Sulphur spiking agents for hydroprocessing units activation (ERTC)
The hydroprocessing process employs catalysts in which the active species are molybdenum 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 presence of hydrogen at elevated pressure and temperature.
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H2S is a colourless, heavier than air, explosive and toxic gas. In refineries, for obvious safety reasons, liquid sulphur spiking agents are used instead. A very common application is hydroprocessing catalyst activation, where the most used sulphur spiking agents are dimethyl disulphide (DMDS) and di-tert-butyl polysulphide (TBPS). Both spiking 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 typically 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.
• TBPS is much more viscous than DMDS, which increases the pressure drop and energy required for injection. This drawback is exacerbated at low temperatures.
Sulphur spiking agents decomposition
Sulphur spiking agents heated in the presence of hydrogen and catalyst decompose into H2S. 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 providing H2S at around 170°C, which suits the decomposition temperature requirement for primary safe sulphiding. Apart from this similar property, their decomposition pathways vary a lot.
Using DMDS as a sulphiding spiking agent:
• H2S 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 anticipated. Fortunately, the sulphur supplier or catalyst manufacturer are able to offer advice to mitigate this risk.
- Light by-products of the DMDS decomposition 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 elemental sulphur when decomposing at intermediate temperature. It can precipitate and lead to pressure drop. An additional problem can result from the recombination of olefins and sulphur, forming a solid compound called “carsul”. Carsul formation 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 residual sulphur at the outlet and for different temperatures. Figure 2 shows the activity result from an academic laboratory on conventional catalysts, using the same normalised 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.
The choice of sulphiding agent is the primary step, but proper sulphiding also requires an experienced team, dedicated equipment and the best monitoring to optimise the sulphiding process. For hydroprocessing 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 standard on the market. It is the sulphiding agent with the highest sulphur content and provides 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.
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This short article originally appeared in the 2021 ERTC Newspapers, produced by PTQ / DigitalRefining.
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