Upflow and downflow reactors

Upflow and downflow reactors are often used in laboratory-scale studies, however, few comparative studies between these two systems are reported in the literature. Dudukovic et al [1] summarized some of these reports. They have found that the upflow reactor behavior can be better than downflow reactor and viceversa depending on gas and liquid velocities, level of... 

In order to imderstand the differences between upflow and downflow reactors for a given reaction, more systematic studies need to be done. 

The hydrotreating was performed in a fixed-bed upflow and downflow reactor, which was operated in isothermal mode by independent temperature control of a three-zone electric furnace. 

Goto, S. and K. Mabuchi. Oxidation of Ethanol in Gas-Liquid Cocurrent Upflow and Downflow Reactors. Can. J. Chem. Eng. 

Failing to identify the limiting reactant can lead to failure in the scale-up of trickle-bed reactors (Dudukovic, 1999). Gas-limited reactions occur when the gaseous reactant is slightly soluble in the liquid and at moderate operating pressures. For liquid-limited reactions, concurrent upflow is preferred (packed bubble columns) as it provides for complete catalyst wetting and thus enhances the mass transfer from the liquid phase to the catalyst. On the other hand, for gas reactions, concurrent downflow operation (trickle-bed reactors), especially at partially wetted conditions, is preferred as it facilitates the mass transfer from the gas phase to the catalyst. The differences between upflow and downflow conditions disappear by the addition of fines (see Section 3.7.3, Wetting efficiency in trickle-bed reactors). 

F. Larachi, A. Laurent, G. Wild and N. Midoux, Some experimental liquid saturation results in fixed-bed reactors operated under elevated pressure in cocurrent upflow and downflow of the gas and the liquid, Ind. Engng. Chem. Res., 30 (1991) 2404-2410. 

Kawakami et al. [ 18] studied mass transfer from the gas to the liquid in a biochemical process at upflow and downflow operation. They compared volumetric mass transfer coefficients for monoliths with those for trickle-bed reactors, based on equivalent geometric surface area. The finding was that mass transfer coefficients for monoliths are several times higher than for trickle beds (see Fig. 5). 

Wu Y, Khadilkar MR, Al-Dahhan MH, Dudukovic MP. Comparison of upflow and downflow two-phase flow reactors with and without fines. Ind. Eng. Chem. Res. 1996 35 397. 

Recently Fixed Bed Reactors with a cocurrent liquid and gas upflow have gained increasing interest. In this case the liquid is entrained by the gas in a system with high interaction between both phases that absolutely cannot be hydrodynamically similar to the usual TBR. An analogy occasionally exists when both systems are operated with high gas and liquid flow rates. Nevertheless upward flow reactors will be considered here too. Table 3 shows criteria for the choice between Upflow and Downflow Fixed Bed Reactors. 

Criteria governing the choice between Upflow and Downflow Fixed Bed Multiphase Reactors... 

The model was developed from experiments conducted in a downflow reactor, where gas residence times are greater than solid residence times. The model may not be able to predict residence time effect in a fluid bed or upflow reactor where solid residence times are greater than gas residence times. But it does give useful information for pressure and temperature effects in these reactors. 

In the once-through studies reported in the literature, a downflow reactor scheme was used for catalytic hydrocracking (9) in contrast to an upflow reactor scheme used in this study. It has been reported in the literature that an upflow reactor scheme is superior to the usual trickle-bed operation for residual feedstocks (18,19). Desulfurization, denitrogena-tion, and demetallization conversions were better in an upflow reactor. 

Experimental apparatus used in this study consisted of a 12.5 mm internal diameter stainless steel tubular reactor with a length of 864 mm (34 in). It was originally constructed and used as a trickle bed downflow reactor. With proper modification of the plumbing it was used to regenerate the catalyst in either an upflow or downflow mode. 

At low liquid flow rates, upflow will provide better distribution of liquid and, thus, in many cases, better performance of the reactor than the downflow reactor under similar operating conditions. 

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. 

Cocurrent downflow with slug or Taylor flow has been most widely used. Other possible designs, e.g., cocurrent upflow and froth flow, have to our knowledge been tested only in laboratory and pilot plant reactors. Consequently, we will focus on downward slug flow, and the main areas of interest are scale-up, liquid distribution, space velocity, stacking of monoliths, gas-liquid separation, recirculation, and temperature control. 

The initial application of fluidized beds in the petroleum industry was the upflow dilute-phase reactor (M45). The obvious disadvantage of this design is that all of the flowing catalyst passes overhead and must be removed in dust-removal equipment. Later, the basic design which finds widest application is the downflow dense-bed reactor (M2), which has the following major advantages (K26) ... 

In the present La Porte reactor (Figure 2) the basic flow pattern is an upflow at the center and downflow at the walls. This type of a flow pattern mixes the product with the reactants at the bottom of the reactor and causes poorer production of products. 

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