Editor's Choice


Case History 150: Mystery in distillation column temperature control.

September 2016 Editor's Choice

The longer I work in the field of control loop optimisation, the more I learn that things often do not work the way one expects. I have spent over 30 years optimising literally thousands of feedback control loops and have gained tremendous confidence and experience. However, I still frequently encounter new and sometimes strange happenings, which need a lot of thought and experimentation to try and understand how to control them in the best possible way. A case in point being when I was recently requested to optimise the main temperature control on a distillation column.

Distillation column dynamics

A portion of the fluid that collects at the bottom of a distillation column is recycled through a steam-heated heat exchanger generally known as a ‘reboiler’. The temperature at the bottom of the column is kept constant by regulating the amount of steam passing through the reboiler. Figure 1 shows a few of the controls normally present on distillation columns.

Figure 1.
Figure 1.

I have in the past optimised the controls on quite a few columns, and generally, the dynamics in a reboiler temperature control process consists of a simple self-regulating process, with a large first order lag and a small deadtime. Like the vast majority of slow temperature controls, it takes time and patience to do the necessary testing for successful tuning of the temperature controller. However, it is not difficult if you have the right analytical and tuning equipment.

This time it was not so straightforward

Figure 2 shows the open loop step test which is used for tuning on such a process. (As one can only make very small changes on the temperature when performing the test, it is necessary to draw a ‘best-fit’ line through the noise of the PV signal to establish the true dynamic constants.)

Figure 2.
Figure 2.

Nearly all the reboiler temperatures I have tuned have worked extremely well after optimisation. However, the one under discussion here proved to be very difficult. Figure 3 shows the open loop test performed on the temperature.

Figure 3.
Figure 3.

Firstly, for some unknown reason, the process behaved more as an integrator than as self-regulating, and although the test was repeated quite a few times, the temperature just seemed to keep on integrating when a step change was made. This does not make much sense, and I had some in-depth discussion with several experienced process engineers, but nobody has been able to explain why this particular column reacted in this way.

Secondly, immediately the output of the controller was stepped, there was a very fast rise in temperature which eventually slowed down and then came slowly back until the ramping temperature started it moving up again. This is a most unusual response, and we repeated the test many times, and this time trended most of the other controls. It was then found that for some reason the pressure, measured at the top of the column, reacted immediately when the output of the temperature control moved. This again does not make much sense, as the temperature itself has not had time to change under the influence of the change in steam, but the pressure change caused the temperature to ‘jump’ virtually immediately. Again, no one has been able to offer a rational explanation for this.

Michael Brown.
Michael Brown.

Tuning as an integrator solves the problem

We finally tuned the temperature as a ‘straight’ integrator as shown by the dotted line in Figure 3. The final tuning worked extremely well. There is definitely some interaction between column pressure and temperature, but it is not serious.

This is not the first time we have come across mysterious and unexpected happenings in the control world for which no rational explanation seems to exist. I would welcome any readers’ comments on this. Possibly someone else has experienced a similar phenomenon?

Michael Brown is a specialist in control loop optimisation with many years of experience in process control instrumentation. His main activities are consulting, and teaching practical control loop analysis and optimisation. He gives training courses which can be held in clients’ plants, where students can have the added benefit of practising on live loops. His work takes him to plants all over South Africa and also to other countries. He can be contacted at Michael Brown Control Engineering cc, +27(0)82 440 7790, michael.brown@mweb.co.za, www.controlloop.co.za



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Case History 191: The weakest link.
Michael Brown Control Engineering Valves, Actuators & Pump Control
Which is the weakest link in a control loop? The answer, without any doubt is that, in most cases, the final control element is the weakest link.

Read more...
Bringing Industry 4.0 to a castings foundry for heavy industries
Editor's Choice
Moving to Industry 4.0 takes time and determination, especially for an established company in a heavy industry. Castings foundry, POK in Mexico has moved toward Industry 4.0 in a series of steps over several years, changing from manual to automated systems for more available, immediate and reliable data.

Read more...
SMOM – the future is here now
Iritron Editor's Choice IT in Manufacturing
In his presentation at the recent MESA Africa conference, Neels van der Walt, business development manager at Iritron, revealed the all-encompassing concept of smart mining operations management (SMOM), and why it is inextricably linked to the future of worldwide mining operations.

Read more...
Navigating disruption in manufacturing
Editor's Choice IT in Manufacturing
When considering IT in manufacturing, the underlying assumption is twofold: first, a wave of valuable maturing technologies can be harnessed to create new business value, and second, the environment in which these technologies will be applied will be relatively predictable, with change following a manageable, evolutionary path. However, recent disruptions have shattered these assumptions.

Read more...
The fascination of movement
Editor's Choice Motion Control & Drives
A motor from Faulhaber provides gentle motion for the finest watches in the world.

Read more...
Complete system for transparent energy monitoring
Beckhoff Automation Editor's Choice Electrical Power & Protection
Transparent energy monitoring reduces both machine downtime and the necessity to oversize the corresponding components. Added to these advantages are simplified preventive maintenance, and increased production efficiency. The wide range of PC-based control technology from Beckhoff offers a solution that can be optimally adapted to individual applications.

Read more...
Iritron’s year of consolidation
Iritron Editor's Choice System Integration & Control Systems Design
Despite the multiple challenges faced by businesses in South Africa, the buoyancy of the technology sector worldwide has produced some green shoots for automation specialist, Iritron.

Read more...
Five edge opportunities for SIs to maximise revenue in 2024
Editor's Choice System Integration & Control Systems Design
System integrators continue to face the challenge of doing more with less – supporting complex operations, while meeting production schedules with limited resources, and innovating to increase efficiency, maximise safety and reduce risk.

Read more...
WEG Africa applauds energy standards for motor efficiency
WEG Africa Editor's Choice Electrical Power & Protection
In a country where most electric motors are not energy efficient, the announcement by South Africa’s government of its plans to introduce Minimum Energy Performance Standards is not a day too soon.

Read more...
World first planetary gearbox customisation
SEW-Eurodrive Editor's Choice Motion Control & Drives
When a mining operation in Mpumalanga found that one of its planetary gearboxes on a high-torque conveyor was failing, it called SEW-EURODRIVE for a solution. Leveraging the latest intelligent technology, the solution was a pioneering innovation providing a cost-effective replacement that will ensure reliable performance.

Read more...