Brent Sleep, Magdalena Krol, University of Toronto Kevin Mumford, Queen s University Richard Johnson, Oregon Health and Science University

Transcription

1 Brent Sleep, Magdalena Krol, University of Toronto Kevin Mumford, Queen s University Richard Johnson, Oregon Health and Science University

2 Electrical Resistance Heating (ERH) Power Control System Vapour Recovery System Contaminated Zone Electrodes 2

3 Outline Examine ERH impacts on subsurface flow and mass transport during heating process including buoyant flow and the effects on contaminant transport effects of soil heterogeneity and groundwater flux on energy and mass distribution bubble movement and impact on contaminant transport Demonstrated with lab experiments and modeling 6

4 Thermally Induced Buoyancy Lab Expt

5 Thermal Effects on Buoyancy 1.5 hrs Flow 3 hrs

6 Modelling Impacts of Electrical Resistance Heating Necessary to simulate Alternating current flow and heating with temperature dependent electrical conductivity Energy transport Water flow with temperature dependent density, viscosity Aqueous phase transport Bubble formation and transport with temperature dependent, solubilities, vapor pressures

7 Electro Thermal Model (ETM) Ohm s Law: Conservation of charge: J J 0 Voltage: 0 cos t Electrical Field: Power: E U E 2

8 Temperature Dependence Energy transport in the subsurface: t 2 nc T (1 n) ct c ( qt ) K T U 0 w p b p w H Mass transport in the subsurface: t nc CK ( C q) ( nd C) 0 w w b d w w

9 Voltage Distribution Experiment Model Initial Initial Final Final

10 Power Power (W) Experiment Model Elapsed time (min)

11 Modelling the Tracer Movement Experiment Model 1.5 hrs 1.5 hrs 1.5 hrs Flow 3 hrs 3 hrs 3 hrs

12 Buoyant Flow When are buoyant flow and effects on contaminant transport significant?

13 Buoyancy Ratio Electrodes q in Ra K TL Temp Diff = T Rayleigh # Peclet # Pe L q in L K Buoyancy Ratio Ra Pe T i

14 Types of Flow Forced Flow Natural (Buoyant) Flow Mixed Flow

15 Impact of Buoyancy Ratio on Flow Ra/Pe=1 T ( C) Ra/Pe=10 Ra/Pe=100 Ra/Pe=500

16 Impact of Buoyancy Ratio on Mass Buoyant flow Change in mass distribution Ra/Pe >1 Ra> No buoyant flow Ra/Pe <1 Permeability Decreases Ra/Pe >1 Ra< Buoyant flow Gradient Increases No change in mass distribution 7 0.7

17 Impact of Heterogeneities Flow through high permeabity lenses was directed upwards with high buoyancy ratios Before heating After 10 days of heating to 80 C

18 Bubble Formation Examine the potential for bubble movement and impact on contaminant transport

19 Bubble Formation in Porous Media DNAPL Pool Effect of gas bubbles in DNAPL source zones

20 A Simple Proof-of-concept Set up Initially VOC free air bubble Placed in inverted 1.5 ml vial Outer vial open to atmosphere DNAPL PCE: low volatility (0.03 atm) DCE: high volatility (0.41 atm) Control: no pool 0 days

21 0 days 5 days 10 days 14 days 19 days Expansion

22 Expansion 2.5 Bubble radius (mm) PCE vials DCE vials Control vials Time (days)

23 Mechanism Multi component partitioning Partitioning of VOC lowers partial pressure of other gases Steady transport of VOC and other gases to gas phase Results in expansion Atmospheric gases P g P w P c g P i KH ici, g dn dt i D i, z L A D D Pi C i i, g C i, g Dalton s Law Henry s Law Mass Transport VOC

24 Partitioning Model Bubble radius (mm) PCE vials DCE vials PCE model DCE model Control vials Control model Time (days)

25 Flow cell experiment

26 Effect on Pools Flow cell cm mm dia. sand Horizontal flow DNAPL 1,1,1-TCA 17-cm long pool Initial gas Residual saturation

27 Gas Migration 3 days 4 days 6 days 11 days 18 days 21 days 40 days 70 days

28 Transport of VOCs C/CS S z=22 cm z=32 cm Time (days) cm C/CS S z=7 cm Common model Time (days)

29 Enhancement of Vertical Contaminant Movement days 30 z (cm) 20 C/C S x (cm)

30 23.1 days 23.2 days 23.3 days z (cm) 20 z (cm) 20 z (cm) days x (cm) x (cm) 23.5 days x (cm) 23.6 days 23.7 days z (cm) 20 z (cm) 20 z (cm) 20 z (cm) x (cm) x (cm) x (cm) x (cm)

31 Heating of CT Pool

32 Simulation of Bubble Movement

33 MIP-MT approach Macroscopic invasion percolation with mass transfer (MIP-MT) 1. Continuum approach to model solute transport fully implicit block-centered finite difference 2. Discrete approach to model gas movement Macroscopic invasion percolation 3. Linked to ETM Model by gas-liquid partitioning

34 Model Parameters Solute transport Discretized domain Entry thresholds Withdrawal thresholds Intrinsic permeability Gas movement

35 Gas Movement by MIP a) Initial bubble b) Expansion c) Critical length d) Mobilization e) Fragmentation Critical Length

36 Effect of Temperature and Soil Properties on Bubble Migration Three soil types with different permeabilities and pore radii were simulated at 70 C, 80 C, and 90 C Two different inlet groundwater velocities were examined Soil Type Reference Permeability (cm 2 ) Mean pore radius (mm) Displacement Pressure (cm) #25 Ottawa Sand 2.00E #50 Ottawa Sand 5.30E #75 Ottawa Sand 8.20E

37 Aqueous Concentration in High Permeability Soils 70 C 90 C C (mg/l) No Gas Phase Modelled 70 C 90 C Gas Phase Modelled

38 Aqueous Concentration in Low Permeability Soils 70 C 90 C C (mg/l) No Gas Phase Modelled 70 C 90 C Gas Phase Modelled

39 Aqueous Concentration in Low Permeability Soils, Low Velocity 70 C 90 C C (mg/l) No Gas Phase Modelled 70 C 90 C Gas Phase Modelled

40 Impact of Capillary Barriers

41 Conclusions Low temperature ERH may produce buoyant flow Significance of buoyant flow is a function of buoyancy ratio Bubble generation and migration can be significant in permeable soils Bubbles can transfer mass away from the heated zone, particularly vertically Bubble movement in low permeability soils, under low groundwater velocities may result in concentrations over solubility limit of contaminant

42 Thank you