Feature Report
FIGURE 3a. In a plug flow reactor, material is only retained in the reactor while it is reacting, and it cannot be discharged until the required residence time has elapsed
Plug flow Plug flow describes a flow pattern where the fluid velocity across the face of a channel is uniform (Figure 2). It also implies no back mixing and all fluid elements having the same residence time. While plug flow is required for most applications, ideal plug flow cannot be achieved in practice due to the effects of molecular diffusion, surface drag, bends and obstructions in the channel. Microreactors typically have channel diameters of less than 0.5 mm. In these systems, the plug flow characteristics benefit from low fluid velocities [2]. This is a useful characteristic as the reaction time can be varied from less than a second to many minutes in relatively short channels. In larger diameter channels, good plug-flow characteristics benefit from high fluid velocities [2], and the channel length has to be varied to suit the reaction time. Good radial mixing is also desirable to prevent the fluid from separating into zones of fast — and slow — moving fluid. Radial mixing can be achieved by means of baffles, static mixing elements, dynamic mixing and turbulent flow. A good approximation to plug flow can also be achieved by using multiple stirred tanks in series, and the higher the number of tanks used the better. Above ten stages, however, the cost of additional stages starts to outweigh the incremental benefits. Plug flow is determined by experimental methods such as dye injection tests. These data need to be treated with care, because the quality of plug flow required will change with the reaction rate. The observed quality of plug flow can also change as the physical properties of the fluid changes. Plug flow provides the means for separating reacted and unreacted material (Figure 3a). Without plug flow, 36
some product is retained for longer than required in the reactor and some unreacted material is discharged prematurely (Figure 3b). Good plug flow is the key to minimizing reactor size and optimizing yield and purity. A common misconception is that plug flow is not required for zero order reactions (where the reaction rate is independent of reactant concentration). This is not correct. Plug flow is important for virtually all processes, even for zero order reactions. The exception to this is fast zero-order reactions (typically a few seconds or less) where reactor size is not a significant cost issue and there are no consecutive reactions. For many applications, the benefits of good plug flow are not limited to residence time control. Where the reaction rate is dependent on reactant concentration (nth order reactions), low back mixing reduces dilution of unreacted material giving faster reaction rates and higher yields per unit volume. Plug flow also provides the means for preventing material from over reacting, and reducing reactions between reactants and product. Where the outcome of a reaction is particularly sensitive to plug flow, high fluid velocities and long channels are desirable. From the perspective of pressure drop and reactor cost, however, there are practical advantages to minimizing channel length. A good compromise is to use long small-diameter tubes where the reaction rate is high (and therefore more sensitive to back mixing) and larger diameter tubes where the reaction rate is slow.
Mixing Mixing can be characterized in many ways, such as applied mixing energy, uniformity of mixing, time to achieve a uniform blend, Reynolds number or mass transfer characteristics for
CHEMICAL ENGINEERING WWW.CHE.COM OCTOBER 2012
FIGURE 3b. An example of a system that is not plug flow is the continuously stirred tank reactor (CSTR), which is fully back-mixed. In the absence of plug flow, reacted material is retained for longer than required, and unreacted material is discharged prematurely. It is therefore inherently less efficient than either a plug reactor or an ideal batch reactor
two phase mixtures. There are also different mechanisms for promoting mixing, such as molecular diffusion, turbulent flow or splitting and folding of fluid streams. In a flow reactor, the optimum mixing condition is good radial mixing (mixing across the tube) with low axial mixing (mixing along the tube). Scale is important for mixing. For example, a small flow reactor can blend a fluid in a fraction of a second, whereas a 1,600-L batch reactor can take 10 to 20 seconds [3]. Microreactors operate under laminar flow conditions with Reynolds numbers of typically less than one hundred [4]. The flow streams are parallel, and these systems rely principally on molecular diffusion for radial mixing. Microreactors have a reputation for good mixing that is not always deserved. For demanding mixing applications, such as with two phase fluids, it is often necessary to employ static mixing elements within the channel to promote radial mixing. The need for good radial mixing becomes increasingly important as the diameter of a flow reactor is increased. Without radial mixing, large tubes bring problems of poor blending, poor plug flow and reduced heattransfer performance. There are two general methods for promoting radial mixing: dynamic