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Calculation Routines Supporting the use of Diesel Fuel Cetane Improver
2-ethyl hexyl nitrate.
Laboratory Projects can be completed to support the refinery blending process and enhance the overall accuracy of targeting the cetane specification.
Development of blend models for the planning process.
Blender control systems effective in adjusting for cetane.
Reliable IQT calibration and measurement accuracy.
The planned blend formulation needs to be realistic for the base cetane number and the dosage of cetane improver calculated to move the result from the base to the fuel specification. Blender control systems can correct minor errors but need to make a ‘good start’ to be reliable. The component cetane will vary significantly for different feed sources and the formulation of blends to a cetane target requires reliable blending indices and rules. The predicted blend cetane index (CI) is often used for the base cetane. The 2 and 4 variable Cetane Indices are correlations based on the density and distillation of the blend. These assume an even character across the distillation range but can be modified to be more accurate for specific blends types. Sampling components and checking the way they blend for cetane can help to establish blending rules. The Excel download above gives an example of such a study.
On-Line Controller & Cetane Improver Dosing
Advanced blenders are equipped with fast loop sampling and line analysers. These measure diesel properties as blends are being produced and control the blender in real time, optimising the blend formulation and additive dosages. Direct measurement of the cetane number is not usual, but spectroscopic methods such as NIR are available, requiring a continuous calibration support to maintain accuracy. Density and distillation points are usually available to follow the cetane index.
‘Splash Blending’ to a batch tank may allow time for rundown and tank analysis in the laboratory to help set additive dosages. The introduction of the IQT instrument has greatly improved the speed of analysis. Supporting laboratory studies can be run to measure and model the cetane improver effect and the non-linear shape of the response curve be taken into account when setting and adjusting dosages. The Excel download study above, shows response curves for different fuel character and the accuracy of the blend models in predicting the cetane result.
Batch Release Measurement of dCN
The quoted reproducibility of the IQT method is significantly higher than the repeatability (3.2 vs 0.76), which indicates different laboratories tend to vary as instruments drift out of calibration. As such, studies are of value in checking the refinery instruments are remaining in calibration. Batch samples can be sent to a central laboratory for routine checking of the dCN by IQT and compared across the industry. Normal industry ‘Round Robin’ programmes provide a good basis for calibration, but more regular checks can be of benefit especially when a refinery only runs one IQT instrument. A weekly batch sample run on a second machine can highlight any drift in the calibration.