| Technologies for CO2 absorption with high removal efficieny and low energy consumption are the hot spot and trend for biogas upgrading process development,one of which is CO2 absorption by an aqueous monoethanolamine(MEA)solution.In this thesis,the systematic investigation combining numerical simulation and experimental methods were carried out to study the absorption properties of highly-concentrated CO2 from biogas,the kinetics and mass transfer performance of CO2 absorption process in packing column.On these bases,the circulating processes for biogas upgrading were optimized by Aspen Plus.The effect of MEA concentration on absorption performance and the effect of desorption temperature on desorption performance were studied experimentally.How the gas and liquid inlet flow rate,CO2 concentration,absorption temperature and pressure influence the absorption properties in the packing column was also explored.The results show that with increasing MEA concentration,CO2 absorption rate increases but the CO2 loading of rich solution decreases.The CO2 desorption rate increases with the increase of desorption temperature,and the optimal desorption temperature of is 105? for the MEA solution saturated with CO2.In the packing column,the order if the significance of the influence factor is:liquid inlet flow rate>gas inlet flow rate>absorption temperature>CO2 concentration>absorption pressure.The mass transfer kinetics parameters including the Hatta number,Ha,enhancement factor,E,and volumetric overall mass transfer coefficient,KGae,were determined,on the basis of experimentally measuring the variation characteristics of CO2 concentration along the height of the counter-flow packing tower,the gas-liquid effective specific surface area during the CO2 adsorption process,and the physical diffusion coefficient and the Henry's coefficient obtained through the analogy method.It is found that the changing rules of the mass transfer performance varying with the reaction conditions are:1)increasing the MEA concentration is conducive to improving the mass transfer performance;2)increasing the gas flow rate and the CO2 concentration have the opposite effects.3)with increasing absorption temperature,the mass transfer performance first increased and then decreased.A mass transfer mathematical model for high concentration CO2 in biogas by MEA solution was established,receiving the distribution of CO2 concentration in the packed column.This model can accurately predict the mass transfer process of CO2 adsorption by MEA in the packed column.Meanwhile,based on the second-order reaction kinetics equation,the mathematic model of the enhancement factor was established.The results show that the enhancement factor of the adsorption process is less than the values of Hatta number.The reaction process can be described by the second-order irreversible reaction kinetic equation.The experimental apparatus of CO2 absorption recycling process by MEA solution was established,and a series of investigation were carried out to inspect the influence of process parameters on CO2 removal rate and regeneration energy consumption during the purification process.The results show that increasing absorption temperature appropriately is conducive to achieving high CO2 removal rate with reduced desorption energy;the optimal temperature of the feed liquid is 50?.2)CO2 removal rate is initially increased and then decreased with increasing flow rate of absorption solution,and the optimal value of flow rate is 30L/h,while the desorption energy consumption decreased with increasing liquid flow rate.It should be noticed that reducing the flow rate of absorption solution appropriately could improve the economic benefits of the device.3)Absorption pressure has no significant effect on CO2 removal rate and desorption energy consumption.4)The optimal set point of heating temperature in desorption column was 170?,the corresponding temperature of absorption solution in column bottom is 111.3?.5)When the desorption temperature was increased,CO2 removal rate and desorption energy consumption per unit CO2 increased first and then decreased,while the overall desorption energy consumption increased gradually,and then tended to be stable;the optimum value of desorption pressure was 34kPa:On the basis of above results,simulation optimization of the biogas purification by Aspen Plus was carried out,and new biogas upgrading processes,re-circulating process on the absorption column and split flow process on the desorption column were developed.Re-circulating process can improve CO2 removal efficiency up to 98.5%,while reducing desorption energy consumption per unit CO2 by 3.9%,reaching the minimum value of 3.69GJ/t.The product gas with high CH4 concentration can be obtained by the split flow process,with the removal efficiency of CO2 of 99.9%,and the desorption energy reduced by 4.95%.The results of this study can provide theoretical and technical references for the development and optimization of biogas upgrading processes. |
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