(226a) Rapid Thermal Swing Adsorption in Microchannels

Separating mixtures of gases is the most challenging separation process due to the tendency of gases to form homogenous mixtures, which are difficult to separate into their component parts. A few approaches to separate gases have proven commercially viable, but there remains a large margin for improvement. Application of microchannel process technology to thermal swing adsorption (TSA) offers the potential to split small-scale, concentrated gas mixtures economically, which has proven to be a vexing problem for existing commercial gas separation technologies.

The three most popular methods of separating gas mixtures are cryogenic distillation, absorption, and pressure swing adsorption (PSA); however, each of these has its unique shortcomings and challenges. Cryogenic systems overcome the gas-gas separation challenge by cooling the mixture to extremely low temperatures, thereby turning them into liquids. The liquids are then separated by fairly conventional distillation techniques. The equipment required to cool and separate the gases is very expensive and does not scale-down economically, which relegates cryogenic systems to only the largest applications. Absorption scrubs one gas from another in a liquid-gas contact tower. This is effective for relatively dilute solutions but less appropriate for concentrated mixtures, where the quantity of absorbent fluid becomes a processing challenge. PSA is the technique most commonly used for separating relatively small-scale, non-dilute gas mixtures, but it also has problems. PSA affects separation by pressurizing and depressurizing a series of adsorbent beds, so a large pressure penalty is imposed. These depressurized gases must be compressed to reenter the process, which is a capital and energy intensive unit operation.

Thermal Swing Adsorption (TSA) is another alternative that has many advantages, and historically, one major disadvantage. By swinging the gas mixture's temperature, instead of the pressure, compression costs can be avoided. However, the time to swing adsorbent beds over a temperature range sufficient to affect the separation can be relatively long, which means the equipment must be very large and therefore economically unattractive. Microchannel architecture can overcome this historic shortfall by rapidly swinging temperatures, thereby greatly reducing the size of TSA processing equipment. The microchannel approach to TSA is based on using this technology's enhanced heat and mass transfer capabilities to achieve rapid temperature swings of the adsorbent bed. Enhanced heat transfer allows TSA cycle times of seconds, compared to several minutes or hours for conventional systems. Improved mass transfer allows sufficient adsorption to occur during the shorter cycles.

This presentation will describe the application of rapid TSA to the separation of nitrogen from methane, which is one of the most significant challenges in recovering low-purity methane streams. Results from laboratory experiments show that microchannel adsorbent beds can be swung over the desired temperature range in cycles less than 5 seconds. It has also been demonstrated that flowing and purging gas mixtures through the device concentrates the methane product stream. Successful development of this application could enable the recovery of 3.5 trillion standard cubic feet of sub-quality natural gas.