Hybrid Separation

Enhanced distillation designates special techniques for separating nonideal azeotropic mixtures, or zeotropic mixtures with very low relative volatility, based on the use of a dedicated mass-separation agent that has to be recycled. Sometimes, a component already present in the mixture can play this role. 

The extractive distillation profits from the capacity of an entrainer (solvent) to modify selectively the relative volatility of species. Normally, the entrainer is the highest boiler, while the component to be separated becomes heavier, being carried out in bottoms. For this reason, this operation may be regarded as an extractive absorption. Extractive distillation can be used for separating both zeo-tropic and azeotropic mixtures. The entrainer is fed near the top for a zeotropic mixture or a minimum-boiling azeotrope, or mixed with the feed for a maximumboiling azeotrope. The separation sequence normally has two columns, for extraction and solvent recovery [5]. 

Chemically enhanced distillation is similar to extractive distillation, but this time the solvent is an ionic salt or a complex organic molecule. As an example, acetone separates easier from a solution with methanol in the presence of a concentrated solution of calcium chloride. More modern solvents are based on ionic liquids. 

When the composition of an azeotrope is sensitive to moderate changes in pressure, then pressure-swing distillation may be used to separate almost pure components. The first column separates pure A in top or in bottoms, if this is a minimum or maximum boiler, respectively. Then the second column makes possible the separation of pure B while recycling the A/B azeotrope. 

By a hybrid separation, distillation is combined with other separation methods, such as L-L extraction, adsorption, crystallization and membranes. It is mainly 

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Hamad, A. A., Garrison, G. W., Crabtree, E, W., and El-Halwagi, M. M. (1996), Optimal Design of Hybrid Separation Systems for Waste Reduction, Fifth World Congress of Chemical Engineers, Vol. Ill, pp. 453-458, San Diego, California. 

Novel processing methods, such as integration of reaction and one or more unit operations in so-called multifunctional reactors and integration of two or more separation techniques in hybrid separations Use of alternative forms and sources of energy for chemical processing Novel methods of process/plant development and operation... 

Among hybrid separations not involving membranes, adsorptive distillation (87) offers interesting advantages over conventional methods. In this technique a selective adsorbent is added to a distillation mixture. This increases separation ability and may present an attractive option in the separation of azeotropes or close-boiling components. Adsorptive distillation can be used, for instance, for the removal of trace impurities in the manufacturing of fine chemicals (it may allow for switching some fine chemical processes from batchwise to continuous operation). 

Reactive and Hybrid Separations Incentives, Applications, Barriers... 

Extractive distillation is probably the oldest and most widely applied type of hybrid separation, particularly useful in close-boiling-point problems or in systems in which components form azeotropes. In the method, an extra component (solvent) is added to the system, which does not form azeotropes with feed components. The solvent alters the relative volatility of original feed components, allowing one to distill overhead. The solvent leaves the column with the bottom products and is separated in a binary column. Energy savings represent the most important advantage of extractive distillation over the conventional (nonhybrid) separation methods (168,169). 

Originally, extractive distillation was limited to two-component problems. However, recent developments in solvent technology enabled applications of this hybrid separation in multicomponent systems as well. An example of such application is the BTX process of the GTC Technology Corp., shown in Figure 6, in which extractive distillation replaced the conventional liquid-liquid extraction to separate aromatics from catalytic reformate or pyrolysis gasoline. This led to a ca. 25% lower capital cost and a ca. 15% decrease in energy consumption (170). Some other examples of existing and potential applications of the extractive distillations are listed in Table 6. 

Although considered by some authors a novel process, adsorptive distillation is a relatively old hybrid separation, originating in the early 1950s (196). It is a... 

Despite an almost 50-year history, no large-scale commercial processes using adsorptive distillation have been reported so far. Some potential application fields for this hybrid separation are listed in Table 7. 

Despite many ongoing research activities in the field and a number of successful commercializations, there still exist numerous technical and nontechnical barriers that hinder a wider introduction of reactive and hybrid separations into industrial practice. Two workshops held in 1998 by the Center for Waste Reduction Technologies of AIChE (305) identified some of the barriers for reactive separations and divided them into three categories ... 

Stankiewicz and Moulijn further divide PI into two areas equipment and methods. They give an extensive list of equipment examples. The methods are subdivided into multifunctional reactors, hybrid separations, alternative energy sources, and other methods. 

In the above equation, the hybrid is clearly broken down into a real part (second line), and an imaginary part (third line). We have found it convenient to analyze these two parts of the hybrid separately because of the earlier mentioned property that the real part of a hybrid will not mix with the imaginary part when computing expectation values and densities. 

Figure 28.9. RNA-DNA Hybrid Separation. A structure within RNA polymerase forces the separation of the RNA-DNA hybrid, allowing the DNA strand to exit in one direction and the RNA product to exit in another.

Sommer S and Melin T. Design and optimization of hybrid separation processes for the dehydration of 2-propanol and other orgnanics. Ind. Eng. Chem. Res. 2004 43(17) 5248-5259. 

Ray R, W) tcherley RW, Newbold D, McCray S, Friesen D, and Brose D. Synergistic, membrane-based hybrid separation systems. JMem Sci, 1991 62(3) 347-369. 

S. Sommer, T. Melin, 2004, Design and Optimization of Hybrid Separation Proeesses for the Dehydration of 2-Propanol and Other Organics, Ind. Eng. Chem., vol. 43, 5248-5259. 

RNA—DNA hybrid separation by a structure within RNA polymerase Figure 29.9... 

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