Portable Cooling Water Sampler

One difficult measurement to make in the domain of emissions control is that of fugitive volatile material, usually hydro carbonaceous, escaping from cooling tower water. Such measurements are necessary not only to allow plants to conform to laws limiting such emissions but also because sudden appearances of some materials in the cooling tower water are sometimes indicative of process leaks. The root of the analytical problem is that it is difficult to sample water in the field then to analyze it in the laboratory without some loss of the volatile material. Compounding the problem is the fact that the measurement of interest is the amount of material that volatilizes while the water is in the cooling tower. This measurement may not necessarily coincide with the total carbon in the water since some of it may be nonvolatile. Described herein is a sampling device, built specifically for use in the field, that can measure the volatile organic carbon content of cooling tower water.

Description:

The main vessel is a thick walled glass pipe thirty-six inches in length and three inches in diameter. It is packed by the end user to a depth of twenty six inches with 1/4" beryl saddles. All the lines to and from the main vessel are 1/4" stainless steel tubing, except 1/4" PFA tubing joining dryer and Erlenmeyer flask. The ends of the main vessel are sealed with threaded Teflon stoppers of the appropriate size. Gas exiting the main tube is passed through a glass tube, six inches long and one inch in diameter, packed with indicating Drierite furnished by the end user. This attachment dehumidifies the air stream with little or no absorption of hydrocarbon. The dried air the flows into a 250ml Erlenmeyer flask into which the probe of a detector (TLV Sniffer, Bacharach Instrument Company) has been sealed. Excess air not sucked into the Sniffer probe is vented through a bubbler which serves to indicate that such sufficient flow has been established. Water and air are metered into the system through rotometers.

Operation:

Before each series of runs, the Sniffer is calibrated by having a commercially available air-hydrocarbon mixture of know composition pass first through the drying tube then into the flask housing the probe where it is analyzed. Best results are obtained when the Sniffer has been allowed to warm up at least thirty minutes prior to calibration. Periodically the rotometers used to measure the air and water flow should be recalibrated. The air rotometer is calibrated on the volume of air delivered at ambient temperature and pressure, and the water rotometer is calibrated on water volume at ambient temperature. The ultimate accuracy of measurements made by this apparatus depends on the accuracy of these calibrations.

The initially empty apparatus is filled by having cooling tower water fed to it at a measured rate until the packing is just submerged. At that time the drain valve is opened just enough to maintain that water level. Then air from either a cylinder or a portable pump is bubbled through the packing from the bottom at a measured rate. In practice, we found that an air : water volume ration of 20:1 gives the best results. Since the Sniffer requires 2500 ml of air per minute, we routinely set the air flow slightly above 2500 ml per minute and the water flow at 125 ml per minute.

The Sniffer is read after the air and water streams have flowed at least ten minutes. The concentration of hydrocarbon in the water is calculated according to Equation 1:C=MPbc/RTa

C = weight ppm of hydrocarbon in the cooling water

M = molecular weight of the hydrocarbon, g/mol

P = pressure of air in the main tube in the atmospheres

a = water flow, ml/min

b = air flow, ml/min

c = volume ppm of hydrocarbon in the air (Sniffer reading)

R = 82.054 ml-atm/mol-K

T = temperature in kelvins

If M is not known precisely, or if the volatile material is a mixture, a convenient number can be substituted to yield reasonable results.