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HYSYS Number of Stages

HYSYS Number of Stages

Dear Engineers,

I have got one basic question on defining the number of stages in HYSYS simulation for a distillation column.

As an example,
- on the design datasheet of the column it says that the the number of trays is 22 and the feed enter at tray 10.

My question is why when we sepcify the number of stages in column in HYSYS, sometimes we reduce the number of trays (i.e. from design datasheet: 22 trays, but inHYSYS it specified as 11 trays). Is that because of the correlation of the tray efficiency (we give some assumption of the tray efficiency)?

And what is the difference between actual trays and theoretical trays? Which is actual and theoretical stages if I refer to the design data datasheet trays and in HYSYS trays?

I am not really sure with this concept. Hope someone can feed me with your explanation on my questions? Appreciate for your help.


RE: HYSYS Number of Stages

Simulations model stages, which means complete ("perfect") thermodynamic equilibrium between the up-flowing vapor and the down-flowing liquid. Trays are the hardware that's installed in an actual column. Tray designs vary because the internal vapor and liquid traffic varies. It varies within a given column, and from one distillation application to the next. The tray design attempts to maximize efficiency (maximize attainment of perfect vap-liq equilibrium) while at the same time handling the specific vapor and liquid loads. No tray achieves complete vap-liq equilibrium (no tray is 100% efficient). Therefore the number of trays must be greater than the number of simulation stages. Assuming the simulation has a sufficiently accurate physical property package, the simulation will accurately tell you the number of equilibrium stages required to do a given separation. Then you evaluate the tray efficiency to determine how many trays are needed to actually achieve that number of stages. The tray efficiency will depend on the particular tray design, the vapor-liquid loading, and the properties of the fluid.

RE: HYSYS Number of Stages

Thank you very much don19080 for your response on my question.

Further question on that, if i have 22 trays and then if i want to specify the number of stages in HYSYS (i.e. I specified as 11 stages foe a column in HYSYS), why 11 stages is used instead of 10,11,12 or any numbers? is there any reason to select the number as i mentioned?

RE: HYSYS Number of Stages

If you are trying to model an existing column, then you enter the number of stages that matches the actual performance of the column. You know the chemical analysis of the overheads and bottoms streams, and you know the reflux ratio. So, vary the Hysis stages in the rectifying and stripping sections until you match the actual column performance. Then you have the correct number of stages in the simulation model.

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