| The present Ph.D. dissertation considers the simultaneous effects of reaction and diffusion in FCC catalysts. This research is developed on the basis of both experimental and modeling studies.; Regarding the modeling work, a significant aspect of this research deals with kinetic of models with different levels of complexity, namely a four and a seven lump model. Both models include a modified Thiele modulus and an effectiveness factor. Using these models it is shown that the effectiveness factor is a function of the following parameters: (a) gas oil conversion, (b) intrinsic rate constant and (c) zeolite crystal size. The proposed models are able to predict that changes in the 0.13 μm to 1.3 μm crystal size affect the relative importance of both primary and secondary catalytic cracking reactions and as a result the product distribution. It is found that the smaller zeolites produced more gasoline and less coke and light gases than the larger crystals. In addition, the seven lump model allows to establish the role of hydrogen transfer in product distribution. For instance, it is predicted that the small crystals, less affected by diffusional constrains; produce more olefins and naphthenes and less aromatics and paraffins than the larger crystals. Moreover, according to both the four and the seven lump models it is expected that the smaller crystals will yield less coke.; The effect of Y-zeolite crystal size on the performance of FCC catalysts was investigated experimentally in the novel CREC Riser Simulator, using two sets of FCC catalysts. These two catalysts have the same chemical and physical properties except the crystal size: CAT-LC with 0.9 μm and CAT-SC with 0.4μm. Catalytic cracking runs were developed using several alkyl-benzene compounds with different critical molecular diameters. Reaction runs were conducted varying systematically a number of operating conditions such as reaction time (3, 5, 7, 10 seconds) and temperature (350–550°C). Regarding the rates of cumene cracking, they were essentially identical for both catalysts and as a result no important effect of crystal size was observed. However, the rates of cracking of 1,3,5-tri-iso-propylbenzene (1,3,5-TIPB) were significantly affected by the zeolite crystal size. At 350–450°C, higher 1,3,5-TIPB conversions were obtained using CAT-SC. Differences on 1,3,5-TIPB conversions were, however, less significant at 500–550°C. On this basis it is argued that the 1,3,5-TIPB cracking displayed two regimes: (a) at 350–450°C the overall cracking rate is controlled by diffusional transport, (b) at 500–550°C the overall reaction rate is controlled by the intrinsic cracking kinetics. Cumene and benzene were found to be the main product of 1,3,5-TIPB cracking with their yields decreasing and increasing respectively with both crystal size and temperature.; Experimental runs with model compounds in the CREC Riser Simulator were simulated using two decay models: (a) novel catalyst decay model based on converted reactant, and (b) a more empirical model based on time-on-stream. Kinetic and deactivation parameters were estimated using non-linear regression analysis and these parameters, together with estimated species diffusivities, were employed to calculate modified Thiele modulus and effectiveness factors. It was found that at 500–550°C both catalysts displayed an effectiveness factor close to unity. At 350–450°C, however, the CAT-SC showed higher effectiveness factors than CAT-LC with the CAT-LC showing an unambiguous change of regime, from diffusional controlled to chemical controlled, in the range of temperatures studied. |
