Flow diagrams simulation flowsheet

Fig. 6.5 Comparison between process flow diagram and simulation flowsheet for an HPLC plant.

Transformation of the process diagram into a corresponding simulation flowsheet is illustrated in the lower part of Fig. 6.5. In principle all plant elements may be represented by a separate model. For practical applications, though, it is sufficient to take into account only a time delay as well as the dispersion of the peak until it enters the column. This can be achieved by a pipe-flow model that includes axial dispersion. The detector (including some connecting pipes) can be represented by a stirred-tank model. 

In this way, the layout of a complete flowsheet can be done. It may be observed that the flowsheet for simulation is somewhat different from Fig. 2.4 A first conclusion might be drawn the technological flowsheet, commonly called Process Flow Diagram, must be translated in a diagram compatible with the capabilities of the simulator. This new flowsheet will be called here Process Simulation Diagram. 

A real Process Flow Diagram (PFD) must be translated in a scheme compatible with the software capabilities and with the simulation goals. The flowsheet scheme built up for simulation purposes will be called in this book Process Simulation Diagram (PSD). PSD is in general different from PFD. For example, some simple units, as for pressure or temperature change, may be lumped in more complex units (from simulation viewpoint). Contrary, complex units, as distillation columns or chemical reactors, may need to be simulated as small flowsheets. Hence, a preliminary problem analysis is necessary. The steps in defining a simulation problem are ... 

As mentioned throughout the discussion of preliminary process synthesis and the creation of the process flow diagram, the process simulator usually plays an important role, even if a simulation model is not prepared for the entire flowsheet. When parts of a simulation model exist, it is common for the design team to assemble a more comprehensive model, one that enables the team to examine the effect of parametric changes on the entire process. In other cases, when the process simulators have not been used for design, a simulation model is often created for comparison with the pilot-plant data and for parametric studies. 

Finally, the IPE Block Flow Diagram shows the simulation flowsheet. It is displayed using the View pulldown menu and clicking on Block Flow Diagram to give ... 

We use a simple process as a numerical example to illustrate moving from a steady-state simulation in Aspen Plus to a dynamic simulation in Aspen Dynamics. Figure 4.15 shows the flowsheet and the control stmcture. The flash drum is the same as the one sized in Section 4.1.2. It is a vertical vessel 2 ft in diameter and 4 ft in height. Figure 4.16 shows the Aspen Plus process flow diagram. 

The most reliable way to input the topology of the process flow diagram is to make a sketch on paper and have this in front of you when you construct the flowsheet on the simulator. Contrary to the rules given in Chapter 1 on the construction of PFDs, every time a stream splits or several streams combine, a simulator equipment module (splitter or mixer) must be included. These phantom units were introduced in Chapter 5 and are useful in tracing streams in a PFD as well as being required for the simulator. Most... 

Several options will be available to display the results of a simulation. Often, a report file can be generated and customized to include a wide variety of stream and equipment information. In addition, a simulation flowsheet (not a PFD), T-Q diagrams for heat exchangers, vapor and liquid flows, tenperature and composition profiles (tray-by-tray) for multistaged equipment, scheduling charts for batch operations, environmental parameters for exit streams, and a wide variety of phase diagrams for streams can be generated. The user manual should be consulted for the specific options available for the simulator you... 

Recycle streams cutting, 585 description, 142-146 input/output models, 585 in process simulation BFD (block flow diagram), 442 overview, 441-443 sanple flowsheet, 426-427 tear streams, 426 27 tearing, 585... 

Process simulation software typically will produce a flowsheet, but this is probably not a true process flow diagram and should not be used as one in a report. There are two reasons for this. First, the simulator-produced flowsheet is unlikely to follow all the conventions of PFDs given in Chapter 1 regarding equipment symbols, line crosses, labels, and so on. Second, the process simulated is not the true process. One unit in the process simulator (such as a distillation column) maybe several pieces of equipment (tower, condenser, condensate tank, etc.) that need to be shown on a PFD. It is common to simulate a sin e unit (such as a process-process heat exchanger) as two units to decouple the recycle calculations. Some units, such as a storage tank, require no calculations in the process simulator and thus are not shown on its flowsheet. However, the simulator flowsheet is essential in the appendix of the report if the simulator report is included. Care should be taken to use the same stream numbers in the simulation flowsheet as in the PFD whenever possible. 

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