Technology Type

The Riedl-Pfleiderer Process, also known as the Anthraquinone Auto-Oxidation (AO) Process, is the predominant industrial method for hydrogen peroxide (H₂O₂) production worldwide, accounting for over 95% of global production. Developed by Georg Pfleiderer and Hans-Joachim Riedl at IG Farben between 1935 and 1945, this process was first commercialized by E.I. DuPont de Nemours in 1953. The process has since been continuously refined and remains the technological backbone of the hydrogen peroxide industry.

Historical Development

The development of the Riedel-Pfleiderer process represented a significant advancement over earlier hydrogen peroxide production methods. Prior to this innovation, hydrogen peroxide was primarily produced through the Thenard process (using barium peroxide) or electrochemical methods, both of which were limited in scale and efficiency. The breakthrough came from Pfleiderer's work on polynuclear hydroquinones, building upon earlier research by Riedl on autoxidizable compounds. German company BASF developed the process to achieve a monthly output of 30 MT (based on 100% H₂O₂) by 1945.

Process Overview and Chemistry

Fundamental Principle

The Riedel-Pfleiderer process operates on a cyclical reduction-oxidation mechanism involving alkylated anthraquinones as organic carriers. The overall net reaction is:

H₂ + O₂ → H₂O₂

This is achieved through a two-step cycle:

1. Hydrogenation (Reduction):

   • Alkylanthraquinone + H₂ → Alkylanthrahydroquinone
     
     

2. Oxidation (Auto-oxidation):

   • Alkylanthrahydroquinone + O₂ → Alkylanthraquinone + H₂O₂
     
     

Working Solution Composition

The working solution is the heart of the process, consisting of:

Active Compounds:

• 2-Ethylanthraquinone (EAQ): Most commonly used (30% of active quinones)
• 2-Ethyl-5,6,7,8-tetrahydroanthraquinone (THEAQ): (70% of active quinones)
• Alternative quinones: 2-tert-butyl-, 2-tert-amyl-, or 2-sec-amylanthraquinones

Solvent System:

• Quinone-friendly solvents: Aromatic hydrocarbons (C₉-C₁₂), trimethylbenzene (TMB), methylnaphthalene, tert-butylbenzene
• Hydroquinone-friendly solvents: Trioctyl phosphate (TOP), nonyl alcohols, tetrabutylurea (TBU), alkylcyclohexanol acetates
• Typical ratio: 50-80% aromatic solvents, 20-50% polar solvents

The solvent system must satisfy multiple requirements: low water solubility, chemical stability, good dissolving properties for both quinone forms, low volatility, and appropriate density for phase separation.

Detailed Process Description

Simplified Riedl-Pfleiderer Process Block Flow Diagram

Step 1: Hydrogenation (Reduction)

Reactor Design:

• Fixed-bed reactors: Trickle-bed or packed-bed configurations with immobilized Pd catalyst
• Slurry reactors: Fluidized-bed or stirred-tank reactors with suspended catalyst particles
• Continuous stirred-tank reactors (CSTR): For laboratory and pilot-scale operations

Operating Conditions:

• Temperature: 40-70°C (optimal around 50°C)
• Pressure: 2-5 bar H₂ pressure
• Conversion: Limited to 50-70% to minimize side reactions
• Residence time: 5-30 minutes depending on reactor type
• Catalyst System:
• Active metal: Palladium (preferred) or Raney nickel
• Support: Activated alumina, activated carbon, or ion-exchange resins
• Particle size: 1-5 mm for fixed-bed, 50-200 μm for slurry systems
• Loading: 0.1-1.0 wt% Pd on support

Reaction Mechanism:
The hydrogenation follows a Rideal-Eley mechanism where hydrogen dissociatively adsorbs on palladium, while tetrahydroanthraquinone (THEAQ) reacts from solution:

1. H₂ + 2 Pd → 2 Pd-H (surface)

2. THEAQ + 2 Pd-H → THEAQH₂ + 2 Pd

Side Reactions and Catalyst Deactivation:

• Reversible poisoning: Water adsorption on Pd sites (30% activity loss in 1-2 days)
• Irreversible poisoning: Condensation of partially hydrogenated anthraquinones
• Ring hydrogenation: Formation of octahydro derivatives (H₈EAQH₂) which are inactive
• Catalyst regeneration: Required every 3-6 months in industrial operations

Post-Reaction Working Solution Handling:

• Cooling: After the hydrogenation reactor, the working solution is cooled before proceeding to catalyst filtration
• Filtration: The working solution passes through a three-stage filtration train — primary, secondary, and polish/safety filters — to remove Pd catalyst particles before the solution enters the oxidation section (Pd carry-over into the oxidation stage promotes H2O2 decomposition and must be controlled to near-zero levels)
• Reversion Unit: A small side stream of the cooled working solution is continuously diverted to a Reversion Unit, where labile degradation products formed during hydrogenation — primarily epoxides and partially hydrogenated quinone derivatives — are reconverted to active quinone species. This continuous in-line reversion maintains working-solution health between periodic regeneration cycles and reduces the load on the standalone regeneration unit (Step 5).
  Filtration and reversion units

Step 2: Oxidation (Auto-oxidation)

Reactor Design:

• Bubble column reactors: Continuous countercurrent air-liquid contact
• Packed columns: Multi-stage cascade systems (e.g., BASF's four-column series)
• Co-current oxidation: Single-column systems with air and solution flowing together

Operating Conditions:

• Temperature: 30-80°C (optimal 40-70°C)
• Pressure: Atmospheric to 5 bar
• Oxygen partial pressure: 0.2-3 atm (optimal 0.5-1.5 atm)
• Residence time: 1-3 hours
• Air flow: High enough to maintain good mixing

Reaction Kinetics:
The oxidation follows second-order kinetics:

• Rate law: r = k[THEAQH₂][O₂]
• Enhancement factor: ≈ 1 (moderately slow reaction)
• Mass transfer: Consecutive with chemical reaction

By-product Formation:

• Epoxides: Formed during oxidation, particularly at higher temperatures
• Anthrones/Dianthrones: Result from tautomerization reactions
• Oxanthrones: Formed through side oxidation pathways

Step 3: Extraction

Process Design:

• Countercurrent extraction: Pulsating packed towers or mixer-settler units
• Extraction efficiency: 98% H₂O₂ recovery
• Solvent: Demineralized water
• Product concentration: 15-35 wt% H₂O₂ solution

Operating Conditions:

• Temperature: 25-40°C
• Pressure: Atmospheric
• Water-to-organic ratio: 1:3 to 1:5 (volume basis)
• Number of stages: 3-8 theoretical stages

Step 4: Purification and Concentration

Purification Steps:

• Activated carbon treatment: Removal of organic impurities
• Ion exchange: Removal of metal ions and stabilization
• Distillation: Concentration to 50-70 wt% H₂O₂

Concentration Methods:

• Vacuum distillation: Primary concentration method
• Fractional distillation: For high-purity grades
• Crystallization: For ultra-pure applications

Step 5: Working Solution Regeneration

Drying Step:

• Objective: Remove water content to 0.1-0.5 wt%
• Methods: Vacuum drying, molecular sieves, or azeotropic distillation

Regeneration Process:

• Catalyst: Alkaline alumina or silica-alumina (4% NaOH on Al₂O₃)
• Operating conditions: 100-150°C, packed-bed reactor
• Reactions promoted:
  1. THEAQ dehydrogenation: Converts degraded quinones back to active forms
  2. Epoxide reconversion: Regenerates active quinones from epoxides
  3. Acid neutralization: Removes free acidity from working solution

Regeneration Reactions:

• Dehydrogenation: H₆THEAQ → THEAQ + 3 H₂
• Epoxide conversion: Epoxides → Active quinones
• Tar removal: Preferential adsorption on alkaline catalyst

Technology Owners and Licensors

Historical Development

• Original developers: Georg Pfleiderer and Hans-Joachim Riedl (IG Farben)
• First commercial plant: E.I. DuPont de Nemours (1953)
• Early industrial adopters: BASF, Degussa, Solvay

Current Technology Providers

• Solvay SA: Major licensor and operator of AO process technology
• BASF SE: Continues to operate and license the technology
• Evonik Industries: Provides catalyst and process technology
• Chinese companies: Multiple domestic technology providers for local markets

Equipment Manufacturers

• Reactor systems: Specialized chemical engineering companies
• Catalysts: Solvay, BASF, Johnson Matthey, Evonik
• Process equipment: Alfa Laval, Sulzer, Koch-Glitsch

Process Variants and Improvements

All-Tetra Process

A specialized variant using a mixture of 30% EAQ and 70% THEAQ, designed for:

• Higher selectivity: Reduced side reactions
• Improved stability: Better catalyst performance
• Enhanced efficiency: Higher H₂O₂ productivity per cycle

Fixed-Bed vs. Fluidized-Bed Reactors

• Fixed-bed: More common, easier operation, lower maintenance
• Fluidized-bed: Better heat transfer, uniform catalyst distribution, higher productivity
• Slurry systems: Easier catalyst handling, better temperature control

Modern Improvements

• Micro-packed-bed reactors: 25× higher space-time yield than conventional reactors
• Membrane separation: Enhanced extraction efficiency and organic retention
• Process intensification: Reduced equipment size and energy consumption

Economic and Environmental Considerations

Capital Investment

• Plant capacity: 40,000-330,000 MT/year typical industrial scale
• Equipment costs: Dominated by reactor systems, extraction columns, and distillation units
• Working capital: Significant inventory of working solution and catalysts

Operating Costs

• Raw materials: Hydrogen (60-70%), oxygen/air (10-15%), quinones and solvents (15-20%)
• Utilities: Energy for compression, heating, and cooling
• Catalyst replacement: Regular Pd catalyst makeup and regeneration
• Maintenance: Working solution purification and regeneration

Environmental Impact

• Waste streams: Spent working solution, catalyst disposal, organic emissions
• Energy consumption: Significant for hydrogen production and compression
• Safety considerations: Hydrogen handling, organic solvent management

Process Advantages and Limitations

Advantages

• High efficiency: 95%+ conversion of H₂ and O₂ to H₂O₂
• Scalability: Proven from pilot to mega-scale (330,000 MT/year)
• Product purity: Achievable grades from 35% to 70% H₂O₂
• Technological maturity: Decades of industrial experience and optimization

Limitations

• Complex chemistry: Multiple side reactions and catalyst deactivation
• High capital investment: Large-scale equipment requirements
• Operating complexity: Requires skilled operators and process control
• Environmental concerns: Organic working solution management and disposal

References

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