Conclusions
(1) The optimum conversion to m-toluenesulfonic acid found in this study is 19.3% which is far above the few per cent previously reported for a liquid-phase process. The optimum operating conditions are 88’ reaction temperature, time of 3.7 hr, a mole ratio of sulfuric acid to toluene of 1.0, 97% sulfuric acid, and addition of the sulfuric acid to the toluene over the entire reaction time. (2) Searly complete conversion of toluene to combined toluenesulfonic acids is achieved under the conditions of high meta isomer conversion. Thus the yields are essentially the same as the reported conversions. (3) Only 20 experiments were performed to locate the maximum in conversion t o m-toluenesulfonic acid. Six more experiments were used t o estimate error and confirm the location of the maximum in the principal response. The systemized method of determining the optimum in the dependent variable using factorial experiments to determine a path of steepest ascent resulted in an economical use of the experiments. (4) The method of analysis using multivariable, linear regression analysis on ultraviolet spectra of samples containing mixtures of the isomers of toluenesulfonic acid proved to be a fast, convenient, and precise method of determining isomer distribution and overall conversion. ( 5 ) Sulfuric acid strengths above 97% have potential to increase the conversion to m-toluenesulfonic acid above 19.3%. Acknowledgment
The authors thank Vilas S. Patwardhan for some computations and for reviewing the manuscript.
C = conversion to all isomers of toluenesulfonic acid C, = acid strength in weight per cent = mole ratio of sulfuric acid to toluene R -. T = temperature in degrees Centigrade y = conversion of toluene to m-toluenesulfonic acid in per cent based on toluene 0 = reaction time in hours v = degrees of freedom of the numerator in the F ratio literature Cited
American Cyanamid Co., British Patent 949852 (1964); U. S. Appl. (1960). Arends, J. M., Cerfontain, H., Herschberg, I. S., Prinsen, A. J., and Wanders, A. C. M., Anal. Chem., 36,1802 (1964). Box, G. E. P., Youle, P. V., Biometrics, 11, 287 (1955). Brooks, B. T., Kurtz, S. S., Boord, C. E., Schmerling, L., “The Chemistry of Petroleum Hydrocarbons,” Vol. 111, Reinhold, New York, N. Y., 1955. Cerfontain, H., Sixma, F. L. J., Vollbracht, L., Recl. Trav. Chem. Pays-Bas, 82, 659 (1963). Cerfontain, H., personal communication, 1967. Chemetron Corp., British Patent 967894 (1964); U. S. Appl. (1960). Englund, S.W., Aries, R. S.,and Othmer, D. F., Ind. Eng. Chem., 45, 189 (1953). I. G. Farbenindustrie A. G., U.S. Dept. Commerce, OTS Report No. PB 91,355 (1945). Lion, P. H. R., French Patent 1397422 (1965); U. S. Appl. (1964) .-,. I
~
_
Meerwein, H., Dittman, G., Gollner, R., Hofner, K., Mensch, F., Steinfort, O., Berichte, 90, 841 (1957). Norwood. S. L.. SauLs. T. W. (to Tennessee Corn). - U.S. Patent 2838333 (1958). ’ Spryskov, A. A., Yakovleva, T. I., Zh. Obshch. Khim., 27, 239 . I
(19.57). ,. \_.__
Spryskov, A. A., Zh. Obshch. Khim., 30, 2449 (1960). Sternberg, J. C., Stillo, H. S., Schwendeman, R. H., Anal. Chem., 32, 84 (1960). Witco Chem. Co., Netherlands Patent Appl. 290738 (1965), U. S. Appl. (1963). Wylie, L. M. (to Tennessee Corp.), U.S. Patent 2841612 (1958).
Nomenclature
A
= time of addition of sulfuric acid t o toluene in per cent of the total reaction time
RECEIVED for review July 24, 1972 ACCEPTEDMarch 9, 1973
A Direct Contact Cooled Crystallizer Ruth Letan Chemical Engineering Department, University of the Negev, Beer-Sheva, Israel
A method and an apparatus are described for crystallization of salts from their solutions by direct cooling with an immiscible coolant dispersed into drops. The heat absorbed by the coolant in the crystallizer i s partly rejected in a direct contact heat exchanger, by contact with the cold depleted solution discharged from the crystallizer. The heat rejection is accomplished in an indirect cooler. The smaller the portion of heat rejected in the cooler, the more efficient thermally i s the process. In comparison to other crystallizers, the described process shows the advantage of continuous operation and high thermal economy; it has, however, the disadvantage of operating an additional thermal unit.
I n crystallizing of salts from their solutions several procedures may be utilized such as vacuum, evaporation, reaction, or cooling. Crystallization by cooling is applied to solutions of salts which show a decreasing solubility with reduced temperature. The crystallization can be brought about by direct or indirect 300
Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 3, 1973
heat exchange between the crystallizing solution and a colder fluid: a gas or a liquid. In indirectly cooled crystallization processes the crystallizing solution is cooled in a heat exchanger of metallic surfaces through which heat is extracted out of the solution. The s o h tion becomes supersaturated at the cooling surface, crystals
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Figure 1 . Schematic shape of a crystallizer
deposit on the surface, and the deposited layer grows. The deposition reduces the heat transfer rates and therefore the production rates. If the operation continues on for a long time, the apparatus becomes encrusted and blocked. It has to be subjected to periodic cleaning and discontinuous operation. A modification and improvement of the operation of a n indirect heat exchanger was achieved by the Gas Council (1963) by the installment of two coolers in the apparatus and their alternate operation. Another method of cooling a crystallizing solution is by contacting it directly with an immiscible coolant. The coolant, a gas such as air or a liquid such as petroleum, can be applied to brines, while a brine can be applied for cooling crystallizing organic solutions. I n direct cooling two methods have to be distinguished: cooling the solution to its saturation by operating a heat exchanger and a crystallizer or cooling it through crystallization by operating a crystallizer only. I n the first method, the heat exchanger is either a tank in which cold air is bubbled or it is a liquid-liquid spray column, as described by Letan and Kehat (1968) and Kehat and Letan (1968, 1969). The crystallizer can be of any common design, as, for example, the one operated by Zmora (1970). I n the directly cooled crystallizer the cooling and crystallization are achieved in a single unit. This method was applied by French (1963) to crystallization of benzene, by impinging jets of benzene and cold brine inside a centrifuge which was subsequently used for the separation of the product. Another kind of a directly cooled crystallizer was applied by Cerny (1963). The Cernyls crystallizer was operated in cocurrent flow of an immiscible organic coolant and an aqueous crystallizing solution through the central tube of the unit, while the annular space served for flow and recirculation of the crystallizing solution. The organic coolant rejected its heat in a n external refrigerator. Several features of the above described process are later compared with the crystallizer to be described below. Process and Apparatus
Our process (Negev University, 1972) as well as the Cerny's process relate to direct contact cooled crystallizers and to a
Figure 2. Crystallizer operated with an ejector
process which provides heat exchange between fluids of different temperature and specific gravity. The apparatus may thus be utilized in connection with any process in which crystallization is achieved by cooling, however, presently it is described particularly for crystallizing inorganic salts. The units involved in the process are a crystallizer, a direct contact heat exchanger (one or two), an indirect cooler (or refrigerator) , and a filtration unit (Figures 4 and 5). The crystallizer (Figure 1) is a vertical column equipped with an enlarged top and a double conical section a t the bottom. The top section may be either cylindrical or conical as required for the appropriate mode of operation, described by Kehat and Letan (1968). The column proper enlarges a t the bottom into a conical section followed by a cylindrical section which contracts again into a cone. A steep slope of the lower cone prevents accumulation of solids on its surface. I n the bottom cylindrical section a disperser is vertically installed to avoid clogging of nozzles by the settling crystals. The crystallizing aqueous solution is fed a t the top of the column proper. It flows downward countercurrently to the organic coolant. The cooled saturated solution deposits crystals. The crystals entrained in the solution flow down, growing in size and quantity. The cold suspension of crystals and saturated solution is discharged a t the bottom of the lower conical section and transferred to a filtration unit (Figure 2). The organic coolant which is of lower specific gravity than the aqueous suspension is introduced into the crystallizer through the disperser a t the bottom. It is dispersed into drops which rise through the aqueous suspension, heat up, and coalesce a t the interface held within the enlarged top section of the crystallizer. The heated organic liquid is passed then to the bottom of the direct contact heat exchanger (No. 2, Figure 4) to be cooled. The suspension of crystals and solution discharged out of the crystallizer is fed into a n insulated filtration unit, which provides adequate drainage of the solution from its crystals. The type of filtration unit used in desalination and described by Barak and Dagan (1970) for the separation of ice-brine Ind. Eng. Chem. Process Der. Develop., Vol. 12, No. 3, 1973
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Figure 3. Filtration unit
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Figure 4. Flow diagram of a crystallization process (three heat exchangers)
suspensions and wash of the ice crystals may be applied in this process as well (Figure 3). The cold saturated solution separated in the filtration unit is fed to the top of the direct contact heat exchanger (No. 2, Figures 4 and 5). The crystals discharged from the filtration unit are conveyed away as a product. A suspension with a low fraction of solids (
