Page 107 - Design of Simple and Robust Process Plants

92 Chapter 4 Process Synthesis and Design Optimization
. Purity is another main point for discussion. The raw materials may come
from different suppliers, or from ªownº sources as existing processes at loca-
tion. The overall input and output balance of the process as proposed by Dou-
glas (1988) is a key factor at this stage. It is vital to understand the effect of an
impurity on the reaction(s), as well as its potential for accumulation in the
process. There may be significant advantages in using less pure streams as
reactant or as product. An example is the use of lower-grade ethylene and
propylene for alkylation reactions. Lower purity options are particularly
important for components and its impurities that are difficult (high cost) to
separate (see Section 4.2.3.2 Process integration).

It is not always practical to evaluate these materials at the beginning of the process
synthesis step, but evaluation and selection should be ensured during the concep-
tual design.

4.2.1.2 Evaluate reactor type, excess of reactants, and options to shift the equilibrium,
including simplification
Reactor system synthesis 1 inputs include:
. Chemistry
. Catalyst system
. Phase(s) of the reaction
. Reactor models

The generic factors influencing a reaction are:
. Temperature on kinetics and equilibrium.
. Pressure on equilibrium.
. Diluent on equilibrium, by lowering partial pressure and as heat carrier.
. Reactant ratio on kinetics, equilibrium, as heat carrier.
. Catalyst concentration on kinetics.
. Time on conversion.
The objective is to compare different reactor configuration as CSTR(s) and PFR or
combinations based on the variable operational margin to enable discrimination
between different configurations. The variable operational margin is the difference
between the revenues minus the costs of the raw material and the energy. In the
case of a small difference occurring between alternatives, it is possible to proceed
with two or three configurations for reactor systems.
The optimization requirements should be determined as:
. Economic prices of inlet and outlet (process and utility) streams (see Section
4.1.4).
. Constraints
. Preliminary energy exchange system in reactor section.
. Process conditions to be fixed. Depending on the effect on the reactor config-
uration selection, those conditions are fixed that have a low impact, for exam-
ple reactor pressure for liquid or pressure-neutral reactions, or systems