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Mar-2020

A reliable solution to measure chlorine in catalyst

Over the years, refineries have fine-tuned their production methods to maximise efficiency while ensuring a higher-quality product.

Satbir Nayar and Leslie McHenry
XOS
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Article Summary
One such example is an increase in the use of catalysts which speed up reactions as crude oils work their way toward becoming finished products. As the use of catalysts became more commonplace, refineries quickly realised that these reaction-inducing substances were rapidly deactivating due to the naturally occurring metals commonly found in crude oil, decreasing the efficiency of their operations. To mitigate this and save time and money, refinery labs are assessing the content of these metals in crude oil to ensure that catalyst fouling is kept to a minimum. However, another important aspect of assessing catalyst efficiency requires a closer look at another element: chlorine.

Challenge
In addition to keeping catalyst fouling under control, refiners are also challenged with determining catalyst lifespan. Chlorine content, depending on its level and the type of feed in question, will impact the effectiveness of the catalyst, and thus, the refining process. Therefore, one can determine how long their catalysts can be used before becoming spent or needing regeneration by monitoring chlorine concentration over time. Essentially, refiners can optimise their catalyst quality by measuring metals in crude, and then assess the payoff of those optimisations in real-time by measuring chlorine concentration trends in the catalysts themselves.

Solution
UOP 979 is a well-accepted test method for determining the total chloride content of fresh, regenerated, and spent alumina-supported catalysts by wavelength dispersive X-ray fluorescence (WDXRF) spectroscopy. UOP 979 also includes energy dispersive XRF (EDXRF) spectroscopy, provided the EDXRF instrument meets the precision criteria (as stated in Note 3 of the method). Petra MAX is powered by monochromatic EDXRF (HDXRF®) and meets the precision requirements for UOP 979, and is therefore a viable solution for determining chloride content in alumina-supported catalysts.

In this paper, we will demonstrate that Petra MAX meets the repeatability and site precision criteria of UOP 979 based on real-world data.

Sample Preparation
XRF analysers function best when analysing homogeneous samples, as the measurement results come from a relatively small focal point on the sample. For finished liquid hydrocarbon fuels, such as diesel or gasoline, this is generally not an issue as they tend to be homogeneous.

Unfortunately, catalyst tends to be inherently non-homogeneous, which is why it is necessary to grind the sample to a fine powder and mix as best as possible before analysis. For best results, use a laboratory grinder to grind catalyst samples to 325 mesh. XOS also recommends tapping the ground catalyst in the sample cup prior to analysis, as this helps to compress the powder which minimises air gaps between particles and makes a more homogeneous sample at the analyser focal point.

However, even grinding and thoroughly mixing the sample followed by tapping the sample cup may not be enough to ensure consistent results if the chlorine is still not homogeneously distributed in the sample. For this reason, XOS also recommends repeat analysis of the sample with multiple focal points using the following procedure:

• Prepare a catalyst sample using the best practices described above. Tap the filled sample cup on its side to compress the powder, then introduce into Petra MAX in the correct orientation (using a vent clip for autosampler analysis). Measure for 100s.
• Prepare a second sample following the same process as above, or, using the first sample, shake to mix the powder then re-tap the sample as before. Insert into Petra MAX and measure for 100s.
• Prepare a third sample or re-analyse the first sample again using the procedure above.
• Report the average of the three determinations as the measurement result. This will ensure that the user gets a more accurate value, that is, a value that is more consistent with the true value of the entire sample.

Experiment
Four catalyst samples—two spent samples and two regenerated samples—were prepared and analysed on two Petra MAX instruments at one location by one operator. Each of the samples were measured twice on each analyser for two consecutive days. Note that per XOS best practices, a single test measurement result is the average of three 100s determinations. Between each determination, the sample cup was removed from the analyser, shaken, and re-tapped before measuring for a second and third 100s determination. Before we look at the results, let’s review UOP 979 method precision.

The precision of the UOP test method was developed by a multi-analyser study run on a spent, a regenerated, and a fresh catalyst sample. (Note: A fresh sample was not available for the Petra experiment and so further discussion is limited to spent and regenerated catalyst samples.) The precision is presented in the method as a series of allowable differences based on the wattage of the WDXRF analyser and sample type. These differences are summarised in Table 2.

To comply with the precision of UOP 979, the difference between sample measurements must be less than or equal to the maximum allowable difference. Note that the lowest allowable difference is not a minimum specification. Instead, it is the lowest allowable difference reported in the method precision study. Therefore, differences between sample measurements that are lower than this lowest allowable difference indicate precision that is better than the UOP 979 method.

Results
As seen from the data in Table 3, the repeatability for all four samples run on both Petra MAX analysers was less than or equal to the lowest allowable difference, indicating that the data not only meets UOP 979 but has better repeatability than the method. Site precision data for Analyser 1 was also less than or equal to the lowest allowable difference in the method 100% of the time, indicating better precision than the method. In addition, site precision data for Analyser 2 was lower than the maximum allowable difference stated in the method 100% of the time as well, indicating that the analyser precision is compliant with UOP 979.
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