Application of the Dilution Principle to the Measurement of Gas Flow Rates in Large-Scale Chemical Processes

The value of radiotracer techniques is briefly discussed in relation to the accurate measurement of gaseous flow in continuous chemical processes with special reference to determinations which would be difficult by conventional instrumental methods. The application of the dilution principle to the measurement of particularly large rates is considered and a method involving an activity concentration technique is described in detail. In the method, carrier and 133Xe are introduced into a gas cylinder which is pressurized with nitrogen. The tracer is injected at a constant accurately known rate into the gas stream to be measured. Samples taken from the stream are brought to equilibrium with known conditions of temperature and pressure and subjected to the concentration technique which involves circulation of the sample gas through a refrigerated trapping arrangement. Extraction of the active xenon from the gas sample is taken to completion by raising the partial pressure of xenon in the sample by the injection of inactive xenon at suitable intervals during the process. In this way a concentration factor of 103 is readily achieved and the small volume of 133Xe and carrier resulting may be assayed under conditions of high efficiency. Flow rates in the region of 106 m3/h can be measured with care to an accuracy of ± 1%, using a total activity of approx. 100 mCi 133Xe. Examples of the application of the dilution method - with and without the concentration technique - are discussed and the adaptation of the method to detect very small internal leakage rates between gas systems in chemical processes is described. (author)