Catalysis 2022 issue

with low to moderate feed sulphur can typically achieve >95% SOx emissions reduction relative to the uncontrolled SOx. In absolute terms, this generally corresponds to stack SOx concentrations <25 ppm. Meanwhile, this level of SOx reduction is not typically achieved in partial-burn units, though SOx reduction of 30-70% can be achieved. In both full- and partial-burn applications, SOx additives are often used for offset - ting caustic opex, and the balance of flue gas sulphur is removed at a wet gas scrubber to ensure emissions compliance. The choice of SOx additive itself also impacts SOx emissions. Emisscian is Grace’s latest SOx additive development and is delivering higher pickup factors than alternative technologies in both full-burn and partial-burn applications, as described in recent publi - cations. 1 For more detailed information, please refer to the Grace Guide To Fluid Catalytic Cracking . 1. Baillie C, Improved SOx reduction in partial burn FCC, PTQ Q4 2021, 51. A Rick Fisher, Senior FCC Technical Service Engineer, rick. The answer to this question depends on several factors. Is the FCC full burn or partial burn? Does the FCC run in gasoline mode or diesel mode? A SOx additive has three basic components. 1. The sorbate: this is what captures the SO 3 , which is a mixed Mg/Al oxide. 2. The oxidation package, normally a Cerium oxide which converts SO 2 to SO 3 so that it can be captured by the Mg/Al oxide (the Mg/Al oxide will only capture SO 3 ). 3. The S-release package: various transition metals to facilitate the reduction of the MSO 4 back to MO, con - verting the SOx to H 2 S. The sorbate and oxidation reactions both occur in the regenerator and are thus dependent on the regenerator operation and conditions. The S-release reactions occur in the riser/reactor and are thus dependent on the riser/ reactor conditions. SOx reduction in full-burn regenerators is generally very effective. It is favoured by the higher partial pres - sure of SOx and O 2 , higher catalyst circulation rates, higher catalyst replacement rates (younger inventory), lower regenerator temperatures, good air/catalyst mix - ing in the regenerator, and higher riser/reactor tempera - tures. Johnson Matthey’s Super SOxGetter II typically achieves SOx removal rates in excess of 80% for most full-burn units, and over 95% SOx removal has been maintainable in numerous full-burn units, and com - plete SOx elimination has been achieved in some full- burn units. SOx reduction in partial-burn regenerators is limited by the availability of oxygen for the oxidation reac - tions, converting reduced sulphur species (COS, H 2 S) to SO 2 and then to SO 3 . The higher the CO content of the regenerator flue gas, the less effective a SOx additive will be. A rule of thumb to estimate the maximum SOx reduction achievable in a partial-burn regenerator is:

For a partial-burn regenerator with a flue gas CO content of 6%, the maximum achievable SOx reduction would be approximately 40%. If higher SOx reduction is desired/required, the unit operation will most likely need to be moved to shallower partial burn (lower CO). LO-SOx PB XL is Johnson Matthey’s SOx addi - tive specifically developed for use in FCCs operating in mid-deep partial burn. LO-SOx PB XL can substan - tially reduce the amount of additive required to attain a desired SOx target compared with standard SOx addi - tive technologies. Units with two-stage regeneration (units with both a full-burn and partial-burn regenerator) will experience both scenarios described above, and the SOx reduction limit is almost always set by the SOx reduction that can be achieved in the partial-burn regenerator. Due to the limits of the partial-burn regenerator, LO-SOx PB XL is normally used in these applications. A few units experience issues with the S-release in the riser/reactor. When this occurs, it is almost always due to operating at lower riser/reactor temperatures (such as diesel mode). This is most usually seen at tempera - tures below 950°F (510°C). Johnson Matthey has devel - oped super SOxGetter II DM to alleviate these issues and achieve comparable efficiency to Super SOxGetter II at higher riser temperatures. As you can see, many factors will ultimately deter - mine the amount of SOx emissions reduction that can be achieved with the SOx additives available in the market today. Q Should we expect some return on our original purchase cost if we send spent catalyst for metals recovery? A Francis Humblot, New Business Developer for Oil & Gas Market, Arkema, Thiochemicals, francis.humblot@ Vijay Srinivas, Principal Scientist, Arkema, Thiochemicals R&D, The catalyst manufacturer has a prescribed procedure to sulphide the catalyst. The catalyst manufacturer has a prescribed procedure to sulphide the catalyst, and they should review and approve any deviation from this procedure. Spiked feed sulphiding of a metal oxide hydrotreat - ing catalyst is a process where a spiking agent such as dimethyl disulphide (DMDS) in the presence of H 2 converts to hydrogen sulphide (H 2 S) in-situ, which then reacts with the metal oxide to generate an active metal sulphide catalyst. This exothermic process has to be controlled by appropriate heat removal to main - tain temperatures prescribed by the catalyst manu - facturer. Dimethyl disulphide (DMDS) is one of the most efficient sulphiding agents. With a high active sulphur content (68% w/w), it decomposes at rela - tively low temperatures to generate H 2 S that reaches stoichiometric levels in the DMDS decomposition at around 240⁰C.


Maximum achievable SOx reduction in partial burn = 100% - %CO*10

 2 CH 4 + 2H 2 S {Exclusively}

CH 3 SSCH 3 + 3H 2

14 Catalysis 2022

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