Abstract
The overall aim of this research is to successfully fabricate a functional polyimide (PI) foam structure comprising high proportion (80 wt%) of adsorbent and antibacterial agents for the removal of pollutant gases and microbes from air. The need for such a filter arises due to the lack of existing filters that can be regenerated for reuse and also the ease of being able to use existing infrastructure for the production of these foam filters on a large scale.The foaming process involves CO2 generation (blowing) and polymerisation reactions occurring simultaneously. Foam structures containing sorbents namely 13X, HiSiv 3000, HiSiv 1000 and potassium bicarbonate were tested against a range of gases of different molecular sizes such as CO2, butane, cyclohexane and hexane to determine the adsorption performance of the foam filter. PI foams containing antibacterial agents such as copper, nickel and copper - nickel mixtures were synthesized and tested against a model bacterium, Erwinia carotovora (gram-negative), to determine the antibacterial efficacy and filtration efficiency of the air filter.
Highly loaded PI/13X (80 wt%) adsorbent foams were characterised in the context of removing CO2 from air. Thermal analysis of pure PI and 13X powder showed that the foams can be regenerated at 300 ℃. Polyvinylpyrrolidone (PVP) of different molecular weights (10 kDa, 40 kDa and 58 kDa) were used as pore former to enable more adsorption sites to be exposed to CO2. In dynamic adsorption breakthrough experiments at 101.325 kPa and 20 ℃, 10k PVP foams demonstrated an equilibrium loading of 0.05 g g-1 for CO2 (at 40,000 ppmv in air), showing the longest equilibrium time and highest adsorption capacity. The adsorption loading of the foams (0.094 g g-1) were found to be comparable in performance to commercial 13X beads (0.098 g g-1) at 40 mbar, when tested using pure CO2 in an Intelligent Gravimetric Analyser (IGA). At pressures beyond 10 kPa, only 1.3 - 4.5% reduction in adsorption capacity was observed due to some of the zeolite being covered by the polymer binder. The adsorbent foams showed superior CO2/N2 selectivity compared to other adsorbent structures in literature and was comparable in selectivity to commercial 13X beads. However, the incorporation of PVP into the foam structure did not result in drastic increment in CO2 adsorption. On the other hand, PI foams incorporated with potassium bicarbonate replacing 13X and PVP, improved the CO2 adsorption capacity of the foam by approximately 74% and it also reduced the regeneration temperature from 300 ℃ to 165 ℃.
The butane adsorption performance of HiS-3 (PI/HiSiv 3000 (80 wt%)), HiS-1 (PI/HiSiv 1000 (80 wt%)) and HiS-3,1 (PI/HiSiv 3000 (40 wt%) - HiSiv 1000 (40 wt%)) foams were compared against the PI/13X (80 wt%) adsorbent foams. HiS3 foams displayed the highest adsorption capacity of 6.7 wt% for butane whilst HiS-1 foams were the worst performing with 0.97 wt%. The butane study also showed that the PI/13X foams were not only efficient in removing CO2 but were also able to achieve a 3.7 wt% adsorption capacity for butane as well. The cyclohexane and hexane isotherm studies showed that the HiS-3 foams were limited by the shape of the zeolite structure and the size of the zeolite pore diameter whilst HiS-1 foams achieved a high adsorption capacity and thus, were found to be ideal for removing cyclohexane and hexane. In order to deal with multiple pollutants in the environment, a combination of zeolite types could be incorporated into the foam structure to successfully remove the pollutants.
PCu80 (PI/copper (80 wt%)) displayed a high log reduction value (LRV) of 99.996% and thus exhibited a bactericidal effect. PNi80 (PI/nickel (80 wt%)) displayed a lower LRV of 99.4%. However, the LRV values were higher compared to the control, 95.5% (PI foam without antibacterial agent) and thus, confirmed bacteriostatic effect. PCu64Ni16 (PI/copper (64 wt%) - nickel (16 wt%)) exhibited and sustained exceptional microbe removal efficiencies of 99.9997% for 24 hrs at high humidity levels and demonstrated the highest zone of inhibition (ZOI) value of 33.90 ± 0.16 mm compared to PCu80 (27.5 ± 1.1 mm). Nickel strongly inhibited the proliferation of bacteria whilst copper killed the bacteria on the foam filters. Therefore, such functionalized filters can potentially overcome the inherent limitation in conventional filters and imply their superiority for controlling indoor air quality.
Computational Fluid Dynamics simulations were successfully implemented in order to understand the CO2 adsorption behaviour on the new foam filter. Such modelling proved to be invaluable in understanding the adsorptive behaviour through the complex foam structures as this is difficult to achieve experimentally. Experimental pressure drop data obtained from the foams, were successfully reverse engineered to achieve a correlation between porosity and the Forchheimer drag coefficient. As such, interpolation can be done using the correlation to predict the pressure drop of the prepared polyimide foams based on their porosity.
| Date of Award | 28 Apr 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Semali Perera (Supervisor) & John Chew (Supervisor) |