Unisulf process

Applying the Stretford or Unisulf processes usually means there is no need for a Claus process. These processes are especially suited to small applications where complete removal of H2S is sufficient and removal of COS is not needed. 

Thus the combination of olefins (in trace quantities) with a high concentration of CO2 in the acid gas may make a Claus-type process unusable. The path of least resistance may be to use the Stretford or Unisulf process, which tolerate both CO2 and olefins. 

Perox Process, 762 Takahax Process, 765 Stretford Process, 769 Hiperion Process, 794 Sulfolin Process, 797 Unisulf Process, 802... 

The Unisulf process, a homogeneous liquid redox catalytic process for oxidizing H2S to sulfur, was developed by Unocal in 1981 and was licensed jointly by Unocal and The Parsons Corporation. It was intended for clean-up treatment of Claus tail gas, Rectisol or Selex-ol off-gas, and oil shale retort off gas. Because of problems encountered in commercial operations, the process is no longer offered by Unocal (Bingham, 1992). 

The Unisulf process is quite similar to the Stretford process, as is apparent from a cursory inspection of the flow diagram shown in Figure 9-29. The principal difference is the absence of quinone compounds in the Unisulf solution. Instead, carboxylated complexing agents (for example, sodium l-hydroxybenzene-4-sulfonate and sodium 8-hydroxyquinoline-5-sul-fonate) are used (Fenton and Gowdy, 1981). 

The overall composition of the aqueous solution employed in the Unisulf process includes the following generic components ... 

Figure 9-29. Schematic flow diagram of Unisulf process Fenton and Gowdy, 1981). Hydrocarbon Processing, April, 1982...

Unisulf [Unocal sulfur removal] A process for removing sulfur compounds from petroleum fractions, similar to the Stretford process, but including in the catalytic solution vanadium, a thiocyanate, a carboxylate (usually citrate), and an aromatic sulfonate complexing agent. Developed by the Union Oil Company of California in 1979, commercialized in 1985, and operated in three commercial plants in 1989. 

In another case, when the acid gas is very rich in CO2 (i.e. 90 percent or higher) and contains olefins, the author knows no suitable process other than Stretford or Unisulf. An example is Rectisol offgas downstream of a Lurgi gasifier the Stretford process has been used, although with some problems. 

Several liquid redox processes have been developed for smaller-scale (ca. 0.25-20 t sulfur day ) applications, achieving 99.9-t- % recovery. For example, SuIFerox and ARI-LO-CAT use complexed Fe species to oxidize absorbed H2S in acidic solutions, and Stretford, Unisulf, and Sulfolin use vanadium (V) in slightly alkaline solutions as the oxidant for absorbed HS ions. Atmospheric oxygen is the ultimate oxidant in all such processes, as it reoxidizes the reduced form of the dissolved metal species, the concentrations of which are not so limited as that of dissolved oxygen ... 

The section closes with a discussion of three new processes introduced in the 1980s they include the Unisulf, Sulfolin, and Hiperion processes. Unisulf and Sulfolin are vanadium-based processes that were designed to minimize or eliminate the need for spent solution... 

Unocal built a total of four Unisulf plants. The first unit treated Rectisol off-gas and had a sulfur recovery capacity of 26 TPD. The second Unisulf unit was commissioned as part of a now shutdown oil-shale synthetic fuels plant located at Parachute Creek, CO. The first application in a petroleum refinery was a 6 TPD unit, constructed at Unocal s Santa Maria, CA, refinery in 1986. This unit processed the tail gas from two 60-ton/d Claus plants. The fourth and last plant was a Beavon-Stretford unit conversion. 

Unisulf [Unocal sulfur removal] A process for removing sulfur compounds from petroleum fractions similar... 

---