6 replies to this topic

[Alikhani]

dear all,
one of the several ways to control Column pressure is minipulating air cooler (as condenser) fan pitch.
question is:
how is this acted in hysys dynamic?

[Zauberberg]

Hello Alikhani,

Controlling distillation column pressure by manipulating air fan pitch is not an effective and efficient method, because of high dead-time and highly non-linear interrelationship between controlled and manipulated variable.

Depending on system characteristics, it is best to choose between flooded condenser, hot vapor bypass or split-range control - which can be easilly simulated in Hysys.

Regards

[Alikhani]

dear Zauberberg,
thanks alot for response.
using air cooler as primary distillation condenser and water cooler as trim, is common.
and normally pressure sensing is transfered to air cooler.
changing fan pitch, like using variable speed motor, changes air mass flow and as a result the amount of heat dissipated.
when air mass flow changes the outside heat transfer coefficient, ho, also changes; so the UA parameter changes.
but in hysys varying the fan speed does not change the UA and this is the problem i have with.

[Art Montemayor]

What Zauberberg is expounding is actual, factual, empirical fact. One of the main draw backs or negative points of an air-cooled exchanger acting as an overhead condenser is that it can't be relied upon as a dependable source of column pressure control directly. The lag time, as Zauberberg has pointed out, is just too long to be effective and to be relied upon. This is a major problem with varying flow rates or column flow variations.

The main engineering point here is that it is folly to plan on such a control (column pressure control with variable pitch fans) when the acknowledged and practical way to do it is with flooded condenser, hot vapor bypass or split-range control. I believe Zauberberg even gave some references that could be downloaded on this in a previous thread. Am I getting old and forgetting, Zauberberg? If I'm correct, can you mention or upload those references again? If I'm wrong, then also please let me know.

In my opinion, to use Hysys to simulate a process that can't control itself efficiently is a waste of good simulation time. Although this is a common error made by simulators who don't bring in the hands-on experience, it would be wise to heed the Zauberberg advice and simulate in a proven and operable manner. There is no personal criticism meant here. However, there is professional criticism on what constitutes the best, proven, and practical manner of controlling a column pressure when proposing the use of a slow-reacting final control element (the fan's blades).

[Zauberberg]

Of course, Mr. Montemayor.

The article you mentioned is Andrew Sloley's "Effectively control column pressure", if I remember well.
Good reference materials can also be found in H. Kister's "Distillation Operation" and "Working guide to process equipment" by N. Lieberman, as well as in the literature cited in the last page of Andrew Sloley's article.

Many thanks to Mr. Sloley, who shared his great engineering skills through published papers.

[jitendraprocess]

Dear sir,

Please attached Andrew Sloley's "Effectively control column pressure", articles or if possible please send to my email id.
jainjvs@gmail.com

Thanks & regards,
Jitendraprocess

[pavanayi]

Jitendra
Here you go. A very useful article for anyone working with distillation columns.

Attached Files

•  Distillation Column Pressure Control.pdf   2.06MB   200 downloads