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TRACERCO Diagnostics™ - Residence Study

Measurement of Bypassing in a Feed/Effluent Exchanger

A frequently encountered problem on refineries is the decision as to whether or not poor conversion in a catalytic refining unit is due to a leak in an associated feed/effluent heat exchanger (which is an inexpensive repair) or due to catalyst deterioration.

Tower scan

Tracerco were called to a major refinery to carry out a TRACERCO Diagnostics™ Residence Study on a feed/effluent exchanger. A pulse of radioisotope was injected into the gas feed to the exchanger and a radiation detector was positioned on the effluent gas exit line, as shown to record the residence time distribution of the gas.

In the absence of bypassing, the residence time distribution would have been a single peak corresponding to the plug flow of the tracer through the system. However, examination of the detector response curve revealed the presence of a subsidiary peak prior to the main peak. This peak corresponded to the fraction of the gas, which had taken a shortened route through the system. The presence of internal bypassing was thus established. From the ratio of the areas of the subsidiary and main peaks it was possible to calculate the fraction of the feed gas which was bypassing the reactor and to show that this was sufficient to account for the observed loss of conversion efficiency.

Residence flow study

As a result of the TRACERCO Diagnostics™ Residence Study, the plant was brought off-line to effect repairs to the exchanger – a relatively rapid operation compared to the lengthy shutdown, which would have been required to enter the reactor and inspect and test the catalyst. Estimated savings were in excess of $500,000.

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