With the increasing strictness of environmental regulations and the growing public awareness of environmental protection, the importance of vapor recovery technology in the petrochemical industry has become increasingly prominent. Activated carbon adsorption, as an efficient and economical vapor recovery technology, has been widely applied in fields such as oil storage facilities. However, during actual operation, abnormal temperature rise in the activated carbon adsorption tank frequently occurs, posing a serious threat to the safe operation of the system and recovery efficiency, necessitating urgent resolution. This study aims to thoroughly investigate the causes of abnormal temperature rise in activated carbon adsorption vapor recovery systems and propose effective prevention and control strategies to provide references for the optimized design and safe operation of vapor recovery systems.
1 Overview of Activated Carbon Adsorption vapor recovery Devices
Activated carbon adsorption vapor recovery devices primarily utilize the high adsorption efficiency of activated carbon to adsorb and recover volatile organic compounds (VOCs). Due to the advantages of activated carbon adsorption technology in vapor recovery devices, such as high adsorption efficiency, strong adaptability, economic feasibility, and stable operation, they are widely applied in finished oil storage facilities, yielding significant environmental and economic benefits.
1.1 Process Flow
The activated carbon adsorption oil vapor recovery system primarily consists of a pretreatment system (filters, coolers, etc.), an adsorption system (adsorption tanks, activated carbon bed layers, inlet systems, etc.), a desorption system (vacuum pumps, etc.), a recovery system (scrubber towers, etc.), and a control system (PLC, sensors, and instruments, etc.).
1) Exhaust gas enters the pretreatment system, where it is filtered and cooled before entering the adsorption system.
(2) The exhaust gas enters the adsorption system, where VOCs are adsorbed by activated carbon in the adsorption bed, and the purified gas is emitted.
(3) The exhaust gas enters the desorption system. When the activated carbon reaches adsorption saturation, it switches to desorption mode to desorb VOCs.
(4) The exhaust gas enters the recovery system, where the high-concentration VOCs gas after desorption is recovered.
(5) The system control system monitors and adjusts parameters in real-time across all stages to ensure efficient and stable operation of the system.
1.2 Working Principle
The core working principle of the activated carbon adsorption oil vapor recovery system is to selectively adsorb oil vapor mixtures using activated carbon, separating VOCs from the exhaust gas, and then recovering high-concentration VOCs through the desorption process. The process primarily consists of adsorption, desorption, and recovery stages.
(1) Adsorption stage.
Based on activated carbon's unique physical and chemical structure, which enables the capture and fixation of impurities in gases (such as organic compounds and odor molecules), when exhaust gas containing VOCs passes through the activated carbon bed after filtration and cooling, the activated carbon effectively adsorbs VOC molecules, and the purified gas is discharged in compliance with standards. The core mechanism of activated carbon adsorption can be divided into two types: physical adsorption and chemical adsorption. Physical adsorption is achieved through van der Waals forces between molecules. Adsorbate molecules
(such as VOCs, pigments, etc.) are captured by the porous structure and large specific surface area of activated carbon. Chemical adsorption is dominated by surface reactions. Adsorbate molecules form chemical bonds (such as covalent bonds, hydrogen bonds, or ion exchange) with functional groups on the activated carbon surface, commonly observed in the adsorption of polar molecules (such as heavy metal ions or acidic gases). The primary pollutants in petroleum storage facilities are VOCs, where physical adsorption dominates, but chemical adsorption should not be overlooked, especially under high-temperature conditions or in the presence of specific functional groups.
(2) Desorption stage.
After activated carbon reaches adsorption saturation, the process of desorbing adsorbed substances from the activated carbon surface using physical or chemical methods is a critical step for activated carbon regeneration and reuse. Common methods include thermal desorption, pressure desorption, and displacement desorption. Pressure desorption is commonly used in petroleum storage facilities. This involves reducing system pressure using equipment such as vacuum pumps to decrease the amount of adsorbate on the activated carbon surface. Once pressure is reduced, the adsorption equilibrium is disrupted, and adsorbate molecules desorb from the activated
carbon surface.
(3) Recovery stage.
The high-concentration VOCs gas after desorption enters the recovery system, where it can be recovered using methods such as absorption, condensation, or membrane separation to reduce environmental pollution and resource waste. Physical absorption is commonly used in finished oil storage facilities, where high-concentration VOCs gas comes into countercurrent contact with an absorbent. The separation is achieved by utilizing the difference in solubility of VOCs in the absorbent (such as gasoline), causing the VOCs to dissolve into the absorbent.
2 Abnormal temperature rise in activated carbon and its hazards
The normal operating temperature range for activated carbon is typically 40–60°C. Within this temperature range, activated carbon maintains high adsorption capacity and rapid adsorption rates. However, in actual use,
the temperature of the activated carbon bed may significantly exceed the normal operating temperature. Based on the extent and rate of temperature increase, abnormal temperature rise can be classified into two types: slow temperature rise and rapid temperature rise. Slow temperature rise is often caused by activated carbon aging or the accumulation of trace impurities, typically manifesting as the activated carbon bed layer temperature gradually increasing at a rate of 1–3 °C/h, potentially reaching dangerous levels within hours or days; Rapid temperature rise is often caused by localized adsorption heat concentration or uncontrolled exothermic reactions (such as olefin polymerization or sulfur compound oxidation), characterized by a temperature increase of over 10°C within minutes, with continued rapid rise, potentially reaching dangerous levels within minutes or hours.
Regardless of the form of abnormal temperature rise in activated carbon, when the temperature exceeds a certain threshold, it is accompanied by abnormal phenomena such as a decrease in activated carbon adsorption capacity, fluctuations in bed pressure, and excessive exhaust gas emissions, and may even lead to safety issues such as spontaneous combustion. The hazards are primarily manifested in the following three aspects.
(1) Decline in operational efficiency.
Temperature increases reduce the adsorption capacity of activated carbon. According to adsorption thermodynamics theory, physical adsorption is an exothermic process, and adsorption capacity decreases with increasing temperature. Experimental data statistics show that for every 10°C increase in bed temperature, the adsorption capacity of activated carbon for VOCs such as benzene and toluene decreases by 10% to 17%, which can lead to reduced recovery efficiency of the system and even exceed emissions standards,
increasing the risk of environmental penalties.
(2) Equipment damage.
Temperature increases can cause localized overheating in adsorption tanks (e.g., at weld seams), leading to thermal stress corrosion, reduced pressure resistance, and inability to achieve vacuum levels during desorption, or even structural deformation and cracking of the tank; during desorption, heated oil vapors from the bed layer can damage vacuum pumps; temperatures exceeding 200°C can cause collapse of activated carbon microporous structures, reducing specific surface area by 40%–
60%, and a decrease in iodine value to 400 mg/g, leading to the deactivation of activated carbon.
(3) Increased safety risks.
When the bed layer temperature exceeds the autoignition point of activated carbon, it may cause open flames. If the vaporconcentration inside the tank reaches the explosion limit, it may also lead to a deflagration accident. Hazardous chemical production and storage companies have experienced similar typical accidents due to abnormal temperature increases in activated carbon. For example, in 2021, a refinery experienced an accident due to an incorrect desorption temperature setting (set to 180°C, with a design value of 120°C), causing the activated carbon bed temperature to rise to 250°C within 3 hours, resulting in tank cracking, VOC leakage of 1.2 tons, and direct economic losses exceeding
5 million yuan. In 2019, sulfur compounds accumulated and oxidized on the surface of activated carbon at a petroleum storage facility, triggering spontaneous combustion at a local hotspot of 310°C. The fire spread to adjacent storage tanks, and it took 72 hours to extinguish.
