Synthesis, separation systems

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

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. 

Malone, M. F. and Doherty, M. F. (1995). Separation system synthesis for nonideal liquid mixtures. AlChE Symp. Ser., 91(304), 9-18. 

Chapter 9 Distillation-based Separation Systems Synthesis... 

To illustrate the nature of the distillation-based separation system synthesis problem, let us consider its generic definition shown in Figure 9.1, which is as follows ... 

It should be noted that the distillation-based separation system synthesis problem corresponds to a combinatorial problem, since the number of sequences and configurations increase with the... 

This chapter discusses mixed-integer nonlinear optimization applications in the area of distillation-based separation system synthesis. Section 9.1 introduces the reader to the generic definition of separation system synthesis and provides a classification of the different types of separations based on distillation column sequencing. For further reading on separation system synthesis the reader is referred to the reviews by Westerberg (1985) and Nishida et al. (1981)... 

REACTION-SEPARATION AND REACTIVE-SEPARATION SYSTEM SYNTHESIS... 

Barnicki, S.D., Fair, J.R. Separation System Synthesis A Knowledge Based Approach 1. Liquid Mixture Separations, Ind. Eng. Chem. Res., 29, 431-439, 1990... 

Barnicki S.D., J.J. Siirola, Separation system synthesis, in Kirk-Othmer... 

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. 

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... 

Bamicki, S.D., J.R. Fair, 1990, Separation System Synthesis A knowledge based approach 1. Liquid Mixture Separations, Ind. Eng. Chem. Res., 29,431-439 Bamicki, S.D., J.R. Fair, 1992, Separation System Synthesis A knowledge based approach 2. GasA apour mixtures, Ind. Eng. Chem. Res., 31, 1679-1694 Bamicki S. D., J. J. Siirola, 1997, Separation system synthesis, in Kirk-Othmer Encyclopedia of Chemical Technology, 4 edition, vol. 21, 923-962 Crowl, D., J. F. Louvar, 1989, Chemical Process Safety Fundamentals with Applications, Prentice Hall,... 

Malone, M. F., M. F. Dogerthy, 1996, Separation system synthesis for non-ideal liquid mixtures. Foundations of Computer Aided Process Design, p. 9-18 Perry s, 1997, Chemical Engineers Handbook, 7th edition... 

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