Environmental Engineering Reference
In-Depth Information
Subprocess 3
NG
syn
gas
Fischer-Tropsch
conversion
(H
2
/CO = 2.1)
Recovery of
unconverted
syngas
Purification
of
products
Hydrocarbon
fuel
H
2
/CO = 3
Waste
C
Biomass
syngas
Heat
Power generation
system
Power
H
2
/CO = 0.7
FIGURE 7.11
Expansion of subprocess 3 in Figure 7.8 with sequential and parallel order
of processing functions.
Suitable specific technologies to perform the functions
Active agents to support processing functions, such as catalysts and solvents
Mode of operation (continuous, semibatch, etc.) and operating conditions
Control over levels for inert and trace components by venting and bleeding
Alternative arrangements in the ordering of units and their connections by
streams
In setting targets for the duties of the various functions, economic trade-offs
occur. For instance, a decision to have a low conversion of the feed per pass
through a reactor will lead to a smaller reactor and a lower investment cost.
However, this decision will often give rise to much higher costs in separation
for the recovery of the unconverted feed while attaining high product purity.
Increasing the conversion and product yield in the reactor will increase the reac-
tor cost but may greatly decrease separation costs and so lower the overall cost.
d.
Analysis
Having synthesized a functional block diagram, a model can be developed to
obtain the flow of resources and analyze how the flows change when design
variables are altered. The process model is made up of the models of the indi-
vidual blocks, connected by the streams as in the functional block diagram. The
models of the functional blocks are of the input
-
output type. One cannot go
beyond the input
output type of model because at this level of design, the inner
structure of the process functional units is not known yet. It is only the target for
a duty that is set per functional block, such as a desired degree of conversion of a
key reactant. Thus, for each block, one can easily set up the species balances and
determine the main heat effects associated with chemical reactions and phase
-
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