Question 12.4: A CFB boiler is operating at a temperature of 850°C and pres......
A CFB boiler is operating at a temperature of 850°C and pressure of 1 bar. Solid particles have an average diameter d of 3 mm. The average bed porosity ε is 0.8, bed height H is 7 m, and particle density \rho_{p} is 2400 kg/m³. The fluidizing fluid is air. At 1 bar and 850°C, air properties are density ρ = 0.31 kg/m³ and kinematic viscosity \nu = 151 \times 10^{−6}\, m^{2}/s. The empirical parameter a for calculation of Reynolds number is 33.7.
Calculate (a) the pressure drop across the fluidized-bed and (b) the minimum fluidization velocity of the fluidizing fluid (air).
1. Pressure drop across the fluidized-bed
\Delta\rho=g\;{ H}\;r_{\mathrm{p}}\;(1-e)=9.81\times7\times2400\times(1-0.8)=32,962\;\mathrm{Pa}\;\approx\;0.33~\mathrm{bar}2. Archimedes number at 1 bar is given by
\mathrm{Ar}=\mathrm{g}d^{3}(\rho_{p}-\rho)/(\rho \nu^{2})={9}.81\,{\times}\,0.003^{3}\times(2400-0.31)/[0.31\times(151\times10^{-6})^{2}]=89,935
3. Reynolds number at minimum fluidization can be calculated using the fol-lowing empirical formula:
\mathrm{Re}_{\mathrm{mf}}=(a^{2}+0.0408\times A r)^{0.5}-a=(33.7^{2}+0.0408\times89,935)^{0.5}-33.7=35.64. Minimum fluidization velocity of the air
u_{\mathrm{mf}}=\mathrm{Re}_{\mathrm{mf}}\;{ \nu}/\mathrm{d},\;u_{\mathrm{mf}}=35.6\times{151}\times{10}^{-6}/0.003=1.79\mathrm{~m/s}