(81b) Fixing a Critical 2nd Stage Pygas Hydrogenation Reactor ?P Problem

From the initial start-up of the 2^nd stage Pygas hydrogenation reactor at a US Gulf Coast ethylene cracker, the reactor suffered short cycle lengths due to rapid ∆P growth. This paper illustrates how a close collaboration between CRI and the Plant Site developed a comprehensive technical solution that significantly improved cycle times and eliminated the need to purchase multiple batches of replacement catalyst. This paper will describe the operational and catalyst changes that significantly improved the performance of this reactor.

The typical catalyst cycle time of the 2^nd stage reactor, until the reactor reached a ∆P limit and required full catalyst replacement, ranged from 6 to 12 months’. The root cause for the rapid fouling and ∆P growth was the presence of a high boiling tail in the feed to the vapor phase reactor. Insufficient vaporization resulted in liquid mist carryover into the reactor that caused rapid fouling. Once this understanding was reached, the site worked a variety of operational changes to reduce the amount of heavy feed components but could not totally eliminate them. Through close collaboration the plant incorporated a new reactor catalyst loading scheme that included a catalyst grading system of high void space inert and active catalyst components at the reactor inlet. The collaboration also used comprehensive kinetic models to optimize the operation of the entire reactor system to help mitigate the liquid carryover. The combination of operational changes and the new reactor loading scheme increased the cycle time to ~28 months, far exceeding any previous cycle length. Since the initial optimization the unit has been further optimized yielding a cycle life of ~40 months.