Adsorption, Liquid Separation

Abstract

AbstractLiquid‐phase adsorption has long been used for the removal of contaminants present at low concentrations in process streams. In most cases, the objective is to remove a specific feed component; alternatively, the contaminants are not well defined, and the objective is the improvement of feed quality defined by color, taste, odor, and storage stability. In contrast to trace impurity removal, the use of adsorption for bulk separation in the liquid phase on a commercial scale is a relatively recent development; this article is devoted mainly these liquid‐phase bulk adsorptive separation processes. A significant advantage of adsorbents over other separative agents lies in the fact that favorable equilibrium‐phase relations can be developed for particular separations; adsorbents can be produced that are much more selective in their affinity for various substances than are any known solvents. A practical adsorbent has four primary requirements: selectivity, capacity, mass transfer rate, and long‐term stability. The requirement for adequate adsorptive capacity restricts the choice of adsorbents to microporous solids with pore diameters ranging from a few tenths to a few tens of nanometers. Zeolites, crystalline aluminosilicates containing an assemblage of SiO4and AlO4tetrahedra joined together by oxygen atoms to form a microporous solid, offer increased possibilities for exploiting molecular‐level differences among adsorbates. The versatility of zeolites lies in the fact that widely different adsorptive properties may be realized by the appropriate control of the framework structure, the silica‐to‐alumina ratio (Si/Al), and the cation form. Industrial‐scale adsorption processes can be classified as batch or continuous. The efficiency of an adsorption process is significantly higher in a continuous mode of operation than in a cyclic batch mode. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, whereas a continuous mode allows more degrees of freedom with respect to these functions, and thus a better overall operation. Continuous processes have wide application in different areas of the chemical industry. However, in some applications the cyclic‐batch process may be preferred because of the complexity of design and the difficulty of controlling the continuous processes. Examples of commercial cyclic‐batch adsorption processes include the UOP methanol recovery (UOP MRU) and oxygenate removal (UOP ORU) processes. Conventional liquid chromatography has not attained great commercial significance in the area of large‐scale bulk separations from the liquid phase. Liquid adsorption processes hold a prominent position in several applications for the production of high purity chemicals on a commodity scale, and they continue to increase in value as improvements are made.