Co-current downflow

Taking the upward direction as positive, when both Fy < 0 and Fs<0, represents a co-current downflow operation, if Fs < 0, but Fy > 0 means a counter-current operation. The rest of the equation defining the system can be seen in Berty (1997). Integration was done by the Romberg method as is used in Mathcad PLUS 6 (1996) software. 

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... 

A heterogeneous tubular reactor that incorporates three phases where gas and liquid reactants are contacted with the solid catalyst particles, is classified as a trickle-bed reactor. The liquid is usually allowed to flow down over the bed of catalyst, while the gas flows either up or down through the void spaces between the wetted pellets. Co-current downflow of the gas is generally preferred because it allows for better distribution of liquid over the catalyst bed and higher liquid flow rates are possible without flooding. 

In ideal trickle flow reactors, all particles in the catalyst bed take part in the overall reactor performance, since each is surrounded (wetted) by the liquid phase that flows around it. Situations in which the liquid flows preferentially through a certain part of the bed, while the gas phase flows predominantly through another part, should be avoided [23]. In this case, part of the bed is not contacted by the liquid reactant at all and docs not contribute to the overall conversion. To avoid this maldistribution, Gierman [20] proposed the following criterion for the wetting number Wtx for co-current downflow operation ... 

Typical properties of slurry reactors, and of packed bed co-current downflow trickle flow reactors, are summarized in Table 1. Most properties indicated for slurry reactors also hold for three-phase fluidized beds. These properties can be advantageous or disadvantageous, depending on the application ... 

Property Slurry reactors Co-current downflow trickle flow reactors... 

Figure 1. Slurry reactors classified by the contacting pattern and mechanical devices (a) slurry (bubble) column (b) countercurrent column (c) co-current upflow (d) co-current downflow (e) stirred vessel (C) draft tube reactor (g) tray column (h) rotating disc or multi-agitated column reactor (i) three-phase spray column — liquid flow —> gas flow.

Aubert, E., Barreteau, D., Gauthier, T., and Pontier, R. Pressure Profiles and Velocities in a Co-current Downflow Fluidized Reactor, in Circulating Fluidized Bed Technology IV" (Amos A. Avidan, ed.), pp. 490-495. Somerset, Pennsulvania (1993). 

The modeling and design of a three-phase reactor requires the knowledge of several hydrodynamic (e.g., flow regime, pressure drop, holdups of various phases, etc.) and transport (e.g., degree of backmixing in each phase, gas-liquid, liquid-solid mass transfer, fluid-reactor wall heat transfer, etc.) parameters. During the past decade, extensive research efforts have been made in order to improve our know-how in these areas. Chapters 6 to 8 present a unified review of the reported studies on these aspects for a variety of fixed bed columns (i.e., co-current downflow, co-current upflow, and counter-current flow). Chapter 9 presents a similar survey for three-phase fluidized columns. 

Fig. 30. Contacting patterns and contactor types for gas-liquid-solid reactors, (a) Co-current downflow trickle bed. (b) Countercurrent flow trickle bed. (c) Co-current downflow of gas, liquid, and catalyst, (d) Downflow of catalyst and co-current upflow of gas and liquid, (e) Multi-tubular trickle bed with co-current flow of gas and liquid down tubes with catalyst packed inside them coolant on shell side, (f) Multi-tubular trickle bed with downflow of gas and liquid coolant inside the tubes, (g) Three-phase fluidized bed of solids with solids-free freeboard, (h) Three-phase slurry reactor with no solids-free freeboard, (i) Three-phase fluidized beds with horizontally disposed internals to achieve staging, (j) Three-phase slurry reactor with horizontally disposed internals to achieve staging, (k) Three-phase fluidized bed in which cooling tubes have been inserted coolant inside the tubes. (1) Three-phase slurry... 

Fig. 35. Flow regime map for co-current downflow of gas and liquid through packed beds.

Pangarkar K, Schildhauer TJ, van Ommen JR, Nijenhuis J, Moulijn JA, Kapteijn F. (2010) Heat transfer in structured packings with co-current downflow of gas and liquid. Chem. Eng. Sci., 65 420-426. 

Three phase reactions comprise gas-liquid-solid and gas-liquid-liquid reactions. Gas-liquid reactions using solid catalysts represent a very important class of reactions. They may be carried out in either slurry (such as bubble column, stirred tank and gas-liquid fluidized configurations) or fixed-bed reactors (trickle bed with co-current-downflow or co-current-upflow, segmented bed and counter-current gas-liquid arrangements). 

Figure 2 - Schematic of the co-current, downflow trickle bed packed with carbon coated Sulzer Static Mixer Packing T = thermocouple location, P = pressure tap...

Only a few recent results for co-current downflow packed bed and well stirred tank will be presented in the present review for the illustration of the previous considerations and because of the complex and simultaneous mechanism involved. A similar and complete review for the case of bubble columns may be obtained in the texts published by Deckwer et al. (72) and by Kastanek et al. (73). 

---