Impurities crystallization conditions influence

Moreover, scale up of the data obtained is virtually impossible, mainly due to different hydrodynamic conditions in the pi-lot-scale plant and the technical plant, impurities in the technical crystallizer which influence nucleation and crystal growth, and different heat transfer coefficients caused by different degrees of incrustation (or fouling) in pilot-scale and technical plants. Different models plug flow, macromixing, etc.) are discussed in [7.38] which consider deviations from ideal behavior in an MSMPR crystallizer. Results of scale up for this type of crystallizer are given in [7.48]. 

The incorporation of impurities into the lattice by the formation of partial or complete solid solutions cannot be influenced by process conditions. In this respect, this is the limit of the purification effect attainable. Contrarily, the incorporation of impurities by 3D inclusion or adhesion to mother liquor in or onto the crop can be influenced by modifying the grovrth, that is, crystallization conditions or the workup conditions in downstream processes. In the case of partial or complete solid solutions, the purification of a single-step crystallization is limited, higher purities can be obtained by fractional crystallization techniques. 

To account for deviation from thermodynamics, under real conditions an effective distribution coefficient ke F is defined (Equation 7.3). The equation is similar to Equation 7.2, but the effective distribution coefficient results from parameters measured under real crystallization conditions. That is, the parameters Xir,ip s as the impurity content in the solid phase and ximp.i, as the impurity content in the liquid phase are values obtained from the separation process performed. In contrast, the parameters in Equation 7.2 are directly related to the phase diagram. Thus, the effective distribution coefficient also comprises the influence of the crystallization kinetics, in particular the crystal growth rate and mass transfer limitations. 

As already stated, the crystallization conditions have a significant influence on the product purity achieved. For example, the crystallization conditions that control the growth rate particularly affect the content of impurities originating from liquid inclusions and lattice incorporation. 

In development of a new manufacturing process, studying and minimization, that is management of the impurities, is an important issue since the purity of the final product has to meet the purity specifications defined. Therefore, the influence of the crystallization conditions and the downstream processing techniques on product purity have to be studied. This issue has been addressed in Ref [14], which will be used here to suggest a workflow for impurity management in process development. The tasks that have to be handled are as follows ... 

The equilibrium concentration of free valencies in the crystal and on its surface depends not only on the nature of the crystal, but also on external conditions it increases with a rise in temperature and may be artificially increased or decreased under the influence of external agents (illumination, impurities, etc.). 

Factors that influence growth of sucrose crystals have been listed by Smythe (1971). They include supersaturation of the solution, temperature, relative velocity of crystal and solution, nature and concentration of impurities, and nature of the crystal surface. Crystal growth of sucrose consists of two steps (1) the mass transfer of sucrose molecules to the surface of the crystal, which is a first-order process and (2) the incorporation of the molecules in the crystal surface, a second-order process. Under usual conditions, overall growth rate is a function of the rate of both processes, with neither being rate-controlling. The effect of impurities can be of two kinds. Viscosity can increase, thus reducing the rate of mass transfer, or impurities can involve adsorption on specific surfaces of the crystal, thereby reducing the rate of surface incorporation. 

The formation of the nickel excitons results in the lattice distortion near them and the induced lattice vibrations. Conditions of their occurrence are defined by the charged impurity because the removed hydrogen-like type carrier practically does not influence the deformation of the lattice near the charged impurity. Thus, our analysis of the vibrational background of the zero phonon line of the EA spectrum of the nickel exciton is based on results of a simulation of the lattice dynamics of the ZnO crystal with NE or Ni ions. 

The study of crystallization and alteration processes of natural and artificial snow crystals has been a subject of interest for several decades. Nakaya was the first who investigated the relations between growth forms and experimental conditions (temperature and water vapour saturation relative to ice). Although many laboratory" " and theoretical studies on ice crystals have been carried out since his seminal work, snow crystal growth is still not completely understood. Experimental studies are challenged by the complexity of the physical processes influencing the crystal growth, which are further complicated by effects of chemical impurities. ... 

Under most conditions, the shape (i.e., the habit) of a crystal is determined by kinetics rather than thermodynamics and the resulting habit is termed the growth, as opposed to the equilibrium, habit. The kinetics of each crystal face can be influenced by such external factors as supersaturation, temperature, and mixing. However, the complexity of crystallization operations arises from the fact that the mass transfer processes at the crystal interface, as opposed to the bulk, are often rate-determining. Thus, without consideration of the role of the interface during crystallization it is impossible to predict the influence of impurities or solvents on crystallization, or explain such diverse processes as secondary nucleation and inclusion formation. 

With the use of the Burton-Slichter-Prim model, the influence of crystallizer operating conditions on the amount of substitutional impurity uptake can be assessed. Namely, for values of K less than unity, the quality of separation tends to improve with lower growth rate, increasing diffusivity of the impurity, and decreasing diffusion film thickness. Thus, stronger convection (which reduces the boundary layer thickness 6) is expected to improve separation in these cases (Rosenberger 1986). 

These influences make for uniformity and continuity in the distribution of matter, but an opposing tendency is also observable. Ordinary crystals consist, in general, not of single perfect blocks but of a mosaic of smaller ones with planes of easy fracture between them. This structure is partly the result of the conditions prevailing during growth. A minute inclusion of impurity— which is never... 

Controlled crystallization, coupled with some form of classifying action in the crystallizer, helps to produce crystals of uniform size. The production of granular crystals, however, may demand the careful control of other conditions of crystallization to modify the crystal habit the rate of cooling, the degree of supersaturation and the pH of the crystallizing solution can exert considerable influence. The deliberate addition of traces of impurity, in the form of active ions or surface-active agents, may also help to produce the right type of crystal. 

The challenge to the theories is to be able to predict the various polymorphs— different morphologies that can arise when crystals are grown under different conditions.The primary factors influencing polymorphism are solvent, temperature and impurities, i.e. habit modifiers. The recent use of computer predictions has allowed the development of a greater insight into the detailed factors that control the formation of particular morphologies. 

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