Louisiana Chemical Equipment Company

The Tail Gas Treating Unit (TGU#2) treats the sulfur recovery unit tail gas using a reduction process to convert any sulfur species present to hydrogen sulfide (H2S) for subsequent recovery using an amine-based solvent. The recovered H2S is recycled back to the sulfur recovery unit for conversion to elemental sulfur. Treated tail gas, or "vent gas", containing a maximum of 130 ppmv H2S is sent to an incinerator for destruction. 

PROCESS CHEMISTRY 

Sulfur recovery unit tail gas contains several sulfur species such as hydrogen sulfide (H2S), sulfur dioxide (S02), sulfur (S), carbon disulfide (CS2) and carbonyl sulfide (COS). Since amine solvents will only absorb H2S, the other species must be converted to H2S in a catalytic reactor. Conversion to H2S is achieved through both hydrolysis and hydrogenation type reactions. Compounds react with water to form H2S and carbon dioxide (C02) The remaining sulfur species react with hydrogen (H2) ) to form H2S and water . The converted sulfur species are then removed from the tail gas as H2S. This is accomplished through absorption into a 45 wt% methyl-diethanolamine (MDEA) based solvent solution. The mechanism for absorption is actually a reaction of H2S with the methyl-diethanolamine which proceeds according to the 

Following generalized equation. MDEA also reacts with CO2 which is present in the tail gas. 

A key design consideration of the TGU#2 is to absorb as much H2S as possible while absorbing only minor quantities of CO2• This reduces undesirable recycle of CO2 to the sulfur recovery unit. The TGU#2 is designed to reject approximately 75% of CO2 in the feed gas to the absorber.