Experimental investigation of the behavior of gas phase simulated moving beds

The preparative, continuous gas chromatographic separation of the enantiomers of the inhalation anaesthetic enflurane has been studied on a chiral stationary phase based on octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin, dissolved in polysiloxane SE-54 and coated on Chromosorb particles. This has been carried out in a gas chromatographic simulated moving bed (GC-SMB) unit, equipped with eight columns, yielding a total volume of 1.128 l. With reference to this particular system, the separation performance and the behavior of GC-SMB units have been analyzed in depth. We have varied the internal flow-rates in the four sections of the unit in the range between about 1 and about 5 std l/min, the temperature between 20 and 45 degrees C, the switch time between about 2 and about 14 min, and the feed concentration in the range 0.32-0.90 mol%. Similarities and differences in the behavior of GC-SMBs as compared to conventional liquid phase SMBs have been described, and discussed. Operating conditions leading to more than 99% purity in one or both outlet stream have been identified, together with those achieving optimal throughput. Under such optimal conditions, about 20 g of each enflurane enantiomer with enantiomeric purity larger than 98% have been prepared.