Bio De Reactores

D. Moutafchieva, D. Popova, M. Dimitrova, S. 4, 2013, 351-356Journal of Chemical Technology and Metallurgy, 48,Tchaoushev

EXPERIMENTAL DETERMINATION

OF THE VOLUMETRIC MASS TRANSFER COEFFICIENT

D. Moutafchieva, D. Popova, M. Dimitrova, S. Tchaoushev

University of Chemical Technology and Metallurgy

8 Kl. Ohridski, 1756 Sofia, Bulgaria

E-mail: dmoutaf@uctm.edu

Received 16 April 2013

Accepted 15 May 2013

ABSTRACT

The volumetric mass transfer coefficient (kLa) is often used in order to compare the efficiency of bioreactors

and as an important scale-up factor. In bioreactors, a number of methods are available for estimation the overall

volumetric oxygen mass transfer coefficient: the dynamic method, the steady-state method based on a preliminary

determination of the oxygen uptake rate, the gaseous oxygen balance, the carbon dioxide balance. Each method

provides a different estimation of the value of kLa. We examined the sensitivity of the various methods and compare

the kLa values obtained in order to select the most suitable method as a function of the type of bioreactor used. In

this work we applied dynamic gassing-out method for measuring the volumetric mass transfer coefficient kLa in

three type gas-liquid reactors (stirred tanks, bubble columns and airlift). Тhe aim of this work is on the basis of our

experimental data to obtain several correlations for evaluation of kLa.

Keywords: volumetric oxygen transfer coefficient, bubble columns, CSTR, airlift reactors.

INTRODUCTION

Aeration and agitation are important variables to

provide effective oxygen transfer rate during aerobic

bioprocesses. Hence, the knowledge of the volumetric

mass transfer coefficient (kLa) is required. The deter�

mination of kLa in a bioreactor is essential in order to

establish its aeration efficiency and to quantify the effects

of operating variables on oxygen supply.

Oxygen transfer in aerobic bioprocesses is essential

and any shortage of oxygen vastly affects the process

performance. Therefore, oxygen mass transfer is one

of the most important phenomena in the design and

operation of mixing�sparging equipment for bioreactors

[4]. It can be described and analyzed by means of the

volumetric mass transfer coefficient, kLa. The values of

kLa are affected by many factors, such as geometrical and

operating characteristics of the reactor (type of impeller,

the geometry of the bioreactor, the agitation speed and

the air flow rate), media composition and properties,

concentration and microorganism’s morphology and

biocatalyst’s properties [3].

In aerated systems the critical limiting factor in pro�

viding the optimal environment is the oxygen transfer

rate (OTR). The mass balance for the dissolved oxygen

in the well�mixed liquid phase can be written as:

dC L∗= k L a (C L − C L ) − rO2 = OTR − OUR

dt

(1)

When oxygen uptake rate, OUR = 0, the oxygen

mass balance in the liquid phase can be simplified to:

dCL∗= k L a (CL − CL= OTR)

dt

or

(2)

the oxygen mass transfer rate can be described as

proportional to the concentration gradient. For aerobic

fermentation the maximum value of the concentration

gradient is limited due to the low solubility of oxygen.

Therefore, the maximum mass transfer rate from the gas

to the liquid in the bioreactor can be estimated by kL a.CL

,

∗

351

Journal of Chemical Technology and Metallurgy, 48, 4, 2013

as CL is the saturation concentration in the liquid phase.

Integrating:

CL 2

CL1

∗

Hence a plot of of

∗ CL − CL1 

ln  ∗

CL − CL 2 

vs. t should result in a straight line of slope kL a .

∫(

1

dC = kL a ∫ dt∗CL − CL0

t

)

(3)

∗ CL − CL1 

ln  ∗ = kL a.t

 CL − CL 2 

(4)

METHODS

The methods for measuring the kLa can be classified

depending on whether the determination is made in the

Table 1. Methods for volumetric mass transfer coefficient determination [2].

Measurement

method

kLa.102

[s-1]

Assay

time

Scale

applied

Assumptions/Drawbacks

The rate of reaction is assumed to be zero

order in sulfite. Alteration of driving

force,diffusioncoefficient,and

coalescence properties; complex kinetics

boundary layer reduction. This method is

fairly labor intensive.

Assumptions about kinetic reaction must

be made. Possible alteration of the

driving force. Change of the coalescence

behavior.

Assumptions about kinetic reaction must

be made. Salt addition does not alter the

mass transfer rate of CO2.

Hydrazine does not accumulate. No

chemical enhancement.

Available of oxidative enzyme; limited to

small scales.

A nonrespiring system can be employed

to simulate the fermentation broth. The

response time of the electrode, τr, is a

critical parameter. Gassing time can be

significant at larger scales

High DO concentration is necessary.

Nongassing period must be short and

OUR independent of DO concentration.

Invasive probes are necessary and

response time must be considered.

Hydrodynamic changes may disturb the

microbial metabolism.

OUR is independent from DO

concentration. Invasive probes are

necessary and response time must be

considered

For large scales, the assumptions of well�

mixed gas and liquid phase may not be

valid. This method may not be the best

choice in case of small bioreactors, where

the difference between Fin and Fout may

be very small because of the short contact

time The accuracy depends on the

precision of oxygen analyzer

Sulfite

oxidation

0 � 0,3

Hours

Laboratory

scale

Chemical

Absorption

of CO2

Dynamic

measure of

pH

...