Development of Novel PVAm/PVA Blend FSC Membrane for CO2 Capture

Abstract

As membrane technology is energy-saving, environment-friendly and at lowcost with a small footprint, it has great potential to become a green and efficient CO2 separation technology. Membranes with both high CO2 permeance and selectivity are desired for membrane separation processes to be an alternative to the currently most commonly used amine adsorption process in medium to large scale applications.

In this work, polyvinyl amine / polyvinyl alcohol (PVAm/PVA) blend fixed-sitecarrier (FSC) CO2-selective membranes were developed, and composite membranes with the homogeneous PVAm/PVA blended selective layers (0.3-25 μm) on PSf porous supports were prepared to meet the requirements for competitive CO2-selective membranes. Different preparation conditions, such as the blend ratio of PVAm/PVA, molecular weight and degree of hydrolysis of the polymers, cross-linking conditions, selective layer thickness and membrane support material, were evaluated to optimize the PVAm/PVA blend membrane preparation procedure and improve CO2 separation performance. The operating parameters were investigated with respect to relative humidity (0-95%), feed flow rate (1.0-5.0 ml/s), sweep gas flow rate (0.5-1.5 ml/s) and feed pressure (2-15 bar). CO2 permeance up to 0.83 m3(STP)/(m2.h.bar) with CO2/N2 selectivity up to 174 and CO2 permeance up to 0.45 m3(STP)/(m2.h.bar) with CO2/CH4 selectivity up to 50 were documented during the permeation tests at their respective optimized operating conditions. The membrane showed good reproducibility in a 6 repeated test runs and excellent stability during a 5 month test.

The excellent CO2 separation performance, as well as the good mechanical strength and stability of this membrane are believed to be a result of the entanglement of PVAm with PVA chains in the blend membrane and the facilitated transport mechanism. Amino groups in PVAm work as CO2 facilitated transport carriers while PVA offers an enhanced polymeric network with good membrane forming properties. The reversible CO2 hydration reactions, initiated by the weak basic amino carriers in membrane selective layer are believed to facilitate the CO2 transport, therefore CO2 is transported through the PVAm/PVA blend membrane in the form of bicarbonate ions, which results in both high permeability and selectivity of CO2 over other gases.

The process simulation of a biogas upgrading plant (1000 Nm3/h) using membrane separation based on data from the PVAm/PVA blend membrane was conducted. Processes with four different membrane module configurations with or without recycle were evaluated technically and economically. The 2-stage in cascade with recycle configuration was proven to be optimal for this process. The sensitivity of the process to various operation parameters was analyzed and the operation conditions were optimized.

Since swelling of the membrane unavoidably causes the loss of mechanical strength, and the packing effect diminishes the membrane swelling capacity at high pressures, Carbon Nanotubes (CNTs) were incorporated to reinforce the PVAm/PVA nanocomposite membrane and hence improve the membrane performance. The CNTs and PVAm/PVA blend polymer was proven compatible. Experiments showed that the degree of swelling of the CNTs reinforced nanocomposite membranes (1.0-5.0 μm) increased, and the CO2 permeance increased 5-10 times at high pressures (10 bar and 15 bar in this work) while the CO2/CH4 selectivity remained similar to its counterpart without CNTs.