ULTRA-LOW SULPHUR ANALYSIS IN PETROLEUM PROCESS STREAMS USING MWDXRF
Refiners are challenged to produce higher-quality products while trying to maximize efficiency. In the last decade, national regulators in places like the US, Europe, China, and India have implemented or plan to implement requirements for total sulphur in gasoline and diesel levels as low as 10 ppm. Increased hydrotreating and modifying crude slate are some of the levers that can be pulled to help lower sulphur levels in finished products. Simultaneously, there are global initiatives to increase the percentage of biofuels produced using renewable feedstocks. In order to meet these lower sulphur levels, refineries must invest in new or upgraded equipment, modify operations or a combination of both. Regardless, it will increase the cost of producing diesel and gasoline.
Figure 1: 2019 Global Maximum Sulphur Limits
In order to meet renewable fuel usage targets, new standalone biofuel refineries have been constructed to produce biofuel from a variety of feedstocks, and some traditional refineries have been upgraded to allow for biofuel feedstocks to be used, or completely converted into biofuel refineries. Biofuels present some interesting challenges from an online sulphur measurement standpoint. Some biofuels naturally have such low sulphur, there are few or no additional procedures or units required to further reduce the sulphur content, while others require typical sulphur removal units such as a hydrotreater. However, all biofuels must meet regulated sulphur specifications such as EN 14214, and must be measured and certified to such specifications before being sold.
In addition, it can be very valuable to know the specific sulphur content of a given biofuel being used for blending with traditional refinery fuels in order to reduce sulphur giveaway. Of course, it is also highly profitable to measure biofuels where they need to be treated to remove sulphur. A refiner can use a sulphur measurement to reduce hydrogen gas consumption and optimise catalyst usage. The challenge, then, is how to condition and measure biofuels, especially when the feedstock can change and alter the sample conditioning requirements.
Changing feedstocks will always cause complications, as a given analyser will typically be fine-tuned to operate with certain specifications such as pressure, temperature, flow rate, viscosity, density, composition, and filtration. This becomes readily apparent with analyser that require sample conversion before analysis. XRF analyser offer a unique resiliency to this as it is a non-destructive method that does not require the sample to be combusted, mixed with solvents, or adjusted for density to report ppmw properly. In addition to this, due to the nature of XRF, the analyser is resilient to sulphur concentration changes whether it is 10 ppmw, 100 ppmw, or 1000 ppmw, though a calibration and validation program designed for the measurement range of interest will always improve precision. For such a dependable system, the value the analyser provides then comes from its accuracy and precision.
For every part per million of sulphur removed, a refinery spends significant money on capital, hydrogen, catalyst, and energy. The related downtime to catalyst changeout must also be factored into the equation. Catalyst has a finite life, and that length of time is dictated by how the hydrotreater is operated. By varying temperature, space velocity, and hydrogen partial pressure, sulphur removal and catalyst life are impacted. A refiner may choose to vary the specific feedstocks used in order to reduce the total sulphur content entering a hydrotreater for the purpose of minimizing the required use of sulphur removal equipment, though this will come at a cost, either in the form of higher purchase price, or by higher concentrations of other contaminants.
Regardless of what method is utilized to produce biofuels at these lower sulphur levels, monitoring sulphur levels will be critical in controlling costs. Optimisation is dependent on knowing the sulphur levels at all times, accurately and reliably.