Determination of total sulphur in biofuels from feedstock to endproduct according to ASTM D5453

Sulphur is a natural component in both crude oil and biobased feedstocks that will be present in diesel and gasoline unless it is removed during the production process. Sulphur in fuels contributes to air pollution, so lowering the Sulphur content in these products contributes to the reduction of air pollution and further control of emissions.


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Article Summary

Hydrotreatment or hydrocracking is a process used after the pretreatment steps to upgrade the bio feedstocks into biofuels. This process not only converts the relatively large and complex molecules into molecules of the size and boiling point range of conventional fuels, but it is also used for the removal of heteroatoms including Sulphur (S) and Nitrogen (N). The analysis of these species in the feedstocks is used to control the treatment process and determine content in the final product. Final product testing is required for eventual product release.

In general, the first generation of biofuels derived from vegetable oils contain low concentrations of Sulphur. Second generation biofuel feedstocks can come from many different sources, such as animal fats and used cooking oil, and can have widely ranging Sulphur concentrations. To optimise the process and ensure end product compliance, it is very important to obtain analytically correct results. The many different feedstocks and products can complicate the analysis, as a single analyser might not be able to easily handle samples that are solid, liquid, or gaseous without any extra sample preparation.

Measuring principle
Liquid samples are injected, by a fully automated liquid sampler, into a sample boat. Solid samples are weighed into this sample boat and placed on the boat carrier. The sample boat is then introduced into the combustion tube at a controlled speed. The combustion tube is heated by a furnace to 1050°C. The Sulphur bound components are vaporised and combusted. The released Sulphur is oxidised to Sulphur dioxide (SO2) in an oxygen rich atmosphere.

A stream of inert gas (helium or argon) transfers the reaction products, after removal of the water vapour produced, to a reaction chamber. Here the SO2 molecules are excited by the absorption of energy of a UV source and emitting light (fluorescence) while it relaxes to a stable state.

A Photomultiplier tube measures the emitted light and converts it into an electrical signal.

The response signal is integrated to calculate the area. The Sulphur concentration of an unknown product is calculated using the linear regression function of the concentration of standard mixtures versus integrated area.

Horizontal ElemeNtS
In 2018 PAC successfully introduced the Antek ElemeNtS for total Sulphur and Nitrogen analyses in liquids and gases. The standard method requirement of a boat-inlet introduction, as well as the ability to analyse viscous liquids and solid samples, have led to the development of the horizontal configuration of the ElemeNtS platform.

The horizontal ElemeNtS offers the same benefits as the vertical configuration. The ability to use the 749 ALS for high liquid sample throughput and the use of the PAC Accura for accurate gas and LPG injection. The 10” touchscreen on the front offers full control of the instrument in addition to the automated vacuum and pressure tests for easy leak detection. The front maintenance door allows easy access to the consumables, eliminating the need to access the back of the instrument. In addition, the vertical and horizontal configurations share about 90% of their parts, eliminating the need for different stocks of spare parts and consumables.

Analytically the horizontal ElemeNtS is very similar to its vertical counterpart. It has a wide linear dynamic range of up to 104 for Sulphur, allowing for a single calibration curve of 0.1-1000 ppm. The working range is up to 1% mass. Its superb repeatability and excellent precision ensure it meets requirements. Each instrument is factory tested with round-robin samples, covering the range of products as defined in the method scope, and compared to the accepted reference value (ARV). The limit of detection is calculated according to ISO11843 and is <100 ppb for horizontal ElemeNtS.

The Antek ElemeNtS total Sulphur analyser system and methodology is rigorously tested for response linearity, precision, and accuracy, to validate its performance according to ASTM D5453.

Two calibration curves were constructed to cover the complete range of samples. One for liquids, using dibenzothiophene in iso-octane standards, from 0 to 100 mg/L Sulphur, and another for solids using polymer standards from 1 to 500 mg/kg Sulphur. Results are displayed in tables 1 and 2.

The method editor in the IRIS software allows the technician to shorten or lengthen the calibration, depending on the concentration of the sample. Because of this option, only 1 method with corresponding calibration is necessary to quantify a wide range of samples with different concentrations of Sulphur.

Several biofuel feedstock samples ranging from first-generation feedstock such as palm oil, to second generation feedstock such as used cooking oil and waste animal fat are analysed. The 2 used cooking oils were analysed with the 749 autosampler, the palm oil heated and injected as a liquid as well. The solid samples (2 animal fats) were weighed into the sample boat (~20mg) and placed on the boat carrier before analysis.

Samples were analysed at least three times, with two samples analysed 10 times in a row to determine the repeatability standard deviation. This repeatability is compared to the ASTM D5453 repeatability although, strictly taken, solid samples are not included in its scope. Results are displayed in tables 3 and 4.

There are several ways that biofuel feedstocks can be converted to a usable state:
- First generation biofuel feedstocks, like palm oil, are commonly trans esterified to produce fatty acid methyl ester (FAME), which can then be blended into traditional petroleum diesel.
- Hydrocracking is also commonly used to obtain hydrotreated vegetable oil (HVO), which has chemical properties very similar to normal diesel.
- Crops such as corn and sugar cane are used to produce ethanol through fermentation, which can be blended into gasoline.

The samples analysed are all liquid, as can be expected for material that is to be blended into diesel or gasoline. Three FAME’s derived from used cooking oil (UCO), palm oil (PO) and animal fat (AF) were analysed as well as two ethanol samples. The ethanol samples were spiked with either an organic (Org) or an inorganic (InO) Sulphur compound and the recovery determined. Results are displayed in tables 5 and 6.

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