6 replies to this topic

[Sampath123]

Hello all. I am simulating a stabilizer column in hysys. The column got converged with required specs. In column environment the vapor from top tray to condenser is at 75 deg C and vaporfraction is 1. I copied the same composition to a stream outside the column environment but here it is showing vapor & liquid fractions at 75 deg C. At 79 deg C I am getting complete vapor fraction outside the column environment. Because of this variation in temperature the condenser duty calculated by column differs from the extrnal cooler which calculated the duty bases on copied stream. I want to export top stage vapor composition & properties to EDR to design condenser. So I copied top vapor composition from column environment to oitside but it is causing the above mentioned problem. Please help me with it.

[latexman]

What pressure is at top tray to condenser? What pressure is at the stream outside the column environment? Different pressures would explain the temperature difference.

[Sampath123]

@latexman thankyou for the response. Top tray pressure and outside stream pressure is same.

[latexman]

Is it because of the difference between bubble point and dew point at the same composition?

[Pilesar]

Are you modeling the column as theoretical stages or as trays with efficiencies applied? It is better to use theoretical stages so that the top vapor will be at its dew point.

[Sampath123]

@Pilesar thankyou for the response. Yes I modeled with overall efficiencies and actual trays as theoretical trays divided by efficiency. By considering theroretical stages problem got solved. Thankyou once agian for sharing your knowledge.

[Pilesar]

Thanks for letting us know the result. Theoretical stages are so much easier to use in simulations once you can adjust your mind to think that way. There are some situations where tray efficiency can be useful such as two side draws very close to each other, but even then I try to use theoretical stages if at all possible and make adjustments using 'engineering judgment' or additional detailed calculations after the main model converges. When building distillation models, I found it helpful to map actual trays to theoretical stages in a spreadsheet. When trays are numbered from the bottom up and stages numbered from the top down and the condenser is a full stage but not a tray it can get confusing trying to figure out which stage gets the feed and which stages have temperature measurements in the field. I think the software developers might provide a bit more help in this area, but they do a pretty good job with the tools and the answers are only as good as the inputs we give.