Space division method

Abstract

The invention provides an air separation method. In the air separation method, an air separation system includes an air separation apparatus and a nitrogen liquefier. In the air separation apparatus, a plurality of output lines are routed from a rectifier column system through a main heat exchanger. In the nitrogen liquefier, a gas feed line is used to deliver a gas stream expanded through an expander. The air separation system further includes an intermediate line having an inlet end connected to the gas feed line of the nitrogen liquefier and an outlet end connected to at least one of the plurality of output lines of the air separation apparatus at a location between the rectifier column system and the main heat exchanger, such that the intermediate line delivers the gas stream to the at least one output line. The air separation method described above can be started up smoothly without using an external supply of liquid nitrogen.

Description

Technical Field

[0001] This invention belongs to the field of air separation and relates to an air separation method. Background Technology

[0002] Air separation equipment is widely used in various industries. Many users of air separation equipment also equip it with nitrogen liquefaction units to liquefy the nitrogen produced by the air separation equipment, thereby obtaining some liquid nitrogen. At this time, the air separation equipment and the nitrogen liquefaction unit constitute an air separation system.

[0003] Air separation equipment relies on the principle of low-temperature air separation to separate feed air in a distillation column to form various air products. Therefore, it is necessary to provide cooling to the distillation column to ensure that its operating temperature reaches the required low temperature. In particular, the air separation equipment in the above-mentioned air separation system often does not have an expander refrigeration system and usually relies on the liquid nitrogen reflux generated by the nitrogen liquefaction unit to provide cooling during normal operation.

[0004] However, during the start-up phase, the distillation column needs to be cooled from ambient temperature to the required cryogenic temperature. Currently, the common practice for the aforementioned air separation units is to use liquid nitrogen stored in a backup system. When the air separation unit is first started, cryogenic liquid nitrogen is supplied to the distillation column from the external backup system to provide cooling and bring it to the required temperature.

[0005] However, the inventors argue that storing sufficient liquid nitrogen for startup is not only expensive, but also requires the construction or expansion of backup facilities. This increases costs for users. Furthermore, some users are located in remote areas, making it difficult to store cryogenic liquid nitrogen before the air separation unit produces air products such as nitrogen. Additionally, further analysis suggests that using stored liquid nitrogen to cool distillation columns typically results in a rapid cooling rate that is sometimes difficult to control. This can cause problems such as material deformation and even malfunctions in the air separation unit and the entire air separation system, including other equipment, due to sudden cooling.

[0006] Therefore, a solution is needed to replace externally stored liquid nitrogen to provide cooling and ensure smooth startup. Summary of the Invention

[0007] The purpose of this invention is to provide an air separation system that can be started smoothly without the use of externally stored liquid nitrogen.

[0008] Another object of the present invention is to provide an air separation system that can reduce startup costs.

[0009] Another objective of this invention is to provide an air separation system that can prevent the cooling rate of the entire air separation system from becoming too fast during startup, thereby making it more reliable and safer.

[0010] This invention provides an air separation system, including an air separation unit and a nitrogen liquefaction unit. The air separation unit includes a distillation column system, a main heat exchanger, and multiple output lines. The multiple output lines extend from the distillation column system via the main heat exchanger, and include a nitrogen line for outputting nitrogen product. The nitrogen liquefaction unit includes an expander and a gas delivery line. The expander expands at least a portion of the nitrogen product from the nitrogen line, and the gas delivery line delivers the gas stream expanded by the expander. The air separation system also includes an intermediate line, the inlet of which is connected to the gas delivery line of the nitrogen liquefaction unit, and the outlet of which is connected to at least one of the multiple output lines of the air separation unit at a location between the distillation column system and the main heat exchanger, thereby allowing the intermediate line to deliver gas stream to at least one output line.

[0011] In one embodiment, the nitrogen liquefaction unit further includes an internal heat exchanger, a gas-liquid separator, and a throttling valve. A gas supply line delivers gas from the expander to the internal heat exchanger for reheating. The gas-liquid separator is located on the gas supply line and between the expander and the internal heat exchanger. The gas-liquid separator collects the gas flow expanded by the expander and the stream throttled by the throttling valve, and separates all collected fluids into gas and liquid phases.

[0012] In one embodiment, the inlet end of the intermediate pipeline is connected to the gas transmission pipeline at the location between the expander and the internal heat exchanger.

[0013] In one embodiment, the inlet end of the intermediate pipeline is connected to the gas transmission pipeline at a location between the gas-liquid separator and the internal heat exchanger.

[0014] In one embodiment, the nitrogen liquefaction unit further includes a return line and a delivery line. The return line is used to return a first portion of the liquid stream separated by the gas-liquid separator to the distillation column system. The delivery line is used to output a second portion of the liquid stream separated by the gas-liquid separator to a storage tank.

[0015] In one embodiment, in the nitrogen liquefaction unit, the outlet end of the gas delivery line is connected to the nitrogen line, thereby sending the gas flow that has been reheated by the internal heat exchanger to the nitrogen line to merge with the nitrogen product in the nitrogen line.

[0016] In one embodiment, the air separation system further includes an airflow regulating component, a liquid flow regulating component, a controller, and a detection element. The airflow regulating component regulates the flow rate of the airflow in the intermediate pipeline. The liquid flow regulating component regulates the flow rate of the liquid in the return pipeline and / or the delivery pipeline. The controller controls the airflow regulating component and / or the liquid flow regulating component according to a control signal. The detection element sends a detection signal to the controller, and the controller generates a control signal based on the detection signal from the detection element.

[0017] In one embodiment, at least one output line is a waste gas discharge line of the air separation unit for conveying waste gas.

[0018] The present invention also provides an air separation method. This air separation method uses the aforementioned air separation system. Furthermore, in the air separation method, during the start-up phase of the air separation unit of the air separation system, gas flow is supplied from the nitrogen liquefier of the air separation system to at least one output line of the air separation unit via an intermediate pipeline of the air separation system.

[0019] In one embodiment, at the start of the startup phase, the intermediate pipeline is opened, and the return and delivery pipelines of the nitrogen liquefier are closed; during the startup phase, the nitrogen purity in the nitrogen pipeline of the air separation unit and / or the gas-liquid separator of the nitrogen liquefier is detected, and the liquid level in the gas-liquid separator is detected; when the liquid level in the gas-liquid separator reaches a predetermined value, the return pipeline is opened; when the nitrogen purity in the nitrogen pipeline and / or the gas-liquid separator reaches a predetermined value, the startup phase is determined to be over and the operation phase begins; and during the operation phase, the delivery pipeline is opened, and the intermediate pipeline is closed.

[0020] In one embodiment, during the startup phase, the cooling rate of a fixed point in the internal heat exchanger of the nitrogen liquefier is also detected, and the flow rate of the gas supply line sent to the nitrogen line through the internal heat exchanger is adjusted so that the detected cooling rate is within 2°C / min.

[0021] In the aforementioned air separation system and method, the air separation equipment is supplied with air that has been expanded and cooled by the expander within the nitrogen liquefaction unit, utilizing the system's own nitrogen liquefaction unit. This provides cooling capacity for the start-up of the air separation equipment, allowing for smooth startup without the need for externally stored liquid nitrogen. Therefore, there is no need to construct corresponding backup facilities or pre-store expensive liquid nitrogen, resulting in lower startup costs and greater ease of startup.

[0022] Furthermore, in the aforementioned air separation system and method, the gas stream cooled by the expander in the nitrogen liquefaction unit gradually cools down, thus providing cooling capacity to the distillation column system of the air separation equipment in a gradual manner. Therefore, during startup, the cooling rate of the entire air separation system is less likely to be too rapid, making it easier to control and thus more reliable and safer. Failures caused by excessively rapid cooling are less likely to occur. Attached Figure Description

[0023] The advantages and spirit of this invention can be further understood through the following detailed description of the invention and the accompanying drawings.

[0024] Figure 1 This is a schematic diagram illustrating an exemplary air separation system, particularly its air separation equipment.

[0025] Figure 2 This is a schematic diagram illustrating an exemplary air separation system, particularly its nitrogen liquefaction unit. Detailed Implementation

[0026] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, it should be understood that the present invention is not limited to the embodiments described below, and the technical concept of the present invention can be implemented in combination with other known technologies or functions, or with other technologies similar to those known technologies.

[0027] The terms "first" and "second" are used for descriptive purposes only and do not refer to a limitation on time sequence, quantity, or importance. They should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated, but are merely used to distinguish one technical feature from another in this technical solution. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "multiple" means two or more, unless otherwise explicitly specified. Similarly, qualifiers such as "one" appearing in the text do not refer to a limitation on quantity, but describe technical features not mentioned above. Likewise, unless a noun is modified by a specific quantifier, it should be considered in the text to include both singular and plural forms; the technical solution may include either a singular or plural number of that technical feature. Similarly, modifiers such as "approximately" or "about" preceding numerals in the text generally include the number itself, and their specific meaning should be understood in conjunction with the context.

[0028] It should be understood that in this invention, "at least one time" means one or more times. "And / or" is used to describe the relationship between related objects, indicating that there can be three relationships. For example, "A and / or B" can represent three cases: only A exists, only B exists, and both A and B exist simultaneously, where A and B can be singular or plural.

[0029] As used herein, the term "and / or" includes any and all combinations of one or more of the related listed items. Unless otherwise stated, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood in the art to which this invention pertains. It should also be understood that terms, such as those defined in common dictionaries, should be understood to have the meaning consistent with their meaning in the context of this specification and the related art, and should not be interpreted in an idealized or overly formal sense unless expressly stated herein. For the sake of brevity and / or clarity, well-known functions or constructions may not be described in detail.

[0030] As mentioned earlier, the inventors discovered numerous problems with using externally stored liquid nitrogen to provide the cooling required for startup. Furthermore, the inventors found that users without a backup system to store large quantities of liquid nitrogen typically prefer to have a nitrogen liquefaction unit readily available. Therefore, this invention was provided.

[0031] Figure 1 and Figure 2 An exemplary configuration of an air separation system 100 is shown together. The air separation system 100 includes an air separation unit 10 and a nitrogen liquefaction unit 20. Figure 1 An example construction of the air separation unit 10 is shown in more detail, while Figure 2 An example construction of the nitrogen liquefaction device 20 is shown in more detail.

[0032] It is understood that the accompanying drawings in this article are merely examples and are not necessarily drawn to scale, and should not be used as a limitation on the actual scope of protection of this invention.

[0033] like Figure 1 As shown, the air separation unit 10 includes a distillation column system 1, a main heat exchanger 2, and multiple output pipelines 3. The aforementioned multiple output pipelines 3 are led out from the distillation column system 1 via the main heat exchanger 2.

[0034] A distillation column system typically includes at least one distillation column, and can therefore be a single-column or multi-column configuration, such as a twin-column system. In a distillation column system, the feed air is separated into various air products, such as nitrogen, oxygen, and argon, at a cryogenic environment where at least a portion of the temperature is below 150 K. This process is often referred to as cryogenic air separation. The air products can be in liquid or gaseous form. A "distillation column" refers to a distillation or fractionation column or zone in which the liquid and gas phases come into countercurrent contact to effectively separate the fluid mixture.

[0035] In the "main heat exchanger," the feed air typically receives indirect heat from the output stream (also known as the return stream) of the distillation column system, thus cooling the feed air and reheating the output stream. The main heat exchanger can be formed by a single heat exchanger section or multiple heat exchanger sections connected in parallel and / or series. Each heat exchanger section, for example, consists of one or more plate heat exchanger blocks. The plate heat exchanger blocks can have channels that are separated from each other and have heat exchange surfaces, through which different streams flow, thereby being cooled or heated separately. Commonly, the main heat exchanger can be an aluminum brazed plate-fin heat exchanger (BAHX). "Complete cooling" means that the stream being cooled enters the main heat exchanger at the hot end and is then cooled to the cold end temperature of the main heat exchanger; that is, the stream being cooled exits from the cold end of the main heat exchanger. "Partial cooling" means that the cooled stream is cooled to the intermediate temperature between the hot and cold ends of the main heat exchanger, that is, the cooled stream is drawn out from the intermediate position between the hot and cold ends of the main heat exchanger.

[0036] Output line 3 extends from the distillation column system 1 via the main heat exchanger 2. That is, output line 3 delivers the output streams, with different parameters (e.g., composition) formed after the feed air g0 is separated by the distillation column system 1, to the colder end of the main heat exchanger 2 for reheating. Then, the reheated streams from the hotter end of the main heat exchanger 2 are sent out. For example, the output streams may include waste gas, which can be directly discharged into the atmosphere via output line 3; in this case, output line 3 is a waste gas discharge line. Alternatively, the output streams may include the air product required by the user, which can be directly delivered to the required location via output line 3; in this case, output line 3 is a product line.

[0037] The aforementioned multiple output lines 3 include a nitrogen line 31 for outputting nitrogen product g3. It is understood that... Figure 1 In this air separation unit 10, both high-pressure nitrogen product g3a (relatively high pressure) and low-pressure nitrogen product g3b (relatively low pressure) are produced. Here, nitrogen pipeline 31 can refer to both the high-pressure nitrogen pipeline 31a that outputs high-pressure nitrogen product g3a and the low-pressure nitrogen pipeline 31b that outputs low-pressure nitrogen product g3b. However, this does not constitute any limitation. For example, the aforementioned plurality of output pipelines 3 may include only one nitrogen pipeline. In this case, when it is mentioned that the plurality of output pipelines 3 include nitrogen pipeline 31, it naturally refers to that single nitrogen pipeline. As another example, the aforementioned plurality of output pipelines 3 may include multiple nitrogen pipelines, and when it is mentioned that the plurality of output pipelines 3 include nitrogen pipeline 31, it may refer to only one of those nitrogen pipelines, not all of them. Furthermore, in addition to producing nitrogen products, air separation unit 10 may also produce oxygen or argon products. That is, the description of nitrogen line 31 in the text only needs to apply to at least one nitrogen line 31 in the air separation unit 10. It can also be understood that "nitrogen product" g3 in the text refers to gaseous nitrogen product.

[0038] like Figure 2 As shown, the nitrogen liquefaction unit 20 includes an expander 4 and a gas delivery line 5. The expander 4 is used to expand at least a portion of the nitrogen product g3 from the nitrogen delivery line 31. The gas delivery line 5 is used to deliver the gas stream g4 expanded by the expander 4.

[0039] "At least a portion of nitrogen product g3" means at least a portion of the nitrogen product g3 fed from the air separation unit 10 into the nitrogen liquefaction unit 20. Figure 2In this process, the gas flow g4 expanded by expander 4 is part of the nitrogen-enriched gas flow g30, for example, it can account for about half of the mass flow rate of the nitrogen-enriched gas flow g30, such as 40% to 60%. The nitrogen-enriched gas flow g30 includes nitrogen product g3 (including high-pressure nitrogen product g3a and low-pressure nitrogen product g3b). That is, the gas flow g4 expanded by expander 4 includes at least a portion of the nitrogen product g3, and therefore can be described as "expander 4 is used to expand at least a portion of the nitrogen product g3 from nitrogen line 31". Similar to the previous description of nitrogen line 31, nitrogen liquefaction unit 20 may include multiple expanders, and when it is mentioned above that nitrogen liquefaction unit 20 includes expander 4, expander 4 can be considered as one of multiple expanders. For example, Figure 2 In this context, the nitrogen liquefaction unit 20 essentially comprises two expanders, and unless further specified, expander 4 can refer to either of the two expanders. Figure 2 Another expander in nitrogen liquefaction unit 20 is designated as 4c.

[0040] A nitrogen liquefaction unit is a device that converts gaseous nitrogen into liquid nitrogen. It primarily works by compressing the gas to high pressure, removing the heat of compression using an aftercooler (e.g., a water-cooled aftercooler), and then using adiabatic expansion and / or throttling expansion to generate cooling, thus liquefying the gas. A nitrogen liquefaction unit typically includes an expander and commonly also includes a throttling valve, employing a combination of adiabatic and throttling expansion. The aforementioned nitrogen product is liquefied in the nitrogen liquefaction unit to produce a nitrogen-rich liquid stream, which, in principle, has the same molar percentage of nitrogen as the aforementioned nitrogen product.

[0041] Combination Figure 1 and Figure 2 The air separation system 100 also includes an intermediate pipeline 6. The inlet end 61 of the intermediate pipeline 6 is connected to the gas delivery pipeline 5 of the nitrogen liquefaction unit 20. (Back to...) Figure 1 The outlet end 62 of the intermediate pipeline 6 is connected to at least one of the multiple output pipelines 3 of the air separation unit 10 at the location between the distillation column system 1 and the main heat exchanger 2, thereby the intermediate pipeline 6 delivers gas flow to the aforementioned at least one output pipeline 3. In the illustrated embodiment, the aforementioned at least one output pipeline 3 may be a waste discharge pipeline 32 of the air separation unit 10 for conveying waste gas WO, such as waste nitrogen (also known as sludge nitrogen).

[0042] In the aforementioned air separation system 100, by providing an intermediate pipeline 6, the gas flow in the gas delivery pipeline 5 of the nitrogen liquefier 20 can be delivered to the output pipeline 3 of the distillation column system 1. Therefore, when starting up the air separation system 100, particularly its air separation unit 10, the gas flow that has been expanded and cooled by the expander 4 of the nitrogen liquefier 20 can be used to provide cooling to the air separation unit 10, specifically its distillation column system 1, thereby completing the startup process and enabling the air separation unit 10 to smoothly enter the normal operation phase. The aforementioned air separation system 100 is particularly suitable for applications such as… Figure 1 The air separation unit 10 shown is an air separation unit without an internal expander. Furthermore, the intermediate pipeline 6 leads to the exhaust pipeline 32 instead of the product pipeline outputting the air product, such as the nitrogen pipeline 31. This avoids interfering with the composition of the air product, such as nitrogen product g3, thus allowing for smoother production of the required air product and smoother entry into normal operation. The aforementioned air separation system 100 is also particularly suitable for applications such as… Figure 1 The air separation unit 10 shown is an air separation unit without an internal expander.

[0043] Compared to relying on stored liquid nitrogen in a backup system to start up air separation equipment, the aforementioned air separation system 100 utilizes the gas flow cooled by expansion in the expander 4 within the nitrogen liquefaction unit 20 to provide the necessary cooling capacity for startup. This allows for easier control of the cooling rate of the entire air separation system 100. This is because the temperature of the gas flow cooled by expansion in the expander 4 within the nitrogen liquefaction unit 20 gradually decreases during startup. This results in a slower initial cooling of the entire air separation system 100, followed by a gradual acceleration. This is highly beneficial for the various components within the air separation system 100, especially the heat exchanger, reducing the risk of material deformation and malfunctions caused by sudden temperature drops. Furthermore, the aforementioned air separation system 100 fully utilizes the nitrogen liquefaction unit 20, which is commonly found in many factories, significantly reducing the economic cost of storing liquid nitrogen. This is particularly advantageous for users in remote areas where access to stored liquid nitrogen is difficult.

[0044] like Figure 2 As shown, the nitrogen liquefaction unit 20 also includes an internal heat exchanger 21, a gas-liquid separator 22, and a throttle valve 23.

[0045] Figure 2 In the illustrated embodiment, gas pipeline 5 delivers gas flow g4 from expander 4 to internal heat exchanger 21 for reheating. As will be mentioned later, in the illustrated embodiment, the gas flow g5 delivered by gas pipeline 5 to internal heat exchanger 21 is at least a portion of the gas flow g22. When intermediate pipeline 6 does not split the flow, gas flow g5 is also the entirety of gas flow g22; however, when intermediate pipeline 6 splits the flow, gas flow g5 is a portion of, but not the entirety of, gas flow g22. Simultaneously, gas flow g22 includes the gas flow g4 expanded by expander 4. That is, the gas flow g5 delivered by gas pipeline 5 to internal heat exchanger 21 is not entirely the gas flow g4 expanded by expander 4. However, since at least a portion of the gas flow g4 expanded by expander 4 is delivered to internal heat exchanger 21 for reheating via gas pipeline 5, it is still permissible to describe the gas flow g4 as "delivering gas flow g4 from expander 4 to internal heat exchanger 21 for reheating" in this text.

[0046] Figure 2 In the middle, the gas-liquid separator 22 is located on the gas transmission pipeline 5 and between the expander 4 and the internal heat exchanger 21.

[0047] It is understood that the terms "pipeline," "pipe segment," and "pipeline" used in the text all refer to the route through which the flow stream passes, and do not limit the physical form of the corresponding components. Taking "pipeline" as an example, a pipeline can refer to a segment of a complete pipeline. A pipeline can also be a combination of multiple pipelines connected sequentially. These multiple pipelines can be connected by pipe fittings or by other pipeline components such as valves. The space through which the flow stream passes in the pipe fittings or other pipeline components can also be considered part of the pipeline. For example, Figure 2 In this context, the gas transmission line 5 may substantially include a pipe section 5a connecting the expander 4 and the gas-liquid separator 22, a pipe section 5b connecting the gas-liquid separator 22 and the internal heat exchanger 21, a space within the gas-liquid separator 22 through which gas flow passes, and even a space within the internal heat exchanger 21 through which gas flow g5 passes, as well as a pipe section 5c exiting from the hot end of the internal heat exchanger 21. Thus, it can be described that "the gas-liquid separator 22 is located on the gas transmission line 5".

[0048] The gas-liquid separator 22 collects the gas flow g4 expanded by the expander 4 and the flow stream f23 throttled by the throttle valve 23, and separates all the collected fluids (including the gas flow g4 and the flow stream f23) into a gas phase flow g22 and a liquid phase flow l22. It can be understood that the gas phase flow g22 is the gas flow g22 that constitutes the gas transmission pipeline 5 (specifically, pipe section 5b) transported downstream from the gas-liquid separator 22. It can be understood that when the terms "upstream" and "downstream" are used to describe relative positions, they are relative to the flow direction of the corresponding flow stream in the pipeline.

[0049] Figure 2 In this configuration, the inlet end 61 of the intermediate pipeline 6 can be connected to the gas transmission pipeline 5 between the expander 4 and the internal heat exchanger 21. Furthermore, the inlet end 61 of the intermediate pipeline 6 can be connected to the gas transmission pipeline 5 between the gas-liquid separator 22 and the internal heat exchanger 21.

[0050] like Figure 2 As shown, the nitrogen liquefaction unit 20 also includes a return line 71 and a delivery line 72. The return line 71 is used to return the first portion of the liquid stream 171 from the liquid phase stream 122 separated by the gas-liquid separator 22 back to the distillation column system 1. See further details. Figure 1 The first portion of liquid flow l71, transported by the return liquid line 71, merges into the input line p1, which is located at the highest position and supplies the stream to the distillation column system 1. In other words, the air separation unit 10 has multiple input lines that supply streams to the distillation column system 1. For example, in addition to the input line p1 that supplies stream s1, there are also input lines that supply streams s2, s4, etc. Figure 1In this system, among the multiple input lines, the input line p1 of the transport stream s1 has the highest injection height, drawing the first portion of liquid flow l71 into input line p1. The first portion of liquid flow l71 merges with the nitrogen-rich stream s1, which is about to enter the pure nitrogen column 14 as reflux liquid, and together they enter the pure nitrogen column 14, i.e., return to the distillation column system 1. This facilitates a more uniform distribution of the cooling capacity in liquid flow l71 throughout the entire distillation column system 1. Furthermore, the merging point of the return line 71 and the input line p1 is between the throttling valve 91 and the subcooler 18. That is, liquid flow l71 and the nitrogen-rich stream s1 enter the pure nitrogen column 14 after throttling, thereby reducing flash evaporation.

[0051] The inlet line 72 is used to output the second portion of the liquid stream 172 from the liquid phase stream 122 separated by the gas-liquid separator 22 to the storage tank 90. ​​That is, a portion of the liquid stream 171 from the liquid phase stream 122 is returned to the distillation column system 1 as reflux liquid, while another portion of the liquid stream 172 from the liquid phase stream 122 is directly stored in the storage tank 90. ​​This will be described later. Figure 2 In essence, the liquid phase stream l22 exiting the gas-liquid separator 22 undergoes a series of further processing, splitting into liquid stream l70 and stream f26. Liquid stream l70 is further divided into liquid stream l71 and liquid stream l72. In the illustrated embodiment, the composition of liquid stream l70 formed by liquefying nitrogen product g3 in nitrogen liquefaction unit 20 is similar to that of high-purity liquid nitrogen. Taking high-purity liquid nitrogen as an example, "high-purity" in this text refers to a nitrogen-rich liquid with a nitrogen content exceeding 99%, preferably 99.999% molar percentage.

[0052] See also Figure 2 In the nitrogen liquefaction unit 20, the outlet end 52 of the gas supply line 5 is connected to the nitrogen line 31, thereby sending the gas flow g5, which has been reheated by the internal heat exchanger 21, to the nitrogen line 31 to merge with the nitrogen product g3 in the nitrogen line 31. In the figure, the gas flow g5, which has been reheated by the internal heat exchanger 21, is sent to the high-pressure nitrogen line 31a to merge with the high-pressure nitrogen product g3a.

[0053] See also Figure 2 The air separation system 100 may also include an airflow regulating component 30, a liquid flow regulating component 40, a controller 50, and a detection element 60, etc.

[0054] The airflow regulating component 30 can be used to regulate the flow rate of the airflow g6 in the intermediate pipeline 6. The liquid flow regulating component 40 can be used to regulate the flow rate of the liquid flow l71, l72 in the return liquid pipeline 71 and / or the infusion pipeline 72. The controller 50 can control the airflow regulating component 30 and / or the liquid flow regulating component 40 according to the control signal. The detection element 60 can send a detection signal to the controller 50. The controller 50 can generate a control signal according to the detection signal of the detection element 60. That is, the controller 50 can generate a control signal according to the detection signal of the detection element 60, thereby controlling the airflow regulating component 30 and / or the liquid flow regulating component 40. Figure 2 The diagram schematically illustrates the connections between the controller 50 and the airflow regulating component 30, the liquid flow regulating component 40, etc., but does not show the connections between the controller 50 and the detection element 60, etc., to simplify the illustration. It is understood that the transmission of various signals can be achieved via wired or wireless connections.

[0055] The airflow regulating component 30 may be, for example, a switch valve, provided on the intermediate pipeline 6. By switching the switch valve on and off, the intermediate pipeline 6 can be opened or closed, thus switching the flow rate of the intermediate pipeline 6 between zero and maximum flow rates, i.e., regulating the flow rate of the airflow g6 in the intermediate pipeline 6. Alternatively, the airflow regulating component 30 may be a regulating valve whose opening can be adjusted continuously or discontinuously, wherein adjusting the opening allows the flow rate of the airflow g6 in the intermediate pipeline 6 to change continuously or discontinuously. Similarly, the liquid flow regulating component 40 may include, for example, switch valves or regulating valves respectively provided on the return liquid pipeline 71 and the delivery liquid pipeline 72. The detection element 60 may include, for example, a level sensor 68 for detecting the liquid level in the gas-liquid separator 22, or, for example, a component analyzer 63a, 63b for detecting the nitrogen purity in the nitrogen pipeline 31 of the air separation unit 10 and / or the gas-liquid separator 22 of the nitrogen liquefaction unit 20. How control is performed based on the detection signal of the detection element 60 will be described in detail later.

[0056] The present invention also provides an air separation method. The air separation method uses the air separation system 100 described above. In this air separation method, during the start-up phase ST1 of the air separation unit 10 of the air separation system 100, a gas flow g6 is delivered from the nitrogen liquefier 20 of the air separation system 100 to at least one output line 3 of the air separation unit 10 via the intermediate line 6 of the air separation system 100.

[0057] The start-up phase refers to the period when the entire air separation unit 10 has just been turned on but has not yet been able to produce qualified air products such as oxygen and nitrogen. After the start-up phase, the air separation unit 10 enters the operation phase, also known as the normal operation phase. During the operation phase, the air separation unit 10 can run continuously to produce qualified air products.

[0058] In this air separation method, at the start of the startup phase ST1, the intermediate line 6 can be opened, for example by means of a switching valve, which is an example of an airflow regulating component 30. Furthermore, the return line 71 and the delivery line 72 of the nitrogen liquefier 20 can be shut off, for example by means of a switching valve, which is an example of a liquid flow regulating component 40.

[0059] During the start-up phase ST1, the nitrogen purity in the nitrogen line 31 of the air separation unit 10 and / or the gas-liquid separator 22 of the nitrogen liquefaction unit 20 is detected, for example, by means of component analyzers 63a and 63b, respectively. The liquid level in the gas-liquid separator 22 is also detected, for example, by means of a liquid level sensor 68. When the liquid level in the gas-liquid separator 22 reaches a predetermined value, the return liquid line 71 is opened, for example, by opening a switch valve located on the return liquid line 71, which is an example of a liquid flow regulating component 40. When the nitrogen purity in the nitrogen line 31 and / or the gas-liquid separator 22 reaches the predetermined value, the start-up phase ST1 is determined to have ended, and the operation phase ST2 begins.

[0060] During operation phase ST2, the infusion line 72 is opened, for example by opening a switch valve located on the infusion line 72, which is an example of a flow regulating component 40. Meanwhile, the intermediate line 6 is closed, for example by closing a switch valve located on the airflow regulating component 30. At this time, the return line 71 is not closed; that is, the return line 71 remains open, i.e., the switch valve located on the return line 71, which is an example of a flow regulating component 40, remains open.

[0061] In the aforementioned air separation method, during the start-up phase ST1, the gas stream, expanded and cooled by the expander 4 in the nitrogen liquefier 20, is delivered to the distillation column system 1 via the intermediate pipeline 6. This allows for a continuous supply of cooling capacity to the air separation unit 10 from the moment it is started. Furthermore, during the start-up phase ST1, the liquid level is monitored, and liquid stream l71 is promptly returned to the distillation column system 1 to further provide cooling capacity. The cooling capacity supply gradually increases throughout the start-up process, avoiding waste of the cooling capacity generated by the expander 4 in the nitrogen liquefier 20 and preventing sudden cooling at startup that could lead to malfunctions. Moreover, after confirming successful entry into the operation phase ST2 by monitoring nitrogen purity, the supply of gas stream via the intermediate pipeline 6 is promptly stopped, and instead, a continuous supply of cooling capacity is provided to the distillation column system 1 using the high-purity liquid stream l71. Excess liquid stream l72 is then stored in the storage tank 90 via the liquid transfer pipeline 72. This avoids any impact on the composition of the output stream in the output pipeline 3. Therefore, the system can be smoothly switched to the operation phase of the air separation unit 10.

[0062] In this air separation method, during the start-up phase ST1, the cooling rate of a fixed point in the internal heat exchanger 21 of the nitrogen liquefaction unit 20 can also be detected. That is, for example, the temperature of the fixed point can be detected using a temperature sensor built into the internal heat exchanger 21, thus allowing the detection of the cooling rate r1, i.e., the degree of temperature decrease per unit time. Furthermore, during the start-up phase ST1, the flow rate g5 of the gas supply line 5, delivered to the nitrogen line 31 via the internal heat exchanger 21, can be adjusted so that the detected cooling rate r1 is within 2℃ / min. For example, a flow-adjustable regulating valve 51 can be installed upstream of the internal heat exchanger 21 on the gas supply line 5; by adjusting the opening of the regulating valve 51, the flow rate g5 can be controlled. The reception, processing, and control of related signals can all be achieved through the controller 50.

[0063] The cooling rate r1 at a fixed point in the internal heat exchanger 21 of the nitrogen liquefaction unit 20 can, to some extent, represent the cooling rate of any point inside the nitrogen liquefaction unit 20 and even the air separation system 100 including the air separation equipment 10. By detecting the cooling rate r1, malfunctions caused by an excessively rapid cooling rate r1 can be avoided. Furthermore, since the gas flow g5 eventually merges with the nitrogen product g3 and enters the expanders 4 and 4c of the nitrogen liquefaction unit 20 for expansion and refrigeration, reducing the flow rate of gas flow g5 can effectively reduce the flow rate of gas flow entering the expanders 4 and 4c for expansion, thereby reducing the load on the expanders 4 and 4c and thus reducing the cooling capacity or cooling rate. Therefore, this can make the temperature drop at any point in the entire system, such as a fixed point in the internal heat exchanger 21 of the nitrogen liquefaction unit 20, more slow. That is, reducing the flow rate of gas flow g5 can reduce the cooling rate r1. In essence, the gas flow g6 supplying cooling capacity to the distillation column system 1 is drawn from the gas delivery line 5. Therefore, the larger the flow rate of gas flow g6, the smaller the flow rate of gas flow g5 delivered from the gas delivery line 5 to the nitrogen line 31 via the internal heat exchanger 21. Consequently, the load on the expanders 4 and 4c downstream of the nitrogen line 31 is also smaller, resulting in less cooling capacity. Therefore, the cooling capacity supply rate can be easily adjusted according to the distribution of the flow rates of gas flow g6 and gas flow g5, thereby controlling the cooling rate r1. Controlling the cooling rate r1 can enhance the low-temperature resistance of the entire system. In particular, for the internal heat exchanger 21 of the nitrogen liquefaction unit 20, if the temperature drops too quickly, the fins inside the internal heat exchanger 21 may easily become brittle and warp. If the cooling rate r1 is controlled within 2℃ / min, the performance of the entire system in the face of temperature drops can be significantly improved, and the lifespan of each unit can be significantly extended.

[0064] In one embodiment, a temperature detector for detecting temperature can also be installed in the main heat exchanger 2 of the air separation unit 10, thereby accurately obtaining the cooling rate within the main heat exchanger 2. The cooling rate of the main heat exchanger 2 can also be adjusted by regulating the flow rate of the airflow g6. After the liquid flow l71 is generated, the cooling rate of the main heat exchanger 2 can also be adjusted by regulating the flow rate of the liquid flow l71.

[0065] The following will combine Figure 1 and Figure 2 A more detailed description of the exemplary construction and process is provided. See also Figure 1 The distillation column system 1 of the air separation unit 10 may include a high-pressure column 11 and a low-pressure column 12, and may also include a main condenser-evaporator 13 and a pure nitrogen column 14. The operating pressure of the low-pressure column 12 may be 1-2 bara, for example 1.1 bara, and the operating pressure of the high-pressure column 11 may be 4-6 bara, for example 5.5 bara. Figure 1 As shown, the low-pressure tower 12 can be vertically installed on top of the high-pressure tower 11, the main condenser-evaporator 13 can be located at the bottom of the low-pressure tower 12, and the pure nitrogen tower 14 can be located at the top of the low-pressure tower 12 and communicate with it. The main condenser-evaporator 13 can be of various types, including shell-and-tube type, falling film type, and immersion type.

[0066] Figure 1 In this air separation unit 10, in addition to producing nitrogen product g3, oxygen product gx is also produced. Oxygen product gx is drawn from the top of the main condenser-evaporator 13 and reheated via the main heat exchanger 2, and can then be discharged, for example, to the desired location. The air separation unit 10 shown in the figure does not have a usable expander installed, or is said to be internally unequipped with an expander.

[0067] The feed air g0 is fed into the main heat exchanger 2, where it undergoes indirect heat exchange with low-pressure oxygen product g3b, waste gas W0, and high-pressure nitrogen product g3a, which are derived from the top of the pure nitrogen tower 14, the top of the low-pressure tower 12, and the top of the high-pressure tower 11, respectively, after being purified by the distillation tower system 1. After cooling to, for example, approximately -173°C, it is fed into the lower part of the high-pressure tower 11 for further distillation. "Feed air" can refer to a mixture primarily containing oxygen and nitrogen, such as dry air. It is understood that "high-pressure nitrogen product" and "low-pressure nitrogen product" refer to the relatively high and low pressures of these two nitrogen products relative to each other. In practice, in this embodiment, the high-pressure nitrogen product g3a is, for example, 5.5 bara, often referred to as a low-pressure nitrogen product in engineering, while the low-pressure nitrogen product g3b is, for example, 1.1 bara, often referred to as an ultra-low-pressure nitrogen product in engineering. The waste gas W0 is, for example, 1.1 bara of polluted nitrogen, with a nitrogen content generally not less than 95% molar percentage.

[0068] It is understood that when the text describes a stream entering the first and second elements sequentially, or uses similar descriptions, it only indicates the order in which the stream enters the first and second elements. It does not preclude the possibility that the stream passes through a third element between the first and second elements, nor does it preclude the possibility that the stream passes through a third element before or after the first element. For example, before being fed into the main heat exchanger 2, although not illustrated, the raw material air g0 can be compressed by the main air compressor, for example, pressurized to approximately 6 bara, and can further undergo pre-cooling and purification.

[0069] Low-pressure oxygen product g3b and waste gas W0, respectively, from the top of the pure nitrogen tower 14 and the top of the low-pressure tower 12, are reheated in the subcooler 18. Specifically, they enter from the cold end and exit from the hot end, thus providing the cooling capacity required for heat exchange in the subcooler 18. The subcooler 18 can also subcool the oxygen-rich stream S4 drawn from the bottom of the high-pressure tower 11, which, after being subcooled by the subcooler 18, is sent to the middle of the low-pressure tower 12 as reflux liquid. The subcooler 18 can also subcool the nitrogen-rich stream S1 drawn from the top of the high-pressure tower 11, which, after being subcooled by the subcooler 18, is further throttled and sent to the pure nitrogen tower 14 as reflux liquid.

[0070] Figure 1 In addition, the oxygen is drawn from the bottom of the main condenser evaporator 13 at the bottom of the low-pressure tower 12 and subcooled by the subcooler 18 to form low-pressure high-purity liquid oxygen s3, which can be sent to a liquid oxygen storage tank for storage, for example.

[0071] See Figure 2 In addition to the aforementioned units, the nitrogen liquefaction unit 20 typically includes a compressor such as 81a. In the nitrogen liquefaction unit 20, nitrogen product g3 is typically pressurized by at least one compressor such as 81a, the heat of compression is removed by an aftercooler, and then the gas expands and cools down by generating cooling capacity through at least one expander such as 4 and optionally by throttling expansion through at least one throttling valve such as 23. The stream obtained after adiabatic expansion and throttling expansion is typically composed of two phases: gas and liquid. Therefore, gas-liquid separation is performed by at least one gas-liquid separator such as 22 to obtain a nitrogen-rich liquid stream.

[0072] In this embodiment, the nitrogen liquefaction unit further includes a booster 82 and a booster 83. Expanders 4 and 4c provide power to boosters 82 and 83, respectively. The high-pressure nitrogen product g3a produced by the air separation unit 10 is delivered to compressor 81a of the nitrogen liquefaction unit 20. In the nitrogen liquefaction unit 20, the nitrogen-rich gas flow g30 containing the high-pressure nitrogen product g3a is pressurized by compressor 81a, for example, to 30 bara. The nitrogen-rich gas flow g30 after being pressurized by compressor 81a is divided into two parts, namely, a first nitrogen-rich gas flow g31 and a second nitrogen-rich gas flow g32.

[0073] The first part of the nitrogen-rich gas flow g31 continues to enter the booster 83 and booster 82 for pressurization, for example, to 65 bara. After being cooled by aftercoolers 802 and 803, it enters the internal heat exchanger 21 and is divided into two streams, namely stream g4 (directly referred to as gas flow g4 in the text) and stream f23.

[0074] The gas flow g4 is drawn out from the middle position of the internal heat exchanger 21 and enters the expander 4 for expansion, for example, to reduce the pressure to 5.5 bara. The gas flow g4 after expansion by the expander 4 is transported to the gas-liquid separator 22 through the section 5a of the gas transmission pipeline 5.

[0075] After being cooled by the internal heat exchanger 21, the flow stream f23 is throttled by the throttling valve 23, for example, its pressure is reduced to 5.5 bara, and it is also sent to the gas-liquid separator 22.

[0076] The gas and liquid phases of the streams (including gas stream g4 and stream f23) fed into the gas-liquid separator 22 are separated in the gas-liquid separator 22 to form a gas stream g22 and a liquid stream l22. At least a portion of the gas stream g22 (referred directly to as gas stream g5 in the text) is returned to the internal heat exchanger 21 for reheating via a section 5b of the gas delivery line 5, and then combined with the high-pressure nitrogen product g3a and delivered to the inlet of the compressor 81a. An optional portion of the gas stream g22 (referred to as gas stream g6 in the text) can be delivered to the distillation column system 1 of the air separation unit 10 via the intermediate line 6 during the start-up phase ST1.

[0077] The second nitrogen-rich gas stream g32, after being partially cooled by the internal heat exchanger 21, is drawn out from the middle position of the internal heat exchanger 21 and then enters the expander 4c for expansion, for example, depressurization to approximately 5.5 bara. Afterwards, the expanded nitrogen-rich gas stream g32 returns from the middle position to the internal heat exchanger 21, where it merges with the gas stream g5 and is then reheated together. The nitrogen-rich gas stream g30 consists at least of the high-pressure nitrogen product g3a, the gas stream g5, and the second nitrogen-rich gas stream g32. Incidentally, due to the circulation effect, the sum of the mass flow rates of the gas stream g5 and the second nitrogen-rich gas stream g32 is actually greater than that of the nitrogen product g3; therefore, the actual mass flow rate of the nitrogen-rich gas stream g30 can even reach several times, for example, six times, the mass flow rate of the nitrogen product g3.

[0078] Figure 2The nitrogen liquefaction unit also includes another compressor 81b, another internal heat exchanger 25, and another gas-liquid separator 26. The low-pressure nitrogen product g3b produced by the air separation unit 10 is fed to the compressor 81b of the nitrogen liquefaction unit 20. In the nitrogen liquefaction unit 20, a nitrogen-rich gas flow g30c containing the low-pressure nitrogen product g3b is pressurized by the compressor 81b, for example, to 5.5 bara, and then merged with the gas flow g5 reheated by the internal heat exchanger 21, the second nitrogen-rich gas flow g32, and then with the high-pressure nitrogen product g3a, and together they are fed to the inlet of the compressor 81a.

[0079] At least a portion of the liquid phase flow l22, stream f26, is throttled, for example, and depressurized to approximately 1.1 bara, before being conveyed to the gas-liquid separator 26. In the gas-liquid separator 26, the gas and liquid phases are separated to form a gas phase flow g26 and a liquid phase flow l26. The gas phase flow g26 returns to the internal heat exchanger 21 for reheating, and then merges with the low-pressure nitrogen product g3b, together being conveyed to the inlet of the compressor 81b. The liquid phase flow l26, after exchanging heat with the liquid phase flow l22 in the internal heat exchanger 25, merges with the gas phase flow g26 and returns to the internal heat exchanger 21. The nitrogen-rich gas flow g30c consists at least of the low-pressure nitrogen product g3b, the gas phase flow g26, and the liquid phase flow l26 reheated by the internal heat exchanger 25.

[0080] High-pressure nitrogen product g3a and low-pressure nitrogen product g3b are liquefied in nitrogen liquefaction unit 20 to produce a nitrogen-rich liquid stream (mainly composed of liquid phase stream l22). During normal operation, a portion of the liquid stream l71 is returned to the air separation unit 10, specifically the distillation column system 1, while another portion of the liquid stream l72 is introduced into, for example, a storage tank 90 for storage. Considering that the nitrogen-rich liquid stream may lose cooling and vaporize during long-distance transportation to the liquid nitrogen storage tank, an internal heat exchanger 25 and a gas-liquid separator 26 are added, primarily to further cool the liquid stream l72.

[0081] Combination Figure 1 and Figure 2High-pressure nitrogen product g3a (5.5 bara) from the top of high-pressure tower 11 and low-pressure nitrogen product g3b (1.1 bara) from pure nitrogen tower 14 are reheated by the main heat exchanger 2 and then supplied to the inlets of compressors 81a and 81b of nitrogen liquefaction unit 20, respectively. During the start-up phase ST1, a portion of the gas flow g6, cooled by the expander in nitrogen liquefaction unit 20, from the high-pressure nitrogen product g3a and low-pressure nitrogen product g3b, merges with the exhaust gas W0 output from the top of low-pressure tower 12, thus providing cooling when it enters the subcooler 18 and main heat exchanger 2 for reheating along with the exhaust gas W0. Furthermore, after a period of time during the start-up phase ST1, a liquid flow l71 begins to form in nitrogen liquefaction unit 20. This liquid flow l71 merges with the nitrogen-rich stream s1 drawn from the top of high-pressure tower 11 and is sent to the top of pure nitrogen tower 14, also providing cooling. During the subsequent normal operation phase ST2, high-pressure nitrogen product g3a and low-pressure nitrogen product g3b are liquefied in nitrogen liquefaction unit 20 to generate nitrogen-rich liquid stream l70. Part of the nitrogen-rich liquid stream l71 is throttled and merged with nitrogen-rich stream s1 and sent into pure nitrogen tower 14, while the other part of the nitrogen-rich liquid stream l72 is introduced into liquid nitrogen storage tank 90.

[0082] Unless otherwise clearly indicated, each aspect or embodiment defined herein may be combined with any other aspect or embodiment. In particular, any feature indicated as preferred or advantageous may be combined with any other feature indicated as preferred or advantageous.

[0083] The embodiments described in this specification are merely preferred embodiments of the present invention, and are only used to illustrate the technical solutions of the present invention and not to limit the present invention. Any technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation should be within the scope of the present invention.

Claims

1. An air separation method using an air separation system comprising an air separation unit and a nitrogen liquefaction unit, The air separation unit includes a distillation column system, a main heat exchanger, and multiple output pipelines. The multiple output pipelines are led out from the distillation column system via the main heat exchanger, and include a nitrogen pipeline for outputting nitrogen products. The nitrogen liquefaction unit includes an expander, an internal heat exchanger, a gas-liquid separator, a throttling valve, a gas delivery line, a liquid return line, and a liquid delivery line. The expander expands at least a portion of the nitrogen product from the nitrogen delivery line. The gas delivery line transports the gas stream expanded by the expander to the internal heat exchanger for reheating. The gas-liquid separator is located on the gas delivery line and between the expander and the internal heat exchanger. The gas-liquid separator collects the gas stream expanded by the expander and the stream throttled by the throttling valve, and separates all collected fluids into a gas phase flow and a liquid phase flow. The liquid return line returns a first portion of the liquid phase flow separated by the gas-liquid separator to the distillation column system. The liquid delivery line outputs a second portion of the liquid phase flow separated by the gas-liquid separator to a storage tank. Its features are, In the air separation method, the air separation system further includes an intermediate pipeline, the inlet end of which is connected to the gas supply pipeline of the nitrogen liquefaction unit, and the outlet end of which is connected to at least one of the plurality of output pipelines of the air separation unit at the location between the distillation column system and the main heat exchanger. At the start-up phase, the intermediate pipeline is opened to supply gas flow from the nitrogen liquefaction unit to at least one output pipeline of the air separation unit, and the return and delivery pipelines of the nitrogen liquefaction unit are shut off. During the startup phase, the nitrogen purity in the nitrogen pipeline of the air separation unit and / or the gas-liquid separator of the nitrogen liquefaction unit is detected, and the liquid level in the gas-liquid separator is also detected. When the liquid level in the gas-liquid separator reaches a predetermined value, the return liquid pipeline is opened. When the nitrogen purity in the nitrogen pipeline and / or the gas-liquid separator reaches a predetermined value, the start-up phase is determined to be over and the operation phase begins. During the operation phase, the infusion line is opened and the intermediate line is closed.

2. The space separation method as described in claim 1, characterized in that, in, During the startup phase, the cooling rate of a fixed point in the internal heat exchanger of the nitrogen liquefier is also detected, and the flow rate of the gas supply line sent to the nitrogen line through the internal heat exchanger is adjusted so that the detected cooling rate is within 2°C / min.

3. The air separation method as described in claim 1 or 2, characterized in that, This allows the inlet end of the intermediate pipeline to be connected to the gas transmission pipeline at a position between the expander and the internal heat exchanger.

4. The space separation method as described in claim 3, characterized in that, This allows the inlet end of the intermediate pipeline to be connected to the gas transmission pipeline at a position between the gas-liquid separator and the internal heat exchanger.

5. The air separation method as described in claim 1 or 2, characterized in that, In the nitrogen liquefaction unit, the outlet end of the gas delivery pipeline is connected to the nitrogen pipeline, thereby sending the gas flow that has been reheated by the internal heat exchanger to the nitrogen pipeline and merging with the nitrogen product of the nitrogen pipeline.

6. The space separation method as described in claim 5, characterized in that, The air separation system further includes: An airflow regulating component is used to regulate the flow rate of the airflow in the intermediate pipeline; A flow regulating component is used to regulate the flow rate of the return line and / or the delivery line; The controller controls the airflow regulating component and / or the liquid flow regulating component according to the control signal; and The detection element sends a detection signal to the controller, and the controller generates the control signal based on the detection signal from the detection element.

7. The air separation method as described in claim 1 or 2, characterized in that, This ensures that the at least one output pipeline is a waste discharge pipeline of the air separation unit for conveying waste gas.

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