Conceptual Process Design and Techno-Economic Analysis of a Hybrid Zeolite Membrane/Distillation Separation System for Butane Isomer Separation

The application of a hybrid separation process integrating zeolite membranes with distillation column was studied for butane isomer (n-butane/i-butane) separation. Currently, the industrial separation of butane isomers is accomplished using distillation, and due to the close boiling-point of the components, it is considered as one of the most energy-intensive distillation separations. Thus, the implementation of a hybrid process offers a great opportunity for large reductions in energy use and environmental emissions.

In this work, a conceptual process model of a series hybrid configuration, where zeolite membrane is placed before the distillation column to accomplish the initial portion of separation requirements, was developed and analyzed for an industrial case study of butane isomer separation. The permeation through the zeolite membrane was described using the real adsorption solution theory and the Maxwell-Stefan theory for diffusion. The membrane model was simulated in gPROMS v4.0.0 and validated by comparison with results obtained from the experiments. The distillation column was modeled using the McCabe-Thiele method in Aspen Plus v8.0, and was validated with the performance of an industrial n-butane/i-butane fractionator. The n-butane recovery and purity, obtained from the fractionator, were selected as the target for the hybrid system performance, and a techno-economic analysis was performed to this end.

The hybrid process performance was evaluated in terms of the membrane area required and the reduction in energy usage of the distillation column, which represents the additional capital cost and the savings in operational costs, respectively. The membrane area required was found to be one-tenth of that required for stand-alone membrane process, and the reboiler duty was found to be almost half of that required for stand-alone distillation column. This suggests that the hybrid membrane/distillation process has a great potential for industrial implementation. The Net Present Value (NPV) savings projections of the hybrid system were also calculated to generate a set of performance targets in term of the membrane permeance and the membrane cost.