Falling mass-transfer efficiency

Mass transfer efficiency by conversion anaiysis for the falling film micro reactor... 

GL 22] [R 1] [P 23] The mass transfer efficiency of the falling film micro reactor as a function of the carbon dioxide volume content was compared quantitatively (Figure 5.30) [5]. The molar ratio of carbon dioxide to sodium hydroxide was constant at 0.4 for all experiments, i.e. the liquid reactant was in slight excess. 

The mass transfer efficiency of the falling-film microreactor and the microbubble column was compared quantitatively according to the literature reports on conventional packed columns (see Table 4.3) [318]. The process conditions were chosen as similar as possible for the different devices. The conversion of the packed columns was 87-93% the microdevices had conversions of 45-100%. Furthermore, the space-time yield was compared. Flere, the microdevices resulted in larger values by orders of magnitude. The best results for falling-film microreactors and the microbubble columns were 84 and 816 mol/(m3 s), respectively, and are higher than conventional packed-bed reactors by about 0.8 mol/(m3 s). 

Additionally, the surfactant properties of filmers reduce the potential for stagnant, heat-transfer-resisting films, which typically develop in a filmwise condensation process, by promoting the formation of condensate drops (dropwise condensation process) that reach critical mass and fall away to leave a bare metal surface (see Figure 11.2). This function, together with the well-known scouring effect on unwanted deposits keeps internal surfaces clean and thus improves heat-transfer efficiencies (often by 5-10%). 

We included the term r = 0 to indicate that there is no reaction in the gas phase. The mass transfer rates obviously have opposite signs, and we have to multiply the mass transfer flux by [areaA olume], where the volume is that occupied by that phase. Note that the mass transfer term after dividing out becomes proportional to R. Since the reactor volume is proportional to R while the surface area for mass transfer is proportional to R, the falling film column obviously becomes less efficient for larger reactor sizes. This is a fundamental problem with the falling film reactor in that small tubes give high mass transfer rates but low total production of product. 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

Jahnisch et al. used an IMM falling-film microreactor for photochlorination of toluene-2,4-diisocyanate [38] (see also Chapter 4.4.3.3, page 161). As a result of efficient mass transfer and photon penetration, chlorine radicals were well distributed throughout the entire film volume, improving selectivity (side chain versus aromatic ring chlorination by radical versus electrophilic mechanism) and spacetime-based yields of l-chloromethyl-2,4-diisocyanatobenzene compared to those obtained using a conventional batch reactor. 

The minimum wetting rate (MWR) is tbe lower stability limit of packings. It is the liquid load below which the falling liquid film breaks up, and the liquid shortage causes dewetting of the packing surface. The area available for mass transfer diminishes, and efficiency drops (Sec. 8.2.2 points on Fig. 8.16a). 

Lyon, R.K. and Cole, J.A., 1997, Unmixed Combustion for Efficient Heat and Mass Transfer in Chemical Process Systems, American Flame Research Committee Fall Symposium Proceedings. 

In order to achieve the best possible catalyst conversion efficiency at a constant volume, while minimizing the power drain due to excessive pressure drop through the converter, one would maximize the heat and mass transfer with respect to the pressure drop. In other words, in the graph shown in Figure 7 for 100% open frontal area, where the Heat Mass Transfer Factor is on the x-axis and the Pressure Drop Factor is on the y-axis, the slope of the curve should to be as shallow as possible. All of the channel shapes evaluated here tend to fall close to the same hne. However, as the open frontal area decreases from 100%, the Pressure Drop Factor increases while the Heat Mass Transfer Factor remains constant so that the relative attractiveness of some channel structures will be improved as the OFA is taken into account. 

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