Study On Combined Capture Of CO₂and SO₂in Aqueous Ammonia

Aqueous ammonia was widely used in the post-combustion capture of CO₂and SO₂process have been adopted due to its high absorption capacity, lowenergy consumption and potential for combined CO₂and SO₂removal. In thisthesis, five different and interrelated aspects around the aqueous ammonia basedCO₂and SO₂combined capture process were studied, including mass transfercharacteristics, thermodynamic analysis, absorption/regeneration experiments,innovative process and system model development, process optimization andeconomic analysis.The mass transfer characteristics of simultaneous absorption of CO₂and SO₂with aqueous ammonia were studied using a series of wetted-wall column basedexperiments, and the liquid film partition mass transfer model was proposed andbuilt for the calculation of CO₂and SO₂mass transfer coefficient in theinstantaneous reaction zone and the second-order mass transfer coefficient in therapid reaction zone. The absorption features of the absorber were summarized todescribe the simultaneous absorption performance of CO₂and SO₂, which showthat the lower part of the absorber is CO₂and SO₂mixed reaction zone, andupper part of the absorber is CO₂reaction zone.The vapor liquid equilibrium （VLE） data of the NH3-CO₂-SO₂-H2O system isdetermined using a sealed loop apparatus with a low-pressure stirred tankreactor and a Fourier Transform Infrared （FT-IR） analyzer. The VLE and heat ofabsorption prediction models of the NH3-CO₂-H2O, NH3-SO₂-H2O and NH3-CO₂-SO₂-H2O system were developed and validated and the thermodynamic analysisof the absorption/regeneration process involved in the combined CO₂and SO₂capture process were carried out. The results show that adding SO₂to theaqueous ammonia solution can significantly reduce CO₂regeneration energy,and SO₂will not be released in the CO₂thermal regeneration process.The absorption and regeneration experimental studies were carried out in areconstructed absorption/regeneration apparatus with packed column as theabsorber or stripper individually. The effect of ammonia concentration, CO₂and SO₂loading, and solvent temperature on absorption and regenerationperformance were investigated, and the results show that aqueous ammonia witha certain amount of SO₂loading （less than0.2mol SO₂/mol NH3） still has agood absorption properties and the SO₂loading can significantly reduce CO₂regeneration energy consumption.The innovative process of aqueous ammonia based CO₂and SO₂combinedcapture was proposed in which the same absorber is chosen to achieve efficientabsorption of CO₂and SO₂, the thermal regeneration based stripper is used torelease the CO₂loading in the aqueous ammonia, and the salt crystallizationmethod is utilized to achieve the removal of SO₂loading. The aqueous ammoniabased CO₂and SO₂absorption and regeneration process model is developed andvalidated for the further study. The results show that the SO₂mass transfer rateis significantly higher than CO₂at the lower part of absorber, and the ammoniavolatilization operating line almost coincides with the equilibrium line at theupper part of absorber; the decrease of heat of CO₂desorption with the increaseof SO₂loading is the main cause for the reduction of the CO₂regenerationenergy consumption, and there is an optimal value of re-boiler temperature forthe stripper operation.The process optimization studies show that absorber design improvement cansuppress ammonia volatilization, but the effect is not obvious. It is necessary toinstall a washing column at the top of the absorber. The rich solvent spiltmethod is proposed and proved, which can effectively inhibit condensercrystallization at the top of the stripper. The potassium sulfate crystallization isselected as the SO₂content removal method in the aqueous ammonia and can beachieved according to the potassium sulfate crystallization operation lines. Theeconomic analysis of the combined capture of CO₂and SO₂process show thatthis innovative process can effectively reduce the total investment and operatingcosts approximately by17.4%.