Study on the Synergistic Properties of Quaternary Phosphonium

Article pubs.acs.org/EF

Study on the Synergistic Properties of Quaternary Phosphonium Bromide Salts with N‑Vinylcaprolactam Based Kinetic Hydrate Inhibitor Polymers 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: We have evaluated the synergism of five tetraalkyl phosphonium bromide salts with N-vinylcaprolactam based polymers by carrying out high pressure rocking cell experiments with a structure II-forming natural gas mixture. (n-Pe)4PBr was shown to be a superior synergist than (n-Bu)4PBr when used in combination with three different N-vinylcaprolactam based commercial polymers used in kinetic hydrate inhibitor formulations. These are poly(N-vinylcaprolactam) (PVCap), a Nvinylcaprolactam:N-vinylpyrrolidone 1:1 copolymer, and a N-vinylcaprolactam terpolymer. (n-Pe)4PBr and (n-Pe)4NBr were found to have very similar synergistic capacities and to perform better (n-Bu)4NBr when used with PVCap. (n-Bu)4PBr and (nBu)3(iso-Hex)PBr were better synergists than (n-Bu)4NBr with PVCap. They exhibited the same synergistic performances with PVCap even though (n-Bu)3(iso-Hex)PBr had been shown to be a better tetrahydrofuran (THF) hydrate crystal growth inhibitor. (n-Bu)3hexadecylPBr showed a synergistic performance similar to that of (n-Bu)4NBr with PVCap. Phe4PBr showed a poor performance as synergist with PVCap. This was in line with a poor THF hydrate crystal growth performance previously reported. By changing the weight ratios of (n-Pe)4PBr and PVCap at the same total weight concentration of 3000 ppm, it was found that the onset temperatures (To) decreased with increasing amount of (n-Pe)4PBr. The lowest To was reached for the ratio 29:1 ((n-Pe)4PBr:PVCap). Thus, excellent KHI performance can be obtained with as little as 100 ppm of KHI polymer as long as there is sufficient amount of a good synergist.

■

INTRODUCTION Gas clathrate hydrates are solid inclusions consisting of gas molecules (guest molecules) trapped in the empty spaces of hydrogen bonded water molecule cages (the host). Their formation is favored by lower temperatures and elevated pressures such as those found in pipelines during transport of hydrocarbon resources.1 The typical guest molecules found in natural gas hydrates are methane, ethane, propane, and carbon dioxide. In pipelines such blends form preferentially clathrate hydrates of the structure II (SII) type.2 Plugs caused by natural gas hydrates are a costly problem, and their prevention is a major concern for the oil and gas industry. Application of low dosage gas hydrate inhibitors (LDHIs) is a relatively new but now established technology for prevention of gas hydrate plugging of flow lines and wells.3 LDHIs are divided into two categories, kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs) and are applied for different field scenarios and applications. KHIs are generally water-soluble polymers that work by delaying hydrate nucleation and often also crystal growth. In commercial formulations they are often used in combination with synergists aiming to increase the polymers’ inhibition performance, lowering the treatment cost. Most synergists function primarily as hydrate crystal growth inhibitors but can also act as hydrate antinucleators. The potential of quaternary ammonium and phosphonium salts, also known as onium salts, as LDHIs in the battle to prevent gas hydrate blockages of flow lines was first explored by Shell, the oil company, about 2 decades ago.4−7 They first © 2014 American Chemical Society

evaluated the inhibition performance of onium salts on tetrahydrofuran (THF) hydrate crystal growth. THF forms hydrates of the SII type, the same as natural gas mixtures, but at atmospheric pressure. Butylated and pentylated quaternary ammonium salts were found to have the largest effect on THF hydrate crystal growth. The pioneering work of Shell and more recent studies from our group have disclosed the following ranking for the performance of homogeneous tetraalkylammonium salts as THF hydrate inhibitors: (iso-Hex)4NBr > (n-Pe)4NBr) > (isoPe)4NBr) ≈ (n-Bu)4NBr > (n-Hex)4NBr.8 Quaternary ammonium salts and quaternary ammonium based surfactants are now essential constituents of commercial formulations of both KHIs and AAs.9−15 For instance, the synergistic effect of quaternary ammonium salts with poly(Nvinylcaprolactam) (PVCap) is well documented.12,16 It has also been reported that bis-trialkylammonium salts perform better as THF hydrate crystal growth inhibitors than equivalent tetraalkylammonium salts when the alkylene spacer between the nitrogen atoms is between 6 and 8 carbon atoms long.17 Quaternary ammonium and phosphonium salts bearing butyl and pentyl groups have been shown to form clathrate hydrates of different structures where the ionic salts participate in the clathrate hydrate lattice.18−24 Recently, (n-Bu)4NBr (tetrabutylammonium bromide, TBAB) and (n-Bu)4PBr (tetrabutylphosphonium bromide, TBPB) based semiclathrates have Received: July 8, 2014 Revised: September 30, 2014 Published: October 6, 2014 6803

dx.doi.org/10.1021/ef501545p | Energy Fuels 2014, 28, 6803−6810

Energy & Fuels

Article

fully understood. Empirical data obtained by observing THF hydrate crystal growth inhibition of ammonium salts comprising alkyl chains of different lengths has shown that the optimal carbon chain length of n-alkyl groups attached to the nitrogen atom for best inhibition is five carbons. Furthermore, branching at carbon-4 as found in the isohexyl group improves the inhibition performance.8 Longer n-alkyl chains such as n-hexyl or n-heptyl groups reduce the crystal growth inhibition performance significantly as well as shorter chains such propyl or ethyl groups.8 Molecular modeling carried out at the University of Reading for RF-Rogaland Research (now the International Research Institute of Stavanger) in the mid-1990s revealed that (nBu)4NBr and (n-Pe)4NBr penetrate a 51264 cage on the SII hydrate surface driven by van der Waals forces between the hydrophobic alkyl chain and the hydrophobic cavity. Two of the remaining alkyl chains lie in channels where new hydrate cages would otherwise form. As they grow, the alkyl chains become embedded on the hydrate surface causing distortion and finally disruption of newly forming clathrate hydrate crystals.3 The embedding mechanism implies that the pentyl groups in (n-Pe)4NBr fit well in the clathrate hydrate cavities by making stronger van der Waals interactions with the hydrate cages. The isohexyl groups in (iso-Hex)4NBr fit even better in the cavities since they possess the optimum five carbon length alkyl chain, but exert even stronger van der Waals interactions with the empty cavities than n-pentyl groups because of the extra branched methyl group at carbon-4. Our previous work revealed the following ranking for the THF hydrate crystal growth inhibition of phosphonium and ammonium salts:8,28

gained considerable attention from scientists as alternative materials for gas transportation and separation.25−27 Although, the hydrate crystal growth inhibition properties of quaternary ammonium salts are well documented, very little attention has been paid to their quaternary phosphonium analogues. One of the reasons for this could be the generally higher commercial prices of quaternary phosphonium salts in relation to quaternary ammonium salts. Our research group has recently reported the THF crystal growth inhibition performance of several quaternary phosphonium salts and compared them to those of quaternary ammonium salts.28 The better tetraalkyl quaternary phosphonium bromide salts were found to perform better than their equivalent ammonium bromide salts. For instance, (n-Bu)4PBr outperformed (n-Bu)4NBr and tetra-n-pentylphosphonium bromide ((n-Pe)4PBr) was found to be the best homogeneous tetraalkyl quaternary phosphonium salt investigated with a performance close to that of the best ammonium salt, (isoHex)4NBr, previously reported.8 It should be noted that the phosphonium salt (iso-Hex)4PBr has not yet been investigated. In the light of these results we wanted to continue our investigation on the tetraalkylphosphonium bromide salts and evaluate their performances as synergists with the well known N-vinylcaprolactam (VCap) based KHIs on natural gas hydrates. Herein, we report a study on the synergistic properties of several tetraalkyl phosphonium salts with some commercially available VCap based KHI polymers. Performance testing was carried out in high pressure rocking cells with a SII hydrate-forming natural gas mixture. The primary focus of the study was on (n-Bu)4PBr and (n-Pe)4PBr as synergists, although other phosphonium salts were also investigated. For comparison, the synergistic properties of the ammonium salts (n-Pe)4NBr and (n-Bu)4NBr with the VCap based polymers were also evaluated (Figure 1).

■

(n‐Pe)4 PBr ≈ (iso‐Hex)4 NBr > (n‐Pe)4 NBr

THEORY OF SYNERGY WITH QUATERNARY ONIUM SALTS It is generally believed that quaternary ammonium salts are primarily hydrate crystal growth inhibitors and have little effect on hydrate nucleation.2 Their mechanism of action is still not

> (n‐Bu)4 PBr ≫ (n‐Bu)4 NBr ≫ (n‐Hex)4 NBr

The embedding mechanism can also be applied to quaternary phosphonium bromide salts. The (alkyl)4P+ cation differs in several ways from the (alkyl)4N+ cation. For instance, the lower electronegativity and larger size of the phosphorus atom compared to nitrogen accounts for the phosphonium cations’ lower dissociation energies in its ionic interactions and its higher capability to disperse charge over the hydrate surface, making it more hydrophobic.29 Therefore, as shown from the THF hydrate growth inhibition ranking earlier, the quaternary phosphonium salts, especially those bearing butyl and pentyl groups, performed better than their equivalent ammonium salts. KHIs based on N-vinyllactam polymers are one of the major KHI polymer classes used commercially today.30,31 The Nvinyllactam ring can be a 5-ring, N-vinylpyrrolidone (VP), or a 7-ring, N-vinylcaprolactam (VCap) as these N-vinyllactam ring sizes are commercially available globally in large quantities. Their mechanism of KHI action is still not fully understood. They are known to work both on the hydrate nucleation process but also to inhibit hydrate crystal growth. These polyvinyl based polymers have a linear structure with the lactam rings spread out in the aqueous phase.2 The lactam rings can lead to stronger van der Waals interactions with free water molecules in the solution, perturbing the bulk water structure, or can bond to the water molecules on the clathrate hydrate lattice surface.

Figure 1. Ammonium salts (top from left to right): (n-Bu)4NBr and (n-Pe)4NBr. Phosphonium salts (bottom from left to right): (nBu)4PBr and (n-Pe)4PBr. 6804


Energy & Fuels

Article

In general, it is thought that quaternary onium (ammonium, phosphonium, and sulfonium) salts act synergistically with PVCap because of their different geometries.3 The polymer is thought to lay extended in the water phase strongly disturbing water molecules preventing the nucleation of partially formed hydrate cages while the small hydrophobic cations such as in (n-Bu)4NBr with much higher mobility primarily inhibit crystal growth when the hydrates have reached a critical size. PVCap also adsorbs on hydrate crystal surfaces but in a very different way and at different sites to the tetraalkylated onium salts. Water-soluble onium salts with large hydrophobic groups, but poor hydrate crystal growth inhibition properties, such as tetra(n-hexylammonium) bromide, can also contribute to the perturbation of the bulk water structure and thus enhance the KHI performance of polymers such as PVCap.8 This provides a strong argument that the KHI mechanism is not by crystal growth inhibition alone, but involves nucleation inhibition also.

Figure 3. Rocker rig showing the five steel cells in a cooling bath.

■

Germany and has been described previously.4,7,8 The gas composition used was a synthetic natural gas (SNG) mixture given in Table 1.

CHEMICALS INVESTIGATED Figure 2 shows the active polymers in the commercially available KHIs used in this study. Luvicap EG (poly(N-

Table 1. Composition of Synthetic Natural Gas

Figure 2. From left to right: PVCap (PVCap); VP:VCap (1:1 VP:VCap copolymer); INHIBEX 713 (VCap:VP:DMAEMA terpolymer).

vinylcaprolactam, or PVCap, in monoethylene glycol) and VP:VCap (1:1 N-vinylcaprolactam:N-vinylpyrrolidone copolymer or VP:VCap in water) were obtained from BASF and contained 41.1 and 53.8 wt % active low molecular weight polymers (ca. 2000−4000 Da), respectively. INHIBEX 713 (VCap:VP:(dimethylamino)ethyl methacrylate terpolymer, or VCap terpolymer, in ethanol) was obtained from Ashland Chemical Co. and contains 37 wt % of active polymer of fairly high molecular weight (ca. 60,000−80,000 Da). (n-Bu)4PBr (>99%), (n-Pe)4NBr (>99%), and Phe4PBr (97%) were purchased from Sigma-Aldrich. (n-Bu)4NBr (>99%) and (nBu)3hexadecylPBr (97%) were purchased from Fluka, and (nPe)4PBr was obtained from WNCT Ionic Liquid Laboratory. (n-Bu)3(iso-Hex)PBr was prepared in our laboratories according to standard synthetic methods.28 In the present work we investigated, first, the synergism of (n-Bu)4PBr and (n-Pe)4PBr with the three commercial KHI polymers at 3000 ppm total concentration of active polymer (Figure 2). Second, the synergisms of the phosphonium salts: (n-Bu) 4 PBr, (n-Pe) 4 PBr, (n-Bu) 3 (iso-Hex)PBr, (nBu)3hexadecylPBr, and Phe4PBr with PVCap at 3000 ppm total concentration. Finally, we looked into the synergism of (nPe)4PBr with PVCap at different ratios with the same total concentration of 3000 ppm.

component

mol %

methane ethane propane isobutane n-butane N2 CO2

80.67 10.2 4.9 1.53 0.76 0.1 1.84

The test procedure used was a “constant cooling” experiment previously described.8,32 The pressure at the beginning of the experiment was 76 bar. The hydrate equilibrium temperature (HET) 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 Calseṕs PVTSim software.8,32 Other test parameters are as follows: rocking rate, 20 rocks/min; cell rocking angle, 40°; cooling ramp, 1 °C/h. Before the experiment was started the following procedure was applied: the cells were filled with 20 mL of a water phase containing the dissolved compounds to be tested. Then, a vacuum was applied to the cells to remove air followed by 5 bar of SNG, and the cells are rocked for 2 min. After releasing the pressure, a vacuum is applied again and the cells pressurized with SNG to 76 bar. A typical graph of the data obtained according to this procedure using all five cells is shown in Figure 4. There is a pressure drop of about 2 bar due to gas being dissolved in the aqueous phase. The first deviation from the pressure drop due the temperature drop is taken as the time for the first observed onset of hydrate formation, To (Figure 5). Figure 4 shows the data for five identical tests of the same synergistic solution. The Ta value shows the first steepest part of the pressure versus time graph. This part of the graph is characterized by the most rapid hydrate formation. To obtain statistically significant conclusions from such tests, 10−12 tests were carried out. Furthermore, p-values from statistical t tests lower than 0.05 were considered as a strong indication of a significant difference between two sets of To or Ta values.

■

HIGH PRESSURE GAS HYDRATE ROCKER RIG EQUIPMENT TEST METHODS Constant Cooling Tests. Kinetic hydrate inhibition experiments were conducted in five high pressure 40 mL steel rocking cells each containing a steel ball shown in Figure 3. The equipment was supplied by PSL Systemtechnikk, 6805


Energy & Fuels

Article

Figure 4. Pressure and temperature data obtained from constant cooling KHI tests in the multicell rocker rig.

Figure 5. Example of To and Ta calculation in a constant cooling rocker rig KHI experiment.

■

DISCUSSION OF THE HIGH PRESSURE KHI SYNERGISM EXPERIMENTS

As discussed earlier, KHI polymers are known to often inhibit both hydrate nucleation and crystal growth. Polymers of the N-vinyllactam type are believed to disturb the hydrogen bonding between water molecules forming gas hydrate clusters preventing nucleation. In addition, the lactam rings bind to the hydrate surface in two ways, by hydrogen bonding to the hydrate surface via the oxygen atom of the amide functional groups and by interaction of the hydrophobic lactam rings with the hydrate surface cages via van der Waals interactions. Therefore, substitution of a N-vinyllactam ring by a (dimethylamino)ethyl ester function, which offers more options for hydrogen bonding, but is more hydrophilic compared to the lactam rings, is expected to lower the polymer′s inhibition performance.

Table 2 shows the performance of the polymeric KHIs tested in our high pressure gas hydrate rocker rig equipment. The kinetic hydrate inhibition performance was found to increase in the following order: VCap terpolymer < PVCap < VP:VCap at 3000 ppm total concentration of active compounds. PVCap contains the active polymer PVCap, which contains the key VCap monomer used in all of the commercially available KHIs of the vinyllactam type. The lower performance of VCap terpolymer alone compared to PVCap is probably related to the significantly higher polymer molecular weight in VCap terpolymer. 6806


Energy & Fuels

Article

with their performance on THF hydrate crystal growth previously reported by our research group.28 The pentylated phosphonium salt enhanced the performance of the polymers in the following order: VP:VCap < PVCap < VCap terpolymer. Interestingly, this is the reverse order of their inhibition performance without synergist. (n-Pe)4PBr and (nBu)4PBr enhanced the performance of VCap terpolymer more than that of PVCap. We suggest that both salts augment the hydrophobicity of VCap terpolymer to a greater degree compared to VP:VCap, resulting in increased water perturbation. In addition, the quaternary phosphonium salts offer nbutyl and n-pentyl groups that fit better in the 51264 cavities on the SII hydrate surface according to the embedding mechanism, resulting in increased gas hydrate crystal growth inhibition. In contrast, the quaternary phosphonium salts add less hydrophobicity to VP:VCap which has two hydrophobic rings, pyrrolidone and caprolactam rings. However, n-butyl and npentyl groups still enhance the performance of the synergistic mixture with PVCap via the embedding mechanism, but not in such a dramatic way as in combination with VCap terpolymer. We presume that the enhancement in performance of PVCap over VP:VCap by (n-Pe)4PBr may be related to pentylated quaternary phosphonium salt interfering with the different conformations of VP:VCap copolymer making its structure less “open” for interactions with the hydrate surface. The different cloud points of PVCap (30−33 °C) and VP:VCap copolymer (70−80 °C) may also be a contributing factor. Enhancement of kinetic hydrate inhibition performance of a polymeric KHI by adding (n-Pe)4PBr and (n-Bu)4PBr in the way that was carried out in this study suggests that these synergistic salts may have a nucleation hydrate inhibition effect in addition to their hydrate crystal growth inhibition effect. This effect is only seen when they are mixed together in the same solution under test conditions that favor gas hydrate crystal formation. (n-Bu)4NBr and (n-Pe)4NBr have been shown in previous studies to be good synergists for PVCap.3,5,12,13,16,33 The results given in Table 3 confirm this conclusion, and further show that

Table 2. Constant Cooling Tests in High Pressure Rocking Cells: Synergistic Effect of (n-Bu)4PBr and (n-Pe)4PBr with PVCap, VP:VCap, and VCap Terpolymer polym

polym concn (ppm)

onium salt

onium salt concn (ppm)

To (°C)a Ta (°C)b

DI water VCap terpolymer PVCap VP:VCap VCap terpolymer VCap terpolymer PVCap

neat 3000

17.8 9.5

17.4 8.7

3000 3000 1500

1500

7.5 6.1 5.8

7.1 5.0 4.8

1500

5.1

4.2

1500

5.8

4.6

PVCap

1500

1500

4.9

3.5

VP:VCap

1500

1500

5.9

4.9

VP:VCap

1500

1500

5.6

4.4

1500 1500

(nBu)4PBr (nPe)4PBr (nBu)4PBr (nPe)4PBr (nBu)4PBr (nPe)4PBr

a

To, average onset temperature out of 10 tests. bTa, average rapid hydrate formation out of 10 tests.

Although VP:VCap contains the active copolymer VP:VCap in 1:1 ratio, it contains half the amount of VCap units compared to PVCap. The combination of two different lactam ring structures is thought to give a more irregular structure than that of the PVCap homopolymer. VP:VCap copolymer can display more different conformations enabling increased interactions with water molecules and gas hydrate clusters with concomitant higher KHI performance. To assay the synergism of the quaternary butylated and pentylated phosphonium salts with VCap terpolymer, PVCap, and VP:VCap, we decided to carry out a more severe test. Starting from the polymer performance at 3000 ppm concentration of active polymers, the amount of the polymer was reduced by half (1500 ppm) and 1500 ppm of the synergist added keeping the total concentration of inhibitors at 3000 ppm. In this way any improvement of the To for the polymer at 3000 ppm without the quaternary salt will imply synergism. Table 2 shows that the highest differences between To values of the polymers with and without a synergist are for VCap terpolymer and PVCap. This is true for both the butylated and the pentylated phosphonium salts. However, only the (nPe)4PBr and not (n-Bu)4PBr seems to statistically significantly improve the performance of VP:VCap in a 1:1 polymer:synergist ratio. Table 2 reveals that (n-Pe)4PBr and (n-Bu)4PBr lower the To of VCap terpolymer and PVCap significantly. (n-Pe)4PBr and (n-Bu)4PBr lowered the To of VCap terpolymer by 4.4 and 3.7 °C, respectively, and the To for PVCap by 2.6 and 1.7 °C, respectively. However, only the (n-Pe)4PBr lowered statistically significantly the To of VP:VCap (p-value < 0.05 from a t test with 10 tests for each synergistic mixture). (n-Bu)4PBr did not improve the To of VP:VCap giving similar To values with a statistically insignificant difference (p-value > 0.05 from a t test with 10 tests for each mixture). These 1:1 polymer:synergist KHI test results demonstrate the superior capacity of (nPe)4PBr over (n-Bu)4PBr to enhance synergistically gas hydrate inhibition with very well known KHIs. These results are in line

Table 3. Constant Cooling Tests in High Pressure Rocking Cells: Synergistic Effect of Quaternary Phosphonium Bromide Salts Bearing Phenyl Groups and Different Alkyl Groups

a

onium salt added to PVCap (1500 ppm)

onium salt concn (ppm)

(n-Bu)4NBr (n-Bu)4PBr (n-Pe)4NBr (n-Pe)4PBr (n-Bu)3(iso-Hex)PBr (n-Bu)3hexadecylPBr Phe4PBr

1500 1500 1500 1500 1500 1500 1500

To (°C)a Ta (°C)b 8.1 5.8 4.7 4.9 5.8 8.0 12.2

5.8 4.6 3.3 3.5 4.4 7.2 9.6

To, average onset temperature. bTa, average rapid hydrate formation.

the quaternary ammonium bromide salt consisting of pentyl groups is a much better synergist with PVCap (PVCap) than its butylated homologue. This is also in accordance to other reported studies that have confirmed that (n-Pe)4NBr is a superior synergist than (n-Bu)4NBr when used with the KHI PVCap.8,33 Table 3 reveals that (n-Bu)4PBr (To = 5.8 °C) performed better than its ammonium equivalent (To = 8.1 °C). This is in line with their performance on THF hydrate crystal growth.28 6807


Energy & Fuels

Article

as a template for hydrate formation and therefore accelerating hydrate nucleation. (n-Bu)3hexadecylPBr had a synergistic KHI performance similar to that of (n-Bu) 4 NBr with PVCap. (nBu)3hexadecylPBr has a classic surfactant structure, but in our natural gas and water systems (i.e., no liquid hydrocarbon) there seems to be no KHI performance enhancement in using tributyl onium salts with long tails compared to four shorter butyl or pentyl groups.

In that study, their performance as THF hydrate crystal growth inhibitors were evaluated at varying concentrations for the same subcooling (3.8 °C), and at different subcoolings for the same concentration (4000 ppm). In addition, the tetra-n-pentyl phosphonium salt also showed a lower minimum inhibitor concentration (MCI) than its equivalent ammonium salt. Both (n-Pe)4PBr and (n-Pe)4NBr were better synergists than (n-Bu)4NBr when combined with PVCap. Interestingly, the improved performance of (n-Pe)4PBr over (n-Pe)4NBr as THF hydrate crystal growth inhibitors in our previous study does not correlate with their synergistic capability with PVCap revealed in this paper.28 Table 3 shows that To and Ta values for (nPe)4PBr (To = 4.9 °C and Ta = 3.5 °C) and (n-Pe)4NBr (To = 4.7 °C and Ta = 3.3 °C) were very similar to a statistically insignificant difference (p-value > 0.05 for 10 tests of each synergistic mixture). (n-Pe)4PBr and (n-Pe)4NBr enhanced the performance of PVCap with the same magnitude. Previously, mediocre THF hydrate crystal growth inhibitors of the quaternary ammonium and tris(dialkylamino)cyclopropenium salt types, have been found to be better gas hydrate synergists with PVCap than superior THF hydrate crystal growth inhibitors.8,32 For instance, tetra-n-hexylammonium bromide was found to be a better gas hydrate synergist than its n-butyl equivalent salt, although the latter salt was a better THF hydrate crystal growth inhibitor.8 As discussed earlier, higher hydrophobicity of the synergistic salt was suggested to increase disruption of the bulk water structure leading to prevention of gas hydrate nucleation. (n-Pe)4P+ cation has a larger effective ionic radius than (n-Pe)4N+ cation that makes the former more hydrophobic. Therefore, we might have expected (n-Pe)4PBr to outperform (n-Pe)4NBr as a synergist too. However, we speculate that (n-Pe)4PBr is a little larger than the optimum size for best interaction with open hydrate cavities, but for the smaller (n-Pe)4NBr the molecule is at the optimum size due to the smaller N atom. In the present study we have already seen how the gas hydrate inhibition capacity of quaternary phosphonium salts was strongly influenced by the type of KHI polymer present in the synergistic blend. We suggest that the higher hydrophobicity of (n-Pe)4PBr may lead to stronger van der Waals interactions with the hydrophobic part of the ε-caprolactam pendant groups of the polymer in PVCap leading to less interactions of the lactam ring with the hydrate surface and less adsorption onto the hydrate surface at least to some extent. A similar behavior is observed on the synergistic performances of (n-Bu)4PBr and (n-Bu)3(iso-Hex)PBr (Table 3).8 (n-Bu)3(isoHex)PBr had showed a statistically significant lower MIC value on THF hydrate crystals. However, (n-Bu)4PBr and (nBu)3(iso-Hex)PBr gave the same To value, To = 5.8 °C, with PVCap. Even though (n-Bu)4PBr consists of a total of 16 carbon atoms, i.e., two carbon atoms less than (n-Bu)3(isoHex)PBr, we suggest that the longer isohexyl chain could be interfering with the hydrophobic lactam rings to some extent causing a lower performing synergistic blend than expected based on THF hydrate crystal growth inhibition tests. Tetraphenylphosphonium bromide (Phe4PBr) was reported to be a poor THF hydrate crystal growth inhibitor.28 Its flat aromatic ring which lacks both the right size and right hydrophobicity to penetrate into cavities on hydrate crystal surfaces, make it a poor THF hydrate crystal growth inhibitor and poor synergist in combination with PVCap. In fact, Phe4PBr blended with PVCap increased the To value compared to PVCap alone. This might indicate that the salt can be serving

■

KHI EXPERIMENTS WITH VARIATION IN THE SYNERGIST:POLYMER RATIO An interesting behavior was observed when the weight ratios of (n-Pe)4PBr and PVCap were exchanged while maintaining the same total mass concentration of the salt and the active polymer (Table 4). The graph in Figure 6 depicts how To Table 4. Constant Cooling Tests in High Pressure Rocking Cells: Synergistic Effect of (n-Pe)4PBr with PVCap at Different Weight Ratios and 3000 ppm Total Concentration chem concn (ppm) PVCap

(n-Pe)4PBr

To (°C)a

Ta (°C)b

3000 2000 1500 1000 600 100 0

0 1000 1500 2000 2400 2900 3000

7.5 5.7 4.9 3.1 2.7 2.4 8.5c

7.1 4.4 3.5 2.0 1.9 2.1 7.2c

a

To, average onset temperature from 10 tests. bTa, average rapid hydrate formation from 10 tests. cAverage from four tests.

decreases with an increasing amount of the salt and, at the same time, a decreasing amount of the active polymer at 3000 ppm total concentration of active compounds. Even when the amount of polymer was only about 3.4% of the mass of the synergist, the To was still very low, To = 2.4 °C. It is worth mentioning that the To values for (n-Pe)4PBr:PVCap (2400 ppm:600 ppm) and (n-Pe)4PBr:PVCap (2900 ppm:100 ppm) were statistically insignificant (p-values from t tests higher than 0.05). These results imply a strong kinetic inhibition mechanism of the quaternary phosphonium salt in conjunction with the polymer. The data in Table 4 show that the polymeŕs concentration in the synergistic mixture can be as low as 3.3% of the formulation and still exhibit an excellent KHI effect. We speculate that the polymer is spread with the caprolactam pendant groups in the water phase closest to the gas phase. Due to favorable van der Waals interactions between the caprolactam ring pendants and the partially formed hydrate clathrate, hydrate nucleation is prevented in the neighborhood of the caprolactam rings at many places along the polymer strand. The mechanism by which (n-Pe)4PBr enhances the performance of the polymer (Figure 6) and the fact that inhibition is highly dependent on a “catalytic” amount of the polymer, implies that the first step in the synergistic inhibition process may be establishment of lactam ring−hydrate interactions at many sites along the polymer strand and at many places in the testing solution. The increasing amount of the quaternary phosphonium salt must alter the molecular interactions between the polymer and quaternary salt and between the water molecules, resulting in enhanced anti6808


Energy & Fuels

Article

Figure 6. Synergism of (n-Pe)4PBr with PVCap (Luvicap EG) at 3000 ppm total concentration of active components; error bars represent standard deviation (SD).

advantage, in gas−water systems at least, in using quaternary ammonium surfactants with long hydrophobic tails. A very interesting trend was observed when the ratio of (nPe)4PBr and PVCap was changed while maintaining the same total weight concentration of the salt and the active polymer. The optimal (n-Pe)4PBr:PVCap ratio that gives the best synergistic performance was 29:1 with an average onset temperature of 2.4 °C. Thus, excellent KHI performance can be obtained with as little as 100 ppm of KHI polymer as long as there is sufficient amount of a good synergist.

nucleation performance. The optimal amount of polymer needed to trigger the highest inhibition performance at 3000 ppm total concentration is 3.3% of the polymer and 96.7% of (n-Pe)4PBr to give an onset temperature of 2.4 °C.

■

CONCLUSION We have evaluated the synergistic capacity of five quaternary phosphonium bromide salts with commercially available KHI polymers on structure II-forming natural gas hydrates using high pressure rocking cell experiment tests. We focused on (nPe)4PBr and (n-Pe)4PBr which had previously been shown to have an excellent performance as a THF hydrate crystal growth inhibitor. The results were compared to equivalent quaternary ammonium bromide. (n-Pe)4PBr was shown to be a superior synergist than (nBu)4PBr when used in combination with VCap based polymers Luvicap EG (PVCap), Luvicap 55W (1:1 VP:VCap copolymer), or INHIBEX 713 (VCap terpolymer). (n-Bu)4PBr performed better than its ammonium equivalent bromide salt as synergist with PVCap. Both (n-Pe)4PBr and (n-Pe)4NBr were better synergists than (n-Bu)4NBr with PVCap. This was in accordance with their THF hydrate crystal growth performances previously reported. Both (n-Bu)4PBr and (n-Bu)3(iso-Hex)PBr were better synergists than (n-Bu)4NBr with PVCap. Surprisingly, (nPe)4PBr and (n-Pe)4NBr enhanced the inhibition performance of PVCap to the same magnitude. Based on THF hydrate crystal growth inhibition tests, these results were not expected since previously the phosphonium salt was shown to be a better THF hydrate crystal growth inhibitor. However, we speculate that the larger steric size of (n-Pe)4PBr is less optimal for hydrate crystal interaction than (n-Pe)4NBr. (n-Bu)4PBr and (n-Bu)3(iso-Hex)PBr exhibited similar synergistic performances with PVCap even though (nBu)3(iso-Hex)PBr had been reported to be a better THF crystal growth inhibitor. Phe4PBr showed a poor synergistic performance with PVCap. This was in line with a poor THF hydrate crystal growth performance previously reported. (nBu)3hexadecylPBr showed a synergistic performance similar to that of (n-Bu)4NBr with PVCap indicating that there is no

■

AUTHOR INFORMATION

Corresponding Author

* Tel.: +(47)51831823. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Sloan, E. D. J.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; Taylor & Francis: New York, 2008. (2) Koh, C. A.; Westacott, R. E.; Zhang, W.; Hirachand, K.; Creek, J. L.; Soper, A. K. Fluid Phase Equilib. 2002, 194, 143. (3) Kelland, M. A. Energy Fuels 2006, 20, 825. (4) Klomp, U. C.; Kruka, V. C.; Reijnhart, R.; International Patent Application WO9517579, 1995. (5) Klomp, U. C.; Reijnhart, R. International Patent Application WO9634177, 1996. (6) Klomp, U. C.; Kruka, V. C.; Reijhart, R. In Proceedings of the Symposium Controlling Hydrates, Waxes and Asphaltenes; IBC: Aberdeen, Scotland, 1997. (7) Klomp, U. C. International Patent Application WO9913197, 1999. (8) Chua, P. C.; Kelland, M. A. Energy Fuels 2012, 26, 1160. (9) Klomp, U. C.; Le Clerq, M.; Van Kins, S. Proceedings of the 2nd Petromin Conference, Shangri-La, Kuala Lumpur, Malaysia, 2004. (10) Frostman, L. M.; Downs, H. Proceedings of the 2nd International Conference on Petroleum and Gas Phase Behaviour and Fouling, Copenhagen, Denmark, 2000. (11) Kelland, M. A. In Advances in Materials Science Research; Wytherst, M. C., Ed.; Nova Science: New York, 2011; Vol. 8. (12) Duncum, S.; Edwards, A. R.; Osborne, C. G. International Patent Application WO9604462, 1996.

6809


Energy & Fuels

Article

(13) Dahlmann, U.; Feustel, M. U.S. Patent Application No. 20040159041, 2004. (14) Dahlmann, U.; Feustel, M. U.S. Patent 7183240, 2007. (15) Frostman, L. M. Proceedings of the SPE Annual Technical Conference and Exhibition, SPE 63122, Dallas, TX; Society of Petroleum Engineers: Richardson, TX, USA, 2000. (16) Duncum, S.; Edwards, A. R.; James, K.; Osborne, C. G. International Patent Application WO9629502, 1996. (17) Norland, A. K.; Kelland, M. A. Chem. Eng. Sci. 2012, 69, 483. (18) Rodionova, T. V.; Dyadin, Y. A.; Udachin, K. A.; Lipkowski, Y.; Suwinska, K. J. Struct. Chem. 1995, 36, 458. (19) Aladko, L. S.; Dyadin, Y. A.; Rodionova, T. V.; Terekhova, I. S. J. Struct. Chem. 2002, 43, 990. (20) Aladko, L. S.; Dyadin, Y. A.; Rodionova, T. V.; Terekhova, I. S. J. Mol. Liq. 2003, 106, 229. (21) Dyadin, Y. A.; Aladko, L. S.; Zelenina, L. S.; Terekhova, I. S.; Zelenin, Y. M.; Polyanskaya, T. M.; Yakovleva, N. I. In Mezhmolekulyarnoe Vzaimodeistvie i Konformatsii Molekul, Tezisy Dokladov Vsesoyuznogo Simposziuma, 3rd ed.; Poltev, V. I., Ed.; Moscow, 1976; p 35. (22) Deschamps, J.; Dalmazzone, D. J. Chem. Eng. Data 2010, 55, 3395. (23) Mayoufi, N.; Dalmazzone, D.; Fürst, W.; Delahaye, A.; Fournaison, L. J. Chem. Eng. Data 2009, 55, 1271. (24) Oyama, H.; Shimada, W.; Ebinuma, T.; Kamata, Y.; Takeya, S.; Uchida, T.; Nagao, J.; Narita, H. Fluid Phase Equilib. 2005, 234, 131. (25) Shimada, W.; Ebinuma, T.; Oyama, H.; Kamata, Y.; Takeya, S.; Uchida, T.; Nagao, J.; Narita, H. Jpn. J. Appl. Phys. 2003, 42, 129. (26) Gholinezhad, J.; Chapoy, A.; Tohidi, B. Chem. Eng. Res. Des. 2011, 89, 1747. (27) Li, X.-S.; Zhan, H.; Xu, C.-G.; Zeng, Z.-Y.; Lv, Q.-N.; Yan, K.-F. Energy Fuels 2012, 26, 2518. (28) Kelland, M. A.; Gausland, F.; Tsunashima, K. Chem. Eng. Sci. 2013, 98, 12. (29) Ross, M. F.; Kelso, G. F.; Blaikie, F. H.; James, A. M.; Cocheme, H. M.; Filipovska, A.; Da Ros, T.; Hurd, T. R.; Smith, R. A. J.; Murphy, M. P. Biochemistry (Moscow) 2005, 70, 222. (30) Sloan, E. D.Jr. U.S. Patent 5432292, 1995. (31) Angel, M.; Stein, S.; Neubecker, K. WO2001066602, 2001. (32) Kelland, M. A.; Reyes, F. T.; Trovik, K. W. Chem. Eng. Sci. 2013, 93, 423. (33) Sefidroodi, H.; Chua, P. C.; Kelland, M. A. Chem. Eng. Sci. 2011, 66, 2050.

6810


Study on the Synergistic Properties of Quaternary Phosphonium

Recommend Documents