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Thermosyphon Reboiler Design

Thermosyphon Reboiler Design

(OP)
I'm working on a project that has a couple of thermosyphon reboilers. Does anyone have any good design guidelines? From your experience, is there anything I should be aware of while designing one?

I've been told that the hydraulics on a thermosyphon reboiler is something that requires special attention. This is one of my first big projects out of school and I want to make sure I understand how they are designed.

RE: Thermosyphon Reboiler Design

(OP)
Thank you dogtop. This will get me started.

RE: Thermosyphon Reboiler Design

Are you in a MegaChemCorp? If so, ask their heat transfer guru(s) for company manuals, references, and training materials to support your quest/project.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Thermosyphon Reboiler Design

(OP)
Latexman

This is a decent sized company and they have multiple plants throughout the US. I've requested company information on existing equipment but I'll have to wait on that.

RE: Thermosyphon Reboiler Design

Note thermosyphon reboilers have poor flow turndown - if there is an overheads condensor, this condensor will need to be operated at near design rate, even when feed flows to the column are low, to keep the thermosyphon reboiler hydraulics in a stable region. So check how much it costs to provide the excess cooling medium to the ovhd condensor for this option.

RE: Thermosyphon Reboiler Design

There are 6 different tips on vertical thermosyphon design on www.gulleyassociates.com

RE: Thermosyphon Reboiler Design

These are also some good resources:

http://chem.engr.utc.edu/Student-Files/x2008-Fa/43...
http://documents.mx/documents/reboiler-design-5584...
http://www.cambchemtech.com/files/9112/8984/5743/U...
http://www.eng-tips.com/viewthread.cfm?qid=95533
https://www.academia.edu/3642019/Selection_of_Rebo...

In my earlier days the Distillation Group Inc. website was very active and containing a lot of interesting discussions and technical papers. The site was managed by Andrew Sloley. It was possible to send an email with Tech Paper No. in the email subject and to receive the related tech paper within a few days. I think it doesn't work since long time ago but perhaps worth trying. Some of the Q&A are still there anyway:

http://www.distillationgroup.com/questions.htm

Dejan IVANOVIC
Process Engineer, MSChE

RE: Thermosyphon Reboiler Design

Txheat,

The hydraulics are very important, but the vendor is getting paid to address this via the design and it is best not to try and do his mechanical design job. Concentrate on the process design, and then check his proposal to see if it is reasonable.

In the case of a vertical thermosyphon reboiler, you will fill out a TEMA sheet with process data, and provide some elevation details (operating liquid levels, return nozzle elevation, etc). You will already know the composition, tower bottoms pressure, and duty required. You can calculate the other necessary process data (usually by simulation) but there are two common decision pitfalls on the process data to be aware of:

1) The vapor generated by the reboiler is typically only 10% to 20% of the total circulation through the reboiler. You need to specify the total circulation requirement at the reboiler inlet (from outlet vapor flow and vapor fraction matching the duty). The vendor will design the needed length of exchanger, tube count and diameter, and piping size needed to achieve this circulation rate from the elevations you provided.
2) If the heating medium is steam, you must decide a condensing (chest) pressure. This is usually less than the supply pressure (pitfall) because the steam rate is controlled by a control valve. You should choose a condensing pressure which corresponds to a reasonable value above the process temperature. This decision affects the exchanger size, control valve size, amount of flashing in the condensate system, actual steam rate, etc.

A thermosyphon reboiler assignment is where new process engineer can really demonstrate his skill. You will understand more than half of all process engineers I have met if you can explain: what process side vapor rate corresponds to a specified duty, how the vapor rate relates to the total circulation rate by hydraulics, how to calculate the steam rate needed for a specified duty and chest pressure, and how the chest pressure affects the exchanger area (via Q=U*A*MTD).

best wishes always,
SShep

RE: Thermosyphon Reboiler Design

Hello Txheat,

FYI, Aspen-Tech is offering a free 4hr "virtual training" on this subject. The title is: EHX903 Reboilers - Designing and Troubleshooting Thermosiphon Reboilers. A USA course is set for 7-Jun. You can register on their website and download training materials.

best wishes,
sshep

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