Reaction influence reactor dimensions

Laminar flow reactors are equipped with microstructured reaction chambers that have the desired low Reynolds numbers due to their small dimensions. Mass transport perpendicular to the laminar channel flow is dominated by diffusion, a phenomenon known as dispersion. Without the influence of diffusion, laminar flow reactors could not be used in heterogeneous catalysis. There would be no mass transport from the bulk flow to the walls as laminar flow, in contrast to turbulent flow, cannot mix the flow macroscopically. 

The reaction is also influenced by the heat of reaction developing during the conversion of the reactants, which is a problem in tubular screening reactors. In microstructures, the heat transport through the walls of the channels is facilitated by their small dimensions, which allows the development of isothermal reaction conditions. Thus, by decoupling the heat and mass balance, an analytical description of the flow in the screening reactor is achievable. 

In macroscopic reactors, knowledge of the velocity profile in the channel cross-section is a necessary and sufficient prerequisite to describe the material transport. In microscopic dimensions down to a few micrometers, diffusion also has to be considered. In fact, without the influence of diffusion, extremely broad residence time distributions would be found because of the laminar flow conditions. Superposition of convection and diffusion is called dispersion. Taylor [91] was among the first to notice this strong dominating effect in laminar flow. It is possible to transfer his deduction to rectangular channels. A complete fluid dynamic description has been given of the flow, including effects such as the influence of the wall, the aspect ratio and a chemical wall reaction on the concentration field in the cross-section [37]. 

The activities are at present related to two major topics, namely chemical production and mass screening [3, 8, 25]. While miniaturization in the first field focuses on new process regimes due to enhanced heat and mass transfer, the second field of application results in an increase of reaction and detection units per reactor volume. Thus, miniaturization - the reduction of characteristic dimensions - directly influences the process performance in the first case, while in the last case this is only indirect by an increase in flexibility and multiplicity. 

The packed bed reactor is used to contact fluids with solids. It is one of the most widely used industrial reactors and may or may not be catalytic. The bed is usually a column with the actual dimensions influenced by temperature and pressure drop in addition to the reaction kinetics. Heat limitations may require a small diameter tube, in which case total through-put requirements are maintained by the use of multiple tubes. This reduces the effect of hot spots in the reactor. For catalytic packed beds, regeneration is a problem for continuous operation. If a catalyst with a short life is required, then shifting between two columns may be necessary to maintain continuous operation. 

With the aid of the kinetics, dimensioning of the reactor can be performed. Other important quantities that influence the economics of the entire process are the selectivity and the conversion. If the kinetics are known, both quantities can be optimized and thus the yield (= selectivity x conversion) maximized. First we must define these quantities. Consider the reaction of starting materials A and B to give product P (Equation 3.1.4-1) ... 

In upflow bubble operation the consumption of the gas phase by reaction must also be considered in the model if the reactor operates under lower pressure (<20 bar) and if the reactor length is of technical dimensions (L>2 m) additionally gas phase dispersion (radial and axial) may have an influence on conversion [65]. As this reactor type is also used in waste water treatment as well as in fermentation processes, the possible non-Newtonian behavior of the liquid phase as well as the coalescence behavior of the system must be taken into account. Finally, it should be remembered that - comparable to fluidized bed reactors - results from laboratory reactors with small column diameter and/or particle sizes smaller than 0.2 cm usually cannot be regarded as representative for technical upflow units, because capillary force as well as lare scale circulation in the liquid phase may be significantly different. 

For reactor scale-up for a given system, one may proceed as follows. First, a stirred bench scale reactor should be chosen with standardized dimensions. The net superficial gas flow rate should be kept within the indicated limits. Mass transfer measurements (with or without chemical reaction) should be carried out under realistic conditions. The influence of the stirrer speed should be measured accurately over a wide range. The superficial gas flow rate may also be varied, e.g., between 1 and 4 cm/s. With these experiments the constants in eq. (4.62) can be determined. The resulting equation may be used with confidence for the larger scale. The variables e and w, on the larger scale, should be within the limits of the bench scale tests ... 

In true single phase batch reactors the reactants are mixed before the reaction starts. It would seem that transport of matter could never influence the course of chemical reactions then, since a mixture remains mixed. The performance of the reactor could then not be scale dependent. The ideal models presented in section 3.2.1 would then always be applicable, as long as the reactor can be considered isothermal. That is mostly true, but deviations from the ideal situation may occur when the reaction approaches complete conversion. The concentrations of the reactants may become so low, that the average diffusion path becomes large in relation to molecular dimensions, so that diffusion times can no longer be neglected. This is only of practical importance in exceptional cases. Interesting examples are certain polymerizations and polycondensations, see sections 13.3.1 and 13.7. 

The previous variables are dominant and are called thermal variables. Other variables exist, however, that it is sometimes appropriate to examine. This is, for instance, the wavelength and the light intensity in photochemical reactions, which are influenced by hght, or the electric potential in electrochemical reactions, which involve electrons as a reactant or a product of the reaction. Extensive variables should be added to these intensive variables, such as the amounts of substance, the shapes and dimensions of reaction zones and reactors. 

---