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OFM: Enhancement mass transfer

Gas - Liquid Contact

The effect of flow oscillations on mass transfer enhancement has been well characterised in recent years, both for reciprocating plate columns (see Baird et al, 1996) as well as for oscillatory flow in baffled tubes.

The effect of oscillatory flow on gas dispersion in a reciprocating plate column is shown in figures 1 to 4 below: the sequence of four figures show the efficient dispersion of gas and increased hold-up due to the oscillation of the baffle plates. Figure 5 shows the region around a baffle plate. In these figures, gas was sparged from the bottom of the column, and it can be seen how gas is distributed into small, relatively uniformly sized bubbles by the sinusoidal (up and down) motion of the baffle plate, which contains three orifices.

Gas sparged OFM column, no mixing Gas sparged OFM column, start of oscillation
Figure 1 - Gas sparged from base. No oscillations Figure 2 - Start of plate oscillation
Gas sparged OFM column, one complete oscillation Gas sparged OFM, many oscillations

Figure 3 - After one complete oscillation

Figure 4 - After many oscillations

 

Visualisation of bubble dispersion

Figure 5 - Flow visualisation of bubble dispersion in oscillatory flow.
Note the small, uniform size of the bubbles

In a baffled tube system, the effect of flow oscillations is very similar. In a recent study by Hegwill et al (1992), the effect of oscillatory flow in baffled tubes for the oxygen/water system has been investigated. The apparatus consisted of a vertically mounted 26 mm i.d. perspex tube of length 0.6m. Fluid oscillations were provided by a rubber diaphragm and electromagnetic oscillator. Air was sparged from the base of the tube, and the oxygen-water mass transfer coefficient, kLa, was determined for different oscillatory conditions and three different types of baffles: helical, central (disk-type), and wall mounted orifice-type. Photographic visualisation of fluid mechanics and gas bubble behaviour was also performed, which revealed that the oscillatory baffled flow was able to modify the bubble trajectories, which seen to move around the inside the volumes of the baffle cavities. At very intense levels of oscillation, the bubbles could be seen trapped within each cavity for at least several seconds.

In a system without baffles, mass transfer with oscillatory flow was similar to a bubble column. The application of fluid oscillations in the absence of internal baffles had little effect on the measured kLa values. However, it was shown that if orifice-type baffles (spaced at 1.5 tube diameters) and fluid oscillations were present, up to six-fold increase in kLa relative to a bubble column was obtained. Experimental kLa values at various superficial gas flowrates, coupled with power density calculations using a quasi-steady model were used to fit a correlation for kLa, which is given as follows:

 

Oxygen-water mass transfer correlation

 

where εv is power density (w.m-3), and Ug is the superficial gas velocity (mm/s). This correlation can be compared to Linek's correlation for stirred tanks (Linek et. al., 1987).

If these two correlations are compared graphically, as depicted in the graph below, it can be seen that the oscillatory flow system is more energy efficient than gas sparged stirred tanks. For a given power density, the kLa value obtained for oscillatory flow is of order twice that of the stirred tank value, for all values of power density over 200 w.m-3.

 

Plot of mass transfer verses power density

Conclusions

The use of oscillatory flow for mass transfer enhancement, coupled with the fine control over mixing intensity suggests that an oscillatory flow mass transfer device would have potential for certain shear sensitive applications where a stirred tank would be inappropriate, such as fermentation reactions. A preliminary study (Harrison and Mackley, 1991) has demonstrated the feasibility of an oscillatory flow bio-reactor. The capability of oscillatory flow to effectively mix two phases also suggests its use in gas-liquid reactions where diffusion is performance limiting, and scale-up difficulties of stagnant zones would be minimised, which can occur in stirred tank devices.

References

Baird MHI, Rama Rao NV, and Stonestreet P. "Power dissipation and hold-up in a gassed reciprocating baffle plate column." IChemE., 74, Part A, 1996, pp 463-469.

Hewgill, M.R., Mackley, M.R., Pandit, A.B. and Pannu, S.S. "Enhancement of gas-liquid mass transfer using oscillatory flow in baffled tubes." Chem. Engng. Sci. 48, pp799-803, 1993.


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