The Research On Improving Traditional Propylene Carbonate Washing Process For CO₂ Separation

While the use of fossil fuels promotes social progress,the emission of large amounts of CO₂ has aggravated the global“greenhouse effect”.Efficient and low-energy consumption decarbonization technology has become a research hotspot in the international community.The propylene carbonate（PC）washing process is widely applied in decarbonization industry due to a mass of advantages like low material cost and ease of operation.However,the realistic problems of low CO₂ solubility and selectivity in PC limit the development of this process.This study improves the PC washing process by adding 2-methylimidazole（mIm）to PC with ethylene glycol（EG）and water as cosolvents to form two kinds of physical-chemical coupling complex absorbents.It not only greatly improves the solubility and selectivity of CO₂,but also maintains the desorption heat at a low level.（1）Study on CO₂ capture by mIm-EG-PC complex solvent.The viscosity curves with temperature of seven complex solvents with different compositions are measured,and then the absorption isotherms of CO₂ in them and pure PC are measured systematically,and their desorption heats are calculated.After that,the absorption isotherms of CH₄,N₂ and H₂ in the complex solvent,mIm-EG solution and PC are compared to investigate the CO₂ selectivity improvement.The above experimental results show that when the content of mIm is greater than 20 wt%,the viscosity,CO₂solubility and desorption heat of the complex solvent will increase rapidly.And the PC/EG mass ratio close to 1:1 is beneficial to increase the solubility of CO₂ and reduce the desorption heat appropriately.Compared to pure PC,the solubility of CO₂ in the optimal solvent（24 wt%mIm+36 wt%EG+40 wt%PC）can be doubled,and the viscosity and desorption heat are maintained at low levels.The selectivities of CO₂ over CH₄,N₂ and H₂ are increased by 0.7-2.8 times,4.0-6.3 times and 3.5-7.7 times,respectively.The regeneration experiment verifies that the complex solvent can be regenerated only by vacuuming it for half an hour.（2）Study on CO₂ capture by mIm-water-PC complex solvent.The ternary phase diagram of mIm-water-PC at 298.15 K is drawn firstly.Then the viscosities of four complex solvents with different compositions and pure PC are measured at 298.15 K.Subsequently,the CO₂ absorption isotherms and desorption heats in the complex solvents are compared,and the optimal complex solvent（16.7 wt%mIm+8.3 wt%water+75 wt%PC）is selected to evaluate the selectivity of CO₂ over other gases.And the kinetic behavior of the absorption of CO₂ by the above two kinds of complex solvents is investigated experimentally.The experimental results show that majority PC,a small amount of water and saturated mIm is the best solvent mass ratio.Compared to pure PC,the solubility of CO₂ in the optimal mIm-water-PC complex solvent can be more than doubled,and the viscosity is lower than 3.5 m Pa·s and desorption heat are maintained at a low level.The selectivities of CO₂ over CH₄,N₂,and H₂ are 2.1-3.3,1.9-2.5,and 2.4-3.7 times that of PC,respectively.The kinetic experiment of CO₂ absorption by each solvent shows that adding the appropriate amount of mIm and water can have a positive effect on the absorption rate of the complex solvent,and the CO₂ absorption rate of mIm-EG-PC complex solvent will not decrease in the early stage compared with PC.Finally the cyclic regeneration experiments indicate that the regeneration methods of mIm-water-PC complex solvent and mIm-EG-PC complex solvent are the same and both have good regeneration performance.