distillation column simulation 2

[fifi121]

Hello,

I have a distillation column and the overhead stream is to be condensed, pumped and some %age of the pumped liquid is to be sent back to the column as Top feed stream. As to converge the column all the feed streams to the column must be already converged, I can not converge the column if I dont specify the pump discharge stream. and by specifying the pump discharge stream, it shows inconsistency message in the stream and can not converge...
so anyone can please guide how to simulate the column overhead that is sent back to the column as reflux stream after condensation???

 

[Homayun]

What simulation program are you using? Provision, ASPEN, HYSYS ??

Usually the program will determine how much reflux is needed and you don't have to specify a pump for that...If there is a message about inconsistencies, it's probably due to the fact that the amount flow you are sending back to the tower is less than what the tower needs to meet the specifications.

 

[fifi121]

I am using Hysys. Actually I am trying to simulate the condensor and reflux with pump separately, not the ones available with the column Palette. so that I can vary the coolent medium for condensor etc. When I condense the overhead outside the column and then feed it back to the column top tray as reflux it does not converge.
can you please have a look at the attachment to get better idea. Thanks

 

[mfelzien]

fifi121,

I think your column may be under-specified. Check the DOF in the HYSYS window (everything should be green). If not that sometimes adding another specification and rebooting your computer helps. HYSYS can be kludgy when you have played around with a simulation for a time. You may need to specify something on the bottoms. Steam flow rate etc.

Yours,

Mike F.

 

[fifi121]

no the column is not underspecified.
I am having trouble simulating overhead condensor and pump outside the column.

 

[Zoobie]

If you are simulating the overheads outside of the column unit then you need to have a Recycle operation installed in your simulation. Then you put in a best guess for the pump discharge (or other stream that you may choose for the recycle). You may need to play around with iteration parameters on the column and recycle if they start to fight each other.

If you already have the recycle installed then another less likely cause of your problem may be the flash calcs between the column subflowsheet and the main flowsheet...I think I only had this happen to my once when I was trying to get overly creative with my simulation.

A screen cap from HYSYS might be able to help us figure out the problem...otherwise we are just speculating here.

 

[fifi121]

Please have a look at the column T-1002 (the 2nd one)in the attached screen shot. I want to put the condenser outlet stream (OVHD reflux) back to the column in place of ‘Reflux return’ stream and I want to add a pump in between. Now as I try to replace the ‘Reflux return’ stream with the ‘OVHD reflux’ stream, the column can not converge because the ‘OVHD reflux’ stream is not converged at the beginning of simulation.
I guess the ‘Recycle’ option is installed but I never used it before. Is it the proper way to do the reflux by using Recycle button?

 

[Zoobie]

Yes, you need to use the recycle operation (diamond with an R in it). Read the AspenTech help on how it works as you may need to tweak it to get it to converge. You should probably install it in a place where you have the best guess at the stream parameters.

 

[dcasto]

Here, try this, run the column as a refluxed column, it will tell you exactly what the composition is and the re simulate with your system.

Hysys allows you to use reflux condensor duties to be passed on to your "condensor" in the model.

 

[Zoobie]

dcasto's suggestion is very helpful. It also points to a valuable lesson for people that use process simulators. Simulating is not simply an exercise in connecting a bunch of things together to make it look like your actual PFD/P&IDs. You need a sound understanding of the underlying fundamentals (heat and mass transfer, fluid mechanics, thermodynamics, etc.). I have simulated processes where the flowsheet looked absolutely nothing like reality, but this was required for the nuances of the vapor-liquid equilibrium (crude vacuum tower). The other thing that a good modeler needs to know is some of the math in the machine (software)...simple stuff like what math is used to get a recycle to converge. After all, a process simulator is just a big dumb calculator.

 

[dcasto]

Zoobie

You reminded me of a process engineer setting up a simulation once. The plant had platefin flooded chillers with external vessels that you controled the level of the refrigerant in. He litterally put a flash drum in front of the exchanger and then spent days trying to fiquire out how he could change the level in the exchanger with the simulation package. He was not doing a full exchanger analysis with dimensional data.

But, he had to make the process sheet look just like the P&ID.