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Effect of Porous Media and SDS Complex System on Methane Hydrate Formation Zhiming Liu, Zhen Pan, Zhien Zhang, Peisheng Liu, Liyan Shang, and Bingfan Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00041 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018
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Effect of Porous Media and SDS Complex System on Methane Hydrate Formation Zhiming Liua, Zhen Pana,*, Zhien Zhangb,*, Peisheng Liua, Liyan Shangc, Bingfan Lid
a. College of Petroleum Engineering, Liaoning Shihua University, Fushun 113001, China; b. School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, China; c. College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun 113001, China; d. Shandong Provincial Key Laboratory of Oil and Gas Storage and Transportation Safety, China University of Petroleum (East China), Qingdao 266580, China;
ABSTRACT Hydrates are mainly stored in the pores of the sedimentary layer, so their formation and storage characteristics in pores are particularly important for the exploitation and utilization of hydrates. Meanwhile, in order to study the rapid formation methods of hydrate, 300 ppm Sodium Dodecyl Sulphate (SDS) solutions mixed with alumina and silica particles respectively were used for investigating the hydrate formation in the pores of porous media under the conditions of 275.15 K and 7 MPa. The particle sizes of alumina and silica were 2, 4 and 6 mm. The experiments results indicated porous media with smaller size could shorten the induction time of the hydrate formation and increase the final gas uptake. In dissolution period and induction period, gas consumption of hydrate formation in alumina particles was larger than that in silica, but reverse in the growth period. The hydrate formed in alumina particles had a better gas storage ability. In the used SDS solution, the positive charges produced by hydrolysis on alumina surface and the negative charges produced by polarization and hydration on silica surface affected the distribution of SDS active group around *Corresponding Authors：E-mail: [email protected] (Z. Pan); [email protected] (Z. Zhang). 1
their surface, which leaded to the difference of gas storage density between the hydrates formed in these two particles. Finally, with the formation of the hydrate on the wall of the porous medium, the stronger capillary force caused by the decreasing pore size leaded to a migration of water, which further resulted in the hydrate form in the pores without SDS solution originally. KEYWORDS: Methane hydrates; Porous media; Grain size; Surfactant; Particle distributions; Electric double layer model.
1 Introduction As a new energy source, nature gas hydrate has the advantages of large reserves, high calorific value and small combustion pollution, which is considered to be the most valuable environment-friendly fossil energy in 21st Century. It is generated by water molecules and some small molecular gases called "guest molecules" at low temperature and high pressure. Previous studies have shown that many kinds of gases such as methane, ethane, carbon dioxide, and nitrogen as well as tetrahydrofuran, and cyclopentane in liquid phase could all form hydrates with water molecules. Hydrates can be classified into three categories based on the microscopic arrangement of water molecules in crystal nucleus and the size of the gas molecules: structure I (sI), structure II (sII), and hexagonal structure (sH)
[1-3]
. In nature, about 97% of the hydrate is distributed in the seafloor
sediments, and only 3% is distributed in the pores of the terrestrial permafrost [4]. It is estimated that the hydrate reserves could reach 7.6 ×1018 m3, which means a great mining and application prospect [5,6]
. Under the standard conditions, 1 volume of hydrate can store 164-180 volumes of methane
[7]
.
Therefore, due to the excellent gas storage capacity and safe characteristics, hydrates play an 2
increasingly important role in industrial applications such as natural gas storage and transportation, carbon dioxide capture, desalination and refrigeration
[8-11]
. On the other hand, the exploitation of
hydrate with huge reserves in nature is of great significance to improve the world energy supplies and ensure the energy security. Thus, it has been received great interests by the researchers to observe the characteristics of hydrate formation and storage under the natural conditions. However, more than 90% hydrate is stored in the pores of the sedimentary layer in nature, and according to the study of Yan et al. [12], the existence of porous media interface has a positive effect on the nucleation process and the rapid formation of hydrate. Thus, the investigation of the hydrate formation and storage characteristics in porous media is of great significance to the further exploitation and utilization of hydrate resources and promote the application of hydrate in industry. Previous studies have shown that the improvement of the heat and mass transfer conditions in the hydrate formation process could significantly accelerate the formation rate
[13-15]
. On one hand,
the addition of porous media can improve the heat transfer condition and the 2D nucleation helped hydrate formation compared to the slower 3D nucleation
[16,17]
. On the other hand, the addition of
surfactant could reduce the mass transfer resistance by reducing the surface tension of liquid, which could improve the mass transfer conditions [21]
[18-20]
. With the respect to the porous media, Linga et al.
carried out the hydrate formation experiments in silica sand and pure water. The results showed
that the hydrate formation rate in silica sand was much higher than that in water. Bagherzadeh et al. [22]
conducted the hydrate formation experiment in silica sand with different particle sizes and
observed the formed hydrate by NMR (Nuclear magnetic resonance). It was found that the presence of silica sand promoted the hydrate formation especially with small particle sizes. Dai et al.
[23]
demonstrated that the presence of the porous media could shorten the hydrate induction time and 3
promote hydrate formation by adding minerals such as kaolinite, sediment and calcium carbonate with different interface properties to the solution. Moreover, in the aspect of hydrate formation imrpovements by adding surfactant, Kalogerakis et al.
[24]
found that the presence of surfactants
increased the gas solubility and formation rate, which greatly improved the formation of hydrates compared to the pure water system. By studying the hydrate formation in SDS solution, Veluswamy et al.
[25]
believed that anionic surfactant could significantly promote hydrate formation, and studied
its influence on the morphology of the formed hydrate. Hayama et al. [26] reported the crystal growth dynamic and crystal morphology of methane hydrate by visual observation. It was found that the crystal growth and morphology were dominated by the wettability and capillary force. Kumar et al. [27]
studied the effects of Tween 80, dodecyltrimethylammonium chloride (DTACL) and SDS on the
hydrate formation kinetics in a fixed crystallization reactor. The results indicated that SDS showed the best performance in accelerating hydrate formation rates and reducing the induction time among the three studied surfactants. Han et al.
[28]
found that SDS solution with the concentration of 300
ppm had the most remarkable effect on reducing the surface tension of gas and liquid at 275.15K, which could improve the mass transfer condition in hydrate formation. In addition, considering the improvement of porous media and surfactants on the heat and mass transfer conditions in the process of hydrate formation, many scholars combined the porous media and surfactant to study the promoting effect of this complex system on hydrate formation, which showed a better hydrate formation effect than the single system. Mohammadi et al. [29] carried out the hydrate formation experiments in nano-silver particles, SDS solution and their complex system respectively. It revealed that the complex system had the most obvious effect on enhancing the hydrate gas storage capacity. Moreover, compared to pure water, the presence of SDS and 4
nano-silver particles could increase the gas consumption by 93.9% and promote water to hydrate conversion at high pressures. Najibi et al.
[30]
studied the formation of hydrates at different
nanoparticle concentrations by mixing CuO nanoparticles with SDS. The results showed that the gas consumption, the consumption rate and the water to hydrate conversion rate were all increased, and the degree of increase was proportional to the concentration of the nanoparticles. In addition, the complex system could reduce the induction time, but had less effect on the storage capacity. Zhou et al.
[31]
investigated the effect of the graphite nanoparticles and SDS on the induction time. It
indicated that the induction time of hydrate formation was shortened greatly compared with a pure water system, and the effect of the mixture was better than the single additive. Furthermore, Kakati et al. [32] studied the effect of nanoparticles and SDS complex system on the CH4 + C2H6 + C3H8 hydrates. The results indicated that the system could increase the hydrate formation rate and shorten the induction time. In nature, hydrates are mainly stored in the pores consisted of soil particles, and the complex soil components in hydrate storage areas may affect the hydrate formation, which in turn affects the exploitation and utilization of hydrate resources. Therefore, the influence of surface properties on the hydrate formation and the characteristics of hydrate formation in pores should be clearly defined. However, there are few studies on the influence of surface properties and porous media distributions on hydrate formation in the presence of surfactant for now. In this study, it is considered that hydrate is mainly formed and stored in the pores of the sedimentary layer
[33]
. We adopted silica and alumina particles with different sizes mixed with 300
ppm SDS solution to figure out the characteristics of hydrate formation in the pores and the influence of different porous media surface on hydrate formation in the presence of surfactant. The effect of particle size on the hydrate formation and the gas consumption in the three stages of the hydrate 5
formation were also investigated. Besides, the mechanism of hydrate formation under the synergistic effect of SDS and alumina or silica surface was discussed by comparing the hydrate formation in porous media with pure water and SDS solution. Finally, according to the hydrate formation in porous media with different distributions, the influence of the capillary force on the migration of the liquid phase and the hydrate distribution in porous media were investigated.
2 Experimental methodology 2.1 Materials Sodium dodecyl sulfate (Analytical reagent grade) was supplied by Tianjin Dingshengxin Chemical Industry Co. Ltd. Methane (purity 99.9%) was supplied by Shenyang Keruite Gas Co. Ltd. Deionized water was made in the lab. Alumina particles and silica particles with average particle sizes of 2, 4 and 6 mm were used in experiments. The cylindrical plastic container with a volume of 300 ml was made of polyethylene terephthalate. Both silica and alumina particles had a hydrophilic surface, and the contact angle was less than 90º. The alumina particles had a huge amount of micropores on the surface and the volume of the micropores was very large (0.38-0.40 cm3/g), whereas silica particles were solid without micropores on the surface. The weight of all three sizes of 100 ml (true volume) alumina particles was about 198 g. Although there were a lot of micropores inside alumina particles, not all micropores were connected. In the SDS solution, the unconnected micropores should also be regarded as a part of the alumina volume. Therefore, the value of 198 g was smaller than the real weight of 100 ml alumina. The relevant parameters of both particles are summarized in Table 1. 6
2.2 Experimental apparatus and procedure The experiments for hydrate formation were performed using a KDSD-II type hydrate kinetic experimental apparatus, as shown in Fig. 1. It was consisted of a temperature-control system, a pressure-control system, a data acquisition system and a reactor, which could simulate the hydrate formation and dissociation process by adjusting the temperature and pressure in the reactor. The reactor was made of stainless steel with a volume of 350 ml, which could be operated up to 25 MPa. The temperature was controlled by the thermostatic water bath and measured by two platinum resistance sensors (Pt100) with an error of 0.1 K inside the reactor. The pressure was measured by a pressure transducer with an accuracy of 0.01 MPa. All the obtained data were timely recorded by the data acquisition system. The volume of the porous media and liquid phase were all kept constant (100 ml) to ensure the same amount of methane gas used in each experiment at 7 MPa (the volume of the reactor was 350 ml and temperature was kept the same in all experiments). To obtain the same volume of porous media with different particle sizes, we put the porous media into 200 ml pure water gradually in the measuring cylinder. When its volume reached 300 ml, the porous media was taken out and then dried. By this procedure, the 100 ml porous media with particle sizes with 2, 4, and 6 mm could be obtained. In the experiment, we put the formed hydrate inside a plastic container for better observations. The reactor wall and the plastic container were washed and dried using deionized water repeatedly to 7
avoid the interference caused by the original impurities and the residual water. Then the plastic container with porous media was placed into the reactor. After the sealing of the reactor and its air tightness test, the reactor was evacuated into a vacuum state by vacuum pump, and then 100 ml SDS solution with the concentration of 300 ppm or pure water was injected into the reactor through the liquid inlet. The reactor was placed into the water bath with a constant temperature of 275.15 K. When the temperature in the reactor dropped to the experimental temperature, adding methane gas into the reactor at the rate of 0.1 MPa/s through the intake port until the pressure reached 7 MPa. Recording the temperature and pressure in the reactor by data acquisition system. When the pressure was stable within 120 min, the experiment was completed. Three repeated trails under each condition were conducted to ensure the validity of the experiments.
3 Results and discussions 3.1 Effect of particle size on hydrate formation In the porous medium, hydrate is mainly formed in the pores. Different particle size leads to different pore size which creates a different hydrate formation environment. Therefore, it has been pointed out that the particle size of porous media has an effect on the hydrate formation
[34]
.
Moreover, the hydrate is mainly deposited in the pores of the sedimentary layer in nature, its pore size directly determines the maximum hydrate storage potential in this area. Thus, the study of the pore size effect on the hydrate formation is of great value to the hydrate exploration. Generally, a typical hydrate formation process includes three stages: gas dissolution period, induction period and growth period [35]. The temperature and pressure in the reactor exhibit different characteristics in the corresponding periods. In dissolution period, gas is dissolved in the liquid
through the gas-liquid interface, so the gas uptake of the system increases slightly and the pressure has a certain reduction, the degree of the reduction varies with the gas solubility. However, the temperature remains substantially constant. In induction period, the dissolved gas molecules interact with the liquid and generate small-sized nuclei gradually in an unstable state. These unstable nuclei may decompose, or continue to grow up into stable state with a critical size. When the stable nuclei reach a certain number, the induction period ends, and the hydrate begins to generate quickly
[36]
.
Thus, the induction period is a process which is starting from the formation of the unstable hydrate nuclei to the generation of stable nuclei with a critical dimension. Since the hydrate is continuously generated and dissociated in this stage, and only the hydrate nuclei are formed, the temperature, pressure and gas uptake of the system remain almost constant [37]. In the last period, the stable nuclei formed in the induction period grow and coalesce, which macroscopically reflects a large amount of formed hydrates. In this process, a lot of heat are released, so the temperature has fluctuations in the system. Moreover, due to gas being consumed constantly in this period, the system pressure drops significantly and the gas uptake increases accordingly until the hydrate formation ends. Consequently, the three periods could also be divided based on their characteristics reflecting on the pressure drop and gas uptake curves. In this study, all the hydrate formation experiments were conducted at 275.15 K with an initial pressure of 7 MPa. The results obtained in the process are summarized in Table 2.
Fig. 2 shows the gas uptake curves of the hydrate formation in the complex system of SDS and particles. The average sizes of the alumina particles and silica particles were 2, 4 and 6 mm. It can be
seen from Fig. 2 that the gas uptake process was well consistent with the three stages of the hydrate formation as discussed above. The gas uptake increased slightly due to the dissolution of the gas, after that, the gas uptake basically remained the same due to the presence of induction period. Then during the dissolution period, a large amount of hydrates were formed, so the gas uptake increased greatly until the the formation ended. However, different particle sizes in the experiment resulted in differences in the gas uptake curves, which mainly focuses on the following two points: (i) the final gas uptake of the hydrate was different in each experiment after the reaction; (ii) the duration of the induction stages of hydrate formation varied with the particle size. Therefore, in this experiment, the influence of particle size on the hydrate formation was obvious. For the first difference mentioned above, according to the experimental data, the final gas uptake of the hydrate formed in particles with average particle sizes of 2, 4 and 6 mm were 0.097, 0.093, and 0.089 (mol of gas/mol of SDS solution) for alumina and 0.092, 0.090, and 0.087 (mol of gas/mol of SDS solution) for silica, respectively. Thus, it can be found that the smaller the particle size, the larger the gas uptake after the hydrate formation, which meant a larger gas storage capacity and density (due to the same SDS solution volume) of the formed hydrate. This may be due to the fact that for the two kinds of porous media, the smaller particle size provided larger specific surface area which could be the nucleation sites, and the presence of the nucleating sites played an important role in promoting the generation of the hydrate nuclei during hydrate formation
[38]
. Besides, the
larger specific surface area could make the gas and liquid molecules contacted more fully, which could greatly increase the percentage of gas in water molecule cage, thus increasing the gas storage density of the hydrate formed in the smaller size particles. The residual pressure of the hydrate formed in 2, 4 and 6 mm particles were 3.32, 3.49, and 3.67 MPa for alumina and 3.55, 3.65, and 10
3.76 MPa for silica, respectively. According to the characteristics of hydrate formation, the pressure remains constant at the end of hydrate formation, and the temperature and pressure could be under the phase equilibrium condition of hydrate. Based on the existing studies, only when the hydrate is formed in the pores with the diameter less than 65 µm, the phase equilibrium condition changes obviously due to the effect of capillary force on the liquid phase activity [39]. However, the pore diameter used in this experiment was much larger than 65 µm, so the difference in the residual pressure between the experiments should be due to the different gas consumption caused by the porous media, rather than the change in the hydrate equilibrium pressure. And for the second difference mentioned above, different particle size led to different duration of the induction period in hydrate formation. It can be seen from Fig. 2, in dissolution period, there was little difference in gas uptake between the different-sized particles. This was because the dissolution period mainly occurred the dissolution of methane gas, and the degree of gas consumption was mainly determined by the solubility of the gas, which was not directly related to the particle size. Therefore, the dissolution period of the experiment was basically the same under the conditions of different particle sizes. But for the induction period, there was an obvious difference that the induction period was shorter in the smaller particle size. This may be because the induction period mainly occurred the formation of hydrate nuclei, so the duration of the induction period has a direct relationship with the generate rate of nuclei. In smaller particles, the larger nucleation sites provided by the surface area and the promotion effect of the hydrophilic surface could greatly enhance the formation of hydrate nuclei, and shorten the induction period. The induction time represented the duration required for the formation of a suitable-sized hydrate nucleus, the formation of nucleus was stochastic and dependent on many parameters, including impurities, driving forces, 11
contact areas, etc. Therefore, the induction time was apparatus-dependent and the values were generally dispersed. In the repeat experiments, the three induction times were scattered and there was a difference of dozens of minutes, as listed in Table 2. However, it was not difficult to find that with the decrease of the particle size, the average induction time reduced accordingly. Thus, it can be concluded that the porous media with smaller size could reduce the induction time of the hydrate formation. For the growth period, the most notable feature was the gas uptake increased significantly. The different duration of the induction period caused the hydrate formation reaching the growth period at different time. The smaller the particle size was, the earlier the growth period will be reached, and the less time the hydrate formation required. Two temperature sensors were placed to measure the temperature at the top and bottom of the reactor, in the process of hydrate formation, a lot of heat was released resulting in fluctuations in the temperature. Fig. 3 shows the pressure drop and the corresponding temperature profiles of hydrate formation in alumina and silica particles. At the beginning of the experiments, the temperature in the reactor was kept at 275.15 K by the water bath and the pressure dropped slightly due to the dissolution of methane gas. Then, the pressure dropped sharply which meant the hydrate was formed rapidly and a lot of heat were released. The temperature in the reactor increased significantly and because the released heat was conducted outside of the reactor continuously by the cooling water, a new thermal equilibrium was achieved, so the temperature could fluctuate near a higher value, as shown in Figure 3. At last, the hydrate formation was completed and the temperature dropped back to 275.15 K.
In the process of hydrate formation, only two temperature sensors were installed. The two measured temperatures increased and dropped almost simultaneously, so it could only reflect the overall characteristics of the hydrate formation, which was an average information over many local hydrate formation instead of the localized information of the fixed bed, while the latter is more meaningful in understanding the characteristics of hydrate nucleation and formation
[22]
. So two
temperature measuring points are not enough to reveal the local hydrate formation, besides, magnetic resonance imaging (MRI) is also an effective way to study the localized information of hydrate formation in a fixed bed, which should be studied in the future work [40,41]. 3.2 Comparison of Hydrate Formation in Alumina and Silica Particles with SDS According to the characteristics of the pressure drop curves in the process of hydrate formation, the three periods could be distinguished. In dissolution period, pressure dropped slightly due to the dissolution of the gas. In induction period pressure kept almost constant, and in the growth period pressure decreased significantly due to the large amount of formed hydrates. Therefore, the start and end time of each period could be obtained from the pressure drop curves. Then based on the pressure values and the computational methods, the gas consumption in each period could be quantified. Fig. 4 shows the gas consumption in the three periods during hydrate formation. Each point in the figure represents the gas consumption in each period of the hydrate formation at the corresponding particle size, which can be used to compare the gas consumption in the three periods when the hydrate was formed in alumina and silica particles. In the gas dissolution period, it can be found that the gas consumption in the alumina particle
system was significantly larger than that in the silica particle system due to the difference between the two porous media. Alumina particle was a kind of porous material with a large surface area, and the pore volume on its surface was up to 0.38-0.40 cm3/g showing a good gas adsorption capacity. Therefore, in the dissolution period, the methane gas diffused through the upper layer of alumina particles, was dissolved into SDS solution, and finally a portion of the gas was absorbed in the micropores on the surface. However, the silica particles had no ability to adsorb the gas, therefore, the gas consumption in the alumina particles was better than that in the silica particles. The induction period mainly occurs the formation of hydrate nuclei, the dissolution of the gas into the liquid is close to saturation, and the pressure drop of the system changed slightly. But according to the experimental data, the pressure drop during the hydrate formation in silica particles was about 0.1 MPa, while that in the alumina particles was 0.2-0.3 MPa. Since only a small amount of gas was consumed in the hydrate nucleation, it was indicated that there was still an adsorption of gas in the alumina during the induction period. It was also found that the gas consumption of alumina in the induction period was less than that in the dissolution stage showing a weaker gas absorption capacity. This may be because the hydrate generated a thin film on the surface of the particles, preventing the gas adsorption of alumina particles. In growth period, a large amount of hydrates were generated in the particle pores and resulted in a great pressure drop. The difference in this period was that the gas consumption in the silica system was greater than that in the alumina system, indicating that there was more gas involved in the hydrate formation. This could be caused by the following two reasons. The first one was at the beginning of the growth period, the pressure in the two kinds of particles were different. Because the gas consumption of alumina system was higher in the first two periods, so when the growth period 14
started, the residual pressure in silica system was higher. According to the relevant studies, the initial pressure of hydrate formation has a significant effect on the hydrate formation. The higher the initial pressure is, the more favorable the hydrate formation is [42,43]. The second reason is due to the different surface properties of the two particles, the presence of the oxhydryl on the surface of silica surface could form hydrogen bonds with water molecules. These bonds made the arrangement of water molecules on the silica surface more orderly, which promoted the formation of water molecules cage required by the hydrate formation. Thus, the gas consumption in the silica particles was larger during the growth period. Fig. 5 shows a comparison of the final gas uptake in the same-sized alumina and silica particles after the hydrate formation. It can be found that the gas uptake in the alumina particles was larger than that in the silica particles, which indicated a larger amount of gas were consumed in the alumina particles. The effect of the alumina particles surface on the hydrate formation as well as the enhancement of the gas absorption produced by its own microporous structure greatly improved the gas storage performance of the mixture system. Therefore, using porous materials as hydrate formation media was an effective way to increase the gas storage capacity. 3.3 Effect of SDS on Hydrate Formation in Porous Media 3.3.1 Hydrate formation in SDS solution and pure water mixed with porous medium The presence of surface tension and the hydrophobicity resulted in a small solubility of methane molecules. Moreover, hydrates tended to form a thin film at the gas-water interface which would block the mass transfer between the gas and water phases. So just a little hydrate could form in pure 15
water and the hydrate was difficult to generate quickly. SDS molecules ionized with a hydrophilic group oriented towards the aqueous phase and a lipophilic group oriented towards the gas phase [44]. Since the surface tension was caused by the difference in the molecular interaction between the molecules on both sides of the gas-liquid interface. Therefore, when a large amount of SDS active groups were adsorbed on the interface, the molecular interactions of the gas-active groups and the water-active groups were greater than those between the original gas and water molecules, which could enhance the gas-liquid phase interaction and reduce the surface tension of the gas-liquid interface significantly [45,46]. At the same time, a large number of methane molecules adhered to the surfactant hydrophobic groups, and were brought into the liquid phase owing to the Brownian movement of the hydrophobic group, which increased the solubility of methane molecules greatly so that the hydrates could be quickly formed. Fig. 6 illustrates the gas uptake of the hydrates formed in pure water and 300 ppm SDS solution mixed with alumina or silica particles. It was shown that the presence of SDS promoted the hydrate formation dramatically. In pure water, the gas uptake increased slightly, after which the hydrate formation ended and the pressure remained at 5-6 MPa. However, in the SDS solution, the gas uptake increased greatly, and the residual pressure was maintained around 3.5 MPa. This meant that more gas was stored in the hydrate formed in SDS solution. Moreover, it can be observed that the hydrate formed in SDS was denser and harder than that formed in pure water. For natural gas hydrate, the ideal gas storage ratio can reach 1:180 in theory, which indicates that the hydrate has a great potential in the natural gas storage and transportation. According to the
calculated by Eqs. (1) and (2): P0V0 PV − t t z0 RT0 zt RTt nt = P ∆Vm 1− t zt RTt 1.6 4.2 Tc PtTc Tc PtTc z = 1 + 0.083 − 0.422 × + ω 0.139 − 0.172 × Tt PcTt Tt PcTt
(1)
(2)
where n is the mole of gas consumption; P, V and T are the pressure, gas volume and temperature in the reactor respectively; R is the universal gas constant; m is the hydration number; ∆V is the difference of molar volume between hydrate and water; z is the compressibility factor; the subscripts 0 and t denote the parameter at the beginning of the experiment and time t in the hydrate formation process, c represents the parameter is a constant, Pc is 4.599MPa and Tc is 190.6 K for methane; ω is a coefficient, 0.012 for methane.
c s = nt ×
Vmgas × Vmw
(Vi − Vu ) × (Vmw + ∆V )
(3)
where c s is the gas storage density; Vmgas and Vmw are the molar volume of the gas and water respectively; Vi is the volume of the solution before the reaction; Vu is the residual solution volume after the hydrate formation.
Tables 3 and 4 demonstrate the gas storage capacity and density of hydrate formed in 300 ppm SDS solution and pure water with different particle sizes, which are calculated by Eq. (1) and (3). It
was found that the gas storage capacity and gas storage density of the hydrate in pure water were much smaller than those in the SDS solution. On the other hand, because of the adsorption of alumina particles, the gas storage capacity and storage density of the hydrate formed in its pores were obviously larger than those in silica particles. For the same kind of porous media, particles with small size provided a larger area of nucleation sites, which could promote hydrate nucleation and formation. Thus, the smaller the particle size was, the greater the gas storage capacity and gas storage density were when the hydrate was formed. Based on the calculated data, in SDS solution, the gas storage capacity and density of hydrate formed in alumina and silica particles showed a minor difference. The gas storage capacity were all about 0.09 mol of gas/mol of SDS solution, and the gas storage densities were about 115 in both particles. But when hydrate was formed in pure water, the gas storage capacity and density of hydrate formed in alumina and silica particles had a huge difference, as depicted in Fig. 7. The columnar height represented the difference of the gas storage capacity and the gas storage density of the hydrate formed in the two particles. It can be seen that in pure water, the difference in gas storage capacity and gas storage density of the hydrate formed between the two porous media was much larger than that in the SDS solution. Combined with the analysis of the experimental process, it was indicated that the above phenomenon may be caused by the adsorption of micropores on the surface of alumina particles. In pure water, only a small amount of hydrate was generated, and a lot of gas remained in the reactor, which caused less hydrate attached on the wall of the alumina particles. Therefore, the microspores
could absorb gas through the loose hydrate layer; on the other hand, more residual gas made the system pressure higher, which was contributed to the gas adsorption of alumina particles. However, the silica particles had no ability to adsorb gas. Therefore, the gas storage capacity and density of the alumina particles were much larger than those of silica in pure water. On the contrary, in SDS solution, there was a larger amount of hydrate formed both in alumina and silica particles, the gas inside the reactor would be consumed in large quantities. Besides, the hydrate attached on the particle surface was relatively dense in SDS solution, preventing the gas adsorption of the alumina particles. So in the SDS solution, the gas storage capacity and density of the hydrate formed in alumina and silica particles had a minor difference. 3.3.2 Mechanism of hydrate formation under the synergistic effect of surfactant and porous media The chemical composition of the experimental particles is alumina and silica. The study of surface chemistry shown that the solid surfaces could undergo hydrolysis, polarization and hydration in liquid environment, which causes the surface to generate electric charge, and the kind of charge is determined by the surface proprieties [49]. For the surface of alumina, its zero charge point was about pH=9. If the pH value in the liquid phase was higher than 9, the surface was negatively charged, otherwise it was positively charged. Meanwhile, the zero charge point of silica surface was about pH=2.5. If the pH of the liquid phase was lower than 2.5, the surface was positively charged, otherwise it was negatively charged. In the experiments, the pH value of 300 ppm SDS solution was about 7.5, which meant the surface of the alumina particles was positively charged by the hydrolysis, and the surface of the silica was negatively charged due to the combined effects of polarization and hydration.
The charged surface in the liquid environment could affect the surrounding ions distribution because of the Coulomb force, and the macro distribution of charge was electrically neutral. One side of the interface distributed a kind of charge, and the other side distributed the opposite electric charge, resulting in a double electric layer structure on the surface. So far, many scholars have put forward the relevant theory for the charge distribution of the double electric layer
[50,51]
. A relatively optimal
model was put forward by Stern based on the existing double electric layer model
[52]
. It was
regarded that the liquid environment around the particles included two parts: one was the tight layer where the counter ion was strongly bound by the charged surface, called the Stern layer, the counter ion in the Stern layer was fixed at the specific position of the surface; The other part is the diffusion layer of counter ions, where its distribution was loose and the potential decreased slower as the distance increased. The electric charge distribution diagram of the electric double layer is shown in Fig. 8. In the experiment, most of the charged ions in the liquid phase were negatively charged active groups and positively charged sodium ions ionized by SDS. When the electric charge was generated on the surface of alumina and silica in the liquid environment, the distribution of charged ions around the surface would be affected. For the alumina particles, there was Coulomb force between the positively charged surface and the negatively charged SDS active group, which caused the active group accumulating in the vicinity of the particle surface; for the silica particles, the negative charge on the surface attracted sodium ions, causing sodium ions to accumulate around the surface, as shown in Fig. 8. The surfactant molecules could be adsorbed on the uncharged solid-liquid interface in a hydrophobic bonding manner so that the particle surface and the plastic container wall could adsorb 20
a large amount of SDS molecules. In addition, due to the micellar solubilization and the non-polar adsorption between the SDS hydrophobic group and the methane molecules, more methane molecules were accumulated around the particle surface and the container wall, which promoted the nucleation and further growth of hydrate. On the other hand, as mentioned above, the alumina and silica surfaces had different electrical properties in SDS solution. Therefore, compared with the silica surface, the negatively charged SDS active groups would tend to aggregate around the positively charged alumina particles surface because of the Coulomb force. The aggregation caused more methane molecules to dissolve around the alumina particles surface, and then increased the percentage of gas in water molecule cage. So the gas storage capacity of hydrate formed in the alumina particles was larger, and the residual pressure in the system was correspondingly smaller at the end of the experiment. 3.4 Hydrate Formation in Porous Media with Different Distributions In the experiments, hydrates were not distributed uniformly in pores when they were formed in the porous media, even some pores were hollow with no hydrate distributed. Therefore, we designed two kinds of porous media distributions in the reactor to study the characteristics of hydrate distribution in porous media. By this way, the distribution of the formed hydrate in the porous media could be controlled to facilitate the separation of the hydrate and porous media, which could reduce the transport costs of gas storage and transportation by hydrate in industrial applications. In order to study the influence of the surface properties and the capillary force of pores on hydrate formation, two kinds of porous media particle distributions are depicted in the experiment, as 21
shown in Fig. 9 (a) and (b). The particles were distributed in two layers in Fig. 9 (a). A clapboard with holes was placed in the middle of the container, the upper particles were placed on the clapboard, and the lower particles were placed at the bottom of the container. Only the lower particles were immersed in SDS solution, and the solution interface was as the same height as the clapboard. The diameter of the clapboard was exactly the same as that of the plastic container so that it could be stuck in the middle of the container. The diameter of the hole in the clapboard was 1 mm to allow water to pass through, and the spacing between the holes was 2 mm. In Fig. 9 (b), we put the particles into the plastic container gradually, built them into a slope structure, and due to the mutual support between the particles, this slope structure would remain stable in the reactor. A part of particles were above the surface of the SDS solution with no liquid phase in its pores, and the particles at the bottom of the container were immersed in SDS solution. Due to the experiments conducted in this section were to study the migration of the liquid phase and the hydrate distribution in the pores only, so the volume of SDS solution was 50 ml, which could make sure that there were void pores for the liquid phase to migrate, and the amount of porous media was identical in all experiments. Fig. 10 shows the hydrate formation in different particles distribution. As shown in the Fig. 10(a), a part of particles was above the surface of the SDS solution to form a slope without liquid phase in the pores. The experimental results showed that a large amount of hydrates were formed in the upper pores of the porous media without SDS solution before the hydrate formation, and the particles was consolidated together by the formed hydrate. In the process of extracting hydrate, it was found that the pores in the middle of the porous medium were hollow and no hydrate formation 22
could be observed. The same phenomenon was also found in the experiments carried out using silica particles in Fig. 10(b). In order to further investigate the above experimental phenomena, the alumina and silica particles were stratified in the experiments (see Fig. 10(c) and (d)). Only the lower porous media were immersed in the SDS solution, the bottom of the upper layer particles was in the same height as the liquid surface of the SDS solution and was non-wetted with the solution, as shown in Fig. 9. When the hydrate formation was completed, it was found that a large amount of hydrates were formed in the pores of upper particles, which were not filled with the SDS solution before the experiment. However, in the middle of the container, there was little hydrate formed, and most of the hydrates were distributed in the pores around the container wall. In these two different distributions, there was no hydrate formation in theory due to the absence of water in the slope (Fig. 10(a) and (b)) and upper layers (Fig. 10(c) and (d)). However, after the hydrate formation, a large amount of hydrate could be found in those places. It was supposed that the distribution may be caused by the migration of liquid in the porous media, at the solid-liquid interface, a liquid layer whose thickness equaled to the radius of molecular attraction was called the attachment layer. The force of the liquid molecules in the attachment layer was different from the molecules inside the liquid, which was affected by the cohesion of the liquid molecules and the adhesion of the solid molecules. When the adhesion was greater than the cohesion, the resultant force of the liquid molecules in the attachment layer will point to the solid side. As a result, the liquid molecules in the attachment layer were tended to be stretched, which was also the reason for wetting phenomenon [53].
When the liquid was in the pores, the surface tension and the wetting effect between the solid and liquid phase would increase the liquid surface, which was called capillary phenomenon. Its mechanism was that when the solid and liquid phases contacted, the interface would generate surface energy, which was related to the shape of the liquid phase surface
[54]
. When the temperature and
pressure of a stable system remained the same, its Gibbs free energy was tended to be minimum. Although the increase of the liquid surface to a certain extent led to the gravitational potential energy increased, while the solid-liquid contact area increased at the same time, reducing the surface energy of the interface [55,56]. Therefore, there was a critical state which made the total energy of the system minimum and stable. The rise height of the liquid column under the capillary force was studied by many scholars based on the pressure balance
[57]
, the force balance [58] and the liquid flow [59]. It is believed that the
rise of the liquid in the capillary is related to the additional pressure generated by the curved liquid surface formed in the hydrophilic capillaries. In the early 19th century, Young and Laplace deduced the relationship between the additional pressure and the curvature radius of the liquid surface [60], as shown in Fig. 11. Based on the fluid mechanics, the pressures at points A and B are the same for the same horizontal position, thus the capillary force formula can be obtained:
h=
2σ cos θ ∆ρ gD / 2
(4)
where θ is the solid-liquid contact angle, ∆ρ is the density difference between the liquid and the gas, D is the diameter of the capillary, and g is the acceleration of gravity. It indicates the rising height of the liquid column is proportional to the surface tension and the contact angle of the solid-liquid 24
interface, but inversely proportional to the pore radius and the density difference between the gas and liquid phases.
Fig. 12 is the schematic diagram of the hydrate formation process in the pores. Before the hydrate formation, the surface of SDS solution increased to a certain extent due to the capillary force of the porous media pores, and then the hydrate began to generate. According to the research of Bai et al.
[61]
, the hydrate was formed on the wall of the porous medium firstly and then continuously
expanded into the middle of the pores, which was consistent with the promotion effect of porous media on hydrate nucleation. As the hydrate grew, the pore radius decreased with the increased thickness of the hydrate film. As mentioned above, the rising height of liquid column was inversely proportional to the pore radius and the density difference between gas and liquid. Therefore, with the hydrate formation, the liquid column was continuously raised, which led to the hydrate formed in the pores without water originally. It was noteworthy that the hydrate was also formed at the gas-liquid interface in the middle of pores. But according to the research of Gayet et al.
[62]
and Botimer et al.
[63]
, the hydrate formed in SDS solution was relatively loose and contained a lot of pores inside, so it
would not stop the upward migration of liquid. On the other hand, due to the consumption of gas in the hydrate formation process, the gas phase density decreased, and the difference of gas and liquid densities increased, which may affect the rising height of the liquid column. However, the calculation results showed that the change in the density difference was small and the influence on the column height was negligible, so its effect could be ignored. Moreover, the effect of the surface tension and contact angle could be also ignored because they remained almost constant. Therefore, it could be concluded that the above-mentioned experimental phenomenon was due to the fact that the hydrate 25
was continuously formed on the wall of the porous medium, resulting in a stronger capillary force. This led to a migration of water from the lower part to the upper part, and the migration of water in turn resulted in the formation of hydrates in the pores where should be no hydrate formed theoretically. As for the phenomenon that a large amount of hydrate was formed on the container wall. On one hand, this is probably due to that the SDS could be adsorbed to the solid-liquid wall by hydrophobic bonding, resulting in more gas molecules dissolve in the liquid so that the hydrate could be rapidly generated around the wall. On the other hand, it may be because of the better heat and mass transfer conditions around the wall. The temperature was controlled by water bath, the direct contact between the reactor wall and cooling water allowed the generated heat to be transferred outside in a timely manner, which ensured the low temperature required for hydrate formation. Moreover, the presence of pores in porous media allowed water to migrate to the wall, and thus provided sufficient water for the formation of hydrates. So the better heat and mass transfer conditions could lead to the hydrate formation on the container wall.
4 Conclusions In this paper, alumina and silica particles mixed with the 300 ppm SDS solution were used to study the hydrate formation in the pores of porous media under the conditions of 275.15 K and 7 MPa. The effects of the particle size, the type of porous media and the synergistic effect of SDS molecules and the surface properties of medium on the formation of hydrate were analyzed. Finally, the position of hydrate formation in porous media with different distributions was introduced. The major conclusions obtained can be summarized as follows: 26
The smaller the particle size of the porous medium, the shorter the induction period of the hydrate formation process, and the larger the final gas uptake of the hydrate formed in the complex system. In the induction and dissolution periods, the gas consumption of hydrate formation in alumina particles was larger than that in silica, while it showed a reverse trend in growth period. In addition, the alumina particles system had a better gas storage capacity because of the promoting effect of its surface on the hydrate formation and the adsorption capacity of its own micropores. The presence of SDS in porous media could increase the gas storage density by 2 to 4 times compared to pure water, and the enhancing effect was related to the type and particle size of the porous media. On the other hand, the difference in hydrate storage capacity and storage density caused by the type of porous media in pure water was much greater than that in SDS solution. In experimental solution, the positive charges produced by hydrolysis on alumina surface, and the negative charges produced by polarization and hydration on silica surface would affect the distribution of SDS active group around their surface, which led to the difference of gas storage density in these two kinds of particles. And with the hydrate formed on the wall of the porous medium, the stronger capillary force caused by decreasing pore size led to a migration of water, which in turn resulted in the hydrate form in the pores where did not exist SDS solution originally.
Acknowledgment This work was supported by the Natural Science Foundation of Liaoning Province (201602470) and Open Fund of Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry 27
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Tables and Figures Tbale 1. Summary of silica and alumina particles used in the experiments. Table 2. Summary of experimental results for hydrate formation in porous media with 300 ppm SDS solution or pure water.
Table 3. Gas storage capacity of hydrate formed in SDS solution and pure water with porous media. Table 4. Gas storage density of hydrate formed in SDS solution and pure water with porous media. Fig. 1. Experimental apparatus for kinetics study of hydrate formation. Fig. 2. Gas uptake of hydrate formation in (a) alumina and (b) silica particles with SDS solution. Fig.3 Pressure and corresponding temperature profiles of hydrate formation in alumina and silica particles with particle sizes of 2, 4 and 6 mm.
Fig. 4. Comparison of gas consumption in three periods of hydrate formation in alumina and silica particles.
Fig. 5. Comparison of final gas uptake in alumina and silica particles with the same particle size. Fig. 6. Gas uptake of hydrate formation in porous media under the condition of pure water and SDS solution.
Fig. 7. Difference of hydrate (a) storage capacity and (b) storage density between the hydrate formed in alumina and silica particles mixed with pure water or SDS solution.
Fig. 8. Ions distribution around the surface of (a) alumina and (b) silica particles. Fig. 9. Two kinds of porous media distributions in experiments. Fig. 10. Hydrate formation in different porous media distributions. Fig. 11. The relationship between the radius of curvature (r), the diameter of capillary (D) and the wetting angle(θ).
Table 1. Summary of silica and alumina particles used in the experiments. Grain size(mm)
Volume of micropores
porosity
(cm3/g)
Contacet angle
Zore charge point
Silica particles 2
38.6%
—
0-4°
pH=2.5
4
40.1%
—
0-4°
pH=2.5
6
44.6%
—
0-4°
pH=2.5
2
39.9%
0.38-0.40
60-70°
pH=9
4
42.3%
0.38-0.40
60-70°
pH=9
6
47.9%
0.38-0.40
60-70°
pH=9
Alumina particles
Table 2. Summary of experimental results for hydrate formation in porous media with 300 ppm SDS solution or pure water. Experiment category
Volume of liquid (ml)
Volume of particles (ml)
Induction time approx.(min)
Hydrate forming duration approx. (min)
Residual pressure (MPa)
Storage density
Hydrate formed in particles mixed with SDS solution Si(2)
100
100
100/80/70
400/330/320
3.55
114.2
Si(4)
100
100
120/110/90
380/360/300
3.65
111.3
Si(6)
100
100
140/170/160
420/460/360
3.76
108.1
Al(2)
100
100
60/40/20
400/360/320
3.32
120.7
Al(4)
100
100
80/100/70
320/300/380
3.49
115.9
Al(6)
100
100
120/140/110
280/300/320
3.67
110.7
Hydrate formed in particles with pure water Si(2)
100
100
160/110/130
360/320/300
5.96
38.7
Si(4)
100
100
200/150/170
520/500/400
6.23
28.5
Si(6)
100
100
200/160/170
360/320/370
6.31
25.6
Al(2)
100
100
100/160/150
480/490/460
4.95
72.1
Al(4)
100
100
180/190/150
500/460/450
5.39
57.4
Al(6)
100
100
200/160/170
440/460/370
5.74
46.4
Si: experiments used silica particles; Al: experiments used alumina particles;(2)(4)(6):the grain size of the particles.
Table 3. Gas storage capacity of hydrate formed in SDS solution and pure water with porous media.(mol of gas/mol of liquid) Alumina particles Grain size/mm
Fig.3 Pressure and corresponding temperature profiles of hydrate formation in alumina and silica particles with particle sizes of 2, 4 and 6 mm.(Temperature 1 and 2 represent the temperatures at the top and bottom of the reactor respectively)
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0.16
0.14
0.14
-0.027
-0.053
0.4
-0.071
0.12
0.12
0.10
0.10 0.04
0.08
0.5
0.3
0.08 0.044 0.054
0.06
0.06
0.04
0.2
0.04 0.1 0.032
0.029
0.017
0.02
0.02 0.00
0.00 2
4
Gas consumption in growth period (mol)
Gas consumption in dissolution period (mol)
Gas consumption in induction period (mol) 0.16
0.0
6
Grain size(mm)
Fig. 4. Comparison of gas consumption in three periods of hydrate formation in alumina and silica particles. (The triangle, circle and square in this figure represent the gas consumption in dissolution period, induction period and growth period, respectively. The solid and hollow symbols represent the hydrate formed in alumina particles and silica particles, respectively. The vertical lines represent the difference of gas consumption in two kinds of particles.) 0.10 0.08
Fig. 5. Comparison of final gas uptake in alumina and silica particles with the same particle size.
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0.12
0.12
2mm alumina particles
(mol of gas/mol of liquid) Gas uptake
(a)
0.12
(b)
0.10 0.08
0.08
0.08
0.06
0.06
0.06
0.04
0.04
0.04
0.02
0.02
0.02
0.00
0.00 200
400
200
400
600
0
2mm silica particles
(e)
(f)
4mm silica particles
0.08
0.08
0.06
0.06
0.06
0.04
0.04
0.04
0.02
0.02
0.02
400
6mm silica particles pure water SDS solution
0.00
0.00 200
600
0.10
pure water SDS solution
0.08
0.00
400
Time(min)
0.12
0.10
pure water SDS solution
0
200
Time(min) 0.12
(d)
pure water SDS solution
0.00 0
600
Time(min)
0.10
6mm alumina particles
0.10 pure water SDS solution
0.12
(mol of gas/mol of liquid) Gas uptake
(c)
4mm alumina particles
0.10 pure water SDS solution
0
600
0
Time(min)
200
400
600
0
200
Time(min)
400
600
Time(min)
Fig. 6. Gas uptake of hydrate formation in porous media under the condition of pure water and SDS solution. ((a) (b) (c) are the gas uptake of hydrate formation in SDS solution and pure water with 2, 4 and 6 mm alumina particles respectively. (d) (e) (f) are the gas uptake of hydrate formation in SDS solution and pure water with 2, 4 and 6 mm silica particles respectively. )
Hydrate formed in particles with SDS solution Hydrate formed in particles with pure water
2
4
6
Grain size(mm)
Particle size(mm)
(b)
(a)
Fig. 7. Difference of hydrate (a) storage capacity and (b) storage density between the hydrate formed in alumina and silica particles mixed with pure water or SDS solution. (The green columns mean the hydrate formed in alumina and silica particles with SDS solution, and the cyan columns mean the same but with pure water.)
(a) (b) Fig. 8. Ions distribution around the surface of (a) alumina and (b) silica particles.
Particles Clapboard with holes
Particles
SDS solution
SDS solution
(a) (b) Fig. 9. Two kinds of porous media distributions in experiments. (a) Particles distribution in two layers. (b) Particles distribution in the form of a slope.
(d) Fig. 10. Hydrate formation in different porous media distributions. ((a) The photographs of hydrate formed in alumina particles with a slope distribution, (b) silica particles with a slope distribution, (c) alumina particles with a two-layer distribution, and (d) silica particles with a two-layer distribution.)
r θ θ h
A
B
D
Fig. 11. The relationship between the radius of curvature (r), the diameter of capillary (D) and the wetting angle(θ).
hydrate
particles
SDS solution
(1)
(2)
(3)
Fig. 12. Process of hydrate formation in the pores of porous media.
