Part 3: Addressing the Impacts of Changing Regulations and Feedstocks in Sulfur Recovery Units

Part Three of a five part series.

Tail gas/air demand analyzer (AT3)

A tail gas or air demand analyzer is located after the final condenser and is present in nearly every SRU. Measurements here are often referred to as ‘feedback control’, as they provide feedback on the (nearly) final product. This feedback control is responsible for approximately 10% of the air/oxygen that is entering the reaction furnace – the remaining 90% is typically designed into the system. Measurement at this point is the most reliable way to ensure the proper ratio of H₂S to SO₂ has been maintained after the reaction furnace and provides final component concentrations before the TGTU or thermal oxidizer. With such a high installation and utilization rate, and so many articles readily available on the purpose of the tail gas/air demand analyzer, it is only necessary to briefly touch on some key items to consider:

1. As previously mentioned, the ideal ratio of H₂S:SO₂ in the SRU is typically 2:1. Most plants operate with up to 2 – 4% H₂S and 1 – 2% SO₂ expected at the condenser outlet, but some require lower or higher ranges based on engineered design or sulfur recovery requirements. It should be kept in mind that it may be necessary to use ranges that cover upset conditions, so that operators can get back to proper operations after something unexpected occurs prior to or at the reaction furnace.
2. Ensure proper thermal management of the sample gas throughout the analyzer system – probe to analyzer to return location. SX is unique in that its phase can change very easily when sample handling temperatures are not properly maintained. If the sample temperature is not maintained high enough, the SX can condense as a solid, resulting in blocked sample gas analyzer inlet and outlet lines. If sample temperatures are too high, the SX can go from the desired gas phase to a gelatinous one. This too can result in blocked inlet and outlet sample lines, or contamination of the gas analyzer optical cell. Money spent on a well-proven design will pay for itself in analyzer uptime and reduced maintenance.
3. Focus on measurement requirements and not just the technology used in the analyzer. It is important to select an analyzer system that provides the measured components and concentrations that are required and that a team can operate and maintain. It is recommended to search for references within the sulfur community.

Part One - Introduction
Part Two - Feed Gas Analysis 
Part Four - Tail Gas Treatment 
Part Five - Continuous Emission Monitoring Systems

Learn more about AMETEK's solutions for tail gas/air demand.