Q&A

(2010) Question 9: What experience do refiners have regarding fired heater stainless steel tubes OD/external polythionic acid attacks due to sulfur in fuel gas such as the ones in hydroprocessing units? What criteria are refiners using to decide when to neutralize the external side of the tubes with soda ash during turnarounds?

If it has been determined that sulfide scale is present, then the heater tubes can best be protected by maintaining a balanced set of small fires (or pilots, as applicable) in the heater box at all times, even when there is no process circulation

(2010) Question 10: What are refiners' experience with respect to unit availability, catalyst performance and product quality when co-processing "renewable" feedstocks in a ULSD unit? Is there a big variation in operability with different sources of renewable feedstocks?

The co-processing of renewable feedstocks in a ULSD unit means introduction of feed components that are completely foreign to a diesel hydrotreater. These oxygenates will react quite readily with hydrogen to form normal paraffins in the diesel range but in doing so consume high amounts of hydrogen and cause high heat release.

(2010) Question 11: Are any operators still using salt dryers for ULSD or Jet? If so, do you have any related product quality or corrosion issues?

Sunoco has a few ULSD units that employ salt drying of their ULSD rundown product. These units produce an on-specification product that meets the D4176 Haze spec of 2. As far as corrosion, no major issues have been identified to date

(2010) Question 12: Now that ULSD production has seen several cycles, what are the SOR and EOR operating conditions? What catalyst formulations are you using (NiMo, CoMo, regen, various blends)?

The operating conditions for ULSD vary and will depend on the feed to be processed as well as specific requirements to the product quality. This is apart from the typical US and European standards for ULSD, such as low aromatics, color, cold flow properties or other additional specifications.

(2010) Question 13: Severe fouling of diesel and gas oil hydrotreating preheat exchangers has been a growing problem. In your experience, what are the causes and how can these be prevented? Have you tried antifoulant injection in this service?

Fouling of the heat exchanger train is sometimes a problem particularly when processing cracked feed stocks. The fouling is often caused by polymer like compounds (gums) that form when petroleum distillates come in contact with air.

(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 21: Silica uptake on gas oil and diesel hydrotreating units is an increasing problem. In your experience, what is the source of silica in these feeds? Do you have effective ways to manage this silica?

The main source of silicon in Hydrotreaters is polysiloxane compounds (such as polydimethylsiloxane) used to control foaming in delayed coker units. It has also been reported that indigenous silicon is present in some heavy oils.

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