Process simulation—steady state design specifications

For the specific jacket-cooled CSTR process considered in this section and the next, a simple heuristic approach can be used to incorporate quantitatively the limitations of controllability into the steady-state design. The idea is to specify a design criterion that ensures good controllability. In the reactor temperature control problem we use the criterion of a specified ratio of the maximum heat removal rate to the heat removal rate at design conditions. This simple approach is easily understood by designers and operators, and it requires no dynamic simulation or control analysis. We illustrate its usefulness in the following section to determine the besf reactor operating temperature. 

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. 

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]. 

The proposed control structures may be analysed by steady state simulation. In commercial simulators, unit operation input data or flow rate and composition of feed streams can be varied in order to meet some design specifications . Although the design specifications are apparently similar to SISO feedback controllers, it does not mean that the final control structure has to be decentralized or the I/O pairing must be preserved. This approach is advantageous because the nonlinear character of the process remains intact. 

As in the steady-state simulation of continuous processes, it is convenient to convert from a process flowsheet to a simulation flowsheet. To accomplish this, it is helpful to be familiar with the library of models (or procedures) and operations provided by the simulator. For example, when using SUPERPRO DESIGNER to simulate two fermentation reactors in series, the process flowsheet in Figure 4.25a is replaced by the simulation flowsheet in Figure 4.25b. In BATCH PLUS, however, this conversion is accomplished without drawing the simulation flowsheet, since the latter is generated automatically on the basis of the recipe specifications for each equipment item. 

The formulation of the NLP for application to large process design problems begins with the steady-state simulation of the process flowsheet for a nominal set of specifications or decision variables. As described in Section 4.2, during the creation of the simulation model (involving the material and energy balances, kinetic equations, etc., for the process units), a degrees-of-freedom analysis is performed. For the simulation model, the number of variables, normally exceeds the number of equations, N uaiions. with... 

The standard state-of-the-art steady-state process simulation is based on stream flows, which do not change with time. Therefore, flash calculations with other variables like constant specific volume or entropy are not provided. For the design of vessels and safety valves, the so-called isochoric flash is important. It uses the specific volume as the second fixed variable and determines the pressure which... 

The application of rigorous simulation packages, such as HYSYS, provides a valuable basis for the design and overall evaluation of advanced process control applications to the simulated processes. Steady state and dynamic simulations help in the process development by analysing and validating the design and/or ideas before their implementation to avoid costly modifications and to ensure safe operation. The simulation of the VCM plant is developed in HYSYS.PLANT in both steady state and dynamic modes and could be used for further economical, environmental and operational evaluations. Table 2 shows the characteristics of the VCM plant model and the detailed data and specifications of the main processes. Fig. 10 shows the process flowsheet of the simulated VCM plant in HYSYS including the main reactors and distillation columns. 

In this chapter, a suitable entrainer for this acetic acid dehydration system will be selected from several candidate acetates. Steady-state tray-by-tray column simulation will be used to determine the best entrainer with minimum total annual cost (TAC). Optimum process design and operating conditions will be determined to keep a high-purity bottom acetic acid composition and also to keep a small acetic acid loss in top aqueous draw. The overall control strategy of this column system will also be proposed to hold both bottom and top product specifications in spite of feed rate and feed composition disturbances. In the control study, conventional control strategy using only tray tempaature measurements will be considered so that the result of this study can easily be used directly in industry. 

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