Interfacial gas-liquid

Later publications have been concerned with mass transfer in systems containing no suspended solids. Calderbank measured and correlated gas-liquid interfacial areas (Cl), and evaluated the gas and liquid mass-transfer coefficients for gas-liquid contacting equipment with and without mechanical agitation (C2). It was found that gas film resistance was negligible compared to liquid film resistance, and that the latter was largely independent of bubble size and bubble velocity. He concluded that the effect of mechanical agitation on absorber performance is due to an increase of interfacial gas-liquid area corresponding to a decrease of bubble size. 

The interfacial gas-liquid area a is a function of the size of the gas bubble dispersion ... 

Bubble hold-up, interfacial gas/liquid mass transfer area ... 

With such low concentrations of components available to form critical nuclei, hydrate formation seems unlikely in the bulk phases. However, at an interface where higher concentrations exist through adsorption (particularly at the vapor-liquid interface where both phases appear in abundance) cluster growth to a supercritical size is a more likely event. High mixing rates may cause interfacial gas + liquid + crystal structures to be dispersed within the liquid, giving the appearance of bulk nucleation from a surface effect. 

Now, if we can get catalyst carriers that adhere to the gas-liquid surface and we become successful in easily separating particles of dp < 5 fan, we can reduce on catalyst concentrations down to 0.5 x 10 4 < es < 5 x 10 3 and still realize an enhancement in conversion rate, typically by a factor of 5. This way, the reactor volume is reduced by a factor of 5 and the catalyst holdup by several orders of magnitude. The improvement might be even more spectacular because it is known that small particles of dp < 100 fim for Es < 0.5 x 10 2 may cover gas liquid surface, thus prevent bubble coallescence which results in smaller bubbles, leading to still higher interfacial gas liquid areas and thus to still smaller reactors [109]. 

At high <7-values a reduction of the interfacial gas-liquid area also reduces the mass transfer and slows down the reaction rates of SLPC [97]. This argument applies in the same way to SAPC as the water content approaches the upper limit that is given by the total water uptake of the support. 

It is convenient to compare the features of mass transfer in a trickle-bed (completely wetted pellet) and in catalyst pellets under condition of capillary condensation. In trickle-bed reactors, the interfacial gas-liquid surface (5j) is slightly less (due to porosity) than the external surface of pellet 5r, Si < S sq, and is proportional to (1 — s) sq/R. 

Effective interfacial gas-liquid mass transfer must occur in gas-liquid and gas-liquid-liquid systems. In an unsparged tank, gas-liquid mass transfer occurs primarily through the bulk gas-liquid surface unless extreme vortex formation occurs. If a sparger is used, gas-liquid mass transfer occurs through the surface as well as through the surface area associated with the dispersed gas... 

The equation for computing the interfacial gas and liquid compositions in concentrated systems is... 

Flow Reactors Fast reactions and those in the gas phase are generally done in tubular flow reaclors, just as they are often done on the commercial scale. Some heterogeneous reactors are shown in Fig. 23-29 the item in Fig. 23-29g is suited to liquid/liquid as well as gas/liquid. Stirred tanks, bubble and packed towers, and other commercial types are also used. The operadon of such units can sometimes be predicted from independent data of chemical and mass transfer rates, correlations of interfacial areas, droplet sizes, and other data. 

Decreased liquid-liquid interfacial tension (when compared with a gas-liquid system) results in higher liquid-liquid interfacial areas, which favor solid-particle droplet collisions. 

TABLE 23-9 Mass-Transfer Coefficients/ Interfacial Areas and Liquid Holdup in Gas/Liquid Reactions... 

Gas/Liquid Interfacial Area This has been evaluated by measuring absorption rates like those of CO9 in NaOH. A correlation by Charpentier (Chem. Eng. Journal, 11, 161 [1976]) is... 

A non-ideal MSMPR model was developed to account for the gas-liquid mass transfer resistance (Yagi, 1986). The reactor is divided into two regions the level of supersaturation in the gas-liquid interfacial region (region I) is higher than that in the main body of bulk liquid (region II), as shown in Figure 8.12. 

A complicating factor in this process is the formation of finely divided carbon, which causes an increase of liquid viscosity and promotes bubble coalescence whereby the gas-liquid interfacial area is reduced. Also observed was a effect of reactor height, which may be attributed to bubble coalescence. 

El), Oldshue (Ol), and Johnson et al. (J4)] have been concerned with the determination of the volume transfer coefficient KtAb (liter/hr), where Kx is the mass-transfer coefficient and Ab is the total gas-liquid interfacial area. The results obtained using a turbine impeller and an open pipe sparger can be correlated in terms of the nominal gas velocity wg(ft/hr) and the horsepower input to the impeller HP by an expression of the following form ... 

Westerterp et al. (W5) measured interfacial areas in mechanically agitated gas-liquid contactors. The existence of two regions was demonstrated At agitation rates below a certain minimum value, interfacial areas are unaffected by agitation and depend only on nominal gas velocity and the type of gas distributor, whereas at higher agitation rates, the interfacial areas are... 

Kramers et al. (K21) measured gas residence-time distribution in a mechanically agitated gas-liquid contactor of 0.6-m diameter for various gas velocities and agitator speeds. In the region where agitation has an effect on the gas-liquid interfacial area (cf. the study by Westerterp et al. (W5), Section V,D,1), the residence-time distribution was found to resemble closely that of a perfect mixer. 

The absorption rate increased with increasing nominal liquid velocity for all particle sizes and decreased with increasing particle size for all liquid velocities. The absorption rates were lower than those measured in an equivalent gas-liquid system with no solid particles present. The difference is explained as being due to a higher rate of bubble coalescence and, consequently, a lower gas-liquid interfacial area in the gas-liquid fluidized bed. 

Adlington and Thompson (Al) measured the gas-liquid interfacial area in beds of particles of from 0.3- to 3-mm diameter by oxygen absorption in a sodium sulfite solution. They found that the interfacial area decreased with decreasing bed porosity, and was less sensitive to changes in particle size. 

The results of Massimilla et al., 0stergaard, and Adlington and Thompson are in substantial agreement on the fact that gas-liquid fluidized beds are characterized by higher rates of bubble coalescence and, as a consequence, lower gas-liquid interfacial areas than those observed in equivalent gas-liquid systems with no solid particles present. This supports the observations of gas absorption rate by Massimilla et al. It may be assumed that the absorption rate depends upon the interfacial area, the gas residence-time, and a mass-transfer coefficient. The last of these factors is probably higher in a gas-liquid fluidized bed because the bubble Reynolds number is higher, but the interfacial area is lower and the gas residence-time is also lower, as will be further discussed in Section V,E,3. 

These results are, however, only valid for the particle sizes referred to. Lee (L3) has reported measurements of average bubble diameter and gas-liquid interfacial area for gas-liquid fluidized beds of glass beads of 6-mm... 

In addition, it was concluded that the liquid-phase diffusion coefficient is the major factor influencing the value of the mass-transfer coefficient per unit area. Inasmuch as agitators operate poorly in gas-liquid dispersions, it is impractical to induce turbulence by mechanical means that exceeds gravitational forces. They conclude, therefore, that heat- and mass-transfer coefficients per unit area in gas dispersions are almost completely unaffected by the mechanical power dissipated in the system. Consequently, the total mass-transfer rate in agitated gas-liquid contacting is changed almost entirely in accordance with the interfacial area—a function of the power input. 

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