Stirred-reactor regime

The use of a monolithic stirred reactor for carrying out enzyme-catalyzed reactions is presented. Enzyme-loaded monoliths were employed as stirrer blades. The ceramic monoliths were functionalized with conventional carrier materials carbon, chitosan, and polyethylenimine (PEI). The different nature of the carriers with respect to porosity and surface chemistry allows tuning of the support for different enzymes and for use under specific conditions. The model reactions performed in this study demonstrate the benefits of tuning the carrier material to both enzyme and reaction conditions. This is a must to successfully intensify biocatalytic processes. The results show that the monolithic stirrer reactor can be effectively employed in both mass transfer limited and kinetically limited regimes. 

FIGURE 1.8 Flov regimes of gas-liquid stirred reactor (from Middleton, 1992). 

The recent progress in experimental techniques and applications of DNS and LES for turbulent multiphase flows may lead to new insights necessary to develop better computational models to simulate dispersed multiphase flows with wide particle size distribution in turbulent regimes. Until then, the simulations of such complex turbulent multiphase flow processes have to be accompanied by careful validation (to assess errors due to modeling) and error estimation (due to numerical issues) exercise. Applications of these models to simulate multiphase stirred reactors, bubble column reactors and fluidized bed reactors, are discussed in Part IV of this book. 

Van Elk et al. [27] used a similar mathematical model, based on the penetration model for three reactants in an ideally stirred reactor, to study the dynamic behavior of the gas-liquid homogeneous hydroformylation process. The influence of mass and heat transfer on the reactor stability in the Idnetically controlled regime was analyzed and it brought to mind the existence of a dynamically unstable (limit circle) state under certain operating conditions. This model needs to be extended to account for the presence of a second liquid phase in biphasic hydroformylation. 

Thermal regime in stirred reactors is characterized by adiabatic rise of the temperature in the reaction zone [1,8] ... 

Here n - the only parameter of cellular model equal to cells (reactors) number in cascade of ideal stirred reactors, ideal stirring regime is achieved at n —> oo [3], It is accepted [22], that if cells number in reactor n S 8, then such apparatus can be calculated as plug-flow reactor with enough for industrial practice accuracy. 

The other extreme case (local torch regime) is realized at relatively high values of R (type B) (tank stirred reactor). Active sites deactivates having no time for diffusion into reaction volume peripheral parts that in this case are the slip zones of non-reacted monomer. As a consequence specific, complex in configuration fields of monomer, active sites concentrations and temperature are formed. Reaction doesn t reach reactor s walls and product yield due to monomer slipping is always lower than 100% [38,39]. 

Consider now the case of a CSTR. As indicated in section 9.1.1, the behavior of an ideal CSTR is observed when the dispersion inside the reactor is very fast with respect to the residence time. The average concentration, c, is thus kept homogeneous irrespective of time inside the reactor. For a perfectly stirred reactor, we do not distinguish the axial dimension, L, from the lateral dimension. Dr, as we did for the tubirlar reactor (Dr L), because the two scales are usually comparable. Using the dispersion time (eqiration [9.2]) for a single dimension L of the reactor, the hydrodynamic regime of a perfectly stirred reactor therefore corresponds to the following condition ... 

Condition [9.15], which is associated with the regime of a CSTR, implies a high level of turbulence and/or a long residence time. If turbulence is only produced by the mean flow (friction on walls or head loss), the turbulence level produced is usually insufficient to verify [9.15]. For this reason, perfectly stirred reactors usually embody stirrers as the main source of turbulence, as explained in the following section. 

Figure 11.3. Power injected as mechanical energy and dissipation in a perfectly stirred reactor in a steady-state regime...

In the example a gas-liquid reaction with particulate solids (e.g., a catalyst) operating in regime 11 in a stirred reactor with a Rushton turbine is to be scaled up. The primary process requirement is for the same degree of reaction conversion at each scale, which means the same number of moles of gas transferred per mole of liquid fed ... 

When the reaction is performed in an open system, the concentration of the substance changes due to both the chemical reaction and the income of reactants into the reactor and removal of products. In a well-stirred reactor in the steady-state regime of the work, the reaction rate is the following ... 

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