In recent years there has been a push to set very low specifications for carbon disulfide (CS2) in petrochemical grade naphtha (PCN). Although it is only one of the many components that make up the total sulfur of a naphtha stream, the tendency of the relatively stable CS2 to pass through a steam naphtha cracker and negatively impact a polymer production unit has led to this shift in position. Some buyers of PCN have proposed setting CS2 targets of less than 0.1 ppmw. In order to meet this change, refiners and petrochemical producers are changing operating conditions in facilities and searching for economic solutions. This article describes various scenarios and what might drive a refinery to choose one solution over another.
Carbon disulfide in PCN
There are several issues, which are discussed in literature, concerning why CS2 is a problem. One issue has been the isoprene producers in Pacific Rim countries. In addition, other polymer producers, outside of the isoprene market, have recognised CS2 as a catalyst poison and precursor of polymer chain defects. Within the isoprene market, the main effect is that CS2 poisons both the Ziegler-type catalysts and the nickel octoate (used to remove acetylene and cyclo-pentadiene from the process). The traditional alkali wash process used to remove sulfur specifies from these processes cannot remove CS2. Therefore, to maintain acceptable process performance, isoprene manufacturers must replace the expensive catalyst and scavengers at higher rates than expected, resulting in high operational costs, which the petrochemical industry will no longer overlook.
CS2 is found in hydrocarbon reserves and is produced during many refinery reactions. In the majority of these, hydrogen sulfide (H2S) is integral to the reaction, which suggests that in both natural sources and reducing atmospheres, feedstocks rich in H2S will also be rich in CS2. Given the stability of CS2 at high temperatures, akin to its analog carbon dioxide, it is difficult to convert to a more easily extracted molecule once formed.
CS2 has a relatively low boiling point, which results in it tracking both the C5s and C6s through the refinery. With these components being the major constituents of light naphtha, it is unsurprising that the CS2 would become concentrated in the naphtha product. Given the thermodynamics of the compound in question, the issue for a refinery becomes more about removing the CS2 from the light naphtha, rather than preventing it initially.
While CS2 levels in light naphtha have been as high as 300 ppmw, in the majority of refinery process streams it exists at much lower levels and has traditionally been classed with other sulfur species. However, the growing concern over CS2 has led some industry consortiums to undertake detailed analysis of the problem. It is estimated that the average level of CS2 in PCN, in loads that exceed 1 ppmw, is approximately 3.6 ppmw. Therefore, to meet a 1 ppmw specification, many suppliers of PCN would be required to remove between 50% and 80% of the CS2 prior to shipping. However, for certain sources the levels of CS2 removal exceed 95%.
There is a need for a solution that removes CS2 to ultra-low levels without the risk of producing naphtha that becomes ‘off-spec’ in other necessary quality parameters, such as octane number and boiling point range.
Written by John Griffiths, Trevor Smith, Stephen Caskey, Emily Harrell, James Gaspar and Abhishek Kadam, Honeywell UOP, USA.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/03082017/meeting-expectations/