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What harm does N2O do to air separation equipment?
A: The molecular formula of nitrous oxide is N2O, also known as nitrous oxide, commonly known as laughing gas.The concentration of nitrous oxide in the atmosphere is about 3 times 10-9.With the deterioration of ecological environment, its content increases by 0.2% ~ 0.3% per year.
The amount of nitrous oxide generated by soil microorganisms during their oxidation and denitrification activities in soil and ocean accounts for one-third of the atmospheric nitrous oxide content, and another two-thirds is generated artificially.For example, the combustion of fossil fuels, organisms, wastes, sewage treatment, fermentation sources, automobile exhaust, etc. all lead to the generation of N2O.In the vicinity of the N2O generating source, the N2O content in the atmosphere can be more than 3 times 10-6.Although N2O is chemically inert and neither corrodes nor explodes, its physical properties are hazardous to air separation.The critical temperature is 309.7k, the critical pressure is 7.27mpa, and the three phase points are 182.3k and 0.088mpa.It has sublimation property under the pressure and temperature of air separation device.At atmospheric pressure, the boiling point is 185K, higher than that of N2, O2 and Ar. Therefore, in the separation process of oxygen and nitrogen, it will be concentrated in liquid oxygen.
The solubility of N2O in water is very small. After the processed air passes through air filters, compressors, coolers and water separator, N2O cannot be separated and removed.Most N2O will be brought into the molecular sieve purifier. The adsorption capacity of the molecular sieve to N2O is lower than that to CO2.N2O enters the distillation column through the adsorption bed layer, and in the co-adsorption process of H2O, CO2, C2H2 and other hydrocarbons by the molecular sieve, CO2 can replace the N2O molecules already absorbed by the molecular sieve.Therefore, the molecular sieve cannot clear N2O.In the main heat exchanger, the processed air is cooled to near the liquefaction temperature, and N2O first condenses into a solid, causing the air passage to block.When the air pressure of processing is 0.6mpa and the N2O content is 1 x 10-6, the setting temperature of N2O is 113K.
In the distillation column, because N2O is divided into high boiling point components relative to N2, O2 and Ar groups, it will be dissolved in liquid oxygen, making it impossible to obtain high purity liquid oxygen and gas oxygen products at the bottom of the upper column.When the purity of oxygen products was 99.5%, the average N2O content was 1.4x10-5.In addition, when the discharge of liquid oxygen is insufficient, N2O accumulates continuously in the liquid oxygen. When the N2O content in the liquid oxygen is greater than 50 x 10-6, it will precipitate out in a solid state and block the main condensation evaporator channel.
In the production of rare gas krypton and xenon, with the concentration of krypton and xenon, N2O is also concentrated.The content of N2O can reach 100 x 10-6 ~ 150 x 10-6.N2O itself does not burn, but it can be thermally decomposed.This will affect the elimination of catalytic combustion of crude krypton, CH4 in xenon and the adsorption of water and carbon dioxide generated by using molecular sieves.
Due to environmental problems, the concentration of N2O in the air keeps increasing.Moreover, industries such as electronics require higher and higher levels of oxygen product purity (99.99% ~ 99.9999%), so the removal of N2O from processed air is more important than in the past.A better cleaning method is to find the appropriate molecular sieve. In the molecular sieve purifier, H2O, CO2, C2H2 and N2O in the processed air are absorbed and removed.
What is liquid suspension?
A: in the distillation column, the liquid flows down the column plate through the overflow bucket, contacting with the steam at higher temperature on the column plate, and the process of heat transfer and partial evaporation and partial condensation occurs.If the liquid on the tray is difficult to flow down the overflow bucket, causing the liquid level in the overflow bucket to rise higher and higher until it is level with the liquid surface on the tray, and the liquid cannot flow down, it is called "liquid suspension" or "liquid flooding".As shown in figure 56, when the liquid level in the overflow bucket exceeds 50% of the height of the overflow bucket, slight flooding is considered to begin.
When the gas and liquid in the distillation column flow normally, as the steam flows from bottom to top, the resistance of the column plate should be overcome.Therefore, the pressure at the top (P2) is lower than the pressure at the bottom (P1).The pressure difference (p1-p2) reflects the resistance of each plate.The liquid flow from the bottom to the top of the pressure is from the bottom. Therefore, the liquid level in the overflow tank must be higher than that on the tower plate to flow out.At the same time, when the liquid flows through the overflow bucket, the liquid also has to overcome the resistance at the outlet. Therefore, only when the pressure in the overflow bucket is able to overcome the pressure difference between the upper and lower pressure on the tower plate and the resistance of the overflow bucket can the liquid flow smoothly and the liquid level remains stable.
When the resistance of the tower plate increases, the pressure difference between the top and bottom of the tower plate increases, or the resistance of the overflow bucket increases, the original liquid height in the overflow bucket is not enough to overcome the pressure difference and resistance, so the liquid cannot flow down temporarily.When the liquid level in the overflow bucket rises to a certain height, a new balance is reached.When the resistance of the tray or overflow bucket is too high, the liquid level in the overflow bucket will continue to rise until it is level with the liquid level of the upper tray, and the liquid level on the tray will also rise.When the liquid on the tray rises to the point where the vapor cannot hold it, it flows down the screen.If the cause of flooding is not eliminated, the process is repeated.
It can be seen that when there is liquid flooding in the tower, the resistance and liquid level will fluctuate greatly.At the same time, the rectification process in the tower is destroyed, the product purity is often not up to the requirement, and fluctuates greatly, and the normal production cannot be maintained.In the operation should try to avoid the occurrence of flooding and timely processing.
What is the extraction rate of oxygen?
A: in the use of air separation method to produce oxygen, always hope to process as much as possible in the air as a product separation.In order to evaluate the degree of separation perfection, the concept of oxygen extraction rate was introduced.
he rate of oxygen extraction is expressed by the ratio of the total oxygen content in the product to the total oxygen content in the air processed in the tower.namely
In the formula, the milk - oxygen extraction rate;
Vo2, Vk-- oxygen production and processed air volume, m3/h;
Yo2, yk-- volume fraction of oxygen contained in product oxygen and air.
As can be seen from the above equation: for a certain place, the oxygen content in the air is basically unchanged.When the air intake and product oxygen purity are certain, the level of oxygen extraction rate depends on the amount of oxygen yield.And the amount of oxygen production, for the total low-pressure oxygen production machine under a certain amount of air intake, mainly depends on the level of oxygen in the pollution nitrogen.Taking the air separation unit of 3200m3/h as an example, when the air entering the tower is 18100m3/h, the air pollution nitrogen is 60.2% of the processed air, and the volume fraction of oxygen in the air pollution nitrogen is 5.5%, the oxygen yield is 3200m3/h, and the oxygen purity is 99.6%.Thus, the oxygen extraction rate can be calculated
At the same time, it can be calculated that the amount of oxygen lost with pollution nitrogen is 18100(m3/h) x 60.2/% x 5.5%=599.2m3/h.If the oxygen content in the sewage nitrogen increases to 7.5%, the oxygen quantity that runs with the sewage nitrogen is:
18100 (m3 / h) * 60.2% * 7.5% = 817.2 m3 / h
Thus it can be seen that oxygen production will decrease (817.2-599.2)m3/h=218m3/h, that is, oxygen production is (3200-218)m3/h=2982m3/h.At this time, the oxygen extraction rate was 1 bar =2982 x 99.6/(18100 x 20.9)=0.718=71.8%.
Therefore, efforts should be made to reduce the oxygen content in the pollution nitrogen, so as to produce more oxygen and improve oxygen extraction rate.
The oxygen extraction rate of the whole low-pressure distillation column was only 80% ~ 85% before, now it has been raised to 90% ~ 95%, the most advanced is even up to 99% or so.
Why some of the distillation column under the tower pumping liquid nitrogen, some of the lower tower does not pump liquid nitrogen?
A: in order to achieve the self-cleaning of water and carbon dioxide, the oxygen generator of the switching heat exchanger requires a large amount of nitrogen from the air pollution to guarantee the non-freezing condition. Therefore, the volume of pure nitrogen products is small, and the maximum can only be up to 1.3 times of the production of oxygen gas.As a result, the lower tower provides the upper tower with less pure liquid nitrogen, can draw a stream of dirty liquid nitrogen to the upper tower.In this way, the reflux ratio of the rectification section of the upper tower can be increased, so that it has greater rectification potential, so as to allow more expansion air into the upper tower, so as to prevent the pass-through of expansion air and affect the oxygen extraction rate.
For the oxygen generator of the pressurized expansion process of molecular sieve purification, the volume of pure gas nitrogen product is greatly increased because it is not limited by self-cleaning.In addition to ensuring the amount of waste nitrogen used for regeneration of molecular sieve purifier, the rest can be sent out as pure gas nitrogen products, and the ratio of pure nitrogen yield to oxygen yield can be as high as 3 ~ 3.5.In this way, the lower tower needs a larger reflux ratio to guarantee the amount and purity of pure liquid nitrogen, which is then sent to the upper tower for reflux.
In addition, since the process is pressurized and expanded, the unit refrigerating capacity of the expanded working medium is relatively high. Under the premise of compensating the same cooling loss, the required expansion amount is relatively small, which generally does not exceed the amount of air allowed in the upper tower.Therefore, it is not necessary to directly increase the reflux ratio of rectification section by pumping liquid nitrogen.At the same time, by not pumping the liquid nitrogen, the lower tower reduces the number of outlets, pipes and valves, and the flow is simplified.
In conclusion, the production of pure gas nitrogen products under the oxygen machine tower generally does not pump liquid nitrogen.In the case of pressurized expansion and less expansion, the lower tower pumping liquid nitrogen is more unnecessary.