US3508339A - Drying apparatus and process - Google Patents

Description

'April 28, 1970 1W, NEBL fT ET AL 3,508,339

' DRYING APPARATUS AND PROCESS I Filed Nov. 30, 1965 2 Sheets-Sheet 1 51 /1. /v 55 zrmkw A7 O/I'W') J. w. NEBLETT ET'AL.

DRYING APPARATUS AND PROCESS April 28, 1970 2 Sheets-Sheet 2 Filed Nov. 50, 1965 vwavrazm: J4M5 M N53! [/72 20/2440 5 DFEAC/vffi, 5/14 r 4; assa fir j ram [r United States Patent 3,508,339 DRYING APPARATUS AND PROCESS James W. Neblett, Donald E. Debacher, and Billy A.

Nussel, Mount Vernon, Ind., assignors to General Electric Company, a corporation of New York Filed Nov. 30, 1965, Ser. No. 510,554 Int. Cl. F26b 3/10 US. Cl. 34-10 3 Claims ABSTRACT OF THE DISCLOSURE A drying apparatus capable of providing a feed material in the form of dense, discreet particles wherein the apparatus has at least one drying stage which has in combination a jet mixer and an elongated drying pipe in communication therewith. The jet mixer has a hot gas entry duct, a jet nozzle having a Reynolds number of at least 10,000, a mixing chamber at the discharge end of the jet nozzle which has means for the introduction of a feed material to the discharge end of the jet nozzle at an angle converging with the discharge end thereof. At the discharge end of the mixing chamber is a diffusion zone, which diifusion zone diverging at an angle that allows limited expansion of a gas stream passing therethrough while maintaining contact between the gas stream and the walls of the diffusion zone. The gas entry duct, jet nozzle, mixing chamber and diffusion zone are in axial alignment. Further the invention is directed to a process for drying a material in the form of dense, discreet particles, which process comprises continuously passing a stream of hot gas through a jet nozzle at acoustical velocity, angularly projecting a feed material into the stream of hot gas as it discharges from the jet nozzle to form a suspension thereof in the hot gas, passing the suspension into a confined diffusion zone in order to allow limited expansion thereof while maintaining contact with the walls of the diffusion zone, advancing the suspension through an elongated pipe and then separating volatiles from the formed particulate material.

This invention relates to a rapid drying apparatus and method wherein a dry material is obtained in the form of discreet, dense particles of predetermined particle size. More particularly, this invention relates to a rapid drying apparatus consisting of one or more mixing and drying stages and to a method wherein a wet particulate solid or solution is projected into a high velocity stream of a hot gas or vapor in a confined chamber and thereafter passed through a drying pipe to form discreet, dense particles of predetermined particle size.

An important operation in many plants related to the chemical and plastics industry is the separation of a dissolved, solid material from its solvent while at the same time controlling the resultant particle size and particle form so that the precipitated powder will be suitable for further processing. Many processes and apparatus have been developed for such operations but in general, the dried powder resulting from such operations is usually in the form of small, porous agglomerates that are difficult to handle. One such prior art operation identified as spray drying involves atomizing a solution of the material to be dried into very small droplets dispersed in an atmosphere of hot gasses in a large open chamber. The solvent is rapidly removed due to the extremely small "ice size of the droplets and the resulting large total drying surface. One difficulty encountered with the process is that many materials, such as resin solutions, become case hardened when spray dried. On drying, the resins form a tough outer skin which is subsequently blown apart as the interior liquid vaporizes. This forms hollow, fragile spherical particles which have low bulk density. Also, the particles are often very small, resulting in a material that is too dusty for subsequent material handling equipment.

Another frequently utilized drying operation is that of prilling. This operation is a combination of spray drying and crystallization. Large droplets of a hot concentrated solution are sprayed into a tower and allowed to cool on falling through hot air blown through a tower. The solids crystallize into agglomerates or prills which are generally spherical in shape. This method is limited to materials which can be highly concentrated in hot solution and which readily crystallize on cooling. Many materials, such as plastic resins, do not crystallize readily and their concentrated solutions are too viscous to be sprayed at temperatures below degradation levels. Materials of this nature require relatively concentrated solutions for efficient handling. Dilute solutions would require infinitely large prilling towers.

A third procedure frequently employed to recover a solid from its solution is the precipitation technique. In this procedure, a non-solvent for the solid to be collected is added to a solution. The non-solvent is substantially more miscible with the solvent than with the solute and thus causes the solute to separate from the solution as a solid phase. The resulting slurry can then be filtered or centrifuged to recover the solids as a wet cake. This technique has two distinct disadvantages. First, a small fraction of the solids will remain dissolved in the resultant liquid phase and generally, will be lost from the product yield. Second, the original solvent and the non-solvent must be separated if they are to be resused in the process. This generally involves several additional operations such as evaporation, fractional distillation, and extraction. In addition to this, the solute is generally collected in the form of a wet filter cake which must be crushed, dried and sieved to obtain a powder which may be utilized in subsequent operations.

Another approach, known as jet spray drying, involves the introduction of a solution through a water-cooled tube axially aligned with a gas or steam discharge nozzle. The gas or steam leaves the nozzle at high velocities causing the solution to be atomized and entrained by the hot gas or steam. The nozzle discharges into a large open area and the solvent is evaporated. The solids phase is separated from the gas phase in an apparatus such as cyclone separator. A major disadvantage of the jet spray dryer is that the particles of solid are very small, usually less than 10 microns. This offsets the efficiency of the cyclone separator and subsequent handling equipment and necessitates an additional agglomeration step to re-group the tiny particles into larger, more handleable ones. Usually, this involves the reintroduction of a small amount of solvent as a spray to cause the smaller particles to agglomerate.

It has now been discovered that the disadvantages of the various drying procedures noted above can be substantially eliminated by the practice of the present invention which comprises passing a stream of a material to be dried through one or more drying stages wherein each drying stage has in axial alignment a jet mixer and an elongated drying pipe. By employing the apparatus and procedures of this invention, it is possible to form dry, discreet, dense particles of a material having a predetermined particle size from its solution. It is also possible to rapidly form dry, discreet, dense particles from a wet particulate solid. In addition, materials may be dried that have low decomposition temperatures because residence time in the apparatus is usually less than 60 seconds. A further advantage of this system is that the elongated drying pipe may be used to simultaneously dry and convey material from one part of a plant to another.

Accordingly, one object of this invention is to provide a rapid process for forming discreet, dense particles of a material from its solution.

Another object of this invention is to provide a rapid process for drying a wet particulate solid.

A third object of this invention is to provide a drying process wherein the particle size of the resultant dry solid may be controlled and predetermined.

A further object of this invention is to provide a drying apparatus capable of forming discreet, dense solid particles of predetermined particle size and consisting of a jet mixer and a drying pipe.

An additional object of this invention is to provide a drying apparatus consisting of a plurality of stages.

Other objects and advantages of this invention will be in part apparent and in part pointed out in the description which follows.

Briefly stated, the objects and advantages of this invention are achieved by passing a stream of a material to be dried through one or more drying stages wherein each drying stage consists of a jet mixer in axial alignment with an elongated drying pipe. The material to be dried may be in the form of a solution or wet particles. For brevity, the material to be dried will hereinafter be referred to as the feed material which is defined to include both solutions and wet particles.

The jet mixer for each drying stage consists of a jet nozzle in axial alignment with a confining mixing chamber and a diffusion zone. A stream of high velocity gas is passed through the jet nozzle and into the mixing chamber. The feed material is forced into the mixing chamber at an angle converging with the hot gas stream. When the feed material is in the form of a solution, it is sheared or cut into small slugs or droplets which are suspended and intimately dispersed in the hot gas stream. This suspension is passed through the diffusion zone and into an elongated pipe. The suspended droplets are twisted, stretched and torn apart by the turbulent action of the high velocity gas stream. This treatment exposes considerable surface area of the droplets to the hot gas and promotes rapid vaporization of the solvent. It also tends to offset case hardening of the particle since the continually changing shape of the droplets allows internal vapors to escape readily. At some point downstream, the droplets of solution will have lost enough solvent to become tacky particles which reagglomerate into large dense particles. These dry further downstream and can be separated in a collecting apparatus such as a cyclone separator.

During the course of the drying process, the hot gas stream becomes saturated with volatiles and drying efficiency is substantially reduced. To avoid this, it is desirable to collect the particles in a moist-non-tacky condition and pass them through a second jet mixer and into a second drying pipe. This procedure can be repeated as many times as is necessary to effect complete drying.

'When the feed material is in the form of a solution, particles of predetermined size can be obtained by adjustment of the ratio of the rate of flow of the hot gas to the rate of flow of the feed material and/or by control of the percent solids in the feed material. It has been 4 found that decreasing the .above noted ratio or increasing the concentration of dissolved solids favors the formation of large particles.

This invention can be better understood by reference to the drawing wherein:

FIGURE 1 is a sectional elevation of one embodiment of a jet mixer suitable for forming a suspension of a feed material in solution form and a hot gas stream consisting of a jet nozzle, a mixing chamber and a diffusion zone;

FIGURE 2 is an elevation of the apparatus of this invention including the jet mixer of FIGURE 1, a jacketed drying pipe and a cyclone separator; 7

FIGURE 3 is a sectional elevation of another embodiment of a jet mixer designed for mixing wet particulate material with a stream of high velocity, hot gas, and

FIGURE 4 is an elevational view of a two-zone drying apparatus in accordance with this invention and includes a first and second stage jet mixer, a first and second stage drying pipe and a volatile recovery system. The jet mixing apparatus depicted in FIGURE 1 includes a gas entry duct 1, a convergent-divergent jet nozzle 2, a mixing chamber 3, and a diffusion zone 4 consisting of a diffusion throat 5, and diffuser 6. The hot gas stream enters the mixing apparatus through the gas duct and its velocity is substantially increased as it passes through the jet nozzle and into the mixing chamber. The feed material 7 is forced into the mixing chamber 3 through entry duct 8. It is necessary that the solution enter the mixing chamber along an axis converging with that of the hot gas stream. In other words, the solution may not enter the mixing chamber in axial alignment with the hot gas stream. The angle formed between the axis of the gas stream and that of the solution preferably varies from approximately 30 to As the feed enters the mixing chamber, it is sheared into small slugs or droplets by the high velocity gas stream and a suspension of the liquid in the gas stream is formed. A suspension of this nature does not form when the solution enters the mixing chamber in axial alignment with the hotgas stream. The droplets of entrained solution are twisted, stretched and torn apart by the turbulent action of the hot, high velocity gas stream and a considerable surface of the droplet is exposed to the hot gas. This promotes rapid vaporization of the solvent and prevents case hardening of the particle since the continually changing shape of the droplets allows internal vapors to escape. readily. In order to insure that sufficient surface area of the droplets is exposed to drying, the hot gas must be maintained at a high velocity and it is necessary that the Reynolds number for the jet nozzle exceed 10,000. The diameter of the jet nozzle at its narrowest point may vary between broad limits and is dependent upon thecapacity and size of the entire drying system. It has been found that the diameter may vary between 0.10", and 1.50" for a system having a 3" inside diameter gas entry duct 1.

In order to prevent sedimentation formation in the inlet 8, it is necessary to position a heat insulating material 9 between the jet nozzle- 2 and the feet inlet 8. Sedimentation formation can force shutdown of the system due to blocked passages and also causes oversized lumps in the power product. Any heat insulation known to those skilled in the art may be employed provided it is non-reactive with both the solution to be dried and the hot gas stream. An excellent heat insulation material has been found to be tetrafluoroethylene.

The hot gas stream with its entrained solution expands in the diffusion zone 4. It is necessary that the diffusion zone confine the gas stream at all times while allowing for limited expansion. Therefore, the diffusion zone diverges at an angle which provides for limited expansion while maintaining the hot gas stream in contact with the walls of the chamber. For most systems, it is desirable that the angle formed between the axis along which the gas stream passes and the walls of the diffusion zone vary between 3 and 12. It is through the maintenance of high turbulence and velocity caused by the confined area of the diffusion zone that the discreet, dense particles of predetermined particle size are formed.

The suspension of hot gas and entrained solution next passes from the discharge end of the jet mixer into a drying pipe 10. The boundary between the pipe and the diffusion zone should be free of geometric irregularities and the diameter of the pipe should preferably be of the same diameter as the discharge end of the diffusion zone. This diameter may vary between board limits depending upon the capacity of the system employed. If geometric irregularities occur between the diffusion zone and the pipe, a viscous material may be trapped and solute will precipitate in the form of lumps that eventually collect with the dried product necessitating a separate screening operation for removal.

FIGURE 2 represents a single stage dryer consisting of a jet mixer 11, an elongated drying pipe 12 and a cyclone separator 13. The jet mixer discharges into the 'elongaged pipe which, in a preferred embodiment, is

equipped with jacket 14 having inlet 15 and outlet 16. The diameter of the pipe preferably is the same as that of the discharge end of the diffusion zone. This diameter should be small enough to maintain the high velocity of the hot gas through the length of the drying pipe. The velocity of the gas must be sufiicient to maintain the feed material in suspension and the Reynolds number of the drying pipe should exceed 5000. The high velocity of the gas also causes considerable contact between the tacky particles as they form and the walls of the pipe. This forces the particles to roll over each other to some extent and form larger, more rounded particles. In addition, the high velocity prevents the particles from actually adhering to the walls of the apparatus. The jacketed pipe can be heated by steam or other external heat source to prevent condensation of the solvent vapors on the walls of the pipe and to conserve the amount of hot vapors used in the jet mixer by providing heat through the pipe wall to offset that used to vaporize the solvent. Additionally, the pipe may contain more than one jacket so as to'allow for careful regulation of the temperature in the drying pipe along its length. For example, the first stage of a dryer section may be heated to remove volatiles while a second stage may be cooled so that the product may be packaged directly.

At the discharge end of the pipe, a device such as a paratus of this invention suitable for drying a wet particulate materlal. The apparatus comprises a gas entry duct 18, a jet nozzle 19, a mixing chamber 20, and a diffusion zone 21. The hot gas enters the mixing chamber through duct 18 and its velocity is substantially increased as it passes through jet nozzle 19. The wet particulate material is passed into the mixing chamber at an angle converging with the stream of hot gasses. An angle of 90 is preferred. Any method known to those skilled in the art may be used to pass the particulate material into the mixing chamber. For example, a vibrator, not shown, may be fastened to hopper 22. Alternatively, gravity feed may be used. The wet particles enter the mixing chamber and are entrained in the hot gas stream. The suspension of particles and hot gas move into diffusion zone 20 wherein a more homogeneous mixture is obtained. The mixture is then passed into a drying zone such as the one illustrated in FIGURE 2. The dry particles are collected in the same manner as the particles obtained from solution.

For many operations, it is desirable to employ a multistaged drying apparatus such as the one embodied in FIGURE 4 wherein a two-stage apparatus is shown. In this embodiment, feed material, either in the form of solution or wet particles enters a first-stage jet mixer through duct 101. The hot gas stream enters through duct 102 and a suspension of the feed material in the hot gas if formed. This suspension is passed into a first-stage drying pipe 103. This drying pipe may be equipped with a jacket 105 to regulate temperature. The suspension travels at high velocity through the drying zone wherein the solvent is vaporized. The prodnot from this stage is collected in a cyclone separator 107. The hot gas and vaporized solvent leave the cyclone separator through exhaust line 108. The particulate material is further dried by passage through a second jet mixer 109 modified for particulate material as shown 7 in FIGURE 3. The wet particulate material is suspended in a stream of hot gas which enters the second jet mixer through line 110. This suspension is passed into a second-stage dryer 111 which may be equipped with jacket 112. The dry particulate material along with the vaporized solvent and hot gas passes into a cyclone separator 113. The dry powder is collected at the bottom of the cyclone separator employing a suitable discharge valve 114 if necessary. The hot gas and vaporized solvent leave the cyclone separator through line 108. If the solvent is water, it may be vented to the atmosphere. However, if the solvent is a valuable organic solvent, it may be recovered in condenser 115 or any other manner known to those skilled in the art and collected in a condensed solvents tank 120.

There are many advantages to the use of a multistaged drying system such as that of FIGURE 4. One major advantage is that drying efliciency is much higher as the hot gas stream can be renewed periodically to avoid saturation with solvent. A third or additional stage may be used as well as a final stage wherein a coolant is added to the jacket rather than steam so that the final product may be cooled. When the final stage is used to cool the product, a cool carrier gas should be used to convey the powder. A system of this nature is applicable for materials that require a short exposure to high temperature for drying but must be packaged at a low temperature. Additionally, the drying pipe may be used to convey the product from the manufacturing portion of the plant to its packing section.

The process and apparatus of this invention can be employed to dry a wide variety of materials provided they are able to withstand elevated temperatures for short periods of time. The temperature of the hot gas stream should be as high as possible as this promotes greater drying efficiency. However, the temperature used is dependent upon the material to be dried. For example, a material dissolved in a low boiling solvent may be dried at a temperature lower than that which could be used for a material dissolved in a high boiling solvent. In general, the temperature of the hot gas stream entering the first jet mixer should exceed the boiling point of the liquid to be volatized by at least 25 F. and preferably, by at least 50 F.

The gas used as the hot gas stream is dependent upon the material to be dried. If a material such as a resin dissolved in a water insoluble solvent is to be dried, superheated steam at a pressure of between 50 p.s.i.g. and 200 p.s.i.g. is the preferred hot gas stream as it condenses readily and forms a second layer with the solvent which is easily'removed. If the material to be dried is one that is water soluble, air or nitrogen may be used as the drying gas provided it is non-reactive with the material to be dried.

When the feed material is in solution form, the particle size of the product can be regulated'by either adjustment of the ratio of hot gas to feed material or control of the concentration of solids in the feed solution or a combination of the two. It has been found that decreasing the ratio of hot gas to feed or the increasing concentration of solids in solution favors the formation of large particles.

The ratio of hot gas to feed material may vary over broad limits dependent upon the material to be dried, the solids content of the feed, the temperatures of the hot gas stream, etc. The lower limit for this ratio should be such that the flow of hot gas in suflicient to (1) volatize enough solvent to form a non-tacky particle and (2) rapidly drive the feed material through the entire length of the drying pipe. In general, there is no upper limit for this ratio, though at high ratios20 to 1, the particles at quite small and hard to collect.

The solids content of the feed material can also vary over very wide limits depending upon the material to be dried, the drying conditions, etc. In general, for most resin solutions, solids contents of 1 to 20 percent, by weight, are preferred.

That particle size can be controlled using the process and apparatus of this invention is quite unexpected as jet spray drying, which bears the closest resemblance to the present invention, does not provide for control of the particle size. In the normal jet spray drying operation, the product collected is normally in the form of a fine, dust-like powder having particle sizes in the submicron range.

The following are examples of the drying method and apparatus of this invention. All percentages expressed in the examples are by weight. The examples are set forth merely for purposes of illustration and are not to be considered limiting in any way.

EXAMPLE 1 This example is designed to illustrate how particle size of a polycarbonate resin may be controlled through adjustment of the ratio of the hot gas stream to the feed stream.

The apparatus employed was similar to that depicted in FIGURE 2. The jet mixer had about A" diameter nozzle which discharged into a mixing chamber and diffusion zone which had a diameter of A" at the entry end and A" diameter at its exit end. The drying stage consisted of a 16 foot length of 1" inside diameter steel schedule 40 pipe. A three foot length of this pipe had a steam jacket. The hot gas employed was steam which entered the jet nozzle through a 1" inside diameter schedule 40 steel pipe. The drying zone discharged into a cyclone separator and the resin was collected from the cyclone separator in a steel drum.

Steam was used as the hot gas. A 15% solution of a polycarbonate derived from 2,2-bis-(4-hydroxyphenyl)- propane dissolved in methylene chloride was used as the feed. Three runs were made. In each run, the steam flow rate was maintained at approximately 837 lb./hr. The feed flow rate was varied as follows:

Run No Lb./hr. 1 64.9 2 232 The three runs corresponded to steam to feed weight ratios of 12.9/ 1, 3.6/1 and 1.2/1 respectively. The product was collected for each of the runs and a sieve analysis performed. The following results were obtained:

TABLE I.SIEVE ANALYSIS FOR DRY POLYCARBONATE POWDER Rate of hot gas to feed U.S. sieve No. Percentage retained on screen 8 It can be readily seen that as the ratio of the steam to feed decreases, the particles become larger in size.

EXAMPLE 2 Using the apparatus of Example 1, three additional runs were made to demonstrate how particle size of the product can be regulated through adjustment of the con centration of dissolved solids in the feed material. As in the previous example, steam maintained at approximately 837 lb./hr. was used as hot gas. The feed. consisted of a solution of a polycarbonate derived from 2,2'-bis-(4-hydroxyphenyl)-propane dissolved in methylene chloride. The feed flow rate was maintained at approximately 697 lb./hr. for run 1 and 644 lb./hr. for runs 2 and 3. This corresponded to steam to feed ratios of l.2/1.0 for run 1 and 1.3/1.0 for runs 2 and 3. The percent dissolved polycarbonate in the feed was as follows:

Run No Percent 1 15.0 2 11.8 3 4.9

The product was collected for each run and a sieve analysis performed. The following results wereobtained:

TABLE II.SIEVE ANALYSIS FOR DRY POLYCARBONATE POWDER Percent dissolved solids in feed U.S. sieve No. Percentage retained on screen From the above, it is apparent that particle size decreases as the percentage of dissolved solids in the feed solution decreases.

EXAMPLE 3 Using the apparatus and procedure of Example 1, six more runs were performed. The polycarbonate employed was the same as in Example 1, but the solvent consisted of 95.8 parts methylene chloride and 4.2 parts heptane. The solution contained 9.3 percent dissolved polycarbonate. For each run, steam flow was maintained at 837 lbs./ hr. The feed flow rate was varied as follows:

Run No. Lb./hr. 1 418 2 779 The six runs corresponded to steam to feed Weight ratios of 2.0/1.0, 1.1/1.0, 0.9/1.0, 0.8/1.0, 0.7/1.0, and 0.5/1.0. The product was collected for each run and a sieve analysis performed. The following results were obtained:

TABLE III.-SIEVE ANALYSIS FO R D RY POLYCARBONATE POWDER Rate of hot gas to feed 2.0/1.0 1.1/1.0 (1.9/1.0 0.8/1.0 0.7/1.0 0.5/1.0 U S. sieve 0. Percentage retained on screen The results of this example again illustrate how particle size can be controlled by regulation of the ratio of hot gas to feed.

9 EXAMPLE 4 Run No. Lb./h-r.

The three runs corresponded to steam to feed ratios of 6.6/1.0, 3.4/1.0, and 2.8/1.0 respectively. The product was collected for each run and a sieve analysis performed. The following results were obtained:

TABLE IV.SIEVE ANALYSIS FOR DRY POLYCARBONATE POWDER Rate of hot gas to feed Percentage retained on screen U.S. sieve No.

H n-uk NM F PP DKQWGHNQDO EXAMPLE 5 Using the apparatus and procedure of Example 1, two additional runs were performed with a feed solution comprising 9.1 percent polystyrene in methylene chloride. The steam flow was maintained at 837 lb./hr. for both runs and feed flow was maintained at 239 lb./hr. for the first run and 558 lb./hr. for the second run. These rates corresponded to steam to feed ratios of 3.5/1.0 and 1.5/1.0, respectively. Sieve analysis was performed for each of the runs and the following results obtained:

TABLE V.-SIEVE ANALYSIS FOR DRY POLYSTYRENE POWDER Rate of hot gas to feed U.S. sieve No. Percentage retained on screen EXAMPLE 6 This example is designed to show the use of a system consisting of three stages similar to that depicted in FIGURE 4. The first stage consisted of a jet mixer that delivered 1150 lb. of steam per hour from nominal 1S0 p.s.i.g. utility saturated steam service. The diffusion throat of the jet mixer had an inside diameter of A and diverged to discharge smoothly into a 2" schedule 40 drying pipe 80 in length. This drying pipe was heated with a steam jacket using 150 p.s.i.g. saturated steam. The drying pipe vented into a cyclone separator which vented the vapor and discharged the particles into a hopper for sampling and subsequent drying in a second stage.

The second stage consisted of a jet mixer for handling solid particles similar to that shown in FIGURE 3. The particles [flowed by gravity through a funnel shaped inlet which tapered to an inside diameter of 3" at the inlet to the mixing chamber. The hot gas was 600 lbs. of steam per hour supplied from the 150 p.s.i.g. utility saturated steam service. The dilfuser expanded from a throat diameter of 1 /2 to discharge smoothly into a nominal 3 diameter schedule 40 drying pipe which was 360' long and heated along its length with steam jackets. The drying pipe discharged into a second cyclone separator which vented the vapor and fed the particles to a third stage similar to the second stage. The product from the third stage was collected in a hopper for sampling. All cyclone separators were vented through a condensing system wherever solvent was recovered.

The feed to the first stage consisted of a polycarbonate resin dissolved in a solvent consisting of methylene chloride and 25% heptane (by volume) to form a solution containing 9% solute. A feed rate of 6.2 gallons per minute (4000 lbs. of solution per hour) was maintained for an eight-hour run. This provided a. steam to feed ratio of 6.29/ 1.0. Four random samples were taken after the first stage and four after the third stage with the following results:

First-stage product, Third-stage product, percentage volatile percentage volatile 30.0 1.3

What is claimed is:

1. An apparatus capable of drying a feed material to the form of discreet, dense particles of predetermined particle size, said apparatus having at least one drying stage, said drying stage having in combination a jet mixer and an elongated drying pipe in communication therewith, said jet mixer having a hot gas entry duct, a jet nozzle having a Reynolds number of at least 10,000 and capable of substantially increasing the velocity of a hot gas stream passing through said gas entry duct, a mixing chamber at the discharge end of said jet nozzle having means for introduction of a feed material to the discharge end of the jet nozzle at an angle converging with said discharge end, and a diffusion zone at the discharge end of said mixing chamber, said diffusion zone diverging at an angle that allows limited expansion of a gas stream passing therethrough while maintaining contact between the gas stream and the walls of the diffusion zone, said gas entry duct, jet nozzle, mixing chamber and diffusion zone being in axial alignment.

2. A process for drying a material in the form of dense, discreet particles which comprises continuously passing a stream of a hot gas through a jet nozzle at acoustic velocity, angularly projecting a feed material into the stream of hot gas as it discharges from the jet nozzle to form a suspension of feed material in the hot gas, passing said suspension into a confined diffusion zone wherein the suspension is allowed limited expansion while maintaining contact with the walls of the diffusion zone, advancing said suspension through an elongated pipe and separating volatiles from the formed particulate material.

3. A process for drying a material in the form of dense, discreet particles which comprises continuously passing a stream of a hot gas, through a jet nozzle to substantially increase its velocity, angularly projecting a feed material into the stream of hot gas as it discharges from the jet nozzle to form a suspension of feed material in the hot gas, passing said suspension into a confined diffusion zone wherein the suspension is allowed limited expansion while maintaining contact with the walls of the diffusion zone, advancing said suspension through an elongated pipe, separating volatiles from the formed particulate material, passing the particulate material into a second drying 1 1 12 stage wherein the particulate material is angularly pro- 2,639,132 5/ 1953' Bradford. jected into a hot gas stream travelling at acoustical veloc- 2,746,735 5/ 1956 Bradford. ities, passing the particulate material through a difiusion 3,056,212 10/1962 Jamison 34-10 zone and thereafter continuously passing the particulate 3,309,785 3/1967 King. material through a drying pipe.

5 EDWARD G. FAVORS, Primary Examiner References Cited US. Cl. X.R.

UNITED STATES PATENTS 34 57 Re. 17,212 2/1929 Stockton.

2,297,726 10/1942 Stephanofi. 10

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