Aromatics Enrichment in Refinery Streams Using Hyperfiltration

In petrochemical or refinery operations, feed streams containing aromatics such as benzene, toluene, or xylenes can be selectively enriched or depleted in aromatics content by hyperfiltration. The development program for STARMEM included production of asymmetric Lenzing P84 polyimide membranes with various permeate fluxes and aromatics selectivities. Both 5-cm-diameter and commercial-scale 20-cm-diameter spiral-wound elements were designed and built containing these membranes. Tests of these modules at 55 bar (800 psi) and 50 °C for up to 2000 h with a toluene recycle stream from a refinery showed stable operation in hot hydrocarbons. The high efficiencies of these modules can allow for high stage cuts (50% and greater) from aromatic process streams. By combining existing refinery operations with this membrane process, favorable cost savings through better purity, recovery, or throughput can be expected. With these membranes acting as organic solvent nanofiltration devices, the permeation properties can be interpreted through a solution-diffusion equation for molecular transport. The different levels of nonaromatics rejection for the various membranes suggest that different distributions of nanopore dimensions are set during initial membrane formation.