Experimental determination of the solubility of aromatic compounds in aqueous solutions of various amines

Fluid Phase Equilibria 210 (2003) 257–276

Experimental determination of the solubility of aromatic compounds in aqueous solutions of various amines A. Valtz a , M. Hegarty b , D. Richon a,∗ a

b

Lab de Thermodynamique, Ecole Nationale Supérieure des Mines de Paris, CENERG/TEP, 35 Rue Saint Honoré, 77305 Fontainebleau, France H2 W United, LLC, 3900 S. Wadsworth Blvd., Suite 500, Lakewood, CO 80235, USA Received 6 August 2002

Abstract A new apparatus to measure solubility data of aromatics in aqueous solutions has been designed. It is based on a static-analytic method with RolsiTM pneumatic samplers for on line gas chromatograph analysis. Operating pressures and temperatures are between 0.3 and 10 MPa and between 293 and 393 K. Solubility measurement results are reported for several aromatic compounds (benzene, toluene, ethylbenzene and xylene) in different amine aqueous solutions (monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diglycolamine (DGA)). Several influent parameters are studied (temperature, total pressure, etc.). © 2003 Elsevier Science B.V. All rights reserved. Keywords: Apparatus; Solubility; Methane; Benzene; Toluene; Carbon dioxide; MEA; DEA; MDEA; DGA

1. Introduction The Clean Air Act has specified the limiting amounts of heavy hydrocarbons (volatile organic compounds), which may be emitted from a facility to 250 t per year. Some aromatic compounds such as benzene, classified as hazardous air pollutants, are limited to 25 t per year total aromatics and 10 t per year of any individual aromatic. The benzene, toluene, ethylbenzene, xylene (BTEX) regulations obviously have considerable effect on most amine systems. Units as small as 50 m3 /h circulation treating gas containing aromatics may be well above the allowable amounts of aromatics in the vent stream. The heavy hydrocarbons and particularly aromatics have harmful effects [1–3] in a sulfur plant feed. Petro-Canada’s Hanlan Robb Gas Plant is a large sour natural gas treating facility with a significant quantity of aromatics in the feed gas. The amine-treating unit absorbs an appreciable quantity of aromatics Abbreviations: MEA, monoethanolamine; DEA, diethanolamine; MDEA, methyldiethanolamine; DGA, diglycolamine Corresponding author. Tel.: +33-164694965; fax: +33-164694968. E-mail address: [email protected] (D. Richon). ∗

0378-3812/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-3812(03)00173-0

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from the feed gas. Due to the relatively lean acid gas, the aromatics have caused rapid loss of sulfur plant recovery efficiency, necessitating frequent replacement of catalyst. A substantial amount of time, money and resources have been spent on efforts to cope with, and reduce, the impact of aromatics in the sulfur recovery system. A substantial portion of the aromatics in the inlet gas is absorbed by the circulating amine, and is released to the acid gas stream during amine regeneration. Key factors that affect sulfur catalyst deactivation are the amount of aromatics absorbed in the amine plant and the efficiency of aromatic destruction in the reaction furnace. The Hanlan Robb facility has focused on the criteria and techniques required to improve aromatic destruction in the reaction furnace. In addition, there has been substantial investigation of methods to reduce aromatic absorption in the amine plant. Unfortunately, a lack of good quantitative data to predict vapor–liquid equilibria (VLE) behavior of aromatics in amines has made optimization of the amine plant extremely difficult. In order to address this type of problem, the GPA has recently sponsored a project to determine aromatic solubility in various amines. These data will be used to develop amine models that will be capable of optimizing the amine plant for minimal aromatic absorption. Solubility measurements presented in this paper indicate that the solubility of BTEX in aqueous amines increases substantially as the molar concentration of the amine increases. This paper presents new experimental solubility data of benzene, toluene, ethyl benzene and xylene in water and aqueous amine solutions. A special apparatus was designed, built, and used to measure the solubility of hydrocarbons in aqueous amine systems in the range: 298–393 K. Analyses of liquid samples are performed using a gas chromatograph. A lot of different analytical methods and procedures are necessary to carry on the whole campaign; they are described herein. The reliability of the measurements has been successfully tested by measuring the solubility of benzene, toluene, ethylbenzene and xylene in pure water and by comparing the results to literature data. Only comparison for benzene is reported herein. Two experimental methods (synthetic and analytic) have been tested. Solubility data from both methods are in good agreement within the experimental uncertainties confirming the high level of reliability of the new data. The results of the studies are reported, at different temperatures for various aqueous amine solutions. The influence of a number of parameters on the solubility of the aromatics in the amine solutions has been studied. These parameters include: • • • • • • •

temperature; amine concentration; aromatic partial pressure; amine structure; aromatic carbon number; CO2 loading in the amine solution; pressure (different methane partial pressures from 0.5 to 10 MPa).

2. Experimental equipment The cell is made of a sapphire cylinder pressed between two Hastelloy C22 flanges. The upper flange holds two pneumatic samplers (one for the vapor phase and one for the liquid phase (the denser phase in the

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Table 1 Origin and purity of compounds Compound

Formula

Furnisher

Purity

Methane Carbon dioxide Benzene Toluene Ethylbenzene Xylenesa Monoethanolamine (MEA) Diethanolamine (DEA) Methyldiethanolamine (MDEA) Diglycolamine (DGA)

CH4 CO2 C 6 H6 C 7 H8 C8 H10 C8 H10 C2 H7 NO C4 H11 NO C5 H13 NO2 C4 H11 NO2

Messer Griesheim Air Liquide Prolabo Prolabo Fluka Prolabo Aldrich Prolabo Fluka Merck

>99.995% vol. >99.995% vol. >99.8% GC >99.9% GC >99% GC >99.8% GC >99% GC >99.5% GC >98% GC >99% GC

a

Mixture of ortho-, meta- and para-xylene.

case of two liquids), a heating resistance, one 100  platinum probe, one thermocouple, a 1/16 in. tubing for connection to pressure transducers and one feeding valve (non-rotating stem, micro-dead volume). The bottom flange holds one 100  platinum probe. Two pressure transducers allow covering a large range of pressures with the best accuracy. They are maintained at a constant temperature higher than the highest temperature of the solubility study by means of an oven monitored by a PID regulator. A liquid bath is used to regulate the temperature of the equilibrium cell within 0.01 K. 2.1. Purities of the compounds The liquids chemicals have been used after distillation and careful degassing under a vacuum. The origin and purity of compounds are reported in Table 1. The compositions of the studied amine aqueous solutions, determined gravimetrically and by GC, are listed in Table 2. Gravimetrical measurements and gas determinations are in agreement within the corresponding uncertainties. This confirms the quality of gas the chromatograph calibrations. 2.2. VLLE measurements The feeding circuit is composed of: a vacuum pump, a methane containing cylinder and two variable volume cells fitted with displacement transducers (1 ␮m resolution) one for water or aqueous amine solution and one for liquid hydrocarbons. A flow diagram of the corresponding equipment is given in Fig. 1. The methane cylinder and pneumatic samplers represented on the figure are not used. 2.2.1. Synthetic method The evacuated cell is partially loaded with distilled water. The quantity of water introduced into the equilibrium cell is known from the value of the piston displacement in the variable volume cell, VVCW, at controlled pressure. The vapor pressure of pure water is measured and small quantities of hydrocarbon are added progressively through the variable volume cell, VVCHC. After each introduction of benzene,

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Table 2 Compositions of the aqueous amine solutions Aqueous mixture

14.6 wt.% MEA 35 wt.% DEA 25 wt.% MDEAa 25 wt.% MDEAb 50 wt.% MDEAa 50 wt.% MDEAb 50 wt.% MDEAc 35 wt.% DGAa 35 wt.% DGAb 52.5 wt.% DGA 70 wt.% DGAa 70 wt.% DGAb

Amine composition Gravimetric determination (wt.%)

Gravimetric determination (mol%)

Determined by GC (mol%)

14.61 ± 0.05 34.77 ± 0.05 24.74 ± 0.02 25.47 ± 0.02 50.58 ± 0.05 49.55 ± 0.05 49.98 ± 0.05 34.98 ± 0.05 35.01 ± 0.05 52.46 ± 0.05 70.69 ± 0.05 69.63 ± 0.05

4.81 ± 0.05 8.37 ± 0.05 4.74 ± 0.01 4.91 ± 0.01 13.40 ± 0.05 12.93 ± 0.02 13.13 ± 0.02 8.44 ± 0.05 8.45 ± 0.05 15.91 ± 0.05 29.25 ± 0.05 28.21 ± 0.05

4.8 ± 0.2 8.3 ± 0.2 4.7 ± 0.15 4.9 ± 0.1 13.73 ± 0.25 13.1 ± 0.3 13.2 ± 0.3 Not measured 8.4 ± 0.2 15.4 ± 0.3 Not measured 28.0 ± 0.5

(a) Composition corresponding to data appearing in reference [23]. (b) Composition corresponding to VLLE measurements with benzene and toluene independently. (c) Composition corresponding to VLE measurements with toluene. (d) Composition corresponding to VLLE measurements with ethylbenzene, VLE and VLLE measurements with mixture benzene–toluene–ethylbenzene and CO2 influence. (e) Composition corresponding to VLLE measurements with benzene and ethylbenzene independently. (f) Composition corresponding to VLLE measurements with toluene.

total pressure is measured. The “pressure versus hydrocarbon amount” curve displays a break point at saturation. After saturation has been reached, total pressure remains constant and is independent of the new hydrocarbon additions. Solubility value is deduced from the position of the break point. The accuracy of the solubility determinations depends directly on the accuracy of the different determinations of the amounts of hydrocarbon added at each step. This synthetic method has been used to measure the solubility of benzene in water at 298 K. 2.2.2. Analytic method The analytic method is carried out with the same equipment. The evacuated cell is loaded with about 5 cc of solvent (distilled water or aqueous amine solution). About 2 cc of hydrocarbon are added. In these conditions two liquid phases are present in the cell. At equilibrium, the total pressure is measured. Then the total pressure is increased up to about 5 bar using methane. The solution is stirred vigorously for more than 1 h to achieve phase equilibrium. Stirring speed is reduced for 1 h and samples of aqueous phase are withdrawn and analyzed by gas chromatography. Several samples are analyzed to evaluate the repeatability of the measurements. 2.3. VLE measurements In order to measure vapor–liquid equilibria of a mixture involving: methane and one hydrocarbon or a mixture of hydrocarbons dissolved in an aqueous amine solution, we used the following procedure: An aqueous amine+hydrocarbon-saturated mixture is first prepared inside a thermoregulated container (TC). Then a given amount of the saturated mixture is transferred into the equilibrium cell shown in Fig. 1. A given amount of hydrocarbon-free aqueous amine solution is then introduced to dilute the saturated

A. Valtz et al. / Fluid Phase Equilibria 210 (2003) 257–276 Fig. 1. Flow diagram of the equipment. C: carrier gas; DA: degassed amine solution; DDD: displacement digital display; DHC: degassed hydrocarbon; DT: displacement transducer; EC: equilibrium cell; FV: feeding valve; LB: liquid bath; LS: liquid sampler; Me: methane cylinder; MS: magnetic stirring; PN: pressurized nitrogen; PP: platinum probe; PD: pressure display; PTh: pressure transducer for high pressure values; PTl: pressure transducer for low pressure values; RB: regulated bath; SM: sampler monitoring; ST: sapphire tube; SV: special valve; TC: thermoregulated container; TR: temperature regulator; VS: vapor sampler; VSS: variable speed stirring; VVCHC: variable volume cell for hydrocarbons; Vi: shut-off valve; VP: vacuum pump; VVCAS: variable volume cell for amine solution. 261

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aqueous solution and reach the required partial saturation conditions (10, 25 and 50% of the saturation pressure of the hydrocarbon). The vapor–liquid system is stirred and samples of both liquid and vapor phases are withdrawn for composition measurements. These measurements provide the partial pressure of the aromatic as a function of its concentration in the aqueous phase. Methane is used to reach the desired total pressure. The thermoregulated container designed for preparing the saturated solutions is composed of a double glass envelope to allow seeing inside it and performing thermal regulation. Efficient stirring is used to achieve rapid saturation. The transfer of the saturated solution into the equilibrium cell is done using methane pressurization. The variable volume loading cell is used for addition of the “hydrocarbon-free” amine solution to obtain the desired solute partial saturation. 2.4. Analytical circuit The analytical circuit and the analytical conditions (characteristics of the temperature programming, commutation time, etc.) have been optimized for each system. Two typical flow diagrams of the analytical circuits are given in Figs. 2 (simplest) and 3 (most complex). In Fig. 2, two detectors, thermal conductivity detector (TCD) and flame ionization detector (FID) are in series, the analytical column is Porapak R, 80–100 mesh, 1/8 in. stainless steel, 2 m. Carrier gas is helium. The circuit given in Fig. 3 was developed to analyze the VLLE systems in which CO2 has been added. Carrier gas is helium. The CH4 , CO2 , and H2 O pass quickly through the primary column, A1 (Porapak

Fig. 2. Flow diagram of the analytical circuit used for BTEX solubilities in water. A: analytical column; FID: flame ionization detector; I: injector; LS: liquid sampler; O: oven; Ref: reference channels; TCD: thermal conductivity detector; TR: thermal regulator; VS: vapor sampler.

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263

Fig. 3. Flow diagram of the analytical circuit used for VLE measurements on CH4 –CO2 –hydrocarbon(s)–MDEA–water systems. A1: primary column; A2: secondary column; AG: auxiliary gas; C: commutation valve; FID: flame ionization detector; I: injector; LS: liquid sampler; O: oven; Ref: reference channels; TCD1 and 2: thermal conductivity detectors; TR: thermal regulator; VS: vapor sampler.

Q, 60–80 mesh, 1/8 in. silcosteel tube, of 10 cm length. These light components are then separated in the secondary column, A2 (Porapak R, 80–100 mesh, 1/8 in. silcosteel tube, of 200 cm length) and analyzed on TCD1. The gas chromatograph used is from Varian, model 3800. The heavy components (toluene and MDEA) spend more time in the first column and are commutated to TCD2 before they reach A2. The MDEA is analyzed on TCD2. The helium flow carries the stream on to the downstream FID detector where the toluene is analyzed. An additional helium flow (AG) sweeps the light components to A2 and then up to the TCD1. As methane passes only through the TCD1, its very low composition in liquid samples cannot be quantified. 3. Calibrations The Pt 100  probes and thermocouples are calibrated, following ITS 90, against a 25  reference platinum probe (TINSLEY Precision Instruments) fitted with an eight-digit multimeter (Hewlett-Packard 34420A). Uncertainty is estimated to 0.01 K for the Pt 100  platinum probes and to 0.1 K for the thermocouples.

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Table 3 Vapor pressure for the benzene–water system T (K)

P (MPa)

298.03 313.21 373.25 393.09

0.0160 0.0316 0.2815 0.4960

Three pressure transducers have been used for this work (Druck, model PTX 611) (nominal loads: 0.6, 4 and 6 MPa). They can be used up to 0.7, 5 and 9 MPa, respectively. They are maintained at a fixed temperature (higher than the working temperature to avoid any condensation phenomena). Below 0.6 MPa, the calibration is performed against a pressure calibration device (Desgranges & Huot, model 24610). Between 0.6 and 9 MPa, it is done against a dead weight balance (Desgranges & Huot, model 5202 S CP). Uncertainty below 0.6 MPa is ±0.0001 MPa. Uncertainty between 0.6 and 9 MPa is ±0.002 MPa. The pressure transducers, Pt probes and thermocouples are connected to a HP 34970A data acquisition unit. The detectors: The flame ionization detector and the thermal conductivity detector are calibrated using given amounts of compounds injected through syringes. Mole fractions are calculated from numbers of moles determined for all components. Methane is injected as a pure compound. The BTEX are diluted in methanol. Water is used pure or mixed with amines (gravimetrical determination of the synthesized composition).

Fig. 4. Vapor pressure over liquid–liquid water–benzene mixture vs. inverse temperature: (+) this work; (䊉) from [4].

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All the calibrations are freshly performed for each system under study to check for any change in the detector response coefficients. Methane contained in the liquid phase samples (in negligible amounts even at the highest pressure) is analyzed on FID (except for measurements including CO2 ), whereas methane contained in the vapor phase samples is analyzed on TCD. In the vapor phase samples, only the aromatics and methane can be detected and therefore quantified (the concentrations of amine and water are considered to be negligible). 4. Results To validate the experimental equipment described herein several tests have been performed. After loading water and benzene, vapor pressure over the liquid–liquid mixture has been measured as a function of temperature. The results are given in Table 3 and shown in Fig. 4 along with literature data. A very good agreement is found with Anderson and Prausnitz’ data [4]. Table 4 Benzene solubility in water, T = 298 K (synthetic method) P (MPa)

m (mg)

0.032 0.055 0.071 0.092 0.113 0.136 0.160 0.159 0.160

0 6.4 11.4 18.4 24.3 29.4 35.0 43.9 55.0

Table 5 Benzene solubility in water (analytical method) T (K)

P (MPa)

298.11 298.10 298.09 298.08 298.08

0.5254 1.025 2.997 6.505 9.931

313.19 313.18 313.19 313.18 373.11 393.09 393.10

x

2σx

5 5 6 6 8

3.98E−04 4.02E−04 3.97E−04 3.87E−04 3.80E−04

6E−06 3E−06 3E−06 4E−06 4E−06

0.5177 1.020 2.995 6.514

7 7 7 10

4.58E−04 4.47E−04 4.35E−04 4.19E−04

4E−06 4E−06 5E−06 2E−06

0.5137

16

9.16E−04

2E−05

6 15

1.16E−03 1.26E−03

1E−05 2E−05

0.496 0.720

n

a

n: number of samples analyzed, σx = ((n a Value without methane.







x2 − (

x)2 )/(n(n − 1)))1/2 .

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Fig. 5. Solubility of benzene in water vs. total pressure: (+) 298 K; (䊉) 313 K.

Measurements have been done with the synthetic method. For these purposes, the equilibrium cell contains 18.181 g of water at the beginning of the experiment. Temperature is 298 K. Given quantities of benzene are successively added in the mixture. The resulting data “pressure versus mass of benzene added” are given in Table 4. At saturation, a benzene mole fraction of 3.86 × 10−4 has been found from the occurring break point. The estimated error due to uncertainties on all measured quantities is: ±0.20 × 10−4 . From the analytic method, we have obtained the solubility data that are reported in Table 5. The influence of total pressure (methane partial pressure) is shown in Fig. 5. Table 6 Benzene solubility in water at 298 K comparison with a literature data x

Reference

4.02E−04 4.20E−04 4.13E−04 4.16E−04 4.08E−04 4.10E−04 4.11E−04 3.97E−04 4.05E−04 3.98E−04 3.86E−04

[5] [6] [7] [8] [9] [10] [11] [12] [13] This work (analytic method) This work (synthetic method)

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Fig. 6. Comparison between our experimental benzene solubility in water and literature’s data vs. temperature: (a) (䊉) this work; (䊐) from [14]; () from [15]; (䉫) from [16]; (+) from [17]; (䊊) from [18]; (b) (䊉) this work; (䊐) from [19]; () from [20]; (䉫) from [21]; (+) from [3]; (䊊) from [22].

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Table 7 Solubility data for the toluene–MDEA–water (25/75) wt.% mixture T (K)

P (MPa)

n

x

2σx

298.07 298.06

0.4997 2.483

12 18

3.22E−04 2.70E−04

1E−05 9E−06

333.03 333.01 333.00

0.5005 2.488 8.143

12 9 18

5.67E−04 5.26E−04 4.85E−04

1E−05 8E−06 4E−05

363.12

0.5045

12

1.08E−03

3E−05

393.30 393.31 393.25 393.27

0.5058 2.496 2.535 8.045

9 20 6 11

2.26E−03 1.78E−03 1.78E−03 1.64E−03

8E−05 7E−05 5E−05 9E−05

Comparison of our values at 298 K with literature’ is done in Table 6. Other comparisons are done over the temperature range of interest in this work. Literature data appear a little dispersed, but around our values (see Fig. 6a and b). Solubility data obtained with the analytic method in a MDEA–water (25/75) wt.% mixture at different temperatures and pressures are given in Table 7 (in presence of toluene organic phase). Solubility increases significantly with temperature. In Fig. 7, we show solubility of aromatics increases logarithmically with temperature. From Fig. 8, we see that increasing methane partial pressure leads to a moderate decrease in aromatic solubility.

Fig. 7. Solubility of benzene, toluene and ethylbenzene in DGA aqueous solutions vs. temperature at 0.5 MPa total pressure: (䊐) benzene in 35 wt.% DGA; (䊏) benzene in 70 wt.% DGA; () toluene in 35 wt.% DGA; (䉱) toluene in 70 wt.% DGA; (䊊) ethylbenzene in 35 wt.% DGA; (䊉) ethylbenzene in 70 wt.% DGA.

Fig. 8. Solubility of benzene in 70.69 wt.% DGA aqueous solution vs. methane total pressure: (䊉) 298.22 K; (䊐) 333.11 K; () 363.23 K.

Fig. 9. Solubility in 35 wt.% DGA aqueous solution at 0.5 MPa total pressure vs. number of carbon atoms: (䊉) 298 K; (䊐) 333 K; mol% 363 K; (+) 393 K.

Fig. 10. Solubility in DGA aqueous solutions at 298 K and 0.5 MPa total pressure vs. number of carbon atoms: (䊉) 0 wt.% DGA; (䊐) 35 wt.% DGA; () 70 wt.% DGA.

Fig. 11. Solubility in various amine aqueous solutions at 298 K and 0.5 MPa total pressure vs. number of carbon atoms: (䊊) pure water; (䊏) 25 wt.% MDEA; (䊐) 50 wt.% MDEA; (䉱) 35 wt.% DGA; () 70 wt.% DGA; (䊉) 35 wt.% DEA; (+) 14.6 wt.% MEA.

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271

Fig. 12. Solubility of toluene at 298 K and 0.5 MPa total pressure vs. amine mol%: (䉫) pure water; (䊊) 4.8 MDEA mol%; (䊐) 13.1 mol% MDEA; () 26.1 mol% MDEA; (䊉) 8.45 mol% DGA; (䊏) 15.9 mol% DGA; (䉱) 29 mol% DGA; (×) 4.8 mol% MEA; (+) 8.4 mol% DEA.

For a given amine concentration, the aromatic solubility decreases logarithmically with its number of carbon atoms, this is valid for each studied temperature (see Fig. 9). The same behavior is found for the different amine compositions (see Fig. 10) and whatever the amine involved (see Fig. 11). The effect of amine concentration on the solubility of toluene in the aqueous solution is shown at two temperatures in Figs. 12 and 13. At moderate amine concentration (less than 10 mol% amine in aqueous phase), toluene’s solubility increases logarithmically with mole fraction amine. However, from measurements at higher concentrations (above 10 mol% amine) we note that this logarithmic behavior does not continue. In addition, at the moderate amine concentrations (less than 10 mol% amine), toluene’s solubility at a given mole fraction of amine is similar for all of the amines examined. Again, however, at higher concentrations, the toluene solubility in different amines tends to diverge from one another. The solubility of the aromatics depends almost linearly on their partial pressure especially at low partial pressures. The slope of this line begins to change slightly at higher concentrations. However, the line extends smoothly to the maximum partial pressure of the aromatic vapor over its own liquid phase (see Fig. 14). Another study carried out on a mixture of three aromatics in a MDEA–water (50/50) wt.% solution at 333 K and pressurized by methane at 0.5 MPa leads to similar conclusions. The mixture of aromatics

Fig. 13. Solubility of toluene at 333 K and 0.5 MPa total pressure vs. amine mol%: (䉫) pure water; (䊊) 4.8 MDEA mol%; (䊐) 13.1 mol% MDEA; () 26.1 mol% MDEA; (䊉) 8.45 mol% DGA; (䊏) 15.9 mol% DGA; (䉱) 29 mol% DGA; (×) 4.8 mol% MEA; (+) 8.4 mol% DEA.

Fig. 14. Partial pressure of toluene in the toluene–MDEA–water–methane mixture vs. liquid mole fraction of toluene (P = 0.5 MPa; T = 333 K).

Fig. 15. Partial pressure of benzene in the BTE–MDEA–water–methane mixture vs. liquid mole fraction of benzene (P = 0.5 MPa; T = 333 K): (䊉) VLE data; (䊐) VLLE data; (—) linear regression (VLE).

Fig. 16. Partial pressure of toluene in the BTE–MDEA–water–methane mixture vs. liquid mole fraction of toluene (P = 0.5 MPa; T = 333 K): (䊐) VLLE C7 in BTE mixture; (䊏) VLLE C7 alone; (䊉) VLE C7 in BTE mixture; (+) VLE C7 alone; (—) linear regression.

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Fig. 17. Solubility of toluene in the MDEA–water–CO2 mixture vs. carbon dioxide mole fraction in liquid phase at 333 K and 0.5 MPa total pressure.

(benzene, toluene and ethylbenzene) was prepared in order to have one third of each in an aqueous amine mother solution. In Fig. 15, we have the same quasi-linear behavior. The saturation concentration of each of the aromatics in the BTE–MDEA–water solution is reduced from its solubility when it is alone in solution due to its dilution in the hydrocarbon phase. However, a comparison of the partial pressures over solutions with toluene alone and toluene with benzene and ethylbenzene (Fig. 16) indicates that its behavior is essentially the same. Another important study is the effect of loading the amine with carbon dioxide on the aromatic solubility. In fact, the absorption of the aromatics in an amine absorber is always determined by the conditions in the bottom of the absorber where the solution is loaded with CO2 and/or H2 S. Fig. 17 shows the influence of CO2 concentration on toluene solubility in presence of an excess toluene organic phase. The aqueous mixture is: MDEA–water (50/50) wt.% at 333 K pressurized by methane at 0.5 MPa. Within experimental uncertainty we can state that the solubility of the aromatic compounds decrease linearly with the carbon dioxide loading. The same behavior has been found with benzene in the same aqueous solution [23]. 5. Discussion and conclusion The solubility of aromatics was found to increase logarithmically with temperature in a manner similar to its behavior in water. However, the addition of an organic amine to water results in substantially higher solubility of the aromatic over that of water.

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The solubility of aromatics in amine solutions decreases logarithmically with the number of carbon atoms. At moderate amine concentration the toluene solubility increased logarithmically with mole fraction amine for all of the aqueous amine solvents investigated. In addition, all of these moderately concentrated amine solutions have approximately the same toluene solubility at a given mole fraction amine. Therefore, we can conclude that they all have the same tendency to absorb aromatics. However, at higher concentrations, the toluene solubility in different amines tended to diverge from one another. The measurement of vapor–aqueous amine equilibrium shows a nearly linear relationship between the aromatic partial pressure and its solubility in the aqueous phase. This relationship extends smoothly to the maximum partial pressure of the aromatic vapor over its own liquid phase. This behavior was observed in a wide range of pressures, between 0.5 and 7 MPa. Thus, for a given total system pressure, knowing the saturation solubility of an aromatic in the aqueous phase and its partial pressure in the presence of the liquid aromatic phase will allow us to easily predict the solubility of the aromatic at lower partial pressures. As the aqueous amine solution is loaded with CO2 , the solubility of the aromatic is decreased in a linear fashion. Finally, while total pressure has a significant effect on the partial pressure of an aromatic over its own liquid phase, total pressure has very little effect on the liquid–liquid equilibrium observed between 0.5 and 10 MPa. List of symbols C6 benzene C7 toluene C8 ethyl benzene n number of samples analyzed N number of carbon P pressure (MPa) PPCi partial pressure of component i T temperature (K) xi mole fraction relative to component i in aqueous phase Greek letter

σ ((n x2 − ( x)2 )/(n(n − 1)))1/2 Acknowledgements The investigators would like to thank the Gas Processors Association for its financial support and the members of Enthalpy Steering Committee; Dr. Patricia Guilbot for its support and help and Dr. Kai Fischer to have provided us data resulting from the DDB. References [1] K.P. Goodboy, J.C. Downing, H.L. Fleming, Oil Gas J. November 4, (1985) 89–98. [2] A. Johnson, T.R. Edwards, M.F. Miller, Oil Gas J. October 26, (1987) 33–40.

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