NOVELTY - Isomerizing a normal butane-containing isomerization feed containing normal butane involves isomerizing the feed in catalyst; distilling the resultant isomerization effluent to provide a lower boiling fraction and a higher boiling, normal butane-containing fraction; contacting the normal butane-containing fraction with a retentate-side of a selectively permeable membrane to provide a retentate fraction containing isobutane, and to provide across the membrane at a permeate-side, a permeate fraction having normal butane; and withdrawing the retentate fraction.    USE - For isomerizing normal butane-containing isomerization feed containing normal butane to provide an isomerate comprising isobutane (claimed).    ADVANTAGE - The process is improved process for isomerization of normal butane to isobutane using a deisobutanizer in combination with a membrane that is selectively permeable to normal butane as compared to isobutane; and has reduced capital and operating costs.    DETAILED DESCRIPTION - Isomerizing a normal butane-containing isomerization feed containing normal butane (at least 50 mass%) to provide an isomerate comprising isobutane involves: (A) isomerizing the isomerization feed under isomerization conditions including the presence of isomerization catalyst to provide an isomerization effluent containing normal butane but in a concentration less than that in the feedstock; (B) distilling at least a portion of the isomerization effluent to provide a lower boiling fraction containing isobutane and lighter paraffins (where at least 80 mass% of the lower boiling fraction is isobutane) and a higher boiling, normal butane-containing fraction containing normal butane and isobutane (at least 10 mass%); (C) contacting at least a portion of the normal butane-containing fraction from step (B) with a retentate-side of a selectively permeable membrane under conditions including membrane surface area and pressure differential across the membrane to provide a retentate fraction containing isobutane (at least 80 mass%), and to provide across the membrane at a permeate-side, a permeate fraction having an increased concentration of normal butane; and (D) withdrawing from step (C) the retentate fraction. INDEPENDENT CLAIMS are included for the following:    (1) a normal butane isomerization processes involving contacting at least a portion of a normal butane-containing feedstock also containing at isobutane (least 5 mass%) with a retentate-side of a selectively permeable membrane under conditions including membrane surface area and pressure differential across the membrane to provide a retentate fraction containing isobutane (at least 80 mass%), and to provide across the membrane at a permeate-side, a permeate fraction having an increased concentration of normal butane, the permeate fraction preferably containing normal butane (at least 80 mass%) contained in the feedstock contacted with the membrane; and subjecting the permeate fraction to isomerization conditions including the presence of isomerization catalyst to provide an isomerization effluent containing normal butane but in a concentration less than that in the feedstock; and    (2) an apparatus for the isomerization of feedstock containing normal butane comprising (a) an isomerization reactor adapted to isomerize normal butane to an isobutane-containing isomerate having an inlet and an outlet for isomerate; (b) a distillation column having an inlet in fluid communication with the outlet of isomerization reactor a lower boiling fraction outlet conduit and a higher boiling fraction outlet conduit, (where the distillation column is adapted to distill at least a portion of the isobutane-containing isomerate to provide a lower boiling fraction richer in isobutane than the isomerate and a higher boiling fraction richer in normal butane than the isomerate and the fraction containing isobutane); and (c) a membrane separator having a feed side inlet in fluid communication with the higher boiling fraction conduit, a feed side outlet in fluid communication with line from the lower boiling outlet of distillation column, and a permeate outlet in fluid communication with the inlet of the isomerization reactor.