VOC Removal from Water by Pervaporation

Abstract

Pervaporation (PV) is a new technology for removing volatile organic compounds from groundwater and other aqueous waste streams. This technology is of particular interest for separating compounds that have low water solubilities (less than 20,000 ppm) [3]. These compounds include petroleum-based solvents such as benzene and toluene and chlorinated solvents like trichloroethylene.

Pervaporation allows organics to be selectively separated from aqueous sys tems. The process is typically carried out by placing a liquid stream, containing organic species, in contact with one side of a nonporous selective membrane while a vacuum is applied to the other side. The selected organics in the liquid stream sorb into the membrane, permeate through it, and evaporate into the vapor phase. The vapor is then condensed. Since various components in the liquid feed have different affinities for the membrane and different diffusion rates through the membrane, an organic at low feed concentration can have a significantly higher concentration in the vapor phase and when condensed will result in a highly concentrated liquid [1].

In a demonstration, the system successfully removed an average of 94 percent TCE

from a wastewater. In addition to effectively removing VOCs, this technology does not require replacement or disposal of an adsorbent such as activated carbon. Only negligible amounts of VOCs are release to the outside air [3].

Process Description

A full-scale demonstration of the ZENON cross-flow pervaporation technology was performed at a former waste disposal plant at the Naval Air Station, North Island (NASNI), in Coronado, California. Groundwater at the site contains a variety of contaminants, mainly trichloroethene (TCE). In this study, the pervaporation system was operated at influent flow rates ranging from 2 to 11 gpm with trichloroethene concentrations up to 250 mg/L [3].

Figure 1: Pollutants diffuse through the membrane as vapor

Typically, contaminated water is pumped from an equalization tank through a prefilter that remove debris. After which, it enters a heat exchanger which raises the water temperature to about 165°F (75°C) [3]. The heated contaminated water then flows

Permeate Vapor (containing selected VOCs)

Waste Water Membrane

Liquid (selected VOCs have been removed)

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into a pervaporation module containing the organophilic membranes. The organics sorb into the membranes. A vacuum applied to the system causes the organics to diffuse through the membrane as vapor. Components of the liquid stream that do not move through the membrane are discharged from the system, Figure 1. The vapor (permeate) is then passed through a condenser generating a highly concentrated liquid. The condensate separates into aqueous and organic phases. The aqueous phase permeate can be sent back to the pervaporation module for further treatment, while the organic phase permeate is discharged to a receiving vessel [3].

Analysis of the demonstration samples showed the ZENON pervaporation system removed on average 97.3 % of the TCE from groundwater. Treatment efficiency remained fairly consistent throughout the demonstration. However, efficiency decreased at various times due to mineral scaling problems [3]. Key Parameters: The efficiency of PV separation is determined by properties of the VOC being targeted, the membrane, and operating variables. The evaluation and understanding the PV operating variables is critical for VOC removal from water. These variables include permeate pressure, feed temperature and concentration, module geometry, liquid feed velocity, etc. A change in one parameter can result in a change in separation efficiency [1].

The physical properties of VOCs to be considered include: diffusivity in water, affinity for specific membrane material, permeability in the membrane, and the saturation vapor pressure [1].

Membranes: PV membranes used to separate VOCs from water are generally fabricated with non-porous hydrophobic polymeric materials [1]. An analysis of the target VOCs in the wastewater stream is necessary to determine the appropriate membrane material. Benefits

q Can be applied to aqueous waste streams such as groundwater, lagoons, and rinse waters that are contaminated with VOCs such as solvents, degreasers, and gasoline

q Environmentally friendly and an essentially pollution-free operation. q No additional chemicals are needed for fluid separation. Disadvantages

q Permeate flux and selectivity of a solute is dependent on membrane material. q Periodic cleaning of the membranes is necessary to maintain the treatment

efficiency (scaling on the membrane) [3]. q Lowering concentrations to below maximum contaminant levels may require multiple

passes through the pervaporation module. This can be economically impractical when compared to other technologies [3].

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Vendors

Listed below are a few vendors that can provide pervaporation systems

Canzler, LLC http://www.canzler-pt.de Petro Sep Membrane Technologies Inc http://www.petrosepmembrane.com Sulzer Chemtech Ltd http://www.sulzerchemtech.ch Zenon Environmental http://www.zenonenv.com/ Literature Cited

1. Ming Peng, Leland M. Vane, Sean X. Liu, “Recent advances in VOCs removal from water by pervaporation”, Journal of Hazardous Materials, B98 (2003) 69–90.

2. “Zenon Cross-Flow Pervaporation Technology: An Innovative Method for Treating TCE in Ground Water,” Navy Environmental Leadership Program, NELP Fact Sheet No. 5, September 1996.

3. “Zenon Cross-Flow Pervaporation Technology: Innovative Technology Evaluation Report,” US Environmental Protection Agency, National Risk Management Research Laboratory Office of Research and Development, (http://www.epa.gov/ORD/SITE/reports/zenon.html).