Mass Transfer and Reaction in a Packed Bed

The Mears criterion, like the Wiesz-Prater criterion, uses the measmed rate of reaction, — r j, (kmol/kg cat. s) to learn if mass transfer from the bulk gas phase to the catalyst smface can be neglected. Mears proposed that when 

= bulk concentration kmol/m kc = mass transfer coefficient, m/s 

The mass transfer coefficient can be calculated from the appropriate correlation, such as that of Thoenes-Kramers, for the flow conditions through the bed. When Equation (12-62) is satisfied, no concentration gradients exist between the bulk gas and external surface of the catalyst pellet. 

Mears also proposed that the bulk fluid temperature, T, will be virtually the same as the temperature at the external surface of the pellet when 

We shall perform a balanee on A over the volume element AK negleeting any radial variations in coneentration and assuming that the bed is operated at steady state. The following symbols will be used in developing our model  

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We conduct the reaction in a packed bed reactor, assuming that the heat transfer is sufficiently fast for the reactor to be isothermal. The mass W of catalyst in a region of volume V in the reactor is W = ps — solid catalyst and void fraction of the bed. Let T a(IT) be the flow rate (moles per unit time) of A passing through the particular surface in the reactor for which the mass of catalyst in the region between this surface and the inlet is W. The mole balance on A for the region between Wand W + SW is... 

FIG. 16-9 General scheme of adsorbent particles in a packed bed showing the locations of mass transfer and dispersive mechanisms. Numerals correspond to mimhered paragraphs in the text 1, pore diffusion 2, solid diffusion 3, reaction kinetics at phase boundary 4, external mass transfer 5, fluid mixing. 

The absorption of reactants (or desorption of products) in trickle-bed operation is a process step identical to that occurring in a packed-bed absorption process unaccompanied by chemical reaction in the liquid phase. The information on mass-transfer rates in such systems that is available in standard texts (N2, S6) is applicable to calculations regarding trickle beds. This information will not be reviewed in this paper, but it should be noted that it has been obtained almost exclusively for the more efficient types of packing material usually employed in absorption columns, such as rings, saddles, and spirals, and that there is an apparent lack of similar information for the particles of the shapes normally used in gas-liquid-particle operations, such as spheres and cylinders. 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

In dealing with chemical process engineering, conducting chemical reactions in a tubular reactor and in a packed bed reactor (solid-catalyzed reactions) is discussed. In consecutive-competitive reactions between two liquid partners, a maximum possible selectivity is only achievable in a tubular reactor under the condition that back-mixing of educts and products is completely prevented. The scale-up for such a process is presented. Finally, the dimensional-analytical framework is presented for the reaction rate of a fast chemical reaction in the gas/liquid system, which is to a certain degree limited by mass transfer. 

A wide class of forced unsteady-state processes have already been realized on the commercial scale using specific dynamic phenomenon, that takes place during performance of an exothermic reaction in a fixed bed of catalyst. This phenomenon is referred to in the literature as wrong-way behavior of a fixed bed reactor [20]. Substantial differences in characteristic times of heat and mass transfer in a packed bed reactor result in a surprising rise of temperature inside the reactor after... 

External Mass Transfer In a reactor, the solid catalyst is deposited on the surface of narrow tubes (such as monolith or foams), is packed as particles in a tube, or is suspended in slurry or in a fluidized bed as fine particles. For these systems, the bulk concentration of the gas phase approaches that on the catalyst surface if the mass-transfer rate from bulk to surface is substantially larger than the reaction rates on the surface. This, however, is often not the case. The mechanism of mass transfer and reaction on the external catalyst surface includes the following consecutive steps ... 

Kinetic parameters and mass-transfer effects of an immobilized enzyme in a packed-bed reactor and non-isothermal, heterogeneous reactions in a denatur-able, immobilized enzyme catalyst have been examined from theoretical viewpoints. It appears that thermal inactivation (denaturation) has not been considered in previous investigations of the effectiveness factor of an immobilized enzyme catalyst. It was pointed out that it is important to consider the transient... 

The same reaction was performed in a packed bed microreactor using supercritical CO2 [148]. The phase studies confirmed a single phase reaction mixture at 25 and 50 °C during isobaric reaction conditions of 136 bar. The single phase behavior of the reaction mixture avoids the gas/liquid mass transfer resistance. This further enhances productivity, so that the comparison with larger scale systems indicated an increase of about one order of magnitude in space time yield [148]. 

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