Pushing the limits of FCC gasoline desulphurisation
The goal of FCC gasoline desulphurisation to desulphurise FCC gasoline with minimum octane loss. The process uses conventional naphtha hydrotreating and selective catalysts to minimize olefin saturation and octane loss.
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This process has been studied since the 1970’s, but it didn’t get off the ground commercially until 1999 with the introduction of clean fuels mandates in the United States. There are now 300 such units in the world. Most of them are Scanfining or Prime G units licensed by ExxonMobil and Axens. The units are often called “gasoline post-treaters”. There is continued interest in increasing the performance of these units, especially with Tier 3 gasoline regulations coming into effect in the United States. Tier 3 regulations require most refiners to make an average 10 ppm S gasoline starting in January 2017.
FCC gasoline desulphurisation catalysts are designed for olefin retention selectivity. Figure 1 is an excerpt from an ExxonMobil presentation that shows the idea nicely. It is a chart of % olefin saturation versus % desulphurisation for two catalysts. You want to get your target desulphurisation with minimal olefin saturation, so that more high-octane olefins remain in the desulphurised product.
This chart shows that the RT-235 catalyst is more selective, that is it gives lower % olefin saturation for a given level of desulphurisation. This kind of chart pretty much defines the performance of a selective desulphurisation catalyst, and we will be looking at this kind of data through this whole article.
Independent Catalyst Test Report 2015 is the annual report on the work we did in 2015 which involved side by side pilot plant testing of competitive catalysts for this process. In it, we have ranked the catalysts on activity and selectivity for use in catalyst selection.
Our report also includes competitive analysis based on study of open literature on this process, and it includes new insights on the process that will be valuable to those interested in meeting Tier 3 gasoline requirements. Now we will show you some of the results from our pilot plant tests.
Figure 2 shows product sulphur versus time for the first 12 days of our pilot plant test. Data are shown for two catalysts, one in green and another in white. The test is a 20 day test using a sequence of five test conditions, each test condition is held for four days. The first 4 days are at a low temperature which we call the cold condition, then we increase the temperature to the warm temperature for days 5-8, and then hot temperature for days 9-12.
You see here, as we go from cold to warm to hot temperature, the green catalyst goes from 80 ppm product sulphur on day 4, to 40 ppm on day 8, to 20 ppm on day 12 at the hot temperature. The white catalyst is much more active for desulphurisation. At the same 3 temperatures, it produces product sulphur of 40 ppm, 15 ppm and 8 ppm.
So you see our tests covered a wide range of product sulphur, from nearly 100 ppm product sulphur to under 10 ppm product sulphur, as we varied temperature at 3 levels, for test catalysts that have different activities.
This chart is all about desulphurisation activity, what about olefin retention?
We again see the curves from the ExxonMobil presentation (Figure 3), for the RT-225 catalyst and RT–235 catalysts, they show the % olefin saturation charted against % desulphurisation.
We converted our product sulphur data from the last chart to percent desulphurisation, and plotted our four data points as percent desulphurisation on the horizontal axis, and we show here the % olefin saturation we measured, and these fall along the RT-225 curve. The white data point, for the more active catalyst at the cold temperature also falls along the same RT-225 curve. So we say the green and white catalysts have the same olefin retention selectivity, even though they have very different desulphurisation activity.
With the white catalyst we tested, you can get the same desulphurisation, and the same olefin retention selectivity at much lower temperature, compared to the green catalyst.
Now if you’re still with me, (ASK), you realize we also have data for the white catalyst at the higher temperature. Where is that data? Well it falls much higher up and to the right on this chart, and we end up getting squashed in the corner. So we will show those data points shortly but they will be plotted on a different kind of chart.
We call Figure 4 a landscape chart because it shows a broader perspective on activity and selectivity. Instead of plotting % desulphurisation on the x axis, we are showing the product sulphur, in ppm on a logarithmic scale. This gives plenty of space to the left to see what happens as we move deeper in desulphurisation. On the vertical axis, we have the %olefins remaining in the product. This is not percent olefin saturation, it is the absolute amount of olefins remaining in the product. This is how we will look at activity and selectivity data from now on. On this chart, going left means higher conversion, and a higher curve means more olefins retained, means a more selective catalyst.
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