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Jan-2018

Ultra low sulphur analysis in liquid petroleum using MWDXRF

Tougher regulations are challenging refiners to produce higher quality products while trying to maximise efficiency.

Kyle Kuwitzky
XOS
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Article Summary
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. Hydrotreating catalyst life depends on the feed and operation of the unit. Increased monitoring will be critical in meeting these requirements and maximising efficiency. WDXRF has proven to be a fast, easy, and precise method to measure sulphur in hydrocarbon streams. 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.

The availability of different crude types has increased in recent years. Heavy, sour crudes provide a cost advantage as these are priced lower. Refiners have modified equipment and operations to accommodate this lower-cost crude. Refiners modified equipment and operations to accommodate the cheaper crude. Figure 1 illustrates the trend of US sulphur content and API Gravity from 1985 through 2015. Crude trended heavier and sulphur content increased as technology improvements were made to suit these cost-effective feedstocks. Advances in drilling techniques led to readily available light tight oils (LTO) to serve the refining market and lower dependence on less-stable crude sources. Although these feeds were lighter and had little sulphur, they pose other challenges to refiners. Specifically, they tend to be high in waxes and are prone to fouling.

As crude sulphur levels increased, refiners invested heavily in sulphur removal. This was done to capitalise on the cheaper high sulphur crude as well as meet increasingly stringent sulphur specifications on finished products. Between 1985 and 2015, desulphurisation capacity nearly doubled from 8.9 million barrels per stream day (BPSD) to over 17.3 million barrels per stream day. Figure 2, shows the relationship between US crude sulphur levels and US refinery desulphurisation capacity.

For the implementation of low sulphur regulations, refiners are looking at increasing desulphurisation at the fluid catalytic cracking unit (FCCU). The entire feed can be pre-treated or the gasoline can be post-treated, or a combination of both. Careful consideration must be given to factors such as hydrogen availability, heat balance, catalyst type, incoming sulphur content, feed nitrogen content, and planned cycle life. Pre-treating feed provides several advantages for sulphur removal. Pre-treating will remove metals and nitrogen, which are poisons to the FCCU catalyst. Additional hydrogenation from pre-treating will increase conversion in the FCC process. Conversely, pre-treating can be very expensive and may not be possible due to heat or hydrogen limitations. Post-treating the gasoline stream may be an easier option, although there is significant reduction of octane in the process. FCCU gasoline contains valuable olefins that contribute to the octane. Post-treatment will reduce the octane number of the gasoline by conversion of valuable olefins, which must be supplemented by reformate.

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. Crude slate can be modified to reduce total sulphur content and reduce strain on sulphur removal equipment. Crude swaps come at a cost, either in the form of higher purchase price, creating problems on other refinery units, or product yield.

Regardless of what method is utilised to produce gasoline 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.

Total sulphur methodologies and technologies

There are a number of different technologies available on the market for testing sulphur in liquid petroleum products, due to regulations and requirements around the world. Table 1 outlines the different relevant technologies and their correlating methods. Process analysers based on these technologies typically correlate to the respective laboratory method, or in some cases may have a method of their own.

In this paper, we will discuss the performance and precision of the D7039 method using MWDXRF technology. This technique utilises high-performing doubly curved crystal (DCC) optics coupled with a low-power X-ray tube creating a low maintenance, highly precise technology. MWDXRF is a simplified and highly robust X-ray technique which provides sub-1 ppm sulphur detection.

An MWDXRF analyser engine (Figure 3) consists of a low power X-ray tube, a point-to-point focusing optic for excitation, a sample cell, a second focusing optic for collection and an X-ray detector. The first focusing optic captures a narrow bandwidth of X-rays from the source and focuses this intense, monochromatic beam to a small spot on the sample cell. The monochromatic primary beam excites the sample and secondary characteristic fluorescence X-rays are emitted. The second optic collects only the characteristic sulphur X-rays and focuses them on the detector. The analyser engine has no moving parts and does not require consumable gasses or high temperature operations.

MWDXRF removes the scattered background peak created by the X-ray tube increasing the signal-to-background ratio (S/B) by a factor of 10 compared to conventional WDXRF technology. The S/B is improved by using the monochromatic excitation of the X-ray source characteristic line. Additionally, the focusing ability of the collection optic allows for a small-area X-ray counter, which results in low detector noise and enhanced reliability.

The WDXRF technique has been an accepted practice for measuring sulphur in petroleum liquids for many years. However, when the first regulations requiring sulphur in diesel moved to 15 ppm or less in 2006, improvements to the analytical instruments and a revision of the method was required to meet the need of new regulations. Similar evolution of the UVF method has taken place while EDXRF has not yet established itself as a viable ultra-low sulphur measurement technique. MWDXRF, on the other hand, was developed specifically to address the need of refiners and petroleum distribution partners for a simple measurement technique, ideally suited for single element, ultra-low sulphur measurements.

The D7039 method (MWDXRF) is essentially a subset of D2622 (WDXRF) with some important distinctions. The excitation X-ray beam of a WDXRF instrument is polychromatic whereas the MWDXRF excitation beam is monochromatic. For both, the output of the X-ray tube comprises the characteristic energy of the target element and the Bremsstrahlung spectral energy associated with the production X-rays by electron acceleration in a vacuum tube. The target element is chosen for a characteristic X-ray just high enough in excitation energy to produce X-ray fluorescence of the element of interest (sulphur), but low enough to minimise background scattering.

WDXRF instruments aim the multi-energy beam at the sample and the resulting beam is typically collimated and aimed towards a diffraction crystal, where it is then diffracted to a detector. Acting as a filter, the diffraction crystal is selected and physically arranged to direct the characteristic X-rays of the element(s) of interest towards the detector. The detector sees a spectral background with distinct peaks associated with the element(s) of interest rising above the background.
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