Numerical And Experimental Study On Absorption Of Carbon Dioxide From Flue Gas In The Hollow Fiber Membrane Contactor

Global warming and frequent extreme weather have become the climate issues ofglobal concern. These issues are caused by the increasing greenhouse effect.Greenhouse gases consist of carbon dioxide (CO2), methane (CH4), nitrogen oxide(N2O), chlorofluorocarbons (ClFCs) and ozone (O3) in the atmosphere. Especially theCO2concentration is increasing, the greenhouse effect gets worse due to the excessiveexploitation and use of fossil fuels and untreated flue gases emitting into the atmosphere.Therefore, human beings realize the seriousness of the problem, and take somemeasures to control and reduce the CO2emissions. At present, the carbon capture andstorage (CCS) technology is the most effective carbon reduction method. Meanwhile,the method of CO2absorption from flue gases by hollow fiber membrane contactors isconsidered to be one of the most advantageous and potential techniques.The hollow fiber membrane contactor absorption of CO2is a novel method ofdecarburization, which has the advantages of both membrane separation and chemicalabsorption technology. The membrane only plays the role of separating the gas andliquid phases. Because of the difference of CO2concentration in the gas and liquidphases, and the selectivity in CO2with absorbents, the CO2is transferred to liquidphases from gas phase through the membrane porous. Thus, the CO2is removed fromflue gas. This method ensures the high CO2removal efficiency and minimizes thephenomenon of flooding, overflow and the mist entrainment occurring in the chemicalabsorption process. It also has the advantages in simple operation, small volume, smallspace and low cost. Therefore, the polypropylene (PP) hollow fiber membrane contactoris selected in this paper. The performance and characteristics of absorption of CO2fromflue gas through the numerical simulation and experimental study are investigated.A comprehensive two-dimensional axisymmetric mathematical model is developedfor absorption of CO2from flue gas through a PP hollow fiber membrane contactor byusing the finite element method, which is considered under the non-wetting condition.The model simulated the decarburization effect and mass transfer performance ofethyl-ethanolamine (EEA), ethylenediamine (EDA), and piperazine (PZ) under differentoperating conditions and membrane structure parameters. The simulation resultsshowed that the influence of CO2removal and mass transfer of the gas phase parameters are more significant than those of liquid phase parameters. With increasing the gas flowrate, CO2concentration and gas temperature, the CO2removal efficiency will decrease,respectively. While, the CO2mass transfer rate will raise with increasing gas flow rateand CO2concentration. But, the CO2mass transfer rate will decrease with increasinggas temperature. However, the CO2removal efficiency and mass transfer rate increasewith increasing suitable liquid velocity, absorbent concentration and temperature. But ifthe liquid velocity, absorbent concentration or liquid temperature is too high, the masstransfer resistance and wetting possibility increase and the solubility reduces. Therefore,the CO2removal rate may reduce and the energy consumption will increase. The CO2removal efficiency and mass transfer rate of PZ solution are the strongest. The sequenceof CO2removal performance is in turn PZ, EDA, and EEA from high to low. The CO2removal efficiency and CO2removal efficiency will reduce with increasing the wallthickness, diameter and tortuosity factor of membrane. However, the performances arethe opposite of change with increasing membrane fiber length, filling rate and porosity.But, the variation of CO2removal efficiency is very small when the tortuosity andporosity are changing. The absorption CO2effect of membrane structure is excellentwhen the membrane wall thickness is30～50μm, inside diameter is90～110μm, fiberlength is more than150mm, and packing density is21～25%.An experiment system for CO2absorption is designed and built up. Using distilledwater, diethanolamine (DEA), monoethanolamine (MEA), EDA, PZ and some kinds ofmixed amine solution as absorbent, the performances of absorption CO2are tested whenchanging the gas-liquid operation parameters. The results showed the CO2removalefficiency will reduce with increasing flow rate and CO2concentration of raw gas, butthe performances is contrary with increasing the absorbent velocity and concentration.Meanwhile, the effect of the gas phase parameters is more significant than liquid phaseparameters’ in CO2absorption efficiency. Therefore, the results verified that the modelis reliable and accurate for prediction of performance absorption CO2. It is noted theCO2removal efficiency of PZ solution is the strongest in the single absorbentexperiments. The CO2absorption efficiency of distilled water is the worst. Theperformance of five kinds of absorbents is as follows: PZ>EDA>MEA>DEA> distilledwater. When a small amount of EDA or PZ is added to DEA or MEA solution, theblend absorbent has the better removal CO2performance than that of single solution.And the increase amount of CO2absorption efficiency from0to100mol/m3is bigger with the more adding quantity. The comparison of experimental results showed that theCO2removal efficiency of MEA and PZ mixed absorbent solution is higher.