Pumping argon and producing high-purity gas, the secret of cryogenic air separation!

In addition to pumping argon and producing high-purity gas, there are no secrets in cryogenic air separation! Of course, there are some little secrets. The heat pump argon pumping process has been briefly described. Next, we will talk about how to produce high-purity oxygen and high-purity nitrogen from the perspective of heat pump distillation.

From the perspective of effective energy, the effective energy difference between high-purity oxygen and ordinary oxygen is very small. In fact, it is a low-energy process to produce high-purity oxygen from ordinary oxygen. But this is not the case. From the perspective of rectification, there are only two degrees of freedom among product purity, theoretical plate number, and reflux ratio (liquid-gas ratio in the distillation section of low boiling point components and gas-liquid ratio in the distillation section of high boiling point components). Theoretically, in Under the condition of infinite theoretical plate number, as long as the actual reflux ratio is greater than the minimum reflux ratio, pure component products can be obtained. From a distillation point of view, the difficulty in producing high-purity products lies in the limited number of theoretical plates available. In the case of a limited number of theoretical plates, how to increase the reflux ratio with the minimum energy consumption to achieve the product purity index.

The impurities in common oxygen are mainly nitrogen and argon (there are also components with a higher boiling point than oxygen, but they are rare and not considered for now). They are light components with a lower boiling point than oxygen. When producing high-purity oxygen, it is critical to use argon rather than nitrogen. The difference between the boiling points of oxygen and nitrogen is much greater than the difference between the boiling points of oxygen and argon. The top separation tower should theoretically be an oxygen and nitrogen separation tower, more precisely an oxygen, argon and nitrogen separation tower. However, the air separation tower of the current double-tower process is a nitrogen-argon-oxygen rectification tower, so although it is theoretically feasible to directly produce high-purity oxygen in the air separation tower, it is very difficult in practice. At present, the highest oxygen purity of the twin-tower process can reach 99.8%. At this time, all the air has entered the lower tower, the low-temperature expander in the basic process of the double-tower process has been canceled, and the liquefaction of the reflux gas in the stripping section of the upper tower has reached the maximum under the double-tower process.

When producing high-purity oxygen, whether it is a new single-tower process or a double-tower process, the purity of the oxygen product depends on the theoretical plate number and gas-liquid ratio column (upper column) from the outlet of the argon fraction to the bottom of the air separation! The higher the number of theoretical trays, the higher the gas-liquid ratio, and the higher the oxygen purity! The number of theoretical plates from the bottom of the argon distillate outlet to the bottom of the air separation column (upper column) is limited by engineering conditions and cannot be increased indefinitely (generally it can only reach 30). When the number of theoretical plates cannot be increased, the only way is to increase the gas-liquid ratio from below the outlet of the argon distillate to the bottom of the air separation column (upper column). Whether it is a double-tower process or a new single-tower process, if the standard process plan is adopted, it can only be realized by increasing the air intake of the lower tower or increasing the nitrogen decompression to increase the air intake and output.

Air enters the condenser and reduces the amount of air in the expander. Energy consumption costs too much! In order to increase the purity of oxygen to more than 99.8% in the double-tower process, all the air enters the lower tower (of course, there will be no low-temperature expander in this case, it is only feasible in the case of an air booster and a high-temperature expander !), this is of course the limit, so no cryogenic air separation expert can achieve oxygen purity above 99.9%, otherwise there will be a danger of explosion!

What do people say?

When the open heat pump rectification and argon extraction process is adopted (the crude argon condenser in the double-tower process is set in the main condenser, and the crude argon condenser in the new single-column process is set in the bottom of the air separation tower), as long as the crude argon compression rate is increased, the It can increase the gas-liquid ratio from the outlet of the argon distillate to the bottom of the air separation column (upper column), and increase the output of crude argon. On the premise of ensuring the oxygen content index of the crude argon, at the same time, the purity of the oxygen product is improved at a lower energy cost.

There are also great difficulties in the production of high-purity nitrogen by the double-tower method.

The problem lies in the lower tower. The upper limit of nitrogen purity in the upper tower is determined by the purity of liquid nitrogen in the lower tower. The lower tower is actually a nitrogen condensation tower, because the pressure inside the lower tower is about 5b α r, it is impossible to use structured packing. The theoretical number of pallets is limited. The minimum reflux liquid-gas ratio of the lower tower is 0576, and now the actual reflux liquid-gas ratio of the lower tower is about 0.61, which is about 5% higher than the minimum reflux liquid-gas ratio. In order to improve the purity of liquid nitrogen in the lower tower, if the number of theoretical trays cannot be increased, the only way is to increase the reflux liquid-gas ratio of the lower tower. The amount of liquid nitrogen is reduced, which will significantly reduce the oxygen extraction rate, which is unacceptable! Of course, the double-tower process can also be realized by adding an open heat pump with high-purity nitrogen as the circulating working fluid in the rectification section of the upper tower.

The condenser of the open heat pump can be arranged at the inlet of the lower tower oxygen-enriched liquid air or at the inlet of the lower tower liquid nitrogen. Although this can produce high-purity nitrogen, it has already entered the scope of patent protection of the new single-tower process v8 M2 k2\! I4 x2 D5 G

The biggest difficulty in the production of high-purity oxygen and nitrogen by the double-tower process is that under the condition of limited number of available theoretical plates, because the cycle volume of the open heat pump with air as the circulating working fluid is limited by the delivery volume of the distillation feed gas, which limits the available It is possible to improve the product purity by increasing the actual reflux ratio under the fixed number of theoretical plates! In fact, in the dual air compressor single tower nitrogen production process scheme, the circulation volume of the air heat pump is greater than the transmission volume of the air raw material through the cryogenic compression of liquid air (the second air compressor)! The production of high-purity oxygen products by the twin-tower method can also be used by analogy. But the cost of energy consumption is still too high.

Using the multi-stage heat pump rectification process, under the condition of a given number of theoretical plates, the production of high-purity oxygen and nitrogen products can achieve lower energy consumption. This will be explained in detail later.