Hydrodynamics, monolithic catalysts

In monolithic catalyst carriers with wider channels, the hquid forms a film on the channel walls, whereas in the core of the channel a continuous gas phase exists. As shown by Lebens [10], countercurrent gas-liquid operation is now possible, and shows certain advantages over the countercurrent trickle bed operation. Typical channel diameters are 3-5 mm, and the geometric surface areas are between 550 and 1000 m2 m 3. Below the flooding point, almost no hydrodynamic interaction between the gas and hquid can be observed for example, the RTD is the same for both co-current and countercurrent operation. Apart from some surface waves, the film flow is completely laminar. 

In summary, it can be concluded that the monolithic stirrer reactor is a convenient reactor type both for the laboratory and the production plant. It is user-friendly and can be used to compare different catalysts in the kinetically limited regime or hydrodynamic behavior in the mass transfer controlled regime. Stirrers or monolith samples can be easily exchanged and reloaded to suit the desired enzyme and/or reaction conditions. 

Magnetic resonance imaging permitted direct observation of the liquid hold-up in monolith channels in a noninvasive manner. As shown in Fig. 8.14, the film thickness - and therefore the wetting of the channel wall and the liquid hold-up -increase nonlinearly with the flow rate. This is in agreement with a hydrodynamic model, based on the Navier-Stokes equations for laminar flow and full-slip assumption at the gas-liquid interface. Even at superficial velocities of 4 cm s-1, the liquid occupies not more than 15 % of the free channel cross-sectional area. This relates to about 10 % of the total reactor volume. Van Baten, Ellenberger and Krishna [21] measured the liquid hold-up of katapak-S . Due to the capillary forces, the liquid almost completely fills the volume between the catalyst particles in the tea bags (about 20 % of the total reactor volume) even at liquid flow rates of 0.2 cm s-1 (Fig. 8.15). The formation of films and rivulets in the open channels of the structure cause the further slight increase of the hold-up. 

Activity tests were performed with these monoliths in order to compare the preparation procedure with the powder catalysts. As the objective of this paper is to determine if with these two different preparation procedures (monolith and powder) the same catalyst could be obtained, it was decided to crush monoliths with different metal loading and test them in the thiophene HDS, in the same conditions as the powder catalysts. In this way, the hydrodynamics of the system is maintained and a fair comparison of the preparation technique is done. Monoliths with different metal loadings were obtained by varying the metal concentrations of the impregnation solutions. In Figure 3 the rate constants of the thiophene HDS are shown as a function of the metal content for the powder catalysts and monoliths. The wei t of metal was used to calculate the rate constants instead of weight of catalyst, so that an honest comparison can be made between using monoliths or powder. 

Experiments with laboratory monoliths of small cross-section area can lead to biased results due to an uneven flow distribution in the channels, especially close to the reactor wall. The wash-coat of the outer broken chaimels should be scraped away, and the void between the reactor wall and the monolith should be carefully plugged. To minimize wall effects, the diameter of the monolith should be ten tunes the chaimel diameter at least. Plug flow must prevail in a packed bed of crushed catalyst. The bed length and radius should be more than 50 and 10 particle diameters respectively, the flow resistance of the bed support must be unifonn throughout its cross-section, and the particle size distribution must be as narrow as possible. Otherwise, there can be oy-passes or dead vohunes. These hydrodynamic problems are overcome in a recycle loop reactor because the same physical and chemical conditions prevail everywhere. 

Application of the pressure-based solver via the SIMPLE algorithm can be explained in the context of transport equations that quantify hydrodynamics, mass, and heat transfer inside a single porous catalyst-coated channel of a monolith reactor shown in Figure 11.3 [17]. The relevant transport equations are given in their generalized form as follows ... 

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