Article pubs.acs.org/EF
Tetra(iso-hexyl)ammonium BromideThe Most Powerful Quaternary Ammonium-Based Tetrahydrofuran Crystal Growth Inhibitor and Synergist with Polyvinylcaprolactam Kinetic Gas Hydrate Inhibitor Pei Cheng Chua and Malcolm A. Kelland* Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ABSTRACT: Tetraalkylammonium salts, particularly with several n-butyl or n-pentyl or iso-pentyl groups, have previously been shown to be excellent structure II (SII) gas and tetrahydrofuran (THF) hydrate crystal growth inhibitors. We have investigated the ability of quaternary ammonium salts with isoalkyl groups with 5 to 7 carbons atoms to inhibit the growth of tetrahydrofuran (THF) hydrate crystal. Two new quaternary salts were synthesized for the first time: tetra(iso-hexyl)ammonium bromide (TiHexAB) and tetra(iso-heptyl)ammonium bromide (TiHepAB). It was found that tetra(iso-pentyl)ammonium bromide (TiPeNB) gave poorer crystal growth inhibition than isomeric tetra(n-pentyl)ammonium bromide (TPAB) but similar performance to tetra(n-butyl)ammonium bromide (TBAB). However, TiHexAB gave better inhibition than any quaternary ammonium previously reported, including TPAB. TiHepAB was a poorer THF hydrate crystal growth inhibitor, similar in performance to isomeric tetra(n-hexyl)ammonium bromide. We believe the reason for these results is related to the optimal length of the n-alkyl/isoalkyl groups and the improved van der Waals interaction with open SII hydrate cages with the isoalkyl branching at the end of the chains, compared to a straight alkyl chain. The superior inhibition performance of TiHexAB was illustrated by testing its ability as a synergist for the well-known kinetic hydrate inhibitor (KHI) polyvinylcaprolactam (PVCap). In high pressure rocking cell tests using a SII-forming natural gas mixture, TiHexAB clearly outperformed TBAB, TPAB, and all the other quaternary ammonium salts tested. In addition, tetra(n-hexyl)ammonium bromide (THexAB) gave KHI synergism with PVCap superior to that of TBAB, even though TBAB was a better THF hydrate crystal growth inhibitor. We speculate that adsorption onto hydrate crystal surfaces may not be the only synergistic mechanism operating and that the more hydrophobic THexAB is perturbing the nucleation of hydrate more than the less hydrophobic TBAB. In addition, it was investigated whether replacing a carbon atom with an oxygen atom (ether linkage) in the alkyl chains of TPAB would affect the THF hydrate crystal growth inhibition. Thus, tetra(alkoxyethyl)ammonium bromides were prepared for the first time where the alkoxy group is ethoxy or methoxy. Both of these quaternary ammonium salts gave negligible THF hydrate crystal growth inhibition.
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INTRODUCTION It has been known for about two decades that certain onium salts (quaternary ammonium and phosphonium salts) are capable of perturbing and inhibiting the growth of structure II (SII) clathrate hydrates.1−4 Shell, the oil and energy company, was the first to discover this fact from tests with a study on SII tetrahydrofuran (THF) hydrates. The most active inhibitors of THF hydrate were found to have either n-butyl, n-pentyl, or iso-pentyl groups on the quaternary N or P atom (Figure 1). Shell’s patents on this
Figure 1. Structures of alkylated quaternary ammonium, phosphonium, and sulphonium salts.
examples of quaternary salts that are good SII hydrate crystal growth inhibitors are tetra(n-butyl)ammonium bromide (TBAB) and tetra(n-pentyl)ammonium bromide (TPAB). If alkyl groups other than n-butyl, n-pentyl, or iso-pentyl were placed in the quaternary headgroup, the THF hydrate crystal growth inhibition performance dropped drastically. Others besides Shell have also confirmed the THF hydrate growth inhibition properties of TBAB.5,6 Bis-quaternary and polyquaternary ammonium salts have also been studied as tetrahydrofuran hydrate crystal growth inhibitors.7 Quaternary salts in these classes have been used in two ways to control gas hydrate formation in the upstream oil and gas industry in low dosage hydrate inhibitor formulations: as synergists for kinetic hydrate inhibitor polymers8−14and as antiagglomerants when modified into surfactants.8,9,15−20 Small quaternary ammonium surfactants have previously been shown to be excellent synergists for N-vinyl caprolactam polymers that function as kinetic hydrate inhibitors (KHIs),
subject also cover sulphonium salts, which can have a maximum of three alkyl groups attached to the sulfur atom, although no results have ever been reported on this subclass. Typical
Received: November 23, 2011 Revised: January 7, 2012 Published: January 12, 2012
© 2012 American Chemical Society
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interactions with more than one cavity on the surface and will therefore detach easily. In fact, from modeling studies, it has been shown that THAB cannot easily get embedded in the surface of a structure I or II hydrate.9 We were interested in expanding the range of tetra(alkylammonium) salts with a view to testing their ability to prevent tetrahydrofuran (THF) hydrate crystal growth and as KHI synergists for polymers such as PVCap. In this work, we describe the synthesis and hydrate inhibitor testing of two new tetra(iso-alkyl)ammonium salts, tetra(iso-hexylammonium) bromide (TiHexAB) and tetra(iso-heptyl)ammonium bromide (TiHepAB) (Figure 3). In addition, we investigated whether
such as the homopolymer poly(N-vinyl caprolactam) (PVCap).11−14 Such synergistic blends have been used in commercial KHI formulations (Figure 2).
Figure 2. Poly(N-vinylcaprolactam) (PVCap).
When used alone, quaternary salts such as TBAB or TPAB do not inhibit SII gas hydrate nucleation, rather they can promote hydrate formation.9 This is probably related to the fact that these ammonium salts form clathrate hydrate structures of their own which have some common cluster features of SII hydrates.21,22 Thus, they can act as “templates” for the first formation of hydrate nucleii. It has been shown different polyhydrates are known to exist for TBAB or TiPAB, and also shown for the intermediate butyl/iso-pentyl species. The type of hydrate depends on the anion, also (e.g. carboxylates, halides, hydroxides, oxalates, etc.).23,24 It has also been shown that the clathrate framework better fits the iso-pentyl group compared to the n-butyl group.25,26 As far as we are aware, no research on quaternary ammonium hydrates containing iso-hexyl groups has been reported. Bis-quaternary salts have also been shown to form clathrate hydrates, such as salts of the hexamethylenebis(tributylammonium) ion.27 It is thought that TBAB (or TPAB) works synergistically with PVCap because of their different geometries. Thus, TPAB and PVCap should attach to different sites on the hydrate crystal surface. Molecular modeling carried out at the University of Reading for RF-Rogaland Research (now The International Research Institute of Stavanger) in the mid-1990s showed that TBAB or TPAB penetrates a 51264 cavity on the 1,1,1 structure II hydrate surface.9 Two of the other butyl or pentyl groups lay in channels on the hydrate surface where new 51264 cages would normally be formed. It therefore seems possible that these cages could partially form, trapping or imbedding the butyl or pentyl groups in the hydrate surface. Below the critical nuclear size, growth of the nuclei is energetically unfavorable (ΔG is positive). So, TBAB or TPAB will not be embedded in the surface of the nuclei, but they will more easily detach. Above the critical nuclear size, TBAB or TPAB can become embedded in the hydrate surface as partial hydrate cages form around the butyl or pentyl groups, but further structure II growth is prevented by the remaining butyl or pentyl groups. The embedding mechanism for the quaternary salts explains why tetra(n-propyl)ammonium bromide (TPrAB) and tetra(nhexyl)ammonimum bromide (THAB) are significantly poor SII hydrate growth inhibitors. TPrAB has an even weaker van der Waals interaction with cavities and channels on the hydrate surface than TBAB. TPrAB will more easily detach from the surface even if some water molecules start to build cavities at the ends of the propyl groups; that is, it is less quickly embedded in the surface. On the other hand, THAB has alkyl groups that are too long to give strong van der Waals
Figure 3. Tetra(iso-hexyl)ammonium bromide (TiHexAB) (left) and tetra(iso-heptyl)ammonium bromide (TiHepAB) (right).
tetra(n-alkyl)ammonium salts substituted with ether linkages in the alkyl chains would be just as effective as THF hydrate crystal growth inhibitors as their tetra(n-alkyl)ammonium “allcarbon chain” counterparts. Synthesis of Tetraalkylammonium Salts. All chemicals were purchased from either VWR or Sigma-Aldrich except isohexylamine, which was purchased from Otava Chemicals, Ukraine. PVCap was made using radical polymerization in 2propanol as solvent, and the solvent was removed to give a colorless powder. The Mw was found by GPC to be approximately 18000 Da (DMF solvent and polyethyleneoxide standards). The synthesis of the quaternary ammonium compounds was carried out by standard quaternization procedures, from either a primary amine (with added base) or the tertiary trialkylamines using alkyl bromides in a polar solvent.28−31 The 1H and 13C NMR spectra of triiso-pentylamine was checked for purity, as some commercial samples are mixtures containing both npentyl and iso-pentyl groups. A typical preparation is given here for tetra(iso-hexyl)ammonium bromide (TiHexAB): 3.3 mol equiv of iso-hexyl bromide, 3 mol equiv of powdered K2CO3, and 1 mol equiv of iso-hexylamine hydrochloride were refluxed in isobutyronitrile for 5 days. The solution was filtered, and the solvent was removed to leave an off-white solid (52% yield). Crystals of tetra(iso-hexyl)ammonium bromide (TiHexAB) were grown from an ethyl acetate: diethyl ether mixture cooled to −30 °C over 24 h. NMR spectroscopy data were recorded on a 300 MHz Varian instrument. 1H NMR (CDCl3): 0.92 (doublet, 24H), 1.31 (quartet, 8H), 1.66 (multiplet, 4H), 1.68 (multiplet, 8H), 3.39 (multiplet, 8H). 13C NMR (CDCl3): 20.14, 22.27, 27.44, 35.16, 59.45. The crystal structure of TiHexAB has been determined.32 Tetra(iso-heptyl)ammonium bromide (TiHepAB) was made similarly with 40% yield after recrystallization. 1H NMR (CDCl3): 0.89 (doublet, 24H), 1.23 (multiplet, 16H), 1.64 1161
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(multiplet, 4H), 1.67 (multiplet, 8H), 3.38 (multiplet, 8H). 13C NMR (CDCl3): 22.53, 22.67, 24.34, 27.92, 38.36, 59.33. We also synthesized and tested two tetra(alkylether) quaternary ammonium compounds to see if replacing a methylene group (−CH2−) in the butyl or pentyl groups of TBAB or TPAB would enhance the performance.33,34 Preparations of the two compounds synthesized are as follows: • CH3CH2OCH2CH2Br (3 equiv) was refluxed with CH3CH2OCH2CH2NH2 (1 equiv) and K2CO3 (2 equiv) in acetonitrile for 8 days. The solvent was stripped, and the residue was recrystallized from a mixture of dichloromethane and diethyl ether at −30 °C to give white crystals of (CH3CH2OCH2CH2)4N+Br− in 80% yield. 1H NMR (CDCl3): 1.18 (triplet, 12H), 3.54 (multiplet, 8H), 3.93 (multiplet, 16H). 13C NMR (CDCl3): 14.93, 61.40, 64.03, 66.87. • (CH3OCH2CH2)4N+Br− was prepared similarly, from CH3OCH2CH2Br (3 equiv) and CH3OCH2CH2NH2, except the reflux lasted only 2 days and the product was recrystallized at 2−3 °C to give a 70% yield of white crystals. 1H NMR (CDCl3): 1.20 (singlet, 12H), 1.73 (multiplet, 8H), 2.82 (multiplet, 8H), 3.55 (multiplet 8H). 13C NMR (CDCl3): 33.11, 39.60, 64.12, 67.76. These compounds can also be made from trichloroalkylamines.35 THF Hydrate Crystal Growth Experimental Method. THF hydrate crystal growth rate experiments have been reported previously.1−4,36−39 Our method, which is very similar to that of other workers, has also been reported previously.5,7,40−42 An outline of the procedure is as follows: A THF/water solution of 1:17 molar ratio containing 36000 ppm NaCl was used. 80 mL of this solution was cooled in a 100 mL glass beaker placed in a cooling bath (accuracy ±0.05 °C) set to −0.5 °C (giving a theoretical subcooling for THF hydrate of about 3.4 °C) for 20 min with stirring every 5 min. Lower temperatures were also used from some chemicals. Then, a glass tube containing crushed ice crystals at the tip was inserted into the solution to nucleate THF hydrate formation. The rate of growth of THF crystals (g/h) at the tip of the glass tube was then measured by weighing the crystals, usually after 1 h. The average growth rate of 4−8 experiments was recorded, in which the spread in growth rates was about 20−25%. Crystals growing on the glass tube and not connected to crystals at the tip were ignored. Sometimes crystals grew to the edge of the beaker and therefore could not be weighed properly. These are discussed in the next section. High Pressure Gas Hydrate Rocker Rig Equipment Test Methods. Kinetic hydrate inhibition experiments were conducted in five high pressure 40 mL steel rocking cells, each containing a steel ball. (Figure 4) The gas composition used was a synthetic natural gas mixture given in Table 1. The test procedure was a constant cooling test method similar to that used in our autoclaves. 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.43,44 Five repeat tests were carried out, which gave 20.2 °C ± 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.
Figure 4. The rocker rig showing the five steel cells in a cooling bath.
Table 1. Composition of Synthetic Natural Gas (SNG) component
mol %
methane ethane propane iso-butane n-butane N2 CO2
80.67 10.20 4.90 1.53 0.76 0.10 1.84
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 to 2 °C, at a rate of 1 °C/h. 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, was logged on a computer. A typical graph of pressure and temperature data versus time from all five cells is shown in Figure 5. The pressure drops about 2 bar due to gas being dissolved in the aqueous phase. The temperature drops at a constant rate until it reaches the minimum of 2 °C after 1120 min. During this time, the pressure also drops at a constant rate, as it is a closed system, until the rate of pressure drop increases due to hydrate formation. The first deviation from the pressure drop due the temperature drop is taken as the time of 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 5, which shows data from five identical KHI tests, To is in the range 7.5−8.5 °C. This degree of scattering is typical of the range observed in this multicell rocker rig, and it is never more than 15−20% and reflects the stochastic nature of gas hydrate formation. A more thorough investigation of the reproducibility at various test conditions in this multicell rocker rig has been performed.45 At some point rapid, hydrate formation ensues as detected by a rapid pressure drop in the cells. In Figure 6, which shows a single experiment, this occurs after about 1000 min. The temperature, at which 1162
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Figure 5. Typical pressure and temperature graphical data obtained from five identical constant cooling tests in the multicell rocker rig.
Figure 6. Example of To and Ta calculation in a constant cooling rocker rig KHI experiment.
rapid hydrate formation occurs, Ta, is determined for each cell. Ta is determined from when hydrate growth is at its most rapid, that is, the steepest part of the pressure versus time graph. Generally, we find that there is less scattering in the Ta values (
