Page 128 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 113
not the most economical solution in all cases, although it is the author's opinion
that a design should be optimized from a low-cost alternative, and not from an
extended flowsheet.

4.2.2.5 Separation step 2
The input for the separation synthesis step 2 are the results of the overall synthesis
step 1 on reaction separation and integration. The objective is to develop one or two
competing flowsheets, these being developed on the basis of the pre-selected
sequences of separations. The simplification alternatives [as a result of brainstorm-
ing sessions (see Chapter 10) and individual clever thinking] need to be involved in
the optimization.
Optimization is based on the simulation of selected flowsheet alternatives. Only
very definite heat integrations are included, and other options are not exploited at
this stage. The energy prices used should be related to the site energy prices as
applicable for different energy levels. The flowsheet needs to be robust to enable
optimization, and to be divided into sections like reaction and separation to reduce
the size of the problem. In Sections 4.1.4 and 4.1.5, it has already been explained
how the intermediate prices must be determined and updated for each optimization
level. The optimizations are carried out at different levels. The flowsheet description
increases in detail (as explained previously) at each higher level of optimization, and
the number of alternatives and DOFs are adapted. At this stage (separation step 2)
the second layer of optimization is applied. The optimizations are made for each
process section based on NPV calculations, with a limited number of DOFs. To per-
form these optimizations the simulations need to be completed with a cost section
and an economic section. The cost functions need to be adapted to avoid any discon-
tinuities. The selected units are not yet heat integrated at this stage. Distillations are
described as short-cut models to eliminate any mixed integer problem, and to assure
a more robust simulation. The number of DOFs is reduced by setting those condi-
tions which at this stage have limited impact. So, the separation specifications and
process conditions as pressure of operation might be fixed arbitrarily.
The best two to three flowsheets with the highest NPV are selected for process
integration step 2.

Summary
Separations are, in most cases, an implicit part of the process. In some batch pro-
cesses the finishing of products (including the separation) is carried out in the reac-
tor vessel, but for a continuous process (as is being discussed here), a separation
train is designed.

. The process design of the separation train starts with an input and output
stream diagram of the overall process. In the stream diagram it is essential
next to the major components, to indicate specifically the impurity formation
and its removal. This is particularly important as recycle streams may accu-
mulate impurities in the process, and cause problems. Occasionally, specific
purge streams need to be envisaged for impurity removal.