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Energy & Fuels 2005, 19, 584-590
Hydrate Plug Prevention by Quaternary Ammonium Salts Marie L. Zanota, Christophe Dicharry,* and Alain Graciaa UMR 5150, Laboratoire des Fluides Complexes, Universite´ de Pau et des Pays de l’Adour, BP 1155, 64013 Pau Cedex, France Received July 21, 2004. Revised Manuscript Received December 13, 2004
The use of low-dosage inhibitors is an alternative to thermodynamic inhibitors to prevent gas hydrates from plugging oil production pipelines. In this work, quaternary ammonium salts (QAs) with different structures were tested as hydrate plug inhibitors on model systems containing 1/1/4/X proportions (by weight) of water/THF/oil/QA systems. The experimental results suggest that the presence of both small (CH3) groups in their polar moiety and two long alkyl chains in their hydrophobic part has a beneficial effect on their ability to adsorb onto the hydrate surface and form a steric barrier around the hydrate crystals, which limits their agglomeration to larger masses. Above a minimum concentration, the concentration of the double-tailed QAs has no appreciable effect on their ability to prevent hydrates from plugging. Their effectiveness as hydrate plug inhibitors is not dependent on the chain length of the oil.
Introduction The thermodynamic conditions prevailing in gas and condensate pipelines may be favorable to the formation of crystalline inclusion compounds (the so-called “gas hydrates”). These compounds result from the ability of water molecules to form lattice structures through hydrogen bonding, which are stabilized by guest molecules (e.g., CH4, C3H8) under high pressure and low temperature.1 Their agglomeration in pipelines may form plugs and cause production shutdowns. The conventional treatment for the prevention of hydrate plug formation is based on the use of thermodynamic inhibitors such as methanol and ethylene glycol, which shift the hydrate equilibrium conditions to lower temperature and higher pressure.2,3 The concentration required for these inhibitors to be effective is 10-40 wt % of the water mass. Such a large amount of additives increases the cost of exploration and requires the reprocessing of wastewater. The oil and gas industry has responded to both the economic and environmental concerns by identifying alternative low-dosage hydrate inhibitors, which are effective for concentrations of 0.3-3 wt % of the water mass. The low-dosage hydrate inhibitors can be divided in two groups: kinetic inhibitors and antiagglomerants. Kinetic inhibitors are usually polymers with surfactant properties; they do not change dissociation conditions but do delay the formation of nuclei and decrease the hydrate formation rate.4-6 Anti-agglom* Author to whom correspondence should be addressed. Tel.: + 33 5 59 40 76 82. E-mail address:
[email protected]. (1) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd Edition, revised and expanded; Marcel Dekker: New York, 1998. (2) Behar, E.; Delion, A. S.; Durand, J. P. Rev. Inst. Fr. Pet. 1995, 50 (5), 611-625. (3) Dholabhai, D.; Parent, J. S.; Bishnoi, P. R. Fluid Phase Equilib. 1997, 141 (1-2), 235-246. (4) Christiansen, R. L.; Sloan, E. D. International Conference on Natural Gas Hydrates. Ann. N. Y. Acad. Sci. 1994, 283-305.
erants are surfactants that often form stable water-inoil emulsions; although they do not prevent hydrate formation, they do prevent hydrate particles from agglomerating to larger masses, which can plug pipelines.7-9 Therefore, hydrates are transported as a dispersion of particles in a hydrocarbon phase. Phillips and Kelland10 have mentioned that quaternary ammonium salts (QAs) are good candidates as anti-agglomerants, because their positive charge enables them to adsorb onto the negatively charged surface of a hydrate. Arjmandi et al.11 and Karaaslan and Parlaktuna,12 on the other hand, have reported that QAs may increase the induction time and/or decrease the hydrate crystal growth rate. In other words, these additives may also have a kinetic inhibition quality. The main purpose of this work was to investigate QAs that may act as hydrate plug inhibitors. The influence of their structure on both their kinetic inhibition and anti-agglomerant abilities was studied by measuring the hydrate formation/dissociation conditions in model water(5) Lederhos, J. P.; Long, J. P.; Sum, A.; Christiansen, R. L.; Sloan, E. D. Chem. Eng. Sci. 1996, 51 (8), 1221-1229. (6) Jussaume, L. Etude d’Inhibiteurs Cine´tiques d’Hydrates de Gaz: Me´thode Expe´rimentale et Mode´lisation Nume´rique, Ph.D. Thesis, Institut National Polytechnique de Toulouse, Toulouse, France, 1999. (7) Kelland, M. A.; Svartaas, T. M.; Dybvik, L. A. In Proceedings of the SPE 69th Annual Technical Conference and Exhibition, New Orleans, LA, September 25-28, 1994, SPE Paper No. 28506, pp 431438. (8) Urdahl, O.; Lund, A.; Mørk, P.; Nilsen, T. N. Chem. Eng. Sci. 1995, 50 (5), 863-870. (9) Huo, Z.; Freer, E.; Lamar, M.; Sannigrahi, B.; Knauss, D. M.; Sloan, E. D. Chem. Eng. Sci. 2001, 56 (17), 4979-4991. (10) Phillips, N. J.; Kelland, M. A. Industrial Applications of Surfactants IV; The Royal Society of Chemistry, Cambridge, U.K., 1999; pp 244-285. (11) Arjmandi, M.; Ren, S. R.; Yang, J.; Tohidi, B. In Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, Japan, May 19-23, 2002, pp 516-519. (12) Karaaslan, U.; Parlaktuna, M. Energy Fuels 2000, 14 (5), 11031107.
10.1021/ef040064l CCC: $30.25 © 2005 American Chemical Society Published on Web 02/05/2005
Hydrate Plug Prevention by Quaternary Ammonium Salts
Figure 1. Experimental setup.
in-oil emulsions and observing the agglomeration state after hydrate formation. Tetrahydrofuran (THF) was used as the guest molecule. THF forms structure II hydrates, which are of major interest to the oil industry, because most pipeline hydrates are structure II hydrates. Moreover, THF has the advantage of forming hydrates at atmospheric pressure;13,14 however, contrary to natural gas, it may partition between organic and aqueous phases significantly.15 The solutions to be tested contained 1/1/4/0.015 proportions (by weight) of water/THF/oil/QA systems. In the first part of the work, 2,2,4-trimethyl pentane was used as the oil phase. Finally, the most effective QAs, as hydrate plug inhibitors, was tested at different concentrations and then with different linear alkyl oils.
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Figure 2. Method for determining the crystallization temperature (Tc), the time for crystallization (tc), and the dissociation temperature (Td) during a cooling/heating cycle. Table 1. Candidates from Quaternary Ammonium Salt Family
Experimental Section Apparatus. The experimental setup consisted of 12 test tubes mounted in a stainless-steel rack that was rocked at an angle of 150° on one side every 5 s by a motor (Figure 1). The test tubes had an internal diameter of 1 cm and a length of 10 cm. Each test tube contained a stainless-steel cylinder with a diameter of 0.8 cm, a height of 1.7 mm, and a weight of ∼7 g, and the cylinder moved as the tube was rocked back and forth. The rack and the samples were submerged into a bath whose temperature was controlled by a Huber Cryothermostat. Each test tube was fitted with a thermocouple (with an accuracy of ((0.2 °C) that was connected to a data acquisition system (Hewlett-Packard model HP 34970A). The temperature was recorded every 20 s throughout this study. A personal computer was used to process the data. Chemicals. The reagents used in the first part of the experiments were as follows: deionized water, produced by the Millipore Milli-Q 185 E system (conductivity of 24d
6.1 4 0.5
4.1 8.5 0.05
a ∆T ) T - T . b Concentration in the system: 0.25 wt %. c Time for separation of 60 vol % of the initial aqueous phase. d c separated after 24 h.
Figure 3. Tetrahydrofuran/water (THF/H2O) hydrate equilibrium diagram at atmospheric pressure.
d
No water
with both the (rocking) agitation of the sample and the presence of a stainless-steel cylinder, which constituted a preferred site for nucleation, the system was less stochastic. As a result, the crystallization temperature Tc for the first cycle did not differ significantly from that of the other cycles. The width of the crystallization peak determined the time for crystallization, tc. The uncertainty in tc was approximately (1 min. The change in the temperature profile that occurred during the heating period made it possible to detect the dissociation temperature, Td. This temperature was located at the point of the diagram where the slope of the temperature profile changed to meet the temperature of the bath (see Figure 2). This rupture in the slope was assumed to be located at the temperature at which almost all the amount of hydrate has melted. Because it was much more difficult to locate the melting than the formation of hydrate in the temperature curve, Td and Tc were determined within accuracies of (0.5 °C and (0.2 °C, respectively. The THF/H2O hydrate equilibrium diagram obtained with this method for the systems that contained 8-50 wt % THF is consistent with those obtained by Erva13 and Hanley et al.14 (Figure 3). In what follows, the crystallization temperature (Tc), the time for crystallization (tc), and the dissociation temperature (Td) were averaged for 12 experiments (four cycles for three samples with the same composition) and the “agglomeration state" resulted from the visual observation of three samples with the same composition.
Results and Discussion
Figure 4. Partitioning of THF between water and 2,2,4trimethyl pentane: influence of the temperature on the concentration of THF in the aqueous phase. System: 1/1/4/0 proportion (by weight) of water/THF/oil/QAs. follows. Each test tube was filled with ∼8 mL of solution (1/ 1/4/0.015 proportions (by weight) of water/THF/oil/QAs) and agitated by hand to homogenize the system. The stability of the emulsion formed at this stage was determined by transferring the sample to a graduated cylinder and measuring the time for separation of 60 vol % of the initial aqueous phase. As shown in Table 2, the systems without QAs or those containing S50, DA50, or MS50 separated rapidly to two distinct phases. The most-stable emulsions were obtained for the systems that contained M2SH1; no water was separated after 24 h. After submerging the rack with the test tubes into the temperature-controlled bath, the system underwent four cooling-heating cycles. The initial temperature of each cycle was fixed at +7 °C. The samples were kept at this temperature for 10 min and then the temperature was decreased to -7 °C at a cooling rate of 4.7 °C/h. The system was kept at -7 °C for 10 min and then heated to +7 °C at a heating rate of 14 °C/h. Because crystallization is an exothermic phenomenon, hydrate formation was detected by the sharp increase of the sample temperature during the cooling period (see Figure 2). Hydrate nucleation is known to be a stochastic phenomenon. However,
Partitioning of THF between the Organic and Aqueous Phases. Figure 4 shows that the concentration of THF in the aqueous phase increased from ∼9.5 wt % to 15.5 wt % when the temperature decreased from 20 °C to -5 °C (the samples contained a solid phase (hydrate + ice) for temperatures of MS50 > DA50 > S50 ≈ M2SH1. This classification suggests that the presence of small groups (CH3 for M2C and MS50) instead of larger groups (CH2C6H5 for DA50 and S50) in the polar moiety of the molecule had a beneficial effect on the adsorption of the molecule to the hydrate surface. A possible explanation of this behavior is that the CH3 group acts as a pseudoguest within the partially completed cavities at the hydrate surface and the nitrogen of the QAs molecule bonds to the hydrate at the top of the partial cavities. Therefore, the QAs with a polar moiety that contains CH3 groups only could establish van der Waals interactions (between the CH3 and the vacant open cages) and electrostatic interactions (between the nitrogen and an oxygen at the top of the partial cavity) with the hydrate surface more easily than those which contain a CH2C6H5 group. The reason M2SH1 was less effective than M2C, although they had similar polar moieties, is that, because of its lesser solubility in water, a lesser amount of M2SH1, compared to M2C, was available to adsorb onto the surface of the propagating hydrate crystals in the aqueous phase. Except for M2C, the kinetic inhibition quality of the QAs is rather low, compared to that of conventional kinetic inhibitors, such as PVCap, for which a supercooling up to 12 °C may be observed.1 The large measurement uncertainty for tc ((1 min) did not allow us to show clearly any influence of the QAs structure on the hydrate crystal growth. The shorter time for crystallization observed for M2C resulted from the fact that hydrate formation occurred at a higher supercooling; i.e., the driving force was higher for this system than for the others. The Anti-agglomerant Ability of QAs. The cylinders in the samples without QAs or with MS50 were blocked from traveling. Those in the systems that contained DA50 and S50 were not blocked; however, the presence of large clusters of hydrate crystals prevented them from moving throughout the test tube. The shutdown of the rocking agitation led to rapid sedimentation and agglomeration of the clusters at this temperature. The cylinders in the systems that contained M2C and M2SH1 could move throughout the test tubes, and continuous rocking could prevent the hydrate crystals from agglomerating at least 2 days, at T ) -7 °C (which corresponds to a supercooling of ∆T* ) Td - T ) 10.8 °C). At this temperature, it was possible to redisperse the “agglomerated” hydrate crystals up to 10 min after the shutdown of the rocking agitation for the system that contained M2C and at least 30 min after the shutdown of the rocking agitation for those which contained M2SH1. Effective anti-agglomerants have been determined to possess both a polar group, which adsorbs strongly to (22) Clausse, M. Encyclopedia of Emulsion Technology. Vol. 1; Marcel Dekker: New York, 1983; pp 481-715.
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Figure 7. Influence of the M2SH1 concentration on the crystallization temperature (Tc), the time for crystallization (tc), and the dissociation temperature (Td). The symbols refer to the state of the samples observed just before the latest heating period, at -7 °C: ([) hydrate plug, (2) presence of large clusters of hydrate crystals, and (]) no cluster of hydrate crystals.
the hydrate surface, and an hydrophobic group, which separates the hydrates particles by “oil-coating” and dispersing them into the oil phase. As mentioned previously, the QAs with CH3 groups in their polar moiety are likely to adsorb more strongly to the hydrate surface than those containing a CH2C6H5 group. Because of their higher affinity for the aqueous phase than for the oil phase, the single-tailed S50, DA50, and MS50 showed poor or no anti-agglomerant quality. As a result, regardless of the structure of the polar group, it was not possible to show clearly any influence of the chain length of the hydrophobic group for these QAs. Nevertheless, the results obtained for the double-tailed M2C and M2SH1 showed that the anti-agglomerant ability of QAs increased with the chain length of the hydrophobic groups. The comparison of the results obtained for the single-tailed MS50 and the double-tailed M2SH1 showed that the presence of two alkyl chains also had a beneficial effect on the anti-agglomerant ability of the QAs, suggesting that the steric property of the hydrophobic part of the adsorbed QAs is an important factor for obtaining good anti-agglomerant property. It is also worth noting that M2SH1 and M2C gave the moststable water-in-oil emulsions, whereas the other QAs could keep the systems poorly emulsified only with continuous mixing. The Influence of the Concentration of QAs. We investigated the influence of the concentration of M2SH1 on its hydrate plug inhibition ability (Figure 7 and Table 3). The presence of M2SH1 increased both the ∆T value and the tc value, compared to the same parameters determined for the case without M2SH1. However, the increase of the concentration of M2SH1 did not change ∆T and tc significantly. Visual observations of the state of the samples at -7 °C showed that the cylinders could move throughout the test tubes for M2SH1 concentrations of >0.25 wt %. At 0.13 wt % M2SH1, the cylinders could not move throughout the test tubes, and continuous rocking could not prevent the hydrate crystals from agglomerating. The reason M2SH1 failed at this concentration may result from the formation of less-stable emulsions (see Table 3). The Influence of the Oil Phase. 2,2,4-Trimethyl pentane was replaced by different linear alkyl oils for a M2SH1 concentration that was fixed at 0.25 wt % (see
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Table 3. Influence of the Concentration of M2SH1 on Both the Supercooling for Hydrate Formationa and the Time for Crystallization, and Stability of the Water-in-Oil Emulsions Formed by Manual Agitation of the Water/THF/Oil/M2SH1 Systems, at T ) 20 °C Value parameter
0 wt % M2SH1
0.13 wt % M2SH1
0.25 wt % M2SH1
0.50 wt % M2SH1
1.50 wt % M2SH1
supercooling, ∆T (°C) time for crystallization, tc (min) emulsion stabilityb (h)
2.3 6 0
3.7 9 24
3.3 7 >24c
3.4 7 >24c
3.3 9.5 >24c
a ∆T
) Td - Tc. b Time for separation of 60 vol % of the initial aqueous phase. c No water separated after 24 h.
Table 4. Influence of the Oil on Both the Supercooling for Hydrate Formationa and the Time for Crystallization, and Stability of the Water-in-Oil Emulsions Formed by Manual Agitation of the Water/THF/Oil/M2SH1 Systems, at T ) 20 °C Value parameter
C12 oil
C10 oil
C9 oil
C8 oil
C7 oil
C6 oil
supercooling, ∆T (°C) time for crystallization, tc (min) emulsion stabilityb (h)
2.8 8 9
3.5 6.5 10
4.1 10 14
4.5 8.5 20
5 8 >24c
5.7 9 >24c
a ∆T
) Td - Tc. b Time for separation of 60 vol % of the initial aqueous phase. c No water separated after 24 h.
Figure 10. Microscopic observation of the emulsions: (a) water/THF/C12 oil/M2SH1 system and (b) water/THF/C8 oil/ M2SH1 system. Picture size is 675 µm × 490 µm.
Figure 8. Influence of the oil phase on the crystallization temperature (Tc), the time for crystallization (tc), and the dissociation temperature (Td). System: 1/1/4/0.15 proportion (by weight) of water/THF/oil/M2SH1. The symbols refer to the state of the samples observed just before the latest heating period, at -7 °C: ([) hydrate plug, (2) presence of large clusters of hydrate crystals, and (]) no cluster of hydrate crystals.
Figure 9. Concentration of THF in the aqueous phase resulting from its partitioning between water and oil at 20 °C. System: 1/1/4/0 proportion (by weight) of water/THF/oil/ QAs.
Figure 8 and Table 4). The shorter the alkane, the higher the supercooling for hydrate formation. Figure 9 shows that the concentration of THF in the aqueous phase, obtained from the measurements of the refractive index of the aqueous phase for the systems with the different oils and without QAs at 20 °C, decreased as the alkane carbon number decreased. It is known that the decrease of the oil-phase chain length and/or the
addition of a polar oil (here, THF) increases the interactions between the surfactant molecules and the oil phase.23 Under these conditions, M2SH1 was expected to give lower supercooling for the systems containing the shortest alkanes, because the higher the affinity of the surfactant for the oil phase, the lower the amount of the surfactant available in the water phase to adsorb onto the surface of the propagating hydrate crystals. Visual observations of samples emulsified under gentle agitation showed that the stability of the water-in-oil emulsions increased as the length of the oil decreased (Table 4), suggesting the presence of smaller water droplets. This was confirmed by microscopic observations of the water droplets for the systems containing the C12 oil and the C8 oil, respectively (Figure 10a and b). For the systems that contained the C7 or C6 oils, the size of the water droplets was submicrometric and their observation with a simple optical microscope was not possible. Consequently, the higher degree of supercooling observed for the systems that contained the shortest alkanes was probably due to the presence of smaller water droplets. The time for crystallization (tc) was not dependent significantly on the length of the alkane (see Table 4). The fact that the C6 and C12 systems had approximately the same tc value, although the supercooling of the former was twice as great as that of the latter, suggests that M2SH1 decreased the hydrate crystal growth rate. Visual observations of the state of the samples at -7 °C showed that the cylinders could move throughout the (23) Bourrel, M.; Chambu, C. Presented at the 3rd Joint Symposium, SPE/DOE on Enhanced Oil Recovery, Tulsa, OK, 1982, SPE Paper No. 10676.
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test tubes, for all oils tested. Therefore, the effectiveness of M2SH1 as an anti-agglomerant was not dependent on the length of the (linear alkyl) oil. Conclusion All the quaternary ammonium salts (QAs) tested in this work showed some kinetic inhibition and/or antiagglomerant qualities. The most-effective QAs (M2C and M2SH1), in regard to preventing hydrate plug formation, were double-tailed and contained small (CH3) groups in their polar moiety. Their higher effectiveness was probably due to their higher ability both to adsorb to the propagating hydrate crystals and to form a steric barrier around the hydrate crystals. A series of experi-
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ments with M2SH1 were conducted to investigate the influence of the QA concentration and the nature of oil on its effectiveness as an hydrate plug inhibitor. Although M2SH1 failed below a minimum concentration (0.25 wt %), no beneficial effect was observed if it was used at a higher concentration. M2SH1 was effective for all the linear alkyl oils tested in this work. Although M2SH1 worked well on model systems, further experiments, such as the use of a natural gas instead of THF as a guest molecule and the use of a crude oil instead of pure alkanes, are necessary to know if M2SH1 can be applied to field production. EF040064L
