Flowsheet codes

Typical process modules used in sequential modular-based flowsheeting codes with their subroutine names. 

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

Inso r as this book is concerned, if you are not familiar with a filowsheeting code, you will find it will be more trouble to solve a material balance problem using a flowsheeting code than it will be to use a standard equation-solving code such as one of those in the disk pocket in the back of the book. 

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

In this chapter we first examine how to make sure that a problem is properly and completely specified by extending our earlier treatment in Chap. 2 of the analysis of the degrees of freedom. Then we review how computer codes and, in particular, flowsheeting codes can be employed to solve complex problems. Figure 5.0 shows the relationships among two topics to be discussed in this chapter, and the relationships with topics in previous chapters. 

Sec. 5.2 Solving Material and Energy Balances Using Flowsheeting Codes 551... 

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. 

Figure 5.2 Information flow in a typical flowsheeting code.

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. 

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. 

EXAMPLE 5.8 Solution of Material and Energy Balances Using a Sequential Modular-Based Flowsheeting Code... 

Figure E5.8 is a hypothetical process used for demonstration by Diamond Shamrock Co. of their flowsheeting code PROVES. Makeup gas is compressed, combined with recycle gas, and fed, together with liquid raw material, into a three-phase, suspended bed catalytic reactor. The reactor is cooled by recirculating liquid through a heat reclamation steam generator. Reaction products are condensed and the pressure of the exit stream reduced in two stages. The gas from the first-stage separator is recirculated, whereas the liquid from the second-stage separator is fed into a distillation column. Pure product is withdrawn from the bottom of the column. The distillate is a by-product that is pumped to another plant.

Wegstein s method, which is used in many flowsheeting codes, accelerates the convergence of the method of successive substitutions on each iteration. In the secant method, the approximate slope is... 

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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