Effect of Poly Vinyl Caprolactam Concentration on the Dissociation

Jul 6, 2017 - Because residual hydrate structures known as hydrate precursors will exist in the liquid water phase after dissociation, the risk of met...
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Effect of Poly Vinyl Caprolactam Concentration on the Dissociation Temperature for Methane Hydrates Ann Cecilie Gulbrandsen*,†,‡ and Thor Martin Svartaas† †

Department of Petroleum Engineering, Faculty of Science and Technology, University of Stavanger, 4036 Stavanger, Norway S Supporting Information *

ABSTRACT: The use of KHI in conventional gas production has become common, but its applicability to methane hydrate production has not yet been extensively studied, particularly in the presence of residual hydrate structures. Methane gas from marine hydrate deposits can be produced by methods like depressurization, thermal stimulation, and the injection of hydrate inhibitors. Because residual hydrate structures known as hydrate precursors will exist in the liquid water phase after dissociation, the risk of methane hydrate reformation has to be evaluated during the production and transportation of methane gas through offshore pipelines. Even though one viable option to avoid hydrate reformation is injecting hydrate inhibitors before transporting the fluids through pipelines, one must consider the effect that these chemicals have on hydrate plug dissociation. In the research literature, results regarding dissociation phenomena are few. This study reports the effect on dissociation for methane hydrates formed with the kinetic hydrate inhibitor Poly Vinyl Caprolactam (PVCap). Kinetic inhibitor concentrations ranging from 40 to 6000 ppm have been examined. Results reveal that PVCap concentrations of 750 and 1500 ppm result in nearly equal displacement of the dissociation temperature, compared to noninhibited hydrates. Furthermore, 3000 and 6000 ppm PVCap give an identical increase in the dissociation temperature. Even PVCap concentrations as low as 40 ppm produce a higher dissociation temperature than corresponding hydrates formed without an inhibitor. Thus, more energy is required to remove hydrates formed under conditions where the kinetic inhibitor PVCap is applied. This is a factor that must be considered in the field of flow assurance.

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INTRODUCTION The eventual depletion of fossil fuels, coupled with the effects of greenhouse gas emission on the global climate, has called for exploration of alternative technologies to address the increasing energy demand. Clathrate hydrates, crystalline solids composed of gas and water, are a class of materials with enormous potential both as an energy source and as a medium for gas storage. Gas hydrates can hold approximately 160 volumes of gas at STP, and in this context, gas storage of hydrogen and methane is being actively pursued.1 Gas hydrates occur naturally in the permafrost and the deep oceans and are estimated to contain more carbon than all the oil found on Earth. Research involving natural gas hydrates has lately ramped up tremendously, especially to improve resource estimation and to develop recovery methods.2 Identification of possible geohazards related to decomposition with respect to global climate change is important.3,4 Gas hydrate research therefore includes earth and ocean science in addition to other fundamental sciences and engineering disciplines. Gas hydrates can also form at pressures and temperatures commonly encountered in oil and gas production systems.5 They can block flow lines, valves, wellheads, and pipelines, thereby resulting in large production losses in addition to representing problematic issues with respect to safety. The conventional treatment for prevention of hydrate plug formation is based on injection of thermodynamic inhibitors (methanol or ethylene glycol), which shift the hydrate equilibrium curve to higher pressures and lower temperatures. However, as the search for oil and gas transfers to colder and deeper regions, the quantity of THI required to prevent plugging increases. For the past decade, kinetic hydrate inhibitors (KHIs) have been increasingly used as a cost-effective © 2017 American Chemical Society

technology for gas hydrate control in the oil and gas industry, offering significant CAPEX/OPEX advantages over traditional thermodynamic inhibitors.6−8 The key ingredients in a KHI product are polymers or copolymers containing primarily vinyl lactam monomers, specifically the monomers vinylpyrrolidone and vinyl caprolactam. It is believed that KHI polymers delay the hydrate nucleation and/or hydrate growth process by surface adsorption on nuclei, resulting in an increased induction time.9−12 If the KHI-induced induction time is longer than the pipeline fluid residence time at a given condition, then the KHI is able to prevent hydrate formation. PVCap and its copolymers contain seven-membered lactam rings attached to the polymer backbone. It is believed that the hydrophilic lactam group plays an important role in inhibiting hydrate growth and that the hydrogen bonding between the functional group and water molecules leads to binding of the inhibitors on the hydrate surface, which blocks the transport of gas to the hydrate surface and disrupts the hydrate formation.10 It has been observed that hydrates formed with KHIs dissociate at higher temperatures compared to hydrates formed without inhibitors present.13−21 This behavior has implications for hydrocarbon transport pipelines as hydrate plugs from water containing KHI may be more difficult to decompose. There is a problem with respect to performing induction time measurements; nucleation is probabilistic, and the test results are consequently often highly stochastic and poorly transferReceived: December 28, 2016 Revised: May 24, 2017 Published: July 6, 2017 8505

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Energy & Fuels able.6−8,22−26 This has contributed to issues regarding low operator confidence in KHIs, restricting uptake by the industry. In an effort to overcome the problem of poor reproducibility of test results TOTAL developed a “Second Germination” (SG) procedure.22 The SG test protocol was reported, with results supporting significant improvements over traditional approaches in terms of repeatability and transferability of KHI test results.24,25 In summary, the SG method involves first forming hydrate at high subcooling and then dissociating this hydrate before then recooling in order to performing the actual measurements. By maintaining the temperature close to the hydrate phase boundary during the dissociation step, it was concluded that “nuclei” of some form − related to the phenomenon of “hydrate history” − were preserved and that these aided more consistent nucleation/growth patterns when recooling to form hydrate (“second germination”), resulting in much more repeatable data. It has been observed, however, that in laboratory experiments accelerated hydrate growth called catastrophic growth can occur. This may be a serious problem if it occurs in a field application of kinetic inhibitors. It has been observed that sI hydrate formation with PVCap in water which has previously experienced hydrate formation and dissociation was outstandingly fast, resulting in a smaller subcooling than a corresponding system with fresh water.27 Increasing the PVCap concentration from 0.5 wt % to 3.0 wt % resulted in a higher degree of subcooling.27 The mechanism of such accelerated hydrate growth in the presence of KHIs is still not understood. Sharifi and Englezos28 used a high pressure microdifferential scanning calorimeter to study the accelerated hydrate growth in the presence of chemical and biological inhibitors. They proposed a hypothesis suggesting that capillary action facilitates the transport of water molecules across the formed hydrate layer from the bulk of the liquid water phase to the gas−liquid interface. This in turn might be the governing mechanism for catastrophic hydrate growth in the presence of KHIs. A hydrate catastrophic index was introduced as a parameter to quantify the phenomenon based on laboratory data and the type of experiment conducted. There are many papers that address issues regarding retarding nucleation and/or growth for inhibited systems. In comparison, there are few that examine dissociation effects. This is especially the case for different KHI concentrations. Examinations of the influence of different KHI concentrations on dissociation can point toward interactions that the inhibitor molecules have with the hydrate.

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Figure 1. Sketch of the experimental setup.

±0.02 °C. The cell systems were equipped with 4-wire lead 1/10 DIN Pt-100 temperature sensors (accuracy ±0.03 °C) and Rosemount 3051 TA absolute pressure transmitters. The cell pressure is measured via the inlet tubing, and the temperature is measured inside the cell (in the vapor phase). The temperature was measured to an accuracy of ±0.10 °C, and pressure was measured to an accuracy of ±0.25 bar. Data were sampled on a computer using the LabView data acquisition program. The experimental progress was continuously monitored on the computer screen during the experiments. At the end of the experiment data were transferred to office PC for analysis and graphical presentation. The PVCap used in the experiments was originally supplied as 50% solution in isopropyl alcohol (producer: BASF, Germany, weightaverage molecular weight Mw = 6000 according to the producers spec.). The isopropyl alcohol had previously been removed (2005) through a two-stage process with vacuum distillation at 20 °C followed by a final vacuum treatment at approximately 10−5 Pa overnight to produce pure crystalline and dry PVCap powder. Below is a description of the general experimental procedure. 1. The desired concentration of PVCap-6k solution (Mw = 6000) was prepared for the experiment. 2. The magnet house was filled with the aqueous solution, and any air residue was squeezed out of the magnet section during mounting of the magnet house into the bottom end piece. Any residues of the solution on the top surface were removed prior to mounting the bottom end piece into the cell cylinder. 3. 70 mL of the aqueous solution was filled into the cell, and the top end piece was mounted. 4. The temperature of the heating/cooling unit was adjusted to approximately 20 °C prior to cell pressurization. 5. Prior to loading the cell to the experimental pressure it was purged twice with methane gas by pressuring the cell to 60 bara. This was done to remove (dilute) any residues of air in the cell. 6. At approximately 20 °C the cell was loaded to the desired pressure and the inlet valve was closed. 7. The experiment was then run under isochoric conditions (i.e., constant volume). 8. In another study29 from our laboratory it has been shown that the effects of heat and mass transfer restrictions are negligible at stirring rates of 500 rpm and above in the present cell setup, and in the present work the stirring rate was kept constant at 750 rpm during the experiments. Hydrate formation was induced by magnetic stirring. Hydrate formation normally took place over a region of temperatures rather than at a specific formation temperature. The hydrates were dissociated by gradually increasing the cell temperature at preset heating rates. In a first stage the system was heated relatively fast to a temperature of

EXPERIMENTAL PROCEDURES

The experimental setup is shown in Figure 1 and incorporates a cell consisting of a steel cylinder with top and bottom end piece. A stirrer blade is connected to a magnet house in the bottom end piece via an axle. An outer rotating magnetic field created by a laboratory stirrer bar drive was used to regulate the stirrer speed. The stirrer motor can be regulated to maintain speed in the range 0−1200 rpm. The free volume between the top and end pieces is 197 mL, the free volume (dead volume) around the stirrer magnet inside the magnet house is 8 mL, and the total cell volume is 205 mL inclusive of the dead volume and the inlet tubing between the inlet gas valve, the pressure sensor, and the cell. The dead volume is accounted for in experimental performance and calculations and should not affect the integrity of the experimental setup. The cell is equipped with a cooling/heating jacket connected to a JULABO F34 HL refrigerated circulator, and temperature control is obtained by circulating water from the circulator. The desired temperature profile of the experiments is set on the temperature control unit, and the temperature of the heating/cooling unit is regulated within a stability of 8506

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Energy & Fuels minimum 5 °C below the estimated equilibrium dissociation temperature. From this point the heating rate applied was 0.2 °C/h. With reference to Figure 2: The experiment is initiated at point 1 at the right end of the PT baseline, from where the sample is cooled down

dissociation point is the hydrate equilibrium point, where the initial conditions are resumed after a hydrate formation/decomposition cycle. After the first cycle the hydrates were formed with hydrate precursors present, to achieve improved repeatability of the hydrate growth patterns, according to the second germination test protocol. Measured hydrate dissociation temperatures were compared to hydrate equilibrium temperatures for corresponding noninhibited systems using the CMSHYD program.5 CSMHYD was chosen as the prediction program over CSMGEM, due to the former having better agreement with the experimental equilibrium values. The displacement of the hydrate dissociation point, ΔT′dissoc, discrepancies (ΔTdissoc) between experimental dissociation temperatures TEq,expr and predicted equilibrium temperatures TEq,calc at the experimental pressure were calculated as ΔT ′dissoc = TEq,expr − TEq, calc

(1)

On the basis of the experimental results (ΔT′dissoc versus PVCap concentration) a best fit curve was computed. The best fit curve function appeared as logarithmic for this matter, because ΔTdissoc showed a fast increase with increasing concentration in the lower concentration range (i.e., conc. ≤ 750 ppm) and a slow increase as a function of concentration at the higher concentrations (i.e., conc. > 750 ppm). The best fit, or mean curve, is assumed to be linear in the logarithmic plane

Figure 2. Typical pressure versus temperature plot from the formation− dissociation experiment in the system with PVCap in the solution.

̂ , where the symbol “caret” denotes estimate with Y ̂ = Â + BX

to the desired hydrate formation temperature at point 2. The pressure and temperature conditions in the cell were frequently sampled during the experiment. At point 2, a pressure drop and an increase in temperature indicate hydrate formation, and the growth process proceeds along path 2 - 3 until the process stops having reached the equilibrium curve at point 3. The pressure drop is due to gas molecules getting enclathrated into the hydrate lattice, and the temperature increases due to release of energy (formation enthalpy). Along path 2 - 3, the driving force will decrease as a function of decreasing pressure. After the liquid has been transformed into hydrate, the temperature is increased in correspondence with the program outlined above. The final hydrate dissociation point is where the dissociation curve intersects the cooling curve (baseline). The region to the right of this point represents PT conditions for the system without hydrates present. The hydrate

Y = temperature shift X = log(concentraton) ̂ ̅ Â = Y ̅ − BX k

B̂ =

∑i = 1 (Xi − X̅ )(Yi − Y ̅ ) k

∑i = 1 (Xi − X̅ )2

where the symbol overbar denotes average, and k is the total number of samples The unbiased variance is estimated by the following expression

Figure 3. PVCap concentration versus displacement of the hydrate dissociation temperature for methane hydrates. Each table point represents the average of 7 measurements, except for 40 and 80 ppm, where the average is based on 8 measurements and 3 measurements, respectively. Error bars display one standard deviation. 8507

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k ∑i = 1 (Yi − Yî )2

k−2

and the estimated standard deviation is the square root of the above value

StdevlogN =

σ̂2

Amount of Hydrates in the System. The amount of hydrates formed in the isochoric system can be calculated on the basis of the amount of gas consumed and the amount of water initially present in the system. For a monogas system such as methane hydrate, such calculations are relatively simple since the gas compositions in the vapor and hydrate phases are the same. In a multicomponent gas system such calculations must consider the compositional changes in the vapor phase due to the fraction of components consumed by the hydrate. For the vapor phase we estimated the moles of gas by the PVT relation PV = ZnRT, and for the hydrate we assumed a hydration number of 6 at 90 bara pressure. We have used the AGA-8 compressibility calculator for estimation of methane Z-factors at the experimental P1,T1 and P2,T2 conditions. The theoretical hydration number for sI methane hydrate is 5.75 at a fill fraction of 100% occupancy of cavities. For sI methane hydrate the fill fraction will be around 97% at 90 bara according to simulations by CSMHYD. This corresponds to a hydration number around 5.92, and the hydration number of approximately 6 from the literature30−34 seems reasonable taking nonperfect behavior during experiments into consideration. The following relation was used to calculate the amount of water converted into hydrates at the start of the dissociation process

Fw =

Δng · 6 nw

· 100%

Figure 5. PT dissociation curves for the system with 750 ppm PVCap-6k showing the effect of the amount of hydrates formed prior to the start of dissociation.

(2)

where Δng is the number of moles of gas consumed during hydrate formation, nw is the number of moles of water in the solution initially loaded and assuming a hydration number of 6, and Δng*6 is the number of moles of water converted into hydrates during the process. The number of moles of gas consumed is calculated as Δng =

Preg ·Vg Z P,reg ·R ·Texpr

−

Pexpr·Vg Z P,expr·R ·Texpr

=

⎛ Preg Pexpr ⎞ ⎜⎜ ⎟ − R ·Texpr ⎝ Z P,reg Z P,expr ⎟⎠ Vg

Figure 6. Final PT dissociation points for 750 ppm PVCap-6k in systems with different amounts of hydrates present prior to the start of dissociation.

(2a) where Texpr is the experimental temperature during the hydrate formation process, Preg is the pressure in the system without hydrates at Texpr (e.g., a point on the PT baseline in Figures 2, 4, 5, 6, and 7), Pexpr is the final system pressure at the end of the hydrate formation just prior to the start of dissociation, ZP,reg is the gas compressibility factor at

Figure 7. PT dissociation curves for methane hydrate with 1500 ppm PVCap-6k in systems.

Figure 4. PT dissociation curves for methane hydrate with different concentrations of PVCap-6k in systems.

(Texpr,Preg) as calculated by AGA8, and ZP,expr is the gas compressibility factor at (Texpr,Pexpr) as calculated by AGA8. Gas compressibility factors 8508

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In Figure 4, the PT dissociation paths at 40 and 100 ppm PVCap follow the hydrate PT equilibrium curve up to a certain residual amount of hydrates in the solution (Fw) defined by the pressure drop and the residual gas in the hydrates at points 1 and 2, respectively. The amount of water remaining as hydrates at these points were Fw = 2.93% ± 0.72% and 7.57% ± 2.12%, respectively. Figure 4 indicates that the PT curve for 1500 ppm PVCap approaches the equilibrium point around point 3. The PT curve for 750 ppm PVCap approaches the equilibrium curve in between points 2 and 3.

for the set of calculations conducted during the present study are given in the Supporting Information. To evaluate the effect of PVCap on hydrate dissociation we defined a surface adsorption factor for PVCap as nPVCap nPVCap SPVCap = = Δng · 6 n w,hyd (3) where nPVCap is the number of moles PVCap adsorbed on the hydrate surface (or in other way bound to the hydrate), nw,hyd is the amount of water converted to hydrates in moles, and nw,hyd = Δng*6 assuming a hydration number of 6 for the formed methane hydrate.

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DISCUSSION All hydrates formed with PVCap resulted in elevated dissociation temperatures compared to predicted values, at a heating rate of 0.2 °C/h. Even at 40 ppm (i.e., 0.04 wt %) the final dissociation temperature was displaced 1.3 °C above the equilibrium temperature predicted for the uninhibited system (Figure 3 and Table 1). Observations suggest that the effect of PVCap on the hydrate is a function of concentration and the amount of hydrates formed versus the total amount of inhibitor present. Dependent on the initial KHI concentration the residual water solution will be depleted of the inhibitor, and new hydrates may form without PVCap on their surface. The latter will be the first to dissociate, and the portion of the hydrates covered with PVCap will be the last to dissociate. Such behavior can explain the PT dissociation curves at 40 and 100 ppm concentration prior to and post points 1 and 2 depicted in Figure 4, respectively. Prior to points 1 and 2 these PT dissociation curves follow the PT equilibrium curve of the uninhibited system, and post points 1 and 2, a deflection on the PT curves suggests increased hydrate stability and increased energy requirement to dissociate the hydrates. Prior to points 1 and 2 at 40 and 100 ppm concentration, we assumed that there are hydrates with no or a minimal amount of PVCap adsorbed on the surface and that these dissociate. To the right of points 1 and 2, the hydrates with PVCap adsorbed/bound to the surface start to dissociate. If we assume that all of the PVCap initially present in the solution is adsorbed on the surfaces of the residual hydrates at points 1 and 2 for the 40 and 100 ppm solutions in Figure 4, then the respective surface adsorption factors, SPVCap, would be 4.106 × 10−5 and 3.966 × 10−5. These values are quite close, but we would need experiments at other low concentrations in the region between 100 and 750 ppm to determine a surface adsorption factor for PVCap-6k as a function of PVCap concentration. In such study, lower heating rates during dissociation should also be considered to improve resolution and accuracy of measurements. In the present work, the focus has been on the demonstration of the effect of PVCap concentration on hydrate dissociation, but we note that additional information on hydrate−PVCap interactions may be revealed by experiments with low concentrations of the KHI present in the solution. The conducted experiments showed that an increased amount of energy is required to dissociate hydrates formed in solutions with PVCap and that the final dissociation point is displaced toward higher temperatures as a function of inhibitor concentration, but at concentrations above 1500 ppm the effect of PVCap on dissociation temperature appeared to approach a threshold level. Results by Tohidi et al.35 have shown that, unlike nucleation, the effect of KHIs on growth is completely repeatable, i.e. data for a wide variety of different gas−water systems have consistently shown that aqueous KHI polymers induce a number of fixed, repeatable (and transferable between different setups) crystal growth pressure−temperature zones delineated by quite welldefined “phase boundaries” at specific subcoolings. There is a slow growth rate region where hydrate growth can occur but is

EXPERIMENTAL RESULTS

For sI hydrates formed without PVCap, all measured hydrate dissociation temperatures were within 0.15 °C agreement with CSMHYD predicted values. The results from performed dissociation experiments for methane hydrates, formed in the presence of different concentrations of the kinetic inhibitor PVCap, are reported in Figure 3 and Table 1, and more details are available in the Supporting

Table 1. Summary Table for Dissociation Experiments Performed with sI Hydrates Formed with Different PVCap Concentrations no. of expts

PVCap [ppm]

Pexp [Bara]

Texp [°C]

ΔTdissoc [°C]

8 3 7 7 7 7 7

40 80 100 750 1500 3000 6000

93.3 ± 0.1 94.0 ± 0.1 94.8 ± 0.0 94.9 ± 0.2 95.5 ± 0.1 95.3 ± 0.1 95.0 ± 0.2

14.1 ± 0.1 15.0 ± 0.3 15.2 ± 0.1 15.9 ± 0.2 15.9 ± 0.2 16.4 ± 0.3 16.3 ± 0.1

1.3 ± 0.1 2.1 ± 0.3 2.1 ± 0.1 2.8 ± 0.1 2.7 ± 0.2 3.2 ± 0.3 3.2 ± 0.1

Information. PVCap concentrations of 40, 80, 100, 750, 1500, 3000, and 6000 ppm dissociated at temperatures higher than the CSMHYD predicted equilibrium values. 40 ppm resulted in an average dissociation value of 1.3 °C above the predicted CSMHYD value. The gas regained in the cell at the predicted dissociation temperature was on average 98.6% of the initial value. 80 ppm resulted in an average dissociation value of 2.1 °C above the predicted CSMHYD value. The gas regained in the cell at the predicted dissociation temperature was on average 95.8% of the initial value. 100 ppm resulted in an average dissociation value of 2.1 °C above the predicted CSMHYD value. The gas regained in the cell at the predicted dissociation temperature was on average 94.6% of the initial value. 750 and 1500 ppm resulted in an average dissociation value of 2.8 and 2.7 °C above the predicted CSMHYD value, respectively. The gas regained in the cell at the predicted dissociation temperature was on average 91.6% and 89.9% of the initial value for 750 and 1500 ppm, respectively. 3000 and 6000 ppm resulted in an average dissociation value of 3.2 °C above the predicted CSMHYD value. The gas regained in the cell at the predicted dissociation temperature was on average 86.2% and 85.2% of the initial value for 3000 and 6000 ppm, respectively. Figures 4, 5, 6, and 7 show the PT dissociation paths for different concentrations of PVCap in the solution (Figure 4) and for various amounts of hydrates formed prior to dissociation (Figures 5, 6, and 7). The PT paths of the dissociation curves demonstrated increased hydrate stability with increased PVCap concentration as shown in Figure 4. In addition, the PT dissociation paths at concentrations of 750 ppm and higher appeared to be a function of the initial amount of hydrates (Fw) at the start of dissociation, as shown in Figures 5, 6, and 7. This is because the system may have been too close to the equilibrium curve at the start of dissociation, or the equilibrium temperature of the system is less than the initial temperature for the start of dissociation as illustrated by points 3 and 4 in Figure 2. Displacement of PT dissociation paths to the right and toward higher temperatures shows an increased energy requirement to dissociate the hydrates. However, the influence of the amount of hydrates (Fw) on the final dissociation temperature was low, as shown in Figures 6 and 7. 8509

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CONCLUSION Methane gas from marine hydrate deposits can be produced by methods like depressurization, thermal stimulation, and the injection of hydrate inhibitors. Because residual hydrate structures known as hydrate precursors will exist in the liquid water phase after dissociation, the risk of methane hydrate reformation has to be evaluated during the production and transportation of methane gas through offshore pipelines. Even though one viable option to avoid hydrate reformation is injecting hydrate inhibitors before transporting the fluids through pipelines, one must consider the effect that these chemicals have on the dissociation. In this study, the effect on dissociation for methane hydrates formed with the kinetic hydrate inhibitor PVCap has been examined for concentrations ranging from 40 to 6000 ppm. Results reveal that PVCap concentrations of 40, 80, 100, 750, 1500, 3000, and 6000 ppm dissociated at temperatures higher than the CSMHYD predicted equilibrium values. These resulted in an average dissociation value above the predicted CSMHYD value of 1.3, 2.1, 2.1, 2.8, 2.7, 3.2, and 3.2 °C, respectively. The gas regained in the cell at the predicted dissociation temperature was for the hydrates formed with PVCap concentrations of 40, 80, 100, 750, 1500, 3000, and 6000 ppm on average 98.6%, 95.8%, 94.6%, 91.6%, 89.9%, 86.2%, and 85.2%, respectively. Gas regained in the experimental cell, at the CSMHYD predicted dissociation temperature, seems to be inversely proportional to the inhibitor concentration except for the case of 6000 ppm PVCap. The experimental results indicate that there is a concentration limit, which beyond the temperature displacement is not further increased. An increase in PVCap concentration beyond this value will not have any additional influence on the temperature displacement or apparent hydrate stability. The experimental findings indicate that remediation of hydrates in the presence of KHIs requires higher melting temperatures, even for concentrations as low as 40 ppm. Thus, more energy is required to remove hydrates formed under conditions where the kinetic inhibitor PVCap is applied. This is a factor to be considered in the field of flow assurance.

varyingly inhibited in terms of growth rate, from almost fully inhibited (orders of magnitude reduction in growth rates) to steady but still polymer-moderated growth. Furthermore, there exists a rapid growth region where catastrophic hydrate growth occurs upon nucleation with growth rates being largely unaffected by the polymer, i.e., where the latter ceases to inhibit to any measurable extent and therefore there is risk of hydrates to form and create a plug in a pipeline. The extent of these regions is dependent on polymer properties and concentration among other factors. Experiments in our laboratory were run in a manner so that for each PVCap concentration, the hydrate growth took place over regions rather than at a specific temperature. This should cancel out any effect of the hydrate growth formation temperature, when comparing one PVCap concentration regime to another. However, for certain experimental runs, the hydrate growth did take place in a narrower band of temperatures, either at a large degree of subcooling or at a low degree of subcooling. Results indicate that for PVCap (Mw = 6000), the growth region does not affect the final dissociation temperature. Consequently, this indicates that the driving force does not influence the degree to which the PVCap (Mw = 6000) polymer chains interfere with the hydrate. Previously in our laboratory, there have been experimental indications that the dissociation temperature for the longer polymer chain PVCap (Mw = 10000) is influenced by the driving force. Bruusgaard et al.36 performed experiments with hydrate covered droplets for PVCap, PVP, and antifreeze proteins. This resulted in large differences in morphology, translucency, and dendrites. PVCap has been suggested to bind to hydrate crystals and thereby preventing growth in the respective plane.12 Bruusgaard et al.36 however found that the inhibitor does not necessarily fully cover the surface of the droplet at a uniform concentration or bind to all the resulting hydrate. The experimental results regarding 40, 80, and 100 ppm PVCap indicate that even these extremely low inhibitor concentrations have the ability to shift the hydrate dissociation temperature by 1.3, 2.1, and 2.1 °C higher than the program predicted values, respectively. 750 and 1500 ppm PVCap have a somewhat higher dissociation temperature than 100 ppm, an increase of 0.7 and 0.6 °C, respectively. The small increase in dissociation temperature which relates to a relatively large increase in inhibitor concentration implies that the inhibitor binds or interacts with the hydrate in a complex manner. Consequently, this supports the idea that the inhibitor does not bind in a uniform manner to all of the resulting hydrate. The technique of contrast variation to perform a small-angle neutron scattering measurement of PVP adsorbed onto crystalline hydrate surfaces has previously been used.37 From that work, an adsorbed layer was deduced, with the unusual properties of a thickness much larger than the polymer radius of gyration and a profile that does not resemble the standard de Gennes adsorption model.38 In the de Gennes model, surface adsorption sites are saturated, and all the adsorbed chains contact the surface on at least one point.39 A possible explanation for the formation of the thick adsorbed layer was suggested to be caused by a polymer self-aggregation effect. The fact that 750 and 1500 ppm PVCap result in approximately the same experimental dissociation temperature and that 3000 and 6000 ppm PVCap result in an identical experimental dissociation temperature indicates that there is an asymptotic regime to which inhibitor concentration affects the hydrate stability.

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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.6b03487. Table Suppl. S1, experimental data; Tables Suppl. S2−S4, compressibility factors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +47 97115592. E-mail: [email protected]. ORCID

Ann Cecilie Gulbrandsen: 0000-0001-6231-4839 Present Address ‡

Statoil ASA, Forusbeen 50, 4035 Stavanger, Norway.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank The Norwegian Ministry of Education and Research and the University of Stavanger for their support to this work. The authors also thank StatoilHydro and British 8510

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Energy & Fuels Petroleum for their financial support to the hydrate laboratory at the University of Stavanger.

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DOI: 10.1021/acs.energyfuels.6b03487 Energy Fuels 2017, 31, 8505−8511