4 replies to this topic

[nrxbra001]

Hi all,

 

I have a question that may seem "simple" but it is kind of baffling me:

 

Why do we need to control a distillation column sump level? 

 

In my specific case, we have a pumped reboiler (hence level doesn't influence boil-up rate). The column does have low level switches to protect the pumps and high level alarms. What role does a strict column sump level play in the separation and/or bottoms rundown temperature?

 

Appreciate your responses.

[Bobby Strain]

What is your proposed alternate control? The control objective is to take from the bottom as necessary to maintain some specification. And to take from the distillate as needed to control a spec. And to operate the column in such a manner to produce said specifications.

 

Bobby

[nrxbra001]

Let's use an example for better illustration:

 

I have a column and i operate both the distillate product and the bottoms product on flow control. The bottoms flow control is not cascaded to the column bottoms level. As a result, any fluctuations in the column operation lead can lead to a fluctuation in column bottoms level. What is the detriment of having the column bottoms level fluctuate by say 20-30%?

 

Let's maintain the argument to a forced reboiler.

[Bobby Strain]

You cannot operate in the manner you describe and maintain product specification, or inventory in response to feed changes. If you do a careful analysis of degrees of freedom, you will find that you need to control other things, too. Best for you to get a book by Shensky on distillation control. Otherwise, we can chase this topic for months.

 

Bobby

[Art Montemayor]

Bobby is absolutely correct.

 

And without a detailed, accurate sketch depicting what you are trying to say you may never generate an accurate recommendation.  Even the great Greg Shinskey wouldn't attempt to explain a control problem without an adequate, specific, detailed sketch.  Beg, Borrow, or Buy Greg Shinskey's book, "Distillation Control for Productivity and Energy Conservation", 2nd Edition, McGraw-Hill Book Company; ISBN 0-07-56894-4.  It's worth more than the price.