Samuel J. Friedman, of the D u Pont Company, Wilmington, Del., was born in Cleveland, Ohio, in 1918. H e was educated a t Case School of A p p l i e d Science, where he received his B.S. degree in chemical engineering in 1939 and his M.S. degree in 7941. H e has been a member of the staff of the Chemical Engineering and Metallurgical Laboratory of the Engineering Department, E. 1. du Pont d e Nemours & Company, Inc., since 1 9 4 1 , where he has warked primarily in the fields of heat transfer and drying. Friedman is a member of the American Chemical Society and the American Institute of Chemical Engineers.
DRYING Samuel
D
EMANDS on the unit operation of drying in the past two years have arisen primarily as a result of the war effort. As such, they are reflected in developments in the drying of foods, blood plasma and pharmaceuticals, paints, and heavy chemicals. The four types of drying which have received the greatest impetus have been drying by sublimation, infrared drying, dielectric drying, and spray drying. DRYINGBY SUBLIMATION. Sometimes termed "lyophilization", this type may be defined as the process of dehydrating a frozen substance. Materials dried in this manner are frozen, and the water is removed by sublimation, usually into vacuum. Although this method has been known for a long time, it was not until the large scale production of dried blood plasma and penicillin were demanded that commercial units were developed. A number of different methods of drying blood plasma by sublimation have been reported (22, @, 66, 99),but all of them consist of rapidly freezing the plasma and then removing the water by vacuum. Heat must be supplied to the frozen material during the operation, and final drying can be done at temperatures in excess of the freezing point of water. Most penicillin is dried by this method, and the processes employed are extensively described in the literature (14, 28, 40, 61, 88). During the latter stages the drying can occur a t temperatures in excess of 32" F. Both infrared (88) and dielectric heating (12) have been used, It is claimed that dielectric heating reduces the drying time more than 50y0 (12). Some attempts to dry foods by this method have been described (37, 41, @, 68). Orange juice (41) and meat (59) dehydrated in this manner possess better storage qualities, suffer less degradation, and reconstitute to a better product. This method costs from 0.2 to 2.0 cents per pound of water removed and, therefore, appears attractive only for the drying of materials such as pharmaceuticals, foods, and expensive chemicals in which final quality is of major importance. Unfortunately this means of drying must still be considered an art, since no data have been published which permit estimation of drying cycles or equipment sizes from the characteristics of the material to be dried and the operating variables. INFRARED DRYING,This method utilizes radiant energy of the infrared region to supply heat to the materials being dried. In the last two years it has been used primarily for the drying of paints and as a booster heat supply for other methods of drying. Developments have been reported with both gas and electricity as the source of radiant energy. Infrared radiation has been used for some time for the drying 22
J. Friedman and baking of paints. Recent articles are concerncd mainly with performance data (2, 3, 5, 6, 8, 89, 98) and with comparisons of electric energy with gas energy as the source of radiation (38, 104). Considerable controversy has existed as to whether convection ovens or infrared ovens are better adapted for the drying of paints (34, 56). The general consensus of opinion is that a combination of the two methods is probably,the best solution. This type of drying has aroused some interest in the textile industry. It is reported that infrared radiation can be used for singers, tenter frames, slashers, and dryers (13). It is recommended as a booster for the last three operations. Gas-fired radiant burners can be used for complete operations by utilizing both the radiant heat and the heat in the products of combustion by convection (44, 50). It is claimed that a large proportion of the heat is transferred by convection (50). The ceramic industry also reports successful use of radiant energy for drying pottery, glazes, and bricks (52, 103). In drying china coming from an automatic jigger, the drying time has been reduced from 2 hours to 12-15 minutes (49) and in some cases an eighteen fold decrease in drying time has been reported (56). Drying of sawdust (7), smokeless powder (V), fish (%e),and penicillin (88) with infrared lamps has also been described. This type of heat energy is more expensive than other forms, and it would appear that infrared drying is applicable only where heat must be or can be applied rapidly to the material without danger of overheating. Advances in the theory underlying drying by infrared heat in the past two years are presented by Ernst and Schumacher and by Stout, Caplan, and Baird. The first article (93) is concerned with drying of paints by infrared and convection ovens. The authors conclude that there is no significant difference in the drying behavior of the same enamels with the two different types of drying at the same rate of heat flux. For fast-drying enamels infrared drying may prove superior since it is possible to apply heat to the material more rapidly by this method. Stout and eo-workers (100) compare the mechanism of infrared drying of sand, soap, and magnesium stearate to that of atmospheric convection drying and vacuum drying. They conclude that the infrared type permits higher drying rates at equal material temperatures than the other two methods, although the mechanism of water flow to the surface is believed to be the same for all three. As in most other types of drying, the falling-rate period can be expressed by an exponential relation. An increased
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air rate past the surface decreases the rate of drying by increasing convection losses, as could be predicted from the work of Tiller and Garber (106). DIELECTRIC DRYING. The material t o be dried is placed in a n ultrahigh-frequency electric field. The heating and consequent drying which take place in the interior of the material are the result of dielectric loss within the material, This is the only method of drying in which heat can be applied t o the inside of B material without having t o diffuse through it. A detailed review of this method of heating is given by Grimstead (64). Dielectric drying has been used extensively for wood, particularly gunstocks, and has reduced the drying time as much as tenfold. Nonken (74) discusses the application of dielectric drying in the paper industry. He concludes that it probably would not be feasible to dry paper by dielectric heating since it is extremely di5cult t o dry thin sections by this method. The best applications are for thick sections of materials. The initial cost of a dryer of this type is two t o five times that for an oven, and the efficiencyof power conversion is Sherman (97) discusses the applicabi to vegetables. Partially dehydrated beets, carrots, cabbages, and onions were compressed into blocks and subjected to a field of 29,000 cycles per second, Drying was more rapid by this method than by vacuum drying. A cost of one cent per pound of water evaporated is advanced as the probable cost of this method, Rushton, Stanley, and Scott (93) discuss similar experiments. Power consumption was about 50% greater in this work than that of Sherman. Drying occurred slightly faster when the material was subjected to vacuum at the same time it was being heated dielectrically. The drying time with dielectric heating was about one fifth to one tenth that required for convection drying. This method should prove applicable for removing the last traces of moisture from food without injuring it. Dielectric drying has also been reported for drying paints (IO), rayon (23), and penicillin (12). The underlying theory of this type of drying is little known, and much work is needed in this field. SPRAY DRYINQ. This process consists of contacting a liquid or slurry which has been finely atomized with a hot gas, usually in a large chamber, and subsequently of separating the resultant solid, dry material from the gas. Although this method had been used extensively in Europe for a number of products and in this country for drying milk before the war, it was not until the large demands of the armed forces for dried milk and eggs were made that spray drying received the attention it deserves. A large proportion of the egg (29) and milk powder produced in this country during the war was dried by this method. Attempts to spray dry potatoes have also been described (BY). Spray drying has proved adaptable to large quantities of liquids in fields other than foods, and the process industries are turning more and more toward this method. References have drying (16, 25) and been made t o the drying of resin powder (4), spray concentrating (94) of magnesium chloride, and drying of soap powders (87). The majority of the work in this field is reflected only in the patent literature (89, 55, 57, 68, 67, 78, 86). There is a complete lack of fundamental performance and design d a t a on this type of drying. DRYINGOF FOODS.Well over half the articles in the field of drying during the past two years have dealt with foodstuffs, a direct reflection of the demands of war. Probably the most comprehensive general treatment of the subject is a publication (16) of the Department of Agriculture for plant operators. It describes the types of dryers available for the dehydration of fruits and vegetables; characteristics and probable performance data are given, as well as a brief but sound presentation of the theory underlying dehydration. The accompanying operations of blanching, packaging, storing, rehydration, and testing are also discussed. Another publication (69) compares the drying January, 1946
of meats by eight processes, including drying in rotary dryers, drying by sublimation, and tray drying. The vacuum rotary dryer was concluded t o be the best method of dehydrating meat, although sublimation gave an equally good product. Most fruits and vegetables have been dried in tunnel-type dryers. General descriptions are given by Eidt and MacArthur (38) and Phaff and co-workers (82). Finishing dryers are described by Bouyoucas and Marshall ($4). Fruits and vegetables can also be finished by vacuum dehydration (61) and by dielectric dehydration (93, 97). Drum dryers have been used for dehydrating tomatoes (63). Since the work of Van Arsdel (106) and Marshall (70) few data have been presented on design methods for tunnel- and tray-type dryers. However, some basic drying research has been done in the fields of food dehydration which can be translated to commercial design. Perry (81) presents data on heat and vapor transfer in the dehydration of prunes. Extensive temeasurements were made during the drying process, same time moisture distribution within the prune red. This work indicates that drying is governed by the rate of diffusion of moisture through the prunes, and moisture diffusivity appears to be a function of moisture content. Mass transfer and heat transfer coefficients were calculated for all runs. The drying rate was proportional to the moisture content of the prunes except a t low moisture contents. This work is probably the most scholarly approach to the drying of foods available in the literature. The effects of air temperature and air rate on wheat drying by forcing air through a bed of wheat were investigated by Mounfield, Halton, and Simpson (73). No attempt was made to correlate the variables involved, although it was noted that the rate of drying doubled with an increase in air temperature from 160" to 240' F. Complete data are available for correlation purposes. Ede and Partridge (SI)studied the drying rate of minced meat by blowing air both over the meat in trays and through the meat. The data were correlated by plotting moisture content against the product of drying time and wet-bulb depression a t constant air rate. For all air rates and drking temperatures a correlation waa obtained by plotting the moisture content of the stock against the ratio of drying time to total drying time to dry completely. As may be predicted by the work of Powell (86), the drying rate decreased with increasing fat content of the meat. Some data on the pressure drop through beds of meat particles are given. MISCELLANEOUS DRYERSAND MATERIALS.The drying of clayware has received some attention in the past two years; drying by infrared radiation has already been discussed. IZoehler (66) and Gardner (46) report the general types of drying operations encountered in the ceramic industries and point out the recommended types of dryers. Macey and Wilde (69) compare the safe drying rates for bricks when dried .(a) in a chamber dryer with a preheating period, ( b ) in a chamber dryer without preheating, (c) on a hot floor with forced air circulation, and ( d ) on a hot floor without air circulation. The relative drying rates were found to decrease in the order given. Formulas are also presented relating the drying rate on a hot floor without air circulation t o the difference between the vapor pressure of water at the floor temperature and the vapor pressure in the ambient air, and to the dimensions of the brick. Flash drying, which consists of dispersing moist solids into a hot.gas stream and subsequently recovering the dried solids from the gas stream, has received little attention in the literature. Senseman (96) reviews the developments in this field, tracing the present flash-drying equipment from airswept grinding equipment. Some performance data are given as well as the limitations of this method. Unfortunately no data of a fundamental nature are presented which will permit calculation of dryer sizes or the quantitative (Continued on page 36)
INDUSTRIAL AND ENGINEERING CHEMISTRY
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FLOTATION CONTINUED FROM PAQE 21
on the idea that generalizations re ing flotation are dangerous. Fahrenwald (7) discusses the information that may be gained from batch frothers help or hinder flotati engineer (16)was revised an an up-to-date section on the theory and LITERATURE CITED
(1) Am. Inst. of Mining and Met. Mining Tech.. March, 1944. ( 2 ) Barr, J. A., Zbid., Sept., 1946.
s., Flotation Bymposium,
(3) Bowles, Oliver, Milting and Met., Feb., 1944, q6. (4) Clemmer, J. B., et at., U, 8. Bur. Mines, Rept. fnvestigation 3799, 42 (1945).
(5) Eng. Mining J., March, 1945,93 (“Searles Lake, Major Lithium
Source”).
(6) Ibid., Nov., 1945 (“Cerro de Pasco Entbrprise”) (7) Fahrenwald, A. W., Zbid., Jan., 1945, 74-6. (8)Zbtd., Oct., 1945, 94. (9) Mining Congr. J., “New Kona Feldspar Mine and Mill”, Oct., 1945, 81.
(10) Nevada-Massachusetts Go., Eng. Mincng J.. July, (11) Pallanoh, R. A,, Mining and Met., 26, 167-70 (19 (12) Ramaey, R.33..8 % Mining ~. J., Dec., 1944,74-88; Sept., 1945, 78-8.
(13) Rubert, C . D., and Parton, W. J., Deco Trefozl, June, 1944. (14) Runke, S. M., and O’Meara, R . G., Trans. Am. f n s t . &fin& Met. h’ngrs., 159, 218-26 (1944). (15) Swainson, 8. J., Eng. M i n i n g J., Oct., 1944,469-74. (IS) Taggart, A. F., “Handbook of Mineral Dressing”, New York, John Wiley & Sons, 1945. (17) Wright, F. C., Jr., Mining Conur. J., Feb., 1945,78.
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effect of the variables on capacity. Some performance data for this type of equipment are available on t sludges (1,79,80). Although some information is presented on the operation of kilns ( 2 2 , 17,18,30,47,60, 76,83,84,loa), no data are available which would allow ready design of such units. Bayard (91) recorrelated the data of Sullivan, Maier, an present expressions for estimatirlg the time o kiln under different operating conditions. The d y i n g of inks has received some attention (19, 90). Bead (90)found that tlie rate of drying is affected by the pH of the coating on the paper. The only articles on rotary dryers describe the operation of the Roto-Louvre type (9,.@), M in the field of rotary drying, GENERALDBYING.Farmer ( views available drying methods and discusses the relative merits of steam, gRS, electricity, and direct fiting. The importanbe of dewatering mechanically in SO far as possible, of providing adequate insulation, and of preventing leaks is stressed. A nontechnical review of dryers, principally for (86‘). An introductory art principles of drying solids s for drying, the types of by Marshdl (71)discusses moisture mdvement in a selection of dryers. A lengthy paper by of convection drying and illustrates t h e calculations involved in January, 1946
designing a convection-type dryer. Dryer design is based on moisture pickup of the air rather than on drying rates. The author stresses the importance of the proper amount of air recirculation to steam economies. I n another paper (107) the author g,oes into more detail regarding the optimum amount of air recirculation. He concludes that, with drying temperaturea in excess of 260” F., the drying rate when removing unbound moisture will actually increase with an increased amount of recirculation. This is open to some question. RELATEDSTUDIES,A few papers not directly involving drying are worthy of mention. The drying characteristics of materials are sometimes related to their vapor transmission characteristics. The latter property was measured for a number of different insulating materials by Rowley and Lund (92). Waddams (109) describes a method for easily determining the thermal conductivity of granular solids. The water retention characteristics of a solid are of importance in understanding the basic phenomena relevant to drying. Wadsworth (110) interprets the moisture-vapor pressure curves for some soils and concludes that the sigmoidal shape of the curve is thc result of two independent processes. At low vapor pressures the moisture retention is caused by unimolecular adsorption which follows the Williams-Henry law. A t high vapor pressures the moisture retention is caused primarily by capillary condensation. These two phenomena have been separated for a few soils, Nutting (76) develops a thermodynamic relation between the amount of fluid retained by a solid at any temperature and pressure and the energy required to transfer it to the vapor phase. He indicates that the heat required to liberate the last traces of moisture from a solid is considerably in excess of the heat of vaporization of the water. Experimental data on the moisture retention of minerals are presented and interpreted in the light of the thermodynamic theory. The mass transfer in the of gases through granular solids at low Reynolds numbers studied by Wilke and Hougen (111). This work supplements that of Gamson, Thodos, and Hougen (46) and of Hurt (64,and places the study of drying by through circulation on a firm fundamental basis. The evaporation of water from a free water surface was studied by Hickox (62),and many of the data from meteorologiof mass transfer to the cal sources are correlated b process of’heat transfer. M data are in line with the correlation proposed by Shepherd, Hadlock, and Brewer (96‘) for drying during the constant-rate period. The effect of pan length on the rate of evaporation is discussed. Similar experiments on the evaporation of water from cloth surfaces arc reported by Rees (91). He found that the rate of evaporation from waterproofed fabrics was considerably less than that For nonwater-repellent fabrics, Osburn and I h t z (78) studied the effect of structure as a variable in the application of diffusion theory to extraction. Most of the results and conclusions obtained can be directly adapted
