Continuously operated crystallizer

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. 

If crystallization occurs in a steady state so that supersaturation is kept constant—which can, e.g., be realized in a continuously operating crystallizer—the following equation for the number frequency of crystals of radius r will hold ... 

For a complete description of the crystal size distribution (CSD) in a continuously operated crystallizer it is necessary to quantify the nucleation and growth processes and to apply the three conservation laws of mass, energy and crystal population. The importance of the population balance, in which all particles must be accounted for, has been a central feature in the pioneering work of Randolph and Larson (1962, 1988). 

As with other unit operations the design of crystallizers is based on mass and energy balances. Here, the balances for a continuously operated crystallization apparatus at steady state are formulated for a stirred vessel crystallizer. 

In crystallization technology it is common to use mass concentrations c in kg/m apart from mass fractions and loadings. In Fig. 8.3-1, a continuously operated crystallizer is shown. At steady-state conditions, the mass flow Lq of the solution with the concentration Cq entering the crystallizer is equal to the sum of the mass... 

Often, the solution fed into a continuously operated crystallizer is free from crystals and only a single volume flow V is drained continuously which is containing a particle size distribution representing the distribution in the crystallizer. In this case, the population balance can be further simplified to... 

Continuously operated crystallizers with lower limit range from approximately 80-400 kg/h, have the advantages of a high space-time yield, a lower degree of operating effort and a constant product quality over batch operation... 

In the following chapters, the basic concepts of the modern understanding of crystallization will be discussed, such as the internal structure of the crystals and their growth mechanisms or the phase diagrams. Attention will be directed to the purification by crystallization and to effects of polymorphism. Next, the basic methods to carry out a crystallization, from both the solution and the melt, are discussed. Finally, the concepts of mass crystallization in continuously operated crystallizers will be shown. 

A crystallization process has to be designed to meet a number of requirements, for example, particle properties such as particle size and purity. As concerns the purification, a continuously operated crystallization has to deal with the accumulation of impurities in the mother liquor. The drain point for these impurities has to be carefully chosen. It is one of the most underestimated aspect of the process design activity and the source of many errors. Moreover, the process yield is strongly connected with the planning of the process drain points, as is the specific energy consumption and, thus, the success of the process. However, die separation attribute of the unit operation crystallization may not be the best available and one should remain open for competing alternatives. That could be recommendable, for example, in cases of solid solutions, especially with distribution coefficients close to 1, which are not feasible to separate by crystallization at all. 

Figure 16.1 Drain point for impurities in a continuously operated crystallizer.

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. 

Mathews and Rawlings (1998) successfully applied model-based control using solids hold-up and liquid density measurements to control the filtrability of a photochemical product. Togkalidou etal. (2001) report results of a factorial design approach to investigate relative effects of operating conditions on the filtration resistance of slurry produced in a semi-continuous batch crystallizer using various empirical chemometric methods. This method is proposed as an alternative approach to the development of first principle mathematical models of crystallization for application to non-ideal crystals shapes such as needles found in many pharmaceutical crystals. 

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...

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. 

---