Extractive Distillation for Azeotrope Breaking

Reference ID: MET-899A | Process Engineering Reference Sheets Calculation Guide

Introduction & Context

Extractive distillation is a critical unit operation in process engineering used to separate components with close boiling points or those that form azeotropes, such as the ethanol‑water system; for a deeper understanding of how to identify and characterize azeotropes, see our guide on azeotrope identification and characterization. By introducing a high‑boiling, non‑volatile solvent—known as an entrainer—the relative volatility of the components is altered, effectively “breaking” the azeotrope.

In this process, the main column separates the ethanol from the water‑entrainer mixture. A secondary recovery column, described in our wine and spirits distillation column design, is subsequently employed to separate the water from the entrainer (glycerol), allowing the solvent to be recycled back to the main column. This configuration is essential for achieving high‑purity anhydrous products while maintaining process economy through solvent recovery.

Methodology & Formulas

The design methodology relies on the Fenske equation to determine the minimum number of theoretical stages (N_min) required for a specified separation. The relative volatility (α) is modified by the presence of the entrainer, which is determined via activity coefficient models such as NRTL or UNIQUAC.

The minimum stages for the main column are calculated based on the ethanol-water separation:

\[ N_{min,main} = \frac{\log\left[ \left( \frac{x_{D,E}}{x_{B,E}} \right) \cdot \left( \frac{1 - x_{B,E}}{1 - x_{D,E}} \right) \right]}{\log \alpha_{E/W}} \]

The minimum stages for the recovery column, separating water (light key) from glycerol (heavy key), are calculated as follows:

\[ N_{min,rec} = \frac{\log\left[ \left( \frac{x_{D,W}}{1 - x_{D,W}} \right) \cdot \left( \frac{x_{B,G}}{1 - x_{B,G}} \right) \right]}{\log \alpha_{W/G}} \]

The solvent flow rate is determined by the specified solvent-to-feed ratio:

\[ \dot{n}_{G} = \dot{n}_{F} \cdot \left( \frac{S}{F} \right) \]

Parameter	Constraint/Regime
Relative Volatility (α_E/W)	3.0 ≤ α_E/W ≤ 10.0
Relative Volatility (α_W/G)	α_W/G ≥ 100.0
Solvent-to-Feed Ratio (S/F)	0.3 ≤ S/F ≤ 1.0 (molar basis)
Recovery Column Temperature	T < 200°C (to prevent thermal degradation of glycerol)

To select an effective solvent for breaking an azeotrope, process engineers should evaluate the following criteria:

High selectivity to increase the relative volatility of the key components.
Low volatility to ensure the solvent remains in the liquid phase and can be easily recovered.
Thermal stability at the operating temperatures of the reboiler.
Low viscosity to minimize heat transfer resistance and pumping costs.
Compatibility with the feed mixture to prevent unwanted side reactions or phase separation.

Managing the solvent-to-feed ratio is critical for maintaining product purity. Key challenges include:

Maintaining a consistent solvent concentration across the trays to prevent the azeotrope from reforming.
Managing the recycle loop dynamics between the extractive column and the solvent recovery column.
Controlling the temperature profile to ensure the solvent does not carry over into the distillate product.

The solvent-to-feed ratio directly influences the heat duty of the system in several ways:

Higher ratios generally improve separation efficiency but increase the sensible heat required to heat the solvent.
Increased solvent flow rates require larger reboiler duties in the solvent recovery column to strip the solvent from the product.
Optimizing the ratio is a trade-off between capital expenditure for column height and operational expenditure for steam consumption.

Worked Example: Sizing an Extractive Distillation System

Given:

Feed: 100 kmol/h ethanol-water mixture near the azeotropic composition
Feed composition: \(x_{F,E} = 0.85\) (mole fraction ethanol), \(x_{F,W} = 0.15\)
Required distillate ethanol purity: \(x_{D,E} = 0.995\)
Maximum allowable ethanol in bottoms: \(x_{B,E} = 0.05\)
Relative volatility with entrainer (ethanol-water): \(\alpha_{E/W} = 5.0\)
Solvent-to-feed ratio (molar): S/F = 0.5
Recovery column distillate water purity: \(x_{D,W} = 0.999\)
Recovery column bottoms glycerol purity: \(x_{B,G} = 0.998\)
Relative volatility for water-glycerol: \(\alpha_{W/G} = 500\)

Step 1: Minimum Theoretical Stages for Main Column

Using the Fenske equation:

\[ N_{min,main} = \frac{\log\left[\left(\frac{x_{D,E}}{x_{B,E}}\right) \cdot \left(\frac{1 - x_{B,E}}{1 - x_{D,E}}\right)\right]}{\log \alpha_{E/W}} \]

Substitute values:

\[ \frac{x_{D,E}}{x_{B,E}} = \frac{0.995}{0.05} = 19.9 \qquad \frac{1 - x_{B,E}}{1 - x_{D,E}} = \frac{0.95}{0.005} = 190 \]

\[ N_{min,main} = \frac{\log(19.9 \times 190)}{\log(5.0)} = \frac{\log(3781)}{\log(5.0)} = \frac{3.5777}{0.6990} = 5.12 \;\text{stages} \]

Step 2: Solvent Flow Rate

\[ \dot{n}_G = \dot{n}_F \cdot \left(\frac{S}{F}\right) = 100 \times 0.5 = 50 \;\text{kmol/h} \]

Step 3: Minimum Theoretical Stages for Recovery Column

Using the Fenske equation for water (light key) and glycerol (heavy key):

\[ N_{min,rec} = \frac{\log\left[\left(\frac{x_{D,W}}{1 - x_{D,W}}\right) \cdot \left(\frac{x_{B,G}}{1 - x_{B,G}}\right)\right]}{\log \alpha_{W/G}} \]

Substitute values:

\[ \frac{x_{D,W}}{1 - x_{D,W}} = \frac{0.999}{0.001} = 999 \qquad \frac{x_{B,G}}{1 - x_{B,G}} = \frac{0.998}{0.002} = 499 \]

\[ N_{min,rec} = \frac{\log(999 \times 499)}{\log(500)} = \frac{\log(498501)}{\log(500)} = \frac{5.6979}{2.6990} = 2.11 \;\text{stages} \]

Summary of Results:

Parameter	Value
Minimum stages, main column (N_min,main)	5.12
Glycerol solvent flow rate (ṅ_G)	50 kmol/h
Minimum stages, recovery column (N_min,rec)	2.11

Note: These are minimum theoretical stages. Actual stages will be higher after applying stage efficiency factors. The Fenske equation assumes constant relative volatility and total reflux conditions.

"Un projet n'est jamais trop grand s'il est bien conçu." — André Citroën

"La difficulté attire l'homme de caractère, car c'est en l'étreignant qu'il se réalise." — Charles de Gaulle

Extractive Distillation for Azeotrope Breaking

Introduction & Context

Methodology & Formulas

Worked Example: Sizing an Extractive Distillation System

🚀 Skip the Manual Math!