Page 406 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Chapter 13 Synthesis of a Process Using a Simulator and Simulator

Troubleshooting

The advancement in computer-aided process simulation over the past generation has been nothing short of
spectacular. Until the late 1970s, it was rare for a graduating chemical engineer to have any experience in

using a chemical process simulator. Most material and energy balances were still done by hand by teams
of engineers. The rigorous simulation of multistaged separation equipment and complicated reactors was
generally unheard of, and the design of such equipment was achieved by a combination of simplified
analyses, shortcut methods, and years of experience. In the present day, however, companies now expect
their junior engineers to be conversant with a wide variety of computer programs, especially a process
simulator.

To some extent, the knowledge base required to simulate successfully a chemical process will depend on
the simulator used. Currently there are several process simulators on the market, for example,
CHEMCAD, ASPEN PLUS, HYSYS, PRO/II, and SuperPro Designer. Many of these companies
advertise their product in the trade magazines—for example, Chemical Engineering, Chemical
Engineering Progress, Hydrocarbon Processing, or The Chemical Engineer—and on the Internet. A
process simulator typically handles batch, semibatch, and continuous processes, although the extent of
integration of the batch and continuous processes in a single PFD varies between the various popular
simulators. The availability of such powerful software is a great asset to the experienced process
engineer, but such sophisticated tools can be potentially dangerous in the hands of the neophyte engineer.
The bottom line in doing any process simulation is that you, the engineer, are still responsible for
analyzing the results from the computer. The purpose of this chapter is not to act as a primer for one or all
of these products. Rather, the general approach to setting up processes is emphasized, and we aim to
highlight some of the more common problems that process simulator users encounter and to offer solutions

to these problems.

13.1 The Structure of a Process Simulator

The six main features of all process simulators are illustrated in the left-hand column of Figure 13.1.
These elements are:
1. Component Database: This contains the constants required to calculate the physical properties
from the thermodynamic models.
2. Thermodynamic Model Solver: A variety of options for vapor-liquid (VLE) and liquid-liquid
(LLE) equilibrium, enthalpy calculations, and other thermodynamic property estimations are
available.
3. Flowsheet Builder: This part of the simulator keeps track of the flow of streams and equipment
in the process being simulated. This information can be both input and displayed graphically.
4. Unit Operation Block Solver: Computational blocks or modules are available that allow energy
and material balances and some design calculations to be performed for a wide variety of
process equipment.

5. Data Output Generator: This part of the program serves to customize the results of the