The Ins and Outs of Tail Gas Treating
Tail gas treatment is often a required step within a Claus sulfur recovery facility. Tail gas treatment significantly reduces sulfur emissions, which mainly occur as SO2. Sulfur recovery turns an acid gas waste byproduct into sulfur, which can be sold to reap financial benefits. Tail gas treatment at the end of sulfur recovery makes such recovery much more environmentally friendly. Financial benefits and being environmentally friendly = a win-win. JEPCO is here to shed some light on the most commonly used tail gas treatment methods from our Sulfur 101 white paper.
What is Tail Gas?
Tail gas is waste gas produced as a byproduct of a manufacturing process. In this case, we’re specifically referring to the Claus process, the method used to produce the greatest quantity of elemental sulfur.
What is Tail Gas Treatment Processing?
In order to minimize sulfur compound emissions, tail gas exiting a Claus sulfur plant typically undergoes further processing. This processing can be done either before or after the tail gas is incinerated. Although dozens of tail gas treatment processes have been suggested over the last 50 years, the two primary methods of tail gas treatment are the incinerate and scrub method and the react, absorb and recycle method.
The Incinerate & Scrub Method
With the incinerate and scrub method, the sulfur plant’s tail gas is incinerated to oxidize sulfur compounds to SO2. Then it’s contacted with a caustic solution process to remove the SO2 prior to the venting process.
Some may scrub incinerated tail gas without any heat recovery prior to the scrubbing process. Others incorporate a waste heat boiler ahead of caustic scrubbing in order to produce high-pressure steam and reduce cooling and circulating flow requirements for the scrubbing solution. Incineration at higher temperatures for the destruction of CO generally includes a waste heat boiler ahead of scrubbing.
Up to 3% of the SO2 formed when incinerating sulfur compounds can oxidize further to SO3. This SO3, along with the water vapor present in any SRU tail gas, can result in an acidic dew point as high as 350°F. Diluted sulfuric acid could begin condensing on surfaces cooler than that dew point. Steam at 120 PSIG condenses at 350°F. For a 50°F margin above the acidic dew point, it’s commonly recommended that the waste heat boiler produces steam no lower than 400°F (235 PSIG) to protect the boiler tubes and outlet piping from sulfuric acid corrosion.
The React, Absorb & Recycle Method
The react, absorb and recycle method is typically applied to tail gas exiting a sulfur recovery unit (SRU). To convert various sulfur compounds to H2S, the SRU tail gas is heated, a reducing gas is added and the tail gas is finally routed through a catalyst bed.
SRU tail gas is commonly heated through direct firing, indirectly heating with hot oil or indirectly heating with electric resistance. The tail gas unit catalyst is usually in a separate, refractory-lined reactor vessel. Common feed temperatures for this bed may range from 485°F to 525°F, and the catalyst is similar to a naphtha hydrotreating catalyst.
Direct Fired Feed Heating
Indirect Reactor Feed Heating
Hot gas exiting the reactor goes through a waste heat boiler. Gas exiting the waste heat boiler is too hot for contact with an amine absorber, so further cooling is needed. This gas is then typically scrubbed with water to absorb any last traces of SO2, which will degrade the downstream amine. Cooled gas from a quench tower is routed to the bottom of a trayed or packed absorber tower. A selective amine solution enters this absorber and absorbs H2S while allowing CO2 to escape to the absorber overhead gas. After the lean amine is filtered, the amine is regenerated and acid gas is recycled to the SRU.
Tail gas treatment is a critical step in the sulfur recovery process, ensuring that emissions are minimal while generating additional revenue. By understanding the most commonly used tail gas treatment methods, you can choose the most suitable approach for your project’s needs. If you want to dive deeper into all things sulfur, check out our Sulfur 101 white paper.
