Translation o f spray dfying theoy to practical design and the increased use offluid bed dyers mark progress in the past year
Drying
his review covers the period from the first quarter of the first quarter of 1966, during which time several trends became apparent. Although the study of spray drying theory continued, emphasis appeared to shift toward practical design and application problems in efforts to develop more economical processes. Fluid bed dryers now seem to be finding a place in large scale drying processes-Le., processes which would require two or more rotary-type dryers. The developing worldwide problem of food production and distribution offers a major challenge to drying engineers and equipment suppliers. Efficient methods for preparation of high quality, readily preservable, and easily transported food products are becoming increasingly important as the need to move large quantities of food overseas develops. There is increasing attention being given to microwave energy sources as larger output units become available ; however, current equipment costs appear likely to limit applications to special cases involving extended diffusion-drying processes. The application of sophisticated mathematics to drying problems is limited by gaps in fundamental knowledge
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AUTHOR Paul Y. McCormick is on the staf of the Chemical Engineering Section, Engineering Service Division, of the D u Pont Co., Wilmington, Del. 58
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more than by lack of ability to solve complex equations. For example, Sharples (70) presents a complete and systematic series of relationships which permits design of rotary dryers by stepwise integration on a digital computer. T o compute heat and mass transfer and material flow rates, however, empirical relationships, neither fully demonstrated nor universally accepted, are needed. As a result, the computer solution is not safe unless one already has available complete drying data for the product in question. Likewise, Ahn ( 1 ) describes a method for optimum design of a moving bed grain dryer, based on a version of the “maximum principle.” By mathematically dividing the air supply into a number of independently controlled channels, difference equations were employed to design a three-stage dryer and achieve optimization by apportioning the air ideally to each section. Correspondence appearing subsequently indicates lack of agreement among mathematicians as to the validity of employing the maximum principle for difference equations. Spray Drying
Dlouhy and Gauvin previously studied heat and mass transfer characteristics of 11-38p droplets in a spray dryer. They suggested that the evaporation rate can be calculated by assuming a stagnant condition for the droplet in air, Le., employing in the Ranz and Marshall correlation, a Nusselt number of 2, and a Reynolds number of 0-except in the immediate zone of atomization. Bose and Pei (6) report results of subsequent studies on a cocurrent spray dryer where it was found that there is indeed a measurable effect of gas velocity relative to the falling droplet. They, suggest that the relative velocity in a down-flow dryer is equal to the terminal settling velocity of the droplet. I n the latter work, the range of droplet sizes was 40-125p which is probably closer to commercial experience than previously. Also, as the data indicate, the apparent departure from the stagnant condition increases with increasing drop size. The results are consistent with the earlier work of Ranz and Marshall and others, who studied evaporation rates of water droplets falling through still air. Ranz and Marshall suggested a droplet size of 100p as being the smallest which would indicate a departure from stagnant drying conditions. Commenting later on the work of Bose and Pei, Harriott (47) discusses their use of the Sauter mean diameter, Znd3/8nd2, for calculating the Nusselt number. If we use the surface-mean diameter, Znd2/8nd, for calculating the Nusselt number, with the heat transfer coefficient, however, still based on the Sauter mean
diameter, the values would be reduced by 5-20% from the range of 2.0-2.6 reported. Pei suggests the best approach would be to divide the spray into several size classes to determine mean terminal velocity with reasonable accuracy. The two mean diameters approach equality, of course, as the droplets approach uniform size. I t should be stressed here that this work applies to true cocurrent dryers. I n countercurrent units, as well as disk-type dryers, a relative velocity difference between the particles and gas is obviously more likely, particularly as regards the largest particle fraction. Also, it is the largest particle that controls chamber size because it requires the longest drying time. Paste-type spray dryers are discussed by Turba and Nemeth (80) and Baran ( 4 ) . The former describe a screw-fed, two-fluid nozzle with an annular slot discharge. They studied the effects of slot width, screw design, and air placement. Drying of 40% solids chalk paste is reported. Baran’s nozzle was also a twofluid, annular-slot design, capable of atomizing materials with viscosities up to 100,000 C.P.S. I t was an internal mixing type, with a series of expansion mixing stages for contacting gas and solids efficiently before atomization. Prototype nozzles with capacities of 8000 lb./hr. are described. A useful feature of these slot-type nozzles is that scaleup is accomplished by increasing annulus diameter a t constant annulus width. Thus, scaleup retains uniform particle characteristics. This is not the case for center orifice nozzles, where an increase in orifice diameter results in a coarser spray. This change can be compensated only partially by increasing air pressure. The use of sonic energy in spray nozzles to minimize and control droplet sizes has received attention in the past. Wilcox and Tate (84) investigated a Hartmann whistle, stem and cavity atomizer, and a sonic atomizer with an elliptical reflector. I n all cases, air pressure and turbulence appeared to be the predominant atomizing forces. Relationships between air/liquid mass ratios and droplet size were studied. Although finer droplets were produced at reduced liquid rates, the authors detected no advantages for the sonic atomizers employed over conventional two-fluid atomizers. Quinn (65) analyzes spray-drying costs, while Cook (22) gives a general description of spray- drying processes, component selection, costs, instrument ation, and typical applications with special reference to drug and cosmetic products. Entwistle (26) describes spray dryers and spray chillers employed in food processing, while Silvis (77) describes processes for mixing, feeding, and spraydrying detergent formulations.
A flashing-tube evaporator and dryer for soap is reported (72)in which evolved vapor was employed to entrain and convey soap particles through the heat exchanger. Improved quality because of short hot-zone retention is claimed. Zilske (87) describes another type of liquid-phase soap dryer and stresses the importance of velocity and steam-soap ratio for preventing impaction of soap particles on hot tube walls. Spraydryer installations for various melamine and urea formaldehyde resins are reported (21,641, as well as a disk-type spray dryer employed for antibiotics drying (57). Food and Pharmaceutical Drying
The employment of microwave energy in food processing is discussed by Decareau (24). Of the two available, practical frequencies-91 5 mc. and 2450 mc.-the lower frequency is reported to have the greater penetrating power. This energy source appears applicable to freeze drying because many food solids have very low dielectric loss factors after the moisture has been removed. The article includes discussion of several other applications for microwave energy in food processing, such as heating, defrosting, and moisture control at low levels. Coady (20)reviews the status of freeze drying in the pharmaceutical field where drying temperatures appear headed downward toward -60' to -90' C. Routine pressures in the future will be less than 1 . 0of ~ mercury and materials such as liquid nitrogen may become necessary to reach temperatures required for cell preservation during drying. Hutchison (45) describes freeze-drying equipment suitable for preparing small samples for electron microscopy. I n the area of new equipment, a continuous tunneltype freeze dryer is reported (30)and a blast-type freezer and dryer employed by the U. S. Army is described (32). Rockwell (67)studied a rotary steam-tube dryer enclosed in a vacuum chamber employed for freeze drying ; Kaufman (50) describes the construction and performance of a rotating drum dryer incorporating internal tubes and fins to provide extended heating surface for freeze-drying piece-type products. These dryers offer advantages in that they provide for mixing during drying to enhance heat transfer. Also, they can be scaled up without incurring the loss in surface/volume ratio experienced with conventional rotating vacuum dryers. Marich (56) reviews in two articles current design criteria for freeze-drying plants, while Miner (58) reviews freeze-drying problems of particular interest to the refrigeration engineer. Cost summaries for several products at the 4-ton-per-day and 32-ton-per-day operating levels are included. Freeze-drying processes are discussed also by Hughson (44). Investigations were conducted by Carl (72) to determine the effects of freezing rates on the structures and drying rates of mushrooms and potatoes. Freezing rates of 1 minute, 40 minutes, and 180 minutes were studied. As shown by photomicrographs, the slower rates yielded larger ice crystals which caused structural damage to the products. Although it was believed that fast-frozen 60
INDUSTRIAL A N D ENGINEERING CHEMISTRY
samples would dry faster, the opposite proved true. I t is suggested that rupturing of the cell walls during freezing provided open paths for vapor flow during drying and, therefore, that mass transfer rather than heat transfer controlled the drying rates. Kamanowsky (49) applied foamed whole-milk concentrate at 40-46% solids in the form of ropes on a continuous belt, cross-flow dryer. Drying curves indicated no constant rate period, and drying time varied with approximately the square of rope diameter. The drying data correlated well with the diffusion equation for mass transfer through one face of a slab. The product had an oxidized flavor, but was readily redispersible. Berry (5) studied the belt-type foam-mat dryer for dehydration of grapefruit concentrates. Foam temperatures ofup to 180' F. and drying times to 18 minutes yielded acceptable products without heat damage. Soya protein was employed as the foaming agent. I n the area of new equipment, a high capacity foam-mat dryer design is reported (29). Regarding specific food products, Strolle (74) described a USDA technique for producing dried cranberry juice in a thin-film evaporator, including a method for recovering volatilized natural flavors and back-mixing of these before product freezing. Neumann (61) studied the effects of drying temperatures on the initial quality and shelf life of dehydrated frozen peas. Gentry (36)investigated prune drying in cocurrent and countercurrent tunnel dryers, while Turkot (81) reviewed process equipment, investment, and operating costs involved in drying pumpkin powder on drum dryers. Drying rate curves were employed by Hague (37) to calculate diffusion coefficients of water through porous copra, an example of unsteady-state diffusion through a spherical shell. A solution complete with Eigen values and Fourier coefficients was developed. McBean (55) studied the quality of SO2 retention in fruit tissue during drying. A continuous macaroni dryer was described by La Rosa (5,3). Evans (27) has published a general review of food drying methods in use currently, including drum drying, spray drying, and conveyor drying. Recent developments in freeze dryers, stirred fluid beds, puff dryers, and low humidity spray dryers were discussed. European developments in food dryers were reported by Nair (60). Lieberman (54) discussed tray drying of pharmaceutical products, covering general theory and including product examples to illustrate the effects of temperature, air velocity, bed depth, and material type. From India, a combination mechanical dewatering-drying process was reported which yielded substantial drying time reduction and improved reconstitution character, compared with air-drying alone, of various fruits and vegetables (33). Fibers and Fabrics
Norton (62) developed a mathematical model which permitted calculation of water concentration and temperature changes in wool fiber beds during throughcirculation drying as humidity and temperatures were
varied in the air stream. An important assumption was that mass transfer is proportional to the difference between partial pressure of water in the air and the equilibrium pressure in the fiber. Data indicated that the assumption may be in error a t low fiber moisture concentrations where internal diffusion is limiting. A significant behavior difference between adsorption and desorption is reported also. Brock (9) studied through-flow drying of tufted carpets and showed drying times roughly 20% as long as those required in conventional impingement dryers. A fair correlation of the experimental results with the Reynolds analogy for heat and momentum transfer was obtained, and this yielded a tentative approach for correlating drying rate with pressure drop during the constant-rate period. Takeda (75) discussed the influence of drying temperature and humidity on the internal structure of acrylic fibers. Garland (35) discussed the use of gas-fired ceramicand screen-type radiant heaters in textile fabric dryers. Infrared units for carpet dryers and Thermos01 fixation are described also (2, 77). Jarvis (48) discussed the application of radiation pyrometers and controls on a gas-fired radiant fabric dryer to correct side-to-side squeeze roll pressure, fabric speed, and high and low burner heat levels while maintaining uniform drying quality across the face of the fabric normal to web direction. A high velocity, gas-fired tenter-frame oven, having flexibility for handling spandex fabrics of various fibers, weaves, and meshes, relaxed and under tension, was described ( 3 ) , as was a Saco-Lowell drum for nylon and acrylic carpet yarn skeins (78) and a gas-fired, high velocity slot dryer (76). Paper
Rounsley (68) studied vapor transmission through paper by pore diffusion and adsorbed flow-i.e., Knudsen flow. A vapor permeability equation was developed for computer solution and demonstrated. Nonequilibrium effects, such as heat transfer, appeared to place limitations on the practical employment of adsorbed flow in commercial drying. Recent work by Cowan (23),which suggests saturated surface drying during the so-called constant-rate period, does not describe adequately the true drying mechanism. The concept is advanced that hot-surface evaporation forces vapor back through the sheet toward the open surface. Internal cooling and saturation cause condensation. At some plane near the surface, reconstant-rate drying is a evaporation is initiated-i.e., summation of internal and surface evaporation in porous sheets. Han (38) further analyzed Cowan’s data and stressed the importance of pressure. The results indicated normal diffusion accounts for roughly 4Oy0 of the drying rate, involving both air and water. If surface transport and intermediate diffusion are controlling mechanisms, they should be favored by increasing pressure.
Thode (79) discusses instrumentation for continuously monitoring moisture in paper. No completely acceptable instrument is available commercially ; however, microwave absorption and infrared devices offer promise for circumventing interference from chemical composition which seriously affects resistance and impedance instruments. Findley (28) discusses instrument-response lag time which will become a problem with increased dryer speeds and reduced retention times. The dynamic response to heat input changes in a threesection infrared sheeting dryer with countercurrent air flow was investigated, from which Findley obtained a semiempirical correlation between Ziegler-Nichols constants and drying variables. For accurate control, however, precise knowledge of the interrelated effects of vaporization rate, moisture content, and sheet temperature are needed. Janett (47) described standards for high speed nozzle-type and infrared dryer designs where solvent vapors are evolved, and emphasized current underwriter requirements to ensure safe operation and eliminate fire hazards. White (83) investigated the effect of a high intensity acoustic whistle employed to accelerate the mechanical displacement of water from paper and wool felt while they undergo through-circulation drying under pressure. At air pressures < 5 inches of mercury, a substantial reduction in the time needed for air breakthrough was found; at greater pressures, acoustic effects were less pronounced. From the results, it appeared that acoustic energy did have a significant effect in displacing capillary moisture mechanically. By developing a general formula for evaporation on Yankee drums, Gardner (34) investigated the potential of high velocity hoods installed above these drums. A 40-50% increase in drying rate was reported from mounting a 12-18,000 ft./minute hood, 0.25-0.50 inch above the web. An overall energy efficiency of 6OY0 is claimed for a small 30-kw. dielectric dryer installed near the dry end of a 91-inch wide paper machine by Fitchburg Paper Co. (76). Use of quartz-tube lamps to obtain drying rates of up to 13 Ib. of water per hr. per sq. ft. was reported by Hansen (39), while Janes (46) investigated the effects of gas-fired radiant heaters installed before and after the press rolls on a paper machine; beneficial effects were found in both locations. Effects of velocity, volume, and temperature variables in high velocity dryers were studied by Chase (74) and a much-needed comparison of water removal costs among conventional drums, high velocity hoods, and infrared dryers was made by Holt (43). There appeared to be relatively little difference among the three; the latter two offer advantages for improving capacity of existing drums because they are compact. Quality factors were not evaluated. The importance of felt drying was emphasized by Yih (86) who studied water removal in the press roll nip, Cassirer (73) who described the Madeline hot-air felt drying roll, Ramsdell (66) who discussed commercial experience using the Madeline roll, and Burnham (70) VOL. 5 8
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who reviewed the development and operation of the Beloit felt-drying and pocket-ventilating roll. Cirrito (79) completed a comprehensive series of articles on paper drying covering falling-rate drying and various radiant heaters. Dyck (25) described a n experimental paper drying drum which includes a motion picture camera and closed-circuit TV setup installed to observe steam and condensate flow and siphon operation inside the drum. Stockman (73) studied the effects of heat treatment on the mechanical properties of papermaking pulps. General
Kirkwood (57) investigated the effects of drying variables, such as temperature, humidity, air mass flow, and bed depth, on drying several representative materials, namely, coke, ceramic granules, and brewers' spent grain, with the objective of developing methods to predict drying time, critical moisture, and falling rate constants under any conditions within the limits of his experiments. Cross-flow and through-air circulation were studied. Although a fairly large number of experiments was involved, the methods appeared applicable to similar materials, and on the assumption these behave the same, the number of tests could be reduced. For example, he reported that only loading depth affects the critical moisture level of porous materials during cross-flow drying. For all materials tested, the slope of the falling-rate curve was affected only by loading and air temperature. Landsberg (52) describes the production of fine tungsten and tungsten-rhenium alloy powders by freeze drying of metal-salt solutions, followed by reduction to the pure metals. Tungsten powder was produced by freeze drying ammonium tungstate and by hydrogen reduction. During reduction of the alloy, solid solutions were formed indicating the degree to which the freeze-dried salts maintained their homogeneity. The alloy was likewise produced from the ammonium salts of the two metals. An increasing interest in the applications of microwave energy in many drying processes was indicated by development of commercial batch and continuous test dryers ( 3 7 ) . Calus ( 7 7 ) provides a review of European drying developments with 49 references. Other general discussions of European drying equipment and applications also appeared (7, 8,59, 82, 85). Russian drying equipment concepts received attention, particularly with respect to fluid beds (42,69). Hardin (40)described design methods for screen baffles to obtain staging and improve the capacity of fluid beds, while a fluid bed dryer for polymer chips was described by Cherrett (78). The installation of a 30-ton-per-hr. combination fluid bed dryer and classifier for limestone was reported. The bed velocity was designed for a 52mesh cut-point when a maximum particle size of "16 inch is handled (63). Current purchase costs for various standard dryer types were published (77), and a new extruder-dryer for natural rubber was described (75). 62
INDUSTRIAL A N D ENGINEERING CHEMISTRY
REFERENCES (1) Ahn, Y . K., I K D . END. CHEM. P R O C E S S DESIGN DEVELOP. 3 ( Z ) , 96-100 (1964). ( 2 ) A m . Dyestufj Reptr. 53, 22, 942 (1964).
(3) Ibid. 53, 7276-7 (1964). (4) Baran, S. J.? IND. E m CHEM.56 (lo), 34-6 (1964). (5) Berry, R. E., Food Technoi. 19 (3), 426-8 (1965). (6) Bose, A . K . , Pei, D . C . T., Can. J . Chem. E n g . 42 (6), 259-61 (1964). (7) Bowmer, G. C., Chem. Ind. 39, 1638-48 (Sepr. 26,1964). (8) Brit. Chem. Eng. 9 (81,543; (9)619 (1964). (9) Brock, J. D . , Gorton, C . W., Textzie Res. J . 34 (12), 1031-9 (1964). (10) Burnham, L. A . , Tappi 48 (7), 82.4-5A (1965). (11) Calus, W.F., Chem. Proc. E n g . 45 (lo), 596-673 (1964). (12) Carl, K. R., Stephenson, K . Q., A S A E Trans. 8 (3), 414-16 (1965). (13) Cassirer, F. JV., Tappi, 47 (91, 175.4-7.4 (1964). (14) Chase, R. C., Perrault, R. R., Ibid., 48 (3), 117A-20.4 (1965). (15) Chem. Eng. Neles 43 (381, 75-6 (Sept. 20, 1965). (16) Chem. Eng. 71 (19), 86-8 (1964). (17) Ibid., 73 (3), 101 (1966). (18) Cherrett, N., Brit. Chem. En!. 9 ( 1 2 ) , 843-4 (1964). (19) Cirrito, A. I., PuperInd. 46 (21, 115-18; (31, 203-6; (51,407-9; (61,499-501 (1964). (20) Coady, ?vi. G., Res. Deu. 16 (41, 65-7 (1965). (21) Comprassedrlir M a g . 70 (lo), 16 (1965). (22) Cook, E. M., Drug Cosmetic Ind. 96 ( l ) , 45-8 (1965). (23) Cowan, W.F., Tappi 47 (IZ), 808-11 (1964). (24) Decareau, R . V., Food Eng. 37 (7), 137-9 (1965). ( 2 5 ) Dyck, A . W.J., PnperInd. 46 (I), 25-8 (1964). (26) Entwistle, R . E , , Food Eng. 38 (2), 81-6 (1966). (27) Evans, G . , Chem. I n d . 4, 173-9 (Jan. 23, 1965). (28) Findlev M E. Malone, M. LV,, IND,ENC.CHEY.PROCESS DESIGN DEVELOP. 3 (21, 89% (i964). Food Engr. 36 (12), 94-5 (1964). Ibid.,37 (3), 146-7 (1965). Ibid., p. 148. Ibid., 38 (2), 142 (1966). Food Technol. 18 (a), 1193 (1964). Gardner, T., Toppi 47 (4); 210-14 (1964). Garland, G. I V . , Texfile I n d . 128 ( l ) , 124 (1964). Food Technoi. 19 (9), 1427-31 (1965). Gentry, J. P., Hague, S. hl. M . , Eubank, P. T., Ibid.,18 (3), 390-2 (1964). Han, S. T., Matters, J. F., T a p p i 49 ( l ) , 1-4 (1966). Hansen, D . , Wright, B. R.: Ibid., 48 ( Z ) , 85-8 (1965). Hardin, J. E., Chem. Eng. 73 (4), 175-6 (1966). Harriott, P., Can. J. Chem. Eng. 43 (2), 92 (1965). Herz, A , , Ij~tern,Chrm. En!. 5 (2), 276-80 (1965). Holt, S. G . , Knapp, K . K., Tap@ 47 (9), 130tG5A (1964). Hughson, R. V., Cham.Eng. 71 115), 155-60 (1964). Hutchiscn, J. D., Anal. C h m . 3 7 (13), 1784-5 (1965). Janes, R . L., Tappi 49 (2), 82.4-3A (1966). Janett, L. G., Ibid.: ( l ) , 88A-90A (1966). Jarvis, C. \V,: T~xtileI n d . 129 (3), 109-11 (1965). Kamanowsky, M . ? IND.ENC. C H E s l P R O C E S S DESIGK DEVELOP. 3 (3), 193-7 9641
Kaufman, V. F., Food Eng. 36 (7), 58-61 (1964). Kirkwood, K. C., Mitchell, J . J.; , J , Agpi, Chem. 15 (6), 256-80 (1965) Landsberg, A , , Campbell, T. T . , J. M e t a l s 17 (a), 856-60 (1965). L a Rosa, P. P., Porianda. A . K., Food E n g . 3 7 ( 7 ) , 60-2 (1965). Lieberman, H. A , , Drug Cosmetic Ind. 97 (6): 836-8 (1965). McBean, D. M,: Food Technol. 19 (9), 1447-51 (1965). Marich, F., Food Eng. 37 (4), 52-5; (6),85-9 (1965). .U,fg. Ciieirirst 35 (5), 47 (1964). Miner, S. hi.,A S H R A E J. 7 (6), 92-9 (1965). Moss, A . A . H.: Cheni. I n d . 1, 40-3 (Jan. 2, 1965). Nair, J. H., Food E n g . 37 ( 7 ) , 63-9 (1965). Keumann, H. J., Food Technol. 19 ( l l ) , 1729-32 (1965). Norton, P., In!em. J . Heat M o s s Transfer 7 (6), 639-51 (1964). Pzt Quarry 5 7 (5): 114-16 (1964). Plnrfrcs World 22 ( 7 ) , 12-13 (1964). , 5 7 ( l ) , 35-7 (1965). Quinn, J. I., I K D ENG.CHEM. Paper Ind. 46 (5), 402-6 (1964). Ramsdell, E. It'.? Rockwell, W. C . : Food E n g . 37 (41, 49-51 (1965). Rounsley, R . R., Tappi 47 (2), 95-8 (1964). Sazhin, B. S., Intern. Chrm. Eng. 5 (1), 26-30 (1965). Sharpies, K . , Inst. Chrm.Eng. Trans. 42 (7): T275-84 (1964). Silvis, S. J , , J . A m . Oil Cicernzrts' SOC.41 (12), 4-6 (1964). Soap Chem. Specialties 40 ( 7 ) , 145-9 (1964). Stockman, L,,, Teder, A , , Pupdr I n d . 46 (l), 47 (1964). Strolle, E. O., Food Eng. 37 ( l ) ? 70-2 (1965). Takeda, H . , Kukushina, Y . , T e x h l e Res. J . 34 ( 2 ) , 173-5 (1964). Textiie I n d . 128 (7), 101-2 (1964). I t t d . ., 129 (5). . . . 166-7 (1965). I t i d . > (71, p. 114. T h o d e , E. F., Control Eng. 11 ( l l ) , 67-73 (1964). T u r h a , J., Nemeth, I.,Brit. Cham.E n g . 9 ( 7 ) , 457-60 (1964). Turkot, V. A , , Food E n g . 37 (7), 78-81 (1965). White. A , . Chem. Pmc. En
