Introduction
By the trends in social development, focusing on safety, environmental protection, energy conservation, cost reduction, and profit creation, as well as the investment and benefits sharing associated with the implementation of triple oil vapor recovery at gas stations, many gas station operators have proactively sought the installation of oil vapor recovery devices.
I. Significance of Oil Vapor Recovery at Gas Stations
1.1. Clean and Environmental Necessity
Oil vapor recovery effectively prevents much oil and gas from infiltrating the atmosphere, reducing environmental pollution and improving air quality.
1.2. Safety Requirement
Oil depots, oil handling and dispatching areas, and gas stations have become focal points for safety supervision and fire prevention. Oil vapor recovery enhances the safety coefficient of gas station operations.
1.3. Energy Conservation Requirement
Oil vapor recovery involves the liquefaction and reuse of recovered oil and gas, saving resources and bringing substantial economic benefits to businesses.
1.4. Demonstrative Effect
The addition of oil vapor recovery facilities, as an ecological cycling measure, holds significant implications for current energy conservation and emission reduction efforts.
II. Relevant Knowledge of Oil Vapor Recovery
2.1 Definition of Oil Vapor Recovery
Oil vapor recovery refers to the collection of volatile gasoline vapors during the loading and unloading of tanker trucks and refueling vehicles at gas stations. Through absorption, adsorption, or condensation processes, among others, the collected gasoline vapors are transformed from a gaseous to a liquid state, allowing for their reclamation and reducing gas pollution.
2.2 Components of Gas Station Oil Vapor Recovery Systems
Unloading Oil Vapor Recovery System (First-level Vapor Recovery System)
Refueling Oil Vapor Recovery System (Second-level Vapor Recovery System)
Online Detection System (Station Diagnosis System)
Gas Emission Treatment System (Third-level Vapor Recovery System)
Gas-Tight Storage
2.3 Main Locations of Gas Station Vapor Generation
Places in gas station operations where volatile organic compounds are generated include the vapor emissions during unloading from tanker trucks and refueling with fuel nozzles, as well as leaks from tank overfills or fueling mishaps. From the oil depot to tanker trucks, underground storage tanks, fuel dispensers, and the vehicles requiring refueling, all these stages contribute to gas emissions.
2.4 Techniques for Oil Vapor Recovery
Current methods employed in oil vapor recovery equipment include adsorption, membrane separation, condensation, absorption, or a combination of these approaches.
2.5 Key Areas of Oil Vapor Recovery
Studies indicate that in an unreformed gas station without oil vapor recovery, gas leaks during unloading and refueling each account for about 50%. Therefore, the efficiency of first and second-level vapor recovery forms the foundation for gas stations to undergo oil vapor recovery upgrades. The effectiveness of first and second-level vapor recovery fundamentally determines the overall efficiency of the gas station’s oil vapor recovery.
2.6 Principles of First and Second-level Vapor Recovery
The principles of first and second-level vapor recovery systems are based on a balance principle, with the volume of output gasoline equaling the volume of input gasoline at a ratio of 1:1. The first-level vapor recovery system can achieve balance using natural pressure. The principle of a vacuum-assisted second-level system involves using a vacuum pump during vehicle refueling to draw gasoline vapors from the vehicle’s gas tank through the vapor recovery nozzle, hoses, and vapor lines back to the underground gasoline storage tank.
III. Oil Vapor Recovery Process at Gas Stations
3.1 First-level Oil Vapor Recovery System — Unloading Oil Vapor Recovery System
3.1.1 Definition: The system collects gasoline generated during tanker truck unloading in a closed manner, directing it back into the tanker truck.
3.1.2 Retrofitting: Install a vapor return pipeline between the gasoline tank and the unloading port, adding a throttle valve, sealed quick coupling, and cap at the outlet. During unloading, connect the vapor recovery port of the tanker truck with the hose to ensure simultaneous unloading and vapor recovery into the tanker truck.
3.1.3 Technical Requirements: Underground pipeline DN100, equipped with DN100 quick coupling (with locking device) and throttle valve.
3.1.4 Underground pipeline slope not less than 1%, sloping towards the gasoline tank.
3.1.5 Install pressure vacuum valves at all vapor discharge points.
3.2 Second-level Oil Vapor Recovery System — Refueling Oil Vapor Recovery System
3.2.1 Definition: The system collects gasoline vapors generated during the refueling process in a closed manner, directing them into an underground storage tank.
3.2.2 Retrofitting: Bury a secondary vapor recovery pipeline between the lowest-grade gasoline tank and the fuel dispenser, simultaneously retrofitting the fuel dispenser with a vapor recovery vacuum pump, vapor recovery nozzle, hoses, vapor-liquid separation joint, breakaway valve, and other complete equipment.
3.2.3 Technical Requirements:
Vacuum-assisted closure for vapor generated during refueling.
Underground pipeline diameter not less than DN50, slope not less than 1%, sloping towards the gasoline tank.
Fueling hoses equipped with breakaway shut-off valves to prevent spills and drips during fueling.
Install pressure vacuum valves at all vapor discharge points.
3.2.4 Types: Can be classified as centralized or decentralized
3.2.4.1 Centralized Refueling Oil Vapor Recovery involves collecting vapors during refueling through a vapor return line, pumped back by one or more vacuum pumps into a low-grade gasoline tank. It can be powered by self-powered oil vapor recovery (with a motor and vacuum pump) or non-powered oil vapor recovery (using a submersible pump motor to provide vacuum pump power).
3.2.4.2 Decentralized Refueling Oil Vapor Recovery involves using a vacuum pump built into the fuel dispenser to pump vapors into a return line, which is then routed to a central return line connected to a low-grade gasoline tank. It can be powered by self-powered oil vapor recovery (with a motor and vacuum pump) or non-powered oil vapor recovery (using a self-priming pump motor to provide vacuum pump power).
3.3 Third-level Oil Vapor Recovery System — Vapor Emission Treatment Device
3.3.1 Definition: Building upon first and second-level vapor recovery at gas stations, this system processes vapors emitted during minor breathing (converting vapors into gasoline or temporarily storing them).
3.3.2 Types: Current equipment for third-level oil vapor recovery at gas stations primarily utilizes adsorption, membrane separation, condensation, absorption, or a combination of these methods.
3.3.3 Retrofitting: Connect a pipeline from the first-stage recovery port or gasoline tank vent pipe to the treatment device. Install third-level oil vapor recovery equipment, implement tank pressure monitoring, and activate the third-level oil vapor recovery equipment when pressure increases, treating the vapors. The treated liquid gasoline and vapors return to the lowest-grade gasoline tank.
3.3.4 Technical Requirements:
Pipeline diameter not less than DN50, slope not less than 1%, sloping in the opposite direction to the equipment.
The exhaust pipe height of the treatment device is greater than 4 meters.
Gasoline tank vent pipes should use pressure valves with pressure-retaining functions (P/V valves).
3.4 Online Detection System (Station Diagnosis System)
3.4.1 Definition: A system that monitors the gas-liquid ratio, airtightness of the oil vapor recovery system, and the normalcy of pipeline liquid resistance during the refueling oil vapor recovery process. It alerts operators to take appropriate measures when anomalies are detected and can record, store, process, and transmit monitoring data.
3.4.2 Technical Requirements: Must store data for at least one year, have remote transmission and exceedance warning functions, and analyze the airtightness of the oil vapor recovery system, liquid resistance of the vapor recovery pipeline, and the operational status of the treatment device through the data.
3.4.3 Installation Requirements: Depending on the online detection system used by each gas station, reserve power and signal control conduits in the retrofit design.
3.5 Gas Tight Storage
Includes all components affecting the airtightness of oil and gas storage, such as gas pipelines and all connected flanges, valves, quick couplings, and other related components. These components should ensure no gas leakage at pressures below 750Pa.
3.6 Selection Process for Gas Station Oil Vapor Recovery Equipment
When determining the oil vapor recovery solution for a gas station, consider factors such as the specifications and models of existing fuel dispensers, the process type of the fuel dispenser (self-priming pump or submersible pump), internal space of the fuel dispenser, prices, and operating costs of oil vapor recovery equipment, etc. Choose second and third-level oil vapor recovery equipment accordingly.
IV. Potential Issues in Oil Vapor Recovery Retrofit at Gas Stations
4.1 Inadequate System Airtightness
4.1.1 Insufficient sealing at any part, including fuel dispensers, process pipelines, tanks, and vent pipes, can lead to failed inspections.
4.1.2 Response Plan: Before equipment installation, pressure testing is mandatory. Conduct concealed tests for system airtightness in the process system, and choose thicker sealing gaskets for tank manhole covers.
4.2 Improper Slope and Liquid Accumulation in Vapor Recovery Pipelines Resulting in Excessive Liquid Resistance
4.2.1 A 1% slope towards the tank may generate significant elevation differences, especially in larger gas stations with limited tank burial depth, making it challenging to achieve the required pipeline slope, leading to pipeline crossovers.
4.2.2 Response Plan: Address pipeline cushioning, use supports when necessary, opt for rigid pipes, optimize process design to reduce elbows, install liquid collection pipes, or employ other appropriate solutions.
V. Trends in Oil Vapor Recovery Development
Existing drawbacks of oil vapor recovery equipment mainly include high construction and operational costs with relatively low output. Using the mature technology of activated carbon adsorption as an example, for gas stations with monthly gasoline sales exceeding 20 tons, the converted gasoline from recovered vapors can achieve a breakeven point. Profitability is attained for stations with monthly gasoline sales exceeding 40 tons. If more costly oil vapor recovery methods, such as membrane separation, are chosen, the payback period will be longer.
Therefore, the future trend is to produce oil vapor recovery equipment that combines multiple methods, integrating condensation, absorption, adsorption, membrane separation, and other techniques into one device, complementing each other to enhance recovery efficiency. Anticipated combinations include condensation + membrane separation, absorption + membrane separation + pressure swing adsorption, low-temperature cooling absorption + heat storage oxidation, absorption + adsorption, condensation + adsorption, condensation + biofiltration. This approach maximizes the advantages of each method.
VI. Outlook
Oil vapor recovery technology is a national and public-interest technology closely related to people’s daily lives. The evaporation of oil products is widespread in various stages of oil circulation, particularly during the refueling process at gas stations. Therefore, the development of oil vapor recovery technology is receiving increasing attention from countries and petroleum industry enterprises.
Actively promoting and developing oil vapor recovery technology is the responsibility and duty of everyone in the petroleum and energy industries.
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