absorption of aromatics compounds

8/3/2019 Absorption of Aromatics Compounds

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Absorption of Aromatics Compounds

(BTEX) in TEG Dehydration Process

BTEX stands for benzene, toluene, ethylbenzene, and xylene, a group of compounds all that alsobelong 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 nervoussystem 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 aminesweetening 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 fromthe 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. Themost 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 toproduced water disposal.

2) Route the regenerator overhead vapors to another process stream in the facility. This istypically 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 thisTOTM is to reproduce similar diagrams covering wider ranges of pressure and temperature. First
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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 generatedresults are presented graphically.

Verification of Thermodynamic Model:

A series of flash calculations for the reported experimentally measured pressures, temperaturesand 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 400F (25 to 204C). These ranges

cover the normal operating conditions of contactor, flash tank, and regenerator in a TEGdehydration 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. 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:
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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 was99.0 weight % TEG, and it was assumed the TEG temperature was 5F (2.8C) 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/lb m 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 werestudied. 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

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
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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 lowerpressure, 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 thecirculation 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 glycoldehydration 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 thedense 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. Absorption of benzene and toluene in TEG at 500 psia (3447 kPa)


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



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Figure 6 shows the effect of pressure on the absorption of each BTEX component at 95F (35C)

at 0.2 US GPM TEG/MMSCFD of gas (1.6 m3/h TEG/10

6Sm

3/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 stateregulatory 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 increases2. 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


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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 glycolcirculation 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 outcomeis to meet the water content specification for the outlet gas, e.g. 7 lbs H 2O/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)andG5 (Gas

Conditioning and Processing-Special).

John M. Campbell Consulting (JMCC) can provide thermodynamic expertise for gas processing

projects to ensure that the developed process model is as accurate as possible. For moreinformation about services offered by JMCC, visit our website at

www.jmcampbellconsulting.com.

By Mahmood Moshfeghian and Robert A Hubbard

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

Table 2. Comparison between GRI-GLYCalc and ProMax BTEX absorption at
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1000 psia (6,895 kPa), 99.0 weight % lean TEG, and 3 theoretical trays

* 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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