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Solubility of in Aqueous Solutions of Monoethanolamine

Fang-Yuan Jou and Alan E. Mather* Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada

Received November 5, 2018; Accepted November 6, 2018

Abstract: The of benzene in aqueous solutions of monoethanolamine has been measured at temperatures ranging from 25 °C to 100 °C.

Keywords: Liquid-liquid equilibria, aromatic ,

Aqueous solutions of alkanolamines are used for the removal of gases from gas mixtures or liquid hydrocarbons. Some hydrocarbons, especially BTX (­benzene, toluene, xylenes) are absorbed in the solution. These compounds have a deleterious effect on sulfur plants and so knowledge of the amount of aro- matics absorbed is important for the design of treating systems, and in evaluating environmental contamination. Another problem with aromatic hydrocarbons is their tendency to induce foaming in gas treating solutions. Few data are available for the solubility of aromatic compounds in alkanol- solutions. Critchfield et al. [1] presented data for the solubility of a number of hydrocarbons in various alkanolamine solutions. A few points for the solubility of benzene in monoethanolamine (MEA) solutions were obtained. Valtz et al. [2] reported data for aromatic compounds in various alkanolamine solutions. Some data were obtained for benzene in MEA solutions. Valtz et al. [3] presented data for the solubility of benzene in methyldiethanolamine (MDEA) and diglycolamine® (DGA) solutions. Valtz et al. [4] also presented data for the solubility of toluene in alkanolamine solutions. This work was undertaken as part of a study of the solubility of aromatic com- pounds in aqueous alkanolamine solutions.

*Corresponding author: [email protected] DOI: 10.7569/JNGE.2018.692506

J. Natural Gas Eng., Vol. 3, No. 2, December 2018 109 Jou and Mather.: Solubility of Benzene in Aqueous Solutions of Monoethanolamine

Experimental Work The experimental apparatus and procedure were similar to that used by Jou et al. [5] which has been used for equilibria in this laboratory. Hence only a brief description follows. The apparatus consisted of an equilibrium cell equipped with windows so that the contents were easily visible. The contents were mixed using a magnetic pump, described by van Doorn & Mather [6]. The cell and pump were housed in a temperature-controlled air bath. The temperature of the contents of the cell was measured by a Type J thermocouple. The thermo- couple was calibrated against a platinum resistance and found to have an uncertainty of ± 0.1 K. The pressure was measured using digital Heise gauges. They were calibrated against a dead-weight gauge and found to be accu- rate within 0.5% of full scale. The chemicals were obtained from the following sources and were stated to be of the given purity: benzene (Aldrich, HPLC 99+%), n-heptane (Aldrich, HPLC 99+%), glycol (Aldrich, anhydrous, 99.8%), (double distilled). Prior to the introduction of the fluids, the apparatus was brought to the desired temperature and purged with to remove traces of . Approximately 50 cm3 of the monoethanolamine (MEA) solution was fed by gravity to the equilibrium cell and a vacuum drawn to remove the meth- ane. The benzene was added and the pump started to recirculate the upper phase through the . At equilibrium, as established by a constant cell pressure, the pump was stopped and the phases analysed. The pressure was maintained by a spindle press at a level to ensure that no vapour phase was present. Because water

100 Yb

80

60 2-phase region Xb

Yb Xb 40

20

0 0 20 40 60 80 100

Xa (B=0)

Figure 1 Benzene concentration in the co-existing phases versus initial amine concentration.

DOI: 10.7569/JNGE.2018.692506

110 J. Natural Gas Eng., Vol. 3, No. 2, December 2018 Jou and Mather.: Solubility of Benzene in Aqueous Solutions of Monoethanolamine

Table 1 LLE Data for Benzene-MEA solutions (All concentrations in mole per cent) T = 25 °C, P= 150 kPa

Xa (B=0) Xw Xb Xa Yw Yb Ya 98.819 1.007 14.728 84.265 0.060 98.300 1.640 97.463 2.200 13.274 84.526 0.061 98.301 1.638 85.469 13.278 8.623 78.099 0.097 98.687 1.216 63.188 35.620 3.238 61.142 0.165 99.227 0.608 52.746 46.353 1.908 51.739 0.220 99.343 0.437 40.423 58.958 1.038 40.004 0.249 99.504 0.247 18.872 80.894 0.288 18.818 0.293 99.649 0.058 8.988 90.912 0.110 8.978 0.343 99.639 0.018

T = 60 °C, P = 150 kPa

Xa (B=0) Xw Xb Xa Yw Yb Ya 74.756 4.298 18.049 77.653 0.233 94.682 5.085 71.247 26.709 7.108 66.183 0.358 97.276 2.366 62.259 35.972 4.687 59.341 0.407 97.646 1.947 50.199 48.489 2.633 48.878 0.526 98.217 1.257 33.886 65.409 1.066 33.525 0.750 98.593 0.657 15.500 84.260 0.284 15.456 0.890 98.918 0.192 7.695 92.176 0.140 7.684 0.877 99.059 0.064

T = 100 °C

P/kPa Xa(B=0) Xw Xb Xa Yw Yb Ya 220 94.175 1.290 77.860 20.850 - - - 220 84.876 11.568 23.514 64.918 1.349 86.529 12.122 220 77.260 19.108 15.972 64.920 1.444 89.738 8.818 230 63.243 33.935 7.678 58.387 1.519 92.904 5.577 250 35.954 62.844 1.883 35.273 2.206 95.391 2.403 260 26.147 73.108 1.009 25.883 2.259 96.361 1.380 270 16.188 83.408 0.482 16.110 2.407 96.916 0.677 275 9.860 89.885 0.283 9.832 2.488 97.168 0.344 280 3.912 95.944 0.150 3.906 2.560 97.310 0.130

DOI: 10.7569/JNGE.2018.692506

J. Natural Gas Eng., Vol. 3, No. 2, December 2018 111 Jou and Mather.: Solubility of Benzene in Aqueous Solutions of Monoethanolamine is a minor component by weight and its molecular weight is much smaller than that of benzene and MEA, all three components must be individually determined. A sample of the aqueous phase was taken into a 40 ml bomb equipped with a magnetic stirrer. The bomb contained 2 g of n-heptane to extract the benzene. A sample of the benzene-rich phase was taken into a bomb containing 2 g of ethylene glycol. The ethylene glycol (EG) extracted the water and MEA from the benzene. An HP 5890A gas chromatograph was used for the analysis. It contained a 2 m long, 3.175 mm OD column packed with 80/100 mesh Chromosorb 104. The oven was maintained at 200 °C for the heptane sample and 200 °C for 3.5 min and then ramped to 250 °C for 20 min for the EG sample. The heptane completely extracts the benzene and contains less than 0.01% of water or amine. The aqueous sample was then taken for to determine the exact amount of amine. The experimental procedures were developed in previous experiments of measure- ments of the solubility of aromatic compounds in water [7].

Results and Discussion The results are presented in Table 1. The first column gives the concentration of the original MEA solution (no benzene) as mole per cent MEA. The concentration of the three components (water, benzene, MEA) in the aqueous phase is denoted by X. The concentration of these three components in the benzene-rich phase is denoted by Y. All concentrations are given in mole per cent. The results for 100 °C are presented in Figure 1, where the compositions of the coexisting phases are plotted versus the concentration of the original MEA solution. The upper curve, labeled Yb, gives the concentration of benzene in the benzene-rich phase. The lower curve, labeled Xb, gives the concentration of the benzene in the aqueous phase. The two curves converge in the upper right-hand corner. This point is called the plait point and is the last point in the two-phase region.

Acknowledgement The financial assistance of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

Refernces

1. J. Critchfield, .P Holub, H.-J. Ng, A.E. Mather, F.-Y. Jou, and T. Bacon, Solubility of hydro- carbons in aqueous solutions of gas treating . LRGCC Conference Proceedings 51, 199–227 (2001). 2. A.Valtz, M. Hegarty, and D. Richon, Experimental determination of the solubility of aromatic compounds in aqueous solutions of various amines. Fluid Phase Equilibria 210, 257–276, (2003).

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112 J. Natural Gas Eng., Vol. 3, No. 2, December 2018 Jou and Mather.: Solubility of Benzene in Aqueous Solutions of Monoethanolamine

3. A.Valtz, C. Coquelet, and D. Richon, Solubility data for benzene in aqueous solutions of methyldiethanolamine (MDEA) and of diglycolamine (DGA). Thermochimica Acta 443, 245–250 (2006). 4. A.Valtz, C. Coquelet, and D. Richon, Solubility data for toluene in various aqueous alkanolamine solutions. J. Chem. Thermo. 39, 426–432 (2007).

5. F.-Y Jou, A.E. Mather, and F.D. Otto, The solubility of CO2 in a 30 mass percent monoeth- anolamine solution. Can. J. Chem. Eng. 73, 140–147 (1995). 6. J. van Doorn and A.E. Mather, High pressure circulating pump. Rev. Sci. Instrum. 73, 4037–4039 (2002). 7. F.-Y. Jou and A.E. Mather, Liquid-Liquid Equilibria for Binary Mixtures of Water + Benzene, Water + Toluene, and Water + p-Xylene from 273 K to 458 K. J. Chem. Eng. Data 48, 750–752 (2003).

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