US3364982A - Process for cooling high temperature gases - Google Patents

Description

Jan. 23, 1968 J, JAFFE 3,364,982 I PROCESS FOR COOLING HIGH TEMPERATUREGASES Filed Nov. 13. 1964 INVENTOB.

JAM ES JAFFE ATTORNEY United States Patent Ofilice 3,364,982 Patented Jan. 23, 1968 James Jatfe, Summit, NJ., assignor to Allied Chemical Corporation, New York, N.Y., a corporation of New York Filed Nov. 13, 1964, Ser. No. 410,927 9 Claims. (Cl. 165-1) This invention relates to an improved means for cooling high temperature gases, such as the effluent gases from a sulfuric acid decomposition unit.

Commercial gas cooling apparatuses commonly employ brick-lined gas cooling towers. Such cooling towers suffer from the disadvantages of being space, time. and cost consuming in construction, operation and mainten'ance.

Attempts have been made to cool high temperature gases using wateror acid-cooled shell and tube-type heat exchangers, but such attempts have not generally proved to be satisfactory. Such devices are subject to frequent tu'be failure by reason of the wide temperature diiferential created between the upper portions of the tubes and upper tube sheet, and the cooled portions of the tubes. The upper portions of the tubes are not cooled effectively due to the release of non-condensable gases from the cooling Water or cooling acid stream and due to the difficulty of completely venting the space immediately under the upper tube sheet.

It is an object of this invention to provide an efiicient means for cooling high temperature gases which does not suffer from the disadvantages previously encountered by the art.

More specifically, it is an object of the invention to provide a means for cooling high temperature gases in a shell and tube-type heat exchanger in such a manner as to avoid thermal-shock and consequent tube failure.

It is another object of the invention to provide an efiicient means for cooling high temperature gases which is economical in construction, space and operation.

Other objects and advantages of the invention will become apparent from the following description when taken in conjunction with the drawing and claims.

In accordance with the invention, I have found that the above objects can be achieved by causing the high temperature gases, which are to be cooled, to pass downwardly through the tubes of a tubular shell and tube-type heat exchanger, while concurrently circulating a liquid coo ing medium as a film in the tubes from a layer of said liquid cooling medium maintained on the top tube sheet. The liquid cooling medium exiting the tubes is then preferably recirculated to the liquid cooling medium layer maintained on the top tube sheet.

The accompanying drawing shows, in schematic form, an embodiment of the apparatus which may be employed according to the present invention. The shell and tube-type heat exchanger is shown in elevational partial section.

The apparatus, as shown in the drawing, essentially comprises a shell and tube-type heat exchanger 14, comprising a tubular shell 1 within which is disposed a bank of heat-exchange tubes 2 pressed into, or otherwise secured to upper and lower tube sheets 3 and 4 respectively. The tubes 2 project upwardly and beyond upper tube sheet 3, are open at the top and are provided with weirs 5 at their upper extremities. Tubular shell 1 is provided with a head 6, which may be cooled by circulating cooling fluid through inlet 19 and outlet 20. Head 6 is further equipped with a gas inlet 7 and an internal cooling liquid medium inlet 8. Shell 1 is also provided with an external cooling fluid medium inlet 9 at its lower extremity and a corresponding cooling fluid medium outlet 10 is provided subadjacent to upper tube sheet 3. Base 11 of the tubular shell is provided with a gas outlet 12 for the cooled gases and an internal circulating cooling medium outlet 13. Acid reservoir and pump tank 15 and circulating pump 16 are connected with a suitable conduit 17 which connects outlet 13 with inlet 8.

In accordance with the invention, the high temperature gases to be cooled enter the top of the shell and tube-type exchanger 14 through gas inlet 7. The high temperature gases could be any one or a mixture of a variety of organic or inorganic gases at temperatures as high as about 3,500 F. and upwards. The eflluent gases of sulfuric acid decomposition units are typically at about 5002,300 F. and are particularly suitable for use according to the process of the invention. Other illustrative gases and gas mixtures which may be advantageously cooled according to the invention include: Mixtures of nitrous oxide, nitrogen, oxygen and water vapor resulting when ammonia is burned with air at 1,750 F.; mixtures of chlorinated hydrocarbons, hydrochloric acid and chlorine resulting when methane is burned with chlorine and mixtures of sulfur, methane, hydrogen sulfide and carbon bisulfide'resulting when methane and sulfur are reacted catalytically at 1,200" P. Other high temperature gases, or mixtures of high temperature gases which may be cooled according to the process of the invention will readily occur to those skilled in the art.

The gas or mixture of gases to be cooled is caused to flow downwardly through the heat exchange tubes 2 in contact with a film of a cooling liquid which falls, preferably continuously, along the inner peripheral surfaces of tubes 2. The cooling liquid is circulated from a layer 18 of the same maintained on the top tube sheet by the weirs 5, located at the top of each heat exchange tube. The cooling liquid is withdrawn from outlet 13 in base 11 of the cooling unit and is recirculated by means of pump 16 and conduit 17 through inlet 8 to layer 18 at the top of the unit. Pump 16 is provided with means to increase or decrease circulation, thus controlling flow rate. Reservoir or pump tank 15 aifords the advantage of providing temporary storage for the circulating internal cooling liquid. The cooled gases are withdrawn through outlet 12 from the base 11 of the cooling unit.

For increased etficiency, a second cooling medium is circulated along the inner surface of shell 1 and along the outer surfaces of heat exchange tubes 2. This circulation is accomplished through inlet 9 and outlet 10. The second cooling medium may be liquid, such as water, or gaseous, such as air. In the event that a second cooling medium is not employed in this manner, it is desirable to pre-cool the internal circulating cooling liquid medium prior to reentry to the cooling unit via inlet 8, by some external cooling means. Further efficiency is achieved when head 6 is insulated in some manner. For example, the walls of head 6 may be lined with brick or may be liquid-, e.g. water-cooled as shown in the drawing.

The cooling capacity of the system is afiected by the nature of the gases to be cooled, the circulating mediums employed, materials of construction of the components of the cooler, size of the components of the cooler, flow rate of the gases to be cooled and flow rates of the various cooling mediums.

The internal circulating liquid medium employed must, of course, be compatible with the gases to be cooled, as well as being non-corrosive to the material of construction of the upper tube sheet and the heat exchange tubes. It also should be phase-separable from any organic or inorganic products which may be condensed. In most cases of acid manufacture, various chlorination processes and many other processes, water may be used as the internal circulating cooling liquid medium. In most cases, however, and particularly those of acid manufacture, there is an equilibrium amount of acid present in the hot gases which will be absorbed by the water, thus forming weak or dilute acid. In a preferred embodiment, the acid corresponding to that which will be formed in the heat exchange tubes is used initially as the internal circulating cooling liquid medium. Such a choice will facilitate design of the materials of constnuction of the cooler apparatus. Other acids, organic or inorganic, meeting the above-described tests of compatibility, non-cor rosiveness and phase-separability may be employed however, even though not produced inherently in the heat exchange tubes. Acids, and particularly inorganic acids are preferred, in view of their generally higher boiling points than water. The higher boiling point allows more flexibility in operation since, in the event of failure of the circulating pump, for example, there would be less chance of the cooling liquid vaporizing and damaging the top tube sheet of the cooler. Other materials, such as chlorinated hydrocarbons, could be employed, provided they meet the indicated tests. To illustrate the above, in the case of cooling sulfuric acid combustion gases, the hot efiluent gases will contain some S and water. The former will form dilute H 80 with water, if used as a circulating medium; the latter will form dilute sulfuric acid if more concentrated sulfuric acid is used as the circulating medium. Thus it is expedient to use sulfuric acid as the internal circulating medium in this case and it is seen that no matter what strength of H 80 is initially used, some equilibrium strength of dilute H 80 will eventually be formed. The material of construction of the tube walls and top tube sheet will depend upon the strength of. the acids with which it comes in contact. For example, use of H 50, in excess of 93% will prohibit the use of impervious carbon and dilute H 80 say below about 75% strength, will prevent the use of commercial steel. Since dilute H SO will eventually be formed, it is expedient to employ dilute H 50 initially and design the materials of construction accordingly. Similarly, when the gas cooler is used to cool the combustion gases of ammonia and air, it is expedient to employ an internal circulating medium of dilute nitric acid which would be inherently produced in the heat exchange tubes. Dilute nitric acid requires stainless steel construction. Organic cooling fluids would not be suitable for use in such a process since they would be oxidized by the HNO formed. In the case of cooling combustion gases of methane with chlorine, an internal circulating cooling medium of dilute hydrochloric acid may be employed, or a mixture of chlorinated hydrocarbon liquids would be satisfactory.

The term dilute acid is to be understood as referring generally to aqueous acids as opposed to anhydrous acids. The term is a relative one and the percentage strength of the acid, qualifying it as dilute, will vary depending on the particular acid in question, which in turn will depend upon what strength is commonly regarded as being concentrated for that acid. For example, for the purpose of this discussion, anything less than about 75 H 50 may be considered dilute. HNO will form an azeotrope at about 70% strength. Dilute HNO may be considered as being anything less than about 50% strength. HCl forms an azeotrope at a strength of about 23%. Dilute HCl may be considered as being anything less than about 20% strength. It is to be emphasized, however, that the term dilute is an extremely relative one and that consequently the above exemplary values are flexible.

The internal cooling liquid medium is circulated from a layer of the same maintained one the top tube sheet by the weirs 5, located at the top of each heat exchange tube. The weirs serve to meter the flow of cooling medium within the tubes. The notches in the weirs may be a variety of shapes depending'upon choice of design, such as triangular or V-shaped. V-shaped notches are preferred since they permitmore easily regulated flow and provide a continuous flow of cooling medium down the inner peripheral surfaces of the tubes in a swirling type motion, which insures more eflicient contact of the cooling medium with the inner peripheral surfaces of the heat exchange tubes. The medium depth of the internal cooling medium layer, which is required to be maintained on the top tube sheet, is that depth needed to afford protection from impingement of hot gases. This will ordinarily be about /2". There is no maximum limitation on depth of this layer except the practical one of economy and convenience. A layer depth of greater than 6" would afford no particular operating advantages. Cooling medium layer depths of about /2" to about 6" are generally employed with a preferable depth of about 1" to about 3".

The number of heat exchange tubes in the cooler may vary according to design anywhere from one to several thousandths. The cooling capacity of the unit will generally increase with the number of heat exchange tubes employed. Accordingly, it is desirable, as a practical matter, to use a relatively large plurality of heat exchange tubes. The maximum number is limited only by design considerations, i.e. the fabrication facilities of the manufacturer. With steel heat exchange tubes, for example, it is feasible to construct a unit containing between about 1,000 and 2,000 tubes. Size of the heat exchange tubes is more or less standard and could range in normal design depending upon the material of construction from about A ID. to about 6" ID. or more. In the event that impervious carbon is employed as the construction material, for example, a preferred size for such a tube is about 1%" CD. by ID.

The flow rate of circulating internal cooling medium required is interrelated to such factors as the cooling capacity required, number of heat exchange tubes in the cooler and flow rate of the gases to be cooled. Generally, the higher the cooling capacity which is required, the higher should be the internal cooling medium flow rate. An increased flow rate of internal cooling medium will contribute to more efficient heat exchange and consequent higher cooling capacity. As a minimum, an internal cooling medium flow rate of about 0.3 lb./hr./inch of tube circumference is required in order to maintain a film of coolant on the inner peripheral surfaces of the tubes. This figure may be increased to about 300 lbs./hr./inch of tube circumference, or higher if required. Normally, flow rates in the range of about 50 to about 150 lbs./hr./ inch of tube circumference would be employed. Optimum flow rates ordinarily lie in the range of about to about lbs./hr./inch of tube circumference. The optimum flow rate values are determined on a practical basis by a trial and error procedure whichinvolves comparing the economics of increased pumping costs to obtain the higher flow rates versus the larger heat transfers accomplished by the greater coolant circulation. V

The flow rate of the circulating external cooling medi um does not have as large an effect upon the cooling capacity of the unit as does the flow rate of the circulating internal cooling medium. Enough coolant should be circulated to dissipate most of the heat picked up by the heat exchange tubes. In those cases where a liquid external cooling medium is employed, e.g. water, it is generally desirable to maintain a flow rate in the range of about 20 to about 120 gallons/hr./inch of tube circumference.

Selection of the optimum number, size and wetted area of the heat exchange tubes to be used is based upon such considerations as the amount of heat to be transferred, the temperature limitations of the materials of construction of the tubes and the economics of tube sizes versus the cost. These factors may be interrelated in a manner such as follows: In an illustrative case wherein it is desirable to cool a gas stream from 1,000 F. to 250 F. using dilute sulfuric acid as the internal circulating cooling medium, the total heat Q which is to be removed from the gas stream amounts to about 1,000,000 B.t.u./hr. The log mean temperature difference AT, between the hot gas stream and the acid coolant film, will be about 400 F. There are two overall coefficients of heat transfer involved, one being from the acid to the cooling medium outside the tube, the other being from the hot gas stream to the circulating acid coolant. Assuming that the acid to the cooling medium is controlling at approximately the coefiicient of 100 B.t.u./hr./sq. ft./ F., hereinafter designated as the symbol U, the wetted area required equals A Q =w UAT 400x100 wherein Q, U and AT are as defined above. The 25 sq. ft. value for the required wetted area is now, by a trial and error procedure, distributed among various combinations of tube size and length and number of tubes. The various combinations are priced out and graph plotted in order to arrive at that combination with the lowest net cost. The optimum tube size and number for the required wetting area would, of course, be that combination with the lowest net cost.

In an illustrative embodiment of a typical cooling cycle employing the process of the invention, a mixture of sulfuric acid decomposition effluent gases at about 690 F. is circulated, at a flow rate of 1,347.7 lbs./min., downwardly through heat exchange tubes 2 of cooler 14, as shown in the drawing. The sulfuric acid decomposition efiiuent gas mixture has the following composition.

=25 sq. ft.

The apparatus employed is of the type shown in the draw ing, the heat exchange portion of which consists of a bank of 600 Karbate (impervious carbon or graphite) tubes, each of which tubes is 12' in length and 1%" CD. by /3 ID. The internal circulating cooling medium employed is about 5% sulfuric acid which is maintained at a depth of about 3" on the top tube sheet 3 by weirs 5 and is circulated downwardly as a falling film along the inner peripheral surfaces of the tubes at a rate of about 600 gallons/min. Cooling water at 85 F. is circulated at a rate of about 2,400 gallons per minute, between the inner surfaces of shell 1 and the outer surfaces of the tubes 2, through inlet 9 and exits through outlet 10 at about 105 F. The cooling Water also contacts the undersurface of top tube sheet 3. The cooled gas mixture is withdrawn through gas outlet 12 at a temperature of about 120 F. The sulfuric acid internal cooling medium is withdrawn through outlet 13 at about the same temperature, viz. 120 F. and is recirculated to the acid layer on top tube sheet 3.

In the above illustration, if the sulfuric acid efiluents are at a higher temperature when passed through the heat exchange tubes, a larger number of heat exchange tubes would be required in order to maintain the same throughput of gases and recover the same at the 120 F. level. For example, if the sulfuric acid efiluents are at about 1,800 F., about 1,200 heat exchange tubes of the same size would be needed. Two units of 600 tubes each, in parallel, could be used advantageously if desired.

An important design consideration in choosing the materials of construction for any shell and tube-type heat exchanger is the upper temperature limitation which the top tube sheet and tube walls will have to endure. The upper temperature limitation of a tube sheet is normally determined by the pressure differential across the tube sheet and the temperature differential between the shell and the tube fluids at the tube sheet. One of the advantages afforded by the process of the invention is that the top tube sheet is kept at temperatures below about F., resulting in very low temperature differentials at the tube sheet, which circumstance permits the use of cheaper materials of construction for the top tube sheet than would be needed if the same were exposed to very high temperature differentials. For example, with low temperature differentials, ordinary steel and impervious carbon may be substituted for the costly high temperature resistent metals, such as the Hastelloy alloys, for use under other favorable conditions. Similarly, since by adjusting the flow rate of the circulating internal cooling medium the tube wall temperature can be kept below about 200 F.; cheaper materials may also be used in the tube wall construction than would otherwise 'be possible.

A further important advantage of the process of the invention is that the layer of internal circulating cooling medium, which is maintained on the top tube sheet, surrounds the projecting uppermost extremities of the heat exchange tubes and thus serves to prevent thermal-shock to the heat exchange tubes.

Additionally, the cooling medium, being circulated internally of the tubes, keeps the tubes clean and minimizes fouling of the tubes by carbon dust and various other hydrocarbon materials. This feature makes the invention process admirably suited to the cooling of dirty gas mixtures.

Although the invention has been described with some particularity and since others skilled in the art will readily be able to make modifications and innovations over the embodiments described; it should 'be understood that I do not wish to be limited except by the scope of the appended claims.

I claim:

1. A method for cooling high temperature gases which comprises passing the gases downwardly through the tubes of a tube sheet-type heat exchanger comprising one or more heat exchange tubes and a top tube sheet, maintaining a layer of a first cooling medium on the top tube sheet, circulating the first cooling medium from said layer as a falling film along the inner peripheral surfaces of the heat exchange tubes, in concurrent flow with the gases and subjecting the outer peripheral surfaces of the heat exchange tubes to heat exchange with a second cooling medium.

2. The method of claim 1 wherein the second cooling medium is water.

3. The method for cooling gases at temperatures up to about 3,500 P. which comprises passing the gases downwardly through the tubes of a tube sheet-type heat exchanger comprising one or more heat exchange tubes and a top tube sheet, keeping the top tube sheet at a temperature below about 150 F. by maintaining thereon a layer of a first cooling medium, circulating the cooling liquid medium from said layer as a continuously falling film along the inner peripheral surfaces of the heat exchange tubes in concurrent flow relationship with the gases and cooling the outer peripheral surfaces of the heat exchange tubes with a second cooling medium.

4. The method of claim 3 wherein a plurality of heat exchange tubes are employed.

5. The method of claim 4 wherein a dilute inorganic acid is used as the first cooling medium.

6. The method of claim 5 wherein the dilute inorganic acid is dilute sulfuric acid.

7. The method of claim 5 wherein the medium is water.

8. The method for cooling the efiluent gases from a sulfuric acid decomposition unit which comprises passing the gases downwardly through a plurality of heat exchange tubes of a tube sheet-type heat exchanger, including a top second cooling tube sheet, keeping the top tube sheet at a temperature below about 150 F. by maintaining thereon a first circulating cooling medium consisting of a layer of dilute sulfuric acid, circulating the dilute sulfuric acid from said layer as a continuously falling film along the inner peripheral surf-aces of the heat exchange tubes concurrently and in direct heat exchange relationship with the et'fiuent gases and cooling the outer peripheral surfaces of the heat exchange tubes with a second circulating cooling medium.

9. The method of claim 8 wherein the second circulating cooling medium is Water.

8. References Cited UNITED STATES PATENTS EDWARD J. MICHAEL, Primary Examiner.

0 ROBERT A. OLEARY, CHARLES SUKALO,

Examiners

Claims (1)

1. A METHOD FOR COOLING HIGH TEMPERATURE GASES WHICH COMPRISES PASSING THE GASES DOWNWARDLY THROUGH THE TUBES OF A TUBE SHEET-TYPE HEAT EXCHANGER COMPRISING ONE OR MORE HEAT EXCHANGE TUBES AND A TOP TUBE SHEET, MAINTAINING A LAYER OF A FIRST COOLING MEDIUM ON THE TOP TUBE SHEET, CIRCULATING THE FIRST COOLING MEDIUM FROM SAID LAYER AS A FALLING FILM ALONG THE INNER PERIPHERAL SURFACES OF THE HEAT EXCHANGE TUBES, IN CONCURRENT FLOW WITH THE GASES AND SUBJECTING THE OUTER PERIPHERAL SURFACES OF THE HEAT EXCHANGE TUBES TO HEAT EXCHANGE WITH A SECOND COOLING MEDIUM.

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