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
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