Continuously operated crystallizer fractionation

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... 

Crystal growth is a layer-by-layer process, and the retention time required in most commercial equipment to produce crystals of the size normally desired is often on the order of 2 to 6 h. Growth rates are usually limited to less than 1 to 2 pm/min. On the other hand, nucle-ation in a supersaturated solution can be generated in a fraction of a second. The influence of any upsets in operating conditions, in terms of the excess nuclei produced, is very short-term in comparison with the total growth period of the product removed from the crystallizer. A worst-case scenario for batch or continuous operation occurs when the explosion of nuclei is so severe that it is impossible to grow an acceptable crystal size distribution, requiring redesolution or washout of the system. In a practical sense, this means that steadiness of operation is much more important in crystallization equipment than it is in many other types of process equipment. 

Kao [48] has reported an elaborate, extractive process to produce 96 wt% MAP containing only 2.3% phosphoric acid and <1% each of DAP and unreacted alcohol. The process uses a fivefold molar excess of polyphosphoric acid (105%) to alcohol, thereby minimizing unreacted alcohol in the product. Because of the high viscosity of the intermediate product, the reaction is carried out in hydrocarbon (hexane) solvent. On completion of the reaction, additional hexane is added and the mixtnre is extracted with isopropanol. The hexane and aqueous layers are then separated. The hexane layer containing the MAP ester is extracted for the second time with aqueous isopropanol, then any residual water and isopropanol are removed by azeotropic distillation with continuous hexane addition. The hexane fraction is cooled, allowing the monoalkyl ester to crystallize. It is filtered and the residual hexane is distilled for recycle. The water-isopropanol extracts containing phosphoric acid are then stripped to recover the phosphoric acid for recycle. This batch process can be adapted to a continuous operation. Several refinements of this process have been published [49-51]. 

Some processors have employed solvent fractionation systems to produce salad oil, which has three major advantages over the product obtained by traditional winterization. These are (i) a considerably lower viscosity, which allows a faster crystal growth for more rapid stearin separation (ii) the salad oil produced has a better resistance to clouding at cool temperatures for longer cold tests and (iii) less liquid oil is trapped in the stearin component giving higher salad oil yields. An operational continuous solvent process was described by Cavanagh (1961) for winterization of cottonseed oil. 

Analysis of complex mixtures often requires separation and isolation of components, or classes of components. Examples in noninstrumental analysis include extraction, precipitation, and distillation. These procedures partition components between two phases based on differences in the components physical properties. In liquid-liquid extraction components are distributed between two immiscible liquids based on their similarity in polarity to the two liquids (i.e., like dissolves like ). In precipitation, the separation between solid and liquid phases depends on relative solubility in the liquid phase. In distillation the partition between the mixture liquid phase and its vapor (prior to recondensation of the separated vapor) is primarily governed by the relative vapor pressures of the components at different temperatures (i.e., differences in boiling points). When the relevant physical properties of the two components are very similar, their distribution between the phases at equilibrium will result in shght enrichment of each in one of the phases, rather than complete separation. To attain nearly complete separation the partition process must be repeated multiple times, and the partially separated fractions recombined and repartitioned multiple times in a carefully organized fashion. This is achieved in the laborious batch processes of countercurrent liquid—liquid extraction, fractional crystallization, and fractional distillation. The latter appears to operate continuously, as the vapors from a single equilibration chamber are drawn off and recondensed, but the equilibration in each of the chambers or plates of a fractional distillation tower represents a discrete equihbration at a characteristic temperature. 

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