Pd-based membranes performance under hydrocarbon exposure for propane dehydrogenation processes: Experimental and modeling

In this work, a novel Pd–Ag double-skinned (DS-) membrane is used for the first time in conditions typical of propane dehydrogenation (PDH). This membrane presents a protective layer on top of the H₂-selective one, which acts as shield against chemical deactivation and mechanical erosion under reaction conditions. While the protective layer is already been proven as an efficient barrier against membrane erosion in fluidized beds, there is no validation yet under PDH reaction. The DS- membrane performance is compared with a conventional (C-) Pd–Ag membrane under alkane/alkene exposure, at 400–500 °C and 3 bar, to investigate whether the incorporation of the protective layer would be suited for H₂ separation in PDH systems, and if coking rate would be affected. The novel membrane shows a H₂ permeance of 2.28 × 10⁻⁶ mol∙m⁻² s⁻¹∙Pa⁻¹ at 500 ᵒC and 4 bar of pressure difference, overcoming the performance of the conventional PdAg one (1.56x∙10⁻⁶ mol m⁻² s⁻¹∙Pa⁻¹). Both membranes present a stable H₂ flux under alkane exposure, while deactivation occurs under exposure to alkenes. A model able to describe the H₂ flux through Pd-based membranes is presented to fit the experimental data and predict membrane performance. The model includes mass transfer limitations in the retentate and a corrective inhibition factor to account for the competitive adsorption of hydrocarbon species in the H₂ selective layer. The experimental results obtained under alkene exposure deviates from model predictions; this can be attributed to carbon deposition on the surface of the selective layer, as further detected on the DS-membrane by Scanning Electron Microscopy (SEM)/Energy Dispersive X-Ray Analysis (EDX), which is the main factor for membrane deactivation.