Design of Fluid-Solid Extractions

In the food industry, commercial plants with supercritical fluids have so far only been used with supercritical CO2 as solvent. The first plant was opened in 1981 in Bremen (Germany) for decaffeination of coffee, by a process invented in the 1970s by Zosel (1973). Plants for the production of hop extract and for the decaffeination of tea are today also in operation, for example, in Germany, England, and Australia (Voeste et al., 1997). Supercritical hydrocarbons such as propane are also used, for example, for deasphalting of heavy oils or for the removal of triglycerides from fish oils. 

To design such a process, the McCabe-Thiele method may be used to determine the number of theoretical separation stages, as examined in Sections 3.3.2-3.3.4 for distillation, absorption (gas scrubbing), and liquid-liquid-extraction. Thus, we obtain the number of theoretical extraction stages of a countercurrent extraction column based on the equilibrium curve (solubility of extract in the solvent for a given content in the solid) and the operating line. The latter depends on the extract content of the solid feed and residue, and on the in- and outlet extract concentration in the solvent The extract content of the feed is fixed, and the value of the residue is specified by the required degree of extraction. The inlet content of the extract in the solvent is also fixed, as either pure solvent is used or the value is specified by separation of the extract from the used solvent after the extraction. Therefore, the only parameter that is left is the outlet concentration of the extract in the solvent, which depends on the ratio of the solvent flow to the feed rate of the solid feedstock (mass balance). 

800 mm, effective length 12 m, operating pressure 500 bar). Courtesy of Uhde, Germany. 

Crystallization is mainly used for separation as an alternative to distillation, if the involved compounds are thermally unstable (e.g., acrylic acid), have a low or practically no vapor pressure (like salts), if the boiling points are similar, or if the system forms an azeotrope. Crystallization is used for the production and purification of various organic chemicals ranging from bulk chemicals (p-xylene and naphthalene) to fine chemicals like pharmaceuticals (e.g., proteins). Further examples of industrial crystallization processes are sugar refining, salt production for the food industry, and silicon crystal wafer production. 

The basic thermodynamic equation of crystallization for an ideal hquid and solid phase is  

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