Q&A

(2010) Question 30: What process parameters can affect alkylate T90? What are the critical variables you monitor in both sulfuric and HF units? Discuss processing schemes, feed impacts and operating variables.

The type of feed is very significant for T90. Amylenes make alkylate with higher T90 in both sulfuric and HF units. Propylene generally makes lower T90 than butylene in HF units. However, with sulfuric-catalyzed technologies, propylene can increase T90 as discussed below.

(2010) Question 31: In your experience, when sampling the HF Alky iso-recycle stream, how and where is the sample neutralized prior to analysis? Can this approach be used for online GC analysis as well?

The isobutane recycle sample can be neutralized at the sample location using a chamber filled with alumina or KOH pellets. If using a KOH chamber, it is best to add a filter downstream to filter out any fines.

(2010) Question 32: In your experience, what contributes to Monel denickelification in the HF Acid Regenerator circuit? What are the potential problems associated with this?

Oxygen is a major cause of monel denickelfication. Oxygen can enter the circuit during loading operations. Care should be taken to avoid pressuring air contained within loading pipes/hoses into the unit.

(2010) Question 33: How do refiners avoid De-isobutanizer (DIB) column/reboiler fouling in sulfuric acid alkylation? What process conditions on the column do you use to detect this fouling? What process modifications do you take to minimize the impact of this fouling?

Fouling in the DIB column is almost always caused by salt deposits. These salts are typically sodium sulfate and sodium sulfite but can also contain calcium or magnesium if the effluent treating water is not demineralized. If these water-soluble salts are present in the DIB feed, the water will evaporate once inside the column leaving the solids behind.

Question 1: Historically, instrument air was used to purge FCC reactor instruments. More recently, dry gas or nitrogen is typically used for this service. Please explain the reasons for moving away from air and provide examples of operating upsets that have occurred when using air to purge instruments.

Gas purge streams for instrumentation in FCCU reactor-regenerator service are required to ensure their functionality. For example, the reliability of level- and pressure-measuring devices relies on clear impulse lines. The utilization of a continuous-purge gas stream ensures that catalyst particles are prevented from entering or are swept from impulse lines to keep them clear and reliable.

Question 2: Which type of valve technology or design is typically utilized in units with high catalyst withdrawal rates? Do you continuously withdraw catalyst? From a reliability and safety perspective, what type of hardware are you using for control? What is the best withdrawal line design?

Valve selection for FCC catalyst withdrawal services is dictated by temperature and erosion considerations.

Question 3: Carbonate stress corrosion cracking (CSCC) has been identified as a cause of failure in FCC main fractionator overhead systems. What changes in feed quality, unit operation, or configuration would lead to increased risk of CSCC? What parameters do you monitor to determine whether a system is susceptible to CSCC? Has the problem been significant enough to warrant either comprehensive PWHT in potentially affected areas or localized PWHT when problem areas are identified?

Carbonate stress corrosion cracking, CSCC, is characterized by inter-granular, sometimes branchy, scale-filled cracks. It is believed that ammonium carbonate is the main contributor to the cracking mechanism.

Question 5: What is the shortest possible time between oil out and entry for maintenance on large inventory, high capacity FCC units? How is this achieved?

With FCCs at 27,000 bpd and 9,000 bpd capacity, neither of our units really qualifies for high throughput or large capacity. However, in discussions amongst the members of the panel, we found that throughput and inventory are not necessarily good indicators of how quickly you can move oil-out to maintenance-in.

Question 6: Some CO and waste heat boilers operate with bypass stacks separated by seal pots or isolation valves. Maintenance of these seal systems can be expensive and these seal systems can be sources of poor reliability. What design upgrades and operating practices have enabled you to eliminate these bypass systems?

Holly Refining continues to use bypass stacks with diversion valves at both of our refineries. Those stacks and diversion valves are allowed by permit and we use them during startup/shutdown manning upset conditions. Both our Operations and Maintenance groups are assigned weekly preventative maintenance tasks to ensure operability when we do have to throw these valves.

Question 7: Is your company either considering, or actually implementing, FCC projects that include reduced CO2 emissions (greenhouse gas reduction-GHGR) as an offset/credit?

We do not have any current FCC projects that plan on utilizing some form of CO2 offset or credit in the emissions analysis. ConocoPhillips has performed a limited modeling study to evaluate the effect of operational changes on CO2 emissions from the FCC. Also, the study indicated that there is very little you can do to reduce overall emissions without also overall significantly reducing unit throughput or conversion.

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