In many aerobic industrial bioprocesses, oxygen is an important nutrient that
is used by microbes for growth, maintenance and metabolite production (Garcia-Ochoa
and Gomez, 2008). Oxygen is a soluble substrate, but its solubility in aqueous
media is very low (8-10 ppm) (Doran, 1995). Consequently,
actively growing cells can consume all the dissolved oxygen very fast. Therefore,
oxygen has to be supplied continuously by mass transfer from air to the growth
Many studies indicate that the oxygen mass transfer can be enhanced with the
addition of a second liquid phase in which oxygen solubility is high. Compounds
such as hydrocarbons (Galaction et al., 2004;
Clarke et al., 2005), PFC (Elibol,
1998; Amaral et al., 2008) and vegetable
oil (Rols and Goma, 1991), which are non-toxic to microorganisms,
were used as the second liquid phase. The advantage of using these organic phases
in the system is that they can increase the oxygen transfer rate from gas phase
to the microorganism without the need of extra energy supply (Amaral
et al., 2008). In contrast to these studies, it is also known that
addition of antifoam (normally are also organic phase) can reduce the oxygen
In this research, palm oil (type RBD palm olein) which is available abundantly
in Malaysia was used as the second liquid phase. It has high oxygen solubility
(47.7 mg L-1 at 30°C) (Allen and Hamilton,
1994). Palm oil is non-toxic towards the microorganisms and it has very
low solubility in water (below 100 mg dm-3 at 28°C) (Ahmad
et al., 1996). Palm oil can also be used as an antifoam agent. This
study has been done in order to evaluate the effect of palm oil on the oxygen
transfer rate, in view of the conflicting reports on the use of organic phase
in the media.
MATERIALS AND METHODS
Experiments were carried out in a computer-coupled 5 L bench top scale bioreactor (Biostat B, Sartorius BBI Systems) with a working volume of 4 L. The glass vessel has a height/diameter ratio of about 2:1. Two types of impeller were used, Rushton turbine and InterMIG impeller. A ring sparger was situated below the bottom impeller. The system was agitated at two different speeds viz., 200 and 400 rpm. The effect of aeration rate was studied at 0.25, 0.75 and 1.25 vvm. The experiments were performed at atmospheric pressure and the temperature was controlled at 30°C.
The model media used were distilled water and xanthan gum solution, in order to represent aqueous solutions of different viscosities. Food grade xanthan gum was used in these studies. Xanthan gum solutions were prepared at two different viscosities, viz 140 cP and 290 cP and were measured by Brookfield viscometer (model LVDV-II+Pro, using SC25 spindle at 100 rpm). In order to study the effect of palm oil dosage, experiments were carried out at different volumetric fractions of palm oil in the media viz., 0.05, 0.1, 0.15 and 0.2.
Dissolved oxygen in the liquid was measured by using a polarographic dissolved
oxygen probe (InPro 6820 Series, Mettler Toledo). For kLa value determination,
unsteady state, i.e., dynamic method has been used (Clarke
et al., 2005). This method was performed by first sparging the nitrogen
gas through the system until the dissolved oxygen falls to zero. Then, continue
with aeration at different operating conditions of aeration and agitation and
monitor the dissolved oxygen concentration (CL) until it reaches
a steady value. The following Eq. 1 is used to determine the
which on integrations yields:
The kLa value can be determined from the slope of ln(1-CL/CL*) versus t graph where CL* is the equilibrium dissolved oxygen concentration.
RESULTS AND DISCUSSION
Experiments with palm oil in water: The experimental data shows that
at an agitation speed of 200 rpm using Rushton impeller, the kLa
values in the presence of palm oil in the system are very much lower compared
to those without oil. As shown in Fig. 1, kLa at
5% oil fraction dropped more than 2 times compared to those without oil and
it went on decreasing as palm oil fraction was increased. At the time these
experiments were carried out, it was observed that the diameters of air bubbles
and oil droplets were higher than those at higher agitation rate. This observation
was also reported by Clarke et al. (2005) in
their study with alkane. It was also observed that some of the oil tends to
stick at the glass wall of the bioreactor than to disperse with water. This
event might be due to poor mixing at low agitation rate (200 rpm).
Experiments were also carried out at an agitation speed of 400 rpm using Rushton impeller. The results obtained are shown in Fig. 2 and they indicate similar behavior as seen at an agitation speed of 200 rpm using Rushton impeller. The Nevertheless, the kLa values did not drop as much as at 200 rpm with the addition of oil. This time, air bubbles and oil droplets size were smaller than those at 200 rpm and less oil was found to stick to the glass wall of the bioreactor. Even though there was a slight increase of kLa values at 15% of palm oil addition, it was not high enough to enhance the oxygen transfer as much as without oil condition.
||Effect of palm oil on kL a for water at 200 rpm
at different aeration rate
||Effect of palm oil on kL a for water at 400 rpm
at different aeration rate
Amaral et al. (2008) observed similar effect
of oil on the oxygen transfer in the medium when they used olive oil as second
organic phase in their study. They reasoned that the decrease of kLa
by using olive oil was due to poor dispersion of oil. This could be caused by
the properties of olive oil, such as higher viscosity and lesser density than
water. They suggested that operation at higher agitation rates could enhance
the dispersion. This might explain the steep decrease in oxygen transfer with
palm oil addition at low speed of agitation in this study.
Experiments with palm oil in xanthan: For xanthan gum solution, experiments
were carried out at 400 rpm and 0.75 vvm with different palm oil fractions.
For 140 cP viscosity of xanthan gum solution, experiments were carried out with
Rushton turbine and for 290 cP, InterMIG impeller was used, as it is more suitable
for high viscosity solution. Result in Fig. 3 showed that
kLa values decreased as the oil fraction is increased for both impellers.
||Effect of palm oil on kL a for xanthan gum solution
at different aeration rate
However, it can be seen that kLa values were higher for high viscosity
(290 cP) xanthan gum solution when using InterMIG impeller compared to low viscosity
of xanthan gum solution using Rushton turbine. This in contrast to the findings
of Garcia-Ochoa and Gomez (1997), who found that kLa
values decreases as the viscosity of liquid increasing, which happened due to
decrease in the degree of liquid flow turbulence. It could be due to the use
of a more effective impeller i.e., InterMIG.
Oxygen transfer in bioprocesses is one of the major parameters that determine the productivity. There have been many studies to increase the oxygen transfer rate in the process adopting various strategies. One of the strategies is to add an organic phase, which has higher oxygen solubility to the system. But, in these studies, it has been found that the addition of an organic phase like palm oil decreased the oxygen transfer rate. Nevertheless, oxygen transfer rate was found to be higher even in higher viscosity solutions if an InterMIG impeller is used.
The authors are grateful to UiTM, for the financial support provided to carry out this project.