Lecture 5: Soil Vapor Extraction - Memorial University of ...baiyu/ENGI 7718-9621_files/lecture...
Transcript of Lecture 5: Soil Vapor Extraction - Memorial University of ...baiyu/ENGI 7718-9621_files/lecture...
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ENGI 7718 – Environmental GeotechniquesENGI 9621 – Soil Remediation Engineering
Spring 2011 Faculty of Engineering & Applied Science
Lecture 5: Soil Vapor Extraction
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2Source: Suthersan, 1997
Schematic of SVE implementation in the field
5.1 Introduction
Vapor extraction wells + blowers or vacuum pumps
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An accepted, recognized, and cost-effective technology for remediating soils contaminated with volatile and semi-volatile organic compounds
known in the industry by various other names, such as soil venting and vacuum extraction
Inducing airflow in the subsurface with an applied vacuum enhancing the in situ volatilization of contaminants extraction of air laden with contaminant vapors can be achieved with vertical extraction wells or horizontal extraction pipes
(1) Soil vapor extraction
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Usage of a similar approach to soil vapor extraction, but with a different objective
The intent of bioventing to induce airflow to provide oxygen to maximize the aerobic biodegradation of the compounds, in contrast to volatilization
Usage of the same blowers used in SVE systems provide specific distribution and flux of air through the contaminated vadose zone stimulate the indigenous microorganisms to degrade the contaminants
e.g. degradation of benzene
(2) Bioventing
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An in-situ technology Can be implemented with a minimum disturbance to
site operations Very effective in removing the volatile contaminant
mass present in the vadose zoneHas the potential for treating large volumes of soil at
acceptable costsThe system can be mobilized and installed very
quicklyCan easily integrate with other technologies for site
cleanup
(3) Advantages
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5.2 Governing Phenomena
5.2.1 Airflow characteristics
Soil air permeability
describes how easily vapors flow through the soil pore space
the airflow and the air permeability are linearly dependent a higher air permeability will result in a higher flow rate at the same vacuum
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Source: Suthersan, 1997
Typical Airflow patterns and vacuum distribution during SVE
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5.2.2 Contaminant properties
Source: Suthersan, 1997
Contaminant distribution into various phases
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(1) Solubility, vapor pressure, and Henry’s law constant
Source: Suthersan, 1997
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(2) Soil adsorption coefficient
Difference in adsorption potential under wet and drier soil conditionsSource: Suthersan, 1997
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(3) Biodegradability of contaminant
The ease of biodegradation affect on efficiencies of bioventing operations
Alkynes > Alkenes > Carboxylic Acids >Alcohols > Straight Chains > Aromatics > Chlorinated Aromatics > Branched Chains
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Weathering Process of changing in organic contaminant mixture characteristics
During SVE initially remove primarily the more volatile, lighter-end fractions then the extracted vapor will likely be composed mostly of heavier compounds
The degree of weathering impact the overall SVE effectiveness
(4) Weathering
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Sand ==> Silt ==> Claydecreased sizedecreased porosity increased available surface decreased SVE efficiency
5.2.3 Soil properties(1) Soil porosity
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(2) Moisture content
increased moisture content decreased of Kddecreased adsorptive capacity increased SVE efficiency
Adsorption of contaminants as a function of moisture content
Source: Suthersan, 1997
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(3) Soil heterogeneity
Removal of contaminants from heterogeneous soilSource: Suthersan, 1997
Dead zone molecular diffusionHomogenerous subsurface better for SVE
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(4) Surface seals
Effect of surface seal on vapor flow pathsSource: Suthersan, 1997
usage of air inlet wells to control the subsurface air flow
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(5) Depth of water table
Water table rise during SVE operation
Source: Suthersan, 1997
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5.3 Applicability
SVE applicability monograph Source: Suthersan, 1997
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SVE more applicable where the contaminated unsaturated zone is relatively permeable and homogenerous
A candidate site for SVE
Source: Suthersan, 1997
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5.4 Pilot testing
SVE pilot testing schematic
Source: Suthersan, 1997
It’s important to conduct pilot testing before field system design and application
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21Typical SVE well
Source: Suthersan, 1997
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22Monitoring well
Source: Suthersan, 1997
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5.5 System design
Most widely used approach in industry today
The extraction flow rate vacuum, number of wells and their locations selected based on the information obtained from a field pilot test
The pilot test provide steady state vacuum vs. distance relationships for a given site
Radius of influence approach
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(1) Determination of the radius of influence of the extraction well
Vacuum vs. distance plot on a semi-log paperSource: Suthersan, 1997
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(2) Determination of the required number of wells from the radius of influence
Source: Suthersan, 1997
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(3) Estimation of the total extraction airflow rate
Airflow generation plot
Source: Suthersan, 1997
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Operation of two smaller-capacity blowers in series or parallel instead of a single large-capacity blower cost-effective way
Use of blowers in series or parallelSource: Suthersan, 1997
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5.6 Vapor treatment technologies
(1) Thermal oxidation sufficiently heating a VOC in the presence of oxygen to convert VOC to harmless end products
Schematic diagram of a recuperative thermal oxidizerSource: Suthersan, 1997
Key parameters: combustion T + residence time + degree of mixing
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(2) Catalytic oxidation combines a heat exchanger with a catalyst. the catalyst inside the combustion unit lowers the activation energy for combustion
Catalyst Pt, Pd, CoO, CuO, MnO2 …Catalyst deactivators Pb, Ar, S, Si, P, Hg, Zn… Source: Suthersan, 1997
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(3) Adsorption gaseous VOC molecules contact a solid adsorbent and bond via weak intermolecular forces replacement/regeneration desorption
Schematic diagram of a fixed regenerative bed carbon adsorberSource: Suthersan, 1997
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(4) Condensation remove VOCs from a non-condensable gas stream by lowering gas stream temperature at constant pressure or increasing gas stream pressure at constant temperature
Schematic diagram of a refrigerated condenser
Source: Suthersan, 1997
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(5) Biofiltration
In a biofilter => microorganisms grow on materials such as soil or compost + supplemented with synthetic materials including activatedcarbon, which provides bulking and structural stability
Source: Suthersan, 1997
Components of a biofiltration system
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33VOC removal in air streams by membrane separationSource: Suthersan, 1997
(6) Membrane filtration use of a semi-
permeable membrane to separate VOCs from an air stream VOC-laden air contacts one side of a membrane that is permeable to organic vapors but is relatively impermeable to air
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5.7 Cost considerations
Costs equipment cost + operation cost$10 to $50 per yd3 of contaminated subsurface material
Source: Suthersan, 1997