There are three argon extraction schemes in the tower process, one is the crude argon condensing tower scheme, the other is the conventional oxygen argon distillation tower scheme, and the third is the open heat pump oxygen argon distillation scheme. The second and third schemes are both oxygen argon distillation schemes, while the first scheme is condensation separation scheme. What are the advantages and disadvantages of the two schemes?
The common point of the three argon extraction schemes is that the argon fraction containing 10% argon is extracted from the middle of the upper tower stripping section. There are also great differences between them. In the crude argon condensing tower scheme, the amount of argon fraction is 1-2 to 1-3 times of oxygen output, while in the second and third technical schemes, the amount of argon fraction is 0-4 times of oxygen output, which is about 1 / 3 of that in the crude argon condensing tower technical scheme. In addition, the crude argon condensing tower technical scheme also has the same amount of liquid argon rich fraction returned to the upper tower. The second and third technical solutions do not have this problem.
Now compare the pros and cons between crude argon condensing tower and oxygen argon distillation scheme with conventional layout.
Whether the first or second scheme is adopted, for the air separation unit, the amount of air passing through the expander remains unchanged, the cooling capacity balance of the unit remains unchanged, the liquid nitrogen in the upper tower remains unchanged, and the oxygen output and oxygen extraction rate remain unchanged. But the argon extraction part changes greatly.
Firstly, if the second scheme is adopted, the purity of oxygen products will be greatly improved when the number of trays remains unchanged. The reason is that the actual gas-liquid ratio of the second scheme is much larger than that of the first scheme, whether from the bottom of the upper tower to the argon fraction outlet or from the argon fraction outlet to the top of the stripping section. Of course, the reasons are different, From the bottom of the upper tower to the argon fraction outlet, the gas-liquid ratio increases due to the decrease of the amount of oxygen at the bottom of the upper tower and the corresponding increase of the amount of rising gas. From the argon fraction outlet to the top of the stripping section, the return amount of oxygen enriched air is greater than the amount of argon distillation outlet.
Secondly, the problem of nitrogen resistance can be avoided to a great extent. Even if nitrogen resistance occurs, it is very simple to deal with. Firstly, due to the crude argon condensing tower scheme and the crude argon rectifying tower scheme, when the nitrogen content in the crude argon is the same, the nitrogen content in the corresponding argon fraction is twice the same, that is, the nitrogen content in the argon fraction extracted from the crude argon rectifying tower scheme can be twice higher than that of the crude argon condensing tower. At the same time, the actual gas-liquid ratio in the stripping section of the crude argon tower scheme is greater than that of the crude argon condensing tower scheme, If the number of trays remains unchanged, the nitrogen content in the argon fraction with the crude argon tower scheme is lower than that with the crude argon condenser scheme. The joint action of the two factors greatly reduces the possibility of nitrogen resistance. Even if nitrogen resistance occurs, it is very simple to adopt the second scheme, i.e. crude argon tower scheme, as long as the extraction quantity of argon fraction is reduced.
If the first scheme is adopted, the argon extraction rate of the whole unit can only reach about 85%, the argon extraction rate of the crude argon condensing tower is only about 33%, while the argon extraction rate of the whole unit can reach more than 90%. Argon is a high-value product of air separation, and the extraction rate is a very important index.
A stripping section is added in the second scheme, which generally requires about 30 theoretical trays. The engineering condition limit refers to the height of the argon extraction tower. In order to make the liquid argon rich fraction flow back to the upper tower, the crude argon condensation tower is suspended. After adopting the second scheme, keep the height of the argon extraction tower unchanged, and there is sufficient space to arrange the stripping section from the suspended position of the crude argon condensation tower to the ground. What really needs to be considered is that when the argon extraction rate increases and the argon output increases, it is necessary to increase the number of theoretical trays in the distillation section, and this problem can not be solved by increasing the condensation amount of crude argon, because when the condensation amount of crude argon reaches a certain amount, increasing the condensation amount of crude argon condenser can only bring a small increase in the liquid-gas ratio in the distillation section, and the energy consumption will be greatly increased. The solution is simple.
The first is to extrude part of the theoretical plate number from the stripping section to the distillation section under the condition of keeping the height of the argon stripping column unchanged. Because the crude argon distillation scheme does not have the problem of argon rich liquid returning to the upper tower, and the number of theoretical trays that can be arranged from the suspended part of the crude argon condensing tower to the ground exceeds the number of theoretical trays required in the stripping section. The second is that the number of theoretical plates in the distillation section can be increased by segmenting the argon extraction column. Thirdly, multi heat pump technology can be used to control the number of theoretical trays required in the distillation section. Of course, the first method is the simplest. The second method has been used in the process of crude argon condensing tower. The third method will inevitably lead to the complexity of the process. It is necessary to use it only when preparing high-purity argon products (for example, when the oxygen content in crude argon is required to be less than 0.01ppm).
In a word, the second scheme is much more advantageous than the current crude argon condenser scheme. As for why the crude argon condensing tower scheme is still the only practical technical scheme, the reason can only be explained by path dependence.
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