Determinations of volumetric mass transfer coefficient were conducted in a three-phase internal-loop airlift reactor with an enlarged degassing zone. The effect of parameters such as the airflow rate (riser superficial gas velocities between 0.01 and 0.5 m/s), solids loading (up to 30%, v/v), solids density (1023 and 1048 kg/m 3) and the liquid-phase properties on k L a was studied. It was observed that the increase of the airflow rate and the introduction of ethanol enhanced the volumetric mass transfer coefficient in the system. On the contrary, the progressive introduction of solids and a small increase on solids density were responsible for the diminishing of the mass transfer rate. Correlations for the volumetric mass transfer coefficient with the riser superficial gas velocity and solids loading were determined for the two solids density and the two liquid-phases. A good agreement between experimental data and the calculated values was obtained.
The hydrodynamic behaviour of a 60 l threephase airlift bioreactor, of the concentric draught tube type, with an enlarged degassing zone has been studied. Ca-alginate beads were used as the solid phase. Air¯ow rate (from 1.9 to 90.2 l/min), solids loading (0% to 40% (v/v)) and solids density (1016 and 1038 kg/m 3 ) were manipulated and their in¯uence on solids and gas holdup, circulation and mixing times and in the interstitial liquid velocity was determined. Riser and downcomer solids holdup was found to decrease with the increase of air¯ow rate and to increase with solids loading and density. On the contrary, gas holdup in the riser and in the downcomer increased with air¯ow rate and decreased with solids loading and density. By increasing air¯ow rate, a decrease in circulation time was observed while the effects of solids loading and density were negligible. Mixing time decreased with air¯ow rate, increased with solids density, in the studied range, and presented a maximum for solids loading of approximately 20% (v/v). List of symbolsA d m downcomer cross-section area A r m riser cross-section area d m vertical distance between two points in the riser and in the downcomer H 1 À H 2 cmH 2 O pressure difference between two points of the riser and of the downcomer t c s circulation time t m s mixing time u l m/s interstitial liquid velocity u ld m/s downcomer interstitial liquid velocity u lr m/s riser interstitial liquid velocity v gr m/s riser super®cial gas velocity
The e!ect of the distributing plate ori"ce diameter, air#ow rate, solids loading and solids density on the hydrodynamic characteristics * gas holdup, circulation time and liquid velocity * of a three-phase external-loop airlift reactor was characterized. It was observed that the gas distributor has a small e!ect on riser gas holdup, circulation time and downcomer liquid velocity. On the contrary, the air#ow rate, solids loading and solids density signi"cantly a!ect the hydrodynamic characteristics of the external-loop airlift reactor. A previously described model was used to estimate simultaneously both the riser gas holdup and the downcomer linear liquid velocity. The model simulated with high-accuracy experimental data obtained with three di!erent distributing plate ori"ce diameters, two solids density and solids loading up to 30% (v/v).
The effect of the addition of ethanol (10 g/l) to the liquid-phase on gas and solids holdup, circulation and mixing times and interstitial liquid velocity in a threephase airlift reactor was investigated. The airlift reactor (60 l) is of the concentric draught-tube type with an enlarged degassing zone. Ca-alginate beads were used as solid-phase and air¯ow rate (from 1.9 to 90.2 l/min) and solids loading (0±30% (v/v)) were manipulated. Riser and downcomer gas holdup were found to increase with the addition of ethanol, leading to a decrease on the relative solids holdup. The presence of ethanol seems to have no in¯uence on the circulation time. On the other hand, mixing time variation depends on the solids loading and air¯ow rate. Riser and downcomer interstitial liquid velocity are lower for ethanol solution than for water. List of symbolsA d downcomer cross-section area (m) A r riser cross-section area (m) d vertical distance between two points of the riser and of the downcomer (m) H 1 AH 2 pressure difference between two points of the riser and of the downcomer (cmH 2 O) t c circulation time (s) t m mixing time (s) u ld downcomer interstitial liquid velocity (m/s) u li interstitial liquid velocity in section i (m/s) u lr riser interstitial liquid velocity (m/s) v gr riser super®cial gas velocity (m/s) V s solids volume measured in each sample (l) V T sample total volume (l) Dt time required by the tracer to travel between the two acquisition points in the riser and in the downcomer (s) e gd downcomer gas holdup e gi gas holdup in section i e gr riser gas holdup e sd downcomer solids holdup e si solids holdup in section i e sr riser solids holdup q l liquid density (kg/m 3 ) q s solids density (kg/m 3 )
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