Thermodynamics of solvent selectivity in extractive distillation of hydrocarbons

Abstract

P J P O = dimensionless. temperature r. for nonreacting system, = dimensionless velocity, U (r,/p")l'z = dimensionless distance, ( r J p n ) d P 0 Y Subscripts a n d Superscripts b 0 = subscript denoting bulk mean = subscript denoting wall con-conditions ditions -= molal basis = value in reacting systemThe selectivity of a polar solvent is expressed in terms of an approximate theory of solutions. The theoretical results are insufficiently precise for the accurate prediction of activity coefficients, but the analysis shows that i n the absence of chemical effects selectivity depends primarily on the difference i n molar volumes of the hydrocarbons to be separated and on the polar energy density of the solvent. The effectiveness of o solvent i s related to its polarity (which should be large) and t o i t s molecular size (which should be small). In cases where chemical effects are important or where the molar volumes of the hydrocarbons t o be separated are only slightly different, selectivity olso depends on the relative ability of the hydrocarbons i n acting as electron donors and on the ability of the solvent t o act as an electron acceptor in forming acid-base complexes. The theoretical conclusions, which are based on modern thermodynamics and on the theory of intermolecular forces, are i n agreement with experimental observations. The results obtained i n this work provide theoretical criteria for the selection of an optimum solvent for a given separation and phenomena related to extractive distillation.

Extractive Distillation: A Review

“…Therefore, the addition of a polar solvent enables facile separation by distillation of certain mixtures which otherwise can only be separated with difficulty. The Prausnitz and Anderson theory (Prausnitz and Anderson, 1961) tries to explain the solvent selectivity in extractive distillation of hydrocarbons from the viewpoint of molecular thermodynamics and intermolecular forces. The interaction forces between the solvent and the component are broadly divided into two types, i.e.…”

Solvent Selectivity for Hydrocarbons with Close Molar Volumes

“…For such systems it has been recommended (Prausnitz and Anderson, 1961) that selectivity could be enhanced through chemical effects.…”

“…The solvent selectivity is the result of two effects: physical and chemical (Prausnitz and Anderson, 1961). The Physical effects result from polar, dispersion and induction contributions.…”

Physical and chemical forces in solvent selectivity for hydrocarbons

“…On the basis of a simplified model it was shown (13) that when the polar solvent is present in excess, the selectivity (and hence the relative volatility) can be expressed in terms of a polar, a dispersion, and an induction contribution. It was further shown that in the absence of chemical forces the polar term was the dominant one whenever there was a significant difference in the sizes of the two hydrocarbons to be separated; the model predicts that in the absence of chemical forces the larger hydrocarbon will have the larger activity coefficient and that the logarithm of the selectivity is proportional to the square of the polar solubility parameter of the extracting solvent.…”