Desorbent Critical Characteristics

Like the criteria for adsorbent selection, there are criteria for desorbent selection. For the Molex process this is covered by the following five main performance 

Selectivity is a relative term that means a desorbent must have balanced strength or selectivity for the adsorbent relative to the desired feed normal paraffins. A desorbent that is more strongly held by the adsorbent than the feed normal paraffins will be difficult for the desired feed normal paraffins to displace from the adsorbent This results in poor adsorbent utihzation due to a loss in selective volume. At the same time, a desorbent carmot be too weakly attracted to the selective volume (relative to the feed normal paraffins) or excessive desorbent circulation is needed to displace the feed component from the adsorbent. In this situation, excessively high desorbent rates are needed to recover feed normal paraffins from the adsorbent These high desorbent flush rates result in non-economical process conditions due to excessively high utility consumption in post-Sorbex product fractionation. 

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The properties of fluids under supercritical conditions are considered ideal for extracting substances from exhausted activated carbons. Two supercritical fluids are of particular interest, carbon dioxide and water. Carbon dioxide has a low critical temperature of 304 K and a moderate critical pressure of 73 bar, while water has a critical temperature of 647 K and a critical pressure of 220 bar. The character of water at supercritical conditions changes from one that supports only ionic species at ambient conditions to one that dissolves paraffins, aromatics, gases and salts [65]. These supercritical fluids exhibit densities similar to those of liquids (high solvent strengths) and diffusion coefficients similar to those of gases (excellent transport characteristics), enabling them to effectively dissolve and/or desorb contaminants from the carbon surface and to easily enter/exit even the smallest pores and carry away any... 

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