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Azeotropic distillation is a special type of distillation for separating components of azeotropic mixtures by adding an entrainer that changes the relative volatility of the substances.
Also called constant boiling mixtures, azeotropes have the same vapor and liquid concentration — which means no one component is more volatile than the other. Hence, simple or fractional distillation alone cannot purify azeotropic mixtures.
Azeotropes are non-ideal mixtures that deviate from Raoult’s Law. At the azeotropic point, the mixture reaches maximum purity because it distills like a single compound. A classic example would be a 95.6 wt% ethanol solution which boils at 78.2°C.
Distilling an azeotropic mixture beyond the azeotropic point is possible, although this process utilizes alternative cost-intensive separation techniques like pressure swing distillation.
When an entrainer changes the relative volatility of the azeotropic mixture, the new azeotrope either forms either a maximum boiling (left) or minimum boiling azeotrope (right).
A minimum boiling azeotrope is a positive deviation from Raoult’s Law. The lowest point on the VLE phase diagram means the boiling point of the azeotrope is lower than the boiling point of the pure components. For example, ethanol (bp at 78.5°C) – water (bp at 100°C) solution boils at 78.2°C.
Meanwhile, a maximum boiling azeotrope is a negative deviation from Raoult’s Law. The highest point on the VLE phase diagram means the boiling point of the azeotrope is higher than the boiling point of the pure components. For example, nitric acid (bp at 83°C) – water (bp at 100°C) solution boils at 120.5°C.
Adding an entrainer can further enhance the purity of an azeotropic component. An entrainer is a substance that breaks the feed azeotropic mixture and forms a new azeotrope with the less volatile component.
There are 2 types of azeotropic distillation depending on how the entrainer interacts with the feed mixture:
Comparing the two, homogenous is much easier to operate and can outperform heterogeneous azeotropic distillation [1]. The latter shows high sensitivity to operating conditions which can cause the column to fail with just small pressure fluctuations or shutdown with small losses in the decanter. However, heterogenous is used more industrially, such as denatured or fuel-grade ethanol production.
Azeotropic distillation is fractional distillation with pre and post-treatment processes.
First, the entrainer mixes with the feed mixture as it enters the fractionating column. The minimum boiling azeotrope that forms with the entrainer vaporizes first as the distillate. Otherwise, if a maximum boiling azeotrope forms with the entrainer, the other more volatile component of the original azeotrope (feed) vaporizes first.
The post-treatment includes purifying the new azeotrope with another separation method (usually a decanter for heterogeneous azeotropic distillation) to further strip the entrainer for recycling.
The most common application of azeotropic distillation is the dehydration of ethanol. The process flow goes like this:
In this process flow scheme, the feed mixture consists of 92.4 wt% ethanol [2]. The entrainer, usually benzene or toluene, mixes with the feed azeotrope. Toluene is preferred since benzene has carcinogenic properties. Toluene forms a new minimum boiling azeotrope with water at 20.2 wt% that boils at 80.5°C.
The ternary azeotrope containing water vapor, toluene, and small amounts of ethanol exits at the top of the fractionating column as the distillate and proceeds to the decanter for further purification. Meanwhile, the purified ethanol (> 99.5 wt%) exits as the bottoms product
The ternary azeotrope separates into 2 layers — the organic and aqueous phases. The organic layer (toluene-ethanol stream) at the top returns back to the azeotropic column. Meanwhile, the aqueous layer (toluene-water stream) exits at the bottom of the decanter and proceeds to the stripping column.
Through fractional distillation, the aqueous layer (toluene-water stream) is separated for recovery of the distillate and bottoms. The toluene-rich stream distillate is recovered and recycled back to the azeotropic column. The bottoms product is mostly water which can proceed to the wastewater treatment plant to be safely discharged into the environment.
Overall, the critical step in the azeotropic distillation is choosing the entrainer. It must be specific to the azeotropic mixture to be separated. Aside from benzene and toluene, other suitable entrainers for ethanol-water azeotrope are cyclohexane, diethyl ether, acetone, pentane, hexane, heptane, and isooctane [3].
Choosing the appropriate entrainer and its specific amount dictates the design process of the azeotropic distillation. There are 3 types of entrainers:
Below is just a general checklist guide of what process engineers usually look for in an entrainer:
Analysis tools can further screen the list of suitable entrainers and find which one is the most feasible. VLE phase diagrams are essential to determine the behavior of the azeotrope, particularly the azeotropic point.
Then, another important screening analysis is the residue curve map (RCM) analysis. According to Doherty & Malone, RCM is a geometric representation of VLE phase behaviors for ternary mixtures [4]. It is used to determine whether the selected entrainer can feasibly separate the azeotrope.
The main advantage is achieving the highest purity achievable for either azeotropic components where conventional distillation is limited.
On the other hand, there are disadvantages especially when it comes to capital and operations costs because of the added purification and recovery steps. Also, another disadvantage is the high specificity of the entrainer’s use. Its boiling point doesn’t only have to be in between the pure components of the azeotrope; it must also form a new azeotrope within the system.
Azeotropic and extractive distillation are similar in the way that a third component is added to separate the feed mixture through distillation. But the main difference is the purpose of the said third component.
In azeotropic distillation, the third component is called the entrainer which must form an azeotrope with one of the feed components. On the other hand, extractive distillation introduces a solvent that doesn’t have to form an azeotrope with any of the feed components.
Azeotropes are also called constant boiling mixtures and they can be separated through azeotropic distillation. It can either be done homogeneously or heterogeneously.
Benzene (bp at 80.1°C) forms a minimum boiling azeotrope with water at 91.1 wt% benzene at a constant boiling temperature of 69.4°C. It’s a good candidate entrainer for the ethanol-water solution because vaporization starts with lesser heat requirements. Also, it is widely available in the market because benzene has a lot of industrial uses.
There is only one caveat — benzene is carcinogenic. To avoid unfavorable health effects, alternatives like toluene are being used instead.
The azeotropic point is the intersection point found in the VLE phase diagram of the binary mixture. This point represents the constant boiling temperature at a fixed composition. Beyond this point, conventional distillation cannot separate the mixture anymore. It already calls for special separation techniques such as changing the pressure conditions or adding an entrainer.
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