Process Technology: An Introduction - Haan A.B. 2015

9 Adsorption and ion exchange
9.5 Ion-exchange processes

Ion-exchange cycles consist of two principal steps: adsorption and regeneration. During adsorption impurities are removed or valuable constituents are recovered. Regeneration is in general of much shorter duration than the adsorption step. Industrial systems may be batch, semicontinuous, or continuous. They vary from simple onecolumn units to numerous arrays of cation and anion exchangers, depending upon the required application and quality of the effluent. A single-column installation is satisfactory if the unit can be shut down for regeneration. However, for the processing of continuous streams, two or more columns of the same resin must be installed in parallel. Most ion exchanger columns are operated concurrently with the process stream and regeneration solution flowing down through the resin bed to avoid possible fluidization of the resin particles.

Deionization processes require two columns, one containing a cation exchanger and one an anion exchanger. The cation exchanger unit must be a strong acid-type resin, and it must precede the anion exchange unit. Placing the anion exchanger first generally causes problems with precipitates of metal hydroxides. A column containing a mixture of cation and anion exchanger resins is called a mixed bed. The majority of the used resins consist of strong acid cation and strong base anion exchangers. Mixed bed systems yield higher quality deionized water or process streams than when the same resins are used in separate columns. However, regeneration of a mixed bed is more complicated than of a two-bed system.

Fig. 9.19: Higgins moving bed contactor. Adapted from [7].

Fig. 9.20: Himsley fluidized bed contactor. Adapted from [7].

To increase resin utilization and achieve high ion exchange reaction efficiency, numerous efforts have been made to develop continuous ion exchange contactors. Two examples of them are shown in Figs. 9.19 and 9.20. The Higgins loop contactor operates as a moving packed bed by using intermittent hydraulic pulses to move incremental portions of the bed from the contacting to the regeneration section. In other continuous systems, such as the Himsley contactor, columns with perforated plates are used. The liquid is pumped into the bottom and flows upward through the column to fluidize the resin beads on each plate. The flow is reversed periodically to move incremental amounts of resin to the stage below. The resin at the bottom is lifted via the wash column to the regeneration column. A more recent approach involves the placement of a number of columns on a carousel that rotates constantly at an adjustable speed.

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