(602d) CFD Modeling and Design of a Gravity Settler for Algae Harvesting

Future
world energy needs necessitate the search for alternative and sustainable
energy sources.  Biodiesel is one such
source with many upsides, but current production methods are flawed. Existing
large scale biodiesel production facilities rely primarily on soybean and
canola oil as the lipid source for the biodiesel. The production of soybean and
canola oil directly competes with food crops and is not viable in the long term
as a transportation fuel lipid source.

Algae
are the future of lipid sources for biodiesel production. Algae can be grown in
large open pools on non-arable lands where they will not compete with food crops
for space. Biodiesel production per acre for Algae dwarfs soybean or canola
based biodiesel production; 20,000 gallons per acre of algae cultivation
compared to 38-100 gallons per acre. The cells use carbon dioxide as a carbon
source and thus can be used to offset carbon dioxide production of facilities,
namely power plants.

The
problem with Algae as a lipid source for biodiesel is the cost of dewatering of
the cells. Algae concentrations in the best conditions in bioreactors will not
surpass 20 g/L due to light limitations. The water must be removed before the
lipids can be used in the transesterification process that produces the
biodiesel.  Typical methods for
dewatering the algae involve using settling ponds to concentrate the algae a
little and then using centrifugation to do most of the dewatering. The costs of
this centrifugation step limit the commercial viability of algae based
biodiesel.

A
patented a novel gravity settler design at Cleveland State University, see
Figure 1, is analyzed in this study. Preliminary results obtained with this
design show promise to help the commercial viability of the algae biodiesel
process by lowering the dewatering costs. The inlet of the gravity settler is
at the top unlike traditional gravity settlers which have intakes at the bottom. Two prototypes have been tested and experiments
indicate that the technique is effective at dewatering algae.

Figure
1: Novel Algae Settler

In order to benefit the
design process, a computational fluid mechanics model for the system has been
developed using a finite element based multi-physics modeling environment (COMSOL?,
formerly known as FEMLab). This study shows how the
powerful environments of a detailed CFD modeling interfaced wiht
a commercial CAD software can be used to optimize the design of gravity
settlers. Comparative studies carried out with alternative designs and results
from validation experiments are presented.

 The model formulation is based on the mixed model.
The mixture (or mixed) model considers a system where there is a dispersed
phase of either solids or liquid droplets/bubbles in a continuous liquid phase.
It was developed for hard spherical molecules, so some adaption is required to
correctly model algae cells which can clump and deform, these modifications will
be discussed.

The geometrical model has
been developed based on the simplified geometry of the gravity settler shown in
Figure 1. After validation experiments are completed and results with
simplified relations for settling velocities are evaluated, the simulation
environments is to be updated to the more advanced geometry of the prototypes by
means of Solidworks® and COMSOL? LiveLink? for
Solidworks®. This LiveLink enables to import complex
geometries into COMSOL? as well as formulation of parametric studies where the
CAD software is used to interactively modify geometric parameters that may show
relevance in the performance of the settler. Solidworks® parameters can also be
used in COMSOL? to perform parametric sweeps.

Using this LiveLink? functionality, parametric sweeps will be demonstrated
as tools to optimize the settlers design and operation by analyzing geometric (such
as length, angle of inclination, and height for dewatering) as well as
operating variables (such as inlet and outlet flow rates and velocities).The
optimized design will be physically prototyped and experimented on to validate
the CFD modeling.