Two-phase downflow fixed-bed reactors

Two-phase downflow fixed-bed reactors operate in a variety of flow r )mes ranging from gas-continuous to liquid-continuous patterns. They usually fall into two broad categories referred to as low and high interaction r )mes. The low interaction r )me (trickling flow) manifests at low gas and liquid flow rates and is characterized by a weak gas-liquid interfacial... 

As the catalytic reaction taking place inside the pellets is usually accompanied by heat effects, the particle-liquid heat transfer coefficient becomes a fundamental ingredient to be estimated for the assessment of the efficacy of the heat withdrawal from the particle level away to the reactor wall leveL In particular, when highly exothermic reactions are in play, impediment of liquid replenishment over the dried spots on the catalyst surface may favor inception of hot spots that are responsible for reactor runaway. As a result, evacuation of heat across the liquid-covered pellet spots becomes a critical issue. Not many studies in literature deal with particle-liquid heat transfer rates in three-phase fixed-bed reactors. The main reason is probably the difficulty to find an accurate experimental method. The following current trends emanate from the analysis of the particle-liquid heat transfer two-phase downflow fixed-bed literature (i) the transition from trickle to pulsing flow is accompanied by a... 

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... 

Medeiros, E.B.M., Petrissans, M., Wehrer, A., and Zoulalian, A. (2001), Comparative study of two cocurrent downflow three phase catalytic fixed bed reactors Application to the sulphur dioxide catalytic oxidation on active carbon particles, Chemical Engineering and Processing, 40(2) 153-158. 

Whereas in a fixed bed reactor with a single fluid phase there exist only two modes of operation, either downflow (which is used in most cases) or upflow, and only two different flow regimes, either laminar or turbulent flow, which can be observed and characterized by a Reynolds number as the single relevant dimensionless group, the fluiddynamics in multiphase catalytic fixed bed reactors are much more complex. 

Standard two-fluid models for two-phase downflow and upflow in three-phase fixed-bed reactors... 

In two-phase downflow and upflow fixed-bed reactors, gas-liquid mass transfer resistance can be detrimental to the overall reactor performance [14, 32]. Therefore, accurate estimation of gas-liquid mass transfer parameters is important for achieving successful reactor design or scale-up. The overall gas-liquid mass transfer coefficient may be expressed, according to the two-film concept, in terms of the liquid-side and the gas-side mass transfer coefficients ... 

The Simulation or Design of a Packed Bed Absorption Tower The Absorption of GO2 into a Monoethanolamine (MEA) Solution 14.3.2 Two-Phase Fixed Bed Gatalytic Reactors with Gocurrent Downflow. Trickle Bed Reactors and Packed Downflow Bubble Reactors... 

Two-Phase Fixed Bed Catalytic Reactors with Cocurrent Downflow. Trickle Bed Reactors and Packed Downflow Bubble Reactors... 

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... 

To provide an illustration, the flow sheet of the IFP process shown in Fig. 3.12 comprises two possible variants. The simpler corresponds to the direct use of the etherified solution in the gasoline pool, without separating e excess methanol contained. Operations are conducted with two reactors in series the first with an upflow stream and expanded bed with recycle of part of the previously cooled effluent for better control of the temperature rise, and the second with a downflow stream and a fixed bed. The more complex involves the recovery of excess methanol, first by azeotropic distillation in a depentanizer with part of the unconverted hydrocarbons, and then by water washing of this raffinate. The hydrocarbon phase is added to the bottom of the depentanizer. The water/methanol mixture is distilled to recover and recycle the alcohol to the etherification staee. 

As indicated in Table 17.1, there are essentially three main classes of three-phase fixed-bed catalytic reactors. The class of reactors characterized by cocurrent downflow of gas and liquid is called the trickle bed reactor (TBR). We shall be concerned here only with these reactors, for they are more commonly used in organic technology than the other two variations. 

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