US3241248A

US3241248A - Drying method and apparatus

Info

Publication number: US3241248A
Application number: US269232A
Authority: US
Inventors: Gerlach Richard; Foller Ernst; Wiloth Fritz
Original assignee: Glanzstoff AG
Current assignee: Glanzstoff AG; Vereinigte Glanzstoff Fabriken AG
Priority date: 1960-07-23
Filing date: 1963-03-25
Publication date: 1966-03-22
Anticipated expiration: 1983-03-22
Also published as: CH385744A; AT248343B; DE1191299B; BE605649A; GB954311A

Description

March 22 1966 R. Gl-:RLACH ETAL 3,241,248

DRYING METHOD AND APPARATUS Filed March 25, 1963 ATT YS i however, such dryers have very large dimensions.

United States Patent O s claims. (bl. 34-22) This is a continuation-in-part of application Serial No. 125,840, led July 21, 1961, now abandoned.

The present invention relates to a method and apparatus for drying various granular substances. More particularly, the subject invention is directed to a method and apparatus tor drying heat sensitive high molecular weight polymers.

The methods of drying granular materials that are practiced in industry today are both time-consuming and expensive. This is especially true if the iinal moisture content of the compositions is to be low and if the moisture content must be kept within narrow limits. If the material is also easily damaged by temperature fluctuations an added diiculty arises in keeping the drying temperature within a workable range.

As an example, high molecular weight polymers which are delivered to spinning heads in cut form may vary only slightly in moisture content from an optimum level of about 0.05%. Inasmuch as the cuttings have a considerably higher moisture content, the granules must be dried. The drying step is usually carried out in socalled tumbler dryers. Due `to the fact that the axis of rotation of the tumbler .is placed at an `angle to the tumbler axis, the granular material can be thoroughly mixed. The tumbling action plus vacuum conditions make it possible to provide a uniform heating of the material placed in the dryer. Due to the nature of the drying operation it is necessary that the material be processed in batches.

The cost of tumbler dryers is very high, especially when one considers the relatively ineiicient utilization of energy which occurs with this equipment. The heat- .ing of the material is accomplished by the container wall. For this reason and because ot the low heat conductivity of the material being dried, as well as the small contact surface, such drying operations require a considerable amount of time. Accordingly, a total drying time per batch is needed which cannot be reduced much below 15 to 25 hours with dryers of conventional size. A lessening of the total drying time is possible only it the drying temperature is raised. In many cases this is impossible because above a certain temperature the cuttings or other material react with atmospheric oxygen and, accordingly, must be cooled before the vacuum is discontinued.

Despite the long heating period which is required with tumbler dryers, continuous drying processes have not been favorably received in the industry. Each one of the known continuous methods has drawbacks which have caused processors to return to batch methods. As an example, the How-bed or fluidized layer dryer has a very broad stay-time spectrum and, consequently, an extreme variation in the degree of drying and in the solution viscosity of the polymer occurs. Moreover, considerable abrasion takes place in such a method. Large quantities of the drying agent are required so that the regeneration of the agent also is relatively expensive.

In contrast, belt dryers operate substantially without abrasion and with constant staying time. Because the material .to be dried must be maintained in a thin layer,

Fur-

3,Z4l,248 Patented Mar. 22, 1966 ICC thermore, considerable quantities of drying agent are required in such lmethods which adds additional regeneration expense to the process.

In drying grains, especially cereals, tubular or sprinkling shaft dryers are known which are generally operated with hot air in counterflow. It would seem to be an obvious suggestion to use such a dryer ior cuttings of synthetic polymers which are used for the spinning of threads wherein the air which is harmful because of its oxygen content could be replaced -by an inert gas such as nitrogen. Inasmuch as it is not possible to exceed a maximum input temperature of the gas (determined by .the material) of, for example, 160 C., however, and also because the amount of gas must be so great that on the discharge of moist gas the dew point will not be exceeded, there results, for example, where nitrogen is the drying agent, :su-ch an amount ot gas that an economical operation is no longer possible. This is especially true because of the regeneration cost involved.

In continuous drying methods it is necessary to operate at atmospheric pressure in order to keep the size of the apparatus and its economical operation within tolerable limits. In drying cut granules of high molecular weight organic polymers (for example, polyamides) the upper drying temperature range is from about C. to about C. Correspondingly, .the heating and drying of the -cuttings should be carried out in as short a time as possible. It has been found that it would be necessary to use an inert gas having a dew point of about 20 C. .in order to achieve a product dryness of approximately 0.05%. By dew point is meant the temperature for a given gas at which the condensation of the vapor dissolved in the gas begins. In the regeneration of a gas stream this state is achieved in a simple manner by cooling the gas to the dew point and thereby allowing the condensed vapor to separate fout. It follows that with a :starting w-ater content of 15% approximately live tons of nitrogen per ton of a material to be dried would have to be cooled constantly to approxi-mately 20 C. and then heated to about 160 C. in which process, moreover, the oxygen originating from the water of the cuttings or adhering to the surface of the cuttings would have to be removed.

Another obvious suggestion for drying processes of this type would be to pass superheated steam through the cuttings or other material. At the given `upper temperature limit, however, it would be impossible to dry the product to a moisture content of less ,than about 0.6% to 1%.

It is an object of the present invention to provide an improved drying process which avoids the ditliculties of the prior art methods which are set forth above.

Another object of the invention -is to provide apparatus for carrying out an improved drying process.

Another object of the invention is to provide an economical process and apparatus for drying various materials.

Another object is to provide a drying method in which the stay-time spectrum is equal or approximately equal and in which the degree of drying of the individual particles varies little, if any.

Another object of the invention is to provide an apparatus and method which dries polymer cuttings and other materials with little, if any, abrasion.

Still another object is to provide a method and apparatus which is capable of drying cuttings of high molecular weight polymers in -a relatively short -period whereby troublesome afterapolymerization is avoided.

In general, the present invention comprises the discovery that an economical process for continuously drying various materials with nitrogen or other gases,

preferably inert, is possible if the drying process is divided into at least two circuits in the manner described in the following paragraphs. With heat sensitive products the temperature of the drying process is of critical importance. The subject invention solves this problem by the use of two separate circuits which are connected to a shaft dryer of special design whereby two zones result.

In the process a very small amount of pure, unheated and oxygen-free nitrogen is passed into the lower Zone of the dryer and then through the heated material to be dried in the upper zone. In passing from the lower to the upper zone the nitrogen is heated and simultaneously absorbs moisture. On entering the upper zone the nitrogen which is then heated, for example, to about 150 C. mixes with a second partial circuit consisting of a large amount of nitrogen or other inert gas laden with vapor which was drawn from the material to be dried. The said second partial current enters the lower end of .the upper zone at a temperature of, for example, about 160 C. The temperature will vary widely in accordance with the nature of the material being dried and the amount of moisture in the material. In general, temperatures of from about 135 C. to about 200 C., and more particularly from about 140 C. to about 165 C., are satisfactory in most instances. The nature and origin of the gas mixture of the second circuit will subsequently be explained in detail. The merging stream of nitrogen from the lower Zone and of heated moist atmosphere (gas mixture) in the second circuit pass together through the material to be dried in counterflow and in so doing heat the material to a temperature of, for example, 150 C.

The gas stream after leaving the dryer is again divided into two circuits. The one circuit carrying the smaller amount of gas is designated as the small partial current. The small partial current is passed through a condenser and thereupon through an apparatus which removes the oxygen which is carried along with the nitrogen. Initially -in the regeneration process, the vapor carried along with the gas is condensed and separated whereupon the dried nitrogen is then freed of oxygen. This small partial current then is again passed to the lower zone of the dryer in a cold state. The second current, designated as .the large partial current, is conducted through a passage heater in which it is heated to a starting temperature of, for example, 160 C. whereupon it is then supplied to the lower end of the upper zone of .the dryer.

In the process, the allocation of functions is such that the large partial current circulating without regeneration serves mainly to heat, possibly to predry, and to condition the material to be dried, while the constantly regenerated small partial current serves to remove mois ture or to finally dry the material. By selecting appropriate operating conditions it is possible by the subject method to provide in a simple manner a self-adjusting equilibrium in the moisture content of the nitrogen in the upper zone. The same amount of water is precipitated in the condenser as was absorbed in the drying of the material. The volume of the small partial current is regulated according to the amount of nitrogen which is necessary for the final drying `of the product. This amount of nitrogen can easily be determined by calculation or by experiment and ordinarily is very slight. In general, the ratio of the large partial current to the small partial current will be in the range of 20:1 to 300:1, preferably 75:1 to 225:1. Due to the constant thorough mixing of the two currents, a concentration of harmful `ad-mixtures such as oxygen is avoided in the large partial current Icirculating without regeneration.

The manner in which the process is carried out is as follows. Prior to commencing the process the dryer filled with predried cuttings is rinsed with nitrogen until all 'of the air is removed. Thereupon the nitrogen is cir- Nitrogen Steam carried along- Nitrogen in the small partial current Amount of evaporated water The process and apparatus of the present invention can best be understood by reference to the attached diagrammatic drawing which is the apparatus of the invention.

The material to be dried is introduced into Y-pipe 1 at arrow A and on its emergence from openings 2 is distributed with the aid of agitator 3 over the entire dryer cross-section. Base 5 which is subdivided in honeycomb fashion closes the upper space of the dryer downward at the level of ange 4. The honeycomb type plate 5 is formed of funnels 6 which at their broader ends are hexagonal and which are connected to each other, while at their lower ends they assume a round crosssection. A cylindrical pipe section 7 is attached to each of the funnels 6. Pipe section 7 passes through the intermediate oor 8 of the dryer and is sealed gastight with said oor 8, for example, by fillet welding on the underside. A hollow space 21 is defined by the walls of the funnels 6 of pipe section 7 and intermediate floor 8. The heated large partial current enters space 21 through ange 20. Pipes 10 which are bent at ya right angle are connected by means of openings 9 to hollow space 21. Pipes 10 extend within pipe sections 7 to a point above base plate 5. Pipes 10 are covered by caps 11 which allow free passage of gas flowing through pipe 10 and which conduct said gases downwardly. In the space beneath pipe 7 there is arranged a cooling coil 12 which is designed to cool the material after it has been dried. Cooling coil or heat exchanger 12 cools the cuttings leaving the second stage of the dryer to a temperature suiciently low so that contact with yatmospheric oxygen is not hazardous. A wide variety of coolers can be used for this purpose, for example, the cooler can consist of two or more layers of crossed pipes having a large surface area and having interspaces allowing the cuttings to trickle through the structure. The dried material passes to funnels 13 and then through openings 14 whereupon it is pushed through openings 16 by means of distributor rake 15. The dried material ultimately collects in cone 17 and is drawn off. at B over a suitable sufliciently gas-excluding device such as, for example, a bucket wheel. The distributor rake 15 is a grate with a level which can correspond about to the distance between the lower edge of the funnels 13 and the upper edge of the perforated plate. The rake is moved back and forth during the discharge to even the cuttings. The material emerging from the funnels is pushed back and forth far enough so that it can emerge through the holes 16, The rake 15 can be of very simple design, say, of strips of sheet metal standing upright (on edge), which are joined in the form of a grid with square or otherwise shaped apertures. The amount discharged then depends on the amplitude of the swinging movement and the frequency.

The diameter of the dryer can be selected in such a manner that the velocity of ow of the gas in the upper zone is suiciently low to prevent so-called vortex bed formation, while a considerably smaller cross-section of the lower zone formed by parts 6 and 7 provides a controllable flow resistance. The hollow space 21 in conjunction with caps 11 makes possible a uniform distribution of the gas flowing in at 20, While in the hollow space 19 formed between pipe 7 the nitrogen entering at 1S can be distributed. The cold nitrogen passes at attratta 18 into space 19 and in consequence of a slight excess pressure rises through pipe 7. In so doing it flows through the heated drying material in pipe 7 and funnels 6 whereby the material is dried in accordance with the subject process to a desired final moisture content. In this process the gas is heated.

The gas current is divided at point 24. That portion of the current which is not subjected to regeneration flows through throttle valve 25 and heater 26 where it is brought to the desired temperature. At 20 it enters hollow space 21 formed on the one hand by base 5 and on the other hand by the intermediate floor 8. From space 21 the current passes through openings 9 into pipe 10. Caps 11 prevent the gas current from passing through the drying material in the shortest path. These caps deflect the current downwardly so that the material is uniformly traversed by the heated gas. The heated gas mixes with the nitrogen rising through pipe 7 and funnels 6. At 46 the gas, enriched with steam, is withdrawn with the aid of suction fan 23 and the temperature of the gas is noted by means of thermometer 46a. In cyclone 22 it is purified of suspended particles and at 24 it is subdivided according to the subject process. The gas current to be regenerated is cooled in cooler 28 and in separator 29 the water is removed from the gas. In apparatus 34 which is suitable for this purpose the oxygen carried along with the gas is bound with hydrogen flowing from hydrogen reservoir 33. A suitable device can be used for the extraction of the oxygen. Depending on the operating conditions which are selected, the cooled nitrogen which is freed of oxygen passes over bypass line 44 to connecting flange 18 or is cooled still further in cooler 37 and is redried in separator 38 whereupon it flows through valve 40 to connecting flange 18. Throttle valves 25 and

valves

27 or 45 or 40 serve to produce the desired distribution of the gas currents. Working in cooperation,

throttle valves

25 and 27 control both the distribution of the two gas streams as well as the total amount and thereby the dew point and temperature at outlet 46. The regulating can be accomplished in accordance with the pressure in the dryer. Unavoidable leakage losses of nitrogen are replaced from nitrogen reservoir 42 through valve 43 which is controlled by 'a throughput metering apparatus which is not shown. The feed may, however, take place above the discharge connecting piece B.

It is clear that the constriction of the cross-section of the lower zone with a simultaneous provision for as constant a flow velocity as possible over the entire crosssection can be achieved by other known means such as rod-form installations which could possibly be crisscrossed with rhomboid cross-section. Likewise, the uniform distribution of the gas currents can be varied. For example, the feed can take place over hollow installations with rhomboid cross-section whereby the regenerated nitrogen, for example, would be in the lowest portion and the unregenerated nitrogen in the uppermost row. Instead of cyclone 22, any other device can be used for removing the particles carried along by the gas stream from the material to be dried. The regulation of the amount of regenerated nitrogen fed into the system can also be accomplished by other means. According to such a process the amount can be regulated in dependence on the state data of the gas leaving the dryer. Another possibility consists in maintaining constant the amount of the regenerated gas by any means in a manner in itself known and in regulating the amount of the gas in the large partial current in dependence on the temperature of the gas leaving the dryer. Thermometer 46a indicates the temperature of the gas leaving the dryer. This temperature then controls the choke valve 25 in such a way that the temperature of the gas stream laden with vapor which was drawn from the material in the upper zone f lies, on leaving the dryer, above the dew point of the gas (for example, 100 C.). The regulation of the two '6 partial streams may also be accomplished by expedient combination of the possibilities mentioned.

The following examples illustrate the subject invention.

Example I In this example, nylon-6 cuttings of ca. 3 mg. granule weight were dried by the subject process. The cuttings had an initial moisture content of about 20% by weight. The throughput was maintained at about 167 kg./hr. with reference to the dry cuttings. The circulation of gas through the rst zone was maintained at about 4,000 m.3/hr. The temperature of the gas as it entered zone 21 was 139 C. and the moisture content was approximately 70%. Twenty (20) m.3/hr. of gas was circulated through the second zone (zone 19). The nitrogen had a temperature of 25 C. as it entered the zone. Nitrogen in the amount of 2.5 m/hr. was added to compensate for nitrogen loss. The cuttings remained in the first zone for 5 hours and in the second zone for 4 hours. As they left the dryer the cuttings had a moisture content of 0.08% by weight and a temperature of 20 C. The after-polymerization of the nylon-6 amounted to 17ml 0.04 unit measured in 1% weight per volume solutions in formic acid.

Example ll In this example, the process shown in Example I was modified by a more intense cooling of the circulation gas for the second zone. The moisture content of the gas circulating to the second zone was lowered by one-third. This modification made it possible to reduce the temperature of the gas circulating through the first zone (zone 21) by from 6 to 7 C. while still reducing the moisture content of the cuttings to about 0.08% by weight.

Example III Nylon-6 cuttings having a granule weight of ca. 3 mg. were passed through the apparatus described above with a throughput of 333 kg./hr. The cuttings had an initial moisture content of ca. 20% by weight. Gas was circulated through the first zone (zone 21) at a rate of 6,500 m.3/hr. at an entry temperature of 147 C.. and the moisture content was at ca. 74%. Nitrogen was circulated through the second zone (zone 19) at a rate of 33 m.3/hr. at an entry temperature of 30 C. 4.0 m.3/hr. of nitrogen was added to the gas circulated to the second zone in order to make up for nitrogen loss. The staying time of the cuttings in the first zone was 2.5 hours, and the staying time of the cuttings in the second zone was 2 hours. As the cuttings left the dryer they had a moisture content of 0.09% by weight and a temperature of 35 C. The after-polymerization amounted to 77ml 0.05 unit measured in 1% weight per volume solutions in 90% formic acid.

Example 1V This example illustrates the importance of the heating and conditioning which takes place in the first zone of the drying apparatus. Nylon-6 cuttings having a granule size of about 3 mg. and an initial moisture content fluctuating between 7 and 15% by weight were passed through the apparatus described above at the rate of 11.5 tato (480 kg./hr.) with reference to the dry cuttings. The gas throughput in the iirst zone was 4,000 m.3/hr. at an entry temperature of 152 C. The gas throughput in the second zone was less than m.3/hr. at an entry temperature of 25 C. The steam content in the first zone was adjusted by regulating the amount of gas added to the second zone as is explained above. Despite the substantial fluctuations in the initial moisture content of the cuttings, the conditioning of the cuttings in the first Zone made it possible to control the final moisture content of the cuttings to a level of between about 0.05% and 0.06% by weight.

7 Example V This example illustrates the fact that the after-polymerization of the cuttings can be influenced to an appreciable extent by the establishment of the moisture content of the gas circulating in the first zone of the apparatus. To a very great degree a sorption equilibrium is produced in the lirst zone between the cuttings and the moisture of the atmosphere. Thus, for example, in the process described in Example I, at an entry temperature of 139 C. and a moisture content of ca. 70% by volume, the moisture in the cuttings is reduced to ca. 0.8% by weight (in the first zone). If the moisture content of the circulating gas stream is reduced to ca. 20% by volume, the cuttings leaving the irst zone still contain ca. 0.2% by weight of moisture. In order to reduce in the rst zone the moisture content of the circulating gas stream to 20% by volume, it may be necessary in Example I for the hourly throughput amount for the second zone to be raised to about 180 m.3/hr.

During the staying time of hours at 139 C., as given in Example I for the first Zone, cuttings with 0.2% by weight of moisture polymerize up to an 17ml of 0.02 more than do cuttings which leave the first zone with 0.8% by weight of moisture. The complete after-polymerization in Example I amounted to 0.04 unit of the relative viscosity, from which fact it follows that the controllable degree of after-polymerization amounts to up to 50% of the total after-polymerization.

Example V1 This example (as well as Examples VII and VIII) was carried out using apparatus having smaller dimensions than the apparatus used in connection with Examples I to V. In this example, nylon-6 cuttings having a granule weight of ca. 3 mg. were passed through the drying apparatus at a rate of 0.75 tato with reference to the dry cuttings. The initial moisture content of the cuttings was ca. 20 to 24% by weight. The gas throughout maintained in .the first zone was ca. 500 m.3/hr. at an entry temperature of 165 C. The moisture content of the Iirst zone was ca. 67 volume percent. The nitrogen circulated through the second zone was about 4.8 m.3/hr. at an entry temperature of 25 C. An additional 0.45 m.3/hr. gas was added to the second Zone to make up for the lost nitrogen. The staying time of the cuttings in the first Zone was 1.2 hours, and the staying time of the cuttings in the second Zone was 1.7 hours. The final moisture content of the cuttings was 0.045 to 0.067% by weight at a iinal temperature of ca. 20 C.

Example VII The material used in this example was nylon-66 having la mean cutting weight of ca. 25 mg. The initial moisture content of the cuttings was 1% and less by weight. The cuttings were passed through the subject apparatus at a rate of 0.77 tato. Gas was circulated through the first zone at a rate of about 280 m/ hr. at an entry temperature of 145 C. and a moisture content of less than 14 volume percent. Nitrogen Was circulated through the second zone at a rate of 2.8 m.3/hr. at an entry temperature of about 25 C. Additional nitrogen was added at the rate of about 1 m.3/ hr. The staying time of the cuttings in the lirst zone was about 1 hour, and the staying time in the second Zone was about 1.3 hours. The linal moisture content of the cuttings was about 0.08% by weight and the final temperature of the cuttings was about 20 C.

Example VIII The material dried in this example was polyethylene -terephthalate having a cutting weight of about 30 mg. The initial moisture content of the cuttings was about 0.5% by weight. The gas throughput in the first zone was ca. 450 m.3/hr. at an entry temperature of 185 C. and a moisture content of less than 10 volume percent. The gas throughput in the second zone was ca. 5 m.3/hr. at

an entry temperature of 25 C. Additional gas was added to the second Zone at the rate of 0.75 m.3/hi'. The staying time of the cuttings in the first zone was ca. 0.7 hr. and the staying time of the cuttings in the second Zone was ca. 0.9 hr. The mean moisture content of the cuttings on leaving the dryer was ca. 0.0095% by weight, and the temperature was about 20 C. In view of the high temperatures involved, it was found to be expedient to use an inert gas such as nitrogen rather than air in connection with the drying of the polyethylene terephthalate.

As was mentioned above, the upper zone of the dryer functions to bring the `drying material rapidly to the intended drying temperature. Additionally, the upper zone also may predry and condition or merely condition the material depending upon the moisture content of the material. The conditioning of the material is an important feature of the process. In certain cases even an increase of the moisture content of the drying material in the upper zone is possible, as, for example, if a moist dust separator is used instead of the cyclone 22.

In general, with a suitable selection of conditions it is usually possible to achieve simultaneously a conditioning and predrying of the material. With the aid of the small partial current the level of the moisture content off the drying gas can be easily regulated in the upper part of the dryer, whereby the dryer can to a great extent absorb iiuctuations in the initial moisture content of the material. A special `advantage of the subject process, therefore, is that as a result of the above described characteristics of the process a continuous and economical drying is possible even when one is treating substances of great adsorption capacity.

The subject process is not limited to situations which require the use of an inert gas. Under corresponding circumstances as, for example, with high moisture contents, the process can be used with considerable saving where hot air is employed for the drying. In this instance, however, the small partial current would be replaced by a corresponding fresh air feed with a simultaneous release of the part of moist air at the upper end of the upper zone.

The process `of the subject invention is also applicable to all desorption processes with inert gases or superheated vapors as, for example, in the regeneration of active carbon, silica gel, and the like.

Obviously many modifications and variations of the invention as hereinbef'ore set forth kmay be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A continuous process for drying granular material which comprises: passing said material downwardly through a shaft dryer in a continuous flow, contacting said material with two circulating currents of inert gases which currents merge within said dryer and pass upwardly through said material removing moisture from said granular material, said currents thereafter leaving said dryer laden with vapor drawn from said material, said currents subsequently being divided, the smaller of said currents 4being cooled and treated to lower its moisture content and to remove oxygen and then being returned unheated to a lower zone of said dryer, the amount of moisture removed from said smaller current being the amount of moisture absorbed in the drying of said granular material, the larger of said currents being heated to a predetermined temperature and then returned to an upper zone of said dryer in a moist condition, the ratio of the larger of said currents to the smaller of said currents being in the range of from about 20:1 to 300:1, said granular material being heated to drying temperature -by said larger, moist current in said upper zone, and said material being iinally dried by contact with said smaller current as it flows from said lower zone upwardly through said dryer.

2. A process as in claim 1 wherein nitrogen is used as the circulating gas.

3. A process as in claim 1 wherein said granular material is a high molecular weight organic polymer.

4. A process as in claim 1 wherein the ow distribution within said dryer and the composition of the circulating gas are adjusted in accordance with the temperature of the gas leaving the dryer.

5. A process as in claim 1 wherein the amount of gas in the smaller partial current is regulated in accordance with the moisture content of the larger partial current.

6. A continuous process for drying granular material which comprises: lpassing said material downwardly through a shaft dryer in a continuous ow, contacting said material with two circulating currents of inert gases which currents merge within said dryer and pass upwardly through said material removing moisture from said granular material, said currents thereafter leaving said dryer laden with vapor drawn from said material, said currents subsequently being divided, the smaller of said currents being cooled and treated to lower its moisture content and to remove oxygen and then being returned unheated to a lower zone of said dryer, the larger of said currents being heated to a temperature of from 135 to 200 C. and then returned to an upper zone of said dryer, the amount of moisture removed from said smaller current being the amount of moisture absorbed in the drying of said granular material, the ratio of the larger of said currents to the smaller of said currents being in the range of `from about 75:1 to 225:1, said granular material being heated by said larger, moist current in said upper zone, and said material being dried by contact with said smaller current as it flows from said lower zone upwardly through said dryer.

7. A continuous process as in claim 6 wherein polyamide granules are ybeing dried and wherein the temperature of the larger of said currents is from about 140 to 165 C. as it enters said upper zone of said dryer.

8. Apparatus for drying granular material which comprises: a shaft dryer, said shaft dryer being divided into an upper and a lower zone, the lower end of each of said zones including an inlet opening designed to receive a current of gas; circuits exterior to said dryer for carrying currents of gas connecting each of said inlet openings with a common exit opening for said currents at the upper end of said dryer, one of said circuits connecting the inlet opening at the lower end of the upper zone of said dryer and said exit opening at the upper end of said dryer including heating means for regulating the temperature of the gas flowing within said circuit; the other of said l References Cited by the Examiner UNITED STATES PATENTS 2,410,309 10/1946 Simpson 34-168 X 2,475,984 7/1949 Owen 34-10 2,701,758 2/1955 Danulat et al. a 34-10 3,102,795 9/1963 Andrews et al. 34-36 3,112,188 11/1963 Zehnder 34--77 FOREIGN PATENTS 262,866 11/1949 Switzerland. 263,637 12/ 1949 Switzerland.

WILLIAM F. ODEA, Primary Examiner. NORMAN YUDKOFF, Examiner.

Claims

1. A CONTINUOUS PROCESS FOR DRYING GRANULAR MATERIAL WHICH COMPRISES: PASSING SAID MATERIAL DOWNWARDLY THROUGH A SHAFT DRYER IN A CONTINUOUS FLOW, CONTACTING SAID MATERIAL WITH TWO CIRCULATING CURRENTS OF INERT GASES WHICH CURRENTS MERGE WITHIN SAID DRYER AND PASS UPWARDLY THROUGH SAID MATERIAL REMOVING MOISTURE FORM SAID GRANULAR MATERIAL, SAID CURRENTS THEREAFTER LEAVING SAID DRYER LADEN WITH VAPOR DRAWN FROM SAID MATERIAL, SAID CURRENTS SUBSEQUENTLY BEING DIVIDED, THE SMALLER OF SAID CURRENTS BEING COOLED AND TREATED TO LOWER ITS MOISTURE CONTENT AND TO REMOVE OXYGEN AND THEN BEING RETURNED UNHEATED TO A LOWER ZONE OF SAID DRYER, THE AMOUNT OF MOISTURE REMOVED FROM SAID SMALLER CURRENT BEING THE AMOUNT OF MOISTURE ABSORBED IN THE DRYING OF SAID GRANULAR MATERIAL, THE LARGER OF SAID CURRENTS BEING HEATED TO A PREDETERMINED TEMPERATURE AND THEN RETURNED TO AN UPPER ZONE OF SAID DRYER IN A MOIST CONDITION, THE RATIO OF THE LARGER OF SAID CURRENTS TO THE SMALLER OF SAID CURRENTS BEING IN THE RANGE OF FROM ABOUT 20::1 TO 300:1, SAID GRANULAR MATERIAL BEING HEATED TO DRYING TEMPERATURE BY SAID LARGER, MOIST CURRENT IN SAID UPPER ZONE, AND SAID MATERIAL BEING FINALLY DRIED BY CONTACT WITH SAID SMALLER CURRENT AS IT FLOWS FROM SAID LOWER ZONE UPWARDLY THROUGH SAID DRYER.