Tubular reactor dynamic response

SJi. The initial startup of an adiabatic, gas-phase packed tubular reactor makes a good example of how a distributed system can be lumped into a series of CSTRs in order to study the dynamic response. The reactor is a cylindrical vessel (3 feet ID by 20 feet long) packed with a metal packing. The packing occupies 5 percent of the total volume, provides 50 ft of area per of total volume, weighs 400 ib yft and has a heat capacity of 0.1 Btu/lb °F. The heat transfer coefficient between the packing and the gas is 10 Btu/h It "F. 

The second difference is the dynamic response to disturbances or changes in manipulated variables. In a perfectly mixed CSTR, a change in an input variable has an immediate effect on variables in the reactor. In a tubular reactor it takes time for the disturbance to work its way through the reactor to the exit. Therefore there are very significant dynamic lags and deadtimes between changes made at the inlet of the reactor and... 

Openloop Response The openloop responses of a single adiabatic tubular reactor system to +20% step changes in recycle flowrate FR are shown in Figure 6.9. The solid lines represent increases in recycle flow and the dashed lines, decreases. The results show that the system produces limit cycle behavior, alternating between high temperatures and low temperatures. This type of dynamic response is called openloop-unstable behavior in this chapter. 

The dynamics of four different tubular reactor systems have been explored. The cooled reactor has the best dynamic response because of the internal heat transfer helps to provide some self-regulation. As discussed in Chapter 5, the cooled reactor is also the best from the standpoint of steady-state economics. This combination of both superior steady-state and dynamic performance is quite unusual in chemical processes. The typical situation is that there is a conflict between economics and dynamics. 

These results illustrate the drastic effect of catalyst on the dynamics of tubular reactors. It is clear that a control structure that attempts to control reactor outlet temperature by adjusting reactor inlet temperature will exhibit poor performance because of the inverse response. 

Hudgins 17) has used a sinusoidally varying input in concentration to analyze the frequency response of a reactor for the catalytic dehydrogenation of ethanol using a conventional tubular reactor. Leder and Butt 18) have successfully used frequency response to analyze the dynamic behavior of a fixed-bed catalytic reactor used for the hydrogen-oxygen reaction over a supported platinum catalyst. Hydrogen inlet concentration was varied sinusoidally over frequencies from 2 cycles/ hour to 120 cycles/hour. 

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