The hydrocarbon-containing gas generated during storage and transportation passes through an activated carbon adsorbent bed, where the hydrocarbons are adsorbed by the adsorbent. The adsorption process occurs at ambient temperature and pressure. Once the adsorbent reaches a certain saturation level, vacuum regeneration is carried out. The desorbed vapor during regeneration is then absorbed by oil, and the lean gas after absorption is returned to the adsorption process for further adsorption.
The main process units include: vapor collection, adsorption process, regeneration process, compression process, absorption process, heat exchange, and sealing. The greatest advantage of the adsorption method is that the concentration of hydrocarbons in the outlet gas can be controlled by changing the operating conditions of adsorption and regeneration. The disadvantages are: complex process, the adsorption bed is prone to high-temperature hot spots (as proven by laboratory tests). Benzene, toluene, and xylene can easily deactivate activated carbon; and the issue of disposing deactivated activated carbon. There are no domestically produced industrial units in operation in China; four imported units are operating at oil depots, with high equipment acquisition costs.
Process flow: Vapor generated at the loading point enters the vapor recovery unit through a sealed loading arm. Before entering the unit, the vapor passes through a drain tank to ensure that vapor particles without gasoline enter the carbon bed. Additionally, a PVV (vacuum/pressure valve) emergency outlet is installed on the main vapor line to ensure that vapor in the main line can be released when the unit is shut down. The PVV emergency outlet or other emergency outlets should be equipped with appropriate flame arresters.
The recovery unit consists of two carbon beds: one is connected to the vapor inlet line via valves and is in the "adsorption" state, while the other undergoes "regeneration" via a vacuum pump. The two carbon beds work simultaneously to ensure timely recovery of continuously incoming vapor. That is, one carbon bed adsorbs hydrocarbons from the vapor, while the other discharges the adsorbed hydrocarbons through the vacuum pump; when the first carbon bed reaches saturation, it immediately switches to "regeneration" operation (i.e., desorption phase), while the second carbon bed, which has been evacuated earlier, enters the next "adsorption" state.
The regeneration of activated carbon is completed in two stages. First, the activated carbon container is evacuated, and the adsorbed hydrocarbons are separated from the carbon bed, allowing most hydrocarbons to be desorbed. Then, to ensure that the hydrocarbons in the carbon bed are removed as thoroughly as possible, it is necessary to introduce a small amount of air to purge any residual hydrocarbons on the carbon bed. The vacuum pump used in this unit is a liquid ring pump. A liquid-gas separation tank and a heat exchanger are required. The sealing liquid for the vacuum pump is a mixture of ethylene glycol and water. The standard optional medium for the heat exchanger is gasoline or other types of condensate.
In the separation tank, the high-concentration hydrocarbon gas enters the absorption spray tower. Gasoline is pumped from the gasoline storage tank and sprayed from the top of the tower, mixing with the pure hydrocarbon gas rising from the bottom, thereby achieving absorption of hydrocarbons into the gasoline.
The entire unit has automatic energy-saving functions: if loading stops, all equipment is in standby operation mode. The standby equipment can be started at any time. The vacuum pump automatically starts at intervals to keep the carbon bed clean and maintain the activity of the activated carbon. When the next loading begins, the entire unit automatically starts.
The activated carbon adsorption vapor recovery unit is a popular technology in Europe and America. Its greatest feature is that by changing the operating conditions of the unit, the concentration of hydrocarbons in the outlet gas can be controlled to meet different emission standards.
Each liter of gasoline recovered consumes 0.15 to 0.2 kWh of electricity. The average annual operating cost is 160,000 RMB. According to laboratory adsorbent screening studies, the activated carbon is specially manufactured and is not ordinary activated carbon. Activated carbon available on the market cannot meet the adsorption and desorption performance requirements. The adsorption process is a physical exothermic process. When adsorbing high-concentration vapor, the temperature of the carbon layer rises quickly and reaches high levels, as demonstrated by laboratory adsorbent screening tests. An adsorption column of L×D 250×40 mm, when adsorbing at room temperature, reaches a carbon layer temperature of 80-90°C within just a few minutes. Therefore, the Japanese government strictly prohibits the use of combustible activated carbon as an adsorbent for vapor recovery from a safety perspective. Furthermore, using vacuum desorption to regenerate activated carbon poses problems with the desorption of benzene, toluene, and xylene; their adsorption on activated carbon will eventually lead to deactivation. Using the adsorption method to recover vapor does not allow direct visual observation of the recovered material. How to dispose of deactivated activated carbon is also a problem that will be faced. Since the height of the carbon layer causes pressure loss for vapor passing through it, higher requirements are placed on the sealing of the loading arms.
