by Limin Qiu, Xiaobin Zhang, Jianye Chen, and Xuejun Zhang, all from the Institute of Refrigeration and Cryogenics, Zhejiang University; Lei Yao, Hangzhou Hangyang Co. Ltd.; and Yonghua Huang, Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University
Industrial gases such as oxygen (O2), nitrogen (N2) and argon (Ar) can be regarded as the “blood” of modern industries like the steelmaking and chemical product industries, which are the world’s primary users of the products of air separation units (ASUs). At present, the production of large quantities of high purity industrial gases still mainly depends on a large-scale cryogenic air separation method. Here, the terminology “large-scale” means that the O2 production of a single ASU is beyond 60,000 Nm3/h.
Development of China’s Cryogenic ASU Technology
At the beginning of 1953, the production capacity of an ASU was only 20 m3/h O2 in China. After 60 years of development, China now has the ability to produce a series of commercial ASU products with a capacity ranging from 20,000 to 100,000 m3/h. Since 2011, the Hanyang Group, China’s largest air separation enterprise, has been manufacturing a 120,000 m3/h ASU for Iran Carvedilol Group.
Figure 1 presents the evolution of the technology of the cryogenic air separation industry in China during the past years. In spite of its late development, China has been catching up with the pace of international development through the independent innovation of ASUs since about 1986. From then on, air booster and high pressure gas expansion processes with molecular sieve purification and distributed control system (DCS) were developed and applied. The extraction rate of O2 and Ar reached 93-97% and 54-60% respectively in that period. Significant progress was made in 1996 with the application of the structured packing column, which has become the most common-used facility nowadays. Compared with the traditional sieve plate column, the structured packing column has the advantages of lower operation pressure in the upper column and a higher efficiency in the production of pure Ar; thus it assists in lowering the overall power consumption and increases the extraction rate of O2 to 97-99% and Ar to 65-84%. Since the beginning of the 21st century, the internal compression process with liquid oxygen (LO2) pumps has begun to be widely used to replace the traditional gaseous O2 compressor at room temperature to increase high pressure O2 production. The internal compression process has the advantages of flexibility, reliability and safety.
State-of-the-Art ASU in China
Presently, air separation processes comprise the following basic characteristics: molecular sieve adsorber; structured packing column; fully distilled Ar production without hydrogen, external or internal compression of N2 and O2 products; internal compression of liquid argon; and DCS control. All of the above technologies are currently being applied to China’s modern large-scale ASU with the features of high loads, multi-conditions, automation, high efficiency and reliability.
In China, the mature 60,000 Nm3/h ASU, which was produced by Hangyang for the coal-to-alkene project in the city of Baotou, can be taken as a chief example to illustrate the present technological advancement of ASUs. In this unit, the structured packings are used in the main upper and Ar columns, and the compressed air is efficiently cooled by an evaporative cooling technology without the freezer. Other technologies embedded include double bed adsorption and two floors of condenser/ evaporator. The device successfully went into operation in 2010 and is now operating to produce O2 and LO2 with purity higher than 99.6%, and O2 contents in liquid nitrogen (LN2) less than 10 ppm. The absolute pressure of O2 and N2 is 8.6 MPa and ≥ 0.9 MPa, respectively, and the extraction ratios are > 99% for O2 and > 82% for Ar. Although the specific power consumption of 0.38 kWh/m3 O2 is slightly higher than the international consumption of 0.28~0.3 kWh/m3, its advantages of proprietary intellectual property and low price show great significance to the industry, as in the past, ASUs at this level being assembled in China had to rely on imports.
Problems and Prospects
The domestic design and manufacture of large-scale ASU has made tremendous inroads in China, and the gap between international and domestic technological knowledge has been greatly reduced. However, comprehensive technological differences still exist compared to abroad, especially with regard to the construction of 80,000 to 100,000 m3/h ASUs. For example, China has already qualified independent design and production of static equipment and some dynamic equipment, such as large-flow air compressor units, but for other dynamic equipment—such as turbo expanders, key cryogenic valves of high pressure or large pressure difference, large-flow high pressure cryogenic liquid pumps and instrumentation—the reliance on imported equipment persists. For reliable operation, the upper column and Ar column of the 60,000 m3/h ASU are still designed by the Swiss Sulzer company and equipped with its PLUS packing for better performance and reliability. Overall, there is still much room to improve the scientific and technological innovation capability.
Currently, new technologies in air separation equipment are still emerging, such as the large double-layer radial flow adsorber. Recently, the internal thermally coupled approach has been extensively studied, and its application to the distillation column air separation process has also been proposed. Based on theoretical calculations, when compared to a conventional thermal coupling column, the energy-saving effects of the new method are very significant. These technological innovations are all based on a deep understanding of the basic hydrodynamic and thermodynamic theories, as well as advanced experimental methods. Therefore, in order to be on par with international standards, independent innovation should be encouraged. On one hand, basic theoretical and experimental studies, such as two-phase flow and heat and mass transfer in complex structures, need to be further strengthened. On the other hand, enterprises should actively change their management concept to become suppliers of both facility and gas, rather than only device providers. This will lay the foundation for applications of new technologies, because as gas suppliers, the risk of damaged reputations and commercial disputes from setbacks or failures can be minimized.
After more than two decades, and especially in the last decade or so, through independent research, technology introduction, co-production and the improvement of old equipment and operating practices of over 10 sets of large-scale ASUs, Chinese enterprises have successfully resolved a large number of key technical equipment design and manufacturing issues. These technologies primarily include process design to meet needs, equipment design with specified performance, and so on. China is now able to independently manufacture 60,000 Nm3/h ASUs and has the capabilities to design and manufacture larger ASU systems up to 120,000 m3/h. Among these, most static equipment can be designed and made in China, while some special devices, such as large-flow expanders, high pressure cryogenic liquid pumps and cryogenic valves, generally have to be imported. From the perspective of cost, this combination is currently considered to be an optimal solution—namely, ensuring that the device is advanced and reliable with a lower investment and operating cost. Specifically, as the device-supporting enterprise, Shaangu Power Company Ltd. has the ability to independently design and manufacture air compressor units for 120,000 Nm3/h air separation system after accumulating sufficient research development and technology over the years. It also provides system solutions and services for units in the air separation industry.
In general, power consumption approaches 0.38 kW/m3 O2 with the domestic large-scale 60,000 m3/h ASU, which features long-term safety, reliable and easy operation and good appearance. Thanks to technological advances, Chinese air separation firms not only account for more domestic market share, but also show greatly enhanced international competitiveness. For example, Sichuan Air Separate Group, one of the four air separation enterprises in China, exported a 10,000 Nm3/h ASU to Turkey; Hangyang exported a 20,000 Nm3/h ASU to Spain; and a 120,000 Nm3/h ASU was exported to Iran in 2014, which is the largest ASU exported by China by far.