Page 416 - Analysis, Synthesis and Design of Chemical Processes, Third Edition
P. 416

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
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