Separation systems synthesis method

In general, the steps of this separations system synthesis method for nonideal mixtures involving azeotropes include examination of the RCM representation (overlaid with vapor-liquid equlibria (VLE) pinch information, liquid-liquid equlibria (LLE) binodal curves and tie lines, and. solid-liquid equlibria (SLE) phase diagrams if appropriate) determination of the critical thermodynamic features to be avoided (e.g., pinched regions), overcome (e.g., necessary distillation... 

There are numerous other features of the separations system synthesis method based on the RCM representation that space permits only brief mention. Residue... 

As one last example of the separations system synthesis method, a process was being developed to produce the chemical diethoxymethane (DEM), the ethanol acetal of formaldehyde. The raw materials are paraformaldehyde and ethanol, and the reaction by-produces water. Diethoxymethane, ethanol, and water form a homogeneous ternary azeotrope that is readily removed from the reaction mass (the paraformaldehyde being relatively nonvolatile). The species allocation... 

The economics of the various methods that are employed to sequence multicomponent columns have been studied. For example, the separation of three-, four-, and five-component mixtures has been considered (44) where the heuristics (rules of thumb) developed by earlier investigators were examined and an economic analysis of various methods of sequencing the columns was made. The study of sequencing of multicomponent columns is part of a broader field, process synthesis, which attempts to formalize and develop strategies for the optimum overall process (45) (see Separation systems synthesis). 

Distillation (qv) is the most widely used separation technique in the chemical and petroleum industries. Not aU. Hquid mixtures are amenable to ordinary fractional distillation, however. Close-boiling and low relative volatihty mixtures are difficult and often uneconomical to distill, and azeotropic mixtures are impossible to separate by ordinary distillation. Yet such mixtures are quite common (1) and many industrial processes depend on efficient methods for their separation (see also Separation systems synthesis). This article describes special distillation techniques for economically separating low relative volatihty and azeotropic mixtures. 

Constraint propagation also reduces the search space size. This method looks at the problem from every view conceivable and, in each of these views, develops and propagates constraints on the solution space. In a separation system synthesis problem, one might look at the mixture and decide that the species D and E should be separated alone and by using distillation. This constraint dramatically reduces the search space size. Decisions not leading to D and E being isolated into a stream that will then be separated by distillation need not be considered in any enumeration scheme. 

B. Separ.ations System Synthesis Method for Nonideal Mixtures... 

This chapter presents two applications of MINLP methods in the area of separations. Section 9.1 provides an overall introduction to the synthesis of separation systems. Section 9.2 focuses on sharp heat-integrated distillation sequencing. Section 9.3 presents an application of nonsharp separation synthesis. 

L. Pibouleau, P. Floquet, and S. Domenech. Optimal synthesis of reactor separator systems by nonlinear programming method. AlChEJ., 34 163,1988. 

A more promising approach for the synthesis of hydrophobic substances with ADHs is published by Kruse et al. [159, 238], They use a continuously operating reactor where the enzyme containing water phase is separated from the hydrophobic substrate-containing organic phase by a membrane. The hydrophobic product is extracted continuously via a hydrophobic membrane into an hexane phase, whereas the coenzyme is regenerated in a separate cycle, that consists of a hydrophilic buffer system. This method decouples advantageously the residence time of the cofactor from the residence time of the substrate. Several hydrophobic alcohols were prepared in this way with (S)-ADH from Rhodococcus erythropolis (Table 16). 

The next section will reflect on common conceptual process design practices for overall flowsheet development. The remainder of this chapter reviews recent developments in process synthesis methods for reaction and separation systems that systematically guide the design decision-making process toward novel and improved designs. 

Particularly strong and complex interactions prevail among reaction and separation systems that are generally not at all or not fully exploited as a result of the application of the available synthesis methods for reactor networks and separation systems in isolation. The lack of generality in the synthesis methods is a tribute to the nonlinear process models required to capture the reaction and separation phenomena as well as to the vast number of feasible process design candidates. These complexities even make it difficult to synthesize the decomposed subsystems, which are typically reactor networks, separation systems, reactor-separator-recycle systems, and reactive separation systems. The development of reliable synthesis tools for these sub-systems is still an active research area. 

The basic flowsheet structure is given by the reactor and separation systems. Alternatives can be developed by applying process-synthesis methods. Use computer simulation to get physical insights into different conceptual issues and to evaluate the performance of different alternatives. 

For the synthesis of separation systems the strategy is decomposing the overall problem into subsystems handling essentially gases, liquids and solids, for which systematic methods exist. Environmental protection can also be handled by taking into account the tasks defined early in the above procedure. 

Chapter 3 deals with the Synthesis of the Separation System. A task-oriented approach is proposed for generating close-to-ophmum separation sequences for which both feasibility and performance of splits are guaranteed. Emphasis is placed on the synthesis of distillation systems by residue curve map methods. 

From exhaustive application of alternative simple distillation operators to all possible separations for this four-component system, or from application of ranked-list-based separations synthesis methods, it is easily shown that there are five different separation train structures for this four-component problem. Each can be generated systematically or since this pattern of solutions is already well known, each can be written down immediately or design heuristics can be used to generate one or more of the structures expected to be most suitable. After each structure is synthesized, its performance can be analyzed and evaluated with a flowsheet simulator. 

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