In many developed countries, papermaking is one of their 10 pillar industries of national economy, whereas its wastewater treatment is intractable. The chlorine-containing bleaching waste liquids from paper mills include not only COD and BOD, but also other highly toxic substances. Replacing the traditional CEH three-stage pulp bleaching method with the new totally chlorine free (TCF) or elemental chlorine free (ECF) bleaching is an effective solution to the environmental pollution of the pulp and paper industry. Since oxygen itself is not toxic or pollutant to the environment, the amount of bleaching agent and wastewater in the latter sections can be reduced by 50% after oxygen delignification. Therefore, oxygen pulp bleaching has become an essential stage in the development of clean bleaching processes. In recent years, there has been a growing interest in oxygen bleaching in the global papermaking industry in view of the stricter requirements for environmental protection.

Commonly used industrial oxygen generation technologies include the cryogenic air separation and VPSA (Vacuum Pressure Swing Adsorption) methods. Cryogenic air separation unit has high investment and energy consumption as well as mass operation and maintenance tasks, which is not an ideal oxygen source option as the supporting bleaching equipment for pulp enterprises due to its inconvenience in start-up and shut-down and high overall costs. VPSA oxygen plant has the advantages of simpler composition, easier operation and maintenance, relatively smaller investment and lower operation cost with only 20-minute startup and 93% oxygen purity, which is fully able to meet the requirements of pulp bleaching process due to the fact that the oxygen consumption range of pulp mills is generally 200-5000Nm3/h, which is regarded as a small and medium VPSA oxygen production capacity. What’s more, the energy consumption of VPSA oxygen systems is 0.29-0.32kWh/Nm3, which is 30% lower than that of cryogenic air separation units. Now VPSA oxygen generation technology has been applied in more than 30 pulp mills in China, and even extended to global well-known paper enterprises such as Double A (Thailand) PLC and a pulp mill in Belarus.

1. VPSA-O2 System Technical Process

The technical principle of the VPSA & PSA oxygen unit is to separate oxygen on the basis of the different adsorptive capacities of solid adsorbents to gas components. When porous solid adsorbents contact with gases, the phenomenon of accommodating gases inside or on its surface is called the adsorption of solid adsorbent to gas, while the returning of atoms or molecules that have been adsorbed by the adsorbent to the air is called desorption.

By utilizing the principle that the molecular sieve’s adsorptive capacity of nitrogen and oxygen in the air differs under various pressures, the adsorbers of VPSA & PSA oxygen generator realize the separation of oxygen and nitrogen through the cyclic switching of adsorption, depressurization, vacuum desorption, purging and repressurization steps. When the air passes through the adsorption bed equipped with the zeolite molecular sieve adsorbent under certain pressure, the nitrogen is adsorbed by the molecular sieve while the oxygen is enriched in the gas phase and flows out of the adsorption bed due to less adsorption. When the molecular sieve is close to saturation, stop feeding the air and lower the pressure of the adsorption bed, then the nitrogen adsorbed in the molecular sieve can be desorbed, thus achieving the regeneration and reusage of the molecular sieve. Oxygen can be generated continuously when more than two adsorption beds are operated in turn. The continuously obtained product oxygen is discharged from the top of the adsorbers into the buffer tanks, and then sent to the oxygen pulp bleaching section after being pressurized by the oxygen booster. In the above process, each adsorber is in different operating steps at the same time, and the valves are switched at regular intervals under the control of the PLC system to manufacture the product oxygen that meets the customer's requirements.

2. Adsorber (Adsorption Vessel)

VPSA-O2 unit of PKU Pioneer, the largest VPSA oxygen generation equipment provider in China, adopts parallel two-adsorber process, making it available to flexibly customize the oxygen capacity according to customer needs. PKU Pioneer's latest VPSA oxygen generating process uses vertical radial adsorption towers, compared with initial axial ones, it has thinner adsorption beds, smaller resistance and air flow depressurization, thus helping to reduce the exhaust pressure of power equipment and the energy consumption by 10%-15% with remarkable energy saving efficiency. Additionally, the cylindrical-bed structure covers a smaller area, which effectively saves the construction cost as well.

2.1 Selection and Application of Molecular Sieve

Commonly used molecular sieve adsorbents are 5A, 10X, 13X, N-2, CaA, NaX, CaX, LiX, CNA-198, Li.X.RE, HX5A-980, etc. Taking the common CaA molecular sieve as an example, when the air passes through the adsorption tower equipped with CaA molecular sieve, N2 is adsorbed preferentially so that the oxygen flows out of the adsorption tower as the product gas because the quadrupole moment of nitrogen is much larger than that of oxygen and the Ca2+ on the surface of the CaA molecular sieve micropore is more effective in adsorbing N2 than adsorbing O2.

However, the adsorptive capacity and selectivity of CaA molecular sieve adsorbent for nitrogen are still not high enough, resulting in low oxygen yield and high energy consumption of VPSA oxygen generation units. Compared with CaA and NaX molecular sieve adsorbents, LiX molecular sieve has higher oxygen production efficiency. Less LiX molecular sieve could be used for the same oxygen capacity, thus reducing the energy consumption and size of the VPSA and PSA oxygen generator. LiLSX molecular sieve's capacity for adsorbing nitrogen is greater than its capacity for adsorbing the oxygen component of air with a higher degree of Li-ion exchange and smaller radius of Li+. The number of Li+ in liLSX molecular sieve is twice that of Ca2+ in CaA molecular sieve adsorbent and nitrogen & oxygen separation coefficient is 2-5 times that of the traditional oxygen molecular sieves, which enables lithium molecular sieve to adsorb more nitrogen. When applied to VPSA & PSA oxygen units, the consumption of liLSX molecular sieve is only 1/4-1/5 that of CaA molecular sieve, which is conducive to lowering the total investment, increasing the oxygen yield and greatly reducing the power consumption.

PKU Pioneer High-Efficiency Lithium-Based Oxygen Molecular Sieve Adsorbent PU-8 for Industrial and Medical Uses

2.2 Internal Structure of Adsorbers

Irrational design of the internal structure of VPSA oxygen adsorbers may lead to excessive displacement or even pulverization of the molecular sieve loaded into the adsorbers due to the impact of air flow. Especially, after the molecular sieve has been in service for a long time, the gap between molecular sieves will gradually decrease and the adsorption bed will descend. PKU Pioneer has made a lot of optimization and improvement in the structural design of the adsorber through over 20 years of engineering experience, and originated a more reasonable air flow distribution system, which can reduce the inoperative zone in the adsorption bed, avoid the direct impact of air stream on the molecular sieve and prolong the service life of molecular sieves.

The adsorber is loaded with molecular sieve whose adsorption and desorption are directly influenced by the airflow velocity. The slower the airflow is, the more propitious it is for molecular sieve adsorption and desorption. Compared with the axial flow from bottom to top in the axial adsorption vessel, the adsorbent in radial adsorber is in full contact with the airflow whose direction is more conducive to adsorption and desorption. In the adsorption stage, N2 is gradually absorbed and gas volume lowers as the airstream flows from the outside to the inside. The cross section of the radial adsorption tower flow is also progressively narrowed from the outside to the inside. This structure increases the comprehensive utilization efficiency of the adsorbent and improves the stability of the adsorption bed. The direction of air is perpendicular to that of gravity, which effectively reduces the washing-away to the molecular sieve and increases its service life.

Reasonable compacting equipment can prevent the molecular sieve from pulverizing due to mutual collision. PKU Pioneer’s patent technology, i.e., molecular sieve mechanical homogeneous spin loading and automatic membrane compacting system, ensure uniform force on the adsorbent so that the molecular sieve is evenly compacted and flattened and the adsorption bed upper end is not easily to pulverize, which can avoid the oxygen purity decrease or affecting the production capacity of oxygen generator caused by the adsorbent powder pumped out by the vacuum pump to assure the stable operation of the VPSA oxygen equipment and further improve the utilization efficiency of the molecular sieve adsorbent. At present, PKU Pioneer has the most oxygen production projects that have been in operation for more than 10 years in China with excellent indicators, providing a solid guarantee for customers' stable oxygen supply.

3. Other Equipment and Components in the Process

3.1 Blower

The centrifugal blower with constant pressure makes the oxygen flow rate varies continuously with the pressure. Although the capacity can be adjusted, the efficiency will degrade quickly whenever it deviates from the optimal design value. In VPSA-O2 plant, in order to provide pressurized air for the adsorber, the constant airflow Roots blower is generally used to pressurize the air, as a result, the oxygen capacity is basically stable when the pressure changes, which casts less impact on the adsorption bed and is beneficial to the molecular sieve adsorption. In VPSA oxygen system, the efficiency of Roots blower is higher than that of centrifugal blower, and the practical operation has proven that its energy consumption index is also the lowest.

3.2 Vacuum Pump

Roots vacuum pumps have fast start-up and high ultimate vacuum. The pump transmission components are of robust anti-backlash configuration, allowing optimum vacuum levels to be achieved in a short time. There are no sliding parts in the pump chamber and oil lubrication is unnecessary, preventing contamination to the system by oil vapor. It also has significant advantages such as lower power consumption, less operation and maintenance cost, higher pumping speed and efficiency, more stable running, etc.

3.3 Oxygen Booster

In VPSA oxygen generating system, piston, Roots and centrifugal oxygen booster are applied, among which the piston oxygen booster is generally used. Oxygen is sensitive to temperature, oil and sparks, so the oxygen booster failure will affect the operation of adsorbers. If a piston type oxygen booster is used, it is necessary to prevent the sudden rise of cylinder exhaust temperature as well as interlock shutdown caused by mixing of lubricating oil and oxygen. If using Roots type oxygen booster, the unbalanced double mechanical seal can be realized from both radial and axial directions.

4. Control System

4.1 Switching Valves

Switching valves are also called PLC control valves. Air blower outlet valves, vacuum blower inlet valves, product oxygen outlet valves, purging valves, balance valves (pressure equalization valves), etc. enable the adsorbers to normally undergo the adsorption, depressurization, forward & reverse desorption and purging sections. They’re generally pneumatic and able to be controlled by computer programs when connecting with DCS or PLC with not only a short switching cycle, but also a long service life of up to 15 years, and can be switched for millions of times after continuous operation for a year.

4.2 Instrumentation

The instrumentation control system of VPSA oxygen equipment is controlled by the PLC system. Safe, reliable and advanced instruments are crucial to meet the monitoring and control requirements of smooth operation.

5. Conclusion

With the advancement and development of VPSA oxygen production technology, VPSA-O2 unit has been gradually recognizing by many paper mills due to its less investment, lower power consumption, more reliable and stable operation, effective benefits and higher economical efficiency. As the largest global supplier of VPSA and PSA oxygen production equipment, PKU Pioneer has provided economical and reliable oxygen generation solutions for world-leading paper enterprises such as Sun Paper Group and Double A PLC to help clients save energy and reduce costs substantially to improve the efficiency of the overall paper manufacturing process, which is of great practical significance to promote the resource recycling and sustainable development of the papermaking industry.