molecules 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