Equipment Bubble-column reactor

External and internal loop air-lifts and bubble column reactors containing a range of coalescing and non-Newtonian fluids, have been studied (52,53). It was shown that there are distinct differences in the characteristics of external and internal loop reactors (54). Overall, in this type of equipment... 

The most frequently used contactors in full-scale waste water ozonation systems are bubble column reactors equipped with diffusers or venturi injectors, mostly operated in a reactor-in-series counter-current continuous mode. Many full-scale ozone reactors are operated at elevated pressure (2-6 barabs) in order to achieve a high ozone mass transfer rate, which in turn increases the process efficiency. 

The list is merely suggestive. Complexity of reactive flows may greatly expand the list of issues on which further research is required. Another area which deserves mention here is modeling of inherently unsteady flows. Most flows in engineering equipment are unsteady (gas-liquid flow in a bubble column reactor, gas-solid flow in a riser reactor and so on). However, for most engineering purposes, all the details of these unsteady flows are not required to be known. Further work is necessary to evolve adequate representation of such flows within the CFD framework without resorting to full, unsteady simulations. This development is especially necessary to simulate inherently unsteady flows in large industrial reactors where full, unsteady simulations may require unaffordable resources (and therefore, may not be cost effective). Different reactor types and different classes of multiphase flows will have different research requirements based on current and future applications under consideration. 

With the batch reactors used in the fine-chemical industry, the rate of the catalytic reaction is generally not decisively important. The number of catalyst particles per unit volume of the liquid to be treated is one of the experimental factors determining the apparent activity of the catalyst. Because the size of the catalyst particles usually affects the apparent activity of the catalyst only, the size is not critical, provided the particles are no smaller than ca 3 pm. When the size of the particles is below this, separation of the catalyst from the reaction product(s) is difficult, and with still smaller sizes even impossible. The requirement to avoid particles smaller than ca 3 pm imposes fairly severe requirements on the mechanical strength of catalyst particles employed in slurry-phase reactors. When the catalyst particles are liable to attrition, which leads to particles smaller than 3 pm, it is difficult to purify the reaction product(s) completely from the catalyst. Especially with fine-chemicals to be used in the food or pharmaceutical industry, contamination of the reaction product with the catalyst is usually not acceptable. Either mechanically strong catalyst particles must therefore be employed with slurry-phase catalysts or the reactor must be adapted to minimize attrition. With a bubble-column reactor the attrition of suspended catalyst particles is much smaller than with a reactor equipped with a stirrer that vigorously agitates the suspension. 

There is a definiti relationship between the coalescence suppressing effect of salt solutions, their ion strength and their position in the lyotropic (Hof-meister) series (19). At the same ion strength, the salt has a stronger effect which exhibits a greater tendency to flocculate proteins. Since no relationship is known for complex cultivation media the bubble coalescence behavior of media was experimentally determined by means of the volumetric mass transfer coefficients, kj a, which were measured in a standard bubble column reactor. These kLa values were compared with the kj a values measured in the same equipment under the Scime operational condi-... 

The internals of the bubble column reactor may have a dramatic impact on the flow patterns of the bubbles and the liquid. Companies have not divulged details about the internals to date. Some details of the US DOE pilot plant (22.5 inch 0.57 m diameter) have been published [ 106]. In this report the dimensions of the cooling tubes, their location, and their number are provided. These cooling coils occupied about 10% of the total volume of their commercial reactors slurry volume. The gas holdup and bubble characteristics as well as their radial profiles were determined in a column that was about the size of the US DOE reactor [107-109]. Dense internals were found to increase the overall gas holdup and to alter the radial gas profile at various superficial gas velocities. The tube bundle in the column increased the liquid recirculation and eliminated the rise of bubbles in the wall region of the column. These results indicate that further studies of bubble column hydrodynamics are directed toward larger scale units equipped with heat exchange tubes. 

In this chapter, we focus on our efforts to model dispersed multiphase flows in which a discrete phase (consisting of solid particles, gas bubbles, or liquid droplets) is moving through, or is moved by, a continuous Newtonian fluid phase. Such flows appear frequendy in process equipment in the chemical, metallurgical, pharmaceutical, and food industries. Examples include fluidized bed reactors, spouted bed reactors, pneumatic conveyors, bubble column reactors, slurry reactors, and spray driers. Figure 1 shows a schematic overview of typical dispersed multiphase systems. 

Values of the ratio V(IVR given in Table 24.1 emphasize that most of the volume in a tower reactor (apart from a bubble column, data for which would be similar to a sparger-equipped tank) is occupied by the gas phase, and conversely for a tank reactor. This means that a, a in a tower and a, - a t in a tank. For mass transfer-controlled situations, a, is the more important quantity, and is much greater in a tower. For reaction-controlled situations, in which neither ai nor a is important, a sparger-equipped tank reactor, the cheapest arrangement, is sufficient. 

Reactions involving gaseous and liquid reactants are carried out in various types of equipment. Packed columns, spray columns and bubble columns, as well as agitated tanks are all used (Fig. 2). Trickle-bed reactors are widely used in the petroleum industry for hydrodesulphurisation and related processes. In this type of reactor, liquid and gas both flow down through a bed of catalyst particles. The liquid flows around the particles as a thin film, thereby keeping the liquid residence time short and reducing undesirable side reactions. 

The IL airlift reactor shown in Figure 7.11a is a modification of the bubble column equipped with a draft tube (a concentric cylindrical partition) that divides the column into two sections of roughly equal sectional areas. These are the central riser for upward fluid flow and the annular downcomer for downward fluid flow. Gas is sparged at the bottom of the draft tube. In another type of IL airlift, the gas is sparged at the bottom of the annular space, which acts as the riser, while the central draft tube serves as the downcomer. 

Stirred tank reactors (STR) are the most frequently used reactors in lab-scale ozonation, partially due to the ease in modeling completely mixed phases, but they are very seldom used in full-scale applications. There are various modifications with regard to the types of gas diffusers or the construction of the stirrers possible. Normally lab-scale reactors are equipped with coarse diffusers, such as a ring pipe with holes of 0,1-1.0 m3 diameter. The k/ a-values are in the range of 0.02 to 2.0 s (see Table 2-4 ), which are considerably higher than those of bubble columns. From the viewpoint of mass transfer, the main advantage of STRs is that the stirrer speed can be varied, and thus also the ozone mass transfer coefficient, independently of the gas flow rate. 

Gas-liquid reactions form an integral part of the production of many bulk and specialty chemicals, such as the dissolution of gases for oxidations, chlorin-ations, sulfonations, nitrations, and hydrogenations. When the gaseous reactant must be transferred to the liquid phase, mass transfer can become the rate-limiting step. In this case, the use of high-intensity mixers (motionless mixers or ejectors) can increase the reaction rate. Conversely, for slow reactions a coarse dispersion of gas, as produced by a bubble column, will suffice. Because a large variety of equipment is available (bubble columns, sieve trays, stirred tanks, motionless mixers, ejectors, loop reactors, etc.), a criterion for equipment selection can be established and is dictated by the required rate of mass transfer between the phases. 

Class 1 equipment are also called column-type equipment. Under this category, there are the various multiphase contactors. Gas-liquid contactors include bubble columns, packed bubble columns, internal-loop and external-loop air-lift reactors, sectionalized bubble columns, plate columns, and others. Solid-fluid (liquid or gas) contactors include static mixers, fixed beds, expanded beds, fluidized beds, transport reactors or contactors, and so forth. For instance, fixed-bed geometry is used in unit operations such as ion exchange, adsorptive and chromatographic separations, and drying and in catalytic reactors. Liquid-liquid contactors include spray columns, packed extraction... 

The mass transfer characteristics of bubbles determine the efficiency and the size of process equipment used to carry out a range of chemical processes. Such processes may involve the use of bubble columns and three-phase fluidized bed reactors. A range of polymer processes such as those associated with the manufacture of foamed plastics would also benefit from a better understanding of bubble dynamics and mass transfer. Consequently, some research effort has been directed at elucidating the role of the rheological complexities of the ambient medium on mass transfer to/from stationary and moving bubbles as well as of bubble swarms. 

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