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Liquid chromatography and adsorption processes are based on the differential affinity of various soluble molecules for specific types of solids. In these processes, equilibrium is approached between a solid phase, often called the resin, or stationary phase, and the soluble molecules in a liquid phase. The solid phase is “stationary” because it is often packed in a fixed column. Since the liquid phase is often flowing past the solid phase, it is referred to as the mobile phase. Chromatography and adsorption are related unit operations. In chromatography, typically multiple solutes are separated from each other, with the target product solute being one of many that might be recovered at the end of the process step. In adsorption, there are typically only three groups of solutes: those that do not adsorb to the stationary phase (sometimes called “flow through”); secondly, those that adsorb and then are subsequently recovered by an elution step; and thirdly, those solutes that are nearly irreversibly bound and can only be removed from the adsorbent by regenerating the adsorbent, which usually results in the chemical destruction of these solutes. The word “adsorption” is used both to describe the physical adherence of a solute to a stationary phase, and as the name of the unit operation described above. Adsorption is a subset of the “sorption” phenomena, absorption (transfer of a solute from one phase into another) and ion exchange (exchange of a counter-ion between two opposing co-ions) being the other two sorption phenomena. The unit operations chromatography and adsorption can rely on any of these three sorption processes individually or in combination. In chromatography and adsorption, a mixture of solutes in a feed solution is introduced at the inlet of a column containing the stationary phase and separated into zones of individual solutes over the length of the column. The solutes are carried by the convective action of an elution solvent that is continuously fed to the column after the feed solution has been introduced. |
