Desorption is the creation of conditions corresponding to low load, the introduction of substances or energy to make the adsorbent molecules and activated carbon between the force weakened or disappeared, to remove reversible adsorbent. Traditional activated carbon desorption methods include water vapor, hot gas desorption, variable pressure desorption, solvent replacement, and in recent years, new desorption methods such as electrothermal method, ultrasonic regeneration, microwave irradiation, etc. have emerged.
1. Water vapor, hot gas desorption method is suitable for desorption of low boiling point of low molecular hydrocarbons and aromatic organics, water vapor enthalpy is high and easy to obtain, economy, safety, but for the high boiling point of the material desorption ability is weak, desorption cycle is long, easy to cause corrosion of the system, the material performance requirements are high; recovery of the material of the higher water content, desorption of easy to hydrolyze pollutants (such as halogenated hydrocarbons) when the recovery of the quality; water vapor desorption system needs a long time to cool and dry before re-use, there is also the problem of secondary pollution of the condensate. Quality; water vapor desorption, the adsorption system needs a long time of cooling and drying to be put into use again, there is also the problem of secondary pollution of condensate. Compared with water vapor desorption, the condensate of hot gas desorption has little secondary pollution, the recovered organic matter has low water content (for water-soluble organic matter is more advantageous), it is convenient for further refining and recovery, the regeneration drying and cooling time is short, and the material requirements for the adsorption system are low.The disadvantage of hot gas desorption is that the gas heat capacity is small and the area required for gas heat exchange is relatively large. If hot air desorption is used directly, there may be some danger, and the presence of oxygen affects the quality of the recovered substances, so it is necessary to control the content of oxygen in the regeneration gas and increase the recovery cost. Some scholars have proposed improvements to hot gas desorption: in 2002 Reiter proposed the method of regeneration vapors and the contaminated gas stream to be adsorbed in the downstream flow to improve the desorption efficiency and extend the service life of activated carbon, and the use of peripheral air rather than the traditional purified gas as the drying gas.Flink used a mixture of air, inert gas for the cycle of desorption.
2. Solvent displacement method is represented by pharmaceutical elution and supercritical fluid regeneration method, which desorbs the adsorbent by changing the concentration of the adsorbed components, and then heats up and excludes the solvent to regenerate the adsorbent. Pharmaceutical elution method is suitable for desorption of high concentration, low boiling point of organic matter, so that the adsorbent reacts with the appropriate chemicals to regenerate the activated carbon, targeted, often a solvent can only desorb certain pollutants, the application range is narrow. The price of organic solvents used is high, some have toxicity, which will bring secondary pollution, activated carbon regeneration is not complete, easy to block the microporous activated carbon, and the adsorption performance of activated carbon is obviously reduced after regeneration for many times. Supercritical fluid regeneration method uses supercritical fluid as solvent, dissolves organic pollutants adsorbed on activated carbon in supercritical fluid, and then utilizes the relationship between fluid properties and temperature and pressure to separate organic matter from supercritical fluid to achieve the purpose of regeneration, and CO2 is generally used as the extractant.In 1979, Modell first used supercritical CO2 to regenerate phenol from activated carbon, the method operates at low temperature, does not change the physical and chemical properties of the adsorbent and the original structure of activated carbon, activated carbon is basically lossless, facilitates the collection of pollutants, facilitates the reuse of adsorbent, cuts off secondary pollution, and can realize continuous operation, regeneration equipment occupies a small area, and consumes little energy. However, the organic pollutants studied in this method are very limited, and it is difficult to prove the broadness of application.
3. Electrothermal desorption method
Fabuss and Dubois in 1970 utilized the electrical conductivity of adsorbent materials by applying an electric current to the adsorbent after adsorption saturation, using the Joule effect to generate heat and provide energy for desorption. Currently, there are two ways to generate current: direct current generation by electrodes and indirect current generation by electromagnetic induction. Compared with the traditional variable temperature resolution method, the regeneration gas flow of electrothermal desorption method can be reduced by 10-20%, with high efficiency, low energy consumption, and fewer limitations on the treatment object. However, there will be over-hot spot when direct heating, which affects the control of adsorption bed temperature and is difficult to be enlarged, in addition, the electrode arrangement connection and insulation are still to be further studied in depth.
4. Microwave desorption method
Activated carbon can absorb microwave energy for desorption of adsorbent. Microwave heating is fast, only 1/100-1/10 of the conventional method can be completed, and the heating is uniform, only the material absorbing microwave heating effect, low energy consumption, equipment, operation is simple, regeneration efficiency is high, easy to automatic control, but due to the microwave heating process is closed, desorption of substances can not be excluded in time, the effect of the regeneration will have a certain effect.Ania et al. regenerated activated carbon saturated with phenol using 2450MHz microwave and traditional electric heating methods, and found that microwave can significantly shorten the desorption time and reduce the loss of adsorption capacity of activated carbon. Ning et al. used microwave irradiation to regenerate activated carbon adsorbed with toluene waste gas and condensed the desorbed gas, and the recovery rate of toluene was more than 60%, which was close to chemical purity. Wang Baoqing used microwave desorption to regenerate ethanol-loaded activated carbon, and the desorption rate reached more than 90% after 3-4 min.
5. Ultrasonic regeneration of different scholars on the mechanism of ultrasonic desorption of different explanations: Yu, Bässler, Hamdaoui, etc. that is the acoustic cavity generated by high-speed micro-jet and high-pressure shock waves lead to adsorption of adsorbent desorption, Breit-bach, etc. that is the thermal effect of ultrasonic accelerate desorption of adsorbent. Our scholars believe that ultrasonic waves and different phase interfaces or other ultrasonic wave peaks meet, will produce a huge compression force, with the wave rebound to form a tiny “cavitation bubble”, “cavitation bubble” burst when the explosion point of the temperature and pressure rises steeply, the energy transfer to the adsorbed material, the energy transfer to the adsorbed material, the energy transfer to the adsorbed material, the energy transfer to the adsorbed material, the energy transfer to the adsorbed material. energy transfer to the adsorbed material, intensifying its thermal movement, detached from the adsorbent surface. Since ultrasound only applies energy locally, it consumes less energy, has less carbon loss, and is a simple process. Hamdaoui’s results show that ultrasound can significantly increase the desorption rate of p-chlorobenzene, and the desorption rate is accelerated with increasing frequency in the range of 21 to 800 kHz, and the stability of the activated carbon is not affected until the ultrasound reaches 38.3W.
