Separation of Butane Isomer Mixtures Using Dense 6FDA-DAM Membranes

SEPARATION OF BUTANE ISOMER MIXTURES USING DENSE 6FDA-DAM MEMBRANES

Omoye Esekhile, William J. Koros

oesekhile3@gatech.edu,  bill.koros@chbe.gatech.edu

School of Chemical and Biomolecular Engineering

Georgia Institute of Technology, Atlanta, GA

The glassy 6FDA-DAM polymer has a relatively high fractional free volume, thus allowing for high permeabilities of butane isomers compared to most other polymers. The separation performance of this membrane has been studied previously under ideal conditions of single gas feeds with upstream pressures up to 2atm at 100°C, and downstream under vacuum. These ideal conditions may give overly optimistic separation performance estimates.

Mixture gas permeation studies introduce factors such as sorption and transport competition, and so-called bulk flow or “frame of reference” nonidealities. These factors may significantly influence the separation performance of the membrane even in the absence of plasticization. The dual mode transport model has been extended to account for competition in the Langmuir environment of the glassy polymer, and modified to account for the bulk flow effect. Current work with butane isomers, however, further suggests a possible breakdown in local equilibrium in the transport of this mixture system. This adds a previously unobserved effect that may be a new feature for glassy membrane-based separations of large penetrants

The breakdown in local equilibrium is hypothesized to result from the slow jump rate of isobutane from the Langmuir to dissolved environments, relative to the diffusion time scale of nC4 into the Langmuir environment. An apparent affinity constant for the faster moving n-butane isomer can be used to describe this breakdown.

In this presentation, I will discuss the behavior of this mixture system over various feed compositions and discuss further the possible breakdown of local equilibrium. I will also suggest ways of enhancing the separation performance of the membrane.