Countercurrent flow reactors

Ammonia (see also under ammonium perchlorate). Perchloric ac vapor and anhyd ammonia were reacted in a countercurrent flow reactor with the object of obtaining a stable flame. The vapors were found to react vigorously with the formn of gaseous prods and copious deposits of amm chloride, but a stable flame could not be established (Ref 39)... 

Flooding is not a problem. Pressure drop is lower than in cocurrent-upfiow and countercurrent-flow reactors. 

Nonagitated three-phase countercurrent-flow reactors (spouted-bed reactors)... 

Several researchers have experimentally demonstrated the inhibiting influence of hydrogen sulfide (H2S) on HDS. This inhibiting influence is expected from simple and kinetic and equilibrium considerations. Refiners take great care to keep H2S in commercial hydrotreaters at an optimum level. For example, hydrogen—used in excess in a hydrotreater—is recirculated after scrubbing out the H2S by-product carefully. The recycle stream needs to contain an optimum level of H2S to keep the catalyst as a sulfide and thus maintain its activity and selectivity. Sie has described other process options to minimize inhibition effects by H2S, e.g., countercurrent flow reactors and monolithic catalyst systems. ... 

Various reactors were developed to handle different sluny polymerization processes. One model, a countercurrent-flow reactor for production of polyethylene, is shown in Fig. 5.2. The slurry is maintained in suspension by ethylene gas. The gas rises to the top and maintains agimtion, while the polymer particles settle to the bottom where they are collected. 

FIGURE 5.2. Countercurrent-flow reactor for slurry polymerization of ethylene with Ziegler catalysts (as illustrated in Koppeis Co. Inc., British Patent 826,563). 

The unit Kureha operated at Nakoso to process 120,000 metric tons per year of naphtha produces a mix of acetylene and ethylene at a 1 1 ratio. Kureha s development work was directed toward producing ethylene from cmde oil. Their work showed that at extreme operating conditions, 2000°C and short residence time, appreciable acetylene production was possible. In the process, cmde oil or naphtha is sprayed with superheated steam into the specially designed reactor. The steam is superheated to 2000°C in refractory lined, pebble bed regenerative-type heaters. A pair of the heaters are used with countercurrent flows of combustion gas and steam to alternately heat the refractory and produce the superheated steam. In addition to the acetylene and ethylene products, the process produces a variety of by-products including pitch, tars, and oils rich in naphthalene. One of the important attributes of this type of reactor is its abiUty to produce variable quantities of ethylene as a coproduct by dropping the reaction temperature (20—22). 

Fig. 8. Combined flow reactor models (a) parallel flow reactors with longitudinal diffusion (diffusivities can differ), (b) internal recycle—cross-flow reactor (the recycle can be in either direction), comprising two countercurrent plug-flow reactors with intercormecting distributed flows, (c) plug-flow and weU-mixed reactors in series, and (d) 2ero-interniixing model, in which plug-flow reactors are parallel and a distribution of residence times dupHcates that...

There are essentially three types of coal gasifiers moving-bed or countercurrent reactors fluidized-bed or back-mixed reactors and entrained-flow or plug-flow reactors. The three types are shown schematically in Eigure 2. 

In the first class, the particles form a fixed bed, and the fluid phases may be in either cocurrent or countercurrent flow. Two different flow patterns are of interest, trickle flow and bubble flow. In trickle-flow reactors, the liquid flows as a film over the particle surface, and the gas forms a continuous phase. In bubble-flow reactors, the liquid holdup is higher, and the gas forms a discontinuous, bubbling phase. 

Babcock et al. (Bl) examined the hydrogenation of a-methylstyrene catalyzed by palladium and platinum catalysts in a reactor of 1 -in. diameter under countercurrent flow. Flow rates were above 1500 kg/m2-hr for the liquid phase and above 15 kg/m2-hr for the gas, and it was concluded from the experimental results that mass transfer was not of rate-determining influence under these conditions. 

The CHF in vertical upward and downward, countercurrent flow was recently studied by Sudo et al. (1991) in a vertical rectangular channel. Sudo and Kaminaga (1993) later presented a new CHF correlation scheme for vertical rectangular channels heated from both sides in a nuclear research reactor. 

Compare T (z) and T Cz) trajectories of a wall-cooled PFTR with cocurrent and countercurrent flows. Which configuration is more likely to produce more problems with a hot spot in the reactor ... 

In multiphase reactors we frequently exploit the density differences between phases to produce relative motions between phases for better contacting and higher mass transfer rates. As an example, in trickle bed reactors (Chapter 12) liquids flow by gravity down a packed bed filled with catalyst, while gases are pumped up through the reactor in countercurrent flow so that they may react together on the catalyst surface. 

An example of an incredibly complex multiphase chemical reactor is iron ore refining in a blast furnace. As sketched in Figure 12-22, it involves gas, liquid, and solid phases in countercurrent flows with complex temperature profiles and heat generation and removal processes. 

To retain consistency throughout this presentation, we will consider a general nonadiabatic, packed bed reactor, as shown in Fig. 1, with a central axial thermal well and countercurrent flow of cooling fluid in an exterior jacket.1 We focus on the methanation reaction since methanation is a reaction of industrial importance and since methanation exhibits many common difficulties such as high exothermicity and undesirable side reactions. 

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

The countercurrent-flow fixed-bed operation is often used for physical absorption or for gas-liquid reactions rather than gas-liquid-solid processes. Shah [1] gives a comparison between a gas-liquid-solid (catalytic) fixed bed reactor and a gas-liquid-solid (inert) fixed-bed reactor. The major difference between these two types of reactors are the nature and the size of the packing used and the conditions of gas and liquid flow-rates. 

Fig. 4.17. Flow regimes in three-phase fixed-bed reactors, (a) Gas and liquid in co-current downwards flow (trickle-bed operation). (b) Gas and liquid in co-current upwards flow (liquid floods bed), (c) Gas and liquid in countercurrent flow (not often used for catalytic reactors)...

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

Hasselt BWv, Lebens PJM, Calis HP, Kapteijn F, Sie ST, Moulijn JA, Bleek CMvd. A numerical comparison of alternative three-phase reactors with a conventional trickle-bed reactor. The advantages of countercurrent flow for hydrodesulfurization. Chem Eng Sci 1999 54 4791-4799. 

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