Hydrate Growth Inhibition by Poly(vinyl caprolactam) released from

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Hydrate Growth Inhibition by Poly(vinyl caprolactam) released from Microcarriers under Turbulent Mixing Conditions Juwoon Park, Sang Seok Lee, Hyunho Kim, Shin-Hyun Kim, and Yutaek Seo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01390 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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Hydrate Growth Inhibition by Poly(vinyl caprolactam) released from Microcarriers under Turbulent Mixing Conditions Juwoon Park a, Sang Seok Leeb, Hyunho Kim a, Shin-Hyun Kimb, and Yutaek Seo a* a

Department of Naval Architecture and Ocean Engineering, Research Institute of Marine

Systems Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea b

Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST, Daejeon, 34141 Republic of Korea

CORRESPONDING AUTHORS: Shin-Hyun Kim and Yutaek Seo

AUTHOR EMAIL ADDRESSES: [email protected] and [email protected]

TELEPHONE: SHK +82-42-350-3911; YS +82 42-880-7329

FAX: SHK +82-42-350-3910; CDW +81-2-888-9298 1 ACS Paragon Plus Environment

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ABSTRACT Core-shell microcarriers were microfluidically prepared by using water-in-oil-in-water double-emulsion drops as a template. The aqueous core contained a kinetic hydrate inhibitor (KHI) of poly(vinyl caprolactam) (PVCap), and the shell was made of a crosslinked polymer. To make the microcarriers selectively release PVCap at temperatures where hydrate formation occurs under a constant shear flow, the relative shell thickness to the radius of the microcarrier was set to 0.11. The hydrate inhibition performance of PVCap released from the microcarriers was investigated using continuous cooling and constant subcooling in a high-pressure autoclave. Longer hydrate onset times were observed for the PVCap microcarriers compared to bulk water, suggesting that hydrate nucleation was inhibited by PVCap released from the microcarriers. The obtained subcooling temperature for the PVCap microcarriers was 11.3 °C, which was close to that of the PVCap bulk solution at 10.8 °C. The hydrate growth was faster for the PVCap microcarriers than for bulk water, but the effective growth period was shorter, resulting in a lower hydrate fraction in the liquid phase. Although the PVCap microcarriers performed successfully under continuous cooling, limited performance was observed with constant subcooling. Successful hydrate inhibition was sometimes observed, but fast hydrate formation was also observed over repeated experiments. This result is because microcarriers are designed to rupture under a constant shear flow. Thus, more studies are required to improve the design of microcarriers to release the inner KHI solution, even in cold-restart operations. Nevertheless, microcarriers provide a flexible way to inject KHI into subsea flowlines, as many different types of KHIs can be simultaneously delivered at a proper dose using a set of distinct microcarriers.

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KEYWORDS: Gas hydrates, flow assurance, kinetic hydrate inhibitor, growth inhibition, microcarriers


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1. Introduction Gas hydrates are nonstoichiometric crystalline compounds in which water molecules encage gas molecules via hydrogen bonding under low temperature and high pressure conditions.1-3 Such conditions are found in subsea flowlines that transport hydrocarbons such as methane, ethane, and propane with produced water to processing facilities. As the formation of gas hydrates in subsea flowlines may lead to solid blockages that cause costly production stoppages and remediation processes, the energy industry has been injecting alcohol- or glycol-based hydrate inhibitors into these lines to thermodynamically shift the hydrate formation conditions 3-6. Mono ethylene glycol (MEG) has been a popular choice of inhibitor for offshore gas fields due to its negligible loss to the hydrocarbon phase, while methanol has been used in offshore oil fields due to its low cost. However, these conventional inhibitors require a large infrastructure for storage with regeneration and high operational expenditures due to their large injection volume, which can be as high as 40~60 wt%, to the aqueous phase. Therefore, many studies have sought alternative solutions to manage the risk of hydrate formation in subsea flowlines. Based on the concept of hydrate risk management, kinetic hydrate inhibitors (KHIs) have been widely investigated, as they are able to delay hydrate formation during the residence time of hydrocarbon fluids in subsea flowlines, and require only relatively small doses of 0.5~3.0 wt% to the aqueous phase containing

homo-

or

7-9

. The well-known KHIs are water-soluble polymers

copolymers

isopropylacrylamide (NIPAM)

16-18

of

N-vinylcaprolactam

, and N-vinylpyrrolidone (VP)

19,20

(VCap)

10-15

,

N-

. Their structures are

composed of a polyethylene backbone connected to a pendant group containing hydrogen bonding sites, which are able to defer nucleation of hydrate crystals. Recent studies sug4 ACS Paragon Plus Environment

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gested that KHI molecules not only delay the nucleation of hydrate crystals but also reduce hydrate growth by adsorbing onto the hydrate surfaces22. Hydrate plugs are formed by deposition and annealing of dispersed hydrate particles on the pipeline wall. Therefore, their reduced growth rate in the presence of KHI would extend the amount of time before plug formation. Evaluations of the performances of KHI candidates have been carried out by measuring the onset time of hydrate particles and the required subcooling temperature. Anderson et al. determined the inhibition abilities of KHIs by focusing on hydrate growth using subcooling temperature versus hydrate growth rate 22. Modification of the base polymer, PNIPAM-coAA, with tert-butylamine, PNIPAM-co-C4t, showed slower growth, suggesting its role as a crystal growth inhibitor

19

. The inclusion of a corrosion inhibitor group such as imidazole

into a PVCap-based polymer resulted in slower growth than that in pure water, whereas the addition of quaternary ammonium into PVCap showed a negative effect on the growth inhibition performance, indicating that functional groups attached to the polymer backbone have an important role in the inhibition13. However, these experiments were mostly carried out under continuous cooling of the hydrocarbon fluids with a constant mixing rate. The KHI solution was loaded into the high-pressure autoclave before beginning the cool-down process, and therefore, the KHI molecules were well dispersed in the aqueous phase during the cool-down process. This is a conventional approach to evaluate the performance of a KHI candidate. However, hydrate plug formation may occur during the cold-restart operation, where the liquid phase would be cooled without mixing until the temperature reached the minimum temperature of seawater. Sohn and Seo studied the hydrate formation process during the cold restart operation and observed fast growth into hydrate plugs within 40 5 ACS Paragon Plus Environment

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minutes 23. When 1.0 wt% of a Luvicap solution stayed below the hydrate formation temperature for 10 hours without mixing and was instantly mixed at 600 rpm, the hydrate growth rate was reduced by 50% and hydrate plug formation was not observed. However, when the Luvicap solution was mixed at 200 rpm, slow hydrate formation eventually resulted in hydrate plug formation 300 min after mixing. Previous research on KHIs has focused on improving their performance, biodegradability, and compatibility with other chemicals such as corrosion inhibitors24. However, there have been few studies on the injection of KHI into subsea flowlines. Current industrial practice suggested KHIs were injected into flowlines through umbilicals, sometimes with carrying fluids such as MEG. Once KHI is injected into the flowlines, it will mix with the free water phase until it reaches the offshore processing facilities. In our previous work 24,25, microcarriers were designed to encapsulate a PVCap solution and selectively release the solution at temperatures where hydrate formation occurs under constant shear flow. We proved that these microcarriers were able to release the encapsulant on demand, and the released PVCap effectively delayed hydrate formation under shear flow. This result suggested that the KHI solution could be reused if the microcapsules did not experience hydrate formation and were collected in the processing facilities. Further investigation on the performance of the released PVCap was required, including the growth rate and hydrate fractions, to better understand the role of PVCap in the hydrate formation process. In this work, experiments were carried out to investigate the performance of PVCap released from microcarriers under both continuous mixing and cold-restart conditions, and the results were compared with those of bulk water and free PVCap solutions.


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2. Experimental Section 2.1 Materials The synthetic natural gas used in this work was composed of 90.0 mol% CH4, 6.0 mol% C2H6, 3.0 mol% C3H8, and 1.0 mol% n-C4H10 and was supplied by Special Gas (Korea). Deionized water with 99.9% purity and decane with 99.5% purity were supplied by OCI and Sigma-Aldrich, respectively. Poly(n-vinylcaprolactam) (PVCap, Mn = 5100) was provided by BASF. All the chemicals were used without further purification. 2.2 Preparation of temperature-sensitive microcarriers Using a capillary microfluidic device, water-in-oil-in-water (W/O/W) double-emulsion drops were prepared. The device was comprised of two tapered cylindrical capillaries assembled in a square capillary, as illustrated in Figure 1a. One tapered cylindrical capillary has an orifice with a diameter of 350 µm and a hydrophobic surface, and the other tapered capillary has a an orifice with a 450 µm and hydrophilic surface. The two capillaries were coaxially aligned in a tip-to-tip configuration with a separation of 300 µm. As the innermost phase, an aqueous solution of 20 wt% PVCap and 1 wt% poly(vinyl alcohol) (PVA, Mw = 13000–23000, Sigma-Aldrich) was used, where PVA was added as a surfactant. In addition, green-colored dye was dissolved in the solution to monitor the release of the encapsulants from the microcarriers. The middle phase was ethoxylated trimethylolpropane triacylate (ETPTA, Mn = 428, Sigma-Aldrich) containing 1 wt/wt% photoinitiator of 2hydroxy-2-methylpropiophenone (97%, Sigma-Aldrich). The continuous phase was a 10 w/w% aqueous solution of PVA. In our previous study, the shell thickness relative to the radius of the microcarrier was optimized to 0.11, which resulted in predominant rupturing of the microcarriers below 20 °C 7 ACS Paragon Plus Environment

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under agitation at 600 rpm, while retaining most microcarriers intact above 20 °C;25 the temperature-dependent stability of the microcarriers originated from the temperaturedependent brittleness of the polymer shell. In this work, we prepared the microcarriers with the same shell-to-radius ratio of 0.11 by setting the flow rates of the innermost, middle, and continuous phases to be 330, 800, and 3300 µl/h, respectively, where the flow rates were independently controlled by syringe pumps (KdScientific, Inc.). The double-emulsion drops were exposed to UV light (Innocure 100N, Lichtzen Co.) during collection in a vial to form a solid shell made of polymerized ETPTA. The resulting microcarriers has a radius of 160 µm and a shell thickness of 17.5 µm, such that the shell thickness relative to the radius is approximately 0.11, as shown in Figure 1b–d.


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Figure 1. Preparation of PVCap-loaded microcarriers. (a) Schematic of a capillary microfluidic device for generating monodisperse microcarriers containing the kinetic inhibitor of PVCap. (b) Schematic of the microcarrier with a shell thickness relative to the radius, t/R. (c, d) Optical microscope and scanning electron microscope (SEM) images showing microcarriers with t/R = 0.11.

2.3 Hydrate formation experiments with and without microcarriers Hydrate formation characteristics including onset time, growth rate, and hydrate fraction were determined using a high-pressure autoclave. The autoclave was made using 316 stainless steel, with a 4.0 cm internal diameter and a 7.8 cm height. The inner volume of the autoclave was 100 ml. A 12 ml bulk water suspension of the PVCap-loaded microcarriers and 18 ml decane (total liquid volume 30 ml; water cut 40%) was loaded into the autoclave, which was agitated using a magnetic stirrer and an anchor type impeller with a 3.8 cm diameter. Approximately 9300 microcarriers with t/r0 = 0.11 containing 20 wt% PVCap solution in the core were suspended in the water phase. As the weight fraction of the core to the microcarriers is 0.66, full release of PVCap solution can lead to a PVCap concentration of 0.25 wt% in 12 ml of water. Pure water, bulk water suspended with empty microcarriers, and a 0.25 wt% PVCap solution were selected as control groups to compare the hydrate inhibition performance. The autoclave was placed in a water bath connected to an external water chiller (Jeiotech RW2025G, Korea) to control the experimental temperature. The temperature of the liquid phase was measured by PT-100 Ω (accuracy: ± 0.15 °C) and the pressure of the gas phase was monitored by an Omega pressure transducer (accuracy: ± 0.1 bar from 0–200 bar). Temperature and pressure data were recorded with a LabVIEW data acquisition system during the entire experiment. The detailed experimental procedure are described in detail in the Supporting Information. 9 ACS Paragon Plus Environment

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3. Results and Discussion 3.1 Hydrate formation under continuous cooling The hydrate formation characteristics can be analyzed by the onset time, subcooling temperature, hydrate growth rate, and hydrate fraction in the liquid phase. As the primary function of KHI is to delay the nucleation of hydrate crystals during the residence time of hydrocarbon fluids in subsea flowlines, tests of its performance test have focused on detecting the onset times and subcooling temperatures. Our previous work suggested that variations in the onset time were large dependent on the cooling rate: The onset time is 6 to 10 times longer with slow cooling than with fast

19

. However, variation in the subcooling tempera-

ture was relatively small, and only a 1.4 °C difference was observed between fast and slow cooling. This suggested that it would be better to assess the performance of KHI by the subcooling temperature. The final set point for the water bath temperature was fixed at 4.0 °C, and the cooling rate was 0.25 °C /min. The liquid phase was agitated with the impeller at 600 rpm to apply a shear stress with a Reynolds number of 30,000. This agitation rate allows complete mixing of the water and decane mixture, whereas water and decane were partially mixed at 400 rpm and completely segregated at 200 rpm. The microcarriers were designed to rupture by more than 80% at 600 rpm or above, and less than 20% below 600 rpm, as shown in Figure S1 in the Supporting Information. Hydrate formation was detected at temperatures ranging from 10.0 to 7.0 °C with or without PVCap in the water phase.


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Table 1. Hydrate onset conditions and growth rates during the effective growth period under continuous cooling.

ΔTsub (°C)

tonset (min-1)

rmax (min-1)

teffect

φeffect

Pure water

7.45 (0.78)

29.67 (3.05)

0.011 (0.003)

14.09 (4.1)

0.12 (0.05)

Empty microcarriers

7.57 (0.66)

30.08 (2.68)

0.01 (0.001)

20.5 (1.65)

0.18 (0.01)

Bulk PVCap solution

10.78 (0.48)

45.37 (0.65)

0.014 (0.003)

10.8 (0.74)

0.13 (0.01)

PVCap microcapsules

11.34 (0.45)

44.08 (1.34)

0.014 (0.01)

12.8 (0.96)

0.14 (0.02)

System

Table 1 presents the mean values and standard deviations of the subcooling temperatures, onset times, maximum growth rates, effective growth periods, and hydrate fractions over repeated trials. All data for the repeated trials are given in Table S1 in the Supporting Information. During continuous cooling of the water and decane mixture, the temperature linearly decreased from 24 to 9.14 °C with a constant slope, which then spiked up to 10.5 °C, indicating exothermic heat from hydrate formation (see Figure S2 in the Supporting Information). The pressure also decreased faster due to gas consumption to form hydrate cages. The hydrate equilibrium temperature was predicted at the measured pressure using the CPA equation of the state model in Multiflash 4.2 28. The subcooling temperature, ΔTsub, the difference between the measured temperature at the hydrate onset and the hydrate equilibrium temperature were estimated to be 7.5 °C for the water and decane mixture. When the empty microcarriers with no PVCap solution inside were suspended in the water phase, the subcooling temperature was 7.6 °C. Both polymerized ETPTA and PVA


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did not show a kinetic inhibition effect and just dispersed into the liquid phase without interacting with the hydrate nuclei. However, when the microcapsules carried a 20 wt% PVCap solution, hydrate formation occurred at a much lower temperature of 9.7 °C than that for pure water and empty microcarriers; therefore, the subcooling temperature was 11.3 °C. As the shell of the microcarriers was designed to rupture and release the inner fluid below 20.0 °C, the PVCap solution was expected to release and mix with the water phase. The PVCap molecules dissolved in the water phase served as kinetic hydrate inhibitors during continuous cooling, and thus prevented the nucleation of hydrate crystals. When 0.25 wt% of PVCap was added to the water phase to make a bulk PVCap solution, the subcooling temperature was 10.8 °C , which was comparable to that of the PVCap microcarriers. These results suggested that the released PVCap from the microcarriers were fully used to defer hydrate nucleation, confirming the rupture of the microcarriers to release PVCap. The role of the released PVCap during hydrate growth was studied by analyzing the growth rate during the effective growth period. The hydrate fraction increased fast for 40 min after the onset of hydrate formation, and then plateaued with a decreasing growth rate, as shown in Figure 2. The growth curve was obtained from the average values over four cycles of hydrate formation experiments. The results of each experiment are shown in Figure S3 in the Supporting Information. Time zero indicates the onset of hydrate formation. The hydrate growth rate was presented as the slope in the growth curve, r=dϕ/dt. The slopes showed a scattered pattern, and nonlinear regression of the data with a Gaussian equation was carried out in order to find the regressed growth rate values. The maximum growth rate and the effective growth period were determined from the regressed curve, and indicated


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how long hydrate growth continued. Afterward, the hydrate fraction during the effective growth period was calculated, as summarized in Table 1. For the bulk water and decane mixture (Figure 2 (a)), the initial growth rate was approximately 0.0019 min-1 at 7 min, and then it surged up to 0.011 min-1 at 17 min from the onset. Then, the growth rate decreased to 0.0015 min-1 at 28 min. The hydrate fraction quickly reached 0.13 during this effective growth period. A similar growth pattern was observed for empty microcarriers suspended in a bulk water and decane mixture (Figure 2 (b)). Initially, hydrate growth proceeded slowly, but sped up at 10 min until the growth rate reached 0.010 min-1 at 22 min. The growth rate decreased to 0.0026 min-1 at 38 min with a hydrate fraction of 0.20. Most growth occurred during this effective growth period beginning at 28 min after the onset of hydrate formation. The empty microcarriers showed a negligible effect on the hydrate growth characteristics. When PVCap was dissolved in bulk water (Figure 2 (c)), fast hydrate growth was observed in the early stage, rather than a slow growth period. The growth rate quickly surged to 0.014 min-1 at 4 min, then decreased to 0.0018 min-1 at 20 min when the hydrate fraction reached 0.16. For the PVCap microcarriers, the hydrate growth pattern was similar to that of the bulk PVCap solution. The growth rate reached 0.014 min-1 at 5 min, and then decreased to 0.0014 min-1 at 23 min when the hydrate fraction reached 0.16. This fast growth of hydrate in the initial stage after onset is attributed to the high subcooling temperature of 10.8 °C, which provides a strong driving force for hydrate growth. These results suggested that the PVCap solution released from the microcarriers was wellmixed with the water phase. Because the PVCap molecules interact with hydrate nuclei, they successfully serve as kinetic hydrate inhibitors, as evidenced by the increased subcool14 ACS Paragon Plus Environment

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ing temperature. The PVCap molecules also participated in the hydrate growth period. Although fast growth was observed in the early stage of hydrate formation due to the increased subcooling temperature, it is likely that the PVCap molecules adsorbed into the growing hydrate surface, resulting in a fast reduction of the growth rate. The effective growth period for the bulk PVCap solution and PVCap microcarriers shown in Figure 2 (c) and (d) was approximately 12 min, while the period for the system without PVCap was approximately 24.5 min. A relatively low hydrate fraction was observed for the microcarriers containing a PVCap solution.


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(a)

0.18

0.014 Hydrate volume fraction Growth rate Regression of growth rate

0.16

0.012

0.010 -1

0.12

Growth rate (min )

Hydrate volume fraction

0.14

0.10

0.008

0.08

0.006

0.06 0.004 0.04 0.002 0.02 0.00

0.000 0

10

20

30

40

Time (min) 0.012

0.30 Hydrate volume fraction Growth rate Regression of grwoth rate

0.010

0.20

0.008

0.15

0.006

0.10

0.004

0.05

0.002

0.00

-1

0.25

Growth rate (min )

(b)


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0.000 0

10

20

30

40


Page 17 of 37

0.30

(c)


0.25

0.014

0.20

0.008 0.10 0.006 0.05

-1

0.010 0.15

Growth rate (min )


0.012

0.004

0.00

0.002 0.000

0

10

20

30

40

0.25


(d)

0.014

0.20

0.008 0.10

0.006

-1

0.010

0.15

Growth rate (min )

0.012


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0.004 0.05 0.002 0.00

0.000 0

10

20

30

40

Time (min)

Figure 2. Average hydrate fraction, hydrate growth rate, and growth rate trends under continuous cooling conditions. (a) Pure water, (b) empty microcarrier, (c) bulk PVCap solution, and (d) PVCap microcarrier.

The results described above suggest that the subcooling temperature increases from 7.5 °C to 11.3 °C by adding PVCap to the aqueous phase. Because the subcooling temperature is a 17 ACS Paragon Plus Environment

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key indicator for the performance of KHIs, additional experiments were performed to study hydrate formation at low sub-cooling temperatures. Instead of 4 °C, we chose 16 °C as the target temperature for the continuous cooling experiments; that is, the subcooling temperature was 5 °C. Figure 3 shows the changes in the hydrate growth curve and growth rate for the bulk water and decane mixture. Time zero indicates the moment at which the temperature became lower than the hydrate equilibrium temperature. Hydrate formation occurred at 13 min for the bulk water and decane mixture, indicating a hydrate onset time of 13 min. In contrast, no hydrate formation was observed with the PVCap microcarriers. Although the subcooling temperature was only 5 °C, the maximum growth rate was 0.007 min-1 and the effective growth period was 22 min in pure water. Interestingly, the growth rate remained at 0.0006 min-1 for 30 min after the onset, suggesting a slow linear growth of hydrate crystals. For PVCap microcarriers, a slight decrease in pressure was observed 150 min after the onset, indicating that the hydrate fraction was less than 0.002 in the liquid phase. The PVCap released from the microcarriers effectively inhibited the formation of hydrate crystals at a low subcooling temperature. Figure 4 shows images of the PVCap microcarriers before and after the hydrate formation experiments. Before the experiments, the microcarriers maintained their spherical shape with the encapsulated PVCap solution . After performing the experiments, the aqueous phase turned green in color because the dye was released from the microcarriers along with the PVCap. The aqueous phase was filtered to collect the fractured microcarriers, as seen in the right panel of Figure 4b. Almost all microcarriers were ruptured. These images suggested that the PVCap molecules were released from the microcarriers under turbulent mixing conditions and were important in inhibiting the nucleation and growth of hydrate crystals. 18 ACS Paragon Plus Environment

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As desired, the microcarriers ruptured and released the internal PVCap solution during continuous cooling.

0.05

0.008

Hydrate volume fraction Growth rate

0.04

-1

Growth rate (min )

0.006


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0.03 0.004 0.02

0.002 0.01

0.00

0.000 0

10

20

30

40

50

Time (min)

Figure 3. Hydrate volume fraction and growth rate for the water and decane mixture under low subcooling conditions. Time zero indicates the moment at which the system temperature passes through the hydrate equilibrium point.


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Figure 4. (a, b) Photographs and optical microscope images of the microcarriers (a) before and (b) after hydrate formation experiment.


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3.2 Hydrate formation in constant subcooling The microcarriers were designed to release the PVCap solution when the temperature decreased below 20 °C and the liquid phase was in a turbulent mixing condition, where the brittle microcarrier shell would rupture. The constant subcooling method evaluates the performance of KHI candidates at constant temperature, where the system temperature was decreased to the target temperature without agitating the liquid phase. In our previous work21, 1.0 wt% of a Luvicap solution gave a hydrate onset time of 4.2 min when the solution was at a subcooling temperature of 13.3 °C for 10 hours. Although Luvicap could not delay the hydrate nucleation for longer, it reduced its growth rate by 55% compared to that of bulk water. In this work, we performed constant subcooling experiments to investigate the possibility of releasing a PVCap solution from microcarriers during cold-restart operations and its efficacy for hydrate growth inhibition. The concentration of PVCap is 0.25 wt% if the PVCap solution is fully released from the microcarriers. The system was set at 4 °C and 105 bar for approximately 4.5 hours with a subcooling temperature of 16 °C before commencing agitation at 600 rpm. Table 2 shows the results for the bulk water and decane mixture, PVCap solution and decane mixture, and suspended PVCap microcarriers mixture. Although the liquid phase was not agitated during the shut-in period, hydrate formation was stochastically observed, as seen in Figure S4 of the Supporting Information. The hydrate onset time and hydrate fraction during the shut-in period are presented in Table 2. The presence of PVCap in the aqueous phase could not delay hydrate formation for 4.5 hours under a subcooling temperature 16 °C. The hydrate fraction in the liquid phase reached approximately 0.02, indicating only ~5% water conversion into the hydrate phase for the 3rd exper21 ACS Paragon Plus Environment

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iment, while less hydrate formation was observed in other experiments. This is because the hydrate formation occurred at the water-decane interface by consuming dissolved gas molecules. However, without agitation of the liquid phase, the hydrate layer covers the interface and limits mass transfer from gas phase. PVCap molecules dissolved in the water phase might not be able to interact with the growing hydrate crystals at the water-decane interface. When agitation begins, the hydrate layer at the water-decane interface can break into small hydrate pieces and disperse into the liquid phase.

Table 2. Hydrate onset conditions and growth rates during the effective growth period under constant subcooling.

ΔTsub

tonset

rmax

(°C)

(min)

(min-1)

Pure water

16

350

0.15 (0.01)

8.54 (0.56) 0.13 (0.01)

Bulk PVCap solution

16

350

0.10 (0.03)

6.47 (1.05) 0.03 (0.00)

PVCap microcapsules

16

350

0.16 (0.04)

System

teffect

10.73 (4.61)

φeffect

0.14 (0.02)

Because the hydrate fragments grow as they intake gas molecules transferred from the gas phase, catastrophic hydrate growth was observed, as seen in Figure 5. Time zero in Figure 5 indicates the moment agitation begins. The maximum hydrate growth rate for the bulk water and decane mixture was 0.15 min-1 under constant subcooling conditions, which is similar to that observed with continuous cooling. However, the effective growth period was approximately 8.5 min, suggesting catastrophic formation resulted in fast growth, but over a short duration. The thermal driving force for hydrate growth almost doubled, and the dis22 ACS Paragon Plus Environment

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persed hydrate fragments served as a catalyst for hydrate grow upon mixing. Figure 5 (a) shows the hydrate growth curve for the bulk water and decane mixture. Due to the catastrophic hydrate formation, exothermic heat of formation increased the liquid phase temperature to 8.8 °C. The limited heat transfer leads to a reduction of the thermal driving force for hydrate formation from 16 to 11.2 °C (see Figure S6 in Supporting Information). Thus, the growth rate also decreased when the hydrate fraction reached 0.05 in Figure 5 (a). Hydrate formation slowly continued until the hydrate fraction reached 0.22 at the end of the experiment.


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Figure 5. Hydrate formation for the water and decane mixture under constant subcooling conditions. (a) Difference in hydrate growth and temperature for equilibrium and experimental conditions as a function of time. (b) Regression of growth rate. 24 ACS Paragon Plus Environment

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When PVCap is dissolved in the water phase (Figure 6) the maximum hydrate growth rate was observed earlier than that in pure water. The growth rate reached 0.005 min-1 and then quickly decreased after the inflection point at a hydrate fraction of 0.04. Figure S6 in the Supporting Information shows a decrease in the subcooling temperature due to the exothermic heat from hydrate growth in the bulk PVCap solution. The released heat was quickly removed due to the low hydrate growth rate. The effective growth period was 6.5 min. It is likely that the PVCap molecules adsorbed onto the hydrate surface and further inhibited the crystal growth, as witnessed in the continuous cooling experiments. Figure 7 shows the hydrate growth curves for the mixture of suspended PVCap microcarriers. As there was no shear flow during the cool-down process and shut-in period, it is unlikely that the microcarriers released the encapsulated PVCap solution into the liquid phase. Instead, the microcarriers ruptured upon restarting agitation, which would cause maldistribution of PVCap in the liquid phase. The catastrophic hydrate formation aggravated the spacial distribution of PVCap molecules, and thus, the PVCap released from the microcarriers showed limited performance as a crystal growth inhibitor, as seen in Figure 7 (a). In the case of the well-distributed PVCap from the microcarriers, in the first experiment, hydrate growth slowed after the hydrate fraction reached 0.06. However, in case of the maldistribution of PVCap from the microcarriers in the second and third experiments, the inflection point was observed at a hydrate fraction of 0.10 or larger, resulting in a higher hydrate fraction. The effective growth period varied from 5 to 14 min. Figure S7 in the Supporting Information shows changes in the subcooling temperature, where a slight change was observed between the first and second experiments, but a large drop was observed for the other experiments. These results suggested that the microcarriers may not be effective 25 ACS Paragon Plus Environment

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for managing hydrate formation during cold-restart operations. More studies are required to improve the design of microcarriers and find the effective way to release the encapsulants considering the operation of subsea flowlines. For example, the shell materials for the microcarriers is composed of the polymer materials with the lower critical solution temperature (LCST) at around 20oC while increasing the mechanical strength of the microcarriers. It would be possible to release the inner KHI solutions through the swollen polymer shell of the microcarriers below the LCST, instead of rupturing via turbulent mixing. Improving the mechanical strength of the microcarriers can be coupled with the design of high pressure cartridge to carry the microcarriers via umbilical lines into subea flowlines. The production rate of microcarrier can be improved using an integrated device containing a parallel array of tens or hundreds of drop generators. Recently, pioneering works on the parallelization for a production scale-up of double-emulsion drops have been reported, which induces Romanowsky et al.30, Nisisako et al.31, and Eggersdorfer et al.32. From these studies, it was revealed that the production of microcapsule can be scale-up. Moreover, we believe that the microfluidic technology further makes progress on the high-throughput production. By controlling the number of microcarriers and the concentration of the inner KHI solution, it would be possible to control the concentration of KHI in the bulk aqueous phase at the corresponding temperature. During normal production, the fluid temperatures approach those of the hydrate equilibrium conditions. The microcarriers can be adjusted to release the inner KHI solution to the fluids inside the subsea flowlines at the hydrate equilibrium temperature. Turbulent mixing of the fluids under steady-flow conditions and the fluid temperature would result in rupture of the microcarriers. If the temperature is higher than the hydrate equilibrium temperature, the microcarriers would remain intact. Thus, the microcarri26 ACS Paragon Plus Environment

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ers would indicate the fluid temperature while mitigating the risk of hydrate formation. During extended shut-in, the flow regime during the restart operation may not be turbulent enough to break the microcarriers. Moreover, thin hydrate films can form during the shut-in period, which enhances hydrate growth during the restart operations. We observed this phenomenon in this work, and realized the design of these microcarriers must be improved. To release the KHIs during the extended shut-in period, the shell materials of the microcarriers can be designed with a polymer with an LCST near the hydrate equilibrium temperature, i.e., a more temperature-sensitive material, while maintaining the mechanical stability necessary injecting and collecting the materials in the processed system. The newly designed cartridge and filter system can be used to inject KHIs into the subsea flowlines and re-collect the microcarriers from the produced fluids. In addition, it is possible to simultaneously inject many different types of KHIs through a single pipeline without cross-contamination, as the microcarriers can encapsulate their own KHIs. Therefore, the microcarriers would provide a flexible way to release KHIs into the liquid phase to manage the risk of hydrate formation under various subcooling conditions.


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Figure 6. Hydrate formation for a bulk PVCap solution under constant subcooling conditions. (a) Hydrate fraction in the liquid phase. (b) Regression of growth rate. 28 ACS Paragon Plus Environment

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Figure 7. Hydrate formation for PVCap microcarriers under constant subcooling conditions. (a) Hydrate fraction in liquid phase. (b) Regression of growth rate.


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4. Conclusion The hydrate inhibition performance of PVCap released from the microcarriers was investigated using continuous cooling and constant subcooling methods in a high-pressure autoclave. A longer hydrate onset time was observed for the PVCap microcarriers compared to that of bulk water during continuous cooling under a constant flow, suggesting that hydrate nucleation was inhibited by PVCap released from the microcarriers. The obtained subcooling temperature for the PVCap microcarriers was 11.3 °C, which was close to that of the PVCap bulk solution at 10.8 °C. The hydrate growth rate was similar for both the PVCap microcarriers and bulk water, but the effective growth period was shorter in the presence of PVCap, leading to a lower hydrate fraction in the liquid phase. Although the PVCap microcarriers performed successfully under continuous cooling, limited performance was observed with constant subcooling, i.e., cold restart operation conditions. In five repeated experiments, successful hydrate inhibition was observed twice, but fast hydrate formation was also observed in other trials. This finding suggested that the released KHIs show limited performance under catastrophic hydrate formation during cold restart operations. To be more efficient in the inhibition of hydrate formation, the shell materials of the microcarriers should be modified to release the internal KHI solutions during static shut-in conditions. More studies are required to improve the design of microcarriers and to find an effective way to release encapsulants while considering the operation of the subsea flowlines. When the microcarriers were applied with low subcooling temperatures, negligible hydrate formation was observed over 5 hours, proving the performance of the KHI released from microcarriers at low subcooling temperatures. The microcarriers would provide a flexible way


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to inject KHIs into subsea flowlines by controlling the number of microcarriers and the composition of the internal KHI solution.


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Author Contributions The manuscript was written with contributions from all the authors. All the authors have given approval of the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Technology Innovation Program (10060099), which is funded by the Ministry of Trade, Industry & Energy (MI, Korea).


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Hydrate Growth Inhibition by Poly(vinyl caprolactam) released from

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