Flowsheeting programs, e. g. Aspen Plus, HYSYS and ChemCad, may be used to perform rigorous material and energy balance calculations, with the use of detailed equipment models, to determine the flow rates, composition and energy flow for all streams in the process. Because of their flexibility, the programs have many advantages when comparing different process configu­rations or scenarios in terms of overall efficiency, minimum energy demand or lower production cost. Also, they serve as a powerful tool when performing sensitivity analyses, due to the ease of changing a certain parameter. All flow­sheeting programs are based on a modular approach where each module is a mathematical model of a unit operation. The fundamental equations needed to accurately describe standard process equipment, such as columns (distil­lation, absorption, etc.), heat exchangers, pumps, reactors and splitters, are normally available as part of the program. The actual simulation is performed by arranging different unit operation modules into a complete flowsheet that represents the process to be simulated.

Construction of a process model in a flowsheeting program can be sum­marized in the following three steps.

• Flowsheet definition: The flowsheet defines the process configuration. It shows all streams entering the system as well as all unit operations and their interconnecting streams. The flowsheet also indicates all product streams that will be determined by the simulation program.

• Chemical components: The user must specify all the chemical compo­nents to be used in the system. All necessary physical and thermodynamic properties must be defined for each component. Normally, a database con­taining these properties for a large number of chemical compounds is included in the flowsheeting software. In general, the size of this database, which varies greatly between different simulators, determines the cost of the flowsheeting program. If data for some compounds are missing the user has to define them.

• Operating conditions: For every unit operation the user has to specify the operating conditions, such as temperature, pressure, heat duties, etc. In addition, all input streams have to be completely defined. Enough infor­mation has to be provided to result in a single steady-state solution based on material and energy balances coupled with phase equilibrium equa­tions.

2.1