Dear all,

We have a Natural Gas compressor system in which hydrate formation is predicted by HYSYS below 15 deg C in the third stage cooler outlet at about 49 barg g. The compressor is a three stage centrifugal machine with intercoolers and scrubbers.

I wanted to know how the hydrates can be removed once they are formed. Will they vapourise once the system is shut down and depressurised? or will they remain in solid form even after reducing the pressure? Has anybody got practical experience on handling hydrates once they are formed.

Does methanol injection help after hydrates are formed?

Can we expect a complete blockage of discharge pipe of 16" size with a normal velocity of 10 m/s due to hydrates? Does the phenomenon of hydrate formation happen gradually over a period of time once the conditions are established or is it a sudden onset?

Please help with your guidance.

Regards
Benoy

I am a bit curious and surprise that hydrate formed downstream of cooler during NORMAL operation. Do you mind to advise the composition and normal operating temperature ?

Benoy,

I have some practical experience with hydrate fprmation, but of course, every situation is unique, and you need apply yourself to the situation carefully. There are different sorts of hydrates, and I would suggest reading the GPSA book for the latest theory and practice.

a) Once hydrates form they are very, very difficult to get rid off. More often than not it means taking the equipment down and cleaning up the filter or the line mechanically. They will definitely not vaporise and solve the problem for you.

Yes, methanol injection helps up to a point. I do not know the theory on this, but practically I have observed that injecting methanol will dissolve the hydrates ( as observed by a reduction pressure difference across a filter), but till a limit. After that it is the good old hammer and tongs approach.

c) I have seen large diameter pipeline completely blocked with a plug of hydrate. The conditions may or may not be same as yours.

d) I have observed that a pressure drop across a filter rising slowly with time , presumably, due to hydrate formation. The pressure drop then suddenly shoots up. So my conclusion is that hydrate formation is gradual, but certain factors could percipitate the hydrate formation.

Regards
Rajiv

Hello Benoy and welcome to Muscat if you joined here recently,


1. Hydrate predictions in HYSYS are not very accurate and reliable, especially in two-phase or three-phase systems. If possible, sample of condenser stream should be submitted for laboratory analyses and estimation of hydrate equilibrium conditions.

2. Pluggage of large diameter pipeline with hydrates is not likely to occur, if other type of "obstacles" are not present: filters, coalescers etc. High process velocities also inhibit hydrate formation: precipitation of hydrates is a function of system kinetics as well.

3. Once hydrates are formed, injection of CH3OH is not of much help - if we are talking about permanent and complete removal. I have seen high-pressure methane being injected into pipelines in order to remove plugs of hydrates.

There is a lot of information available on the internet, I suggest you to look for these materials.
Best of luck,

Dear Zauberberg,
Could you please provide some useful links so that people like me who are new to gas processing will get the benefit of it.

In my experience, Dr. John Carroll of Gas Liquids Engineering, Ltd. is the one acknowledged expert in natural gas hydrates that catches everyone’s attention when he writes on the subject. This Canadian is well known for his expertise in this field and anyone dealing with natural gas dehydration, transport and sales should be right on top of each of his books – the best of the lot which is :

Natural Gas Hydrates - A Guide for Engineers; 2003 Elsevier
This book provides an accurate review of what hydrates are and under what conditions they will form, and it provides the engineer with the methods to predict the occurrences of hydrates.

The hard cover copy sells for approximately US$100 and is more than worth it.

Mr Carroll is already a legend in this business. He has presented several methods for sour gas hydrate formation predictions by using hand calculations and different softwares available in the market. I remember his article (also available on Gas Liquid Engineering website, http://www.gasliquids.com/papers/gpa_dublin_2004.pdf) where he stated that HYSYS simulator gives very accurate hydrate formation points.

However, I encountered large discrepancies in actual (field) vs. software data in several occasions, which convinced me that Hysys cannot be used in 100% cases. The more I think of it, the more I feel sure there cannot be any software which can predict hydrate conditions in sufficiently accurate manner.

Nevertheless, Mr Montemayor's reference is a very good one and it is highly recommended.
Best regards,

Dear all,

Thanks a lot for sharing your knowledge on the subject.

I am attaching the stream composition. (for JoeWong) The normal operating temperature is 60 deg C. However abnormal operations (winter conditions) may lead to hydrates as the predcited hydrate formation temperature (by HYSYS) is about 15 deg C.

Thanks and regards
Benoy

According to Hysys, you have liquid phase present in the system (vapor fraction is 0.99).
Which hydrate equilibrium model have you used for hydrate prediction?

Benoy,
Ok...No hydrate formation in normal operation and will experience hydrate formation during winter operation downstream of cooler. This is pretty common at site experiencing low ambient temperature. Personally, hydrate formation temperature of 15 degC is pretty low. In many events, i have experienced hydrate formation temperature around 25 degC.

Looking at composition, i guess it is wet gas (pretty high water content). Those high potential of hydrate formation if vapor temperature is allowed to drop below hydrate formation temperature.

Looking at the CO2 (with No H2S) level, pretty low, don't see much deviation from the normal prediction by GPSA or HYSYS.

There are many ways to tackle this kind of common issue.

1) You may consider to provide a temperature transmitter with low temperature alarm downstream of air cooler outlet Set it a bit high (say >10 degC above hydrate formation temperature). During winter, stop some fans.

2) You may consider to provide temperature transmitter downstream of cooler and Variable Speed Drive for air cooler motors (may be 50% subject to your analysis) so that the vapor temperature will be maintained at some point above hydrate formation temperature with some margin e.g. 5 degC. VSD is a pretty good option. Other than above mentioned benefits, there are other benefits as discussed in Variable Frequency Drive (VFD) helps in Many Aspects (click here)



Once hydrate is formed, it is difficult to remove but it is possible by heating it up.

You can imagine hydrate like a CO2 solid. In atmosphere it slowly vaporise when it absorb heat from ambient.



It helps if the methanol is contacted with the hydrate but inefficient as compare to condition before hydrate is formed.



The velocity is pretty low. So hydrate will slowly form. How fast the hydrate is subject to temperature, quantity of hydrate former, etc.

Once it formed, it will slowly reduce the cross sectional area, pressure drop increased, etc, experienced operator would be able to know by monitoring plant performance parameters.

You scenario is pretty common. Identify some performance parameters and provide sufficient instruments to ensure operator is capable of identify potential hydrate formed.


Another thing to share with you. Nowadays, hydrate formation temperature is normally included in the datasheet and submitted to air cooler vendor. Don't forget this. Read more.

Hope above help.

As a matter of general interest attached is a photo of a big hydrate plug. In the literature I read that plugging 14" lines with hydrates is not unknown.

Regards
Rajiv