How to calculate the unavoidable exergy destruction of demethanizer and deethanizer in the attached figure? Some paper said using minimun reflux ratio to calculate unvoidable exergy destruction of columns, but I find it impossible used it in complex demethanizer and deethanizer because infinite stages cannot be achieved in simulation. So I think wether it is right that I just increase the theoretical stages to a certain value and I define it as unavoidable condition.
Dear sir
It is true you can assume maximum efficieny and after that you subtract optimum exergy destruction from real.
I almost answered this several times hoping somebody else would point out that real distillation columns are very different than simulated tray to tray columns - they usually require 20-50% more trays than the simulation suggests are needed. There are many possible reasons for this. Liquid carryover with the vapor and vapor entrained in the liquid are a few of many reasonable explanations. Flashes in large drums with distinct liquid and vapor zones often behave more like the simulations suggest they should - commercial sized columns usually do not. You could set up a bunch of flash drums in series each representing a tray of the column and send liquid down towards the flash on the bottom and vapor going up to the flash on the top with cooling to produce "reflux" to do your Gibbs free energy/exergy calculations
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I hope this answer helps. Let me know if you want to explore this or other routes in more detail.
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