Stage Efficiency VS Overhead Vapor Fraction

[EmmanuelTop]

Dear members of Eng-Tips,

Recently, when I was performing a series of sour water steam stripper simulations in Hysys (absorber template), I've noticed strange results when altering top tray efficiency (from 1.0 to 0.5). The stripper is fed to the top tray, live steam is injected at the bottom of the tower, and there is no reflux.

1. As soon as I reduced top tray efficiency from 1.0 to 0.95, stripper overhead stream vapor fraction dropped from 1.0 to 0.56.

2. Further reduction of top tray efficiency resulted in even lower vapor fraction of stripper overhead stream.

3. The total number of stages in the model and their efficiencies have no influence on overhead stream vapor fraction. You can have efficencies as low as 0.1 and as high as 0.95, regardless of the number of stages - but if top tray efficiency is number smaller than 1.0, there is a liquid in the overhead stream as simulation output.

4. The same thing happens in the crude atmospheric distillation column model (also modeled as absorber-type).


Does anyone know why is this happening and how to overcome this phenomenon?


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[EmmanuelTop]

Just to mention, this is not an issue in refluxed absorber and distillation column templates/models.


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"Our Music is New Black Polished Chrome That Came Into Summer Like Liquid Light"

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[CJKruger]

EmmanuelTop, it sounds like a bug. But I have not seen it myself, probably as I always had a reflux or used theo trays.

What did Hyprotech/Aspentech say ? For know I think you would have to use theoretical trays (100 % eff) for the simulation.

 

[Jason5000]

I've seen this before too. In general if a tray efficiency is less than 1 the vapor and liquid leaving the tray are not in equilibrium; some liquid is entrained with the vapor and vice versa.

Aspentech has a some info in their knowledge base on why this happens and how to deal with it:

http://support.aspentech.com/webteamcgi/SolutionDisplay_view.cgi?key=110523

You'll have to sign up for an account to get in, but there is some good stuff in there. I've found it to be worth getting the occasional product e-mail.

 

[gedomin]

As a general practice, condenser efficiency shoud not be set different to 1, as it is a flash operation (V/L equilibrium). The problem you are finding is just as expected.
As far as I know, experts prefer not using tray individual efficiencies. Better to set your model on theo trays. No. of theo always < No. of actual trays.
If fluid properties vary along the tower, estimate an overall efficiency for each section i.e. stripping zone, wash zone, etc. A key property is liquid viscosity. Then section theo trays = section actual trays x efficiency, round down to the next entire number.

 

[EmmanuelTop]

I have never had a problem simulating distillation towers with their actual number of trays, although the process can be simplified by using theoretical plates. When modeling multi-product columns (including pumparounds, side-draws etc.) it is best to use actual number of trays and then seak for tray-to-tray efficiency until plant data is matched.

This is an absorber-type column, simple steam stripper and there should not be any liquid in the overhead vapor stream if model has converged. Regardless of components transfer efficiency, vapor should remain vapor (only with different composition, but not with liquid phase in it). But this is exactly the case.

I agree with CJ Kruger - it looks as a bug. I was just wandering if anyone of you had similar experiences and what could be the possible solutions. For now, I set the efficiency of top tray = 1 and I think the results I obtained are very close to reality (this is a design case).

Thanks,

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