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Why is the liquid oxygen temperature lower than the gas nitrogen temperature in the condensing evaporator of the air separation plant manufacturer for endothermic evaporation?

2021-05-08 H:45:56

When the heat transfer surface of condensing evaporator is insufficient, the air separation plant manufacturer must increase the heat transfer temperature difference by increasing the down tower pressure to ensure a certain heat load. The characteristic curve of the air compressor shows that with the increase of the discharge pressure, the air quantity and oxygen production decrease. If the air temperature in front of the expander increases, the unit cooling capacity will increase, while the expansion capacity will decrease, and the total cooling capacity will remain unchanged, that is, the temperature changes within the allowable range, which has little impact on the operation of the air separation equipment. If the temperature of expansion air increases and the cooling capacity of expander decreases, the liquid air will decrease and the liquid oxygen level will decrease because the temperature before throttling also increases.

The better the ability of liquid oxygen of the air separation equipment manufacturer to wash the wall of the channel, the more difficult it is for acetylene and other hydrocarbons to precipitate on the wall and carbon dioxide particles to block the channel section; From the point of view of heat transfer, because the boiling heat transfer coefficient of the liquid oxygen side of the plate fin condensing evaporator is related to the fluid flow, the better the flow is, the greater the heat transfer coefficient is. The refrigeration capacity of throttling effect of air separation equipment manufacturer is that the air with higher pressure enters the air separation equipment and expands by reducing the pressure of throttling valve, pipeline and equipment. Generally speaking, the throttling process will cause the temperature to drop and the ability of the gas to take away the heat, that is, the heat that the low-pressure gas can take away when it returns to the same inlet temperature when it leaves the equipment. In other words, at the same temperature, the energy (enthalpy) of high-pressure gas is less than that of low-pressure gas, and the difference between the two energy (enthalpy) is the heat absorbed, which is the so-called throttling effect refrigeration capacity.

When the outlet temperature of expander drops below - 150 ℃ to - 165 ℃, the amount of carbon dioxide precipitation is large. Therefore, when the temperature does not drop to this value, the large bypass valve can be opened; When the temperature reaches - 150 ℃, close the bypass valve and the circulating current to make it pass through the CO2 freezing zone quickly to avoid blocking the expander. In the production process of synthetic ammonia, in addition to nitrogen, oxygen is used in high temperature cracking of heavy oil and gasification of pulverized coal, so as to strengthen the process and increase the output of chemical fertilizer. Generally speaking, a 100000 t / a ammonia plant needs 10000 m3 / h oxygen generator. In addition, a large amount of oxygen is also needed for desulfurization and recovery when natural gas is recombined to produce methanol, ethylene, propylene and oxidized to produce oxides. The oxygen consumption of a ton of product is in the range of 300 ~ 1000m3 / T, and an oxygen generator of 10000 ~ 30000m3 / h must be equipped.

In the air separation stage, with the cooling of the switching heat exchanger, in order to ensure self cleaning, the switching cycle must be shortened. The shortening of the switching period is helpful to remove the frozen water and carbon dioxide. However, the air switching loss is very large. When the heat transfer surface of the condensing evaporator is insufficient or deteriorated, the temperature difference of the air separation unit manufacturer increases, which reflects the increase of the lower tower pressure. The temperature of liquid oxygen and gas nitrogen can be controlled according to its pressure and purity. In practical operation, the pressure of upper and lower towers, the purity of gas nitrogen and liquid oxygen and the height of liquid oxygen surface are controlled, rather than the temperature difference of condensing evaporator.

After the air compressor stopped automatically, the pressure in the air cooling tower decreased, and when the air compressor started again, the molecular sieve purifier water inflow accident occurred. The reason is due to the failure of the automatic water level control valve and the check valve of the return water system. When the air compressor stops automatically, the air of the air compressor enters into the check valve at the outlet of the air compressor and is filled with water, and the air compressor starts again. Some liquid air from the manufacturer of air separation equipment is completely evaporated after entering the condensing evaporator of the crude argon tower, and then the steam is sent to the upper tower. Its characteristics are opposite to the first one. Generally, the third way between the two ways is adopted, that is, part of the liquid air passes through the crude argon tower, but not completely evaporated. This can not only ensure the normal operation of the crude argon tower, but also reduce the impact on the main tower.

Air separation plant production