Common mistakes in process design/simulation

Guys,

through my carreer, I had to mentor some new process engineers and I noticed that most of them make almost the same mistakes... Here are some:

1- When designing scrubbers or separators, people take the Standard Flow for Actual flow and they calculate huge volumes
rule of thumb : Actual volume =~ Standard volume / pressure (barg)

2- When dealing with gas compressors (especially reciprocating), people have the tendence to believe that compressor discharge pressure is set by the compressor it self. Which is totally wrong!
Discharge pressure is the backpressure exerced by downstream equipment.
(In case the downstream equipment is a control or a globe valve, the discharge pressure changes with flowrate, which is controllable on the compressor it self)

3- When using HYSYS for designing distillation columns, people have the tendency to use short-cut column and to inject the resulting tray number, reflux ratio, feed position, reboiler temperature,... into the column model and run it...!!!
They spend a week trying to converge the column... Waste of time!!!
I recommend you to take only the number of trays out of the short-cut method + 2 to 5 trays and take the feed position.
Then, you should specify the light key content in the bottom product and the heavy key content in the overhead product, and if necessary, specify one of the product's flowrate based on the inlet composition.
This will converge immediately... then it's up to you to OPTIMIZE!

4- Some people rely on HYSYS to determine the H2S content of a stream... WRONG! HYSYS doesn't deal correctly with H2S solubility in water and in HC. Be Careful!


Feed us with your experience guys... Tell us about the common mistakes you know!