14A-53

Xanthan gum is a microbial polysaccharide with extensive industrial applications because of its unique rheological properties.  However, the high broth viscosity causes agitation and aeration in the conventional stirred-tank reactor (STR) to be extremely difficult and consequently limits its productivity.  We have developed a novel, centrifugal, packed-bed reactor (CPBR) for viscous xanthan gum fermentation.  The xanthan productivity achieved in CPBR was found significantly higher than that of STR.  We hypothesize that the arrangement of the centrifugal, packed bed (CPB) and the recirculation loop (RL) may enhance overall OTR in CPBR.

Our objective was to characterize the aeration efficiency in CPBR with respect to the effects of CPB arrangement and the RL.

Xanthan fermentation broth was produced from a glucose medium by batch fermentation using Xanthomonas campestris.  The oxygen transfer efficiency of CPBR was evaluated using the volumetric mass transfer coefficient (k_La) in comparison with STR and water-in-oil emulsion (STR-WIO).

STR-WIO showed the highest k_La (0.038 s^-1 at 2%) among all systems studied, yet practical difficulties exist in product recovery.  It was found that, at 3.5% xanthan the k_La in CPBR (0.018 s^-1) was higher than that of STR (0.005 s^-1) and close to that of STR-WIO (0.020 s^-1), indicating improved OTR at such xanthan concentration.  A gas-continuous arrangement, in which the CPB was kept above the broth, was able to elevate k_La to 0.023 s^-1, higher than that of STR-WIO.  The RL was found to play an important role in providing better gas-liquid contact to enhance CPBR aeration.

With the improved oxygen transfer character in CPBR at high xanthan concentration, the CPBR system is practically the preferred system for xanthan fermentation in terms of oxygen transfer ability.  The characteristic roles of CPB arrangement and the RL should be considered primarily during scale-up operation.