This book written by one of the most pretigious Lehigh Chemical Engineering professor Bill Luyben is like the Holy Bible to distillation column control. It’s colloquially known as “the blue book”, and it’s been read and passed on from hands to hands. Similarly to its infamous brother, “the yellow book” which was used in the first semester in process design, it covers a ton of detail and everything you can learn so much from going through page to page. Here I would like to share with you some insights on distillation column controls strategies.
There’re tons of distillation column control schemes out there. It’s very hard to decide which control scheme to use, especially choosing the ones that works with your column and ones that work correctly. Most importantly, you must choose what you like to control to meet your control objectives. For my project, I have to ensure the product meets a certain required purity, and certain required vapor distillate flow rate. My process can be considered a binary process because the main components in this separation processes are Ethylene Oxide and water.
Generally, an effective operation of a distillation column needs to consider the control of the following variables:
- Composition of the distillate stream, xD to ensure product quality
- Composition of the bottoms stream, xB to also ensure product quality
- Liquid level in the condenser reflux drum to maintain inventory and satisfy material balance
- Liquid level in the sump to also maintain inventory and satisfy material balance
- Column pressure to ensure equilibrium.
These variable should always be controlled in a distillation column. However, how you want these to be controlled is up to you. However, there’s a few rules that applies to most cases. For example, it’s always a good idea to flow control the smallest product flow. In my process there is a relative small distillate flow rate in comparison to a large bottoms flow rate. The material balance must satisfy around the condenser drum. In order to make sure the liquid level remains constant, a level controller must be used in the reflux drum. For the same reason, a level controller is required on the sump with the manipulated variable being the bottoms flow rate.
It’ll make a lot of sense to use a energy balance control scheme when there’s a small reflux ratio. If the reflux rate is small in comparison to the distillate rate, then a relatively small change in the distillate rate will ensure a good condenser level control. To control the composition of the product, a on-line composition control is not often used because it’s very expensive and unreliable. One typical way to do this is to control the temperature of the stripping section of the column.
Fortunately, my column falls into one the standard categories, which has made it easier to choose from. However, right now it’s still in progress. I’ll upload it once it’s finished and ready.