US3583895A - Evaporation using vapor-reheat and multieffects - Google Patents

Abstract

VAPOR-REHEAT MULTISTAGE FLASH (MSF) EVAPORATION MAY BE ADVANTAGEOUSLY COMBINED WITH THE USUAL EVAPORATION ONE ONE SIDE OF A METAL SURFACE, OFTEN OPERATING IN MULTIEFFECT (ME) UNITS. IN COMBINING VAPOR REHEAT MSF WITH A ME TO GIVE A HYBRID EVAPORATOR, SOME OF THE STEAM FROM A FLASH STAGE IS PASSED TO CLOSED CONDENSATION IN A HEATING UNIT OR REBOILER BUILT INTO OR COMBINED WITH A VAPORIZING ZONE OF ONE OF THE LOWER STAGES. STEAM ARISING FROM THAT STAGE THUS COMES FROM THE FLASH EVAPORATION DUE TO THE SENSIBLE HEAT OF THE LIQUID COMING FROM THE NEXT HIGHER STAGE, PLUS THE SURFACE EVAPORATION DUE TO THE LATENT HEAT OF THE STEAM COMING FROM A HIGHER STAGE. THIS ARRANGEMENT MAY BE COMPOUNDED; AND THE ME SURFACEEVAPORATOR MAY BE WORKED INTO THE VAPOR-REHEAT MSF, TO OBTAIN, AT LEAST IN PART, THE ADVANTAGES OF EACH. USUALLY THERE IS A LESSER NUMBER OF EFFECTS THAN STAGES, SINCE THE SUM OF THE SMALLER TEMPERATURE DROPS OF TWO OR MORE STAGES MAY BE NECESSARY TO GIVE AN ADEQUATE TEMPERATURE DROP FOR ONE EFFECT.

Description

EVAPORATION USING VAPOR-REHEAT AND MULTIEFFECTS Filed May 20, 1969 D. F. OTHMER Juhe 8, 1911 4 Sheets-Sheet 1 INDIRECT HEAT EXCHANGE SEA WATER SEA WATER INDIRECT HEAT I NVENT 0R.

DONALD F. OTHM ER June '8, 1971 n. F. OTHMER 3,583,895

EVAPORATION USING VAPOR-REHEAT AND MULTIEFFECTS Filed May 20, 1969 FIG?) SEA WATER FEED FLUID FUEL AND OXYGEN s2 SUBMERGED I comausnou 4 Sheets-Shoot 2 INTERMEDIATE HEAT EXCHANGE SEA WATER I NVEN'IUR.

DONALD F. 0TH MER June 8, 1971 Filed May 20, 1969 FIGS ORGANIC MATERIALS IN ORIGINAL SOLUTION SEA WATER FEED f 2| k 20 C -i FLUID FUEL AND OXYGEN] D. F. OTHMER EVAPORATION USING VAPOR-REHEAT AND MULTIE FFECTS ISUBMERCED COMBUSTION 4 Sheets-Sheet s INDIRECT HEAT I I I I I I SUBMERGED l COMBUSTION INVENTOR.

DONALD F. OTHMER June 8, 1971 D. F. OTHMER 3,583,895

EVAPORATION USING VAPOR-REHEAT AND MULTIEFFECTS Filed May 20, 1969 4 Sheets-Sheet 4.

I FIG? SEA WATER INDIRECT HEAT FEED EXCHANGE l7 I N VENIOR DONALD F. OTHMER United States Patent Oflice 3,583,895 Patented June 8, 1971 Claims ABSTRACT OF THE DISCLOSURE Vapor-reheat multistage flash (MSF) evaporationmay be advantageously combined with the usual evaporation on one side of a metal surface, often operating in multielfect (ME) units. In combining Vapor Reheat MSF with a ME to give a Hybrid Evaporator, some of the steam from a flash stage is passed to closed condensation in a heating unit or reboiler built into or combined with a vaporizing zone of one of the lower stages. Steam arising from that stage thus comes from the flash evaporation due to the sensible heat of the liquid coming from the next higher stage, plus the surface evaporation due to the latent heat of the steam coming from a higher stage. This arrangement may be compounded; and the ME surfacee'vaporator may be worked into the vapor-reheat MSF, to obtain, at least in part, the advantages of each. Usually there is a lesser number of effects than stages, since the sum of the smaller temperature drops of two or more stages may be necessary to give an adequate temperature drop forone effect.

This is a continuation in part of my application Ser. No. 639,989 filed May 22, 1967 and issued as U.S. Pat. No. 3,446,712 on May 27, 1969, which was a continuation in part of my application Ser. No. 252,473, filed'lan. 18, 1963, issued July 4, 1967, as U.S. Pat. 3,329,583, both entitled Method for Producing Pure Water from Sea Water and Other Solutions by Flash vaporization and Condensation.

This invention relates to a combination or Hybrid evaporation process for removing a part of the water in dilute aqueous solution such as, particularly, sea water, waste pulp liquors, sewage and sludges therefrom, using both: (a) vapor-reheat multiple stage flash (MSF) evaporation, with an open condensation of the vapors; and (b) surfaceboiling by heat transfer through a metal wall to give vapors, which, by repeating the'operation, gives multiple effect (ME) evaporation. It has been found possible to interwork these two systems to use a minimum of heat in evaporation between the highest and the lowest temperatures available in'the system.

More specifically, the method flash evaporates a part of the aqueous solution'in a series of steps which may comprise, besides the steps of the usual ME system, and of the vapor reheat MSF evaporation, as described in above mentioned U.S. Pat. 3,329,583, some of the following:

(a) Evaporating a small amount of the dilute solution in the prime heater; the steam formed being passed to what is described in U.S. Pat. 3,329,583 as a half-stage to heat the recycle stream of condensate to a higher temperature than any of the flash evaporations;

(b) Supplying external heat to the recycle stream of condensate water leaving the top condensating zone of the vapor reheat system before passing to the heat exchanger in a half-stage, so that this prime heat, as well as that absorbed in the ladder of condensing zones, is transferred to the original solution by the heat exchanger as described in U.S. Pat. 3,288,668;

(c) Supplying external heat directly to the dilute solution in the prime heater, or to the recycle stream of condensate water in a half-stage by a submerged combustion, using added oxygen, of an organic material in the solution or of an added fluid fuel.

(d) Cooling the heated stream of fresh water condensate while preheating the dilute solution before entering the prime heater bya direct contact liquid-liquid-liquid heat exchanger, or by a heat exchanger which uses multiple flash evaporation for cooling. Both are described in U.S. Pat. 3,329,583;

(e) Withdrawing a part or all of the vapors formed in one of the lower of the MSF evaporation, compressing them to a pressure at least as high as that of they prime heater or the half-stage, to use as the source of Prime Heat, as described in U.S. Pat. 3,288,686. Either the mechanical cycle of compression, or the absorption cycle of compression described in that patent may be used;

(f) Withdrawing steam from one or more stages above the bottom one, possibly also from the prime heater; passing each steam flow to heat by closed condensation heat transfer, the dilute liquid on a stage of lower temperature, so as to combine Vapor Reheat and multiple effect in a Hybrid evaporation process.

Water is the usual solvent in dilute solutions or suspensions of the fine solids which is to be purified either by distillation or by chemical action at elevated temperatures on impurities after heating and before cooling by the method herein described. However, other solvents may also be purified in the same manner. Sea water often is used as a type of dilute aqueous solution, but the method is not limited to it or to other saline waters, although sea water may be referred to hereinafter as a general example of a dilute solution. Wastes such as sewage or flowable sludges coming from sewage treatment, black liquors from pulping wood to make paper, and other wastes from food and other industries may be handled. The production of distilled or twice-distilled water, of Water purified by removal of impurities therefrom, of concentrated solutions, of products separated after chemical treatment, of steam for other uses, and of power-all are possible with different embodiments of this method.

Solutions and suspensions in flowable form of inorganic, or of organic materials, or of both, may be processed by methods to be described; particularized for the liquid to be processed, the reasons for processing, and the products desired.

Hereinafter, recycling of solutions being evaporated in the flash stages is not considered, although it usually reduces heat requirements of evaporation both by the conventional MSF, and by vapor-reheat MSF. This may be a saving of as much as 25% to 40% if only a few stages are used in many flow sheets. However, the advantages of the present invention improve the operation without recycling; and a correct balance of the relative amounts of condensate formed in the vapor-reheat MSF and in the ME surface may give a minimum thermal requirement without any recycle.

The multiple-stage flash (MSF) evaporation process, described in U.S. 3,329,583 and elsewhere, has no metallic heat transfer surfaces for condensing the vapors formed during the cooling of heated sea water accomplished in the multiple flash vaporizations. These vapors are used, not to preheat directly the stream of solution being handled, as in the usual multiple flash evaporator; but instead, the vaporous heat is used to reheat a cycling stream of colder fresh water condensate (sometimes called distillate) by direct contact of the vapors with and condensation of them on the surface of the liquid of the fresh water stream. There is a ladder of stages with hot sea water descending one side as it cools by flash vaporization in each stage; and colder pure water condensate is ascending the other side to be heated correspondingly by flash or open condensation. Vapors pass from hot brine to colder condensate across each stage; thus the metallic heat transfer surface for condensation is eliminated.

Inherently, this, the vapor reheat process, is economic of heat, particularly with an optimization of the operating variables, as: (a) number of stages, (b) inlet top temperature of sea water feed, (c) ratio of fresh water to sea water fed to the ladder, and (d) ratio of amount of sea water recycled to the prime heater. Minimum heat requirements may be in the low range of 40 to 100 B.t.u. per pound of fresh water produced by this system, depending on the variables, even without the present improvements which reduce these requirements still lower, as noted hereinafter.

Vapor reheat has a fundamental thermal advantage. The condensate formed on each stage is added to the cycling fresh water stream at its temperature, thus securing an additional cooling and condensation on each higher temperature stage. All sensible heat so accumulated in this stream is then recovered in the single heat exchanger in preheating the feed sea water.

FIG. 1 is a schematic flow sheet of a Hybrid Evaporator as defined a'bove. Here is heat reject and a half-stage in a vapor reheat evaporator combined with a built-in and interconnected multiple effect evaporator having the heat transfer surfaces of its eifects adding evaporation on the flash stages.

FIG. 2 is a flow sheet similar to FIG. 1 and illustrating an alternate manner of transferring heat from an external source to an original solution;

FIG. 3 is a flow sheet similar to FIG. 1 and illustrating an alternate manner of transferring heat from a condensate liquid to an original solution; and

FIGS. 4-6 are flow sheets similar to FIG. 1 and illustrating different manners of heating an original solution from an external source.

FIG. 1 diagrams a combination of the vapor reheat MSF evaporator with a multieffect evaporator using metallic surfaces to transfer heat for the boiling. In this case, the condensing-heating, or right side 11, substantially corresponds to the vapor-reheat system as described in U.S. Pat. 3,289,583 and elsewhere. Pumps 40 are provided between adjacent stages of the right side 11 to transfer liquid condensate and/ or coolant to the next higher stage. There is in addition to some or all of the flash evaporation zones on the left side 10, of the conventional MSF ladder, a coil, calandria, tube bundle, or other heat transfer surface on which there is condensation of steam from previous evaporations as in a usual evaporator having surface type boiling. Several of these calandria and vaporizingcondensation systems in the usual manner give a multipleefiect which, in combination with the vapor reheat MSF gives a Hybrid Evaporation system comprising both types, along with some other features.

The familiar heat exchanger 18 is above the vapor reheat ladder, transferring the heat from the fresh water stream leaving the half-stage 19 to the sea water feed 14. This may be of the standard shell and tube or other standard type. Alternately, as shown in FIG. 3, it may be of the liquid-liquid-liquid type comprising a pair of direct contact liquid-liquid heat exchangers 18a and 18b between which an intermediate heat transfer liquid is recirculated. The heat exchanger may also be of the MSF chilling type. Both of these types of heat exchanger are described in U.S. Pat. No. 3,329,583. In each event, heat is transferred indirectly to the sea water feed 14.

The prime heater 12 is heated by an external source, here shown to be prime steam supplied through line 33 to the closed coil 105. The sea water, heated to its highest temperature, is passed into the first flash evaporator stage having a pressure P, and then successively by lines 13 to the flash evaporation stages of successively lower pressure, finally discharging from the lowest stage P", through line 32 as the concentrated brine.

Water vapor formed by normal boiling in the prime heater, as described in U. S. Patent 3,329,583, passes through line 31 to the half-stage 19, which is desirable, but in some cases may not be used. This water vapor is formed by the evaporation of a part of the sea water in the prime heater, as previously described. The condensate from the coil 105, in the prime heater is passed to the steam trap 4, which discharges in the half-stage 19 to give up its available heat in flashing to the temperature of the half-stage. (Alternately, the steam trap 4 may be located outside and pass its condensate back to the boilers, as is usual practice with condensate from the prime steam supplied to evaporators.) An amount of vapors is withdrawn from the top of the prime heater, through the curved vapor line 103 and passed to a heat transfer evaporator surface in the top or other flash evaporation zone. This surface is indicated as a submerged steam coil 5 but any tubular or other suitable type of heat transfer surface for causing evaporation or boiling may be used. The vapors are condensed and the condensate from the coil 5 discharges through the steam trap 4.

The condensate from each evaporator heat transfer surface, here diagrammed as a steam coil 5 is somewhat warmer than the liquid on the condensing stage on which it operates; and some flash evaporation takes place as this fresh water discharges from 4 to the lower temperature and pressure of the condensate on this stage. The latent heat of the vapors so formed is recovered in the system. Alternately, if desired, condensate from 4 may discharge outside of the system.

Flash evaporation takes place as usual, from the hot sea water entering the top stage with pressure P, and this continues through each successive stage.

The combined vapors from the flash evaporation, plus those formed by the evaporation due to the closed condensation of vapors and heat transfer through the coil 5 are combined and pass as shown by the arrow 24 from the flash evaporating zone into the condensing zone of the stage.

The sea water from each stage goes by the discharge line 13 to the next lower pressure stage for subsequent flash evaporations at the lower pressures P", 'P', etc. There is also withdrawn from each stage by the curved vapor line 3, fresh water vapors which are condensed in the heating coil 5 of the successively lower stage. In each flash stage which is also a surface evaporator effect, there is a mixture of vapors from: (a) the flash evaporation of the sea water coming from the next higher stage inlets, 13; and (b) the evaporation as in the ordinary evaporator by heat transfer in condensing steam in the coils 5. Condensate from the coils 5, in each case, goes through the trap lines 4 to discharge into the adjacent, condensing zone of the respective stage. Combined vapors from flash evaporation and from heat transfer surface evaporation, pass partly as shown by arrows through 24 into the condensation zone, and partly through the curved vapor lines 3 to the coil in the next lower flash evaporation zone.

Fresh water produced as condensate is recycled and withdrawn as usual on the right side 11 of the ladder of stages in the vapor reheat evaporator-and in this case, eflects of the usual multielfect evaporator. In this Hybrid Evaporator, combining stages and effects, the fresh Water is the sum of the condensate in the condensation-rebating cycle, and the total of that trapped from the several heat transfer steam coils through the trap lines 4.

With this Hybrid Evaporator, it may be desirable to operate without a recycle of any part of the concentrated brine discharging from line 32 back to the inlet of sea Water 14 to the heat exchanger, and to accomplish the desired concentration of the sea water before discharge in a single pass. This has been found to have certain fundamental economy, if the ratio of the total amount of flash evaporation to total amount of surface evaporation is at an optimum ,level. The condensate from the trap lines 4 is in each case used at the next higher stage to condense vapors at a higher temperature and then finally this heat is given up to heat the incoming liquid. This is a thermally efficient process. Also, all of the numerous heat exchangers usually used by a multieflect system are combined in the single exchanger 18.

Boiler steam or other heating fluid supplied through line 33 may usually be the most convenient source of heat for this Hybrid Evaporator, as for other ME or M SF evaporators. Usually in an aqueous system, the steam may be supplied through line 33. v

However, in other cases, it has been found economic to supply the external energy needed through thermocompression. Thus, vapors may be taken from those formed by flash evaporation in a low pressure stage-or the lowest pressure stage-of the right side of the ladder 11 of FIG. 1 and be compressed to the higher temperature and pressure of the steam supply 33 to the prime heater 12.

This is not shown in FIG. 1.. Alternatively, and usually better, vapors are Withdrawn, as shown in FIG. 7, through pipe 7, from a lower stage of 11 to a thermocompressor 9 and compressed 'to flow through pipe 8 into the half-stage 19. Here they would condense by direct contact with the liquid if in open flow, or they may pass their heat to the feedliquid through a tubular surface in condensing on one of its surfaces,,if the liquid is inclosed flow.. This compression of vapors from a lower stage to the half-stage by either a mechanical or an absorption cycle is explained in 11.8. Pat. 3,288,- 686 of Nov. 29, 1966. i

The Hybrid Evaporator of FIG. 1 may utilize the methods of preventing heat reject as defined in thempending Application Serial Number 639,989 of May' 22, 1967 now US. Pat. No, 3,446,712 of May 27, 1969. It combines some of the advantages of a multietfect evaporator with those of a conventional multiflash evaporator, and, in particular, those of the vapor reheat-evaporator. In this combined unit, each stage of the multi-flash system operates, as usual, by flash evaporation on the left or evaporator side 10 of a succession of stages. Vapors from the flash evaporation are condensedon the right or con.- densing side 11 on tubes preheating. the feed or. inthe familiar circulating stream of fresh water of the vapor reheat system, always at a lower temperature than the liquid which is flash evaporating.

In addition to, this flash evaporation and either closed or open condensation of vapors,v there is withdrawn through the curved vapor lines 3,0f FIG. 1', vapors which are fed to aheat transfer surface, or coil, of any desired type located on a lower stage, which transfers heat as in any ordinary evaporator, by condensing'lthe steam on oneside of a metallic surface with the boiling liquid on the other; The temperature of the steam rising from a,

the flash'evaporation in any one stage, is obviously higher than the liquid on the next lower stage; and there is thus'the necessary temperature drop'to give the heatflow to cause evaporation, as is usual intan'evaporator with heat transfer surface. FIG. 1, diagrams 5 stages; and with the half-stage, this gives a total of 6 condensing zones, with direct contact with fresh water being. circulated in each, to condense the respective vapors formed. The closedcondensation of the conventional multi-flash system is not shown; but it would becomparable to -Ethe right side 11, of FIG. 1. p s

The operation of FIG. 1 is exactly that of a vapor reheat evaporator, as previously described, with a multipleetfect evaporator of the same number of stages superimposed thereon. There is a combination of the steam flows, also of the condensate flows, as in a vapor reheat unit. In this Hybrid Evaporator, the operation of each of the separate components is improved over what it would be alone. The multieffect evaporator component operates in a forward feed manner, and it has the advantages only otherwise achieved with a large number of heat exchangers for preheating the feed up to the highest temperature of the system. Thus, by this super-imposition of the ME upon the vapor reheat ladder, it achieves an efficiency equivalent to a multietfect, with a line of heat exchangers for preheating the feed, and another line for recovering the heat in the condensate due. It may in this may become much more economic evaporator for its service than the usual multiple-effect evaporator taken alone.

Similarly, the vapor reheat ladder may be operated at its greatest efficiency, which is where there is no recycle or reflux of concentrated solution back to join the feed liquor. This eliminates the high elevation of boiling point which must be overcome in the top stage, when there is large reflux. It is always necessary to supply the temperature difference equal to this boiling point elevation, plus the other temperature differences which are involved, by heating in the prime evaporator. If the elevation of boiling point is minimized by having the lowest concentrated feed, that of the dilute solution, without recycle of higher concentration brine-normally blow-downthis is an important heat economy.

Therefore, the vapor reheat aspect in the hybrid evaporator is also simpler, as well as more efiicient thermally since it does not require the recycle or reflux; and more eflicient in that the boiling elevation at the top of the ladder is minimized. I For these reasons, the Hybrid Evaporator has been found to give advantages to both of the systems over their individual operations; i.e., the vapor reheat MSF is simpler and more economic than it would be alone, as is also the multietfect unit.

While the heat transfer surfaces 5 are indicated diagrammatically as ordinary submerged heating coils, it is recognized that this is not the most efficient type of heat transfer surface for an evaporator. Hence, the design of the heat transfer surface may be any one of the many types of natural or forced circulation, falling film, or other type of surface familiar in evaporator design, and such types may be incorporated in place of the coil 5, which is indicated here by any suitable mechanical design. One preferred type might be a falling film evaporator, with a long tube, to obtain the high heat transfer coefiicients obtained with this type of evaporator surface.

'. A large number of effects, from 12 to 20, would be indicated in any commercial unit for desalination use. However, the number of multiflash stages to show minimum productive costs when the cost of thermal energy and the cost of equipment are balanced, may usually be in the range of 35 to 50.

Thus, it may be quite often that the most economical number of stages for the operation may be a much greater number than the optimum number of effects in the multielfect evaporator with heat transfer surfaces, which is a part of this hybrid unit. Therefore, the number of flash stages may bemost economic with 2 or even 3 times the number of the effects with heat transfer surface. The design then might allow the vapor line 3 leading from the top stage with pressure P, to go instead to a coil on the stage of pressure P"., and then the vapors formed at this pressure, P, may go to the heat transfer surface in a stage P", or even P (not shown). In this way the temperature differential, say for most economicheat transfer to the coils 5 may be that of several or more of the difbe balanced out at the same number of effects and stages.

It has been found that the spacing of the surface boiling units or effects with regard to the flash stages may not be uniform throughout the whole ladder. For example, there may be more stages per effect in the lower pressure range, to give the usual higher temperature drops associated with the low pressure effects in the usual multieffect. Or the effects may be in only a part of the range of stages; and they often may be found most advantageous in the higher pressure range with only multiflash stages in the lower pressure range. Because the viscosities of solutions increase in the lower pressure range of stages, due to lower temperatures, and particularly increased concentration, particularly for some concentrated solutions, heat transfer through metallic surfaces becomes less efficient. Thus, there may be no effects below some particular pressure; and above this pressure, the heat transfer surfaces of the effects may give the larger part of the evaporation. Indeed, in some eflicient designs for particular liquids, there may be little or no flash evaporation in the upper temperature ranges.

It is, of course, possible to take off a greater amount of vaporization in any one stage than would be passed to the condensing side of that stage, and it is therefore possible to withdraw some of those vapors for the heat transfer in the lower stage, through the condensation in closed tubes, as previously noted. This means that vapors may be withdrawn from the prime heater through the line 103, to the closed coil in the top stage P. This temperature drop usually will be substantial. However, it may be desired to remove vapors from P and bring them to a stage below the next stage, e.g., P. There may be no condensing coil or effect in the stage P, but vapors may be withdrawn from P to P", where there may be a heat transfer coil. In that case, the vapors may then be withdrawn from P" to P, the stage below P". Then each effect will actually show the temperature drop of 2 stages. Or, in the system shown, there may be only coils in the stages P, P', and P", in which case, the vapor lines 3 would go only from P to P, P" which would have the coils, and P", P" would merely act as flash stages.

Other combinations of vapor withdrawals from stages to supply the condensation zone of the same stage and at the same time to supply a heating coil or calandria of a lower stage, are possible.

The evaluation of the optimum relation may be made through knowledge of: (a) the heat transfer coefficients which are to be expected in the particular type of coil or calandria; (b) the overall temperature which is available from the highest temperature which can be reached in the prime evaporator, based on steam pressure limitations; (c) steam costs at different pressure; (d) scaling problems of the water itself at higher temperatures; (e) the number of stages which may be desired; (f) the cost factors of mechanical design; and (g) some other variables.

The advantages of this Hybrid Evaporator, either with conventional multiflash or with vaporreheat, are realized especially well when there is considered the system of recovery of heat which otherwise is rejected; since this allows the maintaining of the optimum or near-optimum thermal efliciency of the multiflash ladder, as above explained. With the integration of the multieflect evaporator into the multiflash evaporation ladder-either conventional or vapor reheat ladder in the Hybrid evaporator, there is an increased complexity of controls; and the possibility of an upset in operation is increased. The heat balance between the two sides of the ladder may be promptly effected with a minimum of heat rejected if the same systems are used as before described, each giving their relative advantages. Thus, in the Hybrid evaporator, with either vapor reheat or conventional multiflash, a part of the dilute feed may be diverted around the prime heater 12 by valved lines 20 and 22; while in the conventional multi-flash, a valved line 8 and in vapor reheat a valved 38 may serve as before to divert part of the liquid being heated to a lower stage with advantages already explained in co-pending application 639,-

8 989, filed May 22, 1967, and issued as US. Pat. No. 3,446,712 on May 27, 1969.

As described above, vapors withdrawn from a lower stage and compressed to the pressure of the half-stage 19 may serve as the prime heat. Similarly, as illustrated in FIG. 2, boiler steam may be supplied through line 333 as the source of external heat, again to the half-stage, for heating the recycling stream of condensate water in open flow to a temperature sufficiently higher than the feed solution, so that the heat from the boiler steam added to the condensate stream may be transferred to the feed solution by the heat exchanger 18. In such case the feed solution passes directly from the heat exchanger 18 to the first evaporation stage.

Also, as described more fully in co-pending application Ser. No. 639,989, now US. Pat. No. 3,446,712 of May 27, 1969, and in its continuation-in-part co-pending application Ser. No. 826,135 of May 20, 1969, now abandoned, and as illustrated in FIGS. 4 and 6, the vapor reheat MSF evaporator may be supplied with the prime or external heat by means of a combustion in a chamber 50 of organic materials which may be present in the original feed, or in a fluid fuel which is added through a feed line 52. In either case, oxygen as such, or more usually in the form of air, is also added. If a fluid fuel is used, it must be ignited. If the feed liquid is a solution or suspension-solution of organic materials, which has been preheated to 300 to 400 F or even higher, addition of air or oxygen gives an exothermic wet combustion. Non-condensable gases may be vented through an exhaust line 54.

Similarly, in this hybrid modification of the vapor reheat system, external heat may be supplied in the prime heater 12 by the addition of air or pure oxygen to combust added fluid fuel or organic materials in the dilute liquid feed. Such heat supply, diagrammed by line 52, and the necessary accesories, are well known in the art and are explained in the copending applications.

Similarly, as illustrated in FIG. 5 the external heat may be supplied by a submerged combustion of a fluid fuel in the half-stage 19, to supply heat to the condensate stream being recycled, before this stream goes to the heat exchanger 18. As shown, the fuel and oxygen are supplied via a line 56 to the half-stage 19. A vent 54' for the resulting gases and vapors is provided in said halfstage. In this case, as in others where external heat is supplied to the half-stage, the total of the pre-heat obtained by the condensation of vapors in the condensing-heating side of the vapor reheat ladder, and the external or prime heat supplied by submerged combustion is added to the stream leaving at 29, to go through the heat exchanger 18. The total heator as large an amount as possibleis thus transferred to the dilute solution. The mechanics of such submerged combustion is familiar to the art and is described, to some extent, in co-pending application 639,989 and now US. Pat. No. 3,446,712 of May 27, 1969, and US. application Ser. No. 826,135 of May 20, 1969.

In any such oxidation, provision must be made by well known means to remove the combustion gases from the vapors, to allow their condensation. In this case, particularly, but also in other systems, a system must be made for exhaust of non-condensible gases from the condensing vapors.

What is claimed is:

1. In the process of heating and evaporating solutions, and condensing vapors formed therefrom, the steps of:

(a) driving a stream of condensate water so that it passes in dispersed flow through the open condensing zones of a series of plural stages a stream of condensate water being maintained at a temperature below its boiling point at the pressure prevailing in the respective condensing zone of each of said stages, said flow being in the direction of increasing temperatures of said condensing zones; (b) transferring heat from the said heated stream of condensate water leaving the said condensing zone of highest temperature to preheat the said original aqueous solution;

(c) further heating via a source of external heat at least a major portion of the said original solution to the highest temperature it attains in the system;

((1) passing said original solution at its highest tempera ture in open flow through the evaporating zones of the said'series of plural stages in counter-flow relation to that of the said stream of condensate water, but out of contact therewith said open flow of solution through said evaporating zones being in the order of their successively lower pressures and lower boiling point temperatures corresponding to the respective pressures, the temperature and pressure of any evaporating zone being greater than the temperature and pressure of its corresponding adjacent condensing zone;

(e) flash vaporizing a part of the water from the original solution in each of said evaporating zones to obtain water vapors and a solution which is somewhat more concentrated as it leaves each successive evaporating zone:

(f) withdrawing from each of the evaporating zones of the said plural stages some part of the said water vapors formed therein;

(g) passing at least a portion of said withdrawn part of said water vapors to a closed condensing element which is in contact with the said somewhat more concentrated solution in the evaporating zone of another one of the said plural stages at a lower pressure than that from which the said vapors are withdrawn;

(h) condensing in said closed condensing element the said part of said water vapors and transferring the heat of condensation to the said somewhat more concentrated solution on the said stage of lower pressure, so as to cause evaporation of Water from said more concentrated solution and to form additional water vapors which combine with those formed by the flash evaporation at said stage of lower pressure;

(i) directing the water vapors which are formed in the evaporating zones of the respective stages, and which are not withdrawn and passed to a closed condensing element in a stage of lower pressure, to the respective open condensing zones of said stages; and

(j) condensing in the respective open condensing zones the water vapors directed from the evaporating zone of that respective stage, by direct contact with the said stream of condensate water passing in dispersed flow through the open condensing zones in the direction of increasing temperature thus heating the said stream of condensate water.

2. In the process of claim 1, wherein the said transferring of heat from the heated stream of condensate water leaving the said condensing zone of highest temperature to heat the said original aqueous solution comprises:

(a) contacting directly by a counter-current, liquidliquid relation the said heated stream of condensate water leaving the said condensing zone of the highest temperature with an intermediate cooler stream of a substantially water-insoluble liquid, which is being heated thereby and (b) cooling said stream of substantially water-insoluble liquid, after being so heated, by a direct liquidliquid contacting with the said original aqueous solution which is being heated thereby.

3. The process of claim 1, wherein the external heat supplied to heat at least a major portion of the said original solution to bring it the highest tempreature it attains in the system, is added in a prime heater after heat from 10 the condensate Water has been transferred to preheat the said orginal aqueous solution.

4. The process of claim 3, wherein the heat from said external source is used to evaporate in the prime heater some part of the water in the original solution to give fresh water vapors which are passed to and condensed in an open condensing zone at a higher temperature and pressure than prevails in the condensing zones of any of the series of plural stages, while heating the said stream of condensate water in said open condensing zone.

5. The process of claim 3, wherein the said external heat comes from the submerged combustion of a fluid fuel, the combustion products being in direct contact with the said original solution.

6. The process of claim 3, wherein the said external heat comes from the combustion with added oxygen of organic materials contained in the original aqueous solution, the combustion products being in direct contact with the said original solution.

7. In the process of claim 1 wherein the external heat supplied to heat at least a major portion of the original solution to bring the solution to the highest temperature it attains in the system, is transferred from the external heat source to further heat the condensate water after said condensate water has been heated in said open condensing zones, to its maximum temperature prior to receiving heat from said external heat source, and thereafter transferring heat indirectly from the condensate water, which has been heated from said external heat source, to said original solution.

8. The process of claim 7, wherein the said external heat comes from steam supplied at a higher temperature than that of the said stream of condensate water leaving the said condensing zone of highest temperature, said stream being directly contacted with the stream of condensate water leaving the condensing zone of highest temperature.

9. The process of claim 7, wherein the said external heat comes from the submerged combustion of a fluid fuel with added oxygen, the combustion products being in direct contact with said condensate stream which has passed the said open condensing zones.

10. The process of claim 7, wherein the external heat supplied comes from the compression of water vapors drawn from one of the said plural stages having a lower pressure and temperature;

said water vapors being compressed to a sufliciently high temperature to heat the stream of condensate water to a temperature higher than the highest temperature which the said original solution attains in the system.

References Cited UNITED STATES PATENTS 2,665,249 1/1954 Zimmerman 210-2 2,750,999 6/ 1956 DeVries 15918 2,894,879 7/ 1959 Hickman 203-27 3,152,053 10/ 1964 Lynam 159-2MS 3,215,189 11/1965 Bauer 15916A 3,249,517 5/ 1966 Lockman 202--159 3,298,932 1/1967 Bauer 203-11 3,299,942 1 1967 Jacoby 15947WLX 3,351,120 11/1967 Goeldner et a1 15913B 3,461,460 8/ 1969 McGrath 202-173X NORMAN YUDKOFF, Primary Examiner J. SOFER, Assistant Examiner US. Cl. X.R.

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