US2920120A - Heat transfer method for solvent recovery and aromatic crystallization - Google Patents

Description

1960 M. R. FENSKE ETAL 2,920,120

HEAT TRANSFER METHOD FOR sowmw RECOVERY AND AROMATIC CRYSTALLIZATION Filed Jan. 23, 1957 3 Sheets-Sheet 1 P Lu 1| 6 m 2 D 4. i o (D I 0 a r n m 0 6 (O T 4 E D 9 u.

J, f v

f n to Merrell R. Fenske I ventors Walter G. Braun By 1 1%? H-MKAflorney 7 Jan. 5, 1960 F s ET AL 2,920,120

HEAT TRANSFER METHOD FOR SOLVENT RECOVERY AND AROMATIC CRYSTALLIZATION Filed Jan. 23. 1957 3 Sheets-Sheet 2 209 FIGURE-2 309 FIGURE-3 401 O 50/ J 4I5 l -4o2 L 4l4 /-K(--D 5n v 512 FIGURE 4 509m FIGURE 5 Merrell R. Fenske Walter 6. Braun Inventors By E/C Attorney 960 M. R. FENSKE ETAL 2,920,120

- HEAT TRANSFER METHOD FOR SOLVENT RECOVERY AND AROMATIC CRYSTALLIZAT'ION Filed Jan. 23, 1957 3 Sheets-Sheet 3 CRYSTALS y Merrel! R. Fenske Walter G. Braun '"Venfors Y (E/C- H Attorney HEAT TRANSFER METHOD FOR SOLVENT RE- COVERY AND AROMATIC RYSTALLIZATION Pa., assignors to Esso Research and Engineering Company, a corporation of Delaware Application January 23, 1957, Serial No. 635,868 20 Claims. c1. 26 -674) This invention relates to an improved method of cooling a liquid solution containing a substantial portion of a;volatile solvent. More particularly, the invention relates to the transfer of heat from one liquid stream to another liquid stream using a circulating inert gas. In a specific embodiment, the invention relates to the liquid ammonia extraction of durene range hydrocarbons from a catalytic naphtha wherein cooling of the extract phase Merrell R. Fenske and Walter G..Braun, State College,

for solvent recovery and durene crystallization as well as the extractor temperature control is efiected by circulationof an inert gas such as nitrogen therethrough.

Cooling of a solution to precipitate a solute is a com-. 7

surfaces which can foul or on which the solute can pre-- cipitate. A relatively large temperature difierence must be maintained between the two fluid streams to transfer heat because of the heat resistances of the wall and the fluid films.

When 'one of the fluids is a gas and the other a liquid the wall is often not necessary, particularly when there is no problem of mutual contamination, e.g., in water cooling towers. Temperaturedifierences between the two streams at any given point in such a direct-contact apparatus can be less'than if the streams were separated by a wall. If the liquid stream is non-volatile the heat interchanged with the gas is essentially sensible heat. If the liquid stream is volatile there may be an exchange of both sensible heat and also latent heat because of evaporation or condensation of some of the volatile liquid.

It is an object of'this invention to provide an improved method of heat exchange using direct contact between a liquid and a gas. It is a further object to provide a method of transferring heat from one liquid stream to another by employing direct contact between one or more circulating inert gas streams and each of the liquid streams. It is also an object to provide a heat exchange method between two fluid streams in which a minimum of energy is spent and comparatively little of the heat 'the invention.

Figure 7 ;is:,a schematic flow sheet of a process em- ,bodyingzthepresent invention.

Other objects will become apparent 2,926,120 Patented Jan. 5, 1960 In accordance with the present invention, heat transfer between two liquid streams is effected by contacting a relatively warm volatile liquid countercurrent to a stream of a dryand cool inert gas, whereby some of the liquid evaporates into the gas phase and the liquid is cooled. The extent of the cooling is dependent on the amount of liquid evaporated and on the heat of vaporization. .The amount of liquid evaporated is, in turn, dependent on the vapor pressure-temperature relationship of the liquid and on the total pressure of the system. The gas phase is warmed and picks up more volatilized liquid as it passes through the countercurrentcontacting unit.

In a companion step, a relatively cold stream of the volatile liquid flows countercurrent to the warm inert gas which was essentially saturated with vapors of the volatile liquid in the previous step. As a result, the vapors will condense out of the gas phase and into the countercurrent flowing, relatively cool liquid phase. The heat of vaporization thusreleased will tend to warm the liquid phase and thereby in effect transfer heat thereto from the first mentioned, relatively warm liquid stream. The gas phase is cooled and denuded of its vaporized liquid content as it flows through the countercurrent contacting unit, and

fmay eventually be recycled to the first step of the process.

Figure 1 illustrates an apparatus in which a volatile, liquid .can be cooled to some low temperature in order,

for example, to precipitate asolute, and-then be reheated. Referring-to Figure 1, a warm volatile liquid 1, e.g.,asolventextract containing a normally solid solute, enters the top of a vapor liquid contactor 2 such as a packed or a packed or a bubble-cap tower operating in essentially an adiabatic manner. Passing countercurrently to this liquid stream -.is a relativelycool, dry stream of an inert gas which is continuously recirculated through the contactor by means of-blower 3. Some of the volatile liquid evaporates into the circulating gas stream as it passes down through the upper part of ,the contactor and the liquid stream is thereby cooled while the gas is warmed and humidified by the vaporized liquid. Approximately in the center of the contactor a partition is placed which traps the liquid passing down the unit but allows the free upward passage of vapor. The partition as illustrated in the diagram consists essentially of a plate or diaphragm 6 into which vertical lengths of-pipe 7 have been fixed. Small umbrellas 8 cover each of the tubes to prevent liquid from dropping through them. As cold liquid collects on the plate and around tube 7 it is removed via line 9 to a pump 10 and then to a heat exchanger 11 where it can be further cooled to the desired precipitation temperature. In the example chosen, 12 represents a vessel, e.g., a basket-type centrifuge, wherein the solute can precipitate and from which it is removed at 13. The cold liquid solvent, denuded of its solute, then returns via line 14 to the contactor where it is introduced, just below the liquid separator plate 6. The cold liquid stream then flows countercurrently to the warm and humid gas stream and is thereby reheated while the gas is cooled and dried. The liquid, reheated and freed of solute, leaves the contactor via line 15 and may be reused, for instance, in the principal extraction stage, not shown.

The gas stream, warm and essentially saturated with volatilized liquid, leaves the top of the contactor by line 5 and is returned by means of blower 3 and line 4 to the bottom where it meets cold descending liquid which cools it and condenses out much of the volatilized liquid, as already described. The dry gas then passes from the lower to the upper section of the contactor, through the vapor-liquid separator 6, at the same, or at a slightly higher temperature than the liquid stream entering via- 14. In passing upward through the upper section of the is transferred through an impermeable wall in heat exchanger 11. As will be illustrated later, a liquid stream such as liquid ammonia can be cooled from 60 F. to 20 F. in this manner. Figure 1 and the modifications shown in Figures 2, 3, 4, 5 and 6 illustrate the vaporliquid contactor for the liquid cooling and liquid heating operations as a single unit. However, it is obvious that the heating and cooling operations could be effected in separate units suitably-connected by vapor and liquid lines.

It is an important requirement of this invention that.

in the upper or liquid cooling section of the contactor the temperature of the liquid' be above the saturation temperature of the gas stream at any point in order for liquid to evaporate. In the bottom section of the contactor the liquid temperature must be below the saturation temperature of the gas stream at any point in order for liquid to condense out of the gas. To accomplish this, it is necessary to reduce the enthalpy of either the liquid stream as it passes from the liquid cooling section to the liquid heating section, or the gas as it passes from The enthalpy of the liquid stream can be reduced by cooling. The enthalpy of the gas stream can be reduced by cooling or by reducing the concentration of vaporized the liquid heating section to the liquid cooling section."

liquid in the gas, or by'compression and expansion, or-

by any suitable combination of the foregoing expedients. In the arrangement shown in Figure l, exchangerll cools the liquid, as it passes from the upper to the lower section of the apparatus, to a temperature below the saturation temperature of the gas stream at that lower section of the contactor. The arrangements shown in Figures 2-5 employ some of the other methods of meeting the above-mentioned requirement.

Referring to Figure 2, 'a warm volatile liquid extract 201 comes into the top of a vapor-liquid contactor 202 and flows downward countercurrently to a gas stream. The contactor is divided into two sections by means of an impermeable plate 203, which prevents the passage of liquid or vapor. The liquid is cooled in the upper section of the contactor and is sent via line 204 through pump 205 directly to a vessel 206 where a precipitated solute can be removed via 207 and the cold solvent returned via line 208 to the lower section of the contactor. The liquid is reheated by the circulating gas in this lower section and leaves via 209. The warm inert gas containing a large portion of vaporized liquid leaves the top of the contactor via line 210 and is recirculated into the bottom by means of blower 211 and inlet line 212. The gas stream leaves the lower section of the contactor via line 213, is cooled in exchanger 214, and returns to the contactor just above the dividing plate 203 via line 215. Thus the enthalpy of the gas stream is reduced as it passes from the lower to the upper section of the contactor. Some liquid is condensed in exchanger 214 and may be returned to the contactor via line 216.

A third method of operation is illustrated in Figure 3. It is essentially similar to that described previously with reference to Figure 2 except that instead of refrigeration of the gas stream between the upper and lower sections of the contactor, the gas leaving the contactor via 313 is at least partially denuded of its volatilized solvent in an appropriate unit 314 by a process such as water scrubbing, adsorption on charcoal, or similar processes such as chemical removal that will be referred to herein by the generic expression physical absorption. The dried or essentially pure inert gas is returned to the contactor via "pressure than the upper section.

4 315 and the recovered volatile liquid removed via line 316.

A fourth method of operation is illustrated in Figure 4 wherein the liquid extract 401 to be cooled enters the contactor 402 and flows downward countercurrently to a circulating gas stream., In approximately the middle of the tower the cooled liquid is collected on plate 404 and removed via line 407. The liquid is pumped by pump 408 directly to a settling vessel 409 where the precipitated solute can be removed as 410. The collecting plate 404, vertical pipes 405 and umbrellas 406 have been previously described with reference to Figure l. The denuded solvent is returned to the contactor via 411 and is reheated and leaves as liquid stream 412. The gas stream passes through the contactor countercurrently to the liquid phase, entering the bottom as stream 413. Immediately above the gas-liquid separator plate 404 a stream of dry inert gas enters via line 414 and is added to the gas stream in the contactor. The gas leaving the top of the contactor is split into two streams, one being recirculated to the gas dehumidifying or bottom portion of the contactor by blower 403, and the other leaving via line 415, is denuded of its vaporized liquid by some process such as gas washing or adsorption in zone 416 and returned as stream 414, The purpose of adding a dry inert gas streamin the upper or cooling section of the contactor is to reduce the vaporized liquid concentration, and thus the enthalpy, of the gas stream so that evaporation, and hence cooling, can take effect.

Still another method of operation of the heat exchanger is illustrated in Figure 5. Warm volatile liquid as stream "501 enters the top of contacts 502 and flows downward counterourrently to a circulating gas stream. The contactor is divided into two sections by plate 503 which prevents the passage of liquid or vapor and allows the lower section of the contactor to operate at a higher The liquid having been cooled in the upper section of the contactor is removed via line 504 through pump 505 to a vessel 506 where a precipitated solute can be removed at 507. The cold liquid phase returns to the lower section of the contactor via line 508, flows downward countercurrent to the gas stream, and leaves as stream 509. The gas stream leaves the top of the contactor via line 510, is compressed by compressor 511, e.g., to a pressure between about 175 and 250 p.s.i.a., and recirculated to the bottom of the contactor via line 512. Immediately below the dividing plate 503, which separates the upper and lower sections of the contactor, the cold gas stream is removed by line 513 and allowed to expand through an expansion engine such as a turbine 514 to a lower pressure, e.g., between about 100 and 150 p.s.i.a. In other words, the required enthalpy offset is provided by operating the lower section of the contactor at a high pressure, eg 200 p.s.i.a., while operating the upper section at a lower pressure, e.g. 125 p.s.i.a. The gas in then passed by line 515 into the contactor immediately above the dividing plate 503 and passed upward countercurrently to the descending warm liquid. The expansion is accomplished through the expansion engine 514 in order to reduce the enthalpy of the gas stream 7 and at the same time produce work to ofiset some of the work required by compressor 511.

Figures 1 to illustrate modifications suitable for maintaining the circulating gas stream or the liquid stream properties such that liquid will evaporate into the gas stream in the liquid cooling section of the apparatus and such that liquid will condense from the circulating gas stream into theliquid stream and heat In Figure 1 the liquid stream enthalpy is reduced in exchanger 11 while in Figure 2 the gas stream enthalpy is reduced by cooling in exchanger 214. In Figures 3 and 4 the gas stream enthalpy per unit weight of inert gas is reduced, in the former case by reduction of vaporized liquid content of the gas, and in the latter case by dilurise or the gas st eam with r jessent'iallydry inertga's. In Figure .5, the gas stream-en lpy is reduced by expansion through an engine. ,Theenthalpy of the liquid referred to here istaken in the sense as the heat content per unitweight of liquidrelativ eto a chosen reference temperature. The enthalpy of the gas, in the sense used here, is the enthalpy per unit weight of inert gas relative to a chosen reference temperature plus the enthalpy of the weight of vaporized liquid associated with the unit weight of inert gas';

A certain quantity of energy 'is required to maintain "the necessary enthalpies of the liquid orgas streams such that evaporation and condensation will take place. This energy is supplied either to cool the liquid or gas streams (Figures 1 and 2), to reduce the vaporized liquid content of the gas stream (Figuresd and 4), or to drive the compressor over and, above the energy that is recovered by the expansion engine (Figure 5).

In order to reduce the energy requirement it has been found advantageous to maintain a higher ratio of'gas flow to liquid flow at the cold section of the, contactor, i.e., the middle section extending from a point just above the cold liquid withdrawal line to a point just beloW the cold liquid return line, than at the warmer sections, i.e., the'sections near the top of the liquid cooling zone and near the bottom of the liquid heating Zone.

Thus, the enthalpy of the gas stream is maintained relatively close. to its equilibrium enthalpy throughout the ..,L.. I The temperatures prevailing in the contac'tors of Figures 1 through depend? uponthe temperature of the entering liquid and the temperature to which this liquid is to belcooled". In general, thesetemperatures lie belength of the contactor, andthe enthalpy requirement to produce the enthalpy offset between the upper and lower cold section of the contactor is illustrated in Figure 6.

The basic arrangement of the liquid flow has'been previously described With Figure 1. The gas, however, is

circulated in two or more streams. As shown in Figure 6, the primary gas stream leaves the top of the contactor via line 610 and is recirculated to the bottom via line 612 by means of blower 611; A secondary gas stream leaves the liquid cooling section of thecontactor via line 613 at some point between the primary gas stream take-01f 610 and the point of cold liquid withdrawal 603. This secondary gas stream is recirculated by means of blower 614 and return via line 615 at a point between the cold liquid return line 608' and the primary recirculating gas inlet 612 which is approximately at the same temperature level as the aforesaid point of secondary gas withdrawal. The ratio of gas 'in the" secondary gas stream to that in the primary stream is maintained between aboutl to 1 and 3 to 1.

For cooling liquids to very low temperatures and where a minimum of energy is to be expended it may be advantageous to use additional oirculatin'g'ga's streams.

skilled in the art simply by maintaining a liquid to gas ratio at all points in the contactor so that the enthalpy of the gas and its equilibrium enthalpy differ just enough to "allow the transfer of heat to take place.

In the same manner that Figure 6 is a variation of Figure l, the use of multiple recirculatinggas streams can also be applied. to the arrangements illustrated in Figures'Z, 3, 4 and 5 in order to reduce the energy requirements.

The general principle regarding the total pressure on the contactors described in Figures 1 through 6 is that the pressure is composed of the partial pressure ofthe liquid phase plus the partial pressure of the inert gas. Accordiiigly, the total pressure on the contractor is the partial on the liquid flow rate. These can be calculated by those pressure of the solvent at the warmest point plus at least and several modifications tween about '40 F; and about +300 F.

Inconnection with-Figures- 1 through 6, when liquid ammonia comprises the bulk of the liquid phase and when the liquid phase enters at about 60 to F. and is cooled to about ,-4,0 to '+60'T" F., pressures in the range of to 400 p.s.i. have been found suitable.

c Having described. the basic principles of our invention ere'of, the invention will be further illustrated with referents-m Figure 7 wherein durene is extracted from" catalytic naphthas. Dure'ne, 1,2,4,5-tetramethyl benze becoming an increasingly important petrochemical at finds uses in the synthesis ofQfiber-fo'nning menus-,ae; Much of the description herein will concern the extraction of durene with a liquid ammonia solvent using nitrogen as" the inert gas in the heat transfer operation's, bi'1t it should be understood that this description is being given' primarily-for purposes of illustration rather than limitation. It is to be under- I Figure 7 shows a lilarifliior recovering durene in accordance with the. invention: one inventive feature relates to continuously cycling a'stream of nitrogen through some extractor stages to fevaporate'" some of the solvent and reduce the temperature;'and their contacting the nitrogen stream countercurre'ntly with cold solvent to reduce the solvent vapor content offthe nitrogen. ,Anotherffeature relates to cycling another portion of inert' gas to cool the extract phasev of theextraction, by evaporating some of the liquid therefrom, for purposes of solvent recovery and durene crystallization. [The remaining op-' erations of the process are essentially conventional, but the over-all combination provides an improved, economical process of extraction; The most conservative measures are taken to lower external energy and heat-transfer requirements.

Durene is one of the aromatic hydrocarbons found in catalytically cracked. naphthas and 'it'can be concentrated to 2 to 15 percent by careful fractional distillation. Preferred feed stocks may have a boilingrange from 370 to 400 F.,' though wider or narrower cuts may be used;- Referring to Figure 7, the naphtha feed 702 enters on a stage near the middle of liquid extractor 701 which is operated at a pressure betweenabo'ut 150 and 400 p.s.i.a. A solvent containing 60 to"1 00'% ammonia is introduced as a liquid through line 703. Bo'thtli'e' naphtha and the enter evaporating drum 705. The ratio of solvent to feed is preferably maintained between about2.5 to 1 and 5 to 1. The extractor should have nine to twenty theoretical stages.

Maintenance of the proper hydrocarbon solubility in the extract phase in the extractor is important, particularly at the extract or stripping end where the concentration of aromatics is high. The reduction of the temperature in the upper or stripping stages is one method of maintaining the desired solubility. An important phase of this invention is the transfer of heat from the liquid on these stages to a cold solvent stream by means of a circulating nitrogen gas stream. This nitrogen together with some solvent vapor enters these extraction stages, that are to be cooled, at a temperature 5 to 20 F. cooler than extract stream 704, through lines 707, 708,

.704, containing from 10 to 30% hydrocarbons, is about 30'to 50 F. cooler than feed stream 702. The nitrogen, containing from 0.3 to pounds of ammonia per pound of nitrogen, leaves the extractor via lines 711, 712,

713 and 714 to vapor-liquid contactor 718 which serves as a solvent heater. This may be a packed-or bubble cap tower having eight to ten vapor-liquid contact stages and operating in an essentially adiabatic manner. Within the vapor-liquid contactor the gas stream flows countercurrently to a descending stream of cold solvent, at a temperature between about 30 to 80 F., entering by line 719. Solvent vapors associated with the inert gas condense into the coldliquid and heat the liquid to about 80 to 130 F. Thewarmed liquid solvent leaves via line 720, and the nitrogen'stream, now containing from 0.1 to 2 lbs. NH /lb. N leaves via line 721 at a temperature between about 50 and 90 F. to blower 722 which recirculates the gas. Makeup gas may be introduced through line 766.

Within the evaporating drum 705, operated at a pressure between about 50 and 200 p.s.i.a., sufiicient selfevaporation of the solvent occurs to cool the extract phase about another to 30 F. to a temperature be tween about 40 and 80 F., thus precipitating some of the dissolved hydrocarbons. These hydrocarbons are returned through line 715 to the top of the extractor to comprise about 80 to 99% of the reflux. Solvent vapors generated in drum 705 are passed via line 716 into compressor 723 and after compression are condensed in 725 and returned to the main solvent stream entering the extractor. The remaining extract layer in drum 705 leaves through line 717 and enters the top of the liquid cooling section of the direct contact exchanger 726, which operates at a pressure of about 150 to 400 p.s.i.a. Contactor 726 is a second important phase of the embodiment and is a direct application of the invention illustrated in Figures 1 and 6. It has a total of to vapor-liquid contact stages, about half in the liquid heating section and half in the liquid cooling section.

In the vapor-liquid contactor 726 the extract phase 717 is cooled about 60 to 100 F., e.g. from 60 F. to 20 F., by countercurrent contact with a circulating stream of nitrogen and solvent vapor. The nitrogen and solvent vapor are recirculated in lines 763 and 764 by means of blowers 744 and 745 as described previously with reference to Figure 6. Makeup nitrogen may be introduced through line 765. The cold liquid stream leaves to cooling section of the contactor via line 727 and proceeds to a self-evaporation chiller 728, which operates at about atmospheric pressure, Where some of the solvent is evaporated, reducing the liquid temperature about another 5 to 30 F., e.g. to between 30 and ,50 F. The non-vapor material 728B in chiller 728 is a slurrycontaining about 5 to hydrocarbons and consisting of three phases: a liquid hydrocarbon phase which is essentially aromatic in nature, a liquid solvent phase containing about 2% dissolved hydrocarbon, and a solid crystalline phase consisting largely of durene. The slurry is sent via line 730 to a filter, centrifuge or other mechanical separation device 731 where the solid durene crystals 732 are removed. The filtrate is removed-vialine 733 to a settling drum 734 where a solvent-rich layer 734A and an extract hydrocarbon layer 734B are formed. The solvent-rich layer, 5 to F. cooler than exit stream 727, is sent via line 736 to the liquid heating section of the contactor 726 where it descends countercurrently to the nitrogen-solvent vapor stream and is warmed about 75 to 115 F., e.g. from 40 to 45 F. The ratio of liquid solvent to nitrogen-solvent gas is maintained between about 5 to 1 and 8 to 1 in the warmer upper and lower extremity sections of the contactor and between aboutl to 1 and 3 to 1 in the colder middle sections for reasons set forth hereinbefore with reference to Figure 6.

The solvent removed by line 739 is sufiiciently pure, i.e., containing less than 2% hydrocarbon, when mixed with the other solvent streams from compressor 740 and distillation column 755 to be returned to extractor 701 via line 703. The hydrocarbon-rich phase 734B is removed from drum 734 via line 735, is passed through an indirect heat exchange coil 737 in contactor 726, and is sent via line 738 to water washing tower 747. 5

Drum 728 is autorefrigerated by evaporation of solvent from the solvent extract phase 728A. The solvent vapors pass through line 729 to compressors 740 and 741 in a proportion between about 1 to 2 and 2 to 1. The compressed solvent vapor 742 from compressor 740 is combined with the cold liquid solvent stream 739 from contactor 726, thereby condensed, and introduced into solvent heater 718 via line 719 and finally returned to extractor 701 as aforesaid. The other part of the solvent vapor, compressed in compressor 741, is condensed in condenser 725 and returned to the solvent stream 703 entering the extractor.

Towers 746 and 747 operate identically to remove solvent from the rafiinate and extract phases by water washing. Water at a temperature between about 50 and 200 F. and at about atmospheric pressure enters the top of both towers through line 748 and flows downward with the. hydrocarbons, removing any remaining solvent from the hydrocarbons by dissolving them in the water. Two liquid phases collect in each of the two settling drums 749 and 750 at the base of towers 746 and 747. The hydrocarbon products 7 49A and 750A are removed, at temperatures between about 50 and 200 F. and at about atmospheric pressures, as raffinate product 751 and extract product 752. The solvent-containing water 7493 and 750B is removed from each drum via 7.53, passed through heat exchanger 754, and entered'into distillationcolumn 755'.v This column 755 operatescom ventionally at a pressure between about 200 and 400 p.s.i.a. with a reboiler 756 at the base of the column containing a heating coil 757 wherein steam or other suitable heating fluid flows. The column is provided with a condenser 758 and some method of proportioning reflux 759 to the column. Solvent distillate is added via line 760 to the solvent stream 703 entering the extractor. The liquid water at about 300 to 500 F. and 200 to 400 p.s.i..a. leaving the reboiler via line 761 is cooled in heat exchangers 754 and 762; this water enters via line 748, and flows to the water scrubbing towers 746 and 747.

The invention with respect to the extraction of durene fromheavy catalytic naphthas will now be described more specifically in the following illustrative embodiment.

EXAMPLE Using a process identical to that described with reference to Figure 7, the following data shown in Tables 1 and2 illustrate how an yield of durene crystals can be obtained from a heavy catalytic naphtha, containing 11% durene and boiling from 370 to 400 F., using as a solvent a mixture of percent ammonia and '10 percent monomethylamine, with a solvent to feed ratio of 5 ml. The various streams are identified in the tables by symbols corresponding to the respective v numerals appearing in Figure 7.

quired 425 HP. The coolingwater requirement'sand the number. of indirect heat exchangers, were. also cane is'pondingly greater. In general, "the method of indirect hea 'axchan e herein disclosed provides an attractive, economical operation when applied to a liquid extraction process. The invention may be applied, however, not ny to. naphtha extraction, but also to such processes as dewaxing to cool the oil-wax -solvent slurry and to removing water wherein water is frozen out of a solvent. It may. be'used essentially wherever it is desired to cool a v ola tile liquid streamv 'at one point and, by the transfer 16 poneiit is often the principal solvent or extractant, such as ammonia, liquid sulfur dioxide, and the like. In this case, it, is preferable to use a suitable modifying agent iri admixture with the volatile liquid; such as the methyl amines, ethylamines, aniline, pyridine, methanol, lower alcohdls and ethers, which increase the solvent power of ammonia; or water, ethylene glycol, ethylene diamine; forirrainide, and low melting paraffinic hydrocarbons, whichdecrease the solvent power of ammonia. Particularly preferable in this invention is a solvent comprising 60 to 100% ammonia in admixture with 40 to 0% mono- Table 1 nYDRooAnBoN AND SOLVENT GLOW Wt. l er- Flow, 1,000s-I 7bs. Per Hour 7 cent I, Wt. Per- Temp, Press, Symbol Name Hydrocent F. p.s.1.a.

' carbon Solvent Hydro- Solvent Total carbon Fee l 100 0 5 120 300 15.5 0 15.5 Solvent Feed-' 5 1.2 98.8 120 300 0.9 77.5 78.4 18. 7 81.3 80 300 16. 4 71.8 88. 2 95.1 5 4.9 120 300 7.8 0.4 8.2 95.1 4.9 60 300 7.8 0.4 8.2 0v 100 60 107 0 3.5 3.5 11.3 -88.7 60 107 8.7 67.9 76.6 1.4 98.6 55 300 0.9 66.7 67.6 1.3 98.7 1 105 300 0.9 172.3 73.3 Extract Phase 12.8- 87.2 175. 8.7 59.0 67.7 Solvent Vapor 0 100 10' 0 2.3 2. 3 Extract Phase 13.3 87.0 40 10 8. 7 56. 7 65. 4 Durene Crystals (80% Yield) 100.0 0 40 10, 1. 39 0 1. 39 Extract Phase 11.4 88.6 40 10 7. 3 56.7 64.0 Extract Product .98; 0 2. 0 -40 10 6. 4 0.1 6. 5 Solvent 1.6 98.4 -40 10 0.9 56.6 57.5 Extract 'Product 98.0 q 2.0 35 10 6.4 0.1 6.5 Solvent- 1.4 98.6 175 0.9 65.5 06.5 0 100 290 98 0 1.2 1.2 0 100 430 300 0 1.1 1.1 v ,..0 0 100' 20 100 0 5 .100 20 7.5 o 7.8 100 0 100 20 6.4 0 6.4 0, 100 20 -0 100 90 300 Solvent 0 100 90 300 0 0.5 05 Fresh Water Y, 0 O Y 417 '300 Table 2 40 methylamine. An extract phase leaving the extractor INERT GAS RECYOLE containing 60 to 90% ammonia, 0 to 18% monomethylyamine, and 10 to 25% hydrocarbons is a highly prefer- Flow, able condition. Symbol g n, g 6 0 The solvent may also comprise a non-volatile princier 2 lll pal solvent-such asaniline, glycols, furfural, or phenols,

, in admixture with a minor proportion of a volatile com- 80 300 M32 7A ponent such as ammonia, propane, orone of the Freon- 38 288 g type chlorofiuoro alkanes. Multi-component solvent no I 1 mixtures containing liquid sulfur dioxide, such as sulfur 70, 300 0.457 -5 dioxide and. benzene, are also suitable. In addition, the 48 175 0.585 11.0 50 11 175 0,176 23,0 volatlle component can be one of the components belng separated, such as propane in the separation of propylene of the heat thereby lost, reheat the same liquid atanother point in the process.

Thus, in the dewaxing of oils, propane, butane, or ammonia may be used as the volatile component in the liquid phase and the process would operate essentially described, with wax crystals being produced from filter 731, and a propane-wax free oil solution would leave via line 739 of Figure 7. The warm propane-waxy oil solution would enter tower 726'via line 717 of Figure'7.

Other important petrochemical separations where this process can be used are in the liquid extraction of naphthas containing cyclohcxane, p-xylene, naphthalene, and

styrene, or any one of them. The extraction and 0178- tallization steps to separate these hydrocarbons would be essentially as described in Figure 7. It should be understood, however, that the crystallization step isf'not necessary to the operation of this invention. The process disclosed is well suited to therecovery of solvent in a liquid extraction process by chilling, as described in US. Patent 2,728,708.

It has already been pointed out that the liquid phase to be cooled by the present invention must comprise, at least in part, a volatile component. This volatile comfrom propane, or butane in the separation of butylenes by liquidv extraction with. a high boiling solvent such as glycol oraniline. 1

The term inert gas is intended to mean a gas that is. non-:condensable under the operating conditions, but may be soluble in the liquid to be cooled. It is inert towards the ordinary materials of construction, and the 'fluidingredients used. in the process. Examples are nitrogen, hydrogen, the low boiling hydrocarbons such as methane and ethane, helium, low boiling chlorofl-uoro- -methanes suchas dichlorodifiuoromethane, and the like. The term dry inert gas means that the non-condensable "gas has a low'content 'of the vaporized volatile liquid, that this dry gas has been produced by'reducing its content of volatile liquid in a preceding step.

The process can be operated in reverse. That is, insteadof starting with a warm liquid, first cooling it, and then reheating it; the same process can be used starting "with 'a'c'ool'liquid, by first heating it to perform some desired physical or chemical change, and then cooling it back'to the approximate temperature level at which it entered the process.

Having thus presented a general description and il- 11 lustrative embodiments of the present invention, the true scope is now set forth in the appended claims.

The claimed invention is:

1. A process for transferring heat in a heat transfer system from a relatively warm liquid stream containing a substantial fraction of a volatile liquid to a second portion of said stream, which comprises passing a dry insert gas stream countercurrently to the liquid stream in a cooling zone wherein the liquid is at a temperature above the saturation temperature of the gas, thereby evaporating a portion of said volatile liquid into the gas stream and cooling the liquid, passing the resulting vaporcontaining gas stream countercurrently to the cooled liquid stream in a heating zone wherein the liquid is at a temperature below the saturation temperature of the gas, thereby condensing the vapors into said liquid stream and heating the liquid, continuously removing heated liquid from said system, returning the vapor-denuded gas stream to the cooling zone, and reducing the enthalpy of one of the streams at a stage between the cooling and heating zones sufliciently to maintain the temperature of the liquid above the saturation temperature of the gas in said cooling zone and below the saturation temperature of the gas in said heating zone.

2. A method according to claim 1 wherein the enthalpy of the liquid is reduced by further cooling said liquid at a point after leaving said cooling zone and before entering said heating zone.

3. A method according to claim 1 wherein the enthalpy of the gas stream is reduced by cooling said gas stream at a point after leaving said heating zone and before entering said cooling zone.

4. A method according to claim 1 wherein the enthalpy of the gas stream is reduced by physical absorption of the volatile liquid vapors from said gas stream at a point after leaving said heating zone and before entering said cooling zone.

5. A method according to claim 1 wherein the enthalpy of the inert gas stream is reduced by diluting said gas stream with an additional amount of at least partially dried inert gas at a point after leaving saidheating zone and before entering said cooling zone.

6. A method according to claim 1 wherein the enthalpy of the gas stream is reduced by reducing the pressure of said gas stream at a point after leaving said heating zone and before entering said cooling zone, and increasing the pressure of said gas at a point after leaving said cooling zone and before entering said heating zone.

7. A process for the transfer of heat which comprises passing a stream of a relatively warm liquid solution containing a solute and a substantial fraction of a volatile liquid solvent into a liquid cooling zone, countercurrently contacting with said solution a relatively dry inert gas stream, thereby evaporating an amount of said volatile liquid into said gas stream and cooling said liquid stream, removing the cooled liquid stream from said cooling zone, recoveringprecipitated solute from said cooled liquid stream, passing said cooled, relatively solute-free liquid stream into a heating zone, countercurrently contacting said cooled, relatively solute-free'liquid in said heating zone with the vapor-enriched gas from said cooling zone, thereby condensing the volatile liquid vapors from said gas into and heating said. liquid stream, recycling the resulting relatively dry gas stream to said cooling zone, continuously recovering relatively warm liquid solvent, and reducing the enthalpy of at least one of the streams between their withdrawal from their respective cooling zones and their introduction into their respective heating zones so that the temperature of the liquid is maintained above the saturation temperature of the gas in said cooling zone and below the saturation temperature of the gas in said heating zone.

8. A process according to claim 7 wherein said inert gas circulates between said heating and cooling zones in a plurality of streams such that a higher gas to liquid flow ratio is maintained at the cold sections of the contacting zones than at the warmer sections. 7

9. An extraction process which comprises extracting relatively soluble constituents from a hydrocarbon feed in a multistage liquid extraction zone with a liquid solvent containing a substantial fraction of a volatile liquid, passing a relatively dry inert gas through several stages of the said extraction zone wherein a portion of the liquid solvent evaporates into the inert gas stream, thereby cooling said solvent and humidifying said gas, withdrawing a liquid solvent extract from said extraction zone, cooling the withdrawn extract until at least a part of the hydrocarbon dissolved therein is precipitated, separating the precipitated hydrocarbon, countercurrently contacting the cooled hydrocarbon-denuded solvent with the humidified inert gas from the extraction zone in a gas-liquid contacting zone, thereby condensing the vapors into the liquid solvent and reheating said solvent, and recycling the resulting relatively dry gas and reheated solvent to the extraction zone.

10. A process according to claim 9 wherein the hydro- I carbon precipitated from'the solvent comprises both a liquid phase and an aromatic crystalline hydrocarbon phase, each phase is separated from the other and from the entrained solvent therein, and the separated solvent is recycled to the extraction zone.

11."A process according to claim 9 wherein said solvent comprises liquid ammonia and wherein sufficient ammonia is evaporated into the inert gas in the extraction zone to maintain theextract stream leaving said extraction zone at a temperature 30 to 50 F. cooler than the hydrocarbon feed stage.

12. A process according to claim 9 wherein said liquid solvent comprises 60 to weight percent ammonia and 40 to 0 weight percent of a methylarnine.

13. A process according to claim 9 wherein said inert gas circulates between said heating and cooling zones in a plurality of streams such that a higher gas to liquid flow ratio is maintained at the cold sections of the contacting zones than at the warmer sections.

14. A process which comprises extracting relatively soluble aromatic constituents from a hydrocarbon feed in a multistage liquid extraction zone with a liquid solvent containing a substantial fraction of liquid ammonia, passing a plurality of inert gas streams through a plurality of stages of said liquid extraction zone so as to evaporate gradually of sufiicient portion of the liquid ammonia into said inert gas streams to cool said solvent to a temperature 30 to 50 F. cooler than the hydrocarbon feed stage, withdrawing an extract stream, precipitating a portion of the hydrocarbons contained in the withdrawn extract in a separation zone, recycling said precipitated hydrocarbons to the extraction zone as retlux, passing the remaining solvent extract layer from said separation zone through an extract cooling zone countercurrently to another relatively dry inert gas stream whereby a portion of volatile solvent evaporates from said extract into the gas stream cooling said extract, passing the resulting humidified gas stream from said extract cooling zone into a first solvent heating zone, where it is countercurrently contacted with a relatively cool liquid solvent phase, removing the cooled extract from said cooling zone, further chilling the removed extract in a chilling zone so as to separate it into a liquid solvent phase and a precipitated hydrocarbon phase and so that the temperature of the liquid is maintained above the saturation temperature of V the gas in said extract cooling zone and below the saturation temperature of the gas in said first solvent heating zone, recovering the chilled solvent phase, a liquid hydrocarbon phase and a solid hydrocarbon phase from the chilled mixture, passing the chilled solvent phase from said chilling zone to the aforesaid first solvent heating zone for reheating by countercurrent contact with said humidified gas stream, returning the resulting relatively dry gas stream to the aforesaid extract cooling zone, passing the resulting reheated solvent to a second solvent heating zone and there countercurrently contacting it with the vapor-containing inert gas previously withdrawn from said extraction zone, returning the resulting reheated solvent from said second solvent reheating zone to said extraction zone, and likewise recycling the resulting s01- vent-denuded gas stream to said extraction zone.

15. A process according to claim 14 wherein the cyclic crystalline hydrocarbon phase consists essentially of durene and the inert gasis nitrogen.

16. A process according to claim 14 wherein the extract phase leaving the extraction zone comprises 60 to 90 wt. percent ammonia, to 18 wt. percent monomethylamine, and 10 to 25 wt. percent hydrocarbons, and wherein the weight ratio of solvent to feed in the extraction zone is between 2.5 to 1 and 5 to 1.

17. A process for solvent recovery which comprises passing an ammonia-hydrocarbon liquid extract through an extract cooling zone in countercurrent contact with a relatively dry inert gas whereby ammonia evaporates into said gas and cools said liquid extract; passing the resulting humidified gas from said extract cooling zone into an ammonia heating zone wherein said humidified gas is countercurrently contacted with a chilled liquid ammonia phase; recycling the resulting dehumidified gas from said ammonia heating zone to said extract cooling zone in a plurality of streams so that the gas-to-liquid ratio is higher at the colder sections of said cooling and heating zones than at the warmer sections thereof; removing the cooled liquid extract from said extract cooling zone to a chilling zone wherein ammonia evaporates from said liquid extract and thereby further cools said liquid extract to a temperature below the saturation temperature of the gas in said ammonia heating zone; re covering the evaporated ammonia vapors; removing the resulting chilled slurry of a liquid ammonia phase, a liquid hydrocarbon phase and a solid hydrocarbon phase to a mechanical separation zone; recovering said solid phase from said chilled slurry; removing the remaining chilled mixture of liquid ammonia and liquid hydrocarbon to a phase separation zone; separating and recovering a liquid hydrocarbon phase and a liquid ammonia phase from said separation zone; and recycling the chilled liquid ammonia phase to the aforesaid ammonia heating zone wherein it is heated by the aforesaid contact with the humidified gas; and recovering the heated liquid ammonia solvent.

18. A process for solvent recovery which comprises passing a durene-containing ammonia hydrocarbon liquid extract of a temperature between about 40 and 90 F. in countercurrent contact with substantially dry nitrogen gas through an extract cooling zone operating at a pressure of between about 150 and 400 p.s.i.a., whereby ammonia evaporates into said nitrogen cooling said liquid extract by about 60 to 100 F.; passing the resulting humidified nitrogen from said extract cooling zone into an ammonia heating zone operating at a pressure of between about 150 and 400 p.s.i.a. wherein said humidified gas is countercurrently contacted with a chilled liquid ammonia phase; recycling the resulting dehumidified nitrogen from said ammonia heating zone to said extract cooling zone in a plurality of streams so that the gas-to-liquid ratio is higher at the colder sections of said cooling and heating zones than at the warmer sections thereof; removing the cooled liquid extract from said extract cooling zone to a chilling zone wherein ammonia evaporates from said liquid extract and thereby further cools said liquid extract to about -30 to 50 F. and to a temperature below the saturation temperature of the gas in said ammonia heating zone; recovering the evaporated ammonia vapors; removing the resulting chilled slurry of a liquid ammonia phase, a liquid hydrocarbon phase, and a solid crystalline durene phase to a mechanical separation zone wherefrom said solid durene phase is recovered as product from said chilled slurry; removing the remaining chilled mixture of liquid ammonia and liquid hydrocarbon to a phase separation zone; there separating a liquid hydrocarbon phase from a liquid ammonia solvent phase; recycling the chilled liquid ammonia phase to said ammonia heating zone wherein it is heated by about to F. by the aforesaid contact with the humidified gas; recovering the reheated liquid ammonia solvent; passing said liquid hydrocarbon phase from said separation zone to a scrubbing zone wherein said liquid hydrocarbon phase is contacted with water thereby removing remaining ammonia from said liquid hydrocarbon phase; separating the ammonia-denuded liquid hydrocarbons from an ammoniacontaining water phase in a separation zone; recovering said hydrocarbons; passing said water phase from said scrubbing zone to a distillation zone wherein the ammonia is stripped from said water phase; recovering said ammonia; and recycling the solvent-denuded water to said water-washing zone.

19. A process in accordance with claim 1 wherein said heating and cooling zones are maintained under substantially adiabatic conditions.

20. A process for extracting an ammonia soluble hydrocarbon from a hydrocarbon feed which comprises contacting said feed with warm liquid ammonia in an extraction zone to form an extract, concomitantly passing into said extraction zone a stream of inert gas at a temperature below saturation, evaporating a portion of the liquid ammonia into said inert gaseous st-ream thereby cooling the extract, removing the extract from said extraction zone and separating cold liquid ammonia from the ammonia soluble hydrocarbon components by further cooling, recovering the ammonia soluble hydrocarbon components, passing a wet gas stream containing volatiles from said extraction zone to a solvent heating zone countercurrent to the cold separated liquid ammonia thereby condensing volatiles from said wet gas and warming said cold liquid ammonia, recovering warm liquid ammonia from said solvent heating zone for use in said extraction zone.

References Cited in the file of this patent UNITED STATES PATENTS Stephens Dec. 11,

Claims (1)

14. A PROCESS WHICH COMPRISES EXTRACTING RELATIVELY SOLUBLE AROMATIC CONSTITUENTS FROM A HYDROCARBON FEED IN A MULTISTAGE LIQUID EXTRACTION ZONE WHICH A LIQUID SOLVENT CONTAINING A SUBSTANTIAL FRACTION OF LIQUID AMMONIA, PASSING A PLURALITY OF INERT GAS STREAMS THROUGH A PLURALITY OF STAGES OF SAID LIQUID EXTRACTION ZONE SO AS TO EVAPORATE GRADUALLY OF SUFFICIENT PORTION OF THE LIQUID AMMONIA INTO SAID INERT GAS STREAMS TO COOL SAID SOLVENT TO A TEMPERATURE 30* TO 50*F. COOLER THAN THE HYDROCARBON FEED STAGE, WITHDRAWING AN EXTRACT STREAM, PRECIPITATING A PORTION OF THE HYDROCARBONS CONTAINED IN THE WITHDRAWN EXTRACT IN A SEPARATION ZONE, RECYCLING SAID PERCIPITATED HYDROCARBONS TO THE EXTRACTION ZONE COUNTERCURRENTLY TO AN REMAINING SOLVENT EXTRACT LAYER FROM SAID SEPARATION ZONE THROUGH AN EXTRACT COOLING ZONE COUNTERCURRENTLY TO ANOTHER RELATIVELY DRY INERT GAS STREAM WHEREBY A PORTION OF VOLATILE SOLVENT EVAPORATES FROM SAID EXTRACT INTO THE GAS STREAM COOLING SAID EXTRACT, PASSING THE RESULTING HUMIDIFIED GAS STREAM FROM SAID EXTRACT COOLING ZONE INTO A FIRST SOLVENT HEATING ZONE, WHERE IT IS COUNTERCURRENTLY CONTACTED WITH A RELATIVELY COOL LIQUID SOLVENT PHASE, REMOVING THE COOLED EXTRACT FROM SAID COOLING ZONE, FURTHER CHILLING THE REMOVED EXTRACT IN A CHILLING ZONE SO AS TO SEPARATE IT INTO A LIQUID SOLVENT PHASE AND A PRECIPITATED HYDROCARBON PHASE AND SO THAT THE TEMPERATURE OF THE LIQUID IS MAINTAINED ABOVE THE SATURATION TEMPERATURE OF THE GAS IN SAID EXTRACT COOLING ZONE AND BELOW THE SATURATION TEMPERATURE OF THE GAS IN SAID SOLVENT HEATING ZONE, RECOVERING THE CHILLED SOLVENT PHASE, A LIQUID HYDROCARBON PHASE AND A SOLID HYDROCARBON PHASE FROM THE CHILLED MIXTURE, PASSING THE CHILLED SOLVENT PHASE FROM SAID CHILLING ZONE TO THE AFORESAID FIRST SOLVENT HEATING ZONE FOR REHEATING BY COUNTERCURRENT CONTACT WITH SAID HUMIDIFIED GAS STREAM, RETURNING THE RESULTS RELATIVELY DRY GAS STREAM TO THE AFORESAID EXTRACT COOLING ZONE, PASSING THE RESULTING REHEATED SOLVENT TO A SECOND SOLVENT HEATING ZONE AND THERE COUNTERCURRENTLY CONTACTING IT WITH THE VAPOR-CONTAINING INERT GAS PREVIOUSLY WITHDRAWN FROM SAID EXTRACTION ZONE, RETURNING THE RESULTING REHEATED SOLVENT FROM SAID SECOND SOLVENT REHEATING ZONE TO SAID EXTRACTION ZONE, AND LIKEWISE RECYCLING THE RESULTING SOLVENT-DENUDED GAS STREAM TO SAID EXTRACTION ZONE.

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