Liquid-solid crystallization

All these treatments exert their effect by controlled crystal growth, e.g. larger, fewer crystals adsorb less liquid fat and there is less formation of mixed (liquid-solid) crystals due to reduced supercooling. 

Liquid-liquid and liquid-solid equilibria also find industrial applications in liquid-liquid extraction and fractional crystallization operations. 

Richard C. Bennett, B.S., Ch.E., Registered Professional Engineer, Illinois Member, American Institute of Chemical Engineers (AIChE) President of Crystallization Technology, Inc. Former President of Swenson Process Equipment, Inc. (Section 18, Liquid-Solid Operations and Equipment)... 

Joseph D Henry/ Jr./ Ph.D./ P.E./ Senior Fellow, Deportment of Engineering and Public Policy, Carnegie Mellon University Member, American Institute of Chemical Engineers, American Society for Engineering Education. (Section Editor, Alternative Solid/Liquid Separations, Crystallization from the Melt)... 

Totally enclosed impellers are designed with the blades between two support shrouds or plates. These impellers are for totally clean liquids because tolerances are tight at the eye and the housing, and there is no room for suspended solids, crystals or sediment, see Figure 6-18. 

Sample requirements Very flexible liquids, gases, crystals, polycrystalline solids, powders, and thin films... 

Crystallization is an important separation proeess that purifies fluids by forming solids. Crystallization is also a partiele formation proeess by whieh moleeules in solution or vapour are transformed into a solid phase of regular lattiee strueture, whieh is refleeted on the external faees. Crystallization may be further deseribed as a self-assembly moleeular building proeess. Crystallo-graphie and moleeular faetors are thus very important in affeeting the shape (habit), purity and strueture of erystals, as eonsidered in detail by, for example, Mullin (2001) and Myerson (1999). In this ehapter the internal erystal strueture and external partiele eharaeteristies of size and shape are eonsidered, whieh are important indieators of produet quality and ean affeet downstream proeessing, sueh as solid-liquid separation markedly. Larger partieles separate out from fluids more quiekly than fines and are less prone to dust formation whilst smaller partieles dissolve more rapidly. 

One of the factors that influences the rate of dissolving of solid is the area, A, of the crystal surface that contacts the liquid. If many crystals (with large A) are dissolving simultaneously, the rate of dissolving is faster than if only a few crystals (with small A) are in the solvent. The rate of dissolving is proportional to this liquid-solid surface area, A. 

Liquid crystals have a degree of order characteristic of solid crystals, but they can flow like viscous liquids. They are mesophases, intermediate between solids and liquids their properties can be modified by electric fields and changes in temperature. 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

Figure 6. Vapor-liquid-solid (plastic crystal) phase diagram of adamantane. The phase transition from plastic crystal to rigid crystal phase occurs at 208.6K (l/T = 0.004794K ). This diagram is based on the data of Table II.

Although the crystals are likely to be pure, the mass of crystals will retain some liquid when the solid crystals are separated from the residual liquid. If the adhering liquid is dried on the crystals, this will contaminate the product. In practice, the crystals will be separated from the residual liquid by filtration or centrifuging. Large uniform crystals separated from a low-viscosity liquid will retain the smallest proportion of liquid. Nonuniform crystals separated from a viscous liquid will retain a higher proportion of liquid. It is common practice to wash the crystals in the filter or centrifuge. This might be with fresh solvent, or in the case of melt crystallization, with a portion of melted product. 

The sessile drop method has several drawbacks. Several days elapse between each displacement, and total test times exceeding one month are not uncommon. It can be difficult to determine that the interface has actually advanced across the face of the crystal. Displacement frequency and distance are variable and dependent upon the operator. Tests are conducted on pure mineral surfaces, usually quartz, which does not adequately model the heterogeneous rock surfaces in reservoirs. There is a need for a simple technique that gives reproducible data and can be used to characterize various mineral surfaces. The dynamic Wilhelmy plate technique has such a potential. This paper discusses the dynamic Wilhelmy plate apparatus used to study wetting properties of liquid/liquid/solid systems important to the oil industry. 

Liquid-solid distributions are involved in ion-exchange and other adsorption-based separation processes, separation processes based on crystallization or precipitation, flotation processes for ore dressing, and smelting processes. 

When a solid is in liquid solution it behaves according to its liquid state properties because it is in a liquid mixture. When applying Raoult s Law or similar expressions, the pure substance property is that of the liquid. Liquids such as crude oils and PCB mixtures consist largely of solid substances, but they are in the liquid state and generally unable to precipitate as solid crystals because of their low individual concentrations. 

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