Binary eluent adsorption behavior

It is essential that while setting the conditions for the differential mass-balance equation we did not define the function of the excess adsorption isotherm. We can now use the expression (2-46) for measurement of the model independent excess adsorption values. It is convenient to use it for the study of the adsorption behavior of binary eluents [22]. 

Equation (2-46) is applicable for the description of adsorption behavior of binary eluent in the column. In the isocratic mode, the column is equilibrated at given composition of the eluent. Any small volume of the mixture of eluent... 

A chromatographic system consisting of the binary eluent (e.g., acetonitrile-water) with liophilic salt added in low concentration (not more than lOOmM), along with basic analyte, is considered here. Adsorption behavior of acetonitrile in the column has been discussed in Section 2.12, and we assume that low concentrations of liophilic salt additives and injection of small amount of the analyte does not noticeably disturb its adsorption equilibrium. 

As mentioned above, in each section of the SMBR, a specific task must be fulfilled and certain binary selectivities can be chosen as key adsorption parameters to explain the separation behavior in each section. In section I, the heavy product (water) must be desorbed by the eluent (methanol) from the ion-exchange resin, and therefore a low selectivity between water and methanol is desirable in order to reduce the methanol flow rate in this section as much as possible. In section IV, the fight component (methyl acetate) is removed from the eluent stream, and consequently a low-selectivity methanol/methyl acetate is advantageous. In sections II and III, the products water and methyl acetate are separated and a high-selectivity water/ methyl acetate is therefore beneficial. The selectivity of acetic acid to methyl acetate and the water/acetic acid selectivity should also be high in order to permit an easy separation of acetic acid from the products - which in turn allows the residence time of acetic acid to be increased and the conversion to be raised. 

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