Simplified Biofuels Measurements with Mid Infrared Analysis

By Joseph Boyd¹ and Sandra Rintoul²
June 2007

  1. Lab Manager with Imperial Western Products, Inc. in Coachella, California.
  2. Applications Specialist with Wilks Enterprise, Inc. in South Norwalk, Connecticut.

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The biofuels industry is growing rapidly and so is the need for analysis to ensure quality product. Start-up biodiesel producers are lured by claims from instrument manufacturers that their analyzer will do it all. Unfortunately, there is no one instrument, or even type of instrument, that can make all the measurements to ensure product quality. The parameters for quality biodiesel require different types of instruments and measurement systems, such as gas chromatographs (GC), titrators, centrifuges, infrared spectrometers.

Product quality begins at the production facility and extends to consumer filling station. Some of these analyses are for process control or quality verification and can be done with quick and simple verification tests while others require a more complete test according to the ASTM or EN approved methods. Many crucial measurements can be performed by mid-infrared (IR) analysis. These measurements include free fatty acids and water in the incoming feedstock, total glycerin during transesterification in production, to measuring blend ratios (biodiesel in diesel or ethanol in gasoline).

Biodiesel Production

There are three main areas where analysis is useful or necessary in the production of biodiesel; testing incoming feed stock, monitoring the reaction process and verifying the quality of the finished product.

Incoming Feed Stock

Biodiesel is typically made from animal fats or vegetable oils that are chemically reacted with an alcohol (methanol or ethanol) and a catalyst (sodium or potassium hydroxide) to produce an ester or biodiesel. The process is called transesterification. Knowing the amount of Free Fatty Acid (FFA) and water in the incoming feedstock helps the producer to adjust the amount of alcohol and catalyst for a complete reaction. FFA’s in oil react with the alkaline catalyst to form soap and can cause a reduction in yield. Water deactivates the catalyst.

FFA and water measurements are for the producer and do not need to be approved methods. The approved method is a non-aqueous potentiometric acid-base titration to determine the acid number. Moisture is typically measured by Karl Fischer Titration. Both of these analyses can also be done with infrared spectroscopy. Water can be extracted from the biodiesel with acetonitrile and measured in the 6 micron region. Measuring FFA involves adding a weak base to form a salt whose carbonyl absorption band is shifted away from that of the biodiesel ester. Even with sample preparation, either measurement takes under 5 minutes and does not require a skilled technician.

Transesterification of Feed Stock

VFA-IR Spectrometer

Imperial Western Products (IWP) is a manufacturer of alkyl methyl esters, better known as biodiesel. IWP utilizes multiple feedstocks, including yellow grease, soybean oil and corn oil. IWP has been producing biodiesel for over 5 years and is a BQ-9000 accredited manufacturer. They recently pioneered using a small portable mid infrared analyzer with a variable filter array, the Wilks InfraSpec VFA-IR Spectrometer (shown in the picture to the right), for measuring bound glycerin during the transesterification reaction. The InfraSpec incorporates a patented design consisting of an Attenuated Total Reflection (ATR) sample plate with an electronically modulated source on one end and a linear variable filter (LVF) coupled with a detector array on the other. The result is a compact spectrometer with no moving parts and no optical path exposed to air that is portable and rugged.

There are several reactors used in a batch process in which the feedstock oil is transesterified into biodiesel. The transesterification reaction in their process takes 3-4 hours. To determine when the reaction is complete, a sample must be taken from the reactor and the mass % of bound glycerin (mono-, di- and triglycerides) determined on the sample. The ASTM standard D 6751, specifies that the total glycerin, which is the sum of free and bound glycerin, must be below 0.240 mass %. Since virtually all of the free glycerin is removed downstream they are concerned only with the bound glycerin. Therefore, if bound glycerin is below 0.240 mass % the reaction is considered to be complete. If greater than 0.240 mass % the reaction is allowed to continue possibly with the addition of more reactant or catalyst.

Before testing the sample, the excess methanol and sodium hydroxide catalyst are removed by washing the sample several times with water and then drying on a hotplate. After the sample has been washed and dried it is ready for testing by either gas chromatography or the InfraSpec VFA-IR Spectrometer. Running a gas chromatogram takes approximately 45 minutes. The sample can be run on the InfraSpec Spectrometer in less than 5 minutes.

IWP found that the use of the InfraSpec Spectrometer for making pass/fail determinations can significantly reduce analysis time, by about 40 minutes, and thereby decreasing wait time and increasing throughput. It is estimated that use of the InfraSpec increased reactor throughput by 15 -20 %. Although the InfraSpec instrument can not achieve the performance of a gas chromatograph, it can be useful for quickly giving a pass/fail determination for biodiesel while in-process. Testing by gas chromatography per ASTM specifications is still necessary for finished lots of biodiesel.

Biofuel Blend Ratios

Filter based Analyzer

Knowing the biodiesel/diesel blend and the gasoline/ethanol blend is important to distributors, engine manufacturers, fleet operators, and regulatory agencies. Many engine warrantees are not valid above a specified biodiesel percentage and fleet operators need to know the blend to ensure compliance with the warrantee terms. The Volumetric “Blend” Tax Credit at the blender level makes the IRS concerned about the percent blend. Many regulatory Weights and Measures agencies are also required to know the blend ratio.

In the mid infrared region, the biodiesel ester has characteristic absorption due to the carbonyl band (5.73 µm or 1745 cm‾¹). Ethanol had an absorption at 9.6 µm (1042 cm‾¹) that is unique from gasoline. Infrared absorption increases with concentration therefore quantitative measurements can be made by determining the absorbance changes at the wavelength specific to the analysis. Inexpensive filter based analyzers (such as the InfraCal Filtometer shown in the photo on the right) can select the 5.73 µm wavelength for biodiesel or 9.6 µm wavelength for ethanol allow for portable and easy analysis. A simplified operator interface allows non-technical personnel to perform the analysis onsite. EN 14078 and the ASTM method currently in progress, both specify mid infrared for the biodiesel blend ratio.


No one method can satisfy all the measurement criteria to ensure that a biofuel meets the required standard. Mid infrared analysis is often a quicker method that can be used for production monitoring, to verify product quality on the buyers end, and to determine biodiesel/diesel or ethanol/gasoline blend ratios. Filter based mid infrared analyzers offer the ease of use, portability and ruggedness that is required for in plant or field analysis. More time consuming methods such as GC and titration are sometimes required to meet ASTM or EN standard requirements. Determining whether your analysis has regulatory requirements or time, cost and labor requirements will help you decide what equipment you need for your specific analyses.


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