Kettle Reboiler Design 1

[villaflora]

In our plant we have a distillation tower (column A) whose energy is provided by a thermosifon reboiler which is fed with steam at 6,5 Kgf/cm² g .
The column A bottom temperature is kept at 90ºC ( pressure 0.2 kgf/cm² g). Bottom flow rate is 2200 kg/h ( enthalpy of vaporization is around 80 kcal/kg , heat capacity 0.5 kcal/kgºC ). Boilup rate is 5000 kg/h.
In order to reduce steam consumption in column A, I am studying the possibility of installing, at the bottom of colum A, a second reboiler ( I thought a Kettle could be a good choice). The hot stream of this kettle would be vapor supplied by the top of another distillation tower (column B).
In other words I intend to replace the condenser of colum B by a kettle instaled at colum A. This kettle would condense (totally or partially) vapor from top colum B, and would supply energy to the liquid at bottom of colum A. Thus, existing reboiler duty would drop.
Vapor from top of colum B flowrate is around 4500 kg/h, latent heat of vaporization is around 80 kcal/kg / heat capacity of liquid is also 0.5 kcal/kgºC.
The problem is how to design a reboiler (kettle for these conditions). The main doubt is which temperatures I should consider for getting started with preliminar design?
Please somebody could help me with a step by step procedure, or a good source of information?

 

[srfish]

Hi,

You need to decide how much reduction in steam you need. Then this would set the heat load of the new kettle. Once the kettle heat load is set, you can back into the other information like outlet temperatures, amounts of condensate from the tubeside and amount of flows from the kettle side.

For more information on kettle design, I like Chapter 10 for reboilers in Process Heat Transfer by Serth.

 

[dcasto]

the kettle reboiler be difficult to control because you'll need a level control on it. Then the level control on the tower would be in conflict.

Usually when interconnecting heat loads, a second reboiler is used several trays higher so that it can maximize energy cature and then the lowe reboiler with steam trims the heat and the purity.

 

[villaflora]

Dcasto, thanks...I haven't thought about level control yet...but I agree with you that level control could be a problem...instead, if I decided to use a second reboiler (several trays higher) would I need a special tray ( with some kind of collector for example)?

Srfish, thanks for the source...Serth is indeed an excellent book!

 

[dcasto]

yes, you would need a hat tray with sufficient height to run the thermal syphon.