Azeotropic distillation maximum

All these distillations are conducted in the presence of a polymerization inhibitor (200 ppm of hydroquinone or its monoethyl ether), introduced at the irifiix level, and in moderate bottom temperature conditions (100 to 110°C maximum), and hence under reduced pressure if necessary. The different aqueous phases, especially the water extract of the alcohol, are stripped to recover tbe ternary acrylate/alcohol/watcr azeotrope at the top, which is recycled to the extraction step (case of butanol in particular), and a water/alcohol mixture at the bottom, which may be separated by azeotropic distillation. The molar yield of esterification is 90 to 95 per cent in relation to acryfic arid. 

If a mixture of ethanol and water is distilled, eventually it will form a solution that is 95 % ethanol regardless of the starting composition. Hydrochloric acid and water will form a 20.22 % HCl solution, and chloroform and acetone will form a 65.5 % CHCI, solution. These solutions are called azeotropes (Gr a + zeo to boil, trope more at). Webster s dictionary defines an azeotrope as a liquid mixture that is characterized by a constant maximum or minimum boiling point which is lower or higher than that of any of the components and that distills without change in composition. An azeotropic distillation involves the formation of an azeotrope with at least one of the components of a liquid mixture, which then can be separated more readily because of the resulting increase in the difference between the volatilities of the components of the mixture. Figure 4-1, p. 44, shows the water-ethanol system (A) and the HCl-water system (B). 

Athermal solution, 184 Auxiliary operations, 3 Availability function, 685 Azeotropes, distillation, 9 hetoogeneous, 101 homogeneous, 101 maximum-boiling, 99,208-209 minimum-boiling, 98,100,207-208 thermodynamic activity coefficients, 208 Azeotropic distillation, 9,13... 

Mixtures forming an azeotrope with maximum, boiling point distillate the comj)o-nent in excess, pure bottom product azeotropic mixture of the two components. 

For the production of fuel-grade ethanol, the ethanol has to be dried . Anhydrous ethanol cannot be produced by simple distillation because ethanol forms an azeotropic mixture with water. The maximum ethanol content achievable by distillation is approximately 97.2vol.%, which is usually not sufficient for the application as fuel-ethanol. The residual water can be removed either by azeotropic distillation by the addition of, e.g., cyclohexane or by the application of molecular sieves. Today, state-of-the-art plants operate with molecular sieves which provide considerable advantages in terms of investment and operating costs. 

Since the decanter forms part of the reflux loop it is important to keep the distillate s residence time in it as small as possible in batch distillation. As Fig. 7.1 shows, the droplet size generated by azeotropic distillation including condensation and subsequent cooling processes is very small and without accelerated coalescing an undesirably large decanter is needed to get the maximum phase split. 

Some indication of at least the maximum size of the silicate species in a solution of sodium silicate of 3.3 ratio SiOjrNajO has been obtained from the nature of the corresponding silicic acid ester (69). Silicic acid of low molecular weight is so unstable in aqueous solutions that any attempt to isolate it by evaporation of water, even at ordinary temperature, results in rapid polymerization to a gel. The direct esterification of silicic acid therefore remained impractical until the discovery of Kirk (70) of a method for transferring silicic acid of low molecular weight from aqueous solution to solution in an alcohol. This transfer is accomplished by extraction of the acid with a suitable polar organic solvent, simultaneously saturating the aqueous phase with sodium chloride in order to salt the silicic acid into the organic phase. An alcohol such as rt-butyl alcohol is then added, and esterification is effected by azeotropic distillation of water from the alcohol solution (29). 

Luyben W. L., Control of maximum-boiling acetone/chloroform azeotropic distillation system, Ind. Engng. Chem. Res., 47, 6140-6149 (2008). 

Some azeotropic systems exhibit the desirable property that pressure has a strong effect on the composition of the azeotrope. When this occurs, a two-column system can be used to achieve the desired separation. The basic idea is to operate one column at low pressure and a second column at high pressure. One of the components comes out from one end of the low-pressure column. If the azeotrope is minimum boiling, the azeotrope will come out of the top and the product stream out the bottom. The composition of this distillate stream will be close to that of the azeotrope at the low pressure. If the azeotrope is maximum boiling, the azeotrope will come out the bottom and the product stream out the top. The azeotrope is then fed to the second high-pressure column in which a similar separation occurs, except now the other component is removed from one end of the column and a stream with composition close to the high-pressure azeotrope is removed from the other end. This azeotropic stream is fed back to the low-pressure column. 

Generally more favorable for maximum boiling azeotrope because the recycles between columns are bottoms streams, pure products are distillates recycle not as energy-intensive, products distilled once. 

All extractive distillations correspond to one of three possible residue curve maps one for mixtures containing minimum boiling azeotropes, one for mixtures containing maximum boiling azeotropes, and one for nonazeotropic mixtures. Thus extractive distillations can be divided into these three categories. 

Fig. 10. Residue curve map for separating a maximum boiling azeotrope using a high boiling solvent where (-----------------) represents the distillation boundary and...

An azeotrope limits the separation that can be obtained between components by simple distillation. For the system described by cui ve B, the maximum overhead-product concentration that could be obtained from a feed with X = 0.25 is the azeotropic composition. Similarly, a feed with X = 0.9 could produce a bottom-product composition no lower than the azeotrope. 

---