Q&A

(2010) Question 17: What are the best practices to manage ammonium chloride fouling ? What methods are used to set wash intervals? What are the potential pitfalls?

It is not unusual that NH3, H2S, and HCl are all present in the reactor effluent stream. Since ammonium chloride (NH4Cl), and ammonium bisulfide (NH4HS) form above the dew point of water, water is injected in reactor effluent train, upstream of the effluent air cooler.

(2010) Question 18: What are your key strategies to maximize the heavy diesel barrels in the diesel pool without cracking? Do you consider blending and dewaxing etc. to meet product specifications?

Generally maximizing heavy diesel barrels in the total refinery diesel pool would be based on distillation cut points the diesel processing units, mainly crude atmospheric columns. The objective is to maximize barrels by increasing distillation cut points up to distillation cut point maximum or to product quality specifications.

(2010) Question 19: In your experience, what are the effects on ULSD hydrotreaters when FCC operation is adjusted to maximize diesel?

There are a number of approaches to maximize the diesel yield from FCC units, such as catalyst optimization, process modifications, and changing the FCC product cut points. These approaches can be used independently or in combination - the ultimate objective being to maximize the production of light-cycle oil (LCO) from the FCC unit for subsequent conversion to diesel.

(2010) Question 20: How do refiners quantify the impact of sodium on hydroprocessing units, specifically those processing either residuum or VGO feeds?

Sodium generally enters a hydrotreater due to upstream addition of caustic soda or desalter operational problems. Feed sodium content of more than 3-5 ppm should be avoided. Sodium has a significant deactivation effect; 1-3 %wt results in a 50% loss of catalyst activity.

(2010) Question 23: The liquid recycle rate to a second stage of a hydrocracker can shift conversion, light end yields, cycle length and/or the required temperature to achieve a desired conversion. What strategies do you employ to reach optimum conditions

In two-stage hydrocracking design there are several variables that need to be balanced for optimum performance. Desired product yields or selectivity can be affected by conversion per pass in each stage. Lower conversion per pass is desirable to maximize heavier product selectivity.

(2010) Question 25: Besides high reactor temperatures and flow maldistribution, what are other causes of high gas and LPG yields in a hydrocracker have you experienced?

Hydrocrackers typically process heavy gas oils into distillate-range material. The gas oils are catalytically cracked at high pressures in the presence of hydrocracking catalyst and hydrogen. The reaction is exothermic and consumes a relatively large quantity of hydrogen. High gas and LPG yields would be generally undesirable in a properly operating hydrocracker.

(2010) Question 49: What testing procedures do you use for emergency shutdown valves? What are the parameters you measure and what are acceptable values?

The appropriate required testing of emergency shutdown valves is included in each Marathon refinery’s Mechanical Integrity (MI) program, complying with OSHA’s Process Safety Management regulation and EPA’s Risk Management Plan regulation.

(2010) Question 50: Chemical cleaning of towers and vessels prior to entry is being used to reduce time to entry. What practices are you employing and how much time is saved?

Our chemical cleaning practices for Crude/Vacuum and Coking units have evolved over many years. The starting point in most cases, after feed is out and the unit has circulated down, cooled and pumped out, is a flush with diesel or LCO followed by a good steamout.

(2010) Question 52: What areas of a delayed coker are susceptible to naphthenic acid corrosion? How do you determine the maximum allowable TAN for these areas?

The primary areas of concern include: •Coker feed circuit above 450 °F •Bottom section of the fractionator, including internals •Heater charge pumps and associated piping •Heater tubes.

(2010) Question 53: During the coking cycle, how are drum skin temperatures used to monitor drum wall condition?

Monitoring drum wall conditions during the drum cycle is best performed using strain gauges. Each strain gauge location will typically be fitted with gauges in both the axial and hoop directions and have an associated skin thermocouple.

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