(430f) Characterization of Stable Flow Conditions in Fixed Bed Reactors with Downwards Directed Flows

In this work fixed bed reactors with downwards directed liquid flows were investigated. Maldistribution and instable flow conditions in fixed bed reactors result in hotspots and the shift of the conversion towards unwanted byproducts or even in a runaway of the reactor. To reduce the probability of the formation of maldistribution by ensuring stable downwards directed flow conditions, the dynamic pressure drop must exceed the buoyancy force which results from the local density differences. Otherwise, the liquid flow in the reactor becomes instable [1-3].

In this paper experimental and theoretical investigations are shown to determine instable flow regimes for the exothermic oligomerization of 1-Butene on a Nickel catalyst in a fixed bed reactor (D_i = 68 mm L = 1,000 mm). In experiments the reactor feed was reduced to provoke instable flow conditions starting from stable conditions. The occurrence of the transition between stable and instable flow conditions depends on more than the residence time respectively the velocity. Furthermore, the concentration of the reactant and feed temperature also showed an influence on the occurrence of instable flow conditions.

A CFD model (3D) and an Aspen Plus model (1D) were developed based on the experimental setup and results. At stable flow conditions both models showed good agreement with the experimental results (see Figure). While the Aspen (1D) model was not sufficient to predict the occurrence of instable flow conditions, the higher modelling depth of the CFD model (3D) gave the required information. Sensitivity studies with the complex CFD model were carried out to derive stability criteria for stable flow conditions.

[1] Nguyen, D., & Balakotaiah, V. (1995). P R Soc London A, 450(1938), 1-21.

[2] Luss, D., & Sheintuch, M. (2005). Catal today, 105(2), 254-274.

[3] Agrawal, R., West, D. H., & Balakotaiah, V. (2007). Chem Eng Sci, 62(18-20), 4926-4943.