Technology 0002: 15000 Series - Liquid Oxygen Liquid Nitrogen Cryogenic Air Separation Plant
1. working principle
The working principle of the air separation plant is to use the different boiling points of the various components in the air, compress, pre-cool, purify, and use the cooling provided by the turbo expander to partially liquefy it and then carry out rectification to obtain the required oxygen and nitrogen. Product.
The main function of the purification system is to receive the processed air from the compressor, first lower the air temperature in the pre-cooling unit, and then enter the adsorber for adsorption. Its specific structure and operation method are as follows:
2. main equipment and structure:
Pre-purification equipment the purification system consists of two adsorbers, an electric heater, valves and instruments.
Each adsorber is equipped with 13x-apg4×8 spherical molecular sieve, activated alumina and inert alumina. The boundary surface between molecular sieve and activated alumina, activated alumina and inert alumina should be flat and separated by two layers of wire mesh. Molecular sieves and alumina are filled on site, and please follow the requirements of the adsorption cylinder drawing. The air enters from the lower part, is adsorbed by activated alumina and molecular sieve, removes water, carbon dioxide and acetylene, and goes to the fractionation tower through the top.
Electric heaters. The regenerative gas enters the heater barrel from the inlet of the electric heater connection, then passes through the electric heating tube, passes through the baffle plate in the middle to fully heat the gas, and finally discharges from the outlet connection.
The outlet temperature of the electric heater is controlled at 175°c. When the outlet temperature of the adsorber reaches 100°c, the regeneration ends. The control unit will automatically cut off the power to the electric heater. If the outlet temperature of the electric heater exceeds 200°c, the control equipment will also automatically cut off the power supply of the electric heater.
Start:
Working conditions:
The air compressor has been started and is operating normally.
The air precooling system is put into normal operation.
All valves are closed before starting.
Operation and maintenance:
Impurities such as water and carbon dioxide in the air at the outlet of the adsorber exceed the specified value, indicating that the adsorption effect is not good. The reason should be found out in time, and the molecular sieve should be supplemented or replaced.
Molecular sieve or alumina replacement:
When the molecular sieve or alumina needs to be replaced, the air pressure in the adsorber should be exhausted. If the temperature in the container is high, cold air should be used to blow it to normal temperature.
Contact the previous and subsequent processes, and take corresponding measures to close the valve of the adsorber.
Remove the top flange of the adsorber and take out the upper distributor for cleaning. Take out the molecular sieve and alumina from the hand hole, blow out the inside of the cylinder with high-pressure clean air, and refill the alumina and molecular sieve.
Before molecular sieve or alumina is installed, check whether there is any powder or powder, if there is, it must be cleaned and then loaded into the adsorber.
Molecular sieves or alumina should be loaded gently so as not to break and increase resistance.
Since the molecular sieve has a strong ability to absorb water, special care should be taken during storage, and it should not be placed in the atmosphere and should be stored in a sealed container.
Use and maintenance of electric heaters:
Always check the use of the electric heating tube during work, whether the wiring is good, and whether the insulation meets the requirements.
Regularly remove debris such as dust and scale.
Always check whether the safety film is in good condition.
Always check whether the grounding equipment is reliable.
Instrument air, expander bearing /seal air supply system
3. process characteristics
In the air-cooling system, the pre-cooling unit is used, and the unit is simple and reliable.
The molecular sieve adsorption method is used to purify the air, the process flow is simple, the start-up is easy, the operation is convenient, the operation is safe, the switching loss is small, the rectification working condition is stable, the product extraction rate is high, the molecular sieve adsorber is automatically switched, and the heating is regenerated.
The turbo expander is used, which is stable in operation and easy to operate, ensuring smooth driving.
The temperature adjustment flow path before the expander is set to meet the working conditions of the start-up process and normal operation.
4. brief description of the process
This equipment is a set of liquid air separation equipment with supercharged turbo expander for normal temperature molecular sieve adsorption process.
The air passes through the self-cleaning filter to filter out dust and mechanical impurities, then enters the raw material air compressor, and enters the pre-purification equipment system after compression. The air is cooled in the pre-purification equipment system, and then enters the molecular sieve adsorber to absorb the moisture in the air, carbon dioxide and acetylene and other hydrocarbons; the two adsorbers are switched, and when one is working, the other is taken from the fractionation tower. The contaminated nitrogen gas is regenerated after being heated by an electric heater. Moisture, carbon dioxide, hydrocarbons, etc. In the air in the adsorption cylinder are removed after being adsorbed by activated alumina and molecular sieves. The purified raw air is divided into two streams, the first stream enters the cold box after being cooled, and the second stream enters the supercharger coaxially driven by the expander, and also enters the cold box after being pressurized and cooled.
The first air enters the main heat exchanger of the cold box, is cooled by the return gas, and the humid air enters the bottom of the lower tower after throttling. The air in the lower tower is rectified to obtain liquid air and liquid nitrogen. The liquid air and liquid nitrogen are respectively exported from the bottom of the lower tower and the main condensing evaporator, and enter the subcooler, where they are cooled by the dirty nitrogen and nitrogen gas from the upper tower. It is cooled, and then enters the upper tower as the feed for the upper tower. After rectification in the upper tower, liquid oxygen and nitrogen are obtained at the bottom of the condensing evaporator and the top of the upper tower respectively.
The second air pressurized by the expander supercharger also enters the main heat exchanger after entering the cold box, and is drawn out when it is cooled to -120°c, and enters the expander for refrigeration. After expansion, part of the air is introduced into the middle of the upper tower to participate in rectification, and the other part enters the backflow dirty nitrogen channel of the main heat exchanger, while cooling and condensing the raw material air, it is reheated by itself and then exits the cold box.
The nitrogen and dirty nitrogen extracted from the top of the upper tower enter the subcooler and the corresponding channel of the main heat exchanger successively as the cold source of the heat exchanger, and then exit the cold box after reheating itself. The exhaust gas from the cold box is used as the regeneration gas and cold blowing gas of the purification system.
5. start-up operation of air separation plant
Conditions that must be met before starting
The installation and construction of the air separation equipment are all completed, the installation records are verified to be correct, the system is blown off, pressure tested, exposed to cold, insulation materials, adsorbents are filled, and the drying is qualified.
The air filter, air compressor, and pre-purification equipment have been successfully tested on a single machine and meet the requirements. The installation records have been verified to be correct and meet the conditions for a single machine test.
The instrumentation and control commissioning work are all completed, meets the requirements, and the installation records are verified to be correct.
Guarantee the supply of water, electricity and gas.
The operation site is smooth, with complete safety facilities, eye-catching signs, and necessary communication facilities.
Preparations before starting the equipment
This process is mainly to lay a good foundation for the start-up and long-term operation of the equipment, so material and technical preparations must be made. That is to say, in addition to doing what they should know and understand, and having a full understanding of the process flow and safety technology, the personnel in each position should also do the following work carefully in order to drive smoothly.
Check that the connections between the systems are correct.
According to the requirements of the operating instructions of each unit, check whether the filter, air compressor, expander, pre-purification equipment, etc. Are normal, and make preparations for starting, so that each unit is in a standby state.
Check that the air separation unit is completely dry, no liquid moisture is allowed in this area, and close all valves.
All instrument valves are open except the analytical instrument.
The temperature recording and measuring instruments must be in the open state, and check whether they are normal.
Cooling water system starts
Open the inlet and outlet valves of the cooling water pipe.
The main cooling water pipe supplies water.
Check the instrument gas source and molecular sieve adsorber automatic control system, seal the gas supply system and put it into operation.
Adjust the instrument air source pressure to ~0.5mpa to provide instrument air.
Start all indicating instruments except analytical instruments.
Switch on the program controller of the molecular sieve adsorber.
Put the pneumatic regulating valve into dry operation, check whether the valve action program is normal, and carefully observe whether the valve action is lagging or other abnormal conditions.
Start the air compressor oil circuit system
Start the oil pump of the air compressor and adjust it to the normal value to make the lubrication circulation line normal.
Start the air compressor
Start the air compressor according to the requirements of its operation and maintenance manual, and slowly increase the discharge pressure of the air compressor to about ~1.0mpa(g) to make it run.
Preparation for start-up of the air precooling system.
Start the pre-cooling unit according to the requirements of the pre-cooling unit operation and maintenance manual.
Start the purification system.
Equipment cooling in the cold box of fractionation tower
Replace the regeneration gas source of the molecular sieve adsorber: slowly close the air to the purifier regeneration gas system through the valve to ensure the stability of the regeneration gas flow and maintain the regeneration gas pressure constant.
Other considerations at this stage
At the beginning of cooling, all the air in the air compressor cannot enter the fractionation tower, and the excess compressed air is discharged into the atmosphere through the vent valve, thereby keeping the discharge pressure of the compressor constant. As the temperature of each part of the fractionation tower gradually decreases, the amount of air inhaled will gradually increase. At this time, it can be adjusted by gradually closing the vent valve of the air compressor.
As the temperature of each part of the fractionation column continues to drop, when the temperature of the cold end of the main heat exchanger is close to the temperature of the air liquefaction point, the cooling stage ends.
It should be noted that the temperature difference of each part in the cold box should not be too large during the cooling process, otherwise it will cause temperature difference stress. The cooling process should be carried out slowly in order to ensure that the temperature of each part is uniform.
Adjust the valve opening of each flow path in time according to the working conditions to control the pressure of the upper and lower towers.
Maintain air compressor discharge pressure.
Switch the regeneration gas route of the molecular sieve purifier from the air flow path to the dirty nitrogen flow path.
It is necessary to pay attention to and control the flow rate of each flow path, so that the temperature of each part drops evenly, and no large temperature difference occurs.
Turboexpander control:
In the cooling stage, the cooling capacity of the turboexpander should be kept at the maximum, and the outlet temperature zone of the expander should be as low as possible, but not lower than the liquefaction temperature.
6. normal operation and maintenance of equipment
Normal operation:
After the start-up of the air separation unit, it tends to be normal after cooling, liquid accumulation and working condition adjustment. However, in the initial stage of equipment operation, the insulation material has not yet reached the required temperature and external factors change, which have some influence on the working conditions. In order to obtain the most qualified products with the least consumption, the equipment must be comprehensively adjusted frequently so that the equipment is always in the best working condition.
Air precooling system operation
According to the pressure and temperature changes of the air-cooling tower, the water volume\pressure and temperature of its cooling water should be adjusted in time.
For details, refer to "pre-cooling unit instruction manual".
Purification system operation
Please refer to "purifier instruction manual" for details.
Adjustment of working condition of fractionation tower:
Adjustment of refrigeration working conditions: on the premise of ensuring that the expander does not exceed the speed, adjust the opening of the inlet valve of the expander to keep the main cooling liquid level stable.
Adjustment of distillation conditions:
Purpose: to make the rectification system in the best working condition, so that the product can reach the highest possible extraction rate.
Method: the adjustment of the rectification working conditions is mainly the distribution of the flow in the fractionation tower, that is, the adjustment of the reflux liquid, the control of the liquid level, and the adjustment of the output and purity of the product.
Ensure the amount of air entering the tower and the air pressure.
Ensure that the lower tower and the main cooling liquid level are stable at the design value.
On the premise of ensuring the nitrogen purity at the top of the lower tower, adjust the opening degree of the throttle valve to make the reflux liquid of each section of the upper tower most suitable.
The adjustment should be carried out gradually and slowly according to the characteristics of the process and the problems existing in the working conditions. Otherwise, one will lose sight of the other, which will worsen the working conditions and fail to achieve the expected adjustment effect.
Equipment maintenance
Here is an explanation of the maintenance of the main units of this equipment: normal maintenance work is an important link to ensure long-term, stable and safe operation of the equipment, and must not be taken lightly.
General operations management:
Each post should set up an operation record sheet and a maintenance history book, and keep records.
The operation record is recorded every hour, and its content includes temperature, pressure, resistance, flow, liquid level, purity, etc.
Blow off the measuring instrument pipeline once a week (cut off the instrument when blowing off), calibrate the zero point of the instrument, and check whether the switching procedure of the purification system is correct.
Normal maintenance of main equipment:
Main heat exchanger:
It is advisable to adjust the distribution of the various streams in the main heat exchanger to ensure that the temperature difference at the hot end of the main heat exchanger is not greater than 3°c.
Of the heat exchanger is mainly to pay attention to the change of resistance and temperature. Abnormal conditions, usually due to ice, dry ice and powder blockages. This is often caused by improper operation of the equipment and can be eliminated by heating and blowing off.
In addition, pay attention to whether there is leakage in the heat exchanger, which can be judged by analyzing whether there are differences in the components of the inlet and outlet of the heat exchanger. A number of analysis points are provided in the equipment for sampling and analysis.
Up and down the tower:
Frequently check the resistance of each tower to judge whether there is gas crossing, liquid leakage or plate blockage, liquid flooding and other phenomena. The first start to adjust the resistance of normal pressure can be used as the basis for operation.
Analyze the content of contaminated nitrogen components to judge the separation efficiency of the tower.
Molecular adsorber:
An important aspect of the management of the molecular sieve purifier is the management of the "switching equipment", which can refer to the instrument control manual.
Check the purifier once a week to see whether the specified temperature is reached during regeneration and cold blowing, and whether the switching time is in compliance with the regulations. If there is any abnormality, it should be adjusted.
After the purifier has been used for 1 to 2 years, it is necessary to measure the particle breakage of the molecular sieve. If necessary, all the fillers should be taken out and sieved to remove the particles. It is necessary to carefully blow and brush the sieve to remove the particles and powder deposited on it.
Molecular sieves should be added or replaced according to regulations, and unidentified molecular sieves should not be selected, and the adsorption layer must reach the specified height.
Valve:
Cryogenic valves and valves on oxygen lines must be free of oil and grease. Special care must be taken to ensure this when maintaining and repairing these valves. If it has been in contact with oil and grease, it should be degreased.
Gaskets and sealing rings should also be free of oil and grease and must be made of non-combustible materials suitable for oxygen valves.
The management and maintenance of the valve also includes keeping the visible surface of the valve stem clean and checking the leakage of the valve. The surface of the valve should be regularly coated with a lubricant suitable for oxygen valves.
For detailed management and maintenance requirements of valves and special valves, please refer to the valve maintenance instructions.
Measurement and control system:
The management and maintenance of various instruments must be carried out in accordance with the provisions of the "instrument and control instruction manual". The measurement pipeline should be specially maintained to ensure that there is no leakage, otherwise, it will affect the accuracy of the instrument measurement, or even fail to work. The same blockage of the measuring line is not allowed and should be removed by heating and blowing.
Electric heaters:
When the electric heater is put into operation, it must be ventilated first and then powered to avoid damage to the electric heating element.
Regularly check the insulation condition, and it should meet the drawing requirements.
7. downtime _
Sudden shutdown
Due to power failure, water supply or other factors, the equipment suddenly shuts down, and the shutdown steps are operated according to the "planned short-term shutdown". First, the product output valve should be closed to stop the product delivery.
Plan long-term downtime
Its operation steps are the same as "planned short-term shutdown", in which it must be done:
Drain all the liquid in the tower in time after shutdown.
Check whether all valves of the air separation unit are closed, including liquid discharge valve, blow-off valve, switching valve, remote control valve, counter valve , etc.
After the liquid removal operation of the fractionation tower is completed, the fractionation tower is left to stand for 2 hours to allow it to heat up naturally, and then it can be fully purged.
Restart of equipment after short-term shutdown
The method and steps of restarting the air separation plant after a short-term shutdown should be determined according to the length of the shutdown time and the specific conditions in the tower. When the main cooling liquid level drops to 20% of the normal operating value, the liquid must be discharged to prevent the concentration of acetylene and other substances, which will cause danger.
Condition: after the short-term or temporary shutdown of the air separation plant, the inside of the system is still in a low temperature state.
Method: refer to the steps of cold start-up, that is, the start-up operation of the air separation unit, and operate according to the principles of start-up, but the following problems should be paid attention to during the start-up process:
Make preparations according to the cold start preparation procedure. .
When restarting the equipment after temporary shutdown, the operation steps should be determined according to the temperature in the cold box. When restarting after temporary shutdown, the equipment and pipelines in the cold box do not need to be purged. The start-up of the equipment outside the cold box is carried out according to the above-mentioned method. Before the air enters the fractionation tower, the purification system must first perform a switching cycle, and the subsequent operation steps are carried out according to the above-mentioned method.
8. ( heating ) purge of equipment
Comprehensive purging of fractionation column
After installation, overhaul and long-term shutdown of air separation equipment, dust, oil, water and other impurities will accumulate inside. The existence of these substances will not be conducive to the start-up of the equipment. At the same time, after long-term operation of the equipment, there will also be moisture , co2 and hydrocarbons accumulate, which not only increases the resistance of the equipment, affects the operation of the equipment, and even threatens the safe operation of the equipment, so the equipment should be fully purged once before the large-scale operation to achieve the removal of accumulated purpose.
Under normal circumstances, it is stipulated to shut down and purge once every 1 year.
Check whether all the valves of the air separation unit are closed, especially for the intermodal transport of multiple sets of air separation units, it is necessary to check whether the valves at the associated places are closed.
Before comprehensive purging, check whether the liquid in the equipment has been discharged, and it must be allowed to stand for 2 hours to allow the equipment to heat up naturally before purging with dry gas.
A process flow chart and a red pencil should be prepared before comprehensive purging to mark the purging route.
Start the various systems before the fractionation tower according to the procedures and requirements in "start-up operation of the air separation unit", and make them run stably, and adjust the air volume to about 60% of the normal operating value.
The gas used for purging and heating comes from the air compressor. When purging a certain part, the entire detection point of the part should be purged at the same time.
If the outlet gas has no free water, is dry, or the dew point of the blown gas is below -42°c (100ppm), the purging can be ended.
Finally, stop the air compressor, pre-cooling system, and molecular sieve purification system, and close all valves of the equipment.
Local heating purge
Valve heating
Valves in the low temperature section may leak (usually the stuffing box freezes, and these valves cannot be opened or closed forcefully, because this will seriously damage the valve).
To freeze the valve, use water or hot steam to spray the valve body, but care must be taken not to allow water to enter the stuffing box cover .
After the valve is thawed, the leakage point should be checked and dealt with in time.
Warming purge of turboexpander.
Close all valves in the turbine section.
Open the heater line outlet purge valve.
Open the heating line inlet valve, including the relevant valve in front of the electric heater.
When the inlet and outlet temperatures of all the hot gases are close, and the outlet gas is dry, then the heating can be ended.
Then close the valves on the heating line and all valves in the turbine section.
9. safety technology
Air separation equipment is gradually popularized, and its characteristics are gradually known and mastered by people. However, due to various reasons, its safety in many aspects has not yet attracted people's attention and full understanding. Undue danger.
Aspects that are prone to harm:
High-voltage electricity: high voltage and large current, so it is easy to generate discharge, breakdown insulation, severe heating of electrical appliances, and great impact on the power grid when starting and stopping.
Low-voltage electricity: it is responsible for the power supply of water pumps, oil pumps, instruments and other power sources, so its influence is very wide, and it is likely to cause the unit to fly, shut down by itself, burn out, lose control and other hazards.
Water: the interruption of water will cause the equipment to lose cooling and be out of service. The water in the out of service equipment will not be discharged completely, and the equipment will be frozen and damaged in low temperature weather.
Inadvertent blow-by of high- and low-pressure air flow media will damage low pressure equipment.
High-concentration oxygen is a strong combustion aid, which will lower the ignition point of combustibles and make the fire source that is originally a safe distance unsafe.
High concentrations of nitrogen can cause suffocation and death.
The running air separation plant is a low-temperature cold source. Any improper operation may cause the leakage of low-temperature gas and liquid, causing a series of visual obstacles, causing burns, suffocation, strong burning, freezing equipment, and rapid death to the human body. The volume expansion will cause the equipment to burst and other dangers.
The thermal insulation material in the cold storage box has strong fluidity and large volume, which can make people sink and suffocate to death.
The high concentration of co2, c2h2 and other hydrocarbons accumulated in the equipment will make the equipment unable to operate, and it will explode if not careful.
The high protruding cold box building is easy to discharge in thunderstorm weather.
Precautions for safe work:
Safety issues have been basically considered in equipment design and engineering design, but due to many risk factors, there are hidden dangers in the operation of air separation equipment, especially the operator's awareness of equipment performance and whether the management system is perfect or not will seriously affect safe job.
The user must closely monitor the operating status of the equipment to discover hidden dangers and deficiencies, and take corresponding remedial measures.
Operators must be trained and assessed before they can start working.
Strictly prohibit illegal operation and operation beyond the position.
All safety signs should be eye-catching and accurate.
Strict safety management system must be established.
Fireworks are strictly prohibited around and near the air separation equipment, and storage of combustibles, explosives, grease, gasoline, etc. Is strictly prohibited.
Necessary and complete fire-fighting equipment must be installed near the air separation plant.
There must be good lighting facilities. The passages around the workshop on the construction site should not be stacked with items and should be kept unobstructed. The operation room must have good ventilation equipment to keep the air fresh to prevent the concentration of harmful gases.
The air separation plant must have a reliable lightning protection equipment installed.
Discharging liquid oxygen, liquid nitrogen, and liquid air, special attention should be paid to the container path and place specified in the engineering design to avoid unsafe hidden dangers.
For safety work, this manual will not describe them one by one, and the user unit shall establish a system based on strict requirements based on relevant information.
Calibration of safety valve
Calibrate once a year and seal with lead after calibration.
10. naked chill
After the air separation equipment is installed or overhauled, it must be frozen naked before starting up to check the installation and maintenance quality of the equipment at a low temperature. At the same time, through this work, it is necessary to check whether the equipment pipeline connections are correct and whether the pipelines and valves are leaking.
Generally, naked cooling is to make the equipment and pipes in the fractionation tower frosted, and the temperature will not drop until the equilibrium is reached, and the naked cooling is considered to be over.
Bare cold according to the following steps
The operating steps of naked cooling are basically carried out according to the procedures and requirements in the start-up operation of the air separation unit.
During the cooling process, when the temperature of the equipment reaches equilibrium at about -70°c (that is, the temperature does not drop anymore), keep it for 2 to 3 hours (the temperature of the cooling equipment in hot regions will be higher in season).
After the equipment is started, the changes in working conditions should be closely monitored to see if there is any abnormality in each link during the entire operation, and relevant records (pressure, temperature, etc.) Should be made every hour. In the cold state, check whether there is any deformation in each part, and can judge whether there is leakage according to the frosting situation, and mark the location of the leakage point. Take the time to tighten all the connecting bolts again before defrosting after shutdown.
After warming up, deal with problems found or work that needs to be improved.
Found by naked cooling, decide whether to perform another naked cooling.
The naked cold test run, a comprehensive blow-off must be carried out.
Problems that should be paid attention to during normal operation and maintenance
Carry out an operation check and parameter record every hour;
Each supporting unit is operated and maintained according to its manual.
Equipment safety operation measures:
In order to prevent humid air from penetrating into the cold box and dangerous gas from concentrating in the cold box, the cold box needs to be filled with sealing gas. Its gas source comes from the backflow of dirty nitrogen, which is filled into the cold box.
Under normal circumstances, if the air volume of the air seal is too large, causing the pressure of the cold box to rise, the exhalation tube on the cold box can seep out to maintain a certain pressure. The exhalation tube on the cold box should be checked regularly, and no objects should be placed in it. On the exhalation tube, at the same time, prevent the exhalation tube from being frozen by ice and snow.
When starting or restarting after shutdown, check whether the position of the inlet and outlet valve switch of the molecular sieve adsorber is correct, otherwise it should be adjusted. The opening action of the valve should be carried out slowly, so as not to cause impact on the molecular sieve bed.
When the air separation unit starts to be fully heated after the cold start-up and shutdown, it must be noted that the heating gas volume should be small and the speed should be slow. It is absolutely impossible to use a large volume of heating at the beginning, and the heating gas is dry air at room temperature.
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Key Words: Liquid Air Separation Plant, Liquid Oxygen, Liquid Nitrogen Air Separation Plant, Full Liquid Air Separation Plant, Low Pressure Air Separation Plant, Air Separation Plant
