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Aspen Hysys - Hydrocarbon Dewpoint Modification

hysys gas simulation dewpoint topside process model phase envelope

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#1 roblar03

roblar03

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Posted 05 September 2022 - 05:51 AM

Hello everyone! I have a challenge at work that I would love some insight on. I have recently started in my new position as junior process engineer, and to develop a grasp at process modelling I am currently tasked with creating and simulating a fictive and generic topside oil and gas handling process in Aspen HYSYS. I have reached a point where the challenge is to modify the hydrocarbon dew point of the gas to meet export specifications. I have attached a PDF of the flowsheet diagram of my simulation here:

 

Attached File  PFD.pdf   187.99KB   14 downloads

 

I have also uploaded the phase envelope of the gas export stream here:

 

Attached File  envelope.PNG   150.24KB   1 downloads

 

If I could, I would upload the HYSYS file, but the uploader does not allow me to. Please forgive my ignorance if there are other pieces of information that should be added here. Just let me know and I will try to provide.

 

The challenge is to make the gas export stream have a hydrocarbon dewpoint of -2C at 70 bar (a). The actual pressure of the gas export stream also needs to be at 180 bar. As it stands, the current dewpoint of my export gas mixture is approximately 27C at 70 bar. My guess is that I would have to condense some of the heavier components in the stream in order to lower the dewpoint of the mixture, and to do this, I would have to lower the temperature somewhere in the process. I have tried two solutions so far:

 

1) I tried to place a cooler at the end of the gas product stream to cause condensation and then I tried removing the condensate with an additional scrubber (these are not shown in the uploaded PDF). The problem with this solution is that the stream is pressurized to 180 bar, causing it to be in a supercritical state. As far as I can tell, this raises a couple of issues. Firstly, a phase separator unit at this point in a realistic setting seems to me to be highly impractical and unnecessarily difficult. Also, HYSYS forces the streams out of the separator to be either 100% liquid or 100% vapor (a temperature cutoff point at approximately 18C seems to determine this. If the temperature is higher, the total molar flow goes out of the separator as gas, if the temperature is lower the total molar flow goes out as a liquid), making phase separation at this stage pointless in the context of designing a good model, as it does not even seem possible given the properties of the stream. I have tried to modify the dense phase tuning factor to see if this would cause a change in stream behaviour, but it does not. Also, I am unsure about the actual physical nature of such a stream, so troubleshooting becomes extra difficult. A screenshot of the stream properties is shown here:

 

Attached File  gasproduct.PNG   233.91KB   1 downloads

 

2) From this, I wanted to try adjusting the temperatures in the coolers between the compressors and scrubbers further back in the system. I don't know the practical limitations of topside coolers, but I assume that seawater is used for cooling and that stream temperatures lower than 30C would be at the very least impractical. Also, the phase problem, as mentioned in the paragraph above, causing 100% stream recycle below a certain temperature also makes the simulation seem quite fragile.

 

I have made a spreadsheet of the different process parameters that would be adjustable in the system. This is shown here:

 

Attached File  processvariables.PNG   491.31KB   1 downloads

 

Since these attempts have not been fruitful, I am sure that there are a ton of things to consider that I am unaware of. I am also pretty sure that my model has plenty of flaws, but I would love some advice as to how I should approach a possible solution. One again, I apologize for my ignorance, but I am trying to learn to the best of my abilities. Please let me know if there is something I could do better. If you want the HYSYS file to have a look at, just point me to somewhere I would be able to upload it/give it to you, and I will do so as fast as possible.

 

Thanks for your attention and I hope you have a wonderful day!

 

Best Regards

Robert A.G. Larsen - Junior Process Engineer, Aker Solutions


Edited by roblar03, 05 September 2022 - 06:42 AM.


#2 roblar03

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Posted 05 September 2022 - 08:06 AM

Okay, so I got some valuable feedback from a talented colleague. He told me to insert a cooler after the water remover and remove the condensate before compressing the gas to 180 bar. Of course it was silly to do any operation of the stream *after* it had been pressurized so much. The result was a PFD looking like this:

 

Attached File  PFD2.pdf   187.82KB   16 downloads

 

This allowed a reduction of the temperature in the two-phase region of the fluid since this happens below the cricondenbar. Now I have a phase separation such that I can remove the condensate and ensure that the gas meets specifications. The solution is so simple that of course I feel like should have understood this from the beginning, but it is what it is! Maybe someone else learns from this also.






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