Dehydratian of Heat-Sensitive Materials H. W. SCHWARZ NATIONAL RESEARCH C O R P O R A T I O N , C A M B R I D D E 42, M A S S ,
T h e pilot plant phases of t h e development of dry orange juice powder and a dry coffee powder are discussed. Fundamental similarities between these products, as illustrated by drying ratesand vapor pressure data, require specialized types of dryers, Furthermore, t h e economics of drying a t pressures below 1 mm. makes i t desirable t o feed a concentrated solution t o t h e dryer. In t h e orange juice process, concentration is carried o u t under vacuum and t h e concentrate is then dried i n semicontinuousvertical dryers. In t h e coffee process, concentration is obtained during t h e extraction step, and t h e concentrate is fed t o a continuous belt dryer. 4 s a means of removing water vapor, solid phase ice condensers and liquid phase absorption systems have been tried. Equipment and control design was directed towards easily operated units. Manpower requirements were low, and no particularly complex operations were required.
F
OR many years processing of heat-senbitive materials has been steadily improved. During the recent war vacuum dehydration received great attention because of the part i t played in the production of penicillin. Vacuum dehydration as a technique is old, (2, 3 ) and over the years appears to have come to mean the drying of materials at very low air pressures. Thus vacuum dehydration may be done from the frozen or liquid state and a t a variety of pressures, depending on the physical properties of the material being handled. X review of this field will be found in articles by Flosdorf and Friedman (4,5). Within certain limits three main advantages are obtained by going to vacuum dehydration for the processing of heat-sensitive materials: (1) Thermal decomposition is reduced, (2) a highly active and greatly expanded (lyophilic) strueturf: is produced, and (3) substantially all the water (or solvent) can be removed and this usually results in better storage stability for the product. For many substances good results can be obtained without resorting to vacuum dehydration; for others this special treatment is necessary (3, 6). This paper describes the pilot plants developed to produce a dry orange juice powder and a dry coffee powder by vacuum dehydration from the liquid state. The limits for food processing are to a large extent determined by their effect on quality. On the other hand, the production economics generally requires some compromise A ith the quality standards. I n the ideal case reduction to the dry state would be carried out a t extremely low ternperatui es, arid usually require correspondingly low pressures, but the costs of water vapor removal rise sharply as the pressure is reduced below 500 microns (8). It was apparent that most of the water should be removed a t as high a preseure as possible, so that the material enters the vacuum dehydration step with a minimum amount of water. The development programs for orange juice and coffee followed the outline given above. Work previously reported ( 7 ) showed that drying rates from the liquid state were substantially greater
than from the frozen state, while quality for these particular materials was not significantly impaired. Laboratory studies also demonstrated that concentration of the dilute solutions was feasible and that these concentrates could be readily dried, at least on small scale equipment. It was found that the materials should be dried to 2% moisture or below for good storage results Finally, a complete technique was worked out which involved drying from thin films (6 to 40 mils) at pressures below 1 mm. D R Y I N G RATES
When the development program moved into the pilot plant stage it was found that the concentration step was relatively straightforward, but that the drying operation was complex. The philosophy in regard to drying equipment was that it should be continuous with throughput as laige as possible and labor a t a minimum. A study of the drying curves for these substances will help explain some of the difficulties involved. Figure 1 shows the instantaneous drying rates (rate of water removal) plotted against moisture content. (All moistur c determinations were made by the Karl Fischer method, IO.) The drying temperatures indicated are those which had been established as the upper limits by laboratory quality control. The data presented are typical; variation of the physical and chemical properties of' the materials will displace the family of curves ab a whole and will not change their general shape or relation to each other. Although the plot does not show it, both materials pass through a stage between 10 and 470 moisture, in which they tend to form gels if the drying film is disturbed or allowed to collapse. If this happens the material will coalesce t o a sticky mass which cannot readily be dried further. This means that methods that agitate the material while i t dries are not likely to bc successful. The data also show that the rate of water release drops off lapidly as the moisture content is decreased. However, the effect of layer thickness of the drying material is not so important as might be a t first supposed, for it can be seen that the rates of water release for loadings as different as twofold draw together and approach each other closely as the moisture content falls to 1%. (In this range of moistures, per cent moisture and moisture content as pounds of water per pound of bone-dry solids are nearly equivalent.) For example, t o dry a 357, (2' = 1.86) solids coffcc film of 0.017 pound of solids per square foot (equivalent to a a e t film of approximately 8 mils) to 3.5% moisture would take 2.7 minutes, while to dry it to 1% moisture would take 10.6 minutes. For a film of 0.032 pound of solids per square foot the times to dry t o 3.5 and lmo moisture would be 7.4 and 17.6 minutes, respectively. I n contrast, for 5 0 7 , ( T = 1.0) solids orange juice concentrate, t o dry a film of 0.046 pound of solids per square foot (ivet film thickness 14 mils) would take 9.5 minutes, while to dry to 1% moisture takes 390 minutes, For a film of 0.092 pound solids per square foot the times for the above moistures would be 26 and 450 min-
utes.
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Here again, the importance of starting the vacuum dehydration with a concentrated solution can be seen, when it is realized t h a t a dilute starting material must pass through the long drying periods in the lower moisture ranges where i t will follow approximately t h e drying curves presented for the concentrates. Difficulties in handling highly concentrated and usually viscous materials set upper limits on the amount of concentration t h a t can be done before the dehydration step. If the rate a s pounds of water/(pounds of bone-dry solids) (hour) of Figure 1 is multiplied by the loading factor, the pounds of water/(hour) (square foot) will be obtained. I n these low moisture ranges the Vaporization rate per square foot is greater for thick films than for thin. At present there are not enough data to explain this anomaly, and any remarks made here would be speculative. Figure 2 presents two vapor pressure isotkerms for the materials at their drying temperatures. As would be expected, coffee powder has t h e higher vapor pressure, and although not shown, coffee powder has a higher vapor pressure than orange juice even at the same temperature. T h a t the equilibrium pressures are so much higher than the actual drying pressures (1 mm. or less) is not too surprising when one remembers the extremely low rates of water release portrayed in Figure 1. Attempts t o dry these materials a t pressures over 5 mm. have produced powders markedly inferior to those dried below 1 mm. Other efforts t o dry the materials in two stages with various methods of heating, including radio-frequency, have resulted in the general conclusion t h a t nothing will materially improve the drying rate without causing serious quality and appearance changes. The requirements so far enumerated for the vacuum dryer appear t o be as follows:
It should be continuous in operation. It must be vacuum-tight.
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The drying surface must be available for a fairly long period (because of the long drying cycles). T h e design must be such t,hat a uniform film can be deposited on t h e surface and allowed to dry undisturbed. It should be reasonable in initial cost and require little operating labor.
t
I
2.0
001
I
I
I
1
002
1
O W
AVERAGE MOISTUR~%ONT.LNT LB. H.O/LBS
Figure 2.
BONC DRV S O L ~ D S
Vapor Pressure Isotherms
The many possibilities t h a t meet the above requirements may be divided into two general classifications: those in which t h e film is deposited on a stationary surface, and those in which the film is deposited on a moving surface. This arbitrary division admittedly excludes such techniques as vacuum spray drying, never intensively studied in the low pressure range because of the complex heat transfer and entrainment problems. One dryer from each group was chosen for study i n the pilot plant stage as part of the general dehydration program on orange juice and coffee. ORANGE JUICE PILOT PLANT
The orange juice dryer (shown in Figure 6 of 8) was of the stationary surface type. It was described in some detail in a n earlier paper (8),b u t for purposes of comparison is briefly reviewed here. This dryer contained 282 square feet of drying surface, and was 6 feet in diameter and 25 feet over-all. With the dryer a t 500 microns or less, the charge, usually orange juice concentrate of 50% solids, was sprayed on the walls through spray nozzles. T h e concentrate adhered to the walls, forming a viscous film which rapidly puffed out as much as 3 inches as the water vapor escaped. This film was allowed to dry undisturbed until i t reached the desired moisture content, at which time it was scraped from the walls and allowed t o drop into a removable dolly. A flapper valve made it possible t o remove the dolly from t h e dryer without breaking t h e vacuum, so t h a t the dryer could be charged immediately after the scraping cycle had been finished.
INITIALLY 50% SOLIDS
O'o10l1
I
'
0 oe I
I
'
I " 0 [4 3 I
I
' ' ' ' 0 04
AVERAGE M O I S T U R E CONTLNT LB. H&O/ LB. DRY SOLIDS
Figure 1.
Instantaneous Drying Rates
While not continuous in operation, this dryer could be duplicated, so t h a t concentrate could be charged and powder removed continuously t o and from t h e system. To handle the water vapor load and maintain the vacuum, a combination of a rotary condenser and four-stage steam ejector was used. T h e condenser was designed to handle 25 pounds of water vapor per hour at 100 microns water vapor pressure, condensing the vapor to the solid phase on the chilled wall from which it was continuously scraped. It was provided with 2.5 tons of Tefrigeration at -65 F. During the charging operation the system pressure rises sharply, owing t o the flash of vapor from the liquid concentrate. The t s p total pressure may be as high as 1500 microns, and generally occurs 10 to 20 minutes after charging starts. The
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Figure 3.
Vol. 40, No. 11
P i l o t P l a n t for Dehydration of Orange Juice
pressure then drops rapidly to 600 t o 700 microns, decreasing slowly to 200 t o 300 microns over the rest of the drying cycle. Figure 3 shoivs the pilot plant incorporating this unit, which !?as installed at Plymouth, Fla., in 1944, The black equipment in the upper foreground is the rotary condenser-ice receiver, the dryer is partially visible directly in back of the condenser, while the concentrating equipment is just discernible behind the dryer. The refrigeration compressors are on the floor a t the right. The rest of the building contained a cold storage room, a dehumidified room, laboratories, and offices. Total space occupied was 80 X 120 feet with a maximum headroom of 35 feet required. The pilot plant was operated 24 hours per day during the fruit season. Fresh juice was obtained from a nearby cannery, brought to the pilot plant in a 100-gallon portable tank, filtered, and chilled. It \Tas then deaerated, concentrated to 50% solids, and charged to the dryer. T n o men per shift were required t o operate the plant, escludiiig the research and control chemists. Proces3 controls were of a very simple nature, and consisted mostly of self-contained temperature and pressure regulators. The refrigeration was separately controlled to maintain a temperature of -65' F. a t the rotary condenser. The main process control as indiiect, in that the results of a series of runs were analyzed as to taste, chemical and physical properties, and storage stability. The results from operation of this plant are discussed below.
COFFEE PILOT PLANT
The coffee dryer shown in Figure 4 is representative of the second type of dryer, in which the surface moves. The moving surface was provided in the form of a slov-moving belt, and the Elm was deposited by an oscillating spray nozzle. The belt shown was in the form of close-fitting plates 2 1 inche~! wide, which were carried on standard roller chain. The belt was about 18 feet long, with the pulleys on 9-foot centers, and had 25 square feet of effective drying area. The powder was scraped from the belt a t the end of the return travel directly below the point a t which the film was deposited. The pori-der fell downward into a removable powder dollv similar to the one used on the orange juice dryer; when full the dolly could be removed without in any way disturbing the drving operation. The belt was provided with a variable rpeed drive and an automatic time cycle control, so that the drying cycle could be varied from a few minutes up t o several hours (by starting and stopping the belt). The oscillating spray arm was synchronized with the belt tinier and fitted with a solenoid spray nozzle, so t h a t the film was deposited only when the belt i u s moving. Best results were obtained for coffee when the belt \vas in continuous motion and the drying cycle varied from 3 to 15 minutes at 1mm. of mercury total pressure. Although the dryer is shown fitted with a plate belt and spray arm, many variations will achieve the same result. For example, this dryer can be provided with a solid belt and a tank or roller type of feed, so that the film is continuously wiped onto the belt surfaces (I).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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over t o a n open spray column 14 inches in diameter, where i t was absorbed at about 1 mm. total pressure by the brine. The brine was then pumped out of the bottom of the column by a conventional turbine type pump and delivered to the cooling or regenerating equipment as required. Here again controls were of a simple nature. Vacuum systems are more or less self-regulating with load, if they have been designed correctly. T h e film-loading characteristics in the dryer can be accurately controlled by the proper choice of spray nozzles, and by variation of the spray pressure and belt speed. The brine and refrigeration systems had control instruments t o regulate the brine temperatures and flows through the various pieces of equipment. Pressures in the dryer were conSCRAPLRBLADE. tinuously indicated by an Alphatron (radium source ionization type) gage. As before, final control resided in a taste panel and analytical group. Figure 6 is a general view of the pilot plant. The extraction columns are on the left, the deaeratoicL_/' concentrator is the thin cylindrical tower supported Figure 4. Moving Belt Dryer for Coffee above the belt dryer, and a t the extreme right the brinc absorption column and recirculating pump can be seen. The refrigeration and heat exchanging The pumping system for this plant consisted of a brine absorpequipment are not shown but the total floor area occupied was tion system similar to the type described by Tucker and Sherapproximately 65 X 25 feet with headroom of 20 feet. T h e wood (9). Residual air was removed by a mechanical vacuum plant normally operated 24 hours per day with two men per shift. pump handling 100 cubic feet per minute. I n this type of system the water vapor is absorbed continuously in a stream of refrigerated brine. The brine is recirculated; a portion is withdrawn C O M P A R I S O N O F TWO D R Y E R S continuously, from which the absorbed water vapor is boiled The over-all purpose of the pilot plants described was, as usual, off at atmospheric pressure. A variety of brines can be used, t o test the general feasibility of a process. Studies were made but usually lithium chloride, lithium bromide, calcium chloridc, of the effect of variables on over-all yield, production rate, costs, or mixtures of these salts are recommended. The refrigeration physical and chemical properties of the product, and storage required is slightly less than the tonnage for the solid phase constability. Most of this work was similar to all pilot plant operadensers, and the temperatures required are not so low. For this tion. However, a comparison of the two types of dryers used plant the brine was controlled a t -20 ' F., the refrigerating load may be of interest to others working on allied problems. being carried by a single 10-h.p. compressor. The two dryers have the same drying rates per unit area when Figure 5 is an elevation flow sheet for thc coffecpilot plant. drying the same materials under identical conditions. T h e most Roasted and ground coffee bean was extracted in the columns; significant results from a plant design viewpoint are contained i n a n extract of about 35% solids was obtained. The extract was Figure 7, which shows the average drying rate-Le., production deaerated'and concentrated further under about 10 mm. of merrate-versus the loading factor. As might be inferred from t h e cury pressure and then fed to the dryer. The water vapor passed
1
BRINE.
ABSORPTION
Figure 5.
Elevation Flow Sheet for Coffee Pilot P l a n t
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 6.
Pilot P l a n t for Dehydration of Coffee
instantaneous rates, the production late wlicii plotted against layer thickness goes through a maximum. The mavima shift tov ards higher loadings as the moisture content is decreased, but the rate also rapidly decreases, so t h a t t h e net production is
Figure 7.
Vol. 40, No. 11
Average Drying Rates
very low when a n attempt is made to di y to rnoisturc, contcxnts below 1%. The data indicate that the orange juice diyer u i t h 282 .quare feet of drying area vould take a 56-pOUIld load ol solid, nhich would require 3.9 hours of drying time when drying to 2y0 moist,urc. Adding 0.3 hour for charging and discharging, the hourly production rate would have been 12.8 pounds of orange juice solids per hour. The coffee dryer (with 25 square feet) for the same moisture content would take a continuous film representing about 0.014 pound of solids per square foot, requiring a belt or drying cycle of about 3.6 minutes. Total hourly production would be about 6 pounds of coffee solids per hour. However, the dryers are not necessarily operated a t the poinl representing maximum production, as special characteristics of the dry product can be obtained at, different. loading factors. The two dryers have features t,hat fit them for uso on products of different drying properties. I f thr: belt dryer %yereto be used for a material t h a t requircs a long drying cycle, t,he belt would be excessively long or the travel very slow. The vertical dryer, on the other hand, requires a complex internal mechanism if the drying cycle is short, and charging and discharging must be done continuously. There is also one rather important fundamental difference betwccn the dryers. The batch or semicontinuous dryctr operates on a drying cycle which can produce a final
November 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
pressure considerably lower than the normal running pressure, while the belt dryer maintains a steady operating pressure set by the capacity of the pumping system. Some materials, therefore, which require a final drying period a t pressures lower than those needed in the first part of the cycle, may be more readily handled in the semicontinuous type of dryer. A variety of heat-sensitive materials such as fine chemicals, plastics, and other food products, have been studied with results substantiating the conclusions reached above. P A C K A G I N G FACl L I T 1ES
d n integral part of these pilot plants was the packaging areas. In each case a vapor-tight room which could be maintained at relative humidities of 15% or lower was required. Under these conditions the powder can be stored for several hours, and handled without undue difficulty from moisture reabsorption. No sperial packaging machinery was required, although i t is desirable to use net-weight filling machines because the container often weighs more than the powder. CONCLUSION
The basic difficulties in the vacuum dehydration of liquid materials have been outlined. There are, at present, limitations on the results t h a t can be obtained. However, the probable process requirements for different materials can be predicted from laboratory measurements. Two vacuum dryers of different characteristics were incorporated in pilot plants to test
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the feasibility of processes for drying orange juice and coffee. An analysis of their performance indicates t h a t each has a range where i t is best adapted for use. ACKNOWLEDGMENT
The author wishes to acknowledge the valuable assistance of other members of the company in preparing this paper. T h e data presented were based on work done by R. H. Cotton, A. DiNardo, E. G. Hellier, A. L. Schroeder, and G. A. Schroeder. LITERATURE CITED (1) Campbell, W. L., Proctor, B. E., and Sluder, J. G., “Research Reports on Quartermaster Contract Project,s,” July 1, 1944, t o Oct. 31, 1945, Massachusetts Institute of Technology, Food
Technology Laboratories, Cambridge, Mass.
d’Arsonval and Bordas, Bull. assoc. SUCT. dist., 24, 917 (1907). Falk, K., Frankel, E. M., and McKee, R. H., IND. ENG.CHEM., 1 1 , 1036-40 (1919). Flosdorf, E. W., FoodInds., 17, 92 (January 1945). Friedman, S. J., IND.ENG.CHEM.,40, 19 (1948). Moore, E. L., Atkins, C . D., Wiederhold, E., MacDowell, L. G., and Heid, J. L., Proc. Inst. Food Technol., 1945, 160-8. Schroeder, A. L., and Cotton, R. H., IND. ENG.CHEM.,40, 803 (1948). Schwarz, H. W., and Penn, F. E., Zbid., 40, 938 (1948). Tucker, W. H., and Sherwood, T. K., Ibid., 40, 832 (1948). Wernimont, Grant, and Hopkinson, F. J., IWD. ENG. CHEM.. ANAL.ED., 15, 272-4 (1943). R E C E I V ~April D 26, 1948.
LANTS
FLUIDIZED SOLIDS PILOT PLANTS A l t h o u g h t h e m a i n commercial application t o date of t h e fluidized solids technique has been in t h e field of t h e catalytic cracking of petroleum oils, this extremely flexible and versatile technique will undoubtedly be applied t o a n ever increasing number of processes. T h e technique is particularly adaptable t o mixed phase (solid-gas) processes requiring t h e addition or removal of large amounts of heat, especially where isothermal conditions are desirable. T h e fluidized solids m a y be catalysts, reactants, inert heat transfer media, or adsorbents. This article presents t h e steps necessary for t h e practical design of a pilot plant utilizing t h e fluidized solids technique. An apparatus which has been used for fluidization studies essential t o unit design is illustrated. T h e translation of data obtained in such equipment t o t h e design of reactors, circulating equipment, etc., is outlined. Recommendations are made as t o t h e design of specific features common t o most fluidized solids pilot plants, such as t h e solids-gas distributors and t h e solids recovery equipment. Methods of measurements and control of temperature, pressure, solids flow rate, and space velocity are discussed.
E. W. NICHOLSON
AND
J. E. MOlSE
E S S O LABORATORIES. BATON R O U G E . LA.
R. L. H A R D Y ESSO LABORATORIES. ELIZABETH, N . J
HE use of the fluidized solids technique first became of general
nique to many other processes and fields soon were investigated on pilot plant and commercial scales. One of the most interesting of these new developments is in the field of hydrocarbon synthesis, and commercial plants employing the fluid technique are planned in the near future. As has been pointed out (5,6, 6),the essence of the fluidized solids technique is the suspension of finely divided solid particles in a rising stream of gas. The gas separates and supports the particles and provides mobility and fluidity for the mass of suspended particles. This mass of suspended particles has the appearance, behavior, and many of the properties of a true fluid; these similarities have resulted in the name fluid being given t o the process. I n applying the fluidized solids technique in different fields, the process role of the solids has varied considerably, as may be seen from the following general classification of these applications :
interest during the recent war when i t was applied extensively in the fluid catalytic cracking process. It quickly be-came apparent t h a t this new engineering technique was a n extremely flexible and versatile one, and applications of the tech-
The fluidized solids may be a catalyst, as in catalytic cracking or hydrocarbon synthesis. The fluidized solids may be a reactant, as in the fluid coal gasification process.
T
