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The different cuts then go through hydrotreating processes. It is a catalytic treatment in the presence of hydrogen to remove unwanted heteroatoms or metals such as sulfur or nitrogen from the feedstock for three main objectives:
  • To remove heteroelements in order to meet specifications, for example EURO VI standards limit to 10 ppm the maximal sulfur in diesel.
  • Remove heteroelements so that they are not problematic in downstream units.
  • To start hydrogenating the aromatic molecules in order to make them more reactive in downstream units.
The hydroprocessing processes employ catalysts in which the active species are molybdenum or tungsten sulfides promoted by cobalt or nickel. Sulfide catalysts are pyrophoric. Therefore, they are delivered as oxides and activated in situ by reductive sulfiding in the presence of hydrogen at elevated pressure and temperature.
 
H2S is a colorless, heavier than air, explosive and toxic gas. In refineries, for obvious safety reasons, liquid sulfur spiking agents are used instead. A very common application is hydroprocessing catalyst activation, where the most used sulfur spiking agents are dimethyl disulfide (DMDS) and di-tert-butyl polysulfide (TBPS).

What are the differences between TBPS et DMDS?

  • TBPS contains 54% sulfur compared to DMDS 68% sulfur. Thus, an extra 25% of TBPS is required to complete the sulfiding, which complicates the logistic requirement for delivery, near the unit and increase the costs.
  • DMDS has a lower flash point, it typically requires 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®, who is performing these injections on a regular basis, comes with dedicated and safe equipment (magnetic drive pumps, dry break coupling…).
  • The higher volatility of DMDS ensures a good vaporization 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 event, TBPS need to be injected at high pressures, as close as possible to the process and should not be used in gas phase sulfidation.
  • TBPS is much more viscous than DMDS, which increases the pressure drop and the energy required for injection. This drawback is exacerbated at low temperature.

Sulfur spiking agents decomposition

Sulfur spiking agents, heated in presence of hydrogen and the catalyst, decompose into H2S. An important criteria for the selection of a sulfiding agent is that its decomposition temperature must be low enough to ensure a primary sulfiding at 220-230 °C.

Partially sulfided catalysts are robust enough to withstand secondary sulfiding at or above 300 °C, without any concomitant over-reduction of the metals, causing activity loss. Both sulfiding compounds start providing H2S around 170 °C, which suits the decomposition temperature requirement for primary safe sulfiding. Except this similar property, their decomposition pathways bring a lot of differences.
Figure 1 Sulfur spiking agents decomposition pathways on hydrotreating catalyst (LHSV: 1 h-1, P: 35 b), determined by Arkema.

Using DMDS as a sulfiding spiking agent, the beneficial alternative:

  • H2S is the main sulfur product at 210 °C. Once, the primary sulfiding step is reached (between 220 and 230 °C), the clean decomposition of DMDS means that every sulfur atom is contributing to the sulfiding.
  • The by-products are lighter:
  1. They may accumulate in the recycle loop, which can bring unwanted SOx emissions if the procedure is not fully optimized and the purge timing not anticipated. Fortunately, the sulfur supplier or the catalyst manufacturer are able to bring advice to mitigate this risk.
  2. Light by-products of the DMDS decomposition do not cause coke.

Using TBPS as a sulfiding agent:

 
  • TBPS decomposition is less clean than DMDS and comes with elemental sulfur 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 sulfur when decomposing at intermediate temperature. It can precipitate and lead to pressure drop. An additional problem can result from the recombination of olefins and sulfur, forming a solid compound called “carsul”. Carsul formation downstream 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 sulfur at the outlet and for different temperatures. See on Figure 2, the the activity result coming from an academic laboratory, on conventional catalysts, using the same normalized activation procedure for each sulfur compound. To reach the same performance, the unit start-up using TBPS requires to run at higher temperatures than unit start-up using DMDS.

Arkema, a premier solution for a flexible and safe sulfiding operation

The choice of the sulfiding agent is the primary step, proper sulfiding also require experienced team, dedicated equipment and the best monitoring in order to optimize the sulfiding process. For hydroprocessing catalysts activation, the two main sulfur compounds are DMDS and TBPS. Both compounds have their own advantages and disadvantages. In terms of benefit, DMDS is well headed. It is the sulfiding agent with the highest sulfur content and providing the highest activity. Some drawbacks can easily be avoided or at least diminished by selecting the right sulfiding service company, such as Carelflex® from Arkema.

Dimethyl Disulfide Applications

See also

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