Study On The Performance And Simulation Of CO₂ Removal From Biogas By A Rotating Bed Based On MEA-ethanol Absorption

At present,the large-scale biogas projects in China are increasing,but due to the high content of CO₂ in the biogas,resulting in its low calorific value,so that biogas can only be used for lighting,cooking and other fields.In order to improve the caloric value of biogas and then broaden the application range of biogas,it is necessary to remove CO₂from biogas and make it become biomethane with CH₄ content above 97%.Biogas upgrading technologies are varied,which MEA absorption technology with its high degree of CO₂ removal efficiency,CH₄ almost no losses,atmospheric operating advantages gained widely used,but there is also an problem of high renewable energy consumption,and the current absorbent regeneration technology process is relatively complex,leading to a certain extent absorbent concentration changing and absorbent high temperature degradation,regeneration degree of absorbent is not very high.Traditional mass transfer equipment,such as packed tower and plate tower,has the disadvantages of low mass transfer efficiency,large space occupation,high investment,difficult maintenance and so on.In view of these short comings,this study used ethanol instead of water as the solvent of MEA absorbent,combined MEA-ethanol absorbent with RPB reactor and applied to CO₂ remove from biogas,and introduced a new batch regeneration method for absorbent regeneration.The main work is followed as:(1)The absorption capacity and regeneration capacity of MEA-ethanol-water system were studied.The experimental platform of absorption and regeneration was set up,and the absorption and regeneration properties of MEA-ethanol-water absorbent were studied under different temperature and different concentration.It was found that absorption properties of each absorbent didn't change in the temperature range of 20?45?.Compared with MEA-water absorbent,the absorption capacity of MEA-ethanol absorbent decreased,and the absorption rate increased.With the increase of MEA concentration,the CO₂ absorption capacity and absorption rate of MEA-ethanol absorbent increased,but the CO₂ loading of saturated absorbent decreased,but the CO₂ loading of saturated absorbent decreased,especially when MEA concentration exceeded 30 wt.%.The regenerative performance of the absorbent was enhanced with the increase of temperature and reached the best at boiling temperature.The regeneration degree of MEA-ethanol absorbent increased by nearly 6 percentage points compared with MEA-water absorbent,the regeneration temperature decreased by about 20?,and the average regeneration rate increased by 41.72%in the first 20 minutes.The unit regenerative energy consumption decreased by 80.26%.With the increase of MEA concentration,the regeneration degree of absorbent changed little,but the regeneration rate increased.(2)The mass transfer performance of MEA-ethanol absorbent for CO₂ removal from biogas was studied.And the experimental study of batch regeneration of absorbent was carried out by heater and RPB as regeneration reactor.In rotating bed reactor for mass transfer test apparatus,the performance of CO₂removal from biogas by MEA-water absorbent and MEA-ethanol absorbent under different gas flow rate,CO₂ content,liquid flow rate,MEA content,rotation speed was studied under pilot scale.The results showed that CO₂ removal efficiency became worse with the increase of inlet gas flow rate and inlet CO₂ content.With the increase of and MEA content,the CO₂ removal efficiency became better.With the increase of rotation speed,the CO₂ removal efficiency changes little at first(below 600 r/min),then got better rapidly(600?1000 r/min),and finally tended to be stable(above 1000 r/min),and the optimal speed was 1000r/min.Under the optimum rotation speed,the best CO₂ removal efficiency could be achieved by the total MEA/total CO₂>2.5(molar ratio)(CO₂content of product gas was closed to 0).In regeneration study,it was found that the performance of the batch regeneration of heater was much better than that of the batch regeneration of RPB.For MEA-ethanol absorbent with concentration of 3.28 mol/L,the RPB regeneration could reduce the CO₂ loading of regenerated liquid to 0.07 mol/mol in 5 hours;the heater regeneration could reduce the CO₂ loading of regenerated liquid to0.03mol/mol in 40 minutes.In the process of heater regeneration,when MEA concentration was 4.92 mol/L,the minimum value was 3.17 MJ/kg CO₂,the regeneration liquid loading was 0.16 mol/L,and the regeneration time was 20 minutes.(3)The mass transfer process of biogas decarbonization in a rotating bed based on MEA-ethanol absorption was modeled.Model taking into account the mass transfer process of gas liquid phase flow rate and temperature change on the influence of the mass transfer process,to improve the gas-liquid equilibrium constant instead of the enhancement factor to describe the mass transfer occurs in the process of chemical reaction,respectively based on the double membrane theory and surface renewal theory to determine the model parameters,introducing the hyperbolic tangent function on the mass transfer model based on the theory of double film is optimized.The results showed that the maximum errors of the two models were within±15%,and the prediction effect of the surface renewal model(average error 5.82%)was better than that of the double membrane theory model(average error 7.89%).(4)CH₄ and CO₂ were used to prepare real biogas,several absorption-regeneration cycle experiments were performed;the process was simulated by Aspen Plus software.It was found that the experimental results of CO₂removal from real biogas and absorbent regeneration were similar to that of simulated biogas.When under the optimum operating conditions,after five times of absorption-regeneration cycles,the CO₂ content of the product gas was close to 0,and the CO₂ loading of the regenerated liquid was between 0.15 and 0.17 mol/mol.The performance of the absorbent would not deteriorate with the increase of the number of cycles.The simulation results of Aspen Plus showed that when the CO₂ content of the product gas was less than 3%,the lowest unit power consumption was 3.42MJ/kg CO₂.With the deepening of the degree of regeneration,the flow rate of absorbent circulation gradually decreased,the power consumption of pump and RPB decreased significantly,and the regeneration power consumption increased significantly.Among the power consumption components,the proportion of regeneration power consumption in the total power consumption was 0.9?0.923.The ratio of pump power consumption to total power consumption was 0.0009?0.012,the ratio of RPB power consumption to total power consumption was 0.076?0.086.(5)According to the mass transfer model and simulation,the industrial process of CO₂ absorption from biogas by MEA-ethanol in RPB and absorbent regeneration by the heater was designed and equipment parameters of a large-scale biogas project with a daily output of 20000 Nm³was determined.The operation cost was calculated and compared with the normal operation of high-pressure water washing biogas upgrading technology in Chifeng City.It was found that the industrial application potential of this process was related to the electricity price and biogas production cost.The higher the electricity price and the lower the biogas production cost,the higher the industrial application value of this process.