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In two stage hydrocracker units, as the catalyst ages the HPNA at the second stage feed increases and gets heavier. Is this going to cause a catastrophic blockage on the second stage CFEs or fractionator bottom network heat exchangers?
 
Answers
29/10/2020 A: Joel Kaye, Retired from UOP & Tesoro, kaye13@gmail.com
Increased HPNA will deposit in lower temperature CFE and bottoms exchangers. You can expect to see decreasing U-values and increasing pressure drops. I would not label it as catastrophic, but in severe cases it may drive your TA cycle length.
20/10/2020 A: Sridhar Balakrishnan, Bharat Oman Refineries Limited , laksrid@yahoo.com
HPNA ( heavy polynuclear aromatics ) are getting settled, not converted due to ageing of the catalyst. Check the asphaltene content ( ASTM D 6560 /IP 143 methods ) in the feedstock reguarly to maintain the HPNA content . Also check the polycyclic aromatic content of the products after conversion. This will give an idea of degree of conversion. Avoid accumulation of HPNAs (check with the technology supplier for any suitable method) . Also reduce the cracked material in the feed to the units to avoid formation of HPNAs.
13/10/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
You are right that the PNA content of the second stage feed can increase due to first stage deactivation and the resulting increase in reactor temperature. If no other change is made, this will increase deactivation of the second stage catalyst, and could also foul the effluent exchangers or air-cooler. However, a bleed of unconverted oil from the fractionator bottoms is used to control this problem. Most two-stage recycle units operate with a low-bleed, but also with close monitoring of the PNA content and colour of the unconverted oil and increase the bleed slightly to manage the situation. If the operators and process engineer are monitoring the situation and following the guidelines from the licensor or catalyst supplier, it is very unlikely that a catastrophic blockage would occur.
13/10/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Remember the 'product' of the first stage in a 2 stage hydro-cracker has already been severely hydrotreated. As such, it will be very stable with a very low 'fouling index'