Permeation Modified Gas Separation Membranes

Objective

A method was desired to allow selective modification of the permselectivity of hollow fiber membranes for gas separations using the same base polymer and without significantly decreasing flux. Permselectivity in this case was defined as the ratio of the permeability of gas A to gas B where it is desired to separate gas A from a mixture with gas B. Examples of important gas separations include the removal of hydrogen from a mixture containing carbon monoxide, oxygen from nitrogen, and carbon dioxide from methane.

Project Background

It was known in the literature that some low-molecular-weight organic compounds when blended in low concentration with some polymers, principally poly(vinyl chloride) (PVC), cause an antiplasticization effect whereby there is a selective densification with an increase in some mechanical properties such as modulus. Higher additive concentrations result in plasticization whereby modulus and the glass transition temperature (Tg) decrease. It is believed that densification occurring during antiplasticization can result in a selective loss of free volume and, thereby, modify permeability. For example, incorporating ester plasticizers had been shown to enhance the permselectivity of PVC membranes for carbon dioxide. Unfortunately, the flux of PVC is too low for commercial separations compared to other polymers such as polysulfone.

Consultant Approach

A systematic study was pursued to look at the effect of adding different low-molecular-weight additives to a number of different polymers (e.g., polysulfone, polyphenylene oxides, and polyimides) that are important polymers for use in hollow fiber gas separations. In preliminary studies, potential permeation modifiers were added to polymer solutions which were then cast and dried to thin films whose permeability for different gases could be easily evaluated by gas chromatography.

Consultant Results

The thin-film studies indicated that a large variety of additives had potential as permeability modifiers. Those with the best performance (i.e., lowest decrease in flux and highest gain in permselectivity) were further evaluated in actual membrane modules that were assembled by potting the ends of hollow fiber bundles in epoxy seals. The fibers were dip coated in a solution containing a permeation modifier for different times and the concentration of modifier in the fiber bundle determined. It was found that low concentrations of such permeation modifiers, 0.01 to about 1.0 wt % of the membrane, introduced in this way to the outer skin of an asymmetric hollow fiber membrane was effective to improve module performance without a loss in mechanical and thermal resistance (i.e., Tg) of the membranes. This dip coating procedure provided an advantage over pre-blending the additive with the polymer dope by enabling newly assembled hollow fiber modules to be selectively modified for a given application as needed. In addition, dip coating eliminated the possibility that the permeation modifier could be leached from the membrane polymer into the coagulation bath during solution spinning of the hollow fiber and also enabled the modifier to selectively added to the thin skin of the hollow fibers where permselectivity is controlled. It was found that a wide range of organic compounds could be used in this way to modify the permselectivity of hollow fiber modules. Permselectivity could be significantly increased in many cases. Although, membrane flux (ratio of permeability coefficient over membrane thickness) is reduced upon modifier addition, such reductions can be tolerated in the case of highly permeable polymers such as polysulfones and poly(phenylene oxides) and still offer economic advantage in commercial gas separations. As an example, it was shown that 4,4'-(1-methylethylidene)bis(2-methylphenol) could be used to increase the perselectivity of hydrogen/carbon monoxide from 35 to 80 with about a 60% loss in flux. This work resulted in a patent awarded to the company.

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