4 replies to this topic

[freeman]

Dear All,

I have a question about distillation design. Is thermosiphon reboiler considered as an equilibrium stage (like kettle reboilers) in distillation design?

 

Thank you in advance.

 

Freeman

[serra]

(in general) reboilers are equilibrium stages

but for the specific I think it depends from the way you solve the column.

[Pilesar]

Your question is more complicated than it appears. If you were doing the calcs by hand, you should assume a thermosiphon reboiler provides only a partial theoretical stage. If you are using a commercial simulator for the calcs, the thermosiphon reboiler and sump portion of the column is calculated as the last stage or even the last two stages depending on the configuration of the sump. (Sometimes the sump is divided into sections and the hot liquid from the thermosiphon return may cause additional vaporization from one of the sections.)

[freeman]

Thank you so much for the reply!
Please consider the determination of equilibrium stages through the Mccabe Thiel binary distillation method by hand. In the kettle reboiler the vapor and liquid are separated in the heat exchanger. The vapor is returned to the column and the liquid is exited from the exchanger as the bottom product. so that the vapor and liquid are in equilibrium.

In thermosiphon reboilers the liquid is partially vaporized in the heat exchanger and two phase fluid is returned to the column. Of course in thermosiphon reboilers, the vapor and liquid are separated after entering the column.

What is the reason to consider the thermosiphon reboiler as an equilibrium stage.

Thank you in advance.

[Pilesar]

For a column with a thermosiphon reboiler, make sure in your hand calculations that you have enough theoretical stages in the column while not including the reboiler. Not all thermosiphon configurations are alike. Some thermosiphons are 'once through'. Some columns have a baffle in the sump that divide the bottom liquid into two different compositions after the thermosiphon liquid is preferentially returned to one side of the baffle. The liquid product may be from only one of these sides of the sump. In that configuration, the vapor to the bottom tray comes both from the thermosiphon directly as well as vapor flashed when the hot returned liquid mixes with the cooler sump liquid. Unless you know the configuration, for hand calcs just do not count the thermosiphon reboiler as a stage. In other words, counting the thermosiphon as 'zero equilibrium stages' is safe and conservative. 

Most engineers use some sort of commercial steady-state software instead of hand calcs. The software keeps track and reports the calcs for the thermosiphon by using the last stage or last two stages of the distillation column. Since that is the way the engineer interacts with the design, it is common to refer the the reboiler as the 'last theoretical stage' because that is where the results are found.