5 replies to this topic

[Amit Sonpal]

My job responsibilities includes performance evaluation of refinery columns like Product Fractionator, Stripper based on field data using Aspen Plus. I used to simulate the column with actual stages using Murphree efficiency. What should be the draw stage like column top/bottom, PA, side product efficiency specified in simulator?

As per my understanding, stream leaving the column will be in equilibrium hence all draw stages efficiency should be always 1, similar to condenser / reboiler stage. Is it valid to tune the draw stage efficiency to simulate actual field operation?

[Zauberberg]

Yes, that is the way to simulate actual tower operation. Efficiencies vary from tray to tray.

[Chellani]

Sonpal Bhai,
I would avoid doing that. The reason has already been quoted by you i.e. if draw stage doesn't have efficiency of 1; you may not get vapor fraction of 0 or 1 which may give you other problems in the simulation and other guys dependent on you for other equipment design e.g. pump can get upset because of this. Apart from this, temperature of draw stage is also questionable.

You need not match every data in the plant as plant data itself can be incorrect. If you are really deperate to do so, tune efficiency of stages without any draw.

All the best.

[Zauberberg]

I'm working on the simulation of a cryogenic plant where there are several vapor/liquid draw offs from distillation tower, and the only way to obtain the same temperature/compositional profile was to "adjust" stage efficiencies across the column. I really can't see some other way of modeling in order to match actual tower performance with the model.

Yes, there are issues with top/bottom stage phase fraction if one uses efficiencies less than 1 - there has been an interesting thread on the same subject a few years ago: http://www.cheresour...-vapor-fraction

Back to the original question: you may try to play with stage efficiency across the tower while keeping the top/bottom tray efficiency equal to 100%, and see if that can bring you closer to the actual performance. When troubleshooting an existing unit, working with ideal plates is probably not the best way to go - except for conventional binary distillation systems.

[Amit Sonpal]

Chelu,

Thanks for reply.

I found difference in methodolody w.r.t. draw stage efficiency among vairous process engineer but I am sure we should take it as "1".
Rest is fine.

[Amit Sonpal]

Mr. Zauberberg

Thanks for your post.

I learnt that changing efficiency of stages other than draw stages has insignificant impact on top temp / reflux rate.