Vapor recovery is an energy-saving and environmentally friendly new technology that recovers oil vapor naturally lost during the storage, transfer, and loading and unloading of petroleum products. It prevents air pollution caused by vapor evaporation, eliminates safety hazards, and reduces product loss, thereby delivering considerable economic returns.
Most petroleum products exhibit high saturated vapor pressures at ambient temperature and therefore evaporate and diffuse readily in the open atmosphere. In China, the storage and transportation of crude oil and refined products result in substantial losses every year as oil vapor escapes into the air. Take, for example, the vehicle refueling stations located throughout urban and rural areas: gasoline and diesel are both highly volatile liquids, and oil vapor escapes at every stage of storage, loading and unloading, transportation, and refueling. Vapor emissions not only cause product losses and considerable economic damage, but also pose major safety hazards such as fire and explosion. In addition, oil vapor exerts an anesthetic effect on the human central nervous system and irritates the skin and mucous membranes, and may even cause dermatitis and eczema. Under the action of sunlight, the constituents of oil vapor also undergo photochemical reactions with nitrogen oxides in the atmosphere, generating the far more toxic photochemical smog and polluting the ambient air. For these reasons, the promotion of vapor recovery technology is imperative.
An adsorption-type vapor recovery system is composed of the following main parts: two adsorbers and one absorption tower as the principal equipment, together with a vacuum pump, a gas-liquid separator, gasoline pumps, piping, valves, and other auxiliary components. Its working principle and process flow are as follows. Each of the two adsorbers contains an adsorbent bed made of activated carbon or silica gel, and the two units alternate between adsorption and regeneration modes by means of valve switching. While a fuel nozzle is dispensing product into a vehicle, the vapor recovery pipe draws in a mixture of evaporated gasoline vapor and air at a rate of 1.2 times the dispensed volume and delivers it to the adsorber that is in the adsorption mode (adsorber A); the gasoline vapor in the gas stream is captured by the adsorbent bed, while the stripped air is discharged through the vent pipe. When the vapor adsorbed in an adsorber approaches saturation, the valves are switched to connect that adsorber to the vacuum pump, and the unit enters the regeneration mode (adsorber B in the diagram). Under the low pressure created by the vacuum pump, the gasoline vapor desorbs from the adsorbent bed and is drawn off by the vacuum pump, then discharged into the absorption tower after passing through the gas-liquid separator. To ensure that the hydrocarbons held in the bed are removed as completely as possible, a small quantity of air is introduced to purge any residual hydrocarbons from the adsorbent bed. The absorption tower may be a spray tower or a packed column: a gasoline feed pump introduces liquid gasoline from the tank farm at the top of the tower, where it meets the gasoline vapor entering from the bottom, absorbs it, and condenses it back into liquid; the mixture is discharged from the bottom of the absorption tower by the gasoline return pump and, after being cooled in a heat exchanger, is conveyed back to the tank farm. The residual vapor and air at the top of the tower are piped to the adsorbers for adsorption and recovery once again. Regeneration continues until the vapor held in the adsorber has been fully released, preparing the unit for the next adsorption cycle; in this way the system works in a continuous, repeating cycle.
A dry screw vacuum pump is extremely well suited to vapor recovery systems and serves as the key equipment in them. Because the pumped gas consists of highly volatile gasoline and diesel vapor, the pump is generally required to operate at a temperature below 80 °C. Casing-jacket cooling alone, however, cannot bring a running pump down to such a low temperature; moreover, cooling water is unavailable at most sites, and the only coolant that can be supplied is gasoline. To lower the pump operating temperature, in addition to feeding gasoline through the pump casing jacket, the most effective method is to use a seal-liquid circulation pump to inject an appropriate amount of gasoline directly into the pump chamber (at the point of highest temperature), where its heat of vaporization is absorbed to reduce the chamber temperature. At the same time, the liquid present in the pump chamber also provides a sealing effect, which helps to prevent back-migration of the highly diffusive oil vapor and thus enables a higher vacuum to be attained. To prevent condensed gasoline from accumulating at the pump discharge port, which would cause abnormal vibration and noise, any accumulated liquid must be discharged in a timely manner. The following improvement measures can be adopted for this purpose: position the pump discharge pipe below the level of the discharge port and slope it downward so that the liquid flows into the gas-liquid separator; install a circulation pump at the lowest point of the discharge port or of the silencer to eliminate liquid accumulation; and fit a check valve at the discharge port to prevent condensate from siphoning back into the pump chamber when the pump is shut down.
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