Mass transfer monoliths

Because of the low black pressure and fast mass transfer, monolithic columns can be run at very high speed (see Figure 7.14). As a result. 

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particularly active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). Eor these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

Much effort has been devoted to the development of a multi-step swelling polymerization technique using water as suspension medium [98]. This has resulted in polymers showing similar selectivities but slightly improved mass transfer characteristics compared with the corresponding monolithic polymers. Of particular rele-... 

The three principal catalyst bed configurations are the pellet bed, the monolith, and the metallic wire meshes. An open structure with large openings is needed to fulfill the requirement of a low pressure drop even at the very high space velocities of 200,000 hr-1. On the other hand, packings with small diameters would provide more external surface area to fulfill the requirement for rapid mass transfer from the g .s stream to the solid surface. The compromise between these two ideals results in a rather narrow range of dimensions pellets are from to 1 in. in diameter, monoliths have 6 to 20 channels/in., and metallic meshes have diameters of about 0.004 to 0.03 in. 

The rates of catalytic bed warm-up from a cold start and of destructive overheating are governed by the rate of heat transfer from the gas phase to the solid surfaces. In the highest flow rate of gases, the rate of mass transfer of pollutant molecules to the catalytic walls is inadequate in the monolith. 

Provided that the catalyst is active enough, there will be sufficient conversion of the pollutant gases through the pellet bed and the screen bed. The Sherwood number of CO is almost equal to the Nusselt number, and 2.6% of the inlet CO will not be converted in the monolith. The diffusion coefficient of benzene is somewhat smaller, and 10% of the inlet benzene is not converted in the monolith, no matter how active is the catalyst. This mass transfer limitation can be easily avoided by forcing the streams to change flow direction at the cost of some increased pressure drop. These calculations are comparable with the data in Fig. 22, taken from Carlson 112). 

Naturally, there are two more Peclet numbers defined for the transverse direction dispersions. In these ranges of Reynolds number, the Peclet number for transverse mass transfer is 11, but the Peclet number for transverse heat transfer is not well agreed upon (121, 122). None of these dispersions numbers is known in the metal screen bed. A special problem is created in the monolith where transverse dispersion of mass must be zero, and the parallel dispersion of mass can be estimated by the Taylor axial dispersion theory (123). The dispersion of heat would depend principally on the properties of the monolith substrate. Often, these Peclet numbers for individual pellets are replaced by the Bodenstein numbers for the entire bed... 

In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

While conventional monoliths contain parallel channels, in practice, systems are often made from alternate layers that allow lighter structures with better mass transfer characteristics in gas-phase applications, see Figure 9.6 showing interconnected flow paths. They are usually made from metal, mostly Fecralloy , Kanthal , or stainless steel, and widely used in autocatalysts and in environmental... 

Describe the advantages and disadvantages of the following reactor types with reference to heat and mass transfer. For each reactor discuss one reaction for which it may be appropriate to use that reactor, (a) fluidized bed reactor, (b) A continuous counter-current flow reactor, (c) A monolith reactor. 

This is explained by a possible higher activity of pure rhodium than supported metal catalysts. However, two other reasons are also taken into account to explain the superior performance of the micro reactor boundary-layer mass transfer limitations, which exist for the laboratory-scale monoliths with larger internal dimensions, are less significant for the micro reactor with order-of-magnitude smaller dimensions, and the use of the thermally highly conductive rhodium as construction material facilitates heat transfer from the oxidation to the reforming zone. 

The difference in reactor performance is due to the difference in hydraulic diameters of the reaction channels, i.e. related to varying mass-transfer limitations. The micro channels of the p-gauze platinum catalyst amount to 70 pm, whereas the monoliths have channel/pore diameters of 500-1200 pm. 

Mass transfer-limited processes favour slurry reactors over monoliths as far as the overall process rates are concerned. Moreover, slurry reactors are more versatile and less sensitive to gas flow rates. However, the productivity per unit volume is not necessarily higher for slurry reactors because of the low concentration of catalyst in such reactors. There also is no simple answer to the selectivity problem, and again, each process should be compared in detail for both reactors. 

The situation is completely different for mass transfer within the pore network of monolithic compounds. Here mass transfer can occur both on the pore surface or in the pore volume and molecular exchange between these two states of mobility can occur anywhere within the pore system, being completely uncorrelated with the respective diffusion paths. As a consequence, Eq. (3.1.11) is applicable, without any restrictions, to describing long-range diffusion in the pore space. Equation (3.1.14) is thus obtained,... 

Hatziantoniou, V., Andersson, B., and Schbon, N.-H., Mass transfer and selectivity in liquid phase hydrogenation of nitro compounds in a monolithic catalyst reactor with segmented gas-liquid flow. Ind. Eng. Chem. Process Des. Dev. 25, 964-970 (1986). 

Miyabe, K., Cavazzini, A., Gritti, F., Kele, M., Guiochon, G. (2003). Moment analysis of mass-transfer kinetics in Ci8-silica monolithic columns. Anal. Chem. 75, 6975-6986. 

Because the monoliths allow total convection of the mobile phase through their pores, the overall mass transfer is dramatically accelerated compared to conventional porous structures. Based on the morphology and porous properties of the molded monoliths, which allow fast flow of substrate solutions, it can be safely anticipated that they would also provide outstanding supports for immobilization of biocatalysts, thus extending the original concept of monolithic materials to the area of catalysis. 

The positive effect of convection of the substrate solution on mass transfer can be observed even better with macromolecular substrates that undergo processes such as protein digestion. For example, Fig. 9 compares reversed-phase chromatograms of cytochrome c digests obtained by cleavage with trypsin immobilized in both packed and molded column reactors, and clearly demonstrates the much higher activity of the monolithic device under otherwise similar circumstances [90]. 

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