Transfer, mass gas-liquid

A summary of the available experimental data for gas liquid mass transfer under trickle-flow conditions is given in Tabic 6-8. A significant portion of this table is derived from Table 4 of Charpentier.12 The liquid-phase mass-transfer coefficient is affected by both gas and liquid flow rates. At high gas and liquid rates, the values of fcLaL may exceed 1 s l, a value normally not achieved in any other type of gas-liquid contactor. When the liquid is trickling over the packing, kLaL values for the cocurrent operation are of the same order of magnitude as those obtained in countercurrent operation under similar working conditions. 

Just as for the liquid holdup, the correlations for the k,aL are reported in two ways. Some investigators correlated kLaL to liquid and gas velocities by either dimensional30,34,35 or dimensionless34 correlations. The dimensional correlations assumed kLaL a Ui U . The values of r and s for various types of packings reported by various invesiigators are summarized in Table 6-9. Goto and Smith34 have correlated Sherwood numbers to the liquid-phase Reynolds and Schmidt numbers. 

Sato et al.1 Glass spheres diameters from 2.50 through 12.17 mm 0.80 0.80 

Charpentier,12 however, suggested that, in pulsed and spray flow one should use the relation 

In a trickle-bed reactor, due to a very thin liquid film, gas-liquid and liquid solid mass-transfer coefficients are sometimes combined as 

In two-phase downflow and upflow fixed-bed reactors, gas-liquid mass transfer resistance can be detrimental to the overall reactor performance [14, 32]. Therefore, accurate estimation of gas-liquid mass transfer parameters is important for achieving successful reactor design or scale-up. The overall gas-liquid mass transfer coefficient may be expressed, according to the two-film concept, in terms of the liquid-side and the gas-side mass transfer coefficients  

In most cases, mass transfer resistance in the gas film is considerably smaller than the liquid-side resistance therefore, the study of gas-liquid mass transfer has concentrated on the investigation of the mass transfer in the liquid film of the gas-liquid interface. Usually, gas-liquid mass transfer takes place across gas-liquid interfaces where the liquid can be stagnant or dynamic. While for slow reactions stagnant liquid affects very little the global reaction rate, fast reactions are characterized by a net contrast in reactant concentrations in the two regions resulting in reactionally ineffective stagnant zones. 

In spite of the extensive studies on the gas-liquid mass transfer in two-phase upflow fixed-bed reactors, summarized by Shah [27], Ramachandran and Chaudhari [64], Gianetto and Silveston [65], and Molga and Westerterp [62], only few correlations are reported in the Uterature to evaluate gas-Uquid mass transfer parameters under low and high pressure conditions [59, 66]. 

This section is concerned mainly with predicting or scaling-up the mass transfer rate between gas and liquid, in which the controlling factor is film diffusion on the liquid side of the interface, as described by the mass transfer coefficient, kt. Ideally, perhaps, this should be done from a basis of predicting local bubble sizes and gas fractions, using perhaps CFD, but this is not established within the realms of process engineering. The traditional method is (as for gas fraction) to use empirical correlations for the mass transfer factor kLa, and to use this in mass balance equations  

This has the advantage of not reqniring knowledge of bubble sizes, but also has some inherent disadvantages which are set out later in this section. Evidently, it will also be necessary to use an appropriate value of the mean for (C — Cl), which, as discussed in Section 11-3, will in general be between those for the ideal backmixed and plug flow cases. It should be noted that this is important also for the extraction of kLa values from laboratory concentration measurements and may not have been observed correctly in the derivation of some older correlations. 

The homogeneous region correlations for kLa (again like those for gas fraction) for the turbulent regime are best expressed in the form 

For extension into transitional Reynolds numbers (range 100 to 10 ), Cooke et al. (1988) obtained 

Although it is commonly assumed that when agitation conditions are sufficiently intense for effective gas-liquid dispersion, the liquid nfixedness will be 

In the above-described set-up (Fig. 8.16), the physical absorption of oxygen in water was used to measure the gas to liquid mass transfer. Thin-film fluorescence quenching-based sensors were installed to determine the oxygen concentration at the inlet and exit of the reactor. From the measured oxygen concentrations, the mass transfer coefficient was calculated based on the reactor-based liquid velocity  

Two basic trends can be determined. By comparing the 25 cpsi monoliths, channel rounding clearly leads to better mass transfer performance. The slightly rounded channels (RC) show a small increase, but a significant improvement is found for the more rounded channels (MRC). As expected, a higher S/V ratio or cell density also 

8 Reactive Stripping in Structured Catalytic Reactors Hydrodynamics and Reaction Performance 

However, the mass transfer coefficients found are clearly lower than those reported for distillation packings [26, 27]. This can be explained by the flow patterns in distillation packings, where the films constantly are disturbed and remixed, and therefore a completely developed laminar profile is never present. The mass transport is dominated by convection, not diffusion. It would be expected that remixing of the film layers, as accomplished by the stacking of monoliths (see Section 8.23) improves not only the RTD but also the mass transfer performance of monoliths. 

To compare the mass transfer behavior under realistic stripping conditions, a set of experiments was carried out at elevated temperatures and pressures. In a 2 m-long column (diameter 5 cm), nitrogen was brought into contact with a mixture of ca. 14 mol% ester (octyl-hexanoate) and 86 mol% cumene, enriched with 2000-2500 ppm water, which was representative for the reactive experiments in these studies. 

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Z. 5-25-Y, large huhhles = AA = 0.42 (NG..) Wi dy > 0.25 cm Dr luterfacial area 6 fig volume dy [E] Use with arithmetic concentration difference, ffg = fractional gas holdup, volume gas/total volume. For large huhhles, k is independent of bubble size aud independent of agitation or liquid velocity. Resistance is entirely in liquid phase for most gas-liquid mass transfer. [79][91] p. 452 [109] p. 119 [114] p. 249... 

In this section emphasis is placed on the transfer of mass. Typical gas-liquid mass-transfer systems are ... 

Qualitative and, hopefully, quantitative estimates of how the process result will be measured must be made in advance. The evaluations must allow one to estabhsh the importance of the different steps in a process, such as gas-liquid mass transfer, chemical reac tion rate, or heat transfer. 

Normally there must be a way of determining whether the mass-transfer rate with the solids is the key controlling parameter or the gas-liquid mass transfer rate. 

FIG. 18-28 Usually, the gas-liquid mass-transfer coefficient, K, is reduced with increased viscosity. This shows the effect of increased concentration of microbial cells in a fermentation process. 

Gas/Liquid Mass Transfer This topic has been widely investigated for gas absorption in packed beds, usually countercurrent. One correlation for cocurrent flow in catalyst beds is by Sato et al. (First Pacific Chemical Engineering Congre.s.s, Pergamon, 1972, p. 187) ... 

For gas/liquid mass transfer, a data point is taken at a particular power level and gas rate. This point is obtained from a similar application or from laboratory or pilot plant data. 

F = Function of the molecular volume of the solute. Correlations for this parameter are given in Figure 7 as a function of the parameter (j), which is an empirical constant that depends on the solvent characteristics. As points of reference for water, (j) = 1.0 for methanol, (j) = 0.82 and for benzene, (j) = 0.70. The two-film theory is convenient for describing gas-liquid mass transfer where the pollutant solute is considered to be continuously diffusing through the gas and liquid films. 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

Several reported chemical systems of gas-liquid precipitation are first reviewed from the viewpoints of both experimental study and industrial application. The characteristic feature of gas-liquid mass transfer in terms of its effects on the crystallization process is then discussed theoretically together with a summary of experimental results. The secondary processes of particle agglomeration and disruption are then modelled and discussed in respect of the effect of reactor fluid dynamics. Finally, different types of gas-liquid contacting reactor and their respective design considerations are overviewed for application to controlled precipitate particle formation. 

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. 

Sada, E., Kumazawa, H., Lee, C. and Fujiwara, N., 1985. Gas-liquid mass transfer characteristics in a bubble column with suspended sparingly soluble fine particles. Industrial and Engineering Chemistry Process Design and Development, 24, 255-261. 

Wachi, S. and Jones, A.G., 1991b. Effect of gas-liquid mass transfer on crystal size distribution during the batch precipitation of calcium carbonate. Chemical Engineering Science, 46, 3289-3293. 

The mass transfer coefficient is expected to relate gas power per unit volume and gas terminal velocity. Measurement of gas bubble velocity is troublesome in the experimental stage of aeration. Extensive research has been conducted for an explanation of the above correlation. Gas-liquid mass transfer in low viscosity fluids in agitated vessels has been reviewed and summarised as stated in (3.5.1.7)—(3.6.2) 3... 

Only one publication on gas-liquid mass transfer in bubble-column slurry reactors has come to the author s attention. However, a relatively large volume of information regarding mass transfer between single bubbles or bubble swarms and pure liquid containing no suspended solids is available, and this information is probably of some relevance to the analysis of systems... 

The remaining studies reviewed in this section are concerned with gas-liquid mass transfer for single bubbles or bubble swarms in clear liquids. 

Kolbel et al. (K16) examined the conversion of carbon monoxide and hydrogen to methane catalyzed by a nickel-magnesium oxide catalyst suspended in a paraffinic hydrocarbon, as well as the oxidation of carbon monoxide catalyzed by a manganese-cupric oxide catalyst suspended in a silicone oil. The results are interpreted in terms of the theoretical model referred to in Section IV,B, in which gas-liquid mass transfer and chemical reaction are assumed to be rate-determining process steps. Conversion data for technical and pilot-scale reactors are also presented. 

Calderbank and Moo-Young (C5) have studied gas-liquid mass transfer in systems characterized by high viscosities and high diffusion coefficients, and have on the basis of data obtained in this and other studies developed correlations for the mass-transfer coefficients. 

Ogl gas-liquid mass transfer area to reactor volume relation c concentration D diffusion coefficient... 

To avoid gas-liquid mass transfer Hmitation, which would have a negative influence on productivity, in correctly operated bioreactors there are turbulent flow conditions with more or less pronounced turbulence, for which the Reynolds stress formula (Eq. (2)) can be used. Whereas, as a rule there is fully developed turbulent flow in technical apparatuses (see condition (6) and explanations in Sect. 8), this is frequently not the case in laboratory fermenters. Equations (3) and (4) are then only valid to a limited extent. 

The volumetric gas-liquid mass transfer coefficient ki a) has been obtained by fitting the concentration profile of dissolved oxygen to the axial dispersion model [8, 18]. The value of... 

Evaluation of reactor hydrodynamics by measuring gas-liquid mass transfer coefficient... 

This research used mechanically agitated tank reactor system shown in Fig. 1. The reactor, 102 mm in diameter and 165 mm in height, was made of transparant pyrex glass and was equipped with four baffles, 120 mm in length and 8 mm in width, and six blades disc turbine impeller 45 mm in diameter and 12 mm in width. The impeller was rotated by electric motor with digital impeller rotation speed indicator. Waterbath thermostatic, equipped with temperature controller was used to stabilize reactor temperature. Gas-liquid mass transfer coefficient kia was determined using dynamic oxygenation method as has been used by Suprapto et al. [11]. 

The measurement of liquid side gas - liquid mass transfer coefficient kia, showed that the value of kia increase with increasing rotation speed (V) and gas flow rate (Qg). hi the present research, the effect of impeller rotation on mass transfer coefficient was more significant than the effect of gas flow rate. The following correlation was obtained kia =1.7 x 10 ... 

Case B. Gas-Liquid Mass Transfer to a Batch Tank... 

For single-solute, gas-liquid mass transfer, the component balances are as before... 

The experiments were conducted at four different temperatures for each gas. At each temperature experiments were performed at different pressures. A total of 14 and 11 experiments were performed for methane and ethane respectively. Based on crystallization theory, and the two film theory for gas-liquid mass transfer Englezos et al. (1987) formulated five differential equations to describe the kinetics of hydrate formation in the vessel and the associate mass transfer rates. The governing ODEs are given next. 

The ongoing work on sludge-blanket and draft-tube reactors requires demonstration of sufficient gas-liquid mass transfer to provide the necessary oxygen needed in high cell density reactors. 

Gas-Liquid Mass Transfer. Gas-liquid mass transfer within the three-phase fluidized bed bioreactor is dependent on the interfacial area available for mass transfer, a the gas-liquid mass transfer coefficient, kx, and the driving force that results from the concentration difference between the bulk liquid and the bulk gas. The latter can be easily controlled by varying the inlet gas concentration. Because estimations of the interfacial area available for mass transfer depends on somewhat challenging measurements of bubble size and bubble size distribution, much of the research on increasing mass transfer rates has concentrated on increasing the overall mass transfer coefficient, kxa, though several studies look at the influence of various process conditions on the individual parameters. Typical values of kxa reported in the literature are listed in Table 19. 

Nore, O., Briens, C., Margaritis, A., and Wild, G., Hydrodynamics, Gas-Liquid Mass Transfer and Particle-Liquid Heat and Mass Transfer in a Three-Phase Fluidized Bed for Biochemical Process Applications, Chem. Eng. Sci., 47 3573 (1992)... 

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