A method and apparatus for testing gas hydrate phase equilibrium conditions at fixed points and pressures

Abstract

The application discloses a gas hydrate phase equilibrium condition fixed-point constant-pressure testing method and equipment, and belongs to the technical field of gas hydrate testing. The method comprises the following steps: injecting a reaction liquid and gas into a reaction cavity, controlling a piston to keep the cavity pressure constant to a target pressure P1; then, cooling to a temperature below a predicted phase equilibrium temperature, keeping the constant pressure P1 during the process and recording a volume change amount AV to obtain a cooling curve; after hydrate is generated, keeping the constant pressure to make it completely decompose, recording the AV to obtain a heating curve; determining a phase equilibrium temperature T cal under the pressure P1 according to a tangent point of the cooling and heating curves to obtain the gas hydrate phase equilibrium condition. The application can quickly and accurately obtain the gas hydrate phase equilibrium temperature under any specified pressure, and is simple to operate, and can reduce the cost and time of measurement.

Description

Technical Field

[0001] This invention relates to a method and equipment for fixed-point and fixed-pressure testing of gas hydrate phase equilibrium conditions, belonging to the field of gas hydrate testing technology. Background Technology

[0002] Gas hydrates are ice-like solid compounds formed by the combination of gas and water under certain temperature and pressure conditions (usually high pressure and low temperature). They have applications in natural gas exploration and development, oil and gas pipeline transportation, hydrogen storage, CO2 capture and storage, and seawater desalination. Measuring the phase equilibrium conditions of gas hydrates is one of the important tasks in the basic and applied research of gas hydrates. Therefore, how to efficiently and accurately obtain phase equilibrium curve data of gas hydrates under different systems has always been a focus of attention.

[0003] Hydrate phase equilibrium refers to the state of dynamic equilibrium between gaseous hydrates formed under certain temperature and pressure conditions and the gas, liquid, and solid phases (such as ice phase and gaseous hydrate phase). This phase equilibrium state is affected by factors such as temperature, pressure, gas composition, the properties of water, and the presence of additives. A change in any of these factors can disrupt the original equilibrium, leading to a change in phase state.

[0004] Currently, the main methods for determining the phase equilibrium of gas hydrates include direct observation, graphical methods, and calorimetry. Direct observation utilizes a transparent high-pressure reactor or a special observation window to directly observe the formation and decomposition of gas hydrates, determining the state at phase equilibrium with the naked eye or with the aid of a microscope. However, reactor windows or transparent reactors are usually expensive, and it is difficult to accurately distinguish the formation of hydrates in less clear systems, especially within sediments. Graphical methods are suitable for reaction systems with poor visibility and are not easy to observe, mainly including three methods: constant pressure, constant volume, and constant temperature. This involves controlling one parameter of the system (temperature, pressure, or volume) to remain constant while changing the other two parameters to induce hydrate formation and decomposition. Currently, the constant volume method is mainly used, plotting the two easily measurable changing parameters (pressure and temperature) on a two-dimensional graph, and using the intersection (inflection point) of hydrate formation and decomposition to find the phase equilibrium condition. The constant pressure and constant temperature methods still rely on visually observing the dissolution of hydrated particles to determine whether the phase equilibrium point has been reached.

[0005] CN101055276A discloses a natural gas hydrate phase equilibrium simulation experimental device, which mainly includes: a phase equilibrium simulation unit, a pressure control unit, a temperature control unit, a pressure-stabilized liquid supply unit, a pressure-stabilized gas supply unit, a metering unit, and a data acquisition unit. The hydrate phase equilibrium simulation experimental device, composed of these units, can conduct phase equilibrium experiments on hydrates of different gases and determine the phase equilibrium conditions of hydrates in the presence of different chemical reagents. With minor modifications to the device, phase equilibrium experiments in porous media can also be conducted, and the phase equilibrium conditions of hydrates in porous media with different pore diameters can be determined. The main method used with this device is direct observation for determining the phase equilibrium of natural gas hydrates.

[0006] CN101377478A discloses a method for determining the phase equilibrium conditions of gas hydrates, which mainly includes the following steps: the gas to be tested is introduced into a reaction vessel equipped with a temperature measuring, pressure measuring, and exhaust system to generate hydrates; the gas phase space inside the reaction vessel is compressed; the temperature of the system inside the reaction vessel is adjusted to the temperature of the equilibrium condition to be measured, and the pressure is reduced by exhaust until the pressure inside the vessel immediately rises due to the cessation of exhaust, at which point the exhaust system is closed, making the reaction vessel a closed system; the pressure change inside the vessel is observed, and if the pressure remains constant for a long time, it is considered that three-phase equilibrium has been reached, and the corresponding temperature and pressure at this time are a set of phase equilibrium conditions for the hydrate. The system temperature can then be further increased to the next equilibrium temperature to be measured, and the pressure change continues to be recorded until the hydrate inside the reaction vessel is completely decomposed, thereby obtaining a series of phase equilibrium condition data for the hydrate. This method can be used to measure the phase equilibrium conditions of gas hydrates in free water systems or porous media. This method employs the constant volume temperature-increasing decomposition equilibrium method.

[0007] The above experiments are quite complex, requiring precise control of experimental conditions such as temperature and pressure, and demanding high-quality experimental equipment. Data processing and graphing require certain professional knowledge and skills, and human error may occur during visual observation, necessitating repeated experiments and measurements.

[0008] Calorimetry is based on the thermal effects during the formation or decomposition of hydrates, determining the phase equilibrium state by measuring the heat absorbed or released by the system. Hydrate formation is an exothermic process, while decomposition is an endothermic process, and the change in heat is closely related to the phase transition. However, the calorimeter used is a precision instrument, expensive, and somewhat difficult to operate. Furthermore, due to device limitations, it is generally only suitable for low-pressure systems, making it difficult to measure high-pressure systems.

[0009] It is known that currently, the phase equilibrium temperature (pressure) at a specified pressure (temperature) can be roughly determined by observing the formation and decomposition of hydrates with the naked eye. It is difficult to quickly find the accurate phase equilibrium temperature (pressure) at a specified pressure (temperature), and the phase equilibrium conditions of hydrates cannot be directly measured by the changes in volume and temperature parameters.

[0010] Therefore, existing technologies cannot quickly and accurately obtain the equilibrium temperature of gas hydrate phases under any specified pressure. They can only rely on visual judgment (disappearance of hydrate crystals), which has problems such as high experimental costs, low measurement accuracy, and large amount of repetitive work. Summary of the Invention

[0011] To address at least one of the aforementioned technical problems, the present invention aims to provide a method and apparatus for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions. This invention can quickly and accurately obtain the gas hydrate phase equilibrium temperature at any specified pressure, and is simple to operate, reducing measurement costs and time.

[0012] To achieve the above objectives, the first aspect of the present invention provides a method for fixed-point and fixed-pressure testing of gas hydrate phase equilibrium conditions, comprising the following steps: S1: A high-pressure reactor with a movable piston inside is used. The movable piston divides the high-pressure reactor into an upper pressure control chamber and a lower reaction chamber, so that the reaction liquid and the reaction gas to be tested are mixed in the reaction chamber. Then, the movable piston is controlled to pressurize the reaction chamber, so that the pressure in the reaction chamber is constant to the target pressure P1. After the pressure in the reaction chamber stabilizes, the volume change in the reaction chamber at this time is recorded as ΔV0. S2: Cool the temperature inside the reaction chamber to below the phase equilibrium temperature of the gas hydrate corresponding to the target pressure P1, and record the volume change ΔV inside the reaction chamber during the process to obtain the cooling curve, which is the curve of the volume change ΔV as a function of temperature; the gas hydrate begins to form, causing the temperature inside the reaction chamber to change. After the temperature inside the reaction chamber stabilizes, the formation of the gas hydrate is complete, and the volume change ΔV inside the reaction chamber during the process is recorded; during the cooling process and the formation of the gas hydrate, control the movable piston to keep the pressure inside the reaction chamber constant at the target pressure P1. S3: After the gas hydrate is formed, the reaction chamber is heated until the gas hydrate is completely decomposed. The volume change ΔV in the reaction chamber is recorded during the process to obtain the heating curve, which is the curve of volume change ΔV versus temperature. During the heating process, the movable piston is controlled to maintain the pressure in the reaction chamber at a constant target pressure P1. The temperature T1 corresponding to the point of tangency between the cooling curve and the heating curve is the phase equilibrium temperature T of the gas hydrate corresponding to the target pressure P1. cal The equilibrium conditions for the gas hydrate phase were obtained.

[0013] According to a specific embodiment of the present invention, preferably, in step S1, the reaction liquid is first injected into the reaction chamber, then the reaction chamber is evacuated and the temperature inside the reaction chamber is controlled to the initial temperature T0, then the gas to be tested is injected into the reaction chamber to make the pressure inside the reaction chamber the initial pressure P0, and then the reaction liquid and the gas to be tested in the reaction chamber are mixed.

[0014] According to a specific embodiment of the present invention, preferably, in step S1, a limiting ring is provided in the reaction chamber, and the volume of the reaction liquid injected into the reaction chamber is greater than 1.5 times the volume of the reaction chamber below the limiting ring.

[0015] According to a specific embodiment of the present invention, preferably, in step S1, the initial temperature T0 is higher than the phase equilibrium temperature of the gas hydrate corresponding to the initial pressure P0, and the initial pressure P0 is below the maximum working pressure of the high-pressure reactor.

[0016] According to a specific embodiment of the present invention, preferably, in step S2, ΔV is calculated by the following formula: ΔV = ΔX × S, where ΔX is the displacement of the movable piston and S is the area of ​​the cross-section of the movable piston.

[0017] According to a specific embodiment of the present invention, preferably, in step S3, the rate at which the reaction chamber is heated is in the range of 0.5℃ / 4.0 h to 0.5℃ / 0.5 h.

[0018] According to a specific embodiment of the present invention, preferably, the above method further includes: S4: increasing or decreasing the pressure in the reaction chamber to the next target pressure, and repeating steps S2-S3 to obtain the phase equilibrium temperature corresponding to different target pressures.

[0019] According to a specific embodiment of the present invention, preferably, step S4 further includes: plotting the phase equilibrium pressure and temperature curve of gas hydrate based on the phase equilibrium temperature corresponding to different target pressures.

[0020] According to a specific embodiment of the present invention, preferably, the target pressure is below the maximum operating pressure of the high-pressure reactor.

[0021] According to a specific embodiment of the present invention, preferably, the gas to be tested includes one or more of methane, ethane, propane, ethylene, carbon dioxide, and hydrogen sulfide. More preferably, the gas to be tested includes carbon dioxide.

[0022] According to a specific embodiment of the present invention, preferably, the reaction solution comprises one or more of water, and water containing soluble additives and / or salts and / or solid materials. More preferably, the reaction solution comprises seawater.

[0023] A second aspect of the present invention provides a fixed-point, fixed-pressure testing device for gas hydrate phase equilibrium conditions, which is used to implement the above-mentioned fixed-point, fixed-pressure testing method for gas hydrate phase equilibrium conditions. The device includes: a circulating temperature control system, an air inlet device, a temperature sensor, a pressure sensor, a pressure control volume calculation and reaction system, a magnetic stirring device, a magnetic rotor, a heat insulation jacket, and a data monitoring, acquisition, and processing system; the pressure control volume calculation and reaction system includes a high-pressure reactor and a high-pressure oil injection device. The high-pressure reactor is equipped with a movable piston that divides it into an upper pressure-controlled chamber and a lower reaction chamber. A high-pressure oil injection device is connected to the pressure-controlled chamber for injecting hydraulic oil. A heat-insulating jacket is installed on the outer wall of the high-pressure reactor. A circulating temperature control system is connected to the high-pressure reactor. An air intake device is connected to the reaction chamber. Temperature and pressure sensors are used to monitor the temperature and pressure within the reaction chamber in real time. A magnetic rotor is installed within the reaction chamber, and a magnetic stirring device is located below the reaction chamber. A data monitoring, acquisition, and processing system is communicatively connected to the temperature sensor, the pressure sensor, the circulating temperature control system, and the high-pressure oil injection device.

[0024] According to a specific embodiment of the present invention, preferably, the maximum working pressure of the high-pressure reactor is 35 MPa.

[0025] According to a specific embodiment of the present invention, preferably, a limiting ring is provided in the reaction chamber.

[0026] According to a specific embodiment of the present invention, preferably, the maximum working pressure of the high-pressure oil injection device is 35 MPa.

[0027] According to a specific embodiment of the present invention, preferably, the temperature sensor has an accuracy of ±0.01 K, and the pressure sensor has an accuracy of ±0.01% FS.

[0028] According to a specific embodiment of the present invention, preferably, the data monitoring, acquisition and processing system is equipped with a constant pressure injection volume calculation program.

[0029] The present invention has at least the following beneficial effects: This invention develops a constant-pressure volume measurement method, which can accurately measure the volume change data of the reaction chamber caused by hydrate formation and decomposition during the reaction process while maintaining a constant target pressure within the chamber. This directly obtains the key parameter of the constant-pressure process, the volume change (ΔV), and combines it with temperature (T) change data to directly fit and output a ΔV-T image. Based on this image, the phase equilibrium temperature of the gas hydrate at any specified pressure can be obtained, leading to a complete gas hydrate phase equilibrium curve. This invention is applicable to the measurement of hydrate phase equilibrium conditions in liquid-liquid (gas) systems, systems containing various reagents, and solid-phase materials. This invention can quickly and accurately obtain the phase equilibrium temperature of gas hydrates at any specified pressure. It is simple to operate, requires a small measurement volume, has high accuracy, and is applicable to a wide temperature and pressure range. It eliminates the need for visual observation, avoiding potential human error associated with visual observation. It also eliminates the need for repeated disassembly of the reaction apparatus and loading of experimental materials, reducing repetitive workload and significantly lowering consumable and time costs, thus reducing both measurement costs and time. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a gas hydrate phase equilibrium condition fixed-point and fixed-pressure testing device in a specific embodiment of the present invention.

[0031] Figure 2 This is a schematic diagram of a high-pressure reactor in a specific embodiment of the present invention.

[0032] Figure 3 This is a schematic diagram of the ΔV-T image in a specific embodiment of the present invention.

[0033] Figure 4 This is a schematic diagram of the ΔV-T image of the continuous measurement process in a specific embodiment of the present invention.

[0034] Figure 5 This is a ΔV-T image of the CO2 hydrate formation-decomposition process in the pure seawater-CO2 system of Example 1.

[0035] Figure 6 The curves showing the phase equilibrium conditions of the pure seawater-CO2 system-high-pressure liquid CO2 hydrate in Example 1 are shown.

[0036] Figure 7 This is a schematic diagram of the experimental setup for Comparative Example 1.

[0037] Figure 8 The curves show the changes in temperature and pressure during the formation / decomposition of hydrates in Comparative Example 1.

[0038] Figure 9 This is a schematic diagram of the experimental setup for Comparative Example 2.

[0039] Figure 10 This is the PT curve in Comparative Example 2.

[0040] Explanation of icon numbers: 1-Circulating temperature control system; 2-Air inlet device; 3-Temperature sensor; 4-Pressure sensor; 5-High-pressure reactor; 6-High-pressure oil injection device; 7-Magnetic stirring device; 8-Magnetic rotor; 9-Insulation jacket; 10-Data monitoring, acquisition and processing system. Detailed Implementation

[0041] To provide a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the present invention will now be described in detail below, but this should not be construed as limiting the scope of the invention.

[0042] It should be noted that, unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0043] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0044] It should be understood that the terms “comprising,” “including,” and / or “containing” as used herein specify the presence of the stated features, integers, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

[0045] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0046] In the description of this invention, it should be noted that the terms "upper", "lower", "top / bottom", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0047] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installed", "equipped", "sleeved / connected", "connected", etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances.

[0048] According to a specific embodiment of the first aspect of the present invention, the present invention provides a method for fixed-point and fixed-pressure testing of gas hydrate phase equilibrium conditions, which includes the following steps: S1: A high-pressure reactor with a movable piston inside is used. The movable piston divides the high-pressure reactor into an upper pressure control chamber and a lower reaction chamber, so that the reaction liquid and the reaction gas to be tested are mixed in the reaction chamber. Then, the movable piston is controlled to pressurize the reaction chamber, so that the pressure in the reaction chamber is constant to the target pressure P1. After the pressure in the reaction chamber stabilizes, the volume change in the reaction chamber at this time is recorded as ΔV0. S2: Cool the temperature inside the reaction chamber to below the phase equilibrium temperature of the gas hydrate corresponding to the target pressure P1, and record the volume change ΔV inside the reaction chamber during the process to obtain the cooling curve, which is the curve of the volume change ΔV as a function of temperature; the gas hydrate begins to form, causing the temperature inside the reaction chamber to change. After the temperature inside the reaction chamber stabilizes, the formation of the gas hydrate is complete, and the volume change ΔV inside the reaction chamber during the process is recorded; during the cooling process and the formation of the gas hydrate, control the movable piston to keep the pressure inside the reaction chamber constant at the target pressure P1. S3: After the gas hydrate is formed, the reaction chamber is heated until the gas hydrate is completely decomposed. The volume change ΔV in the reaction chamber is recorded during the process to obtain the heating curve, which is the curve of volume change ΔV versus temperature. During the heating process, the movable piston is controlled to maintain the pressure in the reaction chamber at a constant target pressure P1. The temperature T1 corresponding to the point of tangency between the cooling curve and the heating curve is the phase equilibrium temperature T of the gas hydrate corresponding to the target pressure P1. cal The equilibrium conditions for the gas hydrate phase were obtained.

[0049] It should be noted that in step S3, the point of tangency between the cooling curve and the heating curve refers to the point where the two curves share a common tangent. This means that at that point, the two curves not only intersect but also have the same trend of change.

[0050] In some embodiments, in step S1, the reaction liquid is first injected into the reaction chamber, then the reaction chamber is evacuated and the temperature inside the reaction chamber is controlled to the initial temperature T0. Then the gas to be tested is injected into the reaction chamber to make the pressure inside the reaction chamber the initial pressure P0. Subsequently, the reaction liquid and the gas to be tested in the reaction chamber are mixed.

[0051] In some embodiments, in step S1, a limiting ring is provided in the reaction chamber, and the volume of the reaction liquid injected into the reaction chamber is greater than 1.5 times the volume of the reaction chamber below the limiting ring. This ensures precise pressure control throughout the experiment and provides sufficient capacity to handle possible volume changes during the phase transition, thereby guaranteeing the reliability of the experiment and the accuracy of the data. If the volume of the reaction liquid and the reaction gas is too small, the piston may not reach the target pressure before the piston contact surface directly hits the limiting ring, preventing further pressurization and thus failing to reach the target pressure.

[0052] In some embodiments, in step S1, the initial temperature T0 is higher than the phase equilibrium temperature of the gas hydrate corresponding to the initial pressure P0, and the initial pressure P0 is below the maximum operating pressure of the high-pressure reactor. The initial temperature T0 can be predicted using a gas hydrate phase equilibrium model. This gas hydrate phase equilibrium model can be a model from the prior art, and the present invention does not impose any special limitations on it.

[0053] In some embodiments, during step S1, the process of pressurizing the reaction chamber controls the maximum safe volume change ΔV. max This represents the volume change that occurs when the movable piston moves from its initial position to the point of contact with the limiting ring.

[0054] In some implementations, in step S2, ΔV is calculated using the following formula: ΔV = ΔX × S, where ΔX is the displacement of the movable piston and S is the cross-sectional area of ​​the movable piston.

[0055] In some embodiments, in step S3, the rate at which the reaction chamber is heated ranges from 0.5℃ / 4.0 h to 0.5℃ / 0.5 h.

[0056] In some embodiments, the above method further includes: S4: increasing or decreasing the pressure in the reaction chamber to the next target pressure, and repeating steps S2-S3 to obtain the phase equilibrium temperature corresponding to different target pressures.

[0057] In some embodiments, step S4 further includes: plotting the phase equilibrium pressure and temperature curves of gas hydrates based on the phase equilibrium temperatures corresponding to different target pressures.

[0058] In some embodiments, the target pressure is below the maximum operating pressure of the high-pressure reactor.

[0059] In some embodiments, the gas to be tested includes one or more of methane, ethane, propane, ethylene, carbon dioxide, and hydrogen sulfide. Preferably, the gas to be tested includes carbon dioxide.

[0060] In some embodiments, the reaction solution comprises one or more of water and water containing soluble additives and / or salts and / or solid materials. Preferably, the reaction solution comprises seawater.

[0061] According to a specific embodiment of the second aspect of the present invention, the present invention provides a gas hydrate phase equilibrium condition fixed-point and fixed-pressure testing device, which is used to implement the above-mentioned gas hydrate phase equilibrium condition fixed-point and fixed-pressure testing method, such as... Figure 1 As shown, the equipment includes: a circulating temperature control system 1, an air intake device 2, a temperature sensor 3, a pressure sensor 4, a pressure control volume calculation and reaction system, a magnetic stirring device 7, a magnetic rotor 8, a heat insulation jacket 9, and a data monitoring, acquisition, and processing system 10; the pressure control volume calculation and reaction system includes a high-pressure reactor 5 and a high-pressure oil injection device 6; like Figure 2 As shown, a movable piston is installed inside the high-pressure reactor 5, dividing the reactor 5 into an upper pressure control chamber and a lower reaction chamber. A high-pressure oil injection device 6 is connected to the pressure control chamber for injecting hydraulic oil into it. A heat-insulating jacket 9 is installed on the outer wall of the high-pressure reactor 5. A circulating temperature control system 1 is connected to the high-pressure reactor 5. An air inlet device 2 is connected to the reaction chamber. A temperature sensor 3 and a pressure sensor 4 are used to monitor the temperature and pressure inside the reaction chamber in real time. A magnetic rotor 8 is installed inside the reaction chamber, and a magnetic stirring device 7 is installed below the reaction chamber. A data monitoring, acquisition, and processing system 10 is communicatively connected to the temperature sensor 3, the pressure sensor 4, the circulating temperature control system 1, and the high-pressure oil injection device 6.

[0062] In some embodiments, the maximum operating pressure of the high-pressure reactor 5 is 35 MPa. The maximum effective volume of the high-pressure reactor 5 can be 50.89 mL (Φ36×50 mm).

[0063] In some embodiments, a limiting ring is provided in the reaction chamber. For example... Figure 2 As shown, the reaction chamber is also equipped with an injection port and an outlet port.

[0064] In some embodiments, the maximum operating pressure of the high-pressure oil injection device 6 is 35 MPa. The high-pressure oil injection device 6 is connected to the pressure control chamber at the top of the high-pressure reactor 5 via a high-pressure pipeline, and the two are completely sealed together.

[0065] In some embodiments, the temperature sensor 3 has an accuracy of ±0.01 K, and the pressure sensor 4 has an accuracy of ±0.01% FS (Full Scale). For the aforementioned Φ36×50mm high-pressure reactor 5, the temperature sensor 3 and pressure sensor 4 can be installed at a distance from the bottom of the high-pressure reactor 5 (i.e., Figure 2 (20 mm from the upper surface of the vessel lid). It is understood that temperature sensor 3 and pressure sensor 4 are located below the limiting ring.

[0066] In some embodiments, the data monitoring, acquisition, and processing system 10 is equipped with a constant-pressure injection volume calculation program. This program is used to maintain the pressure within the reaction chamber at a target pressure during cooling, gas hydrate formation, and heating processes. Specifically, the constant-pressure injection volume calculation program can control the measurement accuracy of the injection volume of the high-pressure oil injection device 6 to ±0.01 mL. This constant-pressure injection volume calculation program can use existing programs, and the present invention does not impose any special limitations on it.

[0067] Specifically, the method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions includes the following steps: S1: A high-pressure reactor 5 with a movable piston inside is used. The movable piston divides the high-pressure reactor into an upper pressure control chamber and a lower reaction chamber. After the reaction chamber is rinsed and dried, the reaction liquid is injected into the reaction chamber through the injection port, making the volume of the reaction liquid more than 1.5 times the volume of the reaction chamber below the limiting ring. Then, the reaction chamber is sealed and evacuated, and the temperature inside the reaction chamber is controlled to the initial temperature T0 using the circulating temperature control system 1. Then, the gas to be tested is injected into the reaction chamber through the injection port using the gas inlet device 2, making the pressure inside the reaction chamber the initial pressure P0. Then, the magnetic stirring device 7 is turned on to mix the reaction liquid and the gas to be tested in the reaction chamber. Then, the movable piston is controlled to pressurize the reaction chamber, and the pressure inside the reaction chamber is kept constant to the target pressure P1 using the constant pressure injection volume calculation program. After the pressure inside the reaction chamber stabilizes, the volume change inside the reaction chamber at this time is recorded as ΔV0 (i.e., Figure 3 Point A in the middle); S2: Use the circulating temperature control system 1 to cool the temperature inside the reaction chamber to below the phase equilibrium temperature of the gas hydrate corresponding to the target pressure P1 (i.e., the temperature below the target pressure P1). Figure 3 Point B in the diagram), using a constant pressure injection volume measurement program to record the volume change ΔV within the reaction chamber during the process, yields the cooling curve (i.e., the... Figure 3The curve from point A to point B in the diagram represents the change in volume ΔV as a function of temperature. As gas hydrates begin to form, the temperature inside the reaction chamber changes (the data monitoring, acquisition, and processing system 10 can detect a rapid temperature rise). Once the temperature inside the reaction chamber stabilizes, the formation of gas hydrates is complete. The volume change ΔV inside the reaction chamber during this process is recorded using a constant pressure injection volume measurement program. Figure 3 From point B to point C, the volume changes due to the consumption of the gas and liquid being tested in the reaction (during the cooling process and the formation of gas hydrates), the movable piston is controlled and the constant pressure injection volume calculation program is used to keep the pressure in the reaction chamber constant at the target pressure P1. S3: After the gas hydrate is formed, the reaction chamber is heated at a rate of 0.5 ℃ / 0.5 h using the circulating temperature control system 1 until the gas hydrate is completely decomposed (i.e., ...). Figure 3 Point E in the diagram is used to record the volume change ΔV within the reaction chamber during the process using a constant pressure injection volume measurement program, thus obtaining the heating curve (i.e., point E). Figure 3 The curve from point C to point E in the graph represents the change in volume ΔV as a function of temperature. During the heating process, the movable piston is controlled, and a constant pressure injection volume calculation program is used to maintain the pressure in the reaction chamber at the target pressure P1. The ΔV-T graph is composed of the cooling curve, the curve of gas hydrate formation, and the heating curve. The point of tangency between the cooling curve and the heating curve (i.e., the...) Figure 3 The temperature T1 corresponding to point E in the diagram is the phase equilibrium temperature T of the gas hydrate corresponding to the target pressure P1. cal The equilibrium conditions for the gas hydrate phase were obtained.

[0068] The method may further include: S4: increasing or decreasing the pressure within the reaction chamber to the next target pressure, and repeating steps S2-S3 to obtain different ΔV-T images, thereby obtaining the phase equilibrium temperature corresponding to different target pressures, such as... Figure 4 As shown, the equilibrium pressure and temperature curves of the gas hydrate phase can then be plotted.

[0069] This invention develops a constant-pressure volume measurement method. A reaction liquid and gas are injected into a reaction chamber, and the piston is controlled to maintain the pressure inside the chamber at a constant target pressure P1. Subsequently, the temperature is lowered to below the expected phase equilibrium temperature, maintaining a constant pressure P1 and recording the volume change ΔV to obtain a cooling curve. After hydrate formation, the temperature is then raised again at a constant pressure until complete decomposition, and ΔV is recorded to obtain a heating curve. The phase equilibrium temperature T at that pressure P1 is determined based on the tangent point of the cooling and heating curves. calThis invention obtains the phase equilibrium conditions of gas hydrates. While maintaining a constant target pressure within the reaction chamber, it accurately measures the volume changes in the reaction chamber caused by hydrate formation and decomposition during the reaction process in real time. This directly obtains the key parameter of the constant-pressure process, the volume change (ΔV), and combines it with temperature (T) change data to directly fit and output a ΔV-T image. Based on this image, the phase equilibrium temperature of the gas hydrate at any specified pressure can be obtained, thus yielding a complete gas hydrate phase equilibrium curve. Due to the incompressibility of hydraulic oil, this invention can maintain a constant pressure in the reaction chamber while accurately measuring and recording the precise volume (piston displacement) changes in the reaction chamber in real time. Using a data monitoring, acquisition, and processing system, a complete ΔV-T image can be directly output, directly obtaining the precise gas hydrate phase equilibrium temperature at a specified pressure. This invention is applicable to the measurement of hydrate phase equilibrium conditions in liquid-liquid (gas) systems, systems containing various reagents, and solid-phase materials. This invention can quickly and accurately obtain the equilibrium temperature of gas hydrate phase under any specified pressure. It is simple to operate, has a small measurement volume, high accuracy, and a wide applicable temperature and pressure range. It does not require visual observation, thus avoiding the human error that may occur with visual observation. It does not require repeated disassembly of the reaction device and loading of experimental materials, nor does it require repeated measurements, thus avoiding a large amount of repetitive work and greatly reducing the cost of consumables and time. Therefore, it can reduce the cost and time of measurement.

[0070] The technical solutions of the present invention are specifically illustrated by the following embodiments, but the present invention is not limited to these embodiments. Of course, various modifications can be made within the scope of the key points of the present invention.

[0071] Example 1

[0072] This embodiment provides a fixed-point, fixed-pressure testing device for gas hydrate phase equilibrium conditions, such as... Figure 1 and Figure 2 As shown, its structure has been described above and will not be repeated here.

[0073] In this embodiment, the maximum working pressure of the high-pressure reactor 5 is 35 MPa. The maximum effective volume of the high-pressure reactor 5 is 50.89 mL (Φ36×50mm). The maximum working pressure of the high-pressure oil injection device 6 is 35 MPa. The high-pressure oil injection device 6 is connected to the pressure control chamber at the top of the high-pressure reactor 5 via a high-pressure pipeline, and the two are completely sealed together. The accuracy of the temperature sensor 3 is ±0.01 K, and the accuracy of the pressure sensor 4 is ±0.01% FS. The temperature sensor 3 and the pressure sensor 4 are installed at a distance from the bottom of the high-pressure reactor 5 (i.e., Figure 2The pressure sensor 3 and pressure sensor 4 are located 20 mm above the upper surface of the vessel lid. The data monitoring, acquisition, and processing system 10 is equipped with a constant pressure injection volume calculation program. This program can control the measurement accuracy of the injection volume of the high-pressure oil injection device 6 to ±0.01 mL.

[0074] Example 2

[0075] This embodiment provides a method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions. The method uses the equipment provided in Embodiment 1 and includes the following steps: S1: After cleaning the reaction chamber of the high-pressure reactor 5 with deionized water 2-3 times and drying it, inject 25 mL of seawater sample (containing 2.5 wt% NaCl) into the reaction chamber through the injection port, making the volume of the seawater sample more than 1.5 times the volume of the reaction chamber below the limiting ring. Then, seal the reaction chamber and evacuate it. Use the circulating temperature control system 1 to cool the temperature inside the reaction chamber to the initial temperature T0, T0 = 11.00 ℃. Then, use the gas inlet device 2 to inject CO2 gas into the reaction chamber through the injection port, making the pressure inside the reaction chamber the initial pressure P0 (at this time, the displacement of the movable piston is considered to be 0), P0 = 12.025 MPa (the phase equilibrium temperature corresponding to 12.025 MPa predicted by the Chen-Guo gas hydrate phase equilibrium model is 9.85 ℃). Then, turn on the magnetic stirring device 7 (speed 1000). The seawater sample and CO2 gas in the reaction chamber are mixed at a speed of r / min. Then, the movable piston is controlled to pressurize the reaction chamber. The pressure in the reaction chamber is kept constant to the target pressure P1 using a constant pressure injection volume calculation program. P1 = 20.360 MPa (the phase equilibrium temperature corresponding to 20.360 MPa predicted by the Chen-Guo gas hydrate phase equilibrium model is 10.39℃). After the pressure in the reaction chamber stabilizes, the volume change in the reaction chamber at this time is recorded as ΔV0. S2: The temperature inside the reaction chamber is cooled down to below the phase equilibrium temperature of the gas hydrate corresponding to the target pressure P1 using the circulating temperature control system 1, specifically 8.5 ℃. The volume change ΔV inside the reaction chamber during the process is recorded using the constant pressure injection volume calculation program, resulting in a cooling curve, which is a curve showing the change in volume ΔV with temperature. Gas hydrate begins to form, causing a change in temperature inside the reaction chamber (the data monitoring, acquisition, and processing system 10 can detect a sharp temperature rise). After the temperature inside the reaction chamber stabilizes, the formation of gas hydrate is complete. The volume change ΔV inside the reaction chamber during the process is recorded using the constant pressure injection volume calculation program. During the cooling process and the formation of gas hydrate, the movable piston is controlled, and the pressure inside the reaction chamber is kept constant at the target pressure P1 using the constant pressure injection volume calculation program. S3: After the gas hydrate is formed, the reaction chamber is heated at a rate of 0.5 ℃ / 0.5 h using the circulating temperature control system 1 until the gas hydrate is completely decomposed. The volume change ΔV in the reaction chamber during the process is recorded using a constant pressure injection volume measurement program, resulting in a heating curve, which is a curve showing the change in volume ΔV with temperature. During the heating process, the movable piston is controlled, and the pressure in the reaction chamber is kept constant at the target pressure P1 using the constant pressure injection volume measurement program. A ΔV-T graph is composed of the cooling curve, the curve of gas hydrate formation, and the heating curve, as shown in the figure. Figure 5 As shown, the temperature T1 corresponding to the point of tangency between the cooling curve and the heating curve is the thermodynamic phase equilibrium temperature T of the gas hydrate corresponding to the target pressure P1. cal (20.360 MPa, 10.43 ℃, i.e.) Figure 5 Point E in the equation is used to obtain the thermodynamic phase equilibrium condition of the gas hydrate. After the heating process, the temperature is directly reduced from this state. The experiment is repeated 3 times to prevent errors caused by experimental randomness.

[0076] The phase equilibrium data obtained from the repeated experiments under the same pressure condition P1 are shown in Table 1 below.

[0077] Table 1. Phase equilibrium conditions of CO2 hydrate in the pure seawater-CO2 system

[0078] This verifies the high accuracy of the present invention.

[0079] The method may further include: S4: using a constant pressure injection volume calculation program to increase or decrease the pressure in the reaction chamber to the next target pressure, and repeating steps S2-S3 to obtain different ΔV-T images, thus obtaining the phase equilibrium temperature corresponding to different target pressures. The resulting high-pressure CO2 hydrate phase equilibrium curve is shown below. Figure 6 As shown, this is the equilibrium pressure and temperature curve for CO2 hydrate phase. This verifies that the present invention has a wide applicable temperature and pressure range.

[0080] Comparative Example 1

[0081] Thermostatic pressure method: This comparative example uses the isothermal method, and the experimental apparatus used is as follows: Figure 7 As shown, the specific operation steps are as follows: The preprocessing stage plan is as follows: S1: Clean the sapphire reactor with pure water / salt water; S2: Add 4 mL of pure water / seawater to the sapphire reactor; the amount of water should not be too little, but enough to observe the formation and decomposition of hydrates; at the same time, the amount added should not be too much, to avoid the formation of a large amount of hydrates, which would change the composition of the mixed gas. S3: Connects the reaction vessel and the equilibrium vessel, and tests the airtightness of the device; S4: Set the air bath temperature, and after the temperature stabilizes, evacuate the entire system. S5: Purge three times with CO2 to remove air; S6: Pretreatment of deionized water in the reactor to shorten the induction time required for hydrate formation: (a) Rotate the hand pump to move the piston inside the balance vessel upwards, rapidly increasing the system pressure to a level much higher than the CO2 hydrate formation pressure; observe whether hydrates appear through the sapphire reactor. (b) When the hydrate is rapidly formed, retract the piston and gradually reduce the pressure inside the reactor at a rate of 0.5 MPa / min until the hydrate is completely decomposed; (c) Repeat steps (a) and (b) twice.

[0082] The procedure for determining the hydrate phase equilibrium conditions is as follows: S7: During the measurement of CO2 hydrate formation conditions, the experimental temperature remained constant and was controlled by an air bath; S8: Increase the system pressure slightly above the estimated hydrate formation pressure; S9: If a small amount of hydrate begins to appear in the reactor, reduce the system pressure at a rate of 0.2 MPa per adjustment, with an interval of approximately 2 hours between each adjustment; the system pressure and temperature must be stable before adjusting the pressure. S10: By visual inspection combined with pressure changes, when only trace amounts of hydrates exist and the system temperature and pressure can be kept constant for 4-6 hours, the system is considered to have reached equilibrium. At this time, the pressure in the system is the phase equilibrium condition of the mixed gas hydrates at that temperature. The temperature and pressure can be read directly from the computer.

[0083] S11: Repeat the above steps to determine the hydrate formation conditions at different temperatures.

[0084] Figure 8 The pressure change curve during the formation / decomposition of hydrates is shown. The thermodynamic equilibrium point was measured to be (8.6 ℃, 7.653 MPa).

[0085] The results show that the operation of this comparative example is too cumbersome, and it requires visual observation of the hydrate formation and decomposition process, which introduces human measurement errors and requires high precision equipment.

[0086] Comparative Example 2

[0087] Constant volume method: This comparative example uses the traditional constant-volume method to measure the phase equilibrium conditions of the pure seawater-CO2 system, and tests were conducted according to this method. The experimental setup is as follows: Figure 9 As shown, the remaining equipment and Figure 1 Similarly, the specific steps involved in conducting the experiment are as follows: S1: Before the experiment, rinse the inside of the vessel with deionized water 2-3 times and dry it. Then, put in 35 mL of seawater sample, seal it, draw a vacuum, and cool it down to the initial experimental temperature of 11 ℃. S2: Then, inject CO2 into the reactor to the initial pressure of 20.798 MPa using a high-pressure injection pump, turn on the electromagnetic stirring device (1000 r / min), and wait for the temperature and pressure inside the reactor to stabilize before starting the experiment (if the pressure drops too much, inject more CO2 to replenish the pressure). S3: Use a circulating temperature control device to cool the inside of the reactor (ΔT=3 ℃), wait for the hydrate to form, and then control the temperature inside the reactor to rise at a rate of 0.5 ℃ / 0.5 h until the hydrate is completely decomposed; S4: Plot the temperature and pressure data of this process on a PT curve as shown below. Figure 10 As shown, the thermodynamic equilibrium point A (10.35 ℃, 20.157 MPa) under this condition can be obtained. The results show that the constant volume method in this comparative example cannot accurately measure the equilibrium temperature and pressure point of the specified phase, but can only obtain a series of non-equidistant phase equilibrium points by roughly controlling the initial pressure step.

[0088] As can be seen, compared with Comparative Examples 1 and 2, the embodiments of the present invention can quickly and accurately obtain the equilibrium temperature of gas hydrate phase under any specified pressure. The operation is simple, the measurement volume is small, the accuracy is high, and the applicable temperature and pressure range is wide. No visual observation is required, which avoids the problem of human error that may exist in the visual observation method. There is no need to repeatedly disassemble the reaction device and fill the experimental materials, and no need to repeat the measurement, which avoids a lot of repetitive work and greatly reduces the cost of consumables and time. Therefore, the cost and time of measurement can be reduced.

[0089] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions, comprising the following steps: S1: A high-pressure reactor with a movable piston inside is used. The movable piston divides the high-pressure reactor into an upper pressure control chamber and a lower reaction chamber, so that the reaction liquid and the reaction gas to be tested are mixed in the reaction chamber. Then, the movable piston is controlled to pressurize the reaction chamber, so that the pressure in the reaction chamber is constant to the target pressure P1. After the pressure in the reaction chamber stabilizes, the volume change in the reaction chamber at this time is recorded as ΔV0. S2: Cool the temperature inside the reaction chamber to below the phase equilibrium temperature of the gas hydrate corresponding to the target pressure P1, and record the volume change ΔV inside the reaction chamber during the process to obtain the cooling curve, which is the curve of the volume change ΔV as a function of temperature; the gas hydrate begins to form, causing the temperature inside the reaction chamber to change. After the temperature inside the reaction chamber stabilizes, the formation of the gas hydrate is complete, and the volume change ΔV inside the reaction chamber during the process is recorded; during the cooling process and the formation of the gas hydrate, control the movable piston to keep the pressure inside the reaction chamber constant at the target pressure P1. S3: After the gas hydrate is formed, the reaction chamber is heated until the gas hydrate is completely decomposed. The volume change ΔV in the reaction chamber is recorded during the process to obtain the heating curve, which is the curve of volume change ΔV versus temperature. During the heating process, the movable piston is controlled to maintain the pressure in the reaction chamber at a constant target pressure P1. The temperature T1 corresponding to the point of tangency between the cooling curve and the heating curve is the phase equilibrium temperature T of the gas hydrate corresponding to the target pressure P1. cal The equilibrium conditions for the gas hydrate phase were obtained.

2. The method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions according to claim 1, wherein, In step S1, the reaction liquid is first injected into the reaction chamber, then the reaction chamber is evacuated and the temperature inside the reaction chamber is controlled to the initial temperature T0. Then the gas to be tested is injected into the reaction chamber to make the pressure inside the reaction chamber the initial pressure P0. Then the reaction liquid and the gas to be tested in the reaction chamber are mixed. And / or, in step S1, a limiting ring is provided in the reaction chamber, and the volume of the reaction liquid injected into the reaction chamber is greater than 1.5 times the volume of the reaction chamber below the limiting ring.

3. The method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions according to claim 2, wherein, In step S1, the initial temperature T0 is higher than the phase equilibrium temperature of the gas hydrate corresponding to the initial pressure P0, and the initial pressure P0 is below the maximum working pressure of the high-pressure reactor.

4. The method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions according to claim 1, wherein, In step S2, ΔV is calculated using the following formula: ΔV = ΔX × S, where ΔX is the displacement of the movable piston and S is the cross-sectional area of ​​the movable piston. And / or, in step S3, the rate at which the reaction chamber is heated ranges from 0.5℃ / 4.0 h to 0.5℃ / 0.5 h.

5. The method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions according to claim 1, wherein, The above method further includes: S4: increasing or decreasing the pressure in the reaction chamber to the next target pressure, and repeating steps S2-S3 to obtain the phase equilibrium temperature corresponding to different target pressures.

6. The method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions according to claim 5, wherein, Step S4 further includes: plotting the phase equilibrium pressure and temperature curves of gas hydrates based on the phase equilibrium temperatures corresponding to different target pressures.

7. The method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions according to claim 1, wherein, The target pressure is below the maximum operating pressure of the high-pressure reactor.

8. The method for fixed-point, fixed-pressure testing of gas hydrate phase equilibrium conditions according to claim 1, wherein, The reaction gas to be tested includes one or more of methane, ethane, propane, ethylene, carbon dioxide, and hydrogen sulfide; And / or, the gas to be tested includes carbon dioxide; And / or, the reaction solution includes water, and one or more of water containing soluble additives and / or salts and / or solid materials; And / or, the reaction solution includes seawater.

9. A device for fixed-point and fixed-pressure testing of gas hydrate phase equilibrium conditions, used to implement the method for fixed-point and fixed-pressure testing of gas hydrate phase equilibrium conditions according to any one of claims 1-8, the device comprising: The system includes a circulating temperature control system, an air intake device, a temperature sensor, a pressure sensor, a pressure control volume calculation and reaction system, a magnetic stirring device, a magnetic rotor, an insulation jacket, and a data monitoring, acquisition, and processing system; the pressure control volume calculation and reaction system includes a high-pressure reactor and a high-pressure oil injection device. The high-pressure reactor is equipped with a movable piston that divides the reactor into an upper pressure control chamber and a lower reaction chamber. The high-pressure oil injection device is connected to the pressure control chamber and is used to inject hydraulic oil into it. The heat-insulating jacket is disposed on the outer wall of the high-pressure reactor; the circulating temperature control system is connected to the high-pressure reactor; the air inlet device is connected to the reaction chamber; the temperature sensor and the pressure sensor are used to monitor the temperature and pressure inside the reaction chamber in real time; the magnetic rotor is disposed inside the reaction chamber, and the magnetic stirring device is disposed below the reaction chamber; the data monitoring, acquisition and processing system is communicatively connected to the temperature sensor, the pressure sensor, the circulating temperature control system and the high-pressure oil injection device.

10. The fixed-point, fixed-pressure testing equipment for gas hydrate phase equilibrium conditions according to claim 9, wherein, The maximum working pressure of the high-pressure reactor is 35 MPa; And / or, a limiting ring is provided in the reaction chamber; And / or, the maximum operating pressure of the high-pressure oil injection device is 35 MPa; And / or, the temperature sensor has an accuracy of ±0.01 K, and the pressure sensor has an accuracy of ±0.01% FS; And / or, the data monitoring, acquisition and processing system is equipped with a constant pressure injection volume calculation program.

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