Hydrogen Permeation Performance Of Pd Membrane Modules And Separator Design Theory

Palladium membranes are regarded as a better choice for producing ultra-pure hydrogenin a relatively small scale as compared with conventional processes. It is generally regardedthat the permeation of hydrogen through palladium or palladium alloy membranes follows the‘‘solution–diffusion’’ mechanism, which can be described by Sieverts’ Law.An efficiency (or efficiency factor) has been introduced by many researchers to denotethe difference between the actual permeability and those predicted from Sieverts’ Law.Extensive experiments were carried out in an electrically heated vessel to study the hydrogenpermeation flux and permeation efficiency of Pd–Ag membrane modules with porousstainless steel substrates. The influence of operation conditions on the membrane permeationflux and efficiency was examined. It was observed that the hydrogen permeation flux throughthe module increased by increasing the temperature and hydrogen pressure in the vessel,while the permeation efficiency increased by increasing the hydrogen pressure in the vesselside and decreasing the membrane temperature. The permeation efficiency was correlated tothe vessel temperature and hydrogen pressure difference (PH0.5PL0.5) for the conditionsstudied.Thin palladium membranes are easy to form defects during the preparation andapplication processes. The inter-metallic diffusion offers a method to repair membrane defectssuch as winkles and cracks by pasting a membrane patch on the defect area. A repairprocedure was developed which includes:1) Clean the membrane surface.2) Cover the defectwith a membrane patch.3) Press the membrane patch on membrane module surface withflanges.4) Heat the membrane module under hydrogen environment for at least12hours.Two membrane modules with defects were successfully repaired by this method. The integrityof the modules after repair is confirmed and their performance was studied.In a membrane separator, hydrogen flowrates and partial pressures may vary along theflow directions on both sides of the separator. Sieverts’ Law itself cannot be directly used fordesigning membrane separators. Considering the flow pattern, flow direction and theexistence of sweep gas in a membrane separator, seven flow scenarios are considered. Aprocedure is recommended for designing such membrane separators. A typical design case isstudied. New concepts of economical permeation capacity and hydrogen production rate areproposed and defined.Based on the membrane separator design theory, a modular membrane separator wasdesigned for separating the hydrogen from syngas produced by ethanol reforming. The separator consists of end membrane module, end syngas module, middle membrane modules,blind flanges, gas manifolds, electric heaters etc. The design offers the benefit for easyscale-up. The hydrogen product rate can be easily varied with changing the numbers of themiddle membrane module. A2Nm3h-1separator was fabricated, with the design temperature620℃and the design pressure10atm.