Analytical Instrumentation & Environmental Monitoring


Measuring sulphur content of fuel

May 2006 Analytical Instrumentation & Environmental Monitoring

The enactment of the US Clean Air Act Amendment (CAA) in 1990 set in motion changes the dimensions of which were unparalleled in the history of air quality control.

What this legislation in effect did was place a duty on motor vehicle fuel manufacturers and resellers to reduce refuelling, evaporative and exhaust emissions of ozone-forming compounds, carbon monoxide and other air toxins from motor vehicles.

A principal target of the legislation was the sulphur level in fuel. Sulphur compounds are natural components of the crude oil from which petroleum-based fuels are made. Studies have shown that sulphur compounds can diminish the efficiency and limit the oxygen storage capacity (the measure of catalyst performance) of an automobile's catalytic converter. This can lead to increased emissions of hydrocarbons, NOx, carbon monoxide, other air toxins and particulate matter. The detrimental health effects of the poor air quality created by many of these pollutants is well documented.

California shows the way

Most petrol sold in the United States has a relatively high sulphur level. According to a recent petroleum industry survey, the national average of sulphur in conventional (non-reformulated) petrol was approximately 350 ppm, with one quarter of samples containing levels greater than 500 ppm. Sulphur levels in diesel fuels are even higher. Typical levels in reformulated petrol are 150 ppm.

To combat these high levels, some regions of the world have introduced limits. California has the strictest standards, with average sulphur levels of 30 ppm for petrol (with a maximum cap of 80 ppm) and below 500 ppm for diesel fuels. Refiners and automakers are in agreement that hydrocarbon-based fuels must become cleaner. The debate between these two industries is over what the specific sulphur limits should be. It is becoming clear that sulphur limits in the USA of 5-50 ppm in both petrol and diesel fuel will become reality in the next few years. A further aspect has focused interest on sulphur levels in fuel: 'sweet' crude oil (oil with a low sulphur content, which tastes sweet) is the refiner's raw material of choice. However, as sources of sweet crude become harder to find, more and more 'sour' crude finds its way onto the market. The higher sulphur levels found in this oil contribute to the increasing average sulphur levels in the crude oil reaching the refineries. These factors, the higher sulphur level in the crude supply and the lower levels mandated for the refined products, have led to an increase in demand for reliable on-line measurement of low sulphur levels in fuel.

Petrol as well as diesel products are affected, and operations in North America, Europe, and Asia have similar requirements. Until recently, it was usual for only offline laboratory test methods to be used to determine low sulphur levels in fuel. This was because very few on-line process analysers can measure the total sulphur accurately and reliably at the low levels now being proposed or enforced. The technology used in the ABB PGC2007 total sulphur analyser changes that.

Choosing the right technology

For many years, during which most parts of the world had mandated levels in the hundreds or even thousands of parts per million (ppm), many of the measurement methods in use were quite capable of quantifying sulphur in fuels. However, the new, much lower limits present a significantly greater challenge, and with environmental regulations continually evolving even more stringent limits for total sulphur in petrol and diesel fuel may be expected in the future. In response, various technologies for petrol and diesel fuel analysis have appeared on the market. These feature technical approaches as varied as X-ray energy dispersion, lead acetate tape, UV fluorescence (UVF) and gas chromatography, each with its specific advantages and limitations.

Most of these measurement techniques are laboratory methodologies that have not been easily or reliably adapted to on-line measurement use. In 2002, ASTM round-robin tests were performed on X-ray fluorescence, microcoulometry, UV fluorescence and the electrochemical methods of sample analysis. Seventy labs were involved and over 5600 data points were collected. What this very extensive test showed was that two of these methods were unable to meet the requirements and the other two were not as precise as hoped for. Obviously, another technique was required if refiners are to ensure low sulphur levels in their products.

ABB and sulphur analysis

In 1975, ABB Analytical introduced the first process gas chromatograph incorporating flame photometric detection. In the meantime, hundreds of sulphur applications (targeting both species and total sulphur) in complex stream matrices have used this basic hardware design. Less than two years after the US Clean Air Act set the limit for total sulphur in reformulated petrol, ABB introduced the Vista Model 3107 Fuel Sulphur Analyser for on-line analysis of total sulphur in petrol. In 1997, the Model 2007 replaced the Model 3107.

Given all this experience, designing an apparatus to take care of these new low-level measurements might be considered a relatively simple affair. However, this is not so. ABB has also had to invest substantially in the refinement of its hardware and measurement methods in order to achieve the required performance at these low levels. The investment has nevertheless paid dividends, having produced an updated analyser and method capable of measurement ranges as low as 0 to 10 ppm.

An example of a chromatogram from the PGC2007 showing the separation of CO<sub>2</sub> and SO<sub>2</sub>
An example of a chromatogram from the PGC2007 showing the separation of CO2 and SO2

The problem

Measuring such a straightforward substance, even at these low levels, hardly seems difficult. But it is. For a start, sulphur compounds seem to be everywhere and in everything, so minimising carry-over or cross-contamination in samples is no easy matter. Special attention must be paid to the selection of construction materials. Not only in those parts of the analyser that are exposed to the sample, but even in the sample transport tubing and sample conditioning hardware. Care must be taken during the design phase to minimise dead spots or tiny spaces that may hold up even the smallest amounts of sample. These small pockets of 'old' sample might contain higher levels of sulphur, thus contaminating the current measurement and producing wildly inaccurate results.

As for the analyser itself, extra diligence is necessary when handling parts, like the components of the injection valve, columns and detector, during assembly. Some metal surfaces are specially treated in a process known as silco-treating in order to make the surfaces that come into contact with the sample more inert or passive. Analysers must be calibrated with a known standard. To measure 10 ppm of sulphur in diesel fuel, a calibration standard for a hydrocarbon of similar density to diesel containing about 8-10 ppm sulphur, guaranteed, is needed. That means that a sulphur-free hydrocarbon that can be dosed with the appropriate amount of sulphur to produce the guaranteed standard is required. There is just one small problem: as mentioned above, there is not much in this world that does not contain sulphur. Finding these sulphur-free bases from which to make standards is a difficult task. The finished product has to be tough, too: often, a process analyser will have to operate continuously and unattended close to the sample point in hazardous and variable environments, and meet strict safety regulations (NEC, CSA, ATEX, etc). And there has to be full communication with the plant's process control system.

Simplicity: injection, oxidation, separation, measurement

The PGC2007 from ABB uses gas chromatography and an oxidation furnace for total sulphur measurement. Unlike some of the other more hardware-intensive methods mentioned, the ABB device uses the field-proven flame photometric detector (FPD), well known for its reliability and simplicity. The features and performance of the complete analyser system make ABB Analytical the leader in process chromatography for total sulphur.

The platform is familiar to refinery technicians; in addition, its simple furnace converter and proven FPD technology provide excellent analytical stability. An electronic pressure control (EPC) module helps the device meet the exacting demands made on it.

A sulphur analysis with the PGC2007 comprises three simple steps and takes about 5-6 minutes to complete:

ABB's liquid sample valve (LSV) is used to inject a fixed volume of liquid fuel stream. An air carrier transports the sample to the furnace, where it oxidises to carbon dioxide, water and sulphur dioxide. Specially packed columns separate these components, which then pass into the FPD for measurement. The flow rate through the LSV is varied, being at first low and then increasing over time to minimise tailing. Use of hydrogen or helium as the valve sweep gas improves the vaporisation of heavy samples.

The PGC2007 features a new method of flow control: the flow through the sample valve can now be controlled separately from the column, which improves response time, stability, detectability and linearity. Besides increasing sensitivity by optimising the hydrocarbon flow rates, the PGC2007 also eliminates the mixing chamber used to reduce peak sample delivery rate, and the separator valve used to compensate for poor separation of CO2 and SO2. Cross interference due to variation in sample composition is also reduced.

LSV features include a one-piece vaporiser assembly and low-tension load adjustments that allow for months of operation without additional adjustments. In a chromatograph, proper separation of the components in the sample prior to measurement is critical.

Repeatability

The measure of a process analyser's performance is its demonstrated repeatability. To obtain this a known sample is injected and measured repeatedly for several hours. The standard deviation of the measured value is an accurate representation of analyser performance and gives a good idea of how it will perform on process samples, in other words how close the analyser reading will be to the actual, correct value when used to monitor the level of sulphur in blended petrol or diesel.

It also tells us how confident an operator can be that the analyser will repeatedly give the same answer for the same actual level in the sample, whether the next day or the next week. The lower the measuring range (ie, 0 to 10 or 0 to 100 ppm), the more difficult it is to analyse with good repeatability.

For example, a very good analyser should be able to exhibit a repeatability of ±1-2 % of the full-scale range. On a 0 to 100 ppm range, the expectation would be 1 to 2 ppm shows repeatability data for both petrol and diesel with the PGC2007. The ABB PGC2007 performs with superior repeatability and reproducibility. In fact, work is currently under way to obtain ASTM designation of the application method used in the device. This would assure refiners of the capabilities required of low-level, reliable measurement and of a recognised measurement method.

Industrial IT enabled

Currently installed at over 100 locations in 18 different countries, the PGC2007 is gaining widespread acceptance as an accurate and reliable process analyser for total sulphur measurements. ABB's IndustrialIT architecture is being used increasingly in process plants around the world. This means that the PGC2007, being industrial IT enabled, can be easily integrated into any process plant where this architecture is used. Mandated low-sulphur specifications in hydrocarbon-based fuels have presented many technical challenges to both the refining industry and instrument manufacturers. The process gas chromatographic technique with flame photometric detection has not only met these challenges but also proved to be a reliable solution for the measurement of low total sulphur levels in fuels. The distinct advantage of this approach is its simplicity of operation, its reliability and repeatability, and its demonstrated capability to meet even today's low-level sulphur measurement requirements.

Gas chromatography

Gas chromatography is a technique used to separate or analyse mixtures of gases. The apparatus consists of a fine tube containing a stationary phase that may be an adsorbing solid or a non-volatile liquid coating on a solid support (gas-liquid chromatography).

For some analyses, the wall of a very fine column itself can act as the support medium. This is the packed column referred to in the text. In our case, the sample is swept through the column by an air stream, with appropriate pressure regulation. The sample components have different affinities for the immobilised stationary phase and so they are adsorbed to different extents and, therefore, pass through the column at different rates. The various peaks of the components are detected as they exit the column, for example by a flame photometric detector. Under known conditions, the components can be identified by the time it takes them to pass through the column. This detector signal, when recorded as a function of time, produces the familiar chromatogram.

Flame photometric detection

Different detectors are used in gas chromatography to achieve selective and/or highly sensitive detection of specific compounds. Invented over 30 years ago, the flame photometric detector (FPD) is the detector of choice for sulphur-containing compounds. In an FPD, the sample is burned in a hydrogen-rich flame (this is a different combustion from that occurring in the PGC2007 furnace). The optical emission from the excited chemical species, which emit in the region of 320-460 nm upon decay to the ground state, in the flame (chemiluminescence) are picked up and amplified by a PMT (thermoelectrically cooled to reduce thermal noise from heat or the dark current) and passed on to a computer for processing. The output signal is the familiar chromatograph curve, and reveals how much sulphur was in the original sample. The low-level sensitivity and linearity are enhanced by standard addition of sulphur.

For more information contact ABB South Africa, 011 236 7623.





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