PHYSICAL PRETREATMENT OF BIOMASS - Biomass as a Sustainable Energy Source for the Future - page 255

TABLE 8.3 Selection of dryer types
Feedstock
nature !

Liquids:
slurry

Liquids:
paste

Solids:
cakes

Solids:
powder

Solids:
granules

Solids:
pellets

Solids:
fibers τ (min)

Convection dryers

Belt 10-60 x x x

Flash 0-0.1 x x x x

Fluid bed 10-60 x x x x x x

Rotary 10-60 x x x x x

Spray 0.1-0.5 x x

Tray 10-60

60-360
(batch)

x x x x x

Conduction dryers

Drum 0.1-0.5

Steam-
jacketed
rotary

10-60 x x

Steam tube
rotary

10-60 x x x x x

Tray 60-360

(batch)

x x x x x

Source: Adapted from Mujumdar (2006).

the driving force for heat supply from the gas to the wet surface is enhanced. The
so-called wet-bulb temperature, T wb , can then be reached at equilibrium, realizing
that heat transfer to the vaporizing surface must equal mass transfer times heat of
vaporization. The rate of heat transfer is given by

Q = h AT

ð − T wb Þ ð Eq : 8 : 14 Þ

The vaporization rate follows from

φ m , w = k A MW w p sat

− p w

k A MW a
R u T

w sat sat

f ð p − p w Þ ω ð − ω Þ g ≈ k A ρ a ω ð − ω Þ

≈

R u T

: :

ð Eq 8 15 Þ

The associated heat flow then is

Q = k A

sat

ρ a ω ð − ω Þ h fg ð Eq : 8 : 16 Þ

Necessarily, the heat flows given by Equations (8.14) and (8.16) are equal at equilib-
rium; thus,