Biomedical Engineering Reference
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for synthesis reactions but, unless they can be efficiently recovered, are
expensive and hazardous to dispose of.
Owing to its unique properties, SCW can act as a solvent for some reac-
tions. Based on their studies of the following reactions Krammer et al.
(1999) noted that many hydration, dehydration, as well as hydrolysis reac-
tions can take place in SCW with good selectivity and high space/time yield,
with no acids or bases as support materials.
Dehydration of 1,4-butandiol and glycerine
Hydrolysis of ether acetate, acetonitrile, and acetamide
Reaction of acetone cayanohydrine
Production of useful chemicals from biomass is another use for SCW
gasification. During its degradation in SCW, biomass produces phenols.
Phenol production increases with feed concentration (Kruse et al., 2003).
Because phenol is an important feedstock for the green resin, wood compos-
ite, and laminate industries, SCW provides an effective medium for green
chemistry.
9.6 REACTION KINETICS
Limited information is available on the global kinetics of SCW gasification.
Lee et al. (2002) studied the kinetics of glucose (used as the model biomass)
in SCWG with a plug-flow reactor.
C
6
H
12
O
6
1
6H
2
O
6CO
2
1
12H
2
(9.5)
5
We define the reaction rate, r, as the depletion of the biomass carbon
fraction, C, with time. Assuming pseudo-first-order kinetics, we can write:
dC
d
r
52
τ
5
k
g
C
(9.6)
where k
g
is the reaction rate constant.
The fraction of carbon converted into gas, X
c
, may be related to the cur-
rent carbon fraction, C, and the initial carbon fraction, C
0
, in the fuel:
C
C
0
X
c
5
1
2
(9.7)
Now replacing the carbon fraction in
Eq. (9.6)
and integrating, we get:
ln
ð
1
2
X
c
Þ
τ
k
g
52
(9.8)
Table 9.4
presents some data on the global kinetics for SCWG of model
compounds. The rates measured by Mettanant et al. (2009b), Lee et al.
(2002), and Kabyemela et al. (1997) show how the reaction rate decreases
with increasing solid carbon in the feed.
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