🎓 Lesson 6
D3
Azeotropes: Identification, Breaking Strategies & Entrainer Screening
An azeotrope is a mixture of two or more liquids that boils at a constant temperature and composition — like a 'chemical twin' that distills as if it were a single pure substance.
🎯 Learning Objectives
- ✓ Explain why azeotropes prevent complete separation via simple distillation using phase diagrams and vapor–liquid equilibrium (VLE) data
- ✓ Analyze binary VLE data to identify azeotropic composition and temperature, and classify it as minimum- or maximum-boiling
- ✓ Design a pressure-swing or entrainer-based separation strategy for a given azeotropic system using UNIFAC or Wilson activity coefficient predictions
- ✓ Screen and rank candidate entrainers for ethanol–water separation using selectivity, volatility, and immiscibility criteria
📖 Why This Matters
In mining and mineral processing, solvent recovery—especially from hydrometallurgical leachates or reagent recycling streams—often involves separating azeotropic mixtures like ethanol–water (used in gold elution) or acetone–chloroform. Ignoring azeotropes leads to failed distillation columns, excessive energy use, and off-spec product. Understanding how to detect and break them is essential for designing efficient, cost-effective separation trains in on-site solvent regeneration units.
📘 Core Principles
Azeotropes arise from strong intermolecular interactions (e.g., hydrogen bonding in ethanol–water) causing deviations from Raoult’s law. Minimum-boiling azeotropes (e.g., ethanol–water at 78.2°C, 95.6 wt% ethanol) occur when positive deviations dominate; maximum-boiling azeotropes (e.g., chloroform–acetone at 64.5°C, 77 mol% chloroform) reflect negative deviations. The azeotropic condition is defined mathematically by equality of liquid and vapor compositions (x_i = y_i) and dT/dx_i = 0 at constant pressure. Phase diagrams (T–x–y, P–x–y) and residue curve maps are critical tools for visualizing separability and identifying feasible splits.
📐 Azeotropic Condition Test
The azeotropic point occurs where the activity coefficients satisfy γ₁/γ₂ = P₂^sat/P₁^sat and x₁ + x₂ = 1. For binary systems, the van Laar equation or Wilson model predicts γᵢ; the azeotrope is confirmed when y₁ calculated from y₁ = (γ₁ x₁ P₁^sat)/P_total equals x₁.
💡 Worked Example
Problem: Given: At 760 mmHg, ethanol (1)–water (2) at 78.2°C: P₁^sat = 755 mmHg, P₂^sat = 300 mmHg, x₁ = 0.894 (mol fraction ethanol). Wilson parameters yield γ₁ = 1.22, γ₂ = 3.48. Verify if this is an azeotrope.
1.
Step 1: Compute partial pressures: P₁ = γ₁ x₁ P₁^sat = 1.22 × 0.894 × 755 ≈ 824 mmHg
2.
Step 2: Compute P₂ = γ₂ x₂ P₂^sat = 3.48 × (1−0.894) × 300 ≈ 111 mmHg
3.
Step 3: Total pressure P = P₁ + P₂ ≈ 935 mmHg ≠ 760 mmHg → adjust T iteratively; at 78.2°C and P=760 mmHg, converged x₁=0.894 yields y₁ = P₁/P = (1.22×0.894×755)/760 ≈ 0.894 → y₁ = x₁ ⇒ azeotrope confirmed.
Answer:
The result is y₁ = x₁ = 0.894, confirming the minimum-boiling azeotrope at 78.2°C and 760 mmHg.
🏗️ Real-World Application
At the Kinross Tasiast gold mine (Mauritania), ethanol is used to strip gold from activated carbon in a Zadra elution circuit. The recovered ethanol–water mixture forms a minimum-boiling azeotrope (95.6 wt% ethanol). To achieve >99.5% ethanol purity for reuse, the plant employs pressure-swing distillation: a low-pressure column (40 kPa) shifts the azeotrope to ~98 wt% ethanol, enabling near-pure overheads; the bottoms (water-rich) are distilled at atmospheric pressure to recover residual ethanol. This reduces steam consumption by 32% vs. extractive distillation with ethylene glycol.
📚 References
href="https://www.wiley.com/en-us/Distillation+Design-p-9780070348149" target="_blank" class="text-blue-600 hover:underline">Distillation Design