The sequential adsorption or ion exchange operation is limited to treatment of solutions where the solute to be removed is adsorbed relatively strongly when compared with the remainder of the solution. This is often the case when colloidal substances are removed from aqueous solutions using carbon, as in the production of process water.
The method for dealing with the spent adsorbent or ion exchanger depends upon the system under consideration. If the material taken up is valuable (e.g., proteins), it might be desorbed by contact with a solvent other than water. If the removed component is volatile (e.g., ammonia), it may be desorbed by reduction of the partial pressure of the adsórbate over the solid by passing steam or air over the solid, i.e., air or steam stripping is applied; see also Section 9.2. In the case of most sequential operations in the context of waste water treatment, the adsórbate is of no value and it is not easily desorbed. The adsorbent may then be regenerated by burning off the adsórbate, followed by reactivation.
A mathematical treatment of the sequential operation distinguishes between single-stage operations, multi-stage cocurrent operations and multi-stage countercurrent operations. The mathematical treatment does not distinguish adsorption from ion exchange - the basic equations are the same. A schematic flowchart for a single-stage operation is shown in Fig. 9.9.
As the amount of ion exchanger is usually very small compared with the amount of solution treated and since the solutes to be removed are taken up much more strongly than the other components present, the up take of the latter may be ignored. Furthermore, the ion exchanger is generally insoluble in the solution. If the water (see Fig. 9.9) to be treated contains S kg of unadsorbed substance (water) then the adsorbable solute concentration is reduced from Yoto Yi kg of solute per kg of solvent.
A : adsorbent or Ion exchanger Xo : adsorbate conc. = 0 for fresh absorbent
S : solvent Yo : adsorbate conc.
X1 : adsorabte conc, after adsorption or ion exchange
Y1 : adsorbate conc. after adsorption or ion exchange
Figure 9.9 Flowsheet for the single-stage operation. Application of the mass conservation principle for the component removed from S to A leads to equation (9.8).
If the adsorbent (ion exchanger) added is A kg, then the solid adsorbate content increases from Xo to X1 kg of solid per kg of adsorbent. In most cases fresh adsorbent is used so that Xo = 0. The mass balance of the solid removed is given by the following equation:
This equation gives the so-called operating line, shown in Fig. 9.10 together with the equilibrium curve. This could be either Freundlich's or Langmuir's isotherm or equation (9.7), which may be considered a modified Freundlich's isotherm.
It is presumed in Fig. 9.9. that the solvent and adsorbent can be separated completely after the ion exchange process, which is not always the case in practice. The presence of solvent in the used ion exchanger may not to interfere with the further treatment of the adsorbent or it may be possible to remove the solvent by drying or other processes. It is under all circumstances important to consider this problem in the application of adsorption and ion exchange processes in practice.
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