US2537045A - Cooling gases containing condensable material - Google Patents

Description

Jan. 9, 1951 p, W @ARBO 2,537,045

COOLING GASES CONTAINING CONDENSABLE MATERIAL Filed Feb. 8, 1949 jg CULD H/R 175g/ Patented Jan. 9, 1 951 COOLING GASES CONTAINING CONDENSABLE MATERIAL Paul W. Garbo, Freeport, N. Y., assignor to Hydrocarbon Research. Inc., New York, N. Y., a corporation of New Jersey Application February 8, 1949, Serial No. 75,220

(Cl. (iz-122) i4 Claims.

This invention relates to processes involving heat exchange and more particularly to processes for cooling gaseous streams in which condensation occurs during the contact of the gasecus stream with the cooling surfaces.

in heat exchange procedures, condensible components of a gaseous stream are deposited on the surfaces of the cooling means when the gaseous stream is cooled to a temperature below the condensation points of the condensible components. Often the cooling surface is cold enough to cause freezing or solidication of the condensed material so that there is a progressive building-up of condensed material on the cooling surface with result that the heat transfer rate progressively decreases and the heat exchange apparatus eventually is stopped up or choked with the solidified deposits. Several proposals have been made to prevent or remove such solidied deposits.

It has been suggested that the gaseous stream to be cooled be pretreated to remove condensible components prior to admittance of the gaseous stream into the cooling exchanger. For example,

in the production of oxygen by the liquefaction i and rectification of air where the air feed is cooled by passage in indirect heat exchange relation vwith the cold outgoing products of rectication, the air which invariably contains about 0.03% by volume of carbon dioxide and varying quantities of moisture, is pretreated in driers and caustic scrubbers to remove the water and carbon dioxide, respectively, prior to admittance of the air into the heat exchangers. Even with such pretreatment which is costly and cumbersome. the exchangers have to be thawed out periodically to remove solidified deposits of water and carbon 'dioxide which are present in traces in the pretreated air.

More recently there has been developed the use of reversing exchangers by which the gaseous stream to be cooled and the cooling fluid passed in indirect heat exchange relation therewith are reversed, i. e., the flow paths are switched periodieaily so that the cooling fluid flows through the path previously traversed by the gaseous stream to be cooled and the gaseous stream to be cooled flows through the path previously traversed by the cooling uid. By such reversal, the deposited components are re-evaporated or sublimed and carried out of the heat exchanger by the cooling fluid. In other words, the material deposited by the process stream during one period of operation is reevaporated and purged from the exchanger by the reverse ow of the coolant stream during the succeeding period. The use pressure results in more costly operation from the standpoint of horsepower requirements because, upon every reversal, the volume of the compressed process gas in the heat exchanger is lost and must be replaced.

Itis a primary object of this invention to provide a simple and improved process for cooling a gaseous stream to a temperature at which condensation normally occurs on the cooling surfaces.

Another important object of this invention is to prevent the deposition on cooling surfaces of condensible components of a gaseous stream undergoing cooling.

It is a'further object of this invention to provide a truly continuous method of cooling a gaseous stream containing condensible material without pretreatment of the stream.

Other objects and advantages of this invention will become apparent from the description which follows.

In accordance with this invention, moving solid particles and a gaseous stream containing condensible matter are brought into contact with the cooling surfaces of a heat exchanger to effect cooling of the gaseous stream to a temperature at which the condensible matter is condensed. The gaseous stream containing the condensible matter ilows through the heat exchanger in countercurrent flow and indirect heat exchange relation to a gaseous cooling stream. As the gaseous stream is cooled, the condensible matter therein is substantially prevented from accumulating as condensate on the cooling surfaces by the solid particles contacting these surfaces, the condensible material being picked up by the moving solid particles and conveyed therewith out of the cooling exchanger. The cooled particles and the condensible material which has become associated therewith are separated from the gaseous stream which has been cooled to the desired low temperature and are brought into contactwith the coolant stream while the coolant stream is passed in indirect heat exchange and countercurrent flow relation to the gaseous stream being cooled. Under these circumstances, the condensible material is re-eva'oorated or otherwise released from the solid particles into the gasiform coolant so that the solid particles are then separated from4 the gasiform coolant substantially freed of the condensible material which was associated with the particles before they were brought into contact with the coolant stream. The solid particles thus purged of condensible material are again introduced into the heat exchanger in direct contact with the gaseous stream containing condensible material and undergoing cooling to repeat the cyclic operation just described.

Since a gaseous stream containing one or more condensible components can be cooled by this invention substantially without condensation on the cooling surfaces, it is evident that the rate of heat transfer will not diminish in the course of operation which, incidentally, does not need to be interrupted because of stoppage of the exchanger or reversal of the streams passing therethrough to effect purging of condensed material deposited on the cooling surfaces.

While the solid particles used in the process of this invention may be such common materials as sand, crushed stone, powdered coke or metal, and the like, it is frequently preferred to employ solid particles which preferentially adsorb the condensible matter in the gaseous stream undergoing cooling. Thus, where air containing moisture and carbon dioxide is cooled in accordance with this invention to a temperature below that at which solid carbon dioxide would normally be deposited on the cooling surfaces, particles of adsorptive silica gel may be passed through the heat exchanger in direct contact with the air stream to adsorb preferentially the moisture and carbon dioxide therein and thus further ensure that the solid particles keep the cooling surfaces of the exchanger substantially free of deposited condensible matter. Other illustrative situations include the cooling of hot gases containing tarry vapors in which case adsorptive carbon may be used to prevent the deposition of tar on the cooling surfaces, and the cooling of natural gas containing condensible impurities like hydrogen sulfide and carbon dioxide in which case adsorptive bauxite and silica gel may be used, respectively, to obviate the accumulation of these components on the cooling surfaces.

In some cases, it may be advisable to use two or more different adsorbents to remove different condensible components from the gaseous stream undergoing cooling. The different adsorbents may be used as a particulate mixture or separately in different zones of the heat exchanger system particularly where the different components are condensed in different temperature ranges. Sometimes, one particulate adsorbent may be used to take up two or more condensible components of a gaseous stream which is being cooled to a temperature below the condensation points of these components.

The present invention is particularly valuable in the cooling of a gaseous stream containing at least one condensible component to a temperature at which the condensed material freezes or solidiiies on the cooling surfaces. A prominent case in which the condensible matter of a gaseous stream is normally solidified on the cooling surfaces of a heat exchanger is the cooling of air preparatory to its liqueiaction and rectification for the production of oxygen. For convenience, further description of the invention will be in terms of ordinary air which is cooled for an oxygen plant, but it will be understood that the invention is not limited to this illustrative application.

For a clearer and more detailed understanding of this invention, reference is now made to the drawing forming a part of this specification, of which:

Figure 1 is a diagrammatic representation of apparatus suited for the process of the invention: and

Figure 2 is a fragmentary diagram showing a modification of the lower or warm end of the heat exchange system of Figure l.

It is to be understood the invention is not limited to these illustrative embodiments nor to others herein discussed.

The apparatus of Figure 1 comprises heat exchanger I0 which may be of any well-known type that will permit the simultaneous passage therethrough of solid particles and a gaseous stream on each of the opposite sides of the heat transfer walls of the exchanger. In the embodiment illustrated herein, the exchanger II) consists of an outer shell Il and an inner shell I2, disposed in concentric relation to each other. Thus, exchanger IIl has an outer annular' flow path A and an inner cylindrical flow path B separated from each other by the heat transfer wall or shell I2. It is desirable to have fins of heat conducting material, e. g., fins of copper, aluminum or the like, attached to the opposite sides of heat transfer wall I2 to promote rapid and efficient heat exchange between the gaseous streams flowing through paths A and B. The fins are preferably arranged parallel to the line of flow so as not to interfere substantially with the movement of the particles, particularly where the particles are suspended in the gaseous streams passing through paths A and B.

In Figure 1, pipe I3 empties into path A of heat exchanger II) for the admission of a warm air stream containing moisture and carbon dioxide. Solid particles are introduced from separator Il through pipe I5 into the air stream flowing through pipe I3. The air stream has sufficient velocity, usually at least about 5 feet per second and preferably at least about 20 feet per second, to carry the solid particles in suspension during its passage through path A of exchanger I0. The air stream and the suspended moving particles are in direct contact with one another and with the outer surface of heat transfer wall I2 and pass upwardly through path A in indirect heat exchange and countercurrent now relation to the gaseous coolant stream, i. e., a cold rectification product of air, flowing downwardly through path B. The solid particles now up through path A in rubbing contact with heat transfer wall I2 and thus prevent the build-up of solidified deposits of ice and carbon dioxide thereon; these condensible components of air become associated with the moving solid particles and are carried therewith out of exchanger I0 by way of pipe I1 which discharges into separator I8 wherein the chilled air, substantially free of moisture and carbon dioxide, is separated from the solid particles and associated deposits of moisture and carbon dioxide.

Separator I8 which may be of the cyclone or other conventional type has a pipe I9 for withdrawing the chilled air, which may then be passed to the liquefaction and rectification plant, and a rotary bucket-type valve 20 or equivalent device for transferring separated solids from separator I8 to pipe 2l. The solids containing moisture and carbon dioxide pass from pipe 2I into the cold rectification product stream flowing through pipe I6 which supplies the gaseous coolant to path B of exchanger I0. Thus, the solid particles containing the condensible material originally present in the air stream are transported into path B and pass downwardly therethrough. During the passage of the particles through path B, carbon dioxide and moisture are released from the solid particles to the gaseous coolant stream in contact therewith so that by the time the particles leavek path B by way of pipe 22 together with the warmed-up coolant stream a substantial portion or all of the moisture and carbon dioxide have been revaporized and freed from the solid particles.

The solid particles and warmed-up coolant stream discharge from pipe 22 into separator I4 which may be the same as separator I8. The warm gaseous stream exits from separator I4 through pipe 23, substantially free of solid particles but containing the revaporized moisture and carbon dioxide. The separated particles pass from separator I4 by way of rotary valve 24 to pipe I5 and thence into the air stream flowing through pipe I3 to repeat the cyclic operation hereinbefore described.

Figure 2 shows a modification of the lower or warm end of the exchanger system of Figure 1. This modification is of particular value where the solid particles used in the exchanger system also function in preferentially adsorbing condensible matter in the gaseous stream undergoing cooing and do not release all of the adsorbed matter while in contact with the gaseous coolant passing through the heat exchanger. Under such circumstances, the adsorptive particles passing from path B of exchanger I0 through pipe 22 to separator I4 would still contain some adsorbed matter.

According to Figure 2, adsorptive particles containing some adsorbed matter may pass from separator I4 to stripping chamber 25 provided with heating coil 26 to heat the particles and thereby effect volatilization of the adsorbed matter which is then vented through pipe 21. Thus further stripped of adsorbed matter, the warm particles continue' their downward movement into and through cooling chamber 28 provided with cooling coil 29, which may be supplied with ordinary cooling water, to cool the adsorptive particles to a temperature approximating that of the gas entering exchanger I0. It is generally advisable to cool the particles to a temperature somewhat below, say about to 20 F. below, the gas feed temperature. The adsorptive particles are transferred from cooling chamber 28 to pipe I5 by means of rotary valve 24. The stripped or regenerated adsorptive particles discharge from pipe I5 into the gas stream containing condensible material and flowing through pine I3 to perform the cyclic operation of this invention In some cases, the additional stripping of adsorbed material from the solid adsorbent, which is effected in the added equipment of Figure 2. may be conducted only periodicaly as needed: in such event, heat would be supplied through coil 26 to stripping chamber 25 only at predetermined intervals. Incidentally, stripping may be carried out by injecting steam or other heated gas through pipe 26a directly into contact with the adsorptive particles in chamber 25 instead of passing a heating uid through coil 26. The steam or other heated stripping gas together with the condensible matter stripped from the solid adsorbent would be withdrawn through pipe 21.

Alternatively, instead of interrupting the flow of heat or stripping gas into chamber 25, the solid adsorbent containing some adsorbed matter may leave separator I4 as two streams: one stream of adsorptive particles passing through stripping chamber 25 and the other through pipe I5a controlled by rotary-valve 24a. Both streams of particles would discharge into the gas flowing through pipe I3. By regulating the relative pro`I portions of the two particle streams passing from separator I4 to pipe I3 of Figure 2, the adsorptive particles may be introduced into the gaseous stream containing condensible matter with as low an average content of residual adsorbed matter as may be desired.

For further clarification of the invention and its advantages, a specific example will now be presented illustrating one embodiment of the invention as conducted in the apparatus of Figure 1.

In the production of oxygen by the liquefaction of air and its rectification at low temperatures, the rectification is preferably conducted in two stages at different pressures. The refrigeration necessary for liquefaction may be supplied to the air after it has been compressed and water cooled to approximately room temperature, by indirect heat exchange with the eilluent products of rectii'lcation. An additional amount of refrigeration is usually supplied to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks in the system.

Fine sand particles of about mesh are fed continuously from hopper I4 by way of valve 24 to the incoming warm air stream in pipe I3. The sand particles are added at a rate of about 0.1 pound per pound of air. The warm air containing moisture, carbon dioxide and other condensible constituents like acetylene is supplied to pipe I3 at a gauge pressure of about 75 pounds per square inch and a temperature of about '70 F. and in flowing through pipe I3 carries the sand particles in suspension into and through path A of exchanger I0 at a velocity of about 50 feet per second. Cold nitrogen from the low pressure stage of the rectification system flows continuously through path B countercurrently and in indirect heat exchange relation to the air stream. The cold nitrogen enters path B at a gauge pressure of about 5 pounds per square inch and a temperature of about 275 F. The nitrogen travels down through path B at a velocity of about 25 feet per second. The air and suspended sand particles passing upwardly through path A are cooled and during this passage the condensible constituents of the air undergo solidiflcation but are prevented from building up as deposited material on heat transfer wall I2 by the sand particles passing and rubbing thereover. The solidified moisture and carbon dioxide thus be-v come associated with the moving sand particles and are transported therewith. The cooled air and the sand particles together with solidified condensibles leave the top of path A through pipe I1 and discharge into separator I8. The cooled air is separated from the sand and the associated solidified material and leaves separator I8 at outlet I9 at a temperature of about -270 F., whence it flows to the rectification system. Not only is the air thus cooled but also it is now substantially free of condensible constituents. The sand particles and solidified condensibles are continuously transferred from separator I8 through valve 20 and pipe 2I to the cold nitrogen rectification product stream flowing through pipe I6. The particles are suspended in the cold nitrogen stream and carried thereby into and through path B. During the heat exchange between the upwardly flowing air of path A and the downwardly flowing nitrogen of path B, the'suspended particles in path B are warmed and the condensible constituents are revaporized. The nitrogen 7 stream and the entrained particles leave path B at its warm end through pipe 22 which leads into separator I4. The nitrogen together with the revaporized carbon dioxide and otherVY condensibles originally present in the incoming air is separated from the fine sand and leaves separator I4 through outlet 23 at a temperature of about 65 F. The sand purged of the condensibles is refed to the incoming air stream of pipe I3 to continue the cyclic operation.

The foregoing illustrative operation can be conducted with a particulate adsorbent like silica. gel which tends to adsorb preferentially the moisture, carbon dioxide and acetylene in the air supplied to the exchanger. If a particulate adsorbent is chosen, the exchanger system may be modified as shown in Figure 2 so that the arsorbent separated from the effluent nitrogen may be more completely stripped of adsorbed condensibles in chamber 25 before being reintroduced into the air stream flowing through pipe I3. For instance, hot flue gas may be passed by way of pipe 2a up through a bed of the silica gel enter ing chamber 25 from separator I4. A temperature of about 300 F. is thus established in chamber 25 with the result that residual adsorbed condensibles are removed from the particulate adsorbent and carried away with the flue gas issuing from pipe 21. The hot purged adsorbent is then cooled in chamber 28 by passing cooling water through coil 29 and thence returned to pipe I3 to repeat the cycle of operation. Under such circumstances, the rate of adsorbent addition to the air stream entering the exchanger may be only about 0.01 pound per pound of air.

If desired, the apparatus may be arranged so that the air stream passes downwardly through the exchanger and the nitrogen stream passes upwardly therethrough. While it is preferable to locate the air stream in the outer path of the exchanger and the cooling nitrogen stream in the inner path, the reverse arrangement may be employed.

The present invention permits the use of coolant and air stream paths of different cross-sectional area which arrangement is not possible with reversing type exchangers because of the change in pressure drop that would be realized on reversal of the streams. Different sized paths permit different velocities for the gaseous streams which are passed in indirect heat exchange relation with each other. As an illustration of some of the advantages of such diiference in velocity, it may be pointed out that a high air velocity ensures good rubbing contact of the cooling walls by the particles suspended in the air stream and consequently ensures the prevention of accumulation of condensed material on these walls. A high air velocity also improves the rate of heat transfer. At the same time, a comparatively low nitrogen velocity involves a low pressure drop in the nitrogen stream and this tends to reduce the energy consumed at the air compressor. in another way, for a given nitrogen velocity, this invention permits increased air velocity without appreciably increasing the energy consumed in compressing the air.

A noteworthy feature of the invention is that the amount of refrigeration abstracted from the gaseous coolant to condense and even freeze the ccndensible matter in the gaseous stream undergoing cooling is not lost because the condensibles which become associated with the moving solids in the exchanger path in which the gas is cooled are conveyed to and revaporized in the exchanger Stated path for the gaseous coolant. A further advantage of the invention over previous methods for clearing heat exchangers of material condensed therein is that the operation is truly continuous. Therefore, there is no loss of refrigeration as occasioned by the periodic thawing out of the exchanger.

Many modifications of the basic invention will suggest themselves to those skilled in the art. For example, the exchanger with its two countercurrent paths or zones may comprise a large cylin drical vessel in which the inner path is made up of a multiplicity of small tubes distributed within the vessel; one path, preferably for the process stream, is that surrounding the several tubes and the other path, preferably for the gaseous coolant, is provided by the tubes. The exchanger may be placed in any desired position, providing, of course, the velocity of the gaseous stream to be cooled is sulllcient to move the solid particles in rubbing contact with the cooling walls of the exchanger to prevent the accumulation thereon of matter condensible under the operating conditions. The arrangement must also provide for the movement of the solid particles through the coolant path of the exchanger. Accordingly, the exchanger may be placed horizontally or when in a vertical position its warm end may be above the cold end. Frequently, it is advisable, Where adsorptive particles are not being used, to use particles having the same composition as the exchanger walls, e. g., for an aluminum-walled exchanger, aluminum particles are suggested and similarly copper particles for copper-walled exchangers.

In the liquefaction and rectification of air there are two principal rectification products, namely, nitrogen and oxygen. As discussed hereinbefore, only the nitrogen rectification product has been used to cool the air stream. However, the oxygen rectication product stream may be similarly employed. In the case where it is desirable to use both the oxygen and nitrogen rectification product streams, the nitrogen may be employed as hereinbefore discussed and the oxygen may be passed through a third path in the exchanger. It is optional whether or not solid particles move through the third path.

In view of the various modifications of the invention which will occur to those skilled in the art upon consideration 0f the foregoing disclosure Without departing from the spirit or scope thereof, only such limitations should be imposed as are indicated by the appended claims.

What is claimed is:

1. The method of cooling a gaseous stream comprising ccndensible material to a temperature which is effective forV the condensation of said ccndensible material, which comprises passing said gaseous stream and solid particles in contact with one side of a heat transfer wall and thereby cooling said gaseous stream to a temperature at which said ccndensible material becomes associated with said particles and is prevented from accumulating on said wall, separating the thus cooled gaseous stream from said particles with the associated ccndensible material, passing a gaseous coolant and said particles with the associated ccndensible material in contact with the opposite side of said wall and thereby releasing said associated ccndensible material from said particles, separating said gaseous coolant containing ing the released ccndensible material from said particles, and again passing the separated ticles in contact with the first said side of said wall.

2. The method of claim 1 wherein said gaseous stream and said gaseous coolant are passed countercurrently on the opposite sides of said heat transfer wall.

3. The method of claim 1 wherein said solid particles comprise a preferential adsorbent for said condensible material.

4. The method of chilling air containing moisture and carbon dioxide to a temperature below the freezing points of moisture and carbon dioxide and of separating said moisture and said carbon dioxide from said air, which comprises passing said air and solid particles suspended therein in contact with one side of a heat transfer wall and thereby cooling said air to a temperature at which said moisture and said carbon dioxide become associated with said particles and are prevented from accumulating on said wall, separating the thus cooled air from said particles with the associated moisture and carbon dioxide, passing countercurrently to said air a cold gaseous rectication product of said air and said particles with the associated moisture and carbon dioxide suspended therein in contact with the opposite side of said wall and thereby releasing said associated moisture and carbon dioxide from said particles, separating said rectification product containing the released moisture and carbon dioxide from said particles, and again passing the separated particles in contact with the first said side of said wall.

5. The method of claim 4 wherein said solid particles comprise adsorptive silica gel.

6. The method of claim 5 wherein said adsorptive silica gel is treated to strip residual moisture and carbon dioxide therefrom before being again passed in contact with the rst said side of said wall.

7. The method of claim 4 wherein said cold gaseous rectification product is nitrogen at an initial temperature .below about 270 F.

8. The method of claim 4 wherein said air is passed in contact with said wall at a higher velocity than said rectification product.

9. An improved heat exchanger comprising two ow paths on opposite sides of a heat transfer wall, means for feedinga gaseous stream and solid particles into one end of one of said flow paths, means for withdrawing said gaseous stream and said particles from the other end of said first flow path, means for separating the withdrawn gaseous stream and particles, means for feeding the separated particles and a gaseous coolant into oneend of the second flow path, means for withdrawing said coolant and said particles from the other end of said second flow path, and means for separating the withdrawn gaseous coolant and particles.

10. The heat exchanger of claim 9 wherein the two flow paths are arranged for countercurrent ilows therethrough.

11. The method of cooling a gaseous stream comprising condensible material to a temperature below the freezing point of said condensible material, which comprises passing said gaseous stream and solid particles suspended therein in contact with one side of a heat transfer wall and thereby cooling said gaseous stream to a temperature at which said condensible material freezes on said particles and is prevented from accumulating on said wall, separating the thus cooled gaseous stream from said particles with the frozen condensible material, passing a gaseous coolant and said particles with the frozen condensible material suspended therein in contact with the opposite side of Said wall and thereby releasing said frozen condensible material from said particles, separating said particles from said gaseous coolant and the released condensible material, and again passing the separated particles in contact with the first said side of said wall.

12. The method of claim 11 wherein said gaseous stream comprising condensible material is air comprising carbon dioxide.

13. The method of claim 11 wherein said gaseous stream and said gaseous coolant are passed countercurrently on the opposite sides of said heat transfer wall.

14. The method of claim 13 wherein said condensible material comprises carbon dioxide.

PAUL W. GARBO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,378,607 Watts June 19, 1945 2,475,255 Rollman July 5, 1949

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