BACKGROUND OF THE INVENTION

The invention relates to a process for removing oxygen from a hydrogen stream. More specifically, the invention relates to the use of a catalyst in a temperature swing or a pressure swing adsorption unit to remove oxygen from a gas stream such as from hydrogen product gas from a water electrolyzer.

Oxygen removal is difficult in pressure swing adsorption (PSA) hydrogen units due to the low oxygen capacity of conventional adsorbents (including activated carbon, molecular sieve zeolites, activated alumina, and silica gel). In situations where oxygen removal is required, PSA bed volume increases substantially and hydrogen recovery drops by several percentage points. For example, in propane dehydrogenation units (such as UOP's Oleflex™ process), a small amount of air ingress is possible due to low-pressure (vacuum) operation, leading to the presence of oxygen (50 to 100 ppmv) in net gas feed to PSA unit. In cases where the PSA hydrogen product is exported, a typical oxygen pipeline specification is <1 ppmv. However, in prior art processes oxygen removal causes a large drop in PSA hydrogen recovery. A similar issue is found in other situations that require similar oxygen removal requirements (e.g., refinery off-gas and coke oven gas) and these processes also suffer from low PSA hydrogen recovery and large PSA bed volume due to low oxygen capacity of conventional adsorbents.

In addition, it has been found that a significant source of hydrogen is from water electrolysis including the use of electricity produced by renewable fuels in which the gas stream is mostly hydrogen with relatively small amounts of oxygen, water and nitrogen that need to be removed before the hydrogen is used for other purposes.

Accordingly, an improved oxygen adsorbent or catalyst is needed for PSA applications that require oxygen removal. It has now been found that an activated alumina catalyst comprising copper is particularly effective in removing oxygen without impacting the recovery of hydrogen. Other catalysts such as palladium and platinum may be substituted if commercially viable.

SUMMARY OF THE INVENTION

A catalyst material such as a copper, palladium, or platinum material has been found to be effective to remove oxygen with a very high working capacity in a pressure swing adsorption cycle, resulting in much improved PSA performance and cost. The catalyst material results in the reaction of oxygen with hydrogen to produce water. Hydrogen product gas from a water electrolyzer is generally saturated with water. The presence of water can be problematic in the oxygen removal system due to inhibition of the oxidation reaction and due to very high water concentrations in the adsorption bed and possible condensation of liquid water. A method is needed to cost effectively remove the water in hydrogen gas upstream of the oxygen removal system. Accordingly, a layer of dehydration adsorbent such as silica gel, activated alumina, or 13X zeolite can be used in the PSA or temperature swing adsorption (TSA) unit for water removal at the feed end of the bed upstream of the oxidation catalyst. Additional water formed by the oxidation reaction can be removed in a downstream desiccant layer of silica gel, activated alumina, or 13X zeolite. A layer of CaX zeolite or 5A zeolite may be added at the product end of the bed when needed to remove nitrogen.

Based on experimental results to date, it is believed that the catalyst material functions by an oxidation/reduction mechanism: i.e., oxygen is chemisorbed during the high-pressure feed step in the PSA cycle, and then copper is reduced (thereby forming water) during the regeneration/counter-current purge step. Incorporating water removal, oxygen removal, and nitrogen removal in a single bed by suitable arrangement of functional adsorbent/catalyst layers allows one to maintain high hydrogen recovery, small PSA or TSA bed volume, and a single unit operation for complete purification of hydrogen gas from a water electrolyzer.

