Help please.. The actual reboiler temperature is higher than the simulation

[Theophile]

Dear everyone

Now, I'm simulating the pilot extractive distillation column which employs NFM to capture toluene.
The reboiler is an internal reboiler and electrical wire is used to provide heat instead of heating fluid.

The bottom outlet temperature is 183 C in the simulation that give the mass fraction of toluene 0.15

but in the real world its temperature is at around 190 C that give mass fraction toluene 0.15 (using GC)
and in GC, it found only 2 components NFM and Toluene

I'm curious that is there a possibility that the bottom outlet temperature is not saturated ?

Thank you very much for your help.

 

[PaoloPemi]

question is not clear,
if you have a plant working and try to obtain the same results from a software, check thermo models etc. etc. (every software may require different settings, and you may contact assistance for details) the same if you are simulating a plant and obtain strange results...
Finally, if you observe strange conditions (such as temperatures or pressures) in a working plant, it is not a matter of software, may be someone can help but you must provide a more detailed description of the problem...

 

[don1980]

Your results (183c vs 190C) is not at all surprising. For distillation column bottoms streams, it's very common (typical in my experience) to find such deviations between the actual bottoms temperature and the simulation value. The actual temperature is usually hotter than the simulated value. The explanation is simple - the bottoms composition is slightly different from what you have simulated. In most cases there's an array of heavy components, each being a very small fraction of the total stream, which are not in the simulation. The net effect is that the actual stream is a little heavier (lower vapor pressure, higher temperature) than what you've simulated in the model.

You can eliminate this deviation by spending a lot of time getting a more detailed analysis of the stream, and then refining the thermo properties in the simulation. Or you can spike the simulation bottoms stream with a heavy pseudo component to narrow this temperature gap. In my experience, the latter option is best - much faster. I've designed many reboilers and bottoms exchangers using this method. As long as you understand what you're doing, this isn't an obstacle for successfully completing a process design.

 

[georgeverghese]

In a process simulation for this non ideal mix of NFM ( n-methyl formamide?), toluene and some light component, a lot depends on the thermo model and additional data you may have had to provide to get some reasonable VLE that matches actual lab information in the pressure range of interest - did you run this "calibration check" before you started this simulation ?

If you have and they match well, then maybe there is poor vapor liquid separation in the reboiler sump, so the reboiler has to operate at a slightly higher temp to counter the effect of the contamination with the lighter component.

In real practice, the reboiler sump usually feeds the adjacent product sump through an underweir that prevents vapor bubbles from carrying under in to the product.

 

[MortenA]

Is the pressure the same as in your simulation?

 

[mbt22]

Have you validated the thermodynamic method against actual experimental data?

Matt