Fast-fill-and-hold

A simpler (and faster) way to achieve a bumpless startup is to fast fill and hold. In the limiting case, the fill is instantaneous and the reactor acts in a batch mode until the steady-state concentration is reached. 

Example 14.4 Compare the fed-batch and fast-fill-and-hold methods for achieving a bumpless startup. 

The fast-fill-and-hold method instantaneously achieves a full reactor, the concentration in which follows batch, first-order kinetics until the desired Uout is reached. Equation (14.5) is equated to exp(—An analytical solution is possible for this case ... 

Obviously, the fast-fill-and-hold method is preferred from the viewpoint of elapsed time. More importantly, the fed-batch method requires an accurate process model that may not be available. The fast-fill-and-hold method can use a process model or it can use a real-time measurement of concentration. 

Devise a fast-fill-and-hold startup strategy for the reaction of Problem 14.1. 

Suppose there are two parallel, first-order reactions in a steady-state CSTR. Show that neither the fed-batch nor fast-fill-and-hold strategies can achieve a bumpless startup if the reactions have different rate... 

Suppose the reactor in Example 14.8 remains in batch mode after the fast-fill-and-hold startup. Will the temperature control system still work A preliminary answer based on the approximate kinetics of Example 14.7 is sufficient, but see the next problem. 

Suppose the reactor has been started using the fast-fill-and-hold method and has reached a = 0.65 at 7 = 420 K. Continuous flow is started with ain= 1, Y , = 375 K, and Ts,i = 404K. Figure 14.4 shows the response. The temperature response is very rapid, but the conversion increases slightly during the first seconds of operation. Without temperature control, the reaction would have run away. The concentration is slowly evolving to its eventual steady-state value of about 0.26. There is a small offset in the temperature because the controller has no reset term. 

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