Abstract
Shell operates at its refinery in Cologne three reactors for the selective hydrogenation and desulfurization of a Pyrolysis gasoline stream (Pygas/C5 to C11), using a Ni-based catalyst. The hydrogenation reaction to saturate di-olefins and olefins taking place in these trickle flow reactor-beds is highly exothermic. Part of the hydrogenated liquid stream and unused hydrogen is recycled to the inlet of the reactor, to quench and regulate the temperature in the catalyst bed. A Reactive Hazard Assessment indicated that further improvements could be implemented to mitigate the risk to As Low As Reasonably Practicable (ALARP). Loss of liquid-recycle or hydrogen recycle or maldistribution of the liquid/gas hydrocarbon flow through the reactor lead to temperature increases which in turn can facilitate secondary chemical reactions that generate excessive heat. The heat formation can cause substantial local temperature excursions and subsequent runaway reactions (coking, polymerization). Localized temperature excursions, so-called hot-spots are considered a threat, especially if this formation occurs close to the reactor walls, because it increases the probability of vessel rupture and loss of containment. Several corrective measures were determined that will mitigate these risks. The new and innovative safety approach includes a redesign of reactor internals with new distributor-trays based on extensive research, the installation of over 20 temperature sensors per reactor-bed, a new safeguarding concept with low-rate emergency depressurization and measures to ease safe maintenance and construction. At the same time, the new customized a "Shell' reactor internals significantly improve the performance of reactors. The modifications will be installed at the turnaround between June and November 2018. In addition to the above, lessons-learned from the implementation and the start-up of the four reactors will also be included in the presentation.
