Gas Hold-Up

Gas hold-up is one of the most important parameters characterizing the hydrodynamics in a fermenter. Gas hold-up depends mainly on the superficial gas velocity and the power consumption, and often is very sensitive to the physical properties of the liquid. Gas hold-up can be determined easily by measuring level of the aerated liquid during operation ZF and that of clear liquid ZL. Thus, the average fractional gas hold-up H is given as 

Gas Sparging with No Mechanical Agitation In a two-phase system where the continuous phase remains in place, the hold-up is related to superficial gas velocity Vs and bubble rise velocity V, (Sridhar and Potter, 1980)  

Akita and Yoshida (1973) correlated the gas hold-up for the absorption of oxygen in various aqueous solutions in bubble columns, as follows  

Gas Sparging with Mechanical Agitation Calderbank (1958) correlated gas hold-up for the gas-liquid dispersion agitated by a flat-blade disk turbine impeller as 

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This form is partieularly appropriate when the gas is of low solubility in the liquid and "liquid film resistanee" eontrols the rate of transfer. More eomplex forms whieh use an overall mass transfer eoeffieient whieh ineludes the effeets of gas film resistanee must be used otherwise. Also, if ehemieal reaetions are involved, they are not rate limiting. The approaeh given here, however, illustrates the required ealeulation steps. The nature of the mixing or agitation primarily affeets the interfaeial area per unit volume, a. The liquid phase mass transfer eoeffieient, kL, is primarily a funetion of the physieal properties of the fluid. The interfaeial area is determined by the size of the gas bubbles formed and how long they remain in the mixing vessel. The size of the bubbles is normally expressed in terms of their Sauter mean diameter, dj, whieh is defined below. How long the bubbles remain is expressed in terms of gas hold-up, H, the fraetion of the total fluid volume (gas plus liquid) whieh is oeeupied by gas bubbles. 

To apply the mass transfer equation for design, the interfacial area, a, and mass transfer coefficient kL must be calculated. The interfacial area is dependent upon the bubble size and gas hold-up in the mixing vessel as given by ... 

Wachi, S., Morikawa, H. and Ueyama, K., 1987. Gas hold-up and axial dispersion in gas-liquid concurrent bubble column. Journal of Chemical Engineering Japan, 20, 309-316. 

Gas hold-up is defined as Ha = Bubble volume/Reactor volume, which is the volume of gas per unit volume of reactor. Assume the system is an agitated vessel. Let us use Richard s data to define gas hold-up 3... 

Calculate the gas hold-up for an agitated and aerated system with power input of 18 hp in an 80 m3 vessel with gas superficial velocity of 2.6 m-rniif1. 

The gas hold-up can be defined by the above definition using the gas height per volume, where H is 0.6 m for aeration... 

Calculate mass transfer, gas hold up, gassed and ungassed power for the fermenter with the given data ... 

Fig.l Schematic diagram of ALR Fig.2 Liquid velocity profile Fig.3 Gas hold-up profile...

Fig.2 and Fig.3 show the typical liquid velocity and gas hold up distribution in the ALR. From the figures, one notices that the cyclohexane circulates in the ALR under the density difference between the riser and the downcomer. An apparent large vortex appears near the air sparger when the circulating liquid flows fi om the downcomer to the riser at the bottom. In the riser, liquid velocity near the draft-tube is much larger than that near the reactor wall, the latter moved somewhat downward. The gas holdup is nonuniform in the reactor, most gas exists in the riser while only a little appears in the dowmcomer. 

Effect of draft-tube horn-mouth diameter on liquid velocity and gas hold-up... 

In the Fig.4, it can be seen that the gas hold-up in both riser and downcomer decreases with increasing the draft-tube horn-mouth diameter and approaches the maximum when the draft-tube hom-mouth diameter is 1.05m. However, due to the gas hold-up decreases more in the downcomer, the gas hold-up difference between the downcomer and the riser increases. Therefore, the apparent density difference between the riser and the downcomer enhances, causing higher liquid superficial velocity in the downcomer and in the riser With increasing the hom-mouth diameter. Fig.5 also shows that the existence of hom-mouth promotes the ability to separate gas from liquid and decreases the amount of gas entrained into the downcomer. 

Fig.6 and Fig.7 illustrate the effect of draft-tube diameter on liquid superficial velocity, liquid circulating flowrate and gas hold-up. Results show that the liquid superficial velocity in the riser increases with increasing the draft-tube diameter while the liquid velocity in the... 

Since the local turbulence intensity and the energy dissipation in the reactor are lower, under the condition that the gas hold-up keeps approximately constant in the riser and in the... 

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

SG gas hold-up (volumetric fraction of gas in reaction mixture) dimensionless 0.02-0.4... 

The presence of a gas in the suspension results in an increase of the stirrer speed required to establish the state of complete suspension. The propeller usually requires a higher speed than the turbine. Furthermore, a critical volume gas flow exists above which drastic sedimentation of particles occurs. Hence, homogenisation of the suspension requires an increase of the rotational speed and/or a decrease of the gas flow rate. The hydrodynamics of suspensions with a solid fraction exceeding 0.25-0.3 becomes very complex because such suspensions behave like non-Newtonian liquids. This produces problems in the scale-up of operations. Hydrodynamics, gas hold-up, mass-transfer coefficients, etc. have been widely studied and many correlations can be found in literature (see e.g. Shah, 1991). 

A well-substantiated correlation for air-water systems taken from the trickle bed literature (Morsi and Charpentier, 1981) was used for the volumetric mass transfer coefficients in the / , and (Rewap)i terms in the model. The hi term was taken from a correlation of Kirillov et al. (1983), while the liquid hold-up term a, in Eqs. (70), (71), (74), (77), and (79) were estimated from a hold-up model of Specchia and Baldi (1977). All of these correlations require the pressure drop per unit bed length. The correlation of Rao and Drinkenburg (1985) was employed for this purpose. Liquid static hold-up was assumed invariate and a literature value was used. Gas hold-up was obtained by difference using the bed porosity. 

Both reactions are slow compared to the film diffusion in the liquid phase13-15. Hence, the reactions can be assumed to take place predominantly in the bulk phase of the liquid. The rate of mass transfer can be calculated using Equation 7.2. The interfacial concentration can be calculated using Henry Law. Mass transfer coefficients, interfacial area and gas hold-up data are required. Gas hold-up is defined as ... 

Mixing characteristics of phase Practical device Gas hold-up (%) kL A (s )... 

Table 7.4 gives the hydrodynamic data used for the calculations. Gas hold-ups were taken from Table 7.3. 

The draft-tube airlift bioreactor was studied using water-in-kerosene microemulsions [263], The effect of draft tube area vs. the top-section area on various parameters was studied. The effect of gas flow rates on recirculation and gas carry over due to incomplete gas disengagement were studied [264], Additionally, the effect of riser to downcomer volume was also studied. The effect of W/O ratio and viscosity was tested on gas hold-up and mass transfer coefficient [265], One limitation of these studies was the use of plain water as the aqueous phase in the cold model. The absence of biocatalyst or any fermentation broth from the experiments makes these results of little value. The effect of the parameters studied will greatly depend on the change in viscosity, hold-up, phase distribution caused due to the presence of biocatalyst, such as IGTS8, due to production of biosurfactants, etc., by the biocatalyst. Thus, further work including biocatalyst is necessary to truly assess the utility of the draft-tube airlift bioreactor for biodesulfurization. 

Mehrnia, M. R. Towfighi, J. Bonakdarpour, B., and Akbarnejad, M. M., Gas hold-up and oxygen transfer in a draft-tube airlift bioreactor with petroleum-based liquids. Biochemical Engineering Journal, 2005. 22(2) pp. 105-110. 

In the early 1990s, Bakker and Van den Akker (1991, 1994) introduced an approximate but effective Euler Euler approach (see also A. Bakker s PhD Thesis, 1992) on the basis of a single-phase RANS flow field calculated by FLUENT, a code named GHOST calculated local and averaged values of bubble size db, gas hold-up a, and specific mass transfer rate kfl. 

In spite of all the simplifications Bakker and Van den Akker applied and given the black box approach for the impeller swept domain, their simulations resulted in values for the bubble size just below the liquid surface, overall holdup, and average kfl values which are in good agreement with their experimental data (see Table II). The major step forward they made was the acquisition of the different spatial distributions of average bubble size (see Fig. 13), bubble holdup and kfl as effected by three common impeller types. As a matter of fact, their approach may be restricted to low values of the gas hold-up. 

Fig. 14. Spatial distribution of the gas hold-up in a turbulent stirred vessel filled with a 0.075% Keltrol solution in water and agitated by a Rushton turbine (a) experimental data obtained by means of an optical probe (b) computational result from DAWN. Overall hold-up amounts to some 3.1% in the simulation and 3.7% in the experiment. Reproduced with permission from Venneker...

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