Stream process synthesis

The CHARMEN synthesis problem can be stated as follows Given a number Nr of waste (rich) streams and a number Ns of lean streams (frtiysical and reactive MSAs), it is desired to synthesize a cost-effective network of physical and/or reactive mass exchangers which can preferentially transfer certain undesirable species from the waste streams to the MSAs. Given also are the flowrate of each waste stream, G,, its supply (inlet) composition, yf, and target (outlet) composition, y/, and the supply and target compositions, Xj and jc for each MSA. In addition, available for service are hot and cold streams (process streams as weU as utilities) that can be used to optimize the mass-exchange temperatures. 

Process simulation, 20 710, 728-730 Process specific piping system, in fine chemical production, 11 429-430 Process-stream purification, 13 620 Process synthesis, 13 218 26 999... 

In most chemical processes reactors are sequenced by systems that separate the desired products out of their outlet reactor streams and recycle the unconverted reactants back to the reactor system. Despite the fact that process synthesis has been developed into a very active research area, very few systematic procedures have been proposed for the synthesis of reactor/separator/recycle systems. The proposed evolutionary approaches are always based upon a large number of heuristic rules to eliminate the wide variety of choices. Many of these heuristics are actually extensions of results obtained by separately studying the synthesis problem of reactor networks or separator systems, and therefore the potential trade-offs resulting from the coupling of the reactors with the separators have not been investigated. 

Early work in process synthesis focused on the solution of specific problems, such as the best sequence of distillation columns to perform separation of components in feedstreams into product streams. Another early problem was the synthesis of heat-exchanger networks. 

Figure 2.14 illustrates the overall approach by pinch-point analysis. The first step is extraction of stream data from the process synthesis. This step involves the simulation of the material-balance envelope by using appropriate models for the accurate computation of enthalpy. On this basis composite curves are obtained by plotting the temperature T against the cumulative enthalpy H of streams selected for analysis, hot and cold, respectively. Two aspects should be taken into account ... 

Process synthesis tries to find the flowsheet and equipment for specified feed and product streams. We define process synthesis as the activity allowing one to assume which process units should be used, how those units will be interconnected and what temperatures, pressures and flow rates will be required [2.15, 2.16[. 

The components involved in this example are proprietary, but the results are general (Siirola, 1981). During the species allocation stage of the process synthesis procedure, it was determined that each species of a particular four-component stream was required to be relatively pure at four different destinations. The components are liquids at ambient temperatures, have about equal relative volatility differences, and form no azeotropes. Distillative separation methods were selected to resolve all composition property differences. The feed stream composition was dominated (about 70%) by the heaviest component (D). 

Bretzel, W. et al. (1999) Commercial riboflavin production by recombinant Bacillus suhtilis down-stream processing and comparison of the composition of riboflavin produced by fermentation or chemical synthesis. J. Ind. Microbiol. Biotechnol, 22 (1), 19-26. 

For commodity chemicals, of the above conditions, the scale (i.e., production level or flow rate) of the process is a primary consideration early in the design process. Working together with the marketing people, the scale of the process is determined on the basis of the projected demand for the product. Often the demographics of the most promising customers have an important impact on the location of the plant and the choice of its raw materials. As the scale and the location are established, the composition, phase, form, temperature, and pressure of eaeh produet and raw material stream are considered as well. When the desired states of these streams have been identified, the problem of process synthesis becomes better defined. As shown in Figure 3.3, for the production of vinyl chloride, it remains to insert the process operations into the flowsheet. 

Flowsheets are rarely acyclic, as in Figure 4.4. In process synthesis, most distributions of chemicals involve recycle streams as in Figure 4.1. For the simpler distributions, where the fractional conversions or the extents of reaction are known, the split fractions are specified, and no purge streams exist, as in the vinyl-chloride process (Figures 3.8 and 3.19), the flow rates of the species in the recycle streams can be calculated directly (without iteration). 

The next two alternatives do not affect the distribution of chemicals and are usually considered for moderately exothermic reactions, later in process synthesis— that is, during heat and power integration, when opportunities are considered for heat exchange between high-and low-temperature streams. 

In all of these alternatives, the design team selects acceptable temperature levels and flow rates of the recirculating fluids. These are usually limited by the rates of reaction, and especially the need to avoid thermal runaway or catalyst deterioration, as well as the materials (rf construction and the temperature levels of the available cold process streams and utilities, such as cooling water. It is common to assign temperatures on the basis of these factors earily in process synthesis. However, as optimization strategies are perfected, temperature levels are varied within bounds. See Chapters 10 and 18 for discussions of the use of optimization in process synthesis and optimization of process flowsheets, as well as Example 6.3 to see how constrained optimization is applied to design an ammonia cold-shot converter. 

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