Bubbling gas-liquid reactors

In both the gassed (aerated) stirred tank and in the bubble column, the gas bubbles rise through a liquid, despite the mechanisms of bubble formation in the two types of apparatus being different. In this section, we shall consider some common aspects of the gas bubble - liquid systems in these two types of reactors. 

The value of the Thiele modulus is calculated from Equation 7.21. 

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The correlations for as discussed above are for homogeneous liquids. Bubbling gas-liquid reactors are sometimes used for suspensions, and bioreactors of this type must often handle suspensions of microorganisms, cells, or immobilized cells or enzymes. Occasionally, suspensions of nonbiological particles, to which organisms are attached, are handled. Consequently, it is often necessary to predict how the values for suspensions will be affected by the system properties and operating conditions. In fermentation with a hydrocarbon substrate, the substrate is usually dispersed as droplets in an aqueous culture medium. Details of... 

The correlations for kLa as discussed above are for homogeneous liquids. Bubbling gas-liquid reactors are sometimes used for suspensions, and bioreactors of this type must often handle suspensions of microorganisms, cells, or immobilized... 

In general, the gas holdups and kLa for suspensions in bubbling gas-liquid reactors decrease substantially with increasing concentrations of solid particles, possibly because the coalescence of bubbles is promoted by presence of particles, which in turn results in a larger bubble size and hence a smaller gas-liquid interfacial area. Various empirical correlations have been proposed for the kLa and gas holdup in slurry bubble columns. Equation 7.46 [24], which is dimensionless and based on data for suspensions with four bubble columns, 10-30 cm in diameter, over a range of particle concentrations from 0 to 200 kg m 3 and particle diameter of 50-200 pm, can be used to predict the ratio r of the ordinary kLo values in bubble columns. This can, in turn, be predicted for example by Equation 7.41, to the kLa values with suspensions. 

The interfacial area AtV usually excludes the contact area between the vapor space and the liquid at the top of the reactor. The justification for this is that most gas-liquid reactors have gas bubbles as a dispersed phase. This gives a much larger interfacial area than the nominal contact area at the top of the reactor. There are exceptions—e.g., polyester reactors where by-product water is removed only through the nominal interface at the top of the reactor— but these are old and inefficient designs. This nominal area scales as while the contact area with a dispersed phase can scale as S. 

Mechanism of mass transfer from bubbles in dispersions Part II Mass transfer coefficients in stirred gas-liquid reactor and bubble column... 

M. Bouaifi, G. Hebrard, D. Bostoul, M. Roustan, A comparative study of gas hold-up, bubble size, interfacial area and mass transfer coefficients in gas—liquid reactors and bubble columns, Chem. Eng. Proc. 40 (2001) 97-111. 

Usually, the typology of batch reactors also includes the semi-batch gas-liquid reactors, in which a gaseous phase is fed continuously in order to provide one of the reactants. A typical example is given by the reactors used both in different oxidative industrial processes and in the active sludge processes for the treatment of wastewater. It is possible to distinguish between the bubble columns (Fig. 7.1(c)), in which the gas rises undisturbed in the liquid phase, and the bubble stirred reactor, in which a mechanical mixer is added. Finally, the slurry reactors can be considered, in which the liquid phase contains a finely dispersed solid phase as well, which can act as a reactant or as a heterogeneous catalyst these reactors assume in general the features of Fig. 7.1(d). 

A spray tower is a continuous gas-liquid reactor. Gases pass upward through a column and contact liquid reactant sprayed into the column. The spray tower represents the opposite extreme from a bubble tower. The spray tower has greater than 90% of the volume as gas. This allows for much reduced liquid-handling rates for highly soluble reactants. 

FIG. 19-26 Types of industrial gas-liquid reactors, (a) Tray tower, (b) Packed, countercurrent, (c) Packed, co-current, (d) Falling liquid film, (e) Spray tower. (/) Bubble tower, (g) Venturi mixer. (h) Static in-line mixer, (t) Tubular flow. / Stirred tank. /, Centrifugal pump. (l) Two-phase flow in horizontal tubes. 

The discussion is centered around gas-liquid reactors. If the dissolved gas content exceeds the amount needed for the reaction, the liquid may be first saturated with gas and then sent through a stirred tank or tubular reactor as a single phase. If the residence times for the liquid and gas are comparable, both gas and liquid may be pumped in and out of the reactor together. If the gas has limited solubility, it is bubbled through the reactor and the residence time for gas is much smaller. Figure 19-29 provides examples of gas-liquid reactors for specific processes. 

As is shown in Figure 2, in the two-phase model the fluid bed reactor is assumed to be divided into two phases with mass transfer across the phase boundary. The mass transfer between the two phases and the subsequent reaction in the suspension phase are described in analogy to gas/liquid reactors, i.e. as an absorption of the reactants from the bubble phase with pseudo-homogeneous reaction in the suspension phase. Mass transfer from the bubble surface into the bulk of the suspension phase is described by the film theory with 6 being the thickness of the film. D is the diffusion coefficient of the gas and a denotes the mass transfer coefficient based on unit of transfer area between the two phases. 6 is given by 6 = D/a. 

Much more information is available on the product ky a than on kl and a separately. For low solids concentrations it may be assumed that the solids do not affect the value of A a, so that the existing relations for two-phase gas-liquid reactors can be applied. For reviews on these relationships, see Lee and Foster [76], for draft tube slurry reactors Goto et al. [77], for bubble columns Deckwer and Schumpe [78] and Deckwer [79], and for stirred tank reactors Mann [80] and Schluter and Deckwer [81]. Despite of much research published on the influence of solids on k a there is still no universally applicable relation describing the influence of all types of particles in any weight fraction in any liquid. 

Conventional mechanically agitated gas-liquid reactors, wherein gas and liquid make contact in batch, semibatch, or continuous mode, are widely used in processes involving chlorination, sulfonation, hydrogenation, selective absorptions in amine solutions, etc. (Doraiswamy and Sharma, 1984). These reactors are popular for laboratory studies because of their simplicity in construction and low cost. As a rule of thumb with noncorrosive liquids, the mechanically agitated reactor is most economical when the overall reaction rate is five times greater than the mass transfer rate in a bubble column. If a... 

Commercial reactors are non isothermal and often adiabatic. In a noniso-thermal gas-liquid reactor, along with the mass dispersions in each phase, the corresponding heat dispersions are also required. Normally, the gas and liquid at any given axial position are assumed to be at the same temperature. Thus, in contrast to the case of mass, only a single heat-balance equation (and corresponding heat-dispersion coefficient) is needed. Under turbulent flow conditions (such as in the bubble-column reactor) the Peclet number for the heat dispersion is often assumed to be approximately equal to the Peclet number for the mass dispersion in a slow-moving liquid phase. 

MODELS FOR THE PACKED-BUBBLE-COLUMN GAS-LIQUID REACTOR... 

The specific surface area of contact for mass transfer in a gas-liquid dispersion (or in any type of gas-liquid reactor) is defined as the interfacial area of all the bubbles or drops (or phase elements such as films or rivulets) within a volume element divided by the volume of the element. It is necessary to distinguish between the overall specific contact area S for the whole reactor with volume Vr and the local specific contact area 51 for a small volume element AVi- In practice AVi is directly determined by physical methods. The main difficulty in determining overall specific area from local specific areas is that Si varies strongly with the location of AVi in the reactor—a consequence of variations in local gas holdup and in the local Sauter mean diameter [Eq. (64)]. So there is a need for a direct determination of overall interfacial area, over the entire reactor, which is possible with use of the chemical technique. 

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