Ethanol dehydration, entrainer

Fig. 28. Ethanol dehydration Mass balance lines for entrainer stripper and water column.

One method for ethanol dehydration is heterogeneous azeotropic distillation, which has been used for many decades. A suitable light entrainer component (benzene, cyclohexane, isooctane, ethylene glycol, and so on) is added to modify the relative volatilities. The water is driven overhead with the entrainer and a high-piu ity ethanol bottoms stream is produced in the azeotropic column. The overhead vapor is condensed and fed to a decanter. The organic phase is refluxed back to the column. The aqueous phase is fed to another column that produces a bottoms product of high-purity water and a distillate that is recycled back to the azeotropic column. A third column in the front end of the process is used to preconcentrate the low-concentration stream from the fermenter up to a concentration closer to the azeotrope before feeding this into the azeotropic column. 

Although benzene is a more favorable entrainer for this separation or similar ethanol dehydration process, its use is prohibited because of the carcinogenic nature of this entrainer. Having insight into what constitutes a good entrainer will help to identify other potential candidates as entrainers. 

The methods used here to give the phase equilibria are reviewed, and the Azeotropic Distillation Program ADP/ADPLLE is described. Application of the program to calculate an azeotropic distillation problem is shown and discussed, and a sample computer output is given and is briefly discussed. Finally, calculated azeotropic distillation results are compared for dehydrating aqueous ethanol for the three entrainers, n-pentane, benzene, and diethyl ether. 

Dehydration of Aqueous Ethanol Using n-Pentane as Entrainer... 

For the azeotropic dehydration of aqueous ethanol mixtures approaching the constant boiling mixture, a brief comparison is shown for the entrainers, n-pentane, benzene, and diethyl ether. Since water is most non-ideal in n-pentane, the driest ethanol is expected to be produced if n-pentane is used. 

Figure 7. Comparing the entrainers, n-pentane, benzene, and diethyl ether, for reboiler and condenser loads in dehydrating aqueous ethanol...

Distillation, under nitrogen, with toluene as entrainer. This leads to a low-boiling azeotrope, the effect being similar to the well-known dehydration of ethanol with benzene or cyclohexane. 

These developments will have a wide impact. Reaction enhancement will be a major beneficiary, but a look at the simpler field of solvent dehydration shows that the innovation process is very application dependant. Pervaporation (with vapor permeation) is progressively displacing other techniques in solvent dehydration. Replacing entrainer distillation for drying ethanol and isopropanol, pervaporation at initial stages is always now preferred to techniques, where a third component must be added to shift equlibria. The handling of entrainers and/or calcium chloride or caustic with the attendant environmental risks and costs is no longer a viable option. 

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. 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

Isopropanol, ethanol Standard applications for pervaporation, typically dehydrated from their azeotropes to fractions of a percent of water. De-bottlenecking of entrainer plants. 

It is desired to locate a solvent for the liquid-liquid extraction of ethanol from its azeotrope with water. This dehydration has been carried out principally by heterogeneous azeotropic distillation using benzene, now known to be a carcinogen, as an entrainer. If such a solvent can be located, liquid-liquid extraction could become the preferred processing technique. 

Figure 732 Dehydration of ethanol using toluene as an entrainer (a) process flow diagram (b) temaiy composition diagram (c) T-x-y agram at 1 atm (d) x-y diagranal 1 atm (Stichlmair et al., 1989).

In Chapters, the steady-state design of a heterogeneous azeotropic distillation process for the dehydration of ethanol using benzene as a light entrainer was studied. The process consisted of two distillation columns, one decanter and two recycle streams. One of the recycle streams was successfully closed, but the second would not converge using steady-state Aspen Plus. 

FIGURE 5,5-1 Dehydration of alcohol by azeotropic distillation, using benzene as entrainer to form i low-boiling ethanol-water-benzene azeotrope. 

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