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What Refiners Need to Know About Diesel Cetane

By Process Pro Eric

Sep 07, 2015
 

Good tips on what you should know for Diesel Cetane.

 
 

Let’s face it… while maximizing diesel production trumps gasoline production these days, not many refinery operators, engineers, nor managers really understand diesel specs that well.  The most commonly misunderstood spec is diesel cetane. 

 

Cetane Number (CN) is a measure of a fuel’s ignition delay in an engine.  A higher cetane number indicates shorter ignition delay.  Since diesel engines act somewhat oppose to gasoline engines, shorter ignition delay (i.e. higher cetane number) fuels provide better performance.  This is why many refiners refer to cetane as the opposite of octane.  U.S. cetane number spec is set at 40, while fuels in EU and elsewhere can have cetane number specs of 51 or higher.


               


Like most other specs, diesel cetane number is a specification that is measured with lab equipment – in this case a Cooperative Fuel Research (CFR) engine.  Unlike other specs, cetane number measurement by CFR engine is rather difficult and costly.  Many refinery labs are not equipped to measure cetane number by engine, so many refiners rely on correlations.

 

Refiners who rely on cetane correlations can use the calculated Cetane Index (CI) to estimate cetane number.  In the U.S. there are two ASTM test method correlations, D976 and D4737, both of which rely on ASTM D86 distillation and density. 

 

  • D976 is an older correlation that relies on two-variables – API and T50
  • D4737 is an updated correlation that uses four variables - API, T10, T50, and T90

 

 

While D976 (two-variable) is often used as a surrogate test method for setting aromatic specs in diesel fuel, it is often not used as a preferred means of calculating Cetane Index these days.  This is because a larger offset generally exists between the measured Cetane Number (CN) and the calculated Cetane Index (CI) when using ASTM D976. 

 

D976 worked fairly well when the correlation was first developed and most diesel formulations consisted primarily of straight run diesel components.  As oil refining technology advanced, diesel fuels had a higher composition of cracked and aromatic stocks from Cokers and FCCs.  Furthermore, in today’s environment where synthetic and oil sand crudes distort “straight run” diesel qualities, more robust correlations became necessary to better predict cetane.

 

For those engineers interested to know the equations behind the two-variable and four-variable correlations, the formulas below show the calculation of CI by D976 and D4737.  While neither are exactly straight-forward, you can see that the four variable equation has a higher complexity.

  

            

 

Since many refineries do not have cetane engines, most engineers do not appreciate the offset between D976 & D4737 with the measured cetane number.  The data below shows various diesel streams and compares estimated cetane index with measured cetane

 

 

number.  In this first data set we compare calculated cetane index using both the 2-variable and 4-variable equations to the measured cetane.

 

While this is a straight run kero stream, the D4737 (4-variable) correlation actually has a lower offset as compared to D976.  This kero stream has a 550 deg F 90% and a 37 API.

 

 

In the next data set we have D4737 and D976 on a straight run diesel stream compared to the measured cetane number.

 

For this stream the D976 (2-variable) correlation is a better predictor of cetane number as compared to the
D4737.  This straight run stream has a 90% distillation of 660 deg F and 31

API.  As you can see, The 2-variable equation does a fairly decent job of cetane prediction for traditional diesel properties.

 

 

In this last data set, the two cetane correlations are compared to the cetane number for a FCC LCO stream.

 

Both correlations match rather poorly, and interestingly enough there’s a lower offset between the 2-variable equation and the CN. 

 

The LCO stream has a distillation of 700 deg F 90% and API of 9…yes 9. 

Although D4737 better predicts CI for diesels containing cracked stocks, the very low API of this LCO breaks the correlation for an individual stream assessment.

 

To summarize the information above, each refiner should rigorously check the bias between measured cetane number and calculated cetane index on an occasional basis.  Depending on the quality of your diesel stream, D976 and D4737 cetane index correlations will provide vaying levels of accuracy. 

 

A savvy process engineer should also be mindful of refinery configuration changes.  It can pay-out big bucks to scrutinize diesel giveaway when unit shutdowns occur.  When FCC units go offline and LCO production declines, one should re-assess the cetane balance and not blindly use the ASTM CI correlations to predict cetane.

 

As one last tip to note, many refineries use cetane improver to increase the cetane number of finished diesel fuel.  While additives can truly improve Cetane Number (CN), they do not affect Cetane Index (CI).  Depending on whether your efinery is cetane number or cetane index limited, improver may or may not make a difference.  Also be mindful that cetane improver has a response curve decline.  Beyond an optimal dosage rate, you receive limited benefit for incremental additive usage.

 
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  • Process Pro Eric :   Note that I have updated the equation above to include the missing parenthesis in the D4737 equation. Thanks for pointing out the error.

    Nov 18, 2013

  • Lionel :   D4737 still does not work Please check the formula before publishing it.

    Nov 27, 2014

  • Process Pro Eric :   Lionel, If you wish to send me your spreadsheet I will confirm if you have the calculation copied correctly. You can send the file to support@refinerlink.com.

    Dec 01, 2014

  • Janis :   If I know cetane index is over 40, is there a correlation or calculation for aromatic percentage - for Title V air permit when we get diesel deliveries I have to document both and suppliers only show cetane index.

    Aug 05, 2015

  • Process Pro Eric :   Janis, unfortunately i'm not aware of any correlation between CI and Aromatics. I know of other refiners in your situation and they run lab analysis to confirm the aromatics content.

    Aug 09, 2015

  • Tony Evans :   I used to run Cetane Engines for Chevron back in the day, (ASTM D613), together with RON, & MON engines, still involved now with Inspectorate in the UK. A lot of refiners do not have the expertise on hand for D613, and have now plumped for the IQT testing apparatus http://www.biofueltesting.com/product.asp?ID=3455 As a process engineer, I had to optimize diesel production in a short diesel world, and to do this used every componant available, to meet the CI spec, and using iso octyl nitrite to get to CN spec. I have some blend optimisation programs available if anyone needs

    Sep 08, 2015

  • Tony Evans :   Janis, the ignition quality of a diesel fuel is boosted by paraffin content, and reduced by aromatic content. So, to optimize diesel blends in a full conversion refinery, need to introduce LCGO to diesel blend pool. This component reduces cetane, and to overcome the neg effects to cetane when using highly aromatic LCGO, companies will use a cetane improver additive.

    Sep 08, 2015

  • Marc Valleur :   This is a very good illustration of the caution required when using regressions. They are only valid in a specific operating window and any extrapolation must be avoided. It was interesting to use Diesel Cetane Index as an example because I feel that blending middle distillates has become more complex than blending gasoline: a) A Euro V Diesel quality certificate has typically 22 quality determinations b) An optimal blend recipe for diesel can be 4 - 6 components out of a dozen in refineries with very complex process scheme c) FAME of various grades used to produce Bio Diesel is bringing new chemical species, impacting cold properties. In comparison, adding ethanol to gasoline is easy. d) Typically more than a dozen of additives, tracers, dyes, etc are used to produce the final commercial product, much more than gasoline e) There are many grades of middle distillates, including specialty products, such as arctic diesel In consideration of the above, the operating window for diesel is very large which is difficult to accomodate with a set of regression coefficients. My belief is that more elaborate methods can fight complexity and I am convinced that we need to track chemical species in order to have robust predictions of quality determinations. Much progress has been made over the last 20 years using lab and on-line spectrometers (NIR, MIR, Raman) for that purpose.

    Sep 20, 2015

  • Antony Evans :   Aside from the obvious implications to cold properties, FAME as a blend component to Diesel, can also affect the Cetane, filterability,and oxidation stability of the finished blend.

    Sep 22, 2015

  • Marc Valleur :   It is true that FAME improves many Diesel quality specs and degrades cold flow proerties and oxydation stability. It also impacts density and viscosity. Many of these commercial specs are controled using additives such as cetane boster or lubricity improver. We need a good representation of the non linear response to injection rates under various blend compositons. This is an area where we still need progress to achieve operational excellence, for instance performing in-line quality certification.

    Sep 22, 2015

  • S Bose :   Am going thru' the article for the first time. A question that comes - tat the three illustrative graphs used in the text, what is the parameter plotted on x-axis? Shall much appreciate a quick answer.

    Apr 15, 2016

  • Process Pro Eric :   S Bose - there is no meaningful parameter plotted on the x-axis (i.e. it is only time). The graphs are meant to illustrate the variation between the calculated cetane index for the two equations compared to the cetane number derived by engine.

    Apr 16, 2016

  • Facundo :   Hi! For D4737 method, the B parameter formula should be B = exp(-3.5*DN) - 1

    Sep 18, 2017

  • Tommy A :   Hey guys, can you send me calculatation sheets as discussed above that you don’t mind sending me? Tommy.arencibia@camincargo.com Thanks guys.

    Nov 17, 2017

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    Mar 06, 2018

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