sition after iridium as the last precious metal to leave the annular column, thebase metals were stripped of the cation-exchange resin by using a step-eluent(2–3 mol/l HCl). In this eluent the base metals were not retained by either ofthe two stationary phases. Therefore a fraction consisting of the sum of the basemetals could be collected finally at the end of the annular column (Fig. 10).

It has been found from batch experiments that the base metals Fe, Ni and Coare fully adsorbed by the cation-exchange resin when the hydrochloric acid con-centration of the eluent does not exceed 0.4 M. If the concentration exceeds 0.4 Mthe base metals start to break through. The same thing happens when the hy-

Continuous Annular Chromatography 253

Fig. 10. Photograph of the separation of the PGM and base metals in a two-phase (mixed mode)P-CAC system

Fig. 11. Experimental chromatogram of a separation of a solution containing PGMs and basemetals using a mixed mode P-CAC system1

Rhodium

Palladium

Platin

Iridium IV

base metals(Fe, Ni, Co)

base metals

elution angle [°]

drochloric acid concentration of the feed solution exceeded 4 M. The minimumheight of the cation-exchange resin in the P-CAC depends on the concentrationof the base metals present in the feed solution. The height is directly proportionalto the maximum capacity of the resin. The maximum capacity of the resin for themixture of all three cations was calculated from the adsorption isotherm. The ad-sorption isotherm represents the equilibrium of a compound between the liquidand the solid phase in chromatography; isotherms can be estimated by batchshaking experiments.

It was also shown that the feed inlet band of the PGMs broadens when it passesthrough the cation-exchange resin layer. This means that the concentration of theplatinum group metals in the sample decreases accordingly, which depending onthe exact conditions results in dilution factors between 2 and 10. Figure 11 showsthe experimental chromatogram of the separation of a mixture used in the stud-ies.

7Conclusion

Several applications throughout the last two decades have shown that startingfrom batch chromatography experiments a scale-up to a continuous annularchromatograph is easy and straightforward. It has also been shown that many op-erating modes, including isocratic, step and displacement elution are possible ona CAC. The apparatus retains its relative mechanical simplicity in comparisonwith fixed-bed processes. No precise timing of a valve system for the introduc-tion of feed and the product removal are needed. The key advantages of annularchromatography over fixed-bed operations are likely the simplicity of the appa-ratus, its productivity and resolution improvement, and its truly continuous op-erational capabilities.

A very promising application of the P-CAC technology, which at the time thisarticle was written was undergoing intensive studies, is to couple the continuouschromatograph to a continuous fermenter system. Continuous bioreactors are re-ceiving attention as an efficient method of producing biochemicals. For this ap-plication it was necessary to develop a P-CAC unit where the column can be au-toclaved by steam. The coupling of a continuous fermentation to a continuouscapturing step promises a significant improvement in terms of throughput andproduct yield.

Compared to the SMB system the annular chromatography allows the con-tinuous separation of a multicomponent mixture as it is most often the case inbiopharmaceutical separations.

8References

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Received: July 2001

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