CO₂ Absorption In A Hollow Fiber Membrane Contactor And Its Sorption Characteristics In A PVA Facilitated Transport Membrane

Recently, with deterioration of the global greenhouse effect, dealing with the greenhouse gas emissions has been a hot topic all over the world. As a most promising emissions-reducing method, carbon capture and storage(CCS) technology could solve the carbon reduction problems in the field of power industry and is also a most promising way to tracle global climate change. Currently, chemical absorption is the commonly technique used in the coal-fired power plants. However, it has some disadvantages like large equipment and occupation area, heavy weight and hard to separately control gas and liquid phases. Therefore, membrane technology is regarded as a novel and potential method for CO2 capture due to the advantages like small occupation area, light weight, easy to control both phases and large gas and liquid contacting area.CO2 capture is the main objective of this study. An experimental system of CO2 abosrption and desorption with use of heat recovery technology using a hollow fiber membrane contactor is designed and the setup is conducted. Polypropylene(PP), CO2/N2 gas mixture and deionized water are used as the membrane material, gas phase and absorbent, respectively. The effects of gas velocity, absorbent velocity, CO2 content in gas mixture, and number of membrane modules on CO2 absorption are investigated. On the other hand, the software of COMSOL Multiphysics on the basis of finite element method(FEM) is used for analyzing the CO2 absorption process in membrane contactor. Under the given conditions, CO2 removal shows a downward trend with increasing gas velocity or CO2 content in feed gas. When absorbent velocity increases, CO2 removal is improved. And when the experiments are carried out with serial membrane modules, CO2 absorption performance is enhanced. In addition, a developed 2D mathematical model is reliable for predicting the characteristics of CO2 transport and absorption in membrane contactor by comparing experimental data with modeling results using H2 O as the physical absorbent.Moreover, the blends of methyldiethanolamine(MDEA) and 2-(1-piperazinyl)-ethylamine(PZEA) are used as the absorbent to improve the CO2 absorption performance and reduce the energy comsuption while considering a single absorbent. Then, 3D dimensionless concentration distributions of CO2 and absorbent are depicted. A comprehensive study on the effects of gas velocity, absorbent velocity and concentration, CO2 content in flue gas, relative concentration in the blend, operating pressure and temperature, flow direction and condition, membrane module in series, inner fiber diameter, membrane thickness, membrane length, number of fibers, inner module diameter, membrane porosity and tortuosity, and membrane wettability on CO2 removal and mass transfer rate is conducted and discussed in details using the developed membrane gas absorption mathematical model in Chapter 3. It shows that as increasing gas and absorbent velocities,z both of CO2 mass transfer rates increase. However, CO2 removal efficiencies for those two velocities increases and decreases, respectively. When the concentration of absorbent increases, CO2 removal process is enhanced. It is also found that increasing gas temperature and decreasing absorbent temperature within 298 – 313 K weaken the membrane performance. Moreover, CO2 absorption is improved while the operating pressure increases from 0.1 MPa to 5.0 MPa. The membrane contactor provides better CO2 removal with a countercurrent flow or a turbulent flow inside the fibers due to the enhancement in gas and liquid disturbance. In terms of number of membrane contactors, serial modules exihibit better CO2 removal. Regarding the effects of membrane contactor dimentsions, increasing hollow fiber membrane length, number of fibers, membrane porosity and tortuosity ratio, and decreasing inner fiber diameter, membrane thickness, inner contactor diameter and membrane wettability are good for CO2 removal from flue gas. In particular, when the relative concentration of absorbent β = 1.5(i.e. 0.4 mol/L MDEA and 0.6 mol/L PZEA), CO2 removal efficiency achieves above 95%. Considering the input costs effect, it provides the best economic benefit.Based on the membrane gas absorption technology for CO2 capture from power plant flue gas, CO2 capture from biogas was investigated using MGA method in Chapter 5. Physical absorbent H2 O and chemical absorbents monoethanolamine(MEA), diethanolamine(DEA), triethanolamine(TEA), and potassium argininate(PA) are utilized as absorbents. The influences of gas and liquid parameters, system operating conditions and membrane contactor structure on CO2 removal and CH4 recovery are investigated. Results show that the absorption performance order of these absorbents is PA > MEA > DEA > TEA > H2 O. Thus, PA could be a novel absorbent for biogas purification using a membrane contactor. As increasing absorbent velocity and concentration, and operating pressure, and decreasing gas velocity, CO2 content in biogas, and serial membrane modules are helpful to the purification process. It is also indicated that turbulent flow provides better biogas purification results than laminar flow. And countercurrent flow is better than concurrent flow for purifying biogas. In addition, a membrane contactor with a small fiber diameter or membrane thickness or long membrane promotes the purification process of biogas. Particularly, the contactor embedded with 500 fibers shows the best purification results.In order to improve the high heat and pressure resistance properties of membrane material, the freestanding facilitated transport membrane with a membrane composition of 50.0 wt% poly(vinyl alcohol)(PVA), 18.3 wt% porassium hydroxide(KOH), 20.7 wt% 2-aminoisobutyric acid-potassium salt(AIBA-K) or potassium glycinate(PG), and 11.0 wt% polyethylenimine(PEI) is investigated in Chapter 6. The experiments on CO2 sorption are carried out in a high-pressure thermal gravimetric analyzer(HPTGA) for the first time. It is found that an acetal linkage is formed at the peak of 1142 cm-1 by the C-O-C stretch which presents both before and after the heat-treatment. However, it does not occur at this point for pure PVA membrane. With a decrease in operating temperature or an increment in pressure, the CO2 sorption results show a non-linear increase trend. In addition, using PG as one of the mobile carriers shows better CO2 sorption performance than that of AIBA-K. The results of CO2 sorption in the facilitated transport membrane with a 3% water vapor content in feed gas are twice that with a dry feed gas of CO2.