Tailored Analysis for the Biofuels Industry

Introduction: Keen Interest in Biogenic Fuels

Recently, there has been greater interest placed on biofuels, not only due to their neutral CO2 balance but also due to their high microbiologically degradable characteristics. This is the reason for their use in ecologically sensitive areas. As these alternative fuels can be produced locally, the over-reliance on imported raw materials is reduced.

Biodiesel and Bioethanol Production

Animal fats or vegetable oils are normally used to produce biodiesel. The triglycerides contained in the fat or oil are transesterified with methanol in a base-, acid-, or enzyme-catalyzed reaction to form fatty acid methyl esters (FAME) that are commonly known as biodiesel.

Bioethanol is produced from materials containing starch or sugar. The steps involved in the bioethanol production process are as follows:

(a) use of yeast cultures to carry out microbial fermentation of the sugar in the biomass
(b) purification and concentration of the acquired raw ethanol through rectification/distillation
(c) dehydration of raw ethanol, for example, using molecular sieve technology

The entire process is well established and is extensively used for commercial purposes. Cellulosic ethanol produced from lignocellulose-containing waste materials will find more attention in the near future, specifically due to its favorable carbon footprint. However, it has been produced only in pilot plants to date.

High Quality Standards Help to Prevent Damage

The fuel system of a vehicle is commonly affected when the fuel quality degrades due to the presence of a high amount of water and also due to the presence of alkaline earth metal or alkali metal ions as well as chloride and sulfate ions. Biofuels that are based on fatty acids exhibit a heightened risk for polymerization at higher temperatures due to the presence of nonferrous metals or in the absence of antioxidants.

To eliminate such issues, the motor vehicle and biofuel manufacturers should strictly stick to the quality guidelines specified in international standards, which set minimum requirements in the manner of limits for impurities and test procedures for pure biofuels, biofuel blended with fossil fuels, and for biofuels used as a blend component. A considerable number of such test procedures can be performed by using Metrohm instruments and Metrohm application techniques, not only in the laboratory but also at-line and online in the process environment.

Table 1 and 2 for biodiesel and bioethanol indicate the capability of Metrohm instruments. Here, the symbol “√” denotes an alternative test method for monitoring the limits.

Table 1. Requirements and test methods for biodiesel

Test method Method EN 14214
EN 14213
ASTM D 6751
ASTM D 7467
EN 590
Acid number Titration EN 14104 ASTM D 664
Iodine number Titration EN 14111
Water content KFTcoulometric EN ISO 12937 EN ISO 12937
Oxidation stability Oxidation stability EN 15751
(EN 14112)
EN 15751
(EN 14112)
EN 15751a
Free and total glycerol content Ion chromatography ASTM D XXXXa
Alkali metal and alkaline earth metal content Ion chromatography
Antioxidant content Ion chromatography recommended recommended recommended
Sulfur content CIC

KFT = Karl Fischer Titration, CIC = Combustion ion chromatography
a Applies to pure biodiesel and blends that contain at least 2% (v/v) biodiesel
b Test method is currently being validated in a round robin test and is only referenced in ASTM D 6751

Table 2. Requirements and test methods for bioethanol

Test method Method EN 15376 ASTM D 4806
ASTM D 5798
pHe value pHe value EN 15490 ASTM D 6423
Conductivity Conductivity DIN 51627-4
Total acid number and acidity Titration EN 15491 ASTM D 1613
Total inorganic sulfate content Titration EN 15484 ASTM D 512
Water content KFTcoulometric ASTM D 7318
Water content KFTcoulometric
EN 15489
ASTM E 1064
ASTM E 203
Inorganic chloride content Ion chromatography EN 15492 ASTM D 7319
ASTM D 7328
Total and potential inorganic sulfate content Ion chromatography EN 15492 ASTM D 7319
ASTM D 7328
Sulfur content CIC
Copper content Voltammetry

KFT = Karl Fischer Titration, CIC = Combustion ion chromatography

Minimum Requirements and Test Methods for Biodiesel

Biodiesel is sold not only as a pure fuel but also in blends with fossil fuels. The standards EN 14214, for pure fuel and blend stock, and ASTM D 6751, for only blend stock, indicate the minimum requirements for biodiesel. EN 14213 specifies the minimum requirements for biodiesel that is used as heating oil. EN 590 relates to diesel fuels comprising up to 7% biodiesel. ASTM D 7467 relates to diesel fuels comprising 6%-20% biodiesel.

Minimum Requirements and Test Methods for Bioethanol

The EN 15376 and ASTM D 4806 standards specify the minimum requirements for the use of bioethanol as a blend component in gasoline. ASTM D 5798 pertains to ethanol-gasoline blends E75-E85.

Water Determination According to Karl Fischer

The test methods enlisted in Tables 1 and 2 are explained in greater detail elsewhere. This article describes the method of water determination using Karl Fischer titration. The calorific value of biofuels is reduced and the corrosion rate is increased due to the presence of water. Metrohm’s novel 915 KF Ti-Touch can also be used for water determination.


Water as a Contaminant

The presence of water initiates hydrolyzation of the ester bonds of the fatty acid methyl esters, causing fatty acid formation. These fatty acids consume additional NaOH and form soaps, which make the elimination of glycerol highly difficult.

Biodiesel with high water content obviously has lower oxidation stability, which consequently increases the probability of oxidation products forming in the long run. Such oxidation products form deposits and as a result damage the engine, specifically the fuel injection system.

Maximum Permissible Water Content

The aforementioned drawbacks are eliminated by the EN 14214 standard, which limits the water content in biodiesel to about 500 mg/kg. The test method EN ISO 12937 indicated in EN 14214 reports the coulometric Karl Fischer titration to determine the amount of water.

Coulometer and Titrando

Metrohm’s 756 and 831 KF Coulometers and its 851 and 852 Titrando (Figure 1) easily satisfy all of the specifications put forward in the standard and are highly appropriate for this application. In most of the instances, the sample can be directly injected into the reaction solution. Xylene (dimethylbenzene) is added to the KF reagent to enhance the solubility of the samples.

Figure 1. 852 Titrando with coulometric and volumetric titration cell for determining trace amounts of water.

Karl Fischer Oven Method

In certain biodiesel fuels, there are additives that can participate in side reactions during the direct coulometric Karl Fischer titration. For such fuels, Metrohm suggests that the biodiesel sample should not be injected directly into the reaction solution.

Conversely, a Karl Fischer oven should be used to remove the water contained in the biodiesel. For this purpose, a 860 KF Thermoprep is appropriate. The water is removed at 120°C and the biodiesel fuel is sent to the titration cell of the KF Coulometer in a stream of carrier gas, such as inert gas or dry air. This procedure can be entirely automated using the 874 USB Oven Sample Processor (Figure 2).

Figure 2. 874 USB Oven Sample Processor with 851 Titrando for the automatic determination of the water content in samples containing oil. Side reactions and contamination of the KF coulometer cell are avoided.


Gasoline-Alcohol Blends

Bioethanol is blended with gasoline at different ratios to reduce the demand for gasoline as well as the environmental pollution. This gasoline-alcohol blend is known as gasohol in the United States and as Gasolina Tipo C in Brazil. In the United States, the commonly used mixtures are E10 and E85, which respectively comprise 10% and 85% bioethanol. However in Brazil, most of the blends contain 21%-23% of bioethanol.

Test methods

The ASTM E 1064 is the standard test method for water in organic liquids by coulometric Karl Fischer titration, and EN 15489 standards specify the coulometric Karl Fischer titration to determine water content. These processes can be performed using Metrohm’s 901 Titrando (Figure 3).

Figure 3. 901 Titrando with 900 Touch Control and 803 Ti Stand.

If the water content is greater than 2%, then volumetric titration as per the ASTM E 203 standard is the recommended test method. Metrohm’s volumetric KF titrators satisfy all of the specifications put forward by these standards and are highly appropriate for this application.

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.

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