Feb-2006

Troubleshooting ZSM-5 in the FCC

While using a ZSM-5 additive to increase propylene, one refiner experienced increases in FCC dry gas, coke and slurry oil, even at a reduced temperature

Solly Ismail, BASF Catalysts

Article Summary

With the increasing acceptance of additives as a means of providing added value to FCC operations, there occasionally occur operational issues that require advanced problem solving. The refiner in question was seeking to rapidly expand propylene production to take advantage of favourable market conditions. Upon incorporating ZSM-5 additive into their FCC, the unit began experiencing conflicting symptoms that were attributed to the additive. Variables that were in conflict at this FCC included:
• Increased propylene make
• Decrease in total liquid volume yield
• Increased slurry
• Increased dry gas
• Increased coke.

With such a deviation from normal operations, the refiner suspended use of the ZSM-5 additive and called on Engelhard to troubleshoot the unit. After a thorough analysis, a variety of procedures were isolated, which led to the conflicting variables. Several process improvements were recommended, which the refiner has since adopted. These changes have returned the unit to normal operations.

Problem description and analysis
With two operating FCC units, this Asian refinery sought to maximise revenues by increasing propylene production. Through a structured plan, the refiner wanted to trial a ZSM-5 additive in the smaller of the two FCC units and then, when advantageous, expand to the larger FCC using the knowledge gained from the first trial.

Upon starting the trial in the smaller unit, engineers became vexed, as they saw an anomaly that was completely unexpected. Their expectations were that they would see a direct correlation between ZSM-5 additions and resultant LPG make with no other side reactions. While the published literature supported their expectations that ZSM-5 cracks gasoline olefin components in the C7 to C9 range to LPG with insignificant coke and dry gas make, they were not witnessing that in practice. In addition to a reduction in gasoline, which was expected, they noted an increase in dry gas and coke make, while also experiencing a lower liquid volume and an increase in higher slurry in spite of a feed quality that was relatively constant throughout the balance of the trial. To help troubleshoot the situation, the engineers generated the results shown in Table 1.

The simultaneous increases in dry gas and coke with the accompanying increase in unconverted oil were their points of greatest concern. An increase in dry gas and coke make indicate increasing conversion, while the increase in unconverted oils points to a decrease in conversion. Although this is not commonly seen, there is a reasonable potential for it to occur in any FCC unit.

Reactor temperatures and absorber system
As the feedstock was relatively constant, it was first hypothesised that the refiner may have increased reactor temperatures and that the metering of the FCC bottoms might be incorrect. An increased reactor temperature would have explained the increase in dry gas and coke make as a result of enhanced thermal and catalytic conversion. Upon asking about this possibility, the operators reported that they had actually decreased reactor temperatures during the ZSM-5 trial. They reduced the temperatures in response to the dry gas yield, which was excessive.

However, it was also observed that the temperature reduction did little to decrease the dry gas output. This provided the clues necessary to question whether the dry gas was as high as unit operators thought. To answer that question, the dry gas flow through the absorber system had to be traced. The purpose of the absorber is to remove most of the dry gas (C2 and lighter) before recovering the LPG and stabilised gasoline in the gas concentration section. In most cases, there are two absorbers: a primary absorber that uses unstabilised gasoline as a solvent and a secondary absorber that utilises LCO (lean oil) as a solvent.

The primary control parameters of the absorber system are the amount of solvent that measures the L/G ratio, the temperature of the solvent and the absorber pressure. In this example, the refinery did not have a secondary absorber and held solvent rates, temperature and absorber pressures constant. Additionally, the refiner had no dry gas sample for compositional analysis. Although not usually necessary during a ZSM-5 trial, occasional sampling of the dry gas will help identify if LPG is being lost in the dry gas system, thus reducing profitability. Therefore, in the absence of a dry gas sample, it became impossible to determine whether the dry gas during the trial contained any LPG. However, the fact that there was a higher level of dry gas make while the same amount of liquid flowed to the absorber provided the impression, based on experience, that the absorber was not only letting out (not absorbing) dry gas, but also had the potential for LPG to leak into the refinery’s fuel gas system.

The investigation proceeded to the next part of the problem: clarified slurry production. Before the ZSM-5 trial, the refiner was producing virtually no slurry, as they were recycling to destruction. However, during the trial period, slurry oil increased substantially to approximately 4% of the feed rate. Concurrently, engineers had increased heavy cycle oil recycle to the FCC reactor. It was surmised that both the increase in clarified slurry and heavy cycle oil might be the cause of decreased outlet temperatures in the reactor.

Additive base loading and fresh catalyst additions
While investigating the factors around the reactor outlet temperature, another issue was brought to Engelhard’s attention with data provided by the refiner (Table 2).