Nitrogen emission rate control system and method for air separation device

Abstract

The invention discloses a nitrogen emission rate control system and method for an air separation device, and belongs to the technical field of air separation. The system comprises a product nitrogen outlet header pipe, a nitrogen backflow branch, a nitrogen backflow adjusting module (comprising a flow meter, an electric adjusting valve and a bypass manual valve), a nitrogen circulating fan unit (comprising at least one main fan and one standby fan), a mixed gas connecting pipe section and a liquid nitrogen gasification gas supplementing device (comprising a liquid nitrogen storage tank, a vaporizer and a control valve). And a DCS automatic control system and a safety emptying device. The control method comprises the steps that nitrogen backflow is started according to the operation load, the backflow amount is adjusted in real time through the DCS, liquid nitrogen supplementation is started when the backflow amount is insufficient, and safe emptying is triggered when the backflow amount is abnormal. The system can remarkably reduce the nitrogen emission rate, save energy consumption and enhance the low-load operation stability of the air separation device, and has high economic value and engineering practicability.

Description

Technical Field

[0001] This invention relates to the field of air separation technology, and in particular to a control system and method for nitrogen emission rate in an air separation unit. Background Technology

[0002] In cryogenic air separation, product nitrogen is typically obtained at the top of the distillation column. Air separation units are usually designed to withstand maximum load, which can lead to a significant decrease in nitrogen demand when the main unit is operating at low load. This results in the release of a large amount of product nitrogen, which not only wastes resources but also causes problems such as air compressors running unloaded, increased power consumption, and increased unit noise.

[0003] Furthermore, when the air compressor operates at reduced capacity or the amount of fresh air decreases, the empty column velocity, tray load, and reflux ratio of the distillation column are prone to deviating from the design conditions, leading to stability issues such as flooding and leakage. Therefore, a system solution is needed that can recover and utilize excess nitrogen while maintaining normal operating conditions within the column. Summary of the Invention

[0004] The purpose of this invention is to provide a nitrogen emission rate control system and method for an air separation unit, which reduces the nitrogen emission rate, saves energy, and enhances the operational stability of the air separation unit under low load conditions by utilizing an external nitrogen reflux method.

[0005] This invention provides a nitrogen emission rate control system for an air separation unit, comprising: The product nitrogen outlet manifold is used to collect high-purity nitrogen generated at the top of the distillation column; A nitrogen reflux branch is led out from the nitrogen outlet main pipe of the product and connected to the air separation unit inlet path to realize the reflux and utilization of nitrogen; A nitrogen reflux regulating module, located in the nitrogen reflux branch, is used to regulate the nitrogen reflux flow rate. It includes a flow meter, an electric regulating valve, and a bypass manual valve. The nitrogen circulation fan unit includes at least one main fan and one standby fan, which are used to drive the return nitrogen into the intake system and overcome the system pressure drop; The mixing connection pipe section, located between the return branch and the fresh air supply pipe, is used to achieve the mixing of nitrogen and air; The liquid nitrogen vaporization and replenishment device is used to replenish gas when the system gas consumption increases instantaneously or the nitrogen return flow is insufficient. The device includes a liquid nitrogen storage tank, a vaporizer, and control valves. The DCS automatic control system is used to monitor the operating parameters of the air separation unit in real time (including tower load, pressure, flow rate, etc.) and automatically adjust the operating status of various valves and fans to achieve linkage control of nitrogen reflux and gas replenishment. A safety venting device is installed at the end of the return branch. It can automatically vent nitrogen in case of abnormal overpressure in the system or fan failure, and is equipped with a silencer to ensure system safety.

[0006] In the control method of the system, the nitrogen reflux function is first activated based on the operating load of the air separation unit. If the activation conditions are met, a portion of the product nitrogen is introduced into the inlet path through the nitrogen reflux branch, mixed with fresh air, and then sent to the precooler and molecular sieve before entering the distillation column. The DCS control system automatically adjusts the nitrogen reflux flow rate and whether to activate liquid nitrogen replenishment based on real-time monitoring data. If the system pressure is abnormal or the fan fails, the safety venting device is triggered to release excess nitrogen. The entire process is automatically adjusted and protected under the unified control of the DCS system.

[0007] Through the above system configuration and operation control methods, this invention can not only significantly reduce the release of nitrogen resources and improve energy utilization efficiency, but also effectively expand the low-load operation capability of air separation units, enhance process stability and automation level, and has significant economic value and engineering applicability. Detailed Implementation

[0008] The following will describe the technologies in the embodiments of the present invention clearly and completely with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention. It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present invention can be combined with each other. The following will describe in detail the nitrogen emission rate control system and control method for air separation units proposed in this invention, combined with typical engineering scenarios, to demonstrate the complete implementation and operating effect of the invention in actual industrial plants.

[0009] This implementation method is applied to a unit with a rated capacity of 20000 Nm. 3 The cryogenic air separation unit has a capacity of [number] h. To address issues such as severe nitrogen venting, increased energy consumption, and unstable distillation column operation during low-load operation of the main unit, a nitrogen reflux control system was added to the unit, along with automated control methods.

[0010] Specifically, the distillation column of the air separation unit is equipped with a main product nitrogen outlet pipe at the top for outputting high-purity nitrogen. A nitrogen reflux branch is opened on this main pipe, through which a portion of the product nitrogen is led out and delivered to the air path leading into the column. The reflux branch is made of stainless steel with a diameter of DN100. The branch is equipped with a reflux regulation module consisting of a flow meter, an electric regulating valve, and a manual bypass valve to precisely control the nitrogen reflux flow rate and support online adjustment and maintenance bypass operations.

[0011] To ensure stable flow and pressure of the reflux nitrogen, the system is equipped with two nitrogen circulation fan units consisting of variable frequency fans, serving as the main unit and standby unit. The selected fans meet flow rate requirements ranging from 3000 to 12000 Nm³. 3 The gas delivery capacity is 15 kPa / h, with a maximum outlet pressure of not less than 15 kPa, and is equipped with variable frequency speed control function, which can dynamically adjust the gas delivery capacity according to the load.

[0012] Nitrogen gas, pressurized by the blower, enters the mixing connection pipe section, mixes with the fresh air output from the air compressor, and then enters the precooler and molecular sieve system of the air separation unit. After purification, it is sent to the distillation column for separation.

[0013] When the system experiences a sudden increase in gas consumption, insufficient product nitrogen, or limited reflux capacity, the automatic control system will activate the liquid nitrogen replenishment device. This device includes a 5 cubic meter liquid nitrogen storage tank and an evaporator with an evaporation capacity of 200 Nm³. 3 The vaporizer and automatic control solenoid valve assembly, with a capacity of / h, can respond and replenish nitrogen within seconds, ensuring that the inlet pressure and nitrogen concentration remain stable.

[0014] To achieve intelligent regulation of the nitrogen reflux and replenishment processes, the system is equipped with a DCS automatic control system. This system uses a Siemens S7-300 platform to collect key operating parameters in real time, such as pressure, load, reflux ratio, and air compressor flow rate within the air separation tower, and determines whether the system has entered a low-load operating state. If the main unit load drops below a set threshold (e.g., 50%), the system initiates the nitrogen reflux procedure.

[0015] After the reflux procedure is initiated, the DCS system outputs commands to control the opening of the electric valves, allowing some of the product nitrogen to enter the blower through the reflux branch, and then be sent to the mixing section by the blower. To maintain stable operating conditions inside the tower, the system adjusts the valve and blower operating parameters in real time by feeding back changes in flow and pressure, ensuring that the composition of the mixed gas meets the separation requirements inside the tower and avoiding problems such as flooding, overcooling, or abnormal reflux.

[0016] When gas demand fluctuates drastically, if the refluxed nitrogen cannot meet the actual demand, the system immediately activates the liquid nitrogen replenishment device to inject vaporized nitrogen into the mixing pipe section to make up the shortfall. At the same time, the control system dynamically determines when to activate and deactivate the replenishment to avoid oversupply and waste of resources.

[0017] To ensure safe operation of the system under extreme or fault conditions, a safety venting device is also installed at the end of the return system. This device includes a safety valve with a set pressure of 20 kPa and a venting pipe with a silencer. When the system outlet pressure exceeds the limit or the fan trips, it can automatically and quickly vent excess nitrogen to prevent equipment damage and reduce operating noise.

[0018] This system can ensure efficient nitrogen recovery and stable system operation even when the main air separation unit operates at a load as low as 10%. Practical application tests have shown that the system can reduce the nitrogen emission rate from 12.3% to 1.6%; it can shut down one air compressor during low-load operation, saving approximately 1600 kWh of electricity per hour; and it reduces venting noise from 84 decibels to 64 decibels. The entire process achieves fully automatic adjustment and emergency response, significantly improving the system's intelligence and operational economy.

[0019] Based on the nitrogen venting control method of the above system, and based on the real-time operating load of the air separation unit, the nitrogen reflux and replenishment process is regulated in a closed loop by an automatic control system, thereby achieving the dual objectives of minimizing venting and stabilizing the operating conditions inside the tower.

[0020] During normal operation of the air separation unit, the DCS system continuously monitors several key parameters, including tower top pressure, tray load, fresh air flow rate, and product nitrogen production. When the DCS system detects a decrease in the main unit load and that the product nitrogen demand is lower than the production volume, the system will automatically enter the "nitrogen reflux control mode".

[0021] In this mode, the DCS system first adjusts the opening of the electric valve on the nitrogen reflux branch by outputting commands, controlling a portion of high-purity nitrogen to be drawn from the product nitrogen pipeline and then pressurized by a blower before being sent into the intake system. The entire nitrogen reflux flow rate is monitored in real time by a flow meter. If the reflux rate deviates from the set target, the control system will automatically correct the valve opening to maintain a stable predetermined flow rate.

[0022] At the same time, the DCS system also monitors the air compressor outlet pressure and mixed gas flow rate. When the gas dilution ratio deviates from the set value or the distillation column reflux ratio fluctuates, the system will adjust the fan frequency accordingly to maintain a reasonable ratio of reflux nitrogen and fresh air, ensuring the stability of the distillation column's heat load and material balance.

[0023] If the system detects a sudden increase in gas demand (such as bypass startup or increased external nitrogen supply), and the return nitrogen cannot temporarily meet the total demand, the control system will automatically activate the liquid nitrogen replenishment device. Liquid nitrogen is converted into gaseous nitrogen through a vaporizer and then fed into the mixing pipe section under the action of a control valve to ensure that the inlet pressure and nitrogen concentration remain stable.

[0024] When abnormal situations occur during system operation, such as fan failure or excessively high pressure in the return pipeline, the control system will issue an alarm and automatically close the return valve according to the preset interlock logic. At the same time, the safety venting device will be activated to discharge excess nitrogen through the silencer, ensuring the overall safety of the system.

[0025] The entire control strategy has a closed-loop characteristic of "prediction-judgment-response-correction", which can dynamically match the nitrogen treatment method according to the main unit load, minimize the emission loss, and ensure that the various operating parameters do not deviate from the set operating conditions.

[0026] This method has been applied to a certain 50000 Nm 3 Successfully deployed on a large-scale air separation unit, the measured data shows that even when the main unit is operating at less than 20% load, the system can still stably control the reflux, maintain the flow rate and liquid level in the tower, and keep the nitrogen venting rate below 1.5%. The energy-saving effect and automatic response capability are better than the traditional passive venting method.

Claims

1. A nitrogen emission rate control system for an air separation unit, characterized in that, Includes the following parts: The product nitrogen outlet manifold is used to collect high-purity nitrogen generated at the top of the distillation column; A nitrogen reflux branch is led out from the nitrogen outlet main pipe of the product and connected to the air separation unit inlet path to realize the reflux and utilization of nitrogen; A nitrogen reflux regulating module is located in the nitrogen reflux branch and is used to regulate the nitrogen reflux flow rate; Nitrogen circulation fan unit is used to drive the return nitrogen into the intake system and overcome the system pressure drop; The mixing connection pipe section, located between the return branch and the fresh air supply pipe, is used to achieve the mixing of nitrogen and air; The liquid nitrogen vaporization and replenishment device is used to replenish gas when the system gas consumption increases instantaneously or the nitrogen return flow is insufficient. The device includes a liquid nitrogen storage tank, a vaporizer, and control valves. A safety venting device is installed at the end of the return branch, which can automatically vent nitrogen in case of abnormal overpressure in the system or fan failure.

2. The nitrogen emission rate control system for an air separation unit according to claim 1, characterized in that, The nitrogen reflux regulating module includes a flow meter, an electric regulating valve, and a bypass manual valve.

3. The nitrogen emission rate control system for an air separation unit according to claim 1, characterized in that, The nitrogen circulation fan unit includes at least one main fan and one standby fan.

4. The nitrogen emission rate control system for an air separation unit according to claim 1, characterized in that, The liquid nitrogen vaporization and replenishment device includes a liquid nitrogen storage tank, a vaporizer, and control valves.

5. The nitrogen emission rate control system for an air separation unit according to claim 1, characterized in that, The system also includes a DCS automatic control system, which is used to monitor the operating parameters of the air separation unit in real time and automatically adjust the operating status of various valves and fans to achieve linkage control of nitrogen reflux and gas replenishment.

6. A method for controlling the nitrogen emission rate of an air separation unit, employing the control system according to any one of claims 1 to 5, characterized in that, The method is as follows: First, determine whether to start the nitrogen reflux function based on the operating load of the air separation unit. If the start-up conditions are met, introduce some product nitrogen into the air intake path through the nitrogen reflux branch, mix it with fresh air, and then send it into the precooler and molecular sieve before entering the distillation column. The DCS control system automatically adjusts the nitrogen return flow rate and whether to activate liquid nitrogen replenishment based on real-time monitoring data. If the system pressure is abnormal or the fan fails, the safety venting device will be triggered to release excess nitrogen; the entire process is automatically adjusted and protected under the unified control of the DCS system.