Input/output structure

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. 

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. 

Milestone 1. Synthesize and evaluate plant complex structure, i.e., identify the number of abstract plants each of which is formalized around a set of reactions, which take place under the same conditions (Fig. 4a). Milestone 2. Design the input/output structure for each plant, i.e., identify the set of distinct input and output streams associated with each plant of the overall structure (Fig. 4b). 

Fig. 9. Goal structure for the goal G,, , Design an input/output structure for plant-i. ...

Hi) Evaluation. Economic potential is computed by subtracting the costs for the reaction system and gas compressor from the economic potential computed at the input/output structure level. To compute the costs for those units, several assumptions, such as the reactor type and the kinetic model, should be made. More detailed algorithms and example applications are available in Douglas design text (Douglas, 1988). 

Fio, 14. Schematic used to indicate propagation of constraints from the input/output structure (a) to the recycle structure (b). 

Fig. 15. Context-based design. This figure illustrates the scoping relationships among design contexts. The design decisions at the recycle structure are within the scope of the design decisions at the input/ output structure level.

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