w.L. Badger was born in Minneapolis in 1886 and attended the University of Minnesota
(B.A. 7907, B.S. 7908, M.S. 1909). In 7909 he worked for Great Western Sugar Company. In 1 9 1 0 he joined the chemical division of the U. S. Bureau of Standards where he remained for two years. H e then went to the University of Michigan to teach chemical engineering, became a professor in 1917, and continued teaching there until 7937. H e was in charge of research on water purification for Detroit Edison Company from 7974 t o 1976, and has been director of research and consulting engineer for the Swenson Evaporator Company since 1917. In 1936 he assumed the position of manager of the consulting engineering division of The Dow Chemical Company, which he resigned in 7944 to become a n independent consulting engineer. Badger is the author o f “ H e a t Transfer and Evaporation’: coauthor (with E. M. Baker) of “lnorganic Chemical Technology”, and coauthor (with W. 1. M c C a b e ) of “Elements of Chemical Engineering”. H e i s a member of the American Chemical Societyf the American lnstitute of Chemical Engineers, and the Executive Committee on H e a t Transfer of the Engineering Division of the National Research Council.
EVAPORATI W. L. Badger and R. A. Lindsay EVERAL inforriilttive review articles on thc subject of evaporation have been published recently. Deere and Brooks (5)trace the development and history of evaporators in the sugar industry. Waeser (39) reviews recent progress in the heavy chemical industry; unfortunately this German article, which has several references, is unavailable at present. Semple (91) covers multiple-eff ect evaporators but this English article is also difficult to secure. Wulfinghoff (96) presents a short discussion of evaporation. Rubush and Seavoy (26) report on the operation and performance of waste liquor evaporators in the paper industry. During the past few years there have been no important changes in standard evaporator types. The war has accelerated the change to the outside-heating-element, forced-circulation design for salting operations and to the long-tube vertical design for nonsalting operations. The short-tube vertical evaporator with propeller agitation is losing favor as a salting evaporator, as a result of the higher coefficients and generally longer salting cycle for, the submerged-heating-element, forced-circulation evaporator. The war plants in the light metals industry served to emphasize this trend. I n the aluminum plants, where evaporators were required t o concentrate tremendous quantities of spent liquor (alumina mother liquor), long-tube vertical cvaporators mere used exclusively. This application is interesting in that a certain amount of silicate scale is encountered in this operation; nevertheless, the long-tube vertical is very successful. I n the magnesium industry, where magnesium chloride liquors required concentration, the horizontal-tube outside heating element forced-circulation evaporator was used. Some minor trends have been noted in the design of standard types. I n the long-tube vertical evaporator there is a tendency to dispense with the normal vapor head. Where this is done, the heating element discharges tangentially into a liquid-vapor separator. This makes the tubes more accessible, an important factor when scaling liquids are being evaporated. Further, it makes the construction considerably cheaper. The design of the forced-circulation evaporator is in a state of flux. The original design, which consisted of an internal vertical heating element, is being replaced by the outside heating element, which may be either horizontal or vertical. This newer design is intended to keep the liquid level high enough so that no vaporization occurs outside of the vapor head. The prototype of this machine is used a t Trona, Calif., by The American Potash and Chemical Company. I n these units four heaters are provided
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4
for each body, and they can be cleaned individually without shutting down the machine. iinother tendency in these machines is to increase the tube size. Smaller tubes may be blocked by pieces of salt that have d’ropped off the side of the evaporator body. Larger tubes increase the pumping load a t similar velocities, so there is a tendency toward lower velocities. In t h e calandria machine there is a tendency to increase the amount of artificial agitation; i t is common knowledge that the liquor distribution is not uniform, and some work has been done t o overcome this disadvantage (94). Because of the scarcity of corrosion-resistant metals during the past few years, many substitute materials were tried. Brick lining and plastic coatings were used with some success, but proved to be both expensive and unreliable. The best substitute is Karbate for tubes and rubber for lining. Karbate tube sheets have been used, but they present a serious problem when field repairs are required. A successful application consists of a rubber-lined tube sheet and Karbate tubes. Rubush and Seavoy (26) discuss tube life in paper waste-liquor evaporators. Unfortunately they have been unable t o publish their information but do state that there is no way to correlate the data. Indications are that a chrome plate inside the top of the tube (long-tube vertical evaporator) increased the life of the tube enough to warrant its additional cost. Several special types of evaporators have been used. Probably the most notable is the submerged combustion evaporator developed by the Ozark Chemical Company some years ago. This equipment was developed to handle sodium sulfate liquors and has found considerable application in the evaporation of concentrated magnesium chloride liquors (20). It has a fuelto-evaporation efficiency comparable to a double-effect evaporator; inasmuch as it eliminated the need for steam-generating equipment, it offered considerable advantage during the war. The principal shortcoming of the unit is its high entrainment. Kokatnur and Jacobs (16) patented a method for concentrating solutions, using kerosene to permit partial pressure evaporation of the water. The Dow Chemical Company developed a spray-type direct-fired evaporator (IO, 28) similar in principle and operation t o a spray dryer. The concentration of penicillin liquors presented a difficult problem of evaporation from heatsensitive material; i t was accomplished.by very high vacuum and vaporization from the frozen solid. There has been some work on the use of radiant heat in the evaporation of heat-sensitive liquids (26). This system is interesting in that, with the liquid opaque t o the radiation and the
INDUSTRIAL AND EPGINEERING CHEMISTRY
Vol. 38, No. 1
R. A. Lindsay was born M.S. in 1940 from the
in Detroit, Mich., in 1915. H e received his B.S. in 1939 and his University of Michigan. In 1 9 4 1 he was employed as a chemical engineer by the Ann Arbor laboratory of The Dow Chemical Company, and since that time has been working with W. L. Badger on heat transfer and evaporation. W i t h coauthors, Lindsay has published severe1 papers in Industrial and Engineering Chemistry: “Equilibria between Liquid and Vapor in System Ethanol-CellosoIve-Water”, page 1263 (1939); “System Aniline-Chlorobenzene. Equilibria between Liquid and Vapor a t Pressures below Atmospheric”, page 1 2 5 1 ( 1 9 4 7 ) ; “Equilibria in Ethanol-Water System at Pressures Less Than Atmospheric”, page 1 5 0 1 (1942); and “Design Calculations for f l a t 6 Columns”, page 4 1 8 ( 7 9 4 3 ) . H e is a member of the American Institute of Chemical Engineers.
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container transparent to It, the IS reversed. This assures that temperature above its boiling p tion. The development of evaporators for the manufacture of distilled‘ water had great impetus during the war. Thousands of these units both stationary and portable, were built for the armed forces. The principle of all the designs is similar in that they use mechanical recompression with the compressor driven by an internal combustion engine. The make-up heat required is supplied by the waste heat of the engine. The economy of these units has increased tremendously; at the beginning of the development they required as much as l gallon of fuel for 10 gallons of distilled water; the present units require only a fraction of a gallon. Meier (21) discusses the application of thermocompression evaporators to milk, including the compression of vapors from the evaporator for other uses. The operation of evaporators has shown a marked trend toward higher temperatures. The principal reason has been higher fuel costs. More effects are being used; in some installations all the effects have been operated under pressure, and the vapor from the last effect has been used as process steam. Probably the highest temperature unit is a Dowtherm-heated caustic. evaporator a t the plant of Reichhold Chemicals, IRC.,a t Tuscaloosa, Ala. (36). This unit is designed t o carry 50% caustic t o approximately 85%. A similar unit is now in progress of construction at the Pittsburgh, Calif., plant of The Dow Chemical Company. Claasen (3)presents a study of the optimum number of effects and temperature range for a sugar evaporator. H e arrives at the conclusion that a four-bodied triple-effect operated above atmospheric pressure is the best arrangement. Rapid strides were made during the war i n the control and instrumentation of evaporators. The control of the level in the unit has received considerable study. Clayton (4) discusses the control of the level in sugar evaporators and reaches the conclusion t h a t overflow is the best method. T h e use of floats and valves introduces the complications .of electrical and air circuits plus the mechanical features of the valve. I n this connection some manufacturers are producing valves in which the stuffing box is supplanted with a bellows. This eliminates the difficulty sometimes experienced with crystallization on the valve stem and subsequent failure of the packing. A level control of the float and valve type has been described (1.9). This unit consists of a float in a level tank outside the body which controls directly the position of the valve governing the air pressure on the motoroperated valve. Much emphasis has been placed on the need for instruments in the control and operation of evaporators. Ziegler (37)reviews such control for the sugar industry and explains the principles of the instruments required. Rubush and Seavoy (26, 90) explain the need for instruments in evaporators. They point out the particular value of the temperature difference a t each January, 1946
cate that without adequate inetermine which effects in a mulcausing difficulty. Further, the proper boil-out cycle cannot be determined. Schiebl (29) describes a method of controlling the effluent concentration by controlling the evaporation with a fixed feed composition. Galainea y Quesada (8) describes a control for the specific gravity of sugar juices. The rise in boiling point of salt solutions can be used for composition control. A differential thermocouple, with one connection in the liquid and the other measuring the saturated vapor temperature, can be used as the motivating force for the control. This control is suitable for solutions having a high boiling point rise but must be used with care. Where the characteristics of the system are such t h a t during the salting cycle the pressure in the evaporator being controlled varies considerably, the boiling point rise-concentration relation will change and the instrument must be reset. There has been considerable interest in the formation, prevention, and removal of scale from evaporator heating surfaces. Scale is generally defined as deposition of a material which has an inverted solubility curve. Thus, maximum concentration occurs in the stagnant film at the heating surface. Scale differs from salt precipitation in that it is generally quite hard and smooth where salt is porous and rough. Rubush and Seavoy (26) report methods for the removal of scale from both the inside and outside of tubes in paper waste liquor evaporators. The outside scale which has a composition of 25.2% FeSOr, 38.491,FeS, and 36.4% FezOa should be treated with hot caustic for 24 hours, boiled out, and then treated with 3-501, HsPO4 for 1-2 hours. They suggest that sodium sulfate scale can be removed by boiling out with water. Calcium carbonate scale may be removed by circulating 5% inhibited hydrochloric acid. The bothersome sodium-aluminum sulfate scale can be removed by circulating a 20% solution of sodium acid phosphate a t 160’ F. for 3 4 hours. After washing out, the residual material can be removed by mechanical means. Kichigan (16) reports t h a t silicate scale in distilled water evaporators can be diminished by keeping the water basic and by adding phosphate. I n sugar evaporators McCleery (19) suggests that silicate scale can be removed with sodium fluoride and hydrochloric acid. Calcium sulfate scale can be removed by using ,ammonium chloride with molasses as an inhibitor. For the removal of oil and carbon deposits on the steam side of evaporators, Dittmar (6) recommends that the surface be treated overnight with a solution of 1.5% permanganate and 2% caustic at 212’ F. The surface should then be rinsed and treated with a 1% ferrous sulfate solution containing 3-575 hydrochloric acid at 212” F. for 0.5 t o 1 hour. Finally, it should be washed with hot water. This treatment is reported to cost about $0.01 per square foot of heating surface; Dittmar discusses the economics and indicates t h a t considerable saving results in distilled water evaporators using exhaust steam. (Contznued on page 30)
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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EVAPORATION CONTINUED F R O M P A G E
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Several patents have been issued covering designs for the handling of salt and salt accumulations. Pendl ($3) presents a method which involves the use of dilute liquor to remove incrustations from the heating surface. This treatment has been common practice in the operation of forced-circulation evaporators and is called a "rinse" t o differentiate it from the normal boil-out. Phillips (93) covers a method for the prevention of salt accumulations on evaporator side walls. H e installs a cooling section around the body above the liquor level, and the resulting condensate keeps the walls washed clean. George (9) recommends a truncated cone baffle on the top tube sheet on an annular-downtake calandria. The top opening of the cone is approximately equal t o the sum of cross-sectional areas of the tubes. Iiermer (13) describes a complete flow sheet for the concentration of, and salt removal from, salt solutions with particular emphasis on caustic cell liquor. Buch and Lilleike (9) suggest that foam in sugar evaporators can be controlled by the use of sulfonated phosphatides. The problem of entrainment has become increasingly important as plant boiler pressures increase. It is common practice to use evaporator condensate for boiler make-up; during the war when many plants were far removed from a source of good water, many evaporators were counted on, not only for boiler feed water, but for water for process uses. This served to emphasbe the need for entrainment prevention. The conductivity cell proved to be of considerable assistance in the handling of evaporator condensate (11) and served t o warn the operator of unsatisfactory performance. Arnold (1) discusses the cause and effect of entrainment in the sugar industry, and describes the principles and operation of twelve entrainment separators. Unfortunately this Indian article is not easily secured so that it is not possible to summarize the material presented more completely. Kermer (14) describes an entrainment separator consisting of a special bafding arrangement. The war has seriously curtailed the amount of mork done on the theoretical aspects of evaporation problems. Staub ( J d ) presents some data on the concentration of sodium chloride solutions in a long-tube vertical evaporator. Coefficients for both the boiling and nonboiling sections of the tubes are reported, However, no indication is given of the relative lengths of the sections. The unit was run at constant steam pressure with varying feed rates ,and salt concentrations, and coefficients as high as 1500 B.t.u./(sq. ft.) (hr.) ( " F.) are reported. Vapor velocities out of the tube are reported t o be as high as 120 ft./ sec. SBvavskg (97) discusses the coefficients obtained in the concentration of sugar in a pressure evaporator. He reports that the juice level is kept very low and the coefficient is relatively independent of this level. The density of the juice has an appreciable effect on the coefficient. H e further states that the use of high inlet steam velocity increases the coefficient. Lukomskii (18) presents a summary of boiling coefficients. He states that, in general, they can be expressed as
U = where
c At'J
U C
= heat transfer coefficient = constant At = temperature difference
H e reports that the ratio of heat flux t o maximum heat flux (Q/Qmlas.) can be plotted against the ratio of the temperature difference to the critical temperature difference ( A t / Atem.); further, Qmo../r0.8d= constant 30
where r = latent heat d = density of liquid
A table of these constants i s presented for various liquids and metal surfaces. Larian (17') derives heat balance equations for multiple-effect evaporators. While these equations are simple and neglect some heat effects such as heat of solution, they are of interest in the teaching of such calculations. Ray and Carnahan (94) present a rather complete and thorough method for the calculation of heat, balances for multiple-effect evaporator problems. This method depends on the r factor, which is the ratio of heat effects other than evaporation to the heat required for evaporation. It is shown that
where p = heat transferred through heating surface h = heat required for eva oration h, = heat released on con&nsation From Q / Q Z = U l A l A t l / U 2 A 2 A t z and the coefficients, it is possible t o calculate the At values. The evaporation per unit time can also be expressed:
Thus the material balance can also be ascertained. This method has much to recommend it, but the T factor has to be evaluated a t each effect. Inasmuch as T is variable with temperature, the solution still involves trial-and-error calculations. Egli and Love11 ( 7 ) present information on tho growth and habit of crystals in an agitated evaporator. They conclude that the amount of mechanical agitation has no effect on crystal size; the agitation due to boiling controls. They report that supersaturation results in small crystals. LITERATURE CITED
(1) Arnold, IC. S., Proc. 11th Ann. Convention Sugar Tech. Assoc. India,1942, I, 29-52. (2) Buch, A., and Lilleike, E., German Patent 715,109 (Nov. 20. 1941). (3) Claasen, H., Centr. Zuckerind., 49, 517-19 (1941). (4) Clayton, J. L., Proc. Queensland SOC.Sugar Cane Tech., 12, 2534 (1941). (5) Deere, N., and Brooks, A., Intern. Sugar J.,43, 368-70 (1941). (6) Dittmar, J. H., Chem. & M e t . Eng., 50, No. 2, 137-8 (1943). (7) Egli, P. H., and LoveI1, C. L., Purdue Univ. Eng. Expt. Sta.. Research Series 80, 2, 5-19 (1941). (8) Galainea y Quesada, M. J., U. S. Patent 2,314,822 (March 23 1943). (9) George, J. E., Ibid., 2,326,024 (Aug. 3, 1943). (10) Heath, S. B., Ibid., 2,327,039 (Aug. 17, 1943). (11) Hewlett, A. M., I n t e r n . Sugar J., 46,102 (1944). (12) Hughes, B. S., U. S. Patent 2,339,167 (Jan. 18, 1944). (13) Kermer, M. J., Ibid., 2,330,221 (Sept. 28, 1943). (14) Ibid., 2,338,117 (Jan. 4, 1944). (15) Kichigan, M. A,, Khim. Referat. Zhur., 4, No. 5 , 113 (1941). (16) Kokatnur, V. P., and Jacobs, J. J., Jr., U. S. Patent 2,326,099 (Aug. 3, 1943). (17) Larian, M. G., Chem. 6: M e t . Eny., 49, No. 10, 90-1 (1942). (18) Lukomskii, S. M., Khimicheskava Prom., 1944, No. 6, 9-14. (19) McCleery. W. L., Rept. Ann. Meeting Hawaiian Sugar Planters Assoc., 61, 117 (1941). (20) Manning, P. D. V., and Kirkpatrick, S. D., Chem. & M e t . Eng.. 51, NO.5, 92-6, 137 (1944). (21) Meier, C. A., Modern Power and Eng., 39, No. 2, 41-3 (1945). (22) Pendl, H., U. S. Patent 2,263,703 (Nov. 25, 1941). (23) Phillips, J. F., Ibid., 2,261,486 (Nov. 4, 1941). (24) Ray, H. S., and Carnahan, F. J., Trans. A m . Inst. Chem. Engrs., 41, 253-64 (1946).
INDUSTRIAL AND ENGINEERING CHEMISTRY
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(25) Reavell, J. A,, Ind. Chemist, io, 119-30 (1944). (26) Rubush, J. P., and Seavoy, G. E., Paper Trade J.,116, No. 24, 25-30 (1943). (27) Shvavskf, V., Chem. Zentr., 1941, 11, 123. (28) Schambra, W. P., Trans. A m . Zmt. Chem. Engrs., 41, 35-51 (1945). (29) Schiebl, K., Centr. Zuckerznd., 50, 63-4 (1942). (30) Seavoy, G. P., Pulp Paper Mag. Can., 45, 455-7 (1944). (31) Semple, D. M., Trans. Znst. Chem. Engrs. (London), Chem. En&. Group, Glasgow Sect., advance copy, March 26, 1943. (32) Staub, S., Intern. Sugar J., 45, 40-3 (1943). (33) Waeser, B., Chem. Tech., 16, 252-4 (1943). (34) Webre, A. L., Intern. Sugar J., 47, 65-8 (1945). (35) Weiss, J. M . , Heat Engineering, 19, 110-15 (1944). (36) Wulfinghoff, M. F. A., Southern Power and Znd., 62, No. 2, 107-12, NO. 4, 100-4 (1944). (37) Zieder, J. G., Sugar, 38, No. 7, 22-4, 27, No. 8, 22-4 (1943).
SEDIMENTATION AND HYDRAULIC CLASSIFICATION CONTINUED FROM P A m
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opening against a controlled flow of hydraulic water into a dewatering device. Operating data are not yet available for publication. Also in the field of classification, the Bird continuous centrifugal has been further proved during the past few years. The advantages of centrifugal classification are best realized when particle size cuts are made in the micron range, particularly finer than 20 microns. For example, a considerable tonnage of titanium dioxide and lithopone is classified in continuous centrifugals. In the case of titanium pigment, the slurry is fed a t ball mill consistency or with limited dilution, and the fine fraction which is 100% -5 microns is discharged at about 20% solids. The coarse fraction is returned t o the ball mill in the conventional manner. An operation of this type permits direct filtration of the slurry comprising the fine fraction. Other materials such as clay, calcium carbonate, and other coating or filling materials are similarly classified. The continuous centrifugal has been used in the same manner in the cement industry for closed-circuit grinding; i t produces a thick slurry suitable for direct blending and calcination. I n this instance the slurry is ground t o 85% -200 mesh. I n the cement industry it has also been used t o classify cement slurry into two fractions, the cuts being made from 10 to 30 microns, and the adjustment in performance is controlled by variable-speed motor drive. Approximately one million tons of solids have been processed in two units, 54 inches in diameter and 70 inches long during the past two years. A still newer use for centrifugal classification is the desliming (rejection of 2-3 micron particles) of the slurry feed t o froth flotation cells. Another application is in the separation of magnesium from water softening sludge where the sludge is burned for reuse in the system. During the 1944 season on the Iron Range there was an interesting development in hydraulic classification. The mining companies were pressing to ship the maximum tonnage of ore to the blast furnaces. Washing (concentrating) plant tailings were examined for the purpose of determining how t o recover additional tonnage of blast-furnace grade quickly and economically. In one typical case the tailings from a washing plant at -35 mesh contained about 22-23% iron and 63-65% silicon and represented about one third of the crude ore treated. ‘ Normal concentrate from this plant contained about 61% iron and 9% silicon. January, 1946
A tailings retredtment plant was developed and proved; it involved a three-step operation: (1) dewatering and desliming the tailings, (2) hindered sizing of the deslimed tailings, and (3) dewatering the concentrates produced by selected pockets of the Dorrco sizer. Briefly, this sizing device consists of a number of rectangular pockets, each fitted with a perforated bottom through which is introduced a controlled volume of hydraulic water for sizing. Feed is introduced into pocket No. I, and all particles which will settle through the controlled water velocity are automatically discharged at the bottom. Particles carried over by the hydraulic water pass t o succeeding pockets with diminishing upward velocities. The operation is controlled by pressureregulating instruments set in accordance with the particle sizing desired. I n hydraulic sizing of iron ore and silica gangue, it has been established that a silica particle of a certain size and an iron ore particle 2-2.8 meshes finer will overflow a t substantially the same rate. The commercial significance of this fact is shown by the following figures which represent plant performance over a 24hour period: Feed
No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8
% Fe 41.83 60.02 62.82 63.15 61.45 62.18 60.56 59.11 32.78 %Si 36.42 7.63 6.90 6.53 8 64 8 12 10.28 12.54 48.76
Products from pockets 1 to 7, inclusive, were combined to produce a shipping grade of concentrate containing 61.45% iron and 8.92% silicon. The weight recovery for the washing plant aa a whole was increased from 66.6 t o 71%, representing ore which otherwise would not have been delivered t o the blast furnace during the war. Hydraulic classification was also used in a war industry for grading abrasives in the micron size range. Beginning in 1942 and continuing through the war, some grades of abrasives for lens and crystal grinding became so scarce t h a t it was necessary t o recover the abrasive grains by the same method used on iron ore tailings. There may be other developments along the lines covered in this review, but any omission is due t o the fact that they have not been brought t o our attention or have not been available for publication.
HIGH TEMPERATURE DISTILLATION CONTINUED FROM P A Q E 9
are charged into the retort. The open end is then sealed, and a vacuum of 0.05 t o 0.15 mm. mercury is applied. Eight hours later the vacuum is broken, the lid opened, and a “crown” of magnesium is removed in the condenser sleeve. A new sleeve is inserted and the retort is ready for recharging. From 200 pounds of charge, about 37 pounds of magnesium may be recovered. Some magnesium oxide does distill over with the magnesium metal, and this is removed by fluxing during molding. An interesting side light on this operation is the technique of extending the life of the retorts by “reblowing”. At temperatures in this range the steel in the retorts flows under the differential pressure in the furnace proper and inside the retort. It has been found possible, when a retort collapsed, t o apply pressure t o the inside of the retort and thus cause i t to resume a shape suitable for continued operation. I n this manner the useful life of retorts has been extended from a few days t o well over a year,
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