9 replies to this topic

[Zauberberg]

When calculating hydrate formation conditions for a given process stream, knowing its pressure and composition, Aspen software output defines a “hydrate formation temperature” – which is a fixed value for that particular pressure and composition.
Searching through literature resources, I found that for each operating pressure only one temperature corresponds to the hydrate formation point.

1. If process stream is flowing at temperatures that are below hydrate formation temperature, will hydrate formation occur?

2. If two- or three-phase fluid is involved in hydrate formation study, which hydrate prediction model should be used? Usually there are “Symmetric”, “Asymmetric”, “Vapor only” or “Free water present” correlation models. The use of these predictive models is based not on the number of phases present in the system, but on the phase that is in equilibrium with hydrate solids. How to establish which is the correct equilibrium model, since each of these comes with a different hydrate temperature as calculation output?

3. When developing a P-T hydrate curve for a given stream, is there any way to switch between different hydrate prediction models within the P-T envelope – since the fluid may exist as single phase (vapor), two-phase (vapor + hydrocarbon liquid) or three-phase (vapor + hydrocarbon liquid + aqueous phase) system, or it has to be done manually?


Thanks in advance,

[benoyjohn]

Zauberberg,

The HYSYS envelope utility now comes with options to select the hydrate prediction model.

However I have noticed some mismatches between the results from the hydrate formation utility and the envelope utilty.

In one case, the hydrate formation utility predicted hydrate formation at 30 deg C and 1 barg for a stream of natural gas which I thought was not correct. The envelope utility however did not plot the points at this low pressure. The curve provided with the envelope utility ended at about 7 barg and 0 deg C.

Did you have any simillar experience?

regards,
Benoy

[Zauberberg]

Hello Benoy,

I have had similar issues when using Aspen 2006+ versions; I have also observed inaccuracies with phase envelopes and critical points - and I never went back to AspenTech in order to investigate the problem further. I guess I was simply too lazy to do so.

Does your system have any hypothetical components or "unusual" compounds?

[benoyjohn]

Hi Zauberberg,

Yes. The system has hypothetical components.

Any clues?

regards,
Benoy

[Zauberberg]

Try to delete those and replace them with corresponding true components (usually n-paraffins available in HYSYS library). This may not give you 100% accurate results, but it will give you at least the sense of hydrate formation parameters that are not too far from the real values. Finally, we'll never see hypos anywhere out in the real world.

[benoyjohn]

Thanks Zauberberg.

I tried after deleting the hypos and substituting with n-parrafins. But the result is the same.

I have reffered the problem with the case file to ASPEN. They have admitted that there is something wrong, I guess with the software. They have forwarded it to their development team.

I will keep you updated on the results.

regards,
Benoy

Thanks Zauberberg.

I tried after deleting the hypos and substituting with n-parrafins. But the result is the same.

I have reffered the problem with the case file to ASPEN. They have admitted that there is something wrong, I guess with the software. They have forwarded it to their development team.

I will keep you updated on the results.

regards,
Benoy

Hello

I've experienced similar problems with the hydrate formation curve, but i think I've found the problem. In the graph I noticed that the axis doesn't contain any units. I usually work with my own preference set, where I use bar_g as the standard pressure unit. After a long time of troubleshooting, a realized that if the unit on the pressure axis was bar_g, the small, "compressed" hydrate curve I got was in fact correct. So I loaded the default preference set, with kPa as the pressure unit, and tried to make a new curve, witch then became as I expected it to be.

So I guess the bug in the software is a missing conversion between kPa and other units for pressure, and the way to handle it, at least for now, is to make sure your pressure unit is kPa when making the hydrate curve.

I hope this was of help.

Best regards, Øystein

[benoyjohn]

Øystein, Zauberberg,

I tried what Øystein told but it does not help.

Meanwhile I have got reply from AspenTech that their development team confirmed that there is a bug/problem. As per them the correction will be made available in version 7.2 due out early 2010.

regards,
Benoy

benoyjohn

Attached is an example of what I experienced. The first curve I created with the envelope utility while using my own preference set, where I use barg as the unit for pressure. Then I changed the preference set back to the standard one, the one witch is automatically loaded when I open HYSYS, where kPa is the unit for pressure. Then I again utilized the envelope utility on the same stream, and got the second curve.

Regards,
Øystein

 Hydrate_curves.doc   142.5KB   77 downloads

[VikingUK]

You are better off simply calculating the temp/press formation points
see attached DOS programme

Attached Files

•  CSMHyd.zip   186.25KB   66 downloads