Fluorinated Quaternary Ammonium Bromides: Studies on Their

Aug 16, 2013 - Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger,. Norway. â...
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Fluorinated Quaternary Ammonium Bromides: Studies on Their Tetrahydrofuran Hydrate Crystal Growth Inhibition and as Synergists with Polyvinylcaprolactam Kinetic Gas Hydrate Inhibitor Mohamed F. Mady†,‡ and Malcolm A. Kelland*,† †

Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ Department of Green Chemistry, National Research Centre, Dokki, Cairo 12622, Egypt ABSTRACT: Fluorinated low-dosage hydrate inhibitors (LDHIs) have been investigated for the first time. Three novel quaternary ammonium salts with monofluorobutyl groups were synthesized and characterized, N-(4-fluorobutyl)-N,Ndipentylpentan-1-aminium bromide (2), N,N-bis(4-fluorobutyl)-N-pentylpentan-1-aminium bromide (3), and N,N,N-tris(4fluorobutyl)pentan-1-aminium bromide (4). It was investigated whether these fluorinated ammonium salts, in which a fluorine atom replacing an end methyl group in the alkyl chains of tetra(n-pentyl)ammonium bromide (TPAB), would affect tetrahydrofuran (THF) hydrate crystal growth inhibition. We determined that the monofluoro ammonium salt N-(4fluorobutyl)-N,N-dipentylpentan-1-aminium bromide (2) was the best THF hydrate crystal growth inhibitor, showing better performance than either TPAB or tetra(n-butyl)ammonium bromide (TBAB). However, THF hydrate crystal growth inhibition decreased as the number of fluorobutyl groups on the quaternary nitrogen atom was increased to two or three. In addition, we observed that the monofluoro ammonium salt (2) was a better kinetic hydrate inhibitor synergist than TPAB in combination with poly(N-vinyl caprolactam) (PVCap) under high-pressure rocking cell tests using a structure-II-forming natural gas mixture. The difluoro and trifluoro salts (3 and 4) were poorer synergists in keeping with the trend from the THF hydrate crystal growth experiments. A mechanism to explain these results is proposed. 1).13,14 The application of quaternary ammonium salts (QAs) in the offshore field to inhibit the gas hydrate was divided into two

1. INTRODUCTION Gas hydrates are clathrates in which water molecules form a hydrogen-bonded network enclosing roughly spherical cavities that are filled with gas molecules.1,2 The guest molecules are necessary to support the cavities in the water lattice.3 The problem of hydrate formation is critical in pipelines transporting natural gas and liquid hydrocarbons. This is because gas hydrates are solids and will leave deposits in the line. The solid deposits reduce the effective diameter of the line and can therefore restrict the flow of fluids.4 It is well-known that low-dosage hydrate inhibitors (LDHIs) play an efficient role to prevent gas hydrates from plugging pipelines.5,6 Kinetic hydrate inhibitors (KHIs) are one type of LDHIs that have been applied successfully in the upstream oil and gas production industry for almost 2 decades.7 Probably the most well-applied and studied class of KHIs are N-vinyl lactam polymers, such as poly(N-vinylcaprolactam).5 Some onium salts (quaternary ammonium, phosphonium, and sulfonium salts) have found applications in the development of KHI formulations.8−10 At relatively low concentrations, the onium salts interfere with the growth of hydrate crystals11 and, therefore, are useful in KHI formulations, preventing the growth of structure II (SII) clathrate hydrates. In 1995, the Klomp group12 reported independently the effect of quaternary ammonium salts to inhibit the growth of SII tetrahydrofuran (THF) hydrates, in which the most powerful inhibitors of THF hydrates were found on n-butyl, n-pentyl, or isopentyl groups linked to the quaternary nitrogen atom.8−10 Tetrabutylammonium bromide is an important quaternary ammonium salt for THF hydrate growth inhibition (Figure © 2013 American Chemical Society

Figure 1. (Left) TBAB and (right) TPAB.

classes: one is synergists5,15−20 in KHI formulations, and the other is surfactant anti-agglomerants (AAs).21−26 For example, tetra(n-butyl)ammonium bromide (TBAB) has been used as an efficient synergist for N-vinyl caprolactam polymers and became used in the first offshore field applications of KHIs on southern North Sea gas fields.27 Tetra(n-pentyl)ammonium bromide (TPAB) is a more efficient KHI synergist than TBAB (Figure 1).30 In more recent years, our group has continued with efforts toward synthesis and study of potentially new types of KHI synergists, such as hexaalkylated guanidinium salts,28 trisReceived: July 9, 2013 Revised: August 13, 2013 Published: August 16, 2013 5175

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Figure 2. Some of the promising onium salts synthesized by our research group used as synergists for KHIs.

(dialkylamino)cyclopropenium chlorides,29 tetra(isohexyl)ammonium bromide,30 and tetraalkylphosphonium salts.31 These are powerful THF hydrate crystal growth inhibitors and KHI synergists (Figure 2). It occurred to us that fluorocarbons are capable of forming gas hydrates at temperatures higher than that of hydrocarbons. For example, at any given pressure, fluoromethane hydrate has a higher equilibrium temperature than both methane structure I hydrate or a typical hydrocarbon gas mixture forming SII gas hydrate.32 This is illustrated in Figure 3 using hydrate equilibrium

Table 1. Composition of Synthetic Natural Gas (SNG) component

mol %

methane ethane propane isobutane n-butane N2 CO2

80.67 10.20 4.90 1.53 0.76 0.10 1.84

To explore this idea, we set out to synthesize a series of ammonium salts with fluoromethyl end groups and compare their hydrate crystal growth inhibition properties to known nonfluorinated tetraaalkylammonium salts. The novel mono-, di-, and trifluoro quaternary ammonium derivatives were evaluated for their ability to inhibit THF hydrate crystal growth and as synergists for the KHI poly(N-vinyl caprolactam) (PVCap).

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were purchased from VWR, Nippon Chemical Industrial Co., Ltd., Tokyo Chemical Industry Co., Ltd., and Sigma-Aldrich. PVCap (Mw = 2000−4000 Da) was kindly supplied as 41.1 wt % Luvicap EG in monoethyleneglycol by BASF (Germany). The synthesis of the quaternary ammonium compounds was carried out by standard quaternization procedures from amines (with added base) using alkyl bromides in a polar solvent.28−31 Nuclear magnetic resonance (NMR) spectra were recorded on a 300 MHz Varian NMR spectrometer in CDCl3 using tetramethylsilane (TMS) as an internal standard. 2.2. Synthesis of Fluoroammonium Salts. 2.2.1. Synthesis of Monofluoro Quaternary Ammonium Bromide (2). In a round-bottom flask equipped with a magnetic stirring bar were placed 1.3 mol equiv of 1-bromo-4-fluorobutane and 1.0 mol equiv of tri-n-pentylamine. The reaction mixture was refluxed in 25 mL of isobutyronitrile for 4 days. The solvent was removed under vacuum to leave an off-white solid (60% yield). Crystals of N-(4-fluorobutyl)-N,N-dipentylpentan-1-aminium bromide (2) were grown from an ethyl acetate solution cooled to −30 °C over 24 h. NMR spectroscopy data were recorded on a 300 MHz Varian instrument. 1H NMR (CDCl3) δ: 0.92 (triplet, 9H), 1.39 (multiplet, 6H), 1.71 (multiplet, 6H), 1.93 (multiplet, 2H), 2.29 (multiplet, 8H),

Figure 3. Hydrate equilibrium curves for methane (CH4) (calculated), fluoromethane (CH3F) (experimental), and a synthetic natural gas mixture (UiS SII SNG, composition described in Table 1) (calculated).

curves for methane (CH4) (calculated), fluoromethane (CH3F) (experimental data32), and the calculated curve for our synthetic natural gas mixture used in this study (UiS SII SNG, a synthetic natural gas mixture described in Table 1). The higher equilibrium temperatures for fluoromethane suggest that it has a stronger van der Waals interaction with the hydrate cavities than methane or a natural gas mixture. Therefore, we wondered if chemicals with fluoromethyl groups would be more powerful hydrate crystal growth inhibitors than chemicals with just methyl groups. 5176

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Figure 4. Synthetic routes for the preparation of fluoroammonium salts. 3.34 (triplet, 8H), 4.48, 4.64 (triplet, 2H, CH2F). 13C NMR (CDCl3) δ: 13.82, 19.12, 21.99, 22.23, 23.16, 28.45, 58.96, 59.32, 81.73, 84.45. 2.2.2. Synthesis of Difluoro Quaternary Ammonium Bromide (3). In a round-bottom flask equipped with a magnetic stirring bar were placed 2.1 mol equiv of 1-bromo-4-fluorobutane, 2.0 mol equiv of powdered K2CO3, and 1.0 mol equiv of di-n-pentylamine. The reaction mixture was stirred under reflux in 25 mL of isobutyronitrile for 4 days. The solution was filtered, and the solvent was removed under vacuum to leave an off-white solid (51% yield). Crystals of N,N-bis(4-fluorobutyl)N-pentylpentan-1-aminium bromide (3) were grown from an ethyl acetate solution cooled to −30 °C over 24 h. 1 H NMR (CDCl3) δ: 0.86 (triplet, 6H), 1.33 (multiplet, 4H), 1.64 (multiplet, 4H), 2.24 (multiplet, 4H), 3.36 (multiplet, 8H), 3.79 (triplet, 8H), 4.43, 4.58 (triplet, 4H, CH2F). 13C NMR (CDCl3) δ: 13.73, 19.63, 21.79, 22.11, 23.15, 28.32, 59.41, 62.90, 81.75, 84.72. 2.2.3. Synthesis of Trifluoro Quaternary Ammonium Bromide (4). A total of 3.3 mol equiv of 1-bromo-4-fluorobutane, 3.0 mol equiv of powdered K2CO3, and 1.0 mol equiv of pentylamine were refluxed in 25 mL of isobutyronitrile for 4 days on a 50 mL round flask. The solution was filtered, and solvent was removed under vacuum to leave an offwhite solid (70% yield). Crystals of N,N,N-tris(4-fluorobutyl)pentan-1aminium bromide (4) were grown from an ethyl acetate solution cooled to −30 °C over 24 h. 1 H NMR (CDCl3) δ: 0.80 (triplet, 3H), 1.26 (multiplet, 2H), 1.71 (multiplet, 2H), 2.17 (multiplet, 6H), 3.37 (multiplet, 8H), 3.70 (triplet, 8H), 4.35, 4.50 (triplet, 6H, CH2F). 13C NMR (CDCl3) δ: 13.57, 19.74, 21.79, 22.95, 29.97, 28.15, 59.54, 62.89, 81.82, 84.01. The route for synthesis of fluoroammonium salts is shown in Figure 4. 2.3. THF Hydrate Crystal Growth Experimental Method. A mixture of THF and 3.5 wt % sodium chloride was used as a hydrateforming solution. The SII THF hydrate can be formed at about 4.4 °C under atmospheric pressure. The method that we used for studying the inhibition of THF hydrate crystal growth has been illustrated previously.6,8,12,33−41 NaCl (26.28 g) and THF (99.9%, 170 g) are mixed, and deionized water is added to give a final volume of 900 mL. This gives a stoichiometrically correct molar composition for making SII THF hydrate, THF·17H2O. With this added salt, the equilibrium temperature for THF hydrate formation is approximately 3.3 °C. The test procedure is as follows: (1) A total of 80 mL of the aqueous THF/NaCl solution is placed in a 100 mL glass beaker. (2) The test chemical is dissolved in this solution to give the desired concentration; for example, 0.32 g of additive in 80 mL of solution gives a 0.4 wt % (4000 ppm) solution of the additive. (3) The beaker is placed in a stirred cooling bath preset to a set temperature, e.g., −0.5 °C (±0.05 °C), which represents about 3.8 °C subcooling. (4) The solution is briefly stirred manually with a glass rod

every 5 min, without touching the glass beaker walls, while being cooled for 20 min. (5) A hollow glass tube with an inner diameter of 3 mm was filled at the end with ice crystals kept at −10 °C. The ice crystals are used to initiate THF hydrate formation. (6) The glass tube was placed almost halfway down in the cooled additive/THF/NaCl solution after the solution had been cooled for 20 min. (7) THF hydrate crystals were allowed to grow at the end of the glass tube for 60 min. (8) After this time, the glass tube was removed and the amount of THF hydrate crystal formed at the tip was weighed. The shape and morphology of the crystals both in the beaker (if any) and on the end of the glass tube were also visually analyzed and recorded. Sometimes crystals grew to the edge of the beaker and, therefore, could not be weighed properly.30 The scattering in amount of crystal growth of over 1 h was a maximum of 20−25%. If no salt is added to the mixture, THF hydrate formation becomes too fast because of a high subcooling. This makes it hard to rank inhibitors. At temperatures above 0 °C, the ice in the glass tube (used to initiate THF hydrate formation) would melt and you would obtain no THF hydrate. With added salt, it makes it possible to carry out tests at a suitable subcooling and without melting the ice in the tubes. 2.4. High-Pressure Gas Hydrate Rocker Rig Equipment Test Methods. Kinetic hydrate inhibition tests described herein were carried out by a constant cooling test method. Tests were conducted in five high-pressure 40 mL steel rocking cells each containing a steel ball, which is shown in Figure 5. The equipment was manufactured by PSL Systemtechnikk, Germany. The gas composition used was a SNG mixture given in Table 1. Details of the test procedure are reported below. At the start of each constant cooling experiment, the pressure was approximately 76 bar. The equilibrium temperature (Teq) at this pressure was determined by standard laboratory dissociation experiments warming at 0.025 °C/h for the last 3−4 °C.42,43 Five repeat tests were carried out, which gave 20.2 ± 0.05 °C as Teq. This value agrees very well with a calculated Teq value of 20.5 °C at 76 bar using Calsep’s PVTSim software. The constant cooling test procedure was as follows: (1) Each cell was filled with 20 mL of distilled water, in which the various additives had been dissolved. (2) Air in the cells was removed with a combination of vacuum pumping and filling with SNG to 2 bar and then repeating the procedure. (3) The cell was pressurized to 76 bar and rocked at 20 rocks per minute at an angle of 40°. (4) The cells were cooled from 20.5 °C at a rate of 1 °C/h to 2 °C. If rapid hydrate formation has not occurred during this time, as judged by a large fast pressure drop, the temperature was held at 2 °C until it had occurred. (5) The pressure and temperature for each individual cell as well as the cooling bath were logged on a computer. 5177

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did not observe any bias between the five cells; i.e., no one cell gave consistently worse or better results than the others.

3. RESULTS AND DISCUSSION 3.1. THF Hydrate Crystal Growth Experiments. The synthesis of fluoroammonium salts (2, 3, and 4) involved the reaction between primary, secondary, and tertiary n-pentylated amines with 1-bromo-4-fluorobutane (1) under reflux for 4 days. The salts were obtained as white crystals with yields ranging from 51 to 70%. The results of the THF hydrate crystal growth inhibition test are listed in Table 2. Six tests on each chemical were conducted at −0.5 °C over 1 h, and the growth rate over this time was determined. The scattering was not more than 20−25% in any set of tests for any one additive. According to the Shell Oil Company hydrate research group findings, we have used the same crystal morphology descriptions of the shape of the THF hydrate crystals.8 “Regular pyramidal crystals” (RP) as obtained with no additive occurred often, especially with poor inhibitors or at low concentrations of inhibitor. “Rounded edges” (RE) on the crystals were seen as a sign of some effect on perturbing the normal THF hydrate crystal morphology. “Crumpled sheet of paper” (CS), is an extreme form of the “rounded edges” result, where no clear crystal structure can be defined. This occurred only for a few very good crystal growth inhibitors, such as quaternary ammonium salts with several butyl or pentyl groups, e.g., TBAB and TPAB. We tested three novel synthesized additives for THF hydrate crystal growth inhibition compared to known tetraalkylammonium salts, TBAB and TPAB. As outlined in Table 2, the results of THF hydrate crystal growth inhibition tests shows that the most effective chemical is the monofluoro derivative N-(4-fluorobutyl)-N,N-dipentylpentan-1-aminium bromide (2). The growth rate and crystal morphology was similar to that found for the quaternary ammonium salt TPAB and more efficient than TBAB. It is not

Figure 5. Rocker rig showing the five steel cells in a cooling bath. A graphical representation of results from five rocking cells with a KHI blend is shown in Figure 6. The pressure drops about 2 bar because of gas being dissolved in the aqueous phase. The temperature drops at a constant rate until the minimum of 2 °C after 1120 min. According to the graph, the pressure decreases constantly because of the decrease in the temperature. The first deviation from the pressure drop because of the temperature drop is taken as the time for onset of hydrate formation, although nucleation on an undetectable scale may have occurred earlier. The onset temperature, To, at this time is determined. In Figure 6, To is in the range of 3.8−5.2 °C (this gives an average for To of 4.5 °C, which gives a spread of ±15%, which is the maximum spread that we observed in this and other similar constant cooling studies). At some point later during the experiment, the rate of hydrate formation will reach a maximum. The temperature at which this happens is known as the rapid hydrate formation temperature, Ta. As shown in Figure 7, the graph explained the onset temperature (To) and the temperature at which fast hydrate formation is observed (Ta). We ran 10 tests for each synergist to obtain enough data to give a statistically significant ranking of the performance of the chemicals. We

Figure 6. Pressure and temperature data obtained from five parallel constant cooling tests with the same KHI blend in the multi-cell rocker rig. 5178

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Figure 7. Example of To and Ta calculations in a constant cooling rocker rig KHI experiment.

bromide (3) and decreases further with three fluorine atoms in N,N,N-tris(4-fluorobutyl)pentan-1-aminium bromide (4). For example, with 4000 ppm of difluoro salt (3) with two fluorine atoms, we obtained an average of 0.65 g of THF hydrate crystal growth after 1 h at −0.5 °C compared to almost negligible growth with the monofluoro derivative (2). This is also a worse result than with either TPAB or TBAB at the same test conditions. A possible mechanism for the lower inhibition performance of N,N-bis(4-fluorobutyl)-N- pentylpentan-1-aminium bromide (3) and N,N,N-tris(4-fluorobutyl)pentan-1-aminium bromide (4) compared to N-(4-fluorobutyl)-N,N-dipentylpentan-1-aminium bromide (2) may be according to a theory put forward for hydrate crystal growth inhibition by quaternary ammonium salts. It is known that quaternary tetraalkylammonium salts with npentyl and isohexyl groups are better THF hydrate crystal growth inhibitors than TBAB.11,12,30 These results shows that the optimum carbon chain length on the quaternary nitrogen is five carbons. The theory also suggests that two or three alkyl chains become embedded in the hydrate crystal surface, preventing it from being lost so easily from the crystal surface. The fourth alkyl group presumably protrudes from the crystal surface, disrupting further crystal growth. However, in this work, we replaced two or three methyl carbon atoms by fluorine atoms at the end of the chain. The fluorine atom is about the same size as a methyl group;

Table 2. THF Hydrate Growth Rates in g/h for Various Quaternary Ammonium Salts at Varying Concentrations and Crystal Morphologies chemical

concentration (ppm)

average growth rate (g/h)

morphologya

no additive TBAB TPAB (Pe)3[F(CH2)4]1NBr (2) (Pe)2[F(CH2)4]2NBr (3) Pe[F(CH2)4]3NBr (4) TBAB TPAB (Pe)3[F(CH2)4]1NBr (2) (Pe)2[F(CH2)4]2NBr (3) Pe[F(CH2)4]3NBr (4)

4000 4000 4000 4000 4000 2000 2000 2000 2000 2000

1.75 0.19 0.01 0.014 0.65 1.24 0.68 0.12 0. 15 1.45 1.75

RP CS CS CS CS CS RP/RE CS CS CS CS

a

RP, regular pyramidal crystals; CS, crumpled sheet of paper; and RE, rounded edges.

surprising that some of the fluorinated ammonium salts also give crumpled sheet effect because they are structurally very similar to TBAB and TPAB. Interestingly, the THF hydrate crystal growth inhibition decreases when the fluorination is increased to two fluorine atoms in N,N-bis(4-fluorobutyl)-N-pentylpentan-1-aminium

Table 3. Constant Cooling KHI Tests in the High-Pressure Multi-cell Rocker Rig chemical

concentration (ppm)

average onset temperature, To (°C)

average rapid hydrate formation temperature, Ta (°C)

no additive PVCap PVCap PVCap + TPAB PVCap + (Pe)3[F(CH2)4]1NBr (2) PVCap + (Pe)2[F(CH2)4]2NBr (3) PVCap + Pe[F(CH2)4]3NBr (4)

2500 5000 2500 + 2500 2500 + 2500 2500 + 2500 2500 + 2500

17.9 9.9 7.2 5.1 4.3 6.1 7.9

17.5 8.8 6.2 3.3 4.1 5.3 7.4

5179

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therefore, we assume a fluorobutyl group is about the same size as a pentyl group. Because the difluoro (3) and trifluoro (4) derivatives gave poorer crystal growth inhibition than either the monofluoro derivative and TBAB or TPAB, it appears that two more terminal fluorine atoms give a worse van der Waals interaction with the THF hydrate crystal surface. Because the monofluoro derivative gave a better crystal growth inhibition than TPAB or TBAB, and assuming that the mechanism is correct, this would suggest that the single fluorobutyl group is pointing away from the surface, because increasing to two or more fluorobutyl groups is detrimental to crystal growth inhibition performance. We are investigating ways to study this mechanism using computer simulations. In addition, the results suggest that substitution of one n-pentyl group in TPAB with other small groups besides the fluoro group may also be beneficial for THF hydrate crystal growth inhibition. 3.2. High-Pressure KHI Synergist Experiments. Table 3 and Figure 8 summarize the average kinetic hydrate inhibition

used. Test results using TPAB and PVCap as a reference system show that this quaternary ammonium salt is an effective synergist, in which the average To and Ta values are 5.1 and 3.3 °C, respectively. This is lower than using 5000 ppm PVCap. This result correlates well with the strong THF hydrate crystal growth inhibition observed with TPAB. However, using 2500 ppm of (Pe)3F(CH2)4NBr (2) gave clearly the lowest To values of any quaternary ammonium salt synergist investigated. The average value from 10 tests was 4.3 °C, statistically better than the results with TPAB from t tests (p value = 0.05). Interestingly, the average Ta value of 4.1 °C was higher than the value found for TPAB. This would suggest that TPAB is better at inhibiting gas hydrate crystal growth, although this does not fit with the THF hydrate crystal growth experiments. However, the low To values using (Pe)3F(CH2)4NBr (2) as a synergist may mean nucleation occurred at a lower temperature than with TPAB as a synergist. This, in turn, would mean there are more particles present of critical size with (Pe)3F(CH2)4NBr (2), which give a faster uptake of SNG and, hence, a smaller difference between To and Ta values. Both (Pe)2[F(CH2)4]2NBr (3) and Pe[F(CH2)4]3NBr (4) were tested at 2500 ppm as synergists with PVCap and showed a weaker KHI effect than the monofluorinated salt (Pe)3F(CH2)4NBr (2). For compounds 3 and 4, the average To values increased to 6.1 and 7.9 °C, respectively, and Ta values increased to 5.3 and 7.4 °C. For example, the p value in t tests between the To values for compounds 2 and 3 is 0.01, indicating a clear statistically different result. This KHI synergism data fits the trend seen from the THF hydrate crystal growth results with the three fluoroammonium salts. The high-pressure results indicate weaker synergy as the number of fluorobutyl groups is increased above 1 (Figure 9). We were also interested in testing tetra(4-

Figure 8. Average To, Ta, and To − Ta calculation in a constant cooling rocker rig KHI experiment at 2500 ppm.

performance results for three new fluoro quaternary ammonium salts used a synergists with PVCap. The KHI results were compared to a blend of PVCap with TPAB.5,8,17,19 Their performances as KHI synergists were examined using the highpressure rocker rig with five steel cells in a cooling bath. A total of 10 experiments were carried out for each of the chemical conditions listed to obtain statistically significant differences in the results. The three fluorinated ammonim salts were not tested by themselves. TBAB, TPAB, and tetraisohexylammonium bromide have previously been shown to perform very poorly as gas hydrate anti-nucleator KHIs when tested by themselves at concentrations up to 5000 ppm, and therefore, we assume the fluorinated varieties would also be poor KHIs when used alone.5,12 As shown in Table 3, with no additive, the onset of hydrate formation occurred at an average To value of 17.9 °C with rapid hydrate formation at 17.5 °C. With 2500 ppm of PVCap polymer (using Luvicap EG), the average To and Ta values dropped to 9.9 and 8.8 °C, respectively, showing good kinetic hydrate inhibition by this polymer as expected. At 5000 ppm, the performance was markedly higher, with average To and Ta values now at 7.2 and 6.2 °C, respectively. The remaining tests were conducted with non-fluorinated and fluorinated quaternary ammonium salts as synergists with PVCap. Blends of 2500 ppm of PVCap and a synergist were

Figure 9. KHI test results for fluorinated ammonium salts with PVCap.

fluorobutyl)ammonium bromide to complete the series, but the synthesis was less straightforward. The trend seen in Figure 9 suggests that this ammonium salt would be an even worse synergist than the trifluoro derivative. The statistical differences between the synergists are determined by comparing To values using t test to 10 experiments for each KHI, when the p value is less than the predetermined significance level, which is often 0.05 or 0.01, indicating that the observed result would be highly statistically significant. The results of t tests show that p values of mono- and difluoro ammoinium salts are highly statistically significant. The comparison of To for (Pe)3F(CH2)4NBr and TPAB gives a p value of 0.05; in addition, the p value of (Pe)2[F(CH2)4]2NBr compared to TPAB gives 0.01. In contrast, Pe[F(CH2)4]3NBr compared to TPAB gave the worst p value at 1.68. 5180

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4. CONCLUSION Fluorinated LDHIs have been investigated for the first time. Three novel quaternary ammonium salts were synthesized and characterized. It was investigated whether replacing a carbon atom with a fluorine atom in the alkyl chains of TPAB would affect the THF hydrate crystal growth inhibition. We determined that the monofluoro ammonium salt N-(4-fluorobutyl)-N,Ndipentylpentan-1-aminium bromide (2) was the best THF hydrate crystal growth inhibitor, showing better performance than TPAB. THF hydrate crystal growth inhibition decreased as the number of fluorobutyl groups was increased to two or three. In addition, we observed that the monofluoro ammonium salt (2) was a better synergist than TPAB in combination with PVCap under high-pressure rocking cell tests using a SII-forming natural gas mixture. The difluoro and trifluoro salts (3 and 4) were poorer synergists in keeping with the trend from the THF hydrate crystal growth experiments. We are currently investigating other fluorinated LDHIs and ways to model the interaction of the quaternary ammonium salts on hydrate surfaces using computer simulations. We are also investigating alternatives to replace one or more fluoro groups in pentylated quaternary ammonium salts to determine if one fluorobutyl is crucial to the improved hydrate inhibition performance. These studies may help confirm the hydrate crystal growth inhibition embedding mechanism for these salts outlined in this paper.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +47-51831823. Fax: +47-51831750. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ef401292m | Energy Fuels 2013, 27, 5175−5181