Separation Sequencing for Solid-Fluid Systems

Absorption of components of a gas mixture into a solvent may take place by physical or chemical means. When no chemical reaction between the solute and absorbent occurs (physical absorption), the separation factor is given by Eq. (7.2). Thus, if component 1 is to be selectively absorbed, a small value of SF is desired. Alternatively, Bamicki and Fair (1992) suggest that consideration of physical absorption should be based on a selectivity, 5i 2, defined as the ratio of liquid-phase mole fractions of the two key components in the gas mixture. This selectivity can be estimated firom the partial pressures of the two components in the gas feed and their K values for the given solvent. For components whose critical temperatures are greater than the system temperature, 

The previously discussed separation techniques for gas mixtures all involve a mass separating agent. Alternatively, thermal means is employed with partial condensation and cryogenic distillation. Bamicki and Fair (1992) recommend that partial condensation be considered for enrichment when the relative volatility between the key components is 7. For large-scale ( 10-20 tons/day of product gas) enrichment and sharp separations, cryogenic distillation is feasible when the relative volatility between the key components is greater than 2. However, if the feed gas contains components, such as carbon dioxide and water that can freeze at the distillation temperatures, those components must be removed first. 

The final product from many industrial chemical processes is a solid material. This is especially true for inorganic compounds, but is also common for a number of moderate- to high-molecular-weight organic compounds. Such processes involve the separation operations of leaching, evaporation, solution crystallization (solutes with high melting points 

In both of the processes just described, a crystallizer produces a solid and, following a solid-liquid phase separation, a dryer removes the moisture. In some cases, all three of these operations can be carried out in a single piece of equipment, a spray dryer or a drum dryer, but at the expense of increased utility cost because all of the solvent is evaporated. Such dryers are used extensively to produce dried milk and detergents. For these products, spray dryers are particularly desirable, because the drying process produces porous particles that are readily dissolved in water. Spray dryers can also handle slurries and pastes. 

As discussed by Bamicki and Fair (1990), melt crystallization is an alternative to other separation techniques for liquid mixtures, including ordinary distillation, enhanced distillation, liquid-liquid extraction, adsorption, and membrane permeation. Melt crystallization should be considered only when ordinary distillation is not feasible, but may be an attractive alternative when the melting-point difference between the two key components exceeds 20°C and a eutectic is not formed. If a eutectic is formed, high recovery may not be possible, as discussed by King (1980). Methods for circumventing the eutectic limitation are discussed by Dye and Ng (1995a). 

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