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(2013) Question 73: What is the current design philosophy in the lower section of a coker main fractionator (from tower bottoms up to first product draw) for controlling product quality and coke fines buildup?

The main objective is to keep the coke fines agitated and then efficiently remove them from the bottom of the fractionator. A properly designed coke drum with low vapor velocity helps minimize the coke fines carryover to the fractionator. Proper C factor for tower sizing is critical to achieving the HCGO quality in low pressure cokers.
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(2017) Question 28: Under what conditions will you strip sulfur from hydrotreating/hydrocracking catalysts?

Base-metal hydrotreating and hydrocracking catalysts are susceptible to sulfur stripping at high temperature in the presence of flowing hydrogen and in the absence of H2S. For that reason, we advise unit operators to avoid prolonged exposure of sulfided catalysts to flowing hydrogen at temperatures exceeding 500°F if the H2S content of the hydrogen is below 50 ppmv (parts per million by volume).
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(2017) Question 30: What are common mechanical defects that occur to the weld overlay material in hydroprocessing reactors? What are the most common locations for defects, and does the location play a factor in the mechanical integrity of the equipment? How do you detect and repair the defects? How often do you conduct Remaining Life Analysis (RLA) and/or Fit For Service (FFS) Assessment on critical equipment?

The common mechanical defects that occur are disbonding and cracking. Non-vanadium-modified 2¼ Cr-1 Mo material is more susceptible to disbonding than vanadium-modified 2¼ Cr-1 Mo-V material. V-modified 2¼ Cr-1 Mo has a much higher solubility of hydrogen compared to conventional 2¼ Cr -1 Mo. 
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(2017) Question 31: What are the potential impacts to hydrocracking units [i.e., deactivation rate, HPNA (heavy polynuclear aromatics) formation, etc.] as heavy coker gasoil (HCGO) rate/endpoint are increased?

Processing heavier and cracked feedstocks poses many challenges to the hydrocracking unit. Thermally-cracked feedstock such as HCGO, apart from being unsaturated, has relatively lower API, higher sulfur, and nitrogen content, higher proportion of C7 insoluble, and Conradson carbon residue (CCR). An increase in the HCGO distillation endpoint results in a significant increase in the proportion of polynuclear aromatics (PNA) and asphaltenes, both of which are coke precursors, which results in an exponential increase in catalyst deactivation rates. 
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