Understanding the Software Options

Lozowski
FIGURE 2. The �at-plate reactor contains a
series of parallel �at plates
channels that use paddlewheels for
mixing. These are the most common
algae cultivation systems in use today.
A main advantage is that they are
inexpensive to build. Disadvantages
include evaporation of water, and
difficulties in keeping out unwanted
species and controlling the process.
The open structure of these systems
makes them susceptible to infections
and limits the choice of algae to fastgrowing,
resistant strains. Another
disadvantage is the large footprint
needed for scaleup.
Horizontal tube reactors (Figure
1) are single layers of clear horizontal
tubes through which the algae are
circulated. Advantages are that the
productivity per surface area is higher
than in an open pond, process control
is better because it is in a closed loop,
and scaleup can be fairly straightforward
by extending the length of
the tubes. A main disadvantage with
horizontal tubes is the high intensity
of light on the tube surface, which can
be detrimental to algae growth. Additional
disadvantages are the cost of
circulating the fluid in the tubes and
the O 2 buildup in the closed loop that
can slow algae growth.
Vertical tube reactors are basically
vertical rows of horizontal tubes.
This configuration greatly decreases
the very high-intensity light associated
with horizontal tubes, and decreases
the footprint needed with
horizontal tubes.
Flat-plate reactors (Figure 2) consist
of a series of flat, parallel plates
in a closed reactor system. These large
polymeric “bags” are filled with water
and form a big heat sink so that temperature
control is an advantage in
this system. The flat-plate reactors
also afford some of the same advantages
of the other closed systems,
namely better process control and purity.
Good mass transfer is obtained
TABLE 1. ADVANTAGES AND DISADVANTAGES
OF THE FOUR ALGAE REACTOR TYPES
Raceway Horizontal Vertical Flat-plate
Ponds Tubes Tubes reactors
Inexpensive to build yes unknown unknown unknown
Easy to scale up yes yes yes no
Good process control no yes yes yes
Maintain purity no yes yes yes
Optimum light intensity yes no yes yes
Low footprint
Maintain water level (no
no no yes yes
evaporative losses)
Low-cost to circulate
no yes yes yes
fluids
Maintain good gas bal-
yes no no no
ance (no O2 buildup) yes no no yes
by sparging air in the panels, so O 2
buildup is not a problem. Scaleup of
this system is, however, complex.
The tube diameter of the systems in
AlgaePARC is 5.5 cm — “large enough
to prevent blockages and small enough
for good heat transfer”, Wijffels explains.
Density measurements are used
to detect when the algae are ready for
harvesting, which is typically done by
circulating the biomass to a centrifuge.
While fouling can be a factor in all
of these systems, Wijffels says that it
depends on the type of algae used and
how the algae are treated. Stresses,
such as large temperature swings
can cause a sticky substance to be secreted.
Beads, however, can be used in
circulation loops to help clear fouling.
Within a couple of years, the AlgaePARC
team hopes to have learned
enough from these four reactors to
build a fifth, optimized system.
Lactic acid and PLA
Polylactic acid (PLA) is a bio-based
plastic that is a sustainable alternative
to oil-based polymers, and the demand
for PLA’s monomers, lactic acid
and lactides, is rising (see Bio-Based
Chemicals Positioned to Grow, Chem.
Eng., March 2011, pp. 19–23). Rop Zoetmeyer,
CTO of Purac (Gorinchem,
the Netherlands; www.purac.com),
says that the market for PLA is estimated
to reach over three million tons
in the next ten years. Purac, a subsidiary
of CSM N.V. (Diemen, the Netherlands;
www.csm.nl) is a market leader
in lactic acid production.
Zoetmeyer says that Purac’s technologies
comply with the cradle-to-cradle
concept, and that sustainability is key
for bio-based chemicals. Purac is currently
working on two projects toward
lowering its eco-footprint — which
Zoetmeyer explains means more than
just a favorable CO 2 footprint — for
lactic acid production: a gypsum-free
lactic acid process; and a process using
alternative, non-food raw materials.
A gypsum-free process. In the current
process, carbohydrates are fermented
to produce lactic acid. Lime
is added for pH control, and then sulfuric
acid is added to convert calcium
lactate into lactic acid and gypsum
(CaSO 4) as a byproduct. A separation
step removes the gypsum and biomass
and a subsequent purification
separates out the lactic acid residue
and yields the purified lactic acid. In
the new process, neither lime nor sulfuric
acid are used, eliminating the
formation of gypsum and the associated
steps to remove it. The result is a
much simplified process.
This patented, gypsum-free process
has been run on a demonstration
scale for two years, and scale up to a
production-scale plant is planned. The
site of the new plant is expected to be
chosen by the end of 2011.
Non-food substrates. In order to
move away from competing with potential
food sources for carbohydrates
and sugar, Purac is working on using
non-food substrates, such as corn stover
or bagasse. The company’s goal is
to have a commercial plant for producing
PLA monomers using non-food
substrates in 2015. The location of
the plant will, in part, depend on the
availability of the substrate chosen.
New bioplastics
Avantium (Amsterdam, the Netherlands;
www.avantium.com) has developed
a patented process to produce
furanic building blocks, tradenamed
YXY, that can be used for new, biobased
plastics. Using high-throughput
experimentation technology, the company
has identified catalysts that can
convert carbohydrates into furanic
molecules in a very selective and fast
way, it says. Since the process is a catalytic
one, it can utilize existing CPI
CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2011 15