Software flowsheeting simulators

The calculation in this example can be conveniently carried out in spreadsheet software. However, many implementations are available in commercial flowsheet simulation software. 

Throughout this book, we have seen that when more than one species is involved in a process or when energy balances are required, several balance equations must be derived and solved simultaneously. For steady-state systems the equations are algebraic, but when the systems are transient, simultaneous differential equations must be solved. For the simplest systems, analytical solutions may be obtained by hand, but more commonly numerical solutions are required. Software packages that solve general systems of ordinary differential equations— such as Mathematica , Maple , Matlab , TK-Solver , Polymath , and EZ-Solve —are readily obtained for most computers. Other software packages have been designed specifically to simulate transient chemical processes. Some of these dynamic process simulators run in conjunction with the steady-state flowsheet simulators mentioned in Chapter 10 (e.g.. SPEEDUP, which runs with Aspen Plus, and a dynamic component of HYSYS ) and so have access to physical property databases and thermodynamic correlations. 

The fouling factor has to be determined from actual heat exchanger performance based on online measurements taken from a process unit test run. Heat exchanger clean performance is obtained from process flowsheet simulation software (e.g., Hysys by Aspen Tech or Unisim by Honeywell), while dirty performance from exchanger rating software (e.g., HTRI by Heat Transfer Research Institute). 

First, heat exchanger heat balance calculations are conducted in a flowsheet simulation software, which has adequate thermal data and can describe process streams according to their physical properties and operating conditions. By providing measured temperatures, the simulation can determine the heat transfer duty from Q = m Cp AT. At the same time, the simulation calculates transfer capability by lumping overall heat transfer coefficient and surface area together as U - A = 2/ATlm> where ATlm is defined in equation (6.7) in Chapter 6. 

Second, heat exchanger performance calculations are performed in a rating software. The thermal and physical property data for process streams are transferred from the flowsheet simulation to rating software. The dimensions and geometry of the heat exchanger are entered based on the manufacturing datasheet. 

Figure 5.1 Simplified flowsheet showing the role of computer software and simulations in the selection, sizing and optimisation of solid/liquid separation equipment.

Incorporation of Rigorous Models into Flowsheet Simulators and Putting Mathematical Models into User-Friendly Software Packages... 

INCORPORATION OF RIGOROUS MODELS INTO FLOWSHEET SIMULATORS AND PUTTING MATHEMATICAL MODELS INTO USER-FRIENDLY SOFTWARE PACKAGES... 

The capacity-based approach is a practical and effective method for incorporating dynamic controllability into the design of a chemical plant. All the tools needed for the job are available in the commercial flowsheeting simulation software. 

Commercial flowsheet simulation software is now sufficiently user-friendly that undergraduates can produce steady-state and dynamic simulations of fairly complex processes. Figure 17 shows a typical flowsheet with all the controllers installed that our senior would develop and study. Computer speed has increased to the point that dynamic simulations of these complex flowsheets can be run in reasonable times. 

Optimize each application of PUREX and the overall process to attain sufficiently improved performance by refinement of flowsheet conditions using reliable and accurate software (i.e., database and simulation code) and by sophistication of process-control methods. 

Since the advent of efficient and robust simulation and optimization solution engines" and flowsheeting software packages that allow for relatively easy configuration of complex models, numerous integrated, high fidelity, and multiscale process model applications have been deployed in industrial plants to monitor performance and to determine and capture improvements in operating profit. 

Flowsheets may be drawn by hand at preliminary stages of a project, but with process simulators and CAD software packages, it is a simple matter to develop flowsheets with a consistent set of... 

If a flowsheet is not converged, or if the process simulation software runs and gives a statement converged with errors, then the results cannot be used for design. The designer must take steps to improve the simulation so that a converged solution can be found. 

Using available flowsheeting software, it is possible to produce a computerized tool that will permit us to learn or even mirror the plant behaviour under different operating conditions or with different raw materials and product specifications. Such as tool is called the steady state plant simulation model. The steady state model, whieh is simpler to build, and has a wide variety of applications in its own right, it can be used directly in revamping and a wide variety of other engineering projeets. 

This problem has been formulated from AppHcation Briefs of Process, the user manual for the computer simulation software package of Simulation Sciences, Inc., and is conveniently solved using Process, but other flowsheeting packages can be used. 

In this paper, we present a detailed process analysis of the Cu-Cl cycle as a potential alternative of the S-I cycle. Thermodynamic feasibility of the reactions involved in tliis cycle has been evaluated by HSC Chemistry 5.11 (commercially available thermodynamic database software). Simulation flowsheet has been developed by using chemical analysis simulator ASPEN PLUS 12.1. 

In contrast to these generic tools, the second group of domain-specific software tools addresses certain tasks during the design process in the chemical engineering domain. They can roughly be classified as data retrieval, synthesis, and analysis tools. A typical example for data retrieval tools are physical property systems [1048, 1053[. Synthesis tools include flowsheet synthesis [951, 1039], plant layout and pipe routing [955], model development ]54, 558] or even solvent selection [7.37]. The most prominent examples of analysis tools are process simulators for steady-state ]518, 556, 1046] and dynamic simulation ]288, 518, 916]. 

Alternatively, closed-form model representations require a modular simulation approach, where each closed-form model is computed using the internal solver of the software tool the model is implemented in. The algorithm sets the model inputs, performs control over the simulation, and retrieves the outputs of each model through the commonly defined interface of the closed-form model representation, independently of the specific implementation. These outputs are propagated to the inputs of downstream units, and the simulation continues until all the units are computed. If the flowsheet contains recycles, an iterative strategy is performed until convergence of the flowsheet variables in tear streams is achieved. 

The two basic flowsheet software architectures are sequential modular and equation-based. In sequential modular, we write each unit model so that it calculates output(s), given feed(s), and unit parameters. This is the most commonly used flowsheeting architecture at present, and examples include Aspen+ plus Hysys (AspenTech), ChemCAD, and PROll (SimSci). In equation-based (or open-system) architectures, all equations are written describing material and energy balances as algebraic equations in the form/(x) = 0. This is the preferred architecture for new simulators and optimization, and examples include Speedup (AspenTech) and gPROMS (PSE pic). Each is discussed in turn. 

Note that in a larger extent process simulation should include other computer-based activities, as Molecular Simulation and Computer Fluid Dynamics (CFD). However, in this book we will limit the presentation to the capabilities offered only by the flowsheeting software, commonly called process simulators. 

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 ... 

It may be concluded that Sequential-Modular approach keeps a dominant position in steady state simulation. The Equation-Oriented approach has proved its potential in dynamic simulation, and real time optimisation. The solution for the future generations of flowsheeting software seems to be a fusion of these strategies. The release 11.1 of Aspen Plus (2002) incorporates for the first time EO features in the environment of a SM simulator. 

From the above description, we may conclude that flowsheeting software is a very sophisticated computer-based system, and not a collection of algorithms for solving different unit operations. A process simulator must be designed with computer science development and management tools. It is interesting to note that in the total cost the software maintenance (typically more than 70 %) is by far more important than the cost of programming (typically under 10 %). 

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