Crystallization continuously operated

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

Batch crystallizers are widely used in the chemical and allied industries, solar saltpans of ancient China being perhaps the earliest recorded examples. Nowadays, they still comprise relatively simple vessels, but are usually (though not always) provided with some means of agitation and often have artificial aids to heat exchange or evaporation. Batch crystallizers are generally quite labour intensive so are preferred for production rates of up to say 10 000 tonnes per year, above which continuous operation often becomes more favourable. Nevertheless, batch crystallizers are very commonly the vessel of choice or availability in such duties as the manufacture of fine chemicals, pharmaceutical components and speciality products. 

The product crystals were agglomerates of needles or dendrites. Loose floes of dendroid strontium carbonate are compacted by agitation, which is an important factor in controlling the habit of product particles. Semi-batch operation produces larger particles compared to batch or continuous operation. 

Franck, R., David, R., Villenuaux, J. and Klein, J.P., 1988. Crystallization and precipitation engineering - II. A chemical reaction engineering approach to salicylic acid precipitation Modelling of batch kinetics and application to continuous operation. Chemical Engineering Science, 43, 69-11. 

In batch operations, mixing takes place until a desired composition or concentration of chemical products or solids/crystals is achieved. For continuous operation, the feed, intermediate, and exit streams will not necessarily be of the same composition, but the objective is for the end/exit stream to be of constant composition as a result of the blending, mixing, chemical reaction, solids suspension, gas dispension, or other operations of the process. Perfect mixing is rarely totally achieved, but represents the instantaneous conversion of the feed to the final bulk and exit composition (see Figure 5-26). 

Example 10.7 A solution of sucrose in water is to be separated b. A mass balance on the solvent gives by crystallization in a continuous operation. The solubility of... 

Figure 4.18 Enzyme membrane reactor synthesis of L-tert-leucine from trimethylpyruvic acid in an continuously operated enzyme membrane reactor with ultrafiltration followed by a crystallization step...

Crystal nucleation and growth in a crystalliser cannot be considered in isolation because they interact with one another and with other system parameters in a complex manner. For a complete description of the crystal size distribution of the product in a continuously operated crystalliser, both the nucleation and the growth processes must be quantified, and the laws of conservation of mass, energy, and crystal population must be applied. The importance of population balance, in which all particles are accounted for, was first stressed in the pioneering work of Randolph and Larson1371. ... 

Inherent in any reaction system where solids are deposited at a distance from the initial reaction is the question of the origin of the nuclei from which they are produced. In continuously operating commercial reactors, this is usually the result of attrition between existing crystals of the... 

The suspension of crystals of PETN contained in the diluter flows into one of a batch of vacuum filters (4). As each filter is filled with PETN, the stream is directed to another filter, while from the first acid is removed as completely as possible by suction, after which an outlet in the centre of the filter-floor, closed by a conical stopper is opened. The PETN is washed out of the filter with water and carried to a battery of continuously operated washers. 

Natural circulation evaporators like those shown on Figure 8.16 may be equipped for continuous salt removal and thus adapted to crystallization service. For large production rates, however, forced circulation types such as the DTB crystallizer of Figure 16.10(g), with some control of crystal size, are the most often used. The lower limit for economic continuous operation is l-4tons/day of crystals, and the upper limit in a single vessel is 100-300 tons/day, but units in parallel can be used for unlimited capacity. 

In addition to continuous operation, the impinging stream crystallizer can also be operated in batch mode to suit the crystallization of substances with very low crystal-growth rates, e.g., some of the organic compounds. 

Evaporated solution is then pumped into the stirred continually operating crystallizers where it is cooled from 90° to 25 °C. All impurities and salt crystals are then allowed to separate in settling vessels. The clear solution is afterwards pumped into storage tanks, where a small amount of sodium hypochlorite is added in order to decolorize the lye. 

Materials that have a tendency to grow readily on the walls of the crytallizer require periodic washout, and therefore an otherwise continuous operation would be interrupted once or even twice a week for the removal of these deposits. The impact that this contingency may have on the processing-equipment train ahead of the crystallizer must be considered. 

Continuous crystallizers must operate steadily at equilibrium to achieve the design requirements. This means the feed rate, production rate, slurry density, operating temperature, liquid level, and so on, should held constant as a function of time. To accomplish this result requires the crystallizer to be isolated from upstream or downstream variations and the instruments need to be continuously calibrated. To help accomphsh this objective, a 12- to 24-hour agitated feed tank needs to be installed before the crystallizer. 

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