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• What level of SOx emissions from the FCC should we expect from current catalyst additives?

  Mar-2022

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Answers

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• Victor Batarseh, W. R. Grace & Co, victor.batarseh@grace.com

  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.

   

  Mar-2022