VisSim for the Refinery and Plant Process Control Engineer
Edward Dilley MIChemE, Ceng
Chartered Chemical Engineer, Peterborough, UK
Ive been a VisSim user for nearly 15 years now, and although
Im one of its greatest fans, I felt somewhat daunted when asked
to write about my experiences. Most articles on VisSim that Ive
read describe huge simulation projects staffed by teams of simulation
experts who continually push forward the frontiers of applied mathematics.
Im at the other end of the spectrum. Im a Chemical Engineer
who has specialised in process control application. Ive frequently
been working alone, and Im by no means a natural mathematician.
Back in the 1980s I found myself trying to improve distillation control
at an oil refinery. I read the textbooks of the time, used the tuning
formulae to the best of my ability, and usually managed to make matters
worse. Surely there must be a better way, I thought.
Controller-tuning technology improved throughout the 80s, but for me
the arrival of VisSim was a breakthrough. At last I was able to see
rather than hope, and to demonstrate to others before trying out new
controller-tuning. I used Personal Version 1.5, which is still on my
laptop as I type this. VisSim is now at Version 5; the mathematical
power and graphics capabilities have advanced dramatically, but VisSim
got the principles correct from the very start.
What Attracted Me to VisSim?
I make no apology for saying that VisSim was easy to learn, especially
for an engineer who had forgotten most of his college mathematics. What
makes it easy? The main factors are:
- Its Intuitive
The GUI could hardly be easier. Function-blocks are selected by clicking,
positioned by dragging and then wired together to force
the arithmetic or logic to do what you want. The model is always on
display, and you can build it up gradually, checking operation section-by-section.
A model doesnt have to be complex to be useful.
- No Prior Programming Knowledge
You dont need to be a computer programmer to use VisSim
just click, drag and wire. Nor do you need to be a mathematician,
though it will help if you know what dy/dx means.
Many of VisSims function blocks are mathematically powerful, with
several options for integration or optimising methods. However I use
just a few function-blocks regularly, and for those with options, Ive
found that VisSims default choices work just fine.
- Plain English
The VisSim manual is a credit to its authors. Its refreshing to
have a manual which helps readers rather than confusing them. A hard
copy of the manual comes with the boxed package, though everything is
supplied in .pdf format for on-screen use. (The hard copy
does make learning VisSim easier for the novice user.)
The supplied tutorials illustrate the principles of simulation and the
principles of VisSim.
The examples have been chosen with care to kick-start beginners
without swamping them with detail. You can be configuring a simple simulation
minutes after loading.
- Function-Block Reference
VisSim has over 100 standard function-blocks. Each is clearly described
in the Block Reference chapter of the manual. Most importantly, the
manual gives one or more practical examples of how best to use each
block. Thats user-friendly.
- Worked-Examples Library
VisSim provides an extensive library of working VisSim models from a
wide range of industries. Most examples demonstrate rigorous ways to
simulate process equipment; other examples use techniques such as curve-fitting
or economic optimisation. Browsing through the library soon had me thinking
of other simulation ideas.
- Excellent Value
It was affordable, and paid for itself the first time I used it for
How Has VisSim Helped Me to Resolve Process Problems?
I have over 100 VisSim models in my file, mostly small (less than 100
blocks), but some with several hundred blocks. Ive used VisSim for
formal controller design, for training operators and technicians, as a
control scratch-pad to quickly check out ideas, and most importantly,
for process troubleshooting. VisSim has been equally
useful for continuous, batch and semi-batch processes.
As a Process Control applications engineer, Ive often used VisSim
to simulate, tune and optimise PID controllers. VisSim is well-suited
to modelling the discrete arithmetic of Digital Control Systems (use
the pulse function and unit delay), so I was able to build the exact
PID algorithm used by the DCS.
The example below demonstrates my first attempt to control the top- and
base-temperatures on a refinery Pentane-splitter tower. Although the two
controllers were tuned optimally (look at the straight lines
for temperature), several hours after commissioning control, the tower
ceased to fractionate. The reflux and reboiler steam flows had wound to
minimum! VisSim showed that the cause was severe interaction
between the two controllers.
Figure 1, Interaction causing reflux/steam wind-down
Control was stabilised by replacing the top TC with a reflux-ratio controller.
Below are the results of a before and after study
of the behaviour of an inventory controller. The controller was intended
to limit the loading of material on some mechanical conveyors. The operators
reported erratic control with unacceptable peak-loading, such that they
normally ran the loop in manual mode:
Figure 2, Erratic control and excessive peak-loading
Site inspection and then
modelling with VisSim showed that the problems were caused by the compressible
nature of the material in the exit chute; the compressibility introduced
some unexpected dynamics into the loop.
VisSim confirmed that the solution was to use sufficient derivative
action to quieten the loop without overloading the electric motors and
Figure 3, Derivative action stabilises control, reduces peak
The above examples show how VisSim was used
to optimise control loops. But how many times is a Controller
Tuning problem in reality a process problem, such as a sticking
control-valve or a heat-exchanger thats fouled? VisSim has helped
many times in identifying the real problem.
At one site, pH control problems with a
reaction vessel had halved the process throughput. VisSim helped track
down the real culprit:
Suspect #1 was the pH controller (standard
PID). Process step tests and simulation with VisSim showed that the
controller-tuning was optimal even though control was in fact unacceptably
Suspect #2 was the chemistry. Were we trying
to control at the wrong part of the pH curve? We couldnt be sure,
because nobody had a pH curve for the di-protic weak acid vs. di-basic
weak base used. A chemistry reference book and VisSim produced the pH
curve, and laboratory tests confirmed we werent trying to do the
Figure 4, VisSim-derived pH curve used in troubleshooting
Suspect #3 was then highlighted: process conditions
had changed such that the reaction vessel was now undersized for the throughput
required and the feedstock composition. The vessel was modified to double
the capacity, following which pH control was excellent.
For the Process Control Engineer, a powerful and easy-to-use simulation
tool is as essential as a good spreadsheet and word processor. VisSim
meets the needs, is excellent value, and has proved its worth. And whats
more, its fun!
About the author
Ed Dilley is a chartered chemical engineer with 30 years experience
of applied process control. He has worked in the oil, petrochemical,
polymer and food industries. He lives in Peterborough, UK.