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 levels.

Sentry 2000 Design Evaluation

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.

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