Kinetic Hydrate Inhibition of Poly(N-isopropylacrylamide)s with

Jun 29, 2012 - Juwoon ParkSang Seok LeeHyunho KimShin-Hyun KimYutaek Seo ... Juwoon Park , Hyunho Kim , Qi Sheng , Colin D. Wood , and Yutaek Seo...
5 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/EF

Kinetic Hydrate Inhibition of Poly(N-isopropylacrylamide)s with Different Tacticities Pei Cheng Chua,*,† Malcolm A. Kelland,† Tomohiro Hirano,‡ and Hiroaki Yamamoto‡ †

Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ Department of Chemical Science and Technology, Institute of Technology and Science, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan ABSTRACT: Poly(N-isopropylacrylamide)s (PNIPAMs) and poly(N-isopropylmethacrylamide)s have been investigated recently as kinetic hydrate inhibitors (KHIs). Now, poly(N-isopropylmethacrylamide) is commercialized. These are usually made by standard radical polymerization methods, which do not allow for control over the polymer tacticity. For this study, PNIPAMs were synthesized using new polymerization methods, giving a fairly high degree of tacticity control. We report here results on the performance of different tacticities of PNIPAMs with similar molecular weights in KHI tests with natural gas in stirred autoclaves and on tetrahydrofuran (THF) structure II hydrate crystal growth. From the results, we can conclude that the polymer tacticity does affect the KHI performance of PNIPAMs. PNIPAM with a higher syndiotactic percentage performed better than PNIPAM with a lower syndiotactic percentage. Both polymers demonstrated some kind of crystal surface adsorption by affecting the morphology of the THF hydrate crystals.

■

INTRODUCTION Natural gas hydrates are crystalline solids, in which gas molecules (guest molecules) are trapped inside hydrogenbonded water cavities. Typical guest molecules include carbon dioxide and small hydrocarbons, such as methane, ethane, and propane. In the mid-1930s, it was discovered that thermodynamic conditions (elevated pressure and low temperature) favoring hydrate formation occur in pipelines and that natural gas hydrates were blocking gas transmission lines.1 Natural gas hydrate plugging is one of the costly and challenging problems for the oil and gas industry, especially for deepwater fields. The prevention of gas hydrate formation can be accomplished in a few methods. Among them is the chemical treatment, which includes the usage of kinetic hydrate inhibitors (KHIs). They are generally water-soluble polymers. The mechanism is to delay the nucleation and crystal growth. The main classes of KHIs that have been in commercial use are polymers based on homo-polymers and co-polymers of vinyl caprolactam (VCap) as well as hyperbranched polyesteramides.2 A group of KHIs based on the polymer of alkylacrylamide was developed. It was also pointed out that poly(N-monoalkyl(meth)acrylamide)s are also known to perform well as KHIs, especially when isopropyl serves as the alkyl group.3 Therefore, we are interested in studying the KHI performance of this new KHI class. One of the important properties of polymers are their tacticity, which describes the relative stereochemistry of adjacent chiral centers within a macromolecule.4 As shown in Figure 1, there are three types of tacticities, namely, isotactic with all pendant groups located on one side of the backbone, syndiotactic with alternating orientated pendant groups, and atactic with randomly orientated pendant groups. In a previous work, the KHI performance of poly(N,N-dialkylacrylamide)s was studied to determine the effect of polymer tacticity of this KHI class.5 The results show that syndiotactic poly(N,N© 2012 American Chemical Society

dialkylacrylamide)s perform better than other tacticities. Commercial synthesis of KHIs by radical polymerization of vinyl monomers using azo or peroxy initiators gave an atactic polymer structure. Therefore, we have to use other synthesis methods. For this project, we wanted to investigate poly(Nisopropylacrylamide)s (PNIPAMs) that would have a good effect as KHIs (Figure 2). It is important that the obtained polymers of different tacticities have similar a molecular weight because the KHI performance is influenced by the molecular weight.2 Isotactic PNIPAM is known to be insoluble in water at all temperatures.6 Therefore, for this paper, we are comparing the KHI performance of stereoregulated syndiotactic polymers. The polymers that were synthesized are summarized in Table 1.

■

SYNTHESIS OF PNIPAMS7 A typical polymerization procedure was as follows: NIPAM (1.062 g, 9.4 mmol) and 3-methyl-3-pentanol (3.835 g, 37.5 mmol) were dissolved in toluene to prepare a 20 mL solution. A total of 16 mL of the solution was transferred to a glass ampule and cooled to −60 °C. Polymerization was initiated by adding n-Bu3B solution (5.0 mL) in tetrahydrofuran (THF) (1.0 M) to the monomer solution. The reaction was terminated after 24 h by adding a small amount of a solution of 2,6-di-tbutyl-4-methylphenol in THF at the polymerization temperature. The reaction mixture was poured into a large amount of diethyl ether, and the precipitated polymer collected by filtration then dried in vacuo. The polymer yield was determined gravimetrically. Received: February 1, 2012 Revised: June 26, 2012 Published: June 29, 2012 4961

dx.doi.org/10.1021/ef300178u | Energy Fuels 2012, 26, 4961−4967

Energy & Fuels

Article

Figure 1. Vinyl polymers of various tacticities: isotactic (I), syndiotactic (S), and atactic (A). molecular-weight polyvinylcaprolactam in ethylene glycol (PVCap in EG) (supplied as Luvicap EG by BASF, Germany). High-Pressure Autoclave Tests with Synthetic Natural Gas (SNG). A high-pressure sapphire and stainless-steel autoclave with a

Figure 2. Structure of PNIPAM.

Table 1. PNIPAM Samples Synthesized for This Study sample ID

polymer

syndiotactic percentage

molecular weight (Mn) (×10−3)

PDI

v50 v70

PNIPAM PNIPAM

50 70

13.3 13.3

1.55 1.35

The 1H nuclear magnetic resonance (NMR) spectra (400 MHz) were obtained using an EX-400 spectrometer (JEOL, Ltd., Tokyo, Japan). The dyad tacticity of the PNIPAMs was determined from the 1H NMR signals of the methylene groups in the main chain, in deuterated dimethyl sulfoxide (DMSO-d6) at 150 °C. The molecular weight and molecular weight distribution of the PNIMAMs were determined by sizeexclusion chromatography (SEC), using polystyrene samples as molecular-weight standards. SEC was performed with a HLC 8220 chromatograph (Tosoh Co., Tokyo, Japan) equipped with TSK gel columns [SuperHM-M (6.5 mm inner diameter × 150 mm) and SuperHM-H (6.5 mm inner diameter × 150 mm), Tosoh Co.]. Dimethylformamide containing LiBr (10 mmol L−1) was used as the eluent at 40 °C with a flow rate of 0.35 mL min−1. The initial polymer concentration was 1.0 mg mL−1. The increase in syndiotactic percentage increases the cloud point slightly from 32 °C (for atactic PNIPAM) to 35 °C. This makes sense because the amide groups are less congested and have more room to interact with the water.

■

Figure 3. Sapphire/stainless-steel autoclave high-pressure test equipment. volume of 23 mL (Figure 3) were used for the KHI performance tests. This equipment setting has been used by our research group in previous tests.8 We carried out two kinds of test methods, namely, the “standard constant cooling” test and the “superheating constant cooling” test, which are described below. Table 2 shows the SNG mixture that was used in all of the experiments. The aqueous phase consists of distilled water. This experiment setup will give us structure II hydrates. The initial pressure in the cell is 78 bar for “standard constant cooling”, giving an equilibrium temperature of 19.6 °C (calculated using Calsep’s PVTSim software). Standard slow hydrate dissociation experiments were used to determine the equilibrium temperature.9,10 Experiments conducted previously gave very good agreement with the predicted equilibrium temperature, with an accuracy within 1 °C.5 Therefore, the equilibrium temperature for our SNG−water system at 78 bar is assumed to be 19.6 °C. For “superheating constant cooling” experiment, the cell was charged to 100 bar. The calculated equilibrium temperature using Calsep’s PVTSim software is approximately 21.0 °C.

EXPERIMENTAL METHODS: KHI PERFORMANCE TESTS

Two commercial KHI products were used for comparison to the PNIPAMs. These are a 1:1 VCap/vinyl pyrrolidone (VP) polymer in water (supplied as Luvicap 55W by BASF, Germany) and a low4962

dx.doi.org/10.1021/ef300178u | Energy Fuels 2012, 26, 4961−4967

Energy & Fuels

Article

Table 2. Composition of 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

Table 3. Average Values of Onset and Fast Hydrate Formation Temperatures with PNIPAMs

polymer no polymer low-molecular-weight PVCap in EG 1:1 VCap/VP polymer PNIPAM v50 PNIPAM v70

A total of 0.5 wt % of PNIPAM was dissolved in deionized water to prepare the 5000 ppm test solution. For standard constant cooling tests, we used 8 mL of the aqueous solution. The superheating constant cooling tests were carried out using 3 mL of aqueous solution and 5 mL of decane. Further details and the test procedure of both test methods could be found in our previously published work.8 Standard Constant Cooling Test Procedure. The cell was cooled with 600 rpm stirring from 20.5 to 2 °C over 18 h. In the closed system, we can observe a pressure drop because of the temperature decrement of the cell. Figure 4 illustrates the pressure and temperature plot versus time. Because of the slow cooling rate, we are able to determine the start of hydrate formation (To) and the fast, catastrophic hydrate formation (Ta). Another indication for the fast hydrate formation is the exothermic peak at the temperature curve and the increased torque measurement of the stirrer blade after hydrate plugging. After 907 min (To = 5.8 °C), a detectable amount of gas is being used to form gas hydrates. This is indicated by the pressure drop deviation from the pressure drop because of the temperature decrease. The pressure drop curve became almost vertical after 990 min (Ta = 4.3 °C), indicating the occurrence of fast hydrate formation. A total of 8−10 repetitions of the standard constant cooling test were conducted for each polymer. The results are shown in Figure 9. The average onset temperatures [To(av)] and the catastrophic hydrate formation temperatures [Ta(av)] are given in Table 3. Superheating Constant Cooling Test Procedure. The cell was cooled with 600 rpm stirring from 20.5 to 2 °C over 18 h. The cell content is then melted at 23 °C and held for 1 h before being cooled under a constant rate again. The procedures are repeated to produce at least four superheating constant cooling test cycles. Figure 5 shows an example of the pressure and temperature versus time for a complete series of superheating constant cooling test, consisting of a fast cooling procedure to make hydrate, followed by four repetitions of the constant cooling procedure. Figure 6 is enlargements of one of the

standard constant cooling

superheating constant cooling

To(av) (°C)

Ta(av) (°C)

To(av) (°C)

Ta(av) (°C)

8.4 2.9

8.3 2.6

20.9

18.0

5.6 5.4 4.2

4.8 4.4 3.4

13.2 13.9 13.2

9.0 9.0 7.9

cycles to determine To and Ta more accurately because hydrate formation could be very slow initially. To and Ta values are evaluated from the pressure and temperature plots using the same method as in the standard constant cooling test. Because of the slow hydrate formation, it is difficult to determine the To value accurately. After 3840 min, catastrophic hydrate formation occurs at Ta = 9.3 °C. Further enlargement of part of the third cycle (Figure 7) shows the first detectable gas uptake at 3601 min and To = 13.4 °C. THF Hydrate Crystal Growth Experiment. The study on the inhibition of hydrate crystal growth can be carried out using THF, which forms structure II hydrates at about 4.4 °C under atmospheric pressure in THF/water mixtures. This method used has been reported previously.11−13 A total of 900 mL of THF/water test solution (equilibrium temperature of about 3.2 °C) is prepared by mixing 26.28 g of NaCl and 170 g of THF in distilled water. The test polymer was dissolved in the aqueous THF/NaCl solution to the desired concentration. The beaker with the test solution is then placed in a stirred cooling bath preset to −0.5 °C (±0.05 °C), which represents about 3.8 °C subcooling. Before conducting THF hydrate crystal growth experiments, we had to first check the solubility of the PNIPAMs in the test solution. The polymers were not totally soluble in the solution, even at 0.2 wt % (2000 ppm), but formed a cloudy solution, also with sediment if sufficient polymer was used. It was difficult to know how much polymer had actually dissolved, and the structure of the soluble fraction compared to that of the insoluble fraction. Tests carried out with these cloudy solutions of unknown concentration did affect the growth of THF hydrate crystals, forming slushy wet lumps that grew around the tip of the glass tube (Figure 8). Tests with no additive give pyramidal crystals, as reported previously.13−15

Figure 4. Pressure and temperature plot during an 18 h standard constant cooling test using 5000 ppm PNIPAM v50. 4963

dx.doi.org/10.1021/ef300178u | Energy Fuels 2012, 26, 4961−4967

Energy & Fuels

Article

Figure 5. Complete cycle of superheating constant cooling test experiments with 5000 ppm PNIPAM v50.

Figure 6. Enlargement of the third cycle of the superheating constant cooling test with 5000 ppm PNIPAM v50.

Figure 7. Further enlargement of part of the third cycle shown in Figure 6. These slushy wet lumps release liquid when crushed and appeared to be made of fine plates, which incorporate the aqueous solution

trapped between the plates. The slush lump formed with both polymers (whether high or low syndiotactic percentage) and varied in 4964

dx.doi.org/10.1021/ef300178u | Energy Fuels 2012, 26, 4961−4967

Energy & Fuels

Article

variation went up to almost 30%. For example, with no additive, To varied from 6.6 to 9.9 °C, with an average value of 8.4 °C, over 5 experiments. For low-molecular-weight PVCap in EG, the To value varied between 2.3 and 3.6 °C and the Ta value varied between 1.9 and 3.3 °C. For PNIPAM v70, the To and Ta values varied from 2.9 to 5.2 °C and from 2.6 to 3.8 °C, respectively. Previous tests in our laboratories gave percentage variations of similar magnitude.15 Stochasticity of hydrate formation in a small cell leads to these scatterings. In addition, there is one result in some of the test series that lies outside the distribution, which could be neglected. For example, tests with no additive and PNIPAMs v50 and v70 gave much higher To values of 11.3, 8.2, and 7.5 °C, respectively. Systematic errors, such as hydrate initiation by rust or worn particles from the ball bearing, could lead to higher To and Ta values. The statistical differences between the polymers are determined by comparing the To and Ta values using independent sample t tests with equal variances assumed. The result is said to be statistically significant when the p value is less than 0.05.16 The results of the standard constant cooling tests using no polymer gave the highest onset temperatures with a clear statistical difference (p value < 0.05) to all of the polymers tested. The best polymer tested was the low-molecular-weight PVCap in EG. It gave consistently lower To values than all other polymers. The performance of PNIPAM v70 lies between the two commercial KHIs. PNIPAM v70 gave a lower average To value than PNIPAM v50 and 1:1 VCap/VP polymer. An independent sample t test for differences in onset temperatures for PNIPAM v50 and v70 gave a p value of 0.004; i.e., there is a clearly significant difference in the onset temperature results for the two polymers. There is no clear statistical difference (on the basis of the p values) between the average To values of 1:1 VCap/VP polymer and PNIPAM v50. The KHI performance of the polymers ranked using the To values is as follows: low-

Figure 8. Slush lump of THF hydrate crystals incorporating free solution between fine plates.15 size considerably between tests. Therefore, even though both polymers gave the same perturbation of the THF hydrate crystal structure, we could not satisfactorily compare their THF hydrate crystal growth inhibition performance quantitatively.

■

RESULTS AND DISCUSSION The PNIPAMs were first tested using the standard constant cooling test at 0.5 wt % dissolved in distilled water cooling from 78 bar and 20.5 °C to 2 °C over 18 h with 600 rpm stirring. The results from 6 to 8 individual experiments on each polymer are compared to those from distilled water without KHIs as well as using the 1:1 VCap/VP polymer and the low-molecularweight PVCap in EG. The tests with these commercial polymers are used to ascertain the KHI performance of the PNIPAMs. The results are shown in Figure 9, and the average To and Ta values are listed in Table 3. The percentage variation in the To and Ta values was about 3−21% for most of the tests. In two of the test series, the

Figure 9. To and Ta values from the standard constant cooling test. 4965

dx.doi.org/10.1021/ef300178u | Energy Fuels 2012, 26, 4961−4967

Energy & Fuels

Article

Figure 10. To and Ta values from the superheating constant cooling test.

to compare to the standard constant cooling test. The initial pressure of 100 bar was chosen for superheating experiments as decane lowers the equilibrium temperature. This gives the same subcooling as in the standard constant cooling experiments at 70 bar using hydrocarbon gas alone. The test using lowmolecular-weight PVCap in EG is not carried out, because its hydrates were not fully melted at 23 °C. In one of our previous studies, the pressure after the melting attempt lies lower than the maximum temperature before hydrate formation. The percentage variation of the To and Ta temperatures is considerably improved in comparison to the standard constant cooling test method to about 1−10%, with the exception of 1:1 VCap/VP polymer (10−15%). The To value varied from 11.8 to 14.6 °C, and the Ta value varied between 7.6 and 10.2 °C, over 4 cycles. The ranking of the KHI performance of the polymers by the superheating constant cooling test method, using the To values was less clear: PNIPAM v70 = PNIPAM v50 = 1:1 VCap/VP polymer > no additive. It was not possible to determine, with statistical significance, any difference in the KHI performance of the polymers. However, there is a clear statistical difference (p values = 0.004) between the average Ta values of PNIPAM v50 and v70: PNIPAM v70 (=1:1 VCap/VP polymer) > PNIPAM v50 (=1:1 VCap/VP polymer) > no additive. The results suggest that PNIPAM with 70% syndiotactic percentage is a better nucleation inhibitor than PNIPAM of lower syndiotactic percentage. It clarifies the effect of polymer tacticity on structure II gas nucleation inhibition with this class of polymer. According to Pyun and Fixman, the frictional coefficient, kf,0, indicates the measure of coil interpenetration for polymer molecules in a solution, with 7 as low interpenetrating coils and 2.23 as fully interpenetrating coils.19 The kf,0 of PNIPAM was reported to be 5.3 in water, which means that PNIPAM is a

molecular-weight PVCap in EG > PNIPAM v70 > PNIPAM v50 = 1:1 VCap/VP polymer > no additive. In addition, the standard constant cooling test allows us to rank the KHI performance of the polymers using the Ta values as follows: low-molecular-weight PVCap in EG > PNIPAM v70 > PNIPAM v50 > 1:1 VCap/VP polymer > no additive. t tests gave statistical p values of less than 0.05 for comparisons between all polymers. The comparison of Ta for PNIPAM v50 and v70 gives a p value of 0.001. Again, we can observe a clearly significant difference in the performance for the two polymers with different tacticities. Because of the scattered results from the standard constant cooling tests, it was decided to carry out the superheating constant cooling tests. It is a test procedure based on the “memory effect” of water molecules first described by workers at TOTAL and the University of Pau.17,18 When gas hydrates are melted at a moderate temperature, there will be some residual structures remaining in the fluids. They act as hydrate precursors in subsequent formation to improve the reproducibility of results. The superheating test method was used in earlier studies from our research group to increase the reproducibility of To values or induction times in standard constant cooling and isothermal tests, respectively.8 The ranking of KHIs using the superheating constant cooling test method is consistent with those from the standard constant cooling method, provided that the correct test conditions and test methods are used. Dependent upon the chosen hydrate dissociation temperature and duration, some systems (especially those of different KHI classes) might contain different amounts of non-melted hydrates. As a result, the relative position of the KHI performances could be different from that using the standard constant cooling (non-superheating) test method.18 The superheating constant cooling results are presented in Figure 10, and the average To and Ta values are given in Table 3 4966

dx.doi.org/10.1021/ef300178u | Energy Fuels 2012, 26, 4961−4967

Energy & Fuels

■

fairly flexible polymer, whose coil dimension is influenced by tacticity.20 Previous research on the effect of tacticity on chain dimension of polypropylene showed that the statistical segment length (i.e., repeating tactic unit) of syndiotactic polypropylene is substantially higher than isotactic polypropylene.21 Accordingly, one can rationalize these results in that PNIPAM v70 with a higher syndiotactic percentage has a higher segment length than PNIPAM v50. Subsequently, the pendant groups in PNIPAM v70 are farther away from one another than those in PNIPAM v50, which increases the water perturbation and interaction with hydrate cavities. This helps to disrupt the hydrate nucleation, assuming that perturbation of the water structure is the primary nucleation inhibition mechanism operating for this polymer class. Increasing the syndiotactic percentage of a polymer also provides a higher surface/volume ratio and enhances the hydrogen bonding with water molecules.

■

NOMENCLATURE av = average EG = ethylene glycol Mn = number average molecular weight Mw = weight average molecular weight PVCap = polyvinylcaprolactam St-1 = slow growth time (min) Ta = catastrophic hydrate formation temperature (°C) ta = rapid hydrate formation time (min) ti = induction time (min) To = onset temperature (°C) VCap = vinyl caprolactam VP = vinyl pyrrolidone REFERENCES

(1) Sloan, E. D., Jr.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (2) Kelland, M. A. Production Chemicals for the Oil and Gas Industry; CRC Press: Boca Raton, FL, 2010. (3) Kelland, M. A.; Svartaas, T. M.; Øvsthus, J.; Namba, T. Ann. N. Y. Acad. Sci. 2000, 912, 281−293. (4) Young, R. J.; Lovell, P. A. Introduction to Polymers, 2nd ed.; CRC Press: Boca Raton, FL, 2010. (5) Del Villano, L.; Kelland, M. A.; Miyake, G. M.; Chen, E. Y.-X. Energy Fuels 2010, 24 (4), 2554−2562. (6) Ray, B.; Isobe, Y.; Matsumoto, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2004, 37 (5), 1702− 1710. (7) Hirano, T.; Okumura, Y.; Kitajima, H.; Seno, M.; Sato, T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44 (15), 4450−4460. (8) Del Villano, L.; Kelland, M. A. Chem. Eng. Sci. 2011, 66 (9), 1973−1985. (9) Gjertsen, L. H.; Fadnes, F. H. Ann. N. Y. Acad. Sci. 2000, 912, 722−734. (10) Tohidi, B.; Burgass, R. W.; Danesh, A.; Ostergaard, K. K.; Todd, A. C. Ann. N. Y. Acad. Sci. 2000, 912, 924−931. (11) Klomp, U. C.; Kruka, V. C.; Reijnhart, R. WO Patent Application 95/17579, 1995. (12) Larsen, R.; Knight, C. A.; Sloan, E. D. Fluid Phase Equilib. 1998, 150, 353−360. (13) Kelland, M. A.; Del Villano, L. Chem. Eng. Sci. 2009, 64, 3197. (14) Klomp, U. C.; Kruka, V. C.; Reijnhart, R. WO Patent Application 95/17579, 1995. (15) Ajiro, H.; Takemoto, Y.; Akashi, M.; Chua, P. C.; Kelland, M. A. Energy Fuels 2010, 24 (12), 6400−6410. (16) Myers, R. H.; Myers, S. L.; Walpole, R. E.; Ye, K. Probability and Statistics for Engineers and Scientists; Pearson Education International: Upper Saddle River, NJ, 2007. (17) Duchateau, C.; Peytavy, J. L.; Glénat, P.; Pou, T.-E.; Hidalgo, M.; Dicharry, C. Energy Fuels 2009, 23 (2), 962−966. (18) Duchateau, C.; Glénat, P.; Pou, T.-E.; Hidalgo, M.; Dicharry, C. Energy Fuels 2010, 24, 616−623. (19) Pyun, C. W.; Fixman, M. J. Chem. Phys. 1964, 41, 937. (20) Kubota, K.; Hamano, K.; Kuwahara, N.; Fujishige, S.; Ando, I. Polym. J. 1990, 22 (12), 1051−1057. (21) Jones, T. D.; Chaffin, K. A.; Bates, F. S.; Annis, B. K.; Hagaman, E. W.; Kim, M. H.; Wignall, G. D.; Fan, W.; Waymouth, R. Macromolecules 2002, 35 (13), 5061−5068.

■

CONCLUSION We have synthesized two PNIPAMs, with different syndiotactic percentages but similar molecular weights. These polymers were tested for their structure II gas hydrate KHI performance in a high-pressure gas hydrate autoclave and compared to two commercial VCap-based KHI polymers. From the results, we can conclude that the polymer tacticity does affect the KHI performance of PNIPAMs. PNIPAM v70 with 70% syndiotactic percentage performed better than PNIPAM v50 with 50% syndiotactic percentage. In the standard constant cooling test, PNIPAM v70 performed significantly better than a commercial 1:1 VCap/VP polymer. Further tests would be required to clarify any small performance differences between the commercial polymer and PNIPAM v50. Experiments using the superheating constant cooling test method gave better reproducibility of the To and Ta temperatures, which were significantly higher than values obtained with the standard constant cooling test method. However, we are not able to show any statistically significant difference in their KHI performance of the PNIPAMs using To values by the superheating constant cooling method. The ranking of the PNIPAMs using Ta values by the superheating method was in accordance with the standard constant cooling test method. With the assumption that the primary nucleation inhibition mechanism is perturbation of the water structure, increasing the segment length of a polymer by increasing the syndiotactic percentage will improve the water perturbation and, therefore, has a greater effect on disrupting hydrate nucleation. Both NIPAM polymers were investigated for their ability to inhibit THF hydrate crystal growth. Both polymers gave a similar effect on the morphology of the THF hydrate crystals, indicating some kind of crystal surface adsorption. However, it was not possible to quantify which polymer gave the greatest inhibition because both polymers were only partially soluble in the THF/water/NaCl mixture at the test conditions. R e s u l t s o n t h e K H I p e r f o r ma n c e o f p o l y ( Nisopropylmethacrylamide)s of different tacticities will be reported shortly.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Telephone: +47-51831767. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 4967

dx.doi.org/10.1021/ef300178u | Energy Fuels 2012, 26, 4961−4967