Heat and mass transfer limitations in monolith reactor simulation with non uniform washcoat thickness (original) (raw)
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Latin American applied research Pesquisa aplicada latino americana = Investigación aplicada latinoamericana
A simple, precise and fast procedure to simulate monolith reactors is presented. The method allows the estimation of effectiveness factors (η) in monolith with washcoat of irregular geometries and arbitrary catalytic activity distribution. Catalytic washcoat with the same quantity of active material, deposited in different manners, are compared in their influence on monolith reactor performance. Intrinsic effectiveness factor estimations, with the approximate method, for first order reaction gave results very close to the rigorous 2D calculation. It is shown that differences between η values can be as much as 54% when non uniform catalytic activity distribution is considered. It is also shown the influence of different catalyst distribution on the behavior of a monolith reactor where the isothermal NO decomposition on Cu/ZSM-5 washcoat with complex Langmuir-Hinshelwood kinetic expression, is carried out. Estimated results are in close agreement with experimental findings. The influe...
Modelling of Mass Transfer Resistances in Non-uniformly Washcoated Monolith Reactors
Emission Control Science and Technology, 2021
There are various methodologies to account for mass transfer within non-uniformly distributed washcoats in monolith reactors in 1D models (axially). However, 1+1D models (axially/radially) fail to capture local variations in mass transfer from different coating thicknesses or cracks. In this paper, we present a novel way to account for local material properties in a washcoated monolith reactor. The suggested method uses an existing 1+1D modelling framework and sectionalizes the washcoat into multiple tangential segments which are solved independently. Intelligent gravimetric analysis and scanning electron microscopy are used in combination to calculate local effective diffusivity as an input for each simulation. The new model is compared to the original 1+1D model using NO light-off simulations. The new model predicted increased conversion at elevated temperatures, where mass transfer limitations are present, due to the higher porosity in the corners. The simulation time for each mo...
Some Critical Issues in the Analysis of Partial Oxidation Reactions in Monolith Reactors
Studies in Surface Science and Catalysis, 2001
In the catalytic oxidation of ethane on platinum-containing monoliths, the high rates of the surface reactions coupled with their high exothermicities lead to a very large axial temperature gradient over a distance comprising only a minor fraction of the reactor length. The temperature reaches a level such that homogeneous gas-phase reactions occur rapidly enough to make a large contribution to the overall conversion occurring in the remainder of the reactor. The gas temperatures generated by the surface reactions are high enough so that ignition delay times for the gas-phase reactions are small compared to total residence times in the reactor. For a quantitative assessment of the extent to which homogeneous gas-phase reactions contribute to the overall conversion, the most important considerations are having a reliable estimate of the temperature gradient generated by the surface catalyzed reactions at the front of the reactor and being careful to utilize a kinetic scheme for the gas-phase reactions that is appropriate for the reaction conditions.
Mass transfer in monolith catalysts–CO oxidation experiments and simulations
Chemical Engineering Science, 1998
From CO oxidation measurements in monoliths, and subsequent three-dimensional CFD simulations, the gas-solid mass transfer in square monolith channels with rounded corners was studied. Generally, the measured conversions were higher than the simulated conversions, especially at high flow rates. The following expression for the mass transfer rate, expressed as the average Sh number, was obtained from the experiments:
Model-based analysis of reactor geometrical configuration on CO preferential oxidation performance
International Journal of Hydrogen Energy, 2009
Modeling studies have been conducted on Preferential Oxidation (PROX) Reaction, covering chemical kinetics and heat/mass transfer phenomena that occur in a shell and tube system, to be used in a beta 5 kWe hydrogen generator for Polymer Electrolyte Fuel Cells (PEFCs). The critical issue in the PROX reactor design is to achieve temperature control along the catalyst bed, because poor selectivities primarily result from excess reactor temperature. Aim of the model is to investigate the effects of the reactor dimensions on process performance, in order to obtain high CO conversion and high selectivity with respect to the undesired H2 oxidation. The CO removal from simulated reformate was examined, by evaluating the temperature and the gas concentration profiles along the reactor. The sensitivity analysis showed that the overall performance is strongly dependent upon the geometrical configurations examined.
Reaction and mass transfer effects in a catalytic monolith reactor
Reaction Kinetics and Catalysis Letters
Zeolite-based monoliths (Cu/ZSM-5 on cordierite) are prepared and used to catalyze direct decomposition of nitrogen monoxide. Two-dimensional heterogeneous model is applied to describe the behavior of the monolith reactor, with the emphasis on the features introduced due to coupling of flow, mass transfer and chemical reaction. The proposed model has been verified by comparing computer simulation data with laboratory experimental data. It is shown that both inter- and intraphase diffusion limitations have to be considered when modeling complex reactor configuration, such as monolith reactors, especially when monolith with thicker catalytic layer are used at higher temperatures.
2020
The optimisation of complex geometries such as that of monolith reactors can be supported by computation and simulation. However, complex boundaries such as those found in multi-channel monoliths render such simulations of extremely high computational expense. Adding to the computational expense is the strong coupling among reaction kinetics, heat and mass transfer limitations in these channels. This severely limits the possibilities for geometric optimisation. In the first step toward developing a fast-solving hybrid simulation, a detailed CFD simulation was used to obtain the unsteady state, spatial temperature and concentration (and hence reaction rate) profiles for a range of input conditions. The results of the CFD simulation were then accepted as the benchmark to which fastersolving models were measured against to be considered as viable descriptions. A modified plug flow with effectiveness factor correction for wall mass-transfer was developed and evaluated as the first step towards the development of a multi-channel model. However, the modified plug model is only applicable to single channel monoliths and cannot account for heat transfer across high-density multi-channel beds. For multichannel simulations, the modified plug flow model is embedded into a hybridmodel framework. The hybrid model is based on the principle that, due to the high density of channels in a monolith, there will exist an equivalent homogeneous cylindrical model that approximates the behaviour of a bundle of channels acting as axial heat sources. This model entails the coupling of analytical solutions to single channel mass and momentum transfer with heat transfer across the single-shell extramulti-channel space. Due to the application of effectiveness-factor type approaches, it is shown that the model can be represented by algebraic models that accurately represent the partial differential equations (PDEs) that describe monolith reactors. A close agreement between both temperature and species mole fraction profiles predicted from the modified plug flow model and a detailed CFD model was found with R 2 values of 0.994 for temperature. The time needed to find a converged solution for plug flow model on an Intel(R) Core(TM) i5-5300U CPU @ 2.30GHz workstation was found to be 53 seconds in comparison to 1.3 hours taken by a CFD model. The hybrid ii model was itself validated against the CFD multichannel model. The hybrid model axial temperature and species concentration profiles at various radial positions were found to be in a close agreement with CFD simulations, with relative error found to be in the 0.35 % range. The clock time on an Intel(R) Core(TM) i5-5300U CPU @ 2.30GHz workstation was found to be 38 hours for a CFD multi-channel simulation which when compared with the 53 seconds clock time of the hybrid model implies the suitability of hybridisation for the application to geometric optimisation in the design of monolith reactors. The hybrid-model is developed to facilitate geometric optimization with the view of reducing hot spot formation, pressure drop and manufacturing costs. This is because monolith reactors applied in catalytic partial oxidation of methane are coated with precious metal catalysts, significantly contributing to capital costs. By isolating regions of high catalytic activity, it becomes possible to reduce the amount of precious metal coating required to achieve high conversion. The fast-solving hybrid model was used in the economic analysis of the catalytic partial oxidation of methane to syngas. Due to the low computational expense of the hybrid model, it was possible to investigate a wide range of design geometry and operating condition .It is shown that, for methane oxidation over a Platinum gauze catalyst, the channel diameter could be optimised to the 0.8 mm level resulting in the highest syngas revenue (R 65754.14 /day). The distribution of the catalytic material on the monolithic walls was found to influence the reactor performance hence the process profitability. The non-uniform distribution was found to significantly reduce the cost of fabrication while maintaining a high syngas productivity. In general, a method is proposed to optimise design and operation of catalytic monolith reactors through the application of fast-solving models.
Catalysis Today, 1999
Effects of periodic switching between lean and rich combustion conditions on CO, HC and NO x conversion on a monolithic catalyst with NO x storage were simulated by mathematical model. The model includes description of oxygen and NO x storage on the washcoat. Parametric study showed possibility to reach much improved time-averaged NO x conversion on a single monolith in comparison with steady-state operation, but lower CO and HCs conversions under reducing (rich) conditions. Sequence of monoliths, the first one with NO x storage catalyst and the second one with oxidizing catalyst, then enables to obtain satisfactory conversions of all pollutants.
3-D modeling of monolith reactors
Catalysis Today, 1997
Dynamic behavior of a monolith catalytic reactor formed by ceramic body with the system of parallel channels with square crossection has been studied experimentally and by numerical simulation. Gaseous reaction mixture flows through channels without mass exchange among them. Inner channel surface is coated with catalyst and strongly exothermic reaction of CO oxidation takes place on the catalyst surface. Constructed experimental monolith reactor enables measurement of temperature profiles along 24 chosen channels. The used three-dimensional (3D) model considers heat conduction and accumulation in the solid phase (Fourier equation) and mass and heat balances both on the surface of the catalyst and in the gas phase. Proper discretization of concentration and temperature fields leads to a system of several tens of thousands of ODE's which are then integrated on a fast workstation. Mutual interaction of heat accumulation, heat transfer and nonlinear heat generation gives rise to propagating temperature fronts. Evolution of temperature fields for spontaneous ignition and extinction and after electric preheating of small part of the monolith are studied experimentally and by mathematical modeling.