Two-phase Flow in Trickle-Bed Reactors

Lynn F. Gladden, Laura D. Anadon, Matthew H. M. Lim, and Andrew J. Sederman 

Introduction to Magnetic Resonance Imaging of Trickle-bed Reactors 

Superficial velocities are defined as the respective volume flow rates divided by the cross-sectional area of the reactor. In subsequent figures the superficial liquid velocity is simply referred to as a liquid velocity. 

The MRI measurement is made by positioning the model trickle-bed reactor within the bore of the superconducting magnet, with the imaging section located 

MR signal intensity associated with them and Voxel resolution is 195 pm x 195 pm x 195 pm. are identified as black voxels. Flow velocities in 

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Morsi, B.I., Midoux, N, Laurent, A. and Charpentier, J.C., "Hydrodynamic and gas-liquid interfacial parameters of co-current two-phase downward flow in trickle-bed reactors". Communication presented at CHISA Congress, (1981). 

In conventional industrial multi-phase reactors, the heterogeneous catalyst can be organized as a packed (or fixed) bed of catalyst particles (e.g., in trickle-bed reactors or in submerged up-flow reactors), as catalyst particles suspended or fluidized in one of the two phases (in the hquid phase of a three-phase reactor, as for example in a slurry-stirred reactor and a slurry-bubbling reactor) or finally as a structured catalyst (e.g., monolith and membrane reactors). Structured catalysts are regular solid structures which reduce randomness through a well-defined structure and shape at a reactor level. The selection of the most appropriate traditional multi-phase... 

Two-phase flow in three-phase fixed-bed reactors makes the reactor design problem complex [12], Interphase mass transfer can be important between gas and liquid as also between liquid and catalyst particle. Also, in the case of trickle-bed reactors, the rivulet-type flow of the liquid falling through the fixed bed may result (particularly at low liquid flow rates) in only part of the catalyst particle surface being covered with the liquid phase. This introduces a third mass transfer process from gas to the so-called gas-covered surface. Also, the reaction rates in three-phase fixed-bed catalytic reactors are highly affected by the heat transfer resistances resistance to radial heat transfer and resistance to fluid-to-particle heat transfer. As a result of these and other factors, predicting the local (global) rate of reaction for a catalyst particle in three-phase fixed-bed reactors requires not only... 

Multiphase catalytic reactors are employed in nearly 80% of industrial processes with annual global sales of about 1.5 trillion, contributing around 35% of the world s GDP [17]. Microreactors for multiphase reactions are classified based on the contact principles of gas and liquid phases continuous-phase contacting and dispersed-phase contacting [18]. In the former type, the two phases are kept in continuous contact with each other by creating an interface. In the latter case, one fluid phase is dispersed into another fluid phase. In addition, micro trickle bed operation is reported following the path of classical chemical engineering. The study of mass and heat transfer in two-phase flow in micro trickle bed reactors still remains as a less... 

Mixed phase downflow fixed bed reactors can operate in several flow regimes. These bear resemblance to flow regimes in two phase flow in pipes. These flow regimes are the gas continuous ("trickle bed"), bubble flow, pulse flow and gas continuous blurring regime or spray flow. Many maps have been proposed in the... 

Turek, F.and R. Lange. Mass Transfer in Trickle-Bed Reactors at Low Reynolds Nuntoer. Chem. Eng. Sci. 36 (1981) 569-579. Turpin, J. L. and R. L. Huntington. Prediction of Pressure Drop for Two-Phase, Two-Component Concurrent Flow in Packed Beds. AICHE J. 13 (1967) 1196-1202,... 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

In the first class, the particles form a fixed bed, and the fluid phases may be in either cocurrent or countercurrent flow. Two different flow patterns are of interest, trickle flow and bubble flow. In trickle-flow reactors, the liquid flows as a film over the particle surface, and the gas forms a continuous phase. In bubble-flow reactors, the liquid holdup is higher, and the gas forms a discontinuous, bubbling phase. 

In connection with the engineering content of the book, a large number of reactors is analyzed two- and three-phase (slurry) agitated reactors (batch and continuous flow), two-and three-phase fixed beds (fixed beds, trickle beds, and packed bubble beds), three-phase (slurry) bubble columns, and two-phase fluidized beds. All these reactors are applicable to catalysis two-phase fixed and fluidized beds and agitated tank reactors concern adsorption and ion exchange as well. 

Trickle-bed reactors usually consist of a fixed bed of catalyst particles, contacted by a gas liquid two-phase flow, with co-current downflow as the most common mode of operation. Such reactors are particularly important in the petroleum industry, where they are used primarily for hydrocracking, hydrodesulfurization, and hydrodenitrogenation other commercial applications are found in the petrochemical industry, involving mainly hydrogenation and oxidation of organic compounds. Two important quantities used to characterize a trickle-bed reactor are... 

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

Two-parameter time-delay model (Buffham,14 Buflham and Gibilaro,15 and Buflham el al.18) This model is based on the concept of fluid elements being randomly delayed in time on their passage through the bed. The model has been mainly applied to the liquid-phase backmixing in a trickle-bed reactor. The model assumes that the liquid would flow in plug flow except for the fact that molecules... 

Three-parameter PDE model (Van Swaaij et aL106) This model is largely used to correlate the RTD curves from a trickle-bed reactor. The model is based on the same concept as the crossflow or modified mixing-cell model, except that axial dispersion in the mobile phase is also considered. The model, therefore, contains three arbitrary parameters, two of which are the same as those used in the cross-flow model and the third one is the axial dispersion coefficient (or the Peclet number in dimensionless form) in the mobile phase (see Fig. 3-11). 

Jacobs, G.E. Milliken, A.S. Evaluating liquid distributors in hydroprocessing reactors. Hydrocarbon Processing, International Edition 2000, 79, 75. Jiang, Y. Khadilkar, M.R. Al-Dahhan, M.H. Dudukovic, M.P. Two-phase flow distribution in 2D trickle-bed reactors. Chem. Eng. Sci. 1999, 54, 2409. 

Two-phase flow over a packing is given in Section 16.11.6.15 for trickle bed reactors. 

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