Solubility of Hexane in Aqueous Solutions of Methyldiethanolamine

Article pubs.acs.org/jced

Solubility of Hexane in Aqueous Solutions of Methyldiethanolamine Emad Alhseinat,* Rana Danon, Cornelis Peters, and Fawzi Banat Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE ABSTRACT: This paper presents new experimental solubility data of hexane in aqueous amine solutions. A set of simple thermodynamics equilibrium cells were used to measure the solubility of hydrocarbons in aqueous amine systems in the range 298 K to 333 K. Analyses of aqueous liquid samples are performed using a gas chromatograph. It was observed that hexane solubility in methyldiethanolamine (MDEA) solutions increased with increasing temperature. Also, increasing amine concentration in the test solution increased the solubility of hexane. A simple model is developed in order to represent the activity coefficient of hexane in the aqueous MDEA solution in the range of 40 wt % to 50 wt % MDEA.

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INTRODUCTION Aqueous solutions of alkanolamines are commonly used in the natural gas industry to absorb acid gases (CO2 and H2S) from natural gas and hydrocarbon liquids. Sour feed gas contains 0.5 % to 7 % H2S and 3 % to 6 % CO2, which needs to be removed in order to meet the design specification of 4 ppm to 20 ppm of H2S and < 3 % CO2.1 One of the advantages of using alkanolamines solutions is that they dissolve selectively more acid gases than hydrocarbons. However, hydrocarbons have low solubility in alkanolamines solutions. This low solubility represents lost product in natural gas treating solvents. The accurate knowledge of the solubility of hydrocarbons in aqueous amine solutions is consequently very important to determine the magnitude of hydrocarbon losses due to the treating process.2 Indeed, hydrocarbon losses are more significant for light hydrocarbons, that is, C1 (methane), C2 (ethane), C3 (propane), and C4 (butane), than heavy liquid hydrocarbons, that is, C5 (pentane), C6 (hexane), and C7 (heptane). Most of the available data in literature is focused on light hydrocarbon solubility.2−5 However, the solubility information for heavy hydrocarbons is of high importance not just to determine the magnitude of hydrocarbon losses but also for the influence of the hydrocarbons on alkanolamine foaming. Foaming during gas sweetening is a serious operational problem. Foaming can be induced by various contaminants including liquid hydrocarbon.6 Indeed, foaming generally leads to serious consequences such as loss of absorption capacity, reduced mass transfer area and efficiency, and carryover of amine solution to the downstream plant.7 Foamed alkanolamines may carry large amounts of hydrocarbons which contribute to hydrocarbon losses, which is far in excess of what would be expected from solubility alone. A foam problem is enhanced when the solution has sufficient liquid organics to increase the solution viscosity and density and reduce the solution surface tension.6 This study arises as a part of the ongoing project studying the deterioration of solvent quality and foaming problems in a Habshan Gas Sweetening Unit (HGSU), © XXXX American Chemical Society

GASCO, Abu Dhabi. In GASCO 40 wt % to 50 wt % MDEA is used for acid gas removal. This work was undertaken to measure the solubility of hexane in MDEA amine solutions and to quantify the effect of composition and temperature on hexane solubility. Thermodynamic of the System. Because of very low mutual solubilities in aqueous solvent−hydrocarbon systems, it is very easy to reach vapor−liquid−liquid equilibrium (VLLE) conditions.8,9 Thus, it can be assumed that the organic phase is composed mainly of hexane. The aqueous phase is mainly composed of water and MDEA. The same above assumptions have been followed by Valtz et al.8−11 to model the solubility of aromatics in amine solutions Thus, for hexane, equilibrium thermodynamic equations are v o aq f hexane = f hexane = f hexane

(1)

where; v, o, and aq denote the vapor phase, organic phase, and aqueous phase, respectively. xhexane and yhexane are mole fractions of hexane in liquid and vapor, respectively, The equilibrium between the aqueous phase and the organic phase can be expressed by aq o o aq xhexane γhexane = xhexane γhexane

(2)

o where; γaq hexane and γhexane are activity coefficients of hydrocarbon species, in the aqueous and hydrocarbon rich liquid phase, respectively. Since the hydrocarbon phase is assumed pure hexane:

aq aq xhexane *γhexane =1 aq γhexane =

1 aq xhexane

(3)

(4)

Received: March 12, 2015 Accepted: September 29, 2015

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DOI: 10.1021/acs.jced.5b00240 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Article

EXPERIMENTAL PROCEDURE FOR N-HEXANE SOLUBILITY MEASUREMENTS Apparatus and Experimental Procedures. Simple equilibrium cells immersed in an oil bath at fixed temperature were used to reach thermodynamics equilibrium. Figure 1 shows a scheme for the used equilibrium cell. The origin and purity of used chemicals are reported in Table 1.

Figure 2. Kinetic data.

30 m, 0.32 mm i.d., 0.25 μm film thickness. The split injection mode was used and adjusted to achieve the best reproducibility; the best split ration was found to be 2 %. To avoid or reduce the possibility of troubles and losing hydrocarbon due the split mode and adsorption throughout the column, a blank injection of the pure hydrocarbon was used frequently between samples measurements. For hexane, a five-point calibration line was generated, as can be seen in Figure 3.

Figure 1. Schematic for equilibrium cell used for the experiments.

Table 1. Chemical Origin and Purity chemical name and formula n-hexane commercial MDEA a

C6H14 C12H17NO2

source

purity from sourcea

Merck GASCO−Dow

≥ 96.0 % 99.0 %

No further purification was carried out.

Three MDEA concentrations, (40, 45, and 50) wt % concentrations were prepared from commercial MDEA treating solvent provided by GASCO. The used commercial MDEA was compared with 99.8 % analytical grade MDEA from Merck using a DSA−MS instrument and showed the same spectrum. The equilibrium cell was loaded with about 25 mL of solvent (aqueous MDEA solution) and about 15 mL of hexane. In these conditions two liquid phases were present in the cell. The aqueous phase was stirred vigorously for a long time, more than 12 h to achieve phase equilibrium. Kinetic study was performed to ensure that equilibrium status was achieved, as can be seen in Figure 2. Stirring was stopped for 1 h and samples of aqueous phase were withdrawn and analyzed by gas chromatography. Several samples were analyzed to evaluate the repeatability of the results. Selected experiments were duplicated to test the reproducibility of the results. The reliability of the measurements was successfully tested by comparing some of the results to comparable literature data, as well will be discussed later. The standard uncertainty in the mole fraction of hexane based in our repeatability and reproducibility measurements is ∼5·10−06. All analyses were performed using Agilant 6890 N gas chromatograph equipped with a flame ionization detector. Chromatographic separation was accomplished with an HP-5,

Figure 3. GC hexane calibration line.

The vapor phase was assumed to be pure hexane, and thus the pressure in the equilibrium cell is the vapor pressure of pure hexane at each experimental temperature. The vapor pressure of pure hexane can be calculated from Antoine equation.12 Indeed, in this study no measurements were conducted at elevated pressures. However, a new designed static analytical apparatus for high-pressure phase equilibrium measurements13 can be used to extend this study further for elevated pressures.

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RESULTS AND DISCUSSION The influence of temperature and amine concentrations on the solubility of hexane in aqueous MDEA solutions has been studied. The results of this study are reported in Table 2 and plotted in Figure 4. The optioned solubility data for hexane in 50 wt % MDEA are in excellent agreement and fit well with the available data in ref 2, confirming the high level of reliability of the new data. B

DOI: 10.1021/acs.jced.5b00240 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 2. Solubility of Hexane in (40, 45, and 50) wt % MDEA at Different Temperaturesa 40 wt % MDEA (gravimetric determination (GT) 40.2 wt %)

45 wt % MDEA (gravimetric determination (GT) 44.99 wt %)

T/Kb

xhexane

standard deviationc

298.15 308.15 318.15 328.15

15.5·10−06 17.4·10−06 30.0·10−06 31.0·10−06

5.68·10−06 1.43·10−06

T/Kb 298.15 308.15 318.15 328.15

T/Kb

standard deviationc

xhexane

298.15 21.2·10−06 308.15 23.2·10−06 318.15 42.0·10−06 328.15 53.0·10−06 50 wt % MDEA (gravimetric determination (GT) 49.99 wt %) xhexane

standard deviationc

−06

43.7·10 59.0·10−06 70.3·10−06 81.3·10−06

−06

4.32·10

2.47·10−06 7.98·10−06

4.62·10−06 1.54·10−06

T/Kd

xdhexane

298.21 298.34 313.24 323.21

44·10−06 46·10−06 64·10−06 90·10−06

a Standard uncertainty: u(x) ≈ 5·10−06; u(T) = 0.3; u(GT) = 0.1. bThe system assumed to be under the corresponding vapor pressure of pure hexane at each T. cStandard deviation s = ((∑i n= 1(xi − x)̅ 2)/(n − 1))1/2, n is number of samples. dData from ref.2

reproducibility of the results. Hexane−50 wt % MDEA equilibria was duplicated to test the reproducibility. Table 3 and Table 4 show the results of the repeatability and reproducibility studies, respectively. Table 3. Results of the Repeatability Studya

40 wt % MDEA 45 wt % MDEA 50 wt % MDEA

a

Figure 4. Solubility of hexane in different wt % MDEA as a function of temperature.

(temperature, vapor pressure of the system)

number of samples

average hexane mole fraction

standard deviation

298.15 308.15 298.15 308.15 298.15 318.15 328.15

3 2 2 2 3 3 2

15.5·10−06 17.4·10−06 21.2·10−06 23.2·10−06 43.7·10−06 70.3·10−06 81.3·10−06

5.68·10−06 1.43·10−06 4.62·10−06 1.54·10−06 4.32·10−06 2.47·10−06 7.98·10−06

Uncertainty of data is the same as reported in Table 2

Table 4. Results of the Reproducibility Study

The results show that the solubility of hexane increases with increasing temperature and MDEA concentration. Figure 5 shows the effect of MDEA concentration in hexane solubility. Repeatability and Reproducibility Studies. Several samples were analyzed to evaluate the repeatability and

50 wt % MDEA

temperature

number experimental repetition

hexane mole fraction

standard deviation

298.15 308.15 318.15 328.15

2 2 2 2

44.0·10−06 69.0·10−06 71.0·10−06 80.0·10−06

6.64·10−06 6.83·10−06 7.64·10−07 5.8·10−07

Simple Activity Coefficient Model. Activity coefficient of hexane in the aqueous phase can be calculated through eq 4 as discussed before. 1 aq γhexane = aq xhexane (4) Table 5 and Figure 6 show the calculated activity coefficients of hexane at different temperatures and MDEA concentrations. A simple model is developed in order to represent the activity coefficient of hexane in the aqueous solution as can be seen in eq 5. aq γhexane = Á exp(BT )

(5)

Taking the natural logarithm leads to Figure 5. Effect of MDEA concentration in hexane solubility at different temperatures.

aq ln γhexane = ln Á + BT = A + BT

C

(7)

DOI: 10.1021/acs.jced.5b00240 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 5. Activity Coefficient of Hexane in Aqueous Phase Using the Measured Solubilities activity coefficient γaq hexane T/K

40 wt %

45 wt %

50 wt %

298.15 308.15 318.15 328.15

64474 57537 33636 32669

47148 43122 23787 18932

22910 17016 14221 12296

Figure 8. Hexane activity coefficient parameter A as a function of MDEA wt %.

Figure 6. Effect of temperature in hexane activity coefficient at different MDEA wt %.

The values of these parameters are shown in Table 6. Table 6. Parameters of the Fitting Equation solvent

A

B

40 wt % MDEA 45 wt % MDEA 50 wt % MDEA

18.421 20.723 16.118

−0.026 −0.033 −0.020

Figure 9. Hexane activity coefficient parameter B as a function of MDEA wt %.

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CONCLUSION Simple thermodynamics equilibrium cells were used to obtain hexane solubility data in aqueous alkanolamines solutions. It has been observed that hexane solubility in all MDEA solutions increased with increasing temperature. Moreover, increasing amine concentration in the test solution increased the solubility of hexane. Finally, a simple model is developed in order to represent the activity coefficient of hexane in the aqueous MDEA solution in the range of 40 wt % to 50 wt % MDEA.

Combining eqs 4 and 5 will allow calculating hexane solubility in any aqueous amine solution for the amine−water system in the 0.4−0.5 weight fraction range at a given temperature. Figure 7 shows the effect of MDEA concentration on the activity coefficient of hexane in aqueous MDEA solution. Figures 8 and 9 show the effect of MDEA concentration on the activity coefficient parameter A and B of hexane in aqueous MDEA solution, respectively.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

The authors are grateful to the Petroleum Institute Gas Processing and Materials Science Research Center, Abu Dhabi for funding the project (GRC006). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 7. Effect of MDEA wt % in hexane activity coefficient at different temperature. D

DOI: 10.1021/acs.jced.5b00240 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jced.5b00240 J. Chem. Eng. Data XXXX, XXX, XXX−XXX