Gas separation through the looking glass
15 Jan 2000
It's like Alice Through the Looking Glass meets chemical engineering. American industrial gas company Air Products has developed a membrane whose pores allow large hydrocarbon molecules to pass through, while leaving minuscule hydrogen molecules behind.
The technique of using membranes to separate gases is not new, but all membranes let small particles through while blocking larger ones. The Air Products invention, known as a selective surface flow (SSF) membrane, works `backwards' because it combines a sieving action with adsorption.
SSFs consist of a thin layer of nanoporous carbon, with pores around 5-7u across, supported on porous alumina. When a mixture of hydrocarbons and hydrogen is passed along the nanoporous side at an elevated pressure, the large hydrocarbon molecules are adsorbed into the pores while the hydrogen is rejected. The adsorbed molecules diffuse through the pores and through to the low-pressure side.
According to Phil Cook, product manager for the SSF project, the adsorbed hydrocarbon molecules restrict the permeation of the smaller, less strongly-adsorbed hydrogen molecules. Working on mixtures containing 20-50 per cent hydrogen, the membranes can recover 60 per cent of the hydrogen content, and remove over 90 per cent of the hydrocarbons, he claims.
The SSF doesn't require pressures as high as other membranes, as the energy barriers for surface diffusion are very small for physically adsorbed species. The membranes work well for bulk gas separation, but can be combined with other purification processes where finer separation is needed, adds Cook.
The versatility of SSFs makes them suitable for a wide variety of processes, claims the firm. Most hydrogen-consuming operations, such as hydrocracking and isomerisation, generate an off-gas stream containing hydrogen. Fluid catalytic cracking units also generate hydrogen. The gas is usually not recovered, because the best technology currently available - pressure-swing adsorption - requires costly equipment and high pressures.
SSFs could make hydrogen recovery economic, giving low-cost access to a valuable feedstock, explains Air Products.
Another opportunity for SSFs is in natural gas gathering and compression stations. The compressors at such sites are fuelled by the natural gas within the system, but this can cause problems. Generally, the gas contains some heavy molecules (greater than C3) which don't burn efficiently, leading to poor fuel consumption and higher emissions. An SSF, installed before the compressor, will keep the light fractions on the high-pressure side, which can be diverted into the compressor engine. The heavier molecules will pass through to the low-pressure side, where they can be recycled straight into the compressor.
Air Products is currently working on a semi-commercial scale SSF unit in New Orleans, expected to be completed in the coming months. It hopes to offer the process commercially from next year, initially to natural gas stations, and eventually to chemicals manufacturers.
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