Hydrodynamic Behavior of a Liquid-Lift, External-Loop Bioreactor Using a Spinning Sparger

Abstract

The liquid-lift bioreactor is conceptually similar to the familiar air-lift bioreactor. A liquid is sparged into the base of a column containing a second, immiscible liquid of higher density. The two phases rise cocurrently to the top of the column, where they are separated. The dense phase is then recycled to the base of the riser, whereas the light phase is removed from the bioreactor. The hydrodynamic characteristics of a 12 L, liquid-lift, external-loop bioreactor have been investigated using water and oleic acid as the continuous and dispersed phases, respectively. The experimental unit had a working height of 1.7 m and a downcomer to riser area ratio of 0.43. A spinning sparger consisting of six, 1 mm diameter orifices spread evenly on a 4.4 cm diameter circle was incorporated near the base of the riser to allow for enhanced control of the produced droplets. Experimental studies were undertaken at superficial dispersed-phase velocities up to 4 cm/min and sparger spinning speeds up to 350 rpm (maximum orifice tangential velocity of 0.8 m/s). Uniform droplets were produced at diameters ranging from 1 to 5 mm, while liquid holdups and circulation velocities reached up to 2% and 3 cm/s, respectively. The droplet size data were best fit to an empirical model, and the well-known drift-flux theory of Zuber and Findlay was used to predict the dispersed-phase holdup. The circulation velocity of the continuous phase was predicted using an energy balance around the loop. The model was found to provide reasonable predictions of droplet diameter, dispersed phase holdup, and circulation velocity as functions of both the dispersed-phase superficial velocity and the spinning speed of the sparger.

Mass Transfer and Bioethanol Production in an External-Loop Liquid-Lift Bioreactor

“…The oleic acid entering the column did not detach in uniform droplets from the spinning sparger. This was the same problem reported earlier by Modaressi et al so that in this investigation no measurements were made below 150 rpm. Unfortunately, slow coalescence at the top of the column can cause carryover of the continuous fluid out of the top of the riser at high oleic acid flow rates.…”

“…With the hydrodynamic conditions known, 13 masstransfer characteristics inside the ELLB studied in this work, and knowledge of the growth characteristics of S. cerevisiae fermenting glucose to ethanol, 18 it was possible to develop a model incorporating both mass transfer and fermentation. For the purpose of this model, it was assumed that the liquid phase was well mixed because mass transfer and the growth of the cells were slow (measured in minutes or hours) relative to the rates at which the aqueous phase circulated about the column (circulation time of less than 10 s).…”

“…Oleic acid has a high selectivity for ethanol over water (ratio of the number of ethanol to water molecules in the oleic acid phase >100 at 20 °C), is nontoxic to Saccharomyces cerevisiae, and is not miscible with water. Modaressi et al 13 investigated the use of a new extractive fermentation vessel: a spinning-sparger, external-loop, liquidlift bioreactor (ELLB). They studied the hydrodynamic properties of an oleic acid-water system including drop size, gas holdup, and circulation velocity.…”