US3501921A

US3501921A - Method of operating heat exchangers in cryogenic systems

Info

Publication number: US3501921A
Application number: US692856A
Authority: US
Inventors: James R Muenger; David L Alexander
Original assignee: Texaco Inc
Current assignee: Texaco Inc
Priority date: 1967-12-22
Filing date: 1967-12-22
Publication date: 1970-03-24
Anticipated expiration: 1987-03-24

Description

METHOD oF OPERATING 'EfAT'mccm'mmms` 'IN -cRYoGENIc SYSTEMS Filed gec. 22 1967 2 sheets-sheet 1 Tlcfl.

Mmh 24, 1970 J. R. @Emea am. 3,501,921

METHOD OF OPERATING HEAT EXCHANGERS IN CRYOGENIC SYSTEMS Filed Dec. 22, 1 96'7 Tlll. ik

2 Sheets-Sheet 2 n H f AB AB CD A3 .48m U H H United States Patent 3,501,921 METHOD 0F OPERATING HEAT EXCHANGERS IN CRYOGENIC SYSTEMS James R. Muenger, Beacon, and David L. Alexander,

Fishkill, N.Y., assignors to Texaco Inc., New York,

N.Y., a corporation of Delaware Filed Dec. 22, 1967, Ser. No. 692,856 Int. Cl. FZSj 3/00 U.S. Cl. 62-13 2 Claims ABSTRACT OF THE DISCLOSURE In the operation of reversing heat exchangers or thermal regenerators, contamination of a relatively pure high pressure process gas stream by a low pressure purge gas stream is avoided by isolating two sections of the heat exchanger, bleeding gas from the one end of the high pressure section of the heat exchanger to one end of the low pressure section thereof thereby equalizing the pressure between the sections, thereafter depressuring the high pressure section at the opposite end of the high pressure section and bleeding gas from the opposite end of the low pressure section into the depressured high pressure section whereby purge gas from the low pressure section is substantially completely replaced with high purity gas from the high pressure section prior to introduction of relatively pure, high pressure, process gas to the purged section of the yheat exchanger.

This invention relates to improvements in the method of operating heat exchange apparatus of the type known as reversing heat exchangers in systems i-n which a relatively pure process gas stream is at a pressure considervthe process gas mixture.

In cooling gaseous mixtures to cryogenic temperatures, the problem of frost and ice formation resulting from solidication of higher melting point components of said gaseous mixtures and the accumulation of solid deposits or frost on the surfaces of heat exchange elements is often encountered. Accumulations of solid deposits on heat transfer surfaces interfere with the flow of heat so that the rate of heat transfer from the gas stream undergoing cooling to solid heat exchanger surfaces is substantially reduced. Such accumulations also tend to restrict the flow of gas through the passageways of the heat exchanger, sometimes plugging gas passages in exchanger with solid deposits.

The operation of reversing heat exchangers, heat accumulators (sometimes called cold accumulators) or thermal regenerators, commonly employed in air separation systems is well known. Solid deposits accumulating on the heat exchange surfaces, or packing material, customarily are periodically re-evaporated into a purge gas stream. The operation as conventionally carried out is a cyclic one in which a gaseous mixture undergoing cooling, hereinafter termed the process gas stream, is passed over solid packing material at a temperature lower than the temperature of the process gas stream in one section of the regenerator and a stream of colder purge gas is passed over solid packing material at a temperature higher than the temperature of the purge gas stream in another section of the regenerator. The ow paths of the process gas stream and the purge gas stream are periodically reversed. It is particularly important in those situations in which the process gas stream is relatively pure, and it is desired to produce a substantially pure product gas stream, to avoid contamination of process gas with purge gas trapped within a regenerator when llow paths are switched.

In a commonly used system, a plurality of packed regenerators or heat accumulators are employed so that the process and purge gas streams may be switched from one to another to maintain continuous ow of both the process and purge gas streams. Regenerators of this type are well known and are described in the literature so that it is not necessary to describe them in detail herein. In normal operation, such regenerators retain or trap some of the purge gas in the interstices of the packing material. Packing material, such as metal or ceramic shapes, is warmed or cooled by the gas streams and serves to accumulate heat from the process gas and give up heat to the cooler purge gas.

Upon reversing the flow of the process gas and purge gas in such heat exchange systems so that the purge gas stream passes over the packing material formerly contacted by process gas stream and vice versa, some of the purge gas is picked up in the process gas stream. Similarly, some of the process gas is picked up -by the purge gas stream each time the streams of process gas and purge gas are interchanged. In addition, vapor from the deposits evaporated by the purge gas stream is picked up by the purge gas stream so that, in net effect, the more readily condensible components of the process gas stream are transferred to the purge gas stream. Losses due to stream mixing or contamination of the process gas stream with the purge gas are greater when the system is operated at high pressure than when operated at moderate pressure. In other Words, as the operating pressure is increased, the amount of contamination of process gas with residual purge gas retained on the packing at each cycle is increased. For this reason, the use of regenerators or reversing heat exchangers is usually limited to low or moderate pressure applications and to those situations in which the contamination of the process gas stream can be tolerated.

Problems which arise in connection with removal of deposits by use of a purge gas stream at lower pressure are common to various types of heat exchangers including both the reversing heat exchanger and accumulator types. In the general description of the problem and the operation of this invention, the terms heat exchanger, reversing heat exchanger, accumulator and regenerator are employed Without distinguishing one over the other.

This invention provides a method by which the amount of contamination of a process gas stream supplied to regenerators or reversing heat exchangers at a higher pressure than the pressure of the purge gas stream can be avoided or substantially reduced. The method of this invention may be applied to either regenerators (heat accumulators) or to combination indirect and heat accumulator type heat exchangers, as will be more evident from the detailed description hereinafter.

As used herein, the term heat exchanger is used in a generic sense to include both regenerators or heat accumulators (sometimes also called cold accumulators) as well as apparatus for directly effecting heat transfer from a warmer stream to a cooler stream without deliberate and cyclic storage of heat. The term regenerator is used herein in the sense commonly employed in this art. The term condensation is used herein in a comprehensive sense to include the formation of solid deposits which subsequently melt or sublime as well as liquid condensate. The term more readily condensible component is used in comprehensive sense to include higher melting point materials contained in process gas stream. The lterm process gas is used to designate a gaseous mixture which is subjected to cooling to a temperature below the dew point of the gas mixture or below the solidication temperature of its more readily condensible or higher melting point constituents.

Reversing heat exchangers, as contrasted with regenerators, are also used for the transfer of heat from a proeess gas stream to a colder gas stream to effect cooling of process gas. The use of reversing heat exchangers in place of regenerators permits purging of solid deposits from heat exchange surfaces by a minor fraction of a colder gas stream. For example, in the separation of oxygen and nitrogen from air in cryogenic air separation systems, a minor portion of the nitrogen may be passed through the heat exchanger for the removal of solid reposits from the heat exchange surfaces. The major part of the nitrogen may be passed over uncontaminated heat exchanger surfaces and delivered as clean product stream. The problem of loss of product gas or contamination of product gas or of purge gas occurs also in the operation of reversing heat exchangers even though such losses are generally lower than the losses in a comparable regenerator type system, primarily because the necessity for frequent switching is lessened.

In the purification of ammonia synthesis feed gas streams, i.e. mixtures of hydrogen and nitrogen, liquid nitrogen is frequently employed as a wash for the final clean-up of the hydrogen-rich gas stream. The nitrogen wash effects the removal of impurities, such as carbon monoxide, methane and argon, which are condensed from the gas stream and removed from the system as a solution in liquid nitrogen. At the same time, some liquid nitrogen is evaporated into the hydrogen-rich gas stream, supplying part of the nitrogen required for the ammonia synthesis reaction. Prior to cooling the process gas to a cryogenic temperature suitable for feeding it to the nitrogen wash tower, it is customary to substantially completely remove carbon dioxide, and hydrogen sulfide, carbon disulfide, and carbonyl sulfide, if present, from the gas stream by an absorption system. Absorption in methanol, acetone, low boiling hydrocarbons, polyglycol ethers, sulfanol, water, or in an aqueous solution of monoor diethanol amine or potassium carbonate, may be used for removal of various impurities from hydrogen.

Alternatively, carbon dioxide may be condensed in part by cryogenic cooling of the gas stream, the liquefied carbon dioxide separated from the hydrogen-rich gas stream, and carbon dioxide still remaining in the hydrogen stream subsequently removed therefrom by one of the absorption processes.

The method of this invention will be readily understoor from the following detailed description with reference to the accompanying drawings which illustrate diagrammatically specific examples, of a preferred mode of operation of the process of this invention.

FIG. l is a flow diagram illustrating a specific example of the method of this invention as applied to the purification of ammonia synthesis feed gas.

FIG. 2 is a diagrammatic representation of the method of this invention utilizing a set of four thermal regenerators and illustrating one complete cycle of regenerator switching operations.

Referring to FIG. l of the drawings, the

numerals

1, 2, 3 and 4 designate identical heat accumulators or thermal regenerators, for example Frankl-type regenerative heat exchangers, comprising a series of vessels containing packing material of high heat absorptive capacity. For convenience in describing the process ow, these ves- 4 sels are designated also by the letters A, B, C and D, respectively.

Although four heat accumulators are illustrated in the drawing, it is to be understood that more may ybe llSed if desired. Neither lthe specific arrangement of heat exchangers illustrated in the figures nor the specific number of vessels illustrated is to be taken as a limiting arrangement or number.

For convenience in describing the method of switching heat exchangers in accordance with this invention, a specific example is illustrated and described in which an impure stream of hydrogen is purified and blended with nitrogen as feed for ammonia synthesis. Such process gas streams are encountered commercially in plants generating hydrogen by partial oxidation of hydrocarbons or by reforming hydrocarbons from steam. Following shift conversion, i.e. reaction of carbon monoxide with steam and removal of carbon dioxide and water vapor, a hydrogen rich mixture containing minor amounts of methane, carbon monoxide, argon, and nitrogen is usually obtained. The process gas stream may also contain more or less carbon dioxide, depending upon the effectiveness of the carbon dioxide removal step.

High pressure process gas, for example shifted synthesis gas comprising hydrogen with minor amounts of carbon dioxide, carbon monoxide, methane, hydrogen sulfide, nitrogen, water vapor, argon and other rare atmospheric gases, at elevated pressure in the range of 300` to 2500 p.s.i.g., for example, 815 p.s.i.g., enters the system through line 5. The entering process gas, for example, at a temperature of' 80 F., is directed into a selected thermal regenerator by means of valves SA, 5B, 5C and 5D which direct the ow of gas to regenerators A, B, C and D, respectively. In the thermal regenerators, the process gas is cooled to a low temperature, for example a temperature o'f the order of 296 F. by direct contact with previously chilled packing material contained in the thermal regenerator. In passing over the cold packing material, the process gas stream is cooled to a temperature well below the solidication temperature of any carbon dioxide contained in the gas stream.

Chilled process gas discharger from the thermal regenerators is passed through

appropriate valves

6A, 6B, 6C and 6D into line 6 through which it flows into a nitrogen wash column 7. As the process gas passes down through the regenerator it is cooled and deposits carbon dioxide plus hydrogen sulfide and traces of water vapor as a solid on the surfaces of the cold packing material contained in the regenerator. Other relatively high melting point contaminants may also be condensed as solids or liquids in the regenerators.

While the process gas is undergoing cooling and cleaning in passing through one of the accumulators, for example, accumulator A, a stream of cold purge gas (which serves as a refrigerant) is passed through another of the accumulators, for example, accumulator B. In this specific example, dry air is employed as cold purge gas at a pressure of about 15 p.s.i.g. Purge gas from a suitable source enters the system through line 8 and is passed through heat exchanger 10, where it is cooled to a low temperature, e.g. 300 F., to line 11. Chilled low pressure purge gas from line 11 may be introduced into an appropriate thermal regenerator through one of the valves 11A, 11B, 11C or 11D to regenerator A, B, C, or D, respectively. In passing upward through a thermal regenerator, for example, regenerator B, the cold purge gas stream serves to chill the packing material in the thermal regenerator, and at the same time any deposits on the packing material are sublimed into the purge gas stream. Purge gas is discharged from the thermal regenerators through appropriate valve 12A, 12B, 12C, or 12D into line 12. The expended purge gas, comprising in this example air and carbon dioxide, leaves the system through line 12 at about 84 F. and is used for refrigeration external to the system of FIG. 1.

The partially purified process gas entering the nitrogen wash column at approximately 296 F. in this example comprises hydrogen together with small amounts of methane, carbon monoxide, nitrogen, and argon. In this example, the nitrogen wash tower is operated at about 800 p.s.i.g. Liquid nitrogen is supplied to the upper part of the nitrogen wash tower 7 through line 13. In the nitrogen `wash column, the hydrogen-rich feed gas stream entering through line 6 is brought into intimate countercurrent contact with liquid nitrogen whereby components in the feed gas stream other than hydrogen and nitrogen are condensed and accumulated in the lower part of column 7.

Bottoms from the nitrogen wash column comprising liquid nitrogen containing condensed methane, carbon monoxide, and argon, and dissolved hydrogen, typically at a temperature of about 307 F., is discharged from the bottom of the nitrogen wash column through line 14 and conducted to heat exchanger 15 where it is used to liquify nitrogen by countercurrent indirect heat exchange in two passes. In its first pass, which does not traverse the entire length exchanger 15, the bottoms stream is warmed to a suitable temperature for turbine expansion to achieve desired pressure and temperature of the expanded stream.

Typically, the desired pressure is that required for pressure drop in the second pass through exchanger 15 plus the pressure required to introduced the warmed bottoms stream into a tail gas furnace, and the desired temperature of the, expanded stream is near the lowest temperature of the system. The expansion thus furnishes low temperature refrigeration required by the System. In the specific example, the bottoms stream from line 14 in its first pass through exchanger 15 is warmed to approximately 98 F. before being conducted to expansion turbine 17 through line 16. Turbine 17 expands the bottoms stream to approximately l p.s.i.g. and 307 F. into line 18. The expanded bottoms stream after making its second pass through heat exchanger 15 enters line 19 as tail gas at essentially atmospheric temperature, and leaves the system. This gas, containing, principally, carbon monoxide, methane, hydrogen, nitrogen, and argon is used as a fuel external to the system of FIGURE 1.

Relatively pure nitrogen, e.g., nitrogen of 99.9-{- volume percent purity available from a' Suitable source, c g., from the rectification of air (not illustrated) is supplied to the system through line 20 at approximately 830 p.s.i.g. The flow of nitrogen to heat exchanger 15 through 'line 22 is regulated by valve 21. This nitrogen stream is cooled to approximately 296 F. in exchanger 15 at which temperature it is compressed liquid. The major portion of this compressed liquid nitrogen stream is used in the nitrogen wash column 7 as has been described. A

' small amount of the compressed liquid nitrogen of line 13 is bled into line 24, which carries purified overhead process gas from the nitrogen wash column 7 to heat exchanger 10. The bleed stream is regulated by valve 23. In this manner the refrigeration requirements of heat exchanger 10 are balanced. The remainder of the nitrogen being supplied to the system through line 20 is regulated by valve 25 into line 26 through which it is fed into the warmed purified process gas of line 27. When thus combined, the streams of

line

26 and 27 form the purified product gas of proper 3:1 hydrogen to nitrogen content (by volume) for ammonia synthesis. The product gas leaves the system through line 28. Purified hydrogen, containing a minor amount of nitrogen, for example at a temperature of about 307 F. is discharged overhead from the nitrogen wash column 7 to line 24. A small amount of liquid nitrogen is bled into this stream through valve 23 as has been described in order to increase its refrigeration capabilities. The combined stream is warmed to approximately ambient temperature in heat exchanger 10 by indirect heat exchange which protects the purity of the combined purified process -gas stream. The combined stream leaves heat exchanger 10 in line 27 and becomes product gas when supplied additional nitrogen from line 26 as has been described. The countercurrent stream which warms the purified gas in indirect heat exchange in exchanger 10 is dry low pressure purge gas which must be cooled to

service regenerators

1, 2, 3, and 4 as has been described. The required cooling of the purge gas is thus accomplished by the warming of the purified process gas. Heat exchanger 10 is a suitably compact brazed aluminum, plate-fin heat exchanger of the type normally used for cryogenic service.

Traces of contaminants may be found in available purge gases such as air used in this specific example. Drying of air normally will not lower the dew point below F. In addition, air normally contains 0.03 volume percent of carbon dioxide. Water and carbon dioxide and other trace components will gradually build up in the purge gas passageways of exchanger 10 and a periodic deriming of the exchanger is necessary. For this reason a duplicate exchanger 9 (shown dotted) is provided for use when exchanger 10 is being derimed. Derming is accomplished by warming the exchanger with high pressure gaseous nitrogen from line 20 passing through the process gas passages of the exchanger before being directed to line 22. No nitrogen is thus lost nor is there danger of contamination of the process gas. Purging of the purge gas side is then accomplished by diverting warmed bottoms gas from line 19 so that it passes through the exchanger carrying away vaporized contaminants. Finally the exchanger is rechilled by diverting a small portion of the effluent of turbine 17 from line 18 through the purge gas side until the exchanger is properly cooled for use. The deriming operation is accomplished with no loss of gas from the system. The refrigeration capability of the bottoms stream with its turbine expansion is adequate for the requirements of exchanger 15 plus the alternate recooling after deriming recooling of

exchangers

9 and 10.

A valved line 29 at the upper end of regenerators A and B permits fiow of gas between regenerators A and B. Similarly valved line 30 permits flow of gas between regenerators A and B at their opposite or lower ends. Valved lines 31 and 32 perform the same functions for regenerators C and D. The purpose of these valved lines will be explained hereinafter.

Operation of the thermal regenerators is illustrated in FIG. 2. In operation, the warm high pressure process gas is passed through a previously cooled, clean thermal regenerator while cold purge gas is passed through another which has been warmed by the process gas, and which contains solid deposits, e.g. solid carbon dioxide from the process gas. The cold purge gas is passed over the packing material in the reverse direction from the direction of flow of warm high pressure process gas in a' preceding step in the process. The purge gas thus functions both as purge and as a refrigerant. The purge gas stream becomes contaminated or diluted with vaporized solid deposits and with process gas remaining in the spaces surrounding the packing material in the regenerator. The method of this invention prevents the purge gas from contaminating the process gas upon reversal of ow through the packing material.

A complete cycle is illustrated diagrammatically in FIG. 2, beginning with the situation in which regenerator A has just completed its service of cooling the process gas stream and regenerator B has just been purged and cooled by the cold purge gas stream and is ready for service. At this point in the process, high pressure process gas is passing through regenerator D7 i.e. valves SD and 6D are open to permit the process gas stream to pass from line 5 to line 6 through regenerator D. At the same time valves 11C and 12C of FIG. l are open to permit low pressure purge gas from line 11 to pass through regenerator C into line 12.

Valves

5A, 5B and 5C, 6A, 6B, and 6C of FIG. l are closed as are valves 11A, 11B, 11D, 12A, 12B, and 12D. This situation is illustrated 7 in the following table in the column M, representing the end of a previous cycle in which the valve positions are indicated as O (open) or C (closed).

plied to a specificexample are used as terms of description and not ofy limitation. There is no intention in the use of such terms and expressions of excluding any equiva- SEQUENCE OF VALVE OPERATIONS, STEP NUMBER IN CYCLE Valve lllOlllll'llllllIllll M-End of previous cycle. l-Beginning of cycle. cycle.

In the table, the valve positions are plete cycle starting at the end of a previous cycle and continuing to the first step at the beginning of the next cycle. Dashed lines in the table indicate no change in valve position from the position of the valve in the previous step in the cycle.

In the rst step (step l) of the cycle illustrated in FIG. 2 and indicated in the following table valve 29 is opened to permit high pressure process gas to flow from the upper end of regenerator A to the upper end of regenerator B, equalizing the pressure in the two regenerator vessels and displacing low pressure purge gas from the upper part of the regenerator B to the lower part of regenerator B. In the next step in the process (step 2), valve 29 is closed and valve 30 opened to permit compressed purge gas from the lower part of regenerator B to tiow into the lower part of regenerator A. At the same time, valve 12A is opened to vent residual process gas from the upper portion of regenerator A into line 12. Valve is then closed (step 3), valve 5B opened to permit high pressure process gas to enter regenerator B and bring the regenerator up to the operating pressure of the process gas stream and valve 11A is opened permitting cold purge gas to flow through regenerator A. With regenerator B repressured, valve 6B is opened permitting the process gas to flow through regenerator B to line 6 (step 4). At the same time valves 5D, 6D, 11D and 12D are closed; regenerator D now contains high pressure process gas and regenerator C now contains low pressure purge gas.

A similar sequence of equalizing pressure, depressuring the regenerators and transferring process gas from regenerator D to regenerator C (step 5) followed by transfer of purge gas from regenerator C to regenerator D (step 6) are carried out as illustrated in the second line of FIG. 2. Thereafter regenerator C is repressured with process gas and ow of purge gas established through regenerator D (step 7) after which regenerators A and B are isolated (step 8). The sequence of steps is continued so that, in turn, the process gass passes through regenerator C while regenerator D is cooled and purged (steps 9-12) and then through A while B is cooled and purged (steps 13-16). In this manner, each of the regenerators in turn serves to cool the process gas stream and each in turn is depressured and purged and recooled by the cold purge gas stream.

The terms and expressions employed throughout the above description of the method of the invention as apl-End of cycle. N-Beginning of next indicated for a com- 25 lents of the features shown and described, or portions thereof. It is recognized that various modications are possible within the scope of the invention as described and as hereinafter claimed.

We claim:

1. A method of switching flow in reversing heat exchangers with minimum contamination of process gas with purge gas wherein said process gas in a first section of said heat exchanger is at greater pressure than said purge gas in a second section of said heat exchanger, lwhich comprises:

(a) isolating both of said sections from said process gas and purge gas streams;

(b) passing residual high pressure process gas from one end of aid rst section into a first end of said second section displacing residual low pressure lpurge gases toward the opposite end thereof and thereby substantially equalizing the pressures in said sections;

(c) isolating said rst section from said second section;

(d) discharging the process gas from said one end of said first section and passing said equalized gas from said opposite end of said second section to the opposite end of said rst section, and

(e) thereafter establishing flow of purge gas through said first section at relatively low pressure and establishing flow of process gas through said second section at relatively high pressure.

2. A process according to claim 1 wherein the initial pressure in said irst section is at least one and one-half times the equalized pressure of said sections and the final discharge pressure from said rst section is not more than one-half said equalized pressure.

References Cited UNITED STATES PATENTS 3,056,269 10/1962 Melchior 62-12 3,073,128 1/1963 Becker 62-13 3,251,189 5/1966 Jakob 62-13 3,375,672 4/1968 Jakob 62--13 3,421,332 1/1969 Becker 62l3 WIIJBUR L. BASCOMB, JR., Primary Examiner A. PURCELL, Assistant Examiner U.S. C1. X.R.