Testing and Optimization of PVAm/PVA Blend Membranes for Biogas Upgrading
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Membrane technology is an energy saving, environment friendly and low cost separation technology. This master’s thesis has focused on testing and optimization of polyvinylamine/polyvinylalcohol (PVAm/PVA) blend fixed-site-carrier (FSC) membranes on polysulfone supports for separation of carbon dioxide (CO2) and methane (CH4). Permeation tests, process simulation and cost estimation were applied to evaluate CO2/CH4 separation performance and process feasibility for biogas upgrading. Utilization of biogas as a natural gas substitute or as vehicle fuel can contribute to reduced greenhouse gas emissions. Orthogonal experimental design (OED) was employed to study the influences of membrane preparation conditions on the transport properties of flat sheet PVAm/PVA blend FSC membranes. The conjoint analysis method was applied for the statistical analysis of OED results using SPSS software, and the importance of the investigated membrane preparation condition parameters on the CO2/CH4 separation performance was found to be: polymer concentration in casting solution > heat treatment temperature > heat treatment duration > content of carbon nanotubes (CNTs) in polymer. The optimized membrane preparation conditions in the interval investigated were: 1 wt% polymer in the casting solution, containing 3 wt% of CNTs, heat-treated at 105 °C for 0.5 h. It was found that a membrane with a very thin selective layer (375 nm) was able to achieve both high CO2/CH4 selectivity and CO2 permeance. Reinforcing the PVAm/PVA membrane with CNTs was investigated, but no significant effect was found within the range of investigation. SEM analysis has shown that CNTs gathers in large aggregates, and that an even distribution of well-dispersed CNTs is needed to secure a defect free selective layer. Permeation tests were performed in an advanced mixed gas permeation rig and operating conditions were optimized on the basis of OED and conjoint analysis by SPSS software. The relative importance of the operating condition parameters investigated in this work was in the following order: relative humidity > sweep gas flow rate > feed gas pressure > feed gas flow rate. The optimized operating conditions were found at a feed gas pressure of 2 bar with a relative humidity of 80 % and a feed gas flow rate and sweep gas flow rate at 12.3 cm3/s and 0.18 cm3/s, respectively. CO2/CH4 selectivity of 31 with a CO2 permeance of 0.16 m3(STP)/(m2.h.bar) was obtained at optimized conditions. A conceptual design of a biogas upgrading process with a feed gas flow rate at 300 Nm3/h (60 vol% CH4 and 40 vol% CO2) was conducted. Two different process designs with a feed gas pressure of 2 and 5 bar were simulated in UniSim. In a two-stage membrane module separation system with recycle it was possible to purify biogas up to 99.3 vol% CH4 (vehicle fuel quality), and obtain a CH4 recovery of 98 %. The total membrane area was reduced a lot by increasing the feed gas pressure from 2 to 5 bar. The capital cost of the most promising process design was estimated to US$4.622 million, and the running costs were estimated to be US$0.603/Nm3 upgraded biogas. The total membrane area was 7900 m2. The most important economic parameter for upgrading biogas is the price of upgraded biogas as vehicle fuel, and a price of US$1.22/Nm3 is necessary to secure a positive net present value of the project after 10 years.