Solvent reclaiming by sulfate precipitation for CO2 capture

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Rafique, Humera Abdul

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Sulfate accumulates in the post-combustion CO₂ capture system and must be removed to re-use amine efficiently. Removal of sulfate from the amine-based postcombustion CO₂ capture system through a solvent reclaiming process may reduce CO₂ capture costs. This work determines the solubility of K₂SO₄ and Na₂SO₄ in 2 to 8 m PZ loaded with CO₂ and develops a thermodynamic and process model for the reclaiming process. At 40°C the solubility of Na2SO₄ in 8 m PZ with a CO₂ loading of 0.3 is 0.3 m Na2SO₄ and that of K₂SO₄ is 0.1 m K₂SO₄. Sulfate solubility in PZ solutions is represented by the empirical models: K₂SO₄: ln(Ksp) = 10.53I[superscript 0.3] - 0.98[PZ][subscript T] -3440/T - 2.42 ; Na₂SO₄: ln(Ksp) = 2.137I[superscript0.3] - .6505[PZ][subscript T] -826/T + 265 where [PZ][subscript T] = 2*(molality of PZ). A K₂SO₄ and Na₂SO₄ solubility thermodynamic model was developed in the eNRTL framework in the Fawkes model for PZ/CO₂/H₂O in Aspen Plus[trademark]. The energy cost of the Na process when removing the equivalent of 100 ppm SO₂ from the flue gas, ranging from $0.1-0.5/ton CO₂, was practically the same as the K process(ranging from $0.1-0.8/ton CO₂). The K₂SO₄ recovered in the process can be used as fertilizer. However, the KOH will still cost $0.6/tonne CO₂. If it is not possible to sell the K₂SO₄ as fertilizer because of the impurities that may be present on the K₂SO₄crystals, the chemical cost of the process would increase to $2/tonne CO₂. The chemical cost for the Na case is $0.7/tonne of CO₂.



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