Recovery of gasoline vapor by a combined process of two-stage dehumidification and condensation

The emission of gasoline vapor containing volatile organic compounds (VOCs) from gasoline service stations is becoming a significant environmental problem especially for the population densed area. A combined process of two-stage dehumidification and condensation (TSDC) is proposed as a method of recovery of VOCs from the emitted gasoline vapor. A two-stage dehumidification procedure, consisting of using a first-stage dehumidifier (at 4°C with removal efficiency of water of vapor, R_wi to about 67 wt%) and a second-stage adsorber (enhancing the R_wi to about 95 wt%), can be employed to remove most of water of vapor thus avoiding the possible icing problem for the subsequent condensation. Salable liquid gasoline can then be recovered by operating the condenser at the desired low temperature T_c depending on the emission and/or reduction regulation of VOCs. Aspen-plus, a software package for thermodynamic computation developed by Aspen Technology Inc., is used to simulate the condensation removal or recovery efficiencies of condenser (R_c, based on the inlet gasoline vapor entering the condenser) of VOCs at various T_c. The simulation results indicate that the values of R_c are about 73, 85, and 90 wt% at T_c of -40, -60, and -73°C, respectively. Thus, the operation of condenser at T_c of -60°C would meet the non-destructive reduction of VOCs at 85 wt% level. However, should the stringent regulation on the oil storage tank of chemical plant be applied to the gasoline service station, then the condenser operated at T_c of -60°C would have to recycle part of the outlet vapor from the condenser back into the undergrounded gasoline tank. This is to ensure that the emission of VOCs is less than 35 mg for refueling 1 L gasoline into the fuel tank of vehicle.