BTEX stands for benzene, toluene, ethylbenzene, and xylene, a group of compounds all that also belong to the broader category of Hazardous Air Pollutants (HAPs). Benzene is a known carcinogen, and has also been shown to cause blood disorders and to impact the central nervous system and the reproductive system.  Toluene may affect the reproductive and central nervous systems.  Ethylbenzene and xylene may have respiratory and neurological effects [1]. BTEX is present in natural gas streams and is being picked up in glycol dehydration and amine sweetening units.

In the United States HAP emissions from glycol dehydration units are regulated under 40 CFR, Part 63, Subpart HH.  Glycol dehydration units processing more than 3 MMscfd (0.85 106 Sm3 per day) and having benzene emissions greater than 900 kg/year (1 ton/year) are required to control HAP emissions.

This problem is one which requires careful attention in the design phase. The purpose of this Tip of the Month (TOTM) is to discuss the primary factors affecting the absorption of BTEX components in glycol dehydration systems.

In gas dehydration service, triethylene glycol (TEG) will absorb limited quantities of BTEX from the gas. Based on the data from reference [2], predicted absorption levels for BTEX components vary from 5-10% for benzene to 20-30% for ethylbenzene and xylene. Figure 18.18 in reference [2] shows approximate ab­sorption percentages for BTEX components as a function of TEG circulation rate and contactor temperature at 6895 kPa (1000 psia). Absorption is fa­vored at lower temperatures, higher pressure, increasing TEG concentration and circulation rate.

The bulk of absorbed HAPs will be vented with the water vapor at the top of the regenera­tor. The most common emission mitigation strategies are to:

1) Condense the regenerator overhead vapor in a partial condenser and combust the remaining vapor.  The uncondensed vapors are typically routed to an incinerator or, if a direct-fired reboiler is used, routed to the reboiler fuel gas. The liquid hydrocarbons are collected and disposed of by blending into a crude oil or condensate stream.  The condensed water is typically routed to produced water disposal.

2) Route the regenerator overhead vapors to another process stream in the facility.   This is typically a low pressure stream such as flash vapors from the last stage of a crude or condensate stabilization system.

In this TOTM, we will revisit Figure 18.18 of reference [2] for estimating absorption of BTEX in the glycol dehydration systems using the experimental vapor-liquid equilibrium data reported in the Gas Processors Association Research Report 131 (GPA RR 131) [3]. The objective of this TOTM is to reproduce similar diagrams covering wider ranges of pressure and temperature. First we demonstrate the accuracy of ProMax [4] and the Peng-Robinson [5] equation of state (PR EOS) of the same software to generate the required data. Finally, for ease of use the generated results are presented graphically.

Verification of Thermodynamic Model:

A series of flash calculations for the reported experimentally measured pressures, temperatures and synthetic feed gas compositions were performed. The mixtures consisted of methane, benzene, toluene, ethylbenzene, o-xylene, TEG and water. The pressure ranged from 20 to 1000 psia (138 to 6895kPa) and temperature ranged from 77 to 400°F (25 to 204°C). These ranges cover the normal operating conditions of contactor, flash tank, and regenerator in a TEG dehydration plant. The calculated liquid (x) and vapor (y) phase compositions for the four BTEX components are compared with the corresponding experimental values and presented in Figure 1.

Figure 1

Figure 1. Comparison of calculated BTEX mole fractions in the liquid and vapor phases  by ProMax with the experimental values reported in GPA RR 131.

Results and Discussion:

For the purpose of this study, a contactor column with three theoretical stages and with the feed composition shown in Table 1 was simulated. The concentration of the lean TEG stream was 99.0 weight % TEG, and it was assumed the TEG temperature was 5°F (2.8°C) warmer than the feed gas. The feed gas was saturated with water at feed conditions. For each contactor pressure and temperature, the lean TEG circulation ratio was varied from 1 to 7 US gallon of TEG/lbm of water removed (8.3 to 58.4 liters of TEG/kg of water removed).

Three temperatures and three pressures, covering typical contactor operation ranges were studied. Figures 2 to 5 present the results of simulations using ProMax. Absorption of BTEX components is plotted as a function of temperature, pressure and glycol circulation rate.

Table 1. Dry-basis composition of feed gas

Table 1

Figure 2

Figure 2. Absorption of benzene as a function of temperature, pressure, and circulation ratio

In Figure 2, benzene absorption is plotted as a function of circulation ratio (liquid volume rate per gas standard volume rate) for two temperatures (77 and 122 °F or 25 and 50 °C) and two pressures (500 and 1000 psia or 3447 and 6895 kPa). Absorption increases with decreasing temperature and increasing circulation ratio. The effect of pressure on absorption is small but is more pronounced at 500 psia than at 1000 psia.  The likely reason for this is that at the lower pressure, the water content of the feed gas is higher and the heat of absorption effect increases the gas outlet temperature which, in turn, decreases the solubility of benzene in the TEG. This effect will be not as significant at higher pressures.

In TEG dehydration process, the common unit of circulation ratio is in gallons of TEG per pound of water absorbed (liters of TEG per kilogram of water absorbed). In Figures 3, 4, and 5 the circulation units on the x-axis were changed to these units.

Figures 3 to 5 can be used to estimate the absorption of BTEX components in a glycol dehydration system for a given pressure, temperature and circulation ratio.

Experimental solubility data for BTEX components in TEG at pressures greater than 1000 psia (6895 kPa) are not available in open literature. Figure 5, which presents BTEX absorption at 1500 psia (10344 kPa) has not been validated with experimental data. In addition, 1500 psia (10344 kPa) is above the cricondenbar of the feed gas used in this study and hence falls in the dense phase region. The solubility behavior of dilute vapor components in solvents such as TEG can be significantly different in the dense phase; therefore, caution should be taken in extrapolating these correlations above 1000 psia (6895 kPa).

Figure 3A

Figure 3A. Absorption of benzene and toluene in TEG at 500 psia (3447 kPa)

Figure 3B

Figure 3B. Absorption of ethylbenzene and o-xylene in TEG at 500 psia (3447 kPa)

Figure 4A

Figure 4A. Absorption of benzene and toluene in TEG at 1000 psia (6895 kPa)

Figure 4B

Figure 4B. Absorption of ethylbenzene and o-xylene in TEG at 1000 psia (6895 kPa)

Figure 5A

Figure 5A. Absorption of benzene and toluene in TEG at 1500 psia (10342 kPa)

Figure 5B

Figure 5B. Absorption of ethylbenzene and o-xylene in TEG at 1500 psia (10342 kPa)

Figure 6 shows the effect of pressure on the absorption of each BTEX component at 95°F (35°C) at 0.2 US GPM TEG/MMSCFD of gas (1.6 m3/h TEG/106 Sm3/d of gas). Be reminded that high this work has not been experimentally validated at pressures above 1000 psia (6895 kPa).

 

Comparison with the GRI-GLYCalc Software:

GRI-GLYCalc [6] is a relatively simple and easy-to-use software package that is widely used by operators for the estimation of BTEX emissions from glycol units.  It is accepted by most state regulatory authorities.  Table 2 shows the ProMax results in this work compared to GLYCalc for each BTEX component at 3 different operating conditions.

 

Conclusions:

As shown in Figure 1, PR EOS can be used to estimate VLE of BTEX compounds in glycol systems.

In reviewing Figures 2 to 5, one can conclude that the absorption of the BTEX components decreases as:

  1. Temperature increases
  2. Circulation ratio decreases

For pressures between 500 (3450 kPa) and 1000 psia (6895 kPa), the effect of pressure on BTEX absorption is not large.

From operational point of view, minimizing circulation ratio is the most effective way of decreasing the absorption of BTEX components. This also minimizes reboiler duty and the size of the regeneration skid.  Lower TEG circulation rates require more theoretical stages in the contactor to meet outlet water content specifications, but the additional cost of a taller contactor is often offset by savings in the regeneration package.  Care should be taken that the glycol circulation rate is sufficient to ensure adequate liquid distribution over the packing.  Packing vendors can provide minimum circulation guidelines.

Finally, it should be noted that in the operation of a glycol dehydration unit, the desired outcome is to meet the water content specification for the outlet gas, e.g. 7 lbs H2O/MMSCF (111 kg/106 Sm3).   When using the graphs in this TOTM, different operating points (T, P and circ ratio) will produce different outlet water contents. Make sure that the operating points you are using to estimate BTEX absorption are can also meet the water specification.

To learn more about similar cases and how to minimize operational problems, we suggest attending the John M. Campbell courses; G4 (Gas Conditioning and Processing) and G5 (Gas Conditioning and Processing-Special).

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 your consulting needs to consulting@jmcampbell.com.

 

By Mahmood Moshfeghian and Robert A Hubbard

 

 

Figure 6

Figure 6. Impact of pressure on BTEX absorption at 95 F (35 C) and 0.2 US GPM TEG/MMSCFD of gas (1.6 m3/h TEG/106 Sm3/d of gas)

Table 2. Comparison between GRI-GLYCalc and ProMax BTEX absorption at

1000 psia (6,895 kPa), 99.0 weight % lean TEG, and 3 theoretical trays

Table 2

* gallons TEG/lbm of water removed (liters TEG/kg of water removed)

 

Reference:

  1. http://www.earthworksaction.org/BTEX.cfm, 2011.
  2. Campbell, J. M. “Gas conditioning and processing, Volume 2: The Equipment Modules,” John M. Campbell and Company, Norman, Oklahoma, USA, 2001.
  3. Ng, H. J., Chen, C. J., and Robinson, D.B.: RR-131, “The Solubility of Selected Aromatic Hydrocarbons in Triethylene Glycol,” Gas Processors Association (Dec. 1991).
  4. ProMax 3.2, Bryan Research and Engineering, Inc, Bryan, Texas, 2011.
  5. Peng, D. Y., and Robinson, D. B., Ind. Eng. Chem. Fundam., Vol. 15, p. 59, 1976.
  6. GRI-GLYCalc 4.0, Gas Research Institute, Des Planes, Illinois, 2000

 

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