EVAPORATION W. 1. BADGER, 309 South State St., Ann Arbor, Mich. R. A. LINDSAY, The Dow Chemical Co., Midland, Mich.
HE literature during the past year reveals that currently there is considerable work being done on the theory of evaporation and evaporation processes. A s far as evaporation itself is Concerned, the investigators are showing considerable interest in the actual mechanics involved as a sufficient amount of heat is transferred a t some surface t o a liquid to result in massive vapor formation. In addition to this work, there apparently is considerable current interest in the problem of entrainment. Perhaps the most unique of these interests is the demonstrated need for entrainment control in areas where radioactive materials are being concentrated. I n this case, the carryover is of concern, not only for the contamination it may introduce into the condensatc in the form of scale forming materials, but actually the contamination that may result in a health haaard. Again, the literature of these 12 months does not reveal any outstanding patent disclosures. It appears that the patent literature is primarily involved with minor modifications that have only a limited field of interest and make no real contributions to the design of evaporator equipment. As far as evaporator types are concerned, evidence of renewed interest is seen in the submerged combustion evaporator.
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Theory and Experimental Work The current interest in such things as nuclear reactors, rockets, jet engines, and others, has resultcd in considerable need for knowledge regarding high rates of heat tiansfer. The subject of boiling heat transfer is covered by Jens (18) and boiling phenomen3 are reviewed for different conditions of temperature and pressure. The effect of these on the maximum heat transfer is such that the maximum possible heat flux (with boiling water) appears to occur a t around one third of the critical pressure or about 1000 pounds per square inch. This maximum heat flux of around a million and a quarter B.t.u.'s per hour per square foot is about three t:mes the maximum heat flusa t atmospheric pressur?. Experimental data here are reviewed covering heat fiux prior to burn out arid the experimental procedure indicates the extreme need for positive liquid f l o ~in situations of this kind. Somewhat similar n-ork by Bogdanov ( 5 ) indicates thc kind of variation of heat transfer coefficient that occurs n-ith pressuie. I t is reported that betneen 1 and 40 atmospheres the heat transfer coefficient is equal t o some constant times the pressure. At an intprmediate pressure range between 40 and TO atmospheres the coeflicierit varies as the pressure taken to a power, and above i o atmosphercs the relationship again becomes constant. This data n a s taken from a study that used a aO-mm.-diameter tube under pressures of 2 to 160 atmospheres and heat loads of 50,000 to 400,000 K. calories per squarc meter per hour with circulation rates of 0.5 t o 2.5 meters per second. One of the better known contributors to the litrrature, Iiirschbaum (go), has provided us with R dimensionless number correlation of heat transler for vertical tube evaporation. The value of all physiral characteristics is established at the arithmetic mean of the tube wall temperature and the boiling temperature in the vapor
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space. The experimental work covered operations with distilled water and salt and sugar solutions of various concentrations. A considerable range of temperature drops and boiling temperatures were covered in a copper tube 40-mm. inside diameter, 48-mm. outside diameter, and 4 metcrs long. The results and data extending over the last decade could be correlated semi-empirically by means of a modified Nusselt number so that with the two equations given rather complete correlation was possible. The liquid level plays a considerable part in the evaluation of these and an empirical relationship for the constants in the equations is presented as a function of the apparent liquid levelthe liquid level always being measured from the bottom of the tube. Yamragata and others (47) investigated, by the use of an optical method and thermocouples, nucleate boiling of a pool of water on a horizontal heating surface. The technique was used to determine the thickness of the boundary layer and the temperature difference between the heating surface and the boiling liquid. Photographic evidence of bubble formation on the heating surface indicated that four types of bubbles are possible in boiling liquid. A t low heat flows corresponding to the free convection region (below 9800 K. calories per square meter per hour) the bubbles formed were uniform in size and either spherical or bell shaped. There was little turbulence, only a small number of vapor columns, and very little stirring effect from the rising steam bubbles. In the forced convection region (heat fluxes greater than 15,000 K. calories per square meter per hour), the stirring effect of the bubbles formed was extrcmcly important and in the boundary layer there was evidence of considerable liquid turbulence. Large bubbles, withasmall bubble trailing aftcr each, and multiple or tandem bubbles, formed by the coalescence of single bubbles, constituted two types formcd in the forced convection region. It n-as concludcd that the most important factor controlling the coefficient of heat transfer for boiling liquids v,-as the stirring effect of the vapor bubbles being formed and not the temperature difference or heat flux between the heating surface and the boiling liquid. The data indicate that the Susselt number is proportional t o the cube root of the number of vapor columns formed in the boiling liquid. Three types of boiling are observed by TT'estwater (44). These are nuclear, marked by large rates of heat transfer and rapid formation of bubbles from fixed points, transition, exhibiting the poorer heat transfer in random bursts of vapor; in film, shon-ing the poorest heat transfer and complete blanketing of the heat transfer surface with a film of vapor. The maximum rate of heat transfer is obtained just a t the transition point between nuclear and transition boiling, or perhaps best described as in the highest range of the transition boiling just prior to the occurrence of nuclear boiling. hlotte (25) covers very extensivcly the phenomenon of film boiling on the heating of flowing subcooled liquids. Hcat transfer coefficients, H , across the vapor film are evaluated for upward flow, forced convection, and heating on the outside of a single, horizontal, electrically heated tube. The liquids evaluated wcre ethyl alcohol, benzene, hexane, and carbon tetrachloridc.
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UNIT OPERATIONS REVIEW Graphite tubes varying from a/*- t o 6/ginch outsidc diameter were used and since the heat transfer coefficient is greater a t :he bottom of the tube, the walls of the larger tubes were made thicker to prevent a temperature gradient between the top and the bottom. If the liquid is subcooled, the heat is transferred from the vapor-liquid interface into the liquid by eddy conduction and the effect of thermal conduction is negligible. The increased values of the heat transfer coefficient are found to approach those of nucleate boiling. The author presents two theoretical parameters which correlate the data. Heat transfer coefficients secured in practice are presented by Saranin and Jenkins (57) and cover operations on sugar solutions. The heat transfer coefficient varies considerably, with the sugar concentration, dropping as the concentration increases to 65' Brix. If the data ES extrapolated the coefficient appears to fall to 0 a t 75" Brix. Then the coefficient improves as the temperature differential and the boiling temperature are increased and the writers reached the conclusion that in this operation backward feed should increase the capacity of equipment. Probably more liquids are actually evaporated in jacketed vessels than in the so-called standard evaporators. Marek and Rod (26)discuss heat transfer coefficients during this type of evaporation. The authors have derived dimensionless relations for computing the amount of evaporation as a function of the type of bottoms. Plots of these relationships are given t o facilitate calculations a t different presewes and temperatures. A hydromechanical model is dencribed hy Kutateladze (23) for evaluating heat transfer by free convec:ion and in the boiling of liquids. Four simple assumptions are verified and data are presented for the type of heat transfer That occurs from both smooth and rough surfaces. The process of boiling refrigerants is covcred by Danilova and Mazyukevich 19). Both ammonia and Freon 12 were studied in a compound tBoiler and condenser apparatus using vertical steel tubes in the boiler section. It was discovered that the sites of boiling are more numerous in the case of Freon than in the case of ammonia. Consequently, the heat transfer rate achieved a t any particular heat loading is higher for ammonia than for Freon. Equations xre given for Freon 12 covering the heat loading area from 2500 t o 11,OOO K. calories per square meter per hour and similar ones are presented for ammonia in the range of 4000 t o 24,000 K. calories per square meter per hour. Linning (24) presents a theoretical invcstigation of the ff ow of evaporating fluids in tubes of uniform diameters. The conclusions are supported by experimental evidence on water. By evaluating published data the \%ritersreached the conclusion that an evaporating fluid flow conforms to one of three modes: annular, separate, and frothing. I n the annular and separate flow the liquid and vapor phascs flow a t different velocities. Starting from usual assumptions on fluid flow, a theory is developed which enables the mean liquid and vapor velocities and also the associated critical outlet conditions to be calculated. In the frothing operation, a network of liquid maintained by surface tension operates to prevent differential motion, and the standard thermodynamic theory is applicable. In experiments n ith water the flow is found t o be annular. By measuring the Auid momentum a t the tube outlet an experimental determination of the relative velocity factor (mean vapor velocity divided by mean liquid velocity) was obtained. Using data of their own, and accumulated by others, the writers reached the conclusion that measured values of critical outlet pressures and relative velocity factors are in good agreement rrith those that are predicted by the theory presented. A chart is providcd relating initial pressure, critical outlet pressure, and mass flow per unit area over a range of initial pressures from 8 to 100 pounds per square inch absolute for water. Evaluation of evaporation from flat surfaces, water at rest, is covered by Hickman and Torpey (17) and further by Hickman (16). The variation and the rate of emission of vapor from the water surface and the turbidity of the surface were examined.
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-4 vacuum still is used to demonstrate the turbidity of the water, and this turbidity was found to be due to a layer of impurities on its surface, possibly from the surface concentration by evaporation and lack of diffusion. It was noted that a considerable decrease in the rate of the evaporation could be accomplished by adding chemical impurities often prcsent in very low concentrations. .4n extension of the \vork indicates that with a clean, new surface of water it is possible to approach coefficients that are 25 to 100 times greater than any previously reported. Rates ranging from 38 to 7800, as compared to the best previous 150 to 270, are reported. The maximum evaporation cocficient was realized when a clean surface of water was continuously exposed to a high vacuum and instantly removed. A modified flowing stream jet tensimeter q-as used for the experimental work. Averin (1) reports that when operated under similar conditions of heat load, pressure, and others, the heat transfer coefficient for water boiling off of a steel, nickel-plated copper, and aluminum surface, are all substantially equal. Appreciable effects of the material and of the surface roughness of the heat transmitting surface are only observed when the heat flux rearhes 100,000 K. calories per square meter per hour. A t this point, apparently the roughness factors are destroyed becausc of the blanketing effect of bubble formation. It is also noted that when the surface is hydrophobic, the heat trensfer coefficient decreases with increasing load. I n such c a a s , the limiting heat load equals 100,000 X. calories per square meter per hour.
Equipment S&zavsk$ (58) presents a new theory for calculating the heat transfer area required in pressure evaporators. Considering evaporators operated a t temperatures in the neighborhood of 125' C. the proper dimensions depend on factors such as hourly heat input, heat transfer rate, scaling heat load, and heatirig surface. I n an experimental study of evaporation cquipmmt, as reported by Dubourg and others ( I I ) , vertical tubular evaporators such as are employed in such different industries as sugar, paper, salt, alkali, and others, were studied. Although the operation of the equipment is simple, the fundamental theory of evaporation knowledge is incomplete. The study --as conducted on a pilot scale n-ith equipment consisting of the vertical tubular evaporator which i s described in considerable detail. From the data obtained, several equations are derived t o calculate the heat transfer in sugar juice operation. Factors such as static level, emulsions, and liquid level on sugar decomposition were also investigated. Superheating of the vapoi a t the heat transfer area actually blinds the surface and inhibits heat transfer. A very careful analysis of the state of the liquid and vapor on the interior of the tubes is presented and the various factors that modify the ronditions are presented. Operating data and structural details are given for a new type of film evaporator in an article by Bchnnder (39). The unit designated as a Sambay evaporator maintains a uniform liquid film on the wall of the evaporator tube by means of a central rotor with hinged wipers. Badger and Standiford ( 2 ) present a comparison hetwcen present day and older practice in the manufacture of salt. Currently a much greater production is possible because of the development of new equipment. r u r ther, some of the older problems encountered in evaporatois have been eliminated. Coriosion, for example, has been licked by the replacement of cast irou with stainless steel or hlonel, with Monel being the favorite material. The increased use of the forced circulation evaporator has eliminated the problem of tube scaling but has highlighted a new problem in the form of combing. This is a terminology used to dmcribe the growth of salts on the side wall of the evaporator body. The combing originates from brine splashes or spray As long as the combs remain on the body they cause no problem but as they break off in chunks they can plug up the equipment,
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EVAPORATION or in the event of larger pieces falling off, actually damage the equipment. The problem of combing has apparently been solved by the use of Monel, since these deposits do not adhere to the smoother surface. In view of the higher heat transfer coefficients obtained in the forced circulation evaporator, it is possible to squeeze more effects in the total available temperature difference. I n addition to the foregoing, the article covrrs many of the newer and less orthodox processes. h very similar artirk, discussing salt manufacture, is presented by Hester and Diamond (15). In this excellent discussion complete treatment of the use of multiple-effect evaporators is presrnted indicating the design and operation currently in use bh- the Morton Salt Go. Included is complete design data. Some mention is made of the experimental use of submerged combustion evaporators for salt. The Texas Brine Corp. operated a submerged combustion unit for several years which produced esperimental quantities of high quality salt. The authors feel, however, that despite the loxer initial investment compared with vacuum pan equipment the long term increase in natural gas prices makes the process appear lcss favorable than it appeared only a few years ago. An interesting sidelight on the use of the submerged combustion unit is the fact that niagriesiuni chloride in the brine is converted to carbonate by the carbon dioxide. Since magnesium carbonate is added to thc salt anyway, its presence i n the brine is not detrimental arid a means is provided for cheap addition of this material to the final product. Currently, it is indicated that Texas Brine Corp. is working on a unit which is neither a true vacuum pan nor a submerged combustion unit. A further project involving submerged combustion evaporators is reported on hy Kobe and Hall ($1). The project covered the design, construction, and operation of a submerged combustion unit to supply quantities of hot water as for laundries or boiler feed prehcaters. The burner was set vertically in an criclosed 500-gallon tank and a perforated distribution header was bent at right angles to the burner plate leading the combustion gases evenly into a 300-gallon charge of water. Natural gas and compressed air were used in the burner and heated water was withdrawn from the tank bottom with the exhaust vapors bcing led to a pack tower sprayed n7ith feed water. The unit utilized about 90% of the groas heating value of the gas, heating large quantities of water to 190” F. T r s t pieces of pipe and coupling were submerged in the water and the average calculated corrosion rate was 0.03 inch per year. Submerged combustion evaporation of neutral sulfite spent cooking liquor is covered by Owen and Moggio (30). The evaporation was carried out in a 10-inch-diameter stainless steel tank with a ll/l-inch stainless steel burner. Presented are a flow diagram, time of operation, weights of charge, and the analysis A complete heat balance on two runs is also presented. maters and Bergstrom (43) present further information in the form of the dilution curve useful in the evaporation of liquors in the paper industry. The dilution curve is made up by plotting chemical losses in washed
W. L. BADGER, author of “Heat Transfer and Evaporation” and coauthor of “Inorganic Chemical Technology” and ‘%Elements of Chemical Engineering,” i s a native of Minneapolis. H e attended the University of Minnesota, receiving his M.S. in 1909. Badger is a consulting chemical engineer and a member of the American Chemical Society and American Institute of Chemical Engineers. March 1956
pulps against the “dilution factors,” and by means of this subsequent calculations can be made fixing the feed concentration of the evaporators, The dilution curves provide the base for securing a correlation between washing and evaporation and thus permit the determination of the most economical number of washings and evaporator effects. Although this type of determination is of some importance i r , soda and draft mills, it is of greatest importance in sulfite milla where corrosive liquors demand expensive construction materiald. The writers claim that the proper use of thcse curves can result in marked economies in the mill. The subject of kraft black liquor evaporation is covered hy Bergstrom and Waters (3 They present design, operation, and application information on the use of a long-tube film-type evaporator and discuss in particular the design of various types of condensers for use with this piece of equipmcnt. Further work on evaporation in the paper industry is presented by Craig and others ( 7 ) in a discussion on a thermal compression evaporator for spent sulfite liquor. Operating data are presented on a long tube vortical evaporator made out of stainless steel, The unit is designed t o evaporate 30,000 pounds per hour and uses a steam driven turbine compressor. Operations a t evaporation rates of 16.7 pounds of water per pound of steam are described. The article presents a complete description of equipment and the interesting side-note that it was possible to control scaling in the unit by daily cleaning with distillate recovered from the evaporation. Another presentation on thermal compression evaporators is given by Pouit (34). The unit is described wherein it i s possible to maintain a constant evaporation rate in spite of large fluctuations in withdrawal of vapor by maintaining only small fluctuations in the flow to the recycling compressor. This i s possible by combining mechanical and thermal recompression and by injection of new steam. The method is applicable to multieffect evaporators with regulation and control based on the flow of the varying bleed vapor. Ponomarenko (33) presents an application of the thermal compression system t o a multi-effect evaporator in the sugar industry. A review on evaporators in the sugar industry is covered by Tonn (4.2). Although the author is primarily interested in heat exchange operations, conaidcrable emphasis is placed on the evaporation problems. Case histories on brass evaporator tubes in the sugar industry arc presented by Whaley and others (46). Brass tubes commonly fail by longitudinal cracking, dexincification, or abrasion. The writers claim that the best brass is a 70 to 30 containing 0.02 to 0.06% arsenic and that with proper heat treatment and careful mechanical cleaning the tube-life can be prolonged considerably. The corrosion problems encountered in a boiler feed water evaporator are shown by Wickert (46‘). The corrosion of the raw water surfaces in a vertical tube evaporator are due t o operations with low level. I n view of the fact that feed water treatment includes the use of phosphates arid sodium hydroxide, these materials can be concentrated on tube surfaces wet by splash from the main body of the liquid. Definite evidence
R. A. LINDSAY obtained a B.S. (1939) and an M.S. (1940) from the University of Michigan. Since 1941 Lindsay has worked with W. L. Badger on heat transfer and evaporation a t Dow Chemical Co. In 1952 he was transferred to technical service and development a s assistant manager. With coauthors, Lindsay has published several papers on heat transfer. H e is a member of the AIChE.
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UNIT OPERATIONS REVIEW
Recent work adds to understanding of the phenomenon of actual vapor formation at heat transfer surface.
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of this is the pattern of corrosion indicated in the tubes where apparently this more concentrated material ran down the tube surface. The author's experience indicated that even distillate water, when treated comparably with caustic and phosphate, could cause the difficulty. Operations a t maximum possible water level practically eliminated the corrosion problem. Several miscellaneous types of equipment and operations are described that are worthy of note. Bruce (6) describes the operation of sea water cvaporators at Morro Bay wherein several possible sources of water were studied. The evaporation of sea water proved to be the most economical. The continuous direct evaporation of cellulose lye has been patented by Palm and Nylof (SI). Apparatus is described wherein cellulose !yes are evaporated in two or more stages in series by the direct addition of superheated steam. Dalin (8) describes an apparatus for the evaporating or spray drying of liquids. Various pieces of standard unit processing equipment, including evaporators, were duplicated in glass by Meade (37). A very excellent Iaboratory type evaporator is presented by Beroza ( 4 ) .
Entrainment A very comprehensivc review of the entrainment problem is presented by Reynolds (35). Although the article is not specifically directed a t evaporators, it does cover the ~7holearea of the separation of liquids from gas or vapor phases. A complete review of the control equipment available and a procedure for its selection is presented. A very complete and concise study of the liquid entrainment covering the mcchanism of drop formation from gas or vapor bubbles is presented by hTervitt and others ($9). By means of extremely high speed photography the evolution of bubbles from liquid surfaces was observed. The bubble, as it emerges from the liquid surface, forms a hemispherical dome with its internal pressure producing a depression at the interface. Liquid drains from the dome until the upper part is so weakened that the internal pressure causes the formation of a secondary cap. This cap subsequently disintegrates, perforating the bubble dome and giving rise to liquid droplets of a few microns in diameter. These droplets are carried away b y the high velocity gas issuiiig from the perforated dome. A system of standing waves is set up on the liquid surface surrounding the crater left by the collapsed bubblc. As the crater fills in, the momentum of the inflowing liquid produces a jet which rises a t high velocity and detaches one or more comparatively large droplets a t its apex. I t is these droplets, having a diameter of as great as 0.1 cm., that are responsible for the innin losses by entrainment. Garner and others ( I S ) have made another complete study of the entrainment problem covering the size distribution in a pilot plant evaporator and in a 4-inch glass evaporator, evaporating both water and potassium nitrate solutions. Droplet formation and projection into the vapor space occur by three means, either splashing, bursting of the vapor bubbles, or foaming. I n the cxpcriment described, samples of the entrained droplets were collected in a tn-o-stage cascade impactor and the entrainment was calculated from the size and total number of droplets. The cascade impactor consisted of one straight rectangular jet and onc tapered rectangular jet in wries, each containing a microscope slide coated with magnesium oxide. A droplet striking the magnesium oxide layer penetrated the surface giving a mll-defined circular impression that appeared as a bright circle of light against a dark background when viewed through a microscope. The ratio of droplet diameter to the
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impression on the slide is a constant which i s independent of the nature of the liquid and the velocity of impact. Entrainment was also determined by measuring the concentration of salt in the condensed vapor from the potassium nitrate solution. Approximately 95% of the droplets entrained in the vapor space of the evaporators were smaller than 20 microns, but because of their low mass they formed only a very small fraction of the total weight of entrained liquid. Droplets larger than 100 microns contributed the main quantity of the entrainmcnt. A study of the size distribution and cntrainment of droplpts from bursting air bubbles emerging from water, ethyl benzene, and ethyl alcohol, has shown that droplets are formed by the collapse of the bubble dome and by disintegration of the liquid jet arising from the bubble crater. The relative magnitude of these two effects depends upon the size of the bubbles. Droplets from the rupture of bubbles larger than 0.5 em. in diameter were almost entirely produced from the bubble dome. Stabilization of a bubble by the presence of dissolved or suspended solids and increases in liquid viscosity or surface tension tend to reduce the size and number of liquid droplets produced upon diuintegration of the bubble dome and, thus, decrease entIainmcnt. Considerable contribution to the problem of entrainment is made by Shotton and IIabeeb (40). They report that when the Reynolds number for vapor flowing through a container is below 2000, indicating stream line or laminar flow, the entrainment is directly proportional to the rate of evaporation in the diameter of the vessel. -4s the Eeynolds number is increased to 6-8000 the entrainment approaches a constant value irrespective of the diameter for a given height of vapor space. Entrainment decreases as the length of vapor space increases from 5 to 15 inches for each diameter, but is highest a t 30 inches. Gross carry-over (climbing film effect) occurred in 5- , 10- , and 15-inch vessels a t an approximate constant value for Reynolds number for each length and was unaffected by diametcr. For 30-inch vapor heads, gross carry-over n7as not achieved for diameteis of l 1 / * and 2 inches, and with 6 / / 8 - and 1-inch-diameter tubes gross carry-over was more nearly related to vapor velocity. A very careful study of the problem of entrainment and its relation to the concentration of radioactive contaminatrd material is presented by hfanowitz and others ( 2 5 ) . The problem of concentrating radioactive maastes for disposal has long been recognized by Brookhaven National Laboratories and ~ o i k in the area was done even before the nuclear reactor v a s under conPtruction. During their xork, two evaporators mere studied and it appeared that the amount of entrainment in both nas comparable a t similar boiling rates. In presenting their n w k the writirs refer to the evaporator decontamination factor nThich is defined as the ratio of activity pcr milliliter of the stlll pot solution to the activity per millilitm of the condenscd vapor leaving the evaporator. A t Brookhaven they w r e able to achieve decontamination factors of 10 to 100,000. Care must be taken in the way the evaporation is handled because it was noted that excessive foaming or splashing in the evaporator can result in much lower decontamination factors. The writers tried bubble caps and raschig ring packed columns but theae proved to be ineffective in removing submicron radioactive particles. Fiberglass packed columns, on the other hand, proved t o be the best available entrainment removal device. For evaporation economy, a vapor compression unit, coupled with a glass fiber filter, were studied on a semiworks scale (50 to 80 gallons per hour). This system proved very satisfactory on both a tcchnical and economic basis and, therefore, a similar
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EVAPORATION system with a capacity of 300 gallons per hour was erected to handle the full-scale liquid n-aste streams. Actual over-all decontamination factors of 1 million or greater are now being currently achieved after some two years of operating experience on the larger unit,. Perk ($2)discusses the prevention of entrainment and the equipment available. Particular attention is presented to the design of save-alls or catch-alls.
that the use of ion exchange in sugar liquor treatment can cause some scaling difficulties. When ion exchange is used the silica content of the scale can increase, perhaps as much as 10 times. As a result the scale is much less soluble and complications are introduced into the cleaning problem.
Bibliography Scale The prevention of scale formation in the evaporation of sea water in brine is the subjert presented by Sugimoto and Y a w ataya (41). The patent is primarily concerned with the ammoniasoda process and claims a method of sodium chloride recovery from waste liquor containing 1.1 grams per liter of calcium sulfate, 120 grams per liter of calciuiii chloride, and 50 grams per liter of sodium chloride. This is accomplished by evaporation a t 560 mni. and 72" C. The solution in the evaporating vessel is introduced into a closed heater situated about 7 meters below the evaporator; in the heater it is superheated to 75" C. n ithout boiling, and transferred to an evaporating vessel or flash tank filled with limestone granules. At this point, the pressure is released by the change in head and the calcium sulfate precipitated on the limestone or in suspension form. The use of dehydrated alfalfa to prevent scale in sea water evaporators is shown by Kail and others (19). The euperiments were conducted with synthetic sea water in an experimental evaporator with 50% blow down. The addition of 0.1% powdered alfalfa prevented the scale formation, with evidence that as little as 0.035% would be sufficient. The writers claim that the effert WVAS not due t o a change in pH, or to alfalfa constituents soluble in ether, alcohol, or water, but appeared to be primarily due to materials that were soluble in alkaline saline solutions. Kubelka (22) has investigated the scale problem in black liquor ev:tporators and explained the cause of the incruatations discovered The writer shows that the scale investigated mas primarily a mixed sodium sulfate-sodiurn carbonate and was deposited at temperatures higher than 35" C. from a saturated solution of sodium sulfate and sodium carbonate. It is claimed that by maintaining sodium sulfate concentrations in the 7 t o 8% range a t 100" C. it is possible to minimize the scale formation. The use of chelating agents such as T'ersene has recently stirred considerable interest in the sugar industry for evaporator cleaning or scalc prevention. This subject was investigated by Hamilton (14) and his tests indicate that 10 to 20 p.p.m. of Verscnc added to the evaporator feed reduced scale formation. IKth 10 p.p.m. a t pH 7 the reduction was 13% and a t pH i . 0 a reduction of 33% in the scale formation Tyas observed. Factory experiments were carried out over a four-meek period in hIay 1952 but they only showed a slight inhibition in the scale formation. The writer repeats the knowledge that due to t,he price of chelating agents such as Versene they do not yet appear to be commercially applicable. The calcium ovalate scale encountered in sugar refineries is discussed by Roche (36). The literature available on the subject is reviewed and the writer feels that calcium oxalate as an offender in this problem has beeu somewhat overlooked. He quotes a typical analysis of a deposit from the body of an evaporutor indicating that calcium oxalate can run as high as 97.3%. IIe suggests either the addition of sodium hexametaphosphate or an ion exchange operation to remove calcium oxalate. Some mention is made of the possibility of Veersene in this operation also. Cleaning evaporators in the sugar industry is coverod by Drinnen (IO). Data on a commercial operation using 1% solution of a mixture of aluminum and ferric sulfates (pH 3.4) are presented. The unit is first boiled with the mixture for one hour and then this is followed by boiling with a 2% caustic solution for another hour. Fivian and Manx ( l a ) point out the fact r
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March 1956
Averin, E. K., Imest. Akad. Nauk S.S.S.R., Otdel. Tekh. Nauk 1954, KO.3, 116-22.
Badger, W. L., Standiford, F. C., Chem. Eng. 62, No. 3, 173--7, NO. 4, 180-3 (1955). Bergstrom, R. E., Waters, H. K., Paper Mill News 77, Xo. 33, 12-33 (1954).
Beroza, Sf., Anal. Chem. 26, 125P-2 (1954). Bogdanov, F. F., Imest. akad. Nauk S.S.S.R., Otdel. Tekh. Nauk. 1954, No. 6 , 137-44. Bruce, A. W.,Combustion 26, No. 6 , 65-8 (1954). Craig, D., Elgee, E., Russell, J. K., Tappi 38, 69-74 (1955). D a h , D., Swedish .Patent 145,287 (May 18, 1954). Danilova, G., hlazyukevich, I., Kholodil'naya Tekh. 31, KO.2, 62-5 (1954).
Drinnen, L. D., Proc. Queensland SOC.Sugar Cane Technol., 21st Ann. Conf., Mackay, Qzaeensland 1954, 93-101.
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