Analysis and selection of VOCs treatment technologies for oil storage tanks 2

A method. Combustion method (also known as thermal oxidation method) is commonly used in oil and gas treatment technology. It uses the flammable nature of VOCs to treat oil and gas
CO2 and H2O. According to different combustion processes, it is divided into direct combustion (TO), thermal storage 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 inject VOCs gas, air and auxiliary fuel directly into the furnace. There is no heat recovery device. The combustion temperature is about 1100°C. It is suitable for treating VOCs waste gas with high concentration and high calorific value. Regenerative combustion absorbs and stores heat from the treated gas through a bed of regenerative ceramics or high-density inert materials, and releases the heat to the low-temperature exhaust gas at the entrance. The VOCs exhaust gas is heated to 760~870°C, and the VOCs are burned and decomposed; After the generated high-temperature gas passes through the ceramic regenerator, it heats up and accumulates energy, which is used to preheat the subsequent incoming 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 concentrations. VOC exhaust gas. Catalytic combustion is a gas-solid phase catalytic reaction that uses a catalyst to reduce the activation energy of the reaction and significantly reduce the reaction temperature. In the adsorption stage, VOCs molecules are adsorbed to the surface of the catalyst and enriched, thereby increasing the concentration of reactants; in the oxidation stage, the catalyst reduces
It increases the activation energy of the reaction and increases the reaction rate. The combustion temperature is around 300-500°C, and it is suitable for treating low-concentration VOC waste gas.

Analyzing the characteristics of the above treatment technologies, combustion technology usually has open flames and needs to meet the relevant requirements of the specifications for fire protection distances and measures. It poses great challenges to the overall layout of oil storage depots and the maximization of land use, especially for the VOCs treatment of existing oil storage depots. It is difficult to find a suitable location to install combustion facilities.

1) The direct combustion process is relatively simple, and the equipment operation and maintenance are convenient, but the working temperature of the combustion process is high, the energy consumption is relatively high, and the combustion process is accompanied by the generation of NOx, causing secondary pollution, which is not in line with the original intention of environmental protection.

2) Thermal storage combustion is suitable for treating medium-concentration VOCs waste gas, but for low-concentration VOCs waste gas with large air volume, adsorption-concentration is required for treatment. The treatment technology has a high working temperature, a large amount of waste gas treatment, high treatment efficiency and heat recovery rate, low energy consumption, long life of thermal storage ceramics, and easy maintenance of equipment, but the safety risk is relatively high, and safety risk protection needs to be strengthened.

3) Catalytic combustion usually uses precious metal catalysts such as platinum and palladium. If necessary, other precious metals can be added for modification. Although the catalyst investment is high and the catalyst has a service life problem, the combustion treatment technology has low temperature and low energy consumption, which can effectively reduce or even eliminate NOx generation. Comparing the characteristics of the above three combustion technologies and the supporting conditions of the oil storage depot, compared with catalytic combustion technology, direct combustion technology and heat storage combustion technology require the use of fuel for combustion, resulting in higher energy consumption and greater difficulty in implementation. Therefore, it is more feasible for oil storage depots to choose catalytic combustion as the terminal treatment technology.

The VOCs emission of oil storage depots mainly comes from the large and small breathing of storage tanks and the loading and shipping operations, which are characterized by non-steady-state conditions caused by non-continuous operation. Characteristics of oil and gas in non-steady-state conditions: large fluctuations in gas volume, and the elasticity of running or operating conditions is between 0 and 100%; high oil and gas concentration, large fluctuations, often within the explosion range; complex oil and gas components, from C2 to C10 alkanes, olefins, cycloalkanes, benzene and other substances exist, and the medium types are many and complex; complex working conditions, the collected oil and gas have various working conditions such as oxygen-containing and oxygen-free, which puts forward higher requirements for oil and gas treatment. Because oil storage depots are non-production petrochemical enterprises, especially third-party commercial storage depots, due to the limited sources of auxiliary fuels and raw materials in VOCs treatment work, and the treatment of components recovered from oil and gas is difficult, the selectivity of technical solutions has become smaller.

In view of the 25 g/m3 emission requirement of GB 20950 "Emission Standards for Air Pollutants in Oil Storage Depots" and the working characteristics of VOCs waste gas in oil storage depots, the feasibility of technical application is summarized, and combined with market research and technical development trends, it is believed that liquid nitrogen cryogenic technology and mechanical condensation + adsorption technology are the preferred technical routes for VOCs treatment in oil storage depots. Liquid nitrogen cryogenic technology uses liquid nitrogen as a refrigerant. It absorbs heat during the vaporization process of liquid nitrogen to reduce the temperature of oil and gas to below the dew point, 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 precipitation. Usually, the oil and gas can be condensed to -110 ℃, and the emission can be directly up to standard. Mechanical condensation + adsorption technology uses heat exchange in a refrigerator to condense the oil and gas to -75°C. Some large molecular hydrocarbons are converted from gas to liquid and condensed, but they cannot achieve direct emission standards. Adsorption technology is needed to adsorb small molecular hydrocarbons on activated carbon to meet emission standards for discharge.

For VOCs treatment in oil storage depots, two technical routes, liquid nitrogen cryogenic technology and mechanical condensation + adsorption technology, were selected as VOCs treatment processes, and key operating indicators such as treatment effect, emission concentration, operating cost, and equipment maintenance were comprehensively compared. Liquid nitrogen cryogenic VOCs treatment process: The designed waste gas treatment capacity is 1,500 m3/h, which is used to treat the oil and gas generated by the ship loading operation, and the minimum condensation temperature is -110°C; Mechanical condensation + adsorption VOCs treatment process: The designed waste gas treatment capacity is 1,500 m3/h, which is used to treat the oil and gas generated by the ship loading operation, and the minimum condensation temperature is -75°C. The condensed VOCs waste gas enters the adsorption system for adsorption and then is discharged. 

Based on the in-depth study of the principle of VOCs treatment technology, the liquid nitrogen cryogenic technology and mechanical condensation + adsorption technology were used to treat the VOCs waste gas in the oil storage depot according to the working conditions of the VOCs waste gas in the oil storage depot. The VOCs emission concentration after testing and treatment was below 10 g/m3, which was far lower than the 25 g/m3 required by the national emission standard. The feasibility of using liquid nitrogen cryogenic technology and mechanical condensation + adsorption technology to treat VOCs in the oil storage depot was verified from both theoretical and practical aspects. However, during the application of the technology, it was also found that the operating cost was high, which increased the operating burden of the enterprise and seriously restricted the promotion and application of VOCs treatment technology. It is urgent to study low-energy VOCs treatment technology to achieve the purpose of meeting the emission standards and reducing the operating costs of enterprises.