PROCESSES: BASICS OF PROCESS DESIGN - Biomass as a Sustainable Energy Source for the Future - page 219

TABLE 7.5 Stream table for FT process
Stream (flows:
kmols −1 )

Main product
(C 4+ lump) CO H 2 H 2 O 4 CO 2

1. Fresh feed 0.792 1.584 0.000 0.051 0.077 0.026

2. Reactor in 1.440 2.572 0.005 0.225 0.828 0.564

3. Reactor out 0.720 1.097 0.055 0.907 0.835 0.598

4. Recycle 0.648 0.988 0.005 0.204 0.752 0.539

5. Outlet 0.072 0.110 0.050 0.703 0.084 0.060

table (Table 7.5). The stream index corresponds with the numbering of the
streams in Figure 7.12.
The total flow into the conversion block (stream 2) is more than twice as
large as the fresh feed intake (stream 1). The molar product (C 4+ ) flow is very
small (0.05) relative to water (0.703). However, the C 4+ lump has a large aver-
age molecular weight, MW C 4+ = 184, compared to that of water (18). Thus, on a
mass basis, the outlet flows are comparable in size: 9.2 kg s −1 C 4+ and 11.2

s −1 H 2 O.
10. Closing Remarks on the Analysis Model
The molar balance model discussed is an input

kg

output one, primarily intended to
get an overview of the order of magnitude of the species flows in the streams and
the suitability of the target variables, such as conversion and separation factors.
Such a simplification comes at a price: an inherent weakness of this input

-

output
model is the lack of internal mechanisms. For example, the conversion and
extents of reaction have not yet been mechanistically obtained from the internal
rates and operating conditions in the reactor. These missing links can be intro-
duced at a deeper level of design. At this level, first principle models are used
to determine internal reactor geometry and conditions, such that the target con-
version and selectivity are closely met. Another weakness is the absence of
energy and momentum balances. However, the rate of change in enthalpy due
to all reactions, which is a major contribution to an enthalpy balance, can easily
be computed. One takes for each reaction the product of its extent and its corre-
sponding standard reaction enthalpy and sums up over all reactions:

-

h i

H 0 = X

n reactions

R con ð Þ
Δ

e con ð Þ
j

Δ r H j

ð Eq : 7 : 16 Þ

j =1

A simplified momentum balance would cover pressure changes over the proc-
ess units. Pumps and compressors are needed to overcome pressure drops in
process units. The amount of work done by compressors is often another dom-
inant term in the energy balance over the process.
11. Evaluation of Economic, Ecological, and Technological Performances
For the evaluation of economic performance, a simplified economic potential
function is formulated: EP 3 , the economic potential function at design level 3.