چكيده به لاتين
Conversion of light hydrocarbons to the more valuable products such as aromatics is the one of the most important subjects in the petrochemical industry. In this regard, the catalytic conversion of light hydrocarbons into aromatics has been received considerable attention within the industrial and academic fields. The most serious disadvantage of aromatization reactions is coke formation that leads to the catalyst deactivation. Two-zone fluidized bed reactor (TZFBR) is a multifunctional reactor which the main reaction in reaction zone as well as the coke deposits on the catalyst surface oxidation in regenerative zone can occur simultaneously in a single fluidized bed.
Therefor, the TZFBR is suggested for propane aromatization reaction. In the present work, the propane aromatization reaction performance over Zn/HZSM-5 catalyst was studied in the fixed bed and TZFBRs. The structure and amount of carbon deposited on the catalyst surface were characterized by XRD, BET, SEM and TGA. The results of the fixed bed reactor showed that the propane conversin 64.8% and selectivity to aromatics 66.9% were obtained at high temperature of 560°C and low gas hourly space velocity 500 cm3/g.h. Moreever, The effects of the reaction parameters such as: space velocty, temperature and time on stream rection were studied.A lumped kinetic model consisting of six lumped components and six reaction steps was proposed to describe aromatization of propane. The kinetic model involves 18 kinetic parameters and one catalyst deactivation constant. The reaction steps orders were obtained by power law model. Frequency factors and apparent activation energies of reaction steps were calculated based on the Arrhenius equation. An exponential function depending on time-on-stream was applied for the catalyst deactivation model and the kinetic parameters were calculated via genetic algorithm. The kinetic results indicated that lumped kinetic model can well estimate the products yields of propane aromatization. Propane conversion to aromatics was carried out in a TZFBR over Zn/HZSM-5 catalyst. At the steady state condition the coke formation in the TZFBR was counteracted with continuous catalyst regeneration by oxygen. In the lower part of the reactor, the oxidation and combustion of deposited coke on the catalyst surface were achieved with a necessary amount of oxygen. The aromatization reaction takes place in the upper part. In order to compare and to ensure the stability of the TZFBR performance, the propane aromatization was carried out in the conventional fluidized and fixed bed reactors in the absence of oxidant. The higher propane conversion and aromatics selectivity were obtained in the TZFBR compared with conventional fluidized and fixed bed reactors under the same operation conditions. The effects of the main operating variables such as: O2 mole percentage, height of the propane feed entry point, bed temperature and relative velocity were studied and the optimum experimental conditions were considered. The results of the TZFBR showed that the yield of aromatics 22.94% was obtained at relative velocity of 1.6, high temperature of 560°C, O2 mole percentage of 3% and propane feed entry point of 5cm. To model TZFBR a simple two-phase for the first time and the Menendez three-phase model were used. Both of them showed a satisfactory agreement with observed experimental data. Because of simplicity of two-phase model, it is suggested to use in TZFBR modeling.
Keywords: Propane aromatization, Zn/HZSM-5 zeolite catalyst, Two-zone fluidized bed reactor, kinetic modeling, Two-zone fluidized bed modeling
