(79g) A Two-Bed Simulated Moving-Bed Adsorber for the Fractionation of Gas Mixtures

The ?Hypersorption Process' is a moving-bed adsorption process similar to distillation. Its stripping and enriching zones are identical to their counterparts in distillation. Its regeneration and presaturation sections are functionally the same as the condenser and the reboiler. Here, it is possible to obtain clean separation as in distillation. However, due to the problems associated with solid handling the moving beds are less viable and therefore not commercialized. To overcome this problem, simulated moving-bed (SMB) technology has been developed. In a simulated moving-bed the countercurrent contact is achieved by the switching of the inlet and outlet ports of streams located along the length of the bed. Earlier, Rao et al. (2005) described simulated moving beds embedded with ?moving-port systems'. These moving-port systems can be used to introduce a fluid into the bed, or withdraw a fluid from the bed (A patent pending). These simulated moving-beds can be used for the clean separation of binary gas mixtures. It has been shown that an ideal simulated moving-bed adsorber can fractionate a mixture of propane and propylene into two product streams with 99 % purity and with productivities in the range of 100-300 mol/kg/h (Rao et al. 2005). In this paper, to realize the SMB operation in practice, we propose a two-bed SMB adsorber for the fractionation of gas mixtures. A similar one has been used to exchange heat between two gas streams (Murthy et al. 2004). Each bed is embedded with three moving-port systems. The proposed adsorber is operated in such way as one bed is effecting separation; the other bed is under regeneration. In the former bed the stripping and enriching zones are formed at either sides of the moving-port used for feed injection. The raffinate is withdrawn as a product from the moving-port in front of the stripping zone. A part of the extract product withdrawn from the bed under regeneration is refluxed using the moving-port behind the enriching zone. The stripping and enriching zones are carried along the bed from one end to the other by the moving-ports. The zone lengths can be adjusted by adjusting the port position at the beginning of the operation. When the enriching zone breaks through the bed, the bed is fully saturated with the heavy component and is ready for regeneration. After regeneration by pressure swing, the bed is saturated with the raffinate component at the adsorption pressure. After saturation, the bed is ready for effecting separation. The separation and regeneration steps are synchronized between the two beds using solenoid valves. To evaluate the performance of the two-bed adsorber, a mathematical model has been developed. The stripping and the enriching sections have been modeled as steady state countercurrent contactors while the regeneration is modeled as a transient process. The fractionation of air using zeolite 13X was studied. Preliminary studies indicate that it is possible to obtain oxygen and nitrogen purities in excess of 99 mole percent as in distillation. The productivity is about 20 times higher than that obtained in conventional PSA.

Reference: 1. D.P. Rao, S.V. Sivakumar, Susmita Mandal, Sridevi Kota and B.S.G. Ramaprasad, Journal of Chromatography A, Volume 1069 (2005), 141. 2. D.S. Murthy, S.V. Sivakumar, K. Kant and D.P. Rao, ASME Heat Transfer / Fluids Engineering Summer Conference, North Carolina, USA, 2004.