Multiphase Extraction for Soil and Groundwater Remediation

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Multiphase Extraction for Soil and Groundwater
Remediation

• Nick Swiger
• Spring 2007

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Introduction
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Contaminant Releases

• When a contaminant is released into the
  environment, it will partition into four phases.
• Bulk liquid if insoluble (LNAPL or DNAPL)
• Adsorb to soil particles
• Vapor phase in soil gas
• Dissolve into soil moisture
• To effectively remediate the environment from a
  chemical spill, all phases will need to be
  recovered.
• Types of remediation systems have greatly evolved
  in last 20 years as everyone is always looking
  for a more effective and efficient process.
• Multiphase extraction is a relatively new
  enhancement to soil vapor extraction.

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What is Multiphase Extraction?

• Extractions of multiple phases of fluids from the
  subsurface.
• Fairly recent enhancement for the increased
  recovery and efficiency (in right subsurface
  conditions) of Soil Vapor Extractions.
• Two Types
• Dual Phase Extraction (Not focused on in
  paper/presentation)
• Extraction of multiple phases of fluids utilizing
  separate pumps and conduits. For example,
  installing a submersible pump in a well to lower
  the elevation of the groundwater to enhance an
  existing soil vapor extraction system.
• Multiphase Extraction
• Extraction of multiple phases of fluids utilizing
  one pump and one conduit.

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Typical Schematic of Multiphase Extraction
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Multiphase Extraction vs. Soil Vapor Extraction

• Both vapor and hydraulic conductivity are a
  function of moisture content in the vadose zone.
• Vapor conductivity decreases with increasing
  moisture content.
• With many chemical releases, they will migrate
  vertically until a lower permeability unit is
  reached or the surface of the saturated soil is
  reached (capillary fringe above the groundwater).
• Chemicals with a lower density than water (LNAPL)
  with stop at the saturated soil.
• Chemicals with a higher density than water
  (DNAPL) with stop at a lower permeability unit or
  force balance (i.e. capillary, hydrostatic, etc.)

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Multiphase Extraction vs. Soil Vapor Extraction
cont.

• In both cases, the areas typically with the
  majority of the contaminants have a high moisture
  content.
• The decreased vapor conductivity in the areas
  with the majority of the contaminants makes soil
  vapor extraction less efficient.
• The increased moisture content also limits the
  mass transfer of contaminants from the liquid
  phase to vapor phase.

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Multiphase Extraction vs. Soil Vapor Extraction
cont.

• The mass transfer limited systems are less
  efficient than flow limited systems.
• Mass transfer in high moisture content dominated
  by Henrys Law
• KH Cv/Cl (conc. vapor/conc. liquid)
• Exponential Decrease vs. Linear Decrease

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Multiphase Extraction vs. Soil Vapor Extraction
cont.

• By the removal of multiple phases of fluids it
  forces systems to be flow limited.

Monthly Mass Removed
Time
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Typical Uses of Multiphase Extraction

• As with soil vapor extraction, the volatile
  compounds that tend not to adhere to the soil are
  amenable to multiphase extraction. The following
  chemical properties are typically used for
  indicators.
• Relatively low distribution coefficient (ratio of
  concentration in soil to concentration in liquid)
• Kd Cs/Cl
• Relatively high vapor pressure (pressure exerted
  by vapor in equilibrium with bulk fluid)
• Indicative of the compounds volatility or
  tendency to exist in gaseous phase.
• Multiphase extraction is most efficient in porous
  media with moderate permeability allows removal
  of both phases with the greatest remedial
  influence.
• Highly permeable media, the dominant phase
  removed will be water and there will be less
  influence on the soil gas. Near the aquifer,
  mostly groundwater removed with very little soil
  gas.
• Low permeable media, the dominant phase removed
  will be gas, but the water will have much less
  influence. Near aquifer, there will be much
  drawdown, with little influence on the vadose
  zone.

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Contaminant Removal with Multiphase Extraction

• Phase removal can be accomplished by two
  mechanisms
• Overcoming hydrostatic pressure (static lift)
• Entraining the liquids in the vapor
• Dependant on vapor velocity

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Contaminant Removal with Multiphase Extraction

• For the static lift/hydrostatic pressure, the
  vacuum applied must be able to lift the most
  dense phase fluid (water with an LNAPL spill and
  contaminant with a DNAPL spill) to the surface
  and overcome major and minor losses.
• P ?gh (h height of lift head loss)

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Contaminant Removal with Multiphase Extraction

• For the liquid to be removed via vapor
  entrainment, the vapor velocity forces drag (Fd)
  and vapor surface forces (Fv) must overcome
  gravity forces (Fg)
• Fg lt Fd Fv
• Fg ?wVg mg
• Fd 0.5Cd ?av2A
• Fv ?aAv2

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Contaminant Removal with Multiphase Extraction

• Three different schemes of multiphase flow in
  well (based on velocity of the vapor)

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Multiphase Extraction Operations

• Two extraction configurations are typically
  utilized and are based on the location of the
  applied vacuum.
• Vacuum can be applied down in the well with the
  use of a stinger or drop tube.
• Vacuum can be applied to the top of the well.

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Multiphase Extraction Operations

• The stinger tube configuration would be most
  applicable very near or at the water table.
• The vacuum configuration would be more applicable
  the high moisture content soil in the vadose zone

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Multiphase Extraction Operations

• The fluid removal is provided by a fan,
  compressor blower, or pump depending on the
  subsurface conditions (i.e. flow rate needed,
  vacuum needed, etc.).
• With all fans, blowers, and compressors the fluid
  stream must be routed through a tank, commonly
  called a knockout tank, as they are not designed
  for liquid movement.
• Liquid ring pumps do not need to have a knockout
  tank designed for multiple phase fluids.

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Multiphase Extraction Operations

• All phases of fluids will have to be treated
  prior to discharge (for the most part depending
  on regulations) however, with non aqueous phase
  contaminants, there will be minimal treatment of
  water as the vapor acts as in situ air stripping.
• Multiple ways to treat the fluids such as
  chemical (i.e. oxidation with ozone or peroxide),
  physical (i.e. carbon adsorption), and biological
  (i.e. bioreactors)

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Determining Treatment Area

• Each extraction point will have a set area of
  influence or radius of influence if radial
  flow is assumed.
• Darcys Law can be used in both the saturated and
  unsaturated media to approximate radius of
  influence (capture).
• Integrated, steady state Darcys Law for
  unconfined aquifers
• Q K2p (Ho2-Hw2) / ln (Ri/Rw)
• Where Ho original water elevation
• Hw water elevation in extraction well
• Ri radius of influence
• Rw radius of extraction well

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Determining Treatment Area

• For the vadose zone, the Darcys Flux, related to
  pressure changes (instead of head) is
• q -k/µ (dP/dx)
• Using the above and flow equations for radial
  flow to wells, Jeff Kuo in Practical Design
  Calculations for Soil and Groundwater Remediation
  presented the following
• uw (k/2µ)Pw/ Rw (ln (Rw/Ri)1-(Pri/Pw)2
• where uw - vapor flux at the extraction well
• Pw - pressure at the extraction well
• Pri - pressure at the radius of influence
• Rw - radius of the extraction well
• Ri - radius of influence
• ur (k/2µ)Pw/ r (ln (Rw/Ri)1-(Pri/Pw)2/
  1 1-(Pri/Pw)2 (ln (r/Rw)/ln (Rw/Rri)0.5
• where ur - vapor velocity at r
• r radial distance r

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Determining Treatment Area

• The two equations can be utilized to find the
  capture area in the vadose zone and vapor
  velocity at certain radial points with in the
  vadose zone.
• May soil and groundwater remediation engineers
  utilize only the radius of influence as the
  soil gas capture.
• This provides the extent of capture, but not
  necessarily the extent of remediation!!!!!

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Summary

• Multiphase extraction is a enhancement that can
  be made to soil vapor extraction system to
  increase efficiency of remediation of volatile,
  moderately soluble, low soil adsorption,
  chemicals in the vadose zones with higher
  moisture contents.
• The most efficient use of multiphase extraction
  is with moderately permeable porous media, so
  both phases of fluids can be extracted.
• Can apply vacuum to the top of well (mostly in
  high moisture content vadose zone) or with a
  stinger pipe (mostly near the surface of the
  aquifer/capillary fringe).
• The extraction can be accomplished with many
  types of fluid pumps, but must have a knockout
  tank with all except a liquid ring pump. The
  fluids will have to treated prior to discharge.
• The extent of remediation can be determined by
  Darcys Law. In the vadose zone, caution must be
  used as the zone of vapor capture is not
  necessarily the zone of remediation.

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Questions or Comments?

• swig2380_at_uidaho.edu or
• swigern_at_michigan.gov