Multi-phase extraction systems are the broadest possible category of pump-and-treat systems in which fluid materials are extracted from the subsurface and treated in aboveground systems. Multi-phase extraction systems are essentially dual phase extraction systems with an additional capability to remove and collect free-phase liquid contamination. Soil vapour extraction systems can be thought of, conceptually, as a multi-phase extraction of one single phase fluid. There are some engineering and design differences in the implementation of a soil vapour extraction, however, the idea is similar to the multi-phase extraction process.
Therefore, multi-phase extraction systems will pump groundwater, vapours, and immiscible liquids (ie, LNAPL, DNAPL, greases, oils, etc.). Dual phase systems are confined to any two of the above, however, in practice, it usually refers to groundwater and soil vapours. Soil vapour extraction (SVE) is limited to soil vapours and does not have the capabilities to treat contaminated liquids.
Subsurface Considerations
The ability of a contaminant to transfer phases from liquid to vapour is known as volatilization and is dependent on several quantities such as pressure, temperature, degree of saturation of the pore space, among others. Therefore, contamination present in the saturated zone of an aquifer is less likely to volatilize than if that contamination is present in the vadose zone or the capillary fringe. Thus, the applicability of extraction systems would depend on where the contaminant plume is with respect to the water table.
Extraction of Vapours
The extraction will extract both contaminated groundwater and contaminant vapours from the same extraction wells using one single vacuum pump.
A high vacuum is applied to an extraction tube placed at the desired drawdown elevation, below the static groundwater table. The top of the well casing is sealed to the atmosphere. The vacuum is applied to the sump-head and the groundwater is extracted from the well until the groundwater is drawn down to the bottom of the extraction tube. At the point where the groundwater is just below extraction tube, the suction on the water will break at this point and the vacuum pressure will be applied to the air space in the sump surrounding the extraction tube until the point where enough groundwater migrates to create a suction force at the extraction tube again. This will repeat causing an alternating extraction of contaminated groundwater and contaminated vapours out of the well and into the processing equipment.
Phase Separation Processes
The extraction will draw out a two-phase fluid (or three-phase depending on the site) with a vapour phase, thus an air/liquid separation step is required. This recovered fluid (containing groundwater, vapours, and potentially free-product) will pass through an air/liquid separator constructed of stainless steel with explosion proof high and low level control switches. The separation of vapour from liquid is accomplished using cyclonic action in the air/liquid separator. An explosion proof centrifugal pump will be employed to move water from the knockout drum to a filtration vessel to remove silt and sediment prior to a carbon filtration step.
From here, the liquid is pumped into a sump where it is in turn pumped into an oil/water separator (if free-product is present). The oil (or product) is directed into a holding tank, while the extracted water is pumped through an air stripper with the resultant liquid treated using activated carbon and discharged. The vapours from the air/liquid separator and from the air stripper are treated with a variety of treatment options and then discharged.
Groundwater Treatment
Two high pressure, corrosion and UV resistant precision moulded vessels with liquid phase carbon will remove CVOC contaminants with low solubility. The filtered groundwater will be collected in a polishing tank with explosion proof high and low level switches.
The news come http://www.bossgoo.com
