Study On Absorptive Desulfurization Of FCC Diesel Over Activated Carbon Based Material

The diesel fuel has been a very important engine fuel in the world. But sulfur in diesel fuel leaded directly to emission of SOx and sulfate particulate matter (PM) which endangered public health and welfare. Almost all the countries have released the increasingly stringent environmental regulation requiring the use of low-sulfur diesel fuel for environmental protection. Using adsorbents to selectively remove the sulfur compounds in fixed-bed adsorption system was one of the promising approaches for producing ultra-clean fuels that not only meeted the stringent fuel specification for FCC diesel, but also removed sulfur compounds at low temperature and atmospheric pressure.The desulfurizers were prepared with carbon based material activated with H2O vapor, HNO3, H3PO4, KOH and (NH4)2S2O8 respectively as well as supported CuO or ZnO. Then the proximate analysis was carried out for desulfurizers and the content of the surface alkaline and acid functional groups was measured. The optimum preparation and desulfurization parameters were gained through investigating the breakthrough sulfur capacity and the desulfurization efficiency. The content of sulfides in crude and desulfurized FCC diesel was analyzed qualitatively through GC-FPD chromatograms. Furthermore, the spent desulfurizers were regenerated circularly with thermal, H2O vapor, ethanol vapor or n-heptane, and another purpose was to gain the optimum regeneration technique.The results of proximate analysis and the content of the surface alkaline and acid functional groups indicated that: 1) The volatile content of semi-coke and precursor activated carbon was enhanced when they are improved, and the ashy content decreased when they were modified with HNO3 or (NH4)2S2O8, but increased when modified with H3PO4, KOH. 2) The content of the surface alkaline and acid functional groups was enhanced greatly when carbon based material was modified by all above depicting methods, but one the surface of desulfurizer was acid when modified with H3PO4, HNO3 or (NH4)2S2O8, the other was alkaline.Under the optimum condition: space velocity 2h-1, L/D of fixed-bed 7.5, fixed-bed temperature 120℃, loaded with CuO 1%, calcination temperature 600℃, the desulfurization efficiency of semi-coke activated with H2O vapor could reach 55.95%, but the desulfurization efficiency of the raw semi-coke was only 17.60%.The optimum preparation, desulfurization parameters and the desulfurization efficiency of the precursor activated carbon modified by above depicting methods were obtained as follows:1) Precursor activated carbon loaded with ZnO 1%, calcining at 350℃for 2.0h, fixed-bed temperature 100℃, and desulfurization efficiency 69.13%.2) Precursor activated carbon loaded with CuO 5%, calcining at for 2.0h, fixed-bed temperature 140℃, and desulfurization efficiency 81.73%.3) Precursor activated carbon was modified with H2O vapor, calcining at 600℃for 1.5h, amount of loaded CuO 5%, desulfurization efficiency 80.03%.4) Precursor activated carbon was modified with 65% HNO3, reaction temperature 80℃, calcination temperature 450℃, amount of loaded CuO 3%, fixed-bed temperature 120℃, desulfurization efficiency 88.52%.5) Precursor activated carbon was modified with 55% H3PO4, reaction temperature 95℃, calcination temperature 450℃, amount of loaded CuO 7%, fixed-bed temperature 120℃, desulfurization efficiency 83.12%.6) The desulfurization efficiency of precursor activated carbon modified with solid KOH was higher than that modified with KOH solution, proportion between solid KOH and precursor activated carbon 2:1, calcining at 700℃for 2.0h, fixed-bed temperature 100℃, desulfurization efficiency 81.15%.7) Precursor activated carbon was modified with 45% (NH4)2S2O8, reacting at 80℃for 1.0h, calcination temperature 250℃, amount of loaded CuO 1%, fixed-bed temperature 160℃, desulfurization efficiency 90.08%.8) Precursor activated carbon was modified with (NH4)2S2O8 followed solid KOH under the optimum conditions respectively, desulfurization efficiency 91.71%.The desulfurization efficiency of desulfurizer prepared by above depicting methods and optimum parameters declined with time. Then the breakthrough sulfur capacity was calculated, but they were no more than 1.0%. In addition, except semi-coke activated with H2O vapor, the higher the first desulfurization efficiency, the higher the breakthrough sulfur capacity.Through analyzing qualitatively GC-FPD chromatograms for the FCC diesel before and after desulfurization, the result showed that the benzothiophenes (BTs) and the alkyl dibenzothiophenes (DBTs) could be removed effectively.The spent desulfurizers were regenerated circularly with thermal, H2O vapor, ethanol vapor or n-heptane. Both the desulfurization efficiency and the breakthrough sulfur capacity declined slowly with time and regeneration cycles. What's more, clean diesel yield more than 80% and overall oil yield over 86% were obtained.