I. Brief introduction
Oxygen is widely used in industry and daily life: for example, the electric furnace steelmaking of black metallurgy, in addition to shortening the melting time by blowing oxygen, it strengthens the reaction of removing impurities and saves electric energy; in Blast Furnace Ironmaking, adopting "oxygen-enriched coal injection" technology can reduce coke consumption and production cost, and increase blast furnace productivity; oxygen-enriched combustion in the melting furnace of non-ferrous metal smelting (copper, lead, zinc, aluminum, etc, it can effectively improve productivity, save energy and reduce consumption, and extend the service life of the Furnace. At the same time, due to the sharp decrease of flue gas, the emission of NOx and other harmful substances is greatly reduced, which is beneficial to environmental protection; oxygen can improve the quality and efficiency of powder coal oxygen-enriched gasification, chemical oxygen-enriched gas, light industrial fermentation, carbon black production and so on in chemical production; in addition, oxygen is also widely used in metal cutting and welding, water treatment, fish cultivation, ozone production, paper industry pulp bleach, solid waste incineration, glass manufacturing, medical oxygen and family health care, etc. The air contains 21% (dry state, volume concentration) of oxygen, which is the cheapest raw material for oxygen production. Therefore, oxygen is usually produced by air separation. The process of air separation and oxygen generation can be divided into two types: deep cooling separation process and non-low temperature separation process.
The cold separation process is the traditional method of air separation and oxygen generation. The first cold air separation oxygen generator in the world was born in 1903 and invented by German Carl Linde. It has a history of more than 100 years. Before 1970s, the subcooling separation process occupied an absolute monopoly position in the field of air separation and oxygen generation, and until now, due to the characteristics of high purity of oxygen and various kinds of products, it still occupies a dominant position in the large air separation oxygen generation unit. The process of non-low temperature air separation and oxygen generation includes variable pressure absorption method and membrane separation method. At present, the application of membrane separation method to air separation and oxygen generation is not mature and basically has not been applied in industry. In recent twenty years, the technology of oxygen generation by pressure conversion adsorption air separation has been developed rapidly. The technology is becoming more and more mature, more and more widely used, and the scale is becoming larger and larger, in many applications, the cooling separation oxygen generation device can be replaced by the pressure conversion absorption oxygen generation device, which is currently the mainstream of non-low temperature air separation oxygen generation technology.
II. The principle of pressure conversion and absorption air separation and oxygen generation
Both nitrogen and oxygen have four-pole moment, but the four-pole moment (0.311) of nitrogen is much larger than that of oxygen (0.10, therefore, the adsorption capacity of nitrogen on the zeolized molecular sieve is stronger than that of oxygen (nitrogen has strong interaction with ions on the surface of the molecular sieve, as shown in Figure 1 ). Therefore, when the air passes through the adsorption bed equipped with zeolized molecular sieve adsorbents under pressure, nitrogen is adsorbed by the molecular sieve, and oxygen is enriched in the gas phase and flows out of the adsorption bed due to less adsorption, separate oxygen and nitrogen to obtain oxygen. When the molecular sieve adsorbs nitrogen until it is nearly saturated, stop air ventilation and reduce the pressure of the adsorption bed, then the nitrogen adsorbed by the molecular sieve can be desorbed, and the molecular sieve can be regenerated and reused. More than two adsorption beds are switched to work in turn, thus oxygen can be produced continuously.
The boiling point of argon and oxygen is close, and it is difficult to separate them, which can be enriched together in the gas phase. Therefore, the pressure-changing adsorption oxygen generation device can only obtain the concentration of 90% ~ 95% oxygen (the limit concentration of oxygen is 95.6%, and the rest is argon), which is also called oxygen-enriched compared with oxygen whose concentration is above 99.5% in the deep cooling and air separation unit.
III. The development process of pressure-changing adsorption air separation and oxygen generation technology
At the beginning of 1970s S, the process of oxygen generation by pressure conversion adsorption air separation was successfully developed by United States United Carbon Corporation and German AG company successively, and began to be applied to sludge aeration treatment and steelmaking in Japan. In 1975, American air and Chemicals Corporation (APCI) successfully developed the pressure swing adsorption oxygen generation process (VSA or VPSA) in the vacuum desorption process, which improved the efficiency of oxygen-nitrogen separation and reduced the energy consumption of oxygen generation. Therefore, the process of oxygen generation through variable pressure adsorption can be divided into two types according to the different methods of desorption: PSA process, namely the process of adsorption under higher pressure and desorption under normal pressure; VSA or VPSA process, that is the process of adsorption and vacuum desorption under normal pressure or slightly higher than normal pressure. Under the same scale of oxygen generation, PSA process is simpler and less invested than VSA (or VPSA) process equipment, but the energy consumption of PSA process is higher than VSA (or VPSA) process. When the scale of oxygen production is relatively small, the investment is dominant, and PSA process is relatively appropriate. While when the scale of oxygen production is relatively large, the operating cost caused by power consumption is more important, and it is more economical to adopt VSA or VPSA process.
Throughout the development of the technology of oxygen generation by pressure-changing adsorption air separation over the past two decades, we can say that we are always focusing on two key points: The first is the development and utilization of new adsorbents, and the second is the improvement and perfection of technological process. The application of new-type adsorbents and the continuous improvement of the technological process make the performance index of the oxygen generation unit of pressure swing adsorption air separation continuously improve, and the scale of the unit becomes larger and larger. Currently, the scale is 6000 ~ 10000Nm3For most oxygen-using occasions below/h, the pressure-changing adsorption oxygen generation device is more economical and applicable than the oxygen generation device adopting the cooling process because of its advantages of low energy consumption, low investment and flexible operation, the competitiveness is increasing day by day, which promotes the continuous improvement of the deep cold air separation process, forming a situation in which the two promote each other and compete for development.
1. Development of absorbents
The performance of the adsorbents is of vital importance to the oxygen generation device by pressure conversion, and its performance level determines the performance limit that can be reached by the oxygen generation device by pressure conversion, each great development of pressure-changing adsorption oxygen generation technology is always associated with the application of new adsorbents.
The adsorbents mentioned here refer to the main adsorbents used for separation of nitrogen and oxygen in the oxygen generation unit, whose absorption of nitrogen takes precedence over oxygen. In the early stage, the absorbents used in the VSA oxygen generation unit were NaX type zeolite (so far, NaX type zeolite is still used in PSA unit), and then CaA type zeolite became the most commonly used VSA-O2The main adsorbents (there are still some manufacturers of pressure-changing adsorption oxygen generation equipment using CaA-type zeolites in China until now), and the modified kind of adsorbents is CaX-type zeolites, in recent years, LiX type zeolite has been applied more and more due to its excellent performance in the VSA oxygen generation process. The performance comparison of these typical VSA oxygen-making adsorbents is shown in Table 1 data.
It can be seen from the comparison of the data that LiX has the largest air handling capacity and the highest oxygen yield, while the vacuum pump capacity required under the same pumping volume is lower than CaX, which is basically the same as CaA, it shows that under the same scale of oxygen production, LiX uses the least amount, needs the least air volume (the minimum load of blower), needs the minimum amount of air and vacuum pump load, therefore, it has the best performance for oxygen generation by pressure conversion and adsorption.
In order to reduce the cost and further improve the performance of oxygen generation, some companies have tried to use hybrid ionic LiX (such as CaLiX, SrLiX and LiX with partial exchange of trivalent ions) for oxygen separation, it has applied for a patent and has industrial applications, but according to the evaluation result of the invention patent of a foreign company, The LiX type still has the best performance.
Note a. For all cases, the operating conditions are: at the beginning of vacuum pumping, P = 101.3kPa, T = 25℃, YN2 = 0.79. At the end of vacuum pumping, P = 20.3kPa, T = 25℃, YN2= 1. The void ratio of bed layer = 0.37.
B. Commonly known as "13X molecular sieve"
C. Commonly called "5A Molecular sieve"
So far, the adsorbents used by the pressure conversion absorption air separation oxygen generation unit are all zeolized molecular sieves, among which LiX can almost be said to be the top-level oxygen generation adsorbents, the possibility of developing the oxygen-making adsorbents which are much better than LiX from the zeolized molecular sieve basically does not exist. Therefore, in the future, the research and development of oxygen-making absorbents should be different. It is said that a foreign company is researching and developing a new type of absorbents whose oxygen absorption is stronger than nitrogen absorption, but there has been no industrial application report so far.
2. Technological process development
The advancement of the technological process is also very important for the pressure conversion absorption oxygen generation unit, which determines the utilization efficiency of the oxygen absorption, the investment of the unit, the reliability of long-term operation and the actual level of oxygen generation power consumption. Therefore, the development of technological process also runs through the development process of pressure-changing adsorption air separation oxygen generation technology.
The transformation from normal pressure desorption process to vacuum desorption process is the most important technological innovation of pressure-changing adsorption oxygen generation technology, which makes it possible to reduce energy consumption of pressure-changing adsorption oxygen generation unit and enlarge the unit. The subsequent development of technological process is based on vacuum desorption, making various efforts in simplifying process, reducing energy consumption, reducing investment and expanding the scale of oxygen production. Major progress includes:
▲ Remove CO from the air from the pretreatment adsorption bed2And H2O) combine with the main adsorption bed into one, that is, mix the pre-treatment absorbents and the main absorbents in the same bed, which greatly simplifies the process and effectively reduces the investment.
▲ Introduce the steps such as average pressure, vacuum cleaning, air intake, average pressure and air extraction, etc. in the operation procedure, and develop shallow adsorption bed (reduce the accumulation height of absorbents, reduce bed resistance), improve the utilization efficiency and oxygen yield of blower and vacuum pump, and reduce oxygen consumption.
▲ If possible, adopt the minimum number of adsorption beds, simplify the process of four beds and three beds into the process of two beds or even single beds, and reduce the number of valves at the same time, moreover, the process of two-bed and single-bed can adopt a short operation cycle, and the utilization frequency of the absorbents increases, thus reducing the dosage of the absorbents and further reducing the investment.
▲ Choose the most suitable power unit according to the characteristics of process flow to ensure the reliability of long-term operation of the unit and reduce energy consumption. For example, the device adopting two-bed or single-bed flow can effectively reduce the impact of the air intake on the absorbents due to its high fluctuation range and high frequency of air intake pressure and the characteristics of the Roots blower with stable flow rate, it is especially beneficial for the long-term stable operation of the device. The vacuum degree of the adsorption bed of the VPSA oxygen generation unit also fluctuates frequently and substantially. Compared with the water ring vacuum pump, the adoption of roots vacuum pump can reduce the energy consumption of Desorption by 20 ~ 40%, therefore, except for a few domestic manufacturers, the roots vacuum pump is widely used at home and abroad as the desorption power equipment of the oxygen generation unit.
▲ The operating pressure ratio (adsorption pressure/desorption pressure
