Key Technology Development Of A Dual-Membrane Reactor System For Autothermal Reforming Of Natural Gas

Membrane reactor is one of the key technologies to realize efficient and green production, integrating reaction and separation processes into one single unit thus can promote process intensification. Membrane reactors can be used for small-scale hydrogen production. A dual-membrane reactor system for autothermal reforming of natural gas was proposed by the research group of professor Kuipers in the Netherlands. The system mainly includes a palladium membrane reactor in the top section and a perovskite membrane reactor in the bottom section. Palladium membranes are used to remove product H₂, shifting the reaction equilibrium towards the desirable direction. Perovskite membranes are used to separate O2 from air to combust partial CH4, suppling the heat needed by the whole reaction system and avoiding costly air separation facilities. Autothermal operation can be realized by adjusting feed ratios and the system acquires higher energy efficiencies. Only the palladium membrane reactor section was simulated and experimentally studied by the research group of professor Kuipers, because of the constraints of perovskite membrane stability and sealing technology. In this research, BaCo_0.4Fe_0.4Zr_0.2O_3-δ（BCFZ）capillary membranes were prepared and the membrane modules were successfully sealed. Modeling and experimental work were carried out, focusing on the bottom section including perovskite membranes.Sealing of perovskite membranes and connectors is one of the key technologies for system integration. BCFZ capillary membrane tubes were prepared by phase inversion spinning method and successfully sealed with Al2O3 connectors using gold paste and schott glass under high temperatures. Sealed membrane modules have high O2 selectivities and O2 permeation rates. N2 volume fractions detected were lower than 0.71%. The effects of operation conditions on O2 permeation rates of different types of membrane modules were investigated. High temperatures, high pressures, and high flow rates of sweep gas can improve O2 permeation rates. A one-dimensional model was built for BCFZ capillary membranes, validated by comparing experimental O2 permeation rates and corresponding calculated values.Component concentration distributions are influenced by mass transfer within the reactor, having an impact on final reaction results. Operation parameters can be optimized by investigating the rate determining steps of the whole mass transfer process. Two three-phase models were built to simulate the perovskite membrane reactor section, having different mass transfer descriptions for O2 between membranes and three phases（bubble, cloud, and emulsion）, which can investigate mass transfer extent among three phases. Oxygen to carbon ratios needed to realize autothermal operations under different temperatures were calculated by enthalpy balance. Higher oxygen to carbon ratios are needed at higher temperatures and lower pressures. The influences of operation conditions on reaction results were studied. CH4 conversions and CO selectivities increase with increasing temperatures. With a higher superficial velocity, a higher H₂ production rate can be obtained at the expense of a lower CH4 conversion.For a palladium membrane reactor, reaction and separation processes are integrated into one single unit. The influences of operations conditions on H₂ permeation rates can be investigated to improve H₂ production capacity. Planar Pd-Ag membrane modules were designed and studied. Palladium membranes are easy to form defects during preparation and application processes. Based on metallic-diffusion bonding, a repair method was developed for millimeter membrane defects. H₂ selectivities and H₂ permeation rates were tested after repair. Repair factor was proposed to compare H₂ permeation rates after and before repair for the whole membrane area. In the case without sweep gas, repair factor is related to membrane parameters. In the case with sweep gas, gas flow pattern should be considered to calculate repair factor.For a fixed H₂ production rate, palladium membrane area needed for H₂ separation should be calculated. H₂ permeation rates are related to membrane permeation capacities and gas flow patterns. Sieverts’ Law cannot be used to calculate H₂ permeation rate directly when H₂ partial pressures are not constant. Several models were built based on basic gas flow types. The relationship beween H₂ production rate and membrane permeation capacity was investigated. When H₂ production rate is fixed, the membrane capacity needed can be calculated using a proper model, and vice-versa. When the gases are plug flow, H₂ permeation rate is higher than that of complete mixing flow for a fixed membrane permeation capacity. H₂ production rates will not increase linely with increasing membrane permeation capacities. Economic membrane permeation capacity was proposed and defined, and a typical case was calculated, aiming at realizing a shorter payback period.