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Jul-2011

Preventing environmental excursions on the FCC unit

Guidelines to enable the FCC engineer to identify and eliminate the root cause of unexpected flue gas emissions

Ray Fletcher and Martin Evans
Intercat (Johnson Matthey)
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Article Summary
Significant attention has been given to the reactor, riser and product recovery sections of the FCC unit. Attention has also been given to the regenerator when necessary due to high catalyst losses, when afterburning arises and when environmental regulations require. The FCC world is presently undergoing a paradigm shift, in which the regenerator now needs to be as well tuned as the reactor riser vessel.

Optimisation of the regenerator is required to minimise emissions 
to comply with environmental 
regulatory permits. Fortunately, optimisation of the regenerator is often less complicated than that of the other sections of the FCC unit. Close attention to regenerator operations will enable the FCC process engineer to efficiently and effectively avoid unexpected emissions excursions and the possibility of large penalties being imposed by regulatory environmental bodies.

This article provides a list of the most common causes of unexpected SOx, NOx and CO emission exceedances from FCC units today. Simple and effective techniques for diagnosing and troubleshooting these problems are embedded within each section. There have been many articles published over the decades relating to particulate emissions, hence troubleshooting catalyst losses will not be addressed within this article.

SOx emissions control
SOx emissions from the FCC regenerator have long been monitored and regulated by environmental regulatory bodies. Furthermore, the technology for controlling SOx emissions from the regenerator has been well defined and developed. SOx additives are very effective at controlling SOx emissions (see Figure 1). The guidelines provided here will enable the process engineer to swiftly troubleshoot the most likely causes for unanticipated shifts in SOx emissions.

Change in feedstock sulphur content and source 

SOx emissions from the FCC unit will frequently, but not always, correlate directly with feedstock sulphur concentration (see Figure 2). A more accurate correlation for predicting SOx emissions has been developed using the Gulf correlation, which is based on slurry sulphur concentration. Most FCC engineers track feed sulphur to predict required rates of addition of SOx additive. However, it is well known that the amount of feed sulphur going to coke can vary with different feedstock types. The Gulf correlation1 uses slurry sulphur concentration to predict coke sulphur levels. This has been found to be a better fit for most FCC units. The Gulf correlation was developed by the Gulf Research & Development Company and relates sulphur in coke to sulphur in the slurry oil. This is the preferred correlation used by Intercat to evaluate SOx additive efficiency. This correlation is defined as:

Coke sulphur = UF * (Sulphur in slurry)1.265
where UF is the unit factor.

The dilemma faced by FCC engineers is that feed sulphur content can be easily predicted (via the projected feed slate), whereas slurry sulphur content is not easy to predict.

However, most FCC units process a limited number of feedstocks. The prudent engineer will evaluate which feedstocks have the largest impact on SOx emissions. Regression techniques are useful for identifying such feeds. This enables feed forward recommendations to operations prior to feedstock sulphur content increases.

Additionally, units operating with FCC feed hydrotreaters may observe a gradual increase in SOx emissions as the catalyst’s age increases, even though the total sulphur content may remain constant. This change in sulphur distribution within the FCC feedstock is a result of catalyst deactivation leading to a different distribution of sulphur between coke and heavy oils. Even the Gulf correlation can fail to predict this effect adequately. It is important to anticipate this trend. These increased emissions are easily controlled by adjusting the SOx additive injection rate.

Equilibrium catalyst iron step change 
A step change increase in feedstock iron may have an immediate and negative impact on SOx emissions.2 There is a direct relationship between SOx emissions and contaminant iron concentration on the equilibrium catalyst (see Figure 3). Unexpected and unexplained step changes in SOx emissions have been related to equilibrium iron concentrations in many cases. The half-life for this iron activity is very short (typically in the range of two to three days), but the magnitude of the effect may be very large. Temporary increases in SOx additive injections will, in most cases, be sufficient during these step change events. Identification of the feed source is recommended, with consideration given to controlling the rate of injection in the future.

Additive loader failure
An additive loader failure can, in some cases, result in a SOx excursion. It is strongly recommended that the FCC operator uses an additive loader supplier with a proven record of high reliability.3 Furthermore, the additive loader should be capable of manual operation in the event of a power outage. It is recommended that the additive loader control system be connected to the distributed control system on the FCC unit. A shutdown alarm will then sound in the control room, enabling the operators to swiftly reinitiate injections should a problem occur.

Flue gas analyser failure 
Regular preventive maintenance of the flue gas analysers is strongly recommended, with close attention being applied to the flue gas sample conditioning system. Flue gas conditioning systems seem to be the most common cause of poor analyser readings. This critical system may often be overlooked by instrumentation groups. Two critical elements of a successful conditioning system are the removal of steam condensate and catalyst fines. Unexplained step changes in SOx concentrations should be followed by a spot check of the analyser with standardised calibration gases.
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