Environmental Engineering Reference
In-Depth Information
not evident that at these lower temperatures also high concentrations of organic
matter can be treated. This subsection is focused mainly on gasification, without
or with catalysis, at about 600
C.
2.
High Pressures.
It is crucial for the process that the heat content of the reactor
effluent is utilized as far as possible to preheat the feedstock stream (mainly
water) to reaction conditions (see Figure 10.10). However, heat exchange
between these streams is not practical at low pressure, because of the high heat
of evaporation at nearly isothermal and isobaric conditions. Heating of the feed-
stock stream to the desired gasification temperatures in a heat exchanger with-
out evaporation requires operation at high pressures. This is the true incentive of
the high pressures involved in wet gasification. The efficiency of the heat
exchange in relation to the applied pressure can be calculated from the heat bal-
ance for a countercurrent shell and tube heat exchanger. The result is presented
in Figure 10.13, in which the heat exchanger efficiency is plotted as a function
of the operating pressure and the available area per unit throughput. In case of
an infinite surface area (no overall transport limitations), the efficiency is
given by
H
vap
H
hot
,
in
−
Δ
η
HE
=1
−
ð
Eq
:
10
:
20
Þ
H
cold
,
in
1.0
∞
100
0.9
50
0.8
25
0.7
0.6
A
HE
= 10
0.5
0.4
0.3
0.2
H
cold,out
-
H
cold,in
Supercritical
pressure
η
HE
=
0.1
H
hot,out
-
H
hot,in
0.0
0
5
10
15
20
25
30
35
Pressure (MPa)
FIGURE 10.13
Calculated efficiencies of a water
water countercurrent heat exchanger
plotted versus the operating pressure for different surface areas; AHE = area (m
2
) per unit
throughput (kgs
−1
). The flow rates (kg
-
s
−1
) on both sides were assumed to be equal. U =
1000 Wm
−2
K
−1
). Inlet conditions: T
hot,in
= 600
C, and T
cold,in
=25
C.
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