The effects of sulphur pollution are well known: acid rain, and the damage it causes to the environment, is due mainly to anthropogenic emissions (emissions caused by human activity) of sulphur and nitrogen compounds. In addition, elevated levels of atmospheric sulphur also have a direct impact on human health.
In efforts to address these issues, governments across the world have acted to implement stringent legislation. For the oil industry this has demanded significant change. Routine use of desulphurisation technology during refining, for example, has contributed to the steady decline in total global sulphur emissions since around 1990.
However, there is no room for complacency and regulations are set to become even more rigorous from 2009 onwards. Accurate and precise measurement of sulphur in petroleum products has never been more important.
Already well established for the analysis of a broad range of elements in light matrices such as petroleum products, XRF now offers the sensitivity and resolution to monitor sulphur down to 1 ppm (1 mg/kg). Importantly, technological advances in X-ray fluorescence (XRF) spectrometry have expanded the role of this powerful technique.
Presented below are data obtained from a new energy dispersive X-ray fluorescence (EDXRF) spectrometer - demonstrating its effectiveness for ensuring compliance with the relevant standards for sulphur in automotive fuel.
The impact of sulphur on climate change and human health underlines the importance of monitoring its level in petroleum products. Accurate assessment of sulphur provides valuable data that will help improve our understanding of climate change mechanisms. A thorough knowledge of all variables that have a role, either positive or negative, is essential for the basis of future policy decisions.
Changes in the oil industry have resulted in dramatic reductions in sulphur emissions over recent years.
Petroleum desulphurisation during refining, which includes catalytic hydrotreatment and other chemical and physical processes, has had a major impact through the production of low sulphur fuels.
In parallel, car manufacturers have been developing engines with advanced emissions control systems that require the new low sulphur fuels.
A decline in the use of fuel oil for furnaces and power stations and for industrial and domestic heating has also been an important factor.
In the UK for example, sulphur emissions from petroleum sources alone have fallen by 94 per cent since 19701. Globally, the World Mineral Exchange estimates that sulphur dioxide emissions have decreased by 71 per cent from 1987 to 2001.
Today, government regulations impose extremely low limits for sulphur in fuel. For example, in the US, the Environmental Protection Agency (EPA) has mandated the use of ultra low sulphur diesel (ULSD) from model-year 2007. The allowable sulphur content for ULSD in the US is now set at 15ppm, down from 500 ppm.
The EU is in transition - from the current Euro IV standard of 50 ppm to a proposed Euro V regulation (Directive 2003/17/EC) which is due to come into force in January 2009. This will bring the limit down to just 10 ppm. In Asia, many national environmental bodies, those in Taiwan and Singapore for example, have adopted the Euro IV standard.
PANalytical's compact MiniPal 4 Sulphur EDXRF spectrometer (Fig.1) has been designed specifically for the non-destructive analysis for sulphur in oils and fuels. It is ideal for quality control and to ensure compliance with international norms, including ASTM, ISO, DIN and EPA.
The system is optimised for the analysis of sulphur, phosphorus and chlorine but it is also very capable of analysing other elements, ranging from sodium to lead.
It is equipped with a new silicon drift detector, which offers the best resolution available in its class, enabling improved sensitivity, high maximum count rates and lower limits of detection.
The data below demonstrate the use of the system to analyse low sulphur content in automotive fuels.
Preparation of standards
A series of commercially available oil standards was used to set up the calibration for the determination of sulphur in automotive fuels.
Five grams of each standard were put into a 15 ml liquid cell, assembled with a 2.5µm Mylar supporting foil.
Measurements were conducted using a MiniPal 4 Sulphur EDXRF spectrometer, equipped with a silver anode X-ray tube, three tube filters, a helium purge facility and a high-resolution silicon drift detector. Typical detection limit for sulphur in automotive fuel is 1ppm.
The measurement conditions are listed in Table 1. All analyses were performed in a helium atmosphere. The total counting time per sample was 10 minutes.
Fig. 2. shows the XRF spectra of six gasoline standards with sulphur concentrations of 0, 5, 10, 30, 50 and 75 ppm.
Even 5ppm sulphur in the gasoline standard can be distinguished from the blank.
The calibration curve for determining low sulphur content was set up using the regression model of the MiniPal software.
The calibration results are shown in Table 2 and illustrated graphically in Fig. 3. The root mean square (RMS) error, listed in Table 2, is an indication of the magnitude of difference between the measured concentration and the certified chemical concentration.
The results obtained show good correlation between the certified concentrations and the intensities. As shown in Table 2, also the lower limit of detection (LLD) determined with 3[g18] is in excellent agreement with the calibration RMS determined with 1[g18].
To illustrate the repeatability of the MiniPal 4 Sulphur, a sample with 10 ppm sulphur was measured 15 times. Each measurement used a freshly prepared sample and liquid cell. The average measured concentrations are plotted in Fig. 4.
These results emphasise the excellent analytical precision of the system at these low sulphur levels.
Standard methods from the ASTM and ISO define three distinct sulphur calibration ranges: 0-0.1 wt per cent, 0.1-1.0 wt per cent and 1.0-5.0 wt per cent. Conventionally, users select one of three calibrations for use when quantifying a particular unknown sample.
Now, the latest MiniPal software includes automatic program selection (APS). This automatically selects the best calibration program, without the need for operator interaction, depending on the sulphur concentration of the unknown sample.
Scientific evidence proves beyond doubt that elevated levels of atmospheric sulphur cause many harmful effects to human health and the environment.
The need to reduce emissions is therefore vital, and the oil industry has been quick to respond, achieving dramatic reductions in a relatively short space of time.
However, anthropogenic sulphur emissions in general are still high enough to cause concern and governments are continuing to press for legislation that sets ever lower sulphur limits, presenting a challenge to all industries concerned.
Results show that the MiniPal 4 Sulphur EDXRF spectrometer is well suited for the analysis of low sulphur content in automotive fuels.
Resolution and sensitivity
The high resolution and sensitivity of the detector result in a low detection limit of 1 ppm, making the system suitable for current and future regulation.
Powerful software matrix correction and automatic program selection features harness the power of the detector and contributes to the accurate and precise results presented here.o
Taco van der Maten is Product Manager XRF with PANalytical BV, Almelo, The Netherlands.
1. Part of the UK National Atmospheric Emissions Inventory. Maintained by AEA Energy & Environment on behalf of Defra and the Developed Administrations.