Reactor Temperature Control
Figure 1. Cascade SISO logic, diagram b, provides much faster response than more obvious option.
However, this approach poses a problem that may not be apparent. It combines two dominant lags in series, making the system respond relatively slowly to disturbances. The first large lag is that changes in Reactant B must change the composition before reaction changes occur. The second lag is that the composition change then has to change the reaction rate before heat generation changes take place. Both of these have relatively long time constants in the system. Reactant B feed rate is small compared to the volume in the reactor. Second, the desired product forms at a relatively low temperature — hence at a relatively low reaction rate. Excursions, especially to higher-than-desired temperature, take the control system too long to correct.
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One proposal was to use an advanced predictive controller to simultaneously change both the cooling-water and reactor rates. This would require extra instrumentation on both the cooling-water-supply pressure and temperature and a much more complicated control system. My personal experience is that most advanced control applications get turned off and abandoned sooner rather than later. Control groups simply don't have time to continuously adjust them for process and plant changes.
We needed a simpler alternative and came up with a solution that uses cascade single-input single-output (SISO) logic (Figure 1b). Reactor temperature controls cooling water supply. Cooling water valve position then cascades to Reactant B feed rate control. This provides much more rapid response. Position changes in the cooling water valve occur instantaneously with water flow rate. The cascade approach acts as fast as any advanced control system that attempts to simultaneously move both the Reactant-B and water-supply control valves. And, unlike advanced control methods, the logic is clear and maintenance requirements are minimal.
Choosing a reasonably high setting (90%) for the cooling water valve position gives a cooling water rate very close to the maximum possible. Reactor temperature control is stable; both reactant and utility changes are quickly dampened.
Over the past 20 years, development of really good valve positioner technology has created many opportunities to improve plant control with straightforward SISO logic. The solution here is still simple, just different from the obvious.
Andrew Sloley is a Chemical Processing Contributing Editor. You can e-mail him at [email protected].