Flowsheet simulation

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

Each chapter presents several detailed studies illustrating the application of various optimization techniques. The following matrix shows the classification of the examples with respect to specific techniques. Truly optimal design of process plants cannot be performed by considering each unit operation separately. Hence, in Chapter 15 we discuss the optimization of large-scale plants, including those represented by flowsheet simulators. 

Mahalec, V. H. Kluzik and L. B. Evans. Simultaneous Modular Algorithm for Steady State Flowsheet Simulation and Design. Paper presented at the 12th European Symposium on Computers in Chemical Engineering. Montreaux, Switzerland (1979). 

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). 

Kaijaluoto, S., Process optimization by flowsheet simulation, Technical Research Center of Finland, Publication 20(1984). 

Since this book is concerned primarily with one kind of equipment at a time, all that need be done here is to call attention to the existence of the abundant literature on these topics of recycle calculations and flowsheet simulation. 

Basically, DESIGNER can use different physical property packages that are easy to interchange with commercial flowsheet simulators. For the case considered, the vapor-liquid equilibrium description is based on the UNIQUAC model. The liquid-phase binary diffusivities are determined using the method of Tyn and Calus (see Ref. 72) for the diluted mixtures, corrected by the Vignes equation (57), to account for finite concentrations. The vapor-phase diffusion coefficients are assumed constant. The reaction kinetics parameters taken from Ref. 202 are implemented directly in the DESIGNER code. 

By the early 1990s, advances in user interfaces made the simulation packages much easier to use. HYSIM from Hyprotech was the first package to be developed specifically for a personal computer (PC) platform. All commonly used process simulators (including PRO/II from Simulation Sciences) are now available on a PC platform, with interfaces that allow flowsheet simulations to be formulated rapidly. Icons representing feed and product streams and unit operations are placed directly on a sim-... 

Parker, A. L., "Chemical Process Optimization by Flowsheet Simulation and Quadratic Approximation Programming", Ph.D. Thesis in Chemical Engineering, U. of Wisconsin, 1979. 

Kaijaluoto, S., "Experiences of the Use of Plex Data Structure in Flowsheeting Simulation",... 

Mahalec, V. Kluzik, H. Evans, L. B., "Simultaneous Modular Algorithm for Steady State Flowsheet Simulation and Design", CACE 79, EFCE Montreux, April 8-11, 1979. 

With all it features, the PROSIM program needs further improvements, especially in the convergence procedures for both single-and multiple-stage calculations. Work is proceeding on this at the present time. Another logical extension of the program is to flowsheet simulation, as well as reduced crude properties prediction. 

For the purpose of conceptual design of the bioethanol plant, Aspen Plus will be used as the flowsheet simulator. However, most of the key components involved in the process are not defined in the standard Aspen Plus property databases, and therefore their physical property data are not available. The National Renewable Energy Laboratory (NREL) has developed a database that includes a complete set of properties for the currently identifiable compounds in the ethanol process [28]. 

In early attempts at optimization, pattern search based methods put yet another loop around these programs. Fifteen hundred flowsheet simulations to find the optimum were not uncommon the computation often failed. 

The options of using available general purpose process flowsheet simulators, or formulating special purpose models were considered. The latter was chosen to satisfy the need for detailed, accurate, and flexible models, which are suitable for routine use. 

Flowsheet simulators consist of unit operation models, physical and thermodynamic calculation models and databanks. Consequently, the simulation results are only as good as the underlying physical properties and engineering models. Many steady-state commercial simulators [2.1, 2.2] have some dynamic (batch) models included, which can be used in steady-state simulations with intermediate storage buffer tanks. 

Clearly define, in your own words, the terms design variables and state variables, sequential modular flowsheet simulation, equation-based flowsheet simulation, tear stream, convergence block, and design specification. 

Ihese difficulties vanish if the system equations are simply collected and solved for all unknown variables. Several powerful equation-solving algorithms are available in commercial programs like Maple , Mathematica , Matlab , Mathcad , and E-Z Solve that make the equation-based approach competitive with the sequential modular approach. Many researchers in the field believe that as this trend continues, the former approach will replace the latter one as the standard method for flowsheet simulation. (Engineers are also working on simultaneous modular methods, which combine features of both sequential modular and equation-based approaches. We will not deal with these refinements here, however.)... 

Calculates mi, m2, and m3. Notice that in previous flowsheet simulation examples, our MIX block could only handle two inlet streams. If that were the case here, we would need two block calls— one to mix SI and S14, and the next to mix the product of the first stage with SIO. 

Equation-based flowsheet simulators have not reached the level of commercialization of their sequential modular counterparts as of the time of this writing, and the development of efficient equation-solving algorithms is the subject of much continuing research. 

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. 

---