Abstract: Plate-and-frame membrane modules, assemblies and processes for separating components of a fluid mixture. The assemblies comprise of a pressure vessel filled with, and able to hold, pressurized fluid being processed. Lightweight membrane plate-and-frame modules are contained inside the vessel. Fluid directing conduits direct the fluid streams being processed into and out of the vessel and across the surface of the separating membrane. Because the modules are surrounded by high pressure fluid, the forces acting on the module are small. This means the modules can be made of lightweight, inexpensive materials, such as plastic. The design of the assemblies is such that it allows for modules to be easily replaced as needed. The assemblies are also designed for pressurized feed fluid separations and separation using a sweep fluid on the permeate side of the membrane. The pressure vessel can contain one or several membrane modules.