Oxidative destruction of monocyclic and polycyclic aromatic hydrocarbon (PAH) contaminants in sulfur recovery units

Abstract

The requirement of low sulfur content in fuels and the stringent environmental regulations has led to an increased production of acid gas (H2S and CO2) as byproducts in oil and gas industry. Consequently, the need for economic and efficient treatment of acid gas has surfaced. Sulfur recovery units (SRU), consisting of a furnace and catalytic reactors, are widely deployed to recover sulfur and thermal energy from acid gas. The contaminants in acid gas such as benzene, toluene, ethylbenzene, and xylenes (BTEX) and frequent changes in its composition cause flame instability and produce unwanted byproducts such as polycyclic aromatic hydrocarbons (PAHs), CO, COS, and CS2, which reduce process efficiency and increase operational cost through frequent catalyst deactivation. In this paper, a detailed reaction mechanism is presented for SRU that includes reactions for the combustion of acid gas and its contaminants, and the formation and oxidation of large PAHs by several oxidants (O, O2, OH, and SO2). The mechanism is validated with different sets of experimental data, and is used to investigate the process conditions that triggers the oxidative destruction of aromatics (BTEX and PAHs) in the furnace. With decreasing acid gas flow rate, and increasing feed preheating temperatures and oxygen concentration in air, the aromatics concentration was found to decrease substantially due to their enhanced oxidation by SO2 and O2, which indicates that the destruction of harmful aromatics in the high-temperature zone of the furnace is possible with optimized SRU process conditions. The developed reaction mechanism provide viable means of optimizing SRU to achieve aromatics destruction, enhanced catalyst life, and reduced sulfur production cost.