Cocurrent downflow

T. R. Melli, W. B. Kolb, J. M. deSantos, and L. E. Scriven, "Cocurrent Downflow in Packed Beds Microscale Roots of Macroscale Plow Regimes,"... 

Larkins et al. (L2) visually observed flow patterns and measured pressure drop and liquid holdup for cocurrent downflow of gas and liquid through beds of spheres, cylinders, and Raschig rings of diameters from 3 mm to f in. in experimental columns of 2- and 4-in. diameter, as well as in a commercial unit several feet in diameter. The fluid media were air, carbon dioxide, or natural gas and water, water containing methylcellulose, water containing soap, ethylene glycol, kerosene, lubricating oil, or hexane. 

Zabor et al. (Zl) have described studies of the catalytic hydration of propylene under such conditions (temperature 279°C, pressure 3675 psig) that both liquid and vapor phases are present in the packed catalyst bed. Conversions are reported for cocurrent upflow and cocurrent downflow, it being assumed in that paper that the former mode corresponds to bubble flow and the latter to trickle-flow conditions. Trickle flow resulted in the higher conversions, and conversion was influenced by changes in bed height (for unchanged space velocity), in contrast to the case for bubble-flow operation. The differences are assumed to be effects of mass transfer or liquid distribution. 

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

The TBR consist of a column that may be very high (above 10-30 m) equipped with a fixed bed of solid catalyst, through which a gas-liquid cocurrent downward flow occurs. In this cocurrent downflow configuration, the liquid flows mainly through the catalyst particles in the form of rivulets, films and droplets. 

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

Most often, at atmospheric pressure the interfacial area correlations use pressure drop as a variable because of their similarities. For example, Midoux et al. [56] proposed the following correlation for the cocurrent downflow in TBR ... 

G. Tosun, A study of cocurrent downflow of non-foaming gas-liquid systems in packed beds, Ind. Eng. Chem. Proc. Des. Dev., 23 (1984) 29-35. 

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.

Reactive absorption can be realized in a variety of equipment types, e.g., in him absorbers, plate columns, packed units, or bubble columns. This process is characterized by independent how of both phases, which is different from distillation and permits both cocurrent (downflow and uphow) and countercurrent regimes. 

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. 

Figure 3. Particle scale modeling of gas-liquid cocurrent downflow through a packed bed.

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. 

Wei, F., Chen, W., Jin, Y., and Yu, Z. Axial solids mixing in cocurrent downflow circulating fluidized bed, in Proceeding of 5th National Meeting on Chemical Reaction Engineering (in Chinese), Tianjing (1993). 

FIGURE 11 Hydrodynamic regimes encountered in gas-liquid flow in tubes. The liquid flow rate is constant while the gas flow rate increases, going from left to right. Gas and liquid are in cocurrent downflow. 

Achieving identical flow in each charmel can be difficult at low flow rates. A careful analysis of fhe hydrod)mamic sfabilify teaches that a monolith system is intrinsically unstable in cocurrent upflow. Conversely, cocurrent downflow is sfable when fhe flow velocity exceeds a critical value. A first approximation of fhe crifical flow velocify is fhe velocity that a liquid plug has when it falls under fhe influence of gravity inside the channel when it is open to the atmosphere at both ends. 

The most extensively investigated mode of flow in monoliths is cocurrent downflow. It can be realized in two ways, either with a controlled flow of gas or with a free recirculafion in bofh cases, fhe gas flow through the channels is caused by entrainment by the liquid at the entrance of fhe monolith (Figure 16). 

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.

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

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. 

---