Page 106 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 91
4.2.1
Reaction

4.2.1.1 Evaluate raw materials, reactants and products and the chemical reaction
The first decision to be taken is that of which feed streams are to be selected for the
main reaction, in addition to other purposes such as solvent, anti-oxidant, inhibitor,
neutralizing agent, and the physical state of the materials. All of these chemicals
must to be evaluated on:

. Purpose
. Safety ± internal as well as external
. Environmental
. Transportation mode, storage, and its physical state
. Integration in relation to other processes
. Purity and concentration
These steps are often under estimated, although a large proportion of the costs are
logistic cost which are determined ± but not yet quantified ± at this stage. Some of
these materials are not selected at this point, though it is possible to discuss them
here as they all have to pass the same rationale.
. The purpose often leaves us alternatives: for example, if we want to neutralize
with a base, there are often more alternatives such as caustic solution or pel-
lets, lime, NH 3, a weak inorganic or an organic base. Similar choices are to
be made for acid neutralization. In most applications alternatives are avail-
able, including the main reactants. List all these alternatives from economic,
safety and environmental perspectives ± and do not forget to list the form/
concentration of supply.
. Apply the inherently safer and environmentally sound design philosophies at
this stage to their full extent. It is at this point when there is the major impact
on the safety and environmental aspects of an operation. A striking example
of this is the selection of a chemical: in an operational facility, seawater was
used for direct cooling of process streams, but the seawater needed to be trea-
ted to overcome the growth of mussels within the exchangers. Although the
solution was to add chlorine, it was unnecessary to highlight the inherent
danger of having chlorine on site; rather, a solution of sodium hypochlorite
was found to be a suitable replacement.
. Transportation and storage each have major impacts on the design. The exam-
ple of the batch plant in Figures 1. 4±1.7 in Chapter 1 clearly shows the
increase and decrease of storage facilities during the process' evolution. The
practice of transporting a dosing agent in dedicated containers in a liquid
form at a concentration level for easy handling is to be preferred. It should be
recognized that handling operations can lead to mishaps and are to be mini-
mized.