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Elution of compounds such as penicillin with either acids or bases will render the penicillin inactive. Although aqueous salt solutions can elute penicillin without inactivating it, the large volumes required make this option impractical. Wolf and co-workers'100' developed an elution solvent combination of organic solvent, water and salt that can elute the penicillin with a minimum volume and no inactivation. The mixture of organic solvent and salt is chosen so that the salt is soluble in the resulting organic solvent-water mixture and the organic substance eluted from the resin is soluble in the elution mixture. Table 19 shows the elution volume required to recover the indicated amount of antibiotic when the elution solvent is 70% methanol and 5% or 7.5% ammonium chloride in water.

Table 19. Amount of Antibiotic Recovered with Increasing Volumes of Methanol in Aqueous Ammonium Chloride'1001

Antibiotic Eluant Total Volume Amount of of Eluant Antibiotic Recovered (Bed Volumes) (%)

Dihydronovobiocin 70%MeC)H 1 50.0

with 5%NH4C1 2 83.3

Novobiocin 70%Me()H 3.5 99.8

with 5%NH4C1

Penicillin 70%MeC)H 0.5 11.0

Regeneration alone is not sufficient to prevent fouling or microbial growth on ion exchange resins. If the resin is left standing in the regenerant during nonoperatingtimes, itis possibleto suppress the microbial growth.'101' The regenerant in this instance was 10or20%NaCl. When the initial microbe count was 10 per milliliter, at the end of three weeks in 20% NaCl, the count had risen to just 800/ml compared to 200,000/ml for the resin stored in water. When an alternate regenerant is used (NaOH or HC1), it is preferable to change the storage medium to 20% NaCl since extended time in an acid or base media can adversely affect the resin matrix.

Since most fluids contain some suspended matter, it is necessary to backwash the resin in the fixed bed column on a regular basis to remove any accumulation of these substances. To carry out a backwashing operation, a flow of water is introduced at the base of the column. The flow is increased to a specific rate to classify the resin hydraulically and remove the collection of suspended matter. Figures 40 and 41 show the types of flow rate which provide certain degrees of expansion of cation and anion resins, respectively. Since the anion resins are significantly less dense than the cation resin shown, it would be necessary to have different amounts of freeboard above the normal resin bed height so that backwashing may be accomplished with only a negligible loss of ion exchange resin. Typically, an anion resin bed may be expanded by 100% during backwashing, while a cation resin bed will only be expanded by 50%.

It is also necessary that the water used for the backwashing be degassed prior to use. Otherwise, resin particles will attach themselves to gas bubbles and be carried out of the top of the column to give an unacceptable increase in resin losses.

When treating fermentation broth filtrates, frequent backwashing of the resin bed is necessary to prevent accumulation of suspended matter. In such cases, the column height should be designed of such a size that the bed is regenerated at least every 10 hours. Shorter columns have been designed to be regenerated at least every hour.[102] These shorter beds can then use finer resins and achieve a high level of efficiency with lower capital costs. This may be taken to the point of using very fine resins, as with the Powdex® system'103' which discards the powdered resin after a single use.

After a bed is backwashed, unless it is air-mixed as the level of water is drained down to the surface, the beads or particles classify according to size. The fine beads end up on top and the large beads on the bottom of the column. In cocurrent operations, the regenerant first contacts the top of the bed. The fast kinetics ofthe fine particles gives a high regeneration efficiency. However, the large beads on the bottom will regenerate more slowly and may end up only partially regenerated. Thus, when the feedstream is next passed through the resin bed, leakage of undesirable ions may occur from the large beads in the bottom of the column. This may be overcome by using a countercurrent flow arrangement described earlier or by using an air-mixing system during the post-backwash draining.11045

BACKWASH FLOW RATE (ML/MIN /CM*)

Figure 40. Backwash expansion characteristics of a macroporous strong acid cation resin, Dowex 88.

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