Chemistry: physical processes – Physical processes – Crystallization
Reexamination Certificate
1994-12-19
2002-11-12
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry: physical processes
Physical processes
Crystallization
C023S303000, C023S3130FB, C423S499400
Reexamination Certificate
active
06478828
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the crystallization of an inorganic substance, using a bed of crystals through which passes a solution which is supersaturated with the substance to be crystallized.
2. Description of the Related Art
The “Oslo” crystallizer is a well-known apparatus for the crystallization of inorganic substances (British Chemical Engineering, August 1971, Vol. 16, No. 8, pages 681 to 685; The Chemical Engineer, July/August 1974, pages 443 to 445; British Patent GB-A-418,349). This known apparatus comprises a vertical cylindrical vessel and a vertical tube which is arranged axially in the vessel and which opens in the immediate vicinity of the bottom of the latter; a vertical annular chamber is thus defined between the axial tube and the cylindrical wall of the vessel. In making use of this known apparatus, a bed of crystals is employed in the annular chamber, through which passes a solution supersaturated with the substance which it is desired to crystallize (for example an aqueous solution supersaturated with sodium chloride). This solution is introduced into the apparatus via the axial tube, so that it enters the annular chamber radially, near the bottom of the latter, and subjects the crystals in the bed to a general rotation comprising an upward translation along the wall of the vessel and a downward translation along the axial tube.
In this known apparatus the intention is to produce uniform crystalline particles of spherical shape, whose mean diameter it should be possible to control by an appropriate choice of the dimensions of the apparatus and operating conditions. In practice, however, this known apparatus is ill-suited for obtaining large spherical particles, especially because of the attrition produced within the bed of crystals and of the general rotation to which these are subjected. In particular, in the case of sodium chloride, it does not permit spherical particles greater than 2 or 3 mm in diameter to be produced. Furthermore, an effect of the attrition phenomenon is to give rise to the formation of fines which are entrained out of the bed by the crystallization mother liquor and which must consequently be separated from the mother liquor before the latter is recycled into the apparatus.
The invention is aimed at overcoming this disadvantage, by providing a new process which makes it possible, on the one hand, to crystallize particles which are spherical and of larger diameter and, on the other hand, to reduce the formation of fines.
SUMMARY OF THE INVENTION
Consequently, the invention relates to a process for the crystallization of an inorganic substance, in which use is made of a bed of crystals through which passes a stream of a solution supersaturated with the substance to be crystallized, the bed of crystals being fluidized by passing the supersaturated solution through a distributor which is arranged below the bed of crystals and which is maintained at a temperature at which the concentration of the supersaturated solution does not exceed the concentration corresponding to saturation.
In the process according to the invention, the crystals in the bed act as seeds for the crystallization of the inorganic material by desupersaturation of the supersaturated solution. They are generally small uniform crystals of the inorganic substance which it is intended to crystallize.
The degree of supersaturation of the supersaturated solution depends on various parameters, especially on the nature of the inorganic material, on its temperature and on the possible presence of solid or dissolved impurities. In practice, everything else being equal, it is advantageous to produce a maximum degree of supersaturation, although this must be limited to avoid accidental crystallization on the walls of the crystallization plant, upstream of the crystal bed, as well as primary and secondary seeding within the solution.
The means employed to obtain the supersaturated solution are not critical. The latter may be obtained, for example, by changing the temperature or by partially evaporating a solution saturated beforehand with the substance to be crystallized.
The solvent in the solution is not critical, and water is generally preferred.
The temperature of the supersaturated solution is not critical. In practice, however, it has been observed that the rate of growth of the crystals in the bed is proportionally greater the higher the solution temperature. It is advisable, however, that the solution temperature should remain below its boiling point at the pressure prevailing in the crystallization chamber. For example, in the case where the process is applied to the crystallization of sodium chloride, it is possible advantageously to employ aqueous solutions of sodium chloride with a degree of supersaturation between 0.3 and 0.5 g/kg, at a temperature of between 50 and 110° C. The degree of supersaturation expresses the excess mass of inorganic substance relative to the mass corresponding to the saturation of the solution.
According to the invention, the crystal bed is a fluidized bed, in accordance with the generally accepted definition (Givaudon, Massot et Bensimon—“Précis de génie chimique” (A summary of chemical engineering)—volume 1—Berger-Levrault, Nancy—1960, pages 353 to 370). To fluidize the bed, the stream of the supersaturated solution is passed through a distributor arranged under the crystal bed, in accordance with the usual technology of fluidized bed reactors. The distributor is a fundamental member of the fluidized bed reactors. Its function is to divide the stream of solution into thin jets, preferably parallel and vertical, while furthermore imposing onto it a defined pressure drop, controlled as a function of the bed dimensions, of the nature of the particles forming the bed and of the solution (Ind. Eng. Chem. Fundam.—1980—19—G. P. Agarwal and others—“Fluid mechanical description of fluidized beds. Experimental investigation of convective instabilities in bounded beds”—pages 59 to 66; John H. Perry—Chemical Engineers' Handbook—4th edition—1963—McGraw-Hill Book Company—pages 20.43 to 20.46). It may be, for example, a horizontal plate pierced with uniformly spaced orifices, a grid or a horizontal mesh, or an assembly of vertical nozzles.
In accordance with the invention, the distributor is maintained at a uniform temperature at which the concentration of the supersaturated solution is lower than or equal to the concentration corresponding to saturation. In other words, in the process according to the invention, the temperature of the distributor is different from the temperature of the supersaturated solution upstream of the distributor, and it is chosen so as to make the concentration of the said supersaturated solution lower than or at most equal to the concentration of the saturated solution which is stable at the temperature of the distributor (for the same single inorganic substance and the same single solvent as the supersaturated solution). The crystallization of the inorganic substance on the distributor is thus avoided. The choice of the distributor temperature is critical and depends on the inorganic substance which it is intended to crystallize, the solvent in the solution and the degree of supersaturation. Thus, in the case of a substance whose solubility in the solvent increases with temperature (for example an aqueous solution of sodium or potassium chloride) the temperature must be higher than that of the supersaturated solution. In the case of a substance whose solubility in the solvent varies inversely with temperature (for example an aqueous solution of sodium carbonate monohydrate), the temperature must be lower than that of the supersaturated solution. Furthermore, the choice of the distributor temperature is conditioned by the need to prevent the supersaturated solution coming into contact with it from undergoing an excessive temperature change, which would result in an exaggerated drop in its degree of supersaturation. The temperature of the distributor mu
Detry Leopold
Ninane Leon
Nguyen Ngoc-Yen
Solvay & Cie (Societe Anonyme)
Wells Ashley J.
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