Membrane Transfer, Sample Permeation, Permeation Transfer, Volatile Organic Carbon (VOC), Analyzer, VOC Remediation, Emissions Reporting, Calibration

The use of a permeable membrane to monitor volatile hydrocarbons including chlorinated hydrocarbons, alcohol and other chemicals from low PPB to percent in a liquid or gas phase is practical. This alternative to the sparger or purge and trap methods reduces maintenance for GC's or other analyzers.

Permeation has been around for many years however, with the in depth research and development of the transfer unit, Figure 1, there are many ways to use the unit that have not been recognized in the past.

The main instrumentation use of the transfer unit is to allow a hydrocarbon tainted liquid or gas to pass on one side of a membrane and the hydrocarbon will permeate through the membrane where a clean flowing gas will evaporate the hydrocarbons from the membrane surface resulting in a clean dry sample suitable for an analyzer. Detectability is 1 PPB range to percent (1).

This eliminates use of heated injection valves to vaporize a sample and also eliminate need for heat traced lines. The clean, dry carrier gas prevents condensation in the flow lines.

Transfer units can operate for long periods of time with a minimum of maintenance, as shown by results in petrochemical plant testing.

Membrane tests are listed below, Table I. Various GC analyzers were employed and flowrates were not adjusted for optimum results. Detection limit was determined by multiplying the PPM weight by the average noise divided by response at that PPM.

SPARGER
RESPONSE

Figure 2, is a chart recording showing calibration of a Keco R & D, Inc., Model 204, benzene in cooling tower water analyzer. The actual cooling tower water was scrubbed with a charcoal filter which provided a zero benzene reading. A permeation tube designed to give 20 PPB by weight, read as 19.5 PPBw was used. A second calibrator mechanically injects benzene into clean water to the analyzer to provide 10 PPBw as shown on the right. The 7.7 PPBw reading correlates very well to the permeation tube calibration. This data was obtained after shipping and installation, additional adjustments after the original factory calibration were unnecessary.

The micro injection method of calibration is a new development which allows over 100 to 1 ranging for linearity checking and requires only a contact closure for automatic calibration. The sample is prepared at the time of use eliminating degradation with time as can result from bacterial action or absorption.

Figure 3, is a recording of cooling tower water with the ethylene plant in normal operation. Both recordings show the GC reading of benzene every four minutes shown as points on the long term graphs. Slight fluctuations of benzene levels of 2.25 PPBw and 1.1 PPBw are shown over a 3 day run. These fluctuations were caused by heat exchanger leaks. Disconnecting the leaking heat exchanger corrected the problem as shown by the recording.