Experimental Study and Modeling of Methane Hydrate Formation Induction Time in the Presence of Ionic Liquids

Transcription

1 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University Experimental Study and Modeling of Methane Hydrate Formation Induction Time in the Presence of Ionic Liquids Ali Rasoolzadeh, Jafar Javanmardi * Department of Chemical Engineering, Shiraz University of Technology, , Shiraz, Iran * Corresponding author. address: Javanmardi@sutech.ac.ir Abstract Gas hydrate formation has been referred as unfavorable phenomenon since it leads to blockage of pipelines. To prevent formation of these compounds, several methods are normally pursued including system heating, depressurization, water removal, and use of gas hydrate formation inhibitors. The latter technique may be the most practical method for this purpose. Two types of inhibitors are generally used in the industry: thermodynamic inhibitors and kinetic ones. Thermodynamic inhibitors (such as ethylene glycol and methanol) shift the hydrate-liquid-vapor- (HLV) equilibrium curve to lower temperature and higher-pressure conditions. Kinetic inhibitors (such as PVP, PVCap) delay the hydrate nucleation and growth rates. There are some evidences that ionic liquids have dual inhibition effects. In this communication, we use three ionic liquids including (BMIM-BF 4), (BMIM-DCA), and (TEACL). Methane hydrate formation induction time in the presence of different concentrations of these three ionic liquids is kinetically investigated in this work. Consequently, the effects of initial pressure and ionic liquids concentration on the induction time can be evaluated. In addition, a three parameter semi-empirical model is developed on the basis of chemical kinetics theory. Finally, it is shown that the proposed semi-empirical model has a good accuracy in comparison with the experimental data. Keywords: Methane hydrate, Kinetics, Ionic liquids, Induction time, Inhibitors Research Highlights Methane hydrates induction time measurement in the presence of ionic liquids. Proposing a three parameter semi-empirical model on the basis of chemical kinetics theory for representing experimental data. Optimization of model parameters by some experimental data and prediction of all results by the model. 1. Introduction Gas hydrates are belonging to the clathrates families, which are composed of water molecules and some guest molecules like methane, ethane, propane, etc. Under appropriate conditions which are high pressures and low temperatures conditions, some guest molecules which are gases with appropriate sizes and shapes are trapped in the cavities which are formed by water molecules that are connecting together by hydrogen bonding and gas hydrates are formed 1

2 Experimental Study and Modeling of Methane Hydrate [1,2]. From the discovery of hydrates in 1810 until present, gas hydrates are one of the interesting fields for scientists to study. Gas hydrates have many advantages and some disadvantages. The major problem in hydrate formation is leading to gas pipelines. In 1934, Hammerschmidt expressed that in high pressures and low temperatures in pipelines gas hydrates are formed that cause pipeline blockage, safety problems, pressure losses and huge economic losses [3]. Since 1934, hydrates were known as harmful phenomena and for avoiding hydrate formations in pipelines several methods such as: system heating, depressurization, water removal and use of inhibitors were developed. Adding inhibitors to the systems is the most possible and flexible way to prevent hydrate formation [4,5]. Two types of inhibitors are used today: thermodynamic inhibitors and kinetic inhibitors. Thermodynamic inhibitors (such as ethylene glycol, methanol) shift the HLV equilibrium curve to lower temperature and higher pressure and make the hydrate formation region smaller. Kinetic inhibitors (such as PVP, PVCap) delay the hydrate nucleation and hydrate growth rates by increasing the induction time but they do not shift the HLV equilibrium curve. Thermodynamic inhibitors are used in concentrations more than 10 wt% but kinetic inhibitors are used in concentrations about 1 wt% [6-9]. Xiao (2009, 2010) showed that ionic liquids, which he was used in his work, have dualfunction inhibition effects. It means that those ionic liquids not only shift the HLV equilibrium curve to lower temperature and higher pressure but also delay the hydrate nucleation and hydrate growth rates by increasing the induction time. Ionic liquids are green solvents and are liquid in a wide temperature range, they have high thermal stability, they do not decompose in high temperatures, they are not flammable, they are safe for reactions, and they have tuning abilities through which we can choose various cations and anions [10-12]. 2. Experimental 2.1. Materials The ionic liquids that we have used are listed in Table 1. All of the ionic liquids are purchased from Merck KGaA [13]. Methane with purity of 99.95% was used to form hydrate with deionized water. We used BMIM-BF4 in 1 wt%, 10 wt%, 15 wt% and 20 wt%, BMIM-DCA in 10 wt% and TEACL in 10 wt% for measuring induction time. Table 1. Ionic Liquids properties studied in this work Symbol Molar mass Solubility Density Provider Purity (g/mol) (g/l) (g/cm 3 ) BMIM-BF Soluble 1.18 Merck 99% BMIM-DCA Soluble 1.06 Merck 99% TEACL Solid Merck 98% 2

3 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University Table 2. Ionic Liquids chemical name and chemical structure studied in this work. Symbol Chemical name Chemical structure BMIM-BF 4 1-Butyl-3-methylimidazolium tetrafluoroborate BMIM-DCA 1-Butyl-3-methylimidazolium dicyanamide TEACL Tetraethyl ammonium chloride 2.2. Apparatus The experimental apparatus is consisted of a high-pressure Stainless Steel SS-316 reactor with total volume of 90 cm 3. The reactor can sustain a pressure of 15 MPa. A thermocouple (Pt- 100) with a division scale of 0.1 K was connected to the reactor to measure the temperature. Also a pressure transducer (P-2) is connected to the reactor to measure the pressure. The reactor is inserted in the ethanol-cooling bath and it has some valves for injecting and venting the gas. The temperature is adjusted by using a controllable circulator (TCS-1) with ability of scheduling (Julabo TP-50). Fig. 1. Shows the schematic diagram of experimental apparatus. Fig.1. Schematic diagram of experimental apparatus Fig. 2. Shows the schematic diagram of the equilibrium cell that is used in this work. 3

4 Experimental Study and Modeling of Methane Hydrate Fig. 2. Schematic diagram of equilibrium cell Procedure The reactor was washed with deionized water and was dried completely. 20 cm 3 aqueous solution of our ionic liquids or pure water was charged into the reactor in the start of each experiment. The reactor was filled with methane and it was pressurized with methane to desired pressures (like 5 MPa, 6 MPa, and 7 MPa) at K. After remaining the system in K for 1 hr to avoid the memory effect for each experiment the system was cooled to K and remaining for 1 hr in this temperature. The mixer was started at a rate of 1000 rpm. After reaching equilibrium at this temperature, the system was cooled to K by the rate of 1 K/hr and remains in for 15 hr to complete the step of hydrate formation. For measuring induction time we must know the time of hydrate formation temperature at the initial pressure and the time that pressure reduction occurred in the pressure-time diagram. Induction time is the period of time between reaching the hydrate formation conditions and formation of hydrate. Fig. 3. shows the induction time and the solid point is represented for hydrate formation condition Experimental data Equilibrium point Pressure (bar) Time (hr) Fig. 3. Pressure-time diagram during hydrate formation 4

5 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University We calculate the hydrate formation temperature by writing a code in Matlab that based on the Van der Waals-Platteuw basis and we calculate the water activity in the presence of ionic liquids by using UNIQUAC and NRTL equations. Table 3., Table 4. and Table 5. are parameters that we used in UNIQUAC and NRTL equations. Table 3. Parameters that we used in UNIQUAC equation [14]. Component r q aij (Optimized parameter) Water BMIM-BF Table 4. Parameters that we used in NRTL equation [15]. Component Δgij (J/mol) αij Water BMIM-DCA Table 5. Parameters that we used in NRTL equation [15]. Component Δgij (J/mol) αij Water TEACL We can also recognize the induction time from pressure-temperature curve of hydrate formation. From fig. 3. induction time is the difference between the hydrate equilibrium conditions and the time that the pressure suddenly decreased. Pressure (bar) Experimental Data Equilibrium calculation Temperature (Centigrade) Fig. 4. Pressure-temperature diagram during hydrate formation. 5

6 Experimental Study and Modeling of Methane Hydrate 3. Modeling On the basis of kinetics theory in crystallization, the nucleation rate formula is: πσ VM N a B = C exp (1) 3υ ( RT ) ( lnα) In the equation (1) B 0 is the nucleation rate, Na is the Avogadro s constant, R is the universal gas constant, VM is the molar volume of crystal, σ is the average surface tension on the solidliquid interface, υ is number of ions per dissolved molecule, α is the mole ratio of supersaturated solution to the saturated solution, C is a constant and s is Supersaturation ratio [16,17]. The relation between α and s is: 1 s (2) By using of mathematical estimation, we have: ln 1 s (3) Then we have equation (1) in the following form: B V M N C exp ( RT ) s a 2 (4) That we can rewrite the above equation in the following form: 0 b B k exp( ) s n (5) Natarajan et al. [18] have said that the relation between induction time and nucleation rate is like the following form: t i B 0 r (6) By rearranging the above equation, the following equations have gained: t i n b k exp s r 1 r k b exp s nr (7) r k (8) 6

7 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University m nr t i b exp( ) s m We have defined the subcooling as the driving force of supersaturation. Then our equation is changed to the following form: (9) (10) t i bt T s m exp( ) (11) Table 6. Optimized parameters for different ionic liquids Ionic liquid λ (min) m b BMIM-BF BMIM-DCA TEACL In the above equation λ, m and b are the constants that have gained by optimization from some experimental data. In this work, we have used a few experimental data points for optimizing λ, m and b. After optimization of the constants, we have predicted all the induction time points. Table 7. Number of data points for optimization of constants. Ionic Liquid Total Induction Time Data Induction Time Data that have used for optimization of constants BMIM-BF BMIM-DCA 7 3 TEACL 8 4 7

8 Experimental Study and Modeling of Methane Hydrate 4. Results 200 Predicted Induction Time (min) Experimental Induction Time (min) Fig. 5. Comparison between experimental induction time data and predicted induction time Solution Table 8. Experimental and predicted induction time for different ionic liquid solutions. Initial Pressure (bar) Experimental Induction Time (min) Predicted Induction Time (min) Solution Initial Pressure (bar) Experimental Induction Time (min) Predicted Induction Time (min) Pure Water wt% BMIM-BF Pure Water wt% BMIM-BF Pure Water wt% BMIM-BF Pure Water wt% BMIM-BF Pure Water wt% BMIM-BF wt% BMIM-BF wt% BMIM-BF wt% BMIM-BF wt% BMIM-BF wt% BMIM-BF wt% BMIM-DCA wt% BMIM-BF wt% BMIM-DCA wt% BMIM-BF wt% BMIM-DCA wt% BMIM-BF wt% BMIM-DCA wt% BMIM-BF wt% BMIM-DCA wt% BMIM-BF wt% BMIM-DCA wt% BMIM-BF wt% BMIM-DCA wt% BMIM-BF wt% TEACL wt% BMIM-BF wt% TEACL wt% BMIM-BF wt% TEACL wt% BMIM-BF wt% TEACL wt% BMIM-BF wt% TEACL wt% BMIM-BF wt% TEACL wt% BMIM-BF wt% TEACL wt% BMIM-BF wt% TEACL wt% BMIM-BF NP=45 AAD% 1 =5.77 NP 1 1 texp - tcal AAD% = ( ) 100 NP t i= 1 exp 8

9 2nd National Iranian Conference on Gas Hydrate (NICGH) Semnan University 5. Conclusions The effects of three ionic liquid solutions have been investigated in this work. In addition, a three parameter semi-empirical model is developed on the basis of chemical kinetics theory for representation of the obtained kinetic parameters. The results indicate that the developed semi-empirical correlation should be used for pressures below 8 MPa, ionic liquids concentrations up to 20 wt% and cooling rate of 1 K/hr. Finally, it is shown that the proposed semi-empirical model leads to a good accuracy in comparison with the experimental data. References [1] Sloan, E. D., Clathrate Hydrates of Natural Gases, 3rd ed., CRC Press, New York, [2] Carroll, J. J., Natural Gas Hydrates: A Guide for Engineers, 1st ed., Gulf Professional Pub, [3] Hammerschmidt, E. G., Preventing and removing hydrates in natural gas pipe lines, Gas Age., Vol. 52, pp , [4] Deaton, W. M., Frost, E. M., Gas Hydrates and Their Relation to the Operation of Natural Gas Pipe Lines, U.S. Bureau of Mines Monograph 8, pp. 101, [5] Lederhos, J., The Transferability of Hydrate Kinetic Inhibitor Results between Bench Scale Apparatuses and a Pilot Scale Flow Loop, Ph.D.Thesis, Colorado School of Mines, Golden, CO, [6] Freer, E. M., Sloan, E. D., An engineering approach to kinetic inhibitor design using molecular dynamics simulations, Annals of New York Academy of Sciences, Vol. 912, pp , [7] Katz, D. L., Cornell, D., Kobayashi, R., Poettmann, F. H., Vary, J. A., Elenbaas, J. R., Weinaug, C. F., Handbook of Natural Gas Engineering, McGraw-Hill, New York, pp. 802, [8] Karaaslan, U., Parlaktuna, M., A new hydrate inhibitor polymer, Energy & Fuels, Vol. 16, pp , [9] Kobayashi, R., Withrow, H. J., Williams, G. B., Katz, D. L., in Proc. Natural Gasoline Association of America, Vol. 27, San Antonio, TX [10] Wasserscheid, P., Welton, T., Ionic Liquids in synthesis, Second edition, Vol. 1, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [11] Suojiang, Z., Xingmei, L., Qing, Z., Xiaohua, L., Xiang ping, Z., Shucai, L., Ionic Liquids Physicochemical Properties, First edition, Amsterdam, The Netherlands, [12] Xiao, C., Adidharma, H., Dual function inhibitors for methane hydrate, AICHE Annual Meeting, Philadelphia, PA, November pp , [13] Merck KGaA,Darmstadt, Germany, customer. Service (at) merckchem.co.uk,

10 Experimental Study and Modeling of Methane Hydrate [14] Santiago R. S., Santos, G. R., Aznar, M., UNIQUAC correlation of liquid liquid equilibrium in systems involving ionic liquids: The DFT PCM approach, Fluid Phase Equilibria, Vol. 278, pp , [15] Ge, Y., Zhang, L., Yuan, X., Geng, W., Ji, J., Selection of ionic liquids as entrainers for separation of water-ethanol, J Chem. Thermodynamics, Vol. 40, pp , [16] McCabe, W. L., Stevens, R.P., Rate of Growth of Crystals in Aqueous Solutions, Chemical Eng. Prog. Vol. 47, p. 168, [17] Mullin, J.W., Crystallization, 3rd Edition, Butterworth-Heinemann, Oxford, U.K., [18] Natarajan, V., Thermodynamics and Nucleation Kinetics of Gas Hydrates, Ph.D. Thesis, University of Calgary, Alberta,