Effects Of Nucleation And Growth Process Of Particles And Its Strengthening Means On Separation Performance Of Cloud-Air-Purifying Technology

Industrial emissions of PM_2.5 are one of the major sources of atmospheric pollution.A large number of fine particles entering the environment will threat human health and do great harm to social and economic development.Due to the small particle size and light weight of PM_2.5,the existing technology is not ideal for its removal efficiency.The Cloud-Air-Purifying（CAP）technology is a new type of fine particle removal technology,which is based on the natural principle of"cloud formation and rainfall",allowing particles to play a role as a nuclei and grow in a supersaturated water vapor environment before being efficiently collected by a supergravity field.The CAP technology shows great potential for the removal of fine particle with high efficiency.And,it has been applied in many industrial fields and is worth further development.The key to high efficiency of the CAP technology is the nucleation and growth process,which is affected by particle properties such as particle hydrophilicity and environmental conditions such as temperature.However,the mechanism that how these important factors affect the nucleation and growth process of particles and hence the removal efficiency in the CAP technology is still not clear.Therefore,this research studied the effect of particle hydrophilicity and temperature on the particle nucleation and growth process and then the removal efficiency of the CAP technology.Furthermore,some strengthening methods such as adding surfactants and vortex-broken wings to enhance the particle nucleation and growth were proposed and discussed.Firstly,the effect of particle hydrophilicity on the removal efficiency of the CAP technology was investigated.Six kinds of particles with different hydrophilicities were tested in the CAP cyclone.The improvement of separation efficiencies was more significant for particles with better hydrophilicity。For example,the efficiency increase is 13.4%for calcium carbonate with a contact angle of 0^o,while for S-zorb（54.5 ^o）is8%.This can be attributed to the particle growth in the supersaturated vapor and the gas flow in the cyclone.Additionally,theoretical analysis shows that the nucleation rate and activation probability of heterogeneous condensation are both higher for the particles with better hydrophilicity,indicating that these particles can be activated more easily as condensation nucleus and hence grow in size.Surfactants were added to improve the solid-liquid wetting process.Four different types of surfactants and two particles with different properties were selected and tested.The results of experiments showed that the removal efficiency of CAP technology can be improved by adding surfactants.However,the improvement varies depending on the electric property of particles.For Si O₂-OH particles,CTAB（90.51%）>OP-10（87.82%）>SDBS（84.86%）>SDS（81.42%）>H₂O（79.73%）,while for Si O₂-CH₃ particles,OP-10（89.94%）>SDBS（88.84%）>CTAB（84.22%）>SDS（80.77%）>H₂O（79.34%）.Aided by the Molecular dynamics software,the interaction energy during the wetting of the particles with the surfactant solution was analyzed to study the relationship between surfactant and particles with different electric properties.It was obtained that for strongly charged particles,the electrostatic effect of the surfactant played a dominant role,while for weakly charged particles,the dominant influence became similar to the phase solubility effect.The effects of the two temperatures on the performance of the CAP cyclone were studied.The experimental results indicated that higher water vapor temperature can improve the removal efficiency.There was a 2.6%increase in removal efficiency when the water vapor temperature increased from 293 K to 323 K.The reason was that the surface tension of the liquid phase decreased as the water vapor temperature raised,causing a decreased in the critical supersaturation of the nucleated particles,which makes the particles easier to nucleate and grow in size.On the contrary,an increase in flue gas temperature led to a decrease in particle removal efficiency,the efficiency declined from 92.8%to 81.9%as the flue gas temperature increased from 288 K to 328K.This was because the increase in flue gas temperature led to a significant reduction in water vapor saturation and condensable water vapor,making it difficult for particles to nucleate and grow effectively.A vortex-broken wing was added into the CAP cyclone in order to diminish the nonuniform distribution of water vapor caused by the complex flow field structure of the cyclone.The results were compared to that in the original cyclone by both experiments and numerical simulations.Through experiments,the total removal efficiency of the CAP technology can be further increased from 90.24%to 93.23%with the addition of the vortex-broken wing.This was also testified by the online laser particle size analyzer in experiments.The median particle size at the spigot of CAP cyclone increased from 12.95μm to 15.35μm.Furthermore,the mechanism for such improvement was revealed through the computational fluid dynamics simulations.It was observed that the inner vortex can be effectively destroyed by the vortex-broken wing,which promoted the diffusion of water vapor and made the vapor distributes more uniformly.As demonstrated in the simulation,the improvement of the vapor distribution can enlarge the supersaturated region and enhance the condensational growth of fine particles.Overall,the mechanisms of particle nucleation and growth processes related to particle hydrophilicity and temperature were studied,and the enhancements by the addition of surfactants and vortex-broken wing were explored,which can provide theoretical guidance for the further development of CAP technology.