Chapoy, Antonin

Coquelet, Christophe

Hajiw, Martha

English

As more new fields contain high concentrations of carbon dioxide and hydrogen sulphide, there is a requirement for accurate equilibrium data for systems containing CO2 and/or H2S. Acid gases are undesirable and they are removed from the hydrocarbon stream after gas processing. They can be re-injected into the reservoir to enhance gas/oil recovery. However some hydrocarbons (e.g. methane and ethane) and aromatic compounds (e.g. benzene and toluene) may remain in the acid gases stream, becoming undesirable impurities leading to operation issues. Accurate experimental measurements of hydrate dissociation points are essential to validate predictions of thermodynamic models. Although there is a significant amount of data available for sweet gas there is still a requirement for measurements for sour gas systems. Experimental measurements, vapour-liquid and vapour-liquid-liquid equilibria or hydrate dissociation conditions, have been conducted for different mixtures of hydrocarbons containing CO2 and/or H2S in high concentration. The GC-PR-CPA and SRK72-CPA were used in this work to predict the different phase equilibria of systems with acid gases and the hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The results reveal that our model is in good agreement with experimental data<br/

Heriot Watt Pure

Impact of Aromatic Compounds on Acid Gas Injection: Experimental Measurements and Predictions with the GC-PR-CPA Equation Of State  M. Hajiw1, C. Coquelet1, A. Chapoy1,2* 1Mines ParisTech ,PSL Research University, CTP – Centre of Thermodynamics of Processes , 35 rue Saint Honoré, 77305 Fontainebleau Cedex, France,  2Hydrates, Flow Assurance & Phase Equilibria Research Group, Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, United Kingdom  Corresponding author: a.chapoy@hw.ac.uk  Abstract  As more new fields contain high concentrations of carbon dioxide and hydrogen sulphide, there is a requirement for accurate equilibrium data for systems containing CO2 and/or H2S. Acid gases are undesirable and they are removed from the hydrocarbon stream after gas processing. They can be re-injected into the reservoir to enhance gas/oil recovery. However some hydrocarbons (e.g. methane and ethane) and aromatic compounds (e.g. benzene and toluene) may remain in the acid gases stream, becoming undesirable impurities leading to operation issues.  Accurate experimental measurements of hydrate dissociation points are essential to validate predictions of thermodynamic models.  Although there is a significant amount of data available for sweet gas there is still a requirement for measurements for sour gas systems. Experimental measurements, vapour-liquid and vapour-liquid-liquid equilibria or hydrate dissociation conditions, have been conducted for different mixtures of hydrocarbons containing CO2 and/or H2S in high concentration. The GC-PR-CPA and SRK72-CPA were used in this work to predict the different phase equilibria of systems with acid gases and the hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The results reveal that our model is in good agreement with experimental data.  Introduction  About 40% of the untapped world’s natural gas resources contain significant concentrations of carbon dioxide and hydrogen sulphide. Acid gases are undesirable and stripped from the hydrocarbon stream. A way to remove them is chemical absorption with various types of alkanolamines. After the amine regeneration process, acid gases come out at low pressure. Acid gases obtained are mainly made up of carbon dioxide and hydrogen sulphide but some light hydrocarbons and aromatic compounds might remain, becoming undesirable impurities leading to operation issues. Acid gases are also saturated with water at these conditions, leading to potential hydrates formation in pipelines. Sulphur recovery is usually not the best solution because of costly processes and environmental concerns make the practice of flaring acid gases undesirable. Therefore, acid gases injection into a suitable underground zone becomes the best solution. However, the impact of the different impurities on process operations is not very well known, due to the complexity of the mixtures. Indeed, a bibliographic study reveals that existing data are limited especially for multi-component systems containing acid gases. Thus, experimental measurements, vapour-liquid equilibria and hydrate dissociation conditions, have been conducted for different mixtures containing CO2 and/or H2S. Hydrate dissociation points have been measured using the isochoric step-heating method for eight systems. The ratio between H2S and CO2 varied from 0% of H2S to 75%. Different concentrations of hydrocarbons (methane, ethane, propane, benzene, toluene, m-xylene and cyclopentane) were added to evaluate their impact on the hydrate dissociation conditions (ratio gas to liquid from 25:1 to 1:1). Measurements were conducted in a cell made of Hastelloy C276 with a volume of 120 cm3. The setup can operate up to 40 MPa and between 203 and 323 K. The cell is immersed in a temperature controlled bath. To achieve a fast thermodynamic equilibrium device high pressure magnetic stirrer is used.  Comparing the different mixtures, the following observations can be made • The presence of H2S promotes hydrates formation and displaces the hydrate dissociation curve to higher temperatures (closer the pure H2S curve) • The presence of aromatic compounds tends to displace hydrate dissociation curves to lower temperatures. There is a difference of about 5 K between a ratio gas to liquid 5:1 and 1:1. Accurate phase diagrams are of a great importance for process design. These diagrams can be predicted by equations of state. In this work, the GC-PR-CPA and SRK72-CPA equations of state (EoS) were used to predict the different phase equilibria of systems with acid gases [1,2]. These models result from the sum of two terms: a cubic equation of state and an associative term to take into account hydrogen bonding. There are thus particularly adapted for systems with water. In the GC-PR-CPA EoS, the Peng-Robinson cubic equation of state has been implemented. The second difference between both models is binary interaction parameters. While for the SRK72-CPA EoS, binary interaction parameters are polynomial functions adjusted for each binary system, the GC-PR-CPA is a group contribution method, where parameters are adjusted for different groups. As for the hydrate phase modelling, these models were coupled with the van der Waals and Platteuw model [3], as implemented by Parrish and Prausnitz [4]. Both GC-PR-CPA and SRK72-CPA models are found to be in good agreement with experimental data.  Experimental Measurements  Hydrate dissociation points have been measured using the isochoric step-heating method. The apparatus is presented in Figure 1. The equilibrium setup consists of a cell made of Hastelloy C276 with a volume of 120 cm3. It can operate up to 40 MPa and between 203 and 323 K. The cell is immersed in a temperature controlled bath. To achieve a fast thermodynamic equilibrium device high pressure magnetic stirrer is used. A full description of the experimental setups and procedures can be found in the GPA Research-Report 230 [5].    Figure 1: Schematic flow diagram of the apparatus. DW: degassed water; DAU: data acquisition unit; EC: equilibrium cell; GC: Gas cylinder; LPT: low pressure transducer; LPT: high pressure transducer; LB: liquid bath; PP: platinum probe; SD: stirring device; TR: temperature regulator; VP: vacuum pump; VVC: variable volume cell; PF: pressurizing fluid; DT: displacement transducer.    Thermodynamic Modelling   Two predictive models are used to predict phase equilibria for acid gases. Both have been developed on data for binary systems. Their capacity to predict phase equilibria for multi-component systems is tested in this paper. Group interaction parameters for the PPR78 EoS can been found in the paper by Xu et al. [6]. As for the GC-PR-CPA EoS, it is first necessary to adjust PR-CPA parameters for pure water. They have been adjusted on vapour pressure and saturated liquid density data. They are recall listed in Table 1. The PR – CPA EoS is expressed here, in term of pressure: 𝑃 =𝑅𝑇𝑣 − 𝑏𝑖𝑃𝑅 −𝑎𝑖(𝑇)𝑣(𝑣 + 𝑏𝑖𝑃𝑅) + 𝑏𝑖𝑃𝑅(𝑣 − 𝑏𝑖𝑃𝑅)−12𝑅𝑇𝑣(1 + 𝜌𝜕𝑙𝑛(𝑔)𝜕𝜌) ∑ 𝑥𝑖 ∑(1 − 𝑋𝐴𝑖)𝐴𝑖𝑁𝑖=1       Group interaction parameters for hydrocarbon – water systems have been presented in the paper by Hajiw et al. [1]. The one for acid gas-water systems have been adjusted and are given in Table 2. The binary interaction parameter is given by: 𝑘𝑖𝑗(𝑇) =−12 × 𝑆𝑢𝑚𝑃𝑃𝑅78 − (√𝛼𝑖(𝑇)𝑏𝑖 –√𝛼𝑗(𝑇)𝑏𝑗)22√𝛼𝑖(𝑇)𝛼𝑗(𝑇)𝑏𝑖𝑏𝑗 With sumPPR78 defined for non-associating sysyem as 𝑆𝑢𝑚𝑃𝑃𝑅78 = ∑ ∑(𝛼𝑖𝑘 − 𝛼𝑗𝑘)(𝛼𝑖𝑙 − 𝛼𝑗𝑙)𝐴𝑘𝑙 (298.15𝑇)(𝐵𝑘𝑙𝐴𝑘𝑙−1)𝑁𝑔𝑙=1𝑁𝑔𝑘=1  Or for associating systems 𝑆𝑢𝑚𝑎𝑠𝑠𝑜 = ∑(𝛼𝑖𝑘 − 𝛼𝑗𝑘)(𝛼𝑖𝐻2𝑂 − 𝛼𝑗𝐻2𝑂)(𝐶𝑘𝐻2𝑂𝑇2 + 𝐷𝑘𝐻2𝑂𝑇 + 𝐸𝑘𝐻2𝑂)𝐻2𝑂𝑘=1+ ∑(α𝑖𝐻2𝑂 − 𝛼𝑗𝐻2𝑂)(𝛼𝑖𝑙 − 𝛼𝑗𝑙)𝐻2𝑂𝑙=1(𝐶𝐻2𝑂𝑙𝑇2 + 𝐷𝐻2𝑂𝑙𝑇 + 𝐸𝐻2𝑂𝑙) With Ck,H2O=CH2O,l, Dk,H2O=DH2O,l, Ek,H2O=EH2O,l.    a0 bar.L².mol-² b L.mol-1 C1 ε bar.L.mol-1 β 103 Temperature Range / K ΔP % Δρ  % Water 2.174 0.015 0.639 146.39 68.31 273 – 643 1.1 2.7  Table 1: PR-CPA parameters for pure water according to Hajiw et al. [1]    Comp. Ck,H2O / 103 Pa.K-2 Dk,H2O / 106 Pa.K-1 Ek,H2O / 109 Pa CO2 -6.5 5.6 -8.4 H2S 0.067 0.556 0.184  Table 2: Group interaction parameters with water (Ck,H2O, Dk,H2O and Ek,H2O)   To predict hydrate phase equilibria, both equations are coupled with the solid solution theory of van der Waals and Platteuw (1959) [3], as developed by Parrish and Prausnitz (1972)[4].       Results and Discussions  Binary systems To evaluate the accuracy of the GC–PR–CPA model for systems of interest, it is first applied to binary systems containing either acid gas or aromatics with water. Figures 2 and 3 are respectively showing carbon dioxide and hydrogen sulfide solubility in water at different temperatures. For the CO2-H2O system, both GC-PR-CPA and PPR78 models are able to represent carbon dioxide solubility over a large range of pressure with 6% and 5% deviation respectively. However, the PPR78 Eos shows larger discrepancies for the H2S-H2O system with a relative deviation of 16% against 2% for the GC-PR-CPA EoS.  Figure 2: Carbon dioxide solubility in water at 298.15 K (♦, Lucile et al.[7]), 323.15 K (■, Lucile et al. [7] and □ Hou et al. [8]), 348.15 K (, Lucile et al. [7] and ○ Hou et al. [8]) and 373.15 K (▲, Lucile et al. [7] and Δ Hou et al. [8]). Solid lines: GC-PR-CPA EoS. Dashed lines: PPR78 EoS.  Figure 3: Hydrogen sulfide solubility in water at 323.34 K (♦), 344.33 K (▲) and 377.42 K () (Yu et al. [9] ). Solid lines: GC-PR-CPA EoS. Dashed lines: PPR78 EoS. Concerning aromatic solubility in water, limited data are available in the literature, because they are sparingly soluble in water. Therefore, solubility data presented in figures 4 and 5are given at atmospheric pressure. Both benzene and toluene show a minimum of solubility at 286 K and 289 K respectively. For the benzene-water system, the GC-PR-CPA and PPR78 EoS are in good agreement with experimental data with 2% and 3% deviation respectively. However, the PPR78 Eos does not reproduce the minimum of solubility. It also displays larger discrepancies when representing toluene solubility in water (7% deviation against 1% for the GC-PR-CPA EoS) but shows the minimum of solubility. 00.010.020.030 5 10 15x CO2(mole fraction)Pressure (MPa)00.0050.010.0150.020 1 2x H2S (mole fraction)Pressure (MPa)  Figure 4: Benzene solubility in water at 0.1 MPa. Experimental data: (▲) (Alexander [10]) and (♦) (Maczynski et al.[11]). Solid lines: GC-PR-CPA EoS. Dashed lines: PPR78 EoS.  Figure 5: Toluene solubility in water at 0.1 MPa. Experimental data: (♦) (Bohon and Claussen [12]). Solid lines: GC–PR–CPA EoS prediction. Dashed lines: PPR78 EoS.  Multicomponent systems Two multicomponent systems containing acid gas have been chosen to evaluate the accuracy of the thermodynamic models for more complex systems with water. The first mixture (MIX 1) contains two light hydrocarbons plus acid gas [13]. The second one (MIX 2) is a mixture studied in the GPA Research Project 082 [5]  . Their compositions are presented in Table 3. Composition of MIX 1 is close to the one of natural gas before treatment. Both GC-PR-CPA and PPR78 EoS have been applied to calculate the composition of the vapour and liquid phases at 322 K. Deviations are given in Table 4. The phase behaviour of MIX 2 has been experimentally studied in the presence of water at 313 K and 3.6 and 5.6 bar. It exhibits a vapour-liquid-liquid equilibrium that both models are able to represent. As seen in Table 5Table 5, they give similar results for the compositions of the hydrocarbon-rich liquid phase (HC liquid) and the vapour phase. For the aqueous phase, the PPR78 EoS shows larger discrepancies in the representation of H2S solubility but is better for methane solubility. Large deviations between experimental data and predictions can be observed for the aromatics solubility in the aqueous phase because of their small values.     Component MIX 1 (Ng et al. [17]) MIX 2 Carbon dioxide 18.77 58.80 Hydrogen sulphide 6.25 18.00 Methane 71.21 1.61 Ethane - 0.187 Propane 3.77 0.093 Cyclopentane - 0.66 Benzene - 12.34 Toluene - 6.82 m-Xylene - 1.49 Table 3: Composition of acid gases  EoS Phase CH4 CO2 H2S C3H8 H2O GC-PR-CPA Liquid 3 5 20 11 0.07 Vapour 0.5 0.9 8 0.2 38 PPR78 Liquid 7 5 17 17 0.06 Vapour 0.8 0.7 11 0.4 21 Table 4: Average Absolute Deviation (AAD %) between experimental data for MIX 1 and EoSs at 322 K.  EoS Phase CO2 H2S CH4 C2H6 C3H8 GC-PR-CPA Aq Liquid 18 2 86 25 82 HC Liquid 24 46 265 117 41 Vapour 5 17 176 89 22 PPR78 Aq Liquid 16 10 62 41 86 HC Liquid 23 45 263 116 40 Vapour 5 16 176 89 23 Table 5: AAD % between experimental data for MIX 1 and EoSs at 3.6 bar and 313 K.  EoS Phase C5H10 C6H6 C7H8 m-C8H10 H2O GC-PR-CPA Aq Liquid 89 88 96 99 0.47 HC Liquid 24 18 20 29  Vapour 34 35 43 57  PPR78 Aq Liquid 94 90 97 99 0.41 HC Liquid 24 17 20 29  Vapour 33 35 43 57  Table 6: AAD % between experimental data for MIX 2 and EoSs at 3.6 bar and 313 K.  Hydrate stability in presence of free water Different mixtures with acid gases in presence or not of aromatic liquids have studied and compared to see the influence of aromatics on hydrate phase equilibria of acid gases in the GPA Research Project 082 [5].  Results for some selected mixtures are presented herein (Table 7). Experimental measurements and predictions with both models are plotted in Figure 6. The PPR78 and GC-PR-CPA models are able to predict hydrate dissociation curve for the three mixtures presented within the experimental errors. Hydrate stability zone of MIX 4 is very close of the one of pure CO2. The presence of H2S (MIX 3) promotes hydrates formation and displaces the hydrate dissociation curve to higher temperatures. Addition of aromatic compounds either promotes (no or low H2S concentration in the acid gas mixture) or inhibit the hydrate stability zone.    Component MIX 3  MIX 4 MIX 5 CO2 46.5 99.0 50.61 H2S 50.0 - 22.03 Methane 3.5 0.85 1.66 Ethane - 0.10 0.18 Propane - 0.05 0.10 Cyclopentane - - 1.79 Benzene - - 0.76 Toluene - - 8.13 m-Xylene - - 14.74 Table 7:  Compositions (mole %) of mixtures for hydrate measurements    Figure 6: Hydrate dissociation points for different acid gas mixtures. ▲ MIX 3,  MIX 4 and ■ MIX 5 [18]. Solid lines: GC-PR-CPA EoS. Dashed lines: PPR78 EoS. Dotted lines: PT envelope of MIX 5.  Conclusions  Acid gas injection is the preferential method for dealing with unwanted acid gases. However, mixtures obtained after treatment contains different impurities including aromatics. Considering the different compositions of mixtures that can be obtained, as well as the operating conditions, reliable thermodynamic models are of great importance. The predictive GC-PR-CPA model is able to predict phase behaviours (VLE, VLLE, LLE and hydrate stability zone) of multi-component systems.  From the results obtained in the GPA Research Project 082 [5], it has been seen that the presence of aromatics has small impact on the hydrate stability zone, but it can be of great importance when considering the compression step before injection. Indeed, there is likely that a liquid phase appears at conditions usually applied for gas compression. It would be interesting to extend experimental measurements to transport properties such as density, to see the impact of aromatics and to evaluate thermodynamic models on their ability to predict such data.  Acknowledgment  The authors wish to gratefully acknowledge Eric Boonaert and Alain Valtz for their expertises and technical advices.     0510152025270 275 280 285 290 295 300 305Pressure (MPa)Temperature (K)References  [1] M. Hajiw, A. Chapoy, C. Coquelet, Hydrocarbons - water phase equilibria using the CPA equation of state with a group contribution method, Can. J. Chem. Eng. 93 (2015) 432–442. doi:10.1002/cjce.22093. [2] M. Hajiw, A. Chapoy, C. Coquelet, G. Lauermann, Prediction of methanol content in natural gas with the GC-PR-CPA model, J. Nat. Gas Sci. Eng. 27 (2015) 745–750. doi:10.1016/j.jngse.2015.09.021. [3] J.H. van der Waals, J.C. Platteeuw, Clathrate solutions, Adv. Chem. Phys. 2 (1958) 1–57. doi:10.1002/9780470143483.ch1. [4] W.R. Parrish, J.M. Prausnitz, Dissociation Pressures of Gas Hydrates Formed by Gas Mixtures, Ind. Eng. Chem. Process Des. Dev. 11 (1972) 26–35. doi:10.1021/i260041a006. [5] M. Hajiw, E. Boonaert, A. Valtz, E. El Ahmar, A. Chapoy, C. Coquelet, Impact of Aromatics on Acid Gas Injection, GPA Res. Rep. 230 (2016) 1–111. [6] X. Xu, J.N. Jaubert, R. Privat, P. Duchet-Suchaux, F. Braña-Mulero, Predicting Binary-Interaction Parameters of Cubic Equations of State for Petroleum Fluids Containing Pseudo-components, Ind. Eng. Chem. Res. 54 (2015) 2816–2824. doi:10.1021/ie504920g. [7] F. Lucile, J. Serin, D. Houssin, P. Arpentinier, Solubility of Carbon Dioxide in Water and Aqueous Solution Containing Sodium Hydroxide at Temperatures from (293.15 to 393.15) K and Pressure up to 5 MPa: Experimental Measurements, J. Chem. Eng.Data. 57 (2012) 784–789. [8] S.-X. Hou, G.C. Maitland, J.P.M. Trusler, Measurement and modeling of the phase behavior of the (carbon dioxide+water) mixture at temperatures from 298.15K to 448.15K, J. Supercrit. Fluids. 73 (2013) 87–96. doi:10.1016/j.supflu.2012.11.011. [9]  et al. Yu, Q., Vapor liquid equilibria of hydrogen sulfide - water system, Chem. Eng. 4 (1980) 1–7. [10] D.M. Alexander, The Solubility of Benzene in Water, J. Phys. Chem. 63 (1959) 1021–1022. doi:10.1021/j150576a608. [11] A. Maczynski, V. Editor, IUPAC-NIST Solubility Data Series . 81 . Hydrocarbons with Water and Seawater — Revised and Updated . Part 2 . Benzene with Water and Heavy Water, 34 (2005). doi:10.1063/1.1790006. [12] R.L. Bohon, W.F. Claussen, The Solubility of Aromatic Hydrocarbons in Water, J. Am. Chem. Soc. 73 (1951) 5883–5884. [13] H.-J. Ng, C.-J. Chen, H. Schroeder, Water Content of Natural Gas Systems Containing Acid Gas, RR-174 (2001).   

Impact of Aromatic Compounds on Acid Gas Injection: Experimental Measurements and Predictions with the GC-PR-CPA Equation Of State

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Impact of Aromatic Compounds on Acid Gas Injection: Experimental Measurements and Predictions with the GC-PR-CPA Equation Of State

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Heriot Watt Pure

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