To non-process control people, it can be difficult to explain why reducing process variation matters (it is even more difficult to explain why sometimes you should allow variation, but that will be another topic). Reducing variation in the process is the key improvement that the installation of instrumentation and a process control system, such as a distributed control system (DCS), provides. There are substantial financial benefits that can be derived and this needs to be translated into the common language between engineers and accountants, money!
What is process variation?
To understand why reducing process variation can reduce cost (through increased production or decreased raw material usage) we need to explain variation in a process plant.
A shower is a great example of simple process control. If you are in the shower at a temperature you like, life is good. This can be regarded as steady operation at the set point. Someone then turns on the hot water tap in the kitchen. This reduces the hot water in your tap, in turn decreasing the water temperature. Your skin (the instrumentation) detects that you are cold and you, the control system, compensates for this by either increasing the hot water or decreasing the cold water. I think we have experienced this.
How is this related to saving inputs or increasing production you might ask? Hot water costs you money. There is the cost of the water itself and the cost of the electricity or gas used to heat up the water. Let’s say that you are a little quick in turning your tap, that will make the shower too hot, thus wasting valuable hot water, probably causing you to turn the cold tap on too much, making the water too cold. You then keep on adjusting the taps until eventually you find the right temperature. This changing of temperature also reduces the ‘quality’ of your shower which with a six-year-old, is one of the few times I get to relax.
If I was in the shower and I suspected that someone was going to turn on a hot water tap in another room I may deliberately have my shower at a slightly hotter temperature than I would like so that I don’t get too cold if someone turns a hot water tap on. This reduces my enjoyment of the shower but I also don’t want to get too cold at any point as this would be REALLY bad (I hate cold showers).
It is easier for me to control the temperature of the water if I can always feel how hot the water is as I can make little adjustments to get the water to that perfect relaxing temperature. Now imagine your skin only felt the temperature of the water every few minutes. This would make it more difficult to keep the temperature constant and there would bigger swings in water temperature which would waste hot and cold water and reduce your enjoyment of the shower.
Enough about showers, we work on complex chemical plants and what I need is money to reduce process variation on my plant. Let’s connect the analogy to a section of a chemical processing plant.
Process variation in the Bayer process
In the Bayer Process (Figure 1) one of the key process parameters is the alumina to caustic ratio (A/C ratio) at the back end of digestion. This parameter is controlled by varying the ratio of spent liquor and bauxite slurry entering the digestion train (my hot and cold taps in the shower). If the A/C ratio is too high precipitation of alumina occurs in the settler which causes process problems (me not wanting to get too cold). To compensate for the variation in the process the set point is reduced so that the A/C ratio doesn’t reach a point where precipitation occurs in the settler. There is a cost to this. If the A/C ratio set point is reduced, there is reduced production. If the set point can be increased without precipitation occurring in the settler (due to the reduced variation) then there will be an increase in production. The easiest way to do this is to measure the A/C ratio continuously. In many plants the A/C ratio is measured and variation compensated every six hours. In this time there can be many changes which means the variation in the A/C ratio will be larger. To avoid precipitation this means that the A/C ratio set point will need to be reduced meaning lower production rates.
The easiest way to reduce A/C ratio variation is to install instrumentation that can measure the A/C online and set up the DCS to make better changes to reduce variation. Precision Light and Air (PLA) supplies accurate and reliable instrumentation systems for measuring A/C which can then be utilised by the control system to reduce variation. This will allow you to increase the A/C set point and make more money. This is something both engineers and accountants can agree upon!
In our next blog we will discuss in more detail the ideal instrumentation set up to measure and control A/C.
For more information contact our technical team at sales@plapl.com.au or call us on +61 3 9786 1711.
Figure 1. The Bayer Process
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Category: Process Control