AirFlow Mechanisms During AirSparging Operations

1
Air-Flow Mechanisms During Air-Sparging Operations

• Patricia J. Culligan
• Associate Professor, MIT
• INEEL Workshop, March 2003

2
Acknowledgements

• Dr. Catalina Marulanda
• Mr. Michael Paonessa
• Dr. John Germaine
• Mr. Stephen Rudolph
• National Institute for Environmental Health
  Sciences
• National Science Foundation

3
Outline of presentation

• Background and Research Objectives
• Experimental investigation
• Methods, Procedure, Results
• Numerical investigation
• New model formulation
• Model validation
• Conclusions

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Insitu Air-Sparging (IAS)

• Technique for the remediation of groundwater
  contaminated with VOCs

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Efficiency of IAS

• Performance of an IAS system dependent on extent
  of contact between injected air and contamination

• Overall system effectiveness limited by
• patterns of air flow
• zone of influence of air sparging well, ZOI
• air saturation within boundaries of air sparging
  plume

• Understanding mechanisms controlling air-flow
  during IAS are crucial to designing efficient IAS
  systems

6

• IAS used in the US since 1990s
• Now demonstrated EPA technology
• In engineering practice
• Current state of IAS design largely empirical and
  based on pilot studies at site
• Research community has developed numerous
  theoretical models for IAS design - solve full
  two-phase flow problem
• Generally not available to practitioners
• Require parameters not typically measured in
  field (a m)

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Air-entry into a soil pore

• For air-entry into a saturated soil, the air
  pressure must exceed the sum of the hydrostatic
  pressure of fluid and the capillary pressure that
  exists as a result of surface tension between air
  and the pore fluid

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Capillary pressure

• Magnitude of the capillary pressure at a pore
  throat in a saturated soil is generally
  approximated by
• Pc 2scosq/r
• s surface tension
• q contact angle between fluids and soil phase
• r pore throat radius
• (soil pore throat is approximated as a capillary
  tube of diameter r)
• Thus, for air to invade a soil pore (i.e., get
  past the restriction offered by a soil pore
  throat)
• Pair gt Phydro 2scosq/r
• (Where it is assumed that the water is not
  moving)

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Ratio of hydrostatic to capillary pressure

• Ratio Pcap / Phyd is important to the problem
• Pcap / Phyd gtgt 1, soil fracture possible
• Pcap / Phyd 1, local capillary pressures
  likely to influence air- movement
• Pcap / Phyd ltlt 1, local capillary pressures less
  likely to influence air-movement
• For most practical applications of IAS, Pcap /
  Phyd ltlt 1

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Pcapillary/Phydrostatic vs Depth of Injection
D


1
0

m
m

1
0
D


1

m
m

1
0
D


0
.
1

m
m

• Pinj Phydro Pcapillary

1
0
D


0
.
0
1

m
m

1
0
P(cap)/P(hydro)
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Research Objectives
Experimental Investigation of Air-Flow Patterns
as f(Pcap/Phydro)
Parameters Controlling Air-Flow Propagation
During IAS
Model Validation
Model to Predict ZOI of Sparge Well
Model Validation
Predictions in Field
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Past Experimental Studies

• Laboratory scale tests - used small models
  less than 0.5 m high
• Tank tests - larger scale models up to 1.5 m
  high
• Do not capture the ratio between P(cap)/P(hydro)
  typically encountered in the field
• Field tests
• Cannot conduct parametric study. Often hard to
  interpret

13
Experimental investigation

• Two unique experimental tools used during this
  investigation
• Geotechnical centrifuge
• use centrifugal acceleration to vary Phydro (vary
  g and hence rgh)
• testing over range of P(cap)/P(hydo)
• Immersion method
• matching index of refraction of granular material
  and saturating fluid creates transparent
  saturated porous medium
• flow patterns could be observed and characterized
  during testing

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MIT geotechnical centrifuge
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Detail of centrifuge platform and air sparging
sample
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Porous media tested
Crushed Pyrex and Pyrex glass beads
saturated with glycerol or immersion liquid

• Index of refraction
• of saturated Pyrex
• n 1.471

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Experimental procedure

• Experiments conducted under accelerations ranging
  from 1-g to 100-g

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Captured images
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Constant pressure propagation at 1-g
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Constant pressure propagation at 40-g
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Effect of g-level on air propagation under
constant flow rate injection
Q 0.66 cm3/s
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Sudden increase in injection flow rate at 1-g
Q 10.83 cm3/s
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Mechanisms of air propagation
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Important parameters governing air propagation

• Hydraulic conductivity
• Air flow cannot occur until pore fluid has been
  displaced. Rate of pore fluid outflow (measured
  by q/K) critical to air propagation
• Three scenarios
• low q/K pore fluid outflow does not constrain
  air inflow ? narrow plumes develop
• high q/K rate of pore fluid flow not fast enough
  to accommodate all air inflow ? plume propagates
  laterally, wider plumes result
• v.high q/K ? medium fractures

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Dimensionless RZI of a Sparge Well
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Compilation of Data
Sa low
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New numerical model and assumptions

• Darcys law is valid and governs propagation of
  air front
• Air propagation takes place as a uniform front
  which displaces fluid present in the pores of the
  medium
• Porous medium within plume boundaries is fully
  saturated with air
• Ideal gas law applies

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Finite difference mesh
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Axisymmetric mesh
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Input Parameters
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Air propagation
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Model fit to experimental data
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Model fit to experimental data
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Model Fit to Experimental Data

• Q 0.66 cm3/s

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Comparisons with existing model
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Conclusions

• Experimental investigation
• Behavior in the field is strongly influenced by
  ratio P(cap)/P(hydro)
• Ratio of air-injection rate to soil hydraulic
  conductivity important to problem
• Numerical model
• A simple model was developed that accurately
  predicts the geometric characteristics of
  observed air sparging plumes without the use of
  fitting parameters
• Model currently being tested against field data
• New model could considerably improve the
  efficiency of the IAS emerging technology in
  practice