NOVELTY - Selective purging of light hydrocarbons from a reactor recycle loop involves separating the effluent from the reactor into liquid and vapor portions, selectively purging at least some of the vapor portion and returning the contaminant-depleted stream to the reactor.    USE - For contaminant removal and hydrogen reuse in reactors, especially in a refinery or petrochemical plant (claimed).    ADVANTAGE - The polymeric membranes are hydrogen rejecting i.e. all hydrocarbons including methane permeate the membrane preferentially leaving a residue stream on the feed side which is concentrated in the slower-permeating hydrogen. The membrane provides selective purging capability. The contaminated purge stream is depleted in hydrogen, thus the proportionate loss of hydrogen per volume of contaminant purged can be reduced several fold as compared with the conventional loop process. The membranes are easy and inexpensive to prepare. The process provides improved contaminant removal. The process achieves a slightly higher hydrogen partial pressure which is beneficial in increasing the catalyst life and suppressing the low-value products.    DETAILED DESCRIPTION - Selective purging of light hydrocarbons (containing at least 4C) in a reactor recycle loop comprises:    (a) withdrawing an effluent stream comprising hydrogen and hydrocarbons from a reactor;    (b) separating a vapor phase comprising hydrogen and light hydrocarbon from the effluent stream;    (c) passing at least a portion of the vapor phase as a feed stream across the feed side of a polymeric membrane having a feed side and permeate side selective for light hydrocarbon over hydrogen;    (d) withdrawing from the permeate side a permeate stream enriched in the light hydrocarbon compared with the vapor phase;    (e) withdrawing from the feed side a residue stream enriched in hydrogen compared with the vapor phase; and    (f) recirculating at least a portion of the residue stream to the reactor.    The steps (c), (d), and (e) are carried out at a stage cut not more than 50%.