Vaporizer is a new type of high-efficiency, environmentally friendly and energy-saving gasification device that uses the characteristics of decompression gasification of liquefied petroleum gas (LPG), liquefied natural gas (LNG) and other gases to achieve gasification.
LNG Vaporizer
Natural gas is mainly used for power generation, chemical industry, urban residential gas and industrial gas, automobile fuel, etc. Other uses include the cold energy utilization of LNG and the need for safe gas supply of natural gas strategic reserves. LNG vaporizer is a heat exchanger specially used for liquefied natural gas gasification. It is mainly used in large coastal LNG receiving stations, LNG liquefaction plants, LNG gasification stations, LNG filling stations, etc. It is a key equipment for realizing gasification function.
Open-frame Vaporizer (ORV)
Open-frame vaporizer is a water-heating vaporizer that uses seawater as a heat source. Its maximum gasification capacity can reach 180t/h. It operates within the load range of 0~100%, and the gasification capacity is adjusted remotely according to changes in demand. It is characterized by large investment, low operating costs, easy operation and maintenance, and is more suitable for basic load LNG receiving stations. As the heat source is seawater, the quality of seawater is required to be high, and anti-corrosion measures such as coating protection should be taken on the outer surface.
The basic unit of the open-frame vaporizer is an aluminum alloy heat transfer tube, which has a star-shaped section inside and fins outside. At the same time, there is a spiral rod inside the tube to increase the heat transfer of LNG; after a number of heat transfer tubes are arranged in a plate shape, the two ends are welded to the gas collecting pipe or liquid collecting pipe to form a tube sheet, and then a number of tube sheets form a vaporizer. The working principle is that LNG flows upward in the tube, and the seawater sprayed by the spray device at the top of the vaporizer flows from top to bottom on the outer surface of the tube sheet to vaporize the LNG in the tube.
When the open-frame vaporizer is running, ice will form on the lower part of the plate-type tube bundle, especially on the outer surface of the liquid collecting pipe, which affects the heat transfer performance of the vaporizer. Osaka Gas and KobelSteel jointly developed a heat transfer tube with a partial double-layer structure, which effectively improved the freezing condition and was called Super ORV.
Submerged combustion vaporizer (SCV)
Submerged combustion vaporizer is a fuel-heated vaporizer. It uses the flue gas generated by fuel combustion as the heat source. Since the flue gas is in direct contact with water, the water is heated and stirred violently. The hot water then exchanges heat with the LNG in the tube, greatly improving the heat transfer efficiency. The thermal efficiency of the submerged combustion vaporizer is about 98%. It can be started quickly and can respond to sudden changes in load. It is suitable for emergency or peak gas use. Its characteristics are low overall investment and installation costs, small footprint, and flexible operation; but the operating cost is very high.
The structure of the submerged combustion vaporizer is that the heat exchange tube bundle is immersed in the pool water, the lower part is welded to the LNG main pipe, and the upper part is welded to the NG main pipe. LNG enters from the bottom and exits from the top. During operation, the fuel gas and air are mixed evenly in proportion and then enter the burner for full combustion. The high-temperature flue gas generated is sprayed into the pool water through the small holes on the distribution pipe. The flue gas enters the pool water to form a large number of bubbles, which rise rapidly, heat and violently stir the pool water, effectively heating the LNG in the tube bundle to vaporize it.
Intermediate medium vaporizer (IFV)
Intermediate medium vaporizers have different forms, including shell and tube vaporizers (STV) and intermediate fluid vaporizers (IFV). The common point is that they use intermediate medium as the heat medium. The intermediate medium can be propane or alcohol (methanol or ethylene glycol) aqueous solution. The heating medium can be seawater, hot water, air, etc., which can improve the impact of icing.
(1) Shell and tube vaporizer (STV)
A shell and tube heat exchanger is used as a vaporizer, and water or methanol (or ethylene glycol) aqueous solution is used as the intermediate heat medium to vaporize LNG. The initial heat source can be hot water, seawater or air. First, the intermediate heat medium is heated with the initial heat source, and then the heated intermediate heat medium is used to gasify LNG through a shell and tube vaporizer. The intermediate heat medium needs to be forced to circulate with a circulating pump, so the energy consumption is relatively high.
(2) Intermediate fluid vaporizer (IFV)
The working principle is to use seawater or hot water from a nearby factory as a heat source, and use this heat source to heat the intermediate medium (propane) and gasify it, and then use propane vapor to gasify LNG. The vaporizer consists of three parts: ① Heat exchange between seawater (or other heat source fluid) and propane; ② Heat exchange between propane and LNG; ③ Natural gas superheating, that is, using seawater to heat the NG after LNG gasification. This vaporizer solves the freezing point problem of seawater (or other heat source fluid) and is widely used in offshore floating storage and gasification, circulation heating, cold energy power generation, etc.
Wound tube vaporizer
The working principle of the wound tube vaporizer is that the heating medium goes through the shell side, and the LNG goes through the tube side. The tube side is a spiral wound heat exchanger structure; the LNG entering from the bottom tube box is heated and vaporized in the tube side and then discharged from the top, and the circulating water heats the natural gas in the tube side in the shell side. The capacity of the vaporizer is mainly determined by the temperature and flow rate of the shell side medium. The main feature is that it has a large heat exchange area in the same volume, and can realize heat exchange of multiple fluids at the same time.
Air-temperature vaporizer
The air-temperature vaporizer is a heat exchange device that uses natural convection of air to heat low-temperature LNG to vaporize it into normal temperature gas. It is composed of aluminum fin tubes connected at a certain distance, generally a single-pass type; in order to increase the heat exchange area on the air side, star fins are installed on the outside of the heat exchange tube. The most commonly used structure is the 8-fin structure, and there are also 12-fin and 4-fin structures.
LNG air-temperature vaporizer is also called air source vaporizer (AAV), which can be divided into two types: indirect contact type and direct contact type.
Indirect contact type uses air to heat the heat transfer fluid in the closed pipeline, and then exchanges heat with LNG for vaporization; direct contact type uses air to directly vaporize LNG. Indirect contact type applications include air fin heat exchange vaporization and air cooling tower vaporization technology; direct contact vaporization includes ambient air vaporization and forced ventilation vaporization.
Although the heat exchange efficiency of LNG air-temperature vaporizer is low, its manufacturing cost and operating cost are also low. It is often used as the main vaporizer in LNG satellite stations (referring to small natural gas receiving stations and gasification stations in small and medium-sized cities and far away from the gas source). The disadvantage is that it is very sensitive to ambient temperature and easy to freeze in winter, so its use in northern my country is subject to certain restrictions; at the same time, it occupies a large area and is also restricted in areas with less land resources.