Refineries everywhere have deployed hydroprocessing, specifically hydrotreating techniques, for decades. These techniques remove sulfur and nitrogen from finished gasoline and diesel fuels, and typically involve combining the hydrocarbon feedstock with hydrogen inside a reactor. The reactor is filled with a catalyst (most commonly based on cobalt molybdenum or nickel molybdenum) and must then be continuously sulfided while the reactor is in operation.
To complete this process, a sulfiding agent is commonly required – but traditional options pose significant challenges and limitations and can compromise environmental health and safety. This is particularly true of renewable diesel refining operations, where traditional sulfiding agents are kept on site 24/7.
The good news is that alternatives are available that can simplify this process, and they are worth refiners’ thorough investigation. This article will examine traditional agents’ and their complexities, reliable alternatives, and why such alternatives are ideal for the burgeoning renewable diesel industry.
A quick examination of the sulfiding process
Within the hydroprocessing reactor, the catalyst starts in an inert form with active metals in their oxide form. The sulfiding process begins as a reaction between the active metals within the hydrogen sulfide and sulfur. Naturally occurring sulfur may be available within the feedstream, but is more commonly provided by a sulfiding agent that is added to the reactor. While it is possible to feed the catalyst with the feedstream’s hydrogen sulfide, this risks coke accumulation on the catalyst surface, reducing its potential lifespan. Meanwhile new feedstocks, including those used to produce renewable diesel (a promising and sustainable fuel source) contain no naturally occurring hydrogen sulfide at all.
Thus, a sulfiding agent is typically introduced into the process as an essential part of modern refining and production, but it is not always simple for refiners.
DMDS and associated challenges for refiners
Dimethyl disulfide (DMDS) has been the most common choice as a sulfiding agent for a number of reasons. It contains high sulfur content (68%) that is optimal for sulfiding and is typically available at a low cost. Most refinery operations view DMDS as the default choice, accepting some of its challenges as simply the cost of doing business.
But those challenges are significant. Most critically, DMDS maintains a very low flash point of 61°F (16°C), making it a fire hazard that must be handled with care. Usually, DMDS is stored under nitrogen pressure in closed containers, and personnel responsible for handling the material must wear specialised personal protective equipment (PPE) to mitigate safety hazards. And because of these risks, DMDS is heavily regulated by the US Department of Transportation (DOT). Drivers of DMDS transport vehicles are required to remain with the vehicle at all times, enabling the vendor to maintain a closed chain of custody.
DMDS presents a few issues within the reactor system as well. DMDS’s hydrocarbon byproduct methane has the potential to accumulate within the hydrogen and hydrogen sulfide recycle gas stream in the reactor. This can cause the hydrogen within the recycle gas stream to become diluted during the sulfiding process, requiring purging of the methane during the process. The flaring of the purged gas, which contains hydrogen sulfide, can produce undesirable sulfur oxide emissions from the hydroprocessor. Further, this may require the addition of supplemental hydrogen to maintain the required hydrogen partial pressure within the reactor.
Then there is perhaps the greatest challenge associated with DMDS: its odour. Sometimes likened to the smell of rotting cabbage, DMDS emits an extremely unpleasant odour that can create suboptimal working conditions, even in an open-air environment. Any potential spills on-site can even have ramifications throughout the surrounding community.
Considering these challenges, the typical refinery will work to minimise the time that DMDS must remain on-site for the cata-lyst sulfiding process. Traditional applications involving crude oil typically require catalyst activation every three-to-five years, with DMDS required on-site during these intervals to activate a new catalyst when it is added to the reactor. However, most refineries have multiple processing streams and associated catalysts that require individual activation, meaning that DMDS may be required on-site more frequently. For mid-to-large-sized operations, DMDS may be required once or twice an-nually for a period of about a week, typically occurring in the spring or autumn.
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