Study On Concentration Control Of VOCs In Vacuum Or Hot Nitrogen Desorption Of Activated Carbon

Volatile organic compounds(VOCs)are a kind of important gaseous pollutants that affect the air quality.As adsorbent,activated carbon is often used to purify VOCs.Adsorption-saturated activated carbon needs to be regenerated for the purpose of activated carbon recycling.During thermal regeneration of activated carbon,problems such as low VOCs desorption efficiency,and high fluctuation of desorption concentration often lead to higher energy consumption and explosion in the subsequent disposal equipment.Here,two pilot plants were designed and built in order to explore strategies for efficient and stable desorption VOCs.High temperature-vacuum desorption and circulating hot nitrogen desorption for saturated activated carbon were carried out using toluene and ethyl acetate as target VOCs pollutants.Based on the pilot experimental results,two demonstration projects at engineering application level,using vacuum thermal desorption and circulating hot nitrogen desorption,respectively,were designed and implemented.In the high-temperature vacuum desorption experiment,the effect of desorption temperature on the concentration of desorbed VOCs was studied.It was found that the VOCs concentration can be effectively stabilized by using stepwise heating mode.We also investigated the effect of vacuum mode on the desorption efficiency of VOCs,and observed that the highest desorption rates of toluene and ethyl acetate(95%and 87%,respectively)were achieved under the mode of"3+1"(i.e.vacuumizing for three minutes and then restoring to normal pressure for one minute).Furthermore,even more efficient and stable desorption of toluene and ethyl acetate was achieved by combining the desorption mode of"3+1"with stepwise temperature increase.Based on the pilot experimental results,a demonstration project of mobile desorption-regeneration was designed and investigated.It was found that the best desorption effect can be achieved by adopting a desorption mode of"7+1"(i.e.vacuumizing for seven minutes and restoring to normal pressure for one minute)and stepwise heating.The average desorption rate of activated carbon was as high as 86%,the desorption concentration fluctuated little The pilot experiment of circulating hot nitrogen desorption was carried out with ethyl acetate as target VOCs pollutant.The effects of stepwise heating and nitrogen flow rate on the desorption concentration were studied,under the precondition of high VOCs desorption rate(more than 85%).The desorption concentration was found to be high in the early stage and fluctuated greatly.The peak values of VOCs concentration were as high as 5.2×10~4 mg/m~3 and 5.5×10~4 mg/m~3 under stepwise heating and flow rate-varying conditions,respectively.We then explored the synergistic effect of stepwise heating and changing nitrogen on the desorption concentration.The results showed that that the desorption concentration was basically stable in the safe range(below the lower explosion limit of ethyl acetate at 45345.83 mg/m~3)in the early stage by using step heating and reducing the nitrogen flow in the early stage of desorption.Based on the pilot experimental results,we designed and built a regeneration project of circulating hot nitrogen desorption.In real tests,we found that the desorption concentration of VOCs could be better stabilized by stepwise heating and reducing nitrogen flow in the early stage of desorption.The average desorption rate of activated carbon was 85%,and the desorption concentration fluctuated little.According to cost-effectiveness calculation,investment on the mobile desorption and regeneration project was 617900 yuan,and the operation cost was 65551.6 yuan/year.While the investment on the circulating hot nitrogen desorption and regeneration project was 707500yuan,and the operation cost was 87587.2 yuan/year.Compared with similar engineering cases,the two demonstration projects in this article not only had better desorption rate and concentration stability,but also lower cost and operation cost,which could reduce the investment of enterprises in pollution control,met the demands of enterprises for adsorbent regeneration,and showed excellent industrial application prospects.