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Understanding catalytic cracking reactions

Fluid catalytic cracking (FCC) is a major refinery process which converts heavy oil feedstocks known as vacuum gasoil, into more valuable products such as gasoline, liquid petroleum gas (LPG) and light cycle oil as well as dry gas and coke. Until now, the catalytic cracking of gasoil in a riser reactor has been modelled in terms of lumped species and lumped reaction pathways. Several products, reactants and intermediates are lumped together so as to reduce the size of the simulation models. The lumped models do not reflect the underlying chemistry and this means that the rate parameters for cracking depend on the feedstock composition. Consequently, each new feedstock requires extensive experimental effort.

To develop improved and more efficient catalytic cracking reactors, the detailed kinetics of the cracking process needed to be better understood. In order to accurately design and optimize the performance of an FCC unit, an advanced riser reactor model is necessary. Such a model involves a detailed description of cracking rates and of the hydrodynamics of the riser reactor. The work centred around developing this model. The model is fully based on the chemistry of carbenium ions, the intermediates of cracking reactions. It leads to kinetic parameters which do not depend on the feedstock composition. Laboratory experiments were performed with model compounds to study the catalytic cracking of 3 types of hydrocarbons: paraffins, alkylaromatics and alkylnapthalenes. A detailed analysis of the vacuum gasoil was developed. The 3-dimensional modelling of the riser reactor was based on a 2 level approach: phase mathematical model accounted for the simulation of flow, heat transfer and reaction inside a riser reactor. The model provides valuable insight into the phenomena occurring in the catalytic cracking processes, especially in the complex inlet zone of the reactor.

The model was validated against experimental data from 2 industrial test runs, showing conclusively that the model gives a detailed insight into the complex phenomena, particularly in the inlet zone of the reactor. The model was used to assess the influence of several variables in the cracking process. These included temperature, catalyst to oil ratio, nozzle geometry and contact time. Significant increases in the yield of gasoline as an end product was also shown to be possible by increasing the number of nozzles on the reactor. This project considered the problems of analysing the reactions in a novel way and developed a model to simulate the chemical reactions in the cracking reactor. With these improvements, the effects of altering variables such as temperature and reaction geometry can be analysed and predicted.

Contact

Gilbert FROMENT
Tel.: +32-9-2644516
Fax: +32-9-2644999
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