Solving Material and Energy Balances Using Flowsheeting Codes

Frequently process plants contain recycle streams and control loops, and the solution for the stream properties requires iterative calculations. Thus efficient numerical methods for convergence must be used. In addition, appropriate physical properties and thermodynamic data have to be retrieved fi om a data base. Finally, a master program must exist that links all the building blocks, physical property data, thermodynamic calculations, subroutines, and numerical subroutines, and that also supervises the information flow. You will find that optimization and economic anafy-sis are really the ultimate goal in the use of flowsheet codes. 

Two extremes are encountered in flowsheeting software. At one extreme the entire set of equations (and inequalities) representing the process is written down, including the material and energy balances, the stream connections, and the rela- 

Solving Material and Energy Balances via Computer Codes Chap. 5 

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. 

Sets of linear and/or nonlinear equations can be solved simultaneously using an appropriate computer code (see Table L.l) by one of the methods described in Appendix L. Equation-based flowsheeting codes pertaining to chemical engineering can be used for the same purpose. The latter have some advantages in that the physical property data needed for the coefficients in the equations are transparently transmitted from a data base at the proper time in the sequence of calculations. 

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Sec. 5.2 Solving Material and Energy Balances Using Flowsheeting Codes 565 TABLE E5.7... 

In this section we examine briefly how equation solvers, generic codes, spreadsheets, and flowsheeting codes can be used to solve material (and energy) balances. 

Solving Material and Energy Balanc >s Using Flowsheeting Codes... 

Whatever the code used to solve material and energy balance problems, you must provide certain input information to the code in an acceptable format. All flowsheeting codes require that you convert the information in the flowsheet (see Fig. 5.4) to an information flowsheet as illustrated in Fig. 5.5, or something equivalent. In the information flowsheet, you use the name of the mathematical model (subroutine for modular-based flowsheeting) that will be used for the calculations instead of the name of the process unit. 

Chapter 5 on the simultaneous solution of material and energy balance problems has been completely revised. It now focuses on the use of flowsheeting codes to solve problems. The problem set for Chapter 5 emphasizes the use of FLOWTRAN and PROCESS for complex problems, codes that have clear manuals and are available at most departments of chemical engineering. 

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