PTQ Q3 2024 Issue

Controlling FCC SOx emissions with SOx reduction additive technology

An additive with optimised metal dispersion and metal-support interaction achieved a lower daily operating cost of SOx removal from an FCC unit – a refinery case study

Hongbo Ma, Xunhua Mo, Marie Goret-Rana, Charles Kanyi and Carl Keeley Johnson Matthey

G overnment agencies continue to introduce stricter legislation to reduce the amount of sulphur oxide emissions. Sulphur oxides (SOx) are pollutants that contribute to the formation of acid rain, as well as partic- ulate pollution. 1 Sulphur present in feedstock is the main source of SOx emissions from oil refineries, and the fluid catalytic cracking (FCC) unit is responsible for up to 90% of oil refinery SOx emissions.2 Typically, 2-10% of FCC feed sulphur ends up as sulphur contained in the coke on spent catalyst.² When the spent catalyst is regenerated, the sul- phur is oxidised to SO₂, SO3 , and so on. Left untreated, SOx emissions are emitted into the environment. FCC SOx emission reduction solutions Process technologies and high-performance catalysts have been developed to reduce FCC SOx emissions. 2,3,4,5 The main solutions used are:  Feed selection and/or pretreatment to reduce FCC feed sulphur. v Hardware solutions which remove SOx from the FCC flue gas (such as flue gas wet gas scrubbers). w Innovative SOx reduction additives. Hardware solutions often require considerable capital investment. On the contrary, SOx reduction additives only require a small capital investment, such as the addition of an additive addition system (AAS).⁶ Furthermore, SOx reduction additives are suitable for a wide range of operat- ing conditions. Thus, SOx reduction additives are the pre- ferred solution for many FCC units. Whichever solution is used, a key benefit often overlooked is the opportunity to use the SOx reduction technology to process high-sulphur, low-cost feeds, which unlocks the potential to increase FCC margin. FCC SOx reduction additive chemistry SOx reduction additives are injected into the FCC regen- erator, where they mix with the circulating catalyst. SOx is captured by the additive in the regenerator, and the circulat- ing catalyst transfers the captured SOx to the reactor. This captured SOx is converted to hydrogen sulphide (H₂S) in the reactor. H₂S is removed from the FCC fuel gas and LPG product in the unsaturated gas plant. H₂S is eventually con - verted to elemental sulphur, which the oil refinery can sell.

Typical SOx reduction additive levels in the catalyst inventory range from 1 to 10%, although some FCC units are being required to use additives at the 20% level.2 In FCC units with full-burn regenerators, SOx reduction additives can achieve and maintain SOx reduction levels of >95%.2 In partial-burn regenerators, the achievable SOx reduction depends on the availability of SO₃ in the regenerator.2 As the SOx additive injection rate increases, suppliers should provide detailed guidelines to enable small adjust- ments to be made in the FCC and unsaturated gas plants, the sulphur recovery system, the product handling area, and procurement systems. Therefore, a global supplier with addi- tive production, laboratory support, quality control, FCC, oil refinery, and supply chain experience is highly desirable. The SOx removal process has multiple steps. Step 1: Oxidation Under typical regenerator operating conditions, the ratio of SO₂ to SO₃ is about 9 to 1 or greater. An oxidation package is needed to convert SO₂ to SO₃ because the SOx reduc - tion additive is more effective at capturing SO₃ than SO₂. Reaction 1 shows the overall reaction.

SO₂ + ½ O₂ _ SO₃

(1)

Step 2: Sorption SO₃ is captured by the SOx reduction additive. The SO₃ is chemisorbed onto the additive as a metal sulphate (where M represents the metal site). Once the additive has picked up the SO₃ and adsorbed it as MSO₄, it circulates along with the catalyst to the reactor. Reaction 2 shows the overall reaction.

SO₃ + MO _ MSO₄

(2)

Step 3: Release In the reducing environment in the reactor, the additive releases sulphur as H₂S. Reaction 3 shows the overall reaction.

MSO₄ + 8 [H] _ MO + H₂S + 3 H₂O

(3)

The active metal oxide site of the additive is regenerated and capable of repeating the sorption-desorption cycle many times.

PTQ Q3 2024