Page 415 - Analysis, Synthesis and Design of Chemical Processes, Third Edition
P. 415
When attempting to do a simulation on a process for the first time, it is recommended that you provide the
minimum data required for a Level 1 simulation. When you have obtained a satisfactory, converged
solution, you can go back and provide more data to obtain desired design parameters, that is, a Level 2
solution.
When first simulating a process, input only the data required to perform the material and
energy balances for the process.
The structure of the process simulator will determine the exact requirements for the input data, and such
information will be available in the user manual for the software or on Help screens. However, for Level
1 simulations, a brief list of typical information is presented below that may help a novice user prepare
the input data for a process simulation.
Pumps, Compressors, and Power Recovery Turbines (Expanders). For pumps, the desired pressure
of the fluid leaving the pump or the desired pressure increase of the fluid as it flows through the pump is
all that is required.
For compressors and turbines, the desired pressure of the fluid leaving the device or the desired pressure
increase of the fluid as it flows through the equipment is required. In addition, the mode of compression
or expansion—adiabatic, isothermal, or polytropic—is required.
Heat Exchangers. For exchangers with a single process stream exchanging energy with a utility stream,
all that is required is the condition of the exit process stream. This can be the exit pressure and
temperature (single-phase exit condition) or the exit pressure and vapor fraction (two-phase exit
condition).
For exchangers with two or more process streams exchanging energy (as might be the case when heat
integration is being considered), the exit conditions (pressure and temperature or vapor fraction) for both
streams are required. The user must be aware of the possibility of temperature crosses in heat-exchange
equipment. The simulator may or may not warn the user that a temperature cross has occurred but will
continue to simulate the rest of the process. The results from such a simulation will not be valid, and the
temperature cross must be remedied before a correct solution can be obtained. Therefore, it is
recommended that the user check the temperature profiles for all heat exchangers after the simulation.
Fired Heaters (Furnaces). The same requirements for heat exchangers with a single process fluid apply
to fired heaters.
Mixers and Splitters. Mixers and splitters used in process simulators are usually no more than simple
tees in pipes. Unless special units must be provided—for example, when the fluids to be mixed are very
viscous and in-line mixers might be used—the capital investment of these units can be assumed to be zero.
Mixers represent points where two or more process streams come together. The only required information
is an outlet pressure or pressure drop at the mixing point. Usually, the pressure drop associated with the
mixing of streams is small, and the pressure drop can be assumed to be equal to zero with little error. If
feed streams enter the mixer at different pressures, then the outlet stream is assumed to be at the lowest