CDU Design Detail

[Milutin]

Hi all,

In previous Petro0707 post thread483-197804 we have seen interesting detail. Part of liquid drawn from chimney partial draw off tray is routed to tray below without any cooling, red line on attached picture.
My question is what is purpose of this stream?

Regards,

Milutin

P.S. Question is asked in separate post because I didn't want go off topic in original post.

 

[Petro0707]

Hi Milutin,

The purpose is to measure the IR flow which is going towards below tray,From another end...I can think off that whatever deposits,foulants are there in top section can go towards below & donot accumulate on top trays and donot create furthur depositions and under deposit corrosion..

Regards,

 

[0707]

It Think the main purpose of the refered stream is to control the level of the chimney tray to avoid that the liquid light fractions can go down flooding trays bellow.

 

[25362]

Petro0707, if deposits are carried down, why shouldn't the regular overflow, which is almost twice the naphtha product, do the same ?

 

[0707]

i agree with 25362 but the pumping down purpose is to avoid excess overflow.

 

[Milutin]

Hi all,

According to original drawing, draw tray is designed as partial draw off tray. It means if you route less liquid by pump to tray below, draw tray will overflow.
From that reason, it is not possible to control quantity of liquid going to tray No 6 by manipulating flow through line from pump to No 6 tray.

In may opinion that line was design error, deleted later, with only real purpose to waste electrical energy.

Regards,

Milutin

 

[Milutin]

Hi 0707,

Purpose of that line not to avoid excess overflow or prevent light fraction to go down and flood trays below.
Liquid going down from that chimney tray is simply called internal reflux.
That internal reflux is set by: crude type, unit throughput, product drawing, side pumprounds duty and top pumparound duty.

On this tray if you draw less liquid it will overflow to tray below, if this is not case (in case that total draw tray is installed) liquid will go up and flood trays above.

Regards,

Milutin

 

[25362]

Milutin,

In case of total draw:

...or overflow through the chimneys in c/c to the rising vapors.

 

[Milutin]

25362,

Yes, one part for sure will go down, but in such case because of large quantity of vapor, one part will be entrained up.