Study Of CO₂ Absorption Characteristics Using Gas-Liquid Membrane Absorption Technology And Its Performance Enhancement

Ever since the industrial revolution,fossil fuel such as oil and coal has played an important role in the sustainable development of global economy.At the maentime,environmental problems caused by the mining and combustion of fossil fuels,such as ecological degradation and climate change,are also having a big impact on human society.In recent years,biogas has been developed as a renewable energy and also been consibered as an alternative to traditional fossil fuel.The main components of biogas are methane（CH₄）,carbon dioxide（CO₂）,nitrogen（N₂）,oxygen（O₂）and other trace components.The presence of incombustible and acid gases,such as CO₂ in biogas,significantly lowers energy converting efficiency and reduces the possibility of compressing and transporting the gas product over long distances.Therefore,removing CO₂ and other impurities from raw biogas to increase CH₄ content is an essential step for utilizing biogas as a high-valued energy source.Gas-liquid membrane absorption process is a novel gasmixture separation technology and attracts attentions of researchers in recent years.It combines the features of conventional gas-liquid absorption process（high selectivity,good endurability）as well as the membrane separation process（operational flexibility,high surface to volume ratio,linear scale-up）.In this paper,a hollow fiber membrane contactor（HFMC）has been designed and build up,combined with manully potted hydrophibic PTFE hollow fiber membrane module,to removal CO₂ from biogas.First,Selexol and water have been selected as CO₂ physical absorbent.Applied physical absorbents in HFMC to study the biogas purification performance at different operating pressures.The results showed that,incrasing operating preesure can enhance the CO₂ absorption efficiency in HFMC.Comperaded to water,Selexol exihibted better CO₂ revoal efficiency.Based on the experimental data,a diffusion-convection model and a resistance-in-series model had been developed and solved by COMSOL Multiphysics 5.2 software to study the biogas purification process in HFMC.The model validation results showed that,when operating HFMC under 0.1 Mpa,the fiber membranes were not wetted by the physical absorbents including water and Selexol.The overall mass transfer resistance is dominated by the liquid phase as it contributed more than 90%of total.However,with the increasing operating pressures,the membrane weeting occurred in the HFMC.The membrane material suffered more from membrane weeting when using selexol as absorbent,due to its lower surface tension.When operating the HFMC at 1.2 Mpa,the membrane wetting ratio was 1.0%for Selexol and 0.35%for water.For selexol absorbent at 1.2 Mpa,the mass transfer resistance in gas,non-wetted membrane and wetted membrane phase significately increased,which decrease the liquid phsed mass transfer resistance to under 50%already.To prevent the PTFE hollow fiber membrane from wetting and cause the mass transfer efficiency decrease,the fiber membrane surface was modified by a spray deposition technology.Fumed hydrophobic SiO₂ nanoparticles were selected as modification precursor and PDMS was used as curing agents.After surface modification,the hydrophobic of membrane surface was enhaced.With increasing the SiO₂ nanoparticles concentration in spray solution,the deposition layer on the membrane surface become much denser.When the SiO₂ concentration reached 1.5 wt.%,the nanoparticles deposition layer was able to cover all the membrane surface including membrane pores.After surface modification,the membrane surface was superhydrophobic as the water contact angle and sliding angle reached 158.4°and 1.3°,respectively.Moreover,the depositon layer on the membrane surface did not hinder the gas permeation ability.Applied the superhydrophobic membrane in HFMC with K₂CO₃solution as absorbent,the experimental results showed that,there was no apparently differences between the original and modified membrane under 0.1 Mpa operation for biogas purification.However,with increasing operating pressures,the modified membrane outperformed the original one.Based on the experimental data,a diffusion-convection model has been developed to study the gas absorption process for modified membrane.The results showed that the membrane model can operate under non-wetted condition at pressures range between 0.1-1.0 Mpa after surface modification.To further increase the biogas absorption performance in HFMC.Three amino acids salts（AASs）namely,potassium glycinate（PG）,potassium arginate（PA）and potassium lysinte（PL）were studied to blend with K₂CO₃ solution as CO₂ absorbent.Compared to 1 M K2CO3 solution,by adding 0.3 M AASs,the CO2 absorption rates increased from 15.4 mol L^-1 s^-1 to 33.3,39.8 and 32.0 mol L^-1 s^-1,respectively,and the CO₂ desorption rates increased from 10.4 mol L^-1 s^-1 to 15.2,17.6 and 15.4 mol L^-1 s^-1.Combined the membrane absorption experiment and mathematic modeling,the effects of various operational parameters on biogas purification with blened absorbents in HFMC have also been investigated.In the lower range of gas/liquid ratio,K₂CO₃-PG and K₂CO₃-PA solution exhibited higher CO₂ absorption flux and removal efficiency.With increasing gas/liquid ratios,the differences between the three blended absorbents diminished.In general,increasing the ASSs addition amount,absorption temperature,operating pressure and liquid absorbent flow rate,while decreasing the gas mixture flow rate and CO2 initial concentration can effectively enhance the biogas purification performance in a hollow fiber membrane contactor.