Large air separation FAQ

Generally, the actual cooling capacity of expander is always less than the theoretical cooling capacity. Under the same inlet and outlet pressure and inlet gas temperature, the lower the efficiency of expander, the smaller the unit refrigerant cooling capacity and the smaller the temperature drop efficiency of expander. The efficiency of expander depends on the size of various losses inside the expander. Due to the existence of various losses, the ability of gas to do external work is reduced in varying degrees. To sum up, the losses mainly include the following aspects: 1. When the air flow passes through the work wheel and the deflector, the loss caused by friction and air flow impact on the runner surface is called flow loss. 2. When the impeller is rotating, the friction heat generated by the gas around the wheel disc will increase the temperature of the gas, which is called friction blast loss. 3. When the gas passes through the deflector, the leakage from the gap bet

The load regulation range of low pressure air separation unit is related to the regulating performance of feed air compressor, the regulating performance of expander and the structural characteristics of distillation column. At present, the flow regulation range of turbine air compressor with inlet guide vane is 75% to 100%, and that of turbocharged expander with adjustable nozzle is 65% to 100%. Next, the most important thing is to adjust the residual of the distillation column. At present, the load regulation range of distillation column with structured packing can reach 50% - 100%, while that of traditional sieve tray column is 70% - 100%. If the load is too low, it will cause liquid leakage because the speed of steam passing through the sieve is too slow. When oxygen is sufficient and the oxygen output needs to be reduced, first reduce the output of oxygen products, then reduce the air flow accordingly, and adjust the expansion air volume according to the main cooling level. The

In the distillation column of air separation, the air is usually pre separated in the lower column, and then distilled into oxygen rich liquid air and pure nitrogen, and then further distillation in the upper column to obtain oxygen and nitrogen products. As a condensing evaporator connecting the upper and lower columns, the pressure nitrogen of the lower column is used to heat the liquid oxygen of the upper column. When the liquid oxygen is evaporated, the gas nitrogen is condensed. The two-stage distillation column can make oxygen and nitrogen products obtain higher extraction rate and higher purity. The working principle of single-stage distillation column is similar to that of the lower column of two-stage distillation column. Oxygen enriched liquid air is at the bottom of the column, and pure nitrogen gas can be obtained at the top of the column. The single-stage distillation column can't separate the air perfectly, so it is seldom used in practice. However, because of its simple

Distillation is using the different boiling points of the two substances to condense the mixed steam and vaporize the mixed liquid repeatedly to achieve gas separation. For a mixed liquid composed of two substances with different boiling points, when the heat absorption part evaporates, the volatile components will evaporate more, while the mixed steam will give off heat and partially condense, the non volatile components will condense more. If the saturated steam with higher temperature is mixed with the saturated liquid with lower temperature, the steam will release heat and partially condense, and the liquid will absorb heat part of it evaporates. If after a round of partial evaporation and partial condensation, the steam and liquid with higher concentration will contact with the steam and liquid with different temperature respectively, and then partial condensation and partial evaporation will occur again. This process will be repeated for many times, and finally gas separation

In the fractionating column, the separation of oxygen and nitrogen is realized through the process of two-stage distillation in the upper and lower columns. The distillation process must have ascending air and downflow liquid, and the main function of the condensing evaporator is to exchange heat between the liquid oxygen returned from the bottom of the upper column and the nitrogen rising from the top of the lower column. Condensing evaporator is an indispensable heat exchange equipment in air separation distillation system. In a sense, condensing evaporator is the link between upper column and lower column. The condensing evaporator is also called "main cooling". In the condensing evaporator, the liquid oxygen returned from the upper column absorbs heat and evaporates the gas oxygen; the rising nitrogen in the lower column is condensed into liquid nitrogen due to heat release. A part of the gas oxygen is sent out of the column as the product, and the rest is sent to the upper colu

At present, there are two types of common condensing evaporators in domestic air separation units, plate fin condensing evaporator and tubular condensing evaporator. Tubular condensing evaporator is divided into three types: long tube, short tube and coil. The long tube and short tube are all in row. The tube is made of red copper and the tube sheet is made of brass. It is generally used in small and medium-sized air separation plants. Coil type is generally used for auxiliary condensing evaporator. Because there is no fixed liquid level in the coil, the heat transfer coefficient is small, so it has been gradually eliminated. The plate fin condenser evaporator is made of all aluminum structure, and the plate unit is arranged in vertical star shape. Its characteristics are compact structure, light weight, small volume and easy to manufacture. It is widely used in large and medium air separation plants. Due to the gradual enlargement of the air separation unit, the heat excha

The air is mainly composed of 78.03% nitrogen and 20.93% oxygen and other gases. Air separation is to cool the air to a certain low temperature firstly and liquefy it into liquid air. Reuse two liquids oxygen and nitrogen with different boiling points (At atmospheric pressure, the boiling point of oxygen is - 183.98°C and that of nitrogen is - 195.8°C), separation is carried out in an air fractionator equipped with sieve plates. The air separation column is also called distillation column. Air distillation column can be divided into single-stage distillation column and double-stage distillation column, and single-stage distillation column can only produce one kind of pure product. Two stage distillation column is used to produce high purity nitrogen and oxygen in common air separation unit.

The purified air enters the fractionator and exchanges heat with the returned polluted nitrogen, oxygen and nitrogen through the main heat exchanger. After cooling, a small part of it is liquefied, and the gas-liquid mixed air is throttled to the pressure of 0.55 MPa. After distillation, 99.999% pure nitrogen is obtained at the top of the lower column, and then it enters the main cooling column, and is condensed into liquid nitrogen by evaporation of liquid oxygen in the upper column. A part of liquid nitrogen is returned to the lower column as reflux liquid, and the remaining liquid nitrogen is throttled after cooling. Enter the sprayer at the top of the upper column. The bottom residue is liquid air containing 38% oxygen. After passing through the liquid air subcooler, it enters the middle of the upper column to participate in distillation. At the same time, the polluted liquid nitrogen flow is drawn out from the middle of the lower column and then enters the upper column as reflux l

Temperature difference is the power of heat transfer. As long as there is temperature difference between two materials, they will carry out the heat or cold transfer spontaneously. The heat is always transferred from the fluid with higher temperature to the lower temperature fluid. Although the heat transfer process of various heat exchangers is not identical, the heat transfer law is basically the same. The following is an example of the most widely used dividing wall heat exchanger: heat is transferred from hot fluid to cold fluid through the wall and flows in the opposite direction. The whole heat transfer process is divided into three stages. In the first stage, convection heat transfer: the heat flow body transmits heat to the wall surface. This process not only has heat transfer caused by the relative displacement of each part of the fluid, but also has heat conduction in the fluid, which is the comprehensive function of convection and heat conduction. In the second

In order to reduce the air temperature in the low temperature air separation unit, various heat exchange equipment are needed for cold and heat exchange in various air separation systems, such as nitrogen water precooler, switching heat exchanger, main heat exchanger, condensing evaporator, subcooler, liquefier, vaporizer, electric heater, etc. These heat exchange equipments are installed in each system of the air separation unit, which are necessary for air liquefaction and gas separation. According to the principle of heat transfer, the heat exchanger of air separation unit can be divided into three types: 1. Dividing wall type: the cold and hot fluid is separated by the heat transfer wall, the medium does not directly contact in the heat transfer process, and the hot fluid transfers heat to the cold fluid through the heat transfer wall. For example, tube heat exchanger and plate heat exchanger. 2. Heat storage type: the cold and hot fluid alternately flows through the solid

The water supply source of air cooling column in precooling system equipment is cooling water pump, and water cooling column is supplied by chilled water pump. Their structures and principles are described as follows: 1. Design structure: the most basic components inside the centrifugal pump are high speed impeller and fixed volute pump case. Generally, the impeller with several (4 ~ 12) backward curved blades is fixed on the pump shaft and driven by the motor to rotate with the pump shaft at high speed. The work done by these impellers will directly act on the liquid in the pump body, which is the energy supply source device of centrifugal pump. 2. Principle: after the centrifugal pump is started, the pump shaft directly drives the reverse curved impeller to move at high speed, squeezing the pre filled liquid between the reverse curved spaces of the blade to rotate. Under the high-speed centrifugal force formed by the movement of the pump shaft, the liquid moves radially from th

Due to improper adjustment of seal gas pressure during operation, liquid oxygen pump can not pump liquid or leak.   From the quality point of view, when the sealing pressure is too high, there will be gas leakage into the pump through the labyrinth seal, resulting in the impeller with gas or even idling, resulting in liquid injection or pressure drop; when the air pressure before sealing is too low, there will be liquid oxygen leakage.   From the quantity point of view, when (p1-p3...