Activated carbon has developed pores, large specific surface area, easy surface modification, high treatment efficiency and relatively low price, and as a broad-spectrum adsorption material, it has become the most commonly used adsorbent for VOCs purification. However, in industrial applications, the adsorption capacity of saturated activated carbon cannot be fully recovered after desorption or regeneration; in the treatment of industrial waste gas containing styrene, butadiene and other reactive VOCs in the chemical industry, glass fiber reinforced plastic and other industries, there is even a phenomenon of sharp decline in the adsorption performance of the activated carbon when it is desorbed. This not only shortens the cycle life of activated carbon and increases the cost of pollution control, but also generates a large amount of hazardous solid waste, posing a new threat to the environment.
The reason for the incomplete reversible adsorption-desorption of VOCs on activated carbon is that some of the VOCs will be retained in the pores of the activated carbon during the desorption process to form a buildup, and the amount of the buildup increases with the increase in the number of adsorption-desorption cycles, and the pores of the activated carbon are gradually clogged with a decrease in the ability to adsorb VOCs.
Phenomena and effects of significant decay of adsorption performance of activated carbon after (thermal) desorption
The ideal regeneration process is one that restores the adsorption capacity of the activated carbon while keeping the porous structure of the activated carbon intact. However, the adsorption performance of VOCs adsorbed on activated carbon in industrial applications is not fully restored after thermal desorption.
The phenomenon of activated carbon performance not recovering after desorption first appeared in the application of liquid phase adsorption of activated carbon.
Suzuki et al discovered this phenomenon during thermal desorption of activated carbon adsorbed with phenol, suggesting that the organic matter undergoes a cleavage reaction during thermal desorption, and that the by-products of the cleavage, or carbon deposits, are retained within the pores of the activated carbon.
Grant et al found that the difficulty in removing phenolic compounds from activated carbon is due to the irreversible adsorption of phenolic compounds by activated carbon.
Later Chatzopoulos et al described irreversible adsorption as the formation of chemical bonds between the adsorbent and the adsorbent (chemisorption) or the chemical reaction of the adsorbent within the pores of the adsorbent.
These phenomena are mainly focused on the adsorption of organic matter in the liquid phase by activated carbon, especially the irreversible adsorption of phenolic compounds.
Lashaki et al for the first time referred to the substances produced by the irreversible adsorption of organic gases by activated carbon as pore buildup, the presence of which shortens the service life of the adsorbent.
For VOCs emitted from the FRP industry, the adsorption performance hardly recovers when the desorbed adsorbent is put back into service, regardless of whether the adsorbent is zeolite or activated carbon.
Although activated carbon adsorption and regenerative catalytic oxidation device (RCO) investment and operation and maintenance costs are not high, is the glass fiber reinforced plastic industry preferred purification and treatment programs, but activated carbon desorption / regeneration process of pore blockage, adsorption performance of the phenomenon of sharp decline, will lead to activated carbon is difficult to reuse to make the enterprise management costs increased dramatically, which is the current VOCs in the glass fiber reinforced plastic industry, the pain, the difficulty in the treatment of the pain point.
The formation of buildup of VOCs in the desorption process of activated carbon is the cause of the significant decay of the adsorption performance of activated carbon after thermal desorption, and the slowing down or elimination of the buildup is a difficult problem faced by activated carbon adsorption treatment of industrial exhaust gases, which needs to be solved urgently.
Accumulation and formation mechanism in activated carbon
Identifying the type of buildup and clarifying the mechanism of buildup formation is the first step in solving the buildup problem and extending the life of activated carbon.
Studies have shown that pore buildup during activated carbon desorption may be chemisorbed species, semi-coke/coke and polymers.
Based on the similarities and differences between stacked and adsorbed VOCs, stacks can be categorized into two groups.
One is the accumulation of VOCs in the form of chemisorption in the pores of activated carbon to form chemisorbed species.
Chemically adsorbed species are usually difficult to desorb because the adsorbent forms a chemical bond with the surface of the activated carbon, which generates a very high heat of adsorption, and in order to break this chemical bond, high temperatures or reduced pressures are required for desorption and desorption.
Using 1,2,4-trimethylbenzene (TMB), a typical emission from the automotive coating industry, as an adsorbent, Hashemi et al. found that the buildup may be a chemisorbed species, and that at higher desorption temperatures, the TMB strongly interacts with the C-C bonds on the activated carbon surface.
The other is that VOCs undergo reactions such as pyrolysis, coupling, polymerization or thermal oxidation to form new substances such as semi-coke/coke or polymers.
Pyrolysis of VOCs usually occurs during regeneration of activated carbon at high temperatures (above 300°C), where VOCs are pyrolyzed to form semi-coke or coke, which clogs the pores of the activated carbon and reduces its adsorption capacity.
VOCs pyrolysis is mainly influenced by the regeneration conditions (temperature, oxygen, heating rate, etc.) and the nature of the VOCs molecules.
Niknaddaf et al found that the higher the desorption temperature, the more the TMB undergoes pyrolytic reactions, which can clog the adsorbent pores.
Lashaki et al used activated carbon to adsorb VOCs generated from automotive paint spraying operations. After cyclic adsorption/desorption experiments, the deactivated and discarded activated carbon was leached, and the leachate was analyzed, and it was found that there was a great difference in the compositions of leachate and adsorbate, which suggested that the VOCs had undergone a chemical reaction during the desorption process of the activated carbon and speculated that it was the pyrolysis of hydrocarbons induced by the free radicals.
It is worth noting that VOCs adsorbed in activated carbon react to coke and are difficult to be leached out after several pyrolysis cycles.
The formation of polymers is mainly attributed to the coupling, polymerization or thermal oxidation of VOCs within the pores of the activated carbon.
For phenolic compounds in the liquid phase, oxygen in the adsorption medium and oxygen-containing groups on the surface of the activated carbon affect their oxidative coupling on the activated carbon. The oxidative coupling mechanism is that phenol loses a proton to form a phenoxy group, and the phenoxy group generates larger compounds such as dimer or trimer through coupling, which are difficult to desorb due to their large size.
Grant et al suggested that at higher desorption temperatures, the oxygen-containing functional groups on the surface of the activated carbon can induce oxidative coupling of phenolic compounds to form polymers. However, oxidative coupling of compounds other than phenols has not been reported in the literature.
Thermal oxidation is a free radical reaction involving oxygen at a certain temperature. Liu et al. found that 1,3 butadiene reacts with oxygen to form peroxides, which are thermally decomposed to produce free radicals that initiate rapid polymerization to form high molecular weight polymers.
Hashemi et al concluded that TMB reacts chemically with oxygen in the regeneration gas and the reaction products are adsorbed on the surface of the activated carbon, but did not identify the reaction products.
Lashaki et al used activated carbon to treat VOCs generated from automotive paint spraying, and when the deactivated and discarded activated carbon was leached, it was found that most of the buildup (75%, mass fraction) still could not be leached out, probably because the VOCs were generated into high-molecular-weight polymers through several thermo-oxidation reactions involving oxygen, and the substances that could not be leached out would either form strong interactions with the chemical bonding of the activated carbon, or would be difficult to desorb due to their high boiling point and large size, and would continue to build up inside the pores of the activated carbon, which would ultimately lead to the loss of the activated carbon’s adsorptive performance.
Bhat et al demonstrated that VOCs (n-butanol, ethanol butyl ester, and TMB) from the automotive coating industry polymerize to form polymers that are difficult to desorb at higher desorption temperatures and in the presence of oxygen.
For the adsorption of VOCs emitted from the glass fiber reinforced plastic (FRP) industry, the adsorption performance of activated carbon does not recover after desorption, mainly due to the production process of FRP is rich in styrene, butadiene and other reactive VOCs containing unsaturated bonds, in the process of activated carbon desorption, this kind of VOCs in the role of oxygen and heat is very easy to form free radicals, polymerization reaction occurs, the plugging of adsorbent pores, resulting in the adsorption of the activated carbon performance of the drastic attenuation.
In summary, the mechanism of pile formation is shown in Fig. 1, and there are several possible explanations.
(1) Chemisorption, where there is a strong interaction between the adsorbent and the activated carbon surface.
(2) Pyrolysis reaction, where the adsorbent pyrolyzes to form semi-coke or coke.
(3) Coupling reactions, in which adsorbates (mainly phenolic compounds) are oxidatively coupled to produce polymers.
(4) Polymerization reaction in which monomers are polymerized to form high molecular weight polymers.
(5) Thermal oxidation reaction, a free radical reaction involving oxygen at a certain temperature, in which oxidation is followed by polymerization.
