Page 114 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 99
. Separation selection 1
. Process integration 1
The objective is selection of the final reactor configuration and preliminary reactor
size for the process; this is based on cost, DFC, and NPV results. In order for selec-
tion of the final configuration to be made, a careful evaluation of simplification alter-
natives must take place, incorporating:

. Updated economic prices for inlet and outlet stream after separation 1.
. Constraints.
. Updated energy exchange system after process integration 1.
. Cost estimation updated for local situation and provided with continuous
function in the working area to avoid discontinuities resulting in converging
problems.
. Fix reactor conditions which have a limited impact on the reactor system to
reduce the sizeof the optimization problem.
. The DOFs to be selected might be reactor volume(s) temperatures, reactants
ratios, diluent ratio, and catalyst concentration, or amount.
The results are the selected reactor system, including preliminary dimensions under
restricted optimized conditions, restricted by the DOFs selected for optimization.

Summary
The selection and design of reactor configuration is the most essential step of the
process synthesis, as this section largely determines the economics of the process
with respect to the selectivity, conversion, and capital cost. The factors that play a
major role in the design of the reactor section are:

. The safety aspects are largely dominated by the selection and inventory of the
chemicals involved.
. The simplicity of the design will have a major impact on the capital and ease
of operation.
. Simpler reactor designs evolve into the direction of adiabatic plug flow
designs with maximum benefit of reaction heat, (see Chapter 5).
. Large single reactor train systems are evolving which benefit from the eco-
nomics of scale, while the design is supported by detailed reactor, flow and
mechanical modeling.
. The combination of reaction and separation is driving down the cost of pro-
cess design, and leading to higher conversion systems.
. Synthesis stage 1 activities lead to the pre-selection of reactor configuration
based on operational cost.
. Synthesis stage 2 selects the final reactor configuration and preliminary size
based on the advantages of simpler designs and optimization of the system
on NPV.