This document discusses the rheological properties of algal slurries at different concentrations that are produced during the algal biodiesel production process. Samples of Cyclotella algal slurry from a photobioreactor were concentrated to various levels and then tested using a rheometer. The tests showed that yield stress and viscosity increased with higher algal concentrations, causing the slurries to exhibit more elastic and gel-like properties. Understanding these rheological changes is important for determining the optimal handling and processing methods for concentrated algal sludges used in biodiesel production.

1. ABSTRACT
Nutrient contamination can cause toxic algal
blooms in lakes and rivers, but algae can also be
turned into biodiesel. Rheology is the study of how
different materials, including biological materials,
react to stress and strain. During the algal biodiesel
production process, the raw effluent from the
photobioreactors or algal raceways are successively
concentrated to a level adequate for chemical or
mechanical extraction of the oils and lipids. Based
on photobioreactor culture of Cyclotella,
each concentration of the algal sludge changes its
rheological response. Therefore, when processing
the algal sludge, we must take into account the
increasing interaction between the diatoms
resulting in non-Newtonian viscoelastic behavior
that will dictate the optimum handling method for
the concentrated sludge.
METHODOLOGY
Algae
• Sample of high concentration Cyclotella slurry
obtained from algal photobioreactor experiment
by Altan Ozkan
• Original (1X) algal slurry concentrated to 1.5X,
2X, 3X, 4X, and 10X concentration by
centrifuging and removing excess supernatant
• % solids determined from drying the slurry in an
oven at 60oC
Rheology
• TA Instruments Discovery Hybrid Rheometer 3
• Concentric cylinder bottom and recessed bob
upper geometry
STRESS SWEEP
• Stress sweep: 0.001 Pa to 10 s-1 by 0.001
increments
• Yield stress of slurry increased with slurry
concentration which corresponded with solids
content of slurry
Algal Biodiesel
Acknowledgements
Altan Ozkan, Greg Rorrer, Kristin Marshall, and Skip
Rochefort.
SHEAR VISCOSITY
• Flow sweep: 0.1 – 1000 s-1, 25oC
• All algal slurries were shear thinning fluids except the
supernate was a Newtonian fluid
• These shear thinning behavior was seen in other algal
sludge studies
• With increasing concentration the viscosity increased
TEMPERATURE RAMP
• Temperature ramp: 10 – 40oC and 40 – 10oC, 100 s-1
• All algal slurries decreased in viscosity as temperature
increased (1X: 0.009 Pa.s at 10oC to 0.005 Pa.s at 40oC)
• Increasing the algal concentration to 10X increased the
viscosity by 100X
RHEOLOGICAL CHANGES IN ALGAL SLUDGE
CHARACTERISTICS WITH SUSPENSION
CONCENTRATION URANBILEG DAALKHAIJAV AND TRAVIS W. WALKER
Results and Analysis Image of
Cyclotella
species (10-20
µm diameter).
Nikon Eclipse
Ti-S inverted
microscope
using 40X
objective.
0.001
0.01
0.1
1
10
100
0.1 1 10 100 1000
Viscosity(Pa.s)
Shear rate (1/s)
0.0001
0.001
0.01
0.1
1
10
100
1000
0.1 1 10 100
Modulus(Pa)
Strain (%)
0.001
0.01
0.1
1
10
100
1000
0.01 0.1 1 10 100
Frequency (rad/s)
0.001
0.01
0.1
1
10
10 20 30 40
Temperature (oC)
AMPLITUDE SWEEP
• Amplitude sweep: 0.1 rad/s, 0.1 – 100% strain
• Linear viscoelastic region (LVR) extending past 1%
strain for all samples
FREQUENCY SWEEP
• Frequency sweep: 1% strain, 0.05 – 100 rad/s
• Elastically dominant, gel-like
• Increase in viscoelasticity of the slurry corresponds
with increasing concentration
• The elastic modulus ranged from 0.05 to 250 Pa
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4 6 8 10 12
Yieldstress(Pa)
% dry solids
[Algae] factor % Dry solids Yield stress (Pa)
Supernate na none
1 4.284 0.0339
1.5 4.545 0.0748
2 4.886 0.1494
3 5.412 0.3165
4 5.974 0.5433
10 11.313 3.3557
CULTIVATION
HARVESTING
EXTRACTION
Open pond raceway
• 0.1% dry solids
• Most practical
• Hard to control
Photo-bioreactor
• 1% dry solids.
• Best yield
• Expensive
Primary dewatering
• Flotation, flocculation, settling
• 2-7% dry solids
Secondary dewatering
• Centrifugation, filtration
• 15-25% dry solids
Direct drying
• Spray, drum, solar
• 90-95% dry solids
Most energy intensive: 45 – 90%
Oil
Water
Algae
Algae: 10 – 75% wt. oil content
Solvent
• Hexane, acetone etc.
• 99% oil recovery
• Most common
Mechanical press
• 75% oil recovery
• Long extraction time
Supernate 1X 1.5X 2X 3X 4X 10X
Filled - elastic modulus (G’) Unfilled - viscous modulus (G”) Unfilled circle - viscosity