Hydrate Formation

[EmmanuelTop]

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,

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[dcasto]

1. Yes they can, hydrates involve kenetics too (time).

 

[JoeWong88]

EmmanuelTop,

My 2 cents of input...

I guess the "Aspen software" that you mentioned is ASPEN HYSYS.

1. I agree with dcasto. As long as the operating temperature below the hydrate formation temperature with right mixture such as lattice molecule (e.g. water) and hydrate former (i.e. C1, C2,Co2, etc). It also needs proper settling surface, time to form, etc. GPSA section 20 has better discussion.

As per my limited knowledge, conventionally water molecule with sufficient quantity is required for hydrate formation. Those gas is dehydrated (conventionally lower than 7 lb/mmscf) before it is feed to cryogenic section. Nevertheless, some study have shown that hydrate may still form even without water (HYSYS is predicting it). Other Che Jedi who has more input, please advise.

2. HYSYS by default will choose right model for the condition (vapor only, liquid-liquid, free water). If you read section 14.12 in HYSYS Operations Guide, Vapor only model is for system without free water. Symmetric model is for system with free water. Asymmetric model will choose either Vapor only or Symmetric. The main distinction here is present of free water (aqueous phase).

3. I guess not for time being.

Hope above helps.

JoeWong