Oil and gas recovery and an introduction to four methods

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Oil and vapor recovery refers to the process of collecting the volatile gasoline vapors that are generated during the loading and unloading of gasoline and while refueling vehicles. Through one or a combination of methods such as absorption, adsorption, or condensation, this process either reduces the pollution caused by these vapors or converts them from a gaseous state to a liquid state, thereby turning them back into gasoline for reuse. 　　Oil and gas recovery is an energy-saving and environmentally friendly high-tech solution. By using this technology to capture the oil and gas released during storage, transportation, and loading/unloading processes, it prevents air pollution caused by the evaporation of such gases, eliminates safety hazards, and reduces economic losses through improved energy efficiency, thereby yielding significant benefits. Common methods currently include adsorption, absorption, condensation, and membrane separation. Introduction to several oil and gas recovery technologies 1. Adsorption method The separation of oil and gas from air is achieved by utilizing the adsorption capacity of adsorbents such as activated carbon, silica gel, or active fibers. The oil and gas pass through adsorbents such as activated carbon; the components of the oil and gas get adsorbed on the surface of these adsorbents. Subsequently, desorption is carried out under reduced pressure or using steam, and the concentrated oil and gas is pumped into oil tanks using vacuum pumps or liquefied by other methods ； Adsorbents such as activated carbon have very low adsorption capacity for air, and the unadsorbed exhaust gases are released through the exhaust pipe. 　　Advantages: The adsorption method can achieve a high treatment efficiency ； 　　The emission concentration is low, and can reach very low values. 　　Disadvantages: Complex manufacturing process, requires secondary processing ； 　　Adsorption beds are prone to developing high-temperature hot spots, posing safety risks ； 　　Triphenyls can easily deactivate activated carbon, and deactivated activated carbon poses a problem of secondary pollution ； 　　The adsorption capacity of domestically produced activated carbon is generally only around 7%, and its lifespan is short – it usually needs to be replaced every 2 years or so. The cost of replacing activated carbon is quite high. 　　At present, only the adsorption equipment designed and manufactured by Japan System Engineering Services Co., Ltd. (SES) is capable of achieving not only the high recovery rates specified in the latest regulations but also exceeding the emission standards required by those regulations. This company uses special hydrophobic silica gel and mesoporous carbon activated carbon as adsorption materials, **which increases the recovery rate; moreover, the service life of these adsorption materials is 10 years and over 5 years respectively. The company’s equipment is well-designed; it operates at normal temperature and pressure, and the adsorption material does not generate high temperatures nor static electricity, resulting in very high safety levels. It is currently the most advanced oil and gas recovery technology in the world. 　　2. Absorption method: Separation of oil and gas from air is carried out based on the differences in solubility of various components in the mixed oil and gas within the absorbent. Generally, lean fuels such as diesel are used as absorbents. Generally, countercurrent contact is employed between the oil and gas and the absorbent sprayed from the top of the absorption tower; the absorbent selectively absorbs the hydrocarbon components, while the unabsorbed gas is discharged through a flame arrester. The absorbent then enters a vacuum desorption tank for desorption, after which the enriched oil and gas are absorbed using oil products. 　　Advantages: Simple process and low investment cost. 　　Disadvantage: The recovery rate is too low, typically only around 80%, failing to meet the current **standards ； 　　The equipment requires a large amount of space ； 　　High energy consumption ； 　　The absorbent is consumed in large quantities, so it needs to be replenished continuously ； 　　The pressure drop is too high, around 5000 pascals. 　　3. Condensation method: This method utilizes refrigeration technology to remove the heat from oil and gas, enabling a direct conversion of their components from the gas phase to the liquid phase. The condensation method is a technique for recovering oil and gas by taking advantage of the differences in vapor pressure of hydrocarbons at different temperatures; by cooling, the vapor pressure of certain hydrocarbons in the oil and gas is brought to a supersaturated state, and the supersaturated vapor condenses into a liquid state. A multi-stage continuous cooling method is generally used to lower the temperature of the oil and gas, causing them to condense into a liquid for recovery. The minimum temperature of the condensation device is determined based on the composition of the volatile gases, the desired recovery rate, and the allowable concentration limits of organic compounds in the exhaust gases released into the atmosphere. 　　It is generally achieved through steps such as pre-cooling and mechanical refrigeration. The pre-cooler is a single-stage cooling device. To reduce the operational energy consumption of the recovery unit, a technology for reusing cold energy has been developed, which lowers the temperature of the gas entering the recovery unit from the ambient temperature to around 4°C, allowing most of the water vapor in the gas to condense into water and be removed. After leaving the pre-cooler, the gas enters the shallow cooling stage. The gas temperature can be cooled to –30°C to –50°C, and as desired, nearly half of the hydrocarbons in the oil and gas can be recovered. The slightly cold oil and gas proceed to the deep-cold stage, where they can be cooled to temperatures ranging from –73°C to –110°C. The temperature is set according to specific requirements, and the compressors are configured accordingly. 　　Advantages: Simple process principle ； 　　The recycled liquid oil can be seen visually ； 　　High security ； 　　High level of automation. 　　Disadvantage: The single-condensation method requires temperatures to be reduced to very low levels in order to meet the standards. 　　4. Direct combustion method: This method involves directly oxidizing and burning the hydrocarbon-containing gases generated during storage and transportation; the carbon dioxide, water, and air produced as a result of combustion are released directly as purified gases after treatment. This process flow serves only as a measure to control oil and gas emissions; it cannot recover oil products and has no economic value. 　　5. Membrane separation method: Utilizing the property of special polymer membranes to allow hydrocarbons to pass through more easily, this method involves forcing a mixture of oil and gas along with air under certain pressure; as a result, the oil and gas molecules pass through the polymer membrane first, while the air components are retained and removed. The concentrated oil and gas are then transferred back to the oil tank or liquefied using other methods. 　　Advantages: Advanced technology, relatively simple process ； 　　Low emission concentration and high recovery rate. 　　Disadvantage: High investment cost ； 　　The membranes have not yet been manufactured domestically; they are expensive, and their lifespan is short ； 　　Membrane separation devices require stable-flow, stable-pressure gas, and have high operational requirements ； 　　In conditions of low oil and gas concentrations and high air volume, the membrane is prone to forming a discharge layer, posing a safety hazard. Research and Current Application Status of Several Oil and Gas Recovery Technologies 1. Adsorption Method Activated carbon adsorption units represented by Jordan Company in the United States and KUSO Deep Company in Denmark, as well as silica gel + activated carbon adsorption units and silica gel adsorption units developed by System Engineering Services Co., Ltd. in Japan. The regulations of Tokyo Metropolitan Government in Japan stipulate that when the concentration of oil and gas is ≥1 vol%, the use of flammable activated carbon adsorbents is prohibited; moreover, oil and gas recovery technologies based on membrane separation and activated carbon adsorption are banned in Japan. 　　Currently, 6 activated carbon-based oil and vapor recovery units have been installed in China, 5 of which are imported products; aside from the one used at a refinery in North China for oil and vapor recovery during railway loading, the performance of the rest is fairly satisfactory. Three sets of equipment designed for oil depots failed to achieve the expected performance levels due to issues with the integrity of the dip pipes and the oil and gas collection systems. A unit at a refinery in the northwest has been idle for over 5 years since its installation. The only set of domestically produced adsorption units in use has been operational for only a short time. Due to the frequent need to replace the activated carbon, and the large amount of carbon that needs to be replaced at each time, the operating costs are too high; as a result, these units are currently not in use. 　　2. Absorption method 　　Due to its fatal flaws, the absorption method is now rarely used on its own. The use of absorption-based oil and vapor recovery equipment is extremely rare in Europe and the United States. Three absorption-based oil and vapor recovery units have been installed in China, including two using specialized absorbents and one using diesel-based absorbents. Based on the operational performance of devices currently in use, among various oil and gas recovery technologies, the absorption method has the lowest recovery rate. The first oil and gas recovery equipment developed independently in China used the \"absorption method.\" Between 2004 and 2005, under the direct supervision of the Science and Technology Department of Sinopec, Professor Huang Weiqiu from Jiangsu University of Technology led his team – the industry-university-research base at Jiangsu University of Technology – to successfully develop such oil and gas recovery equipment using a specialized absorbent called AbsFov-97. This equipment was installed at Jiujiang Petrochemical Company and has been operating properly ever since. However, with the implementation of the **Emission Standards for Air Pollutants from Oil Storage Tanks**, this system can no longer meet the requirements. 　　3. Condensation method The advantages of the condensation method are very evident, and it is also widely used around the world. As early as 1997, the American company Edwards installed over 400 units of \"condensation method\" oil and gas recovery equipment in major oil companies around the world, and has since transferred the patent to the Australian company Thermo-Wave. In 1989, our country also introduced an oil and gas recovery unit using the \"condensation method\" from Edwards Company, which was installed at Zhenhai Refinery; this unit is still in operation today. All 24 oil depots in Taiwan, China, use condensation-based oil and gas recovery equipment. 　　In China, there has also been considerable research on the condensation method; Huang Weiqiu from Jiangsu University of Technology and Sun Yonglin from Sinopec Fushun Research Institute have conducted in-depth studies on this method as well. The selling price of domestically produced condensation-based devices is about half that of the activated carbon adsorption method. A domestically produced condensing oil and gas recovery unit with a capacity of 300 m3/h has also been installed and put into use at the Huangpu oil depot in Guangdong. Gas station condensing oil and gas recovery equipment with a domestic production capacity of 30 m3/h has been installed on a pilot basis at gas stations in Xi’an, Yinchuan, Suzhou and other places; the recovery rate is around 80% at a condensation temperature of –45°C, while it exceeds 90% at a condensation temperature of –70°C. 　　Currently, the oil and gas emission control systems in regular gas stations use the well-established method of \"condensation + adsorption\". First, the oil and gas are condensed to around -40 degrees, causing most of it to liquefy. The remaining oil and gas is then adsorbed by an adsorption tank; since adsorption enables a high recovery rate, the emission concentration remains low, meeting **the standards. Furthermore, the condensed low-temperature oil and gas also effectively prevents the problem of high-temperature hot spots forming in the activated carbon adsorption bed. It also avoids the problem of excessive cryogenic energy consumption. 　　The industry-university-research base of Jiangsu University of Technology – Jiangsu Huilite Company – has developed a mature oil and gas recovery system of the \"condensation + adsorption\" type with a processing capacity of 400 m3/h, which has been put into operation at CNOOC Huizhou Refinery. 　　4. Direct combustion method The oxidation incineration method has been phased out due to its inability to recover valuable hydrocarbon components. 　　5. Membrane separation method: The process is relatively simple, but the initial investment cost is high. Liquid ring compressors and membrane modules are the core equipment of this technology. Compressors require extremely high explosion-proof standards, and only a few companies in Germany and the United States are capable of producing them. The pressure during the compression process is 3.5 bar, which poses a safety hazard. Industrial facilities for oil and vapor recovery in oil depots and refineries have not yet been established in the country; however, such facilities are in use for the recovery of olefins and other substances. There are only demonstration tests of very small membrane separation units at gas stations, as oil and gas emission treatment devices. Comparison of Several Oil and Gas Recovery Technologies and Their Comprehensive Complementation Currently, the main methods for oil and gas recovery include adsorption, absorption, condensation, and oxidative combustion; all of these methods were used in the early stages of oil and gas recovery. As understanding of oil and gas recovery technologies has improved, the absorption method was phased out due to its high exhaust emission concentrations, while the oxidation and combustion method was discarded because it fails to recover valuable hydrocarbon components. In the 1980s, adsorption and condensation methods became the mainstream technologies for oil and gas recovery. The investment costs for the adsorption and condensation methods are roughly comparable; however, within reasonable levels of investment and energy consumption, if the exhaust emission concentration from the condensation method is to be kept below 25 mg/l, both the investment costs and operating expenses will increase significantly. The adsorption method can easily meet the emission standards of less than 10 mg/l, but it also poses safety risks. 　　The various current oil and gas recovery processes all have their own advantages and disadvantages. No single method, whether it is condensation or adsorption, can be considered perfect; only by combining several processes and leveraging their respective strengths can the advantages of each process be fully utilized. 　　The combination of condensation and adsorption is currently a popular method that is also recognized by most people. Generally, secondary condensation is first used to cool the oil and gas to -40 degrees to -50 degrees; after this secondary condensation, over 85% of the oil and gas is liquefied. The oil and gas with a lower concentration that has not been condensed into a liquid state then passes through an adsorption system, which is used to concentrate the oil and gas, thereby increasing its concentration while reducing its volume (the exhaust gas that meets the standards after passing through the adsorption system is then discharged). The concentrated oil and gas subsequently enters a tertiary condensation system for further cooling, at which point the power required by the tertiary condenser is reduced. Advantages of this process: (1) It effectively combines the advantages of the condensation method and the adsorption method ； (2) Due to the enrichment of oil and gas using the adsorption system, the amount of oil and gas that needs to be processed in the three-stage condensation is **reduced, which also lowers energy consumption ； (3) The oil and gas after secondary condensation are of medium and low temperature; the activated carbon bed does not generate high-temperature hot spots, and the adsorption system also eliminates safety hazards.