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II/ECI Unit Operations
Review
Drying by Paul Y. McCormick, E . I . d u Pont de Nemours & Co., Inc., Wilmington 98, Del.
Development of a heavy paste atomizing nozzle suggests new potential uses for spray dryers
B
Important studies cover mechanism of spray drying
R E P O R T S DEVOTED TO THE SUBJECT of spray drying were prominent in the drying literature published since the last review. Of special interest, because of its potential commercial value, was the reported development of a pneumatic atomizer capable of producing fine particulate sprays from heavy pastes and slurries. One of the most severe limitations, which in many cases now discourages use of spray dryers, is the need to dilute solids-containing feeds with water or other low-viscosity liquid vehicles to make them suitable for dispersion in the dryer atmosphere. The economics of evaporating a large quantity of water, frequently required solely to produce good spray characteristics, are rarely favorable when compared with alternative mechanical dewatering and conventional solids drying processes. I t is for this reason that most of the successful spray drying applications to date have come about where spray drying has yielded a desirable product quality value unobtainable by other drying methods, and dryer selection was not a function simply of drying costs. This review covers the final quarter of 1959 and all of 1960.
Process Installations The heavy paste atomizer is a joint development of Proctor and Schwartz. Inc., Philadelphia, Pa., and the J. S. & W. R. Eakins Co., Brooklyn, N. Y . (71). The first commercial installation was in the Eakins plant where a continuous, solid bowl centrifuge-spray dryer installation has replaced a conventional batch filtration, tray drying, and mechanical pulverizing operation for the manufacture of inorganic pigments. The atomizer, mounted on top of a conventional cocurrent dryer chamber, is a two-fluid nozzle employing steam at 100 to 150 p.s.i.g. or compressed air for solids dispersion. I t is reported to be capable of dispersing pigment pastes containing 55 to 65% total solids at viscosities up to 100,000 cp. into particles averaging 4,microns in diameter. The paste is pumped to the nozzle through a special refuse filter by a constant-displacement metering pump.
About 0.17 pound of steam is consumed per pound of paste. Employing a 4.5foot-diameter by 16.0-foot-high drying chamber for the evaporation of 300 pounds per hour of water, Eakins reports a total manufacturing cost for dewatering, drying, and packaging of 70 cents to $1.50 per 100 pounds of finished pigment, exclusive of investment and depreciation charges. Another significant development in the equipment area was the successful operation of a continuous vacuum dryerextruder for the production of low-ash content styrene polymers for electricalgrade rubber (73, 14, 43, 48). The dryer employs a tapered auger a t the feed end to expel water mechanically and provide a vacuum seal. The auger then forces the squeeze-dried rubber through an adjustable slot to form a thin sheet. In the vacuum chamber, which is 1.0 foot in diameter and 22.0 feet long, the sheet is conveyed by a multiflight screw conveyor. The chamber is divided into three sections in series, operating a t less than 27 inches of mercury vacuum and at a temperature of approximately 250" F., in which the residual moisture is flashed off. After drying, the rubber is forced by the conveyor screw through a pelleting die, which serves also as the end seal on the vacuum chamber. The screw section is cored and the housing jacketed for temperature control. The unit is reported to have a drying capacity of 4.0 tons per hour of rubber. Drying time is 5 to 10 minutes, compared with 2.0 hours required in a conventional, air-heated conveyor dryer. Because approximately 80% of the total moisture is removed by squeezing, most of the inorganic impurities initially present are extracted from the rubber in the tapered auger section. Final moisture in the pelleted rubber is less than 0.50%. Other new spray dryer installations include a unit for the drying of 50 to 80 tons per day of lignosulfonates (8) and a Bowen Engineering, Inc., spray dryer for S i c fines. The latter replaces a settling tank-drying kiln process and employs a tool steel disk-atomizer with boron carbide inserts at the wear points.
A 200-kw. dielectric dryer developed by the New Rochelle Tool Co. has a reported efficiency of 2.0 pounds of water evaporated per kilowatt-hour of power input when drying rayon cakes (2). The drying cycle is approximately 7 minutes, with a drying cost of approximately 60 cents per 100 pounds of water evaporated. The dryer employs a 30Mc. generator.
Spray Drying An investigation of air flow through a countercurrent, detergent-type spray dryer, 21 feet in diameter and 50 feet high, was conducted by Place and others (45). They used a helium tracer injected into the chamber a t various points to measure residence timr of gas in the chamber and explore flow patterns. I n other experiments, they employed a l/Both scale model of the large dryer with water as the fluid and dye injection for visual observation of flow patterns. Both sets of experiments revealed a zone of stagnant or reversed flow in the swirling fluid. This is believed to be the primary cause of back-mixing, accounting for apparent uniformity of gas temperatures in detergent dryers. The existence of an isolated core of air with a high vertical velocity, channeling upward along the dryer axis, was indicated also in both the model experiment and velocity survey. Back-mixing appears to take place in an annular ring surrounding this core, about a half radius from the wall. The upward velocity increases again near the wall. Source of gas in the core may be entering air, which swirls downward next to the wall of the bottom cone and reverses direction at the bottom, as in a cyclone collector. The experiments indicated that flow patterns are not stable even under constant operating conditions in chambers of the design investigated. Heat- and mass-transfer coefficients during evaporation of water sprays were measured by Dlouhy and Gauvin (79) using a vertical cocurrent spray dryer 8 inches in diameter and 14 feet high. Progressive evaporation down the chamber was followed by sampling of drying air and water droplets a t various distances from the pneumatic atomizing VOL. 53, NO. ,7
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nozzle. For droplets ranging in mean diameter from 11.5 to 38.5 microns, the results indicated that the rate of evaporation could be calculated by assuming that individual droplets evaporate a t a rate corresponding to stagnant conditions, i.e., by assuming the Nusselt number to be equal to 2.0, as predicted
by the Ranz and Marshall equations for R e = 0. A method is outlined for calculation of the residence time required in the evaporation zone to vaporize the largest droplets produced. I n further work (20)>the same authors investigated drying of calcium-spent sulfite liquors, Lignosol, in the same
(75) ( 76) (3)
chamber. Evaporation and drying time could again be predicted accurately by stepwise calculations assuming a Nusselt number equal to 2. To carry out calculations of this nature, a knowledge of the droplet size distribution from the atomizer, equilibrium vapor pressures. particle density, heat capacity, and heat of solution (if significant) musr be available. I n the case of Lignosol, spray drying was compared with tray drying. In the former, no significant falling-rate drying period was detected, probably because of the high surface-to-weight ratio of the small particles less than 30 microns which retarded dry crust formation until near completion of drying. Heat- and mass-transfer rates in the nozzle lone of a spray dryer were studied by Manning and Gauvin (47) using pneumatic and hollow cone pressure nozzles with both cocurrent and crossflow air patterns. Equations proposed by Froessling and Ranz and Marshall for stationary drops suspended in moving air were found to be generally applicable for correlating the Xusselt numbers in terms of (Pr)l:3. In general, the amount of evaporation occurring in the nozzle zone was comparatively small. Although the rate of evaporation is high, because of a high droplet velocity relative to the air, retention in the first few inches from the nozzle is too short to permit a large amount of evaporation. A study of the evaporation of water from single droplets containing dissolved solids and suspended in an air stream was conducted by Charlesworth and Marshall ( 7 0 ) . During drying, droplet temperatures and weight changes were recorded. Data included the time of first appearance of a solid phase and time of completion of a full crust. A theory was advanced for predicting the time for initial appearance of the former. The first period of drying was characterized by evaporation, as from a free liquid surface, at a solute concentration increasing from the initial value to saturation. After formation of a solid crust, a reduction in drying rate was observed; however, its effect was not uniform, appearing to be a function of solids characteristics, rate of drying (temperature), and initial solute concentration.
(7)
Fibers Drying
NEW EQUIPMENT Ofher equipmenf developments and new commercial installations reported in the literafure are of interesf in specific fields. These are outlined here.
Type of Equipment
Intended Use
“Expansion” dryer
Soap
Gas dehydration unit and regeneration system Dehydration plant Air drying systems Controlled temp. humidity dryer Tow-drying systemperforated drum and tow folding device Stenter dryer Air dryer Improved dryer Rotary dryer Flash drying systems Radiant burner Fluidized solids dryer
Recovery of heavy hydrocarbons, drying Gas drying Wind tunnels Clay tile
Cotton packages Seed cotton GroundJvood pulp Wood pulp Paper drying Limestone, coal, slag, etc.
European drying equipment Rotary dryers
Fertilizer production Burning offorganic Oven design compounds in wire enameling Drying and baking Organic finishes oven Bexh-scale pilot rotary dryer Freeze-drying plant
Roto-Louvre dryer Pneumatic dryer Rotary kiln dryer Automatic, relaxing cylinder dryer Drying and curing machine Electric heating system Drying oven Dryer
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Fruits and vegetables - 10 mesh coal 2 in. and finer coal Fabrics
Printed fabrics
Remarks
Standardized glycol unit Criteria for selection Kiln exhaust is primary heat source Design and operation
(58) (40) (28) (37)
Operation
Practical uses Recent developments
(38)
Countercurrent, direct heat Platinum catalysts
(35)
Tungsten filament lamps radiating in near infrared-1-2 microns 4.0-in. diameter, 2.0 ft. long 5.0 tons/day; Vickers Armstrong design Indirectly heated 200 tons,/hr. Drying with or Lvithout tension Gas-fired design
Circulating heat- Design transfer oils Enameled steel 62-kw. infrared pontoons Textile fabric General design Clay-coated paper Design methods Crops Data useful for des;gn Fine coal Design Wood finishes Review of requirements
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ref.
Soap and water vapor (59) separated in final expansion chamber Dehydration unit employs (22) silica gel
(49)
(42) (72)
(25’) (24)
(37) ( 23) (4)
(5454) (30) (36) (32)
Continuing work previously reported, Bell and Nissan (7) studied the mechanism of wool drying, giving special attention to the adsorbed moisture retained by the fiber in the air-dry region. A bobbin of wool, roughly 3 inches in diameter, wound on a 1-inch core, was saturated with water and dried in an air stream. During the second falling-rate period, it was established that adsorbed moisture was evaporated very near the
an pore-water boundary, leaving the basic theory of the second falling-rate period unchanged. During this period, a balance exists between heat transfer and vapor diffusion through the retreating pore-water boundary, while the waterwet region remains a t a uniform “psuedo wet-bulb temperature.” The solids-tovoid ratio is low, so that the fraction of water absorbed by the fiber is low. I t has apparently little effect on the general equilibrium. Drying a mass of wool fibers by through-circulation was discussed by Downes and McMahon (27). They pointed out that a “drying front’’ proceeds through the mass in the direction of air flow. The drying process must continue until the front reaches the downstream side of the mass if uniformity of moisture distribution among the fibers is to be achieved. Air of an appropriate humidity is necessary to prevent overdrying upstream from the drying front. Reversal of air direction is undesirable on the grounds of efficiency, and in the case of wool, excessive air temperatures are of no benefit because of the higher humidity necessary to avoid over-drying. Moisture pick-up to adiabatic saturation of the air remains practically constant, except when drying to extremely low regain. Data showing the effects of temperature and other variables when drying wool under vacuum were presented by Watt and others (57). The effect of air temperature and humidity on the rate of drying textile fabrics and the effects of air recirculation on drying rates and thermal efficiency were discussed (56).
General Mathematical formulas suitable for computer solutions, presented by Bates (5), predict the surface area of solids which must be exposed in a direct-air dryer to achieve evaporation of a given quantity of water under given conditions of air temperature, humidity, and velocity. Variables which must be known are the critical free-moisture content, the drying rate, and the rate change during the falling-rate drying periods. Equations are derived for spheres, cylinders, and slabs. Race (46) published results of a survey concerned with the use of synthetic fiber felts in paper mills. The construction and operation of a freeze-dryer for drying suspensions of micro-organisms a t controlled temperatures were described by Mellor and Ohye (44). Smithies and Blakley (50) described a freeze-drying process employing a vacuum chamber fitted with closely spaced, blackened shelves either electrically or hot-water heated. The product being dried is supported on an
aluminum tray suspended between, but not in contact with, the heated shelves. Heat is transferred by radiation alone. The rate of drying can be regulated by a thermostat controlling the tray temperature. Meat slices ranging from 3/8 to 0.75 inch thick were dried in 5 to 12 hours, depending on thickness. Results of a study dealing with the application of mass-transfer principles to vapor adsorption from gases flowing through fixed beds were published by Carter (9). Methods for predicting the outlet concentration with time are evaluated employing data from the adsorption of water vapor from air on silica gel and activated alumina. The operation of an adsorption dryer installed in a refrigerating system was reviewed by Krause and others (33). A considerable period of operation is required to bring the moisture level in the system to equilibrium with the adsorbent. This time will vary with refrigerant flow, system size, and dryer size. Because the part of the system immediately downstream from the dryer will be reduced to low moisture content first, the dryer should be installed as close upstream as possible to the expansion valve. Use of activated alumina for removal of water from gas containing unsaturated hydrocarbons was discussed by Dieter (78). H e mentioned a method for burning off polymerized, adsorbed hydrocarbons in a short time to minimize damage to the adsorbent surface. Methods for testing and selecting adsorbents for specific applications were reviewed by Gregory (29), who also discussed recent developments in hygrometry. Mabery (39) discussed air drying systems employing compression, cooling, and solid desiccants. Methods employed for drying poly(vinyl chloride) were described by Labine (34). Other investigations dealt with the effect of wetting and drying on wool fiber strength (26) and the waterholding capacity of cotton fabrics (57). literature Cited (1) Am. Dyestuff Reptr. 48, No. 25, 142 (1959). (2 ’ Zbid:, 49, No. 6, 209 (1960). (31 Zbid., No. 10, p. 379. (4) Andrews, B. R., Jr., Textile Znd. 123, No. 11. 99 (1959). (5) Bates; H.’ T., ’Chem. Eng. Progr. 56, No. 7, 52 (1960). (6) Beeken, D. W., Brit. Chem. Eng. 5, 484 (1960). (7) Bell, J. R., Nissan, A. H., A.Z.CI2.E. Journal 5. 344 11959). ( 8 ) Can. ChLm. Pric. 43,’No. 11, 41 (1959). (9) Carter, J. W., Brit. Chem. Eng. 5 , 472, 552, 625 (1960). (lo), Charlesworth, D. H., Marshall, W. R., Jr., A.1.Ch.E. Journal 6, 9 (1960). 11) Chem. Eng. 67, No. 6, 8 3 (1960). 12) Zbid., No. 13, p. 88.
Unit Operations Review
(13) Zbid., No. 16, p. 63. (14) Chem. Eng. Progr. 56, No. 8, 80 (1960). (15) Coal Age 64, No. 8, 102 (1959). (16) Zbid., No. 11, p. 118. (17) Craig, R. P., Am. Dyestuff Reptr. 48, No. 20, 50 (1959). (18) Dieter, J., Chem. Eng. 67, No. 22, 115 (1960). (19) Dlouhy, J., Gauvin, W. H., Can. J . Chem. Eng. 38, 113 (1960). (20) Dlouhy, J., Gauvin, W. H., A.Z.Ch.E. Journal 6. 29 (1960). (21) Downes, J. G.; McMahon, G. B., Textzle Research J . 29, 1006 (1959). (22) Drake, Q. S., Dawson, E. R.. J . Petrol. Technd. 12, No. 7, 1 9 (1960): (23) Elec. World 152. No. 22. 93 11959\ - , (24) Engineering 189,’ 495 (1960). 25) Zbzd., 190, 292 (1960). 26) Feughelman, M., Textile Research J . 29. 9 6 7 (1959). (27) Gagnon, L., Stangl, K., Tappi 43, No. 1 (Suppl.), 246A (1960). (28) Garve, T. W., Bull. Am. Ceram. Soc. 38, 708 (1959). (29) Gregory, S. A., Znd. Chemist 36, 428, 479-84 (1960). (30) Hedlin, C.P., Air Conditioning, Heating, and Ventilation 57, No. 2, 93 (1960). (31) IND. ENG. CHEM.51, No. 12, 104A (1959). (32) Znd. Finishing 36, No. 5, 105 (1960). (33) Krause, W. O., Guise, A. B., Beacham, E. A., A S H R A E Journal 2, No. 10, 59 (1960). (34) Labine, R. A., Chem. Eng. 66, No. 23, 166 (1959). (35) ‘Lands’iedel; W., Am. City 75, No. 6, 92 (1960). (36) Levin, P., Mining Congr. J . 46, No. 4, 79 (1960). (37) Lill, W., Modern Textiles Mag. 40, No. 10, 56 (1959). (38) Lomas, J., Mfg. Chemist 31, No. 2, 58 (1960). (39) Mabery, H. N., Plant Eng. 14, No. 8. 112 (1960). (40) Macios, T. W., Trans. Am. Soc. Mech. Engrs. 82, Ser. B, No. 3,277 (1960). (41) Manning, W. P., Gauvin, W. H., A.I.Ch.E. Journal 6, 184 (1960). (42) Marotta, G. J., Metal Finishing 57, No. 12. 69 (1959). (43) Matthew;, D.’ W.. Phelps, H. E., Rubber World 142, No. 4, 76 (1960). (44) Mellor, J. W., Ohye, D. F.. Vacuum ‘ 10, No. 3, 245 (1960).‘ (45) Place, G., Ridgeway, K., Danckwerts, P. V., Trans. Znst. Chem. Engrs. (London) 37, 268 (1959). (46) Race, E., Tappi 42, 827 (1959). (471 Richards. A.. Textile Inds. 124. No. ‘ f , 135 (1960). ’ (48) Rubber Age 87, 664 (1960). Y Wire Prods. 34, (49) Ruff, R. E., Wire t 1346 (1959). (50) Smithies, W. R., Blakley, T. S., Food Technol. 13, 610 (1959). (51) Steele, R., Textile Research J . 29, 960 (1959). (52) Sternberger, R. M., Tappi 43, No. 1 (Suppl.), 244A (1960). (53) Uthwatt, T. A., Woollatt, J. S., J . SOC. Dvers Colourists 75. 445 (1959). (54) Vander Velde, M. H.; Tappi 42, No. 11 (Suppl.), 155A (1959). (55) Van Kamven. G. R.. Zbid.., 42., No. ’ fl (Suppl.), f 5 l A (1959)’. (56) Wadsworth, P., J . Textile Inst., Proc. 51, 552 (1960). (57) Watt, I. C., Kennett, R. H., James, J. F. P., Textile Research J . 29, 975 (1959). (58) Zapffe, F., Oil Gas J . 58, No. 11, 180 (1960). (54) Ziiske, H., Soafi Chem. Specialties 36, No. 7, 139 (1960). ’
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