Separation, energy requirement crystallization

Crystalhzation is important as an industrial process because of the number of materials that are and can be marketed in the form of crystals. Its wide use is probably due to the highly purified and attractive form of a chemical solid which can be obtained from relatively impure solutions in a single processing step. In terms of energy requirements, crystallization requires much less energy for separation than do distillation and other commonly used methods of purification. In addition, it can be performed at relatively low temperatures and on a scale which varies from a few grams up to thousands of tons per day. 

In terms of energy requirements, crystallization requires much less energy for separation than do distillation or other commonly used methods of purification. It may be carried on at relatively high or low temperatures on a scale that varies from a few kilograms up to thousands of tons per day. 

The theory of Benedek35) also must be regarded as semi-empirical. The authors treat the ys of alkali halide crystals as a sum of three terms, namely y+, y, and yb. The first component represents the energy required to separate the positive ions, and the second the analogous work for the anions. Both are calculated more or less ab initio. On the other hand, the expression for yb, i.e., the thermal contribution, has no theoretical foundation. It is... 

The purpose of seeking a concentrated strip solution is to reduce the energy required to recover the product from the strip solution. In the case of metal salts, precipitation, electrolysis, direct reduction, and a host of other techniques may be used to generate the final product. In the case of the extraction of organic compounds, distillation, crystallization, or similar separation methods are used. In each case, the more concentrated the strip solution, the less energy is required to recover the desired components. 

D is the dissociation enthalpy of Cl2,1 is the ionization potential of Na, E is the electron addition enthalpy of Cl (which is the negative of the electron affinity), and U is the lattice energy. The Born-Haber cycle shows that the lattice energy corresponds to the energy required to separate a mole of crystal into the gaseous ions, and forming the crystal from the ions represents -U. 

Substituting this value for B in Eq. (3.6) and changing signs because U is defined as the energy required to separate a mole of the crystal into gaseous ions (so it has a positive sign) yields the Born-Mayer equation,... 

Among the most energy-intensive processes in the chemical industry, the purification and separation processes stand out, whether they are carried out through distillation, re-crystallization, or ultrafiltration. By designing a process that minimizes the need for separations of this kind, one also ensures that the energy requirements (thermal, electric, or others) will decrease considerably. 

The free energy in a crystal is a minimum with respect to the arrangement of molecules and ions within it. A useful but approximate indication of the forces between components in a crystal is given by the energy needed to evaporate crystals into their separate molecules or ions. Typical values are the following for the ionic crystals lithium fluoride and sodium chloride, the energies required to break up the crystal into component anions... 

The lattice energy of a crystal is the energy required to separate the crystal into its component atoms, molecules, or ions at 0 K. In this section we examine the calculation and measurement of lattice energies for molecular and ionic crystals. 

The van der Waals lattice energy of a crystal is the energy required to disperse the structure into an assemblage of widely separated molecules and may therefore be directly compared with its heat of sublimation. Some values of this latter quantity are given in table 6.02 and it will be seen that on the whole agreement is satisfactory, especially when it is remembered that we have taken no account of the effect of the repulsive forces which must operate to confer on molecules their characteristic sizes. 

The thermodynamic requirement for crystallization in a miscible blend is that the blend exhibits a free energy on crystallization that is more negative than the free energy of the liquid-liquid mixture. A liquid-solid phase separation can occur when the miscible melt is cooled to a temperature between the glass-transition of the blend and the equilibrium melting point of the crystallizable component(s) (section 3.3.1). 

The two empirical constants b and n are determined by two conditions. First we require that the energy have a minimum value at Vq, the equilibrium separation in the crystal (dU/dr =r = 0- Differentiating Eq. (28.3), we have... 

The cohesive energy of an ionic crystal is the energy of the crystal relative to the infinitely separated ions, the energy required for the reaction... 

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