Immobilised solvent systems for carbon capture
AffiliationChemical and Biomolecular Engineering
Document TypePhD thesis
Access StatusOpen Access
© 2019 Dr. Thomas Charles Reading Moore
Microencapsulated solvents (MECS) are a novel approach to carbon capture, with the potential to reduce unit operation volumes by 1-2 orders of magnitude, and to allow a wide range of solvents to be contacted with a flue gas stream in a practical way. In this technology, small droplets of a solvent which selectivity absorbs CO2 are encapsulated inside thin polymer shells, which immobilise the liquid but allow CO2 to easily pass through. The capsules have diameters of 100 - 600 μm, which corresponds to a surface area 1 - 2 orders of magnitude greater than the specific area of a fluid flowing over random or structured packing in a traditional absorber. A fluidised bed containing fine MECS could plausibly be over 10 times smaller than a traditional absorber, and may allow solvents with slow absorption kinetics to capture CO2 in a practical way. In Chapter 3, it is found that microencapsulation is unlikely to affect the gas flux for concentrated chemical solvents, while for physical solvents it may increase the gas flux under some circumstances, as the reduction in spatial scales increases concentration gradients within the fluid. Overall, microencapsulation may increase the gas absorption rate by an order of magnitude for chemical solvents, and by 2 orders of magnitude for physical solvents. In Chapter 4, a novel material for carbon capture, Solvent Impregnated Polymers (SIPs), is proposed. SIPs have many of the favourable properties of MECS, but are more scalable to manufacture. SIPs were manufactured containing various solvents for CCS, including K2CO3 solutions, an ionic liquid, and a Nanoparticle Organic Hybrid Material. A validated mass transfer model found that SIPs could increase the gas flux by a factor of 2-4 when immobilising solvents in the pseudo-first order reaction regime, and by 1-2 orders of magnitude for solvents in the instantaneous reaction regime. A 50-fold increase in flux was experimentally observed in a SIP containing a Nanoparticle Organic Hybrid Material. If such an increase in flux were combined with an increase in surface area of 1-2 orders of magnitude, this could plausibly lead to a 3-4 order of magnitude increase in the specific gas absorption rate, which may enable slower solvents to be used in a practical way. In Chapter 5, the energy savings which SIPs could provide were analysed. Because of the remarkable efficiency of modern amine-based CCS processes, it is found that even SIPs or MECS containing `optimal' solvents are unlikely to provide significant energy savings, especially if steam regeneration is used. On the other hand, it is possible that SIPs or MECS containing solvents which may be regenerated using low-grade waste- or electrically-generated heat could be used in novel processes at reduced cost. Furthermore, it is found that an optimal solvent for CCS requires both a reasonably large enthalpy of absorption balanced by a sufficiently large entropy of absorption. In the development of novel solvents for CCS, the significance of the entropy of absorption has largely been ignored, and this should be considered more carefully into the future.
Keywordsabsorption; carbon capture; mass transfer; thermodynamics; CCS
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