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.
Other converter issues:
- Exposure to large amounts of condensed water will result in immediate destruction of the catalyst pellets.
- 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 1400F 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.