Oxygen bleaching has become a necessary process for developing cleaner pulp production. Since oxygen itself is not toxic or pollutant, the oxygen delignification liquid waste can be merged into the black liquor extraction section and enter the alkali recovery system. After oxygen delignification, the amount of bleaching agent and wastewater pollutants in the following sections could be reduced by about 50%. Oxygen delignification, as a measure to control pollutants in newly built pulp and bleach mills, can greatly reduce the BOD, COD, chromaticity and total organic chlorine in bleaching effluent, which plays an important role in pollution reduction of pulping & bleaching wastewater. And some developed countries have stipulated that new pulp & bleach plants must adopt oxygen delignification process.

The oxygen purity required for pulp oxygen delignification, oxygen-reinforced alkali extraction, effluent treatment, and pure oxygen aeration is not too high (90%～93%). If supplying oxygen for pulp mills by cryogenic air separators, oxygen, nitrogen and rare gases could be generated at the same time, whereas N2 and rare gases are hardly needed for pulp & bleach mills. In addition, the cryogenic air separator operates at low pressure and low temperature, requiring high safety conditions. For the same oxygen production capacity, the vacuum pressure swing adsorption (VPSA) oxygen unit is more competitive than the air separation unit with advantages like lower investment, safer operation, adjustable capacity according to the user's working conditions, less power consumption, less cooling cycle water, and smaller footprint, etc.

VPSA & PSA oxygen production technology is based on the principle that the adsorption capacity of molecular sieves for nitrogen and oxygen in the air is different at different pressures. The separation of oxygen and nitrogen is achieved through the cyclic switching of atmospheric pressure adsorption and vacuum desorption. In the late 1990s, the VPSA-O2 system gradually showed its competitiveness in terms of investment and operating costs and began to be used in pulp mills. VPSA & PSA oxygen plant is quite competitive due to its simpler process, fewer components, more reliable performance of molecular sieve, more stable oxygen quality, easier start-up and shut-down, less maintenance and lower cost, which makes it widely used in the global papermaking industry.  Not only preferred by China's leading paper mills, but also PKU Pioneer's VPSA  & PSA oxygen unit has been exported to many leading paper enterprises in Belarus, Thailand, Indonesia and other countries, such as DOUBLE A and a Belarusian papermaking giant, all of which have been well recognized by the customers.