Chlorine Stack Emissions for the Sentry 2000
The Sentry 2000 is a patented horizontal cross flow packed bed scrubber system with
major applications in chlorine and sulfur dioxide emissions control. These systems
have been installed for the emergency scrubbing of chlorine and sulfur dioxide container
rooms should leakage of the containers or piping systems occur.
The major design criterion applied to scrubber systems is the stack discharge concentration
during emergency operation. This concentration is specified by the Uniform Fire
Code (UFC) not to exceed one half of the Immediately Dangerous to Life and Health
(IDLH) level for a toxic gas. From the late 1980’s to 1995 the IDLH level, expressed
in parts per million (PPM), had been set at 30 PPM for chlorine. During 1995, however,
the chlorine IDLH level was changed to 10 PPM.
In 1993, Powell Fabrication & Mfg. Inc. in conjunction with Mr. Ralph Strigle,
a world-recognized expert in packed tower scrubber systems, designed the Sentry
2000 to neutralize chlorine and sulfur dioxide. Based on mass transfer efficiency
calculations, the Sentry 2000 system utilized packed bed technology developed over
more than 50 years. Mass transfer efficiency calculations for packed beds have proven
to be accurate when compared to actual systems. Additionally, results for packed
beds, unlike spray chambers or venturi scrubbers, can be scaled upward or downward
reliably. Enclosed is a technical paper by Mr. Strigle presenting the calculations
used to predict and corroborate the performance of the Sentry 2000. System performance
is verified using two independent methods consisting of mass transfer driving forces
with inlet and outlet concentrations.
The Sentry 2000 was designed to operate at an inlet flow rate of 3,000 ACFM for
contaminant release rates in excess of 78 lbs/min. This leakage rate is specified
by the UFC and represents an overfilled one ton chlorine or sulfur dioxide cylinder
emptying in 30 minutes. The design was originally based on the 1993 IDLH of 30 PPM
for chlorine and 100 PPM for sulfur dioxide but is easily adaptable to reduced IDLH
Full scale testing was carried out to verify the design and demonstrate the performance
of this scrubber system for emergency operation. The Sentry 2000 was successfully
tested in 1993, achieving chlorine discharge levels of less than 5 PPM for a 3,000
ACFM inlet gas flow. Tests were conducted with full one ton releases of chlorine
at leakage rates in excess of 78 lbs/min throughout the entire release periods.
For these tests, a caustic flow rate of 400 gallons per minute (GPM) was selected
for chlorine neutralization based on Mr. Strigle’s calculations using the 1993 UFC
15 PPM outlet stack limit. Due to the importance of accurate test procedures, Entropy,
Inc., the largest gas testing company in the industry, was contracted to conduct
the performance evaluation of the Sentry 2000. Dr. John Richards of Entropy, a nationally
recognized chemical engineer specializing in air pollution control equipment, was
on-site and in charge of all data collection using EPA referenced test methods (EPA
Method 26). These referenced test methods are the only means of chlorine emissions
testing recognized by the EPA.
In addition to the EPA reference test methods, Drager test tubes and a continuous
electronic monitor were utilized during testing of the Sentry 2000. The EPA test
methods provided average stack discharge concentrations which were used for performance
review. Drager test tubes provided an immediate check of the system performance.
The continuous monitor provided an indication of the outlet concentration trends
but was not used to provide quantitative chlorine levels. As detailed in the letter
located in the appendix by Dr. John Richards of Air Control Techniques, P.C., currently
there is no EPA-approved performance specification test for chlorine electrochemical
sensors nor is there any work in progress to develop one. In addition, the letter
located in the appendix by Mr. Phillip Juneau of Emission Monitoring, Inc., states
that continuous chlorine sensors can suffer from calibration drift, non-linearity
and temperature effects that produce erroneous results.
According to Mr. Juneau, EPA validation tests have not been conducted successfully
for continuous electrochemical chlorine sensors and thus, the EPA does not recognize
them as acceptable chlorine measurement systems. The continuous chlorine monitor,
while not suitable for absolute chlorine concentration measurements, does provide
a relative indication of changes in concentration. This indication of concentration
change allows for the stimulation of peak chlorine levels relative to the average
chlorine concentrations provided by the EPA Method 26 results. Addition of the difference
between peak and average continuous monitor readings to the EPA Method 26 average
provides an estimate of peak concentrations during the chlorine release period.
This article is available as a PDF.
Click here to download it. For more information about chlorine scrubbers,
visit the Technical