Converting From Steady State To Dynamic Simulation

We are now ready to get into the details of converting a steady-state simulation into a dynamic one. Basically the additional information that must be provided is the physical dimensions of the various pieces of equipment.

It is important to remember that pumps and control valves have already been installed in the steady-state simulation. These are not necessary for steady-state simulation, but they are vital for a realistic dynamic simulation. Providing sufficient pressure drop over a control valve at design conditions with the valve at some fraction opening (typically 50%) is crucial for dynamic controllability. If valve pressure drop is too small, changing the valve opening from 50% to 100% will, in piping systems with other equipment taking pressure drops, result in only a fairly small increase in flowrate. If a valve saturates, controllability is lost.

The propane/isobutane column developed in Chapter 3 is used in this chapter as a numerical example. The control valves all have pressure drops of ^3 atm. The column has 32 stages, is fed on stage 14, operates at 16.8 atm, and produces distillate and bottoms products with impurity levels of 2 mol% isobutane and 1 mol% propane, respectively.

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