May 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
P, but falls rapidly when the forepressure exceeds this value. As t h e discharge rate decreases the boiler temperature rises until the rate begins to increase and normal jet action is temporarily restored. If one records the upper and lower limits of oscillation on the Pirani gage for a given leak rate and plots both values, as well as the mean, on the graph of speed vs. high vacuum, it will usually be found that all of the points fall nearly on the same smooth curve regardless of what pressure is chosen within the range of oscillation on the gage. This is merely a coincidence arising from the fact t h i t the speed is computed,from the formula
where t is constant during the measurement while the actual variation of speed with pressure in this region follows the law
FC
#=P where FC is nearly constant over a range of p corresponding to‘ the range of the oscillations.
Literature Cited Alexander, P., J . Sci. Instruments, 23,11-16 (1946);21,216-18 (1944). Blears, J., Nature, 154,20 (1944); Proc. Roy. S O L ,188, 62-76 (1946). Clausing, P., 2. Physik, 66,471 (1930). Copley, M., Simpson, O., Tenney, H., and Phipps, T., Rev. Sci. Instruments, 6, 265-7 (1935). Czerny, M., and Murmann, H., Physik. Z . , 30,462-3 (1929). Downing, J. R., private communication from National Research Corp. Dunoyer, L., “Vacuum Practice,” pp. 1-4, New York, D. Van Nostrand Co., 1926.
803
(8) Dushman, S., “High Vacuum,” pp. 38-40, Schenectady, N. Y., General Electric Co., 1922; J . Franklin Inst., 211, 691-6
(1931). (9) Ebert, H., 2. Instrumentenk., 51, 337 (1931). (10) Ellett, A , , Phys. Rev., [2]37, 1699 (1931). (11) Eltenton, G.C., J. Sci. Instruments, 15,415 (1938). (12) Farkas, A.,and Melville, H. W., “Experimental Methods in Gas Reactions,” London, Macmillan Co., 1939. (13) Gaede, W., Ann. Phys., 41,337 (1913). (14) Hickman, K. C.D., and Sanford, C. R., Rev. Sci. Instruments, 1, 154-9 (1930). (15) Ho, T.L.,Ibid., 3,133 (1932);Physics, 2,386-95 (1932). (16)Howard, H.C., Rev. Sci. Instruments, 6,327 (1935). (17) Kapff, S. F.,and Jacobs, R. B., Ibid., 18,581-4 (1947). (18) Kaye, G. W.C., “High Vacua,” p. 162,New York, Longmans, Green and Co.,1927. (19) Kennard, E. A., “Kinetic Theory of Gases,” pp. 6-9, 306-8, New York, McGraw-Hill Book Co., 1938. (20) Korsunsky, M.,and Vekshinsky, S., J . Phys., U.S.S.R., 9,399404 (1945). (21) Langmuir, I.,Gen. Elect. Rev., 19, 1062-6 (1916). (22) Loeb, L. B., “Kinetic Theory of Gases,” 2nd ed., pp. 301-10, New York, McGraw-Hill Book Co.,1934. (23) Matricon, M., J . phys. radium, 3, 127-44 (1932). (24) Mills, P. J., Rev. Sci. Instruments, 3,309 (1932). (25) Monch, G., “Vakuumtechnik im Laboratorium,” Ann Arbor, . Mich., Edwards Bros., Lithoprint, 1944. (26)Sohwarz, H., 2. Physik, 122,437-50 (1944). (27) Sinel’nikov, K. D., Val’ter, A. K., Gumenyuk, V. S., and Taranov, A. Ya.,J. Tech. Phys. (U.S.S.R.), 8,1908-22 (1938). (28) Sivertsen, J., Instruments, 20, 333-4 (1947). (29)Strong, J., “Procedures in Experimental Physics,” pp. 97-101, Ne* York, Prentice Hall, 1938. (30) Wachter, H., and Scheer, J. van der, 2. tech. Physik, 24, 287-91 (1943). (31) Westin, S.,and Ramm, W., Kgl. Norske Videnskab. Selskabs, Skrifter, 1936, No. 9. (32)Yarwood, J., “High Vacuum Technique,” Chap. 3, hfew York, John Wiley & Sons, 1945. RECEIVEDNovember 3, 1947. Communication No. 128 from the Laborstories of Distillation Products. Ino.
Dehydration of Orange Juice A. L.
Schroeder
NATIONAL RESEARCH CORPORATION, CAMBRIDGE, MASS.
R. H. Cotton‘ NATIONAL RESEARCH CORPORATION, PLYMOUTH, FLA.
-4 study of the drying of orange juice is presented. The general approach and technique are applicable to the drying of most dilute solutions. Data are presented regarding the variation of drying rates under different conditions of temperature, pressure, and charge in the dryer. The method of drying from viscous thin films is outlined by which maximum production rates and product quality may be obtained. The effect of processing temperature on production rates is discussed, and a brief rBsum4 of the storage stability of orange juice powder is given.
T
HE dehydration of orange juice has been of great interest for many years. The economic advantages are obvious. Suffice it t o say that the citrus crop is growing larger every year, particularly in Florida, and the citrus industry may be confronted with a serious distribution problem. Development of processes and techniques for handling t h e crop willopen the road for stabilizing t h e industry. The large volume of canned citrus juice and the relatively new concentrate illustrate the current trend. The production of a satisfactory citrus powder, such as orange juice pow1
Present address, Holly Sugar Corporation, Colorado Springs, Colo.
der, will add another means for obtaining a broader market domestically and eventually throughout the world. Numerous investigators have been interested in drying orange juice. Some excellent work was done at the Massachusetts Institute of Technology during the recent war by t h e staff of the D e partment of Food Technology (I).‘ Flosdorf has described equipment and summarized a drying technique (2, 3)and Moore et al. (6) have reviewed t h e general developments of drying and concentrating in practice u p t o 1945. During t h e past seven years, this laboratory has been working on t h e general problem of drying heat-sensitive liquid materials, of which orange juice offers a good example. The work was instituted in Boston on a small scale and in 1944 was transferred t o a Citrus Regearoh Laboratory at Plymouth, Fla. Hayes, Cotton, and Roy (4) reviewed some of t h e problems and results in 1946. The present paper presents additional data regarding the drying of orange juice: Initial data comparing the drying of whole orange juice and a concentrate of orange juice. Results of an investigation of temperature ?nd moisture conditions existing within a drying mass of orange juice. A curve t o illustrate the “falling rate” period of drying. The effect of temperature on production rate for different final moistures.
INDUSTRIAL AND ENGINEERING CHEMISTRY
804
Figure 1.
PERCENTSOLIDS Freezing Curve for Orange Juice
Except for Table I, all the data were obtained from a pilot plant dryer, which actually was of such a size that it could be considered as a complete unit in itself for plant scale operations. For a detailed description of the dryer, the reader is referred t o the paper by Schwarz and Penn ( 6 ) .
Problem The dehydration of any foodstuff has as its ideal goal a product which when rehydrated is equal in quality t o the original raw material. A complementary requirement is t h a t t h e product be stable during storage. From the practical standpoint, the problem becomes one of determining the best combination of conditions t h a t will result in a product of high quality consistent with the economics of manufacturing cost.
Dehydration
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Vol. 40, No. 5
stantial lowering of the freezing point. A liquid film represents a film in which the temperature is maintained well above the point a t which ice would separate. The data in Table I m-erc obtained very early in small laboratory equipment, when the full significance of final moisture content was not fully appreciated. The final moisture is of paramount importance from a storage standpoint, a s well as the production rate. Later work made it evident t h a t absolute moisture contents for orange juice powders could be obtained most satisfactorily by use of the Karl Fischer method ( 7 ) . The moisture data given in Table I were determined by t h e vacuum oven method, which accounts for roughly 75% of the water found by the Fischer method. T.T'iih this point in mind, Table I should be used in a comparative sense only. The data are given t o show the basis for t h e development of the final drying technique. Except for Table I all moisture data reported in this paper were obtained by the Fischer method. Experiments with concentrating indicated that both low pressure-low temperature concentrating and freeze centrifuging would produce a satisfactory product for drying. Test panels showed that the flavor change was very slight by either method. Current commercial practice, using either method, gives a product very close to whole fresh juice of good quality. Evidently, the use of concentrate offers several advantages. The production rate based on pounds of solids per hour per square foot can be increased about four t o nine times over that for frozen whole juice. The range of four t o nine depends upon whether frozen or liquid film3 are dried. The cost for complete drying is greatly reduced. This point IT ill be specifically stressed in a report on pilot plant studies. Concentrate can be more easily handled, particularly in loading of dryers. Introducing a dilute solution into a low pressure region
The initial work was done with whole orange juice-that is, juice containing approximately 127, solids. Florida Valcncia oranges were utilized as the source fqr the orange juice as the startTable I. Initial Drying Data on Whole Orange Juice us. ing raw material. The ratio of soluble solids to acid had a range of Orange Juice Concentrate values between 11.0 and 14.0. The first drying runs with orange Film Loading, Production juice demonstlated that a powder of high quality and satisfactory ThickLb. Rate, ness, Solid/ Cycle, % Lb./Hr.: for storage at room temperature could be produced, providing Sample Inch Sq. Pt. Hrs. XIolsture Sq. Ft, the final moisture content was 1.57,. During these experiments, Frozen film 2 1.9 0.025 the finishing drying temperature was 45" t o 50" C. Little or 50% concn.a 1/81 0 05 50% c0ncn.a 1/22 0 10 2 2.14 0.05 no change in taste was found for powders that had been dried 0.011 '/a2 0.021 2 2.2 12% juicea Liquid film at any temperature up t o 50' C. I 1 . 6 0.05 1 / 0 4 0 . 0 5 50% concn.Q 50% concn.a 1/82 0.10 1 2.9 0.10 Table I shows t h a t for juice with 12y0 solids a production rate of about 0.01 pound per square foot per hour was obtained. a Concentrate prepared from portion of original lot of juice. Obviously, a n increase in production rate was desirable, if the quality did not suffer. Taste comparison showed that the quality of powders from concentrate and whole juice was almost identical. Table I indicates that use of concentrate with 507, solids gives a production rate approximately five times t h a t for whole juice, when equal weights of juices, dried t o approximately the same moisture content, a r e compared. The increase in production rate follows from the fact t h a t concentrate with 50% soluble solids contains 4.16 times as much solids as juice with 12Oj, soluble solids. When the time required for drying equal weights of whole juice and concentrate is the same, obviously concentrate should be used in preference t o whole juice. Table I also compares frozen and liquid films. As can be seen, the drying rate for liquid films is double t h a t of frozen films. A frozen film dries by sublimation; the sample is cooled t o a point where a solid mass is attained and then dried. With concentrate containing fi070 soluble solids, TIME-HOURS it is difficult to dry from theso-called frozen state because the high solid content causes a subFigure 2. Drying Curve for 50% Orange Juice Concentrate
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1948
of 1 mm. is very difficult because of the rapid flashing of water and cooling t h a t take place, which ultimately result i n freezing of the material. Use of a concentrate of 50% solids and consequent depressing of the freezing point eliminate this trouble.
805
PERCENT SOLID&
Phase Study for arange Juice. Figures 1 to 4, give some idea of the conditions existing during drying of a film of orange juice concentrate. When a series of cooling curves, using apparatus described in any standard text on physical chemis0 W A L L TnERMOCOUPLL try, is determined for orange juice of varying concentration, a freezing curve such as that shown in I " O F F WALL Figure 1 is obtained. I n a sense then, Figure 1can be taken as one leg of a phase diagram for orange -20: I I I 1 cI 1 I 4 5 juice. Any point along the curve establkhes the TIME - HOURS conditions for equifibrium between ice and orange juice solution, Definite plateaus or changes in Figure 3. Temperature Curves for Orange Juice slope of the cooling curves were found for the points given in Figure 1. No eutectic was found on afiv of the cooline curves down t o a temaerature of -40" C. Figure 4 rearesents a composite of time-temperature and time Schwarz and Penn (6) present a family of curves showing the moisture conditions during drying. The freezing curve is inaverage production rate (pounds of solids per square foot per cluded to indicate t h a t the film is never below freezing temperahour) versus the loading factor (pounds of solids per square foot) tures. The three curves show average point conditions at any for drying t o different final moisture contents. Figure 2 repretime during the drying cycle. The per cent solids value given for sents a typical drying curve €or liquid films of concentrate coneach temperature value is the average value throughout the whole taining 50% solids. This loading factor does not result in a n opfilm. Actually the water content varies through the film. This is timum production rate, but i t will serve t o present a n idea of the partially evident from the temperature values as recorded at indrying conditions existing within a drying film of orange juice creasing distances from the source of heat. The insulating effect of the porous film plays a n important role. However, several concentrate. The rate is fairly constant during the early part of the cycle. I n this case the loading factor was 0.40 pound of solid careful determinations of t h e water content of nearly dry films per square foot and the production rate was 0.08 pound of solid gave the following results: water content of film directly on wall, per square foot per hour, dried t o a final moisture of 3.070. The 0.9%; 1 inch off wall, 1.8%; 3 inches off wall, 3.5%; average temperature of the drying surface was maintaines at 45 O C. The water content of well mixed charge, 2.1%. These data arg valid moisture measurements were done by the Karl Fischer method only for the concentration, temperature, pressure, and loading for these particular experiments. For Figures 2, 3, and 4 the initial ( 7 ) . The absolute pressure i n the drying chamber was 0.2 t o 0.7 mm. of mercury as measured by a n Alphatron vacuum gage. concentrate in all cases was 5070 solids. The drying surface was Figure 3 shows some average temperature measurements within maintained at 45 C. The total pressure varied from 1t o 0.1 mm. the drying film of orange juice, made by copper-constantan therin the drying chamber as measured by a n Alphatron vacuum gage. mocouples placed a t different points within the film. SuperimThe loading on t h e drying surface was 0.40 pound per square foot. posed on the time axis are corresponding values €or moisture content taken from Figure 2. These curves give a n approximate idea of the temperature profile through the orange juice film. The idea of the point measurements with thermocouple junctions was t h a t each junction would be covered by a globule of concentrate during the loading operation. The assumption was then made t h a t the temperatures at the point of the junction would closely approximate the temperature of a n y nearby point. If, by chance, any junction did not receive a coating of concentrate, the temperature would stay a t some fairly constant value approximating the wall temperature. Determinations taken for fifty-five trials gave results closely following t h e pattern of temperatures as shown in Figure 2. Figure 4. Freezing and Drying Curves for Orange Juice
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4
806
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 5
orange juice has a thermal conductivity (0.01 to 0.02 B.t.u. per square foot per O F.) coniparable t o cork. Of perhaps greater importance in considering the slow release of watei are the factors of diffusion and absorption. The difficulty of removing the last traces of mater is further illustrated by Figure 5, which gives the portion of a drying curve bet\veen 2.t 3 and 1% water. One of the desired characteristics of orange juice powder is that the dry product should be readily soluble. T o satisfy this requirement a porous and friable product of low density is desired. Sublimation drying techniques produce this type of structure. However, 1.0 it was found that the same or very similar structure was obtained when viscous thin filni~ of. concentrate were dried. For purposes of reference the viscosity of orange juice conFigure 5. Drying Curve-Falling Rate Period for Orange Juice PQwder centrate with 60% solids is 800 centiDoises a t 0 " C. The equivalent wet film thickness ranges between 0.5 and 0.75 mm. I n order t o obtain t h i b The curve in Figure 5 illustrates the difficulty of removing water from a nearly dry product. I n this particular case, 15 pounds structure the film has to remain undisturbed until neaily dry of powder at 3% water were agitated at a pressure of 200 microns At a moisture content higher than 5% the film is very sticky Anything that is done t o disturb the film, such as agitation, and exposed to a heat source maintained at 45" C. It was not or raising the pressure, will allow the film t o collapse and possible to measure the temperature of the powder itself; however, the average temperature of the powder was probably slightly give a dense sticky mass. less than 45" C. To compare the effect of pressure on the falling Temperature and Time Effects. Figure 6 shows the effect of rate period of drying, two charges of the same weight and with temperature upon the average drying rate for a loading of 0 10 the same moisture content (15 pounds and 37,) were dried a t 40 pound of solid per square foot. As is readily apparent, the late and 800 microns. No change in rate of water removal was found. increases rather rapidly as the temperature is increased. The While 50% concentrate is deposited for a loading factor that three curves in Figure 6 likewise indicate the decrease in drving will give the optimum production rate by spraying upon a warm rate as lower final moistures are obtained. This difference in late platea(45" C.), boiling and bubbling are evident from visual obis what one wouM expect, although the magnitude of difference is much greater than would be suspected a t first thought. For the servation. Within a short time, the film swells and puffs t o as loading factor cited, the time required to dry t o 2.57, moisture i i much as 3 inches. This gives a very porous and fragile structdre after the film is dried. The low density of the film on the drying about half that required to dry to 2.0% moisture, and about one quarter t h a t required t o dry t o 1.5% moisture. Based on pilot surface of 4 t o 5 pounds per cubic foot and temperaturr limitaplant studies, a n interesting family of curves is shown by Schu a r i tions partly explain the falling rate of drying during the latter and Penn ( 6 ) , which complement the curves in Figure 6. part of the cycle. I n addition, the porous film has a low coefficient of heat transfer. Campbell, Proctor, and Sluder ( 1 ) report that For a slight increase in temperature on either of the curve5 N marked increase in drying rate will result. However, caution must be exercised before settling on a temperature that will give a high rate. The final control on this point is product qualitv. With this in mind, Figure 7 shows the change in product quality ~ ( J I two drying temperatures-52 and 60 C. The taste scale is ail arbitrary standard and h & the usual weakness of the human taste factor. A taste panel of five men, who through experience with orange juice tasting gave very reproducible results on any set of samples, made the judgments on the powders after each sample was rehydrated. The definitions of the numerical ratingwere taken as followvq: 3.1
/-
100
4.3
44
5w
zp
66
"C,
110
120
130
140
150
"F
OF DRYINGEffect of Temperature on Average Drying Rate of Orange Juice Powder
-TCMPERAYURL
Figure 6.
Loading factor of 0.10 pound per square foot
10-excellent, equivalent to good whole juice 9-a very good reconstituted juice 8-a good reconstituted juice 7-a juice of fair quality 6-poor quality A marked difference in taste is evident for these t a o tempeiatures up to 250 minutes. Beyond 250 minutes the curves appear to merge and what little difference is shown probably falls xithin expeiimental error. For the loading factor of 0.10, the film as dried t o 1.5% moisture in 240 minutes. Therefore, on the basic of the taste definition a drying temperature of 60" C. would result in a n inferior dry product. On the basis of the taste scale and experience of the panel, a final drying temperature of 50 C n as selected for drying operations. I n the near future, it is planned t o present a discussion of stoiage stability as a p p p l e m e n t to the present drying work. That the stability of the powder is good over a period of a vear a t t ~ m -
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1948
807
Drying from a liquid film is possible. Removing the last traces of water is difficult. T o minimize the effect of the falling rate period of drying, thin liquid films appear to be the logical approach. Temperature and moisture level have an appreciable effect on production rate. A final moisture of 1,5y, is required for satisfactory storage. Because of the effect of temperature and time on initial taste it is recemmended that 50 C. be the maximum temperature tp which the product is exposed, This factor should be decided ultlmatelv on the results of a market survev which will show what is acceptable to the consumer.
9
a 7
O
100
2.00
Figure 7.
300
DRYING T I M E .
’
400
- MINUTLS
500
600
Acknowledgment
Effect of Time-Temperature of Product on Initial Taste of Orange Juice Powder
This paper is the result oT the combined efforts of many people. The authors would like t o express their appreciation for the cooperation of N. V. Hayes, H. w. Schwarz, E. G. Hellier, A. DiNardo, W. R. Roy, 0. McDuff, and C. W. Brokaw.
Loading factor of 0.10 pound per square foot, based on same concentrate
peratures below 32 O C. is shown by the results of seventy stability experiments lasting from six months to one year at temperature levels of 4 O, 21 O, 32”, and 40 O C. The physical appeayance, taste, and ascorbic acid content of powder with 1.5% water are excellent a t the end of one year. At a temperature of 40” C. or over the storage is difficult, the taste and physical appearance changing rapidly within two or three months.
Literature Cited (1) Campbell, W. L., Proctor, B. E., and Sluder, J. G., “Research Reports on Quartermaster Contract Projects,” July 1, 1944,t o Oct. 31, 1945, Massachusetts Institute of Technology, Food
Technology Laboratories, Cambridge, Mass. (2) Flosdorf, E. W., Chem. Eng. Progress, 43,No.7,343-8 (1947). (3) Flosdorf, E. W., J . Chem. Education, 22, No. 10 (October 1945). (4) Hayes, N.V., Cotton, R. H., and Roy, W. R., Proc. Am. SOC. Hort. Sci., 47,123-9 (1946). ( 5 ) Moore, E. L., Atkins, C. D., Wiederhold, E., MacDowell, L. G., and Heid, J. L.,Proc..Inst. Food TechnoZ., 1945,160-8. (6) Schwarz, H. W., andPenn, F.E., IND. ENG.CHEM.,40,938(19481. (7) Wernimont, Grant, and Hopkinson, F. J., IND.ENQ.CHEM., ANAL.ED., 15, 272-4 (1943).
Summary On the basis of the data presented, the following conclusions appear warranted : The production rate of orange juice powder can be increased fivefold if 50% concentrate is dried instead of frozen whole juice. Drying from a liquid film doubles the production rate when compared to a frozen film.
RECEIVm
January 8 , 1948.
Vacuum Distillation of Petroleum Residues W. W. Kraft,
THE LUMMUS COMPANY, NEW YORK, N. Y.
T h e use of vacuum distillation in the petroleum refining industry is discussed and the development of present day commercial units reviewed. The unusual characteristics of performance requirements and design data employed are presented. The major elements of a vacuum distillation unit processing petroleum residues are separately covered in considerable detail to show the type of processing and engineering studies involved and to indicate the most successful methods of solving the problems encountered in designing equipment for this type of operation. Tabulated data and charts are supplied in support of the methods used and the conclusions reached.
B
Y COMPARISON with other fields, the vacuum distillation of petroleum residues cannot properly be classified as a high
vacuum operation. The commercial importance of this type of operation in petroleum refining is indicated by its use in producing all straight reduced asphalts and road oils, providing the optimum material for charging to most catalytic cracking operations, and preparing stocks for the production of all grades of lubricating oils. For the last mentioned application, it is estimated that total charging capacity installed or under construction in the United States alonie amounts to 600,000 barrels per day or over 1,000,000 gallons per hour. Capacity figures for the other two applications are difficult to obtain but are of the same order of magnitude.
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The type of equipment employed is dictated by the very large volumes of material being processed and the relatively low value of charging stock and products. This combination requires that both initial investment and operating cost per unit volume of capacity be moderate. For the simplest type of vacuum flashing operation, investment costs of $35 per hourly gallon charged and direct operating costs of 0.1 cent per gallon may be reached. Such figures make i t apparent that in this field of distillation no higher vacuum will be used than is necessary to protect product quality and avoid deterioration or cracking of the material being distilled. This paper is concerned with presentation of the data required in vacuum unit design, a general summary of the important calculation methods used, a discussion of the results of such calculations, and the engineering method of analysis to translate such results into physical plant design details. The data and process calculation methods uaed in designing this type of equipment are peculiar to the composition of the raw material ?nd the traditional characteristics of the products. Topped or reduced crude oil is a mixture of all varieties of hydrocarbons, including paraffins, isoparaffins, naphthenes, and aromatics plus complexes ad infinitum. Some of these latter may be ring compounds with long side chains controlling the physical characteristics, whereas others may have the ring structure controlling. As the molecular weight and boiling point increase, the complexity increases.
