11 replies to this topic

[Lyne]

Hi everyone,

 

I'm designing a Kettle reboiler with an overflow weir. The client asked for 5 min retention time in the kettle reboiler. I'm using HTRI for my design But I couldn't adjust the kettle length (or distance from weir to the shell end) to have the required retention time. any insights on how to change this length in HTRI ? your help is really appreciated.

 

Thank You

[Pilesar]

You can add the extra liquid holdup volume after your HTRI calc if you like. It will not affect the heat transfer. Consider the kettle as just a horizontal vessel with a stab-in tube bundle.

[Bobby Strain]

What is the 5 minutes for? Maybe you can provide a sketch of the reboiler in relation to the column. and level control.

 

Bobby

[Lyne]

This is a screenshot of the PFD. The retention time is requested to ensure a better disengagement and level control.... But how vendors will ensure the same thing if HTRI can't do it ? There's not another way with a different simulator?

Edited by Lyne, 26 February 2024 - 02:46 PM.

[Bobby Strain]

There are lots of things wrong with this design! Including the column/reboiler configuration. Is this part of an alkylation process?

 

Bobby

[Lyne]

This an existing Isobutane production unit that's currently working, we will be replacing the kettle reboilers and the heating medium

[Lyne]

There are lots of things wrong with this design! Including the column/reboiler configuration. Is this part of an alkylation process?

Bobby

Please, can you elaborate more about what's wrong about this design?

[Bobby Strain]

If it is working, I should ignore this PFD, because some of the controls won't work as shown. You should provide the requested residence time on the outlet side of the reboiler weir. So, just extend the shell beyond that calculated with HTRI. What is the source of the feed to the unit? You client would be well advised to have some knowledgeable engineer review the controls and other things. Operations could be greatly improved. What is the capacity of the unit? And the isobutane is sold as product?

 

The column pressure control as shown can only work by flooding the condenser. So, the liquid temperature cannot be controlled as shown. Reboil control is best accomplished with a lower tray temperature, not the reboiler outlet temperature.

 

Bobby

Edited by Bobby Strain, 26 February 2024 - 05:47 PM.

[Pilesar]

I am surprised the operators can control the columns this way. How frustrating for them! There is such great opportunity here for a process engineer at this plant.  Changing control schemes would do so much more good than blaming the reboiler. If that is not your job and you are just the heat exchanger designer, then figure out what volume of liquid you need after the weir and make room for it. HTRI is a great tool but does not replace the engineer.

[shvet1]

As far as I know HTRI is not rated for pressure parts integrity design, forummembers can correct me. Chamber behind overflow weir relates to pump intake engineering and is irrelevant to heat transfer. This fact means you are able to provide whatever design you need and assure that in a software dedicated to PV parts calculations.

 

Note that in case of incorrect design of the chamber discussed personnel will face with bubble entrainment and instability of control valve downstream (at the min) or even seat/cage erosion (at the max).

Edited by shvet1, 27 February 2024 - 12:20 AM.

[Lyne]

If it is working, I should ignore this PFD, because some of the controls won't work as shown. You should provide the requested residence time on the outlet side of the reboiler weir. So, just extend the shell beyond that calculated with HTRI. What is the source of the feed to the unit? You client would be well advised to have some knowledgeable engineer review the controls and other things. Operations could be greatly improved. What is the capacity of the unit? And the isobutane is sold as product?

 

The column pressure control as shown can only work by flooding the condenser. So, the liquid temperature cannot be controlled as shown. Reboil control is best accomplished with a lower tray temperature, not the reboiler outlet temperature.

 

Bobby

 

The isobutane is sold to another petrochemistry company near the plant. I'm responsible only on heat exchanger and heating medium replacement but with this changes we will get to the control loops soon or later ... Another point is the new heat exchangers will be placed near the existing ones to avoid long period shutdowns but this makes me worry about the pressure drop especially if we add isolation valves.

 

[Bobby Strain]

You need not worry. Just do a good job engineering the new installations. Pay attention to the kettle vapor return line.

 

Bobby

Edited by Bobby Strain, 27 February 2024 - 11:55 AM.