hydrodynamic properties of scene des mus obliquus
TRANSCRIPT
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Investigation of the Effect of Growth from Low to High Biomass
Concentration inside a Photobioreactor on Hydrodynamic Properties of
Scenedesmus obliquus
Le, Evan1; Park, Chanwoo2; Hiibel, Sage.3
Abstract
An investigation on the effect of Scenedesmus obliquus’s growth from low to high
biomass concentration inside a horizontal tubular photobioreactor to determine the impact that it
has on hydrodynamic performances which will affect cost and production efficiency was done.
As the biomass concentration increased, the algal culture was found to remain Newtonian.
Additionally, the biomass concentration (expressed in optical density at 600 nm, OD600) was
found to have lower viscosity even at highest possible concentrations at OD600: 0.404 (1.372+/-
0.132 cp) compare to the Modified 3N Bold medium (1.408+/-0.0941 cp).Furthermore, the
total energy consumption does not appear to depend on the Scenedesmus obliquus biomass
concentrations, but on the medium it lives off of. The rheological properties of autotrophic algae
will not have significant impact on energy requirement until technology improves so that the
concentrations reach those of heterotrophic algae.
Subject Headings: Energy Systems Analysis, Alternative Energy Sources, Energy From
Biomass, Energy Storage Systems. 1Student, Dept. of Mechanical Engineering, University of Nevada, Reno, 89557 . E-mail: [email protected]
2Professor, Dept. of Mechanical Engineering, University of Nevada, Reno, 89557 . E-mail: [email protected]
3 Post-Doc, Dept. of Biochemistry and Molecular Biology, University of Nevada, Reno, 89557. E-mail:
Table of Contents
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I. Introduction…………………………..…………………………………………….…..…….4
II. Theory…………………………………………………………….…………………..…….5-6
III. Experimental Methods and Materials………………..……………….….………….….6-7
IV. Results………...………………………………………………………………….……….7-9
V. Discussion…………………………………….………………………………..………....10-11
VI. Conclusion……………………………………………………………… ……….................11
VII. 1omenclature………………………..…………………………………………………….12
VIII. References………………………….………...………….…………..……………………13
List of Figures
Figure 1: Behavior of non-1ewtonian Fluids…….……………………………………..5
Figure 2: Schematic of the Photobioreactor Used in the Investigation………….……..6
Figure 3: Biomass growth over time in Modified Bold 31 medium………..……….….7
Figure 4: Comparison of varying algae culture densities along with their associated
biomass concentrations……………....………………………….……………7
Figure 5: Comparison of varying algae culture viscosities along with their associated
biomass concentrations……………..……..……………..……………..…….7
Figure 6: Total Power consumed by the fluid through the acrylic tube at different
Reynolds 1umber assuming pump efficiency is 60%. …………………..…..9
Figure 7: Dependence of the Reynolds number on the algae culture flow rate at
different biomass concentration .........................................………………….9
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Introduction It is today’s biggest challenges to develop clean alternative renewable energy fuel.
Solar, wind, fuel cells, and ethanol based sources all have severe limitations that prevent
them from meeting projected energy needs [1]. Algae biodiesel has been identified as the most
promising due to its quick growth rate, large oil yields, non-toxic, renewable, biodegradable,
and carbon neutral, and it does not compete with arable farm land compared to other
alternatives such as ethanol made from corn or sugarcane. It can also be grown in
environments unsuitable for agricultural crops such as the seashore, because of its greater
tolerance for salt and heat. Algae’s remarkably efficient physiology means that it requires less
sunlight, grows faster, and has more potential for genetic engineering.
In order to be a competitive alternative fuel, the algae biodiesel produce must cost less
than petroleum diesel. Currently, the price of crude oil is around $113 per barrel. At this price,
microalgal biomass with an oil content of 55% will need to be produced at less than around
$450/ton, which will depend mainly on the cost of producing the algae biomass[2]. One of the
main factors affecting the production cost is the selection of the host organism. Current criteria
in selecting the host organism for a successful algae biodiesel plant includes: (1) photosynthetic
efficiency to obtain high biomass yield from light, (2) biomass growth rate, (3) oil content of the
biomass, (4) temperature tolerance, (5) the value of biomass residue and byproducts, (6) the
ease of extraction, purification, and conversion process, sensitivity to high oxygen
concentration (7), resistance to photoinhibition (8), response to diurnal fluctuations (9),
amount of dark respiration(10), sensitivity to osmotic stress (11) [1, 3]. However, biodiesel
production at the moment is not profitable with the main challenge being scale up. Most of the
production cost in algae biodiesel plant utilizing photobioreactors comes from the energy
required to power the pumps and harvesting [4]. As a result, solutions to reduce the required
energy can significantly make algae biodiesel production more economically feasible. There
have been few researches on the maximum cell density and morphology’s impact on the
demanding auxiliary energy require for an algae biodiesel plant. Instead, the solution to the
auxiliary energy problem is focus mainly on the photobioreactor design[5]. Using the
microalgae strain that fulfill the traditional criteria in host selection as well as favorable
rheological properties in a photobioreactor with high biomass concentration can significantly
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reduce the pump power consumption but maintain the algae overall productivity. This can
make algae biodiesel a more competitive alternative fuel.
The aim of this work is to investigate the effect of Scenedesmus obliquus’s (UTEX 1450)
growth from low to high biomass concentration inside a horizontal tubular photobioreactor,
one of the most widely used and investigated photobioreactors, to determine the impact that
they have on hydrodynamic performances (such as liquid flow rate, culture velocities, power
consumed, etc.) which will affect cost and production efficiency. The study will compare algae
cultures with varying biomass concentrations with the performances of the same reactor with
only water. Scenedesmus obliquus has been identified as promising source for algae oil content
[6]. Furthermore, it has a unique morphology, growing characteristics, and relatively large size
compare to other biofuel algae which are the main factors in affecting the viscosity of the
system [7].
Theory If a fluid is Newtonian, then the shear stress, ��� , should be proportional to the rate of
deformation (shear rate) ����:
��� ∝ ���� Eq. 1
The constant proportionality in Eq. 1 is the absolute (or dynamic) viscosity, . Thus in terms of
the coordinates of Eq.1, Newton’s law of viscosity is given for one-dimensional flow by:
��� = ���� Eq. 2
For gases, viscosity increases with temperature, whereas for liquids, viscosity decreases with
increasing temperature. Fluids in which shear stress is not directly proportional to deformation
rate are non-Newtonian. Non-Newtonian fluids commonly are classified as having time-
independent or time-dependent behavior. Some examples of non-Newtonian fluid is given
below (Fig 1). The curves shown indicate a few of the many types of non-Newtonian fluid
behavior which have been observed experimentally. The slope of these curves is often called
apparent viscosity, and denoted by ���. One of the simplest laws describing non-Newtonian
fluid behavior is the Ostwald-de Wael model (“power-law” fluid):
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� = � ������
��� ����
Eq.3
When n=1, Eq. 3 reduces to Newton’s law viscosity with � = . If n < 1, it describes a
pseudoplastic fluid, and if n >1, the behavior is dilatant. Besides non-Newtonian and Newtonian
fluid, another way to identified types of fluids is through laminar and turbulent flows. Laminar
flow is characterized by smooth motion of one lamina of fluid past another, while turbulent
flow is characterized by an irregular and nearly random motion superimposed on the main
motion of the fluid. The two types of flow can be identified through the Reynolds number:
��� = ���� Eq. 4
The transition from laminar flow to turbulent in a pipe was found to be around 2100-4000. The
power to the fluid can be calculated with the following equation:
�� = ��ℎ Eq. 5
Where � the specific weight (kg/m3), Q is the volume flow rate (m3/s), and h is the water head
(m). By multiplying Eq. 5 by a coefficient of 9.81, Pc is expressed in watts (W).
Experimental Methods and Materials Horizontal turbular photobioreactor description
The experiment was carried out using a customized liquid loop (holding approximately
600 mL of liquid) with an inner diameter tube of 0.5 inches equipped with a flow meter and a
differential pressure transducer to collect pressure drops between the horizontal acyclic tube at
varying flow rates (Fig 2). Algae culture flow is produced using a mechanical pump. A
positive displacement pump is recommended to be used when circulating algae culture with a
mechanical pump as it minimized the hydrodynamic stress that would significantly reduced
algae productivity [9]. However, for the purpose of this experiment, a centrifugal pump is
suitable since the algae are not growing in the liquid loop. Biomass sedimentation in tubes is
prevented by maintaining highly turbulent flow. The liquid flow rate chosen to be study will be
based on the Reynolds’s number ranging from 0 to 5000. Increasing the Reynolds’s number
within the range was found to increase algae productivity [10].
Algal strain and cultivation conditions
Scenedesmus obliquus (UTEX 1450) was first cultivated in Modified Bold 3N medium
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in 250 mL Erlenmeyer flasks (100 mL of culture) maintained on a flask shaker at a temperature
of 25℃ , under continuous illumination ( m2/s) provided by daylight fluorescent tubes. This was
cultivated for about two weeks before transferring into 2L flasks (1.5 L of culture). The 2L flasks
were grown under similar conditions without the shaker and under 18:6 light: dark cycle at
26:20 ℃ in a growth chamber. Additionally, sterile air was continuously pumped into the
culture. Biomasses concentrations were determined daily by measuring the optical density of
samples at 600 nm (OD600).
Hydrodynamic properties measurement
Density measurements of the algae culture for each corresponding OD600 were taken
along with the viscosity using a Gilmont® falling ball viscometer. Using the algae culture that
were grew in the growth chamber, the pressure drop was then measured using the liquid loop
with the OMEGA’S PX2300 Differential Pressure Transmitters and recorded at flow rates
ranging from 0 to 5 L/min in increments of 0.5 L/min.
Results Daily biomass growth rate of Scenedesmus obliquus in modified bold 3N were measured
by taking optical density reading at 600 nm (Fig 3). Based on the figure, there is a linear
increase in growth rate over the 26 days period instead of the ideal sigmoidal growth phase.
Poor mixing, and hence the slow growth, is likely the reason. Ten varying densities and
viscosities along with their associated biomass concentration (OD600 reading) were measured
and compared. Fig. 4 show a linear dependence of decreasing densities with increasing biomass
concentration. However, Fig. 5 shows no correlations between biomass concentration and
viscosities, which is unexpected. The higher biomass concentration is expected to have higher
viscosities. Despite this, the viscosities were always higher than that of water at 20 ℃. The
viscosities and densities of the Modfied Bold 3N medium and the lowest and highest collected
OD600 readings were also compared. Based on the graphs (Fig 3 & 4), there is little difference
in density even at high biomass concentration, with a slight decrease in density. The viscosities,
however, of the modified bold 3N medium appears to be higher than most of the algae culture
biomass concentrations including the highest OD600 reading (0.407). Ten measureable flow
rates for were taken ranging from 0.0 L/min to 5.0 L/min along with their associated pressure
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drop and energy losses for biomass concentrations ranging from 0 to 0.407 OD600 Reading.
Based on Fig. 6, the comparison does not show any significance differences in power
consumption as the biomass concentrations increased. Any differences are most likely due to
errors. However, both the medium and the algae culture require more energy for the same
Reynolds number than water. In Fig. 7, a plot of Reynolds number against fluid velocity is
reported. Increasing the fluid velocity resulted in a higher Reynolds number in both the algae
cultures and the Modified Bold 3N medium as expected. Since this is a linear relationship, the
algae culture fluid remains Newtonian even as the biomass concentrations increased.
Discussion
The horizontal tubular PBR can usefully satisfy only medium level production demands.
A study by the University of Wageningen found that when comparing types of photobioreactors,
the main contributor to biomass production cost for the horizontal tubular photobioreactor was
the energy cost associated with the pump (46% of the overall cost) [11]. In fact, it requires the
most energy for its pumps compare to a raceway pond and a flat plate PBR. This type of
photobioreactor was used in the investigation to examine how significant the impact the
rheological properties of algae cultures can affect the overall energy consumption of an algae
biodiesel plant. Based on Fig. 7, increasing biomass concentrations does not affect the increase
in power consumption significantly or change the fluid to non-Newtonian. Any differences are
likely due to errors. Since Scenedesmus obliquus does not produce any byproducts, and any
possible aggregations due to its unique morphology and growing characteristics were quickly
dissociated due to the high turbulence, the largest influences on the power consumptions is due
to the medium it lives off. Even at high biomass concentration attainable of OD600: 0.404
(1.372+/-0.132 cp) with the current growing conditions, the viscosity was actually found to be
slightly lower than the pure medium (1.408+/-0.0941 cp). A different medium which has higher
viscosity or higher attainable biomass concentrations could be used to compare the effects the
mediums has on energy and overall cost and algae productivity. When comparing the highest
biomass concentrations with that of water at Reynolds number of 6000, it was found the
modified 3N bold medium requires 234.7% more energy overall. The difference becomes
significantly higher as the Reynolds number increase. Additionally, since the diameter of the
liquid loop is 0.5 inch, the energy required for pumping becomes more significant. The solar
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collector tubes are generally 0.1 m or less in diameter. Tube diameter is limited because light
does not penetrate too deeply in the dense culture broth that is necessary for ensuring a high
biomass productivity of the photobioreactor [1]. However, a study found that for tube diameters
greater than 0.05 m, the mixing energy becomes a significant or dominant energy input, but
when the diameter is less than 0.25m it becomes negligible [12]. Increasing the size of the tubes
within the desire range can help reduce the energy consumptions but maintained the overall
desired algal productivity. Furthermore, the uses of static mixer can improve gas-liquid mass
transfer inside a turbular photobioreactors and give cells appropriate light/dark cycle frequency
without the using high energy input [13]. A study by Sánchez Mirón, A., et al. has even
suggested that the design of the horizontal turbular photobioreactor is inherently flawed, and
should be replace with the more energy-efficient vertical bubble columns [14]. Recent renewed
interest in closed photobioreactor technologies has increased the optimum biomass concentration
and therefore the flow resistance of the system. However, the rheological properties of
Scenedesmus obliquus, and most other popular biofuel algae species whose densities are often
similar or lower and whose morphologies are spherical (therefore, lower viscosity), does not yet
appear to be the main obstacle in reducing the energy consumption. Until the biomass
concentrations for autotrophic algae species is as high as those heterotrophic algae grown in
fermentors, the main culprit in energy pump consumptions will be the overall design of the
photobioreactors.
Conclusions The objective of this investigation was to determine whether the types of algal species
along with their maximum cell densities, sizes, and morphologies have significant affect on the
energy consumptions of the horizontal photobioreactor. By growing Scenedesmus obliquus with
varying biomass concentrations, it was possible to examine how the concentrations affects the
density, viscosities, and therefore the energy consumption required to keep the flow rate at
recommended Reynolds number for high algal productivity. As the biomass concentration
increased, the algal culture was found to remain Newtonian. Additionally, the biomass
concentration (expressed in optical density at 600 nm, OD600) was found to have lower
viscosity even at highest possible concentrations at OD600: 0.404 (1.372+/-0.132 cp) compare to
just the Modified 3N Bold medium (1.408+/-0.0941 cp). Furthermore, the total energy
consumption does not appear to depend on the Scenedesmus obliquus biomass concentrations,
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but on the medium it lives off of. Hence, the main obstacle in reducing the energy consumption
is through the design of the photobioreactor as suggested in literatures [5, 15]. The rheological
properties of autotrophic algae will not have significant impact on energy requirement until
technology improves so that the concentrations reach those of heterotrophic algae. Through error
analysis, the largest source of error for the energy consumption calculation came from the
differential pressure transducer.
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249-270.
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Figure 1
Figure 1. Behavior of non-1ewtonian fluid.
Figure 2
Figure 2: Schematic presentation of the experimental set
pump, differential pressure transducer, and an excess tank. The inner diameters of
the tubes are 0.5 inches and the length between the two sensors is 15 inches. The
total volume is ~600 mL and the tube between two pressure sensors is acrylic, a
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Figure 2: Schematic presentation of the experimental set-up showing the centrifugal
pump, differential pressure transducer, and an excess tank. The inner diameters of
the tubes are 0.5 inches and the length between the two sensors is 15 inches. The
olume is ~600 mL and the tube between two pressure sensors is acrylic, a
common photobioreactor material.
up showing the centrifugal
pump, differential pressure transducer, and an excess tank. The inner diameters of
the tubes are 0.5 inches and the length between the two sensors is 15 inches. The
olume is ~600 mL and the tube between two pressure sensors is acrylic, a
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Figure 3
Figure 2: Scenedesmus obliquus biomass growth in Modified Bold 31 Medium over time
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 5 10 15 20 25 30
OD
60
0 R
ea
din
g
Time (day)
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Figure 4 and 5
Figure 5: Comparison of viscosities at ! ℃ with their
corresponding biomass concentration through OD 600
readings
1.2
1.25
1.3
1.35
1.4
1.45
1.5
1.55
1.6
0.2 0.25 0.3 0.35 0.4
Vis
cosi
ty (
cp)
OD Reading (600 nm)
Figure 4: Comparison of densities at ! ℃ with their
corresponding biomass concentration through OD 600
readings
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
1.3
1.35
0.2 0.25 0.3 0.35 0.4
De
nsi
ty (
g/m
L)
OD Reading (600 nm)
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Figure 6
Figure 7: Total Power consumed by the fluid through the acrylic tube at different Reynolds 1umber assuming pump efficiency is 60%. Cell
concentrations were expressed as OD600 reading.
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0 2000 4000 6000 8000 10000
To
tal
Po
we
r C
on
sum
ed
(J)
Reynolds Number
OD600 Reading: 0.239
OD600 Reading: 0.270
OD600 Reading: 0.323
OD600 Reading: 0.341
OD600 Reading: 0.407
Modified Bold 3N Medium
Water
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Figure 7
Figure 8: Dependence of the Reynolds number on the algae culture flow rate at different biomass concentration measure in OD600 readings.
0
1000
2000
3000
4000
5000
6000
7000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Re
yn
old
s N
um
be
r
Scenedesmus Obliquus Culture (m/s)
OD600 Reading: 0.239
OD600 Reading: 0.270
OD600 Reading: 0.323
OD600 Reading: 0.341
OD600 Reading: 0.407
Modified Bold 3N Medium