The hydrocarbon-containing gas generated during storage and transportation passes through the activated carbon adsorbent bed, where the hydrocarbons are adsorbed by the adsorbent. The adsorption process is carried out at normal temperature and pressure. After the adsorbent reaches a certain saturation level, vacuum regeneration is performed. The oil and gas desorbed during the regeneration process are then absorbed by oil, and the lean gas after absorption is returned to the adsorption process for further adsorption.
The main process units include : oil and gas collection, adsorption process, regeneration process, compression process, absorption process, heat exchange, and sealing. The biggest advantage of the adsorption method is that the concentration of oil and gas 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; the issue of handling deactivated activated carbon. There are no domestically produced industrial units in operation in China, and there are four imported units operating in oil depots, with high equipment acquisition costs.
Process flow: The oil and gas generated at the loading point enters the oil and gas recovery unit through a sealed loading arm. Before entering the unit, the oil and gas passes through a drain tank to ensure that gasoline-free oil and gas particles enter the carbon bed. In addition, the oil and gas main pipe is equipped with a PVV (vacuum/pressure valve) emergency outlet to ensure that the oil and gas in the main pipe can be released when the unit is shut down. The PVV emergency outlet or other emergency outlets should be equipped with corresponding flame arresters.
The recovery unit consists of two carbon beds. One is connected to the oil and gas inlet pipe via valves and is in the "adsorption" state, while the other is in the "regeneration" state through a vacuum pump. The two carbon beds work simultaneously to ensure timely recovery and treatment of the continuously incoming oil and gas. That is, one carbon bed is used to adsorb hydrocarbons from the oil and gas, while the other carbon bed discharges the adsorbed hydrocarbons through a vacuum pump; when the first carbon bed reaches saturation, it immediately switches to the "regeneration" operation (i.e., desorption stage), while the second carbon bed, which has been evacuated earlier, enters the next stage of "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, causing most of the 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 of 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 drawn from the gasoline storage tank is sprayed down from the top of the tower and mixes with the pure hydrocarbon gas rising from the bottom, thereby achieving the absorption of hydrocarbons in gasoline.
The entire unit has automatic energy-saving functions: if loading stops, all equipment is in standby operation mode. The equipment in standby mode 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 method for oil and gas recovery is a popular technology in Europe and America. Its biggest 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 research, the activated carbon is specially manufactured, not ordinary activated carbon. The activated carbon available on the market cannot meet its adsorption and desorption performance. The adsorption process is a physical exothermic process. When adsorbing high-concentration oil and gas, the temperature of the carbon layer rises quickly and reaches high levels, as proven by the adsorbent screening test results in the laboratory. An adsorption column of L×D 250×40mm was used for adsorption at room temperature, and within just a few minutes, the temperature of the carbon layer reached 80 to 90°C. Therefore, the Japanese government strictly prohibits the use of combustible activated carbon as an adsorbent for oil and gas recovery from a safety perspective. In addition, using vacuum desorption to regenerate activated carbon has issues with the desorption of benzene, toluene, and xylene. The adsorption of these compounds on activated carbon will ultimately lead to the deactivation of the activated carbon. Using the adsorption method to recover oil and gas does not allow for direct visual observation of the recovered material. The handling of deactivated activated carbon is also a problem that will be faced. Since the height of the carbon layer causes pressure loss for the oil and gas passing through, higher requirements are placed on the sealing of the loading arm.
The Tokyo Metropolitan Ordinance stipulates that when the oil and gas concentration is ≥1 vol%, the use of combustible activated carbon adsorbents is prohibited. In Japan's adsorption method oil and gas recovery units, initially a single silica gel adsorbent was used, then it was changed to filling different silica gel adsorbents in the bed, and then to filling the adsorption tower with layers of silica gel and activated carbon adsorbents.
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