Concentration Of Volatile Organic Compounds By Adsorption And Desorption In Multistage Fluidized Beds

Volatile Organic Compounds(VOCs)are crucial precursors for the formation of PM2.5 and O3,which seriously damage the ecological environment and affect human health.The control of VOCs emissions in China relatively lags behind.Hence,it is very urgent and crucial to develop efficient technologies to control VOCs emissions.To control the VOCs emissions with dilute concentration and large flow rate,the efficient technology is that the VOCs are first pre-concentrated by adsorption and desorption cycle and then oxidized to H2O and CO2 by catalytic oxidation process or recovered by condensation process.Fluidized bed stands out for the advantages of uniform temperature distribution,fast heat and mass transfer,high handling capacity,and easy scale-up,etc.The multistage shallow fluidized bed has the additional advantages of inhibiting the solid particles back mixing and the formation of bubbles,which ensures high mass transfer efficiency and good gas solid contact.A concentration technology with double multistage shallow fluidized beds that realizes the enrichment of dilute VOCs by the adsorption and desorption cycle is proposed.The adsorption and desorption of VOCs in multistage shallow fluidized beds are studied by experiments and numerical simulations.In the thesis,the HCP-5 hypercrosslinked polymer was used as adsorbent.The physical and fluidization properties of the adsorbent were measured.The adsorption isotherms of HCP-5 adsorbent and o-xylene,ethyl acetate,acetone system at different temperatures were measured.And,the Langmuir isotherm was used to describe the experimental results in the experimental concentration and temperature ranges.A mathematical model of desorption breakthrough curves in fixed bed was established.By comparison of the experimental and calculated breakthrough curve,the internal mass transfer coefficients of o-xylene,ethyl acetate and acetone at different temperatures during desorption process were obtained by using the least square method By comparing the internal and external mass transfer coefficients of the fluidized bed in adsorption and desorption process,it is found that in the adsorption process,the mass transfer process is mainly controlled by the external one.But in the desorption process,the limiting step of mass transfer process is the internal one.Compared with the adsorption process,the contribution of surface diffusion to the internal mass transfer is reduced in the desorption process,resulting in a significant decrease of the internal mass transfer coefficient.The VOCs adsorption and desorption experiments in the multistage fluidized bed were carried out,and the effects of bed height,gas solid flow ratio and total stage number on gas outlet VOCs concentration were studied.A mathematical model based on internal and external mass transfer dynamics was proposed,which was used to describe the adsorption and desorption process in multistage fluidized bed.Through the mathematical model,the effects of effluent gas VOCs concentration,total stage number,gas solid flow ratio,desorption gas temperature on gas outlet VOCs concentration of adsorber and desorber were studied,and the effects of effluent gas VOCs concentration and species of VOCs on concentrating ratio were also investigated.It is found that in the desorption process,when the VOCs concentration of gas phase in the upper stage is greater than the equilibrium VOCs concentration corresponding to the adsorbent in this stage,adsorption phenomenon will occur in the upper stage and the VOCs concentration will decrease.Preheating the adsorbent can eliminate this phenomenon and increase the gas outlet VOCs concentration of the desorber.With the change of gas solid flow ratio of the multistage fluidized bed desorber,there is a maximum value of the gas outlet VOCs concentration of the desorber,which increases with the increase of total stage number,desorption gas temperature,adsorbent inlet temperatureA demonstration device of concentration of VOCs by adsorption and desorption in multistage fluidized beds was designed for the effluent gas VOCs concentration and flow rate of 300 mg/Nm3 and 20,000 Nm3/h respectively.According to the established mathematical model,the parameters such as total stage number,adsorbent flow rate,desorption gas flow rate and temperature were determined.The device has already been constructed and put into use,with the total VOCs removal efficiency up to 98%and concentrating ratio up to 17,meeting the requirements of VOCs emission standards and catalytic oxidation.The technology of concentration of VOCs by adsorption and desorption in multistage fluidized beds provides a feasible technical approach for the purification of VOCs emissions with dilute concentration and large flow rate.