Research On Prediction Model Of Adsorption Filter Efficiency Under The Condition Of Dynamic Concentration

At present,adsorption is still the most effective technique for removing indoor gaseous pollutants.And there have been many studies on the mass transfer model of the adsorption filters,but it is still impossible to quickly predict the efficiency of adsorption filters in its design stage.One of the reasons is that the breakthrough experiment for obtaining the distribution coefficient in the model takes a long time,which can be up to hundreds of hours,so the method is not convenient to engineering applications.Secondly,study on the adsorption capacity and the adsorption characteristics of porous materials in the common concentration range of indoor gaseous pollutants（ppb-ppm level）is still insufficient.In particular,it lacks the desorption data of the loaded porous materials when the concentration of pollutant in environment was reduced.And the applicability of the existing models in the indoor dynamic concentration environment have not been verified.Thirdly,the honeycomb-shaped adsorption filter filled with granular adsorbent is one of the most commonly used form,but there are few studies on the correlation between its structure and the convective mass transfer coefficient.In order to build a practical method that can predict the efficiency of the commonly used adsorption filters,considering the influences of the changing concentration of the pollutants in the indoor environment,the following points was studied in this work:（1）Benzene（C₆H₆）was used as the target gaseous pollutant,for experimentally measuring the equilibrium adsorption capacity and the desorption amount by purging of a variety of commercial adsorbents（the granular activated carbons,an activated carbon fiber felt,an activated carbon polyurethane sponge,the 13X zeolite molecular sieve,the activated alumina and a composite porous mineral material）in a low-concentration environment（50-6000 ppbv）.Considering there is a lack of research on the reversibility of adsorption at low concentrations,an in-depth study was made on the desorption laws of various materials when the concentration of environmental pollutants decreased（0-6000 ppbv）.（2）For the adsorbent-adsorbent combination of activated carbon-benzene,the gas adsorption analysis method was used to characterize the surface of the various activated carbons（9 granular activated carbons,an activated carbon fiber felt and an activated carbon polyurethane sponge）.And the critical pore size range that affects the benzene adsorption capacity and the desorption amount was analyzed on this basis,combining with the differences of the meacured adsorption capacity and reversible adsorption capacity between different activated carbons.（3）The correlation between the structure of the honeycomb-shaped adsorption filters and the convective mass transfer resistance was studied.Firstly,based on the analogy between mass exchange and momentum transfer in the fluid,a number of studies on the relationship between the structural parameters and the friction coefficient of the fixed bed（with structure similar to the honeycomb adsorption filters）were analyzed.And the pressure drop in the honeycomb adsorption filters was experimentally measured.The main structural parameters that affecting the friction coefficient were determined on this basis.Using the picked parameters as the main variables,the single-pass efficiency of different honeycomb adsorption filters in the initial stage of adsorption was measured experimentally.The key structural parameters that affecting the convective mass transfer resistance were studied,and a quantitative relationship was obtained.（4）A prediction model of adsorption filter efficiency under dynamic concentration was established.On the one hand,based on the research results of the adsorption of benzene on activated carbon and the research results of the relationship between the structure of honeycomb adsorption filters and their convective mass transfer resistance in this work,the empirical formulus were introduced in the model:the distribution coefficient in the model is calculated according to the pore characteristics of the material.And the convective mass transfer resistance is calculated according to the structural parameters of the adsorption filter.Therefore,it can avoid the time-consuming and breakthrough experiments and can improve the accuracy of the convective mass transfer resistance of the honeycomb adsorption filter obtained through empirical method,when using this the model.Thereby,it achieves the purpose of predicting the efficiency of the adsorption filter.On the other hand,the parameters that characterize the reversibility of adsorption enable this adsorption efficiency model to reflect the influence of dynamic concentration on efficiency.（5）The prediction model of adsorption filter efficiency under dynamic concentration was verified experimentally.Firstly,the efficiency of two commercial honeycomb adsorption filters at constant benzene concentration and dynamic benzene concentration was measured respectivly,and the measured data was compared with the predicted results of the model.Secondly,the benzene concentration in the test chamber with adsorption filter inside were measured,when the pollution source is intermittently released.And the test data was compared with the coupling calculation results of the adsorption filter efficiency prediction model and the well-mixed zone model.In the above-mentioned works,by studying the adsorption and desorption laws of a variety of commercial adsorption materials on low-concentration benzene,it has been observed that the microporous and the mesoporous materials including activated carbon,zeolite molecular sieves,activated alumina,etc.can produce a desorption amount which can not be ignored compared with the adsorption capacity.And desorption may have a significant impact on indoor air quality.The adsorption capacity of the activated carbons on benzene has the strongest correlation with the micropore volume of the 0.6-0.9 nm pore size segment（molecular probe CO₂,measuring temperature 273K,calculated using NLDFT non-localized density functional method）,and the proportion of the reversible adsorption capacity in the whole adsorption capacity has the strongest correlation with the proportion of the surface area of pore width above 1 nm（molecular probe N₂,measuring temperature 77K,calculated using NLDFT method）.By studying the correlation between the structure of the honeycomb adsorption filters and the convective mass transfer coefficient,it was concluded that the key parameters affecting the convective mass transfer coefficient and the initial single-pass efficiency of the honeycomb adsorption filters are:the face air velocity,the void ratio,the filling rate and the thickness of the filter.On this basis,an empirical method for calculating the convective mass transfer coefficient using the structure of the adsorption filter was built.A prediction model of adsorption filter efficiency under dynamic concentration was established and tested by experiment.It is proved that this model can obtain a predicted efficiency curve as a useful guideline,when the surface characteristics of the adsorption material and the structure parameters of the honeycomb adsorption filter are avalible.When the model is coupled with a well-mixed indoor zone model,it can accurately predict the time it takes for the concentration of benzene in the zone rise to the same level as when the adsorption filter is not installed.In conclusion,in this work the adsorption and desorption characteristics of benzene on the porous materials at low concentrations were investigated,and the correlation between the structure of honeycomb adsorption filter and its convective mass transfer resistance was also studied.On this basis an adsorption filter efficiency prediction model considering dynamic concentration was established.The relationship between the surface pore characteristics of activated carbon and the benzene adsorption capacity and the reversible benzene adsorption capacity obtained in this work can provide the data for the development and selection of adsorption materials for the indoor applications.The quantitative correlation between the structure of the honeycomb adsorption filters,the friction coefficient and the convective mass transfer coefficient can provide the calculation basis for the structural design and the optimization of the energy efficiency of the adsorption air purification devices.And in this method,the easily measurable void ratio can be used to calculate the convective mass transfer coefficient instead of the particle diameter of the adsorbent,which is beneficial for engineering applications.The prediction model for adsorption filter efficiency considering the dynamic concentration can provide the prediction data for the design,selection,operation and maintenance of the adsorption filters.And it also provides the possibility for the coupling calculation of the adsorption air purification device and the indoor VOCs concentration.