Mechanical stirring, relative

Bubble columns and mechanically stirred reactors are the most common reactor types for slurry systems in laboratories, but they have many disadvantages from an industrialization perspective. Mechanically stirred reactors usually used for laboratorial studies are difficult to scale-up. In order to achieve good mixing and mass transfer between the gas and slurry phases, bubble column must be operated at a high space velocity, which leads to a relative low one-through conversion of the syngas. 

Table 5. Iron content in the C2-Ms and C2-Us/Ms catalysts found after different amounts of each catalyst were subjected to the digestion process. It is observed that relatively higher iron content was found in the catalyst prepared by mechanical stirring...

On each of these, random and structured reactors behave quite differently. In terms of costs and catalyst loading, random packed-bed reactors usually are most favorable. So why would one use structured reactors As will become clear, in many of the concerns listed, structured reactors are to be preferred. Precision in catalytic processes is the basis for process improvement. It does not make sense to develop the best possible catalyst and to use it in an unsatisfactory reactor. Both the catalyst and the reactor should be close to perfect. Random packed beds do not fulfill this requirement. They are not homogeneous, because maldistributions always occur at the reactor wall these are unavoidable, originating form the looser packing there. These maldistributions lead to nonuniform flow and concentration profiles, and even hot spots can arise (1). A similar analysis holds for slurry reactors. For instance, in a mechanically stirred tank reactor the mixing intensity is highly non-uniform and conditions exist where only a relatively small annulus around the tip of the stirrer is an effective reaction space. 

Make-up cobalt enters the process via carbonyl generator (2) and is combined with the olefin stream from extraction column (8), which already carries the recycled HCo(CO)4. With syngas from the purification section (1), the hydro-formylation takes place under the usual conditions (160-190°C 25-30 MPa 0.1-0.5 % cobalt relative to olefin) in the reactor (3) equipped with an external loop. No mechanical stirring is applied circulation and mixing are provided by the stream of liquid and gaseous reactants (mammoth pump principle) and by the heat of reaction. 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

A relatively early work came from Sarikaya and Akinc [175] who used a mineral oil with Arlacel 83, a non-ionic surfactant (Sorbitan sesquioleate) as the support solvent (continuous phase) and an aqueous solution of aluminum nitrate as the water phase to form the emulsion by mechanical stirring. The most stable emulsions were obtained with W/O ratio of about 0.45-0.55 (with 5% emulsifier). The emulsion thus produced was added dropwise to a hot (240 C) mineral oil to obtain precipitates of an alumina precursor. 

When the evolving gas flow is relatively small, the nucleation of bubbles may become a limiting factor, particularly in deep liquid layers. This may be a problem when the chemical reaction is reversible (e.g., esterifications). Mechanical stirring or stripping with an inert gas may be helpful (section 4.6.1.3). Another alternative is the use of a liquid film reactor (section 4.6.3.1). 

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