Melt crystallization process

The economy of melt crystallization processes depends on the product purity, which is normally increased by an additional cleaning step. The application of gases under pressure is investigated to show possibilities of product quality improvement. Experimental devices for the determination of the freezing curve under gas pressure and for a solid layer crystallization process are shown. The influence of gas and pressure in respect to the freezing curve are explained on the basis of two binary mixtures (trioxane/water and para-/meta-dichlorobenzene) under CO2- and N2- pressure are presented. Furthermore the results of solid layer crystallization experiments with naphthalene/biphenyl and para-/meta-dichlorobenzene mixtures are shown. 

Slaughter, D. W., and Doherty, M. E, Calculation of solid-liquid equilibrium and crystallization paths for melt crystallization processes. Submitted for publication (1994). 

Test Variables. Examining the melt crystallization process, there are three variables that should be optimized. The variables and the ranges used for this study are ... 

Another commercial melt crystallization process is the MWB rystem depicted schematically in Fig. 

The just-mentioned advantage of the melt crystallization processes concerning temperature level of the product component do not exist if the comparison is made to the crystallization from solutions (see Chapter 1 in this volume) since the product is crystallized in solution at temperatures lower than its melting point. However, the solvent has to be evaporated, usually in larger amounts, and this fact makes up for the disadvantages in many cases. 

Melt crystallization processes are dominated by heat transfer. In the case of melt crystallization in suspension processes, the crystals are surrounded by melt and the system is almost isothermal at the crystallization temperature. The heat of the solidification process is only transported away through the melt. Such a system is often called adiabatic grown, which results in moderate growth rates with rather pure crystals. 

For the design and optimization of melt crystallization processes it is vital to have a complete understanding of the process. To this, a detailed knowledge about the crystallization kinetics is essential. Nevertheless, there is only little theory available to describe melt crystallization processes mathematically or to predict their separation efficiency. This is mainly a consequence of the complex heat and mass transfer processes prevailing in the crystallizers which lead to a non-linerar system of differential equations for the transfer processes. These equations can only be solved numerically and even then require a considerable number of simplifying assumptions and boundary conditions. 

Finally, in some cases it can also be a disadvantage for the product to leave the apparatus in liquid form and to be solidified again. The last point as well as the third, could well be avoided someday if it becomes possible to build continuously operating processes in one plant for solid layer melt crystallization processes. 

An optimization of all three purification steps is needed for all types of melt crystallization processes in order to make melt crystallization more economically competitive. For such an optimization, a mathematical description of the phenomena is necessary. When such a description becomes reality, a great deal of progress for the unit operation melt crystallization will be made. Research in the field continues, therefore this technique is likely to have a bright future. 

The presence of HOCP considerably slows down the melt crystallization process of PB-1. Therefore, the adopted values, lowered by increasing the HOCP fraction, provided similar rates of crystallization for pure PB-1 and blends. Previous calculations from the spherulite growth rate and from the overall kinetic rate constant showed that the number of nuclei per unit volume was similar for samples crystallized at equal undercoolings. Had we used a constant value of T, there would have... 

Simple agitated vessels, such as those commonly used in solution crystallization (section 8.4), rarely find application in melt crystallization processes. One of the... 

For many polymers, crystal growth can take place either from the melt or from dilute solution to yield single crystals. Crystal formations in polymers were studied intensively almost from the time of recognition of their existence in macromolecules. As a result, certain basic principles were established (1) The melt crystallization process is a first-order phase transition (see Section 1.4.5). (2) Crystallization from a molten polymer follows the general mathematical formulation for the kinetics of a phase change.Equilibrium conditions, however, are seldom if ever attained and complete crystallinity is not reached. 

The incorporation of amide functions in the polyester backbones result in semicrystalline materials with melting points of 22-127°C and a complex melting behavior due to pol5mciorphism and melting crystallization processes (1). 

Laakso, P.H., Nurmela, K.V.V., and Homer, D.R. (1992) Composition of the Triacylglycerols of Butterfat and Its Practions Obtained by an Industrial Melt Crystallization Process, /. Agric. Food Chem. 40, 2472-2482. 

The melting-crystallization process of a system of small molecules is formally described as a first-order phase transition. Appropriate laws then follow that can be applied to a variety of problems. For a one-component system at constant pressure, the transition temperature is independent of the relative abundance of either of the two phases that are maintained in equilibrium. Melting is very sharp. The characteristic temperature of equilibrium is defined as the melting temperature. For the above conditions to be experimentally satisfied an almost perfect internal... 

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