Hydrocarbon (VOC) analyser for heat exchanger leak detection

Continuous on line analysis measures hydrocarbons (VOC) in cooling tower water, wastewater or steam.  Weld ruptures or corrosion leaks in heat exchangers can result in massive quantities of VOC being dissolved in cooling tower water.  Approximately 99% of VOC in cooling tower water is striped out in the cooling tower.  Continuous on line monitoring can pay for itself in savings of lost product and labor.  An average refinery of 330 000 bpd, without VOC monitoring, was found to leak 1600 lb/day, approximately 700 ppb by weight (ppbw).  With monitoring and control, leaks are to be limited to 84 ppb or less for all VOCs.² Obviously equipment corrosion or rupture could result in leaks totaling many thousands of lbs/day of VOC vapour from a cooling tower which will be invisible in the air above the tower.  In addition cooling towers have reached the LEL and exploded in some cases.

System description

This study describes equipment installed in an air-conditioned instrument enclosure.  This allows low ppbw measurement.  A sample preparation system was selected to prevent contamination of the analyzer system.  The sample passes by one side of the sample transfer stripper containing a proprietary permeable membrane. VOCs dissolve in the membrane then permeate through into a clean dry carrier gas flowing past the other side of the membrane.  To avoid deterioration of grab samples, used in laboratory analysis, the system includes, an automatic permeation tube validation systems to verify correct analyzer response. RS232/485 MODBUS, local digital readout, as well as 4 - 20 mA signal for control room monitoring, were incorporated.  The complete system is fully automatic with validation, alarm and monitoring by only the control room operator.  Redundant readout solid state sensor and GC was used.  This gives a total VOC as benzene by sensor and benzene only readout by GC.  This helps to identify the leaking heat exchanger. Other detection technologies can be used for the readout including UV spectroscopy.

Validation of data

The analyzer system, called the 204 Hydrocarbon in Water Analyzer, was pre fabricated and calibrated for drop in place installation, was delivered to an ethylene plant in Corpus Christy Texas.  Both the solid state sensor readout and the GC readout in the analyser system were operational when power was applied.  After installation, a test of the two calibration systems, built into the analyser system, were activated.  The solid state sensor and the GC readout had been factory calibrated using a sample prepared by mixing pure benzene in distilled water as reference sample.  The chart recording of the base line is within one ppbw.  The permeation tube furnished as 20 ppbw reference read within one ppbw.  The design was to control benzene level not to exceed 5 ppbw, the allowable benzene limit.

Online data

The figure illustrates a three day on line measurement of benzene level in cooling tower water as it enters the cooling tower.  The GC readout is specific for benzene VOC only.  The solid state tin oxide sensor, which was also installed, responds to all VOCs.  The recording is of benzene concentration by only GC.  Background noise is seen to be approximately +/- ½ ppbw.

Ethylene plant cooling tower water

A chart recording of process water benzene concentration, during a three day period, is shown in the figure.  After twelve hours running time a leak began to develop in a heat exchanger.  At a level of 2 ppbw corrective action had been taken and benzene concentration began to decrease.  As is apparent from the recording less than 30 minutes elapsed from the time the leak was detected and the maintenance action was completed.  Within one and one half-hours all benzene had been stripped by the cooling tower from the cooling tower water.  The steady state size of this leak cannot be determined from the record.

A second leak, probably in another heat exchanger was found about 19 hours later.  This leak was developing more slowly.  Corrective action was again taken at the one ppbw level with recovery to the baseline in about the same length of time.  As shown by the subsequent lowering of the base line, this heat exchanger apparently had a one half ppbw long term leak that was also eliminated.

Conclusion

This analyser system, utilising ASI Membrane Technology from Analytical Systems Keco, proved to more than meet the design requirements, which was to control benzene in cooling tower water at the 5 ppbw level.  Validation of data was accomplished by incorporated of calibration method, automatic or manually activated. A solid state readout provided a measure of total VOC as benzene and a gas chromatograph measured benzene directly.  Fourteen years of continuous on line operation in an operating ethylene plant is proof that it is practical to measure VOC in cooling tower water or waste water treatment systems at low ppbw levels.

References

1. 1. Compilation of Air Pollutant Emissions Factors, Volume 1, Chapter 5, Tables 5.1-2 and 5.1-3, US Environmental  Protection Agency, Office of Air Quality, 1995.
2. 2. TCEQ formally Texas Natural Resources Conservation Commission (TNRCC).  Equivalency of method determination for measurement of Volatile Organic Compounds (VOC) by membrane transfer.
3. 3. Principle of Operation 204 Hydrocarbon in Water Analyzer.
4. 4. “VOC in Water Analyzer  ASI Model 204 Advantages”.
5. 5.  Texas Natural Resources Conservation Commission (TNRCC) [now TCEQ] determined that this installation conforms to “Best Available Control Technology” (BACT).



Article provided by Analytical Systems International, and adapted for the web by Emma McAleavey.

Read the article online at: https://www.hydrocarbonengineering.com/refining/05022015/hydrocarbon-analyser-017/