1. Deterioration of coal activated carbon
As the reasons for the decline of adsorption performance of activated carbon in use, it can be considered as follows: the accumulation of trace macromolecular substances or polymeric substances contained in adsorption gas in activated carbon; Impurities mixed from desorbed water vapor; As can be seen in the recovery of carbon disulfide, the accumulation of sulfur decomposed by oxidation; And the accumulation of polymer substances generated by the adsorbed solvent in activated carbon.
Especially when the target adsorbed substances generate those high boiling substances that cannot be removed in the normal desorption operation through chemical reactions such as oxidation, decomposition and polymerization, sometimes the performance of the adsorption device will decline sharply because these substances are also generated in the activated carbon.
2. Heating of coal activated carbon layer
The disastrous accident in solvent recovery is mainly the ignition of activated carbon. It can be considered that most ignition accidents are caused by natural ignition caused by abnormal temperature rise due to the accumulation of the adsorption heat of activated carbon to solvent or the reaction heat generated by the oxidation reaction of solvent in the activated carbon layer. Activated carbon is a porous structure with poor thermal conductivity, which is easy to cause local heat storage.
During operation under normal conditions, the heat generated by adsorption is in equilibrium with the heat release of adsorbed gas. However, when the adsorbed solvent is oxidized and decomposed, the balance is destroyed, which further accelerates the oxidation and decomposition reaction, and finally leads to the abnormal rise of temperature. Especially when acetone, methyl ethyl ketone, cyclohexanone and other ketone solvents are recovered, there are many fire accidents.
In order to master the situation during ignition, the air with cyclohexanone concentration of 2000ppm (volume) is introduced into the activated carbon filling layer with temperature maintained at 85 ℃, and the time-varying changes of temperature in the activated carbon layer and carbon monoxide concentration in exhaust gas are measured.
3. Desorption performance of coal based activated carbon for adsorbing solvent
To completely desorb the adsorbed solvent will increase the consumption of water vapor and improve the recovery cost; When the amount of water vapor is controlled, the amount of residual solvent increases and the effective adsorption capacity of activated carbon decreases. When there is a drying process after desorption, the solvent will escape, resulting in the decrease of recovery rate and the increase of solvent concentration in exhaust gas.
Generally, a large amount of desorption water vapor is required when desorbing high boiling solvents. It not only increases the desorption cost, but also increases the cost of subsequent refining process due to the mutual solubility of solvent and water. The heat recovery of desorbed steam has been carried out before. In the aspect of activated carbon, activated carbon with good pore size distribution and particle size desorption performance, which is especially suitable for high boiling solvents, has also been developed. Adsorption isotherms of activated carbon suitable for general solvent adsorption such as cyclohexanone and activated carbon suitable for high boiling solvent adsorption at 25 ℃ and 100 ℃.
4. Adsorption properties of coal activated carbon in wet state
When coal activated carbon contains water or circulating gas, the adsorption rate of activated carbon to organic solvents will generally decrease. However, by using activated carbon that can maintain considerable adsorption capacity in the wet state, it can still be used for recovery under the conditions that are not suitable for recovery so far, and the amount of water vapor can be saved.
Especially in the case of solvent recovery that may be heated due to oxidation and decomposition, humidifying the adsorbed gas can inhibit the temperature rise of the activated carbon layer, inhibit the occurrence of reaction and prevent ignition. When ketone solvents are recovered, this becomes an important condition for the selection of activated carbon. Breakthrough curve of acetone adsorption by dry activated carbon and wet activated carbon. Penetration time is also one of the indicators of activated carbon adsorption capacity. This shows the influence of water on the shape of the suction belt.
