Desorption is a method to remove reversible adsorbent by creating conditions corresponding to low load and introducing substances or energy to weaken or disappear the force between adsorbent molecules and activated carbon.
1. Desorption by Water Vapor and Hot Gas
This method is suitable for the desorption of low molecular hydrocarbons and aromatic organic compounds with low boiling point. The enthalpy of water vapor is high and easy to obtain, it's economical and safe. However, the desorption ability of high boiling point substances is weak, the desorption cycle is long, and the system corrosion is easy to cause, and the material performance is high. The water content of the recycled material is high, and the quality of the recycled material will be affected by the desorption of easily hydrolyzed pollutants (such as halogenated hydrocarbons). After water vapor desorption, the adsorption system needs a long time to cool and dry before it can be put into use again, and there is the problem of secondary pollution of condensed water. Compared with water vapor desorption, hot gas desorption condensate has less water secondary pollution, the recovered organic matter water content is low (for water-soluble organic matter is more advantageous), convenient for further refining. Recovery, regeneration, drying, cooling time is short, it has lower requirements on materials.
The disadvantage of hot gas desorption is that the heat capacity of the gas is small and the area required for gas heat exchange is relatively large. If hot air is directly used for desorption, there may be a certain danger. Moreover, the existence of oxygen will affect the quality of recycled materials, so it is necessary to control the content of oxygen in the recycled gas, which will increase the recycling cost. Some scholars put forward improvements to the desorption of hot gases: in 2002, Reiter proposed the method of regenerated steam and polluted air to be adsorbed in order to improve the desorption efficiency and prolong the service life of activated carbon, and used the surrounding air instead of the traditional purified gas as the drying gas. Flink uses a mixture of air and inert gases for cyclic desorption.
2.Solvent Replacement
The method is represented by reagent elution and supercritical fluid regeneration. The adsorbent is desorbed by changing the concentration of adsorbent components, and then the solvent is removed by heating to regenerate the adsorbent. The reagent elution method is suitable for desorbing high concentration and low boiling point organic matter, so that the adsorbent reacts with appropriate chemicals, and the activated carbon is regenerated. It is more targeted, often a solvent can only desorption some pollutants, the scope of application is narrow. However, the organic solvents used in this method are expensive and some are toxic, which will bring secondary pollution. Activated carbon regeneration is not complete, easy to plug the activated carbon micropores, and the adsorption performance of activated carbon is significantly reduced after multiple regeneration.
Supercritical Fluid regeneration uses supercritical fluid as solvent to dissolve organic pollutants adsorbed on activated carbon in supercritical fluid, and then uses the relationship between fluid properties and temperature and pressure to separate organic matter from supercritical fluid to achieve the purpose of regeneration. CO2 is generally used as extraction agent. In 1979, Modell first used supercritical CO2 to regenerate phenol from activated carbon. This method did not change the physical and chemical properties of adsorbent and the original structure of activated carbon in low operating temperature. Activated carbon basically had no loss. And this way is easy to collect pollutants, is conducive to the reuse of adsorbed materials. It cut off the secondary pollution, achieving the continuous operation, the recycling equipment occupies a small area with less energy consumption. However, the organic pollutants studied by this method are relatively few, so it is difficult to prove its wide application.
3.Electrothermal Desorption
In 1970, Fabuss and Dubois used the conductivity of adsorbent materials to apply current to adsorbent after adsorption saturation, and use Joule effect to generate heat to provide energy for desorption. Currently, there are two ways to generate current: directly from electrodes and indirectly from electromagnetic induction. Compared with the traditional variable temperature analytical method, the electric thermal desorption method can reduce the flow rate of regenerative gas by 10%-20%, with high efficiency, low energy consumption and less limitations on the treatment object. However, there will be hot spots during direct heating, which will affect the temperature control of the adsorption bed and make it difficult to amplify. In addition, the electrode arrangement, connection and insulation need to be further studied.
4.Microwave Desorption
Activated carbon can absorb microwave energy for desorption of adsorbent. Microwave heating speed is fast, it can be completed in 1/100-1/10 of the time of the regular method and the heating is uniform. It only has heating effect on microwave absorbing materials, low energy consumption, simple equipment, operation, high regeneration efficiency and easy to control automatically. However, due to the closed process of microwave heating, the desorption materials can not be excluded in time, which will have a certain impact on the regeneration effect. Ania et al. used 2450MHz microwave and traditional electrothermal method to regenerate the phenol saturated activated carbon, and found that microwave could significantly shorten the desorption time, and the adsorption capacity loss of activated carbon was less. Ning Ping et al. used microwave irradiation to regenerate the activated carbon adsorbed toluene waste gas and condense the desorption. The recovery rate of toluene reached more than 60%, close to chemical purity. Wang Baoqing used microwave desorption to regenerate activated carbon loaded with ethanol, and the desorption rate reached more than 90% after 3-4 minutes.
5.Ultrasonic Wave Regeneration
Different scholars have different explanations for the principle of ultrasonic desorption: Yu, Bassler, Hamdaoui et al. believe that the high-speed micro jet generated by acoustic holes and high-pressure shock wave lead to adsorbate desorption, while Breit-bach et al. believe that the thermal effect of ultrasonic wave accelerates adsorbate desorption. Chinese scholars think that the ultrasonic with different phase interface or other ultrasonic wave when they met, will produce a great compression force, as the wave of rebound to form tiny "cavitation bubble", "cavitation bubble burst point when the temperature and pressure rise abruptly, could pass the energy to be adsorption material, increase its thermal motion, from the surface of the adsorbent. Because the ultrasonic wave only applies energy locally, the energy consumption is small, the carbon loss is small, and the process equipment is simple. Hamdaoui's results showed that ultrasonic wave could significantly increase the desorption rate of P-chlorobenzenes. In the range of 21 to 800kHz, the desorption rate increased with the increase of frequency, and the stability of activated carbon was not affected until the ultrasonic wave reached 38.3W.
