Output Structure

For more generality we will consider a process involving two reaction stages  

Hydrogenation of phenol to a cyclohexanone/cyclohexanol mixture in a ratio given by the catalyst selectivity. 

Dehydrogenation of cyclohexanol to cyclohexanone to improve the overall yield. 

At the input/output level the key design decision regards the conversion of reactants. This should obtain the optimal selectivity in useful products, whilst minimizing the occurrence of harmful species for safety and the environment When several reactants are involved, the first heuristic recommends the complete conversion of the reactant that is the most expensive or the most difficult to recycle (see Chapter 2). Thus, the complete conversion of phenol would be desirable, but 

as the first design decision we assume partial conversion of phenol to both cyclohexanone and cyclohexanol. As a result, the unreacted phenol must be recycled. Then cyclohexanol is converted to cyclohexanone in a separate reactor, 

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Batch versus continuous Flowsheet input-output structure Crystallizer and recycle considerations Separation systems specification Product drying Energy systems... 

Heuristic based approaches are more relevant for structured products. The most well known heuristic based approach is the hierarchical decomposition method developed by Douglas (Douglas, 1988). In the first level of this method one only looks at the input-output structure of the process. In subsequent levels more detail is added, finally ending with the entire flowsheet. Design decisions are made by using heuristics and short-cut models. An alternative method is due to Siirola (1996) means end analysis. In this method the properties of the feedstock and the desired products are compared. Tasks are defined to eliminate the property differences between the feedstock and the desired product. 

Using a guanidino group as the seed, output structures similar to several known DHFR inhibitors were generated, such as 74 which may be compared to trimethoprim (2). 

The input/output structure defines the material balance boundary of the flowsheet Often it is referred as the inside battery limit envelope. A golden rule requires that the total mass flow of all components entering the process must be equal with the total mass flow of all components leaving it. It should be kept in mind that the recycles affect only the internal process streams, but not the input/output material balance. 

Figure 2.3 displays the input/output structure, which includes inlet and outlet streams, as well as large external recycles that requires separate plants and storage facilities. Design decisions formulated as heuristics, are given in Table 2.1, mostly... 

Figure 2.4 Input/output structure of the acrylonitrile manufacturing.

Figure 2.11 Input/output structure by toluene hydrodealkylation process.

The hierarchical approach is a simple but powerful methodology for the development of process flowsheets. It consists of a top-down analysis organized as clearly defined sequence of tasks aggregated in levels. Each level handles a fundamental conceptual problem input/output structure, reactor design, structure of separa-... 

As the reactions involve the same components, these may be handled by a common separation system. Figure 5.8 presents the input/output structure and the main recycles. 

The input-output structure of the flowsheet is presented in Figure 9.1. Butene (feed rate FA,0) and isobutane (feed rate FB-0) are the raw materials. The butene feed is impure with quite large amounts of propane (FI 0). The main product is the alkylate C8Hi8, at the rate FP. The selectivity of the process is not 100%, therefore heavy products are formed at the rate FR. The inert fed into the process must also leave the plant, the flow including light byproducts that are formed in secondary reactions. Often, significant quantities of n-butane are mixed with the isobutane fresh feed. For this case, development of the flowsheet and the design of the main units is left as an exercise for the reader. 

Input-output analysis focuses on the overall structure of the flowsheet, and the recycle streams are not considered here in as much as they do not appear in the overall flow streams. Hence, this structure is essentially an overall mass balance for the entire process. A simplified representation of the input-output structure is shown in Fig 6.5. The main purpose of this analysis is to identify the amount of raw materials used, useful products, and waste formed. 

Disadvantage Most output structures are not available from commercial reagents or known synthetic methods. 

By using SEEDS for output structures generated through de novo design programs, we can obtain the derivatives with minimal chemical efforts and can eluci-... 

Output Structures Obtained from de Novo Design Program(s)... 

Extract an essential part of each output structure... 

Fig. 6.6 Rules for extracting essential parts and making queries for searching compound databases. 1 An example of output structures...

First, we obtained 1000 output structures, however, most of these structures were not commercially available or synthetically feasible. As previously mentioned, this problem appears to apply to most de novo design programs that build structures sequentially. 

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