Group:
1. Rose
2. Allison
3. Adam
Table of Contents
Executive summary
Through many years of operation, gasoline spillage has occurred at a large fuel
distribution facility in central Minnesota. A system design will be implemented in the soil
to remove volatile components of gasoline by vapor-extraction techniques. An electric
motor pump will provide the flow of air for the vapor extraction system. The amount of
energy necessary to operate the pump will be calculated and reported for the extraction
period. Once extracted, the volatile chemicals will be passed through a bio-filter to
remove any extracted contaminants (benzene). The bio-filter will be designed and
dimensioned using a pump and specific materials for the cleanup technique. The amount
of energy and costs needed to operate the extraction and bio-filter will be calculated. The
primary objective of the design is to reduce carcinogenic benzene concentrations over a
six-month period to less than 1 ppb (MCL for benzene in water). A comparison will be
discussed regarding dissipation time under natural treatments vs. the design system
dissipation time for 6 months. Safety, risk, and economics of the design will be
considered. A report will be prepared for customer approval.
Problem introduction (description of design data)
Residual gasoline product exists in the unsaturated zone overlying the groundwater and
below the water table to a depth of 1.5m. Ground water at this site is four meters below
grade. The area contaminated by the spill is 1.5 hectare (15,000 m^2). Residual gasoline,
which contains the benzene, occupies the porous volume of the sandy soil. The
volumetric content of the gasoline in the sand at residual level is 8%. A pump will be
utilized to bring water table levels down to a level so that the contaminated soil can be
de-saturated.
Design parameters (assumptions):
Volume of spill: 15,000 m^2 by 5.5m
Ground water recharge rate: 20cm
Benzene Properties:
Molecular Mass: 78.1121 g/mol
Vapor Pressure: 44.35 mmHg
Water solubility (ambient temperature):
Half Life in soil water (ambient temperature):
Sorption of benzene to organic matter fraction of soil:
Average water content of contaminated soil (field capacity of medium grade sand):
Organic matter fraction for unsaturated material: 0.1%
Dry contaminated soil bulk density: 1600 kg/m^3
Mean temperature of contaminated soil: 45 degrees Fahrenheit.
Site’s mean wind velocity: 7km/hr
Reynolds Number:
Electric Pump specifications: Pump motor system for 85% efficiency.
Electricity Costs: $0.068/kw-hr
Bio-filter specifications:
Cross sectional AreaLengthBooster pump sized to provide for calculated pressure drop needed to discharge vapor
stream through this bio-filter.
Efficiency – 90%
Electricity Costs: $0.068/kw-hr
Description of design parameters (equations/relationships, source of data)
Results
Safety Considerations
Economic Considerations
Social Considerations
Design Specifications and Dimensioning
Appendix
SVE - Soil Vapor Extraction (soil venting, vacuum extraction)
A technology that reduces volatile constituent concentrations in petroleum
products that have been absorbed in the unsaturated zone of soil. A vacuum is applied to
the soil system that creates a negative pressure gradient causing vapor movement to
extraction wells. Volatile chemicals are removed from the subsurface through these
extraction wells. These vapors can be treated and released into the atmosphere or returned
to the subsurface. SVE is effective when applied to lighter volatile petroleum products
(gasoline) rather then diesel, kerosene, or heating oil.
Soil Vapor extraction has proven performance for cleanup of gasoline. The system is
easily obtained and installed. The system poses minimal disturbance to site
operation. Under optimal conditions, treatment time is approximately six months to
two years. The average cost of the treatment is $20-$50 per ton of contaminated soil.
Other technologies include: air sparging, bioremediation, and vacuum enhanced
duel phase extraction.
There are several unfavorable situations for using SVE. If concentrations are
greater than 90%, achieving extraction is difficult. Low permeability soils prevent
effectiveness of airflow. Air emission permits are required for use the SVE. This
technique is for unsaturated zone soils, and other methods are needed to clean
saturated soil zones and groundwater.
This site has sand and gasoline spillage, making SVE an effective method for
cleanup.
Sand has a high permeability (k) due to pore space and fluid movement.
With a groundwater depth between 3 and 10 feet, the ground water must be
pumped or horizontal wells must be implemented.
The vapor pressure of benzene at 20 degrees C = 76 mm Hg
SVE effectiveness
1. Permeability of Soil (intrinsic parameter -> ability for soil to transmit fluids)
2. Constituent volatility
These factors are used to determine the mass extraction rate.
Hydraulic conductivity can be used to estimate permeability
High soil moisture content in soils will reduce permeability. And the effectiveness of
SVE by restricting airflow through pore spaces. When combined with pumping
technology, the water table can be lowered to increase the performance of SVE
systems.
This design will require groundwater pumping.
Vapor pressure is a definitive characteristic of potential effectiveness of an SVE
system. Vapor pressure is a measure of evaporation tendency (pressure that a vapor
exerts when in equilibrium with its pure liquid or solid form). Higher vapor
pressure are easier to extract with SVE systems (>0.5 mm Hg are effective).
Injection of heated air can also be used to enhance volatility of these products
because vapor pressure increases with temperature. (Heating energy requirements
may be too large to be economically possible)
Benzene (mm Hg at 20 degrees C) = 76 (fits range)
Due to complex constituent composition, the boiling point is a measure of volatility.
The measure of applicability for a petroleum product can be measured from its
boiling point range.
Gasoline boiling point range (degrees C)= 40 to 225
Petroleum products with boiling points less than 250 to 300 are sufficiently volatile
for removal by SVE. The SVE can remove nearly all constituents at this boiling
point range.
Henry’s Law Constant is an indicator of volatility of a constituent.
This constant is a partitioning coefficient that relates concentration of a constituent
dissolved in water to its partial pressure in the vapor phases under equilibrium
conditions. The constant describes the tendency for a dissolved constituent to
partition between dissolved phase and vapor phase. The measure will give the
degree to which constituents are dissolved in soil or groundwater will volatilize for
removal by SVE systems. Constituents with Henry’s law constants greater than 100
atm are considered removable by SVE systems
Henry’s Law Constant for Benzene: 230
Other site-specific considerations for SVE removal systems. SVE is appropriate for
cleanup near a building to prevent vapors from entering the building.
Bio-venting
Longer pilot studies (6months) utilize more than one extraction technique will be
appropriate for larger sites. To determine optimal operating conditions, different
extraction rates and wellhead vacuums are applied. Vacuum influence is measured
using a vapor probe or existing wells induced in the subsurface. Filed pressure
measurements are used to define the design radius for the soil vapor extraction
system. Vapor concentrations are measured to determine an estimate for the fullscale system. Vapor extraction rate, vapor concentration, and vacuum data are used
to select extraction and treatment equipment.
The EPA’s HyderVentilate is a screening model to determine SVE effectiveness.
-Site data, site appropriateness, air permeability, and estimation of minimum
number of wells needed.
SVE has been determined to be applicable to our specific site, and the system design
can now be determined.