Thermal imaging of catalyst surface during catalytic dehydrogenation of cyclohexane under spray-pulsed conditions

Infra-red thermographic imaging, a tool widely used for screening of active catalytic materials in the field of combinatorial chemistry, has been used during dehydrogenation of cyclohexane on Pt catalyst supported on active carbon cloth and alumite supports. A spray-pulsed reactor with injection of atomized cyclohexane and to create alternate wet and dry conditions on the catalyst surface was used in this study. Since the production rate of hydrogen via endothermic dehydrogenation reaction is greatly dependent on the temperature of the catalyst surface, the temperature profile of the catalyst is important to observe. The observed change in the temperature profile at wet and dry conditions with varying pulse injection frequency and corresponding product gas analysis reveals that the spray-pulse mode is useful in improving the catalyst activity. Further, the comparison between activated carbon support and a more conductive support such as alumite is reported using thermal imaging for more suitable support in this endothermic reaction.     

环己烷气相氧化脱氢催化剂的研究进展

为了尽可能多地获得环己烯和充分利用苯选择加氢反应的副产物环己烷,环己烷氧化脱氢反应在近 20 年内得到迅速发展.环己烷氧化脱氢反应催化剂体系的选择对于获得较高的环己烯产率和选择性、较低的氧化脱氢反应温度以及最优的反应路径尤为重要.详细阐述了阳离子沸石催化剂、复合金属氧化物催化剂以及贵金属丝网催化剂等在环己烷氧化脱氢制取...     

A COMPARISON OF A NOVEL RECUPERATIVE CONFIGURATION AND CONVENTIONAL METHANOL SYNTHESIS REACTOR

Coupling energy-intensive endothermic reaction systems with suitable exothermic reactions improves the thermal efficiency of processes and reduces the capital cost of the reactors. In this study, a steady-state heterogeneous model for a novel thermally coupled reactor, containing methanol synthesis reactions and cyclohexane dehydrogenation, was developed. This heat exchanger reactor consists of two fixed beds separated by a wall, where heat is transferred across the surface of the tube from the exothermic into the endothermic side. The co-current mode is investigated, and the simulation results are compared with corresponding data for an industrial methanol fixed bed reactor operated at the same feed conditions. The results show that although methanol productivity in the thermally coupled reactor is not higher than that in the conventional methanol reactor, benzene is also produced as an additional valuable product in a favorable manner, and autothermality is achieved within the reactor. This novel configuration can increase the methanol synthesis temperature at the first part of the reactor for higher process rates and then reduce the temperature at the second part of reactor for increasing thermodynamic equilibrium; those are two key issues in methanol reactor configurations. The influence of inlet temperature, molar flow rate, and shell diameter of the endothermic stream on reactor behavior is investigated. The results suggest that coupling of these reactions in co-current mode could be feasible and beneficial. Experimental proof-of-concept is needed to establish the validity and safe operation of the novel reactor.     

Computerized Solution of the Dynamic Sorption Process for a Ternary System in a Heterophase Medium

A mathematical model equation for the ternary adsorption–reaction process was developed and illustrated for the catalytic dehydrogenation of cyclohexane to benzene with the adsorption of hydrogen atoms as a monomolecular species on platinum–rhenium/alumina catalyst in inert and active carrier gases using pulse and continuous flow techniques. An optimization routine of the Nelder–Mead simplex method was used to estimate the surface reaction rate constant and adsorption equilibrium constant at different temperatures. These constants were then used to determine activation energies and adsorption equilibrium energies for cyclohexane dehydrogenation in inert (argon, helium) and active (hydrogen) carrier gases using pulse and continuous flow techniques. Numerical solutions for the ternary adsorption–reaction scheme were compared with the binary adsorption–reaction case where hydrogen adsorption is ignored. The predicted results for the ternary adsorption–reaction revealed that hydrogen adsorption during cyclohexane dehydrogenation is significant.     

A comparison of conventional and optimized thermally coupled reactors for Fischer–Tropsch synthesis in GTL technology

In this research, the conditions at which a thermally coupled reactor – containing the Fischer–Tropsch synthesis reactions and the dehydrogenation of cyclohexane – operates have been optimized using differential evolution (DE) method. The proposed reactor is a heat exchanger reactor consists of two fixed bed of catalysts separated by the tube wall with the ability to transfer the produced heat from the exothermic side to the endothermic side. This system can perform the exothermic Fischer–Tropsch (F–T) reactions and the endothermic reaction of cyclohexane dehydrogenation to benzene simultaneously which can save energy and improve the reactions' thermal efficiency. The objective of the research is to optimize the operating conditions to maximize the gasoline (C₅⁺) production yield in the reactor's outlet as a desired product. The temperature distribution limit along the reactor to prevent the quick deactivation of the catalysts by sintering at both sides has been considered in the optimization process. The optimization results show a desirable progress compared with the conventional single stage reactor. Optimal inlet molar flow rate and inlet temperature of exothermic and endothermic sides and pressure of exothermic side have been calculated within the practicable range of temperature and pressure for both sides.     

环己烷在碳化钼催化剂上的脱氢研究

以环己烷为碳源,采用程序升温还原法制备碳化钼催化剂,对前驱物和中间物及催化剂的晶相结构进行XRD表征,在固定床反应装置上进行环己烷的脱氢反应实验。固定反应压力为0.1 MPa,考察反应温度、空速、氢烃体积比和压力对碳化钼脱氢活性的影响。结果表明,催化剂活性相为高活性的β-Mo₂C,在反应温度450 ℃、空速2 h^-1、压力0.1 MPa和氢烃体积比200∶1条件下,环己烷转化率达88%,脱氢反应的选择性为79%。     

Non‐Isothermal Non‐Adiabatic Dehydrogenation of Cyclohexane in Catalytic Membrane Reactors

Abstract

This study focuses on modeling and analysis of the non‐isothermal, non‐adiabatic, dehydrogenation of cyclohexane in membrane catalytic reactors. The dehydrogenation reaction is endothermic with a low equilibrium conversion of 0.06 at a temperature of 473 K and pressure of 101 kPa. The membrane reactor removes hydrogen from the reaction mixture and results in increase of the reaction conversion. The analysis is made as a function of feed flow rate, feed temperature, feed composition, inert flow rate in the feed stream, flow rate of sweep gas, pressures of the tube side and shell side, permeability constant of hydrogen, and tube diameter. The analysis also includes a study of the co‐current and the counter‐current flow modes. The results show lower conversion for the counter‐current flow mode, because of the decrease in the driving force for permeation. A comparison of model predictions against previous literature studies shows good agreement.     

固定床乙苯脱氢制苯乙烯工艺条件的研究

采用固定床反应器,在自制铁系催化剂的作用下,研究了乙苯脱氢制苯乙烯反应的工艺条件,应用气相色谱技术测定了不同工艺条件下的产物组成。结果表明,影响乙苯脱氢制苯乙烯反应的主要因素有反应温度、水与乙苯物质的量比及催化剂活性。反应温度的影响最为显著,当反应温度较低时,催化剂活性较低,产物平衡组成中苯乙烯含量低、未反应的乙苯含量高;当反应温度过高时,系列副反应加剧,催化剂表面结焦倾向加大,导致催化剂活性明显下降。确定了最佳的反应条件为:水与乙苯物质的量比(8~15)∶1、反应温度580~590℃。在此条件下,乙苯的转化率为89.29%、苯乙烯收率为64.59%、苯乙烯选择性为72.34%。     

A novel reverse flow reactor coupling endothermic and exothermic reactions: Part II: Sequential reactor configuration for reversible endothermic reactions

The new reactor concept for highly endothermic reactions at elevated temperatures with possible rapid catalyst deactivation based on the indirect coupling of endothermic and exothermic reactions in reverse flow, developed for irreversible reactions in Part I, has been extended to reversible endothermic reactions for the sequential reactor configuration. In the sequential reactor configuration, the endothermic and exothermic reactants are fed discontinuously and sequentially to the same catalyst bed, which acts as an energy repository delivering energy during the endothermic reaction phase and storing energy during the consecutive exothermic reaction phase. The periodic flow reversals to incorporate recuperative heat exchange result in low temperatures at both reactor ends, while high temperatures prevail in the centre of the reactor. For reversible endothermic reactions, these low exit temperatures can shift the equilibrium back towards the reactants side, causing ‘back-conversion’ at the reactor outlet.The extent of back-conversion is investigated for the propane dehydrogenation/methane combustion reaction system, considering a worst case scenario for the kinetics by assuming that the propylene hydrogenation reaction rate at low temperatures is only limited by mass transfer. It is shown for this reaction system that full equilibrium conversion of the endothermic reactants cannot be combined with recuperative heat exchange, if the reactor is filled entirely with active catalyst. Inactive sections installed at the reactor ends can reduce this back-conversion, but cannot completely prevent it. Furthermore, undesired high temperature peaks can be formed at the transition point between the inactive and active sections, exceeding the maximum allowable temperature (at least for the relatively fast combustion reactions).A new solution is introduced to achieve both full equilibrium conversion and recuperative heat exchange while simultaneously avoiding too high temperatures, even for the worst case scenario of very fast propylene hydrogenation and fuel combustion reaction rates. The proposed solution utilises the movement of the temperature fronts in the sequential reactor configuration and employs less active sections installed at either end of the active catalyst bed and completely inactive sections at the reactor ends, whereas propane combustion is used for energy supply. Finally, it is shown that the plateau temperature can be effectively controlled by simultaneous combustion of propane and methane during the exothermic reaction phase.     

催化裂化过程中的热裂化与催化裂化

研究了催化裂化过程中热裂化反应和催化裂化反应的特点和影响因素 .结果表明反应温度、剂油比和反应时间能显著地影响热裂化反应和催化裂化反应 .采用适当的催化剂和短反应时间能抑制催化裂化过程中不利的热裂化反应 ,达到调整产品分布和产品组成的目的 .     

Multiple Regression Analysis of Catalytic Dehydrogenation of Isopropanol in a Chemical Heat Pump System

Chemical heat pump is a clean technology developed to upgrade the low‐level thermal energy to upper levels and to store energy without losses caused by temperature differences. Multiple regression analysis of catalytic dehydrogenation of isopropanol was performed. The endothermic dehydrogenation of isopropanol was carried out under continuous boiling and refluxing conditions in order to study the enhancement effects of the presence of an alkaline compound and different types of catalysts at various concentrations in the reaction medium on the evolution rate of hydrogen. The factorial experimental design method was applied to understand better the coupled influences of both catalyst and alkaline additive concentrations to discuss and evaluate statistically the results for different catalysts and to develop the related models.     

A novel reverse flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation. Periodic gas flow reversal incorporates regenerative heat exchange inside the reactor. The reactor concept is studied for the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst.Two different reactor configurations are considered: the sequential reactor configuration, where the endothermic and exothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the endothermic and exothermic reactants are fed continuously to two different compartments directly exchanging energy. The dynamic reactor behaviour is studied by detailed simulation for both reactor configurations. Energy constraints, relating the endothermic and exothermic operating conditions, to achieve a cyclic steady state are discussed. Furthermore, it is indicated how the operating conditions should be matched in order to control the maximum temperature. Also, it is shown that for a single first order exothermic reaction the maximum dimensionless temperature in reverse flow reactors depends on a single dimensionless number. Finally, both reactor configurations are compared based on their operating conditions. It is shown that only in the sequential reactor configuration the endothermic inlet concentration can be optimised independently of the gas velocities at high throughput and maximum reaction coupling energy efficiency, by the choice of a proper switching scheme with inherently zero differential creep velocity and using the ratio of the cycle times.In this first part, both the propane dehydrogenation and the methane combustion have been considered as first order irreversible reactions. However, the propane dehydrogenation is an equilibrium reaction and the low exit temperatures resulting from the reverse flow concept entail considerable propane conversion losses. How this ‘back-conversion’ can be counteracted is discussed in part II Chemical Engineering Science, 57, (2002), 855-872.     

草酸铜-草酸镍催化工业双戊烯脱氢合成对伞花烃的研究

研究了并流共沉淀法制备的草酸铜-草酸镍催化剂在工业双戊烯脱氢合成对伞花烃气相反应中的催化性能。主要考察了催化剂铜镍摩尔比、还原温度、催化剂固定床的布置方式和脱氢次数对反应结果的影响。采用TG-DTA、FTIR测试手段对催化剂的热分解过程及其结构进行了表征。结果表明,草酸铜-草酸镍制备简单,在催化合成对伞花烃的气相反应中表现出了良好的活性。实验得到最佳合成条件为:催化剂铜镍摩尔比为1∶3,还原温度为500°C,催化剂床层采用竖向布置,经二次脱氢反应基本完成,在最佳条件下制得产物中对伞花烃含量最高为79.38%。     

Heterogeneously catalyzed oxidative dehydrogenation of isobutyric acid to methacrylic acid: A new application of periodic flow reversal

The heterogeneously catalyzed oxidative dehydrogenation of isobutyric acid in a fixed bed reactor using molybdenum (Mo) heteropoly acids as catalysts shows a loss of Mo into the gas phase due to the formation of volatile Mo-complexes under reaction conditions. To avoid this loss of catalyst and to keep the catalytic material in the fixed bed and thus increase the catalyst's lifetime, the process has been performed under periodic flow reversal within the reactor. In this work, periodic flow reversal is tried in a semi-pilot test reactor as a method to fix the Mo-compounds in the catalyst bed. The influence of this mode of operation on the temperature profile in the reactor, on conversion, selectivity and yield of the product methacrylic acid is investigated in comparison with the process without periodic flow reversal.     

Cyclohexane dehydrogenation kinetics on a platinum-rhenium/aluminium oxide catalyst with hydrogen and inert diluents

The dehydrogenation of cyclohexane to benzene on a platinum-rhenium/aluminium oxide (Pt-Re/Al₂O₃) catalyst was investigated using both pulse and continuous flow techniques with hydrogen and inert (N₂, He, Ar) diluents at the following conditions: 1 and 4 atm (1 atm = 101 kPa) total pressure and 432–623 K. Experiments performed with hydrogen as a carrier in the pulse mode showed anomalous behaviour as cyclohexane conversion was observed to increase with pulse size/pulse width and decrease with temperature and contact time. The dehydrogenation reaction was first-order with respect to cyclohexane for both the pulse and continuous flow experiments. Lower activation energies were observed for the inert carriers; activation energies were between 37.7–54.4 kJ mol<^?1 for inert diluents. With a hydrogen carrier, the activation energies were found to be 87.9 and 121.3 kJ mol^?1 for T<553 K and <553 K, respectively. The difference in activation energy between hydrogen and the inert carriers was attributed to a change in mechanism on the coked catalyst surface. Surface conversion of cyclohexane to adsorbed cyclohexane was the slowest step in a hydrogen carrier. For the inert carriers, it was postulated that surface diffusion of adsorbed intermediate for for subsequent conversion was rate determining.     

Dehydrogenation of C3–C4 paraffin's to corresponding olefins over slit-SAPO-34 supported Pt-Sn-based novel catalyst

The objective of this work is to discuss the performance of Pt-Sn/slit-SAPO-34 novel catalyst for selective C₃–C₄ dehydrogenation to corresponding light olefins. The metallic contents, acidity, active metallic sites and metallic dispersion were determined using a number of physico-chemical techniques as it gives a justification for superior catalytic activity for dehydrogenation reaction. The Pt-Sn/slit-SAPO-34 catalyst was analyzed for dehydrogenation activity under optimized operating conditions; at atmospheric pressure, hydrogen to alkane (feed) molar ratio is 0.2, weight hourly space velocity 5 h^?1 and temperature 585 °C. Around 40% light alkane conversion and above 95% of total olefins selectivity with 94% propene, 92% n-butene and about 84% iso-butene selectivity were achieved over Pt-Sn/slit-SAPO-34 novel catalyst. The catalyst was parametrically characterized over the above said operating conditions and effects of operating conditions on product distribution were discussed. The coke formation was inherently related to catalyst activity in dehydrogenation reaction and related to surface intermetallic ensemble effects; and ultimately the prominent stakeholder in catalyst deactivation. The novel catalysts also showed very good hydrothermal stability in a continuous reaction–regeneration cycles due to silica-based acidic structure of support. The results obtained over Pt-Sn/slit-SAPO-34 novel catalyst were compared with other Pt-Sn-based ZSM-5 and SAPO-34 supported catalysts of similar active metallic content under identical operating conditions.     

Production of hydrogen-rich gas via reforming of iso-octane over Ni–Mn and Rh–Ce bimetallic catalysts using spray pulsed reactor

The conversion of hydrocarbon fuels such as gasoline and diesel is a potential source for hydrogen production towards various fuel cell systems. A novel spray pulsed mode reactor to create alternate wet and dry conditions on the catalyst surface has been used in this study to enhance the rates of hydrogen production compared with the solid-gas phase reaction due to the improvement of the catalyst reactant contact. The production of hydrogen-rich gas by reforming of iso-octane (2,2,4-trimethylepentane) in the presence of steam and air has been studied at 600 and 700 °C over Ni–Mn and Rh–Ce bimetallic catalysts supported on alumina mesh. The feed rate of iso-octane was varied from 0.553 to 5.53 m mol min^–1 by controlling pulse injection in terms of the width and frequency of injection of iso-octane. Based on the product analysis optimized condition for higher hydrogen production and high H₂/CO ratio has been deduced.     

催化裂化过程中的热裂化和二次反应研究

研究了催化裂化过程中热裂化反应和催化裂化反应的特点和影响因素。结果表明,反应温度、剂油比和反应时间能显著地影响热裂化反应和催化裂化反应。采用缩短反应时间的方法,能抑制催化裂化过程中不利的热裂化反应和二次裂化反应,达到调整产品分布和产品组成的目的。     

Cyclohexane dehydrogenation on a nickel catalyst: Kinetics and catalyst fouling

The main reaction and deactivation kinetics of cyclohexane dehydrogenation in the presence of hydrogen has been investigated at atmospheric pressure over a nickel kieselguhr catalyst in the temperature range 583–623 K. The rate of reaction for the fresh catalyst increased with increasing temperature, cyclohexane and hydrogen partial pressures whereas it decreased with an increase in the benzene partial pressure. The experimental data could be adequately modelled by a power law rate expression. The catalyst activity decreased with run time due to catalyst fouling by coke deposition. The rate of deactivation was independent of cyclohexane partial pressure, increased with increasing benzene concentration and decreased with increasing hydrogen partial pressure. It is postulated that coke is most likely formed by the successive dehydrogenation of benzene.