JP4638715B2 - Hydrate decomposition method and hydrate generation method - Google Patents

Description

  The present invention relates to a hydrate decomposition method and a hydrate generation method, and in particular, a hydrate decomposition method capable of greatly reducing the amount of energy supplied from the outside when decomposing hydrate, and a hydrate that does not require a cooling heat source. It relates to a generation method.

  Natural gas is expected to increase in demand in the future because of its low environmental impact.

  Until now, natural gas has been transported in the form of liquefied natural gas (LNG) when transported by sea or stored in production or consumption areas.

  However, there are a large number of small and medium-sized gas fields in the world where the LNG system cannot be applied, mainly in Southeast Asia.

  A gas field to which the LNG system can be applied can be suitably applied to an ultra-large-scale field, but it has been difficult to apply to such a medium-sized gas field. Therefore, in small and medium-sized gas fields, the natural gas produced is often used only in the surrounding area or released into the atmosphere.

  In recent years, as a natural gas storage and transport method different from the LNG method, a method of storing and transporting natural gas in a hydrated manner has attracted attention because it can be easily applied to small and medium-sized gas fields.

  The hydrate of natural gas is generally a tank of water and natural gas at a pressure of 1 MPa (9.869 atm) to 5 MPa (49.345 atm), a temperature of 273 (0 ° C.) to 283 K (10 ° C.), etc. It is manufactured by contacting inside.

  The hydrate produced in this manner is transported to a consumption place by a dedicated ship, where it is decomposed again into water and natural gas, and natural gas is recovered.

As a method for decomposing hydrate, there is a method in which the hydrate is poured into warm water (Patent Document 1).
JP 2001-279281 A

  However, in order to decompose hydrate into natural gas and water, it is necessary to supply a heat amount of 440 kJ / kgf from the outside. Further, since hydrate has a property of self-cooling at the time of decomposition, it is necessary to crush the ice formed on the surface in order to decompose the hydrate continuously.

  In addition, when the hydrate is decomposed into natural gas and water, the cold heat originally possessed by the hydrate is lost. This cold heat can be recovered by a heat recovery device such as a heat pump, for example, but it is difficult to efficiently recover the cold heat because the temperature of the hydrate, which is a cold heat source, has reached the decomposition temperature.

  The present invention has been made to solve the above-mentioned problems, and hydrate decomposition that can greatly reduce the amount of energy supplied from the outside when decomposing hydrate and can efficiently recover the decomposition heat as cold heat. It is an object of the present invention to provide a method and a hydrate generation method that does not require a cold source.

The invention according to claim 1 relates to a hydrate decomposition method in which a hydrate of natural gas or methane gas is maintained in a pressure vessel at a temperature of 173 to 243 K and a pressure of 1 atm to decompose into natural gas or methane gas and ice .

  When the hydrate is held in the decomposition region, which is a region of temperature and pressure that decomposes into ice and hydrate-forming gas, the hydrate begins to decompose from the surface layer and releases hydrate-forming gas into ice. To do.

  However, when decomposed to some extent in the initial stage, the surface of the hydrate is covered with ice, which is a decomposition product, so that the internal hydrate is protected and the decomposition rate is slow. This is called a self-preserving effect.

  According to the study by the present inventors, when the temperature is 258 K (−15 ° C.) to 243 K (−30 ° C.), the initial decomposition amount is small, the subsequent decomposition rate becomes extremely slow, and a stable state is obtained. In particular, the stability is high when the temperature is in the range of 253 K (−20 ° C.) to 243 K (−30 ° C.). This is called a metastable region.

  However, at this time, the present inventors have increased the decomposition rate at a temperature lower than the metastable temperature 243 K (−30 ° C.) to 253 K (−20 ° C.) in which the hydrate is in a metastable state. Decomposition occurred rapidly at a temperature of 40 ° C. or less, and the decomposition rate was maximized around 208 K (−65 ° C.), leading to the hydrate decomposition method of the present invention.

  According to the hydrate decomposition method, since hydrate is directly decomposed into ice and hydrate-forming gas, the generated heat is efficiently cooled by cooling the generated ice with the cold heat used to decompose the hydrate. Can be recovered well.

  Moreover, the cooling heat at the time of decomposition | disassembly is remarkably small compared with 440 kJ / kgf at the time of decomposing | disassembling a hydrate into water and a hydrate with 150 kJ / kgf. In addition, since the hydrate decomposition rate increases as the temperature decreases up to 208K (−65 ° C.), the cold can be used to promote the hydrate decomposition. Accordingly, the heat supplied from the outside world may be little or zero, and the decomposition can be promoted only by the heat entering the decomposition apparatus, so that it is not necessary to heat the decomposition apparatus by special heating. Therefore, the hydrate-forming gas can be taken out with much less energy than when hydrate is decomposed into water and hydrate-forming gas.

  For example, the metastable temperature of natural gas hydrate at 0.1013 MPa (1 atm) is about 248 K (−25 ° C.), and the equilibrium temperature is about 173 K (−100 ° C.). When the hydrate is decomposed, it may be kept at a temperature lower than 248K (−25 ° C.) and higher than 173K (−100 ° C.).

Examples of the hydrate forming gas include natural gas and methane gas.

The invention according to claim 2 relates to the hydrate decomposition method according to claim 1, wherein the hydrate is maintained at a temperature of 198 to 223K.

  According to the study by the present inventors, methane hydrate decomposes rapidly at a temperature of 233 K (−40 ° C.) or less, and becomes most active at around 208 K (−65 ° C.).

  In the hydrate decomposition method, hydrate is decomposed at a temperature in the above range, so that the hydrate can be efficiently decomposed into ice and hydrate-forming gas.

According to a third aspect of the present invention, the ice obtained by the hydrate decomposition method according to the first or second aspect and the hydrate-forming gas are subjected to a predetermined temperature and a pressure higher than the equilibrium pressure at the temperature. It is related with the hydrate production | generation method characterized by making it contact with.

The ice obtained by the hydrate decomposition method according to claim 1 or 2 is considered to be porous. Moreover, it can cool to about 173K (-100 degreeC) with the cold heat at the time of hydrate decomposition | disassembly. Therefore, according to the hydrate generation method, although ice is used instead of water, not only can the hydrate be formed at a high speed, but the reaction heat generated by the hydrate conversion is the cold heat of ice. By adjusting the supply rate of ice and hydrate forming gas and the pressure in the system, hydrate can be generated without cooling the reaction tank. Therefore, energy consumption can be greatly reduced.

  Moreover, there is no need to manufacture ice. In addition, if the raw material ice is formed in a pellet shape in advance, it is sufficient to replenish the portion damaged during transportation (considered to be about 1% of the total), so conventional water and hydrate forming gas are used. Compared with the case of producing hydrate, the energy required for pelletization can be greatly saved, and the pelletizing apparatus can be of a small capacity.

  As described above, according to the present invention, when the hydrate is decomposed, the amount of energy supplied from the outside can be greatly reduced, and a heat recovery device for recovering the decomposition heat is not required, and A method for producing a hydrate that does not require a cold heat source is provided.

  1. Embodiment 1

  An example of a hydrate decomposition apparatus used in the hydrate decomposition method of the present invention is shown in FIG.

  As shown in FIG. 1, a hydrate decomposition apparatus 100 includes a pressure vessel 2 in which natural gas hydrate (hereinafter abbreviated as “NGH”) is stored, and a natural gas extraction line provided in the pressure vessel 2. 4 includes a gas delivery line 6 detachably provided, and a precooling jacket 8 for precooling the pressure vessel 2 to a decomposition temperature. The gas delivery line 6 is provided with a compressor 10 for increasing the pressure to a pressure necessary for supplying natural gas to consumers.

NGH to be decomposed by the decomposition apparatus 100 can be transported from the production site by a dedicated ship together with the pressure vessel 2. Further, NGH has a metastable region in the range of 253 K (−20 ° C.) to 243 K (−30 ° C.) at 0.101325 MPa (1 atm) as shown in FIG. 3, and as shown in FIG. , The decomposition rate is as small as 1 to 3 × 10 ⁻⁶ (% / s). Therefore, the transport and storage of NGH is performed by placing the pressure vessel 2 at 253 K (−20 ° C.) with the gas extraction line 4 closed. ) To 243K (-30 ° C).

In order to decompose NGH in the consumption area, as shown in FIG. 1, a pre-cooling jacket 8 is attached to the bottom of the pressure vessel 2 and the refrigerant is circulated through the pre-cooling jacket 8 to pre-cool the inside of the pressure vessel 2 to −65 ° C. To do. Degradation rate of NGH, as shown in FIG. 4 increases rapidly when the temperature inside the pressure vessel 2 is below -233K (-40 ℃), 208K at ^{(-65 ℃) 1~2 × 10 -3} (% / s ) To reach the maximum value.

  When the inside of the pressure vessel 2 is sufficiently cooled, the decomposition of NGH becomes active, and the inside of the pressure vessel 2 can be maintained at around −65 ° C. only by the cooling heat when NGH is decomposed. To stop.

  After the cooling by the precooling jacket 8 is stopped, the decomposition of NGH is continued by the heat entering the inside of the pressure vessel 2 from the outside.

  As the decomposition of NGH proceeds, most of the NGH in the pressure vessel 2 is changed to ice, and at the same time, it is cooled to about −100 ° C. (173 K) by the cold heat during decomposition.

  The cold heat thus obtained can be used to remove the heat generated when the ice obtained by NGH decomposition is brought into contact with natural gas to become NGH.

  As an operation for converting the ice obtained by the decomposition of NGH into NGH, for example, the natural gas supply line 4 is supplied to the natural gas supply line 4 while the pressure vessel 2 that has completed the decomposition of NGH is held at −65 to −100 ° C. An operation of connecting to a source and maintaining the inside of the pressure vessel 2 at a pressure necessary for converting the ice into NGH is mentioned.

  Since the hydrate decomposition apparatus 100 of Embodiment 1 does not require cooling after pre-cooling, the energy consumption when decomposing NGH can be significantly reduced.

  Further, by using the cold generated when NGH is decomposed as a cold heat source when the ice inside the pressure vessel 2 is converted to NGH again, significant energy saving can be achieved as the entire system.

  Furthermore, since the ice produced in the pressure vessel 2 is directly brought into contact with natural gas to form NGH, cold heat (330 kJ / kgf) for converting the water into ice becomes unnecessary. Therefore, it is also energy saving in that respect.

  2. Embodiment 2

  Another example of the hydrate decomposition apparatus used in the hydrate decomposition method of the present invention is shown in FIG.

  The hydrate decomposition apparatus 102 is a continuous hydrate decomposition apparatus, and as shown in FIG. 2, a raw material hopper 12 to which NGH to be decomposed is supplied, and a decomposition tank 14 positioned below the raw material hopper 12 And an ice storage tank 16 positioned below the decomposition tank 14.

  The decomposition tank 14 is provided with a pre-cooling jacket 15 for pre-cooling the inside, a pressure gauge 26 for detecting the internal pressure, and thermometers 28, 30 and 32 for detecting the temperature inside the decomposition tank 14. The thermometer 28 is a thermometer that detects the temperature of the upper part of the decomposition tank 14, and the thermometer 30 is a thermometer that detects the temperature of the central part of the decomposition tank 14. And the thermometer 32 is a thermometer which detects the temperature of the decomposition tank 14 lower part. The decomposition tank 14 is further provided with a gas delivery line 34 for delivering natural gas produced by NGH decomposition to consumers. A compressor 36 is interposed in the gas delivery line 34.

  A rotary valve 18 for supplying NGH in the raw material hopper 12 to the decomposition tank 14 is provided between the raw material hopper 12 and the ice storage tank 14, and the decomposition tank 14 is provided between the decomposition tank 14 and the ice storage tank 16. A rotary valve 20 is provided for dropping ice particles generated by the decomposition of NGH into the ice storage tank 16.

  The rotary valve 18 includes a cylindrical valve body 19 that is fitted in a bulging portion 23 formed in the middle of a raw material supply pipe 22 that communicates the raw material hopper 12 and the decomposition tank 14 and rotates in the direction of arrow a. Have. The valve body 19 is formed airtight with respect to the bulging portion 23, and four recessed portions 19 </ b> A are formed at equal intervals on the outer peripheral portion of the valve body 19.

  Similarly, the rotary valve 20 is a cylindrical valve body that is fitted into a bulging portion 25 formed in the middle of an ice discharge pipe 24 that communicates the decomposition tank 14 and the ice storage tank 16 and rotates in the direction of arrow b. 21. The valve body 21 is formed airtight with respect to the bulging portion 25, and four recessed portions 21 </ b> A are formed at equal intervals on the outer peripheral portion of the valve body 21.

  The ice storage tank 16 is detachably formed at the lower end of the ice discharge conduit 24.

  Hereinafter, the operation of the hydrate decomposition apparatus 102 will be described.

  Pellet NGH to be decomposed is supplied to the raw material hopper 12 from the previous step. Here, when the rotary valve 18 is rotated, the NGH in the raw material hopper 12 falls to the concave portion 19A facing upward, and the concave portion 19A rotates 180 degrees so that the concave portion 19A faces downward. NGH in the part 19A falls into the decomposition tank.

  Here, the decomposition tank 14 is cooled by the pre-cooling jacket 15 at the start of operation. As cooling progresses and the temperature of the wall surface at the center of the decomposition tank 14 approaches 208 K (−65 ° C.), NGH near the wall surface is actively decomposed, and NGH itself while taking heat of decomposition 150 kJ / kgf from the surroundings Continues to disassemble. Thereby, since it cools to the center part vicinity of the decomposition tank 14, NGH comes to decompose | disassemble actively in the decomposition tank 14 whole.

  The ice produced by the decomposition of NGH in the decomposition tank 14 falls into the ice storage tank 16 by rotating the rotary valve 22.

  In the upper part of the decomposition layer 14, the temperature rises when NGH having a high temperature is supplied from the raw material hopper 12. As NGH descends in the decomposition layer 14, it is cooled by heat absorption due to the decomposition reaction, and the temperature decreases.

  Therefore, by adjusting the number of rotations of the rotary valve 18 and the rotary valve 20 to control the supply amount of NGH and the discharge amount of ice, the inside of the central portion of the decomposition layer 14 can be controlled to 208K (−65 ° C.). If the temperature cannot be controlled only by supplying NGH and discharging ice, temperature control using a heater or precooling jacket 15 may be added as necessary.

  Even in the process of proceeding from the central portion to the lower portion of the decomposition layer 14, the undecomposed NGH continues to decompose, whereby the NGH is further cooled. Thereby, since the ice collect | recovered with the ice storage tank 12 is cooled to -100 degreeC, cold heat is collect | recovered more effectively.

  The change in the decomposition rate of methane hydrate when the decomposition temperature was changed in the range of −5 ° C. (268K) to −115 ° C. (158K) was examined. As the methane hydrate, 3 g of methane hydrate pellets having a diameter of 2 cm and a guest molecule inclusion rate of about 80% were used. The methane hydrate was charged into a sealed container, the release weight of methane gas released from the sealed container was measured while maintaining the sealed container at a predetermined decomposition temperature, and the guest molecule inclusion rate of the following formula methane hydrate Asked.

Guest molecule inclusion rate (%) = (w1-w2) × (5.75 × 18) / (16 × w2)
w1: Sample weight (g)
w2: Weight of residual water after complete sample decomposition (g)

The results are shown in FIGS.

  FIG. 4 shows the relationship between the storage temperature and the decomposition rate obtained by taking the slopes of the graphs of FIGS.

As is apparent from FIG. 4, methane hydrate has a decomposition rate that decreases as the temperature decreases from 273 K (0 ° C.) to 253 K (−20 ° C.). The decomposition rate is lowest in the range from 253K (−20 ° C.) to 243K (−30 ° C.). The decomposition rate is as extremely low as 1 to 3 × 10 ⁻⁶ (% / s).

  When the temperature further decreases, the decomposition temperature slightly increases up to around 233 K (−40 ° C.). Therefore, it is a region with relatively high stability (slow decomposition rate) here. Particularly, 243K (−30 ° C.) to 253K (−20 ° C.) having a high stability is called a metastable region.

When the storage temperature falls below 233 K (−40 ° C.), the decomposition rate of methane hydrate increases rapidly, and reaches a maximum value near 208 K (−65 ° C.). The decomposition rate at this time is 1 × 10 ⁻³ (% / s) or more, and a decomposition rate of 1000 times or more of the metastable region can be obtained.

When the temperature falls below 208K (−65 ° C.), the decomposition rate decreases again. However, the decomposition rate is 1 × 10 ⁻⁵ (% / s) or more up to 173K (−100 ° C.).

  When the temperature further decreased to 168 K (−105 ° C.) or less, the methane hydrate entered an equilibrium temperature in an equilibrium state, and the decomposition rate was almost zero.

  From these results, the decomposition temperature of methane hydrate is preferably in the range of -30 ° C (243K) to -100 ° C (173K), and particularly preferably in the range of 223K (-50 ° C) to 198K (-75 ° C). I understand.

FIG. 1 is a block diagram showing an outline of an example of a hydrate decomposition apparatus used in the hydrate decomposition method of the present invention. FIG. 2 is a block diagram showing an outline of another example of the hydrate decomposition apparatus used in the hydrate decomposition method of the present invention. FIG. 3 is a phase diagram of methane hydrate. FIG. 4 is a graph showing the relationship between the decomposition rate of methane hydrate and temperature. FIG. 5 is a graph showing the relationship between time and guest molecule inclusion rate of methane hydrate when methane hydrate is decomposed at various decomposition temperatures. FIG. 6 is a graph showing the relationship between time and guest molecule inclusion rate of methane hydrate when methane hydrate is decomposed at various decomposition temperatures. FIG. 7 is a graph showing the relationship between time and guest molecule inclusion rate of methane hydrate when methane hydrate is decomposed at various decomposition temperatures.

Explanation of symbols

2 Pressure Vessel 4 Natural Gas Extraction Line 6 Gas Distribution Line 8 Precooling Jacket 10 Compressor

Claims (3)

A hydrate decomposition method in which a hydrate of natural gas or methane gas is charged into a pressure vessel and is maintained at a temperature of 173 to 243 K and a pressure of 1 atm to decompose into natural gas or methane gas and ice. The hydrate decomposition method according to claim 1, wherein the hydrate is maintained at a temperature of 198 to 223K. The ice obtained by the hydrate decomposition method according to claim 1 or 2 is contacted with natural gas or methane gas under a temperature of 173 K or lower and a pressure higher than an equilibrium pressure at the temperature. Hydrate generation method.

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