Processes for Separating Azeotropic Mixtures

Azeotropes form a barrier to distillation as the concentrations of vapor and liquid are the same at the azeotropic point. Thus, no driving force for intetfacial mass transfer exists at the azeotrope. Fractionation of azeotropic mixtures is only possible if either the concentration of the azeotrope is changed by any means, or alternative separation processes, which can overcome azeotropes, are combined with distillation. 

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PV has gained widespread acceptance in the chemical industry as an effective process for separating azeotropic mixtures, for example, separation of... 

Because pervaporation is suitable for separation azeotropic mixtures, such as dehydration of an azeotropic mixture of ethanol-water (ca. 94%), economic comparison of the process with distillation has been reported.228 Besides separation of azeotropic mixtures of organic solvents, dehydration of nitric acid (azeotropic point ca. 68 wt.%) has been tried using a perfluorocarbon ion exchange membrane for the chlor-alkali process nitric acid is concentrated up to... 

Pervaporation is a separation technique that has several advantages over other separation methods such as distillation, extraction, and sorption. It is an especially good process for certain azeotropes and mixtures that have components with similar boiling points. Also, pervaporation can be performed at low temperatures and the membranes involved generally do not need to be regenerated in any way.[161]... 

Fullarton and Schlunder (1983) investigated the process of diffusional distillation for separating liquid mixtures of azeotropic composition. The process is shown schematically in Figure 8.8. A liquid mixture is evaporated at a temperature below its boiling point, diffuses through a vapor space filled with inert gas and condenses at a lower temperature. The inert gas functions as a selective filter that allows preferential passage of those components that diffuse more quickly. Thus, the condensed liquid has a composition different from that of the original mixture. 

In Example 8.3.2 we determined the rates of mass transfer in diffusional distillation, a process described by Fullarton and Schliinder (1983) for separating liquid mixtures of azeotropic composition. Estimate the heat flux through the gas/vapor mixture under the conditions prevailing in the experiment described in Example 8.3.2. 

Except for pressure swing distillation, which makes use of the pressure dependence of the azeotropic composition, all technologies require an entrainer to separate azeotropic mixtures. For some systems the azeotrope vanishes at certain pressures. This means that ordinary distillation at a different pressure (pressure or vacuum distillation) may circumvent the azeotrope. Although pressure swing distillation does not require an additional component in the process, it is not necessarily environmentally advantageous when compared to the alternatives, since additional... 

Knajqi, JJ., and M.F. Doherty, A New Pressure-Swing Distillalioi Process for Separating Homogeneous Azeotropic Mixtures, bid. Ei. Chem. Res., 31,346 (1992). 

The recovery of pure aromatics from hydrocarbon mixtures is not possible using distillation process because the boiling points of many non-aromatics are very close to benzene, toluene, etc. Also, azeotropes are formed between aromatics and aliphatics. Three principle methods are used for separation azeotropic distillation, liquid-liquid extraction, and extractive distillation. Three major commercial processes have been developed for separation Udex, Sulpholane, and Arosolvan. Over 90% plants now use one of these processes. Each use an addition of solvent such as a mixture of glycols, tetramethylene sulfone, or N-methyl-2-pyrrolidone to aid in the extraction of aromatics. This occurs with high precision and efficiency. Pure benzene, toluene, and xylene are produced by these processes. 

In Fig. 2-30, this rectification separation process, in two columns operated at two different pressure levels, is explained as a tv/o pressure process for a binary mixture. The binary mixture consists of components 1 and 2, with mole fraction Xp of the low-boiling component 1. In the first column, operated at a lower pressure Pqj, the binary mixture is separated into component 2 as the bottom product, and an azeotropic mixture of composition, as an overhead product. In the second column, operated at a pressure Pg2 > Pgi l he azeotropic mixture is separated into component 1 (at the bottom) and azeotropic mixture x 2 the top). The azeotropic mixture of the second column is then fed into the side of column 1 at an appropriate location. 

We examine separation of the mixtures, concentration space of which contains region of existence of two hquid phases and points of heteroazeotropes. It is considerably easier to separate such mixtures into pure components because one can use for separation the combination of distillation columns and decanters (i.e., heteroazeotropic and heteroextractive complexes). Such complexes are widely used now for separation of binary azeotropic mixtures (e.g., of ethanol and water) and of mixtures that form a tangential azeotrope (e.g., acetic acid and water), adding an entrainer that forms two liquid phases with one or both components of the separated azeotropic mixture. In a number of cases, the initial mixture itself contains a component that forms two liquid phases with one or several components of this mixture. Such a component is an autoentrainer, and it is the easiest to separate the mixture under consideration with the help of heteroazeotropic or heteroextractive complex. The example can be the mixture of acetone, butanol, and water, where butanol is autoentrainer. First, heteroazeotropic distillation of the mixture of ethanol and water with the help of benzene as an entrainer was offered in the work (Young, 1902) in the form of a periodical process and then in the form of a continuous process in the work (Kubierschky, 1915). 

Besides the methods illustrated so far in this book, there are other ways for separating azeotropes. One way is to react the azeotrope away in a reactive distillation column to form other useful products. The design and control of various reactive distillations have been extensively studied in a recent book by Luyben and Yu. Another way commonly used in ethanol dehydration is to use the hybrid distillation-adsorption process. In this process, distillation is used to purify the mixture to a composition near the ethanol-water azetrope, and then an adsorption unit (e.g., molecular sieves) is used to adsorb the remaming water so that anhydrous ethanol can be obtained. The key technology in this process is the performance of the adsorbent material in removing water from the mixture and is beyond the scope of this book. 

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. 

Even though the simple distillation process has no practical use as a method for separating mixtures, simple distillation residue curve maps have extremely usehil appHcations. These maps can be used to test the consistency of experimental azeotropic data (16,17,19) to predict the order and content of the cuts in batch distillation (20—22) and, in continuous distillation, to determine whether a given mixture is separable by distillation, identify feasible entrainers/solvents, predict the attainable product compositions, quaHtatively predict the composition profile shape, and synthesize the corresponding distillation sequences (16,23—30). By identifying the limited separations achievable by distillation, residue curve maps are also usehil in synthesizing separation sequences combining distillation with other methods. 

Fig. 17. Column sequence for separating a binary heterogeneous azeotropic mixture, and B, where represents the process feed mole fraction, (a)...

A mixture of acetone and chloroform is to be separated into pure products [Hostrup et al. (1999)]. Since they also form an azeotrope, one alternative to satisfy the separation objective is to find a suitable solvent for separation by extractive distillation. This type of problem in product design is usually encountered during the purification or recovery of products, by-products, reactants or removal of undesirable products from the process. Also, it can be noted that failure to find a suitable solvent may result in the discard of the product. Alternatively, a functional chemical product manufacturer may be interested to find, design and develop a new solvent. In this case, the solvent is the chemical product. 

Another field in which azeotropic distillation is finding application is in the separation of the complex mixtures of organic acids, aldehydes, ketones, and alcohols produced by the Hydrocol process. As petroleum stocks are utilized more and more for the production of chemicals, processing of azeotropic mixtures and the use of azeotropic separations should assume increasingly greater importance. 

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