TEG Dehydration: Stripping Gas Correlations for Lean TEG Regeneration

Glycol dehydration is the most common dehydration process used to meet pipeline sales specifications and field requirements (gas lift, fuel, etc.). Triethylene glycol (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 98.7 mass percent.   This represents the maximum lean glycol concentration that can be produced in a reboiler operating at 1 atm. If the lean glycol concentration required at the absorber to meet the water dew point specification is higher than 98.7 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.

In the June 2013 Tip of The Month (TOTM) [1], the effect of stripping gas rate on the regenerated lean TEG concentration for several operation conditions was studied. A series of charts for quick determination of the required stripping gas rate to achieve a desired level of lean TEG concentration was presented. The charts were based on the rigorous calculations performed by computer simulations and 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 2013 TOTM.

In this TOTM, based on the charts presented in the June 2013 TOTM, a correlation is presented to estimate the stripping gas requirement as a function of the lean TEG mass fraction, the reboiler temperature, and the number of theoretical trays used in the gas stripping section. In addition, a summary of the error analysis is presented.

Proposed Correlation:

As discussed in the June TOTM, the stripping gas rate requirement is a function of the lean TEG mass fraction/percent, the reboiler temperature and the number of theoretical trays in the gas stripping section, N_S. In the same TOTM, three charts for 0, 1, and 2 theoretical stages were presented. Each chart contained three isotherms of 182°C (360°F), 193°C (380°F), and 204°C (400°F) for lean TEG mass percent of 98.50 to 99.95. In order to estimate the stripping gas requirements using software such as excel, or by hand calculation, an attempt was made to develop a simple correlation. Several models were tested and evaluated, among which the following correlation was chosen:

Where:

x: The required lean TEG mass fraction

y: (g-T)x

T: Reboiler Temperature, °C (°F)

g: 193°C (380°F)

Using a non-linear regression computer program and the data generated by ProMax [2] and reported in the June 2013 TOTM [1], the correlation parameters were obtained by minimizing the residual error in the objective function, defined in Eq 2.

Where NP is the number of data points. For this analysis 72 points for each theoretical tray in the gas stripping section were used.

Using the objective function defined in Eq 2, the values for parameters “ a ” through “ f ” in Eq 1 were determined while the value of “g” was set equal to 193°C (380°F). The resulting parameter values for the international system of units (SI) and engineering system of units (FPS, representing Foot, Pound and Second) are presented in Table 1. The summary of error analysis is presented in Table 2. Figures 1 through 3 present comparison of the results obtained by the proposed correlation shown in Eq 1 and the results obtained by ProMax [2] for N_S = 0, 1, and 2, respectively.

Table 1. The proposed correlation parameters in SI and FPS

Fig 1. Comparison of the proposed correlation (solid curves) with the ProMax (symbols) [2] results for N_S=0, number of theoretical trays in the gas stripping section

Table 2. The error analysis for the proposed correlation in comparison with ProMax [2]

Fig 2. Comparison of the proposed correlation (solid curves) with the ProMax (symbols) [2] results for N_S=1, number of theoretical trays in the gas stripping section

Conclusions:

In this TOTM, in continuation of the June TOTM, a correlation to estimate the stripping gas rate requirement was presented as Eq 1. The correlation parameters for use in the SI and/or FPS system of units are presented in Table 1. The proposed correlation can be used for quick determination of the required stripping gas rate to achieve a desired level of lean TEG concentration at specified reboiler temperature and the number of theoretical trays in the gas stripping section. The summary of the error analysis in Table 2  and the graphical comparison between the results of Eq 1 and those obtained by ProMax [2] shown in Figures 1 through 3 indicate that the proposed correlation is accurate enough for facilities type calculations for evaluation and trouble shooting of an operating TEG dehydration unit.

Fig 3. Comparison of the proposed correlation (solid curves) with the ProMax (symbols) [2] results for N_S=2, number of theoretical trays in the gas stripping section

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. Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2013/05/teg-dehydration-stripping-gas-charts-for-lean-teg-regeneration/, June 2013.
2. ProMax 3.2, Bryan Research and Engineering, Inc., Bryan, Texas, 2012.