Recuperative Thermal Oxidizer with HCL Scrubber to Treat Chemical Processing Emissions
Executive Summary
Epcon delivered a custom-designed recuperative thermal oxidizer (RTO) combined with an acid-gas (HCl) industrial scrubber system to a chemical manufacturer producing MDI and handling halogenated VOC emissions. The system was engineered to destroy 99.9% of hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) — including chlorinated compounds like chlorobenzene — while neutralizing corrosive hydrochloric acid by-products and protecting plant infrastructure. The integrated solution ensures environmental compliance, process reliability, and safe handling of aggressive exhaust streams.
Project Overview
The customer, a chemical-processing equipment plant producing MDI (methylene diphenyl diisocyanate) — used widely for insulation and automotive applications — required an emissions-control solution for a very challenging exhaust stream. The exhaust contained high concentrations of chlorinated VOCs and HAPs, lacked oxygen, and produced acid gases upon combustion. In particular: the primary released compound was Chlorobenzene, which generates a highly exothermic reaction when oxidized; exhaust flow was variable and low (220–378 SCFM), with very high VOC load (up to 655 lb/hr), and produced concentrated acid gases upon oxidation.
Given those conditions, Epcon was commissioned to engineer a Recuperative Thermal Oxidizer + HCl Scrubber system — capable of oxidizing VOCs/HAPs and neutralizing acid gases, while resisting corrosion and handling variable flow and chemical loads.
The Challenge
The project had to overcome several serious challenges:
- Highly concentrated halogenated VOCs/HAPs (chlorobenzene) — traditional catalytic or simple oxidizers risk poor performance, poisoning, or failure when dealing with halogenated organics.
- Acid-gas formation (HCl) during oxidation — combustion of chlorinated compounds generates hydrochloric acid, which can corrode oxidizer internals, ductwork, and downstream equipment. System required acid-resistant materials or protective coatings, plus an effective scrubber to neutralize acids.
- Absence of oxygen in the incoming exhaust — the raw exhaust stream lacked oxygen, so combustion would be impossible without adding dilution air. The design had to supply sufficient dilution air to enable oxidation and to control the exotherm safely.
- Excessive exothermic reaction of chlorobenzene oxidation — uncontrolled heat release could damage equipment; need to control temperature, residence time, and ensure safe venting and acid-gas neutralization.
- Corrosion, acid-gas handling, and wastewater discharge — acid gases must be scrubbed, neutralized, and scrubber blowdown water cooled and managed; materials must resist acid corrosion while maintaining long service life.
Regulatory compliance & reliability under variable loads — the system needed to guarantee ≥ 99.9% destruction efficiency (DRE) for VOCs/HAPs while operating under variable flow rates and exhaust conditions typical of batch / chemical-processing operations.
Engineering Requirements
To satisfy the customer’s needs and ensure safe, efficient, durable operation, Epcon defined these critical design specifications:
- Use a recuperative thermal oxidizer (shell-and-tube or equivalent metal-heat-exchanger type) to oxidize halogenated VOCs/HAPs at high temperature and recover heat to improve fuel efficiency. Recuperative design helps reduce auxiliary fuel consumption compared to direct-fired oxidizers.
- Provide dilution air capacity: because the exhaust stream lacked oxygen, additional air was required to support combustion and control the exotherm — ensuring safe flame stability and compliance with safety limits (e.g. keeping the stream below 50% lower explosive limit, LEL).
- Construct oxidizer internals with acid-resistant coating / materials, to resist corrosion by HCl and other acid gases generated in the process.
- Integrate a wet-scrubber (HCl scrubber / acid-gas absorber) downstream of the oxidizer to neutralize acid gases (HCl / chlorides), prevent corrosion and meet emissions limits for acid gases. Scrubber design to include proper quenching, chemical neutralization, pH/ORP monitoring, blow-down management, and cooled discharge water handling.
- Ensure system sizing accounts for maximum contaminant load (VOC/HAP throughput, acid gas generation, dilution air, exhaust flow) and variable operating conditions. For this project, although raw exhaust flow was 220–378 SCFM, the system was sized to 6,000 SCFM to allow dilution air and safe handling of exotherm during chlorobenzene oxidation.
- Provide turnkey scope: design, fabrication, protective coatings, oxidizer + scrubber + heat exchanger, controls, instrumentation, installation, commissioning, training and maintenance plan — to minimize plant disruption and maximize safety and long-term reliability.
The Solution
Epcon engineered and supplied a fully integrated Recuperative Thermal Oxidizer + HCl Scrubber System for the chemical-processing client, including:
- A metal-heat-exchanger–based recuperative oxidizer (shell-and-tube design), capable of handling chlorinated-organic exhaust, with internal surfaces coated to resist acid-gas corrosion. The design recovers heat to improve energy efficiency, reducing auxiliary fuel needs.
- A dilution-air addition system, ensuring the oxidizer receives sufficient oxygen for combustion and dilutes the incoming exhaust to safely manage the exothermic reaction from chlorobenzene oxidation — necessary for safe operation and compliance with explosion-limit regulations.
- An acid-gas scrubber (HCl scrubber) downstream of the oxidizer — a wet scrubber/packed-tower-type absorber that neutralizes HCl and other acid gases formed during oxidation, protecting exhaust stack, environment, and downstream equipment. The design includes quenching, water cooling (liquid-to-liquid heat exchanger for blow-down), chemical neutralization (e.g. caustic addition), pH/ORP monitoring, and mist-elimination to handle acid aerosol.
- Full engineering, installation, and commissioning, with quality controls and process-specific adaptations — turning a challenging, halogenated exhaust problem into a compliant, reliable emissions-control solution.
The delivered system successfully destroyed 99.9% of the halogenated VOCs/HAPs, neutralized acid gases, protected plant infrastructure from corrosion, and enabled safe, compliant emissions for a previously problematic exhaust stream.
Technical Specifications
Oxidizer Type: Recuperative Thermal Oxidizer (metal shell-and-tube heat exchanger) — designed for halogenated VOCs/HAPs, corrosive exhaust, acid-gas potential
Flow Capacity / Sizing: System sized for 6,000 SCFM (despite raw exhaust 220-378 SCFM), to include dilution air for combustion support and exotherm control
Contaminants Treated: Halogenated VOCs/HAPs (e.g. Chlorobenzene), other organics from MDI-manufacturing exhaust; acid gases including HCl generated during oxidation
Destruction Efficiency (DRE): Targeted 99.9% VOC/HAP destruction — achieved as per project goals
Materials / Construction: Acid-resistant coating/material on oxidizer internals (to resist HCl and acid-gas corrosion)
Scrubber Type: Wet acid-gas scrubber (packed/tower or similar), including quench, chemical neutralization (e.g. caustic), sump & recirculation, pH/ORP monitoring, mist eliminator, blow-down handling, heat-exchanger to cool discharge water
Controls & Safety: Dilution-air control, LEL monitoring, flame/combustion controls, acid-gas handling, exhaust treatment, instrumentation, safety interlocks, corrosion-resistant design — complying with NFPA and industry standards
Delivery Scope: Full turnkey: evaluation, design, fabrication, installation, commissioning, after-sales service & maintenance support by Epcon
The Results
After commissioning, the oxidizer + scrubber solution delivered substantial value:
- High-efficiency VOC / HAP destruction — 99.9% removal of halogenated VOCs and hazardous compounds ensured compliance with emissions regulations and reduced environmental impact.
- Effective acid-gas neutralization & corrosion prevention — the HCl scrubber removed acid gases, protecting downstream equipment, ductwork and exhaust stacks from corrosion and reducing maintenance/repair needs.
- Safe handling of exothermic halogenated‐gas combustion — dilution-air system and robust oxidizer design managed exotherm from chlorobenzene oxidation, maintaining safe combustion conditions below LEL thresholds.
- Lower operational cost & fuel savings — recuperative design recovers heat, reducing auxiliary fuel demand versus direct-fired systems; integrating scrubber avoids catalyst replacement costs common in catalytic oxidizers dealing with halogenated streams.
- Reliable, long-term operation under challenging chemical exhaust conditions — system materials and design resist acid-gas corrosion, ensuring longevity; turnkey delivery minimized plant downtime during retrofit.
- Regulatory compliance & environmental performance — combination of high VOC destruction and acid-gas neutralization allowed the client to meet stringent hazardous-air-pollutant (HAP) emissions standards despite difficult exhaust composition.
