Sulphur spiking agents for hydroprocessing units activation
Svetan Kolitcheff ARKEMA
The hydroprocessing process employs cat- alysts inwhich the active species aremolyb- denum or tungsten sulphides promoted by cobalt or nickel sulphides. Sulphide catalysts are pyrophoric. Therefore, they are delivered as oxides and activated in- situ by reductive sulphiding in the pres- ence of hydrogen at elevated pressure and temperature. H 2 S is a colourless, heavier than air, explosive and toxic gas. In refineries, for obvious safety reasons, liquid sulphur spik- ing agents are used instead. Avery common application is hydroprocessingcatalyst acti- vation, where the most used sulphur spiking agents are dimethyl disulphide (DMDS) and di-tert-butyl polysulphide (TBPS). Both spik- ing agents have their pros and cons: • TBPS contains 54% sulphur compared to DMDS 68% sulphur. Thus, an extra 25% of TBPS is required to complete the sulphiding, which complicates the logistic requirement for delivery, near the unit and increases the costs. • DMDS has a lower flash point and typi- cally needs to be stored under nitrogen pressure in closed containers. However, this drawback does not imply a new risk in the application, as processed feedstocks and products present an average flash point. This risk can also be taken care of by injection service companies. For example, Carelflex, which performs these injections on a regular basis, comes with dedicated, safe equipment (magnetic drive pumps, dry break coupling, etc). • The higher volatility of DMDS ensures a good vaporisation and distribution upstream of the bed reactor. DMDS is also thermally stable, whereas TBPS decomposes in reactive material (see decomposition paragraph below). To avoid such an event, TBPS needs to be injected at high pressures, as close as possible to the process, and should not be used in gas-phase sulphidation.
Light by-products of the DMDS decom- position do not cause coke.
Using TBPS as a sulphiding agent: • TBPS decomposition is less clean than DMDS and comes with elemental sulphur as an intermediary up to 250°C. • The by-products are heavier: Final by-product is isobutane, which exits the separator with liquid hydrocarbon. TBPS decomposition can form elemen- tal sulphur when decomposing at inter- mediate temperature. It can precipitate and lead to pressure drop. An additional problem can result from the recombina- tion of olefins and sulphur, forming a solid compound called “carsul”. Carsul forma- tion downstream of the reactor can plug the heat exchanger and increase the pressure drop at the entry of the reactor. Hydrotreating catalyst activity Once the catalyst has been activated, the hydroprocessing unit can start to treat the feedstock. The activity of the hydrotreating process is obtained by measuring the resid- ual sulphur at the outlet and for different temperatures. Figure 2 shows the activity result from an academic laboratory on con- ventional catalysts, using the same normal- ised activation procedure for each sulphur compound. To reach the same performance, the unit start-up using TBPS needs to run at higher temperatures than the unit start-up using DMDS. Conclusion The choice of sulphiding agent is the pri- mary step, but proper sulphiding also requires an experienced team, dedicated equipment and the best monitoring to opti- mise the sulphiding process. For hydropro- cessing catalysts activation, the two main sulphur compounds are DMDS and TBPS. Both compounds have their advantages and disadvantages. In terms of benefit, DMDS is well headed, which is why it is the stand- ard on the market. It is the sulphiding agent with the highest sulphur content and pro- vides the highest activity. Some drawbacks can easily be avoided or at least diminished by selecting the right sulphiding service company, such as Carelflex from Arkema.
+H 140 - 170˚C
CH–S–CH + HS
160 - 200˚C
SH + S
+ 2 H
170 - 230˚C
+ 2 H 200 - 230˚C
« CARSUL » Solid deposit
170 - 250˚C
2 HS + 2 CH
250 - 400˚C
Figure 1 Sulphur spiking agents decomposition pathways on hydrotreating catalyst (LHSV: 1 h -1 , P : 35 b), determined by Arkema
decomposition temperature requirement for primary safe sulphiding. Apart from this similar property, their decomposition path- ways vary a lot. Using DMDS as a sulphiding spiking agent: • H 2 S is the main sulphur product at 210°C. Once the primary sulphiding step is reached (between 220 and 230°C), the clean decomposition of DMDS means that every sulphur atom is contributing to the sulphiding. • The by-products are lighter: They may accumulate in the recycle loop, which can bring unwanted SOx emissions if the procedure is not fully optimised and the purge timing not antic- ipated. Fortunately, the sulphur supplier or catalyst manufacturer are able to offer advice to mitigate this risk.
Did you know Sulphur spiking agents heated in the presence of hydrogen and catalyst decompose into H 2 S?
TBPS is much more viscous than DMDS, which increases the pressure drop and energy required for injection. This draw- back is exacerbated at low temperatures. Sulphur spiking agents decomposition Sulphur spiking agents heated in the pres- ence of hydrogen and catalyst decom- pose into H 2 S. An important criteria for the selection of a sulphiding agent is that its decomposition temperature must be low enough to ensure a primary sulphiding at 220-230°C. Partially sulphided catalysts are robust enough to withstand secondary sulphiding at or above 300°C without any concomitant over-reduction of the metals, causing activity loss. Both sulphur spiking agents come with their own decomposition pathways on hydrotreating catalyst (see Figure 1 ). Both sulphiding compounds start provid- ing H 2 S at around 170°C, which suits the
Physical properties comparison: DMDS vs TBPS DMDS
Sulphur content, wt/wt%
Flash point, °C Boiling point, °C
375 370 365 360 355 345 350 0
Viscosity at 20°C, cP
Figure 2 Residual sulphur coming from hydrotreating catalyst activation with different sulphur agents
* Start of thermal decomposition
Powered by FlippingBook