Page 417 - Analysis, Synthesis and Design of Chemical Processes, Third Edition
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sophisticated one that allows the desired design calculations to be performed. It should also be noted that
                    a common error made in setting up a reactor module is the use of the wrong component as the limiting
                    reactant  when  a  desired  conversion  is  specified.  This  is  especially  true  when  several  simultaneous
                    reactions occur and the limiting component may not be obvious solely from the amounts of components in
                    the feed.


                    Flash Units.   In simulators, the term flash refers to the module that performs a single-stage vapor-liquid
                    equilibrium calculation. Material, energy, and phase equilibrium equations are solved for a variety of
                    input  parameter  specifications.  In  order  to  specify  completely  the  condition  of  the  two  output  streams
                    (liquid  and  vapor),  two  parameters  must  be  input.  Many  combinations  are  possible—for  example,
                    temperature and pressure, temperature and heat load, or pressure and mole ratio of vapor to liquid in exit
                    streams. Often, the flash module is a combination of two pieces of physical equipment, that is, a phase
                    separator and a heat exchanger. These should appear as separate equipment on the PFD. Note that a flash
                    unit can also be specified for batch operation, in which case the unit can serve as a surge or storage

                    vessel.

                    Distillation Columns.   Usually, both rigorous methods (plate-by-plate calculations) and shortcut methods

                    (Fenske and Underwood relationships using key components) are available. In preliminary simulations, it
                    is advisable to use shortcut methods. The advantage of the shortcut methods is that they allow a design
                    calculation (which estimates the number of theoretical plates required for the separation) to be performed.
                    For preliminary design calculations, this is a very useful option and can be used as a starting point for
                    using the more rigorous algorithms, which require that the number of theoretical plates be specified. It
                    should be noted that, in both methods, the calculations for the duties of the reboiler and condenser are
                    carried out in the column modules and are presented in the output for the column. Detailed design of these
                    heat exchangers (area calculations) often cannot be carried out during the column simulation.
                          Shortcut  Module:  The  required  input  for  the  design  mode  consists  of  identification  of  the  key
                          components to be separated, specification of the fractional recoveries of each key component in the
                          overhead product, the column pressure and pressure drop, and the ratio of actual to minimum reflux
                          ratio to be used in the column. The simulator will estimate the number of theoretical plates required,
                          the exit stream conditions (bottom and overhead products), optimum feed location, and the reboiler

                          and condenser duties.
                          If the shortcut method is used in the performance mode, the number of plates must also be specified,
                          but the R/R min  is calculated.
                          Rigorous Module: The number of theoretical plates must be specified, along with the condenser and
                          reboiler type, column pressure and pressure drop, feed tray locations, and side product locations (if
                          side stream products are desired). In addition, the total number of specifications given must be equal
                          to the number of products (top, bottom, and side streams) produced. These product specifications are
                          often a source of problems, and this is illustrated in Example 13.2.
                          Several rigorous modules may be available in a given simulator. Differences between the modules
                          are the different solution algorithms used and the size and complexity of the problems that can be
                          handled. Tray-to-tray calculations can be handled for several hundred stages in most simulators. In

                          addition,  these  modules  can  be  used  to  simulate  accurately  other  equilibrium  staged  devices,  for
                          example, absorbers and strippers.
                          Batch Distillation: This module is similar to the rigorous module, except that feeds and product
                          draws  are  on  a  schedule  (not  continuous).  Therefore,  the  start  and  stop  times  of  the  feeds  and
                          products must be specified, and a time series of tray concentrations and temperatures is generated by
                          the simulator.
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