Kinetic modeling for catalytic cracking of paraffinic naphtha

Abstract

Thermal steam cracking is the major process to produce ethylene and propylene using paraffin-rich light naphtha from refinery. They are the most energy consuming processes in the petrochemical plants. Recent global warming crisis and increasing market demand for propylene urge petrochemical industries to develop new processes with low specific energy consumption and high propylene yield. Catalytic cracking process can be a good candidate to meet the social and market needs. To scale-up catalytic cracking process, a kinetic model is necessary. However it is very difficult to model catalytic cracking process as the process has complex chemical reaction network with many components. The purpose of this work is to develop a systematic methodology and a kinetic model for the catalytic cracking of paraffinic naphtha. The catalytic cracking of paraffinic naphtha requires higher temperature than that of olefinic naphtha. In the high temperature catalytic cracking, thermal cracking also occurs simultaneously. As the mechanisms of thermal and catalytic cracking are different, it needs to be analyzed which mechanism is dominant. The degree of thermal cracking which is defined as the yield ratio of ethylene and propylene between thermal and catalytic cracking is experimentally investigated using MAT (micro activity test) and CFBR (continuous fluidized bed reactor) units. As the degree of thermal cracking is over 50% at $670^\circ C$, both thermal and catalytic cracking mechanisms have to be simultaneously considered in the kinetic model. An approximate approach based on the transition state theory is proposed to model the catalytic cracking of paraffinic naphtha which has complex chemical reaction network. The pertinent parameters of the developed kinetic model are estimated by a genetic algorithm. Additionally, an integrated modeling toolbox with graphic user interface is developed. The efficacy of the proposed approach is shown with its application to the industrial catalytic cracking of paraffinic naphtha in the circulating fluidized bed reactor system. The kinetic model is assessed by investigation of some basic assumptions. This approach will be particularly effective for modeling the complex chemical reaction network systems.