disulfide oils (RSSR or DSO), which are then removed from the caustic. The regenerated (or lean) caustic is then returned to again interact with the hydrocarbon, extracting more acidic sulfur species. The extraction and oxidation reactions of mercaptan (RSH) and caustic (NaOH) are shown here: Caustic Extraction:
RSH+NaOH ⇒ NaSR+H2O
Oxidative Regeneration: 2NaSR +
O2 + H2O
Catalyst
RSSR + 2naOH
While it is relatively easy to reduce extractable RSH in the product to 1 – 2 ppm as sulfur (S), one of the largest challenges to caustic treating is preventing the resulting DSO from returning to the hydrocarbon feed. The DSO is typically both entrained and dissolved in the oxidised caustic. The colloquial term for the DSO which returns to the treated hydrocarbon is ‘re-entry sulfur’ or ‘back extraction’. To achieve the low sulfur requirements, Merichem Company offered its REGEN® ULS technology, which reduces DSO back extraction allowing total sulfur product specifications below 10 ppmw. Merichem designed the REGEN ULS technology by combining two separate existing technologies in such a way as to force the DSO to separate more completely from the regenerated caustic compared to a traditional REGEN system. Pairing a high-efficiency RSH removal system (THIOLEXTM) with a specially designed REGEN ULS, where total DSO back extraction is reduced to only 1 – 2 ppm as S, allows for a much lower total S product specification compared to a standard REGEN. The first REGEN ULS unit was installed in Taiwan over 10 years ago and has since been successfully operating and meeting product specifications.
Technology advantages The THIOLEX technology utilises caustic soda as the treating reagent to remove acid gas and mercaptan compounds from liquid hydrocarbon streams. The non-dispersive FIBER FILM® Contactor used in the technology enables reduced CAPEX and less plant space requirements compared to most treating alternatives. The large interfacial surface area, microscopic diffusion distance and continuous renewal of the aqueous phase when using the Contactor combine to yield greater mass transfer efficiencies. Because the aqueous phase adheres to the fibres in the Contactor rather than being dispersed into the hydrocarbon phase, aqueous carryover is virtually eliminated. The FIBER FILM® Contactor was designed as an improvement to conventional caustic/hydrocarbon dispersive mixing designs such as trays and mixing valves. It reduces caustic carryover with the hydrocarbon after the contacting and reduces the size of the separation vessels. The patented REGEN ULS achieves low total S in the lean caustic by using up to three DSO-removal steps: The bulk free DSO is first decanted. Next, the caustic is solvent washed to remove most of the dissolved DSO. Finally, for DSO polishing, the caustic flows through a DSO adsorption bed. This three-step process allows a near total elimination of DSO re-entry sulfur in the treated product. In many cases, butane can be treated pre-fractionation in a single train compared to conventional technologies requiring treatment of multiple trains post-fractionation. In this case study, total mercaptan sulfur content of the BB feed was reduced from 222 ppmw to less than 2 ppmw for combined mercaptan and DSO.
Description of the process
Figure 1 contains a schematic flow diagram of Merichem’s THIOLEX/REGEN ULS treating unit with untreated BB. The hydrocarbon stream is treated in a THIOLEX system using lean caustic from the REGEN ULS. The untreated BB stream has a design throughput of 13 000 bpd and total mercaptans of 222 ppmw as sulfur. After treatment in the THIOLEX unit, the sum of remaining mercaptan plus DSO as sulfur is reduced to 2 ppmw. The mercaptide-rich caustic is directed to the REGEN ULS system, where it is oxidised on a fixed catalyst bed. The resulting DSO is then removed from the caustic using first bulk phase separation and decanting, then solvent washing, and lastly adsorption on a fixed bed (not indicated) to yield the ultra-low sulfur (ULS) lean caustic that is recirculated to the THIOLEX unit for continuous treating. The technology super-regenerates the mercaptide-rich caustic and returns a lean caustic almost free of DSO. This allows ultra-low sulfur levels to Figure 1. Schematic diagram of the THIOLEXTM and REGEN® ULS technology. be obtained in the treated butane product.
May 2021 28 HYDROCARBON ENGINEERING
