The Development Situation and Application Innovation of PSA Oxygen Generation Technology

1. Preface

In the past 20 years, China’s economy has been in a steady and rapid development stage when the production processes of industries such as iron & steel manufacturing, non-ferrous metallurgy, chemical industry, furnace energy saving, environmental protection, glass and paper making have been continuously upgraded and innovated. The process optimization and production capacity expansion led to increased demands for oxygen in various areas, which in turn drove the technological progress of industrial O2 plants, as a result, improvements have been made in the traditional oxygen generating technologies including cryogenic air separation, PSA oxygen generation and membrane separation processes, especially PSA oxygen manufacturing technology made huge progress. After the breakthrough made in the development of the new lithium-based adsorbent and radial adsorbers, PSA oxygen making technology has been widely used and greatly recognized by the end-users.

2. Progress in PSA Oxygen Generation Technology

The development of PSA oxygen generation technology began in the 1960s, and the U.S.A. and Japan achieved industrialization successively in the early 1980s. In the early 1990s, Praxair developed a lithium molecular sieve adsorbent for oxygen generation and the VPSA oxygen generating process based on the adsorbent's characteristics. The maximum capacity of its double-vessel PSA oxygen unit exceeded 3000Nm3/h, and the power consumption was reduced to 0.35 kWh/m3, which led to the rapid growth of PSA oxygen production technology and laid a solid foundation for its wide application.

In recent years, with the development of permanent magnet motors and optimization of the oxygen generation process, the lowest power consumption of PSA oxygen plant has been lowered to less than 0.3kWh/m3 while the maximum capacity of double-adsorber unit has surpassed 6000Nm3/h. The reduced cost and increased product oxygen volume of PSA oxygen equipment both promoted the application expansion of PSA oxygen generation process.

China began to study PSA oxygen generation technology in the late 1980s, and it was not until the early 1990s that small industrialized equipment was available. The initial PSA oxygen system used CaA molecular sieve adsorbent, and the capacity reached the largest 1000Nm3/h in the late 1990s from the original 20Nm3/h, 50Nm3/h, and 100Nm3/h, the power consumption (pure oxygen) was more than 0.5 kWh/m3. Many enterprises, especially those in the iron and steel industry using PSA oxygen equipment based on its advantages of the short construction period, stable operation and fast start-up & shut-down replaced it with cryogenic separation equipment due to the heavy workload and high cost of PSA unit maintenance. In sum, there is a clear gap with the international advanced level of the same period because the PSA equipment at that time cannot bear longtime running, and also, its maintenance is too much and the capacity is limited.

At the beginning of 2000, the PSA oxygen generation technology made rapid progress and was widely popularized thanks to the high-efficiency lithium-based adsorbent production represented by PKU Pioneer and the industrialization of the PSA oxygen production process using the adsorbent. At present, the capacity of PSA plants built by PKU Pioneer has reached 40,000Nm3/h and the pure oxygen power consumption is also close to 0.3kWh/m3.

With the solutions such as the stable production of high-efficiency lithium-based adsorbents, the R&D of radial adsorbers and large-diameter butterfly valves with reliable high switching frequency to some key issues of PSA oxygen generation technology, the capacity of China’s PSA oxygen systems increased year by year while the energy consumption was gradually reduced and the reliability was steadily improved. The capacity of a single set of double-tower PSA oxygen device has also grown from less than 1000Nm3/h to the current 6000Nm3/h, and even more than 40,000Nm3/h after multi-tower connection. In the meantime, the unit oxygen power consumption has lowered to less than 0.32 kWh/m3, and the annual operating rate has gone up to more than 98%. The blower noise has dropped to below 85dB (through silencing measures, the noise at 1m outside the workshop can reach less than 70dB), the non-failure rate of large-diameter butterfly valves mostly reached more than 8000h, and the service life of molecular sieve is lengthened to more than 5 years. End-users had a new understanding of PSA oxygen production equipment and its application was broadened. In 2018 alone, more than 70 PSA oxygen units exceeding 1000Nm3/h were set up in China.

PKU Pioneer has made continuous efforts to change the reliance of PSA molecular sieves on imports and at the same time, made breakthroughs in lithium molecular sieve and other similar products, and achieved the industrial application of those new molecular sieve products.

With excellent development and improvement, PSA oxygen generation technology has formed enormous unique advantages compared with cryogenic technology, which further promotes the wide application of PSA technology in various industries.

3. Characteristics of PSA Oxygen Generation Technology

① Low Energy Consumption and Operating Cost

In oxygen production process, power consumption accounts for more than 90% of the total operating cost. With the continuous optimization of PSA oxygen generating technology, the pure oxygen energy consumption has decreased from 0.45kWh/m3 in the 1990s to less than 0.32kWh/m3 today, comparatively speaking, the lowest pure oxygen power consumption of large-scale cryogenic air separator stays at about 0.42kW·h/m3, which means PSA oxygen generation has the advantage of obvious lower cost on the condition that customers have no nitrogen demand and don't require too high oxygen purity and pressure.

② Simple Process, Flexible Operation, Fast Start-up & Shut-down

Compared with cryogenic air separation, PSA oxygen generating process with the operating pressure of -0.05～0.05MPa is relatively simpler. The main power equipment consists of Roots blower and Roots vacuum pump, which are both simple to operate and easy to maintain. Since the PSA oxygen generator does not include the cooling and heating process when it is starting up or shutting down, it only needs 30 minutes to start and produce target oxygen, furthermore, after short-term intermissions, the system can continue to generate oxygen in just a few minutes, in addition, even easier, the shutdown is committed only by turning off the power equipment and control program. Compared with cryogenic equipment, it is more convenient for PSA oxygen unit to start up and shut down, which greatly cuts the operating cost of the device.

③ Low Investment and Short Construction Period

The PSA oxygen generator is mainly composed of the power system, adsorption system and valve switching system. Small number of components enables the users to save their one-time cost. Meanwhile, the small plot area also helps to reduce construction & land investment. Besides, it takes less than 4 months to complete erecting its main units and under normal circumstances, oxygen production can be achieved within 6 months, so the manufacturing & setting-up period is shorter compared with cryogenic air separator construction.

④ Simple Equipment & Maintenance

Components of PSA oxygen generator such as blowers, vacuum pumps, and program-controlled valves have highly mature supply chains, leading to the easy replacement of its spare parts and contributing to reduce the cost and easily control the construction period. Moreover, the PSA equipment maintenance is simple with convenient after-sales services. Compared with the large-scale centrifugal compressor maintenance in cryogenic air separators, PSA oxygen plants don't require to invest too much maintenance funds or hire professional employees.

⑤ High Turndown Ratio

Compared with cryogenic liquid oxygen technology, PSA oxygen systems achieve swift regulation of product gas volume and purity if there is little change in power consumption (pure oxygen). Generally, the capacity can be adjusted within the range of 30%-100%, and the purity of 70%-95%, especially, the load regulation is rather easier when several sets of PSA oxygen generators are connected in parallel.

⑥ High Operational Safety

Since the PSA oxygen plant operation is undergone under normal atmospheric pressure and temperature without liquid oxygen or acetylene enrichment, etc., it is safer than the cryogenic equipment which is usually operated at low temperature and high pressure.

4. Main Applications of PSA Oxygen Generation Technology

As the capacity of PSA oxygen units increases, the reliability improves while the power consumption gradually decreases, at the same time, with the outstanding advantages of flexible operation, simple load regulation, low power consumption, short construction period and high safety, PSA oxygen production technology can undoubtedly be an alternative process to cryogenic air separation for industries that require flexible utilization of enriched oxygen. The PSA oxygen generation process has recently also been widely used in iron & steel smelting, non-ferrous metallurgy, chemical industry, furnace energy saving, cement rotary kilns, environmental protection, glass & paper making, etc.

① BF Oxygen Enrichment

With the development of blast furnace oxygen enrichment technology, blast furnace has become one of the main oxygen sources in steel mills, so they can be treated as the oxygen supply regulator for the entire steel plant in the early stage of the BF oxygen enrichment application- the BF oxygen enrichment rate is high when the oxygen volume is large, and the rate is low with insufficient oxygen flow. As enterprises gradually become more aware of the BF oxygen enrichment importance in ironmaking processes, the stability of the oxygen enrichment rate becomes a crucial parameter for low-cost and efficient ironmaking operations. The oxygen supply in steel mills requires many procedures, and the load fluctuates every week or even every day, in this case, If cryogenic unit with inferior load regulation and long startup and shutdown time is adopted, the superfluous oxygen needs to be stored after liquefaction for later use or sell as a commercial product if the oxygen consumption is too low, which results in oxygen evacuation at times. In view of the low pressure and purity requirements of BF enriched oxygen, many iron & steel enterprises can erect PSA oxygen generators near the blast furnaces, thus directly providing BF with oxygen and serving as the regulator for the entire mill's oxygen supply. When there is surplus or insufficient oxygen, the PSA oxygen generator can be started or shut down at any time to increase or decrease the volume to provide stable oxygen for the blast furnace. At present, many iron and steel enterprises adopt PSA oxygen generation technology to supply oxygen for blast furnaces with great oxygen cost reduction achieved. It has been a consensus among most iron & steel companies that the PSA oxygen system is a dependable enriched oxygen source for blast furnaces.

② Electric Furnace Steelmaking

About 60% to 70% of electric furnace steelmaking enterprises in Japan use PSA technology to produce 93% oxygen for steelmaking. In theory, electric furnace steelmaking mainly relies on EAF to smelt and manufacture steel, on which oxygen only has an auxiliary effect, therefore, 93% oxygen produced by PSA plants can be used in electric furnace steelmaking. Enterprises in China such as Chizhou Guihang Metal Products Co., Ltd., Zunyi Changling Special Steel Co., Ltd. and Luzhou Yixin Iron & Steel Co., Ltd. have all begun to use PSA oxygen generators to provide oxygen for electric furnace steelmaking. According to actual feedback from those enterprises, the PSA oxygen generation technology not only does no harm to the steel quality but also can significantly cut the production cost of electric furnace steelmaking. After adopting the technology, the unit oxygen cost can be controlled below $0.0447.

③ Non-ferrous Metallurgy

In the past 10 years, PSA oxygen generation technology has obtained high recognition by copper, lead, and zinc smelting enterprises. Most non-ferrous metal smelting process with great load variations hardly needs nitrogen and generally requires 24%-90% oxygen. Owing to the characteristics of simple operation and low power consumption, PSA oxygen generation technology is well suitable for non-ferrous metal smelting. Now most of China's non-ferrous smelting corporates such as Tongling Nonferrous Metals Group, Zijin Mining Group and Yunnan Copper Co., Ltd., have selected PSA oxygen plants as their enriched oxygen sources. For example, Baotou Huading Copper Development Co., Ltd. has successively built 4 sets of PSA oxygen units due to its copper output increasing and copper smelting processes improvement, the total oxygen capacity reached more than 25,000Nm3/h. Likewise, Yunnan Chuxiong Copper Smelter has successively built 3 sets of PSA oxygen plants, with a total oxygen capacity of 30,000Nm3/h.

④ Chemical Industry

Nowadays, the continuous oxygen-enriched generation process is used to improve the previous intermittent oxygen production in small and medium-sized nitrogenous fertilizer mills, and the source of enriched oxygen is mostly from PSA oxygen production equipment. The oxygen-enriched continuous gasification technology has high applicability to coal, which is conducive to improve the production capacity of the oxygen production device. It has a wide application prospect.

⑤ Paper Making

In papermaking, oxygen is mainly used in the pulp manufacturing section such as oxygen delignification and whitening in oxygen bleaching. Since the papermaking process neither claims high oxygen purity nor needs nitrogen, most paper mills currently choose PSA technology to generate oxygen.

⑥ Other Industries

Currently, PSA oxygen generation technology is also widely used in other fields, for instance, glass fiber manufacturing, the oxygen-enriched combustion in float glass, cement kilns and waste incineration, oxygen-rich aeration and ozone generator in sewage treatment, etc.

5. Vision for PSA Oxygen Generation Technology Development

PSA oxygen production technology, a newly emerged technology that has been developing rapidly in the past 20 years, has gradually been recognized by many corporates with its technological progress and application frontier expansion. To reduce the energy consumption of oxygen production, it's the important R&D direction to expand larger application areas of future PSA oxygen production technology by developing new-type adsorbents and attempting to couple with membrane separation or cryogenic processes. For example, combining PSA oxygen production technology with membrane separation technology can obtain 99%+ oxygen, which can replace low-temperature air separation in remote areas or at some mobile requests. PKU Pioneer, as a pioneering leader in the oxygen generation industry, hopes to seize the initiative in future development by continuously increasing effective investment and attaching great importance to R&D of PSA technology. As the PSA oxygen generation technology being increasingly improved, it's bound to have multiple advantages and huge potential to win a brighter application prospect in the future.