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Determining Hydrate Formation Temperature




Determining Hydrate Formation Temperature Hydrates are crystalline solid compounds formed from water and smaller molecules in hydrocarbon fluids such as methane, ethane, propane, nitrogen, carbon dioxide and hydrogen sulfide.

Hydrates are a nuisance, since they can block pipelines leading to stoppage of pipeline transportation operations and hence are an important aspect of pipeline flow assurance that needs to be addressed.

Hydrate formation in pipeline requires three conditions to exist:

1. The right combination of temperature and pressure. Hydrate formation is favored by low temperature
and high pressure.

2. A hydrate former must be present. Hydrate formers are the hydrocarbons mentioned as above.

3. A sufficient amount of water – not too much, not too little.

Certain other conditions in the pipeline enhance the formation of hydrates and are listed below:

A. Turbulence:
This can be either due to high velocity or agitation of the process fluid. High velocities in pipelines
can occur at any sudden restrictions in the line such as a choke valve. In gas flow a large pressure
drop across the choke valve causes the temperature to drop due to the Joule-Thomson effect which
favors hydrate formation

B. Nucleation Sites:
In general terms, a nucleation site is a point where a phase transition is favored, and in this case the
formation of a solid from a fluid phase.

Good nucleation sites for hydrate formation include an imperfection in the pipeline, a weld spot, or a
pipeline fitting (elbow, tee, valve, etc.). Silt, scale, dirt, and sand all make good nucleation sites as
well.

C. Free Water:
Free-water is not necessary for hydrate formation, but the presence of free-water certainly enhances
hydrate formation.

“HYSYS” has a utility called “Hydrate Formation Utility” which predicts the hydrate formation temperature of any defined stream for a given stream pressure and the hydrate formation pressure for a given stream temperature. Most process engineers having access to “HYSYS” would find it convenient to use this utility to determine hydrate forming conditions.

In addition to “HYSYS” an old DOS based program with the name “CSMHyd” developed by the “Colorado School of Mines” also predicts the hydrate formation pressure for a given temperature. This program is available for free download at:

http://hydrates.mines.edu/CHR/Software_files/CSMHyd.zip

In addition to "HYSYS" and "CSMHyd", I had done some of my own investigation and compilation on the subject of hydrate formation conditions in natural gas. The focus of this investigation was to find out whether there were some empirical methods to determine the “hydrate formation temperature” given only the natural gas pressure and the molecular weight / specific gravity of the gas. My investigation was successful considering that I found not one but several empirical methods to determine the “hydrate formation temperature” given only the natural gas pressure and the specific gravity or molecular weight of the gas.

An important point to note is that while both “HYSYS” and “CSMHyd” require that the gas composition be known, whereas the empirical methods I investigated do not require natural gas composition as such. Just the natural gas molecular weight or the natural gas specific gravity allow the determination of the “hydrate formation temperature”.

The end result of this detailed investigation resulted in the generation of an excel workbook where these empirical methods have been represented with example calculations. This blog entry shares the excel workbook.


Download the MS Excel Spreadsheet here

Any comments and observations would be welcomed from the readers and members of “Cheresources”.

References for the hydrate formation mechanism are as follows:

Natural Gas Hydrates – A Guide for Engineers by John Carroll

Section 20- GPSA Engineering Data Book, 11th Ed.

Regards,
Ankur





Dear Shri Ankurji,
Thanks for publishing the subject"Hydrate Formation". I was going through both the DOS program as well as the Spreadsheet. My only question is in your spreasheet, why the CO2 and N2 are not included. Generally they are present in Hydrocarbons. I have the compositions for one of the installations as follows:
1. C1 0.48408
2. C2 0.15373
3. C3 0.16773
4. iC4 0. 04095
5 iC5 0.02040
6. nC5 0.02333
7. nC6 0.01860
8 CO2 0.03280
9 N2 0.0009
10 H2S 0.00067
The operating pressure is 14.7926PSIA and the operating temperature is 104Deg F.
Would you please throw some light on this please?
Sncerely,
samrat
Samrat,

Spreadsheet is not based on compositions. It is based on either NG specific gravity or NG molecular weight. The inputs in the first threee worksheets are "pressure" and "specific gravity" and in the last worksheet are "pressure" and "molecualr weight" to obtain the hydrate formation temperature. This is also clearly explianed in my blog entry on the subject.

Regards,
Ankur.
Hi Ankur,

This is a great effort. what about the accuracy of your spreadsheet? Are the results accurate enough to compare with Hysys?
P. Engr,

The empirical equations consider the commonly found hydrocabons and impurities in NG such as C1, C2, C3, CO2, H2S, N2. Considering these the results are fairly accurate, specially the last worksheet based on the paper by "Bahadori". For a few values that I checked the deviation comes out to be less than 10% from HYSYS.

Most of the excel programs that I develop are for the benefit of those engineers who don't have access to sophisticated process simulation software such as HYSYS. Having said that, I would still recommend that if any process engineer has access to simulation software such as HYSYS, UNISIM, PROII, PROSIM and has sound chemical engineering fundamentals and a good understanding of modeling using these software, he or she should use it because of the extensive effort gone into building of a very exhaustive thermodynamic and kinetic databank into these software which enhances the accuracy of the results obtained from them compared to manual or excel calculations.

Regards,
Ankur.
Hi Ankur,

I really want to appreciate your efforts for your work on this spreadsheet. I was aware of the correlations used in the spreadsheet. But i never thought that these correlations could be so accurate. I have checked your spreadsheet for at least five different gas compositions/conditions and compared the results with Hysys. The difference comes out to be 5-7% and even lesser for Bahadori method. I would like to call this difference to be insignificant when the results are being compared with a rigorous simulator.

Thanks & Regards,
Haseeb Ali
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Vicky Motwani
Mar 20 2012 02:41 PM
Dear sir, I have also followed ur last discussion when you were enquiring abt hydrate formation calculations and even someone said that hysis calculations are conservative. I m asking just for my knowledge tht for actual industry use for initial design using these methods(including hysis) is fine ? Thanks for your collective information abt hydrates Regards Vicky
Thanks Mr. Ankur!
I'll appreciate your effort and thank you for sharing it.
Best regards,
Sebastián
Photo
Debasish Chowdhury
Jul 25 2012 09:52 AM
Dear Sir,
Very much helpful spreadsheet.I really appreciate your effort on that. I am doing by undergraduate thesis on LPG hydrate formation. Sir can you please tell me how can i model empirical relation for commercial LPG mixtures especially contain propane,butane, mercaptan and water exactly in the same way?? .Sir i also found that you mentioned a book name on your excel worksheet.Can you please email me that book on dc_buet_07@yahoo.com. I really am so much grateful to you if you help me in that case.
With Regards
Debasish Chowdhury
Debasish,

Books are copyrighted material and any unauthorized transfer is illegal. Buy yourself a used copy of "Natural Gas Hydrates - A Guide for Engineers" from

http://www.amazon.co...ords=0750684909

Regards,
Ankur
Hello Ankur,
I was wondering if you could shed some light on stable and unstable hydrates based on your experience? My understanding is that Hysys, ProMax and other software predict Hydrates with naming the specific type of hydrate.
Should the design engineer look in to the type of hydrate and its stability criteria? Thank you in advance.

Regards,
ChemG

Hello Ankur,
I was wondering if you could shed some light on stable and unstable hydrates based on your experience? My understanding is that Hysys, ProMax and other software predict Hydrates with naming the specific type of hydrate.
Should the design engineer look in to the type of hydrate and its stability criteria? Thank you in advance.

Regards,
ChemG


I cannot claim to be an authority on hydrates although I have prepared a calculation sheet for determining the hydrate temperature. I would ask you to read John Carroll's book on Natural Gas Hydrates to get a clear picture on types of hydrates. Additionally I would recommend you to look at the thesis document available at the link below:

http://repository.ta....pdf?sequence=4

Regards,
Ankur.

ChemG, on , said:

Hello Ankur,
I was wondering if you could shed some light on stable and unstable hydrates based on your experience? My understanding is that Hysys, ProMax and other software predict Hydrates with naming the specific type of hydrate.
Should the design engineer look in to the type of hydrate and its stability criteria? Thank you in advance.

Regards,
ChemG

I cannot claim to be an authority on hydrates although I have prepared a calculation sheet for determining the hydrate temperature. I would ask you to read John Carroll's book on Natural Gas Hydrates to get a clear picture on types of hydrates. Additionally I would recommend you to look at the thesis document available at the link below:

http://repository.ta....pdf?sequence=4

Regards,
Ankur.


Thank you very much for the reply. I will look in to the suggested references.
Photo
Debasish Chowdhury
Jul 30 2012 11:16 PM
Dear Sir,
Can you please explain why it is not necessary to have water for hydrate formation.It will be very helpful for me if you refer some book regarding that fact.I If you kind enough to refer some website or online pdf . I will be greatful.

Regards
Chemical Engineering Student
Debasish,

Are you not reading what has been written in the blog entry and the book reference provided both in the blog entry as well as in the subsequent comments provided by the readers of the blog? I will once again repeat the book reference:

Natural Gas Hydrates – A Guide for Engineers by John Carroll

I hope you don't have to come back asking the name again. Besides when I googled using the key words "Natural Gas Hydrates" I got 354,000 hits. Obviously you will have to do the effort of typing this in the google search bar and then looking up the literature on the subject and finding what is relevant to you. You don't expect me to do this for you, do you?

Regards,
Ankur.

in addition to Ankur's comments and information I would mention

NIST HYDRATE DATABASE
http://gashydrates.nist.gov/

one of the best sources for hydrate phase equilibria and thermophysical data.


Prode Properties
http://www.prode.com/en/properties.htm

you can download a free version of this tool which works in Excel, Matlab, Mathcad and allows to calculate hydrate pressure and temperature formation curves solving a multiphase flash operation (no free water assumed).

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Zapfenzieher
Aug 09 2016 07:40 PM

In another similar topic, here

 

http://www.cheresour...ction-in-hysys/

 

someone asked about the Kobayashi-Song-Sloan hydrate temperature method for sweet natural gas published in Bradley (1987).

 

I'm sorry -- I'm a bit late to the party. The reason that the correlation published in Bradley doesn't work is because the mantissas in the expression were exactly reversed. Where ln(SG)^n was shown, ln(p)^n should be used instead and where ln(p)^n was shown, ln(SG)^n must be used (where 1 <= n <= 4).

 

To my knowledge, this correction was first published by Elgibaly and Elkamel (Fl Phs Eq 152, 1998, 23) and has been confirmed by Ghayyem et al. (JNGScE 21, 2014, 184). Also, I have tested it myself and it seems to work as well as (or at least no worse than) any other empirical correlation for sweet natural gases.

 

And you were correct: Ameripour guessed wrongly at the units. P should be in psia and T will be in deg R.

 

Cheers,

Rudisaurus

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