Brent Sleep, Magdalena Krol, University of Toronto Kevin Mumford, Queen s University Richard Johnson, Oregon Health and Science University
Electrical Resistance Heating (ERH) Power Control System Vapour Recovery System Contaminated Zone Electrodes 2
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
Thermally Induced Buoyancy Lab Expt
Thermal Effects on Buoyancy 1.5 hrs Flow 3 hrs
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
Electro Thermal Model (ETM) Ohm s Law: Conservation of charge: J J 0 Voltage: 0 cos t Electrical Field: Power: E U E 2
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
Voltage Distribution Experiment Model Initial Initial Final Final
Power Power (W) Experiment Model Elapsed time (min)
Modelling the Tracer Movement Experiment Model 1.5 hrs 1.5 hrs 1.5 hrs Flow 3 hrs 3 hrs 3 hrs
Buoyant Flow When are buoyant flow and effects on contaminant transport significant?
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
Types of Flow Forced Flow Natural (Buoyant) Flow Mixed Flow
Impact of Buoyancy Ratio on Flow Ra/Pe=1 T ( C) Ra/Pe=10 Ra/Pe=100 Ra/Pe=500
Impact of Buoyancy Ratio on Mass Buoyant flow Change in mass distribution Ra/Pe >1 Ra>60 70 700 700 7 70 7 70 0.1 0.001 7 0.2 0.7 No buoyant flow Ra/Pe <1 Permeability Decreases Ra/Pe >1 Ra<0.6 700 70 Buoyant flow Gradient Increases No change in mass distribution 7 0.7
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
Bubble Formation Examine the potential for bubble movement and impact on contaminant transport
Bubble Formation in Porous Media DNAPL Pool Effect of gas bubbles in DNAPL source zones
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
0 days 5 days 10 days 14 days 19 days Expansion
Expansion 2.5 Bubble radius (mm) 2 1.5 1 0.5 0 PCE vials DCE vials Control vials 0 50 100 150 200 250 Time (days)
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
Partitioning Model 2.5 2 Bubble radius (mm) 1.5 1 0.5 PCE vials DCE vials PCE model DCE model Control vials Control model 0 0 50 100 150 200 250 Time (days)
Flow cell experiment
Effect on Pools Flow cell 70 60 1 cm 3 1.1 mm dia. sand Horizontal flow DNAPL 1,1,1-TCA 17-cm long pool Initial gas Residual saturation
Gas Migration 3 days 4 days 6 days 11 days 18 days 21 days 40 days 70 days
Transport of VOCs C/CS S 0.0009 0.0006 0.0003 0 z=22 cm z=32 cm 0 20 40 60 Time (days) 0.16 0.12 10 cm C/CS S 0.08 0.04 0 z=7 cm Common model 0 20 40 60 Time (days)
Enhancement of Vertical Contaminant Movement 40 23.1 days 30 z (cm) 20 C/C S 10 10 6 10 4 10 2 10 0 0 0 10 20 30 40 x (cm)
23.1 days 23.2 days 23.3 days 40 40 40 30 30 30 z (cm) 20 z (cm) 20 z (cm) 20 10 10 10 23.4 days 0 0 10 20 30 40 x (cm) 0 0 10 20 30 40 x (cm) 23.5 days 0 0 10 20 30 40 x (cm) 23.6 days 23.7 days 40 40 40 40 30 30 30 30 z (cm) 20 z (cm) 20 z (cm) 20 z (cm) 20 10 10 10 10 0 0 10 20 30 40 x (cm) 0 0 10 20 30 40 x (cm) 0 0 10 20 30 40 x (cm) 0 0 10 20 30 40 x (cm)
Heating of CT Pool
Simulation of Bubble Movement
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
Model Parameters Solute transport Discretized domain Entry thresholds Withdrawal thresholds Intrinsic permeability Gas movement
Gas Movement by MIP 3 2 1 a) Initial bubble b) Expansion c) Critical length d) Mobilization e) Fragmentation 4 5 1 2 1 3 5 1 5 Critical Length
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 06 0.257 4.43 #50 Ottawa Sand 5.30E 07 0.077 13.5 #75 Ottawa Sand 8.20E 08 0.034 33.1
Aqueous Concentration in High Permeability Soils 70 C 90 C C (mg/l) No Gas Phase Modelled 70 C 90 C Gas Phase Modelled
Aqueous Concentration in Low Permeability Soils 70 C 90 C C (mg/l) No Gas Phase Modelled 70 C 90 C Gas Phase Modelled
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
Impact of Capillary Barriers
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
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