Hi,
I am am trying to simulate a distillation tower in Aspen plus.
The data I have is a GC for a mixture of hydrocarbons C-5 to C-36.
So I've built a TBP curve using the relative area of each hydrocarbon, and its boiling point. I picked C5 boiling point as my IBP, and then picked the C6 boiling point as its percentage and so on.
I measured the solution density and tried simulating this in Aspen plus, do distill the solution to several hydrocarbon fraction.
No matter what I did, the simulation will not converge, streams keep disappearing at one stage or the other of the tower.
I think it may be because I did not add API gravity data, or relative density as well.
You think this related?
Any though on how to converge the simulation? Any specific method to approach this?
Thanks,
Ronen
It is hard to tell what your problem is from the information you have provided. If you think the stream definition is a problem (or even if you don’t) try sending it to a series of flashes instead of a column model. Try flashing the column feed at the feed tray temperature and pressure. If you get a vapor and a liquid product it should be a reasonable feed for the column if it is 100% liquid or vapor at flash zone conditions that might explain your problem.
You can try setting up a series of flashes with recycles between them similar to two or three perfect trays on the column it may give you some insight into your problem. I suspect setting up a few flashes with temperature and pressures specified similar to a few trays of a column will work well. If it doesn't take a closer look at your streams to see if there is a problem.
You can use the information you learn from the flashes to help set up the column and make sure you have both vapor and liquid traffic at the flash zone. Take baby steps set up a column with very easy constraints to hit and slowly adjust the constraints to achieve the desired distillation column conditions and performance.
I hope this helps.
Thank you for the help Rick Manner .
Seems like my issue was not having enough reflux in the tower because I tried keeping the temperature lower so I will not have cracking, a kettle model solved this, but I guess may need to start the tower with some light Naphtha at the overhead tank. Since I have around 50% with higher then 400C(those will go to a vacuum tower). Or adding some vacuum.
I have another question, I am trying to estimate the flash point of the product, and it keep give me the message - Petro Prop cannot be calculated because Assay data were not entered. What data the sim need?
Choose a data set in ASPEN that contains all of your components and enter them as separate components. Then ASPEN will calculate the boiling curve. In your distillation you need to decide how to control the column. Most natural is to define a mass or mole separation and any side streams you want, plus any recycle streams. It should converge if you are only running one unit operation with reflux but no other recycles.
If you have a problem converging a column with side streams and external recycles, then you can try opening up the recycle loops and showing them as one side stream going out and another coming in a s new feed.
Thank you Jerry Decker , I have no issue with the boiling curve, aspen calculated them when I chooses property set --> new --> and added D86, or TBP curve. When I try to choose flash point though, it will not calculate it.
Found it, chose the wrong FP property. I do not know what the other option means thou.
Calculating the flash point of streams containg H2 can be a problem. Make sure you strip out all of the H2.
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