مدل سازی و تجزیه و تحلیل حساسیت فرایند تولید منومر استایرن و بررسی رفتار کاتالیزوری

In this work, a fundamental kinetic model based upon the Hougen–Watson non-porosity formalism was derived and used to simulate dehydrogenation and oxidation axial flow reactors. In addition, partial pressure profiles of components during styrene production process inside porous catalyst were obtained using Dusty-Gas model. The preservation equations are adopted to calculate temperature and flow profiles in the reactors filled with iron–potassium promoted catalyst pellets. The presented mathematical model for ethylbenzene dehydrogenation consists of nonlinear simultaneous differential equations with multiple dependent variables. Simulation results such as selectivity and operating temperature for different conventional catalysts have been presented and compared with those of a new introduced catalyst based on Fe2O3. Comparison of simulation results with experimentally observed ones shows that the model can precisely predict behavior of the industrial unit. Furthermore, the obtained results show that application of the new introduced catalyst increase ethylbenzene conversion and decrease necessary inlet temperature.

Styrene (ST) is the second most important monomers in the chemical industries. In 2002, more than 2.5 × 107 MT/year of styrene monomer was produced worldwide (Product Focus: Styrene, 2002). The styrene production process was originally developed in the 1930s by BASF (Germany) and Dow Chemical (USA). The major commercial process for the production of styrene is the dehydrogenation of ethylbenzene (EB), which accounts for 85% of the commercial production (James & Castor, 1994). The dehydrogenation process mostly consists of the catalytic reaction of ethylbenzene. This process involves a highly endothermic reaction carried out in the vapor phase over solid catalyst particles. Steam, supplies the necessary reaction heat, prevents excessive coking or carbon formation, shifts the equilibrium of the reversible reaction toward the products, and cleans the catalyst from any existing carbon. The potassium-promoted iron oxide catalyst has been extensively used for styrene production (Coulter, Goodman, & Moore, 1995). There are several common dehydrogenation methods for the production of styrene monomer from ethylbenzene including adiabatic dehydrogenation of the ethylbenzene, isothermal dehydrogenation of the ethylbenzene, simultaneously producing styrene and oxidation of propylene, membranous process for dehydrogenation of ethylbenzene and dehydrogenation and oxidation of ethylbenzene using carbon dioxide. However, adiabatic dehydrogenation of the ethylbenzene is the most widely used method (Chon, 2003, Mcketa, 1996, Sun et al., 2004 and Tabriz Petrochemical, 1993). Thanks to the recent enhancements in technology, nowadays further developments in the efficiency of the above-mentioned processes have occurred. In a recent study, Gonzalez and Moronta (2004) reported the dehydrogenation of ethylbenzene over natural clay and its pillared aluminum form impregnated with either cobalt nitrate or cobalt acetate. Although the total conversion was less than 20% for these catalysts in their unreduced forms, the co-impregnated natural clay showed a higher conversion than natural Al-pillared and co-impregnated aluminum-pillared clays. Moreover, those catalysts derived from the cobalt nitrate salt were more active than those obtained using the cobalt acetate salt. Gonzalez and Moronta has demonstrated that incorporation of cobalt has been significantly influenced by the nature of the starting clay, being the Co content in T1 natural clay higher than Al-PILCT1 clay and this strongly effect governed the catalytic process via Lewis–Bronsted acidity more than gallery access. The most active catalysts were those prepared using cobalt nitrate, but due to coke formation those prepared from cobalt acetate were less active than the others. However, all catalysts presented good styrene selectivity. Preparation of SnO2–ZrO2 nanocomposite catalysts by simplified and environmentally acceptable conditions has been achieved by Burri and his co-workers. Whereat, the formation of mixed oxide nanocomposite catalysts, consequent enhancement in the acid–base functional behavior and the augmentation of catalytic performance for the oxidative dehydrogenation of ethylbenzene to styrene, has been accomplished (Burri, Choi, Han, & Jiang, 2008). Preparation of highly active Cr2O3–SiO2 catalyst by sol–gel method for ethylbenzene dehydrogenation in the presence of CO2 was investigated by Huiyun to illustrate influence of catalyst particles characteristics upon dehydrogenation process yield (Huiyun et al., 2006). Abo-Ghander et al. (2010) determined optimal design for an auto thermal membrane reactor coupling the dehydrogenation of ethylbenzene to styrene with the hydrogenation of nitrobenzene to aniline. In their work, the Pareto optimal design frontier of a catalytic membrane reactor coupling dehydrogenation of ethylbenzene to styrene with hydrogenation of nitrobenzene to aniline has been obtained. To achieve this goal, a bi-objective optimization problem with linear and nonlinear constraints has been formulated. Their objective function has been on simultaneous maximizing styrene production and nitrobenzene hydrogenations yields. Bounds and constraints with real industrial values have been imposed on both operational and design variables (Abo-Ghander et al., 2010). The kinetics of EB dehydrogenation have been widely investigated (Bird, 2007 and Pars Petrochemical Complex Operating Manual, 2010) but seldom in a fundamental way, developed empirical polynomial correlations for the optimization of the commercial unit (Ellis, 1999 and Sulzer Chemtech Ltd., 2010). Obviously, a small improvement in the plant operation has led to a substantial increase of economical returns. In other words, most styrene producers and researchers have not pursued the research toward the fundamental kinetic model such as using Hougen–Watson modeling approach. Furthermore, the reaction rates published in the most of papers are not really intrinsic but rather are effective. Recently, Bennet (2000) derived a mechanistic model using a single-crystal unpromoted iron oxide film. His model included EB dehydrogenation into ST, but does not consider benzene (BZ) and toluene (Tol) formation and effects of reaction heating on conversion and catalyst activity. In this work, with a new approach, the thermal and mass differential equations, which are applied in dehydration–oxidation reactors, will be solved by Hougen–Watson method as a non-porosity model. In addition, the mass transfer equation inside the catalyst particles will be solved using Dusty-Gas method for evaluating partial pressure of components in porous catalyst. For evaluation of modeling approach, the simulation results will be compared to actual data of an industrial plant.

This paper modeled the styrene monomer production process in adiabatic dehydrogenation and oxidation reactors and investigated various catalyst effects upon energy consumption and ethylbenzene conversion percent. Mass and heat preservation equations were solved using both porosity and non-porosity models. The modeling results corresponded to the operation data that obtained from a real industrial unit. Also, the steam/hydrocarbon and steam/oxygen ratios were examined and some optimization tips were considered. The EB conversion percent gets high with increasing of these ratios. On the other hand, according to related parameters such as consumed energy during hot vapor injection and reactors volume, there is an optimization limit for the ratio of vapor/hydrocarbon. Using Dusty-Gas porosity model, diffusion variations in the reactors and the products within the catalyst particles were investigated and partial pressure profiles were obtained to present catalyst particle effects on the system components. Furthermore, G-84C catalyst in this research needs to less energy consumption. While the catalyst converts remarkable EB conversion percent that denotes most EB% rather than other prevalent catalysts in the styrene production process.