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Profitable resid processing

FCC catalyst technology can help refiners processing resids overcome any negative effects on unit operation and operate more profitably and efficiently

Larry Hunt, Grace Davison
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Article Summary
In order to help refiners run their FCC operations more efficiently and profitably, Grace Davison routinely analyses more than 
2000 FCC feedstock samples per year from the majority of FCC units around the world. 
This large database indicates that nearly 50% of the FCC/RFCC units worldwide currently 
process some percentage of heavy, residual feedstocks.

This increasing trend is set to continue, driven by the significant decrease in light, sweet crude supply and the widening light/heavy crude differential. Resid processing is challenging, due to the additional concarbon, metals contaminants and higher boiling point distribution these feedstocks possess. While some refiners are well equipped to handle the additional processing demands of heavy, sour crudes, others are not.

Maximum bottoms upgrading
To process resids more profitably, Grace Davison has developed the Midas catalyst series, designed specifically for maximum bottoms upgrading of heavy resid feedstocks. Extensive research shows that bottoms upgrading occurs via three mechanistic pathways.1 Midas technology is designed to be effective in two of the three pathways to bottoms reduction: namely, feed vapourisation and recracking reactions and naphtheno-aromatic cracking reactions. The primary attributes of the technology are:1
• Optimised mesopore design and matrix activity for maximum conversion of bottoms to LCO
• Latest high-stability, high-activity zeolite technology
• Premium metals tolerance functionality for unsurpassed coke selectivity compared to conventional bottoms cracking catalysts.

Unit-specific applications
The processing of heavy, residual feeds impacts unit operations in several ways: elevated regenerator temperature, reduced cat/oil ratio, loss in conversion, large percentage increase in slurry make, slurry exchanger fouling, circulation instability and longer term riser coking due to nozzle plugging. Commercial experience shows that FCC catalysts from the Midas series can successfully help to mitigate, or at least minimise, these negative effects on unit operation. In one recent application, a refiner ran heavier feed to the FCC unit than the feed nozzles were capable of vapourising. Poor feed vapourisation resulted in riser coking that increased the pressure drop over the riser and caused the refiner to shut down the FCCU every six months to decoke the riser. Midas was recommended to the refiner, since the high pore volume and activity of the matrix helps vapourise feedstock and facilitates pre-cracking reactions that are important in heavy feedstock cracking. These attributes helped keep the FCCU running efficiently and boosted bottoms cracking compared to the conventional bottoms cracking catalyst the refiner was using previously. After one year of operation with this catalyst, the refiner shut down to examine the riser (pressure drop was not a significant problem). The refiner still found some evidence of riser coking on one side of the riser, but the problem was reduced dramatically through the use of the Midas catalyst.

In a second documented application, the refiner was experiencing significant slurry exchanger fouling resulting from the processing of heavier, resid feed in the RFCC unit. Chemical treatment in the bottoms circuit showed only minimal improvement. The refiner dropped LCO into the slurry circuit to get the volume and gravity up in an attempt to minimise the exchanger fouling issue. However, this had very limited effect on the frequency of exchanger fouling and was costly to the economics of the operation. Midas was recommended for this unit specifically to minimise the naphthenic polynuclear aromatics (PNA) compounds in the slurry that were suspected of being a major cause of the fouling problem. Within four to six weeks on Midas, bottoms cracking was significantly improved and, although bottoms API dropped, slurry exchanger fouling was minimised greatly.

In a third commercial refinery application, the refiner began to experience a significant decline in conversion and bottoms cracking when the feedstock became heavier through additional atmospheric tower bottoms (ATB) processing. The primary feedstock changes and FCCU selectivity changes are indicated in Tables 1 and 2.

Midas was recommended to increase matrix cracking and to provide additional mesoporosity for pre-cracking and improved catalyst stripping. This recommendation resulted in a large improvement in coke-selective bottoms cracking, allowing the regenerator temperature to drop back to typical levels and allowing for increased C/O cracking. From the net results shown in Table 3, it is apparent that the catalyst was able to reduce bottoms on the heavier feedstock even more than the lighter feed base case operation and increased conversion to gasoline+LPG, providing a significant increase in FCCU profitability.
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