Wang Lifeng Lu Jing Liang Jinqiang
(Guangzhou Marine Geological Survey Guangzhou 5 10760)
:E-mail:charles.wlf@gmail.com Wang Lifeng (1978—), male, doctor, engineer, mainly engaged in marine geology and gas hydrate research.
Natural gas hydrate, as a new energy growth point in the future, has long been concerned by scientific research and industry, and the research on this substance additive has gradually attracted the attention of all parties. In this paper, the importance of additives in the study of natural gas hydrate is demonstrated in the form of a review, and the research direction of simple systems and surfactants is emphatically introduced. The former has broad application prospects in industrial production, which can not only effectively reduce pipeline blockage, but also increase gas storage; The latter explains one of the modes of gas hydrate accumulation and accumulation in geological exploration, that is, the surfactant secreted by abundant autotrophs in the underwater world has the special function of accelerating hydrate formation and stability independently.
Natural gas hydrate additive; Simple system; surfactant
1 preface
Natural gas hydrate, referred to as hydrate for short, is a cage-like skeleton composed of water molecules wrapped in other gases. It usually exists in high-pressure and low-temperature environment and is widely distributed in nature, especially on the submarine continental shelf [1]. The analysis of the chemical composition of the hydrate samples found at present shows that the gas contained in them is mainly methane molecules, and the reserves are huge. It is estimated by the scientific community that these energy sources may be twice as much as the conventional energy sources that have been discovered, which is of great significance to the study of energy strategy and is a hot spot in the development of contemporary earth science and energy industry [2]. Hydrate research involves new energy exploration and development, greenhouse effect, global carbon cycle and climate change, ancient marine environment, marine geological disasters, natural gas transportation, oil and gas pipeline blockage and military defense, which may have a far-reaching impact on the development of geology, environmental science and energy industry.
The research on natural gas hydrate additives is a basic work, which provides basic physical and chemical parameters for the subsequent development and utilization of hydrates. The main research content is to determine the influence of additives on the stable equilibrium of hydrate. As early as the 1960s, the industry paid attention to hydrate only to solve the blockage of oil and gas pipelines and equipment, but the hydrate blockage caused by gas storage equipment and pipelines affected production, and even caused pipelines and even the entire oil well to be scrapped. With the deepening understanding of hydrate in recent years, the research on hydrate additives has gradually become one of the core research issues, and the future exploitation, storage and transportation of hydrate minerals will be based on its basic research [3]. Therefore, it is of great practical significance to control hydrate formation conditions (temperature, pressure, etc.). ) Inhibit or accelerate the formation of hydrate by additives. At present, the research on natural gas hydrate additives can be divided into two categories, namely, simple systems based on alkanes and surfactant systems based on laboratory or natural biosynthesis derivatives. The former is the focus of industrialization research, which is conducive to solving the blockage of natural gas hydrate pipeline and the breakthrough of long-distance storage and transportation technology of natural gas; The latter is the focus of scientific research, which provides a feasible explanation scheme for solving the hydrate submarine enrichment and integration system.
2 Simple system
As a new energy material, methane molecules wrapped in natural gas hydrate are the most important economic products, so the artificial objects in scientific research and industrial experiments and exploration and mining design are mostly alkane hydrates. However, simple methane hydrate is rare in nature, especially the hydrate from deep pyrolysis gas source, which will contain other hydrocarbons or simple molecules. This section mainly summarizes the research on the formation and stability of methane and other hydrocarbon gases.
2. 1 alkane system
Englezos et al. [4] studied the three-phase equilibrium conditions of methane-ethane mixtures with three different composition ratios (figure 1). When the pressure is less than 5.0M Pa, with the increase of methane content in the mixture, the equilibrium temperature of methane hydrate decreases at the same pressure and increases at the same temperature, indicating that the equilibrium temperature of methane hydrate is lower than that of ethane hydrate at low pressure, while the equilibrium pressure of methane hydrate is higher than that of ethane hydrate. Sugahara et al. [5] also confirmed this in the hydrate equilibrium of a single ethane component. When the pressure is > 10.7 MPa and the temperature is > > 290 K, the formation temperature of ethane hydrate is lower than that of methane hydrate under the same pressure.
Figure 1 three-phase equilibrium diagram of methane+ethane hydrate (cited in reference [4]) 1 phase equilibrium diagram of methane+ethane hydrate
Holder et al. [6] found that with the increase of propane mole fraction in ethane-propane mixture, the equilibrium pressure of hydrate gradually decreased at the same temperature. At low pressure, with the increase of molar mass of alkanes, the equilibrium temperature of alkane hydrate gradually increases and the pressure decreases. That is, the greater the molar mass of alkanes forming hydrates, the larger the range of stable existence of hydrates. Among them, when the mole fraction of propane is greater than 32.2%, the hydrates generated are all structure II; When the mole fraction of propane is less than 15%, it is structure I; When the mole fraction of propane is between the two, the hydrate formed at low temperature is structure II, and at higher temperature is structure I, but when the mole fraction of propane is 27. 1%, an abnormality occurs, and all the hydrates formed are structure II.
2.2 carbon dioxide system
When studying the phase equilibrium of methane-carbon dioxide hydrate, Adisasmito et al. [7] found (Figure 2): When the pressure is less than < 9 MPa, with the increase of the molar fraction of carbon dioxide contained in the mixture, the pressure of hydrate formation decreases at isothermal time. Sug-ahara et al. [5] found that when the pressure is about > > 7.0 MPa and the temperature is > > 28 1.5 K, the hydrate formation temperature of carbon dioxide is lower at the same pressure, while the hydrate formation temperature is lower than that of ethane and higher than that of methane. This shows that the difficulty of forming hydrate between carbon dioxide and methane is closely related to pressure, and the pressure factor should be considered when exploiting natural gas hydrate with carbon dioxide instead of natural gas hydrate.
Adisasmito et al. [8] also studied the effect of carbon dioxide on the phase equilibrium of ethane, propane, isobutane and butane to form hydrate. With the increase of carbon dioxide content in gas phase when the system is in equilibrium, the equilibrium pressure of hydrate gradually decreases at the same temperature. When the isothermal system is in equilibrium and the content of carbon dioxide in the gas phase is equal, the order of pressure required for hydrate equilibrium is ethane > propane > isobutane > butane, which is consistent with the change law of simple system without carbon dioxide, indicating that although carbon dioxide moves the formation of hydrate to high temperature and low pressure, it does not change the change law of temperature and pressure for alkane to form hydrate.
Fig. 2 Equilibrium isotherm diagram of methane+carbon dioxide hydrate (cited from reference [7]) Fig. 2 Isothermal diagram of methane+carbon dioxide hydrate
2.3 Other organic systems
Jager et al. [9] found that the hydrate phase equilibrium stability of water-methane-1, 4- dioxane (C4H8O2) system is closely related to the concentration of 1, 4- dioxane in aqueous solution under the pressure of 2 ~ 14 MPa (Figure 3). The simulation is carried out with the theory of van der Waals and Platte. The liquid phase activity coefficient depends on the gas-liquid equilibrium of water-1, 4- dioxane. The experimental results are in good agreement with the simulation results. When the concentration of 1 4- dioxane in the solution is between 1%-7%, the equilibrium pressure of methane hydrate is the lowest at the same temperature. The concentration of 1, 4- dioxane increased from 7% to 30%, and the equilibrium pressure of methane hydrate increased at the same temperature. This shows that when dioxane is used as a hydrate formation inhibitor, its addition concentration must be higher than 7% to achieve the expected effect, otherwise it will not only inhibit hydrate formation, but will promote hydrate formation.
Alcohol is a commonly used hydrate inhibitor, and methanol is the most common inhibitor because it is soluble in hydrocarbon liquids and cheap. Ng et al. [10] studied the effects of methanol on the equilibrium conditions of methane, ethane, propane and carbon dioxide, and found that with the increase of methanol concentration in the solution, the equilibrium temperature of hydrate formation decreased, the pressure increased, and the stable existence range of hydrate gradually decreased.
Fig. 3 Equilibrium isotherm diagram of methane+1, 4- dioxane hydrate (cited from reference [9]) Fig. 3 Phase equilibrium of methane+1, 4- dioxane hydrate.
Mooijer et al. [1 1] found that the additives are cyclic substances, such as tetrahydropyran (THP) and cyclobutanone (CB), forming structural II hydrate; Methylcyclohexylamine (MCH) with structure H and fluorinated alkanes, such as trifluoromethane and tetrafluoromethane, can form hydrates with structure I and structure II, which affects the stable equilibrium of hydrates. The results show that the use of additives reduces the equilibrium pressure of hydrate, and with the decrease of equilibrium pressure, the gas storage capacity of hydrate is also greatly reduced. The decreasing order of additives to hydrate equilibrium pressure is tetrahydropyran > cyclobutanone > methylcyclohexylamine (MCH) > trifluoromethane > tetrafluoromethane, and the decreasing order of additives to gas storage is tetrahydropyran > cyclobutanone = methylcyclohexylamine (MCH) > tetrafluoromethane > trifluoromethane.
3 surfactant
Surfactant is a new type of chemical that emerged with the rapid development of petrochemical industry after the Second World War. Because of its wide application in industry, it is called "industrial monosodium glutamate". Properties that can reduce the surface tension of solvents are called surface activity, and substances with surface activity are called surface active substances. When some substances are dissolved in water, the surface tension of the aqueous solution changes, which changes the interface state of the system, thus producing functions such as wetting, emulsification, foaming and solubilization, which meet the requirements of practical application.
In order to increase the output of natural gas hydrate and reduce the waiting time of experimental synthesis, mechanical stirring device will be used as an auxiliary means in indoor research to shorten the experimental period of hydrate. Hydrate gas is mostly insoluble small molecular substances, such as methane, which is insoluble in water and mainly concentrated at the water-gas interface. With the formation of hydrate during the experiment, a thin curtain will be formed at the interface to prevent hydrate gas from entering the liquid phase, thus hindering the formation of hydrate and delaying the formation time. Although mechanical stirring can speed up the formation of hydrate, its production cost, efficiency and technical difficulty of deep-sea mining limit its application. First of all, in the natural state, hydrate will not accelerate the stirring process to promote a large number of growth; Secondly, in the gas storage process of industrial projects, it is difficult to separate the filtered hydrate from the ice produced under the condition of simultaneous supercooling, so it will take a lot of time to increase this process; Finally, due to the existence of surface ice, the energy consumption of mechanical stirring will inevitably increase, resulting in additional costs. Therefore, whether it is energy exploration or gas storage and environmental protection, a more suitable acceleration method for hydrate synthesis is needed.
Kalogerakis et al. [12] conducted a study to increase the amount of gas hydrate by using anions as surfactants, thus improving efficiency. Since then, the research in this field has developed rapidly. The research on surfactants has gradually shifted from synthetic surfactants to natural biosurfactants.
3. 1 chemical surfactant
Rogers et al. [13] conducted a comparative experimental study on chemical surfactants. In the container without surfactant, no large-scale hydrate was formed after 5 days. However, after adding specific surfactants, the situation has changed greatly, which greatly promoted the formation speed of natural gas hydrate. After adding SDS and sodium dodecyl sulfate (SDS) with the concentration of 282 ppm in the experiment, in the same experimental environment, the natural gas hydrate filled the whole container in only 3 hours, with very high timeliness (Figure 4).
Fig. 4 hydrate container after 5 days without surfactant (left); Hydrate container containing surfactant for 3 hours (right) (quoted from literature [13]) Figure 4 Hydrate formed after 5 days without surfactant (left); Hydrate formed after using surfactant for 3 hours (right)
In order to track the experimental progress and recording efficiency, in addition to testing instruments such as temperature and pressure, the growth of natural gas hydrate in solution was studied by using an optical aperture detector (boroscope) made of optical fiber. By judging the intensity of reflected light, it can be seen that hydrate can be generated anywhere inside the liquid surface. Because its density is less than that of water, many rapidly formed small particles float on the water surface. When the charge fades, the aggregation intensity is accelerated, and they are continuously aggregated on the inner surface of stainless steel to form larger flocculated solids, and the whole growth system presents a symmetrical form. Most of the generated hydrates float on the liquid surface in flocculent structure, and natural gas can continuously penetrate into these tissues and continue to react with pore water, thus increasing the formation quality. The later calculation shows that almost all pore water reacts with natural gas. The above experimental observations show that surfactants effectively adsorb structural water and natural gas in their hydrophilic and lipophilic regions respectively, and actually form gas hydrate nodule centers.
3.2 Biosurfactant
Although chemical surfactants can catalyze the formation of natural gas hydrate in the laboratory, geologists have begun to pay attention to whether benthic autotrophic bioactive agents can also accelerate the formation of in-situ hydrate. Studies have shown that aquatic microorganisms can secrete biosurfactants, which can not only degrade toxic substances in soil, but also aggregate insoluble carbon organic substances in water. It is found that the natural gas adsorbed in hydrates in some areas comes from organisms, and most microbial communities live and multiply near hydrate mounds [14].
At present, there are three kinds of biosurfactants worth studying:
1) surfactant This substance is secreted by Bacillus subtilis, which is a very effective antibiotic in medicine and has antithrombotic effect.
2) R ham nolipid secreted by Pseudomonas aeruginosa is used as facial cleanser in chemical production.
3) E·m·ulsan secreted by Acinetobacter calcoaceticus is a good oil emulsifier, which can remove oil pollution in the ocean in a large area.
Among them, surfactin and rham nolipid can form micelles when their solubility in aqueous solution is low; Em ulsan (molecular weight is almost 106) does not form micelles. All the above three substances are anions, which can improve the formation rate of natural gas hydrate in porous media in different ways and shorten its synthesis induction time.
Fig. 5 diagram of surfactant promoting hydrate growth (cited from literature [13]). Fig. 5 Hydrate of Rham bentonite without lipid.
Biosurfactants can effectively accelerate the formation of gas hydrate without external stirring. Fig. 5 shows the formation of natural gas hydrate after adding rhamnolipid to bentonite. White hydrate can be seen on the side with surfactant added, and there is almost no hydrate on the side without surfactant added. This experiment shows that under the same experimental conditions, surfactants really accelerate the synthesis speed and output of hydrate.
Bacillus subtilis and Pseudomonas aeruginosa were identified in the mud layer containing natural gas hydrate obtained in the Gulf of Mexico, and their acceleration in the synthesis of weather hydrate was also confirmed. In order to evaluate the effect of these microorganisms on natural gas hydrate in deep-sea environment, Bacillus subtilis obtained a large number of surfactants in the culture concentration after a period of time in the laboratory. This substance is the most effective biosurfactant at present. Experiments show that it can reduce the surface tension of water from 72mN/m to 27m N/m, and the critical micelle concentration (CMC) can be formed only when the concentration reaches 25ppm. When the concentration is lower than CMC, the surface tension of water is extremely sensitive to the concentration of surfactant in water. Therefore, by detecting the surface tension of water in the culture medium, the concentration of surfactant protein in water can be tracked by the method of Weilhelnlv plate.
The surface tension of water and the concentration of surfactant in the culture medium gradually increased. When the concentration of surfactant reaches CM·C, the surface tension of water drops to 30mN/m, and the whole experiment lasts for 4 days. After the completion of culture, the surfactant was separated from the culture medium and diluted to CMC concentration with distilled water. The experimental container is divided into three compartments, which contain three kinds of porous media, namely sand, kaolin/sand and bentonite/sand (Figure 6). After sealing and pressurizing with 90% methane, 6% ethane and 4% propane as gas sources, it can be observed that a large number of natural gas hydrates are generated in bentonite/sand porous media, while the other two media are relatively few. This experiment shows that biosurfactant is helpful to generate natural gas hydrate when the concentration requirement is not too high, and the growth and habitat medium of natural gas hydrate are selective, and the mechanism of this characteristic needs further study.
Fig. 6 the trend of hydrate formation in porous media (cited from the literature [13]). Fig. 6 H hydrate shows the surface specificity of bentonite.
At the same time, as a comparative experimental study, the equipment and technology used in the experiment are basically the same, except that the natural gas component is changed to inorganic carbon dioxide. As expected, due to the incompatibility of carbon dioxide with lipophilic biosurfactants, surfactants can't concentrate carbon dioxide, so carbon dioxide can't be effectively synthesized into gas hydrate.
3.3 Promotion mechanism
Surfactants are substances in which one or several hydrogen molecules on hydrocarbon molecules are replaced by polar groups. Therefore, its molecular composition is generally composed of polar groups and nonpolar groups, which is asymmetric. Polar groups are soluble in water and hydrophilic, so they are called hydrophilic groups. Non-polar groups are insoluble in water, but soluble in non-polar solvents, such as oils, and are lipophilic, so they are called lipophilic. Surfactant molecules have "amphiphilic structure" and water is a very strong liquid. When the surfactant is dissolved in water, it shows a special adsorption phenomenon Seddon[ 15].
The mechanism of surfactant promoting the formation rate and quantity of natural gas hydrate has the following microscopic explanations:
Effect of (1) surfactant on nucleation of natural gas hydrate
The early stage of hydrate reaction is the nucleation stage, and the formation speed is reflected by the length of induction time. The induction time is short and the nucleation speed is fast, whereas the nucleation speed is slow. As a whole, surfactants promoted the rapid formation of hydrate nuclei and greatly shortened the induction time. This phenomenon is explained by the theory of secondary nucleation and the properties of surfactants: at the initial stage of nucleation, it is impossible to split because of the low concentration and small size of nuclei, but at the later stage of nucleation, the concentration of nuclei increases, especially at the gas-liquid interface, which not only promotes their growth, but also has a high concentration of reactants.
At the same time, because the surfactant can significantly reduce its surface tension at low concentration, hydrate crystals will not stay at the two-phase interface and be forced to leave the interface. The existence of this disturbance will lead to violent collisions between crystals and between crystals and the reactor wall, and these crystals will split into hydrate nuclei again, thus dramatically increasing the growth rate of crystal nuclei and shortening the induction time, sometimes only a few minutes or even longer. At the same time, because the nature of surfactant has a certain relationship with its concentration, the induction time of hydrate has a certain relationship with its concentration.
(2) Surfactants improve the growth rate of natural gas hydrate.
After adding surfactant, the crystal growth rate of hydrate is greatly accelerated. Under normal circumstances, natural gas and water are immiscible two phases, and the formation of natural gas hydrate is formed by the fusion of natural gas molecules in the inclusion crystal of water. Before the hydrate reaction, natural gas and water are immiscible two phases, and only at the interface can they interact to realize the hydration reaction. How to reduce the surface tension, so that the gas-phase substances can be better dissolved in the liquid phase, and gas molecules enter the envelope crystal of water, which is the key to the formation of gas hydrate. Surfactants can play this role, and adding appropriate surfactants can produce solubilization in two immiscible phases, thus greatly expanding the effective contact area of the two phases and accelerating the crystallization speed.
On the other hand, because the hydrate reaction occurs at the two-phase interface, the surfactant has a small surface tension, which makes it play an important role: the later the hydrate nucleation, the greater the nucleation rate caused by the secondary nucleation, and this effect continues to act on the early stage of hydrate growth. Under the action of surfactant, the natural gas hydrate crystals formed on the interface leave the interface in time, and at the same time transfer the latent heat released by the reaction, thus reducing the heat transfer resistance.
3.4 Similarities and differences between chemical catalysis and biocatalysis
Their catalytic mechanism is the same. Surfactants are composed of asymmetric groups, which form micelles in an environment suitable for the formation of natural gas hydrate, and natural gas is enriched in the spherical space formed by micelle chains. Generally, alkyl groups form lipophilic groups pointing inward, while hydrophilic groups form outside micelles. Micellar forms the growth point of natural gas hydrate and is the liquid phase place for high concentration and concentration of natural gas. Both of them accelerate the formation of natural gas hydrate by changing the effective concentration and shortening the induction time.
Chemical surfactants are the focus of natural gas storage and transportation or carbon dioxide waste gas deep burial in the future. Large-scale investment in production can achieve economic benefits, but the research and implementation of deep-sea gas hydrate catalysis is not mature enough. The discovery of biosurfactants provides a very meaningful explanation for the formation of deep-sea natural gas hydrate. The biodiversity of deep-sea creatures not only takes the micro-biochemical environment where natural gas hydrate is formed as its habitat, but also its ability of biological evolution to adapt to the environment leads to the accelerated formation of natural gas hydrate. This interesting phenomenon provides a new research idea for the exploration and development of this mineral.
4 conclusion
The research of natural gas hydrate additives is a basic subject, and it has gradually developed into a characteristic field with the research of hydrate. In industrial application, the multi-parameter experiments of simple system in additives show that additives can effectively change the conditions and rate of industrial hydrate synthesis. When surfactant is added to natural gas hydrate, the formation of natural gas hydrate is promoted without external mechanical stirring. In addition, the discovery of surfactants related to submarine microbial secretions highlights the complexity and variability of natural gas hydrate formation. Comparative experimental research shows that biosurfactants can improve the formation speed and output of natural gas hydrate without external mechanical stirring. This phenomenon is of positive significance to the industrial production of this ore source or the future seabed exploration.
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Review on the influence of additives on the formation of natural gas hydrate
Wang Lifeng, Lu Jingan, Liang Jinqiang (Guangzhou Marine Geological Survey, Guangzhou, 5 10760)
Abstract: Natural gas hydrate is regarded as a new type of energy in the future, which has attracted more and more attention from academia and industry, and the research on its additives has become more and more important. In this paper, the forms of additives in natural gas hydrate are reviewed, and the importance of simple systems and surfactants in this process is emphatically introduced. The former is widely used in industrial production, which can not only effectively reduce pipeline blockage, but also increase gas storage; The latter can be used as a way to explain the accumulation mode of natural gas hydrate in geological exploration, that is, autotrophic surfactants can independently accelerate the formation of natural gas hydrate and maintain its stability in various underwater environments.
Keywords: gas hydrate; Additives; The system is simple; surfactant