ENVE 436 Project
Fall 1998
Steam Stripping
Presented to :
Dr. Sczechowski
Presented by:
Duane White
Ryan Piner
Linda Louie
INTRODUCTION
Industrial chemical production of benzene, vinyl chloride, and polychlorinated biphenyl, has increased over
the past 50 years. At that time, it was often the case that these chemicals were disposed in the most simple and
cheapest way. This usually meant storing the chemical on the company's property or sending them to a landfill. Little
did the chemical producers realize that eventually these chemicals would migrate through the soil and into the
groundwater. Several years later, the chemicals would be discovered in the local water supply. Once the public was
aware that industrial chemicals were found in the water supply, causing low birth rates and early deaths, Congress
passed the Resource Conservation and Recovery Act (RCRA) in 1976, and later the Comprehensive Environmental
Response Compensation and Liability Act of 1980 (CERCLA). CERCLA's objective was to clean up contaminants at
hazardous waste disposal sites. In response to CERCLA, several technologies were developed to treat different
contaminants at Superfund sites across the country 1. Steam stripping was one of the technologies developed. It was
developed at the University of California at Berkeley to remove volatile organic compounds (VOC’s) and reduce the
level of semi-volatile organic compounds from the groundwater and soil.
APPLICATIONS
Steam stripping, which is also known as hydrous pyrolysis/oxidation and in-situ steam extraction, has been
used for oil recovery and is now being used as a potential method to treat soils contaminated with non-aqueous phase
liquid (NAPL), as well as light non-aqueous phase liquids (LNAPL)
2
. It is a physical and chemical process where
steam, at a temperature of about 170 F to 180 F, is forced into an aquifer through injection wells to vaporize volatile
and semi-volatile contaminants. These vaporized components rise to the vadose zone, where they are removed by
vacuum extraction and then are treated. The steam stripping process is applicable to deep (i.e. the saturated zone), as
well as shallow (i.e. up to five feet in the vadose zone) contaminated areas. They are often used at manufactured gas
plants, wood-treating sites, petroleum-refining facilities, and other sites with soils containing organic liquids
3
(Figure 1). Although there are several slightly varied types of steam strippers available, we will focus our attention to
the physical characteristics of an in-situ steam/hot air-stripping unit. We will discuss the physical process and
theory of steam stripping. Then we will look at two different case studies, and lastly we will discuss the cost of
steam stripping.
1
2
3
United States Environmental Protection Agency, Toxic Treatments, In-Situ/Hot Air Stripping Technology (Washington: ORD, 1991) 3.
“Hot Water or Steam Flushing/Stripping,” www.ftr.gov/matrix2/section4/4_43.html. 15 Nov 98.
“Hot Water or Steam Flushing/Stripping,” www.ftr.gov/matrix2/section4/4_43.html. 15 Nov 98
APPARATUS
The steam stripper is a mobile unit that can be transported from treatment area to treatment area. It is
composed of the two major constituents: 1) the process tower and 2) the process train. The process tower includes
the treatment shroud, the kelly bars, the cutter bits, the rotary table, and the crowd assembly (Figure 2).
Collectively, these components loosen the soil, inject the steam, and collect the VOC from the soil. At the end of
the kelly bars are the cutter bits, which has nozzles for injecting the steam. The rotary table and crowd assemble
provides power to the kelly bars. The steam raises the temperature of the soil to 170 - 180 F, which increases the
vapor pressure of the organic compounds and allows them to travel to the surface by a vacuum. At the surface, the
volatized vapors and hot air are collected beneath the treatment shroud and then travel to the gas treatment system of
the process train.
The gas treatment includes the scrubber, the cyclone separator, cooling system, carbon adsorption system,
and compressor. The air first goes through the scrubber, which removes particulate matter in the air stream. Then
the air stream travels to the cyclone separator, which removes water droplets from condensing steam. The water
collected is sent to the distillation system. Afterwards, the air stream passes three stages of cooling by heat
exchangers, which removes water vapor and volatile compounds from the air stream by condensation.
The
condensation that is accumulated is directed to the distillation process. The air stream then goes through two
alternately used carbon beds. The first bed is used for adsorption of the volatile organics, while the second
undergoes a regeneration process. The liquids from the regeneration are sent to the distillation system. The air
stream is then drawn through a filter of a compressor. The compressor increases the air from atmospheric to 250
pounds per square inch gauge (psig) and temperature to 275 F. The compressed air is sent back down through the
kelly bars to remove more contaminants in the soil.
The condensates generated from the cyclone separator, cooling system, and regeneration of activated carbon
travels through the distillation system into a condensed organics collection tank (Figure 2) 4. The contaminated
water enters the distillation column at the feed point and flows downward, while steam passes upwards through the
column. The steam causes the volatile organics to exert a higher pressure than at ambient conditions, causing the
organics to transfer from the liquid phase to the gas phase. The steam is now contaminated with vaporized volatile
components. The steam is then compressed at an elevated pressure, causing the gas to condense to a liquid in the
4
LaGrega, Micheal, D., et al., Hazardous Waste Management (New York: McGraw-Hill, Inc., 1994) 488-490.
compressor. The compression of the steam creates a vacuum on the compressor inlet side that creates the vacuum
conditions. The compression releases heat which is recovered by vaporizing part of the liquid to create the stripping
steam. The organics will have a different density than water, which will cause the organic phase to float or sink in
water, called the organic phase in the diagram. This aqueous phase returns to the top of the distillation column.
LIMITATIONS
When evaluating the feasibility of using a steam stripper, there are certain conditions to follow. First, only
soils containing VOC can be remediable since this is what this apparatus is designed to work with. Soils that are
comprised of a high clay content will promote binding of the contaminants and will delay the time of treatment. Clay
soils also promote resistance to penetration by the drill augers. Therefore, sandy soils are ideal for shorter
remediation times. Extreme weather conditions should also be avoided. Not only will cold temperatures freeze the
soil, making for difficult penetration, they would increase heat loss from the equipment, as well, thus, demanding
large quantities of energy to operate. Conversely, hot weather conditions decrease the cooling capacity of the cooling
tower and lowers its effectiveness. Furthermore, volatile organic compounds with lower boiling points require less
treatment time than those compounds with higher boiling points. In general, the temperature of the site should be
between 35 to 95 F. Studies show that compounds with boiling point below 175 F, were more likely to be removed
to undetectable levels.
5
When selecting a suitable site for steam stripping usage, surface and subsurface materials such as rock,
concrete, and trash metal with diameters greater than 12 inches, must be removed to prevent damage to the stripper.
The ground area to be treated must be flat and level, gradeable to less than 1 % slope. It must be able to support the
rig, which can apply 25 psi, so it so will not sink or tip. A minimum treatment area of 0.5 acres is and a buffer zone
is necessary for easy maneuverability of the steam stripping unit. Trailers transporting the stripper, weighing 80,000
pounds should be able to drive on the roads leading to the area. The stripping process requires 8 – 10 gallons per
minute at 30 psi for a water supply.
6
CASE STUDY 1
In 1991, the Environmental Protection Agency (EPA) published a report on the removal efficiency of the in
situ steam / hot air stripping technology.
7
Testing showed that the stripper averaged a removal efficiency of 85%
for VOC and 50 % for semivolatiles, although where the semivolatiles went was not determined. Sampling of the
5
6
United States Environmental Protection Agency, Toxic Treatments, In-Situ/Hot Air Stripping Technology (Washington: ORD, 1991) 8.
United States Environmental Protection Agency, Toxic Treatments, In-Situ/Hot Air Stripping Technology (Washington: ORD, 1991) 12.
site took place in a 12-block area down to the water table, which is about 5 feet in depth, and in an alternative
6-block test area down to the saturated zone, which was approximately 8 to 11 feet deep. Pretreatment data showed
that both testing blocks were heavily contaminated with volatile and semivolatile contaminants and the soil had high
clay content. The predominant VOC in the 12-block area were chlorbenzene, trichloroethene, and tetrachloroehene,
and in the 6-block area, there were ketones such as acetone, 2-methyl-4-pentanone, and 2-butanone, which are more
difficult to remove in the 12-block area because of their increased water solubility. The semivolatiles found in both
areas were bis (2-ethylhexyl) phthalate, phenol, napthalene, and phenanthrene. In the pretreatment 12-block area, the
average levels of VOC ranged from 28 parts per million (ppm) to 1,130 ppm and post treatment ranged from 12 to
196 ppm. In the same 12-block area, the semivolatile compounds found in the pretreatment ranged from 336 to 1310
ppm and for post treatment, the range was 49 to 818 ppm (Table 1 and 2). In the 6-block area, only posttreatment
data of VOC were collected because the study was to evaluate contaminant concentration after treatment and not
removal efficiency. Values for these concentrations ranged from 16 to 119 ppm (Table 3).
The objective of the study was to get post-treatment concentrations down to the 100 ppm federal standard.
But, results showed that one out of every six cores used in the 12-block area still had concentrations above that
standard. Experts believe that the reason for this is that “the augers may have passed below the maximum treatment
depth of 5 feet and brought up contamination from below...without allowing sufficient time for treatment of this
contaminated soil.”8 This issue could be resolved by treating all zones below the contamination.
CASE STUDY 2
In another study, Korfiatis and Shah published laboratory experiments that were conducted to evaluate the
effects that steam injection pressures, soil grain size distribution, and LNAPL type, would have on steam stripping. 9
The experiments were performed with two steam injection pressure settings, two soil types (1. poorly- graded,
uniform-grain size and 2. well-graded, multiple-grained soils) of different distribution slopes (Tables 4 and 5), and
two LNAPLs, No 2 heating oil and jet fuel (Tables 6 and 8).
Data showed that as the steam injection pressure increased in the same grain size, the LNAPL recovery
efficiency increased. However, it was shown that when the grain size decreases, the effect of the steam inlet
pressure on LNAPL recovery efficiency is reduced because the steam flow rate for the same increase in pressure is
7
United States Environmental Protection Agency, Toxic Treatments, In-Situ/Hot Air Stripping Technology (Washington: ORD, 1991)
5-10.
8
United States Environmental Protection Agency, Toxic Treatments, In-Situ/Hot Air Stripping Technology (Washington: ORD, 1991) 6.
G.P. Korfiatis and Hadim, F.H. Shah, “Laboratory Studues of Steam Stripping of LNAPL-Contaminated Soils,” Journal of Soil
contamination.
(1993): 1-22.
9
reduced (Tables 6 and 8). Reducing the flow rate also reduces the rate of energy, which saves money.
To study the effect of grain size, data was collected with No. 2 heating oil , four coil types, and the steam
pressure was fixed at 44.8 kPa. A mean grain size of 1.22 mm gave maximum LNAPL recovery efficiency while a
soil grain of 0.22 mm took more than 30 hours to achieve the same recovery efficiency. This indicates that as mean
grain decreases, the LNAPL recovery efficiency does as well. According to the results of this lab sudy, “Soils with
finer grain sizes have a lower porosity, which reduces the total amount of steam occupying the pore volume, leading
to reduced heat transfer rate between the steam an dthe LNAPL ganglia.”
10
For the effect of grain-size distribution slope, four soil types, having different slopes were tested with No. 2
heating oil and the steam injection pressure was 44.8 kPa (Tables 7). Data showed that as the slope decreases, the
recovery efficiency decreases due to the associated decrease in the porosity and permeability.
Furthermore, conclusions showed that compounds with great volatility have higher recovery efficiency. In
this experiment, data showed jet fuel had higher recovery efficiency than No. 2 heating oil due to its volatility, higher
vapor pressure and lower boiling point (Tables 6 and 8). In essence, the experiments showed that time required to
reduce the contaminants in soil is a function of the pressure that is used, properties of the soil, and the volatility of
chemicals.
COST ANALYSIS
In 1991, the EPA did an economic analysis of a steam stripper.
11
There were several assumptions made in
calculating the cost of using a steam stripper. One assumption made was that the estimated costs are typical charges
a vendor would charge a client, without including a profit. Another assumption made was that cost, such as a
preliminary site preparation, permits and regulatory requirements, waste disposal, sampling and analysis, and site
clean up are the owner’s responsibility and weren’t included in the estimate. For a 100 % on-line condition (i.e. the
ratio of the operation time to the time spent on-site each workday), the treatment rate is assumed three cubic yards an
hour. Operations are assumed to be 16 hours a day with one supervisor, two health and safety engineers, four
operators, and five mechanics. The total volume of soil treated in the report was 8,925 cubic yards. A 65 %
utilization factor is incorporated to account for depreciation, which occurs during maintenance, marketing, and
regulatory delays, while the equipment is not on site. And finally, wastewater is assumed to meet local water quality
standards.
G.P. Korfiatis and Hadim, F.H. Shah, “Laboratory Studues of Steam Stripping of LNAPL-Contaminated Soils,” Journal of Soil
contamination. (1993): 12.
10
The attached table (Table 9) shows the estimated costs ranging from 70 % and 266 treatment days, which is
considered the average time, to 90 % and 207 treatment days for 12 different categories of. These twelve categories
include site preparation costs, permitting and regulatory costs, equipments costs, startup and fixed costs, labor costs,
supplies costs, consumables costs, effluent treatment and disposal costs, residuals and waste shipping costs,
analytical costs, facility modification, repair and replacement costs, and site dmobilizations costs. In general, the
report indicates that the cost ranges from $57 to $317 per cubic yard (Table 9). This is consistent with other findings
that report cost ranges from $50 to $300.
12
The economic analysis also shows that 47% of the costs is attributed to labor, which incorporates living
expenses and salaries of workers. However, as the treatment rate increases, the steam stripper becomes less labor
intensive, which decreases the cost. That is, if VOC reduction is done at a faster rate less time is needed for operation
and consequently, less labor is needed. Labor costs are followed by setup and fixed costs, which reflect
transportation and assembly of the unit, at 30% of the total cost (Figure 3). This cost may be decreased by treating
sites with few surface and subsurface obstacles and using power from a local electric company rather than by an
on-board generator.
CONCLUSIONS
This report describes and explains the process and theory of a steam stripper. In addition, two studies and
cost analysis is included. When selecting a steam stripper, post treatment results will vary from project to project.
There are several other technologies developed for treating hazardous waste sites. The engineer’s responsibility is
to research other technologies for the project.
11
United States Environmental Protection Agency, Toxic Treatments, In-Situ/Hot Air Stripping Technology (Washington: ORD, 1991)
15-24.
12
“Hot Water or Steam Flushing/Stripping,” www.ftr.gov/matrix2/section4/4_43.html. 15 Nov 98
Table 1. Demostration Test Results for Volatiles
Table 2. Demostration Test Results for Semivolatiles
12-Block Test Area
Block
Number
A-25-e
A-26-e
A-27-e
A-28-e
A-29-e
A-30-e
A-31-e
A-32-e
A-33-e
A-34-e
A-35-e
A-36-e
Avg
Std Dev
PreTreatment
(ug/g)
54
28
642
444
850
421
788
479
1133
431
283
153
466
457
PostTreatment
(ug/g)
14
12
29
34
82
145
61
64
104
196
60
56
71
80
12-Block Test Area
Percent
Removal
%
74%
57%
95%
92%
90%
66%
92%
87%
91%
55%
79%
63%
85
N/A
Table 3. Demostration test Results for Volatiles
6-Block Test Area
Block
Number
A-26-n
A-27-n
A-28-n
A-29-n
A-30-n
A-31-n
Avg
Std Dev
PreTreatment
(ug/g)
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A Not applicable
PostTreatment
(ug/g)
16
22
36
80
119
45
53
73
Percent
Removal
%
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
Block
Number
A-25-e
A-26-e
A-27-e
A-28-e
A-29-e
A-30-e
A-31-e
A-32-e
A-33-e
A-34-e
A-35-e
A-36-e
Avg
Std Dev
PreTreatment
(ug/g)
595
1117
1403
1040
1310
1073
781
994
896
698
577
336
902
407
PostTreatment
(ug/g)
82
172
439
576
726
818
610
49
763
163
192
314
409
407
Percent
Removal
%
86%
85%
69%
45%
45%
24%
22%
95%
15%
77%
67%
7%
55
N/A
Table 4.
Physical Properties of Soils with uniform Grain Size
Mean grain
Hydraulic
U.S. std
size (D50)
conductivity
sieve no.
(mm)
Slope
(cm/s)
Porosity
20-Oct
1.2
265.4
0.764
0.39
20-40
0.61
265.4
0.114
0.36
40-60
0.31
265.4
0.0665
0.33
60-100
0.22
265.4
0.0166
0.31
Table 6.
Data for Experiments with No 2. Heating oil as LNAPL for
Various Pressures and Soils of Uniform Grain Size
Mean grain Steam inlet
Inlet
Flow
Recover Max.
y
size
Pressure
temp.
rate
eff. After recovery
(mm)
(kPa)
(C)
(ml/min)
1 PV
eff. (%)
(%)
1.2
44.8
110.0
116.6
95.0
99.8
1.2
24.1
105.0
66.6
89.0
99.0
1.2
12.4
102.2
33.3
86.0
98.7
0.6
44.8
110.0
41.6
90.9
99.4
0.6
24.1
103.9
20.2
88.2
99.0
0.3
44.8
110.0
12.8
87.2
99.3
0.2
44.8
110.0
7.7
82.8
98.2
Table 7.
Data for Experiments with No 2. Heating Oil Lnapl
for Various Well Graded Soils
Steam inlet
Inlet
Flow
Max.
Pressure
temp.
rate
recovery
Slope
(kPa)
(C)
(ml/min)
eff. (%)
265.4
44.8
110
122.7
99.6
95
44.8
110
56.5
99.1
56.6
44.8
110
4
97.3
41.3
44.8
110
2.8
96.8
Table 8
Data for Experiments with Jet Fuel as NAPL for Various
Uniform Grain Size Soils and Pressures
Recover Max
y
Steam inlet
Inlet
eff. After recovery
Mean grain Pressure
temp.
Flow rate
1 PV
eff.
size (mm)
(kPa)
(C)
(ml/min)
(%)
(%)
1.2
44.8
110.0
116.6
96.4
99.5
1.2
24.1
105.0
66.6
94.2
98.9
0.61
44.8
110.0
41.6
91.8
99.2
0.61
24.1
105.0
20.2
89.3
98.8
Table 5.
Physical Properties of Well-Graded Soils
Mean grain Hydraulic
size (D50) conductivity
Slope
(mm)
(cm/s)
Porosity
265.4
1.2
0.764
0.39
95
1.2
0.0097
0.37
55.6
1.2
0.00255
0.37
41.3
1.2
-----0.21
Table 9: Summary of estimated Costs in $/Cubic Yard for Various Treatment and On-line Operating Factors
3 cubic Yards/Hour
0.80
0.90
3.67
3.67
0.70
3.67
10 cubic Yards/Hour
0.80
0.90
3.67
3.67
0.70
3.67
20 cubic Yards/Hour
0.80
0.90
3.67
3.67
Site Preparation
Permitting and
Regulatory Costs
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Equipment Cost
14.24
12.53
11.19
7.37
6.51
5.85
4.62
4.11
3.71
2.56
2.30
2.10
Startup and
Fixed Costs
93.62
83.74
76.05
54.05
49.14
45.29
38.26
35.30
32.98
26.39
24.92
23.77
Labor Costs
151.37
132.45
117.73
75.68
66.22
58.87
45.41
39.73
35.22
22.71
19.87
17.66
Supplies Costs
10.70
9.41
8.84
5.54
4.90
4.40
3.48
3.09
2.79
1.93
1.74
1.59
Consumables Costs
26.64
23.33
20.75
13.38
11.72
10.43
7.99
7.08
6.30
4.09
3.60
3.21
Effluent Treatment
and Disposal Costs
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Residual and Waste
Shipping, Hangling
and Transport Costs
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Analytical Costs
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Facility Modification
Repair and
Replacement Costs
13.62
11.91
10.59
6.81
5.96
5.30
4.08
3.57
3.18
2.04
1.79
1.59
Site Demobilization
3.21
3.21
3.21
3.21
3.21
3.21
3.21
3.21
3.21
3.21
3.21
3.21
317.07
280.25
252.03
169.71
151.33
137.02
110.72
99.76
91.06
66.60
61.10
56.80
Total Costs
0.70
3.67
6 cubic Yards/Hour
0.80
0.90
3.67
3.67
0.70
3.67
This cost analysis does not include profits of the contractors involved
The American Association of Cost Engineers defines thre types of estimates: order of magnitude, budgetary, and definitive. This estimate would most closely fit an
order of magnitude estimate with an accuracy of +50% to - 30%