ANNUAL REVIEW
PAUL Y . MoCORMICK
DRYING A N D DRYERS New applications of many dzfeerent ppes of d y e r s have been described his review encompasses the two-year period from the Tfirst quarter of 1962 through the first quarter of 1964. The greater part of the literature pertaining to drying published during this time consisted of articles of a descriptive nature. Very little new technical data were reported. The food industry continues to lead in the development and application of new drying techniques. There has been steady growth in the use of vacuum freeze drying for food processing, and new uses for spray dryers are being found. A commercial installation of the Canadian developed spouted-bed dryer is reported, while the unique characteristics of the foam-mat drying process, employed for the production of instant foods, receives continuing attention. I n paper drying, major emphasis has been placed on the development of radiant heaters and high velocity air jets impinging on the paper surface, to augment conventional cylinder drying. Additional installations of large-scale fluid bed dryers are reported. The development of efficient small fluid beds specifically for batch drying appears to offer promise for replacement of many of the tray and truck driers now employed for drying small lots of granular solids. Freeze Drying
I n two articles, Maguire (29) reviews freeze drying process requirements and the economics of a commercial freeze drying plant for meat. A plant of 50 to 75 tons per day, wet weight (equal to 18 to 27 tons per day of dry product) is said to be the most economical size. T h e total plant investment of about $2,000,000 will include eight 2000 sq. ft. vacuum chambers, as well as handling equipment, refrigeration, heating and condensing facil-
ities, and controls. Power requirements of 2.5 lbs. of steam and 0.28 kw. per pound of water evaporated, are cited. Labor is estimated at 32 man-hours per 3-shift day. Total drying cost is calculated at $1.98 per 100 lbs. water evaporated. I n connection with a freezedrying plant recently started up in Sweden (76), drying data are published citing product temperatures and moisture contents us. drying time for steak, chicken, halibut, and asparagus. Barrett (4) discusses the relative merits of steam jet boosters, and refrigerated condensers combined with mechanical vacuum pumps, for use on freeze dryers operating at 1.0 mm. H g pressure. No cost data are included. Harper (22) investigated experimentally the mechanisms of heat and mass transfer while drying frozen materials in the 0.1 to 1.0 mm. H g pressure range. Vacuum Drying
The development, in England, of a new vacuum tray dryer having an individual vacuum compartment for each tray is reported ( 8 ) . The trays are bottom-heated and need not be removed completely from the cabinet for recharging. Fischer (75) reviews the operation of the double-cone rotating vacuum dryer, emphasizing the desirability, during the initial drying stage, of maintaining the internal pressure in equilibrium with the liquid saturation temperature, as determined by solids temperature. Prevention of rapid surface-drying of solids prevents case-hardening, and improves the over-all cycle. Additional benefits contributed by heat-flow from wall to vapor to solids are claimed also. The commercial use of a. semicontinuous belt vacuum dryer for drying fruit juices, such as orange juice, is described by Conley (13), and the development of a 1/200th scale model of this dryer type, suitable for laboratory-scale studies, is reported (77). A laboratory apparatus designed for drying very fine powders is described by Solomons (49). Drying is accomplished by alternately evacuating and flushing the chamber with dry gas. A filter and bellows VOL. 5 6
NO. 1 0
OCTOBER 1 9 6 4
57
The food industry continues to lead the way in development and
seal arrangement prevents solids from entering the vacuum manifold during the gas flushing cycle, while the pressure in the container approaches atmospheric. Foam-Mat Drying
The foam-mat drying process involves through circulation drying of thin layers of stabilized foam b) heated air at atmospheric pressure. Foam is prepared from liquid foods or food concentrates by mixing with gas. Where required, a small amount of edible foam stabilizer is added. Rockwell (44) describes the design and operation of a 15-lb. hr. pilot-size foam-mat dryer which incorporates the recently developed “cratering” technique. Cratering is accomplished by blowing thin jets of air u p through the foam immediately after loading on the perforated drying trays. Drying rates are improved because of the increased porosity of the foam contributed by the cratering operation. Foam-mat drying has been employed successfully for tomato paste, citrus juices, apple sauce, and other food products. Ginnette (79) describes the drying of tomato paste by the foam-mat process. Commercial applications are discussed by Sjogren (47). Pneumatic and Spray Drying
A pneumatic conveyor-dryer employing inert, recycle gas for the recovery of solvent vapors is described by Hokinson (24). This “dispersion” dryer was developed originally for application to polyolefin products, to prevent oxidation and to recover flammable solvents. Plugflow of solids is claimed. The drying process is reported to be analogous to spray drying except for the differences in feed characteristics. The use of roller mills, cage mills, and high energy impact mills to speed pneumatic drying by more complete particle dispersion in the gas stream is reviewed by Aldrich ( 2 ) . Hannan (21) describes a specific cage mill “flash” dryer application. The equipment, in this case, is employed to predry filter cake and recover waste heat in a lime kiln installation. Gluckert (20) presents theoretical correlations of spray dryer performance, relating heat transfer to dryer geometry, temperature driving force, atomizer type, and spray particle size. Duration of flight and the heattransfer rate necessary to dry the largest particles form the bases of the derivations. These methods appear to yield conservative dryer designs-as the author concedes. I n three articles, Milwidsky (36) discusses spray drying of detergents. This discussion covers the effects of centrifugal pressure nozzles and disks on drop size distribution, the merits of cocurrent and countercurrent operation, and alternative meany for the control of moisture content by automatic instrumentation. Various detergent atomizing techniques are reviewed anon58
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
ymously (48). Speaking qualitatively, co-current drying yields large hollow beads. Countercurrent drying gives finer, more dense particles. O n the other hand, countercurrent operation promotes agglomeration, and also is preferable for materials which are difficult to dry. Spraying detergents from high-speed disks yields small hollow particles. Pneumatic atomizers produce particles of high specific gravity and wide size distribution; while a single fluid pressure nozzle gives a narrow size distribution of hollow beads. Patsavas (39) discusses spray drying in the food industry, including the application of cocurrent, countercurrent, and mixed flow spray dryers. Jet-spray drying, which achieves very fine droplets by use of a high velocity, two-fluid atomizer, is described by Bradford (6). Savings in dryer size and a reduction in drying time, compared to normal spray drying, are claimed. The atomizer cited appears to be a high power consumer, however, no economic comparisons are included in this article. Successful modification of a semiworks spray dryer was accomplished by Harper (23) for drying condensed whole milk and skim milk products. The unit gives product characteristics similar to those obtained in commercial size equipment and permits use of the dryer for research. Production of commercial-like products in small scale spray dryers is extremely difficult because small chambers usually preclude drying of the larger particle fractions produced by commercial atomizers. The successful employment of a centrifugal disk spray dryer for drying dye slurries, which yields products that can be packed directly from the dryer, is reported in another article ( 3 ) . T o pinpoint reasons for its recent wider acceptance in the chemical industry, Belcher (7) discusses spray drying in a series of two articles. Chamber designs, atomization techniques, and equipment costs are reviewed in detail. Fluid Bed Drying
The approach to perfect mixing achieved in fluid beds makes calculation of drying rates based upon the individual particle of little help in predicting performance, according to Quinn (43). The fluidized solids are maintained essentially in the dry state. High inlet gas temperatures can be employed because rapid heat transfer to the solids near the bottom of the bed results in immediate quenching of the gas. The heat is evenly distributed through the bed by intense solids mixing, and close control of bed temperature can be maintained. This thorough discussion of the fluid bed drying problem
AUTHOR Paul Y . McCormick i s Senior Engineer in the Chemical Engineering Section, Engineering Dept., E. I . du Pont de Nemours and Co., Wilmington, Del.
application of new drying techniques
includes design procedures, basic data requirements, gives approximate cost data, and discusses auxiliary requirements in detail. Agarwal (7) describes the installation of two 375-ton per hour fluid bed coal dryers. The units handle 0.25 inch to 100 mesh coal, initially containing 9% to 14% water. Inlet gas temperatures of 900' to 1200'F. are employed. The article describes the various start-up problems encountered, their solutions, and includes complete operating data from the equipment. A 200-ton per day lime drying and calcining plant, employing a five-stage "Fluo-Solids" reactor, is described by Trauffer (50). Base1 (5) reviews the development of a fluid bed polypropylene dryer which employs superheated solvent as the drying medium to recover hexane and isopropyl alcohol. Control of particle size in fluid beds by the simultaneous introduction of feed solution and seed particles, and the continuous withdrawal of agglomerates, is reported by Metheny (35), T o accomplish particle classification, the lower bed section was reduced in cross-sectional area; the nominal bed diameter employed was 12 inches. Considerable attention is devoted to feed atomization. A steam jacketed pneumatic atomizer, a whirljet nozzle, and a DeVilbiss atomizer were employed. Data included pertain to the drying and agglomeration of ammonium sulfate, sodium cyanide, and calcium chloride. The best results were achieved when treating ammonium sulfate. Particle size appears to be a function of seed size, bed diameter ratio, and the solution-toseed ratio. Very little attention had been given previously to the use of fluidized beds for size enlargement, although the classification application is well known. The recent availability of small, batch, fluid bed dryers has created interest in the pharmaceutical industry which has been, in the past, a large user of batch tray dryers. Scott (46) reports the results of tests comparing the benefits of fluid beds and tray dryers for drying tablet granulations. The former was found to give drying rates 15 times faster than tray dryers. Data are included giving total drying times, rates, and over-all heat transfer coefficients, obtained in each unit. Spouted Bed Drying
Investigations continue employing this specialized equipment, suitable for solids which are free flowing, but too coarse to be truly fluidized. Malek (37) investigated bed-to-wall heat transfer in 3-inch and 6-inch spouted columns, using a variety of materials. The heat transfer coefficient was found to be independent of air flow; it increases with increasing particle heat capacity, but unlike behavior in fluid beds, it decreases as particle diameter decreases. Under similar operating conditions, the heat transfer rate in a spouted bed was found to range
from 10% to 66% of that obtained in a fluid bed. A generalized correlation is derived, expressing heat transfer in terms of modified Nusselt and Reynolds numbers. Air was the spouting fluid employed in all cases. Malek (30) also measured spout diameters with 8 different materials in two semicontinuous spouted beds, 4 inches and 6 inches in diameter. He found that the spout diameter is proportional to the square root of the air mass rate, and he developed an empirical correlation to express spout diameter as a function of bed diameter and air flow. The first commercial spouted bed dryers have been installed in Canada, and are described by Peterson (40). The beds are 2 ft. in diameter and 7 ft. deep, and have been used to dry peas, other lentils, and flax. Air temperatures up to 800' F. are employed, but in no case does product temperature exceed 175" F. Currently, most work is being directed toward drying of vegetables. No serious particle attrition problems have arisen. Direct Heat Rotary Drying
Current knowledge regarding the mechanism of drying in direct heat rotary dryers is reviewed by Saeman (45). Theoretical and practical equations describing material transport, heat transfer, and pressure drop are discussed and scale-up procedures proposed. Heat transfer rates in direct heat rotary dryers, as influenced by gas velocity, are discussed by McCormick (32). Conclusions reached by these two authors are not in complete agreement. Porter (42)in a third article on the same general subject emphasized the fact, which is true in most cases, that moisture movement and thermal diffusion rates within the solids play a significant role in most rotary dryer processes. In his study, this author employed the dryer as a cooler, assuming that the heat transfer problem during cooling is approximately analogous to mass transfer during drying. The development of an internal screw conveyor for solids recycle in rotary dryers is reported also (9). Paper and Textiles
Maahs (28) describes the results of a digital computercalculated, multiple regression analysis employed to examine drying data taken from 26 commercial Yankee dryers. The work illustrates the use of this statistical method for data analysis. I t also emphasizes the need for more carefully controlled and standardized data collecting procedures. Hultgreen (25) discusses the benefits of combined radiant heat and jet impingement drying for paper. Radiant energy of 3 to 6 p wavelength is recommended for paper preheating and the removal of the final 20y0moisture, while heated cylinders and jet impingement are recommended for evaporation VOL. 5 6
NO. 1 0
OCTOBER 1 9 6 4
59
of the bulk of the water during constant rate drying. The importance of employing radiant energy of the proper wavelength, and limiting the heat flux during final drying are emphasized. I n connection with the use of air jets to augment the capacity of Yankee dryers, Gardner (78) developed relationships correlating evaporative heat demand, heat transmission from the cylinder, and mass transfer rates in the air system. The relationships were evaluated on an analog computer. The addition of high velocity air jets is claimed to have the potential for increasing paper drying rates as much as 50%. Based upon field tests, Villalobos (52) considers various methods for evaluating high velocity jet impingement dryers installed on paper machines, including a heat balance on air flow, output rate at constant moisture, and evaporation across the high velocity sections. Means (34) summarizes performance characteristics from 16 high-velocity jet impingement dryers. Drying rate dependence on nozzle velocity is clearly indicated, although the data are confused by the fact that several of the high velocity hoods were not properly placed on the dryer sections so as to achieve the maximum advantage of impingement effects. Methods for calculating heat flux on steam heated cylinder dryers employing the temperature-drop across the cylinder face are discussed by Cirrito (70) who, in a different article ( 7 7), emphasizes the effect of heat storage in cast iron cylinders for minimizing cylinder temperature change during each revolution. I n a very general and qualitative manner, Cirrito (72) also reviews the effects of temperature and air velocity on heat and mass transfer rates in jet impingement dryers. A series of eight articles on the drying of paper and paperboard, summarizing most current knowledge, was initiated anonymously (37). Comprehensive literature references are included. A fluidized bed of sand employed to dry paper has been successfully operated in England (74). The expectation that wet paper would be damaged by immersion in a bed of “boiling” sand was not realized; the paper surface was undamaged. There was no sand adhesion, and the paper was thoroughly dried. Pilot scale development is continuing. General
I n a comprehensive article, Parker (38) outlines routes leading to selection and specification of solids drying equipment. Although it is general in nature, the important considerations which must be Siven to the solids material handling problem during dryer selection are properly emphasized. A dryer selection guide is included in tabular form, which is useful for high-spot evaluations. Safety aspects in the operation of coal dryers are reviewed by McMorris (33). The use of recycle gas to reduce oxygen levels, installation of fog nozzles, and the maintenance of clean, dust-free conditions are among the items stressed. Specifically, in connection with coal drying, Leonard (27) presents nomographs which permit the rapid calculation of the relative humidity and 60
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
dew point of the dryer exit air on the basis of feed rate, air rate, and fuel consumption. Use of the nomographs is recommended to achieve minimum bed temperatures and maximum exit humidity for maximum thermal efficiency and operating safety. Uh1 (57) summarizes up-to-date knowledge in connection with indirect drying in agitated vessels. Pan dryers, horizontal ribbonagitator dryers, screw conveyors, and double cone dryers are discussed. Information pertaining to specific applications, scale-up and operating procedures, and power requirements are included, together with specific operating data, including heat transfer rates, from a number of different products. Lazar (26) reports the results of partial drying of fruits before steam blanching, which renders them similar in appearance and palatability to their sun-dried counterparts. Latest developments in textile sheet goods dryers, including the use of drying cylinders, high velocity jets, and radiant heat are reviewed by Pinault (47). LITERATURE CITED (1) Agarwal, I. C., Davis, W. L., Jr., King, D . T., Chem. Eng. Progr. 58, 1185-
90 (1962).
( 2 ) Aldrich, R. J., Ibid., 6 , 62-6 (1962). (3) Am. Dyestuff Reptr. 52, 5, 185 (1963).
(4) Barrett, J. R., Laxon, R., Webster, P. H . N., Food Tech. 18, 1, 38-41 (1964). (5) Basel, L . , Gray, E . , Chem. Eng. Progr. 58,6, 67-70 (1962). (6) Bradford, P., Briggs, S. W. Ibid., 59, 3, 76-80 (1963). (7) Belcher, D. W., Smith, D . A,, Cook, E. M., Chem. Eng. 70, 20, 83-8 (1963); Ibid., 70, 21, 201-8 (1963). ( 8 ) Chem. Eng. 69, 5, 86 (1962). ( 9 ) Chem. Eng. 69, 24, 64 (1962). (10) Cirrito,A. J., PaperZnd. 44, 7, 397-401 (1962). (11) Cirrito,.4. J., Zbid., 44 8,469-70 (1962). (12) Cirrito,h. J., Zbid,,45, 8, 433-5 (1963);’10, 548-50 (1964); 11, 599-601 (1964) (13) Conley, W., Griggs, T. W., Food Can. 22, 6, 26-9 (1962). (14) Engineering 193, 5005, 415 (1962). (15) Fischer, J. I., IND.ENC.CHEM.5 5 , 2 , 18-24 (1963). (16) Food Eng. 35,11, 54-6 (1963). (17) Ibid., 36,2, 100 (1964). (18) Gardner, T., Tnppi 47,4, 210-14 (1964). (19) Ginnette, L. F., Graham, R. P., Miers, J. C., Morgan, A . I., Food Tech. 17, 6, 811-13 (1963). (20) Gluckert, F. A,, A.I.Ch.E. J. 8,4,460-6 (1962). (21) Hannan, P. J., Toppi 47, 2, 162A-3A (1964). (22) Harper, J. C., A.Z.Ch.E. J . 8, 3, 298-302 (1962). (23) Harper, J. M., Food Tech. 16, 9, 140-4 (1962). (24) Hokinson, A. E., Baran, S. J., Chem. Eng. Progr. 59, 1, 92-3 (1963). (25) Hultgreen, O., T a p p i 4 5 , 6 , 178A-86A (1962). (26) Lazar, M . E., Barta, E. J., Smith, G. S., Food Tech. 17,9, 120-2 (1963). Sandy, E. J., Coal Age 67, 3, 94-8 (1962). (27) Leonard, J. W., (28) Maahs, H . G . , Johanson, L. N., Tappi 45,10,760-5 (1962). (29) Maguire, J. F., Food Eng. 34,8, 54-5 (1962); Zbid., 12, 90-1 (1962). (30) Malek M. A. Madonna, L. A,, Lu, B. Y . , Ind. Eng. Chem. Process Desi,cn Develop. 2: 1, 30-4(1963). (31) Malek, M, A,, Lu, B. C.-Y., Can. J . Chem. Engr. 42,1, 14-20 (1964). (32) McCormick, P. Y . , Chem. Eng. Progr. 58, 6, 57-61 (1962). (33) McMorris, W. L., Coal Age 68, 1, 88-90 (1963). (34) Means, J. A,, Tappi 45, 6, 527-8 (1962). (35) Metheny, D. E., Vance, S. W,, Chem. Engr. Prog. 5 8 , 6, 45-8 (1962). (36) Milwidsky, B. h?., Soap Chem. Specialties 38,10, 59-62 (1962); 11, 56-60 (1962); 12,73-76 (1962). (37) Paper Trade Journal, 35-43 (Feb. 17, 1964). (38) Parker, N. H., Chem. Eng. 70, 13, 115-19 (1963). (39) Patsavas,A. C., Chem. Eng. Progr. 59, 4, 65-70 (1963). (40) Peterson, W. S., Con. J . Chem. Eng. 40,5,226-30 (1962). (41) Pinault, R . W., Textile World 113, 12, 98-106 (1963). (42) Porter, S. J., Znst. Chem. Eng. Trans. 41,272-80 (1963). (43) Quinn, M. F., INn. ESC.CHEM.55,7, 18-24 (1963). (44) Rockwell, TV. C., Lowe, E., Morgan, A. I., Jr., GraLam, R. P., Ginnette, L. F., Food Eng. 34,8, 86-8 (1962). (45) Saeman, W. C., Chem. Eng. Progr. 58, 6,49-56 (1962). Lieberman, H. A,, Rankel, A . S., Chow, F. S., Johnston, (46) Scott, M. W,, G. W., J. Pharm. Sci. 52, 3, 284-91 (1963). (47) Sjogren, C. N., Food Eng. 34, 11, 44-7 (1962). (48) So@ Chem. Specialties 38,2, 147-9 (1962). (49) Solomons, C., Schlosser, G., Rev. Sci. Instr. 33, 11, 1287-8 (1962). (50) Trauffer, W.E., Pzt Quarry 54, 11, 126-8 (1962). (51) Uhl, V. W., Root, W.L., Chem. Eng. Projr. 58, 6 , 37-44 (1962). (52) Villalobos, J. A,, Tappi 45, 5, 429-32 (1962).
