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 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.

(2010) Question 22: In your experience, how are ULSD units maximizing catalyst life/ cycle length? Do you use feedstock or catalyst analysis to locate sources of contaminants, especially arsenic?

Many factors impact the cycle length in a ULSD unit, and in order to ensure the longest possible cycle length in such units it is important to: Have an optimal flow distribution and gas mixing using latest generation reactor internals designed for the actual operating conditions. This is very important in order to ensure maximum catalyst utilization with no channeling in the catalyst bed.

(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.

Question 24: For a hydrocracker with a debutanizer/stabilizer column, what corrosion issues do refineries experience in the feed and/or overhead systems? What have you done to mitigate the corrosion? What are your key considerations in optimizing these parameters

This answer assumes that the Debutanizer column is the first fractionation column design. In such columns, corrosion of the column bottoms, bottom outlet piping, reboiler tubes, fractionator feed heater tubes and the downstream fractionator is possible due to poor stripping of H2S.

(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 26: What are the best practices for entering the vapor space above an internal floating roof in a gasoline tank

Entering the vapor space above an internal floating roof tank creates a set of somewhat unique safety concerns that must be addressed in a facility’s safe work procedures. The primary hazard is entry into an air atmosphere with some level of hydrocarbon vapor or toxics, and liquid hydrocarbon (gasoline for this discussion) beneath the floor, with wiper seals, pontoons, etc. creating a barrier to prevent conditions within the confined space from changing.

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