4 replies to this topic

[Shashank Gujale]

There is extensive literature available on the design of a vertical thermosiphon reboiler.
What must be the design basis for a forced circulation reboiler?
How does one go about designing a vertical forced circulation reboiler where the liquid involved is very fouling?
What must be the velocity through the tubes in case of very fouling liquids like fermented wash of a distillery?
And/OR
What must be the dirt factor?
How to determine the capacity of the pump required for the forced circuation?
Is it correct to design a forced circulation reboiler on the lines of a vertical thermosiphon reboiler by assuming very large recirculation ratio?

[siretb]

You will need to refer to specific litterature about heat exchangers. A forced circulation reboiler is easier to design than a natural thermosiphon reboiler.
The specific velocity should be quite high, say 20 ft/s in the tubes. Since you have a very fouling application, the vapor fraction should be quite low, probably less than 15%. You may consider flashing AFTER the tubes, that is heating in the heat exchanger (no phase change in the boiler itself due to the overpressure genberated by the pump) , and using a RO lowering the pressure and flashing. It may reduce fouling, in some cases. I would be concerned about fouling the RO, though.
I think that a better approach than a design based on a natural thermosiphon would be to handle the unit as a regular heat exchanger, with phase change, of course. You do not care that much about the tube pressure drop, do you?
As for the fouling factor, I cannot help you.
For the capacity of the pump, first you calculate the heat exchangers, its pressure drop and its vapor fraction. This gives the liquid flow, and then the pump(s).
I hope this helps

[Shashank Gujale]

Actually this design is for a proposed reboiler to a distillation column of a distillery which is distilling a fermented wash containing 8% v/v ethyl alcohol where currently live steam is fed.
This steam gives its heat to the wash and condenses, this condensed steam adds greatly to the effluent.
To reduce the effluent it is proposed that instead of adding live steam to the column, a reboiler, with proper design must be used. Prior experience on this indicates that the reboilers choke after some time and there is no sufficient heat transfer.
Keeping this in mind the only reboiler that would suit this service is obviously a forced circulation reboiler.
If velocity through the tubes alone is kept as the basis for design, then doesnt it directly give the capacity of the pump required?
How do we calcualte exactly the vapours produced?

siretb: Pressure drop in the tube is not an issue at all! What is RO?