Wetting Mechanism And Process Of Hydrophobic Microporous Membrane In The MgA Process

As we know, the traditional absorption technology has its inherent shortcomings, for example, needing a great deal of operating space, high equipment investment and operating instability. Therefore, lots of researchers have paid sufficient attention to a new type of carbon dioxide capture technology-membrane gas absorption, which is of economic, reasonable, stable, safe and high efficient. Membrane gas absorption （MGA） is a coupling process that combines the conventional technique of gas absorption with a membrane contactor as a mass transfer device. In a membrane contactor, the miroporous membrane provides a support for the gas-liquid interface, making the two phases have an effective contact without phase dispersion. The existence of membrane module significantly increases the gas-liquid contact area at the same volume of equipment, and avoids the problems that often occur in the equipments of conventional chemical absorption. At the same time, it is estimated that the mass transfer efficiency of HFM contactor is 30 times higher than that of traditional tower equipment, while its volume is 65% less than the latter’s.However, in a HFM contactor, the liquid may penetrate into membrane pores under the capillary pressure and trans-membrane pressure difference. This phenomenon is called membrane wetting. The existence of membrane in a HFM contactor not only increases the mass transfer resistance, but also makes the membrane resistance increase with operating time as a result of the membrane wetting. It has been reported that the mass transfer flux would be declined sharply when just a little part of membrane pores were wetted; and when all of the membrane pores were filled with liquid, the membrane resistance would be the controlling transfer resistance.To study this wetting kinetics, we take the spontaneous capillary imbibitions as the micro model. In the capillary, a concave meniscus surface occurs due to the presence of capillary pressure. The study of the imbibitions process is by considering the changes of solution concentration on the meniscus due to the adsorption and diffusion.This paper mainly includes the following contents:1. The hydrophobic microporous membranes of PVDF and PTFE were taken into dodecyl benzene sulfonate （SDBS） solution, and the weight of membranes was automatically recorded through a data collection program in real time. So the weight of liquid penetrated into membrane pores was got, and the volume of wetted pores could be figured out. Then the membrane wetness, the ratio of the volume of wetted pores to the total volume of membrane pores, was obtained.The experimental results revealed the process of membrane wetting, and the effect of solution properties and temperature on membrane wetting. With consideration of the capillary imbibitions mechanism and the membrane pore size distribution, a kinetic equation was set up to analyze wetting process and evaluate the time-dependent membrane wetness. And we have studied the influence of solution concentration, temperature and membrane material and structure on the membrane wetting. The membrane wetting and the effects in diethanol amine （DEA） aqueous solutions has been also studied. Just the seem as in SDBS solutions, the membrane wetness was smaller when the hydrophobicity of membrane was stronger, and increasing solution temperature greatly promoted membrane wetting; however, increasing concentration could accelerate the membrane wetting in SDBS solution, but decelerate the wetting in DEA solution. That is because if increasing temperature, the viscosity of DEA solution increases remarkably, which will slow down the penetration of liquid in membrane pores.2. Based on the capillary imbibitions mechanism, kinetic equations of membrane wetting for microporous hydrophobic membrane were proposed in this study. These equations show how membrane wetting proceeds as wetting time passes and the way to calculate membrane wetness as a function of wetting time. These kinetic equations include membrane characteristics as factors affecting membrane wetting process, such as pore size distribution, membrane thickness and tortuosity. The membrane wetting experimental data was fitted to the proposed kinetic equation of membrane wetting. The results indicate that the proposed kinetic equation successfully described the membrane wetting process. Therefore, this equation might provide support for the design of membrane contactor suffering from membrane wetting.3. In this study, membrane absorption experiments were conducted, using the （DEA） solution as absorbent to absorb CO₂ in PVDF, PP and PTFE hollow fiber membrane contactors. We observed the membrane wetness, calculated from membrane resistance, with operating time. At the same time, we have investigated the influence of some operating conditions （such as temperature, liquid and gas flow rate, trans-membrane pressure difference） on membrane wetting process in a hollow fiber membrane contactor. The rank of these membrane wetting rates from larger to smaller are:PVDF, PP and PTFE. The average membrane wetness decreases with the increasing of solution concentration, while membrane wetting proceeds faster under higher temperature and liquid velocity.4. Based on the forces balance among capillary pressure, trans-membrane pressure difference and the viscous friction, a dynamic model was developed to predict local and mean membrane wetness as a function of time for hollow fiber membrane contactor. As indicated in the model, the membrane wetness increases along the membrane module from liquid outlet to inlet due to the changes of trans-membrane pressure difference. The experimental data of membrane wetting were fitted to the proposed kinetic model of membrane wetting. The results indicate that the proposed kinetic equation can successfully describe the membrane wetting process. Therefore, this equation may provide support for the design of membrane contactor suffering from membrane wetting. Model prediction shows that, with the increase of membrane wetness, the contribution of membrane resistance to the overall mass transfer resistance becomes more and more significant, and finally the membrane absorption process may be dominated by the mass transfer through the membrane. This indicates that the prevention of membrane wetting is very important in maintaining the high performance of CO₂ absorption in membrane contactor.