CO2 absorption in microstructured membrane reactors.
Doctoral thesis, UCL (University College London).
The objective of this work is to study experimentally and theoretically novel multiphase microreactors and characterize them in relation to hydrodynamics and mass transfer, in order to evaluate, understand and improve their performance. In order to achieve this CO2 absorption in sodium hydroxide and amine solutions an example of a fast gas-liquid reaction has been investigated in a single microstructured metallic mesh reactor, CRL reactor, PTFE single channel membrane reactor and the silicon nitride mesh reactor. CO2 absorption in sodium hydroxide solution was initially studied experimentally and theoretically in a metal microstructured mesh reactor. The differential mass balances to describe the concentration profiles of components in the three domains (gas/membrane/liquid), were solved with Comsol Multiphysics (modeling software for finite element analysis of partial differential equations). The model indicated that the carbon dioxide is consumed within few microns from the gas – liquid interface, and the dominant resistance for mass transfer is located in the mesh because it is wetted by the liquid reactant. In order to overcome the limitation of the extra resistance to the mass transfer in the metallic mesh, PTFE membranes were used in the single channel reactor, which are considered as hydrophobic to aqueous solutions of NaOH and amines. Monoethanolamine solution (MEA) absorbed more CO2 than diethanolamine (DEA) since the reaction rate constant for MEA is higher than DEA. 8 channel (PTFE) microreactor showed much higher CO2 removal efficiency than the metallic mesh microreactor. Furthermore the model indicated partial-wetting of the PTFE membrane when NaOH solution was used as an absorbent. In order to enhance mass transfer staggered herringbones were used on the floor of the liquid side of the single channel PTFE microreactor. No enhancement of mass transfer was observed with the use of staggered herringbones. A possible reason for that is that a limit for the fast second-order reaction is reached for enhancement and that the apparent reaction rate is independent from mass transfer for our case, or that the herringbones are far away from the reaction zone and cannot create the appropriate stirring for enhancement. In order to increase throughput, carbon dioxide absorption in sodium hydroxide solution was performed in the metallic mesh ‘scale-out’ reactor (with 4 meshes). CO2 removal efficiency for the ‘scale-out’ reactor was significantly lower than the single mesh reactor, which is probably due to breakthrough of liquid in the gas phase (stagnant liquid) or uneven flow distribution in each plate of the ‘scale-out’ reactor. Finally a silicon nitride mesh reactor developed by Bayer Technology Services and FluXXion was used for CO2 absorption in aqueous solutions of NaOH and DEA. The silicon nitride mesh reactor showed better performance than the PTFE single channel reactor, the metallic 8 channel reactor and the CRL mesh reactor when NaOH was used, due to the very thin membrane of 1 μm thickness, which makes the resistance to mass transfer very small.
|Title:||CO2 absorption in microstructured membrane reactors|
|Open access status:||An open access version is available from UCL Discovery|
|UCL classification:||UCL > School of BEAMS > Faculty of Engineering Science > Chemical Engineering|
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