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Interphase diffusion reactors

The second scale which determines the relation between the selectivity and conversion is the diffusion of the reactants through the catalyst poes. Model calculations conducted by McCarty indicated that at 10 atm die coupling of methyl radicals occurs preferentially inside the pores in a particle of 25 mm in diameter. The effect of this time scale is shown in Figure 10(a) in terms of the intraphase and interphase profiles for methane and ethane inside a catalyst pore. Clearly, higha C2 selectivities are obtained on catalysts with an open pore structure and low surface area. A majority of the literature results have been obtained using powdered catalysts in which diffusional effects are not in rtant however, such effects could be relevant at high pressure in fixed-bed reactors requiring the use of catalysts in a pelletized form. [Pg.176]

Both radial and axial diffusion can be taken into account and the final equations to be solved are relatively simpler than those of the continuum model. Although, the equations of the model at steady state are algebraic equations, the dimensionality of the system increases considerably. McGuire and Lapidus (1965) used this model for the study of the stability of a packed bed reactor which included both interphase and intraparticle mass and heat transfer resistances. [Pg.148]

The presence of two phases, namely gas and liquid, is characteristic for non-catalytic or homogeneously catalysed reaction systems. Components in the gas phase diffuse to the gas-liquid interphase, dissolve in the liquid phase and react with components in the bulk liquid phase. The liquid phase may also contain a homogeneous catalyst. Some of the product molecules desorb from the liquid phase to the gas phase and some product molecules remain in the liquid. The processes taking place in a gas-liquid reactor are displayed in Figure 9.5. [Pg.345]

Step 1. Reactants enter a packed catalytic tubular reactor, and they must diffuse from the bulk fluid phase to the external surface of the solid catalyst. If external mass transfer limitations provide the dominant resistance in this sequence of diffusion, adsorption, and chemical reaction, then diffusion from the bulk fluid phase to the external surface of the catalyst is the slowest step in the overall process. Since rates of interphase mass transfer are expressed as a product of a mass transfer coefficient and a concentration driving force, the apparent rate at which reactants are converted to products follows a first-order process even though the true kinetics may not be described by a first-order rate expression. Hence, diffusion acts as an intruder and falsifies the true kinetics. The chemical kineticist seeks to minimize external and internal diffusional limitations in catalytic pellets and to extract kinetic information that is not camouflaged by rates of mass transfer. The reactor design engineer must identify the rate-limiting step that governs the reactant product conversion rate. [Pg.383]

The performance of trickle-bed reactors may be affected by many factors, such as interphase mass transfer, intraparticle diffusion, axial dispersion and incomplete catalyst wetting. Therefore, knowledge about these influenced factors is important for their mathematical description by an unsteady-state reactor model. Until now, the literature analysis shows the experimental and theoretical understanding of trickle-bed reactors under unsteady-state-operation conditions has improved, but not considerably. The following studies are focused on the trickling regime under unsteady-state-operation conditions. [Pg.82]

In comparison to liquid-liquid systems, liquid-gas systems are characterised by relatively higher values of the volume-surface diameter of dispersed phase particles (Figure 2.25). The effect of substantial differences in densities of flows and surface tension at the interphase boundary is obvious. However, differences between the size of particles in liquid-liquid and liquid-gas flows reduce when we move from a cylindrical device to a diffuser-confusor device (Figure 2.25). In particular, when other conditions are equal, the total volume of bubbles is more than 3 times larger than the volume of liquid of a two-phase system in a cylindrical reactor. The diffuser-confusor channel provides the formation of a dispersed system with a comparable size of bubbles and droplets. [Pg.65]

Dynamic analysis of a trickle bed reactor is carried out with a soluble tracer. The impulse response of the tracer is given at the inlet of the column to the gas phase and the tracer concentration distributions are obtained at the effluent both from the gas phase and the liquid phase simultaneously. The overall rate process consists the rates of mass transfer between the phases, the rate of diffusion through the catalyst pores and the rate of adsorption on the solid surface. The theoretical expressions of the zero reduced and first absolute moments which are obtained for plug flow model are compared with the expressions obtained for two different liquid phase hydrodynamic models such as cross flow model and axially dispersed plug flow model. The effect of liquid phase hydrodynamic model parameters on the estimation of intraparticle and interphase transport rates by moment analysis technique are discussed. [Pg.834]

A trickle bed reactor (TBR) consists of a fixed bed of catalyst particles, where liquid and gas phases flow cocurrently downward through the bed. Although its wide application in chemical and petrochemical industry it is one of the most complicated type of reactor in its design and scale-up. Essencially, the overall rate can be controlled by one or a combination of the following processes mass transfer between interphases, intraparticle diffusion, adsorption and surface reaction. The hydrodynamics, solid-liquid contacting efficiency and axial mixing can also affect the performance of TBR. [Pg.834]

In this section, we formulate a ID model with interphase mass and heat transfer coefficients. These lumped models [103] describe the axial variation of concentration and temperature (which are averaged over the channel cross section). The diffusion processes in the transverse directions (represented by differential terms) are replaced by a transfer term, associated with a given driving force. The use of ID models is widespread throughout the literature on monoUth reactor modeling. Chen et al. [3] reviewed some specific appUcations including simulation of simultaneous heat transfer in monofith catalysts... [Pg.194]

However, in other cases internal diffusion limitation can be significant even with very thin washcoat thicknesses [127], when temperature is high (> 700°C). This refers, for example, to catalytic combustions, which are extremely fast. Hayes et al. [135] evaluated the extent of intraphase and interphase resistances to the catalytic conversion of low concentrations of carbon monoxide in air in a tube wall reactor (coated with a platinum-alumina deposit). Above 610 K there was strong evidence of both intraphase and interphase resistances to catalytic conversion. In Sections 8.3.2, 8.3.3, and 8.3.4, we provide a systematic analysis for prediction of the extension of external and internal diffusion limitations. [Pg.199]

Hayes RE, Kolaczkowski ST, Thomas WJ, Titiloye J. Intraphase diffusion and interphase mass transfer effects during the catalytic oxidation of CO in a tube wall reactor. Proceedings—Royal Society of London, A 1995 448 321-334. [Pg.212]

In the Cora code, the corrosion product layers outside the reactor core are rather arbitrarily subdivided into two layers, a transient one and a permanently deposited one. Supply to the transient layer occurs via deposition of suspended particles from the coolant, release from it includes erosion of particles back to the coolant as well as transport into the permanently deposited layer and partial conversion into dissolved species. In a comparable manner, the supply of nuclides to the permanent layer is assumed to result from transfer from the transient layer and the exchange equilibrium with the dissolved species present in the coolant. The deposition coefficients of suspended solids can be calculated on the basis of particle size and flow characteristics. The coefficients of relevance for the permanently deposited layer, including ionic transfer mechanisms between liquid and solid phases, can be derived from theoretical considerations as well as from laboratory studies of corrosion product solubilities. Finally, diffusion rates of nuclides at the interphase layers are needed, from the oxide layer to the coolant as well as in the reverse direction. These data can be obtained in part by theoretical considerations and by measurements at the plants. [Pg.329]

The model species, total mass, momentum, and energy continuity equations are similar to those presented in Section 13.7 on fluidized bed reactors. Constant values of the gas and liquid phase densities, viscosities, and diffusivities were assumed, as well as constant values of the interphase mass transfer coefficient and the reaction rate coefficient. The interphase momentum transfer was modelled in terms of the Eotvos number as in Clift et al. [1978]. The Reynolds-Averaged Navier-Stokes approach was taken and a standard Computational Fluid Dynamics solver was used. In the continuous liquid phase, turbulence, that is, fluctuations in the flow field at the micro-scale, was accounted for using a standard single phase k-e model (see Chapter 12). Its applicability has been considered in detail by Sokolichin and Eigenberger [1999]. No turbulence model was used for the dispersed gas phase. Meso-scale fluctuations around the statistically stationary state occur and were explicitly calculated. This requires a transient simulation and sufficiently fine spatial and temporal grids. [Pg.830]

Optimal reactor design is critical for the effectiveness and economic viability of AOPs. The WAO process poses significant challenges to chemical reactor engineering and design, due to the (i) multiphase nature of WAO reactions (ii) temperatures and pressures of the reaction and (iii) radical reaction mechanism. In multiphase reactors, complex relationships are present between parameters such as chemical kinetics, thermodynamics, interphase/intraphase intraparticle mass transport, flow patterns, and hydrodynamics influencing reactant mass transfer. Complex models of WAO are necessary to take into account the influence of catalyst wetting, the interface mass-transfer coefficients, the intraparticle effective diffusion coefficient, and the axial dispersion coefficient. " ... [Pg.266]


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Interphase

Interphases

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