Polyvinylamine/polysulfone composite membrane with excellent performance by facilitated transport for CO2 capture from exhaust gas of power plant
Advancing the chemical engineering fundamentals
Membranes and Membrane Science (T2-8P)
Keywords: CO2, exhaust gas, membrane, separation, facilitated transport
Introduction
There is increasing concern over the global warming by the increasing CO2 concentration in the atmosphere, and electricity generation from fossil fuel combustion produces almost 35% of total world anthropogenic CO2 emissions.
The removal of CO2 by absorption into aqueous alkanolamine solutions is widely used in the chemical industry. However the membrane application for CO2 capture is getting more attention because of the simplicity, lower energy cost compared to the conventional alkanolamine absorption method.
Especially facilitated transport membrane is attractive with high permeance and selectivity at the same time overcoming the drawbacks of the previous liquid membrane or ion exchange membrane.
Over the last 5 years MEMFO (membrane research group at NTNU, Norway) has been developing fixed-site-carrier (facilitated transport) membranes using polyvinylamine, one of the simplest amine group containing polymer forms.
CO2 is transported through this membrane by both the solution-diffusion and the fast “hops” between reactive fixed-site-carriers while N2 is transported only by the solution-diffusion.
Normal membranes without carrier sites may lose separation ability when swollen by the water vapor in the system, while this polyvinylamine membrane shows even more enhanced CO2 transport in the presence of water molecules by reversible formation of bicarbonates.
Methodology and results
Polyvinylamine solution of carefully adjusted concentration to obtain a required thickness was cast onto polysulfone support. The dried polyvinylamine/polysulfone composite membrane was then cross-linked with ammonium fluoride solution.
Permeance and selectivity of the membrane were measured with an experimental setup equipped with humidifiers to study the effect of water vapor in the system as well. This humidified gas system study is important because combustion gas may always contain water vapor which can affect the performance of normal membranes seriously.
The membrane performance was tested at various conditions (pressure of 2-15bar and temperature of 25-55˚C and relative humidity of 30-95%).
It was found out that both CO2 permeance and CO2 selectivity over N2 were dependent on humidity change in the system. The permeance increased continuously with humidity increase and it even exceeded over about 0.3m3(STP)/m2•bar•hr, while the selectivity (CO2/N2) showed convex curves with maximum of approximately 120-250 depending on temperature and pressure. This performance data (especially CO2 permeance) is much more advanced result compared to the performance data first reported in our previous work [1] or the works reported so far by others.
Selectivity increased with increased pressure while permeance decreased, which needs complicated explanation by the typical facilitated transportation characteristics and membrane compaction caused by pressure.
Conclusions
The novel polyvinylamine fixed-site-carrier membranes showed excellent CO2 capture performance (high permeance, excellent selectivity and durability) under the presence of water vapor in the system, which implies a possibility of omitting water removal unit from the CO2 capture process when applying this membrane for real power plant process.
The excellent CO2 selectivity over N2 (150–250) even at relatively high pressure (10-20 bar) has a great potential of replacing the conventional CO2 capture process using alkanolamine solutions.
Reference
Taek-Joong Kim, Baoan Li, May-Britt Hägg, Journal of Polymer Science: Part B: Vol. 42, 4326-4336 (2004)
Presented Tuesday 18, 13:30 to 15:00, in session Membranes and Membrane Science (T2-8P).
