locations throughout the re®nery plant and used to measure ¯ow velocities,
RTDs and mixing patterns for separate process streams through key elements
of the plant. For example, a radiotracer injected into the riser can be used to
provide information on the following processes:
feed and catalyst distribution in the riser feed zone;
catalyst and vapour velocities in the riser (slip factors);
determination of the ef®ciency of the riser termination device;
RTDs through the reactor and stripper;
mixing and ¯ow characteristics in the reactor and stripper; and
cyclone ef®ciencies, ¯ow distribution and residence times.
These are among the many tests which may be used to investigate a range
of generic issues including: (a) reasons for reduced plant yield; (b) the impact
of changes in operating conditions designed to enhance yield; and (c) the
accuracy of process modelling and simulation.
8.4.3 Radiotracers in the iron and steel industry
Applications of radiotracers to the iron and steel industry will be illustrated
here by an investigation of blast furnace operation. Blast furnaces are used
to reduce iron ore to iron using coke to which is added a ¯ux such as
limestone or quartzite. They comprise three sections: (a) the shaft which
takes the iron ore, coke and ¯ux; (b) the tuyeres or water-cooled nozzles
located around the furnace below the shaft through which air is blown into
the furnace at high pressure; and (c) the hearth at the base of the furnace
from which the molten iron and slag are tapped (Figure 8.9(a)). The
reduction of the iron ore occurs primarily through reactions with carbon
monoxide (CO). The ef®ciency with which the furnace works depends on
many factors including the quality of the coke, the uniform mixing of the
carbon monoxide gas with the iron ore (i.e. the absence of channelling) and
the ef®ciency of the hearth drainage.
The example quoted here describes an investigation of reduced yield due, it
was believed, to the development of a cold spot within the hearth furnace.
Two radiotracers,
198
Au and
60
Co were injected through two air blast nozzles
(tuyeres) located symmetrically with respect to the tap hole (Figure 8.9(b)).
During the tapping process, samples were taken every two minutes and
assayed simultaneously for the two isotopes using a g ray spectrometer. If the
furnace is behaving in accordance with speci®cation, the response curves
should overlap, re¯ecting similar residence time distributions in symmetri-
cally located regions of the furnace. If this is not so, the response curves
Applications of tracer technology to industry and the environment262