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PEOsA New Hydrate Inhibitor Polymer Ugˆur Karaaslan* and Mahmut Parlaktuna† Department of Petroleum and Natural Gas Engineering, Middle East Technical University, 06531, Ankara, Turkey Received February 14, 2002

The hydrate inhibition potential of four different polymers has been tested in a high-pressure apparatus at high pressures and low temperatures. Aqueous polymer solutions were prepared at 1 wt % concentration. Methane (99.5% purity) was used as hydrate former gas. Hydrate formation rates of the tests were calculated to decide the best inhibitors. A known hydrate inhibitor, poly(N-vinylpyrrolidone) (PVP) and a water-soluble thermoplastic polymer (PEO) were determined the best kinetic inhibitors among them.

Introduction Gas hydrates are solid compounds that are formed by the combination of gases and volatile liquids with a large excess of water. Water forms a lattice structure with cavities that are occupied by gas molecules at highpressure and low-temperature conditions. Water molecules are strongly hydrogen bonded with each other and the gas molecules stabilize this lattice through van der Waals type dispersion force.1 In the oil industry, there is an increasing tendency to exploit hydrocarbon reserves at increasing depths in the oceans with increasing distances for pipeline transportation in both horizontal and vertical directions. As a result of this extended exposure of the water-containing unprocessed hydrocarbon fluid to low temperatures and high pressures, the handling of the hydrate problems in such transport lines has become a more integral part of the petroleum industry. Since large percentages of alcohol have to be added, an economic incitement toward more effective inhibitors has evolved. The focus over the past few years has been directed toward slowing down the process of hydrate formation, kinetic inhibition. In this context, studying the fundamental kinetics of hydrate formation is important. Kinetic inhibition is a new means of preventing hydrate blockage of flow channels. The system is allowed to exist within the hydrate thermodynamic stability zone, so that chemical crystals are stabilized for a time exceeding the residence time of free water in a pipeline. The concept of using polymeric surfactants as kinetic inhibitors was derived from the discovery of fish anti freeze proteinssAFPs, which are able to kinetically prevent the formation of ice in fish, preventing them from freezing. This finding prompted researchers to investigate the possibility of developing effective chemical structures that could kinetically inhibit hydrates produced in oil and gas recovery.2 The inhibition mechanism of polymers and surfactants is proposed as follows. Active molecules are found * Corresponding author. Fax: 90 312 2101271. E-mail: ugurk@ metu.edu.tr. † E-mail: [email protected]. (1) Sloan, E. D., Jr. Energy Fuels 1998, 12, 191-196.

to adsorb strongly to the surface of a propagating hydrate crystal or pre-nuclear hydrate-like clusters. In this process, they change the energy of the surface of the hydrate crystal or cluster and so change its growth characteristics. This can be likened to a lock-and-key mechanism, with part of the inhibitor showing a very specific and strong interaction for the microstructures that characterize the hydrate surface or interface, while another part of the inhibitor then interferes with the continued growth process.3 Some examples of known and patented inhibitors are poly(N-vinyl pyrrolidone) (PVP), poly(N-vinylcaprolactam) (PVCap), poly(N-methyl-N-vinylacetamide) (VIMA), poly(N-vinylvalerolactam) (PVVam), poly(acryoylpyrrolidine) (PAPYD), and poly(acryloylmorpholine) (PAMOR).4 In this study, several polymers were tested experimentally to determine their inhibition efficiency on hydrate formation. Hydrate formation rates were calculated from experimental data to decide the best hydrate inhibitors among them. Experimental Setup and Procedure A high-pressure system, whose schematic diagram is given in Figure 1, 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 of 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 (2) Kelland, M.; Svartaas, T. M.; Dybvik, L. SPE 30420. In Proceedings of the SPE Offshore Europe Conference; Aberdeen, 1995; pp 531539. (3) Phillips, N. J.; Kelland, M. A. Industrial Applications of Surfactants IV; Karsa, D. R., Ed.; Royal Society of Chemistry, 1999. (4) Freer, E. M.; Sloan, E. D. Ann. N.Y. Acad. Sci. 2000, 912, 651657.

10.1021/ef0200222 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/02/2002

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Karaaslan and Parlaktuna

Figure 2. Methane/water hydrate equilibrium line5 with experimental conditions.

Figure 1. Experimental setup. Table 1. Experimental Conditions chemical name

Tinitial, °C

Pinitial, psig

Trocking, °C

Procking, psig

pure water PVP PEO HEC HECE

15 15 15 15 15

700 700 700 700 700

3 3 3 3 3

662 662 662 662 662

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 % polymer solutions were prepared in pure water. Poly(N-vinyl pyrrolidone (PVP), a water-soluble thermoplastic poly(2-ethyl-2-oxazoline) (PEO), hydroxyethyl cellulose (HEC) ,and hydroxyethyl cellulose ethoxylate (HECE) were used as hydrate-inhibitor testing polymers. 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 cell and the cell is pressurized by methane gas. After reaching 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 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. The hydrate equilibrium line of methane/water mixture obtained from CSMHYD5 is given in Figure 2 with the schematic representation of the hydrate formation procedure. Point 1 on this figure shows the initial conditions of the system before cooling and Point 2 represents the conditions at which rocking is started with a degree of subcooling of 2.8 °C. All tests of this study were carried out with the same degree of subcooling to eliminate its effect on formation kinetics. Details of the experimental procedure can be found from Karaaslan.6

Results and Discussion A sample plot of recorded cell pressure and temperature data for a test carried out by only pure water (5) Sloan, E. D. Clathrate Hydrates of Natural Gases; Marcel Dekker Inc.: New York, 1990. (6) Karaaslan, U. Ph.D. Thesis, Middle East Technical University, Ankara, Turkey, 2001.

Figure 3. Temperature and pressure data of pure water experiment with time.

during the hydrate formation process is shown in Figure 3. 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. Cooling and rocking ceased after hydrate formation, and then the system is allowed to heat up by ambient temperature. This period is designated as hydrate dissociation in Figure 3. A pressure-temperature plot of experimental data of the hydrate formation-dissociation cycle is known as the hydrate hysteresis curve in the literature.The same data of Figure 3 is given as the hydrate hysteresis curve in Figure 4. Figures 5 to 8 show the hysteresis curves of the tests carried out by 1 wt % aqueous solutions of PVP (patented inhibitor), PEO, HEC, and HECE, respectively. As observed from Figure 4 and Figure 5, although the pressure in the cell decreases drastically in pure water test after the start of rocking (about 125 psig), the decrease in pressure with PVP is very low (about 9 psig). These pressure drops occurred within a period of 3 h rocking. This result shows that PVP is an effective methane hydrate inhibitor. As seen from Figure 6, PEO has the same effect as PVP on methane hydrate formation; therefore, it can

PEOsA New Hydrate Inhibitor Polymer

Figure 4. Hysteresis curve for methane hydrate with pure water test.

Figure 5. Hydrate hysteresis curve for 1 wt % PVP.

Figure 6. Hydrate hysteresis curve for 1 wt % PEO.

Figure 7. Hydrate hysteresis curve for 1 wt % HEC.

be a good candidate to be a hydrate inhibitor. Polymers HEC (Figure 7) and HECE (Figure 8) can also be treated as inhibitors for methane hydrate since the pressure drop during methane hydrate formation with their 1 wt

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Figure 8. Hydrate hysteresis curve for 1 wt % HECE.

Figure 9. Moles of free gas in the high-pressure cell as a function of time for pure water, PVP, PEO, HEC, and HECE tests.

Figure 10. Cartesian plot of free gas versus time for the test with pure water.

% solutions are less compared to a pure water test. However, pressure drops during hydrate formation process with these polymers are higher than PVP and PEO. Pressure-temperature data of each test through the real gas law are used to calculate the moles of remaining free gas in the cell during methane hydrate formation. Moles of free gas in the high-pressure cell as function of time for pure water, PVP, PEO, HEC, and HECE are given in Figure 9. As is seen from the figure, the number of moles of free gas decreases with time in the case of pure water as hydrate-forming media. On the other hand, there is almost no change in moles of free gas when 1 wt % of PVP and PEO solutions are used as hydrate-forming media. HEC and HECE with their 1 wt % solutions are also possible candidates of being hydrate inhibitors according to Figures 9. Although they exhibit hydrate inhibition character-

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Karaaslan and Parlaktuna Table 2. Methane Hydrate Formation Rate Constants (k) and Rates chemical name

No (lb-mol)

k (s-1)

rate (lb-mol/s)

PVP PEO HEC HECE pure water

4.00E-04 4.00E-04 4.01E-04 4.00E-04 4.00E-04

2.42E-06 2.83E-06 2.80E-05 2.93E-05 3.65E-05

0.096E-08 0.11E-08 1.07E-08 1.11E-08 1.37E-08

reaction equation is defined with the following expression: Figure 11. Semilogarithmic plot of free gas versus time for the test with pure water.

N ) N0e-kt

where N is the total number of moles at any time t, N0 is initial number of moles, k is a rate constant (s-1), and t is time (s). When the natural logarithm of eq 1 is taken, the slope of the graph of ln(N) versus time is equal to a rate constant k. When the derivative of N with 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 ) -N0ke-kt dt Figure 12. Hydrate formation rates of pure water, PVP, PEO, HEC, and HECE.

istics, their efficiencies seem lower compared to PVP and PEO. The 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 are plotted as the natural logarithm of free gas mole versus time, the resulting line should fit a straight line. Figure 10 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

Figure 13. Chemical formulas of PVP, PEO, HEC, and HECE.

(1)

(2)

A semilogarithmic plot of the data of Figure 10 is presented in Figure 11 and shows 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 in this case, is the rate constant with a unit of s-1. Thus, the hydrate formation rate can be calculated at a specified time by eq 2. The first 30 min data of all experiments 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 12. Analyses of the results show that the most effective hydrate inhibitors are PVP and PEO among the tested polymers. The methane hydrate formation rate is 14.2 and 12.5 times less compared to pure water

PEOsA New Hydrate Inhibitor Polymer

test with PVP and PEO, respectively. HEC and HECE with their 1 wt % solutions decreased the hydrate formation rate of methane, compared to pure water experiment. However, their inhibition efficiencies are not as good as PVP and PEO. Poor hydrate inhibition efficiency of HEC and HECE can be based on their chemical structures. It is seen from Figure 13 that HEC and HECE have a different structure compared to that of PVP while PEO shows a similarity to that of PVP. The common part of PVP and PEO is the N-CdO bond and it exists in most of the hydrate-inhibiting polymers.4 Conclusions A known, patented hydrate inhibitor, PVP, was used to test the performance of the experimental setup and procedure of this study. The results showed that PVP is an effective methane hydrate inhibitor and the experimental setup and procedure are suitable for testing hydrate inhibition analysis. Tested polymers, PVP, PEO, HEC, and HECE, prevent the methane hydrate formation. PVP is a patented

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inhibitor but is known as a carcinogenic chemical. Though HEC and HECE decrease the methane hydrate formation rate, they are not as effective inhibitors as PVP. PEO exhibits hydrate inhibition characteristics at a concentration of 1 wt % and has not been known as a hydrate inhibitor until this study. This study shows that PEO prevents the methane hydrate formation within the first 60 min period from the start of experiment. After that period of time, the hydrate formation rate is still very low with respect to the experiment carried out with pure water and methane gas. As a result, PEO is a promising inhibitor candidate of methane hydrate formation. 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. EF0200222