A hydrate in natural gas system is a physical combination of water and other small molecules to produce a solid which has an “ice-like” appearance but has a different structure than ice. It resembles dirty ice but has voids into which gas molecules will fit. Most common compounds found in gas hydrate are water, methane, and propane, or water, methane, and ethane.
Why is hydrate control necessary? The formation of gas in natural gas will cause several problems, such as:
- Accumulation of gas hydrates will cause restriction in flowlines, chokes, and valves, and instrumentation. It can accumulate in liquid collection section of vessels
- Gas hydrate plug and retard line capacity, causing physical damage to chokes and instrumentation, and cause separation problems
The primary conditions promoting hydrate formation are the following:
- Gas must be at or below its water (dew) point with “free” water present
- Low temperature
- High pressure
In this post I want to share how to predict hydrate formation. There are at least two methods used to predict hydrate formation.
Pressure-temperature correlation is used when composition of stream is not known. It should be used for first approximation only.
Vapor-solid equilibrium constants is used when composition of the stream is known. Its result is reliable until 1000 psia.
This post will particularly explain about hydrate prediction using pressure-temperature correlation.
How to estimate hydrate formation pressure or temperature? Flow chart below shows the steps.
Pressure-temperature curve below is used to predict hydrate formation pressure or hydrate formation temperature for natural gas at certain specific gravity.
Let say a natural gas has specific gravity 0.7 and operate at 50oF. What is hydrate formation pressure?
From pressure-temperature curve intersect 50oF and 0.7 specific gravity curve and read 320 psia.
Spreadsheet below shows example on how to predict hydrate formation based on steps above.
I hope you find this post useful.
- Abdel-Aal, H. K., Aggour, M., & Fahim, M. A. (2003). Petroleum and Gas Field Processing. New York: Markel Dekker.
- Stewart, M., & Arnold, K. (2011). Gas Dehydration Field Manual. Gulf Professional Publishing.
- GPSA Engineering Data Book
- Sami, Nagham & Sangwai, Jitendra & Subramanian, Bala. (2013). GAS HYDRATE APPLICATIONS AND PROBLEMS IN OIL AND GAS INDUSTRY.