CN1467268A - Method for continuous preparation of solid natural gas - Google Patents

Abstract

The present invention relates to natural gas solidification technology in the field of natural gas storage and transportation, in particular to a method for continuously preparing solid natural gas, which includes the following steps in sequence: (1) water is continuously and evenly entered into natural gas at a certain pressure to wet the gas; 2) The water-containing natural gas enters the reactor, is ejected from multiple nozzles of the reactor, throttling and expanding, and rotates at a high speed in the reactor to rapidly generate solid hydrate; (3) The gas flow containing solid hydrate enters the separation (4) The unreacted gas flow enters the compressor from the upper overflow pipe of the separator, and is mixed with raw natural gas in the compressor to boost the pressure and humidify After that, enter the reactor to continue the reaction. The method is simple and easy to implement, low in cost, high in production efficiency, good in solid-gas separation effect, and at the same time the material is recycled, so that raw material natural gas can be fully utilized.

Description

A method for continuously preparing solid natural gas

technical field

The invention relates to natural gas solidification technology in the field of natural gas storage and transportation.

Background technique

Solid Gas Technology (GTS) is to convert natural gas from gaseous state into solid hydrate form, making it easy to transport and re-conversion into gaseous natural gas. Because it significantly reduces the cost of natural gas storage and transportation, and improves the economy and safety of natural gas storage and transportation, it has received widespread attention from the industry and is a new technology that is being researched and developed. Compared with other forms of storage and transportation of natural gas, such as pipeline transportation, CNG (compressed natural gas) technology, and LNG (liquefied natural gas) technology, solidified natural gas technology has unique advantages, mainly in:

(1) Easy to prepare. The pressure and temperature conditions for preparing solid natural gas are relatively mild (natural gas hydrate can be prepared at 2-6MPa, 0-20°C), avoiding the ultra-low temperature (-162°C) of liquefied natural gas and the high pressure (22MPa) of compressed natural gas. Technically easy to implement;

(2) High energy density. 1M ³ of natural gas hydrate (NGH) can carry up to 150-170M ³ (under standard conditions) of natural gas;

(3) Easy to transport. Solid natural gas can be transported by road, railway, and waterway, avoiding the huge investment of pipeline transportation (4 million yuan per kilometer);

(4) Easy to keep warm. Since the thermal conductivity of solid natural gas is 18.7W/M°C, which is lower than that of general insulation materials, no special insulation measures are required for solid natural gas storage containers;

(5) Easy to store. Solid natural gas can be stored stably at normal pressure and at a temperature above -15°C, and the requirements for storage tank materials are not high;

(6) Very safe. Solid natural gas is a solid compound formed by adsorbing gas molecules in the cage compound pores formed by water molecules. The release of the trapped gas must be based on the melting of the ice crystal skeleton. At 0°C, the latent heat of melting per gram of water in ice is At 0-20°C, the latent heat of melting per gram of water in natural gas hydrate is 0.5-0.6KJ, which is much higher than that of ice, which makes the decomposition of hydrate need to absorb a lot of heat energy. In addition, due to the adiabatic effect of the hydrate itself, the release rate of the gas is slow, and the combustion is slow even if it is ignited with an open flame, which fundamentally prevents explosion accidents caused by a large amount of natural gas leakage;

These outstanding advantages make solid natural gas technology have broad development prospects in the field of natural gas storage and transportation. Around the industrialization of solid natural gas technology, people have done a lot of research on a series of technical issues, mainly including: efficient hydrate production process; determination of hydrate gas storage capacity per unit volume; optimization of hydrate gas storage pressure and temperature conditions to improve hydration The economy of natural gas storage; the effective separation method of hydrate, etc., and the design and research of efficient hydrate production and preparation process is the basis for industrial application.

In 1944, American scientist A.J.L. Hutchinson designed a hydrate production and storage method (U.S. Patent 2,356,407), the core of which is to mix gas and cooling water in a container, adjust the pressure, convert gas into hydrate, and use kerosene fraction as a carrier. The solid hydrate is sent to storage tanks for storage. However, since the basic research on gas hydrates was still immature at that time, this technology did not consider the reaction rate and the effective gas content of hydrates from the perspective of kinetics. During the reaction process, due to the lack of sufficient disturbance of the gas flow and the limited degree of gas supercooling, the reaction speed is extremely slow, and the effective gas content of the formed hydrate is also very low, which is difficult to meet the requirements of industrial applications.

With the deepening of basic research on hydrates, Norwegian scientist Gudmundsson proposed a process designed for large-scale NGH production in 1990 (US Patent 5,536,893), with a production capacity of 41×10 ⁸ M ³ per year. The working pressure is 5.0MPa and the temperature is 10°C. Finally, a solid-liquid mixture containing 30% hydrate is obtained, and the hydrate is separated from solid and liquid by a separator and stored in a storage tank. Although this process technology has been greatly improved, there are still many deficiencies, mainly as follows: ①Under the pressure conditions set by the process, the reaction temperature is too high, the gas lacks enough supercooling space, and the reaction rate remains low. limited, the effective gas content of the generated hydrate is not high, which affects the economic performance of the process; ②In the case of large-scale production, it is difficult to obtain sufficient disturbance of the gas-liquid mixture in the reactor, which will inevitably affect the reaction. The mass transfer and heat transfer process of the system delay the reaction speed, making the production efficiency of the process not high; ③ There are additional equipment such as refrigeration and stirring, which consume high energy and require a large investment; ④ Since the density of solid hydrate and liquid water is similar ( Hydrate density is about 0.9g/cm ³ ), it is difficult to completely separate the two phases in ordinary separators, resulting in a large amount of additional water in the finished product, which increases the transportation cost;

In addition, Sassen and Macdonald of Texas A&M University in the United States designed a method of hydrate storage and transportation of subsea natural gas: a special container is used to collect methane bubbles emerging from the seabed. When the container is full, under low temperature and high pressure conditions on the seabed, Methane easily forms hydrates, and then the container is dragged to the surface of the sea, and the hydrates begin to melt and release methane for fuel use. This method makes full use of the natural conditions of the seabed and saves energy consumption, but due to its special application background, it is not universal. Swiss scientist Ehrsam proposed a technical process suitable for natural gas storage and peak shaving (U.S. Patent 4,920,752): water is made into an ice-water mixture, and natural gas and water are reacted in a container at 0°C and 3Mpa to form hydrates . Hydrates are sent to the storage chamber by natural gas purge or belt transfer. Decomposes and releases gas through heat exchange between water and hydrate. The process of this method is relatively complicated, and the production scale is limited, so it is difficult to realize large-scale industrial application. The technological process proposed by Cahn, Guo, Chersky et al. (U.S. Patent 3,514,274 U.S. Patent 5,473,904 U.S. Patent 3,888,434) is also worthy of reference. However, these technologies have special application backgrounds, do not have universal adaptability, and have limited production scale, making it difficult to meet the needs of large-scale natural gas solidification storage and transportation.

Contents of the invention

The object of the present invention is to provide a method for continuously preparing solid natural gas, which is simple and easy to implement, low in cost, high in production efficiency and good in solid-gas separation effect.

The industrialization of solid natural gas requires an efficient and economical solidification process, and the solidification reaction speed directly determines its production efficiency. Classical kinetic theory and experiments show that the reaction speed is related to various factors such as pressure, degree of subcooling, cooling time, gas-liquid contact area, and system disturbance. More, the more severe the system disturbance, the faster the reaction speed. In addition, the effective gas content rate of hydrate products is closely related to the economic performance of the process. Experiments show that: under a certain reaction pressure, the greater the degree of subcooling, the greater the overpressure (the so-called overpressure means that the pressure of the reaction system is raised first. The higher the degree, the more severe the system disturbance, the higher the effective gas content of hydrate. Therefore, an efficient production process must increase the degree of subcooling, shorten the cooling time, and strengthen the disturbance of the gas-liquid system under the condition of a certain reaction pressure.

The technical scheme of the present invention is based on nozzle throttling, water-containing high-pressure natural gas is ejected from a group of specially designed nozzles, throttling and expansion occurs, the temperature of the airflow is greatly reduced, and a larger supercooling space is obtained for the reaction. The supercooled gas flow rotates at high speed in the reactor to gain time for the formation of hydrates. Under the pressure conditions in the reactor, hydrates can be formed within 10-20 seconds or less. The formed solid hydrate particles pass through the separator along with the gas flow to achieve efficient gas-solid separation. The unreacted low-pressure gas is returned to the compressor for pressurization to realize circulation.

A method for continuously preparing solid natural gas, comprising the following steps in sequence:

(1) Water continuously and evenly enters natural gas at a certain pressure to humidify the gas;

(2) The water-containing natural gas enters the reactor, is sprayed out from the nozzle of the reactor, throttles and expands, and rotates at a high speed in the reactor to rapidly generate solid hydrate;

(3) The unreacted gas flow brings the generated solid hydrate into the separator for solid-gas separation, and the solid product is released from the lower outlet of the separator;

(4) The air flow enters the compressor from the overflow pipe at the upper part of the separator, and after being pressurized and humidified, it enters the reactor for recycling.

The pressure of the raw natural gas from the wellhead or pipeline is stabilized within a reasonable range after passing through the pressure regulating valve. High pressure is conducive to a greater temperature drop after the nozzle is throttled. It brings some other problems, such as higher pressure resistance requirements for equipment, and water is difficult to inject into the airflow. Considering various factors comprehensively, the pressure range before the airflow enters the reactor (that is, before the nozzle is throttled) is designed to be 5-10MPa.

Water enters into the airflow, becomes steam or water mist, contacts the gas evenly and fully, and humidifies the gas. A typical natural gas hydrate contains 15% natural gas and 85% water, and the amount of water added can be appropriately controlled according to the mass flow rate of natural gas, so that the reaction can proceed under optimal conditions.

The reactor is in the shape of a cylinder, and there are several nozzles evenly distributed around it at a certain angle to its radial direction. The nozzles can be 4 to 6 Lafal nozzles, and the angle is 60-90 degrees. The air flow through the nozzles is throttled. Afterwards, the temperature drops sharply, which provides an ideal low temperature and high pressure environment for the formation of solid hydrates. The high-speed rotation of the gas flow prolongs the residence time of the reactants in the reactor, creating conditions for the growth of hydrate crystal nuclei. A large number of hydrates can be formed in it. The formed solid hydrate is swept by the high-speed airflow, so it cannot solidify on the nozzle, but rotates with the airflow at high speed in the reactor, and collides with each other violently to form fine particles. Throw it to the wall of the device, and then form an overflow with the unreacted gas flow, and enter the separator through the connecting pipeline. The gas flow can also continue to form hydrates in the connecting pipeline and separator.

Since the solid-gas separation of the present invention has a large scale, the cyclone separator can well meet the technical requirements. After the airflow enters the cyclone separator, the centrifugal force of high-speed rotation throws the solid hydrate particles to the wall of the cylinder, and releases them from the outlet. The hydrate solid particles are efficiently separated.

The airflow after solid-gas separation can first enter a bag filter from the upper overflow pipe of the separator to further separate fine solid hydrates. The bag filter is double-layer, and the outer steel cylinder is connected to the compressor inlet. isolated from the atmosphere. The gas filtered through the cloth bag enters the compressor, then mixes with raw natural gas, and after pressurization and humidification, enters the reactor for recycling.

The powdery solid hydrate collected by the cyclone separator and bag filter can be easily processed into dense block solids, further improving energy density and gas content rate, and saving a lot of freight at the same time.

Since no obvious liquid water occurs in the production process of the present invention, the reaction is carried out continuously in the gas phase, so the method is called gas phase method. Compared with the traditional gas, liquid and solid three-phase method, the gas phase method has obvious advantages:

①. Without refrigeration and mixing equipment, the pressure energy of the pipeline or wellhead can be fully utilized, which greatly reduces energy consumption and equipment investment;

②. After the water-containing gas passes through the nozzle, due to the Joule-Thomson effect, the cooling range is large, and a large supercooling space is obtained, which increases the driving force for the positive reaction of hydrate and suppresses the negative reaction. Thereby speeding up the reaction speed. ;

③. After the air flow is ejected from the nozzle, it rotates at supersonic speed in the inner plate of the reactor at high speed, blowing the nozzle violently, which not only effectively prevents the nozzle from being blocked by ice, but also enhances the disturbance of the system and increases the gas-liquid contact area. The degree of convective heat transfer of the system is accelerated, that is, the degree of mass transfer and heat transfer of the reaction is enhanced. While the moisture in the natural gas forms ice crystals at low temperatures, it absorbs natural gas and forms hydrates, which improves the effective gas content of the finished product;

④. The gas-solid separation is quick and easy, which realizes the efficient separation of hydrates and enables the reaction to proceed dynamically, continuously and efficiently.

⑤. After the compressor pressurizes and humidifies the unreacted low-pressure gas, it is sent back to the reactor to continue the reaction, forming a material cycle, so that the raw material natural gas can be fully utilized.

Description of drawings

Fig. 1 is a process flow diagram of the present invention

Fig. 2 is the sectional structural drawing of reactor of the present invention in the figure: 1 shut-off valve; 2 pressure regulating valves; 3 flow regulating valves; 4 reactors; 5 cyclone type separators; 6 outlets; 41 air inlet; 42 nozzle; 43 jet; 44 reaction zone; 45 centrifugal zone

Detailed ways

Referring to Fig. 1, Fig. 2, a kind of method for continuously preparing solid natural gas comprises the following steps successively:

(1) The raw natural gas from the wellhead or the pipeline passes through the shut-off valve 1 and adjusts the pressure through the pressure regulating valve 2, and the water enters the natural gas continuously and evenly through the flow regulating valve 3 to moisten the gas;

(2) The water-containing natural gas enters the reactor 4 through the gas flow inlet 41, and a plurality of nozzles 42 that form a certain angle (60-90 degrees) with its radial direction are evenly distributed around the cylindrical reactor 4, and the water-containing natural gas flows through the nozzles. Jet flow 43 is formed, and throttling expansion occurs at the same time. The reaction zone 44 in the reactor rotates at a high speed to rapidly generate solid hydrate, and then enters the centrifugal zone 45;

(3) The airflow containing solid hydrate particles enters the cyclone separator 5 through the connecting pipeline for solid-gas separation, and the solid hydrate particles are released from the lower outlet 6 of the separator;

(4) The unreacted gas entrains a small amount of tiny hydrate particles, enters a bag filter 7 from the upper overflow pipe of the separator, and the tiny particles are captured and released from the outlet 6 of the dust collector. The gas enters the compressor 8, is pressurized and then mixed with the raw natural gas, and after humidification, enters the reactor 4 to continue the reaction.

Claims (6)

1. A method for continuously preparing solid natural gas, comprising the following steps in sequence: (1) Water continuously and evenly enters the natural gas at a certain pressure to wet the gas; (2) Natural gas containing water enters the reactor, is sprayed out from multiple nozzles of the reactor, throttling and expanding, and rotates at a high speed in the reactor to rapidly generate solid hydrate; (3) The gas flow containing solid hydrate enters the separator for solid-gas separation, and the solid hydrate particles are released from the lower outlet of the separator; (4) The unreacted gas flow enters the compressor from the upper overflow pipe of the separator, mixes with raw natural gas in the compressor, increases the pressure and humidifies, and then enters the reactor to continue the reaction. 2. The method according to claim 1, characterized in that the natural gas pressure in step (1) is 5-10MPa. 3. The method according to claim 1, characterized in that the nozzles in step (2) are evenly distributed around the reactor, and form 60-90 degrees with its radial direction. 4. The method according to claim 3, characterized in that, the nozzles are 4 to 6 Lafal nozzles. 5. The method according to claim 1, characterized in that the separator in step (3) is a cyclone separator. 6. The method according to claim 1, characterized in that the unreacted airflow enters the bag filter from the upper overflow pipe of the separator in step (4), and enters the compressor after filtering.