Technology 0004: 11000 Serioes-GAN/LIN Cryogenic Air Separation Plant
1. Working procedure
Basic principle is to use the different boiling point of composition in air for separation using distillation. For such a purpose, the working procedure contains the followings:
Compression of air
Pre-cooling of compressed air
Removal of moisture and CO2
Air cooling
Refrigeration recovery
Liquefaction of air
Distillation
Removal of impurities
Compression of air
Air is compressed into the required pressure through self-cleaning filter. Air compressor provides required energy for air separation.
Pre-cooling of compressed air
Due to the fact that there is less moisture in low-temperature air, capacity of molecular sieve can be reduced. Additionally, low-temperature air can enable molecular sieve to adsorb moisture and carbon dioxide. Therefore, the above purposes can be met through regulating pre-cooler unit.
Removal of moisture and CO2
Because condensing point of moisture and CO2 in processing air is very high, they may block passage after entering cryogenic plant, so it will affect normal operation. Due to this, moisture and CO2 in processing air must be removed and hydrocarbon like acetylene is also removed at the same time. Air temperature of entering purifier is ～10℃，the air outlet temperature will increase into about 20℃ because of heat adsorption of molecular sieve.
Purifier has two adsorbers for operation by turns. While one adsorber is working, the other is regenerating.
Cooling air to its liquefaction temperature
The air is cooled in the main heat exchanger, where the air is cooled by the gas returning from the rectification column to close its liquefaction temperature and meanwhile the cold returning gas is reheated.
Preparation of refrigerating effect
To ensure normal production and required refrigeration and to overcome refrigeration loss resulted from insulation, insufficient reheating and cryogenic liquid discharge from cold box, refrigeration is continuously supplied. Required refrigeration is produced from isothermal throttling effect and working of insulated expansion in expander.
Liquefaction of air
Liquefaction of air is the principal condition for separation of oxygen and nitrogen. Processing air exchanges heat with reflux expanded air, nitrogen and WN through main heat exchanger. Then it goes to distillation column for partial liquefaction. For the same substance, under different pressure, its saturated temperature is also different. The higher the pressure is, the higher its saturated temperature is, i.e. the higher the pressure is, the easier liquefaction is, vice versa.
Distillation
Air rectification is occurring in rectification column. It uses different boiling point of each composition in air, i.e. under same temperature, vaporization pressure of each composition is different, thus to make partial condensation and evaporation of rising vapor and decreasing liquid for many times for separation purpose. When air under condensation temperature attaches and passes through liquid air under lower temperature, heat exchange and mass exchange are occurring between vapor phase and liquid phase. The rising gas is partly condensed to be liquid and releases condensation heat, at the same time liquid will partly vaporize because of heat adsorption. In rectification column, this process is finished on column trays. Because of different boiling point of oxygen and nitrogen, nitrogen is easier to vaporize than oxygen, oxygen is easier to condense than nitrogen, when gas passes through column trays from lower to upper, nitrogen thickness is increased continuously. As long as column trays are sufficient, high purity nitrogen is got at the top of the column, and condensed oxygen flows into the column bottom as the oxygen rich liquid with high oxygen contents.
After it being throttled, oxygen rich liquid enters condenser-vaporizer to exchange heat with nitrogen. Nitrogen is condensed in the condenser/vaporizer. A part of it acts as reflux liquid; the other part acts as liquid product to deliver out and sent to liquid nitrogen tank.
Removal of dangerous impurities
Because of molecular sieve purification process, most hydrocarbon etc dangerous particles have been removed in the purifier. The remainder may enters column and accumulates at the bottom of column, so when plant being operated normally or stopped in a short time, acetylene content in liquid air should be analyzed periodically and should be less than 0.4cm3/L.
2. Process description
Process brief introduction
Feed air is compressed to about 0.8MPa by air compressor after filtering and separate dissociated water in water separator after pre-cooling of pre-cooler, then it enters adsorber of purifier. The moisture, carbon dioxide and hydrocarbon such as acetylene are removed away by adsorption of molecular sieve purifier. The purified compressed air is sent to fractionating tower.
The adsorbed air enters main heat exchanger to exchange heat with reflux oxygen-enriched gas and nitrogen until it cooled down close to liquefaction point. Then into fractionating tower, high purity nitrogen is got at the top of column. High purity nitrogen out of top of rectification column is separated into two parts. One part passes through main heat exchanger to be reheated and out of the cold-box to user. The other part enters condenser/vaporizer to be condensed to liquid nitrogen, one part of it is drawn out as product, and the other part is as reflux for rectification column. The cold source of condenser-evaporator is oxygen rich liquid taken from tower bottom and evaporated in it after throttle.
Oxygen rich liquid is evaporated in condenser-evaporator and reheated in main heat exchanger. Then it enters turbo expander to make it close to atmospheric pressure for generating refrigeration capacity. Expanded oxygen-enriched gas enters main heat exchanger to be reheated to be regeneration gas and blow-off gas for purifier.
3. Other specifications：
This plant uses separate instrument air and bearing gas system. A stream of air taken from purified air general pipe is removed of impurities of molecular sieve powder etc. through AF2001 instrument air filter, and then as instrument air source. A stream of air taken from instrument air source is through AF2002/AF2003 precise filter and then as gas source of bearing and seal gas.
This plant adopts PLC control system for auto regulation and control. Details shall refer to its operation manual.
Rectification column
Structure: whole-set sealing cold box. It consists of main heat exchanger, rectification column, condenser-evaporator, sub-cooler, gas bearing turbo-expander unit, etc.
Function: Compressed air, which has been cleaned and purified by purifier, is cooled and partly liquefied by main heat exchanger in rectification column, finally separated into product gaseous nitrogen, liquid nitrogen and oxygen-enriched gas in rectification column. Expander provides refrigeration capacity.
Main heat exchanger
Structure: Multi-layer plate-fin structured
Function: Heat exchange takes place between flows. Cold and hot gas flows in passages exchange heat by fin and partition. Cold gas flow is reheated to normal temperature, and heat gas flow is cooled to needed temperature.
Application mode: The purified compressed air enters main heat exchanger from upper to lower, and then exchanges heat with reflux oxygen–enriched gas and nitrogen. Oxygen-enriched gas is reheated in main heat exchanger (lower) and then sent to purifier as regeneration gas and blow-off gas. The cryogenic product nitrogen drawn out of the top of rectification column enters main heat exchanger from lower to upper to be reheated to normal temperature.
Distillation column
Structure: Vertical cylinder type, with sieve tray inside. The tray is single counter flow structured.
Function: It uses different boiling point of each composition in air, to separate air into needed composition by rectification.
Application mode: During rectification, liquid flow passes every tray from upper to lower. Because of the overflow weir, some liquid level is formed on the tray. When gas passes through openings of sieve tray from lower to upper to contacting with liquid, bubble is produced, so it increases contacted area between air and liquid to make heat and mass exchange in high efficiency. During heat exchange and mass exchange, composition with lower boiling point is vaporized and composition with higher boiling point is condensed gradually. As long as the tray is sufficient, higher pure nitrogen with low boiling point compositions can be got at the top of column, and certain purity oxygen rich liquid air is got at the bottom of column.
Condenser-evaporator
Type: Dry type
Structure: Multi-layer plate-fin structure
Function: Application of condensing nitrogen and vaporizing oxygen rich liquid to maintain distillation process.
Application mode: A part of nitrogen drained from the top of rectification column enters nitrogen passages of condenser-evaporator (plate-fin heat exchanger) where the nitrogen is condensed to liquid nitrogen by oxygen rich liquid. A part of liquid nitrogen is drawn out as product; the most part is as reflux. Oxygen rich liquid air enters the evaporating side of condenser-evaporator, reheated by main heat exchanger (lower) and then goes to turbo-expander for expansion refrigeration.
Sub-cooler
Structure: Multi-layer plate-fin structure
Function: Expansion oxygen rich gas is used to cool oxygen rich liquid air so that the temperature of oxygen rich liquid air can be lower.
Application method: Oxygen rich gas reaches a lower temperature through expanding refrigeration in turbine expander, which is used to exchange heat with oxygen rich liquid air and cool it.
Turbo-expander
Type: Horizontal single-stage axial inflow counter-strike type
Structure: Gas bearing expansion turbine
Function: Reflux oxygen-enriched gas is expanded and refrigerated in turbo expander to provide refrigeration capacity for fractionating tower.
Application mode: After being reheated in main heat exchanger (lower), oxygen-enriched gas from condenser/vaporizer enters turbo-expander for expansion and refrigeration. Expanded oxygen-enriched gas then is reheated to normal temperature in main heat exchanger and goes out of rectification column.
4. Starting and operating procedure
General description
The operation of high purity nitrogen plant is composed of two steps:
Single unit trial of high purity nitrogen plant, i.e. the testing, adjustment and blow off of single units before operation of the packaged unit. This is to make sure the interconnection between each unit is correct.
High purity nitrogen plant is started and put into operation.
To make sure the safe, reliable and continuous operation of the high purity nitrogen plant, the operators shall be familiar with the operation and maintenance manual of each unit and the operating function of each unit. They shall also know the operating method and troubleshooting methods. On the premise that the product purity meets requirement, try to increase output and cut down energy consumption and cost.
The trial method and procedure for each unit refer to the installation technical manual and the operation and maintenance manual for each unit.
Starting and operation procedure
During the operation period of complete high-purity nitrogen equipment, starting is the key step. Flow, temperature and pressure of system are changed greatly during starting period. These changes will affect normal production of high purity nitrogen equipment, period for starting and production period, etc.
Starting of high purity nitrogen plant
Preparation before starting
High-purity nitrogen equipment can be started after the following works.
Installed test of single units for this complete set equipment has been finished and test should be qualified.
Commission of collocating machines is finished according to technical documents rules of single unit and qualified after inspection.
Thaw and blow out thoroughly and make sure it is clean, dry, oil-free and without impurities. For parts working under cryogenic state (all vessels, valves and pipelines inside cold box, including each analyzing & measuring pipeline), there shall be no liquid.
All equipment and pipelines shall be thoroughly blown out.
Operation of the purification system shall be correct and one adsorber shall have finished regeneration and ready for use.
Proofread completely purity nitrogen equipment system to assure that process connection is right according to process charts of each collocating single machine and whole purity nitrogen equipment. Confirm flow connection is correct and installation quality meets standard requests. Completely blow off all the pipelines before column. The entire blank flange should be disassembled.
Check and proofread each electric appliance, instrument and system circuitry to assure that installation and adjustment is correct and application is sensitive.
Check safety valves etc. to assure that they are in good condition and in working state.
Air filter is under working condition.
Cooling water pipe is open and smooth and in working state.
Recording paper and necessary tools are prepared.
Starting of each machine
All the valves (process valves) of purifier nitrogen equipment should be in close state. Valves connected with indication instrument excluding analysis and measure instrument must be open and temperature measure instrument should be switched on. Then following steps are carried out.
Start air compressor.
Start per-cooler unit
Start molecular sieve purifier.
Start instrument air and bearing gas system.
Blow off equipment and pipeline
Start air compressor.
Refer to Application and maintenance manual for air compressor.
Cooling rectification column system
At beginning of cooling, the compressed air is unable to completely enter the fractional column. With gradual decrease of temperature of each part of the fractional column, the intaken air volume will gradually increase. It can be adjusted by compressor itself.
As temperature of each part of rectification column keeps decreasing, temperature before expansion reaches -148℃. When the cold end air temperature in the main heat exchanger (lower) gets close to the liquefaction temperature（-168℃）, the cooling stage is finished.
Note: During cooling process cares shall be taken to avoid excessive difference between sections inside the cold box, otherwise, excessive thermal stress would occur. Cooling process should be undertaken slowly to ensure even temperature of each station.
Open valves of cooling passages in sequence.
Keep the discharge pressure of air compressor constant.
The flow rate to each passage shall be controlled to keep even temperature of each section without excessive temperature difference.
Control of turbine expander
During cooling period, coldness preparation of turbo-expander should be the largest. The lower outlet temperature of expander is, the better it is, but it cannot be below liquefied temperature. In this stage, we should do our best to increase inlet pressure of turbo-expander, thus we can turn down to increase pressure of compressed air into high purity nitrogen plant. Attention shall be paid to whether each unit is working normally after pressure building up. In this way, time for cooling stage can be shortened. When pipeline of each vessel inside cold box is preliminarily cooled and the air temperature at cold end of main heat exchanger reaches liquefaction temperature, the cooling stage finishes.
Liquefaction and regulation period
When all equipment in cold box is cooled more deeply, air begins to be liquefied and liquid air level appears in the rectification column. Then liquid level appears in condenser vaporizer. Condenser vaporizer continues to accumulate liquid after beginning to work; rectification process begins to be set up, at the same time, product purity can be regulated and nitrogen output may be given at 70%~80% of designed output. During liquefaction period, outlet temperature of expansion turbine should be still kept lower, but it can’t be below liquefaction temperature.
5. Maintenance
This manual only introduces application and maintenance of main machine of purity nitrogen equipment. Application and maintenance of pre-cooler, molecular sieve purifier; turbo-expander, air compressor etc. can be seen in corresponding application and maintenance manual.
General operation management
Operation data should be recording an hour including temperature, pressure, resistance, flaw, liquid level, analysis result etc. Every shift should record the resistance of main heat exchanger and molecular sieve purifier once.
Maintenance entry should be recorded the following working conditions:
Measure pipelines should be blown off every week. Date and process of blow-off should be recorded. Controller on pipeline should be closed and calibrated again when necessary before purity nitrogen equipment being blown off. Record if the function of switchover device and control instrument is normal. Check air filter and gas source filter every month, and clean it and make a record of check condition when necessary.
Main heat exchanger
Maintenance of main heat exchanger is mainly to pay attention to changes of resistance and temperature. The abnormal condition usually is caused by blockage of dry ice and powder and error operation. Thawing and blowing-off can solve this. Moreover, pay attention to whether there is any leakage on main heat exchanger, which can be judged by analyzing composition of access of heat exchanger.
Rectification column.
Pressure differential gauge is fixed on column for measuring resistance during rectification process of nitrogen column. The measured resistance valve should be as normal running data when equipment is started for the first time and being regulated to normal condition. When resistance reduces, it shows that liquid leakage occurs or liquid level on trays is too low or airflow is reducing, at this time reasons should be found out. If resistance increases, blocked trays usually cause this situation. Heating and blowing off rectification column can solve the problem. If load is too big, air feed volume and nitrogen output should be analyzed and regulated according to actual condition. When liquid level at the bottom of rectification column rises too high and leads to the lowest tray to be flooded, the column will be flooded, at this time resistance inside column rises obviously, and the liquid should be discharged and rectification condition should be regulated again.
Valves
There should be no oil, grease and contamination on cryogenic valve. When it is applied, maintained or repaired, pay attention to make sure it not contacted with flammable materials like oil, grease. Degreasing should be done if it did.
There should be no oil and grease on valve gasket, seal padding and seal ring.
Keep visible surface of valve and valve stem clean, and coat with no grease valve lubricant on surface of valve stem periodically.
Check leakage condition of valve. If leakage exists, press padding gland tightly and change padding.
Cryogenic valve should be cleaned and degreased before being installed. The solvent used to clean and degrease should be free from oil, grease and contamination. Not allow using dissociated solvent. All the solid materials such as slag, oxide sheet iron, welding slag including grease and antiseptic etc. must be removed away when valve is installed. Detailed maintenance requests for valves refer to related information of valves.
Measurement and control system
Management and maintenance of each instrument should be carried out according to instrument control manual. Measure pipeline should be specially maintained to assure no leakage or blockage, otherwise correctness of instrument measure will be affected, and even normal working will be affected. Measure pipelines are not allowed to be blocked and solve it by thawing and blowing off.
Temporary stop.
The generator will be stopped for a short time because of troubles. Normally, if stopping time is more than 24 hours, system needs to be thawed, then to restart.
Restarting after temporary stopping.
After temporary stopping, restarting steps ate decided according to temperature inside cold box. When cold box is under cold keeping status, if restart nitrogen plant, cold box needn’t be thawed, and the generator is easier to resume to normal status.
Start air compressor
Start pre-cooler
Start molecular sieve purifier.
Gradually supply gas to fractionating tower.
Start and regulate turbo-expander.
Regulate rectification column.
Regulate product production and purity to specified index.
Thawing:
After air separation part is operating for a long time, cryogenic equipment such as heat exchanger and rectification column and pipeline inside fractionating tower system will produce ice, dry ice and hydrocarbon accumulation etc. Inevitably make resistance gradually increase, and affect production and purity of product. So, after purity nitrogen plant is working for one year, normally full thawing to remove away these accumulations.
During purity generator running, if resistance of heat exchanger and purity of product can’t reach specified index. In this condition, full thawing should be carried out in advance for equipment inside fractionating tower. This condition occurs because of erroneous operation and maintenance.
Gas used for thawing fractionating tower is adsorbed and normal temperature dry air. Air is sent to each heating valve to heat the heat exchanger and distillation column. During thawing, temperature of each part of purity nitrogen generator should resume gradually and even to prevent producing too large stress and damaging equipment and pipeline. When thawing, all instrument tube-lines should be thawed and blown off at the same time.
Thawing for valves.
All cryogenic valves will be frozen because of leakage. This situation occurs because sealing of padding gland is not tightly. For frozen valves, don’t open and close forcedly to prevent valves being damaged, and it can use warm gas or vapor to thaw frozen part of valves. However, in usual use, note not to let water come into padding gland. After thawing, find out the leakage points and solve it.
Thawing for turbo-expander.
Stop turbo-expander. Close all the valves. (Note: keep supplying bearing gas).
Open thawing valves and blowing-off valves.
When gas discharge temperature of blowing-off valve approaches to gas inlet temperature, thawing ends.
Close thawing valves and blowing-off valves.
Thawing for rectification system.
Discharge all liquid. Close all valves.
Start air compressor, pre-cooler and molecular sieve purifier.
Open every valve according to warm flow.
When outlet temperature of thawing gas rises to 0℃, open valve of temperature tube-line connected with blowing-off pipeline.
When inlet and outlet temperature of thawing gas are close, thawing ends.
Stop pre-cooler and purifier, and close all valves.
Adsorbing and regeneration of molecular sieve purifier
Refer to the Operation Instruction for molecular Sieve Purifier and Instrument Control. Here is only the brief introduction.
This molecular sieve purifier of high purity nitrogen plant is composed of two adsorbers. While one is working, the other is regenerating. The working procedure of purifier includes five steps: heating, cold blow, pressure equalizing, switching to adsorbing and pressure relief.
Safety rules
According to suffocation of nitrogen, burn of cryogenic liquid, safety of pressure vessel, possibility of explosion of acetylene existing in liquid air and electric safety, following safety specifications are listed out for users to use in editing safety rules.
Before workers enter nitrogen vessels, analyze and test oxygen content inside vessels to confirm nitrogen is not become dense under the supervision of safety staff to prevent suffocation. Nitrogen inside air separation plant is not permitted to be discharged indoors, and it should always pay attention to surrounding oxygen analyzer, keep alarm device well. Oxygen respirator should be worn well if lots of nitrogen exists. During checking nitrogen vessel, first use air to replace it and then work after qualification of oxygen content and also under the supervision of special staff.
Liquid nitrogen and liquid air cannot be discharged freely. Operators who discharge cryogenic liquid should wear protection clothes to prevent being frozen.
Supply necessary fire protection condition and equipment.
There is convenient water tap in purity nitrogen generator area with special spraying device; water can spray out as long as man presses it.
Fit out whit long enough fire protection tap.
Fit out with convenient portable fire extinguisher.
There is safe and reliable warning system.
No smoking or flame in purity nitrogen generator area.
Special anti-dust mask and groove should be worn to prevent workman being injured when fill insulation material
Non-operators cannot freely enter purity nitrogen generator area.
Liquid air at bottom of rectification column is discharged once a week, each time 10 litters.
A gas-sealing cylinder is fixed at top of cold box at this high purity nitrogen generator to keep gas sealed pressure inside cold box stable. Gas-sealing cylinder should be always checked. Do not put anything on gas-sealing cylinder and safety valve.
For more enquiry, welcome to contact by email sinocom6@163.com
Keywords: Nitrogen Generator, Liquid Nitrogen Generator, Gas Nitrogen Generator, Cryogenic Nitrogen Generator
