US3212197A

US3212197A - Drying method and apparatus

Info

Publication number: US3212197A
Application number: US115710A
Authority: US
Inventors: James R Crawford
Original assignee: Individual
Current assignee: Individual
Priority date: 1961-06-08
Filing date: 1961-06-08
Publication date: 1965-10-19
Anticipated expiration: 1982-10-19

Description

Oct. 19, 1965 J. R. CRAWFORD 3,212,197

DRYING METHOD AND APPARATUS Filed June 8. 1961 2 Sheets-Sheet 1 on 9 4 E Nm hv INVENTOR JAMES R. CRAWFORD l 1 I I KwnEOI BLAIR AND BUCKLES ATTORNEYS.

Oct. 19, 1965 J. R. CRAWFORD DRYING METHOD AND APPARATUS 2 Sheets-Sheet 2 Filed June 8. 1961 INVENTOR. JAMES R. CRAWFORD BLAIR AND BUCKLES ATTORNEYS.

United States Patent Office 3,212,?97 Patented Oct. 19, 1965 3,212,197 DRYING METHOD AND APPARATUS James R. Crawford, 42 Cedar Lane, New Canaan, Conn. Filed .lun'e 8, 1961, Ser. No. 115,710 4 Claims. (CI. 3410) This invention relates to a novel method and apparatus for drying reaction products which are wet with solvents, low-boiling reactant materials, and possibly water. More particularly, the invention relates to the removal of solvents, particularly organic solvents and low-boiling reactants, such as monomeric material, or water from a polymer mix or from a reaction product, by a method and an apparatus whereby superheated vapor of the solvent present in the mix or reaction product is used to dry it as the material is fluidized.

The removal of liquids or solvents such as water, benzene, ether, etc., from solids or semi-solids has been generally accomplished by driers of the spray, flash, drum, rotary, tray, truck, tunnel, belt, pan, or vacuum types. Each type of drier is particularly suited for a specific drying operation, but each type also has certain disadvantages.

Spray driers generally include a vertically positioned flume in which the material is sprayed into a current of hot air or hot flue gas to dry the droplets. Dried material drops to the floor of the flume where it is removed. This drier is used for continuous operation techniques, and its short heating period permits its use with heatsensitive materials. Unfortunately, however, it cannot be used for drying sticky materials, such as synthetic polymers, which readily adhere to the apparatus. The need for accurate control over the temperature of the drying gas to prevent redeposition of the evaporated liquid on the dried product also lessens its usefulness.

Flash driers are similar to spray driers in that the material is dried by dropping it into the heated air stream which then carries it to a hammer mill or a high-speed agitator. A cyclone separator then separates the dried particles from the air stream. Continuous operation and rapid removal of solvent without drastic heating of the material are possible, but the disadvantages of the spray drier are also present in flash driers.

Drum driers utilize an internally heated, closed rotary drum which dips into a solution or slurry of the material to be dried. The material is dried on the drum, and a scraper then removes the dried material from the drum surface. Drying time, however, is governed by the diameter of the drum used, and where a long drying time is necessary, drum driers are uneconomical. Further, since these driers are easily clogged, they are not applicable to the drying of sticky materials.

Rotary driers utilize hollow rotating cylinders which are slightly inclined to the horizontal, or the cylinder may be stationary with a rotating agitator scraping the inner walls of the cylinder. In either case, the wet material is fed in at the upper inclined end and advanced progressively to the lower end where it is discharged as dry material. The rate of feed, the residence time in the cylinder, the volume of heated air or gas passed through the cylinder, etc., control the speed of drying. It should be evident that very long rotary driers are frequently necessary to effect complete drying.

Vacuum driers utilize a closed heated chamber in which the pressure is reduced. Only batch operation is possible because of the need for a closed chamber to obtain and then maintain the vacuum.

Probably the most serious objection to the prior art driers is the fact that economical operation generally necessitates recovery of the solvent from the drying air or gas which is usually passed over the material. Such recovery is frequently diflicult because of the severe dilution of solvent in the drying medium.

Another very serious disadvantage is the fact that the apparatus will frequently clog because the material adheres to the walls, etc., of the drying chamber. This necessitates disassembly and thorough cleaning.

A further disadvantage is the intimate and direct contact of the material being dried with the heated surface of the drier. This frequently causes at least some slight decomposition which might create clogging and gumming in the drier.

Reference is now made to the drying of specific material to illustrate the problems encountered with driers of the above type.

In the compounding or polymerizing of natural and/ or synthetic polymers and copolymers, a solvent reaction medium is generally used. After polymerization, the polymer is precipitated by the addition of a second solvent in which the polymer is insoluble. The polymer is then filtered or centrifuged to form a wet cake, which is then dried. Generally, the wet cake has a ratio of about one part solvent to one part polymer. Unreacted monomer and even possibly water or other impurities are also present. Since the solvent is a valuable material, the reaction product cannot simply be heated to evaporate the solvent. A current of air was heretofore used to dry the polymer, and the very dilute mixture of drying air and solvent was fractionated. Costly oil absorption or activated carbon absorption techniques were also used.

Recently, to avoid severe dilution of solvent in air, superheated vapor of the solvent(s) of the wet material was proposed in an article by In Chin Chu, et al., entitled Evaporation of Liquids into Their Superheated Vapors, appearing in the July 1953 issue of Industrial and Engineering Chemistry, pages 1586-1591. The drying of brown coal, lignite, wood, silica gel, calcium silicate, and vegetables with superheated steam or extracted soybean flakes with superheated hexane vapor follows along the line of this proposal.

In drying with a gas other than the vapor of the liquid in the material being dried, this vapor must diffuse from the interface of each particle through an adjacent layer of vapor-laden gas. At all times, the vapor in the adjacent layer is at a higher partial pressure than the vapor in the main vapor stream. This difference in partial pressure is a measure of the driving force for mass transfer. Obviously, the higher the driving force, the faster the drying. When superheated vapor of the wetting solvent is used as the drying gas, the gas film is not present, making possible significantly higher drying rates.

Accordingly, an object of this invention is to provide a novel method and apparatus for the drying of a reaction product wet with solvent, unreacted material, and possibly water with a high mass transfer rate.

Another object of this invention is to provide a method and apparatus of the above character which is both economical and safe.

Still another object of this invention is to provide a method and apparatus of the above character which lends itself to continuous operation.

A further object of this invention is to provide a method and apparatus of the above character wherein the heat transfer medium is the superheated vapor of the solvent which wets the material.

Still another object of this invention is to provide a method and apparatus of the above character in which optimum drying is achieved by utilizing a fluidized bed technique.

Another object of this invention is to provide a method and apparatus of the above character which acts to cultivate the formation of certain physical forms for the material being dried.

Another object of this invention is to provide a method and apparatus of the above character which may, with some slight modification, be incorporated into certain types of existing driers.

Another object of this invention is to provide a method and apparatus of the above character wherein severe dilution of the drying medium is avoided.

Another object of the invention is to provide a method and apparatus of the above character wherein introduction of impurities is avoided.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements, and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic flow sheet of one embodiment of the apparatus and method of this invention; and

FIGURE 2 is a front elevation view of apparatus which incorporates the embodiment illustrated schematically in FIGURE 1.

Similar reference characters refer to similar parts throughout the several views of the drawing.

In accordance with the objects enumerated above, the new technique and apparatus of the present invention involves the drying of particulate material, such as a reaction product, while the latter is fluidized by a stream of superheated vapor of the wetting solvent. More particularly, the several objects are accomplished by a technique and apparatus wherein such material to be dried is continuously fed to a drying chamber 10. The chamber is fitted with means to fluidize the material with a forced draft of counter-flowing superheated vapor of the wetting solvent. After the material is dried, the spent vapor is directed to a solids removal means 12, while the dried material is continuously removed from the fluidized bed 11 through a product discharge orifice 26.

Any solids removed from the spent vapor are returned to the drying chamber 10. The residual vapor may then, if desired, be subjected to additional cleaning before reuse, or merely returned directly to the drying system, and dilution of the solvent vapor is substantially avoided. Thus, expensive recovery techniques for the solvent are not necessary, and a noteworthy decrease in cost is thereby effected.

Should additional cleaning of the vapor be desired after initial removal of the solids, the vapor may be passed to a scrubber 14 where a shower of scrub liquid, which may be fresh solvent, washes and partially condenses the vapors to re-dissolve any fine particulate material carried therein. The washed vapor is then recycled to blower 16, and it is then heated in heater 18 prior to re-use. The scrub liquid may be purified and also reused.

It is evident that a simple and economical drying system has now been created. Not only are drastic localized heat conditions avoided, but also there is good heat transfer because the fluidization insures uniform distribution of heat throughout the entire mass. Further, the enveloping region of superheated vapor about each particle provides optimum mass transfer conditions so that a fast drying rate results. And the flotation of each particle in a stream of vapor cultivates a condition which favors the formation of the desired physical forms of the material being dried.

Another noteworthy davantage is that the drying medium can be continuously cleaned and rc-used by a simple solids removal step. And as a further refinement, the vapor may be scrubbed with fresh solvent prior to re-use. Hence, there is little dilution, if any, of the drying medium, and no expensive recovery steps need be incorporated into the system. Even with this refinement, the apparatus is fairly simple. The vapor compressor or blower remains as the prime mover. The refinements of a feed conveyor, a gas cyclone, a scrubber, and a recycle circuit do not complicate this basic apparatus. Furthermore, the apparatus has no inaccessible areas, so that maintenance is fairly simple. And it may, with relatively slight modification, be incorporated into existing driers which utilize a closed tank for the drier component.

As indicated briefly above, the invention has rather wide utility even though it is particularly adapted for the drying of polymeric material, especially where solvent is used during polymerization as the reaction medium.

The plastic materials capable of being dried using the technique and apparatus herein are polyvinylchloride, polystyrene, polycarbonate, polyethylene, polypropylene, polyvinyl alcohol, and others. These plastic materials generally contain solvents such as benzene, toluene, pentane, hexane, heptane, butyl ester, propyl ester, methylethylketone, methylene chloride, chlor-oethylene, etc.

Referring to the drawings for a detailed disclosure of this invention, there is shown in FIGURE 1 a schematic flowsheet, illustrating a continuous process flow wherein particulate material to be dried is introduced into hopper 20. From the hopper, the material is conveyed by feeder 22 into drying chamber 10. The feeder may consist of a screw conveyor, a belt conveyor, or similar conveying means.

As the particulate material enters the drying chamber 10, it distributes itself over the upper layer of a fluidized bed 11 of previously added material. Fluidization is accomplished by a counterflowing stream of superheated vapor of the solvent entrained in the material. Grid 24 within the drying chamber 10 disperses the fluidizing stream equally over the entire fluidized bed 11.

Material in the fluidized bed has a downward flow. The upward flow of superheated vapor is controlled as to rate, so that upon complete descent through bed 11, the material is dried. Such dried material is continuously or, if desired, periodically removed through dry-product discharge outlet 26.

Spent vapor is removed through vapor port 28. The

' vapor flows through conduit 30 to a solids removal means, such as a centrifuge or gas cyclone 12. Any solids removed by the cyclone are returned to the drying chamber via solids return conduit 32. This arrangement minimizes loss of material. It also purifies the spent vapor to a substantial extent so that it may be directly re-used as the fluidizing and drying medium. In the preferred form, however, the spent vapor is subjected to additional cleaning before re-use.

Additional cleaning is accomplished by the use of scrubber 14. As the spent vapor leaves the cyclone, it

travels through cyclone exhaust conduit 34 into a central portion of scrubber 14. Here, the scrubber subjects the spent vapor to a shower or spray 36 of scrub liquid, preferably fresh solvent. The shower not only washes the spent vapor, but also removes impurities in the vapor.

A portion of the scrub solvent liquid is passed through cooler 52 through conduit 53 and then introduced into the top of the scrubber 14 where it flows downwardly. Grid 37 disperses the cooled scrub liquid evenly over the cross-sectional area of the scrubber and promotes intimate contact of the scrub liquid with the superheated vapor. The lower temperature of this body of descending scrub liquid condenses a portion of the scrubbed vapor. The solvent so formed dissolves substantially all of the fines that may have eluded the solids removal treatment of the cyclone. Fouling, which would be possible by the decomposition of carried-over fines in heater 18, is thereby avoided.

The cooled scrub solvent liquid also acts partially as a fractionator to dissolve any residual extraneous solvents present in the vapor of the principal solvent. Additionally, the cooled scrub liquid acts to control the solvent vapor pressure within the system. If too high a pressure is produced within the system by heater 18, an increased rate of introduction or a cooler scrub liquid will increase the condensation of the scrubbed vapor. This offsets the higher vaporization caused by heating so that a balanced system is created. Obviously, this balanced system can be made fully automatic by incorporation of appropriate controls into the apparatus.

Spent scrub liquid is removed via outlet port 44. A portion of the spent scrub liquid so removed may be routed to a separate purification via take-off conduit 47. Excess vapor, such as that recovered from the particulate material by drying, can conveniently be removed via the take-off conduit 47. Fresh or clean scrub liquid to replace that portion removed may be introduced via pump 46 from liquid supply conduit 49.

The main body of scrub liquid is recycled, without purification, through shower conduit 48 to the shower head 36. A portion is passed through conduit 50 leading to cooler 52 where it is chilled and then reintroduced via conduit 53 to scrubber 14 above grid 37. The cooler is cooled with cold water or other coolant liquid supplied and removed by

conduits

54 and 56, respectively.

After being scrubbed in scrubber 14, the vapor is recycled to the drying chamber through recycle conduit 38. Blower 16 supplies the vapor with additional velocity so that it can act as a fluidizing medium. The scrubbed vapor is first re-superheated by passage via heater conduit 40 through heater 18. The reheated vapor is then passed into the drying chamber through vapor supply conduit 42. Here, it repeats its fiuidizing and drying function as described above.

Heater 18 is actually a heat exchanger which utilizes high-pressure steam or other heating medium. Steam is supplied and removed via conduits 58 and 60, respectively.

FIGURE 2 shows a pictorial representation of apparatus which embodies the schematic flowsheet arrangement just described. Product material to be dried is fed via

hoppers

120 and 121 and feeder 122 into drying chamber 110. Here it is subjected to an upwardly flowing fluidizing stream of superheated vapor of the solvent which wets the material. Grid 124 disperses the fluidizing stream so that the fluidized bed 111 is uniform.

Glass-covered ports 113 permit visual inspection of the fluidizing stream and bed.

As the product is dried, it is removed through dryproduct outlet 126. Auxiliary apparatus, such as a vaned rotor driven by motor 127, may be used to assist in the removal. The product may be deposited and packaged in bags or boxes (not shown) affixed to or placed immediately below product exit port 129.

The dome of the drying chamber 110 contains a vapor port 128. The spent fluidizing vapor exits through this port and travels through spent vapor conduit to solids removal means, such as cyclone 112. Here, the vapor is subjected to centrifugal force to remove the fine solids inadvertently separated and carried over from the fluidizing bed. These solids are returned through solids return conduit 132 to the fluidized bed 111, the top of which is generally indicated by line 133.

After the cyclone treatment, the vapor passes through cyclone exhaust conduit 134 into an intermediate portion of scrubber 114. Here, as the vapor ascends within the scrubber body, a shower 136 of fresh solvent Washes it. Such washing removes substantially all fines which may have eluded the cyclone treatment. It also may, by proper selection of solvent and added constituents, remove trace amounts of impurities from the vapor. This could include residual solvents other than the main solvent being removed. Indeed, with proper selection, the scrub liquid may remove other impurities which are vaporized along with the superheated vapor.

Grid 137 within the scrubber supplements the washing action of shower 136. As indicated above, it disperses cooled scrub liquid evenly over the cross-sectional area of the scrubber so that more effective washing by the scrubber results. Simultaneously, it condenses a portion of the vapor being scrubbed so that the liquid solvent thereby formed will dissolve substantially all the fines which may have eluded the cyclone treatment. This prevents subsequent fouling of heater 118 during the resuperheating of the recycled vapor. In addition, the cooler descending body of scrub liquid in scrubber 114 acts to fractionate out undesirable vaporous material and also banks the pressure Within the system, to any desired extent. Indeed, the pressure within the entire system may be controlled by varying the amount or the temperature of cool scrub liquid supplied to the scrubber.

Spent scrub liquid is removed from the scrubber 114 via outlet port 144, and pump 146 driven by motor 145. It is returned as a shower or as a scrubbing medium.

In the preferred form, a portion of the scrub liquid is removed and subjected to cleaning to remove the dissolved fines and/or the residual solvents. Take-off conduit 147 effects such take-off. Replacement scrub liquid is supplied via a liquid supply conduit (not shown).

The balance of the scrub liquid is recycled through shower conduit 148 to the shower head 136 and through conduit 150 to cooler 152. Here it is cooled and then it is recycled via conduit 153 to the top of the scrubber.

Ports

154 and 156 provide and remove coolant to and from the cooler.

The scrubbed vapor is removed from the scrubber through recycle conduit 138. Blower 116, powered by motor 117, provides the necessary suction for such removal. The blower also acts to compress the solvent vapor so as to convey it via heater conduit to heat exchanger 118. Here, the vapor is again superheated. After superheating, it is returned as the fluidizing and drying medium to the drying chamber 110 via vapor supply conduit 142.

The heat exchanger is supplied with steam at high pressure so as to superheat the vapor. Such high-pressure steam is supplied by steam supply conduit 158 and spent steam is removed via steam exhaust conduit 160.

To better illustrate the mode of operation, reference is now made to specific examples which illustrate the drying of certain polymeric material.

EXAMPLE I In a plant designed for the production of a certain resin, the reaction product produced had the following composition:

47.8% solids 23.6% volatile A] 13.3% volatile B} 52.2% solvent 15.3% volatile Cy This damp mixture of resin was passed through a drier plant similar, in all respects, to the one shown in FIG- URE 2.

The following conditions existed during one run- Fluidizing vapor used:

50% volatile A 25% volatile B 25% volatile C Temperature at vapor supply conduit 142 312 F.

Fluidized bed temperature 240285 F. Vapor velocity through the fluidized bed 2 ft./ sec. Fluidized bed depth 1 ft.

Dried product removed from the drying chamber contained 0.23 to 0.33% total volatiles (mostly solvent). There obviously was good separation of solvent from the particular resin.

There was no decomposition of the resin and hence no gummy deposit was formed on the interior surface of the drier.

EXAMPLE II In another run using resin product having the same wet composition as in Example I, the following conditions existed Fluidizing gas:

50% volatile A 25% volatile B 25% volatile C Temperature at vapor supply conduit 142 .312 F. Fluidized bed temperature 265-285 F. Vapor velocity through the fluidized bed 2 ft./sec. Fluidized bed depth 1 ft.

Dried product removed from the drying chamber, at various times, analyzed for solvent content as follows- Product sample: Percent total volatiles Here again, excellent removal of solvent (volatiles) occurred with no fouling of the drying apparatus by reason of decomposition.

EXAMPLE III In still another run resin product having the same wet composition as in Example I, the following conditions were used Fluidizing gas:

50% volatile A 25% volatile B 25% volatile C Temperature at vapor supply conduit =142 312 F. Fluidized bed temperature 265 F. Vapor velocity through the fluidized bed 2 ft./ sec Fluidized bed depth 1 ft.

Feed rate (wet) Detention time 25 gms./ min. 150 minutes Dried product removed from the drying chamber, at various times, analyzed for solvent content as follows- Product sample: Percent total volatiles Again, satisfactory drying occurred. No fouling of the apparatus was evident,

8 EXAMPLE IV "Since water constitutes a major portion of the solvent reaction medium, consideration was also given to the use of superheated steam alone as the fluidizing and drying agent. The following conditions were used:

Fluidizing gas steam. Temperature at vapor supply conduit 142 312 F. Fluidized bed temperature 255 F. Vapor velocity through the fluidized bed 2 ft./sec. Fluidized bed depth 1 ft.

Feed rate (wet) Detention time 25 gms./minute. minutes.

Dried product removed from the drying chamber at various times and analyzed for solvent content exhibited these results Product sample: Percent total volatiles EXAMPLE V To determine the effect of varying detention times on the final dryness of the product, various samples were taken at different times after drying under the following conditions Fluid-izing gas:

50% volatile A 25% volatile B 25% volatile C Temperature at vapor supply conduit 142 395 F. Fluidized bed temperature 275 F. Vapor velocity through the fluidized bed 2 ft./sec. Fluidized bed depth 1 ft. Detention times 30180 min.

Samples withdrawn after progressively longer detention times exhibit successively increasing dryness Detention time, min.: Percent total volatiles It is evident that the longer the detention time, the better the effectiveness of drying. It is also evident that no redeposition of vapor occurs for prolonged periods of residence in the drying chamber.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illus- (A) delivering said wet particulate material at a first level to an internally unobstructed drying chamber 50 that said material descends in said chamber,

(B) fluidizing and directly heating said material with a plurality of counterfiowing streams of superheated vapor having the same chemical composition as said solvent ascending through said drying chamber,

(C) controlling the flow rate of said counterflowing vapor to retard the descent of said particulate material so that it is dry when it has descended to a second level below said first level,

(D) removing said dry material from said drying chamber at said second level,

(E) recovering spent vapor that has passed through said bed of material,

(F) centrifuging said recovered vapor to remove fines of said material therefrom,

(G) recycling said fines to said fluidized material in said chamber,

(H) scrubbing said centrifuged spent vapor with a cool scrub liquid to further clean said spent vapor of fines and other impurities,

(I) controlling the flow rate and temperature of said scrub liquid by cooling and recycling a selected proportion of said scrub liquid to control the solvent vapor pressure within the drying system,

(J) reheating the spent vapor output from said scrubber, and

(K) recycling the reheated vapor through said drying chamber.

2. Apparatus for drying particulate material which is wet with solvent without any substantial decomposition including means forming a closed drying chamber unobstructed by internal members with (1) a supply feed means connected to supply particulate material to the upper part of said drying chamber, and a dry product outlet near the bottom of the drying chamber, and

(2) means to fluidize said particulate material within said drying chamber with superheated vapor of the wetting solvent including means near the bottom of the drying chamber to introduce and disperse said superheated vapor in a plurality of upwardly counterflowing streams and a spent vapor outlet near the top of the drying chamber,

said apparatus comprising in combination therewith (A) a closed, vapor-cleaning, recycling and resuperheating system to conduct spent vapor back to said drying chamber for reuse as superheated vapor of said wetting solvent, said superheated vapor acting as the fluidizing, drying and stripping gas in said drying chamber, including (B) a fines removal means in communication with said spent vapor outlet for removal of fines from the spent vapor,

(C) a spent vapor scrubber in communication with said fines removal means with scrub liquid supply means and spent scrub liquid take-01f means, said scrubber containing a sprayer to apply said scrub liquid for washing said spent vapor, and

(D) vapor-recycle conduit means connecting said scrubber with said introducing and dispersing means and returning the cleaned and scrubbed vapor to said drying chamber,

said spent liquid take-oil means including means to cool a portion of said spent liquid, a liquid replacement means whereby a portion of said spent liquid is replaced with fresh clean liquid, and recycle conduits to said scrub liquid supply means, whereby the cooled recycled scrub liquid will act to condense a portion of said spent vapor, thereby dissolving fines which may elude the fines removal means.

3. The combination defined by claim 2, wherein said closed vapor-cleaning, recycling and resuperheating system includes a blower and a heater serially arrayed and operatively connected together between the spent vapor scrubber and the drying chamber, with the heater supplying resuperheated recycled vapor of the wetting solvent to the dispensing means including a dispersing grid introducing the superheated vapor into the drying chamher.

4. The combination defined by claim 2, wherein said spent vapor scrubber includes a sprayer and a perforated grid above the sprayer, with conduit means supplying scrub liquid to the scrubber above the grid and to the sprayer to provide maximum exposure of the spent vapor to the scrub liquid.

References Cited by the Examiner UNITED STATES PATENTS 2,427,302 9/47 Reich 34---75 2,571,143 10/51 Leslie 34-37 2,586,818 2/52 Harms 3410 X 2,775,551 12/56 Nathan et a1.

2,813,352 11/57 Payne et a1. 34-10 2,843,942 7/58 Whistsel 34-10 2,874,480 2/59 Todd 34-10 2,901,402 8/59 Plltman 3475 PERCY L. PATRICK, Primary Examiner.

CHARLES OCONNELL, NORMAN YUDKOFF,

Examiners.

Claims

1. A METHOD CHARACTERIZED BY HIGH HEAT TRANSFER FOR DRYING SOLVENT FROM WET PARTICULATE MATERIAL COMPRISING THE SUCCESIVE STEPS OF (A) DELIVERING SAID WET PARTICULATE MATERIAL AT A FIRST LEVEL TO AN INTERNALLY UNOBSTRUCTED DRYING CHAMBER SO THAT SAID MATERIAL DESCENDS IN SAID CHAMBER, (B) FLUIDIZING AND DIRECTLY HEATING SAID MATERIAL WITH A PLURALITY OF COUNTERFLOWING STREAMS OF SUPERHEATED VAPOR HAVING THE SAME CHEMICAL COMPOSITION AS SAID SOLVENT ASCENDING THROUGH SAID DRYING CHAMBER, (C) CONTROLLING THE FLOW OF SAID COUNTERFLOWING VAPOR TO RETARD THE DESCENT OF SAID PARTICULATE MATERIAL SO THAT IT IS DRY WHEN IT HAS DESCENDED TO A SECOND LEVEL BELOW SAID FIRST LEVEL, (D) REMOVING SAID DRY MATERIAL FROM SAID DRYING CHAMBER AT SAID SECOND LEVEL, (E) RECOVERING SPENT VAPOR THAT HAS PASSED THROUGH SAID BED OF MATERIAL, (F) CENTRIFUGING SAID RECOVERED VAPOR TO REMOVE FINES OF SAID MATERIAL THEREFROM, (G) RECYCLING SAID FINES TO SAID FLUIDIZED MATERIAL IN SAID CHAMBER, (H) SCRUBBING SAID CENTRIFUGED SPENT VAPOR WITH A COOL SCRUB LIQUID TO FURTHER CLEAN SAID SPENT VAPOR OF FINES AND OTHER IMPURITIES, (I) CONTROLLING THE FLOW RATE AND TEMPERATURE OF SAID SCRUB LIQUID BY COOLING AND RECYCLING A SELECTED PROPORTION OF SAID SCRUB LIQUID TO CONTROL THE SOLVENT VAPOR PRESSURE WITHIN THE DRYING SYSTEM, (J) REHEATING THE SPENT VAPOR OUTPUT FROM SAID SCRUBBER, AND (K) RECYCLING THE REHEATED VAPOR THROUGH SAID DRYING CHAMBER.