Transformation adsorb ASU project introduction
Transformation adsorb ASU project introduction
Main process and comparison of oxygen producing by air separation
Oxygen has a wide using range in industrial production and common life. Air is the cheapest raw materials for producing oxygen, which has 21% (volume consistency) of oxygen. As a result, oxygen is usually made by air separation.
Main process of oxygen producing by air separation
1. Cryogenic separation process: traditional oxygen producing technology, oxygen purity is high and various products, which suits large scale oxygen production.
2. PSA: new technology, low investment and low energy consumption, which suits non-high purity requiest of oxygen and middle or small scale oxygen production.
3. membrane separation process: not used in industry for non-mature technology
PSA technical features——compared with cryogenic separation technology
The process is simple, which doesn’t need complex pretreatment equipment;
The product oxygen purity can reach 95%, nitrogen is less than 1%; the rest is argon;
The power consumption is lower and investment is less when the oxygen production is less than 10000m3/h
High automation and convenient running or stopping the equipment;
High independence and safety;
Simple operation and flexible operation(part loading is superior, loading converting is fast)
Low cost of running and maintenance;
Low construction cost and small footprint
Cryogenic separation and PSA comparison
|
item |
Cryogenic separation |
PSA |
|
Theory of separation |
Liquefy the air and separate the oxygen and nitrogen by their boiling point |
Pressurize and adsorb, vacuumize and desorb, separate oxygen and nitrogen by their adsorb ability |
|
Main features of equipments |
The process is complex, and the equipments are many, including air compress system, pre-cooling system, PPU system, expander unit, heat exchange system and fractionating system etc. |
Simple process and few equipments, including blower, vacuum pump and adsorbing column etc. |
|
Features of process |
Running in condition of -160～-190℃ |
Running in normal temperature |
|
Feature of operation |
The starting period is long, usually 15~40 hours, which should run without stop. Returning to work period is long after short stop. |
The starting period is short, usually no more than 30min, it can run continuously run or dis continuously. |
|
Features of maintenance |
Equipment structure is complex, and processing precision is high, the maintenance technology is difficult and cost is high |
Equipment structure is simple, he maintenance technology is easy and cost is low |
|
Feature of construction and fabrication |
It covers a big area, requirement of building and foundation is high, the project manufacturing cost is high, fabrication period is long, technology is difficult, fabrication cost is high |
It covers a small area, no requirement for building, project manufacturing cost is low, fabrication period is short, fabrication cost is low |
|
Oxygen producing cost |
0.5～1.0KW/Nm3
Power consumption of middle and small scale equipment is high, about 0.5～1.0KW/Nm3 |
0.32～0.35KW/Nm3
Power consumption is low, about 0.32～0.35KW/Nm3 |
|
safety |
The equipment is limited by the pressure vessels standard, which may lead a part gather of hydrocarbon, and there is a explosive possibility |
The running pressures is low, which is not limited by pressure vessels standard, and it will not lead any part gather of hydrocarbon |
PSA theory：
The main component of air is nitrogen and oxygen. Design proper process to separate oxygen by choosing different sorbent for nitrogen and oxygen. Nitrogen and oxygen both have quadrupole moment, but the quadrupole moment of nitrogen（0.31Å） is much higher than oxygen（0.10 Å）, as a result nitrogen adsorbing ability is higher than that of oxygen in zeolite molecule sieve.
So, in pressurizing condition, when air goes through a adsorbent bed with zeolite molecule sieve sorbent, the nitrogent is adsorbed by molecule sieve, oxygen will gather and go out of adsorbent bed for less adsorb, and the oxygen is separated. When molecule sieve nearly saturates with adsorbing nitrogen, the air going through adsorbent bed will stop and the around pressure will reduce. The nitrogen adsorbed by molecule sieve will be desorbed, and molecule will regenerate and be used again. Oxygen will be made continuously with the alternative working of two more adsorbent beds.
Argon and oxygen boiling points are similar, so they are hard to be separated. And the vapor-phase will enrich when these two kinds of gas are together. As a result, only oxygen of purity of 90%~95% could be made by PSA(oxygen max purity is 95.6%, the other is argon), which is also called enriched air compared with oxygen with purity more than 99.5% which is made by cryogenic separation unit.
PSA introduction
We could know that from the above, PSA adsorbent bed should have at least two steps: adsorbing and desorbing. So, when there is only one adsorbent bed, the production oxygen is made non-continuously. To get the production gas continuously, two more adsorbent beds are usually made, and some other necessary steps are also made considering of energy saving and steady operation.
Each adsorbent bed will usually go steps of adsorbing, consequent pressure reducing, vacuation or pressure reducing and regenerating, washing permutation and pressure equalizing and boosting etc. which are repeated circularly. In the same time, each adsorbent bed will be in different step, and they are switched regularly with computer controlling. Each adsorbent bed cooperates with each other, and make the PSA run steadily so as to get production gas continuously.
PSA is divided into two processes per different desorbing ways(see chart 1):
PSA process: pressure boosting adsorbing（0.2～0.6MPa）, normal pressure desorbing. Low investment, simple equipment, but the power consumption is high, which is suitable for small scale production.
VPSA: normal pressure or a little higher than normal pressure （0～50KPa） adsorbing, vacuumizing and desorbing. The equipment is complex, but the efficiency is high, and power consumption is low, which is suitable for large scale production.
Chart 1. PSA and VPSA comparison
|
process |
Suitable scale
m3/h |
Adsorbing pressure
KPa |
Desorbing pressure
KPa |
Oxygen purity
% |
Power consumption of oxygen production
KWh/m3 |
Oxygen production
% |
|
PSA |
≤200 |
200～600 |
barometric pressure |
80～93 |
0.7～2 |
30～45 |
|
VPSA |
100～10000 |
0～50 |
－45～－80 |
80～95 |
0.3～0.5 |
46～68 |
For actual separation, the other small air components should be considered. Adsorbing ability of CO2 and H2O is usually much higher than nitrogen and oxygen in normal sorbent, which could be adsorbed and cleaned out by filling proper sorbent in adsorbent bed.
The fractionating column quantity required is per oxygen production scale, sorbent specification and design. Multi- column operation is better in stabability, but the investment is high. Now the tendency is that using high efficient sorbent, reduce adsorbing tower quantity and use short operation period, so as to increase efficiency and save investment. Back >>
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