Assessment of a Commercial Gas Hydrate Inhibitor Using the 3-in-1 Technique

Luna Saray Andrea Bonilla Gomez

An undergraduate thesis completed at the Royal Military College of Canada (RMCC) and submitted to Universidad de los Andes (Colombia), in partial fulfillmentof the requirements of the degree of Chemical Engineer at Universidad de los Andes

Abstract

The ability of the novel 3-in-1 reactor to assess a commercial hydrate inhibitor was evaluated. Methane hydrates were formed usinga mass fraction of 0.1 % of an unknown commercial inhibitor at constant pressure and different driving forces. Phase equilibria,kinetics, and morphology of the methane hydrates in the inhibited system were studied. The technique provided reproducible dataon these three different aspects. The technique allowed to identify that the unknown inhibitor behaves as a kinetic hydrate inhibitor(KHI) for subcoolings more than 3.0 K. The uncertainty of the film growth rate for a specific subcooling varies from 0.97 µm·s−1 to 5.8 µm· s−1. In addition, it was possible to confirm the KHI nature of the inhibitor A1 since the value of THLV = 280.3 K± 0.03 K is within the 95 % of prediction interval of the equilibrium temperature for the uninhibited system at the experimentalpressure. Also, the experimental set up enabled morphological studies of the inhibited system which suggested that dendrites areless densely packed as subcooling decreases. Furthermore, one feature throughout the experiments could be noticed correspondingto the decrease of initial growth points and guest bubbles from cycle to cycle. Besides, the coverage time of hydrate in the surfaceof the droplet increases each reformation as a result of fewer initial growth points. Based on the above, the 3-in-1 technique wasfound to provide quick and reproducible information on the inhibition performance and could be used for further inhibition studieson gas hydrates.

Keywords: Hydrate inhibitor, Methane hydrates, Morphology, Kinetics, Growth mechanism, Phase Equilibria

1. Introduction

Gas hydrates are crystalline, non-stoichiometric compoundsof small molecules ( < 0.9 nm ) and water. A hydrogen-bondedwater framework encloses the small molecular species withincavities forming a host-guest system (Englezos, 1993). Thehost ( water) and guest interact through weak van der Waalsforces that stabilize the hydrate structure. Typically, gas hy-drates comprise 85 % water and 15 % guest on a molar basiswhen all of the cavities are occupied. They form when theircomponents come into contact at temperatures below 300 K andpressures above 0.6 MPa (Sloan, 2003). The crystal structurethat might form depends on the size of the guest gas molecules.For instance, methane hydrates are known to form cubic struc-ture I (sI) (Koh et al., 2011).

Natural-gas hydrates could be considered as a future energyresource since the substantial amount of naturally occurringmethane hydrate deposits in the ocean bottom and permafrostregions is at least twice that of all other fossil fuels combined(Paull et al., 2010).Besides, the innate capacity of hydrates tostore gas may be a solution to the carbon sequestration and nat-ural gas transport (Sloan, 2000) (Gudmundsson and Borrehaug,1996).

Despite the possible applications of gas hydrates, these com-pounds have long been a major concern for the oil and gas in-dustry since Hammerschmidt (1934) discovered that formationof gas hydrates in natural gas pipelines was one of the causes ofplugging. Actually, operational conditions within its transmi-

tion pipelines remains conducive to hydrate formation and thisflow assurance concern continues leading to significant opera-tional upsets, safety hazards, and production loss.

In response to this matter, there are different strategies toprevent or control hydrate formation. First, heating and insu-lation around pipelines are techniques to circumvent hydrate-forming conditions (Carroll, 2014); however, those additionalfacilities involve more consumption of energy and higher oper-ational costs. Second, the injection of additives into the flow-ing stream has been used widely as means to provide flow as-surance. These additives, also known as inhibitor species, areclassified as thermodynamic hydrate inhibitors (THIs) and lowdosage hydrate inhibitors (LDHIs). The latter can be dividedinto two categories; anti-agglomerants (AAs) and kinetic hy-drate inhibitors (KHIs).

Traditionally, inhibition has been performed through the ad-dition of THIs. These are hydrogen bonding fluids such as al-cohols and glycols which cause the shift of the hydrate equilib-rium conditions to a lower temperature and a higher pressurezone, preventing hydrate formation under the prevailing condi-tions in the pipeline (Koh et al., 2002). In general, the massfraction required of THI varies from 20 % to 50 % for an effec-tive inhibition, leading to significant financial and environmen-tal implications (Kelland, 2006). Indeed, over USD 200 millionis spent annually in methanol to prevent hydrate formation in oiland gas pipelines (Sloan, 2003).

A more cost-effective alternative than THIs corresponds tothe use of LDHIs. Both AAs and KHIs can be added in concen-

1

trations as low as 0.01-5% since these compounds act by a moretargeted, mechanistic approach (Perrin et al., 2013). AAs act bypreventing the aggregation of small hydrate crystals into large,pipe-blocking masses. In the case of KHIs, they interfere withthe nucleation and growth rate of hydrate crystals (Andersonet al., 2016) through an inhibitor-adsorption mechanism (Sloanand Koh, 2007). Among the components that constitute AAsand KHIs are quaternary ammonium salts and water-solublepolymers from the poly(vinyl lactam) group respectively (Kel-land, 2006).

Current hydrate inhibition technology can be improved bystudying the hydrate formation, and dissociation mechanismsin the presence of inhibitor species since research in this areamay increase our understanding of inhibited hydrate systemsand the possibility to develop more effective additives. (Kohet al., 2002). This type of systems have been studied in dif-ferent aspects: kinetics, morphology, phase equilibria, rheol-ogy and surface effects. Broadly, kinetics measures the filmvelocity of the advancing hydrate front that forms on the wa-ter surface in the presence of inhibitor. Morphology gives aclear picture of the events that occur upon hydrate crystalliza-tion and determines the type of model to use for the system(Beltran and Servio, 2010). Phase equilibria explain the ther-modynamic behavior by finding the temperature and pressureconditions for hydrate formation and dissociation under inhibi-tion. The hydrate-liquid-vapor temperature, THLV , of the inhib-ited system is defined as the average of the temperatures beforeand after of complete hydrate dissociation.

Each of the above - mentioned aspects gives insight into thetype of inhibitor under study and its performance. Moreover,gas hydrates are studied with an index of experimental devi-ation from equilibrium. Since these compounds are generallyformed by subcooling or pressurization beyond the equilibriumconditions (N., 2016), it is required an index that correlates thecorresponding driving force to the observed morphology andkinetics. The index ∆Tsub is used for subcooling which repre-sents the difference between the experimental temperature andequilibrium temperature at the experimental pressure (Ohmuraet al., 1999).

To study the hydrate inhibitors, various types of reactors havebeen developed such as flow loops, pipe wheels, and variousother kinds of high-pressure cells. Notwithstanding the effec-tiveness of these methods, data acquisition is slow and in theorder of one to two data during a 10 to 50 hour period (Ke andKelland, 2016). Recently, DuQuesnay et al. (2016) developeda novel 3-in-1 reactor that allows assessment of phase equilib-ria, crystal morphology and apparent kinetics in a single exper-iment which lasts 12 hours . The design provides tight con-trol of the crystallization temperature and minimal experimen-tal uncertainties, offering fast, reproducible assessment of gashydrate systems.

The 3-in-1 reactor has suitably assessed uninhibited andinhibited methane hydrate systems. First, the method wasproved that it works for uninhibited methane hydrate systemsby DuQuesnay et al. (2016). In fact, reproducible film mor-phologies agreed with data reported by others respect to thesame driving force. Additionally, hydrate-liquid-vapor equilib-

rium conditions are within the 95 % prediction interval of thedata compiled by Sloan and Koh (2007) and the order of mag-nitude of the hydrate film velocity uncertainties varies from -1 to 0. Then, the technique was tested and proved with non-colligative inhibitors such as antifreeze proteins (Udegbunamet al., 2017) and traditional inhibitors as NaCl, MEG, PVP,and TBAB (Beltran, 2017). Recently, “unknown composition”commercial hydrate inhibitors have been characterized usingthe 3-in-1 reactor to prove its ability to assess this type of in-hibitors. (Ovalle et al., 2018)

The objective of this work is to evaluate the ability of the3-in-1 technique to assess a commercial, gas-hydrate inhibitorof “unknown composition”. The present research seeks to givefurther insight into the novel technique to provide fast and re-producible data on morphology, kinetics, and phase equilibriaof methane gas hydrate under the inhibition conditions.

2. Experimental

2.1. Apparatus

P

T

Stage TCTC

Gas InGas Out

Chiller

DAQ

(c)

(b)(a)(g)

(d)

(f)

(e)

Figure 1: Experimental apparatus schematic. (a) Pressure vessel. (b) Sapphirewindows. (c) PCO.edge Camera (d) Refrigerated circulator. (e) Bi-polar PIDtemperature controllers. (f) cold light source. (g) Cooling jacket. (DuQuesnayet al., 2016)

Figure 1 shows a schematic of the 3-in-1 gas hydrate reactor.The apparatus consists of a pressure vessel made of 316 stain-less steel with several radial ports for pressure and temperaturemonitors as well as gas inlet and outlet. It contains a high-pressure bilateral temperature control stage (HP-BTCS) to pro-vide precise temperature control of the sample. The vessel is fit-ted with two sapphire windows (Rayotek Scientific, CA, USA)on the top and bottom which enable the illumination and obser-vation of the sample held on a sapphire slide and immersed in apressurized methane atmosphere. The sample stage was illumi-nated from the bottom window with a Schott KL2500 LCD coldlight source (Optikon, ON, Canada). A PCO.edge 5.5 cMOScamera (Optikon, ON, Canada) was placed above the top win-dow to acquire images and recordings of the sample. A Nikon,AF-Micro-Nikkor 60 mm lens (Optikon, ON, Canada) was usedat low magnification, and an Infinity KC microscope and IF-3.5lens (Optikon, ON, Canada) were used for high magnification.

The temperature of the vessel was controlled by a ThermoScientific AC200 refrigerated chiller (Fisher Scientific, Canada)

2

which circulated 50/50 (v/v) mixture of ethylene glycol and wa-ter through a copper coil wrapped around the reactor. Two lay-ers of insulations were placed surrounding the coil to achievea steady-state with the environment. The ambient temperatureinside the vessel was monitored with a Platinum RTD probe(Omega Engineering, QC, Canada) and the pressure was mea-sured with a Rosemount 3051s pressure transmitter (Lauren-tide Controls, QC, Canada). Instrumental standard uncertain-ties were as follows : for vessel temperature uTRT D = 0.32 K andfor pressure up = 0.005 MPa.

2.1.1. High-pressure bilateral temperature control stage

(b)

TE

TIC

TE

TIC

(c)

(a)

(d) (e)

(f)

Figure 2: High-pressure bilateral temperature control stage (HPBTCS)schematic. (a) Thermoelectric cooler module. (b) Bi-polar PID temperaturecontrollers. (c) Fast-response thermistor. (d) Sapphire substrate. (e) Sampledroplet. (f) Heat sink. DuQuesnay et al. (2016)

.

Figure 2 shows a schematic of the bilateral temperature con-trol stage, which controlled precisely and independently thetemperature on opposite sides of the stage (DuQuesnay et al.,2016). The stage has two ends, each equipped with a thermo-electric cooler module (TEM) (TE Technology, MI, USA). Inorder to uniformly distribute temperature, copper plates wereattached to the top and bottom of each TEM. Two bi-polar PIDtemperature controllers (TE Technology, MI, USA) controlledthe TEMs. A sapphire substrate, also known as a slide, wasplaced on the top copper plates to hold the sample and connectboth ends of the stage. Temperatures of both top copper plateswere measured by fast-response thermistor elements (TE Tech-nology, MI, USA) with an instrumental uncertainty of uTthermistor

= 0.01 K.

2.2. Materials

The commercial hydrate inhibitor (A1) used for the assess-ment is composed principally of a fatty amidoammonium salt“A” which general structure can be observed in Figure 3 . Oneend is linked to a fatty acid chain as an amide, while the rest ofthe molecule is composed of quaternary ammonium salts thatcontain carboxylate groups. (Nalco, 1963). Table 1 presentsthe approximate composition of inhibitor A1 and Table 2 sum-marizes reagents used in this study.

2.3. Experimental procedure

First, the substrate was thoroughly cleaned three times withliquid detergent and deionized water. Second, it was dried withdust-free compressed air and submerged in acetone for a pe-riod of five minutes in a sonication bath. Then, the slide was

Table 1: Approximate composition of commercial hydrate inhibitor A1

Component (102) Mass fractionFatty amidoammonium salt “A” 60

Methanol 10Fatty alcohol 30

R1

o

N

HN

R2

R2

R3

n

+

A-

Figure 3: General Structure of the fatty amidoammonium salt ”A”

submerged in isopropanol at the same previous conditions andlastly dried with dust-free compressed air.

Afterward, the slide was placed and pressed on the copperplates already covered with thermal paste making sure the pastespread evenly along both ends of the slide. A 20 µL waterdroplet containing a mass fraction of 0.1 % of inhibitor A1was placed on the center of the sapphire substrate with a mi-cropipette. The vessel was then sealed and pressurized threetimes to 4 MPa with nitrogen to remove any air inside the reac-tor. Subsequently, the exact purging procedure was performedwith methane.

2.3.1. Crystal formation History

P/

MP

a

T / K

0.1

273.15

5.1

I-H-L

(a)

(b)(d)

(c)

I-L-V

Figure 4: Pretreatment of a water droplet in the presence of inhibitor A1 over-laid on partial phase diagram for the system CH4 + water. (a) Ice formation bycooling down the droplet to 253.2 K: ice-vapor region at 0.1 MPa, no hydratespresent. (b) Hydrate formation from ice by pressurizing the vessel to 5.1 MPa:ice-hydrate region. (c) Stable hydrate phase from ice by heating the droplet to276.2 K: hydrate-liquid region at 5.1 MPa. (d) Hydrate dissociation by heatingup the droplet to 281.2 K: liquid-vapor region at 5.1 MPa.

Figure 4 illustrates the pre-treatment of the sample to createcrystal formation history. Primarily, the vessel was cooled to271.2 K at 0.1 MPa and the stage was cooled to 253.2 K (Fig-ure 4, step a). Once the sample was frozen in the ice-vapor

3

Table 2: Reagents used in this study. Resistivity was used as an indicator ofwater purity: at the source, it was measured to be 18 MΩ·cm.

Chemical Name Source Purity Purity UnitsDistilled Water In-house see caption see caption

Nitrogen Air Liquide 99.99% mole fractionMethane Air Liquide 99.99% mole fraction

region, the vessel temperature was increased above the H-L-Vequilibrium temperature at the experimental pressure, 5.1 MPa,while pressurizing to this value (Figure 4, step b). In this man-ner, the surroundings of the droplet were above the equilibriumtemperature and did not interfere with the subcooling applied tothe sample in the control temperature stage. After pressuriza-tion, the sample was in the ice-hydrate region and hydrates wereformed from ice. To seed hydrate crystal growth in the sample,it was taken to the hydrate-liquid region to form a stable hydratephase from ice by heating the stage to 276.2 K (Figure 4, stepc). Finally, this initial hydrate is dissociated by increasing thetemperature above the equilibrium condition to 281.2 K (Fig-ure 4, step d). The sample droplet was held at these conditions,liquid-vapor region, for 2 minutes before reforming hydrates atthe experimental temperature. Each reformation after the dis-sociation of hydrate from ice is considered as a cycle.

2.3.2. Hydrate formation

𝑥

𝑇𝐿 𝑇𝐻

𝑇HLV

𝑇L = 𝑇H

𝑇

∆𝑇sub

𝑥

Figure 5: Hydrate formation using a uniform surface temperature across theslide. Both TEM are held at the same Texp below the THLV

As the HP-BTCS can be configured with both ends at thesame or different temperature, two types of experiments wereperformed: uniform surface temperature experiment ( Figure 5)and constant temperature gradient experiment (Figure 6). Forthe former, both TEMs were set at the same experimental tem-perature below the THLV which means one driving force in thesample. For the latter, one of the TEMS was cooled near theTHLV while the other TEM was cooled 6 K under it to achieve a

𝑇𝐿 𝑇𝐻

𝑥

𝑇HLV

𝑇

𝑇H

𝑇L∆𝑇sub1

∆𝑇sub2

𝑥

Figure 6: Hydrate formation using a constant temperature gradient across theslide. One TEM is held at a lower temperature and the other TEM at a highertemperature, both below the THLV

constant temperature difference along the slide. Thus, differentdegrees of subcooling were observed through the water droplet.

2.3.3. Hydrate dissociation

𝑇𝐿 𝑇𝐻

𝑥

𝑇HLV

𝑇

𝑇L = 𝑇H

𝑥

Figure 7: Hydrate dissociation using a uniform surface temperature across theslide. Both TEM are held at the same Texp

For hydrate dissociation and measurement of the equilibriumtemperature of the inhibited system, two different temperatureconfiguration of the TEMs were used. Figure 7 shows a uni-form surface temperature dissociation profile, both TEMs wereheld at the same temperature starting below the THLV for theuninhibited system. Then, the temperature was increased 0.2 Kevery 30 minutes until hydrate dissociation was observed. Af-ter that, the temperature increase was 0.1 K with a waiting timeapproximately of 1 hour and 30 minutes until complete hydratedissociation was observed.

For constant temperature gradient dissociation (Figure 8), adifference of 6 K was maintained at all times between the hotand the cold ends. The hot end was kept above the THLV whilethe cold end held under the THLV . Then, the setpoint temper-

4

𝑇𝐿 𝑇𝐻

𝑥

𝑇𝐻𝐿𝑉

𝑇

𝑇H

𝑇L

𝑥

interface

Figure 8: Hydrate dissociation using a constant temperature gradient across theslide. One TEM is held at a lower temperature and the other TEM at a highertemperature. The TEM with higher temperature is held at a temperature aboveTHLV and the other TEM is held below THLV . The calculated temperature at theHydrate-Liquid interface is the THLV

atures of the ends of the gradient were increased simultane-ously in 0.2 K increments. The hydrate-liquid-vapor interfacewas easily observed where the temperature of sapphire slidecorresponded to the THLV at the experimental pressure. Eachstep increase in the setpoint temperatures displaced the H-L-Visotherm toward the cold side of the stage.

2.4. Experimental conditions

Table 3: Experimental conditions for gas hydrate formation from inhibited wa-ter droplets immersed in a methane atmosphere.THLV , calcualated H-L-V equi-librium temperature for uninhibited system at experimental pressure. TExp, ex-perimental temperature in the stage. ∆Tsub, subcooling in the water droplet. Inthe experiment column the numeral in the ones represents a set of conditions,while the numeral in the tenths represents the replicate number.∆Tsub will de-pend on the exact location of the droplet on the sapphire slide

Experiment THLV / K TExp / K ∆Tsub / K1.1 279.8 275.9 3.91.2 279.8 275.9 3.92.3 279.8 274.5 5.32.4 279.8 274.5 5.32.5 279.8 274.5 5.33.1 279.8 274.2-280.2 -3.2 279.8 273.2-279.2 -3.3 279.8 273.2-279.2 -

Table 3 shows the experimental conditions used in this work.Experiments 1 and 2 were uniform surface temperature exper-iments; two experiments were run at low driving force, ∆Tsub

= 3.9 K and three experiments were run at high driving force,∆Tsub = 5.3 K. Experiments 3 were run at a constant tempera-ture gradient with a difference between the TEMs of 6 K. Forall the experiments, the mass fraction of inhibitor A1 was 0.1%, and the pressure was 5.1 MPa.

3. Results

3.1. Hydrate morphology3.1.1. Uniform surface temperature

1 mm1 mm

(a) (b)

∆TSub = 3.9 K ∆TSub = 5.3 K

Figure 9: Methane hydrate formed at ∆Tsub = 3.9 K and ∆Tsub = 5.3 K onwater droplets, in the presence of inhibitor A1 and subject to uniform surfacetemperature. Both subcoolings form a dendritic morphology, each with a dif-ferent arrangement of dendrites. Hydrate at low driving force (a) forms fewergrain boundaries than hydrate at high driving force (b) and exhibits less denselypacked dendrites. (a) TExp = 275.9 K, ∆Tsub = 3.9 K; (b) TExp = 274.5 K, ∆Tsub= 5.3 K; (a-b) Cycle 2, THLV = 279.8 K, Pexp = 5.1 MPa, xA1 = 0.1 %.

Figure 9 shows methane hydrate formed on water droplets,in the presence of inhibitor A1 and subject to uniform surfacetemperature. Two subcoolings are presented, ∆Tsub = 3.9 K (Figure 9a ) and ∆Tsub = 5.3 K ( Figure 9b ). Both (a) and (b)had a dendritic morphology, each with a different arrangementof dendrites; for hydrate formed at low driving force, the den-drites were less densely packed. Hydrate formed at low sub-cooling exhibited fewer grain boundaries than hydrate formedat high subcooling. Similar behavior was observed in the otherreplicates and cycles for each subcooling.

Figures 10 and 11 present several cycles of methane hydrateformation at ∆Tsub = 3.9 K and ∆Tsub = 5.3 K, respectively.The hydrate films were formed in the presence of inhibitor A1and subjected to uniform surface temperature. For ∆Tsub = 3.9K (Figures 10 a-c), hydrate completely covered the droplet atdifferent times each cycle. From cycle 1 to cycle 3, the cover-age time ranged from 57 s to 84 s. Similar behavior was ob-served at ∆Tsub = 5.3 K (Figures 11 a-c) as the coverage timeranged from 27 s to 35 s between cycles. Following completehydrate coverage for both driving forces, an increase in cover-age time was observed as the cycles progressed. Also, coveragetime decreases as subcooling increases. Another effect of cy-cles observed on the hydrate morphology for both subcoolingscorresponds to fewer grain boundaries and amount of methanebubbles throughout the hydrate film from cycle to cycle. Fur-thermore, grain boundaries increase as subcooling increases.

Figure 12 shows methane hydrate formed at ∆Tsub = 5.3 Kon water droplets, in the presence of inhibitor A1 and subject touniform surface temperature. Figure 12a is the film morphol-ogy of experiment 2.1. Figure 12b and Figure 12c are replicatesof the hydrate formed at exact experimental conditions. Repro-ducible morphologies are obtained since the number of grain

5

∆TSub = 3.9 K

(a) (b) (c)

1 mm

t = 57 s t = 76 s t = 84 s

Cycle 1 Cycle 2 Cycle 3

Figure 10: Several cycles of methane hydrate formed at ∆Tsub = 3.9 K on a water droplet, in the presence of inhibitor A1 and subject to uniform surface temperature(a) First hydrate formation cycle, methane bubbles and grain boundaries are present throughout the hydrate film. The complete coverage of the droplet with hydrateis accomplished in 57 s. (b) Second hydrate formation cycle, fewer guest bubbles, and grain boundaries compared with cycle 1 (a). Bubbles can only be seen at thecenter of the hydrate film. The coverage time is 76 s. (c) Third hydrate formation cycle, methane bubbles are not present in the hydrate film, and only one grainboundary can be observed. The coverage time is 84 s. (a-c) TExp = 275.9 K, THLV = 279.8 K, ∆Tsub = 3.9 K, Pexp = 5.1 MPa, xA1 = 0.1 %.

1 mm

(a) (b) (c)

∆TSub = 5.3 K

t = 27 s t = 32 s t = 35 s

Cycle 1 Cycle 2 Cycle 3

Figure 11: Several cycles of methane hydrate formed at ∆Tsub = 5.3 K on a water droplet, in the presence of inhibitor A1 and subject to uniform surface temperature.(a) First hydrate formation cycle, methane bubbles and grain boundaries are present throughout the hydrate film. The complete coverage of the droplet with hydrateis accomplished in 27 s. (b) Second hydrate formation cycle, fewer guest bubbles compared with cycle 1 (a). Bubbles can only be seen at the center of the hydratefilm. The coverage time is 32 s. (c) Third hydrate formation cycle, methane bubbles are not present in the hydrate film and fewer grain boundaries compared withcycle 1 (a) and cycle 2 (b) are observed. The coverage time is 35 s. (a-c) TExp = 274.5 K, THLV = 279.8 K, ∆Tsub = 5.3 K, Pexp = 5.1 MPa, xA1 = 0.1 %.

6

1 mm

(a) (b) (c)

∆TSub = 5.3 K

Experiment 2.1 Experiment 2.2 Experiment 2.3

1 mm1 mm

Figure 12: Methane hydrate formed at ∆Tsub = 5.3 K on water droplets, in the presence of inhibitor A1 and subject to uniform surface temperature. Experiments 2.1,2.2 and 2.3 keep the exact experimental conditions as listed in Table 3. (a-c) correspond to the hydrate film formed the first cycle of the corresponding experiment.Crystal habit of replicates are very similar. In each experiment, grain boundaries are observed with the presence of bubbles in the center of the droplet. (a-c) Cycle1, TExp = 274.5 K, THLV = 279.8 K, ∆Tsub = 5.3 K, Pexp = 5.1 MPa, xA1 = 0.1 %.

boundaries was comparable between replicates and the pres-ence of methane bubbles was consistent between replicates.

2 mm

∆TSub = 5.3 K ∆TSub = 5.9 K

(a) (b)

2 mm

Figure 13: Methane hydrate formed on water droplets, in the presence of in-hibitor A1 (a) and an uninhibited system (b) (Ovalle et al., 2018) subject touniform surface temperature. (a) Hydrate that forms in the inhibited system at∆Tsub = 5.3 K presents a dendritic morphology with multiple grain boundariesfrom multiple initial growth points. (b) For hydrate that forms in an uninhibitedsystem at ∆Tsub = 5.9 K , no grain boundaries are observed and a uniform film,from a single growth point, covered the water droplet. Also, hydrate in a free-inhibitor system presents a smooth and shiny hydrate surface with small grains.(a) TExp = 274.5 K, THLV = 279.8 K, ∆Tsub = 5.3 K, Pexp = 5.1 MPa, xA1 = 0.1%. (b) TExp = 273.6 K, THLV = 279.5 K, ∆Tsub = 5.9 K, Pexp = 5.1 MPa

Figure 13 shows methane hydrate formed on water droplets,in the presence of inhibitor A1 (Figure 13a) and an uninhibitedsystem (Figure 13b) subject to uniform surface temperature.Hydrate formed in the inhibited system at ∆Tsub = 5.3 K pre-sented a dendritic morphology with multiple grain boundariesfrom multiple initial growth points, whereas, no grain bound-aries were observed for the uninhibited system at ∆Tsub = 5.9

K since only one single growth point covered the water droplet.The uniform radiating film formed in a free-inhibitor systemwas smoother and brighter with a smaller crystal size than theone that forms in the inhibited system under similar conditions.

3.1.2. Constant temperature gradientFigure 14 shows methane hydrate formed on a water droplet,

in the presence of inhibitor A1 and subject to constant gradienttemperature. Figure 14a presents the first hydrate formationcycle. Figure 14b shows the second hydrate formation cycle.A temperature difference of 6 K across the slide set differentsubcoolings across the droplet, from low driving forces as 0.4K to higher driving forces as 4.3 K, allowing the observation ofdifferent morphologies in the hydrate film.

It can be observed in Figure 14 that toward the low drivingforce side, the crystal size increased. Large faceted crystalsformed approximately from ∆TS ub = 0.4 K to ∆TS ub = 1.5 K.On the other side, hydrate film toward higher driving forcesfrom ∆TS ub = 1.6 K and ∆TS ub = 4.3 K exhibited a dendriticmorphology with numerous grain boundaries. The decrease ofgrain boundaries as subcooling decreases observed in the con-stant gradient temperature agreed with the results obtained atuniform surface temperature for ∆TS ub = 3.9 K and ∆TS ub = 5.3K (Figure 9). It can also be seen in Figure 14 that as hydrate for-mation cycles progressed, methane bubbles in the middle of thedroplet and grain boundaries for ∆TS ub = 1.6 K and higher sub-coolings decreased. This behavior is also comparable with theone at uniform surface temperature for both subcoolings (Fig-ure 10 and Figure 11).

Figure 15 shows a time sequence of methane hydrate agingon a water droplet, in the presence of inhibitor A1 and sub-ject to constant gradient temperature. The different subcoolings

7

∆T s

ub

/ K

1

2

3

4

Cycle 1 Cycle 2

275.8

T/ K

278.8

277.8

276.8

279.4

275.5

0.4

4.3 1 mm

(a) (b)

Figure 14: Methane hydrate formed on a water droplet, in the presence of inhibitor A1 and subject to constant gradient temperature. (a) First hydrate formationcycle. (b) Second hydrate formation cycle. (a-b) Different subcoolings across the droplet from ∆TS ub = 0.4 to ∆TS ub = 4.3 K. The hydrate film exhibits differentmorphologies in which the crystal size changes with the driving force. Crystallization of large faceted crystals toward low driving forces and dendritic crystalstoward higher driving forces. TTEM1 = 274.2 K, TTEM2 = 280.2 K, THLV = 279.8 K, ∆TS ub = 0.4 - 4.3 K, Pexp = 5.1 MPa, xA1 = 0.1 %

held across the droplet remained constant over time. Figure15a and Figure 15d corresponds to time when the droplet wascompletely covered with hydrate, t = 0 min, for the first andsecond hydrate formation cycle, respectively. Figure 15b andFigure 15e shows the hydrate aging after 10 minutes of hydrateformation for each cycle. In the same manner, Figure 15c andFigure 15f shows the hydrate aging but after 18 minutes of hy-drate formation. As time progressed in a time span of only 18minutes, the hydrate surface smoothed and grain boundaries, aswell as, differences in morphology observed for the differentdriving forces across the droplet, became less evident.

3.1.3. Hydrate growthFigure 16 illustrates over a time sequence and hydrate for-

mation cycle, the growth mechanism for methane hydrate for-mation at ∆Tsub = 5.3 K on a water droplet, in the presence ofinhibitor A1 and subject to uniform surface temperature. t = 0 s,time when the first observable hydrate crystallite appeared. t =

6 s, time when the amount of initial growth points in the dropletis observable. t = 12 s, time when the hydrate grows and t =

18 s, time for complete hydrate coverage of the inhibited watersurface for the first hydrate formation cycle.

Figure 16, (a-d) presents the growth mechanism for the firsthydrate formation cycle where methane bubbles were presentin the middle of the droplet. Numerous initial growth pointsformed on the periphery and within the inhibited water film canbe seen in Figure 16b. In Figure 16c, hydrate growth occurredfrom the periphery towards the center of the water film for ini-tial points formed on the periphery and growth towards the pe-riphery was observed for the initial points formed within thefilm. Besides, new growth sites appeared within the inhibited

water film. Finally, Figure 16d depicts the droplet completelycovered with a hydrate film.

Figure 16, (e-h) presents the growth mechanism for the sec-ond hydrate formation cycle where fewer methane bubbleswithin the droplet were present compared with the first cycle.Figure 16f displays fewer initial growth points formed than cy-cle 1. Hydrate growth pattern in Figure 16g is similar to cycle 1.In Figure 16h, hydrate had not covered completely the inhibitedwater film at 27 s.

Figure 16, (i-l) provides the growth mechanism for the thirdhydrate formation cycle where no guest bubbles were present inthe droplet. Fewer initial growth points than the first and secondhydrate formation cycle can be observed in Figure 16j. In thesame fashion as the previous cycles, a similar growth pattern isobserved in cycle 3 (16k); however, considerably less area wascovered with hydrate at that time. Lastly, Figure 16l, hydratehad not covered completely the inhibited water film at 27 s,and uncovered area with hydrate is significantly larger than theprevious cycle.

The reproducibility of the observed crystal growth behaviorin the inhibited system at high driving force is shown in Figure17. The first hydrate formation cycle is considered to evaluatethe replicability of the experiments. Consistently reproduciblegrowth mechanism was found as initial growth points of Fig-ure 17, (b,f,j) appeared on the periphery or within the inhibitedwater film in each replicate. Also,the hydrate growth patternand coverage time were similar between replicates. The samebehavior of growth mechanism was observed between experi-ments for the second and third hydrate formation cycle.

8

1 mm

t = 0 min t = 10 min

Cycle 1

Cycle 2

t = 18 min

1 mm

∆Tsub =4.3 K

∆Tsub =0.4 K

∆Tsub =4.3 K

∆Tsub =0.4 K

(a) (b) (c)

(d) (e) (f)

Figure 15: Time sequence of methane hydrate aging on a water droplet, in the presence of inhibitor A1 and subject to constant gradient temperature. t = 0 min,time when the droplet was completely covered with hydrate. (b-c) and (e-f), aging of the hydrate surface over 10 and 18 minutes, for the first and second hydrateformation cycle, respectively. After 18 minutes of hydrate formation, hydrate surface smooths and differences in morphology, as well as grain boundaries, becomeless evident. (a-f) THLV = 279.8 K, ∆TS ub = 0.4 - 4.3 K, Pexp = 5.1 MPa, xA1 = 0.1 %

9

Cycle 1

t =6 s t = 27 st = 0 s t = 12 s

Cycle 2

Cycle 3

∆TSub = 5.3 K

(a) (b) (c) (d)

(e) (f) (g) (h)

(i) (j) (k) (l)

1 mm

Figure 16: Growth mechanism for methane hydrate formation at ∆Tsub = 5.3 K on a water droplet, in the presence of inhibitor A1 and subject to uniform surfacetemperature. t = 0 s, time when the first observable hydrate crystallite appears. t = 6 s and t = 12 s, 6 and 12 seconds after the first observable hydrate crystalliteformed on the inhibited water film. t = 27 s, time for complete hydrate coverage of the inhibited water surface for the first hydrate formation cycle. (a-d), (e-h) and(j-l) growth mechanism for the first, second, and third hydrate formation cycle, respectively. (a), (e), and (i), decrease of guest bubbles from cycle to cycle. (b), (f), and (j), fewer initial growth points as cycles progressed.(c), (g), and (k), hydrate grows in a similar pattern.(d),(h) and (l), coverage time increases as the systemgoes through cycles. TExp = 274.5 K, THLV = 279.8 K, ∆Tsub = 5.3 K, Pexp = 5.1 MPa, xA1 = 0.1 %.

10

Experiment 2.1

t = 18-27 st = 12 s

Experiment 2.2

Experiment 2.3

∆TSub = 5.3 K

(a) (b) (c) (d)

(e) (f) (g) (h)

(i) (j) (k) (l)

1 mm

t = 6 st = 0 s

1 mm

1 mm

Figure 17: Reproducibility of growth mechanism for methane hydrate formation at ∆Tsub = 5.3 K on a water droplet, in the presence of inhibitor A1 and subjectto uniform surface temperature. t = 0 s, time when the first observable hydrate crystallite appears. t = 6 s and t = 12 s, 6 and 12 seconds after the first observablehydrate crystallite formed on the inhibited water film. t = 18-27 s, time span for complete hydrate coverage of the inhibited water film. Replicates follow a similargrowth mechanism. Initial growth points of (b), (f) and (j) appears on the periphery and within the inhibited water film. The hydrate growth and coverage time aresimilar between replicates. (a-l) Cycle 1, TExp = 274.5 K, THLV = 279.8 K, ∆Tsub = 5.3 K, Pexp = 5.1 MPa, xA1 = 0.1 %.

3.1.4. Hydrate dissociationFigure 18 shows the dissociation of the hydrate film under a

constant temperature gradient. It is possible to clearly observethe equilibrium interface at every dissociation step. In addition,it was possible to observe propagation of the hydrate surfaceoutside the original water boundary of the droplet. This phe-nomenon is known as halo (Beltran and Servio, 2010). Figure18a showed a primary halo towards the cold side. In Figure18, (b-f) it seems that two growing halos, a primary translu-cent halo and a secondary dark halo, towards the cold side ap-peared. Upon complete dissociation of the hydrate film, the wa-ter droplet took the form of the secondary halo and what seemedto be a primary halo remained on the shapphire substrate.

Figure 19 shows the dissociation of the hydrate film under auniform surface temperature.

3.2. Phase EquilibriaFigure 20 compares the equilibrium temperatures obtained

for A1 between the uninhibited system studied by Kumar at the

same conditions as this study. Both constant gradient dissocia-tion and uniform temperature dissociation results are shown.

When dissociation is performed under a constant temperaturegradient, the THLV is calculated by measuring the hydrate-liquidinterface distance respect to the cold side of the stage. De-tail of the calculations are described by DuQuesnay, Diaz and,Beltran, see reference DuQuesnay et al. (2016). The gradientacross the slide used for the calculations is the one determinedby (Ovalle et al., 2018), with a value of 0.41 K· mm−1with anuncertainty of ug =0.03 K· mm−1 as it was determined in thesame stage used for this study.

Equilibrium temperatures obtained for inhibitor A1 usingconstant gradient dissociation over 5 dissociation steps can beseen in Figure 20. The values for the THLV ranged from 280.0K to 280.5 K between steps.

Equilibrium temperature using uniform surface temperaturedissociation, the THLV = 280.3 K with an uncertainty of 0.01 Kfrom three replicates of dissociation.

Figure 21 is a partial phase diagram for the system CH4 +

11

THLV = 280.3 K THLV = 280.2 K THLV = 280.1 K THLV = 280.0 K THLV = 280.0 K

(a) (b) (c) (d) (e) (f)

1 mm

(g)

Figure 18: Dissociation steps at constant temperature gradient of methane hydrate in the presence of inhibitor A1. (a) methane hydrate before dissociation, growinghalo toward the cold side. (b-f) dissociation steps, THLV present slightly changes every step, growing halo and a traslucent layer toward the cold side. (g) Completedissociation of the methane hydrate. (a-f) Pexp = 5.1 MPa, xA1 = 0.1 %.

TTEM : 280.0 K

TTEM : 280.1 K TTEM : 280.4 K TTEM : 280.0 K

TTEM : 274.2 K TTEM : 279.7 K

(a) (b) (c)

(d) (e) (f)

1 mm

Figure 19: Hydrate dissociation at uniform surface temperature. (a-f) Pexp =

5.1 MPa, xA1 = 0.1 %.

279.0

279.5

280.0

280.5

1 2 3 4 5 Uniform

T HLV

/ K

Dissociation Step

Uninhibited (G) Uninhibited (U) A1 (G) A1 (U)

Figure 20: Phase equilibria temperatures N. (2016)

water. The equilibrium temperature determined using a uniformsurface for each inhibited system is plotted with its uncertaintiescalculated from the results of three replicates.

3.3. KineticsHydrate film growth velocity was used as a measure of ap-

parent growth kinetics. In Figure 22, film velocity of the ad-vancing hydrate fronts as a function of subcooling can be seen.Lower film growth rates were observed in the presence of in-hibitor A1 for subcoolings greater than 3 K, compared to purewater-methane system reported in the literature (N., 2016). For

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

277.0 278.0 279.0 280.0 281.0

P/

MPa

T / K

Lit. Data Uninhibited

A1

Lit. Regression

95% Prediction Interval

Figure 21: Partial phase diagram of the water - methane system N. (2016)

lower subcoolings that 3 K, the system acted as a system with-out inhibitor.

The film velocity calculated for the high driving force andlow driving force under a uniform surface temperature had anuncertainty of 0.97 µmm· s−1 and 5.8 µmm· s−1, respectively.

The film velocities under a constant gradient temperaturewere determined following the calculations of DuQuesnay et al.(2016)

Figure 23 and 24 present the film velocity for each cycle atlow driving force and high driving force, respectively.

12

0

50

100

150

200

250

300

350

400

450

500

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

v/µ

m·s

-1

ΔTsub /K

Uninhibited methane system (Uniform) (Kumar,2016)

Uninhibited methane system (Gradient) (Kumar,2016)

w(Inh. A1) = 0.1% (Gradient) (Bonilla,2018)

w(Inh. A1) = 0.1% (Uniform) (Bonilla,2018)

Figure 22: Methane hydrate film velocity as a function of subcooling. Pexp =

5.1 MPa, xInhibitors = 0.1 %.

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4

v/µ

m·s

-1

Cycle

Experiment 1.1

Experiment 1.2

Figure 23: Film velocity of the advancing hydrate front on the inhibited watersurface per cycle at low driving force using uniform surface temperature. TExp= 275.9 K, THLV = 279.8 K, ∆Tsub = 3.9 K, Pexp = 5.1 MPa, xA1 = 0.1 %.

50

60

70

80

90

100

110

0 1 2 3 4

v/µ

m·s

-1

Cycle

Experiment 2.1

Experiment 2.2

Experiment 2.3

Figure 24: Film velocity of the advancing hydrate front on the inhibited watersurface per cycle at high driving force using uniform surface temperature. TExp= 274.5 K, THLV = 279.8 K, ∆Tsub = 5.3 K, Pexp = 5.1 MPa, xA1 = 0.1 %

.

13

4. Discussion

4.1. Hydrate morphology

A specific crystal morphology is a dependant on thermody-namics as well as heat and mass transfer. Therefore, differ-ent morphologies may be obtained as a result of variations inthe experimental conditions DuQuesnay et al. (2016). Sincethe technique provided tight control of the system conditions,reproducible morphologies and growth mechanisms were ob-tained for the inhibited system at a specific driving force as itis shown in Figure 12 and Figure 17. For 3 experiments, ∆TS ub

= 5.3 K, at their first hydrate formation cycle, grain boundariesand methane bubbles in the center of the droplet were foundto be consistently reproducible. Also, multiple initial growthpoints and similar hydrate growth pattern were consistent be-tween replicates.

In Figure 17, methane bubbles already present in the dropletbefore the first hydrate formation cycle for each replicate, givesevidence of the undissolved methane once the conditions oftemperature and pressure are within the hydrate stability regionwithout previous formation. Figure 16, Figure 10 and Figure11 show a decrease in these guest bubbles in the hydrate filmas cycles progressed being an independent feature of the sub-cooling. Increasing dissolved methane from cycle to cycle is aplausible explanation for this phenomenon.

Fewer initial growth points formed next to the guest bub-bles agreed with less amount of bubbles as cycles progressed.Hence, one explanation of the reduction of these initial growthsites could be that the existence methane bubbles in the liq-uid phase is affecting the quiescent of the system inducing thenucleation of crystals in various sites and then various initialgrowth points on the film. However, decrease for the initialgrowth points formed on the periphery as cycles progressedcould be the result of more interference of the inhibitor A1 tothe nucleation process every reformation.

Since the decrease of grain boundaries as subcooling de-creases observed in the constant gradient temperature experi-ments followed the same results obtained at uniform surfacetemperature for ∆TS ub = 3.9 K and ∆TS ub = 5.3 K, it can besaid that the ’3-in-1’ technique effectively provides tigh controlof the temperature across the droplet.

A dendritic hydrate film morphology was observed for theinhibited system in both uniform surface temperature and con-stant gradient temperature. A dendritic morphology indicatesthe existence of transport limitations Beltran and Servio (2010).Since film velocities were lower than the ones of the uninhib-ited system at subcoolings greater than 3 K and dendrites wereobserved around the same subcoolings, the inhibitor is poten-tially affecting the diffusion of methane to the already formedcrystals. Therefore, a reduced mass transfer at H-L could bean explanation for this phenomenon. Posteraro has studied theeffect of KHIs on the methane former hydrate in the bulk liquidduring the growth period and has demonstrated that the pres-ence of KHIs decreases the total surface area of reaction dueto adsorption to crystals. This interference is then decreasingthe diffusion of hydrate former to the crystals. Posteraro et al.(2015)

Aging of methane hydrate on a water droplet, in the pres-ence of inhibitor A1 and subject to constant gradient tempera-ture was observed in Figure 15. Regarding aging of methanehydrate in an unhibited system studied by Beltran and Servio(2010); after 20 to 72 h of methane hydrate formation, the hy-drate film smoothed. Aging of methane hydrate under inhibi-tion of A1 could be considered as fast aging since the hydratesurface rapidly started to smooth and differences in morphologybecame less evidents over a time span of 18 minutes. One pos-sible explanation to this surface phenomena could be that as theinhibitor A1 seems to be a surfactant based on its approximatecomposition, it might accelerate the reduction of energy of thesystem. This is a special feature that this system presents as noother inhibitors studied in the group have this behavior.

The growth mechanism presented in Figure 14 of the inhib-ited system at high driving force under a uniform surface tem-perature followed specific trends. Two of them were the de-crease of guest bubbles and initial growth points as cycles pro-gressed. Also, hydrate grew in a similar pattern each cycle.While hydrate grew from the periphery towards the center ofthe water film for initial points formed on the periphery, growthtowards the periphery was observed for the initial points formedwithin the film.

4.2. Phase Equilibria

The 3-in-1 technique allowed the measurement of the equi-librium temperature at the experimental pressure with an uncer-tainty of 0.03 K.

There is not a significant effect of step in the THLV since thevalues for it ranged from 280.0 K to 280.5 K between stepsof the constant gradient dissociation. The possibility of the in-hibitor to be a THI is discarded since the THLV is not lower thanthe one for the uninhibited system, which would mean a shiftin the equilibrium conditions to lower temperatures and higherpressures. Besides, for the step that THLV = 280.0 K, this valueis within the 95% prediction interval for an uninhibited system,see Figure 21. Therefore, the inhibitor can potentially be a KHIas it is not changing the equilibrium conditions of the water -methane system.

From equilibrium temperature resuts using uniform surfacetemperature dissociation, the THLV = 280.3 K with an uncer-tainty of 0.01 K. However, the instrumental uncertainty is 0.03K. In this manner, the THLV can be within the 95% predictioninterval for an uninhibited system, see Figure 21. Both tech-niques confirms that the inhibitor is a KHI.

4.3. Kinetics

Figure 11 and 10 present an increased coverage time of hy-drate on the droplet as cycles progressed. This phenomenonmight be because of an increase in the film velocity or fewer ini-tial growth points formed each cycle. Figure 24 and 23 showsthat film velocity remained approximately constant between cy-cles. Thus, the cause to an increase in coverage time was thedecrease in initial growth points from cycle to cycle as images(b), (f) and (j) from Figure 16 proves it.

14

5. Conclusions

The 3-in-1 technique had the ability to assess a commer-cial hydrate inhibitor in terms of morphology, phase equilib-ria, and kinetics. The technique allowed the reproducibility ofmorphologies and growth mechanisms of hydrate films formedunder both uniform surface temperature and constant gradienttemperature. In addition, it was possible to calculate film veloc-ities at different subcoolings with the constant gradient temper-ature and obtain film velocities uncertainties at a specific sub-cooling from 0.97 µm· s−1 and 5.8 µm· s−1. Finally, the equilib-rium temperature measured with the technique under uniformsurface temperature had an uncertainty of 0.03 K. In conclu-sion, the 3-in-1 technique is a suitable tool to assess gas hy-drates inhibitors.

6. Acknowledgments

I am grateful to my parents Carlos and Sandra for mak-ing this experience possible and for their endless love; to myboyfriend Omar for his great love and support. My advisorDr. Juan Beltran has my gratitude for his continuous guidanceduring my research and for giving me lessons for my profes-sional life. I would like to thank Sebastian, Andres, and Camilofor their help and friendship. Financial support was providedby the Natural Sciences and Engineering Research Council ofCanada, The Canadian Foun- dation for Innovation and RMCC.I thank the Chemistry and Chemical Engineering Department atRMCC for welcoming me to their facilities and for cleri- cal andtechnical support. I would also like to thank the Universidad deLos Andes for allowing me to complete my undergraduate the-sis at RMCC.

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