Air-Flow Mechanisms During Air-Sparging 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

4. Pressure Regulator Pressure Gauge Flowmeter Air Compressor Vacuum Gauge Vapor Vacuum Treatment Pump Unit VADOSE ZONE VAPOR FLOW VAPOR EXTRACTION WELLS SATURATED ZONE AIR FLOW AIR CONTAMINANT INJECTION PLUME WELLS (After Reddy et al., 1995) Insitu Air-Sparging (IAS) • Technique for the remediation of groundwater contaminated with VOCs

5. 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 1990’s 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)

7. P(fluid) = rfgH Height to wt, H Capillary pressure due to interfacial tension Air Injection Port P(air) 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

8. 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> Phydro +2scosq/r (Where it is assumed that the water is not moving)

9. Ratio of hydrostatic to capillary pressure Ratio Pcap / Phyd is important to the problem • Pcap / Phyd >> 1, soil fracture possible • Pcap / Phyd ≈ 1, local capillary pressures likely to influence air- movement • Pcap / Phyd << 1, local capillary pressures less likely to influence air-movement For most practical applications of IAS, Pcap / Phyd << 1

10. 1 0 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 0 1 2 3 4 5 D e p t h o f i n j e c t i o n [ m ] Pcapillary/Phydrostatic vs Depth of Injection Pinj = Phydro + Pcapillary D = 1 0 m m 1 0 D = 1 m m 1 0 D = 0 . 1 m m 1 0 D = 0 . 0 1 m m 1 0 P(cap)/P(hydro)[%]

11. 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

12. 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

14. MIT geotechnical centrifuge

15. Detail of centrifuge platform and air sparging sample

16. Porous media tested Crushed Pyrex and Pyrex glass beads saturated with glycerol or “immersion” liquid Index of refraction of saturated Pyrex n = 1.471

17. Constant pressure injection conditions Constant flow rate injection conditions ranging from 0.26 cm3/s to 28 cm3/s Operational parameter: injector geometry Porous medium parameters: grain-size and pore fluid Experimental procedure Experiments conducted under accelerations ranging from 1-g to 100-g

18. Captured images

19. Constant pressure propagation at 1-g

20. Constant pressure propagation at 40-g

21. 15-g 9-g 30-g 70-g 1-g 6-g Effect of g-level on air propagation under constant flow rate injection Q = 0.66 cm3/s

22. Sudden increase in injection flow rate at 1-g Q = 10.83 cm3/s

23. 2. Uniform propagation 3. Fracturing 2. Uniform propagation 1. Fingering 3. Fracturing Mechanisms of air propagation

24. 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

25. Dimensionless RZI of a Sparge Well P h r e a t i c s u r f a c e S u r f a c e o f r e v o l u t i o n = e l e v a t i o n R Z I 2 p 2 H H=rmax A i r H s p a r g i n g p l u m e I n j e c t i o n p o r t

26. RZI 4Drgr2/vm Compilation of Data Sa low

27. New numerical model and assumptions • Darcy’s 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

28. Constant head boundary No flow boundary Impermeable boundary Finite difference mesh

29. Axisymmetric mesh

30. Input Parameters

31. Pore fluid ( V ) = Assumed air volume air 1 n = Moles of air injected D = (Q . t)/ Molecular weight of air (P ) = (n . R. T)/( V ) air 1 air 1 (Ideal Gas Law) j +1 j Air Q i +1 i Air propagation

32. Model fit to experimental data

33. Model fit to experimental data

34. Model Fit to Experimental Data Q = 0.66 cm3/s

35. Comparisons with existing model

36. 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