Carbon dioxide absorption with non-porous hollow fiber membrane contactors - Module fabrication and characterisation
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Carbon capture and storage (CCS) is recognised as one of the key technologies to reduce greenhouse gas emissions from industrial plants. Currently, the most established technology is chemical absorption with amines, but it has yet to be widely deployed mainly due to high energy and space requirements. Membrane contactors offer technology that can overcome these challenges by combining the high selectivity of absorption, and compactness and modularity of membranes. Third generation absorbents such as 3D3M have significantly lower regeneration energy than the conventional 30 wt% MEA absorbent in post-carbon capture applications. However, they are highly volatile which can cause problems for commercial applications. A solution can be to use non-porous membrane contactors to prevent wetting of the membrane and reduce amine emissions. The aim of this work was to prepare hollow fiber modules with sufficiently large mass transfer areas for testing in the membrane contactor. The modules were prepared with polypropylene as porous support and AF2400 as dense layer. AF2400 was chosen due to its ability to prevent amine evaporation. To add the dense layer to the porous support, bore-coating of modules was chosen as the preferred method. Since no relevant procedure was found in literature, a procedure was developed as the first part of this work.A procedure for modules with two hollow fibers was successfully developed producing thin (1um), defect free dense layers with good coverage and no polymer pore penetration. The procedure was not easily scalable to 20-fiber modules, but by adjusting the technique non-porous membranes were produced for testing in the membrane contactor. Porous modules with mass transfer area of ca. 50 cm2 were tested in the membrane contactor with 1M NaOH as absorbent, and non-porous modules were tested with 1M NaOH, the benchmark absorbent 30 wt% MEA and the third generation absorbent 3D3M. The effect of gas velocity, liquid velocity, temperature and CO2 in the feed were carefully examined to optimise the separation process. It was found that the overall mass transfer coefficients decreased with 23\% for coated compared to uncoated modules at 25oC and 13 vol% CO2 in the feed gas stream. The resistance offered by the liquid phase was also examined finding that it increased as a function of CO2 concentration in the feed gas. This indicated the formation of a boundary layer at the membrane/liquid interface. In addition, 3D3M was found to have lower overall mass transfer coefficients compared to 1M NaOH and 30 wt% MEA which is probably due to its high viscosity. A simplified resistance in series model was also developed to validate the results. By including the mass transfer resistance offered by the boundary layer due to absence of reactive moieties at the membrane/liquid interface, the models corresponded well to the experimental results.