Article pubs.acs.org/EF
Breakthrough in Synergists for Kinetic Hydrate Inhibitor Polymers, Hexaalkylguanidinium Salts: Tetrahydrofuran Hydrate Crystal Growth Inhibition and Synergism with Polyvinylcaprolactam Malcolm A. Kelland,* Nina Moi, and Michelle Howarth Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ABSTRACT: A series of hexaalkylguanidinium salts have been synthesized, either from urea using the well-known Vilsmeier route or in a one-pot alkylation reaction using guanidinium chloride. The hexaalkylguanidinium salts are water-soluble up to the hexa-n-butyl derivative but sparingly soluble for the hex-n-pentyl or hexa-isopentyl derivatives. The ability of hexaalkylguanidinium salts to inhibit the growth of a tetrahydrofuran (THF) hydrate crystal has been investigated. The hexabutylated derivative gave excellent crystal growth inhibition, superior to the performance of the quaternary ammonium salt tetra(n-butyl)ammonium bromide (TBAB) and close to that of tetra(n-pentyl)ammonium bromide (TPAB). As the alkyl group is reduced in length to propyl and ethyl, the inhibition performance drops off radically. The superior inhibition performance of hexabutylguanidinium bromide was illustrated by testing its ability as a synergist for the well-known kinetic hydrate inhibitor (KHI), poly(N-vinylcaprolactam) (PVCap). In high-pressure steel rocking cell and steel autoclave experiments using a natural gas mixture giving preferentially structure II gas hydrate, hexabutylguanidinium bromide clearly outperformed both TBAB and TPAB as synergists. Guanidinium chloride and hexabutylguanidinium bromide were shown to be poor antinucleator KHIs when used alone.
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INTRODUCTION Quaternary ammonium and phosphonium salts with the correct alkyl groups are known to inhibit the growth of structure II (SII) clathrate hydrates. This was first established using tests on SII tetrahydrofuran (THF) hydrate crystal growth.1−4 The preferred alkyl groups for best inhibition were found to be nbutyl, n-pentyl or isopentyl, and isohexyl, with isohexyl giving the strongest inhibition of all (Figure 1).5−7 Quaternary ammonium salts with two or more quaternary groups have also been reported for their effect on THF hydrate crystal growth.8
KHIs. Most work has been reported on the homopolymer poly(N-vinylcaprolactam) (PVCap) (Figure 2).12−15 Such synergistic blends have been used in commercial KHI formulations.
Figure 2. Poly(N-vinylcaprolactam) (PVCap).
Quaternary ammonium salts, such as TBAB or tetra(npentyl)ammonium bromide (TPAB), do not inhibit SII gas hydrate nucleation when used by themselves but have been shown to promote hydrate formation.10 This may be because such salts form clathrate hydrate structures of their own, which have some of the same structural features of SII hydrates.22,23 This can lead to these salts being templates for the initiating of gas hydrate formation. We were interested in investigating other organic cations with multiple alkyl groups as potential synergists for KHI polymers and as AAs. It occurred to us that amidinium and guanidinium salts, which contain two and three nitrogen atoms,
Figure 1. Tetra(n-butyl)ammonium bromide (TBAB) (left), tetra(npentyl)ammonium bromide (TPAB) (middle), and tetra(isohexyl)ammonium bromide (TiHexAB) (right).
Quaternary ammonium salts have been used in low-dosage hydrate inhibitor (LDHI) formulations in two application types: (1) as synergists for kinetic hydrate inhibitor (KHI) polymers9−15 and (2) as anti-agglomerants (AAs) when modified into surfactants.9,10,16−21 Small quaternary ammonium salts, such as tetrabutylammonium bromide (TBAB), have been reported to be strong synergists for N-vinylcaprolactam polymers that function as © 2013 American Chemical Society
Received: October 20, 2012 Revised: January 3, 2013 Published: January 9, 2013 711
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The ether extracts were dried over sodium sulfate, filtered, and stripped to give a pale yellow oil of penta-n-butylguanidine. A mixture of 1.7 g (5 mmol) of penta-n-butylguanidine (made as described earlier), 0.686 g (5 mmol) of 1-bromobutane, and 10 mL of isobutyronitrile was heated under reflux for 20 h. Upon vacuum stripping, a pale yellow oil was obtained, which crystallized to a pale brown solid upon standing. Upon recrystallization from a mixture of hexane and ethyl acetate, the desired hexa-n-butylguanidinium bromide was obtained as a white solid in approximately 40% yield. Scaling up the Vilsmeier synthesis to make hexaaalkylguanidinium salts from tetraalkylureas is less practical on a large scale. A search of the chemical literature suggests that direct alkylation of guanidine with fairly large groups, such as butyl or pentyl, is difficult and proceeds in low yield.27 This is also true for the alkylation of diguanidinium salts.28 Biphasic systems and phase-transfer catalysts have been used. The onepot procedure that we have used with a mild base, potassium carbonate, represents an easier way to produce hexaalkylated guanidinium salts in reasonable yields. The preparation of hexaalkylguanidinium chlorides is illustrated here for hexa-nbutylguanidinium chloride (Figure 6).
respectively, could be used to increase the number of alkyl groups around a cationic center relative to tetra(alkyl)ammonium salts. Thus, pentaalkylamidinium and hexaalkylguanidinium salts could potentially have greater effect on hydrate inhibition than the four alkyl groups present in tetra(alkyl)ammonium salts (Figure 3). In this work, we describe the
Figure 3. Pentaalkylamidinium salts (left) and hexaalkylguanidinium salts (right).
synthesis of a series of hexaalkylguanidinium salts and their ability to prevent THF hydrate crystal growth and as KHI synergists for PVCap. Pentaalkylamidinium-based LDHIs will be described in later publications.
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EXPERIMENTAL SECTION
Synthesis of Hexaalkylguanidinium Salts. All chemicals were purchased from either VWR or Sigma-Aldrich. PVCap used in the high-pressure synthesis studies was a 41.1 wt % solution of a lowmolecular-weight PVCap in monoethyleneglycol (MEG), kindly supplied by BASF, Germany. Nuclear magnetic resonance (NMR) spectra were recorded on a 300 MHz Varian NMR spectrometer at the University of Stavanger and a 500 MHz Bruker by Ewos AS, Dirdal, Norway. Tetraalkylureas were synthesized according to a literature procedure but using triphosgene instead of phosgene (Figure 4).24
Figure 6. One-pot alkylation of guanidinium chlorides. A mixture of 3 g (31.4 mmol) of guanidinium choride, 27.336 g (197.8 mmol) of potassium carbonate, and 27.104 g (197.8 mmol) of butyl bromide were refluxed for 16 h in isobutyronitrile (30 mL). Volatiles were removed, and the residue was refluxed for 1 h with excess 6 M hydrochloric acid. Volatiles were removed under high vacuum to leave a pasty white solid, which was pure hexa-nbutylguanidinium chloride by NMR spectroscopy. Other acids can be used to make salts with other anions. 1H NMR (CDCl3) δ: 0.89 (t, CH3), 1.25 (m, CH2), 1.43 (m, CH2), 3.07 (t, CH2). 13C NMR (CDCl3) δ: 13.8, 20.1, 30.1, 47.9, 165.4. THF Hydrate Crystal Growth Experimental Method. THF hydrate crystal growth rate experiments have been reported previously.1−4,29−32 Our method, which is very similar to other workers, has also been reported previously.5,7,33−36 Essentially, THF hydrate crystals are grown from a THF/water solution of 1:17 molar ratio containing 36 000 ppm NaCl at −0.5 °C using ice to initiate crystallization. After 1 h of growth, the crystals are weighed and their morphology is examined. A total of 4−8 experiments were carried out on all chemicals at any one concentration (1000, 2000, or 4000 ppm). High-Pressure Gas Hydrate Rocker Rig Equipment Test Methods. Kinetic hydrate inhibition experiments were conducted in
Figure 4. Synthesis of tetraalkylureas.
The synthesis of hexalkylguanidinium salts via tetraalkylureas using the Vilsmeier reaction has been reported previously and is illustrated here for hexa-n-butylguanidinium bromide (Figure 5).25,26 A mixture of 5.69 g (20 mmol) of tetra-n-butylurea, 3.22 g (21 mmol) of phosphorus oxychloride, and 15 mL of acetonitrile was heated at 75 °C in a nitrogen atmosphere for 1 h. The mixture was then cooled to 0 °C, and 3.36 g (46 mmol) of n-butylamine was added over 15 min with stirring. The mixture was warmed to 60 °C for 1 h and again cooled to 0 °C, quenched with 5 mL of 25% (by weight) aqueous sodium hydroxide solution, and extracted with diethylether.
Figure 5. Synthesis of hexaalkylguanidinium bromides via the Vilsmeier reaction. 712
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five high-pressure 40 mL steel rocking cells each containing a steel ball (Figure 7). The equipment was supplied by PSL Systemtechnik, Germany, and has been previously reported.7 The gas composition used was a synthetic natural gas mixture given in Table 1.
agrees very well with a calculated Teq value of 20.5 °C at 76 bar using Calsep’s PVTSim software. Our constant cooling test procedure has been reported previously, and only a brief outline is given here.7 Thus, 20 mL of the chemical solution in deionized water in each cell was rocked and cooled from 20.5 °C at a rate of 1 °C/h down to 2 °C. The initial pressure was 76 bar. The pressure and temperature in each cell were recorded. A graph of the logged data versus time from one set of experiments is shown in Figure 8. An explanation of our interpretation of such data has been reported.7,39 Figure 9 shows results from one rocking cell only. From this data, we derive the onset temperature (To) and the temperature at which fast hydrate formation is observed (Ta). At least 10 experiments on each chemical or synergist formulation were carried out to enable statistically significant rankings of the performance of the chemicals. High-Pressure Gas Hydrate Steel Autoclave Test Methods. Besides the use of rocking cells, kinetic hydrate inhibition experiments were also conducted in a stirred 23 mL steel autoclave, which we have used in previous studies (Figure 10).40,41 The gas composition used was the same synthetic natural gas mixture used in the rocking cell experiments and given in Table 1. The test procedure is similar to that outlined for the rocking cells. A total of 8 mL of the aqueous solution containing dissolved inhibitor was placed in the autoclave. The starting pressure was 76 bar, and the temperature decreased at 1 °C/h from 19.5 to 2 °C while stirring at 600 rpm. In the same way as for the tests in rocking cells, the onset temperature (To) for hydrate formation was recorded as the first drop in pressure and not because of the temperature drop in a closed system. The temperature at which rapid hydrate formation occurred, Ta, was also recorded. The degree of scattering in the data for To and Ta for any one chemical was of a larger range, 20−25%, than that observed in the rocking cells. It appears that smaller, stirred autoclaves may give a greater stochasticism to the gas hydrate formation process. This has been confirmed in a recent study with varying sizes of stirred autoclaves and rocking cells.42
Figure 7. Rocker rig showing the five steel cells in a cooling bath.
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
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RESULTS AND DISCUSSION THF Hydrate Crystal Growth Experiments. Table 2 lists the results for the average crystal growth rates of THF hydrate with various hexaalkylguanidinium salts. A minimum of six tests on each additive were conducted over 1 h. The scattering was not more than 20% in any set of tests for any one additive. The table gives results for no additives and for some tetraalkylammonium bromide salts for comparison.
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.37,38 Five repeat tests were carried out, which gave 20.2 ± 0.05 °C as Teq. This value
Figure 8. Typical pressure and temperature graphical data obtained from five identical constant cooling tests in the multi-cell rocker rig. 713
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Figure 9. Example of To and Ta calculations in a constant cooling rocker rig KHI experiment.
two. The crystal growth inhibition was superior to that found for the quaternary ammonium salt TBAB with four butyl groups but not quite as good as for TPAB. The morphology of the THF hydrate crystals was also clearly affected. With no additive, we observe the well-known regular pyramidal (octahedral) crystals. With 4000 ppm of hexabutylguanidinium chloride or bromide, we observed very distorted crystals as first seen by Shell for the best tetraalkylammonium salts.1−3 They described the THF hydrate crystals formed with TPAB or TBAB as looking like scrunched up pieces of paper. This is the same as we observed with the hexabutylguanidinium salts. At lower concentrations, crystal distortion is still seen but not so prominent. Thus, we observed crystals with rounded edges using 2000 ppm hexabutylguanidinium salts. Again, this is also seen with the same concentration of TPAB or TBAB. When the size of the alkyl group is decreased to n-propyl or ethyl, the crystal growth inhibition drops significantly. This follows the same trend seen for tetra(alkyl)ammonium salts, where the n-propylated and ethylated salts exhibited a much worse inhibition performance than TBAB.1−3 Both hexapentylguanidinium chlorides (normal or isopentyl) were poorly soluble in water or the THF/NaCl/water mixture. Nevertheless, the hexa-n-pentylguanidinium chloride was tested, giving poor THF hydrate crystal growth inhibition as expected on the basis of its solubility. Oily droplets of the insoluble guanidinium salt could be seen on the surface of the liquid in the glass beakers used in the tests. Attempts to make partially pentylated water-soluble guanidinium salts that might have improved crystal growth inhibition performance have thus far been unsuccessful. We are in the processing of carrying out computer-modeling studies on the interaction of hexaalkylguanidinium salts with hydrate crystal surfaces. Our preliminary results suggest a wider range of interactions with the crystal surfaces than with tetraalkylammonium salts. This is probably due to the larger number of alkyl groups available for interacting with open cavities on the crystal surface, leading to improved adsorption. KHI Experiments with a Hydrocarbon Gas Mixture. Quaternary ammonium salts with butyl, pentyl, or isohexyl groups are synergists for PVCap.2,9,11−13 We knew from experience with tetraalkylammonium salts and trialkylamine
Figure 10. Stainless-steel autoclave high-pressure test equipment.
Table 2. Average THF Hydrate Crystal Growth Rates in g/h after 1 h of Growth concentration
a
chemical (synthetic method)
4000 ppm
no additive tetrabutylammonium bromide tetrapentylammonium bromide guanidinium chloride hexaethylguanidinium bromide hexapropylguanidinium bromide hexabutylguanidinium bromide hexabutylguanidinium chloride hexapentylguanidinium chloridea
1.75 0.23 0.05 1.38 0.79 0.65 0.04 0.03 0.81
2000 ppm
1000 ppm
0.68 0.15
1.19 0.42
0.37 0.18
0.79
Sparingly soluble in a THF/NaCl/water mixture.
The results in Table 2 show that the most effective hexaalkylguanidinium salts for THF hydrate crystal growth inhibition are the hexabutylguanidinium salts. At 4000 ppm, the bromide and chloride salts gave similar results, but at 2000 ppm, the chloride salt showed the higher performance of the 714
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Table 3. Constant Cooling KHI Tests in High-Pressure Rocking Cellsa chemical no additive tetrabutylammonium bromide guanidinium chloride hexabutylguanidinium bromide PVCap PVCap PVCap + TBAB PVCap + TPAB PVCap + hexapropylguanidinium bromide PVCap + hexabutylguanidinium bromide a
concentration (ppm) 5000 5000 5000 2500 5000 2500 2500 2500 2500
+ + + +
average To (°C)
subcooling at To (°C)
average Ta (°C)
subcooling at Ta (°C)
18.0 17.8 17.7 16.5 8.7 6.6 6.5 5.6 7.6 2.2
2.5 2.7 2.8 3.9 11.6 13.6 13.5 14.6 12.6 17.9
18.0 17.1 17.6 15.1 8.1 6.4 6.3 2.9 3.9 18.1
2500 2500 2500 2500
Average of 10 experiments.
Table 4. Constant Cooling KHI Test Results in a Steel Autoclave chemical no additive tetrabutylammonium bromide guanidinium chloride hexabutylguanidinium bromide PVCap PVCap PVCap + TBAB PVCap + guanidinium chloride PVCap + hexabutylguanidinium bromide
concentration (ppm)
To (°C)
subcooling at To (°C)
Ta (°C)
subcooling at Ta (°C)
5000 5000 5000 2500 5000 2500 + 2500 2500 + 2500 2500 + 2500
11.5 11.5 11.9 10.7 7.9 4.8 6.2 7.8
