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a matter of design, since it is a function of the booster characteristic curve and increasing specific volume of vapor as the vacuum increases. As previously indicated, high vapor space is more economical than the use of auxiliary entrainment separators. Vacuum cooling serves to remove dissolved and entrained gases effectively from the liquor. This is frequently des i r a b l e f o r instance, the removal of carbon disulfide and hydrogen sulfide from viscose spin bath.
Uses of Vacuum Crystallizers Up to a few years ago the only vacuum crystallizer installations in this country were on potash and borax in one western plant. Commercial installations now operating on copperas, Glauber salt, Epsom salt, ammonium sulfate, ammonium chloride, sodium potassium ferricyanide, ammonium fluotitanate, zinc sulfate, citric acid, and various other organics will give some idea of the acceptance by the
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chemical industry of vacuum type crystallizers. Figures 11 and 12 show approximate flow sheets of typical applications of batch and continuous crystallizers.
Bibliography A11 references are to United States Patents. (1) Caldwell, H. B., 2,067,043 (Jan. 5, 1937). (2) Connell, G. A , , Cramer, T. M.,and Caldwell, H . B., 1,972,730 (Sept. 4, 1934). (3) Heath, S. B., 1,815,735 (July 21, 1931). (4) Howard, Henry, 1,559,703 (July 5, 1923), 1,560,473 (Nov. 3, 1925) : Meynardie, Pierre, 1,661,489 (March 6, 1928) : Mumford, R. W., 1,676,277 (July 10, 1928); Caldwell. H. B., 1.865.614 (July 5, 1932); Grill, H. E., and Feldstein, H . H., 2,097,208 (Oct. 26, 1937). (5) Isaachsen, Isak, 1,478,337 (Oct. 12, 1921), 1,573,716 (Feb. 16, 1926), 1,646,454 (Oct. 25, 1927), 1,693,786 (Dec. 4, 1928): Jeremiassen, Finn, 1,704,611 (March 5, 1929), 1,151,740 (March 25, 1930).
“KRYSTAL” CLASSIFYING CRYSTALLIZER HANS SVANOE A/S Krystal, Box 134, Kennett Square, Penna.
To separate chemicals in a crystalline form from solutions, the driving force as a supersaturated solution must first be established. If the crystals are to be formed under controlled conditions, the solution must be supersaturated within the metastable field during the process of crystallization, To maintain this condition, the supersaturated solution must be exposed efficiently to a large quantity of crystals which will bring the solution back to the saturation point before the solution is again supersaturated. The features incorporated in the “Krystal” equipment (also called the “Jeremiassen” or “Oslo crystallizer”) are based upon these principles, and the result is close control of the crystallization process. The type of equipment used depends mainly upon the solubility characteristics of the chemical to be recovered. For a salt with a flat solubility curve, supersaturation of the solution is produced
T
HE separation of chemicals in the crystalline state from solutions has been used for centuries. It was recognized
early that the process of crystallization m-as in many cases the only practical method of manufacturing pure chemicals with economy This outstanding feature of crystallization-to be able t o produce pure chemicals from relatively impure solutions-has therefore been utilized by the chemical industry for numerous products on an extensive scale.
Metastable Field of Supersaturation In order to separate chemicals from solutions by crystallization, a driving force in the form of a supersaturated solution must first be established. To be able to control the process of crystallization, it is of prime importance that this factorsupersaturation of the liquor-be controlled during the entire process.
by evaporation. For a large-capacity installation several units can be connected to form a multipleeffect system. For a salt with a steep solubility curve, supersaturation can be produced by water cooling or more efficiently by the combined effect of vaporization and vacuum cooling. Regarding cost factors in a crystallization problem, i t is important to consider that crystallization is only a part of the total-to separate a chemical from a solution as a dry, marketable product. The efficiency of centrifuging and drying operations depends upon crystal size. Screening of the product to meet trade demands can be avoided when the desired grain size is produced in the crystallization process. Therefore in the crystallizer operation a grain size should be produced that meets the highest standard of efficiency for the entire process.
An important work regarding supersaturation of solutions was published by Miers some thirty years ago. Miers stated that under certain conditions solutions can be supersaturated to a considerable extent beyond the saturation point without the formation of any nuclei. The field where no nuclei are formed Miers called the “metastable field of supersaturation”. If a solution is saturated beyond this field the labile field of supersaturation is reached where nuclei will appear in a clear solution. I n the metastable field crystal growth takes place on nuclei or crystals already present in the solution. If a solution can be maintained in the metastable degree of supersaturation during the entire process, a most important step towards control of crystallization is made. The metastable field is not sharply defined, and factors such as the pH of the liquor or the presence of other chemicals
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and impurities have a marked influence on the.extent of the metastable field. The metastable field as determined in the laboratory may extend 5 to 20 grams of salt per lit'er above saturation or even higher. The degree of supersaturation used in commercial equipment should be only a fraction of these figures in order to minimize the effect of the appearance of new crystal surfaces, either from crystal breakage or excessive local supersaturation. I t should also be noted that the formation of new seed crystals or nuclei requires time. A solution may appear to be in the metastable field, but after a certain length of time nuclei are formed, particularly if the solution is passed over rough metal surfaces. To subject a solution to a slight degree of supersaturation calls for a cyclic system, where large quantities of liquor are supersaturated uniformly. The solution must then be brought back to saturation before feed liquor is allowed to enter the system, and the mixture is again supersaturated in the next cycle. To remove the metastable supersaturation is a comparatively slow process. A large crystal surface is required, and the crystals must be brought in contact with the solution in an effective manner. Krystal equipment is designed with the above considerations in mind, the purpose being to keep the liquor a t all times within the metastable field where nuclei formation and crystal growth can be controlled. The main elements of design in a crystallizer unit will then be as follows: (a) provision for producing supersaturation within the metastable field, and (b) provision for removal of supersaturation on a crystal suspension in a cyclic system.
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One of the more important features of the Krystal classifying crystallizer is that the container for crystal growth has certain elements of design similar in all Krystal units for vacuum crystallizers, cooling crystallizers, and evaporator-crystallizers. I n these units a supersaturated solution of uniform temperature and concentration is conducted upward through a dense suspension of crystals, and the crystals are kept in suspension by this upward flow of liquor. The classifying action in the crystallizer container keeps the large crystals suspended in the bottom layer of the suspension and the smallest crystals in the top layer, with the intermediate sizes suspended between.
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Crystallization Equipment The type of equipment to be used in a crystallization process depends primarily upon the solubility characteristic of the solute. Industrial chemicals dissolved in water or in solutions where water is the main solvent can be classified as follows: 1. Chemicals where a change in solution temperature has little or a negative effect on the solubility-for instance, sodium chloride and anhydrous sodium sulfate; supersaturation is produced by removing solvent by evaporation. The equipment needed here, generally known as salting-out evaporator, can be classified more properly as evaporator-crystallizer. 2. Chemicals wilh a large increase in solubility with an increase in solution temperature; cooling is used to produce supersaturation. This cooling can be effected either by circulating the solution through outside tubular water coolers in equipment classified as cooling crystallizers, or the cooling can be done by evaporation in a vacuum chamber in a vacuum crystallizer. 3. Chemicals with a moderate increase in solubility with increasing temperature; a combination of evaporation and cooling is used to produce supersaturation, an operation most effectively performed in vacuum crystallizer with a tubular heater installed in the circulating system. 4. Supersaturation may also be produced by the addition of another solute with a common ion or by the addition of an acid or base to bring about a considerable change in pH of the solution. The type of equipment t o be used depends upon the change of heat content in the system and upon operating temperatures.
Most industrial solutions used as feed liquor in crystallizer operations contain certain amounts of foreign salts and impurities. Such foreign salts affect the solubility of the product to some extent, and if several salts are present in appreciable concentrations, the solubility relation for the salt system must be determined over the operating range, in order to control the crystallization steps efficiently and economically. The process of crystallization is often influenced to a marked degree by the presence of minor amounts of impurities, with the effect of repressing growth on certain crystal faces. This characteristic is sometimes used to advantage where it is desired to produce a certain crystal form-for instance, changing the appearance of the crystal from long needles to shorter crystals resembling cubes.
%* FIGURE 1.
EVAPOR.4TOR-CRYsT.4LLIZER
EVAPORATOR-CRYSTALLIZER. Figure 1 indicates the main elements of design of an evaporator-crystallizer. Feed liquor entering a t T is mixed with circulating mother liquor, forced by pump F through heater H and conducted to vaporizer A. There the solution is supersaturated, and the supersaturated solution flows down pipe B and up through the dense crystal suspension in container E. The heating surface in the heater must be sufficiently large to take care of the heat requirements of the crystallization process. The heating surface needed can, in most cases, be calculated according to equations available in the literature for turbulent flow in vertical pipes. The increase in liquor temperature will generally amount to a few degrees centigrade only. Vaporizer A is designed so as to keep liquor entrainment through the vapor outlet U down to a minimum, and at the same time provide efficient release of the superheat in the liquid. I n crystallizer container E sufficient crystals are suspended to achieve almost complete release of supersaturation. T h e individual crystals must be in constant motion to prevent their growing together, but the motion must not be too vio-
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lent or new crystals will be formed by attrition. The amount of crystals carried in suspension will vary for different chemicals, and will also depend upon the character and amounts of impurities present and the crystal size desired.
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curve, such as anhydrous sodium sulfate or sodium carbonate monohydrate. The initial high rate of heat transfer is maintained, and the individual units in a multiple-effect system can be operated economically on low steam-to-liquor temperature differential. For the salts mentioned, therefore, multiple-effect installations can be operated with more effects than formerly, or exhaust steam can be used in multiple effect. VACUCMCRYSTALLIZER. A vacuum crystallizer unit is outlined in Figure 2. The elements of design are the same as for the evaporator-crystallizer except that a heater is not needed. The operating features are also similar. The feed is mixed with circulating mother liquor and conducted up to the vaporizer where the sensible heat of the feed and heat of crystallization are utilized to vaporize water and cool the solution t o the required temperature. If desired, a steamjet vacuum booster can be attached to the vacuum chamber to secure a very low operating temperature. The crystallizer container is an open tank not subjected to vacuum and connects with the vaporizer through the barometric leg, B. The supersaturated solution enters underneath the crystal suspension in tank E , which has the same general features as outlined for the evaporator-crystallizer. Mother liquor overflows a t N . If it is desired to vaporize more water than the sensible heat of feed liquor and heat of crystallization will permit, a tubular heater can be installed in the circulating line between pump and vaporizer.
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“4 FIGURE2: VACUUM CRYSTALLIZER The discharge of the finished crystals can be arranged t o suit the conditions required for efficient separation of mother liquor and crystals. The crystals can be discharged by opening valve M ,and the crystal slurry conducted to a filter or a continuously operated centrifuge. The size of the suspended crystals can be watched through sight glasses arranged a t different heights on the side of the container. Where production is large, several evaporator-crystalliaers can be arranged in series in a multiple-effect system. The vapor released in the vaporizer of an earlier effectis admitted to the steam chest of the next effect; and the vapor from the last effect, where the absolute pressure is lowest, is conducted t o the condensing plant. A multiple-effect system may offer several alternatives regarding steam and liquor flow combinations, such as countercurrent and cocurrent. Often, however, some foreign salts will accumulate in the mother liquors, in which case the evaporation in any one effect can be carried only t o the limit where the product is the stable crystalline phase and the liquor flow in the system must be regulated accordingly. By operating in the metastable field of supersaturation, the equipment can be operated for long periods without formation of salt deposit on the metal surfaces. This is particularly striking when producing salts with an inverted solubility
FIGURE3. COOLING CRYSTALLIZER COOLINQCRYSTALLIZER.Figure 3 shows the general arrangement of a cooling crystallizer. The crystallizing chamber is similar in design to the units outlined above, but the supersaturated solution is produced differently. The mixture of mother liquor and feed is pumped through a vertically arranged shell-and-tube heat exchanger. To secure high rates of heat transfer a pump, I , is used to force the cooling medium over the tubes at a given velocity. The temperature drop in the cooler is comparatively small. The solution is supersaturated within the metastable field, control of the crystallization process is thereby secured, and salt deposit on the cooling surfaces is avoided. Crystal discharge can take place in small batches-for instance, once an hour-or can be arranged continuously. Water, brine, or some other cooling liquid can be used as the cooling medium.
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SEPARATION OPERATIONS
SPECIAL ADAPTATIONS. If a particularly large crystal size is desired, or if conditions are such that too many new crystals are formed by attrition or other influences, the excess nuclei can be removed in a fine salt separator. Such an attachment is indicated as G in Figure 3. A portion of the main liquor flow is passed through a separate container, the liquor velocity being sufficiently low to allow the very small crystals to settle. By removing only a small percentage of the production as fine crystals, the remainder is left in the crystallizer proper t o grow to the desired size. A fine salt separator can sometimes be used where a small quantity of another salt is crystallized out as separate crystals along with the main product. The growth of the undesired crop is prevented by collecting these crystals in the fine salt separator before they become too large, and the main product can be discharged from the crystallizer proper as pure crystals uncontaminated by the other crystals. Crystallization in all the continuous units described above takes place a t one definite temperature and not through a fairly wide temperature range. This is important when a salt is produced that forms several hydrates or crystallizes in several different crystal forms. The feed liquor can be a concentrated solution a t a considerably higher temperature than the temperature of operation. When the feed liquor is mixed with circulating mother liquor and subjected t o supersaturation, the resulting mixture is practically a t the temperature of crystallization, where the desired crystal form or hydrate is the stable phase. The capacity of a Krystal unit can be arranged to suit local conditions in obtaining the most economical layout; that is, from a productive capacity of a few tons per day the design can be adapted to capacities of several hundred tons per day per unit. I n many contemporary chemical engineering problems we can make use of a more or less standardized formula for calculating the size of the equipment needed for a particular job. This applies only in a limited sense as far as equipment for crystallization is concerned. Considering the first objectto obtain a supersaturated solution, either by evaporation or cooling-the size of the metal surfaces needed t o transmit
TORQ THICKENERS AT GRAND COULEE DAM S A N DP R E P A R A T I O N PLANT, MASON CITY, WASHINGTON (See article on page 663.) Courtesy, The Dorr Company
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the heat quantities involved can be treated as a problem in h e a t transmission. I n considering the production of a material with a specific crystal size, it is difficult if not impossible, to set down in a formula all the factors affecting crystal growth and to use such a formula as a basis for equipment design. In some cases fairly simple tests can be undertaken which, together with accumulated experience of similar problems, will prove sufficient to formulate the design of the equipment. I n other cases, however, pilot-plant test operations have t o be undertaken to obtain sufficient information regarding the magnitude of the controlling factors.
Cost Factors In considering the cost factors in connection with a crystallization problem, it is important to remember that crystallization itself is only a part of the total-to separate a chemical from a solution as a dry, marketable product. Materials produced in a crystallizer are usually subjected to the following operations: (a) Crystals and mother liquor are separated in a filter or centrifuge; (b) the crystals are dried down to a moisture content of 0.1 per cent water or less; (c) quite often the material is screened in order to produce an even grain size; (d) the crystals are packed in containers, in some cases stored in bulk before packing, or sometimes stored and also shipped in bulk. All of these operations are affected by a change in crystal size. When an even size is produced by properly controlling the crystallization process, screening is eliminated, dust losses are avoided during the drying operation, and crystal caking during storage is reduced to a minimum. The water content of the centrifuged crystals may be reduced one half t o one third as compared with the results obtained with an uneven product, which means that the drying operation is simplified and a purer product is obtained. Few figures are published for industrial chemicals regarding the effect of crystal quality on centrifuge or filtering operations. Whenever available such figures indicate clearly the advantage of even crystal size and also specify the actual crystal size required to separate chemicals from solutions in the most ,economiral manner.
