SMB chromatography has been increasingly applied for separations of pure substances in the pharmaceutical, fine chemistry and biotechnology industries. It has many advantages with respect to discontinuous batch chromatography: higher product purity, less solvent consumption, and higher productivity per unit stationary-phase.
The SMB is a practical way of implementing a counter-current chromatographic process. N identical chromatographic columns are connected in series to build a closed loop. By moving the input/withdrawal ports one bed ahead, in the direction of fluid flow, at fixed intervals, the counter-current contact adsorbent/fluid is simulated. The SMB has been mainly employed for liquids, but the same principle can be applied to separate a gas mixture of two competing adsorbates using a weakly adsorbed gas as eluent. However, SMB gaseous applications have been limited.
Recently, we have 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 setup was employed to demonstrate the process feasibility, explore the effect of its major operating parameters and illustrate the performance enhancements that are obtained by asynchronous and time-variable flowrate schemes for gas-phase SMB.
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. Hence, the adsorbates and eluent quantities required to experimentally reproduce the periodic state of the SMB process can be greatly reduced. This may be an economic, optimal manner of experimentally testing a set of operating conditions to achieve a given separation performance for new continuous chromatographic applications.
Our implementation was successfully applied to separate CO2/CH4 gas mixtures on activated carbon in the Henry's law region using N2 as purge gas. The work is currently being extended to the nonlinear region of the adsorption equilibrium, where the process operates under non-isothermal conditions with non-negligible changes in fluid velocity across the mass transfer zone. The use of pressure swing to induce both desorption and flow of gas through the columns is also being evaluated.
Acknowledgements. FCT/MCES (Portugal) for Post-doctoral grant SFRH/BPD/14910/2004.
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