Automatic control method for main heat exchanger of low-pressure inner compression process air separation device

Abstract

The application discloses a kind of low-pressure inner compression process air separation device main heat exchanger automatic control method, it is related to computer processing technical field, comprising the following steps: S01, equipment is in initial state, whether it is satisfied cold box start step condition is judged automatically;S02, when it is satisfied cold box start step condition respectively executes corresponding step;S03, in positive flow low-pressure air and return flow dirty nitrogen gas heat exchange flow path stable establishment, automatic step starts expander refrigeration system;S04, after expander starts completion, automatically put into product material flow path, and according to product load variation, based on preset pinch point temperature difference model, dynamically adjusts the operating parameter of positive flow high-pressure air.The application is based on pinch point temperature difference, average integral temperature difference and other parameters dynamically adjust, realizes heat exchange optimum working condition point tracking, and links with front-end process system, ensures safe recovery of cold quantity while improving heat exchange efficiency, avoids cold brittle accident.

Description

Technical Field

[0001] This invention relates to the field of computer processing technology, and in particular to an automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit. Background Technology

[0002] In air separation units with low-pressure internal compression processes, the main heat exchanger is one of the key core components. Its function is to achieve heat exchange between the forward-flowing high-pressure air and the refluxing cryogenic liquid (such as liquid oxygen, liquid nitrogen) and cryogenic gas, liquefying the forward-flowing air and simultaneously reheating the refluxing fluid to recover cold energy. The hot-end process systems of the air separation unit (such as air compression and purification units) typically use carbon steel piping, while the cold-end process systems (main heat exchanger, distillation column, etc.) are located in a cold box. Because carbon steel has a cryogenic brittle transition temperature, when the temperature of the process gas flowing through the piping falls below this critical value, the piping is at serious risk of brittle fracture, posing a significant threat to equipment and personnel safety. During unit start-up, shutdown, or unstable operation phases, excessively low reflux gas temperature is the main cause of cryogenic brittle fracture accidents in the carbon steel piping at the cold box outlet.

[0003] Furthermore, the heat exchange efficiency of the main heat exchanger directly determines the energy consumption and product output of the unit. Under traditional operating modes, both the commonly relied-upon manual operation in China and the fixed protection value interlocking methods used in some foreign countries have significant shortcomings. Manual operation demands extremely high levels of experience and responsibility from operators. With numerous valves and complex timing sequences during startup, improper operation (such as incorrect use of high-pressure air for pressure equalization) can easily lead to an imbalance in the material heat exchange ratio of the main heat exchanger. This can not only cause pipeline embrittlement accidents but also fail to ensure the heat exchanger operates at its optimal design point, resulting in wasted cooling capacity and increased energy consumption. On the other hand, fixed interlocking protection strategies that do not consider environmental factors and dynamic operating conditions may be overly conservative, affecting the normal start-up of the unit, or failing under extreme conditions.

[0004] Therefore, how to fundamentally avoid the risk of low-temperature embrittlement of carbon steel pipes at the cold box outlet due to operational errors or fluctuations in operating conditions throughout the entire life cycle of an internal compression process air separation unit, while ensuring that the main heat exchanger always operates near the optimal heat exchange point designed in order to achieve a balance between safety and energy efficiency, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention provides an automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit, comprising the following steps: S01. After the equipment is in the initial state, it automatically determines whether the cold box start-up step conditions are met. The conditions include that the positive flow raw material air pressure at the cold box inlet is greater than the lower tower pressure of the cold end process system. S02. Once the cold box start-up stepping conditions are met, execute the following sub-steps respectively: S21. The solenoid valves of the relevant regulating valves of the main heat exchanger are automatically energized to control the positive flow low-pressure air and the return flow sludge nitrogen. S22. Automatically control the regulating valve for the sludge nitrogen gas flowing back from the main heat exchanger to the hot-end process system to stabilize the pressure at the top of the tower; S23. Automatically control the opening of the raw material air bypass valve at the cold box inlet at a set rate to equalize the pressure in the lower tower; S24. When the pressure difference between the lower tower pressure and the raw material air pressure at the cold box inlet is detected to be less than the set threshold, the main air flow path valve of the cold box is automatically controlled to open at the set rate. S25. When the main air flow path valve opening reaches the set value, the regulating valves of each process flow path from the lower tower to the upper tower are automatically controlled to open at the set rate to realize the output of materials from the upper tower. S03. After the heat exchange flow path between the positive flow low-pressure air and the return flow polluted nitrogen is stably established, the expansion mechanism refrigeration system is automatically started in a stepwise manner. S04. After the expander starts up, it automatically enters the product material flow path and dynamically adjusts the operating parameters of the positive flow high-pressure air according to the product load changes and based on the preset pinch temperature difference model, so as to maintain the optimal heat exchange conditions of the main heat exchanger.

[0006] Preferably, the cold box start-up step condition in step S01 also includes a logical judgment to prevent the backflow of low-temperature materials in the distillation column, ensuring that the pressure in the distillation column is not higher than the pressure of the forward air flow before the forward air channel is established.

[0007] As a preferred embodiment, a dynamic correction step for the low-temperature brittleness interlock protection value is also included: S100. Based on the ambient temperature and device startup process obtained from the detection, dynamically adjust the low-temperature interlock protection setting value for the return gas exiting the cold box; S101. When the ambient temperature is lower than the preset value or the device is in a specific start-up mode, the interlock protection value is corrected to a safer temporary value through the RS trigger logic. S102. After all heat exchange material flow paths of the main heat exchanger have been established and steady-state heat exchange conditions have been reached, the interlock protection value is reset to the normal operating value.

[0008] Preferably, the step S04, which involves dynamically adjusting the operating parameters of the positive-flow high-pressure air based on a preset pinch temperature difference model, includes: S41. Real-time monitoring of the product oxygen outlet pressure Po2 in the cold box; based on the design pinch point temperature difference dTo2, calculate the corresponding target pressure P of the positive flow high-pressure air. air ; S42, the target pressure P airThe outlet pressure of the front-end booster is automatically adjusted as a set value.

[0009] Preferably, the target pressure P of the positive flow high-pressure air air It is obtained through calculation using the following relation: S421. Calculate the target saturation temperature of positive-current high-pressure air using the oxygen bubble point equation. ; S422. Calculate the target pressure using the bubble point equation for air. .

[0010] Preferably, when the main heat exchanger also includes a high-pressure nitrogen product flow path, step S04 further includes: S43. Real-time monitoring of the product nitrogen outlet pressure Pn2 in the cold box; based on the design pinch point temperature difference dTn2, calculate the target pressure P of another positive-flow high-pressure air. airN ; S44. Take the target pressure PairO2 calculated from the oxygen product pressure and the target pressure P calculated from the nitrogen product pressure. airN The highest value in the value is used as the final set point for the booster.

[0011] Preferably, the process also includes a real-time monitoring step for the heat exchanger's operating conditions: real-time acquisition of the hot and cold end temperatures of the forward-flowing high-pressure air and the reverse-flowing high-pressure oxygen; calculation of the integral average temperature difference (LMTD) of the main heat exchanger; comparison with the design value to evaluate the current heat exchange conditions; and issuance of an alarm when the temperature deviates from a preset threshold. The formula for calculating the integral average temperature difference (LMTD) is as follows: ; Wherein, Thotair is the temperature of the positive flow high-pressure air entering the main heat exchanger, Thoto2 is the temperature of the negative flow high-pressure oxygen exiting the main heat exchanger, Tcoldair is the temperature of the positive flow high-pressure liquid air exiting the main heat exchanger, and Tcoldo2 is the temperature of the negative flow high-pressure liquid oxygen entering the main heat exchanger.

[0012] Preferably, in step S04, while maintaining the optimal heat exchange conditions of the main heat exchanger, the positive flow high-pressure air flow rate F is also monitored based on a preset material balance coefficient K. air The ratio of the return product flow rate Fproduct to the design pinch point temperature difference dT is ensured to match.

[0013] The present invention has at least the following beneficial effects: 1. By automatically matching the flow path establishment and valve opening of the forward air and the return waste nitrogen, it is ensured that the material ratio is as close as possible to the design value during the initial non-steady-state heat exchange stage, effectively preventing the risk of cold embrittlement caused by insufficient heat exchange and excessively low temperature of the return gas.

[0014] 2. It can automatically and in real-time calculate the optimal target high-pressure air pressure based on fluctuations in the oxygen or nitrogen pressure of the product, and control the booster compressor in a closed loop, thereby ensuring that the main heat exchanger always operates near the design pinch temperature difference. This not only maximizes the recovery of cold energy and reduces the energy consumption of the unit, but also maintains the stability of the internal temperature field of the main heat exchanger, ensuring that the temperature of each return gas exiting the cold box is always within a safe and reasonable range, thus balancing operational economy and reliability.

[0015] 3. It enables real-time monitoring, automatic optimization, and anomaly warning of the main heat exchanger's operating conditions, freeing operators from tedious and high-risk manual operations and transforming them into process monitors. Furthermore, by providing comprehensive performance indicators such as the integral average temperature difference, it offers a more comprehensive and accurate assessment than traditional methods that only focus on the hot-end temperature difference, providing operators with a scientific basis for evaluating and maintaining heat exchanger performance. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A flowchart provided for an embodiment of the present invention; Figure 2 The logic function control diagram provided for embodiments of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "including," "having," and any variations are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or server that includes a series of steps or units is not necessarily limited to those explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or devices.

[0020] Example 1

[0021] This embodiment provides an automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit. The method includes the following steps: Figure 1 As shown: S01. After the equipment is in the initial state, it automatically determines whether the cold box start-up step conditions are met. The conditions include that the positive flow raw material air pressure at the cold box inlet is greater than the lower tower pressure of the cold end process system. Furthermore, the start-up conditions for the cold box also include a logical judgment to prevent the backflow of low-temperature materials in the distillation column, ensuring that the pressure inside the distillation column is not higher than the pressure of the forward air flow before the forward air channel is established.

[0022] Specifically, the system operates by acquiring two key pressure values ​​in real time via a pressure transmitter: one is the pressure inside the positive flow raw material air pipe at the cold box inlet (P... airin Secondly, the pressure at the bottom of the distillation column in the cold-end process system (P) lowercol The system internally configures a comparison logic module, which only compares when P... airin Greater than P lowercol This basic condition is only met if this state remains stable for a certain period of time (e.g., 3-5 seconds to prevent signal jitter).

[0023] However, simply comparing pressure is not enough; it is also necessary to prevent the backflow of low-temperature materials due to incorrect operating sequence or internal leakage of valves.

[0024] To prevent backflow of the low-temperature material within the distillation column, the pressure trend in the lower column is checked before opening the main valve (usually a pneumatic shut-off valve or regulating valve) for the positive air inlet to the cold box. If the system detects that the lower column pressure is not only higher than the positive air pressure but is also continuously rising (which may indicate that the liquid inside the column is evaporating rapidly or that high-pressure gas has accidentally entered), or if the valve position feedback of some key valves connected to the lower column is abnormal, the system will determine that there is a risk of backflow, thereby locking the opening command of the main valve and potentially triggering an alarm.

[0025] For example, suppose an air separation unit is being restarted after a temporary shutdown. If the operator mistakenly opens a valve on the pressurized air line prematurely, high-pressure air (e.g., 8 barg) will enter the lower column of the distillation column, causing the pressure to instantly rise to 6 barg. At this time, the positive flow feed air pressure at the cold box inlet is only 5 barg. When the judgment in step S01 is executed, it will detect P... airin (5 barg) <P lowercol (6 barg) Therefore, the pressure is not met, and the program will remain at step S01 without proceeding to the next step. Simultaneously, the system's internal backflow protection logic detects that the lower tower pressure is significantly higher than the forward air pressure and is at a high level, identifying this high-pressure source as a potential reverse drive source. Because the startup conditions are not met, the solenoid valve of the forward air inlet main valve remains de-energized and closed, thus completely cutting off the physical channel for the cryogenic medium to backflow out of the cold box through the forward air pipeline. Only after the operator corrects the misoperation and the lower tower pressure is reduced to 4.5 barg through normal venting and other means, and stabilizes below the forward air pressure (5 barg), will the conditions be deemed met, allowing the program to proceed to the next startup step.

[0026] S02. Once the cold box start-up stepping conditions are met, execute the following sub-steps respectively: S21. The solenoid valves of the relevant regulating valves of the main heat exchanger are automatically energized to control the positive flow low-pressure air and the return flow sludge nitrogen. S22. Automatically control the regulating valve for the sludge nitrogen gas flowing back from the main heat exchanger to the hot-end process system to stabilize the pressure at the top of the tower; S23. Automatically control the opening of the raw material air bypass valve at the cold box inlet at a set rate to equalize the pressure in the lower tower; S24. When the pressure difference between the lower tower pressure and the raw material air pressure at the cold box inlet is detected to be less than the set threshold, the main air flow path valve of the cold box is automatically controlled to open at the set rate. S25. When the main air flow path valve opening reaches the set value, the regulating valves of each process flow path from the lower tower to the upper tower are automatically controlled to open at the set rate to realize the output of materials from the upper tower. Specifically, such as Figure 2 As shown, during the start-up of the air separation unit, after the safety interlock conditions are met, the DCS system automatically and orderly establishes the initial heat exchange flow path of the main heat exchanger, namely the circulation path of the forward low-pressure air and the return waste nitrogen. This process completely replaces traditional manual operation to ensure the safety and rationality of heat exchange matching during the start-up phase. When the DCS system determines that the cold box start-up stepping conditions (such as the forward air pressure being higher than the lower column pressure) are met, the system immediately enters the S02 stepping stage.

[0027] First, sub-step S21 is executed, which automatically energizes the solenoid valves of the relevant regulating valves for the positive flow low-pressure air and the return flow waste nitrogen exiting the main heat exchanger. In the DCS sequential control program, this step sends a 24V DC signal to the solenoid valve of the designated valve via the digital output module, enabling the valve to receive regulating signals. Taking a 60,000-class air separation unit as an example, the key valves involved include the solenoid valves HV-101 (positive flow air inlet to cold box shut-off valve) and HV-102 (waste nitrogen outlet to cold box regulating valve). After the solenoid valves are energized, the air path is opened, preparing for subsequent valve actions; this is a prerequisite for establishing the entire flow path.

[0028] The process then proceeds to sub-step S22, which involves automatically controlling the regulating valve for the refluxed nitrogen gas from the main heat exchanger to the hot-end process system to stabilize the pressure at the top of the column. The DCS system, using a PID control algorithm, automatically adjusts the opening of the regulating valve (e.g., PV-102) for the refluxed nitrogen gas exiting the cold box based on real-time feedback from the pressure transmitter (PT-201) located at the top of the column. For example, if the setpoint for the top column pressure is 45 kPa(A), when the actual pressure rises to 48 kPa, the DCS will automatically calculate and output a 4-20 mA signal to open the valve by 5%, allowing more refluxed nitrogen gas to escape, thus maintaining stable pressure at the top column. This step ensures that the core pressure parameters of the distillation system do not fluctuate drastically during subsequent material introduction.

[0029] Next, sub-step S23 is executed, which automatically controls the cold box inlet raw material air bypass valve to open at a set rate to equalize the pressure in the lower column. The DCS controls the bypass valve (such as HV-103) actuator through an analog output module, opening it slowly according to a preset rate ramp function. Typically, the opening rate is set to 10% per minute, increasing by 1% in each step. For example, during a cold start-up of the unit, the lower column pressure is 0 kPa(G), while the forward air pressure is 520 kPa(G). Directly opening the main air valve would cause a huge impact. By slowly opening the bypass valve, the lower column pressure rises steadily at a rate of approximately 50 kPa / min, achieving gradual pressure equalization with the forward air network and preventing airflow impact on the column internals and heat exchanger.

[0030] When the pressure equalization process reaches a critical point, it enters sub-step S24. Specifically, when the pressure difference between the lower tower pressure and the raw material air pressure at the cold box inlet is detected to be less than a set threshold, the main air flow path valve of the cold box is automatically controlled to open at a set rate. The DCS continuously collects data from the lower tower pressure transmitter (PT-301) and the air inlet cold box pressure transmitter (PT-101) to calculate the pressure difference. The set threshold is typically 120 kPa (for high-pressure start-up) or a more stringent value (e.g., 50 kPa for low-pressure start-up). Taking low-pressure start-up as an example, when the DCS detects that the pressure difference has dropped below 50 kPa, the program automatically triggers, controlling the main air flow path valve (e.g., HV-104) to begin opening at a rate of 15% per minute. For example, in a certain start-up, the valve opening smoothly increases from 0% to 30% within 50 seconds, achieving a large flow rate in the main flow path while keeping pressure fluctuations within 20 kPa.

[0031] Finally, sub-step S25 is executed. Once the main air flow path valve opening reaches the set value, the automatic control of the regulating valves in each process flow path from the lower to the upper column opens at a set rate, enabling material output from the upper column. The DCS monitors the opening feedback of the main air valve (HV-104). When this value exceeds the set value (e.g., 80%) and remains stable, the program automatically progresses. At this time, the DCS begins to apply control signals to the liquid-air regulating valves (e.g., LV-201) and waste-liquid nitrogen regulating valves (e.g., LV-202) from the lower to the upper column. These valves also open gradually at a preset rate (e.g., 5% per minute), sending the initially separated liquid material from the lower column into the upper column. For example, during the process of LV-201 opening from 0% to 30%, the temperature monitoring point (TI-401) in the middle of the upper column shows the temperature gradually decreasing from -170℃ to -178℃, indicating that the material has been normally introduced and the distillation conditions have begun to be established.

[0032] S03. After the heat exchange flow path between the positive flow low-pressure air and the return flow polluted nitrogen is stably established, the expansion mechanism refrigeration system is automatically started in a stepwise manner.

[0033] Specifically, firstly, the DCS automatically detects the status of the expander inlet valve, sealing gas pressure, lubrication system, and other auxiliary conditions to confirm that startup is permitted. Subsequently, the DCS automatically opens the expander inlet guide vane or regulating valve at a set rate, introducing high-pressure air from the booster end. The expander starts operating under pressure differential, converting the internal energy of the high-pressure air into mechanical energy (driving the booster end). At the same time, the gas undergoes adiabatic expansion, generating a large amount of cooling energy, causing the temperature to drop sharply. This portion of low-temperature expanded air is introduced into the upper column of the distillation column to participate in the distillation process. Simultaneously, its cooling energy, through the contact between the rising vapor and the flowing liquid in the column, effectively suppresses the violent evaporation of the liquid caused by thermal imbalance in the initial startup phase. The DCS monitors parameters such as expander speed, bearing temperature, outlet temperature, and pressure throughout the process to ensure that the equipment operates within a safe range.

[0034] It should be noted that in step S02, although a heat exchange channel is established between the forward air and the reflux nitrogen, a stable temperature and concentration gradient has not yet been established within the distillation column. The low-temperature liquid (from the previous cooling stage) in the column bottom and main condenser / evaporator will continue to evaporate due to external heat intrusion or pressure fluctuations, generating a large amount of gas. If an external cold source is not introduced in time to suppress this evaporation process, it will lead to increased column pressure, unbalanced heat exchanger load, drastic temperature fluctuations in the reflux gas, and even trigger the safety valve to trip. After the expander starts, the low-temperature gas it generates enters the upper column, directly supplementing the cooling capacity within the column and promoting the condensation of the rising vapor, thus stabilizing the gas-liquid balance within the column. At the same time, the introduction of expanded air also changes the material ratio of the forward and reflux in the main heat exchanger—a pressurized air flow path is added to the forward flow, and an expanded air flow path is added to the reflux, allowing the main heat exchanger to gradually transition to its design operating conditions.

[0035] S04. After the expander starts up, it automatically enters the product material flow path and dynamically adjusts the operating parameters of the positive flow high-pressure air according to the product load changes and based on the preset pinch temperature difference model, so as to maintain the optimal heat exchange conditions of the main heat exchanger.

[0036] The above-mentioned dynamic adjustment of the operating parameters of the positive flow high-pressure air based on the preset pinch temperature difference model includes: S41. Real-time monitoring of the oxygen outlet pressure P of the product in the cooling box. o2 Based on the design pinch temperature difference dTo2, the target pressure P of the corresponding positive flow high-pressure air is calculated. air ; S42, Target pressure P air The outlet pressure of the front-end booster is automatically adjusted as a set value.

[0037] 5. An automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit according to claim 4, characterized in that the target pressure P of the positive flow high-pressure air is... air It is obtained through calculation using the following relation: S421. Calculate the target saturation temperature of positive-current high-pressure air using the oxygen bubble point equation. ; S422. Calculate the target pressure using the bubble point equation for air. .

[0038] Furthermore, when the main heat exchanger also includes a high-pressure nitrogen product flow path, step S04 further includes: S43. Real-time monitoring of the product nitrogen outlet pressure P in the cold box. n2 Based on the design pinch temperature difference dTn2, the target pressure P of another positive-flow high-pressure air is calculated. airN ; S44. Take the target pressure PairO2 calculated from the oxygen product pressure and the target pressure P calculated from the nitrogen product pressure. airN The highest value in the value is used as the final set point for the booster.

[0039] Specifically, the DCS system collects the pressure Po2 of the oxygen outlet in the cold box in real time. According to thermodynamic principles, under gas-liquid equilibrium, there is a one-to-one functional relationship between the saturation temperature and pressure of pure oxygen, i.e., the oxygen bubble point equation. Substituting the detected pressure Po2 into the oxygen bubble point equation, the temperature of the liquid oxygen evaporation section can be calculated at the current pressure. To maintain the pinch temperature difference at the design value dTo2, the liquefaction temperature of the positive-flow high-pressure air in the phase change zone must be higher than this evaporation temperature by dTo2. Therefore, adding the calculated liquid oxygen evaporation temperature to the design pinch temperature difference dTo2 yields the target saturation temperature Tpsatair required for the positive-flow high-pressure air. After obtaining this crucial target temperature, the control logic then calls the air bubble point equation to deduce the target pressure Pair that the positive-flow high-pressure air needs to reach to liquefy at this temperature Tpsatair. This Pair value is the high-pressure air pressure necessary to maintain the design pinch temperature difference. Finally, the DCS system uses this calculated Pair value as a new setpoint and outputs it to the control system of the front-end booster, automatically adjusting its outlet pressure to form a closed-loop optimized control.

[0040] For more complex operating conditions, when the main heat exchanger simultaneously handles the high-pressure nitrogen product flow path, the control strategy is further upgraded to multivariate coordinated optimization. This is achieved by the system executing the two sets of calculation logics mentioned above in parallel. In addition to calculating a target pressure PairO2 based on the oxygen pressure Po2, it also calculates the target high-pressure air pressure PairN required to meet the heat exchange requirements of the nitrogen product, based on the detected product nitrogen outlet cold box pressure Pn2 (high-pressure nitrogen pressure) and the corresponding design pinch point temperature difference dTn2, using the bubble point equations for nitrogen and air. Since high-pressure air is a common heat exchange medium, it must simultaneously meet the heat exchange requirements of both oxygen and nitrogen. According to the heat exchanger design principle, the flow path with the higher required pressure represents a more stringent heat exchange requirement (requiring a higher condensing temperature to match a higher evaporating temperature). Therefore, a high-pressure selector is set up within the DCS system to automatically compare PairO2 and PairN, and take the larger value as the final setpoint of the booster compressor. This ensures that while meeting the heat exchange requirements of all products, the overall temperature field distribution of the main heat exchanger remains within the design range.

[0041] Example 2

[0042] Based on the above embodiment one, this embodiment also includes dynamic correction of the low-temperature brittleness interlock protection value, the steps of which are as follows: S100. Based on the ambient temperature and device startup process obtained from the detection, dynamically adjust the low-temperature interlock protection setting value for the return gas exiting the cold box; S101. When the ambient temperature is lower than the preset value or the device is in a specific start-up mode, the interlock protection value is corrected to a safer temporary value through the RS trigger logic. S102. After all heat exchange material flow paths of the main heat exchanger have been established and steady-state heat exchange conditions have been reached, reset the interlock protection value to the normal operating value.

[0043] Specifically, the DCS system collects data from ambient temperature monitoring instruments in real time and monitors the startup process status of the device, such as whether it is in the stage of establishing the flow path between the positive flow low-pressure air and the return flow polluted nitrogen in step S02, and whether the expander has started. When the ambient temperature is detected to be lower than the preset value (e.g., -18℃) or the operator activates the winter startup cold box mode through the human-machine interface, the system determines that it is currently in a high-risk startup condition. At this time, the DCS uses the internally set RS trigger logic to dynamically correct the low-temperature interlock protection value of the carbon steel pipelines for the return flow polluted nitrogen and atmospheric pressure nitrogen exiting the cold box from the design value under normal operating conditions (e.g., -20℃) to a safer temporary value (e.g., -30℃). The reason for temporarily lowering the protection value is that, during the initial startup phase, a stable process gas flow has not yet been established in the relevant carbon steel pipelines, which are under normal or slightly positive pressure. Based on the mechanical properties of materials, the minimum allowable operating temperature of carbon steel under no-pressure or low-pressure conditions is typically lower than the brittle transition temperature under pressure. Therefore, temporarily adopting a lower protection value provides a greater safety buffer against unpredictable cooling fluctuations during startup, while preventing frequent erroneous shutdowns due to overly sensitive interlocks. Once the DCS system confirms through steps S02 and S03 that all heat exchange material flow paths in the main heat exchanger (including subsequently introduced high-pressure oxygen and high-pressure nitrogen) have been established and reached steady-state heat exchange conditions, the RS trigger resets, and the system automatically restores the low-temperature interlock protection value to the normal operating value (e.g., -20℃) that matches the designed pressure conditions, thus continuing to provide accurate and reliable safety protection during normal production.

[0044] Example 3

[0045] Based on the above embodiment one, this example also includes real-time monitoring of the heat exchange condition of the main heat exchanger, the steps of which are as follows: real-time acquisition of the hot end temperature and cold end temperature of the forward high-pressure air and the reverse high-pressure oxygen, calculation of the integral average temperature difference (LMTD) of the main heat exchanger, and comparison with the design value to evaluate the quality of the current heat exchange condition, and issuance of an alarm when it deviates from a preset threshold, wherein: The formula for calculating the integral average temperature difference (LMTD) is: ; Wherein, Thotair is the temperature of the positive flow high-pressure air entering the main heat exchanger, Thoto2 is the temperature of the negative flow high-pressure oxygen exiting the main heat exchanger, Tcoldair is the temperature of the positive flow high-pressure liquid air exiting the main heat exchanger, and Tcoldo2 is the temperature of the negative flow high-pressure liquid oxygen entering the main heat exchanger.

[0046] Specifically, through Example 1, using the above-mentioned high-pressure oxygen and high-pressure air bubble point equations, P will be established. air The functional relationship of =f(PO2) is set in the internal calculation function block of DCS. After the product oxygen pressure changes, the working pressure required for positive flow heat exchange high pressure air will be obtained based on the designed temperature difference correlation, thereby realizing energy-saving automatic control of heat exchange unit and compression unit.

[0047] As long as the above-mentioned correlation can be ensured during normal operation of the unit, and the oxygen production from reheating and the high-pressure air flow rate of the forward heat exchange satisfy the relationship Fair=K*Fo2, but the prerequisite is that the designed pinch temperature difference must be met, otherwise the high-pressure air flow rate of the forward heat exchange and the return oxygen flow rate will deviate from the designed flow rate ratio. The corresponding designed heat exchange material flow rate K value will correspond to the pinch temperature difference dT. If the above conditions are met, it can be ensured that the integral temperature difference of the main heat exchanger is within the design range. To ensure that the process control is in a closed loop, the DCS is set to monitor the integral average temperature difference.

[0048] The instruments used for integrating average temperature difference (LMTD) monitoring in DCS include the hot-end and cold-end temperatures of the forward-flowing high-pressure air and the reverse-flowing high-pressure oxygen. The following relationship is then used for calculation:

[0049] Thotair is the temperature of the positive flow high-pressure air entering the main heat exchanger, Thoto2 is the temperature of the negative flow high-pressure oxygen exiting the main heat exchanger, Tcoldair is the temperature of the positive flow high-pressure liquid air exiting the main heat exchanger, and Tcoldo2 is the temperature of the negative flow high-pressure liquid oxygen entering the main heat exchanger.

[0050] By using LMTD to monitor the heat exchange conditions during normal operation of the heat exchanger, and ensuring that the pinch temperature difference of the main heat exchanger is within the design value, the high-pressure air pressure of the heat exchange material is correlated with the oxygen product pressure in real time. By maintaining the design ratio of the flow rates of hot and cold materials, the LMTD can be kept within the design range, thereby achieving the optimal operating conditions of the heat exchanger.

[0051] Example 4

[0052] Based on the above embodiment three, this embodiment points out that the DCS system collects the hot and cold end temperatures of the forward high-pressure air and the reverse high-pressure nitrogen in real time, and calculates the integral average temperature difference LMTD2 of the main heat exchanger. The calculation formula is as follows: ; Thotair is the temperature of the positive flow high-pressure air entering the main heat exchanger, Thoto2 is the temperature of the negative flow high-pressure oxygen exiting the main heat exchanger, Tcoldair is the temperature of the positive flow high-pressure liquid air exiting the main heat exchanger, and Tcoldo2 is the temperature of the negative flow high-pressure liquid oxygen entering the main heat exchanger.

[0053] Specifically, if the main heat exchanger includes a high-pressure nitrogen flow path with a large flow rate, the high-pressure air pressure of the positive flow needs to be calculated by associating the high-pressure nitrogen pressure and oxygen pressure of the product with the corresponding design pinch temperature difference, and then using the high value of the high-pressure air pressure as the set point of the booster.

[0054] ; ; Where: P air dTn2 is the pressure of high-pressure air, Pn2 is the pressure of high-pressure nitrogen, and dTn2 is the temperature difference between the nitrogen and the high-pressure air at the design pinch point.

[0055] The corresponding heat exchange flow rate for high-pressure air and high-pressure nitrogen is Fair = Km * Fn2 After matching the pressure and flow rate of the associated high-pressure nitrogen and high-pressure air, the overall integral temperature difference of the main heat exchanger will be within the design range.

[0056] ; Thotair is the temperature of the positive flow high-pressure air entering the main heat exchanger, Thoto2 is the temperature of the negative flow high-pressure oxygen exiting the main heat exchanger, Tcoldair is the temperature of the positive flow high-pressure liquid air exiting the main heat exchanger, and Tcoldo2 is the temperature of the negative flow high-pressure liquid oxygen entering the main heat exchanger.

[0057] The method provided in this example is applicable to air separation units with internal compression processes, and where the high-pressure oxygen pressure is below 20 bar and the high-pressure air pressure is below the critical pressure in the main heat exchanger. This is primarily based on the presence of a phase-change hot and cold heat exchange stream within the main heat exchanger below the critical pressure. During the start-up phase of the air separation unit, the DCS uses an automatic sequential control program to connect the forward low-pressure air and the return waste nitrogen flow paths associated with the main heat exchanger, achieving a matching of the corresponding heat exchange stream height with the design. After the unit's expander starts up, the heat exchange conditions of the main heat exchanger tend to stabilize. Based on subsequent customer requirements for the air separation unit, and the subsequent demand for high-pressure oxygen and high-pressure nitrogen products, the high-pressure air booster in the front-end process system is associated with the steady-state hot material (high-pressure air) and the cold product (high-pressure oxygen and high-pressure nitrogen) of the main heat exchanger, and the designed pinch point temperature difference, to establish the optimal heat exchange temperature field for the heat exchanger during normal unit operation. The entire process is executed automatically by the DCS computer program.

[0058] Example 5

[0059] Based on the above embodiment one, in step S04 of this example, while maintaining the optimal heat exchange conditions of the main heat exchanger, the positive flow high-pressure air flow rate F is also monitored based on the preset material balance coefficient K. air The ratio of the return product flow rate Fproduct to the design pinch point temperature difference dT is ensured to match.

[0060] Specifically, to maintain optimal heat exchange conditions for the main heat exchanger, the DCS system dynamically adjusts pressure parameters based on the pinch temperature difference model, while also introducing a material balance coefficient K to monitor and correct the flow matching relationship in real time. For the main heat exchanger undergoing phase change, the heat exchange between the forward high-pressure air and the reverse product (such as high-pressure oxygen or high-pressure nitrogen) depends not only on the temperature driving force (i.e., the pinch temperature difference dT) but also strictly on the mass flow ratio of the two. Given a fixed heat exchanger structure, when the actual operating ratio of hot and cold streams deviates from the design value K (i.e., Fair / Fproduct ≠ K), even if the nominal pinch temperature difference is maintained through pressure regulation, changes in flow rate will lead to changes in the heat transfer coefficient or cause the phase change zone to shift within the heat exchange channel. This results in the actual pinch temperature difference deviating from the design value dT, leading to decreased heat exchange efficiency or incomplete cold energy recovery.

[0061] To achieve this control function, the DCS system pre-stores the material balance coefficient K and the corresponding design pinch temperature difference dT, determined based on the process design baseline. During operation, the DCS continuously acquires the flow rate signal F of the positive-current high-pressure air. air The flow rate signal Fo2 of the return product (taking hyperbaric oxygen as an example) is calculated, and the real-time ratio Kreal=F between the two is calculated in real time. air / Fo2. DCS compares K_real with the design value K: If K real If the flow rate is within the allowable error range (e.g., ±2%), the flow rate is considered to be well matched and the heat exchange condition is stable. If K real If the temperature continues to deviate from K and exceeds the threshold, it means that the actual pinch temperature difference inside the heat exchanger has shifted. Even if the hot end temperature difference or the calculated apparent pinch temperature difference is still within the normal range, the DCS will judge that the heat exchange condition has deviated from the design optimal point and issue a process warning, prompting the operator to check the settings of the front-end booster pump guide vanes or anti-surge valve, or automatically trigger the fine-tuning of the flow loop.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. An automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit, characterized in that, Includes the following steps: S01. After the equipment is in the initial state, it automatically determines whether the cold box start-up step conditions are met. The conditions include that the positive flow raw material air pressure at the cold box inlet is greater than the lower tower pressure of the cold end process system. S02. Once the cold box start-up stepping conditions are met, execute the following sub-steps respectively: S21. The solenoid valves of the relevant regulating valves of the main heat exchanger are automatically energized to control the positive flow low-pressure air and the return flow sludge nitrogen. S22. Automatically control the regulating valve for the sludge nitrogen gas flowing back from the main heat exchanger to the hot-end process system to stabilize the pressure at the top of the tower; S23. Automatically control the opening of the raw material air bypass valve at the cold box inlet at a set rate to equalize the pressure in the lower tower; S24. When the pressure difference between the lower tower pressure and the raw material air pressure at the cold box inlet is detected to be less than the set threshold, the main air flow path valve of the cold box is automatically controlled to open at the set rate. S25. When the main air flow path valve opening reaches the set value, the regulating valves of each process flow path from the lower tower to the upper tower are automatically controlled to open at the set rate to realize the output of materials from the upper tower. S03. After the heat exchange flow path between the positive flow low-pressure air and the return flow polluted nitrogen is stably established, the expansion mechanism refrigeration system is automatically started in a stepwise manner. S04. After the expander starts up, it automatically enters the product material flow path and dynamically adjusts the operating parameters of the positive flow high-pressure air according to the product load changes and based on the preset pinch temperature difference model, so as to maintain the optimal heat exchange conditions of the main heat exchanger.

2. The automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit according to claim 1, characterized in that, The cold box start-up step condition in step S01 also includes a logical judgment to prevent the backflow of low-temperature materials in the distillation column, ensuring that the pressure in the distillation column is not higher than the pressure of the forward air flow before the forward air channel is established.

3. The automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit according to claim 1, characterized in that, It also includes a dynamic correction step for the low-temperature brittleness interlock protection value: S100. Based on the ambient temperature and device startup process obtained from the detection, dynamically adjust the low-temperature interlock protection setting value for the return gas exiting the cold box; S101. When the ambient temperature is lower than the preset value or the device is in a specific start-up mode, the interlock protection value is corrected to a safer temporary value through the RS trigger logic. S102. After all heat exchange material flow paths of the main heat exchanger have been established and steady-state heat exchange conditions have been reached, the interlock protection value is reset to the normal operating value.

4. The automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit according to claim 1, characterized in that, The step S04, which involves dynamically adjusting the operating parameters of the positive-flow high-pressure air based on a preset pinch temperature difference model, includes: S41. Real-time monitoring of the oxygen outlet pressure P of the product in the cooling box. o2 Based on the design pinch temperature difference dTo2, the target pressure P of the corresponding positive flow high-pressure air is calculated. air ; S42, the target pressure P air The outlet pressure of the front-end booster is automatically adjusted as a set value.

5. The automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit according to claim 4, characterized in that, The target pressure P of the positive flow high-pressure air air It is obtained through calculation using the following relation: S421. Calculate the target saturation temperature of positive-current high-pressure air using the oxygen bubble point equation. ; S422. Calculate the target pressure using the bubble point equation for air. .

6. The automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit according to claim 4, characterized in that, When the main heat exchanger also includes a high-pressure nitrogen product flow path, step S04 further includes: S43. Real-time monitoring of the product nitrogen outlet pressure Pn2 in the cold box; based on the design pinch point temperature difference dTn2, calculate the target pressure P of another positive-flow high-pressure air. airN ; S44. Take the target pressure PairO2 calculated from the oxygen product pressure and the target pressure P calculated from the nitrogen product pressure. airN The highest value in the value is used as the final set point for the booster.

7. An automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit according to any one of claims 1 or 4, characterized in that, It also includes a real-time monitoring step for the heat exchanger's operating conditions: real-time acquisition of the hot and cold end temperatures of the forward-flowing high-pressure air and the reverse-flowing high-pressure oxygen; calculation of the integral average temperature difference (LMTD) of the main heat exchanger; comparison with the design value to evaluate the current heat exchange conditions; and issuance of an alarm when the temperature deviates from a preset threshold. The formula for calculating the integral average temperature difference (LMTD) is as follows: ； Wherein, Thotair is the temperature of the positive flow high-pressure air entering the main heat exchanger, Thoto2 is the temperature of the negative flow high-pressure oxygen exiting the main heat exchanger, Tcoldair is the temperature of the positive flow high-pressure liquid air exiting the main heat exchanger, and Tcoldo2 is the temperature of the negative flow high-pressure liquid oxygen entering the main heat exchanger.

8. The automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit according to claim 1, characterized in that, In step S04, while maintaining the optimal heat exchange conditions of the main heat exchanger, the positive flow high-pressure air flow rate F is also monitored based on a preset material balance coefficient K. air The ratio of the return product flow rate Fproduct to the design pinch point temperature difference dT is ensured to match.

9. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the steps of the automatic control method for the main heat exchanger of the low-pressure internal compression process air separation unit according to any one of claims 1 to 8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the computer program implements the steps of the automatic control method for the main heat exchanger of a low-pressure internal compression process air separation unit as described in any one of claims 1 to 8.

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