(72f) Gas Separation by Simulated Moving Bed Chromatography

Abstract. Simulated moving-bed (SMB) chromatography has been increasingly applied for the separation of pure substances in the pharmaceutical, fine chemistry, and biotechnology industries, at all production scales, from laboratory to pilot, to production scale (Juza et al., 2000). The SMB has many advantages with respect to discontinuous batch chromatography (Nicoud, 1992), such as higher product purity, less solvent consumption, and higher productivity per unit stationary-phase (Jupke, 2002; Strube, 1998; Wankat, 1986).

The SMB is a practical way of implementing a counter-current chromatographic process. The system consists of N identical chromatographic columns connected in series to build a closed loop. By moving the input and withdrawal ports one column ahead (i.e. in the direction of fluid flow) at fixed intervals, the counter-current contact between the adsorbent and fluid is simulated. The SMB has been mainly employed for liquid-phase applications, but the same principle can be applied to separate a gas mixture of two competing adsorbates using a weakly adsorbed gas as eluent.

It is shown that the cyclic steady-state of continuous multicolumn gas chromatography, operating under isothermal conditions, can be reproduced experimentally using a single-column setup. The experimental procedure is based on a mimetic single-column chromatographic model in which the part of the outlet stream that is not recovered as product is recycled back to the column with a lag of (N ? 1)τ time units, where N is the number of columns of the multicolumn unit and τ is the switching interval. The idea is to reproduce on-line the gas composition profile at column inlet from measurements of the outlet gas composition taken (N ? 1)τ time units before, while at the same time satisfying the imposed mass flow rate profile. In addition to on-line monitoring of the outlet gas composition, the experimental setup requires three mass flow controllers, which feed the column at variable mass flow rate with the three gases: the two adsorbates and the gaseous eluent (Fig. 1). The three mass flow rates are continuously manipulated so that the composition and mass flow rate of the combined inlet stream are the same as those for the ideal single-column chromatographic model. During the first (N ? 1)τ time units of operation the lagged outlet composition profile is pre-computed by process simulation using the model of the chromatographic unit; at later times it is determined from the on-line composition measurements of the gaseous outlet stream.

Our implementation is successfully applied to the separation of carbon dioxide and methane on activated carbon in the Henry's law region using nitrogen as purge gas. It is shown that by correctly selecting the step within the cycle for process start-up, the steady periodic state can be achieved in a minimum number of cycles. When the SMB process parameters are correctly chosen, the state of the column at the end of the switch interval during which it was partially fed with gaseous eluent is very well approximated by that of a clean column; the subsequent switching interval is, therefore, the preferred one for process start-up, because a clean column is already very close to its initial state for that switching interval when the system is under cyclic steady-state conditions. Hence, the quantity of gaseous adsorbates and eluent required to experimentally reproduce the periodic state of the SMB process can be reduced by a factor of about 50. This may be an economic, optimal manner of experimentally testing a set of operating conditions to achieve a given separation performance for a new continuous chromatographic application.