applied sciences

Article Storage in -Hydrate: Theoretical and Experimental Study

Mohammad Reza Ghaani 1,* , Satoshi Takeya 2 and Niall J. English 1,*

1 School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, 4 Dublin, Ireland 2 Research Institute for Material and Chemical Measurement, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan; [email protected] * Correspondence: [email protected] (M.R.G.); [email protected] (N.J.E.); Tel.: +353-1-7161758 (M.R.G.); Fax: +353-1-7161177 (N.J.E.)  Received: 29 October 2020; Accepted: 10 December 2020; Published: 15 December 2020 

Abstract: There have been studies on gas-phase promoter facilitation of H2-containing clathrates. In the present study, non-equilibrium molecular dynamics (NEMD) simulations were conducted to analyse hydrogen release and uptake from/into propane planar clathrate surfaces at 180–273 K. The kinetics of the formation of propane hydrate as the host for hydrogen as well as hydrogen uptake into this framework was investigated experimentally using a fixed-bed reactor. The experimental capacity propane hydrate was found to be around 1.04 wt% in compare with the theoretical expected 1.13 wt% storage capacity of propane hydrate. As a result, we advocate some limitation of gas-dispersion (fixed-bed) reactors such as the possibility of having un-reacted as well as limited diffusion of hydrogen in the bulk hydrate.

Keywords: hydrogen storage; hydrogen/propane hydrate; molecular dynamics; cage occupancy

1. Introduction Hydrogen is considered as an efficient, operationally convenient, and cost-effective resource for a clean future. These features of hydrogen have introduced and hydrogen storage techniques as an attractive field of study. In this regard, it is necessary to leverage and understand the full benefits of hydrogen application in the context of “Hydrogen Economy”. For instance, the excess H2 generated from other resources such as solar energy and wind provide a wide range of economically efficient in possibilities in a “power-to-gas” vision. Clathrate hydrates, also known as gas hydrates, are composite compounds, in which water molecules form a lattice framework that are entrapped guest molecules in. This family of materials crystalize in three main structures [1]: types sI, sII and sH. Each polymorph is made by a different number of water molecules which form a different number of cages. For example, dual propane/hydrogen hydrate with sII type structure has a unit cell 136 water molecules form eight 51264 and sixteen 512 cavities. Propane can only be fitted in large (51264) cavities, while smaller 512 cages are empty and have potential to entrap other small guest molecules, e.g., H2, for physical gas storage application [2]. Hydrogen hydrate crystallises in sII type hydrate form and at maximum can store hydrogen equivalent to 5.3 wt% of its total mass (mass H2 per mass H2O) sII at 250 K and 2–3 kbar pressure [3,4]. Indeed, this high-pressure demand acting as a limiting constraint on its practical application as a hydrogen-storage medium [5–7]. To solve this issue one approach can be employing binary hydrates with a “helper” molecule (liquid or gas) and hydrogen as the secondary compound, those are stable at milder conditions, e.g., at ~30–50 bar and ~280 K but with a substantially lower hydrogen-storage capacity [5–9].

Appl. Sci. 2020, 10, 8962; doi:10.3390/app10248962 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 8962 2 of 10

Apart from well-studied liquid-phase promoters such as THF hydrate [8–10] for sII H2-bearing hydrates, there have been studies on gas-phase promoters facilitation sII H2-containing clathrates. Hasegawa et al. investigated the possibility of having SF6 as the helper compound in forming sII hydrate while hydrogen molecules can be stored in small cages. In their molecular simulation study, they have observed frequent diffusion of H2 molecules between large cages, however, having SF6 molecules in the large cages are limiting the diffusion. They proposed an optimal occupancy of large cages with SF6 in which hydrate can stay stable while the maximum H2 storage capacity can be obtained [11]. Skiba et al. found sII structured propane-/hydrogen-mixed hydrate to be favourable for storing hydrogen with 0.33 wt% capacity at 270 K and 120 bar [12]. In the present study, we have investigated H2- uptake and release kinetics through non-equilibrium molecular dynamics simulations at various temperatures, as well as experimentally forming propane hydrate and evaluating the experimental capacity of hydrogen storage into the formed propane hydrate.

2. Simulation Methodology In this study, the mixed hydrogen-/propane- clathrate was stimulated by pairwise potentials; TIP4P/2005 type models for water “which have been used in many simulation studies on hydrates” [13]. Using TIP4P/2005, Huang et al. showed an accurate sequence of in addition to structural parameters for ice polymorphs, in agreement with dispersion-corrected Density Functional Theory (DFT) [14]. In our previous recent studies [15–17], we showed the melting point of propane and propane/hydrogen hydrate can be predicted close to their experimental values using TIP4P/2005 potential. the General Amber Force Field (GAFF) used to handle Propane [18] and restrained electrostatic potential atomic partial charges (RESP) were sampled for propane from the HF/6-31G(d) electrostatic potential [19]. The model proposed by Alavi et al. was used to parametrise the hydrogen molecules [20]. Smooth particle-mesh Ewald method [21] was applied for long-range electrostatic and the temperature was controlled using Nosé–Hoover-thermostat [22]. Initial coordinates of atoms in sII-clathrate unit cells were constructed from X-ray diffraction data [23]. The unit-cell lengths were 17.32 Å; initial orientations of water molecules were chosen by the Rahman–Stillinger procedure [24] to comply with Bernal–Fowler rules [25] and have a negligible total-system dipole moment. The simulations were done for both uptake and release of hydrogen into/from propane hydrate. A 2 2 2 supercell was constructed and was put in contact in the + and x-directions with the length × × − of ~129.75 each direction by vacuum. For the release simulation, every large cage was occupied by one propane molecule, while one hydrogen molecule located in every small one. The vacuum part is large enough to not avoid any significant pressure built up during the time of the hydrogen release process. In the case of uptake study, the same 2 2 2 supercell loaded only with propane molecules in × × large cages and keeping the small cages empty was sandwiched between relaxed hydrogen molecules with the length of 36 Å along the X-axis (i.e., a total hydrogen-phase length of 72 Å). Assuming the ± hydrogen molecules as idea gas, the added hydrogen molecules can provide a pressure around 1000 bar at 273 K in the gas area. To afford compatibility with experimental data (in which case the cages at the interface are, ipso facto, complete before any thermal-imposed for heating above the melting temperature), extra water molecules were added to have complete cages at the interface [15]; cf. Figure1. Appl. Sci. 2020, 10, 8962 3 of 10 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 10

(a)

(b)

Figure 1. Illustration of two proposed models of propane/hydrogen hydrate uptake (a) and release Figure 1. Illustration of two proposed models of propane/hydrogen hydrate uptake (a) and release (b) In this representation, each tri-group of green octahedra denotes a propane molecule and (b) In this representation, each tri-group of light green octahedra denotes a propane molecule and olive-coloured spheres depict the hydrogen molecule position. Red dashed lines represent the hydrogen olive-coloured spheres depict the hydrogen molecule position. Red dashed lines represent the bond between water molecules. hydrogen bond between water molecules. Before starting the production NVT simulations, to let surrounding gas-phase hydrogen molecules Before starting the production NVT simulations, to let surrounding gas-phase hydrogen fully cover the clathrate surfaces, an initial MD runs at 280 K for 200 ps, with applied position molecules fully cover the clathrate surfaces, an initial MD runs at 280 K for 200 ps, with applied restraint on the water molecules of the hydrate. Subsequent NVT production simulations, at various position restraint on the water molecules of the hydrate. Subsequent NVT production simulations, at temperatures (180–273 K) were performed over 500 ns for release and 2 µs for uptake study. Báez various temperatures (180–273 K) were performed over 500 ns for release and 2 µs for uptake study. and Clancy The clathrate-liquid-ice recognition metrics were employed to distinguish geometrically Báez and Clancy The clathrate-liquid-ice recognition metrics were employed to distinguish between the encapsulated vs. free gas molecules [25]. The gas molecules with minimum 8 hydrate like geometrically between the encapsulated vs. free gas molecules [25]. The gas molecules with water molecules categorized as encapsulated gas inside clathrate hydrate cage. minimum 8 hydrate like water molecules categorized as encapsulated gas inside clathrate hydrate 3. Experimentalcage. Methodology

3.The Experimental experimental Methodology apparatus for hydrate- formation included gas cylinders, a 316 stainless steel pressure vessel with approximately 340 cm3, refrigerator to control the temperature of the vessel, tilting shakerThe experimental to agitate the apparatus water inside for thehydrate vessel- formation and logging included system gas to recordcylinders the, a data 316 duringstainless the steel formationpressure time vessel (Figure with2). approximately Detailed information 340 cm about3, refrigerator the hydrate to systemcontrol isthe presented temperature in our of previous the vessel, worktilting [26]. shaker Before to each agitate experiment, the water the inside vessel the was vessel sterilised and usinglogging an system ethanol to solution record (70the wt%). data during 10 cm3 the of deionisedformation water time (Fig wasure loaded 2). Detailed into the information vessel. The about vessel the closed, hydrate sealed system and is evacuated presented forin our 3 min previous to removework any [26] residual. Before air, each and experiment then pressurised, the vessel to 4 bar was using sterilised pure high using purity an ethanol propane solution gas. The (70 cell wt%). was 10 cm3 of deionised water was loaded into the vessel. The vessel closed, sealed and evacuated for 3 min cooled down to 1.5 ◦C and kept at the desired temperature for 7 days to gauge the yield towards full to remove any residual air, and then pressurised to 4 bar using pure high purity propane gas. The cell was cooled down to 1.5 °C and kept at the desired temperature for 7 days to gauge the yield towards full hydrate conversion. In the second step, 25 bar of hydrogen was loaded on the top of Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 10 Appl. Sci. 2020, 10, 8962 4 of 10 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 10 formed propane hydrate to study hydrogen uptake at (or very near) the desired temperature of 1.5 formedhydrate°C for propane 7 conversion. days .hydrate In theto study second hydrogen step, 25 baruptake of hydrogen at (or very was near) loaded the ondesired the top temperature of formed propaneof 1.5 °Chydrate for 7 days to. study hydrogen uptake at (or very near) the desired temperature of 1.5 ◦C for 7 days.

Figure 2. Schematic of the gas-hydrate rig. The four main sections are gas supplier, distribution FigureFigureterminal, 2. 2.SchematicSchematic reactor and of ofthe refrigerator. the gas gas-hydrate-hydrate rig. rig. The The four four main main sections sections are are gas gas supplier, supplier, distribution distribution terminal,terminal, reactor reactor and and refrigerator. refrigerator. 4. Results and Discussion 4. Results and Discussion 4. ResultsForm and ourDiscussion previous study, the melting temperatures of T propane/hydrogen-clathrate surfaces Form our previous study, the melting temperatures of T propane/hydrogen-clathrate surfaces wereForm expected our previous to be study, around the 2m80elting K [17] temperatures. The evolution of T propane in the /hydrogennumber of-clathrate hydrogen surfaces uptaken were expected to be around 280 K [17]. The evolution in the number of hydrogen uptaken into/released wereinto expected/released to from be the around propane 280 hydrate K [17] w. asThe judged evolution via the in Báez the– Clancynumber criteria, of hydrogen is specified uptaken in Fig ure from the propane hydrate was judged via the Báez–Clancy criteria, is specified in Figure3 for both into3/released for both from release the a propanend uptake hydrate at 180 w, 200,as judged 220, 240 via, 260the Báezand 273–Clancy K. Due criteria, to the is observed specified slow in Fig kineticsure release and uptake at 180, 200, 220, 240, 260 and 273 K. Due to the observed slow kinetics of the uptake 3 forof both the uptakerelease simulation,and uptake the at 180 simulation, 200, 220, for 240 hydrogen, 260 and uptake 273 K. extendedDue to the up observed to 5 µs while slow forkinetics the case simulation, the simulation for hydrogen uptake extended up to 5 µs while for the case of release study of theof uptake release simulation, study a kind the simulation of plateau for was hydrogen observed uptake after extended 500 ns. up The to 5 rate µs while constant for the of case the gas a kind of plateau was observed after 500 ns. The rate constant of the gas uptake/release correlates of releaseuptake/release study aco kindrrelates of dire plateauctly with was the observed simulation after temperate 500 ns. as The the rate thermal constant “driving of theforces gas” for directly with the simulation temperate as the thermal “driving forces” for gas diffusion. uptake/releasegas diffusion. co rrelates directly with the simulation temperate as the thermal “driving forces” for gas diffusion.

FigureFigure 3. 3.Hydrogen Hydrogen uptake uptake (a ()a and) and release release (b (b) in) in non-equilibrium non-equilibrium simulations simulations at at various various temperature. temperature. Figure 3. Hydrogen uptake (a) and release (b) in non-equilibrium simulations at various temperature. TheThe density density map map of of molecules molecules (water, (water, propane propane and and hydrogen) hydrogen) along along the the X-axis X-axis at at di differentfferent time time stagesstagesThe candensity can provide provide map aof a better moleculesbetter insight insight (water, on on the the propane hydrate hydrate and condition, condition, hydrogen) the the along position position the ofX of- theaxis the propane atpropane different molecules molecules time stageswhichwhich can can canprovide also also be a be better interpreted interpreted insight as on as the the large-cage hydrate large-cage condition, occupancy occupancy the and andposition the the cage-filling cageof the-filling propane pattern pattern molecules based based on on whichhydrogenhydrogen can also di diffusionff usion be interpreted in in propane propane as hydrate. thehydrate. large Figure Figure-cage4 a occupancy 4a illustrates illustrates and the the density the density cage map- mapfilling for for hydrogen pattern hydrogen based uptake uptake on in in hydrogenwhichwhich hydrogen diffusionhydrogen densityin density propane in in the hydrate.the range range ofFigure of 3.2 3.2 to 4ato 7.9 illustrates7.9 along along X-axis Xthe-axis density at at the the startingmap starting for timehydrogen time of of the the uptake simulation simulation in which(t(t= = 0 hydrogen0 ns)ns) is zero density while while inwater water the rangedistribution distribution of 3.2 toin in 7.9this this along range range X is-axis well is well at distributed, the distributed, starting due time due to ofthe to the long the simulation long-range-range order (t =order of0 ns) clathrate ofis clathratezero whilehydrate. hydrate. water This distribution Thisorder order also in alsocan this canbe r angeobserved be observed is well in distributed, the in thecase case of due propane of propaneto the molecules,long molecules,-range thoseorder those are of areclathratelocated located inhydrate. inlarge large cages. This cages. orderDuring During also the can thesimulation be simulation observed time, time, inthe the water the case water and of propane andpropane propane molecules,distributions distributions those stay are intact stay locatedintactwhil ine while the large hydrogen the cages. hydrogen During penetrating penetrating the simulation the hydrate the hydratetime, area the and area water filling and and the filling propanefirst the and first distributionssecond and column second stay columnof intactthe small of whilthecagese smallthe inhydrogen hydrate cages in penetratingstructure. hydrate structure. The the existing hydrate The plateau area existing and in plateaufillingFigure the 3a in afterfirst Figure and2503 anssecond after for most 250 column ns of forthe of mosttemperatures the small of the cages in hydrate structure. The existing plateau in Figure 3a after 250 ns for most of the temperatures Appl. Sci. 2020, 10, 8962 5 of 10 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 10 temperaturescan also be can noted also in be the noted similar in the density similar map density of hydroge map ofn at hydrogen 250 and at500 250 ns. and The 500 main ns. reason The main behind reasonthisbehind similarity this issimilarity attributable is to attributable the slow diffusion to the slow rate diofff theusion hydrogen rate of themolecules hydrogen inside molecules the hydrate insidestructure. the hydrate Internal structure. hopping Internal of hydrogen hopping of inside hydrogen a hydrogen inside a- hydrogen-hydratehydrate lattice ha lattices been has previously been previouslyinvestigated investigated by English by Englishand his colleagues and his colleagues [27,28], but [27 in,28 the], but case in of the propane/hydrogen case of propane/ hydratehydrogen, since hydrate,all large since cages all large are cagesalready are occupied already occupiedby propane by propanemolecules, molecules, the only the available only available path for path filling for the fillingwhole the whole hydrate hydrate is the is thesmall small-to-small-to-small cage cage hopping hopping which which has to to be be done don bye by pathing pathing hydrogen hydrogen moleculemolecule through through a pentagonal a pentagonal ring. ring.

(a)

(b)

FigureFigure 4. Density 4. Density map map of propane, of propane, water, water, and and hydrogen hydrogen molecules molecules along along the X-axis the X- foraxis uptake for uptake (a) and (a) and releaserelease (b). (b).

TheThe density density map map during during hydrogen hydrogen release release simulation simulation is illustrated is illustrated in Figure in Figure4b. Since 4b. Since for release for release studystudy the hydratethe hydrate is in is contact in contact with with vacuum, vacuum, some some propane propane from from the outerthe outer layer layer cages cages will will be released be released intointo vacuum vacuum part. part. The The presence presence of propane of propane molecules molecules can becan seen be seen in Figure in Fig4ureb. A 4b. hydrogen A hydrogen molecule molecule leavesleaves its small its small cage, cage, hops hops into into another another small small cage cage in its in neighbour its neighbo andur and finds finds the waythe way out out from from the the hydratehydrate lattice. lattice. The same The limitation same limitation discussed discussed on the di ffi onculty the of difficulty small-to-small of small hopping,-to-small the hydrogenhopping, the existshydrogen in the centre exists of in the the hydrate centre of lattice the hydrate may never lattice be ablemay tonever leave be the able hydrate to leave lattice. the hydrate lattice. To analyseTo analyse the ditheff usion diffusion of the of hydrogen the hydrogen molecules molecules inside inside the hydratethe hydrate structure, structure, 300 300 sets sets of of hydrogenhydrogen molecules molecules remaining remaining in the in hydratethe hydrate structure structure over over the wholethe whole sampling sampling period period (100 (100 ns) werens) were identifiedidentified using using Baez–Clancy Baez–Clancy method, method, and and the MSDthe MSD value value of each of each set wasset was calculated. calculated. The The averaged averaged MSDMSD curve curve at each at each temperature temperature for bothfor both uptake uptake and and release release studies studies is illustrated is illustrated in Figure in Figure5 and 5 theand the diffusiondiffusion coe fficoefficientcient of each of each is listed is listed in Table in Table1. It 1. was It foundwas found that thethat hydrogenthe hydrogen molecules molecules show show a a higherhigher movement movement in the in casethe case of release of release studies studies in which in which the hydratethe hydrate is in is contact in contact with with a vacuum a vacuum in in comparecompare with with the uptakethe uptake study study where where gas gas form form hydrogen hydrogen molecules molecules are are located located on theon the top top of the of the hydratehydrate structure. structure. This This difference difference is caused is caused following following to the to presence the presence of some of emptysome empty large cageslarge duecages to due the migrationto the migration of propane of propane molecules molecules from the from hydrate the structurehydrate structure to the vacuum to the part.vacuum On thepart. other On hand,the other in uptakehand, studiesin uptake fully studies occupied fully largeoccupied cages, large all alongcages, the all simulationalong the simul time,ation limited time, movement limited movement of the infiltratedof the infiltrated hydrogen moleculeshydrogen molecules into hydrate into structure hydrate resultedstructure in resulted a lower in di ffausion lower coe diffusionfficient coefficient in this workingin this condition. working Thecondition. activation The energies activation of eachenergies diffusion of each scenario, diffusion in the scenario, presence in and the absencepresence of and absence of available large cages, release and uptake, are also calculated using the Arrhenius model based on the extracted diffusion coefficients at high temperatures (Table 1). Non-Arrhenius diffusion Appl. Sci. 2020, 10, 8962 6 of 10 available large cages, release and uptake, are also calculated using the Arrhenius model based on the Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 10 extracted diffusion coefficients at high temperatures (Table1). Non-Arrhenius di ffusion behaviour observedbehaviour at observed lower temperatures at lower temperatures can due to an can enhanced due to an local enhanced structural local ordering structura [29l ].ordering In addition, [29]. atIn aaddition, lower temperature, at a lower temperature, the structure the of thestructure diffusion of the layer diffusion formed layer in a solidformed/gas in interface a solid/gas also interface plays a significantalso plays rolea significant in the hydrogen role in the uptake hydrogen/release uptake/release kinetics [30]. Furtherkinetics studies [30]. Further are required studies to are explore required the interfacialto explore phenomenathe interfacial in p gashenomena hydrate systems.in gas hydrate systems.

FigureFigure 5.5. Mean-squareMean-square displacementdisplacement (MSD)(MSD) curvescurves ofof thethe hydrogenhydrogen moleculesmolecules inin hydratehydrate cagescages atat variousvarious temperaturestemperatures forfor bothboth uptakeuptake (a(a)) and and release release ( b(b)) simulation simulation studies. studies.

ThisThis resultresult was was confirmed confirmed in in previous previous reports reports [28 ,31[28,31]] showing showing the lowerthe lower energy energy barrier barrier for a small for a cagesmall to cage a large to a cage large di ffcageusion diffusion in compare in compare with the gaswith di theffusion gas fromdiffusion a small from cage a small to another cage smallto another cage insmall a hydrate cage in structure. a hydrate structure.

Table 1. The diffusion coefficient and the activation energy of the hydrogen molecules in hydrate cages Table 1. The diffusion coefficient and the activation energy of the hydrogen molecules in hydrate at various temperatures for both uptake and release simulation studies. cages at various temperatures for both uptake and release simulation studies. 5 2 DiffusionDiffusion Coefficient Coefficient ( (1010−5 cmcm2/s)/s) TempTemp (K) (K) UptakeUptake Release Release 180 0.0169 0.00002 180 0.0169 0.00002 200 0.0368 0.00003 200 0.0368 0.00003 220 0.0165 0.00066 220 0.0165 0.00066 240 0.0281 0.01226 240 0.0281 0.01226 260 0.0441 0.02897 260 0.0441 0.02897 273 0.0694 0.03813 273 0.0694 0.03813 Activation Energy (kJ/mol) 13.11 ± 1.11 44.80 ± 5.35 Activation Energy (kJ/mol) 13.11 1.11 44.80 5.35 ± ± Hydrogen storage in clathrates was also studied experimentally in two steps. The first involved formationHydrogen of pure storage hydrate in clathrates, and second was loading also studied of pure experimentally hydrogen into in the two formed steps. hydrate. The first There involved are formationtwo main ofcategories pure hydrate, in the and design second of loadingthe reactors of pure used hydrogen for hydrate into theformation, formed hydrate.(I) water Theredispersion are two in mainwhich categories water dispersed in the design in a cold of the high reactors-pressure used gas for chamber hydrate formation,[32–34] and (I) (II) water gas dispersion dispersion in reactors which waterwhere dispersed high-pressure in a coldgas introduces high-pressure to a gascold chamber body of [water.32–34] In and this (II) family gas dispersion gas/water reactorsmixture wherecan be high-pressureenhanced by gas bubbling introduces [35– to37] a coldor different body of water. stirring In techniquesthis family gas[38/water–40]. Themixture designs canbe with enhanced water bydispersion bubbling found [35–37 to] orbe di morefferent suitable stirring for techniques hydrogen [38 storage–40]. The due designs to the formation with water of dispersion the small foundpieces toof behydrates more suitable which will for hydrogenprovide sufficient storage duefree tosurfaces the formation for hydrogen of the uptake small pieces or release. of hydrates Many designs which willare dispersing provide su ffigascient in a freelarge surfaces volume for of hydrogenwater, which uptake is useful or release. for many Many other designs applications are dispersing such as gaswater in atreatment large volume or dewate of water,ring. which The formation is useful for of manya big otherslab of applications hydrate will such make as waterit unsuitable treatment for orhydrogen dewatering. storage The formation application. of a An big slab extra of stephydrate of will hydrate make crashingit unsuitable is forbeing hydrogen used forstorage gas application./desorption An extra step application. of hydrate crashing is being used for gas adsorption/desorption application. FigureFigure6 6aa presents presents the the reactor reactor used used to to hydrate hydrate formation formation in in this this study. study. This This reactor reactor works works based based onon gasgas dispersiondispersion mechanismmechanism forfor hydratehydrate formationformation whichwhich meansmeans thethe gasgas willwill bebe purgedpurged intointo alreadyalready loaded water at low temperature and after a while, the hydrate will nucleate and start growing on the wall and bottom of the reactor and at the end of the formation reaction, a solid, dense and large part of the hydrate will form (Figure 6b,c). The number of adsorbed propane molecules during the first step (hydrate formation) was calculated based on the pressure drop and compressibility factor Appl. Sci. 2020, 10, 8962 7 of 10 loaded water at low temperature and after a while, the hydrate will nucleate and start growing on the wall and bottom of the reactor and at the end of the formation reaction, a solid, dense and large part of the hydrate will form (Figure6b,c). The number of adsorbed propane molecules during the first step (hydrate formation) was calculated based on the pressure drop and compressibility factor of propane moleculeAppl. at Sci. the 2020 temperature, 10, x FOR PEER and REVIEW pressure hydrate formation condition (Figure6d,e). Since 107 mLof 10 of water was loaded into the reactor the expected adsorbed propane molecule after the full formation of of propane molecule at the temperature and pressure hydrate formation condition (Figure 6d,e). type-IISince hydrate 10 m willL of bewater equal was to loaded 0.032 mol.into the This reactor means the althoughexpected adsorbed no water propane in liquid molecule form remained after the in the reactor,full formation only 73% of of type expected-II hydrate gas will was be adsorbed equal to 0.032 during mol. hydrate This means formation. although This no diwaterfference in liquid can be explainedform by remained two main in the reasons: reactor, (1) only a small 73% of minority expected of gas formed was adsorbed large cages during stayed hydrate un-occupied formation. This and (2) some possibledifference water can be in explained liquid form by two is stocked main reasons: in between (1) a small two formedminority hydrateof formed chunks. large cages On thestayed other hand, theun- ratiooccupied between and (2) the some amounts possible of water adsorbed in form is over stocked adsorbed in between propane two formed is around hydrate 2.42, as chunks. On the other hand, the ratio between the amounts of adsorbed hydrogen over adsorbed compared to an expected value 2 based on type II hydrate (assuming one H2 molecule per small cage). This higherpropane adsorption is around of2.42 the, as hydrogen compared isto inan agreementexpected value with 2 based the first on type assumption II hydrate mentioned (assuming one above H2 molecule per small cage). This higher adsorption of the hydrogen is in agreement with the first regarding formation of propane hydrate with a small minority of empty large cages. This means that a assumption mentioned above regarding formation of propane hydrate with a small minority of part of the adsorbed hydrogen fills, in part, the available large cages, e.g., with double occupation a empty large cages. This means that a part of the adsorbed hydrogen fills, in part, the available large likelihoodcages, at e.g., present with pressuresdouble occupation in the general a likelihood range at ofpresent 20–30 pressures bar. in the general range of 20–30 bar.

(a) (b) (c)

(d) (e)

Figure 6. (a) High-pressure apparatus used for hydrogen growth and hydrogen storage study, (b,c) the Figure 6. (a) High-pressure apparatus used for hydrogen growth and hydrogen storage study, (b,c) formed hydrate immediate after pressure release and opening the reactor, (d) Number of propane the formed hydrate immediate after pressure release and opening the reactor, (d) Number of propane moles adsorbedmoles adsorbed during during the formation the formation of the ofhydrate the hydrate in the infirst the 7 days first 7 under days 4under bar pressure 4 bar pressure of propane, of (e) Numberpropane, of hydrogen (e) Number moles of hydrogen adsorbed moles during adsorbed the during storage the step storage in the step second in the second 7 days 7 days under under 25 bar pressure25 of bar hydrogen pressure of at hydrogen 1.5 ◦C. at 1.5 °C .

On the other hand, the maximum hydrogen storage capacity of a fully occupied type II gas hydrate, 8 propane and 16 hydrogen molecules in a unit cell with 136 water molecules will be 1.13 Appl. Sci. 2020, 10, 8962 8 of 10

On the other hand, the maximum hydrogen storage capacity of a fully occupied type II gas hydrate, 8 propane and 16 hydrogen molecules in a unit cell with 136 water molecules will be 1.13 wt%. This value can increase in the case of partial occupancy of propane in the large cages (Table2). Based on the number of adsorbed hydrogen and propane molecules in the hydrate formed using 10 mL of water, the overall hydrogen stored in the formed propane hydrate is about 1.04 wt%. This observation also supports the second assumption regarding the existence of liquid-form water trapped on the surface of between formed hydrate chunks. It is worth mentioning that the higher observed value vis-a-vis the previously reported values of hydrogen storage in C3H8+H2 hydrate, 0.33% at 270 K [41], and 0.04% at 263 K [42] is due to lower selected working temperature. According to our MD results, the diffusion coefficient at 273 K (0.0694 10 5 cm2/s) is approximately 1.5 times larger than that of 260 × − K (0.0441 10 5 cm2/s), suggesting a strong dependence of the uptake rate on temperature. × − Table 2. Estimated-hydrogen storage capacity with respect to the large-cage occupancy.

Large Cages Water Propane Hydrogen Storage Capacity Occupancy (%) Molecules Molecules Molecules (wt%) 100 1360 80 160 1.13% 95 1360 76 172 1.22% 90 1360 72 184 1.31% 85 1360 68 196 1.41% 80 1360 64 208 1.50% 75 1360 60 220 1.60% 70 1360 56 232 1.69%

5. Conclusions In this study, propane/hydrogen mixed hydrate (sII structure) was chosen as a suitable hydrate for hydrogen storage with ideal 1.13 wt% storage capacity. Required non-equilibrium MD simulations were conducted to analyse hydrogen release and uptake from/into propane planar clathrate surfaces at 180–273 K. The observed surface fully treated hydrate models from a simulation study, in addition to, the visual inspection of formed hydrate beside lower hydrogen storage capacity for the formed hydrate (1.04 wt%) in experiments, proved the requirement of forming hydrates with the maximum surface to volume ratio to be able to maximise the storage capacity as well as the gas uptake kinetics. As a result, we advocate the use of liquid-dispersion reactors, featuring a water-injection system into a cold gas chamber at high pressure, as opposed to the use of gas-dispersion (fixed-bed) reactors. In future work, detailed (and possibly time-dependent) measurement and characterisation grain size of hydrate would be useful, in terms of estimating growth rates of the C3H8+H2 hydrate more accurately. These results could be compared with results obtained from molecular dynamics.

Author Contributions: M.R.G., S.T. and N.J.E. conceived the study conceptually. M.R.G. did the experimental and simulations. S.T. and N.J.E. contributed importantly with an interpretation of results. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Irish Research Council (GOIPD/2016/365) and Science Foundation Ireland 15/ERC-I3142. Acknowledgments: M.R.G. and N.J.E. thank the Irish Research Council and Science Foundation Ireland for their funding. Conflicts of Interest: The authors declare no conflict of interest.

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