Recovery of hydrocarbons from mixtures containing C3H6, C3H8 and N2 using NaX membranes

The separation of light hydrocarbon (C3H6 and C3H8)/N2 mixtures was investigated using faujasite membranes synthesized on α-Al2O3 disks. The motivation for studying this separation stems from the existence of vent streams in polyolefin production plants that contain these gases. These streams are usually flared and, as result, their valuable hydrocarbon content is lost. Permeation measurements of equimolar binary mixtures (C3H6/N2 and C3H8/N2) were carried out as a function of permeation temperature, feed mixture composition and total feed pressure. The maximum separation factors for C3H6/N2 and C3H8/N2 equimolar mixtures were ∼28.4 (with C3H6 flux 1.93 mmol m−2 s−1) and ∼8.1 (with C3H8 flux 1.34 mmol m−2 s−1), respectively and have been observed at 90 °C. As the total feed pressure was increased from 1 to 3.5 atm the permeation fluxes of all gases increased and the separation factor decreased. In particular, the separation factor of C3H6/N2 dropped from ∼28 to ∼13 and of C3H8/N2 from ∼8 to ∼5. Permeation experiments were carried out using a ternary mixture having a simulated composition of a typical polypropylene vent stream (12.5% C3H6/12.5% C3H8/75% N2). The permeation results indicate that only C3H6 can be recovered from such a mixture. This result is very important because C3H6 is the valuable monomer and if recovered with suitable purity it can be recycled back to the polymerization reactor.