Column Operation

There are several types of column operation, classified according to the technical design of the apparatus

Figure 4 Batch ion exchange: calculated degree of conversion from the B# form to the A# form as a function of solution volume to exchanger mass ratio (batch factor). Selectivity coefficient Aa/b = 1, ion exchange capacity 2 mmol g 1. The solid line represent the conversion in single batch equilibration. The broken line represents successive batch equilibrations with a constant batch factor of 10 mL g"1.

Figure 4 Batch ion exchange: calculated degree of conversion from the B# form to the A# form as a function of solution volume to exchanger mass ratio (batch factor). Selectivity coefficient Aa/b = 1, ion exchange capacity 2 mmol g 1. The solid line represent the conversion in single batch equilibration. The broken line represents successive batch equilibrations with a constant batch factor of 10 mL g"1.

(e.g. fixed-bed or floating-bed operation) or according to the purpose of the application (e.g. column chromatography or column separation).

Column separation Column separation usually involves elimination of undesirable ions from water (deionization, softening, decontamination). Taking the binary exchange discussed earlier as an example, a solution containing harmful ions (A) is passed through the column that contains an exchanger in the B form. The A ions are then taken up by the exchanger and B ions are released into the solution. Because B ions are constantly removed from the system, the operation is much more efficient than batch exchange in removing A ions from the solution (see eqn [18]). The column effluent is first free of ion A, but when a given amount of solution has been passed through, A starts to emerge in the effluent and its concentration increases gradually to that in the influent solution (Figure 5). The graph of the concentration of A in effluent as a function of effluent volume is called the breakthrough curve.

The area above the breakthrough curve gives the total volume of solution that has been freed from ion A. Dividing this volume by bed mass or volume gives the total processing capacity, or theoretical capacity (Qt), of the column (L kg"1 or L L"1). In this simple example, QT = Q/CA (Q = ion exchange capacity in mmol L "1 or mmol mL "1) since at equilibrium A ions have taken up all of the ion exchange capacity. In general, QT is equal to kd, which can be easily calculated in binary systems from eqn [9] for trace ions to be separated, provided that selectivity coefficients are known. However, because ion A is considered harmful, operation is not continued until total processing capacity has been used, but the feed is discontinued when the concentration of A in the effluent reaches a measurable or a regulated value. The capacity at this point is called the processing capacity, or breakthrough capacity (QB). The ratio Qb/Qt is called the column utilization factor, FU. For efficient separation process FU should be maximized.

Column chromatography In column chromatogra-phy ions are separated from each other for analysis or for chemical production purposes. Considering a simple example in which ions A and C are separated for analysis, a sample solution containing A and C is passed into the column containing an exchanger in the B form. The sample volume is so low that A and C take up only a very small fraction of the column capacity near the inlet. After sample injection, an eluent solution containing ion B is passed through the column. A and C in the exchanger are exchanged for B and begin to move through the column at different velocities. At a given volume, the less preferred ion A first emerges in the eluent as a concentration peak followed by ion C. The eluent volumes at which A and C emerge, i.e. the volumes at the peak maxima, are called the retention volumes (VR) and they can be obtained from the relation:

Figure 5 Examples of column breakthrough curves generated for different number of theoretical plates N(see eqns [22]-[27]). In this example, the capacity of the exchanger (Q) is 1 mmol mL~1 and the exchanger bed volume is 1 mL. The exchanger is initially in the B# form and the feed contains only ion A# at a concentration of 0.001 mmol mL~1 (CA). The total processing capacity Q is thus Q/Ca = 1000 mL mL~1 exchanger and the area above the breakthrough curves is 1000 mL for the 1 mL bed. The breakthrough capacity QB depends on N, which is affected by the operating conditions. Continuous line, N = 30; broken line, N = 10.

Figure 5 Examples of column breakthrough curves generated for different number of theoretical plates N(see eqns [22]-[27]). In this example, the capacity of the exchanger (Q) is 1 mmol mL~1 and the exchanger bed volume is 1 mL. The exchanger is initially in the B# form and the feed contains only ion A# at a concentration of 0.001 mmol mL~1 (CA). The total processing capacity Q is thus Q/Ca = 1000 mL mL~1 exchanger and the area above the breakthrough curves is 1000 mL for the 1 mL bed. The breakthrough capacity QB depends on N, which is affected by the operating conditions. Continuous line, N = 30; broken line, N = 10.

where VS is the volume of ion exchanger bed and VM is the free solution volume in the bed. In analytical separations A and C are present at trace levels, so kd values are again easily calculated from eqn [9]. In analytical work, efficient operation requires that the concentration peaks of A and B are well separated (the peaks are sharp). The retention volumes VR should not be too large, because this leads to a long analysis time and to broadening of the peaks.

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