Figure 1. Process Flow Diagram of McClellan Site
S, OU D SVE System.
Major contaminants in the SVE waste stream and
their maximum inlet concentrations are listed in Table 1 below:
|
Table 1.
Maximum VOC contaminant concentrations. |
|
|
Compound |
Concentration
(ppmv) |
|
1,1,1-Trichloroethane |
188 |
|
1,1,2-Trichloroethane |
0.81 |
|
1,1,-Dichlorethane |
4.1 |
|
1,1-Dichloroethene |
4.37 |
|
1,2,4-Trimethylbenzene |
38 |
|
1,2-Dichlorobenzene |
104 |
|
1,3,5-Trimethylbenzene |
4.03 |
|
1,3-Dichlorobenzene |
4.35 |
|
1,4-Dichlorobenzene |
9.33 |
|
4-Ethyl Toluene |
6.18 |
|
Acetone |
75.9 |
|
Benzene |
3.76 |
|
c-1,2-Dichlroethene |
2.85 |
|
Chlorobenzene |
1.56 |
|
Ethylbenzene |
7.09 |
|
Freon 113 |
1.42 |
|
m,p-Xylenes |
19.2 |
|
Methyl Ethyl
Ketone |
7.27 |
|
Methyl
Isobutyl Ketone |
20.2 |
|
Methylene
Chloride |
6.18 |
|
o-Xylenes |
6.14 |
|
Tetrachloroethene |
77.2 |
|
Toluene |
63.3 |
|
Total
Xylenes |
15.8 |
|
Trichloroethene |
85.6 |
|
Contaminant concentrations were measured by
GC/MS EPA TO-14 and GC Method 8010/8020. |
|
Description
of Technology
The
SDPT generates nonequilibrium nonthermal plasmas to oxidize VOCs in the vapor
stream by dielectric-barrier electrical discharges. Nonthermal plasma is a gaseous state of matter at near ambient
temperatures and pressures in which a part or all of the atoms or molecules are
dissociated to form ions. SDPT produces this
plasma in a planar Pyrex TM glass cell sandwiched between stainless
steel electrodes, using glass as the dielectric medium. Figure 2 below shows a schematic of the
plasma reactor.
Figure
2. SDPT Plasma Reactor
The
dielectric barrier and the application of alternating high voltages, produces
substantial quantities of plasma by a large number of uniformly spread
microdischarges in the gas below the electrode area. Without the
dielectric barrier, localized intense arcs rather than microdischarges would
develop in the gas between the metal electrodes. A mean
electrical field discharge area of 1,236 cm2 and an active discharge
volume of 310 cm3 is given in the planar cell approximate 71
centimeters (cm) in length, 18 cm wide, with a 2.5 millimeters (mm) gap.
As the
gases pass through the airtight planar cells, contaminants are exposed to
high-energy plasmas. These plasmas
generate a series of free radicals such as atomic oxygen and the hydroxyl
radical and free radical reactions. These free radical reactions are responsible for the
oxidation of the halocarbons into basic products of oxidation such as HC1, CO2,
H20 and other reaction byproducts. Field conditions such as
temperature, flow rate, humidity, and type and concentration of contaminants
affect the quantity and concentration of the products of plasma technology.
During this
demonstration, two SDPT reactors were connected in parallel, each having a
capacity of 5 cfm.
Each
reactor consisted of two stacks of planar cells placed in a sealed vessel, with
each stack containing 10 cells. The
entire SDPT system, excluding an oil-cooled heat exchanger to prevent the cells
from overheating, was housed in and operated from a long-bed trailer.
The
plasma cells were energized by a variable frequency oscillator amplifier
coupled to a set of tuning inductors and a step-up transformer. The electrical and gas measuring instruments were
interfaced to a computer-based data acquisition and analysis system The fully loaded trailer (excluding the tow truck)
was estimated to weigh less than 4 tons. The SDPT system schematic and layout
are presented in figure 3 below.
Figure 3. Schematic of the
SDPT.
The
following items were included as part of the SDPT package:
•
Equipment trailer (including air conditioner and gasoline-powered
motor-generator)
•
Computer-based data acquisition and control system
•
Plasma-processor reactor, including plasma cells
•
High-voltage transformer
•
Power-factor correction inductors
•
Variable-frequency power supply
•
Instrumentation and controls
•
Oil pump
•
Heat exchanger
•
Dehumidifier, heater
•
An air preparation/drying unit
•
A gas temperature control/heating unit
•
H2 supply system
The SDPT
system at McClellan AFB was operated for incrementally longer periods of time,
since this system had been operated only intermittently and for less than 6
hours on a continuous basis prior to the McClellan demonstration. The system
was to be operated for up to 4 hours per day during the first week of the
demonstration, 8 hours during the second week, 12 hours during the fourth, and
thereafter on a 24-hour basis for an additional 4 weeks. Additionally, the
effect of dehumidifying the process gas and introducing hydrogen into the cells
would be studied during the demonstration. To compensate for downtime during
troubleshooting, the technology vendor demonstrated the system for an
additional week.
Two different types of
samples were collected and analyzed, namely a relatively inexpensive gas
chromatography (GC) method (modified EPA Method 8010/8020) was used to provide
fast VOC results ( 2-day turnarounds), and EPA Method TO-14, a gas
chromatography/mass spectrometer (GC/MS) method, was used to confirm the VOC
results analyzed by GC. The GC method provided results for 18 of the most
frequently detected VOCs in the SVE gas stream, while method TO-14 also
provided an extended list of VOCs that may be present in the slipstream.
Performance
of Technology and DRE
The DRE was calculated under three separate
operating conditions; first without pretreatment of the gas, second with
hydrogen injection only prior to treatment and finally with hydrogen injection
and dehumidification.
Field Performance Data
Listed in tables 2, 3 and 4 below are the DRE
for the contaminants of concern at the McClellan SDPT demonstration.
|
Table 2.
Average VOC DRE by SDPT (without pretreatment) analyzed by GC/MS EPA
TO-14 and GC Method 8010/8020. |
||||||
|
|
GCMS |
GC
Method 8010/8020 |
||||
|
|
Concentration
(ppmv) |
Average |
Concentration
(ppmv) |
Average |
||
|
Compound |
Inlet |
Outlet |
DRE
(%) |
Inlet |
Outlet |
DRE
(%) |
|
1,1,1-Trichloroethane |
155 |
9.99 |
93.55 |
188 |
28.7 |
84.72 |
|
1,1,2-Trichloroethane |
0.81 |
0.43 |
>47.41 |
NR |
NR |
NR |
|
1,1,-Dichlorethane |
3.33 |
0.11 |
>96.67 |
4.10 |
0.56 |
>86.32 |
|
1,1-Dichloroethene |
4.37 |
0.11 |
>97.44 |
4.09 |
0.52 |
>87.37 |
|
1,2,4-Trimethylbenzene |
10.5 |
0.18 |
>98.28 |
8.91 |
0.47 |
>94.72 |
|
1,2-Dichlorobenzene |
31.4 |
0.92 |
>97.07 |
8.47 |
0.50 |
>94.16 |
|
1,3,5-Trimethylbenzene |
3.98 |
0.10 |
>97.47 |
NR |
NR |
NR |
|
1,3-Dichlorobenzene |
2.30 |
0.11 |
>95.18 |
NR |
NR |
NR |
|
1,4-Dichlorobenzene |
6.04 |
0.21 |
>96.56 |
NR |
NR |
NR |
|
4-Ethyl Toluene |
6.18 |
0.13 |
>97.98 |
NR |
NR |
NR |
|
Acetone |
75.9 |
2.06 |
97.29 |
NR |
NR |
NR |
|
c-1,2-Dichlroethene |
2.42 |
0.11 |
>95.30 |
2.85 |
0.52 |
>81.83 |
|
Ethylbenzene |
2.77 |
0.10 |
>96.36 |
3.80 |
0.57 |
>84.96 |
|
Freon 113 |
NR |
NR |
NR |
1.11 |
0.56 |
>49.70 |
|
Methyl Ethyl Ketone |
7.27 |
2.13 |
>70.70 |
NR |
NR |
NR |
|
Methyl Isobutyl Ketone |
20.2 |
.78 |
>96.12 |
NR |
NR |
NR |
|
Methylene Chloride |
5.95 |
.33 |
94.51 |
6.18 |
0.77 |
87.48 |
|
Tetrachloroethene |
71.0 |
1.04 |
> 96.53 |
64.8 |
1.37 |
>97.88 |
|
Toluene |
52.2 |
0.77 |
> 98.53 |
47.7 |
0.85 |
>98.22 |
|
Trichloroethene |
81.1 |
1.21 |
> 98.51 |
85.6 |
1.55 |
>98.18 |
|
m.p-Xylenes |
NR |
NR |
NR |
9.35 |
0.55 |
>94.18 |
|
o-Xylenes |
NR |
NR |
NR |
3.19 |
0.61 |
>80.94 |
|
Total Xylenes |
15.8 |
0.17 |
> 98.90 |
NR |
NR |
NR |
|
Average VOC DRE |
|
|
> 89.05 |
|
|
>91.39 |
|
GC/MS EPA TO-14 method: 4 samples taken from
11/13/95 to 12/15/95. |
||||||
|
GC Method 8010/8020: 9 samples taken from
11/9/95 and 12/14/96. |
||||||
|
The greater than symbol (>) indicates that
the compound was detected in the inlet, but was not detected in the outlet
sample on at least on of the collection dates. |
||||||
|
NR-not reported |
||||||
|
Table 3.
Average VOC DRE by SDPT with hydrogen addition analyzed by GC/MS EPA
TO-14 and GC Method 8010/8020. |
||||||
|
|
GC/MS
EPA TO-14 |
GC
Method 8010/8020 |
||||
|
|
Concentration
(ppmv) |
Average |
Concentration
(ppmv) |
Average |
||
|
Compound |
Inlet |
Outlet |
DRE
(%) |
Inlet |
Outlet |
DRE
(%) |
|
1,1,1-Trichloroethane |
147 |
18 |
87.76 |
153 |
20.3 |
86.75 |
|
1,1,2-Trichloroethane |
NR |
NR |
NR |
NR |
NR |
NR |
|
1,1,-Dichlorethane |
3.70 |
0.22 |
94.05 |
3.95 |
0.45 |
>88.64 |
|
1,1-Dichloroethene |
3.77 |
0.22 |
94.06 |
2.95 |
0.45 |
>84.80 |
|
1,2,4-Trimethylbenzene |
10.2 |
0.34 |
96.71 |
36.5 |
1.27 |
96.54 |
|
1,2-Dichlorobenzene |
35.6 |
1.85 |
94.80 |
90.6 |
3.81 |
95.79 |
|
1,3,5-Trimethylbenzene |
4.03 |
0.18 |
>95.53 |
NR |
NR |
NR |
|
1,3-Dichlorobenzene |
2.36 |
0.18 |
>92.37 |
NR |
NR |
NR |
|
1,4-Dichlorobenzene |
6.61 |
0.41 |
93.83 |
NR |
NR |
NR |
|
4-Ethyl Toluene |
5.81 |
0.25 |
95.78 |
NR |
NR |
NR |
|
Acetone |
60.9 |
0.90 |
>98.53 |
NR |
NR |
NR |
|
c-1,2-Dichlroethene |
2.51 |
0.18 |
>92.83 |
NR |
NR |
NR |
|
Ethylbenzene |
2.47 |
0.18 |
>92.71 |
5.71 |
0.32 |
94.41 |
|
Freon 113 |
NR |
NR |
NR |
1.33 |
0.20 |
85.05 |
|
Methyl Ethyl Ketone |
NR |
NR |
NR |
NR |
NR |
NR |
|
Methyl Isobutyl Ketone |
19.5 |
0.91 |
95.35 |
NR |
NR |
NR |
|
Methylene Chloride |
5.49 |
0.49 |
91.02 |
5.76 |
0.57 |
90.08 |
|
Tetrachloroethene |
55.0 |
3.13 |
94.31 |
65.4 |
3.84 |
94.12 |
|
Toluene |
46.1 |
2.35 |
94.90 |
59.3 |
2.82 |
95.25 |