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Determining Hydrate Formation Temperature
29 February 2012
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
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