Sunday, September 11, 2011
Sampling VOC's from Processes or Stacks with an IR Sensor
One of the most difficult problems in sampling from stacks is the removal of water vapor without losing the components to be measured. Methods for water removal include permeation driers, refrigeration, etc. These methods work well for removing water from stack gases with inorganic gases like NO, NO2 and SO2. For oxygenated VOC's, however, many of the oxygenated hydrocarbons like alcohols, ketones, esters etc. are either miscible or moderately soluable in water. Gases which are soluable in water are carried through the nafion membrane and exhibit significant losses when passed through a Permapure drier. Water soluable gases can also be lost in refrigeration processes as well because of their solubility in water.
Therefore, a different method was employed for alcohols & ketones, namely heating the components above the dewpoint of the stack gas and making the measurement at an elevated temperature. Our IR sensors are temperature compensated up to 50C so we used a heated & temperature controlled (+/- 2C) oven that housed the sensor and a coalescing microfiber filter. We calibrated the sensor at ambient and recalibrated at an operating temperature of 47C. The difference in calibration was < 5%.
A Photo of the Model 202 is shown below: