Gas mass transfer

Calculate mass transfer, gas hold up, gassed and ungassed power for the fermenter with the given data ... 

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. 

The absorption of ozone from the gas occurred simultaneously with the reaction of the PAH inside the oil droplets. In order to prove that the mass transfer rates of ozone were not limiting in this case, the mass transfer gas/water was optimized and the influence of the mass transfer water/oil was studied by ozonating various oil/water-emulsions with defined oil droplet size distributions. No influence of the mean droplet diameter (1.2 15 pm) on the reaction rate of PAH was observed, consequently the chemical reaction was not controlled by mass transfer at the water/oil interface or diffusion inside the oil droplets. Therefore, a microkinetic description was possible by a first order reaction with regard to the PAH concentration (Kornmuller et al., 1997 a). The effects of pH variation and addition of scavengers indicated a selective direct reaction mechanism of PAH inside the oil droplets... 

Enhanced mass transfer Gas miscibility Enhanced separations... 

Increased activity Enhanced mass transfer Gas miscibility Krocher et al. (1996a, 1996b, 1998, 1999)... 

Solvent tunability Selectivity enhancement Enhanced mass transfer Gas miscibility Hitzler and Poliakoff (1997) Hitzler et al. (1998a, 1998b)... 

Here Sh is the modified Sherwood number defined as Sh = /Csnsdp/fl,D and We is the modified Weber number defined as We = UoLpi.dr/hlci. A graphical illustration of the above correlation is shown in Fig. 6-20. The predictions of Eq. (6-67) also agree fairly well with the data of Lemay el al.so Specchia et al.9i showed that, in a trickle-flow reactor, KLaL and Ksas are essentially of the same order of the magnitudes. They also evaluated the conditions under which the mass-transfer (gas-liquid and liquid-solid) influences significantly the performance of a trickle-bed reactor. 

In this chapter, we review the reported studies on the hydrodynamics, holdups, and RTD of the various phases (or axial dispersion in various phases), as well as the mass-transfer (gas-liquid, liquid-solid, and slurry-wall), and heat-transfer characteristics of these types of reactors. It should be noted that the three-phase slurry reactor is presently a subject of considerable research investigation. In some cases, the work performed in two-phase (either gas-liquid or liquid-solid) reactors is applicable to three-phase reactors however, this type of extrapolation is kept to a minimum. Details of the equivalent two-phase reactors are considered to be outside the scope of this chapter. 

Cas-Liquid Mass Transfer Gas-liquid mass transfer normally is correlated by means of the mass-transfer coefficient K a versus power level at various superficial gas velocities. The superficial gas velocity is the volume of gas at the average temperature and pressure at the midpoint in the taiik divided by the area of the vessel. In order to obtain the partial-pressure driving force, an assumption must be made of the partial pressure in equihbrium with the concentration of gas in the liquid. Many times this must be assumed, but if Fig. 18-26 is obtained in the pilot plant and the same assumption principle is used in evaluating the mixer in the full-scale tank, the error from the assumption is limited. 

Gas-Liquid Mass Transfer Gas-liquid mass transfer normally is correlated by means of the mass-transfer coefficient K a versus power level at various superficial gas velocities. The superficial gas velocity is... 

Note that all these correlations (mass transfer, gas holdup, and dynamic pressure drop) are valid only within the following operating ranges ... 

Because diffusion in the liquid phase is much slower than in the gas phase, the liquid film presents most of the resistance to mass transfer. Gas-liquid film theory defines the mass transfer rate (MTR) in terms of the liquid film, as follows... 

Reactions, Kinetics and Mass Transfer Gas phase species balances ... 

If the liquid film has the controlling resistance to mass transfer, gas-film coefficients could still be used for design calculations following Eq. (22.45). If liquid-film coefficients are used, and if the factor (1 — x)i is introduced to allow for... 

Two-film theory of mass transfer Gas Interface Liquid... 

Mass transfer, gas-liquid Mass transfer Mass transfer, liquid-solid Mass transfer, liquid-solid Mass transfer, evaporation Mass transfer, gas-liquid solid-liquid Heat transfer... 

Drop circulation Empirical, mass transfer gas-liquid liquid-liquid... 

Mass transfer involves establishing a transfer between the elementary regions of the reactor and between individual phases (interfacial mass transfer coefficients gas phase mass transfer, liquid phase mass transfer, mass transfer with reaction, liquid-solid mass transfer), as well as other elementary phenomena and processes connected with mass transfer gas phase phenomena and processes (gas hold-up, bubble size, interfacial area and bubble coalescence/redispersion), volumetric mass transfer and power consumption during mass transfer (2). 

In general, it can be concluded that substantial progresses have been made in the experimental and theoretical analysis of trickle-bed reactors under unsteady-state conditions. But until now these results are not sufficient for a priori design and scale-up of a periodically operated trickle-bed reactor. The mathematical reactor models, which are now available are not detailed enough to simulate all of the main transient behavior observed. For solving this problem specific correlations for specific model parameters (e.g. Hquid holdup, mass transfer gas-solid and liquid-solid, intrinsic chemical kinetic, etc.) determined under dynamic conditions are required. The available correlations for important hydrodynamic, mass-and heat-transfer parameters for periodically operated trickle-bed reactors leave a lot to be desired. Indeed, work for unsteady-state conditions on a larger scale may also be necessary. 

S] = reaction -H mass transfer gas-liquid -h inlets/outlets T>eff 2 , (p) = turbulent dispersion coefficient = z, (p)/Sct... 

Local mass transfer gas-Hquid is again predicted with Eqs. (41) - (44). The kinetic expressions are summarized in Table 5. For further details the reader is advised to study the original papers [78,79]. 

Rotating packed bed liquid-side mass transfer Gas-side mass transfer... 

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