On developing improved modelling and scale-up procedures for pneumatic conveying of fine powders

Abstract

Pneumatic transport of fine powders in fluidized dense-phase mode is becoming increasingly popular in various industries, such as power, chemical, cement, refinery, alumina, pharmaceutical, limestone, to list a few, because of the reasons of reduced gas flow rate and power consumption, decreased conveying velocities, improved product quality control, reduced pipeline sizing and wear rate, increased workplace safety etc. For the reliable design of a pneumatic conveying system, it is important to accurately predict the total pipeline pressure drop. However, accurate prediction of pressure drop from an improved understanding of the fundamental transport mechanism of fluidized dense-phase flow condition has only made limited progress till now because of the highly concentrated and turbulent nature of the gas-solids mixture. Power plant fly ash (median particle diameter: 30 μm; particle density: 2300 kg/m 3 ; loose-poured bulk density: 700 kg/m 3 ) was conveyed through different pipelines (69 mm I.D. x 168 m long; 105 mm I.D. x 168 m long; 69 mm I.D. x 554 m long). 8 different fly ash samples were tested in a fluidizing column for their deaeration characteristics and fluidized bulk densities *Revised Manuscript Click here to view linked References 2 were determined. Governing equations of flow for the dense-phase pneumatic conveying system of fine powders were solved using Runge-Kutta-Fehlberg (RKF45) method for different fluidized bulk densities of fly ash and air flow rates. The results have shown that the particle and actual gas velocities and the ratio of the two velocities increase in the direction of flow, while a reverse trend was apparent for the solids volumetric concentration. The results were compared against the predictions obtained using existing empirical relations for particle velocity. To develop an improved model for solids friction factor, an existing reliable pure dilute-phase model has been modified for dense-phase flow condition by incorporating sub-models for particle to actual gas velocity and impact and solids friction factor. The developed solids friction factor model was validated by using it to predict the total pipeline pressure drops for larger and longer pipelines and by comparing the experimental and predicted pneumatic conveying characteristics.The results have shown improved reliable predictions and that the model is capable of addressing the gradual transition of flow mechanism from dense-to dilute-phase. The accuracy of prediction is similar (in fact better in certain scale-up cases) when compared to a recently developed twolayer based model (developed by some of the authors). The results demonstrate the importance of incorporating particle and actual gas velocity terms in the model of solids friction factor instead of the prevailing techniques that overly depend on using superficial gas velocities.

Stability and phase space analysis of fluidized-dense phase pneumatic transport system

“…along the length of flow). Models developed for solids friction factor [2] using different dimensionless number using P11-P12 tapping point data are given as: λ s = 1.998(m*) -0.36 (Fr) -1.657 (27) λ s = 1.85(m*) -0.353 (Fr) -1.66 (ρ) -0.071 (28) Comparing between equations (25) and (27) and between equations (26) and (28) clearly demonstrate that the models representing the flow condition are different along the flow direction.…”

“…Lu et al [30] provided different values of experimental particle velocities for different line pressures. Using the parametric relations obtained through the phasespace analysis provided above for particle velocity, a relation for particle velocity has been developed [25]. The predicted values of particle velocities have been compared with the experimental particle velocity values given by Lu et al [30] for different operating pressures.…”

“…, , , ⁄ and , are calculated at each small interval of the pipeline section. Fluidized bulk density for this fly ash was experimentally determined as 300 kg/m 3[25].…”

“…The phase portraits obtained converge in every case under study, thus demonstrating that the system is operating under the stable conditions.However, on the contrary, it is indicating that the dunes formed at the initial point of the pipe (blow tank discharge) are deforming gradually as the flow moves towards the exit point of the pipeline. As the gas-solids flow develops along the pipeline, the solids and actual gas velocity increases and the pressure drops down with the reduction in volume fraction of solids[25]. The flow changes from fluidized-dense to dilute-phase.…”

“…, two sets of such models using different dimensionless parameters are provided below. Models developed for solids friction factor [2] using different dimensionless number based on P9-P10 tapping point data are given as: λ s = 20.807(m*) -0.71 (Fr) -2.1131(25) λ s = 14.5(m*) -0.63 (Fr) -2.01 (ρ) -0.207(26) …”