Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Acid Dissociation Constant (pKa) of Common Monoethylene Glycol (MEG) Regeneration Organic Acids and Methyldiethanolamine at Varying MEG Concentration, Temperature, and Ionic Strength Adam Soames,*,† Stefan Iglauer,‡ Ahmed Barifcani,† and Rolf Gubner† †
WA School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Bentley, 6102 W.A. Australia Petroleum Engineering Department, Edith Cowan University, Joondalup, 6027 W.A. Australia
‡
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S Supporting Information *
ABSTRACT: The acid dissociation constants (pKa) of four organic acids (formic, acetic, propanoic, and butanoic) commonly found in monoethylene glycol (MEG) regeneration systems and methyldiethanolamine (MDEA) were measured via potentiometric titration. Dissociation constants were measured within varying concentration of MEG solution (0, 30, 40, 50, 60, 70, and 80 wt %) and at varying temperature (25, 30, 40, 50, 60, 70, and 80 °C). Thermodynamic properties of the dissociation process including Gibbs free energy (ΔG° kJ mol−1), standard enthalpy (ΔH° kJ mol−1), and entropy (ΔS° kJ mol−1 K−1) were calculated at 25 °C using the van’t Hoff equation. Comparison of the reported experimental pKa values and calculated thermodynamic properties in aqueous solution to the literature demonstrated good agreement. Two models have been proposed to calculate the pKa of acetic acid and MDEA within MEG solutions of varying concentration, temperature, and ionic strength. The proposed models have an average error of 0.413% and 0.265% for acetic acid and MDEA, respectively.
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INTRODUCTION The presence of organic acids within MEG regeneration systems poses a potential corrosion risk to natural gas transportation pipelines.1−4 Organic acids such as acetic, propanoic, and butanoic typically enter into the MEG regeneration loop via the condensed water phase where free organic acids are present within the reservoir.5 Alternatively, following the breakthrough of formation water, organic acids may also be introduced alongside mineral salt ions.5,6 The thermal degradation of MEG at high temperature in the presence of oxygen may also lead to the formation of organic acids including glycolic, acetic, and formic acids.5,7−10 The introduction of organic acids into natural gas transportation pipelines will ultimately reduce the pH of the liquid phase and hence pose a corrosion risk through increased solubility of the protective iron carbonate film. 11−14 Furthermore, organic acids such as acetic acid have been demonstrated to increase the rate of top of the line corrosion (TLC) in the presence of carbon dioxide.5,15−20 As such, while operating under pH stabilization corrosion control it is important to ensure sufficient alkalinity is present to neutralize incoming organic acids. Corrosion prevention through pH stabilization can be achieved through the addition of hydroxide or carbonate salts or amine-based compounds including MDEA.17,21 The application of MDEA within closed-loop MEG systems for pH stabilization is advantageous due to its thermal stability, © XXXX American Chemical Society
allowing multiple regeneration cycles before degradation occurs and its ability to be recovered during reclamation at high pH.22,23 However, upon the onset of formation water, the risk of scaling within subsea and MEG regeneration systems is present at high pH. As such, if scaling cannot be alternatively managed through scale inhibitors it may be beneficial to transition from pH stabilization using MDEA to more scaling friendly film forming corrosion inhibitors.6,24 The removal of MDEA from the MEG regeneration loop can be accomplished alongside monovalent cations including sodium via vacuum reclamation.6,25 Removal of MDEA during the reclamation process involves first neutralizing the MDEA to facilitate reaction with anionic species including chlorides, sulfates, sulfides, and organic acid ions to form heat-stable salts in a similar manner to industrial CO2 capture systems using amines.25−29 Upon the evaporation of lean MEG under vacuum, the heat-stable MDEA salts will remain, hence facilitating their removal. Likewise, removal of organic acids via the vacuum reclamation system may also be achieved in a similar manner to prevent their accumulation within the loop. As such, the acid dissociation behavior of a chemical is an important factor dictating the efficiency of its removal during reclamation. Received: March 20, 2018 Accepted: June 13, 2018
A
DOI: 10.1021/acs.jced.8b00221 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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respectively, 0.1 M (±0.002 M) NaOH and HCl supplied by Hanna instruments was utilized as the titrant. Solutions of varying MEG concentration (wt %) were prepared using deionized water (18.2 MΩ cm) and ethylene glycol supplied by Chem Supply (CAS: 107-21-1). The concentration of prepared MEG solutions was verified using an ATAGO PAL-91S refractometer precalibrated for the determination of MEG concentration with an accuracy of ±0.4% (V/V). Prior to titration all solutions were sparged using ultrapure nitrogen (>99.999 mol %) to remove dissolved CO2. Each titration was repeated three times with the average pKa presented. To account for the inherent error associated with pH measurement in MEG solutions, the correction factor described by Sandengen et al.35 given by eqs 1 and 2 was applied.
The acid dissociation constant (pKa) of a chemical is influenced by a variety of factors including temperature, ionic strength, and the dielectric constant (ε) of the solvent.30 Organic solvents such as ethylene glycol have been shown experimentally to influence the pKa of various weak acids and bases, typically increasing pK a compared to aqueous solutions.30−33 The dielectric constant of ethylene glycol (37.7 at 20 °C34) is noticeably lower than that of water (78.54 at 20 °C34) and will hence influence the dissociation behavior of weak acids and bases. The pKa of chemicals within MEG are hence important to MEG regeneration system operators and designers. The removal of MDEA and organic acids via vacuum reclamation is dependent on both system pH and temperature and will ultimately be influenced by the acid dissociation behavior. Likewise, the complete neutralization of incoming organic acids from the well is important while operating under pH stabilization corrosion control. The pKa’s of organic acids commonly found within MEG regeneration systems and MDEA have hence been determined at varying MEG concentrations, temperatures, and ionic strengths.
ΔpH MEG = 0.416w − 0.393w 2 + 0.606w 3
(1)
where w = weight fraction of MEG pH MEG = pH Measured + ΔpH MEG
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(2)
To assess the accuracy of the titration and pK a determination procedure utilized, the pKa of the respective organic acids and MDEA within water at 25 °C was compared to literature values. The measured pKa and comparison literature values are summarized in Table 2 with good
EXPERIMENTAL METHODOLOGY Chemicals. The organic acids and MDEA utilized within this study were procured from Chem Supply and SigmaAldrich, respectively. The purities of the chemicals used and their respective chemical structures are outlined by Table 1.
Table 2. Water pKa Comparison to Literaturea pKa,(water) (25 °C)
Table 1. Chemical Purity and Structures
a
compound
literature
this study
acetic acid propanoic acid butanoic acid formic acid MDEA
4.7636 4.8836 4.8237 3.7537 8.5838
4.76 4.87 4.82 3.76 8.58
Standard uncertainties: u(T) = 0.01 °C and u(pKa) = 0.03.
agreement found. Furthermore, the effect of temperature on the measured pKa of the organic acids and MDEA within aqueous solutions was also compared to published literature studies (Figure 1 to Figure 3).
Apparatus and Procedure. The pKa of organic acids and MDEA within varying MEG concentrations was determined via titration using a HI902 potentiometric automatic titration system. The automatic titration system utilizes a dynamic titration mode whereby the addition of the titrant is reduced as the equivalence point approaches. For this study, a minimum titrant volume step size of 0.025 mL was utilized to ensure accurate identification of the equivalence point (±0.1% dosed volume). pH measurements were performed using a Thermo Scientific Orion 8302BNUMD pH probe (±0.01 pH units) with automatic temperature compensation. Temperature was maintained within the titration cell via an external water heating jacket (±0.1 °C). For the organic acids and MDEA,
Figure 1. Effect of temperature on pKa of acetic and propanoic acid (aqueous solution). B
DOI: 10.1021/acs.jced.8b00221 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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utilized.42,46,47,49,50 Where the A and B terms are constants, ai is the ionic size parameter, zi the valence of the ionic species, and I the ionic strength of the solution (calculated through eq 6). The ionic size parameters reported by Kielland51 were utilized for the organic acids (ai = 3.5 Å47) and MDEA (ai = 4.5 Å42). The values of the A and B constant terms were taken from the works of Manov et al.52 and Robinson and Stokes.53 −log(γi) =
Azi2 I1/2 1 + BaiI1/2
(5)
1 ∑ zi2ci 2 where ci = ion concentration and zi = ion valence I=
(6)
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Figure 2. Effect of temperature on pKa of formic acid (aqueous solution).
EXPERIMENTAL RESULTS Effect of MEG Concentration on pKa. The effect of varying MEG concentration on the pKa of the organic acids and MDEA at 25 °C is presented in Table 3 and Table 4, Table 3. Effect of MEG Concentration on Measured Organic Acid pKa (25 °C)a MEG concentration (wt %) compound
0
30
40
50
60
70
80
formic acetic propanoic butanoic
3.75 4.76 4.88 4.82
3.89 4.96 5.23 5.13
3.99 5.08 5.38 5.28
4.10 5.22 5.55 5.46
4.25 5.42 5.74 5.64
4.40 5.63 6.01 5.90
4.65 5.93 6.29 6.21
Standard uncertainties: u(T) = 0.01 °C, u(MEG %) = 0.2, u(pKa) = 0.04.
a
Table 4. Effect of MEG Concentration and Temperature on Measured MDEA pKa Figure 3. Effect of temperature on pKa of MDEA (aqueous solution).
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MEG concentration
DETERMINATION OF PKA VALUES The determination of pKa from potentiometric titration data is often performed by plotting the pH as a function of titrant volume and estimating the pKa value from the curve’s inflection point.30 At the equivalence point, the concentration of the acid or base and its conjugate form is equal, and hence through eq 3, pKa is equal to pH. However, the measurement of pKa is influenced by the ionic strength of the solution and in turn the dissociation of the acid or base to its conjugate form.30,46,47 As such, calculation of the acid dissociation constants (pKa) reported in this study was performed using eq 447 where the activity coefficient of the ionic species (γ−i ) is incorporated. Through eq 4, pKa was calculated over the entire pH range of the titration and the average pKa reported. The activity coefficient of the undissociated species (γi) was assumed to be one.47−49 ÅÄÅ [A±] ÑÉÑ Å ÑÑ Ñ pK a = pH + logÅÅÅ ÅÅÇ [HA] ÑÑÑÖ (3) ÄÅ É iγy Å 1 − α ÑÑÑ ÑÑ + logjjjj i zzzz pK a = pH + logÅÅÅÅ Ñ j γ− z ÅÇ α ÑÖ (4) k i { where α = V/Veq. To calculate the activity coefficient of the dissociated species (γi−), the Debye−Hü ckel equation given by eq 5 was
temperature (°C)
(wt %)
25
30
40
50
60
70
80
0 30 40 50 60 70 80
8.57 8.53 8.51 8.47 8.43 8.38 8.32
8.45 8.41 8.39 8.35 8.30 8.24 8.18
8.27 8.22 8.19 8.16 8.12 8.07 8.01
8.08 8.03 8.01 7.99 7.94 7.89 7.82
7.90 7.87 7.85 7.82 7.77 7.72 7.64
7.73 7.69 7.67 7.63 7.59 7.54 7.46
7.58 7.54 7.51 7.48 7.43 7.37 7.29
Standard uncertainties: u(T) = 0.01 °C, u(MEG %) = 0.2, u(pKa) = 0.04.
a
respectively. For the organic acids, increasing MEG concentration resulted in an increase in the measured pKa as per Figure 4. The change in measured pKa was found to change linearly with respect to MEG mole fraction (not shown). As a result, the change in pKa directly correlated with the change in dielectric constant of the solution (refer to Figure 5 and Figure 6).30 Dielectric constants of MEG solution are available in the Supporting Information, Table S1. In contrast, the measured pKa of MDEA was found to decrease with respect to increasing MEG concentration (Figure 7), however, again in line with the change in MEG mole fraction and resultant change in dielectric constant of the solution (Figure 8). Effect of Temperature on pKa. The effect of temperature on the pKa of the organic acids and MDEA was evaluated C
DOI: 10.1021/acs.jced.8b00221 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 7. Effect of MEG concentration and temperature on pKa of MDEA.
Figure 4. Effect of MEG concentration on pKa of organic acids (25 °C).
Figure 8. Dielectric constant vs MDEA pKa.
Table 5. Effect of Temperature on Measured Organic Acid pKa in Aqueous and Varying MEG Concentration Solutionsa
Figure 5. Dielectric constant of varying MEG concentration solutions.55,56
temperature (°C)
Figure 6. Dielectric constant vs organic acid pKa (25 °C).
within varying MEG concentration solutions (30, 50, and 80 wt %) and is presented in Table 4 and Table 5. The pKa of the organic acids within MEG solution demonstrated similar behavior with respect to temperature as within water (Figure 9 to Figure 11). A similar response was observed for MDEA whereby the change in temperature induced a large reduction in the measured pKa matching that of water (Figure 12). The results are in line with the findings of Castells et al.54 who suggested a large change in pKa can be expected for amines with a lower temperature response exhibited by organic acids.
compound
25
formic acetic propanoic butanoic
3.76 4.76 4.87 4.82
formic acetic propanoic butanoic
3.89 4.96 5.23 5.13
formic acetic propanoic butanoic
4.09 5.22 5.55 5.46
formic acetic propanoic butanoic
4.65 5.93 6.29 6.21
30
40
50
Aqueous Solution 3.76 3.76 3.77 4.76 4.77 4.79 4.88 4.89 4.91 4.82 4.84 4.86 30 wt % MEG 3.89 3.90 3.90 4.96 4.97 4.99 5.23 5.25 5.26 5.14 5.15 5.16 50 wt % MEG 4.09 4.10 4.11 5.22 5.23 5.25 5.56 5.57 5.58 5.46 5.47 5.49 80 wt % MEG 4.65 4.65 4.66 5.93 5.94 5.96 6.30 6.31 6.33 6.22 6.23 6.25
60
70
80
3.79 4.81 4.94 4.88
3.82 4.84 4.96 4.91
3.85 4.87 4.99 4.94
3.92 5.01 5.29 5.19
3.95 5.04 5.32 5.22
3.99 5.08 5.37 5.27
4.12 5.27 5.61 5.52
4.15 5.30 5.65 5.55
4.09 5.22 5.55 5.46
4.68 5.99 6.35 6.28
4.71 6.02 6.39 6.31
4.76 6.07 6.44 6.36
Standard uncertainties: u(T) = 0.01 °C, u(MEG %) = 0.2, u(pKa) = 0.04.
a
Estimation of Thermodynamic Properties. The measured MDEA dissociation constants were correlated with D
DOI: 10.1021/acs.jced.8b00221 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 9. Effect of temperature on pKa of organic acids (30 wt % MEG).
Figure 12. Effect of temperature on pKa of MDEA in varying concentration of MEG solution (wt %).
thermodynamic properties of the organic acids at 25 °C in varying MEG concentration solutions is presented in Table 8. Comparison of the thermodynamic properties of the organic acids to prior studies of organic acids in aqueous solutions showed good agreement (Table 9). ln(K a) = − ln(10 pKa) = a +
b T /K
ΔG° = ΔH ° − T ΔS°
(8)
ΔH ° = −R × b
(9) (10)
ΔS° = R × a ln(K a) = − ln(10 pK a) = a +
Figure 10. Effect of temperature on pKa of organic acids (50 wt % MEG).
(7)
i 2c zy zz ΔH ° = −R jjjjb + T /K z{ k
b c + T /K (T /K)2
(11)
(12)
ij c yzz ΔS° = R jjja − zz 2 j (T /K) z{ (13) k Effect of Sodium Content and Ionic Strength on pKa. The introduction of mineral salts into MEG regeneration systems is a major process concern due to its effect on regeneration performance and increased scaling potential.6,10 Sodium chloride (NaCl) represents one of the major salt species introduced into MEG regeneration loops following formation water breakthrough.6,9 The effect of ionic strength in the form of NaCl upon the acid dissociation behavior of acetic acid and MDEA at 25 °C was analyzed in varying MEG concentration solutions and is presented in Table 10. The effect of ionic strength on MDEA pKa (Figure 13) was in line with the findings of Hartono et al.38 who concluded that at low ionic strengths (
