Natural gas contains many contaminants, one of them is water. When the gas is transmitted to the surface from processing and finally pipeline transmission, its pressure and temperature reduced naturally in the well string. This reduce the capacity of natural gas to hold water vapor and free water is condensed. The water vapor must be reduced to meet sales gas requirement, which is usually around 2-7 lb/MMscf.
For many years, glycol solutions have been used for natural gas drying. Early glycol dehydration units utilized diethylene glycol (DEG). Triethylene glycol (TEG) came into use around 1950 primarily because its higher boiling point thus gives better separation of water and greater dew point depression without causing thermal decomposition of the glycol. Tetraethylene glycol (T4EG) has been used in some specialized cases, but in majority, triethylene glycol is used.
In this post, I want to share preliminary design of gas dehydration unit using glycol solutions. Calculation and formulae used in this post can be accessed through this link.
To design glycol dehydrator, we need to define the following data:
- Gas flow rate
- Gas specific gravity
- Operating pressure
- Maximum operating pressure
- Gas inlet temperature
- Outlet gas water content required
The output of glycol dehydration design is to get:
- Dimension of glycol-gas contactor (outside diameter, shell height)
- Glycol recirculation rate
- Reboiler duty
- Stripping still dimension
Before we design gas dehydration unit, we need to define two design criteria. Those are glycol-water circulation rate and lean TEG concentration from re-concentrator unit.
For most field dehydrator, glycol-water circulation rate is usually 2.5 to 4 gallon TEG per pound H2O removed. As for lean TEG concentration from re-concentrator is usually 99.5% for most design.
Design of gas dehydration unit involves reading many tables and graph. Therefore, please follow the method as mentioned in references.
For trayed type contactor, we also need to create McCabe-Thiele Diagram. It is like going back to college 😊
Please check my spreadsheet to learn more about designing gas dehydration using glycol solution.
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