Effect of naphthenate formation on the anti-adhesive behavior of

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General Research

Effect of naphthenate formation on the anti-adhesive behavior of clathrate hydrates at a water-oil interface Wonhyeong Lee, Juwon Min, Yun-Ho Ahn, Seungjun Baek, Carolyn A. Koh, and Jae W Lee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05869 • Publication Date (Web): 02 Mar 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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Industrial & Engineering Chemistry Research

Effect of naphthenate formation on the antiadhesive behavior of clathrate hydrates at a wateroil interface Wonhyeong Lee,† Juwon Min,† Yun-Ho Ahn,† Seungjun Baek,† Carolyn A. Koh‡ and Jae W. Lee*,† †Department

of Chemical and Biomolecular Engineering (BK21+ Program), Korea Advanced Institute

of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡Center

for Hydrate Research, Department of Chemical and Biological Engineering, Colorado School

of Mines, Golden, Colorado 80401, United States

ABSTRACT This work investigates the effect of naphthenic acid and naphthenate on the adhesion behavior of a clathrate hydrate probe at a water-oil interface. The addition of 0.5 wt % myristic acid in the cyclopentane (CP)/n-decane mixture reduces the adhesion force acting on the probe by 51.8 % from the lowered interfacial tension between the hydrate probe and aqueous phase. With the basic aqueous solution, the precipitate layer of calcium naphthenate is formed at the water-oil interface. When the concentration of myristic acid rises above 0.1 wt%, the precipitate layer formed at the interface blocks the contact between the hydrate probe and aqueous phase completely. The anti-adhesive force pushing on the hydrate probe is induced toward the positive vertical direction. In this case, the precipitated calcium naphthenate acts like an antiagglomerant (AA) that inhibits the agglomeration of hydrate particles. Our present findings imply that the crude oil of high-acidity may alleviate the agglomeration of hydrate particles in the oil pipeline by inducing naphthenate formation.

Keywords: Naphthenate, Adhesion, Anti-adhesion, Clathrate hydrate, Anti-agglomerant 1

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1. INTRODUCTION Clathrate hydrates are inclusion compounds composed of gas or liquid molecules (e.g. methane, ethane, carbon dioxide, tetrahydrofuran, etc) enclathrated in cages of hydrogenbonded water molecules.1, 2 Clathrate hydrates have a large volume capacity (up to 180 m3 per m3 of hydrate) to store gas molecules, and have the advantage of being safe and eco-friendly. Therefore, many studies have focused on promoting hydrate growth, reducing induction time, and increasing the storage capacity to utilize clathrate hydrates as efficient energy storage materials.3-7 On the contrary, the unexpected formation of clathrate hydrates can cause a significant nuisance in the continuous production of gas and oil. A submarine oil transportation pipeline has preferable conditions for gas hydrate formation: high pressure, low temperature, and the presence of light hydrocarbons. The turbulence flow in the pipeline makes water molecules disperse in the oil phase resulting in the formation of a water-in-oil emulsion. Small hydrate particles can be produced from the emulsion and be agglomerated with other hydrate particles inducing a blockage of flow and damaging the pipeline, which leads to significant economic and environmental losses.8-12 Thus, many studies have suggested various strategies to prevent hydrate blockage in oil/natural gas pipelines.

Thermodynamic hydrate inhibitors (THIs), which shift the phase equilibrium to more harsh conditions (i.e. lower temperature for a given pressure or higher pressure for a given temperature), have been employed. However, a large quantity is required (typically 20 – 40 wt% of the aqueous phase) to effectively inhibit the formation of hydrates.13, 14 For this reason, low dosage hydrate inhibitors (LDHIs) have recently been spotlighted to prevent the agglomeration of hydrate particles.15-17 Anti-agglomerants (AAs) are one example of LDHIs, which keep small hydrate particles from forming large hydrate plugs. Surfactants can also act 2


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as AAs; the adsorption of a hydrophilic head and hydrophobic tail to the hydrate particles decreases the hydrate-oil interfacial tension.16, 18, 19 The low interfacial tension between hydrate and water phase makes the surface of the hydrate oil-wetted and prevents an agglomeration of hydrate particles into large hydrate particles.20, 21

The interfacial dynamic phenomenon at the surface of the hydrate is a key factor in understand the agglomeration of hydrate particles in the oil pipeline system. It has been investigated that surfactants like Span 20 or sodium dodecyl sulfate (SDS) reduce the adhesion force between hydrate particles which is affected by the presence of salts in the aqueous phase.16, 20, 22, 23 The method for direct measurements of contact/detachment force between hydrate particles and water by using a z-directional microbalance has been investigated.24 Also, the effects of the volume of water and surfactant solution on the interfacial behavior between cyclopentane hydrate particles and droplets were studied by the direct measurement method.25 The concept of ‘anti-adhesive force’ was investigated as the maximum pushing force induced by hydrophobic silica nanoparticles at the water-oil interface, and the effect of hydrophobic silica nanoparticles which inhibit the agglomeration of hydrate particles was also studied.26, 27

Naphthenic acid, a term used in the petroleum industry, refers to the carboxylic acid components included in crude oil.28, 29 Naphthenic acids form metal naphthenates with metal ions (e.g. calcium or sodium) at basic conditions, and these naphthenates are precipitated at the water-oil interface and may lead to a pipe-clogging problem.30,

31

However, since the

agglomeration of hydrate particles occurs at the interface of the two phases, the precipitation of naphthenates at the interface may inhibit the growth of hydrate particles. For the first time to our knowledge, we report the effects of naphthenate formation on the anti-adhesion behavior 3



between hydrate particles and water-oil interface through direct force measurements between clathrate hydrate particles and the aqueous phase. The z-directional microbalance is used for analysis of adhesive behaviors at the various concentrations of naphthenic acid when the naphthenate precipitate layer is formed. From these experimental results, this study extends our understanding of the effects of naphthenate precipitation on hydrate agglomeration.

2. EXPERIMENTAL SECTION 2.1 Materials. Deionized (DI) water was produced from a Millipore Direct-Q unit with a resistivity of 18.0 MΩcm-1. n-Decane (purity of >99.5 %) and tetrahydrofuran (THF) (purity of 99.0 %) were purchased from Daejung Chemicals, and sodium hydroxide (purity of >97.0 %) and calcium chloride (purity of >95.0 %) were purchased from Junsei Chemical Co. Ltd. Myristic acid (purity of >99.0 %), one example of a naphthenic acid, was purchased from Tokyo Chemical Industry Co. Ltd. Span 20 and cyclopentane (purity of 98.0%) were purchased from Sigma-Aldrich. Liquid nitrogen was obtained from Special Gas (Republic of Korea).

2.2 Sample preparation. The preparation of THF hydrate probes was performed by using a metallic mold illustrated in Figure 1a. THF forms sII hydrates like cyclopentane at ambient pressure around 4 oC32-34 and is suitable for the experiment of force measurement without being enclosed in a high pressure cell.16 18μL of the THF aqueous solution (5.56 mol %) was placed in a metallic mold at the end of platinum rod. The metallic mold was placed at 243 K in a refrigerator for 10 minutes. Next, the mold was placed in a metallic sieve and quenched with liquid nitrogen to freeze completely. After 20 minutes, the metallic mold was separated and THF hydrates with Pt rods were stored in a 243 K refrigerator. The liquid samples were prepared in 20mL glass vials. The vials were filled with 12 mL of aqueous phase (pH 11.0 of 4


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sodium hydroxide solution with 1 wt % of calcium chloride) and 4 mL of oil phase (CP/ndecane, 50:50 volumetric ratio) containing myristic acid (0-1 wt % of oil phase). Since the amount of precipitated naphthenates increases as the pH value of aqueous phase increases,31 we set the high pH value of aqueous phase to maximize the amount of precipitated naphthenates. These samples were stirred with a homogenizer at 7500 rpm for 1 minute to precipitate calcium myristate at the oil-water interface. The stirred samples were stabilized for 1 day at room temperature for the calcium myristate to settle at the water-oil interface and the emulsions formed during the stirring process to totally disappear.

2.3 Dynamic force measurements. Figure 1b shows a schematic diagram of the experimental setup for the measurement of the time-dependent dynamic force between the THF hydrate probe and water-oil interface. The prepared sample vial was immersed in an ethanol bath of 1°C for 30 minutes to stabilize the temperature and calcium myristate for settling at the water-oil interface. As shown in Figure 1b, the top part of the sample vial was not immersed in the ethanol phase to prevent the influx of ethanol. A THF hydrate probe with a platinum rod was hung up at the z-directional microbalance (KSV instruments, Ltd.) and immersed in the oil phase of the sample. The probe was lowered right above the water-oil interface by using a motorized z-axis stage (Velmex), and the entire system was stabilized for 100 seconds. After the stabilization, the THF hydrate probe was lowered 100 μm every 5 seconds until a capillary bridge formed between the hydrate probe and aqueous phase. The time-dependent dynamic force between the probe and water-oil interface was recorded using the KSV software. If a capillary bridge was formed immediately as the probe make contact with an aqueous phase, the maximum positive adhesion force was recorded. If a capillary bridge was not formed immediately, the pushing force of the THF hydrate probe into the aqueous phase was recorded 5



until the formation of a capillary bridge occurred. The maximum pushing force acted on the THF hydrate probe was recorded as anti-adhesive force.26 If there was no wetting after moving down 15 steps, the process was stopped because moving-down more steps would cause an adherence between the Pt rod and the aqueous phase instead of the THF hydrate probe. To compare the effects of naphthenic acid with that of the surfactant, the same procedure was conducted with the oil phase dissolved with Span 20. All measurements were conducted three times for the same concentration, and the images of the adhesion behavior between the THF hydrate probe and aqueous phase were captured by Microsoft LifeCam studio connected to a PC by LifeCam 3.60.

2.4 Interfacial tension measurements. The interfacial tension between the oil phase and aqueous phase was measured by the pendent drop method with a Phoenix-300 Touch (SEO co., Ltd) at room temperature. The sodium hydroxide solution with 1 wt % of calcium chloride was injected by a syringe with a 0.7mm diameter needle immersed in the oil phase (CP/ndecane, 50:50 volumetric ratio) with various concentrations of myristic acid (0-1 wt %) in the transparent rectangular cell. The droplet of sodium hydroxide solution was hung at the end of the needle, and the captured images of the droplet were analyzed with the software, SurfaceWare 8, to calculate the interfacial tension. The measurements were conducted for 20 different droplets for each concentration of myristic acid.

2.5 ICP-MS measurements. The mass fraction of calcium ions included in the aqueous phase of the sample was measured by Inductively Coupled Plasma Mass Spectrometer (ICPMS) to identify the formation of calcium myristate. The prepared liquid sample with 0, 0.25, 0.5, 1 wt % of aqueous phase of calcium chloride was stirred by a homogenizer with 7,500 rpm 6


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for 1 minute. To measure the remaining Ca2+ concentration, 3 mL of the aqueous phase were extracted from the stirred sample that was stabilized for 1 day at room temperature and measured by ICP-MS. The measurements were conducted three times at each calcium chloride concentration to ensure reproducibility.

3. RESULTS AND DISCUSSION 3.1 Adhesion force measurements. To investigate the effects of naphthenic acids on the adhesion behavior between the THF hydrate probe and the water-oil interface, we measured the adhesion force between the probe and the aqueous phase. Figure 2a represents the captured images at each step and corresponding force acted on the THF hydrate probe for a single cycle of the measurement. The THF hydrate probe was placed right above the water-oil interface (Figure 2a (1)), and stabilized for 100 seconds. When the probe was lowered with a constant velocity (150 μm/5 s), and contacted the aqueous phase, a capillary bridge was formed with the drastically increased adhesion force (Figure 2a (2)). After the adhesion, the THF hydrate probe was raised at the constant velocity (150 μm/5 s), and the adhesion force increased as the thickness of the capillary bridge increased (Figure 2a (3)). When the probe was fully detached (Figure 2a (4)), a single cycle of the measurement was completed, and the aforementioned process was repeated 3 times with different liquid samples with the same mystic acid concentration.

The measurement was also conducted with 0-0.5 wt % of myristic acid or Span 20 which is a commercially used surfactant known for reducing adhesion force. The maximum contact force in a single cycle process was recorded as adhesion force, and Figure 2b represents the adhesion forces at various concentrations of myristic acid or Span 20 in the oil phase. When 7



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there is no additive in the oil phase (i.e., 0 wt % of myristic acid or Span 20 in the oil phase), an average adhesion force between the probe and the water-oil interface was 0.1268 ± 0.0148 mN. But the adhesion force gradually decreased as the concentration of myristic acid or Span 20 increased. At the concentration of 0.5 wt % in the oil phase, the adhesion force decreased by 51.8 % with myristic acid (0.0611 ± 0.0066 mN) and 92.6 % with Span 20 (0.0093 ± 0.0011 mN). Even though the adhesion force decrement with myristic acid was smaller than the Span 20 case, the results show that the presence of myristic acid reduces the adhesion force between the probe and water-oil interface.

To explain the decrement of adhesion force between the probe and the water-oil interface, we also measured interfacial tension between aqueous and oil phases. Figure 3 shows the interfacial tension between an aqueous phase and an oil phase with various concentrations of myristic acid and Span 20, and images of a droplet of sodium hydroxide solution in the oil phase with 0 - 0.5 wt% of myristic acid. The average interfacial tension between the oil and the aqueous phases with 0 wt % of myristic acid was 35.68 ± 1.13 mN/m, and the interfacial tension decreased as the concentration of myristic acid in the oil phase increased. The average interfacial tension decreased to 12.22 ± 0.47 mN/m at 0.1 wt % of myristic acid, and 8.92 ± 0.40 mN/m at 0.5 wt %. With Span 20, the interfacial tension decreased to 8.60 ± 0.34 mN/m at 0.1 wt% of Span 20, and 4.79 ± 0.16 mN/m at 0.5 wt %, respectively. It has been reported that surface active agents such as Span 20, SDS, and polymer KHIs adsorb on the oil-water interface and decreases the adhesion force between the hydrate probe and water-oil interface dramatically by lowering the interfacial tension

10, 16, 26, 35, 36.

Since myristic acid has a

hydrophobic chain of alkyl group like Span 20 and hydrophilic carboxylic acid group (Figure 4a), it can be adsorbed at the water-oil interface and can lower the interfacial tension as well as 8


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adhesion force. Also, the presence of calcium ions in the aqueous phase influences the interfacial tension. At the basic condition, the dissociated carboxylate group can form interfacial complexes (RCOO-Ca+), leading to the generation of interfacial films and lowering the interfacial tension.37 Therefore, due to the similar characteristics of myristic acid with those of surfactants, it can form an interfacial ion film decreasing the adhesion force between the hydrate probe and the aqueous phase.

3.2 Calcium myristate formation. After agitating the oil and aqueous phases with a homogenizer, we can observe the white solid particles precipitated at the water-oil interface. To identify the precipitate, we dried the precipitate and collected it to measure its infrared (IR) spectra. Myristic acid can be ionized to myristate (RCOO-) and hydrogen (H+) ions at basic conditions, and if metal ions coexist with myristate ions, insoluble metal myristate can be produced.30 Since we put calcium chloride in the aqueous phase, there should be a carboxylate (COO-) group and there should not be a hydroxyl (OH) group in the precipitate sample if the precipitate is calcium myristate. Figure 5 shows the IR spectra of neat myristic acid and the dried precipitate in the wavelength range of 500 to 2000 cm-1. The peak around 940cm-1 corresponds to the O–H bond in carboxylic acid (COOH) of myristic acid.38 However, there was no O–H bond in the IR spectra of the precipitate; instead, the peak around 1575 cm-1 was newly observed which corresponds to the carboxylate (COO-) bond. The IR spectrum revealed that calcium myristate was produced in the form of the precipitate at the water-oil interface.

The amount of precipitated calcium myristate was also measured using ICP-MS. The measurement was conducted for the aqueous phase mixed with 0, 0.25, 0.5, 1 wt% of myristic acid in the oil phase. Figure 6 presents the amount of calcium ions in the stirred system with 0, 9



0.25, 0.5, 1 wt% of myristic acid. The concentration of calcium ions with 0 wt % of myristic acid was 1421.4 ± 28.9 mg/kg water, and it reduced to 1344.4 ± 26.7, 1349.3 ± 25.2, and 1359.0 ± 17.8 mg/kg water with 0.25, 0.5, and 1 wt % of myristic acid, respectively. The amount of calcium ions in the aqueous phase that decreased after agitation with the oil phase represents the amount of calcium ions reacted with myristate ions. The concentration of calcium ions in the aqueous phase decreased as the concentration of myristic acid increased from 0 to 0.25 wt %, but the concentrations of calcium ions with 0.25, 0.5, 1 wt % of myristic acid were almost identical. The ICP-MS results indicate that the amount of precipitated calcium myristates does not increase as the concentration of myristic acid increases over 0.25 wt %.

3.3 Anti-adhesive force measurements. To understand the effect of naphthenate precipitation on the adhesion behavior of hydrate particles with the aqueous phase, we observed adhesion behavior of the THF hydrate probe at the water-oil interface. Figure 7a shows force acted on the THF hydrate probe in the presence of calcium myristate at the water-oil interface and images captured at each step of one cycle. Similar to the aforementioned procedure, the THF hydrate probe was lowered 100 μm every 5 seconds. At the adhesion force measurement step, a capillary bridge between the probe and the aqueous phase was formed, and in the absence of the precipitate formation, the positive adhesion force was observed as in Figure 2.

However, the THF hydrate probe did not adhere to the aqueous phase, and the capillary bridge was not created when the probe contacted the interface in the presence of the calcium myristate precipitates (Figure 7a (2)). The curvature of the water-oil interface increased as the number of moving-down steps of the probe increased (Figure 7a (3)). The positive pushing force acting on the probe increased as well because the surface energy also increased. After moving-down 10


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further steps, adhesion between the probe and the aqueous phase occurred (Figure 7a (4)) and the positive adhesion force was detected. The maximum pushing force right before the adhesion was defined as an ‘anti-adhesive force’.26 If there was no more adhesion between the probe and aqueous phase, the anti-adhesive force was recorded with 15 lowering steps. The anti-adhesive force with various concentrations of myristic acid from 0 to 1 wt % is presented in Figure 7b. At a concentration of 0.0125 wt %, the probe and aqueous phase adhered immediately when the probe was contacted. At the concentrations of 0.025 and 0.05 wt %, the adhesion occurred after several moving-down steps. Also, the maximum anti-adhesive force before adhesion was increased from 0.0342 ± 0.0059 mN to 0.0392 ± 0.0028 mN as the concentration of myristic acid increased from 0.025 wt % to 0.05 wt %. Over the 0.1 wt % of myristic acid, the adhesion between the probe and water-oil interface did not occur until 15 moving-down steps, and the anti-adhesive force converged to 0.042 mN at 1 wt % of myristic acid. With a high concentration of myristic acid, there were a few cases that a small increment of anti-adhesive force was detected before the contact occurred between THF hydrate probe and water-oil interface. These force increments were induced by the precipitated naphthenate layer which pushes up the THF hydrate probe before the water-oil interface contacts with the THF hydrate probe.

The anti-adhesive behavior of the THF hydrate probe at the water-oil interface was derived from the calcium myristate layer at the water-oil interface. The agitation of the system with the basic aqueous phase induces the precipitation of calcium myristate at the water-oil interface forming a layer that physically blocks the adherence between the THF hydrate particle and the aqueous phase. The anti-adhesion mechanism of calcium myristate was similar to that of hydrophobic silica nanoparticle, a particle-typed anti-agglomerants (AAs).26 The curvature and 11



the interfacial area of the water-oil interface increased when the number of moving-down steps of the probe increased. As the amount of precipitated calcium myristate increases, the layer formed at the water-oil interface become much denser. This results in the increment of maximum moving-down steps before adhesion, showing the anti-adhesive behavior at a high concentration of myristic acid. With over 0.1 wt % of myristic acid, the amount of precipitation is enough to completely cover the interface. Therefore, there is no adhesion between the probe and an aqueous phase, even with 15 iterations of moving-down steps. Also, the convergence of anti-adhesive force over 0.1 wt % of myristic acid can be explained with the ICP-MS result which indicates that the amount of calcium myristate did not increase as the concentration increased over 0.25 wt %.

The adhesion behavior with various concentrations of myristic acid, Span 20, and calcium myristate are presented in Figure 8. Adhesion force acted on the THF hydrate probe towards the negative direction of the z-axis, and the anti-adhesive force towards to the positive direction of the z-axis. Therefore, the value of adhesion force is noted as a negative value, and the antiadhesive force is noted as a positive value. As shown in Figure 8, the incremented concentration of myristic acid and Span 20 decreases the absolute value of adhesion force. The effect of the myristic acid on the adhesion force is smaller to that of Span 20. However, if the precipitate of calcium myristates is formed by the agitation, the anti-adhesive behavior between the hydrate probe and the water-oil interface is observed. The calcium myristate at the interface blocks the physical adherence between the probe and the aqueous phase.

4. CONCLUSIONS

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This study shows the effect of naphthenic acids in the oil phase on the adhesion behavior between the THF hydrate probe and the water-oil interface in a z-axis microbalance. The addition of naphthenic acid to the oil phase reduces the adhesion force between the hydrate and the aqueous phase because the naphthenic acid reduces the interfacial tension like other surface-active agents. The mixing of the oil and water phases makes the precipitation of the calcium naphthenate layer at the water-oil interface. The calcium naphthenate layer at the interface blocks the adhesion of the hydrate and the aqueous phase, and the anti-adhesive force pushes up the hydrate probe from the interface. Therefore, the precipitation of calcium naphthenate can play a role in preventing hydrate-water contacts, leading to hydrate inhibition. There are many turbulences in real oil pipeline systems, so the results with the precipitation of calcium myristate provides insights into understanding the adhesion phenomena of water droplets and hydrate particles in real oil pipelines.

AUTHOR INFORMATION Corresponding Author E-mail:[email protected] Notes The authors declare no competing financial interest.

Acknowledgement

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The authors are grateful for the financial support from the Mid-career Researcher Program through NRF grants (NRF-2017R1A2B4003586) funded by the Ministry of Science, ICT, and Future Planning, Republic of Korea.

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Figure 1. (a) Schematic of the metallic mold which was utilized to produce the THF hydrate probe. (b) Schematic of the experimental setup for dynamic force measurements between the THF hydrate probe and the aqueous solution.

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Figure 2. (a) A set of images and a graph of forces acted on the THF hydrate probe versus time for a single cycle of the measurement with 0.1 wt % of myristic acid in the oil phase. (b) Adhesion force between the THF hydrate probe and the water-oil interface with myristic acid or Span 20. 16


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Figure 3. The images of a droplet of sodium hydroxide solution in the oil phase with (1) 0, (2) 0.25, and (3) 0.5 wt% of myristic acid and a graph of the interfacial tension versus concentrations of myristic acid and Span 20.

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Figure 4. Chemical structures for the (a) myristic acid, (b) calcium myristate.

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Figure 5. IR spectra of myristic acid (black line) and precipitates (red line) at the wavelength range of 500 to 2000 cm-1.

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Figure 6. Calcium ion concentrations of the aqueous phase determined by ICP-MS with respect to various myristic acid concentrations.

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Figure 7. (a) A set of images and a graph of forces acted on the THF hydrate probe versus time for a single cycle of the measurement with 0.0625 wt % of myristic acid in the oil phase. (b) Anti-adhesive force between THF hydrate and water-oil interface with various concentration of myristic acid.

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Figure 8. The adhesion behavior with various concentrations of myristic acid, Span 20, and calcium myristates

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Effect of naphthenate formation on the anti-adhesive behavior of

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