7 replies to this topic

[Venkat @89]

Italic written matter is from book written by Norman Liberman sir, Effect of Lighter components on Overhead Composition control. I have doubt in it. Can Someone guide me.

 

Effect of Lighter Components on Overhead Composition

Control

I noticed on my depropanizer in Texas City that the butane content of my

propane product was erratic, even though the tower top pressure and

temperature were constant. However, when I studied the lab data closely, I

noticed an interesting trend. When the ethane content of the propane

product was high, so was the butane. As the ethane content of the

depropanizer feed became erratic, so did the butane content of the propane.

But why?

The vapor leaving the top of a tower is at its dew-point temperature and

pressure. As the ethane content of a propane vapor stream flowing at its dew

point goes up, the dew-point temperature will decrease. The ethane makes

the propane more volatile. If the reflux rate is on tower-top temperature

control, the lower dew-point temperature will reduce the reflux to restore the

temperature set point. The reduced reflux flow permits more butane to be

distilled overhead.

The resulting variable butane content of the overhead product reflects not so

much a malfunction of the temperature control, but a natural variation in the

 

overhead ethane content. What I eventually did on my depropanizer was to

integrate the online gas chromatograph (GC) to reset the tower-top

temperature set point for ethane and butane composition. Now, with

advances in computer control, this technique has become common.

 

 

It is very clear, when the Ethane content increases in top vapor containing Propane, it is obvious that dew point temperature decreases.

 

Keeping top temperature of the column set value as same as before, firstly why does reflux flow gets reduced as mentioned in the paragraph, when ethane content in top vapor increases?  

 

Can Someone help me to understand? Thank you.

 

[Vegeta]

Can you describe the overhead control, please?

[Venkat @89]

Can you describe the overhead control, please?

The italic written paragraph is from Norman Lieberman sir's book.

Pressure control philosophy not mentioned.

[SilverShaded]

It's simply saying that at constant dewpoint temperature, increasing ethane also increases butane to keep the same dewpoint.

[halkeshhulk]

It's simply saying that at constant dewpoint temperature, increasing ethane also increases butane to keep the same dewpoint.

Absolutely true, to maintain constant dew point when lighters than distillate gets increased, heaviers that distillate needs to increase to maintain constant dew point. Agree.

But I didn't understand this below statement used in the paragraph.

If the reflux rate is on tower-top temperature
control, the lower dew-point temperature will reduce the reflux to restore the
temperature set point. The reduced reflux flow permits more butane to be
distilled overhead


If the column top temperature is kept constant, why does reflux gets reduced?? Please explain.
Thanks

[SilverShaded]

 

It's simply saying that at constant dewpoint temperature, increasing ethane also increases butane to keep the same dewpoint.

Absolutely true, to maintain constant dew point when lighters than distillate gets increased, heaviers that distillate needs to increase to maintain constant dew point. Agree.

But I didn't understand this below statement used in the paragraph.

If the reflux rate is on tower-top temperature
control, the lower dew-point temperature will reduce the reflux to restore the
temperature set point. The reduced reflux flow permits more butane to be
distilled overhead


If the column top temperature is kept constant, why does reflux gets reduced?? Please explain.
Thanks

 

I think it's because although the bubble point is constant (ie top tray temperature) the dew point reduces, so the condenser and reflux are cooler.  I assume from whats written that as ethane and butane are increased the bubble point stays the same but the dewpoint decreases.  I've not tried it but 5 minutes in a simulator would confirm if its true or not.

[breizh]

Hi,

If you don't have access to simulator, you may use the calculator attached.

https://checalc.com/...stillation.html

 

You will find details in the reference attached to the document.

 

I believe this will help you to solve your problem

Good luck

Breizh

[halkeshhulk]

Hi,
If you don't have access to simulator, you may use the calculator attached.
https://checalc.com/...stillation.html

You will find details in the reference attached to the document.

I believe this will help you to solve your problem
Good luck
Breizh

Thanks sir