Why does catalyst deactivation easily occur in formaldehyde production equipment under low load operation?
Publish Time: 2025-11-13
Formaldehyde, as an important basic chemical raw material, is widely used in industries such as resins, plastics, textiles, and building materials. Its industrial production mainly adopts the methanol oxidation method, with the core process relying on the efficient catalytic action of silver-based or iron-molybdenum-based catalysts in the reactor. However, in actual operation, once formaldehyde production equipment enters a low-load state—i.e., with reduced feed rate, lower reaction temperature, and slower gas flow rate—the catalyst often experiences a rapid decline in activity or even permanent deactivation. This problem not only affects product quality and yield but can also lead to unplanned shutdowns and high replacement costs. The reason for this is that catalyst deactivation is not caused by a single factor but is the combined result of multiple factors including thermodynamics, kinetics, and operating conditions.
1. Insufficient reaction temperature leads to increased side reactions
Formaldehyde synthesis is a strongly exothermic process that is highly sensitive to temperature. Under the designed load, the reaction system can be maintained within the optimal temperature window to ensure efficient conversion of methanol to formaldehyde while suppressing side reactions such as deep oxidation. However, under low load operation, the reduced methanol feed leads to a significant decrease in the heat released from the reaction. If heating or heat balance adjustment is not timely, the temperature in the reaction zone can easily drop below the catalyst activity threshold. At this point, methanol cannot be fully oxidized to formaldehyde; instead, it is more prone to dehydration, polymerization, or the formation of byproducts such as formic acid and carbon monoxide. These byproducts not only reduce the selectivity of the target product but also deposit carbon deposits or acidic substances on the catalyst surface, covering active sites and causing "poisoning" deactivation.
2. Low gas flow velocity causes localized hot spots and coking.
Under normal load, high-speed gas flow not only promptly removes the heat of reaction but also flushes the catalyst bed, preventing localized overheating and byproduct accumulation. However, at low loads, the gas flow velocity decreases significantly, making it difficult to dissipate heat evenly and easily forming "hot spots" in certain areas of the catalyst bed. The abnormally high temperature in these hot spot areas accelerates the thermal decomposition of methanol or formaldehyde, generating tar-like polymers that firmly adhere to the catalyst pores and surface. This coking phenomenon is cumulative and difficult to reverse once it begins, ultimately clogging pores, blocking reactant diffusion paths, and causing complete catalyst failure. Furthermore, low flow rates reduce disturbance to catalyst particles, making them more susceptible to sintering or structural collapse due to prolonged static conditions.
3. Water Vapor and Impurity Concentration Exacerbate Corrosion and Poisoning
Water vapor is inevitably generated during formaldehyde production, and under low load conditions, the relative concentration of water vapor per unit volume of gas actually increases. For silver catalysts, high temperature and humidity promote silver grain migration and agglomeration, leading to a reduction in active surface area; for iron-molybdenum catalysts, excessive moisture may cause molybdenum component loss or the formation of insoluble molybdates, damaging the active phase structure. Simultaneously, trace impurities in the raw material methanol are more likely to accumulate on the catalyst surface under low flow conditions, undergoing irreversible chemical reactions with the active components, causing permanent poisoning. These slow but continuous degradation processes are significantly amplified under low load conditions.
Low load operation is often accompanied by frequent load adjustments, start-stop switching, or parameter fluctuations. Each sudden change in temperature, pressure, or concentration causes thermal or chemical shock to the catalyst. Prolonged exposure to this unstable state causes microcracks to form in the catalyst's internal microstructure due to repeated expansion and contraction, leading to the gradual shedding of active components and a decrease in mechanical strength. While this "fatigue deactivation" is less obvious than poisoning or coking, it is a significant, albeit hidden, cause of shortened catalyst lifespan under low-load operation.
The catalyst in formaldehyde production equipment is prone to deactivation under low-load operation because the reaction conditions deviate from the optimal window, triggering a series of chain reactions including side reactions, coking, corrosion, poisoning, and structural degradation. To mitigate this problem, it is necessary to optimize the control system, improve the catalyst formulation, set minimum operating load limits, and strengthen raw material purity management to ensure stable and long-lasting catalyst operation even under non-full-load conditions.
