A new type of high-purity nitrogen equipment comprising a pre-treatment system, a purification system and a fractionating tower, the fractionating tower comprising a turbine expander, a main heat exchanger and a subcooler for heat exchange, as well as a distillation tower and a condensing evaporator; The distillation column comprises an upper column and a lower column. The new type of nitrogen production device made full use of oxygen and nitrogen cooling capacity, energy saving and environmental protection; At the same time, the oxygen-rich is sent to the expansion machine for cooling, which has reasonable structure, saving materials and simple configuration process. The following is a review of the process design of high purity nitrogen equipment:

The main equipment and principle of air separation

The main equipment of air separation includes air separation equipment and low temperature rectification tower, whose working principle is based on low temperature freezing and rectification technology. ‌ An air separation plant compresses air and deeply freezes it to a liquid state, which is then separated in a rectification column. Low temperature distillation is the most commonly used method to separate inert gases such as oxygen, nitrogen and argon by the difference in the boiling point of different gases

In industrial applications, air separation equipment is widely used in metallurgy, chemical industry, petroleum, machinery, mining, food and military fields. Other separation methods such as membrane separation and pressure swing adsorption (PSA and VPSA) are mainly used to separate single-component gases, while low temperature distillation is suitable for the manufacture of high purity gases, especially the high purity oxygen, nitrogen and argon required for semiconductor devices.

Purification principle of air separation equipment

In a device that uses the principle of low temperature distillation to produce oxygen, the distillation separation of air in the low temperature zone requires pretreatment in the normal temperature zone, such as filtration, precooling and purification. Because the air contains a lot of dust, in the long-term high-speed operation, the dust will wear, corrosion, scaling on the impeller, blade and other parts, thereby reducing the service life of the equipment, so it is necessary to filter the material. Since the temperature of the compressed air exceeds 80 ° C, the subsequent heat absorption and heat transfer cannot be carried out, so the pre-cooling device is installed in the air separation device, which can effectively reduce the temperature of the indoor air.

In the air of raw materials, in addition to dust, it also contains water, carbon dioxide, hydrocarbons, and most of the current air separation equipment uses molecular sieve purification system to improve the cleanliness of the air and avoid the accumulation of water and carbon dioxide, which affects the work of the air separation device. Therefore, the gas filtration, precooling and purification device is an indispensable part of the air separation device.

Air separation equipment mainly includes cryogenic air separation equipment and low temperature rectification tower.

Cryogenic air separation equipment

In order to liquefy the air, different deep freezing cycle devices can be used, mainly based on the Linde cycle and the Claude cycle. The former is cooling by throttling expansion; In addition to the latter, there is still throttle expansion, and a part of the gas is isentropic expansion in the expansion machine. When the gas is undergoing isentropic expansion, the temperature reduction is larger than that of throttling expansion, and part of the compression work can be recovered, so it is economical than throttling expansion. Various other improved deep freezing cycles include double pressure throttling cycle, ammonia pre-cooling throttling cycle, step by step overlapping cycle, etc.
In the various cycles of the deep freezing method, the typical process (see figure) is to first filter the air in the filter to filter dust and other impurities into the compressor, and then by the molecular sieve purifier to remove the air in the low temperature easy to solidified gases, such as water vapor and carbon dioxide, etc., the purified air in the first heat exchanger by the product nitrogen and oxygen cooling. After the first heat exchanger, the air is divided into two ways: one way through the second heat exchanger to continue cooling, and then through the throttle valve to reduce pressure; The other way to reduce the pressure through the expansion machine. The air temperature after the expansion of the two channels drops to about 103K and enters the bottom of the lower tower of the two-stage rectification tower.

Low temperature rectification tower

In the deep freezing process, the main separation process is carried out in a two-stage rectification column. The tower consists of an upper and lower two towers and a condensation evaporator between the towers. The air entering the bottom of the lower tower has been partially liquefied under the temperature and pressure conditions there. Because the boiling point of liquid nitrogen is lower than the boiling point of liquid oxygen, the liquefied gas at the bottom of the lower tower is oxygen-rich liquid air, and the oxygen content is generally 30% to 40%. The operating pressure of the lower tower should be higher than that of the upper tower so that the condensation temperature of nitrogen at the top of the lower tower is higher than the boiling temperature of liquid oxygen at the bottom of the upper tower (see p-V-T relation). Thus, the heat inside the condensing evaporator is transmitted from the tube to the tube, and has a certain heat transfer temperature difference. The condensing evaporator simultaneously plays the role of condensation at the top of the lower tower and heating at the bottom of the upper tower. Air in the lower tower from bottom to top through multi-layer tray rectification, so that the concentration of volatile components of nitrogen gradually increased, and condensed into liquid nitrogen in the condensing evaporator tube. Part of the liquid nitrogen is used as reflux liquid in the lower tower. Part of it is collected in the liquid nitrogen tank and used as the reflux liquid at the top of the upper tower after decompression. The oxygen-rich liquid air at the bottom of the lower tower enters the middle of the upper tower through the throttle valve, and countercurrent contact with the gas evaporated from the condensing evaporator. Thus, the oxygen content in the downstream liquid continues to increase from top to bottom, and finally accumulates in the condensate evaporator tube, the oxygen content can reach more than 99%, and the product oxygen is constantly evaporated here and led out of the tower. The top of the upper tower leads to product nitrogen, and the concentration can reach more than 98%. The temperature of the product oxygen and product nitrogen in the distillation column is very low, and the input air can be cooled by the heat exchanger.
Since the boiling point of argon is between the boiling point of nitrogen and oxygen, pure nitrogen and pure oxygen cannot be obtained at the same time by using a two-stage rectification column. The concentration of nitrogen and oxygen in the product can be increased if the argon rich gas is extracted from the appropriate part of the upper tower as the raw material of argon extraction. Neon and helium with lower boiling points accumulate on liquid nitrogen and can be extracted as raw materials for extracting neon and helium. Krypton and xenon with relatively high boiling points are accumulated in liquid oxygen and gas oxygen at the bottom of the upper tower and can be extracted as raw materials for extracting krypton and xenon.

Air separation process description

Air separation equipment is a set of air separation equipment with pressure turbine expander for adsorption purification of molecular sieve and production of hydrogen free argon at normal temperature, in a conventional packed tower. The process flow is as follows:

1. Filtration, compression, precooling and purification

The original process air is inhaled from the intake port, enters the self-cleaning air filter, filters out dust and mechanical impurities, enters the centrifugal air compressor for compression, and the compressed gas enters the air cooling tower in the air precooling system, which is cooled and washed in the tower. The air cooling tower uses recirculated cooling water and cryogenic chilled water cooled by the water cooling tower and further cooled by the ice machine. The top of the air cooling tower is equipped with an inertial separator and a screen separator to prevent free water from being brought out of the process air.

The process air from the air precooling system enters the air purification system for adsorption and removal of water, carbon dioxide and hydrocarbons. The adsorbers in the purification system are composed of two vertical containers. The two adsorption containers adopt a double adsorption tower structure, with activated alumina at the bottom and molecular sieve at the top. The other is regenerated by heating dirty nitrogen from the cooler through a heater.

2. Air rectification

Most of the cleaning process air coming out of the air purification system enters the main heat exchanger in the cold box and is cooled by the returning gas, and the air near the dew point enters the bottom of the lower tower for the first fractionation. In the distillation column, the rising gas is in full contact with the downstream liquid, and after heat and mass transfer, the nitrogen concentration in the rising gas gradually increases. In the main condensing evaporator, nitrogen is condensed and liquid oxygen is vaporized. The liquid air and liquid nitrogen produced in the lower tower are supercooled by the cooler and throttled to the upper tower as the return liquid from the upper tower. In the upper tower, product nitrogen, product oxygen, liquid oxygen, and dirty nitrogen are obtained after a second distillation.

3. Cold production

Most of the cooling required by the device is provided by the turbine expander.

The rest of the clean air coming out of the air purification system goes into a supercharger driven by a turbo expander to increase its pressure. It is then cooled by a cooler after the supercharger, into the main heat exchanger in the cooler, where it is cooled to a certain temperature, and then into the turbine expander. This expanded air is expanded and cooled in the expander and then enters the upper tower to participate in distillation.

4. Argon purification

Argon extraction adopts the latest full distillation technology to produce argon. In order to produce argon, an argon fraction gas is drawn from the appropriate position of the lower part of the upper column of the fractionating column and sent to the crude argon column I for distillation, so that the oxygen content is reduced. The reflux liquid of the crude argon column I is the liquid crude argon extracted from the bottom of the crude argon column II by the liquid pump. The gas extracted from the top of the crude argon column I enters the crude argon column II, in which the deep argon-oxygen separation is carried out, and after the distillation of the crude argon column II, the crude argon gas with oxygen content ≤1PPm is obtained at the top of the crude argon column II. The top of the crude argon tower II is equipped with a condensing evaporator, and the liquid air extracted from the subcooler is sent into it as a cold source after being throttled, and most of the crude argon is used as the reflux liquid of the crude argon tower after being condensed by the condensing evaporator. The rest is led from the top of the crude argon tower (crude argon with oxygen content ≤1PPm) and sent to the fine argon tower, the bottom of the fine argon tower is equipped with an evaporator, using the middle pressure nitrogen at the bottom of the tower as a heat source to evaporate liquid argon, and the nitrogen is liquefied at the same time. A condenser is installed on the top of the fine argon tower, and liquid nitrogen of the fine argon evaporator is used as a cold source to condense most of the rising gas into the reflux of the fine argon tower. After distillation of the fine argon tower, the 99.999%Ar of the fine argon liquid obtained at the bottom of the fine argon tower is extracted into the cold box as the product liquid argon.

5.Equipment design and technical characteristics

• Molecular sieve adsorption with full low pressure process, supercharged turbine expansion mechanism cooling, full rectification argon, oxygen external compression process. Advanced technology, mature technology, reliable operation, simple operation, safety and low consumption.
• The pre-cooling system uses nitrogen and dirty nitrogen into the water cooling tower to reduce the temperature of the cooling water, and the air-cooling tower structure adopts reliable anti-immersion measures.
• The channel of the main condensing evaporator adopts a special structure to prevent acetylene accumulation in liquid oxygen and ensure the safety of the main condensing evaporator and the system.
• The upper tower, crude argon tower and fine argon tower are all conventional packed towers.
• The equipment has variable operating conditions and equipment variable load capacity, equipment variable load capacity range of 75% ~ 105%.
• DCS centralized control system is adopted.