A Study of the Kinetic Hydrate Inhibitor Performance and Seawater

May 28, 2009 - StaVanger, 4036 StaVanger, Norway, Laboratory of Macromolecular Chemistry and Nanoscience,. EindhoVen UniVersity of Technology, PO ...
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Energy & Fuels 2009, 23, 3665–3673

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A Study of the Kinetic Hydrate Inhibitor Performance and Seawater Biodegradability of a Series of Poly(2-alkyl-2-oxazoline)s Luca Del Villano,† Roald Kommedal,† Martin W. M. Fijten,‡ Ulrich S. Schubert,‡ Richard Hoogenboom,*,‡ and Malcolm A. Kelland*,§ Department of Mathematics and Natural Sciences, Faculty of Science and Technology, UniVersity of StaVanger, 4036 StaVanger, Norway, Laboratory of Macromolecular Chemistry and Nanoscience, EindhoVen UniVersity of Technology, PO Box 513, 5600 MB EindhoVen, The Netherlands, and Laboratory of Organic and Macromolecular Chemistry, Friedrich-Schiller-UniVersity Jena, Humboldtstr. 10, 07743 Jena, Germany ReceiVed February 27, 2009. ReVised Manuscript ReceiVed May 1, 2009

Kinetic hydrate inhibitors (KHIs) have been used successfully in the field for about the last 14 years to prevent gas hydrate formation mostly in gas and oilfield production lines. They work by delaying the nucleation and often also the growth of gas hydrate crystals for periods of time dependent on the subcooling in the system. Poly(2-alkyl-2-oxazoline)s [or poly(N-acylalkylene imine)s] are a known class of KHI, but no work has previously been published detailing a systematic study of structure versus performance. In this paper we report the KHI performance of poly(2-alkyl-2-oxazoline) homopolymers and statistical copolymers with alkyl side groups up to four carbon atoms. The study includes hydrate crystal growth tests on structure II tetrahydrofuran hydrate crystals as well as high pressure nucleation and crystal growth studies on a synthetic natural gas mixture giving structure II hydrates. Seawater biodegradation studies on all the polymers according to the OECD306 procedure indicate that they are all poorly biodegradable (60%) in 28 days for new oilfield chemicals and that the products of degradation are also environmentally friendly. We have endeavored to find new polymer classes that are both high performing and have at least 20% biodegradation in the OECD 306 test. One class of polymer that appears appealing in this context are poly(2-alkyl-2-oxazoline)s [or poly(N-acylalkylene imine)s](Figure 1). These polymers have a nitrogen heteroatom in the backbone bonded, in an amide linkage, to a pendant carbonyl group that is further bonded to an alkyl group R. Well-defined poly(2-alkyl-2-oxazoline)s can be prepared by living cationic ring-opening polymerizations of the corresponding 2-oxazoline monomers.18-20 KHI test results on only two poly(2-alkyl-2-oxazoline)s, namely poly(2-isopropyl-2-oxazoline) (PiPrOx) and poly(2ethyl-2-oxazoline) (PEtOx), have been published previously.21 Even though the molecular weight of these two polymers were not reported, PEtOx was shown to have some weak effect in preventing structure II tetrahydrofuran hydrate crystals from nucleating and growing in a rotating rig in which ball stop times are measured. Both polymers were tested for their ability to prevent structure II synthetic natural gas hydrate formation in temperature-ramped high pressure mini-loop experiments. The temperature at which the loop plugged with hydrates was recorded. PiPrOx performed better than PEtOx, which was not much better than no additive even at a polymer concentration of 0.5 wt.% in the aqueous phase. No biodegradation tests were reported. PEtOx, which appears to be the only poly(2-alkyl-2oxazoline) currently commercially available, has also been shown to act as a synergist to poly(vinyllactam)s, particularly to poly(vinylcaprolactam).22 It is known from previous work on poly(aspartamide)s, N-alkylacrylamide polymers, and amide derivatives of maleic anhydride copolymers, that N-isopropylamide and N-isobutylamide groups gave high performance KHI polymers.1,23 These alkyl groups are of optimum size to interact with structure II 51264 cages. Therefore, we decided to prepare a series of poly(2alkyl-2-oxazoline) homopolymers and statistical copolymers based on 2-methyl-2-oxazoline (MeOx), 2-ethyl-2-oxazoline (EtOx), 2-n-propyl-2-oxazoline (nPrOx), 2-i-propyl-2-oxazoline (iPrOx), and 2-i-butyl-2-oxaozline (iBuOx) with alkyl groups (18) Kobayashi, S. Prog. Polym. Sci. 1990, 15, 751. (19) Aoi, K.; Okada, M. Prog. Polym. Sci. 1996, 21, 151. (20) Hoogenboom, R. Macromol. Chem. Phys. 2007, 208, 18. (21) Colle, K. S.; Oelfke, R. H.; International Patent Application WO96/ 08673. (22) Sloan, E. D. US Patent 5880319, 1999. (23) Del Villano, L.; Kommedal, R.; Kelland, M. A. Energy Fuels 2008, 22 (5), 3143.

The synthesis of well-defined poly(2-alkyl-2-oxazoline)s was performed using our recently optimized microwave-assisted polymerization procedure that reduced the polymerization time to several minutes.24 The optimal conditions for the living cationic ring-opening polymerization of 2-oxazolines were determined to be methyl tosylate as initiator, acetonitrile as solvent, 4 M monomer concentration, and 140 °C in a closed reaction vessel (Figure 2, bottom). These conditions were directly applied in the current study for the polymerization of the commercially available MeOx and EtOx monomers. The polymerizations were quenched by the addition of water, leading to the formation of a mixture of hydroxyl and ester end-groups as reported previously (only the hydroxyl end-groups are shown in Figure 2 for clarity).25 The nPrOx, iPrOx, and iBuOx monomers were prepared by condensation of the corresponding nitrile with 1-amino-2-ethanol in the presence of zinc acetate as catalyst following literature procedures (Figure 2, top).26,27 After extraction with water, the monomers were purified by fractional distillation. Subsequently, the polymerizations of these monomers were also performed as described above. The resulting polymers were dried under reduced pressure at 40 °C for several days to remove the solvent acetonitrile as well as remaining monomer (only traces of monomer had to be removed since the polymerizations were performed up to >99% conversion). After this drying procedure, the resulting polymers were analyzed by 1H NMR spectroscopy (CDCl3) to verify that all solvent and monomer was removed and to determine the monomer composition of the copolymers as well as the degree of polymerization from the remaining tosylate signals at 7.2 ppm and the polymer backbone signals at 3.4 ppm (Table 1). Size exclusion chromatography (SEC; chloroform/triethylamine/ isopropanol (94:4:2 vol%) as eluent; polystyrene standards) was performed to determine the number average molecular weight (Mn) and the polydispersity indices (PDI) of the polymers (Table 1). Kinetic Hydrate Inhibitor Performance Test Procedures High Pressure Autoclave Tests with Synthetic Natural Gas. KHI performance tests were carried out in the high pressure autoclave apparatus shown in Figure 3 and discussed previously.53 The temperature was measured to an accuracy of ( 0.l °C, and the pressure was measured with an accuracy of (0.2 bar. The autoclave consists of two identical sapphire cells. Cell 1 was used for one kind of test method, which we call the “constant temperature” or isothermal method, cell 2 was used for a temperature-ramping method. In all the experiments we used the same standard natural gas (SNG) mixture that gives structure II hydrates (Table 2). The aqueous phase was distilled water or a 3.6 wt % NaCl solution in cell 1 and distilled water in cell 2. No liquid hydrocarbon phases were used. At the onset of each constant temperature experiment, that is, the start of the induction time before gas hydrate began to form, the pressure was 90 bar. The equilibrium (24) Wiesbrock, F.; Hoogenboom, R.; Abeln, C. H.; Schubert, U. S. Macromol. Rapid Commun. 2004, 25, 1895–1899. (25) Hoogenboom, R.; Fijten, M. W. M.; Thijs, H. M. L.; Van Lankvelt, B.; Schubert, U. S. Des. Monomers Polym. 2005, 8, 659. (26) Witte, H.; Seeliger, W. Liebigs Ann. Chem. 1974, 996–1009. (27) Kempe, K.; Lobert, M.; Hoogenboom, R.; Schubert, U. S. ; J. Comb. Chem., 2009, 11, 274-280.

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Figure 2. Top: Schematic representation of the synthesis of 2-alkyl-2-oxazoline monomers from the condensation of the corresponding nitrile with 1-amino-2-ethanol in the presence of zinc acetate as catalyst. Bottom: Cationic ring-opening polymerization of 2-oxazolines with methyl tosylate as initiator and water as terminating agent (TsO- represents the tosylate counterion). Table 1. Poly(2-alkyl-2-oxazoline)s Synthesized and Investigated in This Work name

MeOx

PMeOx PEtOx PnPrOx PiPrOx PMeOx-iPrOx PEtOx-iPrOx PMeOx-iBuOx PEtOx-iBuOx

60

EtOx

nPrOx

iPrOx

iBuOx

Mn (Da)

PDI

MeOx

1.18 1.11 1.17 1.05 1.10 1.06 1.14 1.07

53

30 30

2400c 4800 4800 5400 3700c 5000 4200c 5500

60 60 60 30 30

30 30

composition from 1H NMR spectroscopyb

SECa

theoretical composition (# repeat units)

30 30

EtOx

nPrOx

iPrOx

iBuOx

60 60 26 24 29 28

55 25 28 30 28

a SEC was measured using chloroform/triethylamine/isopropanol (94:4:2 vol%) as eluent, and molar masses were calculated against polystyrene standards. b 1H NMR spectroscopy was measured in CDCl3. c The MeOx containing polymers interact with the column material when using chloroform/ triethylamine/isopropanol (94:4:2 vol%) as eluent, leading to underestimation of the molar mass.

Figure 3. Sapphire cell high pressure test equipment. Table 2. Composition of Synthetic Natural Gas (SNG) component

mole %

methane ethane propane Iso-butane n-butane N2 CO2

80.67 10.20 4.90 1.53 0.76 0.10 1.84

temperature using 3.6% NaCl solution at this pressure was calculated using Calsep’s PVTSim software to be 19.6 °C (and 20.8 °C for distilled water). Laboratory experiments in a larger titanium cell to determine the equilibrium temperature by standard slow hydrate dissociation gave equilibrium tempera(28) Gjertsen, L. H.; Fadnes, F. H. Ann. N.Y. Acad. Sci. , 912, 722– 734.

tures of 19.91-19.96 °C with brine.28,29 The cooling rate near the equilibrium temperature was 0.14 °C/h. From our experience, the equilibrium temperature could have been up to 0.2 °C lower if we had used a cooling rate of 0.05 °C/h, which would have given very good agreement with the predicted equilibrium temperature, but this makes the experiments considerably more time-consuming. For the rest of this paper we will assume that the equilibrium temperature for our SNG-brine system at 90 bar is 19.6 and 20.8 °C with distilled water. The same initial procedure for the preparation of the KHI experiment and filling of the cell was followed in all high pressure experiments in both sapphire cells: (1) The polymer to be tested was dissolved in the aqueous fluid (3.6% NaCl solution or distilled water) to the desired concentration, usually 0.5 wt %. (2) The magnet housing of the cell was filled with the aqueous solution containing the additive to be tested. The magnet housing was then mounted in the bottom end piece of the cell, which thereafter was attached to the sapphire tube and the cell holder. (3) The desired amount of the aqueous solution was filled in the cell (above the cell bottom) using a pipet, the top end piece was mounted, and the cell was placed into the cooling bath (plastic cylinder). (4) The temperature of the cooling bath was adjusted to 2-3 °C above the hydrate equilibrium temperature at the pressure conditions to be used in the experiment. (5) After purging the cell twice with the SNG, the cell was loaded with SNG to the desired pressure while stirring at 600 rpm. The remaining procedure for the experiments using the constant cooling method in cell 1 was as follows: the stirring was stopped and the cell was cooled to the experimental temperature (usually 9 to 10 °C). When the temperature and pressure (90 bar) in the cell had stabilized the stirring was started. The induction time, ti, for hydrate formation was determined using the start of the stirring at the experimental temperature as time zero. The time from the first observed start (29) Tohidi, B.; Burgass, R. W.; Danesh, A.; Ostergaard, K. K.; Todd, A. C. Ann. N.Y. Acad. Sci. 2000, 912.

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Figure 6. Pressure versus time for a ramping experiment with 0.5 wt % Luvicap 55W. The first horizontal hold time is at 12 °C, ramping down 1 °C every 4 h.

Figure 4. Example of a KHI test result: Luvicap 55W at 90 bar, 7 °C, 12.6 °C subcooling, SII hydrate. Induction time (ti) and slow growth phase (st-1) are shown.

Figure 5. Schematic representation of the structure of the 1:1 vinyl caprolactam/vinyl pyrrolidone copolymer (the active component in Luvicap 55W).

of hydrate formation to the time when rapid growth of hydrate occurs (combined with a rapid pressure drop), usually connected with the formation of a hydrate plug in the cell, is called the crystal growth delay time, st-1. The results of all experiments were recorded by plotting the gas consumption (in bars) as a function of time (in minutes). An example of a typical plot is given in Figure 4 for a commercial KHI, Luvicap 55W (a 1:1 vinyl caprolactam/vinyl pyrrolidone copolymer) from BASF tested at 0.5 wt % (Figure 5). The remaining part of the procedure for experiments using the temperature-ramping method in cell 2 was as follows: the polymers were dissolved in distilled water to 0.5 wt % concentration. Stirring was continued, and the temperature in the cell was decreased to 12 °C. This operation took about 50 min. The temperature was kept at 12 °C for 4 h. The cell was subsequently cooled in 1 °C intervals, the cooling taking 10-15 min. The cell was held at each set temperature (11, 10, 9 °C, etc.) for 4 h. By the time 7 °C was reached (after about 24 h) a plug of gas hydrate had normally formed in the cell. The pressure was adjusted at the start to give roughly 90 bar in the cell on cooling to 9 °C (∆T ) 12.8 °C). A typical result is shown in Figure 6 for Luvicap 55W at 0.5 wt.%. As the figure shows, the pressure drops several bars at the start due to cooling from room temperature (approximately 22 °C) to 12 °C in about 1 h (3600 s), and drops further in smaller steps each time the temperature is ramped down 1 °C. At about 63 000 s the temperature is constant at 8 °C, and it is clear that the pressure drop that occurs at this time is due to the onset of hydrate formation (to). Slow hydrate growth continues for about 2000 s

(33 min) before fast, catastrophic hydrate formation occurs (ta) and a plug of hydrate is observed in the cell. THF Tests. Tetrahydrofuran (THF) forms structure II hydrate crystals at about 4 °C under atmospheric pressure. Polymers that show good inhibition of THF hydrate crystals might also show good kinetic inhibition of gas hydrate crystals if the same mechanism of surface adsorption can be assumed. However, conclusions should be handled with care since this is not always the case. Tetrahydrofuran hydrate ball-stop test results on PiPrOx polymers revealed that these polymers perform no better than the tests with no additive, yet they are reasonably good KHIs in mini-loop tests with natural gas mixtures.30 Conversely, it is known that nonpolymeric tetraalkylammonium salts, where the alkyl group is preferably n-butyl, n-pentyl, or iso-pentyl, are good inhibitors of THF hydrate crystal growth, yet they are very poor nucleation inhibitors of natural gas hydrates when used alone.1 The method we used for studying the inhibition of THF hydrate crystal growth has been reported previously for work on poly(vinyllactam)s and antifreeze proteins.31-35 NaCl (26.28 g) and THF (99.9%, 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. The test procedure is as follows: (1) 80 mL of the aqueous THF/NaCl solution is placed in a 100 mL glass beaker. (2) 0.16 g of the test chemical is dissolved in this solution to give a 0.2 wt % (2000 ppm) solution of the chemical. (3) The beaker is placed in a stirred cooling bath preset to a temperature of -0.2 °C ((0.05 °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 an inner diameter of 3 mm was filled at the end with ice crystals kept at -10 °C. The ice crystals are used to initiate THF hydrate formation. (30) Coller, K. S.; Oelfke, R. H. International Patent Application WO96/ 08673. (31) Klomp, U. C.; Kruka, V. R.; Reijnhart, R.; Weisenborn, A. J. US Patent 5460728, 1995. (32) Anselme, M. J.; Reijnhout, M. J.; Klomp, U. C.; WO Patent Application 93/25798, 1993. (33) Makogon, T. Y.; Larsen, R.; Knight, C. A.; Sloan, E. D. J. Cryst. Growth 1997, 179, 258–262. (34) Larsen, R.; Knight, C. A.; Sloan, E. D. Fluid Phase Equilib. 1998, 150–151, 353-360. (35) Zeng, H.; Wilson, L. D.; Walker, V. K.; Ripmeester, J. A.; The Inhibition of THF Clathrate Hydrate Formation by Antifreeze Proteins. Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, May 19-23, 2002.

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Table 3. Average Induction Times before Detectable Hydrate Formation for Poly(2-alkyl-2-oxazoline)s Tested by the Constant Temperature Method at 90 bar and 10 °C in Distilled Water polymer

average induction time, ti (mins)

no additive PMeOx PEtOx PnPrOx PiPrOx PMeOx-iPrOx PEtOx-iPrOx PMeOx-iBuOx

12 17 80 134 237 92 112 337

(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 55-65 min. (8) After this time, the tube was removed, the THF hydrate crystals weighed, and the crystal growth rate in grams per hour determined. The shape of the crystals was visually analyzed. Occasionally we obtained secondary nucleation of THF hydrate in the bulk solution or at the beaker walls. Results from these experiments were discarded. KHI Performance Test Results and Discussion Results in the Sapphire Cells. The poly(2-alkyl-2-oxazoline)s were first tested using the constant temperature method at 0.5 wt % dissolved in either 3.6% NaCl or distilled water. Far more tests were conducted in distilled water because three of the polymers (PiPrOx, PMeOx-iBuOx, and PEtOx-iBuOx) gave cloudy solutions in the brine due to salting out, that is, lowering of the lower critical solution temperature by NaCl.36 In distilled water only polymer PEtOx-iBuOx gave a cloudy solution and some undissolved solid, indicating that the large hydrophobic isobutyl side chain renders the polymer too hydrophobic. Therefore, KHI test results with PEtOx-iBuOx were deemed unreliable and were not recorded, as significantly less than 0.5 wt % polymer was dissolved in the aqueous phase. The test results for experiments carried out at 90 bar and 10 °C giving a subcooling of ∆T ) 10.8 °C are listed in Table 3. The average induction time of 4-5 experiments before first detectable hydrate formation, by a pressure drop, is given. The scattering in the induction times due to stochastic hydrate formation in a small cell was about 30-40%, which is very similar to previous experiments we have carried out.23 The polymer with the highest KHI performance was clearly PMeOx-iBuOx, and the worst was PMeOx. We got the same ranking in experiments using 3.6% NaCl as the aqueous phase but only single experiments on each polymer were performed as several polymer solutions were cloudy in this aqueous medium. It was expected that PMeOx would perform poorly as the pendant hydrophobic group in the polymer has only one carbon atom. In fact, the PMeOx and PEtOx are very hydrophilic polymers as indicated by their hygroscopic character.37 In contrast, the PnPrOx and PiPrOx have a subtle balance between hydrophilicity and hydrophobicity, resulting in a lower critical (36) Lin, P.; Clash, C.; Pearce, E. M.; Kwei, T. K.; Aponte, M. A. J. Polym. Sci., Part C: Polym. Phys. 1988, 26, 603–619. (37) Thijs, H. M. L.; Becer, C. R.; Guerrero-Sanchez, C.; Fournier, D.; Hoogenboom, R.; Schubert, U. S. J. Mater. Chem. 2007, 17, 4864–4871. (38) Uyama, H.; Kobayashi, S. Chem. Lett. A992, 1643. (39) Park, J.-S.; Kataoka, K. Macromolecules 2007, 40, 3599–3409. (40) Hoogenboom, R.; Thijs, H. M. L.; Jochems, M. J. H. C.; Van Lankvelt, B. M.; Fijten, M. W. M.; Schubert, U. S. Chem. Commun. 2008, 5758–5760.

Table 4. Average Induction Times (ti) before Detectable Hydrate Formation for Poly(2-alkyl-2-oxazoline)s Tested by the Temperature-ramping Method in Distilled Water polymer

average induction time, ti (mins)

maximum deviation from average (mins)

no additive PMeOx PEtOx PnPrOx PiPrOx PMeOx-iPrOx PEtOx-iPrOx PMeOx-iBuOx PMeOx-iPrOx-2 Luvicap 55W

1233, and >1416 min required. The last two experiments were stopped before a plug of hydrate had formed. The pressure versus time graph for the last experiment is shown in Figure 7. Hydrate formation begins at about 5000 s (83 min) and continues to form at a fairly even rate until the experiment is stopped at 85 000 s (1416 min). However, in the last half hour of the experiment the pressure begins to drop somewhat faster, indicating that catastrophic hydrate growth will probably occur shortly after. Since we only obtained significantly long hydrate slow growth times with PMeOx-iPrOx, we resynthesized a second batch of polymer, PMeOx-iPrOx-2 with the same 1:1 MeOx/iPrOx composition. The new polymer had Mn ) 5000 and PDI ) 1.14 (determined by SEC using N,N-dimethylacetamide with 2.1 g LiCl/L as eluent and PS as calibration) and was fully watersoluble. We carried out four ramping experiments on this polymer (Table 4). In each experiment we obtained very short

Villano et al.

Figure 8. Pressure vs time graph for a ramping experiment with PMeOx-iPrOx-2. Onset of hydrate formation occurs at to. Catastrophic hydrate formation occurred at about 32 000 s (533 min) at which time the cell plugged with gas hydrates.

induction times, as we had previously obtained with PMeOxiPrOx. In addition, we again observed long slow-growth times with the new polymer, much longer than with any other polymer except polymer PMeOx-iPrOx. Thus, in the four experiments the total delay time before fast, catastrophic hydrate formation occurred was 482, 656, 533, and 782 min, respectively. The graphical result for the third ramping test with PMeOx-iPrOx-2 is shown in Figure 8. We wondered if there was any special structural feature in the polymers PMeOx-iPrOx and PMeOx-iPrOx-2 that might help to understand their different KHI behavior to the other poly(2-alkyl-2-oxazoline)s. The reactivity of the MeOx is known to be higher compared to the linear 2-n-alkyl-2-oxazolines.25 EtOx and nPrOx, resulting in a slight gradient in monomer composition, that is, the beginning of the polymer chain is enriched in MeOx whereas the end of the chain is enriched in EtOx.41 The iBuOx monomer was recently demonstrated to have a slightly lower reactivity than EtOx and thus a similar monomer gradient is expected during copolymerization of iBuOx with MeOx.42 The similar reactivity of EtOx and iBuOx is due to the presence of a primary carbon next to the 2-oxazoline ring in both monomers resulting in similar nucleophilicity of the ring. In contrast, iPrOx has a secondary carbon connected to the 2-oxazoline ring, which lowers its reactivity. As a result, it has been demonstrated that the copolymerization of iPrOx with 2-alkyl-2-oxazolines, such as EtOx, nPrOx, and n-butyl-2oxazoline, results in a more pronounced monomer gradient compared to MeOx-EtOx copolymers.43,44 As such, copolymerization of MeOx with iPrOx is expected to result in an even stronger monomer gradient approaching a block copolymer structure with MeOx as first block followed by a small gradient regime and an iPrOx second block. This anticipated strong monomer gradient structure was confirmed by evaluating the copolymerization kinetics that clearly shows a much faster incorporation of MeOx compared to iPrOx (Figure 9). To investigate the polymerization kinetics, separate polymerization mixtures were heated to 140 °C for different times, and the resulting mixtures were quenched by the addition of water and analyzed by gas chromatography to analyze the monomer conversion. The linear first-order kinetics indicate that the (41) Hoogenboom, R.; Fijten, M. W. M.; Wijnans, S.; van den Berg, A. M. J.; Thijs, H. M. L.; Schubert, U. S. J. Comb. Chem. 2006, 8, 145– 148. (42) Kempe, K.; Lobert, M.; Hoogenboom, R.; Schubert, U. S. J. Polym. Sci., Part A: Polym. Chem. 2009, DOI: 10.1002/pola.23448. (43) Park, J.-S.; Kataoka, K. Macromolecules 2006, 39, 6622–6630.

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Energy & Fuels, Vol. 23, 2009 3671 Table 5. THF Hydrate Crystal Growth Rates

Figure 9. First-order kinetic plot for the copolymerization of MeOx and iPrOx under microwave irradiation at 140 °C in acetonitrile using methyl tosylate as initiator.

polymerization proceeds in a living manner, which is confirmed by the well-defined character of the resulting polymer (Mn ) 6500 Da; PDI ) 1.26; determined by SEC using N,Ndimethylacetamide with 2.1 g LiCl/L as eluent and PS as calibration). From the slopes of the linear fits in Figure 9,45 the apparent copolymerization parameters were determined to be rMeOx ) 3.5 and riPrOx ) 0.3, confirming the formation of a strong monomer gradient. This strong gradient in the monomer distribution of PMeOxiPrOx might, in fact, be the reason why this copolymer behaves differently in our studies compared to the other copolymers, which have a much smaller monomer gradient than the PMeOx-iPrOx. The PMeOx-iPrOx consists of two almost pure blocks of PMeOx and PiPrOx with a gradient middle block while all the other gradient copolymers have a much broader transition regime where both monomers are mixed leading to intermediate properties. So, why do the PMeOx-iPrOx and PMeOx-iPrOx-2 copolymers give short gas hydrate induction times, and why do they give longer slow growth times than the other poly(2-alkyl-2oxazoline)s? We propose that these two gradient polymers are poor nucleation inhibitors, because the only part of the polymer that could exhibit good nucleation inhibition, that is, the end of the polymers that is rich in iPrOx, is too short to be effective. It is known from studies on other KHI polymer classes, such as poly(vinylcaprolactam) and poly(alkylacrylamide)s, that the nucleation inhibition performance drops off significantly below a molar mass (Mw) of about 1000-1500 Da.1,46,47 As the PMeOx-iPrOx gradient copolymers have a molar mass of about 5000 Da, the end of the polymers rich in iPrOx will be rather (44) Huber, S.; Jordan, R. Colloid Polym. Sci. 2008, 286, 395. (45) Puskas, J. F.; McAuley, K. B.; Polly Chan, S. W. Macromol. Symp. 2006, 243, 46–52. (46) Larsen, R. Clathrate hydrate single crystals — Growth and inhibition. Ph.D. Thesis, Norwegian University Of Science and Technology: Trondheim, Norway, 1997. (47) Kelland, M. A.; Svartaas, T. M.; Øvsthus, J.; Namba, T. Ann. N.Y. Acad. Sci. 2000, 912, 281. (48) Kelland, M. A.; Svartaas, T. M.; Øvsthus, J.; Tomita, T.; Chosa, J. Chem. Eng. Sci. 2006, 61, 4048. (49) Kelland, M. A. University of Stavanger, Norway, 2007; unpublished results. (50) Wang, C.-H.; Fan, K.-R.; Hsiue, G.-H. Biomaterials 2005, 26, 2803– 2811. (51) OECD Guideline for Testing of Chemicals, Biodegradability in Seawater, Adopted by the Council on 17th July 1992; p 27. (52) Rittmann B. E.; McCarty P. L.; EnVironmental Biotechnology: Principles and Applications, Int. Ed.; McGraw-Hill Comp. Inc.: Singapore, 2001; p 128. (53) Supplementary Guidance for the Completing of Harmonised Offshore Chemical Notification Format (HOCNF) 2000 for Norwegian sector, Harmonised Offshore Chemical Notification Format OSPAR Recommendation 2000/5.

polymer

growth rate/h (average of 3-4 tests)

comments

no additive Luvicap 55W PMeOx PEtOx PnPrOx PiPrOx PMeOx-iPrOx PEtOx-iPrOx PMeOx-iBuOx PEtOx-iBuOx

1.06 not measurable 0.92 0.29 4.0 0.35 0.97 0.58 1.53 3.3

well-defined pyramidal crystal very thin extended plates pyramidal crystals smaller crystals large, soft, round lumps smaller pyramidal crystals pyramidal crystals smaller pyramidal crystals large, soft, round lumps large, soft, round lumps

short, which means they would be expected to exhibit poor nucleation inhibition and much worse inhibition than the PiPrOx homopolymer with a sufficiently large Mn of 5000 Da (PDI ) 1.10). Further, since nucleation occurs early in our experiments with these gradient polymers, far less supercritical nuclei could be available for crystal growth compared to experiments where nucleation occurs much later, whereby many nuclei are close to the critical nucleation size. In fact, these PMeOx-iPrOx gradient polymers must be able to inhibit crystal growth of the supercritical nuclei since we observe long slow growth delays in hydrate formation. THF Hydrate Crystal Growth Test Results and Discussion The poly(2-alkyl-2-oxazoline)s were tested for their ability to inhibit the growth of THF hydrate crystals. Luvicap 55W was also tested for comparison purposes. All the polymers gave clear solutions in the aqueous THF/NaCl solution except PiPrOx and copolymer PEtOx-iBuOx. We estimate that about 70-80% of both polymers were soluble when added at 2000 ppm to the THF/NaCl solution. Table 5 lists the average THF crystal growth rates per hour, based on four identical experiments, as well as some comments on the form of the crystals obtained. In general we observed a scattering in growth rates of about 20-30% to either side of the average. The THF hydrate crystals, grown under our test conditions with no additives, were octahedral with sharp pyramidal shapes growing away from the end of the glass tubing. This is also what has previously been reported.32,33 Experiments with 2000 ppm Luvicap 55W gave very thin plates, which grew out to the sides of the beaker. When the glass tube was removed from the solution the plates collapsed and it was not possible to weigh them. At higher polymer concentrations the plates became thinner until at about 5000 ppm we observed no plate growth at all. This has also been previously observed with vinyl caprolactam polymers.33,34 In general, we observed two types of crystal growth with the poly(2-alkyl-2-oxazoline)s, commonly observed octahedral growth or the formation of round lumps of wet microcrystals, which have not been reported before, with PnPrOx, PMeOx-iBuOx, and PEtOx-iBuOx. PMeOx and PMeOx-iPrOx had the least inhibitory effect on the THF hydrate crystal growth rate. This was expected for polymer PMeOx, which has the smallest pendant alkyl groups (methyl), giving the weakest van der Waals interactions with cavities on the THF hydrate crystal surface. PiPrOx gave fairly good inhibition of THF hydrate crystals despite not being fully soluble in the THF/ salt solution. However, the result with polymer PMeOx-iPrOx, the gradient copolymer made from MeOx and iPrOx, is somewhat surprising, considering we obtained long slow gas hydrate growth delays in the high pressure tests discussed earlier, which would seem to imply that the polymer should show good THF growth

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Table 6. Measured BOD and Estimated ThOD and ThBOD Values for Tested Chemicals in This Study compound

ThOd28 no growth (mg/ L)

ThBOD28 including growth (mg/L)

BOD28 (mg/L)

% 28 day biodegradation according to OECD306

% 28 day biodegradation including growth

seawater + nutrients glucose + nutrients glucose + nutrients glucose + acid PMeOx PMeOx PEtOx PEtOx PnPrOx PnPrOx PiPrOx PiPrOx PMeOx-iPrOx PMeOx-iPrOx PEtOx-iPrOx PEtOx-iPrOx PMeOx-iBuOx PMeOx-iBuOx PEtOx-iBuOx PEtOx-iBuOx

0 107 107 107 140 140 164 164 212 212 212 212 194 194 204 204 209 209 212 212

0 35 35 0 62 62 75 75 102 102 102 102 93 93 98 98 100 100 102 102

2.9 36.0 35.1 0 4.2 4.2 5.6 5.6 8.6 6.7 3.8 7.6 4.8 13.5 5.8 4.8 3.9 3.9 6.7 4.8

0 29 34 0 3 3 3 3 1 0 -1 0 1 5 1 1 0 0 2 1

0 102 100 0 7 7 7 7 2 0 -3 1 2 11 3 2 1 1 4 2

inhibition. As this is not the case, we presume there must be a different crystal growth inhibition mechanism operating with this polymer on SII gas hydrate and SII THF hydrate. At this time, we are unable to suggest what this mechanism could be. Further, PEtOx-iPrOx gave less inhibition of the growth of THF hydrate crystals than PEtOx and PiPrOx. PEtOx performed better than PMeOx at inhibiting THF hydrate crystal growth. The ethyl groups are expected to interact better with open SII hydrate cages than methyl groups. By the same reasoning the inhibition performances of PnPrOx and PiPrOx would therefore be expected to be even greater but this was not the case. For polymer PiPrOx the lack of solubility up to 2000 ppm may be the reason. However, for polymer PnPrOx we observed a new effect on the growth of THF hydrate crystals. Thus, after one hour of growth we consistently obtained large, soft, round lumps, which weighed several grams. They appeared to consist of a mass of microscopic crystals, holding some of the aqueous solution, such that when you pressed on the lump, liquid would run out. Under a microscope we could not make out the form of the crystals in the lump. We obtained the same behavior in experiments with polymers PMeOx-iBuOx and PEtOx-iBuOx, albeit with different average weights of the lumps. Clearly, PnPrOx, PMeOx-iBuOx, and PEtOx-iBuOx are adsorbing onto the THF hydrate crystals in a different way than either Luvicap 55W, which gave large plates, or the other poly(2-alkyl-2-oxazoline)s, which gave solid crystals similar to experiments with no additives but of a smaller size. We have also observed, but not reported, the formation of a round, “wet” mass of microcrystals with poly(acryloylpyrrolidine).49 These results indicate that differences in the size of the alkyl substituents in poly(2-alkyl-2-oxazoline)s, and thus their threedimensional structure, can cause very different effects on the way they interact with SII THF hydrate crystals. For example, PnPrOx with n-propyl side chains gives an entirely different effect on THF hydrate crystal growth than PiPrOx with pendant iso-propyl groups. Seawater Biodegradation Tests. At the time of writing only one polymer KHI, with a proprietary structure, is allowed for use offshore of Norway. Therefore, we were interested to know if poly(2-alkyl-2-oxazoline)s would show good biodegradability in seawater. As far as we are aware, there is just a single report on the enzymatic degradation of PEtOx to a copolymer of PEtOx and ethylene imine by cleavage of some of the amide bonds.50 Nonetheless, biodegradation of organic chemicals in seawater is generally considered to be slower than in freshwater, which has a higher concentration of bacteria as it can be spiked with sewage.

Test Procedure. Marine biodegradation was experimentally measured according to the OECD 306 Closed Bottle Method.51 Briefly, fresh seawater samples were taken from 70 m depth at Byfjord, Stavanger, transported to the lab within one hour, and kept at room temperature in the dark. 350 or 250 mL (depending on the initial concentration) aliquots were transferred to prewashed 510 mL dark bottles, spiked with 2 mL nonammonium (ammonium replaced by nitrate) Bushnell-Haas nutrient solution (contained per liter: 0.2 g MgSO4, 0.02 g CaCl2, 2.0 g K2HPO4, 2.0 g NaNO3, and 0.05 g FeCl3; adjusted to pH 8.0), and volumes of sample bringing the initial concentration of test compounds to 100 mg/L. Sample bottles were closed by OxiTop-C screw cap measuring heads (WTW, Wissenschaftlich-Technische Werksta¨tten, Germany) equipped with saturated NaOH headspace solution for CO2 trapping, and incubated at 20 °C under continuous mixing. BOD bottles were prepared according to the vendor’s guidelines for a biological oxygen demand (BOD) 28 days test. The sample heads correlate the oxygen consumption with the partial pressure reduction in the closed headspace, providing continuous oxygen readings without opening the system. All samples received nutrient solution. Glucose (initial concentration 100 mg/L) was used for positive control, and acidified (addition of 4 M H2SO4 to pH < 2) and autoclaved sample bottles (containing test sample) served as negative controls. Positive controls and KHIs were tested in duplicates. Theoretical oxygen demand (ThOD) and theoretical biological oxygen demand (ThBOD) was calculated according to the literature method based on the chemical composition of the tested chemical.52 The biodegradation results are listed in Table 6. Figure 10 shows the biological oxygen demand for glucose with and without

Figure 10. Oxygen demand during glucose (positive control) degradation.

Kinetic Hydrate Inhibitor Performance

nutrients. The BOD was as expected for a known compound, with no lag time and giving a 28 day biodegradation of about 29-34% without assimilation included in the calculation and about 100-102% with assimilation. However, none of the poly(2-alkyl-2-oxazoline)s showed significant levels of biodegradation within 28 days. Thus, they are unlikely to be allowed for use offshore Norway, where there is a minimum demand of 20% biodegradation and preferably >60% for new chemicals.53 Conclusions We have synthesized and tested a series of poly(2-alkyl-2oxazoline)s as KHIs both in high-pressure gas hydrate autoclaves and as THF hydrate crystal growth inhibitors. The best polymer for gas hydrate nucleation inhibition was a random copolymer of MeOx and iBuOx consisting of methyl and isobutyl side chains. Its performance was close to the performance of a commercial KHI Luvicap 55W containing vinyl pyrrolidone/ vinyl caprolactam copolymer as the active component. As such,

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the PMeOx-iBuOx is a potential lead candidate for further optimization of the molecular structure for KHI performance, which will be performed in future work. During the current investigations, two different effects on the growth of THF crystals were observed depending on the size of the pendant alkyl groups; either stunting the growth of octahedral crystals on all faces, or formation of wet, round lumps of microcrystals, which has not been reported previously. The mechanism for the formation of these large wet lumps is not yet understood. It was found that none of the poly(2-alkyl-2-oxazoline)s showed >20% biodegradation by the OECD 306 28 day seawater test method. Acknowledgment. L.D.V., R.K., and M.A.K. thank StatoilHydro, Total, and NanoChem Solutions Inc. for financial support of this work. M.W.M., U.S.S., and R.H. thank the Dutch Polymer Institute for funding. EF900172F