September 2021


Deliver high levels of SOx reduction with SOx-reduction additive

The global refining industry continues to move toward more restrictive environmental laws for sulfur oxide (SOx) emissions.

Baillie, C., Cooper, C., W. R. Grace

The global refining industry continues to move toward more restrictive environmental laws for sulfur oxide (SOx) emissions. In addition, many refineries are looking to maximize fluid catalytic cracking unit (FCCU) profitability by processing more challenging feedstocks that are higher in sulfur, or using SOx reduction additives to reduce caustic consumption in the wet gas scrubber for overall OPEX reduction. As such, SOx emissions control remains a crucial topic for refineries to ensure competitiveness in the market. The authors’ company recognizes this and has recently allocated considerable resources to developing the latest generation of SOx reduction additives.

A new FCC additive technologya for SOx reduction has been successfully commercialized. The SOx reduction additive incorporates a more homogeneous distribution of cerium and vanadium across the additive particle, as well as an increase in the magnesium aluminate spinel content of the additive. Since its introduction, several refiners have switched from general SOx reduction additives to the proprietary SOx reduction additivea due to the step-out improvements in performance.

The BP Rotterdam refinery recently began using the proprietary additive. This refinery initially switched from a general SOx additive to another of the authors’ company’s additivesb, followed by the aforementioned SOx reduction additivea. Both additive changes resulted in an increase in SOx reduction.

SOx additive development

In 2018, the authors’ company made a modification at its SOx additive manufacturing facilities, which required significant capital investment. The result is an improved version of its additiveb for SOx reduction, with improvements to both product properties and performance. The new SOx additive was initially referred to by another namec to signify the improvement in cerium and vanadium dispersion across the additive particle. However, based on commercial performance, the new additive technology is being brandeda to better reflect its step-out performance.

Cerium and vanadium dispersion play an important role in SOx additive performance. Improved dispersion of these key active ingredients results in a more effective oxidation of sulfur dioxide (SO2) to sulfur trioxide (SO3), and enhances the additive regeneration step where the sulfate is reduced to hydrogen sulfide (H2S), both of which are key steps in the SOx reduction mechanism.

For the development of SOx reduction additives, the authors’ company utilizes larger scale test equipment, specifically the Davison circulating riser (DCR) pilot plant.1 Such a pilot unit provides valuable information based on a continuous operation. Additionally, compared to bench-scale units, the DCR pilot plant has the advantage in that it mimics all the processes present in a commercial operation and can also operate at the same hydrocarbon partial pressure as a commercial unit. The continuous catalyst regeneration in the DCR allows for the measurement of regenerator SOx and nitrogen oxide (NOx) emissions and testing of environmental additives, experiments that cannot be done in a batch unit.

FIG. 1 shows DCR pilot plant testing comparing SOx reduction performance for SOx additives with less-dispersed vanadium and cerium vs. the same SOx additives with a higher level of dispersion. The pilot plant testing was initially operated without an SOx additive to establish the baseline level of SOx, then at “Time = 0 hr” the SOx additive was introduced in a single dose to observe the initial level of SOx reduction. The subsequent period of time required for the SOx emissions to increase back to the baseline level of SOx was used as a measurement for the additive’s effectiveness for SOx reduction.

FIG.1. Impact of vanadium dispersion on SOx additive performance.

The additives with high and low vanadium dispersion were tested at the standard level of 1 vol% oxygen (O2), and the results highlighted that both additives showed the same initial level of SOx reduction, but the additive with a higher level of vanadium dispersion retained the SOx reduction activity for a longer period of time.

SOx additive performance at BP Rotterdam

The BP Rotterdam refinery uses an SOx additive on an ongoing basis to comply with SOx emissions limits. In March 2018, the refinery elected to change from a general SOx additive to the proprietary SOx additiveb, later changing to the newly developed additivea. The following case study describes how these SOx additive changes impacted SOx emissions control.

The BP Rotterdam FCCU is a challenging operation in terms of SOx additive performance. Specifically, the low oxygen level in the regenerator is likely to be a limiting factor for SOx additive performance due to the oxidation of SO2 to SO3 being a bottleneck in the SOx reduction mechanism. This is reflected in the relatively low pick-up factor (PUF) values observed. It is also worth noting that the Rotterdam FCCU’s highly variable feed properties can make the evaluation of SOx additive performance more challenging. For this reason, a multi-variable regression analysis was performed. This statistical analysis seeks to establish the relationship between dependent and independent variables. Developing a robust multivariable regression allows the evaluation of additive performance despite many changes occurring simultaneously.

FIG. 2 shows the daily SOx additive usage rate from November 2017 through the SOx additivea trial. Using a seven-day (7-d) rolling average is a useful way to look at the SOx additive rate, as it helps to smooth out large differences in the daily addition rate, enabling a more meaningful comparison of PUFs. The average usage rate of the SOx additiveb was slightly higher than the previous additive due to pre-blending with fresh FCC catalyst, which coincided with a period of higher catalyst addition rate. The average addition rate for the latest SOx additivea was 15% lower than the general additive period.

FIG. 2. The 7-d average SOx additive rate (normalized).

The uncontrolled levels of SOx (i.e., the SOx level that would be obtained without using an additive) are calculated using a correlation based on the wt% sulfur in the net slurry product. FIG. 3 shows the uncontrolled SOx levels vs. actual SOx levels observed during the different periods (the values are normalized to the highest uncontrolled SOx level). The actual SOx levels were maintained at low levels during the use of the proprietary SOx additivesa,b. The overall SOx reduction increased from 67% to 70% with the use of one SOx additiveb, then increased further to 73% using the newest SOx additivea.

FIG. 3. SOX emissions level (normalized to highest uncontrolled SOX).

As a performance indicator when assessing SOx additive performance, the PUF is a calculation of mass of SOx captured per mass of SOx additive used, and is a measure of SOx additive effectiveness. A PUF value for a given application can fluctuate depending on various factors. For example, the PUF would be expected to decrease when achieving a higher percentage of SOx reduction as the concentration of available SOx molecules becomes lower.

Therefore, with the higher percentage SOx reduction achieved with the proprietary SOx additivesa,b, a lower PUF might have been expected. However, FIGS. 4 and 5 show that the expected drop in PUF was not observed. In fact, for a given percentage of SOx reduction, the additives even resulted in a slightly higher PUF, which is testament to the SOx reduction performance being achieved.

FIGS. 4 and 5. PUF plotted against time and % SOX reduction.
FIGS. 4 and 5. PUF plotted against time and % SOX reduction.

As previously mentioned, multi-variable regression analysis can be useful for evaluating SOx additive performance, as it considers changes that are occurring in the operation.

Such an analysis was performed to establish a correlation for calculating the expected SOx emissions during the period of using a general additive. The analysis identified three key variables or predictors for SOx emissions (ppm): slurry sulfur (wt%), SOx additive rate (kg/d) and excess oxygen (vol%). The formula established below results in a high R2 value of 90%, indicating that this regression analysis can accurately explain the variation of SOx emissions using the specified predictors. The definition of R2 being the percentage of the response variable variation (calculated SOx) is explained by the three key variables.

These three key variables are well-known in terms of impact on SOx emissions. Slurry sulfur levels typically correlate well with coke sulfur levels, which is the direct precursor for the uncontrolled SOx emissions. The SOx additive rate is an obvious parameter for SOx emissions reduction, while the excess oxygen level typically has an effect on SOx additive performance, particularly at the low oxygen levels at which the Rotterdam FCCU runs.

The correlation for calculating actual SOx emissions using a SOX additive:

  • Calculated SOx (ppm) = –119.33 + 326.72* (slurry sulfur, wt%) –0.41*(addition rate, 7-d average kg/d) –14.21*(O2, vol%)

FIG. 6 shows the SOx emissions calculated using the regression (in black). These data points overlap closely with the actual SOx emissions during the period of using a general additive. The analysis highlights that on average compared to the general additive, the proprietary additiveb resulted in 6% lower SOx emissions, while the newest proprietary additivea reduced SOx emissions by 12%.

FIG. 6. Comparison of actual SOX levels vs. calculated SOX from regression analysis.


Through improved cerium and vanadium dispersion, as well as increased spinel content, the authors’ company’s SOx reduction additivea is delivering improved levels of SOx reduction performance in multiple back-to-back additive trials. More than 15 refiners globally have now successfully trialed the new additive vs. the general additive. The investment made in the commercialization of the new additive highlights the commitment to developing new FCC additive technology as part of the authors’ company’s continued focus on its FCC product portfolio. The case study described at BP Rotterdam demonstrates the high levels of SOx reduction performance that is possible, based on both unit operating data and multi-variable regression analysis.

As described, the new FCC additive technologya has been applied successfully in BP Rotterdam’s full-burn FCCU. This new additive has also been successfully trialed in partial-burn FCCUs, which will be described in upcoming articles. HP


  1. Habib, T. and K. Bryden, “Flexible pilot plant technology for evaluation of unconventional feedstocks and processes,” AFPM Reliability & Maintenance Conference and Exhibition, Orlando, Florida, 2013.


a W. R. Grace & Co.’s EMISSCIAN™
b W. R. Grace & Co.’s Super DESOX®
c W. R. Grace & Co.’s Super DESOX® CV+

The Authors

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