1 Advantages and disadvantages of nitrogen cyclic pressurization internal compression process
Linde company of Germany developed the first air separation equipment for the internal compression process of the liquid oxygen pump in 1970s. But this set of 35100 m used in a fertilizer plant in Navmada, India3/H air separation equipment is not currently commonly used to vaporize high-pressure liquid oxygen in the main heat exchanger by using high-pressure air produced by the Air booster as heat source, but using nitrogen circulating booster, after reheating the gas and nitrogen on the top of the pressure tower through the heat exchanger, and then compressing them into high-pressure nitrogen through the nitrogen circulating booster, the high-pressure nitrogen enters into the vaporized liquid oxygen of the high-pressure heat exchanger, and enters the pressure Tower cyclically after it is liquefied. Three years later, on 1979, Linde company provided 3 sets of 28000 m for Zhejiang Zhenhai refinery, Ningxia chemical plant and Urumqi fertilizer plant3In the/h air separation plant, the internal compression process of the liquid oxygen pump which adopts the nitrogen circulating booster is pushed out again.
At that time, the internal compression process of air booster which is commonly used at present was not adopted, but there were two main reasons for the internal compression process of nitrogen circulating booster: (1) These 3 sets of 28000 m3/H the air separation plant still adopts the procedure of removing moisture and carbon dioxide in the air by the freezing method of switching heat exchanger. Before the air enters into the cold box, there is not a stream of air that is dry and contains a very small amount of carbon dioxide. If the air that is not purified is used as the pressurized air, then when this stream of air exchanges heat with the backflow low temperature medium, the moisture and carbon dioxide in the air will inevitably freeze in the high-pressure air channel. This problem is avoided by compressing the low pressure nitrogen from the cold box to the required pressure and then sending it to the main heat exchanger. (2) due to the production capacity required by these 3 sets of air separation plant: oxygen with a pressure of 9.6MPa is 28000 m3/H and nitrogen with the pressure of 8.ompa 37000 m3/h According to the calculation at that time, the pressure of nitrogen outlet circulating booster reached 12MPa. Therefore, the adoption of nitrogen circulating booster not only solves the heat source of vaporized high-pressure liquid oxygen, but also meets users' requirements for high-pressure nitrogen, which saves 1 high-pressure nitrogen presses with large flow compared with the adoption of air booster, it greatly saves the investment of equipment.
However, if there is a stream of purified air from the raw material air compressor as the heat source of vaporized high-pressure liquid oxygen, then the adoption of nitrogen cyclic pressurization internal compression process has great disadvantages, that is, compared with the internal compression process of air pressurization, it has higher energy consumption, mainly due to the following reasons:
(1) nitrogen is used for heat exchange and condensation, and its performance is worse than that of air. For example, the condensation temperature of air is higher than that of nitrogen. If we want to reach the same condensation temperature, we need to make the pressurization pressure of nitrogen higher than that of air; the condensation heat of air is also higher than that of nitrogen, the amount of nitrogen needed to vaporize the same high pressure liquid oxygen is larger than that of air.
(2) air pressurization process the constant heat loss of gas in the main heat exchanger is less than that of nitrogen circulating pressurization process. Because of the adoption of air pressurization process, this part of Air only enters the main heat exchanger once, so its no-heat loss is only once; and nitrogen cyclic pressurization process, nitrogen enters the main heat exchanger as a part of the raw material air, which forms the first no-heat loss. After the nitrogen of the main heat exchanger is pressurized, it enters the main heat exchanger again, causing the second no-heat loss.
(3) because the nitrogen circulation pressurization process needs to extract a large amount of nitrogen from the distillation column as the air source of evaporating oxygen, the air volume is much larger than the air circulation. The amount of nitrogen extracted directly from the top of the medium pressure tower is too much, which will affect the reflux ratio in the distillation tower and destroy the original good rectification efficiency, leading to the rapid decrease of the extraction rate of oxygen and argon, and the increase of the flow rate of the air compressor, the whole device energy consumption increased. Just because the nitrogen cyclic pressurization process has the shortcoming of high energy consumption, after the normal temperature adsorption purification Air process appears, the internal compression process of air pressurization replaces the internal compression process of nitrogen cyclic pressurization.
