Experimental And Theoretical Studies On Mass Transfer Performance For CO₂ Absorption Into Aqueous Amine Solution In Hollow Fiber Membrane Contactor

In recent decades,a large amount of CO₂ emission into the atmosphere from the combustion of fossil fuels has caused a series of environmental problems,of which global warming is the most representative one.Today,two-fifths of the world's CO₂ emissions come from coal-fired power plants.Therefore,it is urgent for coal-fired power plants to realize CO₂ capture and storage.Currently,the chemical absorption based on amine absorbents has become one of the most mature CO₂ capture technologies.However,there are still some problems urgently to be solved in the application,such as high energy consumption,large space and corrosion of equipment,and operating problems including flooding,bubble,channeling and entrainment in traditional columns,which seriously affects the mass transfer efficiency and reduces CO₂ capture performance.CO₂ separation technology based on hollow membrane contactor fully combines the advantages of chemical absorption and membrane separation.Compared with the traditional chemical absorption technology,its equipment covers compact strcuture,light,high mass transfer efficiency,independent control of gas-liquid,flexibility,which is regareded as a new gas separation technology with good application prospects.However,there are still some problems in the application process,such as high energy consumption for absorbent regeneration,loss of mass transfer performance due to membrane wetting in operation,which has become a major bottleneck for membrane absorption in the application of CO₂ capture technology.Based on this,in order to improve the mass transfer performance in the process of CO₂ capture and reduce wettability and energy consumption for regeneration,in this work,a set of experimental hydrophobic teflon?PTFE?fiber membrane contactor was built to simulate CO₂ removal from flue gas from the coal-fired power plant flue gas,the experiment and theoretical investigation on mass transfer performance of CO₂ absorption into a single and blended amine solutions was conducted.The main research work and conclusions of this paper are as follows:?1?The mass transfer performance of CO₂ absorbion into aquesos 1DM2 AP in PTFE hollow fiber membrane contactor was studied.For 1DMA2P-CO₂ system,the effects of different operating parameters on mass transfer performance were investigated such as liquid velcoty,gas velocity and CO₂ partial pressure,etc.The experimental results show that increasing the liquid velocity,amine concentration and the liquid phase temperature can effectively increase the CO₂ absorption flux in a reasonable range while increasing the gas velocity and CO₂ division pressure can also promote the CO₂ absorption process,and CO₂ absorption flux decreases with the increase in CO₂ lean loading.Compared with the co-current mode,CO₂ absorption flux is higher under the counter-current mode.In addition,in this paper,a mass transfer correlation between the operating parameters and the overall CO₂ mass transfer coefficient?KG?and CO₂ absorption flux?JCO₂?is proprosed in the hollow fiber membrane contactor for the 1DMA2P-CO₂ system.The predicted results are in good agreement with the experimental values with the average relative deviation of only 6.2%.It shows that the semi-empirical model has good correlation.?2?The effects of different operating parameters on CO₂ absoprion flux were investigated in the process of CO₂ absoprion using DMEA in the PTFE hollow fiber membrane contactor experimentally inlucding liquid velocity,gas velocity,CO₂ partial pressure,amine concentration,the liquid phase temperature and CO₂ lean loading The experimental results reveals that increasing liquid velocity,DMEA concentration and the liquid phase temperature,CO₂ partial pressure,gas velocity and reducing CO₂ lean loading in a certain range are beneficial to the process of CO₂ absorption.In addition,based on the finite element analysis,a two-dimensional steady-state mathematical model is established by using COMSOL Multiphysics software to calculate the CO₂ absorption flux and the distribution of CO₂ concentration in the hollow fibe membrane contactor under non-wetted and partially-wetted modes.The research indicated that the experimental CO₂ absorption flux is in good agreement with the simulated value with 10% membrane wetting,and the mass transfer performance significantly deteriorate when membrane is wetted.?3?Based on the two-dimensional steady-state mass transfer numerical model established in the fourth chapter,the mass transfer performance in the process of absorbing CO₂ in the hollow fiber membrane contactor of DEAB solution under nonwetting and partial wetting conditions is studied.It is also compared with the CO₂ absorption performance of MEA,DEA,AMP and MDEA.The simulation results show that DEAB has a good CO₂ absorption performance.Based on the analysis of chemical enhancement factors,the chemical reaction between CO₂ and DEAB under non-wetted mode basically occurred in the intermediate fast-instantaneous regime,and the intermediate regime was closer to the rapid reaction zone,and gradually transitioned to the transient reaction zone with the increase of liquid velocity and membrane wetting degree.The overall mass transfer coefficient increases with the increase in liquid velocity and gas velocity,and is hardly affected by the change of CO₂ pressure.With the increase in membrane wetting,the overall mass transfer coefficient decreases continuously.Under the non-wetted mode,the mass transfer resistance of liquid phase is dominant in the total mass transfer resistance.That is,the mass transfer process is controlled by the mass transfer resistance of the wetted membrane,with the membrane wetting,the influence of the resistance of the wetted membrane on mass transfer process becomes predominant gradually and exceeds the resistance of liquid phase.?4?Due to the limited CO₂ absorption rate of a single N.N-dimethylethanolamine?DMEA?,various activated amines?MEA,PZ,MAE,EAE and AMP?were added into the DMEA solution to further improve the CO₂ capture performance,and their effects on the CO₂ absorption performance were studied.The results showed that compared to a single DMEA,the number of active amino groups in the solution increased after the addition of activated amine,thus improving the CO₂ absorption performance.The type and relative position of amino group,hydroxyl group and alkyl group had significant influence on the CO₂ absorption performance using the blended amine.Moreoever,DMEA+PZ had the best CO₂ absorption performance in the five DMEA-based blended solutions.In comparison with single DMEA,the CO₂ absorption flux increased by 52.44%.