The effects of air velocity, temperature and particle size on low-temperature bed drying of wood chips

  • Niranjan Fernando
  • Mahinsasa Narayana
  • W. A. M. K. P. Wickramaarachchi
Review Article

Abstract

This paper describes a mathematical model for wood chip packed bed drying process with the effects of hot air flow velocity, temperature and particle size. A single-particle drying model was developed by considering impacts of external and internal parameters. External parameters are hot air flow velocity and air temperature. Internal parameters are porosity and particle size. These parameters are incorporated to the present model by introducing two mass transfer coefficients. The model was fine-tuned by comparing simulation results and experimental data. Effects of a factor relating to internal mass transfer coefficient were found for three wood types, and a functional dependence of internal mass transfer on temperature was suggested in this study. The model was implemented in computational fluid dynamics (CFD) to evaluate spatial variation of moisture in the packed bed drying process. The CFD model was validated by results of lab-scale packed bed. Drying performance of the packed bed was estimated by CFD simulations for variations of external hot air flow velocity, flow temperature and particle size. Sensitivity of these parameters for dying performance was evaluated by design of experiment (DOE) method. It was clarified that air temperature is most critical for the drying process. Interaction of between external hot air flow velocity and particle size for dying performance is also significant.

Keywords

Wood chip drying Drying kinetics Modelling optimisation Moisture content 

Nomenclature

A

The specific surface area (m−2)

Ap

Surface area of wood particle (m−2)

C

Sutherland’s constant

Di,g

Diffusivity coefficient of gas species i (m2 s−1)

h

Overall heat transfer coefficient based on wood particle surface area (W/m2K)

kg

Thermal conductivity of gas phase (W m−1 K−1)

ks

Thermal conductivity of biomass (W m−1 K−1)

L

Characteristic length of biomass particle (m)

M

Molar mass of air (kg mol−1)

Nu

Nusselt number

Pr

Prandlt number

R

Gas constant

Re

Reynolds number

Sh

Sherwood number

Sc

Schmidt number

p

Air pressure (Pa)

σ

Stephan Boltzmann constant

Emissivity of solid particles

w

Moisture evaporation flux (kg m−3 s−1)

Cg

Heat capacity of gas phase (J kg−1 K−1)

cs

Heat capacity of solid phase (J kg−1 K−1)

Pw

Saturation vapour pressure

T0

Reference temperature (K)

Tg

Gas phase temperature (K)

Ts

Solid phase temperature (K)

expi

Expected data from the theory

kg

Gas phase thermal conductivity (W m−1 K−1)

ks

Solid phase thermal conductivity (W m−1 K−1)

km,i

Internal mass transfer resistance (m s−1)

km,e

External mass transfer resistance (m s−1)

km

Mass transfer coefficient (m s−1)

md

Dry mass of biomass (kg)

obsi

Observed data

x

Gas phase moisture fraction within wood particle

xg

Gas phase moisture fraction

xFSP

Moisture fibre saturation point of wood particle

εg

Volume fraction of gas phase

εs

Volume fraction of solid phase

μg

Dynamic viscosity of gas phase (Pa s)

ρg

Density of gas phase (kg m−3)

ρj

Cell density of species j (kg m−3)

ρs

Density of solid phase (kg m−3)

σi , g

Average collision diameter (A)

Hv

Evaporation enthalpy of water

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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  1. 1.Department of Chemical and Process EngineeringUniversity of MoratuwaMoratuwaSri Lanka

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