Flowsheet conventions

Let s state some of the conventions for process flowsheets that we implicitly invoked in the preceding designs. First, each unit should represent only one physical or chemical operation. For example, unit 1 below is unacceptable. The flowsheet needs more details than unit 1 provides. In practice some units involve more than one operation. Some reactors are also mixers and some reactors are also separators. However, when designing a process here, draw an individual unit for each operation. 

Second, process streams and units should be labeled. The chemical content of the streams and the physical properties of the unit should be included whenever possible. For example, a process stream might be labeled 1240 mol/min methane at 77°C, and a unit might be labeled reactor, 500 C, 10 atm.  

streams should be drawn to and from units such that matter is neither created nor destroyed. Consider a washer for recycled glass bottles  

Water appears from nowhere in the diagram above. A proper flowsheet for the washer might be  

Reactors are sometimes a source of confusion. For example, a reactor may seem to destroy matter, such as nitrogen and hydrogen in the reactor below. A reactor may also seem to create matter. In this case the reactor seems to create ammonia. 

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In order for flowsheets and P IDs to be understood by people with different job responsibilities such as plant designers, process engineers, instrumentation specialists, and vendors, it is useful to use standardized symbols and conventions on the flowsheets. Standards concerning instrumentation symbols and flowsheet conventions have been developed by technical societies, such as the International Society of Automation (ISA). However, individual companies often use different or additional symbols for particular processes. 

Figure 12-4. Flowsheet and floor plan of conventional equipment. (Source Shabica [2].)...

Figure 7 An outline of the flowsheet for zinc production by the conventional roast/neutral-leach/electro-...

Comparison of metallurgical results using conventional and bulk flotation flowsheets on ore... 

Another widely used safety analysis method in process industry is the Hazard and Operability Analysis, better known as Hazop (Kletz, 1992). The conventional Hazop is developed to identify probable process disturbances when complete process and instrumentation diagrams are available. Therefore it is not very applicable to conceptual process design. Kletz has also mentioned a Hazop of a flowsheet, which can be used in preliminary process design, but it is not widely used. More usable method in preliminary process design is PIIS (Edwards and Lawrence, 1993), which has been developed to select safe process routes. 

The application of simultaneous optimization to reactor-based flowsheets leads us to consider the more general problem of differentiable/algebraic optimization problems. Again, the optimization problem needs to be reconsidered and reformulated to allow the application of efficient nonlinear programming algorithms. As with flowsheet optimization, older conventional approaches require the repeated execution of the differential/algebraic equation (DAE) model. Instead, we briefly describe these conventional methods and then consider the application and advantages of a simultaneous approach. Here, similar benefits are realized with these problems as with flowsheet optimization. 

Compared to adsorption s use in bulk-gas separations, its use in gas purifications is much more frequent (see Table I and references 13, 36 and 37), and the technology is, for the most part, more conventional Temperature-swing adsorption, often combined with inert-purge stripping, is by far the most common process used Two or more fixed beds operated in parallel, typically with one adsorbing and one or more regenerating, constitute the standard flowsheet ... 

Fig. 5. Comparative flowsheet for the conventional and microbial processes for the production of acrylamide...

For illustration purposes, consider the Williams-Otto flowsheeting problem (80). In the conventional design, raw materials A and B are fed to a reactor, where the following reactions occur ... 

A number of variations are possible with such two tiered sytems. Tearing can take place in the conventional way and the torn streams can be estimated. Each module in turn can be calculated as in the sequential modular systems. A linearized model of each module can then be generated which in turn can be used in the linearized flowsheet model. From Equation (1)... 

Lin (100) suggested breaking the process flowsheet into one or more blocks of modules. Each block of modules contains one or more modules and all of the modules in the same block are solved simultaneously. The whole process flowsheet is then solved by conventional sequential modular approach by treating each block as a module. 

The conventional flowsheet to separate a ternary mixture uses two distillation columns in series. It is sometimes more economical to use a single distillation column with a sidestream. This is particularly true when product purities are moderate to low. Consider the case where the ternary mixture contains components A. B, and C, with decreasing relative volatilities. Figure 6.19 shows two common situations a liquid sidestream is withdrawn from a tray somewhere above the feed tray, or a vapor sidestream is withdrawn from a tray somewhere below the... 

To illustrate the very large energy savings that are possible with this complex/heat-integrated system, consider the separation of a benzene, toluene, and xylene mixture. A conventional two-column light-out-first separation flowsheet with no heat integration uses twice the energy7 that the prefractionator-reverse flowsheet uses. 

The classic flowsheet for the production of acetates was unsuitable for adoption for methyl acetate. However, the flowsheet generated by the conventional conceptual process design approach using literature schemes and standard patterns seems especially complicated for such simple chemistry involving so few components. This first flowsheet is a candidate for evolutionary modification. 

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