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May-2020

FCC equilibrium catalyst analysis

Analysis of equilibrium catalyst provides important indications of FCC catalyst performance, cyclone mechanical integrity, and attrition

RAY FLETCHER
The FCC Analyst, LLC

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Article Summary

The fluidised catalytic cracking unit is a highly complex multivariable operation. The conversion of feed to petrochemical feedstocks and/or motor fuels is primarily a function of feed quality, independent operating parameters, and catalyst composition. Perhaps the single most powerful tool for understanding unit performance is the equilibrium catalyst analysis provided by catalyst vendors.

The intent of this article is to provide the process engineer with advanced Ecat analysis techniques to assist in understanding unit performance. These techniques span the range from measuring the resistance to mass transfer in the catalyst circulating inventory to analysing the subtle shifts in catalyst composition imparting measurable impacts on yield selectivities.

Catalyst diffusion
The ability of FCC catalyst to diffuse high molecular weight, sterically hindered molecules into the catalyst particle has been debated and marketed by catalyst suppliers for decades. This article offers no claim that one catalyst technology is superior to other technologies. It is expected that each refiner has selected the best available catalyst for their feed slate, providing the desired product slate. The focus is to assist the FCC operator to understand where an inflection point may exist in their operation at which point a barrier to mass transfer into the catalyst is measurable.

The simplest technique is to plot the slurry yield vs total equilibrium metals. Total metals include nickel, vanadium, sodium, iron, and calcium. The highest accuracy will be achieved by calculating the ‘add-on’ sodium and iron as both sodium and iron are present in the fresh catalyst. However, since the concentration of these elements in fresh catalyst is consistent between batches, the error incorporated by using total sodium and iron is systematic and may be ignored.

As the amount of contaminant metals increases on the Ecat particles, a barrier to mass transfer builds in the form of a crust. The more sterically hindered feed molecules will not be able to access the particle pore structure as the thickness of this crust increases, resulting in an increase in slurry yield. Cross plotting slurry yield vs total metals provides a simple indication of where this barrier begins for your feedstock and catalyst technology. In general, the author has observed that the inflection point is frequently observed at approximately 10 000-12 000 ppm for most units.

Refiner #1 has a typical metal loading of 13000-15000 ppm total metals. The data demonstrate a significant step change increase to 28000 ppm metals beginning at day 600 (see Figure 1). Figure 2 presents the response of slurry yield to the typical metals level observed in the first portion of the curve from day 0 to about day 600. The slurry yield increases by approximately 2.0 wt% for every 1000 ppm increase in contaminant metals. An inflection point appears at approximately 15000 ppm. Figure 3 presents the slurry response for the peak observed beginning at day 600. It is interesting to note that a clear inflection point occurs with this feedstock at approximately 21 000 ppm.
 
Feed quality shifts
Approximately 75% of all yield selectivity shifts are the result of feed quality variation for most units not involved in a catalyst change-out. The two simplest methods to determine the root cause of the yield selectivity shift resulting from feed quality variation is the direct measurement of feed quality and monitoring the Ecat nickel-to-vanadium (Ni-to-V) ratio. Feed quality parameters include density, nitrogen, CCR, UOP K, and so on.

The catalyst circulating inventory is very sensitive to feed quality shifts. The simplest measurement available to the process engineer to monitor feed quality shifts requiring no laboratory testing is the equilibrium catalyst Ni-to-V ratio. A feedstock that becomes more paraffinic will observe an increase in the Ni-to-V ratio while a feedstock becoming more refractive will observe a decrease in the Ni-to-V ratio. This is the result of the catalyst converting the heavier metal bearing components of the feed into lighter molecules, with the metals being deposited on the surface of the catalyst.

The Ecat Ni-to-V ratio is a simple method to monitor subtle changes in feed quality and correlates well with many yield selectivities such as conversion, gasoline, and slurry.

Figure 4 demonstrates that the typical Ni-to-V ratio for Refiner #2 is from 1.0-1.4. However, beginning at approximately day 95, a lower density feed slate was processed in the unit (dark red data points). Figure 5 indicates the normal response of conversion to the Ni-to-V ratio. The lighter feedstock clearly does not follow the same response curve as observed with the refiner’s more typical feeds.

Please note that each unit responds differently to changes in the Ni-to-V ratio. It is essential that the process engineer determines the response, if any, to the typical variation in their unit.

Fresh catalyst composition
Fresh catalyst composition is critical to optimal conversion with desired yield selectivities. However, the fresh catalyst certificate of analysis (COA) often contains limited data, making detailed catalyst quality analysis difficult. It is recommended that the process engineer correlates key Ecat properties and ratios vs important yield selectivities.

For refiners targeting maximum light olefin yield, it is recommended to plot propylene yield or propylene olefinicity vs Ecat sodium. For additional yields, the engineer is recommended to plot the desired yield vs the Ecat zeolite-to-matrix (Z-to-M) ratio. As with all analyses, it is important to verify each unit’s response to these variable shifts. Not all units respond alike. It is also suggested that the refiner negotiate tighter upper and lower limits for those units whose yield selectivities are sensitive to catalyst composition.

Figures 6 and 7 demonstrate the gasoline yield response of Refiner #3 to normal variations in the equilibrium catalyst Z-to-M ratio. It is important to note the degree of gasoline yield variation in this period of normal operations. Analyses such as these enable the FCC operator to understand the root cause in ‘typical’ yield variation or scatter.

Catalyst stability
The engineer is encouraged to monitor the Z-to-M ratio as an indicator of catalyst stability. A decreasing trend in the Z-to-M ratio that is not correlated to decreased catalyst additions but is correlated to an anticipated long-term increase in regenerator temperature or equilibrium catalyst metals is encouraged to discuss potential catalyst reformulation with their catalyst supplier.

As the regenerator increases in temperature, the hydrothermal deactivation of the catalyst increases. The zeolite portion of the catalyst is much more sensitive to regenerator temperature than the active alumina, resulting in dealumination of the zeolite crystal structure. This loss in activity leads to increased slurry yield. The process engineer is encouraged to evaluate the effect of regenerator temperature effects on Z-to-M and yield selectivity on their unit.

Catalyst attrition resistance
The refiner is recommended to regularly monitor and record catalyst fines for surface area and metals content. An increase in Ecat fines surface area combined with a decrease in total metals is an indication of softer catalyst. An increase in catalyst fines production not demonstrating increased surface area or decreased metals is an indicator of a new attrition source in the unit. Please note that reports of soft catalyst by any catalyst manufacturer is very rare.

Cyclone mechanical integrity
It is strongly recommended that regular catalyst fines samples be acquired and measured for particle size distribution (PSD) on a frequency of at least once monthly. The refiner is advised to request a more detailed PSD for the fines than for the circulating inventory. The suggested PSD is a one-micron scale for 0-10 µ particles (1, 2, 3 … 10), every two microns for 12-40 µ and every five microns for particles greater than 40 µ. Plot wt% capture vs PSD using a semi-log plot for each sample taken. Keep a monthly record of these plots through each operating cycle and for the life of the cyclones.


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