Ternary ethanol/water/entrainer

For all these reasons, the separation stage is important when selecting an entrainer. The various settling characteristics of possible ternary azeotropes formed in the ethanol/water/entrainer combination illustrate the problems (Table 7.4). 

Podebush Sequence forPthanol—Water Separation. When ethyl acetate is used as the entrainer to break the ethanol—water azeotrope the residue curve map is similar to the one shown in Figure 21d, ie, the ternary azeotrope is homogeneous. Otherwise the map is the same as for ethanol—water—benzene. In such... 

There are several studies in literature about the phase equilibria of the ternary system of ethanol-water-CC>2 [2-12] and only one study exits for the quaternary system of ethanol-water-CC>2-entrainer [13], In general, these studies showed that the extract concentration was positively influenced with increasing the extraction pressure and initial ethanol concentration and with decreasing the extraction temperature. 

A second example of the use of liquid-liquid immiscibilities in an azeotropic distillation sequence is the separation of the ethanol-water minimum-boiling homogeneous azeotrope. For this separation, a number of entrainers have been proposed, which are usually chosen to be immiscible with water and form a ternary minimum-boiling... 

Purifying ethanol from ethanol/water solution by simple distillation is limited by the formation of a minimum-boiling azeotrope containing 90.37 mole% ethanol at 78.14°C, 760 mmHg. Raising the ethanol concentration at 100 kPa above 90.37 mole%, or dehydrating it, can be accomplished by adding to the distillation column an entrainer to form a ternary azeotrope. 

Using benzene as the entrainer to the ethanol/water solution forms a ternary azeotrope at 100 kPa, 64.86°C, containing 22.81 mole% ethanol, 23.32 mole% water, and 53.87 mole% benzene. By adding benzene to the distillation column, the ternary azeotrope, with its lower boiling point than that of ethanol at 78.4°C, leaves the column in the distillate, and purified ethanol is recovered in the bottoms. 

Azeotropic distillation involves either an embedded azeotrope, present in the feed mixture, or a contrived azeotrope, formed by the addition of an extraneous component called an entrainer. Benzene-water may be separated into high-purity benzene and the benzene-water azeotrope this is frequently practiced to remove water from benzene when very dry benzene is needed for chemical processing. More commonly encountered are distillation separations that are enhanced through the addition of an entrainer to form an azeotrope. Perhaps the best known separation of this type is the production of anhydrous ethanol from the ethanol-water azeotrope. Here, benzene is added as the entrainer, with the result that a low-boiling ternary azeotrope is formed between benzene, ethanol, and water. This permits the higher-boiling ethanol to be taken from the bottom of the column. The distillate condenses to a heterogeneous mixture of benzene and alcohol-water phases. 

That benzene is an effective entrainer can be confirmed by comparing the ethanol/water molar ratio in the binary and ternary azeotropes ... 

The ethanol column (C-1) has practically only stripping zone. Fig. 9.31-left shows composition profile both for liquid and vapour phase. The examination of the composition profiles highlights the role of the entrainer. In the zone close to the top the benzene extracts the ethanol in the liquid phase, and as a result increases the volatility of water, so that on lower stages the water is completely removed. In the lower part practically only the binary ethanol/benzene remains. The distillation trajectory starts from the ternary azeotrope, goes along the ethanol/ benzene saddle and terminates in the ethanol vertex. Because the boiling point of the azeotrope ethanol-water is close to the pure ethanol, the profile could easily jump to the ethanol/water azeotrope. Consequently, the design and operation of the column (C-1) is very sensitive. 

Table 7.4 Settling characteristics of water entrainers in ternary azeotropes with ethanol Density of...

Whether an azeotrope is a binary or a ternary it is desirable that it should be fractionated easily from the other component(s) of the system. In the absence of vapour/liquid data the boiling point gap is the best indication of how easy the split is. The comparative complexity of the column contents can be illustrated by the ethanol/water system with cyclohexane added as a dewatering entrainer (Table 7.5). 

Water and ethanol form a low boiling point azeotrope. So, water cannot be completely separated from ethanol by straight distillation. To produce absolute (100 per cent) ethanol it is necessary to add an entraining agent to break the azeotrope. Benzene is an effective entrainer and is used where the product is not required for food products. Three columns are used in the benzene process. Column 1. This column separates the ethanol from the water. The bottom product is essentially pure ethanol. The water in the feed is carried overhead as the ternary azeotrope of ethanol, benzene and water (24 per cent ethanol, 54 per cent benzene, 22 per cent water). The overhead vapour is condensed and the condensate separated in a decanter into, a benzene-rich phase (22 per cent ethanol, 74 per cent benzene, 4 per cent water) and a water-rich phase (35 per cent ethanol, 4 per cent benzene, 61 per cent water). The benzene-rich phase is recycled to the column as reflux. A benzene make-up stream is added to the reflux to make good any loss of benzene from the process. The water-rich phase is fed to the second column. 

Eresh aqueous ethanol feed is first preconcentrated to nearly the azeotropic composition in column C3, while producing a water bottoms product. The distillate from C3 is sent to column Cl, which is refluxed with the entire organic (entrainer-rich) layer, recycled from a decanter. Mixing of these two streams is the key to this sequence as it allows the overall feed composition to cross the distillation boundary into Region II. column Cl is operated to recover pure high-boihng node ethanol as a bottoms product and to produce a distillate close to the ternary azeotrope. If the ternary azeotrope is heterogeneous (as it... 

Two of the three composition goals, pure DEM and pure water, have been met and by mass balance, sufficient water has been regenerated to serve as the mixing agent for dragging the ternary azeotrope into the two-phase region (actually, the extra water is first put in the system, then continually recycled as is the entrainer in the previous example). A third composition near the ethanol-... 

Assuming perfect distillation, with pure ethanol in the bottoms product and only the ternary azeotrope in the distillate, find the entrainer to feed flow rate ratio if the ethanol solution feed stream contains 90 mole% ethanol and 10 mole% water. 

It may be observed that the ternary azeotrope m falls Inside the heterogeneous region. Thus, an overhead vapour of this composition splits by decantation in two phases, one rich in entrainer o other in water w the ethanol being distributed in both. Moreover, o, and w, are in different distillation regions. By clever mixing with other streams, these streams can produce feasible feeds for ethanol and water recovery columns, by overcoming the constraints of the distillation boundaries. Hence, liquid-liquid decantation creates opportunities for the separation of an azeotropic mixture. 

Ethanol and water cannot be separated by conventional distillation at atmospheric pressure due to the formation of a minimum boiling azeotrope at 172.V°F. However, benzene can be added to this system as an entrainer, which produces a ternary azeotrope boiling at 148.7°F. This ternary azeotrope is condensed and separated into a benzene-rich phase... 

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