acid gas-light alkanes solubility in teg

Simple Correlation Analyzes Solubility of Acid Gas and Light Alkanes in Triethylene Glycol

Alireza Bahadori

Department of Petroleum Engineering, National Iranian South Oil CompanyAhwaz, Iran

Saeid Mokhatab

Gas-Engineering ConsultantTehran, Iran

Abstract

Hydrocarbon and acid gas solubility are major factors when considering the use of a physical solvent. Quantifying this amount of absorption is critical in order to minimize hydrocarbon losses or to optimize hydrocarbon recovery and these present an environmental or safety hazard when they are discharged from the top of the regenerator. Considering this, a new correlation that covers full range of dehydration operating condition has been developed in order to analyze the amount of acid gas and lower alkanes absorbed versus the partial pressure of these components and the absorber temperature in different triethylene glycol (TEG) circulation rates.

This article evaluates the effects of circulated TEG flowrates on the solubility of acid gas and lower alkanes in TEG and provides comparisons with the experimental data for improving the model. Results show the solubility of acid gases is higher than light alkanes, however, since the mole fraction of light alkanes in contactor's feed gas is more than acid gas, the solubility of light alkanes is therefore important and has to be accurately considered in design of TEG dehydration units.

Introduction

Glycol dehydration of natural gas employs ethylene glycol (EG), diethylene glycol (DEG) or triethylene glycol (TEG) to remove water from the gas stream. However, the most commonly used glycol is triethylene glycol. Diethylene glycol and ethylene glycol may also be used in dehydration applications; however, DEG and EG are often not considered due to dry gas water content requirements. TEG has a higher degradation temperature and can be regenerated to a higher lean concentration with no modifications to the standard regenerator reboiler. However, EG and DEG can meet water content specifications when used with enhanced regeneration systems. Enhanced regeneration is any system that improves glycol regeneration to achieve a "leaner" or more concentrated glycol solution. Enhanced regeneration could be the injection of stripping gas into the reboiler, azeotropic regeneration, or other proprietary processes. The costs associated with the use of EG or DEG would be increased glycol makeup and some form of enhanced regeneration to obtain a more concentrated glycol to achieve the dry gas water content (Grizzle, 1993).

More stringent emissions regulations have forced the use of some methods of minimizing hydrocarbon pickup or disposing of the emissions in glycol dehydration units. When natural gas is dehydrated with TEG, some of methane (CH4), ethane (C2H6) and propane (C3H8) and acid gases (H2S, CO2) are absorbed in the TEG. Regeneration of the rich TEG solution liberates the light hydrocarbons and acid gas components. The amount of these compounds absorbed and consequently liberated from the glycol

depends on their concentrations in the wet gas being dehydrated and on the contactor’s pressure and temperature.

The amount of acid gases removed by the glycol depends on several parameters:

Concentration of acid gases in the feed gas. The effect of composition increases with pressure, particularly for a gas that contains CO2 or H2S or both.

Contactor operating pressure and temperature. Glycol circulation rate.

When sour natural gas is dehydrated with TEG, a substantial amount of H2S and CO2 and light alkanes are absorbed in the TEG. Regeneration of the rich TEG solution liberates the acid-gas and light alkanes components. The amount of these compounds absorbed and consequently liberated from the glycol depends on their concentrations in the sour gas being dehydrated and on the contactor’s pressure and temperature.

Emissions from glycol dehydration units are a major environmental concern. In most cases the hydrocarbon and acid gas removal is undesirable, and should be minimized. In some cases, which include hydrocarbon recovery (Bahadori, 2006a), the pickup should be maximized or at least optimized. There are no standard sampling and analytical methods established by regulatory agencies for determining emissions from glycol dehydrators, and the methods initially used by the industry showed significant variability in results (Grizzle, 1993; Jou et al., 1994)

In an actual dehydration facility, the glycol is regenerated to about 99% purity or higher. The absorbed amount of gases depends on the amount of glycol circulated. If the glycol is not regenerated to this purity, then the amount of gas absorbed per unit of lean glycol circulated would be slightly less than in the proposed equations. Both temperature and pressure also affect hydrocarbon absorption. In general, the lower the temperature and the higher the pressure, the more hydrocarbons will be dissolved in the physical solvent. In some cases, however, the hydrocarbon solubility actually increases with temperature (Jou et al., 1994).

Figure 1 illustrates a typical TEG dehydration unit. As can be seen, the regenerated glycol is pumped to the top tray of the contactor (absorber). The glycol absorbs water as it flows down through the contactor counter-currently to the gas flow. Water-rich glycol is removed from the bottom of the contactor, passes through the reflux condenser coil, flashes off most of the soluble gas in the flash tank, and flows through the rich-lean heat exchanger to the regenerator. In the regenerator, absorbed water is distilled from the glycol at near-atmospheric pressure by application of heat. The regenerated lean glycol leaving the surge drum, partially cools in the lean-rich exchanger, and pumps through the glycol cooler before being recirculated to the contactor (Bahadori and Zeidani, 2006).

Glycol will absorb some hydrocarbons and acid gases at the high pressure of the contactor. Glycol has a special affinity for light alkanes, such as cyclic hydrocarbons as benzene, toluene, ethyl benzene, and xylene (BTEX) as well as polar gases like H2S and CO2

(Bahadori, 2006b).

Heavier paraffinic hydrocarbons are essentially insoluble in TEG. Aromatic hydrocarbons, however, are very soluble in TEG, and significant amounts of aromatic hydrocarbons may be absorbed in TEG at contactor conditions. This may present an environmental or safety hazard when they are discharged from the top of the regenerator.

Upon regeneration of the glycol, all absorbed gases are flashed off. From here they can be routed to fuel, flare, or gas-recovery system.

Figure 1 – Schematic of a typical TEG dehydration unit

New Proposed Model

Equation 1 presents a new correlation in which four coefficients are used to correlate solubility and partial pressure:

(1)

Where: (2)

(3)

(4)

(5)

In the above equations, X is the acid gas or light alkane component mole fraction in TEG, Pr and Tr are reduced partial pressure and reduced temperature of each component, respectively. Tuned coefficients in Equations 2 to 4 are also given in Table 1. These tuned coefficients are changed if more accurate experimental data are available by proposed numerical model.

Table 1 Tuned coefficients in Equations 2 to 5

Equations 6 and 7 also show the mole fraction conversion of components to volumetric dimensions.

(6)

(7)

Where, V is the standard volume of acid gas or light alkane component in TEG (cubic meter), M is the molecular weight of acid gas or light alkane components, and VTEG is the volume of TEG (cubic meter).

Considering Equations 6 and 7, TEG circulation flowrate can be calculated as:

(8)

Where, qTEG is circulated TEG (cubic meter per day).

Case Study

Table 2 shows the composition and operating condition of a typical contactor feed. In this study the solubility of acid-gas in TEG in different TEG circulation rates and different contactor’ temperatures and pressures are evaluated and the effect of TEG circulation rate on solubility of acid gases and light alkanes components are analyzed. Some improvements in previous developed equations are finally presented.

Table 2 Typical composition for wet gas inlet to contactor

Tables 3 and 4 present comparisons between some experimental data for H2S and CO2 solubility in TEG with the obtained results of the new developed model. As can be seen, the average absolute deviation percentage of the new model for predicting H2S and CO2 solubility in TEG are 3.0296% and 1.9394%, respectively.

Tables 5 to 7 also give comparisons between some experimental data for methane, ethane, and propane solubilities in TEG with the obtained results of the new developed model. As can be seen, the average absolute deviation percentage of the new model for predicting methane, ethane, and propane solubilities in TEG are 3.3075%, 3.139%, and 2.7903%, respectively.

In lights of the above and considering the simplicity of the proposed correlation, it is recommended to use the new developed model against of using routine graphical methods or conventional commercial softwares.

Table 3 Comparing new model results for predicting the solubility of H2S in TEG with experimental data

PressureKPa (abs)

Temperature °C

H2S Mole fraction at

(experimental) [2]

H2S Mole fraction(calculated)

Absolute Deviation Percent (%)

(ADP)

51.2 25 0.03657 0.0339 7.8761112 25 0.07131 0.0747 4.5382432 25 0.2436 0.2513 3.0641969 25 0.4701 0.4595 2.30691452 25 0.6412 0.6407 0.0781794 25 0.8089 0.817 0.99141919 25 0.8958 0.8988 0.3338

Component Mole %

C1 0.5202

C2 0.1504

C3 0.1180

IC4 0.0167

NC4 0.0462

IC5 0.0098

NC5 0.0090

NC6 0.0006

H2O 0.0025

H2S 0.0282

CO2 0.0856

N2 0.0127

C7+ 0.0002

C7+ MW 81.290C7+ SP. GR. 0.7646

P (KPa) 3700

T (°C) 42

Inlet gas water content(KG/HR) 115.2

1958 25 0.9426 0.92670 1.715860.4 50 0.0225 0.0217 3.6866261 50 0.09328 0.098 4.81631009 50 0.31079 0.3153 1.43041949 50 0.5122 0.5205 1.62052468 50 0.6244 0.6475 3.699685.5 75 0.021 0.0193 8.8083327 75 0.074 0.0756 2.1184971 75 0.1994 0.2055 2.96842000 75 0.36 0.34939 3.036711 100 0.00206 0.0021 1.9048113 100 0.01882 0.0185 1.7297676 100 0.1014 0.1029 1.45771920 100 0.2607 0.2721 4.189629.9 125 0.0039 0.0042 7.1429132 125 0.01657 0.0159 4.2138462 125 0.05338 0.053 0.7171890 125 0.1989 0.1963 1.3245

Average of Absolute Deviation (%) 3.0296

Table 4 Comparing new model results for predicting the solubility of CO2 in TEG with experimental data

PressureKPa

Temperature °C

CO2 Mole fraction

(experimental) [2]

CO2 Mole fraction (calculated) Absolute Deviation Percent (%)

(ADP)

467 25 0.04084 0.04180 2.2967750 25 0.064 0.06717 4.7194

1420 25 0.1152 0.12330 6.56931500 25 0.12 0.13039 7.96843210 25 0.2464 0.26050 5.41274870 25 0.3511 0.35140 0.08546510 25 0.4338 0.41220 5.2402

10420 25 0.4531 0.47130 3.861715670 25 0.4613 0.46200 0.151520250 25 0.4848 0.49140 1.3431

105 50 0.00651 0.00680 4.2647709 50 0.04063 0.04040 0.5693

2560 50 0.1367 0.13960 2.07745710 50 0.2611 0.26620 1.91597840 50 0.3305 0.32670 1.1631

10720 50 0.3906 0.38290 2.01115530 50 0.4328 0.43160 0.277319780 50 0.4582 0.45360 1.0141

129 75 0.005515 0.00520 6.0577710 75 0.03031 0.03030 0.033750 75 0.0315 0.03203 1.6825

2710 75 0.111 0.10570 5.01426160 75 0.2121 0.21320 0.5159

10630 75 0.3164 0.31490 0.476315480 75 0.38 0.38530 1.375619490 75 0.4203 0.41870 0.3821

1290 100 0.04205 0.04270 1.52223180 100 0.09725 0.10030 3.04094930 100 0.1471 0.14750 0.27128260 100 0.2237 0.22290 0.3589

10920 100 0.2749 0.27170 1.177814940 100 0.3347 0.33080 1.17918410 100 0.3759 0.37230 0.95771770 125 0.04902 0.04870 0.65283750 125 0.09805 0.09810 0.0516490 125 0.1582 0.15760 0.38079300 125 0.2104 0.20960 0.3817

11840 125 0.2513 0.25050 0.319414530 125 0.2875 0.28930 0.626118310 125 0.3397 0.33910 0.1769


Table 5 Comparing new model results for predicting the solubility of CH4 in TEG with experimental data

PressureKPa(abs)

Temperature °C

CH4 Mole fraction at

(experimental) [2]

CH4 Mole fraction(calculated)

Absolute Deviation Percent (%)

(ADP)

113.5 25 0.000636 0.00074 14.0541366.1 25 0.002022 0.00200 1.1960 25 0.005212 0.00480 8.58332320 25 0.01193 0.01110 7.47754800 25 0.02028 0.02170 6.54386120 25 0.02776 0.02700 2.81489240 25 0.03921 0.03820 2.64412840 25 0.04907 0.049 0.142916280 25 0.05656 0.0573 1.291419470 25 0.06379 0.0631 1.0935110.7 50 0.000617 0.00057 8.2456335.2 50 0.001859 0.00170 9.35291010 50 0.005324 0.00510 4.39222350 50 0.0119 0.01120 6.254520 50 0.02198 0.02060 6.6997320 50 0.03326 0.03150 5.587310190 50 0.04302 0.04120 4.417413140 50 0.05243 0.0498 5.281116830 50 0.06215 0.061 1.8852109.2 75 0.00059 0.00055 7.2727328.7 75 0.001806 0.00170 6.2353393.4 75 0.002129 0.00204 4.3627905 75 0.004878 0.00469 4.00852910 75 0.015 0.01465 2.38915350 75 0.02638 0.02589 1.89267550 75 0.03505 0.03518 0.369510540 75 0.04654 0.046531 0.019313560 75 0.05692 0.0565 0.743416630 75 0.06586 0.0651 1.167419690 75 0.07432 0.0721 3.0791

112.9 100 0.000627 0.00060 4.5388.8 100 0.002182 0.00210 3.9048940 100 0.005139 0.00500 2.782510 100 0.01345 0.01300 3.46155100 100 0.02557 0.02560 0.11728080 100 0.03923 0.03890 0.848310340 100 0.04807 0.04830 0.476212890 100 0.05818 0.058 0.310315400 100 0.06668 0.0667 0.0319240 100 0.07842 0.0784 0.0255369.9 125 0.002181 0.00200 9.05928 125 0.005305 0.00510 4.01962510 125 0.01379 0.01370 0.65693860 125 0.02094 0.02080 0.67316270 125 0.03321 0.03290 0.94228750 125 0.04453 0.04450 0.067411980 125 0.05865 0.0584 0.428115140 125 0.0708 0.0707 0.141418920 125 0.084 0.0838 0.2387


Table 6 Comparing new model results for predicting the solubility of C2H6 in TEG with experimental data

PressureKPa(abs)

Temperature °C

C2H6 Mole fraction

(experimental) [2]

C2H6 Mole fraction (calculated) Absolute Deviation Percent (%)

(ADP)

110 25 0.0028 0.00270 3.7037156 25 0.00399 0.00390 2.3077305 25 0.00772 0.00790 2.2785983 25 0.02348 0.02470 4.93932840 25 0.06044 0.06050 0.09924260 25 0.08037 0.07790 3.17077700 25 0.08287 0.08400 1.3452116 50 0.002457 0.00250 1.72228 50 0.004919 0.00480 2.4792397 50 0.008381 0.00810 3.4691950 50 0.01769 0.01870 5.40112100 50 0.03841 0.03850 0.23383200 50 0.0557 0.05470 1.82826490 50 0.0866 0.08650 0.11569850 50 0.09348 0.0937 0.2348115 75 0.002121 0.0022 3.5909225 75 0.004044 0.0041 1.3659418 75 0.00761 0.0073 4.2466583 75 0.01036 0.0101 2.57431010 75 0.01688 0.017 0.70592130 75 0.03362 0.0339 0.8264280 75 0.06103 0.0611 0.11467640 75 0.08982 0.0898 0.022310140 75 0.1002 0.1002 0112 100 0.00183 0.0019 3.6842194 100 0.003205 0.0033 2.8788

259 100 0.004074 0.0045 9.4667392 100 0.006103 0.0067 8.9104709 100 0.01068 0.012 111500 100 0.02182 0.0244 10.57383950 100 0.05236 0.0547 4.27795690 100 0.07101 0.069 2.913


Table 7 Comparing new model results for predicting the solubility of C3H8 in TEG with experimental data

PressureKPa

Temperature °C

C3H8 Mole fraction

(experimental) [2]

C3H8 Mole fraction (calculated) Absolute Deviation Percent (%)

(ADP)

15.9 25 0.001001 9.45E-04 5.925946 25 0.002667 0.0027 1.222257 25 0.003475 0.0034 2.2059

122.1 25 0.007085 0.0073 2.9452127 25 0.007138 0.0076 6.0789

231.6 25 0.01381 0.0138 0.0724378.5 25 0.02275 0.0225 1.1111767 25 0.04654 0.0456 2.0614959 25 0.05641 0.0577 2.235719.1 50 0.000855 8.94E-04 4.362435.2 50 0.001525 0.0015 1.6667131.4 50 0.005522 0.0055 0.4289.9 50 0.01231 0.0119 3.4454695 50 0.02939 0.0283 3.85161750 50 0.06963 0.0711 2.067533.2 75 0.001126 1.30E-03 13.3846134.8 75 0.004374 0.0044 0.5909305 75 0.009931 0.0098 1.3367802 75 0.02548 0.0254 0.315960 75 0.03014 0.0303 0.5281


Figure 2 shows the new model' results for acid-gas absorbed in TEG versus contactor total pressure at different TEG circulation flow rates. It also shows the trend of solubility of acid gases in TEG for different TEG circulation rates and contactor pressures.

Figure 3 illustrates the new proposed model' results for acid-gas absorbed in TEG versus the contactor temperature at different TEG circulation rates.

Figure 4 shows the new proposed model' results for acid-gas absorbed in TEG versus the TEG circulation rate at different contactor pressures.

Figure 5 shows new model' results for acid-gas absorbed in TEG versus TEG circulation rate at different total contactor temperature. It also shows the trend of solubility of acid gas in TEG at different temperatures and TEG circulation rates.

Figure 6 shows the new model' results for light alkanes including CH4, C2H6 and C3H8 absorbed in TEG versus total contactor pressure at different TEG circulation rates. It also shows the trend of solubility of light hydrocarbons in TEG for different TEG circulation rates and contactor pressures.

Figure 7 illustrates the new proposed model' results for light alkanes gas absorbed in TEG versus total contactor temperature at different TEG circulation rates.

Figure 8 shows the new proposed model' results for light hydrocarbons absorbed in TEG versus TEG circulation rate at different total contactor pressures.

Figure 9 shows the new model' results for light hydrocarbon absorbed in TEG versus TEG circulation rate at different total contactor temperature. It also shows the trend of solubility of light hydrocarbon gas in TEG at different temperatures and TEG circulation rates.

0 1000 2000 3000 4000 5000 6000

101

102

103

Contactor Pressure, (KPa)

Lib

era

ted

Acid

Gas(

Std

. C

ub

ic m

ete

r p

er

day)

TEG Rate= 30 m3/day

TEG Rate= 40 m3/day

TEG Rate=50 m3/day

TEG Rate=60 m3/day

TEG Rate=70 m3/day

TEG Rate=80 m3/day

TEG Rate=90 m3/day

TEG Rate=100 m3/day

TEG Rate= 110 m3/day

TEG Rate=120 m3/day

Figure 2 New model results for acid-gas absorbed in TEG vs. contactor pressure at different TEG circulation rates

0 10 20 30 40 50 60 70 8010

2

103

Contactor Temperature, °C

Lib

erat

ed A

cid

Gas

( S

td. C

ub

ic m

eter

per

day

)

TEG Rate= 30 m3/day

TEG Rate=40 m3/day

TEG Rate=50 m3/day

TEG Rate=60 m3/day

TEG Rate=70 m3/day

TEG Rate=80 m3/day

TEG Rate=90 m3/day

TEG Rate=100 m3/day

TEG Rate=110 m3/day

TEG Rate=120 m3/day

Figure 3- New model results for acid-gas absorbed in TEG vs. contactor temperature at different TEG circulation rates

30 40 50 60 70 80 90 100 110 120

102

103

TEG Circulation Rate ( cubic meter/day)

Lib

erat

ed A

cid

Gas

(S

td. C

ub

ic m

eter

per

day

)

Contactor P=1000 KPa

Contactor P= 2000 KPa




Contactor=6000 KPa

Figure 4 New model results for acid-gas absorbed in TEG vs. TEG circulation rate at different contactor pressure

30 40 50 60 70 80 90 100 110 120

103

TEG circulation Rate ( cubic meter/day)

Lib

erat

ed A

cid

Gas

(S

td. C

ub

ic m

eter

per

day

)

Contactor T= 20 °C

Contactor T= 30°C

Contactor T=40°C

Contactor T=50 °C

Contactor T=60 °C

Contactor T=70 °C

Contactor T=80°C

Figure 5 New model results for acid-gas absorbed in TEG vs. TEG circulation rate at different contactor temperature

0 1000 2000 3000 4000 5000 6000

102

103

Contactor Pressure, (KPa)

L

iber

ated

Hyd

roca

rbo

n G

as (

std

. Cu

b. m

etr

per

day

)

TEG Rate= 30 m3/day

TEG Rate=40 m3/day

TEG Rate= 50 m3/dayTEG Rate= 60 m3/day

TEG Rate=70 m3/day

TEG Rate=80 m3/day

TEG Rate= 90 m3/dayTEG Rate=100 m3/day

TEG Rate=110 m3/day

TEG Rate=120 m3/day

Figure 6 New model results for light hydrocarbons absorbed in TEG vs. contactor pressure and at different TEG circulation rates

10 20 30 40 50 60 70 8010

2

103

Contactor Temperature, °C

Lib

era

ted

Hyd

rocarb

on

Gas (

Std

. C

ub

. m

ete

r p

er

day)

TEG Rate=30 m3/dayTEG Rate=40 m3/dayTEG Rate=50 m3/dayTEG Rate=60 m3/dayTEG Rate=70 m3/dayTEG Rate=80 m3/hrTEG Rate=90 m3/dayTEG Rate=100 m3/dayTEG Rate=110 m3/dayTEG Rate=120 m3/day

Figure 7 New model results for light hydrocarbons absorbed in TEG vs. contactor temperature at different TEG circulation rates

30 40 50 60 70 80 90 100 110 120

102

103

TEG Circulation Rate (cub. meter/day)

L

ibera

ted

Hyd

rocarb

on

Gas(s

td. cu

bic

mete

r p

er

day)







Figure 8 New model results for light hydrocarbons absorbed in TEG vs. TEG circulation rate at different contactor pressure

30 40 50 60 70 80 90 100 110 12010

2

103

TEG Circulation Rate (cub. meter/day)

Lib

erat

ed H

ydro

carb

on

Gas

(S

td. C

ub

. met

er p

er d

ay)

Contactor T=20 °C

Contactor T=30 °C

Contactor T=40 °C

Contactor T=50 °C

Contactor T=60°C

Contactor T=70°C

Contactor T=80°C

Figure 9 New model results for acid-gas absorbed in TEG vs. TEG circulation rate at different total contactor temperature

As can be seen from the above-mentioned figures, the solubility of acid gases in TEG is more than light alkanes solubility but the mole fraction of light alkanes in contactor's feed gas is more than acid gas, so the solubility of light alkanes is important and has to be accurately considered in design of TEG dehydration units.

In general, the proposed model is very accurate in predicting the solubility of acid gases and light alkanes in TEG. However, since dehydrators usually operate at temperatures of less than 60° C, there was no practical need to include temperatures higher than 75° C in graphs of this work.

References

Bahadori, A., “Calculating Light Alkanes Solubility in TEG”, Petroleum Technology Quarterly, 11 5, 165-171 (2006a).

Bahadori, A., “Solubility of BTEX in TEG”, International Gas Engineering & Management, 46, 7, 15-17 (2006b).

Bahadori, A., and Zeidani, K., “New Equations Estimate Acid Gas Solubility in TEG”, Oil & Gas Journal, 104, 8 (Feb. 27, 2006).

Grizzle, P.L., “Hydrocarbon Emission Estimates and Controls for Natural Gas Glycol Dehydration Units”, paper presented at the SPE/EPA Exploration & Production Environmental Conference, San Antonio, TX, USA (March 7-10, 1993).

Jou, F.-Y., Otto, F.D., and Mather, A.E., “Solubility of Methane in Glycols at Elevated Pressures,” The Canadian Journal of Chemical Engineering, 72, 130-133 (Feb.1994).

acid gas-light alkanes solubility in teg

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