Nov-2019

FCC additive technology for SOx reduction

Advances in metal dispersion and structure delivered a SOx reduction additive that is driving down refiners’ FCC emission levels.

COLIN BAILLIE
W. R. Grace & Co.

Article Summary

The control of sulphur oxides (SOx) is required at an increasing number of oil refineries worldwide. In North America, agreements between the EPA and refiners, known as consent decrees, typically require refiners to reduce FCC unit SOx emissions to less than 25 volppm. In Europe, more stringent environmental legislation started to come into effect in 2018 with BREF regulation, resulting in more SOx additive applications. SOx additives are also used to a lesser extent in Latin America, the Middle East, and Asia Pacific, but as regulations are set in these regions (combined with EU BREF legislation coming into full effect), global demand for SOx additives is expected to increase in the short-to-medium future.

FCC SOx reduction additives are a cost-effective and versatile means of controlling FCC SOx emissions, and are well established as one of the best available technologies. This is highlighted by the fact that more than 130 refiners globally have used Grace’s SOx additives.

Super DeSOx CV+ additive
During the process of SOx additive development, it is important to consider that there are three key steps in the SOx reduction mechanism. The first step occurs in the regenerator, where sulphur contained in the coke on the catalyst is oxidised to about 90% SO2 and 10% SO3. In this regards, cerium plays a critical role in promoting the full oxidation of SO2 to SO3 in the presence of oxygen. In the second step in the reaction mechanism, SO3 is captured by a magnesium species to form magnesium sulphate in the regenerator. The third and final step occurs in the FCC reactor/stripper section, where magnesium sulphate is reduced to hydrogen sulphide. Here, both the magnesium and vanadium functionalities facilitate this additive regeneration step.

In 2018, Grace made a modification in its SOx additive manufacturing facilities, requiring significant capital investment. This has resulted in an improved version of its Super DeSOx additive for SOx reduction, with improvements to both product properties and performance. The improved SOx additives are described as CV+ grades, which signifies an improvement in cerium and vanadium dispersion across the additive particle. Cerium and vanadium dispersion plays an important role in SOx additive performance by enhancing the effectiveness in their key roles described. Figure 1 shows DCR pilot plant testing comparing SOx reduction for SOx additives with lower vanadium and cerium dispersion versus the same SOx additives with higher levels of vanadium and cerium dispersion. The pilot plant testing is initially operated without SOx additive to establish the baseline level of SOx, and then at ‘Time = 0 hours’ the SOx additive is introduced in a single dose to observe the initial level of SOx reduction. The subsequent period of time that it takes for the SOx emissions to increase back to the baseline level of SOx is used as a measurement for the additive’s effectiveness for SOx reduction. The additives with high and low vanadium dispersion were tested at the standard level of 1 wt% oxygen, and the results highlight that both additives show the same initial level of SOx reduction, but the additive with a higher level of vanadium dispersion retains the SOx reduction activity for a longer period of time. When the additives with high and low cerium dispersion were tested at the standard level of 1 wt% oxygen, both additives showed a similar profile for SOx reduction. However, when tested at a lower oxygen level of 0.2 wt%, the additive with a higher level of cerium dispersion shows an improved capability for SOx reduction. The testing highlights the importance of vanadium dispersion in full burn operations, as well as the benefits of improved cerium dispersion for lower oxygen or partial burn applications.

Super DeSOx additives are unique in that they incorporate magnesium-aluminate (MgAl2O4) spinel technology. One of the founding SOx additive developmental publications describes that a stoichiometric amount of magnesium-aluminate spinel with free magnesium oxide (MgAl2O4•yMgO; y = 1) provides the maximum SOx pick up activity (see Figure 2).1 SOx pick up activity starts to drops off when higher than stoichiometric amounts of free MgO are present (MgAl2O4•yMgO; y > 1). In addition, the effect of reduction temperature in the riser on the sulphated additives was studied, and the results show that the reduction of MgSO4 to H2S is easier and requires lower temperature for the spinel based additive compared to the MgO based additive (see Figure 2). This highlights the importance of spinel technology for maximum SOx reduction performance.

Super DeSOx CV+ additive has improved cerium and vanadium dispersion across the additive particle, and has an optimised spinel formation, resulting in improved SOx reduction performance compared to the non-CV+ grades. This is highlighted by the DCR pilot plant testing shown in Figure 3.

Performance at Refinery A
Refinery A’s FCC unit trialled Super DeSOx CV+ additive in March 2019. Figure 4 shows the operating data from January 2018 to June 2019. During the previous period of using a SOx additive from an alternative supplier, SOx emissions were maintained at low levels, with the average value being 9 ppm. Switching to Super DeSOx CV+ not only maintained the very low SOx levels; a reduction to an average value of 4 ppm was observed. This reduction in SOx emissions is even more impressive considering that the SOx additive addition rate was reduced by approximately 50% with the use of Super DeSOx CV+. The base level of uncontrolled SOx (the level of SOx that would be obtained without the use of SOx additive) is calculated based on slurry sulphur levels. This allows the mass of SOx being captured to be calculated, which, when divided by the mass of SOx additive being used, gives a measure of the SOx additive effectiveness, known as the pick up factor (PUF). Figure 5 highlights that the PUF increased considerably with the switch to Super DeSOx CV+, demonstrating the improvement in SOx reduction performance compared to the previous additive being used.

The SOx emissions obtained using the previous additive versus Super DeSOx CV+ at comparable operating parameters is shown in Figure 6. The switch to Grace SOx additive provided lower SOx emissions when evaluated against constant levels of additive rate, excess oxygen, feed sulphur and slurry sulphur.

A summary of SOx additive performance is shown in Table 1. Super DeSOx CV+ is providing lower SOx emissions at a lower additive addition rate, which has resulted in a significant increase in PUF. Some of the key operating parameters that can influence SOx emissions when using additive include feed sulphur, slurry sulphur and excess oxygen. Table 1 highlights that these variables were in a similar range for both additives analysed.

The switch to Super DeSOx CV+ additive has been successful and it continues to be used to control SOx emissions effectively while achieving lower opex costs due to the reduced additive addition rate.

Super DeSOx CV+ performance vs Super DeSOx
Refinery B switched from Grace’s Super DeSOx additive to the improved version (Super DeSOx CV+) in June 2018. As Table 2 shows, average feed sulphur and slurry sulphur levels were very similar during the trial period, resulting in a similar level of uncontrolled SOx as calculated using a refinery correlation. In addition, excess oxygen levels (which can impact additive performance) were similar.

SOx emissions dropped by 22% with the switch to Super DeSOx CV+. This translates to an improvement in SOx reduction levels from 47% to 59% (see Figure 7).