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Aug-2017

Coke protection eliminates skim (TIA)

For refiners operating a hydrotreating unit, the goal is twofold: minimal downtime and maximum profitability.

Chris Alexander
Crystaphase Technologies
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Article Summary
While the focus of a catalyst vendor is usually on catalyst activity, smooth operation depends on a comprehensive solution that integrates feedstock purification. Reducing downtime and increasing profitability means removing particles and poisons that hinder throughput and disrupt performance.

Running thermally cracked feed from a coker or an FCC can create a unique set of problems that limit cycle length. Foulants in these streams take many forms that can compromise a reactor in different ways. To protect the catalyst bed and ensure reactor uptime, a refiner must target these foulants with a carefully designed collection system.

A major Gulf Coast refinery had set a goal of a 24-month production cycle, but had to shut down for a skim after just a few weeks on the system recommended by the catalyst vendor. Rapidly climbing pressure drop was quickly leading to a second skim, making a two-year run seem like an impossible challenge. The refinery brought in Crystaphase to diagnose the issue and suggest a more effective approach.

Technicians began analysing samples from the first skim and recommended loading CatTrap as an interim solution during the imminent second skim. While the CatTrap system tamed the pressure drop right away, Crystaphase studied the samples from both skims more closely to determine the root cause of the pressure drop. The results of that analysis were used to develop a collection system that would put the desired 24-month run back within the refinery’s reach.

The analysis identified coke as the primary foulant. Coke is frequently found in post mortem analysis and has two common sources. The most common is from upstream heat exchangers or furnaces. Less common, but much more problematic, is when soluble coke precursor compounds from the thermal cracking process react with the active sites on the catalyst and polymerise, generating coke within the main bed itself, where it is too late for an inert filtration system to catch it.

Studying the morphology of the foulant under a scanning electron microscope, Crystaphase observed a smooth accumulation pattern consistent with coke polymerisation, which indicated the second case was the issue (see Figure 1).

Being able to characterise the morphology and determine where the coke was being generated was the key to understanding the nature of the problem. Having a material that could combat its precursors was the key to solving it.

Many refiners use active rings to try to deal with precipitable foulants, but they find rings lack the capacity to handle an acute problem like the one this refinery faced. Rings may tackle polymer precursors, but they still promote crust layer formation, which changes the cause of pressure drop but does not eliminate it. This is why Crystaphase developed ActiPhase, first to react with these precursors and then, utilising CatTrap technology, to filter the resulting particles.

The refiner had data from a comprehensive analysis, and Crystaphase had ActiPhase material ready to install. Pressure drop was beginning to rise again on the interim solution, so it was time to put the optimised plan into action.

The ActiPhase filtration system finally helped this refiner reach its cycle length goal, finishing out the remainder of the 24-month cycle with pressure drop at last under control. Not only that, but the optimised loading improved filtration capacity significantly. An upstream upset abruptly brought down the whole unit, but the reactor was on track to run for yet another 
24 months, doubling the predicted cycle length (see Figure 2).
 
This short case study originally appeared in PTQ's Technology In Action feature - Q3 2017 issue.

For more information: optimization@crystaphase.com
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