Bottoms liquid draw off point in a packed distillation column

[Sesquipedalianist]

Hello,
I am enquiring about a design-related aspect in the consideration of packed distillation columns. Is it common to have the bottoms liquid draw off through the reboiler or through a bottoms nozzle on the column itself? I have indicated a screen grab on the reboiler draw-off option but sadly for all the years I have practiced, I had never seen this kind of operation. Is there any code of practice that guides [draw-off points in distillation?



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https://res.cloudinary.com/engineering-com/image/upload/v1682501974/tips/Reboiler-bottoms_ztfwld.png

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[pierreick]

Hi,
From the reboiler.Did you notice the weir to keep constant the level in the reboiler, the overflow will be the residue.
Pierre

 

[georgeverghese]

This is one possible option for a once through kettle reboiler. There should be sufficient holdup in the product compartment to enable adequate degassing and to handle flow upsets, which could affect the operation of the LIC in this compartment. Another option is to install a balance line between this product compartment in the kettle and a dedicated product compartment in the bottom section of the column to gain additional surge holdup.

 

[TiCl4]

Sesqui,

I don't know of any code that guides the decision here - making the reboiler a partial reboiler has the advantage of the reboiler acting as a separation stage, which can slightly reduce the NTU's needed in the packed column. What you've drawn is a bit odd, though, with two takeoffs indicated. One is "sewage" from the column to the reboiler and the other is the "light hydrocarbons" coming off the bottom. I'm not in O&G, but what would be coming out the top if the "light hydrocarbons" are the heavy components?

I will say that another advantage of having a partial reboiler is having buildup control. If the feed to the column has any really heavy particles that don't boil off, then a total reboiler would tend to concentrate those particles in the reboiler, eventually fouling the tubes. Pulling directly out of the reboiler would ensure that any heavy material doesn't accumulate.

For what it's worth, my experience in distillation is limited to reactive distillation with partial reboil and high purity distillation with total reboilers, so make of this advice what you will.