(4k) Advanced Membrane Materials for Refinery Separations by Pervaporation

Refinery streams can contain a mixture of aromatic and aliphatic hydrocarbons, with volumes that can reach 1600-8000 m^3,/day. Many of these mixtures are difficult to separate because they contain close boiling compounds that generally have similar physical and chemical properties with respect to each other. The conventional technologies to perform these separations, such as extractive distillation, azeotropic distillation and liquid-liquid extraction, are energy intensive and expensive. Membrane processes, being more economical, energy-saving and ecologically friendly, constitute a promising alternative to conventional separation technologies. Pervaporation, in particular, has been intensively investigated for the separation of azeotropic, close-boiling and isomeric mixtures.

Aromatic polyimides are well known for their excellent thermal stability, mechanical strength and good chemical resistance. Thus, these polymers have attracted considerable interest as membrane materials for organic-organic pervaporation. Nevertheless, a suitable polymeric membrane material to enable commercial application of this technology in refineries is yet to emerge. The challenge is to develop a membrane with suitable transport properties that, at the same time, can withstand the aggressive feed streams and harsh operating conditions involved.

In this contribution, we report a structure/property study of a series of aromatic polyimides for the separation of toluene/n-heptane and benzene/n-heptane mixtures by pervaporation. Homo- and copolyimides were synthesized by the two-step polycondensation of three aromatic dianhydrides and four diamines (two containing a hydroxyl group relative to the amino moiety, one containing a fluorine moiety, and one containing methyl groups in the aromatic ring). Pervaporation experiments were performed at 80^oC with feed streams containing 40 wt% aromatics. Pure-liquid sorption experiments at 25^oC were also conducted to investigate the solubility selectivity of each polymer. The long-term chemical stability of the polymers was assessed in exposure tests to benzene in a Soxhlet extractor for at least a week.

Contrary to most studies on pervaporation conducted so far, the individual contributions of the membrane material and the driving force to the permeate flux were properly evaluated. It was observed that changes in the chemical structure of the polyimide led to an increase of up to 3 orders of magnitude in the permeability of the hydrocarbons.