How to configure PID structures for distillation columns

Posted by Christiaan Moons on Jun 7, 2021 10:03:33 AM

As a control engineer, you may get called by plant operators when a PID loop is a “bad actor”, causing problems on a distillation column. The process upset caused by this PID loop may have a destabilising impact on downstream process units as well, potentially causing widespread impact. Quite often, the “quick fix” for process operators is to insert these bad acting PID loops in manual. While this mitigates the immediate issue, the cost of such actions is poor performance of the distillation column. The control engineer is tasked with rectifying the problem. Is this a PID tuning issue or are the PID loops on the distillation column configured incorrectly? How would you ensure that the PID structures on a distillation column are configured correctly? You can read it in this blog.

Finding a root cause can be complicated

While investigating an issue with a particular PID loop on a distillation column, the root cause may not be easily apparent. As a control engineer, you need to keep an open mind and investigate the possibility that the PID loop is incorrectly configured from the time the plant was commissioned.

There are many other possibilities such as faulty instruments, poor PID tuning, interacting PID loops, etc. The distillation column designs are also very different, the heat input may be a furnace, waste heat from another unit, steam, or a combination. Temperature control in the distillation column may be the top temperature or sometimes a tray temperature. How do you navigate all these possibilities and configure a PID structure that is best suited for your distillation column?



Configure the correct PID structure

Typically, a distillation column will have a product quality that needs to be controlled tightly. The product quality is well-correlated with a temperature and therefore, it’s essential to control the temperature profile tightly. A test can be conducted to pinpoint the most sensitive tray. This can be done by changing the reboil duty and measuring the change in the temperature profile.

A similar test can be done for the reflux flow. The temperature that responds the most to these changes can be used to form a cascade with either the reflux or reboiler. This approach will identify whether the basic configuration is correct. A control engineer can focus on investigating other issues such as PID tuning, instrument issues, and type of heat input, once this structure is in place.

For example, when using steam as a heat input, it may be worth exploring if the steam header pressure is stable and whether it would be necessary to check and tune the steam system. It is also good practice to have EITHER a top temperature PID loop OR a bottom temperature PID loop, not both. It may be necessary to tune the column pressure, reflux drum level, and top temperature all together as a single, interactive PID tuning problem.


No more operator complaints!

Distillation forms a core activity in many refining, chemical and petrochemical processes. Getting the PID structure correct will stabilise the operation and maintain on-spec product qualities, reduce process alarms, and ultimately reduce operator stress. Control engineers will also be spared a daily dose of complaints!

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Topics: PID tuning