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TRACERCO™ - Smart Density Gauge

Determining Coke Thickness in a FCCU

In large scale continuous operating plant optimisation of operating conditions can lead to significant increases in productivity and a reduction in operating costs. This certainly applies to Fluid Catalytic Cracking Units (FCCU's).

TRACERCO™ Smart Density Gauges, used to determine coke thickness were installed at three critical points along the FCCU reactor transfer line on a refinery in South Africa and yielded significant benefits to the refinery.

Source containersThe FCCU reactor’s transfer line was being scanned using gamma transmission on a monthly basis to determine how much coke had built up within the line. It was difficult to fully optimise the unit based on these monthly scans, which had resulted in excessive coke build-up at certain locations. During this period, the unit was operated at reduced reactor gas exit temperatures in order to maximise the production of Light Cycle Oil (LCO). Under these conditions reactor gas exit temperatures are dropped from the normal order of 525oC to around 510oC to maximise the amount of diesel (C12), which is produced. When these conditions prevail, there is an increased propensity for coke to precipitate out on the transfer line between the reactor vessel and the main fractionating column.

This situation could not be tolerated after the recent revamp of the FCCU reactor. Plant technical personnel had to define new reactor temperature and feed composition limits without the threat of excessive coke build up in the transfer line.

The plant management team became aware of a similar issue at a UK refinery and established that TRACERCO™ Smart Density Gauges had been installed with great success. From time to time the plant management finds it necessary to run their FCCU reactor with a 20% vacuum residue. During such periods a coke build up is noticed within two days of starting to operate in such a mode.

The plant management team decided to install three reliable TRACERCO™ Smart Density Gauges at critical positions along the reactor transfer line, with this ‘continuous’ scanning facility providing the advantage of giving an early warning of problems with the feed nozzles. The gauge closest to the reactor would easily detect catalyst carry-over.

The instrumentation department in South Africa indicated that the plant processes highly flammable products. Safety was therefore an important issue. Tracerco supplies the only non-intrusive solid build-up gauges that are certified intrinsically safe for zones 0, 1 and 2 use..

The coke thickness gauges had to be highly reliable, as their outputs would be routed to the central DCS system. These signals would be continuously monitored in order to optimise the operating parameters of the FCCU reactor.

The TRACERCO™ Smart Density Gauge has an independently assessed Mean Time Between Failures of more than 14 years, therefore frequent, unanticipated system failures would therefore not be a problem. With their permanent scanning facility the plant personnel have easily defined reactor temperature and feed composition limits after the revamp without coke building up in the line. The process engineer that managed the project, reports that the facility has proved to be very useful since the point of temperature control moved from the inside of the reactor to the outside. Thus a new safe operating temperature range has been defined.

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