Environmental Engineering Reference
In-Depth Information
These efficiency factors will play a role in the evaluation of a design.
e.
Evaluation
In general, multiple evaluation criteria play a role (see Section 7.4.4), of which
economic performance is often dominant. One can calculate the economic
potential, according to Equation (7.3), based on the prices of the feed, product,
and waste streams:
"
#
n
o
n
o
n
o
EP
1
=
X
L
prod
L
feed
X
L
waste
X
p
proð Þ
l
ð Þ
j
prod
p
feeð Þ
l
ð Þ
j
feed
c
wastð Þ
l
ð Þ
j
waste
φ
−
φ
−
φ
l
=1
l
=1
l
=1
Criterion
:
"
#
n
o
EP
1
≥
ε
profit
X
L
prod
p
proð Þ
l
ð Þ
j
prod
φ
:
:
ð
Eq
7
3
Þ
l
=1
EP
1
economic potential at level 1 earning potential without capital and
ð
:
overhead costs
Þ
p
l
:
price per unit amount of stream
l
,with
l
=1,
…
,
L
c
l
:
cost per unit amount of waste stream
l
ð
feed
=
prod
Þ
s
−1
or in kg
s
−1
,whatever ismost
φ
flowof stream
l
in kmol
:
l
convenient
Þ
ε
profit
profitabilitymargin typical range
ð
0
:
50
−
0
:
75
Þ
:
:
This potential can be assessed against a profitability criterion. At this input
output
level of design, the setting of the profitability criterion should be high enough to
leave room for the costs of utilities and the capital charge due to process invest-
ments costs and for costs of overhead in operations, to be determined in later
design levels. Another criterion can be atom efficiency:
-
ε
element
≥
ε
element,min
.
For instance, carbon atom efficiency is an increasingly important performance
indicator, defined here as the carbon atoms in the feed that end up in products.
The remainder is often lost as CO
2
in a flue gas stream.
There is a degree of freedom at this design level to minimize the losses
of atom efficiency or energy efficiency by adjusting the ratios of the product
streams
ð Þ
l
prod
φ
within practically feasible limits.
f.
Selecting and (g) reporting
Often, alternative, profitable options can be identified for setting up the process,
e.g., by using different feeds or another mix of products or applying a different
technology. In the end, a choice is necessary on which few options to pass on to
the next level of design. Continuing with all options will soon result in a pro-
hibitive design effort, because at every level the number of design alternatives is
multiplied with the number of new options. So, pruning of alternatives is nec-
essary. It is advisable, though, to report the essence of the disregarded ones, as in
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