Crystallization processes involved

Potassium and ammonium dichromates are generally made from sodium dichromate by a crystallization process involving equivalent amounts of potassium chloride or ammonium sulfate. In each case the solubiHty relationships are favorable so that the desired dichromate can be separated on cooling, whereas the sodium chloride or sulfate crystallizes out on boiling. For certain uses, ammonium dichromate, which is low in alkaH salts, is required. This special salt may be prepared by the addition of ammonia to an aqueous solution of chromic acid. Ammonium dichromate must be dried with care, because decomposition starts at 185°C and becomes violent and self-sustaining at slightly higher temperatures. 

The crystallization process involves a system (which we are interested in) and the surroundings. In terms of the component entropies in this example, we say AS,iSyiSlcm, is the entropy of the solute crystallizing and that A.S liSlllTOimdmgiS I represents the entropy change of the solvent molecules released. 

The design and operation of industrial crystallizers is where developments in the laboratory are confirmed and their practical significance determined. In recent years, crystallization processes involving specialty chemicals and pharmaceuticals have increased. This has led increased interest in batch crystallization operation, optimization and desigrt At the same time, the advent of powerful computers and their routine avaUabilily has stimulated interest in the area of on-line control of crystallization process (both batch and continuous). Progress in batch crystallization is surrunarized in a number of recent papers and reviews 173-801. In this section I will discuss two areas which I think will have an impact in the next decade. 

NH -TPD reveals that the crystallization process involves a post-synthesis insertion of gallium atoms into the zeolite framework (Fig 3b). After a crystallization time of 24 h only about 50 % of the gallium present in the sample is actually incorporated into the framework (Fig. 5), although the... 

Virtually all industrial crystallization processes involve solutions. The development, design, and control of any of these processes involve knowledge of a number of the properties of the solution. This chapter will present and explain solutions and solution properties, and relate these properties to industrial crystallization operations. 

Accurate solubility data is a crucial part of the design, development, and operation of a crystallization process. When confronted with the need for accurate solubility data, it is often common to find that the data is not available for the solute at the conditions of interest. This is especially true for mixed and nonaqueous solvents, and for systems with more than one solute. In addition, most industrial crystallization processes involve solutions with impurities present. If it is desired to know the solubility of the solute in the actual working solution with all impurities present, it is very unlikely that data will be available in the literature. Methods for the calculation of solubility have been discussed previously. These can be quite useful, but often are not possible because of lack of adequate thermodynamic data. This means that the only method available to determine the needed information is solubility measurement. 

In crystallization processes involving a material that displays polymorphism, it is quite common for an unstable polymorph to appear first and then transform into a stable form. This observation is summarized by Ostwald s step rule, sometimes referred to as the Law of Successive Reactions, which says that in any process, the state which is initially obtained is not the stablest state but the least stable state that is closest in terms of free energy change, to the original state. What this means, therefore, is that a crystallization process, the initial solid phase, can be the least stable polymorph that will then transform into successively more stable forms until the stable form, at the conditions of the system, is reached. With some systems this can mean the formation of an... 

Fitch, B. (1976) Design of fractional crystallization processes involving solid solutions. AIChE, Symposium Series, No. 153, 72, 79-86. 

As in the case of small molecules, the crystallization process involves nucle-ation and diffusion of the relevant entity to the surface site. The formation of the critical nucleus will be controlled by thermodynamics. The total change in free energy AG is the sum of contributions from the bulk and surface energies. Using the convention which we introduced in Figure 6.1, where cr,- is the specific surface energy of surface i and At is the area, then the free energy can be described by... 

It has been proposed that the following equation can be used to describe the temperature dependence of both diffusive transport and nucleation. The overall crystallization rate (wc) at a general temperature is a combination of several factors. The crystallization process involves the diffusion (the first... 

The ratios of AojB and AjB are independent of i//. The absolute rate constants are dependent on ij/ and the crystallization process involves subsequent addition of stems 1,2,. .., nto the substrate surface. If it is assumed that a steady-state condition is developed then ... 

The crystallization process involves two basic steps, nucleation and crystal growth. Nucleation starts with small, nanometer-sized areas where, as a result of heat motion, some chains or their segments arrange parallel to one another. Those nuclei, or seeds , can either dissociate, if thermal motion destroys the molecular order, or grow further, if the nuc-... 

The crystallization process is a supramolecular analog of molecular reactions. Zhang etal have considered the thermodynamics of the heterogeneous crystallization process involving a metal ion and organic ligand. 

The discussion of chains in statistical conformation is based on the properties of individual, isolated chains. Except for crystallization from very dilute solution the crystallization process involves a collection of such chains. The question can... 

The crystallization of oriented PET from the glassy state is known as strain-induced crystallization. The crystallization process involves rotation, alignment, and perfection of the internal order of the paracrystalline structures [15]. The purpose of many investigations have been to study the kinetics of strain-induced crystallization of PET during annealing of oriented samples [16,17],... 

L.H. Tung If that s the case, then the random coil should stay as a random coil. There s no folding that should be involved because there s no crystallization process involved - the kinetic effect of crystallization. How could it exist in the random coil, actually, in the liquid state Note that I didn t say amorphous state but, rather, liquid state. 

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