Volatile Organic Compounds (VOCs) not only cause photochemical smog and PM2.5 pollution, but also react with nitrogen oxides, sulfur oxides, etc. to form secondary pollution. VOCs treatment technologies are divided into source control, process control and end-of-pipe treatment. Source control is to take measures to inhibit the volatilization of the medium and reduce the amount of VOCs generated; Process control is to reduce VOCs leakage points through leak detection and repair technology;
End treatment is to achieve standard emission through closed treatment of VOCs recovery and treatment facilities.
In recent years, with the gradual tightening of national environmental protection policies, higher requirements have been put forward for VOCs treatment in oil storage depots.
VOCs in oil storage depots mainly come from large and small breathing of storage tanks and oil loading and unloading operations. Under the premise of meeting the requirements of the specifications, measures such as full liquid floating plates, high-efficiency seals, and low-leakage breathing valves can be used for large and small breathing of storage tanks to reduce the emission of VOCs in storage tanks. However, VOCs generated during the oil loading and unloading process can only be recovered or treated through end-of-pipe treatment to meet the requirements of emission indicators. Through the research on terminal treatment technology, there are many process routes for oil and gas recovery technology, oil and gas treatment technology, and combined with the characteristics of VOCs emission conditions in oil storage depots, liquid nitrogen cryogenic technology and mechanical condensation + adsorption technology are selected as VOCs treatment processes for oil storage depots. The VOCs treatment effects are compared in practice, and the feasibility of the technology is verified.
1 Oil and gas recovery technology
Common oil and gas recovery technologies include condensation, adsorption, absorption, membrane separation, etc. Their common characteristics are based on physical methods to separate VOCs from the air to achieve the purpose of reducing the concentration of VOCs in the air.
The basic principle of the condensation method is to reduce the oil and gas temperature to below its dew point temperature, so that the vapor pressure of some hydrocarbons in the oil and gas at different temperatures reaches a supersaturated state, thereby condensing the high-boiling point components into liquid precipitated oil and gas recovery technology. According to the different principles of condensation, it is divided into mechanical condensation and liquid nitrogen deep cooling, which is suitable for the recovery of high-concentration and single-component VOCs with recovery value. The basic principle of the adsorption method is to separate oil and gas from air based on the different adsorption forces of the adsorbent on hydrocarbons and air, which is suitable for the recovery of medium and low-concentration VOCs. The basic principle of the absorption method is to dissolve hydrocarbons in the absorption liquid according to the different solubility of different components in the oil and gas in the absorbent to achieve separation from the air. The hydrocarbons in the oil and gas can be absorbed by light component gasoline, low-temperature gasoline, kerosene, light diesel, cold ethylene glycol solution and special organic solvents. The basic principle of the membrane separation method is based on the principle of dissolution and diffusion. Since the membrane has selective permeability to oil and gas, the permeation rate of each component when passing through the membrane is different. The hydrocarbon component permeates to the vacuum side, and the air is retained by the membrane on the pressure side. The characteristics of the above oil and gas recovery technologies are as follows:
1) In the condensation method, mechanical condensation uses a compressor to cool the refrigerant, which can usually condense to -75 ℃. Liquid nitrogen deep cooling uses the vaporization of liquid nitrogen to directly cool the oil and gas, which can usually condense to -110 ℃. Due to the different composition of hydrocarbon components in oil and gas, three stages of condensation are usually set. The pre-cooling stage (2-5 ℃) is to condense the water and heavy components in the oil and gas, the shallow cooling stage (-50 ℃ to -30 ℃) is to condense the light components such as C4 and C5 in the oil and gas, and the deep cooling stage (below -80 ℃) is to condense the light components such as C3 in the oil and gas. According to the different saturated vapor pressures, the lower the temperature, the higher the oil and gas recovery efficiency, but limited by the refrigeration effect, liquid nitrogen deep cooling technology can achieve -110 ℃, which is already a relatively ideal solution in the current conventional technology.Since the condensation method has the problem of secondary volatilization of hydrocarbon components after condensation, the components after condensation need to be further processed.
2) The adsorption method is to adsorb VOCs through adsorbents, but the adsorbent will reach saturation after being used to a certain extent, resulting in a significant decrease in the adsorption effect of the adsorbent. In order to regenerate the adsorbent, desorption treatment is required, but the desorbed hydrocarbon components will re-enter the adsorption system and cannot be completely removed. At this time, it is necessary to cooperate with the absorption method for treatment, so that the high-concentration oil and gas analyzed by vacuum passes through the absorption tower. After countercurrent contact, the high-concentration oil and gas are absorbed by the absorption liquid, and the gas components that are not absorbed re-enter the exhaust gas system for adsorption, avoiding the penetration of the adsorbent due to cyclic adsorption, which makes the adsorbent ineffective.
3) The absorption method is divided into normal temperature and normal pressure absorption and normal temperature and low pressure absorption according to different processes. According to the principle of like dissolves like, hydrocarbons in oil and gas can be absorbed by light component gasoline, low-temperature gasoline, kerosene, light diesel oil, cold ethylene glycol solution and special organic solvents. The advantages are simple process, low investment cost and low safety risk, but there are also disadvantages of low recovery efficiency, so it is often necessary to use other processes for treatment. For oil storage depots, the source and treatment of absorbents become a key issue that restricts the long-term application of this technology.
4) The chemical properties and structure of the membrane in the membrane separation method have a decisive influence on the separation performance. The membrane separation material should have high permeability, high mechanical strength, chemical stability and good film-forming processing performance. The key to the application of membrane separation is the service life of the membrane. The use conditions of the membrane are relatively harsh. Impurities in the gas will block the membrane, resulting in a shortened membrane service life.
2 Oil and gas treatment technology
Oil and gas treatment technology commonly uses combustion method (also known as thermal oxidation method), which is a method of oil and gas treatment using the flammable nature of VOCs. VOCs are decomposed to generate CO2 and H2O after combustion. According to different combustion processes, they are divided into direct combustion (TO), regenerative combustion (RTO) and catalytic combustion (CO). Their common feature is that they are based on chemical methods. VOCs react under high temperature conditions to generate CO2 and H2O, achieving the purpose of reducing the concentration of VOCs in the air.
Direct combustion is to directly spray VOCs gas, air and auxiliary fuel into the furnace without heat recovery device. The combustion temperature is about 1100℃, which is suitable for treating high-concentration and high-calorific value VOCs waste gas. Thermal storage combustion absorbs and stores heat from the treated gas through a thermal storage ceramic or high-density inert material bed, and releases the heat to the low-temperature exhaust gas at the inlet. The VOCs exhaust gas is heated to 760~870℃, and the VOCs are burned and decomposed; the high-temperature gas generated passes through the ceramic thermal storage body to increase its temperature and accumulate energy, which is used to preheat the subsequent VOCs exhaust gas, thereby reducing the energy consumption of exhaust gas heating. The combustion temperature is 760~870℃, which is suitable for treating medium and low concentration VOC exhaust gas. Catalytic combustion is a gas-solid phase catalytic reaction. The catalyst is used to reduce the activation energy of the reaction and significantly reduce the reaction temperature. In the adsorption stage, VOCs molecules are adsorbed on the catalyst surface for enrichment, thereby increasing the concentration of reactants; in the oxidation stage, the catalyst reduces the activation energy of the reaction and increases the reaction rate. The combustion temperature is around 300-500°C, which is suitable for treating low-concentration VOC waste gas.
