Nonpolymeric Kinetic Hydrate Inhibitors: Alkylated Ethyleneamine

Sep 1, 2015 - The butylated hyperbranched polyethylenimine oxides (HPEI-Bu-AOs) showed molecular weight KHI performance dependence...
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Nonpolymeric Kinetic Hydrate Inhibitors: Alkylated Ethyleneamine Oxides 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: In this work, we present the results of a rare class of nonpolymeric molecules that show significant performance as stand-alone kinetic gas hydrate inhibitors (KHIs). n-Butylated amine oxides of four nonpolymeric oligoethyleneamines, pentaethylenehexamine (PEHA), tetraethylenepentamine (TEPA), triethylene tetramine (TETA), diethylenetriamine (DETA) (all made from ammonia and ethylene) with molecular weights in the range 103−232 D, as well as four hyperbranched polyethylenimines (HPEIs) (made from aziridine), with molecular weights in the range of 0.3−10.0 kDa, have been synthesized and evaluated for their KHI performance. All the amine oxides were first evaluated by carrying out slow constant cooling experiments (1 °C/h) at 76 bar with a Structure II hydrate-forming gas in steel rocking cells. The butylated hyperbranched polyethylenimine oxides (HPEI-Bu-AOs) showed molecular weight KHI performance dependence. The performance increased with decreasing molecular weight of the polymer with HPEI-Bu-AOs prepared from low-molecular-weight oligoethylenimines (MWs 0.3 kDa and 0.6 kDa) showing the best KHI performances, with average onset temperatures (To) from 10 parallel experiments of 6.4 and 6.7 °C, respectively. Among the oligoethyleneamine oxides, TEPA-Bu-AO, based on TEPA (MW ≈ 190 Da) showed a KHI performance, To = 6.7 °C, similar to that of the best HPEI-Bu-AOs. The best amine oxides from the screening tests were further investigated in long-term isothermal experiments at 4.1 °C and ∼30 bar, giving a subcooling (ΔT) of 7.5 °C. Using 5000 ppm additive, HPEI-Bu-AO-0.3kD and HPEI-Bu-AO-0.6kD were superior KHIs to the best performing nonpolymeric oligoethyleneamine oxide, TEPA-Bu-AO. The HPEI-Bu-AO-0.3kD compound and its 0.6kD analogue showed hold times of 10−13.6 days, while TEPA-Bu-AO showed a hold time of 6.5 days under the same test conditions. All these three oligomeric amine oxides exhibited no cloud point up to 100 °C as 1.0 wt % solutions in deionized water.

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guanidinium salts.11,12 Amine oxides are organic compounds that carry a tertiary amine N-oxide (R3N+−O−) functional group. The functional group accounts for their high polarity, dipole moment of ca. 4.3 D, and amphoteric properties (see Figures 1 and 2). These special features make amine oxides

INTRODUCTION During the last two decades, there has been an increased interest in the use of low-dosage hydrate inhibitors (LDHIs) for combating gas hydrate plugging during the upstream part of the oil and gas production process.1 LDHIs offer several advantages over traditional thermodynamic hydrates inhibitors (THIs), such as methanol and glycols. Although THIs are often dosed as 20−50 wt %, based on the water phase, the active component in LDHIs is usually dosed at between 0.1−1.0 wt %. This can lead to high cost savings, because of the significantly lower volumes required as well as several other benefits.2,3 LDHIs are subclassified into kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs). AAs are surfactants that act by dispersing hydrate crystals in a carrier solvent, as well as preventing pipeline deposition. The main active components in KHIs are water-soluble polymers that act primarily by perturbing and delaying hydrate crystal nucleation and hydrate crystal growth.4 Within a certain range of subcooling, KHIs can totally inhibit hydrate crystal growth.5−8 Examples of commercial KHI polymers include homopolymers and copolymers of N-vinylcaprolactam, N-vinylpyrrolidone and N-isopropylmethacrylamide, as well as hyperbranched poly(esteramide)s and polyester pyroglutamates.1,4,9,10 A wide range of synergists have been found for these KHI polymers.1,4 In addition, different classes of KHI polymers can also work together synergistically. Several of the KHI synergists are nonpolymeric and include quaternary ammonium salts, amine oxides, cyclopropenium salts, and amidinium and © 2015 American Chemical Society

Figure 1. TBAO (left) and TBAB (right).

with long N-alkyl chains highly versatile surfactants.13−15 Based on the structural resemblance of the amine oxide moiety to

Figure 2. Resonance stabilization of amine oxides and amphoteric properties. Received: July 14, 2015 Revised: August 31, 2015 Published: September 1, 2015 6347

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Figure 3. Synthetic route to HPEI-Bu-AO.

quaternary ammonium salts (R4N+X−), and the fact that the amine oxide, tributyl amine N-oxide (TBAO), was known to form clathrate hydrates, their performance as tetrahydrofuran (THF) hydrate crystal growth inhibitors and as synergists for polymeric KHIs, has been investigated.16,17 In our laboratory, an extensive screening of a wide range of amine oxides comprising one tertiary amine N-oxide moiety, disclosed that, those bearing chains of four and five carbon atoms around the N atom performed best as THF hydrate crystal growth inhibitors, and that the tri-n-butyl amine N-oxide (TBAO) was the best overall.16 Interestingly, the same study showed that the pentylated analogue, tri-n-pentyl amine N-oxide (TPAO), performed better than TBAO as a synergist, when combined with poly(N-vinyl caprolactam) (PVCap), using a Structure II (SII) hydrate forming natural gas mixture in high-pressure rocking cell tests.16 The reason for this was suggested to be due to the larger perturbation of the water structure with TPAO, leading to greater gas hydrate nucleation inhibition than with TBAO. A further study on bis- and tris-amine N-oxides revealed that those bearing two dibutylated amine N-oxide groups separated by normal alkyl chains of 5−6 C atoms long were superior THF hydrate crystal growth inhibitors to the best amine oxide bearing only one tertiary amine N-oxide group (i.e., TBAO).17 The AA performance of a series of alkylated bisamine N-oxide surfactants on SII gas hydrates has also been reported, along with some preliminary results on the KHI performance of polyamine N-oxides.18 In order to follow up the polyamine oxide KHI results, we decided to investigate a wider range of molecular weights, including nonpolymeric molecules with as low as three amine groups. As far as we are aware, no stand-alone nonpolymeric KHIs have been reported that show sufficient performance for

potential field applications.4 Amino acids were first reported in patents in the mid-1990s to have weak KHI activity, but further laboratory work was dropped in favor of better performing polymers such as poly(N-vinyl lactam)s.1,19 Recently, amino acids have been revisited but the reported KHI performance on THF or CO2 hydrate is low.20−22 Proteins and peptides and polyamino acids show significantly higher performance as KHIs but even these have not yet reached field application.23−28 Citramides and related unsaturated amides, and optionally esters, have been shown to exhibit some effect as KHIs.29 Imidazolium-based ionic liquids have been investigated as both kinetic and thermodynamic hydrate inhibitors.30,31 A reinvestigation of the two best ionic liquids, at more typical subsea temperatures and subcoolings, showed that they were poor KHIs when used alone. However, some ionic liquids are useful synergists for KHI polymers.32,33 In general, if a nonpolymeric chemical is used in the field in a KHI formulation, it is as a synergist and/or solvent to the KHI polymer; when used as a solvent, it can be a thermodynamic hydrate inhibitor (THI) as well. This study reports the synthesis and KHI performance of a total of seven butylated oligoamine and polyamine oxides, four of them synthesized from hyperbranched polyethylenimines (HPEIs) and three from nonpolymeric oligoethyleneamines. Although tripentylamine oxide is water-soluble, various pentylated (n- or iso-) oligoamines and polyamines that were synthesized were found to have negligible water solubility; therefore, they were not investigated as KHIs. All the watersoluble butylated compounds were tested in a high-pressure rocking cell equipment using slow constant cooling experiments (1 °C/h) with a SII forming natural gas mixture. Long6348

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Energy & Fuels term isothermal tests over 2 weeks were also carried out on the best performing oligoamine and polyamine oxides.

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CHEMICAL SYNTHESIS AND EXPERIMENTAL METHODS

Synthesis of Butylated Oligoamine and Polyamine Oxides. The hyperbranched polyethylenimines (HPEIs), EPOMIN SP-003 (MW = 300 D; 98%), EPOMIN SP-006 (MW = 600 D; 98%) and EPOMIN SP-200 (MW = 10 000 D; 98%) were obtained from Nippon Shokubai Co., Ltd. They are assumed to be 98% pure when used in synthesis. Luvicap 55W (1:1 N-vinylpyrrolidone:N-vinyl caprolactam or VP:VCap copolymer, 53.8 wt % in water) and HPEI Lupasol PR 8515 (MW = 2000 D) were obtained from BASF. Pentaethylenehexamine (PEHA (MW = 232 g/mol), technical grade), triethylenetetramine (TETA; 60% plus impurities) and diethylenetriamine (DETA; 99%) were obtained from Aldrich. Tetraethylenepentamine (TEPA; 85%) was obtained from Fluka. THF (which was subsequently distilled from sodium/benzophenone and assumed to be 100%), potassium carbonate (anhydrous 99.6%), and 2-propanol (IPA) (99+% and assumed to be 100% in all syntheses where it takes no part in any reactions) were purchased from VWR Chemicals. 1Bromobutane (≥98%), hydrogen peroxide (35 wt %), PEG200, and triethylamine (99%) were purchased from Merck. Dequest 2066 (25% sodium diethylenetriamine penta(methylenephosphonate) in water) was obtained from Italmatch Chemicals. Cocoacid (MW = 215 g/mol) was purchased from Oleon in Belgium. Synthesis of Butylated Polyamine Oxides. Synthesis of HPEIBu-AOs. All the HPEI-Bu-AO polymers were prepared in the same way according to published procedures from the corresponding hyperbranched polyethylenimines (HPEI), EPOMIN SP-003 (MW = 300 D), EPOMIN SP-006 (MW = 600 D), PR 8515 (MW 2000 D) and EPOMIN SP-200 (MW = 10 000 D; 98%). A typical example of a synthesis is given here for HPEI-Bu-AO-0.3kD (Figure 3):

Figure 4. Chemical structure showing the main components in PEHABu-AO (top), TEPA-Bu-AO (middle), and TETA-Bu-AO (bottom). • Complete oxidation to the amine oxide product was confirmed by 1H NMR of the resulting 42.1 wt % solution of TEPA-BuAO. The concentrations of the other butylated polyethyleneamine oxides in 2-propanol prepared in the same way were as follows: PEHABu-AO, 31.9 wt %; TETA-Bu-AO, 42.1 wt %; and DETA-Bu-AO, 38.0 wt %. These solutions were used as such in further hydrate inhibition tests. None of them exhibited cloud points up to 100 °C as 1 wt % solutions in deionized water. High-Pressure Gas Hydrate Rocker Rig Equipment and Test Methods. Constant Cooling Test Procedure. The constant cooling experiments were carried out in high-pressure 5 × 40 mL steel rocking cells equipment (RC5), containing a steel ball, obtained from PSL Systemtechnik, Germany (see Figures 5 and 6).12,34 A synthetic natural gas (SNG) mixture was used, and its composition is given in Table 1.

• K2CO3 (24.7 g, 0.179 mol) and 1-bromobutane (24.5 g, 0.179 mol) were added to solution of EPOMIN SP-003 (7.0 g, 0.16 mol) in THF (105 mL), and the resulting mixture gently refluxed with stirring for 48 h. • After filtration of the solids and concentration of the filtrate under vacuum, the resulting butylated adduct, HPEI-Bu-0.3kD (15.7 g, 0.1588 mol) was dissolved in 2-propanol (18.8 g) and treated with a 35% H2O2 solution (0.175 mol, 14.4 mL). • After stirring the solution for 24 h at room temperature, complete conversion to the desired amine oxide was confirmed by 1H NMR spectroscopy. In this way, a 36.0 wt % solution of HPEI-Bu-AO-0.3kD in 2-propanol was obtained and used as such in further hydrate inhibition tests. The concentrations of the other HPE-Bu-AOs in 2-propanol were as follows: HPEI-Bu-AO-0.6D, 36.1 wt %; HPEI-Bu-AO-2000D, 37.7 wt %; and HPEI-Bu-AO-10 000D, 31.3 wt %. Synthesis of Butylated Linear Polyethyleneamine Oxides . All the butylated oligoethyleneamine oxides were prepared from the corresponding oligoethyleneamines, pentaethylene hexamine (PEHA), tetraethylenepentamine (TEPA), triethylene tetramine (TETA), and diethylenetriamine (DETA), in the same way as for the HPEI-Bu-AOs previously described. The general procedure is described for the synthesis of TEPA-Bu-AO (Figure 4), which is described as follows:

Figure 5. Photograph of an open cell of the multirocking cell equipment. The test procedure for the constant cooling test is described 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: 20.2 °C ± 0.05 °C, which is close to the calculated value of 20.5 °C at 76 °C bar, using Calseṕs PVTSim software.12,34,35 The cells are filled with 20 mL of an aqueous solution of the compound to be tested. Then, air is removed under vacuum and the cells charged with SNG to 5 bar, followed by rocking for 2 min. After releasing the pressure, vacuum is applied again and the cells charged immediately with SNG to 76 bar. The constant cooling program is started according to the following test parameters: rocking rate: 20 rocks/min; maximum cell angle, 40°; cooling ramp speed, 1

• K2CO3 (11.2 g, 0.0813 mol) and 1-bromobutane (11.1 g, 0.0813 mol) were added to a solution of TEPA (2.0 g, 0.011 mol) in THF (30 mL), and the reaction mixture gently refluxed for 48 h under stirring. • Filtration of solids and concentration of the filtrate gave the butylated adduct, TEPA-Bu (4.69 g, 0.00806 mol), which was not further purified but dissolved in 2-propanol (5.0 g), treated with a 35% H2O2 solution (5.2 mL, 0.0604 mol) and stirred at room temperature for 24 h. 6349

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Long-Term Isothermal Test. Long-term isothermal tests were carried out for the following amine oxides: HPEI-Bu-AO-0.3kD, HPEI-Bu-AO-0.6kD, and TEPA-Bu-AO. High-pressure 20 mL transparent sapphire rocking cells equipment (RCS20) was used (obtained from PSL Systemtechnik, Germany).35 Each of the six cells contained a steel ball. The test conditions were as follows. The hydrocarbon phase was a clear condensate (mixture of aromatic and aliphatic hydrocarbons) obtained originally from Shell, starting temperature = 20 °C, cooling ramp = 5.3 °C/h, isothermal temperature = 4.1 °C, test gas = SNG (Table 1), test pressure at isothermal temperature = 29−30 bar, test duration = 336 h, rocking rate = 10 rocks per minute, maximum rocking angle = 35°. The test cells were charged with the active compounds dissolved in the aqueous solution and the condensate using the following equations:

volume of aqueous phase (mL) = 0.0776 × V volume of condensate (mL) = 0.1810 × V where V represents the volume of the test cells (for the RCS20, meaning that V = 20 mL here). Note that the liquid volume was doubled in some experiments in order to get the flowing liquids to touch the walls of the sapphire tubes evenly while rocking. The test procedure was as follows. First, the cells were charged with the aqueous phase and condensate. Then, cells were gently 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 continuing the rocking and cooling from 20 °C to 4.1 °C with a cooling ramp of 5.3 °C/h. When a temperature of 4.1 °C was reached, the pressure had dropped to ∼30 bar (giving a subcooling of ∼7.5 °C for this pressure loading, gas-to-oil ratio (GOR), and water cut), and the rocking was continued. The temperature was maintained at 4.1 °C for 336 h (2 weeks). For one test (last entry in Table 3, presented later in this paper), the cell was pressurized again up to 36 bar after reaching the isothermal temperature of 4.1 °C. The pressure and temperature were recorded in a computer. Figure 8 shows an example of the graphical results from an isothermal experiment, in this case, for TEPA-Bu-AO. After cooling

Figure 6. Multirocking cell equipment in the water bath.

Table 1. Composition of Synthetica Nautral Gas (SNG) component

composition (mol %)

methane ethane propane isobutane n-butane N2 CO2

80.67 10.2 4.9 1.53 0.76 0.1 1.84

°C/h. The pressure and temperature for each cell were recorded on a computer. Figure 7 depicts a graph of a constant cooling test for the polyamine oxide, HPEI-Bu-AO-0.3kD, at 5000 ppm concentration of active

Figure 7. Typical graph of a constant cooling test showing To and Ta determinations.

Figure 8. Graph obtained from a long-term isothermal test.

chemical. There is a pressure drop of ∼2 bar, whch is due to gas being dissolved in the aqueous phase. The first deviation from the pressure drop due to the temperature drop is taken as the time for the first observed onset of hydrate formation (To). The Ta value shows the first steepest part of the pressure versus time due to rapid hydrate formation. To obtain statistically significant conclusions from such a test, 10 tests were executed. p-Values from t-tests 0.05). HPEI-Bu-AO-10kD gave the poorest performance (To = 10.3 °C) of the HPEI-Bu-AO compounds tested. A molecular-weight-dependent KHI performance is a known trend of some polymeric KHIs. For instance, it has been demonstrated that KHIs based on N-vinyllactam polymers and other KHI polymers show their best performances when their molecular weights are in the range of 1500−2000 D. Lower or higher average molecular weights (with a monomodal distribution) outside this range led to poorer KHI performances.1,36−38 HPEI-Bu-AO-0.3kD also had a similar performance at 5000 ppm (To = 6.4 °C) as the commercial VP:VCap copolymer at 3000 ppm (To = 6.1 °C). However, increasing the concentration of VP:VCap copolymer to 5000 ppm for a better comparison, gave To = 3.1 °C, demonstrating the superiority of the N-vinyl lactam-based polymer KHI over the best performing butylated hyperbranched amine oxide. The VP:VCap copolymer is commercial and has a cloud point of ∼85 °C. In contrast, 1 wt % solutions in water of HPEI-Bu-AO-0.3kD and HPEI-Bu-AO-0.6kD exhibit no cloud point at all, even at the boiling point of water. Since the best performing HPEI-Bu-AO, HPEI-Bu-AO0.3kD, is a small oligomer containing only seven N atoms on average separated by ethylene groups, this prompted us to synthesize related butylated oligomeric amine oxides based on oligoethyleneamines with even fewer N atoms. It should be pointed out that, apart from the smallest members of the series, ethylenediamine and diethylenetriamine, unfractioned oligoethyleneamines are not single-component chemicals. The

Figure 9. KHI performance of HPEI-Bu-AO of different molecular weights. 6351

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Figure 10. Main components of commercial unfractioned triethylenetetramine (TETA).

mixture with a different average molecular weight, in accordance with a predetermined formulation, which could give a better performance. This is a very well-known method in the development of new KHIs besides the synthesis of KHI polymers of different molecular weights. However, a mixture of HPEI-Bu-AO-0.3kD:HPEI-Bu-AO-10kD (4500:500 ppm) gave a worse performance (To = 7.1 °C), compared to 5000 ppm of HPEI-Bu-AO-0.3kD alone (To = 6.4 °C). The possibility of synergy of TEPA-Bu-AO with diethylenetriaminepentamethylenephosphonic acid (DETPMP), which is a well-known scale inhibitor, and polyethylene glycol (PEG200) was investigated. DETPMP might well be a found in the produced fluids, since it is back-produced after a scale inhibitor squeeze treatment. Polyethylene glycols have been used as synergists for N-vinyl lactam polymers.3,4 Mixing TEPA-Bu-AO with DETPMP in a 5:1 weight ratio slightly worsened the KHI performance (To = 7.1 °C), compared to TEPA-Bu-AO alone. A mixture of 1000 ppm of PEG200 and 5000 ppm of TEPA-Bu-AO did not improve the performance of the polyamine oxide, but, in fact, gave a slightly decreased performance, giving an average To value of 7.4 °C. To further address the gas hydrate inhibition performance of the polyamine oxides, we carried out long-term isothermal tests as previously described in this paper. As a logical step, we tested the best performing polyamine oxides: HPEI-Bu-AO-0.3kD, HPEI-Bu-AO-0.6kD, and TEPA-Bu-AO. These results are showed in Table 3. Repeat experiments were carried out in all cases and only the worst result is given. At a pressure of 30 bar, the equilibrium temperature (Teq) is ∼11.6 °C, giving a subcooling of 7.5 °C when running tests at 4.1 °C. As Table 3 shows, initial loadings varied from 29.1 bar to 30.7 bar, which, from our calculations, means the subcooling only varies by as much as 0.2 °C. With no additive, hydrate formation occurred within 0.05). None of the butylated ethyleneamine oxides based on PEHA, TEPA, TETA, and DETA exhibited cloud points in water as 1 wt % solutions, even at 100 °C. Butylated linear and hyperbranched polyamine oxides such as those presented in this paper exhibit two important structure characteristics associated with their hydrate crystal growth inhibitory and antinucleator properties. The tertiary amine Noxide (R3N+−O−) functional group contains N-alkyl groups of the right length, i.e., butyl groups. As previously mentioned, amine oxides with several N-butylated amine N-oxide (R3N+− O−) functional groups have been shown to be superb THF hydrate crystal growth inhibitors.16,17 The butyl groups trigger the embedding of the polyamine oxides via strong van der Waals interactions with the voids of the 51264 cages of the SII hydrates and, in this way, prevent the formation of adjacent new-forming clathrate hydrates.1 As this study shows, increasing the number of N atoms gives the polyamine oxides their antinucleator properties. The N-butylated amine N-oxide (R3N+−O−) groups can act at many sites at the same time, heavily perturbing the interactions between free water molecules. In this way, polyamine oxides inhibit the hydrate cages from building up and eventually reaching the hydrate critical nuclear size. With the objective of improving the performance of butylated oligoamine oxides, HPEI-Bu-AO-0.3kD was tested combined with a smaller amount of HPEI-Bu-AO-10kD. The idea of mixing these two chemicals was to make a polymer 6352

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oxides exhibited no cloud point up to 100 °C as 1.0 wt % solutions in deionized water. In summary, these results demonstrate a rare nonpolymeric class of KHIs having sufficient performance and cloud point for potential field applicability. We are currently investigating oligomeric amine oxides with variable-size alkyl groups for their KHI performance of Structure I methane hydrates.

Table 3. Long-Term Isothermal Experiments at 4.1 °C KHI identification

composition (ppm)

pressure (bar)

hold time

experiment duration

2500

30.0 29.1