Tertiary-Butyl Polysulfide (TBPS)
Di-Tertiary-Butyl Polysulfide (TBPS) is a mercaptan that can be used for sulfiding hydrotreating catalysts. Produced by ChevronPhillips Chemical Company under the trade name "TBPS 454", TBPS offers some benefits over other sulfiding compounds, although several factors should be considered before selecting it as a sulfiding agent.
One major issue is the cost. TBPS contains 54% sulfur as compared to 68% sulfur for Dimethyl Disulfide (DMDS). The result is 25% more TBPS is required to complete the sulfiding of a hydroprocessing unit versus DMDS.
Other areas where TPBS 454 and DMDS differ are:
- Lower decomposition temperature – TBPS begins to decompose at 320°F versus 360°F for DMDS (in the presence of NiMo or CoMo catalysts).
- Lower vapor pressure – TBPS has a lower vapor pressure than DMDS. TBPS has an odor similar to sour gasoil. However, the DMDS now manufactured by ChevronPhillips is made to a higher purity specification than in the past, greatly reducing its garlic-like odor.
- Non-flammable – TBPS is classified as a non-flammable liquid.
- Decomposition products – The decomposition products of TBPS are H2S, isobutylene, and isobutane, while DMDS decomposes to H2S and methane. Isobutane will typically leave the high-pressure separator on the liquid side, improving the purity of the recycle hydrogen and keeping the gas density low. Note that the methane make associated with the decomposition of DMDS can be controlled by carefully monitoring H2S levels during the sulfiding process with the use of Reactor Resources’ Online H2S Analyzer. In addition, isobutylene is a coke precursor. Catalyst manufactures typically recommend that all olefinic materials (such as isobutylene) be avoided for three to four days at the start of a catalyst cycle. This break-in period allows the hyper-activity of the catalyst to subside, reducing the chance of premature coke formation from highly reactive olefins. The introduction of isobutylene onto a freshly sulfided catalyst bed will exacerbate the formation of coke, gums, and tars.
There are other issues that should be considered when selecting TBPS as a sulfiding agent. TBPS can form elemental sulfur when decomposing at intermediate temperature levels if there is not sufficient amounts of hydrogen available. Any sulfur that precipitates out can lead to pressure drop issues in the hydrotreating reactor. An additional problem can result from the recombination of olefins and sulfur, forming a solid compound called “carsul”. To avoid sulfur precipitation and carsul formation, TBPS is often injected at high pressures and cannot be used in gas phase. The injection point should also be as close to the process as possible to reduce the opportunity for solids formation.