(329c) Novel Membrane Absorption-Based Processes for CO2 Capture

This presentation will deal with novel membrane absorption and stripping based processes and devices for CO₂ absorption and stripping for carbon capture and sequestration. Two feed streams are considered: simulated flue gas at around 40-50⁰C; lower temperature (L-T) post-shift reactor synthesis gas at high pressures and temperature up to 100⁰C where He has been used as a surrogate for H₂.

Conventional methods for CO₂ capture generally employ an absorption and stripping process. The CO₂ is absorbed for example in an aqueous solution of amine e.g., monoethanolamine and then stripped out in a separate tower with steam at 120⁰C. Such a process is highly energy intensive. Further the amine used is volatile. The low partial pressure of CO₂ in the flue gas inhibits the application of CO₂-selective membranes unless methods are employed to increase the CO₂ partial pressure in the flue gas to be treated. Adsorption processes using novel sorbents having high CO₂ adsorption capacity as well as other separation methods are being studied. We will describe novel membrane-based techniques to potentially bypass most of the shortcomings of many existing approaches. We have employed a simulated humidified flue gas containing around 14% CO₂ and demonstrated successful removal of bulk of the CO₂ and its recovery in a CO₂-concentrated stream.

To separate hydrogen from the low temperature (L-T) post-shift reactor synthesis gas and simultaneously obtain a purified CO₂ stream, an advanced pressure swing membrane absorption (PSMAB) technology utilizing a hollow fiber membrane contactor device and a cyclic absorption-desorption process is being developed. Studies have been carried out with around 40%CO₂-rest He feed gas mixture coming in at temperatures up to 100⁰C and pressures varying from 100 to 250 psig (689-1723kPag). The nonvolatile absorbent liquid consists of an ionic liquid 1-butyl-3-methylimidazolium dicyanamide ([bmim][DCA]) containing a suitable amine. Studies were carried out with porous hydrophobized polyether ether ketone (PEEK) hollow fiber membrane modules of small sizes having different characteristics. It was determined that reducing the dead volume on the module tube side and decreasing the fiber packing density greatly enhanced gas absorption and improved product qualities while increasing temperature had a negative impact on product qualities with pure [bmim][DCA] as absorbent. Adding nonvolatile amines to the ionic liquid showed an improvement for CO₂ absorption capability especially at higher temperatures. The measured CO₂ concentration in the CO₂-rich product stream from the less-than-optimum membrane modules has varied between 90.7-92.9% depending on the temperature. A mathematical model has been developed and numerically solved to predict the cyclic process performance of pure ionic liquid as the absorbent. Solubility and diffusion coefficient data for the gases needed in this model were determined for the pure ionic liquid absorbent using a pressure-decay-dual transducer apparatus at temperatures of 323, 353, 363, and 373 K and at pressures up to 1.38 MPa (~200 psig). The solubility selectivity of CO₂ over He was also determined for various amine concentrations in the ionic liquid with or without water.