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
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Posted 03 May 2017 - 07:10 AM
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
Posted 03 May 2017 - 07:39 AM
(in general) reboilers are equilibrium stages
but for the specific I think it depends from the way you solve the column.
Posted 03 May 2017 - 08:59 AM
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.)
Posted 07 May 2017 - 04:59 AM
Posted 07 May 2017 - 12:45 PM
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.
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