The HYSYS column solver may be able to do more than you think

I remember this one from quite a while back, a model was said to be very slow and not very reliable in it's convergence. The model was supposed to be part of a data management system on a plant, but because of the encountered problems the engineers decided not to use it on-site.

Upon inspection the model didn't look particularly large or complex, so there wasn't any obvious reason for it to slow or unstable. There was a pump around external to the column that needed a recycle operation to make things converge. That is usually slowing things down, but not to the slowness of this case. Closer inspection revealed that the flow rate of the column draw was being calculated by a spreadsheet. The recycle operation that was converging the pump around loop was continuously perturbed by the changing draw rate of the column. Analysis of the spreadsheet showed that the intention of this setup was to change the draw rate such that the draw was taking all the liquid from the column. This might have been an ingenious way of using the spreadsheet and the recycle to achieve a goal if there hadn't been a more simple and faster way.

In the column you can specify the flow rate of the liquid stream leaving a tray, so in this case the most efficient solution was to de-activate the specification of the draw rate and to create an active specification of the liquid flowing down and to set the value of the specification to zero. This removes the need for the spreadsheet calculation and provides the recycle on the pump around stream with a much stabler feed. The column also becomes more robust as otherwise the calculated liquid draw rate from the column sometimes is impossible to achieve during the iterations of the recycle operation.

This change in the model along with a couple of other changes reduced the solution time of the model from more than 5 minutes to less than 30 seconds.

So, it is of course ok to be inventive, but do not forget to check out first if there isn't a built-in features that will take care of things in a more direct manner.