ION w, L. BADGER, ANN ARBOR, MICH. R. A. LlNDSAY, THE
DOW CHEMICAL
COMPANY, MIDLAND, MICH.
the liquid temperature as i t varied with the height in the tube When the point of maximum liquid temperature was used to determine the division between boiling and nonboiling sections of the tube and the heat transfer coefficient was calculated for the boiling section, the coefficient increased rapidly as the feed rate increased, while the total evaporation remained constant. The author felt that the coefficients so determined were too high, 12,000 B.t.u./(hr.) (sq. ft.) (’ F.). Furthermore, when he neglected the division of the tube and calculated the average of the heat transfer coefficient from the integrated temperature difference, he found that it was constant. It was stated that boiling occurred, a t least in the film, throughout the length of the tube and the only true manner of expressing the performance was to report the coefficient as one for the full tube length. When runs were made a t varying feed temperatures the coefficient so calculated increased as the feed temperature increased. The cvidence given indicates that at any particular point in the tube where the liquid temperature is below the boiling point, the heat transfer coefficient above that point is higher than that below the point. The writers disagree with the conclusion drawn by this author, particularly in view of work they have seen in a glass unit where there was no evidence of vaporization in the tube below the point of maximum temperature. Furthermore, i t is a generally accepted fact that the heat transferred is principally to the liquid phase and when vapor is present in appreciable volume, the velocity of the liquid particles past the heat transfer surface must be greater than the liquid mass velocity where appreciable amounts of vapor are not present. It is possible that the author has not taken into full account the superheat that appears in the liquid a t the point of maximum temperature and the subsequent liquid flash as the pressure head decreases. Unfortunately, the paper is not clear in this respect and a reply to this question had not been received from the author prior to the appearance of this paper. The experimental work was carried out in a tube 9 feet long by 0.5 inch in diameter; this gives a length-to-diameter ratio of 216 !o 1 just above the 200 to 1ratio generally accepted as the dividing iine for “long”-tube vertical evaporators. The work was prrformed with a temperature difference of 18”to 41 ’ E’. and coefficients for water are reported a t 500 to 700, for ethyl alcohol at 300, for toluene a t 300, and for nitric acid at 125. It is stated that if more than 10% of the feed is evaporated the heat transfer coefficient is constant regardless of the mass velocity.
HERE have been few changes in evaporator equipment or evaporation procedure in recent months. A trend is noticed in the organic chemical and pharmaceutical industries toward usc of equipment and procedures developed in the heavy chemical industries. I n short, these industries have reached a stagc where they are using more engineering equipment rather than enlarged laboratory equipment. Typical of this development has been the increased use of a long-tube vertical evaporator as a reboiler. Such a unit has a considerable advantage over the old tubular still because of the higher heat transfer coefficients possible and the greater ease of cleaning. I n one case the use of such a reboiler decreased the still cost by some 500/i, a substantial saving particularly because a serious corrosion problem made expensive materials necessary. However, many of the evaporation problems in the organic and pharmaceutical fields are probably drying problems. The theory of evaporator design and evaporation practice is discussed by Connell ( I d ) , who presents arguments for increased use of bleed-off steam from various effects in a sugar evaporator. McDonald and Rodgers ( 2 8 )describe a heat balance and set up an equation for the determination of the area of the individual effects and Ihe steam required for particular systems in a sugar refinery. Along this same line, Kulkarni ( 2 5 ) discusses a formula for control of the boiling house. Farher and Scorah ( 1 6 ) have done considerable work on boiling 5lm heat transfer coefficients, which were determined by an electrically heated wire submerged in liquid. Probably the most interesting observation in this work is that the heat transfer coefficient regularly increases as the temperature difference is increased up to a maximum (critical temperature difference), decreases to a minimum, and then increases again. This is the first work to come to the writers’ attention in which any amount of data has been reported a t temperature differences very far above the critical. Although there is probably no particular significance to this work as far as standard evaporators are concerned, i t may be of considerable note in direct-fired evaporators or boilers. McDonald and Rodgers ( 2 7 ) discuss an empirical formula for heat trans€er coefficients in a sugar evaporator:
U
=
COSTS
Bliss ( 7 ) presents an interesting picture on the costs of various pieces of chemical engineering equipment including evaporators. He states that in the case of basket or calandria evaporators the cost per square foot continues to decrease as the area iricreases; unfortunately, the end point of this relationship L not shown. (It probably occurs a t the maximum shippable diameter of the shell or basket.) For long-tube vertical evaporators the cost decreases until the area reaches 5000 to 8000 square feet, as at this point the cost in steel is reported as $4 50 per square foot in 1947. For forced circulation evaporators the cost decreases until the area reaches 3500 square feet; for larger sizes the cost is constant a t $40 per square foot in a steel shell with alloy tubes and vapor head. These costs are presented for evaporators complete with the normal auxiliary equipment such a5 liquid level controls, pumps, etc. Basic work for this article was probably done in 1946 or 1946 and all the charts have notations such as “add 35% for 1947”!
14.5 ( t ~ / l O O ) ’ / ( d ~ )
where U = over-all heat transfer co$Ficient, B.t.u./(hr.)(sq. ft.)(”F,); t, = boilingpoint of the juice, F.; pj = viscosityof the juice a t ti and discharge concentration; Cj = specific heat of the juice as discharged.
I n addition, a method of arriving at the probable radiation loss is shown. LONG-TUBE VERTICAL EVAPORATOR
An interesting paper presented recently by Rumford ( 3 2 ) discusses work on a long-tube vertical evaporator. The purpose of the work was to establish information on nitration plant liquid concentrators. As a result, information is given on ethyl alcohol, toluene. and nitric acid in addition to water.
SPECIAL PROBLEMS
The fundamental purpose of the work is lost, however, in a discussion of the advisability of dividing the tube in a long-tube evaporator into boiling and nonboiling sections. Considerable data are presented on operation a t constant heat flux with varying feed rate. A traveling thermocouple was used to measure
As usual, there are many special designs presented for coping with special liquids or operating problems. Whitehurat (44) discusses high vacuum in chemical engineering processes. This
32
January 1949
INDUSTRIAL A N D ENGINEERING CHEMISTRY
article is of particular interest to the food and pharmaceutical industries, where heat-sensitive liquids must be handled a t low temperatures. Storen and Snersrud (40) discuss the use of vacuum evaporation in the preparation of whey concentrate for cheese making. They show that in the manufacture of cheese from both cow and goat milk there are advantages to vacuum operation and claim that an improved taste is achieved. Further developments in the design of units for heat-sensitive liquids are disclosed by Bracke and Deutsch (8). An evaporator for foods consists of several superposed inclined planes with provision for flow from plane to plane. The planes are heated and control is presented for each separately. A laboratory evaporator for substances subject to thermal decomposition is described by Muirhead (31), along with information on structural features, method of operation, and applications. For the paper industry a Kestner type inclined tube laboratory unit is described by Bjorkman (6). The unit is constructed of glass for observation and is complete with a foam separator. It has a capacity of about 3 liters per hour evaporation. The problem of sulfite waste handling comes in for considerable discussion. Sweden appears to pay particular attention to this problem and with the increasing pressure in the United States to decrease stream pollution that we may learn from their experience. Elding (16)describes spray evaporators for concentrating sulfite waste liquor. These units are iired by burning the dried product and indications are that they have a thermal efficiency of about 65% based on the calorific value of the waste. The article describes the operation and difficulties encountered with some 25 units in operation or being constructed. Samuelson (93) continues the discussion of the relative merits of presssure and vacuum evaporation for sulfite waste liquors. His conclusions are that the pressure unit is preferred but the problem of scale is more serious. A very complete description of the nature and mechanics of formation of the scale encountered is presented. A two-effect pilot plant evaporator is described; the unit is a modified Kestner with a combination heat exchanger and flash tank available for feed treatment. The feed t o the unit was heated to 163" C. and flashed t o temperatures varying from 112" to 145" C. before entering the evaporator. Sulfur dioxide was released in the flash tank and the evaporator, being present in the condensate as or-oxysulfonic acids. During the heating and the evaporation the sulfur dioxide does not form insoluble sulfites nor is it oxidized to sulfates. Pressure heating of the neutralized feed caused precipitation of calcium sulfate, some of which redissolved on cooling (flashing). As much as 80% of the calcium sulfate in the feed can be precipitated by heating to 163" C.-the effect requires some time and there is evidence that a lower temperature in the evaporator is likely to result in redissolving some of the sulfate. The problem of silicate scale is discussed and a method of analysis is shown. The liquor used in the experimental work had a concentration of 0.5 to 1.5 mg. per liter. Hedborg (2%')discloses a scheme for the handling of sulfite waste. Before the evaporation and burning of the waste sodium sulfide along with carbon dioxide is added and the precipitated calcium carbonate is removed. In a modification sodium sulfide is added to a pH of 5 and the sugar present is fermented t o alcohol. More sodium sulfide is added to a p H of 9, the liquor is evaporated, and the waste is burned. Sodium sulfide can be recovered from the ash and reused. Silen (94) discusses the advantages of triple-effect evaporation. The operation of the salt recovery unit a t the Dead Sea is described by Halperin ( 2 1 ) . Solar evaporation is used for concentration. Very little information has been available lately on this method of evaporation and the unit described is of interest. A small amount of dye is added to the liquor to increase the heat absorption. The liquor is pumped from some distance below the surface, as the salt concentration is higher at the lower levels. Sodium chloride, carnallite, potassium chloride, and bromides are precipitated in that order from separate tanks. The bromides are precipitated only in the hottest part of the summer
33
and auxiliary evaporation is generally necessary. The area of the units is 4000 acres. Sun temperatures of 149" to 167" F. are reached and the mean heat absorption is 630 calories per sq. cm. per day. Dubourg (14) discusses sugar manufacture in Sweden with emphasis on the evaporation procedure. Moller (SO) also discusses sugar manufacture, covering the boiling operations in some detail. Frattali et al. (20)preseht the Concentration of liquors in the lime-soda process for alumina for the recovery of sulfate and soda. The liquor is evaporated in a vacuum unit to saturation and on cooling soda is precipitated. On evaporation of the mother liquor a salt with an analysis of 50y0 sodium carbonate and 5Oy0 sodium sulfate is recovered. A patent has been issued to I. G. Farbenindustrie ($3)for a special evaporator for handling foaming liquors. Bertetti (4) discloses a method for the concentration of sulfuric acid. Sulfuric acid is concentrated in the usual manner to 90% and the product is split into two streams, one heated and the other cooled. The _streamsare passed through a sealed vessel in separate liquid channels with a common vapor space. The lower vapor pressure of water over the cooler liquid causes it to absorb vapor, thus concentrating the hotter stream. After processing, the hotter stream is the product and the cooler stream is returned to the first concentrator. In this same vein Bailly ( 8 ) has patented a sulfuric acid concentrator with a special pot-type still. Dowdell (18)presents a special evaporator for feed to a high pressure boiler. The boiler operates at 900' F. with about 2y0make-up and 0.5% blowdown to prevent silicate deposits. The blowdown is run through a flask tank to an evaporator and the flash vapor is used for heating. It is estimated that the unit results in a saving of about $1000 per year. SCALE AND FOAM
Scale and foam receive continuous attention, particularly in the sugar industry, where shutdowns or decreased capacity can be serious; nature dictates the length of operation and lost time or lost production cannot generally be recovered. Stewart (30)discusses the operation of three Louisiana plants and shows that the addition of tetraphosphoglucosate is valuable in scale control. From 0.75 to 1.0 pound per 100 pounds of cane are used and evidence shows the scale forms much more slowly and is more easily removed with the soda and acid treatment. A method of cleaning sugar evaporators with sodium hydroxide is described by Congelosi (11). The sodium hydroxide in 58% concentration is sprayed into the fourth and fifth effects and circulated for 2.5 hours with the steam on. The machine is then boiled out with water and no hydrochloric acid boiling is necessary. Fitzwilliam (1'7)presents 90 references on scale and scale control in the sugar industry. A discussion of the cleaning of sugar evaporators by Floro (19) indicates the use of sodium hydroxide, sodium phosphate, and hydrochloric acid. The equipment necessary and the methods of operation are presented for use with these reagents Springer (97')presents a unique method for scale control by use of an electric potential. In the third effect of a quadruple-effect evaporator a large copper-aluminum electrode was suspended. This electrode was connected t o the positive pole of a direct current generator, the other pole being the shell of the evaporator. The generator was operated at 15 to 20 volts and delivered 15 amperes. About 80% of the area in the evaporator was kept free of scale in a one-week test, the effect being particularly noticeable in the upper section of the tubes. I n addition, there was evidence that the scaling rate in the fourth effect was decreased. Sofronyuk (36) describes a method for scale removal after the campaign is over. A final molasses of 24" Brix is left in the units for 2.5 to 3 months. Toward the end of fermentation, which occurs during standing, some raw water is added. The scale is so softened that water a t 60 to 90 pounds per square inch will successfully wash it out. For the prevention of scale in sulfite waste liquor evaporators
34
INDUSTRIAL AND ENGINEERING CHEMISTRY
Bergstrom and Troluck (3) disclose the use of sodium hexametaphosphate in a recommended concentration of 1 to 10 grams per cubic meter, Brunes et al. (IO)discuss the scale problem in sulfite evaporators-five different flow sheets are shown involving quintuple- and sextuple-effect units, The handling of wash liquors, bleed-off steam, eto., is also discussed. McDaniel ( 2 6 ) discusses the removal of sludge and scale frpm steel equipment of all kinds. Sulfate scale in waste liquor evaporators is covered thoroughly by Sillen (%), who presents the properties of the scale and an analysis of those encountered from different feeds. It appears that most of the calcium sulfate is precipitated on the pulp and some of it is redissolved in the wash liquor. Evidence of the oxidation of sulfur dioxide to sulfate is presented along with data on the solubility of calcium sulfate and the hemihydrate in sulfite waste liquor a t elevated temperatures. It is shown that a t temperatures above 127" C. there is a slow transition from the hemihydrate to the anhydrite, which is very insoluble. The author suggests frequent washing to prevent anhydrite formation and postulates that the scale is always precipitated as the hemihydrate but this is subsequently dehydrated to the anhydride. I n concentrated liquor the equilibrium solid phase is the anhydrite, but this does not seem to precipitate as a scale but merely forms a suspended sludge that causes no difficulty. Foam prevention is the interest of Jacoby (84),who suggests the use of mono or diacyl derivatives of piperazine. The mono should contain a t least 24 carbon atoms. The diacyl if composed of chains of equal length should contain a t least 34 carbon atoms; if the chains are of unequal length they should contain at least 22 carbon atoms. These are brown, waxy solids. Bird ( 5 ) discloses the use of a material RXHCOANHZ, a diamine, where: R = a long-chain alkyl group, h = an alkylene or arylene group, and 2 = a long chain acyl group. The recommended concentration is 0.01 to 1 grain per gallon of feed. The control of sugar vacuum pans is discussed by Webre (43). He suggests the use of the boiling point rise as determined from an absolute pressure gage attached to the dome of the unit and a thermocouple in the central downtake. This system requires readings on the part of the operator and leaves something to be desired when compared to the control discussed in this review in 1946. This article contains in addition to the control a general discussion of the operation and control of vacuum pans in the sugar industry. Brown ( 9 ) describes an automatic control for a quintuple-effect unit. The problem of proper venting of the steam chest may be assisted by a Standard Oil patent (38). An apparatus accurately measuring the condensing temperature compared to the true boiling point is used. The unit was developed for the determination of air leaks in vacuum equipment, but appears to be satisfactory for pressure operation. Meyer (e9) describes insulation, types, and methods of application. The use of vapor recompression for beet sugar evaporators and vacuum pans in Switzerland is described by Tromp ( 4 1 ) . Steam injection as an aid to natural circulation evaporators is discussed by Fitzwilliam and Yearwood (18). It is indicated that this scheme i, of little value except where the circulation is low, owing t o low temperature difference, scale, etc. Experiments were run with an injection of 24% steam by volume and no entrainment was reported. Anderson ( 1 ) describes a method of liquid circulation to improve performance. Vondrake ( 4 8 ) presents a review of sugar operations in Bohemia and Moravia with a discussion of equipment, yields, and methods of operation.
Vol. 41, No. 1
Bjarkman, A,, Svensk. Papperstidn., 50, 529 (1947). Bliss, H., C h m . Eng., 54, No. 5, 126 (1947). Bracke, R., and Deutsch, S., Belgian Patent 449,519 (April 1943).
Brown, B., Sugar, 42, KO.7, 36 (1947). Brunes. B., Samuelson, O., and Ulfsparre, S., Svensk Papperstidn., 50, No. 11B, 29 (1947).
Congelosi, R. A., Mem. asoc. tecnicos ~ Z U C U TCuba, . 20,303 (1946). Connell, C. B. B., Intern. Sugar J . , 49, 152 (1947). Dowdell, S. H., Southern Power and Ind., 65, No. 11, 58 (1947). Dubourg, J., Bull. assoc. chim., 63,351 (1946). Elding, G., Trans. Furl Economy Conf. World Power Conf., 1947.
Farber, E. A., and Scorah, R. L., Trans. Am. SOC.illech. Engrs., 70, 369 (1945).
Fitzwilliam, C . W., Proc. Brit. West Indies Sugar Technol., 13,52 (1947).
Fitzwilliam, C. TV., and Yearwood, R. D. E., Intern. Sugar J . ,
(35) (39) (40) (41) (42) (43) (44)
49, 240 (1947). Floro, M. B., Jamaican Assoc. Sugar Technol. Quart. Bull., 10, No. 3, 33 (1947). Frattali, F. J., Green, S. J., and McLendon, V. I., U.S. B74r. Mines, Rept. Invest. 4126 (1947). Halperin, Z., Chem. Eng., 54, No. 6 , 694 (1947). Hedborg, F. O., Swedish Patent 117,579 (Nov. 5, 1946). I. G. Farbenindustrie, Belgian Patent 450,545 (June 1943). Jacoby, A. L., U. S. Patent 2,428,801 (Oct, 14, 1947). Kulkarni, H. G., Indian Sugar, 10, 89 (1947). McDaniel, B. H., Iron and Steel Enyr., 24, No. 9, 43 (1947). McDonald, J. C., and Rodgers, T., Intern. Suyar J., 49, 205 (1947). McDonald, J. C., and Rodgers, T., Zbid., 49, 264 (1947). Meyer, F. 0. W., Die Technik, 2, 329 (1947). hfoller, C., Intern. Sugar J., 49, 182 (1947). Muirhead, G. S., Chem. Products, 10,78 (1947). Rumford, F., J . SOC.Chem. Ind., 66,309 (1947). Samuelson, o., Svensk Papperstidn., 50, 239 (1947). Silen, P. M., Sakharnaya Prom., 20, No. 1, 13 (1947). Sillen, L. G., Svensk Papperstidn., 50, 339 (1947). Sofronyuk, L. G., Sakharnaya Prom., 20, No. 3, 29 (1947). Springer, H. B., PTOC.Brit. West Indies Sugar Technol., 13, 60 (1947). Standard Oil Development Co., British Patent 556,530 (March 21, 1947). Stewart, C. W., Szigar J., 10, No. 8, 15 (1948). Storen, K., and Snersrud, T., C h i d e & Industrie, 57, 557 (1647). Tromp, L. A., Intern. Sugar J., 49, 289 (1947). Vondrak, J., Listy Cukrovar, 59, 221 (1941). Webre, A. L., Intern. Sugar J., 49, 149 (1947). Whitehurst, B. W., Chem. Eng., 54, No. 10, 98 (1947).
RECEIVED November 25, 1948.
LITERATURE CITED
(1) Anderson, A. L., Danish Patent 65,374 (Feb. 3, 1947). (2) Bailly, L., British Patent 569,606 (May 31, 1945). (3) Bergstrom, H. 0. V., and Troluck, K. G., Swedish Patent 115,913 (May 27, 1947). (4) Bertetti, J. W., U. S. Patent 2,432,136 (Dec. 9, 1947). (5) Bird, P. G . Ibid., 2,428,775 {Oct. 14, 1947).
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