Oil gas recovery
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During the process of loading, unloading, storage, transportation and car re-fueling of the gasoline, a significant amount of oil gas has to be vented to keep the pressure balance of oil tank. Because the storage and unloading of gasoline has not realize the entire airtightly operation by the pressure balance system, oil gas loses especially seriously in our country. Statistical data indicates that gas vapor loss caused by loading and unloading of the gasoline reaches to 350 ton/year for a fuel depot with a daily sale of 600 ton. That means 1,300,000 RMB loss per year calculated by the price of 3750 RMB/ton for 90# gasoline without considering the soaring price of gasoline.Moreover gasoline evaporation causes the deterioration of fuel quality in that oil gas is mainly composed of lighter hydrocarbon. With the emission of light distillate, the initial boiling point of gasoline raises whereas the vapor pressure, octane value and start performance of car decline. There is strict control regulation of gasoline emission (for instance 150mg/m3 by TI Air in Europe) especially for refinery and oil depot where large amount of oil gas is concentrated. In china the relating regulation is being enacted actively.。
Currently the worldwide prevailing design for oil gas recovery integrates membrane with other technologies. The drawing below shows the mostly applied flow scheme to late installed oil gas recovery unit. It combines the principles of compression/condensation, adsorbing and PSA with membrane. Fully utilizing the advantages of all the technologies above but avoiding their disadvantages, the entire process of oil gas recovery is optimized.

This process is composed of three parts. The first part, Liquid-ring compressor and scrubber, constitutes traditional compression/condensation process. As the 2nd part, membrane unit is placed subsequently. The pressure swing adsorption (PSA), as the third part, is optional according to different requirement of emission levels. Relating with temperature, pressure and loading or unloading operation the concentration of oil gas in the feed gas is normally 30~40 V%.Oil gas, after compressed, is sent to the scrubber in which the vapor is absorbed by gasoline. As the overhead of scrubber, the saturated oil gas/air mixture flows to membrane unit for further recovery where the feed gas of the membrane is split into two streams: 1) oil gas enriched permeate, recycled to the inlet of compressor 2) residue (purified air) vented to the air or to PSA unit with an oil gas concentration of 10g/m3 around, less than the criteria of European union 35g/m3 by EU 94/63/EC regulation. Further treated by PSA, the vent gas can be purified to a concentration of 150 mg/m3.
This process integrates the advantages of compression/condensation, membrane unit, PSA. Firstly, the compression/condensation unit recovers part of oil gas and in addition pressurized it, which is in favor of the following absorption and membrane separation operation, because the driving force of membrane separation lies in the partial pressure difference between the feed and permeate side. The larger of the difference, the better the membrane separation performance. After the membrane treatment the concentration of oil gas can be reduced to 10g/m3. If the PSA unit is installed subsequent to the membrane unit the oil gas concentration of the vent gas can be decreased further. Additionally the load of PSA is reduced greatly since most of oil gas has been removed. The investment and maintenance costs of PSA unit are therefore reduced significantly as well as the lifetime is extended likewise. Finally, the whole process of oil gas recovery is optimized.
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