Incorporation of cellulose nano-crystals in polyvinyl amine (PVAm) matrix for biogas upgrading at high pressure

Abstract

Driven by the energy demand, the humanity has consumed the fossil fuels to the brink of depletion and in doing so, damaged the natural ecosystem by global warming. Eventually the fossil fuels will run out. However, the energy demand will keep on increasing due to increase in global population. For this purpose, attention must be shifted towards renewable and green energy sources to fulfill the increasing energy needs with no negative environmental impacts. Biogas is one of the candidates in this regard. To use biogas as high quality fuel it needs to be upgraded. Upgraded biogas has methane contents > 95% which are sufficient to be used as fuel for vehicles and industrial sector. To upgrade biogas, it is necessary that such process be adopted which gives maximum efficiency at low energy consumption. Membrane technology has multiple advantages over other processes. Furthermore, membrane technology is economical, green process and can be easily retrofitted in the existing technologies. Permeance and selectivity are major parameters that defines the separation performance of membranes. High selectivity and permeance are required so that upgraded biogas can be obtained at low cost. Different gas separation membranes have already been investigated for their gas separation performance. The membrane can be made from variety of materials such as carbon, metallic, ceramic and polymeric. Polymeric membranes have advantage over other membrane materials due to their low cost and optimized performance. Carrier mediated facilitated transport membranes are relatively new as compared to other membranes. These membranes make use of fixed site carrier and facilitated transport for gas separation by selectively transporting one component in gaseous mixture across the membrane. Furthermore, the separation performance of these membranes can be enhanced by introducing moisture which provides –OH groups for facilitated transport. This work is focused on developing carrier mediated facilitated transport membranes with polyvinyl amine (PVAm) as main polymeric matrix and cellulose nano crystals (CNC) as additive. The membranes were casted on microporous polysulfone support. The surface morphology and cross-section of the composite membranes were investigated by scanning electron microscopy which showed defect free and smooth surface. Cross sectional images help in determining the thickness of selective layer. It was seen that increasing CNC concentration caused an increase in thickness of selective layer. Furthermore, degree of swelling for membranes with different CNC concentrations have also been investigated and membrane PVAm/1 CNC showed highest degree of swelling of 73%. Data from degree of swelling helped in the study of effect of relative humidity on permeance and selectivity of membranes. XRD analysis was done for determination of crystallinity of membranes. It was seen that increase in CNC concentration caused a rise in crystallinity of membranes. The highest crystallinity was obtained for membrane PVAm/1.5 CNC i.e. 65%. Finally, permeation testing of all membranes was carried out by passing CO2 and CH4 individually at varying pressure of 5, 10 and 15 bar. The effect of pressure, CNC concentration and relative humidity on permeance and selectivity were investigated. It was seen that increasing the pressure deteriorated the membrane performance by showing a decrease in CO2 permeance and selectivity. As for CNC concentration, membrane performance increased up to 1 wt.% CNC concentration and then started to decrease at 1.5 wt.% CNC concentration. The membrane PVAm/1 CNC showed the best results with CO2 permeance of 0.0216m3(STP)/m2.bar.hr. and selectivity of 41. The results showed that PVAm/CNC membranes are one of many options that can be utilized for biogas upgrading. However, further research can lead to better performance of these composite membranes enabling them to be used on commercial biogas plants