Motivation

Steady State Recycling (SSR) Chromatography is a periodic, pseudo-continuous operating mode for binary chromatographic separations. The process requires only a partial separation of the mixture at the column outlet. Unresolved portions of the concentration bands eluting from the column's outlet are recycled back to the inlet (see Fig. 1).

Since it can overcome yield limitations, the SSR concept is particularly attractive for enantioseparations. However, its full potential is yet unexploited, mainly due to a lack of proper design tools. Although the SSR process is commercialized, exisiting design methods are either empirical in nature [1], or hold only for complete separation of the components [2].

Approach & Results

Here, equilibrium theory (e.g. [3]) is applied to devise a comparably straightforward method for analysis and optimal design of SSR chromatography [4]. The approach holds for arbitrary purity or yield requirements and applies to SSR processes in so-called mixed-recycle operation.
The method directly predicts optimal design parameters and the steady state without requiring any dynamic simulations. Furthermore, it allowed to prove that – in comparison to conventional chromatography – the SSR concept always allows for a reduction of solvent consumption and increased product concentrations. This is at the expense of slightly lower overall productivity [4].

Finally, knowing the steady state, the typically long startup period of SSR processes can be shortened drastically (see Fig. 2).

Future work

• Further refinement of the design approach,
• Role of non-idealities,
• Experimental investigations,
• Development of process control scheme.

Acknowledgement

This work is in part supported by the German Academic Exchange Service (DAAD) and the Academy of Finland.

References

1. C.M. Grill (1998). Closed-loop recycling with periodic intra-profile injection: a new binary preparative chromatographic technique. J. Chromatogr. A, 796, pp. 101 – 113.
2. M. Bailly and D. Tondeur (1982). Recycle optimization in non-linear productive chromatography – I Mixing recycle with fresh feed. Chem. Engng. Sci., 37, pp. 1199 – 1212.
3. H.-K. Rhee, R. Aris and N.R. Amundson (2001). First-order Partial Differential Equations. Vol's. I & II, Dover, Englewood Cliffs.
4. T. Sainio and M. Kaspereit (2009). Analysis of steady state recycling chromatography using equilibrium theory. Sep. & Purif. Technol.(in press).