![]() Steady-state simulations can continue to be used for optimization. The matrix relative gains and condition number are also used for model predictive control design. For this and many other reasons, steady-state simulations are preferably used as a starting point to find the best temperature sensor locations (e.g., best tray) and the process gains that are essential for PID feedback and feedforward gains, as well as relative gain analysis (RGA) to determine the best pairing of controlled and manipulated variables and decoupling needed. There has also been a propensity for the need to completely start over in creating a dynamic model and the dynamic model falling short in terms of the intensity of detail in the process and equipment models seen in steady-state models. This is rarely possible because often the dead times and time constants associated with the control system are proportionally reduced and the control tuning changed accordingly (e.g., PID gain and rate time decreased and PID reset time increased). Variable speed up seen in some simulations is not a viable option unless all the control system dynamics have the same speed up. ![]() Consequently, dynamic simulations require patience and some fixed speedup to see the final responses of the control system. ![]() The time to steady state can be hours or days. The user can then concentrate on getting and entering the physical parameters of equipment (e.g., volumes, diameters, number and geometry of trays) and the physical properties, flows and operating conditions of all streams and fluids in the equipment. The use of simulation software with advanced modeling objects for a wide spectrum of equipment designs is especially important. Simulations of distillation columns can offer incredible benefits in finding, implementing, improving and optimizing the best control system, but the effort to achieve the high-fidelity dynamic simulation needed is considerable. Greg: We conclude our discussion of distillation control best practices with Mark Darby of CMiD Solutions and Doug Nicholson of Spiro Control by offering additional knowledge to provide essential guidance on how to make the most out of simulation and optimization opportunities. ![]()
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