TEG Dehydration: How Does the Stripping Gas Work in Lean TEG Regeneration?

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.

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.

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 °C [400 °F] 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].

In the June and July 2013 Tip Of The Months (TOTM) [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].

Vapor-Liquid Equilibrium:

For the sake of simplicity, pure methane is used as the stripping gas; however, the following discussion is valid for any gas mixture.

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’s Law for water (See Eq A5 in Appendix A) is.

According to Raoult’s Law, at a fixed pressure and temperature (i.e. the reboiler condition), the mole fraction of water in the liquid phase (x_water) 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) is the sum of partial pressure of all components. Assuming an ideal mixture, mathematically P is approximated by:

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.

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:

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’s 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.

Fig 1A (SI). Comparison of water [4] and TEG [5] vapor pressure

Fig 1B (FPS). Comparison of water [4] and TEG [5] vapor pressure

The effect of stripping gas can be summarized as follows:

• Higher stripping gas (methane) rate increases methane mole fraction in the vapor phase
• Higher methane mole fraction in the vapor phase increases partial pressure of methane
• Higher partial pressure of methane lowers partial pressure of water (P is fixed, Eq 3)
• Lower partial pressure of water lowers mole fraction of water (Raoult’s Law, Eq 1) in the liquid phase (Raoult’s Law, Eq 1) which, in turn, increases the TEG concentration

In order to demonstrate quantitatively the impact of stripping gas on water partial pressure and lean TEG concentration or boiling temperature and to support the above reasoning, two case studies were investigated. For both cases, ProMax [6] with the SRK (Soave-Redlich-Kwong) [7] equation of state option was used to perform the calculations. In addition, all of the results presented are are for a single equilibrium (theoretical) stage.

Case Study 1:

In this case, the impact of stripping gas on regeneration of lean TEG concentration at reboiler conditions of 101.3 kPa and 199°C (14.7 psia & 390°F) is studied.

Figures 2 A & B present the variation of partial pressures of TEG, water and methane as a function of stripping gas (methane) rate and concentration, respectively. The total pressure is constant and plotted in this figure as well. These figures clearly show that as stripping gas (methane) rate or concentration increases, the methane partial pressure increases and the water partial pressure decreases, approximately in the same amount. Note the TEG partial pressure is relatively constant and very low compared to methane and water. It makes up less than 7 percent of the total pressure.

Figures 3 A & B present the impact of stripping gas rate and concentration in the vapor and liquid phases on lean TEG concentrations. As can be seen in these figures, as the stripping gas rate (or concentration) increases, the lean TEG concentration increases. It should be emphasized that these results are for a single equilibrium stage. As shown in the previous TOTM, increasing the number of stripping stages changes this relationship significantly.

Figures 2 and 3 indicate that variation of methane and water partial pressures or lean TEG concentration with stripping gas (methane) rate is non-linear whereas their variation with stripping gas concentration is linear.

Fig 2A. Impact of stripping gas rate on partial pressure of methane, water and TEG at 101.3 kPa & 199°C (14.7 psia & 390°F)

Fig 2B. Impact of stripping gas concentration on partial pressure of methane, water and TEG at 101.3 kPa & 199°C (14.7 psia & 390°F)

Fig 3A. Impact of stripping gas rate on lean TEG concentration at 101.3 kPa & 199°C (14.7 psia & 390°F)

Fig 3B. Impact of stripping gas concentration on lean TEG concentration at 101.3 kPa & 199°C (14.7 psia & 390°F)

Case Study 2:

This is an academic case and is presented to reemphasize the impact of stripping gas. In this case, the impact of stripping gas concentration on the lean TEG solution boiling (bubble point) temperature for relatively fixed lean TEG concentration of TEG and water at 101.3 kPa (14.7 psia) is studied.

Figure 4 indicates that as the stripping gas (methane) concentration increases, the lean TEG solution boiling (bubble point) temperature drops.

Figure 5 presents a variation of partial pressures of TEG, water and methane as a function of the stripping gas (methane) concentration. The total pressure is constant and plotted in this figure, too. As in case 1, this figure also clearly indicates that as the stripping gas (methane) concentration increases, while methane partial pressure increases water partial pressure decreases, approximately in the same amount. Note that TEG partial pressure decreases due to the lowering of boiling temperature and its value is much lower compared to those of methane and water. Again, it makes up less than 7 percent of total pressure.

Variation of component K-values as a function of TEG solution boiling (bubble point) temperature is presented in Figure 6. This figure indicates that the relative volatility of water with respect to TEG (K_water/K_TEG) is in the order of 100 and that of methane with respect to TEG (K_methane/K_TEG) is in the order of more than 10,000.

Fig 4. Impact of stripping gas concentration on the lean TEG solution boiling (bubble point) temperature at 101.3 kPa (14.7 psia).

Fig 5. Impact of stripping gas concentration on the component partial pressures at 101.3 kPa (14.7 psia) for 99 mass % TEG and 1 mass % water.

Fig 6. Variation of K-values with the lean TEG solution boiling (bubble point) temperature at 101.3 kPa (14.7 psia) for 99 mass % TEG and 1 mass % water.

Example 1:

Determine the reboiler temperature to regenerate a lean TEG concentration of 99.4 mass % at 101.3 kPa (14.7 psia). If the required reboiler temperature is above the maximum allowable of 204°C (400°F), determine the required partial pressure of a stripping gas (methane) to keep the reboiler temperature at 199°C (390°F).

Solution:

Figure 4 indicates that for a lean TEG concentration of 99.4 mass %, the required reboiler temperature without any stripping gas is 220.5°C (428.8°F). Obiviously, the is above the maximum allowable operating reboiler temperature. From the same figure, for a reboiler temperature of 199°C (390°F), the required mole % of striping gas in the vapor phase is 35.6. Therefore, the required partial of stripping gas is:

Example 2:

It is desired to regenerate a lean TEG solution to a concentration of 99.4 mass % at 101.3 kPa and 199°C (14.7 psia & 390°F). How much stripping gas is required in a regenerator with 1 theoretical tray?

Soultion:

Figure 3A indicates that for a lean TEG concentration of 99.4 mass%, the required striping gas rate is 10.9 Std m³/m³ of TEG (1.44 SCF/gal TEG). The coressponding water and methane partial pressures from Figure 2A are 57.8 and 36.5 kPa ( 8.4 and 5.3 psia), respectively. Note that without any stripping gas, the maximum achievable lean TEG concentration is 99 mass % and the coressponding water and methane partial pressures are 94.6 and 0 kPa (13.7 and 0 psia), respectively. The stripping gas reduced the water partial presuure by 38.9 %.

Conclusions:

 To achieve a higher lean TEG concentration at a specified reboiler temperature, i.e. below maximum of 204°C (400°F), and pressure, a stripping gas is commonly used. In this TOTM, the mechanism of stripping gas was reviewed. The impact of stripping gas on lean TEG solution concentration or boiling (bubble point) temperature was further investigated quantitatively. The impact of stripping gas on regeneration of TEG solution concentration can be summarized as:

• Each constituent of a mixture exerts its own partial pressure as a function of temperature and composition.
• Total system pressure is the sum of all partial pressures.
• For a fixed pressure, stripping gas lowers the partial pressure of water in the vapor phase
• Even though this system is not ideal, from Raoult’s Law (equation 1) at a fixed pressure and temperature the concentration of water in the TEG solution decreases as the partial pressure in the vapor phase decreases

To learn more, we suggest attending our G40 (Process/Facility Fundamentals), G4 (Gas Conditioning and Processing), G5 (Gas Conditioning and Processing-Special), and PF81 (CO₂ Surface Facilities), PF4 (Oil Production and Processing Facilities), courses.

John M. Campbell Consulting (JMCC) offers consulting expertise on this subject and many others. For more information about the services JMCC provides, visit our website at www.jmcampbellconsulting.com, or email us at consulting@jmcampbell.com.

By: Dr. Mahmood Moshfeghian

Reference:

1. Campbell, J. M., “Gas Conditioning and Processing”, Vol. 2, The Equipment Module, 8^th Ed., Second Printing, J. M. Campbell and Company, Norman, Oklahoma, 2002.
2. Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2013/05/teg-dehydration-stripping-gas-charts-for-lean-teg-regeneration/, June 2013.
3. Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2013/06/teg-dehydration-stripping-gas-correlations-for-lean-teg-regeneration/, July 2013.
4. Smith, J.M., Van Ness, H.C. and M.M. Abbott, “Introduction to Chemical Engineering Thermodynamics,” 7^th, McGraw-Hill, New York, 2004.
5. http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_004d/0901b8038004d042.pdf, Dow Chemical Company, 2007.
6. ProMax 3.2, Bryan Research and Engineering, Inc., Bryan, Texas, 2013.
7. Soave, G., Chem. Eng. Sci. Vol. 27, No. 6, p. 1197, 1972. 

Appendix A: Vapor-Liquid Equilibrium

The criteria for vapor-liquid equilibrium are the equality of fugacity of each component in the mixture between phases. Applying these criteria to fugacity (f), of component (i) in the vapor (V) and liquid, (L), phases:

For a non-ideal mixture of water, TEG and methane, one can write the following expressions in terms of fugacity coefficients (φ) in the vapor phase and activity coefficients (γ) in the liquid phases.

And,

Where:

Equating equations A2 and A3 and solving for x_i:

Calculation of  involves the use of an equation of state. It is a tedious trial and error procedure; therefore, a computer program should be used. These two terms represent the non-ideality of the vapor phase and if the system is an ideal gas both are set equal to 1. On the other hand, activity coefficient  represents the non-ideality of the liquid phase and is calculated by an appropriate activity coefficient model. For an ideal liquid solution, the activity coefficient is also set to 1.

Assuming the vapor phase is an ideal gas and the liquid phase is an ideal solution, then equation A4 reduces to equation A5, which is Raoult’s Law.