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Biochemical Engineering Journal 22 (2005) 105110

Gas hold-up and oxygen transfer in a draft-tube airliftbioreactor with petroleum-based liquids

Mohammad Reza Mehrniaa,b,, Jafar Towfighib, Babak Bonakdarpourc,Mohammad Mehdi Akbarnejadd

a Chemical Engineering Department, Tehran University, Tehran 11365-4563, Iranb Biotechnology group, Chemical Engineering Department, Tarbiat Modarres University, Tehran, Iran

c Chemical Engineering Department, Amir Kabir University of Technology, Tehran, Irand Iranian Research Institute of Petroleum Industry, Tehran, Iran

Received 11 May 2004; received in revised form 25 August 2004; accepted 3 September 2004

Abstract

Gas hold-up (g) and volumetric gasliquid oxygen transfer coefficient (kLa) in a draft-tube airlift bioreactor (24 103 m3 in volume)

were studied using pure kerosene and diesel and their water-in-oil microemulsions, as the model solutions for petroleum biodesulfurization.

For comparison, experiments were also done with distilled water. The g and the kLa values for kerosene and diesel systems were in most

cases significantly higher than the water system. Increase in water-to-oil phase volume ratio () of the microemulsion systems resulted in

decrease in the values ofg, which was attributed to a decrease in the coalescence-inhibiting tendency of the petroleum liquids. Increase in

the viscosity (v) of the microemulsion systems to around 32 106 m2/s resulted in the occurrence of the churn turbulent regime with an

associated decrease in the value ofg, which could not be solely accounted for by the increase in . Furthermore, the kLa values decreased

with increase in the viscosity of the petroleum-based liquids. However, when churn turbulent conditions prevailed, increase in viscosity alone

could not account for the decrease observed in the kLa values. Empirical correlations were developed that related g and kLa to and ,

respectively, under the bubbly flow regime inside the bioreactor. 2004 Elsevier B.V. All rights reserved.

Keywords: Airlift bioreactors; Gas hold-up; Oxygen transfer; Petroleum; Microemulsions; Viscosity

1. Introduction

Airlift bioreactors are being considered for use in a variety

of biotechnological processes, such as biocatalytic desulfu-

rization (BDS) of petroleum, in which a slurry of bacterial

cells or aqueous biocatalytic agent (as the dispersed phase)is mixed with high-sulfur distillate to produce a homogenous

emulsion, preferably a microemulsion [15].

The majority of hydrodynamics and oxygen transfer data

for airlift bioreactors have beenobtained with air/water-based

systems, with properties different from the real conditions of

the BDS processes, in which petroleum-based media are em-

Corresponding author. Tel.: +98-21-6498982; fax: +98-21-6461024.

E-mail address: jtd [email protected] (M.R. Mehrnia).

ployed that have much lower surface tensions, higher viscosi-

ties and bubble coalescence inhibiting rather than promoting

properties.

It is known that both liquid viscosity and surface tension

affect gas hold-up (g) [68] and volumetric oxygen trans-

fer coefficient (kLa) [912] in airlift bioreactors containingwater-based media. The small bubbles formed in liquids with

reduced surface tension may enhance both g and kLa [9,10].

An increase in liquid viscosity results in the formation of

larger bubbles that reduce g, while at the same time the vis-

cose drag increaseswhich canlead to a decrease in bubble rise

velocity and hence increase in g [12,13]. Some researchers

have, therefore, reported a decrease in the value ofg with in-

creasing liquid viscosity [1215] whereas others have found

that there is no significant effect of viscosity on g [9,10,16].

1369-703X/$ see front matter 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.bej.2004.09.007


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106 M.R. Mehrnia et al. / Biochemical Engineering Journal 22 (2005) 105110

Nomenclature

a specific gasliquid interfacial area (m2/m3)

AN two-pair non-ionic surfactants (Arkopal N100

and N40)

Dcolumn diameter (m)ESF two-pair non-ionic surfactants (Ethoxylated

soya fatty acid 2 and 10 M)

H draft-tube height (m)

kL liquid-side oxygen transfer coefficient (m/s)

kLa volumetric oxygen transfer coefficient (1/s)

TS two-pair non-ionic surfactants (Tween 80 and

Span 80)

ugr riser superficial gas velocity (m/s)

v kinematic viscosity (m2/s)

W/D water-in-diesel microemulsion

W/K water-in-kerosene microemulsion

W/O water-in-oil microemulsion

Greek letters

g gas hold-up

density (kg/m3)

surface tension (N/m)

water-to-oil phase volume ratio

Furthermore, increase in liquid viscosity results in decrease

in the value of kL due to decrease in internal recirculation

of bubbles [11,12]. Therefore, an increase in liquid viscos-

ity generally leads to a decrease in kLa [1015,17]. On theother hand, increase in the bubble coalescence inhibiting ten-

dencies of the liquid phase has been reported to affect the

relationship between g and gas velocity [18,19].

Mehrnia et al. [20,21] used a water-in-kerosene (W/K)

microemulsion with a water-to-oil phase volume ratio () of

20%, as a cold model of the BDS inside a draft-tube air-

lift bioreactor (DTAB) with different geometries and found

somewhat different hydrodynamic and oxygen transfer char-

acteristics of the bioreactor compared to the results reported

for water-based media. In BDS processes, various hydrocar-

bon fractions are used as substrate and also the ratio is

an important operational variable [14,2224]. The latter, to-

gether with the type of hydrocarbon fractions used in making

the microemulsion, determines the viscosity of these systems

[25,26].

Thepurpose of the present work, therefore, hasbeen to ob-

tain gas hold-up and the kLa values inside a DTAB employ-

ing different petroleum liquids and water-in-oil (W/O) mi-

croemulsion systems with the in the range 1030%. Higher

ratioswas notconsideredsince, in BDSprocesses,the useof

microemulsions with low water-to-oil ratios is recommended

to reduce vessel size, lower the cost of downstream process-

ing operations and minimize biocatalyst and water utilization

with respect to the amount of oil [25,2224]. Furthermore,

the effect of the type of surfactant used in the preparation of

the microemulsion system was also investigated. Addition-

ally, correlations describing the effect of the physicochemical

characteristicsof petroleum liquids and W/O microemulsions

on the gas hold-up and the volumetric oxygen transfer coef-

ficient in the DTAB have been obtained.

2. Experimental system

The DTAB was made of Pyrex glass with 0.14 m inside

diameter. The height of draft-tubes was 1.48 m. The inside

diameter of draft-tubes was 0.095 m with a wall thickness

of 0.0025 m, yielding an Ad/Ar ratio of 0.707. There was a

0.024 m annular gapbeneaththe bottomedge of thedraft-tube

for fluid circulation. Air was introduced into the draft-tube

through a set of perforated pipes (ladder-like shape sparger)

with 30 equally spaced 0.001m holes. The sparger was lo-

cated 0.06 m above the lower end of the draft-tube. The dis-

tance between the static liquid surface and the upper edge ofthe draft-tube was 0.1 m.

The liquids employed in the present work were distilled

water, kerosene, diesel and different W/O microemulsion

systems employing different hydrocarbon base and ra-

tios in the range 1030%. Table 1 lists the properties of

those liquids at the experimental temperature, i.e., 25 C.

Three different non-ionic pair of surfactants namely Arkopal

N100 and N40 (Ethoxylated nonylphenol 10 and 4 M) (AN)

[Clariant, Germany], Ethoxylated soya fatty acid 10 and 2 M

(ESF) [Hooman shimi Co., Iran], Tween 80 and Span 80

(TS) [Aldrich Co. Ltd.] were used in the preparation of the

microemulsion systems. These systems were prepared asdescribed previously [19,20]. Surfactant pair AN was also

added to kerosene in one of the experimental runs inside

the DTAB in order to investigate the influence of the ad-

dition of surfactants to petroleum liquids on gas hold-up

and kLa.

The bed expansion method was used to evaluate the over-

all gas hold-up in the airlift reactor [12,27]. This was de-

termined by measuring the difference between the ungassed

and gassed liquid volume. The volumetric oxygen transfer

coefficient was determined by the dynamic physical method

[12,20,27]. Dissolved oxygen concentrations were monitored

continuously using a dissolved oxygen meter (Mettler Toledo

4220X, Germany) with a rapid polarographic dissolved oxy-

gen probe (Mettler Toledo, Inpro 6100/120/T/P, Germany).

3. Results and discussion

3.1. Gas hold-up

In Fig. 1, the gas hold-up data obtained inside the DTAB

with kerosene, diesel, and their various W/O microemulsions

as a function of riser superficial gas velocity (ugr) is pre-

sented and compared with the data for distilled water. As


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M.R. Mehrnia et al. / Biochemical Engineering Journal 22 (2005) 105110 107

Table 1

Properties of liquids at 25 C

Liquid Water/oil

ratio ()

Surfactants Density

(kg/m3)

Kinematic viscosity

(106 m2/s)

Surface tension

(mN/m)

Symbol in the

figuresPetroleum soluble Water soluble

Kerosene 0 801.8 1.74 29.5 Kerosene

Surfactant-added

kerosene

0 AN4 815.5 1.92 29 W/K 0, AN

Water/kerosene

microemulsion

10 AN4 AN10 839.9 3.7 29 W/K 10, AN

Water/kerosene

microemulsion

20 Span 80 Tween 80 862.6 4.87 29.5 W/K 20, TS

Water/kerosene

microemulsion

20 AN4 AN10 869.5 6.52 29.5 W/K 20, AN

Water/kerosene

microemulsion

20 ESF4 ESF10 873.3 9.34 30 W/K 20, ESF

Water/kerosene

microemulsion

30 AN4 AN10 883.8 31.15 30 W/K 30, AN

Diesel 0 826.3 8.07 32 Diesel

Water/diesel

microemulsion

20 AN4 AN10 892.2 32.3 33 W/D 20, AN

Water 997 0.897 72 Water

expected, the value of g for all of the fluids generally in-

creases with increasing gas velocity. The results also show

that most of the values ofg for the petroleum liquids and the

W/O microemulsion systems are significantly higher than

the water system except for the g values obtained for 30%

W/K and the 20% water-in-diesel (W/D) microemulsions at

ugr0.06 m/s. This difference can generally be attributed to

the much lower surface tension and perhaps the higher co-

alescence hindering characteristics of the petroleum liquids

and the microemulsions compared to water, which results

in the generation of smaller sized bubbles in the bioreactorfilled with these systems. Observation of the two-phase flow

in the DTAB with petroleum-based systems and distilled wa-

ter confirmed the presence of smaller sized bubbles in the

former systems. Similar g values are obtained for diesel and

kerosene although the former has a viscosity that is more than

four times higher than the latter (see Table 1). Similarly, re-

sults presented in Fig. 1 also show that there is no significant

Fig. 1. Gas hold-up for kerosene, diesel and their various water-in-oil mi-

croemulsions, and distilled water as a function of superficial gas velocity.

change in the value ofg when the viscosity is increased by a

change in the type of surfactant used in making the 20% W/K

microemulsion systems. However, the addition of water-to-

diesel and -kerosene, with an associated increase in viscosity,

hasresulted in a significant reduction in the value ofg. These

results seem to suggest that for all the petroleum-based sys-

tems, except 30% W/K and the 20% W/D microemulsions,

andwith the bioreactor configuration employed in the present

study, the enhancing effect of viscosity on g due to smaller

bubble rise velocities (which results in higher residence time

of bubbles in the draft-tube and a higher rate of recircula-tion of bubbles into the downcomer) balances the negative

effect of viscosity on g as a result of the generation of larger

bubble sizes. Different results might be obtained in DTABs

that have a larger gas disengagement section, such as those

employed by Mehrnia et al. [20]. The smaller g values ob-

tained with microemulsion systems as compared to the pure

petroleum liquids, with an indirect relationship being found

between g and , can perhaps be attributed to a decrease in

the coalescence inhibiting tendency of these liquids through

the addition of the highly coalescence promoting distilled

water. For all the petroleum-based liquids employed, except

the30% W/Kand the20% W/Dmicroemulsions, bubbly flow

was observed over the whole range of ugr employed in the

present study, and as the results in Fig. 1 show there is no

significant decrease in the rate of rise ofg with ugr over this

range. On the other hand, the results for the 30% W/K and the

20% W/D microemulsions, as well as distilled water, show

a significant lowering of the rate of rise of g with ugr. Vi-

sual observations also indicated the occurrence of the churn

turbulent at ugr around 0.020.04 m/s for distilled water and

around 0.040.06 m/s for the other two microemulsion sys-

tems. Observation of Fig. 1 also shows a significant drop in

theslopeof the curve for distilledwaterat ugr around0.04 m/s

and the two microemulsion systems at around 0.06 m/s.


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Fig. 2. Correspondence between predicted and experimental values of gas

hold-up.

In terms of the above discussion, for conditions employed

in the present study which lead to bubbly flow, the gas hold-

up was affected by the ugr and the water-to-oil phase volumeratio. An exponential multiple regression used on the data

from the gas hold-up experiments in the DTAB containing

different petroleum-based fluids for bubbly flow regime gave

the following correlation:

g = 1.320[ugr]0.927[1+ ]0.673 (1)

A comparison between experimental data and calculated

values of gas hold-up for petroleum-based media is shown

in Fig. 2. It can be seen that Eq. (1) predicts about 83% of

the experimental data with 15% error. The data obtained un-

der conditions in which the churn-turbulent regime prevailed

in the DTAB could not be satisfactorily correlated with theabove equation. However, due to the smaller number of data

points available, a suitable correlation for this regime cannot

be presented here. It should also be noted that change in the

geometry of the gas disengagement section could also have

a bearing on the relationship between g and ugr in DTABs

employing petroleum-based media.

3.2. Volumetric oxygen transfer coefficient

Fig. 3 shows the measured kLa as a function ofugr for the

same liquids employed for gas hold-up measurement pre-

sented in Fig. 1. Again, the value ofkL

a for all of the fluids

increases with increasing gas velocity, the only exception

being the kLa data for the 30% W/K and the 20% W/D mi-

croemulsions in which kLa drops when ugr is increased from

0.04 to 0.06 m/s but further increase in ugr up to 0.08 m/s

again had lead to an increase in kLa. The kLa data for dis-

tilled water also shows a significant decline in the slope of

the plot for ugr > 0.02 m/s. Visual observations of the flow

regime in the DTAB indicated the occurrence of the churn

turbulent regime somewhere between 0.02 and 0.04 m/s for

distilled water and somewhere between 0.04 and 0.06 m/s for

the 30% W/K and the 20% W/D microemulsions. Therefore,

the above-mentioned trends could be attributed to a change in

the flow regime from bubbly to churn-turbulent. It should be

noted that this trend was also observed to some extend with

the g data for these liquids (Fig. 1). Similar observations

have been previously reported in internal loop airlift reactors

employing aqueous sucrose solutions [18] and seawater [19].

Similar to the gas hold-up data, in nearly all cases the

kLa values for all the petroleum-based liquids is higher thandistilled water. However, kLa data for diesel is significantly

lower than those for kerosene at all ugr employed although

similar g values were obtained for these liquids inside the

DTAB. Higher viscosities of solutions generally result in

lower diffusivities of solutes [28], which in turn results in

a lowering of kL [17,29]. This can, at least partly, explain

the difference between the kLa and g results for these two

liquids. The W/K microemulsions in nearly all cases shows

higher kLa values. Increase in the value of viscosity, as a re-

sult of increase in (from 10 to 30%), type of surfactant (in

the order TS, AN and ESF) or type of petroleum base (from

kerosene to diesel), has resulted in decrease in the values of

kLa at constant ugr inside the DTAB. The only exception isthe kLa values at ugr = 0.08 m/s where kLa values for 10% and

20% W/K microemulsion (prepared with AN surfactant) are

similar or higher than those for kerosene even though these

microemulsions have higher viscosities than pure kerosene.

A closer examination ofFig. 3 shows that for kerosene a drop

in the rate of riseofkLa with ugr is observed at ugr 0.02 m/s

whereas for all W/K microemulsions with 20% there is

no significant drop in this rate for ugr up to 0.08 m/s. When

kerosene was employed inside the DTAB, stable foam was

observed at ugr0.02 m/s, with the rate of foaming signif-

icantly increasing with increasing ugr. However, this phe-

nomenon was not observed inside the DTAB when the otherpetroleum-based liquids were employed. With kerosene, the

milky appearance of the fluid inside the DTAB at all air rates

was also more prominent compared to the other petroleum

liquids. Foaming, together with the milky appearance, is in-

dicative of the presence of very small bubbles, and visual

observations seem to suggest that their population increases

with increasing ugr. This can result in a decrease in the values

Fig. 3. kLa for kerosene, diesel and their various water-in-oil microemul-

sions, and distilled water as a function of superficial gas velocity.


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M.R. Mehrnia et al. / Biochemical Engineering Journal 22 (2005) 105110 109

Fig. 4. Correspondence between predicted and experimental values ofkLa.

ofkL [1113,17,29], which might explain the drop in the rate

of rise ofkLa with ugr at ugr 0.02 m/s.

The kLa values for 20% W/D is higher than 30% W/K mi-

croemulsion especially at ugr 0.06 m/s although these twomicroemulsions have similar viscosities. This seems to indi-

cate an additional influence of water content on kLa, inde-

pendent of its effect on viscosity, which probably results in

a decrease in the bubble coalescence-inhibiting tendency of

the microemulsion system. The results show that this phe-

nomenon has a more pronounced effect on kLa in the churn

turbulent regime where due to the high concentration of bub-

bles, bubble coalescence is expected to be more prominent.

A multiple-regression analysis, conducted on the data

from the volumetric oxygen transfer coefficient experiments

for the different petroleum-based fluids in the DTAB, under

conditions in which bubbly flow regime prevailed, gave thefollowing correlation:

kLa = 0.235[ugr]0.757[v]0.11 (2)

Fig. 4 compares the predicted values and experimental

data for kLa in the bioreactor containing petroleum-based

medium. The correlation predicts about 85% of the experi-

mental data with an error of 20% or less. More work is re-

quired to develop a suitable correlation for conditions in the

DTAB in which the churn-turbulent regime prevails.

4. Conclusions

Petroleum fractions (i.e., kerosene and diesel) and their

water-in-oil microemulsions with different water-to-oil vol-

ume ratio and surfactant types were used to study the gas

hold-up (g) and volumetric oxygen transfer coefficient (kLa)

in a draft-tube airlift bioreactor for application in BDS pro-

cesses. The addition of water as the coalescence promoting

liquid to the petroleum-based systems, with an associated in-

crease in and viscosity, resulted in a decrease in g and

kLa, respectively. Moreover, for all petroleum-based liquids

with viscosities up to about 8 106 m2/s, as reported pre-

viously for 20% water-in-kerosene microemulsion [20,21],

the bubbly flow regime prevailed. However, an increase in

viscosity to around 32 106 m2/s either through change in

the petroleum base or the of the microemulsion resulted

in the onset of churn turbulent regime over most of the air

rates employed in present study. Appropriate correlations re-

lating g to and also kLa to viscosity were developed for

bubbly flow regime, which were found to be less able to ac-curately predict the experimental data in the churn turbulent

regime. The correlations obtained in this work can be further

developed for churn turbulent regime and by taking design

parameters effects into consideration.

Acknowledgement

The financial support of Iranian Research Institute of

Petroleum Industry is gratefully acknowledged.

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