Inhibition–Promotion: Dual Effects of Polyvinylpyrrolidone (PVP) on

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Inhibition−Promotion: Dual Effects of Polyvinylpyrrolidone (PVP) on Structure-II Hydrate Nucleation Wei Ke,† Thor M Svartaas,*,† Jan T Kvaløy,‡ and Birgitte R Kosberg†,§ †

Department of Petroleum Engineering, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway § Department of Petroleum Engineering and Applied Geophysics, Faculty of Engineering Science and Technology, Norwegian University of Science and Technology, NTNU, N-7491 Trondheim, Norway ‡

S Supporting Information *

ABSTRACT: Polyvinylpyrrolidone (PVP) is known as a kinetic hydrate inhibitor (KHI) and its inhibiting effect on hydrate nucleation and growth has been widely studied and acknowledged. In this work, the effect of PVP on methane−propane hydrate nucleation was examined in high-pressure autoclave at isothermal conditions. We examined PVP at ultralow concentrations of 50 and 100 ppm to avoid overlapping effects on nucleation and growth and maintain focus on the nucleation stage of the process. Two experimental pressures (Pexp = 6.1 or 9.0 MPa), two cooling rates (2 K/h or 6 K/h), and six subcooling levels (ΔT) ranging from 6.37−10.05 K were applied to induce nucleation. The induction time for hydrate nucleation increased with PVP present at ultralow concentrations, but intriguingly, PVP demonstrated dual effects of inhibition and promotion on hydrate nucleation rate at the experimental pressures examined. The switch from promoting to inhibiting effect on nucleation rate was seemingly not affected by pressure or cooling rate, but rather dependent on the degree of subcooling, ΔT. As compared to systems without inhibitor, PVP reduced the stationary nucleation rate at ΔT > 9 K. At subcooling levels between 6−9 K, PVP tended to promote the process through an increased nucleation rate. This is the first time a promoting effect of PVP alone on hydrate nucleation rate has been experimentally observed and reported. The cause remains unclear. We propose a hypothesis that the docking orientation of PVP polymers relative to the water cages on the surface of hydrate embryos is a function of temperature and applied subcooling level. Subsequently, a switch of the spatial configuration of PVP molecules could either inhibit or promote hydrate nucleation rate. The experimental study gives new insight into KHI working mechanisms. Further investigations are required to improve our understanding of the observed dual effects of PVP on gas hydrate nucleation.

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INTRODUCTION Gas hydrates are nonstoichiometric crystalline compounds of water and gas. They are formed when small gas molecules are enclathrated in a network of hydrogen-bonded water cativites.1 It is a general requirement of relatively high pressures and low temperatures for gas hydrates to stay thermodynamically stable. It was first reported 80 years ago by Hammerschmidt2 that hydrate formation could block gas transport pipelines and cause operation shut-ins and economical losses. Hammerschmidt suggested addition of antifreeze, such as methanol, to prevent hydrate formation. Still, hydrate formation threatens oil and gas transport in industrial practices.3 Traditional thermodynamic hydrate inhibitors, such as methanol and mono ethylene glycol, are associated with high quantity dosing (20−50 wt%) and huge expense.4 There has been continuous research and development of low dosage hydrate inhibitors (LDHIs).5 This has led to applications of novel polymer-based compounds, known as kinetic hydrate inhibitors (KHIs),6 and surfactants as anti-agglomerants (AAs)7 in fields for more than two decades.8,9 Common KHIs like polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap) and their derivatives work by delaying hydrate nucleation and/or growth.6 Consequently, the reservoir fluid flow could be left undisturbed for a period longer than the fluid residence time during pipeline transport. As compared to the large quantities of THIs used, LDHIs are © XXXX American Chemical Society

usually dosed at much lower concentrations, commonly 0.1− 2.0 wt% (active components).7 Details on development and field applications of LDHIs, including both KHIs and AAs, could be found in the literature.5,10 In recent years, the search for biodegradable KHIs, such as antifreeze proteins (AFP)11,12 and amino acids,13,14 has increased. Nevertheless, PVP and PVCap are still commonly used as references for evaluation of the performance of alternative, more biodegradable and greener KHIs.12,15 Similar to ice formation and melting, gas hydrate may undergo phase transitions of three stages, i.e., nucleation,1,16 growth,1,17 and dissociation.1,18 Accumulated research efforts via experimental studies1,19−27 and modeling/simulation work23,28−32 have been made in attempt to gain more understanding of hydrate formation and dissociation phenomena for decades. It is relatively easy to monitor and study hydrate growth and dissociation on a macroscopic scale.25,27,33−38 During nucleation, the water−gas clustering process is an energy battle between the volume excess free energy (favorable, reduced enthalpy) and surface excess free energy (unfavorable, reduced entropy) in the metastable Received: May 31, 2016 Revised: July 27, 2016

A

DOI: 10.1021/acs.energyfuels.6b01321 Energy Fuels XXXX, XXX, XXX−XXX

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system using water from melted hydrate to confine the stochastic nature of hydrate nucleation. Lachance et al.57 examined the effectiveness of PVCap on crude oil system using water-in-oil emulsions. Abay et al.58 tested PVCap on methane hydrate nucleation and growth and found that its performance as KHI was not a linear function of applied concentration. Ke et al.59 reported that PVP had no observable effect on total gas uptake or average hydrate growth rate during methane hydrate formation. Studies in the literature on PVP and PVCap do not point to a same direction that well acknowledges their performance as effective and universal hydrate inhibitors. Unexpected results or even adverse effects/predictions from experimental investigations have been reported from time to time. Abay et al. reported60 that PVCap at ultralow concentrations of 50−500 ppm (0.005−0.05 wt %) promoted structure II hydrate nucleation. Sakaguchi et al.61 confirmed that PVP (up to 0.2 wt%) could promote sII hydrate growth into the aqueous phase. Sharifi et al.44 showed that in the presence of heptane, addition of PVP or PVCap (0.1 mM in saline solution) would not further delay the nucleation process, but rather unexpectedly, promote nucleation by reducing the induction time. It is generally taken for granted that 0.1−1.0 wt % KHIs, such as PVP and PVCap, can effectively inhibit hydrate nucleation. There is yet no experimental proof of any promoting effect on hydrate nucleation with single addition of PVP. The current work presents the first report on the effect of ultralow concentrations of PVP (50 and 100 ppm, i.e., 0.005 and 0.01 wt%) on methane−propane structure II hydrate nucleation. In previous students’ theses work (unpublished) on sI and sII hydrate nucleation, we observed that PVCap concentration down to 100 ppm had an effect on initial hydrate growth, but not with 50 ppm at similar conditions. We did not observe any effect on initial growth in similar systems with 100 ppm PVP. In the present work, ultralow concentrations were adopted to avoid combined effects on nucleation and growth and to limit observed effects to KHI interaction with the nucleation kinetics only. Two different experimental pressures and three degrees of subcooling level at each pressure were applied. The aim of study was to examine the effect of PVP at these varied experimental conditions in attempt to improve understanding of the working mechanisms of polymeric KHIs on hydrate nucleation.

region. Nucleation is stochastic and intractable because initial embryos of hydrate clusters of different sizes would not obtain the critical size to form stable nuclei until the free energy barrier of pushing away the old phases is overcome.1,16 Only a nucleus surpassing such energy barrier would reach critical size for sustainable nucleation and trigger macroscopic growth. The induction time required for the onset of massive growth is also a stochastic variable, subject to the nucleation stage and phase transitions thereafter. With the presence of extra surfaces, for instance for systems in porous media, the induction time could be much shortened.39 The mechanism of the stochastic hydrate nucleation process is not fully understood. In addition to experimental studies with measurement of induction times and deduction of nucleation rates, molecular dynamics (MD) simulations40,41 and density functional theory42 are often utilized to study the nucleation processes. Two major mechanisms have been proposed for explaining the inhibition effect of KHIs on hydrate nucleation. These are adsorption−inhibition mechanism and perturbation−inhibition mechanism, respectively.43−50 The former assumes that the presence of polymeric KHI molecules would adsorb onto and cover the surface of clustering embryos thus able to prevent hydrate nucleation. The latter assumes that KHI molecules would perturb the water phase, preventing water molecules from effectively gathering and forming complete cavities. Support to either of these two mechanisms is available in experimental studies. Posteraro et al.43 studied the effect of PVP on sI methane hydrate formation; Sharifi et al.44 investigated the effect of PVP and PVCap on sII methane− ethane-propane hydrate; Rojas González45 examined the effect of PVP and PVCap on sII hydrate of natural gas mixture. These studies supported the adsorption−inhibition mechanism that the retarded gas enclathration and hydrate nucleation could be a result of PVP/PVCap binding to hydrate lattices. On the other hand, Varma-Nair et al.46 studied how PVP and PVCap would influence sII methane−propane hydrate formation; Talaghat47 investigated sI (methane, carbon dioxide) and sII (propane, isobutane) hydrate formation in the presence of PVP; Villano et al.48 examined the effect of PVCap on sII synthetic natural gas (SNG) hydrate. These studies favored the perturbation−inhibition mechanism. In addition, support to either nucleation inhibition mechanism is also available in MD simulations. Anderson et al.49 simulated methane hydrate formation in the presence of PVP and PVCap indicating the adsorption−inhibition mechanism. In contrast, MD simulations by Kvamme et al.50 showed that PVP and PVCap could trigger dissociation of sI methane hydrate and sII propane hydrate without direct contact with hydrate particles, suggesting that perturbation−inhibition was highly probable. Such disagreements among researchers indicate that the inhibition mechanism of KHIs is not fully understood. More efforts from both experimental studies and computational simulations are required to further our understanding. Sloan’s group is among the earliest to synthesize and test PVP and PVCap.51−53 After that, their effects on hydrate nucleation and growth have been massively studied from various points of view. Lee et al. 54 reported unusual catastrophic growth of methane−ethane hydrate in the presence of PVP. Zeng et al.55 compared the adsorption of PVP and PVCap on silica during tetrahydrofuran (THF) hydrate formation and found that the strength of polymer adsorption affected their performance during hydrate reformation. Duchateau et al.56 studied the effect of PVP on condensate

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EXPERIMENTAL SETUP AND ANALYTICAL METHODS

Experimental Setup. A binary synthetic natural gas mixture (SNG2) composed of 92.5 mol % methane and 7.5 mol% propane was used for the hydrate nucleation study and the experiments were conducted in high-pressure autoclave at isothermal conditions. This SNG2 composition forms structure II hydrate and has been used in previous hydrate studies in our laboratory.22,24,62 The autoclave has an inner volume of 141.3 mL and the experiments were run with 50 mL of the aqueous solution and 91.3 mL of SNG2 at experimental pressure. The experimental setup and procedure for performing a nucleation experiment were identical to that used by Abay and Svartaas22 except for a modification on the stirrer blade allowing the inclusion of a second temperature sensor in the water phase as described by Meindinyo et al.63 A schematic diagram of the experimental setup is shown in Figure 1. PVP was examined at two ultralow concentrations of 50 and 100 ppm in distilled water, and compared to the pure gas−water system as control. The tested PVP polymer (PVP K-15 from Ashland) had a weight-average molecular weight, Mw ≈ 9700. The 50 ppm PVP B

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The accuracies of temperature and pressure monitoring are ±0.1 K and ±0.02 MPa, respectively. At 6.1 MPa, 64 parallels were run with 50 ppm PVP and 68 parallels without. At 9.0 MPa, 60 parallels were run with 100 ppm PVP and 154 parallels without. For each experimental condition with varied P/T/ subcooling level, at least 20 parallel (and an average of 29) experiments were repeated in order to achieve representative data collection for statistical analysis. Part of the baseline data at 9.0 MPa was shared with a recent work by Svartaas et al.62 A total of 346 experimental parallels were produced for the analysis of nucleation rates. The experimentally measured induction time data for all 346 parallels are given in Supporting Information. In addition, SNG2 gas solubility in the presence or absence of PVP molecules was examined with same experimental setup. Solubility data are also presented in Supporting Information. Analytical Method. Penalized maximum likelihood estimation (MLE)64,65 was used for nucleation data analysis in this work. A detailed description of MLE method and its first-ever application to hydrate nucleation data has been given in a recent work.62 For statistical estimation of stationary nucleation rate J and lag time τ0, the penalized MLE estimators take the form: Figure 1. Schematic apparatus diagram for hydrate formation experiments.

J = (m − 1)/[m( t ̅ − t1: m)]

(1)

τ0 = (mt1: m − t ̅ )/(m − 1)

(2)

where m is the total number of experimental parallels, t ̅ is the mean value of the measured induction time distribution, and t1:m is the shortest induction time. Although these estimators give unbiased estimates, in some cases eq 2 leads to a negative estimate of τ0. This can be avoided by imposing the additional restriction in the MLE that τ0 ≥ 0 which implies that in the cases where eq 2 would give a negative estimate τ0 is set to 0 and J = 1/t.̅ With nucleation rate J and lag time τ0 as two input variables estimated by eqs 1 and 2, the cumulative distribution function (CDF) could subsequently be obtained as

solution was examined at 6.1 MPa and at temperatures of 286.15, 284.15, and 282.90 K, separately. This gave degrees of subcooling (ΔT) of 6.37 , 8.37, and 9.62 K for the three series, respectively. A cooling rate of 2 K/h was applied to reach the preset temperature for the experiments conducted at 6.1 MPa. The 100 ppm PVP solution was examined at 9.0 MPa and at temperatures of 287.40, 286.15, and 284.90 K, separately. This gave degrees of subcooling (ΔT) of 7.55 , 8.80, and 10.05 K for the three series of experiments, respectively. A cooling rate of 6 K/h was applied to reach the preset temperature for the experiments conducted at 9.0 MPa. Six series of baseline nucleation experiments were run accordingly with distilled water as control groups for comparison. For each individual experiment, constant pressure and temperature (degree of subcooling) were maintained from start of the experiment until the onset and detection of hydrate formation. The start of stirring was defined as start of the experiment and the induction time was measured as the time elapsed from this point to hydrate onset in the cell. The first sign of sudden pressure drop and temperature peak in the cell indicates incipient hydrate formation as illustrated in Figure 2. Real-time pressure and temperature data was recorded at 3-s intervals.

P(t ) = 1 − exp(− J(t − τ0))

(3)

where P(t) is the probability of nucleation within the time interval [τ0, t]. Uncertainty of Estimation. The uncertainty in the estimated nucleation rates due to the amount of data information is reflected by calculating 95% confidence intervals. In addition, a permutation test was adopted for testing if the rate parameters estimated from two data sets show a statistically significant difference.62 Details, including R scripts for running penalized MLE and permutation test, have been

Figure 2. An example of real-time experimental pressure and temperature recording (from parallel nr. Eleven for SNG2 hydrate nucleation with 50 ppm PVP at 6.1 MPa and 284.15 K). Induction time, ti, was found to be 8172 s, counting from start of stirring to hydrate detection. C

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Figure 3. Effect of lag time estimation on estimated nucleation rate and probability distribution with the lag time estimated as 0 and as the shortest induction time t1:m (300 s in this case), respectively. (a) Cumulative probability distribution; (b) 95% confidence intervals of estimated rate. given as Supporting Information in previous publication.62 In that work, the required number of parallels for reliable estimation of nucleation rate by penalized MLE was determined to be around 20. However, reliable estimation of lag time would require more parallels. Since the average number of parallels performed at 6.1 and 9.0 MPa in this work was 22 and 36, respectively, it may or may not be statistically sufficient to ensure the accuracy of estimated lag time. τ0 could be underestimated by the penalized MLE and sometimes takes negative value.62 Inaccurate lag time may affect the estimation of nucleation rate. Thus, to also quantify the uncertainty in the estimation of nucleation rate due to the estimation of lag time, the following methodology has been applied. To study the impact on the estimated J, we set as requirement that τ0 should fall within its physical probability boundary. Since nucleation prior to start of stirring has not been observed in any experiment, negative lag time values should not exist. If negative τ0 were obtained by eq 2, it is set to zero (0). On the other extreme, the lag time cannot be longer than the shortest induction time measured. Estimating the lag time by the shortest observed induction time actually corresponds to the ordinary MLE estimate.65 Thus, it is reasonable to believe that a good estimate of nucleation rate would be obtained for values of lag

time in the region between 0 and the lowest induction time observed. The inaccuracy in the estimated rate due to uncertainties in lag time estimates can be considered through the minimum and maximum nucleation rates, Jmin and Jmax, corresponding to the lower and upper limit of lag time. Figure 3 took the baseline experiments at 6.1 MPa and 286.15 K containing 22 parallels as an example. The original lag time estimate from eq 2 was negative and the lag time is thus set to 0, while the lowest measured induction time was 300 s. Figure 3a demonstrates the effect of lag time estimation on the estimated probability distribution. The estimated nucleation rate when τ0 = 0 s was found to be 1.41 × 10−4 s−1 and when τ0 = 300 s the estimated rate was 1.47 × 10−4 s−1, i.e., a deviation of merely 4.1%. Figure 3b shows the two estimated J values with their corresponding 95% confidence intervals. The 95% confidence intervals of the estimated J values almost fully overlap. The analysis showed that the effect of estimation method for lag time on nucleation rate estimation is marginal. Data analysis using the penalized MLE with restriction that τ0 ≥ 0 is reported and discussed in the following sections. When performance of baseline and PVP experiments were compared for differences, permutation test with one million D

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Energy & Fuels Table 1. Methane−Propane Hydrate Nucleation with and without 50 ppm PVP at 6.1 MPa aqueous phase DIW 50 ppm PVP DIW 50 ppm PVP DIW 50 ppm PVP

exp. T (K)

subcooling, ΔT (K)

no. of parallels

nucleation rate, J × 104 (s−1)

95% confidence interval ×104 (s−1)

lag time, τ0 (s)

lowest ind. time, ti,min (s)

p value

282.90 282.90

9.62 9.62

24 20

24.0 13.9

[15.2, 34.7] [8.4, 20.8]

9.6 108.0

27 144

0.030

284.15 284.15

8.37 8.37

22 22

7.7 9.4

[4.8, 11.2] [5.9, 13.7]

0 0

54 27

0.327

286.15 286.15

6.37 6.37

22 22

1.4 4.7

[0.9, 2.1] [3.0, 6.9]

0 0

300 48

0.002

Table 2. Methane−Propane Hydrate Nucleation with and without 100 ppm PVP at 9.0 MPa aqueous phase DIW 100 ppm PVP DIW 100 ppm PVP DIW 100 ppm PVP

exp. T (K)

subcooling, ΔT (K)

no. of parallels

nucleation rate, J × 104 (s−1)

95% confidence interval ×104 (s−1)

lag time, τ0 (s)

lowest ind. time, ti,min (s)

p value

284.90 284.90

10.05 10.05

38 20

27.0 20.7

[19.0, 36.4] [12.5, 31.0]

2.3 2.8

12 27

0.345

286.15 286.15

8.80 8.80

76 20

9.8 9.8

[7.7, 12.2] [5.9, 14.7]

7.6 15.0

21 66

0.428

287.40 287.40

7.55 7.55

40 20

3.5 5.8

[2.5, 4.7] [3.5, 8.5]

0 0

39 63

0.151

Figure 4. Methane−propane nucleation induction time vs probability distribution at 6.1 MPa at varied degrees of subcooling. resamplings was used to verify if the nucleation rates for the two situations were significantly different or not. This is particularly necessary and useful when there is overlap in the 95% confidence intervals of the estimated nucleation rates in the compared data sets. A p value ≤0.05 from a permutation test indicates significant difference between data sets.

given in Tables 1 and 2. Figures 4 and 5 show the cumulative distribution function (CDF) of nucleation probabilities based on the nucleation rate J and lag time τ0 estimated by penalized MLE. The obtained cumulative nucleation probabilities using eq 3 were curve fitted by the same equation producing smooth lines. Figures 6 and 7 show the nucleation rate as a function of subcooling levels at either experimental pressures examined. The curves in Figures 6 and 7 were generated by power fit and denote only a trend of the variation in the nucleation rate as a function of the subcooling level. It is worth noting that all the six subcooling levels examined had proper intervals in between, covering a range of 6.37−10.05 K. We consider the nucleation rate as the critical indicator of nucleation processes and more representative for the stochastic behavior. At high nucleation rates the time elapsed until probability of nucleation P(t) equals 1 may be relatively short and at low nucleation rates the time

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RESULTS Tables 1 and 2 summarize the results of structure-II SNG2 hydrate nucleation with 50 ppm PVP at 6.1 MPa and 100 ppm PVP at 9.0 MPa, respectively. These tables show the estimates of nucleation rate J with 95% confidence interval for each series of baseline and PVP experiments. Estimated lag times (τ0) together with shortest induction times measured (ti,min) were also included. In addition, p values from permutation tests between each baseline and corresponding PVP experiments are E

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Figure 5. Methane−propane nucleation induction time vs probability distribution at 9.0 MPa at varied degrees of subcooling.

Figure 6. Rate of nucleation as a function of subcooling in baseline and 50 ppm PVP experiments at 6.1 MPa. Curves were generated by power fit and represent trends only.

strictly, indicating retarded nucleation as expected. As a comparison, the estimated lag time τ0 was not able to reflect such a trend. As mentioned earlier, estimated lag time is probably not an appropriate system parameter to quantitatively describe effects on the nucleation process. The estimated nucleation rate J is a more competent system parameter for such a purpose. The second and more intriguing trend was: ultralow PVP concentration (50 or 100 ppm) showed dual effects on the nucleation stage as a function of subcooling level for the examined sII hydrate system. Similar behavior was observed at both 6.1 and 9.0 MPa, as shown in Tables 1 and 2. As compared to the pure water baseline system at the highest subcooling (ΔT = 9.62 K), 50 ppm PVP gave a significant decrease of 42.1% (p value =0.030) in the estimated nucleation rate from 2.40 × 10−3 s−1 for the baseline to 1.39 × 10−3 s−1 with PVP. With a p value of 0.345, no significant difference could be concluded for the estimated nucleation rate with 100 ppm PVP at the highest subcooling (ΔT = 10.05 K) as

could be relatively long. Thus, the process may appear less stochastic at higher nucleation rates (i.e., at higher driving forces/lower temperatures in the system) than at lower nucleation rates. From Tables 1 and 2 and Figures 4−7, we noticed three major trends of the nucleation rate as a function of subcooling in the system with and without PVP added. These trends are discussed below. The first trend noticed was: despite the variations in system parameters, such as experimental pressure, subcooling level (ΔT), and PVP concentrations, the synchronizing pace between nucleation rate J and ΔT was strictly maintained. The nucleation rate, J, increased with increasing ΔT (thus higher nucleation driving force), as observed in all series of experiments. This is well in accordance with the classic nucleation theory. The shortest induction time ti,min followed a general trend as a function of experimental temperature (and thus ΔT), with or without PVP dosing. Increased experimental temperature (decreased ΔT) gave prolonged ti,min, though not F

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Figure 7. Rate of nucleation as a function of subcooling in baseline and 100 ppm PVP experiments at 9.0 MPa. Curves were generated by power fit and represent trends only.

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compared to baseline. Nevertheless, a similar trend of inhibiting effect was shown with a 23.3% decrease in the estimated nucleation rate from 2.70 × 10−3 s−1 for the baseline to 2.07 × 10−3 s−1 with PVP. On the other hand, with decreasing levels of subcooling between 6−9 K, addition of either 50 or 100 ppm PVP has caused a gradual shifting of their effect on the nucleation rate from inhibiting to promoting. With 50 ppm PVP, no significant difference (p value =0.327) in the nucleation rate could be observed (7.7 × 10−4 s−1 for the baseline and 9.4 × 10−4 s−1 with PVP) at the middle subcooling level (ΔT = 8.37 K). At the highest temperature and lowest subcooling (ΔT = 6.37 K), J increased significantly by a factor of more than 3 (p value =0.002) from 1.4 × 10−4 s−1 for the baseline to 4.7 × 10−4 s−1 with PVP. In a similar manner, with 100 ppm PVP at the middle subcooling level examined (ΔT = 8.80 K), the estimated nucleation rate was at a same level of 9.8 × 10−4 s−1 for both the baseline and tests with PVP. At the highest temperature and lowest subcooling (ΔT = 7.55 K), the estimated nucleation rate increased by 65.7% from 3.5 × 10 −4 s−1 for the baseline to 5.8 × 10−4 s−1 in system with PVP. The difference could not be declared significant for 100 ppm PVP experiments compared to baselines at the middle and lower subcooling levels examined (p value =0.428 and 0.151, respectively). Nevertheless, they showed similar trends of promoting effect on the nucleation rate as observed in 50 ppm PVP experiments at lower subcooling levels. The observed variation in nucleation rate showed a kind of oscillation between inhibiting and promoting effects of PVP at low concentrations. Such an observation, as can be easily seen in Figures 6 and 7, shows dual effects of PVP on the nucleation rate for the examined SNG2 hydrate forming systems. Finally, the third trend noticed: in systems with PVP, the nucleation rate increased more slowly as a function of reduced subcooling in comparison with the pure water baseline experiments. This can be observed from curves illustrating the trends in Figures 6 and 7. It indicates that in general the presence of PVP at low concentrations has retarded the system response to varied driving force.

DISCUSSION

The last trend noted in the Results Section showed that the nucleation rate varied in a slower pace against subcooling levels in the presence of PVP. This could be directly related to the chemical structure and working mechanism of PVP. The pendent ring structure5 of PVP may have affected the properties of newly formed cluster embryos at the growing surface. It is also possible that the PVP molecules could have a long-range effect on water structures and their suitability to form complete cavities and entrap gas molecules. It is not clear why the presence of PVP molecules gave dual effects of promotion and inhibition on the nucleation rate in the examined SNG2 systems at the tested concentrations and subcooling levels. To seek or exclude a possible cause, we have considered the potential effect of PVP concentration on the solubility of hydrate formers in the aqueous phase. Posteraro et al.43 recently conducted experiments to study the effect of PVP on methane hydrate formation. At PVP concentration of 0− 0.07 wt%, they showed that PVP had no significant effect on methane solubility. We have checked the solubility of SNG2 in pure water and PVP solutions at 0.005−0.01 wt% concentrations as used in this work. Data from this check are given in Supporting Information. We observed a slight but not significant decrease in SNG2 solubility in the presence of PVP through this solubility check. This could not explain the observed dual effects of PVP on SNG2 nucleation. Nevertheless, the test offered an insight that could be important for future KHI studies and performance evaluations. If the level of supersaturation was considerably affected by the presence of KHI molecules, the degree of subcooling alone could be less representative as the driving force for hydrate formation. In such case, one should examine the level of supersaturation as a function of KHI dosing prior to further experimental investigations. MD simulations by Moon et al.29 indicated that there is no irreversible chemical bonding between PVP molecules and water lattices. PVP molecules did not adsorb onto the growing crystal planes but held a distance of 5−10 Å from the surface. Monte Carlo simulation by Wathen et al.66 indicated that when PVP was within its working distance to the crystal surface, its G

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CONCLUSION The effect of PVP at ultralow concentrations (0.005 and 0.01 wt%) was experimentally examined for structure-II methane− propane hydrate nucleation in high-pressure autoclave at isothermal conditions. It was reported that PVP alone at ultralow concentrations could either inhibit or promote hydrate nucleation rate. The experimental pressures and cooling rates did not affect the observations. Rather, the observed dual effects of PVP on hydrate nucleation appeared as closely related to the actual subcooling levels applied. The exact reason remains unclear. We propose a hypothesis that PVP’s docking orientation relative to the water cavities on the surface of hydrate embryos could be a function of the subcooling level which in turn may determine its effect on nucleation. Further investigations are required to improve understanding of the mechanisms involved in kinetic hydrate inhibition.

effect could be dual. Whether PVP inhibited or promoted nucleation depends on its spatial configuration relative to the water molecules on the surface of hydrate embryos, i.e., the “PVP−hydrate docking orientation”.66 According to their simulation results, if the polymer chain orients itself and blocks the docking site on the embryo surface, no gas entering into the partially complete water cavities would be possible. In this manner, PVP will inhibit hydrate nucleation; if the oriented spatial configuration of PVP only caps the incomplete water cavities without blocking, the entering of gas molecules would encourage the completion of the water cavities. In this way, PVP may increase hydrate nucleation rate. Combining such indications from simulations with the experimental observations made in the present study, we propose a hypothesis on the working mechanism of PVP. The spatial configuration of PVP at low concentrations in an aqueous environment could be a function of the applied subcooling level as well as the system temperature. With elevated temperature and reduced subcooling level, PVP could change its docking orientation and shift its effect on the nucleation rate from inhibition to promotion. It is interesting that the observed dual effects of PVP caused oscillation in the nucleation rate while at the same time PVP has in general prolonged the induction time. It is possible that while its varied spatial configuration as a function of subcooling has caused the increased or decreased nucleation rate, water perturbation induced by the PVP molecules may have contributed to the increased induction time. In literature, PVP concentrations above 0.1 wt% were never reported with a promoting effect on hydrate nucleation. It is not sure yet if the observed dual effects on hydrate nucleation only exist at ultralow PVP concentrations, such as 0.01 wt% or even lower. Thorough experimental investigations would be required to understand the working range and mechanism of specific KHIs, such as PVP. For the time being, it is not practical to simulate a large quantity of PVP monomers or polymer chains in a computational environment due to the complexity and limitations in computational capacity. Also, molecular simulations generally require much lower temperatures and higher driving force due to the small size of the system and observation of hydrate nucleation within a feasible time length.28 According to Mullin,16 the critical nuclei size decreases as a function of decreasing temperature and observed effects as kinetic inhibitor may be a function of this size. Thus, it is reasonable to assume that KHI effects should preferentially be examined at realistic temperature and pressure conditions. However, molecular simulations with super computers and big data handling in the future may probably facilitate the examination of proposed working mechanisms for PVP or other KHIs. The dual effects of PVP on hydrate nucleation as observed in this work might also be associated with the actual experimental setup and conditions applied. Abay and Svartaas67 have previously observed similar dual effects of methanol at ultralow concentrations (0.00015−0.002 wt%) on structure-I methane hydrate. They also reported promoting effect of PVCap at 0.005−0.05 wt% on structure-II hydrate nucleation of sevencomponent synthetic natural gas (SNG7).60 In their works, the same autoclave was used. Experimental observations made on gas hydrates are not always transplantable from one lab and device to another.10 The observed dual effects of PVP, especially its promoting effect on hydrate nucleation, deserve attention and should be subject for further investigations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01321. Experimentally measured induction times for sII methane−propane gas hydrate nucleation and SNG2 gas dissolution tests in the presence or absence of PVP molecules (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +4751832285; Fax: +4751832050; E-mail: thor.m. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Norwegian Ministry of Education and Research, and University of Stavanger for their financial support of this work.

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REFERENCES

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