Biomass drying in a pulsed fluidized bed without inert bed particles

Published Date
Fuel
15 December 2016, Vol.186:270–284, doi:10.1016/j.fuel.2016.08.100
Full Length Article

• Author 

• Dening Jia ^a
• Xiaotao Bi ^a,,
• C. Jim Lim ^a
• Shahab Sokhansanj ^a,b
• Atsushi Tsutsumi ^c

• aDepartment of Chemical and Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, BC V6T 1Z3, Canada
• bEnvironmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
• cCollaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan

Received 17 June 2016. Revised 17 August 2016. Accepted 23 August 2016. Available online 29 August 2016. 

Highlights

• •
  Fluidization of biomass without bed material is achieved with pulsed gas flow.
• •
  Reducing opening time in a pulsation cycle benefits fluidization of wet particles.
• •
  The addition of fines improves heat and mass transfer.
• •
  Two-phase model can reasonably predict fluidized bed drying.
• •
  Mass transfer rate at different frequencies is reflected by effective diffusivity.

Abstract

Batch drying was performed in the pulsed fluidized bed with various species of biomass particles as an indicator of gas–solid contact efficiency and mass transfer rate under different operating conditions including pulsation duty cycle and particle size distribution. The fluidization of cohesive biomass particles benefited from the shorter opening time of pulsed gas flow and increased peak pressure drop. The presence of fines enhanced gas–solid contact of large and irregular biomass particles, as well as the mass transfer efficiency. A drying model based on two-phase theory was proposed, from which effective diffusivity was calculated for various gas flow rates, temperature and pulsation frequency. Intricate relationship was discovered between pulsation frequency and effective diffusivity, as mass transfer was deeply connected with the hydrodynamics. Effective diffusivity was also found to be proportional to gas flow rate and drying temperature. Operating near the natural frequency of the system also favored drying and mass transfer.

Keywords

Fluidized bed

Pulsation

Modeling

Drying

Mass transfer

Biomass

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Nomenclature

A

cross-sectional area of the fluidized bed column, m²

A′

interfacial area of particle per unit volume of dense phase, m²/m³

d_b

bubble diameter, m

d_b0

initial bubble diameter at multi-orifice distributor plate, m

d_bm

maximum attainable bubble diameter, m

d_p

particle diameter, m

D_eff

effective diffusivity, m²/s

D₀

pre-exponential factor in Arrhenius equation, m²/s

D_t

hydraulic diameter of the column, m

D_v

molecular diffusivity of water vapor in air, m²/s

E_a

activation energy, kJ/mol

G_b

gas flow rate in bubble phase per unit volume of bed, m³/(m³ s)

G_d

gas flow rate in dense phase per unit volume of bed, m³/(m³ s)

f

pulsation frequency, Hz

K_c

mass transfer coefficient across bubble boundary, m/s

K_i

particle surface mass transfer coefficient, m/s

m_wet

water content in wet biomass samples, g

m_dry

water content in dried biomass samples, g

M

mass flow rate of drying air, g/s

N_or

number of orifices in distributor

p

water vapor pressure, Pa

p_s

saturated water vapor pressure, Pa

r

radial distance, m

R

universal gas constant, J/(kg K)

Re_mf

Reynolds number at minimum fluidization, ρ_gd_pU_mf/μ_g

R_p

radius of particle, m

Sc

Schmidt number, μ_g/ρ_gD_v

t

time, s

T

temperature, K

U

superficial gas velocity, m/s

average gas velocity, m/s

U_b

bubble rise velocity, m/s

U_mf

minimum fluidization velocity, m/s

w

weight of the sample, g

W

drying rate, g/s

X

moisture distribution within a biomass particle, dry basis

X_e

equilibrium moisture content of biomass samples, dry basis

X_Exp

experimentally obtained moisture content of biomass samples, dry basis

X_o

initial moisture content of biomass samples, dry basis

average moisture content of the biomass particles at a given time, dry basis

Y_b

absolute humidity in the bubble phase, kg-water/kg-air

Y_d

absolute humidity in the dense phase interstitial gas, kg-water/kg-air

Y_i

absolute humidity of the inlet gas, kg-water/kg-air

Y_o

absolute humidity of the exit gas, kg-water/kg-air

Y_p

absolute humidity at particle surface, kg-water/kg-air

z

height above gas distributor, m

Greek letters

ε_b

bubble volume fraction

ε_mf

bed voidage at minimum fluidization

μ_g

gas viscosity, kg/(m s)

η

amount of water removed

ν

simplified term, K⁻¹

ρ_p

particle density, kg/m³

ρ_g

air density, kg/m³

τ

period of the pulsation, ms

φ

mole fraction of non-diffusing component

Φ

moisture ratio

χ²

reduced Chi square

ω

mass rate of evaporation of water per unit volume of bed, kg/(m³ s)

Fig. 1.

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Table 1.

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Table 2.

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Table 3.

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Table 4.

Fig. 6.

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Table 5.

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 Table 6

Table 6.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 11.

 Table 7

Table 7.

Fig. 12.

Fig. 13.

Fig. 14.

• ⁎ 
  Corresponding author.
  
  

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