Gas 2024 Issue

Optimise sulphur recovery plant emissions during unit upset conditions

Knowledge, understanding, and awareness training are essential to maintain process instrumentation for optimal information gathering

Jochen Geiger, Michael Gaura and Anantha Kukkuvada AMETEK Process Instruments

W orking conditions for refineries are changing, and such changes might be challenging. In several geographical areas, the historical supplier of crude oil has changed, resulting in new crude oil compositions. The method of supply may also have changed from pipe - lines, with stable composition, towards ocean vessel sup - ply, with different compositions. Regardless, final product quality from refineries needs to be stable, and quantities need to be ever increasing. At the same time, environmental impacts are being more strictly regulated, increasing monitoring and reporting require - ments. Emissions always need to be reduced. In summary, more flexibility from refiners is required. All of this puts more load and attention on sulphur recov - ery units (SRUs), which are expected to operate continu - ously and in an optimised manner. As discussed in several papers, such SRUs require some special attention with regard to safety and operational monitoring. On its own, elemental sulphur is not an issue. It is one of the most common elements (by mass) on earth and is essential to life. Sulphur is present in living organisms, including humans, and can be used in fertiliser production. It

even has medicinal applications. However, sulphur can also be harmful to both humans and the environment. Hydrogen sulphide (H₂S) concentrations of just a few parts per million (ppm) in ambient air can lead to illness and death at sus - tained levels greater than 100 ppm. Sulphur dioxide (SO₂) is also toxic to humans at very low concentrations but has received significant attention based on its negative impacts on the environment. Specifically, it can destroy vegetation and wildlife and contribute to the production of acid rain. The basic chemistry of sulphur recovery has been known for more than 90 years. In today’s world, the capacity of modern SRUs can range from tens to thousands of tons of sulphur production per day. The most significant improve - ments have been made to the overall recovery efficiency. Although it was sufficient to operate at 80-85% in the 1970s and between 95% and 99% in the 1990s, the cur - rent requirement is to operate at recovery efficiency rates of 99.9+%. Considering that we rely on the same chemistry as 90 years ago, except for tail gas clean-up units, such efficiency improvements only became possible using reliable process instrumentation (see Figure 1 ).

Stack gas emissions monitoring SO No (mass ow) x

Claus sulphur recovery unit

AT7

Tail gas treating unit

Absorber outlet H HS (COS)

Quench column o-gas H HS

Acid gas feed forward Total hydrocarbons HS NH CO

AT5

AT6

Claus tail gas HS SO (COS CS)

Temperature

AT1

Reduction gas generator

AT3

Cobalt molybdenum reduction reactor

Air

Converters & condensers

Thermal oxidiser

Reactor furnace & Waste heat exchanger

AT4

Quench column

Absorber & stripper columns

Reduction reactor o-gas H SO

Sulphur pit sweep gas HS SO

AT2

Sulphur pit

Figure 1 Claus SRU

Gas 2024