Hydrogen, mass transfer

It should be noted that the reaetion using Et-DuPhos-Rh catalyst is not limited by hydrogen mass transfer sinee the hydrogen mass transfer rate is at least 5 times as fast as the initial reaction rate. Furthermore, the overall reaction time, 700 minutes, remained the same regardless of the size of the reactor. 

Aiken, III, J.D. and Finke, R.G., Nanocluster formation synthetic, kinetic, and mechanistic studies. The detection of, and then methods to avoid, hydrogen mass-transfer limitations in the synthesis of polyoxoanion- and tetrabutylammonium-stabilized, near-monodisperse 40 6 A Rh(0) nanoclusters, J. Am. Chem. Soc., 120, 9545, 1998. 

Undesirable by-products as a result of strong hydrogen mass-transfer control... 

The current process has some disadvantages discontinuous operation, strong hydrogen mass transfer control and low space-time-yields. The nickel on kiesel-guhr catalyst causes also some problems undesirable by-products, formation of nickel soaps in free fatty acid hydrogenation and disposal of nickel residues. 

It was found that experimental conditions such as rate of stirring have a much more dramatic effect than the choice of precatalyst. For example, hydrogenation of the candoxatril precursor 11 was 4.5 times slower with less efficient stirring (Fig. 8), using either the COD or NBD precatalysts. For this class of reaction, hydrogen mass transfer into solution [15] is the most important single process parameter affecting the overall reaction rate. 

The model for slurry hydrogenation is based on the three steps involving hydrogen mass transfer and reaction. The surface reaction is assumed first order to hydrogen, so the rate constant can be combined with the mass transfer coefficients in a simple equation. Step 3 is omitted, since the bulk concentration of B is normally much greater than the hydrogen concentration in the liquid. Step 5 is not included in the model but will be considered later, in the discussion of selectivity with consecutive reactions. Because of... 

Hydrogen mass transfer rate from the gas channel to the anode electrode surface is given in terms of the convective mass transfer coefficient as... 

Shah et al. (ik ) studied the catalytic liquefaction of a sub-bituminous coal using the axial dispersion model. They concluded that a minimum gas flow rate could be found that wo ald eliminate possible hydrogen mass transfer resistance from the gas phase to the catalyst surface. 

Hydrogen can permeate selectively dense metal layers, and, accordingly, this phenomenon is exploited when metal membranes are applied for separating ultrapure hydrogen from gas mixtures [24-27]. The hydrogen mass transfer through dense metal membranes includes several transport mechanisms, which are discussed in the following sections. 

Fabrication of Pd membranes has to take into consideration both the hydrogen/ metal interaction and hydrogen mass-transfer mechanisms through the metal lattice. Particnlarly, the strong hydrogen/metal interaction is responsible for the change of chemical, physical, and mechanical properties of the Pd alloys used. 

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