Dynamics of catalytic reactions on microdesigned surfaces

We report experimental and computational studies of reaction dynamics on Pt/Rh and Pt/TiO₂ microcomposite catalytic surfaces. Reaction fronts initiated at the material interface dominate both steady and dynamic behavior of the composite catalytic material. Our analysis links the transient phenomenon of front initiation to the bifurcations of reaction–diffusion systems with active boundaries.     

Periodic operation of transport reactors for catalytic reactions

Based on plug flow of gas and catalyst particles and concentration dependent deactivation kinetics, the performance of transport reactors under a periodic (rectangular pulse) inlet concentration is analysed for improvement in conversion and extent of catalyst decay. The effects of reaction and deactivation orders, reaction and deactivation constant groups, and γ (cycle split) on the performance of the reactors are evaluated theoretically. For reaction orders greater than one, periodic operation improves conversion. Resonance behaviour is observed for certain combinations of parameters. For identical operating conditions vertical upflow, downflow and horizontal flow reactors are compared. Conversion in upflow reactors is higher than that in either horizontal flow or downflow reactors. However, catalyst decay is the least in downflow reactors.     

Selectivity of partial hydrogenation reactions performed in a pore-through-flow catalytic membrane reactor

The selectivity of partial hydrogenation reactions of unsaturated substrates was studied in a membrane reactor operating at 323 K and 40 bar hydrogen pressure. The reactor system was constructed as a loop of a saturation vessel and a membrane module in which the reaction mixture was resaturated with hydrogen up to 100 times. In a porous membrane made from cross-linked polyacrylic acid palladium nanoparticles were incorporated as catalysts. A well-defined residence time within the membrane was achieved due to a defined pore structure of the membrane and a convective mass flow of the reaction mixture through the membrane. The selectivity for the partially hydrogenated products was investigated as a function of the pore size of the PAA membrane and was compared to commercially available catalysts. Compared to experiments with supported catalysts (Pd/C and Pd/Al₂O₃) in a slurry and a fixed bed reactor the selectivity for the desired products could be increased by 3% (1-octyne) up to 40% (geraniol).     

An investigation of catalytic plate reactors by means of parametric sensitivity analysis

A catalytic plate reactor consisting of closely spaced catalytically coated plates, where endothermic and exothermic reactions take place in alternate channels is studied. The influence of several design parameters on its thermal behaviour and performance is investigated by parametric sensitivity analysis using a two-dimensional reactor model. Parameters considered are: channel size; wall thickness and thermal conductivity; inlet temperature, composition and velocities; and kinetic parameters. It is demonstrated that different catalysts can show similar thermal behaviour and performance but exhibit different sensitivity behaviour. For the system investigated the strongest influence on reactor sensitivity comes from the activation energies of both exothermic and endothermic reactions.     

Numerical assessment of diffusion-convection-reaction model for the catalytic abatement of phenolic wastewaters in packed-bed reactors under trickling flow conditions

Computational fluid dynamics (CFD) modeling of trickle-bed reactors with detailed interstitial flow solvers has remained elusive mostly due to the extreme CPU and memory intensive constraints. Here, we developed a comprehensible and scalable CFD model based on the conservative unstructured finite volume methodology to bring new insights from the perspective of catalytic reactor engineering to gas-liquid-solid catalytic wet oxidation. First, the heterogeneous flow constitutive equations of the trickle bed system have been derived by means of diffusion-convection-reaction model coupled within a Volume-of-Fluid framework. The multiphase model was investigated to gain further evidence on how the effect of process variables such as liquid velocity, surface tension and wetting phenomena affect the overall performance of high-pressure trickle-bed reactor. Second, as long as the application of under-relaxation parameters, mesh density, and time stepping strategy play a major role on the final corroboration, several computational runs on the detoxification of liquid pollutants were validated accordingly and evaluated in terms of convergence and stability criteria. Finally, the analysis of spatial mappings for the reaction properties enables us to identify the existence of relevant dry zones and unveil the channeling phenomena within in the trickle-bed reactor.     

The catalytic heat exchanger using catalytic fin tubes

The characteristics of a catalytic heat exchanger which integrates heat generation and heat exchange into one equipment have been investigated by the experiment and numerical simulation. The surface of the fin tubes was catalyzed by the formation of the oxide layer and the subsequent washcoating of ZrO₂, followed by the impregnation of Pd catalyst. The experimental results showed that the performance of catalytic combustion in the catalytic heat exchanger was more significantly affected by the inlet velocity of the mixture than by its inlet temperature and equivalence ratio. It was also found that the catalytic surface area was a critical parameter to obtain the complete conversion of the mixture. Numerical simulation has been performed with a commercial software FLUENT. The calculated results indicated that the performance of the catalytic combustion was influenced by the catalytic fin configuration as well as the flow pattern of the mixture over the catalytic fins. The results recommend that the number and thickness of catalytic fins should be designed above 6 pieces/inch and less than to achieve the best performance in the catalytic heat exchanger.     

Dynamics and control of autothermal reactors for the production of hydrogen

Autothermal reactors, coupling endothermic and exothermic reactions in parallel channels, represent one of the most promising technologies for hydrogen production. The bulk of existing research work concerning their operation refers, however, to steady state conditions. In the present work, the dynamic behavior of autothermal reactors is analyzed. It is demonstrated that such systems are modeled by systems of equations that are stiff, their dynamics consequently featuring two time scales. Within the framework of singular perturbations, reduced-order, nonstiff models are derived for the transient evolution in each time scale. Furthermore, the challenges posed by the transient operation of autothermal reactors are identified, along with demonstrating the implementation of feedback control in order to improve transient performance and to avoid severe issues (such as reactor extinction) that can arise in the course of operating the reactor. All theoretical concepts are illustrated with numerical simulations performed using the model of a hydrogen production reactor.     

Catalytic reactors based on porous ceramic membranes

This overview discusses some of the developments and outstanding opportunities in the field of catalytic reactors based on porous ceramic membranes, both inert and catalytic. This is an emerging area, where inputs from heterogeneous catalysis, material science and reactor engineering are playing the key roles. Rather than attempting a thorough review of the relevant literature, this work deals with some general concepts and then concentrates on a few selected examples that illustrate the application of membrane reactors.     

流化催化裂化反应器的技术进展

介绍并讨论了近期开发的流化催化裂化反应器,包括两段串联反应器、双提升管反应器、下行式反应器、多段进料反应器以及新型提升管反应器端口结构技术。以新型反应器为核心技术的各种催化裂化新工艺可以有效地提高催化裂化反应的转化率和选择性,减少非理想产品产率,也可以改善产品质量,生产环境友好的清洁燃料油品。此外新型提升管反应器端口结构还可以抑制设备结焦,延长流化催化裂化(FCC)装置的开工周期。     

催化裂化提升管反应器流动反应耦合模型研究进展

催化裂化(FCC)工艺在重质油轻质化过程中发挥着重要作用, 而FCC提升管反应器的模型化是催化裂化新工艺与新装备的开发、催化裂化装置稳定操作与生产调优等常需做的工作。本文首先根据流动模型与反应模型不同的集成方式对提升管反应器流动-反应耦合模型进行了归纳与分类, 并回顾了国内外流动-反应耦合模型的研究历程, 指出了耦合模型的发展趋势;随后对当前研究较多的计算流体力学(CFD)流动-反应耦合模型进行了较为全面的阐述, 包括对耦合模型的应用场合、模型求解解耦方法的研究情况等均作了介绍, 同时还分析了该类耦合模型所存在的不足之处, 并指出工业提升管反应器在线采样技术的开发在耦合模型的验证工作上的必要性;最后, 对FCC提升管反应器流动-反应耦合模型研究进行了总结与展望, 以期能够为FCC提升管反应器模型化新方法的提出以及耦合模型的验证工作等研究给予借鉴和指导。     

Modeling for the catalytic coupling reaction of carbon monoxide to diethyl oxalate in fixed-bed reactors: Reactor model and its applications

A two-dimensional (2D) pseudo-homogeneous reactor model was developed to simulate the performance of fixed-bed reactors for catalytic coupling reaction of carbon monoxide to diethyl oxalate. Reactor modeling was performed using a comprehensive numerical model consisting of two-dimensional coupled material and energy balance equations. A power law kinetic model was applied for simulating the catalytic coupling reaction with considering one main-reaction and two side-reactions. The validity of the reactor model was tested against the measured data from different-scale demonstration processes and satisfactory agreements between the model prediction and measured results were obtained. Furthermore, detailed numerical simulations were performed to investigate the effect of major operation parameters on the reactor behavior of fixed bed for catalytic coupling reaction of carbon monoxide to diethyl oxalate, and the result shows that the coolant temperature is the most sensitive parameter.     

Fuel processor based on syngas production via short contact time catalytic partial oxidation reactors

Short contact time catalytic partial oxidation (SCT-CPO) of natural gas is a promising technology for syngas production, representing an appealing alternative to existing processes. The high conversion and selectivity observed since the earlier works in this field can make this process attractive. Moreover, the SCT-CPO reactors can be autothermally operated and the possibility to use air as oxidant appears a feasible route to reduce syngas production costs: these two issues make possible the use of a SCT-CPO reactor as the reformer of a fuel processor for H₂ production for fuel cells.

The present work refers to an experimental study of syngas production from CH₄ and O₂ via a SCT-CPO reactor made of a fixed bed of Rh/-Al₂O₃ spheres. The main obtained results are: (i) an increase in GHSV produces an enhancement of transport rates and this in turn determines an improvement in CH₄ conversion, despite the reduction in residence time; (ii) the catalyst pellets get hotter than the gas phase thus favouring the H₂ and CO production; syngas formation is in fact both thermodynamically and kinetically promoted at high temperatures; (iii) a similar improvement of conversion was obtained with a reduction of the catalyst particle size, thanks once again to an increase in the heat transport and a higher geometrical surface area of the catalyst itself. By a slight increase of the O₂ fed to the reactor, H₂ and CO yields can be maximised and a complete CH₄ conversion achieved.     

CFD and experimental studies of reactive pulsing flow in environmentally-based trickle-bed reactors

Pulsing flow in trickle-bed reactors (TBRs) has been claimed to promote the averaged heat and mass transfer rates, thereby enhancing the overall conversion and productivity. In this work, we focused on the mineralization of organic matter by catalytic wet oxidation at different liquid flow modulations. The convective nature of the disturbances that lead to pulsing was simulated by an Eulerian Computational Fluid Dynamics (CFD) model and validated with experimental data. In order to evaluate the predicted effects of pulsing on reaction outcome, first the multiphase flow governing equations were detailed with the computational methodology used in the simulation procedure. Prominent numerical parameters were optimized in terms of mesh aperture and time step. Second, to enable objective assessments of the merits of the different cyclic strategies, several computational runs were performed addressing the effect of nominal gas and liquid flow rates as well as the oxidation temperature. Here, we found that the concentration profile computed by the CFD model for pulsing flow conditions demonstrated superior oxidation performance over the trickling flow regime, which has been further corroborated by experimental evidences. Afterwards, the normalized concentration series close to the top of the TBR exhibited sharp fronts and gradually become less intense as they travel downward that reflected the nonisolated nature of the traveling liquid pulsations. Finally, these computational and experimental findings enabled us to intensify the detoxification of high-strength wastewaters and can be further exploited due to advantageous dispersive and convective heat/mass transfer phenomena under pulsing flow conditions.     

用于甲醇制烯烃的非均相催化反应器评述

甲醇制低碳烯烃（MTO、MTP）是煤化工的关键环节,近年来相关技术已经在我国成功工业化,包括中国科学院大连化学物理研究所的DMTO技术、中石化的s-MTO技术、UOP的MTO+OCP技术以及Lurgi的MTP技术。从非均相催化反应角度分析甲醇制烯烃的过程特点,剖析了各类典型的非均相催化反应器用于该过程的优劣,对比了国内外各主流技术的关键指标,评述了甲醇制烯烃和催化裂化的异同。分子筛催化的甲醇制烯烃过程遵循烃池机理,呈现自催化和积炭失活特性,控制催化剂的工作状态和限制返混对提高催化活性及烯烃产物选择性尤为重要。具有多级结构的分子筛催化剂及低返混流化床反应器或反应器串联组合是未来的发展方向。     

A heterogeneous chemical reactor analysis and design laboratory: The kinetics of ammonia decomposition

A laboratory module for senior-level reaction engineering/reactor design students is described. Students use low-conversion experimental data to explore and characterize the kinetics of ammonia decomposition over various supported catalysts at atmospheric pressure in a packed-bed reactor. Each student team is assigned one of four catalyst types, a reactor temperature, and a series of feed flow rates and compositions. Aggregate data from all student groups is then summarily analyzed per catalyst type. In each experimental trial, the reactor conversion is determined by a thermal conductivity measurement applied to the feed (reactor bypass) and reactor effluent gases. An analysis of the reaction rate across a range of temperatures and varying feed gas partial pressures allows students to test various reaction mechanisms, to suggest rate-determining steps, and to statistically determine rate law parameters. Students typically use the Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach to derive rate law expressions, and determine rate constants through application of the Arrhenius equation. High student numbers (ca. 140) are accommodated through the availability of four experimental stations — each sharing a common source of feed gas and equipped with independent flow controllers and gas analyzers.     

Analysis of design sensitivity of flow-reversal reactors: Simulations, approximations and oxidation experiments

The analysis of a flow-reversal reactor, constructed from a catalytic bed imbedded within two inert beds and catalysing an instantaneous reaction, shows that the main parameters that determine the maximal reactor temperature are the thermodynamic parameters, heat loss through the walls and the conductivity and length of the inert zones. We show how these parameters can be easily extracted from the experimental data of very fast reactions, and demonstrate it for ethylene and for propane oxidation. We also derive an approximation for the maximal temperature for fast or slow reactions. These approximations are compared with experimental results obtained during propane (with low feed concentration) or methane oxidation.     

A high-speed method for obtaining kinetic data for exothermic or endothermic catalytic reactions under non-isothermal conditions illustrated for the ammonia synthesis

A detailed analysis of the method for fast acquisition of kinetic data by means of a polythermal-temperature-ramping reactor (PTR) as suggested by Wojciechowski in 1995 was performed. The ammonia synthesis reaction over a commercial Fe catalyst (BASF) was taken as an example. Kinetic data evaluation was based on a kinetic model as reported by Sehested et al. in 1999. As a result of data analysis, the experimental molar rate of change of ammonia could be described adequately over a wide range of N₂ and H₂ partial pressures as well as temperature. The pre-exponential factors, activation energies and adsorption enthalpies are in reasonable agreement with data for promoted iron catalysts reported in literature. Accordingly, the PTR method was evaluated as a efficient tool for kinetic data analysis. The number of experiments including different initial conditions can be significantly diminished since PTR experiments have not to follow the strict sequence of varying one experimental parameter after the other by keeping all others constant. The collected exit concentrations, exit temperature and contact time are correlated by spline-functions which allow the extraction of functional relationships required for derivation of rate expressions. If the steady-state of the catalyst is rapidly attained, temperature ramping allows fast continuous data acquisition. Thus, the PTR method has the potential of significantly reducing the time needed for determining reliable kinetics over a wide range of operating conditions.