A Review of the Effect of Porous Media on Gas Hydrate Formation.

Most gas hydrates on the earth are in sediments and permafrost areas, and porous media are often used industrially as additives to improve gas hydrate formation. For further understanding its exploration and exploitation under natural conditions and its application in industry, it is necessary to study the effect of porous media on hydrate formation. The results show that the stacked porous media affects the phase equilibrium of hydrate formation depending on the competition water activity and large specific surface areas, while integrated porous media, such as metal foam, can transfer the hydration heat rapidly and moderate the hydrate phase equilibrium. A supersaturated metal-organic framework is able to significantly improve gas storage performance and can be used as a new material to promote hydrate formation. However, the critical particle size should be studied further for approaching the best promotion effect. In addition, together with the kinetic accelerators, porous media has a synergistic effect on gas hydrate formation. The carboxyl and hydroxyl groups on the surface of porous media can stabilize hydrate crystals through hydrogen bonding. However, the hydroxyl radicals on the silica surface inhibit the combination of CH4 and free water, making the phase equilibrium conditions more demanding.

Introduction

With the social progress and the development of industry, the world’s demand for energy continues to grow. Energy is not only related to the development of the economy of various countries but also to the future of the world’s ecology and environment. Conventional energy sources such as coal and oil tend to dry up, and China, as the largest developing country in the world, is more dependent on energy. Therefore, seeking and developing new alternative energy is significant, and a large number of funds and human resources have been invested to carry out new energy research. Gas hydrate is a nonstoichiometric cage-like crystal, in which water molecules are connected by hydrogen bonds to form cavities and wrap gas molecules in them. Natural hydrates are widely found in tundra, sedimentary layers hundreds of meters to more than 1000 m below the seabed in water depths greater than 100–250 m (polar regions) and 400–650 m (equatorial areas).[1] It is estimated the total amount of gas hydrates in marine sediments and the terrestrial permafrost is up to 7.6 × 1018 m3, which has a great prospect for exploitation and application.[2] In addition to abundant resources, hydrate also has a wide range of application prospects in industry. Notably, 1 m3 of CH4 hydrate can store 160–180 m3 of CH4 at the standard condition, which can be used as a gas storage and transportation medium. Furthermore, the applications based on hydrate technology are divided into CO2 storage and capture,[3] natural gas storage and transport,[4,5] seawater desalination,[6] and other applications.

The environment of forming hydrates in nature is very complex, and different components may affect the growth rate and formation mechanism of hydrates, especially porous media. In addition to thermodynamic conditions, the properties of porous media also affect the hydrate formation.[7] Meanwhile, a series of conditions such as the transport path of gas and water in porous media, migration characteristic, and pore size affect the hydrate formation characteristics.[8−10] The study of the hydrate formation characteristics in porous media is of great significance for understanding the formation mechanism of natural gas hydrates, inhibiting the secondary generation of hydrates in the process of natural gas hydrate extraction, ensuring the safety of oil and gas pipeline transportation, implementing gas separation by hydrate method and carbon dioxide fixation, and so on, and it can realize the application of hydrate technology in industry at an early date. In order to simulate hydrate formation under natural conditions, porous media with different materials are often used in laboratory. This paper mainly reviews the influencing factors of porous media for hydrate formation, summarizes different research results in the world, and provides reference for future research.

Classification of Porous Media

Over the past 20 years, the use of additives to promote hydrate formation has proved to be an effective way to advance the application of hydrate technology. Researchers have successfully used various kinetic promoters for gas hydrate formation, such as activated carbon,[11] graphite,[12] graphene oxide,[13] carbon nanotubes,[14] and so on. Porous media such as silica sand and silica are in line with the environment of hydrate formation in the natural environment, so the research is never stopped. Current research also involves aluminum foams,[15] Fe3O4,[16] and MOFs,[17] but there is no systematic experimental study on the classification and statistics of porous media. Porous media have different classification methods, and this paper collects a large amount of literatures related hydrate formation under porous media, based on which the porous media can be divided into two types: stacked (sediment) and integrated (porous). The stacked (sediment) includes natural sediment and artificial sediment. The common natural sediments are loess, montmorillonite, clay, and so on; artificial sediments are silica sand, quartz sand, silica, nanoparticles, mental organic framework, molecular sieve, activated carbon, and so on. The overall type (porous materials) is divided into two categories, i.e., uniform pore size distribution and random pore size distribution, among which common porous materials with uniform pore size distribution include porous glass, stainless steel mesh, honeycomb, and so on; porous materials with random pore size distribution include foam plastics, cellulose foam, metal foam, and so on. The classification of porous media is shown in Table for details. Generally speaking, the porosity of porous media is mainly affected by particle size and, water saturation, and storage capacity of hydrate in porous media is also related to particle size,[18,19] pore size, water saturation, surface properties of porous media, and so on.[14,20−22] Here, the schematic diagram is drawn according to different categories, as shown in Figure .

Table 1

Classification and Related Research of Porous Media

material			particle size	guest gas	water saturation	ref
stacked	natural sediment	loess; silicon dioxide	loess: 34.5 μm; silicon dioxide: 25–58 μm	CH4	loess: 18%; silicon dioxide: 100%	(23)
		coarse sand; loess	coarse sand: 1.0–2.0 mm; loess: 34.5 μm		coarse sand: 14%; Loess: 35%	(24)
		montmorillonite	0.5–25 μm		——	(25)
		clay; natural sand	0.25–0.425 mm; 0.15–0.18 mm		unsaturated	(26)
	artificial sediment	silica sand	300–450 μm	CH4	unsaturated	(27)
		silica sand; activated carbon	pore size of silica sand: 0.9 mm; pore size of activated carbon: 2.19 nm		50%; 100%	(28)
		quartz sand	pore size: 13.8, 14.2, 26.7 nm	CO2	saturated	(29)
			14–20 mesh, 35–60 mesh, 80–120 mesh, 400–500 mesh	CH4	——	(30)
			60–80 mesh	N2O	25.06%, 33.23%, 42.13%, 50.81%	(31)
			clay (<4 μm), mealy sand (4–63 μm), fine sand (63–250 μm), medium sand (250–500 μm), coarse sand (500–2000 μm), gravel (>2000 μm)	CH4	unsaturated	(32)
		glass bead	0.105–0.125, 0.177–0.25, 0.35–0.5, 0.5–0.71, 0.991–1.397 mm	THF	unsaturated	(33)
			porosity of 36.8%	CH4	unsaturated	(34)
			3.0 mm, 1.2 mm, 0.4 mm, 0.1 mm	CO2	unsaturated	(35)
		silica gel	0.105–0.150 mm	CH4	saturated	(8)
			——	CO2	saturated	(36)
			——	CO2, N2	saturated	(37)
			——	CH4, CO2	saturated	(38)
			0.105–0.150, 0.150–0.2, 0.3–0.45 mm	CH4	saturated	(39)

Figure 1

(a) Stacked particles and pores; (b) Integrated porous materials and uniform pores; (c) Integrated porous materials and random pores.

Effects of Porous Media on the Phase Equilibrium of Gas Hydrate Formation

Hydrate phase equilibrium refers to a state of equilibrium between the three phases of hydrate-gas–liquid. In 1992, Handa and Stupin[50] measured the formation and decomposition of CH4 and C2H6 hydrates in porous media silica gel, thus opening the study prelude to influence of porous media on the phase equilibrium. The phase equilibrium of hydrates under porous media has been studied for a long time, and researchers concluded different views. Some researchers suggest that the capillary effect generated by pores inhibits the activity of water, requiring higher pressures or lower temperatures to generate hydrates and making the phase equilibrium conditions of hydrate formation more demanding.[51] However, a few studies state that the porous media has large specific surface area, which can provide more nucleation sites, increasing the gas–liquid contact areas, and the existence of the third interface allows water molecules to be arranged in an orderly manner. Therefore, the presence of porous media makes the phase equilibrium conditions milder.[10,52]

Many researchers have reported the effects of stacked porous media on the phase equilibrium of gas hydrate formation. The phase equilibrium curves of CO2, CH4, C2H6, and C3H8 hydrate formation in the presence of porous media (e.g., silica gels, glasses) are illustrated in Figures –Figure ,[18,20,53−58] all of which indicated that the phase equilibrium curve shifts to the low-temperature and high-pressure zone in the presence of the porous material system. In general, it is shown that with the decrease of pore size, the equilibrium curve shifts to the left, which is attributed to the decrease of water activity in porous media due to the strong capillary effect.

Figure 2

Phase diagram of CO2-water in porous medium.[19,53,54] (Note: the curve is the CO2-water phase equilibrium: Larson.[59])

Figure 6

Phase diagram of C3H8-water in porous medium.[57,58] (Note: the curve is the C3H8-water phase equilibrium: Deaton and Frost.[60])

Phase diagram of CH4-water in porous medium.[19,53,54] (Note: the curve is the CH4-water phase equilibrium: Deaton and Frost.[60])

Phase diagram of CH4-water in porous medium.[20,55] (Note: the curve is the CH4-water phase equilibrium: Deaton and Frost.[60])

Phase diagram of C2H6-water in porous medium.[56,57] (Note: the curve is the C2H6-water phase equilibrium: Deaton and Frost.[60])

Metal–organic frameworks (MOFs), in which metal-ion clusters are linked by organic molecules, are highly porous materials, which have large surface areas and exhibit tunable porosity.[61] Kim et al.[62] studied the effect of the MIL-53 metal–organic framework with meso/macro pores on the phase equilibrium of CO2 and CH4 hydrates. In this study, “bulk phase” indicates a phase exhibiting a property similar to a normal gas hydrate. The “confined phase” represents a phase that is relatively inhibited due to the small surrounding pore dimensions. The results show that while “confined phases” of both CH4 and CO2 hydrates in MIL-53 show inhibited phase behavior, the “bulk phases” show a different tendency depending on the type of gases, e.g. inhibition for CO2 but a slight promotion effect for CH4, which are demonstrated in Figure a,b. This is due to a fact that CO2 molecules mainly exist in the micropores and the activity of H2O molecules decrease, which affects the hydrate formation. MOF (MIL-53) is reported to inhibit CH4 hydrates growth thermodynamically. However, compared with other porous materials such as silica or carbon materials, the inhibition of meso/macro pores in MIL-53 is relatively weaker. Therefore, MIL-53 may be used as a new material to promote the formation of natural gas hydrates.

Figure 7

Phase diagrams of (a) CO2 and (b) CH4 hydrates in MIL-53. (Note: the curve is the CO2- pure water phase equilibrium: Larson;[59] the curve is CH4- pure water the phase equilibrium: Deaton and Frost.[60])

In addition, He[63] investigated the formation of CH4 hydrates in a mesoporous metal–organic framework MIL-101 using microsecond molecular dynamic simulations. The simulation results show that a competition in hydration between in the inner of MIL-101 cavities and the outer space, CH4 hydrates form preferentially in the outer space of MIL-101 cavities rather than in the cavities. Large and stable hydrate clusters are easy to form outside; in contrast, only small hydrate clusters form in internal cavities of MIL-101, and the hydrate clusters dissociate immediately once CH4 molecules in the cavities diffuse out driven by the CH4 concentration gradient inside and outside cavities. Therefore, only when the external is nearly saturated, can stable hydrates form in MIL-101 cavities. H2O-saturated MIL-101 greatly reduces gas consumption due to the occupancy of the cavities by H2O, while oversaturated MIL-101 significantly increases gas consumption.

The effect of porous media on phase equilibrium of mixed gases has also been studied by Lee et al.[64] They measured hydrate phase equilibria for the CH4 (90%) + C3H8 (10%) + water mixtures in silica gel pores with nominal diameters of 6.0, 15.0, 30.0, and 100.0 nm. The results indicate that the presence of silica gels cause the phase equilibria curves to shift to the left region represented by lower temperature and higher pressure conditions than those in the pure water (as shown Figure ). It is explained that the activity of water confined in the pores could be depressed by the partial ordering and bonding of water molecules with the hydrophilic surfaces of pores, which inhibited hydrate formation.

Figure 8

Hydrate-phase equilibria of the CH4 (90%) + C3H8 (10%) + water mixture in silica gel pores.[64]

However, the hydrate formation of mixed gases of CH4 (87.2%), C2H6 (7.6%), C3H8 (3.1%), C4H10 (0.5%), n-C4H10 (0.8%), C5H12 (0.2%), n-C5H12 (0.2%), N2 (0.4%) in porous bentonite was studied by Cha et al.[65] It is found that it is easier to form the cage structures required for hydrates’ growth because the water molecules would be orderly arranged on the surface of porous media and thus promote the hydrates’ formation in the presence of a third-interface (i.e., the surface of porous media). From the perspective of dynamics, the third-surface provides more nucleation sites leading to faster and greater crystal growth. Besides, from the perspective of thermodynamics, it may be caused by the ordered absorption of water molecules onto some surface as a structure which approximates a part of a hydrate lattice, and this surface becomes a part of the hydrate structures.

Besides, the effects of stacked porous media on the phase equilibrium of gas hydrate formation is also mentioned, especially analyzing foam materials. The foam metal is porous media possessing large rough surface and excellent thermal conductivity, which can transfer the hydration heat generated by hydrate formation in time and promote hydrate nucleation.[15,42] Under low pressure, relatively severe hydration produced heat fast, and foam metal may remove hydration heat partly in a limited time. At the same time, the conclusion agrees with the findings of Tian,[44] who proposed the SFC could maintain the reaction system at a lower thermal resistance for a longer time at a relatively low driving force. Therefore, it can be considered that the integrated porous media has certain promoting effects on hydrate phase equilibrium.

Effects of Particle Size of Porous Media on Hydrate Formation Rate

One disadvantage of hydrates’ nucleation and growth in pure water is kinetically slow. This is due to the limited gas–liquid interface (interface phenomenon) that inhibits the dissolution/diffusion of CH4 (mass transfer resistance), making it difficult to reach the critical concentration required for nucleation. Besides, the gas diffusion process is also limited by the agglomeration of hydrate crystals at the interface, and the formation rate is inversely proportional to the thickness of the hydrate film. Some scholars believe that compared with a homogeneous nucleation process, the addition of porous media can provide more nucleation sites, thus promoting the hydrate formation.[66,67] This theory can also be supported by the studies of Andres-Garcia[68] and Prasad.[69] Moreover, the hydration process generates a large amount of hydration heat, which seriously hinders the hydrate formation. The addition of porous media can improve this situation and quickly transfer hydration heat.[70] Researchers have different views on the effect of porous media particle size on hydrate formation rate. One view is that the large particle size is helpful for accelerating the formation rate. Researchers supporting this view generally believe that the large particle size of porous media has larger specific surface areas and provides hydrates more nucleation sites. However, smaller particle sizes lead to greater gas–liquid interfacial tension and pore capillary force,[73] which reduces the activity of water and requires the higher pressure or lower temperature to generate hydrates. Furthermore, the small particle size is helpful in accelerating formation rate. It is believed that the connectivity of the pore space formed by the medium with small particle size is better, which promotes the migration of gas and water in the system, allows the system to reach a supersaturation state faster, and creates nucleation quickly.

Figure illustrates the process of hydrate formation in detail with changes in pressure, temperature, and time. The induction time is a very important parameter when studying the kinetics of hydrate formation. The induction time can be used as a standard to evaluate hydrate formation rate and determine the efficiency of hydrate formation to a large extent.[71]

Figure 9

Typical hydrate nucleation and grow plot.

Siangsai et al.[72] conducted the experiment of CH4 hydrate formation at 8 MPa and 4 °C in a quiescent fixed bed crystallizer. Activated carbons with different particle sizes (250–420, 420–841, and 841–1680 μm) were used. The results showed that the activated carbon with the smallest particle size has the lowest induction time of hydrate, and the activated carbon with 841–1680 μm has the longest induction time, as shown in Figure . However, the average water conversion to hydrate in the activated carbon of 841–1680 μm is highest. It can be concluded that the larger particle size of activated carbon, the larger the interstitial space between the particles, which increases the probability of hydrate nucleation and improves the water conversion rate. Additionally, the experimental study indicated a hydrate formation rate with the smaller particle size than that of activated carbon with large particle size, but the highest gas storage capacity appears in the large particle size.

Figure 10

Induction time of CH4 hydrate formation in activated carbon.[72]

The effect of particle size of porous sediments (pumice and fire hardened red clay) on the carbon dioxide hydrate formation kinetics at 3 MPa pressure and 274 K temperature was studied by Bhattacharjee et al.[73] The results observed that hydrate formation kinetics was enhanced with a decrease in the particle size fraction, as shown in Figure . This observation is in good agreement with those made by some earlier studies. Tohidi et al.[74,75] found the hydrate formation occurred at the pores rather than on the surface of the particles for large sediment, while for the smaller grain sizes, the hydrates formed large masses almost completely surrounding the grains. This has been represented in the form of a well detailed schematic in Figure . In conclusion, reducing the particle size of porous media can increase the surface area for gas–water contact, increase the number of nucleating sites, and accelerate hydrate growing rates.[76]

Figure 11

Induction time of CO2 hydrate formation in pumice and fire-hardened red clay.[73]

Figure 12

Mechanism of hydrate formation in systems with different particle size fractions. (a) Large particle size fraction: no interaction between the hydrates and the surface of the particles. (b) Small particle size fraction: hydrates form in large masses almost completely enveloping the particle themselves.[74,75]

Bagherzadeh et al.[77] observed the formation process of CH4 hydrate in silica sand with different particle sizes (210–297, 125–210, 88–177, and <75 μm) by employing the magnetic resonance imaging technique under the condition of 8 MPa and 274.15K, and found that the presence of silica sands can promote the growth of hydrates. The smaller the particle size is, the stronger the promotional effect is, as shown in Figure .

Figure 13

Induction time of CH4 hydrate formation in silica sand.[77]

Generally speaking, the smaller the particle size of porous media, the larger the gas–liquid contact areas and the higher the hydrate formation rate. However, in practical applications, there is more water at the bottom of the reactor. Additionally, with the reaction progress, the hydrate formed at the top and around first, which prevents the gas from spreading to the bottom, and the reaction is difficult to proceed.[78,79] Moreover, it can be seen from Figure and Figure that the particle size is not positively correlated with the hydrate formation rate, and it is necessary to discuss the critical size.

Zhen et al.[80] believe that there is a critical size, and a balance is established between the influence of surface area and capillary force on the formation rate. Page and Sear[81] believed that the ideal nucleation rate can be achieved only when the pore size is close to the critical core. Zhang et al.[82] considered the effect of capillarity on the gas–liquid interfacial tension in the pores of sand beds, and a theoretical model of hydrate nucleation induction time was established on the basis of the Kashchiev model combined with the Arrhenius equation (see eq ). It is concluded that there is a critical size. The range of size should be considered to determine the relationship between the induction time of hydrate formation and the particle size.

At present, there are few studies involving the critical model, and it is difficult to form a theoretical system because these studies are basically in the conjecture stage. Most studies do not have a wide enough particle size range to draw a single conclusion, or the particle size range studied is too large to produce more rigorous results.

Influence of Physical and Chemical Properties of Porous Media on Hydrate Formation

Roughness

Solid surfaces are not smooth, and even surfaces that appear smooth are uneven in the microscopic scale. For the porous media, no wall surface can be considered to be absolutely smooth, so the influence of roughness on liquid flow exists in any case, which differ from the relative roughness size and flow influence degree. Mohammadi et al.[83] investigated the effect of synthesized ZnO nanoparticles on the kinetic and thermodynamic equilibrium conditions of CO2 hydrate formation. The results show that the ZnO nanoparticles slightly act as a thermodynamic inhibitor of hydrate formation compared with the phase equilibrium in pure water. Though studying the effect of MWCNTs on CH4 hydrate formation, Kim et al.[84] found that the thermodynamic conditions for phase equilibrium were little influenced by adding MWCNTs, and even had a slight inhibition effect. Some studies used porous cellulose foam as packing to capture CO2, and experiments had found that adding propane as accelerator in the process of gas separation could reduce the operating pressure of HBGS process.[41] Pei et al.[42] conducted an experimental to study the kinetic characteristics of the formation of CH4 hydrate by metal form in a high-pressure static reactor, and CH4 hydrate was rapidly generated in this system. The research results showed that the copper foam framework can provide sufficient crystallization points for the formation of hydrates, and at the same time, it can be used as a “highway” for hydration heat transfer during hydrate growth. SEM images of porous media are compared here (see Figures –16).

Figure 14

SEM image of ZnO Reprinted with permission from ref (83). Copyright 2016 Elsevier.

Figure 16

SEM image of 15 PPI copper foam Reprinted with permission from ref (42). Copyright 2021 CIESC Journal.

SEM image of MWCNTs Reprinted with permission from ref (84). Copyright 2011 Elsevier.

It can be seen that ZnO copper foam have rough and uneven surfaces, and copper foam has many pores to provide the basis for hydrate formation.

Hydrophilic and Hydrophobic

For porous media, the wettability of the inner surface of the pore has significant impacts on the behavior of adsorbing H2O and CH4 molecules, which is macroscopically manifested as the difference of interface energy between ice and water, water and the pore wall, and hydrate and water, and it directly affects the arrangement form of water molecules on the inner wall at micro level.[85] The wettability inner surface reduces the activity of water molecules in the close water layer on the surface, meaning high pressure or low temperature to form a hydrate. Therefore, hydrate can be formed only in the system with enough water content to produce capillary force. When the inner surface of the pore is hydrophobic, the surface wettability and surface energy state of the solid are easily changed. Even a small amount of organic molecules can significantly affect the stable existence conditions of hydrate.

Using simulation results, Li et al.[86] showed that the hydrophobic groups on the surface of kaolinite play a key role in the hydrate crystal growth. The authors found that the H2O molecules form a regular tetrahedra on cage structure on the mineral surface, which is conducive to the CH4 hydrate formation. He et al.[87] performed microsecond MD simulations to investigate CH4 hydrate formation from gas/water two-phase systems between silica and graphite surfaces. The hydrophilic silica and hydrophobic graphite surfaces exhibit substantially different effects on CH4 hydrate formation. The hydrophilic and hydrophobic characteristics of porous media control the curvature of CH4 bubble, the structure of interface H2O and the surface hydrogen bond, thus affecting the CH4 hydrate formation. In terms of experimental studies, Li et al.[88] used Raman spectroscopy to study the influence of glass particles of hydrophilic, partially hydrophobic and completely hydrophobic surfaces on hydrate formation. The results show that the higher the order degree of water molecules on solid surface, the shorter the induction time of hydrate and the faster the nucleation rate. Another experiment[67] reported that the effects of glass particles coated by N,N-dimethyl-N-octadecyl-3-aminopropyl trimethoxysilyl chloride (DMOAP) (partially hydrophobic) and octadecyltrichlorosilane (OTS) (hydrophobic) on the nucleation of THF, CP, TBAB, and CH4 hydrates. The experimental results show that the hydrophilicity and hydrophobicity of porous media have different manifestations for different guest molecules. For THF and TBAB, partly hydrophobic and hydrophilic glass particles promote hydrate formation. However, for water-immiscible hydrate formers CP and CH4, only encountered hydrophilic glass particle, can detect hydrate nucleation. This is also consistent with the calculation results of Bai,[89] namely that hydrate nucleation is more likely to occur on hydrophobic surfaces. According to Skovborg et al.,[90] the driving force of hydrate nucleation can be defined as the chemical potential difference between water in liquid phase and hydrate phase, and the hydrate formation rate is positively correlated with this driving force. Since the chemical potential of water molecules near the hydrophobic surface is higher than that of pure water molecules,[91] the increase in nucleation rate observed on the hydrophobic surface can be at least partially attributed to the increase in nucleation driving force of cage hydrate.

Wang et al.[92] studied the effects of hydrophobic nanosilica with different grain sizes and concentrations on CH4 hydrate formation. The results find that the gas storage is not change significantly, but induction time and rate of hydrate formation are significantly improved. This is because CH4 molecules are adsorbed on the surface of the hydrophobic nano-SiO2,[93] and surrounded by H2O molecules, which helps to increase the gas–liquid contact areas, and the hydrate formed tightly surrounds the hydrophobic nano-SiO2. Thus, the higher the concentration of nano-SiO2, the shorter the induction time of hydrate formation, and the hydrate formation rate obviously accelerated. Figure shows the mechanism that nano-SiO2 promotes hydrate formation.

Figure 17

Mechanism of hydrate promotion for hydrophobic nano-SiO2.[92]

Xu et al.[94] studied the effect of hydrophilic silica nanoparticles on hydrate formation during upward migration of CH4 gas. The experimental results showed that hydrophilic silica nanoparticles inhibit hydrate formation, and the strongest inhibition effect at 2.0 wt % water molecules will be adsorbed on the surfaces of nanoparticles, which reduces the number of free water and inhibits the activity of water molecules, as shown in Figure . At the same time, only a small amount of hydrophobic nonpolar CH4 molecules can be distributed on the surfaces of hydrophilic nano-SiO2 particles. Both plays a role in delaying the induction time and rate of hydrate formation.[95−97]

Figure 18

Distribution of CH4 molecules and H2O molecules on the surface of hydrophilic nano-SiO2.[94]

However, different conclusions are drawn in terms of numerical analysis. Kashchiev et al.[98] adopted crystallization kinetics theory to obtain the relationship between the interfacial tension of hydrate nucleation power and the wetting angle of matrix surface. The study found that when the interfacial tension keeps constant, the nucleation power is proportional to the wetting angle, and the stronger the hydrophilicity is, the smaller the contact angle is. Therefore, they believe that the more hydrophilic the nucleating matrix is, the easier hydrate formation is.

By summarizing previous research results, it can be found that there are different conclusions about the influence of hydrophilicity and hydrophobicity on hydrate nucleation. The nucleation process of the hydrate on the solid surface is very complex and may be affected by various factors in the specific experimental process. However, the establishment of models for numerical simulations may not be able to take all possible factors into account, resulting in different conclusions based on different research methods. We should aim at the focus of the dispute and continuously optimize the numerical analysis models to include more influential factors and make it closer to the actual situation in the following work. In the experiment, the formation mechanism of hydrate at hydrophilic and hydrophobic interfaces should be studied from more microscopic perspectives, such as contact angle and interfacial tension, to further clarify the influence of the two interfaces on hydrate formation.

Functional Groups

Graphene is a two-dimensional monolayer nanosheet composed of SP2 hybrid and honeycomb-arranged carbon atoms. It has significant properties such as large specific surface area, high transparency, excellent mechanical strength, superior electrical conductivity, and thermal conductivity, which has wide application prospects.[99] The graphene structure is shown in Figure . Original graphene is highly hydrophobic, in the absence of dispersant could not directly dispersed in water, which limits the large-scale production and application. At present, the graphene derivatives, such as graphene oxide (GO) and chemical modification graphene (CMG), have been prepared. Highly oxidized graphene is mainly present in epoxy compounds’ alcohols and carboxylic acid groups, whose structure is shown in Figure . For hydrate formation, the thermal conductivity of GO decreases because the conjugated structure of nanosheets is destroyed by the presence of oxygen-containing groups. However, GO is amphiphilic and can be used as a surfactant, showing superior dispersity in water. Thus, GO may be suitable for promoting the formation of gas hydrates.[100] Rezaei et al.[47] studied the effects of synthesized graphene oxide suspension on kinetics of ethylene hydrate formation. The results found that the graphene oxide significantly decreased the induction time, and with the increase of the concentration, its storage capacity is improved significantly. They suggest that GO provides excellent structure for heterogeneous nucleation and grid structure for nucleation of water and ethylene molecules. In addition, carboxyl and hydroxyl groups on GO can bind hydrogen bonds to further stabilize the hydrate crystals.

Figure 19

Diagram of graphene.

Figure 20

Diagram of graphene oxide.

Sun et al.[101] studied the influence of silica sand power with different particle size on the phase behavior of CH4 hydrates. The authors find that the effect of coarse-grained silica sand on CH4 hydrate phase equilibrium can be ignored. However, the effect of fine-grained silica sand on CH4 hydrate phase equilibrium is significant. The author put forward that the surface of silica particles retain hydroxyl radicals, with strong hydrophilicity, leading to the positively charged end of water molecule on the solid surface strongly being attracted by the negatively charged surface of silica particle, the mechanism of which is shown in Figure . The powerful force is called the Coulomb force, which is stronger than hydrogen bonds or intermolecular forces, but the CH4 molecule cannot freely combine with bound water molecules. Consequently, a higher enthalpy change is required for CH4 molecules to diffuse into the water, which makes the phase equilibrium conditions more demanding.

Figure 21

Surface groups of hydrophilic SiO2.

Effects of Porous Media and Additive Compound System on Hydrate Formation

Previous studies have shown that only a small amount of hydrates can be formed in the pure water, and there is no significant pressure drop in the reaction system.[102,103] This is because the high surface tension at the gas–liquid interface influences the gas dissolution in a pure water system. Furthermore, the first contact with the liquid surface will form hydrate, which hinders the further gas diffusion, reduces the mass transfer efficiency, and makes it different to further form hydrate. The difficulty of mass transfer and heat transfer is also a problem that must be solved in the wide-scale application of hydrates technology.

The additives can be used to change the low-temperature and high-pressure requirements and slow the growing rate of hydrate formation. Currently, the commonly used hydrate promoters mainly include the thermodynamic promoters and kinetic promoters.[104−107] The common thermodynamics promoters include tetrahydrofuran (THF), tetrabutylammonium bromide (TBAB), and cyclopentane (CP), which aim to moderate the phase equilibrium conditions of hydrate formation. The kinetic promoters mainly include surfactants such as sodium dodecyl sulfate (SDS), sodium dodecyl benzenesulfonate (SDBS), and so on, which eventually affect the kinetic parameters, such as gas storage, formation rate, and induction time.[108] Link et al.[109] tested the CH4 absorption rate of surfactants, such as sodium dodecyl sulfate, dodecyl trimethylammonium chloride, dodecylamine, sodium lauric acid, and sodium oleate in the same concentration range. The addition of SDS provided the theoretical maximum CH4 uptake of over 97%. Of the suite of surfactants studied, it appears that SDS may be the most appropriate surfactant to use for hydrate formation.

Jin et al.[110] used Al(OH)3, Zn(OH)2, Fe(OH)3, and Cu(OH)2 particles with different masses to simulate the porous media environment, compounding with SDS. It is found that the surface of amphoteric hydroxide particles would generate electric charge due to hydrolysis, which attracts the active group of SDS in the solution. Since Al(OH)3 particles are trivalent amphoteric hydroxide, they could generate more electric charge and make the gas consumption and gas storage density of hydrates be larger than the other three particles. Liu et al.[11] studied the hydrate formation with porous Al2O3 and solid SiO2 particles of different particle size compounded with SDS solution. The experimental indicate the surface of Al2O3 would be positively charged because of hydrolysis, and the surface of SiO2 would be negatively charged under the combined influence of polarization and hydration; additionally, the charge would form a two-electron layer with the surfactant ions in the solution. The negatively charged active groups ionized by SDS tend to be adsorbed on the surface of positively charged alumina, so the residual pressure after hydrate formation in alumina becomes lower.

Wu et al.[111] studied the formation of natural gas hydrate under the mixing of Al2O3 particles and surfactants and compared the gas storage density and gas consumption of different composite systems, as shown in Figure . Both gas storage density and consumption in the compound system are slightly higher than those of the single surfactant system. This indicates that the Al2O3 particles can improve the gas storage performance of the hydrates, which is of great significance to accelerate the hydrate formation and its production rate. The gas consumption is further increased in the mixed system of Al2O3 and surfactants, indicating that the two mechanisms are synergistic.

Figure 22

Hydrate storage density and gas consumption with different surfactants.

Zhang et al.[112] studied that solutions of sodium lauryl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), and alcohol ethoxylate (AEO) were used with different-sized (2, 4, and 6 mm) porous media to investigate the characteristics of the hydrate formation. The experimental results show that the porous medium with a small particle size is more conducive to hydrate formation. The author believed that compared with the nonionic surfactant AEO, the anion active groups (SDS– and SDBS−) have stronger Coulomb force and can enhance the aggregation around the surface of alumina particles. The gas storage ability and the gas storage density of the mixed system of SDS and SDBS solutions with porous media are about 5 times higher than those of AEO, as is shown in Figure .

Figure 23

(a) Composite system with particle size of 2 mm. (b) Composite system with particle size of 4 mm. (c) Composite system with particle size of 6 mm.

Due to the limited effect of using single additive, several different additive combinations have been used to maximize its benefits.

Su et al.[113] conducted the experiment on the synthesis of low-concentration oxygenated CBM hydrate under porous media + THF + TBAB systems by an orthogonal experimental method and discussed the experimental results by using range and variance analysis. The results show that THF + TBAB compound system can effectively reduce the phase equilibrium temperature of low-concentration CBM hydrate phase under the same pressure and that the existence of porous media has a significant effect on improving the gas storage of low-concentration CBM hydrate. Further, the results show that THF has a significant effect on the gas storage.

Yang et al.[114] investigated the effect of additive mixture (THF + TBAB) on phase equilibrium of CO2 hydrate through several high-pressure reactor experiments. The presence of thermodynamic promoters THF and TBAB can alleviate the hydrate phase equilibrium, but the addition of THF + TBAB at different concentrations has different results. When the mass fraction of THF is 1%, adding TBAB has little effect on the phase equilibrium of the CO2 hydrate, while the mass fraction is 5%, demonstrating that adding TBAB has great effects on the phase equilibrium of CO2 hydrate. In other words, in the presence of a high concentration of THF, increasing the amount of TBAB is more conducive to regulating the phase equilibrium conditions of hydrates, Besides, compared with the mixture of THF + SDS,[115] it is found that THF + SDS has less effect on reducing the phase equilibrium pressure of hydrate than THF + TBAB, as Figure shows. This occurs because SDS is a kinetic additive which can only change the kinetic properties of hydrate, such as induction time and formation rate, but it can not affect the phase balance of hydrates.

Figure 24

CO2 hydrate phase equilibrium curve with mixed additives in porous media. (Note: the curve is the CO2-water phase equilibrium: Larson.[59])

Conclusion

In this review, the effects of the porous media on hydrate phase equilibrium conditions and formation rates are studied in-depth, which is helpful for better understanding the mechanisms of porous media in the hydrate formation process.

There is no consistent on the effect of stacked porous media on the phase equilibrium of single or mixed gases. On the one hand, the activity of water molecules on the surfaces of pores decreases, which inhibits hydrate formation. On the other hand, the third-surface absorbs water molecules which can accelerate the formation of hydrate crystal nuclei leading to faster and greater crystal growth. However, integrated porous media, such as foam metal, can improve the hydrate phase equilibrium conditions because of rapid hydration heat transfer.

The hydrate formation process is closely related to the physical and chemical properties of porous media. The roughness of the porous media surface improves the nucleation sites and changes the flow rate and thus affects the hydrate formation. The hydrophilic and hydrophobic surfaces have different manifestations depending on guest molecules; that is, water-immiscible CP and CH4, hydrophilic porous media ease hydrate nucleation, while water-miscible THF and TBAB, hydrophilic materials enhance hydrate nucleation. It is necessary to clarify the two interfaces on hydrate formation from more microscopic perspectives.

In general, the characteristics of porous media surface, the critical particle size, and the synergistic effect with the kinetic and thermodynamic accelerators are the important issues to further be studied for the high-efficiency industrial applications.