Cocurrent downflow reactor

Low pressure-drop and no flooding (at least for cocurrent, downflow reactors). 

Figure 5-9 Multiple-sphere cocurrent-downflow reactor (simulation of irickle-bcd reactor) lifter Satterfield et al.i0)...

Low investment and operating costs. Low pressure drop and no flooding, for cocurrent, downflow reactors. Possibility of operating partially or totally in the vapor phase by varying the liquid flow rate. 

Figure 5.2-1. Types of gas-liquid-solid fixed bed reactors, (a), countercurrent flow (b), cocurrent downflow (c), cocurrent upflow.

K.M. Ng, A model for flow regime transitions in cocurrent downflow trickle-bed reactors, AIChE Journal, 32 (1986) 115-122. 

G. Wild, F. Larachi and A. Laurent, The hydrodynamics characteristics of cocurrent downflow and cocurrent upflow gas-liquid-solid catalytic fixed bed reactors the effect of pressure, Revue de l lnstitut Franfais du Petrole, 46 (1991) 467-490. 

Figure 21 Configuration of a cocurrent downflow monolith reactor with free gas recirculation. Only liquid is recirculated, and an external heat exchanger can be scaled independent of the reactor to deliver the required heat duty.

A trickle bed is a continuous three-phase reactor. Three phases are normally needed when one reactant is too volatile to force into the liquid phase or too nonvolatile to vaporize. Operation of a trickle bed is limited to cocurrent downflow to allow the vapor to force the liquid down the column. This contacting pattern gives good interaction between the gaseous and liquid reactants on the catalyst surface. 

Trickling and Pulsing Transition in Cocurrent Downflow Trickle-Bed Reactors with Special Reference to Large-Scale Columns... 

A model is presented to predict flow transition between trickling and pulsing flow in cocurrent downflow trickle-bed reactors. Effects of gas and liquid flow rates, particle size, and pressure on the transition are studied. Comparison of theory with published transition data from pilot-scale reactors shows good agreement. Since the analysis is independent of reactor size, calculations are extended to include large-scale columns some interesting observations concerning flow transition and liquid holdup are obtained. 

Monolithic Loop Reactor A novel MLR was developed af Air Products and Chemicals (Figure 17) (144). The reactor contains a monolithic catalyst operating under cocurrent downflow condifions. Because the residence time in the monolith is short and the heat of reaction has to be removed, the liquid is continually circulated via an external heat exchanger until the desired conversion is reached. The concept was patented for the hydrogenation of dinifrofoluene fo give toluenediamine (37). 

Little is known about the fluid wall heat transfer in the case of gas -liquid flow in a fixed-bed reactor. Some research on this subject, however, has been carried out for the specific case of cocurrent downflow over a fixed-bed reactor. This is summarized in Chap. 6. Some work on the slurry-wall heat-transfer rate for a three-phase fluidized bed has also been reported. The heat-transfer rate is characterized by the convective heat-transfer coefficient between the slurry and the reactor wall. Some correlations for the heat-transfer coefficient in a three-phase slurry reactor are discussed in Chap. 9. 

Figure 1-1 Types of gas liquid-fixed-bed-solid reactor, (o) Fixed-bed cocurrent downflow, (fc) fixed-bed countercurrent flow, (c) fixed-bed cocurrenl upflow.

As discussed in Chap. 3, there are a large number of models proposed to evaluate macromixing in a trickle-bed reactor. A brief summary of the reported experimental studies on the measurements of RTD in a cocurrent-downflow trickle-bed reactor is given in Table 6-7. Some of these experimental studies are described in more detail in a review by Ostergaard.94 Here we briefly review some of the correlations for the axial dispersion in gas and liquid phases based on these experimental studies. 

Table 6-7 A summary of experimental RTD studies in cocurrent-downflow packed-bed reactors... 

In a multichannel monolith with cocurrent downflow, each channel will have the same residence time and a residence-time distribution close to an ideal tubular reactor. But due to nonuniform flow distribution, the gas-liquid ratio, the volume reactant per volume catalyst, and, consequently, the conversion can be very different in different channels. 

The differences between the TBR and the MR originate from the differences in catalyst geometry, which affect catalyst load, internal and external mass transfer resistance, contact areas, as well as pressure drop. These effects have been analyzed by Edvinsson and Cybulski [ 14,26] via computer simulations based on relatively simple mathematical models of the MR and TBR. They considered catalytic consecutive hydrogenation reactions carried out in a plug-flow reactor with cocurrent downflow of both phases, operated isothermally in a pseudo-steady state all fluctuations were modeled by a corresponding time average ... 

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. 

In cocurrent downflow the liquid distribution is the most sensitive part, since there is no redistribution of liquid within the monolith. The liquid must be equally distributed to every channel at the top. On the other hand, when the liquid is evenly distributed, there is no maldistribution further down in the monolith. Nonuniform liquid distribution gives nonuniform conversion, resulting in larger reactors and lower selectivities. 

Since the residence time in cocurrent downflow is very short, it is necessary to recirculate the liquid and the gas. Also, the best performance from a mass transfer point of view is when the gas and liquid volume flow rates are about equal, and with a bubble/slug length of about O.S-2 cm. In these cases the molar flow of gas is much less than that of liquid, and the gas component will be consumed before the liquid component reaches complete conversion. Without recirculation, new gas must be added to the liquid further down in the reactor. 

In moving-bed reactors, both the feed and the catalyst move in cocurrent downflow and the catalysts are continuously renewed. The metal content increases along the bed and metal-rich catalysts are withdrawn from the bottom. It can handle the feed having higher metal content. The moving-bed reactor can be used as a first reactor for demetallization and asphaltene disaggregation. Other conversions (HDN, HDS) can take place in a fixed bed downstream. 

Fig. 5 Fixed-bed reactors with gas-liquid flow. (A) Trickle-bed reactor with cocurrent downflow (B) trickle-bed reactor with counter-current flow and (C) packed bubble-flow reactor with cocurrent upflow.

Process intensification is also possible by induced pulsing a liquid flow in trickle beds to improve liquid-solid contacting at low liquid mass velocities in the cocurrent downflow mode [64]. In a trickle bed reactor the liquid and gas phases flow... 

Winterhottom, J. M., Kan, Z., Boyes, A. P., and Raytirahasay, S., 1997, photocatalyzed oxidation of phenol in water using a cocurrent downflow contactor reactor (CDCR), Environ. Prog, 16(2) 125-131. 

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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