Flowsheet calculations presentation

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. 

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. 

The mass and energy balances, which are related to the flowsheet of Figure 4, are presented in Table III. The following assunqjtions have been used in the calculations ... 

From a computational viewpoint, the presence of recycle streams is one of the major impedements in the sequential solution of a flowsheeting problem. Without recycle streams, the flow of information would proceed in a forward direction, and the calculational sequence for the modules could easily be determined from the precedence order analysis outlined above. With recycle streams present, large groups of modules have to be solved simultaneously, defeating the concept of a sequential solution module by module. For example, in Fig. 5.14, you cannot make a material balance on the reactor without knowing the information in stream S6, but you have to carry out the computations for the cooler module first to evaluate S6, which in turn depends on the separator module, which in turn depends on the reactor module. Partitioning will identify those collections of modules that have to be solved simultaneously (termed maximal cyclical subsystems or irreducible nets). 

We have used the scoping flowsheet methodology to calculate Level 1 and Level 2 efficiencies for the CU-SO4, Zn-SO4, and CuCl cycles, as well as for a new cycle, Mg-Cl [1, 4]. All are hybrid cycles i.e., they contain an electrochemical reaction. The methodology is illustrated in detail for the CU-SO4 cycle. Only the results are presented for the other cycles evaluated. 

The two basic flowsheet software architectures are sequential modular and equation-based. In sequential modular, we write each unit model so that it calculates output(s), given feed(s), and unit parameters. This is the most commonly used flowsheeting architecture at present, and examples include Aspen+ plus Hysys (AspenTech), ChemCAD, and PROll (SimSci). In equation-based (or open-system) architectures, all equations are written describing material and energy balances as algebraic equations in the form/(x) = 0. This is the preferred architecture for new simulators and optimization, and examples include Speedup (AspenTech) and gPROMS (PSE pic). Each is discussed in turn. 

Table 15.11 presents data for the estimation of operating costs based on the above decomposition. The basis is the material and heat balance from flowsheeting, as well as the operating labour. This checklist can be used to compute the operating costs for profit and cash flow calculation. Note that the depreciation charge should be excluded, if it is accounted separately. 

In practice we can meet with even more complicated situations - see Fig. 2-7d. Streams 1, 2, 4 and 5 are measured and redundant (one stream can be calculated from the others). Stream 6 is measured, but nonredundant. Streams 3 and 7 are unmeasured and observable. Streams 8 and 9 are unmeasured and unobservable. The general classification of balancing variables is presented in Fig. 2-8. Anyway, we can see that even in a relatively simple flowsheet with the... 

This problem deals with the feed section to the cumene process in Appendix C. Figure C.8. This problem is actually part of the problem presented in Appendix C, and the process flowsheet along with calculations associated with the pumps can be found there. The problem is restated here. 

In computer calculations for distillation or flowsheeting, K-values subroutines may be called thousands of times. Routines based on complex equations of state can make the computation costly. Is this a problem here Have you, or anyone else present, any experience of fitting data, either measured or calculated by an appropriate equation of state, to something simple for use in computer programs For instance, it should be possible to fit the two Redlich-Kwong parameters per component instead of calculating them from critical constants. It would also be possible to incorporate enthalpy data which may be available, so helping thermodynamic consistency. 

For purposes of calculation, it may be imagined that the agitators are not present in the flowsheet and that the first thickener is fed with a dry mixture of the reaction products, CaCOj and NaOH, together with overflow from the second thickener. 

Figure 9.8 shows the flowsheet of an algae-based biorefinery case study. The mass balance was determined using spreadsheet calculations, and the conversion and yield factors of the various process units are presented in Table 9.8. The production of lOOOkgh" of dry algae biomass is used as basis. The composition of algae biomass is shown in Table 9.9. 

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