Separation flowsheet synthesis selection

In ERD in situ separation is used to improve the yield of reaction whereas an entrainer feed is added to make the separation feasible by selectively increasing the relative volatility of one of the products, ERD promises to be advantageous for the synthesis of fatty acid esters. The entrainer increases the relative volatility of water (by-product) compared to the alcohol (reactant), such that during the reaction the water can be continuously removed by distillation. In this way the chemical equilibrium is shifted such that higher conversions can be obtained. In Figure 1 the flowsheet of the desired process is given, in which RS stands for Reactive Section and DS for Distillation Section. 

Choice of Process Configuration. The sequence of processing steps and flowsheet configuration must be selected concurrently with the choice of extraction chemistry. Some of the established heuristics of separation process synthesis may be helpful here. For example, in multimetal fractionation it is probably better to tiy selective stripping from a single organic stream than to do multiple selective-extraction oper-... 

Step 1. Synthesis or selecting the structure of the flowsheet identification of the equipment, interconnecting and specifying the initial design values. In this step, a crude approximate flowsheet is created to consider recycles, purges, and possible separation schemes make a simple, linear model to assess the effects of major parameters and structural variations do necessary laboratory work get more details for physical properties, thermodynamics, utilities, and process units write unit models, if necessary and fix the flowsheet layout. 

Very little process synthesis literature has dealt successfully with the problem of selection between alternative methods of separation. Indeed, most process synthesis literature has dealt with the selection of the flowsheet sequences Tor a single sepamtion method with only simple, sharp separations between components of ndjacent selectivity, without recycle. Furthermore, most of this literature has used only distillation in illustrative epplications. In practice, much use is made of recycle, nonsharp eplits. and complex columns (in distillation) and, of coorse. distillation is not always the separation merited of choice. 

While process sym basis research has not yet produced the so In lion to the problem of generation of die best flowsheet, a number of heuristic rules have been stated by various process synthesis researchers which can be of considerable aid in guiding the selection of the near optimum or base case and Tor systematically choosing those alternate sequences to be investigated. Some of the heuristic roles developed will be compering and in some cases even contradictory, but nevertheless they do serve to reduce the enormous amount of work (hat would be involvnd in evaluating all possible separation sequences. 

When the reaction and separation operations are positioned, the states of their feed and product streams are selected. This is accomplished usually by adjusting the temperature and pressure levels to achieve the desired reaction conversions and separation factors. Subsequently, after the flowsheets have been created, these are often adjusted toward the economic optimum, often using the optimizers in the process simulators discussed in Chapter 18. In this synthesis step, however, the states are assumed to be fixed and operations are inserted to eliminate the temperature, pressure, and phase differences between the feed sources, the product sinks, and the reaction and separation operations. 

In recent years a considerable amount of literature has appeared in chemical engineering journals on the subject of process synthesis. Nishida et al. reviewed the current state of the art as it applies to cheiiucal processes. If process synthesis methodology were developed fully, one could invoke a computer program which, with minimal input, would select not only the separation method of choice but also would determine how the various unit operations should be connected together to form the entire flowsheet. At the present time, however, process synthesis methodology for separation processes is in the earty stages of research. 

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