Mass-transfer-controlled reactions

The microreactor technique described by the authors offered 30 and 50 times higher STY values than the conventional tubular reactor and the slurry reactor, respectively, suggesting microreactors to be a successful technique in mass transfer controlled reactions. 

The prospect of using enzymes as heterogeneous catalysts in scC02 media has created significant interest. Their low viscosity and high diffusion rates offer the possibility of increasing the rate of mass-transfer controlled reactions. Also, because enzymes are not soluble in supercritical fluids, dispersion of the free enzymes potentially allows simple separations without the need for immobilization. 

This says that at low temperatures the reaction of propylene with benzene is reaction rate controlled and is associated with high concentrations of propylene around the active catalyst sites. At higher temperatures the reactions become mass transfer controlled (reaction rate very fast) and this leads to very low concentrations of propylene at the active sites. 

Heitz [22] employed the mass transfer-controlled reaction between zinc and iodine to explore the correlations between mass transport in a pipe, to a rotating disk, and to a rotating cylinder. The correlations were expressed in terms of the equivalent velocity through the tube as (quoted by Poulson [17])... 

Only a small amount of Ca-particles and a hort residence time arc required for the complete removal of SOj. In combination with the high temperatures in the combustion chamber (> 800 C), it seems reasonable to state that the removal of the sulfur is a mass transfer controlled reaction. For simplicity, it is assumed that the reaction of CaO with SOi is first order in SOi, and zero-order in Ca and oxygen ... 

Conceptually, the framework of the theory permits description of interphase heat and mass transfer with reaction occurring in either or both phases. In theory one can use this approach to study the affects of partial mixing of the dispersed phase on extent of reaction for non-first-order reactions which occur in the droplets. Analyses can be made for mass-transfer-controlled reactions and selectivity for complex reactions. Difficulties in the solution of the resulting integro-diflferential equations have restricted applications at present to partial solutions. For example, the effects of partial droplet mixing on extent of reaction were studied for uniform drops. Mass transfer from nonuniform drops for various reactor geometries was studied for dispersions with drop breakage only or drop coalescence only. 

Valentas and Amundson (V3) studied the performance of continuous flow dispersed phase reactors as affected by droplet breakage processes and size distribution of the droplets. Various reaction cases with and without mass transfer were studied for both completely mixed or completely segregated dispersed phase. Droplet size distribution is shown to have a considerable effect on the efliciency of a segregated reaction system. They indicated that polydispersed drop populations require a larger reactor volume to obtain the same conversion as a monodispersed system for zero-order (or mass-transfer-controlled) reactions in higher conversion regions. As the dispersed phase becomes completely mixed, the distribution of droplet sizes becomes less important. These interactions are un-... 

Figure 10.11 Schematic representation of mass-transfer controlled reaction by an ionic species formed through an adsorbed intermediate.

The role of mass transfer effects, whether occurring accidentally or by design, is ambivalent, causing Trevan to ask the question Diffusion limitation - friend or foe [115]. Lower activity as a result of low efficiency indicates that only a minor portion of enzyme is active during operation. The other unused portion may, in simple terms, replace the enzyme as it is inactivated step by step. In other words, mass transfer controlled reactions appear to be much less sensitive to decay of enzyme activity, thus falsely creating an impression of stabilization. Under harsh reaction conditions it may be advantageous to operate under these conditions to keep the reaction rate constant until the diffusion limitation disappears [82,115,116]. 

Cubic/monolithic corrosive liquids, acids, bases, or used as catalyst/heat exchanger for reactors. Usually made of graphite or carbon that has high thermal conductivity, area 1 to 20 m. Ceramic monoliths are used as solid catalyst for highly exothermic gas-catalyst mass transfer-controlled reactions. 

Prefer monolithic confignration for intensification with surface area/volume usually 1.5 to 4 times greater than traditional pellets. Excellent for mass transfer controlled reactions. For gas reacting with solid nsnally heat transfer controlled, because these are highly exothermic or endothermic reactions. Particle size and size distribution are critical. These reactions may follow different patterns ... 

The results of the catalytic activity of monolithic catalysts containing various amounts of unsupported aluminum rhodate particles in a simulated stoichiometric gas mixture are shown in Figure 2. The activity increases with an increase in Rh loading and levels off in the mass transfer controlled region. Comparing the non-mass transfer controlled reaction regime results in Figure... 

Notice that for given reaction conditions (Cq and m), a ring electrode will produce a larger current than a disk electrode of the same area. Thus, the analytical sensitivity of a ring electrode (i.e., the current caused by a mass-transfer-controlled reaction of an electroactive species divided by the residual current) is better than that of a disk electrode, and this is especially true of a thin ring electrode. However, it is usually more difficult to construct a rotating ring electrode than an RDE. 

One of the expected benefits from using enzymes in supercritical fluids (SCFs) is that mass transfer resistance between the reaction mixture and the active sites in the solid enzyme should be greatly reduced if the reactants and products are dissolved in an SCF instead of running the reaction in a liquid phase. It is expected that the high diffusivity and low viscosity of SCFs will accelerate mass-transfer controlled reactions. 

Diffusion problems practically absent present (mass-transfer-controlled reaction)... 

Table 6.9 indicates how the characteristics of the catalyst affect the type of reactor. Prefer monolithic configuration for intensification (H) with surface area/ volume usually 1.5-4 times greater than traditional pellets. The monolithic configuration is excellent for mass transfer controlled reactions. 

Gas-Uquid agitated thin film Holdup liquid holdup per total reactor volume 0.0002-0.2 m Liquid loading 0.06-1.25 L/s m of length. Backmixing liquid plug flow or as a series of at least 5 backmix stages energy needed 1 kW/m film surface. Mass transfer controlled reactions. 

Since Na" ion deposition onto a mercury cathode i.s a mass-transfer controlled reaction, the overpotential for Na" " deposition is (cf. Eq. (46))... 

The proposed reactors for the effluents treatment are focused on these strategies or a combination of both. In case of mass-transfer-controlled reactions, the increase in the kinetic constant can be achieved by two different actions the first is the movement of the electrode and the second one consists in the modification of specific hydrodynamic aspects of the reactor. 

For a mass transfer-controlled reaction k j rearrangement, simplifies to and Eq. (4.14), after... 

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