Chen Yongdong Chen Xuedong
1. Hefei General Machinery Research Institute 2. National Pressure Vessel and Pipeline Safety Engineering Technology Research Center Chen Yongdong et al. Key Technology Analysis of LNG Complete Set Heat Exchanger. Natural Gas Industry, 2010, 30(1): 96-100.
Abstract: Heat exchangers are a key component of LNG plants. Vaporizers and main cryogenic heat exchangers play an important role in LNG receiving stations and liquefaction plants. For this reason, the key technologies of three kinds of typical vaporizers, such as open-frame vaporizer, vaporizer with intermediate medium and coiled-tube heat exchanger, were analyzed from three aspects: structure, material, heat transfer and flow, and the winding was analyzed in combination with the process flow. The tubular heat exchanger and the plate-fin heat exchanger are the characteristics of the main low-temperature heat exchanger of the LNG liquefaction plant. Finally, the following suggestions are made for the localization of the vaporizer and the main low-temperature heat exchanger in the large-scale LNG plant: 1 Strengthening the basic research 2Based on the national technical capabilities, the research on the materials of vaporizers and MCHE is carried out, and the pressure characteristics, surface characteristics and processing characteristics are studied in depth; 3 comprehensively improve the manufacturing process technology and large-scale production capacity of vaporizers and MCHE; Objectively and scientifically select heat exchangers compatible with receiving stations and liquefaction plants; 4 comprehensively track the actual operation of imported heat exchangers, and carry out research work on design and manufacture of heat exchangers for LNG complete sets based on risk and life. .
Keywords: LNG receiving station LNG liquefaction plant vaporizer main low temperature heat exchanger structural material heat transfer and flow technology
At LNG receiving stations and LNG liquefaction plants, carburetor and main cryogenic heat exchangers are important process equipment related to the entire process, and are also the key equipment that affects the energy consumption of the entire plant. To this end, the key technologies of the main low-temperature heat exchangers of LNG receiving station vaporizers and natural gas liquefaction plants will be analyzed from three aspects: structure, material, heat transfer and flow.
1 LNG receiving station heat exchanger
1.1 Main types of vaporizers
The vaporization treatment capacity of the LNG receiving station is very large. Both the air temperature type vaporizer and the forced air type vaporizer [1] require many modules, and the floor space is large and the efficiency is low. Therefore, the liquid heating type vaporizer is mainly selected at present, and the liquid heating type vaporizer is mainly selected. The heat source has chosen seawater according to local conditions.
The liquid heating type vaporizer mainly includes an open frame vaporizer (including ORV and super ORV), a submerged combustion type vaporizer (SCV), a vaporizer (IFV) with an intermediate heat transfer medium, and a coiled tube vaporizer (SWV) [2]. Among them, the submerged combustion type vaporizer is mainly used for peak shaving in other countries, not as a vaporizer under basic load. In the United States, considering that the discharge of cold water into the sea will affect marine life, the US mainly uses submerged combustion type vaporizer as the base load. Vaporizer [3]. Immersion-burning vaporizers are characterized by rapid response [4-6], but because they consume fuel directly, this article does not list them.
1.2 open frame vaporizer
The open-frame vaporizer is a vaporizer that uses seawater as a heat source. It is a large-scale vaporization unit for basic load type, and the maximum natural gas flow rate is 180t/h. The carburetor can operate safely within the load range of 0~100%, and can adjust the vaporization amount remotely according to the change of demand.
The entire vaporizer is fixedly mounted with an aluminum alloy bracket. The basic unit of the vaporizer is a heat transfer tube, which is composed of a plurality of heat transfer tubes in a plate shape, and the two ends are welded by a gas collecting tube or a liquid collecting tube to form a plate type tube bundle, and then a plurality of plate type tube bundles constitute a vaporizer. The top of the carburetor has a seawater sprinkler. The seawater sprays on the outer surface of the plate-shaped tube bundle and flows from top to bottom by gravity. The LNG flows upwards inside the tube, and the seawater transfers heat to the LNG, heating it and vaporizing it. China's Shenzhen Dapeng LNG receiving station uses an open-frame vaporizer.
The key technologies of open-frame heat exchangers are mainly reflected in:
1) Combination of structure and heat transfer and flow process: How to ensure that a large flow of seawater is evenly distributed to each heat transfer tube of each plate type tube bundle, so the ingenious spray structure design is particularly important.
2) Minimize the icing of the lower part of the plate bundle, especially the outer surface of the collector, during ORV operation. When the water film drops, it has a high heat transfer coefficient, but since the thermal conductivity of the ice layer is about 1/40 of the thermal conductivity of the aluminum alloy pipe, the heat transfer performance of the vaporizer is also lowered. Osaka Gas and Kobel Steel jointly developed a two-layer heat transfer tube that effectively improved the icing condition (this open-frame vaporizer is called SuperORV). The LNG enters the inner tube from the distributor at the bottom and then enters the annular gap between the inner and outer tubes [7]. The LNG in the gap is directly heated by the seawater and immediately vaporized, and the LNG flowing in the inner tube is heated by the natural gas which has been vaporized in the gap, so that the vaporization proceeds gradually. Although the gap is not large, it can increase the temperature of the outer surface of the heat transfer tube, thereby suppressing the icing of the heat transfer tube and keeping all the heat transfer areas effective, thereby improving the heat transfer efficiency between seawater and LNG. .
3) Combination of material and heat transfer research: Due to the relatively low heat transfer coefficient of LNG evaporation inside the heat transfer tube, SuperORV design uses some strengthening measures. The heat transfer tube is divided into vaporization zone and heating zone, and the inner fin is used. To increase the heat exchange area and change the shape of the flow channel, increasing the disturbance of the fluid during the flow process. All components in contact with natural gas are made of aluminum alloy and can withstand very low temperatures. All surfaces in contact with seawater are plated with aluminum-zinc alloy to prevent corrosion.
Compared with the traditional ORV (manufactured by Kobel Steel), the super-ORV single heat exchange tube has a three-fold increase in evaporation capacity, a 15% reduction in seawater volume, a 10% reduction in construction costs, and a 40% reduction in installation space.
1.3 Vaporizer (IFV) with intermediate heat transfer medium
The use of intermediate heat transfer fluids can improve the effects of icing. A medium such as propane, isobutane, freon or ammonia is usually used as the medium for the intermediate heat transfer fluid. The IFV can be divided into three parts: the first part is the heat exchange between the seawater (or other heat source fluid) and the intermediate heat transfer fluid; the second part is the heat exchange between the intermediate heat transfer fluid and the LNG; the third part is the natural gas overheating. This vaporizer is remote from the freezing point of the heating fluid and is suitable for use in circulating heating systems, offshore floating storage and vaporization systems [8] and cold energy generation systems.
The key technologies of IFV heat exchangers are mainly reflected in:
1) Structure: How to combine the heat exchange part of intermediate fluid and seawater and the heat exchange part with LNG; whether the heat exchange part of intermediate fluid and LNG is designed as a pullable heat exchange structure; whether the overheated part of seawater to natural gas is designed Independent structure, etc.
2) Material: The heat exchanger embodies the safety and economic harmony in the selection of materials, and it is required to withstand both seawater corrosion and low temperature. The heat exchange tube in contact with seawater is selected from titanium; the heat exchange tube and the tube box in contact with LNG are selected from austenitic stainless steel. The tube plate can be selected as a composite steel plate structure, and the pipe box and the variable diameter cylinder which are in contact with seawater can be either a composite steel plate structure or a lining structure.
3) Heat transfer and fluid flow process: First, select the intermediate heat transfer medium to determine the phase change pressure of the intermediate heat transfer fluid and its corresponding temperature; IFV has a wide temperature range for the heat source fluid, so it can be maximized. The thermal physical properties such as latent heat, select the matching intermediate heat transfer medium; secondly, select and optimize the series flow of the heat source fluid; again improve the surface characteristics of the IFV heat exchange tube to achieve enhanced heat transfer.
1.4 Winding tubular vaporizer
The coiled tube vaporizer is actually a shell-and-tube heat exchanger. It was jointly developed by Hefei General Machinery Research Institute and Zhenhai Petrochemical Jianan Engineering Co., Ltd., and has been successfully applied in small and medium-sized LNG vaporization equipment. The main technical parameters are shown in Table 1. . Under normal circumstances, the heating medium takes the shell and the LNG takes the tube. The capacity of the vaporizer depends primarily on the temperature and flow rate of the shell-side heating medium. The new type of coiled tube vaporizer has a large heat transfer area in the same volume, and can realize heat transfer of multiple fluids in the same equipment. The structure is shown in Fig. 1. The coiled tube heat exchanger has a small shell side resistance, allowing a large flow rate, and the temperature requirement of the shell-side medium is not critical, so that a circulating heating medium can be used as a heat source or a large-flow seawater can be used as a heat source.
The key technologies of the coiled tube vaporizer are mainly reflected in the following aspects:
1) Structure: The structure, process conditions and material selection of the vaporizer are inseparable. If seawater is used as the heat source, a single-strand-wound tubular vaporizer is used; if there is an optional auxiliary heat source, such as a heat engine circulating water, a multi-stream wound tubular vaporizer may be employed. In order to prevent icing, some ducts can be placed in the carburetor to increase gas or steam disturbances.
2) Material: A major feature of the material selection of vaporizers using seawater as a heat source is that the choice of heat transfer tubes and tube sheet materials requires consideration of both low temperature and seawater corrosion, regardless of the LNG pipe or shell side. If the seawater takes the shell and the LNG is taken away, the shell side does not need to consider the low temperature, and only the seawater-resistant corrosion needs to be considered. If the LNG takes the shell, the seawater is taken away, and the difficulty of selecting materials cannot be alleviated. At the same time, due to the LNG shelling process, the shell pressure is high, and it is subjected to low temperature, only thick austenitic stainless steel material can be selected. In addition, the heat exchange tubes of the wound tubular vaporizer are much longer than the conventional heat exchangers. From the perspective of material adaptability, TA2, AL-6XN, B30, HAl77-2, HSn70-1A can be used (recommended degree from high to low) )[9]. The ultra-long heat exchange tubes of these materials are currently mainly dependent on imports. By comparison, the tubesheet and shell of the vaporizer are made of AL-6XN super austenitic stainless steel, and the tube material can be made of ordinary austenitic stainless steel. The heat exchange tubes and the tube sheets are welded by strength and swelled. The tube sheet is adjacent to the homogenous metal of the heat exchange tube (except AL-6XN) on the surface of the tube side of the tube box.
3) Heat transfer and fluid flow technology: The seawater shelling process is to fully exert the strengthening effect of seawater flow on heat transfer and increase the Reynolds number of the shell side. Another reason is the vaporization heat transfer flow mechanism and application of LNG in the tube process. Research has matured. However, if the seawater travels, the large flow of seawater flows in the pipe, which increases the pipe resistance; since the heat exchange pipe is long, it can only be adjusted by increasing the number of heat transfer pipes and reducing the flow rate, which will affect the transmission in the pipe. The heat and the low heat transfer coefficient of the LNG shell side make the whole thermal technology not optimized enough, resulting in an increase in the heat exchange area. Therefore, the overall economic efficiency is not as good as the design scheme of the LNG pipe and the sea shell.
2 Main low temperature heat exchanger (MCHE) of LNG liquefaction plant
2.1 Overview
In natural gas plants, the refrigeration section is where energy consumption is most concentrated. The flexibility and effectiveness of refrigeration section operations directly affects the efficiency of the entire liquefaction plant. The main low temperature heat exchanger (MCHE) is the core of the refrigeration section and the most important heat exchange equipment for the entire LNG liquefaction plant. The role of MCHE is to liquefy natural gas to -162 ° C. Its technological advances will have a significant impact on the overall LNG liquefaction process and operating costs of the plant.
At present, the world's major manufacturers of large-scale LNG plants (generally LNG liquefaction capacity above 300 × 104t / a) are mainly APCI, Shell, ConocoPhillips, Statoil, Linde and Axens. Among them, APCI is the most powerful LNG liquefaction processer. Its main process is propane precooling, mixed refrigerant liquefaction and nitrogen expansion and subcooling LNG liquefaction process. In the selection of the main low temperature heat exchanger, multiple shares are adopted. Flow-wound tubular heat exchanger. Depending on the liquefaction capacity and liquefaction process, Statoil and Linde use a basic single mixed refrigerant process in a small LNG plant, and an aluminum brazed plate fin heat exchanger; an improved mixed refrigerant process in a medium LNG plant. The natural gas is pre-cooled, liquefied and supercooled in the same multi-stream wound tubular heat exchanger. The refrigerant separators with different pressures and temperatures provide refrigerant for the cooling of each section of natural gas (the typical device is China Xinjiang Guanghui Company). LNG plant, 43×104t/a, put into operation in 2004); used a mixed refrigerant step circulation process in a large LNG plant, using three different types of mixed refrigerant circulating compressors, each cycle corresponding to the natural gas cooling process At different temperature stages, an aluminum brazed plate fin heat exchanger was selected as the pre-cooling section heat exchanger, and a multi-strand wound tube heat exchanger was selected as the main low temperature heat exchanger of the liquefaction section and the supercooling section.
2.2 plate fin heat exchanger
The plate-fin heat exchanger is a compact heat exchanger. The aluminum plate-fin heat exchanger is used in the air separation and LNG liquefaction fields. It is characterized by multi-stream heat transfer and hot and cold flow. The number of shares does not need to be strictly limited. Germany Linde uses a plate-fin heat exchanger in its basic single LNG process (natural gas liquefaction capacity is less than 20×104t/a). The schematic diagram of the process is shown in Figure 2. The plate-fin heat exchanger installed in the cold box passes. The two-stage single mixed refrigerant cycle directly cools the natural gas to LNG temperature (typically at the Kollsnes LNG plant in Norway, 4 x 104 t/a, commissioned in 2003). The gas phase mixed refrigerant at the top of the separator is cooled and then throttled into the heat exchanger to provide a subcooling temperature. The liquid phase mixed refrigerant at the bottom of the separator is cooled and then throttled into the heat exchanger to provide pre-cooling and liquefaction temperatures. The American Gas Technology Research Institute also uses aluminum plate-fin heat exchangers as low-temperature heat exchangers in its small LNG liquefaction unit [9]. The world's aluminum plate-fin heat exchangers are used as the main low-temperature heat exchangers for large LNG liquefaction plants. ConocoPhillips (Darwin LNG, Australia, liquefaction capacity 324×104t/a) uses three kinds of refrigerants. In a step cycle, the dried natural gas is finally liquefied through a plate-fin heat exchanger corresponding to different cycle temperatures.
The shortcomings of the aluminum plate-fin heat exchanger are also prominent: the volume of the brazing furnace limits the large-scale liquefaction operation, the two-phase flow distribution technology causes uneven fluid distribution, and the connection pipes often cause complex stress and many leakage points. This is also the main reason why large LNG plants around the world rarely use it as MCHE.
The key technologies of plate-fin heat exchangers are shown in the following aspects:
1) Structure: Studying the high pressure bearing fins can improve the structural bearing capacity of the entire aluminum plate fin heat exchanger. Increasing the fin density not only increases the pressure bearing capacity, but also increases the heat exchange area per unit volume; the optimization technology of the gas-liquid two-phase distribution structure can achieve uniform distribution of fluid under large flow; due to multiple plate fins The heat exchangers are combined in the cold box, so the optimization of the multi-unit parallel piping, the configuration of the overall cold box structure, and the safety analysis of the cold box structure are all key aspects of the design.
2) Materials: The quality of thin aluminum sheets and tubes used in pressure vessels is not stable relative to the quality of steel. Therefore, the quality of raw materials for aluminum plate-fin heat exchangers must be controlled. The selection of brazing materials for different alloyed aluminum materials and the optimization of the brazing process are the primary guarantees for the safety of the entire structure.
3) Heat transfer and fluid flow: improving the geometry of the surface of the plate-fin heat exchanger, enhancing the fin heat transfer capacity, and reducing the fin flow resistance are the research topics of plate-fin heat exchangers in various fields [10-11], It is no exception in the LNG field; what is more interesting is how to achieve a uniform distribution of fluid under high flow, especially in two-phase flow. Therefore, it is very important to study the uniformity of flow path arrangement and fluid distribution; the processing precision of plate-fin heat exchanger has great influence on heat transfer performance; because the flow path length of plate-fin heat exchanger is limited, there are more hot and cold ends. Large temperature gradients, axial heat conduction increase irreversible losses, reduce the thermal efficiency of heat exchangers, should be considered in heat transfer design; in addition, attention should be paid to the influence of the installation position of plate-fin heat exchangers on heat transfer and flow. .
2.3 Winding Tube Heat Exchanger (SWHE)
The winding tubular heat exchanger used as the LNG liquefaction main low temperature heat exchanger (MCHE) is determined by its own characteristics: 1 the medium in the tube flows in a spiral manner, and the shell medium crosses the transverse flow through the coil, and the heat exchanger layer and the layer The heat exchange tube is reversely wound, and the tube and shell-side medium are heat-transferred in a pure countercurrent manner. Even at a low Reynolds number, the flow pattern is turbulent, and the heat transfer coefficient is high; 2 kinds of medium coexist in one winding When the tubular heat exchanger performs heat transfer, since the heat transfer element is a round pipe, the wound tubular heat exchanger requires less pressure difference and temperature difference between different media, which reduces the operation difficulty of the production device and improves the operation. The safety of the equipment; 3 relatively compact structure, high pressure resistance and reliable sealing, thermal expansion can be self-compensated; 4 easy to achieve large LNG liquefaction operations.
Air products is the largest supplier of SWHE in the LNG field. Between 1977 and 2008, 79 sets of LNG plants (with a total liquefaction capacity of 2.3×108t/a) produced coiled tube heat exchangers. Device. In the past 5 years, Linde has produced a multi-strand wound tube heat exchanger with a cumulative metal weight of 3 120 t for use in LNG plants.
The key technologies of the coiled tube heat exchanger are:
1) Structure: The structure and process conditions of the wound tubular heat exchanger are closely linked, and the heat load of the liquefaction section and the supercooling section is reasonably distributed, so that the liquefaction section and the supercooling section are relatively coordinated; combined with the load of the extra large heat exchanger The distribution and the relatively soft nature of the heat transfer tubes use a center cylinder of sufficient rigidity to ensure the uniformity of the windings. The full application of the combined design technology makes the structure of the "cold tower" reasonable; the reasonable selection of the shell and shell and the location of the material inlet and outlet makes the distribution of the fluid more uniform; the application of the multi-tube sheet structure further optimizes the structure.
2) Materials: Since the thermal load of large LNG liquefaction plants is tens or even hundreds of megawatts, plus low temperature requirements, there are only two materials available: austenitic stainless steel and aluminum alloy. The heat transfer tube of the coiled tube heat exchanger with a heat exchange area of ​​2×104 m2 or less may also be considered to be a thin-walled austenitic stainless steel material, and the heat exchange tube of the wound tube heat exchanger of 2×104 m 2 or more is basically made of an aluminum alloy material. The wound tubular heat exchanger of full austenitic stainless steel material is relatively simple to manufacture. If the heat exchange tube adopts aluminum-magnesium alloy tube, it faces several problems: 1 localization of ultra-long aluminum-magnesium alloy heat exchange tube; Selection of other pressure-receiving components of the heat exchanger and its adaptability to the heat exchange tubes; research on the composite technology of the three tube sheets, research on the low-temperature mechanical properties of the composite tube sheets after forming at normal temperature, and study on the material thickness of the transition layer of the tube sheets; Research on the forming technology of precision stamping internals to ensure zero damage to the heat exchange tubes.
3) Heat transfer and fluid flow: 1 Improve the accuracy of physical property calculation. Study on the thermal properties of mixed hydrocarbon media under low temperature and high pressure, especially supercritical conditions; 2 Process design optimization: using existing foreign hydrocarbon processing software, the thermal load of natural gas precooling, liquefaction, supercooling, etc. The temperature relationship is simulated, mainly analyzing the interval of thermal load and the magnitude of the temperature difference dynamics and the relationship between them; 3 fluid distribution and simulation; 4 pairs of pipe-cold natural gas and shell-and-shell mixing in the main low-temperature heat exchanger The study of heat transfer and flow in liquid, gas, two-phase and supercritical pressures under different conditions, including heat transfer at low Reynolds number [12], gives heat transfer factors for different load intervals. And the frictional resistance factor to calculate the heat transfer area of ​​different pipe lengths and different intervals; 5 combined with the economics of heat transfer research under extra large load, the in-depth study on the fouling characteristics of natural gas and mixed refrigerant medium [13].
3 Conclusions and recommendations
The heat exchangers of the LNG plant reflect the technical integration of many disciplines in the mechanical and energy fields: 1) The structure and process of the vaporizer and the main cryogenic heat exchanger are closely linked. The structure of these heat exchangers involves some patented technologies. Should be circumvented.
2) The material of the vaporizer and the main low-temperature heat exchanger is low-temperature high-alloy steel or non-ferrous metal, which is different from the pressure-bearing component materials currently available in China, especially the ultra-long non-ferrous metal heat exchange tubes and the special-shaped aluminum alloy strengthened tubes. Our country is blank.
3) The research on heat transfer and fluid flow in low temperature, high pressure and complex phase transition in China needs to be further strengthened.
With the increasing demand for natural gas applications in China, the related technologies of large-scale LNG complete sets of equipment should make breakthroughs. In order to realize the localization of heat exchangers for large-scale LNG plants, the following are recommended:
1) Intensify basic research: screen some hydrocarbon analysis software for secondary development, simulate and optimize the process flow; increase thermal properties under low temperature and high pressure and supercritical conditions, and improve the accuracy of heat exchanger design input conditions; The CFD technology is used to simulate the heat transfer enhancement mode and the uniformity of fluid distribution. The experimental study on heat transfer and fluid flow is carried out conditionally, and the heat transfer characteristics and fluids between multiple flow media under various working conditions are analyzed. Resistance characteristics and analysis of their compliance.
2) Based on the national technical capabilities, expand the research on vaporizers and MCHE materials, especially non-ferrous materials. In-depth study of its pressure characteristics, surface characteristics, processing characteristics.
3) Comprehensively improve the manufacturing process technology and large-scale production capacity of the vaporizer and MCHE.
4) LNG complete sets of equipment reflect the requirements of both safety and energy saving. It is necessary to select heat exchangers that are compatible with receiving stations and liquefaction plants in a fair, objective and scientific manner.
5) Carry out comprehensive tracking of the actual operation of the imported heat exchanger, and carry out research work on the design and manufacture of heat exchangers for LNG complete sets based on risk and life [14].
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