Apparatus and processes are disclosed for economical separation of fluid mixtures utilizing perm-selective membranes. Broadly, apparatus of the invention comprises a plurality of membrane modules comprising a solid perm-selective membrane and equipment for controlling enthalpy of selected fluids within the apparatus. Advantageously, the membrane modules are disposed in a first product group, a second product group, and at least one intermediate group. Apparatus of the invention is particularly useful for simultaneous recovery of a very pure permeate product, and/or a desired non-permeate stream, from fluid mixtures of two or more compounds which when subjected to appropriately altered conditions of temperature and/or pressure exhibit a bubble point.