[Haichuan’s Best Practices] All refining storage tanks at Yangzi Petrochemical have completed VOCs control measures

Regulatory standards for VOCs control in storage tanks: The \"12th Five-Year Plan for Joint Prevention and Control of Air Pollution in Key Areas\", formulated as part of the \"12th Five-Year Plan for National Economic and Social Development of the People’s Republic of China\", stipulates that it is necessary to strengthen the monitoring and supervision of volatile organic compound leaks during the production, transportation, and storage of petrochemical products； Strict control should be exercised over breathing losses in storage tanks and during transportation; storage facilities for raw materials, intermediate products, and finished products should all use highly efficiently sealed floating roof tanks, or equipped with overhead space connection systems for the recovery of oil and gas (that is, systems to connect and recover the exhaust gases from the storage tanks). It is required that \"raw materials, intermediate products, and finished products should be stored in sealed conditions; control measures must be taken for organic liquid storage tanks with a actual vapor pressure greater than 2.8 kPa and a storage volume of more than 100 m³.\" The **standard \"Emission Standards for Pollutants in the Petrochemical Industry\" (GB31571-2015)**, issued in April 2015, stipulates that for volatile organic liquid storage tanks using internal floating roof designs, efficient sealing methods such as liquid-filled sealing, mechanical shoe-type sealing, or double-seal systems should be employed between the floating roof and the tank wall. In the case of tanks with fixed roofs, a sealed exhaust system should be installed to lead the organic waste gases to recovery or treatment units. The \"Guiding Opinions on the Implementation Plan for the Oil-Gas Connection Process and Safety Measures for VOCs Control Projects in Petrochemical Storage and Transportation Areas\" (Letter No. 127), issued on October 25, 2016 (hereinafter referred to as the \"Guiding Opinions\"), provides specific recommendations regarding the methods for connecting different types of storage tanks, the stable control of emissions from these tanks, the basic configuration of equipment installed on tank tops, the selection of pipeline materials, control measures to prevent safety accidents in tank clusters, as well as the balanced control of nitrogen sealing systems. Section “II-(IV)-4” of the \"Comprehensive Plan for the Control of Volatile Organic Compounds in the Petrochemical Industry\" (Environmental Regulation [2014] No. 177) also requires that hazardous chemicals such as benzene, toluene, and xylene should be equipped with treatment facilities such as vapor recovery systems installed on top of internal floating roof tanks”

Methods for controlling VOCs in storage tanks: The main approach to controlling VOCs in storage tanks is to collect the VOCs that are emitted and then treat them. In short, the main method for controlling VOCs in storage tanks is “top connection.” The VOC emissions from the roof vents of multiple storage tanks in the tank farm are connected via gas pipelines, brought together, and then VOC gas recovery and treatment equipment is installed at the end of these combined gas pipelines. How to install a sealed exhaust system for organic waste gas recovery or treatment devices is an important issue in the field of VOCs management research. On one hand, sealed vapor pipelines are installed outside the breathing vents of the tanks; this adds resistance to the breathing exhaust, which can pose safety risks related to the pressure exerted on the tank walls. On the other hand, when multiple tank tops are connected to each other, it is necessary to prevent accidents such as fires in one tank from affecting the other tanks. It is also essential to control the fire and explosion risks associated with these connections, in order to ensure the safe operation of the tanks and prevent accidents from impacting the safety of the entire tank area.

For the treatment of VOCs in storage tanks, a combined condensation + absorption process is used. The condensation method typically employs a multi-stage (three-stage) continuous cooling approach to separate oil and gas. The condensation process is divided into pre-cooling, primary condensation, secondary condensation, and tertiary condensation. Generally, four levels of condensation temperatures are set, at 5, –35, –75, and –110 °C respectively. Due to the strict requirements for benzene emissions, and since the adsorption efficiency of a single-stage activated carbon tank is between 97% and 99%, it is not sufficient to meet these emission standards. Therefore, an additional adsorption tank is needed; by using a two-stage activated carbon system, the adsorption efficiency for benzene and other hydrocarbons can reach over 99.5%, thus meeting the emission requirements. The combined process of condensation (pre-cooling) and adsorption eliminates the energy-intensive cryogenic stage; the adsorption unit handles gas with lower concentrations, which facilitates the long-term operation of this combined process.

The combined process of condensation and catalytic oxidation for VOCs treatment in storage tanks: The principle of catalytic oxidation is that, at temperatures between 250 and 450 °C and in the presence of a catalyst, VOCs react with oxygen to produce water and carbon dioxide. This method is currently used to treat exhaust gases from wastewater treatment plants in petrochemical companies, emissions from benzene storage tanks, rubber processing emissions, polyether-related emissions, and emissions generated during the loading of chemicals onto ships. More than 20 such treatment systems have been installed in petrochemical enterprises. For example, the high-concentration exhaust gas treatment system at the wastewater treatment plant of Sinopec Jinling Branch utilizes catalytic oxidation combustion technology, achieving an average removal rate of 92.58% for total hydrocarbons; Yangzi Petrochemical – BASF Co., Ltd. collects the exhaust gases generated in the polyethylene production process and sends them to a regenerative oxidation furnace, where the organic gases in those exhaust gases are effectively oxidized and decomposed under high temperatures.

The combined process of condensation + membrane separation + adsorption for VOCs treatment in storage tanks: The condensation method is suitable for the preliminary treatment of gas streams with high concentrations, as it can rapidly reduce the oil and gas concentration; the adsorption process is more effective for dealing with gas streams at lower concentrations, and can be placed at the end of the system; membrane separation works well with various oil and gas concentrations, but it does not produce a liquid product directly, making it suitable as an intermediate step between the condensation and adsorption processes. At present, the benzene tank farm of CNPC Yunnan Petrochemical Co., Ltd., the aromatic hydrocarbons tank farm of Sinopec Sichuan Petrochemical Co., Ltd., and the triphenyl railway loading system of Sinopec Changling Branch Company (hereinafter referred to as Changling Branch) all employ the condensation – absorption – membrane separation – pressure swing adsorption process, which has demonstrated good performance. ,

Collection of heavy benzene/rough benzene vapor: The benzene-containing vapor is collected and then sent to an incinerator for burning. A vapor collection branch is installed at the top of each storage tank, and flame arrestors designed to prevent detonation are placed near these tanks along the VOC vapor lines; such flame arrestors serve to prevent the spread of fire under extreme conditions. Isolation valves are installed at both ends of the flame arrestors. A pressurized liquid ring compressor, in the form of skid-mounted equipment, is installed behind the oil and gas main pipe. The booster fan is equipped with a variable frequency speed control system, which ensures smooth exhaust from the tank top while also helping to save energy.

Collection of heavy benzene/rough benzene vapor: The benzene-containing vapor is collected and then sent to an incinerator for burning. A vapor collection branch is installed at the top of each storage tank, and flame arrestors designed to prevent detonation are placed near these tanks along the VOC vapor lines; such flame arrestors serve to prevent the spread of fire under extreme conditions. Isolation valves are installed at both ends of the flame arrestors. A pressurized liquid ring compressor, in the form of skid-mounted equipment, is installed behind the oil and gas main pipe. The booster fan is equipped with a variable frequency speed control system, which ensures smooth exhaust from the tank top while also helping to save energy.

Analysis of the application of triphenyl recovery technologies: The combination of condensation + membrane separation + adsorption is suitable for treating VOCs in large aromatic hydrocarbon storage areas, and it can meet stringent environmental standards. The nitrogen gas purified by the adsorber can be recycled to reduce nitrogen consumption, although this approach requires significant investment. The combination of condensation + catalytic oxidation enables the recovery of some triphenyl compounds; catalytic oxidation allows for thorough treatment of the oil gases (typically to levels below 20 mg/m3), satisfying strict environmental emission requirements and being suitable for dealing with VOCs at lower concentrations. The combination of condensation + secondary adsorption meets environmental standards, but it requires a large amount of space, and the relatively high concentration of the oil gases after pre-cooling can lead to easier deactivation of the activated carbon. Direct disposal of benzene-containing oil gases in the kiln exhaust system may result in significant pressure fluctuations due to the large volume of gas, and because the concentration is low, more fuel gas is required for combustion.

In the context of VOCs control in storage tanks, ensuring the safety of multiple tanks connected together is a critical issue. The VOCs transported through the connecting pipelines are flammable and explosive gases; therefore, it is essential to find ways to prevent a fire in one of the tanks from spreading to the others, thereby ensuring the safety of all tanks involved. In response to the repeated incidents of leaks, fires, and explosions in chemical tank areas, the Third Department under the State Administration of Work Safety issued Notice No. 68 on 2014/07/16, ordering that the exhaust gas connection and recovery systems used in various chemical storage tanks be suspended immediately, and that they could be put back into use only after passing safety assessments. ” To address the safety risks related to fire and explosion in groups of tanks resulting from connections at the tank tops, and to ensure the safe operation of the tanks while preventing accidents from affecting the safety of the tank area, control measures should be taken from four aspects: First, flame arresters capable of preventing detonations must be installed at locations such as the points where the VOC vapor lines connect to the tanks and at the inlet of waste gas treatment systems; these flame arresters should be made of stainless steel. II. Intrinsically safe technologies should be employed for oil and gas collection, and it is necessary to ensure that such technologies are mature, reliable, energy-efficient, cost-effective, and easy to operate. III. Enterprises should establish positions for operating VOCs treatment facilities, formulate system operation procedures and process cards, and enforce their strict implementation. IV. Establish emergency response mechanisms and measures for the control and treatment of VOCs in the event of abnormalities or accidents in downstream waste gas treatment devices. The selected flame arrestors designed to prevent deflagration must meet the test requirements specified in the current international standard ISO16852 and standard GB/T 13347, with third-party test verification documents provided. Moreover, the pressure drop should not be greater than 0.3 kPa. Cut-off valves should be installed at both ends of the deflagration-resistant flame arrester, and measures to clear blockages and prevent freezing should be put in place based on weather conditions and the properties of the oil and gas.