Heat transport resistance

Controlled elimination of mass and heat transport resistances is an important prerequisite for obtaining intrinsic kinetic parameters of the fast exothermic reaction of partial oxidation of methane to synthesis gas. It has been demonstrated that under conditions of strong transport limitations erroneous conclusions concerning the reaction scheme can be derived [7-9]. It was determined in this laboratory that transport limitations are practically absent over a wide range of operating conditions if one portion of the catalyst (< 40 pm) is diluted with -5 portions of an... 

As one might expect, moisture sorption is controlled by both mass and heat transport resistance. The heat transport model discussed earlier was based on the concept that condensation of water would generate heat, which at steady state would... 

It is very easy to see that Eqs. (48) and (49) have similar form and yet they represent mass transport and heat transport resistance, respectively. What is needed is a connection between the two transport resistances, and this is accomplished by combining the two equations to eliminate RHS. The resulting combined mass-heat transport limited rate moisture uptake (W mh) is given by... 

Referring to the membrane module design, it has a big influence on the membrane-contactor efficiency, because it affects the pressure drops of the streams (and, thus, the operating pressures and flowrates), and their fluidodynamic (which means the mass and heat transport resistances of the phases). Furthermore, for hollow-fiber modules it is essential to ensure a uniform packing, in order to have... 

However, the thermal conductivity of the catalyst matrix is usually larger than that of the gas. This means that the external gas-catalyst heat transport resistance exceeds the thermal conduction resistance in the catalyst particles. The temperature and concentration profiles established in a spherical catalyst are illustrated for a partial oxidation reaction in Figure 4. 

The performance of a trickle-bed reactor may be markedly affected by fluid flow hydrodynamics. That is the reason why an accurate prediction or simulation of the performance of such a reactor involves a detained description of the fluid flow hydrodynamics as well as of the various mass and/or heat transport resistances between gas, liquid and solid. 

For negligible external heat transport resistance, the criterion is... 

The criterion for negligible internal heat transport resistance is... 

The global heat transport coefficient U is calculated from the sum of different heat resistances in a series. For a packed bed reactor, the heat transport resistances concern ... 

In a kinetic investigation, the rate-determining step and, hence, the functional form of the rate model are not known a priori also unknown are the rate constants and adsorption equilibrium coefficients. Hence, the aim of data procurement and correlation is both model discrimination and parameter estimation which are completed in tandem [17]. The critical problem at this point is to obtain reliable experimental data from which kinetic models that reflect steady-state chemical activity can be extracted and evaluated. In order to measure correctly the rates of chemical events only, (i) external and internal mass and heat transport resistances at the particle scale have to be eliminated,... 

The analogous Biot number for heat is defined as the ratio of internal to external heat transport resistances and is used together with (BOm to assess the relative importance of all heat and mass transport limitations ... 

Given an experimental reactor, catalyst pellet size and fluid velocity are usually chosen to eliminate the disguise of intrinsic kinetics by mass and heat transport resistances. To a certain extent, the fluid concentration and temperature can also be used for that purpose. Small pellet sizes and high fluid velocities tend to lower the concentration and temperature differences caused by the resistances. 

Let us examine the combined effects of these two resistances on selectivity. A simple way of accomplishing this is to compare the selectivity affected only by diffusion with that affected by both diffusion and external heat transport resistance. Let (Rg) /(Rg)X be the point yield of a consecutive reaction network... 

For the more realistic case of internal mass transport and external heat transport resistances, the system equations written for a slab-like pellet are ... 

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