When you have a distillation problem that is fully defined, you usually do not need to resort to the below. But sometimes the problem specification is partial or ambiguous. For modelling a column, HYSYS has two simplified models that come in handy when you are having issues. One is the shortcut column and an even simpler model is the component splitter. The aim of using these simplified models is ultimately to get the more rigorous stage by stage calculation to converge, the simplified models help to understand better the nature of the problem you are trying to solve.
Although this description starts from the simplest approach, quite often you’ll end up going from distillation to short cut to component splitter and then back to short cut and then distillation. At times “regressing” to the short cut may be enough, sometimes it is not. In the picture to the left you can see that you can use the same feed for all three unit operations. You will have to set your preferences to allow multiple stream connections!
I’ll use the general description of the problem discussed on LinkedIn:
Feed is a mixture of 10% mass Oil and 90% n-Hexane
The aim is to recover 99% mass of the n-Hexane as a distillate
No info was given about the composition of the oil, so I assumed in my test case a mix of C7 to C16 (10% mass of each).
When you model this using a component splitter, it is pretty easy to specify the 99% recovery of the n-Hexane. But, it will (should …) be quite clear that you need to have some info about what needs to happen with the oil components. In the description above, there is no indication of that info. Preferably you’d find out from the person/company that posed the problem what the specifications are concerning recovery of oil or tolerated amount of oil in the n-Hexane. If no info can be obtained, you would have to come up with something reasonable yourself. In this case, I assumed that 0.1% mass of C7 was acceptable in the n-Hexane. I assumed the other components would not be present in the n-Hexane. You MUST have a non-zero spec for one of the components! The component splitter would allow a zero spec, but the short cut and stage by stage model would not work with a zero spec.
The results of the component splitter will help you specify the short cut distillation. You need the desired mole fraction of the light key (n-Hexane) in the bottoms product and the desired mole fraction of the heavy key in the distillate. These cannot directly be deduced from the original specifications. But once you have specified the component splitter, the required inputs for the short cut distillation are readily available. You might be surprised that in the example case the mole fraction of n-Hexane in the oil that corresponds to a 99% recovery is 13.3% mole .
When you have specified the key component mole fractions and the operating pressure, the short cut model will report a minimum reflux ratio. The actual reflux ratio you will use is an economical decision. Factors that will have a high impact are the cost of the cooling and heating medium you need to use and the operating pressure and the metal of construction. I have seen cases where the selected reflux ratio was as low as 15% above the minimum reflux ratio and sometimes it is more like 50% above the minimum reflux ratio.
Once you have the short cut column working, it is easy to derive the stage by stage model. You could use the same specifications as in the short cut model to start with. Once the column is converged, you can switch to specifications that make more sense to you.