Carbon Disulfide: Differentiating Sulfur Species for Premium Markets

The life quote “If it was easy, everybody would be doing it” partly explains why the number of refining facilities in the U.S. has declined from around 337 in 1981 to the current 119. Aside from transportation fuels, capturing margins opportunities from premium products such as petrochemical grade naphtha (PCN) is why many of these facilities are still expanding. However, it may become more difficult to meet impending PCN specifications in the future. The problem is carbon disulfide (CS₂), which is typically seen with the C₅ and C₆ components in PCN.

Petroleum refinery heaters and fractionators may be sources of CS2.

Certain refiners have found a lucrative market selling PCN to those naphtha-based steam crackers producing a range of olefins. Unlike ethane-based steam crackers that primarily produce ethylene, naphtha crackers can produce a significant amount of propylene and its derivatives for the polymer market. According to chemical processing information available in the public domain, the relatively stable CS₂ passes through naphtha steam crackers and negatively impacts downstream polymer production units. More specifically, the CS₂ poisons isoprene (C₅H₈) reactor Ziegler-type catalysts. High quality isoprene is a precursor to the rubber industry.

A detailed discussion on the kinetics and process aspects revolving around CS₂ is beyond the scope of this discussion, but more detailed information on this processing challenge can be found in two separately published AIChE papers written by subject matter experts at Honeywell UOP (AIChE paper #479799, “Carbon Disulfide Removal to Help Meet New Specifications for Petrochemical Grade Naphtha,” Gorawara et al) and Shell Global Solutions (AIChE paper #442505, “Carbon Disulfide (CS₂): A New Challenge for Steam Cracker Complexes,” P Coenen, Shell Global Solutions, E Anguiera, CRI/Criterion Catalyst Company Ltd).

Against this backdrop, polymer producers, especially those serving Asian markets, have been keen to point out that CS₂ is a precursor to polymer chain defects. Other problems noted in the industry involve CS₂ poisoning of catalysts in BTX extraction units. In fact, panel discussions at the recent 23^rd CFR Far East Open Spec Naphtha Conference in Singapore noted that “cargoes exceeding a CS₂ limit of 2 ppm would be unable to be ‘nominated’ into the open spec naphtha chain.” This is because when such cargoes are cracked in a steam cracker, it can possibly produce off-spec C­₅ derivatives.

More ominously, it’s been noted in at least one industry forum that isoprene producers in the PAC Rim want a PCN quality based on a CS₂ spec below 0.1 ppmv — not an easy task. Are refiners up to the challenge? An April 25, 2014 report in S&P Global Platts noted that, with regard to CS₂ in light distillate/clause 2.1 (c), (i) of the “Open Specification Naphtha Contract requires” that the first seller (typically the refiner) test and report on CS₂ levels.

CS₂ comes from refinery operations (e.g., crude heater/crude column overhead), with a certain amount of it already in crude feedstocks, as CS₂ can be found in oilfield additives. In general, where there’s H₂S, there’s CS₂, but the push by the petrochemical and polymer industry to differentiate CS₂ from other sulfur species begs the question if the refining industry is prepared to invest in the technology, particularly if premiums might be offered for a PCN market with very low levels of CS₂. Unlike other regulatory challenges like Tier 3 and the 2020 IMO Global 0.5 wt% Sulfur Cap, how the competitive refiner will deal with this (e.g., molecular sieve adsorption technology) is yet to be seen.