Fluidization of fine particles and its optimal operation condition in multimedia water filter

摘要

Approach: Backwashing process was implemented using column of deep bed filter with different media characteristics to provide optimal operation condition. Minimum velocity of fluidization (V_(mf)) and porosity at minimum fluidization (e_(mf)) were investigated under different operation condition and grains size. The results were compared with Wen and Yu equation and Richardson-Zaki correlation, respectively. Backwashing model was developed to predict the effect of fluid shear strength (τ_a), velocity grade (G), dissipation rate coefficient (C_a), and parameters of random motion (C_a~(0.5)/Re) on the energy dissipation rate during fluidization process. Results: The simulated result of proposed models gives a good convergence to observed data. It was seen that a smaller size of sand media need a lower value of backwash velocity than a higher size for lifting filter media. Fixed bed porosity increased from 0.510 to 0.704 at bigger size of sand media (1.18 mm), while it increased to 0.680 at smaller size of sand media (0.5 mm). The simulated result showed that Richardson-Zaki model has a good convergence with experimental result of expanded porosity for all effective size of sand media. Conclusion: It was concluded that low superficial velocity produced through the system at higher velocity grade (G). In addition to that the fluid shear strength (τ_a) increased along with expanded porosity. The maximum fluid shear strength (τ_a) was occurred at porosity equal to 0.730 and in large effective size of sand media (1.18 mm). Also, the lighter particles caused a high increasing rate in the random motion parameters compared with heavier particles. The study showed that the effluent suspended solid and hydrodynamics detachment of deposited fine particles reduces with time of fluidization but it rises with backwash discharge. The bigger grain of sand media has a lower peak point of optimum backwash time than smaller grain due to accumulation a large amount of deposit fine particles. The study was shown that the energy dissipation rate and the velocity grade have a dominant mechanism in filter backwashing and increases with increasing particle population in a expanded bed. The result revealed that the random motion parameter increased along with increasing in fraction solids and a higher rate was appeared in the lighter particles that it caused increasing in random motion parameter compared with heavier particles.