One way of reducing sulphur oxide (SOx) emissions from Fluidized Catalytic Cracking (FCC) units is to mix an SOx transfer catalyst with the FCC-catalyst. A basic understanding of the SOx transfer mechanism is needed if optimal SOx transfer conditions are to be found. With the aid of thermogravimetry the reactions of bastnaesite, essentially a mixture of rare-earth-element fluorocarbonates, were studied during SOx transfer to determine its properties as a potential SOx transfer catalyst. The fast absorption of SO2 in air at 700°C gives a rare-earth sulphate compound. However, much of the cerium (III) is oxidized during absorption and CeO2 is formed which is not active in SOx transfer. H2S is the important gaseous species in the slow regeneration of the SOx transfer catalyst. The regeneration of the SOx transfer catalyst in the reactor/stripper has been found to involve the formation of gaseous sulphur and an oxosulphate. The rare-earth oxosulphates are considered to be the active SO2 absorbing compounds in the regeneration of the cracking catalyst. According to the thermogravimetric measurements the rate of regeneration reaches a maximum when approximately one third of the SO2 absorbing capacity has been utilized. Increased H2S partial pressure, higher temperature and longer solids residence time in the (reactor)/stripper, are other factors that should increase the conversion of sulphate to sulphur and oxosulphate. © 1990.
