Systems approach for evaluating dynamics and plantwide control of complex plants

Recycle loops strongly affect dynamics and control of complex-plants. The stability of control structures for component inventories may be influenced by recycle interactions and by connectivity which leads to flowsheet alternatives possessing different control properties. A simulation-based methodology for evaluating these phenomena and finding the best flowsheet structure is reported from a controllability point of view. Steady-state and dynamic simulations are combined with controllability analysis tools, both steady state and in the frequency domain, which extracts more value from simulation than the usual sensitivity studies. The case of a vinyl chloride monomer (VCM) plant with impurities handling as a main issue demonstrates the power of the approach. It lends to an unconventional design alternative where an extra impurity-destroying reactor reduces bad dynamic characteristics, since it prevents impurity buildup in the recycle. The steady-state analysis of the VCM case is confirmed at low frequencies of disturbances, but difficulties may occur at higher frequencies, where the disturbances' period and time constants of the distillation columns are of the same order of magnitude. Controller interactions play a significant role in disturbance rejection. Nonlinear dynamic simulations with closed plantwide control loops confirm the analysis results.