Mass Transport and Fluid Dynamics Properties

The above values are most often determined via correlations, which allow a scale-up (or down) to different operating states. Along these fines, the liquid and gas phase mass transfer coefficients are usually related to Sherwood number (Sh) as a function of Reynolds number (Re), Schmidt number (Sc) and other dimensionless process characteristics [3, 59-61]. It is important that the correlations are applied within the same parameter range in which they are determined as only there can their reliability be assured. 

The selection of a proper correlation is mostly a question of user experience. Basically, the mass transfer correlations must be compared and validated with experimental data because the application of different correlations can lead to different simulation results (e.g., axial concentration profiles). Some correlations which -according to our experience - demonstrated their suitability for reactive absorption processes can be found in [60-62], 

The reaction rate of a homogeneous kinetically controlled reaction 

Measurements of kinetic parameters of liquid-phase reactions can be performed in apparata without phase transition (rapid-mixing method [66], stopped-flow method [67], etc.) or in apparata with phase transition of the gaseous components (laminar jet absorber [68], stirred cell reactor [69], etc.). In experiments without phase transition, the studied gas is dissolved physically in a liquid and subsequently mixed with the liquid absorbent to be examined, in a way that ensures a perfect mixing. Afterwards, the reaction conversion is determined via the temperature evolution in the reactor (rapid mixing) or with an indicator (stopped flow). The reaction kinetics can then be deduced from the conversion. In experiments with phase transition, additionally, the phase equilibrium and mass transport must be taken into account as the gaseous component must penetrate into the liquid phase before it reacts. In the laminar jet absorber, a liquid jet of a very small diameter passes continuously through a chamber filled with the gas to be examined. In order to determine the reaction rate constant at a certain temperature, the jet length and diameter as well as the amount of gas absorbed per time unit must be known. 

Our own dynamic experiments for the determination of the gas-liquid-reaction kinetics have been performed in a stirred-cell reactor (Fig. 9.6). After thermodynamic equilibrium is reached inside the reactor, the gas is introduced rapidly and the pressure decrease recorded as a function of time. From this course, the reaction rate constant at the respective temperature can be obtained. 

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