(357a) Reactor Runaway Due to Statistically-Driven Axial Activity Variations In Graded Catalyst Beds

Edward M. Calverley, Paul M. Witt, Jeff D. Sweeney, Core R&D, the Dow Chemical Company, Midland MI 48674, USA

 In some multi-tubular fixed bed catalytic reactors, a graduated activity profile in the axial direction is created by loading several “zones” of catalyst, each with a different target catalytic activity.  This is often achieved by diluting active catalyst pellets with inert materials and can help balance the heat generation and heat removal capabilities of the reactor.  Partial oxidation reactions where one or more of the reagents undergoes very high conversion often use such strategies and some examples are ethylene oxychlorination to 1,2-dichloroethane, propylene oxidation to acrylic acid and ortho-xylene oxidation to phthalic anhydride.

 Loading a mixture of active and inactive particles into a reactor tube is an inherently statistical process, which produces non-uniform activity profiles within the reactor tubes.  In the present work, we have evaluated the impact of this purely statistical axial activity variation on the performance of a reactor for the conversion of ortho-xylene to phthalic anhydride.

 Simulations of commercial-scale reactors for the partial oxidation of o-xylene to phthalic anhydride suggest that statistically-driven axial activity variations can cause a significant fraction of the reactor tubes to run away, even though the reactor is operating within its stable range when a perfect activity profile is assumed.  These runaways occur for commercially relevant combinations of particle to tube diameter ratios (N) and extents of catalyst dilution.  Activity variations are exacerbated by larger catalyst particles (small N) and to a lesser degree by greater extents of catalyst dilution.

 These observations are relevant for practical systems where improved productivity can be achieved by using a graded activity profile and where increased throughput can be attained by using larger catalyst particles to reduce pressure drop.