INDUSTRIAL AND ENGINEERING CHEMISTRY
38
(14) Dayton, B. B., INU. ENC.CHEM., 40, 795-803, 1948. (15) Distillation Products, Inc., Brit. Patent 595,096 (Nov. 26, 1947). (16) Dryer, W. B., Chem. Eng., 54, 122-4 (1947). (17) Ibid., 127-31. (18) Dushman, S., Chemistry & Industry, 1948, No. 41, p. 53. (19) Dushman, S., IND.ENC.CHEM., 40, 778 (1948). (20) Dushman, S., “Scientific Foundations of High Vacuum Technique,” p. 882, New York, John Wiley & Sons, 1949. (21) Epstein, S., Proc. Conf. Chem., Chern. Inst. Canada, p. 108-16 (1947). (22) Fawcett, E. W. M.,Chemistry & Industry, 1948, No. 41, pp. 527-33. !23) Ferris, S. W., Lampson, E. R., and Smith, D. M. (to Atlantic Refining Co.), U. S. Patent 2,447,746 (Aug. 24, 1948). (24) Fletcher, G. L., Lasalaco, M., Cobler, J. C., and Hodge, H. C., Anal. Chem., 20, 943-8 (1948). (25) Fluke, D., Reu. SCi. Instruments, 19, 665-6 (1948). (26) Fox, K., Stauffer, R. A., and DiPietro, W. O., Iron Age (Feb. 19, 1948). (27) Godley, P.,Ibid. (April 1, 1948). (28) Gold, iM.H., Anal. Chem., 21, 636-7 (1949). (29) Hickman, K.,, IND.ENG.CHSM.,40, 16-18 (1948). (30) Hickman, K. (to Distillation Products, h e . ) , Ti. 8. Patent 2,455,059 (March 29, 1949). (31) Ibid., 2,465,590 (Nov. 30, 1948). (32) Hobby, A. C., Ibid., 2,443,070 (June 1948). (33) Huebler, J., IND. ENG.CHEY.,40, 825-32 (1948). ENG.CHEM., 40, 778-847 (1948). (34) IND. (35) Jacobs, R. B., Ibid., 791-4. (36) Jacobs, R. B., and Kappf, S. E., Ibid., 842-5. (37) Jaeckel, R., and Oetien, G. W., Chsmie-Ingenieur Technick, 21, 169-76 (1949).
PACED
by symposia on drying in the Ketherlands, Great BritainL,and the United States in the past year, the quantity and, in general, the quality of literature on the unit operation oFdrying increased markedly. A number of fundamental investigations were reported as well as the usual number of descriptive and review articles. Because of this increase i t was impossible to cover the literature in this short review article as thoroughly as it has been covered in past years. Only about 35% of the available literature is discussed although every effort was made to cover all of the important developments. For those people who are interested. a, list of the articles on drvine. not covered by this review, has been prepared and filed with the American Documentation Institute. I
Y l
INFRARED DRYING
Once again this method of drying, which employs radiant energy of the infrared wave length for supplying heat to the material being dried, received, in number of words printed, greater attention than any other drying method. Most of the literature was, however, descriptive and still posed the same arguments of: (A) convection against radiant heat drying; (B) gas-fired against electrical installations; and (C) short against long wave-length radiation. It is becoming more generally conceded, however, that except for paints, other surface coatings, and some specialty items, the principal role of infrared energy will be in assisting or boosting the capacity of other drying methods. It is particularly advantageous to combine convection and radiation drying in a single unit. Poeschmann (161) and Malins (140) presented general reviews of the mechanism of radiant heat transfer and some of the
Vol. 42, No. 1
(38) Kraft, W . W., IND. ENG.CHEM.? 40,807-20 (1948). (39) Laurent, P., and Duhavel, M. J., Conapt. rend., 225, 674-6 (1947). (40) Louis, M., Rev. inst. franq pQtrole et A n n . combustibles liquiderr, 3, 192-8 (1948). (41) Madorsky, S . L., and Straus, S.,IND.ENG.CHEY.,40, 848-53 (1948). (42) Mellen, G. L., Ibid., 40, 787-90 (1948). (43) Morse, Richard, Chemistry & Industry, 1948, No. 41, 13-18. 40, 783-6 (1948). (44) Normand, C. E., IND.ENG.CHEM., (45) Olifant, M. L. E., Chemistry & Industry, 1949, No. 6, p. 8h (46) Rose, A., Anal. Chem., 21, 81-4 (1949). (47) Roy-Pochon, C., L e Vide, 2, 333-48 (1947). (48) Sohroeder, A. L., and Colton, E . H., IND.ENG.CHEY.,40, 803-7 (1948). (49) Schroedes, A. L., and Schwartz, H. W., Chem. Eng. Progress 45, 370-6 (1949). (50) Schwartr, €1.w., IND. ENG.CHEM., 40, 2028-33 (1948). (51) Shaler, A. J., and Wolf, W., Ibid., 838-42. (52) Stauffer, R. A., Am. Inst. M i n i n g M e t . Engrs., Tech. Pub 2268, 1-10 (1949). (53) Stauffer, R. A., Chemistry & Industry, 1948, No. 41, pp. S19-26. (54) Stauffer, R. A., Fox, K., and DiPietro, W. O., IND.ENG.CHEM., 40, 820-5 (1948). (55) Sullivan, H. M., Rev. Sci. Instruments, 19$1-15 (1948). (56) Swallow, F. C., and Gourlay, F. J., Chemistry & Industry, 1948, NO.41, pp. 59-12. (57) Tucker, W . H., and Sherwood, T. R.,IND.ENG.CHEM.,40, 832-8 (1948). (58) Weingartner, H. C., Ibid., 40,780-2 (1948). (59) Whitehurst, B. W., Chem. E-ng.. 54, 98 -102 (1947).
RECEIVED October 10, 1949.
general applications. Other general articles on electric infrared heating and drying applications were published by Goodell (92), Dume (Yd), and SIanders (141). Factors influencing electric infrared tunnel design ($5)and safety precautions ( 1 6 ) have been discussed. Perhaps two of the best papers discussed factors inkluericing the infrared drying of textiles. One of these by Paul and Wilhelm (159) reports the results of a study of the effects of air humidity, velocity, and temperature, intensity of radiant eneigyt sample thickness, and optical properties of the material dricd on the drying of woven and matted wool felts, woven wool crepe. and absorbent cotton batting. The results of this study confirmed the results of other workers in the field-namely, that the mechanism of drying by infrared radiation is similar to that of convection drying. Some penetration of infrared rays into the textile samples was noted. Complementing this investigation was another by Wilhelm and Smith ($88) in which the transmittance, reflectance, and absorptance of near infrared radiation by textile materials was studied. Factors influencing these properties were quantitatively evaluated and theoretical relations were developed. Dyes had a pronounced effect on the transmittance. Casey and Feil(57) presented codt information for the infritred drying of textiles based on actual installations and compared these costs with the cost by other drying methods. The advantages of predrying textiles and paper with gas infrared heaters were outlined by Van Kampen ($18). Garland (85) claimed that gas-fired infrared units are an inexpensive means for increasing the capacity of existing paper dryers if they are installed a t the wet end. The vast is said to be much less than electric infrared
January 1950
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INDUSTRIAL AND ENGINEERING CHEMISTRY
or dielectric heat and approaches that of steam. Articles on the use of electric infrared heat for drying paper are also available ($4, 70). Uchastkina (207) presented data on the drying of paperboard. The application of infrared drying to the ceramic industry is apparently restricted to relatively thin pieces. Mackadam (137) concluded from experiments that 1.25 inches is about the maximum thickness of ceramic material that can be dried without %eriousshrinkage or warpage. Yarham (2S4) claimed that in the drying of refractories with infrared heat the moisture seemed to migrate away from the hot surface. This is contrary to the findings of other investigators. He also claimed that only relac tively thin pottery could be satisfactorily dried. Successful drying of tile ( 7 )and bricks (4) was reported. Little information on paint baking and drying by infrared radiation was presented in the past year, The best discussion was that of Lohausen (134). Other articles (12, 15, 18, 23, 144) primarily dealt with specific applications and presented little or no data. Patel, Jenkins, and DeKay (158) presented information on the drying of pharmaceutical tablets and stated that infrared drying appeared to be a promising method. Deribere has described the application of electric infrared heating t o the plastics industry ( 7 l ) ,and to the drying of some chemical products (72). Rugs have been successfully dried with radiant heat ( 6 2 ) and a combination of radiation and convection heating (22). Applications to the drying of glue (143), dry extracts (46), clothes (14), and ink (19, 181) were also reported. SPECIAL DRYING METHODS
b
S o surprisingly new drying methods were reported in the past year. Work in fields of high-frequency dielectric and low-temperature drying was continued, however, in an attempt to increase the possible fields of application for these expensive drying methods. High-Frequency Drying. This drying method, which depends on generation of heat within the solid from dielectric losses when the solid is placed in a high-frequency alternating electrical field, has showed greatest promise for the drying of bulky materials since heat is supplied within the solid without having to diffuse through it from the outside. A number of general articles describing the theoretical basis for this method of drying, the equipment available, and some typical applications were published in the past year (41, 73, 81, 117, 16S, 186, 227, ,931). Perhaps the best are those of Appell (SO), Duryee (76), and Stephens (196). Kinn (122) presented charts for estimating the cost of drying with dielectric heat. In a pioneering paper Mann, Ceaglske, and Olson (142) studied the removal of water and other liquids from trays of sand by dielectric drying. They found the same mechanisms of drying to exist with dielectric drying as with convection and infrared drying-namely a constant- and falling-rate period which could be related to the rate of moisture removal from the surface of the solid being dried. Large grains were found to dry more rapidly than smaller ones. This latter phenomenon agrees with resuIts obtainpd by Jelinek et al. (114) who found that the heating rate of crushed silica depended on the particle size of the sample. Both a minimum and a maximum heating rate were noted over the range of particle sizes which they studied (235 t o 3530 p). More studies like these last two (114,14S) are needed in this field. A number of different applications of dielectric drying were reported. LeBihan and Spanjaard ( B O ) indicated its use in the drying of ceramics and impregnated sheet materials. Story (196) has described actual and possible applications in the textile field. Possible use for predrying on slashers was suggested. Casey and Feil (67) reported costs for its use in the drying of textile fabrics and concluded that it would be very costly. Dielectric drying of foundry cores continues to be reported (8).
39
Bord and Pound (46) found that drying times of cores could be reduced from 45 to 2 minutes with consequent space and inventory savings. Bikin et al. (.@) described the drying of pharmaceutical tablets. The drying of sugar (I.%), varnish (88) rubber latex (MI), and wood (160)was also reported. Sublimation Drying. This method of drying, which depenctp on subliming the moisture from a frozen material under conditions of high vacuum, is best known for its application t o the drying of pharmaceutical products such as blood plasma, peni cillin, and streptomycin. I t s use appears t o be generally r e stricted t o expensive, heat-sensitive materials. Greaves (94) and Chambers (58) have reviewed the progress made in thie drying technique. The only truly technical paper applicable to this method of drying was published by Carman (54), who studied the theoretical aspect8 of evaporation and condensatioo under high vacuums. He concluded that with sublimation drying a t temperatures of -20 to -30' C., the gap between the evaporating and condensing surfaces was not critical, and that extremely low pressures were not necesmrily required to obtain relatively rapid evaporation rates. Schwara (184)has described pilot plant installations for drying orange juice and coffee. Orange juice is sprayed on the wall of a vertical cylinder maintained a t low vacuum, and the dried product is continuously removed by scraper blades. Coffee WE*, dried by spraying the solution on a continuous, moving belt which was heated by radiation. The whole assembly was maintained in a vacuum housing. In another article (182) the probable cost of operation of typical dryers for coffee by the above method was reported. It was claimed that 200 pounds per hour of water could be removed a t a cost of 32 cents per pound of water evaporated. This same dryer with operahing details and costs was described by Hellier (101). Tucker and Sherwood (206) studied liquid desiccant systeniv applicable to sublimation drying units. Wetted-wall absorbers have a higher capacity per unit volume of absorber liquid, but spray towers will cost less. Either is suitable. A emall laboratory freeze dryer has been described (20). O
OF SPECIFIC MATERIALS Drying of Textiles. Except for the work of Wilhelm arid Smith (228) on the transmittance, reflectance, and absorptance of infrared radiation by textiles and that of Paul and Wilhelm (169)on the radiation drying of textiles, the only fundamental work in this field was a study of the moisture determination of textiles with electric meters, by Toner et al. (206). They found it necessary to calibrate every instrument for the material and condition of use to obtain accurate results from any of the meters which were tested. Heddergott (99) studied the effects of different commercial dryers on the physical properties of dried wool. He concluded that a conveying screen dryer was the best dryer. Feubel and Hilgers (78) concluded that it was feasible to dry viscose and cuprammonium rayon a t temperatures as high as 270' C., if the drying is uniform and properly controlled. Morningstar (147) and Seydel (186, 187) have reviewed existing slashing methods. Story (196) has suggested that dielectric drying may be applicable to slashing as well as the drying or setting of cotton, wool, and nylon. A new hot air slasher was briefly reported ($9)along with a fixed-roll sheeting dryer capable of high evapoiative capacities. A direct, gas-fired, festoon dryer, operating at up to 450" F. with slot velocities as high ab 4000 feet per minute, was described by Rabold (166). Van Kampen (212) discussed the use of radiant burners for predrying textiles. Passing heated air through wool felts instead of over them t o accomplish dPying was said t o reduce the drying time from 45 t o 6 minutes (S), One of the most interesting new developments was an all-purpose drying unit for textiles, described by Wall (281). This unit can be mounted almost any place and can be used t o provide high velocity, infrared, or low temperature drying. DRYING
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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
Mcli'arlane (190') brierly discussed the use of tenter dryers for tinishing rayon fabrics. New, two-pass, pin-tenter dryers incorporating all of the latest advances in design were described in the past year (26-68, 164). The results of a survey on the cost of drying textiles materials were published in the past year by Casey and Feil (66, 6 7 ) . Cost information was presented for can *dashers, tenter dryers, infrared, dielectric, and vacuum dryers. Walker (218) and Zeller (856) described the use arid need for controls on textile dryers. Drying of Paper. No truly funda.nienta1work on paper drying was published during the past year. Karpinski (116) reviewed the general theory of drying paper witjli hot air showing how a Mollier diagram can be used to describe the course of drying. Young (856) described different, paper dryers, the mechanics of moisture removal, and stressed the need for low initial moisture content, high steam pressure, and good contact for eEcierit drying. Heat losses during the drying of paper and methods of reducing heating loads were discussed by Remess (169) a,nd Montgomery (146). h good analysis of the possibilities of using h e heat-pump principle in conjunction. with paper dryers was given by \;l'oods (236). The theoretical heat savings are extrem.ely large, but practical methods of accomplishing the savings are not a t the moment available. How high pressure condensate drainage from steam cylinders was effectivc in decreasing ciyying costs was shown by Thus (203). Van Kampen (616) and Garlaiid ( 8 6 ) shoivod how gas-fired infrared burners could be used to boost dryer capacity. A gas-fired burner impinging a flame directly on t,he paper at ~ , h c wet end was said to increase the capacity oi a 26-can dryer by 15% (175). The exhaust gases from the burners were used in other dryers on the plant. Electric infrared units for drying paper were described by Barber ( 8 4 )and Deribere (70). Ucliastkina (207) gave experinaent>alresults on drying paperboard n.it,h electric infrared units. Rutt (179) described an efficient humidifying unit for contlit,ioning paper as i t leaves the dryer. A review of the existing methods for drying coated paper was prepared by Major (159). Techniques for drying paperboard were briefly discussed b r Saechtling (180). Heated air increased the capacity of one installation of cylinder dryers on paperboard by 10% (21). Heated air over the surface of a Yankee dryer used for creped paper increased the capacity as much as 30% (800). Holden (106) gave a, thorough description of the drying of fiberboard in roller dryers. Some operating data were presented. Argy ( 3 2 ) reviewed t,he operation of the "spiral dryer" and suggested its application to the drying of coated paper. Drying of Wood. Hyler (116) described the prohiems involvcd in the drying of veneers, the various types of dryers, and the recent improvements. Costs and drying times of woods in dry kilns of various designs were reported (56). The principles, methods, and equipment applica.hle to the drying of timber wcrc published by Bateson (39). Drying of Food. Food drying received hardly any attention in the past year, indicating that except for some foods not directly purchased by the housewife, this method of preserving foods has met with little success. Heiss (100) revjewed the technological progress in vegetable drying and concluded that t,he processes prior and subsequent to drying had more effect on the quality of the dried product than the drying conditions or method. fIukill (100) reviewed the different types of farm grain dryers and recommended mechanical forced-draft units. Claydon (61) described an electrically heated dryer for grain. The movement of heat and moisture in stored grain was studied by Oxley (164). Steam-tube dryers were reported applicable to the drying of potato flour, meal, and feed (11). Drying of Ceramic Materials. In addition to the application of infrared drying (157, %94), thc drying of ceramic mat'erials
Vol. 42, No. 1
claimed considerable attention in the past. year. An excellent review of the literat,ure on clay-water relations was prepared by Williamson (629). liobinson ( 1 7 3 ) and Garve ( 8 7 ) discussed the theory of drying and types of dryers involved in drying ce'. A very careful and thorough investigation of the movenieirt of moisture in clay columns subjected to a temperature gradient was reported by \-assiliou and White (816). The results of t>hiswork indicated that the moisture moved through the c h y by an evaporation-condensation procedure. This information is applicable to the lnttnr stages of clay drying and to t,he tray drying of most g r a n i h r materials. Chiahski1 (60) developed a,n expression for the maximum safe drying rate of clayware. An expression for the diffusion coefficient of water in clxy was presented. Kovel'inan (126) found that nonkaolinitir materials decreased the drying rat,e of porcelain ware. Addit.ion of steam and sulfur dioxide to the gases used to dry bricks was i.eported to permit the use of higher. drying temperatures wit.h resultant decreased drying timer! (17f ). Factors influencing eb.e crnclting of plates during firing n-ere investigated and reported (1661. Cox ( 6 4 ) investigated the possibility of using preheated clay rather than high humidity ttir 3%-hendrying clayware. Garve (86) described a new continuous chamber kiln for ceramics and Thompson (201) slmved how the effective use of gas heating could materially reduce drying times. Controlled drying was reported to reduce the drying time of zinc retorts from 46 to 4 days (84). Rowden (178) indicated methods for reducing the power required for drying clays and refractories. Proper use of excess air was said t o increase tunnel-kiln production (156)~ Shorter (189) measured t,he theoretical heat required for firing of clays. Examples of the use of radiant heat from the outside of lcilns for useful work were reported ( 8 ) . Forbes ( 7 9 ) presented an over-all review of drying and di,yers for clayware. Drying of Leather. Very little new data on leather drying was made available in the past year but three good reviews of t,he subject were published. Buck (51)and Dempsey ( 6 9 ) reviewed the various methods of drying leather and discussed the effects of teniperaLure and humidity on tlie drying rate. Cos (66: rcviswed the progress in the methods for pasting leather for the drying operation and recommended glass plates as tehc best support. Kanagy and Charles (115) pointed out some of the deficiencies of the exist,ingreconinionded method for determining moisture in leather. Drying of Coal. The best work on coal drying in the past year mas that of Parry et al. (166) who studied the drying of coal iu a pneumatic conveying dryer a,nd a fluidized-solids dryer, both of semiworks size. Considerable data and design information ivsre presented. The pneumatic conveying dryer handled coal 16mesh or smaller at S5y0 thermal efficiency, and the fluidizedsolids dryer handled S-mesh coal at 35 to 90% efficiency. Kramer and McKee (128) reviewed the different methods available for the drying of coal. A brief discussion of some of the theoretical aspects of coa,l drying was prcsented by Vissac (817). A novel coal dryer called a Verti-Vane dryer h a s hcen ciescribed (11). Drying of Finishes. A. few articles in addition to the ini'rured drying of paints (12, 1,5, 18, 25, 134, 144) and printing inks (19, 181) Ivere available in this field. Baird ( 3 3 ) and Rohrnan ( 1 7 4 ) discussed high-velocity, direct gas-fired convection ovens for drying of paints on car bodies; these offer coinpetition to the infrared units. One of the these was installed by the Ford Motor Company which pioneered in the adoption of electric infrared dryers for this service. Wampler (883-886) discussed t,he drying of several types of finishes on wood and the difficulties which may be encountered. Walker (219) stated that the paper on which i t is used has the most marked effect on the initial set and final drying of printing inks. Drying of Foundry Materials, Forslund (80) has reviewed the various types of ovens available for drying cores and molds and discussed the theordcal and economical aspects of each type.
'
January 1950
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INDUSTRIAL AND ENGINEERING CHEMISTRY
41
flow of liquid leaving a rotating disk and determined the factors Roll and Arland (176) studied the effect of air velocity on the which controlled the type of flow to be obtained. compressive strength of cores containing various binders. The Herman et al. (103) and Ridgway et al. (170)described the strength of the core increased with increasing air velocity. spray drying of distillers’ solubles in a spray dryer employing a Dielectric drying of cores has already been discussed ( 6 , g ) . pressure nozzle. Spray drying of rubber latex was reported Morton (148) reviewed the various types of dryers available (14). The theory and practice of spray drying and its applicafor cores, molds, and sand. Smith (191)made some additional tion to the milk industry were reviewed (77). commenta. Kesper (118) discussed the advantage@ and disPneumatic Conveying Drying. I n this method of drying, the advantages of a number of different dryers commonly used for solids are suspended in an air stream and finally removed when drying foundry sand. dried. The drying of coal in an ordinary pneumatic conveying Drying Miscellaneous Materials. Goldstein (91) described dryer and a fluidized solids bed has already been discussed (166). the drying of sewage sludge in a Roto-Louvre dry Gordon (93) has presented a very general article describing one methods of drying sludge were discussed by Howard type of pneumatic conveying dryer commonly used in this Kozma (117). Bixby (44)presented extensive data on the drying country. Barr ( 8 7 )reports a “ring” dryer which is essentially a of synthetic rubber granules in a continuous-sheeting, fixed-roll closed circuit pneumatic conveying dryer which permits material dryer. Extremely high capacities were obtained by using high to be maintained in the air stream for considerably longer periods air velocities. Partridge and Hansen (167‘)illustrated how genof time than are possible with conventional pneumatic conveying eral drying principles could be applied to the drying of dipped dryers. The maximum retention time in an air stream is oblatex forms. Suggestions for ensuring optimum operation of a tained when a fluidized solids bed is used for drying as described continuous flaker and conveying screen dryer for soap chips by Wall and Ash (120). Freiday (86) and Tamblyn (198)dewere made by Thomssen (202). Optimum conditions for the scribed the use of pneumatic conveying drying for bark. spinning of cellulose acetate fibers were established by Araki and Rotary Drying. A great deal of work with this type of dryer Tadenuma (31). Countercurrent drying of sodium sulfate on a was reported in the past year. Friedman and Marshall (83) continuous belt with an inlet air temperature of 300 O C. has been reported the results of an extensive study of the factors influencing described (119). Different types of dryers for clay for installation a t the point of mining were described by Vainrub (110). .the holdup, dusting, heat transfer, and drying in ordinary rotary dryers. Van Krevelen (118,215) studied the rate of drying of Sievers and Clarke (190) found that different drying methods and fertilizer granules in a rotary dryer and concluded that diffusion temperatures for sumac had little effect on the tannin content of moisture from the inside of the granules controlled the drying but did cause variation in product quality. rate. Vahl (108)and Strelov (197)proposed formulas for estimating the time of passage in rotary dryers. DRYING EQUIPMENT The granulation of fertilizers in a rotary dryer and some of ita General Reviews. A number of good general reviews of drying attendant problems were discussed by Sherwin and Steventon equipment appeared during the past year. Papers on drying (188). Leger (181)developed theoretical equations for heat equipment presented a t the conference of the Institution of transfer and granulation inside a rotary dryer and concluded Chemical Engineers early this year have been reviewed (10). that parallel air flow was desirable. A general description of MacTaggart (188), Razous (167), and Schwarz (183) reviewed different types of rotary dryers was presented (13). Bauer the different equipment available for drying. Bauer (40) (40) described the mechanical auxiliaries ordinarily employed classified drying equipment on the basis of heat transfer and with rotary dryers, material-handling characteristics and discussed the fields of Miscellaneous Dryers. Gibbs (89) discussed the factors t o be application of the various types. Dryers useful in t inconsidered in selecting the proper size kiln. Took (8U4)discussed dustry were stressed. At the Netherlands symposiu iger rotary lime kiln design giving formulas for calculating retention (133) presented ,a general survey of the various drying techtime and heat losses through the kiln walls. Khodorov (121) niques, and De Haas (68) discussed the various factors governing gave formulas for estimating the heat transfer occurring in the the operation of different types of dryers. Brown (49) dechain zone of a rotary kiin. In another article (160)he recomscribed some of the newer drying equipment and Redniss (168) mended the optimum kiln sizes and operating conditions to give presented a cursory review of the progress in drying equipment maximum heat transfer. Vainrub (109)discussed the drying over the past year. It is reported (9)that the government is of clay grogs in rotary kilns. A description of a full scale kiln undertaking a survey of drying practices in 26 different industries employing an electrical arc for a heat supply has been given (160). in Great Britain. Spray Drying. During the past decade, the process industries Newcum (151) described a filter dryer. Iles and Sharman have begun to recognize some of the advantages of spray drying (113) developed a novel moisture-testing oven and showed but until this year, very little technical information ooncerning how the ventilation of tobacco samples during drying markedly the process could be obtained. In a thorough article containing affected the measured moisture contents. much experimental data, Etleling (76) has proposed a sound approach t o many of the phenomena of spray drying. He inMOISTURE EFFECTS AND M O V E M E N T vestigated the factors influencing the design of a two-fluid nozzle for atomization and determined the effect of the physical The effect of liquids on the solids from which they are dried, properties of the liquid on the maximum drop size from such an the methods by means of which liquids adhere to solids and move atomizer. Equations for the motion of the spray particles in a through them, and the rate a t which liquids can be evaporated typical idealized dryer were developed and the time required for are all important to a thorough understanding of the phenomea particle to pass from the atomizer t o the dryer wall was calnon of drying. Several good articles in these fields were pubculated for several cases. Methods for estimating the time t o lished in the past year. dry a drop to dryness were presented. Moisture Equilibria. The moisture adsorption of textiles reFor spray dryers employing centrifugal disks for atomization ceived less attention this year than in the past 2 or 3 yeara. Hutthe work of Walton and Prewett (626) and Hinze and Milborn ton and Gartside reported the moisture regah of silk (110) and (104) should be helpful. nylon (111). Abbot and Goodings (1)also studied the moisture very uniform drops could absorption of nylon. Bright and Carson (48)reported that they Lng a t low feed rates. T h had difficulty reproducing their results showing that the water this drop size. Hinze and Milborn (104) found three rggimes of
42
INDUSTRIAL AND ENGINEEXING CHEMIS'rRY
absorption of wool ~ v a - pliotosensitive. Other investigators (65)failed to find the effeci. Barkas (55, 56) reported studies on the absorption of water h y woods. Campbell (62)reviewed available information on the moisture adsorption of paper. Howsmon (108) and Guthrie (96) studied the moisture adsorption of cellulose. Hall and Mitu O ~ l( 9 7 ) found that water absorption of leather took place in two qtages. Adsorption iqothermu of water on collagen and elastin were reported (95). The movement and h c e b existing between water and fine interstices of solids were analyzed by Childs and George (69) and Hirst (106). Shrinkage and Swelling. Coclrrell (63')presented a theory t o explain the anomalous longitudinal shrinkage of wood. Barkas (36,36) and Stamm and Tarkow (294) and Tarkow (199) analyzed the factors contributing to swelling of wood and other cellulosic materials and reviewed existing theory. Preston and Yimkar i162) studied the swelling characteristich oi a number of synthetic and natural fibers in water. The swelling of nylon in water was investigated ( 1 ) . The effect of semperature on the swelling of leathei and collagen was reported by Weir (286). The welling was found to be irreversible a t tpmperatures as low as 50" C. Data on the shrinkage of fiberboard were presented (163'). Moisture Movement. Since moisture must first get to the suriace of a material being dried before it can be removed, the mechanism of this movement is important. Adani ( 2 ) has discussed the principle of penetration of liquids into solids. Barrer (38)analyzed the merhanisms of fluid flow in porous media. 'The method of flow is dependent on the pore radii through which the fluid is flowing Stamrn (196, 195) presented an excellent malysis of the passrge of water through ~ o o dand cellulosic materials. Moisture waves as a result of externally applied preswre, capillarity, and diffusion I esulting from a concentration gradient. Calculated diffusion coefficients were found to agree with measured rait-of-drying and moisture gradient data. Wilson (230) hds obtained analytical solutions for diffusion where the amount of diffusing material is finite. Crank and Henry (67) have studied by numerical approximation methods the effect of a variable diffusion coefficient for diffusion in one dimension on desorption and absorption rates. When the (liffusion coefficient increases uniformly with increasing concentration, absorption is more rapid than desorption. For a decreasing coefficient with increasing concentration, the reverse is true. Hartleg and Crank (98) examined the definition of the cliffusion coefficient. A tentaf ive quantitative theory is proposed incorporating the effect of mass flow, and an expression tor the mutual diffusion coefficient in a thermodynamically cionitleal binary solution is derived. Crank ( 6 6 ) reported the results of diffusion of dye from a finite bath, 90% depleted, into cellulose. The results did not agree with the theory which had been developed. Robinson et nl. (179) measured the permeability of several different leathers t o water and water vapor. The diffusion of dyes thiough cellophane was studied by Neale (149). Henrj (IO?) considered the diffusion of moisture and heat through textiles as Lwo coupled diffusion processes. Drying Rates Allerton et aE. ( 5 ) made a very thorough study oi the through-drying of porous media for the case where the quantity of air passing through the medium is relatively small, with the result that the air leaving the material is always saturated. Drying rates, which were expressed as a peculiar drying coefficient when they could have been expressed just as easily on a transfer unit basis, were related to the physical properties of the porous media Brownell and Crosier (50) have illustrated the use of these datu in the design of a filter dryer. Van Krevelen and Hoftijzer (214) investigated the drying rate of single granules. Their data appear questionable since they conclude that the area for ],eat transfer is only ,5070 of that
Vol. 42, No. I:
for mass transfer wibh a wholly svet sphere. This does not agree with the findings of several competent invest,igators. Diffusion was found to be a,lniost entirely controlling when drying fertilizer granules. Newitt et al. (162) studied the mechanism of air drying of beds of solids by considering and measuring the capillary forces during the drying period. Eykov (136) went into the mathematics of the drying of solids based on the usual three r6gimes of drying. Zomosa (257) described an apparatus and method, which is not significantly different from classical methods, for determining the critical moisture content of solids. Evaporation Rates. For very wet materials or those which have only surface moisture, the rate of evaporation of liquid from the surface determines the drying rate. Van der Held (2f1) has reviewed the theoretical aspects of evaporat,ion during the drying process. Wyllie (235)discussed t'he evaporation of liquids in terms of their surface structure. Alcohols and wat,er were found to have accommodation coefficients of 0.01 to 0.04. Gilby and Heyrnann (90) studied the evaporation of water. through films of oil. As would be expected, they found that the fractional reduction in the evaporation rate of water by covering t,he water with an oil film mas greatest at high air velocities over the surface. Birks and Bradley ( 4 3 ) studied the effect of a change in d,rop temperature and the composition of the surrounding gamson the rate of evaporation from a drop of di-n-butyl phthalate. The evaporation rate obeyed the Langmuir equation. Bradley and Shelland ( 4 7 ) found the same to hold true €or the evaporafian of high molecular weight straight-chain hydrocarbonfi. Carman (54) considered theoretically the evaporat,ion rates of icr and other solid's in vacuum equipment. Marwedel and Hauser (146) reported relatire evapora,tiou rates of a number of commercial solvents into still air from crysta,llizing dishes. The rate of evaporation of mercury dropletlying on a plate exposed t o an air stream wss reported by P'yankov (166). Combined Heat and Mass Transfer. Kirschbaum (189) has redefined the wet-bulb cooling equation to take into amount the heat required to raise the evaporated vapor from the wet-bulb temperature t o the dry-bulb temperature. This c,orrection is very small for water evaporating a t ordinary temperat,ures. The psychrometric ratio of mater-air mixt,ures was eval tiated by using these new equations from experimental data k k e n u p t o 07 C. dry-bulb temperature in a paper by Mirschhaum and Lisr (124). The measured psychrometric ratio divided by t)he humid heat of the air ranged from 0.84 t,o 0.94 over the temperature range of 20" to 67' C. Rossler ( 1 7 7 ) presented theoretical equations expressing the ra,te of heat transfer from air to IZ we: solid. O
CONCLUSIONS
Although considerable new data, new theory, and new information was presented during the past year, there is still a real need for fundamental studies of almost all of the dryer types. In addition, operating informhtion carefully obtained on large scale installations is needed to complement any fundamental data obtained with small equipment under idealized conditions. The trends in these directions in the past year have been promising. It is hoped that they will continue. LITERATURE CITED
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