How are InfraRed Blend Measurements Affected By Biodiesel Feedstocks?

To answer the question of how infrared blend measurements are impacted by biodiesel feedstock, a number of other questions need to be answered. These questions are studied in detail in this article.

How does Infrared Determine Biodiesel Content?

Biodiesel has a unique signature separate from diesel in the infrared range of the spectrum. This means that at a wavelength specific to biodiesel, for instance the carbonyl band at 5.73µm(1745cm^-1), the absorbance intensity increases as the biodiesel concentration increases as seen in Figure 1.

The absorbance change correlates to a percent biodiesel value. Carbonyl infrared absorbance occurs because of the stretching vibration of the carbon-oxygen double bond (C=O) and this absorption correlates directly to the biodiesel concentration result.

Normally the intensity and wavelength of a carbonyl absorption will be impacted by the mass and nature of the atoms attached to the C=O group.

Figure 1. IR spectra from a InfraSpec VFA-IR Spectrometer of different biodiesel concentrations

How does Feedstock Make Biodiesel Different?

The key difference between the fatty acid esters in oils from a number of feedstocks is the hydrocarbon chain length and the position and number of the C=C bonds. Most feedstocks such as canola, soy and yellow grease or waste vegetable oil –WVO have chain lengths in the range of C16 and C22, with C18 predominant.

How do Feedstock Differences Impact Infrared Measurements?

The intensity and wavelength of a carbonyl absorption will be normally impacted by the nature and mass of the atoms attached to the C=O group. It can be observed from Table 1 that the average molecular weight for five different feedstocks is almost the same except for one.

Different chain lengths imply that the chain has different masses. These aliphatic chains have sufficient flexibility such that a few carbon atoms at the end, far removed from the carbonyl, do not have any significant impact on the infrared absorption. The presence of carbon-carbon double bonds near the chain center that is far from the carbonyl group has a minimal impact on the carbonyl absorption.

Table 1. Biodiesel feedstock comparison

How to Ensure Biodiesel Blend Measurements as Accurate?

Table 1 shows the results from a B20 blend with five different feedstocks measured with a InfraCal Biodiesel Blend Analyzer. As shown in the B20 column, most of the feedstock perform well for infrared analysis except coconut oil. The third column of the table shows the average molecular weight of the FAME (fatty acid methyl ester).

Coconut oil is considerably different from other oils with a 180 molecular weight. Based on the molecular weight, the coconut-based FAME will offer a response of about 26.7%. It is possible to specifically calibrate any infrared analyzer for a coconut oil, so that concentration measurement will be correct in spite of the molecular weight difference. Coconut oil biodiesel does not perform well in high temperature or cold climates, hence its use is restricted to more tropical parts in the globe.

Figure 2. Biodiesel blend analyzer

Infrared measurements are impacted little by a large amount of biodiesel feedstock, making infrared analysis a simple and reliable analytical technique for checking biodiesel blend.

Infrared instruments, such as the InfraCal Biodiesel Blend Analyzer and InfraSpec VFA-IR Spectrometer, help fuel distributors enable petroleum terminals, fuel distributors, fleet operators and regulatory agencies to make quick, on-site measurements with little or no technical training.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.