CRYSTALLIZATION bg
C. S. GROVE, JR.,
AND
J. B. GRAY
SYRACUSE UNIVERSITY, SYRACUSE 10, N. Y.
T
H E theory and practice of crystallization processes have advanced relatively little in the two years since the last review (80)of the subject was published. However, fundamental studies of the variables affecting the growth and habits of crystals have been continued. Several modifications of equipment for crystallization have been proposed, but no fundamentally different types have been devised. Numerous articles, which are not covered in this review, have been published on crystal constants and habits of certain crystalline compounds, and on grain structure of metals. Some interesting and potentially valuable fields of research have been suggested. This review summarizes briefly what are felt to be the more significant advances in the field of crystalliaation as a unit operation. Rate and Theory. Considerations of rates of crystallization are usually divided into two phases-nucleation (or initial formation of minute crystalline particles) and growth of these particles. For production of fine crystals a high nucleation rate is desired to obtain many centers for growth of new crystals, whereas for production of large crystals a low rate of nucleation is required so that ordered growth can occur on the faces of the initial seed crystals. The distribution of final crystal sizes may be said to be a balance between rate of nucleation and rate of growth. McCabe (SO) discusses nucleation rate as a function of free energy differences, stating that “neither nucleation nor growth can occur unless the precipitated substance has a lower thermodynamic potential after precipitation than before.” The rate of nucleation, according to Becker (6),is a function of the activation energy for diffusion and of the work required to form the surface of the nucleus. “The growth process is also amenable to the same kind of treatment as that for nucleation on the assumption that crystal growth is essentially a two-dimensional nucleation process.” Laurent (29) has derived mathematical formulas for the number of nuclei formed in a given time and for the velocity of crystallization, based on the energy of the atom. He uses this derived law to explain allotropic transformations. Ramberg (34) defines the force or energy of crystallization mathematically, using thermodynamic principlea and the relation between mechanical pressure and partial vapor pressure (or escaping tendency). At a given supersaturation, different minerals are able to bear and grow against different mechanical pressures. Some of the factors which affect the habits of crystals are discussed by Wells (@, 43). I n many cases interaction with the solvent is important. The Gibbs condition for the stability of a crystal-namely, that for a given volume the total surface free energy shall be a minimum-is confirmed. Evidence of various kinds is adduced to show that there is a definite equilibrium shape for crystals of a given solute crystalliring from a particular solvent. There is no tendency for a macroscopic crystal of nonequilibrium shape to approach an equilibrium shape unless crystal growth is permitted. The relations between the face development and lattice space groups are reviewed for certain aromatic hydrocarbons and metallic halides. Barkhuysen (4) explains the growth of crystals by assuming that the rate of growth depends on the velocity of diffusion. Van Hook (40, 41) studied the kinetics of sucrose crystallization. He concludes that the rate of growth of crystals is determined primarily by 8ome interfacial (homogeneous, chemical) reaction instead of an interboundary (heterogeoeous, physical) reaction.
This was deduced from the observation that distorted sucrose crystals are more frequently produced from impure sirups. Unequal and variable growth of the various faces occurred rather than the uniform and ordered growth which would be expected on the basis of a simple diffusional mechanism. I n general, the presence of impurities in massecuites depresses the normal rate of crystallization. Experimental proof of the pressure exerted by a crystal growing in an open vessel is presented by Hey (22). Harbury (20)discusses some theoretical aspects of crystallization from supersaturated solutions, considering selective adsorption, impurities, mobility, blocking agents, and lattice defects. Badger and Seavoy (3) discuss the hypothesis of Ross (35) that “the process of dissolving a substance in water involves a chemical reaction resulting in the formation of new compounds which can be designated as ‘molten hydrates’ of the solute.” Continuing this reasoning, [‘the formation of a crystal resolves into a process of freezing just as ice is created from water. Upon lowering the temperature of a solution, a molten hydrate freezes to the solid state. The heat effects are considered a combination of the heat of reaction and the heat of fusion or what is normally called the heat of crystallization.” . “Ross rationalizes his hypothesis to account for crystal growth by presenting photographic evidence to the effect that there is a field of attractive force surrounding the original nucleus and nuclei agglomerates which combine to form the small crystals, and these small crystals grow by attracting nuclei which attach themselves in an orderly arrangement to the growing crystals. Each crystal substance appears t o have its own inherent rate of growth which is somehow related to the attractive force on each different face of the crystal with which it builds itself by the above phenomenon.” Mukherjee (32)states that the main factors affecting crystal growth are the degree of supersaturation and temperature of the solution, and local fluctuations of concentration and temperature. Fundamental data required for crystallizer design inolude determinations of heats of crystallization and solution, of work required to form crystal surfaces, and of rate of nucleus formation as a function of supersaturation. Stuckenbruck, Osburn, and Grove (38)used a modified Jolly balance to follow growth of a single crystal of potassium alum from a solution kept supersaturated by constant controlled cooling. The weight increase seemed to be independent of the area of the crystal (under the conditions of the investigation) and to be B straight line function of time. Grove, Montillon, and Mann (18)developed a laboratory crystallizer for use in a study of the growth of potaaeium alum crystals. A rate equation has been derived and experimentally verified. Jang, Montillon, and Mann (961,using a similar crystallizer, have studied the crystallization of magnesium sulfate heptahydrate. The original rate equation has been expanded and experimentally confirmed. Frenkel (16)states that the conception of a crystal face as an ideally even plane, with an orientation corresponding to a small value of the surface tension, is wrong. He presents the idea of vicinal faces, arising because of thermal 5uctuations, which act as terraces on the main faces and thus give rise to natural roughnesses. He develops a mathematical treatment of the energetics of this situation and considers the problem of rate of approach of a crystal to its equilibrium shape. Semenchenko (37) gives equations for the effect of surface tension on the solubility of crystal faces, proving thermodyuamically that the surface ten-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
sion of one-component crystals should be smaller the larger the crystals. Dankov (18)reviews the crystallo-chemical mechanism of the interaction of newly formed crystals of one compound on crystal surfaces of another. He shows that orientation takes place depending on the relevant lattice spacing of the new crystal, its surface tension, and its moduli of elasticity. A review of the identification of crystalline substances is presented by Nelson (33). He discusses ( a ) morphological examination with an optical reflection goniometer; (b) optical examination with a polarization microscope; (c) x-ray diffraction examination with a single-crystal specimen; and ( d ) x-ray diffraction examination with a polycrystal specimen. He lists 159 references. Equipment Development. Crystallization equipment is classified by Badger and Seavoy (3)into batch or continuous types, with or without agitation. The type of refrigeration used is a further means of classification. Crystallhers may be gas-cooled, liquid-cooled, or vacuum-cooled. Mukherjee (92) discusses the theory of crystal growth as it is applied to industrial crystallization. The trend of crystallizer design is to improve control of the rate of crystal growth by controlling temperature and concentration of the crystallizing solution. Although no fundamentally different types of crystallizers have been devised recently, several modifications of familiar designs have been proposed. The use of sprays to cool the solution from which crystals are being obtained is suggested in several patents (11,13, 26). I n the production of crystalline dextrose (26) the supercooled solution is sprayed directly on the growing crystals, which are rotated in a drum and simultaneously subjected to a stream of hot air. The spray evaporation may be carried out in one chamber and crystal growth in another. Substances are crystallized (I) from solution by circulating the solution in streams through a chamber through which also passes a current of gas. The solution is thereby supersaturated and is passed through a layer of crystals before returning to the chamber. The increased industrial importance of single crystals-for example piezoelectiic-has resulted in designs of crystallizing equipment particularly adapted to the production of large single crystals (19,23). Hughes (24) has patented a crystallizing evaporator. This evaporator is designed for continuous operation by defining a series of concentric chambers, so arranged that circulation of the solution is upward in the inner chambers and downward in the outer chambers. Crystals which form must fall through the upward flowing solution and are thus classified as they grow. Vacuum crystallizers are used in the viscose rayon and Cellophane industry to crystallize sodium sulfate from the spinning bath (8). They are also used in making ferrous sulfate, chromates, potassium chloride, nitrates, and in separating sodium chloride from caustic solutions. Caldwell (8) states, “the now well established vacuum crystallizer has gained wider acceptance due to its inherently simple construction and dependable low cost operation. There have been improvements in the details of design such as simplified and more efficient agitators which result in a more uniform crystal size; improved thermo-compressors which reduce steam consumption and virtually eliminate salting up of these parts; and better use of materials of construction to overcome contamination of product and corrosion of equipment.” Industrial Practice. Much of the new information pertinent to the industrial applications of crystallizafion processes is concerned with the equipment and the methods used for agitation, cooling, controllin1 the degree of supersaturation, and separating product crystals of the desired size. There are some aspects of crystallization, other than equipment, which affect the quantity and quality of the product. Operating variables, such as composition and rate of feed to the crystallizer, rates of agitation or circulation within the crystallizer, product withdrawal rate, and pressure, are eigni5cmnt in continuous vacuum crystallizers. In
Vol. 40, No. 1
batch or continuous liquid-cooled crystallizers the composition of the feed, the agitation rate, the rate of cooling, and the period elapsing before discharge of the product determine the nature and yield of crystals. However, there has been relatively little published during 1946 and 1947 on the effect of these variables on crystallization as practiced industrially. There have been, on the other hand, several studies reported on the effect of various substances, such as ions of various types, on the size and shape of crystals and on the rates of crystallization. Some of these substances may be present as impurities in the feed to the crystallizer. Coalstad @,IO) concluded on the basis of studies of the crystallization of sucrose from impure solutions that the failure of sucrose to crystallize further from molasses, which is a highly supersaturated solution, is due to the presence of impurities which are adsorbed on the surfaces of the crystals and thus inhibit their growth. When these impurities are removed by the addition of an excess of adsorbent carbon, the sucrose crystallizes out almost completely. It is shown that the ratio of sucrose to impurities is valuable in appraising the purity of molasses. Erikson and Ryan (14) have patented a process for removing impurities which interfere with the crystallization of sucrose. Lime is added until a p H of 10 is obtained in the sucrose solution, which is then filtered. Phosphoric acid is added to a p H of 4.6 to 5.3 and the precipitate removed. Neutralization with lime (to a pH of 6.3 to 8) and heating above 54” C. may remove more precipitated impurities. The filtrate is then fermented with Torulopsis mcmosa to remove glucose as ethanol. The sucrose is not affected by this treatment. The inhibition of crystallization of calcium salts such as the carbonate and sulfate by the addition of an extract of cacao bean husks is patented by Havelock et al. (62). An extract of cacao bean husks is obtained by treating 1 pound of husks with 1.25 gallons of water and steam a t 30 pounds per square inch gage. The extract is added in the ratio of 1 gallon of extract to 75,000 gallons of water to be treated. Boiler feed water treatment is one application of the process. The habit modifications of several other crystals by various ions have been studied. Kolb and Comer (B8)experimented with the effect of metal ions of gold, copper, magnesium, cobalt, nickel, cadmium, and manganese on the crystal habit of the monohydrate of ammonium oxalate. Til’mans (39) studied the influence of iron, copper, manganese, and cadmium ions on the crystallization of ammonium chloride and bromides. The use of addition agents to obtain finely divided crystalline solids is claimed in two patents. Kokatnur (BY) prepares finely divided sodium or potassium hydroxide by evaporation of solutions to which have been added polychlorinated and -brominated aromatic hydrocarbons, such as tetrachlorobenzene, which coat the small crystals and isolate them from the mother liquor. After the coated crystals are separated, the coating is removed with acetone or benzene. Foster and Williams (15) obtained very small crystals of nitcoguanidine by adding substituted amines to a hot aqueous solution of nitroguanidine. The phase relations of the system sodium acetate-sodium hydroxide-water were investigated by Morgan and Walker (51) to provide a basis for determining the proper operating conditions for the crystallization of sodium acetate from sodium hydroxide solutions. Zil’berman and Ivanov (46) studied the solubility of components of the system magnesium chloridesodium thiosulfate-water and of a similar system in which calcium is substituted for magnesium. Research Possibilities. More fundamental data are needed on rates of crystallization during nucleation and growth. Such studies should include investigations of the effect of amount and kind of impurities, of the degree of supersaturation, of the method and rate of stirring, and of the control and rate of cooling. I n addition, studies of the variables affecting crystal habit are needed. Ways of increasing or decreasing the degree of super-
January 1948
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
saturation in order to operate in the metastable solubility zone, and changes of crystsl form under various conditions, should be investigated further. Elucidation of the relatione among variables such 88 these would aid materially in advancing the knowledge of crystalliaation. The research described in certain articles indicates new a p proaches or new techniques in crystallization processes which appear worthy of further exploration. Berlaga (6) shows that, with adequate cooling to remove the heat due to vibrations, application of a supersonic field t o undercooled salol (phenyl salicylate) increases the rate of crystrtlliaation. Furthermore, he notes an increase in the number of centers of crystallization. Boerboom (7) has studied crystallization of supersaturated solutions by means of an electric current. He states that when supersaturated solutions of electrolytes are electrolyzed, such a high concentration results from the transference of ions at one of the electrodes that crystallization nuclei are formed spontaneously and grow in the supersaturated solution. Gorbunova and Dankov (17) discuss elementary electrocrystallization processes theoretically, by taking as an example the isolated growth of a face of a single crystal forming part of the electrode in an electrolytic cell. They believe that the growth of a crystal from a mixture of gases and solutions is subject to the same relations as exist in electrocrystallization. Schutz ($6) and Bader and Schutz ( 8 ) describe the method and principles of fractionation by adsorption and crystallizatian on foam. West (44, 46) has patented an apparatus for growing single crystals of uniaxial material, such as sodium nitrate. This depends on the use of an added orienting surface such as mica. ACKNOWLEDGMENT
The authors acknowledge, with thanks, the help of W. L. McCabe of the Flintkote Company, East Rutherford, N. J., for supplying a portion of t,he literature references. LITERATURE CITED
(1) Aktieselskaget Krystal., Norwegian Patent 62,374 (1940). (2) Bader, R., and Schutz, F., Trans. Faraday SOC.,42, 571 (1948). (3) Badger, W. L. and Seavoy, G. E., “Heat Transfer and Crystallization,” Harvey, Ill., Swenson Evaporator Co. (1946). (4) Barkhuysen, F. H. C., Chem. Weekblad.,43,234 (1947). (5) Becker, R., Am. Physik., 32,128 (1938).
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(6) Berlaga, R. Ya., J. Exptl. Theoret. Phy8. (U.S.S.R.), 16, 647 (1946). (7) Boerboom, A. J. H., Nature, 159,230 (1947). (8) Caldwell, R. B., C h m . I d . , 61, No. 3, 418 (1947). (9) Coalstad, 8. E.,Intern. Sugar J., 48,319 (1946). (10) Coalstad, S. E., J . Soc. Chem. Ind., 65,206 (1946). (1 1) Corn Products Refining Co.. Brit. Patent 596,951 (1947). (12) Dankov, P. D., J . Phys. C h m . (U.S.S.R.), 20, 853 (1946). (13) Davis, C. P., U. S. Patent 2,396,689 (1946). (14) Erikson, A. M. and Ryan, J. D., ZbM., 2,416,682 (1947). (15) Foster, G. H., and Williams, E. F., ZbM., 2,395,856-60 (1946). (16) Frenkel, Ya. I., J.Phys. U.S.S.R.,9,392 (1945). (17) Gorbunova, K. M., and Dankov, P. D., Compt. rend. acad. 815. U.R.S.S.,48,15 (1945). (18) Grove, C. S., Jr., Montillon, G. H., and Mann, C. A., Ph.D. Thesis, Univ. of Minn. (1941). (19) Haas, W. O., Jr., U. S. Patent 2,424,273 (1947). (20) Harbury, Lawrence, J. Phys. & Colloid Chem., 51, 382 (1947). (21) Havelock, J. R., Parr-Burman, B. A., Elkington, Frank, and Gilpin, W. C., Brit. Patent 587,054 (1947). (22) Hey, M. H., Nature, 158,584 (1946). (23) Holden, A. N., Phvs. Rev., 68,283 (1945). (24) Hughes, J. S., U. S. Patent 2,384,747 (1945). (25) Jang, J. A., Montillon, G. H., and Mann, C. A., Ph.D. Thesis, Univ. of Minn. (1943). (26) Jeremiasaen, Finn, U. S. Patent 2,375,922 (1946). (27) Kokatnur, V. R., Ibid., 2,393,108 (1946). (28) Kolb, €1.J., and Comer, J. J., J . Am. Chem. SOC.,68, 719 (1946). (29) Laurent, Pierre, Rev. d t . , 42,22 (1945). ENG.CHEM.,38,18 (1946). (30) McCabe, W. L., IND. (31) Morgan, R.A., and Walker, R. D.,Zbid., 37,1186 (1945). (32) Mukherjee, N. R., J. Imp. Coll. Chem. Eng. Soc., 2, 68 (1946). (33) Nelson, J. B., Bull. Brit. Coal Utilisation Research Assoc., 10, 257 (1946). (34) Ramberg, Haus, Geol. Fbren. i Stockholm Fbrh., 69, 189 (1947). (35) Ross, E. T., Pacific Chem. Met. Inds., 2 , No. 3 , 9 (1938). (36) Schuta, F., TrUn8. Faraday Soc., 42,437 (1946). (37) Semenchenko, V. K., J. Phya. Chem. (U.S.S.R.), 19,298 (1945). (38) Stuckenbruck, L. C., Osburn, J. O., and Grove, C. S., Jr., M.S. Thesis, State Univ. of Iowa (1947). (39) Til’mans, Yu. Ya., J. Gen. Chem. (U.S.S.R.), 16, 3 (1946). (40) VanHook, Andrew, IND. ENQ.CEEM.,38,60(1946). (41) VanHook, Andrew, PTOC. Am. Sugar Beet Technal., 4,559 (1946). (42) Wells, A. F., Phil. Mag., 37,184 (1946). (43) -Ibid.. 37.217 11946). (44) West,C.’D.,U.S.Patent 2,414,679 (1947). (45) Ibid., 2,414,680 (1947). (46) Zil’berman, Ya. I., and Ivanov, P. T., J . Gen. Chem. (U.S.S.R.), 16,1589 (1946). \--I
RECEIvlDD
November 24,1947.
HIGH TEMPERATURE DISTILLATION Bg T. J. WALSH,
CASE INSTITUTE
OF TECHNOLOGY, CLEVELAND, OHIO
HE future student of distillation will probably find that the year from November 1, 1946,to November 1, 1947,contains more interesting and useful data than any preceding year. Sparked by two symposia, one on distillation in general sponsored by the Division of Industrial and Engineering Chemistry of the AMERICAN CHEMICAL SOCIETYand the other on analytical distillation sponsored by the American Petroleum Institute, the number of articles in this field was double that of 1946. The subjects covered the range from microcolumns with a charge of 10 ml. to industrial bubble-cap stills several feet in diameter; pressures from 0.10 mm. absolute to the critical pressures of the distilling mixtures; from highly theoretical mathematical analyses of the differential equations involved to operational
comment on how a particular problem has been solved. A major trend has developed toward a return to the study of column calculations and how they can best be solved, with less attention directed toward the improvement of column capacities or efficiencies, The revelation of data has continued at a somewhat slower rate than previously. Principles of Distillation. Several reviews of the general principles of distillation have appeared during the year. Reed (479, in introducing the A.P.I. symposium, defines many distillation terms and describes the types into which analytical columns, column heads, and jackets may be classified. Edmister (17) and White (66) describe distillation behavior and outline the standard techniques for calculating column height. Cica-
