Performance Enhancement of N-Vinylcaprolactam-Based Kinetic

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Performance Enhancement of N‑Vinylcaprolactam-Based Kinetic Hydrate Inhibitors by Synergism with Alkylated Guanidinium Salts Carlos D. Magnusson and Malcolm A. Kelland* Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ABSTRACT: The synergistic effect of hexa-n-butylguanidinium chloride (n-Bu6GuanCl) and related compounds in different ratio mixtures with a series of three commercially available N-vinylcaprolactam-based polymer kinetic hydrate inhibitors (KHIs) against structure-II-forming synthetic natural gas hydrates has been evaluated by carrying out constant cooling and isothermal KHI screening experiments in high-pressure rocking cells. Formulations containing n-Bu6GuanCl/poly(N-vinylcaprolactam) with weight ratios of 4:1, 9:1, and 14:1 at 3000 ppm total concentrations were found to give the best gas hydrate inhibitory performances, with average onset temperatures (To) of 3.4, 3.1, and 3.3 °C, respectively. In comparison, 3000 ppm of poly(Nvinylcaprolactam) alone gave an average To of 7.7 °C. Using 9% white spirit lowers the To values of the best synergistic mixtures by 2.1 °C. However, this effect is at least partly due to shifting of the hydrate phase equilibria to a lower equilibrium temperature and higher equilibrium pressure. n-Bu6GuanCl was found to be a better synergist than n-Bu6GuanCH3COO, n-Bu6GuanHCOO, and diethyl-di-n-pentylguanidinium chloride [Et2(n-Pe)4GuanCl] when used with poly(N-vinylcaprolactam) for gas hydrate kinetic inhibition. The synergistic capacity of n-Bu6GuanCl is dependent upon the chemical structure of the N-vinylcaprolactambased polymer, although the molecular weight differences may also play a part. A mixture of 300 ppm of high-cloud-point Nvinylcaprolactam-based terpolymer with 2700 ppm of n-Bu6GuanCl lowered the average To by 8.7 °C compared to using 3000 ppm of this terpolymer alone. In long-term isothermal experiments at 8 °C subcooling and 30 bar, 5000 ppm of the synergistic mixture Bu6GuanCl/poly(N-vinylcaprolactam) in a 9:1 weight ratio showed a hold time of over 13.6 days. Long hold times were also achieved with various ratios of n-Bu6GuanCl/N-vinylcaprolactam/N-vinylpyrrolidone copolymer or n-Bu6GuanCl/Nvinylcaprolactam terpolymer mixtures.

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INTRODUCTION Hydrate plugging of flow lines during transportation of wet hydrocarbon resources is a major concern for the oil and gas industry. This is due to the high costs of plug remediation and decreased revenue from delayed or reduced production.1−3 Natural gas hydrates are crystalline clathrates where gas guest molecules, such as methane or propane, are caged in the cavities of a hydrogen-bonded framework made of water molecules. The optimal conditions for gas hydrates to form are high pressures and low temperatures. Therefore, flow lines and wells located subsea or in cold environments are particularly exposed to hydrate plugs.2 Under these conditions, natural gas originates clathrate hydrate of the structure II (SII) type. Two types of chemical treatments are used to cope with hydrate control. The first method is the use of thermodynamic inhibitors (THIs), such as methanol and ethylene glycol (MEG). They work by shifting the hydrate phase equilibria to lower equilibrium temperature and higher equilibrium pressure. However, their major disadvantages are the high concentrations, often 20−50% based on the water phase, needed for them to work. This results in high costs and high environmental impact in the case of leaks, especially for methanol. The other chemical method is the use of low-dosage hydrate inhibitors (LDHIs). LDHIs are used at much lower concentrations than THIs, usually between 0.1 and 1.0 wt % of active chemicals, and they are intended for hydrate prevention and not remediation.4 They are sub-classified into two subgroups, kinetic hydrate inhibitors (KHIs) and anti© XXXX American Chemical Society

agglomerants (AAs), in connection to their applications in the field. KHIs are based on water-soluble polymers that inhibit gas hydrate formation primarily by retarding hydrate nucleation but often also by hindering hydrate crystal growth to varying extents. Among the commercial KHI polymers are the Nvinylcaprolactam (VCap)-based polymers, as either a homopolymer (PVCap) or copolymerized with more hydrophilic monomers to raise the cloud point.1,4 To improve KHI performance, they are often combined with synergists, both polymers, and non-polymeric chemicals. Examples are homogeneous tetraalkyl quaternary ammonium and phosphonium salts carrying butyl and pentyl alkyl chains, which were some of the first synergists developed in the mid-1990s.5−7 Tetra-nbutylammonium bromide (TBAB) and tetra-n-pentylammonium bromide (TPAB) were found to be very good THF hydrate crystal growth inhibitors and excellent synergists when used with VCap-based polymers.8,9 Today, the main use of quaternary ammonium salts is as AA surfactants containing one to two long alkyl tails.7,10,11 In recent years, our research group has carried out further investigations on hydrophobic cations, such as ammonium and phosphonium cations, and their performance as THF hydrate crystal growth inhibitors and synergists for polymer KHIs. For Received: March 14, 2016 Revised: May 11, 2016

A

DOI: 10.1021/acs.energyfuels.6b00612 Energy Fuels XXXX, XXX, XXX−XXX

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instance, tetra-isohexyl ammonium bromide, tris(tert-heptyl)ammonium salts, and tetra-n-pentylphosphonium bromide have been shown to be the best THF crystal growth inhibitors of the homogeneously alkylated quaternary ammonium and phosphonium salt types, respectively.12,13 Bis-trialkylammonium salts were shown to have improved crystal growth inhibition performance compared to the tetraalkylated ammonium salts, particularly when the methylene chain is of six to eight carbons between the ammonium functional groups.14 More recently, the fluorinated quaternary ammonium salt, N-(4-fluorobutyl)N,N,N-tripentylammonium bromide, was found to perform better than TBAB and TPAB on THF crystal hydrate growth.15 Our research group has also been searching for new hydrophobic cations bearing more than the normal four alkyl groups found in the well-known tetraalkyl ammonium and phosphonium salts. For instance, we have reported that the tris(dialkylamino)cyclopropenium chloride salt carrying six nbutyl groups showed a similar performance as TBAB on THF hydrate crystal growth.16 Another and more interesting group is the hexaalkylguanidinum salts carrying six alkyl groups around the guanidinum cationic center. In this regard, we recently demonstrated that the hexa-n-butylguanidinium chloride (n-Bu6GuanCl) salt had a superior THF hydrate crystal growth inhibitor performance to TBAB and close to TPAB.17 Furthermore, n-Bu6GuanCl was shown to outperform both TBAB and TPAB as a synergist when combined with PVCap (Figure 1).17

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CHEMICALS AND SYNTHESIS

n-Bu6GuanCl and Et2(n-Pe)4GuanCl were synthesized via a two-step synthetic procedure previously reported by our group starting from guanidinium chloride and N,N′-diethylguandinium chloride, respectively.17 n-Bu6GuanCH3COO and n-Bu6GuanHCOO were prepared from the corresponding chloride salt after stirring at room temperature overnight with 2 mol equiv of glacial acetic acid and formic acid, respectively, in deionized water (DI), followed by evaporation of volatile compounds under vacuum. The obtained semi-solid salts were pure by nuclear magnetic resonance (NMR) spectroscopy after comparison to NMR spectra of previously made n-Bu6GuanCl.17 Luvicap EG (41.4 wt % PVCap in water) and Luvicap 55W [53.8 wt % VCap/N-vinylpyrrolidone (VP) copolymer] were obtained from BASF. INHIBEX 505 (38 wt % VCap-based polymer in n-butyl glycol ether) was obtained from Ashland, Inc. White spirit (a mixture of aromatic and aliphatic hydrocarbons) was obtained from Europris, Norway.

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HIGH-PRESSURE GAS HYDRATE ROCKER RIG EQUIPMENT TEST METHODS Constant Cooling Test. The constant cooling experiments were performed in high-pressure 5 × 40 mL steel rocking cell equipment (RC5), containing a steel ball, obtained from PSL Systemtechnik, Germany.12,16 A synthetic natural gas (SNG) mixture was used, and its composition is given in Table 1. The Table 1. Composition of SNG

Figure 1. n-Bu6GuanCl (left) and TBAB (right).

component

mol %

methane ethane propane isobutane n-butane N2 CO2

80.67 10.2 4.9 1.53 0.76 0.1 1.84

test procedure for the constant cooling test is as follows: At the beginning of the experiment, the pressure in the cells was 76 bar. The equilibrium temperature (Teq) at this pressure has also been reported previously and is 20.2 ± 0.05 °C, which is close to the calculated value of 20.5 °C at 76 bar using the PVTSim software of Calsep.12,15,16 This software was also used to determine equilibrium temperatures for the liquid hydrocarbon systems in this study. First, the cells are filled with 20 mL of a water phase containing the dissolved compounds to be tested. Then, vacuum is applied to the cells to remove air, followed by 5 bar of SNG, and is rocked for 2 min. After the pressure is released, vacuum is applied again and the cells are pressurized with SNG to 76 bar. Finally, the constant cooling program is started according to the following test parameters: rocking rate of 20 rocks/min, angle of the cells at 40°, and cooling ramp of 1 °C/ h. The pressure and temperature for each cell are recorded in a computer. Figure 2 shows a graph of a constant cooling test for the synergistic solution n-Bu6GuanCl/Luvicap EG (2700:300 ppm). There is an immediate pressure drop of about 2 bar as a result of gas being dissolved in the aqueous phase at the start of the experiment. The first deviation from the pressure drop as result of the temperature drop is taken as the time for the first observed onset of hydrate formation (To). In Figure 2, To is 11.1 °C. The Ta value indicates the first steepest part of the

Following up on those results, we report here a study on the synergistic performance of n-Bu6GuanCl with PVCap at different weight ratios, using constant cooling experiments with high-pressure rocking cells and a SII-forming synthetic natural gas, to determine an optimal formulation. SII hydrate is most commonly formed in the field, but lean gas mixtures (very high methane fraction) can form structure I (SI) hydrate. Using the same KHI test method, we also investigated the performance of n-Bu6GuanCl with two other commercially available VCap-based copolymers, Luvicap 55 (VCap/VP copolymer) and INHIBEX 505 (VCap terpolymer). In addition, to avoid the corrosive effect of the chloride ion in n-Bu6GuanCl, the corresponding organic acetate and formate salts were prepared and their synergistic KHI performances were evaluated in the same way. In addition, the less symmetric N,N′-diethyl-tetra-n-pentylguanidinium chloride [Et 2 (nPe)4GuanCl] was also prepared, and its synergism with PVCap was tested. Finally, long-term isothermal tests were carried out for mixtures of n-Bu6GuanCl with the VCap-based homopolymer and the VCap/VP copolymer at different ratios and total concentrations to evaluate their gas hydrate inhibitory performances in terms of hold time. B


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Figure 2. Typical graph of a constant cooling test showing To (11.1 °C) and Ta (10.7 °C) determinations.

Figure 3. Graph obtained from a long-term isothermal test, also showing a constant slow leak.

cooling rate was 5.3 °C/h. The isothermal temperature was 4.1 °C, giving a test subcooling of 8 °C. The test pressure at the isothermal temperature was 29−30 bar. The test duration was 336 h. The rocking rate was 10 rocks per minute, with a rocking angle of 35°. The test cells were charged with the active compounds dissolved in the aqueous solution and the NAM condensate using the following equation:

pressure versus time graph as result of stochastic hydrate formation. In Figure 2, Ta is 10.7 °C. To obtain statistically significant conclusions from such experiments, 10 tests were executed. p values from t tests lower than 0.05 were considered as a strong indication of a significant difference between two sets of To or Ta values. Long-Term Isothermal Tests. To evaluate the long-term gas hydrate inhibitory performances of n-Bu6GuanCl combined with Luvicap EG, Luvicap 55W, or INHIBEX 505, we carried out long-term isothermal experiments. The tests were performed in high-pressure 20 mL transparent sapphire rocking cell equipment (RCS20), containing a steel ball, obtained from PSL Systemtechnik, Germany.18 The test conditions were as follows: The synthetic natural gas is the same as for the constant cooling experiments given in Table 1. The hydrocarbon phase was a field condensate (mixture of aromatic and aliphatic hydrocarbons) obtained originally from the Shell Oil Company. The starting temperature was 20 °C. The initial

0.0776V mL of aqueous phase

0.1810V mL of condensate V = volume of test cells (RCS20 = 20 mL)

The volume was doubled in a few experiments to obtain the flowing liquids to touch evenly the walls of the sapphire tubes while rocking. The test procedure was as follows: First, the cells were charged with the aqueous fluid and NAM condensate. Then, C


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Figure 4. Synergism of n-Bu6GuanCl with Luvicap EG (PVCap) at 3000 ppm. Error bars represent the standard deviation (SD).

ratio to 14:1 (To = 3.3 °C) did not decrease the performance statistically (p values from t test > 0.05), however, the higher guanidinium ratio 29:1 (To = 3.8 °C) did slightly decrease the inhibition performance statistically (p values from t test < 0.05). Indeed, the difference in performances of the guanidinium ratios 9:1 (To = 3.1 °C) and 4:1 (To = 3.4 °C) were found to be statistically insignificant (p values > 0.05). In addition, no statistically significant difference was found between the mixtures with ratios of 4:1 and 14:1 n-Bu6GuanCl/PVCap (p values >0.05), implying that similar KHI performances can probably be reached with the mixture n-Bu6GuanCl/PVCap at 4:1, 9:1, and 14:1 ratios. Interestingly, a similar trend has been observed for the synergistic mixture of n-Pe4PBr and PVCap.19 These results suggest that a very large percentage of the KHI polymer can be replaced with the guanidium synergist and still give an improvement in KHI performance compared to using the polymer alone. As observed in Table 2, n-Bu6GuanCl shows poor antinucleator activity when used alone (To = 14.4 °C), meaning

the cells were evacuated of air with a vacuum pump, saturated with 2 bar of SNG, and rocked. After release of the pressure, the cells were evacuated again before pressurizing them with SNG to 36 bar. The test was started by cooling the temperature from 20 to 4.1 °C with a 5.3 °C/h cooling ramp. When 4.1 °C was reached, the rocking was continued and the temperature was maintained for 336 h (2 weeks). The pressure and temperature were recorded in a computer. Figure 3 shows the graph obtained from the long-term isothermal test for the synergistic mixture Bu6GuanCl/PVCap (2250:250 ppm). After cooling from 20 to 4.1 °C, the pressure drops immediately and very slowly with time by about 1 bar at 4.1 °C. We strongly believe this is a leak from the cell and not due to slow gas hydrate formation. We have since seen from other projects at similar conditions and using condensate that constant slow leaks can happen with the type of sealing in the end pieces of the cells.19 To check for a leak, the rocking cells were warmed up again to dissociate all hydrates, back to the initial starting temperature of 20.5 °C. If the cell can be returned to initial pressure−temperature conditions before cooling to form hydrates, then no leak has occurred. However, we observed a lower pressure by as much as 1−2 bar at the starting temperature of 20.5 °C, indicating a leak. The pressure graph is clearly horizontal for long periods in experiments where no leak has taken place. In the test shown in Figure 3, no slow hydrate growth was observed after first pressure drop as a result of to hydrate formation at 17 332 min. Instead, the pressure drops rapidly after this time. Thus, in this experiment, the time to first hydrate formation (hold time) is equal to the time when rapid hydrate formation begins (fail time). This is quite common in experiments where induction times are very long because, by this time, there are very many hydrate particles close to the critical nuclear size when spontaneous crystal growth becomes possible. Discussion Related to the Constant Cooling Tests. Figure 4 depicts To of the synergistic mixture n-Bu6GuanCl/ PVCap as a function of the n-Bu6GuanCl concentration in parts per million (ppm). Interestingly, the To values decreased with an increasing percentage of the alkylated guanidinium salt against PVCap. The increase in kinetic hydrate inhibition performance continued starting from pure PVCap (To = 7.7 °C) and reached its maximum, To = 3.1 °C, with n-Bu6GuanCl/ PVCap (2700:300) or 9:1 ratio. Increasing the guanidinium

Table 2. Synergistic Effect of n-Bu6GuanCl with PVCap (Luvicap EG) at Different Ratios at 3000 ppm PVCap concentration (ppm)

n-Bu6GuanCl concentration (ppm)

3000 2000 1500 1000 600 300 200 100 0

0 1000 1500 2000 2400 2700 2800 2900 3000

To (°C)a Ta (°C)b 7.7 6.6 4.9 4.2 3.4 3.1 3.3 3.8 14.4

7.1 5.4 4.1 3.4 2.4 2.3 2.7 2.9 12.1

a

To = average onset temperature from 10 tests. bTa = average rapid hydrate formation from 10 tests.

that it is mainly working to inhibit the crystal growth when used alone. This has also been shown from THF hydrate crystal growth studies previously reported.17 Thus, the fact that increasing the ratio of n-Bu6GuanCl to PVCap increases the performance of the KHI mixture implies that the guanidinium salt could be acting in two ways. First and primarily, nD


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performance by lowering the average To values of the nBu6GuanCl/PVCap mixtures. The improvement is however probably due to shifting of the hydrate phase equilibria by the added white spirit, resulting in lower equilibrium temperatures and higher equilibrium pressures. A second possibility is that the white spirit contains components that produce an additional synergistic KHI effect. We also evaluated the synergistic performances of nBu6GuanCl with three different commercially available lowmolecular-weight VCap-based polymers using the nBu6GuanCl/polymer ratio previously found to give exceptionally good KHI effect with PVCap (Table 4).

Bu6GuanCl could be inhibiting crystal growth but not in the same way as PVCap or else they would be competing for the same surface sites.20,21 Second, although n-Bu6GuanCl is a poor anti-nucleator by itself, it nevertheless has six hydrophobic groups that can disrupt the bulk water structure, and therefore, in combination with the long disruptive chains of PVCap, the small n-Bu6Guan+ cation can also improve the anti-nucleation processes. The high hydrophobicity of the n-Bu6Guan+ cation is a product of its high capability to disperse charge over its surface. Although the n-Bu6Guan+ cation is highly hydrophobic, it is still soluble in water. This hypothesis correlates with previous studies from our group, where we reported that a mediocre THF hydrate crystal growth inhibitor, such tetra-nhexylammonium bromide, was able to outperform a better crystal growth inhibitor, such as TBAB, when used as synergists with PVCap in gas hydrate KHI experiments.12 Another synergistic mechanism that cannot be discounted is related to molecular interactions between the PVCap polymer and Bu6GuanCl (or tetraalkylammonium salts). Herein, we tentatively suggest that certain water-soluble highly hydrophobic cations, such as the n-Bu6Guan+ cation may be able to interact with PVCap in such a way to affect its structural conformation with ensuing enhancement of KHI performance.22,23 Therefore, in the presence of highly hydrophobic cations, such as n-Bu6Guan+, PVCap may exhibit a more open conformation, resulting in greater water perturbation and increased anti-nucleator performance. For example, it is known that guanidinium chloride is a widely used chemical denaturant that unfolds proteins and can help unfold hydrophobic polymers.24 Interestingly, a thermodynamic study on the interactions between a protein and quaternary ammonium salts showed that the binding entropies and heats increased in the following order Et4NBr < Pr4NBr < Bu4NBr as a result of increasing hydrophobicity of the salt.25 With the aim to further increase the performance of the nBu6 GuanCl/PVCap mixtures, their synergism was also evaluated by adding 2 mL of white spirit to the 20 mL water phase (Table 3). Interestingly, the average lowering of To values containing white spirit was 2.1 °C. In fact, the ratios 4:1 and 9:1 with white spirit showed no pressure drops during the cooling test temperature range (20.5−2.0 °C) as expected from the To values of the same mixtures without white spirit, i.e., 3.4 and 3.1 °C, respectively. Therefore, using 9% white spirit improves the

Table 4. Synergistic Effect of n-Bu6GuanCl with PVCap (Luvicap EG), VCap/VP Copolymer (Luvicap 55W), and VCap Terpolymer (INHIBEX 505) with 9:1 Weight Ratios at 3000 ppm polymer DI water no polymer VCap terpolymer PVCap VCap/VP copolymer VCap terpolymer PVCap VCap/VP copolymer a


3000 2000 1500 1000 600

0 1000 1500 2000 2400

300

2700

0

3000

To (°C)a c

5.3 4.2 3.4 2.3 no hydrate formation no hydrate formation 12.1d


To (°C)a Ta (°C)a

neat 3000

18.0 14.4 12.2

17.6 12.1 11.6

3000 3000

7.7 6.5

7.1 5.0

3000

300

2700

3.5

2.8

300 300

2700 2700

3.1 5.0

2.3 4.2

To and Ta = average onset temperature of 10 tests.

When the To values of the polymers alone at 3000 ppm are compared to the To values of the n-Bu6GuanCl/polymer (9:1) mixtures at 3000 ppm, we find the following enhancement in the KHI performance: VCap/VP (Luvicap 55W) < PVCap (Luvicap EG) < VCap terpolymer (INHIBEX 505)

The guanidinium salt greatly increased the performance of the VCap terpolymer, INHIBEX 505, by lowering To by 8.7 °C. The performance enhancement is also drastic for PVCap but much less for VP/VCap copolymer, lowering the To values by 4.6 and 1.5 °C, respectively. A similar trend has also been reported for another commercial KHI, INHIBEX 713 (from Ashland, Inc.), Luvicap 55W, and Luvicap EG when they are synergistically mixed with quaternary phosphonium salts.19 Interestingly, this enhancement order given above is the reverse for the ranking when the polymers are tested alone without a synergist at the same concentration (Table 4). It should also be noted that the lowest To values after adding the synergist are not for the VCap terpolymer. The lowest To values are obtained with synergistic blends with PVCap, the most hydrophobic polymer of the three polymers. Figure 5 depicts the active polymer components in Luvicap EG, Luvicap 55W, and INHIBEX 505, which are PVCap, VCap/VP copolymer, and VCap terpolymer, respectively. These PVCap-based polymers have been designed to meet different field operating conditions, including subcooling, well head temperature, and salinity. For instance, the VCap/VP

Table 3. Synergistic Effect of n-Bu6GuanCl with PVCap (Luvicap EG) at Different Ratios at 3000 ppm Total Concentration with Added 2 mL of White Spirit PVCap concentration (ppm)

polymer concentration (ppm)

Ta (°C)b 4.7c 3.6 2.7 2.0

10.5d

a

To = average onset temperature from 10 tests. bTa = average rapid hydrate formation from 10 tests. cAverage of 4 tests. dAverage of 8 tests. E


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effect of the chemical molecular weight and the molar concentration of n-Bu6Guan+ cations in the aqueous phase. However, it does raise questions about the role of the anion in the mechanism of the synergistic effect of the quaternary guanidinium salts with PVCap. In this regard, the bromide salt analogue, n-Bu6GuanBr, has been reported to be an inferior THF hydrate crystal growth inhibitor compared to the chloride salt analogue.17 The tetrapentylated guanidinium salt Et2(nPe)4GuanCl also gave a worse synergistic performance than nBu6GuanCl. The ammonium salt TPAB is known to be a better synergist than TBAB. This has been explained from an embedding mechanism in which pentyl groups are the ideal length to fit into neighboring 51264 cavities on the SII hydrate surface.4 These pentyl groups subsequently become embedded because hydrate cavities grow around them to such an extent that removal of the tripodal pentyl legs becomes physically impossible. The reason for the somewhat poorer synergist performance of guanidinium salt Et2(n-Pe)4GuanCl may be due to the two shorter ethyl “legs”, which are less easily embedded in the hydrate surface. To further investigate the gas hydrate inhibition performance of the n-Bu6GuanCl/PVCap (9:1) ratio blend, we carried out long-term isothermal KHI performance tests using a field gas condensate. The subcooling was 8 °C in these tests. Table 6 shows the hold times for the KHI formulation, n-Bu6GuanCl/ PVCap (9:1), at 5000, 3000, and 1000 ppm, carrying out two tests for each concentration. Tests with no additive gave hydrate plugs in under 1 h. Taking the worst case experiment for each mixture, the data in Table 6 show that the hold time increases with an increasing total concentration of the mixture. The shortest hold time for the lowest concentration 1000 ppm blend was 9 h; the hold time for the 3000 ppm mixture was 8.7 days; and the hold time for the 5000 ppm mixture with no pressure drops was observed during the whole test time of 13.6 days. Fast hydrate formation occurred in all tests almost immediately when the hold time (first sign of hydrate formation) was reached. These results are in line with the general trend that increased KHI concentrations lead to increased gas hydrate inhibition performance.4,26 Because the VP/VCap copolymer and VCap terpolymer have high cloud points, making them compatible for a greater range of field applications than PVCap, we carried out a series of KHI isothermal tests on synergistic mixtures of these two polymers with n-Bu6GuanCl. A field gas condensate was also used in these tests. Table 7 gives the performance results for the nBu6GuanCl/VP/VCap copolymer blend at 1:1 (one test), 4:1 (three tests), and 9:1 (three tests) ratios at 5000 ppm total concentration of active compounds. The data show that at least one test at each concentration did not give hydrates within the 13.6 day test period. The worst results for the 9:1 blend gave a hold time of 4.5 days. This was followed by slow hydrate

Figure 5. From top to bottom: PVCap (polymer in Luvicap EG), VCap/VP copolymer (in Luvicap 55W), and VCap/VP/AMPS terpolymer (in INHIBEX 505).

copolymer (ca. 78 °C) and VCap terpolymer (ca. 90 °C) have higher cloud points than PVCap (ca. 30 °C) in pure water, therefore making them useful at higher injection temperatures without the risk of precipitation. In addition, the VCap terpolymer is stated to have a high salt tolerance. To explain the dramatic enhancement of performance of the VCap terpolymer compared to the other two polymers investigated, we suggest that n-Bu6GuanCl adds more hydrophobicity to the VCap terpolymer compared to the other polymers. This argument seems reasonable because the VCap terpolymer is the most hydrophilic of the three, designed with a high cloud point for high-salinity systems, although at the expense of lowering the KHI performance in some systems. The synergism of the n-Bu6Guan+ cation bearing other organic anions and that of the less symmetric Et2(nPe)4GuanCl salt with PVCap were explored (Figure 6). n-

Figure 6. Structure of Et2(n-Pe)4GuanCl.

Pe6GuanCl and (n-Bu)2(n-Pe)4GuanCl were also synthesized, found to be water-insoluble, and not investigated further. The results are given in Table 5 and compared to n-Bu6GuanCl. All three salts showed less synergistic effect with PVCap than nBu6GuanCl, giving statistically significantly higher average To values (p < 0.05 in t tests). The results will at least in part be an Table 5. Synergistic Effect of Other Guanidinium Salt Analogues

a

polymer


guanidinium salt

guanidinium salt concentration (ppm)

To (°C)a

Ta (°C)b

DI water PVCap PVCap PVCap PVCap PVCap

neat 300 600 600 300 600

n-Bu6GuanCl n-Bu6GuanCl n-Bu6GuanCH3COO n-Bu6GuanHCOO Et2(n-Pe)4GuanCl

2700 2400 2400 2700 2400

17.8 3.1 3.4 4.3 3.7 4.2

17.4 2.3 2.4 3.3 3.0 3.2

To = average onset temperature of 10 tests. bTa = average rapid hydrate formation of 10 tests. F


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Table 6. Long-Term Isothermal Experiments at 8 °C Subcooling of the Mixture n-Bu6GuanCl/PVCap (9:1) at 5000, 3000, and 1000 ppm Concentrations polymer KHI



total concentration (ppm)

pressure (bar)

hold time (days)

test time (days)

PVCap PVCap PVCap PVCap PVCap PVCap

500 500 250 250 100 100

4500 4500 2250 2250 900 900

5000 5000 3000 3000 1000 1000

29.7 29.1 29.5 29.3 29.5 29.3

no pressure drop no pressure drop 11.9 8.7 9h 5.7

13.6 13.6 13.6 13.6 13.6 13.6

Table 7. Long-Term Isothermal Tests at 8 °C Subcooling of the Mixture n-Bu6GuanCl/VCap/VP Copolymer at Ratios of 1:1, 4:1, and 9:1 at 5000 ppma polymer KHI




pressure (bar)

VCap/VP VCap/VP VCap/VP VCap/VP VCap/VP VCap/VP VCap/VP

2500 1000 1000 1000 500 500 500

2500 4000 4000 4000 4500 4500 4500

5000 5000 5000 5000 5000 5000 5000

29.3 28.8 29.0 29.2 29.0 29.1 28.9

hold time (days) no pressure 6.0b no pressure no pressure no pressure 4.5c no pressure

drop drop drop drop drop

test time (days) 13.6 13.6 11.7 11.7 11.7 13.6 13.6

a

For 20 mL sapphire cells, 1.55 mL water phase + 3.62 mL condensate. bSlow but no rapid pressure drop. cA total of 10.5 days to rapid pressure drop.

Table 8. Long-Term Isothermal Tests at 8 °C Subcooling of n-Bu6GuanCl/VCap Terpolymer Blend at 1:1 and 9:1 Ratios and VCap Terpolymer Alone at 5000 ppm polymer KHI VCap VCap VCap VCap VCap

terpolymer terpolymer terpolymer terpolymer terpolymer




pressure (bar)

hold time (days)

test time (days)

2500 500 5000 500 500

2500 4500

5000a 5000a 5000b 5000b 5000b

30.4 30.0 28.2 34.9 35.9

no pressure drop no pressure drop 1.1 6.7 no pressure drop

13.6 13.6 8.0 11.7 11.7

4500 4500

a

For 20 mL sapphire cells, 1.55 mL of water phase + 3.62 mL of condensate. bFor 20 mL sapphire cells, 3.10 mL of water phase + 7.24 mL of condensate.

hydrates after 6.7 days. From these results, it is difficult to conclude which ratio of n-Bu6GuanCl/VCap terpolymer is best but it is fairly certain that the blends perform better than the terpolymer alone.

growth for the next 6 days until catastrophic fast growth occurred and the cell became plugged with hydrates. The worst result for the 4:1 blend was a hold time of 6 days, followed by a further 7.6 days with a slow pressure drop of about 1 bar, indicating slow hydrate growth. Although more results would be preferable, given the stochastic nature of hydrate formation, we can only tentatively conclude that the KHI performance improves moving from the 9:1 blend to the 4:1 blend and possibly also the 1:1 blend. Table 8 gives the isothermal KHI test results for nBu6GuanCl/VCap terpolymer blends with ratios of 1:1 and 9:1. Two different volumes of liquids (water and condensate) were used, but the temperature was adjusted to give 8 °C subcooling in all experiments, except the test with VCap terpolymer alone at 28.2 bar. The subcooling was less than the other tests, approximately 7.5 °C, and the hold time was observed to be 1.1 days (ca. 27 h). A shorter hold time would be expected at 8 °C subcooling. In comparison, blends of the terpolymer with n-Bu6GuanCl with the same total concentration always gave longer hold times. However, the varying results underline the stochastic nature of the hydrate formation process. For example, in the last two entries in Table 8, the 9:1 blend of n-Bu6GuanCl/VCap terpolymer gave no hydrates in 11.7 days in one test, while the other identical test gave

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CONCLUSION The synergistic effect of n-Bu6GuanCl and related compounds in different ratio mixtures with a series of three commercially available VCap-based polymer KHIs against SII-forming synthetic natural gas hydrates has been evaluated by carrying out constant cooling and isothermal KHI screening experiments in high-pressure rocking cells. Formulations containing n-Bu6GuanCl/PVCap with weight ratios of 4:1, 9:1, and 14:1 at 3000 ppm total concentrations were found to give the best gas hydrate inhibitory performances, with To values of 3.4, 3.1, and 3.3 °C, respectively. In comparison, 3000 ppm of PVCap alone gave an average To of 7.7 °C. Using 9 vol % white spirit lowers the To values of the best synergistic mixtures by 2.1 °C. However, this effect is at least partly due to shifting of the hydrate phase equilibria to a lower equilibrium temperature and higher equilibrium pressure. n-Bu6GuanCl was found to be a better synergist than nBu6GuanCH3COO, n-Bu6GuanHCOO, and Et2(n-Pe)4GuanCl when used with PVCap for gas hydrate kinetic inhibition. The G


Article

Energy & Fuels

(24) Godawat, R.; Jamadagni, S. N.; Garde, S. J. Phys. Chem. B 2010, 114, 2246−2254. (25) Chen, C.-H.; Berns, D. S. J. Phys. Chem. 1978, 82, 2781. (26) Kelland, M. A. A review of kinetic hydrate inhibitors: Tailormade water-soluble polymers for oil and gas industry applications. In Advances in Materials Science Research; Wytherst, M. C., Ed.; Nova Science Publishers, Inc.: New York, 2011; Vol. 8, Chapter 5, pp 171−210.

synergistic capacity of n-Bu6GuanCl is dependent upon the chemical structure of the VCap-based polymer, although the molecular weight differences may also play a part. A mixture of 300 ppm of high-cloud-point VCap-based terpolymer with 2700 ppm of n-Bu6GuanCl lowered the average To by 8.7 °C compared to using 3000 ppm of this terpolymer alone. In long-term isothermal experiments at 8 °C subcooling and 30 bar, 5000 ppm of the synergistic mixture Bu6GuanCl/ PVCap in a 9:1 weight ratio showed a hold time of over 13.6 days. Long hold times were also achieved with various ratios of n-Bu6GuanCl/VCap/VP copolymer or n-Bu6GuanCl/VCap terpolymer mixtures.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +47-51832295. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Norwegian Research Council FORNY Program and Nalco Champion (an EcoLab company) for partial support of this work.

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REFERENCES

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