Stirred Tank Reactor Volumetric Oxygen Transfer Coefficient Environmental Sciences Essay

Among the agitation processes, there are chief factors to optimise and command to obtain the best output and productiveness such as pH, temperature and dissolved O concentration. The concentration of dissolved O in agitation stock is affected by the O transportation rate ( OTR ) which relates to agitation velocity and aeration rate and the O uptake rate ( OUR ) from the micro-organisms in the stock. The volumetric O transportation coefficient ( kLa ) is basically the agitation parametric quantity which presents the efficiency of aeration and the effects of the operating variables on dissolved O concentration. Therefore, it is used to supervise and command the agitation. Several methods to gauge kLa such as O balance method, sulfite oxidization and dynamic method. Among these methods, dynamic method is the technique that normally use for mensurating kLa during operation. Besides the direct measuring of kLa during civilization, there are many of correlativities, which have been developed to gauge kLa in moved armored combat vehicle reactor. The simplex correlativity for anticipation kLa was developed by Aguiar Jr. et Al. ( 2002 ) . A correlativity of kLa was affected by agitation and aeration, which used as the original theoretical account for farther developing the correlativity of kLa. This was act uponing from agitation velocity, aeration rate and biomass concentration.

Introduction

The aerophilic bioprocesses are normally carried out under optimized conditions such as temperature, pH, force per unit area, commixture, concentrations of biomass, foods and dissolved O. In these procedures, O is an importance substrate that is used by the aerophilic micro-organism for growing, care and production. Furthermore, the lack of O affects the procedure public presentation. Therefore, it is of import to present sufficient O from a gas watercourse to the agitation stock by appraisal of the O transportation rate ( OTR ) which can be described by the volumetric O transportation coefficient ( kLa ) . The purpose of this seminar is to show the appraisal of kLa by the dynamic method and the method to develop kLa correlativity.

1. The volumetric O transportation coefficient ( kLa ) description

kLa is the proportionality invariable to find the magnitude of the OTR. kLa is influenced by many factors such as operational conditions ( agitation velocity and aeration rate ) , physical belongingss of gas and liquid, geometrical parametric quantities of the reactor and the presence of biomass. As shown in Figure 1 is an illustrated position of the factors impacting OTR and kLa at different degrees in a bioprocess. Furthermore, kLa is importance for design the reactor for illustration the distance between two impellers. kLa is used for operate and command the agitation procedure such as the accommodation of agitation velocity or aeration rate to obtain the desire kLa. Finally, kLa is used as the parametric quantity for graduated table up to the big graduated table.

Figure 1 Relationship between OTR, volumetric O transportation coefficient and hydrodynamic parametric quantities in bioreactors at several degrees

Beginning: Garcia-Ochoa and Gomez ( 2009 )

2. Experimental finding of the volumetric O transportation coefficient ( kLa )

The finding of kLa in reactors is indispensable for prove the aeration efficiency in the reactors and to look into the effects of the operating conditions on dissolved O. There are many methods that have been developed to find the kLa in the reactors such as sulfite oxidization method, gas reconciliation method and dynamic method.

Dynamic method is the method to find the kLa base on the mass balance of the dissolved O in the well-mixed liquid stage as shown in equation ( 1 ) and ( 2 )

( 1 )

( 2 )

is the accretion rate of O in the liquid stage, represents the O transportation rate from the gas to the liquid stage which is the equilibrium impregnation concentration of dissolved O, is the concentration of dissolved O, is the specific O ingestion rate and is the biomass concentration. So, the merchandise is the oxygen uptake rate ( or OUR ) .

The methods to mensurating the kLa in a microbic bioprocess can be classified into the absence of micro-organisms or with dead cells and in the presence of biomass that consumes O at the clip of measuring.

2.1 The methods to kLa measurement of without biological ingestion of O or gassing-out method ( =0 )

In the absence of biomass, the O uptake rate by micro-organism is zero ( OUR=0 ) . Therefore, equation ( 2 ) can be simplified to equation ( 3 )

( 3 )

The stairss of experiments consist of soaking up and desorption of O from liquid medium. The desorption of dissolved O is started by agencies of bubbling N gas to extinguish O in medium until the O concentration is equal to nothing. Subsequently, the liquid is contacted once more with air. The O concentration grows up with clip as shown in Figure 2

Figure 2 Conventional description of the dynamic technique desorption-absorption of O for inert status measurings

Beginning: Garcia-Ochoa and Gomez ( 2009 )

So, the kLa is determined by integrating of equation ( 3 ) into equation ( 4 ) as follow:

( 4 )

In this instance, kLa can be obtained as the incline of a secret plan of versus clip as illustrated in Figure 3

Slope= kLa

Figure 3 A secret plan of versus clip to find the kLa value in absence of biomass

2.2 The methods to kLa measurement of with biological ingestion of O ( ? 0 )

In the presence of biomass, the O transportation rate and O uptake rate occur together. Therefore, the mass balance of dissolved O is equal to equation ( 2 ) . In desorption measure or the gas supply to the reactor is turned off ( OTR=0 ) , the dissolved O concentration will cut down at a rate of O ingestion by the respiration of micro-organism ( Figure 4 ) . Under these measure equation ( 2 ) can be simplified to equation ( 5 )

( 5 )

Therefore, or OUR can be acquire as the incline of a secret plan of versus clip. Furthermore, the specific O ingestion ( ) is easy to cipher utilizing the measured of value.

During the aeration measure, the gas is flowed once more. The O is absorbed into the broth medium. Both O transportation and O ingestion occur together. The equation ( 2 ) is rearranged into equation ( 6 ) as follow

( 6 )

For a known biomass concentration ( ) and. kLa is the incline of a secret plan of versus and the y-intercept of this secret plan obtains the value of the equilibrium impregnation concentration of dissolved O ( ) in the stock.

Figure 4 Conventional description of the direct measurement of OTR in bioprocess by the classical moral force technique.

Beginning: Garcia-Ochoa and Gomez ( 2009 )

Figure 5 A secret plan of versus to find the kLa value in presence of biomass

3. Empirical correlativity of kLa values in moved armored combat vehicle reactor

In moved armored combat vehicle reactor and the impeller is the chief gas scattering tool and agitation velocity and design of impeller have an consequence on mass transportation in reactor. Garcia-Ochoa and Gomez ( 2009 ) reviewed the empirical correlativities for the volumetric O transportation coefficient which depend on geometrical parametric quantities and correlated with the combination of agitation velocity ( N ) , superficial gas speed ( VS ) and liquid effectual viscousness ( ?a ) as shown in the undermentioned equation ( 7 )

( 7 )

where the changeless C is the geometrical parametric quantities of the vas and the impeller employed. The mean power input per volume ( P/V ) is the consequence of agitation velocity ( N ) and a, B and degree Celsius are the values to demo a fluctuation scope in the different correlativities of different writers:

0.3 ? a ? 0.7 ; 0.4 ? B ? 1 ; -0.4 ? c ? -0.7

The advocates of different correlativities were reviewed by Garcia-Ochoa and Gomez ( 2009 ) shown in Table 1

Table 1 Advocate values in equation ( 7 ) for stirred armored combat vehicle bioreactors

Furthermore, there are some of the dimensionless correlativities in the literature for Newtonian and non-Newtonian fluids as shown in Table 2 and 3, severally.

Table 2 Dimensionless correlativities for anticipation of kLa in Newtonian fluids in moved armored combat vehicles

Table 3 Dimensionless correlativities for anticipation of kLa in non-Newtonian fluids in moved armored combat vehicles

4. A correlativity of kLa impacting with agitation and aeration ( Aguiar Jr. et Al. ( 2002 ) )

Besides, the empirical correlativities which have been developed for appraisal of kLa in moved armored combat vehicle reactor as described above. The simplex correlativity for anticipation kLa was developed by Aguiar Jr. et Al. ( 2002 ) utilizing gassing-out method to find of kLa. The stairss to develop this correlativity was established as follow

1. The experiments for determine of kLa was conducted in a New Brunswick BioFlow III fermentor with aeration, agitation, temperature, pH and dissolved O control. The surveies were carried out at temperature 30 -C in a 2.5 cubic decimeter capacity civilization vas with 2.2 fifty civilization medium at agitation velocity 100-900 revolutions per minute and air flow at 0.1-4.0 l/min. The dissolved O was measured by Ingold polarographic electrode and kLa was determined utilizing the gassing-out method ( absence of biomass ) .

2. The kLa values were calculated from the dissolved O dynamic balance in absence of biomass ( OUR=0 ) as equation ( 8 )

( 8 )

where is the equilibrium impregnation concentration of dissolved O and is the concentration of dissolved O.

Integration of this equation ( 8 ) obtaining equation ( 9 )

( 9 )

The kLa is incline of this consecutive line between versus clip. The assorted values obtained in the aeration rate at 0.10 to 4 l/min and agitation velocity runing from 100-900 revolutions per minute were used to develop a kLa correlativity. In Figure 6 showed the kLa findings for different agitation velocities at an aeration rate of 1 l/min.

Figure 6 Typical curves for kLa findings at assorted agitation velocity at aeration rate 1.0 l/min

3. Afterward, the kLa values were plotted at different agitation velocities ( N ) in different aeration rate ( Q )

Figure 7 kLa with different agitation velocities ( N ) in different air flows ( Q )

From this Figure 7, the consequences showed the kLa values increase when increasing the agitation velocity and aeration rate.

4. Figure 8 showed the concluding correlativity after secret plan ( the incline of a secret plan of kLa versus agitation velocity ( N ) ) with aeration rate ( Q ) obtaining the correlativity: = 0.0461 ln ( Q ) + 0.1165

Figure 8 kLa/N – air flow-final correlativity

Decision

1. A correlativity for anticipation of kLa which was developed by Aguiar Jr. et Al. ( 2002 ) was related kLa with agitation velocity and aeration rate. The correlativity was used for proctor and controller the agitation to obtaining the desire kLa by changing agitation velocity or aeration rate.

2. This correlativity was used as the original theoretical account for farther developing the correlativity of kLa which act uponing from agitation velocity, aeration rate and biomass concentration.

LITERATURE CITED

Aguiar Jr. , W.B. , L.F.F. Faria, M.A.P.G. Couto, O.Q.F. Araujo and N. Pereira Jr. 2002. Growth theoretical account and anticipation of O transportation rate for xylitol production from d- wood sugar by C. guilliermondii. Biochem. Eng. J. 12: 49-59.

Garcia-Ochoa, F. and E. Gomez. 2009. Bioreactor scale-up and oxygen transportation rate in microbic procedures: an overview. Biotechnol. Adv. 27: 153-176.

Lopez, J.L.C. , E.M.R. Porcel, I.O. Alberola, M.M.B. Martin, J.A.S. Perez, J.M.F. Sevilla, and Y. Chisti. 2006. Coincident finding of O ingestion rate and volumetric O transportation coefficient in pneumatically agitated bioreactors. Ind. Eng. Chem. Res. 45: 1167- 1171.

Suresh, S. , V.C. Srivastava and I.M. Misha. 2009. Techniques for O transportation measuring in bioreactors: a reappraisal. J. Chem. Technol. Biotechnol. 84: 1091-1103.