Synthesis, Characterization And Evaluation Of CuMgAlFe As Sulfur-transfer Additives For Removal Of SO_x From FCC Flue Gas

In the feedstock of fluid catalytic cracking(FCC)process,there exist many different types of sulfur-containing organic compounds.When the feedstock containing organic sulfur is fed into FCC units about 45～55%of the sulfur in the feedstock is converted to hydrogen sulfide in the reactor of FCC units,about 35～45% is still remained in the liquid products,and the rest of the sulfur of about 5～10%is deposited in the coke formed on the FCC catalyst.The sulfur deposited in the coke is transferred to SO_x(SO₂+SO₃)when the coke is combusted in the regenerator of FCC units.With the increase of throughput of FCC units and of sulfur content in crude oil, emission of SO_x from flue gas of FCC units is increased.These gases are among the major atmospheric pollutants and are acid rain precursors,posing a serious problem to the protection of the environment.Both flue gas scrubbing and feedstock hydrodesulfurization are all effective means of SO_x control,but they are laborious and expensive.Compared with the two techniques for controlling SO_x emission from FCC units,sulfur-transfer additive technique is more practical and attractive for the present FCC units.It is very important to design high effective SO_x reducing materials for this purpose.Potential SO_x transfer catalyst must be able to oxidize SO₂ to SO₃ under the FCC regenerator conditions,be able to chemisorb the SO₃ in the form of stable metal sulfates in the FCC regenerator,be able to reduce sulfates to release sulfur as H₂S in the FCC reactor and stripper.Study on sulfur-transfer additive should aim at the following questions(1)to improve the activity and abrasion resistance ability of sulfur-transfer additive,(2)to reduce the toxicity and production costs of sulfur-transfer additive.The CuMgAlFe-HTlcs were prepared by different synthetic methods and different materials,and The CuMgAlFe complex oxides used for the removal of SO_x from FCC flue gas were prepared by decomposition of CuMgAlFe hydrotalcite-like compounds precursors at 700℃for 6 h.These catalysts were characterized by XRD, XRF,TG-DTA,BET,SEM and FT-IR techniques and first evaluated as potential SO_x transfer catalysts by thermogravimetric techniques under conditions similar to those of a typical FCC.The MgAl3-LDOs,8%CuMgAl3-LDOs and MgAl3Fe8%-LDOs mixed oxides were obtained from the hydrotalcite-like precursors which were synthesized by coprecipitation method.The results of SO_x adsorption-reduction tests showed that MgAl3-LDOs displayed the lowest adsorption capacity in the longest saturation time, and MgAl3-LDOs can not be regenerated at 600℃.8%CuMgAl3-LDOs exhibited the highest adsorption rate,and total saturation adsorption capacity with 1.22 g SO_x/g in 6 min,but its sulfates can not be completely reduced at 600℃.MgAl3Fe8%-LDOs reached total saturation adsorption capacity with 0.93 g SO_x/g in 20 min,its sulfate reduction ability was much better than that of MgAl3-LDOs and 8%CuMgAl3-LDOs at 600℃.The results indicated that MgO was the only active material for SO_x adsorption,and formed very stable MgSO₄ compounds,restricting the additive regeneration.Al₂O₃ showed a low SO_x removal capacity because the Al₂(SO₄)₃ formed is very unstable at the regenerator temperature so it releases the sulfate species as produced in the regenerator.But Al₂O₃ contributed to the structure of sulfur-transfer additive,and Al₂O₃ controlled sulfur content of FCC gasoline in synergic effect.Fe₂O₃ played a dual role as an oxidizing and a reducing promoter, CuO was an efficient promoter for oxidating SO₂ to SO₃,but had no significant contribute to the reduction of metal sulfates.Cu-MgAlFe catalysts were prepared by the impregnation with copper on the mixed oxides MgAl5Fe8%-LDOs.First,MgAl5Fe8%-LDOs reached total saturation adsorption capacity with 1.45 g SO_x/g in 25 min,its sulfate reduction ability was much best in all MgAlFe-LDOs with different Mg/Al ratios and Fe contents at 600℃. So MgAl5Fe8%-LDOs was an efficient active material precursor of sulfur-transfer additive.Second,Cu-MgAlFe catalysts showed the higher adsorption rates which are reflected in lower saturation times compared with that of MgAl5Fe8%-LDOs. 1.0Cu-MgAlFe reached total saturation adsorption capacity with 1.60 g SO_x/g in 6 min,its adsorption rate was 4.6 times higher than that of MgAl5Fe8%-LDOs,and 1.0Cu-MgAlFe showed similar adsorption capacities after the eighth SO₂ adsorption-reduction cycle.CuMgAlFe-LDOs complex oxides used for the removal of SO_x from FCC flue gas were prepared by decomposition of CuMgAlFe hydrotalcite precursors synthesized by coprecipitation method at 700℃for 6 h.The influence of different metal salts in the sulfur transfer catalyst was investigated.The results of SO_x adsorption-reduction test showed that CuMgAlFe-LDOs prepared from nitrates achieved high oxidative adsorption rate,short saturation time,and large reductive capacity.CuMgAlFe-LDOs with different Mg/Al ratios,Fe and Cu contents in the sulfur transfer catalyst were found that CuMgAlFe-LDO6 presented the greatest total adsorption capacity(1.62 g SO_x/g)in 6 min,and was regenerated completely in 8 min, which was also remained at constant after the eighth cycles.CuMg(Al)Fe-LDOs complex oxides were prepared by decomposition of CuMg(Al)Fe hydrotalcite precursors with NaAlO₂ as aluminium source synthesized by coprecipitation method at 700℃for 6 h.The influence of different Mg/Al ratios, Fe and Cu contents in the sulfur transfer catalyst was investigated.This indicated that CuMg(Al)Fe-LDO6 showed the greatest total adsorption capacity(1.74 g SO_x/g)in 6 min,and was regenerated completely in 8 min,which was also remained at constant after the eighth cycles.Cu(MgAl)Fe hydrotalcite-likes compounds(includes Mg(OH)₂ and Al(OH)₃) were synthesized with MgO,pseudoboehmite,Fe(NO₃)₃ and Cu(NO₃)₂ by hydrothermal method.Cu(MgAl)Fe-LDOs complex oxides were prepared by decomposition of Cu(MgAl)Fe hydrotalcite-likes precursors at 700℃for 6 h. Cu(MgAl)Fe-LDO8 reached total saturation adsorption capacity with 1.69 g SO_x/g in 6 min,but it exhibited bad regeneration performance.So Cu(MgAl)Fe hydrotalcite-likes compounds as precursors for CuMg(Al)Fe-LDOs complex oxides were prepared by the hydrothermal method need be further researched.All CuMgAlFe catalysts with different methods exhibited excellent adsorption capacities,adsorption-reduction rates,and reduction percentages at 600℃.To compare the CuMgAlFe sulfur-transfer additives performance,CuMg(Al)Fe-LDOs showed the better SO_x absorption-reduction ability than the other catalysts.