Flowsheeting options

The last of the four flowsheet options generated above, which features excess decane in the reactor, is therefore preferred (see Fig. 4.7[Pg.104]

Fig. 8.2. Production costs for a high-boiling ester for various flowsheet options. (From Ref. [6].)...

One central step considered in all flowsheet options, is an entrained flow gasifier operating at a flame temperature up to 1500°C. The hot tar-free syngas generated in the gasifier flame should be quenched with crushed char powder (not shown in the flowsheet) to increase the cold gas efficiency by chemical quenching [16]. 

Discrete variables are also sometimes used in process design, for example, the number of trays or the feed tray of a distillation column, and in process synthesis, to allow selection between flowsheet options, as described later. 

Binary integer variables can be used to formulate optimization problems that choose between flowsheet options. For example, consider the problem of selecting a reactor. We can set up a unit cell consisting of a well-mixed reactor, a plug-flow reactor and a... 

The identihcation of components is shown in Figure 7.2. It is useful to check the connections between the units using the Flowsheet option/Section//GLOBAL, as shown in Figure 7.3. Any streams that were inadvertently not connected to a unit will show up here because those stream numbers will not appear as input stream numbers. 

Once this has been completed, create a new Calculator block in the Flowsheeting Options folder of the navigator pane (it is named CALCMKUP for this example). ... 

Spec/Kebons Q Simiiation Options Q Streem Cass S Substreams S Units-Sets Q Custom Units Q Report Optior s Components Properties Flowsheet Streams Utitoes Btocks Reactions r 1 Chemistry Reactions o R-MTBE>U Convergence Flowsheeting Options Model Analysts Tools EO Configurabon Resdts Summary... 

O R-MTBE-U Convergence Flowsheeting Options Model Analysis Tools EO Configuration Results Summary... 

However, factors such as this should not he allowed to dictate design options at the early stages of flowsheet design because preheating the cold feed hy heat integration with the rest of the process might be possible. 

Indirect heat transfer with the reactor. Although indirect heat transfer with the reactor tends to bring about the most complex reactor design options, it is often preferable to the use of a heat carrier. A heat carrier creates complications elsewhere in the flowsheet. A number of options for indirect heat transfer were discussed earlier in Chap. 2. 

The preceding definitions of economic potential and total annual cost can be simplified if it is accepted that they will be used to compare the relative merits of difierent structural options in the flowsheet and difierent settings of the operating parameters. Thus items which will be common to the options being compared can be neglected. 

Figure 10.16 presents the refining flowsheet of the two decades between 1950 and 1970. An optional unit, visbreaking, appears there. 

After perusal of these process options, the engineer asks the computer to select five designs for further study, and the oomputer produces a paper copy of the flowsheet and design parameters... 

FIGURE 8.2 Example of a flowsheet generated by computer-aided process synthesis, as would be seen on the screen of a computer terminal. The column on the left side of the figure shows options available in the particular design program being run. Courtesy, Peter Piela, Carnegie-Mel Ion University. 

In Chapter 1, two alternative ways were discussed that can be used to develop the structure of a flowsheet. In the first way, an irreducible structure is built by successively adding new features if these can be justified technically and economically. The second way to develop the structure of a flowsheet is to first create a superstructure. This superstructure involves redundant features but includes the structural options that should be considered. This superstructure is then subjected to optimization. The optimization varies the settings of the process parameters (e.g. temperature, flowrate) and also optimizes the structural features. Thus to adopt this approach, both structural and parameter optimization must be carried out. So far, the discussion of optimization has been restricted to parameter optimization. Consider now how structural optimization can be carried out. 

Consider now ways in which the best arrangement of a distillation sequence can be determined more systematically. Given the possibilities for changing the sequence of simple columns or the introduction of prefractionators, side-strippers, side-rectifiers and fully thermally coupled arrangements, the problem is complex with many structural options. The problem can be addressed using the optimization of a superstructure. As discussed in Chapter 1, this approach starts by setting up a grand flowsheet in which all structural features for an optimal solution are embedded. 

Flowsheet optimization is also regarded as a key task in the structural optimization of a flowsheet. As a described in the introduction, structural optimization for process design can be formulated as a mixed integer nonlinear program (MINLP). This then allows for addition or replacement of existing units, and consideration of a number of design options simultaneously. In these formulations individual units are turned on and off over the course of the optimization, as suggested by the MINLP master problem. 

Conceptually, the representation of alternative process flowsheet(s) is based on elementary graph theory ideas. By representing each unit of the superstructure as a node, each input and output as a node, the interconnections among the process units as two-way arcs, the interconnections between the inputs and the process units as one-way arcs, the interconnections between the process units and the outputs as one-way arcs, and the interconnections between the inputs and the outputs as one-way arcs, then we have a bipartite planar graph that represents all options of the superstructure. 

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