Mass transfer in monolith

Figure 6. Visualization of mass transfer in monolith structures (flow from left to right). Top Corrugated packing bottom Square duct monolith [9],...

AA. Gas-liquid mass transfer in monoliths =ai( ) P/V = power/volume (kW/m3), range = 100 to 10,000 [E] Each channel in monolith is a capillary. Results are in expected order of magnitude for capillaries based on 5-21-Z. kL is larger than in stirred tanks. [93]... 

The most characteristic features of structured catalysts are given in Table 1. The main difference between the three types of structured catalysts we have just distinguished consists in the rates of radial mixing in the reactor containing the structured catalyst from zero radial mass transfer in monolithic reactors to a very intense radial mass transfer in reactors with structural catalysts. For the sake of simplicity, reactors containing monolithic or membrane catalysis will be referred to as monolithic or membrane reactors, respectively. 

These equations are identical to the equations describing mass transfer in monolithic reactors. For monolithic reactors it was shown [14] that when the reaction rate is very fast compared to the mass transfer rate in the fluid domain, the boundary condition of Eq. (1 5) becomes identical to the standard heat transfer boundary condition of constant wall temperature when the reaction rate is very slow compared to the mass transfer rate in the fluid domain, the boundary condition of Eq. (IS) becomes identical to the standard heat transfer boundary condition of constant heat flux. The influence of the boundary conditions on the mass transfer coefficient in case of laminar flow is discussed in the following section. 

Votruba J, Mikus 0, Nguen K, Hlavacek V, Skrivanek J. Heat and mass transfer in monolithic honeycomb catalysts-II. Chemical Engineering Science 1975 30 201-206. 

Holmgren A, Andersson B. Mass transfer in monolith catalysts-CO oxidation experiments and simulations. Chemical Engineering Science 1998 53 2285-2298. 

Hayes RE, Liu B, Moxom R, Votsmeier M (2004) The effect of washcoat geometry on mass transfer in monolith reactors. Chemical Engineering Science, 59, 3169-3181... 

The significance of the parameters that govern heat and mass transfer in a monolith may be illustrated most effectively by presentation and manipulation of the differential equations which are supposed to describe the system (56). The governing mechanism for heat and mass transfer within a monolithic structure are ... 

While we have for heat and mass transfer in a porous catalyst explicit relations for parameters giving rise to multiple steady states, there is nothing similar developed for the monolithic catalysts so far. Hence we are forced to investigate, for particular conditions, the region of multiple steady states numerically. 

Figure 4 Shaped channels in metal monoliths in order to increase mass transfer in gas-phase applications.

Heibel AK, Heiszwolf JJ, Kapteijn F, Moulijn JA. Influence of channel geometry on hydrodynamics and mass transfer in the monolith film-flow reactor. Catal Today 2001 69 153-163. 

Comparing the performance of the micro structured reactor with a ceramic monolith at 230 °C reaction temperature and a GHSV of 300 000 h 1, the conversion in the micro reactor was 94%, whereas 86% was found for the monolith, which was attributed to the improved heat and mass transfer in the metallic micro-structures (see Figure 2.89). The GHSV value of 500 000 h 1 corresponds to a dry gas flow rate of440 Ndm3 h 1. However, the stability of the catalyst coated on the monolith was superior to that of the catalyst coated on the micro structured stainless-steel plates. 

In a search for novel column design for proteomics with enhanced mass-transfer properties, monolithic columns have been developed and manufactured which show lower pressure drop than comparable particulate columns and improved column performance at the same time [71]. 

TABLE 2 Gas-Liquid Mass Transfer in Conventional Reactors Compared with That in Monoliths. 

Considerations along the above lines lead to analogous correlations for the Sherwood number for the description of mass transfer in a single channel. The application of the rather simple Nusselt and Sherwood number concept for monolith reactor modeling implies that the laminar flow through the channel can be approached as plug flow, but it is always limited to cases in which homogeneous gas-phase reactions are absent and catalytic reactions in the washcoat prevail. If not, a model description via distributed flow is necessary. 

Other known disadvantages of packed-bed reactors are low external and internal mass transfer rates. For trickle-bed reactors, a representative value for both and k/ig is 0.06 sec [18]. As was pointed out previously, the corresponding values for monolith reactors are higher due to the enhanced radial mass transfer in liquid slugs and to shorter diffusion length in both the liquid film and the solid catalyst. 

A. Cybulski and J.A. Moulijn, Mass Transfer in a Monolithic Catalyst Under Reacting Conditions Oxidation of Carbon Mono.xide, Unpublished works of the Delft University of Technology, Delft, The Netherlands, (1994). 

As discussed before, the transition from laminar to turbulent flow in the PPR channels already occurs at relatively low Reynolds number as a consequence of the roughness of the channel walls. Under typical operating conditions in practice, flow through the channels is quite turbulent, in contrast to the situation generally prevailing in monoliths as used in exhaust convertors, where due to the much smaller channel diameter and smoothness of the wall, flow is generally laminar. Therefore, in a PPR mass transfer in the gas inside the channel is generally relatively fast. 

Kambing [7] has quantified the comparative mass transfer in OCFS and monoliths for the oxidation of propylene. With equal residence times, virtually 100% conversion was achieved in the OCFS at a throughput for which only 79% conversion was achieved in the monolith, although the temperature in the latter was higher and the surface area 1.4 times that of the OCFS. The mass transfer data, in the form of the Sherwood number, Sh, can be correlated with the Reynolds and Schmidt numbers. Re and Sc, respectively, in the form... 

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 ... 

Instead of polarized noble gases, thermally polarized NMR microimaging was used to study of liquid and gas flow in monolithic catalysts. Two-dimensional spatial maps of flow velocity distributions for acetylene, propane, and butane flowing along the transport channels of shaped monolithic alumina catalysts were obtained at 7 T by NMR, with true in-plane resolution of 400 xm and reasonable detection times. The flow maps reveal the highly nonuniform spatial distribution of shear rates within the monolith channels of square cross-section, the kind of information essential for evaluation and improvement of the efficiency of mass transfer in shaped catalysts. The water flow imaging, for comparison, demonstrates the transformation of a transient flow pattern observed closer to the inflow edge of a monolith into a fully developed one further downstream. 

The heat and mass transfer in a monolith have been well described. In this chapter, the phenomena are broken down to a simpler form for easier understanding of each effect. 

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