上海东化环境工程有限公司

Shanghai Donghua Environment Engineering Corp.

Gasoline vapor recovery membrane system

   Industry background

Petroleum consumption in China is higher than before and is more depending on importation day by day with the speedy economic development and the more vehicle possessions. According to statistics, the total quantity of internal petroleum and import petroleum was about 290 million tons in 2004, including 120 million tons by import. However, there are few concerns about the gasoline vapor volatilization in transportation and storage. According to the detect data from gas station (without gasoline vapor recovery system) by CARB(California Air Resources Board), there is 0.84 pound (0.38kg) wastage from oilcan by breath when selling 1000 gallon (2952kg) gasoline, and another 8.4 pound (3.8kg) wastage by volatilization in gasoline supplement . The gasoline vapor withdrawal is big in the tank area: there will be 0.21% volatilization wastage by 1℃ temperature increment, and more gas-phase gasoline in oilcan with the prolonged storage time. Suppose there is 2 million tons gasoline storage in the oilcan with 50% of utility ratio by 300 days, and the average temperature change everyday is 10℃, the volatilization wastage will be 550,000 cubic meters (660 tons). The domestic data indicates, the consumption of gasoline in all country was 50 million tons in 2003, there was at least 280 million cubic meters gasoline wastage (340,000 tons) in transportation、transfer and distribution, and the economic loss reached 1.3 billion RMB. According to incomplete statistics, the economic loss by gasoline volatilization reached as high as 2 billion RMB in 2004.

It brings plenty of saturated gasoline vapors by volatilization, which is easy to explode just by spark or static electricity, causes hidden trouble of safety to tank area and gas station. The greaser with vapor capture system has been installed now in some gas station in downtown area, it has solved the volatilization during the process of oil injection, but the vapor captured has not been reclaimed, and most vented again through breath valve, so it has not cleared up the safety danger radically.
The gasoline vapor consists of benzene, dimethylbenzene, ethyl benzene and other hydrocarbon mostly, carcinogenic. And most gas stations are close to the resident area, the height of venting point is also not beyond people’s breathing space. So, we can not ignore the dangers. The gasoline vapor will react with other nocuousness gas after irradiation by ultraviolet radiation in air, and becomes more toxic contamination, brings more threatens to people’s health.
In Europe and America, it began to recover gasoline vapor in 1970s, and now there are comparatively mature gasoline vapor recovery technology and consummate laws.
U.S. Environmental Protection Agency emended Air Protection Law in 1990, it prescribed that gas station must be installed gasoline vapor recovery equipment if its air condition can not meet the laws. In 2001 CARB developed more strict Enhanced Vapor Recovery law, and it declared that more advanced equipment need be installed since 2002. From 2004 to 2006, all gas stations shall be forced to install the equipment. In China, the legislation preparation on gasoline vapor recovery is beginning. General Administration of Quality Supervision P.R.China and Ministry of Construction P.R.China established Design and Construction Criterion of Gas Station in 2002. It prescribed the installation of gasoline vapor recovery and leakage checking equipment clearly.

  Gasoline vapor recovery technology

There are four kinds of gasoline vapor recovery technology by theory at present, including absorption, condensation, adsorbtion and membrane separation.

   Absorption

At normal pressure and temperature or at normal pressure and low temperature, using the absorption liquid, such as gasoline (or waste oil), coal oil series solvent, light diesel oil, special organic solvent, the gasoline vapor is dissolved and absorbed in the absorption column with contacting spraying. The effect of this process is mostly depending on the solvent, and the recovery of the solvent is generally less than 80%, there are other disadvantages, such as more occupying room and unavailability to batch operation.

  Adsorbtion

Using active carbon to adsorb gasoline vapor, the equipment consists of PSA unit, separator and absorption column. Gasoline vapor gathered by the equipment first comes into PSA unit where adsorption and desorption happen, the desorption gas goes through separator and is absorbed by solvent in the absorption column. It has higher gasoline vapor recovery, but needs more occupying room and the adsorption tower needs frequent adsorption-desorption automatic recycling switching, and service time of active carbon is short, so it is difficult for operation and maintenance. Otherwise, it is difficult to deal with the waste active carbon and secondary pollution happens easily.

   Condensation

Gasoline vapor is condensed in heat exchanger with refrigerant and gasoline is recovered directly. The temperature of the vapor is decreased to 4℃ in precooler and a majority of water vapor of gasoline vapor is condensed into water and discharged. Then the temperature of gasoline vapor is decreased to －40℃ in the first-stage cooler, and then is decreased to below －70℃ in the second-stage cooler. After two stages of coolers, the majority of VOC is recovered in liquid and gasoline vapor in discharge comes up to the standard of 35m/l. But because of indirect heat exchange, the temperature of refrigerant must be very low (－80～－70℃) to guarantee higher recovery. If gasoline vapor in discharge is required to be lower, further refrigeration is required. For condensation, safety and gasoline vapor recovery is high, but installation cost is also great. In addition, gasoline vapor is decreased to －80~－70 ℃ with refrigeration, both energy consumption and operation costs are great. 

   Membrane separation

This process actually is one combination of compression, condensation and membrane, and has grown up rapidly as modern chemical separation technology after 1960’s. As the molecular of hydrocarbon and air in the mixture are different in size, there are great differences in permeability through the thin membrane. Based on the differences, hydrocarbon vapor is separated from air. After mixture is compressed and condensed, gasoline vapor is separated from air by membrane. The gasoline vapor is recirculated into the compressor and purified air is discharged into the atmosphere. The gasoline vapor recovery based the membrane can reach above 99%.
Gasoline vapor recovery membrane systems have many advantages including small floor area, simple operation, easy maintenance, safe running and high rate of return on investment so the systems are widely applied after the systems become commercial. Since 1989, more than one hundred sets of the systems have worked in Europe. The market share of membrane systems among newly-built gasoline vapor recovery systems reaches above 80%. In 2002 the two sets of membrane system were firstly built in America. Because of their excellent performance, 120 sets of the membrane systems were built in 2003. Now their market share grows up rapidly.

  SDEEC's gasoline vapor recovery membrane technology

SDEEC imports world leading gasoline vapor recovery membrane technology from MTR.
The organic vapor membrane of MTR of USA is the world leader, and mainly applied in petrochemical organic vapors recovery and nature gas conditioning. MTR is also successful in gasoline vapor recovery, and has acquired seven patents  and good achievement in gasoline vapor recovery of big oil tank. CARB’s EVR regulation of CA, USA demands that gasoline vapor discharge standard of all gas stations in CA is improved, and MTR’s newly-developed VaporsaverTM can reach new discharge standard.
As for organic vapor membrane of MTR, please refer to Introduction to MTR’s VapSep System.
Gasoline vapor mainly contains C₅~C₉, and can be separated from the air based on MTR’s membrane system. The compressed mixture can be divided into permeate vapor (rich in gasoline) and residual (purified air). The permeate vapor can be reused and the purified air can be discharged into atmosphere directly.