CSTR-PFTR

With two equal-volume reactors, the overall conversion in these two reactors is independent of which reactor is first but in general it is a common strategy to use a CSTR first where the conversion is low and then switch to a PFTR as the conversion becomes high to rninirnize total reactor volume. The total residence time from CSTR+PFTR in series is indicated by the 1/ r plots in Figure 3-1 1. 

Gas residence time 0.5 to 1.3 s gas velocity 3 to 10 m/s Re > 10, L/D > 100. To eliminate backmixing, Pe > 100. Liquid residence time 1 to 6 s liquid velocity 1 to 2 m/s Re > 10, L/D > 100. PFTR is smaller and less expensive than CSTR. PFTR is more efhcient/volume than CSTR if the reaction order is positive with simple kinetics. For fast reactions, nse small-diameter empty tube in turbulent flow. For slow reactions, use large-diameter empty tubes in laminar flow. If reaction is complex and a spread in RTD is harmful, consider adding motionless mixer (Section 16.11.6.10). Examples hydrolysis of corn starch to dextrose polymerization of styrene hydrolysis of chlorobenzene to phenol esterification of lactic acid. Gas-liquid see transfer line. Section 16.11.6.9, or bubble reactors. Section 16.11.6.11. Liquid-liquid see transfer line. Section 16.11.6.9, or bubble reactors. Section 16.11.6.11. 

CSTR + PFTR adiabatic or PFTR if the reaction rate decreases monotonically with conversion... 

A —> P fej C 2 > 1 A- W P - W 2 ikj c Combo CSTR + PFTR PFTR + CSTR PFTR with large recycle series of CSTR ... 

Liquid Residence time 0.4-2000 s with the usual values 1-6 s liquid velocity 1-2 m/s Re > 10", I/D > 100. PFTR is smaller and less expensive than CSTR. PFTR is more efficient/volume than CSTR if the reaction order is positive with simple kinetics. 

The chemical reactor is the unif in which chemical reactions occur. Reactors can be operated in batch (no mass flow into or out of the reactor) or flow modes. Flow reactors operate between hmits of completely unmixed contents (the plug-flow tubular reactor or PFTR) and completely mixed contents (the continuous stirred tank reactor or CSTR). A flow reactor may be operated in steady state (no variables vary with time) or transient modes. The properties of continuous flow reactors wiU be the main subject of this course, and an alternate title of this book could be Continuous Chemical Reactors. The next two chapters will deal with the characteristics of these reactors operated isothermaUy. We can categorize chemical reactors as shown in Figure 2-8. 

We will define these descriptions of reactors later, with the steady-state PFTR and CSTR being the most considered reactors in this course. 

I CSTR and that the residence time and reactor volume required are considerably i smaller in a PFTR than in a CSTR. 

Thus, while the PFTR reactor volume is much smaller than the CSTR for this conversion, the PFTR tube length may become impractical, particularly when pumping costs are considered. 

Obviously, batch and PFTR will give the same conversion, but the CSTR gives a lower conversion for the same reaction time (batch) or residence time (continuous). 

We can immediately see major reactor design considerations between batch, CSTR, and PFTR. Table 3-1 shows the first of many situations where we are interested in the design of a reactor. We may be interested in choosing rninimum volume or many other process variables in designing the best reactor for a given process. 

Comparisons of Possible Advantages (- -) and Disadvantages (—) of Batch, CSTR, and PFTR Reactors... 

Ratio of Residence Times and Reactor Volumes in CSTR and PFTR versus Conversion for a First-Order Irreversible Reaction... 

It is clear that the CSTR quickly becomes extremely large (large volume or large residence time) compared to the PFTR for high conversions for these kinetics. It is instructive to plot Ca/Cao versus z to see how Ca decreases in the two ideal reactors (Figure 3-3). 

Figure 3-3 Plots of Ca (r) and Ca (t) in PFTR and CSTR for a first-order irreversible reaction A -> B, r = Ca. By plotting versus let, the graphs appear identical for any value of k.

Figure 3-5 Residence times in CSTR (shaded rectangle) and PFTR (area under curve) from the 1/r plot.

From these graphs of 1 /r versus Cao Ca w can construct the residence times in PFTR and CSTR, as shown in Figure 3-5. 

Thus it is evident that a PFTR is always the reactor of choice (smaller for greater than zero-order kinetics in an isothermal reactor. The CSTR may stUl be favored for n > 0 for cost reasons as long as the conversion is not too high, but the isothermal PFTR is much superior at high conversions whenever n > 0. 

The question of choosing a PFTR or a CSTR will occur throughout this book. From the preceding arguments it is clear that the PFTR usuaUy requires a smaller reactor volume for a given conversion, but even here the CSTR may be preferred because it may have lower material cost (pipe is more expensive than a pot). We will later see other situations where a CSTR is clearly preferred, for example, in some situations to maximize reaction selectivity, in most nonisothermal reactors, and in polymerization processes where plugging a tube with overpolymerized solid polymer could be disastrous. 

These expressions are appropriate whether or not the density of the fluid varies with conversion. The latter is also valid if the cross section of the tube varies with z. Note the similarity (and difference) between the CSTR and PFTR expressions, the first being Fao times X/r(X) and the second Fao times the integral of dX/r X). 

This expression can be inserted into the CSTR and PFTR mass-balance equations to yield... 

These equations are significantly more complicated to solve than those for constant density. If we specify the reactor volume and must calculate the conversion, for second-order kinetics we have to solve a cubic polynomial for the CSTR and a transcendental equation for the PFTR In principle, the problems are similar to the same problems with constant density, but the algebra is more comphcated. Because we want to illustrate the principles of chemical reactors in this book without becoming lost in the calculations, we win usually assume constant density in most of our development and in problems. 

---