Plug-flow adsorption reactor model

The plug-flow model indicates that the fluid velocity profile is plug shaped, that is, is uniform at all radial positions, fact which normally involves turbulent flow conditions, such that the fluid constituents are well-mixed [99], Additionally, it is considered that the fixed-bed adsorption reactor is packed randomly with adsorbent particles that are fresh or have just been regenerated [103], Moreover, in this adsorption separation process, a rate process and a thermodynamic equilibrium take place, where individual parts of the system react so fast that for practical purposes local equilibrium can be assumed [99], Clearly, the adsorption process is supposed to be very fast relative to the convection and diffusion effects consequently, local equilibrium will exist close to the adsorbent beads [2,103], Further assumptions are that no chemical reactions takes place in the column and that only mass transfer by convection is important. 

Finally, it must be pointed out that the adsorbent when it makes contact with a binary mixture, one component is selectively adsorbed by the solid adsorbent. In the flowing fluid, a trace of an adsorbable species is adsorbed from a relatively inert carrier. In addition, the heat effects can be ignored, as a result, isothermal conditions can be taken [9,103], The flow is fed at the top of the bed at a constant flow rate, and under conditions such that mass-transfer resistance is insignificant [2,103], 

Other parameters characterizing a PFAR are the column breakthrough capacity, Bc, the column saturation capacity, Sc, and the column efficiency, E, of the PFIEBR, which are calculated with the following equations [105]  

At this point, it is necessary to state that the physical process of adsorption is so fast relative to other steps, such as diffusion within the solid. In this case, in and near the solid adsorbent, the general form for the equilibrium isotherm is [99] 

C denotes the solute composition, in moles of solute per unit volume of fluid, which could exist at equilibrium 

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The plug-flow model signifies that the fluid velocity profile is plug shaped, and is uniform at all radial positions, as explained earlier for the plug-flow adsorption reactor [38,49-53] (see Section 6.11.2). The fixed-bed ion-exchange reactor is packed randomly with particles from a solid ion... 

The complete unsteady-state model for adsorption-desorption and surface reactions of the plug flow laboratory reactor was based on the following equations NH3 mass balance on the catalyst suiface ... 

A one-dimensional isothermal plug-flow model is used because the inner diameter of the reactor is 4 mm. Although the apparent gas flow rate is small, axial dispersion can be neglected because the catalj st is closely compacted and the concentration profile is placid. With the assumption of Langmuir adsorption, the reactor model can be formulated as. 

The results confirm that the adsorption of ammonia is very fast and that ammonia is strongly adsorbed on the catalyst surface. The data were analyzed by a dynamic isothermal plug flow reactor model and estimates of the relevant kinetic parameters were obtained by global nonlinear regression over the entire set of runs. The influences of both intra-particle and external mass transfer limitations were estimated to be negligible, on the basis of theoretical diagnostic criteria. 

In the present paper we report kinetic analyses of H2-TPD for two commercial hydrogenation catalysts. The quasiequilibrium adsorption is still assumed in the analysis, but the reactor is now modelled as a plug flow reactor. Preliminary indications on the microkinetic relevance of hydrogen adsorption to the toluene hydrogenation are presented. 

RTD experiments showed that the fixed-bed almost behaves like a plug-flow reactor and the infrared cell like a continuous stirred tank reactor. This fixed-bed is described by the tanks-in-series model, using 9 tanks for the catalyst compartment. The two kinetic models (Equations 1-6) are able to describe the stop-effect experiments at 180 and 200°C, and the considerations made in this work are valid for both temperatures. However, for the sake of clarity, only model discrimination at 180°C will be presented here. In the experimental conditions used here, both models can be simplified the first adsorption step is considered as irreversible, and instantaneous equilibrium is assumed for the second one. With these hypothesis the total number of kinetic parameters is reduced from five (ki, Li, k2, k.2 and ks) to three (ki, K2 and ks), and the models can be expressed as follows ... 

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. 

From the mechanism it can be seen that material is added to or depleted from the gas phase by adsorption/desorption with the exception of hydrogen which is assumed to be consumed directly from the gas phase. In formulating a theoretical model for the system it was assumed that the adsorption/desorption kinetics played an important role in the dynamics of the periodic operation and these kinetics were incorporated into the dynamic equations. Furthermore, it was assumed that there was neither bulk nor pore diffusional heat and mass transfer resistances, that the reactor was isothermal (both in the bulk gas phase and locally) and that the flow pattern in the reactor could be approximated by plug flow. Most of the above assumptions (i.e. plug flow, bulk isothermal conditions, no pore diffusion limitations) could be... 

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