Fundamentals by Michael North Peter Styring

Author:Michael North, Peter Styring
Language: eng
Format: epub
Publisher: De Gruyter
Published: 2019-10-01T16:34:14.491000+00:00

Figure 9.15: Simplified schematic of a membrane gas–liquid contactor separating two gasses.

A significant issue that arises with the highly porous materials used in hollow fibre contactors is wetting [47]. This occurs when the liquid (which is usually aqueous) enters the pores, and can cause significantly increased resistance to mass transfer across the membrane. A common strategy to mitigate this is to increase the surface hydrophobicity of the membrane. PTFE and other fluoropolymers have intrinsic hydrophobicity, but they are relatively expensive. Surface modification of cheaper polymers may provide a more economical alternative. In one study, CO2 absorption by a mixed piperazine/2-amino-2-methyl-propanol aqueous absorbent was investigated using both native polypropylene (PP) and PP that was plasma treated in the presence of CF4 (forming a hydrophobic fluorous surface) [48]. The water contact angle was found to increase with plasma treatment time up to a certain point after which it reached a plateau (ascribed to establishment of equilibrium between making and breaking surface C–F bonds). The wetting ratio was significantly reduced from 0.0674% to 0.027% following plasma treatment. Increases in CO2 transfer rates and improved membrane durability were also observed for the plasma-treated membrane, which was found to have a similar mass transfer coefficient to PTFE membranes.

A related approach to avoid unwanted wetting is to use composite membranes in which the standard membrane material is coated with a thin layer of a denser, hydrophobic polymer material. In a recent example, a standard PP hollow fibre membrane was coated with a thin layer of Teflon AF-2400, a glassy amorphous fluoropolymer that has very high permeability to a range of gasses along with excellent chemical resistance [49]. Surprisingly, the Teflon AF-2400 coating, whilst protecting against wetting, caused almost no additional resistance to CO2 transfer through the membrane (into monoethanolamine solution). For the other two coatings assayed (PDMS and poly(trimethylsilylacetylene)), the composite membrane had significantly poorer CO2 transport properties when compared with the native PP hollow fibre membrane material.

Although this has been only a rather brief survey, and has not included many polymer materials that show great promise in CO2 separation technologies (including Polymers of Intrinsic Porosity, PIMs) [50], it is hopefully clear that polymer membranes have the potential to make a huge contribution to mitigation of the effects of anthropogenic CO2 emissions in future. The next section highlights another aspect of polymer membrane technology, but focuses on CO2 utilisation rather than CO2 separation.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.