A method for preventing and controlling dew point corrosion of precooling heat exchanger in mixed refrigerant liquefaction system

Abstract

The present invention relates to the field of dew point corrosion and corrosion prevention, and discloses a method for preventing and controlling dew point corrosion of a precooling heat exchanger in a mixed refrigerant liquefaction system, which controls the molar fraction of heavy hydrocarbons C8 and above in natural gas to 0.04%~0.1%. The present invention proposes measures to prevent dew point corrosion of the precooling heat exchanger in the system based on the dew point of water and hydrocarbons in natural gas. By adding heavy hydrocarbons C8 and above to natural gas, liquid hydrocarbons are first precipitated during the cooling process to form a hydrocarbon dew point, and an oil-in-water state is formed in the dew point solution, which does not react with CO2, thereby solving the dew point corrosion problem of the precooling heat exchanger in a natural gas single-cycle mixed refrigerant liquefaction system, which is beneficial to improving the liquefaction efficiency of natural gas, and has certain theoretical significance and practical feasibility.

Description

Technical Field

[0001] The present invention relates to the field of dew point corrosion and corrosion prevention, and in particular to a method for preventing and controlling dew point corrosion of a precooling heat exchanger in a mixed refrigerant liquefaction system. Background Art

[0002] There are two types of dew points for natural gas: water dew point and hydrocarbon dew point. The water dew point of natural gas refers to the temperature at which the first drop of water is precipitated from natural gas under certain pressure conditions, while the hydrocarbon dew point refers to the temperature at which the first drop of liquid hydrocarbon is precipitated from natural gas under certain pressure conditions. Natural gas is usually dehydrated before entering the liquefaction system, but when the dehydration of natural gas is not thorough, the trace amount of water in it will be cooled out under certain pressure and temperature conditions, reacting with the carbon dioxide in the natural gas to form acidic substances, corroding related equipment in the natural gas liquefaction system.

[0003] The existing dew point corrosion protection measures are mainly flue gas dew point corrosion protection measures and hydrochloric acid dew point corrosion protection measures. The flue gas dew point corrosion protection measures mainly include optimizing fuel oil properties, reducing fuel sulfur content, controlling the oxygen content of the heating furnace, controlling the water vapor content in the flue gas, and increasing the equipment pipe wall temperature. The hydrochloric acid dew point corrosion protection measures mainly include wet scrubbing, dry steam scrubbing, electrical desalination, ammonia injection, water injection, and corrosion inhibitor injection. At present, there is little research on the protection measures for natural gas dew point corrosion.

[0004] There are three main methods for natural gas dew point control technology: low temperature separation, solid adsorption and solvent absorption. Among them, the low temperature separation method can achieve deoiling and dehydration at the same time, meet the control requirements of water dew point and hydrocarbon dew point, and has a simple process, low investment and low operating costs, and is widely used in the petrochemical industry. In terms of low temperature separation method to control natural gas dew point, in 2014, Zhang Wenchao and others determined the low temperature separation process based on the gas quality characteristics of the produced gas containing a small amount of condensate oil in the Sugeli gas field, combined with the product natural gas index requirements and the specific application of the natural gas dew point control process. The UniSim Design software was used to analyze in detail the method and process of determining the condensation temperature, and the selection of key equipment such as the refrigeration process and pre-cooling heat exchanger and low temperature separator was discussed. Finally, it was determined that the low temperature separation process with propane as an external cold source is completely suitable for the natural gas dew point control in the Sulige gas field. In terms of natural gas dew point control devices, Sun Lei optimized the natural gas dew point control device of a gas field in Qinghai in 2017. The device originally used a propane refrigeration system to cool, dehydrate and dehydrocarbon, but the device had too high energy consumption, outdated equipment and poor economic benefits. Therefore, optimization was carried out from the perspectives of key equipment selection and antifreeze injection. In the original device, no separator was set after the precooling heat exchanger, and antifreeze was injected on demand at one time. After optimization, a two-stage cooling and two-stage alcohol injection process was adopted, which saved the amount of ethanol injection and reduced the energy consumption of ethylene glycol regeneration.

[0005] In summary, the current research on dew point corrosion protection technology mainly focuses on flue gas dew point corrosion protection and hydrochloric acid dew point corrosion protection, and there are few studies on natural gas dew point corrosion protection technology. Most of the research on natural gas dew point control technology is aimed at controlling the dew point of a specific device, and the research on pipeline natural gas dew point control technology is the main focus. The natural gas single-cycle mixed refrigerant liquefaction system has the advantages of a small number of equipment, simple process, and relatively low investment in liquefaction equipment. In the cooling process, the composition of the mixed refrigerant can be adjusted to adapt to the physical property changes of the raw gas. In the liquefaction process, natural gas is mainly cooled step by step through the compression, condensation, throttling and cooling, liquefaction, absorption and gasification of the mixed refrigerant. Its main heat exchange equipment includes precooling heat exchanger, intercooling heat exchanger and cryogenic heat exchanger. If dew point corrosion occurs in the relevant components of the precooling heat exchanger during the liquefaction process, it will accelerate the corrosion rate of the relevant components in the precooling heat exchanger, affect the liquefaction efficiency of natural gas, and not only have an adverse effect on the service life of the precooling heat exchanger, but also seriously threaten the safe production of natural gas enterprises, and bring certain economic losses. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, an object of the present invention is to provide a method for preventing and controlling dew point corrosion of a precooling heat exchanger in a mixed refrigerant liquefaction system.

[0007] During the research process, the inventors found that when the molar fraction of the heavy components i-C5, n-C5, C6, and C7 in natural gas is between 0.02% and 0.1%, the dew point type of the natural gas is all water dew point, and the pH value of the dew point solution is relatively small, which will cause dew point corrosion to the relevant components in the pre-cooling heat exchanger; when the molar fraction of any one or a combination of the heavy components C8 and above in the natural gas is greater than or equal to 0.04%, the content of liquid heavy hydrocarbons in the dew point solution is much greater than the content of liquid water, the dew point type is hydrocarbon dew point, and the dew point solution presents an oil-in-water state, that is, the liquid heavy hydrocarbons in the dew point solution are the continuous phase and the liquid water is the dispersed phase. At this time, although a trace amount of CO2 is dissolved in the dew point solution, it will not cause dew point corrosion to the relevant components in the pre-cooling heat exchanger.

[0008] Based on the above research results, the present invention adopts the following technical solutions:

[0009] A method for preventing and controlling dew point corrosion of a precooling heat exchanger in a mixed refrigerant liquefaction system controls the molar fraction of any one or a combination of heavy hydrocarbon C8 and above components in natural gas to 0.04% to 0.1%.

[0010] Furthermore, the components above C8 include C9, C 10 and C 11 .

[0011] Furthermore, any one or a combination of heavy hydrocarbons C8 and above are added to the natural gas before it starts to liquefy.

[0012] Furthermore, after the natural gas containing any one or a combination of heavy hydrocarbons C8 and above is liquefied by a pre-cooling heat exchanger, the liquefied natural gas is separated into gaseous natural gas, liquid heavy hydrocarbons and liquid water by a three-phase separator, among which the liquid heavy hydrocarbons containing any one or a combination of heavy hydrocarbons C8 and above are recycled.

[0013] Furthermore, the recovered liquid heavy hydrocarbons containing any one or a combination of C8 and above components are directly returned to the natural gas transportation pipeline to be liquefied after being atomized by the heavy hydrocarbon atomizer, and re-enter the precooling heat exchanger for liquefaction.

[0014] Furthermore, the method of the present invention is applicable to a natural gas mixed refrigeration liquefaction system.

[0015] In a preferred embodiment of the present invention, the method of the present invention is applicable to a natural gas single-cycle mixed refrigeration liquefaction system.

[0016] Furthermore, the present invention can also be applied to a propane precooling mixed refrigerant liquefaction system and a double-cycle mixed refrigerant liquefaction system, etc.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] (1) The present invention proposes measures to prevent dew point corrosion of the precooling heat exchanger in the system based on the dew point of water and hydrocarbons in natural gas. By adding any one or a combination of heavy hydrocarbons C8 and above components to natural gas, the dew point corrosion problem of the precooling heat exchanger in the natural gas single-cycle mixed refrigerant liquefaction system is solved, which is beneficial to improving the liquefaction efficiency of natural gas and has certain theoretical significance and practical feasibility.

[0019] (2) The present invention also realizes the recycling of any one or a combination of components C8 and above by adding relevant equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0021] Figure 1 is the change of mass flow rate of each component in the dew point solution with temperature, including (a) the change of mass flow rate of each component in the dew point solution with temperature, (b) the change of CO2 mass flow rate in the dew point solution with temperature;

[0022] Figure 2 is the change of pH value of dew point solution with mass flow rate of dew point solution

[0023] Figure 3 is the variation of natural gas dew point temperature, dew point type a and dew point solution pH value b with i-C5 mole fraction;

[0024] Figure 4 is the variation of natural gas dew point temperature, dew point type a and dew point solution pH value b with n-C5 mole fraction;

[0025] Figure 5 is the variation of natural gas dew point temperature, dew point type a and dew point solution pH value b with C6 mole fraction;

[0026] Figure 6 is the variation of natural gas dew point temperature, dew point type a and dew point solution pH value b with C7 mole fraction;

[0027] Figure 7 is the change of natural gas dew point temperature and dew point type with C8 mole fraction;

[0028] Figure 8 The mass flow rate of each component in the natural gas dew point solution with a C8 mole fraction of 0.04% varies with temperature;

[0029] Fig. 9 The mass flow rate of each component in the natural gas dew point solution with a C9 molar fraction of 0.04% varies with temperature;

[0030] Fig.10 C 10 The variation of mass flow rate of each component in a natural gas dew point solution with a molar fraction of 0.04% with temperature;

[0031] Fig.11 C 11 The variation of mass flow rate of each component in a natural gas dew point solution with a molar fraction of 0.04% with temperature;

[0032] Fig.12 A schematic diagram of a device system for implementing the method of the present invention (Note: all intersecting lines in the figure are not connected).

[0033] Figure numerals: 1 is a gas-liquid separation tank; 2 is a natural gas pre-cooling heat exchanger; 3 is a three-phase separator; 4 is a gas-liquid separation tank; 5 is a natural gas inter-cooling heat exchanger; 6 is a natural gas deep-cooling heat exchanger; 7 is an LNG flash tank; 8 is a refrigerant mixer; 9 is a refrigerant mixer; 10 is a heavy hydrocarbon atomizer. DETAILED DESCRIPTION

[0034] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0035] Taking the natural gas single-cycle mixed refrigerant liquefaction system as an example, the problem existing in the existing liquefaction process is that a small amount of water or liquid hydrocarbons will be cooled out during the cooling of the gaseous natural gas in the pre-cooling heat exchanger. Natural gas contains a small amount of CO2. Under certain pressure and temperature conditions, CO2 will dissolve in liquid water to form a high-concentration acidic solution, causing dew point corrosion to related equipment in the pre-cooling heat exchanger, which will not only have an adverse effect on the service life of the pre-cooling heat exchanger, but also seriously threaten the safe production of natural gas enterprises.

[0036] Through relevant research, it is found that when natural gas is composed of the light components in Table 1 below, the mass flow rate of each component in the dew point solution changes with temperature after reaching the dew point temperature as follows Figure 1 As shown on the left, after reaching the dew point temperature, the mass flow rate of the CO2 liquid component is significantly different from the mass flow rates of other components in natural gas, so the mass flow rate of the CO2 liquid component is expressed as Figure 1 Described on the right.

[0037] Components <![CDATA[C1]]> <![CDATA[C2]]> <![CDATA[C3]]> <![CDATA[i-C4]]> <![CDATA[n-C4]]> <![CDATA[CO2]]> <![CDATA[H2O]]> Raw gas (mol%) 92.00 4.00 0.75 0.12 0.12 3.00 0.01

[0038] pass Figure 1 It is found that the dew point temperature of the natural gas in the precooling heat exchanger of the natural gas single-cycle mixed refrigerant liquefaction system is -6.4℃ at this time, and the dew point solution is mainly composed of liquid water, that is, the dew point type is water dew point. CO2 dissolves in the dew point solution and dew point corrosion occurs. As the temperature decreases, the mass flow rate of liquid water increases. The pH value of the dew point solution changes with the mass flow rate of the dew point solution. Figure 2 It can be seen that the pH value of the dew point solution in the precooling heat exchanger does not change with the increase of the mass flow rate of the dew point solution. The pH value of the dew point solution is 0.8 between -6℃ and -7℃, which will cause dew point corrosion to the equipment. As the temperature decreases, the mass flow rate of the dew point solution increases, that is, the mass flow rate of the acidic solution increases, and the degree of corrosion to the equipment will gradually increase.

[0039] In order to solve the above problems, the inventors studied the influence of the molar fraction of heavy components i-C5, n-C5, C6, C7, and C8 in natural gas on the dew point type of natural gas and the corrosion condition of the precooling heat exchanger. At this time, the composition of natural gas is shown in Table 2 below:

[0040] Components <![CDATA[C1]]> <![CDATA[C2]]> <![CDATA[C3]]> <![CDATA[i-C4]]> <![CDATA[n-C4]]> <![CDATA[i-C5、n-C5、C6、C7、C8]]> <![CDATA[CO2]]> <![CDATA[H2O]]> Raw gas (mol%) 91.98~91.90 4.00 0.75 0.12 0.12 0.02~0.1 3.00 0.01

[0041] Through research, it is found that the natural gas dew point temperature, dew point type and dew point solution pH value change with the molar fraction of heavy components i-C5, n-C5, C6, C7 and C8 in natural gas. Figures 3 to 7 shown.

[0042] pass Figures 3 to 6 It can be seen that when the molar fraction of the heavy components i-C5, n-C5, C6, and C7 in natural gas is between 0.02% and 0.1%, the dew point type of natural gas is water dew point, and the pH value of the dew point solution is 0.84, which will cause dew point corrosion to the relevant components in the precooling heat exchanger. Figure 7 It can be seen that when the molar fraction of the heavy component C8 in natural gas is 0.04%~0.1%, the dew point type of natural gas at this time is hydrocarbon dew point, and the mass flow rate of each component in the dew point solution changes with temperature as shown in the following figure: Figure 8 As shown in the figure, when heavy components above C8 are added to natural gas, the dew point type of natural gas is also hydrocarbon dew point.

[0043] pass Figures 8 to 11 It can be seen that when the molar fraction of heavy components C8 and above in natural gas is 0.04%, the content of liquid heavy hydrocarbons in the dew point solution is much greater than the content of liquid water, the dew point type is hydrocarbon dew point, and the dew point solution presents an oil-in-water state, that is, the liquid heavy hydrocarbons in the dew point solution are the continuous phase and the liquid water is the dispersed phase. At this time, although there is a trace amount of CO2 dissolved in the dew point solution, it will not cause dew point corrosion to the relevant components in the precooling heat exchanger, thereby affecting the liquefaction efficiency of natural gas.

[0044] Based on the above research results, the present invention prevents dew point corrosion of the precooling heat exchanger by adding an appropriate amount of heavy hydrocarbon C8 and above components to natural gas. After the natural gas to which the heavy hydrocarbon C8 and above components are added is liquefied by the precooling heat exchanger, the liquefied natural gas is separated into gaseous natural gas, liquid heavy hydrocarbons and liquid water by a three-phase separator. The liquid heavy hydrocarbons containing C8 and above components are atomized by a heavy hydrocarbon atomizer and directly returned to the natural gas transportation pipeline to be liquefied, and re-enter the precooling heat exchanger for liquefaction. The presence of C8 and above components will prevent the generation of water dew point in the precooling heat exchanger, thereby preventing dew point corrosion of related components in the precooling heat exchanger, and improving the liquefaction efficiency of the natural gas single-cycle mixed refrigerant liquefaction system.

[0045] like Fig.12As shown, in the natural gas single-cycle mixed refrigerant liquefaction system (natural gas processing capacity is 2800kmol / h, pressure is 5MPa, temperature is 40℃), heavy hydrocarbon C8 is added to the natural gas before it starts to liquefy, so that the molar fraction of heavy hydrocarbon C8 relative to the natural gas containing heavy hydrocarbon C8 is 0.04%, and it is cooled to -70℃ by the pre-cooling heat exchanger 2, and enters the three-phase separator 3 to separate gaseous natural gas, liquid heavy hydrocarbons and liquid water, wherein the gaseous natural gas is cooled to -137.5℃ by the intermediate cooling heat exchanger 5, and then cooled to -162℃ by the deep cooling heat exchanger 6, and finally transported to the LNG flash tank 7 after cooling and reducing the pressure to -160.2℃ by the throttle valve, and the flash gas is sent to the gas mixing unit, and the LNG product is transported to the LNG storage tank. The heavy hydrocarbons separated by the three-phase separator 3 are atomized by the heavy hydrocarbon atomizer 10 before entering the natural gas transmission pipeline to be liquefied, and the heavy hydrocarbons containing C8 are re-entrained into the precooling heat exchanger for liquefaction, thereby realizing the recycling of C8 and preventing the occurrence of dew point corrosion of the precooling heat exchanger. During the entire liquefaction process, the mixed refrigerant first enters the gas-liquid separation tank 1 to separate the gas phase and the liquid phase, and both the gas and liquid phases enter the pre-cooling heat exchanger 2 to be cooled to -70°C. The cooled liquid phase is mixed with the mixed refrigerant from the inter-cooling heat exchanger 5 to provide cooling for the pre-cooling heat exchanger 2; the cooled gas phase enters the gas-liquid separation tank 4 to be separated into the gas phase and the liquid phase again, and enters the inter-cooling heat exchanger 5 to be cooled to -137.5°C, wherein the cooled liquid phase is mixed with the mixed refrigerant from the cryogenic heat exchanger 6 to provide cooling for the inter-cooling heat exchanger 5; the cooled gas phase enters the cryogenic heat exchanger 6 to continue to be cooled to -162°C, and then enters the cryogenic heat exchanger 6 through the throttle valve to provide cooling for it, completing the circulation of the mixed refrigerant.

[0046] Those skilled in the art will appreciate that the method of the present invention is not only applicable to natural gas single-cycle mixed refrigeration liquefaction systems, but also to other natural gas mixed refrigerant refrigeration liquefaction systems, such as propane pre-cooling mixed refrigerant liquefaction systems, dual-cycle mixed refrigerant liquefaction systems, etc.

[0047] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preventing and controlling dew point corrosion of a precooling heat exchanger in a mixed refrigerant liquefaction system. It is characterized in that Control of heavy hydrocarbons C 8 and C 8 The molar fraction of any one or a combination of the above components in natural gas is 0.04% to 0.1%; Adding heavy hydrocarbons C to natural gas before it starts to liquefy 8 and C 8 Any one or a combination of the above components; Contains heavy hydrocarbons C 8 and C 8 After the natural gas of any one or a combination of the above components is liquefied by the precooling heat exchanger, the liquefied natural gas is separated into gas phase natural gas, liquid heavy hydrocarbons and liquid water by a three-phase separator. 8 and C 8 Liquid heavy hydrocarbons of any one or a combination of the above components are recycled.

2. According to the mixed refrigerant liquefaction system precooling heat exchanger dew point corrosion prevention and control method according to claim 1, It is characterized in that The C 8 The above components, including C 9 , C 10 and C 11 .

3. According to the mixed refrigerant liquefaction system precooling heat exchanger dew point corrosion prevention and control method according to claim 1, It is characterized in that The recovered 8 and C 8 The liquid heavy hydrocarbons of any one or a combination of the above components are directly returned to the natural gas transportation pipeline to be liquefied after being atomized by the heavy hydrocarbon atomizer, and re-enter the precooling heat exchanger for liquefaction.

4. The method for preventing and controlling dew point corrosion of a precooling heat exchanger in a mixed refrigerant liquefaction system according to any one of claims 1 to 2, It is characterized in that Applicable to natural gas mixed refrigeration liquefaction system.

5. The method for preventing and controlling dew point corrosion of a precooling heat exchanger in a mixed refrigerant liquefaction system according to any one of claims 1 to 2, It is characterized in that Applicable to natural gas single-cycle mixed refrigeration liquefaction system.

6. The method for preventing and controlling dew point corrosion of a precooling heat exchanger in a mixed refrigerant liquefaction system according to any one of claims 1 to 2, It is characterized in that Applicable to propane precooling mixed refrigerant liquefaction system and double-cycle mixed refrigerant liquefaction system.

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