Tetrahydrofuran Hydrate Crystal Growth Inhibition by Trialkylamine

For this reason, the oil industry has sought more environmentally friendly AAs, particularly for use in offshore regions with strict environmental reg...
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Tetrahydrofuran Hydrate Crystal Growth Inhibition by Trialkylamine Oxides and Synergism with the Gas Kinetic Hydrate Inhibitor Poly(Nvinyl caprolactam) Malcolm A. Kelland,*,† Ann Helen Kvæstad,† and Erik Langeland Astad† †

Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ABSTRACT: We have determined the crystal growth inhibition of tetrahydrofuran (THF) hydrates with trialkylamine oxides. Our studies indicate that, within this class, tri-n-butylamine oxide (TBAO), tri-iso-pentylamine oxide (TiPAO), and tri-npentylamine oxide (TPAO) are the best THF hydrate crystal growth inhibitors, the butylated derivative being the best of these three. This contrasts with tetraalkylammonium bromide salts, where the pentylated and not the butylated species gives the stronger THF hydrate crystal growth inhibition. A possible explanation is discussed. The THF hydrate crystal growth inhibition performance of TBAO is reduced when one of the butyl groups is replaced by various other organic groups but still shows significant activity. The hydrate inhibition performance of trialkylamine oxides was illustrated by testing their ability as synergists to enhance the performance of the well-known kinetic hydrate inhibitor, polyvinylcaprolactam. In high pressure rocking cell tests using a SII-forming natural gas mixture, TPAO was found to be a better synergist than either TBAO, the best THF hydrate crystal growth inhibitor, or TiPAO. We speculate that adsorption onto hydrate crystal surfaces may not be the only synergistic kinetic hydrate inhibition mechanism operating and that the more hydrophobic TPAO is perturbing the nucleation of hydrate more than the less hydrophobic TBAO.

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INTRODUCTION Solid plugs caused by gas hydrate formation are a menace in various stages of the upstream oil and gas industry such as in production lines, during drilling (especially in deep water), and in work-over operations.1−4 Methods to avoid hydrate plugs include raising the temperature/heating (e.g., insulation, bundles, electric or hot water heating), lowering the pressure, removing the water, and shifting the equilibrium for hydrate formation by adding antifreeze chemicals. These techniques are often very expensive (such as heated pipelines or the need for methanol or glycol regeneration facilities) or not the complete solution (e.g., subsea water separation). Hence, there is a clear need for cheaper technologies. In the last 18 years low dosage hydrate inhibitors (LDHIs) have been developed which can be significantly cheaper to deploy than other methods just described.1,4−7 In particular, the design of a new field development with LDHI technology can give large CAPEX savings. There are two main classes of LDHI: • Kinetic Inhibitors (KHIs) • Anti-Agglomerants (AAs) Both KHIs and AAs are added at low concentrations, often around 0.1−1.0 wt % active concentration. This can be contrasted with the 10−50 wt % needed for thermodynamic inhibitors or “anti-freezes” such as methanol, glycols, or salts. The two new types of additive have different field application ranges related to performance, field conditions, fluid properties, and the properties of the additives, not the least of which their environmental impact. Whereas KHIs are water-soluble polymers that delay hydrate formation, AAs are surfactants that allow hydrates to form but they prevent the hydrate © 2012 American Chemical Society

crystals from agglomerating and subsequently accumulating into large masses. An AA for use in oil or condensate pipelines enables the hydrates to form as a transportable nonsticky slurry of hydrate particles dispersed in the liquid hydrocarbon phase. All current commercial AAs are quaternary ammonium (“quat”) surfactants and work by a mechanism first proposed by researchers at Shell in the early 1990s (Figure 1).8,9 Quat

Figure 1. Examples of twin-tail and single tail quaternary ammonium surfactant AAs. R is a long hydrophobic tail with at least 10 carbon atoms. X is an anion such as a halide.

surfactants were well-known at the time as particle dispersants, such as for their use in inks. The Shell researchers discovered that if the quaternary headgroup (phosphonium or ammonium) contained two or more n-butyl, n-pentyl, or iso-pentyl groups then the surfactant showed good AA properties (Figure 2). The correct head-groups were those that were best at adsorbing onto the hydrate crystal surface. This was first determined from studies on the inhibition of tetrahydrofuran Received: April 12, 2012 Revised: June 19, 2012 Published: June 26, 2012 4454

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long to give strong van der Waals 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 Structure I or II hydrate.5 Many classes of quaternary surfactants are known to be toxic. Some are even used as household disinfectants. For this reason, the oil industry has sought more environmentally friendly AAs, particularly for use in offshore regions with strict environmental regulations, such as the North Sea. To avoid the use of quaternary, cationic surfactants some research groups have looked at zwitterionc and betaine surfactants.21−23 Others have studied nonionic surfactants such as polypropoxylates and amide surfactants.1,6,24 About 15 years ago some work was carried out on amine oxides, as an alternative class of nonionic surfactant AAs including a small amount of THF hydrate crystal growth inhibition studies (Figure 3).5,25 Tributyl phosphine oxide and

Figure 2. Structure of tetraalkyl-onium salts of nitrogen (M = N) and phosphorus (M = P).

(THF) hydrate crystals with various quaternary ammonium and a few phosphonium salts. THF clathrate hydrate is the same Structure II hydrate as would form with most natural gas compositions found in the field so it is relevant for use in hydrate crystal growth studies. Recently, it was found that tetra(iso-hexyl)ammonium bromide was a superior THF hydrate crystal growth inhibitor than tetra(pentyl)ammonium bromides (iso- or n- isomers) (TPAB) and tetra(n-butyl)ammonium bromide (TBAB) as well as the best quaternary ammonium salt synergist for poly(N-vinyl caprolactam) (PVCap).10,11 If alkyl groups other than n-butyl, n-pentyl, or iso-pentyl were placed in the quaternary headgroup the inhibition performance dropped drastically. Others have also tested the THF hydrate growth inhibition properties of TBAB.12,13 The inspiration for the development of the quaternary AAs came from work on the stability and X-ray crystal structures of clathrate hydrates of small quaternary ammonium salts.14,15 It has been shown different polyhydrates are known to exist for TBAB or TiPAB and also shown for the intermediate butyl/ isopentyl species. The type of hydrate depends on the anion also, e.g. carboxylate anions, halide anions, hydroxide, oxalate, etc.16,17 It was also shown that the clathrate framework better fits the isopentyl group compared to the butyl group.18,19 Bisquaternary salts have also been shown to form clathrate hydrates, such as salts of the hexamethylenebis(tributylammonium) ion.20 It has been shown from molecular modeling studies that butyl or pentyl groups can penetrate a 51264 cavity on the 1,1,1 Structure II hydrate surface.5 One or two of the other butyl or pentyl groups lay in channels on the hydrate surface where new 51264 cages would normally be formed. These cages could partially form, trapping or imbedding the butyl or pentyl groups in the hydrate surface. Below the critical nuclear size, growth of a gas hydrate nuclei is energetically unfavorable (ΔG is positive). So, TPAB will not be embedded in the surface of the hydrate nuclei but will more easily detach. (Thus, quaternary salts are not hydrate antinucleators so they are poor KHIs when used by themselves.) Above the critical nuclear size, quaternary salts with butyl or pentyl groups can become embedded in the hydrate surface as partial hydrate cages form around these groups. Further Structure II hydrate growth is prevented by the remaining one or two groups attached to the quaternary ammonium center. These remaining groups can have long hydrophobic chains, which leads to their AA properties. Both single tail and twin-tail quat AAs have been commercialized.5,6 The embedding mechanism for the quaternary salts explains why for example tetrapropylammonium bromide (TPrAB) and tetrahexylammonimum bromide (THAB) are 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, i.e. it is less quickly embedded in the surface. On the other hand, THAB has alkyl groups that are too

Figure 3. Two ways of drawing an amine oxide structure.

tributyl amine oxide (TBAO) were already known to form clathrate hydrates, like TBAB and TPAB, so it was assumed that they also might make good SII hydrate crystal growth inhibitors.26−29 Amine oxide surfactants with tails of 12 carbon atoms or more have also been tested and claimed as AAs, but few results have been reported in the open literature and not necessarily with optimized structures for best performance.5,30 We have carried out a more thorough study on amine oxides with a view to designing optimized amine oxide surfactant AAs. In this paper we report studies on the inhibition of THF hydrate crystal growth with a wide range of amine oxides. This work has formed the basis for designing optimum amine oxide surfactant AAs, which will be reported later.

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EXPERIMENTAL - SYNTHESIS OF AMINE OXIDES

All chemicals for synthesis and THF hydrate crystal growth studies were obtained from Sigma-Aldrich or VWR International. Samples of coconut oil derived C8−18 amidopropyldimethylamine oxide (Macat Ultra CDO), dodecyl dihydroxyethylamine oxide (Macat AO-12-2), octyldimethylamine oxide (Macat AO-8), and dodecyldimethylamine oxide (Macat AO-12) were kindly donated by Mason Chemical Company. A purity level of at least 95% for all new amines synthesized in this work was determined by 1H and 13C NMR spectroscopy. Poly(N-vinyl caprolactam) was synthesized by polymerization of Nvinyl caprolactam in 2-propanol at 80 °C using azoisobutyronitrile (AIBN) as initiator. The number average molecular weight (Mw) was found to be 18000 by GPC analysis. The synthesis of amine oxides was done by a well-known and common laboratory method by reaction of a tertiary amine with a concentrated solution of hydrogen peroxide (H2O2) in water.31 The reaction goes 100% to completion for tertiary amines (Figure 4). Some amine oxides were prepared in isopropanol or butyl glycol ether (BGE) because the starting amine was too water-insoluble to react with the hydrogen peroxide, but as the results in Table 2 show these small solvent molecules had negligible effect on the growth rate of THF hydrate crystals. A typical example of a synthesis is given here for dimethylbenzylamine oxide: 4455

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Figure 4. General synthesis of amine oxides. Benzyldimethylamine (5.0 g, 0,037 mol) was added to a slight excess of 35 wt.% hydrogen peroxide (3.95 g, 0.0406 mols) in a 1-neck round-bottom flask to give two phases. Isopropanol (5.0 g) was added to give one phase. The reaction mixture was stirred overnight to give a 40 wt.% solution of benzyldimethylamine oxide. Hydroxyethyldibutylamine oxide, for esterification reactions discussed later in this paper, was needed in the pure form. Therefore, after normal reaction of hydroxyethyldibutylamine with hydrogen peroxide in isopropanol, the solvent was evaporated, and the residue was dissolved in the minimum volume of dichloromethane. Diethyl ether was added until the solution was just cloudy and then cooled to −30 °C overnight to give white crystals which were filtered off. This gave a 60% yield of the pure amine oxide checked by 1H and 13C NMR spectroscopy. We also wanted to investigate the THF hydrate crystal growth inhibition of the product of reacting secondary amines such as di-nbutylamine with hydrogen peroxide. Unlike tertiary amines, secondary amines are known to oxidize to a mixture of products including N,Ndisubstituted hydroxyamines and a nitrone as the main product if two equivalents of hydrogen peroxide is used and the reaction is left long enough (Figure 5).31 Use of various catalysts can give almost quantitative yields of either product via kinetic control.32−34 We carried out the reaction of dipentylamine and dibutylamine with 2 equivalents of hydrogen peroxide over 24 h. The 1H NMR spectrum of the two reaction products after this time indicated a mixture of at least two compounds, probably the nitrone and hydroxylamine derivatives.35,36 We did not analyze the mixture further but tested these products, labeled “dialkylamine nitrones”, directly in the THF hydrate crystal growth apparatus. Primary amines react with hydrogen peroxide to form first hydroxyamines (R-NOH), but these react further to form nitroso (R-NO) or nitro compounds (R-NO2) Thus, when we reacted 3(dibutylamino)propylamine with 2 equivalents of hydrogen peroxide we obtained a mixture of products as determined by its 1H NMR spectrum, but it appeared the tertiary dibutylamino group was almost fully converted to the amine oxide. This product we have called “3(dibutylamino)propylamine oxide”. We also wanted to make dibutylamine oxides with ester or amide groups in the side-chain since these were reasoned to be reasonable candidate structures for surfactant AAs with good biodegradability. There are several possible routes to esteramine oxides from cheap, commercially available starting materials, such as the following:

Figure 6. General synthesis of an esteramine oxide from hydroxyethyldibutylamine via an esteramine.

Figure 7. General synthesis of an esteramine oxide from hydroxyethyldibutylamine via hydroxyethyldibutylamine oxide.

Figure 8. Michael addition of an alkyl(meth)acrylate ester to the secondary amine di-n-butylamine (R1 is an alkyl group, R2 is CH3 or H) followed by conversion to esteramine oxide.

A Conversion of hydroxyethyldibutylamine first to an esteramine and then an esteramine oxide (Figure 6).

B Conversion of hydroxyethyldibutylamine first to hydroxyethyldibutylamine oxide and then to an esteramine oxide (Figure 7).

C Michael addition of dibutylamine to the vinyl functionality of

amine oxide center, compared with the products from methods A and B. In theory, methods A and B should give the same product. However, we found that only method A gave a pure product as shown by 1H and 13C NMR, also in quantitative yield. Method B gave roughly 50% of the correct esteramine oxide product in step 2 from pure

an acrylate ester and then conversion to an esteramine oxide (Figure 8). All three methods lead to an ethylene spacer group between the amine oxide and ester groups. However, the ester group is reversed in the products from method C, with the C(O) group closest to the

Figure 5. Probable products of the reaction of trialkylamine with hydrogen peroxide: a hydroxydialkylamine (left) or a nitrone (right). 4456

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hydroxyethyldi-n-butylamine oxide. Given here is a typical synthesis according to method A: Acetyl chloride (2.26 g, 28.8 mmol) was added dropwise over 15 min to a stirred mixture of hydroxyethyldi-n-butylamine (5.0 g, 28.8 mmol), triethylamine (2.92 g, 28.8 mmol) in diethyl ether (50 mL). The voluminous white precipitate of Et3HN+Cl− was filtered off and the solvent evaporated. 1 H NMR showed pure esteramine, Bu2NCH2CH2OC(O)CH3 in 100% yield, 6.20 g. To 4.97 g (23.06 mmol) of this esteramine in isopropanol (5 g) was added a slight excess of 35% hydrogen peroxide (2.46 g, 25.37 mmol), and the mixture was stirred for 18 h. The isopropanol was evaporated to leave a colorless liquid which was shown by 1H and 13C NMR to be pure esteramine oxide B in 100% yield. We also explored other ways to provide a dibutylamino oxide headgroup starting with commercially available dibutylamine, a headgroup which might be useful in an AA surfactant. One general method is Michael addition of dibutylamine to a vinylic compound such as an alkyl acrylate ester, an N-alkylacrylamide, or an N-alkyl vinyl

Figure 10. Synthesis of amidoamine oxides from 3-(dibutylamino)propylamine. quantitative yield as shown by 1H NMR. Isopropanol (10 mL) was added to the amidoamine and treated with 35 wt.% hydrogen peroxide (2.72 g, 27.99 mmol) and stirred for 18 h at 20 °C. The resulting 28.8% of amidoamine oxide was used in THF hydrate experiments. The last class of amine oxide that was synthesized was made from a dialkylamine, as shown in Figure 11.

Figure 9. Synthesis of alkoxyethyldibutylamine oxides (R = ethyl or nbutyl). ether (Figure 9) followed by conversion of the tertiary amine product to an amine oxide with hydrogen peroxide. To keep the Michael addition product water-soluble, so that it could be tested for THF hydrate crystal growth inhibition, we kept the alkyl group fairly short to minimize the hydrophobicity of the final amine oxide product. An example of a synthesis with an alkyl acrylate is as follows: Dibutylamine (2.0 g, 15.48 mmol) was added to methyl acrylate (1.33 g, 15.48 mmol) in isopropanol (3 mL) and heated overnight at 70 °C with stirring in a sealed flask. The resulting esteramine was treated with 35 wt.% hydrogen peroxide (1.65 g, 17.0 mmol) with stirring for 18 h to give a 44.7 wt.% solution of the esteramine oxide C (via Method C). The following is an example of a Michael addition synthesis of an etheramine oxide using an alkyl vinyl ether: Di-n-butylamine (2.33 g, 23.2 mmol) was added to n-butyl vinyl ether (3.0 g, 23.2 mmol) and isopropanol (3 mL) and heated to 90 °C overnight in a sealed flask. The product was cooled, 35 wt.% hydrogen peroxide (2.48 g, 25.5 mmol) was added, and the solution was stirred overnight to give a 47.3 wt.% solution of butoxyethyldibutylamine oxide (BOEDBAO). In the same way, using ethyl vinyl ether, ethoxyethyldibutylamine oxide (EOEDBAO) was also synthesized. Amidoamine oxides can also be synthesized in two steps from commercially available 3-(dibutylamino)propylamine (Figure 10). Hydrogen peroxide cannot be reacted with 3-(dibutylamino)propylamine otherwise both amine groups will react. Therefore, 3(dibutylamino)propylamine must first be converted to an amidoamine and then the tertiary amine converted to an amidoamine oxide. A typical synthesis is as follows: Acetyl chloride (2.1 g, 26.8 mmol) was added dropwise at 20 °C to a solution of 3-(dibutylamino)propylamine (5.0 g, 26.8 mmol) and triethylamine (2.83 g, 26.8 mmol) at 10−20 °C in dichloromethane (20 mL). The solution was filtered to remove NEt3HCl, and the solvent was removed from the filtrate to leave an amidoamine in

Figure 11. Synthesis of a 2-hydroxypropyldialkylamine oxide. Hydroxyethyldi(n-pentyl)amine oxide and hydroxyethyldi(isopentyl)amine oxide were made from from refluxing the ethanolamine and the alkyl bromide with potassium carbonate in toluene according to the literature method (Figure 12).37 These aminoalcohols were also converted into ester amine oxides, as describe earlier for other aminoalcohols.

Figure 12. Bis-alkylation of ethanolamine.

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THF HYDRATE CRYSTAL GROWTH EXPERIMENTAL PROCEDURE Tetrahydrofuran (THF) forms Structure II hydrate crystals at about 4.4 °C under atmospheric pressure. The method we used for studying the inhibition of THF hydrate crystal growth has been reported previously.8,38−40 In summary, NaCl (26.28 g) and THF (170 g) are mixed, and distilled water is added to give a final volume of 900 mL. This gives a stoichiometrically correct molar composition for making Structure II THF hydrate. In this saline solution the equilibrium temperature for THF hydrate formation is about 3.2 °C. The test procedure is as follows: 4457

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1 80 mL of the aqueous THF/NaCl solution was placed in a 100 mL glass beaker. 2 Usually 0.32 g of the test chemical was dissolved in this solution to give a 0.4 wt.% (4000 ppm) solution of the chemical. Some tests were carried out at other concentrations. 3 The beaker was placed in a stirred cooling bath preset to a temperature of −0.5 °C (±0.05 °C) giving a theoretical subcooling of about 3.7 °C. 4 The solution is briefly stirred manually with a glass rod every 5 min, without touching the glass beaker walls, while being cooled for 20 min. 5 A hollow glass tube with inner diameter 3 mm was filled at the end with ice crystals kept at −10 °C. The ice crystals are used to initiate THF hydrate formation. The crystals must stick out beyond the end of the tube otherwise they will interfere with the weight determination of THF hydrate crystals. 6 The glass tube was placed about halfway down in the cooled THF/NaCl solution after the solution had been cooled for 20 min. 7 THF hydrate crystals were allowed to grow at the end of the glass tube, for 55 to 65 min. 8 After this time, the tube was removed. Any drops of liquid on the THF hydrate crystals were removed by touching the crystals, briefly and gently, against an adsorbent cloth. The crystals were removed with a knife from the glass rod and weighed. If there are THF hydrate crystals further up the glass tube, which happens occasionally, to get a correct result it is important to remove only the crystals growing directly out from the ice at the end of the tube. 9 The crystal growth rate in grams per hour was determined. The shape of the crystals was visually analyzed. Experiments were repeated at least 4−5 times to obtain an average growth rate. The spread in growth rates is usually about 20−25% either side of the average. If it is more than this, then further tests are carried out and a new average calculated. Occasionally we obtained secondary nucleation of THF hydrate in the bulk solution or at the beaker walls. Results from these experiments were discarded if the crystals interfered with THF hydrate crystals on the end of the glass tube.

Table 1. Composition of Synthetic Natural Gas (SNG) component

mole %

methane ethane propane isobutane 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 at a rate of 1 °C/h down to 2 °C. 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 one of the five cells is shown in Figure 13. The gray line extending from the blue pressure curve is a line of best fit on the pressure data based on the decrease in temperature. This is why it flattens out at about 900 min as does the temperature curve. This gray line is used to determine the first sign of pressure drop due to hydrate formation (see Figures 19 and 20 also). The graph in Figure 13 is for a test with the poly(N-vinyl caprolactam) used in this study. The pressure drops about 2 bar due to gas being dissolved in the aqueous phase. The temperature drops at a constant rate until the minimum of 2 °C after about 900 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 to the temperature drop is taken as the time for 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 13, To is 10.5 °C. The degree of scattering in To values between the five cells 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 carried out. At some point rapid hydrate formation ensues as detected by a rapid pressure drop in the cells. In Figure 13 this occurs after about 430 min. Ta is determined from when hydrate growth is at its most rapid, i.e. the steepest part of the pressure versus time graph. Generally we find that there is less scattering in the Ta values (