Startup of front-end equipment of air separation unit
Startup of front-end equipment of air separation unit
1. Start the air compressor and pre-cooling system according to the operating procedures.
2. Start the molecular sieve according to the molecular sieve startup procedure, put the carbon dioxide analyzer into use, and confirm that the outlet of the molecular sieve is dry air without carbon dioxide. For the temperature swing adsorption process, it is necessary to determine the appropriate temperature for heating according to the filling material to ensure that the molecular sieve is fully activated; for the pressure swing adsorption process, at least two purge cycles are required before increasing the load. In order to prevent the molecular sieve from fluidizing and destroying the bed, there should be a pressure difference or maximum flow limit, which should not exceed the set value during forward and reverse flow regeneration.
3. When the molecular sieve steam heater is put into use, the steam introduction should be slowly heated and the condensed water should be discharged to avoid water hammer. If you use an electric heater, you should first turn on the heating gas and then turn on the electricity to avoid dry burning
4. After the molecular sieve activation is completed, purge the equipment pipelines in the cold box. It is advisable to switch the pressurized gas in the air separation cold box to a nitrogen pipeline and gradually replace it so that the oxygen purity of the gas in the cold box is less than 5%.
5. Cold box purging
5.1 When driving, the cold box should be purged to ensure that the dew point of each part of the cold box is below -50°C.
5.2 Prepare a process flow diagram in the control room and mark the pipelines through which the gas has been purged.
5.3 The instrument pipeline, discharge pipeline, and sampling pipeline should be disconnected and purged separately. When the sampling line is disconnected and purged, the cold box should be prevented from being overpressured.
5.4 During the purging process of the cold box, attention should be paid to the purging of parallel pipelines. The pipes of different units of the main heat exchanger and the pipes of the cryogenic pumps that serve as backup for each other should be switched and purged separately.
5.5 During the purging process of the cryogenic pump, the pump rotation should be avoided.
5.6 When purging the expander, the sealing gas and lubricating oil systems should be put into use in advance.
5.7 Each liquid pipeline, gas pipeline and liquid filling pipeline should be purged clean and reverse purged if necessary.
5.8 During the purging process, the dew point test can be conducted and recorded, and the purging valve and flow path gas volume can be appropriately adjusted based on the dew point test results.
5.9 Whether the purge is completed should be judged based on the trend of multiple dew points. If necessary, a static dew point test can be performed.
5.10 When the dew point of the cold box equipment, pipes, valves, etc. is qualified, the purge ends. At this time, restore the disconnected instrument pipelines, discharge pipelines, etc., and prepare for cold box cooling.
6. Initial cooling of air separation cold box
6.1 The cooling capacity of the air separation unit is generally provided by the expander. After confirming that the cold box has the cooling conditions, the expander can be started according to the expander startup process.
6.2 The temperature difference between heat exchangers, towers, pipes and separation equipment during the cooling stage should not be too large, otherwise the thermal stress of the materials will cause damage to the equipment. The cooling process should be slow, and the temperature drop displayed by the temperature measurement points of the expander, distillation tower, heat exchanger in the tower and other equipment should not exceed 30°C/h .
6.3 Distillation towers, especially packed towers, should be cooled slowly and evenly. In the early stage of cooling, the cold box is at normal temperature and the device cannot handle the air flow during normal operation. The excess air from the main air compressor will be vented through the vent valve.
6.4 During the cooling process of the cold box, the hot gas can be properly discharged at the farthest end of the container through the bleed valve of the cold box, but the cooling capacity of the expander is mainly recovered through the main heat exchanger. In order to allow more gas to pass through the oxygen, nitrogen, and argon channels of the main heat exchanger, the tower pressure setting value can be adjusted. After cooling is completed, the tower pressure setting value is adjusted back to the normal setting.
6.5 During the cooling stage, the discharge pressure of the main air compressor should be kept stable, especially the pressure behind the molecular sieve adsorber. All adjustments should be made based on parameters such as main air compressor discharge pressure, lower tower pressure, upper tower pressure, and air flow into the tower.
6.6 During cooling, the main cooling neon and helium non-condensable gas discharge valve should be opened.
6.7 When liquid appears in the tower, the valves of each liquid pipeline should be closed or reduced. Equipment such as crude argon condenser and pure argon condenser do not need to accumulate liquid prematurely.
6.8 When the temperature of each container in the cold box reaches approximately -165°C, the cooling stage ends and rapid cooling can be continued for liquid accumulation adjustment.
6.9 When the liquid level is reached, the cryogenic pump can be cooled, and it should be ensured that the sealing gas is put into normal use.
6.10 The pump pipelines (inlet, exhaust, outlet pipes) should be cooled together with the pump body to prevent thermal stress from causing damage to the pipelines. To avoid cooling too fast, use low-temperature gas for cooling first. When cooling with gas, the pump should not rotate. Before starting the pump, use low-temperature liquid cooling to ensure that liquid comes out of the cooling discharge valve before starting.
7. Start-up operation of expander
7.1 The liquid level in the lower tower should be controlled well. If the liquid level is too high, it will flood the air inlet of the lower tower, and the violent flow of gas and liquid will cause damage to the structure inside the tower.
7.2 During the startup operation of the expander, attention should be paid to the oxygen content of the gas in the upper tower to avoid excessive oxygen content during startup.
7.3 The load can be increased by adjusting the nozzle opening, so that the expander can quickly cross the critical speed and improve the refrigeration efficiency.
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8. Cold box purification
8.1 After liquid accumulation in the tower, priority should be given to establishing the liquid level of the main condensing evaporator before putting the main condensing evaporator into operation. As the main condensing evaporator works, the liquid level drops, the pressure in the lower tower decreases, and the amount of inhaled air increases. At this time, operations should be performed to maintain the stability of the pressure in the upper and lower towers.
8.2 The commissioning operation of the main condensing evaporator should be carried out gradually and slowly based on DCS pressure and temperature data.
8.3 When the main condensing evaporator is fully put into use and the liquid level and working conditions stabilize, the purity can be adjusted. For the internal compression process, the final adjustment of the booster and expander should be completed after the internal compression pump is started.
8.4 During the process of establishing the liquid level of the main condensing evaporator, there is a risk of carbon dioxide, nitrous oxide and total carbon accumulating in the heat exchanger channel. It is advisable to use liquid nitrogen backfill to establish the liquid level and reduce the risk. Liquid nitrogen backfilling should be performed after the device is fully cooled to prevent thermal stress in the material and damage to the equipment.
8.5 For devices with argon, after the argon tower working conditions are established, operate each equipment to stabilize the distillation working conditions and produce and output various qualified products.
9. Normal operation and shutdown
9.1 The air separation unit should operate within the design allowable range, and the load should be adjusted according to changes in product demand.
9.2 The alarms and safety interlocks of the air separation unit should not be released without reason.
9.3 When the air separation unit is stopped, when the liquid level in the tower rises to the first tray or packing and submerges the air inlet, the liquid should be discharged until the high liquid level interlock is released.
9.4 During temporary parking, if the liquid level is maintained above the safe level, there is no need to drain the liquid. Otherwise, the liquid should be drained and refilled.
9.5 When the device is shut down for a long time, the main condensing evaporator should be drained, and the liquid in the oxygen channel of the heat exchanger and the high-pressure oxygen product pipeline should also be drained to prevent the concentration and accumulation of hydrocarbons in the liquid oxygen pipeline after the low-temperature liquid evaporates.
9.6 During the shutdown period of the device, the equipment and containers in the cold box should not have overpressure or negative pressure.
10. Unfreezing the device
10.1 The device should be thawed and rewarmed according to the equipment maintenance and operation manual.
10.2 Before thawing the cold box, the liquid in the cold box equipment and pipes should be drained cleanly.
10.3 When rewarming, make sure that the system sealing gas and purge gas are supplied normally, the molecular sieve adsorber is in working condition, and the instrument air required for heating is provided.
10.4 If a separate rewarming pipeline is not set up, rewarming can be carried out through forward flow gas.
10.5 Precautions for rewarming the cold box:
——During rewarming, all parts of the cold box equipment should be rewarmed evenly to avoid thermal stress damage to equipment and pipelines caused by excessive temperature differences;
——When the equipment is in the low temperature stage in the early stage of rewarming, low-temperature gas should not be discharged directly into the atmosphere. The gas should be discharged from the cold box to the drainage system, and the main heat exchanger should be used to ensure the temperature difference and reduce the rewarming time;
——All pipelines should be rewarmed, and if necessary, valves can be opened to reversely purge the pipelines;
——At the end of thawing, open each measuring pipeline to ensure that no alarms and interlocks occur at each measuring point.
10.6 When the temperature indicator in the cold box and the temperature at the outlet of the thawing pipeline is higher than 0°C and continues for more than 2 to 3 hours, the thawing process is completed. The dew point of the relevant sampling points should be below -50°C.
10.7 After the rewarming is completed, shut down the molecular sieve purification system and air compressor according to the operating procedures, and perform subsequent operations.
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Chemical Engineer/Plant Operator (Air Separating Unit-New Oxygen Plant) at African Foundries Limited
8moThanks so much for your sharing