Large air separation FAQ

Cryogenic air separation nitrogen production uses air as raw material, which is compressed, purified and heat exchanged to liquefy gas air into liquid air. Liquid air is mainly a mixture of liquid oxygen and liquid nitrogen. Liquid oxygen and liquid nitrogen have different boiling points and are separated by distillation to obtain nitrogen. 1. Air compression and purification The air passes through the air strainer to remove dust and mechanical impurities, then enters the air compressor, compresses to the required pressure, and then goes to the air cooler to reduce the air temperature. It then enters the air drying purifier to remove moisture, carbon dioxide, acetylene and other hydrocarbons from the air. 2. Air separation The purified air enters the main heat exchanger in the air separation column, and is cooled to the saturation temperature by the reflux gas (production nitrogen gas, waste gas), and is sent to the bottom of the distillation column. The nitrogen is obtained fr

Shell and tube air separation heat exchanger is the most representative wall heat exchanger. It has a long history of application and is still dominant in all heat exchangers. Shell and tube heat exchanger is mainly composed of shell, tube bundle, tube sheet and head. Most of the shell is round, and there are parallel tube bundles inside. Both ends of the tube bundle are fixed on the tube plate. In shell and tube heat exchanger, there are two kinds of fluids for heat exchange. One kind of fluid flows in the tube, and its stroke is called tube side. The other flows outside the tube and its stroke is called shell side. The wall of tube bundle is the heat transfer surface. In order to improve the heat transfer coefficient of the fluid outside the tube, a certain number of transverse baffles are usually installed in the shell. The baffle can not only prevent the short circuit of the fluid and increase the velocity of the fluid, but also force the fluid to flow through the tube bundle man

When the exhaust temperature of the piston compressor is higher than the design value, the most direct impact is that the lubricating oil is carbonized, which leads to the compressor parts burned out because of not getting good lubrication. If the temperature of the compressor is too high during actual operation, the cause should be analyzed in time and corresponding preventive measures should be taken. The following methods can be used for reference in practice. 1. Reducing the inlet air temperature can directly reduce the exhaust temperature. Check the intercooler to ensure the cooling effect of the intercooler. 2. The installation of the valve and the selection of the spring should be standardized to ensure the normal operation of the valve and minimize the loss of inlet and outlet pressure. 3. Strengthen the cooling of cylinder to ensure the normal compression process index. 4. Internal leakage is the main reason for the increase of exhaust temperature. 5.For multist

Generally, the main reason for the vibration of piston type oxygen compressor is the action of alternating load. There are two kinds of alternating loads: one is the unbalanced inertia force, the other is the interference force caused by the strong gas pressure pulse in the gas pipeline caused by the discontinuous gas supply of the compressor. The vibration caused by unbalanced inertia force can be eliminated and reduced from the structure of the machine itself. The specific methods are as follows: reasonably arrange the crank stagger angle; properly select the mass configuration of reciprocating parts. In addition, the basic design and manufacturing process of the compressor should also strictly comply with the technical specifications. For the vibration caused by air flow pressure pulse, a buffer can be installed near the cylinder exhaust pipe nozzle; the appropriate installation position of orifice plate should be selected on the gas flow channel; the shock absorber should be

When the main condenser evaporator leaks seriously, a large amount of high-pressure nitrogen will leak to the low-pressure oxygen side, and the pressure of the upper and lower columns and the purity of the product will change significantly until normal production cannot be maintained. When the main condenser evaporator leaks slightly, the pressure of the upper and lower columns will not change significantly, and the purity of liquid oxygen in the main condenser evaporator will not decrease significantly. The common phenomenon is that the purity of the gas and liquid oxygen in the main condensing evaporator is very different, and the gas concentration is lower than that in equilibrium with liquid oxygen. For example, the concentration of liquid oxygen is 99% and that of gaseous oxygen is 96%, that 7 main cooling pipes were found to be leaking during maintenance. The common causes of leakage are as follows: 1. Pipes are worn due to mutual vibration. For long tube condensing evap

Expander is a kind of mechanical equipment that uses compressed gas to expand and depressurize, so that the gas temperature can be reduced to obtain the cooling capacity. In the air separation unit, the expander is an important equipment to provide cooling capacity for the air separation unit, and its cooling capacity accounts for 80% - 90% of the cooling capacity of the air separation unit. The main structural forms of expander are piston type and turbine type. The piston expander is suitable for air separation process with large pressure drop and small flow rate because of its convenient manufacture and low speed. Turbine type has the characteristics of large gas flow, high efficiency, stable and reliable operation and no friction parts. Due to the low pressure air separation process used in large air separation units, the pressure drop and unit cooling capacity are very small, and because the expansion air is sent to the upper column for rectification, the expander is requi

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