{"id":1759,"date":"2013-08-01T10:46:40","date_gmt":"2013-08-01T15:46:40","guid":{"rendered":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/?p=1759"},"modified":"2013-08-01T10:48:28","modified_gmt":"2013-08-01T15:48:28","slug":"teg-dehydration-how-does-the-stripping-gas-work-in-lean-teg-regeneration","status":"publish","type":"post","link":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/2013\/08\/teg-dehydration-how-does-the-stripping-gas-work-in-lean-teg-regeneration\/","title":{"rendered":"TEG Dehydration: How Does the Stripping Gas Work in Lean TEG Regeneration?"},"content":{"rendered":"

For dehydration of natural gas by triethylene glycol (TEG) process to a lower water content\/water dew point temperature a higher lean TEG concentration is required. To achieve a higher lean TEG concentration at a specified reboiler temperature and pressure, commonly a stripping gas is used. Stripping is defined as a physical separation process by which one or more components are removed from a liquid stream by a vapor stream. Stripping gas lowers the water partial pressure causing more water to vaporize from TEG solution. Normally, stripping is performed at the practical\/possible highest temperature and lowest pressure. This allows for minimum stripping gas flow rate. Any inert gas or a portion of the gas being dehydrated is suitable.<\/p>\n

In this Tip Of The Month (TOTM), the impact of stripping gas on the water partial pressure and lean TEG solution boiling temperature or lean TEG concentration is studied. Two case studies are considered. In case one, at a specified reboiler temperature and pressure, the impact of stripping gas on lowering the water partial pressure and consequently increasing the TEG concentration in the lean TEG solution is demonstrated. Similarly in case two, for a specified 99 and 99.4 mass percent TEG in the liquid phase and atmospheric pressure, the impact of stripping gas on lowering the water partial pressure and consequently lowering the liquid boiling point (bubble point) temperature is demonstrated.<\/p>\n

Glycol dehydration is the most common dehydration process used to meet pipeline sales specifications and field requirements (gas lift, fuel, etc.). TEG is the most common glycol used in these absorption systems. At atmospheric pressure and a maximum reboiler temperature of 204 \u00b0C [400 \u00b0F] the highest glycol concentration of lean TEG that can be achieved is roughly 99 mass percent. This represents the maximum lean TEG concentration that can be produced in a reboiler operating at 1 atm. If the lean TEG concentration required at the absorber to meet the water dew point specification is higher than 99 mass percent, then some method of further increasing the glycol concentration at the regenerator must be incorporated in the unit. Virtually all of these methods involve lowering the partial pressure of water in the glycol solution either by pulling a vacuum on the regenerator or by introducing stripping gas into the regenerator [1].<\/p>\n

In the June and July 2013 Tip Of The Months (TOTM)<\/a> [2, 3], the effect of stripping gas rate on the regenerated lean TEG concentration for several operation conditions was studied. A series of charts and their corresponding correlations for quick determination of the required stripping gas rate to achieve a desired level of lean TEG concentration were presented. The charts were based on the rigorous calculations performed by computer simulations with correlations developed by regressing the same data to a proposed model. These can be used for facilities type calculations for evaluation and trouble shooting of an operating TEG dehydration unit. For further detail refer to the June and July 2013 TOTM [2, 3].<\/p>\n

Vapor-Liquid Equilibrium:<\/strong><\/p>\n

For the sake of simplicity, pure methane is used as the stripping gas; however, the following discussion is valid for any gas mixture.<\/p>\n

The governing equations for vapor-liquid equilibrium of TEG, water, and methane are derived and presented in Appendix A at the end of this TOTM. Even though the system of water, TEG and methane is non-ideal, to show the principle of gas stripping, assume the vapor phase is an ideal gas and the liquid phase is an ideal solution. Under these conditions Raoult\u2019s Law for water (See Eq A5 in Appendix A) is.<\/p>\n

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According to Raoult\u2019s Law, at a fixed pressure and temperature (i.e. the reboiler condition), the mole fraction of water in the liquid phase (xwater<\/sub><\/em>) decreases as water partial pressure decreases. Note that at a fixed temperature, water vapor pressure is constant (see Figure 1). In addition, system pressure (P<\/em>) is the sum of partial pressure of all components. Assuming an ideal mixture, mathematically P is approximated by:<\/p>\n

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Figure 1 presents the vapor pressure of water [4] and TEG [5] for an operating range of reboiler temperature. As can be seen in Figure 1, the vapor pressure (volatility) of water is much higher than that of TEG. In general the vapor pressure of water is approximately 100 to 1000 times greater than that of TEG. This shows why water is vaporized much more than TEG from a TEG solution by heating.<\/p>\n

Since TEG volatility is much lower than that of water and methane, its mole fraction in the vapor phase will be much smaller and for simplicity its partial pressure can be ignored. Therefore equation 2 reduces to:<\/p>\n

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At a fixed system pressure (reboiler condition), equation 3 clearly shows that as the methane partial pressure increases, water partial pressure has to decrease and consequently, based on Raoult\u2019s Law, equation 1, the mole fraction of water in the liquid phase decreases. In other words, increasing the stripping gas rate results in lower water mole fraction in TEG and higher TEG mole (mass) fraction is achieved.<\/p>\n

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Fig 1A (SI). Comparison of water [4] and TEG [5] vapor pressure<\/p>\n

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Fig 1B (FPS). Comparison of water [4] and TEG [5] vapor pressure<\/p>\n

The effect of stripping gas can be summarized as follows:<\/p>\n