Bubble column reactors interface

Figure 8.11 Types of reactors for gas-liquid precipitation, (a) bubbling stirred tank, (b) fiat interface stirred tank, draft-tube bubble column, (d) spray column after Wachi and Jones, 1994)...

The effects of diffusional restrictions on the activity and selectivity of FT synthesis processes have been widely studied (32,52,56-60). Intrapellet diffusion limitations are common in packed-bed reactors because heat transfer and pressure-drop considerations require the use of relatively large particles. Bubble columns typically use much smaller pellets, and FT synthesis rates and selectivity are more likely to be influenced by the rate of mass transfer across the gas-liquid interface as a gas bubble traverses the reactor (59,61,62). 

In most reactor engineering applications, it may not be necessary to include the gas-liquid interface in the solution domain. In an alternative approach, the solution domain is restricted to the height of gas-liquid dispersion. Of course, an exact value of dispersion height is not known a priori. However, in most cases, overall gas volume fractions and therefore, height of the gas-liquid dispersion can be estimated. Even if there is 40% error in the prediction of overall gas volume fraction, it will result in only 10% error in the estimation of height of gas-liquid dispersion, if the volume fraction is about 25%. Except for very shallow bubble columns, fluid dynamics is not... 

In bubble columns and gas-liquid stirred reactors, the estimation of parameters is more difficult than in gas-solid or liquid-solid fluidized beds. Solid particles are rigid, and hence the fluid-solid interface is nonde-formable, whereas the gas-liquid interface is deformable. In addition, the effect of surface-active agents is much more pronounced in the case of gas-liquid interfaces. This leads to uncertainties in the prediction of all major parameters, such as the terminal bubble rise velocity, the bubble diameter, the gas holdup, and the relation between the bubble diameter and the terminal bubble raise velocity. 

The reaction rate can be defined in different ways using a unit volume of liquid, a unit volume of reactor, or unit interfacial area as a basis. Since mass transfer coefficients for tanks and packed columns are often based on the volume of the apparatus, the overall reaction rate will be expressed in units such as Ib-mol-A/hr, ft or kg mol A/hr, m. The volume is the active reactor volume, which is the volume of the gas liquid mixture in a stirred tank or bubble column, the packed column volume, or the chamber volume for a spray reactor. The mass transfer of A to the interface is the first step ... 

Besides bubble columns, other reactor types are investigated. Using a high-shear device creates a high air/oxygen—cumene interface, which increases CHP formation by increased oxygen mass transfer [52]. 

Novel bubble-induced flow designs apply a plethora of mechanisms that help differentiate each specific design from other novel and standard devices. Some changes are structural and include use of different materials and internals. Others include the use of novel methods to excite the bubble interface and induce gas-liquid mass transfer. Novel methods exclude devices that are created to study specific events relating to standard devices. For example, the study by Sotiriadis et al. (2005) using a specially designed bubble column where the phases move downward to specifically study bubble behavior, bubble size, and gas-liquid mass transfer in the downcomer of airhft reactors would fall in the excluded devices. 

Jet injectors may also be combined with monolith reactors. Monoliths are usually tube reactors with channeled flow. The reaction occurs at the gas-liquid interface as well as on the channel wall, which are usually catalytic or coated with catalytic material. Monoliths can be made into vertical (similar to bubble column) or horizontal tubes, airlift devices (whereby the riser would a monolith), or even into a mechanically stirred device. Usually, however, monoliths are designed like bubble columns or airhft reactors (Broekhuis et al., 2001). 

Mixing in microreactors is almost exclusively assumed to be laminar dne to the small flow channel width. Laminar flow through microchannels reqnires the phase flow to be alternated in some fashion in order to create the mixing environment. This operation is important because mass transfer, in this case, is driven only by molecular diffusion. Hence, the creation of larger gas-liquid interfaces is the only practical course of action for gas-limited operations. Miniaturized bubble columns are able to accomplish this task well. For example, a standard reactor can create... 

Let us consider the mass balance of two kinds of three-phase reactors bubble columns and tube reactors with a plug flow for the gas and the liquid phases, and stirred tank reactors with complete backmixing. Modeling concepts can be implemented in most existing reactors backmixing is typical for slurry reactors, bubble columns, and stirred tank reactors, whereas plug flow models describe the conditions in a trickle bed reactor. The interface between the gas and the liquid is supposed to be surroimded by gas and liquid films. Around the catalyst particles, there also exists a liquid film. In gas and liquid films, physical diffusion, but no chemical reactions, is assumed to take place. A volume element is illustrated in Figure 6.15. 

An interesting class of exact self-similar solutions (H2) can be deduced for the case where the newly formed phase density is a function of temperature only. The method involves a transformation to Lagrangian coordinates, based upon the principle of conservation of mass within the new phase. A similarity variable akin to that employed by Zener (Z2) is then introduced which immobilizes the moving boundary in the transformed space. A particular case which has been studied in detail is that of a column of liquid, initially at the saturation temperature T , in contact with a flat, horizontal plate whose temperature is suddenly increased to a large value, Tw T . Suppose that the density of nucleation sites is so great that individual bubbles coalesce immediately upon formation into a continuous vapor film of uniform thickness, which increases with time. Eventually the liquid-vapor interface becomes severely distorted, in part due to Taylor instability but the vapor film growth, before such effects become important, can be treated as a one-dimensional problem. This problem is closely related to reactor safety problems associated with fast power transients. The assumptions made are ... 

Systematic investigations of microbial cell recovery by foam flotation were performed by Hansenula polymorpha [113-117] and Saccharomyces cerevisiae [ 118 -123] in continuous operation. The equipment used for flotation was often identical to that used for protein flotation. The microorganisms were cultivated in laboratory reactors on synthetic media in the absence of antifoam agents in continuous operation and the cell-containing cultivation medium was collected in a buffer storage and was fed into the middle of the colunm, at the top of the interface between the bubble and the foam layers. The height of the interface was controlled by an overflow. The foam left the colunm at the top. The cells were recovered from the foam liquid by a mechanical foam destroyer. The liquid residue left the column through an overflow [113] (Fig. 6). 

Middleton indicates that for his regime 1 (very slow reaction), where Rl is little affected by the chemical reaction, the interface surface area per unit volume, a, is of little importance since the reaction takes place in the bulk liquid phase, so a bubble colunm is the typical reactor of choice. For Middleton s regimes 11, IV, and V—diflfusional control, very fast reaction, and instantaneous reaction, respectively—both high a and k [ are needed, so a stirred tank is the typical reactor recommended. In regime III—reaction in the mass transfer film—the most important variable is the interface area, so a packed column yielding much liquid surface area may be appropriate. 

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