Lowering of Clathrate Hydrate Cohesive Forces by Surface Active

Jul 11, 2012 - Through systematic measurements, the data yield new insight into how some acids may alter hydrate surface wettability. Polynuclear arom...
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Lowering of Clathrate Hydrate Cohesive Forces by Surface Active Carboxylic Acids Zachary M. Aman, E. Dendy Sloan, Amadeu K. Sum, and Carolyn A. Koh* Center for Hydrate Research, Chemical and Biological Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States ABSTRACT: The present work uses a micromechanical force apparatus to directly measure hydrate particle−particle cohesion forces in hydrocarbon systems containing various carboxylic acids. Measured cohesive forces provide fundamental insight to the balance between surfactant adsorption kinetics and interfacial thermodynamics in hydrate systems. These results are essential to the accurate prediction of hydrate aggregation in multiphase flow, as encountered in oil/gas production. The present data support the existence of the water capillary bridge between hydrate particles as an essential mechanism for hydrate cohesion in oil continuous systems. The results indicate that, while all surface active compounds tested decreased the water−oil interfacial tension, only some chemicals were effective at reducing the interparticle cohesion force. Through systematic measurements, the data yield new insight into how some acids may alter hydrate surface wettability. Polynuclear aromatic carboxylic acids were found to be highly effective, reducing cohesion forces up to 87 ± 9% for a four-ring conjugated hydrophobic group.

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Dieker et al. first measured cohesive forces with 0−5 wt % of field hydrocarbon fluids dissolved in a CyC5 bulk phase, concluding that the force decreased by up to 90% for some unmodified crude oils and up to 70% for both deasphalted and pH 14 extracted crude oils.15,20 The results suggested that both asphaltene and acidic components in those crude oils might have a role in naturally dispersing hydrate particles. Borgund et al.21 observed that, through a multivariate analysis of over 22 crude oils with qualitative field observations of hydrate plugging risk, crude oils capable of naturally dispersing water and hydrate tended to be both highly biodegraded with higher total acid content. Further work by Borgund et al.22 suggested that compositional and structural features of the acid fraction may be more important than the quantity of acid present and that the biodegradation process may serve to fundamentally and irreversibly alter the acid profile. The biodegradation process has previously been linked to lower molecular weight acids,23 which shift from acyclic to 1-, 2-, and 3-ring naphthenic acids.24 Further analysis of the acid fraction by Erstad et al.25 suggested that acid extracts of “nonplugging” oils have enhanced ester carbonyl functionality and a higher quantity of polyfunctional and weakly polar compounds. Some weakly polar functional groups, such as aromatics, may be trapped within the asphaltene solvation shell26 and inaccessible depending upon the asphaltene aggregation state.27 Spectroscopic analysis of naphthenic acids by Headley et al.28 suggested carbon number ranges from 20 to 35 and hydrogen deficiency (functionally, the acyclic/cyclic ratio) from 2 to 18. This qualitatively agrees with observations by Barrow et al.,29 Qian et al.,30 Rogers et al.,31 and Clemente et al.;32,33 the latter four citations suggest a compressed hydrogen deficiency range from 0 to 12.

INTRODUCTION Gas hydrates are crystalline inclusion compounds, where molecular water cages enclathrate small molecular components, such as methane or ethane, typically under high pressure and low temperature.1 Gas hydrates may occur naturally below the seafloor, representing a significant unconventional energy resource.1 In conventional transportation of oil/gas in flowlines, hydrate crystals may aggregate or deposit, eventually plugging the flowline.2 For this reason, hydrates are considered to be a primary flow assurance challenge in oil and gas lines.1 Turner proposed a four-step conceptual mechanism (Figure 1) to

Figure 1. Conceptual mechanism for hydrate plug formation in oildominated flowlines, reproduced from Aman et al.6 and developed by Turner (in collaboration with Abrahamson).5

explain the influence of aggregation and deposition on hydrate plugging: (i) water entrainment in the oil phase, (ii) hydrate shell growth at the water−oil interface, (iii) hydrate aggregation,3 and (iv) plugging of hydrate particles and aggregates.4,5 Much time has been devoted to understand water entrainment7 and hydrate shell growth.5,8 Work by Yang et al.,9 Taylor et al.,10−12 Dieker et al.,13−15 and Aman et al.16−19 used a micromechanical cohesion force (MMF) apparatus to directly measure hydrate interparticle cohesive force. Cyclopentane (CyC5) was used as a structure II (sII) hydrate former;1 hydrate particles were stabilized above the ice point and below the CyC5 hydrate melting temperature (7.7 °C at atmospheric pressure).15 © 2012 American Chemical Society

Received: April 26, 2012 Revised: July 10, 2012 Published: July 11, 2012 5102

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Fundamentally, much of the work to date has used a “topdown” approach to identify naturally inhibiting compounds present in “non-plugging” (or naturally hydrate dispersing) field hydrocarbon fluids. In our previous study,18 we revealed that the hydrate cohesion force decreased by 80% with the addition of 0.6 wt % bulk naphthenic acid (Sigma-Aldrich) in CyC5 bulk. The present work builds upon this foundation, with a “bottom-up” approach of directly measuring hydrate interparticle cohesion force deviations over a systematic study of relevant carboxylic acids within the naphthenic acid spectrum.

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which is visually determined through image analysis using ImageJ,36 to obtain

FA = ΔDkspring

(1)

A “pull-off” measurement consists of the top particle (Figure 2) being brought into contact with the bottom particle with an average preload force of approximately 2 mN/m. After a 10 s waiting period, the top particle is raised until the breakpoint. Figure 3 illustrates the cohesion

EXPERIMENTAL SECTION

Interfacial Tensiometer. Water−oil interfacial tension (IFT) was measured at ambient pressure and temperature through a pendant drop technique by a KSV Instruments, Ltd. CAM200 apparatus, with mineral oil as the light hydrocarbon phase and aqueous buffer solution as the heavy aqueous phase (details given below). A droplet of the heavy aqueous phase was ejected with a straight, high-precision needle into the light hydrocarbon phase. The CAM200 apparatus captured an image every 1−5 s. The CAM200 software regresses the curvature of the droplet as a function of time, to automatically determine the droplet surface tension. The software requires input of each phase density; the pure phase densities for mineral oil at ambient pressure and temperature were used, measured experimentally using an Anton Paar DMS60 densitometer connected to an Anton Paar DMA602HT measuring cell. The inclusion of various chemical additives (discussed below) was not found to alter the mineral oil density from its manufacturer-reported value (0.856 g/cm3 at 25 °C from STE Oil Company, Inc.). We use the reported pH 7.0 buffer solution density (1.000 g/cm3 at 20 °C from Fluka). We assume that the finite solubility between aqueous buffer solution and mineral oil will negligibly affect density. In systems with surface active compounds, IFT decreased over time to an asymptotic value;27,34,35 the magnitude of the IFT decrease was related to the surfactant effectiveness, while the time required to reach the asymptotic value is a functional measure of surfactant mobility. The IFT was measured every 5 s up to 300 s. The measurement time was extended in case the IFT had not visibly reached an asymptotic value. Micromechanical Force (MMF) Apparatus. An improved MMF apparatus was used to directly measure the cohesion force between CyC5 hydrate particles. Further information about the development of the apparatus and experimental method may be found elsewhere.6,10,13 A schematic of the experimental cell is shown in Figure 2. The bottom particle (Figure 2) is attached to a calibrated glass cantilever, where the cohesion force (FA) is determined by the product of the cantilever spring constant (kspring) and the particle−particle displacement (ΔD),

Figure 3. Schematic of capillary cohesion for particle−particle geometries, showing relevant parameters: embracing angle (α), capillary bridge width (χ), contact angle (θp), external contact angle (θs), liquid bridge immersion depth (d), particle separation distance (H), bridge radius of curvature (r), and particle radius (R), reproduced from Aman et al.6 force parameters, with an exaggerated liquid bridge. The liquid bridge, measured for ice particles by Döppenschmidt et al.,37 exists to minimize the interfacial energy of the solid−liquid system. Aman et al.,18 Dieker et al.,14,15 Anklam et al.,38,39 Aspenes et al.,40 and Taylor et al.11,12 have provided indirect evidence for the predominance of the capillary bridge theory in determining hydrate cohesive forces. Aman et al.6 showed the fundamental dependence of the capillary bridge equation upon IFT (γ) and surface wetting angle (θp)

2πγ cos θp FA = 2πγ sin(α)sin(θp + θs) + H R* 1 + 2d

(2)

where α and θs both represent the functional width of the liquid bridge (Figure 2), θp is the contact angle of the bridge on the hydrate surface, H is the separation distance between the particles, d is the immersion depth of the particles in the bridge, and R* is the harmonic mean radius of the particle pair, defined by eq 3

1 1⎛ 1 1 ⎞ = ⎜ + ⎟ R* 2 ⎝ R1 R2 ⎠

(3)

where R1 and R2 are the radii of both hydrate particles. To form hydrate, ice particles are formed first by placing a droplet of deionized water on the tip of the glass cantilever and freezing it in liquid nitrogen. The cantilever is then placed in liquid CyC5 maintained below the ice point, and the system temperature is slowly raised to provide a driving force for hydrate formation. The hydrate particles are allowed to anneal for 30 min prior to movement. Each experiment begins with 20 pull-off measurements between two hydrate particles in a pure liquid CyC5 bulk at 3.2 °C and atmospheric pressure. The cohesive force in liquid CyC5 bulk (i.e., without any surfactant) is compared directly to values reported by Dieker et al.,14 and a calibration constant is extracted for each particle pair to account for variable particle surface roughness and air humidity. After each calibration trial, up to three experiments may be performed, with increasing amounts of surfactant in each experiment. For a single experiment, the data points are represented as the average of 40 pull-off trials, with 1 standard deviation error bounds. For multiple experiments (at the same surfactant concentration), the average of all pull-off trials are shown, with 95% confidence bounds of all pull-off trials.41

Figure 2. Schematic of MMF experimental cell (from above), reproduced from Aman et al.6 5103

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Because CyC5 is a mildly volatile organic compound (vapor pressure ∼0.2 bar at experimental conditions),42 the composition of the experimental cell is maintained by bubbling nitrogen gas through liquid CyC5 and into the drybox enclosing the MMF system. An upstream pressure of approximately 0.7 bar is optimal to maintain a complete CyC5 and N2 atmosphere in the 1 m3 drybox, while minimizing the amount of material used. Model Oil Selection. White petroleum mineral oil [STE Oil Company, Inc., 200 cP viscosity, 32.0° American Petroleum Institute (API), 0.856 specific gravity at 25 °C, and 15 °F pour point]43 was used as a chemically inert model for field hydrocarbon fluids, with pH 7.0 buffer solution (Sigma-Aldrich) as a heavy phase. Surface active compounds (discussed below) were mixed directly in the mineral oil. This mixture was tested directly against pH 7.0 buffer solution for IFT and was injected in a cyclopentane bulk for MMF experiments. The latter procedure requires that hydrate be nucleated in the presence of pure cyclopentane, to prevent any morphological or surface roughness effects from surfactants in the bulk phase. IFT results between pure mineral oil and pH 7.0 buffer are shown for five trials in Figure 4, with typical error bounds of ±2 mN/m for this pendant drop technique.

Article

RESULTS AND DISCUSSION

The measured hydrate cohesion force generally decreased with an increasing concentration of each surface-active compound (shown in Figure 5). Figure 6 shows the cohesive force results for the highest measured surfactant concentration in each system (except 1- and 2-napthaleneacetic acids), alongside the water−oil IFT for each surfactant-in-oil mixture (with surfactant-in-oil concentrations ranging from 12.3 to 40.1 mmol/L). The highest acid concentration was reported, to minimize any intermediate water−oil or hydrate−water−oil adsorption effects that may have been present at lower concentrations. Further work is required to directly quantify the adsorption of each surfactant over a wide range of concentrations. The naphthenic acid cohesion force value obtained by Aman et al.18 is provided for comparison at the top of Figure 6, and the mineral oil baseline value (for both hydrate cohesion and water−oil IFT) is marked by the dotted line. Recalling the linear relationship between cohesion force and IFT (eq 2), three trends emerge from the data in Figure 6. First, the decrease in cohesion force observed for complex acids, amino acids, and diacids is consistently less than or equal to (within error bounds) the observed decrease in water−oil IFT. On the basis of eq 2, these data indicate that the reduction in cohesion force may be explained as primarily a result of a decrease in IFT. In some cases (e.g., dicyclohexylacetic acid), the measured force is greater than predicted solely by a decrease in IFT. This may be explained by a significant time required to reach equilibrium IFT compared to the 10−20 s lifetime of liquid bridges in cohesive force measurements (Figure 7). Second, the saturated acyclic and cyclic acids decreased cohesive force by 33% on average compared to a 29% average decrease for water−oil IFT. This discrepancy may be due to mild surface interactions (that is, slight alteration of the cos θ term in eq 2). Third, the branched and conjugated aromatic acids are highly effective at reducing cohesion force and may result in significant surface interaction. 1-Pyreneacetic acid was the most effective acid tested, reducing cohesion force by 87 ± 9% (in comparison to a water−oil IFT reduction of 31%). On the basis of the results in Figure 6, a further investigation was performed into three functional parameters of polynuclear aromatic carboxylic acids: (i) number of aromatic rings, (ii) carbon spacing between the acid and aromatic groups, and (iii) replacement of the carboxyl group with an ammonium salt. Results for these three investigations are plotted in Figures 8−10, respectively. Each figure shows a comparison point of hydrate cohesion force in pure mineral oil.18 On the basis of results in Figure 6, the polynuclear aromatic carboxylic acid may be interacting with both the capillary water bridge (thereby decreasing IFT) and the hydrate surface (i.e., lowering cohesive force more than IFT). The results in Figure 8 show a decrease in cohesion force with the number of polynuclear aromatic rings. The results in Figure 9 indicate that the one-carbon spacing between the acid and aromatic groups is more effective at reducing the hydrate cohesion force than zero- or three-carbon spacing. One possible explanation may be due to packing efficiency and alignment at the water−oil interface of the capillary water bridge. Zero-carbon spacing prevents molecules from stably positioning across the interface, while three-carbon spacing creates more steric hindrance from the interaction between hydrophobic groups. Alternatively, this behavior may

Figure 4. Average of five IFT experiments between pure mineral oil and pH 7 buffer over 5 min. Error bars represent 95% confidence.

The maximum decrease in IFT of 2.5% over 5 min suggests that the mineral oil is inactive at the water−oil interface. This agrees with the conclusion by Aman et al.,18 where the addition of pure mineral oil increased hydrate cohesion force from 3.8 mN/m14,15,40 to 6.1 ± 1.3 mN/m; this increase may be attributed to an increase in the water−oil IFT and bulk fluid viscosity in moving from cyclopentane to mineral oil. Cohesion forces were not observed to depend upon the concentration of mineral oil between 1 and 50 wt %.18 Surface Active Compound Selection. Four chemical families were selected to represent a portion of the naphthenic acid spectrum (Figure 5): saturated acyclic carboxylic acids, saturated cyclic carboxylic acids, aromatic carboxylic acids, and amino acids (with carboxyl functionality). All chemicals were obtained from SigmaAldrich at >99% purity. We assumed a molecular weight of mineral oil of around 350 g/mol, given a density of 0.860 g/cm3 and a liquid form at experimental temperature (3.2 °C). Each surfactant was added to a mineral oil sample, at concentrations ranging from 12.3 to 40.1 mmol/L, and stored in clean glass containers at room temperature; vessels were manually agitated for 3 min before use. IFT measurements were taken for aqueous pH 7 buffer solution and each mineral oil mixture (mineral oil with and without surfactant). Cyclopentane was not added to the mineral oil for IFT measurements, to minimize the effect of increased vapor pressure on the measurement; we assume herein that cyclopentane and mineral oil are equally hydrophobic. MMF measurements were taken with variable concentrations of each mineral oil mixture added to the CyC5 bulk phase and agitated manually for 2 min. 5104

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Figure 5. Carboxylic acids selected for the present investigation, grouped by hydrophobic group classification.

Figure 6. Hydrate cohesion force (dark bars) and water−oil IFT (light bar) for various surfactants. Error bars are ±1 standard deviation. Dotted line marks mineral oil MMF and IFT baseline values.

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Figure 10. Cohesion force for surfactants with different hydrophilic groups. Error bars are 95% confidence for at least five experiments with concentrations between 250 and 1300 ppm, and 1-pyreneacetic acid error bounds are ±1 standard deviation.

Figure 7. Time-dependent water−oil IFT for amino acids and complex carboxylic acids (one trial).

oil. The results in Figure 10 clearly show that the ammonium salt is less effective than the carboxyl (non-ionic) group, which agrees with observations by Zerpa et al.35 This investigation has revealed that some components of the naphthenic acid cut are highly effective at reducing hydrate interparticle cohesion force. Saturated cyclic and acyclic carboxylic acids appear to reduce the cohesion force proportionally to their decrease of water−oil IFT. Polynuclear aromatic acids may be adsorbing to the hydrate surface in addition to the capillary water bridge, and this behavior seems primarily tied to functional groups present in the surface-active compounds, such as spacing between functional groups and number of aromatic rings.

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CONCLUSION Motivated by the effect of various field hydrocarbon fluids on cohesion force,15,18 the current work systematically presents the effects of various carboxylic acid molecules on hydrate interparticle cohesion forces. Acids were selected on the basis of the reported diversity of the naphthenic acid spectra and included four primary chemical families. Overall, 1-pyreneacetic acid (a polynuclear aromatic carboxylic acid) was able to reduce interparticle cohesion force by 87 ± 9%, beyond what is predicted by the measured IFT reduction in the capillary bridge theory (eq 2). These fundamental data yield a new physical insight that 1-pyreneacetic acid may affect cyclopentane hydrate surface wettability. Further investigation revealed that the effectiveness of 1-pyreneacetic acid may be attributed to the number of aromatic rings and optimal (one-carbon) spacing between hydrophilic and hydrophobic groups. Using this novel technique to directly capture cohesive forces, further studies may elucidate a chemical mechanism through analyzing the concentration-dependent cohesion forces with each surfactant, in combination with direct water−oil interfacial and surface tension measurements.47,48

Figure 8. Cohesion force for carboxylic acids with various hydrophobic ring structures. Error bars are 95% confidence for at least five experiments with concentrations between 200 and 1250 ppm, and 1pyreneacetic acid error bounds are ±1 standard deviation. A line is provided through data to guide the eye.

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Figure 9. Cohesion force for carboxylic acids with various hydrophilic−hydrophobic group spacing. Error bars are 95% confidence for at least five experiments with concentrations between 250 and 1600 ppm, and 1-pyreneacetic acid error bounds are ±1 standard deviation.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

be explained by bilayer or micellar formations that are unique to the functional spacing of the molecules.44−46 Further investigation is required to better understand the concentration and temperature dependence of both cohesive force and IFT measurements, with a particular focus on the critical micelle concentrations for each chemical in white petroleum mineral

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the funding and support from the Colorado School of Mines Hydrate Consortium, which is 5106

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currently funded by BP, Chevron, ConocoPhillips, ExxonMobil, MultiChem, Nalco, Petrobras, Shell, SPT Group, Statoil, and Total. Carolyn A. Koh acknowledges partial support by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy (DOE−BES Award DE-FG02-05ER46242).

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