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

9 Adsorption and ion exchange
9.1 Introduction

Adsorption and ion exchange are sorption operations, in which certain components of a fluid phase are selectively transferred to insoluble particles. Adsorption processes use the natural tendency of liquid or gas components to collect at the surface of a solid material. As a result selective concentration, adsorption, of one or more components, adsorbates, occurs at the surface of a (micro)porous solid, adsorbent. An adsorbent is an example of a mass separating agent, which is used to facilitate the separation. In most cases, the attractive forces binding the adsorbate are weaker than those of chemical bonds, allowing the adsorption to be reversed by either raising the temperature of the adsorbent or by reducing the concentration or partial pressure of the adsorbate. This combination of selective adsorption followed by regeneration, shown in Fig. 9.1, is the basis for a separation when more of one component is removed from a gas or liquid mixture than of the other components. In an overall process the desorption or regeneration step is very important. It allows recovery of the adsorbates when they are valuable and permits reuse of the adsorbent for further cycles. The downside of the need to regenerate the adsorbent is that the overall process is necessarily cyclic in time. Only in a few cases is desorption not practical, and the adsorbate must be removed by thermal destruction, another chemical reaction or the adsorbent is simply discarded.

In ion exchange processes, positive (cations) or negative (anions) ions from an aqueous solution are exchanged with cations or anions on a solid ion exchanger. Water softening by ion exchange involves a cation exchanger, in which calcium ions are removed by the following reaction:

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(9.1)

where R is the polymeric backbone of the ion exchanger. The exchange of ions is reversible, which allows extended use of the ion exchange resin before replacement is necessary. Regeneration is usually accomplished with concentrated acid, base, or saline solutions. In demineralization or deionization the ion-exchange concept is extended to complete removal of inorganic salts from water by a two-step process. In the first step a cation resin is used to exchange hydrogen ions for cations such as calcium, magnesium, and sodium. In the second step, an anion resin exchanges hydroxyl ions for sulfate, nitrate, and chloride anions. The hydrogen and hydroxyl ions produce water.

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Fig. 9.1: Schematic of an adsorption/desorption process.

Adsorption processes may be classified as purification or bulk separations, depending on the concentration in the feed fluid of the components to be adsorbed. Nowadays it is the most widely used nonvapor-liquid technique for molecular separations throughout a wide range of industries. Application should be considered when separation by distillation becomes difficult or expensive and a suitable adsorbent exists. By suitable we mean that the adsorbent shows proper selectivity and capacity, can be easily regenerated, and causes no damage to the products. Selectivity between key components should in general be greater than 2. A representative list of industrial applications is given in Tab. 9.1. Industrial applications of ion exchange range from the purification of low-cost commodities such as water to the purification and treatment of high-cost pharmaceutical derivatives as well as precious metals. Tab. 9.1 illustrates that the largest single application is water treatment. Other major industrial applications are the processing and decolorization of sugar solutions and the recovery of uranium from relatively low-grade mineral acid leach solutions.