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On the Dissociation of Natural Gas Hydrates from Surfactant Solutions Ugˇur Karaaslan and Mahmut Parlaktuna* Department of Petroleum and Natural Gas Engineering, Middle East Technical University, 06531, Ankara, Turkey Received March 22, 2000. Revised Manuscript Received September 10, 2000
Natural gas hydrates from different types and concentrations of surfactant solutions were produced and dissociated in a high-pressure cell. The Clausius-Clapeyron equation was utilized to derive the enthalpy related to the phase transition of the gas component from dissociation data. The results indicated that the phase change enthalpy of the nonionic surfactant solution hydrate is higher compared to that of pure-water and anionic surfactant solution hydrates. Experimental results also show that surfactants do not influence the thermodynamic dissociation point.
Introduction Natural gas hydrates are considered nuisances in gas production and processing, since they plug flow channels and impede flow. Consequently, much of the gas hydrate research has been directed toward determining ways of hydrate prevention. On the other hand, high gas storage capacity of hydrate (1:150) makes hydrates as an alternative for gas transportation and storage.1 It is therefore necessary to find ways of promoting hydrate formation for storage and transportation applications. Surfactants are the class of compounds which may find a place in either prevention or promotion of hydrate formation. Several researchers have studied the effect of surfactants in hydrate prevention as antiagglomerants.2-4 To realistically model hydrate prevention and promotion schemes it is necessary (but not sufficient) to know the heat of dissociation (∆Hd). It is the goal of this study to present a method of experimentally determining the phase change enthalpy of the gas component from the dissociation of natural gas hydrate samples produced from surfactant solutions. Theory and Discussion The most accurate way of determining the heat of dissociation (∆Hd) for hydrate formation reaction is to * Corresponding author. E-mail:
[email protected]. Fax: 90312-2101271. (1) Gudmundsson, J. S.; Parlaktuna, M.; Khokhar, A. A. SPE Prod. Facil. 1994, 69-73. (2) Kelland, M. A.; Svartaas, T. M.; Dybvik, L. A. SPE 28506. In Proceedings of the SPE 69th Annual Technical Conference and Exhibition, New Orleans, 1994; Society of Plastic Engineers: Richardson, TX, 1994; pp 431-438. (3) Kelland M. A.; Svartaas T. M.; Dybvik L. A. SPE 30695. In Proceedings of the SPE 69th Annual Technical Conference, Dallas, 1995; Society of Plastic Engineers: Richardson, TX, 1996; pp 529-537. (4) Kalogerakis, N.; Jamaluddin, A. K. M.; Dholabhai, P. D.; Bishnoi, P. R. SPE 22188. In Proceedings of the SPE International Symposium on Oilfield Chemistry, New Orleans, 1993; Society of Plastic Engineers: Richardson, TX, 1994; pp 375-383.
measure it calorimetrically. Such experimental measurements are tedious and limited. However, the enthalpy of dissociation of pure components and naturally occurring gas hydrates has been determined calorimetrically.5-7 A second way of determining the heat of dissociation is the use of Clausius-Clapeyron equation,8 eq 1:
d ln P -∆Hd ) zR d(1/T)
(?)
where P and T are the absolute pressure and temperature of the hydrate formation, R is the universal gas constant, and z is the gas deviation factor. The Clausius-Clapeyron equation indicated that the slope of the logarithm of the hydrate dissociation pressure plotted against reciprocal temperature will give the negative heat of dissociation divided by the product of the gas deviation factor and the gas constant. Skovborg9 reformulated the Clausius-Clapeyron equation as follows, eq 2: v H n(HLw - HH w ) + (Hg - Hg ) dlnP )zR d(1/T)
The first enthalpy difference of the above equation accounts for the enthalpy change involved in the water phase transition from liquid to hydrate. The second term accounts for the enthalpy related to the phase transition of the gas component. It is possible experimentally to estimate the phase change enthalpy of gas component from the Clausius-Clapeyron equation by measuring pressure-temperature data of the gas phase during (5) Handa, Y. P. J. Chem. Thermodyn. 1986, 18, 915-921. (6) Handa, Y. P. Ind. Eng. Chem. Res. 1988, 27, 872-874. (7) Rueff, R. M.; Sloan, E. D.; Yesavage, V. F. AIChE J. 1988, 34, 1468-1476. (8) Sloan, E. D.; Fleyfel, F. Fluid Phase Equil. 1992, 76, 123-140. (9) Skovborg, P. Ph.D. Thesis, 1993, Danmarks Tekniske Hojskole.
10.1021/ef000060q CCC: $20.00 © 2001 American Chemical Society Published on Web 11/05/2000
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Parlaktuna Table 3. Hydrate Formation and Dissociation Conditions hydrate dissociation conditions
Figure 1. Experimental setup for high-pressure test. Table 1. Names and Types of Surfactants Used in This Study commercial name
structure
type
LABSA ethoxalate
linear alkyl benzene sulfonic acid nonylphenol ethoxalate
anionic nonionic
Table 2. Chemical Composition of Gas Used in Experiments CH4a C2H6 C3H8 i-C4H10 n-C4H10 i-C5H12 n-C5H12 N2 89.47 5.39 1.89 a
0.40
0.62
0.28
0.19
CO2
1.58 0.18
The unit for each component is mole %.
hydrate dissociation. It is assumed that hydrate, water, and gas phases are in equilibrium during dissociation and that the Clausius-Clapeyron equation is applicable. Experimental Setup and Procedure A high-pressure system is used to produce gas hydrate. A schematic diagram of the experimental setup is given in Figure 1. A cylindrical high-pressure reactor with dimensions of 3.4 cm in inner diameter, 15 cm in length, and total available volume of 142 cm3 including connections is used to produce gas hydrate. It is made of brass and tested up to 1000 psi. The cell consists of glass windows that are used for visual observation of hydrate formation on two faces. The highpressure cell is placed into a constant-temperature bath made by plexiglass that allows the observation of the system easily. A temperature controller and a refrigerated chiller are connected to the water bath to provide temperature control of the experiments. Also a thermocouple and a pressure transducer are connected into the high-pressure cell to measure system temperature and pressure. They are connected to a datalogger and a personal computer to record the temperature and pressure as functions of time. A magnetic stirrer, rotating a stirring bar within the cell, accomplishes the stirring of the system. The magnetic stirrer has a speed range of 0-1200 rpm and it was kept at 500 rpm throughout this study. Tap water, natural gas and surfactants were used as reagents in experiments. Name of the surfactants and their types are given in Table 1. A natural gas from a gas field of Turkish Petroleum Corporation was used to produce hydrate during this study. The composition of natural gas is given in Table 2. Two different surfactants, a nonionic and an anionic, were used throughout the study to observe the effect of type and concentration of surfactants on the phase change enthalpies of gas component during hydrate dissociation. Nine experiments (four experiments for each type of surfactant and one experiment without surfactant) were carried out. The experimental conditions of these tests are presented in Table 3. The
test no.
surfactant
concentration (wt %)
temperature (°C)
pressure (psig)
1 2 3 4 5 6 7 8 9
nonionic nonionic nonionic nonionic anionic anionic anionic anionic
0.005 0.01 0.1 1.0 0.005 0.01 0.1 1
12.87 12.65 12.93 12.82 13.09 12.48 12.78 13.12 12.88
556.0 554.3 555.6 552.7 557.5 552.6 556.8 558.0 552.8
initial temperature and pressure of each test were tried to be the same at 18 ( 1 °C and 576 ( 2 psig, respectively. Each experiment starts with evacuation of high- pressure cell and injection of surfactant solution followed by gas injection into the cell. After getting temperature and pressure equilibrium, the system is cooled and the temperature and pressure of the high-pressure cell are recorded as a function of time at every 10 s. This period corresponds to hydrate formation, since the aim is to produce hydrates from the surfactant solution and natural gas. After forming hydrates and getting the cell temperature below 4 °C, the cell is heated to 18 °C with the aid of heater/circulator. This is the part in which the hydrate is dissociated. The experiment is ceased when the temperature of the cell is reached to 18 °C. Details of experimental setup and procedure can be found in Karaaslan.10
Results and Discussion A sample plot of the change in pressure and temperature during hydrate formation and dissociation is shown in Figure 2 from Test 4. This test was carried out twice to check the reproducibility of hydrate formation and dissociation. Since water will have some inherent structure and have a well-known “memory” effect after forming hydrates, these two tests were run with different water/surfactant solutions having same concentration. The temperature and pressure records of two tests are almost the same indicating the reproducibility of hydrate formation and dissociation. Hydrate formation-dissociation cycle is started with cooling of high-pressure cell content (surfactant solution and hydrate forming gas under high-pressure). Pressure in the cell decreases slightly due to cooling and dissolution of gas in solution. An increase in the rate of pressure drop with temperature is the indication of the initiation of gas consumption for hydrate formation. After forming hydrate and getting the cell temperature below 4 °C, cooling is stopped and the cell content is heated to dissociate already formed hydrate. As seen from Figure 2, hydrate dissociation does not start immediately after heat application. The increase in pressure is due to expansion of gas, hydrate and water content of the cell with temperature. At a certain temperature the increase in pressure becomes more rapid indicating the start of hydrate dissociation. The end of hydrate dissociation is observed as a change (a decrease) in the slope of pressure-temperature plot. After that point the change in pressure is due to temperature only. (10) Karaaslan, U. MSc. Thesis., 1998, Middle East Technical University, Ankara.
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Figure 2. Pressure-temperature plot of two different experiments with same type of solution during hydrate formation and dissociation (test no. 4).
Figure 3. Change in pressure during heating for hydrate and blank tests.
Although the main reason for pressure increase during dissociation is the gas coming from hydrate structure, the expansion of free gas also contributes. A blank test was run with pure methane and tap water without forming hydrate (which was achieved without stirring the cell content) to quantify the pressure increase due to expansion of free gas only. Figure 3 compares the pressure-temperature data of two tests with and without hydrate formation. As seen from these two plots that the contribution of gas expansion on pressure increase is negligibly small compared to the pressure increase coming from hydrate structure. Hydrate Formation and Dissociation Conditions. Pressure-temperature data records are utilized
to determine the hydrate formation and dissociation conditions. Application of real gas law (PV ) znRT) for each data point with known pressure, temperature and free gas volume gives the change in number of moles of free gas with time. The gas compressibility factor of the real gas law z is estimated by using Lee and Kesler’s11 compressibility factor expression. A sample plot of change in free gas number of moles is given in Figure 4 for Test no 4. At the initial stage of test there is a slight decrease in free gas moles due to solubility of gas with decrease in temperature. The point at which a decrease in number of free gas moles is observed is taken as (11) Lee, B. I.; Kesler, M. G. AIChE J. 1975, 21, 510-527.
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Figure 4. Determination of hydrate formation initiation conditions (test no. 4).
Figure 5. ln P versus 1/T graph for test no. 1.
hydrate formation initiation point and the corresponding pressure-temperature values as incipient hydrate formation conditions. Similarly, increase in free gas moles during heating period is the result of dissociation. The end of point of dissociation is marked as a change in free gas moles. Two vertical lines in Figure 4 are drawn as reference lines showing the hydrate formation and dissociation conditions for Test no 4. Since hydrate formation is stochastic and unpredictable, only hydrate dissociation conditions are presented in Table 3. As observed, there is no measurable influence of surfactants on hydrate dissociation conditions. This observation is in agreement with Kalogerakis et al.4 Dissociation Enthalpy Calculations. Hydrate dissociation data of these tests are utilized to derive the enthalpy change of gas component from the Clausius-
Clapeyron equation during hydrate dissociation. If the system during dissociation is assumed to be in equilibrium, the temperature-pressure data of the dissociation period can be used to determine the enthalpy change of gas component during dissociation. To facilitate this determination, a plot of ln P versus 1/T is drawn and the slope of the best-fit straight line should give ∆H/zR value. A sample plot of ln P versus 1/T data is shown in Figure 5 for Test no 1. Best-fit straight lines of the ln P vs 1/T plots for nonionic and anionic surfactants are given in Figures 6 and 7, respectively. Slopes of bestfit lines of the tests with nonionic surfactant solutions (Exp 2-5) are higher than that of pure water (Exp 1) indicating higher heat of dissociation. Lower slopes of anionic surfactant solution tests compared to pure water test are the indication of lower heat of dissociation.
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Figure 6. ln P versus 1/T graph for nonionic surfactant.
Figure 7. ln P versus 1/T graph for anionic surfactant. Table 4. Enthalpy of Dissociation of Hydrates (kJ/mol) concentration (wt %)
nonionic
anionic
0.0 0.005 0.01 0.1 1.0
16.65 18.65 18.91 21.00 22.15
16.65 12.09 13.91 15.00 13.57
Enthalpy changes of gas component during dissociation obtained with the best fit of ln P versus 1/T plots are presented in Table 4. The gas compressibility factor z was estimated by using Lee and Kesler’s11 compressibility factor expression. As observed, the enthalpy change of gas component lies in the range from 12 to 22 kJ/mol as is also shown in Figure 8. These values are comparable with the measured enthalpy of dissociation for methane (18.13 ( 0.27 kJ/mol) for the process from hydrate to ice and gas.5
Critical micelle concentrations of both surfactants are also located in Figure 8 designated as cmc. The critical micelle concentration (cmc) is the concentration, above which the surfactant might not be expected to be effective. Critical micelle concentrations of both surfactants were determined through the surface tension measurements with air and found to be 0.025 wt %. Enthalpy change of gas component during dissociation of hydrate from nonionic surfactant solutions gave higher values compared to pure water hydrate with an increasing trend with surfactant concentration. On the other hand, dissociation of hydrates produced from anionic surfactant solutions resulted with lower enthalpy changes of gas component for all concentrations of surfactant studied. Enthalpy change of gas component with anionic surfactant solutions shows a maximum around cmc.
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Figure 8. Enthalpy change of gas component during hydrate dissociation.
Conclusions
Nomenclature
A series of experiments were conducted to investigate the influence of surfactants on hydrate formation and dissociation. A method is proposed to estimate the enthalpy change of gas component from experimental data of hydrate dissociation and was applied hydrate samples produced from different surfactant solutions. For surfactant concentrations studied, there is no appreciable effect on hydrate dissociation conditions. The phase change enthalpy of gas component of nonionic surfactant was found to be higher than that of pure water and anionic surfactant solution hydrates.
P: pressure T: temperature R: gas constant Z: gas deviation factor ∆Hd : heat of dissociation n: the number of moles of water per mole of gas present in hydrate equilibrium phase HLw: molar enthalpy of water in the liquid water phase HH w : molar enthalpy of water in the hydrate phase HH g : molar enthalpy of gas in the hydrate phase HVg : molar enthalpy of gas in the vapor phase EF000060Q
