Unit 8: Chlor-Alkali and Heavy Chemicals: Membrane Cells & Heavy Acids

The raw feedstock for the chlor-alkali industry is saturated aqueous sodium chloride ($\text{NaCl}$) brine ($300 - 320\text{ g/L NaCl}$) sourced from solution-mined underground salt deposits or solar marine salt pans.

Impurity Hazards in Modern Membrane Cells

Modern perfluorinated cation-exchange membranes (e.g., DuPont Nafion, Asahi Kasei Aciplex) are irreversibly poisoned by multivalent alkaline-earth cations ($\text{Ca}^{2+}, \text{Mg}^{2+}, \text{Ba}^{2+}, \text{Sr}^{2+}$). Inside the membrane, migrating divalent cations encounter hydroxide ions ($\text{OH}^-$) diffusing from the catholyte:

Insoluble precipitates crystallize inside the microscopic hydrophilic ion clusters ($2 - 4\text{ nm}$), rupturing polymer chains, causing blistering, and escalating electrical cell voltage. Consequently, total hardness ($[\text{Ca}^{2+}] + [\text{Mg}^{2+}]$) must be reduced below $20\text{ ppb}$ ($0.020\text{ mg/L}$).

Two-Stage Industrial Purification Circuit

1. Primary Chemical Precipitation:

• Sodium carbonate ($\text{Na}_2\text{CO}_3$) precipitates calcium:

• Sodium hydroxide ($\text{NaOH}$) precipitates magnesium and iron:

• Barium chloride ($\text{BaCl}_2$) precipitates sulfate:

The effluent is treated with polyelectrolyte flocculants, clarified in rake settlers, and polished through anthracite/sand filters, reducing hardness to $1 - 5\text{ ppm}$.

2. Secondary Purification via Chelating Ion-Exchange Resins:

The polished brine passes through packed columns of macroporous polystyrene resins functionalized with iminodiacetic acid or aminomethylphosphonic acid groups:

The chelating resin exhibits an affinity for divalent alkaline earths over sodium ions exceeding $10,000 : 1$, consistently discharging ultra-pure brine containing $< 10\text{ ppb total hardness}$.

The industrial electrolysis of aqueous sodium chloride has undergone three major technological transitions:

Comparison of Electrolytic Technologies

| Feature | Mercury Cell (Castner-Kellner) | Diaphragm Cell | Modern Membrane Cell | |---|---|---|---| | Anode Reaction | $2\text{Cl}^- \to \text{Cl}_2 + 2e^-$ | $2\text{Cl}^- \to \text{Cl}_2 + 2e^-$ | $2\text{Cl}^- \to \text{Cl}_2 + 2e^-$ | | Cathode Reaction | $\text{Na}^+ + \text{Hg} + e^- \to \text{Na(Hg)}$ amalgam | $2\text{H}_2\text{O} + 2e^- \to \text{H}_2 + 2\text{OH}^-$ | $2\text{H}_2\text{O} + 2e^- \to \text{H}_2 + 2\text{OH}^-$ | | Caustic Purity | Pure $50\%\text{ NaOH}$ directly (from decomposer) | Dilute $12\%\text{ NaOH} + 15\%\text{ NaCl}$ (requires huge evaporation) | Pure $32 - 35\%\text{ NaOH}$ ($< 30\text{ ppm NaCl}$) | | Electrical Energy | $3,100 - 3,400\text{ kWh/t NaOH}$ | $2,700 - 3,000\text{ kWh/t NaOH}$ | $2,100 - 2,400\text{ kWh/t NaOH}$ | | Environmental Hazard | Severe toxic mercury bioaccumulation (Minamata) | Carcinogenic asbestos fiber emission | Completely environmentally benign |

``` MODERN MEMBRANE ELECTROLYSIS CELL Depleted Brine (200 g/L) Water / Dilute NaOH (30%) ▲ ▲ │ │ ┌────┴──────────────────────┐ ┌──────┴────────────────────┐ │ ANODE COMPARTMENT │ │ CATHODE COMPARTMENT │ │ │ │ │ Saturated ──> [ DSA Ti Mesh Anode]│ │[ Nickel Mesh Cathode ] <── Dilute Brine │ 2 Cl⁻ ──> Cl₂ + 2e⁻ │ │ 2 H₂O + 2e⁻ ──> H₂ + 2OH⁻ │ NaOH Feed (300 g/L) │ │ │ │ │ │ │ │ Cl₂ Gas ──> │ Na⁺ │ <── H₂ Gas │ │ │ ──────>│ │ └────────────┬──────────────┘ └──────────────┬────────────┘ │ Nafion Perfluorinated │ │ Cation-Exchange Membrane ▼ │ (Sulfonate / Carboxylate Layers) Pure Product NaOH (32-35%) ```

Membrane Architecture (Bilayer Design)

Modern membranes consist of a reinforced perfluorosulfonic acid (PFSA) backing layer facing the anode (low electrical resistance) coupled to an ultra-thin carboxylate polymer layer facing the cathode. The high fixed charge density of carboxylate groups ($\text{-COO}^-$) creates extreme Donnan exclusion against back-migrating hydroxide ions ($\text{OH}^-$), maintaining caustic current efficiencies above $95 - 97\%$ at $35\text{ wt}\%\text{ NaOH}$.

The overall electrochemical decomposition of aqueous sodium chloride:

Thermochemically, $\Delta G^\circ = +422.3\text{ kJ/mol}$. The reversible thermodynamic cell potential ($E_{\text{rev}}^\circ$) under standard conditions ($298\text{ K}$, $1\text{ bar}$):

Cell Voltage Breakdown ($U_{\text{cell}}$)

The actual operational cell voltage required to drive electrolysis at commercial current densities ($j = 4.0 - 7.0\text{ kA/m}^2$) is significantly higher, expressed by the additive polarization sum:

1. Reversible Potential under Operating Conditions ($90^\circ\text{C}$):

2. Anodic Overpotential ($\eta_{\text{anode}}$):

Chlorine evolution on modern Dimensionally Stable Anodes (DSA: titanium substrate coated with mixed metal oxides $\text{RuO}_2-\text{TiO}_2-\text{IrO}_2$). Catalytic activation lowers overpotential to $\eta_{\text{anode}} \approx 0.05 - 0.08\text{ V}$ via the Butler-Volmer relation:

3. Cathodic Overpotential ($\eta_{\text{cathode}}$):

Hydrogen evolution on activated nickel cathodes coated with ruthenium or Raney nickel. $\eta_{\text{cathode}} \approx 0.08 - 0.12\text{ V}$.

4. Ohmic Voltage Drop Across Membrane ($I \cdot R_{\text{membrane}}$):

Resistance of $\text{Na}^+$ transport across the membrane: $\Delta U_{\text{mem}} \approx 0.35 - 0.50\text{ V}$.

5. Ohmic Drops in Electrolytes and Structural Hardware:

Bubble void fraction (gas dispersion of $\text{Cl}_2$ and $\text{H}_2$) elevates solution resistance by the Bruggeman relation:

Zero-gap cell configurations (where flexible mesh electrodes compress directly against the membrane) compress inter-electrode electrolyte drops to $< 0.10\text{ V}$. Total industrial operational cell voltage sits between $2.95\text{ V}$ and $3.15\text{ V}$ at $6.0\text{ kA/m}^2$.

Complete Electrochemical Parameters of Modern Chlor-Alkali Membrane Cells

| Electrochemical Parameter | Symbol | Industrial Operating Range | Reference Target Value | |---|---|---|---| | Current Density | $j$ | $4.0 - 7.0\text{ kA/m}^2$ | $6.0\text{ kA/m}^2$ | | Operating Temperature | $T$ | $85 - 92^\circ\text{C}$ | $88^\circ\text{C}$ | | Anolyte NaCl Concentration | $[\text{NaCl}]_{\text{ano}}$ | $190 - 220\text{ g/L}$ | $205\text{ g/L}$ | | Catholyte NaOH Concentration| $[\text{NaOH}]_{\text{cat}}$ | $32.0 - 35.0\text{ wt}\%$ | $33.5\text{ wt}\%$ | | Current Efficiency (NaOH) | $\eta_{\text{NaOH}}$ | $95.5 - 97.5\%$ | $96.5\%$ | | Operating Cell Voltage | $U_{\text{cell}}$ | $2.95 - 3.15\text{ V}$ | $3.02\text{ V}$ | | Specific Power Consumption | $w_{\text{spec}}$ | $2,050 - 2,250\text{ kWh/t NaOH}$ | $2,120\text{ kWh/t NaOH}$ | | Membrane Service Life | $\tau_{\text{mem}}$ | $3 - 5\text{ years}$ | $4\text{ years}$ | | Anode Coating Service Life | $\tau_{\text{anode}}$ | $8 - 12\text{ years}$ | $10\text{ years}$ |

The aqueous sodium hydroxide discharged from modern membrane cells has a concentration of $32 - 35\text{ wt}\%\text{ NaOH}$. Standard global merchant commerce requires $50.0\text{ wt}\%\text{ NaOH}$ liquid caustic soda or $99\%\text{ NaOH}$ anhydrous solid pearls/flakes.

1. Multi-Effect Falling-Film Evaporation ($32\% \to 50\%\text{ NaOH}$)

Concentrating caustic soda requires specialized metallurgy (pure nickel $\text{Ni } 200$ or high-nickel alloys) to resist caustic stress-corrosion cracking and embrittlement at elevated temperatures ($> 120^\circ\text{C}$):

• Modern plants employ triple- or quadruple-effect falling film evaporators operating under forward or counter-current feed.
• Boiling Point Elevation (BPE): Saturated caustic solutions exhibit extreme boiling point elevation. While water boils at $100^\circ\text{C}$ at $1\text{ atm}$, a $50\text{ wt}\%\text{ NaOH}$ solution boils at $143^\circ\text{C}$ ($\text{BPE} = 43\text{ K}$). At $73\text{ wt}\%\text{ NaOH}$, the boiling point climbs to $190^\circ\text{C}$. This massive BPE significantly compresses the effective logarithmic mean temperature difference ($\Delta T_{\text{eff}}$) available across successive evaporator effects.

2. Solid Anhydrous Caustic Production ($50\% \to 99\%\text{ NaOH}$)

To produce solid flake or pearl caustic:

• The $50\text{ wt}\%$ caustic is concentrated in a falling film evaporator heated by molten heat transfer salts (eutectic mixture of $53\%\text{ KNO}_3 + 40\%\text{ NaNO}_2 + 7\%\text{ NaNO}_3$) operating at $380 - 400^\circ\text{C}$ under vacuum.
• Molten anhydrous caustic leaves at $320 - 340^\circ\text{C}$ ($< 0.5\%\text{ moisture}$).
• Flaking: The molten liquid is fed onto water-cooled rotating nickel flaker drums, solidifying instantaneously into a crystalline sheet that is sheared off by doctor blades.
• Prilling: Alternatively, molten caustic is sprayed down a counter-current air prilling tower, crystallizing into spherical beads (pearls).

Moist, saturated chlorine gas ($\text{Cl}_2$) discharged from the membrane cell anodes at $85 - 90^\circ\text{C}$ is saturated with water vapor and is intensely corrosive to all conventional structural metals:

``` CHLORINE GAS PROCESSING TRAIN Wet Hot Cl2 Gas (~85°C) from Cells │ ▼ ┌──────────────┐ │ DIRECT WATER │ (Chilled water spray cools Cl2 to 12-15°C; │ COOLER │ condenses >80% of water vapor without hydrate formation) └──────┬───────┘ ▼ Cooled Wet Cl2 ┌────────────────────────────────────────────────────────┐ │ SULFURIC ACID DRYING TOWERS (3-Stage Counter-Current) │ <── Fresh 98% H2SO4 │ Stage 1 (78% H2SO4) ──> Stage 2 (92%) ──> Stage 3 (96-98%)│ ──> Spent Dilute H2SO4 └──────────────────────────┬─────────────────────────────┘ ▼ Bone-Dry Cl2 Gas (Moisture < 5 ppm) ┌────────────────────────────────────────────────────────┐ │ CHLORINE COMPRESSOR (Liquid Ring or Centrifugal) │ ──> Pressurizes to 8-12 bar └──────────────────────────┬─────────────────────────────┘ ▼ ┌────────────────────────────────────────────────────────┐ │ LIQUEFACTION CONDENSER (Refrigerated Freon/Ammonia) │ ──> Pure Liquid Chlorine (-15°C) └──────────────────────────┬─────────────────────────────┘ ▼ ┌──────────────┐ │ STORAGE TANK │ ──> Bulk Pressurized Railcars / Pipeline Distribution └──────────────┘ ```

1. Direct Gas Cooling & Hydrate Prevention

The gas is cooled to $12 - 15^\circ\text{C}$ in packed titanium or polyvinylidene fluoride (PVDF) direct-contact cooling towers using chilled water. Cooling must strictly remain above $9.8^\circ\text{C}$ at atmospheric pressure to prevent crystallization of solid yellow chlorine hydrate ($\text{Cl}_2\cdot 7.3\text{H}_2\text{O}$), which clogs tower packing and transfer pipelines.

2. Multi-Stage Sulfuric Acid Drying

Water must be removed to $< 5\text{ ppm}$ moisture:

• When moisture $< 20\text{ ppm}$, chlorine is non-corrosive to ordinary carbon steel, allowing inexpensive steel pipes, valves, and railcars to be used safely.
• Drying is conducted in three packed towers flowing concentrated sulfuric acid ($\text{H}_2\text{SO}_4$) counter-currently. Fresh $98\text{ wt}\%\text{ H}_2\text{SO}_4$ enters the third stage, cascading to the first stage where spent acid exits at $75 - 78\text{ wt}\%$.

3. Compression & Liquefaction

Bone-dry chlorine is compressed using liquid-ring compressors (employing concentrated sulfuric acid as the sealing fluid) or multi-stage centrifugal compressors to $8 - 12\text{ bar}$. The pressurized gas enters shell-and-tube condensers chilled by refrigerant (ammonia or R-134a) to $-10^\circ\text{C}$ to $-25^\circ\text{C}$, condensing into clear amber liquid chlorine ($\rho = 1.41\text{ g/cm}^3$). Non-condensable inert gases ($\text{H}_2, \text{N}_2, \text{O}_2$) are purged ("sniff gas") to a sodium hydroxide scrubber.

Byproduct chlorine and hydrogen from the chlor-alkali cell room are converted on-site into essential industrial chemicals:

1. Hydrochloric Acid ($\text{HCl}$) Synthesis

Pure gaseous hydrogen and chlorine are reacted in a specialized water-cooled silica or impregnated graphite combustion chamber:

• Combustion Control: To ensure complete consumption of chlorine (which is intensely toxic and corrosive), hydrogen is fed at a $5 - 10\%$ stoichiometric excess. The flame temperature exceeds $2000 - 2500^\circ\text{C}$.
• Adiabatic Absorption: The exiting hot anhydrous $\text{HCl}$ gas enters an isothermal or adiabatic falling-film graphite absorption tower where it dissolves violently in demineralized water:

The maximum concentration at atmospheric pressure is governed by the negative azeotrope ($20.22\text{ wt}\%\text{ HCl}$ at $108.6^\circ\text{C}$), but refrigerated commercial absorbers produce concentrated $33 - 36\text{ wt}\%\text{ Technical Grade HCl}$.

2. Sodium Hypochlorite ($\text{NaOCl}$) Bleach Manufacture

Produced by scrubbing dilute or tail-gas chlorine with refrigerated aqueous sodium hydroxide:

• Temperature & Decomposition Control: The reaction is conducted below $30 - 35^\circ\text{C}$. Above $40^\circ\text{C}$, hypochlorite decomposes rapidly into toxic and inactive chlorate:

• Free Alkali Stabilization: Excess sodium hydroxide ($0.5 - 1.0\text{ wt}\%\text{ free NaOH}$) is maintained to keep the $\text{pH} > 11.5$, preventing decomposition into hypochlorous acid ($\text{HOCl}$) and toxic chlorine gas.

Sulfuric acid ($\text{H}_2\text{SO}_4$, $M = 98.08\text{ g/mol}$) is the world's most widely consumed heavy industrial chemical. Modern production utilizes the Double Contact Double Absorption (DCDA) catalytic contact process:

1. Sulfur Combustion & Gas Conditioning

Molten bright sulfur ($135 - 145^\circ\text{C}$) is atomized with dry air in a refractory burner:

The resulting process gas ($10 - 11.5\text{ vol}\%\text{ SO}_2$, $9.5 - 11\text{ vol}\%\text{ O}_2$) is cooled from $1050^\circ\text{C}$ to $420^\circ\text{C}$ in a waste heat boiler, generating high-pressure steam ($40 - 60\text{ bar}$).

2. Catalytic Oxidation Kinetics ($SO_2 \to SO_3$)

The reversible oxidation is conducted over a cesium-promoted vanadium pentoxide catalyst supported on silica ($\text{V}_2\text{O}_5-\text{K}_2\text{SO}_4/\text{SiO}_2$):

The equilibrium constant is given by:

Because the reaction is strongly exothermic, thermodynamic equilibrium conversion decreases with increasing temperature, while reaction kinetics freeze below $400^\circ\text{C}$. The converter is structured into four sequential adiabatic catalyst beds with inter-bed cooling:

• Bed 1 ($420 \to 600^\circ\text{C}$): Rapid kinetic conversion reaches $60 - 65\%$.
• Bed 2 ($440 \to 510^\circ\text{C}$): Cumulative conversion reaches $85\%$.
• Bed 3 ($430 \to 455^\circ\text{C}$): Conversion reaches $93 - 95\%$.

3. The DCDA Innovation (Interpass Absorption)

In a single absorption plant, Le Chatelier's principle restricts overall conversion to $\le 97.5\%$, discharging thousands of ppm of harmful $\text{SO}_2$ into the atmosphere. In the DCDA configuration:

• Process gas leaving Bed 3 is cooled and passed through an Intermediate Absorption Tower (IPAT) where $> 99.9\%$ of generated $\text{SO}_3$ is absorbed into circulating $98.5\%\text{ H}_2\text{SO}_4$.
• The $\text{SO}_3$-depleted gas is reheated to $420^\circ\text{C}$ in gas-gas heat exchangers and fed to Bed 4.
• Removing $\text{SO}_3$ drives the thermodynamic equilibrium of the remaining gas overwhelmingly forward, raising overall conversion to $> 99.85\%$, cutting stack emissions to $< 100\text{ ppm SO}_2$.
• The gas exits Bed 4 through a Final Absorption Tower (FAT), producing concentrated $98.5\text{ wt}\%\text{ H}_2\text{SO}_4$ and Oleum ($20 - 65\%\text{ free SO}_3$).

The chlor-alkali sector consumes massive electrical energy ($> 2,200\text{ kWh/metric ton NaOH}$). The development of Oxygen-Depolarized Cathode (ODC) technology and hydrogen valorization represents a paradigm shift toward decarbonized heavy chemicals:

1. Oxygen-Depolarized Cathode (ODC) Electrochemistry

In conventional membrane cells, hydrogen gas is evolved at the cathode by water reduction:

In an ODC cell, gaseous pure oxygen ($\text{O}_2$) is introduced through a porous gas-diffusion cathode coated with silver or platinum catalysts, depolarizing the cathode reaction into oxygen reduction:

• Thermodynamic Voltage Reduction:

The cathode standard potential shifts positively by $\Delta E^\circ = 0.401 - (-0.828) = +1.229\text{ V}$.

• Cell Operating Voltage Drop:

Operational cell voltage plummets from $\sim 3.00\text{ V}$ to $2.00 - 2.10\text{ V}$ at $4 - 5\text{ kA/m}^2$.

• Energy Conservation:

Electrical energy consumption drops from $2,200\text{ kWh/t NaOH}$ to $1,500 - 1,600\text{ kWh/t NaOH}$, achieving an extraordinary $30\%$ reduction in grid power consumption.

2. High-Purity Green Hydrogen Valorization

For conventional membrane plants that continue to produce byproduct hydrogen:

• Chlor-alkali hydrogen is ultra-pure ($> 99.999\%$ after moisture condensation and trace oxygen catalytic deoxidation).
• Rather than combusting hydrogen for low-grade process steam, modern complexes feed this chemical-grade hydrogen into fuel-cell vehicles, direct ammonia synthesis, or green methanol plants, capturing immense clean-energy carbon credits.

University Honors Industrial Case Study: Wet Chlorine Crevice Corrosion in Titanium Exchangers

Titanium is the universal metal of choice for handling wet chlorine gas because a stable, self-healing rutile passive oxide film ($\text{TiO}_2$) forms instantaneously in the presence of trace moisture ($> 0.5\text{ wt}\%\text{ H}_2\text{O}$):

• The Dry Chlorine Fire Hazard: In bone-dry chlorine gas ($< 20\text{ ppm H}_2\text{O}$), the protective oxide film cannot self-repair. If mechanical scratching exposes virgin titanium metal, a violent, auto-igniting exothermic chlorination fire erupts:

The titanium metal burns vigorously in dry chlorine at room temperature, releasing dense white clouds of boiling $\text{TiCl}_4$.

• Crevice Corrosion: In gasket joints of plate heat exchangers where brine flow stagnates and temperature exceeds $75^\circ\text{C}$, localized acid buildup ($\text{pH} < 1$) breaks down passivity. Modern chlor-alkali exchangers specify palladium-stabilized titanium (ASTM Grade 7, $\text{Ti}-0.15\%\text{Pd}$) to prevent crevice initiation.