Single Pellet Diffusion Reactor

Figure 5.8 Single pellet diffusion reactor (a) set up for permeability and. .. internal diffusion measurements (b)...

Experimental verification of the models has been carried out using equipment ranging from a thermogravimetric analyzer [11], a gradientless recycle reactor [9], to a single-pellet diffusion reactor [12,13]. 

The single-pellet diffusion reactor can be employed for transient experiments. Cannestra et al. [65] give an example. Gas composition was measured at the center of one-dimensional pellets. The standard single-pellet diffusion reactor was modified to allow continuous gas analysis and miniaturized in order to reduce the time constants of gas flow mixing. A rather simple model for data evaluation used by the authors was not able to predict major features of the response measurments at the pellet center but gave qualitatively correct results of the external concentration responses. This demonstrates the necessity of an elaborate modeling of instationary multicomponent diffusion and porous structure for this type of reactor. 

Figure 5. (a) Single pellet diffusion reactor—concentration measurements (26) (b) single pellet diffusion reactor—temperature measurements (29)... 

More recent work by Wolf and Petersen (37,38) has investigated the influence of the order of the main reaction on single pellet diffusion reactor behavior, using the same model deactivation mechanisms studied before. Experimental application to a reaction with complicated kinetics (and complicated deactivation behavior) was reported for methylcyclohexane dehydrogenation. 

Measurement of Reaction Kinetics and Effective Pellet Diffusivity in a Single-peUet Reactor 215... 

To analyse the performance of exhaust gas catalyst (activity and poisoning) the effective diffusivity of six Rhone Poulenc alumina supports is measured by a physical dynamic method in a single pellet string reactor. 

Experiments for measuring effective diffusivities in catalysts can also be carried out in fixed bed systems such as wide body or single pellet string reactors (SPSR). 

Single pellet reactor for determination of effective pellet diffusivity under reaction conditions... 

A qualified question is then whether or not the multicomponent models are really worthwhile in reactor simulations, considering the accuracy reflected by the flow, kinetics and equilibrium model parts involved. For the present multiphase flow simulations, the accuracy reflected by the flow part of the model is still limited so an extended binary approach like the Wilke model sufEce in many practical cases. This is most likely the case for most single phase simulations as well. However, for diffusion dominated problems multicomponent diffusion of concentrated ideal gases, i.e., for the cases where we cannot confidently designate one of the species as a solvent, the accuracy of the diffusive fluxes may be significantly improved using the Maxwell-Stefan approach compared to the approximate binary Fickian fluxes. The Wilke model might still be an option and is frequently used for catalyst pellet analysis. 

Chapter 5 is dedicated to the single particle problem, the main building block of the overall reactor model. Both porous and non-porous catalyst pellets are considered. The modelling of diffusion and chemical reaction in porous catalyst pellets is treated using two degrees of model sophistication, namely the approximate Fickian type description of the diffusion process and the more rigorous formulation based on the Stefan-Maxwell equations for diffusion in multicomponent systems. 

From the foregoing discussion, it is clear that the catalyst pellet is not only the heart of the catalytic reactor but also the hardest part of the system to model accurately. The difficulties associated with the modelling of the single pellet (especially the porous pellet) is due to the uncertainties associated with the intrinsic kinetics and the precise modelling of diffusion of mass and heat inside the pellet as well as the complex interaction between these two processes. The complex tortuous structure of porous catalyst pellet adds to the complexity. Different trials to estimate the tortuosity factor (which accounts for the complex tortuous structure of the pellet) theoretically have failed to give accurate results and this factor is usually estimated experimentally. 

The dynamics of sulphur uptake in a prereformer is like a fixed-bed absorption as seen in a zinc-oxide bed (refer to Chapter 1). However, in a tubular reformer the pore diffusion restrictions in the sulphur adsorption in a single pellet has a complex influence on the transient sulphur profiles in the reactor and a mathematical model [112] [387] [389] is required to evaluate more exactly the time for fiill saturation and the breakthrough curves of sulphur. 

The single pellet moment technique which was originally developed by Dogu and Smith (1975,1976) for the evaluation of diffusion and adsorption parameters in porous solids, was extended to reaction systems and used for the kinetic analysis of SO 2-activated soda reaction (Dogu et al, 1986). In this technique, an inert carrier gas (helium) flowed over two end faces of a cylindrical soda pellet placed into the single pellet reactor (Figure 11). 

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