Dynamic modeling and operability analysis of a dual-membrane fixed bed reactor to produce methanol considering catalyst deactivation

The goal of this research is dynamic operability analysis of dual-membrane reactor considering catalyst deactivation to produce methanol. A dynamic heterogeneous one-dimensional model is developed to predict the performance of this configuration. In this configuration, a conventional reactor has been supported by a Pd/Ag membrane tube for hydrogen permeation and alumina–silica composite membrane tube to remove water vapor from the reaction zone. To verify the accuracy of the considered model, the results of conventional reactor are compared with the plant data. The main advantages of the dual-membrane reactor are: higher catalyst activity and lifetime, higher CO₂ conversion and methanol production.     

Dynamic modelling of catalytic liquid-phase reactions in fixed beds—Kinetics and catalyst deactivation in the recovery of anthraquinones

Dynamic modelling of catalytic fixed-bed reactors with liquid-phase feed is of crucial importance, since catalyst deactivation often plays a central role in reaction engineering. General dynamic modelling of liquid-phase fixed beds was considered, including complex reaction kinetics and catalyst deactivation. The modelling concept was applied on a catalytic liquid-phase conversion reaction. The model was tested with pilot-plant data and showed a good predictivibility. The model can be used to optimize the production life cycles of fixed beds with catalyst deactivation.     

Prolonging catalyst lifetime in supercritical isomerization of 1-hexene over a platinum/alumina catalyst

A series of experiments were carried out for the isomerization of 1-hexene in the temperature range and pressure range 10-100 bar, representing operating points both above and below the critical point of 1-hexene. At constant temperature of , increasing the pressure from 10 to 100 bar led to a substantial conversion increase up to a maximum of 78%. At each pressure, loss of conversion of 1-hexene was observed over the course of 8 h reaction time, which was attributed to the formation of oligomers and eventually coke upon the catalyst surface. Loss of conversion occurred initially more rapidly at the intermediate pressures of 40 and 70 bar, compared with at 10 and 100 bar, where a sustained but gradual decrease of conversion occurred. With increasing temperature in the range , conversion was highest at the condition , 40 bar, which is closest to the critical point of 1-hexene. Higher concentrations of oligomers, which act as coke precursors, were detected with increasing temperature and pressure of the reaction. The mass fraction of coke deposited upon the catalysts was dependent upon operating conditions and was within the range 0.63-1.36%, whilst the metal dispersion reduced from 26.8% for the fresh catalyst to 2.82-4.61% for the range of coked samples. A detailed examination of the void space structural changes, occurring after coking under both sub-and super-critical conditions, has been made. In particular, the void space structures were characterised in terms of external accessibility using percolation theory. The catalysts which were operated at reaction conditions where rapid initial deactivation occurred displayed a lower apparent connectivity in comparison with catalysts operated at conditions favouring a gradual sustained deactivation. It has also been found that, under certain conditions, the apparent connectivity of the remaining pore network can, unexpectedly, appear to increase following coking. This has been attributed to the initial loss of the most inaccessible pores within the network.     

The role of carbonaceous deposits in the skeletal isomerization of 1-butene over ferrierite zeolites

The inner pores of ferrierite (FER) with a Si/Al ratio of 21 have been modified by plasma-polymerized propylene loading and subsequent thermal treatment under a flow of He at 550°C in order to identify the exact role of carbonaceous deposits (coke) in the skeletal isomerization of 1-butene. It was found that even a small weight loading (0.8 wt%) of carbonaceous deposits within the inner pores of FER zeolite was sufficient to modify the pore structure of the zeolite, leading to a noticeable enhancement of the selectivity to isobutene. This may be attributed to the increased spatial constraints inside the pores which restrict undesired side reactions, such as 1-butene dimerization followed by cracking to light hydrocarbons.     

Methanol synthesis in a novel axial-flow, spherical packed bed reactor in the presence of catalyst deactivation

Since in the foreseeable future liquid hydrocarbon fuels will play a significant role in the transportation sector, methanol might be used potentially as a cleaner and more reliable fuel than the petrochemical-based fuels in the future. Consequently, enhancement of methanol production technology attracts increasing attention and, therefore, several studies for developing new methanol synthesis reactors have been conducted worldwide. The purpose of this research is to reduce the pressure drop and recompression costs through the conventional single-stage methanol reactor. To reach this goal, a novel axial-flow spherical packed bed reactor (AF-SPBR) for methanol synthesis in the presence of catalyst deactivation is developed. In this configuration, the reactor is loaded with the same amount of catalyst in the conventional single-stage methanol reactor. The reactants are flowing axially through the reactor. The dynamic simulation of the spherical reactors has been studied in the presence of long-term catalyst deactivation for four reactor configurations and the results are compared with the achieved results of the conventional tubular packed bed reactor (CR). The results show that the three and four stages reactor setups can improve the methanol production rate by 4.4% and 7.7% for steady state condition. By utilizing the spherical reactors, some drawbacks of the conventional methanol synthesis reactors such as high pressure drop, would be solved. This research shows how this new configuration can be useful and beneficial in the methanol synthesis process.     

Quick identification of a simple enzyme deactivation model for an extended-Michaelis-Menten reaction type. Exemplification for the D-glucose oxidation with a complex enzyme deactivation kinetics

One essential engineering problem when developing an industrial enzymatic process concerns the model-based design and optimal operation of the enzymatic reactor based on the process and enzyme inactivation kinetics. For a complex enzymatic system, the “default” used first-order enzyme deactivation model has been proved to lead to inadequate process design or sub-optimal operating policies. The present study investigates if a complex enzyme deactivation can be approximated with simple 1st, 2nd, or a novel proposed model with variable deactivation constant. The approached complex enzymatic system is those of the oxidation of D-glucose to 2-keto-D-glucose in the presence of pyranose 2-oxidase. The necessary “simulated experimental data” have been generated by means of an extended kinetic model from literature used to simulate a batch reactor under well-defined nominal conditions. The proposed enzyme deactivation model has been found to be the best lumping alternative, presenting several advantages: simplicity, flexibility, and a very good adequacy.     

The skeletal isomerization of C4-C7 1-olefins over ferrierite and ZSM-5 zeolite catalysts

The skeletal isomerization of C₄-C₇ 1-olefins was studied on ferrierite (FER) and ZSM-5 (MFI) zeolites to elucidate the effect of the molecular distribution in zeolite pores on the selectivity foriso-olefin formation. Regardless of the difference in molecular length of 1-olefins, the FER zeolite showed high selectivity foriso-olefins, while the selectivity became slightly low at the skeletal isomerization of long olefin molecules. The drastic decrease in the selectivity of MFI zeolites by increasing the conversion is concurrently observed in the skeletal isomerization of C₄-C₇ 1-olefins. The high selectivity of FER zeolites is explained by their sparse distributions of olefin molecules in pores, which induces a high preference for monomolecular skeletal isomerization.     

Skeletal isomerization of 1-butene over ferrierite and ZSM-5 zeolites: influence of zeolite acidity

Three ferrierite (FER) and five ZSM-5 (MFI) zeolites with SiO₂Al₂O₃ ratio ranging from 27 to 2000 are tested as catalysts for the skeletal isomerization of 1-butene at 350–450°C and atmospheric total pressure in order to study the influence of acidity and pore structure of zeolite on conversion and selectivity. The catalytic and NH₃ temperature-programmed desorption results from FER and MFI catalysts with the same SiO₂/Al₂O₃ ratio reveal that the pore structure of FER zeolite rather than its acidity may play an important role in achieving high selectivity for the skeletal isomerization of 1-butene to isobutene.     

Modeling and simulation of an integrated multi-shell fixed bed membrane reactor with well-mixed catalyst pattern for production of styrene and cyclohexane

A rigorous two-dimensional steady state mathematical model based on the dusty gas model is implemented to investigate the performance of a bench-scale integrated multi-shell fixed bed membrane reactor with well-mixed catalyst pattern for simultaneous production of styrene and cyclohexane. Since the styrene producing reaction is equilibrium limited, significant displacement of the thermodynamic equilibrium is achieved by three simultaneous actions of an auxiliary hydrogenation reaction of benzene using a well-mixed catalyst pattern, the membrane and the multi-shell reactor configuration. The simulation results show that the complete conversion of ethylbenzene is possible at relatively low temperature and shorter reactor length. Effective operating regions with optimal conditions are observed and explanations offered. An effective length criterion for the optimal conditions is presented. The effective operating regions are found to be sensitive to changes of catalyst bed composition, feed temperature, feed pressure and shells ratio. It is also found that the multi-shell configuration is superior to the single shell configuration. Although this investigation is restricted to two catalysts and two shells, some of the rich characteristics of this system have been uncovered.     

Selective kinetic deactivation model for methanol synthesis from simultaneous reaction of CO2 and CO with H2 on a commercial copper/zinc oxide catalyst

A kinetic model for the deactivation of copper/zinc oxide catalyst during the methanol synthesis has been developed. This model is of the Langmuir-Hinshelwood-Hougen-Watson type and considers two types of active sites for the deactivation of catalyst. One of the site types on copper is allocated for the deactivation of the catalyst due to carbon dioxide while another type is assigned for the deactivation of the catalyst due to carbon monoxide. The parameters of the deactivation rate equations based on the above concept have been determined using the experimental data of Hoffmann (1993). The validity of the deactivation model has been checked by comparing the results predicted by the model with experimental data different than of those used to evaluate the parameters of the model. The good agreement that noticed in this comparison confirmed the idea that CO and CO₂ are responsible at different extent for the deactivation of Cu/ZnO catalyst during methanol synthesis.     

Five-lump kinetic model with selective catalyst deactivation for the prediction of the product selectivity in the fluid catalytic cracking process

The effective simulation of the fluid catalytic cracking (FCC) operation requires a good understanding of many factors such as, reaction kinetics, fluid dynamics, and feed and catalyst effects. The different product slates that can be obtained are the consequence of a complex reaction scheme including cracking, isomerization, hydrogen transfer, oligomerization, etc. Furthermore, the catalyst deactivation may affect each one of the reactions in different ways, which creates an additional reason for different variation with time-on-stream of the yield to each product. On the basis of the experimental data of the FCC pilot plant operated in Chemical Process Engineering Research Institute (CPERI, Thessaloniki, Greece), a lumping model was developed for the prediction of the FCC product distribution. The lumped reaction network involved five general lumps (gas oil, gasoline, coke, liquefied product gas, and dry gas) to simulate the cracking reactions and to predict the gas oil conversion and the product distribution. The paths of catalyst deactivation were studied and a selective deactivation model was adopted that enhances the fundamentality and accuracy of the lumping scheme. The hypothesis of selective catalyst deactivation was found to improve the product slates prediction. Models with different assumptions were examined, regarding the behavior of the catalyst, as deactivated, and its effect on the reactions of the lumping scheme. A large database of experiments, performed in the FCC pilot plant of CPERI was used to verify the performance of the models in steady state unit operation. The simulation results depict the importance of incorporating selective catalyst deactivation functions in FCC lumping models.     

Kinetic model and simulation for catalyst deactivation during dehydrogenation of methylcyclohexane over commercial Pt-, PtRe- and presulfided PtRe-Al2O3 catalysts

The catalyst deactivation kinetics for the dehydrogenation of methylcyclohexane were studied with Pt-, PtRe-, and presulfided Presulfided PtRe-Al₂O₃ Catalysts in a fixed-bed differential reactor. The kinetic analysis was made based on a Langmuir-Hinshelwood model assuming that the dehydrogenation of methylcyclohexene into methylcyclohexadiene was the rate-controlling step for the main reaction, and the polymerization of adjacently adsorbed methylcyclohexadiene as coke precursor molecules was the rate-controlling step for the deactivation reaction. The experimental data for the three catalysts at varying partial pressures of methylcyclohexane and hydrogen were well correlated by the hyperbolic deactivation function derived. The number of active sites involved in the rate-controlling step of the deactivation was evaluated to be 2.7 in average. The enhanced activity maintenance by the addition of Re as well as sulfur was attributed to a reduction in the concentration of multiple sites occupied by coke precursors adjacently adsorbed, leading to a decrease in the rate constant of the deactivation reaction.     

列管固定床反应器内CO氧化偶联制草酸二甲酯反应模拟及优化

针对列管式固定床反应器中的单根反应管,采用在接近工业条件下获得的CO氧化偶联制草酸二甲酯动力学方程,建立了一维、二维拟均相模型,并与单管实验结果进行了对比,结果表明一维拟均相反应器模型更能准确描述单管反应器内的CO偶联反应。进一步利用一维拟均相模型模拟计算了操作参数对床层热点温度、反应转化率、产物选择性及床层压降的影响,分析了反应器热点温度对操作参数的敏感性。计算结果表明：冷却介质温度对反应管热点温度、亚硝酸甲酯转化率有较大影响,是需要严格控制的工艺指标;较低的空速容易引起反应器飞温;反应器进口压力、原料气进料温度和反应物组成在计算范围内对反应器热点温度影响相对较小。为了提高偶联反应器的负荷和强化床层内的传热效果,可以将进料空速提高至4000 h^-1,同时,可以通过将反应器进口压力增大至500 kPa来降低压缩机能耗。研究结果可为现有列管式CO氧化偶联反应器的改进和工艺优化提供参考。     

Numerical simulation of the dynamic flow behavior in a bubble column: A study of closures for turbulence and interface forces

Numerical simulations of the bubbly flow in two square cross-sectioned bubble columns were conducted with the commercial CFD package CFX-4.4. The effect of the model constant used in the sub-grid scale (SGS) model, C_S, as well as the interfacial closures for the drag, lift and virtual mass forces were investigated. Furthermore, the performance of three models [Pfleger, D., Becker, S., 2001. Modeling and simulation of the dynamic flow behavior in a bubble column. Chemical Engineering Science, 56, 1737-1747; Sato, Y., Sekoguchi, K.,1975. Liquid velocity distribution in two-phase bubble flow. International Journal of Multiphase Flow 2, 79-95; Troshko, A.A., Hassan, Y.A., 2001. A two-equation turbulence model of turbulent bubbly flows. International Journal of Multiphase Flow 27, 1965-2000] to account for the bubble-induced turbulence in the k-ε model was assessed. All simulation results were compared with experimental data for the mean and fluctuating liquid and gas velocities. It is shown that the simulation results with C_S=0.08 and 0.10 agree well with the measurements. When C_S is increased, the effective viscosity increases and subsequently the bubble plume becomes less dynamic. All three bubble-induced turbulence models could produce good solutions for the time-averaged velocity. The models of Troshko and Hassan and Pfleger and Becker reproduce the dynamics of the bubbly flow in a more accurate way than the model of Sato and Sekoguchi. Based on the comparison of the results obtained for two columns with different aspect ratio (H/D=3 and H/D=6), it was found that the model of Pfleger and Becker performs better than the model of Troshko and Hassan, while the model of Sato and Sekoguchi performs the worst. It was observed that the interfacial closure model proposed by Tomiyama [2004. Drag, lift and virtual mass forces acting on a single bubble. Third International Symposium on Two-Phase Flow Modeling and Experimentation, Pisa, Italy, 22-24 September] performs better for the taller column. With the drag coefficient proposed by Tomiyama, the predicted slip velocity agrees well with the experimental data in both columns. The virtual mass force has a small influence on the investigated bubbly flow characteristics. However, the lift force strongly influences the bubble plume dynamics and consequently determines the shape of the vertical velocity profile. In a taller column, the lift coefficient following from the model of Tomiyama produces the best results.