Electron Beam—Zapit Processing Unit
Current
Status of Technology
Advanced Oxidation Technologies, Inc. owns patent of former Zapit Technology, Inc. Status being determined.
Description
of Technology
Electron
beam technology has been used for industrial applications for over 30
years. Zapit Technology, Inc. of Santa
Clara, California, developed a gas phase VOC destruction system that is a
non-burning, on-site, electron beam technology. In Zapit’s non-thermal oxidation process, a beam of electrons is
passed through a thin window into a reaction chamber where the electrons react
with the vapor stream to create free radicals that break down the complex
organic molecules. Adjusting beam
power, and the addition of oxidizing agents modifies the DRE of the e-beam. The system produces carbon dioxide, water
and acid gases from halogenated hydrocarbons.
These acids can be neutralized in a caustic scrubber.
The
system consists of a prototype Zapit Processing Unit (ZPU), containing the
following components:
·
Sealed-tube
electron beam generator and reaction chamber
·
Sheet
electron beam generator
·
High-voltage
power supply and control panel
·
Multi-tray
air stripper or desorber
·
Packed-bed
scrubber or adsorber
·
Activated
charcoal adsorbers for both liquid and gas phases
·
On-line
analyzers
·
Blower
·
Two
500-gallon polyethylene storage tanks
A
simplified schematic of the system is shown in figure 1 below.
The
reaction chamber is cylindrical and approximately 20 inches long and 8 inches
in diameter, and constructed of stainless steel. A 13 inch by 2.5 inch window system along the top of the reactor
allows electrons to pass from the electron source vacuum chamber into the vapor
stream. Within the chamber the VOC
stream is exposed to 0-20 mA of 170 keV electrons. Approximately 28% of the power provided by the e-beam source is
transmitted into the reaction chamber.
Zapit expects approximately 50% transmission efficiencies in commercial
systems. Gas temperature, pressure, and
composition are measured at the outlet of the ZPU reaction chamber.
The
effluent from the reaction chamber are passed through a caustic scrubber
containing continuously recirculating sodium hydroxide. The scrubber was designed to handle flows up
to 500 cfm, however this demonstration had a nominal flow of 3 cfm. The scrubber effectively removed nitric
acid, which can interfere with detection of NOx by chemiluminescent
NOx analyzers. The effluent
from the scrubber was passed through a vapor-phase carbon canister before being
vented to the atmosphere.
Site
and Contaminants Description
Bench
scale tests of the electron beam technology were conducted to determine if this
technology would be suitable for application at McClellan Air Force base. Under subcontract with Radian Corporation,
the tests were conducted at Zapit Technology at the University of Tennessee’s
Space Institute.
Previously
Zapit Technologies demonstrated a bench scale system capable of achieving 98%
DRE for an entire off gas stream from McClellan’s SVE. Samples of the off gas were placed in Tedlar
bags and transported to Zapit’s test cell.
As a result of this demonstration, McClellan recommended a performance
test of a working, flow-through prototype before installing an e-beam pilot
unit at McClellan. The flow-through
prototype demonstration was designed in two phases. Phase I consisted of a bench scale steady-state test on two soil
gas mixtures, while phase II would involve an on-site, pilot demonstration
following phase I. Phase I failed to
show viability of the technology pilot demonstration at McClellan. The results of the phase I bench scale test
are presented here.
The
simulated soil gas compositions are listed in table 1 below.
|
Table
1. Simulated Soil Gas Inlet Compositions for Zapit E-Beam Technology |
||
|
Compound |
Mix
#1 Concentration (ppmv) |
Mix
#2 Concentration (ppmv) |
|
1, l, 1-Trichloroethane
(TCA) |
8 |
1,657 |
|
1,1,2-Trichloroethane
(TCA) |
30 |
0 |
|
l,l-Dichloroethene
(DCE) |
15 |
1,018 |
|
1,1
-Dichloroethane (DCA) |
0 |
66 |
|
1,2,4-Tnmethylbenzene |
0 |
10 |
|
Acetone |
15 |
0 |
|
Benzene |
10 |
0 |
|
Chlorobenzene |
0 |
4 |
|
Freon®
12 |
0 |
8 |
|
Freon®
113 |
150 |
88 |
|
Methylene
chloride |
0 |
50 |
|
Tetrachloroethene
(PCE) |
300 |
0 |
|
Toluene |
70 |
45 |
|
Trichloroethene
(TCE) |
300 |
300 |
|
cis-
1,2-Dichloroethene (DCE) |
200 |
62 |
|
m,p-Xylene |
40 |
2 |
|
o-Xylene |
95 |
5 |
|
Vinyl
chloride |
120 |
30 |
Performance
of Technology and DRE
For
each of the two gas mixtures, the optimum combination of beam power and
promoter dose was calculated to maximize DRE of the VOCs while minimizing NOx
formation. Optimization tests were run
at three different beam powers at zero promoter dose, and at three different
promoter doses with beam power held constant.
Since the scrubber was oversized the concentration of organic compounds
in the scrubber liquid never achieved steady state, and no make-up caustic
solution was required to maintain a high pH.
The scrubber appeared to be absorbing organic compounds when the
solution was fresh, and desorbing compounds from the liquid to the gas phase
after periods where the scrubber received high levels of organic compounds. Therefore the DREs are calculated based on
the effluent from the ZPU and not the scrubber. Important values for estimating E-beam process emissions are the
NOx, HCl, HF and HNO3 concentrations exiting the
scrubber.
Field Performance Data
Tables
2 and 3 below present the DREs for the e-beam technology bench scale
demonstration of the two different soil gas mixtures.