What level of SOx emissions from the FCC should we expect from current catalyst additives?Mar-2022
Tom Ventham, G. W. Aru, LLC and Unicat Catalyst Technologies, firstname.lastname@example.org
It is a common misconception that FCC additives cannot provide the low level of SOx emissions that a wet gas scrubber (WGS) can achieve. Although for certain FCC operations, such as those in deep partial burn or requiring an ultra-low SOx emission (<5 ppm SOx) WGS may turn out to be a more appropriate choice, SOx reduction additives provide the most cost effective (for Capex and Opex) and flexible option for meeting emissions limits in a large proportion of cases. Moreover, SOx additives can also be used in series with WGS technology to offload fresh caustic costs and the costs of processing spent caustic, as well as allowing a smaller and cheaper WGS to be selected for new-build units.
With confidence that SOx additives are the appropriate tool for achieving SOx emissions targets, the next decision is to choose the best SOx additive option for your operation. The SOx reduction additive market is mature with over 30 years industrial use. However, it is also an area that has not always received the investment, development, and attention deserved by those who supply these materials over the history of this technology. When G. W. Aru, LLC launched to the industry in August 2018 as disruptor in the stale FCC additives market, a spurt in SOx additive marketing and development was the response from the industry. G. W. Aru, LLC, with their partners at Unicat, have proven their SOx additive technology, Ultra SOxBuster, to be superior to incumbent suppliers in numerous commercial FCC trials conducted since the company’s founding.
To explain these benefits further; when considering which SOx additive is most optimum for your FCC unit a key decision is the selection of technology base. SOx additive technologies on the market follow one of two platforms, spinel-based materials or hydrotalcite materials.
Traditionally FCC catalyst or additive suppliers in this area focused their SOx additive offerings on one or other of these opposing platforms, normally based on historical patents filed or limitations of their manufacturing plants. As a supplier of both hydrotalcite (Ultra SOxBuster-M60) and spinel (Ultra SOxBuster-M30) SOx additives G. W. Aru, LLC & Unicat does not have a legacy of needing to bias towards only one of the technology platforms available. However, it is the experience in recent trials that Ultra SOxBuster-M60 (“USB-M60”) has outperformed other commercially available SOx reduction additives by a margin of 30%. It is the well-understood technology of USB™-M60 that results in this step-out performance that should be seen in every commercial case and will be of particular further advantage to those units that are restricted by dust emissions, dilution effects at high SOx additive usage, and units with rapid and dynamic fluctuations in feed quality that result in challenges in maintaining SOx emissions compliance.
This significant and proven performance advantage gives opportunities for refiners to make significant savings in SOx control costs. Further benefit is gained from diverse sourcing of Ultra SOxBuster additives giving security of supply and price stability in an era of intense price disturbances and mandated price increases from other catalyst suppliers. Moreover, Ultra SOxBuster additives have been intensively studied and determined to be not hazardous to human health, as shown from the Safety Data Sheet. SOx additives contain a vanadium component which in some forms presents a significant health hazard (i.e. CMR – carcinogenic, mutagenic, reproductive toxicity) meaning in some regions, such as the UK (COMAH regulations) and mainland Europe (EU mandatory substitution rules), this can be of intense importance to refiners. It is for a refiner’s environmental, health & safety, and management departments to determine how such rules are interpreted for each individual site.
Rick Fisher, Johnson Matthey, email@example.com
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 SO3, which is a mixed Mg/Al oxide.
2. The oxidation package, normally a Cerium oxide which converts SO2 to SO3 so that it can be captured by the Mg/Al oxide (the Mg/Al oxide will only capture SO3).
3. The S-release package: various transition metals to facilitate the reduction of the MSO4 back to MO, converting the SOx to H2S.
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 pressure of SOx and O2, higher catalyst circulation rates, higher catalyst replacement rates (younger inventory), lower regenerator temperatures, good air/catalyst mixing in the regenerator, and higher riser/reactor temperatures. 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 complete 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 reactions, converting reduced sulphur species (COS, H2S) to SO2 and then to SO3. 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:
Maximum achievable SOx reduction in partial burn = 100% - %CO*10
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 additive specifically developed for use in FCCs operating in mid-deep partial burn. LO-SOx PB XL can substantially reduce the amount of additive required to attain a desired SOx target compared with standard SOx additive 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 temperatures below 950°F (510°C). Johnson Matthey has developed 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 determine the amount of SOx emissions reduction that can be achieved with the SOx additives available in the market today.
Victor Batarseh, W. R. Grace & Co, firstname.lastname@example.org
Units using SOx additives can achieve anywhere from 30 to >95% SOx emission reduction depending on unit configuration, operating conditions, additive usage rates, and feed quality. Low feed sulphur, full burn units can achieve the lowest SOx emissions at levels below 25 ppm utilising SOx additive alone, while partial burn units may only be able to achieve a 30-70% reduction in SOx emissions with additives and may require the operation of a wet gas scrubber to attain emissions compliance. While this question mentions expected SOx emissions with additives, it is important to recognise that factors such as feed sulphur and operating conditions play a major role in SOx emissions, and these factors are discussed in detail below.
There are a variety of avenues for controlling SOx emissions from the FCC, and a refiner may select any one or a combination of control options, depending on the crude slate, refinery configuration, and economics. Sulphur in FCC feedstock distributes amongst all FCC products, including coke, which is combusted in the regenerator and ultimately produces SOx emissions. As a result, the expected emissions from an FCC are primarily dependent on the sulphur levels in the feedstock and how this sulphur partitions to coke. SOx additives often represent a flexible and more cost-effective technology to alternative solutions for SOx compliance, including switching to sweeter crudes, FCC feed hydrotreatment, and wet gas scrubbing.
The SOx emission levels obtained when using SOx additives depend on factors that can be split into two broad categories: those that influence sulphur to the regenerator and those that impact additive performance. Factors that influence sulphur to the regenerator are important as they determine the level of SOx emissions for an FCC without SOx additive, or ‘uncontrolled SOx’. The higher the uncontrolled SOx, the more challenging it is to achieve environmentally compliant SOx emissions. These factors include:
• Feed sulphur, type, and degree of hydrotreating
• Product recycle streams
• Reactor stripper conditions
• Coke yield
Any factor that can influence the SOx capture by the additive or the additive regeneration mechanisms in the reactor can impact SOx additive performance. These factors include:
• Oxygen availability in the regenerator (excess O2, full or partial burn operation)
• Regenerator air distribution
• Regenerator temperature
• Reactor temperature
• Reactor stripper conditions
• Additive usage rates
The two most powerful drivers of the achievable SOx emissions levels are the feed sulphur levels and the oxygen availability in the regenerator. Feed sulphur levels directly impact the uncontrolled SOx levels in the regenerator. Feeds that produce higher levels of uncontrolled SOx require a greater percentage of SOx reduction to achieve the same emissions targets.
For a given set of conditions, SOx additive performance can be characterised by a pickup factor, which is defined by lbs of SOx removed per lb of additive utilised. A typical performance curve of pickup factor vs % flue gas SOx reduction is shown in Figure 1.
It can be observed in Figure 1 that the pickup factor is not constant across the range of SOx reduction; this results in a nonlinear increase in the additive required as the targeted SOx reduction percentage is increased. This operational curve for a given set of conditions and additive type set the feasibility of achieving different SOx emissions levels with additive. As the additive required increases, it may not be desirable to further reduce SOx emissions with additive due to performance concerns or logistical constraints. SOx additive does not have the same cracking functionality as FCC catalyst and, when utilised in excess amounts, can negatively impact the unit operation.
While uncontrolled SOx and emissions targets dictate where an operation lies on the pickup factor curve, oxygen availability can dramatically shift the curve up or down, as shown in Figure 2.
As a result of this phenomenon, full-burn FCCs 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 offsetting 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 publications.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.
Corbett Senter, BASF Refining Catalysts, email@example.com
SOx emissions from FCC depend on several factors (not just additive usage and performance), as SOx emission and the mechanism of SOx additives are both multi-step processes. SOx is formed when sulphur in feed enters the regenerator either as coke or due to poor stripping of FCC product from the circulating catalyst after the reactor. The sulphur in the regenerator is then combusted to SO2 and SO3 which can leave the FCC as emissions. SOx additives function by capturing SO3 in the regenerator, reducing the captured S-oxide on the additive in the riser and enabling conversion of S-oxide to H2S in the stripper, which causes the sulphur to exit with effluent from the reactor. Thus, the amount of sulphur in feed, feed type (type of sulphur compounds and metals in feed), stripper performance, catalyst circulation rate, regen operation, and the amount of SOx additive used will all play a role in SOx emissions. The key to understanding the expected emissions levels is to consider all these factors to develop a plan for reducing SOx emissions.