S

Figure 35. Sequence of operation of the Cloete-Streat countercurrent ion exchange process.'931

Solution Itid

Figure 36. Schematic diagram of the USBM ion exchange column.[94]

Solution Itid

Figure 36. Schematic diagram of the USBM ion exchange column.[94]

The principal disadvantage of fluidized bed columns is the mixing of resin in various stages of utilization. This mixing means that breakthrough occurs sooner and the degree of resin capacity utilization is much lower than in packed bed columns. Byplacingperforatedplatesinacolumn(Fig. 37),[95] the resin beads only mix within a restricted area allowing more complete utilization of the ion exchange resin's capacity.

ion exchange bead fine solid particle in feedstream

O'O OOO

Figure 37. The perforated plates in this fluidized bed column allow fine solid particles to flow through the column while the ion exchange beds are, for the most part, confined to individual compartments.1951

Smaller continuous fluid bed systems, like the one shown inFig. 38,[96] have been developed which operate with a high concentration of ion exchange resin and suspended solids. These units are 80% smaller than the conventional resin-in-pulp plants of the type which are used in the treatment of uranium ore slurries.[97] The pilot plant unit, which would probably be the size needed for processing commercial fermentation broths, had dimensions per contact chamber of0.82mx0.82m with a fluid bed height of 0.82 m and an additional 0.16 m for free board. The unit has been successfully operated with 25 to 50% resin and up to 45% suspended solids.

Figure 38. Pilot plant for resin-in-pulp contactor unit.[96]

The effect of the degree of regeneration of the resin on the degree of extraction of a solute was measured by Slater[98] using a seven stage unit. The results are shown in Fig. 39 for the extraction of uranium using a fluidized bed slurry of a strong base resin in a 10% uranium ore leach slurry.

However, the fluidized bed column, even with perforated plates separating it into as many as 25 compartments, may not be appropriate for applications in which there are strict requirements (<1% of influent concentration) on the effluent. For readily exchangeable ions, the optimum utilization of this technique occurs when an actual effluent concentration of 5% of the influent is the breakthrough point. At such times, the ion exchange resin capacity would be 70% utilized.[95] Should it be acceptable that the breakthrough point occur when the average effluent reaches a concentration of 10% of the influent, the utilization of the ion exchange capacity is 90%. If the ions are not readily exchangeable (low selectivity), the resin utilization would be significantly less and fluidized bed operations should not be used.

Figure 39. Effect of residual feedstream ions on the resin on the efficiency of a staged system. QP(l) is the concentration of the feedstream ions on the resin at the end of the cycle; QP(N) is the concentration of the feedstream ions on the resin at the beginning of the cycle; q is the cycle time; x is the fraction of the resin bed volume in a stage which is removed per cycle.[98]

6.4 Elution/Regeneration

Elution of proteins from ion exchangers can be achieve with buffers containing salts such as sodium chloride or ammonium acetate or by an appropriate pH change, provided that the pH change does not result in denaturation of the eluted protein.[99] The elution may be performed with a series of stepped changes or with a continuous gradient change in the eluting power of the eluant. With such changes, it is possible to separate different proteins or protein fractions from each other based on their different affinities for the ion exchange resin.

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