And now to Converters, where the catalytic
reaction takes place. • If the temperature rise across a converter
is observed to be lessening, this indicates that deactivation is occurring. This phenomena
may be accompanied by an increase in the temperature rise in the downstream Converter since it
now has to do ‘more work’. We also need to be aware of an increase in the RF pressure,
which may indicate that one of the Converters has gone ‘sub-dewpoint’ and is plugging
off due to liquid sulfur. There are a number of mechanisms that can
cause catalyst deactivation. They include BTEX and methanol poisoning; sulfation; carbon
fouling, hydrothermal ageing; and normal ageing. BTEX poisoning occurs when the RF is unable
to completely destroy BTEX components in the acid gas feed stream(s) to the SRU. The resulting
effect is a ‘cracking’ or ‘polymerizing’ of these components on the Claus catalyst.
Methanol poisoning is normally due to an SRU with an acid gas by-pass that allows methanol
to by-pass the RF. Both of these poisoning mechanisms are permanent.
Soot deposition and liquid sulfur deposition on the top of the catalyst, results in plugging
of the converter beds, but these can be reversible with a heat soak.
Sulfation of catalyst occurs when excessive free oxygen is carried over from either the
RF or direct-fired reheaters; Hydrothermal ageing results when the catalyst
is exposed to excessive amounts of water vapor over a long period of time. Although the actual
physical mechanism is still not completely understood, it can occur when either excessive
steam is introduced into the process and may also occur due to serious tube or tubesheet
leaks (BFW being on the shell side) from the Wasteheat boiler or Condensers.
Thermal ageing is caused by ‘thermal excursions’ or ‘sulfur fires’ in the catalyst beds.
Temperatures above 1300 F, which are all too easy to obtain during a serious sulfur fire,
can result in ‘fusing’ of the catalyst into large solid pieces.
And then more quick notes; • Exposure to large amounts of condensed
water will result in immediate destruction of the catalyst pellets.
• And If there is a sudden step-change reduction in the temperature rise across the first Converter,
this would indicate that a severe carryover of contaminants such as hydrocarbons and/or
amine has occurred • A sudden increase in temperature in the
Converters will be due to a ‘sulfur fire’. This means that there is free oxygen getting
to the Converters either from the RF and/or the Reheaters. If the problem is not corrected
quickly, it will be necessary to introduce ‘snuffing steam’ or an inert gas to stop
the fire. Do not allow the temperatures to go above 450C in the Converters or there will
be severe damage done to the internals. • Damage in the Converter internals normally
shows up as a failure in the catalyst supporting mesh screen and grating, which will result
in catalyst falling through and ending up in the Condensers, Rundowns, and Seal Legs
(often plugging is the result). In the worst case scenario the carbon steel support beams
will bow downward or collapse. So to quickly sum up the troubleshooting process
for catalyst deactivation, once the specific mechanism has been identified, we need to
eliminate the cause, whether it be air, soot, or poison going onto the catalyst, or if its
just normal aging that we need to minimize with smoother start-up and shutdowns. Once
the cause has been taken care of, we need to assess the performance of the converter,
is the temperature profile still ok, is the conversion reasonable? Then we can take the
necessary planning steps to either do a heat soak to try a reversible, if applicable. Or
to plan a catalyst changeout for the next shutdown.