Fortschritte bei der Modellierung von Festbettreaktoren

Progress in the modeling of fixed bed reactors. Recent work on the modeling of catalytic fixed bed reactors have led to more realistic mathematical models and to a better understanding of problems connected with modeling. Nowadays standard routine computer programs are available for use in the design (and analysis) of catalytic fixed bed reactors for very complex reaction systems. The influence of bed structure on transport parameters was illustrated by experimental determination of axial and radial concentration and temperature distribution. Moreover the problem of multicomponent diffusion coupled with complex reactions in porous catalysts has been solved and can be included in the reactor model. However, the practical application of possible reactor models is limited by the availability of transport parameters for fixed beds.     

Nonuniformly Activated Catalysts

In most theoretical and experimental studies in the area of heterogeneous catalysis, the distribution of catalytically active material within three-dimensional carriers has been assumed to be uniform. An extensive literature exists [1, 2] dealing with interactions between chemical and physical transport phenomena within such porous catalysts. The effects of intraparticle mass and heat diffusional resistances in catalysts exhibiting uniform activity distributions have been extensively studied in terms of point and overall effectiveness, selectivity, and yield, as well as in terms of concentration and temperature profiles within catalyst particles and chemical reactors.     

Nonuniformly Activated Catalysts

In most theoretical and experimental studies in the area of heterogeneous catalysis, the distribution of catalytically active material within three-dimensional carriers has been assumed to be uniform. An extensive literature exists [1, 2] dealing with interactions between chemical and physical transport phenomena within such porous catalysts. The effects of intraparticle mass and heat diffusional resistances in catalysts exhibiting uniform activity distributions have been extensively studied in terms of point and overall effectiveness, selectivity, and yield, as well as in terms of concentration and temperature profiles within catalyst particles and chemical reactors.     

Gas-phase residence time distribution in a falling-film microreactor

Industrial-scale performance of gas-liquid reactors can be difficult to optimise for very rapid or highly exothermic reactions. Microstructured reactors for laboratory measurements offer new opportunities for the study of these reactions by enabling precise heat management and fine control of reactor operating conditions. For accurate experimental study, characterisation of the flow conditions within these new reactor devices is essential.The present study examines experimental residence time distributions for the gas phase through a microstructured falling-film reactor, in order to develop an appropriate flow model for further study of gas-phase mass-transfer characteristics in the system. For the gas-phase residence time distribution experiments, the detection system involves a flow of oxygen containing ozone as a tracer gas with continuous monitoring of the concentration by UV-light absorption. The experimental results are used to model the flow behaviour in the gas volume over the gas-liquid contact zone as a series of continuous stirred tank reactors whose number is a simple function of the gas Reynolds number.The experimental results are compared with computational fluid dynamics calculations of the gas flow within the reactor. The comparison indicates a clear correlation of the flow model behaviour with the appearance of recirculation loops in the reaction chamber and the effect of the gas jet at the entrance of the gas-liquid contact zone.     

The influence of mixing on the distribution of copolymerization compositions

The paper shows theoretically that the composition distribution in a copolymer depends on both the type of continuous reactor used in its production and the molecular and macroscopic mixing patterns within the reactor. Examples are given of the distributions to be expected from a plug flow reactor, a CSTR with either molecular or macroscopic scale mixing, a tubular reactor with backmixing and reactors with several regimes of mixing. The distribution curves are compared with those obtained by tracer injection. A short account is given of the experimental difficulties encountered in reproducing the composition distributions.     

Monoliths in Heterogeneous Catalysis

The use of structured catalysts in the chemical industry has been considered for years. Conventional fixed-bed reactors have some obvious disadvantages such as maldistributions of various kinds (including a nonuniform access of reactants to the catalytic surface), high pressure drop in the bed, etc. Structured catalysts are promising as far as elimination of these setbacks is concerned. Two basic kinds of structured catalysts can be distinguished:

1. Structural packings covered with catalytically active material, similar in design to those used in distillation and absorption columns and/or static mixers. Good examples of catalysts of this kind are those offered by Sulzer, clearly developed by Sulzer column packings and static mixers. As in packed beds, there is an intensive radial convective mass transport over the entire cross-section of these packings. Structural packing catalysts and the reactors containing them are, however, not within the scope of this review.
2. Monolithic catalysts are continuous unitary structures which contain many small, mostly parallel passages. A ceramic or metallic support is coated with a layer of material in which active ingredients are dispersed. An interaction between these passages can occur if walls are permeable. The catalytically active material is present on or inside the walls of these passages. Radial mass transport can occur only by diffusion through the pores of the permeable walls.

     

Simulations of the effect of hydrodynamic conditions and properties of membranes on the catalytic performance of a fluidized-bed membrane reactor for partial oxidation of methane

In a fluidized-bed membrane reactor the selectivity of separation can be controlled by influencing the hydrodynamics of the fluidized bed. In this reactor type, with the mass transport limitation between bubbles and the emulsion phase, even with the non-selective membranes, high selectivity of separation can be achieved. This opens the possibility for applications of membrane reactors for reaction systems for which selective membranes do not exist, e.g. when Knudsen-type membranes or form-selective separation can not be applied. This paper is aimed at explaining the interaction between the selectivity of separation and the hydrodynamics of the fluidized bed by means of simulations that were performed for a fluidized-bed membrane reactor for catalytic partial oxidation of methane.     

Backmixing studies in tubular reactors—I: The middle injection technique

A new technique of tracer injection to evaluate mixing inside tubular reactors with appreciable backmixing has been developed. Three models were tested; the dispersed plug flow model, the finite-stages with backflow model, and a multi-parameter mixed model. The relative response to a middle stimulus is found to be time-independent, independent of type of stimulus but dependent upon reactor geometry and flow conditions. Mixing parameters can be determined from the relative response to a middle stimulus with a minimum of experimental and computational efforts. The decay of the relative response with reactor length is compared to turbulence-decay behind a grid to develop the limits of the use of the middle injection technique. The middle-injection technique shows superiority over moment analysis, or curve fitting in the time or the Laplace domains. This method is also easier than the analysis of steady-state tracer profiles. The technique is useful in predicting the relationship between mixing parameters of different models.     

低压化学蒸气淀积(LPCVD)反应器的模拟和分析

本文利用低压化学蒸气淀积(LPCVD)反应器中衬底间隙内的扩散-反应过程和环形通道内的质量、热量传递过程的数学模型对片内和片间均匀性问题进行了分析。结果表明:片内均匀性h和片内效率因子n均可用体积变化准数和扩散准数的函数表示。合理设置管外加热装置,可改善片间均匀性或提高反应气流的转化率。这些结果可用于指导低压化学蒸气淀积反应器的设计和操作参数的选择 ,     

An efficient algorithm for solving hollow-fiber bioreactor design equations

A simple algorithm for solving the hollow-fiber bioreactor design equations has been presented. This algorithm is quite general and is applicable to any nonlinear reaction occurring in the reactor spongy matrix. This can also be applied to other reactor types having similar configurations, like the wall-coated enzyme reactors.     

Transport properties of graphene oxide nanofiltration membranes: Electrokinetic modeling and experimental validation

There is a need for developing reliable models for water and solute transport in graphene oxide (GO) membranes for advancing their emerging industrial water processing applications. In this direction, we develop predictive transport models for GO and reduced-GO (rGO) membranes over a wide solute concentration range (0.01–0.5 M) and compositions, based on the extended Nernst–Planck transport equations, Donnan equilibrium condition, and solute adsorption models. Some model parameters are obtained by fitting experimental permeation data for water and unary (single-component) aqueous solutions. The model is validated by predicting experimental permeation behavior in binary solutions, which display very different characteristics. Sensitivity analysis of salt rejections as a function of membrane design parameters (pore size and membrane charge density) allows us to infer design targets to achieve high salt rejections. Such models will be useful in accelerating structure-separation property relationships of GO membranes and for separation process design and optimization.     

From bubbling to turbulent fluidization: Advanced onset of regime transition in micro-fluidized beds

Membrane fluidized bed reactors have been proposed and demonstrated as an effective reactor concept for ultrapure hydrogen production with integrated carbon dioxide capture. Recent experimental studies have shown that the hydrogen permeation rate through the membranes and the mass transfer rate from the bubble phase to the emulsion phase are the two main limiting factors in this type of reactors. To this end, we propose the concept of a micro membrane fluidized bed reactor (MMFBR) as a possible method to remove those two limitations. The idea of the MMFBR is that a significantly larger membrane area per unit reactor volume can be accommodated, thereby removing the limitation of the hydrogen permeation rate through the membranes. Furthermore, we numerically show with discrete particle simulations that the onset of turbulent fluidization is advanced significantly in a MMFBR, which allows the bed to be operated at the turbulent fluidization regime at a relatively low gas velocity. This is quite beneficial, since it provides a gentler environment for the membranes, and indicates a significant attenuation or possible removal of mass transfer limitations due to the well-known excellent mass transfer characteristic of turbulent fluidization.     

Reactor modelling for electrochemical processes

This paper presents a relatively straightforward approach to the modelling of electrochemical reactors operated in batch or continuous modes. The models are based on ideal flow assumptions of either well-mixed or plug flow and incorporate reaction rate models based on electrochemical kinetics and mass transport at one electrode. General characteristics of the reactor models are described, particularly with regard to the need for good mass transport in metal recovery applications. An example is given on the use of the model in the recovery of a heavy metal (Cd²⁺) from an acidified solution containing Cd(II) and Fe(III) ions. The reaction rate model is based on experimental data.     

REGENERATION PROTOCOLS FOR FIXED BED REACTORS DEACTIVATED BY COKE

Coked catalyst in fixed bed reactors is regenerated by passing hot air mixed with a non-reacting diluent (like steam(. The concentration of oxygen in the regenerating stream is increased from the beginning of the regeneration following a so called regeneration protocol, in order to minimize the regeneration time but still maintaining the maximum temperature within the reactor below permissible level. In this paper, we have modelled the low temperature regeneration in fixed bed reactors and have suggested general regeneration protocols, suitable for wide range of process conditions

A pseudo-homogeneous model is developed to simulate regeneration of fixed bed reactors. The model accounts for major industrially important issues like, non-uniform coke deposition along the reactor length, hydrogen content (with faster burning of hydrogen) of coke and incomplete combustion of coke. Detailed qualitative understanding of the regeneration process and the role of protocol is developed. Specific guidelines were evolved and discussed to aid process engineers to select better regeneration protocol. One worked example of regeneration of fixed bed reactors is included     

Catalysts for the conversion of hydrocarbon and synthetic fuels for onboard syngas generators

The use of syngas derived on board a vehicle as a supplement to the main fuel fed to engines ensures engine operation using dilute fuel mixtures. This leads to a decrease in emission toxicity and an increase in the fuel efficiency of the engine. The preparation of new types of efficient catalysts for the conversion of hydrocarbon and synthetic fuels for onboard syngas generators requires the use of new approaches to the design of catalysts not only as catalytically active material, but also as a structural component of a chemical reactor. We prepared and tested a set of catalysts for the conversion of hydrocarbons, i.e., natural gas, diesel and biodiesel fuels, biofuels, and alcohols (ethanol, methanol) to syngas. Primary supports for the catalysts were metals grids and porous tapes; secondary supports were oxides of aluminum and magnesium deposited on or sintered to a primary support. The catalysts exhibited high thermal stability and mechanical strength, and were characterized by the conformity of the coefficients of thermal expansion of the support material and the catalytically active bed. The catalysts can be used as structural components of reactors and as a basis for the preparation of monolithic blocks and planar components of radial and planar reactors. The developed catalysts were subjected to laboratory and bench tests and examined as components of onboard generators of vehicles.     

Hollow fiber enzymic reactors for a two substrate process: analytical modeling and numerical simulations

The use of immobilized enzyme reactors in biotechnology and biomedicine is rapidly expanding. This study concentrates on hollow-fiber (HF) enzymic reactors for continuous, single-pass operation. The enzyme, in a soluble form, is physically confined within the shell section of the reactor and the substrate solution flows through the lumen section of it. We consider here a two-substrate reaction proceeding via the Ping-Pong mechanism, with substrates and reaction products diffusing through the fiber wall. The developed analytical model enables to calculate the expected conversion as a function of the volumetric flow rate, kinetic constants, diffusion coefficients, geometric dimensions of the reactor, the flow regimen in the apparatus and substrates concentrations. The model equations are solved by a numerical procedure and the system performance is simulated. Depending on the operation conditions employed, the reactor is controlled by kinetic processes, diffusion processes, or both.     

A transport-oriented approach to residence time distributions and two-phase chemical reactors

This paper focuses on two-phase (e.g., fluid-solid catalyst) chemical reactors where one phase participates in the feed and effluent, first order chemical reactions proceed in the other phase and linear, species-dependent, interphase transport connects the phases. A class of reactors exists in which the Laplace transform of the dynamic reaction/interphase transport equations reduces to essentially that of an effective set of first order reactions amongst effluent phase species only, in an imaginary single phase reactor. This result bears on scale-up: Since the imaginary reactor simply scales with the usual residence time distribution, so too do reactors of this class scale with its analog. This easily measurable analog turns out to be just the non-adsorbing tracer experiment. Significantly, certain reactors outside of this class do not scale likewise, even with first order chemistry; an example illustrates. The class in question includes, but is not limited to, fixed bed and Berty (CSTR) reactors. The analysis allows design inferences for two-phase reactors, including fluidized beds.     

Safety aspects in modelling and operating of batch and semibatch stirred tank chemical reactors

Safety aspects in modelling of batch and semibatch stirred tank reactors as well as a model based safety analysis have been considered. Applicability of two basic types of models – i.e. the perfectly mixed reactor model and the CFD model, both formulated for laboratory scale as well as pilot plant scale reactors – has been discussed. A formulation of the appropriate reactor model, which is adequate to the considered case study has been demonstrated and tested experimentally. Particular attention has been devoted to the formulation of robust CFD models employed to simulate a performance of the stirred tank reactors. It has been found that models for perfectly mixed reactors may have quite wide range of application, while the CFD models should be definitely used in case of fast reactions, high viscosity of the reacting mixture as well as of failure leading to stopping of the impeller. The CFD models are able to predict a dynamic behaviour of reactors at any circumstances, so they can play a significant role in safety analysis carried out for industrial scale reactors, for which experimental safety tests are expensive and dangerous.     

Pulvermetallurgische Bauelemente in der Verfahrenstechnik

Powder-metallurgical components in process engineering. Applications of structural parts made by powder metallurgy can be found in nearly all kinds of industries, especially the automotive industry, and the electrical and electronics industry. This paper describes applications of porous P/M materials for chemical technologies. Quite well known are sintered metal filter elements of different sizes in the form of tubes, plates, and cartridges. Porous P/M materials can also be used as flame resistors and silencers, for the dispersion of gases in liquids, for drying processes, and for homogenisation of various products. Almost all technically interesting alloys are available for these elements. New applications for porous metal structures are under development for the use of catalytically active reactor beds in solid bed reactors. It was found that some reactor bed materials are themselves active as catalysts. In such a reactor, the temperature gradient is very small, so that chemical reactions can be controlled much more efficiently.     

Multiphase catalytic reactors: a perspective on current knowledge and future trends

Conventional and emerging processes that require the application of multiphase reactors are reviewed with an emphasis on catalytic processes. In the past, catalyst discovery and development preceded and drove the selection and development of an appropriate multiphase reactor type. This sequential approach is increasingly being replaced by a parallel approach to catalyst and reactor selection. Either approach requires quantitative models for the flow patterns, phase contacting, and transport in various multiphase reactor types. This review focuses on these physical parameters for various multiphase reactors. First, fixed-bed reactors are reviewed for gas-phase catalyzed processes with an emphasis on unsteady state operation. Fixed-bed reactors with two-phase flow are treated next. The similarities and differences are outlined between trickle beds with cocurrent gas-liquid downflow, trickle-beds with countercurrent gas-liquid flow, and packed-bubble columns where gas and liquid are contacted in cocurrent upflow. The advantages of cyclic operation are also outlined. This is followed by a discussion on conventional reactors with mobile catalysts, such as slurry bubble columns, ebullated beds, and agitated reactors. Several unconventional reactor types are reviewed also, such as monoliths for two-phase flow processing, membrane reactors, reactors with circulating solids, rotating packed beds, catalytic distillation, and moving-bed chromatographic reactors.

Numerous references are cited throughout the review, and the state-of-the-art is also summarized. Measurements and experimental characterization methods for multiphase systems as well as the role of computational fluid dynamics are not covered in a comprehensive manner due to other recent reviews in these areas. While it is evident that numerous studies have been conducted to elucidate the behavior of multiphase reactors, a key conclusion is that the current level of understanding can be improved further by the increased use of fundamentals.