US3313035A - Apparatus for drying particulate material - Google Patents

Description

3 Sheets-Sheet 1 J. R. CRAWFORD ETAL APPARATUS FOR DRYING PARTICULATE MATERIAL INVENTORS BY i/ Il .JNM

April 11, 1967 Filed March 14, 1966 pri l1, 1957 J. R. CRAWFORD ETAL APPARATUS FOR DRYING PARTICULATE MATERIAL vFiled March 14, 1966 (5 Sheets-Sheet 2 NNN mll bl All NWN

pr M, E967 J. R. CRAWFORD ETAL. $313,035

APPARATUS FOR DRYING PARTICULATE MATERIALv Filed March 14, 1966 Z5 Sheets-Sheet 3 r i l 1 r ml n n United States Patent O 3,313,035 APPARATUS FOR DRYING PARTICULATE MATERIAL James R. Crawford, New Canaan, Conn., and Richard Hooker, Jr., Mount Kisco, N.Y., assignors to Crawford & Russell Incorporated, Stamford, Conn.

Filed Mar. 14, 1966, Ser. No. 534,171 4 Claims. (Cl. 34-57) This is a continuation-impart of co-pending application Ser. No. 280,213, led by Richard Hooker, Ir., on May 14, 1963, now abandoned. This invention relates to methods and apparatus for the drying of materials and more particularly to the drying of particulate materials which may be moved by flowing ui-ds.

Particulate or granular material at variousstages of production or processing often has a considerable amount of liquid on the surface of the particles or entrained therein which must be removed before the next processing step for the material. Conventional methods of removing these liquids from the particulate material are generally of three types. These are: (l) mechanical drums wherein the material is tumbled in the presence of heated air to remove the moisture; (2) fluidized beds where drying heat is supplied by heated air or gas moving through the material to be dried; (3) flash drying `systems wherein the material is heated to a high temperature under pressure and then is exposed to air or super heated vapor to flash ott the moisture.

The above systems have a number of disadvantages. The cost of manufacture of equipment for these prior art systems and the operating costs for these methods is excessively high. In rotary dryers and uidized beds in particular, a great amount of heat energy is required per unit weight of dried material. In the case of fluid beds heat is transferred to the wet particles by heated air or gas with a substantial amount of heat energy lost in the recycling of the heated air or gas. Heat transfer from a gas to a solid requires a substantial amount of heated gas to heat and dry a given amount of solids. Granular lumps of polymer require special handling, and particular polymers exhibit diffusion-controlled slow drying rates during vaporization of the last few percentage points of volatiles, which must diffuse outward through capillary crevices to the particle surface before they can be Vaporized.

Mechanical driers employ very large rotating drums which are expensive to manufacture and to maintain in operation. A particular problem with such driers lies in the seals that must be employed on the rotating drum, since seal leakage wastes material and seals are expensive to maintain. Flash drying alsorequires a great amount of heat, and the pressure pumps for the material are very costly.

A further problem exists when drying materials which will decompose or undergo degradation at higher temperatures. In such instances many rotary driers are not practical since the material being dried will be deteriorated by excessive drying heat. These conventional driers also require extended residence time at high temperatures to dry a substantial charge of material. Thus the extended exposure to high temperatures of material'that is already dried also leads to decomposition or degradation of a portion of the material.

Accordingly, it is an object of this invention to provide methods of drying particulate material which are eflicient and economical.

Another object of the invention is to provide methods of the above character wherein the material to be dried is exposed to rapid intermittent contact with a heated surface in the drying apparatus.

A fur-ther object of the invention is to provide methods 3,313,035 Patented Apr. 1l, 1957 ICC of the above character wherein the material to be dried is moved through the drying stages by turbulent air or gas.

Another object is to provide such drying methods accommodating widely varying sizes of granules or particles, and material with widely diierent drying rates.

Another object of the invention is to provide apparatus for carrying out methods of the above character.

A further object of the invention is to provide apparatus of the above character which is economical to manufacture and maintain in operation.

Other and more specific objects will be apparent from the features, elements, combinations and operating procedures disclosed in the following detailed description and shown in the drawings, in which:

FIGURE 1 is a schematic view in partial section of the initial drying units in one form of the invention;

FIGURE 2 is an enlarged partial side sectional view of a drying tube and jacket shown in FIGURE l;

FIGURE 3 is a schematic side View in partial section of an alternate material recovery portion of the invention;

FIGURE 4 is a schematic diagram of a different embodiment of the invention, with several units of apparatus partly broken away for clarity;

FIGURE 5 is a sectional side elevation view of a sifting unit shown in FIGURE 4, where lumps of feed material are broken up into particulate form;

FIGURE 6 is a sectional top plan view of the unit of FIGURE 5;

yFIGURE 7 is an enlarged fragmentary sectional elevation'view of the unit of FIGURE 5, and

FIGURE 8 is a side elevation view of the diffusion dryer unit shown in FIGURE 4, partly cutaway to show its preferred internal construction.

In general the invention involves the drying of particulate material by moving the material with a stream of air or gas through one or more heated tubes of relatively small diameter in proportion to its length, prefera-bly followed by further gas-agitation in one or more heated holding tanks or residence chambers. The velocity of the moving air stream should be high enough to create a great amount of turbulence in the particulate material being moved through the drying tube. It has been found that such turbulence causes the particulate material to impinge upon the heated walls of the drying tube rapidly but frequently enough to cause heat transfer from the drying tube directly to the particles `as well as to the air or gas moving through the tube.

As shown in FIGURE 1, the drying tube` 1t) is supplied with particulate material trom a hopper 12 which is moved through the tube by a partial vacuum created at tube exit 40 by blower 14. The drying tube 10 has a heat exchange jacket 18 for heat transfer to the material. A material separating cyclone 20 is provided at the exit end 40 of the tu'be to deposit Ithe dried material in bit 46 and send the moisture laden air through conduit 42 for venting or vapor recovery.

As shown in FIGURE 2, the materia-l 22 passes through the tube 10 with great turbulence due to the velocity of the air or gas moving the material through the relatively small diameter tube. As the partial vacuum draws the material through t-he tube 10 there is little chance of the tube plugging. The vacuum also lowers the vapor pressure of the moisture present and permits lower tube temperature for moisture removal.

The invention will now be described more specifically. Referring now to FIGURE l it will .be seen that the drying tube 10 is supplied with particulate material from the hopper 12 through Ia rotary gate 24 which controls the feed rate into the drying tube. As 'the particulate material to be dried enters the tube at 26 adjacent air 3 entrance end 16, it is carried rapidly through the tube in a direction of the arrows by high veloci-ty air which is drawn through the tube 10 and cyclone 20 by blower 14.

The tube 10 is preferably made of stainless steel and may vary in length from 20 to 300 feet or more. Tube 10 is enclosed over the greater part of its ilength lby a heat exchange jacket 18 having an inlet 28 and an outlet 30 for steam or hot water which is to =be circulated through the jacket. A valve control 32 may be operated by a thermostat 34 to Acontrol the ow of heat exchange fluid within the jacket 18.

The cyclone 20 may be enclosed by a heat exchange jacket 52 which is connected to steam outlet 30 through regulator 54 and conduit 56. Thus the cyclone 20 is maintained at a temperature below th-at of the drying tube 10 ybut hot enough to -prevent condensation of vapors therein. The regulator 54 is used to reduce steam pressure to the cyclone jacket 52,.as the velocity of material is greatly reduced in the cyclone 'and high cyclone tempei-rature may cause degradation of -some materials as they are separated `from the air-vapor stre-am. Steam return conduits 58, 60 direct the steam from the jackets 18 and S2 respectively to the steam source (not shown).

As vshown in FIGURE 2, the heat exchange jacket 18 is comprised of a steel wall 36 which is preferably covered by heat insulation 3S.

The tube 10 and its heat exchange jacket 1S may have U-shaped bends over its length as shown in FIGURE l, or it may be straight over its length if such a configuration is practical from thestandpoint of space. Sharp bends in the drying tube should be avoided to prevent build up of materiad and consequent plugging of the tube.

At its exit end 40 the tube 10 is `connected to cyclone 20 which separates the dried particles from the moisture laden air or gas which has ybeen drawn through the tube. From the cyclone 20 the moisture laden air or gas passes from the system via conduit 42 where the gases may be vented to the atmosphere or recovered for other uses. The dried material empties from the cyclone 20 at spout 44 into a bin 46 or onto a conveyor (not shown).

A manometer 48 may be connected to conduit 42 between the cyclone 20 and the Iblower 14 for indication of vacuum level in the system. A velometer S may `also be positioned at `the entrance end 16 of tube 10 to measure the velocity olf air being drawn into the tube.

As shown in FIGURE 2, the material 22 in the drying tube is moved with a great amount of turbulence so that the particles of material are vigorously impinged against the inner surface 10a of the drying tube. The heat in the drying tube is thus effectively transferred to the particles during this rapid and successive impingement. The heat thus transferred from the drying tu'be in conjunction with the heated air moving through the tube effectively and efficiently removes moisture from the particles. Because of the rapidity of the particle im pingelment there is no degradation of the material as it passes through the drying tube. v

It has been found that turbulence of the material in the drying tube depends upon the diameter of the drying tube itself in rel-ation lto the velocity of the stream of air moving the material through the tube. rllhus for some applications a number of small diameter tubes, e.g., one inch or less in diameter, may be used instead of a few tubes of larger diameter.

In the following pilot plant test examples the drying tube was 40 feet in length with an O.D. of 1/2 with a .035 wall thickness. The tube materiai was stainless steel. Except as otherwise noted, the cyclone residence time for separation of solids was less than 1 minute. Steam lunder pressure of from 25 to 100 p.s.i. was used in the heat exchange jacket.

EXAMPLE I A batch of particulated polycarbonate material having a moisture content of 59.2% was fed into one end of the tube at the rate of 4 lbs. per hour. A vacuum of 5.0 in. of I-Ig was pulled on the exit end of the tube to draw the particulated material through the 40-fit. tube. During the drying operation the temperature of the exit air from the drying tube was maintained at 330 F. by control of the steam jacket temperature. The polycarbonate tra-versed the 40 feet of drying tube in approximately 1 second and upon analysis was found to contain 6.4% vmoisture after its pass through the drying tube. The condition of the polycarbonate was very good, showing no signs of decomposition or damage after passage through the tube.

EXAMPLE II A second run under the same conditions as in Example I was made with particulated polycarbonate wherein the polycarbonate, after passing through the tube, was retained in the cyclone for a period of l0 minutes. After this IO-minute residence time in the cyclone the moisture content of the polycarbonate material was found to be 1.0%.

EXAMPLE III Granular polypropylene having a 22% moisture content was fed into one end of the 40-ft. tube at the rate of 18 lbs. per hour. A vacuum was maintained on the exist end of the tube equivalent to 4.0 in. of Hg, and the temperature of the exit air was maintained at 249 F. The polypropylene material took slightly over 1 second to traverse the 40-ft. drying tube and contained 1.4% moisture when recovered from cyclone at the exit end of the tube. |Under these conditions the polypropylene showed no degradation or tendency to agglomerate and was in excellent condition for further processing.

EXAMPLE IV Polypropylene was dried under the same conditions as in Example III above but was retained in the cyclone for a period of 3 minutes. 'After this B-minute cyclone residence time, the moisture content of the polypropylene was measured and found to be .15% and the condition of the dried polypropylene was excellent as in Example III above.

EXAMPLE V Particulated polyethylene having a moisture content of 18.8%, the moisture being substantially hexane, was fed into the 40-ft. tube at the rate of 15 lbs. per hour. The material was drawn through the drying tube by a vacuum of approximately 1.5 in. of Hg and the temperature of the air leaving the drying tube was maintained at 240 F. Upon analysis after recovery from the cyclone the polyethylene is found to be completely dried and contains no moisture. The condition of the polyethylene is excellent and ready for further processing.

EXAMPLE VI Particulated polyethylene which contains 25.5% moisture in the form of methanol was fed into the tube at the rate of 25 lbs. per hour and was moved through the tube by a vacuum of approximately 1.5 in. of Hg at the exit end. The air exit temperature was maintained at 240 F. Moisture analysis of the polyethylene passed through the tube indicates a moisture content of .2%, and the polyethylene itself is unchanged physically and chemically.

EXAMPLE VII Granular Teon (tetrafluoroethylene polymer) with a moisture content of 30.0% was fed into the tube at a rate of 8 lbs. per hour. The Teon was moved through the tu-be by a vacuum of 2.5 inches of mercury at the tube exit and the heating jacket temperature was maintained to provide an air exit temperature of 240 F. After passage through the drying tube, the Teflon moisture content is .05% and the condition of the Te-on was unaltered either physically or chemically by the drying process.

Further pilot plant drying tests on polycarbonate and municating with the cyclone or -other separator.

polypropylene were made with a stainless steel drying tube having the same diameter as above, but having 'a length of only 20 feet. Approximately the same steam temperatures and pressures were maintained in the jacket surrounding the tube as were used above in Examples I and III respectively.

EXAMPLE VIII Particulated polycarbonate having a moisture content of 59.2% was fed into the open end of the 20-ft. tube at the rate of 4 lbs. per hour and was drawn through the tube by a partial vacuum equivalent to 5.0 inches of mercury. The temperature of the exit air from the tube was 318 F. The moisture content of the polycarbonate was 12.4% upon recovery.

EXAMPLE IX Granular polypropylene having 22% moisture was fed into the tube at the rate of 18 lbs. per hour and was drawn through the 20-ft. drying tube by a partial vacuum equivalent to 4.0 inches of mercury. The temperature of the exit air from the tube was measured at 238 F. and the recovered polypropylene had a moisture content of 2.5%.

As shown in the above examples, the shorter 20-ft. tube resulted in a slight lowering of the temperature of the exit air from the drying tube. lAlso, from the reduced amount of, contact of the material with the tube walls the amount of moisture left in the material after it passed through the 20-ft. tube was somewhat higher than when passed through the 40-ft. tube under the same conditions. Even after passing through the 20-ft. tube, however, the amount of moisture removed is quite substantial, and the m-aterial thus dried would be suitable for further processing in a great num-ber of instances.

The drawing of the particulate material through the tube by a partial vacuum is preferred for a number of reasons. Creation of a partial vacuum reduces the Vapor pressure of the moisture, aiding in its removal from the material. Further, the vacuum aids in preventing the plugging of the tube as the material is drawn therethrough. It is to be pointed out, however, that for some materials blower pressure at the feed or entrance end of the tube may be used within the scope of the invention.

It has been found that the ratio of length to diameter of the drying tube may be varied for different materials, different particle sizes, and different types of mo1sture which are contained in the material. The length to diameter ratio of the drying tube however should be Ifrom 300 to 2500 to one. A length to diameter ratio -of around 1200 has been found to be generally suitable for most materials. Thus in providing a drying tube for large scale drying operations the tube may have, tfor example, an inner diameter of 3 inches and a length of 300 feet. For many materials a plurality of tubes having an inner diameter of from one-half to `one inch may be employed with a common heat exchange jacket around them. When using a plurality of small diameter tubes, the material may be drawn therethrough by a single vacuum system Icorrlac tube should be fed with material at a substantially uniform rate to provide uniform tube travel time and exposure to drying heat. Such an elongated drying tube may be bent at a number of points along its length to conserve space with the tube bends preferably 'being smooth and gradual to prevent material build up in any sharp bends within the tube.

As pointed out above, the material may take from 1 to 2 seconds to traverse a 40-ft. drying tube with a diameter of slightly under `one-half inch. At these speeds it is preferable to maintain the temperature of the tube itself within F. of the melting point or degradation point of the material being dried.

The amount of vacuum applied to the exit end of the tube should be from l to 6 inches of mercury. The

higher vacuums draw the material through the tube at a greater velocity and therefore expose the material to heat for a shorter period of time. Any reduction in vapor pressure when applying higher vacuums, however, will at least in part compensate for the shorter .period of time the material is exposed and impinged against the inner sur- -face of the drying tube. In the claims the term gas includes air as a conveying medium.

As shown in FIGURE 3 the cyclone separator 20 rnay be replaced by a multi-stage separator comprising a residence chamber 62 (shown in section) which has the drying tube exit 40 emptying into lower end 63. An upstanding tube 64 removes material and vapor from chamber 62 and passes it to cyclone 68 via connection 66. The height of tube 64 may be varied to provide varying residence time in chamber 62 before transfer to cyclone 68.

In cyclone 68 the material is separated from the vapors and is removed through spout 72. Vapors are passed from cyclone 68 through conduit 70 which is connected to the vacuum blower as cyclone 20 is connected to blower 14 in FIGURE l. Heat exchange jackets 7480 preferably surround chamber 62 and cyclone 68 respectively. Heat exchange fluid inlets 7S, 84 and outlets 76, 82 are c-onnected to a heat exchange fluid source to maintain the chamber and cyclone at a temperature above the condensation point of the vapors being removed .from the material.

Thus, for some materials the amount of remaining moisture may be reduced by increasing residence time in the heated chamber 62, before nal separation in cyclone 68. Examples II and IV above are illustrative of cases wherein increased separator residence time may be desirable.

A great number of materials may be dried, in accordance with the invention including, for example, styrene, maleic anhydride, polyvinyl chloride, polychlorouoroethylene and polymerized acrylics as well as the materials used in the above examples.

The impingement of the particles against the walls of the drying tube thus Igreatly increases heat transfer from the tube to the material particles. The quantity of arr required to move the particles through the drying tube is greatly reduced in comparison to prior art drying apparatus and methods. Thus the invention provides not only for inexpensive apparatus but also for substantially increased economy in operation.

A preferred embodiment of the invention in a transport dryer for drying acrylic polymer is shown schematically 1n FIGURE 4. In this dryer, acrylic polymer of 80 to mesh particle size is dried from a feed moisture content of between 7 and 15%, to a delivery moisture content of about 0.5%, in a total handling time -of less than ve minutes. In FIGURE 4, the incoming granulated wet polymer is stored in a vibrator hopper 86 and delivered therefrom through a flexible conduit 87, via a motor-driven screw feed unit 88, to a feed distributor 89 shown in detail in FIGURES 5, 6 and 7.

Distributor 89 breaks up lumps `of granular polymer int-o particulate material of a predetermined maximum particle size which then passes directly into the elongated multi-ple tube drying -unit 91, for turbulent stream drying of surface moisture as described above.

From tube unit 91, the stream of gas carrying the polymer particles enters a heated residence chamber or hol-ding tank 92, where it is agitated in a fully or semi-fluidized state `for a predetermined drying period, thereafter being drawn through a heated conduit 93 to a conventional bag collector '94 discharging dried polymer directly into a delivery container 96 through conduit 97, or into a storage tank 98 from which a continuous stream or suitable batches can be discharged into a diffusion dryer 99, shown in detail in FIGURE 8.

Diffusion dryer 99 is used to provide additional drying time `for vaporizing absorbed `or occluded moisture diffusing through capillary-like interstices within each particle toward the particle surface. Dryer 99 is heated by an external heating jacket 101 and by an internal double- Walled draft tube 102 through which Vthe polymer particles are raised by a helical impeller 163. Dried polymer is discharged through a valved product port 104, and purge air enters the diffusion dryer 99 at the lower end of the draft tube 102, and leaves it via a top outlet conduit 106, preferably leading into an air-cleaned dustcollector 107 incorporating a `porous dust-bag, such as the Flex-Kleen unit manufactured by the Flex-Kleen yCorporation of Chicago, Illinois, which traps the `fine polymer dust in the exhaust air stream and periodically shakes and empties the trapped dust, returning it through conduit 106 to diffusion dryer 99.

Blower 108 draws gas through feed distributor 89, tubes 91, residence chamber 92, conduit 93 and bag collector 94, delivering solvent vapor to a condenser 109 draining into receiver 111.

If atmospheric air is used as the `drying gas, inlet valve 112 is opened, supplying air to feed distributor 89, and outlet valve 113 is opened, venting receiver 111 to atmosphere.

If solvent vapor is to be retained in a closed system, valves 112 and 113 are cl-osed and a vapor return valve 114 is opened, conducting receiver 111 through a return conduit 116 to feed ydistributor 89. Conduit 116 is provided with a pressure relief or safety valve 117 and a flow meter 118.

Feed distributor The feed distributor 89 includes an upright cylindrical housing 119 having a top plate 121 with a central axial aperture accommodating a vertical power driven driveshaft 122. Spanning housing 119 are -a spaced plurality of transverse perforated plates or screens 123-125, preferably having diminishing mesh sizes from top to bottom of distri-butor 89.

As shown in FIGURES and 7, Va corresponding spaced plurality of diametric sifter blades 126-128 are anchored to the shaft 122, one blade being positioned just above each of the screens 12S-125. The uppermost blade 126 preferably has a substantial rake angle, converging with its underlying coarse screen 123. As shaft 122. rotates, blade 126 crumbles the lumps of polymer fed through screw feed unit 38, trapping them between itself and its underlying screen 123, and thus forcing them through screen 123 to succeeding blades, which may have smaller rake angles. The final blade 128 co-operating with the finest screen 125 may be substantially dat, as shown in FIGURE 7.

Granular polymer material entering distributor S9 through feed unit 88 falls by gravity through successive screens 123-125, with blades 126-128 successively reducing its particle size. The downward progress of the polymer particles is also aided `by the drying gas stream entering housing 119 through conduit 116. Gas and particles thus travel through screens 123-125 toward -the multiple tube unit 91, whose open portal ends are welded ush into spaced apertures in a tube plate 129 spanning the lower end of distributor 89.

A feeder blade 131 anchored to the lower end of shaft 122 revolves just above plate 129, serving to distribute the polymer particles continuously into the tubes 91. The distributor unit 89 thus reduces the particle size of fresh feed polymer, breaking any lumps and supplying .a continuous stream of fine polymer particles for rapid drying in subsequent stages of the system.

Multiple tube unit The multiple tube unit 91 enclosed within `a high pressure steam jacket provides a plurality of parallel drying paths along with the polymer particles are tumbled and agitated rapidly by the advancing gas stream drawn through the system by blower 108.

The multiple tube unit is designed to provide sufficient heat transfer area for drying the polymer to a moisture content which is optimum based upon its drying rate curve.' Thus if surface moisture .accounts for all but the last 3% of volatiles, which require diffusion to reach the particle surface, the polymer particles may be dried to 5% moisture content in tube unit 91, leaving the final, diffusion-controlled drying to be performed in subsequent drying stages.

In the multiple tube unit 91, heat is supplied to the polymer both by conduction and by radi-ation from the tubes internal wall surfaces. In most cases, tube temperatures can be higher than the melting point of the polymer because the surfaces of the cooler polymer particles cannot reach this temperature as long as moisture is present on the particle surfaces. Radi-ation from the tube walls has the effect of increasing the wet bulb temperature of the surface of the polymer particles, producing a corresponding increase uof as much as 30% in the rate of mass transfer of moisture from inside each particle to its surface.

Residence chamber In the system of FIGURE 4, Vthe holding tank or residence chamber 92, like chamber 62 in FIGURE 3, serves to arrest the rapid turbulent advance of the polymer particles in a fully or partially duidized mass within the chamber. An adjustable downpipe 132 extends downward into the heated chamber 92, providing an outlet for delivery of the polymer particles through heated conduit 93 to ba-g collector 94, exhausted by blower 108. By raising or lowering the adjustable downpipe 132, the volume of polymer agitated in the heated chamber 92 can be changed, with a corresponding change in the res-idence time of the material. The multiple tubes 91 enter the lower end of chamber 92, and downpipe 132 extends downward from the upper end of the cham-ber. If multiple tubes 91 and downpipe 132 are substantially coaxial or otherwise aligned, fast-moving polymer particles may be carried directly through chamber 92 by the advancing gas stream. To avoid such a wind tunnel effect, a bafiie plate 133 may be positioned by suitable supporting struts in chamber 92 between the entering multiple tubes 91 and the exhaust portal end of pownpipe 132. Baie plate 133 interrupts the gas stream idelivered by tubes 91, Icreating turbulent, billow-ing agita-tion of the polymer particles for extended residence time in `chamber 92, heated by its medium pressure steam jacket 134.

As the advancing stream of gas passes through the system of FIGURE 4, the pressure differential created by the yblower is balanced by the successive pressure drops through conduit 116, feed distributor 89, tubes 91, chamber 92, conduit 93 and bag collector 94. Accordingly, the pressure in chamber 92 is appreciably lower than the pressure within multiple tubes 91.

The high-velocity turbulent motion of the polymer particles through heated tubes 91 combined with their abruptly arrested, agitated residence in the lower-pressure, heated residence chamber 92 produces unexpectedly effective drying of the particles. The high mass transfer rate produced by high temperature heating in the tubes 91 apparently plays a part by Iinitiating migration of internal moisture toward the particle surfaces, where it is vaporized with high effectiveness during the particles residence in chamber 92 at temperatures Ibelow the polymer melting point and `at pressures below those in tubes 91.

Bag collector 94 performs the same function as cyclone 20 or 68 by separating the polymer particles from the advancing gas stream drawn into the blower.

For products dried sufficiently in the foregoing stages, the three-way exit valve 135 beneath bag collector 94 may be turned to direct the particles continuously through conduit 97 to delivery container 96.

Many products require additional drying, particularly those in which the tinal increments of solvent moisture kcan be vaporized only at diffusion-controlled slow drying Final, diffusion-controlled slow drying of polymer particles is performed in diffusion dryer 99, shown in FIG- URES 4 and 8. If three-way exit valve 135 is turned to shunt the conduit 97, and storage tank 14S interposed therein, polymer particles from collector 94 are delivered directly to a storage tank 98 having an exit valve 136 connecting it to dryer 99 for delivering batches of material thereto. For continuous operation valve 135 may be opened to connect collector 94 to tank 98, and valve 135 may be replaced by a continuous screwtype feeder to deliver material against the slight pressure in dryer 99, affording a vacuum seal between tank 98 and dryer 99. The storage tanks 98 and 148 are interposed to provide air locks blocking atmospheric pressure or the pressure in dryer 99 from reversing the normal delivery 110W through exit valve 135.

As shown in FIGURE 4, the diffusion dryer 99 is heated preferably to a temperature lower than the temperature of chamber 92 by a steam-water mix or hot water circulating through its jacket 101, connected to a return sump 137 by an overflow conduit 138 and a circulating pump 139 and supply conduit 141. Water in sump 137 is heated by low pressure steam supplied through a steam conduit 142; accumulating water produced by condensation of this steam is removed from sump 137 by an overow drain 143.

Circulation of polymer particles in diifusion dryer 99 is produced by central helical impeller 103 carrying the material upward inside the double-walled draft tube 102,

which is heated by a sepa-rate supply of low pressure steam or by the steam-water mix or hot water circulating through jacket 101. The lower end of draft tube 102 is open, communicating with the conical-walled lower end 144 of the dryer chamber at` the bottom of which a surge air inlet 146 supplies astream of air to carry away vaporized moisture via air outlet 147 connected to an aircleaned dust collector, such as the Flex-Kleen unit sold by the Flex-Kleen Corporation of Chicago, in which a dust collector bag is periodically shaken to return nes to the dryer 99.

Product is delivered from dryer 99 via valved product port 104 to the delivery container 96, although a reverse iiight helical screw impeller mounted coaxially below impeller 103 may be employed for continuous product delivery.

Highly eicient diffusion-controlled drying of the polymer particles is achieved by the repeated circulating exposure of the material to the heat from jacket 101 and draft tube 102. Residence time is selected to produce the desired degree of dryness.

The effectiveness of the transport drying system shown schematically in FIGURE 4 is demonstrated by the following examples of drying runs performed on acrylic polymer, summarized in Table I, and on vinyl chloride and vinyl acetate, summarized in Table II, Table III and Table IV.

R MULTIPLE TUBE UNIT 91 Tube Polymer, Wt. Heat Run Type Tube Unit Jacket Percent Vol. Product Transfer No. CdzR Transport Temp., Rate, ate,

Dryer F. lbs/hr B.t.u./hr.

Feed Product It.2 F.

20 ft. Single. 307 7.5 0.8 7.5 40 ft. single*. 307 7. 5 0.6 15. 8 20 ft. singl 307 7. 5 0.6 7. 5 40 it. single 281 7. 5 1.0 13. 9 -.d 281 7. 5 0.8 7. 5 do 281 15.0 1.0 6.9 40 ft. multiple* (10) 270 7. 7 0.4 91. 9 -do.* 270 7. 7 0.75 134. 0 270 7. 7 0. 9 167. 5 270 7. 7 1. 1 214. 4

*Fluidizing residence chamber 92 used, as shown in FIGURE 4.

TABLE 11.-VINYL CHLORIDE-VINYL ACETATE COPOLYMER DRYING RUNS USING SINGLE TUBE UNIT 10 Dryer p Feed Feed, Product, Tube Date Run No./ Type Tube Dryer Tube Cyclone Cyclone Rate, Vol. Vol. Unit 1904 Dur. of C&R Length, Jacket Jacket Vacuum, lbs/hr. percent percent Vapor Remarks Run, Min. Dryer Ft. Temp., F. Temp., F. in. Hg Wet Basis Wet Basis Wet Basis Disch.

Temp., F.

10/14 1/5 S.T. 40 174 H2O. 174 HgO..- 2. 5 11. 5 20. 0 12. 0 170-160 Raln Well; cyclone c can. 10/14 2/5 S.T 60 174 1120..... 174 H2O. 4. 5 11. 5 20.0 10.0 a170-165 Same as Run 1. 10/14 3/5 S.T- 60 198 1120.--.. 198 1130.--.. 4. (i 11. 5 20.0 8. 0 196-189 Do. 10/l4. 4/3 S.T. 40 269 Steam. 232 Steam.-- 3. 6 11. 5 20.0 5. 3 258-248 Do. 10/l5 5,/3 S.T 60 d0 d0..--.. 4.5 11.5 20.0 4.6 a258-252 10/15 6/2b S.T 60 .do. .do 3. 7 3. 5 259-255 10/15 7/3 S.'I. 60 4.0 11.5 20.0 n214203 ttl/18..-.. 9/28 S.T 60 5.0 11. 5 20.0 a 21o-197 Air Temp. at start of run before adding feed.F S.T.=Single Tube Unit.

Jacket heat on luidizer.

b Feed crumb slurried-Z water washes-210o Fluidizer on stream. H.U.T. 22 mn. in uidizer. minute Run No. 9 reduced moisture content from 5.0% to 0.2%

Bed vol.=0.148 ft. Use of fluidizing residence chamber 92 in 28- d Straight run tube in luidizeri.e., negligible hold-up time. Feed p0lymer=Escambia lot 102764.

BLE IIL-VINYL CIILORIDE-VINYL ACETATE COPOLYMER DRYING RUNS USING MULTIPLE TUBE UNIT 91 OR TA SINGLE TUBE 10 WITH FLUIDIZING RESIDENCE CHAMBER 92 Run Dryer Dryer Feed Fluidizer Tube Dryer Flnidizcr No./ Dur. Type Tube Tube Flnidizer Cyclone Rate, Feed Vapor Vapor Vapor Disch. Date 1964 of Run, C& R Length, Jacket Jacket .Tacker lbs/hr ol. Disch. Disch. (air) Prod.,

Min. Dryer Ft. Temp., Temp.,o F. Te1np., F. Wet Percent Temp., F. Temp., V01. V01.

F. Basis F. FILE/min. Percent 1&2/15 M.T 40 235 249 35 20. 0 d 233 n 240 28 3.5.

d 219 204 3 M.T 40 247 247 250 34. 7 20. 0 d 228 180 30 4.5. 4 M. 4U 245 245 250 34. 7 20. 0 d 226 230 27 3.9. 5 S,T 66 268 None 11. 5 30. 0 None 6.2-1pass.

0.4-3 passes. 6 S.T 60 247 163 163 11.5 30.0 d 162 226 5.0-1 pass.

approx. 220 7 S.T 60 260 None None 11.5 5.0

See foot-notes at cnd of Table II.

TABLE IV.-VINYL CI-ILORIDEVINYL ACETATE COPOLYMER DRYING RUNS: CONDITIONS MAINTAINED AND OBSERVED DURING BATCH DRYING IN DIFFUSION DRYER 99 Jacket Bed Prod. Date 1964 Run No. Drying Air Temp. Air Flow, Temp., Temp., Disch. Screw, Remarks Time, Hrs. In., F 1n.1 t.3 Hr. F. F. ol., rpm,

Percent 1 0833 180 20 181 170 3.0 110 Screw pumping polymer. l 0.416 180 2() 179 175 1. 0 110 D0. 1 0.917 180 20 180 178 0.7 110 Do.

2 0. 25 160 20 160 155 2. 5 110 Do. 2 0.5 160 20 161 154 1. 8 1l() Do. 2 0.66 166 20. 5 165 159 1.3 110 Do. 2 1.0 160 20.5 165 165 1.1 110 Do. 2 1. 25 160 2o. 5 165 165 0.9 110 Do. 2 1. 75 164 20.5 164 160 0. 9 110 Do.

111/30 3 0 4. 5 0 Do. 3 0.25 173 20. 5 170 156 3.0 110 Do. 3 0.50 173 20. 5 168 161 2.0 110 De. 3 0.75 173 20. 5 168 161 1. 3 116 Do. 3 l. 0 168 20.5 167 160 1. O l1() Do. 3 1. 25 168 20.5 170 167 0.8 110 Do. 3 1.75 169 20.5 165 168 0. 8 110 Do.

4 0. 25 1GO 51. 5 167 135 2.9 110 Diseh. air visually saturated. 4 0.5 175 5l. 5 170 158 l. 5 110 0. 4 1. 0 170 29. 4 170 168 1.0 11() Diseh. air not saturated. 4 1. 5 170 29. 4 178 167 0. 8 110 Do.

The drying systems of this invention dry with high effectiveness and without degradation or discoloration such heat sensitive materials as vinyl acetate copolymers, polyvinyl chloride and ABS polymers. One unexpected advantage of these drying systems is the fact that they require only one-fifth of the `quantities of drying air normally required in conventional, continuous, direct rotary dryers.

While the objects of the invention are efficiently achieved by the preferred forms of the invention described in the foregoing specification, the invention also includes changes and variations falling within and between the deiinitions of the following claims.

We claim:

1. Drying apparatus for removing moisture from particulate material comprising in combination (A) particulate material -feed means,

(B) elongated, open-ended drying tube means surrounded by a heating jacket, having (1) an effective cross-sectional area A, (2) an input end, and (3) a delivery end, (C) a residence chamber surrounded by a heating jacket, having (l) an entrance port centrally positioned in the lower portion thereof and connected to the tube delivery end, and (2) an exit portal,

(D) a blower operatively connected to draw gas through the heated tube means and the residence chamber,

(E) and a separator operatively connected to separate particulate material from the gas drawn through the tube means and the residence chamber by the blower,

(F) with the residence chamber having an effective cross-sectional area substantially greater than A, producing abrupt deceleration of the gas delivered thereto through the entrance port and turbulent iiuidizing agitation of the particulate material throughout its residence in the residence chamber by the gas drawn therethrough by the blower,

(G) with the feed means including a feed distributor incorporating (l) a vertical-elongated housing,

(2) a spaced plurality of perforated, horizontal transverse screens spanning the housing, all having aligned central apertures, and all arrayed from near the top to near the bottom of the housing, with successively diminishing mesh sizes,

(3) a rotatable power-driven drive shaft extending vertically through the aligned central apertures in the screens, and

(4) at least one transverse sifter blade anchored to the shaft above each screen to agitate material entrapped thereon.

2. The apparatus defined in claim 1 wherein the separator is connected to deliver partially dried material to a diffusion dryer sur-rounded by a heating jacket and having a central, open-ended, double-walled, heated draft tube surrounding a power-driver helical impeller, whereby the particulate material is propelled longitudinally through the draft tube and recirculated outside the draft tube for repeated circulating movement within the diffusion dryer.

3. The apparatus defined in claim 2 wherein the diffusion dryer includes an inner chamber having a converging lower end with a dried material product port formed therein.

`4. The apparatus dened in claim 2 wherein the dif- References Cited by the Examiner UNITED STATES PATENTS 6/1930 Soderlund et a1. 34-10 8/1942 `Freund 34-10 X Beardslee 34-10 Gindoc et a1. 34-57 Schaub et al 34-57 X Mark 34-57 Clute 263-21 FREDERICK L. MATTESON JR., Primary Examiner. D. A. TAMBURRO, Assistant Examiner.

Claims (1)

1. DRYING APPARATUS FOR REMOVING MOISTURE FROM PARTICULATE MATERIAL COMPRISING IN COMBINATION (A) PARTICULATE MATERIAL FEED MEANS, (B) ELONGATED, OPEN-ENDED DRYING TUBE MEANS SURROUNDED BY A HEATING JACKET, HAVING (1) AN EFFECTIVE CROSS-SECTIONAL AREA A, (2) AN INPUT END, AND (3) A DELIVERY END, (C) A RESIDENCE CHAMBER SURROUNDED BY A HEATING JACKET, HAVING (1) AN ENTRANCE PORT CENTRALLY POSITIONED IN THE LOWER PORTION THEREOF AND CONNECTED TO THE TUBE DELIVERY END, AND (2) AN EXIT PORTAL, (D) A BLOWER OPERATIVELY CONNECTED TO DRAW GAS THROUGH THE HEATED TUBE MEANS AND THE RESIDENCE CHAMBER, (E) AND A SEPARATOR OPERATIVELY CONNECTED TO SEPARATE PARTICULATE MATERIAL FROM THE GAS DRAWN THROUGH THE TUBE MEANS AND THE RESIDENCE CHAMBER BY THE BLOWER, (F) WITH THE RESIDENCE CHAMBER HAVING AN EFFECTIVE CROSS-SECTIONAL AREA SUBSTANTIALLY GREATER THAN A, PRODUCING ABRUPT DECELERATION OF THE GAS DELIVERED THERETO THROUGH THE ENTRANCE PORT AND TURBULENT FLUIDIZING AGITATION OF THE PARTICULATE MATERIAL THROUGHOUT ITS RESIDENCE IN THE RESIDENCE CHAMBER BY THE GAS DRAWN THERETHROUGH BY THE BLOWER, (G) WITH THE FEED MEANS INCLUDING A FEED DISTRIBUTOR INCORPORATING (1) A VERITICAL-ELONGATED HOUSING (2) A SPACED PLURALITY OF PERFORATED, HORIZONTAL TRANSVERSE SCREENS SPANNING THE HOUSING, ALL HAVING ALIGNED CENTRAL APERTURES, AND ALL ARRAYED FROM NEAR THE TOP TO NEAR THE BOTTOM OF THE HOUSING, WITH SUCCESSIVELY DIMINISHING MESH SIZES, (3) A ROTATABLE POWER-DRIVEN DRIVE SHAFT EXTENDING VERTICALLY THROUGH THE ALIGNED CENTRAL APERTURES IN THE SCREENS, AND (4) AT LEAST ONE TRANSVERSE SIFTER BLADE ANCHORED TO THE SHAFT ABOVE EACH SCREEN TO AGITATE MATERIAL ENTRAPPED THEREON.

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