| The influences of total foam height, HF, and gas flow rate on the axial liquid holdup profile of rising foam was studied systematically and a drainage-enhanced multistage-draining foam fractionator (MDF) was developed based on the new findings. The newly developed device was successfully applied to the extraction of nisin from its fermentation broth on a laboratory scale and the average liquid fraction of the foam leaving the top of the columnεout, was effectively reduced, thus the enrichment was enhanced. The mechanism of the MDF was explained by invoking recent work on hydrodynamics of rising foam and the process was modeled. The data-processing method of the turbidimetric bioassay of nisin was also improved for the convenience of subsequent application experiments of the MDF. The method also represents a good candidate for daily practice in industrial application.Single factor controlled experiments showed that the column resident time of the foam had no deterministic influences onεout, while HF had a significant impact on the axial liquid holdup profile of the foam itself. When HF increased from 20 cm to 60 cm, the average liquid fraction in the lowest column section was enhanced from 1.2% to 2.8%, while only slight decrease was observed inεout (falling from 0.7% to 0.5%). The gas flow rate, G, had more influence onεout than HF did: the effect of a drop of G from 400 cm3/min to 300 cm3/min was found to be equivalent to an increase in HF from 20 cm to 80 cm, thus attempts to increase the extent of drainage of the foam by lengthening the height of the foaming column may not be very productive. A device consisted of a foam riser and a closing plated was designed for enhanced foam drainage and the device was finally developed into an MDF. An MDF with three of such gadgets were tested on laboratory scale using Nisin fermentation broth. It was found that the potency recovery of Nisin obtained from a semi-batch foam fractionation performed on the MDF was less than 95% of that given by a bare column while the residual loss and the activity loss caused by denaturation in foam was more serious (1.26 and 1.16 times higher than a bare column, respectively). In spite of this, the high enrichment factor given by the MDF, which was 2.42 times higher than a bare column operated in a parallel experiment, made the newly developed device very attractive for industrial scale application. Experimental results and theoretical analysis indicated that the gadget developed in the present work could effectively change the axial liquid holdup profile of the foam layer; however this was not the cause for the decreasedεout.εout was found to be mainly dependent on the bubble size at the top of the column when other operating parameters such as superficial gas velocity were fixed.Stevenson's hydrodynamic theory of rising foam (Stevenson P. Hydrodynamic theory of rising foam, Miner. Eng. 2007, 20: 282-289) was extended to the MDF and the draining process was modeled. This is the first time in this filed that a drainage-enhancing device was developed and successfully modeled as well. |
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