Ind. Eng. Chem. Res. 1992, 31, 2741-2748
2741
Evaluation and Kinetics of Commercially Available Additives for SO, Control in Fluid Catalytic Cracking Units Vasilios D. Dimitriadis and Iacovos A. Vasalos* Chemical Process Engineering Research Institute and Aristotelian University of Thessaloniki, P.O.151 7, 54006 Thessaloniki, Greece
Commercially available additives for SO, control containing CeO2, MgO, and MgA1204(spinel) or rare earth oxides on A1203were evaluated in a bench-scale unit simulating the conditions existing in the regenerator of a fluid catalytic cracking unit. The unit uses a fluid bed reactor and it operates in cyclic mode with a synthetic flue gas containing 1500 ppm SO2 during the absorption cycle and hexane during the sulfate reduction cycle. Although the relative SO2 absorbing capacity of the spinel containing additive was found to be 2-3 times higher than the rare earth oxides containing additive, the latter demonstrates easier regenerability toward the reduction to Ha.The effect of unit operating conditions on sulfate decomposition is also discussed. A simple kinetic model of the power law type was used to fit the experimentally taken breakthrough curves from a bench scale fixed bed absorber, and it was found to predict the reactor performance quite well in the temperature range of 600-732 OC showing a n Arrhenius type dependency with temperature for the reaction rate constant.
Evaluation of SO, Additives Introduction. In an integrated modern refinery the fluid catalytic cracking (FCC) process plays an important role in converting heavy boiling fractions (220+ “C) to gasoline and lighter products. Concurrently with the desirable reactions, side reactions produce coke containing sulfur. For high-sulfur feeds the latter can lead to high SO, (SOz + SO3) emissions in the regenerator flue gas (Vasalos et al., 1977). Since these high emissions have become a target of EPA regulations (Fed. Reg., 1984),the petroleum industry and catalyst manufacturers have introduced additives for achieving SO, reductions up to 90% compared with conventional catalysts (Vasalos et al., 1977, 1980, Hirschberg and Bertolacini, 1988, Lowell et al., 1971; Byrne et al., 1984; Rheaume and Ritter, 1987; Bhattacharyya et al., 1987). SO, catalysts convert sulfur oxides, formed during coke bum-off in the regenerator, to sulfate on the catalyst which is subsequently released as HzS in the reactor. This step rejuvenates the catalyst which is then recirculated to the regenerator and the cycle is repeated. Regenerator
S (in coke) + Oz
-
SO2 (>go%) + SO3 (
