Experimental demonstration of a novel control philosophy for process operation near an unstable steady state

Several studies have shown that process performance may be optimal at an unstable steady state. Conventional control schemes for process operation at an unstable steady state may be difficult to implement due to physical constraints or impossible control actions. Following a new control philosophy, stable operation near an unstable steady state can be realized using a relay with hysteresis as a feedback controller. Properly designed, this control scheme produces a stable, small amplitude oscillation in the neighborhood of the desired state. Characteristics of this oscillation can be obtained through process simulation using graphically determined parameters (Van Heerden diagram). Approximate solutions of such oscillation can also be obtained using Tsypkin's method of analysis. This analysis also forms the basis of design principles.

The practical feasibility of the new control policy has been successfully demonstrated. The experimental system used was the isothermal autocatalytic reaction in a CSTR between bis-trichloromethyl-triculfide in methanol and an excess of aniline. A PDP-15 computer was used to implement the control action.

Designed oscillation characteristics were 0.2178 rad./min. and 28.83 min. for frequency and period of oscillation, respectively. Experimentally determined values were 0.3452 rad./min. and 18.20 min. for frequency and period of oscillation, respectively.

Discrepancy of results have been attributed to possible kinetic changes and experimental errors in setting parameters according to design specifications. A major identifiable source of error was the sometimes inaccurate measurement of the process output variable.