Flowsheeting equation-oriented

Process flowsheeting tools can usually be classified within two categories. In the equation-oriented framework, the simulation system creates and... 

The older modular simulation mode, on the other hand, is more common in commerical applications. Here process equations are organized within their particular unit operation. Solution methods that apply to a particular unit operation solve the unit model and pass the resulting stream information to the next unit. Thus, the unit operation represents a procedure or module in the overall flowsheet calculation. These calculations continue from unit to unit, with recycle streams in the process updated and converged with new unit information. Consequently, the flow of information in the simulation systems is often analogous to the flow of material in the actual process. Unlike equation-oriented simulators, modular simulators solve smaller sets of equations, and the solution procedure can be tailored for the particular unit operation. However, because the equations are embedded within procedures, it becomes difficult to provide problem specifications where the information flow does not parallel that of the flowsheet. The earliest modular simulators (the sequential modular type) accommodated these specifications, as well as complex recycle loops, through inefficient iterative procedures. The more recent simultaneous modular simulators now have efficient convergence capabilities for handling multiple recycles and nonconventional problem specifications in a coordinated manner. 

Hutchison, H. P., Jackson, D. J., and Morton, W., The development of an equation-oriented flowsheet simulation and optimization package—II. Examples and results, Comp, and Chem Eng. 10(1), 31 (1986). 

Dynamic simulation, process control, real-time optimization Process synthesis, flowsheet convergence, simultaneous modular vs. equation-oriented... 

Process design for continuous processes is carried out mostly using steady-state simulators. In steady-state process simulation, individual process units or entire floivsheets are calculated, such that there are no time deviations of variables and parameters. Most of the steady-state floivsheet simulators use a sequential modular approach in which the flowsheet is broken into small units. Since each unit is solved separately, the flowsheet is worked through sequentially and iteration is continued until the entire flowsheet is converged. Another way to solve the flowsheet is to use the equation oriented approach, where the flowsheet is handled as a large set of equations, which are solved simultaneously. 

In an equation-oriented simulator, the executive program sets up the flowsheet and the set of equations that describe the unit operations and then solves the equations using data from the unit operations library and the physical property data bank and calling on the file of thermodynamics subroutines. 

Two extremes are encountered in flowsheeting software. At one extreme, the entire set of equations (and inequalities) representing the process is employed. This representation is known as the equation-oriented method of flowsheeting. The equations can be solved in a sequential fashion analogous to the modular representation described below or simultaneously by Newton s method, Broyden s method, or by employing sparse matrix techniques to reduce the extent of matrix manipulations. Refer to the review by Evans and Chapter 5. ... 

Hutchinson, H. P., D. J. Jackson, and W. Morton, Equation Oriented Flowsheet Simulation, Design and Optimization, Proc. Eur. Fed. Chem. Eng, Conf. Comput. Appl. Chem. Eng., Paris, April 1983 The Development of an Equation-Oriented Flowsheet Simulation and Optimization Package, Comput. Chem. Eng., v. 10, p. 19 (1986). 

Perkins, J. D., Equation Oriented Flowsheeting, in Foundations of Computer-Aided Design, A. W. Westerberg and H. H. Chien, eds., CACHE Corp., Austin, Tex., 1983. 

Both sequential and simultaneous calculational sequences have been proposed for the modular approach as well as the equation-oriented approach. Either the program and/or the user must select the decision variables for recycle and provide estimates of certain stream values to make sure that covergence of the calculations occurs, especially in a process with many recycle streams. Reviews by Evans and Rosen point out many of the problems and practices pertaining to flowsheeting. 

More recant efforts to develop computer-aided process synthesis methodologies can be cbeincterized as either sequential modular, simultaneous modular, or equation oriented, Sequeatial-modular approaches are ben for steady-state simulation where process inputs are defined and process para met ara ate available. The best feature of sequential-modular approaches is that they are flowsheet oriented, bot they are not as flexible as the other methodologies in parfonuing design and optimization tasks.39... 

Equation-oriented approaches are based on sets of equations that are written for the units in a particalar flowsheet.36 Unlike the sequential-modular systems, which often contain the necemery information for a veriety of process units, equaiion-orianted synthesizers require the pinctirioner to develop the mode] eqon-lions. These are solved through iterative tecbaiques with staadatd numerical methods. 

In the period 1970-80 s the major engineering bureaus, as well as some large manufacturing companies in refining and petrochemical industries developed in-house flowsheeting programs. Mostly adopted the modular sequential architecture. However, some were based on the equations oriented approach, as SPEEDUP at Imperial College in London (UK) and TISFLO at DSM in The Netherlands. 

In Simultaneous-Modular approach the solution strategy is a combination of Sequential-Modular and Equation-Oriented approaches. Rigorous models are used at units level, which are solved sequentially, while linear models are used at flowsheet level, solved globally. The linear models are updated based on results obtained with rigorous models. This architecture has been experimented in some academic products. 

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. 

Flowsheeting is still dominated by the Sequential-Modular architecture, but incorporates increasingly features of the Equation-Oriented solution mode. A limited number of systems can offer both steady state and dynamic flowsheeting simulators. 

The algorithmic treatment depends on the architecture of the flowsheeting system. In Equation-Oriented mode, the approach consists of solving all the equations describing the problem simultaneously. In Sequential-Modular approach the mathematical solution must take into account the convergence of units and tear streams, as well as of all design specifications. Supplementary equations must be added, so that the general formulation of the optimisation problem (3.10) becomes ... 

J. D. Perkins, Equation-Oriented Flowsheeting, in Proceedings of the Second International Con-... 

A chemical process plant consists of many unit operations connected by process streams. Each process unit may be modelled by a set of equations (ODEs, PDEs, DAEs, algebraic equations), which include material, energy and momentum balances, phase and chemical equilibrium relations, rate equations and physical property correlations. These equations relate the outlet stream variables to the inlet stream variables for a given set of equipment parameters. At present, there are three approaches of flowsheet calculations the sequential modular, the equation oriented approach and the simultaneous modular strategy. 

Sequential modular approach has some clear advantages for process flowsheeting that explain why it still dominates the technology of steady-state simulation over the simultaneous or equation-oriented approach. Table 8.2 shows a list of pros and cons about sequential modular process simulators. In order to cope with the disadvantages, a few process simulators have improved the flow of information and avoid redundant computations. As an example, Aspen HYSYS has implemented the bidirectional transmission of information technology. 

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