4 replies to this topic

[keby9vam]

Hi there,

I've completed my azeotropic distillation column design and proceeding to design the reboiler of its system. Since my column is operating at vacuum pressure, I've noticed from literature (R.K Sinnott) that kettle reboilers are suited for operation of such low pressures.

I've done some calculations in my effort to design the reboiler, but I can't help but to worry the fact that I'm receiving an overall heat transfer coefficient of 5 W/m2.C, which brings me to my doubt; isn't this value really small? Or is it expected for a rather low bottom flow rate of 64 kg/h entering the reboiler? Thank you for your time

Regards

Viknesh

[Art Montemayor]

Viknesh:

You are on the right trail to suspect that 5 W/m²-^oC (0.88 Btu/F^t2-^oF) is far too small to be valid as an Overall "U" in a kettle reboiler. There has to be an error, mistake, or omission in your calculations for the Overal "U".

Regardless of what you have read in the literature (R.K Sinnott) that kettle reboilers are suited for operation of such low pressures, my experience in designing, building some, and operating them indicates that for vacuum service a thermosyphon is more practical and economical. A kettle reboiler has to have a relatively large shell - due to the need to create vapor disengagement - and this necessitates a full vacuum design on a large diameter vessel (something that is costly because it requires a thicker wall). Meanwhile, a thermosyphon type is designed to produce a mixed phase of vapor and liquid droplets and its tube side (not the shell side) is inherently strong against a full vacuum due to the relatively small diameter of the tubes. Unless you want (or need) a kettle for this application, I would employ a thermosphon type.

[keby9vam]

Art:

Thank you for your comments. I have tried iterating the U value has discussed in R.K. Sinnott, and found the value to be about 21 W/m2.C. After getting such value, I realised it's about time I give up on kettle reboilers and try to implement thermosphon reboilers.

In fact, in my kettle reboiler calculations, the procedure is to estimate the U value 1st. Upon calculations, a calculated U will be found, and we're supposed to compare the difference. If the initial guess is far apart from the calculated one, the design doesn't stand (as mentioned in R.K.Sinnott). However, a more plausible U value of 200 was initially estimated but the calculated value obtained later was around 60+, and the number of tubes for this situation was 0.4. My conclusion is, this calculations are going nowhere, despite no mistakes in the method.

I will try out the other design later today and give my feedback on this. Thank you again.

Regards

Viknesh

[keby9vam]

Art:

Forgive me for asking this question out of curiosity, but is it possible to have a reboiler with 250 tubes?

Regards

Viknesh

[Art Montemayor]

Viknesh:

You have to be specific with engineers.

When you discuss a reboiler, what type are you talking about?
If it is a kettle type with a "U" tube (hairpin) bundle what you should refer to is the quantity of hairpins - and how long the bundle is.

If you mean a straight tube thermosyphon, then that is another matter.
You also have to specify the tube OD.

If you are referring to a thermosyphon with 1" OD tubes, 10 feet long, then 250 is not a large number of straight tubes. You should make a tube layout and obtain an estimated shell diameter that corresponds to the tube quantity. You can easily and quickly do an accurate estimate on an Excel spreadsheet. Do that, and let us know.