Page 417 - Analysis, Synthesis and Design of Chemical Processes, Third Edition
P. 417
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.