What should be noted in daily maintenance of VOC catalysts?

The core of VOC catalyst maintenance is "prevention first, monitoring second" – minimizing deactivation risks through rigorous exhaust gas pretreatment and standardized start-stop procedures, while establishing a systematic activity monitoring mechanism to take timely regeneration measures at the early stage of activity decline, rather than waiting until complete failure and then paying high replacement costs. The following five dimensions detail specific precautions for daily maintenance.

1. Exhaust Gas Pretreatment: Protecting the Catalyst at the Source

Exhaust gas pretreatment is the most important yet most easily overlooked aspect of daily maintenance. Industrial exhaust often contains particulate matter, paint mist, oil mist, high-boiling organics, as well as poisons such as sulfur, phosphorus, chlorine, and silicon. If these enter the catalyst bed directly without filtration, they will form a physical covering layer on the catalyst surface or undergo irreversible chemical reactions, leading to rapid loss of active sites.

Key daily maintenance points:

• Check the operating status of pretreatment equipment (baghouse filters, electrostatic precipitators, dry filters, etc.) daily to confirm no damage or clogging. Once filter element damage is found, replace it immediately; otherwise, dust-laden exhaust will directly impact the catalyst.
• For exhaust with high humidity, check whether the demister or condensation unit is working properly to prevent moisture from condensing at low temperatures and forming a water film that covers the catalyst surface.
• If the exhaust contains elements such as sulfur or chlorine, periodically test the residual concentration of these impurities in the gas after pretreatment to ensure they are within the catalyst's tolerance range. If necessary, add deep purification units such as alkali scrubbers or activated carbon adsorbers.

The pretreatment system itself also requires regular maintenance – replace baghouse filter bags every six months to one year, and replace dry filter media based on pressure drop changes. Failure of any pretreatment link will cause irreversible catalyst damage in a short period.

2. Standardized Start-Stop Procedures: Avoiding Human Error

Non-standard start-up and shutdown are the primary human factors leading to abnormal catalyst deactivation. Many users only focus on parameters during operation but neglect protection during start-up and shutdown stages.

Precautions during start-up:

• Preheat the catalyst bed with fresh air first. Only after the temperature has stabilized and reached the designed light-off temperature (typically above 250-300°C, depending on catalyst specifications) can organic exhaust be slowly introduced.
• Never introduce exhaust when the bed temperature is below the light-off temperature. Under such conditions, organics cannot be fully oxidized and will form tarry carbon deposits on the catalyst surface. Once formed, these deposits are extremely difficult to remove and will permanently cover active sites.

Precautions during shutdown:

• First, cut off the exhaust source, keep the heater and fan running, and purge the catalyst bed with fresh air for at least 30 minutes.
• Only after the bed temperature drops below 150°C can the fan and power be turned off. This step prevents residual exhaust from condensing and contaminating the catalyst during cooling, while also avoiding thermal stress cracking of the catalyst due to sudden chilling.

Post these operating procedures on-site and conduct regular training and assessments for operators to prevent random operation based on experience.

3. Operating Parameter Monitoring: Three Key Indicators Cannot Be Ignored

Establish a daily monitoring log, recording at least the following three key parameters each day and comparing them with historical trends.

3.1 Purification Efficiency

Measure the inlet and outlet concentrations at least once a week using a portable VOCs detector or online monitoring system. Initial efficiency is typically above 95%. If efficiency continuously drops below 85% and cannot be restored by adjusting operating temperature or increasing airflow, the catalyst activity has significantly declined.

3.2 Bed Temperature

The normal operating temperature of catalysts is generally between 200-400°C. Check the temperature readings at various points on the bed daily, noting two points:

• If the reaction temperature required to maintain the same purification efficiency has increased by more than 30°C compared to initial operation, this is a typical signal of activity decline.
• Avoid local bed temperatures exceeding 650°C (for noble metal catalysts) or 700°C (for non-noble metal catalysts). High temperatures cause sintering and agglomeration of active components, resulting in permanent deactivation.

3.3 Bed Pressure Drop

Record the pressure drop across the catalyst bed (inlet to outlet) weekly. Under normal operation, the empty-bed pressure drop is typically between 500-1500 Pa. If the pressure drop suddenly rises more than 20% above the initial value, it usually indicates that catalyst channels are blocked by dust, carbon deposits, or high-boiling substances. If the pressure drop abnormally decreases, there may be gas bypass or catalyst damage.

Additionally, once a month open the equipment access port and visually inspect the catalyst surface condition: obvious dust accumulation, color changes (e.g., from dark brown to grayish-white may indicate poisoning), mechanical damage, or sintered agglomerates.

4. Deactivation Cause Diagnosis and Daily Response

When the above monitoring parameters show abnormalities, quickly determine the type of deactivation before taking targeted measures. Three common types of deactivation in daily maintenance:

Physical Coverage (Dust/Coke Deposition)

Manifestation: increased pressure drop, decreased efficiency, but little temperature change. Response: Use low-pressure compressed air at 0.3-0.5 MPa for back-blowing, or remove the catalyst and gently tap to remove loose dust. For carbon deposition, perform thermal regeneration.

Chemical Poisoning

Manifestation: rapid efficiency decline that is difficult to recover even by raising temperature, and abnormal colors appear on the catalyst surface (e.g., blackening may indicate sulfur poisoning; whitening may indicate chlorine or silicon poisoning). Response: For sulfur or chlorine poisoning, mild cases may try high-temperature thermal regeneration (500-600°C) with special atmosphere; severe poisoning cannot be regenerated and must be replaced. The core of daily prevention is to strengthen pretreatment and prevent poisons from entering.

High-Temperature Sintering

Manifestation: permanent efficiency decline, glassy or coarsened particle appearance on the catalyst surface. Response: Cannot be repaired; only replacement. In daily operation, avoid over-temperature operation and install high-temperature alarm interlocks.

If efficiency drops and the cause is unclear, do not blindly perform chemical cleaning or high-temperature regeneration. First contact the catalyst supplier for laboratory diagnosis.

5. Decision Timing for Regeneration and Replacement

In daily maintenance, timely regeneration of reversible deactivation is the most economical means to extend catalyst life.

Thermal Regeneration (Recommended for Carbon Deposits and Mild Poisoning)

Close the exhaust valve, introduce fresh air, and heat the catalyst bed to 250-300°C (for carbon deposits) or 500-600°C (for mild poisoning), hold for 6-8 hours. The heating rate must not exceed 5°C/min. Perform preventive thermal regeneration monthly or quarterly to effectively remove carbon deposits.

Chemical Cleaning (Qualified Personnel Only)

For specific metal ion deposits or alkaline fouling, soak in dilute acid or dilute alkali solutions. However, frequent chemical cleaning is not recommended in daily maintenance because residual cleaning agents may cause secondary damage.

Criteria for Replacement

Replace the catalyst promptly when any of the following conditions occur:

• Purification efficiency remains below 70% of the design value after two thermal regeneration attempts.
• Catalyst breakage rate exceeds 15%, causing downstream pipeline blockage.
• Operating time has exceeded the supplier's design life (typically 2-3 years).
• Severe sulfur, phosphorus, or silicon poisoning occurs and cannot be regenerated.

Note during replacement: spent catalyst is hazardous waste (HW49 category) and must be sealed, collected, and disposed of by a licensed hazardous waste treatment facility. Do not discard arbitrarily. Store new catalyst in a dry, light-protected, moisture-proof environment.

Establish a "one machine, one file" maintenance log, recording daily operating parameters, weekly efficiency test data, each thermal regeneration operation, and replacement records. Through systematic daily maintenance, the actual service life of VOC catalysts can be extended by 30%-50%, significantly reducing the overall environmental treatment costs for enterprises.