7 replies to this topic

[khattabon]

Hello,
I want to calculate the ppm of aromatics that is allowed in a natural gas stream mixture which is about to be liquified so that the aromatics won't freeze and plug the exchanger.
How can I approach this case. Also can it be done in Hysys or not. Please refer to any reference/textbook. Thanks a lot.

[Pilesar]

The hydrates calculation in Hysys is a good one. Other simulators also have hydrate predictors, but using different methods. From my research, Hysys has the most accurate predictor.

[Bobby Strain]

Normally butanes are removed to prevent freezing. At the level they are OK, there are no aromatics to freeze. Yours must be an academic question since all liquefaction processes are licensed technology.

 

Bobby

[khattabon]

-The hydrate utility in Hysys predicts hydrate formation (water) not aromatics freezing.

- It's a licenced technology. The heavies are removed in a dethanizer before going to liquefaction. However the column overhead can still contain heavies that can freeze. So it is required to calculate the maximum allowed conc. of aromatics in the ovhd stream to avoid freezing.

[Bobby Strain]

Your licensor knows the answer.

 

Bobby

[PaoloPemi]

"I want to calculate the ppm of aromatics that is allowed in a natural gas stream mixture which is about to be liquified so that the aromatics won't freeze and plug the exchanger"

you can ask the vendor as suggested or adopt a software which supports multiphase operations,
to estimate solid fractions and freezing points for (hydrocarbons / non-hydrocarbons mixtures) I utilize Prode Properties (see prode.com),
Prode can solve multiphase flash operations (P-T, H-P, H-T, S-P, S-T, V-P, V-T...) for vapor + liquids + solids + hydrates etc. with different models and (with commercial versions of Prode) you can calculate vapor-liquid-solid phase diagrams (should you need these)


 

[Pilesar]

When I investigated similar suspected cryo freezing of aromatics several years ago, I used the TFREEZ option in AspenPlus to find the temperature where a component would begin to freeze out of a stream. The software vendor's technical support team could tell you more about the accuracy of that calc and the references for its source code. Similar calculations may be available in other software also.

[shvet1]

Allowable aromatics content depends on particular LNG process, equipment design and guaranteed non-stop run. For example sometimes mercury interferes this limit (e.g. ~5 ppm Ar @ ~10 ppb Hg). As per my experience freezing point alone is not enough to guarantee proper operation of liquefaction stage and quality of LNG. You should consult a Licensor of your LNG.

 

Limit of most impurities is dictated by experience, not a process feature. The same situation is in air cryo separation, e.g. allowable content of olefins and acetylenes is not obvious as dictated mainly by experience and safety factors.

 

For info

1/ Note that you are not able to check inaccuracy and repeatability of calculations you are looking for and therefore risks for downstream stage.

2/ Check inaccuracy of calculations of impurities in deethanizer reflux. It is doubtful that your method is able to predict 1/1000000 with tolerable accuracy. It is often modelling of fractionation of ultrapure products (ultraimpurities) has no sense.

Edited by shvet1, 03 July 2023 - 05:54 AM.