US3922796A

US3922796A - Drying mill for wet granular material

Info

Publication number: US3922796A
Application number: US517849A
Authority: US
Inventors: Nicholas N Stephanoff
Original assignee: Fluid Energy Processing and Equipment Co
Current assignee: Fluid Energy Processing and Equipment Co
Priority date: 1974-10-25
Filing date: 1974-10-25
Publication date: 1975-12-02
Anticipated expiration: 1992-12-02

Abstract

A drying mill for wet granular material whereby the material is fed into a stream of hot gases whirling through a generally annular mill portion, whereby the wet particles are dispersed and rapidly dried by the hot gases and are centrifugally separated into fully dry and less dry particles, with the fully dry particles being centrifugally exhausted from the mill. Coolant jackets are provided around the nozzles for the hot gases to prevent deterioration of the particles around the nozzles and thereby prevent clogging of the nozzles.

Description

United States Patent 11 1 Stephanotf 1 Dec. 2, 1975 [5 DRYING MILL FOR WET GRANULAR 3,403,451 10/1968 Stephanoff 34/10 MATERIAL 3,814,316 6/1974 Stephanoff....

3,852,108 12 1974 Lindberg 34 10 [75] Inventor: Nicholas N. Stephanoft, l-laverford,

[73] Assignee: Fluid Energy Processing &

Equipment Co., Hatfield, Pa.

[22] Filed: Oct. 25, 1974 [21] Appl. No.: 517,849

[52] U.S. Cl 34/10; 34/57 E; 432/2 [51] Int. Cl. F26B 33/08 [58] Field of Search 34/10, 57 E; 432/2 [56] References Cited UNITED STATES PATENTS 3,070,621 12/1962 Lind 34/10 3,280,472 10/1966 Lorenian 34/10 Primary Examiner-John J. Camby Attorney, Agent, or Firm-Arthur A. Jacobs, Esq.

[57] ABSTRACT.

11 Claims, 3 Drawing Figures US. Patent Dec. 2, 1975 DRYING MILL FOR WET GRANULAR MATERIAL This invention relates to a drying mill for drying wet granular material, and it particularly relates to a drying mill of the aforesaid type which is adapted to the drying of material which has a relatively low melting or deterioration point.

It is known to dry wet granular material by injecting the material into a mill having a generally annular portion, the particles being subjected to the action of hot gases projected into the mill in a tangential direction, whereby the gases disperse the particles and whirl them through a centrifugal path in the generally annular portion while simultaneously subjecting the dispersed particles to rapid drying. The particles which are completely dried become lighter than those not completely dried and the lighter and heavier particles are separated from each other by the centrifugal forces. The lighter particles, being on the inside of the centrifugal path, are automatically expelled from the mill through an exhaust opening at the inner side of the centrifugal path while the heavier particles are returned tothe initial portion of the mill for further drying.

An important aspect for the most effective utilization of the above type mill is the use of relatively high temperature gases. Many materials, such as thermoplastic hydrocarbons, and particularly polyolefins such as polyethylene and polypropylene, soften or melt or may otherwise deteriorate at relatively low temperatures, which are far below the most effective temperature of the gases at the time these gases are propelled into the mill. However, even though the initial temperature of the incoming gases may be as high as 500-600F, and even though the melting point or deterioration point of the material may be far below this temperature, no melting or deterioration takes place because the evaporation of the moisture of the wet particles is an endothermic action that causes rapid cooling of the gases during their contact with the particles in their passage through the mill. Therefore, even though the initial temperature of the gases is between about 500600F, the internal temperature in the mill and the temperature at the exhaust outlet may be only about l40l60F. In other words, complete evaporation takes place at a temperature lower than the wet-bulb temperature. Sufficient hot gas is supplied to maintain the exhaust gas at a relative humidity that is generally well below 90 percent, whereby the gas is still moisture hungry.

For example, polyethylene resins soften at about 230F. Nevertheless, it is possible to utilize gases having an initial temperature close to this softening point or even above it, because, due to the rapid cooling resulting from the evaporation, the internal temperature is quickly reduced to about l40l60F. Since this is below the softening point of the polyethylene resin, and since the cooling is sufficiently rapid to avoid any lengthy subjection of the resin to the high initial temperature, the resin particles are thoroughly dried without being damaged by heat.

It is important that the initial temperature be as high as feasible because the hotter the gases, the more efficient is the drying effect. This is illustrated by the following equations where H total heat required from the heater, H actually utilized heat, H rejected heat passing through the exhaust (H H, H T initial temperature of the gas, T the exhaust temperature, and T, the temperature of the air at the intake:

For example, when T 300F, T l50F and T, 50F, we have:

This means that 1/].666 or only 60 percent of the available heat (H is used for drying while 40% is wasted through the exhaust.

However, when T 650F, with T and T retaining the same values:

This means that 83.3 percent of the available heat (H is used for drying while only 16.7 percent is lost at the exhaust.

The greater efficiency of the hotter gases means that more particles are completely dried on each pass through the mill. Therefore, not only is there a greater production of finished product at each pass but less particles require recycling for a further pass through the mill for additional drying. This means that less gas is required for total production, so that less total heat is required, thereby saving fuel. Furthermore, considerably less power is required for operation of the blower or compressor so that the blower or compressor utilized is much smaller even though the same volume of product is produced. This saving of power and utilization of a smaller blower or compressor results in a considerable decrease in cost.

It has now been found, however, that it is difficult to use this type of mill with such thermoplastic materials as polyethylene, polypropylene, polystyrene, synthetic or natural rubber, etc. because even though there is a rapid cooling of the gases internally of the mill where evaporation occurs, at the area of the gas inlet nozzles where the gas enters the mill, the heat acts to melt the thermoplastic material on the mill wall surrounding the nozzle due to heat conduction. This so-called hotplate effect results from the fact that there is not sufficient time for the heat to become dissipated by vaporization or by convection. This melted material not only clogs the nozzles and the areas immediately adjacent thereto but runs down into the nozzles. The nozzles are therefore constantly being clogged up, requiring frequent stoppage of the mill for cleaning or replacement of the nozzles. Furthermore, the melted or deterioriated material acts as a contaminent in the finished product.

It is an object of the present invention to provide a method and apparatus for drying wet granular material whereby the material may be dried at the most efficient temperatures without causing deterioration of the material being treated even when such temperatures are above the deterioration temperature of the material.

Another object of the present invention is to provide a method and apparatus of the aforesaid type whereby the treatment is effected without clogging of the inlets 3 for the heated gases and without producing contaminents.

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 side view, partly in elevation and partly in section, showing a mill embodying the present invention.

FIG. 2 is a sectional view taken on line 2-2 of FIG. 1.

FIG. 3 is a fragmentary side view, partly in elevation and partly in section, of modified form of the mill of FIG. 1.

Referring in greater detail to the various figures of the drawing wherein similar reference characters refer to similar parts, there is shown in FIG. 1 a mill generally designated 10, comprising an inlet section 12, an upstack section 14, a classification section 16 and a downstack section 18. The upstack 14 and the downstack 18 are illustrated as being formed of two tubular portions joined together at the

respective flanges

20 and 22. However, this is only for ease of manufacture and assembly and these sections may be made of integral construction where desirable or feasible. In either event, the upstack forms a continuous curved extension of the inlet section and the classification and downstack sections form continuous extensions of the upstack, resulting in a generally annular configuration.

Between the classification sections 16 and the downstack 18 is an exhaust duct 24 leading from the inner periphery of the mill to an outlet 26. The outlet 26 may be coupled to a collector or to any desired receiving means for the finished product. An exhaust fan or blower (not shown) may also be connected to the exhaust means for movement of the exhaust particles out of the mill and creating a negative pressure in the dryer.

The inlet section 12 is provided with a feed means, generally indicated at 28. This feed means for the raw material to be processed may be any standard or other type of feed means which can feasibly be used for the purpose. An inspection port, having a closure cap 30, is optionally provided in the inlet section 12 adjacent the feed means 28. A similar inspection port 32 may be provided at one end of a header 34.

The header 34 may be either a manifold for receiving hot, low pressure gases from a source (not shown) or it may, itself, be a combustion chamber to generate such hot gases. Leading from the header 34 are a plurality of inlet ducts 36 having nozzles 38 in communication with the interior of the inlet section, are arranged in tangency to the axis of the inlet section, their angles varying from right to left, as shown in the drawing, so that the streams of gases issuing therefrom not only move longitudinally of the inlet section, but also intersect each other at approximately the elbow portion between the inlet section and the upstack so that the gases travel in a common direction around the elbow portion and into the upstack. The nozzle 38 which leads into the front end of the inlet section provides a straight axial path for the gaseous stream issuing therefrom, but this stream is entrained by and mixed with the streams from the tangential nozzles and flows through the elbow portion together therewith.

In operation, hot gases from the nozzles 38 entrain the raw feed from feed means 28 and carry them along through the inlet section, the varying tangencies of 4 these gaseous streams resulting in a vortex or whirling cloud which acts to disperse the particles of the raw feed. At the same time, the heat from the gases vaporizes the moisture on the exposed surfaces of the dispersed particles.

As the particles and gases are carried through the upstack and around into the classification chamber 16, the centrifugal force on the particles acts to separate the most thoroughly dry, and therefore lighter, particles from the less dry, and therefore heavier, particles, the lighter particles passing along the inner zone of the mill and the heavier particles along the outer zone. As the stream passes down toward the downstack 22, the lighter particles in the inner zone pass through the exhaust duct 24 to the outlet 26, while the heavier particles continue down through the downstack into the inlet section 12 for a further entrainment by the hot gases and another pass through the mill.

As was stated above, the most efficient utilization of the mill is obtained when the gases are very hot. When a thermoplastic such as polyethylene resin is being treated, this temperature can exceed the softening temperature of the particles, which is about 230F. However, the temperature within the mill is rapidly lowered to a temperature safely below the softening point because of the heat required for vaporization, so that the particles do not have an opportunity to soften or melt within the interior of the mill. But this loss of temperature does not take place with such rapidity at the areas of the nozzles 38. At the instant that the hot gases issue from the nozzles, there is not sufficient time for the particles to be sufficiently dispersed to expose all their surfaces to the heat of vaporization. Furthermore, the metal walls surrounding the nozzles are very hot and act as a hot-plate to which the particles tend to adhere. The result is that at the areas of the nozzles, the particles melt and not only form a film which covers the metal walls but flow down into the nozzles themselves and clog them up. This is especially true at the most downstream nozzle because at that area a large proportion of the particles are completely dry and, therefore, are more apt to melt because less heat is absorbed by vaporization of the moisture. Furthermore, the deteriorated particles act as contaminents which are propelled into the finished product. For this reason, it was not, heretofore, possible to use this method of treatment for thermoplastic materials with relatively low melting points with any degree of feasibility.

In accordance with the present invention, the above problem has been overcome by providing a cooling jacket, indicated at 40, around each nozzle 38. This cooling jacket may be supplied with any desired coolant such as circulating water, chilled gas, or any other coolant means in accordance with the particular material being processed and the temperature of the hot gases. The coolant may be circulated through the jackets by means of inlet and outlet ducts such as indicated at 42 and 44.

The above type of coolant jackets 40, which are provided only adjacent the individual nozzles are the most effective and, by far, the most preferable type because,

by being concentrated only at the nozzle area, there is.

no conduction of cooling effect into the other metal wall areas and no significant amount of convection of the cold into the interior of the mill. This is important because if such cooling effect is permitted, it significantly lowers the effective temperature in the mill and seriously impairs the maximum effectiveness. Nevertheless, in some instances, where high efficiency is not a significant factor, or where the melting point of the material being processed is extremely low, it is possible to utilize a single elongated coolant jacket for all of the tangential nozzles. This use of a single coolant jacket is somewhat less expensive and, although not usually recommended, such cost may be a factor where high efficiency is not important. Such single coolant jacket is illustrated in FIG. 3.

As shown in FIG. 3 a mill, generally designated 100, comprises the same type inlet section 102, header 104, inlet ducts 106 and nozzles 108. However, extending substantially the length of the inlet section 102 is an elongated coolant jacket 110 through which each of the nozzles 108 extend. Conduits, indicated at 1 l2 and 114, lead into and out of the jacket 110 from a source of coolant.

The invention has been described with respect to the clogging effect of melted particles particularly, but any type of heat-deterioration, even softening or disintegration, may also result in undesirable clogging of the nozzles.

The invention claimed is:

l. A method of drying wet solid particles which comprises propelling a hot gaseous fluid from at least one gaseous fluid inlet thereof into a circulating path, passing said wet solid particles into said circulating gaseous fluid from a separate particle feed means, said gaseous fluid being initially at a temperature at least approximately the deterioration temperature of said particles, said gaseous fluid acting to simultaneously disperse said particles and apply drying heat to their exposed surfaces upon contact therewith, the particles being thereafter centrifugally separated in accordance with their state of dryness while being moved by said gaseous fluid through said circulating path, and maintaining the area surrounding said fluid inlet at a temperature that is lower than the temperature of said gaseous fluid.

2. The method of claim 1 wherein said gaseous fluid is at a temperature in excess of said deterioration temperature.

3. The method of claim 1 wherein said particles are thermoplastic.

4. The method of claim 1 wherein said particles are polyolefins.

5. The method of claim 1 wherein said particles are selected from the group consisting of polyethylene, polypropylene and polystyrene.

6. The method of claim 1 wherein said particles are selected from the group consisting of natural and synthetic rubber.

7. The method of claim 1 wherein the said gaseous fluid inlet is cooled while said hot gaseous fluid is being propelled therefrom.

8. The method of claim 1 wherein there are a plurality of gaseous fluid inlets, each of said inlets being separately and individually cooled while said hot gaseous fluid is being propelled therefrom.

9. A drying mill comprising an inlet section, an upstack, section, a classification section and a downstack section, said sections forming a substantially annular path, said inlet section having particle feed means for heat sensitive wet, granular particles and at least one gaseous fluid inlet separate from said feed means and arranged tangentially to the axis of said inlet section, means to pass hot gaseous fluid through said fluid inlet into said inlet section, and cooling means for cooling the area around said fluid inlet to a temperature below the melting point of the material being treated while said hot gaseous fluid is being propelled therethrough, whereby clogging of the fluid inlet is prevented.

10. The mill of claim 9 wherein said cooling means is a jacket surrounding said fluid inlet, said jacket being adapted to contain coolant therein.

11. The mill of claim 9 wherein there are a plurality of fluid inlets, each of said inlets having a separate and individual jacket surrounding it, each of said jackets being adapted to contain coolant therein.

Claims

1. A method of drying wet solid particles which comprises propelling a hot gaseous fluid from at least one gaseous fluid inlet thereof into a circulating path, passing said wet solid particles into said circulating gaseous fluid from a separate particle feed means, said gaseous fluid being initially at a temperature at least approximately the deterioration temperature of said particles, said gaseous fluid acting to simultaneously disperse said particles and apply drying heat to their exposed surfaces upon contact therewith, the particles being thereafter centrifugally separated in accordance with their state of dryness while being moved by said gaseous fluid through said circulating path, and maintaining the area surrounding said fluid inlet at a temperature that is lower than the temperature of said gaseous fluid.

3. The method of claim 1 wherein said particles are thermoplastic.

4. The method of claim 1 wherein said particles are polyolefins.