A method for storing and transporting high-capacity natural gas hydrates

Abstract

The present invention discloses a method for storing and transporting hydrates with high natural gas storage capacity, which adopts the method of low-temperature generation and high-temperature storage of hydrates, and specifically comprises the following steps: in a mode where the hydrate reaction tank is also used as a transport tank, the mixed hydrate reaction liquid is placed in a hydrate reaction tank adapted to a transport vehicle, natural gas is introduced, and a hydrate generation reaction is carried out under the condition of a temperature of 273.65-283.15K. After the reaction reaches equilibrium, the temperature is raised to ≤298.15K for storage for long-distance transportation. The present invention adopts the technical scheme of "low-temperature generation and high-temperature storage of hydrates", which can synthesize hydrates with high natural gas storage capacity in a relatively short time, and transport the hydrates to the destination safely, economically and efficiently.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas storage and transportation, and in particular relates to a method for safely, economically and efficiently transporting hydrates with a high natural gas storage capacity to a destination. Background Art

[0002] Coal, oil and natural gas are the three traditional fossil energy sources that play an important role in supporting the development of modern agriculture and industry. However, the environmental pollution caused by the use of the former two has also attracted widespread attention from researchers.

[0003] As a clean and environmentally friendly high-quality energy, natural gas plays an increasingly important role as an "energy protagonist" in today's advocacy of energy conservation and emission reduction, and its share in my country's energy structure is also increasing. The growing demand for natural gas urgently requires an economical and safe storage and transportation method to transport natural gas from the gas source to the user.

[0004] At present, the commercial storage and transportation methods of natural gas mainly include pipeline transportation, LNG storage and transportation (liquefied natural gas), and CNG storage and transportation (compressed natural gas). Among them, pipeline transportation is the current mainstream storage and transportation method. Natural gas is delivered to consumers by laying pipelines and building fixed dedicated stations, and gas can also be stored in pipelines. Pipeline transportation has the advantages of large transportation volume and good sustainability, but this method also has unavoidable problems such as strong specificity, large fixed investment and poor flexibility. LNG storage and transportation is to convert natural gas from gas to liquid, and then transport it by ship, road, railway and pipeline. However, before transportation, a lot of energy is required to liquefy natural gas for storage in low-temperature storage tanks. CNG storage and transportation uses the compressibility of natural gas to store gas in the form of high pressure, and the storage pressure is generally between 15 and 25 MPa. This method has high requirements for storage equipment and consumes a lot of energy during compression.

[0005] Gas hydrate solidification storage and transportation technology is a new type of natural gas transportation technology developed in recent years. It mainly reacts natural gas with water under low temperature and high pressure conditions to form solid hydrates for storage and transportation. The patent "A transport vehicle for transporting combustible ice" (CN103754152A) proposes a design concept for a natural gas hydrate transport vehicle. It mainly installs a generator and a semiconductor refrigerator on the transport vehicle to cool the storage cavity, and then installs insulation materials around the car to maintain the low temperature storage conditions of the hydrate to prevent the decomposition of the hydrate, thereby achieving the purpose of long-distance transportation. The patent "A transport vehicle for transporting natural gas hydrates" (CN102371934A) also uses this principle to achieve the purpose of hydrate transportation. The patent "Hydrate storage and transportation device" (CN101520130A) proposes a hydrate transportation device with a tubular layered structure, and the low temperature of its internal space is mainly maintained by external refrigeration. All of the above schemes use low temperature and normal pressure transportation methods to transport natural gas hydrates. As mentioned above, hydrates are a kind of cage-type complex generated by gas and water under low temperature and high pressure (the gas itself provides pressure in the reactor or tank). As hydrates are generated, gas is consumed and pressure decreases. Limited by the phase equilibrium conditions with hydrates, the low-temperature and normal-pressure storage of hydrates means that after the gas hydrates are generated under low temperature and high pressure conditions, the hydrates together with the unreacted gas are cooled to below -30°C, and then the gas is released to return to normal pressure (directly losing pressure energy) to ensure the stability of the hydrates. After completion, granulation and filling are required, and such low temperature must be maintained during transportation to the destination, which consumes huge energy. It can be seen that although the above technical solutions can achieve stable storage and transportation of natural gas hydrates, the storage method under low temperature conditions requires a large amount of cold energy, which will undoubtedly greatly reduce the economic efficiency of hydrate transportation. Therefore, we urgently need to develop a way and method to safely, economically and efficiently transport and store hydrates with high natural gas storage capacity to the destination. Summary of the invention

[0006] The purpose of the present invention is to provide a method for safely, economically and efficiently transporting hydrates with high natural gas storage capacity based on the existing technology, in view of the shortcomings of gas hydrates in natural gas solidification storage and transportation technology that are easy to decompose and have poor economic efficiency in low-temperature storage and transportation.

[0007] To achieve the above object, the present invention adopts the following technical solution:

[0008] A method for storing and transporting hydrates with a high natural gas storage capacity, the method adopts a method of low-temperature hydrate generation and high-temperature hydrate storage, and specifically comprises the following steps: in a mode in which a hydrate reaction tank is also used as a transport tank, a mixed hydrate reaction liquid is placed in a hydrate reaction tank adapted to a transport vehicle, natural gas is introduced, and a hydrate generation reaction is carried out at a temperature of 273.65-283.15K; after the reaction reaches equilibrium, the temperature is raised to ≤298.15K for storage for long-distance transportation.

[0009] Preferably, the reaction temperature of the hydrate formation reaction is 274.15-283.15K.

[0010] Preferably, the mixed hydrate reaction liquid consists of a hydrate formation promoter and water.

[0011] The hydrate formation promoter includes but is not limited to cyclopentane. Other additives that can promote hydrate formation can also be used, such as tetrahydrofuran, methylcyclopentane, methylcyclohexane, etc.

[0012] Preferably, the mixed hydrate reaction liquid is composed of cyclopentane and water. Preferably, in the mixed hydrate reaction liquid, the molar fraction of cyclopentane is 5.6 mol%, calculated based on the total moles of the mixed hydrate reaction liquid.

[0013] The hydrate reaction tank can be conveniently adapted to a transport vehicle, and can also be directly used as a hydrate transport tank, thus achieving a multi-purpose effect.

[0014] The present invention adopts the technical scheme of "low-temperature hydrate generation and high-temperature storage" to achieve the purpose of synthesizing hydrates with high natural gas storage in a relatively short time, and transporting the hydrates to the destination safely, economically and efficiently. The present invention has found that by controlling the hydrate generation temperature within a certain low temperature range, hydrates with high gas lift can be quickly generated, and the temperature is continued to be raised to a temperature close to room temperature. Under the pressure of unreacted gas, the hydrate can maintain good stability. Therefore, the scheme of the present invention meets the industrial production requirements of "fast hydrate generation, high gas lift, and high transportation temperature", and is the most economical and effective way. Specifically, the present invention controls the temperature in the natural gas hydrate generation process within the range of 273.65-283.15K, preferably the temperature is in the range of 274.15-283.15K, and obtains hydrates with high natural gas storage within 1.0-4.0 hours. The natural gas storage capacity of the hydrate is 100-160V / V.

[0015] The hydrate with high natural gas storage capacity is stored and transported at near room temperature after being generated. The transport vehicle does not need to provide a refrigeration device to refrigerate the hydrate storage chamber during transportation to prevent the decomposition of the hydrate. The ambient temperature during transportation is required to be lower than 298.15K. In fact, China and many other countries and regions in the world are under this temperature condition for a long time in a year.

[0016] After the hydrate with a high natural gas storage capacity is transported to the destination, the hydrate is decomposed by introducing industrial waste heat, thereby releasing natural gas for use, and the hydrate reaction liquid is repeatedly reused.

[0017] The present invention has the following beneficial effects:

[0018] 1. The natural gas hydrate in the cyclopentane-containing system is generated at 273.65-283.15K with an extremely fast gas hydrate generation rate and a high gas storage capacity. The time required to complete the reaction is between 1.0-4.0 hours, and the production efficiency is high.

[0019] 2. The technical solution of "low-temperature generation and high-temperature storage" of hydrates avoids the investment and operating costs of refrigeration equipment during the transportation of hydrates, greatly reducing the transportation cost of hydrates.

[0020] 3. The technical solution of "low-temperature generation and high-temperature storage" of hydrates makes it possible to use the hydrate generation tank and the hydrate transport tank as the same tank, which can reduce the process flow of natural gas hydrate solidification storage and transportation technology, greatly reducing the investment and operating costs of related equipment and improving the economy of the industrial chain.

[0021] 4. After the hydrate is transported to the destination, industrial waste heat is introduced to decompose the gas hydrate to release natural gas. This can not only make full use of this low-grade heat source, but also greatly shorten the unloading time of natural gas and improve the operating efficiency of the hydrate industry chain. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the curve showing the change in the release rate of natural gas in the storage tank as a function of temperature. DETAILED DESCRIPTION

[0023] The main component of natural gas is methane. Therefore, the test gas selected in this laboratory is methane with a purity of 99.0 mol%. The molar fraction of cyclopentane used is 5.6 mol%, calculated based on the total molar number of the mixed hydrate reaction liquid (water + cyclopentane). The formation temperature of hydrate is controlled between the preferred temperature of 274.15-283.15K, and the transportation temperature of hydrate is controlled below 298.15K.

[0024] from Figure 1 It can be seen that the methane release rate of the hydrate in the storage tank does not increase significantly when the hydrate storage temperature is lower than 298.15K, but as the temperature further rises, the methane release rate of the hydrate in the storage tank increases significantly. This means that the storage temperature of hydrates with high methane storage capacity must not be higher than 298.15K. At the same time, it can be seen from the following embodiments that at a temperature of 293.15K and an initial pressure of 8.0MPa, the gas storage capacity of hydrates directly generated is 23.7V / V, while the gas storage capacity of hydrates generated at 274.15K and an initial pressure of 8.0MPa is 148.0V / V. After the hydrate is heated to 293.15K and stored and part of the methane gas is released, the methane storage capacity of the hydrate is still as high as 134.09V / V. This means that storing and transporting hydrates with high gas storage capacity at near room temperature but not higher than 298.15K will not cause hydrate instability, which can greatly reduce the investment and operation costs of providing cold energy to maintain hydrate stability during the transportation of hydrates with high methane storage capacity. However, the efficiency and gas storage capacity of directly synthesizing hydrates under such high temperature conditions are extremely low. It is worth noting that under 293.15K conditions, hydrates were not generated for up to 530 minutes. This shows that the "low temperature generation and high temperature storage" of natural gas hydrates is an effective and reasonable method for industrial application of natural gas hydrate solidification storage and transportation technology.

[0025] The embodiments of the present invention are further described below in conjunction with examples.

[0026] Example 1

[0027] Based on the total volume of 30 mL of the solution, a reaction solution (water + cyclopentane) with a molar fraction of cyclopentane of 5.6 mol% was placed in a 120 mL hydrate reactor, and then methane gas was introduced to reach a set pressure of 8 MPa. The hydrate formation reaction was carried out under 274.15 K conditions. The time for the reaction to reach equilibrium (i.e., the time for the hydrate formation reaction to be completed) was 120 min, and the obtained hydrate gas storage capacity was 148.0 V / V. This system was placed under 293.15 K conditions for storage for long-distance transportation. It was found that after 48 hours of storage, the hydrate gas storage capacity was only reduced from 148.0 V / V to 134.09 V / V, and the reduction ratio was only 9.4%.

[0028] Example 2

[0029] Based on the total volume of 30 mL of the solution, a reaction solution (water + cyclopentane) with a molar fraction of cyclopentane of 5.6 mol% was placed in a 120 mL hydrate reactor, and then methane gas was introduced to reach a set pressure of 8 MPa. The hydrate formation reaction was carried out at 293.15 K. The time for the reaction to reach equilibrium (i.e., the time for the hydrate formation reaction to be completed) was 530 min, and the obtained hydrate gas storage capacity was 23.7 V / V. This system was placed at 298.15 K for storage for long-distance transportation. It was found that after 48 hours of storage, the hydrate gas storage capacity was only reduced from 23.7 V / V to 22.6 V / V, and the reduction ratio was only 4.6%.

[0030] Example 3

[0031] Based on the total volume of 30 mL of the solution, a reaction solution (water + cyclopentane) with a molar fraction of cyclopentane of 5.6 mol% was placed in a 120 mL hydrate reactor, and then methane gas was introduced to reach a set pressure of 8 MPa. The hydrate formation reaction was carried out under 283.15 K conditions. The time for the reaction to reach equilibrium (i.e., the time for the hydrate formation reaction to be completed) was 230 min, and the obtained hydrate gas storage capacity was 142.3 V / V. This system was placed under 293.15 K conditions for storage for long-distance transportation. It was found that after 48 hours of storage, the hydrate gas storage capacity was only reduced from 142.3 V / V to 134.76 V / V, and the reduction ratio was only 5.3%.

[0032] Example 4

[0033] Based on the total volume of 30 mL of the solution, a reaction solution (water + cyclopentane) with a molar fraction of cyclopentane of 5.6 mol% was placed in a 120 mL hydrate reactor, and then methane gas was introduced to reach a set pressure of 7 MPa. The hydrate formation reaction was carried out under 280.15 K conditions. The time for the reaction to reach equilibrium (i.e., the time for the hydrate formation reaction to be completed) was 187 min, and the obtained hydrate gas storage capacity was 137.6 V / V. This system was placed under 293.15 K conditions for storage for long-distance transportation. It can be found that after 48 hours of storage, the hydrate gas storage capacity was only reduced from 137.6 V / V to 135.2 V / V, and the reduction ratio was only 1.7%.

[0034] Example 5

[0035] Based on a total volume of 30 mL, a reaction solution (water + cyclopentane) with a molar fraction of cyclopentane of 5.6 mol% was placed in a 120 mL hydrate reactor, and then methane gas was introduced to reach a set pressure of 7 MPa. The hydrate formation reaction was carried out under 278.15 K conditions. The time for the reaction to reach equilibrium (i.e., the time for the hydrate formation reaction to be completed) was 148 min, and the obtained hydrate gas storage capacity was 144.7 V / V. This system was placed under 288.15 K conditions for storage for long-distance transportation. It can be found that after 48 hours of storage, the hydrate gas storage capacity was only reduced from 144.7 V / V to 138.5 V / V, and the reduction ratio was only 4.3%.

[0036] Example 6

[0037] Based on the total volume of 30 mL of the solution, a reaction solution (water + cyclopentane) with a cyclopentane molar fraction of 5.6 mol% was placed in a 120 mL hydrate reactor, and then methane gas was introduced to reach a set pressure of 7 MPa. The hydrate formation reaction was carried out under 276.15 K conditions. The time for the reaction to reach equilibrium (i.e., the time for the hydrate formation reaction to be completed) was 127 min, and the obtained hydrate gas storage capacity was 142.3 V / V. This system was placed under 283.15 K conditions for storage for long-distance transportation. It was found that after 48 hours of storage, the hydrate gas storage capacity dropped from 142.3 V / V to only 127.22 V / V, and the reduction ratio was only 10.6%.

[0038] The above are only preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be regarded as limiting the present invention, and the protection scope of the present invention should be based on the scope defined by the claims. For ordinary technicians in this technical field, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for storing and transporting high-natural gas storage hydrates, characterized in that: The method adopts the method of low-temperature generation and high-temperature storage of hydrates, and specifically includes the following steps: in a mode in which the hydrate reaction tank is also used as a transport tank, the mixed hydrate reaction liquid is placed in a hydrate reaction tank adapted to the transport vehicle, natural gas is introduced, and the hydrate generation reaction is carried out at a temperature of 273.65-283.15K. After the reaction reaches equilibrium, the temperature is raised to ≤298.15K for storage for long-distance transportation.

2. The method for storing and transporting high natural gas storage hydrate according to claim 1, characterized in that: The reaction temperature of the hydrate formation reaction is 274.15-283.15K.

3. The method for storing and transporting high natural gas storage hydrate according to claim 1, characterized in that: The mixed hydrate reaction liquid is composed of a hydrate formation promoter and water.

4. The method for storing and transporting high natural gas storage hydrate according to claim 3, characterized in that: The hydrate formation promoter is cyclopentane.

5. The method for storing and transporting high natural gas storage hydrate according to claim 4, characterized in that: Calculated based on the total moles of the mixed hydrate reaction solution, the molar fraction of cyclopentane is 5.6%.

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