Separation, energy requirement liquid-solids

Before equations such as Eqs. 6, 7 and 8 can be used, values for the surface energies have to be obtained. While surface energies of liquids may be measured relatively easily by methods such as the du Nouy ring and Wilhelmy plate, those of solids present more problems. Three approaches will be briefly described. Two involve probing the solid surface with a liquid or a gas, the third relies on very sensitive measurement of the force required to separate two surfaces of defined geometry. All involve applying judicious assumptions to the experimental results. 

There is a real opportunity to reduce biodiesel production costs and environmental impact by applying modem catalyst technology, which will allow increased process flexibility to incorporate the use of low-cost high-FFA feedstock, and reduce water and energy requirement. Solid catalysts such as synthetic polymeric catalysts, zeolites and superacids like sulfated zirconia and niobic acid have the strong potential to replace liquid acids, eliminating separation, corrosion and environmental problems. Lotero et al. recently published a review that elaborates the importance of solid acids for biodiesel production. ... 

Unfortunately the authors argue that they were performing mechanochemical reactions with mechanical energy input for the salt formation or complexation to occur, rather than just creating the required contacts between reacting crystals. Furthermore, they did not exclude moisture, reported intermediate liquid phases in various cases, and did not separate out any real solid-state reactions that might have been achieved. It is therefore not possible to discuss the results in more detail here. 

Mindful of the energy requirement for dispersion of a polysaccharide solute in water, syneresis is the slow, spontaneous separation of liquid from a gel, as the solid phase attempts to return to its energy ground state. This phenomenon is a quality defect, because it foreshadows solute sedimentation. 

When a solid or liquid dissolves, the structural units—ions or molecules— become separated from each other, and the spaces in between become occupied by solvent molecules. In dissolution, as in melting and boiling, energy must be supplied to overcome the interionic or intermolecular forces. Where does the necessary energy come from The energy required to break the bonds between solute particles is supplied by the formation of bonds between the solute particles and the solvent molecules the old attractive forces are replaced by new ones. 

Answer (a) Endothermic. It requires heat energy to separate molecules in a solid to form a gas. (b) Endothermic. It requires heat energy to overcome some of the intermolecular forces in a solid to form a liquid, (c) Endothermic, for the same reasons given in (a), (d) Exothermic. Energy is released as intermolecular forces form when the gaseous molecules pack together in the solid. 

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). 

For the separation of solids from liquids, filtration and sedimention can be considered. The energy required for separation can be introduced by a centrifugal field, gravity, and, in the case of filtration, by application of under- or overpressure. 

Prediction of the adhesion forces lies beyond the scope of the present work however, certain observations are possible. Initially, deformation in a pressure drop field occurs, satisfying equation (4). Then, non-spherical deformation can occur whenever a ganglion is touching either the solid spheres or the overlying surface since there is adhesion between the ganglion and the solid. Separation of the two phases and transformation of the air-solid interface into separate air-liquid and liquid-solid interfaces requires an increase in the Gibbs free energy which, at constant temperature and pressure, becomes the work of adhesion per unit area, i.e.. 

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