Air is a mixed gas, and the gas separation process cannot be carried out spontaneously. In order to realize the separation of oxygen and nitrogen, work must be consumed. If the separation process is reversible, that is, without any energy loss, the consumed power is the minimum, which is called the minimum power of separation. Set the effective energy of the environmental state as zero, and separate the air into oxygen and nitrogen, which means that the air is in an unbalanced state with the environment, and it needs to consume energy, that is, the separated products have effective energy.
Since the raw material air is in the environmental state, its effective energy is zero. For the ideal separation process, the power of component separation is all transformed into effective energy. For example: under the condition of environment condition Po = 0.1013MPa, To = 298 K, calculating according To Air oxygen content 20.9%, nitrogen content 79.1%, the effective energy of oxygen eo2 = 173kJ/m3, the effective energy of nitrogen eN2 = 25. 9kj/m3. The sum of effective energy after 1m3 air is separated into oxygen and nitrogen products eB = (0.209 × 173 + 0.791 × 25.9) kJ/m3 = 56. 64kJ/m3. And in order to release air, will processing air compression to 0.6MPa need consumption of work (effective) for eA = 234. 3kJ/m3. Therefore, the effective energy efficiency of the air separation plant δ = eB/eA X 100% = 56.64/234.3X100% = 24.2%. Thus, more than 75% of the energy is lost in the separation process.
These energy losses mainly occur in the compression process, followed by the rectification process. The loss of rectification process is mainly manifested in three aspects:
1) pressure drop when gas and liquid flow;
2) there is temperature difference between gas and liquid in heat transfer;
3) there exists concentration difference when gas and liquid are transferred to mass.
For the distillation column, in order to achieve the purpose of energy saving in the production process, the following points should be paid attention:
1) reduce the tower plate resistance to prevent the tower plate from blocking;
2) the structure design of the tower plate should ensure the full contact between gas and liquid to strengthen the heat and mass transfer, and the gas and liquid concentration should be close to the gas-liquid equilibrium concentration;
3) adjust the working condition of the distillation column to make it have appropriate gas-liquid ratio;
4) reduce expansion air volume and superheat;
5) when designing and manufacturing the distillation tower, use high-efficiency plate to improve the structure of the tower. For example: the sieve Tower enlarges the diameter of sieve hole, develops the guide-oriented Tower plate, ripple Tower plate and so on to make the separation more perfect. At present, sieve tower is changed into regular packing Tower, because the resistance of the packing tower is only 1/6 ~ of that of sieve Tower ~ 1/7, therefore, the rectification of the stuffing tower can significantly save energy than that of the sieve tower.
