Mass transfer limited reactions

FIG. 14-13 Gas-phase and liquid-phase solute-concentration profiles for a liquid-phase mass-transfer limited reaction system in which is larger than 3. 

The diffusion paths for heat and mass transfer are very small, making micro reactors ideal candidates for heat or mass transfer-limited reactions. 

The performance of adsorption processes results in general from the combined effects of thermodynamic and rate factors. It is convenient to consider first thermodynamic factors. These determine the process performance in a limit where the system behaves ideally i.e. without mass transfer and kinetic limitations and with the fluid phase in perfect piston flow. Rate factors determine the efficiency of the real process in relation to the ideal process performance. Rate factors include heat-and mass-transfer limitations, reaction kinetic limitations, and hydro-dynamic dispersion resulting from the velocity distribution across the bed and from mixing and diffusion in the interparticle void space. 

It is important to be able to identify mass transfer limitations that occur when the reaction rate is high compared with the rate of mass transfer. For a heavily mass transfer limited reaction, preliminary experiments in a non optimised laboratory reactor... 

The first of the four characteristic currents to J4 has a prominent position. It indicates the crossover from a charge supply limited reaction to a kmetically and mass transfer limited reaction. This crossover is accompanied by pronounced changes in charge state, chemical dissolution reaction, dissolution valence, pore formation and anodic oxide formation. Therefore its dependence on other parameters, such as crystal orientation, temperature or H F concentration deserves further investigation. In the literature Jt is usually termed /crl JPS or JPSL. In the following the symbol JPS will be used. 

As indicated earlier, the effect of CO pressure is often unpredictable in carbonylations. To optimize this process, the effect of CO pressure was measured at 120°C and 130°C and the results appear in Table 4. With these highly active catalyst systems, there appeared to be an optimum CO pressure and excess CO pressures was deleterious to the reaction. While the presence of CO optima is not unknown in carbonylation chemistry, it is normally observed at significantly higher CO pressures. It is likely that the optimum observed in this study represented the transition from a mass transfer limited reaction to a chemically limited reaction. (The combination of a phosphine optima and rate reductions with increased CO likely indicate a rate determining dissociative process along the reaction pathway.)... 

External mass transfer-limited reactions In the expression (5.191), km has to be known, but it is not necessaiy if the external mass transfer phenomena are very intense. Actually, if strong mass resistance exists, the knowledge of the rate law is not essential, because the rate can be written as... 

The most likely opportunities for exploitation will come from mass transfer-limited reactions and the combination of unit operations in one device. Examples of reactions mentioned earlier include polymerization, condensation reactions, crystallization, and heterogeneous catalysis. Combined unit operations are illustrated by reactive distillation, polymer devolatilization with pelletization, and the use of heat exchangers (reboilers and condensers) with distillation. 

A monolith catalyst has a much higher void fraction (between 65 and 91 percent) than does a packed bed (which is between 36 and 45 percent). In the case of small channels, monoliths have a high geometric surface area per unit volume and may be preferred for mass-transfer-limited reactions. The higher void fraction provides the monolith catalyst with a pressure drop advantage compared to fixed beds. 

In the present case, the Levich equation describes the reaction rate for a mass-transfer limited reaction in an RDE experiment. The Levich equation is... 

Mass Transfer-Limited Reactions in Packed Beds 706... 

Mass Transfer-Limited Reaction on Metallic Surfaces 714... 

In most mass transfer-limited reactions, the surface concentration is negligible with respect to the bulk concentration (i.e, Ca CaJ ... 

Reactor concentration profile for a mass transfer-limited reaction... 

The rate of surface reaction is equal to mass flux to the surface. Taking the surface concentration equal to zero for mass transfer-limited reactions gives... 

As a first approximation, we neglect the effects of small changes in temperature and pressure on mass transfer. We recall Equation (11-64), which gives conversion as a function of reactor length. For a mass transfer-limited reaction... 

The conversion for externally mass transfer-limited reactions ean be found from the equation... 

Mass Transfer Limited Reactions on Metallic Surfaces... 

Consequently, for external mass transfer-limited reactions, the rate is inversely proportional to tlie particle diameter to the three-halves power ... 

This measuring method is, due to its adaptability, very good for investigating coalescence phenomena in any physical system and is suitable for clarifying the chemical kinetics in mass transfer-limited reactions in gas/liquid systems (e.g. hydrogenations, oxidations, phosgenations, etc.) (see also [215]). 

In the previous sections the use of catalysts dissolved in ionic liquids has been documented with a variety of examples from the most recent literature. They were classified are catalytic systems based on the adoption of Strategies A, B and C, when solvent-less conditions were not adopted. In an ideal liquid-liquid biphasic system, the IL must dissolve the catalytic intermediates and, in part, the substrate to avoid that mass transfer limits reaction rates. Moreover, products should have a limited solubility in the IL to allow a facile product removal or extraction, and, possibly, the recycle of the ionic liquid-trapped catalyst. The separation of the catalyst from the products is made easier if solid support-immobilised ILs are used. The preference for a solid catalyst is dictated not only by the easier separation but also, as outlined by Mehnert in an excellent review article, " by (i) the possible use of fixed bed reactors, and (ii) the use of a limited amount of IL, a generally expensive chemical which can limit the economic viability of the process. In this section attention will be focused only on the most recent examples of solid-phase assisted catalysis using ionic liquids, following Strategy D. Examples prior to 2006 are covered in recent reviews and will not be discussed here. " ... 

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