Q & A > Question Details
Why is the cetane index of diesel higher for high sulfur than low sulfur crude?
02/07/2010 A: RENE AGA, SECOT, renelaga@telefonica.net
Cetane index is not a measure for sulfur content. Paraffinic crudes will have a higher cetane index. Naphtenic crudes a lower indezx and aromatic crudes the lowest. Aromatic crudes have generally high contents of heterocyclic sulfur compounds.
25/02/2009 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Cetane number or index of diesel cut from a crude is not directly dependent on sulfur. Generally the heavy crudes have high sulfur and if aromatics and poly aromatics are high cetane is low. We have low sulfur BH crude in India having a diesel cut with high cetane. Another low sulfur Assam crude has diesel with low Cetane. However hydrotreated diesel with aromatic saturation will have improved cetane and low sulfur compared to raw diesel from the same crude. Paraffinic stocks have high cetane, naphthenic intermediate while aromatics lower values
17/02/2009 A: Samuel Sinanan, IPSL, Samuel.Sinanan@ipsl.co.tt
Cetane index and not cetane number is a calculated number based on the ASTM boiling point curve. Deep aromatic saturation improves CI because there is a boiling point shift when aromatics are saturated. The CI of diesel has little to do with the sulphur content of the crude but the boiling point properties of the diesel fraction from the crudes.
16/02/2009 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
For the straight-run / wide boiling diesel fraction [nominal 320F to 662F] distillable from crudes and bitumens, Cetane Number per ASTM D613 engine testing and Cetane Index per ASTM D976 and ASTM D4737-A are driven by the molecular character of the crude oil or bitumen and not by organic sulfur content. Many of the higher sulfur crude oils from the Middle Eastern region exhibit a "waxy" character and thus contain increased content of linear paraffins and mono-aromatics with one alkyl being longer in terms of (CH2)n and linear and mono-cyclo-paraffins with one alkyl being longer and linear. Cetane values will be higher than for "cycloparaffinic" character crude oils with more alkyl poly-cyclo-paraffins and alkyl benzo-cyclo-paraffins and higher than for "aromatic" character crude oils and bitumens with more alkyl polynuclear aromatics.
16/02/2009 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Sulfur content of a whole crude is not an predictor of the cetane number of the straight run diesel from that crude. Cetane number is a measure of how long it takes for the fuel spray to ignite--the longer the delay , the LOWER the cetane number. Normal paraffins exhibit the shortest ignition delay, followed by iso-paraffins..... all the way to multi-ring aromatics Actually sulfur in/on the fuel molecule degrades the ignition characteristics. Fuel oxidation is a 'free-radical' reaction and sulfur is a free radical terminator.
15/02/2009 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
Low Sulphur diesel is low sulphur because it has been though hydrotreating process. In the hydrotreating process, the double bonds are saturated as there is excess hydrogen required for hydrotreating to convert S to H2S. By saturating the double bonds, the cetane no is reduced which is a negative.