Energy & Fuels 2002, 16, 1413-1416
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Promotion Effect of Polymers and Surfactants on Hydrate Formation Rate Ugˇur Karaaslan* and Mahmut Parlaktuna† Department of Petroleum and Natural Gas Engineering, Middle East Technical University, 06531, Ankara, Turkey Received February 15, 2002
The promotion effect of two polymers and three surfactants on methane hydrate formation was investigated in a high-pressure system. For all of the tested chemicals, 1 wt % aqueous solutions were prepared and methane hydrate was formed in those media to detect their effect on hydrate formation rate. It was determined that Igepal-520 is the most effective promoter on methane hydrate formation rate with respect to a reference test carried out by using only pure water.
Introduction Interest in gas hydrates of the petroleum industry started with the discovery of natural gas hydrates as plugs in pipelines. This initiated further studies for finding ways of hydrate formation prevention. Meanwhile, scientists discovered new application areas of gas hydrates in laboratories for scientific and industrial purposes. Desalinization of sea water;1 storage and transportation of natural gas in the form of gas hydrate;2 processing and sweetening of natural gas;3 separation of light ends of oil from black oil;4 CO2 sequestration;5 and enzyme recovery from reversed micellar solution through formation of gas hydrates6 are some examples of gas hydrate applications. Therefore, gas hydrate promotion has become important. It has been known that surfactants have a strong influence on the kinetics of gas dissolution in the water phase as well as on the overall rate of hydrate formation.7 Surfactants such as linear alkyl benzene sulfonic acid (LABSA), an anionic surfactant, increased the hydrate formation rate for all concentrations tested.8 Another example of hydrate promoter surfactant is sodium dodecyl sulfate. It promotes the hydrate formation rate by a factor greater than 700 above the critical micellar concentration.9 * Corresponding author. Fax: 90 312 2101271. E-mail: ugurk@ metu.edu.tr. † E-mail:
[email protected]. (1) Knox, W. G.; Hess, M.; Jones, G. E., Jr.; Smith H. B. Chem. Eng. Prog. 1961, 57 (2), 66-71. (2) Gudmundsson, J. S.; Parlaktuna, M.; Khokhar, A. A. SPE Prod. Facil. 1994, 69-73. (3) Gudmundsson, J. S.; Andersson, V.; Levik, O. I. Proceedings of Offshore Mediterranean Conference, Ravenna, 1997. (4) Ostergaard, K. K.; Tohidi, B.; Danesh, A.; Burgass, R. W.; Todd, A. C.; Baxter T. Ann. N.Y. Acad. Sci. 2000, 912, 832-842. (5) Lund, P. C. Proceedings of the International Polar EngineeringConference, The Hague, 1995; pp 277-28. (6) Nagahama, K.; Noritomi, H.; Koyama, A. Fluid Phase Equilib. 1996, 116, 126-132. (7) Kalogerakis, N.; Jamaluddin, A. K. M.; Dholabhai, P. D.; Bishnoi, P. R. SPE 25188, Proceedings of the SPE International Symposium on Oilfield Chemistry, New Orleans, 1993; pp 375-383. (8) Karaaslan, U.; Parlaktuna, M. Energy Fuels 2000, 14, 11031107.
In this study, five water-soluble chemicals, poly(acrylamide-co-acrylic acid) (PAAA), polyoxyethylene (20) cetyl ether (Brij-58), polyoxyethylene (20) sorbitan monopalmitate (Tween-40), poly(vinyl alcohol) (PVA) ,and polyoxyethylene (5) nonylphenyl ether (Igepal-520), were tested in a high-pressure apparatus to investigate their effect on methane hydrate formation rate. Chemical structures of these chemicals are given in Figure 1. These chemicals are selected because of their wide usage in the petroleum industry, especially for preparation of drilling fluids. It is known that hydrate formation occurs during the drilling operation of gas and oil wells in the colder waters of deep oceans and the use of some of these polymers during the drilling operation may affect the hydrate formation characteristics of drilling fluids (inhibition or promotion of hydrate formation). Experimental Set-Up and Procedure A high-pressure system, whose schematic diagram is given in Figure 2, is used to carry out hydrate formation tests. A cylindrical high-pressure reactor with dimensions of 3.4 cm in diameter and 15 cm in length is the main piece of experimental setup. Its total available volume is 142 cm3 including connections. It is made of brass and tested up to 1200 psi. The high-pressure cell is placed into a constant-temperature bath made by plexiglass, allowing the observation of the system easily. A temperature controller and a refrigerated chiller are immersed into the water bath to provide temperature control during tests. A thermocouple (with an accuracy of (0.2 °C) and a pressure transducer (with an accuracy of (1 psig) are connected into the high-pressure cell to measure cell temperature and pressure. They are connected to a datalogger and a personal computer to record the temperature and pressure as functions of time. A motor at the outside of the bath is attached to the cell to provide rocking, and a glass marble is placed into the cell to stir and agitate the liquid inside. The speed of the rocking system was kept at 30 rpm; temperature and pressure are recorded every 10 s throughout this study. Methane gas that has 99.5% purity was used as hydrate former gas. Aqueous 1 wt % chemical solutions were (9) Zhong, Y.; Rogerrs, R. E. Chem. Eng. Sci. 2000, 55, 4175-4187.
10.1021/ef020023u CCC: $22.00 © 2002 American Chemical Society Published on Web 10/11/2002
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Figure 3. Temperature and pressure data of the pure water experiment with time. Table 1. Experimental Conditions chemical name
Tinitial, °C
Pinitial, psig
Trocking, °C
Procking, psig
pure water (C-1A and C-1B) PAAA Brij-58 Tween-40 PVA Igepal-520
15 15 15 15 15 15
700 700 700 700 700 700
3 3 3 3 3 3
662 662 662 662 662 662
process is started to agitate the solution in the cell to initiate hydrate formation at a predetermined hydrate formation temperature. At that point, the cell pressure already decreased from 700 psig to 662 psig as a result of cooling and dissolution of methane gas at low temperature. Temperature and pressure data during hydrate formation are collected with respect to time and stored by a data acquisition system to calculate formation rates. Details of the experimental procedure can be found from Karaaslan.10 Figure 1. Name and structures of polymers and surfactants.
Results and Discussion
Figure 2. Experimental setup. prepared in pure water and all of the chemicals were supplied from Aldrich. After making the necessary connections (gas, water, and vacuum lines, thermocouple and pressure transducer connections) to the high-pressure cell, the system is evacuated. Then, the aqueous phase is injected into the cell and the cell is pressurized by methane gas. After reaching an equilibrium at initial pressure and temperature conditions (Table 1), the system is cooled to hydrate formation temperature without rocking; meanwhile data collection is started. The rocking
Kinetics. Methane hydrate was formed with only pure water to make a comparison between tested chemicals and water, so a pure water test is the reference test of this study. Figure 3 shows the raw temperature and pressure data change with time during the hydrate formation process. The cooling period of the hydrate formation in the figure represents the cooling of the cell without rocking. As observed, there is a decline in cell pressure while the cell temperature is decreased from 15 °C to 3 °C. The cell is allowed to stand at 3 °C for a while, and no further pressure drop is observed during this period. When the rocking of the cell is started, the cell pressure decreases, indicating formation of hydrate. After forming hydrate, cooling and rocking ceased and the system is allowed to heat-up by ambient temperature. This period is designated as hydrate dissociation in Figure 3. This observation was obtained through all of the tests. In addition to tests with chemicals, two tests with pure water were carried out to check the reproducibility of the experimental data. For this reason, two pure water tests were carried out at the same experimental conditions except the water was renewed for the second test to avoid the memory effect of water. Figure 4 shows (10) Karaaslan, U. Ph.D. Thesis, Middle East Technical University, Ankara, Turkey, 2001.
Hydrate Formation Rate
Figure 4. Reproducibility of the experiments (C-1A and C-1B).
Figure 5. Free gas content in the cell with respect to time for the pure water test.
Figure 6. Efficiency comparison of 1 wt % hydrate formation promoters.
the experimental data of two pure water tests (C-1A and C-1B) as a hydrate hysteresis plot. As observed from the figure, the hydrate formation-dissociation data is satisfactorily reproduced. It indicates the reproducibility of the thermodynamic data for hydrate formation and dissociation. Recorded pressure and temperature data of each test were used to calculate the moles of remaining free gas in the cell during methane hydrate formation through real gas law. The amount of moles of free gas in the high-pressure cell as a function of time for pure water is seen in Figure 5. This plot is obtained after processing the experimental temperature and pressure data in Figure 3 with the use of the real gas law. For all of the tests, free gas contents were calculated and plotted in Figure 6. Figure 6 shows the change of free gas in the cell with respect to time after starting of the rocking. As it is seen
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Figure 7. Cartesian plot of free gas versus time for the test with pure water.
Figure 8. Semilogarithmic plot of free gas versus time for the test with pure water.
from Figure 6, 1 wt % PAAA has a little inhibition effect when their response is compared with that of pure water. On the other hand, there is an early drastic drop in moles of free gas for chemicals Brij-58, Igepal-520, Tween-40, and PVA in their 1 wt % aqueous solutions. In all these tests, the moles of free gas become lower compared to the pure water test. An early and drastic drop in moles of free gas indicates the higher amount of gas consumption for hydrate formationsin other words, an increase in hydrate formation rate. This is a clue for these chemicals being hydrate formation promoters. Hydrate Formation Rates. Rate of change of free gas mole is used as an indicator for the hydrate formation rate. If the Cartesian plot of free gas mole versus time shows an exponential behavior it is an indication of a first-order reaction. If the same data such as pure water data are plotted as the natural logarithm of free gas mole versus time, the resulting line should fit a straight line. Figure 7 gives the Cartesian plot of the experimental data for the test with pure water. As observed, it exhibits an exponential behavior from which a first-order reaction can be assumed. A first-order reaction equation is defined with the following expression:
N ) N0e-kt
(1)
where N is the total number of moles at any time t, N0 is the initial number of moles, k is the rate constant (s-1), and t is time (s). When the natural logarithm of above equation is taken, the slope of ln(N) versus time graph is equal to the rate constant k. When the derivative of the N with
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Table 2. Methane Hydrate Formation Rate Constants (k) and Rates chemical name
No (lb-mole)
k (s-1)
rate (lb-mole/s)
pure water (C-1A and C-1B) PAAA Brij-58 Tween-40 PVA Igepal-520
4.00E-04 4.00E-04 4.01E-04 3.99E-04 3.99E-04 3.99E-04
3.65E-05 3.75E-05 6.85E-05 8.93E-05 9.25E-05 9.91E-05
1.37E-08 1.40E-08 2.43E-08 3.03E-08 3.12E-08 3.31E-08
respect to time is taken, it will give the change in free gas content and it is relative to the hydrate formation rate so after that point it is going to be called hydrate formation rate.
dN ) -N0e-kt dt
(2)
A semilogarithmic plot of the same data of Figure 7 is presented in Figure 8 showing a linear behavior. Its best fit as a straight line has a correlation coefficient very close to unity. The slope of the straight line, 3.653E-05 for this case, is the rate constant with a unit of s-1. Thus, eq 2 can be used to calculate hydrate formation rate at a specified time. The first 30 minutes of data for all of the tests after the start of rocking are used to estimate the rate constants and hydrate formation rates. The results are tabulated in Table 2 and given in Figure 9. Methane hydrate formation rates of tests with 1 wt % PAAA is very close to the pure water test. It can be concluded that this polymer has no effect on methane hydrate formation. On the other hand, 1 wt % of Brij-58, Igepal-520, Tween-40, and PVA promoted the methane hydrate formation rate. The most effective promoter is the Igepal-520 among them; it accelerated the methane hydrate formation rate by a factor of 2.4 compared to methane hydrate formation with pure water.
Figure 9. Hydrate formation rates of pure water, 1 wt % PAAA, Brij-58, Tween-40, PVA, and Igepal-520.
Analysis of chemical structures can give a clue for the reason of the promotion of hydrate formation by these additives. Since all of the additives consist of OH molecules and their presence increases the hydrate formation rates, it can be speculated that OH molecules may play a mission. One other observation is that the more effective chemicals, Igepal and PVA, have smaller molecular size than others. Conclusions Except PAAA, all of the tested polymers and surfactants enhance the methane hydrate formation rate. IGEPAL-520 and PVA, the more effective hydrate promoters, are among tested chemicals. An amount of 1 wt % Igepal-520 accelerates the methane hydrate formation rate by a factor of 2.4 compared to methane hydrate formation with pure water. Acknowledgment. The authors express their thankfulness to The Scientific and Technical Research Council of Turkey (TU ¨ BI˙ TAK) for the financial support of this work. EF020023U
