The basic principle of Pressure Swing Adsorption (PSA) is to use the difference of the adsorption characteristics of the gas components on solid materials and the change of adsorption capacity with pressure regulations, to achieve gas separation or purification by periodic pressure switch process.

At present, pressure swing adsorption technology is widely used in air separation to produce O2 and N2, in separation and purification of other gases containing CO, H2, CO2 .,etc (such as furnace gases and industrial exhausts), in petroleum cracking gases like ethylene, ethane and in concentrating gas into CH4. The application boundary is gradually expanding with technological progress in this field.

In the 1970s, Union Carbide Corporation industrialized pressure swing adsorption (PSA) oxygen generation technology for the first time using normal pressure desorption (PSA) process. The adsorbent was CaA adsorbent with low nitrogen adsorption capacity and high oxygen power consumption.

In the 1990s, vacuum pressure swing adsorption (VPSA) oxygen production technology using LiX adsorbent became the international mainstream process, which is more suitable for the needs of large-scale installations. The research on oxygen production by pressure swing adsorption in China was carried out almost simultaneously with the international counterparts. However, limited by the low domestic productivity of efficient adsorbents and backward technical research on adsorption vessels and overall processes, the development of domestic PSA oxygen technology was slow, meanwhile, the scale of VPSA plants was stuck in bottlenecks and oxygen generation was accompanied by problems like high energy consumption, frequent replacement of adsorbents, etc., leading to a critical impact on production. During this period, domestic large-scale VPSA oxygen plants were almost all imported and a large amount of foreign exchange was used due to the high price.

In the late 1990s, the gas separation and purification center led by Professor Xie Youchang of Peking University took the lead in creating high-efficiency LiX oxygen adsorbent with high nitrogen & oxygen selectivity and nitrogen adsorption capacity. After stable mass production of LiX oxygen adsorbent, China started integrated process design & manufacture of complete sets of industrial PSA oxygen generation plants for the first time. From then on, the PSA oxygen generation plants produced in China and using efficient LiX oxygen adsorbent were widely applied.

Recently, with the gradual standardization, maturity and growth of the gas market, domestic first-line pressure swing adsorption manufacturers have looked beyond plant sales and focused more on entering the professional service market of onsite gas production & supply in line with their specialized service concept. The oxygen generation plant has achieved intelligent unattended operation, marking a new development period for domestic pressure swing adsorption oxygen generation.

The oxygen-producing adsorbents mainly relies on its selective adsorption to nitrogen and oxygen penetrating function. They’re mainly divided into calcium-based CaA and CaX, and lithium-based LiX. CaA and CaX adsorbents are based on traditional molecular sieves used in the 1980s, therefore the cost is lower, but the energy consumption of producing oxygen is higher, therefore, total loading is several times the amount of LiX. Judging from both the plot area of absorption tower or the long-term operation cost, CaA and CaX adsorbents have evident disadvantages, thus they are only used in small-scale pressure swing adsorption (PSA) operations with atmospheric desorption currently.

LiLSX molecular sieve (LiLSX) adsorbent for oxygen production with high lithium ion exchange rate is the best among LiX adsorbents. Its “nitrogen adsorption capacity” and “nitrogen & oxygen selectivity” are far superior to CaA and CaX oxygen production adsorbents. The higher oxygen yield LiX adsorbent has, the less its loading will be, and finally the operating loading of supporting power equipment will also be decreased accordingly. As a result, the direct investment and operating energy consumption can be reduced and the economic indicators of the oxygen plant can be increased. The first PU-8 high-efficiency lithium-based oxygen adsorbent with industrialized stable mass production in China at the earliest has won the first prize for the National Science and Technology Progress Award of the Ministry of Education.

VPSA (vacuum pressure swing adsorption) is separating oxygen from air by vacuum decompression for desorption.

The pressure swing adsorption oxygen generation uses air as the feed gas which is forced by a blower to pass under pressure through the adsorbent bed. The nitrogen, carbon dioxide and water in the air are adsorbed by the adsorbent, and the remaining components pass through the absorbent for richer oxygen. And then as the pressure is being lowered, the nitrogen, carbon dioxide and water adsorbed on the adsorbent are released and the adsorbent can be regenerated in this way. The reciprocating process makes up the basic principle of vacuum pressure swing adsorption oxygen generation.

Vacuum pressure swing adsorption (VPSA) oxygen plants generally utilize the operating steps shown above to separate and enrich oxygen. In one cycle, each adsorption vessel needs to undergo five steps: “adsorption”, “pressure reduction”, “vacuum desorption”, “Purging” and “pressure increasing”.

(1) Adsorption

After mechanical impurities in the air is removed by the filter, it enters the adsorption tower through the Roots blower. The H2O, CO2, and N2 in the air stay in the adsorbent bed. Since O2 is absorbed little in the adsorbent, the O2 exiting in the vessel will be richer than other entering mixture, and it is discharged from the outlet of the tower. A portion of the oxygen produced by this step is sent to the buffer tank, and the remaining portion is reserved for the next step to regenerate and boost the pressure in the adsorption tower.

(2) Pressure Reduction

In the “pressure reduction” step, oxygen-rich gas passes along the vessel outlet into another one in the “pressure increasing” step, and the pressure goes up.

(3) Vacuum Desorption

At the end of the “pressure reduction” step, in order to desorb the impurities as much as possible, the tower must be evacuated and depressurized. The biggest difference between VPSA and PSA lies in this step, that is, the vacuum pump is used to further evacuate the adsorption tower, which causes the pressure in the tower to decrease when the impurities are released and discharged through the vacuum pump outside.

(4) Purging

In order to desorb the impurities of the adsorption tower more thoroughly, at the end of the “vacuum desorption” stage, a small amount of oxygen will be introduced from another high-pressure tower to revitalize the adsorbent in the tower, at which time the partial pressure of oxygen in the tower rises while impurities’ is further reduced so that the adsorbent is more completely regenerated, which is more conducive to the adsorption in the next cycle.

(5) Pressure Increasing

After “vacuum desorption” and “purging”, the adsorbent in the adsoption vessel is regenerated. At this time, the pressure in the vessel is lowered. In order to quickly recover the pressure for adsorption and ensure that the adsorption front does not move up too quickly, it is necessary to introduce enriched oxygen in the other adsorption vessel in “pressure reduction” step to increase the pressure. The pressure of the vessel reaches the requirements and is ready for the next adsorption cycle when the “pressure increasing” step is completed.

The switching of the above steps is mainly done by the control system and switch butterfly valves. According to the sequential order of each step , the control system switches butterfly valves to control the length of time during “adsorption”, “pressure reduction”, “desorption”, “purging” or “pressure increasing” processes in the adsorption vessel, achieving the separation of oxygen from nitrogen and finally obtaining the required oxygen.

The basic principle of PSA-CO technology is to use the adsorption selectivity of the adsorbent to adsorb CO in the mixed gas, and then to desorb CO by decompression or vacuumizing to achieve the CO separation.

It can be seen from the comparison with the adsorption curve of 5A molecular sieve that the adsorption performance of the copper-loaded molecular sieve is more excellent. On one hand, it has higher adsorbing capacity of CO as a result of the complex adsorption of the active Cu+ to CO. On the other hand, almost no other gases can be adsorbed because CuCl covers the original active centers of the molecular sieve on the surface, and it also reduces the adsorption to CO2 which was highly adsorbed before. Therefore, when processing the feed gas with a low CO2 content, it is possible to directly absorb and separate CO without removing CO2, which is the so called one-stage PSA. The superior performance of Cu-based molecular sieves lies in the fact that their adsorption principle combines physical and chemical methods together, utilizing the large specific surface area of the molecular sieve carrier and the complexation between Cu+ and CO. We dispersed a single layer of CuCl on the inner surface of the molecular sieve, and finally produced highly-efficient copper-loaded molecular sieve.

Compared with the cryogenic separation technology, PSA-CO plant has quite a few advantages: simple operation, short start-up and shut-down time, flexible load adjustment and high automation. It only takes tens of minutes to start up. Meanwhile, according to the downstream needs, load adjustment within the range of 30% to 100% can be realized by simple adjusting at short notice, which can greatly save the cost during commissioning and pilot run of the plant, thus lowering the investment indirectly.

The pressure swing adsorption plant is made up by adsorption vessels, vacuum pumps, compressors, program-controlled valves, etc. The plant is simple and easy to operate, and common employees can master the operation by simple training. The supporting equipment can be purchased and manufactured domestically, which assures the safety of the plant. Also, installation is not difficult, and construction can be completed in a short time.

Considering the above advantages, PSA-CO technology is widely used in the treatment of coal chemical synthesis gases and various complex exhausts. It is applied to treat entrained-bed gas, water gas, semi-water gas, natural gas conversion gas, calcium carbide furnace exhaust, acetic acid tail gas and blast furnace exhaust for production of downstream chemical and industrial products such as acetic acid, butanol, TDI, ethylene glycol, etc.

At present, industrial oxygen production methods mainly include cryogenic air separation oxygen generation, pressure swing adsorption oxygen production and membrane separation oxygen production. Pressure swing adsorption is an advanced gas separation technology standing at an irreplaceable position in the field of onsite gas supply in the world today. Main features of pressure swing adsorption oxygen plant are as follows:

1. simple process, compact structure and low investment

2. high degree of automation- full-automatic operation for 24 hours and remote monitoring through communication interface

3. short start-up and shout-down time (usually can produce qualified oxygen within 0.5h)

4. lower cost than that of the cryogenic oxygen production technology (unit power consumption of 0.33-0.35 kWh/m3 for 100% pure oxygen)

5. operating at normal temperature and low pressure with prior safety

6. flexible load adjustment (The pressure swing adsorption oxygen plant can adjust the load according to the changes of production volume. A single plant can achieve 50%-100% load regulation)

Based on the above characteristics of PSA oxygen generation technology, it is generally believed that cryogenic oxygen production technology has certain advantages in large-scale and high-purity oxygen conditions, and pressure swing adsorption oxygen generation technology, with low cost, easy operation, flexible load adjustment and other outstanding features,  is more advantageous in variable and low-purity oxygen use.