Temperature Sensitive Product in Column Bottoms

[gte682n]

Hello,

I have an application where the bottom product leaving a column degrades at elevated temperatures. There are times when we increase the heat input to the vertical thermosyphon reboiler in order to increase the recovery of the light component, at the cost of losing bottom product. I am investigating methods to improve the bottom product recovery. I have attached a sketch of the bottoms arrangement to help with my post.

Some issues to consider are:
1) The 14" diameter standpipe going to pump is necessary in order to maintain suction head as well as ensure the vertical thermosyphon reboiler has enough head. The liquid level is maintained 3" above the standpipe.
2) The height of the stand pipe is 1" above the weir for the reboiler to ensure liquid will preferentially feed the reboiler to maintain column traffic.

The above items appear to exclude the option of reducing residence time by lowering the level.

Another option to reduce volume is to partition off the side of the column below the reboiler return baffle, to the left side of the 14" stand pipe. My concern with this is that I don't know how the hydraulics flowing into the stand pipe will be affected. Does it make a difference if liquid is overflowing over the entire circumference of the stand pipe, or only half?

Modifying the reboiler to a forced recirculation operation has been discussed as well, to allow us to lower the level of the in the sump. With this option we would look for a pump with lower npsh requirements. My concern with this is could the same reboiler that was once used as a thermosyphon be used in a forced recirculation application? What issues might come up?

Any other ideas that you have seen or used to address this type of problem is appreciated.

Tbank you.

 

[Compositepro]

Often heat sensitive materials are distilled under vacuum to reduce the boiling temperature.

 

[gte682n]

Yes, I should have mentioned that this tower operates at 50 mmHg. The bottom temperature ranges from 575 F (ok) to 585 F (bad)

 

[georgeverghese]

Some clarification first:

a)Why do you call this a thermosyphon type reboiler when you are feeding this with a pump?

b)Two nozzles assigned for E53 feed and return - what is E53?

In my experience, the reboiler feed and return were in one compartment, while the product sump is fed by an underweir adjacent to the reboiler feed / return sump - this does not appear to to be the case here.This may explain why you have to fire harder to reduce light key component composition in the product.

 

[gte682n]

We are not feeding the reboiler, E-53, with a pump currently.

The reboiler return and feed locations are on opposite sides of the tower.

 

[moltenmetal]

You'll get lower reboiler heat transfer surface temperatures- where your degradation is no doubt occurring- if you switch from a thermosiphon to a forced recirculation design. Not a cheap change, but an effective one.

Otherwise, your options are limited to perhaps running at an even higher vacuum.

 

[georgeverghese]

So this pump you are feeding through the 14inch standpipe would be the product pump..

There appears to be very little space between the top of the 14inch product exit standpipe and the bottom of the 2phase return nozzle, so not much room for vapor bubbles to disengage before the stream enters the product standpipe.

Can see the rational for this standpipe - one reason would be to minimise contamination of the product with the cold feed from tray above. But to give more room for vapor to disengage, would an alternative be to interpose an overflow weir between the cold feed side and the reboiler return side and product is taken off from the bottom of the dished end downstream of the overflow weir. Dont see a need for this standpipe - the product pump would be operated on level control at the sump, and the standpipe doesnt help with level modulation?