1, recovering oil and gas by adsorption.

Process principle: In the production process, the gasoline volatile gas from the pipeline of the closed crane pipe collection system enters the adsorption tank A after separating free droplets through the condensation tank, and the gasoline in the volatile gas is adsorbed in the pores by the adsorbent, while the air passes through the bed. The tail gas that meets the emission requirements is discharged into the atmosphere through the discharge port at the top of the adsorption tank through the flame arrester. When the adsorption amount of gasoline in the adsorption tank A reaches a certain value and the top of the adsorption tank is about to break through, the PLC program control system will automatically switch to another adsorption tank B for adsorption according to the preset and adjusted time, and at the same time, the adsorption tank A will be transferred to the regeneration stage, and it will be evacuated to below the absolute pressure 15KPa by the desorption vacuum pump. According to the principle of pressure swing adsorption, gasoline adsorbed in the pores of adsorbent will be desorbed [2]. In order to ensure that the gasoline in the bed is removed as cleanly as possible, a small amount of air is introduced to purge the bed in the later stage. The adsorption bed is provided with a plurality of upper, middle and lower temperature measuring points. When the adsorbent adsorbs gasoline, the bed temperature will rise due to the adsorption heat. When the bed temperature rises to a certain value, the control system will give an alarm in time, and automatically switch to another adsorption tank if necessary, or close the oil and gas inlet valve to ensure safety. When the adsorbent desorbs gasoline, it is an endothermic process and the bed temperature will drop again. Both the oil and gas pipeline at the inlet of the adsorption tank and the tail gas discharge pipeline are provided with flame arresters, and the tail gas discharge pipeline can also be provided with combustible gas detection alarms. The vacuum pump unit adopts liquid ring and closed loop system. The desorbed and desorbed high-concentration oil and gas (rich gas) enters the vacuum pump and is separated from the working medium and partially condensed liquid gasoline in the vacuum pump outlet separator. The separator is equipped with liquid level alarm interlock, which can automatically discharge liquid when the liquid level exceeds the high limit; When the working liquid level is too low, the solenoid valve for fluid replacement is opened. The heat generated by the vacuum pump is taken away by the cooling medium in the cooler on the working liquid circulation pipeline. The oil and gas (rich gas) separated by the vacuum pump outlet separator [3] is sent to the lower part of the packed absorption tower and absorbed by normal-temperature gasoline. The gasoline from the tank farm is pumped to the top of the absorption tower by the oil pump, and fully contacts with the bottom-up oil and gas through the top-down packing. Because of the low liquid partial pressure, large flow rate and lower phase equilibrium partial pressure than oil and gas partial pressure, most oil and gas can no longer exist in gas phase, and high concentration oil and gas is absorbed by refined oil. A small amount of low-concentration oil and gas not absorbed by the absorption tower is led from the top of the absorption tower to the oil and gas main pipe in front of the adsorption tank and sent to the adsorption tank for cyclic adsorption.

Key points: The adsorbents suitable for repeated adsorption and desorption of gasoline volatile gases should be carefully screened. Because silica gel absorbs most of the gasoline in volatile gas, the temperature rise is small, and a small amount of activated carbon can further absorb a small amount of gasoline, but the temperature rise is large, which has hidden dangers of ignition and combustion; Therefore, adsorption beds can be divided into silica gel and activated carbon. By layering, the shortcomings of both can be avoided, and the advantage of low concentration of tail gas absorbed by activated carbon can be fully exerted.

Selection of process equipment: The selection of oil and gas adsorption device shall refer to the following standards:

① The pressure of oil and gas inlet pipeline is automatically controlled, and the system is equipped with pressure alarm and interlock;

(2) The absorption tower is equipped with liquid level alarm and interlock;

(3) The system should be equipped with a flame arrester and a cut-off valve, and the gasoline pipeline in and out of the device should be equipped with an automatic control valve to prevent gasoline from entering the device when it fails or stops;

(4) Temperature alarms and interlocks should be set in many places in the adsorption bed to automatically switch to the desorption state before the bed temperature enters the dangerous range to ensure safety;

⑤ All equipment, electrical equipment, instruments and control systems are explosion-proof designed and selected in strict accordance with national petrochemical industry standards.

2. Recover oil and gas by adsorption and condensation.

Process and process principle: Oil and gas (mixture of light hydrocarbon components and air) from the primary and secondary collection systems of gas stations are sent to the carbon adsorption tank, and the light hydrocarbons are adsorbed on the carbon layer. After most of the light hydrocarbons are removed, the tail gas reaching the standard is discharged into the atmosphere from the top of the carbon adsorption tank. The saturated carbon adsorption tank is evacuated by a vacuum pump, and the light hydrocarbons in it are desorbed and sent to the ultra-low temperature condensation unit. First, the ultra-low temperature condensing unit precooled to about +2℃ in the first stage to remove water vapor, and then cooled to -40℃ in the second stage to condense most light hydrocarbons into liquid oil, which was temporarily stored in the oil storage tank. The uncondensed gas is sent to the recovery carbon tank for adsorption, and the tail gas is discharged into the atmosphere from the top of the recovery carbon tank [4]. The optimal combination of adsorption and condensation not only reduces the overall energy consumption of the plant, but also gives full play to the characteristics of strong adaptability and high recovery rate of adsorption method, and the treated tail gas meets the requirements of relevant national standards.

It is noteworthy that gas stations should adopt enhanced oil and gas treatment technology (EVR) as far as possible to improve the efficiency to more than 95%. Accordingly, technologies such as reducing oil dripping from oil guns, on-line monitoring and post-treatment should be adopted as far as possible.

Besides adsorption and adsorption+condensation, we can also choose direct condensation, adsorption+absorption and membrane separation according to the specific situation, so as to find the best oil and gas recovery scheme. Various restoration methods are suitable for different places. Generally speaking, adsorption oil and gas recovery device is suitable for large and medium-sized oil depots, condensation oil and gas recovery device is suitable for small oil depots and tankers, and condensation+adsorption device is suitable for gas stations.