Chemical reaction, mass transfer

Figure 7.5. Concentration of gaseous feedstock 1 in a gas-ionic liquid biphasic reaction for slow (A) and fast (B) chemical reaction - mass transfer resistance on the gas side is neglected...

Simultaneous measurements of the rate of change, temperature and composition of the reacting fluid can be reliably carried out only in a reactor where gradients of temperature and/or composition of the fluid phase are absent or vanish in the limit of suitable operating conditions. The determination of specific quantities such as catalytic activity from observations on a reactor system where composition and temperature depend on position in the reactor requires that the distribution of reaction rate, temperature and compositions in the reactor are measured or obtained from a mathematical model, representing the interaction of chemical reaction, mass-transfer and heat-transfer in the reactor. The model and its underlying assumptions should be specified when specific rate parameters are obtained in this way. 

The spatial distribution of composition and temperature within a catalyst particle or in the fluid in contact with a catalyst surface result from the interaction of chemical reaction, mass-transfer and heat-transfer in the system which in this case is the catalyst particle. Only composition and temperature at the boundary of the system are then fixed by experimental conditions. Knowledge of local concentrations within the boundaries of the system is required for the evaluation of activity and of a rate equation. They can be computed on the basis of a suitable mathematical model if the kinetics of heat- and mass-transfer arc known or determined separately. It is preferable that experimental conditions for determination of rate parameters should be chosen so that gradients of composition and temperature in the system can be neglected. 

Regime 1 slow reactions, controlled by Rate of chemical reaction < mass transfer (phase equilibria)... 

The describing equation for chemical reaction mass transfer is obtained by applying the conservation law for either mass or moles on a time rate basis to the contents of a batch reactor. It is best to work with moles rather than mass since the rate of reaction is most conveniently described in terms of molar concentrations. The describing equation for species A in a batch reactor takes the form... 

The global transformation rate of a gas-liquid reaction catalyzed by a solid material is influenced by the mass transfer at the gas-liquid boundary and the liquid-solid boundary. Mass transfer and surface reaction occur in sequence, and for fast chemical reactions, mass transfer may affect the reactant concentration on the catalyst surface and, as a result, the reactor performance and the product selectivity. For a gaseous reactant, three mass transfer steps can be identified [113] (1) the direct transfer from the bubble through the thin liquid film near the wall to the catalyst surface (characterized by k aQg), (2) the transfer from the caps (i.e., front and back end) of the gas bubbles to a dissolved state in the liquid slug (characterized by and (3) the transfer of dissolved gas... 

Kong et al. [90] applied the electrochemical approach to the study of a two-phase azo coupling facilitated by reverse PTC. Cyclic voltammetry and chronoamperometry were employed to evaluate quantitatively the rate constants for the reaction. The process was interpreted in terms of an EC mechanism, i.e., diffusion-controlled electrochemical charge transfer followed by a homogeneous chemical reaction. The authors highlighted the usefulness of this approach based on the factors that enable the estimation of the contributions of the chemical reaction, mass transfer, partitioning, and the adsorption of reactants at the interface to the overall two-phase reaction. 

The purpose of this question is to find out whether any passive sink for the iodine has been provided to supplement the natural iodine removal mechanisms, like deposition, adsorption, chemical reaction, mass transfer into the water pool or into the droplets, and pool scrubbing, etc. Table 6 presents a summary of the responses. Except the borax used in the ice condenser of the Loviisa units (Finland) no other passive means have been reported. 

Additional functions can be included in the exchanger design, such as chemical reaction, mass transfer, and mixing, optimising the process considerably. In fact, the Heatric PCHF is also marketed as a reactor - the printed circuit reactor (PCR) (see Chapter 5). 

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