Specific contact area

Of course, when designing reactors it is interesting to refer to the specific contact area a (m2 mG+L-3) - that is, the interfacial area per unit volume of gas-... 

In some cases, it can be useful to use other definitions for the specific contact area. For example, some authors use oL, which is the interfacial area per unit volume of liquid (m2 mf3) or aG, the interfacial area per unit volume of gas (m2 mG3). Note that the nomenclature a, aL or oG is not always specified. The specific contact areas are related by ... 

This simplified description of molecular transfer of hydrogen from the gas phase into the bulk of the liquid phase will be used extensively to describe the coupling of mass transfer with the catalytic reaction. Beside the Henry coefficient (which will be described in Section 45.2.2.2 and is a thermodynamic constant independent of the reactor used), the key parameters governing the mass transfer process are the mass transfer coefficient kL and the specific contact area a. Correlations used for the estimation of these parameters or their product (i.e., the volumetric mass transfer coefficient kLo) will be presented in Section 45.3 on industrial reactors and scale-up issues. Note that the reciprocal of the latter coefficient has a dimension of time and is the characteristic time for the diffusion mass transfer process tdifl-GL=l/kLa (s). 

The CFD simulations should be linked with the rate-based process simulator, providing important information on the process hydrodynamics in the form of correlations for mass transfer coefficients, specific contact area, liquid holdup, residence time distribution, and pressure drop. An ability to obtain these correlation via the purely theoretical way rather than by the traditional experimental one should be considered a significant advantage, because this brings a principal opportunity to virtually prototyping of new optimized internals for reactive separations. 

The surface tension is important for the calculation of mass transfer coefficients and the specific contact area (see Section 9.4.4). Depending on the availability of necessary parameters, the surface tension for a molecular species can be determined either with the simplest method of Hakim-Steinberg-Stiel or with a more complex DIPPR-method (see Ref. [52]). The mixture surface tension can be obtained via a mixing rule. A further extension to cover electrolyte mixtures is realized by the method of Onsager and Samaras (see Ref. [44]). The latter uses an additive term which can be estimated using the dielectric constant of the mixture and molar volumes of electrolytes. 

Thermodynamic non-idealities are taken into account while calculating necessary physical properties such as densities, viscosities, and diffusion coefficients. In addition, non-ideal phase equilibrium behavior is accounted for. In this respect, the Elec-trolyte-NRTL model (see Section 9.4.1) is used and supplied with the relevant parameters from Ref. [50]. The mass transport properties of the packing are described via the correlations from Refs. [59, 61]. This allows the mass transfer coefficients, specific contact area, hold-up and pressure drop as functions of physical properties and hydrodynamic conditions inside the column to be determined. 

Mass transfer coefficients and specific contact area are calculated with the correlations of Onda et al. [60]. In order to determine the liquid film thickness, the diffusion coefficient for SO2 is used. 

The rate-based stage model parameters describing the mass transfer and hydrodynamic behavior comprise mass transfer coefficients, specific contact area, liquid hold-up, residence time distribution characteristics and pressure drop. Usually they have to be determined by extensive and expensive experimental estimation procedures and correlated with process variables and specific internals properties. 

In this way, extremely high mass transfer coefficients and specific contact areas between gas and liquid are possible [7],... 

Only the parameters of the first type need to be discussed here. It seems logical to start with a brief discussion on the value of the product of A l and the specific contact area a (k a), because this is often the overall rate-controlling step. 

The True Gas-Liquid Specific Contact Area (a) and the Liquid-Side Mass Transfer Coefficient (ki)... 

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. 

At the outset, we recognize that a technique that measures overall values cannot be used without the restrictions that arise from the results observed with physical methods. For example, the chemical method can hardly be used with fast-coalescing systems, since the presence of a chemical compound may well reduce the coalescence rates. In fast-coalescing systems, as observed with physical methods, the wide variation of specific contact area at different locations in the reactor negates the meaning of an average value. In fact, physical and chemical techniques should be used simultaneously to identify more fully the phenomena that occur in gas-liquid reactors. While chemical methods provide overall values of interfacial area that are immediately usable for design, we must also know the variations in the local interfacial parameters (a, dgM) within the reactor in order to deal competently with scale-up. These complementary data, measured by physical methods, should be obtained from local simultaneous measurements of two of the three interfacial parameters as discussed above. 

To be able to calculate the volumetric drying rate from TLE, one needs to know the voidage e and specific contact area ay in the dryer. 

The development of the specific contact area along the mixing channel is shown in Figure 4.11. Tracer profiles obtained by numerical simulation for different distances from the mixing point are compared with experimental results [14]. 

The problem at hand is to determine the mass transfer coefficient and the air flow needed in an industrial cooling tower that is meant to refrigerate 2000 kg/s of water that enters at 27°C and is cooled down to 20°C to be reused as cooling agent. The ambient air is at 21°C with an RH of 60% and leaves the column with 90% humidity at 22°C. The specific contact area, a, is 250 m and the cross-sectional area of the column, S, is 25 m (see Figure 3.19). The packing height, Z, is 5 m. 

Therefore, in this work a more physically consistent way is used by which a direct account of process kinetics is realised. This approach to the description of a column stage is known as the rate-based approach and implies that actual rates of multicomponent mass transport, heat transport and chemical reactions are considered immediately in the equations governing the stage phenomena. Mass transfer at the vapour-liquid interface is described via the well known two-film model. Multicomponent diffusion in the fdms is covered by the Maxwell-Stefan equations (Hirschfelder et al., 1964). In the rate-based approach, the influence of the process hydrodynamics is taken into account by applying correlations for mass transfer coefficients, specific contact area, liquid hold-up and pressure drop. Chemical reactions are accounted for in the bulk phases and, if relevant, in the film regions as well. 

The external diameter of the water cyhnders is about 2 mm. The distance between cable centers being 8 mm, we can tighten 16 km of cable in a volume of 1 cu. meter. The specific contact area is higher than 100 sq. meter per cu. meter of exchanger volume. 

In this review we will limit our scope to the parameters of type (a), only. Below, we summarize new information on each parameter as collected from open literature published since 1980. However, we start with a short discussion of the product of kj and the specific contact area, a, (kj a) because this often is the overall rate controlling step. Fortunately, this parameter is also relatively easy to measure easier than kQa and kj and a separately. Therefore it is always essential to estimate as early as possible in the progress of the reactor design which resistance(s) might be rate controlling because that dictates the information necessary for a sound design and the type of experiments to be done if the available information turns out to be insufficient. 

Specially for fast reactions, where enhancement at the gas-liquid interface occurs, knowledge of the true gas liquid specific contact area, a, is desired rather than knowledge of the product kj a only. One method is the measurement of both gas hold-up and the Sauter mean bubble diameter 3. 

Then, the specific contact area follows from ... 

For a steady state column reactor the specific contact area, a, can be determined by solving... 

Table VI. Specific contact area in bubble columns according to Akita and Yoshida [107] 10 < T [ c] < 0 790 < [kg/m ] < 1165 ...

For reactive systems where capacity depends on the contact area, a, rather than kj a, more data are needed on the influence of pressure and liquid mixture composition on the value of the specific contact area. In case of uncertainty on the contact area, the second best alternative to measuring the area is measuring gas hold-up and combine it with the existing relations for d according to a ... 

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