Q&A

(2013) Question 64: How do you select the proper metallurgy for crude overhead (OVHD) to increase reliability?

Our belief is that a proper design includes a good waterwash system and less expensive metallurgy. Our tower tops are typically carbon steel with Monel overlay.

(2013) Question 65: Please share experiences in crude tower fouling from organic and/or phosphorous-containing deposits.

We have experience with two types of organic tower fouling. The first was an organic fouling in the top portion of the tower due to rerunning cracked slop oil, coker naphtha, and kerosene material. Typically, we do not want to do that. Since then, we have stopped processing those materials or limited the amount that we are rerunning so it does not happen anymore.

Question 11: What process or catalyst options are available for shifting yield selectivities from gasoline to distillate while minimizing the impact on light olefin yields? How are the product properties impacted? How does change-out rate impact the viability of the catalyst options?

Undercutting gasoline into light cycle is the first option and is widely employed. It is quick, it is easy, and it gives an immediate impact. Reducing riser temperature and/or cat-to-oil ratios reduces conversion, while using a ZSM-5 additive to regain C3-C4 olefins is another option.

Question 12: For FCC units with closed riser termination device (RTD)/cyclone systems, do you operate with the primary separator sealed or unsealed in the stripper bed? What differences in performance do you see between these modes? Which do you prefer?

The answer to this question depends on the specific RTD technology.

Question 13: With the move toward greater utilization of “opportunity crudes” such as Canadian synthetic crudes, what shifts do you expect in FCC product yield and quality, and how will this impact the operation of the FCC unit?

Holly’s choice for opportunity crudes are somewhat limited by our position as an inland refiner and being located far away from many of the crude pipelines. We are making changes in both the way we operate our units, as well as our capital investment, so that we can maintain our current slate with crudes of varying quality.

Question 14: What reactions lead to acetone formation and how can they be mitigated? We have measured acetone concentrations between 100 and 1,200 ppm in the FCC butanes/butylenes stream.

We have very little data on this subject. We did find data on one virgin gas oil operation with 70 ppm to 110 ppm of acetone. If you are getting 1200 ppm, you probably have organic oxygen coming from some type of catalytically converted feedstock or recycle stream. We do not think you can have this level of acetone with pure virgin feedstock.

Question 15: What variables influence gasoline aromatics? In particular, please address feed properties, catalyst, and FCC operating conditions.

Feed properties play an important role, namely the amount of one- and two-ring aromatics in the feed. Single-ring aromatics pretty much go straight to gasoline. Many single-ring aromatics in gasoline, however, are formed via reactions; either via cyclization or the cracking of partially saturated multi-ring compounds.

Question 16: A number of refiners are adding a chloride dispersant to address FCC main fractionator overhead system plugging issues. What is your experience with these products and have you had issues with downstream gasoline product quality?

Processing of increased amounts of imported FCC feed, both gasoline and reduced crude, results in increased chloride salts in the main fractionator, which is usually a result of sea water contamination of transfers. Ammonium chloride salt dispersant is a chemical, which can be used to move or disperse ammonium chloride salts to prevent pluggage of the FCC main fractionator.

Question 17: What minimum nozzle velocities are required in air and steam distributors to prevent catalyst backflow and subsequent erosion? Please consider both upward and downward pointing nozzles.

The FCC has many distributors: combustion air, stripping steam, feed nozzles, and torch oil. Many routine FCC problems are the result of poor distributor operation; for example, poor yields due to poor catalyst/oil contact, poor stripping due to lack of catalyst/steam contact, and excessive attrition due to distributor damage.

Question 18: Some refiners have installed gas injection in FCC secondary cyclone diplegs to increase capacity and avoid defluidization problems. Please describe your experience operating with gas addition in the diplegs and any maintenance issues. What advice would you give to others considering this installation?

Cyclone dipleg aeration is a technique used to improve cyclone dipleg fluidization. It involves adding steam, a stream of purge gas—usually air or steam—to the dipleg just ahead of the trickle valve.

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