Page 416 - Analysis, Synthesis and Design of Chemical Processes, Third Edition
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pressure of the feed streams. This assumption causes little error in the material and energy balance.
However, the correct analysis of the pressure profiles in a system where several streams mix is given in
Section 19.4.
Splitters represent points at which a process stream splits into two or more streams with different
flowrates but identical compositions. The required information is the outlet pressure or pressure drop
across the device and the relative flows of the output streams. Usually, there is little pressure drop across
a splitter, and all streams leaving the unit are at the same pressure as the single feed stream. In a batch
operation, the splitter can be assigned on and off times to divert the inlet flow to various other units on a
schedule.
Valves. Either the outlet pressure or pressure drop is required.
Reactors. The way in which reactors are specified depends on a combination of the input information
required and the reactor category. Generally there are four categories of reactor: stoichiometric reactor,
kinetic (plug flow or CSTR) reactor, equilibrium reactor, and batch reactor. All these reactor
configurations require input concerning the thermal mode of operation: adiabatic, isothermal, amount of
heat removed or added. Additional information is also required. Each reactor type is considered
separately below.
Stoichiometric Reactor: This is the simplest reactor type that can be simulated. The required input
data are the number and stoichiometry of the reactions, the temperature and pressure, and the
conversion of the limiting reactant. Reactor configuration (plug flow, CSTR) is not required because
no estimate of reactor volume is made. Only basic material and energy balances are performed.
Kinetic (Plug Flow and CSTR) Reactor: This reactor type is used to simulate reactions for which
kinetics expressions are known. The number and stoichiometry of the reactions are required input
data. Kinetics constants (Arrhenius rate constants and Langmuir-Hinshelwood constants, if used) and
the form of the rate equation (simple first-order, second-order, Langmuir-Hinshelwood kinetics, etc.)
are also required. Reactor configuration (plug flow, CSTR) is required. Options may be available to
simulate cooling or heating of reactants in shell-and-tube reactor configurations in order to generate
temperature profiles in the reactor.
Equilibrium Reactor: As the name implies, this reactor type is used to simulate reactions that obtain
or approach equilibrium conversion. The number and stoichiometry of the reactions and the
fractional approach to equilibrium are the required input data. In addition, equilibrium constants as a
function of temperature may be required for each reaction or may be calculated directly from
information in the database. In this mode, the user has control over which reactions should be
considered in the analysis.
Minimum Gibbs Free Energy Reactor: This is another common form of the equilibrium reactor. In
the Gibbs reactor, the outlet stream composition is calculated by a free energy minimization
technique. Usually data are available from the simulator’s databank to do these calculations. The
only input data required are the list of components that one anticipates in the output from the reactor.
In this mode the equilibrium conversion that would occur for an infinite residence time is calculated.
Batch Reactor: This reactor type is similar to the kinetic reactor (and requires the same kinetics
input), except that it is batch. The volume of the reactor is specified. The feeds, outlets flows, and
reactor temperature (or heat duty) are scheduled (i.e., they are specified as time series).
As a general rule, one should initially use the least complicated reactor module that will allow the heat
and material balance to be established. The reactor module can always be substituted later with a more