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Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas
Journal of Natural Gas Science and Engineering xxx (2015) 1e5
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas Maria T. Mota-Martinez a, b, Sabbir Samdani a, Abdallah S. Berrouk a, Marisa A.A. Rocha b, Emad Y. Alhseinat a, Fawzi Banat a, Maaike C. Kroon b, Cor J. Peters a, b, * a
The Petroleum Institute, Chemical Engineering Department, P.O. Box 2533, Abu Dhabi, United Arab Emirates Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, Separation Technology Group, Den Dolech 2, 5612 AZ Eindhoven, Netherlands b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2015 Received in revised form 2 September 2015 Accepted 3 September 2015 Available online xxx
A new static analytical apparatus for high-pressure phase equilibrium measurements has been designed and built. The new apparatus enables the measurement of vaporeliquid and liquideliquid equilibria, which can operate at temperatures ranging from 225 K to 475 K and pressures up to 20 MPa. It is constructed in Titanium and alloy C276, being suitable for highly corrosive systems of interest for the gas industry (e.g., for hydrogen sulfide containing mixtures). The apparatus is equipped with two Rapid Online Sampling Injectors (ROLSI™) enabling the withdrawing of micro-samples without disturbing the equilibrium conditions. A gas chromatograph is connected to the apparatus for direct analysis of the phases' compositions. The quality and performance of the new apparatus has been evaluated by measuring a well reported system (carbon dioxide þ methylcyclohexane). © 2015 Elsevier B.V. All rights reserved.
Keywords: Static analytical apparatus High pressure phase equilibria Natural gas mixtures Corrosive mixtures Carbon dioxide
1. Introduction Operations in oil and gas industry frequently require the use of high pressure conditions. Gas reservoirs are encountered at elevated pressures and temperatures. Gas sweetening, gas dehydration or hydrocarbon recovery are examples of high pressure processes in the gas industry. As more sour gas fields are being developed, the need of accurate experimental data involving corrosive mixtures increases. Detailed thermodynamic data are essential for pipe transport and process design. In particular, the availability of experimental data of systems with hydrogen sulfide (H2S) involved are very limited. The highly non-ideal behavior of these mixtures, particularly at moderate and high pressures, and the lack of predictive thermodynamic models (Hendriks et al., 2010), urges the development of reliable and safe experimental equipment for phase equilibrium Abbreviations: F.S., full scale; TCD, thermal conductivity detector; FID, flame ionization detector; PID, proportional-integral-differential; CO2, carbon dioxide; H2S, hydrogen sulfide; MCH, methylcyclohexane; T, temperature; P, pressure; xCO2, molar composition of CO2. * Corresponding author. The Petroleum Institute, Chemical Engineering Department, P.O. Box 2533, Abu Dhabi, United Arab Emirates. E-mail address: [email protected] (C.J. Peters).
measurements. Knowledge of the liquidegas distribution of a component is crucial for gas separation processes. In particular, the accurate description of the phase boundaries is essential to prevent the appearance of undesired phases, i.e., vaporization, (retrograde) condensation of streams during transportation, formation of hydrates, etc. Experimental set-ups for high pressure phase equilibria can be classified as analytical systems, if the composition of the phases is determined, or as synthetic systems, if only the overall composition is known. Synthetic methods are based on the determination of phase transitions. The main advantages of these methods are their simpler design and operation, and the high accuracy of the measurements. Analytical methods, on the other hand, are more advantageous for multicomponent separation systems, because they allow the determination of tie-lines (Fornari et al., 1990; Dohrn et al., 2012). Isothermal analytical methods have been reported to produce reliable data when careful procedures are carried out (Christov and Dohrn, 2002). In this work, the design and test of a new high pressure phase equilibrium apparatus is presented. The brand new analytical apparatus was designed to withstand highly corrosive mixtures at pressures up to 20 MPa and temperatures ranging from 225 K to 475 K. Preventing corrosion is a major challenge and a great
http://dx.doi.org/10.1016/j.jngse.2015.09.008 1875-5100/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
2
M.T. Mota-Martinez et al. / Journal of Natural Gas Science and Engineering xxx (2015) 1e5
concern for equipment integrity as well as operational safety. Therefore, the new apparatus has been constructed in titanium and alloy C276 (nickel-molybdenum-chromium and a small amount of tungsten), both exhibiting excellent corrosion resistance. These materials are commonly used in chemical and petrochemical industry. The main challenge of analytical methods is the way the handling of the sampling is performed (Deiters and Schneider, 1986), which should not cause any disturbance of the equilibrium conditions. Most important is also that the samples should accurately represent the composition of the phases, without noticeably changing the overall compositions. In this new apparatus, Rapid Online Sampling Injectors (ROLSI™) are used for accurate sampling of phases. These injectors have been already successfully used for sampling systems at high pressures (Guilbot et al., 2000; veneau et al., 2006; Narasigadu et al., 2013; Nandi et al., 2013). The The capability of the new apparatus to measure accurate phase equilibrium data has been tested with a binary reference system that was extensively determined at various pressures, temperatures and compositions using the Cailletet apparatus (De Loos et al., 1986). The Cailletet apparatus is recognized as one of the most accurate experimental methods for measuring high pressure phase equilibria, but unfortunately unsuitable for measurement of systems containing H2S. Therefore, we chose carbon dioxide (CO2) þ methylcyclohexane (MCH) as binary reference system (Nasrifar et al., 2003). Knowledge on the solubility of CO2 þ MCH system is crucial for understanding the inhibition of hydrate formation when insoluble organic solvents such as MCH are added to the water (H2O) þ CO2 systems (Mooijer-van den Heuvel et al., 2001). 2. Apparatus and experimental procedure 2.1. Overview The general view of the high-pressure phase equilibrium apparatus is presented in Fig. 1. This system comprises six main parts: (1), gas pressure vessels; (2), variable volume pump; (3), high-pressure equilibrium cell; (4), ROLSI™ IV sampling system; (5), gas chromatograph; and (6), vacuum system. The system is equipped with 19 valves made of alloy C276, provided by Top Industrie. The valves are highly resistance to corrosion and their operating upper limit pressure is 100 MPa.
vessels (Fig. 2). They are designed to operate at pressures up to 20 MPa, with a pressure limit of 30 MPa. Two of the vessels, with an internal volume of 191 cm3, are used to load two different gases. These vessels are connected to a third vessel with an internal volume of 111 cm3, connected to the equilibrium cell. In this vessel, the two gases can be mixed. The gas dosing system can be submerged in a temperature controlled bath (e.g. water, ethanol or oil bath, depending on the working temperature range) to allow thermal stabilization and to prevent condensation when the gas is expanded to another vessel. The pressure of the gas pressure vessels is measured by three pressure transmitters (Keller, PA35X HTT200 bar), with an accuracy of 0.2% full scale (F.S.). 2.1.2. Variable volume pump A high precision variable volume pump (Top Industrie, model PMHP100 e 500) is used to load the liquid solution into the equilibrium cell, and can operate up to 50 MPa. The exact quantity of liquid introduced into the cell is determined by the displacement of the piston with an accuracy of 0.05%. 2.1.3. The high-pressure equilibrium cell The equilibrium cell, constructed in titanium and with an internal volume of 50 cm3, can operate up to 20 MPa and in the temperature range between 225 K and 475 K. The titanium highpressure equilibrium cell is presented in Fig. 3. The cell consists of two separate segments screwed together. This construction enables the maintenance and cleaning of the interior of the cell. Two sapphire windows on the side of the cell allow the visual observation of the phases inside. The equilibrium cell is connected with the mixing vessel, with a gas inlet/drain connection and with the variable volume pump and the vacuum pump. A platinum stirrer, operated by an external magnetic rotor, is used to agitate the sample inside the cell, enhancing the mixing of the phases and reducing the time to attain equilibrium pressure. The capillary tubes from two ROLSI™ IV samplers are inserted from
2.1.1. Gas pressure vessels The gas dosing system is composed of three titanium pressure
Fig. 1. General view of the Titanium high-pressure phase equilibrium apparatus. 1), gas pressure vessels; (2), variable volume pump; (3), high-pressure equilibrium cell; (4), ROLSI™ IV sampling system; (5), gas chromatograph; and (6), vacuum system.
Fig. 2. Two 191 cm3 and one 111 cm3 pressure vessels of the gas dosing system and their connections above a temperature controlled bath.
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
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3
equilibrium conditions in the cell. The sampler chamber and the lines connecting the equilibrium cell and the gas chromatograph are heated by an electric resistance system. 2.1.5. Gas chromatograph The composition of the phases is analyzed using gas chromatography (Agilent, model 7890 A). The chromatograph is equipped with two inlets, one used for manual injection of samples (e.g. calibration) and the other is connected to the ROLSI™ IV valves. The gas chromatograph is equipped with a PORAPAK R 80/100 packed column (polydivinylbenzene copolymers) with a stainless steel shell. Helium or nitrogen can be used as a carrier gas. The gas chromatograph accommodates two detectors: a thermal conductivity detector (TCD) and a flame ionization detector (FID). 2.1.6. Vacuum pump The vacuum pump (Leybold, model TRIVAC D2,5 E) is used to evacuate the system before the start of a new experiment and after the cleaning process. Fig. 3. View of the titanium high-pressure equilibrium cell, including one of the sapphire windows and the connections.
the top of the cell at two different levels, allowing sampling from the lower and upper phases of the system. The cell is installed inside a temperature-controlled chamber (Benchtop, model TMX 55), where the temperature is kept constant by a PID (proportional, integral and differential) controller in the range between 223 K and 473 K. The pressure inside the cell is measured with two pressure transmitters, one for low pressure (Keller, PA35X HTC e 5 bar) and another for high pressures (Keller, PA35X HTC e 200 bar), with an accuracy of 0.2% F.S.. The temperature at the top and at the bottom of the cell is recorded by two Pt-100 sensors with an uncertainty of ±0.1 K. 2.1.4. ROLSI™ IV sampling system The electromagnetically-controlled ROLSI™ IV sampler injector is an advanced version of the sampling system based on the U.S. Patent number 4,688,436 (Richon and Laugier, 1987). It consists of a capillary of 0.1 mm internal diameter (see Fig. 4), connecting the equilibrium cell to a micro-chamber where the sample is swept by the carrier gas of the chromatographic circuit. The ROLSI™ sampler is capable of withdrawing micro-samples without disturbing the
2.2. Experimental procedure The first step of the experimental procedure is to set all the apparatus components and connections, including the equilibrium cell, the pressure and mixing vessels and tubing connections, under vacuum conditions for at least 1 h to degas and to eliminate any trace of any volatile chemical. A well-known composition of liquid sample is prepared and thoroughly degassed prior to its injection into the cell using the variable volume pump. Although analytical methods do not require exact determination of the total amount injected, sometimes it is necessary for mass balances purposes. The variable volume pump allows a highly precise determination of the volume injected by means of an accurate control of the piston displacement. Two different gases can be dosed into the system through the two larger pressure vessels and be mixed in the smaller mixing vessel (Fig. 2). From this vessel, the gas is injected into the equilibrium cell, where the liquid has been previously added, until the required overall composition is reached. If needed, the exact number of moles transferred into the cell can be determined using the ideal gas equation for low pressures, or by the virial equation of state (truncated after the second term) for higher pressures. The volume of the vessels and the equilibrium cell is accurately known, and the pressure and temperature are
Fig. 4. Liquid and gas sampling ROLSI™ IV. Left, the liquid and gas sampling valves assembled on the top of the cell chamber. Right, capillary taken apart from the valve body.
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
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M.T. Mota-Martinez et al. / Journal of Natural Gas Science and Engineering xxx (2015) 1e5
recorded before and after adding the gas in the cell. When higher absolute pressures are required, an inert gas, e.g. nitrogen, can be added to the system following the same procedure. Before loading another gas, the gas-dosing system should be brought under vacuum conditions for at least half an hour so that any traces of the previous loaded gases are eliminated. Determining the amount of moles of the inert gas in not necessary. 2.3. Equilibrium attainment and composition analysis The equilibrium cell is a fixed-volume cell. Consequently, the pressure cannot be controlled independently from the other variables, so temperature and overall composition are the only adjustable variables. Therefore, the most convenient experimental procedure is to determine isothermal lines, starting at low pressures, and increasing the pressure step by step by adding more gas, if necessary. The temperature of the equilibrium cell is kept constant by a PID controller of the chamber. Based on the Gibbs phase rule, once the gas and the liquid have been loaded into the cell (fixed composition), the temperature has been set and there is equilibrium in the constant volume cell. Then, all variables are fixed, i.e. the system is fully defined. In this way, the pressure equilibrates as the gas is being absorbed in the liquid phase (equalizing the chemical potential). A magnetically powered stirrer inside the cell enhances the mixing of the two phases. Eventually, the pressure inside the cell will remain steady, indicating that equilibrium has been attained. The stirring is stopped to let the phases separate. Visual observation through the sapphire window allows determining the time when the two phases are fully split and no bubbles are trapped in the liquid phase. At that moment, samples are withdrawn from the vapor and the liquid phases using the ROLSI™ IV samplers and the composition of the phases is determined. The samples are guided to the gas chromatograph for analysis. The connecting lines between the ROLSI™ IV samplers and the oven of the gas chromatograph should be at such a temperature that the liquid sample vaporizes and to prevent condensation of the vapor phase sample. At least three samples should be taken to flush the lines until two consistent and repeatable peaks are shown in the gas chromatograph. Once the line is purged, the samples are analyzed until three measurements in agreement are obtained. 3. Results and discussion The quality and performance of the new pressure phase equilibrium apparatus were evaluated by measuring the CO2 þ MCH system. The solubility of CO2 in MCH is well reported in the literature (Nasrifar et al., 2003) in a wide range of temperatures and pressures up to 11.5 MPa using the Cailletet apparatus. The experimental data obtained in our work were compared with the Cailletet data. Four isotherms, ranging from 290 to 350 K and pressures up to 4.4 MPa were determined using the new apparatus described in
this work and following the procedure described in the previous section. The composition of CO2 (xCO2) in the liquid phase was analyzed with the TCD. The experimental results are presented in Table 1. The composition of the vapor phase was not determine because (i) the vapor pressure of MCH is very low, and therefore the gas phase was virtually pure CO2 and (ii) only bubble point data, i.e. liquid composition, is reported in the literature (Nasrifar et al., 2003). The associated uncertainty of the CO2 composition was calculated based on the standard deviation of the samples analyzed for each equilibrium point. The four isotherms are graphically compared to the Cailletet data (Nasrifar et al., 2003) in Fig. 5. The compositional uncertainty reported for the literature data (±0.002 (De Loos et al., 1986)) is smaller than the size of the symbol in Fig. 5, so these error bars are not represented. Considering the uncertainty associated, the obtained results by the new apparatus are in good agreement with the literature data. The main source of uncertainty of this apparatus is the analytical procedure. The calibration of the gas chromatograph is one of the most challenging steps of the analytical procedure and a major source of uncertainty. The CO2 used to calibrate the gas chromatograph was withdrawn from the gas cylinder through a septum using a gas-tight micro syringe. Although the calibration was performed in a stable and controlled environment, experimental errors can arise mainly for low volumes of CO2 gas and the human error. Another reason for the uncertainty of the measurements is that a TCD was used to analyze the CO2 composition in the liquid. Despite of being an universal non-destructive detector, TCD presents a lower sensitivity compared to other detectors. Some disadvantages of TCDs are that it detects impurities present in the carrier gas and that it is highly sensitive to variations in the flow rate. Analytical methods present a trade-off between accuracy and applicability. As shown in Fig. 5, the Cailletet apparatus produces highly precise phase equilibrium data, while the results obtained with the new apparatus exhibits a higher uncertainty. However, the Cailletet apparatus is mainly suitable for pure and binary systems, where only phase boundaries can be determined. The new apparatus also allows analytical determination of multicomponent systems and tie lines, which contributes for a higher versatility. Additional advantages of this new apparatus are: (i) precise injection of liquid sample in the equilibrium cell, (ii) isothermal operation in temperature-controlled chamber and prevention of heat losses, (iii) fast, easy and flexible operation, (iv) semi-automatic composition analysis by gas chromatography, and (v) mercury-free apparatus preventing any mercury leaks into the environment. The major objective of designing and building the new facility, as described in this paper, is the need for experimental phase equilibrium data on highly corrosive systems (high concentrations of H2S and/or CO2 and the presence of other corrosive contaminants) by the gas and oil industry. The new facility is able to handle these corrosive systems, while the mercury containing Cailletet facility cannot handle systems with sulfur-containing components.
Table 1 Experimental isotherms of the binary system CO2 þ MCH measured with the new apparatus. T/K
290
310
P/MPa
xCO2
P/MPa
xCO2
1.01 1.98 3.18 4.19
0.131 0.199 0.430 0.655
1.31 2.40 3.45 4.84
0.096 0.160 0.245 0.407
± ± ± ±
0.008 0.039 0.047 0.037
330
± ± ± ±
0.027 0.018 0.003 0.034
350
P/MPa
xCO2
0.92 2.14 3.24 4.14 5.09
0.047 0.140 0.232 0.298 0.402
± ± ± ± ±
0.016 0.026 0.046 0.002 0.002
P/MPa
xCO2
1.05 2.06 3.32 4.36 5.26
0.066 0.126 0.194 0.247 0.304
± ± ± ± ±
0.014 0.013 0.011 0.018 0.018
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
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5
Acknowledgments The authors would like to acknowledge the ADNOC Gas Subcommittee for supporting the project “Improved Performance of Gas-Sweetening Processes” (Grant number 6000.1681). ARMINES and the CTP of MINES ParisTech are acknowledged for their guidance in designing the apparatus and the training of the operators. Maria T. Mota-Martinez is grateful to the Petroleum Institute for the Visiting Graduate Researcher Scholarship.
References
Fig. 5. Comparison of experimental isotherms of the CO2 þ MCH system measured with the titanium cell (solid symbols) and literature data (Nasrifar et al., 2003) (open symbols) at: , 290 K; , 310 K; , 330 K; and , 350 K.
Another application of this new apparatus is the determination of the solubility of medium chain hydrocarbons in aqueous mixtures of alkanolamines and the effect of additives on their solubility (Mota-Martinez et al., 2014), which is of main interest for the local gas industry in the UAE.
4. Conclusions A new static analytic high pressure phase equilibria apparatus has been designed and tested to withstand highly corrosive systems. The apparatus is made of titanium and alloy C276 because of their excellent corrosion resistance. Furthermore, the possibility of analytical determination of diverse multi-component systems makes it possible to apply the new apparatus for phase behavior measurements of a wide range of systems of interest for the oil and gas industry.
Christov, M., Dohrn, R., 2002. High-pressure fluid phase equilibria: experimental methods and systems investigated (1994e1999). Fluid Phase Equilib. 202, 153e218. De Loos, T.W., Van der Kooi, H.J., Ott, P.L., 1986. Vapor-liquid critical curve of the system ethane þ 2-methylpropane. J. Chem. Eng. Data 31, 166e168. Deiters, U.K., Schneider, G.M., 1986. High pressure phase equilibria: experimental methods. Fluid Phase Equilib. 29, 145e160. Dohrn, R., Fonseca, J.M.S., Peper, S., 2012. Experimental methods for phase equilibria at high pressures. Annu. Rev. Chem. Biomol. Eng. 3, 343e367. Fornari, R.E., Alessi, P., Kikic, I., 1990. High pressure fluid phase equilibria: experimental methods and systems investigated (1978e1987). Fluid Phase Equilib. 57, 1e33. Guilbot, P., Valtz, A., Legendre, H., Richon, D., 2000. Rapid on-line sampler-injector: a reliable tool for HT-HP sampling and on-line GC analysis. Analusis 28, 426e431. Hendriks, E., Kontogeorgis, G.M., Dohrn, R., de Hemptinne, J.-C., Economou, I.G., Zilnik, L.F., Vesovic, V., 2010. Industrial requirements for thermodynamics and transport properties. Ind. Eng. Chem. Res. 49, 11131e11141. Mooijer-van den Heuvel, M.M., Witteman, R., Peters, C.J., 2001. Phase behaviour of gas hydrates of carbon dioxide in the presence of tetrahydropyran, cyclobutanone, cyclohexane and methylcyclohexane. Fluid Phase Equilibr 182, 97e110. Mota-Martinez, M.T., Samdani, S., Berrouk, A.S., Kroon, M.C., Peters, C.J., 2014. Effect of additives on the CO2 absorption in aqueous MDEA solutions. Ind. Eng. Chem. Res. 53, 20032e20035. Nandi, P., Moodley, S., Ramjugernath, D., 2013. Isothermal vaporeliquid equilibrium of R170 þ n-perfluorooctane at 308e338 K: measurement, equation of state modelling, and molecular simulation. Fluid Phase Equilib. 344, 84e91. Narasigadu, C., Naidoo, P., Coquelet, C., Richon, D., Ramjugernath, D., 2013. A novel static analytical apparatus for phase equilibrium measurements. Fluid Phase Equilib. 338, 188e196. Nasrifar, K., Mooijer-van den Heuvel, M.M., Peters, C.J., Ayatollahi, S., Moshfeghian, M., 2003. Measurements and modeling of bubble points in binary carbon dioxide systems with tetrahydropyran and methylcyclohexane. Fluid Phase Equilib. 204, 1e14. Richon, D., Laugier, S., 1987. In: Automatic Pressurized Fluid Micro-sampling and Injection Device. Google Patents. veneau, P., Coquelet, C., Richon, D., 2006. Vapoureliquid equilibrium data for the The hydrogen sulphide þ n-heptane system at temperatures from 293.25 to 373.22 K and pressures up to about 6.9 MPa. Fluid Phase Equilib. 249, 179e186.
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas Maria T. Mota-Martinez a, b, Sabbir Samdani a, Abdallah S. Berrouk a, Marisa A.A. Rocha b, Emad Y. Alhseinat a, Fawzi Banat a, Maaike C. Kroon b, Cor J. Peters a, b, * a
The Petroleum Institute, Chemical Engineering Department, P.O. Box 2533, Abu Dhabi, United Arab Emirates Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, Separation Technology Group, Den Dolech 2, 5612 AZ Eindhoven, Netherlands b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2015 Received in revised form 2 September 2015 Accepted 3 September 2015 Available online xxx
A new static analytical apparatus for high-pressure phase equilibrium measurements has been designed and built. The new apparatus enables the measurement of vaporeliquid and liquideliquid equilibria, which can operate at temperatures ranging from 225 K to 475 K and pressures up to 20 MPa. It is constructed in Titanium and alloy C276, being suitable for highly corrosive systems of interest for the gas industry (e.g., for hydrogen sulfide containing mixtures). The apparatus is equipped with two Rapid Online Sampling Injectors (ROLSI™) enabling the withdrawing of micro-samples without disturbing the equilibrium conditions. A gas chromatograph is connected to the apparatus for direct analysis of the phases' compositions. The quality and performance of the new apparatus has been evaluated by measuring a well reported system (carbon dioxide þ methylcyclohexane). © 2015 Elsevier B.V. All rights reserved.
Keywords: Static analytical apparatus High pressure phase equilibria Natural gas mixtures Corrosive mixtures Carbon dioxide
1. Introduction Operations in oil and gas industry frequently require the use of high pressure conditions. Gas reservoirs are encountered at elevated pressures and temperatures. Gas sweetening, gas dehydration or hydrocarbon recovery are examples of high pressure processes in the gas industry. As more sour gas fields are being developed, the need of accurate experimental data involving corrosive mixtures increases. Detailed thermodynamic data are essential for pipe transport and process design. In particular, the availability of experimental data of systems with hydrogen sulfide (H2S) involved are very limited. The highly non-ideal behavior of these mixtures, particularly at moderate and high pressures, and the lack of predictive thermodynamic models (Hendriks et al., 2010), urges the development of reliable and safe experimental equipment for phase equilibrium Abbreviations: F.S., full scale; TCD, thermal conductivity detector; FID, flame ionization detector; PID, proportional-integral-differential; CO2, carbon dioxide; H2S, hydrogen sulfide; MCH, methylcyclohexane; T, temperature; P, pressure; xCO2, molar composition of CO2. * Corresponding author. The Petroleum Institute, Chemical Engineering Department, P.O. Box 2533, Abu Dhabi, United Arab Emirates. E-mail address: [email protected] (C.J. Peters).
measurements. Knowledge of the liquidegas distribution of a component is crucial for gas separation processes. In particular, the accurate description of the phase boundaries is essential to prevent the appearance of undesired phases, i.e., vaporization, (retrograde) condensation of streams during transportation, formation of hydrates, etc. Experimental set-ups for high pressure phase equilibria can be classified as analytical systems, if the composition of the phases is determined, or as synthetic systems, if only the overall composition is known. Synthetic methods are based on the determination of phase transitions. The main advantages of these methods are their simpler design and operation, and the high accuracy of the measurements. Analytical methods, on the other hand, are more advantageous for multicomponent separation systems, because they allow the determination of tie-lines (Fornari et al., 1990; Dohrn et al., 2012). Isothermal analytical methods have been reported to produce reliable data when careful procedures are carried out (Christov and Dohrn, 2002). In this work, the design and test of a new high pressure phase equilibrium apparatus is presented. The brand new analytical apparatus was designed to withstand highly corrosive mixtures at pressures up to 20 MPa and temperatures ranging from 225 K to 475 K. Preventing corrosion is a major challenge and a great
http://dx.doi.org/10.1016/j.jngse.2015.09.008 1875-5100/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
2
M.T. Mota-Martinez et al. / Journal of Natural Gas Science and Engineering xxx (2015) 1e5
concern for equipment integrity as well as operational safety. Therefore, the new apparatus has been constructed in titanium and alloy C276 (nickel-molybdenum-chromium and a small amount of tungsten), both exhibiting excellent corrosion resistance. These materials are commonly used in chemical and petrochemical industry. The main challenge of analytical methods is the way the handling of the sampling is performed (Deiters and Schneider, 1986), which should not cause any disturbance of the equilibrium conditions. Most important is also that the samples should accurately represent the composition of the phases, without noticeably changing the overall compositions. In this new apparatus, Rapid Online Sampling Injectors (ROLSI™) are used for accurate sampling of phases. These injectors have been already successfully used for sampling systems at high pressures (Guilbot et al., 2000; veneau et al., 2006; Narasigadu et al., 2013; Nandi et al., 2013). The The capability of the new apparatus to measure accurate phase equilibrium data has been tested with a binary reference system that was extensively determined at various pressures, temperatures and compositions using the Cailletet apparatus (De Loos et al., 1986). The Cailletet apparatus is recognized as one of the most accurate experimental methods for measuring high pressure phase equilibria, but unfortunately unsuitable for measurement of systems containing H2S. Therefore, we chose carbon dioxide (CO2) þ methylcyclohexane (MCH) as binary reference system (Nasrifar et al., 2003). Knowledge on the solubility of CO2 þ MCH system is crucial for understanding the inhibition of hydrate formation when insoluble organic solvents such as MCH are added to the water (H2O) þ CO2 systems (Mooijer-van den Heuvel et al., 2001). 2. Apparatus and experimental procedure 2.1. Overview The general view of the high-pressure phase equilibrium apparatus is presented in Fig. 1. This system comprises six main parts: (1), gas pressure vessels; (2), variable volume pump; (3), high-pressure equilibrium cell; (4), ROLSI™ IV sampling system; (5), gas chromatograph; and (6), vacuum system. The system is equipped with 19 valves made of alloy C276, provided by Top Industrie. The valves are highly resistance to corrosion and their operating upper limit pressure is 100 MPa.
vessels (Fig. 2). They are designed to operate at pressures up to 20 MPa, with a pressure limit of 30 MPa. Two of the vessels, with an internal volume of 191 cm3, are used to load two different gases. These vessels are connected to a third vessel with an internal volume of 111 cm3, connected to the equilibrium cell. In this vessel, the two gases can be mixed. The gas dosing system can be submerged in a temperature controlled bath (e.g. water, ethanol or oil bath, depending on the working temperature range) to allow thermal stabilization and to prevent condensation when the gas is expanded to another vessel. The pressure of the gas pressure vessels is measured by three pressure transmitters (Keller, PA35X HTT200 bar), with an accuracy of 0.2% full scale (F.S.). 2.1.2. Variable volume pump A high precision variable volume pump (Top Industrie, model PMHP100 e 500) is used to load the liquid solution into the equilibrium cell, and can operate up to 50 MPa. The exact quantity of liquid introduced into the cell is determined by the displacement of the piston with an accuracy of 0.05%. 2.1.3. The high-pressure equilibrium cell The equilibrium cell, constructed in titanium and with an internal volume of 50 cm3, can operate up to 20 MPa and in the temperature range between 225 K and 475 K. The titanium highpressure equilibrium cell is presented in Fig. 3. The cell consists of two separate segments screwed together. This construction enables the maintenance and cleaning of the interior of the cell. Two sapphire windows on the side of the cell allow the visual observation of the phases inside. The equilibrium cell is connected with the mixing vessel, with a gas inlet/drain connection and with the variable volume pump and the vacuum pump. A platinum stirrer, operated by an external magnetic rotor, is used to agitate the sample inside the cell, enhancing the mixing of the phases and reducing the time to attain equilibrium pressure. The capillary tubes from two ROLSI™ IV samplers are inserted from
2.1.1. Gas pressure vessels The gas dosing system is composed of three titanium pressure
Fig. 1. General view of the Titanium high-pressure phase equilibrium apparatus. 1), gas pressure vessels; (2), variable volume pump; (3), high-pressure equilibrium cell; (4), ROLSI™ IV sampling system; (5), gas chromatograph; and (6), vacuum system.
Fig. 2. Two 191 cm3 and one 111 cm3 pressure vessels of the gas dosing system and their connections above a temperature controlled bath.
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
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3
equilibrium conditions in the cell. The sampler chamber and the lines connecting the equilibrium cell and the gas chromatograph are heated by an electric resistance system. 2.1.5. Gas chromatograph The composition of the phases is analyzed using gas chromatography (Agilent, model 7890 A). The chromatograph is equipped with two inlets, one used for manual injection of samples (e.g. calibration) and the other is connected to the ROLSI™ IV valves. The gas chromatograph is equipped with a PORAPAK R 80/100 packed column (polydivinylbenzene copolymers) with a stainless steel shell. Helium or nitrogen can be used as a carrier gas. The gas chromatograph accommodates two detectors: a thermal conductivity detector (TCD) and a flame ionization detector (FID). 2.1.6. Vacuum pump The vacuum pump (Leybold, model TRIVAC D2,5 E) is used to evacuate the system before the start of a new experiment and after the cleaning process. Fig. 3. View of the titanium high-pressure equilibrium cell, including one of the sapphire windows and the connections.
the top of the cell at two different levels, allowing sampling from the lower and upper phases of the system. The cell is installed inside a temperature-controlled chamber (Benchtop, model TMX 55), where the temperature is kept constant by a PID (proportional, integral and differential) controller in the range between 223 K and 473 K. The pressure inside the cell is measured with two pressure transmitters, one for low pressure (Keller, PA35X HTC e 5 bar) and another for high pressures (Keller, PA35X HTC e 200 bar), with an accuracy of 0.2% F.S.. The temperature at the top and at the bottom of the cell is recorded by two Pt-100 sensors with an uncertainty of ±0.1 K. 2.1.4. ROLSI™ IV sampling system The electromagnetically-controlled ROLSI™ IV sampler injector is an advanced version of the sampling system based on the U.S. Patent number 4,688,436 (Richon and Laugier, 1987). It consists of a capillary of 0.1 mm internal diameter (see Fig. 4), connecting the equilibrium cell to a micro-chamber where the sample is swept by the carrier gas of the chromatographic circuit. The ROLSI™ sampler is capable of withdrawing micro-samples without disturbing the
2.2. Experimental procedure The first step of the experimental procedure is to set all the apparatus components and connections, including the equilibrium cell, the pressure and mixing vessels and tubing connections, under vacuum conditions for at least 1 h to degas and to eliminate any trace of any volatile chemical. A well-known composition of liquid sample is prepared and thoroughly degassed prior to its injection into the cell using the variable volume pump. Although analytical methods do not require exact determination of the total amount injected, sometimes it is necessary for mass balances purposes. The variable volume pump allows a highly precise determination of the volume injected by means of an accurate control of the piston displacement. Two different gases can be dosed into the system through the two larger pressure vessels and be mixed in the smaller mixing vessel (Fig. 2). From this vessel, the gas is injected into the equilibrium cell, where the liquid has been previously added, until the required overall composition is reached. If needed, the exact number of moles transferred into the cell can be determined using the ideal gas equation for low pressures, or by the virial equation of state (truncated after the second term) for higher pressures. The volume of the vessels and the equilibrium cell is accurately known, and the pressure and temperature are
Fig. 4. Liquid and gas sampling ROLSI™ IV. Left, the liquid and gas sampling valves assembled on the top of the cell chamber. Right, capillary taken apart from the valve body.
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
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recorded before and after adding the gas in the cell. When higher absolute pressures are required, an inert gas, e.g. nitrogen, can be added to the system following the same procedure. Before loading another gas, the gas-dosing system should be brought under vacuum conditions for at least half an hour so that any traces of the previous loaded gases are eliminated. Determining the amount of moles of the inert gas in not necessary. 2.3. Equilibrium attainment and composition analysis The equilibrium cell is a fixed-volume cell. Consequently, the pressure cannot be controlled independently from the other variables, so temperature and overall composition are the only adjustable variables. Therefore, the most convenient experimental procedure is to determine isothermal lines, starting at low pressures, and increasing the pressure step by step by adding more gas, if necessary. The temperature of the equilibrium cell is kept constant by a PID controller of the chamber. Based on the Gibbs phase rule, once the gas and the liquid have been loaded into the cell (fixed composition), the temperature has been set and there is equilibrium in the constant volume cell. Then, all variables are fixed, i.e. the system is fully defined. In this way, the pressure equilibrates as the gas is being absorbed in the liquid phase (equalizing the chemical potential). A magnetically powered stirrer inside the cell enhances the mixing of the two phases. Eventually, the pressure inside the cell will remain steady, indicating that equilibrium has been attained. The stirring is stopped to let the phases separate. Visual observation through the sapphire window allows determining the time when the two phases are fully split and no bubbles are trapped in the liquid phase. At that moment, samples are withdrawn from the vapor and the liquid phases using the ROLSI™ IV samplers and the composition of the phases is determined. The samples are guided to the gas chromatograph for analysis. The connecting lines between the ROLSI™ IV samplers and the oven of the gas chromatograph should be at such a temperature that the liquid sample vaporizes and to prevent condensation of the vapor phase sample. At least three samples should be taken to flush the lines until two consistent and repeatable peaks are shown in the gas chromatograph. Once the line is purged, the samples are analyzed until three measurements in agreement are obtained. 3. Results and discussion The quality and performance of the new pressure phase equilibrium apparatus were evaluated by measuring the CO2 þ MCH system. The solubility of CO2 in MCH is well reported in the literature (Nasrifar et al., 2003) in a wide range of temperatures and pressures up to 11.5 MPa using the Cailletet apparatus. The experimental data obtained in our work were compared with the Cailletet data. Four isotherms, ranging from 290 to 350 K and pressures up to 4.4 MPa were determined using the new apparatus described in
this work and following the procedure described in the previous section. The composition of CO2 (xCO2) in the liquid phase was analyzed with the TCD. The experimental results are presented in Table 1. The composition of the vapor phase was not determine because (i) the vapor pressure of MCH is very low, and therefore the gas phase was virtually pure CO2 and (ii) only bubble point data, i.e. liquid composition, is reported in the literature (Nasrifar et al., 2003). The associated uncertainty of the CO2 composition was calculated based on the standard deviation of the samples analyzed for each equilibrium point. The four isotherms are graphically compared to the Cailletet data (Nasrifar et al., 2003) in Fig. 5. The compositional uncertainty reported for the literature data (±0.002 (De Loos et al., 1986)) is smaller than the size of the symbol in Fig. 5, so these error bars are not represented. Considering the uncertainty associated, the obtained results by the new apparatus are in good agreement with the literature data. The main source of uncertainty of this apparatus is the analytical procedure. The calibration of the gas chromatograph is one of the most challenging steps of the analytical procedure and a major source of uncertainty. The CO2 used to calibrate the gas chromatograph was withdrawn from the gas cylinder through a septum using a gas-tight micro syringe. Although the calibration was performed in a stable and controlled environment, experimental errors can arise mainly for low volumes of CO2 gas and the human error. Another reason for the uncertainty of the measurements is that a TCD was used to analyze the CO2 composition in the liquid. Despite of being an universal non-destructive detector, TCD presents a lower sensitivity compared to other detectors. Some disadvantages of TCDs are that it detects impurities present in the carrier gas and that it is highly sensitive to variations in the flow rate. Analytical methods present a trade-off between accuracy and applicability. As shown in Fig. 5, the Cailletet apparatus produces highly precise phase equilibrium data, while the results obtained with the new apparatus exhibits a higher uncertainty. However, the Cailletet apparatus is mainly suitable for pure and binary systems, where only phase boundaries can be determined. The new apparatus also allows analytical determination of multicomponent systems and tie lines, which contributes for a higher versatility. Additional advantages of this new apparatus are: (i) precise injection of liquid sample in the equilibrium cell, (ii) isothermal operation in temperature-controlled chamber and prevention of heat losses, (iii) fast, easy and flexible operation, (iv) semi-automatic composition analysis by gas chromatography, and (v) mercury-free apparatus preventing any mercury leaks into the environment. The major objective of designing and building the new facility, as described in this paper, is the need for experimental phase equilibrium data on highly corrosive systems (high concentrations of H2S and/or CO2 and the presence of other corrosive contaminants) by the gas and oil industry. The new facility is able to handle these corrosive systems, while the mercury containing Cailletet facility cannot handle systems with sulfur-containing components.
Table 1 Experimental isotherms of the binary system CO2 þ MCH measured with the new apparatus. T/K
290
310
P/MPa
xCO2
P/MPa
xCO2
1.01 1.98 3.18 4.19
0.131 0.199 0.430 0.655
1.31 2.40 3.45 4.84
0.096 0.160 0.245 0.407
± ± ± ±
0.008 0.039 0.047 0.037
330
± ± ± ±
0.027 0.018 0.003 0.034
350
P/MPa
xCO2
0.92 2.14 3.24 4.14 5.09
0.047 0.140 0.232 0.298 0.402
± ± ± ± ±
0.016 0.026 0.046 0.002 0.002
P/MPa
xCO2
1.05 2.06 3.32 4.36 5.26
0.066 0.126 0.194 0.247 0.304
± ± ± ± ±
0.014 0.013 0.011 0.018 0.018
Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008
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Acknowledgments The authors would like to acknowledge the ADNOC Gas Subcommittee for supporting the project “Improved Performance of Gas-Sweetening Processes” (Grant number 6000.1681). ARMINES and the CTP of MINES ParisTech are acknowledged for their guidance in designing the apparatus and the training of the operators. Maria T. Mota-Martinez is grateful to the Petroleum Institute for the Visiting Graduate Researcher Scholarship.
References
Fig. 5. Comparison of experimental isotherms of the CO2 þ MCH system measured with the titanium cell (solid symbols) and literature data (Nasrifar et al., 2003) (open symbols) at: , 290 K; , 310 K; , 330 K; and , 350 K.
Another application of this new apparatus is the determination of the solubility of medium chain hydrocarbons in aqueous mixtures of alkanolamines and the effect of additives on their solubility (Mota-Martinez et al., 2014), which is of main interest for the local gas industry in the UAE.
4. Conclusions A new static analytic high pressure phase equilibria apparatus has been designed and tested to withstand highly corrosive systems. The apparatus is made of titanium and alloy C276 because of their excellent corrosion resistance. Furthermore, the possibility of analytical determination of diverse multi-component systems makes it possible to apply the new apparatus for phase behavior measurements of a wide range of systems of interest for the oil and gas industry.
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Please cite this article in press as: Mota-Martinez, M.T., et al., Design and test of a new high pressure phase equilibrium apparatus for highly corrosive mixtures of importance for natural gas, Journal of Natural Gas Science and Engineering (2015), http://dx.doi.org/10.1016/ j.jngse.2015.09.008