In this 3-part series on distillation column control basics, we’ll look at traditional control and modern approaches to improve control robustness. For background, you can find a good write up on industrial distillation in Wikipedia.
In the distillation process, a feed mixture is separated by volatility or boiling point into 2 or sometimes more component streams. Energy in the form of heat is introduced at the bottom of the column in the reboiler and removed at the top of the column in the overhead condenser.
Closing the material and energy balances mean that at a minimum, the sum of the product flows must equal the feed flow and the enthalpy introduced with the feed and in the reboiler must be removed with the product streams and in the overhead condenser.
The product streams will not be perfectly pure and don’t need to be. Product purity is specified based on the subsequent use of the product stream. Purity is directly related to the amount of energy that is used in the process. Energy, itself, has a cost, so there is the competing objective of meeting purity specifications with minimum use of energy.
Traditional Control. The traditional approach to dual composition control of continuous distillation is simply to control the overhead temperature with reflux flow and control the bottom temperature with heat (usually steam) to the reboiler. These may or may not be configured as cascade loops with the temperature controller outputs directly controlling valves or providing a setpoint to the flow controllers.
Temperatures are selected as inferential measurements related to product composition, because usually on line analyzers are not available or are impractical. Level in the reflux accumulator and base of the column are typically controlled with the product flows. Again, there may be a cascade between level and flow or the level controllers may manipulate the control valves directly.
This kind of strategy is simple to implement and works for most distillation processes. Pressure can be controlled in different ways depending on whether there is non-condensable gas in the overhead that must be vented or not. Feed to the column is seldom available for control as part of the column controls as it is usually coming from somewhere else.
The feed flow rate, temperature, and composition can all vary creating disturbances to the column. In the case of a side draw product, the side draw flow is often controlled manually or it may be manipulated to control a nearby temperature.
However, there are often better ways to configure the distillation controls and as a workhorse unit operation in the process industries, it is worthwhile to take a deeper look at how to control a column. Distillation is a very interactive process. Changing the reflux flow, for example, will change the composition at both the top and the bottom.
In fact, changing the flow of any of the main loops will result in a change in the levels in the short term and the compositions in the longer term. In fact, the dynamic response of the controlled variables can be quite different for different manipulated variables and even for the same manipulated variable. Reflux flow, for example, will have an immediate effect on overhead composition, but a slower effect on bottom composition.
Some of the key measurement devices include Rosemount 3051S transmitters for pressure and differential pressure and Rosemount 848T high density temperature transmitters and sensors.
Still, the distillation process is fairly forgiving and single loop controls will usually work to some extent. However, improved control can result in increased throughput, reduced unit energy consumption and or yield improvements of the more valuable product. And these have real quantifiable benefits.
Part 2 of our series will look at inferential composition measurement and control design techniques.
Part 3 of the series will look at how to close the material balance and how advanced process control tools such as model predictive control can help in operating the distillation columns closer to constraints.
From Jim: You can connect and interact with other process control experts in the Control & Safety Systems groups in the Emerson Exchange 365 community.