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Structured Adsorbent Bed for Rapid Adsorption and Regeneration Process

Wei Liu, Energy and Environmental Directorate, Pacific Northwest National Lab, 902 Battelle Boulevard, MSIN:K2-12, Richland, WA 99352

In the conventional fixed bed adsorption process, adsorbent materials are typically made into pellets of various shapes such as sphere and cylinder in a hydraulic diameter in the order of 1mm level, and those pellets are packed inside a large adsorbent bed vessel. A major limitation to use such an adsorbent bed for flue gas CO2 capture is pressure drop. Due to huge gas flow rate of the flue gas, the large pressure drop through the pellet-packed bed is directly associated with high energy consumption. A structured adsorbent bed design is presented here to substantially reduce capital cost and energy consumption for CO2 capture and separation from a gas mixture.

In the structured bed, fine adsorbent powder ranged from a few hundred nm to a few um is confined by a gas-permeable medium to form straight flow channels. As the gas mixture flows through the channel, CO2 diffuses across the gas-permeable medium, into the adsorbent layer, and further adsorbs inside individual adsorbent particles. The structured bed provides following advantages over the conventional packed bed:

Fast mass and heat transfer rate between the adsorbent material and convective fluid by minimizing all the transport resistance from bulk fluid onto channel surface, from the channel surface into the adsorbent layer, and from the external surface of individual adsorbent particles into the adsorbent.

102 103 lower DP than the pellet-packed bed at comparable hydraulic diameter under the same flow conditions

Little dead volume

Those critical attributes enable adsorption/regeneration process be conducted at exceptionally fast cycle speed with low energy consumption. Concept of this new adsorbent bed design is demonstrated by using certain zeolite adsorbent powder and conducting multiple adsorption/regeneration cycles in a flow system. The breakthrough curves of N2, H2O, O2, and CO2 during adsorption and desorption for the conventional packed bed and for the present structured bed are measured. A characteristic time, tau, which corresponds time duration for 2/3 change of the CO2 concentration in the adsorption or desorption profile, is used to compare the global adsorption and desorption kinetics. For the adsorption process, the smaller tau means a sharper adsorption front and thus better utilization of the adsorption bed. For the desorption process, tau number measures the regeneration time. Thus, for an efficient adsorbent bed design, small tau number is needed for better utilization and rapid regeneration.

The testing results show that both adsorption and regeneration time is reduced by a few times with the structured adsorbent bed. It is noted that in addition to the adsorption and regeneration kinetics advantage, the structured bed provides much lower pressure drop and other benefits as well. With some optimization of the structured bed, even higher level of kinetics rate enhancement is obtainable.

Fast adsorption/regeneration cycling increases throughput of the adsorbent material and low pressure drop reduces parasitic power consumption during constant adsorption/regeneration cycles. The structured adsorbent bed is a potential breakthrough adsorption technology to reduce the capital cost and energy consumption of an adsorption process.