioxide is absorbed in the sulfuric acid, The acid drains into a pump tank, where water is added to dilute the acid to the desired strength. Both absorption and dilution are exothermic reactions~ and the genera~ed heat is removed in a heat exchanger which is typically located between the pump tank and the absorption tower inlet. The operation of the absorp~ion tower is characterized by the con-centration of the recirculated sulEuric acidr typically 98~, a low maximum acid exit temperature of approximately 120C, and a typical acid inlet temperature of approximately ~0C. A lower 17-21 (5083 jA

~L3U3~14 acid inlet temperature would thermally shock the hot gas stream and often create an undesirable acid mist. A higher acid inlet temperature would increase the acid exit temperature and the corrosion of related piping and heat exchangers It is thus known that the operating temperature of the absorption tower is set by and limited by considerations of the rate of corrosion of the equipment, and the undesirable formation o~ acid mist.
The absorption tower has typically been constructed a~ a brick lined carbon steel tower to limit corrosion. Typi-cally, cast iron or ductile iron pipe has been used around theabsorption tower. Historically, a number of materials have been used for acid cool~rs. These include cast iron pipe or radia'tor sections, alloy C276 plate type heat exchangers, polytetrafluorethylene (PTFE) tank coils and stainless steel shell and tube heat exchangers.
The cast iron coolers are limited in operating temperature ~o approximately 110C by corrosion. They have poor heat transfer and occupy a large amount of space within the sulfuric acid plant. In addition, they have many mechani-cal joints which tend to leak and result in high maintenancerequirements.
Alloy C276 plate type heat exchangers can be cost effec~ive relative to the cast iron coolers. However, this expensive alloy is limited in use to a maximum acid temperature of approximately 90C; thus, the liquid exiting from the absorption tower at approximately 120C is mixed with cold recycle acid before entering the heat exchanger This reduces the thermal driving force and shows ~hat the use of expensive alloys will not provide an easy solu~ion to the problem of heat recovery rom sulfuric acid.

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:13~3~3~4 PTFE tank coils have been used to minimize corro-sion. Small thin wall tubes, which are easily plugged, are required to obtain adequate heat transfer. The PTFE can Withstand temperatures up to 200C; however, in heat recovery applications its low mechanical strength limits the pressure of steam that can be generated. Thus, an intermediate heat tran-sfer ~luid is required for heat exchange with the hot sulfuric acid. A second heat exchanger is then required or heat ex-change between the hea~ transfer fluid and steam: thus, this d~sign is too expensive for use in this hea~ recovery applica-tion.
Stainless steel heat exchangers, typically type 316 stainless steel, haYe been used as acid coolers. These require careful control of the acid temperature and the acid velocity in order ta minimize corrosion. The more recent anodically protected stainless steel acid coolers have proven to be a reliable means of minimizing corrosion; however, practice has been to limit acid operating temperatures to less than 115C.
The equipment to provide the anodic passivation is expensive.
Past practice with the aforementioned types of heat exchangers generally has been limited to rejecting ~he heat into cooling water or recovering heat in a low level form such as hot water for boiler feed or district heating.
E~orts have been made in the past to recover the heat generated ~hen the sulfur trioxide is absorbed into sul-furic acid. ~.S. Patent 2,017,676 describes an apparatus for condensing sulfuric acid. Sulfur trioxide and sulfuric acid fumes are passed through a heat exchanger which has ceramic tu~es to slowly and unifor~ly cool the gases from a temperature of about 350C to about 140C. The ceramic tube material is used in contact wi~h ~he sulfuric acid ~o prevent corrosion;

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~l3¢~3~4 however, a metallic tube is used concentrically about each ceramic tube to prevent mechanical stress and breakage of the ceramic tubes. The cooling medium, a high boiling point oil or boiling hot water, is allowed to become heated to high tempera-tures such as would be present in a steam boiler. When oper-ated in this manner, the patent s~ates that approximately 1.5 tons of steam may be generated per ton of sul~uric acid and thé
sulfuric acid manufacturing c~sts may be reduced.
British Patent 1,175~055 describes a method for the manufacture of sulfuric acid in which the gases are alternately pa~sed through a catalyst bed to convert sulfur dioxide to ~ul-fur ~rioxide and a heat exchanger/condenser in which the gases are cooled in ~he presence of water vapor to condense a part of the sulfur ~rioxide as sulfuric acid. The heat exchangers are lined with, or constructed of, materials which are resistant to corrosion by the hot, concentrated sulfuric acid such as ceramic materials and porcelain, metals such as steel coated with polytetrafluoroethylene or other corrosion resistant materials, or metals such as silicon-iron and nickel alloys.
The heat created by formation of sulfuric ~cid and the heat released by condensation is utilized to create high pressure steam which may be utilized as a source of power~ The 3ritish Patent also discusses recovery of the sulfuric acid in a more concentrated ~orm. By employing a stsichiometric deficiency of steam during the intermediate csndensations, it is possible to obtain sulfuric acid having a concentration of greater than 100~. Only in the final condensation after completion of the conversion of sulfur dioxide to sulfur trioxide is the remain-ing sulfur trioxide condensed in the prese~e of a sufficient excess of steam to insure that substantially all of the sulfur trioxide is removed from the gas stream.

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~3~38~

Both U.S. Patent 2,017,676 and British Patent 1,175,055 teach methods of recovering energy from the sulfuric acid process. However, both patents require the use of exotic materials of ~onstruction and emphasize the use of ceramics, porcelain materials, coated metals, brittle metals such as silicon-iron, and expensive nickel alloys for con~truction to preven~ rapid corrosion and ~ailure of the equipment.
Blumrich et al U~S. patent 4,330,364 describes a process for strengthening dilute phosphoric acid using energy derived from a contact sulfuric acid plant. In one e~lbodiment, heat is transferred from an H2SO4 absorber acid ~tream to a phosphoric acid stream wi~hout the use of any in~ermediate 1uid. In an alternative embodiment, a pressurized water system is interposed between the sulfuric acid and phosphoric acid systems. In the lat~er system the pressurized water is said to be heated to 120C by transfer of heat from 98.5%
absorber acid which is cooled from 140C to 120CC in the process. However, the reference contains no disclosure of the materials of construction to be used in the heat exchanger for ~ransfer o~ heat from absorber acid to the pressurized water.
Sand~r and Beckmann, ~Concentration of Dilute Sul-~uric Acid and Phosphoric Acid With Waste Heat~, paper 25 of ~Making the Most of Sulfuric Acid~, Proceedings of the British Sulphur Corporations's Fifth International Conference, Part II:
Additional Papers and Discussions; London, ~ovember 16-18, 1981, includes a flow sheet for an integrated sulfurio acid plant wi~h a venturi reconcentrator unit which uses waste hsat from a double absorp~ion production unit. In this process, hot acid from each of ~he intermediate and final absorbers is passed through a heat exohanger which ~ransfers heat to spent acid to be concentrated in the Venturi reconcentrator. In the 17-21 (5083) A

'3~

case of the intermediate absorber, absorber acid having a strength of 98.5~ is passed through the heat exchanger where it is cooled ~rom 130 to 110C by transfer of heat to the recon-centratOr acid circulating streain. However, in the Sander system, the heat exchanger utilized contains Teflon rather than metal alloy tubes, and the recirculating acid i8 heated only to a temperature of 70-80C.
In connection with the concentration of phosphoric acid, the Sander et al reference refers to the possibility of running the sulfur trioxide absorption system as high as 130 ~4 140C to generate low pressure steam of 1.2 to 1.5 bar, and the Use of such steam in a phosphoric acid vacuum concentra-tor. Sander et al further report an actual installation of a system apparen~ly patterned on the process of the Blumrich et al patent. In the latter system, Sander et al state that the sulfuric acid is cooled from 110C to 90C in the course of heating demineralized water up to 90C.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method for the recovery in high grade form of heat which is now lost to cooling water in the sulfuric acid process.
It is a further object of this invention to provide a method for recovery of the heat created in the sulfuric acid 25 process when sulfur trioxide is absorbed into sulfuric acid.
It is yet another object of this invention to provide a me~hod ~or the ab~orption of sulfur trioxide into hot, con-centrated sulfuric acid while greatly reducing the corrosive effect of the sulfuric acid~

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It is yet anothec object of this invention to provide a method f~r recovering the heat oE absorption of sulf~r tri-oxide in sul~uric acid at a higher temperature level, and thus in higher grade form, than has heretofore been practicable. A
particular ~bject is to recover ab~orption energy in the form of elec~ricity, steam or process heat.
~ n additional object of this invention is to provide a heat recovery system consis~ing of a heat recovery tower, heat exchanger and associated equipment such as pumps and piping for use in a sulfuric acid plant which may be construct-ed of cost effective alloy materials rather than the porcelain, ceramic, and ooated materials heretofore proposed for high tem-perature operation, all of which have mechanical, heat transfer and economic limitations.
These and other objec~s are ob~ained through a novel process in which the sulfur trioxide passiny from the converter in a sulfuric acid plant is absorbed into hot concentrated sulfuric acid and the heat is recovered for useful purposes through heat exchange with a third fluid.
The process of the invention is implemented utilizing a novel apparatus for recovery of the energy of absorption of sulur trioxide in sulfuric acid. This apparatus comprises a vessel containing an absorption zone in which sulfur trioxide is absorbed in a sul~uric acid stream, the absorption zone comprising m~ans for promoting contact and mass transfer between a gas phase comprising sulfur trioxide and a liquid phase comprising sulfuric acid having a concentration o~
greater than 98~ and less than 101% and a temperature of grea~er than 120QC. The apparatus further includes means for : 30 delivering sulfuric acid to the vessel for passage ~hrough the - absorp~ion zone, means for delivering gas comprising sulfur .

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~3~3~3~L4 trioxide to the vessel for passage through the absocption zone, means for egress o sulfuric acid from the vessel after passage through the absorption zone, means for egress of the gas s~ream from the vessel after passage through the absorption zone, and a heat exchanger wherein the heat of a~sorption is recovered from the sulfuric acid in useful foxm ~y heat exchange with a third flui~.[ The heat exchanger comprises ~eans for transfer of heat from sul~uric acid to the third fluid, the hea~ tran-sfer means being fabricated of an alloy having a low corrosion rate when exposed to hot concentrated sulfuric acid. ~
More particularly, the invention is directed to such apparatus including a heat recovery tower having top and bottom inlets and top and bottom exit~. The sulfur trioxide con~ain-ing yas stream from the converter~ a~ter being cooled, enters lS the heat recovery tower through the bottom inlet and flows up-ward through the tower and the hot sul~uric acid stream enters the heat recovery tower through the top inlet and flows down-ward through the ~ower. At all points in the heat recovery tower and heat exchanger system ~he sulfuric acid has a con-centration greater ~han 98% and less than 101~ and a tempera-ture greater than 120~C. The acid concentration is defined as being the weight percen~ of sulfuric acid. The counterflow of the gas stream and sul~uric acid maximizes the driving force for efficiently àbsorbing the sulur trioxide into the sulfuric acid. Co-current flow of gas and acid can be utilized, but iç
less efficient. The absorption of sulfur trioxide into sul-furic acid is a process which is known to ~hose having experi-ence in the manufacture of sulfuric acid and will thus not be further desc~ibed. This process will be referred ~o herein as t~e absorption of sulfur trioxide into ~ulfuric acid and the 17-21 (5()83) A

:~3~3~4 heat generated by the process will be referred to as the heat of absorption. The heat of absorption includes the heat liber-ated when water is aaded to dilute the recycled sulfuric acid, a process step whic~ may occur within or external to the heat recovery tower. Af~er the absorption of sulfur trioxide, the sulfuric acid stream passes through a hea~ exchanger wherein the heat of absorption is recover~d ~hrough heat exchange with other 1uids. It is desirable that the heat exchanger be ~ab-rica~ed from a metal to facilita~e the transfer of heat from the sulfuric acid stream to other ~luids. It has been dis-covered that by operating the heat recovery tow~r in a very narro,w acid concen~ration range between 9~% and 101%, and preferably between 98.5~ and 100.0~, it is possible to absorb sulfur trioxide efficiently and to dramatically reduce the corrosion rate of certain alloys while operating at tempera-tures heretofore considered impracticable. It has been discovered that certain alloys exhibit excellent corrosion resistance in the concentration range previously defined, and at temperatures of 120C or hi~her. Stainless steel alloys are generally superior to high nickel alloys. Excellent corrosion resistance has been found or certain iron/chromium alloys, iron/chromium/nickel alloys and nickel/chromium alloys having austenitic, erritic or duplex structures. S~ainless steels of such structure have been ~ound especially sui~able. Thirty alloys were tested at service conditions typical of the heat recovery systPm. It has b~en de~ermined that the corrosion resistance of these alloys can be characterized in terms of the percentages of major alloying constituents. ~ he alloys~best suited for service in this heat recovery system~contained iron, chromium, and nickel as the principal alloy constituents, and had compositions which gave a corrosion index ~CI) greater than 3~

17-21(5083~A

~3~3B~L4 CI> 39 as deined by the following equation:
CI - 0.35(Fe+Mn) ~ 0.70(Cr~ + 0.30(Ni) - 0.12(Mo) where:
~e z the weight percent of iron in the alloy, Mn - the weight percent of manganese in the alloy, : Cr - the weight percent of chromium in the alloy J
Ni 2 the weight percent o~ nickel in the alloy, and Mo = the weight percent of molybdenum in the alloy.
, In a conventional sulfuric acid plant, the heat of absorption of sulfur trioxide into sulfuric acid is lost to cooling towers. ~y use o the process and apparatus of this invention, a hish percentage of this previously lost energy may be recovered and profitably used, The heat may be used, for example, to produce low to medium pressure steam for process heating or to power a turbogenerator for the generation of electricity. In a 2700 tonne per day sulfur burning sulfuric acid plant approximately 6 megawatts of additional electrical power can be produced from the heat recovered in the heat recovery tower.

: : DESCRIPTIQN OF THE DRAWINGS
: Figure 1 is a process:flow diagram o~ a sulfuric acid ~la t Wh ch includes t~e ~pparacus f this invention.

; ' .

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~3~1 3~3~4 Figure 2 is a schematic which illus~rates the co~rosion rate of alloys and the degree of absorption of sulfur trioxide into sulfuric acid at a given temperature as the con-centration of the sulfuric acid varies.
Figure 3 is a diagram of the process and apparatus of this invention.
Figure 4 is a graph showing the operating cycle of the heat recovery tower in relation to the operating cycle of a typical interpass absorption tower.
~igure 5 is a process flow diagram allustrating a preferred scheme for implementing the improved process of the invention, Fig. 6 is a sulfuric acid temperatUre/conceQtration diagram having plstted thereon corrosion data and estimated ~ 15 isocorrosion curves for type 304L stainless steel;
Fig. 7 is a plot comparable to Fig. 6 but for type 310S stainless steel;
Fig. 8 is a plot showing both the literature 5 mpy isocorrosion curve for ~ncoloy 825 in sulfuric acid and ~he approximate 5 mpy isocorrosion curve for Incoloy 825 in the 98%-100% acid concentration region as determined in the course of the development of the improved process of the invention;
Fig! 9 is a plot showing both a litera~re 5 mpy isocorrosion curve for type 316 stainIess steel in sulfuric acid and the approximate 5 mpy isocorrosion curve for type 116 stainless steel in the 98% to 190~ concentration region as determined in t~e course of the developmen~ of ~he improved process o the invention;

~ .

~,. . .. .

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~L3U3~

Fig. 10 is a plot showing literature 1 mpy and 50 mpy i~ocorrosion curves for Ferralium Alloy 255 in sulfuric acid and the approximate 1 mpy isocorrosion curve for Ferralium in the 98% to 100% concentration region as determined in the course of the development of the improved process of ~he invention;
~ ig. 11 is a plot showing seYeral isocorrosion curves for Hastelloy C-275 in sulfuric acid, having plotted thereon a point showing the corrosion rate ~or ~lloy C-276 in the 98~ to 100% region as determined in the course of the develop~ent of the process of the invention;
, ~ig. 12 shows anodic potentiodynamic scans for type 304 stainless steel in 99.2% sulfuric acid at a scan ratP of 1.8 v/hr;
Fig. 13 shows anodic potentiodynamic scans for type 304 stainless steel in 99.2% sulfuric acid with a purge gas of nitrogen, 5~ 2 and 0.5~ SO2 at a scan rate of 1.8v/hr.;
Fig. 14 shows anodic potentiodynamic -~cans ~or type 310 stainless steel in 99.2~ sulfuric acid at a scan rate of 1.8 v/hr. and Fig. 15 shows anodic potentiodynamic scans for E-Brite 26~1 in 100% sulfuric acid at a scan rate of 2 v/hr.
Corresponding re~erence oharacters indicate corres-ponding process equipment elements throughout ~he several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT
. .
Figure 1 illustra~es a proc@ss flow diagram for a sulfuric acid plant which includes the apparatus of this i,nvention. The sulfuric acid process is well known; thus, 17-21 (~083~A

313~4 portions of the sulfuric acid plant will not be described in detail herein, The drawing shows a sulfuric acid plant which burns sulfur to supply the gas stream containing sulfur dioxide to the sulfuric acid plant.
Loo~ing now at Figure 1, blower 12 supplies air through drying tower 14 to tne sulfur burner lO in which the sulfur is burned, to provide a gas stream containing sulfur dioxide. The sulfur dioxide laden feed gas stream exits from the sulfur ~urner lO and passes through a ~irst heat exchanger 22, before entering the converter 30. The ~eed gas is cooled in first heat exchange~ 22 to a temperature near the desired inlet temperature to the converter. ~irst heat exchanger 22 is used to generate steam for driving a turbogenerator 23 for the generation of electrical power, but other uses are also practi-lS cal.
Converter 30, a vessel for the catalytic conversionof sulfur dioxide into sulfur trioxide, typically has a plural-ity of catalyst beds which are divided into a first oxidation stage 32 and a second oxidation stage 34. ~etween any ~wo catalyst beds there is heat exchange to remove the heat gener-ated during the oxidation of the sulfur dioxide. These heat exchangers are not shown in Figure l. In a typical sulfuric acid plant, between the first oxidation stage 32 and the second oxidation stage 34 the gas stream passes through an interpass 2S absorption tower to remove the sulfur trioxide from the gas stream to provide a gas stream to the second oxidation stage 34 that i~ lean in sulfur trioxide. As the sulfur dioxide laden ~eed gas stream passes ~hrough the first oxidation stage 32, greater than 90~ of the sulfur dioxide will be converted to sulur trioxide. The oxidation reaction is a reversible reaction which approaches an equilibrium: thus, some 17-21 (50833A

~3~3~4 of the sulfur trioxide must be removed from the gas stream to enable the remaining sulfur dioxide to be oxidized easily.
Esonomizer 54 is used to cool the gas stream exiting from the first oxidation stage 32 to a temperature which is above the dew point of the gas stream.
The sulfur trioxide in the gas stream is then absorbed into a sulfuric acid stream and heat is generated by the process. According to typical conventional practice, the absorption takes place in an absorption tower in which the acid 10 temperature is maintained at a low level, to minimize corrosion of related piping and hea~ exchangers. However, the mainten-ance of the low temperature in the absorption tower makes it dif icult to recover energy in an economically viable manner, that is, in a useful form.
~ 15 In accordance with the embodiment of Fig. 1, a heat recovery tower 60 is provided downstream of the economizer 54.
The cooled sulfur trioxide laden gas stream enters the lower portion of heat recovery tower 60 and flows upward through an absorption zone in the tower, the absorption zone containing means, such as a bed of packing 61, for promoting contact and mass trans~er between the gas and liquid phases. While this description is of a packed tower, it is contemplated that other gas-liquid contacting devices such as tray towers or ve~turi absorbers can be used~ Hot, liquid sulfuric acid is sprayed ~rom the top of the heat recovery ~ower 60 onto the bed of packing 61 and, as the sulfuric acid and sulfur trioxide con-tact one another in the absorption zone, the sulfur trioxide is absorbed in~o the sulfuric acid. As delivered to the eounter-current absorption zone in the hea~ recovery tower 60, ~he s~lfuric acid has a concen~ration greater than 98~ and less than 101%, preferably between about 98~5~ and about 99.3%, and 17-21 ~083)A

~3~3~i~

a temperature greater than 120C. As discussed above, the heat of absorption of the sulfur trioxide into the sulfuric acid i5 released in this process. Throughout ~he absorption zone, and throughout the course of SO3 absorption therein, the strength of the sulfuric acid s~ream is maintained between 98%-and 101~, pre~erably between 98.5~ and 10~.0%, and the acid temperature is maintai~ed a~ greater than 120C. After a~sorbing the ~ulfur trioxide and being heated by the exothermic reaction, the sulfuric a~id èxits the absorption zone and passe3 through ~he bottom outlet of the heat recovery tower. At the exit of the absorption zone and outlet at ~he tower, the acid has a conc~ntration of greater than 98~, preferably greater than 99%, optimally between about 9g.~3 and about 100.0~ and a tempera-ture o~ greater than 1~0C. After leaving the absorption zone, the hot concentrated sulfuric acid stream passes through heat exchanger 62 to remove the heat of absorption of he sulfur trioxide, prior to again baing circulated through the heat recovery tower. Preferably heat exchanger 6~ comprises a means for indirect transfer of heat between two fluids, ordinarily a solid partition such as, for example, the tube wall of a shell and tube heat exchanger, or the plates of a plate type exchang-er. ~he absorption of the sulfur trioxide increases the concentration of the sulfuric acid; therefore, the sulfuric acid must be diluted at some point. The required water may be added within the heat recovery tower 60 or into the piping between the heat recovery tower 60 and the heat exchanger 62;
however, it is preferred that the dilution water be added following cooling of the sulfuric acid in heat exchanger 62 at .

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~3~ 4 a point in which sufficient mixing may occur before the sulfuric acid enters the heat recovery towee 60. The dilution water can also be added as a vapor. This increases the amount of heat recovered and provides a means of upgrading atmospheric pressure steam to a useable higher pressure Alternately, the dilution water can be added as dilute sulfuric acid.
In Figure 1 the heat exchanger 62 is shown external rom the heat recovery tower 60. While this is the preferred arrangement, it is recognized that the heat exchanger 62 may be located within the hea~ recovery tower 60~ In heat exchanger 62 the heat of absorption released in the process is removed by heat transfer to a ~hird fluid. Preferably, th~ cooling fluid is water and the heat of absorption is recovered ~y the generation of low to medium pressure steam, for example, steam having an absolute pressure between apprsximately 150 and 2000 kPa and normally between approximately 300 and 1200 kPa. Where liquid wat~r is fed as the cooling fluid to the acid cooler, all or part of the water may be converted to steam. The steam produced by the heat recovery system is approximately 0.5 tonnes per tonne of acid produced when li~uid water is used for acid dilution. This steam may be used within the manufacturing complex surrounding the sulfuric acid plant or to generate electrici~y. It is a common practice to remove low and/or medium pressure steam from a turbogenerator for process use.
The removal of this steam reduces the electrical output of the turbogenerator. The steam generated in t~e heat exchanger 62 may be used to reduce the amount of steam normally removed from the turbogenerator or to eliminate this removal of steam alto-gether. If additional steam ~s avai~able, it may be injected 17-21(,083)A

~3~J 3B14 into the turbogenerator 23. The cessation of removal of steam from the turbogenerator increases i~s electrical output and the injection of additional steam will also increase the turbo-generator's electrical output. In a 2700 tonne per day sulfur burning sulfuric acid plant, approximately 6 megawatts of addi-tional elec~rical power can be produced as a result of the use of the steam generated in the heat exchanger 62. Alterna-tively, electrical power can be generated by using heat exchanger 62 as ~he boiler for vaporizing an organic liquid for use in driving a generator or otherwise producing work in an organic Rankine cycle. The higher temperatures available through the use of the heat recovery tower have now made such usage economically feasible.
In this way the heat of absorption of sulfur trioxide into sulfuric acid may be removed from the sulfuric acid process in a useful form; that is, in a form which may be utilized to produce a benefit either through use in a process or through the generation of electricity. This is in contrast to the current typical loss of this heat through removal by cooling water and release of the heat to the atmosphere in cooling towers.
Implementation of the improvements illustrated in Figure l provldes a major increase in the overall recovery of energy fro~ a contact sulfuric acid manufacturing plant. Prior to the present invention, developments such as increased converter reaotion gas strength, low tempera~ure economizers, reduced gas stream pressure drop, heating of boiler feed water with low temperature effluent heat, and u~e of suction drying tower had improved to about 70% the recovery of energy in high grade form, as electrici~y, steam or otherwise as a source of process heat. Introduction of the improved process of the 17-21 ~5083~A

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invention increases that high grade energy recovery efficiency to 90-95%.
Following passage of the gas stream through the heat recovery tower, the gas stream exits from the top of the hea~
recovery tower 60 and passes to the absorption zone of an interpass absorption tower 64 where absorption of any sulfur trioxide remaining in the gas stream takes place. Although shown as comprising a separate-tower in Fig. 1, the interpass sbsorber may comprise a separate stage positioned within the same housing as ~he hea~ recovery z~ne. In a typical sulfuric acid pla~t in wbich all of the sulfur trioxide is absorbed into a su~furic acid stream in the interpass ~bsorption tower 64, it is necessary to remove ~he heat of absorption. Thus, an acid cooler 66 is provided for the sul~uric acid recirculating ~ 15 through the in~erpass absorption ~ower 64~ ~owever, in a sul-furic acid plant utilizing the current invention, most of the sulfur trioxide is absorbed into sulfuric acid in the heat recovery tower 60: thus, only a small portion of the sulfur trioxide remains to be absorbed in the interpass absorption tower 64. Only a small temperature rise occurs within the interpass a~sorption tower 64. In the circumstances this small heat load may be removed elsewhere in the system; thu~, the acid cooler 66 is usually unnecessary and may be eliminated.
The dotted lines used to show the acid cooler 66 in Figure 1 indicate that the a~id cooler 66 has been removed from the process.
As a result of the high temperature operation of the heat recovery tower, the gas stream exiting ~he heat recoverY
tower is relatively hot, and is in contact with hot acid. This .
in turn results in stripping of sulfuric acid from the acid 17-21 (5083) A

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stream into the gas stream. On passage through interpass absorption tower 64, the gas stream is cooled, typically to a temperature in the range of between about 7~C and about 120C, preferably between about 75C and about 100C, by contact with the acid stream circulating through the interpass tower. At the gas exit ~rom the interpass tower absorp~ion zone, it is preferred that the acid temperature be below about 120C, preferably below about 100C, and a~ leas~ about 10C lower than the temperature of the acid at the inlet of the heat recovery tower. In a countercurrent interpass tower, th~ acid preferably enters at a temperature of abou~ 75 to about 100C. Where an interpass tower is used, it is preferred that the acid temperature throughout the interpass t~wer be lower than the acid temperature at the inlet of the heat recovery tower.
As the gas stream cool~ during its passage through the interpass tower, sulfuric acid vapor condenses and is either absorbed by the interpass tower acid stream or collected by a mist eliminator positioned in or at the exit of the inter-pass tower. Such lowering of the dew point of the ga~ streamhelps minimize condensation of acid and corrosion of downstream ducts and equipment.
Where there i~ no separate interpass tower, the interstage absorption may be accomplished essentially in a single absorption zone with n heat recovery tower 60. In this instance, the gas leaving the tower is preferably cooled before return to the converter, for example, by passing it through an economizer ~or heating boiler feed water. In this case, She dew point of the gas stream is also preferably reduced ~o 75C
to 120DC by cooling i~ to a temperature in that ran~e. How-ever, in the latter instance, condensation of sulfuric acid in 17-21 ~5083) A

the economizer creates conditions which are corrosive to many metals. Thus, an economizer used to lower the dew psint should be constructed of a metal of the type discussed above for use in the heat recovery tower acid cool~r. It is also important to control the addition of dilution water to the heat recovery tower recirculating stream so that the acid condensed in ~he economizer has a strength of at least 98.~.
Alternatively, the dew point may be lowered by pas-sage of the gas stream through a direct contact cooler, such as a packed tower, countercurren~ to sulfuric acid flowing at a rela~ively low flow rate, i.e., so that th~ temperature of the acid leaving ~he contact cooler approaches ~he temperature of the acid at the inlet of the heat recovery tower. Thus, con tact with the acid is effective in reducin~ the dew point of ~ 15 the gas (and absorbing residual sulfur trioxide), but the acid is heated to a temperature high enough so ~hat i~ may be blended into the acid entering the heat recovery absorption zone withou~ significantly reducing the temperature at which energy is recovered from the heat recovery system. The ~ela-tively small temperature difference between gas and liquid throughout the cooling zone minimizes acid mist formation.
Such a packed tower cooler may be constructed either as a separate vessel or as a separate stage within the same housing that contains the heat recovery absorp~ion zoneO In either instance, it is pre~erred that the temperature of the cooling acid that i9 in contact with the gas leaving the cooling zone be at least 10C lower than the acid temperature at the inlet of the heat recovery absorption zone. Whatever approach is taken, lowering of the gas dew point is important for protect-ing carbon s eel heat exchangers conventionally locateddownstream of ~he converter to which ~he gas stream is returned.

17-21(~083)~

~3¢~3~

The remainder of the sulfuric acid process shown in ~igure 1 is well known The sulfu~ trioxide depleted gas stream is returned to the second oxidation s~age 34 of the converter 30 to complete oxidation of the remaining sulfur dioxide. This final passage through an oxidation stage will complete the conversion of sulfur dioxlde ~o sùlfur trioxide.
The gas stream exits from the converter 30, passes ~hrough an economizer ~8 for cooling, and passes through ~he final absorp-tion tower 70 in which the sulfur trioxide in the gas str~am is absorbed into sulfuric acid. The amount of sul~ur trioxide present to be absorbed is much smaller than that which is absor~ed in the hea~ recovery tower and interpass absorption tower; thus, only a small amoun~ of heat is created by the absorption of the sulur trioxide into sulfuric acid in the - 15 final absorption tower 70. Following absorption of the sulfur trioxide, the gas stream is released to the atmosphere.
While the above descrip~ion is for an interpass plant, i~ is contemplated that the heat recovery tower can be installed upstream of the absorption tower in a non-interpass plant. For some operating conditions, as noted above, it is also contemplated that the heat recovery tower can replace ~he interpass absorption tower in an interpass plant, or the absorbing tower in a non-interpass plant. As further noted above, this is not the preferred mode of opera~ion for a heat r~covery tower located between converter stages, unless the heat recovery tower includes a cooling zone to reduce the high sulfuric acid vapor content of the gas exiting a hea~ recovery tower, which would otherwise lead to corrosion of downstream equipmen~. Without a cooling zone, it is not preferred in the case of final absorption either, since residual sul~ur trioxide 17-21(~083)A

~ 3~33~L4 and sulfuric acid vapor contained in the exit gas from the heat recovery tower constitute undesirable atmospheric emissions if not recovered in a lower temperature final absorption tower or in other emission control apparatus. Moreover, in the case of either an interpass or final absorption system, problems of corrosion and/or emissions would be aggravated by any upsets in the operation of the heat recovery tower.
~ eferring to Figure 2/ it can be seen that at constant temperature, both the corrosion rate of certain alloys and the degree of sul~ur trioxide absorption decrease rapitly as the sulfuric acid concentration increases. It has been deter~ined for this inven~ion ~hat there is a narrow window of operation in which the corrosiveness of the sulfuric acid to certain alloys at high ~emperature is greatly reduced while the absorption of sulfur trioxide into sulfuric acid is maintained at a sufficient level to remove the sulfur trioxide from the gas stream passing through ~he heat recovery tower.
Referring now to ~igure 3, the heat recovery tower S0 of this invention and its associated pipi~g, including the heat exchanger 62 and pump 63 are shown. The sulur ~rioxide laden gas stream, leaving the first oxidation stage 32 of converter 30, enters the heat recovery tower 60 ~hrough bottom inlet 82.
The gas stream passes upward through the absorption zone com-p~ising the bed of packing 61 in which ~he gas contacts the sulfuric acid stream and the sulfur trioxide is absorb~d in~o the ~ulfuric acid~ The sulfur trioxide depleted gas stream exits ~rom the hea~ recovery tower 60 through mist eliminator 89 and top outlet 88. The sulfuric acid enters the heat re-covery tower 60 through top inlet 84 and is sprayed through a plurality of acid distri~utors 85 onto the upper surface of the bed of paoking 61. The sulfuric acid flows downward through .

17-21 (5083) A

~3~3~ ~

the bed oE packing 61 in which it comes in contact with the rising sulfur trioxide laden ~as stream and the sulfur trioxid~
is absorbed into the sulfuric acid. The absorption of the sul-fur trioxide into the sulfuric acid is an exothermic process.
The hot sulfuric acid enters the heat recovery tower 60 at a temperature greater than 120C and, after absorbing the sulfur trioxide and being heated by the exothermic reaction, the sul-furic acid exits from the heat r~covery tower 60 at a temper-ature as high as about 250C. The sulfuric acid exits from the heat recovery tower through bottom outle~ 86 and is pumped by pump 63 ~hrough heat exchanger 62 to remove, by transfer to a third, fluid, the heat generated by the absorption of the sulfur trioxide prior to again being circulated through tower 60.
Preferably, the acid Qtream has a concen~ration of greater than ~ 15 98% and less than 1013 and a temperature of greater than 120C, more preferably greater than 125C ~hroughout the course of heat transfee to the another fluid. It is also preferred that the third fluid be heated to a temperature of at least 120C, more preferably greater than 125C, in its passage through exchanger 62. Corrosion of the heat exchangec is minimized if the acid concentration is at least 99~ throughout the course of heat transfer. Following passage through heat exchanger 62, a portion of the sulfuric acid is removed as p oduct through ~he acid pipe 95. In addition to the rise in the temperature Qf the sulfuric acid, the absorption of the sulfur trioxide increases the concentration of the su}uric acid; therefore, the sulfuric acid must be diluted~ The sulfuric acid may be dilu~ed by ~he addition of dilute sulfuric acid or water in the liquid or Yapor state; and the terms water or dilution water will be used ~o refer to the diluent. The required wa~er for ' 17-21 ~50~3)A

~3~38~

dilution of tne acid is shown being added in line through pipe 90 upstream of ~he absorption zone. It is preferred that the dilution water be added, as shown, in the piping 91 between the heat exchanger 62 and the entry of the sulfur~c acid into the S heat recovery tower 60 th~ough top inlet 84. However, the addition of the dilution water at this point is not required for this inventlon. ~he dilution water may be ~dded prior to passage of th~ sulfuric acid through ~he heat exchanger 62 or may be added to the sul~uric acid within the heat recovery ~ower 60, ei~her upstream, downst~eam or within the absorption æone omprising packing 61. The pref~rred location for the addi~ion o~ dilution water, represented by pipe gO, allows the alloy pump and heat exchanyer to operate at the highest sul-furic acid concentration, which gives the lowest rate of cor-rosion, at any given temperature within the operating range.It is particularly preferred that the acid strength be main-~ained above 9g~ throughout the alloy heat exchanger. Acid strength downstream of the dilution point is monitored by a conductivity probe. Conveniently, ~he rate o~ dilution water addition is controlled by throttling a feed control Yalve in response to a feedback con~roller that in turn operates in response to the conductivity probe downstrea~ of the addition point.
For the purposes of this invention it has been determined that ~he preferred operating conditions for th~ heat recovery tower compri~e sulfuric acid temperatures greater than 120C and concentrations greater than 98% and less than 101~, more preferably 98.5% to 100. n~ . operation of ~he hea~
reco~ery tower under these conditions provides the reduced s~lfuric acid corrosiveness to cer~ain alloys necessary for .

17-21 (5083) A

~3~ 4 operation of the equipment for long periods of time, and a high degree of absorption of sul~ur ~rioxide into the sulfuric acid. A reduced level of sulfur trioxide absorption would reduce the amount of energy recovered in useful form. To pro-vide yood absorp~ion efficiency, the temperature of the acid atthe outlet of the tower, and at the inlet of the acid cooler, is preferably not greater than about 250C. Within that limit, however, the acid temperature may be varied rather widely, depending on the manner in which the absorption heat is ultimately to be utilized. Thus, for examplei the e~ergy may be used ~o generate low pressure steam for concen~ration of phospporic acid in an integrated sulfuric ac~d/phosphoric acid plant, in which case the temperature of the acid should be above 120C through the hea~ exchanger, and consequently some-what higher than tha~ at both the inlet and exit of the heatrecovery ~ower. To provide a working margin to compensate for line pressure drop, phosphoric acid evaporator steam chest fouling, and the like, it is preferred that the acid tempera-ture throughout the heat exchanger ~e above 130C, more preferably above 140C, and most preferably above 150C.
Temperature~ significantly above 150C are generally unnecessary in an integrated sulfuric acid/phosphoric acid plant.
~owever, it has been found that the process of the invention is capable of operating with substantially higher acid temperature~, both at ~he inle~ and exit of the tower and throughout thP heat exchanger. Operation at such higher ~em-pera~ures makes pos~ible the generation of medium pressure steam for such applications as the operation of a tur~ogenera-tor for the production of eleotricity. In such instance, it is 17-21 (5083)A

~3~

preerred that the acid temperature throughout the heat exchanser be above 150C, more preferably above 1759C, and even more preferably above 200C. To provide such temperatures throughout the heat exchanger, it is, of course, necessary to maintain even higher temperatures at both the exit and inlet of the heat recovery tower. For certain alloys, it has been found that at some temperature above 120C the rat~ of corrosion in 98 101% acid actually begins to decrease with temperature, par~icularly at concentrations above 99~. Fsr such alloys, an especially attractive mode of operation may be to maintain an acid temperature entering the heat exchanger at somewhat above the ~oint where the negative relationship between temperature and corrosion ra~e begins to obtain.
The upper acid concentration limit of 101% is estab-lished on the basis of achieving a high degree of absorption in ~he heat recovery tower while opera~ing at conventional acid plant pressures which are near atmospheric. The upper acid concentration limit can be extended to approximately 105% if the heat recovery tower is operated at a pressure up to 1000 kPa. Where the tower is operated under pressures up to 1000 kPa, the preferred acid temperature range in the tower is between about 150C and about 270C.
It has been de~ermined that certain materials are particularly suitable for construc~ion of the heat recovery tower 60, the heat exchanger 67, the pump 63 and the other equipment associated with them. In partioular certain alloys have baen found to be preferable for their corrosion resistance at the conditions under which the heat recov~ry tower, heat exchanger and pump will be operated. Similarly; such alloys can be utilized as materials of construction for other equip-men~ or components exposed to sulfuric acid at ~he above-described concentrations and temperatures, for example, in 17-21 ( ~083) A

~3'~ 4 piping, ductwork, heat exchangers or other equipment loca~ed downstream of the heat recovery tower. These alloys exhibit minimal, industrially acceptable, corrosion at acid concentra-tions of 98-101% and temperatures above 120C. It has been determined that the corrosion resistance of these alloys can be characterized in terms of the percentages o the major alloying constituents. Provided that the acid concentration is main-tained within the ranges specified hereinaboVe, it has been determined that the rates of corrosivn are sutprisingly low for a su~stantial range of iron/chromium, nickel/chromiUm, and iron/chromium/nickel alloys, even at highly elevated tempera-tures substantially above 12~C. As indicated above, the alloys used for hea~ exchanger ~u~es, and other equipment elements exposed to the hot concentrated heat recovery tower - 15 absorber acid, should be consti~uted of either a ferrous alloy containing chromium, an iron-nickel alloy containing chromium, or a nickel alloy containing chromium. In each instance, the alloy should have an austenitic, ferritic, or duplex structure.
Generally suitable stainless steel (ferrous) and iron-nickel alloys may contain between about 16% and about 30 by weight chromium, up to about 33~ by weight nickel, up to about 6~, pre~erably up to about 4%r by weight molybdenum, and between about 35~ and about 83~ t pre~erably about 46~ to about 83%, by weight iron. Other alloying elements such as copper, manganese, silicon, cobalt, tungsten, niobium, titaniuml nitrogen, sulfur, phosphorus, and carbon may also be present in minor proportions.

17-2i!~G83iA

~3~`3~

Nickel base alloys useful in the method of the invention may generally contain between about 32% and about 76%
preferaDly about 37% to about 43~, by weight nickel, be~ween about 1% and ~bout 31%, preferably about 23% to a~out 31%, by weight chromium, between about 2~ and about 463, preferably between about 16% and about 28%, by wPight iron, and up to about 28%, preferably up to about 3~, by weight molybdenum. As in the case of stainless steel type and iron/chromium/nickel type alloys, the nickel base alloys may also contain minor proportions of other allsying elements.
The alloys best suited $or service in this heat reco~ery system have compositions which give a corrosion index (CI) greater than 39, CI ~ 39 l~ as defined by the following equation:
.

CI = 0.35(Fe+Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) where:
Fe 5 the weight percent of iron in the alloy, Mn = the weight percent of manganese in the alloy, Cr = the weight perc~nt of chromium in ~he alloy, Ni - the weight percent of nickel in the alloy, and Mo = the weight percent of molybdenum in the alloy.

For alloys of the above-described type, industrially acceptable corrosion ra~es are a matter oE economics. In addition to varying with general economic factors bearing on the profitability of a given plant, ~he acceptable rate of 17-21(5083)A

13~13t3~L4 corrosion varies with the function of the particular piece of equipment for Which an alloy is considered as a candidate. Por heat exchanger tubes, however, it may be said that an acceptable rate is generally in the range of 0~05 to 0.08 mm/yr. For piping, pump tanks, distributor~, or the shell of an absorption tower, slightly to moderately higher rates of corrosion may be tolerated. Based on corrosion data obtained in connection with the development of the process of the inven-tion, it has been found hat stainless steel type alloys are generally preerred to nic~el allvys for use in equipment exposed to the heat ~ecovery towe~ abso~ber acid at ~empera-turesq above 120C.
A preferr~d material of cons~ruc~ion is 304~ stain-less steel which typically contains about 18-20% by weight chromium, about 8-12~ by weight nickelJ up to about 2~ by weight manganese, up to about 1.0~ by weight silicon, up to about 0.030% by weight carbon, up to about 0.045~ phosphorus, and up to about 0.030~ sulfur and the balance essentially iron. Another preferred materi~l is 309 stainless steel which comprises approximately 20-24% by weight chromium, about 12-15 by weight nickel, up to about 1.0~ by weight silicon, up to about 0.045% phosphorus, up to about 0.03~ sulfur and the balance essentially iron. Especially preferred is 310 stainless steel`which contains between about 24~ and about 26%
by weight chromium, between about 19% and about 22% by weight nickel, up to about 2.0% manganese, up to about 1.5~ by weight silicon, up to about 0~08% by weight carbon, up ~o about 0.045%
phosphorus, up to about 0.030% sulfur, and the ~alance essen~ially iron. Other particularly useful alloys include `

17-21 (5C83)A

~3~ 4 E-Brite Alloy 26-1, which comprises between about 25.0% and about 27.5% by weight chromium, between about 0.75% and about 1~50% by weight molybdenum, between about 0.05% and about 0.20 by weight niobium, up to about 0.50~ by weight nickel, up to about 0.20% by weight copper, up to about 0.40% by~ weight manganese, up ~o about 0.02% by weight phosphorus, up to about 0.02% by weight sulfur, up to about 0.40% by weight ~ilicon, up to about 0.01~ by weight carbon, up to about 0.01S% by weight nitrogen, and the balance essentcially iron; and ~erralium Alloy 10 255, which comprises between about 24.0% and about 27.0% by weight chromium, between about 2O0% and about 4.0% by weight molybdenum, between about 4.5~ and aboutc 6.5~ by weight nickel, between about l.S~ and about 2.5% by weight capper, between about 0.10% and about 0.25% by weight nitrogen, up to about - lS 1.0~ by weight silicon, up to abou~ 1.5% by weight manganese, up to about 0.04% by weight carbon, up to about 0.04% by weight phosphorus; up to abou~ 0.03~ by weight sulfur, and the balance essentially iron. As generally exemplified by Alloy 26-1, Alloy 255, 304L stainless steel, 309 stainless steel and 310 stainless steel, a preferred range of alloys compris~s between about 16% and about 30% by weight chromium, up to about 23~ by weight nickel, up to about 4% by weight molybdenum, and the balance essentially iron~ Other conventional alloying elements may be present in minor proportions.
While the~e alloys of the above-described type are pref2rred for construction, sometimes conventional materials of construction for the hea~ recovery ~ower will be more cost effec~ive. A~ these times the heat recovery tower can be construc~ed of carbon steel and lined with a ceramic ma~erial 17-21 t5083)A

13E~

to protect the carbon steel shell from the corrosive attack of the sulfuric cid. Such construction is ve~y similar to tha~
typically utilized for the interpass absorption tower. In the case of heat exchanger 62, either ~he entire exchanger or only 5 the means for ~ransfer of heat ~rom the acid to the third fluid, for example, the ~ubes, tube sheet, and channel3 of a shell and tube exchanger, are fabricated of the corrosion re-sistant alloy. In the latter case, the acid is passed through the tubes of the exchanger and the shell is constructed of a - 10 rela~ively low cost material such as mild steel for containment of a relatively non-corrosive cooling fluid.
The function of the heat recovery tower is to contain the sulfuric acid and the sulfur trioxide laden gas stream and to provide for the contacting o~ these two streams such that ~ 15 the sulfur trioxide is absorbed into the sulfuric acid. As the abs~rption of the sulfur trioxide into the sulfuric acid is an exothermic reaction, the acid is heated. It is preferred that the sulfuric acid entering ~he heat recovery tower have a temperature greater than 120C. The heat of absorption will raise this temperature, generally within the range of 10-60~C, more ~ypically by 20-40C. The exit temperature ~rom the tower may be as high as about 250C. While this is the preferred operating range or the acid temperature, it is possible to operate at higher temperatures by increasing the pressure or reducing the degree of sulfur trioxide absorption. In the preferred temperature range low to medium pressure ~team, for example, steam having a pressure between approximately 150 and 2000 kPa, pre~erably 300-1200 kP , may be generated. Increas-ing ~he steam pressure will require a cor~esponding increase in the temperature of ~he sul~uric acid entering the heat recovery tower.

17-21 (5083)A

13~:3~

Steam generated by transfer of the absorption heat may be used in a variety of applications. Relatively low pressure steam at a temp~rature greater than 120C, more preferably at a temperature greater than about 125C, is advantageously used as a source of energy for concentrating phosphoric acid in an integrated sulfuric/phosphoric acid plant. As noted above, it may be advantageous to generate steam at somewhat higher temperatures to compensate for such problems as line pressure drop between the heat exchanger for cooling sulfuric acid and the phosphoric acid evaporator, and fouling of the tubes of the phosphoric acid evaporator steam chest. Thus, for such applications, it is preferred that the third fluid be heated to a temperature of at least about 130C, more preferably at least about 140C, and most preferably at - 15 least about 150C.
Alternatively, steam may be generated at higher temperature and pressure for use in operating a turbogenerator or for delivering process heat at temperatures higher than that re~uired, for example, in concentrating phosphoric acid. For such higher temperature applications, it is preferred that the other fluid to which the absorption heat is transferred, whether steam or some other fluid, be heated in the sulfuric acid heat exchanger to a temperature o~ at leas~ about 150C, more prefsrably at least about 175C, most preferably to at least about 200C. It will be understood that the inlet tempera~ure to the sulfuric acid heat exchanger will exceed the temperature ~o which the third fluid is heated by an increment o~ roughly 20-70C, mo-e typicallg 30-50C.

17-21(5083)A

13'J3~

In a particularly preferred mode of operation, the tower is operated at essentially atmospheric pressure, the acid leaving the tower 60 is maintained at a temperature greater than 150C, and steam is generated in heat exchanger 62 at a pressure o~ 440 kPa absolute or greater.
Again, it must be stated tha~ this invention is not limited to the configuration shown in Figure 3. ~he héat exchanger 62 is shown located externally of the heat recovery tower 60~ This is Eor numerous reasons the p~eferred embodi-ment; however, the heat exchanger may also be located withi~the heat recovery tower 60. Similarly, the dilution uater is shown being added through pipe 90 into pipe 91 betwee~ the heat exchanger 62 and the top inlet 84 to the heat recovery tower 60. With this preferred configuration the heat exchanger 62 and pump 63 always contact acid of the highest concentration which provides the lowest corrosion rates and the highest degree of protection to the heat exchanger and pump. It is also contemplated, and is a part of this invention, that the dilu~ion water may be added into pipe 92 which is located between the bottom outlet 36 of heat recovery tower 60 and the heat exchanger 62. It is also contemplated that the dilution water could be added directly to the heat recovery tower 60 or to the gas stream entering the tower. It is noted that the addition of dilu~ion water downstream of ~he absorption 20ne, or in the zone near the acid exit point, will reduce the sulfuric acid concentration beore it passes through the pump 63 and heat exchanger 62 and that this lower sul~uric acid concentration will result in a higher corrosion rate for the heat exchanger 62 and pump 63.

17-21(5083)A

:~3~

Illustrated in Figure 5 is a particularly preferred embodiment of the process of the invention. In this system, boiler feed water is preheated in a heat exchanger 96 by transfer of heat rom final absorption tower circulating acid, and in a heat exchanger 100 by transfer of heat from acid circulating through drying tower 14 and interpass a~sorption tower 64. Preheated boiler feed water is sent to a de-aerator ~no~ snown), and a portion of ~he water exiting the de-aerator i returned to another heat exchan~er 98 where it is further heated by transfer of heat from the produc~ acid stream ~hat is withdrawn from the heat recovery ~ower circulating loop via disc~arge line 95. ~ m~jor portion of the boiler feed water stream leaving exchanger 98 is delivered to h~at exchanger 62 - where it is vaporized to produce steam ~hat may be used for power generation or process heat. However, a side stream of the water leaving exchanger 98 is diverted to line 90 through which it is injected as makeup water into the heat recoverY
tower circulating loop. Depending on the volume of feed wa~er and the particular conditions prevailing in the final absorption tower, the boiler feed water m~y be partially vaporized by the time it reaches pipe 90.
A circulating acid pump tank 99 is provided for drying tower 14 and interpass absorption tower 64. Product acid from exchanger 98 is delivered to tank 99. Some or all of ~he acid returned to ~ower 64 may bypass exchanger 100 through line 101, thereby increasing ~he temperature of the ~cid in ~he pump tank and allowing the boiler feed wa~er passing through the exchanger ~o be heated to a higher temperature than would otherwise be feasible. A product acid stream is removed ~rom tank 99 through boiler ~e~d water preheater 109.

.

17-21 (5083)A

3~

Figure 4 is a graph which shows the operating cycle of the heat recovery tower 60 in terms of the temperature and concentration of the sulfuric acid. Shown on the graph are isocorrosion lines for type 304L stainless steel. Also shown is a line representing the sulfur trioxide equilibrium between typical inlet gas and outlet acid. Thi~ equilibrium line defines the limiting conditions for the absorption of sulfur trioxide in the acid at atmospheric pressure. The operating cycle of the heat recovery ~ower is shown as a triangle A3C.
The loca~ion of points A, ~ and C in ~he process may be found in Figure 3 and Figure 1. Point A of the operating cycle repr~sents the conditions, the temperature and concentration of the sulfuric acid, at the bottom outlet 86 ~rom the heat - recovery tower ~0. Point B represents the Qulfuric acid conditions after pas~age through the heat exchanger 62. Point C represents ~he sulfuric acid conditions as the sulfuric acid is entering the heat recovery tower through top inlet 84, after the addition o~ the dilution water.
Looking at Figures 3 and 4, it is of interest to discuss a complete operating cycle by considering the flow of the sulfuric acid starting at point C. The sulfuric acid enters the heat recovery tower 60 at a temperature of approximately 165C and a concentration of approximately 99~.
As the sulfuric acid passes downward through the heat recovery ~ower and absorbs ~he sulfur trioxide from the gas stream passing upward through the hea~ recovery tower an exothermic reaction takes plaoe. The temperature o~ ~he sulfuric acid rises and the concentration increases. At the outlet of the heat recovery tower, represented by point A, the sulfuric acid has a temp~ratur~ of approximately 200UC and a concentr~tion of approximately ~00%. Following the heat recovery tower, the 17-21 (5083)A

~3~3B14 sulfuric acid flows theough heat exchanger 62 and is cooled.
Re~erence point B is at ~he exit of the heat exchanger 62. At this point the acid has been cooled from approximately 200C
to approximately 157C and the acid concentration has remained constant. Before the sulfuric acid en~ers the heat recovery tower again, dilution water is added. The addition of water to the concentrated sulfuric acid reduces the concentration and causes a rise in temperature. ThereforP, the ~riangle of the operating cycle shows that the concentration of the sul-furic acid is reduced from approximately 100% to approximately99% and that, during this reduction, the temperature of the sulf~ric acid rises from approximately 157C to 165C. At ~his point the sulfuric acid again enters the heat recovery tower and ~he opera~ing cycle is repeated.
lS It is easy to see in Figure 4 the relationship between the operating cycle of the heat recovery tower 60, which includes the heat exchanger 62, the corrosion rates for type 304L stainless steel at di~ferent temperatures and sul-furic acid strengths, the equilibrium line for the absorption of sulfur trioxide into the sulfuric acid, and the ~emperature of the steam that may be generated. This operating cycle is compared to the operating cycle of a typical in~erpass absorp-tion tower represented by the triangle DEF. The lo~ation of points D, E, and F in the process may be found in Figure 1.
Point D represents the conditions o~ temperature and acid concentration leaving the interpass tower. Point E represents typical condition where the acid in the pump tank has been diluted with water and cooled by mixing wi~h colder acid d~aining from the drying tower circuit. Point F represents the temperature and acid concentration lPaving ~he acid cooler and recirculating to ~he interpass absorbing ~ower. It can be 17-21(5083)A

3~

seen in Figure 4 that the heat recovery tower of this inven-tion permits absorbing sulfur ~rioxide at significantly higher temperature than previously practiced, while reducing cor-rosion rates for type 304L stainless steel by a ~actor of ten or more, compared to that obtained at acid t0mperatures and concentrations characteristic of past practices. Significant reductions in corrosion rates were ~ound ~sr the other alloys Whlch have a corrosion index (CI) greater than 39, with the extent of the reduction dependent upon the specific alloy.
It may be noted that the discoveries of the present invention with respect to materials of construction have gene~al application to the storage, transpor~ation or handling of hot concentrated sulfuric acid streams irrespective of the - particular process or other operation that may be involved.
~hus, a basic method has been discovered for storing, trans-porting or handling sulfuric acid having a concentration greater than 98.5~ and less than 101~ at temperatures greater than 120C. This method comprises containing the açid in a conduit or vessel constituted of an Fe/Cr, Ni/Cr or Fe/Cr/Ni alloy having an austenitic, ferritic, or duplex structure and having a corrosion index corresponding to the algorithm set orth hereinabove. Particularly preferred alloys have com-posiitio~s falling within the ranges particularly specified hereinabove.
Further in accordance with the invention, a method has been di-~covered for condensing sulfuric acid vapor from a gas stream satur~ed with sulfuric acid vapo~ at a emperature above 120C. This me~thod is of important value in reducing the dew point of the gas stream so as to reduce its corrosive-; 30 ness to materials or equipment with which it may later come into contac~. It is particularly useful in oooling and reduc-: . ing the dew point of gas exiting the heat recovery zone in the 17-21 (~083)A

13~3B14 heat recovery process of the present invention. In accordance with the novel method for condensing sulfuric acid vapor, the saturated gas stream is brought into contact with a heat transfer surface having a tempera~ure below the saturation temperature of the ~as with ~espect to sulfuric acid vapor, but above 120~C and hiyh enough so that the sulfuric acid which condenses on the heat transfer surface has a concentra-tion greater than 98.5~. The heat transfer surface is com-prised of a errous alloy containing chromium, an iron-nickel alloy containing chromium, or a nickel alloy containing chromium, and has either an austenitic, ferritic, or duplex struc~ure. The composition of the alloy further corresponds to the corrosion index set forth hereinabove.
The novel method for condensing sulfuric acid vapor from a saturated gas stream may be implemented, for example, by passing the gas stream exiting the hea~ recovery tower of the invention through an economizer having tubes constituted o~ the above described alloy, and controlling the temperature of the cooling fluid inside the economizer tubes so that the outside of the tube wall in contact with the gas stream and condensate has a temperature greater than 120C and high enough so that the condensate has a concentration greater than 98.5~.

Sta~ic corrosion immersion ~ests were carried out in hot concentrated ~ulfuric acid over a range o~ temperatures to determine ~he corrosion resis~ance of carbon steel and various alloy me~als whose compositions are set forth in Table 1.

17-21 (5083)A
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17-21(50833A

~3ql3~

For purposes of these tests, samples were cut to a size of approximately 10 mm x 6 mm x 2 mm and the surfaces of the samples ground smoo~h with a 129-grit belt surface grinder.
These samples were immersed in various sulf~ric acid solutions in 80 ml capacity Teflon cups fabricated by Savillex Corpora-tion, The cups were provided with screw caps de~igned to afford a seal. A glass rod grid was inserted in the bottom of each cup to support the sample with minimum contact, so that there would be little difference be~ween the areas of exposure to the acid on the top and bottom of the sample. ~f~er the sample was placed on ~he glass qrid, a portion of sulfuric acid ~50 ml) of the desired stren~th was poured into ~he cup, thus providing a Eatio o acid voluma to sample surface area of 29 ml/cm2, a ratio wi~hin the range of 20-40 ml/cm2, as recom-mended in ASTM Method G-310 The acid solutions used in the corrosion tests were prepared by mixing 98% by weight sulfuric acid (Axton-Cross) with either water, to provide solutions more dilute than 98%, or with 20~ oleum to provide solutions of a strength greater than 9R4, Acid concentrations were determined by measuring the conductivity of the solu~ion at a prescribed temperature. After the acid solu~ion was poured into a cup containing a corrosion sample, the cup was sealed and inserted into an oven at the test temperature and the sample exposed to the acid solution at that temperature for seven days. After exposure, the cup was removed from the oven and allowed to cool, and the sample then removed and rinsed. Most samples required no extensive clean-up after rinsing, but some were cleaned with a rubber era er to remove an adherent film.
Thereafter, ~he samples were weighed ~o detarmine loss of m~ss on corrosion, and from such loss of mass the linear corrosion rates were determined. Set forth in Table 2 is a compilation of the data obtained in Shese static immersion tests. This data provided ~he basis for the co~rosion index ~et forth - hereinabove.

17-~1 (5083) A

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17-21 (~083)A

~3~38~

Collated in Table 3 is data at a variety of acid con-centrations for several alloys, including certain alloys which exhibited favorable corrosion resistance based on the data of Table 2. Among the specific alloys for which corrosion data is presented in Table 3 are E-Brite Alloy 26-1, a ierritic stain-less steel, Alloy 255 duplex s~ainless steel, Alloy 304L, an austenitic stainless steel, and Alloy C276, a high nickel ailoy. Other alloys for which data is presented in Table 3 are 304L stainless, 310 stainless and Alloy 29-4-2. Thi data illustrates thP very significant e~fect o~ small differences in acid concentration on the corrosion ra~es of ~hese alloys.
Thus,. the stainless st el alloys experience as much as a 35-~old increase in the corrosion rate when the sul~uric acid ~ concentration is decreased from 100 weight percent acid to approximately 98%. Alloy C276 shows a reduction in corrosion in the 98-99% concentration range, but the corrosion rate in-creases between 99~ and 100%. If the Alloy C276 data is com-pared with any of the stainless steel data, it can easily be seen that there is a considerable advantage ~or stainless steel alloys when used in accordance with the teachings of this invention.
At the elevated temperatures normally encountered in the heat recovery tower and heat exchanger many alloys become more passive, or resistant to corrosion. Thi~ effect can be seen to prevail for a number of alloys as illustrated by the data given in Table 3.
Set forth in Fiq. 6 are estimated isocorrosion curves based on the data of Table 3 ~or type 304L stainless steel. It may be noted that the isoco~rosion curves of Fig~ 6 are no~
~traight. The data of Table 3 shows ~he corrosion rate 51~

17-21 (~083) A

~3~3~

increasing rapidly with ~emperature in the range of 250C and exposure to 97-9~% acid concentration. However, for acid concen~rations between 99-~00%, the temperature effect was found not to ~e substantial. It may be noted that the isocorrosion curves o~ ~ig. 6 exhibit an ~S~ shape, resulting from an actual decrease in corrosion as a function of temperature in elevated temperature regions typically above 130C.

17-21(5083)A

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~3U.3~

Set forth in Fig. 7 is a plot of corrosion data of Table 3 on a temperature vs. acid concentration diagram for Type 310S stainless. Also set forth in Fig. 7 are estimated isocorrosion curves. Chemical analysis of the Type 310S
stainless steel used in the static immersion tests of this example is set forth in Table 4.

TA~LE 4 Chemical Analysis o~ Type 310S Stainless Steel Used in Static Immersion Tests - ~eight Element Percent .;
C 0.055 Fe 52.5 Cr 24.04 Ni 21.33 ; 15 S~ 0.46 Figures 8 to 10 illustrate the unexpected results observed in the corrosion tests o Example 1. Thus, in the temperature concentration diagram of Fig. 8, curve 1 is the isocorrosion curve at 5 mpy ~or Incoloy 825 as reported in Fig.
38 of International Nickel Company Corrosion Engineering, ~Bulletin CEB-lU (January 1983)~ while curve 2 is ~he approximate 5 mpy (0.13 mm/yr) isocorrosion curve in the 98-100% acid region for Incoloy 825 as derived from data ob-tained in the above-described static immersion tests. In Fig.
9, ourve 1 is based on 5 mpy (0.}3 mm/yr) corrosion data pub-lLshed by Fontana ~or 316 stainless steel, while ~urve 2 is the .

17-21(5083)A

13~3B~4 approximate 5 mpy (0.13 mm/yr) isocorrosion cu~ve as derived from the above-described test. In Fig. 10, curves 1 and 2 are the 1 mpy and 50 mpy isocorrosion curves ~or Ferralium Alloy 255 as published ~y Cabot Corporation, whil~ curve 3 is the approximate 1 mpy (0.03 mm/yr) isocorrosion cu~ve in the 98-100~ sulfuric acid region as deriYed from the above-described corrosion test. In Fi.g. 11, curves 1-4 are isocorrosion curves for Hastelloy C-276 as published at page 75 o Cabot Corporation's ~Corrosion Resistance of Hastelloys~
(198~). Inscribed as point S on Fig. 11 is the da~a obtained for this alloy in the above-described test.

Despite the g2nerally favorable resul~s obtained in the static immersion tests described in Example 1, tests of such nature cannot necessarily be relied upon as a basis for specifying materials of construction for the tubes of a heat exchanger cooling hot abso~ber acid. Static tests do not pro-vide a satisfactory basis for determining whether the alloys involved are or may be subject to active-passive type oor-rosion. In order to provide a further assess~ent of thesuitability of various alloys in absorber acid cooler service at elevated temperatures, ~lec~rochemical tests we~e carried out to establish the stability of the passivation process under specific conditions. In these tests, the voltage was monitored during approach to the freely corroding potential (FCP~. Once the ~CP was reached, potentio-dynamic scans were used to assess stabi}ity. Tests were run under th~ following conditions:

17-21(;083)A

13~?3Bl~

(~) type 304 stainless steel and 99.2% sulfuric acid at 143C, 171C and 199C;

~ B) same conditions as (A) but the acid was purged with a ~as mixture to simulate the absorber atmosphere, i.e., nitrogen containing 5% by volume oxygen and 0~5% by volume sulfur dioxide;

(C) type 310 stainless steel under the same conditions as (A);

(D) E-~rite 26-1 in 100% sulfuric acid at 143C and 171C.

-In test series (A) for 304 stainless steel, the acid was takendirectly from a carboy and no attempt was made to aerate or purge the acid before or during the electrochemical tests.
After an electrochemical cell was set up and the desired temperature achieved, the circuit was left open and the cell was allowed to stabilize at the FCP. Table 5 shows the reely corrGding potential at the various temperatures involved in this test. After the FCP was established at each test temperature, an anodic scan was run at 1.8 volts per hour.
The scans were then reversed and the voltage was allowed to decrease until ~he current reached 103 microa~ps/cm2 in the cathodic direction. The voltage was then turned off and the cell allowed to again reach the ~CP. These last FCPs are also shown:in Table 5. The anodic scans are shown in Fi~. 12.

17-21 (5083~ A

13~3~

TA~LE 5 Freely Corroding Potentials (~CPs~ or Type 304 Stainless Steel in 99.2~ Sulfuric Acid Freely Corroding Pote_tlals, ~olts_~S.C.E.) ~efore ~oan ter scan 143~ (290F) ~.343 ~.309 171C (340F) +.330 199C (390F) ~.334 +.329 The FCPs shown in Table 5 were found to be very stable since all settled out in less than one hour, except for the FCP at 143C after the cathodic scan, in which case two hours were required. Such behavior indicates a system with a strong tendency to passivity. Moreover, ~he shape of the anodic scans indicates that the natural corrosion potential is in the pas-sive region of the potentiodynamic curve. ~ased on the cor-rosion current, and assuming that the corrosion consists of oxida~ion of metallic iron to ferrous ion, the curves indicate corrosion rate~ of less than 0.013 mm/yr at 143C and about . 0.02S mm/yr at both 171C and lg9C. These results are in good agreement with the immersion tests of Example 1.
In the ~lectrochemi~cal tests on 304 stainless steel in 99.2% sulfuric acid purged~with nitrogen, 5~ oxygen and 0.5 sul~ur dioxide (Seri~s (B) as described above) the ~est sequence was as follows: (l) establish ~CP; (2) conduct a : 25 cathodic scan; (3)~ re-establ~sh FCPs; (4) run an anodic s~an;
: and ~4) re-establish F~P. Tnis seguence increased the severity ::
. Sg , 17-21 (5083) A

13~3~

of the test, since the cathodic scan was expected to strip the prot~ctive passive layer, and thus provided an evaluation of the ability of the alloy to repassivate under adverse conditions The FCPs observed in the tests of series ~B) are set forth in Table 6 with their anodic scans ~eing shown in Fig. 13. The results under the conditions of series (B) reveal that at 143C the FCP is beyond the passive range and entering tne ac~ive zone, bu~ the scans at the two higher temperatures show that the type 304 stainless steel specimen is in a passive 10 s~ateO

~ ~reely Corroding Potentials (FCPs) for Type 304 Stainless Steel in 99.2~ Sulfuric Acid wi~h a Nitro~en, 5% 2~ and 0.5~ SO2 Gas Purge Freel~_Corroding Potentials, Volts ~S.C.~.) 3efore Before After anodic scans cathodic scan scan r ........... _ _ _ 143C (290F) -~093 +.11~ ~.115 171C (340F) +.343 ~.344 ~.335 199C (390F) +.350 +.360 ~.350 Table 7 shows the freely corroding potentials and Fig. 14 shows ~he three anodic seans for the tests of s~ries (C) or type 310 stainless steel. The voltage traces for 143C
(290~F) and 171C (340F) showed initial instability and active-passive behavior fcr less than five minutes, but then 17-21 (~083)A

~3~1 3B14 gradually approached their stable FCP. In at least three of the four FCPs measured, one hour was re~uired to achieve a stable voltage. The scans in Fig. 14 indicate higher corrosion ra~es for the passive regions at the two lower temperatures, S approximately 0.13 mm/yr and up ~o approximat~ly 0.23 mm/yr, but only approximately 0.012 mm/yr at 199C (390F).

TA3~E 7 Freely Corroding Potentials ~FCPs) for Type 310 Stainless Steel in 99.~ Sulfuric Acid , (Parentheses Indicate That Value Might be Higher Than Shown.) -Freely Corroding Potentials, Volts (S.~.E.) Before scan After scan 143C ~290~ ~.245 171C (340F)(+.271) ~~~~
199C (390F) +~342 +o317 In series (D), for E-Brite 26-1l only two temperatures were evaluated, i.e., 143C and 171C. The freely corroding potentials are shown in Table 8, and the potentiodynamic scan 29 curves are shown in Fig. 15. These results ~oth reflect a very stabIe passivation situation. Calculated passive corrosion rates Are less than about 0.012 mm~yr and thus in excellen~
agreement wi~h immersion test results shown in Table 3.

17-21(5583)A

13~3~314 Freely Corroding Potentials (FCPs) for E-Brite 26-1 Ailoy in 100~ Sulfuric Acid -Freel~ Corrodin~ Potentials, Yolts (S.C.E.) Before Af~er Ater anodic ~cans cathodlc scan scan 143G t290F) 0.227 171C (340F) 0.408 0.408 0.390 -.: Based on the static corrosion tests of Example 1, as generally corroborated by the electrochemical tests of Example 2, the alloys tested were ranked according to their relative suitability for use as the material of construotion of ~he tubes of a heat exchanger for r~covery o~ the heat of absorp-tion from 98% to lOQ% absorber acid at temperatures of greater than 120C, in accordance with the process of the invention.
In making such ranking, ~he corrosion data was compared for the various alloys at each of the three corners of ~he heat recovery tower operating diagram as shown in Fig~ 4O Given the vicissitudes of ~tartup and process upset conditions, the rank-ing also took into account the corrosion performànce at temper-atures and concentrations adjacent but somewhat outside the operating ranges~ The ranking is set forth in Table 9, to-gether with the corrosion index fo~ each sf the listed alloys ~ as:calculated from the corr~sion index equa~ion se~ forth : ~ 2S :hereinabove. ~ ;

' ~ ~

17-21 (~083)A

~3~31~4 The dotted line on Table 9 divides the alloys con-sidered to be most suitable ~or implementation of the process o~ the invention from those which would not currently be con-sidered for commercial application, at least not for use in the tubes of the heat ~xchanger in which the heat of absorption is recovered by ~ransfer from the absorption acid to another fluid. It should be understood tha~ the corrosion rates for most of the alloys below the dotted line are reasonably satisfactory, and in fact lower ~han would have been expected prior to the corrosion testing program described in Examples l and 2. However, the alloys below the line are not currently consi~ered commercial candidates because o the even more favorable corrosion performance of the alloys listed above the dotted line. I~ should further be noted that ~he ranking set forth in Table 9 is based entirely on corrosion data and does not necessarily reflect the exact current ranking when detailed economic or fabrication considerations are taken into account.
However, regardless of whether the ranking incorporates design optimization considerations or is done strictly on the basis of corrosion results as in Table 9~ the general correlation between a corrosion index greater than 39 and suitability for use in the heat recovery process of the invention remains consistently valid.

17-21 (5083)A

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13~ 3~

A pilot heat recovery tower is insta}led ahead of tne final absorption tower in a non-interpass sulfur ~urning sul-furic acid plant. A process gas slipstream of 5.0 Nm3/min at a temperature of 260C and containing 7.5 volume percent suliur trioxide is ~ed to the heat recovery towerO Sulfuric acid with a concentration of 99 . O weight percent and a temperature of L6~C is ~ed ~o the ~9p of the heat recovery tower at a rate of 35 kg~min. The acid leaves the tower at a concentration of 99.9 weight percent and a temperature of 201C. Overall ab-sorption of sulfur trioxida from the feed gas stream is appr~ximately 96~ Acid flows by gravity from the ~ower to a pump tank, from whence it is pumped to a boiler where 0.8 - kg/min of steam is generated at a pressure of 450 kPa. Acid leaves the boiler at a temperature of 15SC and i5 diluted to a 99.0 weight percent by in-line addition of liquid water. The heat of dilution increases the acid temperature to 162C. This stream is then circulated to the top of the ~ower, completing the cycle.

EXAMPL~ 4 As described in Example 3, a pilot plant heat recovery tower was installed ahead of ~he final absorption tower in a non-interpass sulfu~-burning sulfuric acid plant.
The pilot plant tower was 0.~4 m I.D. and contained an :25 absorption zone comprising No. 2 ceramic intalox saddles with a ~acked height of 1.1 m. A process gas ~lipstream con~aining 7.8 volume percen~ sulfur trioxide at a temperature of 2~C
was fed to the bottom inlet of the heat recovery tower at a volumetric ra~e of 5.8 Nm3 per minute. Sulfuric acid having an average concentration of 9~.9~: and temperature of 161C was .

17-21 (~083)A

~3-~3~

fed to the top inlet of the tower at a rate of 50 kg/min. At steady sta~e, a sulfuric acid stream having an average concen-tration of 99.6% by weight and a temperature of 188C flowed outward through ~he bottom outlet of the tower. During passage through th~ heat rec~very tower; approximately 95% of the sul-fur trioxide con~ent of the feed gas stream was absorbed into the sulfuric acid stream. Suifuric acid leaving the tower ~lowed by gravity to a pump tank from whence it was pumped to a boiler where 1.1 kg/min of s~eam was generated at a pressure of 445 kPa. Acid le~t the boiler at a temperature of 154C, 1.9 kg/min product acid was removed, and the remaining acid ~tream was tpereafter diluted to 98.9 welght percent by inline addi-tion of liquid water prior to recycle to the tower. The heat of dilution increased the acid temperature to 161C. Corrosion probes of type 304~ stainless steel were mounted in the acid line before the boiler. The corrosion rate of type 304L
stainless steel was less ~han 0.03 mm per year.

Operation of the heat recovery process of the inven-tion was continued in the pilot plant described in Example 4,the conditions being varied to demonstrate varying temperatures in the heat recovery tower and boiler.
~ he boiler used for cooling the absorber acid in the operation of the pilot plant was a vertical shell and tube heat exchanger having a shell constructed of 310 stainless steel.
Various ~aterials were used for the tubes of the exchanger.
One tube had an O.D~ of 25 mm and was oonstructed o' ~ype 310 stainless steel, while the remainin~ 22 tubes, all 19 mm O.D., ~ere constructed of type 304L stainless steel (9 tubes), E-Bri~e alloy XM-27 t7 tubes), and Yerralium alloy 255 ~6 tubes) During pilot plant operation, absorber acid having a 11-21(5083)A

~ `3 ~ ~ ~

strength of 99-100% was passed through the shell side and boiler fee~ water through the tube side of the exchanger.
Steam was generated on the tube side and, during the two-month period of pilot plant operation, the acid temperature was varied as required to generate steam at various pressures.
Thus, the acid temperature varied from 144C to 217C at ths ~_ d inlet on the bottom of the ~oiler and between 132~C and 194C at ~he acid outlet at the top of ~he boiler. The highest st~am pressure produced was 1140 kPa corresponding to 181C at saturation. Over the course 9~ the pilot plant operation, measurements were made to determine the effect of the high temperature concentrated acid on the hea~ exchanger tubes.
Additionally, corrosion coupons constituted of various metals were installed at various points throughout the acid recirculation system. Nlne different metals were corrosion tested by means of coupons located under ~he acid distributor within the heat recovety ~ower, and in the pump tank. Pipe spools of type 304L and type 310 stainless steel were located in the acid line at points before and after the boiler and immediately downstream of the dilution sparger, i.e., the Teflon sparger through which water was introduced into a Teflon-lined pipe section in the recirculating system to adjust the acid s~rength in compensation for the sulfur trioxide absorbed Additionally, various distributor parts of type 304L
and type 310 stainless steel were tested, and a type 304L mesh pad was installed at the top of ~he tower.
After the pilot plant run was compl~ted, the heat exchanger was removed and cut up for visual inspection and observation. An attempt was made to directly measure 1QSS in 6~

17-21(,083)A

:~L3V3~

tube diameter, but these measurements proved erratic and unreliable, However, visual observations at up to 40 magnifications of the outside of the surfaces of ~he tubes revealed:

310 - uniform light etch Ferralium 255 - etch, few craters, probably at weld E-Brite 26-1 - uniformly dispersed craters 304L - uniform etch, indications of some in~ergranular attack Water-side observations also indicated that the various tube materials tested were satisfactory under the conditions prevailing. Tube sheet welds were found to be generally in satisfactory condition. There were numerous white deposits on the sufaces within the acid side o~ the exchanger. These were most often found on the tubes at the baffle areas, on the tops of the ba~fles and on the top of weld beads conneoting the baffles to the positioning rods. A tube with such deposits on it was analy2ed by X-ray 1uorescence. Areas with a~d without deposits were compared. The only difference ~ound was in sulfur content, thus indicating that the deposits were primarily sulfates.
Using ~he standard NACE test, the corrosion rate on the Yarious corrosion coupons was determined. Comparable techniques were utilize~ to determl~e ~he corrosion rates on the pipe spools, distributors, and type 304L stainle~s steel mesh pad. The results of th~se corrosion measurements are set orth in~Table 10.

17-21 ( ~0~3) A

~3~3~4 ~q O O
U~
tn ~ ~ ~ ~: ~ ~C -' ~ : ~^
~ ~ z ~
E~ O u~u~ OOO~ 0 : g ~ ~ C~ ~ O ~ _l ~ ~; u ~ m ~
o :~E X~:

~ ~ r~ o ~
E~ ~ . . . . . ~ . u~
:: D o o o _I o 8 co ~ ~
Z ~ N N

;: ~ :
: :::: : ~n E3: ~ ~: : : ~
_I V V ~O ~ , ~ O er a~ ~S ~.1 ~ _I o o ~ '-P o ::

: . - O U~

17-21(~083)A

~3~38~L4 ~ CC
8 -- ~ ~ ~ _, ~
U~ ~ ,¢ ,, ~ _, o ,, I ¦ 2 ~:
c ~ E~ ~ . - - -O kl E~

"~ 2 ~N`

~ ~3 ~ ~ ~ ~ : O
E~ ~ ~ o o ~r O o ~ _I O O
r ~ ~ ~ ~

~ O

' ~ ~ ~, ~ , r;

13U3~

Additional data on the corrosion rate of type 304L stainless steel was taken during the pilot plant run using an electro-chemical device (~CorraterR*sold by Rohrback Instruments,Division of Rohrback Cocporation, 11861 E. Telegraph Rd., Santa Fe Springs, CA 90670) for measuring corrosion rate. -This de-vice was installed in the acid stream on the inlet end of the boiler. Conditions at the inlet side varied as the pilot plant run progressed, and there was also some variation in the cor-rosion rates measured with the Corrater~* Overall, however, the results were favorable. For operation at temperatures varying from 144F to 217F and acid concentrations varying from 99.2% to 100.9~ the Corrater indicated a mean corrosion rate of 0.03 mm/yr, with a standard deviation of 0.03 mm/yr.
-A heat recovery tower is installed ahead of the interpass absorption tower in a sulfur burning sulfuric acid plant.
A process gas stream of 2914 Nm3 per min. at atemperature of 166C containing 11.8 volume percent sulfur trioxide is fed to the bottom of the heat recovery tower.
Sulfuric acid having a concentration of 98.6~ by weight and a temperature of 168C is fed to the top of the tower at a rate of 22670 kg per min.
The acid leaves the tower at a concentration of 99.8 weight percent and a temperature of 198C. Overall absorption of sulfur trioxide from the feed gas stream is approximately 97~.
Acid leaving the tower flo~s by gravity to a pump tank, from whence it is pumped to a boiler where approximately 830 kg~min. of steam is generated at a pressure of 377 kPa.

* ~rade Mark 17-21~5083)A

13~3~

Acid leaves the boiler at a temperature of 152C and, after product removal, is diluted to gB.6 weight percent by in-line addition of liquid water. The heat of dilution increases the acid temperature to 16BC. This ~tream is then recirculated to the top of the tower, completing the cycle, A heat recove~y towe~ is intalled ahead of the interpass absorp~ion tower in an existing sulfur burning sulfuric acid plant.
A process gas stream of 2067 Nm3/min at ~
temp~rature of 232C containing 10.~ volume percent sulfur trioxide is fed to the bot~om o the heat recovery tower.
Sulfuric acid with a concentration of 99.1 weight percent and a temperature of 206C is ~ed to the top of the tower at a rate of 43157 kg/min.
The acid leaves the tower at a concentration of 99.5 weight percent and a temperature of 215C. Overall absorption of sulur trioxide from the feed gas stream is approximately 89%.
Acid leaving the ~ower flows by gravity to a pump tank; from whence it is pumped to a boiler where approximately 507 kg/min of s~eam is generated at a pressure of 1342 kPa.
Acid leaves the boiler at a temperature of 201C and, after product removal, is dilu~ed to 99.1 weight percent by in-}ine addition of liquid water. The heat of dilution increases the acid temperature to 206C. This stream is then recirculated to the ~op o the tower, completing the cycle.

- 1 7 - 2 1 ( 5 0 8 3 ) A

~3~3B14 EXAMI:'LE 8 A heat recovery ~ower is installed ahead of the final absorption tower in a non-interpass metallurglcal sulfuric asid plant.
A process gas stream of 2470 Nm3/min a~ a tempera-ture of 232C containing 9.8 volume percent sulfur trioxide is fed ~o ~he bottom of the hea~ recovery tower. Sulfuric acid with a concentration o~ 98.6 weigh~ percent and a ~emperature of 162C is fed ~o the top o~ the towPr at a rate of 16242 kg/min.
The acid leaves ~he tower at a concentration of 99.8 weig~t percent and a temperature of 206C. Overall absorption of sulur trioxide from the feed gas stream is approximately 97%, Acid leaving the tower flows by gravity to a pump tank where it is diluted to 99.3 weight percent. Dilution is accomplished by addition o~ 1375 kg/min. of 66C, 93.0 weight percent acid.
Ater dilution in the pump tank the resulting acid enters a boiler at 196C where approximately 641 kg/min. of steam is generated at a pressure o~ 377 kPa.
Acid leaves the boiler at a temperature of 153C and, after product removal, is diluted to 98.6 weight percent by in-line additisn of 112 kg/mm of 141C water containing 1.2%
steam. The heat of dilution inceases the acid tempera~ure to 162~C. This stream is ~hen cecirculated to the top of the tower, completing the cycle.

A heat recove~y ~ower is installed ahead of the interpass absorption tower in an exis~ing Culfur burning sul~uric acid plant.

17-21 (~083)A

:~ 3~

A process gas stream of 2542 Nm3/min. at a tempera-ture of 154C contaiing 11.8 volume percent sulfur trioxide is fed ~o the bottom of the heat recovery tower. Sulfuric acid with a concentration of 98.6 weight percent and a temperature of 168C is fed to the top of the tower at a rate of 19871 kg/min.
The acid leaves the tower at a concentration of 99.8 weight percent and a temperature of 197C. Overall absorption of sulfur trioxide from ~he feed gas str~am is approximately 97~.
Acid leaving the tower ~lows by gravity to a pump tank~ from whence it is pumped o a boiler where approximately 691 kg/min. of steam is genera~ed at a pressure o 377 kPa.
- Acid leaves the ~oiler at a temperature of 152C and, after product removal is diluted to g8.6 w~ight percent by in-line additon of liquid water. The heat of dilution increases the acid temperature to 168C. This stream is then recirculated to the top of the tower, completing the cycle.

Example 10 A heat recovery tower is installed ahead of the interpass absorption tower in a sulfur burning sulfuric acid plant.
A process gas stream of 2969 Nm3/min. at a temperature of 162C containing 11~6 volume percent sulfur trioxide is fed to the bottom of the heat recovery tower.
Sulfuric acid with a concentration of 98.8 weight percent and a temperatur~ o~ 206C is fed to the top of the tower at a rate of 29293 kg/min.

17-21 (~083) A

3~3~3~

The acid leaves the tower at a concentration of 99.7 weigh~ percent and a temperature of 222C. Overall absorption of sulfur trioxide from the feed gas stream is approximately 90~ .
Acid leaving the tower flows by g~avity to a pump ~ank, from whence it is pumped to a boiler where approximately 710 kg/min. of steam is generated at a pressure o~ 1136 kPa.
Acid leaves the boiler at a temperature of 194C and, after product removal, is diluted to S8.8 weight percent by in-line addition of liquid water. The hea~ of dilution incr~ases the acid temperature to 206C. Thi~ s~ream is then recir'culated to the op of the tower, comple~ing the cycle.
-

Claims (54)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for the manufacture of sulfuric acid, comprising the catalytic oxidation of sulfur dioxide to sulfur trioxide, absorption of the sulfur trioxide in sulfuric acid, and cooling the sulfuric acid in a heat exchanger by transfer of heat to another fluid, the improvement which comprises:

contacting a gas comprising sulfur trioxide with sulfuric acid having a concentration of between 98% and 101%, thereby absorbing sulfur trioxide in the sulfuric acid and generating the heat of absorption, the absorp-tion being carried out in a heat recovery absorption zone to which said sulfuric acid is delivered at a concentration of at least 98% and a temperature of at least 120°C, and from which said sulfuric acid is discharged at a concentration of at least 99% and a temperature greater than 120°C; and removing said heat of absorption from said sulfuric acid in useful form by transfer of heat to another fluid in said heat exchanger, and thereby heating said another fluid to a temperature greater than 120°C, said sulfuric acid having a temperature greater than 120°C and a concentration of at least 99% throughout the course of transfer of heat to said another fluid.

2. An improved process as set forth in claim 1 wherein said sulfuric acid leaves said zone in a stream having a temperature greater than 120°C, said sulfuric acid stream is passed through said heat exchanger, and at least a portion of the cooled acid stream is recycled to said heat recovery zone.

3. An improved process as set forth in claim 2 wherein said sulfuric acid is diluted during recirculation to a strength greater than 98% and up to about 99.3%.

4. An improved process as set forth in claim 2 wherein dilution is effected by adding a diluent selected from a group consisting of liquid water, steam, and relatively dilute sulfuric acid to said circulating stream after heat transfer to said another fluid and before entry of said acid into said zone.

5. An improved process as set forth in claim 1 wherein said heat transfer comprises means for indirect transfer of heat from said sulfuric acid to said another fluid.

6. An improved process as set forth in claim 1 wherein said sulfuric acid has a temperature greater than 120°C and less than about 250°C as it exits said zone.

7. In a process for the manufacture of sulfuric acid, comprising the catalytic oxidation of sulfur dioxide to sulfur trioxide, absorption of the sulfur trioxide in sulfuric acid, and cooling the sulfuric acid in a heat exchanger by transfer of heat to another fluid, the improvement which comprises:

contacting a gas comprising sulfur trioxide with sulfuric acid having a concentration of between 98% and 101%, thereby absorbing sulfur trioxide in the sulfuric acid and generating the heat of absorption r the absorption being carried out in a heat recovery absorption zone to which said sulfuric acid is delivered at a concentration of at least 98% and a temperature of at least 120°C, and from which said sulfuric acid is discharged at a concentration of at least 99% and a temperature greater than 120°C; and removing said heat of absorption from said sulfuric acid in useful form by transfer of heat to another fluid in said heat exchanger, thereby heating said another fluid to a temperature greater than 120°C, said sulfuric acid having a temperature greater than 120°C and a concentra-tion of at least 99% throughout the course of heat transfer to said another fluid, said another fluid as introduced into said heat exchanger comprising a liquid, and transfer of heat to said another fluid resulting in vaporization of said another fluid so that said another fluid comprises a vapor as it exits said exchanger.

8. An improved process as set forth in claim 7 wherein said another fluid consists essentially of a vapor as it exits said exchanger.

9. An improved process as set forth in claim 8 wherein said another fluid comprises water, and steam is generated in said heat exchanger.

10. An improved process as set forth in claim 7 wherein said heat exchanger comprises means for indirect transfer of heat from said sulfuric acid to said another fluid.

11. An improved process as set forth in claim 7 wherein said heat exchanger comprises means for transfer of heat from said sulfuric acid to said another fluid, said means being constituted of an alloy selected from the group consisting of ferrous alloys containing chromium, iron-nickel alloys containing chromium, and nickel alloys containing chromium, said alloy having a ferritic, austenitic or duplex structure, and the composition of said alloy further satisfying the following relationship:
0.35(Fe+Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) > 39 where:

Fe = the weight percent of iron in the alloy, Mn = the weight percent of manganese in the alloy, Cr = the weight percent of chromium in the alloy, Ni = the weight percent of nickel in the alloy, and Mo = the weight percent of molybdenum in the alloy.

12. An improved process as set forth in claim 7 wherein gas exiting from said heat recovery absorption zone is passed through another absorption zone where it is contacted with sulfuric acid for further absorption of sulfur trioxide and condensation of sulfuric acid vapor contained in said gas.

13. An improved process as set forth in claim 7 wherein a stream of sulfuric acid and a stream of gas comprising sulfur trioxide are passed countercurrently through said zone.

14. In a process for the manufacture of sulfuric acid comprising the catalytic oxidation of sulfur dioxide to sulfur trioxide, absorption of the sulfur trioxide in sulfuric acid, and cooling the sulfuric acid in a heat exchanger by transfer of heat to another fluid, the improvement which comprises:

contacting a gas comprising sulfur trioxide with sulfuric acid having a concentration between 98% and 101%, thereby absorbing sulfur trioxide in the sulfuric acid and generating the heat of absorption, the absorp-tion being carried out in a heat recovery absorption zone to which said sulfuric acid is delivered at a concentration of at least 98% and a temperature of at least 120°C, and from which said sulfuric acid is discharged at a concentration of at least 99% and a temperature greater than 120°C; and removing the heat of absorption from said sulfuric acid in useful form through transfer of heat to another fluid in said heat exchanger and thereby heating said another fluid to a temperature greater than 120°C, said heat exchanger comprising means for transfer of heat from said sulfuric acid to another fluid, said means compris-ing an alloy selected from the group consisting of ferrous alloys containing chromium, iron-nickel alloys containing chromium, and nickel base alloys containing chromium, said alloy having a ferritic, austenitic or austenitic-ferritic duplex structure, the composition of said alloy further satisfying the following relation-ship:
0.35(Fe+Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) > 39 where:

Fe = the weight percent of iron in the alloy, Mn = the weight percent of manganese in the alloy, Cr = the weight percent of chromium in the alloy, Ni = the weight percent of nickel in the alloy, and Mo - the weight percent of molybdenum in the alloy.

15. An improved process as set forth in claim 14 wherein said heat transfer means comprises means for indirect transfer of heat from said sulfuric acid to said another fluid.

16. An improved process as set forth in claim 14 wherein the gas leaving said heat recovery absorption zone is contacted with sulfuric acid in another absorp-tion zone, thereby effecting further absorption of sulfur trioxide and condensation of sulfuric acid vapor contained in said gas.

17. An improved process as set forth in claim 14 wherein said sulfuric acid and said gas are passed countercurrently through said zone.

18. An improved process as set forth in claim 14 wherein said alloy is a ferrous or iron-nickel alloy containing between about 16% and about 30% by weight chromium, up to about 33% by weight nickel, up to about 6% by weight molybdenum, and between about 35% and about 83% by weight iron.

19. An improved process as set forth in claim 14 wherein said alloy is a nickel alloy containing between about 32% and about 76% by weight nickel, between about 1% and about 31% by weight chromium, between about 2 and about 46% by weight iron, and up to about 28% by weight molybdenum.

20. In a process for the manufacture of sulfuric acid comprising the catalytic oxidation of sulfur dioxide to sulfur trioxide, absorption of the sulfur trioxide in sulfuric acid, and cooling the sulfuric acid in a heat exchanger by transfer of heat to another fluid, the improvement which comprises:

contacting a gas comprising sulfur trioxide with sulfuric acid having a concentration between 98% and 101%, thereby absorbing sulfur trioxide in the sulfuric acid and generating the heat of absorption, the absorp-tion being carried out in a heat recovery zone to which sulfuric acid is delivered at a concentration of at least 98% and a temperature of at least 120°C, and from which sulfuric acid is discharged at a concentration of at least 99% and a temperature greater than 120°C;

removing the heat of absorption from said sulfuric acid in useful form through transfer of heat to another fluid in said heat exchanger and thereby heating said another fluid to a temperature greater than 120°C; and contacting the exit gas from said heat recovery zone with sulfuric acid in another absorption zone for removal of residual sulfur trioxide and condensation of sulfuric acid vapor from said exit gas before either exhausting said exit gas from the process or cataly-tically oxidizing sulfur dioxide contained therein to produce additional sulfur trioxide, the temperature of the sulfuric acid at the gas exit from said another absorption zone being at least about 10°C lower than the temperature of the sulfuric acid at the inlet of said heat recovery zone.

21. In a process for the manufacture of sulfuric acid comprising the catalytic oxidation of sulfur dioxide to sulfur trioxide, absorption of the sulfur trioxide in sulfuric acid and cooling the sulfuric acid in a heat exchanger by transfer of heat to another fluid, the improvement which comprises:

contacting a gas comprising sulfur trioxide with sulfuric acid, said sulfuric acid having a concentration between 98% and 101% and a temperature greater than 125°C, thereby generating the heat of absorption; and removing said heat of absorption from said sulfuric acid in useful form through heat exchange with another fluid in said heat exchanger, and thereby heating said another fluid to a temperature greater than 125°C.

22. An improved process as set forth in claim 21 wherein sulfur trioxide is absorbed into sulfuric acid in a heat recovery zone, said sulfuric acid leaves the heat recovery zone in a stream having a temperature greater than 125°C, said sulfuric acid stream is passed through a heat exchanger in which heat is transferred from said sulfuric acid to said another fluid, and said sulfuric acid is circulated between said heat recovery zone and said heat exchanger.

23. An improved process as set forth in claim 22 wherein said sulfuric acid is diluted during circulation to a concentration greater than 98% and up to about 99.3%.

24. An improved process as set forth in claim 23 wherein dilution is effected by adding a diluent selected from the group consisting of liquid water, steam, and relatively dilute sulfuric acid to said circulating stream after heat transfer to said another fluid and before entry into said heat recovery zone.

25. A method as set forth in claim 22 wherein the concentration of said circulating stream is maintained at greater than 98.5% and less than 101% throughout said heat recovery zone.

26. An improved process as set forth in claim 22 wherein the concentration of said circulating stream is maintained at greater than 99% and less than 101%
throughout said heat exchanger.

27. An improved process as set forth in claim 22 wherein said another fluid as introduced into said heat exchanger comprises a liquid and transfer of heat to said another fluid results in vaporization of said another fluid so that said another fluid comprises a vapor as it exits said exchanger.

28. An improved process as set forth in claim 27 wherein said heat exchanger comprises means for the transfer of heat between said acid and said another fluid, said heat transfer means being constituted of an alloy selected from the group consisting of ferrous alloys containing chromium, iron-nickel alloys contain-ing chromium, and nickel alloys containing chromium, said alloy having an austenitic, ferritic or duplex structure.

29. A method as set forth in claim 22 wherein said acid stream and said sulfur trioxide are passed counter-currently through said zone.

30. An improved process as set forth in claim 22 wherein the acid stream and sulfuric acid are passed cocurrently through the zone.

31. An improved process as set forth in claim 22 wherein said sulfuric acid has a temperature greater than 125°C and less than about 250°C as it exits said heat recovery zone.

32. An improved process as set forth in claim 22 wherein the gas stream exiting said heat recovery zone is passed through another sulfuric acid absorption zone for removal of sulfur trioxide and condensation of sulfuric acid vapor from said gas stream, said another absorption zone being operated with an inlet sulfuric acid temperature lower than 125°C.

33. An improved process as set forth in claim 32 wherein said another absorption zone comprises an interpass tower and the gas stream exiting said another contains sulfur dioxide and is delivered to a catalytic converter stage for oxidation of sulfur dioxide to sulfur trioxide.

34. In a process for the manufacture of sulfuric acid by catalytic oxidation of sulfur dioxide to sulfur trioxide and absorption of the sulfur trioxide in a sulfuric acid medium, the improvement which comprises:

contacting a catalytic oxidation reaction gas containing sulfur trioxide in an absorption zone with sulfuric acid having a strength between 98% and 101% and a temperature of at least about 125°C, whereby sulfur trioxide is absorbed by the sulfuric acid thereby generating heat in the sulfuric acid phase; and recovering the heat of absorption in useful form through transfer of heat from the sulfuric acid to another fluid, the temperature of said acid being greater than 125°C throughout the course of heat transfer to said another fluid.

35. An improved process as set forth in claim 34 wherein said another fluid is heated to a temperature greater than about 125°C.

36. An improved process as set forth in claim 35 wherein said sulfuric acid is maintained at a tempera-ture greater than about 130°C throughout the course of heat transfer to said another fluid and said another fluid is heated to a temperature greater than about 130°C.

37. An improved process as set forth in claim 36 wherein said sulfuric acid is maintained at a tempera-ture greater than about 135°C throughout the course of heat transfer to said another fluid and said another fluid is heated to a temperature greater than about 135°C.

38. An improved process as set forth in claim 36 wherein said sulfuric acid is maintained at a tempera-ture greater than about 140°C throughout the course of heat transfer to said another fluid and said another fluid is heated to a temperature greater than about 140°C.

39. An improved process as set forth in claim 38 wherein said sulfuric acid is maintained at a tempera-ture greater than about 150°C throughout the course of heat transfer to said another fluid and said another fluid is heated to a temperature greater than about 150°C.

40. An improved process as set forth in claim 39 wherein said sulfuric acid is maintained at a tempera-ture greater than about 175°C throughout the course of heat transfer to said another fluid and said another fluid is heated to a temperature greater than about 175°C

41. An improved process as set forth in claim 40 wherein said sulfuric acid is maintained at a tempera-ture greater than about 200°C throughout the course of heat transfer to said another fluid and said another fluid is heated to a temperature greater than about 200°C.

42. A method for storage, transportation, or handling of sulfuric acid having a concentration between 98% and 101% and a temperature above 120°C comprising containing said sulfuric acid within a conduit or vessel constituted of an alloy selected from the group consist-ing of ferrous alloys containing chromium, iron-nickel alloys containing chromium, and nickel alloys containing chromium, said alloy having an austenitic, ferritic, or duplex structure, the composition of said alloy further corresponding to the relationship:

0.35(Fe+Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) > 39 where:

Fe = the weight percent iron in the alloy, Mn = the weight percent manganese in the alloy, Cr = the weight percent chromium in the alloy, Ni = the weight percent nickel in the alloy, and Mo = the weight percent molybdenum in the alloy.

43. A method for condensing sulfuric acid vapor from a gas stream saturated with sulfuric acid vapor at a temperature greater than 120°C, comprising bringing said gas stream into contact with a heat transfer surface having a temperature lower than the saturation temperature of said gas stream with respect to sulfuric acid vapor but greater than 120°C and high enough so that the sulfuric acid condensing on said surface has a concentration greater than 98.5%, said heat transfer surface being comprised of an alloy selected from the group consisting of ferrous alloys containing chromium, iron-nickel alloys containing chromium, and nickel alloys containing chromium, said alloy having an austenitic, ferritic, or duplex structure, the composi-tion of said alloy further corresponding to the rela-tionship:
0.35(Fe+Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) > 39 where:

Fe = the weight percent of iron in the alloy, Mn = the weight percent of manganese in the alloy, Cr = the weight percent of chromium in the alloy, Ni = the weight percent of nickel in the alloy, and Mo = the weight percent of molybdenum in the alloy.

44. In a process for the manufacture of sulfuric acid comprising the catalytic oxidation of sulfur dioxide to sulfur trioxide, absorption of the sulfur trioxide in sulfuric acid by contacting the sulfur trioxide with sulfuric acid in an absorption zone, cooling the sulfuric acid in a heat exchanger by transfer of heat to another fluid, and circulating at least a portion of the cooled sulfuric acid back to the inlet of the absorption zone, whereby an acid circulating loop is established comprising said absorption zone and said heat exchanger, the improvement which comprises:
contacting a gas comprising sulfur trioxide in a heat recovery absorption zone with sulfuric acid having a concentration of between 98% and 101%, thereby absorbing sulfur trioxide in the sulfuric acid and generating the heat of absorption, sulfuric acid being delivered to said heat recovery zone at a concentration of at least 99% and a temperature of at least 120°C;
removing the heat of absorption from said sulfuric acid in useful form through transfer of heat to another fluid in a heat exchanger in said circulating loop, thereby heating said another fluid to a temperature greater than 120°C, said sulfuric acid having a temperature greater than 120°C and a concentration greater than 99% throughout the course of heat transfer to said another fluid;
removing a portion of the acid exiting said circulating loop heat exchanger as a discharge stream from said circulating loop; and recovering energy from said discharge stream by passing said stream through a second heat exchanger.

45. An improved process as set forth in claim 44, wherein heat is transferred from said discharge stream to said another fluid in said second heat exchanger, and thereafter said another fluid is passed through the heat exchanger in said circulating loop for removal of heat of absorption from the circulating acid.

46. An improved process as set forth in claim 45, wherein said another fluid as introduced into said circulating loop heat exchanger comprises a liquid, and transfer of heat to said another fluid results in vaporization of said another fluid so that said another fluid comprises a vapour as it exits said circulating loop heat exchanger.

47. An improved process as set forth in claim 46, wherein said another fluid consists essentially of a vapour as it exits said circulating loop heat exchanger.

48. An improved process as set forth in claim 47, wherein said another fluid as introduced into said second heat exchanger comprises boiler feed water, said boiler feed water is preheated in said second heat exchanger, and steam is generated by vaporizing the preheated boiler feed water in said circulating loop heat exchanger .

49. An apparatus for use in the recovery of the heat of absorption in a process for the manufacture of sulfuric acid comprising:
a vessel:

a primary absorption zone within said vessel in which sulfur trioxide is absorbed in a sulfuric acid stream, said primary absorption zone comprising contact means for contacting a gas stream containing sulfur trioxide with sulfuric acid and promoting mass transfer between the gas and liquid phases to effect absorption of sulfur trioxide from the gas stream into the sulfuric acid;
inlet means below said primary absorption zone for inflow to the primary absorption zone of said sulfur trioxide;
exit means associated with said vessel for discharge of gas exiting from said vessel;
inlet means for delivery of sulfuric acid to said vessel for passage through said primary absorption zone;
a secondary absorption and cooling zone within said vessel above said primary absorption zone, said secondary absorption zone comprising means for contacting gas exiting from said primary absorption zone with a relatively cool sulfuric acid stream and promoting mass and heat transfer between the gas and liquid phases for cooling the gas exiting from said primary absorption zone and removing vapour phase sulfuric acid and additional sulfur trioxide therefrom; and outlet means for discharging absorption acid from said vessel.

50. Apparatus as claimed in claim 49, said apparatus including heat exchange means for recovering heat from discharged absorption acid.

51. Apparatus as claimed in claim 49, said apparatus including heat exchange means for recovering heat from discharged absorption acid, wherein said heat exchange means is constructed of an alloy having the following relationships:
0.35(Fe+Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) > 39 where: Fe = the weight percent of iron in the alloy Mn = the weight percent of manganese in the alloy Cr = the weight percent of chromium in the alloy Mo = the weight percent of molybdenum in the alloy.

52. An apparatus as set forth in claim 51, wherein said alloy comprises a ferrous or iron nickel alloy and contains between about 16% and about 30% by weight chromium, up to about 33% by weight nickel, up to about 6%
by weight molybdenum, and between about 35% and about 83%
by weight iron.

53. An apparatus as set forth in claim 51, wherein said alloy comprises a nickel alloy and contains between about 32% and about 76% by weight nickel, between about 1% and about 31% by weight chromium, between about 2%
and about 46% by weight iron, and up to about 28% by weight molybdenum.

54. An apparatus for use in the recovery of the heat of absorption in a process for the manufacture of sulfuric acid comprising:
a vessel;
an absorption zone within said vessel in which sulfur trioxide is absorbed in a sulfuric acid stream, said absorption zone comprising contact means for contacting a gas stream containing sulfur trioxide with sulfuric acid and promoting mass transfer between the gas and liquid phases to effect absorption of sulfur trioxide from the gas stream into the sulfuric acid;
inlet means below said absorption zone for inflow to the absorption zone of said sulfur trioxide;
exit means associated with said vessel for discharge of gas exiting from said vessel;
inlet means for delivery of sulfuric acid to said vessel for passage through said absorption zone;
outlet means for discharging absorption acid from said vessel after passage through said absorption zone; and heat exchange means for recovering heat from discharged absorption acid, wherein said heat exchange means is constructed of an alloy having the following relationships:
0.35 (Fe+Mn) + 0.70(Cr) + 0.30(Ni) - 0.12(Mo) > 39 where Fe = the weight percent of iron in the alloy Mn = the weight percent of manganese in the alloy Cr = the weight percent of chromium in the alloy Mo = the weight percent of molybdenum in the alloy.