Development and testing of inorganic membranes for hydrogen separation and purification in a catalytic membrane reactor.

Alkali, Abubakar

Abstract

Palladium membranes have been identified as the membranes of choice in hydrogen separation and purification processes due to their infinite selectivity to hydrogen when defect-free. Despite their potentials in hydrogen processes, palladium membranes pose challenges in terms of cost and embritllement, which occurs when palladium comes in contact with hydrogen at temperatures below 573 K. The challenges posed by palladium membranes have encouraged research into non-palladium-based membranes such as silica and alumina. This thesis investigates hydrogen permeation and separation in palladium membranes and also the use of non-palladium membranes - silica and alumina membranes - in hydrogen permeation. In this study, hydrogen permeation behaviour was investigated for 3 types of membranes, palladium, silica and alumina. Thin palladium films were deposited onto a 30 nm porous ceramic alumina support using both conventional and modified electroless plating methods. The hydrogen separation and purification behaviour of the membranes were investigated, including the effect of annealing at higher temperatures. Gas permeation through silica and alumina membranes was investigated for five single gases including hydrogen. The silica and alumina membranes were fabricated using the dip-coating method and their hydrogen permeation behaviour investigated at different coatings. A thin palladium (Pd1) membrane with a thickness of 2 μm was prepared over porous ceramic alumina support using the electroless plating method and a maximum hydrogen flux of 80.4 cm3 cm-2 min-1 was observed at 873 K and 0.4 bar after annealing the membrane. The hydrogen flux increased to 94.5 cm3 cm-2 min-1 at the same temperature and pressure for the palladium membrane (Pd2), prepared using the modified electroless plating method. The hydrogen flux increased to 98.1 cm3 cm2 min-1 for the palladium/silver (Pd/Ag) membrane prepared using the co-deposition electroless plating method and the PdAg membrane avoided the hydrogen embrittlement at low temperature. Hydrogen purity for the membrane was also investigated for a reformate gas mixture and a maximum hydrogen purity of 99.93% was observed at 873 K and 0.4 bar. The hydrogen purity was observed to increase as a result of the addition of sulphur, which suppresses the inhibition effect of carbon monoxide in the reformate gas mixture. The presence of CO and CO2 was observed to lead to an increase of the exponential factor n above 0.5 as a result of the inhibiting effect of these compounds on hydrogen permeation. The value of the exponential factor n depicting the rate limiting step to hydrogen permeation in the palladium and palladium-alloy membranes was also investigated. Deviations from Sievert’s Law were observed from the palladium membranes investigated in this work. In the single-gas hydrogen permeation investigation for the Pd1 membrane, prepared using the conventional electroless plating method, the value of the exponential factor n = 0.5 in accordance with Sievert’s Law. However, for the mixed-gas hydrogen separation investigation n = 0.62 at 573 K, which decreased to 0.55 when the membrane was annealed at 873 K. For the Pd2 membrane, prepared using the modified elctroless plating method, n = 1 at 573 K - but the value decreased to 0.76 for the mixed-gas hydrogen separation investigation at the same temperature, which depicts a deviation from Sievert’s Law. In all the investigations carried out for the Pd3 palladium alloy membrane, prepared using the co-deposition Pd/Ag electroless plating method at same conditions with the Pd1 and Pd2 membranes, n = 0.5 in accordance with Sievert’s Law. For the non-palladium-based silica and ceramic alumina membranes, investigations were carried out for hydrogen permeation and five other single gases: He, CO2, CH4, N2 and Ar. For the silica membranes, a maximum hydrogen permeance of 3.12-7 x 10 mol m-2 s-1 Pa-1 at 573 K and 0.4 bar was observed, which increased to 4.05 x 10-7 mol m-2 s-1 Pa-1 at 573 K and 0.4 when the membrane was modified with Boehmite sol prior to deposition of the silica layer. The permeance for hydrogen and the five single gases was investigated for the alumina membrane at five successive coatings. It was observed that the commercial alumina membrane displayed a maximum hydrogen permeance of 9.72 x 10-7 mol m-2 s-1 Pa-1 at 573 K and 0.4 bar, which increased to 9.85 x 10-7 mol m-2 s-1 Pa-1 at the same temperature and pressure when the membrane was modified with Boehmite sol.

Citation

ALKALI, A. 2016. Development and testing of inorganic membranes for hydrogen separation and purification in a catalytic membrane reactor. Robert Gordon University, PhD thesis.

• Thesis Type: Thesis
• Deposit Date: Jul 16, 2019
• Publicly Available Date: Jul 16, 2019
• Keywords: Palladium membranes; Hydrogen separation; Purification processes; Non-palladium membranes; Silica; Alumina; Hydrogen permeation
• Public URL: https://rgu-repository.worktribe.com/output/322182
• Award Date: May 31, 2016