4 FLOWSHEETING
4.3. PROCESS SIMULATION PROGRAMS
The most commonly used commercial process simulation programs are listed in Table 4.1. Most of these programs can be licensed by universities for educational purposes at nominal cost.
Note:Contact the website to check the full features of the most recent versions of the programs.
Detailed discussion of the features of each of these programs is beyond the scope of this book. For a general review of the requirements, methodology, and application of process simulation programs, refer to the books by Husain (1986), Wells and Rose (1986), Leesley (1982), Benedek (1980), and Westerberg et al. (1979). The features of the individual programs are described in their user manuals and online help. Two of these simulators have been used to generate the examples in this chapter: Aspen Plus1(v.11.1) and UniSim DesignTM(R360.1).
Process simulation programs can be divided into two basic types:
Sequential-modularprograms: in which the equations describing each process unit operation (module) are solved module-by-module in a stepwise manner. Iterative techniques are then used to solve the problems arising from the recycle of information.
Simultaneous (also known as equation-oriented) programs: in which the entire process is described by a set of equations, and the equations are solved simul- taneously, not stepwise as in the sequential approach. Simultaneous programs
can simulate the unsteady-state operation of processes and equipment, and can give faster convergence when multiple recycles are present.
In the past, most simulation programs available to designers were of the sequential- modular type. They were simpler to develop than the equation-oriented programs and required only moderate computing power. The modules are processed sequentially, so essentially only the equations for a particular unit are in the computer memory at one time. Also, the process conditions, temperature, pressure, flow rate, etc., are fixed in time. With the sequential modular approach, computational difficulties can arise due to the iterative methods used to solve recycle problems and obtain convergence.
A major limitation of sequential modular simulators is the inability to simulate the dynamic, time-dependent behavior of a process.
Simultaneous, dynamic simulators require appreciably more computing power than steady-state simulators to solve the thousands of differential equations needed to describe a process, or even a single item of equipment. With the development of fast, powerful computers, this is no longer a restriction. By their nature, simultan- eous programs do not experience the problems of recycle convergence inherent in sequential simulators; however, as temperature, pressure, and flow rate are not fixed and the input of one unit is not determined by the calculated output from the Table 4.1. Simulation Packages
Name Type Source Internet Address http//www.—
Aspen Plus steady-state Aspen Technology Inc. Aspentech.com
Ten Canal Park Cambridge, MA 02141-2201, USA
CHEMCAD steady-state Chemstations Inc. Chemstations.net
2901 Wilcrest, Suite 305 Houston, TX 77042 USA
DESIGN II steady-state WinSim Inc. Winsim.com
P.O. Box 1885 Houston, TX 77251-1885, USA
HYSYS steady-state and dynamic Aspen Technology Inc. Aspentech.com Ten Canal Park
Cambridge, MA 02141-2201, USA
PRO/II and DYNSIM steady-state and dynamic SimSci-Esscor Simsci.com 5760 Fleet Street
Suite 100 Carlsbad, CA 92009, USA
UniSim Design steady-state and dynamic Honeywell Honeywell.com 300-250 York Street
London, Ontario N6A 6K2, Canada
previous unit in the sequence, simultaneous programs demand more computer time.
This has led to the development of hybrid programs in which the steady-state simulator is used to generate the initial conditions for the equation-oriented or dynamic simulation.
The principal advantage of simultaneous, dynamic simulators is their ability to model the unsteady-state conditions that occur at startup and during fault conditions.
Dynamic simulators are being increasingly used for safety studies and in the design of control systems, as discussed in Section 4.9.
The structure of a typical simulation program is shown in Figure 4.4.
The program consists of
1. A main executive program that controls and keeps track of the flowsheet calculations and the flow of information to and from the subroutines.
2. A library of equipment performance subroutines (modules) that simulate the equipment and enable the output streams to be calculated from information on the inlet streams.
3. A data bank of physical properties. To a large extent, the utility of a sophisti- cated flowsheeting program depends on the comprehensiveness of the physical property data bank. The collection of the physical property data required for the design of a particular process and its transformation into a form suitable for a particular flowsheeting program can be very time-consuming.
4. Subroutines for thermodynamics, such as the calculation of vapor-liquid equi- librium and stream enthalpies.
5. Subprograms and data banks for equipment sizing and costing. Process simula- tion programs enable the designer to consider alternative processing schemes,
Data output Equipment
sub-routines Library and specials Thermodynamic sub-routines Convergence promotion sub-routines Physical property data files Cost data files
Executive program (organization of the problem)
Data input
Figure 4.4. A typical simulation program.
and the cost routines allow quick economic comparisons to be made. Some programs include optimization routines. To make use of a costing routine, the program must be capable of producing at least approximate equipment designs.
In a sequential-modular program, the executive program sets up the flowsheet sequence, identifies the recycle loops, and controls the unit operation calculations, while interacting with the unit operations library, physical property data bank, and the other subroutines. The executive program also contains procedures for the optimum ordering of the calculations and routines to promote convergence.
In an equation-oriented simulator, the executive program sets up the flowsheet and the set of equations that describe the unit operations and then solves the equations using data from the unit operations library and the physical property data bank and calling on the file of thermodynamics subroutines.
All process simulators use graphical user interfaces to display the flowsheet and facilitate the input of information to the package. The entry of data is usually intuitive to anyone familiar with the MS WindowsTMoperating systems.