Ask Greg McMillan - What role do you see dynamic simulation playing in the future of best distillation column temperature control strategy?

Greg's Response:

Dynamic simulations running faster than real time are key to ensuring the best column temperature control strategy. The manipulation of separation by the adjustment of the energy or vapor per unit feed (V/F ratio) has a dramatically faster response. The process dead time and time constant are seconds rather than hours. Now, the measurement time constant is no longer negligible and may, in fact, be the largest lag in the loop. The manipulation of separation to control composition is not useful except for high purity columns and two-point composition control, because the change in steam and vapor flow required is exceptionally large to get a temperature change. Extreme changes in reboiler steam, which change the column traffic of both vapor and liquid, disrupt column operation and upset the pressure and level loops. Thus, for most columns, temperature is best controlled by the manipulation of split (material balance).

The manipulation of distillate flow by temperature control gives internal reflux control that inherently compensates for ambient temperature changes that can be sudden (e.g., cold rain storms) and minimizes interaction between the material and energy balances but requires high overheads level controller gains (e.g., > 15). A distillate flow much smaller than the reflux flow would cause poor level control and lead to the manipulation of distillate for temperature control.  The manipulation of reflux flow eliminates the dependence upon overheads level control for temperature control but causes interaction between the material and energy balances. The manipulation of reboiler steam for temperature control is fast but is adversely affected by a low bottoms level controller gain, thermowell lag, and severe interaction with the energy balance. The manipulation of bottoms flow offers fast bottoms temperature control, but the inverse response in sump level to steam flow changes can severely disrupt or disable level and separation control.

Process simulation and field test results may also show only one temperature really needs to be controlled. Many two point composition control problems simplify to a single point control system after the best strategy is chosen for the dominant temperature loop and improvements, such as better temperature and flow measurements and feedforward (e.g., ratio control) are implemented.

For much more knowledge, see the ISA book Advanced Temperature Measurement and Control, Second Edition (LIST PRICE REDUCED by 50%. Use promo code ISAGM10 for an additional 10% discount on Greg’s ISA books).