Gasoline Vapor Recovery with Activated Carbon and Silica Gel
Gasoline is a mixture of various hydrocarbons, of which the light components are highly volatile. In the process of gasoline storage and transferring, a part of the light components will volatilize and be discharged out. Vapor recovery is important because a large amount of high-concentration vapors of gasoline and other fuels accumulating in the atmosphere will bring serious safety hazards, environmental pollution, energy waste, and economic losses.
During the operations of oil terminal, vapor emissions mainly occur in the loading process, where gasoline is transferred by loading arms with flexible composite hoses to road or rail tankers. As the gasoline in the tankers increases, the pressure rises accordingly, and the gasoline is continuously squeezed out of the tankers. At this stage, the amount of gasoline vapor emissions is relatively large, and the following vapor recovery solution is used for the capture and treatment of gasoline.
Vapor recovery solution
Vapor recovery methods include condensation, adsorption, membrane separation, etc. For now the most commonly used are activated carbon adsorption method and condensation method. The adsorption process uses the difference in the binding affinity between the components in the mixture and the adsorbent, and the difficult-to-adsorb and easy-to-adsorb components in the mixture can be separated. That is, the affinity of the adsorbent and the hydrocarbon molecules to adsorb the hydrocarbon molecules without adsorbing air. Suitable adsorbents can have high selectivity for the adsorption of each component.
At present, almost all adsorbents used in gasoline vapor recovery processes are activated carbon. The main advantages of the activated carbon adsorption method are the high recovery rate of gasoline components, low exhaust gas emission concentration, and low one-time investment cost. The disadvantage is that the large adsorption heat that does not easily dissipate in a short time will decrease the adsorption rate and may cause the carbon bed to overheat. So the temperature of the carbon bed needs to be strictly controlled.
Activated carbon and silica gel in vapor recovery system
A combination of activated carbon and silica gel is used as a new adsorbent option in vapor recovery system. In the adsorption tower of the recovery system, a double-layer method is adopted in which the lower layer is filled with hydrophobic silica gel and the upper layer is filled with activated carbon.
| Characteristics | Silica Gel | Activated Carbon |
|---|---|---|
| Flammability | Nonflammable. | Difficult to ignite. |
| Heat of adsorption | Average temperature rise of adsorption layers: 5-10°C. | Average temperature rise of adsorption layers: 15-30°C. |
| Adsorption rate | Higher than activated carbon in high concentration applications. | Higher than silica gel in low concentration applications. |
| Moisture adsorption | Absorb less moisture after hydrophobic treatment. | Tend to absorb moisture in high humidity levels. |
| Pressure drop | Less. | More. |
This makes full use of the advantages of the two adsorbents to make up for each other’s defects. The high-concentration part of the vapor is adsorbed by hydrophobic silica gel, which has a larger adsorption capacity in the high-concentration area and is not easy to generate adsorption heat. And the low-concentration part uses activated carbon to adsorb. And because the concentration of gasoline vapor decreases a lot after passing through the silica gel adsorption layer, the activated carbon adsorption layer no longer generates a large amount of adsorption heat, which prevent a rapid temperature rise in the adsorption tower and the system safety is improved. Normally, the working temperature of the adsorption layer can be controlled between 35°C to 45°C.