Biomedical Engineering Reference
In-Depth Information
uniform cross-section shaft of the unit (
Figure 4.11
). Hot gas, W
g
, enters the
bottom of the torrefaction section at temperature, T
gi
, and this along with
other gaseous products leaves the top of the drying section at a temperature
T
g0
. Fresh biomass, W
f
, enters the top of the drier at T
0
, and hot torrefied
biomass, W
t
, leaves the bottom of the torrefier at T
t
.
If the reactor volume is V
r
, the total particle surface area, S
r
, available for
heat transfer, and the reaction can be calculated for known values of the sur-
face to volume ratio S
p
of biomass particles and
ε
p
, the voidage in this
section.
S
r
5
ð
1
ε
p
Þ
V
r
S
p
(4.29)
The average value of surface to volume ratio of solid particles is the ratio
of external surface area and volume of the particles, S
p
5
(A
p
/V
p
), which is a
function of the particle shape.
Predrying Section
We assume that hot gas enters its bottom at temperature,
T
g
3
and leaves from the top at T
g0
. The biomass enters from the top at T
0
and leaves the bottom at 100
C. Gas to biomass particle heat transfer takes
place across the log mean temperature difference, LMTD, which is given as:
5
½ð
T
0
Þ
ln
½ðð
T
g
3
2
100
Þ=ð
T
g
0
2
T
0
ÞÞ
T
g
3
2
100
Þ
2
ð
T
g
0
2
LMTD
(4.30)
gas overall heat transfer coefficient is U
t
, the heat trans-
ferred to biomass, Q
ph
, in the predrying section is
If the particle
Q
ph
5
U
t
S
r
;
ph
LMTD
ph
(4.31)
where the subscript “ph” refers to the values in the preheating section.
The preheat section's volume, V
r,ph
, should be such that it accommodates
at least an amount of biomass that would give the required gas
particle sur-
face area of S
r,ph
. Using the relationship (
Eq. (4.29)
) between particle surface
area and volume occupied by it in this section, we can find the minimum
volume of this section of the reactor, V
r,ph
, as:
Q
ph
U
t
LMTD
ph
3
1
V
r
;
ph
5
(4.32)
S
p
ð
1
2
A
p
Þ
The minimum height of this section, L
ph
, with cross-sectional area, A
r
,is
given as:
V
r
;
ph
A
r
L
ph
5
(4.33)
Besides the volume, there are two other stipulations for such a moving
bed system.
Search WWH ::
Custom Search