DRYING
-W. R. MARSHALL, JR. UNIVERSITY OF WISCONSIN,. MADISON, WIS.
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Two years have elapsed since the last unit operations review of drying. The character of the literature which has appeared in the intervening years has not changed. It has consisted, for the most part, of several hundred descriptive and review articles of drying applications and studies. O n l y a relatively few have contributed new information to the fundamentals of the drying process. In the opinion of this reviewer, “it will always be thus,” for drying as it is treated today scarcely fulfills the definition of a unit operation, and as a consequence the literature will be varied. Thus, the various aspects of drying, which may be of interest to a given group of workers include heat transfer, mass transfer, fluid flow, adsorption, atomization, dust collection, humidification, and dehumidification, etc., aside from the specific relationships between drying and the properties of specific products of manufacture. For these reasons, the literature in drying has the appearance of being nonuniform and heterogeneous, and of embracing unrelated topics.
T
HE references cited in this review represent about 50% of
the total references compiled, and are considered to be the most significant articles. The organization of material in this review is as follows: genera1 articles, drying methods, drying fundamentals, and drying specific materials. The latter category covers papers which cannot be properly - - - included in the first three categories. GENERAL AND REVIEW ARTICLES O N DRYING
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Each year brings forth a crop of review articles on drying, frequently directed toward a specific industry, and usually typified by a general description of drying accompanied by a re-hash of a few principles and a description of available commercial drying equipment. Typical of such articles were those by Slade (185, I@), who reviewed drying systems for technologists in the food industry; Beckemeyer ( I O ) , who reviewed principles and methods of drying in the ceramics industry, but made a common mistake of misunderstanding the difference between the wet-bulb temperature and temperature of adiabatic saturation; Hendry and Scott (86), who reviewed air or direct drying processes. A summary of papers presented a t a symposium on drying held by the Institute of Fuel in London in April 1951 may be found in reference (97’). Papers from a similar symposium held by the Institute of Chemical Engineers in London was published in the Transactions of this institution for 1949. A review concerning the drying of chemicals in the form of pastes, powders, or crystals was presented by Clegg and Jackson (54). A compilation of papers presented a t a symposium in England on freezing and drying was edited by Harris (81). These papers were devoted exclusively to the biological and biochemical applications of freeze drying. Bickle (16) prepared a most ambitious bibliography on drying, covering several hundred pages with abstracts of more than 4600 references. A book by Kroll (125’) devoted to drying processes for free flowing materials appeared during the past 2 years. I n other articles of a general nature, Friedman (61)presented a detailed procedure with examples for selecting drying equipment; a review of the development of drying machinery was presented by MacTaggart (136);a revised U. S. Department of Agriculture publication by Van Arsdel(108) presented rather well the principles of drying with special reference to vegetable dehydration. I n a review article by Walter (216),attention was given to the economics of drying and the types of dryers used in food plants, and an attempt was made to discuss drying fundamentals with the result that new and misleading terms were unnecessarily introduced, such as “suction effect” for vapor pressure difference, and “two capacity” process for drying with air heated indirectly. Boehm (1‘7) presented a review of drying principles and processes.
DRYING METHODS
Spray Drying and Atomization. Interest in spray drying and atomization continued to be high. This is reflected in the numerous articles devoted to fundamental spray drying studies, and to the development of atomizers and studies of atomization. Fundamental heat and mass transfer data for the evaporation from pure liquid drops and from drops containing solids were presented by Ranz and Marshall (169). Ingebo (98) reported a similar study, but his experimental technique made the measurement of the evaporation rate extremely difficult, so that his results and conclusion are open to some question. Topps (209) presented results of a study of the evaporation and combustion of falling fuel drops. A study by Johnstone and Eads (101) on the viporization of small -sulfur drops contributed additional data for correlation with other droplet evaporation studies (169). A thermodynamic and statistical treatment of the abnormal vapor pressure of small drops was prepared by Kuhrt (124). He proposed an additive term to the Thompson-Gibbs equation for the droplet vapor pressure when the drop is moving. Langstroth, Diehl, and Winhold (129) evaporated drops of various pure liquids in still air in a finite surrounding. They verified Fuchs’ theory (66) for the evaporation of drops in a quiescent atmosphere, but claimed to have found a significant difference between the drop surface temperature and the bulk temperature. The former was calculated from the experiments while thelatter was measured. The results conflict with those of Ranz and Marshall (169). Luchak and Langstroth (131) presented a rigorous analysis of evaporating drops to show that the assumption of a quasi-stationary state was valid under most circumstances. Tverskaya (206) also reported on the influence of a current of air on the rate of evaporation of a drop of water. I n a mathematical analysis, Weinberg (218) developed a n expression for the temperature difference between a small water drop and a saturated surrounding atmosphere. His method was based on Kelvin’s equation for the vapor pressure over a convex surface, and he assumed equality between the mass and thermal dzusivity of water vapor. Hughes and Gilliland (96) reviewed the mechanics of drops and presented fundamental conpepts in connection with the effect of acceleration on drag, the equilibrium distortion, and the internal circulation caused by skin friction. This work would apply only to the initial period of spray drying. Atomization studies which may ultimately contribute to spray drying progress included studies of the performance of spinning disk atomizers by Adler and Marshall (2), and by Friedman, Gluckert, and Marshall (62). These studies reported drop size distribution, weight distribution, and power cgonsumption for various types of spinning disks. For the atomization of water, the disk design appeared to have little influence on the drop size distribution. Dixon, Russell, and Swallow (47) observed the mechanism of film formation and breakup from stationary and spinning disks, and they proposed an equation for predicting the film thickness. This equation is obscure because the authors did not take care to clarify and explain the terms involved. They
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INDUSTRIAL AND ENGINEERING CHEMISTRY
introduced the concept of constant breaking thickness to explain why the film shrank back t o the disk as the disk speed increased. I n other atomization studies Bergquist and Stewart ( 1 4 ) investigated the effect of atomization on the quality of spray-dried egg albumen, and developed a special pneumatic nozzle which created a foam prior to atomization. They showed that the shear stresses causing atomization degraded the properties of the dried albumen, Lane (128) reported on the mechanism involved when a drop is shattered by an air blast, and presented striking high speed pictures of the phenomenon. Lewis (130) studied the surface irregularities of small liquid samples when accelerated downward. McEntee (134) reviewed the common methods of atomization used in spray drying. Monk (144) endeavored to show by calculation that when a very fine spray is desired (1 t o 5 microns), the viscous energy loss during atomization can become significant if the atomization is restricted to a single jet of fluid. Mugele and Evans (146) proposed a so-called upper limit equation for expressing the drop size distribution of sprays. Actually, the form they proposed was simply the log-normal distribution with an upper size limit, thereby requiring evaluation of three parameters for its use, instead of two. Kottler (119) discussed this same proposal prior to Mugele and Evans (146) but did not reach the conclusions of these workers that an upper limit form had any special advantages. The effect of turbulence of a liquid jet and its effect on atomization was studied by Panasenkov (168). This study led t o the conclusion that the average drop size after atomization of a turbulent jet was almost independent of the jet Reynolds number, but approximately proportional to the orifice diameter. Sliepcevich, Consiglio, and Kurata ( 186) reported on the development of a vibrating-type atomizing nozzle. Sohngen and Grigull(187) measured the spray angles and flow rates from eighteen swirl type pressure nozzles and compared the results with calculations made for frictionless flow. Their derivation of a theoretical equation relating the air core t o the velocity components appears to be in error, however, since in deriving their Equation 9, it would seem that they differentiated a constant and then divided by zero on both sides of the equation. Taylor (192) developed mathematical formulations to predict under what conditions the liquid efflux in a swirl .type pressure nozzle will reach the orifice by way of the boundary layer. The effect of viscosity on the air core is predicted from this development. Patents relating to spray drying and spray dryers included a British patent by Hutcheson (13)for a new type of spinning disk, ~1 patent for a new type of milk spray dryer by Coulter, Montonna, and Kitzes (39), a process patent for spray drying allyl starch (46), a French patent for spray drying biological materials in an atmosphere of nitrogen or hydrogen (62), and a patent by Hall ( 7 8 ) for spray drying milk and other liquids by first preheating them above their boiling points and then spraying into heated air. General studies of spray drying and spray-dried products included such articles as an improvement in spray drying technique for food (69), maintaining high quality in dried foods by spray drying (82), a n experimental study of spray drying pharmaceuticals (go), and an inconclusive experimental study, due to poor control of operating variables, of factors affecting the physical properties of detergents by Chu, Stout, and Busche (33). Kirschbaum ( 1 1 2 ) prepared a rather comprehensive article on the principles of spray drying and the factors affecting the drying of particles and dryer performance, Marshall and Seltzer (139) prepared a similar paper but included further consideration of atomization and the factors influencing bulk density. Ladisch (126) prepared a general review of spray drying and the economics involved. I n another general review paper on spray drying, Sjenitzer (188) presented theoretical considerations of droplet motion, and of the evaporation of drops containing dissolved solids. He assumed that a drop of dilute solution would evaporate with a continually changing surface temperature until saturation was reached. Ranz and Marshall (169), however, showed that this
Vol. 45, No. 1
was not the case, and that a drop of a dilute solution always evaporated as though it were saturated. The literature indicated that Russian interest in spray drying increased in the past two years. I n addition to the article mentioned above by Panasenkov (168), another by this author ( 15 7 ) was concerned with a particle size study of spray-dried whole milk. Particle sizes ranging from 35 t o 44.5 microns for whole and skim milk were observed, and 36 t o 38 microns for dried cream. No diameter larger than 110 microns was observed. A Russian spray drying process for eggs was described by Pankova and Lyutikova (169), while a Russian milk spray drying plant was described in detail by Polykovskii (166). A general article on spray drying as developed by a Swiss firm was prepared by Piatti (165). From England, a general article on the spray drying of synthetic detergents and soaps was prepared by Smith (186),with consideration given to the characteristics of spinning disk, pressure, and pneumatic atomizers as used in the soap industry. Tracy, Hetrick, and Krienke ($04) reported a study of the effect of spraying pressure (400 t o 3500 pounds per square inch) and orifice size (0.025 t o 0.042 inch) on the physical characteristics and keeping quality of spray-dried whole milk. Their results were inconclusive. Particle size distribution was reported on a number basis which is always misleading. A weight or volume basis is preferable in comparing atomization performance. Experimental studies of spray drying a milk-soybean mixture in China were reported by Tsao et al. (806). The effects of inlet air temperature and the vertical air velocity component on dryer performance were studied. The well-known fact that dryer e5ciency increased with increase in air temperature was reported. Evaporation rates of 0.8 t o 2.5 pounds per hour per cubic feet of tower were reported. A performance study of a pilot model spray dryer was reported by Wallman and BIyth ($13). These workers used sodium silicate as a test material and studied the effect of operating variables on particle size and size distribution. Spinning disk atomization was used. More reports of this type from our industrial laboratories are desirable. The reluctance of the chemical industries t o publish some of their findings with regard to spray drying indicates one of two things, ( a ) either the know-how is regarded as an important trade secret or ( b ) the industrial developments in tbis field are not significantly worth reporting a t this time. Infrared Drying. The continuing large number of articles relative to infrared drying indicate that this method still appeals to many engineers, in spite of the fact that it must be able to overcome the disadvantage of high power costs. The literature on infrared drying appears to fall into two categories: (1)articles which review the principles and fundamentals of radiant energy and sometimes report experimental data, and (2) specific plant applications where infrared drying has been used. Literature in the first category will be cited first. A paper by Broughton (21 ) reviewed the principles of infrared heating with particular reference to paper drying. In a paper by Engel ( 6 1 ) the principles and problems of infrared drying were considered and its application as a supplementary drying method in the paper industry discussed. Faggiani (54) offered some technological considerations of infrared rays, and reported data on the infrared drying time for coated films and fabrics. Kolbe ( 1 1 7 ) reported on the optics involved in infrared drying, such as the effect of wave length on reflectance, absorption, and transmission. Landfermann (187) also considered the effect of wave length in the iufrared drying of paint, and concluded that only transparent paints showed variations in absorption with n-ave length. Landfermann (126) in another paper presented a theoretical comparison of the heat flux possible with infrared and convection heating. Martens (140, 141) prepared two reviews on the principles of infrared drying. The infrared absorption bands for water of crystallization, reported by Matsumura (I@), showed that maxi-
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mum absorption occurred a t a wave length of about 3 microns. Shuman and Staley (181) discussed the radiation absorption properties of materials as a guide in the selection of radiant energy transparent supporting media for a given wavelength distribution. However, they neglected t o discuss the difficulties of measuring transmission for diffuse materials, where transmitted energy is scattered and not detected in the usual spectrophotometers. Yagi and Kunii in a series of papers (887-829) reported results of experimental infrared drying and heating studies, and proposed a design procedure for infrared dryers. They proposed correlating drying data on the basis of the ratio of the volume occupied by water to the volume of voids instead of the usual moisture content on a weight basis, The authors claim that this method permits the prediction of the critical moisture content for thin layers. However, this procedure would require data on the volume of the voids, so that i t appears questionable as to whether this proposal has any real advantage. Wingard and Rozier (184) made experimental comparisons of infrared drying and convection drying. Seven different materials were studied in a modified compartment-type dryer, and the not very startling conclusion was reached that a combination of steam and infrared heating gave the highest drying rates. Zamzow and Marshall (131) reported the results 'of preliminary experiments on the application of radiant energy to freeze drying. Experimental drying time curves were given for an organic dye supported on a radiant energy transparent medium. In the category of specific applications several articles appeared. The use and efficiency of infrared radiation in drying molds and firing cores was reported (68). The use of commercial infrared lamps t o dry cellulosic materials with experimental determinations of product temperature was reported by Fujii and Kageyama (66). I t o and Hoube (99) reported tests on the infrared drying of ceramic pieces for various source temperatures ranging from the temperatures of incandescent lamps to hot plates a t 130" C. Jubitz (108)reviewed progress of infrared drying in Germany and in other countries. Keylwerth (110, 111) presented a general discussion of the use of infrared in the wood industry, and treated the principles of electric and gas radiators Narayanamurti and Prasad (146) also considered the application of infrared to wood drying, and treated veneer drying in particular. They compared the advantages and disadvantages of infrared drying, and decided that i t held promise only for a high cost material such as veneer. They reported energy consumptions per pound of water removed between 0.75 and 2.5 kw.-hr. Efficiency increased with decreasing thickness. Nitta, Sugimoto, and Nakai (163)presented curves showing the rate of loss of moisture by fish of different thicknesses for infrared drying. The usual conclusion wasreached-namely, infraredis not economical for low-priced products. A novel adaptation of infrared lamps to a double screw conveyor dryer for tungsten was described (171). Dryer length was 19.25 feet, and screw speed was 1.25 r.p.9. Operating costs were given. An account of the drying of hard porcelain by infrared, together with the apparatus and cost data, were reported by Vindreau and Ardouin (210). Drying by Sublimation. I n the field of sublimation or freeze drying, interesting and new applications were reported together with the usual crop of review articles. Among the latter were articles by Beckett (11) who surveyed the general aspects of freeze drying, Flosdorf (67)who summarized the general material he has written during the paat 10 years, and Goddard ( 7 1 ) who summarized American practice in large-scale freeze drying. Guess and Burlage (73) discussed freeze drying as applied to pharmaceuticals. A compilation of papers presented in England at a symposium on freezing and drying with special attention to the biochemical and biological field was edited by Harris (81). One of the more interesting aspects of this symposium was the conclusion that very low moisture contents do not appear essential to the preservation of bacteria, which is contrary t o the practice with respect to antibiotics.
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New fundamental data on freeze drying were contributed in a few articles. The value of freeze drying for the preservation of mold cultures waa reported by Fennel], Raper, and Flickinger (66). Results of viability studies covering periods up t o 7 years were reported. Freeze drying appeared to be a superior method of preservation of the several methods studied. Unfortunately, no data were given on the final moisture contents of the various freeze-dried mold cultures. Guigo (74) presented a theoretical basis for sublimation drying of dairy products, and drying rate equations were presented. An equation for the critical moisture content was given as well as an equation for the total drying time. Kramers and Stemerding (190) reported on studies of the rate of sublimation of ice in vacuo, and showed that their data could be correlated by equations proposed by Carman (16). Zamzow and Marshall (831) reported some freeze drying experiments in which radiant energy was transmitted t o the frozen material through a semitransparent retaining medium. Comparisons were made with conduction heating, and i t was observed t h a t with the radiation method significantly higher, sublimation rates were possible. This conclusion should be restricted only t o those materials in which heat transfer controls the drying rate. A number of papers appeared dealing with specific applications and developments of freeze drying. One interesting new development is the process whereby freezing is accomplished by dispersing and freezing the material in a cold solvent (30). The frozen pellets are then freeze-dried. The rate of freezing is very rapid by this method. Gane (69) described an experimental freeze dryer for foods, This unit was comprised of six trays. Results of freeze drying tests on the quality of apple slices, eggs, minced beef, and fish were reported, together with curves of temperature, weight, and heat input us. time. Snap freezing was used. Guigo and Gulyaeva (76) reported results of sublimation drying of casein. The application of radiant freeze drying t o dry adrenalcorticotropic hormone (ACTH) was described (84). Two patents on methods for the removal of water vapor in freeze drying were issued (88, 89). The principle underlying each of these is the removal of the condensed phase as soon rn i t is formed. A Dutch patent (188) proposed supplying the heat for freeze drying by condensing water vapor on the exterior of t h e containers holding the frozen solid. Nickerson, Coulter, and Jenness (161) concluded there were no product advantages in freeze drying milk compared with spray drying. Since this was a preliminary study and the drying variables were all held constant, this appears t o be a premature conclusion. The freeze dryer wrn not described, nor were the experimental procedures well detailed. The results on the properties of milk were of interest, but t o make any final conclusions regarding freeze drying us. spray drying seems improper, The research done on freeze drying milk t o date has been minuscule compared t o that done on spray drying. Therefore, definite conclusions comparing the two methods certainly should await the results of further research. Talburt and Legault (190) reported further data on their new process of drying and then freezing (dehydrofreezing) with specific reference t o peas. High Frequency Drying. As in the case of infrared drying, continued interest was shown in high frequency drying in spite of its high cost-higher than infrared. A study by Alexander and Meek (9) gave an insight into the mechanism of drying textiles by radio frequency, and showed t h a t the usual periods of drying occurred even with this method. The change from the constant-rate t o the falling-rate period was accompanied by a change in the dielectric properties of the textiles. These authors also presented a general discussion of the use of high frequency in heating and drying in the textile industry (4). Billig (16) presented data on the use of high frequency in timber processing, and reported energy concentrations of 20 t o 25 w.-min. per cubic centimeter t o dry thin sections of timber. A patent issued t o Dippel, Lely, and Dikhoff (46)described the application of radio-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
frequency drying to wet photographic film. The electrodes were arranged so that the lines of force passed through the film in the direction of motion. Hagopian ( 7 6 ) discussed the advantages of dielectric heating in the drying of rayon cakes. Iwashita and Koizumi (100) presented results of experiments on the drying of wood by radio frequency. Klages (11S) discussed the techniques of high frequency and the mechanism of the method in the drying of lacquer. A review of the problems, advantages, and disadvantages of drying ceramics by dielectric heating was given by Martin (148). H e discussed the physical problems involved and the electrical measurements required in its study. Nelson (147) made a mathematical development of formulas t o predict temperature gradients in materials undergoing dielectric heating. Results are given for various boundary conditions. A Russian investigation of wood drying by high frequency was reported (148), and a description was given of a 50-kw. high frequency generator developed for wood drying. A fundamental study of high frequency drying of granular solids by Nicol (162) gave results of experiments on the heating of water and a series of salt solutions, and on the drying of glass beads by high frequency currents. Moisture gradients were reported and a mechanism of drying was proposed. Yamamoto et al. (230) reported that cotton yarn could be dried with a frequency of 37 megacycles in 5 to 15 minutes. The yarn had properties similar t o that dried in cool or warm air for 3 t o 8 hours. Miscellaneous Methods. A few applications of drying wherein the wet solid is dwpersed in a hot gas stream were described. Flood ( 6 6 ) described flash drying of sludge for several sewage treating plants. Parry and Wagner (160,161) described pilot plant tests on the drying of low rank coals in fluidized beds. Moisture was reduced by 90 t o 95% in a drying column into which gas jets entered a t a velocity of 80 feet per second. It was shown that drying would be economically feasible when shipping distances were such that the shipping cost exceeded $0.65 per ton. Chapman and Needham (18)also considered the problem of drying washed coal in relation t o cost of transportation and where freezing may occur. I n another article on pneumatic conveying drying, Vranian and A-ickerson (211) described the drying of lime kiln feed. Few articles on rotary drying appeared. An interesting experiment using a radioactive tracer t o establish the retention time of seaweed in a roto louvre dryer was described by Gardener et al. (70). They added 100 microcuries of radioactive o-phosphoric acid t o sea water in which the seaweed was soaked. Results showed that for an average retention time of 41 minutes, some material passed through in 15 minutes while some was retained for 80 minutes. Luethge (132) proposed four short-cut formulas. t o improve the evaluation and operation of rotary kilns. Vacuum drying was given some attention, both theoretically and experimentally. Arnborger (6) presented experimental results and data for the vacuum drying of extracts. Even vacuum drying is not free of the falling rate period, for the material temperature was reported to rise when a critical moisture content was reached. Dunoyer ( 4 8 ) presented a study of vacuum drying as influenced by the usual parameters, and he also prepared two lengthy articles (49) on the theory of vacuum drying. DRYING FUNDAMENTALS
Mechanism of Internal Moisture Flow. Fundamental studies of drying will inevitably involve studies of the mechanism of moisture flow in solids. Because of the fundamental nature of this phase of drying, many of the underlying principles will come from basic studies made in various branches of the physical sciences. Thus, the study of Ananyan (6) on the mobility of water and ice in frozen soil may be of value in the interpretation of a freeze drying mechanism. A study by Booth (19) on the effect of surface conditions on the electrophoresis of solid particles may in time contribute t o studies of internal flow. Christensen ( S I )
Vol. 45, No. 1
and Christensen and Williams (52) contributed to the theory of diffusion in wood and porous media. Crank and Robinson ( 4 0 ) continued t o report experiments on diffusion in cellulosic materials, and described interferometric studies of diffusion to establish the influence of concentration and orientation on diffusion in cellulose acetate, Further studies on diffusion and the effects of concentration on the diffusion coefficient were reported by Fujita and Kishimoto (68). Fujita ( 6 7 ) also presented a numerical solution of the differential equation for absorption-controlled diffusion in solid. Fundamental studies by Klinkenberg (114) on the analogy b e b e e n diffusion and electrical conductivity in porous rocks may be a tool available to chemical engineers for studying the internal mechanism of flow. This method was used earlier by Burr and Stamm ($4)t o verify Stamm's (189)theoretical calculations for the mechanism of flow in wood. Hutcheon and Paxton (96) made measurements of moisture migration and thermal conductivity on moist spruce sawdust enclosed between hot and cold surfaces. A vaporization-condensation phenomenon was observed as the moisture redistributed itself. Steady heat flow was not established until the moisture migration was stabilized. They found the thermal conductivity to be only slightly affected by moisture in the system: Lykov (133) presented a theory of drying ceramic materials based on the internal mechanism, He classified ceramics as colloidal, capillary-colloidal, and colloidal capillary-porous Fundamental studies on the internal mechanism came out of England during the past 2 years. Newitt and Coleman (149,160)reported on the absorption phenomena in drying clays; Pearse, Oliver, and Kewitt (162),Oliver (155),and Oliver and Newitt (166) contributed new data and studies on the forces causing flow in granular beds, on the measurement of capillary forces in drying, and on the moisture gradients resulting therefrom. Peck, Griffith, and Rao (163) studied the surface and internal resistances in the falling-rate period of drying and proposed using an average diffusion coefficient for this period even though the flow mechanism might be one of capillarity. Rogers and Morrison (179) studied convection currents in porous media, and extended their theory of critical gradients. Two methods for the measurement of diffusion coefficients in gels were described by Salvinien (176) and by Salvinien et al. (176). These methods were: ( a ) observing in the gel the advancing boundary of a precipitate as diffusion occurred, and ( b ) utilizing a radioactive tracer in a tube of cast gel which was sliced and analyzed for the tracer concentration exactly as in moisture dietribution determinations Tanner and Hanks (191)reported on studies of moisture hysteresis in so-called gypsum moisture blocks. Drying and wetting curves were measured for a range of moisture contents. An optical investigation of the Soret effect (thermal diffusion) was reported by Thomaes (193). This effect has received little attention as a mechanism for moisture movement in solids and any new data are welcome. A detailed study of the complex internal structure of peat and its influence on the drying of peat was reported by Westlin (281). consideration was given $0 the manner in which water was held in the peat and how this would influence drying by various methods, such as freezing, cataphoresis, pressing, air drying, and drying with superheated steam. Economic considerations were also given. Heat and Mass Transfer. This phase of drying has been associated with the so-called external mechanism-i.e., where heat is transferred t o the surface of the material while mass is transferred away. Studies of drying from this standpoint have proved t o be more fruitful for design and operation purposes than have studies of the internal flow mechanism. Studies important t o this phase of drying include such works as the article by Davies and Walters (44)on the effect of a finite width of area on the rate of evaporation in a turbulent atmosphere, and a report by Fourt et al. (60) on the factors influencing the rate of transport of heat t o fabrics and the transfer rate of water vapor away during drying. I n considering the stationary evaporation of a liquid a t different temperatures of an evaporating and a condensing surface, Gran-
January 1953
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ovskiI (72) presented an analytical solution of the problem for the case of no foreign gas. This work would be expected t o have importance in the field of high vacuum drying, especially freeze drying. Several heat and mass transfer studies involving droplets have previously been mentioned under spray drying. These include the studies of Ingebo (98) for pure liquid drops, Johnstone and Eads' (101)study of the vaporization of small sulfur droplets, and the work of Ranz and Marshall (169) on the evaporation of pure liquid drops and drops containing dissolved and suspended solids. Reference is repeated here t o Langstroth et al. ( l a g ) and Luchak and Langstroth (131) for theoretical and experimental considerations of the evaporation of drops. Kuhrt (124) extended the theory of the vapor pressure of small drops t o include the effect when the drop is moving. Other references for heat and mass transfer to drops as discussed above under spray drying are (202, 106, 218). Timofeev (201) presented equations for evaporation from a water surface into a turbulent air stream, and extended the treatment t o other surfaces such as ice, mercury, and methanol. Knacke et at. (115) made a mathematical study of the theory of evaporation rates. The study, based on liquid and solid models, gave good agreement with data for evaporation of certain crystals. This work is highly fundamental. Wenzel and White (280) reported on a study of drying in an atmosphere of superheated steam, and presented data to show that higher drying ratcs were possible with this process than with air but a t the expense of a greater capital cost. More data of this type are especially desirable. Hygrometry and Equilibrium Mksture. Methods of measuring humidity are fundamentally important in drying studies. Hinzpeter and Meyer (92) described a glow discharge procedure for measuring the humidity of air in vacuum at pressures ranging from 10 to 100 microns. Prudhomme (167) proposed a method for measuring the humidity of air by measuring the temperature rise of 98% sulfuric acid in contact with humid air. This method was compared with the sling psychrometer, and appeared t o be more accurate for large wet-bulb depressions. Prudhomme (168) also prepared a review of physical and chemical methods of measuring the water content of atmospheres, gases, and vapors. A similar review with methods of testing hygrometers was prepared by Wexler and Brombacher (222). Webster (217)investigated the effects of total pressure on the water vapor pressure in a saturated compressed gas Studies of equilibrium moisture content of textile fibers a t high humidities were made by Ashpole ( 7 ) . Bond (18) described the manner in which the submicroscopic pores of coal become filled with moisture, the properties of which differ from those of normal liquid water. Heats of wetting of cellulose and regenerated cellulose, and the adsorption isotherms at 30 C. were determined by Wahba (212). Methods of Moisture Analysis. A fundamental analytical procedure in any drying research or process is the determination of the moisture content of the product. The importance of the methods of moisture analysk is reflected in the number of literature articles bearing on this subject.. Many of the articles compared the application of various well-known moisture analysis methods to specific materials. Others dealt with specific procedures such as conductance and dielectric methods. A large percentage of the articles treated t,he determination of moisture in foods. An instrument for measuring moisture in grains and foodstuffs by a conductance process was given ( 2 7 ) . Barlow (9) concluded that the Karl Fischer method approached stoichiometrical accuracy for water in most foodstuffs. Belanger (12) compared vacuum, forced air, and atmospheric oven methods for determining moisture in meats. Chappin de Janvey and Francois (19)recommended the Karl Fischer method for determining moisture in oil seeds. Common (38) discussed the difficulties encountered in determining the moisture content of food and reviewed the probO
51
lem of determining the amount of bound water. E v a (63) compared methods of distillation, total solids, and electrical characteristics for determining the moisture in cereals and legumes. Barber (8) reported on the determination of moisture in coal. Brochmann-Hanssen and Pong (20) modified the Karl Fischer method with a reagent containing ethylene glycol monomethyl ether instead of methanol t o determine the moisture content of medicinal chemicals and other drug products. Broughton and Hobbs (22),in an investigation of the factors influencing the moisture determination of paper in oven drying, concluded t h a t temperature was the single most important variable t o control. Bryson and Pickering (23) developed a procedure for the determination of moisture in coal by the Karl Fischer method. I n a comparison of the moisture analysis of drugs by solvent distillation, oven drying, and infrared drying, Collett ( 3 7 ) concluded that infrared drying is satisfactory for control purposes. Typical infrared drying curves were reported. Eberius (60) made a critical analysis of the application of the Karl Fischer method t o explosives and related materials. Fryd ( 6 3 ) reviewed methods of moisture analysis for foods and organic substances. Fryd and Kiff ( 6 4 ) discussed the difficulties involved in determining the true moisture content of tobacco. Jalien (103)considered six methods of moisture analysis for dairy products. Kamiyoshi and Miyamoto (104) reported tests on a portable instrument t o measure the dielectric constant of wet materials, and found it t o be independent of the sample used. This conclusion is undoubtedly restricted t o granular materials of the type tested. Kawada and Uohida (106) developed a so-called hot-wire moisture meter which operated, in principle, similar to the thermal conductivity method of the measurement of moisture in gases. From a comparison of oven drying, vacuum drying, infrared drying, toluene distillation, and the Karl Fiecher method for the determination of water in cocoa and chocolate products, Kentie and Barreveld (109) concluded t h a t the Karl Fischer method gave the most reliable results. Makower (137) reviewed and summarized the methods of moisture analysis for dehydrated foods. Sakagami et al. (173) studied the measurement of moisture in textiles by high frequency, and concluded that when the textiles were compressed (174)t o a constant density the dielectric constant became more stable. Another review of methods of estimating moisture was prepared by Ward (216). DRYING SPECIFIC MATERIALS
I n the foregoing sections, the drying literature wm reviewed from the standpoint of methods and principles. Many articles appeared, however, which must be classified under the drying of specific materials. Many more articles appeared than have been reported here. Only those that appear t o have more than temporary value and more than specific interest have been indicated below. Ceramics. A review article by Beckemeyer (10)on the drying of ceramic ware has already been cited under general articles. Clegg (35) described various types of conveyor dryers used for ceramics. Hall (79)discussed the testing of a tunnel kiln. I n a review of drying methods in the pottery industry, Hind (91) discussed methods of regulating the water content of the raw material, and reviewed clay-water relationships and their influence on drying. The effect of the plaster mold on drying speed was considered. Consideration was also given t o the use of high frequency heating and vacuum drying. Keller and Gorazdovskil (108) reported on the problem of the cracking of ceramic shapes during drying. The drying of ceramic bodies electronically and with infrared was considered by Kohler (116). A review of drying ceramic products was prepared by Krause (181). Another review article on the basic problems of drying bricks, tiles, and heavy clay articles, and the types of dryers used was presented by Macey (135). Newitt and Coleman (160) described the mechanism of moisture migration in china clay. Roberts (170) presented drying rate curves and shrinkage data on the drying of
52
INDUSTRIAL AND ENGINEERING CHEMISTRY
refractories, and discussed precautions necessary t o prevent cracking. Walter (814) reviewed the types of dryers used in the ceramic industry. Williams-Gardner (123) added another review article on pottery drying and surveyed the problems of drying, types of dryers, layouts, and the stages of drying in relation t o the methods of heat transfer. Zhukov (132) reported operating data on air velocities and temperatures for drying structural brick. Foods and Agricultural Products. I n addition to articles on food drying discussed above under spray drying, a number of others appeared. Only the more significant ones are given here. Hathaway et al. (85') studied the effect of drying temperature on the nutritive value and commercial grade of corn. They concluded corn should not be dried above 140" F. Aceto et al. ( 1 ) developed a process for producing high-vitamin, high-protein leaf meals from vegetable wastes in a direct-fired rotary dryer. Cost estimates were given for various vegetable wastes. Davidov (43) described a Russian method for the drying of milk a t low temperatures. Harler (80) presented an interesting review of the history and present methods of drying tea. Hobbs (93) gave an informative account of early sugar drying methods, and described current British. practices of sugar drying in a rotary dryer. The heat for drying sugar from 0.95 to 0.03% moisture is provided by the sensible heat of the sugar itself, and the air in passing through the dryer is heated rather than cooled. Operating data were reported. Scott (177)reviewed drying methods in the manufacture of dairy products. Sen Gupta (178-180) made studies of the dehydration of meat. An electron microscope study of spray-dried milk powder by Villanova and Ballarin (209) showed that the milk components were distributed uniformly throughout the particle. Woodforde (225) concluded that the rate of drying grain and seeds was largely controlled by the rate of internal moisture flow t o the grain surface. Drying systems and types of dryers used were also described. Tosello and Veiga (103)compared the drying of starch in vacuum and with hot air. The degree of dextrin formation in hot air drying was held to 0.3%. Textiles and Paper. An informative review of methods and equipment used in the drying of staple fiber was given by Coles (5'6). H e reported that the drying rate for staple was found t o vary as the 6/&h's power of the mass velocity. This is somewhat higher than theory and other experiments have indicated. An 800-ton-per-day kraft board drying machine was described by Crowder (41). Drying of package yarns by alternately blowing hot air or steam from outside t o the inside, and then vice versa was discussed by Hall ( 7 7 ) . Welo et al. (119)made use of the periods of drying and the critical moisture content t o determine the swelling capacities of fibers in water. No contribution was made t o drying theory. Wood Drying. A rather large number of articles appeared on this subject. Many dealt with specific methods, such as the seasoning of wood a t temperatures below the boiling point of water (25),new dry kiln practices (87),and lumber drying by the vapor process in which Hudson (94) compared the use of naphtha and perchloroethylene vapors. Several papers dealt with the theory and mechanism of wood drying. Czepek ( 4 8 ) considered the internal mechanism of drying pine containing 25 % moisture, and reported that temperatures up to 140" C. could be used without degradation. Kollmann et al. (118) studied the discoloration of whole wood and veneer during drying and suggested four causes for the discoloration. Kroll (122) in two articles presented the theory and mechanism for moisture movement in pine or soft wood during drying. Maku (15'8)reported that the diffusion equation could be used with good accuracy t o estimate the drying time and average moisture content for drying below the fiber saturation point, not a new finding. Above the fiber saturation point, agreement was not so good. Ogurta (154) also made studies of the mechanisms of wood drying. A description of lumber dryers used in Russia was given by
Vol. 45, No. 1
Peich (16.4). Tiemann (194-199) presented an extensive series of articles describing the kiln drying of lumber. He also discussed the phenomenon of collapse in wood drying (am). Keylwerth ( 1 1 1 ) reported on following the course of kiln drying by measuring the temperature in different regions during drying. A report by the Forest Products Laboratory (207) reviewed and summarized properties of wood as related to drying in order t o aid in understanding kiln drying problems. Tables of data of important wood properties were provided. Miscellaneous. A few miscellaneous articles are cited. A rapid process for drying motion picture film was reported by Kata (105). The equipment was designed on the basis of theoretical investigations of the drying problem, and the use of turbulent air gave rapid drying with low film distortion. I n a general paper, Kay (107) reviewed methods of drying t o remove surface moisture from fine crystalline chemicals, and concluded that a vibrating conveyor with infrared offered substantial advantages over oven and vacuum methods. Henaker (85) and Wudich (1B6) reported on investigations made of the pasting process in drying leather. The former studied various adhesives, but found no single superior formulation. Wudich concluded that in the pasting process the relative humidity should be in the range of 50 to 60% and the temperature 40' t o 50 ' C . LITERATURE CITED
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,----e-
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