4/7/2012 AIRLIFT BIOREACTORS contents Introduction Fluid Dynamics Mass Transfer Airlift Reactor — Selection and Design 1
4/7/2012 INTRODUCTION • airlift reactor (ALR) covers a wide range of gas – liquid or gas – liquid – solid pneumatic contacting devices that are characterized by fluid circulation in a defined cyclic pattern • Role of gas stream • Difference between ALRs and bubble columns Airlift reactors on the basis of their structure • 1) external loop vessels • 2) baffled (or internal-loop) vessels 2
4/7/2012 All ALRs comprise four distinct sections • Riser • Downcomer • Base • Gas separator Beter heat transfer for large scale Advantages of Airlift Bioreactors • energy dissipation • heat transfer • shear 3
4/7/2012 Gas injected to the Airlift reactors the difference in gas holdup between the riser and the downcomer where ∆ P b is the pressure difference, is the density of the liquid (the density of the gas is considered to be negligible), g is the gravitational constant, and and are the fractional gas holdup of the riser and downcomer efficiency of ALRs Aeration efficiency as a function of pneumatic power of gas input per unit volume in a straightbaffleALR 4
4/7/2012 • Energy economy • The advantages counterbalance the obvious disadvantage of ALRs FLUID DYNAMICS • Design variables : reactor height, riser-to-downcomer area ratio, geometrical design of gas liquid velocity separator, and the bottom clearance in the ALR • Operating variables : gas input rate , top clearance 5
4/7/2012 Flow Configuration • Riser • 1. Homogeneous bubbly flow regime • 2. Churn-turbulent regime Map of flow configurations for gas – liquid concurrent flow in a vertical tube . 6
4/7/2012 • Downcomer. • cross-sectional area ratio of the riser to the downcomer • Gas Separator. • Gas Holdup (2) where the subindexes L, G, and S indicate liquid, gas, and solid, and i indicates the region in which the holdup is considered 7
4/7/2012 Importance of the holdup • 1) the value of the holdup gives an indication of the potential for mass transfer • 2) the difference in holdup between the riser and the downcomer generates the driving force for liquid circulation Gas Holdup in Internal Airlift Reactors. (3) J G is the superficial gas velocity, is the effective viscosity of the liquid,and and a are constants 8
4/7/2012 (4) Q in is the freshly injected gas, Q d is the recirculated gas Some correlations proposed for prediction of gas holdup in the riser of internal-loop ALRs. 9
4/7/2012 External-Loop Airlift Reactors. • By using Drift flux model • Drift velocities of the gas and liquid phases where A is cross-sectional area, C 0 is distribution parameter, J is superficial velocity, J G is superficial gas velocity, U G linear gas velocity, and ϕ is gas holdup 10
4/7/2012 • The value of C 0 depends mainly on the radial profile of the gas holdup • C 0 = 1 for a flat profile and C 0 = 1.5 for a parabolic profile • C 0 = 1.03 – 1.2 for upflow, • C 0 = 1.0 – 1.16 for downflow Drift velocity of a swarm of bubbles Where U 2J is the velocity of the swarm of bubbles, ∆ρ is the density difference, is the surface tension difference between holdup, ϕ , and the flowing volumetric concentration ( β ) 11
4/7/2012 connection between the gas holdup and β J is the superficial velocity, is the terminal gas velocity For riser For downcomer Gas flow holdup ( ϕ ) vs. flowing volumetric concentration (β ). The different zones in the plane ϕ - β identify the two phase flow. 12
4/7/2012 holdup increases with height. Dependence of the riser gas holdup in a 4-m high external-loop ALR with a multiple-orifice sparger ( solid lines) and a single- orifice sparger ( broken lines). gas holdup in the riser where the constant α depends on the friction losses in the loop, and β is usually a value between 0.6 and 0.7 Gas holdup reported by various sources for the riser of airlift reactors under conditions of little or no gas recirculation.The data correspond to different Ad/Ar ratios. 13
4/7/2012 Some correlations proposed for prediction of gas holdup in the riser of external ALRs. The gas holdup is presented as a function of the superficial gas velocity. Effects of Liquid Rheology. • Effects • superficial gas velocity and the global shear rate have direct Proportionality: Where γ is global shear rate is used global viscosity 14
4/7/2012 L R is an effective length that represents the mean circulation path of a bubble in the system considered, P is the power input, Sab is the total surface of all of bubbles, and τ is the shear stress where the subindexes 1 and 2 represent the two extremes of the section considered Effect of Liquid Level. Riser and downcomer gas holdup in an internal-loop ALR for two different top clearances and two liquids 15
4/7/2012 Gas holdup in the riser of an external-loop ALR for several top clearances ? Gas Recirculation Gas recirculation in a split-cylinder ALR. The level indicated corresponds to no-aeration conditions. 16
4/7/2012 The mathematical expression that gives this maximal gas recirculation is: (20) where Q d is the gas flow rate in the downcomer, Q L is the liquid circulation flow rate, Pi is pressure at point i of the reactor (1 is top of the riser, 2 is top of the downcomer, 3 is bottom of the downcomer, 4 is bottom of the riser), C d is the hydraulic resistance coefficient, U L is the linear liquid velocity Liquid Velocity • affects the gas holdup in the riser and downcomer, the mixing time, the mean residence time of the gas phase, the interfacial area, and the mass and heat transfer coefficients. 17
4/7/2012 Modeling of Liquid Flow. • Two main methods for the modeling of two- phase flow in ALRs • energy balances and momentum balances • expression for the average superficial liquid velocity based on energy balance: where K b ,K t are the hydraulic pressure loss coefficients Authors assume that K t , the friction coefficient at the top of the loop, is negligible in concentric-tube type reactors and that in external- loop reactors K t can be taken as equal to K b , the friction coefficient for the bottom of the loop. empirical correlation for K b where A b is the minimal cross section at the bottom of the airlift reactor. 18
4/7/2012 the riser-to-downcomer cross-sectional area ratio and the reactor height are the main parameters that affect the superficial liquid velocity at constant superficial gas velocity. Liquid velocity predicted by some of the proposed correlations Liquid Mixing • t m • degree of homogeneity ( I): where C is the maximum local concentration and C m is the mean concentration of tracer at complete mixing. 19
4/7/2012 • Study of the residence time distribution (RTD) • The axial dispersion model where C is the concentration of a tracer Bodenstein number ( Bo), which is used to describe the mixing in the reactor : where L is the characteristic length the mixing conditions are similar to those Bo of a plug-flow reactor Low Bo no. the reactor can be considered as well-mixed where M is a constant equal to 0.093 or to 0.089 for Bo 50 and a degree of inhomogeneity,I = 0.05 20
4/7/2012 where D z is the dispersion coefficient and D is the column diameter where J L is the superficial liquid velocity and U c is the cell circulation velocity given by: Where is the terminal bubble velocity Mixing in the Gas Phase Energy Dissipation and Shear Rate in Airlift Reactors Extended to mass transfer proposed the expression: 21
4/7/2012 The general energy balance Riser Gas separator Downcomer Bottom Schematic description of the variables in the thermodynamic model for energy dissipation distribution in an ALR. 22
4/7/2012 The shear stress in the liquid of each region of the reactor can be defined as the energy dissipated divided by the mean path of circulation in the region and by the sum of the areas of all the bubbles For the region i in the ALR where t i is the residence time of the liquid, h i is the effective length, and a i is the specific interfacial area, in the region i. A global shear rate ci can be calculated for each region i as MASS TRANSFER • 1) static properties of the liquid • 2) dynamic properties of the liquid • 3) liquid dynamics. Mass Transfer Rate Measurements: • steady-state and nonsteady-state methods valid only for perfectly mixed systems 23
4/7/2012 Bubble Size and Interfacial Area • in a population of homogeneous bubble size where the Sauter mean diameter (d S ) is given by Influence of the superficial gas velocity on overall k L a and on the k L a in each of the regions of an ALR. 24
4/7/2012 for the volumetric mean diameter of the bubbles in the riser of a concentric-tube ALR where d O is the diameter of the sparger orifice and the function f ( N W ) is different for each range of N W Data Correlations for Mass Transfer Rate • Hydrodynamic • thermodynamic Influence of the superficial gas velocity on overall k L a and on the k L a in each of the regions of an ALR 25
4/7/2012 AIRLIFT REACTOR — SELECTION AND DESIGN Scale-up of Airlift Bioreactors • purposes • problems Results for a scaled-up bioreactor with a constant oxygen transfer rate 26
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