Monolithic & modular ozone catalysts for large industrial units

1. Introduction

In large industrial installations, ozone is frequently used as a strong oxidant for exhaust gas scrubbing, advanced wastewater treatment, and electronics cleanroom cleaning processes. However, residual ozone emissions require effective abatement. Traditional packed-bed granular catalyst systems face challenges such as high pressure drop, cumbersome replacement, and a tendency to generate dust. For a single exhaust system handling 50,000 m³/h, a granular catalyst bed can produce a pressure drop of 1500–2000 Pa, increasing fan energy consumption by hundreds of thousands of kilowatt-hours per year. Moreover, when the granular catalyst reaches end-of-life, the entire bed must be unloaded and refilled, typically causing more than 48 hours of production line downtime. Monolithic catalysts combined with modular design, through their honeycomb structure offering low flow resistance and unit‑based flexible maintenance, provide a superior engineering solution for large‑scale ozone abatement.

2. Structure and Characteristics of Monolithic Catalysts

Monolithic catalysts use cordierite honeycomb ceramics as the skeleton, coated with transition metal oxide active components such as manganese and copper, forming an integrated structure with regular through‑channels. Common cell densities range from 200 to 400 cpsi (cells per square inch), with wall thicknesses of 0.1–0.2 mm and geometric surface areas of 1.5–3.0 m²/g.

Compared with traditional granular catalysts (particle size 2–5 mm), the pressure drop of monolithic catalysts is only 1/5 to 1/10 that of granular beds. At the same space velocity of 20,000 h⁻¹, a monolithic bed typically exhibits a pressure drop below 200 Pa, while a granular bed can reach 800–1500 Pa. This significantly reduces fan energy consumption, making monolithic catalysts particularly suitable for large installations handling airflows greater than 20,000 m³/h. Monoliths can accommodate space velocities of 10,000–50,000 h⁻¹. In laboratory tests, a honeycomb catalyst achieved stable ozone decomposition efficiency above 95% for 30 ppm ozone (25°C, 40% relative humidity). Their high mechanical strength and resistance to erosion mean they do not easily pulverize in dust‑laden or pulsed flow conditions, avoiding the risk of dust clogging downstream equipment common with granular catalysts.

3. Engineering Value of Modular Catalysts

Modular catalysts consist of standard‑size monolithic catalyst units (typically 100×100×50 mm or 100×100×100 mm) assembled as modules in stainless steel frames installed into the reactor. Each module is independently supported, with small gaps between units for sealing and flow distribution.

The engineering benefits of modular design are threefold. First, easy online replacement. When a local module deactivates due to poisoning or aging, only that module needs to be removed and replaced, without unloading the entire catalyst bed. For continuous production large plants, modularization can reduce replacement downtime from 2–3 days to 4–8 hours, greatly lowering production losses. Second, flexible scaling. When treatment gas volume increases or emission limits become stricter, additional module layers or parallel modules can be added in reserved reactor space without replacing the entire vessel. Third, quality uniformity. Modular production keeps the deviation of coating thickness and active loading per catalyst unit within ±5%, facilitating batch testing and quality traceability. Life‑cycle cost analysis indicates that modular solutions can reduce average annual maintenance costs by 30–50% compared with bulk‑fill designs.

4. Key Performance Influencing Factors

Humidity

Humidity is the most critical limiting factor. Water molecules compete with ozone for active sites on the catalyst surface. Under high relative humidity (>60% RH), the decomposition efficiency of ordinary manganese‑based catalysts can drop from 98% to below 70% within 24 hours. Deactivation mechanisms include water adsorption blocking micropores and surface hydroxylation reducing the activity of Mn‑O bonds. To address this issue, moisture‑resistant monolithic catalysts with hydrophobic components or “hydroxyl‑oxygen vacancy” dual active sites have been developed. Under test conditions of 75% RH, 15,000 h⁻¹ space velocity, and 5 ppm inlet ozone, such catalysts maintain efficiency above 92% after 1000 hours of continuous operation.

Space Velocity

Space velocity determines reactor volume and pressure drop. Low space velocity (<5,000 h⁻¹) requires large catalyst volume and high equipment cost; high space velocity (>30,000 h⁻¹) reduces reactor size but requires sufficient residence time. Engineering designs typically choose a space velocity of 10,000–25,000 h⁻¹ as a balance point. Monolithic catalysts, owing to their higher mass transfer efficiency than packed beds, can tolerate higher space velocities without sharp efficiency drops.

Temperature

Temperature affects reaction kinetics and catalyst stability. In the range of 10–80°C, ozone decomposition activity increases with temperature; however, above 120°C, some manganese oxides may undergo phase transformation leading to irreversible deactivation. In industrial applications, no additional heating is generally required unless the exhaust gas itself is already hot.

Impurity Poisoning

Sulfur oxides (SO₂), chlorinated hydrocarbons, and organic silicon vapors can chemically adsorb onto active sites or form inert salt layers. Experiments show that when SO₂ concentration exceeds 5 ppm, the catalyst lifespan shortens to one‑third of that under normal conditions. For sulfur‑containing exhaust, sulfur‑resistant formulations (e.g., with cerium or zirconium additives) should be selected.

5. Typical Application Cases

Case 1: Large Printing Plant Exhaust System

A large printing plant used UV photolysis and corona discharge equipment to treat VOCs, but ozone by‑product concentrations reached 2–5 ppm, with exhaust airflow of 50,000 m³/h and ambient relative humidity of 70% RH. The original granular catalyst bed had a pressure drop as high as 1800 Pa, causing frequent fan overload; moreover, the entire bed required replacement every six months, causing two days of shutdown and significant production losses.

The retrofit employed modular monolithic catalysts (200 cpsi honeycomb ceramics, manganese‑based moisture‑resistant formulation). The catalyst modules were arranged in two layers, with a total space velocity of 20,000 h⁻¹. After commissioning: pressure drop stabilized at 220 Pa, fan current dropped by 25%; outlet ozone concentration continuously measured by online monitor remained consistently below 0.05 ppm (emission limit 0.1 ppm). After 18 months of operation, a middle‑layer module was retrieved for activity retesting; decomposition efficiency had decreased from an initial 98.2% to 94.5%, a decay of less than 5%. Maintenance involved only an online inspection of the first‑layer module at month 12, with no significant clogging observed. Expected total service life is 36 months, with average annual maintenance costs reduced by approximately 45% compared with the granular solution.

Case 2: Industrial Park Ozone Catalytic Oxidation Basin Tail Gas Treatment

A wastewater treatment plant used an ozone catalytic oxidation process for advanced COD removal, with an ozone dosage of 20 mg/L. The tail gas from the oxidation basin had ozone concentrations of 10–30 ppm and carried nearly saturated water vapor, with a treatment airflow of 2,000 m³/h. The original system employed a combination of granular activated carbon and granular catalyst, presenting two problems: the activated carbon quickly saturated due to water vapor, requiring monthly replacement; and the granular catalyst exhibited a high pressure drop (600 Pa) and suffered from caking and clogging every six months, requiring complete cleaning.

The retrofit used hydrophobic‑modified monolithic catalyst modules. The catalyst featured a surface hydrophobic coating with a water contact angle >120° compared with the granular substrate. Comparative operating data showed: pressure drop reduced from 600 Pa to 150 Pa; catalyst replacement interval extended from 6 months to 24 months. Additionally, less frequent media replacement reduced hazardous waste (spent adsorbent and catalyst) by approximately 1.2 metric tons per year. Average annual maintenance costs (including labor, spare parts, and disposal fees) decreased by 40%. After two years of operation, module sampling showed an activity retention rate of 82%, with an estimated further service life of one year.

6. Selection and Engineering Considerations

Based on the analysis above, technical professionals should consider the following points during selection.

First, select catalyst formulation according to actual humidity. If exhaust gas relative humidity consistently exceeds 60%, explicitly request a moisture‑resistant or hydrophobic‑modified monolithic catalyst and ask for accelerated life test data under the same humidity conditions.

Second, balance pressure drop and energy consumption. For installations with airflow >30,000 m³/h, higher cell density (e.g., 400 cpsi) monolithic catalysts are recommended to further reduce pressure drop; however, note that excessively high cell density may reduce surface catalytic efficiency, requiring validation through small‑scale testing.

Third, modular frame sealing design cannot be overlooked. Gaps between modules or between modules and reactor walls can cause gas bypass, where gas exits without sufficient catalytic decomposition. Common sealing methods include silicone rubber gaskets, metal expansion joints, and labyrinth seals. Design should aim to keep leakage below 1% of total airflow.

Fourth, it is advisable to commission small‑scale simulated testing from a catalyst supplier. Take actual on‑site exhaust (or prepare a similar composition) and run it in a laboratory reactor for 500–1000 hours to determine the efficiency decay curve. According to publicly available technical information from Minstrong, high‑quality monolithic catalysts should exhibit a half‑life greater than 5,000 hours under accelerated aging tests (conditions: 25°C, 80% RH, 5 ppm O₃, space velocity 20,000 h⁻¹).

Finally, avoid direct price comparisons. Instead, focus on the ozone treatment capacity per unit catalyst volume (g O₃/L·h) and the cost per kg of ozone removed over the full life cycle. This metric integrates efficiency, pressure drop, and replacement frequency, offering greater engineering reference value.

7. Conclusion

Monolithic catalysts, with their honeycomb structure offering low pressure drop and high space velocity adaptability, solve the problems of high pressure drop and energy consumption associated with granular catalysts in large industrial plants. Modular design, through unit‑based assembly, enables online replacement and flexible scaling, significantly reducing life‑cycle maintenance costs. Humidity control, space velocity matching, and impurity tolerance are the three core parameters for engineering selection. Real‑world application cases show that well‑designed monolithic/modular catalytic systems can operate long‑term under high‑humidity, high‑flow conditions, with outlet ozone concentrations consistently below 0.05 ppm and service lives of 2–3 years. Future technology directions include development of materials with even higher moisture resistance, improved low‑temperature activity, and intelligent integration of online catalyst condition monitoring.

author: Gloria
date:2026/06/08