Chemical and Biomolecular Engineering - Theses
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ItemMembrane processes for water recovery from brown coal flue gases( 2014)Victorian brown coal has a significantly higher moisture content compared to black coal. Hence, it is possible that significant quantities of water can be recovered from high temperature brown coal flue gases and recycled in the process given that it is a high enough purity. The presence of CO2 and SOx will result in acidic water that may cause corrosion issues. The use of membranes to selectively permeate water over other flue gas components such as N2 and CO2 has the potential to provide a water stream of high purity. While N2 is the major component in flue gas, in this instance the presence of CO2 also needs to be investigated because of its high acidity. This work considered the separation performance of Nafion 115, Sulphonated Poly (Ether Ether) Ketone (SPEEK) and 6FDA – TMPD (2, 2 – bis (3, 4 – dicarboxyphenyl) hexafluoropropane dianhydride – 2, 3, 5, 6 – tetramethyl – 1, 4 – phenylenediamine) at elevated temperatures. Water, CO2 and N2 permeation properties of these membranes were investigated on a novel high temperature mixed gas rig that utilized mechanical agitation to eliminate concentration polarization. The permeation of water, CO2 and N2 through Nafion 115 was investigated as a function of water activity at 70 – 150 °C. It was found that all permeances increased with increasing water activity but reduced with increasing temperature. This data was supplemented by sorption analysis at lower temperature which conversely showed decreasing solubility as temperature increased. The sorption results were modelled using a modified Dual Mode Sorption Model where Arrhenius expressions were used to model the effect of temperature on the water concentration absorbed into the polymer. The changes in solubility and permeance with water activity were attributed to membrane swelling as water activity increased. A comparison with a thinner perfluorosulphonic acid polymer (Fumapem F - 920) from an alternate supplier suggested that permeance did not scale linearly with membrane thickness, reflecting non – linear water activity gradients within the membrane and thus inhomogeneity in membrane swelling. The effect of temperature on water vapour and CO2 permeation properties of Sulphonated Poly (Ether Ether) Ketone (SPEEK) with two different ion exchange capacities (IEC) were investigated. These were Fumapem E - 630 and Fumapem E - 540 - GF with IEC 1.6 meq/g and IEC 1.9 meq/g, respectively. It was found that both permeabilities increased with increasing water activity due to increased water solubility as water concentration increased. This was supported by FTIR spectra that measured peaks for water sorbed into the SPEEK polymer and confirmed the presence of the water as clusters. Both water vapour and CO2 permeabilities increased as temperature increased up to 50 °C. This was due to the increase in diffusion of the penetrating molecules. However as temperature increased further, increased water uptake appeared to result in significant formation of water clusters that hindered the diffusion of isolated molecules. This decrease in diffusion coupled with a reduction in solubility with increasing temperature resulted in a significant drop in water permeability above 50 – 70 °C. Similar results have been obtained by other researchers and were attributed to the reduction in solubility overcoming the increase in diffusivity with increasing temperature. The permeabilities at 70 – 150 °C were modelled using Arrhenius expressions. Activation energies of permeation of ~ -46 kJ/mol for IEC 1.6 meq/g and ~ -43 kJ/mol for IEC 1.9 meq/g were obtained for this temperature range. SPEEK with IEC 1.9 meq/g exhibited higher permeation and selectivity than IEC 1.6 meq/g. This is due to the increased presence of the sulphonic acid groups that are known to improve the performance of any membrane. Water vapour and gas permeation properties of 6FDA – TMPD from 25 – 150 °C were investigated. The water sorption properties from 25 – 35 °C were also investigated and resulting sorption parameters used to model water permeability at the same temperature range. It was found that the infinitely dilute Fickian diffusion constant is temperature – dependant probably due to water sorbed in the polymer plasticizing the membrane and forming clusters at higher water activities and temperatures. A similar permeability trend to SPEEK was obtained where water vapour, CO2 and N2 permeability increased with temperature up to 50 °C but decreased above this. This was likely due to a significant reduction in solubility as temperature increases. It might also be influenced by the formation of clusters that hinder diffusion. Similar results for this polymer were reported by others with “anti – plasticization” or clustering behaviour observed in other polymers. It is also well known that the presence of water reduces the glass transition temperature. It appears that 6FDA – TMPD transitions from a glassy to a rubber state at a Tg of ~ 55 °C. Furthermore, CO2 and N2 permeabilities decrease with increasing feed water activity which suggests the competitive sorption of water. These competitive sorption and plasticization effects were investigated further at high temperatures by comparing water permeability data from a H2O/CO2 feed mixture to that from a H2O/N2 feed mixture. It was found that at low temperatures, the competitive sorption of water reduces sorption of both CO2 and N2. This results in similar water permeabilities through the membrane for both gas mixtures. However as temperature increases the sorption of water is reduced, resulting in increased water permeability for H2O/CO2 feed mixture. This is due to the increased sorption of CO2 and consequently increased CO2 plasticization effects. The permeance data for water and CO2 at 150 °C were compared for all five polymers. The highest H2O/CO2 selectivity was for Nafion 115 followed by Fumapem F - 920, Fumapem E - 540 - GF and Fumapem E - 630 with 6FDA – TMPD having the lowest selectivity. The permeance data was modelled within Aspen HYSYS and it was found that a permeate stream with pH 5.67 is achievable with Nafion 115 at 150 °C. However, the membrane areas required for this are very large indicating that there exists a trade – off between permeate purity and membrane area. A high purity product is required for reuse in the process therefore pH adjustment of the recovered stream would be needed regardless of the membrane material used.