Emissions from the still columns on glycol reboilers are comprised of water vapor and various sulfur and hydrocarbon compounds. These compounds include substances such as benzene, toluene, ethylbenzene, xylene isomers, formaldehyde and hydrogen sulfide that are subject to state and federal toxic air emissions regulations.

Under the implementation of California's Air Toxics "Hot Spots" Information and Assessment Act of 1987, the local air district required quantification of glycol reboiler still column emissions based on in situ vent testing. Glycol reboilers are particularly difficult to source test due to velocity fluctuations, temperature cycles and low flow, which are inherent in the glycol dehydration process. Reboilers cycle between high and low fire in order to maintain the temperature of the glycol within a specified range. The alternating firing cycle results in variable exhaust temperatures and volumetric flow rates.

Pollutant mass emissions are calculated as a product of the gas concentation and the volumetric flow rate. The variable temperatures and volumetric flow rates common to the reboiler process may adversely affect volumetric flow measurements and, consequently, the resultant emission measurements.

Volumetric flow rates obtained with various instruments were compared to determine the most suitable measurement technique for low-flow reboilers. These instruments included a hot wire anemometer, a vane anemometer, a pitot tube and an orifice meter. The use of a continuously-monitored orifice flowmeter yielded accurate, integratable volumetric flow data and defined a repeatable and quantifiable temperature and flow profile. This procedure also enabled the stack tester to obtain a representative, integrated sample over one or more complete cycles and established a maximum sampling rate.

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