Jun-2015

Measuring metal mobility in FCC equilibrium catalysts (TIA)

In recent years, a number of new techniques have been used to study metal mobility and metal effects in FCC units. These techniques brought new insights into how deleterious metals do their work in the FCC and have created solutions to counter these effects.

Bart de Graaf
Johnson Matthey Process Technologies

Article Summary

One of the recent, more surprising discoveries was that iron from crude, deposited on the FCC catalyst surface, can move from particle to particle. Vitrification of the catalyst surface was extensively documented in the mid 1990s and though the vitrification mechanism was known and widely accepted, one of its effects has been largely ignored: when a catalyst surface is in flux due to temperature and contaminant metals, transfer of contaminant iron oxide crystallites is possible when FCC catalyst particles collide in the regenerator.

The study of this is best carried out by a survey of catalyst particles from a unit suffering from iron poisoning. For every particle, the concentration of contaminant metals on the particle can be analysed, and the distribution over the particles can be derived from this examination. As catalyst is daily added and removed from the FCC unit, it will exhibit a residence time distribution (exponential decay, the highest fraction of the particles is the newest, and only very few very old particles are present). If a contaminant metal cannot move from particle to particle, the metal concentration will show its age and an exponential decay can be observed. For metals that move from particle to particle in the FCC unit, a bell curve type of distribution will be observed.

The new analytical methods, such as imaging and chemical analysis by scanning electron microscope, are straightforward to use, though their proper application requires some attention. What can go wrong? One thing is not using the average concentration of contaminant metal on the FCC particle, but using concentrations at arbitrarily chosen spots on a very limited amount of particles.

Imagine the following example. During a flu pandemic, two doctors were sent out to study whether the flu is contagious. One doctor rounded up four people from within an area and took 250 spot temperatures from each person. He correlated the spot temperatures with the location where he had taken the samples: mouth, ear, every finger, every toe, and so on. His analysis showed the flu cannot transfer from person to person and he could prove this with 1000 temperatures (collected from four people).

The other doctor took the oral temperatures of 1000 people within one area. He correlated the results via distribution of people within the area and concluded that the flu is contagious.

Every study of metal mobility on iron contaminated FCC catalyst has shown that iron moves around. In hindsight this is not a complete surprise knowing what has been discovered in the previous century: contaminant iron exhibits its destructive effects by vitrification of the FCC catalyst surface. Under FCC conditions this glass phase is not completely solid, as the dense outer surface of the catalyst particle testifies. Sticky surfaces can transfer matter upon collision and the distribution of iron on FCC E-cat is testament of this behaviour. Cat-Aid, an effective iron trapping FCC additive that protects the base catalyst from well-known adverse effects, has been proven to alleviate the inter-particle migration of contaminants (see Figure 1).

This short case study originally appeared in PTQ's Technology In Action feature - Q3 2015 issue.
For more information: Bart.deGraaf@matthey.com