• ARID’s PERMEATOR System Earns CARB Certification...
• PERMEATOR Systems to be Installed at ALL H-E-B Houston area sites...
• ARID Receives Certification from Maryland...
• Independent 3rd Party Field Testing Confirms...
• Texas ORVR Certification Approval...
• ARID Announces Completion of Study...
• ARID announces appointment...
• Vapor Recovery Around the World
• Retooling the Vapor Recovery System
• Membranes, Molecules and the Science of Permeation

This system, called PERMEATOR, separates hydrocarbon vapors from air; exhausts the cleaned air to the atmosphere; and returns the enriched hydrocarbon vapors to the storage tank head space. By selectively removing air from the storage tank head space, storage tank pressures are reduced and fugitive and vent emissions are virtually eliminated. The system is installed on manifolded tank vent lines. It can be used at uncontrolled stations (no Stage II) as well as Stage II-compliant facilities using balance or vacuum-assist (dispenser based and centralized) vapor recovery systems.

The selectively permeable membrane material is positioned in a compact module that exhibits very low pressure drop and excellent mass transfer characteristics. The entire system is housed in a relatively small enclosure (measuring four feet W x two feet H x two feet D) that can typically be installed without any excavation. Energy consumption is minimized since all streams enter and exit the membrane in vapor phase. Normal operation of the vent lines is not impeded because the system is installed parallel to the vents. No restriction whatsoever is created in the vapor pathway.

Six PERMEATOR systems are currently in operation (one in the US and the others overseas) as part of test programs. The units are being monitored to verify ARID’s projected overall estimates of vapor recovery efficiency of 95 percent.

Fig 2: How the Membrane Works                

The membrane is made of extremely thin, selectively permeable, polymeric films attached to a porous support structure. Membrane films are integrated into modules to provide maximum surface area per unit volume of pressure housing.

Unlike conventional particle filters that separate materials based on physical size differences, the membranes used by ARID separate compounds based on differences in the solubility and diffusivity of specific molecules. Hydrocarbon molecules pass through, or permeate, the thin polymer film more rapidly than other molecules and are returned to the storage tank (see Figures 1 and 2). Molecules such as oxygen and nitrogen (air) are much slower permeators, and they are "rejected" by the membrane film and vented to the atmosphere. The difference in permeation rates between hydrocarbon and air molecules allows for separation of gasoline vapors from air.

The system operating cycle is described below and illustrated in Figure 2.

1. Air and hydrocarbon vapors fill the space left in the storage tank when liquid gasoline is transferred to an automobile.

2. The pressure in the storage tank head space increases as liquid gasoline in the storage tank evaporates to increase the hydrocarbon concentration in the head space. The PERMEATOR system is actuated by a pressure switch connected to the ullage.

3. The air/hydrocarbon mixture expelled from the storage tank vent line is directed to the membrane module. Here, a vacuum pump creates a differential pressure that causes the hydrocarbon molecules to preferentially permeate, or pass through, the membrane.

4. The hydrocarbon-rich permeate stream is returned to the storage tank while the air-rich retentate stream is vented to the atmosphere. The purity of the exiting air stream that has been depleted of hydrocarbons is determined by feed flow rate, membrane area and the pressure ratio between the feed and permeate streams.