R. Z. Tudose

Mixing studies in external-loop airlift reactors

Abstract The mixing behaviour of the liquid phase in two external-loop airlift reactors of laboratory and pilot scale, in terms of the mixing time and axial dispersion, was investigated. The mixing parameters were determined from the output curves to an initial Dirac pulse, using the classical tracer response technique, and analysed in relation to operating and geometrical parameters, such as vsGR, AD/AR, Hd and DR. Mixing in the external-loop airlift reactors under investigation was essentially correlated with the liquid circulation velocity. The specific sections of the airlift reactor have different mixing behaviour, so that the riser and downcomer can be analysed as plug flow with axial dispersion, whereas the top section approaches ideal mixing behaviour. A simple correlation between the specific mixing time and the operating and geometrical parameters was developed which can be used for design and scale-up purposes.

Residence time distribution of the liquid phase in a concentric-tube airlift reactor

Abstract The residence time distribution (RTD) analysis of liquid phase was performed in a concentric-tube airlift reactor of 0.07 m 3 nominal volume, regarded as a simple unit and discriminating its different sections (riser, downcomer, bottom zone and gas-liquid separator) using the tracer response technique. The reactor was operated in biphasic continuous flow of liquid and gaseous phase. The volumetric liquid inflow rate, Q 1 , and gas superficial velocity in the riser, ν SGR , were chosen as independent variables. RTD functions in the reactor, namely: the RTD distribution functions at the reactor exit, inclusively in normalized form, E ( θ ), the distribution ages inside the reactor, I ( θ ), the intensity of distribution, λ ( θ ), respectively, as well as the mean liquid residence time, and the variance of distributions were essentially used for liquid flow diagnosis in the reactor in two operational modes: with and without recirculation. The experimental results revealed that the investigated airlift reactor had a more uniform flow than the tubular and bubble column reactors and the flow defects proved a significant attenuation. These conclusions were confirmed by the single parameter flow models: axial dispersion and tank-in-series models. This fact has a great practical importance, especially in the biotechnological applications of the airlift reactors, where the aerobic cultures can be affected greatly by the flow deficiencies.

Modelling of liquid circulation velocity in concentric-tube airlift reactors

Abstract A mathematical model for riser liquid superficial velocity in a concentric-tube airlift reactor is proposed. The model is based on an energy balance incorporating acceleration coefficients to quantify deviations from ideal flow. The acceleration coefficients at the draft-tube and downcomer entrance are determined experimentally, based on static pressure profile measurements. The model could predict liquid velocities over a broad range, including an almost 50-fold variation of liquid circulation velocity and a four-fold change in reactor height. The model predictions agreed with the measurements to within ± 28%.

Study of the liquid circulation velocity in external-loop airlift bioreactors

AbstractLiquid circulation velocity was studied in externalloop air-lift bioreactors of laboratory and pilot scale, respectively for different gas input rates, downcomer-to-riser cross-sectional area ratio, AD/AR and liquid phase apparent viscosities.It was found that, up to a gas superficial velocity in the riser vSGR ≈ 0.04 m/s the dependency of vSLR on vSGR is in the following form: vSLR = a vSGRb, with the exponent b being 0.40. Over this value of vSGR, only a small increase in liquid superficial velocity, vSLR is produced by an increase in vSGR.nAD/AR ratio affects the liquid superficial velocity due to the resistance in flow and overall friction.For non-Newtonian viscous liquids, the circulation liquid velocity in the riser section of the pilot external-loop airlift bioreactor is shown to be dependent mainly on the downcomer-to-riser cross-sectional area ratio, AD/AR, the effective (apparent) liquid viscosity, ηeff and the superficial gas velocity, vSGR.The equation proposed by Popovic and Robinson [11] was fitted well, with an error of ± 20%.

Effects of downcomer-to-riser cross sectional area ratio on operation behaviour of external-loop airlift bioreactors

Experiments performed in two external-loop airlift bioreactors of laboratory and pilot scale, (1.880–1.189) · 10−3 m3 and (0.170-0.157)m3, respectively, are reported. The AD/ARratio was varied between 0.111–1.000 and 0.040–0.1225 in the laboratory and pilot contractor respectively.Water and solutions of different coalescence (2-propanol 2% vol, 1 M Na (glucose 50% wt/vol) and rheological behaviour (non-Newtonian starch solutions with consistency index K=0.061–3.518 Pasn and flow behaviour index n=0.86-0.39), respectively, were used as liquid phase. Compressed air at superficial velocities vSGR=0.016–0.178 ms−1 in the laboratory contactor and vSGR=0.010–0.120 ms−1 in the pilot contactor, respectively was used as gaseous phase.The AD/ARratio affect gas-holdup behaviour as a result of the influence of AD/ARon liquid circulation velocity.Experimental results show that AD/ARratio affect circulation liquid velocity by modifying he resistence to flow and by varying the fraction of the total volume contained in downcomer and riser. AD/ARratio has proven to be the main factor which determines the friction in the reactor. Mixing time increases with increasing of the reactor size and decreases with AD/ARdecreasing.The volumetric gas-liquid mass transfer coefficient increases with AD/ARratio decreasing, as a result of variations of the liquid velocity with AD/AR, which affect interfacial areas.Correlations applicable to the investigated contactors have been presented, together with the fit of some experimental data to existing correlation in literature.

Concentric-tube airlift bioreactors

Abstract Liquid circulation velocity was investigated in three concentric-tube airlift reactors of different scales (RIMP, VL=0.07 m3; RIS-1, VL=2.5 m3; RIS-2, VL=5.20 m3). The effects of top and bottom clearance and resistance in flow pathway at downcomer entrance on the riser liquid superficial velocity, the circulation time, the friction coefficient and flow radial profiles of the gas holdup and the liquid superficial velocity in riser, using water-air as a biphasic system, were studied. It was found that the riser liquid superficial velocity is affected by the analyzed geometrical parameters in different ways, depending on their effects on the pressure loss. The riser liquid superficial velocity, the friction coefficient and the parameters of the drift-flux model were satisfactorily correlated with the bottom spatial ratio (B), gas separation ratio (Y) and downcomer flow resistance ratio (Ad/AD), resulting empirical models, with correlation coefficients greater than 0.85.

Residence time distribution of liquid phase in an external-loop airlift bioreactor

The residence time distribution analysis was used to investigated the flow behaviour in an external-loop airlift bioreactor regarded as a single unit and discriminating its different sections. The experimental results were fitted according to plug flow with superimposed axial dispersion and tank-in-series models, which have proved that it is reasonable to assume plug flow with axial dispersion in the overall reactor, in riser and downcomer sections, as well, while the gas separator should be considered as a perfectly mixed zone. Also, the whole reactor could be replaced with 105-30 zones with perfect mixing in series, while its separate zones, that is the riser with 104-27, the downcomer with 115-35 and the gas separator with 25-5 perfectly mixed zones in series, respectively, depending on gas superficial velocity, AD/AR ratio and the liquid feed rate.

EFFECTS OF GEOMETRY ON HYDRODYNAMICS IN EXTERNAL-LOOP AIRLIFT REACTORS

Experimental investigations were carried out in model external-loop airlift reactors. Two reactors of laboratory scale (riser liquid height ranged between 1.16-1.56 m, riser diameter 0.03 m, AD/AR ratio between 0.111-1,000, total liquid volume VT = (1.189-2.446).10−3m3) and pilot-plant scale (riser liquid height of 4.4 and 4.7 m, respectively, riser diameter 0.200 m, AD/AR ratio of 0.1225 and 0.040 m, total liquid volume, VT = (0.144-0.170) m3) were used. The influences of reactor geometry characterized by some parameter as: AD/AR ratio, liquid height in riser and downcomer and liquid height in gas separator, together with the amount of introduced air, on the basic hydrodynamic design parameters: gas holdup and liquid circulation velocity were analysed. The influence of gas sparger design on gas holdup and liquid velocity was found to be negligible. The experimental liquid circulation velocity was correlated using a simplified form of the energy balance in airlift reactors, valid for external-loop airlift...

Mass transfer coefficients in liquid-liquid extraction

Abstract The paper concerns with mass transfer in liquid–liquid extraction, considering that the experimental determination of the individual mass transfer coefficients during liquid–liquid extraction is still considered to be a difficult problem. In this work, the liquid–liquid extraction using a ternary system, water–acetone–carbon tetrachloride, was studied. The investigations were performed in an improved Lewis cell, in continuous and batch operation modes. The immiscible components (water–carbon tetrachloride) were mixed separately, while the acetone transfer was carried out in the two directions through an interface with a constant area of the surface, created between the two phases. The specific fluxes, the driving forces and the individual and overall mass transfer coefficients were determined by measuring the inflow and outflow acetone concentrations in the two phases, together with the equilibrium data. The values of the individual mass transfer coefficients in each phase were correlated and expressed in the form of criterial equation, as Sh = bRe p Sc m .

Modelling mixing parameters in concentric-tube airlift bioreactors

Abstract The mixing behaviour of the liquid phase in concentric-tube airlift bioreactors of different scale (RIMP: VL=0.070u2009m3; RIS-1: VL=2.50u2009m3; RIS-2: VL=5.20u2009m3) in terms of mixing time was investigated. This mixing parameter was determined from the output curves to an initial Dirac pulse, using the classical tracer response technique, and analyzed in relation to process and geometrical parameters, such as: gas superficial velocity, νSGR; top clearance, hS; bottom clearance, hB, and ratio of the resistances at downcomer entrance, Ad/AR. A correlation between the mixing time and the specified operating and geometrical parameters was developed, which was particularized for two flow regimes: bubbly and transition (νSGR≤0.08 m/s) and churn turbulent flow (νSGR> 0.08 m/s) respectively. The correlation was applied in bioreactors of different scale with a maximum error of ±30%.