Environmental Engineering Reference
In-Depth Information
CCD camera
Fiber flow
FIGure 15.2 (see color insert)
Schematic diagram showing the wet-imaging technique and typical
images acquired.
model for specific surface calculation is justifiable for particles with close to unity-aspect ratio, such
as wood sawdust, but is questionable for most forest biomass substrates consisting of fibers. Recently,
we developed a wet-imaging technique for the characterization of forest biomass substrate (Zhu et al.
2009). In this technique, the two dimensions of each substrate fiber are measured in a flowing water
channel by an optical microscopy using a charge-coupled device (CCD) camera (Figure 15.2).
The total surface and volume of the substrate can be calculated using a cylinder model for each
fiber. The volumetric specific surface can be determined by dividing the total surface of a sample
by its volume [eq. (15.1)] (Zhu et al. 2009). For most mechanically derived substrates, the cylinder
assumption is reasonable, as confirmed by scanning electron microscope pictures of the substrates
(Zhu et al. 2009). If the substrate particles are spherical like sawdust, then a spherical model can be
used to determine specific surface [eq. (15.2)].
∑
∑
∑
∑
2
nd dL
(
+⋅ ⋅
2
)
4
nd
(
Ld
+
/2
)
nL d
⋅
2
i
i
i
i
ii
i
i
ii i
A
V
4
f
S
==
i
=
i
≈⋅
4
i
=
(15.1)
f
V
∑
∑
D
L
nd L
2
⋅
nL d
⋅
2
nL d
⋅
2
f
21
ii i
ii i
ii i
i
i
i
∑
∑
nd
2
ii
A
V
6
p
S
==⋅
6
i
=
(15.2)
V
D
nd
3
p
32
ii
i
S
and
A
are the specific surface and total surface area of the substrate, respectively. Subscript
f
and
p
are fiber (cylinder model) and particle (sphere model), respectively.
D
L
21
is fiber-length weighted-
surface-length mean fiber diameter or “width.”
D
32
is often called Sauter mean diameter (SMD).
With this wet-imaging technique, we were able to objectively compare the efficiencies of different
size-reduction and chemical pretreatment processes in terms of size-reduction energy consumption
and cellulose to glucose conversion.
The specific surface can be used to measure the degree of size-reduction. Zhu et al. (2009) used
specific surface to successfully measure the efficiency of various size-reduction processes.
15.6.1.2 Fiberization mechanism
Energy consumption in mechanical pulping of wood depends significantly on how wood chips are
fiberized. Refiner mechanical pulps (RMP) are produced under atmospheric refining conditions,
