Mass transport agitation effect

Modification of silica gel with volatile or gaseous compounds is performed in the vapour phase. Industrial-scale reactors and laboratory scale gas adsorption apparatus have been used. In the industrial field, fluidized bed and fluid mill reactors are of main importance. In fluidized bed reactors,82 the particles undergo constant agitation due to a turbulent gas stream. Therefore, temperatures are uniform and easy to control. Reagents are introduced in the system as gases. Mass transport in the gas phase is much faster than in solution. Furthermore, gaseous phase separations require fewer procedural steps than solution phase procedures, and may also be more cost-effective, due to independence from the use and disposal of non-aqueous solvents. All these advantages make the fluidized bed reactors preferential for controlled-process industrial modifications. 

In three-phase systems, where the solid catalyst is in contact with a liquid reactant or its solution plus a gaseous reactant, efficient agitation is required to effect dissolution of the gaseous molecule into the liquid and its transport to the catalyst surface. Such systems easily become mass-transport limited, especially when a very active catalyst is used. In a batch reactor, rapid shaking or stirring is needed, and catalyst particles must be small it may operate at atmospheric pressure, or at superatmospheric pressure as an autoclave. Large catalyst particles can however be used with liquid reactants either in a trickle-column reactor or a spinning-basket reactor. 

The information required to predict electrochemical reaction rates (i.e., experimentally determined by Evans diagrams, electrochemical impedance, etc.) depends upon whether the reaction is controlled by the rate of charge transfer or by mass transport. Charge transfer controlled processes are usually not affected by solution velocity or agitation. On the other hand, mass transport controlled processes are strongly influenced by the solution velocity and agitation. The influence of fluid velocity on corrosion rates and/or the rates of electrochemical reactions is complex. To understand these effects requires an understanding of mixed potential theory in combination with hydrodynamic concepts. 

Mass transport induced by US is a key aspect of sonoelectrochemistry as confirmed by several authors who have identified the underlying physical processes [140,144-148]. Enhanced mass transport to the electrode displaces the redox process equilibrium and as a result increases the analytical signal. The influence of US is closely related to the use of continuous agitation or rotating disk electrodes [145,146]. The effect was demonstrated by... 

Fig. 5.6 shows a plot of Eqn 5.5 which represents a reaction run with varying quantities of catalyst at a fixed pressure, temperature and agitation rate. When mass transport is an insignificant factor in the reaction, km approaches zero and the rate, r, equals kfX, which is the asymptote of the curve at low catalyst quantities. With larger amounts of catalyst, the curve approaches the second asymptote, r = km, for a reaction controlled by mass transport effects. There is... 

Solution Easier to control heat and mass transport Wide range of accessible molecular weights Easier to transport reagents and products Needs agitation Solvent removal and recycling Chain transfer to solvent may lead to undesirable effects Inefficient heat and mass transport at high conversions and/or concentrations... 

The reactions of linear or branched alkenes containing more than four carbon atoms reveal no types of reaction not already met, but their lower volatility permits their study as liquids or in solution as well as in the vapour phase. Thus for example the relative isomerisation rate r,7r/, of liquid 1-pentene over Pd/C at 290 K was independent of the conditions of agitation, showing there were no mass-transport effects attributable to the hydrocarbons. The presence of solvents was also without effect. ... 

If the rate is independent of the agitation efficiency at sufficiendy high stirring speed (Fig. 10.29), it is conventionally assumed that mass transport effects are minimized. 

As presented in Fig. 10.13, the effect of Triton X-100 at 0.25 CMC was remarkable. Mass transfer coefficients were 5-fold higher than those obtained in absence of surfactant. The dispersion of nonaqueous-phase, leading to an increase in contact area, and the facilitated transport of the pollutant, probably due to reduction of surface tension or interaction of the pollutant with single surfactant molecules, may explain this increase of the mass transfer coefficients at concentrations lower than CMC [ 131 ]. On the other hand, mass transfer coefficients increased slightly when the agitation rate was increased from 200 to 250 rpm, which was coincident with the formation of the emulsion. The effect of increasing to 300 rpm had little impact on the mass transfer coefficients. The value of kt a with Triton at 300 rpm was not determined due to an inefficient separation of both phases. Finally, silicone oil of 10 and 20 cSt led to similar results, while solvent with 50 cSt always led to lower mass transfer coefficients. 

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