4 replies to this topic

[kkts45]

I'm a student using HYSYS for the first time this semester, hence my apologies for possibly sounding ignorant. I had a query regarding the absorber column sub flowsheet.  

 

Based on initial estimates of parameters like reflux ratio and number of stages from the shortcut model, I would set up a rigorous distillation simulation model and then I would set 2 active specifications, which were the purities of the top and bottom product streams. HYSYS would then calculate the required duties of reboiler and condenser for said purities and the product flowrates.

 

However in absorber column since there is no degree of freedom to specify, I'm unsure how to proceed. I specified the composition (that is to be obtained) in the outlet product streams. 

 

I have a case study to show the energy savings by using mechanical vapour recompression of vapour outlet compared to conventional distillation column for the separation of an equimolar mixture of i-butane and n-butane to streams with product specs of 90%

 

As an initial guess I added values for the recycled streams TopIN and BottIN (file attached), thinking it would aid convergence.  In order to let the other equipment converge, I input values of flowrates of product from absorber equal to the values obtained from the rigorous distillation column. But by doing so, I'm seemingly not getting any benefit from the absorber column if Im declaring all the properties of the products.

 

I've clearly made a mistake elsewhere in the flowsheet, but since everything else has converged I'm not sure where. Is it due to less flow back to the column?

 

Any help would be appreciated. Do let me know if more information is needed. I was unable to upload the hysys file

 Capture5.PNG   12.31KB   8 downloads

[thorium90]

Post screens of the column subflowsheet as well as the parameters that you have input. Hysys files can be uploaded by zipping them up.

Edited by thorium90, 03 July 2015 - 11:30 AM.

[Bobby Strain]

If I were a new HYSYS user, I would use a conventional distillation operation to begin. You can export streams from the distillation operation to simulate the heat pump operation. When you are satisfied with this model, then you can create one to your liking. Heat pump systems rarely balance duty for reboil and condenser. So, some supplemental cooling or heating is required. If you start as I suggest, you can then have enough information to build and specify your complex model.

 

Bobby

[Mathi]

kkts45

 

I would suggest you to break the connections to the other units, leave the recycle streams and simply do the absorber column first. After convergence of the absorber column, add the recycle streams and run. 

 

Sometimes the pressure drop across the column might influence the convergence, so try adjusting the pressure drop

 

I hope these tips will help you. 

[kkts45]

Thank you for the tips. I did get the column to converge eventually. I had made a mistake while specifying the heat exchanger's details, causing infeasible temperatures.