Reducing environmental impact

This flexibility will continue to drive the longevity of the FCC in the upcoming decades and through the energy transition. In addition to navigating the dynamic landscape in years to come, many refiners are embarking on, or continuing their sustainability journeys. Grace remains committed to supporting the industry with creative solutions to drive the highest profitability operation while minimising the environmental footprint of FCCs. The case studies included in this article examine Grace’s collaboration with refiners to maintain environmental compliance for SO_x emissions in a more cost effective and sustainable manner.

SO_x emissions are an undesired byproduct of FCC catalyst regeneration in crude oil refineries. These emissions are proportional to the sulfur content of the combined feed to the FCC unit. End of the pipe solutions, such as wet gas scrubbers (WGSs), are a flexible alternative to reduce the SO_x emissions at the stack of FCC units.¹ They typically employ a quench and absorber section and make use of an alkaline reagent, typically caustic, to capture the SO_x in the form of sodium sulfites/sulfates.

To reduce SO_x emissions from FCC units, SO_x reduction additives were developed in the 1980s and have gained widespread application since then.² Such additives are typically bifunctional catalysts incorporating SO_x capturing functionality with the catalytic activity for SO₂ oxidation and additive regeneration.

SO_x reduction additives have long been used in the FCC to limit SO_x emissions to the atmosphere in units without WGSs. Units with WGSs typically do not require SO_x additives to maintain regulatory compliance. However, inflation in caustic soda pricing often impacts operating expenses for FCCs with WGSs. For some units, inflation of caustic prices in recent years has resulted in over US$1 million/yr in increased OPEX.

In response, Grace collaborated with refiners operating WGSs to implement SO_x additive for reducing overall SO_x compliance costs in both full and partial burn applications. Further analysis of this activity reveals that the benefits extend far beyond a simple cost savings activity. Balancing SO_x additive and WGS caustic is not only economically viable, but also reduces the environmental footprint of the FCC. This is done by reducing Scope 3 emissions and converting the sulfur destined for waste from WGS wastewater plants, over to reactor hydrogen sulfide (H2S), which is converted into elemental sulfur and utilised to produce commodities including sulfuric acid and fertilizers.

In 1998, Paul Anastas and John C. Warner published 12 principles to guide chemical product and process design according to green chemistry considerations.³ The American Chemical Society provides reference to the principles on its website. Implementing FCC SO_x additive to avoid large amounts of fresh caustic usage and spent caustic treatment closely aligns with two of these principles: preventing waste instead of treating it and catalytic reagents are superior to stoichiometric reagents.

This article seeks to quantify the economic and sustainability benefits associated with implementing a combination of additive and WGS caustic for maintaining SO_x emissions compliance. The evaluation focuses on two example cases: a full burn FCC at the CITGO Lemont refinery, processing a high sulfur feed, and a deep partial burn FCC at a Shell refinery in the US Gulf Coast region that processes a moderately sour feed. Both of these refiners utilised Grace’s latest SO_x additive, EMISSCIANTM, which achieves excellent performance by effectively balancing the functionalities of cerium, vanadium, and magnesium, while dispersing the components evenly throughout the additive particle.

Case 1: full burn unit with high sulfur feed

The full burn unit at CITGO Lemont examined in this study typically processes 60 000 bpd of feed with an API of 18 and sulfur content of 2.5 weight %. For this operation, the unit’s WGS consumes ~24.5 dry short tons (dst) of caustic (NaOH) per day. As a starting point for the evaluation, the relationship between feed sulfur and caustic consumption at the WGS without SO_x additive was determined. It was found that a simple linear regression utilising the mass flow rate of sulfur in the feed predicted WGS caustic consumption reasonably well over a variety of operating conditions, producing a fit with an R2(adj.) = 77%.

This article was originally published in the December 2023 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.

Written by Andrew Hubbell, CITGO, Brandon Burns, Shell Energy and Chemicals, and Victor Batarseh, W.R. Grace & Co.

References

¹SEXTON, J. A., ‘FCC emission reduction technologies through consent decree implementation: FCC SOx emissions and controls in advances in fluid catalytic cracking: testing, characterization, and environmental regulations’ (2010).

²YALURIS, G., and DOUGAN., T.; Catalagram Europe, Fall 2006, 8-11.

³ANASTAS., P., and WARNER., J., ‘Green Chemistry: Theory and Practice’, (1998).

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/12042024/reducing-environmental-impact/