Section 4.6 Solution Crystallization

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or crystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binary mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirectly cooled, and/or solvent is evaporated to effect crystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt crystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining section of the apparatus. 

In this section, diastereomeric crystallization is presented as a driving force -or internal selection pressure - to resolve dynamic diastereomeric systems. The dynamic diastereomeric systems are generated from reversible covalent bond formation, leading to compounds carrying chiral carbon centers under thermodynamic control. The dynamic systems can represent more variety of the possible diastereomer adducts. The selective diastereomers, A —B, , are subsequently chosen from the dynamic system by self-transformation and/or self-preferential crystallization. When the selective product C , is formed, the ratio of its corresponding diastereomer adducts A -Bm in the dynamic system will be decreased. The equilibrium in the dynamic system will force the reproduction of the intermediate until the resolution has reached completion. In the end, only one diastereomeric product Cnm is selectively crystallized and easily purified from the solution. 

For shell-and-tube boiling approach temperature <25°C to ensure nucleate boiling. Related topics include evaporation (Section 16.11.4.1), distillation (Section 16.11.4.2), solution crystallization (Section 16.11.4.6), and reactors, PFTR nonadiabatic (Section 16.11.6.6). 

Next we come to phase transitions. Chapter 14 mentions the various phase transitions that may occur, such as crystallization, gas bubble formation, or separation of a polymer solution in two layers it then treats the nucleation phenomena that often initiate phase transitions. Chapter 15 discusses crystallization, a complicated phase transition of great importance in foods. It includes sections on crystallization of water, sugars, and triacylglycerols. Chapter 16 introduces glass transitions and the various changes that can occur upon freezing of aqueous systems. 

This section discusses various features of commercial types of solution crystallizers used in industrial processes. In solution crystallization a diluent solvent is ndded to the mixture or is already present as a carrier liquid. The solution is then cooled and/or solvent is evaporated to cause crystallization Another feature of solution crystallization is that the solid phase is formed and maintained below its pure-component freezing temperature. 

The first important part of every crystallization operation, namely the nucleation step, reveals the same mechanisms underlying the kinetics in the fields of melt and solution crystallization. Therefore, it is here referred to Section 2.2 of this book which gives a detailed introduction into the corresponding theoretical background. 

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 shell and tube condensation Related topics evaporation Section 4.1, distillation Section 4.2. Prefer condensation outside horizontal tubes use vertical tubes when condensing immiscible liquids to subcool the condensate. Assume pressure drop of 0.5 of the pressure drop calculated for the vapor at the inlet conditions. Baffle spacing is 0.2 to 1 times the shell diameter with the baffle window about 25 %. Limit pressure drop for steam to 7 kPa on the shell side. U = 0.5-0.85 kW/m °C. For shell and tube boiling approach temperature < 25 °C to ensure nucleate boiling. Related topics evaporation Section 4.1, distillation. Section 4.2, solution crystallization Section 4.6 and reactors PFTR non-adiabatic. Sections 6.8 and 6.12. 

In this chapter we consider the separation of species contained in a homogeneous phase, such as a liquid or gas. The separation is based on exploiting a fundamental difference that exists between the species. Section 4.0 gives some overall guidelines. Methods that exploit differences in vapor pressures are evaporation, in Section 4.1 and distillation, in Section 4.2. Methods that exploit differences in freezing temperature and solubility are freeze concentration. Section 4.3, melt crystallization. Section 4.4 and zone refining. Section 4.5. Methods exploiting solubility are solution crystallization. Section 4.6 precipitation. Section 4.7 absorption, Section 4.8, and desorption. Section 4.9. Solvent extraction. Section 4.10, exploits differences in partition coefficient. 

Here, the compositions which ensure the maximum electron density are presented in terras of the composition of the liquid phase. This is because the composition of the solid phase was not analyzed for In and Sb contents (it is fairly complicated to do this), whereas there was no certainty that the solid solution was not displaced from the corresponding section during crystallization only in the case of compositions taken corresponding to the quasibinary section InSb-InTe. 

Continuous fractionation is carried out in countercurrent flow. For the continuous fractionation of a solution containing two dissolved substances, a split column is used (Fig. 7-30). In the upper column section, the crystallization column, the less soluble component crystallizes during the counter-current contact of the mother liquor and the crystal. In the lower section, the concentrating or enrichment column, the more soluble component is favorably extracted from... 

In Section 11.4, it was shown how suitable solvents can be selected with the help of powerful predictive thermodynamic models or direct access to the DDB using a sophisticated software package. A similar procedure for the selection of suitable solvents was also realized for other separation processes, such as physical absorption, extraction, solution crystallization, supercritical extraction, and so on. In the case of absorption processes or supercritical extraction instead of a g -model, for example, modified UNIFAC, of course an equation of state such as PSRK or VTPR has to be used. For the separation processes mentioned above instead of azeotropic data or activity coefficients at infinite dilution, now gas solubility data, liquid-liquid equilibrium data, distribution coefficients, solid-liquid equilibrium data or VLE data with supercritical compounds are required and can be accessed from the DDB. 

Prom the series of experiments reported above it is evident that chain folding at the interphase plays an important role in packing of the chains within the crystalline lattice. In this section we aim to investigate the influence of the interphase on the melting behaviour of crystals and its implications in the polymer melt. Material investigated for the purpose is a solution crystallized UHMW-PE. Salient features on the material have been summarized in the section 4 of this chapter and details have been provided in [31]. Since the solution crystallized UHMW-PE is made from dilute solutions, the number of entanglements between the crystalline and amorphous regions is reduced to an extent that the material can be drawn in the solid state (Fig. 15.5) by more than 100 times. 

Figure 3-2 shows the cellular membrane microstructure of a P-quartz solid solution glass-ceramic. The individual phases are presented in connection with their effect on the different solid-state reactions to provide a better understanding of this microstructure. As described in Section 2.2.2, the nucleation of P-quartz solid-solution crystals is heterogeneously initiated by ZrTiO nucleating agents. These ZrHO crystals represent the nucleus of the... 

The results summarized above were obtained on solution crystallized lamellas prepared using relatively low degrees of undercooling. Spherulitic bodies are composed of lamellas but these should also include sections of chain connecting adjacent lamellas as well as having chain folds and chain ends. The number of monomer units in these connecting segments has not been determined to date. Also it is not expected that all of the noncrystalline material will be available for reaction. 

As described in previous sections, solution phase interactions play an important role in co-crystal solubility. The influence is greater than for single component crystals (or their hydrates) since each co-crystal component will modify solution behavior to different extents depending on their interactions with the environment. Kinetic studies are useful when informed by co-crystal thermodynamic solubilities and their solution phase dependence. Simply adding a cocrystal to a solution and measuring drug concentration as a function of time may fail to capture important properties of the co-crystal and lead to inaccurate assessment of its performance. 

Further investigations are, therefore, necessary to decide whether this is possible or not. In the second and third sections of the second chapter, methods are described which allow us to determine the basic parameters for a production of a glass ceramic and the development of a glass ceramic based on lithium-alumino-silicate solid solution crystals. 

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