Counter-current separation

U. von Stockar and Xiao-Ping Lu,M Simple andMccurate Short-cut Procedun toMccount forMxialDispenion in Counter-current Separation Columns, Ind. Eng. Chem. Research, 1991, in press. 

Continuous counter-current separation by SMB chromatography improves process economics. Nevertheless, more improved processes have been developed recently. They are all based on the standard SMB technology but operated under variable process conditions to reduce the costs of column hardware and stationary phase as well as for fresh eluent and eluent work-up. 

Stainthorp, F. P. and Whitehouse, P. A., General Computer Programs for Multi-Stage Counter-Current Separation Problems, The Institution of Chemical Engineers Symposium Series No. 23, 181-188, 189-192 (1967). 

The following examples illustrate important aspects of phase equilibrium data with respect to a counter current separation process. Mutual solubilities reveal the extent of the two-phase region with respect to pressure (Fig. 16). Solubilities in a certain range of pressure and temperature have to be determined (Fig. 15). Most important for the separation process are the distribution coefficients for the individual compounds and the separation factors determined from that data. As an example, the separation factor of squalene for a vSeparation from a tocopherol/sterol-mixture is shown in Figure 17 [7]. Furthermore, it is necessary to determine the dependence of the separation factor on concentration of the key-components since these vary substantially over the separation column. 

Figure 18. Simulation of concentration profiles with ASPEN+ in a multistage counter current separation. Separation of fatty acid methylesters with supercritical CO [f 9].

H. R. C. Pratt, Counter-Current Separation Processes. Elsevier, Holland, 1967. 

The SMB technique is based on the technical simulation of a true counter-current separation, for which - in complete contrast to conventional HPLC - the stationary phase is moved in a coimter current to the flow direction of the mobile phase and does not remain stationary (see Eig. 20, top). 

Fig. 20. Top idealized true counter-current separation components to be separated (feed), product streams (raffinate, extract) bottom simulation of the counter current by segregation of the column and change of the positions of inlets and outlets after a pre-defined switch time.

When the content of essential oil is low in the raw material or the aroma constituents are destroyed hy steam distillation or the aroma is lost by its solubility in water, then the oil in the raw material is recovered by an extraction process. Examples are certain herbs or spices (cf. 22.1.1.1) and some fruit powders. Hexane, triacetin, acetone, ethanol, water and/or edible oil or fat are used as solvents. Good yields are also obtained by using liquid CO2. The volatile solvent is then fully removed by distillation. The oil extract (resin, absolue) often contains volatile aroma compounds in excess of 10% in addition to lipids, waxes, plant pigments and other substances extractable by the chosen solvent. Extraction may be followed by chromatographic or counter-current separation to isolate some desired aroma fractions. If the solvent used is not removed by distillation, the product is called an extract. The odor intensity of the extract, compared to the pure essential oil, is weaker for aromatization purposes hy a factor of 1Q2 to 10 ... 

As the models of counter-current separations drawn up so far [1,2] have proven unsuitable or too complex [12] for the evaluation of the multistage cross-flow gravity separation studied here. In order to describe the process efficiency, the separation tapping model of a turbulent cross-flow hydroclassification developed by Schubert [8] and Neesse [5] was supplemented with a model for multistage turbulent cross-flow aeroseparation [16],... 

Dubant S, Mathews B, Higginson P, Crook R, Snowden M, MitcheU J (2008) Practical solvent system selection for counter-current separation of pharmaceutical compounds. J Chromatogr A 1207 190-192... 

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