The Effect of Feedstocks on InfraRed Blend Measurements
IR can be a simple, effective way to measure biodiesel blend ratios from a variety of feedstocks BY SANDRA RINTOUL
With biodiesel hitting a new production record of 823 million gallons through October and expected to reach 1.28 billion by 2013, more biodiesel will ultimately be blended into our diesel fuel.
This boom in production has opened the doors to a number of potential biodiesel feedstock sources from algae and jatro-pha to grease from food production or municipal waste.
More blends mean an increased need to measure the ratio of biodiesel in petro diesel. Infrared technology, used in both the European EN 14078 and the ASTM D737I methods, offers a quick and easy way to make the measurement.
Biodiesel Feedstocks and Measurement Accuracy: A common concern of individuals measuring the percent of biodiesel in diesel is if different feedstock will affect the blend analysis. Knowing more about how mid-infrared analysis detects biodiesel and the chemical structure of the feedstock oils will help show that infrared is a convenient and accurate tool for blend determination.
How Does Infrared Measure Biodiesel Content? Fortunately for manufacturers of infrared instrumentation, biodiesel has a unique signature from diesel
in the infrared range of the spectrum at the carbonyl band (5.73 micrometers or 1745cm1). Carbonyl infrared absorbancc is due to the stretching vibration of the carbon-oxygen double bond (C-O). At this infrared wavelength specific to biodiesel, the intensity of the absorbance increases as the concentration of biodiesel increases (see Figure 1). Calibration standards of different biodiesel blends are used to correlate the infrared absorbance to a percent concentration. In general, the wavelength and intensity of a carbonyl absorption will be affected by the mass and nature of the atoms attached to the C=0 group.
What Effect Does the Feedstock Have on Finished Biodiesel? The primary difference between the fatty acid esters in oils from various feedstocks is the length of the hydrocarbon chains and the number and position of the C-C bonds (carbon-carbon double bond). Most feedstocks such as soy, canola and yellow grease (or waste vegetable oil, WVO) have chain lengths between CI 6 and C22 with CI 8 predominating. The chain lengths and their position affect cold flow properties such as the cold filter plugging point and cloud point.
How Do the Differences in Feedstock Affect Infrared Measurements? As mentioned above, the wavelength and intensity of a carbonyl absorption will generally be affected by the mass and nature of the atoms attached to the C=0 group. Table 1 shows that the average molecular weight (mass) for five different feedstock oils is very similar for all except one. Different chain length means that the chains have different masses. These aliphatic chains, however, arc quite flexible so that a few carbon atoms more or less at the end far removed from the carbonyl group will have no significant affect on the infrared absorption. When C=C bonds are present (unsaturated oils) they are found near the center of the chain, i.e., far removed from the carbonyl group, so they too have little effect on the carbonyl absorption.
How Does This Translate to the Primary Concern of Whether Your Biodiesel Blend Measurements Will Be Accurate? Table 1 shows the results from a B20 blend with five different feedstocks measured with a Wilks InfraCal Biodiesel Blend Analyzer. As indicated in the B20 column, most of the feedstocks arc measured quite easily by infrared analysis with the exception of coconut oil.
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