Molecular Weight and Molecular Weight Distribution on Methane

Article pubs.acs.org/EF

Effects of Poly(N‑vinylcaprolactam) Molecular Weight and Molecular Weight Distribution on Methane Hydrate Formation Seong Deok Seo,†,‡ Hyun-jong Paik,‡ Dong-ha Lim,† and Ju Dong Lee*,† †

Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, 30, Gwahaksandan 1-ro 60 beon-gil, Gasnseo-gu, Busan 618-230, Republic of Korea ‡ Department of Polymer Science and Engineering, Pusan National University, Busan 609-753, Republic of Korea ABSTRACT: Poly(N-vinylcaprolactam) (PVCap) has previously been shown to be an outstanding low-dosage hydrate inhibitor (LDHI). The inhibition performance of LDHI is known to be influenced by molecular weight (Mw) and molecular weight distribution. In this study, PVCaps were synthesized by two different methods. The first method, reversible addition− fragmentation chain transfer (RAFT) polymerization, was used to prepare PVCap with a narrow molecular weight distribution (about Mw/Mn < 1.5), which is close to theoretical values. The second method, free radical polymerization (FRP), was used to obtain a broad molecular weight distribution (about Mw/Mn > 1.5). In a semi-batch stirrer reactor, hydrate inhibition performance of PVCap was evaluated. It was found that the inhibition performance was increased when the PVCap molecular weight decreased. PVCap with the narrower molecular weight distribution also showed the better inhibition performance.

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INTRODUCTION

Two possible hydrate inhibition mechanisms have been proposed for KHIs. The first is an adsorption hypothesis, involving hydrogen bonding to amide groups on the water molecule and van der Waals interactions between aromatic rings and the hydrate surface.15,17 The other is known as the perturbation inhibition hypothesis. It proposes that hydrophilic moieties of the KHIs disrupt the structure of water, hence increasing the barrier to nucleation.18 According to a recently published paper, the reactions of polymer inhibitor between the hydrate and water interface have been proven through molecular dynamics (MD) simulation. As a result, an inhibitor of high molecular weight proved to have strong binding affinity to gas hydrate.19 However, the mechanisms by which KHIs inhibit hydrate formation remain poorly understood. Considering the inhibition mechanisms mentioned above, when KHIs are adsorbed on the surface of the hydrate crystal and/or perturb the local water structure, the KHI mobility or diffusion rate might be an important factor affecting the inhibition performance in the solution. Typically, the conventional radical polymerization process has difficulty controlling molecular weight or achieving a narrow molecular weight distribution.20−22 One category of polymerization methods, known as controlled/living radical polymerization (CLRP), provides unprecedented tools for synthesizing polymers with controlled structures, chain-end fidelity, and narrow molecular weight distribution. Most CLRP methods are transition-metal-catalyzed living radical polymerizations and include atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addit ion−fragment ation chain transfer (RAFT).23−25 For successful RAFT polymerization, the use

Gas hydrates are compounds of water and gas molecules, such as methane or carbon dioxide, that are crystalline solids under conditions of high pressure and low temperature.1,2 Unfortunately, gas hydrates are spontaneously formed in oil and gas transport lines under thermodynamic conditions. This undesirable formation of gas hydrates often results in a hydrate blockage in the transportation pipeline.2−4 Plugs caused by natural gas hydrates are a costly problem, and their prevention is a major concern for the oil and gas industry.5−7 Traditionally, hydrate formation has been prevented by injecting large quantities of thermodynamic inhibitors, such as MeOH or ethylene glycol, into the production pipelines. These inhibitors can shift the hydrate equilibrium condition to a lower temperature and higher pressure. However, successful inhibition with alcohols or glycols requires large amounts (up to 40 vol %) of inhibitor in the free water.8 Kinetic hydrate inhibitors (KHIs) are a type of low-dosage hydrate inhibitor (LDHI), which are well-known for preventing gas hydrate plugging in up- and downstream oilfield operations. In general, the KHIs delay hydrate nucleation and crystallization at low dosage levels (around 1−2 wt %).5,9 Among KHIs, the performance and conditions of polymers such as polyvinylpyrrolidone (PVP), poly(N-vinylcaprolactam) (PVCap), and poly(N-vinylpiperidone) (PVPip) have been tested to investigate their inhibition of structure I (sI) or structure II (sII) hydrate formation.7,10−14 In the case of sII natural gas hydrates, it was determined that inhibition performance was related to the molecular weight of the polymer; higher molecular weight PVCap showed better inhibition performance compared to lower molecular weight PVCap.15 In contrast, in the case of sI hydrate formation, lower molecular weight KHIs showed better inhibition performance.16 © XXXX American Chemical Society

Received: January 31, 2017 Revised: April 24, 2017 Published: April 25, 2017 A

DOI: 10.1021/acs.energyfuels.7b00318 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels of chain transfer agents (CTAs) is crucial. It has been reported that the use of dithiocarbamate was effective for controlling the polymerization of unconjugated monomers.26−29 Xanthates and dithiocarbamate are employed for non-conjugated monomers, such as N-vinylcaprolactam (NVCL) and N-vinylpyrrolidone (NVP). It is reports that, when dithiocarbamate was used as a RAFT agent, the high reactivity of the radicals derived from non-conjugated monomers led to completely inhibited polymerization.30−32 In this study, the inhibition performance of synthesized PVCaps was evaluated with different molecular weights and molecular weight distribution conditions. In addition, the viscosity of the PVCap samples in a water solution system was measured using a rotational rheometer to determine the viscosity influence on hydrate inhibition.

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EXPERIMENTAL SECTION

Materials. NVCL (98%, Sigma-Aldrich) was purified from hexane to remove the inhibitor and then was stored at 4 °C. α,αAzobis(isobutyronitrile) (AIBN, 98%, Sigma-Aldrich) was purified by recrystallization using methanol 3 times before use. Cyanomethyl methyl(phenyl)carbamodithioate (CMPCD, 98%, Sigma-Aldrich) was used as received. Tetrahydrofuran (THF, 99%, Sigma-Aldrich) and nhexane (anhydrous, 95%, Sigma-Aldrich) were filtered under reduced pressure before use. Polymers. Synthesis of RAFT Polymerization. A round-bottom flask was charged with CMPCD (0.016 g, 0.072 mmol), AIBN (3 mg, 0.025 mmol), and NVCL (1 g, 7.2 mmol), and then the mixture was refluxed under nitrogen gas for a predetermined time at 70 °C. The polymer was purified by precipitating, using THF, into petroleum ether [boiling point (bp) of 30−60 °C] 3 times to remove unreacted NVCL and then filtered. Finally, it was dried under vacuum at 40 °C for 24 h. The conversion was determined using gravimetric analysis. Synthesis of Free Radical Polymerization. Polymerization was carried out in a flask bottle. Moisture was excluded from the system. A mixture containing NVCL (5 g, 36 mmol) and AIBN (3 mg, 0.025 mmol) was agitated by a magnetic stirrer. The air in the test flask was replaced with nitrogen. The flask was sealed and put in a bath regulated at the desired temperature. The reaction mixture obtained by polymerization was poured into an excess of petroleum ether. The polymer was filtered off and dried in vacuo at 40 °C for 24 h. Characterization. The synthesized polymers were dissolved in CDCl3, and then 1H nuclear magnetic resonance (NMR) spectra of the polymers were obtained using a Varian 400 (Varian, Palo Alto, CA, U.S.A.) spectrometer. The chemical shifts were calculated relative to tetramethylsilane (TMS). The weight-average molecular weight (Mw) and polydispersity index (PDI) were determined by gel permeation chromatography (GPC, Waters 150-C, Waters, Milford, MA, U.S.A.), calibrated with standard polystyrene and N,N-dimethylformamide (DMF) as a mobile phase at a flow rate of 0.8 mL/min. The viscosities of the different molecular weight polymer solutions were measured with water as a solvent at 25 °C using a rotational rheometer (ARES-G2, TA Instruments, New Castle, DE, U.S.A.). Samples were selected, placed in a sample hopper, and pushed to a cone plate, and their rheological properties were studied. Methane Gas Hydrate Experimental Apparatus and Procedure. The experimental apparatus, as shown in Figure 1, mainly consists of a stainless-steel reactor (372.5 mL), a supply vessel (566.5 mL), a refrigeration system, a water bath, and a data acquisition system. Prior to each hydrate kinetic experimental run, the reactor was filled with 135 cm3 of aqueous test sample (pure water or a liquid solution of the inhibitor), and then the reactor was flushed at least 3 times with methane gas to remove any residual air. Subsequently, the whole system was kept at a desired temperature, and the reactor was filled with methane gas until the experimental pressure was obtained. All experiments were conducted at 5.0 MPa and 274.15 K (5.7 K subcooling). Once the temperature was stabilized, the solution was agitated using a magnetically driven impeller with a rotational speed of

Figure 1. Schematic diagram of the gas hydrate experimental apparatus.

300 rpm, which was determined to be suitable for our experiment and was applied to every experimental run. In a literature review, there was no significant difference in gas consumption between 0.1, 0.5, and 1 wt % performance of the hydrate inhibitor.33 On the basis of this study, the inhibitor performance of PVCap was tested at an economic dosage of 0.1 wt %. As methane gas in the reactor is consumed during the hydrate formation, additional gas was automatically supplied from the supply vessel and the moles of gas uptake over time were calculated from the pressure drop profile in the supply vessel (>5.0 MPa) with respect to the initial pressure. A more detailed description of the experimental procedure is available in a previous report.34

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RESULTS AND DISCUSSION NVCL, as a monomer, was synthesized by two different methods by RAFT polymerization to control molecular weight and free radical polymerization for broad molecular weight distribution. PVCap was polymerized by RAFT processing in THF at 70 °C using CMPCD of the dithiocabamate type as the reversible CTA in the presence of a small percentage of radical initiator (AIBN), as shown in Scheme 1a. The polymerization of NVCL with dithiocabamate as the CTA exhibited a living/ controlled nature. Polymers were synthesized by molar ratio [NVCL]/[CTA]/[AIBN] = 200:1:0.25 in the bulk solution at 70 °C. As shown in Scheme 1b, NVCL was polymerized by free radical polymerization by the solution technique. In the solution polymerization of NVCL, hexane was used as the solvent, because both the initiator and monomer were soluble in it. Hexane has acceptable chain-transfer characteristics and suitable melting and boiling points for the conditions of the polymerization and subsequent solvent removal steps. Polymerization was carried out in an oil bath at 70 °C, and 66% conversions were calculated. The obtained polymer was a white, powder-type polymer. B

DOI: 10.1021/acs.energyfuels.7b00318 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Scheme 1. (a) RAFT Polymerization of NVCL Using Dithiocabamate as the CTA and (b) Free Radical Polymerization of NVCL

Mw and PDI of the obtained polymers were measured using GPC and are summarized in Table 1. DMF was selected as the Table 1. Characteristics of the Synthesized PVCap via RAFT Polymerization and Free Radical Polymerization polymerization free radical polymerization RAFT polymerization RAFT polymerization RAFT polymerization

time (h)

conversion (%)

Mw (g/mol)

PDI

8

45

6300

2.12

200

24

68

5810

1.32

400

24

73

10140

1.47

600

24

79

24360

1.58

[M]/[CTA]

GPC eluent in our study. As shown in Table 1, the CTA exhibited controlled polymerization characteristics, as indicated by the well-controlled molecular weight and narrow molecular weight distribution. On the other hand, a relatively broad molecular distribution (about Mw/Mn ≈ 2.1) was obtained with the free radical polymerization. The PDI is used as a measure of the broadness of the molecular weight distribution of a polymer and is defined by PDI = Mw/Mn. As shown in Figure 2, a complete shift of the trace of the PVCap sample was observed during the chain extension experiment. The molecular weight of the starting PVCap (Mw = 6000) has shifted toward a higher molecular weight (Mw = 20 000). It is easily observed that the GPC traces are symmetric, and no shoulder peak appears at a high molecular weight position, which would be attributed to linear−linear polymer coupling. The GPC traces show a narrow polydispersity (Mw/Mn < 1.5) and a gradually increasing molecular weight with an increasing amount of monomer, demonstrating that the RAFT polymerization proceeded in a well-controlled manner. Therefore, the CMPCD agent turned out to be suitable for the RAFT polymerization of NVCL as a monomer. Figure 3 shows the 1H NMR spectra of PVCap prepared via RAFT polymerization. The bulk radical polymerization of NVCL using CMPCD was characterized. As shown in Figure 2, the presence of the R group as a result of the dithiocabamate moiety at the ends of the polymer chain (Scheme 1) could also

Figure 2. GPC traces of PVCap prepared via RAFT and free radical polymerization.

Figure 3. 1H NMR spectra of PVCap prepared via RAFT polymerization mediated by CMPCD.

be observed in the infrared (IR) spectra of PVCap prepared via RAFT polymerization. In addition, the spectrum of the RAFT PVCap reveals the resonance of the repeat unit: NCCH2CH2CH2 of the C

DOI: 10.1021/acs.energyfuels.7b00318 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels caprolactam ring and CH2 backbone, c, e, f, and g (1.45−1.75 ppm); COCH2, h (2.3−2.5 ppm); NCH2, d (3.22 ppm); NCH3 of dithiocabamate and NCCH2, i and j (3.55−3.8 ppm); and NCH, b (4.4 ppm), respectively. A broad resonation peak of phenyl protons at 7.2−7.4 ppm overlapped with the peak of the solvent CDCl3 at 7.24 ppm. The hydrate inhibition performance of different molecular weight PVCap samples was tested, and the results are shown in Figure 4. The experiments with PVCap were performed in a

Figure 5. Comparison of the induction time for hydrate formation according to the molecular weight of PVCap (for all solutions, except pure water, the PVCap concentration was 0.1 wt % and the hydrate formation was conducted at 5.0 MPa and 274.15 K).

polymer diffusion to the hydrate surface, which is related to inhibitor performance. The experiments with PVCap show significant inhibition performance compared to pure water under the same conditions. The PVCap inhibitors may not only delay the nucleation of hydrates but can generate an inflection point that also affects the hydrate growth pattern after nucleation. With pure water, there was close to full conversion to hydrate, but when the inhibitor was added to water, even after 1000 min, it did not reach 100% conversion to hydrate. In the previous experiments, it was found that low-molecularweight PVCap with a narrow PDI performed best as a KHI, among the different molecular weights that were investigated. Further experiments were necessary to examine how the molecular weight affects mobility and how hydrate inhibition depends upon mobility. To further investigate the mobility of KHIs, samples were measured using a rotational rheometer to confirm their viscosity in the water solution. It was found that, when the molecular weight of PVCap increased, viscosity was also increased. Figure 6 shows shear viscosity versus shear rate and shear stress versus shear rate for PVCap samples whose average molecular weights ranged from 6000 to 20 000 g/mol. As seen here, measurements at 25 °C show that PVCap with the lowest molecular weight exhibits the lowest viscosity, while that with the highest molecular weight has the highest viscosity. Viscosity is an important parameter in determining the flow behavior of polymeric materials. The molecular weight is a major factor and has a direct effect on viscosity in a water solution. We suggest that the perturbation mechanism for PVCap correlates well with the molecular mobility. Before the inhibitor is adsorbed onto the hydrate surface, a low-molecular-weight polymer might have good mobility in the water solution due to its low viscosity. This would be effective for inhibiting nucleation according to the perturbation mechanism. Thus, the low-molecular-weight polymer perturbs the water structure better than higher molecular weight polymers, which might be curled up or entangled due to

Figure 4. Consumption comparisons of CH4 hydrate in the presence of PVCap synthesized at various molecular weights (for all solutions, except pure water, the PVCap concentration was 0.1 wt % and the hydrate formation was conducted at 5.0 MPa and 274.15 K).

semi-batch reactor, and the hydrate nucleation point (induction time) and CH4 gas uptake measurement were compared to a pure water system. All of the hydrate formation experiments were conducted at 5.0 MPa and 274.15 K. From the results in Figure 4, the molecular weights for the polymers were found to range from 6000 to 20 000. The formation rates of CH4 hydrate with the varying molecular weights (Mw = 6000−20 000) of inhibitor polymer were significantly lower than that of the pure water system. The induction time was also considerably delayed in the presence of the inhibitors. The recorded induction time was 22 min for the pure water system and varied 38−278 min for synthesized PVCap solutions at various molecular weights. To obtain accurate results, the experiments were repeated 3 times and the results with standard deviations were shown in Figure 5. The two high-molecular-weight (Mw = 10 000 or 20 000) PVCap samples showed very similar hydrate nucleation and growth pattern. It is obvious that the nucleation inhibition performance of PVCap increased with a decreasing molecular weight in the methane hydrate formation. Although two differently synthesized inhibitors had similar molecular weights (Mw = 6000), synthesized PVCap with the narrower molecular weight distribution showed higher inhibition performance. The enhanced inhibition effect of PVCap with a Mw of 6000 Da synthesized via RAFT polymerization might be related to the characteristics of the polymer, such as a narrow polydispersity, molecular weight, or mobility. This also indicates that a lowmolecular-weight polymer with high mobility achieved better inhibition effect than a larger polymer. The mobility and size of the molecules of the different molecular weights can affect the D

DOI: 10.1021/acs.energyfuels.7b00318 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

To investigate the effects of the inhibitor molecular weight, the kinetics of CH4 hydrate formation were examined for various molecular weights of PVCap at a constant temperature and pressure. From the results of the hydrate kinetic measurements, the presence of a PVCap hydrate inhibitor in the system significantly delayed hydrate nucleation and also prevented further gas hydrate formation. PVCap with the lowest molecular weight (Mw = 6000) had the best nucleation inhibition performance compared to a high-molecular-weight (Mw ≤ 10 000) PVCap at the same conditions. In other words, it was found that, when the molecular weight of PVCap decreased, the inhibition performance was increased. In addition, the induction time of PVCap with the lowest viscosity was higher compared to the high-viscosity polymer. This result suggests that the nucleation inhibition behavior of synthesized PVCap could be related to its mobility (viscosity) and molecular weight (size) in the water solution. However, the detailed mechanisms and theoretical background of the inhibition effects based on polymer characteristics were not fully studied here. Thus, additional experiments and investigations at a molecular level will be needed to develop more powerful inhibitors.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +82-51-974-9311. Fax: +82-51-974-9378. E-mail: [email protected]. ORCID

Dong-ha Lim: 0000-0003-4328-5168 Ju Dong Lee: 0000-0002-9567-8396 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was a part of the projects titled “Development of Module for Shale Gas Flowback Water Treatment Using Clathrate Hydrate Method, Convergence Project (EO170040)” and “Development of Water-Treatment and Recovery of Vital Resources from LNG Cold Energy and Gas Hydrate Process (EO170039)” funded by the Korea Institute of Industrial Technology.

Figure 6. (a) Shear rate versus shear viscosity and (b) shear rate versus shear stress for PVCap samples of various molecular weights in water solution.

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intramolecular hydrogen bonding, thus being less available to perturb the free water structure. In addition, for the two differently synthesized inhibitors with a similar molecular weight (Mw = 6000), it was confirmed that the polymer with low viscosity had a narrow molecular weight distribution. Interestingly, narrow distribution PVCap was found to be more effective at nucleation inhibition than a broadly distributed polymer of the same molecular weight. These experimental results demonstrate that PVCap with low viscosity disrupted the water structure more efficiently because of its mobility and, thus, provided more effective inhibition of hydrate nucleation.

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CONCLUSION In this study, PVCap was successfully synthesized via RAFT polymerization. Optimal reaction conditions (i.e., initiator type and concentration, solvent type and concentration, and temperature) were required to achieve low polydispersity. The “living” nature of PVCap synthesized via the RAFT polymerization method was confirmed by a successful chain control experiment. E

DOI: 10.1021/acs.energyfuels.7b00318 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b00318 Energy Fuels XXXX, XXX, XXX−XXX