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D R Y I N G--. R. MARSHALL, JR.

UNIVERSITY OF WISCONSIN,. MADISON, WIS.Two years have elapsed since the last unit operations review of dry ing . The character of theliterature which has appeared in the intervening years has not changed. It has consisted, for themost part, of several hundred descriptive and revi ew articles of dry ing app lications and studies.On l y a relatively few have contributed new information to the fundamentals of the drying proc-ess. I n the opinion of this reviewer, it wil l always be thus, for dryin g as it i s treated todayscarcely fulfills the definition of a unit operatio n, and as a consequence the literature will bevaried. Thus, the various aspects of drying , which may be of interest to a given group of workersinc lude heat transfer, mass transfer, flu id flow, adsorption, atomi zation, dust col lecti on, humidi-fication, and dehumidification , etc., aside from the specific relationships between drying andthe properties of specific products of manufacture. For these reasons, the literature in dryinghas the appearance of being nonuniform and heterogeneous, and of embracing unrelated topics.

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DRYING METHODSSpray Drying and Atomization. In-

terest in spray drying and atomiza-tion continued to be high. This is re-flected in the numerous articles de-voted to fundamental spray dryingstudies, and to the development ofatomizers and studies of atomization .Fundamental heat and mass trans-fer data for the evaporation fromHE references cited in this review represent about 50 % of pure liquid drops and from drops containing solids were pre-

the total references compiled, and are considered to be the sented by Ranz and Marshall (169). Ingebo (98) reporteda similar study, but his experimental technique made the meas-urement of the evaporation rate extremely difficult, so thathis results and conclusion are open to some question. Topps(209) presented results of a study of the evaporation and com-bustion of falling fuel drops. A study by Johnstone and Eads

T ost significant articles.The organization of material in this review is as follows: gen-

era1 articles, drying methods, drying fundamentals, and dryingspecific materials. The latter category covers papers which can-not be properly included in the first three categories.- - -

GENERAL AND REVIEW ARTICLES ON DRYINGEach year brings forth a crop of review articles on drying, fre-

quently directed toward a specific industry, and usually typifiedby a general descript ion of drying accompanied by a re-hash of afew principles and a description of available commercial dryingequipment. Typical of such articles were those by Slade (185,I@), who reviewed drying systems for technologists in the foodindustry; Beckemeyer ( I O ) , who reviewed principles and methodsof drying in the ceramics industry, but made a common mistakeof misunderstanding the difference between the wet-bulb tem-perature and temperature of adiabatic saturation; Hendry andScott (86), who reviewed air or direct drying processes. A sum-mary of papers presented a t a symposium on drying held by theIns ti tu te of Fuel in London in April 1951 may be found in ref-erence (97). Papers from a similar symposium held by the In-sti tu te of Chemical Engineers in London was published in theTransactions of this institut ion for 1949. A review concerningthe drying of chemicals in the form of pastes, powders, or crystalswas presented by Clegg and Jackson (54). A compilation ofpapers presented a t a symposium in England on freezing and dry-ing was edited by Harris (81). These papers were devoted ex-clusively to the biological and biochemical applications of freezedrying. Bickle (16)prepared a most ambitious bibliography ondrying, covering several hundred pages with abstract s of morethan 4600 references. A book by Kroll (125) devoted to dryingprocesses for free flowing materials appeared during the past 2years.

In other articles of a general nature, Friedman (61) resented adetailed procedure with examples for selecting drying equipment;a review of t he development of drying machinery was presentedby MacTaggart (136); revised U. S. Depar tment of Agriculturepublication by Van Arsdel(108) presented rathe r well the princi-ples of drying with special reference to vegetable dehydration.In a review article by Walter (216), ttention was given to theeconomics of drying and the types of dryers used in food plants ,and an attempt was made to discuss drying fundamentals withthe result tha t new and misleading terms were unnecessarily in-troduced, such as suction effect for vapor pressure difference,and two capacity process for drying with air heated indirectly.Boehm (17) presented a review of drying principles and processes.

,

(101) on th e viporization of small -sulfur drops contributedadditional data for correlation with other droplet evapora-tion studies (169). A thermodynamic and statistical treat-ment of the abnormal vapor pressure of small drops was pre-pared by Kuhrt (124). He proposed an additive term to theThompson-Gibbs equation for the droplet vapor pressure whenthe drop is moving. Langstroth, Diehl, and Winhold (129)evaporated drops of var ious pure liquids in still air in a finitesurrounding. They verified Fuchs theory (66) for the evapo-ration of drops in a quiescent atmosphere, but claimed to havefound a significant difference between the drop surface tem-perature and the bulk temperature. The former was calcu-lated from the experiments while the lat ter was measured. The re-sults conflict with those of Ranz and Marshall (169). Luchakand Langstroth (131)presented a rigorous analysis of evaporatingdrops to show th at the assumption of a quasi-stationary st atewas valid under most circumstances. Tverskaya (206) alsoreported on the influence of a current of air on the rat e of evapora-tion of a drop of water. I n a mathematical analysis, Weinberg(218) developed an expression for the temperatu re difference be-tween a small water drop and a saturated surrounding atmos-phere. His method was based on Kelvins equation for the vaporpressure over a convex surface, and he assumed equality betweenthe mass and thermal dzusi vit y of water vapor. Hughes andGilliland (96) reviewed the mechanics of drops and presentedfundamental conpepts in connection with th e effect of accelerat ionon drag, the equilibrium distortion, and the internal circulationcaused by skin friction. Thi s work would apply only to theinitial period of sp ray drying.

Atomization studies which may ultimately contribute to sp raydrying progress included studies of the performance of spinningdisk atomizers by Adler and Marshall (2), and by Friedman,Gluckert, and Marshall (62). These studies reported drop sizedistribution, weight distribution, and power cgonsumption forvarious types of spinning disks. For the atomization of water,the disk design appeared to have litt le influence on the drop sizedistribution. Dixon, Russell, and Swallow ( 4 7 ) observed themechanism of film formation an d breakup from stat ionary andspinning disks, and they proposed an equation for predicting th efilm thickness. Thi s equation is obscure because the autho rs didnot tak e care to clarify and explain the terms involved. They

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4a I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 45 , No. 1introduced the concept of constant breaking thickness to explainwhy the film shrank back t o the disk as the disk speed increased.

In other atomization studies Bergquist and Stewart ( 1 4 ) in-vestigated the effect of atomization on th e quality of spray-driedegg albumen, and developed a special pneumatic nozzle whichcreated a foam prior to atomization. They showed th at theshear stresses causing atomization degraded th e properties of t hedried albumen, Lane (128) reported on the mechanism involvedwhen a drop is shattered by an air blast, and presented strikinghigh speed pictures of the phenomenon. Lewis (130)studied thesurface irregularities of small liquid samples when accelerateddownward. McEntee (134) reviewed the common methods ofatomization used in spray drying. Monk (144) endeavored toshow by calculation that when a very fine spray is desired (1 o5 microns), the viscous energy loss during atomization can be-come significant if the atomization is restricted to a single jet offluid. Mugele and Evans (146)proposed a so-called upper limitequation for expressing the drop size distribution of sprays.Actually, th e form they proposed was simply the log-normal dis-tribution with an upper size limit, thereby requiring evaluation ofthree parameters for its use, instead of two. Kottl er (119) dis-cussed this same proposal prior to Mugele and Evans (146) butdid not reach the conclusions of these workers th at an upper limitform had an y special advantages. The effect of turbulence of aliquid je t and it s effect on atomization was studied by Panasenkov(168). This study led to the conclusion that the average dropsize after atomization of a turbulent jet was almost independentof t he je t Reynolds number, but approximately proportional tothe orifice diameter. Sliepcevich, Consiglio, and Kurata ( 186)reported on the development of a vibrat ing-type atomizing nozzle.Sohngen and Grigull(187) measured the spray angles and flow ratesfrom eighteen swirl type pressure nozzles and compared the re-sults with calculations made for frictionless flow. Their deriva-tion of a theoretical equation relating the air core to t he velocitycomponents appears to be in error, however, since in derivingtheir Equation 9, it would seem that they differentiated a con-stan t and then divided by zero on both sides of the equation.Taylor (192) developed mathematical formulations to predictunder what conditions the liquid efflux in a swirl .type pressurenozzle will reach the orifice by way of th e boundary layer. Theeffect of viscosity on t he ai r core is predicted f rom this develop-ment.Patents relating to spray drying and spray dryers included aBritish pat ent by Hutcheson (13) or a new type of spinning disk,~1patent for a new type of milk spray dryer by Coulter, Montonna,and Kitzes (39), a process patent for spray drying allyl starch( 4 6 ) ,a French patent for spray drying biological materials in anatmosphere of nitrogen or hydrogen (62),and a patent by Hall(78) for spray drying milk and other liquids by first preheatingthem above their boiling points and then spraying into heated air.

General studies of spray drying and spray-dried products in-cluded such articles as an improvement in spray drying techniquefor food (69), maintaining high quality in dried foods by spraydrying (82),an experimental stu dy of spray drying pharmaceuti-cals (go), and an inconclusive experimental study, due to poorcontrol of operating variables, of factors affecting the physicalproperties of detergents by Chu, Stout, and Busche (33). Kirsch-baum ( 1 1 2 )prepared a rather comprehensive article on the princi-ples of spray drying and th e factors affecting the drying of part i-cles and dryer performance, Marshall and Seltzer (139) pre-pared a similar paper bu t included further consideration of atomi-zation and the factors influencing bulk density. Ladisch (126)prepared a general review of spray drying and the economics in-volved. In another general review paper on spray drying, Sjenit-zer (188) presented theoretical considerations of droplet motion,and of the evaporation of drops containing dissolved solids. Heassumed that a drop of dilute solution would evaporate with acontinually changing surface temperature until saturation wasreached. Ranz and Marshall (169), however, showed that this

was not the case, and that a drop of a dilute solution alwaysevaporated as though it were saturated.

The literature indicated that Russian interest in spray dryingincreased in the past two years. In addition to the article men-tioned above by Panasenkov (168),another by this author ( 1 7 )was concerned with a particle size study of spray-dried wholemilk. Particle sizes ranging from 35 to 44.5 microns for wholeand skim milk were observed, and 36 to 38 microns for driedcream. N o diameter larger than 110 microns was observed. ARussian spray drying process for eggs was described by Pankovaand Lyutikova (169),while a Russian milk spray drying plantwas described in detail by Polykovskii (166). A general articleon spray drying as developed by a Swiss firm was prepared byPiatti (165). From England, a general article on the spray dry-ing of synthetic detergents and soaps was prepared by Smith(186),with consideration given to th e characteristics of spinningdisk, pressure, and pneumatic atomizers as used in the soap in-dustry.Tracy, Hetrick, and Krienke ($04 ) reported a study of theeffect of spraying pressure (400 to 3500 pounds per square inch)and orifice size (0.025 to 0.042 inch) on the physical characteris-tics and keeping quality of spray-dried whole milk. Their resultswere inconclusive. Particle size distribution was reported on anumber basis which is always misleading. A weight or volumebasis is preferable in comparing atomization performance. Ex-perimental studies of spray drying a milk-soybean mixture inChina were reported by Tsao et al. (806). Th e effects of inlet airtemperature and the vertical air velocity component on dryerperformance were studied. The well-known fact tha t dryere5ciency increased with increase in air temperature was reported.Evaporation rates of 0.8 to 2. 5 pounds per hour per cubic feet oftower were reported.A performance study of a pilot model spray dryer was reportedby Wallman and BIyth ($13). These workers used sodium sili-cate as a test material and studied the effect of operating vari-ables on particle size and size distribution. Spinning disk atomi-zation was used. More reports of this type from our industr iallaboratories are desirable. The reluctance of the chemical in-dustries to publish some of their findings with regard to spraydrying indicates one of two things, (a ) either t he know-how is re-garded as an important trade secret or ( b ) the industrial devel-opments in tbis field are not significantly worth reporting a t thistime.

The continuing large number of articlesrelative to infrared drying indicate tha t thi s method still appealsto many engineers, in spite of t he fact tha t it must be able toovercome the disadvantage of high power costs. The literatureon infrared drying appears to fall into two categories: (1)articleswhich review the principles and fundamentals of radiant energyand sometimes report experimental data, and (2 ) specific plantapplications where infrared drying has been used.

Literature in the first category will be cited first. A paper byBroughton ( 21 reviewed the principles of infrared heating withparticular reference to paper drying. In a paper by Engel (61th e principles an d problems of infrared drying were consideredand i ts application as a supplementary drying method in th epaper industry discussed. Faggiani ( 5 4 ) offered some technolog-ical considerations of infrared rays, and reported data on the in-frared drying time for coated films and fabrics. Kolbe ( 1 1 7reported on the optics involved in infrared drying, such as theeffect of wave length on reflectance, absorption, and transmission.Landfermann (187) also considered the effect of wave length inthe iufrared drying of paint, and concluded tha t only transparentpaints showed variations in absorption with n-ave length. Land-fermann (126) in another paper presented a theoretical compari-son of the heat flux possible with infrared and convection heat-ing. Martens (140, 141) prepared two reviews on the principlesof infrared drying. The infrared absorption bands for water ofcrystallization, reported by Matsumura (I@), showed tha t maxi-

Infrared Drying.

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mum absorption occurred a t a wave length of about 3 microns.Shuman and Staley (181 discussed the radiat ion absorption prop-erties of materials as a guide in the selection of radian t energytransparent supporting media fora given wavelength distribution.However, they neglected to discuss the difficulties of measuringtransmission for diffuse materials, where transmitted energy isscattered and not detected in the usual spectrophotometers.Yagi and Kunii in a series of papers (887-829) reported results ofexperimental infrared drying and heating studies, and proposed adesign procedure for infrared dryers. They proposed correlatingdrying da ta on t he basis of the rat io of the volume occupied bywater to the volume of voids instead of the usual moisture contentthe prediction of the critical moisture content for thin layers.However, this procedure would require da ta on the volumeof the voids, so that it appears questionable as to whetherthis proposal has an y real advantage. Wingard and Rozier(184) made experimental comparisons of infrared drying andconvection drying. Seven different materia ls were studied ina modified compartment-type dryer, and the not very startlingconclusion was reached that a combination of steam and infraredheating gave the highest drying rates. Zamzow and Marshall(131) reported the results 'of preliminary experiments on theapplication of radiant energy to freeze drying. Experimentaldrying time curves were given for an organic dye supported ona radiant energy transparent medium.In the category of specific applications several articles ap-peared. The use and efficiency of infrared radiation in dryingmolds and firing cores was reported (68). The use of commercialinfrared lamps t o dry cellulosic materi als with experimental de-terminations of product temperature was reported by Fujii andKageyama (66). It o and Hoube (99) reported tests on the in-frared drying of ceramic pieces for various source temperaturesranging from the temperatures of incandescent lamps to hotplates a t 130" C. Jubitz (108)eviewed progress of infrared dry-ing in Germany and in other countries. Keylwerth (110, 111)presented a general discussion of t he use of infrared in the woodindustry, and trea ted the principles of electric and gas radiatorsNarayanamurti and Prasad ( 1 4 6 ) also considered the applicationof infrared to wood drying, and treated veneer drying in particu-lar. They compared the advantages and disadvantages of in-frared drying, and decided that it held promise only for a highcost material such as veneer. They reported energy consump-tions per pound of water removed between 0.75 and 2.5 kw.-hr.Efficiency increased with decreasing thickness. Ni tta, Sugi-moto, and Nakai (163) resented curves showing the rate of lossof moisture b y fish of different thicknesses for infrared drying.The usual conclusion wasreached-namely, infraredis not econom-

xr ical for low-priced products. A novel adapta tion of infraredlamps to a double screw conveyor dryer for tungsten was de-scribed (171 ) . Dryer length was 19.25 feet, and screw speedwas 1.25 r.p.9 . An account of thedrying of hard porcelain by infrared, together with the apparatusand cost data, were reported by Vindreau and Ardouin (210).Drying by Sublimation. In the field of sublimation or freezedrying, interesting and new applications were reported togetherwith the usual crop of review articles. Among the latt er werearticles by Beckett (11) who surveyed the general aspects offreeze drying, Flosdorf (67)who summarized the general materia lhe has written during the paat 10 years, and Goddard ( 7 1 ) whosummarized American practice in large-scale freeze drying.Guess and Burlage (73) discussed freeze drying as applied topharmaceuticals. A compilation of papers presented in Englandat a symposium on freezing and drying with special attention tothe biochemical and biological field was edited by Harris (81).One of the more interesting aspects of th is symposium was theconclusion tha t very low moisture contents do not appear essen-tial to the preservation of bacteria, which is contrary to the prac-tice with respect t o antibiotics.

4 on a weight basis, The authors claim that this method permits

c

Operating costs were given.*

New fundamental da ta on freeze drying were contributed in afew articles. The value of freeze drying for the preservation ofmold cultures waa reported by Fennel], Raper, and Flickinger(66). Results of viability studies covering periods up to 7 yearswere reported. Freeze drying appeared to be a superior methodof preservation of the several methods studied. Unfortunately,no dat a were given on the final moisture contents of the variousfreeze-dried mold cultures. Guigo (74)presented a theoreticalbasis for sublimation drying of dairy products, and drying rateequations were presented. An equation for th e critical moisturecontent was given as well as an equation for the total dryingtime. Kramers and Stemerding (190) reported on studies of th era te of sublimation of ice in vacuo, and showed that their datacould be correlated by equations proposed by Carman (16).Zamzow and Marshall (831) reported some freeze drying experi-ments in which radiant energy was transmitted t o the frozen ma-terial through a semitransparent retain ing medium. Comparisonswere made with conduction heating, and it was observed thatwith the radiation method significantly higher, sublimation rateswere possible. This conclusion should be restricted only to thosematerials in which heat transfer controls the drying rate.

A number of papers appeared dealing with specific applicationsand developments of freeze drying. One interesting new de-velopment is th e process whereby freezing is accomplished by dis-persing and freezing the material in a cold solvent (30). Th efrozen pellets are then freeze-dried. The rate of freezing is veryrapid by this method. Gane (69) described an experimentalfreeze dryer for foods, This unit was comprised of six trays.Results of freeze drying tests on th e qual ity of apple slices, eggs,minced beef, and fish were reported, together with curves oftemperature, weight, and heat input us. time. Snap freezingwas used.Guigo and Gulyaeva (76) reported results of subl imation dryingof casein. The application of radiant freeze drying to dry ad-renalcorticotropic hormone (ACTH) was described (84). Twopatents on methods for the removal of water vapor in freeze dry-ing were issued (88,89). The principle underlying each of theseis the removal of the condensed phase as soon rn it is formed. ADutch patent (188)proposed supplying the heat for freeze dryingby condensing water vapor on the exterior of t he containersholding the frozen solid. Nickerson, Coulter, and Jenness (161)concluded there were no product advantages in freeze dryingmilk compared with spray drying. Since this was a preliminarystudy and the drying variables were all held constant, this ap-pears to be a premature conclusion. Th e freeze dryer wrn no tdescribed, nor were the experimental procedures well detailed.The results on the properties of milk were of in terest , but to makeany final conclusions regarding freeze drying us. spray dryingseems improper, The research done on freeze drying milk todate has been minuscule compared to that done on spray drying.Therefore, definite conclusions comparing the two methods cer-tainly should await the results of further research. Talburt andLegault (190) reported further dat a on their new process of dryingand then freezing (dehydrofreezing) with specific reference topeas.High Frequency Drying. As in the case of infrared drying,continued interest was shown in high frequency drying in spiteof its high cost-higher tha n infrared. A study by Alexanderand Meek (9) gave an insight into th e mechanism of drying tex-tiles b y radio frequency, and showed tha t the usual periods of dry-ing occurred even with this method. The change from the con-stant- rate to the falling-rate period was accompanied by a changein the dielectric propert ies of the textiles. These authors alsopresented a general discussion of the use of high frequency inheating an d drying in the textile industry (4 ) . Billig ( 1 6 ) pre-sented da ta on the use of high frequency in timber processing, andreported energy concentrations of 20 to 25 w.-min. per cubiccentimeter to dry thin sections of timber. A patent issued toDippel, Lely, and Dikhoff ( 4 6 )described the application of radio-

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50 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 45, No. 1frequency drying to wet photographic film. The electrodes werearranged so that the lines of force passed through the film in thedirection of motion. Hagopian (76)discussed the advantages ofdielectric heating in the drying of rayon cakes. Iwashita andKoizumi (100)presented results of experiments on the drying ofwood by radio frequency. Klages (1 1S) discussed the techniquesof high frequency and the mechanism of the method in the dryingof lacquer. A review of the problems, advantages, and disad-vantages of drying ceramics by dielectric heating was given byMartin (148). He discussed the physical problems involved andthe electrical measurements required in its study . Nelson (147)made a mathematical development of formulas to predict tem-perature gradients in materials undergoing dielectric heating.Results are given for various boundary conditions. A Russianinvestigation of wood drying by high frequency was reported(148),and a description was given of a 50-kw. high frequencygenerator developed for wood drying. A fundamental study ofhigh frequency drying of granular solids by Nicol (162) gave re-sults of experiments on the heating of water and a series of saltsolutions, and on the drying of glass beads by high frequencycurrents. Moisture gradients were reported and a mechanismof drying was proposed. Yamamoto et al . (230) reported thatcotton yarn could be dried with a frequency of 37 megacycles in5 to 15 minutes. The yarn had properties similar to th at dried incool or warm air for 3 to 8 hours.A few applications of drying whereinthe wet solid is dwpersed in a hot gas stream were described.Flood (66) described flash drying of sludge for several sewagetreating plants. Parr y and Wagner (160, 61) described pilotplant tests on th e drying of low rank coals in fluidized beds.Moisture was reduced by 90 to 95% in a drying column into whichgas jets entered at a velocity of 80 feet per second. It was shownthat drying would be economically feasible when shipping dis-tances were such that the shipping cost exceeded $0.65 per ton.Chapman and Needham (18) lso considered the problem of dry-ing washed coal in relation t o cost of transportation and wherefreezing may occur. In another article on pneumatic conveyingdrying, Vranian and A-ickerson (211) described the drying oflime kiln feed.

An interesting ex-periment using a radioactive tracer to establish the retentiontime of seaweed in a roto louvre dryer was described by Gardeneret al . (70). They added 100 microcuries of radioactive o-phos-phoric acid t o sea water in which the seaweed was soaked. Re-sults showed that for an average retention time of 41 minutes,some material passed through in 15 minutes while some was re-tained for 80 minutes. Luethge (132) proposed four short-cutformulas. t o improve the evaluation and operation of ro tarykilns.

Vacuum drying was given some attention, both theoreticallyand experimentally. Arnborger (6) presented experimental re-sults and data for the vacuum drying of extracts. Even vacuumdrying is not free of th e falling ra te period, for the material tem-perature was reported to rise when a critical moisture content wasreached. Dunoyer (48)presented a study of vacuum drying asinfluenced by the usual parameters, and he also prepared twolengthy articles (49) on the theory of vacuum drying.

Miscellaneous Methods.

Few articles on rotary drying appeared.

DRYING FUNDAMENTALSMechanism of Internal Moisture Flow. Fundamental studiesof drying will inevitably involve studies of the mechanism of

moisture flow in solids. Because of the fundamental nature ofthis phase of drying, many of th e underlying principles will comefrom basic studies made in various branches of th e physicalsciences. Thus, the study of Ananyan (6) on the mobility ofwater and ice in frozen soil may be of value in the interpretation ofa freeze drying mechanism. A study by Booth (19) on the effectof surface conditions on the electrophoresis of solid particles mayin time contribute to studies of internal flow. Christensen ( S I )

and Christensen and Williams (52) contributed to t he theory ofdiffusion in wood and porous media. Crank and Robinson (40continued to report experiments on diffusion in cellulosic mate-rials, and described interferometric studies of diffusion to establishthe influence of concentration and orientation on diffusion incellulose acetate, Further studies on diffusion and the effects ofconcentration on the diffusion coefficient were reported by Fuji taand Kishimoto (68). Fujita (67) also presented a numericalsolution of the differential equation for absorption-controlleddiffusion in solid. Fundamental studies by Klinkenberg ( 1 1 4on the analogy bebeen diffusion and electrical conductivity inporous rocks may be a tool available to chemical engineers fo rstudying the internal mechanism of flow. This method was usedearlier by Burr and Stamm ($4) o verify Stamm's ( 1 8 9 ) heoreti-cal calculations for the mechanism of flow in wood. Hutcheonand Paxton (96)made measurements of moisture migration andthermal conductivity on moist spruce sawdust enclosed betweenhot and cold surfaces. A vaporization-condensation phenomenonwas observed as the moisture redistributed itself. Steady heatflow was not established until the moisture migration was sta-bilized. They found the thermal conductivity to be only slightlyaffected by moisture in the system: Lykov (133) presented atheory of drying ceramic materials based on the internal mecha-nism, He classified ceramics as colloidal, capillary-colloidal, andcolloidal capillary-porous Fundamental studies on the internalmechanism came out of England during the past 2 years. Newittand Coleman (149,160) eported on the absorption phenomena indrying clays; Pearse, Oliver, and Kewitt (162),Oliver ( 1 5 5 ) ,andOliver and Newitt (166)contributed new data and studies on theforces causing flow in granular beds, on the measurement of capil-lary forces in drying, and on the moisture gradients resultingtherefrom. Peck, Griffith, and Rao ( 1 6 3 )studied the surface andinternal resistances in the falling-rate period of drying and pro-posed using an average diffusion coefficient for this period eventhough th e flow mechanism might be one of capillarity. Rogersand Morrison (179)studied convection currents in porous media,and extended their theory of critical gradients. Two methods forthe measurement of diffusion coefficients in gels were described bySalvinien (176) and by Salvinien et al. (176). These methodswere: (a)observing in the gel the advancing boundary of a precipi-tate as diffusion occurred, and ( b ) utilizing a radioactive tracerin a tube of cast gel which was sliced and analyzed for the tracerconcentration exactly as in moisture dietribution determinationsTanner and Hanks (191) eported on studies of moisture hystere-sis in so-called gypsum moisture blocks. Drying and wettingcurves were measured for a range of moisture contents. An opti-cal investigation of the Soret effect (thermal diffusion) was re-ported by Thomaes (193). This effect has received little atten-tion as a mechanism fo r moisture movement in solids and anynew da ta are welcome. A detailed study of the complex internalstructure of peat a nd its influence on the drying of peat was re-ported by Westlin (281). considerat ion was given $0 the mannerin which water was held in the peat and how this would influencedrying by various methods, such as freezing, cataphoresis, press-ing, air drying, and drying with superheated steam. Economicconsiderations were also given.Heat and Mas s Transfer. This phase of drying has been as-sociated with the so-called external mechanism-i.e., where hea tis transferred to the surface of the material while mass is trans-ferred away. Studies of drying from this standpoint have provedto be more fruitful for design and operation purposes than havestudies of the internal flow mechanism. Studies importan t tothis phase of drying include such works as the article by Daviesand Walters (44)on the effect of a finite width of area on the rateof evaporation in a turbulent atmosphere, and a report by Four te t al . (60) on the factors influencing the rate of t ransport of hea t tofabrics and the transfer rat e of water vapor away during dryingIn considering the stationary evaporation of a liquid at differentemperatures of an evaporating and a condensing surface, Gran-

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January 1953 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 51ovskiI (72)presented an analytical solution of the problem for thecase of no foreign gas. This work would be expected to have im-portance in the field of high vacuum drying, especially freeze dry-ing. Several hea t and mass transfer studies involving dropletshave previously been mentioned under spray drying. These in-clude the studies of Ingebo (98) for pure liquid drops, Johnstoneand Eads' (101) tudy of th e vaporization of small sulfur droplets,and the work of Ranz and Marshall (169)on the evaporation ofpure liquid drops and drops containing dissolved and suspendedsolids. Reference is repeated here to Langstroth et al . ( l a g ) andLuchak and Langstroth (131) for theoretical and experimentalconsiderations of the evaporat ion of drops. Kuhrt (124) ex-the effect when the drop is moving. Other references for heat an dmass transfer to drops as discussed above under sp ray drying are(202, 106, 218).

Timofeev (201) presented equations for evaporation from awater surface int o a turbulent air stream, and extended the treat-ment to other surfaces such as ice, mercury, and methanol.Knacke et at. (115)made a mathematical st udy of th e theory ofevaporation rates. The study, based on liquid and solid models,gave good agreement with da ta for evaporation of certa in crystals.This work is highly fundamental.

Wenzel and White (280) reported on a study of drying in anatmosphere of superheated steam, and presented da ta t o showthat higher drying ratcs were possible with this process than withair but a t the expense of a greater capital cost. More dat a ofthis type a re especially desirable.

Methods of measur-ing humidity are fundamentally important in drying studies.Hinzpeter an d Meyer (92) described a glow discharge procedurefor measuring th e humidity of air in vacuum at pressures rangingfrom 10 to 100 microns. Prudhomme (167)proposed a methodfor measuring the humidi ty of air by measuring th e temperaturerise of 98% sulfuric acid in contact with humid air. This methodwas compared with the sling psychrometer, and appeared t o bemore accurate for large wet-bulb depressions. Prudhomme (168)also prepared a review of physical and chemical methods ofmeasuring th e water content of atmospheres, gases, and vapors.A similar review with methods of testing hygrometers was pre-pared by Wexler and Brombacher (222). Webster (217) n-vestigated the effects of tota l pressure on the water vapor pres-sure in a saturated compressed gas

Studies of equilibrium moisture content of textile fibers at highhumidities were made by Ashpole ( 7 ) . Bond (18)described themanner in which th e submicroscopic pores of coal become filledwith moisture, the properties of which differ from those of normalliquid water. Heats of wetting of cellulose and regenerated cellu-lose, and the adsorption isotherms at 30 O C. were determined byWahba (212). A fundamental analyticalprocedure in any drying research or process is the determinationof the moisture content of the product. The importance of themethods of moisture analysk is reflected in the number of litera -ture articles bearing on th is subject.. Many of the articles com-pared the application of various well-known moisture analysismethods to specific materials. Others dealt with specific proce-dures such as conductance and dielectric methods. A large per-centage of the articles treated t,he determinat ion of moisture infoods.

An instrument for measuring moisture in grains and foodstuffsby a conductance process was given (27) . Barlow (9) concludedthat t he Karl Fischer method approached stoichiometrical accuracyfor water in most foodstuffs. Belanger (12) compared vacuum,forced air, and atmospheric oven methods for determining mois-ture in meats. Chappin de Janvey and Francois (19) ecom-mended t he Karl Fischer method for determining moisture in oilseeds. Common (38) discussed the difficulties encountered indetermining the moisture content of food and reviewed the prob-

* tended t he theory of the vapor pressure of small drops to include

*

Hygrometry and Equilibrium Mksture.

Methods of Moisture Analysis.

lem of determining the amount of bound water. Ev a (63)compared methods of distillation, total solids, and electricalcharacteristics for determining the moisture in cereals and leg-umes. Barber (8 ) reported on the determination of moisture incoal. Brochmann-Hanssen and Pong (20) modified the KarlFischer method with a reagent containing ethylene glycol mono-methyl e ther instead of methanol to determine the moisture con-te nt of medicinal chemicals and other drug product s. Broughtonand Hobbs (22) , n an investigation of the factors influencing themoisture determination of paper in oven drying, concluded thattemperature was the single most important variable to control.Bryson and Pickering (23) developed a procedure for the deter-mination of moisture in coal by the Karl Fischer method. In acomparison of the moisture analysis of drugs by solvent distilla-tion, oven drying, and infrared drying, Collett (37) concludedthat infrared drying is satisfactory for control purposes. Typicalinfrared drying curves were reported. Eberius (60)made a criti-cal analysis of the appli cation of the Kar l Fischer method t o ex-plosives and related materials. Fryd (63) eviewed methods ofmoisture analysis for foods and organic substances. Fry d andKiff (64) discussed the difficulties involved in determining thetrue moisture content of tobacco. Jalien (103) onsidered sixmethods of moisture analysis for dairy products. Kamiyoshi andMiyamoto (104) reported tests on a portable instrument t o meas-ure th e dielectric constant of wet material s, and found it t o be in-dependent of the sample used. This conclusion is undoubtedlyrestricted to granular materials of the type tested. Kawada andUohida (106) developed a so-called hot-wire moisture meterwhich operated, in principle, similar to the thermal conductivitymethod of th e measurement of moisture in gases. From a com-parison of oven drying, vacuum drying, infrared drying, toluenedistillation, and the Karl Fiecher method for the determination ofwater in cocoa and chocolate products, Kentie and Barreveld(109) concluded that the Karl Fischer method gave the most re-liable results. Makower (137) reviewed and summarized themethods of moisture analysis for dehydrated foods. Sakagamiet al. (173) studied the measurement of moisture in textiles byhigh frequency, and concluded that when the textiles were com-pressed (174) o a constant density th e dielectric constant becamemore stable . Another review of methods of estimating moisturewas prepared by Ward (216).

DRYING SPECIFIC MATERIALSIn the foregoing sections, the drying literature wm reviewed

from the standpoint of methods and principles. Many articlesappeared, however, which must be classified under the drying ofspecific materials. Many more articles appeared tha n have beenreported here. Only those th at appear to have more tha n tem-porary value and more than specific interest have been indicatedbelow. A review article by Beckemeyer (10) n th e drying ofceramic ware has already been cited under general articles. Clegg(35) described various types of conveyor dryers used for ceram-ics. In a re-view of d rying methods in the pottery industry, Hind (91) dis-cussed methods of regulat ing the water content of the raw ma-terial, an d reviewed clay-water relationships and their influenceon drying. Th e effect of th e plaster mold on drying speed wasconsidered. Consideration was also given to th e use of high fre-quency heat ing and vacuum drying. Keller and Gorazdovskil(108) reported on the problem of the cracking of ceramic shapesduring drying. The drying of ceramic bodies electronically andwith infrared was considered by Kohler (116). A review of dry-ing ceramic products was prepared by Krause (181). Anotherreview article on the basic problems of drying bricks, tiles, andheavy clay articles, and the types of dryers used was presented byMacey (135). Newitt and Coleman (160) described the mech-anism of moisture migration in china clay. Roberts (170) pre-sented drying rate curves and shrinkage data on the drying of

Ceramics.

Hall (79) iscussed the testing of a tunnel kiln.

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Vol. 45, No. 12 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Yrefractories, and discussed precautions necessary to preventcracking. Walter (814) reviewed the types of dryers used in theceramic industry . Williams-Gardner (123) added another re-view article on pott ery drying and surveyed the problems of dry-ing, types of dryers, layouts, and th e stages of drying in relationto the methods of heat transfer. Zhukov (132) eported operat-ing dat a on air velocities and temperatures for drying structuralbrick.

In addition to articles onfood drying discussed above under spray drying, a number ofothers appeared. Only th e more significant ones are given here.Hathaway e t al . (85')studied the effect of drying temperature onth e nutr itive value and commercial grade of corn. They con-cluded corn should not be dried above 140" F. Aceto e t al. ( 1 )developed a process for producing high-vitamin, high-proteinleaf 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 lowtemperatures. Harler (80) presented an interesting review ofthe history and present methods of drying tea . Hobbs (93)gave an informative account of early sugar drying methods, anddescribed current British. practices of sugar drying in a rotarydryer. Th e heat for drying sugar from 0.95 to 0.03% moisture isprovided by the sensible heat of the sugar itself, and the air inpassing through the dryer is heated rather than cooled. Operat-ing dat a were reported. Scott (177) eviewed drying methods inth e manufacture of dairy products. Sen Gup ta (178-180) madestudies of the dehydration of mea t. An electron microscopestudy of spray-dried milk powder by Villanova and Ballarin (209)showed that the milk components were distributed uniformlythroughout the particle. Woodforde (225) concluded that therate of drying grain and seeds was largely controlled by th e rateof internal moisture flow to the grain surface. Drying systemsand types of dryers used were also described. Tosello and Veiga(103) ompared the drying of starch in vacuum and with hot air.The degree of dextrin formation in hot ai r drying was held to0.3%. An informative review of me thods andequipment used in th e drying of staple fiber was given by Coles(5'6). He reported that the drying rat e for staple was found t ovary as th e 6/&h's power of th e mass velocity. This is some-what higher than theory and other experiments have indicated.An 800-ton-per-day kraf t board drying machine was describedby Crowder ( 4 1 ) . Drying of package yarns by alternately blow-ing hot air or steam from outside to the inside, and then viceversa was discussed by Hall ( 7 7 ) . Welo e t al. (119)made use ofthe periods of drying and th e critical moisture content to deter-mine the swelling capacities of fibers in water. No contributionwas made t o drying theory.A rather large number of articles appeared onthi s subject. Many dealt with specific methods, such as theseasoning of wood a t temperatures below the boiling point ofwater (25) ,new dry kiln practices (87) ,and lumber drying by th evapor process in which Hudson (94)compared the use of naphthaand perchloroethylene vapors. Several papers dealt with thetheory and mechanism of wood drying. Czepek (48)consideredth e internal mechanism of drying pine containing 25 % moisture,and reported tha t temperatures up to 140"C. could be used with-out degradation. Kollmann et al. (118)studied t he discolorationof whole wood and veneer during drying and suggested fourcauses for the discoloration. Kroll ( 1 2 2 ) in two articles pre-sented the theory and mechanism for moisture movement in pineor soft wood during drying. Maku (15'8) eported that the dif-fusion equation could be used with good accuracy to estimate thedrying time and average moisture content for drying below thefiber saturation point, no t a new finding. Above th e fiber satura-tion point, agreement was not so good. Ogurta (154)also madestudies of th e mechanisms of wood drying.A description of lumber dryers used in Russia was given by

Foods and Agricultural Products.

Textiles and Paper.

Wood Drying.

Peich (16.4). Tiemann (194-199) presented an extensive series ofarticles describing the kiln drying of lumber . He also discussedth e phenomenon of collapse in wood drying (am). Keylwerth( 1 1 1 )reported on following the course of kiln drying by measuringthe temperature in different regions during drying. A report bythe Forest Products Laboratory (207) reviewed and summarizedproperties of wood as related to drying in order t o aid in under-standing kiln drying problems. Tables of da ta of importantwood properties were provided.Miscellaneous. A few miscellaneous articles are cited. Arapid process for drying motion picture film was reported by Kata(105). The equipment was designed on the basis of theoreticalinvest igations of the drying problem, and the use of turbulent airgave rapid drying with lo w film distortion.

In a general paper, Kay (107) reviewed methods of drying toremove surface moisture from fine crystalline chemicals, andconcluded that a vibrating conveyor with infrared offered sub-stantial advantages over oven and vacuum methods.

Henaker (85) and Wudich (1B 6) reported on investigationsmade of the pasting process in drying leather. The formerstudied various adhesives, but found no single superior formula-tion. Wudich concluded th at in the pasting process the relativehumidi ty should be in the range of 50 to 60% and th e tempera-ture 40' to 50' .

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(214) Walter, L., C er amics , 3, 537-42 (1951).(215) Walter, L., F o o d M a n u f . , 25, 105, 155, 197, 239, 289 (1950).(216) Ward, A. H., J . Ins t . Fue l , 24, 16-19 (1951).(217) Webster, T. J. , J . SOC. h e m . I n d . , 69, 343-6 (1950).(218) Weinberg, S. , r i t . J . A p p l . P h y s . , 2, 363-6 (1951).(219) Relo, L. A., Ziifle, H. M., and Loeb, L., Textile Research 6..(220) Wenael, L., and White, R. R., IND .NG.CHEM., 3, 1829-37(221) Westlin, Arne, Tek . Tidskr.,81, 717-23 (1951).(222) Wexler, A.. and Brombacher, W. G., N a t l . B ~ L P .t andar ds ,(223) Williams-Gardner, A., T r a n s . B r i t . C e r a m . Soc., 49, NO. 3,(224) JVingard, R. E., and Roaier, TV. H., Alabama Polu tech . Inst.Bul l . , 47, No. 4; E n g . E x p t . S t a . E'ng. Rzdl. 15, 3-14(1952).(225) Koodforde, J., Engineer ing , 171, 669-73 (1951).(226) Wudich, W., Das Leder , 2, 242-5 (1951).(227) Yagi,S. , nd Kunii, D., Chem. Enp. ( J a p a n ) , 1 5 , 108-16 (1951).(229) Ib id . , pp . 13-17.(230) Yamamoto, I. , Uchida. H., Aoki, A , Tominaga, I ., and Has-umi, S., J . SOC . Textile and C el lu los e Ind . , Japan , 6, 36-41(231) Zamzow, W. ., and Marshall, JT. R., J r . , Chena. Eng. Progr.,(232) Zhukov, D. V., Steklo i K er am. , 1, No. 1, 14-17 (1950).

( L o n d o n ) ,27, 9-18 (1949).

NO.2, 33-5 (1952).22, 254-61 (1952).(1951).974-82 (1950).N O. 2B, 57-66 (1950).

C h e m . E n g r s. ( L o n d o n ) ,27, 1-8 (1949).44, 64-9 (1952). 12 94 0 (1952).

(162) Pearse, J. F., Oliver, T. R., and Newitt, D. &I., Tr ans . I ns t .(163) Peck, R. E., Griffith, R. T. , and Rao, N. K., IND.NG. HEM.,(164) Peich, N. N., Les naya Pr om. , 11, 6, 22-6 (1951).(165) Piatti, L., Sulzer Tech . Rev . ( Sw i t z . ) , 1950, No. 3, 19-26.(166) Polykovskii, A, , Molochnava Pr om. , 10, No. 12, 23-5 (1949).(167) Prudhomme, A., Ann. gdophys., 5 , 293-309 (1949).(168) Ibid. , 6, 117-27 (1950).(169) Ranz, W. E., and Marshall, W. R . , Jr., C h e m . Eng. Prop-., 48,(170) Roberts, A. L., J . Inst. Fuel , 24, So. 137, 126-8 (1951).(171) Rock Products , 52, No. 9,80 (1949).(172) Rogers, F. T., and Morrison, H. L., A p p l . P h y s . , 21, 1177-80(173) Sakagami, T., Eba ta, T., Kaino, I., Tanaka, Y., andTakagishi,(174) Sakaguchi, T., Ebata, T., Kaino, I. , Tanaka, Y . , and Taka-gishi, I., J . Soc. Text i l e and C ellulose Ind . , Jap an , 7, 491-4(1951).(175) Salvinien, J. , J . C h e m P h y s ., 48, 465-70 (1951).(176) Salvinien, J. , Marignan, R., and Cordier, S., J . C h e m . P h y s . , 48,(177) Scott, D., A u s t r al i a n J . D a i r y T e c h n ol . , 5 , 83-94 (1950).(178) Sen Gupta, P. N., J . I n d i a n C h em . S o c ., I n d u s . a n d N e w s E d . ,(179) Ib id . , pp . 125-34.(180) Ibid. , pp . 134-47.(181) Shuman, A. C., and Staley, C. H., Food Technol . , 4 , 481-4

Circ. 512 (Sept. 28, 1951).

141-6, 173-80 (1952). (228) I b i d . , 16, 7-12 (1952).

(1950). (1950).I. , J . Sac. Text i le and C ellu los e Ind . , J apa n , 7,360-72 (1951). 48, 21-32 (1952).

471-3 (1951).

14, 75-84 (1951).

(1950).Sjenitzer, F., C hem. Eng . Se i . , 1, KO. , 101-17 (1952).Slade, F. H., Food, 20, 366-9 (1951).Ibid. , pp . 426-31.Sliepcevich, C. M., Considio, J. il., and Kurata, F., IWD. NG.Smith, M. W., M f g . C hemis t , 22, 186-7 (1951).Sohngen, E., and Grigull, L-., Forsch. Gebiete Ingenieurw., 17,Spaander, J., Mastenbroek, G. G. A., and Seffinga, G., DutchStamm, A. J. , U. S. Dept. dgr., Tech . Bu l l . 929 (194G).Talburt, W. F., and Legault, R. R. , Food Technol . , 4, 286-91

CHEM., 2, 2353-8 (1950).77-82 (1951).Patent 69,176 (Dec. 15, 1951).(1950).Tanner, C. B.. and Hanks, R. J. , Soi l Sei . Soc . A mer . , Pr oc . ,16, 48-51 (1952).129-39 (1950).Taylor, G. I., Q u a r t. J . M e c h . a n d A p p l . M a t h . , 3, Part 2,Thomaes, G., Phvs ica , 17, 885-98 (1951).Tiemann, H. D., Souther n Lumber man , 183, No. 2293, 58, GO,74 (1951).Ib id . , NO.2297, 340-2 (1951).Ib id . , 184, No. 2299, 54, 56 (1952).7h id No. 2301. 52. 54 (1952).- - - - - -Ibid. , No. 2303: 56 , 58 '(1952).Ibid. , No. 2305, 116, 120, 126 (1952).Tiemann. H. D., Wood Wor k ing Dig . , 54, 95-8, 100-1 (1952).Timofeev, M. P. , Uchenye Z a p i s k i Leningrad. Gosudars t .Univ. . A . Zhdanova , Ser. F i z . N auk . , No. 7, 202-40 (1949).

Topps, J. E. C. , J . ns t . Pe t r o leum, 37, 535-53 (1951).Tosello, Andre, andVeiga,A. deA.,Bragantia , 10,357-63 (1950).Tracy, P. H., Hetrick, J . H. , and Krienke, W. -4.,. Dai r y Sc i .,Tsao, P. H., Shen, F., and Tai, H., Eng. Rept s . Xatl. T s i n gTverskaya, N. P., I z ves t i ya Akad . Nauk S.S.S.R., Ser . Geograf .U. S. Dept. Agr., Forest Service, Forest Products Lab., Rept .Van Arsdel, W. B., U. S. Dept. Agr., Bur. Agr.. and Ind. Chem.,Villanova, A . C., and Ballarin, O., Lait, 30, 113-22 (1950).Vindreau and Ardouin, I n d u s tr i e c h a m . , 393, 262-7 (1948).Vranian, H., and Nickerson, R. D., T a p p i , 35,11-13 (1952).Wahba, M., J . P h ys . & Colloid Chem., 5 5 , 1148-60 (1950).Wallman, H., and Blyth, H. A,, IND.NQ.C H E Y . ,43, 1480-6

34,583-92 (1951).Hua Univ . , 4, 115-26 (1948).Geofz . , 14,No . 2, 164-70 (1950).1900-1 (1951).Mimeographed Circ. , Sei-. AIC300 (1951).

(1951).COURTESY BUFLOVAK DIV.. BLAW-KNOX CO.

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