Fluiddynamic characterization of airlift tower loop bioreactors with and without motionless mixtures

Using the heat pulse technique, the local mean ow liquid velocity and the mixing conditions for twophase¯ow in the riser of an airlift bioreactor have been measured and analysed. Xanthan-gum solutions were used as the physical model to some ®lamentous broths reported in the literature. A two-fold decrease of liquid velocity and diffusional mixing regime are predicted for the course of a fermentation process proceeding in a non-Newtonian biomass growth circulation system.

Section: Resultssupporting

confidence: 73%

Section: Introductionmentioning

confidence: 97%

Small-scale liquid mixing in a bioreactor column with xanthan-gum simulated filamentous media

Sisak

Venyige

Martinov

et al. 2000

“…Increases in gas holdup and k L a with a 1% CMC solution but not water were observed by Kawase and Moo-Young [12] using a concentric airlift reactor with a flat six bladed impeller rotating in the top section. Oxygen transfer and riser gas holdup have been improved in airlift reactors by installation of sieve plates and static mixers [13,25,26,31] to disperse large bubbles formed by coalescence. Static mixers suppressed slug formation during viscous fermentations and the resultant increase of oxygen transfer improved cephalosporin productivity [36].…”

Section: Introductionmentioning

confidence: 99%

Influence of a propeller on Saccharomyces cerevisiae fermentations in a pilot scale airlift bioreactor

Pollard

Ison

Shamlou

et al. 1997

The hydrodynamics (sectional gas holdup and liquid velocities) and oxygen transfer performance of a conventionally operated multiconfigurable pilot scale (0.25 m 3 ) concentric airlift bioreactor containing baker's yeast were significantly improved by operating a marine propeller to draw liquid down the draft tube and aid recirculation at the base of the vessel. Propeller operation reduced the severe DOT heterogeneity of the reactor, which gave DOT values below 1% air saturation in the riser, by producing DOTs above 40% around the vessel at maximum energy dissipation rate. As a consequence the overall oxygen uptake rate (OUR) of the baker's yeast increased up to 3 fold with the total energy dissipation rate into the reactor until the lowest DOTs of the vessel were at or above 10%. The different degrees of heterogeneity generated by the two reactor configurations enabled the reactor to be used as a scale down tool to study the impact of heterogeneity on the physiology of fermentation broths. Comparison of the hydrodynamics and oxygen transfer between tall and short reactor heights revealed that the faster circulation times of the short reactor produced a greater improvement in the OUR with propeller operation even though similar DOT changes occurred around both sizes of reactor. This indicated that the yeast cells were responding to the rapid DOT changes around the vessel. List of symbolsA D cross sectional area of the downcomer m 2 A R cross sectional area of the riser m 2 D p propeller diameter m DCW dry cell weight g/l DOT dissolved oxygen tension % air saturation g acceleration due to gravity m/s 2 H D height of the gas -liquid dispersion from the vessel base m H DT height of the top of the draft tube from the vessel base m k L a volumetric mass transfer coefficient s -1 L clearance between the bottom of the draft tube and vessel base m N propeller speed s )1 OUR oxygen uptake rate mmol/l.h P A energy dissipation rate for airlift aeration only operation W/m 3 P g energy dissipation for airlift operation W P IN pressure at the point of gas inlet to the vessel Pa P P energy dissipation rate for propeller only operation W/m 3 P O propeller power number -P T total energy dissipation rate for combined airlift and propeller operation W/m 3 t c liquid circulation time s t m mean mixing time to achieve 90% homogeneity s U LD mean liquid linear velocity in the downcomer m/s U LR mean liquid linear velocity in the riser m/s U sg superficial gas velocity in riser m/s V L liquid volume m 3 L density of liquid kg/m 3 w density of water kg/m 3 gas holdup -D mean downcomer gas holdup -O mean overall gas holdup -R mean riser gas holdup -TS mean top section gas holdup -1 Introduction Many studies of laboratory and pilot scale [7, 10, 22, 30] airlift reactors have considered the hydrodynamics and oxygen transfer but few have examined the impact of the circulatory flow on the metabolism of fermentation cultures. For airlift operation air is introduced into one section of the vessel and bubbles are entrained into the downcomer wh...

“…The in¯uence of the super®cial gas velocity on small bubbles is modest. The small-bubble-¯ow pattern is affected by the liquid phase behaviour [4]. In the case of the highly viscous non-newtonian CMC solution, the radial bubble velocity pro®les present an important variation due to the wide distribution of bubble size.…”

Section: Bubble Sizementioning

confidence: 99%

“…Therefore, it is necessary to develop local experimental studies of the two-phase¯ow in airlift reactors. In spite of their importance, such works are very rare [3,4].…”

Section: Introductionmentioning

confidence: 99%

Local gas hydrodynamics in an external-loop airlift reactor: newtonian and non-newtonian fluids

Bentifraouine

Xuereb

Riba

1999

Measurements of local gas phase characteristics are obtained in an external-loop airlift reactor ®lled with newtonian or viscous non-newtonian liquids. A doubleoptical ®ber probe technique is used. It allows the determination of the axial and radial pro®les of gas hold-up, bubbling frequency, bubble size and velocity. In the case of air-water system, the results show a strong effect of radial liquid velocity variation on the gas¯ow characteristics at the bottom of the riser. In the case of highly viscous nonnewtonian solution, the gas¯ow is strongly affected by the gas distribution just above the gas sparger. This study also points out the bubble coalescence and the break-up phenomena in different liquids and levels in the reactor. Furthermore, the local measurements of bubble size and velocity allows to gain more detailed information on the dynamics of the bubble-¯ow and shows a tendency of large bubbles to circulate in the column center.