Mixing, in slurry reactors

In the cobalt-catalyzed Fischer-Tropsch reaction, oxygen is mainly rejected as water and this will generate high partial pressures of water at the reactor exit for fixed-bed reactors. As a consequence of extensive back mixing in slurry reactors,... 

Here, it has to be noted that for calculating the Peclet number in fixed beds, the actual velocity has to be used, i.e. the interstitial velocity, which influences the degree of mixing. In slurry bubble column reactors, the real velocity of the fluid is the bubble velocity, which is much higher than the gas superficial velocity. The mean bubble rise velocity for a batch liquid is (eq (3.201))... 

Pitch-blade turbine (paddle stirrer with pitched blades) and propeller stirrers provide high mixing with an axial flow pattern. Both of these stirrers are normally used for low-viscosity liquids and in vessels with baffles. They are well suited for providing liquid homogenization and suspension of solids in slurry reactors. The stirrers can also be used in viscous fluids and for vessels with H/dT > 1, which are generally encountered in fermentation processes. For these situations, axial flow is increased with the use of multistage stirrers with pitched stirring surfaces. 

The mixing in agitated vessels has been extensively studied, and this subject is well reviewed by Naga (1975) and Uhl and Gray (1966). In slurry reactors, it... 

In slurry reactors, the liquid phase is completely backmixed, whereas backmixing in the gas and solid phases may not be complete. The gas-phase mixing depends on the design of the impeller and the nature of the bubbles, as well as the superficial gas velocity. The presence of gas reduces liquid-phase mixing however, an increase in gas flow increases the mixing. The mixing is also dependent upon the coalescence rate of the bubbles. 

The scale-up of monolith reactors is expected to be much simpler. This is due to the fact that the only difference between the laboratory and industrial monolith reactors is the number of monolith channels, provided that the inlet flow distribution is satisfactory. In slurry reactors, scale-up problems might appear. These are connected with reactor geometry, low gas superficial velocity, nonuniform catalyst concentration in the liquid, and a significant back-mixing of the gas phase. 

In slurry reactors (Fig. 8.8) small catalyst particles (10-100 pm) are suspended in a liquid phase by mechanical mixing. Stirring also improves the contact between the gas bubbles and the liquid phase and heat exchange with the surroundings. Slurry reactors can be operated in a batch, semi-batch or continuous mode. In the semi-batch mode often the gas-phase is supplied continuously. 

In slurry systems, similar to fluidized beds, the overall rate of chemical transformation is governed by a series of reaction and mass-transfer steps that proceed simultaneously. Thus, we have mass transfer from the bubble phase to the gas-liquid interface, transport of the reactant into the bulk liquid and then to the catalyst, possible diffusion within the catalyst pore structure, adsorption and finally reaction. Then all of this goes the other way for product. Similar steps are to be considered for heat transfer, but because of small particle sizes and the heat capacity of the liquid phase, significant temperature gradients are not often encountered in slurry reactors. The most important factors in analysis and design are fluid holdups, interfacial area, bubble and catalyst particle sizes and size distribution, and the state of mixing of the liquid phase. ... 

Slurry reactors involve the coexistence and intense mixing of gas, liquid, and solid phases in the same volume. The possibility to run slurry reactors in the batch, semibatch, or continuous modes differentiates these reactors from others in terms of operational flexibility. In slurry reactors, the roles of the three phases can be different, that is, liquid can be a reactant, a product, or an inert that serves as a contacting medium for gas and solids. Similarly, dissolved gas can either be a reactant or an inert for inducing mixing of liquid and solids via bubbling. The solid phase usually corresponds to the finely dispersed catalyst particles with diameters lower than 5 x 10 m [20]. 

In slurry reactors (Figure 4.30), a slurry of liquid containing the reactant B mixed with solid catalyst particles is passed through the reaction vessel while the gas containing reactant A is bubbled through the slurry contained in the vessel. Slurry reactor is used for the hydrogenation of vegetable oil in the presence of Ni catalyst particles. The reaction is... 

A catalytic gas-liquid reaction is carried out in a slurry reactor with small catalyst particles, for which the internal mass transfer resistance is negligible. Mixing in the reactor is inefficient, and thus some external mass transfer resistance remains at the outer surfaces of the catalyst particles. The overall stoichiometry is given by... 

Effect of axial dispersion in the liquid and the solid phase Where effects of axial dispersion of the gas phase can usually be neglected in slurry reactor design, the effects of mixing in the... 

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

Topics that acquire special importance on the industrial scale are the quality of mixing in tanks and the residence time distribution in vessels where plug flow may be the goal. The information about agitation in tanks described for gas/liquid and slurry reactions is largely apphcable here. The relation between heat transfer and agitation also is discussed elsewhere in this Handbook. Residence time distribution is covered at length under Reactor Efficiency. A special case is that of laminar and related flow distributions characteristic of non-Newtonian fluids, which often occiu s in polymerization reactors. 

Liquid residence-time distributions in mechanically stirred gas-liquid-solid operations have apparently not been studied as such. It seems a safe assumption that these systems under normal operating conditions may be considered as perfectly mixed vessels. Van de Vusse (V3) have discussed some aspects of liquid flow in stirred slurry reactors. 

In some applications such as catalytic hydrogenation of vegetable oils, slurry reactors, froth flotation, evaporative crystallisation, and so on, the success and efficiency of the process is directly influenced by the extent of mixing between the three phases. Despite its great industrial importance, this topic has received only limited attention. 

In a slurry reactor (Fig 5.4.74), the catalyst is present as finely divided particles, typically in the range 1-200 pm. A mechanical stirrer, or the gas flow itself, provides the agitation power required to keep the catalytic particles in suspension. One advantage is the high catalyst utilization not only is the diffusion distance short, it is al.so possible to obtain high mass-transfer rates by proper mixing. 

A reactor model based on solid particles in BMF may be used for situations in which there is deliberate mixing of the reacting system. An example is that of a fluid-solid system in a well-stirred tank (i.e., a CSTR)-usually referred to as a slurry reactor, since the fluid is normally a liquid (but may also include a gas phase) the system may be semibatch with respect to the solid phase, or may be continuous with respect to all phases (as considered here). Another example involves mixing of solid particles by virtue of the flow of fluid through them an important case is that of a fluidized bed, in which upward flow of fluid through the particles brings about a particular type of behavior. The treatment here is a crude approximation to this case the actual flow pattern and resulting performance in a fluidized bed are more complicated, and are dealt with further in Chapter 23. 

Skeletal catalysts are usually employed in slurry-phase reactors or fixed-bed reactors. Hydrogenation of cottonseed oil, oxidative dehydrogenation of alcohols, and several other reactions are performed in sluny phase, where the catalysts are charged into the liquid and optionally stirred (often by action of the gases involved) to achieve intimate mixing. Fixed-bed designs suit methanol synthesis from syngas and catalysis of the water gas shift reaction, and are usually preferred because they obviate the need to separate product from catalyst and are simple in terms of a continuous process. 

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