Real Reactors and Residence Time Distribution (RTD)

In Chapter 2, the design of the so-called “ideal reactors” was discussed. The reactor “ideality” was based on defined hydrodynamic behavior. We had assumed two flow patterns: plug flow (piston type) where axial dispersion is excluded and completely mixed flow achieved in ideal stirred tank reactors. These flow patterns are often used for reactor design because the mass and heat balances are relatively simple to treat. But real equipment often deviates from that of the ideal flow pattern. In tubular reactors radial velocity and concentration profiles may develop in laminar flow. In turbulent flow, velocity fluctuations can lead to an axial dispersion. In catalytic packed bed reactors, irregular flow with the formation of channels may occur while stagnant fluid zones (dead zones) may develop in other parts of the reactor. Incompletely mixed zones and thus inhomogeneity can also be observed in CSTR, especially in the cases of viscous media. The abovementioned phenomena lead to a nonuniform residence time of the fluid elements in tubular reactors, which may have a detrimental effect on the reactor performance and product yield. In this chapter, residence time distribution (RTD) of ideal and nonideal reactors along with the method of determination are described in detail. The influence of nonideality and RTD on the reactor performance, the target product yield, and selectivity, including complex reactions, is presented.