US3699662A - Drying process for pulverulent material - Google Patents

Abstract

A process for drying wet pulverulent material by utilizing, as the heat-producing medium, the vapor form of the wetting liquid itself at a temperature higher than the deterioration temperature of the material being dried. The system comprises a continuous process whereby the pulverulent material is fluidized and dispersed while being centrifugally propelled through an arcuate mill. The fluidization and dispersion of the particles provides maximum surface exposure thereof to the heat of the liquid vapor. This results in an almost instantaneous flash-drying effect which is endothermic in nature. This endothermic reaction almost instantly lowers the temperature below the deterioration point of the material being dried. In this manner, very high temperatures can be used for rapid drying even though such temperatures may be above the deterioration temperature of the material being dried, while the vapor form of the liquid contaminate, itself, is used both as the source of drying heat and as the motive force.

Description

United States Patent Stephanoff 1 Oct. 24, 1972 [54] DRYING PROCESS FOR PULVERULENT MATERIAL Primary Examiner-Frederick L. Matteson Assistant Examiner-Robert A. Dua [72] Inventor. llillcholas N. Stephanoff, l-laverford, Att0mey Arthur A. Jacobs [73] Assignee: Fluid Energy Processing 8: Equip- [57] ABSTRACT v v em companyflatfieldpa' A process for drying wet pulverulent material by 2] Filed: June 18, 1970 utilizing, as the. heat-producing medium, the vapor form of the wetting liquid itself at a temperature 21 A .N 47232 1 pp] higher than the deterioration temperature of the Related US. Application Data material being dried. The system comprises a continuous process whereby the pulverulent material is [63] ggg t z g My fluidized and dispersed while being centrifugally an one propelled through an arcuate mill. The fluidization and dispersion of the particles provides maximum sur- (g1. face exposure thereof to the heat of the li qui d vapon This results in an almost instantaneous fiashdrying [58] Flew of Search 57 57 feet which is endothermic in nature. This endothermic reaction almost instantly lowers the temperature below the deterioration point of the material being [56] References Clted dried. In this manner, very high temperatures can be UNITED STATES PATENTS used for rapid drying even though such temperatures may be above the deterioration temperature of the 2,297,726 10/ 1942 Stephanofi ..34/10 material being dried, while the vapor form of the 3,190,867 6/1965 Oldwe1ler et al. ..34/10 liquid contaminate, itself, is used both as the Source of 3,212,197 10/1965 Crawford ..34/ 10 drying heat and as the motive force. 3,403,451 10/1968 Stephanoff ..34/l0 3,418,305 12/ 1968 Payne et al. ..260/94.9 4 Claims, 2 Drawing Figures za- I 26 '36 I II I l8 22, /0

PATENTED 0m 24 m2 lNVENW/P ATTORNEY DRYING PROCESS FOR PULVERULENT MATERIAL This is a continuation of co-pending application Ser. No. 748,515, filed July 29, 1968, now abandoned.

This invention relates to a process for drying wet pulverulent material, and it particularly relates to a process of the aforesaid type wherein temperatures are utilized which are higher than the deterioration temperature of the material being dried.

It has, heretofore, been the accepted procedure to dry a material at a temperature that is lower than the deterioration temperature (melting point, decomposition point, etc.) of the material for the obvious reason that such deterioration is to be avoided. As a result, the drying period was unduly extended beyond the time that would have been necessary if higher temperatures could have been used.

Furthermore, since the material required longer processing periods and therefore had to remain in the system for longer periods of time, it was necessary to use larger and more bulky equipment to handle larger batches than would have been necessary if the material could have been dried rapidly enough to pass quickly through the system. Since most plants have space problems, where every bit of available space must be used, the bulk of the drying equipment was economically wasteful.

Another disadvantage of prior drying systems was the waste of the liquid which was removed during the drying process. Obviously, this liquid had to be removed from the system in order to prevent re-wetting of the dried material.

It is an object of the present invention to overcome the aforesaid problems and disadvantages by providing a drying process which is capable of utilizing temperatures that are significantly higher than the deterioration temperatures of the materials being dried, while avoiding any deterioration of the dried material.

Another object of the present invention is to provide a process of the aforesaid type wherein the removed liquid is itself utilized in the process both as a motive force and as a drying medium.

Another object of the present invention is to provide a system and apparatus for effectively and efficiently carrying out the aforesaid process.

Another object of the present invention is to provide a system and equipment of the aforesaid type which is less complex and much smaller in bulk than systems and equipment heretofore generally used for this purpose.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following description when read in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic view of a system for carrying out a process embodying the present invention.

FIG. 2 is a fragmentary schematic view of a modified form of part of the system of FIG. 1. The invention will hereinafter be described for the treatment of a specific material, namely, pulverulent polypropylene wetted with liquid methanol. However, it is to be understood that this is merely one exemplification of the invention and that other pulverulent material wetted with other liquids may be processed in the same manner within the scope of the invention.

In accordance with the present invention, a fluid energy mill is utilized wherein the raw material, such as, for example polypropylene admixed with liquid methanol, is fed into the mill and entrained in methanol vapor passing into the mill in the form of tangentially directed jets. In the specific instance exemplified here, the polypropylene has a softening temperature point of about 275 F, whereas the methanol vapor, as it comes from the heater, is introduced into the mill at a temperature of between about 300 to 310 F. This would ordinarily cause softening and melting of the polypropylene; however, the jets of methanol vapor fluidize and disperse the polypropylene particles and, at the same time, cause them to circulate through the mill which has an elongated oval shape.

The fiuidization and dispersion of the polypropylene particles provides maximum surface exposure of these particles to the heat of the methanolvapor. As a result, there is a flash-drying of the polypropylene whereby the liquid methanol is almost instantaneously vaporized. Since this vaporization is an endothermic reaction, and since the reaction is'almost instantaneous, the temperature in that part of the mill almost instantaneously decreases to about 200 F, well below the softening point of the polypropylene. There is, therefore, insufficient time for any softening of the polypropylene to take place. As the polypropylene and methanol vapor continues to circulate through the mill, additional heat is dissipated so that the actual temperature of the methanol vapor exhausting from the mill is about F. j

The fluid energy mill utilized in this system is of the standard type wherein the tangential jets of gas or vapor carry the fluidized particles through an upstack under centrifugal force. The centrifugal force separates the larger, heavier particles from the smaller, lighter particles, the larger particles being on the outer periphery and the smaller particles on the inner periphery of the circulating stream. An outlet duct is provided at the inner periphery of the mill, adjacent the upper portion thereof, and the smaller particles pass through this outlet duct while the larger particles are recycled for a further pass through the mill. Since the larger particles possess a larger surface area, they are more apt to retain some residual liquid, and the recycling thereof is, therefore, advantageous since it provides further exposure to the flash-drying step.

The above-described fluid energy mill is provided in a closed circuit to effect re-utilization of the methanol vapors and, therefore, increased efficiency as well as saving of space.

Referring now to greater detail to the figures of the drawings wherein similar reference characters refer to similar parts, there is shown a system comprising a fluid energy mill 10 having a plurality of inlet nozzles 12 extending tangentially into the bottom portion of the mill from a header 14. The header 14 is in fluid communication with a duct 16 through which methanol vapor is passed into the header. A hopper 18 having an inlet duct 20 is used to feed the raw materials (in this instance, polypropylene mixed with liquid methanol) into the mill. A screw conveyor 22 is shown within the duct 20, but any other conveying means, including a gravity feed, may be substituted.

The mill 10 is provided, adjacent its upper end and on the inner periphery thereof, with an outlet 24 from which extends an exhaust duct 26. The duct 26 leads into the upper portion of a standard cloth-bag type of dust collector 28. In the collector 28, the solid particles are separated from the vapor, the particles passing down through a rotary valve 30, continuously operated by a motor (not shown), into a duct leading to a storage bin, while the vapors pass upwardly through a duct 34.

The duct 34 leads into the inlet side of a blower 36 and is also connected to a by-pass duct 38.

The blower 36 has a predetermined displacement to take only a predetermined amount of the vapors passing through the duct 34. The excess vapors pass through duct 38 to a standard type condenser 40 having a cooling liquid, such as water, passing therethrough. A storage tank 42 is provided to receive the condensed methanol, and a reflux pump 44 is connected in a reflux line 46 between the tank 42 and the upper end of the condenser tower. A pump 48 is provided to pump the liquid methanol from the condenser to a storage area.

A blow-off valve 50 is provided in the duct 34 to vent the vapors when there is excessive pressure in the duct. Similar blow-off valves 52 and 54 are provided for the condenser 40 and tank 42, respectively. A shut-off valve 56 is provided in the conduit 38. This valve is shown as manually operable but it may be operated by a solenoid or any other desired means.

The blower 36, with a predetermined displacement, as indicated above, is operatively connected to a heater 58, which may be of any feasible type but which is illustrated herein as a steam jacketed heater with an inlet 60 for steam to flow into the jacket and an outlet 62 for removal of condensed steam. The inlet 60 is connected to a'source of steam (not shown) and the outlet 62 is connected to a pump or the like (not shown).

A duct 64 leads from the outlet end of the heater 58 and is connected at an elbow portion 66 with the duct 16. A shutoff valve 68, similar to valve 56, is interposed in the duct 16. Also provided in the duct 16, adjacent the elbow portion 66, is a valved inlet 70 connected to a source of nitrogen or similar inert gas (not shown). A bleed duct 72 having a shut-off valve 74 is also interposed in the duct 16.

In the operation of the above-described system, the process is started by introducing nitrogen gas, heated to a temperature above the vaporization point of methanol and at a pressure slightly greater than ambient, through the inlet 70. At the same time, polypropylene powder, containing 40-50 percent by weight of liquid methanol and at a temperature of 70 F, is continuously fed into the mill by means of a screw conveyor 22.

The fed material is entrained by the nitrogen gas and fluidized and dispersed in the manner described above, the lighter particles passing into the collector 28 and the heavier particles being recycled through the mill. In the collector 28, the solids pass through the rotary valve 30 and through the duct 32 to storage, while the vapors pass up through the duct 34. These vapors include the starting nitrogen gas pulse the vaporized methanol from the raw feed. The solids recovered in this manner have less than 0.5 percent by weight methanol adhering thereto.

The blower 36, being pre-set to a selected displacement, pulls the amount of vapors corresponding to such displacement into the heater 58, the remaining vapors, now cooled and substantially free from solid particles, passing through duct 38 into the condenser 40. At this time the nitrogen flow is shut off, the nitrogen being used only for starting the system, while the methanol vapors, under pressure of the blower 36, circulates through the heater 58, where it is heated to about 300-310 F, and through duct 64, back through duct 16 and into the header 14 to be used as the motive and heating fluid for the next cycle through the mill 10.

The system operates under slight positive pressure, so that at no time is there any danger of introducing air or any other contaminant into the system. In the particular process here described, about 3,300 pounds per hour of polypropylene, wet with 2,200 pounds of methanol, is treated to obtain a polypropylene product containing 0.3 to 0.4 percent by weight methanol. In this process, approximately 6,000 CFM of methanol vapor continuously circulates through the drying system.

Although nitrogen gas is used to start the system, since the specific heat of methanol is considerably higher than nitrogen, the efficiency of the process utilizing methanol is far greater and, therefore, much smaller equipment is necessary than if nitrogen were used. The nitrogen has been exemplified as used to start the system because, being completely inert, it acts to initially flush out the system. It is also less expensive than methanol. However, once the system is in operation, the use of the methanol from the raw feed itself it obviously far more economical than nitrogen would be.

It is, of course, possible to use methanol vapor, air or any other gaseous fluid not only to start the system but even to act to form the recycled heating fluid, if so desired and if the particular process calls for it. For example, superheated steam or steam-air combination vapors may be used for drying heat-sensitive food products such as grains, starches, etc. Where water is the liquid contaminant, steam may be advantageously used so that the vaporized water is itself utilized to vaporize the water in the raw feed. Another type of process requires the elimination of isopropyl alcohol from penicillin. Here, either air or the alcohol vapor itself may be used as the vaporizing agent as well as the motive force.

In general, by means of the present system, the contaminant liquid in any pulverulent raw feed can be removed by the heat generated by the vapor form of the contaminant liquid itself, even if this heat is higher than the deterioration point of the pulverulent material being dried, as long as the removal of the liquid is practically instantaneous (flash-drying) and is sufficiently endothermic to cause corresponding lowering of the temperature below the deterioration point of the material. This can, furthermore, be accomplished in continuous cycle utilizing substantially only the contaminant liquid itself as the vapor source.

The blow-off valves 50, 52 and 54, are utilized in the ordinary manner to vent the vapors of gases when the pressures in the lines are too great. The shut-off valve 56 is closed until sufficient pressure and flow builds up in the duct 34, after which it is opened to permit bypass flow through duct 38. The shut-off valve 68 is used, if desired, at start-up or to stop the action of the system while the bleed valve 74 is used to bleed off the starting nitrogen or any excess fluid.

FIG. 2 shows a slightly modified form of the invention wherein the rotary valve 100, similar to valve 30, is

connected to an outlet duct 102, similar to duct 32, which is in fluid connection with a duct 104 leading to a blower 106. A duct 108 leads from a source of heated nitrogen gas (not shown) into the blower 106.

By means of this mechanism, the nitrogen gas blown through the duct 14 not only entrains the polyurethane particles passing from the collector 28 and conveys them to storage, but also acts to dilute any methanol vapor that is entrapped in the pulverulent product.

Obviously, many modifications of the present invention are possible in the light of the above teachings. It

is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced, otherwise than is specifically described.

The invention claimed is:

1. A method of rapidly removing liquid from wet pulverulent material which comprises simultaneously fluidizing and c'entrifugally dispersing said material in an arcuately moving low velocity circulating stream of gaseous fluid by inserting said material, at substantially ambient pressure, directly into said circulating stream and by flash drying said material during contact with said circulating stream of gaseous fluid while said fluid is at a temperature higher than the deterioration temperature of said material in an endothermic reaction during which the temperature of said fluid drops to below said deterioration temperature, said fluid moving at a velocity which issufficient to convey and disperse said material but which is insufficient to cause significant pulverization thereof, said material being separated into lighter and heavier particles by the centrifugal force generated by said arcuately moving circulating stream of gaseous fluid in such manner that the heavier particles are on the outer periphery and the lighter particles are on the inner periphery of the arcuately moving circulating stream, centrifugally removing the lighter particles and entraining gaseous fluid from said circulating stream, separating the removed lighter particles form the entraining gaseous fluid, reheating the separated gaseous fluid to a temperature higher than the deterioration temperature of said material and intermixing the reheated gaseous fluid with additional pulverulent material while propelling said reheated fluid and additional material into said arcuately moving circulating stream.

2. The method of claim 1 wherein said gaseous fluid is the vapor phase of the liquid being removed.

3. The method of claim 1 wherein said lighter particles, after being separated from the heavier particles, are separated from entraining gaseous fluid by gravity differential.

4'. The method of claim 1 wherein the pulverulent material is polypropylene and the gaseous fluid is methanol vapor.

Claims (4)

1. A method of rapidly removing liquid from wet pulverulent material which comprises simultaneously fluidizing and centrifugally dispersing said material in an arcuately moving low velocity circulating stream of gaseous fluid by inserting said material, at substantially ambient pressure, directly into said circulating stream and by flash drying said material during contact with said circulating stream of gaseous fluid while said fluid is at a temperature higher than the deterioration temperature of said material in an endothermic reaction during which the temperature of said fluid drops to below said deterioration temperature, said fluid moving at a velocity which is sufficient to convey and disperse said material but which is insufficient to cause significant pulverization thereof, said material being separated into lighter and heavier particles by the centrifugal force generated by said arcuately moving circulating stream of gaseous fluid in such manner that the heavier particles are on the outer periphery and the lighter particles are on the inner periphery of the arcuately moving circulating stream, centrifugally removing the lighter particles and entraining gaseous fluid from said circulating stream, separating the removed lighter particles form the entraining gaseous fluid, reheating the separated gaseous fluid to a temperature higher than the deterioration temperature of said material and intermixing the reheated gaseous fluid with additional pulverulent material while propelling said reheated fluid and additional material into said arcuately moving circulating stream.

2. The method of claim 1 wherein said gaseous fluid is the vapor phase of the liquid being removed.

3. The method of claim 1 wherein said lighter particles, after being separated from the heavier particles, are separated from entraining gaseous fluid by gravity differential.

4. The method of claim 1 wherein the pulverulent material is polypropylene and the gaseous fluid is methanol vapor.

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