Energy and Environmental Analysis of Membrane-Based CH4-CO2 Replacement Processes in Natural Gas Hydrates

energies

Article Energy and Environmental Analysis of Membrane-Based CH4-CO2 Replacement Processes in Natural Gas Hydrates

Beatrice Castellani * , Alberto Maria Gambelli, Andrea Nicolini and Federico Rossi

CIRIAF, University of Perugia, Via G. Duranti 67, 06125 Perugia, Italy; [email protected] (A.M.G.); [email protected] (A.N.); [email protected] (F.R.) * Correspondence: [email protected]; Tel.: +39-075-5853914

Received: 29 January 2019; Accepted: 28 February 2019; Published: 5 March 2019

Abstract: Natural gas hydrates are the largest reservoir of natural gas worldwide. This paper proposes and analyzes the CH4-CO2 replacement in the hydrate phase and pure methane collection through the use of membrane-based separation. The investigation uses a 1 L lab reactor, in which the CH4 hydrates are formed in a quartz sand matrix partially saturated with water. CH4 is subsequently dissociated with a CO2 stream supplied within the sediment inside the reactor. An energy and environmental analysis was carried out to prove the sustainability of the process. Results show that the process energy consumption constitutes 4.75% of the energy stored in the recovered methane. The carbon footprint of the CH4-CO2 exchange process is calculated as a balance of the CO2 produced in the process and the CO2 stored in system. Results provide an estimated negative value, equal to 0.004 moles sequestrated, thus proving the environmental beneﬁt of the exchange process.

Keywords: methane hydrate; CO2 replacement; hydrate formation; natural gas hydrate; membrane separation

1. Introduction Global warming has become one of the main issues in the public domain, and its importance is steadily increasing. The temperature increase can be in good part ascribed to the increase in anthropogenic carbon emissions, as a result of mankind’s massive dependence on carbon-based fossil fuels for the majority of energy production. In order to prevent or at least decelerate the advancement of global climate changes, the carbon footprint of human activities needs to be reduced [1]. Combustion of cleaner fuels, increased use of renewable energies and a focus on CO2 capture technologies can all contribute to this ultimate goal [2]. Natural gas use in power generation provides low capital costs and favorable heat rates, for this reason is an attractive fuel [3]. Moreover, the decarbonisation in the energy sector indicates a process of progressive decrease over time of the H/C ratio in the different sources of energy supply. Methane, with a ratio of 4, presents the highest value among fossil energy sources. In addition to conventional gas and oil wells, unconventional deposits such as shale gas, tight gas, or coal bed methane deposits have been studied as huge gas resources. Nevertheless, the largest amount of methane worldwide is trapped in the permafrost and oceanic sediments, as natural gas hydrates (NGH). Gas hydrates are solid crystalline compounds, in which small gas molecules are trapped inside cages of hydrogen-bonded water molecules. They form when the guest gas (i.e., CH4) and water coexist at sufﬁciently low temperatures and high pressures. There are important results in literature about experimental and molecular simulations, which describe in detail the mechanisms of gas hydrate

Energies 2019, 12, 850; doi:10.3390/en12050850 www.mdpi.com/journal/energies Energies 2019, 12, 850 2 of 17 nucleation and crystal growth [4,5] thermal and mechanical properties [6,7], mass and heat transport processes [8–10]. One volume of solid methane hydrate contains 164 volumes of gaseous methane at standard temperature and pressure (STP), which is equivalent to room temperature at 18 MPa [11]. The amount of organic carbon contained in NGH reserves around the globe is estimated to be twice the amount contained in all currently recoverable worldwide conventional hydrocarbon resources [12]. Numerical estimates of world NGH deposits range from the first published number of 3053 × 1015 Sm3 [13] to more conservative values of 15 × 1015 Sm3 [14], 2.5 × 1015 Sm3 [15] and 120 × 1015 Sm3 [16]. NGH exist in those areas where both biogenic or thermogenic gas sources occur under large, cold masses providing high pressure values and low temperatures. These conditions occur beneath quite thick frozen soils such as arctic permafrost. Several areas in the Arctic show potential for having gas hydrate accumulations, in North America, in Russia, in Norway. Another primary hydrate stability zone are regions in the subsea floor sediment, where temperatures and pore pressures are suitable for stable NGH [17]. NGH marine deposits are generally found along the continental slopes and shelves with abundance of organic matter supply and with water depths exceeding 500 m. In the deepest ocean regions, below around 2000 to 3000 m, hydrates are scarcely found because of the limited availability of methane that in the ocean is produced by microbial decomposition of organic matter sinking down from the sunlit zone near the surface. Conversely, no NGHs are found in marginal seas because of the insufficient pressure for their stability [18]. Recent programs of gas hydrate field drilling have assessed gas hydrate accumulations in the marine sands in the Cascadia Margin, Gulf of Mexico and Nankai Trough, and also other types in offshore India, Malaysia and Korea [12]. The natural gas production from oceanic and permafrost sediments is currently being performed using methods such as injection of hydrate inhibitors, depressurization and thermal stimulation. The chemical inhibition method seeks to displace the NGH local conditions of pressure and temperature outside the thermodynamic conditions of hydrate stability zone through the injection of a liquid inhibitor from the surface down to NGH bearing layers [19]. The depressurization method dissociates methane hydrate by decreasing the pressure in the wellbore drilled inside the NGH sediments [20]. When pressure decreases, NGH becomes thermodynamically unstable and dissociates absorbing sensible heat from the surrounding sediments as well as geothermal heat flow. Depressurization method has been extensively investigated at laboratory scales [21]. The thermal stimulation method decomposes NGH by raising temperature beyond the hydrate stability zone. Most of the thermal dissociation tests have been performed via the injection of heat transfer fluids, such as steam, into the hydrate sediment [22]. Other proposed thermal stimulation techniques are microwave heating, electro-magnetic heating, and in-situ combustion [19]. These methods are considered more promising than the conventional heated fluid injection methods, thanks to the direct energy contribution necessary for the dissociation at the hydrate zone [17]. The in-situ combustion process consists in locating a point heat source in the hydrate region, via direct combustion of a liquid fuel and an oxidant [23,24]. Recovering methane from hydrate reservoirs can be also combined with simultaneous permanent CO2 sequestration [23,25,26]. The CO2 injection into the NGH sediments causes the gaseous CH4 release and the formation of CO2 hydrate, even if global conditions of pressure and temperature remain within the CH4 hydrate stability zone. This gives the opportunity to simultaneously recover an energy resource while entrapping a greenhouse gas. Many experimental and numerical studies have been performed to understand the CH4-CO2 replacement [27]. Baig et al. in [28] stated that conversion of CH4 hydrates in CO2 hydrates occurs according to two mechanisms: new CO2 hydrate formation starting from gaseous CO2 and free water surrounding hydrates and a direct conversion in the solid state. Energies 2019, 12, 850 3 of 17

The solid-state exchange is slow because of mass transport limitations. The formation of new hydratesEnergies is2019 a, faster12, x process since it releases heat, assisting the in situ dissociation of the3 methane of 17 hydrate. This is faster, because heat transport is very rapid in these systems [29]. The solid-state exchange is slow because of mass transport limitations. The formation of new hydratesThere is is a a soundfaster process body of since knowledge it releases in heat, literature assisting about the in energy situ dissociation potential ofof NGHthe methane reservoirs, withhydrate. studies This in laboratory is faster, because settings heat and transport some recentis very rapid field trials,in these deeply systems reviewed [29]. in [12]. According to a hydrateThere research is a sound roadmap body of byknowledge the U.S. in National literature Methane about energy Hydrate potential R&D of program NGH reservoirs, [30], the key researchwith studies topics toin belaboratory investigated settings are and the some development recent field technologies trials, deeply of reviewed sampling, in [ characterization,12]. According to and production,a hydrate the research investigation roadmap on by the the role U.S. of gas National hydrate Methane in the globalHydrate climate R&D programchange. Some[30], the production key technologiesresearch topics have to been be recentlyinvestigated proposed, are the suchdevelopment as the use technologies of horizontal of sampling wellbore, characterization, [31]. andThe production, present paper the investigation aims at addressing on the role the of issuegas hydrate of a productionin the global technology climate change. development, Some production technologies have been recently proposed, such as the use of horizontal wellbore [31]. proposing a membrane-based CH4-CO2 replacement process for methane recovery and CO2 storage inThe NGH. present Production paper aims control at addressing during NGH the issue exploitation of a production is considered technology by the development, NGH scientific proposing a membrane-based CH4-CO2 replacement process for methane recovery and CO2 storage community a key priority which involves appropriate drilling and hazard monitoring technologies [32]. in NGH. Production control during NGH exploitation is considered by the NGH scientific Nevertheless, to the best of our knowledge, studies on the CH -CO replacement process, which take community a key priority which involves appropriate drilling4 and hazard2 monitoring technologies into account the separation efficiency and propose a process to recover pure methane, are not present [32]. Nevertheless, to the best of our knowledge, studies on the CH4-CO2 replacement process, which in literature.take into account the separation efficiency and propose a process to recover pure methane, are not presentIn the in present literature. paper, an integrated process with a membrane section is proposed and analyzed from anIn energy the present and paper, environmental an integrated point process of with view. a membrane The use section of mechanical is proposed devicesand analyzed such as membranesfrom an energy [33] is proposedand environmental to separate point the of CO view.2 and The CH use4 which of mechanical have not devices taken partsuch toas themembranes replacement process.[33] is In proposed this way, to pure separate methane the CO can2 and be CH recovered4 which have and COnot2 takencan bepart recirculated to the replacement in the sediment. process. InThe this methaneway, pure methane recovery can and be recovered the CH4 and-CO CO2 replacement2 can be recirculated in the in hydratethe sedime phasent. is studied The methane recovery and the CH4-CO2 replacement in the hydrate phase is studied experimentally in a 1 L lab scale reactor. The CH4 hydrates are formed in a matrix resembling experimentally in a 1 L lab scale reactor. The CH4 hydrates are formed in a matrix resembling geological properties of natural seafloor and are subsequently dissociated by a CO2 stream supplied in geological properties of natural seafloor and are subsequently dissociated by a CO2 stream supplied the upper part of the reactor. Experimental results will be used as input for energy and environmental in the upper part of the reactor. Experimental results will be used as input for energy and evaluations to prove the sustainability of the process. environmental evaluations to prove the sustainability of the process. 2. Materials and Methods 2. Materials and Methods The present paper aims at studying a continuous CH4-CO2 replacement process, which integrates The present paper aims at studying a continuous CH4-CO2 replacement process, which the useintegrates of a membrane the use of a separation membrane sectionseparation for section pure methane for pure recovery.methane recovery. AnAn experimental experimental apparatus apparatus waswas usedused to reproduce NGH NGH marine marine sediments sediments and and perform perform the the CH4CH-CO4-CO2 replacement.2 replacement. The The experimental experimental data data areare usedused togethertogether with the the membrane membrane operation operation data data to obtainto obtain energy energy and environmentaland environmental results results through through anumerical a numerical simulation, simulation, asas shownshown in Figure Figure 11..

FigureFigure 1. 1.Flowchart Flowchart of the the methodology methodology used. used.

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The experimental tests tests consist consist in in a firstﬁrst step of artificialartiﬁcial repreproductionroduction of the NGH sediment insideinside the reactor and in a a second second step step of of CHCH44-CO-CO2 replacementreplacement in in the the NGH. NGH. On On the the basis basis of of the experimental results results obtained, an an energy energy and environmental analysis analysis about about the the integration of of the CH4--COCO22 replacementreplacement with with the the propo proposedsed membrane membrane separation separation section section is is then then discussed. discussed.

2.1. Experimental Experimental Apparatus The reactor is built in 316316 stainlessstainless steelsteel andand hashas aa 0.0730.073 mm internalinternal diameter, diameter, a a 0.2 0.2 m m length length and and a a0.0076 0.0076 m m wall wall thickness. thickness. The The reactor reactor is closed is closed by two by plates two plates of 0.047 of m 0.047 thickness, m thickness, which have which several have severalports to ports connect to connect the input the and input output and output gas lines gas and lines instrumentation. and instrumentation. Flanges Flanges are secured are secured by bolts. by bolts.The vessel The vessel is designed is designed to withstand to withstand a 15 MPa a 15 internal MPa internal pressure. pressure. The reactor The is reactor positioned is positioned inside a insidethermostatic a thermostatic bath to bat ensureh to ensure temperature temperature control, control, as shown as shown in Figure in Figure2. The 1 lower. The lower ﬂange flange has one has onecentral central 3/8” 3/8” port, port, used used for methanefor methane injection. injection. The The upper upper ﬂange flange has severalhas several gates: gates: (i) one (i) centralone central inlet inletis used is used for CO for2 injection,CO2 injection, (ii) gates (ii) gates are used are used for positioning for positioning four thermocouples,four thermocouple (iii)s, one (iii) gate one isgate used is usedfor checking for checking valve valve and pressure and pressure gauge, gauge, (iv) one (iv) outlet one outlet is used is forused gas for recovery. gas recovery.

Figure 2. ExperimentalExperimental apparatus apparatus:: view view from from above above on on the the left, side view on the right right..

1 The CO 2 inletinlet is is equipped equipped with with ¼”4 ” tube to inject the COCO22 inside the hydrate sediment. The The four four 11-mm-mm K type thermocouples are inserted in the reactor at different depths through a high pressure multimulti-sensor-sensor fitting fitting (supplied by Spectite Spectite,, Hillside, Illinois,Illinois, USA).USA). Thermocouples are called T16, T11, T07 and T02 and are located inside the reacto reactorr at different depth: these are respectively 16, 11, 7 and 2 cm cm far far from from the the upper upper flange. flange. A pressure A pressure release release valve valve and a and digital a digital manometer manometer (supplied (supplied by Kobold, by KoboldSettimo, Settimo M.se Milano, M.se Milano, Italy) are Italy) inserted are inserted in the in third the gatethird to gate monitor to monitor the headspace the headspace pressure pressure and andprovide provide depressurization depressurization in case in ofcase pressure of pressure increase increase above above the nominal the nominal value. Thevalue. fourth The gatefourth is usedgate isto used recovery to recovery methane methane after dissociation after dissociation and CH 4and-CO 2CHreplacement.4-CO2 replacement. Gas flows Gas are flows monitored are monitored by mass byflowmeters mass flowmeters (El-flow Series) (El-flow supplied Series) by supplied Bronkhorst by Bronkhorst (Ruurlo, The (Ruurlo, Netherlands), The Netherlands calibrated for), calibrated CH4-CO2 3 −1 forflow CH rates4-CO of2 flow 0–4 Nmratesh of 0.– Instruments4 Nm3h−1. Instruments are connected are connected to a National to a Instruments National Instruments (Austin, TX, (Austin, USA) Texas,data acquisition USA) data equipment acquisition for equipment monitoring for and monitoring recording andexperimental recording data. experi Amental gas chromatograph data. A gas chromatograph(model CP 4900 Micro-GC(model CP Varian, 4900 Micro Agilent,-GC Santa Varian, Clara, Agilent, CA, USA) Santa was Clara, used C foralifornia gas chromatographic, United States) wasanalyses. used Periodicfor gas chromatographic measurements with analyses. certified Periodic mixtures m wereeasurements performed with to evaluate certified the mixtures uncertainties, were performedwhich are always to evaluate lower the than uncer 1%, ensuringtainties, which an excellent are always repeatability lower thanon sequences 1%, ensuring of samples an excellent [34]. repeatability on sequences of samples [34].

2.2. Materials

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2.2. Materials The vessel is ﬁlled with quartz sand grains characterized by the following properties: (i) a mean particle diameter of 200 µm; (ii) sand porosity equal to 34%. For accumulation of gas and pressurization, a 0.01 m headspace is left over the sand deposit. Water is added to the sand to reach the targeted initial water saturation. Gas bottles of methane and carbon dioxide (99% purity) are provided by Air Liquide Italia Service (Lanuvio, Rome, Italy). Internal volumes are given in Table1.

Table 1. Reactor internal volumes.

Headspace volume 109 cm3 Reactor internal volume 949 cm3 Sand sediment volume 744 cm3 Sand sediment porosity 0.34% Sand pore volume 253 cm3 Water pore saturation 50% Water volume 126 cm3

2.3. Experimental Procedure

The experimental investigation starts with CH4 hydrate formation. It is then followed by CO2 injection.

2.3.1. Hydrate Formation The procedure starts when the experimental apparatus has an internal temperature in the 275–277 K range at a pressure of 0.1 MPa. Methane is injected in the closed vessel through the inlet port at the bottom plate and it ﬂows through the sand occupying the sediment pores and the empty head space. CH4 moles injected inside the reactor are measured by CH4 ﬂowmeter. Gas injection is stopped when the pressure reaches the initial value Pi (usually in 40–50 bar range), then the closed system is cooled down to the initial temperature. At these conditions, the system is in the hydrate thermodynamic region and hydrate formation starts. During the hydrate formation, since gaseous methane is entrapped into the hydrate phase, the internal pressure gradually decreases. Reaction heat causes a noticeable temperature increase. The thermostatic bath removes reaction heat and ensures temperature control. The formation test is considered completed when all the thermocouples measure the initial temperature, and the internal pressure goes back to the equilibrium pressure. The number of hydrate moles formed in the system are calculated with the Equation (1) given in [35]: Vpore PiZf − Pf Zi = nhyd P (1) Z RT − f f ρhyd where:

• Pi and Pf are respectively the initial and ﬁnal system pressure;

• Zi and Zf are the compressibility factors, calculated with the Peng-Robinson equation

• ρhyd is the ideal molar density of hydrate, calculated assuming 100% cage occupancy. Pore space saturation is calculated with Equation (2) given in [35]:

n 1 hyd ρhyd sH = (2) Vpore

The final system pressure Pf is the final equilibrium pressure for methane hydrate at the given temperature. Since Pf is fixed, the number of hydrate moles, and then pore space saturation, depends on the initial pressure Pi reached with the gas injection. Energies 2019, 12, 850 6 of 17

To evaluate experimentally the amount of the formed CH hydrates, it is necessary to dissociate Energies 2019, 12, x 4 6 of 17 the formed hydrates and measure the dissociation pressure achieved to establish the number of moles of theTo entrapped evaluate CHexperimentally4. Gas moles the obtained amount of by the the formed hydrate CH dissociation4 hydrates, it is are necessary calculated to dissociate with the Peng Robinsonthe formed equation. hydrates and measure the dissociation pressure achieved to establish the number of moles of the entrapped CH4. Gas moles obtained by the hydrate dissociation are calculated with the 2.3.2.Peng CO Robinson2 Replacement equation. At the end of the methane hydrate formation, when the equilibrium pressure is achieved, the 2.3.2. CO2 Replacement CO2 injection starts with a pure CO2 inlet flow through the CO2 injection valve and a mixed CH4-CO2 outletAt flow the through end of the the methane reactor outlethydrate valve. formation, when the equilibrium pressure is achieved, the CO2 injection starts with a pure CO2 inlet flow−1 through the CO2 injection valve and a mixed CH4-CO2 The CO2 inlet flow is equal to 0.1 Sl s : this low value allows to control pressure and temperature. outlet flow through the reactor outlet valve. During the CO2 flow, temperature is kept constant while pressure is reduced. This allows to The CO2 inlet flow is equal to 0.1 Sl s−1: this low value allows to control pressure and reach suitable thermodynamic conditions for CO2 hydrates formation and to promote the CH4-CO2 temperature. During the CO2 flow, temperature is kept constant while pressure is reduced. This replacement process. allows to reach suitable thermodynamic conditions for CO2 hydrates formation and to promote the CHAt4-CO the2 replacement end of the process. CO2 injection, the valves are closed and the CH4-CO2 replacement process starts.At Before the end the of replacement the CO2 injection process,, the the valves composition are closed of and the the internal CH4-CO gaseous2 replacement phase is process determined viastarts. gas Before chromatographic the replacement analysis. process, Thethe composition process ends of the with internal the pressure gaseous phase stabilization is determined due to the hydratesvia gas formation. chromatographic analysis. The process ends with the pressure stabilization due to the hydratesThe outlet formation. valve is then opened and the internal pressure reaches 0.1 MPa. The vessel is closed The outlet valve is then opened and the internal pressure reaches 0.1 MPa. The vessel is closed again and the hydrate dissociate with the gas release. CO2 and CH4 volume fractions in the released again and the hydrate dissociate with the gas release. CO2 and CH4 volume fractions in the released gaseous phase are assessed via gas chromatographic analysis and the obtained CH4 and CO2 moles aregaseous calculated phase with are assessed the Peng vi Robinsona gas chromatographic equation. analysis and the obtained CH4 and CO2 moles are calculated with the Peng Robinson equation. 2.4. Membrane-Based Gas Separation Process 2.4. Membrane-Based Gas Separation Process In the experimental process, the CO2 injection is performed through the upper central tube which In the experimental process, the CO2 injection is performed through the upper central tube ends in the NGH sediment. which ends in the NGH sediment. In ideal conditions, the replacement occurs with a 1:1 carbon ratio, resulting in the recovery of pure methane.In ideal In real conditions, field conditions, the replacement instead, occurs the result with a of 1:1 the carbon CH4-CO ratio,2 exchangeresulting in is the a CH recovery4-CO2 ofgaseous mixture.pure methane. To pursue In real pure field natural condition gass, recovery instead, the and result a simultaneous of the CH4-CO massive2 exchange CO 2 issequestration, a CH4-CO2 an integratedgaseous mixture.process is To proposed: pursue the pure recovered natural gaseous gas recovery mixture and is sent a simultaneous to a membrane-based massive separationCO2 sectionsequestration, in which an CH integrated4 is separated process from is the proposed: CO2, which the did recovered not take gaseous part in the mixture exchange, is sent and to collected. a membrane-based separation section in which CH4 is separated from the CO2, which did not take part CO2 is recirculated and injected again in the internal sediment for another replacement cycle. Figure3 showsin the a exchange, drawing ofand the collected. proposed CO process2 is recirculated integrated and with injected the in-fieldagain in methane the internal recovery sediment system for from theanother NGH replacement sediment. cycle. Figure 3 shows a drawing of the proposed process integrated with the in-field methane recovery system from the NGH sediment.

FigureFigure 3. MembraneMembrane-based-based separation separation process process for forCH4 CH recovery4 recovery from fromNGH. NGH.

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The purpose of the membrane-based gas separation section is the separation of the recovered The purpose of the membrane-based gas separation section is the separation of the recovered CH4 CH4 from the CO2 which was not encapsulated in the hydrate phase. The schematic of Figure 4 from the CO2 which was not encapsulated in the hydrate phase. The schematic of Figure4 proposes proposesthe integration the integration of the separation of the separation section with section the experimental with the experimental apparatus apparatus (the last one (the already last one in alreadyoperation). in operation).

Figure 4. Schematic drawing of the membrane-based replacement process. Figure 4. Schematic drawing of the membrane-based replacement process. The selected membrane is formed by hollow fibers. They are small silicone micro-tubes with very thinThe walls selected (PDMSXA-10, membrane PermSelect, is formed Ann by hollow Arbor, MI,fibers USA).. They The are selected small silicone membrane micro module-tubes haswith 30 veryhollow thin fibers walls with (PDMSXA a 10 cm-210surface, PermSelect area. Technical, Ann Arbor, data M areI, shownUSA). The in Table selected2. membrane module has 30 hollow fibers with a 10 cm2 surface area. Technical data are shown in Table 2. Table 2. Membrane properties and operating conditions. Table 2. Membrane properties and operating conditions. Material PDMS (silicone) Material PDMS (silicone) Type Dense Hollow Fiber FiberType diameter (µm)Dense Hollow 190 Fiber FiberFiber Wall diameter Thickness (µm) (µm) 55190 Fiber WallFiber Thickness Count (µm) 3055 MembraneFiber Count Area (cm2) 1030 MembraneModule LengthArea (cm (cm)2) 10.910 ModuleModule Length Diameter (cm) (cm) 0.9510.9 MaximumModule Operating Diameter Temperature (cm) (K) 3330.95 MaximumMaximum Operating Shell Side Temperature Pressure (MPa) (K) 0.3333 Gas Flow Rate (Sm3 h−1) 6·10−5–6·10−3 Maximum Shell Side Pressure (MPa) 0.3 Gas Flow Rate (Sm3 h−1) 6·10−5 – 6·10−3 The permeability difference between gases controls the separation in the fibers. The permeability coefficientThe permeability is defined as the difference rate of gas between permeation gases per controls unit area, the per unitseparation membrane in thickness, the fibers. per The unit permeabilitytransmembrane coefficient driving is force. defined The permeabilityas the rate of coefficient, gas permeation at 298 Kper and unit 0.1 area, MPa, per for CHunit4 membraneis 950 barrer thickness,while for per CO 2unitis 3250 transmembranebarrer [36]. The driving CH4 -COforce.2 mixture The permeability is injected coefficient, inside the membraneat 298 K and module 0.1 MPa, via forthe CH inlet4 is port 950 to barrer the tube while side for and CO then2 is 3250 flows barrer through [36] the. The hollow CH4- fibers,CO2 mixture as shown is injected in Figure inside5. the membrane module via the inlet port to the tube side and then flows through the hollow fibers, as shown in Figure 5.

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Figure 5. Membrane gas separation principle (Permselect, available online [37]). Figure 5. Membrane gas separation principle (Permselect, available online [37]).

CO2 has the highest permeability and permeates across the thin silicone walls because of its partialCO2 pressure has the difference highest permeability between the insideand permeates and the outside across of the the thin hollow silicone ﬁbers. walls The because transferred of its gas, partial pressure difference between the inside and the outside of the hollow fibers. The transferred CO2, is called the permeate. The retentate exits the outlet of the tube side and it is fromed by a gas gasmixture, CO2, withis called a higher the permeate. concentration The retentate of methane. exits the outlet of the tube side and it is fromed by a gas mixture with a higher concentration of methane. 3. Experimental Results 3. Experimental Results In this section, results of the experimental tests carried out in laboratory are presented andIn discussed. this section, results of the experimental tests carried out in laboratory are presented and discussed. 3.1. Hydrate Formation 3.1. Hydrate Formation An experimental test with a hydrate saturation equal to 10% was carried out to obtained experimentalAn experimental data useful test for with energy a hydrate and environmental saturation equal analysis to 10% of the was membrane carried out based to integrated obtained experimentalprocess. A 10% data hydrate useful saturationfor energy wasand chosenenvironmental because analysis a good solution of the membrane to well simulate based integrated the hydrate process.deposit A conditions 10% hydrate and saturation effectively was control chosen temperature because a isgood to maintain solution ato low well sand simulate pore saturation.the hydrate In depositTable3 conditionsthe experimental and effectively conditions control and the temperature obtained results is to maintain are showed. a low sand pore saturation. In Table 3 the experimental conditions and the obtained results are showed. Table 3. Experimental tests and results. Table 3. Experimental tests and results. Hydrate Initial Equilibrium Dissociation Injected Released Temperature Test Saturation Pressure Pressure Pressure Methane Methane Hydrate Initial Equilibrium (◦C) Dissociation Injected Released (%) (bar) (bar) Temperature (bar) (mol) (mol) Test Saturation Pressure Pressure Pressure Methane Methane 1 10 42.0 28.3(°C) 1.0 16.5 0.2307 0.0769 (%) (bar) (bar) (bar) (mol) (mol) 1 10 42.0 28.3 1.0 16.5 0.2307 0.0769 Test 1 was carried out at 10% hydrate saturation with an initial pressure and temperature respectivelyTest 1 was of carried 42.0 bar out and at 1.0 10%◦C. hydrate To reach saturation the initial with pressure an initial equal pressure to 42.0 and bar, temperature 0.2307 moles respectivelywere injected. of 42.0 The bar equilibrium and 1.0 °C. pressure To reach achieved the initial at pressure the end equal of the to hydrate 42.0 bar, formation 0.2307 moles phase were was injected.28.3 bar. The At theequilibrium end of the pressure test, the achieved dissociation at the pressure, end of which the hydrate was reached formation inside phase the reactorwas 28.3 because bar. Atof the the end hydrate of the dissociation, test, the dissociation was equal pressure, to 16.5 bar which corresponding was reached to inside CH4 molesthe reactor in the because hydrate of phase the hydrateequal todissociation, 0.0769. Assuming was equal an to ideal 16.5 bar hydration corresponding number to equal CH4 moles to 6 and in the considering hydrate phase the negligible equal to −1 0.0769.methane Assuming dissolved an in ideal water hydration at system numberconditions equal (0.0012 to 6mol and kg consideringH2O ), a total the of negligible 0.948 moles methane of water dissolvedare required in water in the hydrateat system formation. conditions Figure (0.00126 shows mol temperature kgH2O-1), a total proﬁles of 0.948 when moles the methane of water hydrate are requiredstart forming in the inhydrate Test 1, formation. carried out Figure with saturation 6 shows temperature of 10%. profiles when the methane hydrate start forming in Test 1, carried out with saturation of 10%.

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Figure 6. Temperature profiles—Test 1, 10% hydrate saturation. Figure 6. Temperature profiles—Test 1, 10% hydrate saturation. Figure6 shows the temperature vs. time diagram, in which the temperature increase due to the Figure 6 shows the temperature vs. time diagram, in which the temperature increase due to the exothermic nature of the formation is visible. Temperature increase due to hydrate formation is bigger exothermic nature of the formation is visible. Temperature increase due to hydrate formation is in the lower part of the reactor (corresponding to thermocouples T11 and T16), while in the upper part bigger in the lower part of the reactor (corresponding to thermocouples T11 and T16), while in the (T07), temperature starts increasing previously but it then decreases sharper. The thermocouple T02 is upper part (T07), temperature starts increasing previously but it then decreases sharper. The positioned in the headspace, where only gas is present and there is no sand, temperature remains quite thermocouple T02 is positioned in the headspace, where only gas is present and there is no sand, constant, suggesting no hydrate formation in this area. temperature remains quite constant, suggesting no hydrate formation in this area. The temperature increase in the middle part of the diagram shows the exothermic nature of the The temperature increase in the middle part of the diagram shows the exothermic nature of the hydrate formation process, and this is particularly noticeable in a small scale reactor like this. In order hydrate formation process, and this is particularly noticeable in a small scale reactor like this. In to avoid that thermodynamic conditions strays from the equilibrium curve, it is necessary to cool the order to avoid that thermodynamic conditions strays from the equilibrium curve, it is necessary to reactor and then reducing the peak amplitude, achieving the previous temperature value as soon as cool the reactor and then reducing the peak amplitude, achieving the previous temperature value as possible. Monitoring temperature is also necessary to obtain the same value both at the beginning and soon as possible. Monitoring temperature is also necessary to obtain the same value both at the at the ending of the test. beginning and at the ending of the test. CH4 hydrate formation starts in the T07 region and, and then it moves towards the T11 and T16 CH4 hydrate formation starts in the T07 region and, and then it moves towards the T11 and T16 regions. This is consistent with other experimental results. Zhou et al. [38], in a 60 L reactor, found regions. This is consistent with other experimental results. Zhou et al. [38], in a 60 L reactor, found that hydrate formation starts from the gas-liquid interface in the top head space and then the hydrate that hydrate formation starts from the gas-liquid interface in the top head space and then the formation front improves downward, thanks to the larger free water availability in the lower part of hydrate formation front improves downward, thanks to the larger free water availability in the the reactor. From this comparison, it appears that temporal and spatial distribution of the CH4 hydrate lower part of the reactor. From this comparison, it appears that temporal and spatial distribution of formation does not depend on the reactor’s size. Figure7 reports pressure profile during the formation the CH4 hydrate formation does not depend on the reactor’s size. Figure 7 reports pressure profile and dissociation process for Test 1. during the formation and dissociation process for Test 1. The hydrate formation causes a pressure decrease due to the methane adsorption inside the The hydrate formation causes a pressure decrease due to the methane adsorption inside the clathrate solid structure. Once pressure is stabilized, the reactor was opened to eject all gaseous CH4 clathrate solid structure. Once pressure is stabilized, the reactor was opened to eject all gaseous CH4 present inside. Then the vessel was closed again and pressure started increasing, due to the hydrate present inside. Then the vessel was closed again and pressure started increasing, due to the hydrate dissociation and the releasing of methane from solid to gaseous phase. In both situations the pressure dissociation and the releasing of methane from solid to gaseous phase. In both situations the variation happens in the first hours, then it assumes an asymptotic trend until reaching the equilibrium pressure variation happens in the first hours, then it assumes an asymptotic trend until reaching the value. Figure8 shows the pressure-temperature graph, in which the theoretical thermodynamic equilibrium value. Figure 8 shows the pressure-temperature graph, in which the theoretical conditions and the experimental one are compared. thermodynamic conditions and the experimental one are compared.

Energies 2019, 12, x 10 of 17

EnergiesEnergies 20192019,, 1212,, 850x 1010 ofof 1717

Figure 7. Pressure trend in Test 1. Figure 7. Pressure trend in Test 1. Figure 7. Pressure trend in Test 1.

Figure 8. P-T graph, comparison between experimental conditions and theoretical equilibrium proﬁle. Figure 8. P-T graph, comparison between experimental conditions and theoretical equilibrium profile. TheFigure distance 8. P-T between graph, comparison the theoretical between line experimental and experimental conditions points and is theoretical greater in equilibriumthe upper area, profile. corresponding to the hydrate formation beginning, or when reaction has not involved yet the The distance between the theoretical line and experimental points is greater in the upper area, whole reactor internal volume. The temperature increase due to the formation reaction moves the correspondingThe distance to the between hydrate the formation theoretical beginning, line and orexperimental when reaction points has isnot greater involved in the yet u thepper whole area, experimental conditions towards the equilibrium line and during hydrate formation there is an overlap reactorcorresponding internal to volume.the hydrate The formation temperature beginning, increase or when due toreaction the formation has not involved reaction yet moves the whole the between T07, T11 and T16 points and the equilibrium graph until the hydrate formation is completed. experimentalreactor internal conditions volume. towards The temperature the equilibrium increase line due and toduring the formation hydrate formation reaction there moves is the an overlapexperimental between conditions T07, T11 towardsand T16 thepoints equilibrium and the equilibrium line and during graph hydrateuntil the formationhydrate formation there is anis 3.2. CO2 Replacement completed.overlap between T07, T11 and T16 points and the equilibrium graph until the hydrate formation is completed.The CO 2 replacement was carried out by depressurization of the internal environment; the substitution was then started by bringing the reactor from the value of 28.3 bar to a pressure of 20.4 bar. In Table4 values of the experimental parameters are shown.

Energies 2019, 12, x 11 of 17

3.2. CO2 Replacement

The CO2 replacement was carried out by depressurization of the internal environment; the substitution was then started by bringing the reactor from the value of 28.3 bar to a pressure of 20.4 bar. In Table 4 values of the experimental parameters are shown. Energies 2019, 12, 850 11 of 17

Table 4. Experimental parameters for CO2 replacement test.

Table 4. Experimental parameters for CO2 replacementInitial test. Hydrate Initial Equilibrium Injected CO2 Dissociation Injected Internal SaturationHydrate PressureInitial EquilibriumPressure InjectedTemperature CO2 Initial Internal DissociationPressure InjectedMoles Saturation Pressure Pressure Temperature TemperatureTemperature Pressure Moles (%) (bar) (bar) (°C) (bar) (mol) (%) (bar) (bar) (◦C) (◦C)(°C) (bar) (mol) 1010 20.4 16.516.5 15.015.0 1.51.5 6.056.05 0.1190.119

CO2 injection started and continued for 1620 s. After that in the reactor 0.119 CO2 moles are CO2 injection started and continued for 1620 s. After that in the reactor 0.119 CO2 moles are present. At the end of the CO2 injection, the gas composition is 75.4% CO2 and 24.6% CH4. Figure 9 present. At the end of the CO injection, the gas composition is 75.4% CO and 24.6% CH . Figure9 shows the temperature trends2 during the replacement test, starting from internal2 reactor 4conditions shows the temperature trends during the replacement test, starting from internal reactor conditions immediately after the methane hydrate formation. immediately after the methane hydrate formation.

Figure 9. Temperature trend in the reactor during the CO2 replacement phase. Figure 9. Temperature trend in the reactor during the CO2 replacement phase. The test starts at an average internal temperature equal to 1.5 ◦C. Over time, the ﬁrst temperature peakThe occurs test in the starts higher at an part average of the reactor internal at T07, temperature but it is the equal lowest to temperature 1.5 °C. Over value time, reached, the with first antemperature increase of peak about occurs 2 ◦C. Thein the higher higher temperature part of the value reactor is reached at T07, inbut the it loweris the arealowest at T16,temperature which is thevalue second reached, peak with in temporal an increase order. of about This suggests2 °C. The thathigher the temperature zone involved value ﬁrstly is reached with an in exothermic the lower processarea at T16, is the which lower is part the where second the peak CO 2ininjection temporal point order. is located. This suggests This is alsothat anthe area zone of involved the reactor firstly rich inwith water, an exothermic so the higher process temperature is the lower increase part withwhere respect the CO to2 injection the other point thermocouples is located. couldThis is suggest also an newarea hydrate of the reactor formation rich rather in water, than a so replacement the higher process. temperature A second increase phase with follows resp inect the to upper the other part ofthermocouples the sediment, could near thesuggest gaseous new headspace. hydrate formation The last rather peak occurs than a in replacement T11, after few process. minutes, A second in the centralphase follows area of in the the reactor. upper part of the sediment, near the gaseous headspace. The last peak occurs in T11, Theafter comparison few minutes, of in the the obtained central area temperature of the reactor. peaks in the three different areas of the sediment suggestsThe comparison that a CH4-CO of the2 replacement obtained temperature took place peaks in the in central the three part different of the reactorareas of (T07 the sediment and T11) 4 2 wheresuggests the that formation a CH - ofCO CO replacement2 hydrates and took the place simultaneous in the central dissociation part of of the CH 4 reactorhydrates (T07 avoided and T11) the formationwhere the offormation high temperature of CO2 hydrates peaks. and the simultaneous dissociation of CH4 hydrates avoided the formationFigure of 10 high shows temperature the trend peaks. of the pressure during the test. In the left side of the graph, is visible the pressureFigure 10 drop shows made the to trend move of below the pressure the stability during conditions the test. for In methanethe left side hydrates, of the whilegraph, remaining is visible inthe the pressure CO2 hydrate drop formation made to area. move The below constant the pressure stability drop conditions due to the for formation methane of hydrates, CO2 hydrates, while ends with the reactor opening, carried out to leave only the hydrates formed and the gas contained therein inside. The pressure increase in the right side represents the dissociation process of hydrates and the release of CH4 and CO2 contained in them. Energies 2019, 12, x 12 of 17

remaining in the CO2 hydrate formation area. The constant pressure drop due to the formation of CO2 hydrates, ends with the reactor opening, carried out to leave only the hydrates formed and the gas contained therein inside. The pressure increase in the right side represents the dissociation Energiesprocess2019 of, hydrates12, 850 and the release of CH4 and CO2 contained in them. 12 of 17

FigureFigure 10.10. PressurePressure trendtrend duringduring thethe COCO22 replacementreplacement test.test.

Once the CO2 ﬂuxing was completed, the reactor was closed and a sample of gas was withdrawn Once the CO2 fluxing was completed, the reactor was closed and a sample of gas was to make the gas chromatographic analysis. In this way the concentration of CO2 in the reactor was withdrawn to make the gas chromatographic analysis. In this way the concentration of CO2 in the determined.reactor was determined. A second sample A second was sample collected was and collected analyzed and atanalyzed the end at of the the end dissociation of the dissociation phase to evaluatephase to the evaluate new concentrations the new concentrations of the two of gases. the two Knowing gases. pressure Knowing and pressure temperature and temperature conditions, it was therefore determined both the number of moles of CO2 present before the starting of the conditions, it was therefore determined both the number of moles of CO2 present before the starting replacementof the replacement phase, phase, and the and number the number of moles of storedmoles stored in form in of form hydrate. of hydrate. The CH4-CO2 hydrate dissociation causes a pressure increase from 1 to 6.05 bar (see Table3); the The CH4-CO2 hydrate dissociation causes a pressure increase from 1 to 6.05 bar (see Table 3); the gas sample analysis proves that 78.47% of entrapped gas into hydrates is CO , while the remaining gas sample analysis proves that 78.47% of entrapped gas into hydrates is CO22, while the remaining part is CH4. The total gas moles which have formed hydrates are 0.043 mol (0.034 mol of CO2 and part is CH4. The total gas moles which have formed hydrates are 0.043 mol (0.034 mol of CO2 and 0.009 mol of CH ). 0.009 mol of CH44). Two parameters are calculated for the described process, in order to quantify the CO sequestration Two parameters are calculated for the described process, in order to quantify2 the CO2 and the CH -CO exchange. Capture efﬁciency is the ratio of the CO moles stored in the system sequestration4 and2 the CH4-CO2 exchange. Capture efficiency is the ratio2 of the CO2 moles stored in through CO hydrate formation to the total injected moles (Equation (3)): the system through2 CO2 hydrate formation to the total injected moles (Equation (3)): 퐶푂 CO =2푠푡표푟푒푑2stored = 휂푐푎푝푡푢푟푒ηcapture= = 0.280.28 ( (3)3) 퐶푂2푖푛푗푒푐푡푒푑CO2injected

TheThe exchangeexchange factorfactor isis thethe ratioratio ofof thethe COCO22 molesmoles sequesteredsequestered inin thethe hydratehydrate phasephase toto thethe CHCH44 molesmoles producedproduced fromfrom thethe hydratehydrate phasephase andand itit givesgives anan extentextent ofof thethe replacementreplacement processprocess (Equation(Equation (4)).(4)). InIn thisthis case,case, itit isis equalequal toto 0.54.0.54.

퐶푂2푠푡표푟푒푑 휂 = CO2stored= 0.54 (4) 푒푥푐ℎ푎푛푔푒ηexchange퐶퐻= = 0.54 (4) 4푟푒푐표푣푒푟푒푑CH4recovered

At the end of the process, 0.063 CH4 moles were released from the system and 0.034 CO2 moles At the end of the process, 0.063 CH4 moles were released from the system and 0.034 CO2 moles were captured in the hydrate phase. were captured in the hydrate phase.

4. Energy and Environmental Evaluations

An energy and environmental evaluation was carried out considering that the replacement process is integrated with a membrane-based separation process, used for the puriﬁcation of the methane released by the replacement process and for the recirculation of the CO2 separated. Energies 2019, 12, x 13 of 17

4. Energy and Environmental Evaluations An energy and environmental evaluation was carried out considering that the replacement Energiesprocess2019 is, 12 integrated, 850 with a membrane-based separation process, used for the purification 13 of of the 17 methane released by the replacement process and for the recirculation of the CO2 separated. TheThe evaluationevaluation isis basedbased bothboth onon thethe experimentalexperimental datadata obtainedobtained inin laboratorylaboratory andand onon theoreticaltheoretical calculations,calculations, inin whichwhich thethe energyenergy contentcontent associatedassociated toto thethe recoveredrecovered methanemethane isis comparedcompared toto thethe energy consumption spent for its recovery and for CO2 sequestration. energy consumption spent for its recovery and for CO2 sequestration. OnOn the the membrane membrane separation separation section, section, the following the following assumptions assumptions were made: were (i) the made: composition (i) the composition of the gas flow sent to the membrane separation section is exclusively formed by CO2 of the gas flow sent to the membrane separation section is exclusively formed by CO2 and CH4; (ii) and CH4; (ii) with a separation efficiency of 0.98, the retentate gas has a composition in volume of 3% with a separation efficiency of 0.98, the retentate gas has a composition in volume of 3% CO2 and 97% CO2 and 97% CH4; (iii) the permeate is pure CO2, which is recompressed and recirculated in the CH4; (iii) the permeate is pure CO2, which is recompressed and recirculated in the vessel to take part inves thesel replacement to take part process.in the replacement process. SinceSince the the methane methane heatheat of of combustion combustion isis50,000 50,000 kJ kJ kg kg−−11, the the energy energy content content of of the the recovered recovered methane (0.063 mol) is 50.40 kJ. Nevertheless, the CH4-CO2 replacement process presents energy methane (0.063 mol) is 50.40 kJ. Nevertheless, the CH4-CO2 replacement process presents energy consumption due to: (i) CO2 compression for its injection in the sediment, (ii) compression of the consumption due to: (i) CO2 compression for its injection in the sediment, (ii) compression of the separated CO2 to be recirculated in the sediment. These contributions were evaluated considering separated CO2 to be recirculated in the sediment. These contributions were evaluated considering thethe followingfollowing assumptions.assumptions. The compression work work was was calculated calculated considering considering the the compr compressionession as as a amulti multi-stage-stage adiabatic adiabatic process process with with a a compression compression efficiency efficiency of 85% and inter-cooling.inter-cooling. TheThe energyenergy consumptionconsumption forfor thethe coolingcooling operations,operations, inin whichwhich thethe environmentenvironment cancan bebe usedused asas aa heatheat sink,sink, waswas considered zero. CO2 is assumed to be compressed from atmospheric pressure to 42 bar, with an considered zero. CO2 is assumed to be compressed from atmospheric pressure to 42 bar, with an initial temperatureinitial temperature of 298 Kof (25 298◦ C),K (25 through °C), through a 3-stage a 3 compression,-stage compression, with inter-cooling with inter-cooling until 293 until K (20 293◦C) K (20 °C) with the environment as a heat sink. The work of compression for the injected CO2 is equal to with the environment as a heat sink. The work of compression for the injected CO2 is equal to 1.91 kJ 1.91 kJ (361.93 kJ− 1kgCO2−1). (361.93 kJ kgCO2 ). After passing the membrane separation section at 3 bar, the permeated CO2 is recirculated to be After passing the membrane separation section at 3 bar, the permeated CO2 is recirculated to be re-injected in the hydrate-bearing sediment inside the reactor. The permeated CO2 is compressed to re-injected in the hydrate-bearing sediment inside the reactor. The permeated CO2 is compressed to 4242 barbar startingstarting fromfrom 33 barbar andand anan initialinitial temperaturetemperature ofof 277277 K,K, throughthrough aa 2-stage2-stage compressioncompression withwith inter-coolinginter-cooling untiluntil 293293 KK (20(20◦ °C)C) withwith thethe environmentenvironment asas heatheat sink.sink. TheThe workwork ofof compressioncompression forfor thethe recirculation the permeated CO2 is equal to 0.48 kJ (252.81 kJ kgCO2−−11). For the described contributions, recirculation the permeated CO2 is equal to 0.48 kJ (252.81 kJ kgCO2 ). For the described contributions, consideringconsidering a a global global efficiency efficiency of 0.46, of 0.46, the primary the primary energy energy consumption consumption is equal to is 2.39 equal kJ, toequivalent 2.39 kJ, equivalent to the combustion of 0.006 moles of CH4. The primary energy consumption for the to the combustion of 0.006 moles of CH4. The primary energy consumption for the injection of a CO2 injection of a CO2 mass unit (kgCO2) is equal −to1 1336.4 kJ kgCO2−1, which corresponds to the combustion mass unit (kgCO2) is equal to 1336.4 kJ kgCO2 , which corresponds to the combustion of 1.671 moles of of 1.671 moles of CH4. Results are summarized in Figure 11. The ratio between the spent energy and CH4. Results are summarized in Figure 11. The ratio between the spent energy and the stored energy the stored energy in the recovered CH4 is 4.75%. in the recovered CH4 is 4.75%.

FigureFigure11. 11.CH CH44-CO-CO22 replacement energy consumption and thethe recoveredrecovered CHCH44 energy content.

The carbon footprint of the CH4-CO2 exchange process is calculated as a balance of the CO2 produced in the process and the CO2 stored in system. The CO2 produced in the process is due to: Energies 2019, 12, x 14 of 17

EnergiesThe2019 carbon, 12, 850 footprint of the CH4-CO2 exchange process is calculated as a balance of the14 CO of 172 produced in the process and the CO2 stored in system. The CO2 produced in the process is due to: (i) combustion of the recovered CH4; (ii) CO2 related to the primary energy consumption; (iii) CO2 from the(i) combustion membrane ofgas the retentate. recovered As CHthe4 ;CH (ii)4- COCO22 relatedcarbon toratio the is primary 1:1, the energystoichiometric consumption; combustion (iii) CO of2 0.063from therecovered membrane CH4 gas moles retentate. produces As thethe CHsame4-CO number2 carbon of ratioCO2 moles is 1:1,. the On stoichiometric the same basis, combustion the primary of energy0.063 recovered consumption CH4 molesassociated produces to the the compression same number of the of CO injected2 moles. CO On2, brings the same to the basis, production the primary of 0.007energy moles consumption of CO2. CO associated2 from the to membrane the compression separation of the is 0.001 injected mol CO (Figure2, brings 12). to The the amount production of CO of2 moles0.007 moles(0.075 of moles) CO2. CO is 2calculatedfrom the membrane adding the separation moles stored is 0.001 in hydrates mol (Figure to 12the). molesThe amount dissolved of CO in2 water.moles (0.075 moles) is calculated adding the moles stored in hydrates to the moles dissolved in water.

Figure 12. Carbon footprint of the process. Figure 12. Carbon footprint of the process. Also in this second case we can speak of permanent storage, as the thermodynamic conditions of Also in this second case we can speak of permanent storage, as the thermodynamic conditions the environment will lead to the formation of new hydrates. The carbon footprint therefore results in of the environment will lead to the formation of new hydrates. The carbon footprint therefore results an estimated negative value, equal to 0.004 moles sequestrated, thus proving that the exchange process in an estimated negative value, equal to 0.004 moles sequestrated, thus proving that the exchange acts as carbon storage sink with consequent beneficial environmental effects. process acts as carbon storage sink with consequent beneficial environmental effects. 5. Conclusions 5. Conclusions The present paper proposes a process for pure methane recovery and carbon storage in natural The present paper proposes a process for pure methane recovery and carbon storage in natural gas hydrate sediments based on membrane separation. The CH4-CO2 exchange process was studied gas hydrate sediments based on membrane separation. The CH4-CO2 exchange process was studied in a simulated hydrate-bearing sediment in a 1 L lab reactor, where the CH4 hydrates are formed in a in a simulated hydrate-bearing sediment in a 1 L lab reactor, where the CH4 hydrates are formed in a quartz sand matrix, partially saturated with water. The gaseous flow, which results from the CH4-CO2 quartz sand matrix, partially saturated with water. The gaseous flow, which results from the exchange process, is a CH4-CO2 mixture. The separation of the two components is proposed to be CHcarried4-CO out2 exchange by a membrane-based process, is a separationCH4-CO2 section.mixture. The separation of the two components is proposedTest ofto artificialbe carried reproduction out by a membrane of natural-based gas separation hydrates withsection. a hydrate saturation of 10% show that theTest hydrate of artificial formation reproduction goes downward, of natural to gas the hydrates lower part with of thea hydrate reactor. saturation From comparison of 10% show with that the hydrate formation goes downward, to the lower part of the reactor. From comparison with literature data, temporal and spatial distribution of the CH4 hydrate formation is not affected by the literature data, temporal and spatial distribution of the CH4 hydrate formation is not affected by the reactor’s size. The CO2 replacement shows a capture factor of 0.28 and an exchange factor equal to 0.54.

Energies 2019, 12, 850 15 of 17

Energy and environmental evaluations were carried out to assess the viability of the process. The energy analysis takes into account the process energy costs, related to the compression and heating of the injected CO2 and the compression of the CO2 separated in the membrane and recirculated in the sediment. Results show that the spent energy constitutes 4.75% of the energy stored in the recovered methane. The carbon footprint of the CH4-CO2 exchange process is calculated as a balance of the CO2 produced in the process and the CO2 stored in system. Results provide an estimated negative value, equal to 0.004 moles sequestered, thus proving the environmental beneﬁt of the exchange process. Coupling the CH4-CO2 exchange process in the methane hydrate reservoirs with a membrane based separation process to obtain pure gaseous methane is proved to be viable. Further experimental studies to improve exchange and capture efﬁciency are planned. Geological and mechanical properties of the reproduced hydrates reservoir will be monitored, especially during the dissociation and exchange process.

Author Contributions: Conceptualization, B.C. and F.R.; methodology, B.C.; investigation, B.C. and A.M.G.; data curation, B.C., A.M.G. and A.N.; writing—original draft preparation, B.C.; writing—review and editing, A.N.; supervision, F.R. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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