The throttling expansion process of the throttle valve and the expansion process of the expander often encountered in the oxygen generator can be regarded as an adiabatic process. After expansion, the pressure decreases. However, the former is irreversible adiabatic expansion, and the entropy value increases before and after expansion.For the all low pressure air separation unit which uses freezing method to remove moisture and CO2 from the air, because sufficient backflow gas is needed to carry the frozen moisture and CO2 out of the unit, the pure nitrogen (99.999%) yield is only 1.1 times of the oxygen yield. For the molecular sieve purification air separation process of extracting argon fraction, the yield of pure nitrogen is not subject to the above restrictions.According to the characteristics of throttling process, the energy (enthalpy) before and after throttling remains unchanged, but the corresponding saturation temperature also decreases due to the decrease of pressure after throttling. The specific enthalpy (H ′ 2) of low-pressure saturated liquid is smaller than that before throttling. Therefore, some liquid (y) must be gasified into low-pressure saturated steam, and its enthalpy value is h ″ 2, so that the sum of energy of gas and liquid remains unchanged. That is: H1 = (1-y) H ′ 2 + YH ″ 2, y = (h1-h ′ 2) / (H ″ 2-h ′ 2).
When the full low pressure oxygen generator starts up, when it comes to the stage of liquid oxygen accumulation, the surplus refrigeration capacity of the expander should be transferred to the tower as far as possible for liquid accumulation. Since all the liquid in the main cooling system of the upper tower comes from the lower tower, in order to accumulate liquid in the main cooling system, the liquefier should be used to provide as much liquid as possible.For the main heat exchanger of molecular sieve adsorption process, the main reason for the decline of heat transfer performance is the improper operation of the adsorber. Due to the inflow of water into the molecular sieve adsorber, or due to the impact of air flow, the molecular sieve is powdered, which brings the powder into the heat exchanger and adheres to the surface of the heat exchanger channel, affecting the heat transfer performance, resulting in the expansion of the temperature difference at the hot end.In this way, continuous refrigeration can be realized. In the oxygen generator, it is also a refrigeration process to reduce the air temperature to the liquefying temperature. Therefore, there must be a compressor at the cost of power consumption. Only in the oxygen generator, air is used as the working medium, and the purpose of cooling is achieved by first compressing and then expanding the air. Then cool the air itself until it reaches the liquefying temperature.
Scale attached to the cooler wall, nitrogen water precooler packing, nozzle or sieve, not only affect the heat transfer, reduce the cooling effect, but also hinder the circulation of cooling water or air, serious equipment failure will be caused, such as nitrogen water precooler with water, so that the accumulator (or heat exchanger) frozen. The scale is hard and hard to remove.DEAR Air separation plant.When the liquid level of condensing evaporator is too high, part of liquid oxygen can be discharged. This can not only make the liquid level drop rapidly, but also remove some impurities, which is conducive to safe operation. If the equipment is equipped with argon tower, the liquid oxygen level should be raised in advance to accumulate cold capacity, and then the argon tower should be started.DEAR Air separation plant.The switching heat exchanger has supercooling and excess cooling capacity. At the same time solve these two problems, the cold capacity can accumulate in the tower, the liquid level can continue to rise. The "on-off-on" operation refers to the opening and closing of the liquid nitrogen control valve at different stages of plant startup.
