Note: An important consideration in the design of diffused air systems is that the overall mass transfer coefficient changes with changing air flow rate; that is, KLa is not "constant". Under dynamic loading conditions, over periods when the oxygen demand is high, and a higher air flow is required, there is a drop-off in transfer efficiency. This is an important factor when determining blower peak air delivery requirements.
Manufacturers generally provide diffuser performance data in the form of a set of curves for a range of diffuser densities showing SOTE/depth (%/m or %/ft) versus air flow rate per diffuser. Typically, for fine bubble systems the curves start in the region of 8%/m (2.5%/ft) for low air flow rates, decrease with increasing air flow, and level off at higher air flows. An example is shown in the diagram below.
These data reflect the change in the mass transfer coefficient, KLa , with increasing air flow and diffuser density. The data points in the plot below are from aeration tests conducted at two diffuser densities in a 20 foot by 20 foot test tank for diffuser submergences of 9, 14 and 19 feet. [The diffusers used in these tests were 18 inches in diameter, and the numerical values in the plots below should not be seen as typical for other diffuser types]. In the plot the clusters of data points correspond to KLa values for different submergences. Note that the diffuser density, DD%, is defined in terms of the coverage:
where AT = Area of aeration tank.
Therefore:
where AD = Total area of diffusers in aeration tank.
Research has shown that the mass transfer coefficient, KLa correlates with the superficial gas velocity – i.e. the air flow rate per unit aerated tank area. In BioWin the KLa value is estimated using a correlation of the form:
where C = parameter depending on diffuser density (see below), Y = parameter [Default value in BioWin = 0.82], USG = Superficial gas velocity (m3/m2/day) = QAIR / Area of bioreactor, QAIR = Air flow rate (m3/day).
The parameter C is a function of the diffuser density. In BioWin the C value is determined as follows:
where K1 = parameter [Default value in BioWin = 2.5656/day], K2 = parameter [Default value in BioWin = 0.0432].
The diagrams below show how BioWin predicts the relations between (a) KLa and superficial gas velocity, and (b) SOTE (%/m) and air flow per diffuser, for K1 = 1.8 /day and the other parameters presented above.
BioWin allows the user to set up plots for bioreactors showing SOTE/depth (%/m or %/ft) versus air flow rate per diffuser [An example system including an SOTE plot can be downloaded from the downloads section on this website]. The chart is shown below. SOTE data are plotted for the diffuser density of the bioreactor in question, and for four other user-selected diffuser densities. The example includes experimental data for a number of diffuser types]. The plot can be compared to manufacturer data, and used as a basis for selecting appropriate parameters (K1 and K2) for a particular diffuser type. Alternatively users can contact EnviroSim to obtain a spreadsheet to assist in parameter selection.
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