Article pubs.acs.org/EF
Kinetic Hydrate Inhibition of Poly(N-isopropylmethacrylamide)s with Different Tacticities Pei Cheng Chua* and Malcolm A. Kelland Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway
Kenji Ishitake, Kotaro Satoh, and Masami Kamigaito Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
Yoshio Okamoto Nagoya University, Nagoya 464-8603, Japan and College of Material Science and Chemical Engineering, Harbin Engineering University, Nantong St. Harbin, P. R. China ABSTRACT: Poly(N-isopropylmethacrylamide)s (PNIPMAMs) have become commercially available as kinetic hydrate inhibitors (KHIs). PNIPMAMs are usually made by standard radical polymerization, which does not allow control over the polymer tacticity. We have now synthesized PNIPMAMs using stereospecific radical polymerization giving a fairly high degree of tacticity control. In this paper we present results on the performance of different tacticities in KHI tests with synthetic natural gas in stirred autoclaves and on tetrahydrofuran (THF) structure II hydrate crystal growth. The molecular weights of these polymers are almost the same. From the results, we can conclude the effect of polymer tacticity on the KHI performance of PNIPMAMs is noticeable but not very significant. PNIPMAMs with a higher syndiotactic percentage performed slightly better than PNIPMAMs with a lower syndiotactic percentage. Both polymers also gave a similar effect on the morphology of the THF hydrate crystals, indicating some kind of crystal surface adsorption. The PNIPMAMs investigated have a fairly high syndiotactic percentage. In fact, we found that it was difficult to make PNIPMAMs with less than 45% syndiotactic percentage, even using standard radical polymerization methods. This suggests that any method to make commercial NIPMAM-based polymer KHIs via radical polymerization will necessarily give polymers with a fairly high percentage of syndiotacticity.
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INTRODUCTION In pipelines at thermodynamic conditions of elevated pressure and low temperature, water molecules tend to form gas clathrates by trapping low molecular weight natural gas molecules. The typical guest molecules include carbon dioxide, small hydrocarbons such as methane, ethane, propane, and hydrogen sulphide. The problem of natural gas hydrates is a challenging problem for oil and gas industry because it could lead to financial loss due to production interruption as well as property damage and loss of life in the case of a drilling accident. The prevention of gas clathrate formation can be accomplished by avoiding entering the hydrate formation region, which includes keeping the temperature above the hydrate equilibrium temperature by passive heating retention or active heating, dehydration to remove water content, modifying the gas phase with another gas, conversion of water to transportable hydrate particles without chemical usage and chemical treatment.1 In recent years, research on low dosage hydrate inhibitors has been carried out as one of the chemical treatment methods. Kinetic hydrate inhibitors (KHIs) are generally water-soluble polymers that delay the nucleation of gas hydrate and crystal growth. The main classes of KHIs that have been in commercial use in oil and gas field operations are polymers based on © 2012 American Chemical Society
homopolymers and copolymers of N-vinylcaprolactam and various hyperbranched polyesteramides. A group of KHIs based on polymers of alkylacrylamide has been developed. It was also determined that poly(N-monoalkyl(meth)acrylamide)s perform well as KHIs, especially when the alkyl group is isopropyl and there is a methyl group in the polyvinyl backbone.2−6 Polymers and copolymers in this class have been commercialized, although no field applications have yet been reported. Therefore, our interest for this project is to investigate the KHI performance of this new KHI class in more structural detail. Polyvinyl-based polymers can be characterized by their relative stereochemistry of adjacent monomer units, which is known as tacticity.7 The three basic types of tacticities are isotactic, with pendant group placement on the same side of the chain, syndiotactic, with alternating orientated pendant groups, and atactic, with randomly orientated pendant groups (Figure 1). Commercial KHIs are synthesized by radical polymerization of vinyl monomers, which do not normally give tactic control. The resulting polymers usually consist of atactic polymer structures. Received: March 1, 2012 Published: May 14, 2012 3577
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Figure 1. Orientation of pendant groups in isotactic (I), syndiotactic (S), and atactic (A) polyvinyl polymers.
26% monomer conversion). The tacticity was determined by 13 C NMR of the quaternary carbon of the main chain12 and was 74.3% syndiotactic triad (mm/mr/rr = 4.2/21.5/74.3). For the synthesis of IS169, in a graduated tube were placed Yb(OTf)3 (18.6 mg, 0.30 mmol), NIPMAM (7.6 g, 60 mmol), and a solution of a radical initiator (AIBN; 12 mL of 100 mM solution in degassed methanol) at room temperature. Into this solution, degassed methanol was added until the solution volume became 60 mL. The tube was immersed in an oil bath at 60 °C. The polymerization was terminated after 24 h by cooling the reaction mixture to −78 °C. The monomer conversion was 62% by 1H NMR. The solution was evaporated to ca. 30 mL and was slowly poured into 350 mL hot water (70 °C) under stirring to give the precipitated polymer. The polymer was dissolved in methanol (30 mL) and precipitated again into 350 mL hot water (70 °C). The obtained products were dried in vaccuo at 80 °C overnight to give IS169 (2.0 g, 42% yield based on the calculated value for 62% monomer conversion). The tacticity was determined by 13C NMR of the quaternary carbon of the main chain12 and was 47.3% syndiotactic triad (mm/mr/rr = 16.3/36.4/47.3). The number-average molecular weight (Mn) and polydispersity index (PDI, Mw/Mn) of the polymers were determined by size-exclusion chromatography (SEC) in DMF containing 100 mM LiCl at 40 °C on two polystyrene gel columns (Shodex K805 L) connected to Jasco PU-2080 precision pump and a Jasco RI-2031 detector. The columns were calibrated against eight standard poly(methyl methacrylate) samples (Shodex; Mp = 875−1950000; Mw/Mn = 1.02−1.09). The molecular weights of the 2 polymers were almost the same: Mn ∼ 10000, Mw ∼ 25000. The synthesized PNIPMAMs are summarized in Table 1.
In a previous work, the effect of polymer tacticity on the performance of poly(N,N-dialkylacrylamide)s as KHI was investigated.8 It was shown that syndiotactic polymers of this class generally gave better KHI performance than other tacticities. In this work, we have investigated poly(Nisopropylmethacrylamide)s (PNIPMAMs) to determine how the tacticity affects the KHI performance (Figure 2). The aim
Figure 2. Structures of PNIPMAM.
was also to synthesize polymers with similar molecular weight since previous research shows that the performance of some KHIs in the same polymer class varies according to its molecular weight.1,9 In this study, we compare the performance of stereoregulated PNIPMAMs with varying syndiotactic percentage as KHIs.
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SYNTHESIS OF POLY(N-MONOALKYLMETHACRYLAMIDE)S Two polymer samples with different tacticities were prepared by radical polymerization of NIPMAM, which was carried out by the syringe technique under dry nitrogen in a glass tube equipped with a three-way stopcock.10−13 For the synthesis of IS162, in a graduated tube were placed NIPMAM (12.7 g, 100 mmol) and a solution of a radical initiator (V-70: 2,2′-azobis(4-methoxy-2.4-dimethyl)valeronitrile; 20 mL of 100 mM solution in degassed methanol) at 0 °C. Into this solution, degassed methanol was added until the solution volume became 100 mL. The tube was immersed in an oil bath at 20 °C. The polymerization was terminated after 25 h by the cooling of the reaction mixture to −78 °C. The monomer conversion was 26% by 1H NMR. The solution was evaporated to ca. 50 mL and was slowly poured into 350 mL hot water (80 °C) under stirring to give the precipitated polymer. The polymer was dissolved in methanol (50 mL) and precipitated again into 350 mL hot water (80 °C). The obtained products were dried in vaccuo at 80 °C overnight to give IS162 (2.4 g, 75% yield based on the calculated value for
Table 1. Poly(N-isopropylmethacrylamide)s Samples Synthesized in This Work sample ID
syndiotactic percentage
molecular wt (Mn)
PDI
IS169 IS162
47.3 74.3
9800 10700
2.45 2.39
The cloud point of IS169 was found to be 42−43 °C, whereas the more syndiotactic IS162 had a cloud point of 38− 39 °C. In another work, we tested poly(Nisopropylacrylamide)s, PNIPAMs of different syndiotactic percentages. The PNIPAMs without methyl group on the backbone of the polymer have lower cloud points compared to 3578
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PNIPMAMs.14−16 A study by Dybal et al. showed the effect of the methyl group on the backbone in PNIPMAMs on the physical structure of the water solution. Weaker intermolecular interactions between the amide groups were observed as a result of the steric hindrance due to the methyl groups. This raises the cloud points of PNIPMAMs compared to PNIPAMs.17 It has also been shown that the cloud point of PNIPMAM is higher as the methyl groups restrain the intrachain collapse and interchain association.18
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Table 2. Composition of Synthetic Natural Gas
EXPERIMENTAL METHODS: KINETIC HYDRATE INHIBITOR (KHI) PERFOMANCE TESTS
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
78 bar is 19.6 °C. For the superheating constant cooling test, we charge the cell with 100 bar SNG. The calculated equilibrium temperature is approximately 21.0 °C for distilled water. The initial procedures for preparation of the KHI experiment and filling of the cell are the same as previously discussed.14 The 5000 ppm test solutions are prepared by dissolving 0.5 wt % polymer in deionized water. The amount of the aqueous solution used for the tests are as follows:
Two commercial KHI products were used for comparison to the PNIPMAMs. These are a 1:1 N-vinylcaprolactam (VCap)/N-vinylpyrrolidone (VP) polymer in water (supplied as Luvicap 55W by BASF, Germany) and a low molecular weight poly(N-vinylcaprolactam) in ethylene glycol (PVCap in EG) (supplied as Luvicap EG by BASF, Germany). High Pressure Autoclave Tests with Synthetic Natural Gas. KHI performance tests were carried out in high-pressure autoclave
1. Standard constant cooling test, 8 mL of aqueous solution. 2. Superheating constant cooling tests, 3 mL of aqueous solution and 5 mL of decane.
3. Isothermal test, 5 mL of aqueous solution with 6 mg of silica gel as nucleating agent. Standard Constant Cooling Test Procedure. The initial pressure in the cell is 78 bar at 20.5 °C. The cell was cooled from 20.5 to 2 °C over 18 h under stirring of 600 rpm. Pressure drop is observed due to the temperature decrement of the cell, which is a closed system. From the pressure drop curve, it is possible to determine the start of hydrate formation and the fast, catastrophic hydrate formation. Another indication for the fast hydrate formation is the exothermic peak at the temperature curve and an increment of the torque measurement of the stirrer due to hydrate plug. Figure 4 is an example of pressure and temperature curves versus time during a standard constant cooling test. Eight to ten standard constant cooling tests were repeated for each polymer. The onset temperatures (To) and catastrophic hydrate formation temperatures (Ta) are presented in Figure 10. The average To(av) and Ta(av) values are given in Table 3. Superheating Constant Cooling Test Procedure. After the cell is loaded with the given amount of aqueous fluid and decane, the following test procedure are used for superheating constant cooling test: Step 1. The initial pressure in the cell is 100 bar at 21 °C. The cell is cooled from 21 to 1 °C over 2 h under stirring of 600 rpm. The temperature is held at 1 °C for 1 h for further hydrate growth. Hydrate plug should stop the stirring. Step 2. The cell is warmed to 23 °C and held for 1 h to melt the hydrates. Step 3. The cell is cooled from 23 to 2 °C over 20.4 h under stirring of 600 rpm. The To and Ta values are evaluated from the pressure and temperature curves using the same method as in the standard constant cooling test. The cell content is repeatedly melted (step 2) and cooled under constant rate (step 3) in order to produce at least four superheating constant cooling test cycles. Figure 5 is an example of the pressure and temperature versus time curves for a complete series of superheating constant cooling tests. Figure 6 shows a graphical enlargement of one cycle for more accurate determination of To and Ta. The hydrate formation could be very slow initially, so that it is hard to determine when the pressure drop deviation starts. Therefore, it is necessary to further enlarge part of the graph, as in Figure 7. Isothermal Test Procedure. After the cell is charged with 78 bar SNG at 20.5 °C, the fluids were cooled without agitation to the experimental temperature (9 °C). The pressure in the cell should be stabilized at 70 bar. The stirrer is then set to 600 rpm, and the starting time is taken as time zero, ts. From the pressure drop or gas consumption curves, it is possible to determine the induction time, ti, as the start of detectable hydrate formation, and the fast, catastrophic
Figure 3. Sapphire/stainless steel autoclave high pressure test equipment. equipment, as shown in Figure 3. The autoclave consists of a sapphire/ stainless steel cell with volume 23 mL. The same laboratory equipment has been used for experiments conducted previously by our research group.19 Three kinds of test methods were carried out:, the “standard constant cooling” test, the “superheating constant cooling” test, and the “isothermal” test. Further detail of each test methods are described in the following. In all the experiments, the cell was charged with a synthetic natural gas (SNG) mixture (Table 2) and distilled water as aqueous phase, which will form structure II (SII) hydrates. For “standard constant cooling” and “isothermal” experiments, the initial pressure in the cell is 78 bar. The equilibrium temperature for our SNG−water system at this pressure was calculated using Calsep’s PVTSim software to be 19.6 °C. Laboratory experiments were used to determine the equilibrium temperature by standard slow hydrate dissociation.20,21 Experiments conducted previously gave very good agreement with the predicted equilibrium temperature, with accuracy within 1 °C.8 For the rest of this paper we will assume that the equilibrium temperature at 3579
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Figure 4. Pressure and temperature curves of 5000 ppm PNIPMAM IS169 during 18 h standard constant cooling test. First detectable hydrate formation occurred after 930 min at onset temperature, To = 5.4 °C. The temperature for catastrophic hydrate formation, Ta = 4.2 °C, indicates fast hydrate formation after 1005 min. THF Hydrate Crystal Growth Experimental. The method of crystal growth inhibition using THF hydrate has been reported previously.22,23 The procedures for preparation of the test solution and experiment used here are the same as previously discussed.23 26.28 g NaCl and 170 g THF are dissolved in distilled water to make 900 mL of the test solution. This gives SII THF hydrate at an equilibrium temperature of about 3.2 °C. The test polymer is dissolved in the THF/NaCl solution. For example, 0.5 wt.% (5000 ppm) test solution consists of 0.40 g of the test polymer in 80 mL of THF/NaCl solution. The test solution is then placed in a beaker and cooled in a bath preset to −0.5 °C (±0.05 °C). This gives a subcooling of about 3.8 °C. We encountered two problems concerning the comparison of growth rates using the PNIPMAMs. First, the polymers were not totally soluble in the solution, even at 0.2 wt % (2000 ppm). A cloudy solution with sediment was formed if sufficient polymer was used. The actual amount of polymer dissolved and the structure of the soluble fraction compared to that of the insoluble fraction were difficult to determine. However, tests were carried out with these cloudy solutions of unknown concentration. We observed that the PNIPMAM did affect the growth of THF hydrate crystals, forming slushy wet lumps that grew around the tip of the glass tube (Figure 9). Tests with no additive give pyramidal crystals as reported previously.13,21,22
Table 3. Average Values of Onset and Fast Hydrate Formation Temperatures with PNIPMAMs standard constant cooling [°C]
superheating constant cooling [°C]
polymer
To(av)
Ta(av)
Ta(av) minus To(av)
no polymer low molecular weight PVCap in EG 1:1 VCap:VP polymer IS169 IS162
8.4 2.9
8.3 2.6
0.1 0.3
20.9
18.0
2.9
5.6
4.8
0.8
13.2
9.0
4.2
6.9 6.2
3.9 3.4
3.0 2.8
14.4 12.1
9.7 9.9
4.7 2.2
To(av) Ta(av)
Ta(av) minus To(av)
hydrate formation, ta. It should be noted that the nucleation may have started before the induction time. The slow growth time, St − 1, is taken as the time between ti and ta. Figure 8 illustrates the gas consumption and temperature versus time for an isothermal test.
Figure 5. Pressure and temperature curves of 5000 ppm PNIPMAM IS169 in a complete series of superheating constant cooling tests. 3580
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Figure 6. Second cycle with 5000 ppm PNIPMAM IS169 enlarged for more accurate determination of To and Ta. First detectable hydrate formation is hard to be determined graphically due to the slow hydrate formation (see also Figure 7). The temperature for catastrophic hydrate formation, Ta = 9.5 °C, indicates fast hydrate formation after 2480 min.
Figure 7. Enlargement of part of the second cycle shown in Figure 6 shows when first detectable gas uptake due to hydrate formation occurs after 2210 min at onset temperature, To = 14.1 °C. These slushy wet lumps release liquid when crushed, and appeared to be made of fine plates that incorporate the aqueous solution trapped between the plates. The slushy lumps formed with both polymers (whether high or low syndiotactic percentage) varied in size considerably between tests. Therefore, although 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.
The standard constant cooling tests gave 13−37% deviations on either side of the average. For IS169, the To value varied between 4.9 and 8.9 °C and the Ta value varied between 3.4 and 4.5 °C. For IS162, the To and Ta varied from 4.3 to 8.5 °C and 3.1 to 4.1 °C respectively. Similar deviations in To and Ta were observed previously in our laboratories.13 Scatterings of results produced from small autoclaves were expected, due to the stochasticity of hydrate formation. In addition, there is one result in some of the test series that lies outside the distribution, which could be neglected. For example, in tests with no additive, PNIPMAMs IS169 and IS162 gave much higher To values of 11.3, 10.2, and 11.8 °C respectively. Systematic errors like hydrate initiation by rust or worn particles from the ball bearing could lead to higher To and Ta values. Independent sample t-tests with equal variances assumed are used to show the differences in the To and Ta values between the polymers statistically. The result is said to be statistically significant when the p-value is less than 0.05.24 The test results show that the best polymer tested was low molecular weight PVCap in EG. Both the PNIPMAMs IS169 and IS162, 47.3%
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RESULTS AND DISCUSSION We carried out high pressure natural gas hydrate constant cooling experiments with the two different tacticities of PNIPMAMs with number average molecular weight of 9800 and 10700. The onset temperatures (To) and fast hydrate formation temperatures (Ta) are presented in Figure 10. The average values are summarized in Table 3. These results are compared with results from distilled water without polymer addition. In order to determine the KHI performance of these PNIPMAMs, the results with commercial polymers (1:1 VCap/ VP polymer and the low molecular weight PVCap in EG) are also reported here. 3581
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Figure 8. Isothermal test using 5000 ppm PNIPMAM IS162. First detectable hydrate formation occurs after ti = 198 min, as an increment in gas consumption. The period until fast hydrate formation, ta, is 341 min, giving a slow growth time of St − 1 = 143 min.
t-tests of all the samples gave p-values of less than 0.05. For the tactic polymers, the statistical p-value for comparison of Ta of PNIPMAMs IS162 and IS169 is 0.006. In order to eliminate the scattering of results, we decided to examine the polymers using the superheating constant cooling test method. It is a test procedure based on the “memory effect” in the water phase first described by workers at TOTAL and the University of Pau.25,26 By melting gas hydrate at a small superheating temperature above the equilibrium temperature, it is assumed that there is some residual structure remaining in the fluids which can promote a faster second hydrate formation. This subsequent hydrate formation is less stochastic and thus gives improved reproducibility of results. Earlier studies from our research group using this superheating constant cooling test method had increased reproducibility in To values than the standard constant cooling tests. This observation is also valid for induction times in isothermal tests, provided that the test conditions and methods are correct.18,22 The ranking of KHIs using superheating test method must however be made with caution as the hydrate dissociation is influenced by the temperature and duration. Some systems (especially those of different KHI classes) might contain nonmelted hydrates and others not. As a result, the relative position of the KHI performances could be affected, leading to a different ranking from that using standard constant cooling test method.25 The superheating constant cooling test is presented in Figure 11. Table 3 summarizes the average values alongside the results from the standard constant cooling test method. An initial pressure of 100 bar at 20.5 °C was chosen in the superheating constant cooling tests, as decane in the superheating constant cooling tests lowers the hydrate formation temperature compared to using hydrocarbon gas alone. The higher pressure gives the same subcooling as in the 75 bar standard constant cooling tests with no decane. Tests using low molecular weight PVCap in ethylene glycol were not carried out because one of our previous studies shows that 23 °C is insufficient to melt the hydrates completely.18 For PVCap, it was observed that the pressure after the attempt to melt the hydrates by heating to 23 °C was lower than 75 bar (initial pressure at 20.5 °C). For the three chemicals that were tested, the scattering of the To and Ta
Figure 9. Slush lump of THF hydrate crystals incorporating free solution between fine plates.14.
and 74.3% syndiotactic percentage respectively, performed as well as 1:1 VCap/VP polymer. The differences between these three polymers by comparing the To values are not statistically significant, judging from the p-values. The KHI performances of 1:1 VCap/VP polymer, PNIPMAM IS169, and PNIPMAM IS162 are not significantly differentiable: low molecular weight PVCap in EG > IS162 = IS169 = 1 : 1 VCap/VP polymer > no additive
Further tests are necessary to determine whether PNIPMAM IS162 performs better than IS169 as a KHI. However, a significant difference in the Ta values was observed using the standard constant cooling test method, giving the following ranking: low molecular weight PVCap in EG > IS162 > IS169 > 1 : 1VCap/VP polymer > no additive 3582
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Figure 10. To and Ta values from the standard constant cooling test.
Figure 11. To and Ta values from the precursor constant cooling test.
Since the results in the previous two sets of tests comprise certain scatterings, we could only get a significant ranking of the polymers using either To or Ta temperatures, but not both, using any one test method. As our third comparative, we investigated the SII hydrate inhibition of syndiotactic PNIPMAM by using the isothermal test method. Jensen et al. proposed a connection between the heterogeneous nucleation process with the stochastism of gas hydrate formation.27 Different experimental approaches were carried out to determine the induction time for structure I (SI) and SII hydrates, among others, with the addition of small amounts of impurities into the system. It was reported that this increased the induction time reproducibility. Likewise, we hoped that, by adding a small amount of silica gel as impurities, we could
values is considerably reduced to about 5−11% compared with the nonsuperheating test method. However, the Ta values of IS169 varied from 7.5 to 12.4 °C over 4 cycles, which resulted in a deviation of 22−28%. Due to the wide range of Ta values, a significant KHI performance ranking is not obtained using Ta values: IS162 = IS169 = 1: 1 VCap/VP polymer > no additive
Comparing the To of PNIPMAMs, a better inhibition effect is shown using IS162 in the superheating constant cooling test, with p-value = 0.01. This suggests that IS162 inhibits the SII hydrate crystal nucleation more effectively. This corresponds with the expectation according to the results in the previous section. 3583
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hindrance. The pendant-groups, even of those with lower syndiotactic percentage, are hence farther away from one another. It has also been reported that there are two essential hydrogen bonding interactions (between close amide groups as well as between amide groups and hydrating water molecules). The influence of the methyl group on the strength of hydrogen bonding interactions is reported to be significant.16 The influence of the hydrogen bonding interactions in PNIPMAMs might outweigh the effect of tacticity.
reduce the scattering of our test results to a certain degree. In the isothermal test, 0.006 g silica gel/5 mL test liquid was included in the cell. The average induction, slow growth, and total time of the five individual experimental results on each polymer are given in Table 4. The induction time for IS169 Table 4. Isothermal Test Results with PNIPMAMs Using Silica Gel polymer
tI(av)/min
St − 1(av)/min
ta − ts(av)/min
IS169 IS162
256 816
706 155
962 971
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CONCLUSION The effect of polymer tacticity on performance of poly(N,Ndialkylacrylamide)s was shown in earlier work. As poly(Nisopropylmethacrylamide)s (PNIPMAMs) have become commercially available as KHIs, we have synthesized two PNIPMAMs, with different percentage of syndiotactic percentages and similar molecular weights to compare their SII gas hydrate KHI performance. A high pressure gas hydrate autoclave was used for the experiments. From the results, we can conclude the effect of syndiotactic percentage on KHI performance with PNIPMAMs is noticeable, but not very significant. The restricted free rotation due to steric hindrance caused by the methyl group on the backbone leads to a more expended conformation of PNIPMAMs in aqueous solution. As a result, the pendant groups of PNIPMAMs, which are responsible to perturb water and disrupt hydrate nucleation, are farther away from one another. The influence of the methyl group on the backbone on the strength of hydrogen bonding interactions has also been noted. It is therefore predicted that the effect of syndiotactic percentage on spacing the pendant group is minimized. In isothermal tests, IS169 with 47.3% syndiotactic percentage gave shorter induction times than IS162 with 74.3% syndiotactic percentage, which correlates well with our previous explanation on the effect of increased syndiotactic percentage on hydrate nucleation. Yet, the slow hydrate crystal growth period of IS169 is longer than IS162. This shows that the inhibition of hydrate crystal growth is better when the pendant groups are more crowded together along one side of the backbone, which allows the polymer to absorb better onto the hydrate crystal surfaces. Both NIPMAM polymers were also investigated for their performances as THF hydrate crystal growth inhibitors. Both polymers gave a similar effect on the crystal morphology, indicating certain crystal surface adsorption. However, it was not possible to quantify which polymer gave the greatest inhibition, as both polymers were only partially soluble in the THF/water/NaCl mixture at the test conditions. The PNIPMAMs both have fairly high syndiotactic percentage. In fact, we found it was difficult to make PNIPMAMs with less than 45% syndiotactic percentage, even using standard radical polymerization methods. We postulate that this is due to the methyl group on the NIPMAM monomer, which will have a steric effect on the polymerization process, dictating a greater degree of syndiotactic percentage than using the NIPAM monomer as similarly observed for methacrylate and acrylate monomers.29 This suggests that any method to make commercial NIPMAM-based KHIs via radical polymerization will necessarily give a fairly high percentage of syndiotacticity.
varied between 0 and 614 min, whereas for IS162 it varied between 198 and 1991 min in the five tests. The slow growth time for the IS169 and IS162 varied 181−1414 min and 78− 257 min, respectively. Although there is considerable scattering of the test results, we can draw some statistically significant conclusions. For 74.3% syndiotactic IS162, hydrate formation occurred fairly fast immediately after noticeable nucleation was observed. Interestingly, PNIPMAM IS169 with 47.3% syndiotactic percentage had longer slow crystal growth periods. For example, the longest total delay time is 1414 min. This suggests that increased syndiotactic percentage increases nucleation inhibition, but does not work improve the inhibition of hydrate crystal growth. The same trend was also seen for the standard constant cooling and superheating constant cooling tests summarized in Table 3. PNIPMAM IS169 gave a larger temperature difference between the nucleation until rapid formation in both constant cooling test methods. The onset temperature represents the efficiency for hydrate nucleation, whereas the temperature difference Ta − To is an indication of crystal growth inhibition before rapid hydrate formation occurs. The explanation for these observations is that the pendant groups of IS169 is more concentrated on one side of the backbone, allowing the polymer to absorb better onto the growing hydrate crystal surfaces. When comparing the total time before rapid hydrate formation, it seems the two PNIPMAMs with different syndiotactic percentage behave very similarly. A similar observation regarding the effect of tacticity on hydrate crystal growth was reported in earlier work for the KHI performance of poly(N,N-dialkylacrylamide)s.1 In another work of studying the effect of tacticity of poly(Nisopropylacrylamide), where there is no methyl group on the polyvinyl backbone, we found out that the polymer of higher syndiotactic percentage gave the best performance. A similar effect is expected to be observed for PNIPMAMs. However, a promotion of KHI performance with increasing syndiotactic percentage in PNIPMAMs is detectable, but not as significant. The explanation for these observations could be the role of the backbone methyl group on the structural properties of PNIPMAMs in water. In aqueous solution, longer chain polymers might curl up or be entangled with intermolecular hydrogen-bonding. The pendant groups in PNIPAMs with lower syndiotactic percentage are more crowded together. This reduces the water perturbation and, consequently, the ability to disrupt hydrate nucleation. It was found that in aqueous solution PNIPMAM molecules take more expanded structures than PNIPAM molecules as a result of the presence of methyl group to the α-carbon.28 The PNIPMAMs, in which the methyl groups help to open up the polymer to a more expended conformation, have restricted free rotation due to steric
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 3584
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(24) Myers, R. H.; Myers, S. L.; Walpole, R. E.; Ye, K. Probability & Statistics for Engineers & Scientists; Pearson Education Int.: New Jersey, 2007. (25) Duchateau, C.; Peytavy, J. L.; Glénat, P.; Pou, T.-E.; Hidalgo, M.; Dicharry, C. Energy Fuels 2009, 23 (2), 962−966. (26) Duchateau, C.; Glénat, P.; Pou, T.-E.; Hidalgo, M.; Dicharry, C. Energy Fuels 2010, 24, 616−623. (27) Jensen, L.; Thomsen, K.; von Solms, N. Energy Fuels 2011, 25 (1), 17−23. (28) Kubota, K.; Hamano, K.; Kuwahara, N.; Fujishige, S.; Ando, I. Polym. J. 1990, 22 (12), 1051−1057. (29) Satoh, K.; Kamigaito, M. Chem. Rev. 2009, 109 (11), 5120− 5156.
Notes
The authors declare no competing financial interest.
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NOMENCLATURE av = average EG = ethylene glycol Mn = number average molecular weight Mw = weight average molecular weight PVCap = poly(N-vinylcaprolactam) 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 = N-vinylcaprolactam VP = N-vinylpyrrolidone
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REFERENCES
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dx.doi.org/10.1021/ef3006355 | Energy Fuels 2012, 26, 3577−3585
