(HLB) Index Non-Ionic Surfactants on Cyclopentane Hydrates

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Article Rheology Impact of Various Hydrophilic-Hydrophobic Balance (HLB) Index Non-Ionic Surfactants on Cyclopentane Hydrates

Khor Siak Foo 1,2, Cornelius Borecho Bavoh 1,2 , Bhajan Lal 1,2,* and Azmi Mohd Shariﬀ 1,2 1 Chemical Engineering Department, Universiti Teknologi Petronas, Seri Iskandar, Teronoh, Perak 32610, Malaysia; [email protected] (K.S.F.); [email protected] (C.B.B.); [email protected] (A.M.S.) 2 CO2 Research Centre (CO2RES), Universiti Teknologi Petronas, Perak 32610, Malaysia * Correspondence: [email protected]; Tel.: +60-53687619

Academic Editor: Nobuo Maeda Received: 29 May 2020; Accepted: 3 July 2020; Published: 15 August 2020 Abstract: In this study, series of non-ionic surfactants from Span and Tween are evaluated for their ability to affect the viscosity profile of cyclopentane hydrate slurry. The surfactants; Span 20, Span 40, Span 80, Tween 20, Tween 40 and Tween 80 were selected and tested to provide different hydrophilic–hydrophobic balance values and allow evaluation their solubility impact on hydrate formation and growth time. The study was performed by using a HAAKE ViscotesterTM 500 at 2 ◦C and a surfactant concentration ranging from 0.1 wt%–1 wt%. The solubility characteristic of the non-ionic surfactants changed the hydrate slurry in different ways with surfactants type and varying concentration. The rheological measurement suggested that oil-soluble Span surfactants was generally inhibitive to hydrate formation by extending the hydrate induction time. However, an opposite effect was observed for the Tween surfactants. On the other hand, both Span and Tween demonstrated promoting effect to accelerate hydrate growth time of cyclopentane hydrate formation. The average hydrate crystallization growth time of the blank sample was reduced by 86% and 68% by Tween and Span surfactants at 1 wt%, respectively. The findings in this study are useful to understand the rheological behavior of surfactants in hydrate slurry.

Keywords: clathrate hydrates; surfactants; span; tween; cyclopentane; rheology

1. Introduction Clathrate hydrates—also known as gas hydrates—are solid compounds that form when a suitable size substance (gas/liquid) is encapsulated in a 3-dimensional network water cage held together through hydrogen bonding [1–3]. The encapsulating substances are typically light hydrocarbons or small gaseous molecules such as methane, ethane, propane, nitrogen and carbon dioxide. The presence of these small substances known as hydrate formers stabilizes the gas hydrate structure by a weak van der Waals force. High pressure and low ambient temperature favor and enhance gas hydrate formation and stability [1,4]. Depending on the arrangement of the water molecules constructing the hydrate crystals network, hydrates structures can be classified as Type I and Type II—sometimes referred to as Structure I and Structure II. A third type of hydrate that also may be encountered is Type H (or Structure H), but its occurrence is much less common in natural environment. The structure of the Type II hydrates is significantly more complicated than that of the Type I hydrate. The Type II hydrates can be formed from two different types of cage. The small cage of Type II structure is a dodecahedron, a twelve-sided polyhedron where each face is a regular pentagon, whereas the structure of big cage has a hexakaidecahedron, a sixteen-sided polyhedron with twelve pentagonal faces and four hexagonal faces as shown in Figure1.

Molecules 2020, 25, 3725; doi:10.3390/molecules25163725 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 14 pentagon,Molecules 2020 whereas, 25, 3725 the structure of big cage has a hexakaidecahedron, a sixteen-sided polyhedron2 of 14 with twelve pentagonal faces and four hexagonal faces as shown in Figure 1.

(a) (b)

Figure 1. Type II hydrate structures of small cage and large cage. (a) Dodecahedron 12-sided polyhedron Figure(small cage);1. Type (b )II hexakaidecahedron hydrate structures 16-sided of small polyhedron cage and (largelarge cage).cage. (a) Dodecahedron 12-sided polyhedron (small cage); (b) hexakaidecahedron 16-sided polyhedron (large cage). Clathrate hydrates pose major operational threats to oil and gas flow assurance [5]. The increase of deep-waterClathrate oil hydrates and gas pose production major operational poses severe threats hydrate to oil and threats gas flow due assurance to high pressure [5]. The increase and low oftemperature deep-water conditionsoil and gas of production multiphase poses fluid severe flow hydrate in subsea threats production due to wellshigh pressure and pipelines and low [6]. temperatureHydrate formation conditions in pipelines of multiphase may cause fluid economic flow in andsubsea safety production risks due wells to unplanned and pipelines shutdown [6]. Hydrateand high formation cost of removal in pipelines [1]. The may current cause conventional economic and approaches safety risks to hydratedue to unplanned plug prevention shutdown is the anduse high of chemical cost of removal inhibitors [1]. [7 –The16]. current However, convention thermodynamical approaches inhibitors to hydrate such as plug methanol prevention and glycols is the usehave of limitationschemical inhibitors of effectiveness [7–16]. However, and environmental thermodynamic concerns inhibitors [17]. Thus, such eff asorts methanol have been and shifted glycols to havemanage limitations hydrate of formation effectiveness via the and use environmental of low dosage concerns hydrate inhibitors[17]. Thus, (LDHI), efforts whichhave been include shifted the useto manageof anti-agglomerate hydrate formation low dosage via the hydrate use of inhibitor low dosage (AA-LDHI) hydrate [18 inhibitors–22]. This (LDHI), class of which inhibitors include generally the useprevent of anti-agglomerate hydrate agglomeration low dosage and aid hydrate the transportation inhibitor (AA-LDHI) of hydrate [18–22]. as slurry This [23]. class Therefore, of inhibitors a critical generallyunderstanding prevent of thehydrate rheological agglomeration properties ofand hydrate aid the slurry transportation in the presence of ofhydrate AA-LDHI as slurry is needed [23]. to Therefore,efficiently a manage critical hydrateunderstanding formation of the in subsearheologica facilityl properties design and of hydrate operations. slurry in the presence of AA-LDHIIt has is been needed reported to efficiently that cationic manage surfactants hydrate formation are effective in subsea anti-agglomerant facility design low and dosage operations. hydrate inhibitorIt has (AA-LDHI)been reported [24 that,25]. cationic These surfactantssurfactants are control effective hydrate anti-agglomerant growth by dispersing low dosage the hydrate hydrate inhibitorcrystals that(AA-LDHI) form at water-hydrocarbon[24,25]. These surfactants interface co intontrol discrete hydrate suspended growth by particles dispersing without the allowinghydrate crystalsthem to that adhere form and at agglomeratewater-hydrocarbon into big interface particles, into so to discrete remain suspended in a flowable particles and pumpable without multiphase allowing themstream. to Theadhere adsorption and agglomerate of the cationic into surfactant big particles, molecules so to onto remain the hydrate in a flowable particles surfaceand pumpable enhances multiphasethe oil wetting stream. characteristic The adsorption of the hydrateof the cationic particles surfactant so they becomemolecules part onto of the the hydrocarbon hydrate particles phase surfaceand agglomeration enhances the of oil the wetting discrete characteristic hydrate particles of the is inhibitedhydrate particles [25]. so they become part of the hydrocarbonOn the otherphase hand,and agglomeration anionic surfactants of the discrete promote hydrate hydrate particles formation is inhibited by participating [25]. partly in the gasOn the hydrate other clathrate hand, anionic formation surfactants causing promote reduction hydrate in adhesion formation forces by participating among hydrate partly molecules in the gasallowing hydrate for clathrate a larger particleformation surface causing area reduction for hydrate in adhesion growth [26 forces]. It hasamong also beenhydrate proposed molecules that allowingsome anionic for a larger surfactants particle such surface as sodium area for dodecyl hydrate sulfate growth (SDS) [26]. and It has linear also alkyl been benzene proposed sulfonic that some acid anionic(LABSA) surfactants accelerate such hydrate as sodium growth dodecyl through sulfate the formation (SDS) and of hydrophobiclinear alkyl benzene micro-domains sulfonic within acid (LABSA)vicinity ofaccelerate the hydrate hydrate surface growth that increases through gasthe concentrationformation of andhydrophobic intake rate micro-domains for hydrate formation. within vicinityThere are of the substantial hydrate studiessurface onthat the increases morphology gas concentration of surfactants and on intake hydrate rate inhibition, for hydrate the formation. rheological Therebehavior are substantial of surfactants studies in hydrate on the inhibitionmorphology is not of surfactants well understood. on hydrate inhibition, the rheological behaviorRecent of surfactants morphology in hydrate study has inhibition reported is some not well non-ionic understood. surfactants such as Span 80 and Tween 65 asRecent potential morphology additives study to prevent has reported hydrate some formation. non-ionic However, surfactants limited such workas Span has 80 been and doneTween to 65understand as potential the additives effects of to non-ionicprevent hydrate surfactant format concentrationsion. However, on limited hydrate work formation has been and done growth. to understandIn addition, the their effects rheological of non-ionic behaviors surfactant have not co beenncentrations well studied on hydrate in literature formation [27]. Depending and growth. on theIn addition,molecular their structure rheological and characteristics, behaviors have adsorption not been well of non-ionic studied in surfactants literature [27]. would Depending alter the surfaceon the molecularenergy and structure wetting and behavior characteristics, of gas hydrate adsorption particles. of Thisnon-ionic interaction surfactants could changewould thealter hydrate the surface slurry energyrheology and and wetting potentially behavior delay of or gas promote hydrate the partic rate ofles. gas This hydrate interaction formation. could In addition,change the the hydrate effect of slurrytheir concentrationrheology and potentially and HLB values delay onor promote hydrate slurrythe rate is of not gas fully hydrate known. formation. Hence itIn is addition, encouraging the effectto test of the their rheological concentration behavior and of HLB hydrates values in on the hydrate presences slurry of non-ionic is not fully surfactants. known. InHence this study,it is encouragingthe effect of Spanto test and the Tween rheological surfactant behavior on cyclopentane of hydrates hydratein the presences slurry is evaluated of non-ionic in detail surfactants. at varying In thissurfactant study, concentrations.the effect of Span and Tween surfactant on cyclopentane hydrate slurry is evaluated in detail at varying surfactant concentrations.

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1.1. Cyclopentane as Hydrate Former Many gas hydrate studies involve using liquid cyclopentane as hydrate former to test the performance of new compounds [27–33]. This compound could form hydrate crystals at atmospheric pressureMolecules 2020 mode, 25, with x FOR su PEERfficient REVIEW subcooling to overcome the difficulties in testing the rheological behavior3 of 14 of hydrate slurry at high pressure conditions [27]. Therefore, adopting this hydrate model system to1.1. represent Cyclopentane real as gas Hydrate hydrate Former is considered practical and valid due to the low water solubility of the cyclopentaneMany gas hydrate mimicking studies interaction involve ofusing hydrocarbon liquid cyclopentane gas presence as inhydrate water. former More important,to test the cyclopentaneperformance of hydrate new compounds is known to [27–33]. form a This structure compou II (sII)nd could hydrate form which hydrate is one crystals of the mostat atmospheric common hydratepressure crystal mode structure with sufficient found naturallysubcooling [27 ].to overcome the difficulties in testing the rheological behaviorThe sizeof hydrate and structure slurry at of high cyclopentane pressure condit lendsions itself [27]. to e ffiTherefore,cient space adopting filling this of structure hydrate IImodel (sII) cavities,system to as represent shown by real Figure gas2 hydrate. Cyclopentane is considered is at the practical upper sizeand limitvalid for due molecules to the low that water promote solubility the formationof the cyclopentane of sII hydrates, mimicking but it caninte alsoraction form of sHhydrocarbon with helper gas molecules presence such in water. as methane More toimportant, stabilize thecyclopentane structure by hydrate filling is in known small cavities to form [a1 ,structure4]. It was II not (sII) until hydrate 2001 thatwhich Fan is etone al of [30 the]. first most confirmed common cyclopentanehydrate crystal can structure form gas found hydrates naturally in the [27]. absence of any helping-gas.

FigureFigure 2.2. CyclopentaneCyclopentane hydrate.hydrate. HH22O molecules in red cage the guest hydrocarbon moleculemolecule inside.inside.

1.2. SPANSThe size and and TWEENS structure Surfactants of cyclopentane lends itself to efficient space filling of structure II (sII) cavities,Spans as andshown Tweens by Figure are non-ionic 2. Cyclopentane surfactants is at that the are upper used size as emulsifying limit for molecules agents in that the promote preparation the offormation stable emulsion of sII hydrates, systems but for it manycan also applications form sH with [34 ].helper They molecules are also frequentlysuch as methane used into varyingstabilize combinationthe structure proportions by filling in to small produce cavities desired [1,4]. emulsification It was not until stability 2001 to that target Fan di etfferent al [30]. types first of confirmed oil–water systems.cyclopentane Being can non-ionic form gas in hydrates nature, these in the surfactants absence of have any nohelping-gas. inherit ionic interaction affinity and are widely compatible and stable in many fluid systems including fresh water, saline water, mild acids, alkaline1.2. SPANS and and do TWEENS not react withSurfactants ionic ingredients and charged substances. Which makes them good candidatesSpans forand hydrate-based Tweens are applications.non-ionic surfactants Spans are sorbitanthat are estersused thatas emulsifying are manufactured agents from in thethe dehydrationpreparation of of stable sorbitol emulsion through systems esterification for many reaction applications with fatty [34]. acids. They Theare also size frequently and length used of the in fattyvarying acid combination molecules used proportions would decide to produce the hydrophilic–hydrophobic desired emulsification stability balance (HLB)to target and different the solubility types ofof theoil–water surfactant systems. in polar Being and non-io nonpolarnic in solvent.nature, these Further surfactants esterification have no with inherit fatty ionic acids interaction results in polyestersaffinity and [35 are]. The widely HLB compatible value decreases and withstable increasing in many degreesfluid systems of esterification including and fresh size water, of the saline fatty acidwater, molecules. mild acids, Low alkaline HLB Spans and do favor not solubilityreact with in ionic lipophilic ingredients solvent and to producecharged stablesubstances. water-in-oil Which emulsionmakes them system good [35 candidates]. for hydrate-based applications. Spans are sorbitan esters that are manufacturedOn the other from hand, the dehydration Tweens are producedof sorbitol fromthrough ethoxylation esterification of Spansreaction to with increase fatty HLB acids. range The andsize enhanceand length hydrophilic of the fatty solubility acid molecules to yield used stable would oil-in-water decide the emulsion hydrophilic–hydrophobic system [36]. Tweens balance are soluble(HLB) and or dispersible the solubility in water of the and surfactant dilute solutions in polar of and electrolytes. nonpolar Thesolvent. solubility Further of Tweens esterification in aqueous with solutionfatty acids increases results in with polyesters the degree [35]. of The ethoxylation. HLB value decreases However, with for a increasing fixed degree degrees of ethoxylation of esterification and esterification,and size of the aqueous fatty acid solubility molecules. decreases Low HLB as the Span sizes andfavor molecular solubility weight in lipophilic of the fattysolvent acid to increases. produce Spansstable derivedwater-in-oil from emulsion unsaturated system and [35]. branded chain fatty acids act as effective water-in-oil emulsifiers. It is believedOn the other that thehand, above Tweens stated are properties produced of from Span ethoxylation and Tween presentsof Spans themto increase as suitable HLB inhibitorsrange and forenhance gas hydrate hydrophilic management solubility [28 to,29 yield]. Therefore, stable oil- toin-water efficiently emulsion evaluation system the e [36].ffect ofTweens Span andare soluble Tween onor hydratedispersible formation, in water Span and and dilute Tween solutions with di ffoferent electrolytes. HLB values The are solubility selected andof Tweens tested in in this aqueous work. Furthermore,solution increases all the with selected the degree Span and of ethoxylation. Tween surfactants However, are tested for a atfixed diff degreeerent concentrations. of ethoxylation and esterification, aqueous solubility decreases as the size and molecular weight of the fatty acid increases. Spans derived from unsaturated and branded chain fatty acids act as effective water-in-oil emulsifiers. It is believed that the above stated properties of Span and Tween presents them as suitable inhibitors for gas hydrate management [28,29]. Therefore, to efficiently evaluation the effect of Span and Tween on hydrate formation, Span and Tween with different HLB values are selected and tested in this work. Furthermore, all the selected Span and Tween surfactants are tested at different concentrations.

1.3. Sorbitan Ester and Derivatives for Cyclopentane Hydrate Control

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1.3. Sorbitan Ester and Derivatives for Cyclopentane Hydrate Control Delgodo-Linares et al. [37] evaluated the stability of water-in-oil emulsion from mineral oil 70 T and deionized water in the presence of surfactants of non-ionic Span 80 and anionic AOT (di-2-ethylhexylsulfosuccinate). Bottle testing in laboratory at ambient pressure showed that all the emulsion samples prepared in 1% and 5 wt% surfactants were stable against coalescence for at least one week. From the microscopy images of the emulsions, the average water droplet size ranges from 2 to 3 µm. Rheological measurements showed that the viscosity of the emulsion increased with increasing water cut. It was also observed that these emulsions showed shear thinning behavior above certain water cuts (30 vol% at 25 ◦C and 20 vol% at 1 ◦C). Differential scanning calorimetry (DSC) measurements showed that the emulsions prepared with 5 wt% surfactant with less than 50 vol% water were the most stable upon hydrate formation/dissociation. A further micromechanical force (MMF) measurements revealed that the presence of the surfactant mixture of Span 80 and AOT has little impact in the cohesion force of hydrate particles, although a change in hydrate surface morphology was observed. Autoclave cell experiments showed two different types of hydrate formation morphology; large hydrate chunks for the system without surfactant and loose hydrate slurry of the system with surfactant. In addition, Liua et al. [38] studied the interaction between cyclopentane hydrate particles and water droplets using a micromechanical force apparatus [6]. This unique apparatus setup could provide direct measurements of the interaction force between two particles surface in contact while any morphology change in hydrate particles is observed and recorded using an inverted light microscope. The effects of contact time, sub-cooling temperature, contact area and the addition of Span 80 on the water droplet–hydrate particle interaction force were reported. The results indicated that longer contact time and higher sub-cooling lead to more water converting to hydrate, which increased the hydrate particle–water droplets interaction force significantly. The interaction force also increased with hydrate–water droplet contact area. The adsorption of Span 80 induced a morphologic change on the hydrate surface, where hair-like extrusions extended from the surface into the cyclopentane bulk phase. Addition of Span 80 after formation of a hydrate shell appeared to weaken interfacial connection/junction between surface crystals. Internal water then flowed out to contact the external cyclopentane, leading to rapid hydrate formation and consequently an irregular surface growth. Meanwhile, the presence of Span 80 on both the hydrate and water interfaces can form adsorption layers with stronger mechanical strength. This provides more stable interface again particle coalescence, thereby hindering the formation of water bridges. This postulated that Span 80 can prevent hydrate agglomeration. The effects of hydrophilic–lipophilic balance (HLB) number for series of non-ionic sorbitan monoester on equilibrium condition of cyclopentane hydrate in water-in-oil emulsion was reported by Baek et al [31]. The performance of Span 20, Span 40, Span 60 and Span 80 to control formation of cyclopentane hydrate was compared for their effects on droplet size distribution and hydrate dissociation temperature using optical microscope and micro differential scanning calorimetry method. It was found that the mean size of emulsion droplets and the equilibrium temperature of cyclopentane hydrates was proportion to their HLB index. However, the variations of hydrate equilibrium temperature and emulsion size with the HLB index was practically eliminated with the addition of NaCl except for the case of Span 60. The equilibrium temperature of cyclopentane with sorbitan Span 80 and 3.5% NaCl was reduced from 4.75 ◦C to 5.5 ◦C for cyclopentane hydrate with water only. It was believed that the hydrophobic tails of the sorbitan monoester act as a physical barrier to interrupt the coalescence of emulsion droplets and reduced the hydrate dissociate temperature. In case of Span 60, unbalanced activity of the surfactant can be induced by a strong interaction between water and salt ions. However, the rheology of cyclopentane hydrates in the presence of the spans were not evaluated. Karanjkar et al. [27] revealed the rheological properties of cyclopentane hydrate in a water-in-oil emulsion system. Span 80 was studied for its effect to render the effect of viscosity increased during cyclopentane hydrate formation [27]. Liquid cyclopentane hydrate slurry was prepared at atmospheric pressure from a water-in-oil emulsion by quenching it to a lower temperature at a fixed shear rate. It was observed that viscosity increased by several orders of magnitude indicating formation of hydrate Molecules 2020, 25, 3725 5 of 14 on the dispersed water droplets and subsequent agglomeration. They attributed the observation to formation of a hairy and porous hydrate growth combined with enhanced agglomeration due to liquid bridges formed by wetted water films. The development of void space as water converted to hydrate within the hydrate structure network contributed to greater effective dispersed phase fraction that increased the viscosity dramatically. An observed dependence of the effective viscosity of the hydrate slurry on the degree of sub-cooling, with lower slurry viscosity obtained at higher sub-cooling. The possible anti-agglomerate like effect of high Span 80 concentrations, which led to a lower viscosity, was due to the availability of excess amount of surfactant in the oil phase which readily and efficiently adsorbed on the hydrate–oil interface and thus preventing a strong interaction between hydrate particles. Recently, Dann and Rosenfeld [26] studied the hydrate inhibitive performance of Span 20, Span 80, Pluronic L31 and Tween 65 at 2 ◦C, using cyclopentane hydrate and deionized water. He observed morphology shift in crystallization from planar shell growth to conical growth in surfactants. Monitoring the internal pressure of a droplet undergoing planar hydrate crystallization in a hydrate-visualization cell provided a strong correlation of decreasing interfacial tension to the shrinking area of the water–cyclopentane interface. Hydrate growth rate reduced most by Tween 65 (at 0.15 g/100 mL) from 0.59 mm2/min to 0.068 mm2/min (of pure water), nearly an order of magnitude slower than that found for pure water at 0.590 mm2/min. High molecular weight (1845 g/mol) and HLB (10.5) contributed to a large energy of desorption at an interface and are believed to be the sources of Tween 65 hydrate inhibiting properties. They further suggested that hydrate morphology is affected by the type and concentration of surfactants. However, the rheological analysis of surfactants type and concentrations are not well understood and studied in literature. Most hydrate slurry rheology studies are limited to Span 80. There are few studies in open literature that employ rheological approach to study hydrate slurry behavior in the presence of surfactants. In particular, tween-based surfactants. In addition, understanding the effect of surfactants concentration on hydrate slurry is necessary to provided effect guidelines in their practical applications. Therefore, in this article, the rheological slurry of cyclopentane hydrate is evaluation in the presence of three series of Span and Tween surfactants at different concentrations.

2. Materials and Methods

2.1. Materials and Sample Preparation The list of chemicals used in this work is tabulated in Table1. They were used as supplied, without any further purification. The selection of Span and Tween surfactants was based on their HLB values. Allowing an effective evaluation on the impact of the surfactants to enhance the interfacial interaction between gas phase (hydrate former), water phase and solid hydrate particles. The effect of Span and Tween on cyclopentane hydrates were tested at a concentration range of 0.1 wt% to 1.0 wt%.

Table 1. List of chemicals used in this study.

Density @ 25 ◦C Viscosity @ 25 ◦C 1 Chemical HLB 1 MW (gmol− ) Supplier (gcm− ) (cp) Deionized – 1.0 0.890 18.12 Self water Cyclopentane – 0.751 0.419 70.1 Sigma-Aldrich Span 20 8.6 1.032 1000–2000 346.46 Sigma-Aldrich Span 40 6.7 1.075 1000–2000 402.565 Sigma-Aldrich Span 80 4.3 0.986 1200–2000 428.6 Sigma-Aldrich Tween 20 16.7 1.1 370–430 1228 Sigma-Aldrich Tween 40 15.6 1.05–1.10 400–600 1283.63 Sigma-Aldrich Tween 80 15.0 1.06–1.09 300–500 1310 Sigma-Aldrich Molecules 2020, 25, 3725 6 of 14

The tested sample mixture comprised of 50% volume of cyclopentane and 50% water at 25 ◦C. Representing a mixture of 9.0 mL cyclopentane mixed with 9.0 mL of deionized water in a 50-mL glass sample bottle and gently tilted upside-down 50 times. Slow-and-gentle mixing was applied only to well the mixture well but was not intended to produce a tight emulsion with the surfactant present. The sample with Span and Tween were prepared by ﬁrst prepared the desired Span–Tween concentration (0.1 wt%–1.0 wt%). After which the cyclopentane and deionized water were used to dilute the Span and Tween surfactants, respectively before the mixtures were put together in the glass sample bottle.

2.2. Experimental Apparatus and Rheology Measurement Procedure The rheological properties of semi-solid hydrate slurry were studied using HAAKE ViscotesterTM 550. The viscometer is designed with a rotational speed preset and measure the flow resistance of a slurry fluid. The torque maintaining the set speed is proportional to the fluid viscosity, so all final information on the viscosity, shear stress and the shear rate were obtained from the torque required, the set speed and the geometry factors for the applied sensor. The study was performed with a cylindrical concentric geometry. The temperature of the viscometer is controlled by an attached external water bath system. In each experimental run, the desired prepared sample mixture of water and cyclopentane (with or with Span/Tween) is loaded into the viscometer sample holder at a preset temperature of 15 ◦C. The temperature of the water bath is then lowered to the desired experimental 1 temperature 2 ◦C at a rate of 0.21 ◦C/min, with the controlled shear rate of 50 s− . This was done to allow cyclopentane hydrate to form. Due to the long probabilistic nature of cyclopentane hydrate formation, the sample mixtures were seeded at the interface at the experimental temperature about 60 min at the start of the experiment [27]. The time of seeding was taken as the time zero to hydrate formation. A sharp increase in the system viscosity indicated the onset of hydrate crystallization or formation as shown in Figure2[ 39]. Further developing and agglomeration of more hydrate particles continue to increase the slurry viscosity until a point that the amount of hydrate particle was substantial to completely jam and cease and rotational motion of the rotating spindle. The hydrate induction time and crystallization/growth time were estimated to evaluate the effect of Span and Tween on cyclopentane hydrate. The induction time describes the time taken for a noticeable crystal to form. It allows the analysis of how fast or slow it would take for hydrate to form. On the other hand, the cyclopentane hydrate crystallization time is the time taken for a complete hydrate to form and the maximum viscosity achieved. The hydrate induction time (Ti) is estimated from Figure3 as follows: T = T T (1) i h − seed where Th and Tseed are the time taken for hydrate to start forming and when seed where added to the mixture, respectively. The hydrate crystallization/growth time (Gt) is calculated form Figure2 as follows; Gt = T T , (2) G − h where TG is the time taken for complete hydrate formation. Molecules 2020, 25, x FOR PEER REVIEW 6 of 14

Span 20 8.6 1.032 1000–2000 346.46 Sigma-Aldrich Span 40 6.7 1.075 1000–2000 402.565 Sigma-Aldrich Span 80 4.3 0.986 1200–2000 428.6 Sigma-Aldrich Tween 20 16.7 1.1 370–430 1228 Sigma-Aldrich Tween 40 15.6 1.05–1.10 400–600 1283.63 Sigma-Aldrich Tween 80 15.0 1.06–1.09 300–500 1310 Sigma-Aldrich

700

Molecules 2020, 25, x FOR PEER REVIEW Spindle stops spinning, max viscosity 7 of 14 600 (TG)

Ti = Th − Tseed (1) 500 where Th and Tseed are Cyclopentanethe time taken mixture for +hydrate 0.1 g Seed to start forming and when seed where added to the mixture, respectively. The hydrate crystallization/growth time (Gt) is calculated form Figure 2 400 as follows;

300 Gt = TG − Th, (2)

Viscosity (cp) Viscosity Viscosity starts to increase where TG is the time taken for complete hydrate formation. (Th) 200 3. Results and DiscussionSeed added within 50 - 60 mins 100 (Tseed) 3.1. Hydrate Seeds and Volume

Giving sufficient0 time and cooling at atmospheric ambient pressure, cyclopentane would crystallize in distilled0 water 20 to 40form solid 60 hydrate 80 The 100 suspension 120 140 of hydrate 160 solids 180 in 200 liquid slurry Time (min) increases the fluid viscosity until the point when the solid loading in the slurry mixture is packed and viscous enough to stop the rheometer spindle. The induction time, defined as the time interval Figure 3. Typical hydrate rheology profile of cyclopentane mixture (Blank) with 0.1 g seed at 2 ◦C. from theFigure beginning 3. Typical of hydratecooling rheology cycle to profilestart of of soli cyclopentaned hydrate mixture formation (Blank) could with be 0.1 too g longseed atfor 2 °C.practical 3.viscosity Results measurement and Discussion using rheometer. As seen in Figure 4, mixture of cyclopentane–distilled water (CP–water)The hydrate in 50%–50% induction v/v timewhen and aged crystallization/grow at about 2 °C for nearlyth time 48 were h, the estimated mixture to viscosity evaluate remained the effect 3.1. Hydrate Seeds and Volume unchangedof Span and at Tweenaround on 0.5–1.0 cyclopentane cP with shear hydrate. rate 50Th 1/sece induction indicating time no describes hydrate formationthe time takenafter 48 for h. a noticeableGivingKaranjkar crystal suffi etcient al.to form.[27] time recommended andIt allows cooling the at analysis to atmospheric add trace of how of ambient externallyfast or pressure, slow grown it would cyclopentane cyclopentane take forwould hydrate hydrate crystallize to solidsform. inorOn distilledflakes the other that water washand, preprepared to the form cyclopentane solid separately hydrate hydrate The into suspension the crystallization CP–water of hydratemixture time is solidsto the accelerate time in liquid taken hydrate slurry for aformation. increasescomplete theThehydrate fluidhydrate to viscosity form traces and until act the as the maximum hydrate point when seeds viscosity the to trigger solid achieved. loading subsequent The in thehydrate hydrate slurry induction mixture crystallization istime packed (T oni) is and its estimated surface, viscous enoughhencefrom Figure promoting to stop 3 asthe follows: the rheometer speed the spindle.hydrate to The form. induction The quantity time, defined and shape as theof the time hydrate interval seeds from used the beginningshould be optimized of cooling cycleand consistent, to start of solidso results hydrate from formation the Tween could and beSpan too effects long for could practical be compared. viscosity measurementThe induction using time rheometer.and hydrate As crystallization seen in Figure time4, mixture were ofmeasured cyclopentane–distilled repeatedly for waterrepeatability, (CP–water) the instandard 50%–50% deviationv/v when of aged the induction at about 2 ◦timeC for was nearly ±1.5, 48 while h, the that mixture of the viscosity crystallization remained time unchanged was ±0.8. at aroundThese were 0.5–1.0 found cP withto be shear a good rate agreement 50 1/sec indicating and consistent no hydrate for further formation reporting. after 48 h.

3.0

2.5

2.0

1.5 Viscosity (cp) Viscosity 1.0

0.5

0.0 0 500 1000 1500 2000 2500 3000 Time (min)

Figure 4. CP–Water mixture viscosity without hydrate seeds/ﬂakes addition. Figure 4. CP–Water mixture viscosity without hydrate seeds/flakes addition.

Figure 5 shows the acceleration impact to growth the hydrate slurry using externally grown CP hydrate flakes. In all the experiment run, almost constant elongate shape with average length of 2 mm and 1 mm thickness was used. Seeds/flakes were added after 60 min the CP–Water mixture has attained temperature equilibrium to preset temperature at 2 °C. This was assigned as zero time for all the induction time measurement [27]. The result in Figure 5 shows that 0.1 g of hydrate flakes

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Karanjkar et al. [27] recommended to add trace of externally grown cyclopentane hydrate solids or flakes that was preprepared separately into the CP–water mixture to accelerate hydrate formation. The hydrate traces act as hydrate seeds to trigger subsequent hydrate crystallization on its surface, hence promoting the speed the hydrate to form. The quantity and shape of the hydrate seeds used should be optimized and consistent, so results from the Tween and Span effects could be compared. The induction time and hydrate crystallization time were measured repeatedly for repeatability, the standard deviation of the induction time was 1.5, while that of the crystallization time was 0.8. ± ± These were found to be a good agreement and consistent for further reporting. Figure5 shows the acceleration impact to growth the hydrate slurry using externally grown CP hydrate flakes. In all the experiment run, almost constant elongate shape with average length of 2 mm and 1 mm thickness was used. Seeds/flakes were added after 60 min the CP–Water mixture has attained temperature equilibrium to preset temperature at 2 ◦C. This was assigned as zero time for all Moleculesthe induction 2020, 25, x time FOR measurementPEER REVIEW [27]. The result in Figure5 shows that 0.1 g of hydrate flakes8 could of 14 produce more reliable and repeatable viscosity trend. Hence constant amount and size of flakes were couldthen produce used in subsequent more reliable experiment and repeatable to study viscosity surfactants trend. e Henceffects on constant hydrate amount slurry viscosity.and size of flakes were then used in subsequent experiment to study surfactants effects on hydrate slurry viscosity. 1400

Spindle stops spinning, 1200 Seed : 0.1 grams max viscosity Seed : 0.8 grams 1000

800 Spindle stops spinning, max viscosity 600 Viscosity (cp) Viscosity

400 Seed (hydrate flakes) added within 50 - 60 200

0 0 50 100 150 200 Time (min) FigureFigure 5. 5.CP–waterCP–water mixture mixture viscosity viscosity with with hydrate hydrate seeds/flakes seeds/ﬂakes addition. addition.

3.2.3.2. Impact Impact of ofOil-Soluble Oil-Soluble Span Span 80 80 Surfacta Surfactantnt on on CP CP Hydrate Hydrate Slurry Slurry Viscosity Viscosity Giving Giving FigureFigure 66 shows shows the the viscosity viscosity changes changes for for the the cyclopentane cyclopentane hydrate hydrate in in the the presence presence of of oil-soluble oil-soluble SpanSpan 80 80 surfactant. surfactant. Exactly Exactly 0.1 0.1 g gpreprepared preprepared hydrate hydrate flakes flakes was was added added into into the the cyclopentane–water cyclopentane–water mixturemixture after after 60 60 min min the the mixture mixture was was in in equilibrium equilibrium at at 2.0 2.0 °C◦C to to initiate initiate hydrate hydrate formation. formation. It Itwas was notednoted that that the the viscosity viscosity trend trend for for the the blank blank mixture mixture without without oil-soluble oil-soluble surfactant surfactant Span Span 80 80 started started to to increaseincrease after after 11 11 min min hydrate hydrate flakes flakes were were introduced introduced as as the the induction induction time. time. Its Its viscosity viscosity continued continued to to progressprogress gradually gradually and and eventually eventually attained attained maximum maximum value value at at about about 1000 1000 cP cP before before the the rheometer rheometer spindlespindle ceased ceased spinning spinning when when solid solid cyclopentane cyclopentane was was completely completely developed developed in in the the rheometer rheometer cup cup holder.holder. Subsequently Subsequently the the measured measured induction induction time time and and crystallization crystallization time time (time (time taken taken to to achieve achieve maximummaximum viscosity viscosity after after induction induction time) time) for for Span Span and and Tween Tween systems systems at at different different concentrations concentrations are are presentedpresented in in Table Table 22 forand extensive Figure7 forelaborations. extensive elaborations. When Span surfactant was added into the base mixture, it lightly emulsified the cyclopentane and water by the shearing force induced by the spinning spindle set at 50 sec−1 thus increasing the interfacial contact between the 2 liquid phases. [32] The results show that the Span surfactants extended the induction time for CP hydrate formation than Tween surfactants, as shown in Figures 8 and 9 and Table 2. Probably due to the low HLB values of Span, which controls the intensity of emulsification of the cyclopentane and water to favor hydrate formation. This result is encouraging and confirms the potential inhibitive characteristic of non-ionic Spans against hydrate formation. The inhibitive impact of Spans and Tweens are dependent on their concentrations and HLB values.

Molecules 2020, 25, 3725 9 of 14 Molecules 2020, 25, x FOR PEER REVIEW 9 of 14

1100 1000

900 Span80 0.1% 800 Span80 0.5% Span80 1.0% 700 Blank 600 500

Viscosity (cp) Viscosity 400 300 200 100 0 0 20406080100120 Time (min)

FigureFigure 6.6.The The eeffectﬀect ofof SpanSpan 8080 concentrationconcentration (%(%v v/v/v in in oil) oil) on on cyclopentane cyclopentane hydrate hydrate slurry slurry (CP (CP/H/H22O:O: 5050/50;/50; aging aging temperature temperature 2 2◦ °C).C).

Table 2. Measured hydrate induction time and crystallization time. In Figure 8a, the highest hydrate inhibition was observed in Span 20 at 0.1 wt%., however, increasingSystem concentration Conc.significantly (wt%) decreases Induction its inhibitive Time (min) impacta Crystallization by 69% at 1 wt%. Time On (min) theb other hand, theBlank inhibitive impact of –Span 40 and 80 increases 11 with increasing concentration 22 by 166% and 123% at 1.0 wt%. We observed that the hydrate induction time inhibitory efficiency in Span 0.1 45 8 surfactantsSpan with 20 high HLB decreases0.5 with increasing 21 concentrations. While that 14 of Spans with low HLB values (Span 40 and 80) increases1.0 with increasing 14 concentration. High HLB values 7 of surfactants exhibits high hydrophilicity 0.1and enhances the solvation 9 of the surfactants in 20 the mixture thus providingSpan a perturbation 40 effect0.5 in the water structure 25via hydrogen bonding. This 6 perturbation effect prolongs the hydrate nuclei crystallization1.0 time, especially 24 at low concentrations as 3 demonstrated by Span 20 in Figure 8a. Contrary,0.1 the hydrophobic nature 13 of Span with low HLB values 10 increases with concentration,Span 80 hence, increasing0.5 its inhibition impact 19with concentration. 14 In contrary, Tween which1.0 is a water-soluble non- 29ionic surfactant which also 11 lightly emulsified the cyclopentane–water mixture0.1 reduced the hydrate 11induction time significantly4 compared to blank sampleTween as shown 20 in Figures0.5 7 and 8. Tweens generally 9 have high HLB (hydrophilic–lipophilic 5 balance) values compared with1.0 Spans as shown in Table 1 1. Thus, they tend to produce 3 oil-in-water emulsion, i.e., oil droplets are0.1 trapped within continuous 8 water surrounding 4phase. The phases configurationTween 40 with water external0.5 phase promotes hy 18drate crystallization to occur 2 in very short time 1.0 5 4 once the hydrate flakes were added as shown in Figures 7 and 8b. It was believed that the Tweens surfactant adsorbed onto the 0.1cyclopentane–water interface, 5 leading to formation 3of enhanced water- Tween 80 0.5 2 2 wetted hydrate crystals that interacting strongly within each other with the water external phase and 1.0 9 4 eventually agglomerating into larger volume and viscous hydrate slurry. Tween 20 had negligible a Standard deviation for induction time = 1.5; b Standard deviation for crystallization time = 0.8. effect on hydrate nucleation time at 0.1 wt%.± The presence of Tween 40 and 80 reduced± the hydrate induction time by 27% and 55%, respectively (see Figure 8b). when the concentrations of the Tweens were increased to 1.0 wt%, Tween 20 reduced the hydrate nucleation time by 91%, followed by Tween 40, 55% and that of Tween 80 was 18%. Suggesting that the maximum hydrate induction time promotion effect was observed in Tween 20 at high concentration (1.0 wt%) as shown in Figure 8b.

Molecules 2020, 25, x FOR PEER REVIEW 10 of 14

Molecules 2020, 25, 3725 10 of 14

Molecules 2020, 25, x FOR1600 PEER REVIEW 11 of 14 1500 1400 1300Table 2. MeasuredTween80 hydrate 0.1% induction time and crystallization time. 1200 Tween80 0.5% System Conc. (wt%) Induction Time (min) a Crystallization Time (min) b 1100 Tween8 0 1.0% Blank 1000 – Blank 11 22 900 0.1 45 8 800 Span 20 700 0.5 21 14

Viscosity, cP Viscosity, 600 1.0 14 7 500 400 0.1 9 20 Span 40 300 0.5 25 6 200 1.0 24 3 100 0 0.1 13 10 Span 80 00.5 2040608010012019 14 1.0 29 Time, min 11 0.1 11 4 Figure 7. The effect of Tween 80 concentration (% v/v in oil) on cyclopentane hydrate slurry (CP/H2O: Figure 7. The effect of Tween 80 concentration (% v/v in oil) on cyclopentane hydrate slurry (CP/H2O: 50/50; agingTween temperature 20 2.00.5 deg C). 9 5 50/50; aging temperature1.0 2.0 deg C). 1 3 When Span surfactant was0.1 added into the8 base mixture, it lightly emulsified 4 the cyclopentane 1 and waterIt was byTween thealso shearing 40noticed forcethat0.5 once induced the hydrate by the18 spinning crystal started spindle to set develop at 50 2 sec in− thethus Spans increasing and Tweens the interfacialslurry systems, contact betweenthe maximum the1.0 2 liquid viscosity phases. was [32 attained5] The results quicker show compared that the 4 Spanto the surfactants blank sample, extended due to thethe induction surfactant time nature for CP of hydrate Spans0.1 to formation enhance thanhydrate5 Tween crystals surfactants, formation as shown [25,40,41]. 3 in Figures Figure8 and9 depicts9 and the result for time interval from the moment hydrates began to crystalline until full hydrate solid Table2. ProbablyTween due 80 to the low0.5 HLB values of Span,2 which controls the intensity 2 of emulsification of development when the rotational motion of the rotating spindle completely ceased (hydrate the cyclopentane and water1.0 to favor hydrate formation.9 This result is encouraging 4 and confirms the crystallization time). It is noticed that the Span and Tween surfactants accelerated subsequent hydrate potentiala Standard inhibitive deviation characteristic for induction of non-ionic time =±1.5; Spans b Standard against deviation hydrate for formation. crystallization The time inhibitive = ±0.8. impact of Spansformation and once Tweens the arehydrate dependent crystallization on their concentrationswas initiated compared and HLB to values. blank sample. It was suggested that emulsification by Span and Tween has increased surface contact area between cyclopentane and distilled60 water to encourage further agglomeration30 of hydrate particles in slurry and eventually complete(a) solidification was attainedSpan20 [42]. Span surfactants(b) exhibited a fast crystallization time with 25 Tween20 concentrations,50 while the TweenSpan40 surfactants did not, as depicted in Figure 9. ProbablyTween40 due to the small differences in the HLB Span80values of Tween20s (see Table 1). They exhibit similarTween80 hydrate 40 Blank Blank crystallization growth time. Generally, Tweens highly promote hydrate crystallization time than 15 Spans.30 The average hydrate crystallization time of the blank sample was reduced by 86% and 68% by Tweens and Spans surfactants at 1.0 wt%, respectively.10 The best hydrate crystallization time promotional20 effect in the Span systems occurred at 1.0 wt%. The observed efficient hydrate 5 Induction time (min) Induction crystallizationtime (min) Induction time reduction in the presence of Tween and Span can be useful in other application such10 as hydrate-based refrigeration application, gas0 separations and storage [24]. However, Tween and Span has the potentials to provide hydrate slurry0.1 that can be transported 0.5 safely to the 0.9 surface. 0 -5 The0.1 findings in this study 0.5 for Span 80 0.9agrees with Liua et al. [38] whoConcentration reported (wt%) that the presence of Span 80 rapidlyConcentration increases cyclopentane(wt%) hydrate formation which results in an irregular surface growth. However, they further suggested that the effect of Span 80 on cyclopentane hinders hydrate Figure 8. The effect of Span and Tween concentrations (% v/v) on CP–hydrate formation induction time. formationsFigure 8. The and effect agglomerations. of Span and TweenKaranjkar concentrations et al. [27] also(% v/v confirmed) on CP–hydrate the potential formation anti-agglomeration induction (CP/H2O: 50/50; aging temperature 2.0 deg C); (a) Span surfactants, (b) Tween surfactants. behaviortime. (CP/H2O: of Span 50/50; 80 at aging high temperatureconcentrations. 2.0 deg Henc C); e(a considering) Span surfactants, Span ( bsurfactants) Tween surfactants. for gas hydrate risk managementIn Figure8a, in theflow highest assurance hydrate management inhibition is wasrecommended. observed inContrary Span 20 to at the 0.1 morphology wt%., however, study of increasingDann and concentration Rosenfeld [26], significantly the presence decreases of Span its inhibitive20, Span impact80 and byTween 69% at65 1reduced wt%. On cyclopentane the other hand,hydrate the inhibitive growth rate. impact However, of Span we 40 believe and 80 the increases growth withrate reduction increasing is concentration possibly related by 166%to the andability 123%of the at 1.0 surfactants wt%. We observedto form small that the hydrate hydrate crystals induction which time avoids inhibitory or reduces efficiency agglomerations. in Span surfactants On the withother high hand, HLB the decreases reduced with hydrate increasing crystallization concentrations. time observed While that in the of Spanspresence with of low surfactants HLB values in this (Spanstudy 40 can and be 80) interpreted increases withas the increasing ability of concentration.the surfactants Highto quickly HLB reach values critical of surfactants hydrate crystal exhibits size highthat hydrophilicity would prevent and agglomerations enhances the in solvation the hydrate of the slurry. surfactants in the mixture thus providing a perturbation effect in the water structure via hydrogen bonding. This perturbation effect prolongs the

Molecules 2020, 25, 3725 11 of 14 hydrate nuclei crystallization time, especially at low concentrations as demonstrated by Span 20 in

MoleculesFigure8 2020a. Contrary,, 25, x FOR thePEER hydrophobic REVIEW nature of Span with low HLB values increases with concentration,12 of 14 hence, increasing its inhibition impact with concentration.

35 30 Span20 (b) 30 Span40 (a) 25 Span80 25 Blank 20 Tween20 20 Tween40 15 Tween80 15 Blank 10 10 5 5 Hydrate crystalization time (min) time crystalization Hydrate

Hydrate crystalizationHydrate (min) time 0 0 0.1 0.5 0.9 0.1 0.5 0.9 Concentration (wt%) Concentration (wt%) Figure 9. The effect of Span and Tween concentrations (% v/v) on CP–hydrate formation crystallization Figure 9. The effect of Span and Tween concentrations (% v/v) on CP–hydrate formation time. 50/50; aging temperature 2.0 deg C); (a) Span surfactants, (b) Tween surfactants. crystallization time. 50/50; aging temperature 2.0 deg C); (a) Span surfactants, (b) Tween surfactants. In contrary, Tween which is a water-soluble non-ionic surfactant which also lightly emulsified 4.the Conclusions cyclopentane–water mixture reduced the hydrate induction time significantly compared to blank sampleRheological as shown measurement in Figures7 and of8 viscosity. Tweens for generally cyclopentane–water have high HLB slurry (hydrophilic–lipophilic hydrate with and balance)without presencevalues compared of non-ionic with Spanssurfactants as shown has in been Table 1co. Thus,nducted they in tend this to producework as oil-in-water varying surfactant emulsion, concentrationsi.e., oil droplets at are2 °C trapped in a Viscotester. within continuous The results water revealed surrounding that the oil-soluble phase. The Span phases surfactants configuration (Span 20,with Span water 40, externalSpan 80) phase are potentially promotes delaying hydrate crystallization the initialization to occur of hydrate in very formation short time by once extending the hydrate the inductionflakes were time added for hydrate as shown to incrystalline. Figures7 andThe8 b.best It washydrate believed induction that the time Tweens inhibition surfactant impact adsorbed of Span surfactantsonto the cyclopentane–water occurred in Span 20 interface, at lower leading concentration to formation (0.1 ofwt%). enhanced While water-wetted Span 40 best hydrateprolonged crystals the hydratethat interacting crystallization strongly time within at each0.1 wt%. other However, with the water both externalSpan and phase Tween and eventuallypromoted agglomeratingcyclopentane hydrateinto larger crystallization volume and time viscous at all hydrate concentrations slurry. Tweendue to 20their had surfactant negligible beha effectvior. on Hence, hydrate confirming nucleation theirtime potential at 0.1 wt%. anti-agglomeration The presence of Tween impact 40 via and quickly 80 reduced achieving the hydrate critical induction hydrate time size bythat 27% eliminates and 55%, agglomerationrespectively (see and Figure provide8b). wheneasy hydrate the concentrations slurry that ofcan the be Tweens transported. were increased The water-soluble to 1.0 wt%, Tween Tween surfactants20 reduced (Tween the hydrate 20, Tween nucleation 40, timeTween by 80) 91%, demo followednstrate by higher Tween hydrate 40, 55% andpromoting that of Tweenimpact 80 than was Span18%. surfactants. Suggesting thatThe thefindings maximum in this hydrate study will induction help provide time promotion insight into effect the was rheological observed ineffect Tween of surfactants20 at high concentrationon the hydrate (1.0 slurry. wt%) The as shownchanges in in Figure fluid8 b.viscosity following hydrate slurry formation is usefulIt was information also noticed when that performing once the hydratefluid dyna crystalmic calculation started to in develop pipeline in fluid the Spans transmission and Tweens and processslurry systems,design for the gas maximum storage viscosityand potential was attained application quicker to CO2 compared gas sequestration to the blank where sample, the due rate to theof hydratesurfactant formation nature ofand Spans hydrate to enhance slurry hydrateviscosity crystals are critical. formation [25,40,41]. Figure9 depicts the result for time interval from the moment hydrates began to crystalline until full hydrate solid development Author Contributions: Conceptualization, K.S.F.; formal analysis, K.S.F. and C.B.B.; funding acquisition, B.L.; when the rotational motion of the rotating spindle completely ceased (hydrate crystallization time). investigation, K.S.F.; methodology, K.S.F.; resources, A.M.S.; supervision, B.L.; writing—original draft, K.S.F., C.B.B.;It is noticed writing—review that the Span& editing, and TweenC.B.B., K.S.F. surfactants and A.M.S. accelerated All authors subsequent have read hydrate and agreed formation to the published once the versionhydrate of crystallizationthe manuscript. was initiated compared to blank sample. It was suggested that emulsification by Span and Tween has increased surface contact area between cyclopentane and distilled water to Funding: This research was funded by the Ministry of Higher Education, Malaysia through FRGS grant (Grant encourage further agglomeration of hydrate particles in slurry and eventually complete solidification No. FRGS - 0153AB-K77). was attained [42]. Span surfactants exhibited a fast crystallization time with concentrations, while the Acknowledgments:Tween surfactants This did study not, aswas depicted conducted in Figureat the Chemical9. Probably Engineering due to the Department, small di ff Universitierences in Teknologi the HLB PETRONAS.values of Tweens (see Table1). They exhibit similar hydrate crystallization growth time. Generally, ConflictsTweens highlyof Interest: promote The authors hydrate declare crystallization no conflicts timeof interest. than Spans. The average hydrate crystallization time of the blank sample was reduced by 86% and 68% by Tweens and Spans surfactants at 1.0 wt%, Referencesrespectively. The best hydrate crystallization time promotional effect in the Span systems occurred at 1. Koh, C.A. Towards a fundamental understanding of natural gas hydrates. Chem. Soc. Rev. 2002, 31, 157– 167, doi:10.1039/b008672j. 2. Khan, M.S.; Bavoh, C.; Lal, B.; Keong, L.K.; Mellon, N.; Bustam, M.A.; Shariff, A.M. Application of Electrolyte Based Model on Ionic Liquids-Methane Hydrates Phase Boundary. IOP Conf. Series: Mater. Sci. Eng. 2018, 458, 012073, doi:10.1088/1757-899x/458/1/012073.

Molecules 2020, 25, 3725 12 of 14

1.0 wt%. The observed efficient hydrate crystallization time reduction in the presence of Tween and Span can be useful in other application such as hydrate-based refrigeration application, gas separations and storage [24]. However, Tween and Span has the potentials to provide hydrate slurry that can be transported safely to the surface. The findings in this study for Span 80 agrees with Liua et al. [38] who reported that the presence of Span 80 rapidly increases cyclopentane hydrate formation which results in an irregular surface growth. However, they further suggested that the effect of Span 80 on cyclopentane hinders hydrate formations and agglomerations. Karanjkar et al. [27] also confirmed the potential anti-agglomeration behavior of Span 80 at high concentrations. Hence considering Span surfactants for gas hydrate risk management in flow assurance management is recommended. Contrary to the morphology study of Dann and Rosenfeld [26], the presence of Span 20, Span 80 and Tween 65 reduced cyclopentane hydrate growth rate. However, we believe the growth rate reduction is possibly related to the ability of the surfactants to form small hydrate crystals which avoids or reduces agglomerations. On the other hand, the reduced hydrate crystallization time observed in the presence of surfactants in this study can be interpreted as the ability of the surfactants to quickly reach critical hydrate crystal size that would prevent agglomerations in the hydrate slurry.

4. Conclusions Rheological measurement of viscosity for cyclopentane–water slurry hydrate with and without presence of non-ionic surfactants has been conducted in this work as varying surfactant concentrations at 2 ◦C in a Viscotester. The results revealed that the oil-soluble Span surfactants (Span 20, Span 40, Span 80) are potentially delaying the initialization of hydrate formation by extending the induction time for hydrate to crystalline. The best hydrate induction time inhibition impact of Span surfactants occurred in Span 20 at lower concentration (0.1 wt%). While Span 40 best prolonged the hydrate crystallization time at 0.1 wt%. However, both Span and Tween promoted cyclopentane hydrate crystallization time at all concentrations due to their surfactant behavior. Hence, confirming their potential anti-agglomeration impact via quickly achieving critical hydrate size that eliminates agglomeration and provide easy hydrate slurry that can be transported. The water-soluble Tween surfactants (Tween 20, Tween 40, Tween 80) demonstrate higher hydrate promoting impact than Span surfactants. The findings in this study will help provide insight into the rheological effect of surfactants on the hydrate slurry. The changes in fluid viscosity following hydrate slurry formation is useful information when performing fluid dynamic calculation in pipeline fluid transmission and process design for gas storage and potential application to CO2 gas sequestration where the rate of hydrate formation and hydrate slurry viscosity are critical.

Author Contributions: Conceptualization, K.S.F.; formal analysis, K.S.F. and C.B.B.; funding acquisition, B.L.; investigation, K.S.F.; methodology, K.S.F.; resources, A.M.S.; supervision, B.L.; writing—original draft, K.S.F., C.B.B.; writing—review & editing, C.B.B., K.S.F. and A.M.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Higher Education, Malaysia through FRGS grant (Grant No. FRGS - 0153AB-K77). Acknowledgments: This study was conducted at the Chemical Engineering Department, Universiti Teknologi PETRONAS. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Koh, C.A. Towards a fundamental understanding of natural gas hydrates. Chem. Soc. Rev. 2002, 31, 157–167. [CrossRef] 2. Khan, M.S.; Bavoh, C.; Lal, B.; Keong, L.K.; Mellon, N.; Bustam, M.A.; Shariﬀ, A.M. Application of Electrolyte Based Model on Ionic Liquids-Methane Hydrates Phase Boundary. IOP Conf. Ser.: Mater. Sci. Eng. 2018, 458, 012073. [CrossRef] Molecules 2020, 25, 3725 13 of 14

3. Nashed, O.; Dadebayev, D.; Khan, M.S.; Bavoh, C.; Lal, B.; Shariﬀ, A.M. Experimental and modelling studies on thermodynamic methane hydrate inhibition in the presence of ionic liquids. J. Mol. Liq. 2018, 249, 886–891. [CrossRef] 4. Bavoh, C.B.; Lal, B.; Osei, H.; Sabil, K.M.; Mukhtar, H. A review on the role of amino acids in gas hydrate

inhibition, CO2 capture and sequestration, and natural gas storage. J. Nat. Gas. Sci. Eng. 2019, 64, 52–71. [CrossRef] 5. Bavoh, C.B.; Lal, B.; Keong, L.K. Introduction to Gas Hydrates. In Chemical Additives for Gas Hydrates; Green Energy and Technology; Springer: Cham, Switzerland, 2020; pp. 1–20. ISBN 978-3-030-30749-3. 6. Khan, M.S.; Bavoh, B.C.; Mohammad, A.R.; Bhajan, L.; Quainoo, A.K.; Abdulhalim, S.M. Assessing the Alkyl Chain Effect of Ammonium Hydroxides Ionic Liquids on the Kinetics of Pure Methane and Carbon Dioxide Hydrates. Energies. 2020, 13, 3272. [CrossRef] 7. Tariq, M.; Rooney, D.; Othman, E.; Aparicio, S.; Atihan, M.; Khraisheh, M. Gas Hydrate Inhibition: A Review of the Role of Ionic Liquids. Ind. Eng. Chem. Res. 2014, 53, 17855–17868. [CrossRef] 8. Broni-Bediako, E.; Amorin, R.; Bavoh, C. Gas Hydrate Formation Phase Boundary Behaviour of Synthetic Natural Gas System of the Keta Basin of Ghana. Open Pet. Eng. J. 2017, 10, 64–72. [CrossRef] 9. Bavoh, C.; Nashed, O.; Khan, M.S.; Partoon, B.; Lal, B.; Sharif, A.M. The impact of amino acids on methane hydrate phase boundary and formation kinetics. J. Chem. Thermodyn. 2018, 117, 48–53. [CrossRef] 10. Bavoh, C.; Partoon, B.; Lal, B.; Keong, L.K. Methane hydrate-liquid-vapour-equilibrium phase condition measurements in the presence of natural amino acids. J. Nat. Gas. Sci. Eng. 2017, 37, 425–434. [CrossRef] 11. Khan, M.S.; Partoon, B.; Bavoh, C.; Lal, B.; Mellon, N.B. Influence of tetramethylammonium hydroxide on methane and carbon dioxide gas hydrate phase equilibrium conditions. Fluid Phase Equilibria 2017, 440, 1–8. [CrossRef] 12. Bavoh, C.; Lal, B.; Nashed, O.; Khan, M.S.; Keong, L.K.; Bustam, M.A. COSMO-RS: An ionic liquid prescreening tool for gas hydrate mitigation. Chin. J. Chem. Eng. 2016, 24, 1619–1624. [CrossRef] 13. Bavoh, C.B.; Partoon, B.; Keong, L.K.; Lal, B.; Wilfred, C.D. Eeffect of 1-ethyl-3-methylimidazolium chloride and polyvinylpyrrolidone on kineticss of carbon dioxide hydrates. Int. J. Appl. Chem. 2016, 12, 6–11. 14. Bavoh, C.; Lal, B.; Keong, L.K.; Jasamai, M.; Idress, M. Synergic Kinetic Inhibition Effect of EMIM-Cl + PVP

on CO2 Hydrate Formation. Procedia Eng. 2016, 148, 1232–1238. [CrossRef] 15. Khan, M.S.; Cornelius, B.B.; Lal, B.; Bustam, M.A. Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide, Methane and Binary Mix Gas Hydrates. In Recent Advances in Ionic Liquids; IntechOpen: London, UK, 2018; pp. 159–179. 16. Mannar, N.; Bavoh, C.; Baharudin, A.H.; Lal, B.; Mellon, N.B. Thermophysical properties of aqueous lysine and its inhibition inﬂuence on methane and carbon dioxide hydrate phase boundary condition. Fluid Phase Equilibria 2017, 454, 57–63. [CrossRef] 17. Sloan, E.D., Jr.; Koh, C.A.; Sloan, E.D. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press Taylor & Francis: Boca Raton, NY, USA, 2007; p. 758. 18. Bavoh, C.B.; Lal, B.; Ben-Awuah, J.; Khan, M.S.; Ofori-Sarpong, G. Kinetics of Mixed Amino Acid and Ionic

Liquid on CO2 Hydrate Formation. IOP Conf. Series: Mater. Sci. Eng. 2019, 495, 012073. [CrossRef] 19. Bavoh, C.; Lal, B.; Khan, M.S.; Osei, H.; Ayuob, M. Combined Inhibition Effect of 1-Ethyl-3-methy-limidazolium Chloride + Glycine on Methane Hydrate. J. Physics: Conf. Ser. 2018, 1123, 012060. [CrossRef] 20. Bavoh, C.B.; Yuha, Y.B.; Tay, W.H.; Ofei, T.N.; Lal, B.; Mukhtar, H. Experimental and Modelling of the impact of Quaternary Ammonium Salts / Ionic Liquid on the rheological and hydrate inhibition properties of Xanthan gum water-based muds for Drilling Gas Hydrate-Bearing Rocks. J. Pet. Sci. Eng. 2019, 183, 106468. [CrossRef] 21. Yuha, Y.B.M.; Bavoh, C.B.; Lal, B.; Broni-Bediako, E. Methane Hydrate Phase Behaviour in EMIM-Cl Water Based Mud (WBM): An Experimental and Modelling Study. South African J. Chem. Eng. 2020, 34, 47–56. [CrossRef] 22. Bavoh, C.B.; Ntow, T.; Lal, B.; Sharif, A.M.; Shahpin, M.H.B.A.; Sundramoorthy, J.D. Assessing the impact of an ionic liquid on NaCl / KCl / polymer water-based mud ( WBM ) for drilling gas hydrate-bearing sediments. J. Mol. Liq. 2019, 294, 111643. [CrossRef] 23. Kelland, M.A.; Martin, T.; Dybvik, L. Journal of Petroleum Science and Engineering Gas hydrate anti-agglomerant properties of polypropoxylates and some other demulsi ﬁ ers. J. Pet. Sci. Eng. 2009, 64, 1–10. [CrossRef] Molecules 2020, 25, 3725 14 of 14

24. Kumar, A.; Bhattacharjee, G.; Kulkarni, B.; Kumar, R. Role of Surfactants in Promoting Gas Hydrate Formation. Ind. Eng. Chem. Res. 2015, 54, 12217–12232. [CrossRef] 25. Parlaktuna, M. Surfactants as Hydrate Promoters ? Energy Fuels 2000, 14, 1103–1107. 26. Dann, K.; Rosenfeld, L. Surfactant Effect on Hydrate Crystallization at the Oil–Water Interface. Langmuir 2018, 34, 6085–6094. [CrossRef][PubMed] 27. Karanjkar, P.U.; Ahuja, A.; Zylyftari, G.; Lee, J.W.; Morris, J.F. Rheology of cyclopentane hydrate slurry in a model oil-continuous emulsion. Rheol. Acta 2016, 55, 235–243. [CrossRef] 28. Huo, Z.; Freer, E.; Lamar, M.; Sannigrahi, B.; Knauss, D.; Sloan, E. Hydrate plug prevention by anti-agglomeration. Chem. Eng. Sci. 2001, 56, 4979–4991. [CrossRef] 29. Abojaladi, N.; Kelland, M.A. Can cyclopentane hydrate formation be used to screen the performance of surfactants as LDHI anti-agglomerants at atmospheric pressure? Chem. Eng. Sci. 2016, 152, 746–753. [CrossRef] 30. Fan, S.S.; Liang, D.Q.; Guo, K.H. Hydrate Equilibrium Conditions for Cyclopentane and a Quaternary Cyclopentane-Rich Mixture. J. Chem. Eng. Data 2001, 46, 930–932. [CrossRef] 31. Baek, S.; Min, J.; Lee, J.W. Equilibria of cyclopentane hydrates with varying HLB numbers of sorbitan monoesters in water-in-oil emulsions. Fluid Phase Equilibria 2016, 413, 41–47. [CrossRef] 32. Delroisse, H.; Torré, J.-P.; Dicharry, C. Effect of a Hydrophilic Cationic Surfactant on Cyclopentane Hydrate Crystal Growth at the Water/Cyclopentane Interface. Cryst. Growth Des. 2017, 17, 5098–5107. [CrossRef] 33. Sun, Z.-G.; Fan, S.-S.; Guo, K.-H.; Shi, L.; Guo, Y.-K.; Wang, R. Gas Hydrate Phase Equilibrium Data of Cyclohexane and Cyclopentane. J. Chem. Eng. Data 2002, 47, 313–315. [CrossRef] 34. López, A.; Llinares, F.; Cortell, C.; Herráez, M. Comparative enhancer effects of Span®20 with Tween®20 and Azone®on the in vitro percutaneous penetration of compounds with different lipophilicities. Int. J. Pharm. 2000, 202, 133–140. [CrossRef] 35. Boyd, J.; Parkinson, C.; Sherman, P. Factors affecting emulsion stability, and the HLB concept. J. Colloid Interface Sci. 1972, 41, 359–370. [CrossRef] 36. Owusu-Apenten, R.; Zhu, Q.-H. Interfacial parameters for spans and tweens in relation to water-in-oil-in-water multiple emulsion stability. Food Hydrocoll. 1996, 10, 245–250. [CrossRef] 37. Delgado-Linares, J.G.; Majid, A.A.A.; Sloan, E.D.; Koh, C.A.; Sum, A.K. Model Water-in-Oil Emulsions for Gas Hydrate Studies in Oil Continuous Systems. Energy Fuels 2013, 27, 4564–4573. [CrossRef] 38. Liu, C.; Li, M.; Zhang, G.; Koh, C.A. Direct measurements of the interactions between clathrate hydrate particles and water droplets. Phys. Chem. Chem. Phys. 2015, 17, 20021–20029. [CrossRef][PubMed] 39. Bavoh, C.B.; Ofei, T.N.; Lal, B. Investigating the Potential Cuttings Transport Behavior of Ionic Liquids in Drilling Mud in the Presence of sII Hydrates. Energy Fuels 2020, 34, 2903–2915. [CrossRef] 40. Moraveji, M.K.; Ghaffarkhah, A.; Sadeghi, A. Effect of three representative surfactants on methane hydrate formation rate and induction time. Egypt. J. Pet. 2017, 26, 331–339. [CrossRef] 41. Aman, Z.M.; Koh, C.A. Interfacial phenomena in gas hydrate systems. Chem. Soc. Rev. 2016, 45, 1678–1690. [CrossRef][PubMed] 42. Lo, C.; Zhang, J.S.; Couzis, A.; Somasundaran, P.; Lee, J.W. Adsorption of Cationic and Anionic Surfactants on Cyclopentane Hydrates. J. Phys. Chem. C 2010, 114, 13385–13389. [CrossRef]

Sample Availability: Samples of the compounds Tweens and Spans are available from the authors.

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