US2499043A - Heat exchange - Google Patents

Description

V. VOORHEES HEAT EXCHANGE Feb. 28, 1950 Filed March 26, 1947 2 Sheets-Sheet 1 V. VOORHEES HEAT EXCHANGE Feb. 28, 1950 2 Sheets-Sheet 2 Filed March 26, 194'? t e M 5 Patented Feb. 28, 1950 HEAT EXCHANGE Vanderveer Voorhees, Homewood, Ill., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Application March 26, 1947, Serial No. 737,209

This invention relates to improved heat exchangers for cooling fluids and more particularly to an apparatus and method for separating oxygen from ali` by the use of improved heat exchange means. Still more particularly the invention relates to a packing for a regenerativetype heat exchanger which provides a heat exchanger operation giving a substantially constant outlet temperature.

The invention is illustrated by drawings which show in Figure 1 a schematic diagram of an oxygen recovery apparatus and in Figure 2 a detail of the heat-exchanger packing material employed in the process. y

The separation of oxygen and air has heretofore been accomplished chiefly by two rather closely related processes wherein the oxygen and the nitrogen of the air are separated by fractionation in liquefied form. One of these processes, the Linde process, has employed high pressure heat exchangers in coooling the air to liquefaction temperature, whereas the other process, the Frankl modification of the Linde process, has employed regenerative heat exchangers operating intermittently and reversibly to cool the air to liquefaction temperature. In the latter process, the heat exchangers are packed with a heat-retaining packing material presenting an extensive surface,` the temperature of the packing material rising and falling between cycles of :s

charging and discharging through the exchanger, heat being transferred from the charge air to the packing material during the charging cycle and heat being absorbed by the packing material from the cold gases when discharging through the regenerator. For packing, it has been the practice to employ metallic foil, e. g. aluminum foil, suitably crimped and rolled to provide extensive'heat exchanger surface.

The use of the regenerative heat exchanger in the Frankl modilcation has provided numerous advantages over the` Linde exchanger and made it possible to produce oxygen at far lower cost than those obtainable by the Linde method. Theoretically perfect heat 'exchange cannot be obtained in a regenerator, however, inasmuch as the temperature of the packing must rise and fall substantially between cycles `in order for the exchanger to provide the necessary heat capacity in the operation. shortening the cycle time reduces the temperature iluct'uation but introduces other undesirable factors, particularly excessive production of purge gas produced between cycles. It is an object of this invention to increase the heat capacity and the cycle time of a regenerative exchanger in a reversible gas-cooling operation. Another object of the invention is to increase the eillciency of a regenerative exchanger by more closely approaching constant temperature conditions between cycles. Still another object of the invention is to simplify the operation of the Linde-Frankl oxygen process by employing an improved exchanger packing material providing uniform gas temperaturesfor the fractionation operation.

According to my invention, I employ as heatexchange material fusible solids retained within suitable envelopes or seals to prevent contamination of the gases. The fusible solids employed are selected with fusion temperatures substantially equal to the desired gas temperature at the point in the exchanger in which they are employed. Inasmuch as there is a desirable temperature gradient from end to end of each exchanger, I employ a series of fusible solids with progressively increasing or decreasing melting points. In the preferred arrangement, the fusible solid exchange materials are packed in the exchanger in successive layers and it is further preferred that the heat capacity provided by each layer, i. e. the heat absorbed for a given change in temperature, be .substantially the same. and that the fusion points of the solids in each layer be substantially equally spaced on the temperature scale.

Referring to Figure 1, air is compressed in compressor I Il to a pressure of about 50 to 100 pounds per square inch, generally about '75 pounds per square inch, -cooled and charged to two of the twin regenerative exchangers I, 2, 3 and 4. Thus, for example, the air may be charged by lines II and I2 to exchangers I and 3. In passing through the exchangers, the air is cooled by contact with the packing material therein to a temperature near the liquefying point, e. g. about -1'I5 C. at the pressure used. From the exchangers the air passes by lines I3 and I4 into manifold I5 leading through coolers I6 and Il and discharging into fractionator I8. A part of the air, mostly oxygen, is liquefied in coolers I6 and I1 and the resulting liquid air is fractionated in the fractionator operating under pressure substantially equal to the pressure supplied by compressor IIl less a small pressure differential resulting from the resistance to flow through the exchangers.

In fractionator I8, the oxygen is separated from the nitrogen in a manner Well known in the art with the advantage of the difference in boiling points, oxygen boiling at 183 C. and

nitrogen at -195.8 C. under normal pressure. At a pressure of '15 pounds per square inch, gage, the oxygen and nitrogen boil at about -165 and 175 C., respectively. Argon which constitutes about 1% of the air charged has a boiling point just below that of oxygen and may be left with the oxygen as a contaminant, unobjectionable for most industrial uses.

From the base of fractionator i8, liquid oxygen, suitably about 85 to 95 percent purity, is withdrawn by line I9 leading to oxygen exchangers I and 2 to which it is vmaniiolded through expansion valves 20. On expansion at substantially atmospheric pressure, intense cooling oi the oxygen occurs and the resulting cooled mixture of oxygen, gas and liquid, is conducted by line 2| to exchanger 2 through which it ilows in a direction reverse to the arrows passing out by line 22 by which it is discharged from the system at substantially atmospheric pressure and slightly below the inlet temperature of the'air charged at II, the regenerating eiect of the outiiowing gas being transferred to thepacking material in the exchanger.

A part or all of theoxygen leaving'the base oi fractionator I8 is conducted by line 23 to expansion valve 24 where its pressure is reduced to substantially atmospheric and the` temperature to that of liquid oxygen boiling at atmospheric pressure. e. g. about 180 C. The cold mixture oi' liquid and gas flows through reflux coil 25, thereby condensing nitrogen gas in the top of the tower I8 by indirect heat exchange. supplying liouid nitrogen reux for the fractionation. The maximum amount of reflux which-` would be obtainable from this coil is about one-fourth the amount of nitrogen produced but this can be augmented as hereinafter described. Expanded oxygen leaving coil 25 is conducted by line 26 and lines 21 and 28 to the low pressure side'of expansion va1ves'20 for discharge from the system through oxygen exchangers I and 2.

In order to provide reboling in the base of fractionator I8. a part ofthe cold'oxygen gas from line 26 is conducted by line 29 to compressor 30 whereby itis recomnressed to fractionator pressure. e. g. 75 pounds ver square inch', and introduced into the base'of the fracti'onator by line 3|. The temperature ofthe gas is substantially increased by the work of compression and this heat is employed'to reboil the oxygen in the base ofthe fractionator supplying additional vapor for' fractionation. Furthermore, the amount of oxygen recycled vin this'way serves to augment the amount of liduid oxygen available for the rei-lux coil 25, thereby increasing the nitrogen reflux ratio in a desired manner. Heat of compression of oxygen in line 3| may be removed by heat exchange with cold oxygen in lines I9 or 26, or with other suitable cold stream available at about the temperature of the base of the fractionator. thereby serving to'further increase the amount of oxygen which may be recycled to the fractionator in this mannen Nitrogen gas is withdrawn from `the top of the fractionator I8 by line'32 leading to expansion valves 33 where pressure is reduced to vsubstantially atmospheric. lOh expansion oi the cold gas, a portion of the nitrogen is liqueed by the Joule- Thompson effect and is collected in`one of 'the separators 34 and 35. Thus liquid nitrogen is withdrawn from the base' of lseparator 35 by Valved line 36 leading to pump 31 and thence b'y line v3|! the liquid nitrogen is returned to the top I fractlonator I3 to supply additional reiiux therein. In order to obtain further refrigeration for the system, a part of the nitrogen in line 32, for example one-third to two-thirds, is conducted through by-pass line 39 leading through heat exchanger I6 and thence by line 40 to expansion turbine 4I in which the gas is expanded down to substantially atmospheric pressure, the expanded gas flowing by line 42 through exchanger Il and thence by line 63^to one of the nitrogen exchangers 3 or 4, depending on which one is employed for outgoing nitrogen at the moment. Thus, it may pass through valved line 44 and line 45 through exchanger 4 in a reverse direction and thence be discharged by line 46 which may lead to the atmosphere if no use is made of the nitrogen, or 'to suitable storage if the process is being operated to recover'nitrogen. In line 45 the nitrogen stream is augmented by nitrogen gas from separator 35.

In the operation of expansion engine 4I, the temperature of the nitrogen in line 39 is increased, e. g. 5 to 25 CL, by passage through exchanger IB, in order toavoid liqueilcation'in expander 4I. The expander may bea multistage turbine providing useful power for the process, e. g. for compressing air by compressor III. It is desirable to control the operation of the expander, and particularly the temperature of the inlet nitrogen in line 40 so that the outlet nitrogen in line 42 is at about its liquefaction temperature, e. g. about 195 C.

The cold nitrogen gas `passing through exchanger 4 cools the heat-absorbing elements therein as will be hereinafter described. After operating for a brief interval, the ow of gases through the exchangers I to 4'is reversed in order to balance the refrigeration of the outflowing gas with the heating effect of the incoming air. To facilitate this'reversal valves 4l, 48, 49 and 50 leading from the air manifold 5I can be automaticallyoperable by ab cycle timer. Likewise valves 52, 53, 54 and 55 leading to the cold air'manifold I5 are automatically operable. Also valves 56 and v5| leading'to the oxygen outlet are automatically controllable as well'as valves 58 and 59 leading to the'nitrogen'outlet line. By means of an electric orpneumatic controller or other device operating on a time cycle, valves 41, 49, 52, 54, 5l and 59 are in the'open position while valves'48, 50, 53, 55, 56 and 58 arein the closed position, valves 20 and 33 being setto'pass outilowing oxygen and nitrogen through lines 2| and 45, respectively. At the end of'the next time interval, the position of 'these'valvesis instantly reversed, thereby reversing" the ow of gases through the exchangers. If desired, means may be provided for purging vair from the oxygen exchangers. The cycle time, heretofore about 5 minutes, can be increased with the use of my crystallizing packing to 10 or 20 minutes or even more, depending on'the mode of application of the material.

Although all the'exchangers are shown in the drawing as'aboutthe'same size; it isadvisable that the -capacity uof the'nitrogen exchangers be about fourxtimes that offtheoxygen exchangers which may bei effected by'b'uildingl them about twicethediameter.' Likewise,.the volume of air admitted' through'. valvesl 41 and 48 should be about one-'fourth the '-.amount which isadmitted through'valves49 and '50. This'division, however, may'b'e varied somewhat'inl order to best balance` the' refrigeration load.. on .thea different exchangers;

Inuoperati'on, the f-airJcharged-to'the-.processvlll terial in solid form. On the reverse blowthe usually contain considerablemoisture-andabout .03% CO2. On cooling in the exchangers,- water is initially condensed and provision is made for collecting it and discarding it from the system, forexample by valved drain lines I0, 6i, 62 and 63. These lines preferably connect with a water pan located at that point in theexchanger where the liquid water rst condenses, for example at a point part way up in the exchanger. After the air is cooled to still lower temperature, the water vapor contained in it separates in the .form of ice on the heat-exchange packing material. Finally when the temperature is reduced to about 90 C. and below, C: separates on the packing madry oxygen and nitrogen passing in- 'reverse direction evaporates CO: and H2O by ajprocess of sublimation, thus preventing clogging of -the pas- .sages through the packing material. From timeducing liquid air therein from an outside source,.

using line 64 for the purpose.

Referring to Figure 2 of the drawing showing detail of the crystallizing packing material, a fusible solid is contained within a spherical shell as shown at A, the shell being constructed of metal. Suitable dimension of the shell is about one-fourth of an inch to one inch in diameter. Instead of enclosing the fusible solid in a sphere, I may alternatively employ a sealed tube of varyingv length or shape as shown at B, ora coiled tube or torus as shown at C. Spiral tubular coils as shown at D may also be employed providing extensive surface and free passage for gases between the spirals. These spirals may have la gross diametervequal to the diameter ofl the exchanger and successive spirals may be placed one upon the other with provision, e. g. by spacers, for free passage of gases therebetween.

Another structure for the packing element shown at E is a coil or spiral of flattened tube or hollow ribbon presenting a more extensive surface to the gases. A cross-section of one form v of the hollow ribbon construction shown in E is illustrated lin F, the walls of the ribbon being formed of embossed, crimped or corrugated sheets, and the space enclosed within the sheets being filled with the desired freezing liquid. If tubular-spirals D are employed, these may be made of quite small diameter, thin-walled copper tubing, e. g. one-eighth inch diameter wound into relatively small flat spirals which are filled with temperature range of the'exehanger. Thus in the case of the oxygen-exchangers, it isdesirable to divide the. temperaturerange of +20 to about -165 -C. into about eighteen stages employing eighteen different-substances' having fusion temperatures spaced about 10 degrees apart, the lowest melting substance being at the outlet or cold end ofthe exchanger.

As examples-of liquidssuitable for packing material. the following list is illustrative:

Packing Liquid Benzene Water. 0 Dibrombeunne Acetyl acetone anni.

Mercury Amyl amine.

N-butyl alcohol isopropyl alcohol. Toluene It will be noted that many of the low freezing liquids are gases at ordinary temperature and for this reason it is necessary that they be sealed within containers of suillcient strength to withstand the pressure of the exchange liquid when the exchanger is permitted to come to normal temperature. Considering the need for having the exchange liquid deployed with a low maximum distance between the liquid and the walls of the y container, it is readily seen that the container must have a small cross-sectional area and therefore the gross pressure developed within the containers by the vapor of the refrigerating liquid is not large. Accordingly, by making the containers of very small cross-sectional area, the

walls may be of quite thin metal and yet be of adequate strength to withstand the high vapor the desired freezing liquid, sealed and placed As freezing liquids for use in the packing material, I prefer to employ liquids having a high heat of fusion. For maximum eiiiciency it isdesirable to employ a series of liquids having fusion temperatures evenly spaced through the pressure of the filling liquid. Another advantage of thin wall construction lies in the rapid heat transfer through the wall between the gas and the liquid. Rapid heat transfer from the gas to the packing material is facilitated by maintaining relatively high gas velocities and turbulence in the air, nitrogen and oxygen passing through the exchangers.

In order to approach more closely true reversibility in the operation of my regenerative exchangers, I prefer to employ packing liquids which are not entirely pure and which therefore have a spread of crystallizing temperature which suitably equals the dierence between the crystallizing temperature of the liquids in adjacent packing zones. Thus in the case of water, I may add a small amount of alcohol to cause the water to freeze over a temperature range of 0 to 23 C., thereby providing a progressive refrigerating eiIect on the air passing over this section of the exchanger throughout the temperature region of 0 to '-23 C. at which point the melting of acetyl acetone, in this 4example the next adjacent refrigerating liquid, cools the air progressively further. The reverse effect occurs in the reverse flow with cold oxygen or nitrogen.

Having thus described my process, what I claim is:

l.v The method of refrigerating gas which coml agences prises passing a stream of said .gas through a plurality of refrigerating zones packed with refrigerating material of high heat capacity, said high heat capacity being obtained by a plurality of fusible solids, the fusion temperature of said solids being progressively lower in the direction of ovr of said gas.

2. The process of claim 1 wherein the fusible solids employed contain sulcient impurity to spread the fusion temperature over a range approximating the difference in fusion temperature between solids in adjacent refrigerating zones.

3. In the process of producing oxygen from air by the liquid air distillation method, whereby streams of compressed air are cooled in regenerative heat exchangers, liquefied, fractionated into nitrogen and oxygen, expanded to lower pressure, and the cold nitrogen and oxygen are discharged through regenerative heat exchangers, the improvement comprising conducting the gases through said heat exchangers in heat exchange relation with a plurality of crystallizable liquids sealed away from said gases, said liquids being arranged in the order of decreasing melting points from the warm end to the cold end of said exchangers, said liquids being selected to have melting points lying within the range of atmospheric temperature on the warm end to liquid air temperature on the cold end.

4. In a regenerative heat-exchanging process for alternatively heating and cooling iiuids by reversibly contacting with a heat-exchange agent, the improvement comprising passing said uid to be heated in successive heat-exchange relation with a plurality of liquid heat-exchange agents in successive zones, the heat-exchange agent in each zone having a crystallizing temperature approximating but slightly above the temperature of the fluid to be heated in that zone, thereby crystallizing said heatexchange agents during the passage of said uid to be heated and imparting to said uid the latent heat of crystallization of said agents, and subsequently conducting thru said zones in the reverse order a fluid to be cooled, said crystallized heat-exchange agents being reliqueed during the passage of said uid to be cooled thereby abstracting from said last-mentioned fluid an amount of heat substantially equivalent to the latent heat of fusion of said heat-exchange agents.

VANDERVEER VOORHEES.

No references cited.

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