Find the information such as human life, natural resource,agriculture,forestry, biotechnology, biodiversity, wood and non-wood materials.
Blog List
Thursday, 29 December 2016
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
Nomenclature
A
cross-sectional area of the fluidized bed column, m2
A′
interfacial area of particle per unit volume of dense phase, m2/m3
db
bubble diameter, m
db0
initial bubble diameter at multi-orifice distributor plate, m
dbm
maximum attainable bubble diameter, m
dp
particle diameter, m
Deff
effective diffusivity, m2/s
D0
pre-exponential factor in Arrhenius equation, m2/s
Dt
hydraulic diameter of the column, m
Dv
molecular diffusivity of water vapor in air, m2/s
Ea
activation energy, kJ/mol
Gb
gas flow rate in bubble phase per unit volume of bed, m3/(m3 s)
Gd
gas flow rate in dense phase per unit volume of bed, m3/(m3 s)
f
pulsation frequency, Hz
Kc
mass transfer coefficient across bubble boundary, m/s
Ki
particle surface mass transfer coefficient, m/s
mwet
water content in wet biomass samples, g
mdry
water content in dried biomass samples, g
M
mass flow rate of drying air, g/s
Nor
number of orifices in distributor
p
water vapor pressure, Pa
ps
saturated water vapor pressure, Pa
r
radial distance, m
R
universal gas constant, J/(kg K)
Remf
Reynolds number at minimum fluidization, ρgdpUmf/μg
Rp
radius of particle, m
Sc
Schmidt number, μg/ρgDv
t
time, s
T
temperature, K
U
superficial gas velocity, m/s
average gas velocity, m/s
Ub
bubble rise velocity, m/s
Umf
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
Xe
equilibrium moisture content of biomass samples, dry basis
XExp
experimentally obtained moisture content of biomass samples, dry basis
Xo
initial moisture content of biomass samples, dry basis
average moisture content of the biomass particles at a given time, dry basis
Yb
absolute humidity in the bubble phase, kg-water/kg-air
Yd
absolute humidity in the dense phase interstitial gas, kg-water/kg-air
Yi
absolute humidity of the inlet gas, kg-water/kg-air
Yo
absolute humidity of the exit gas, kg-water/kg-air
Yp
absolute humidity at particle surface, kg-water/kg-air
z
height above gas distributor, m
Greek letters
εb
bubble volume fraction
εmf
bed voidage at minimum fluidization
μg
gas viscosity, kg/(m s)
η
amount of water removed
ν
simplified term, K−1
ρp
particle density, kg/m3
ρg
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)
No comments:
Post a Comment