Melt crystallization limitations

Recovery by melt crystallization also is limited by the eutectic composition. 

The horizontal continuous Brodie melt crystallizer is basically an indirectly cooled crystallizer with an internal ribbon conveyor to transport crystals countercurrent to the liquid and a vertical purifier for final refining. Figure 20-8 describes the operation of a single tube unit and Fig. 20-13 depicts a multitube unit. The multitube design has been successfully commercialized for a number of organic chemicals. The Brodie purifier configuration requires careful control of process and equipment temperature differences to eliminate internal encrustations and is limited by the inherent equipment geometry to capacities of less than 15,000 tons per year per module. 

Limited to samples which undergo weight changes, thus melting, crystal phase changes etc. cannot be studied. Complex thermal traces are often difficult to interpret. 

Production rates of melt crystallization of organic materials usually are low enough to warrant the use of scraped surface crystallizers like that of Figure 16.10(a). A major difficulty in the production of crystals is the occlusion of residual liquor on them which cuts the overall purity of the product, especially so because of low temperatures near the eutectic and the consequent high viscosities. Completeness of removal of occluded liquor by centrifugation or filtration often is limited because of the fragility and fineness of the organic crystals. 

Complete recovery of dissolved solids is obtainable by evaporation, but only to the eutectic composition by chilling. Recovery by melt crystallization also is limited by the eutectic composition. 

Neat ADN contains crystals shaped as needles or flakes. No method for re-crystallizing ADN to give more uniformly-shaped crystals has been reported. The ill-shaped crystals limit the amount of ADN in cast-cured compositions due to high viscosity. The only solution to this problem is to reshape the ADN crystals to be more regular. Since the melting point is not directly associated with decomposition, it is possible to use a method whereby ADN is melted. 

There is a potential for the commercial application of supercritical CO2 fluid extraction for fractionation of milk fat. However, the differences between the melting properties are not as pronounced as with melt crystallization, thereby limiting the application range of these fractions. Moreover, this process is more expensive than melt crystallization (Bhaskar et al., 1998). Nevertheless, niche applications could be developed if fractions rich... 

Consequently, Rmax must begin to decrease with a further increase of pn beyond some value of pn to continue to satisfy Equation 6.36. In fact, the decrease of Rmax is likely to begin at a lower value of pn than the upper limit suggested by Equation 6.36, since xc does not increase to 1 with increasing pn. An amorphous fraction always remains in a melt-crystallized polymer. 

In those cases in which complete solid solution with no eutectic formation cecurs, it is possible to separate completely a binary mixture into its components via countercurrent multistage melt crystallization. However, such systems are the rare exception, whereas eutectic formation is the general rule in solid-liquid equilibria. Therefore, in tbe overwhelming majority of cases, the degree of separation attainable by crystallization is limited by the entectic composition. To accomplish a complete separation, it is necessary to couple an auxiliaty separation step to the crystallization process to break the eotectic,... 

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