Page 96 - Design of Simple and Robust Process Plants
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4.1 Process Synthesis 81
. The economic evaluator must agree to the economic calculation set and the
price sets to be used for the evaluation (price sets are ultimately determined
by the business).
. The economics must be based on the net present value (NPV) of the project
over its lifetime. Ultimately, the project needs to meet the criteria for NPV/
investment. Any incremental investment to optimize the facility should meet
these criteria set by the business. Hence, trade-offs between incremental capi-
tal and savings need to meet those economic criteria.
. The price sets used for evaluation need to cover a range. Experience teaches
that price sets are subject to large variations over the lifetime of the process
plant.
. The optimization results are strongly affected by the variation of price sets
(one reason why operation optimization is applied).
Both design optimization and process synthesis are performed in process sections
in order to obtain a better insight of the different DOFs for the optimization (Koolen
et al., 1999). Thus, a process is split into sections and subsections. The methodology
to calculate the intermediate prices is described below. The process taken as an
example is the production of 1-octene from crude C4; this is described in EUR. Pat-
ent 0 461 222 B1 and EUR. Patent 0 561 779 B1.
The main reaction steps are:
Pd catalyst
2C 4 H 6 +CH 3 OH ! C 8 H 13 CH 3 O
(exothermic liquid phase, homogeneous system)
butadiene + methanol ® 1-methoxy 2.7.octadiene (MOD-1) (telomerization)
unwanted by-product isomer(MOD-3)
Pd catalyst
C 8 H 13 CH 3 O+2H 2 ! C 8 H 17 OCH 3 (exothermic gas/liquid phase)
methoxy-octadiene (MOD-1) + hydrogen ® methoxy-octane ether (MOE)
(hydrogenation)
Alumina catalyst
C 8 H 17 OCH 3 ! C 8 H 16 +CH 3 OH
(endothermic gas phase, fixed bed)
methoxy-octane ether (MOE) ®1-octene + methanol (ether cleavage).
The overall process consists of three reaction steps followed by separation sections
(see Figure 4.2). The bottom section of this figure illustrates the eventual split in
subsections.
For the optimization and synthesis of the sections, the direct fixed capital (DFC)
and NPV of the overall process are first calculated based on the raw material and
product prices as supplied by the business and the available unit ratios. Based on
the DFCs of the different sections, the NPV of each section is determined by split-
ting the overall NPV in ratio to the split in DFC.
The intermediate prices (prices between the sections) can now be back-calculated,
starting from the last section forward, to comply with the NPV of the section. In the
