Reducing environmental impact of FCCs with wet gas scrubbers by using SOx additives (ERTC 2023)

Over the last 80 years, the FCC has played a critical role in the refining industry and continues to adapt to shifting industry conditions. The next opportunity to demonstrate the flexibility of the FCC is to re-optimise the unit to reduce its carbon intensity.

Andrew Hubbell, Citgo
Brandon Burns, Shell Energy and Chemicals
Victor Batarseh W.R. Gace & Co

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Article Summary

Grace remains committed to supporting the industry with creative solutions to drive the highest profitability operation while minimising the FCC’s environmental footprint. The case studies below examine Grace’s collaboration with refiners to maintain environmental compliance for SOx emissions in a more cost-effective and sustainable manner.

SOx reduction additives have long been used in the FCC to limit SOx emissions to the atmosphere in units without wet gas scrubbers (WGSs). Units with WGSs typically do not require SOx additives to maintain regulatory compliance. However, they can be implemented to offset caustic usage, reducing operating expenses and limiting the lifecycle CO₂e (carbon dioxide equivalent) of operating the FCC while also converting waste streams to usable products.

In these case studies, lifecycle CO₂e reduction is achieved by limiting the amount of NaOH being produced, shipped to, and utilised by the FCC WGS. The waste avoidance described here is driven by the additive capturing the sulphur, which would be wastewater-dissolved solids and redirecting it to generate elemental sulphur. A full-burn FCC at CITGO Lemont’s refinery and a partial-burn FCC at a Shell refinery on the US Gulf Coast are examined here.

Case Studies
In both case studies, the first step to implementing SOx additives for WGS caustic reduction was to understand the caustic consumption without using additive. To do this, multivariable linear regressions were developed to predict additive-free caustic consumption from parameters, including feed sulphur mass flow rate. For both cases, close alignment between actual and predicted caustic consumption confirms a good fit for the regression model.

Utilising a combination of the baseline data and Grace’s proprietary SOx additive performance model, an evaluation was conducted to determine the optimal balance between additive usage and WGS caustic required to minimise total emissions compliance operating expenses. This type of analysis demonstrates that there is a wide range of SOx additive rates over which operating expenses (Opex) can be reduced.

The full-burn CITGO Lemont FCC unit typically processes 60,000 barrels per day of feed with an API of 18 and sulphur content of 2.5 wt%. For this operation, the unit’s WGS consumes ~24.5 dry short tons (dst) of caustic (NaOH) per day. The CITGO Lemont and Grace teams collaborated to develop a comprehensive analysis to help set SOx additive rates throughout the year as caustic pricing, feed quality, and feed rates shifted.

The evaluation revealed additive rates ranging from 400 to 700 lb per day maximised the total SOx compliance Opex savings by appropriately balancing SOx additive and WGS caustic. The same regression model utilised to describe caustic usage without additive was compared to the actual caustic usage with SOx additive, and the gap between actual and predicted caustic consumption signifies the reduction achieved with the additive.

The refinery averaged 510 lb per day of SOx additive usage and 11.8 dst per day of caustic reduction, realising net SOx control Opex savings of $2.5-3.5 million per year (annualised after excluding a period of unusual operation during turnarounds at other process units). This was calculated assuming a $1,000 per dst caustic price.

The deep partial-burn Shell US Gulf Coast FCC examined here is a residual fluid catalytic cracking unit (RFCC) that typically operates at ~100,000 barrels per day with a 22.5 feed API and sulphur content of 0.7-1.1 wt%. This FCC routinely operates with flue gas CO between 5-7 vol%. Grace collaborated with Shell, leveraging unit data and Grace’s proprietary model to identify the optimum caustic reduction to minimise SOx emissions compliance operating expenses.

Due to operation in partial burn and limited oxygen availability for oxidising SO₂ to SO₃, SOx additive is less effective in this unit than the full-burn FCC studied in the previous case study.

As a result of partial-burn operation and other unit parameters, the target caustic reduction for maximising Opex savings was 20-25% at the Shell refinery. Implementation of 400 lb per day of SOx additive, preblended with fresh catalyst, drove a 21.3% caustic reduction. This resulted in $0.5-1.0 million per year in savings, assuming a $1,000 per dst caustic price. Despite the challenges for SOx additive in partial burn, significant sustainability benefits are still realised through a reduction in lifecycle CO₂e emissions and waste avoidance.

Operation without SOx additive requires greater amounts of caustic to be produced, then shipped to the refiner, and treated as total dissolved solids in wastewater plants. Additional details on data and calculations associated with the lifecycle CO₂e changes in the system when considering this switch from caustic to a combination of caustic and SOx additive are shown in Table 1.

This analysis confirms that implementing Grace’s EMISSCIANTM SOx additive for WGS caustic reduction at both the CITGO Lemont and Shell US Gulf Coast FCCs not only provided significant Opex reduction but also reduced the Scope 3 carbon emissions associated with operating the FCC and diverted sulphur from a waste stream to the sulphur plant, creating a sellable product.

Annualised CO₂e reduction totals were substantial at more than 8,100 tons combined between the two FCCs, which equates to saving the emissions from combusting 135.5 million standard cubic feet (MMSCF) or 140,300 million BTUs of natural gas.

Evaluation of the activities at CITGO and the Shell refinery show that implementing SOx additive for WGS caustic abatement has the potential to deliver sustainability benefits in addition to Opex reduction. The sustainability benefits include a reduction in Scope 3 emissions associated with caustic production and transportation as well as converting the sulphur destined for waste into a usable product in both full- and partial-burn FCC units. Implementing Grace’s SOx additive, EMISSCIAN, allows refiners to achieve these benefits while also minimising SOx emissions compliance Opex by delivering the highest flue gas sulphur reduction per pound of additive across a variety of applications. Grace remains dedicated to continually investing in the development of novel solutions and manufacturing capabilities to better serve its customers.

This short article originally appeared in the 2023 ERTC Newspaper, which you can VIEW HERE

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