HYSYS.Plant dynamic simulation

Unsteady-state or dynamic simulation accounts for process transients, from an initial state to a final state. Dynamic models for complex chemical processes typically consist of large systems of ordinary differential equations and algebraic equations. Therefore, dynamic process simulation is computationally intensive. Dynamic simulators typically contain three units (i) thermodynamic and physical properties packages, (ii) unit operation models, (hi) numerical solvers. Dynamic simulation is used for batch process design and development, control strategy development, control system check-out, the optimization of plant operations, process reliability/availability/safety studies, process improvement, process start-up and shutdown. There are countless dynamic process simulators available on the market. One of them has the commercial name Hysis [2.3]. 

Hysys.Plant steady state and dynamic simulation to evaluate designs of existing plants, and analyse safety and control problems. 

The multimedia CD-ROM that accompanies this book also explains how to use the dynamic simulators. Emphasis is placed on HYSYS.Plant. Using HYSYS.Plant, the design... 

Aspen IPE is also used to calculate equipment sizes and estimate capital costs for the MCB separation process in Section 16.7. Then, a profitability analysis is performed in Section 17.8. In Section 21.5, process controllers are added and their responses to various disturbances are computed using HYSYS.Plant in dynamic mode. Hence, for the MCB separation process, the process simulators have been used throughout the design process, although most design teams use a variety of computational tools to carry out these calculations. 

Many packages are available for steady-state simulation, as discussed in Chapter 4. To manipulate the linearized models in the Laplace, frequency, and time domains, MATLAB and SIMULINK are used commonly, and example scripts are introduced in Section 21.6. The most recent commercial packages permit steady-state and dynamic simulations. These include HYSYS.Plant, CHEMCAD, and ASPEN DYNAMICS, with the former used in this section and in Section 21.5. 

For more details on the implementation and tuning of PI controllers using HYSYS.Plant, the reader is referred to the multimedia CD-ROM that accompanies this text (HYSYS - Dynamic Simulation Tuning PI Controllers). In the following case studies, C R analysis is demonstrated, with results verified using d)mamic simulations of the Pl-controlled processes. 

Given the design decision to use < > = 0.25, based on the steady-state C R analysis, verification is performed by dynamic simulations with HYSYS.Plant. The hot stream of n-octane at 2,350 Ibmol/hr is cooled from 500 to 300°F using n-decane as the coolant, with F2 = 3,070 Ibmol/hr and F3 = 1,200 Ibmol/hr. Note that these species and flow rates are chosen to match the heat-capacity flow rates defined by McAvoy (1983), with F, slightly increased to avoid temperature crossovers in the heat exchangers due to temperature variations in the heat capacities. Additional details of the HYSYS.Plant simulation are... 

To improve the control system in Figure 21.35, controllability and resiliency analysis has two roles. These involve the use of (1) the RGA to aid in selecting the appropriate pairing between the controlled outputs and manipulated variables when interactions are anticipated, and (2) the DC to assist in checking that the operating ranges of the manipulated variables are sufficient to ensure adequate disturbance rejection. To provide data for these two analytical methods, a dynamic simulation of the MCB separation process is developed using HYSYS.Plant. 

Several of the control loops in Figure 21.35 are provided for inventory control, in three level-control loops and two pressure-control loops. Note, however, that the pressure in V-100 is assumed to be constant and loop PC-1 is not simulated by HYSYS.Plant. In contrast, pressure control is crucial to maintain stable internal flows in the column. Finally, because the feed flow rate and temperature controllers are decoupled from the rest of the process, they are not included in the C R analysis. Consequently, the interactions to be analyzed involve the four valves V-7, V-9, V-10, and V-12, and four controlled variables Xdj, JC/u (mole fractions of benzene in the distillate, MCB in the bottoms, and HCl in the absorber overhead stream, respectively) and Tg, the recycle temperature. Note that to improve the dynamic performance, the temperature of tray 4 is controlled rather than the distillate benzene mole fraction. 

As shown in the case studies, it is recommended that dynamic simulation be employed to verify the results obtained by C R analysis. This simulation is routinely performed using HYSYS.Plant and ASPEN DYNAMICS, as demonstrated in this chapter. The reader is referred to the book Plantwide Dynamic Simulators in Chemical Processing and Control (Luy-ben, 2002) for many additional examples in which dynamic simulation assists plantwide controllability analysis. 

Process simulators, steady state, dynamic, and batch, are used throughout the textbook (ASPEN PLUS, HYSYS.Plant, CHEMCAD, PRO/II, BATCH PLUS, and SUPERPRO DESIGNER). This permits access to large physical property, equipment, and cost databases... 

Nowadays dynamic simulation is commonly used during process design, e.g. for compressor surge analysis. Simulation software tools like UniSim (Honeywell), DynSim (SimSci), Hysys, Aspen Dynamics (both AspenTech) and GPROMS (PSE Ltd) have become very powerful and are relatively easy to use. In this way the plant controllability and operability can be tested and its robustness against process upsets verified. 

In the proposed framework, HYSYS. PLANT simulation package is used to validate both the steady state and dynamic models even though the switchability fiom steady state to dynamic mode is not a trivial procedure, as it will be shown in the case study section. 

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. 

The integrated steady state model is switched to the dynamic simulation environment provided by HYSYS.PLANT, where it shares the same physical properties and flowsheet topology as the steady state model. Unlike the steady state model, the dynamic model uses a different set of conservation equations that account for changes occurring over time. Within the dynamic mode, an advanced method of calculating the pressure and flow profile of the simulated model is used as the user states the required number of pressure-flow (P-F) specifications. These equations are solved simultaneously to find the unknown pressure or flowrates. As the last stage in the steady state model before its transition to the dynamic... 

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