Hybrid Water Based Absorption and Membrane for Deep Desulfurization of Claus Unit Tail Gas

A novel approach for the deep removal of sulfur dioxide (SO2) from sulfur recovery unit (SRU) tail gas is presented. This new process combines an absorption process, where SO2 is scrubbed/absorbed by mildly acidic water, followed by a membrane- based enrichment step to concentrate sulfur species before recycling them back to the Sulfur Recovery Unit (SRU). SRUs convert hydrogen sulfide (H2S) into elemental sulfur (S) by the modified Claus process with a yield that can reach up to 99% by partial oxidation with oxygen (O2). To meet stringent SO2 emissions standards, higher recovery rates are imperative. Achieving the required high sulfur recovery efficiency e.g. 99.9% or higher, necessitates the installation of a tail gas treatment. The prevailing technology for tail gas treatment is the reduction absorption, involving hydrogenation/hydrolysis step to convert all sulfur compounds/species exiting SRU into H2S. This is followed by an absorption step with H2S-selective amine base solvent to minimize carbon dioxide (CO2) co- absorption. The amine solution is then regenerated through heating to release H2S and co-absorbed CO2, which are recycled to SRU furnace reactor. However, this regeneration step is energy intensive and significantly contributes to the carbon footprint of the sulfur recovery process. Alternative processes with lower energy intensity, reduced carbon footprint as well as lower capital and operating costs are preferable. In the novel approach, there is no hydrogenation step. Instead, the tail gas from the SRU is sent to a thermal oxidizer, which converts all sulfur compounds and species into SO2 and SO3, collectively referred to as SOx. Downstream of the thermal oxidizer, and heat recovery, SOx are scrubbed from the tail gas by a mildly acidic water in a column at 60 °C. As water is produced during the H2S conversion (i.e. H2S combustion with O2), no water makeup is required, except for the initial water load during the startup. The absorbed SOx are concentrated using Reverse Osmosis and Nano-Filtration membranes, with the membrane rejecting SOx and the concentrated stream sent back to the SRU. Alternatively, SOx stream can be further concentrated to saleable sulfuric acid. The permeate of the membrane can be utilized for further scrubbing, directed to an evaporation pond, or reused within the plant.