AirSurr introduced the working principles and process flows of three nitrogen production processes: cryogenic separation, PSA pressure swing adsorption, and membrane separation, and compared the process performance, main equipment, and relative investment of the three nitrogen production processes; Proposed a method for selecting suitable nitrogen production processes based on different nitrogen requirements.

After the air is pressurized to about 0.8MPa by the compressor, and cooled below 20℃ by the compressor aftercooler and precooling unit, it enters the molecular sieve adsorption used for switching. The carbon dioxide, hydrocarbons and water in the air are adsorbed and purified. The purified air enters the main heat exchanger, and is cooled by the reflux oxygen-rich air to a saturation temperature of about-168℃ before entering the bottom of the distillation tower to participate in the distillation. At the top of the tower, 99.99% purity of the nitrogen is obtained. A part of the nitrogen is reheated by the main heat exchanger and sent out as a product, and the rest into the condensing evaporator is condensed into liquid nitrogen. Most of the liquid nitrogen is returned to the distillation column as reflux liquid to participate in distillation, and a small amount of liquid nitrogen is sent to the liquid nitrogen storage tank for storage. Liquid nitrogen production is about 8% of the gas nitrogen production. At the bottom of the distillation, the oxygen-rich liquid containing about 30% is empty through the throttle and enters the evaporation side of the condensation evaporator for condensing gas nitrogen. Most of the oxygen-rich air extracted from the top of the condensing evaporator directly into the main heat exchanger, and extracted from the middle of the main heat exchanger, the temperature-153℃ into the turbine expander adiabatic expansion to 0.03MPa, the temperature is about-183℃, providing cooling for the cryogenic separation. The expanded oxygen-rich air is mixed with another throttle oxygen-rich air and enters the main heat exchanger with the current air. After the reheating to room temperature, the part is used as the relife of the molecular sieve, and the rest is empty. Flow chart of deep cold separation:

1-air compressor; 2-filter; 3-dryer; 4-filter; 5-PSA adsorption tower; 6-filter; 7-nitrogen buffer tank

After the air is compressed to 0.85MPa by the compressor and cooled to about 40℃ by the compressor rear cooler, it enters the air purification unit to remove the dust, water and oil mist in the compressed air, and then enters the PSA transformer adsorption unit. The unit is equipped with two adsorption towers, one tower adsorption nitrogen production, one tower deattachment regeneration, through the PLC control switch valve switch, so that the two towers cycle alternately. The four working processes of adsorption, homogenization, desorption and purging are as follows.

1-air compressor; 2-filter; 3-dryer; 4-filter; 5-PSA adsorption tower; 6-filter; 7-nitrogen buffer tank

After being compressed to 1.3MPa by the compressor and cooled to about 45 ℃ by the aftercooler of the compressor, the air enters the air purification unit to remove dust, water, and oil mist from the compressed air, and then enters the membrane separation unit. The core component of the membrane separation unit is a set of membrane components with a structure similar to that of a shell and tube heat exchanger. Tens of thousands of small hollow fiber wires are cast into tube bundles and placed inside the pressurized tube shell. Due to the characteristics of the membrane, the temperature of the air entering the membrane separation unit needs to be maintained at 40-50 ℃. Therefore, an electric heater is installed in front of the membrane separation unit to heat and purify the air to maintain stable air temperature. After entering the separator, air flows axially along one side of the fiber, and CO2, O2, and H2O continuously penetrate the membrane wall and accumulate on the other side of the fiber. They are discharged through the permeable outlet, while nitrogen is discharged from the non permeable outlet opposite the gas inlet. The nitrogen gas that meets the purity requirements enters the nitrogen buffer tank, and the pressure is adjusted by the buffer tank outlet regulating valve and sent to the user. The process flow of membrane separation nitrogen production is shown in the following figure:

1-air compressor; 2-filter; 3-dryer; 4-filter; 5-electric heater; 6-membrane assembly

(a) The process of PSA pressure swing adsorption and membrane separation for nitrogen generator is simple, with a small number of equipment and simple operation. It can be stopped at any time and for a long time. Deep cooling nitrogen production not only has a complex process and a large number of equipment, but also needs to operate in a deep cooling and low temperature state. Before the equipment is put into normal operation, there is a pre cooling start-up process, and the start-up time from the start of the expander to the required nitrogen purity is generally not less than 12 hours. Before the equipment enters a major overhaul, there must be a period of heating and thawing, usually 24 hours. Therefore, cryogenic separation nitrogen production is not suitable for occasions with frequent start and stop. Compared with PSA pressure swing adsorption, membrane separation for nitrogen production not only has a simpler equipment structure, but also has no switching valves, making operation and maintenance simpler, and requiring shorter gas production time.
(b) Cryogenic separation for nitrogen production can simultaneously obtain both gaseous and liquid nitrogen, making it suitable for processes that require liquid nitrogen. Liquid nitrogen can also be stored in liquid nitrogen storage tanks as a backup. When there is a short-term surge in nitrogen demand or minor repairs to nitrogen production equipment, the liquid nitrogen in the storage tank can be vaporized and sent to the nitrogen pipeline network to meet the continuity requirements of the process unit for nitrogen. PSA pressure swing adsorption and membrane separation for nitrogen production can only produce nitrogen without backup means, and a single set of equipment is difficult to ensure continuous and long-term operation of the process unit.
(c) When the volume fraction of nitrogen purity is ≤ 97%, the nitrogen extraction rate of PSA pressure swing adsorption and membrane separation nitrogen production processes is basically equivalent; When the volume fraction of nitrogen purity is greater than 99%, the nitrogen extraction rate of the cryogenic separation nitrogen production process is the highest, followed by PSA pressure swing adsorption, and the nitrogen extraction rate of the membrane separation nitrogen production process sharply decreases. At the same time, to produce nitrogen at the same pressure, the outlet air pressure of the cryogenic separation nitrogen production air compressor is equivalent to PSA pressure swing adsorption nitrogen production, while the membrane separation nitrogen production pressure requires a higher air pressure. The main energy consumption of the three nitrogen production processes is in the air compressor. Therefore, when the purity of nitrogen production is high, the air compressor required for membrane separation nitrogen production is large in scale, high in power, and the total energy consumption is the highest. PSA pressure swing adsorption nitrogen production takes the second place, and deep cold separation nitrogen production energy consumption is relatively low.
(d) There are pressure fluctuations in the nitrogen separation adsorption desorption adsorption process of PSA pressure swing adsorption for nitrogen production, resulting in unstable nitrogen pressure; The nitrogen separation process of cryogenic separation and membrane separation for nitrogen production is continuous, and the nitrogen pressure of the product is relatively stable. Therefore, in PSA nitrogen production, it is necessary to add a nitrogen buffer tank at the nitrogen outlet of the PSA adsorption tower to buffer the nitrogen, regulate the gas pressure, and ensure the stability of the nitrogen product pressure.
(e) There are many cryogenic separation nitrogen production equipment, with long processes, large land occupation, and high investment. Compared with PSA pressure swing adsorption nitrogen production, membrane separation nitrogen production requires a large amount of air, a high pressure ratio, a large compressor scale, and corresponding air purification components (filters, dryers, degreasers, etc.) that are larger than PSA pressure swing adsorption, as well as adsorption towers. Therefore, PSA pressure swing adsorption nitrogen production has the lowest investment.