Introduction
Around the world, environmental topics are a source of controversy. One big reason for
this controversy is the increased cost of being environmentally friendly. Furthermore,
two questions must be asked: how much is one willing to pay to be environmentally
friendly, and who pays that price? In recent years, developing a process for
biodegradable plastics made from renewable resources has become important, not only
because of the limited supply of fossil fuels, but also because of decreasing availability of
landfill space. Yet, due to the fact that these processes have traditionally involved
expensive fermentation steps which often created excessive amounts of low value salts
from neutralizing agents within the reactor, they have not been cost competitive. While a
number of biodegradable plastics have been developed, which include poly hydroxyl
butanol (PHB), starch based plastics, and polymer-starch blends, the biodegradable
plastic with the most attention at present is polylactic acid. This process, described
below, involves no by-product formation, and is produced by a prevalent crop: corn.
Historical Perspective
As shown in this section, plastics are causing a huge amount of waste in the United
States, with their use only expected to grow in the future. With recycling efforts failing
to significantly reduce the amount of garbage, a new solution is needed to cut down the
waste. Within the last few years, research has been performed to find a more natural way
to produce plastic materials, which would hopefully decompose after disposal. The
solution may lie in biodegradable plastics made from renewable feedstock. One example
of these new biodegradable plastics is polylactic acid. Polylactic acid, otherwise known
as PLA, is a polymer made from dextrose, a derivative of corn. In a controlled compost
environment, PLA can take 45 to 60 days to breakdown – this may range depending on
the thickness and grade of plastic. PLA degrades down to its monomer, lactic acid, or
into carbon dioxide and water, with the aid of microorganisms. This is a huge advantage
for the facilities that produce PLA, as any off-grade product produced could be converted
back to lactic acid, and then reintroduced to the process at any step without any problems.
The word plastic can be defined as any organic compound, more specifically, a long,
chain molecule comprised of repeating subunits, which can be formed into various shapes
by molding or extruding. Therefore, “plastic” is not a specific raw material, but can be
various different products. Plastic compounds can be traced back to the Old Testament,
where plastic-like compounds were used as adhesives and fillers [19]. The first “official”
plastic compound, cellulose nitrate, was discovered back in 1861 by John Wesley Hyatt.
He was actually searching for a replacement for ivory billiard balls, and ended up
patenting his findings. Today’s current plastics weren’t developed until the 1930’s, when
polyvinyl chloride, low density polyethylene, polystyrene, and polymethyl methacrylate
were all introduced.
The use of plastics has become more prevalent since their inception, due to their wide
range of properties and uses. Unfortunately, this has lead to other problems, as most
plastic products are single-use items, meaning consumers dispose of the item
immediately after using only once. Currently, waste disposal has become a huge problem
1
in the United States. Analyzing the cross section of a landfill gives a better perspective
of how much of a waste source plastic has become. According to [28], plastics constitute
11% of waste in landfills in 2001, as shown in Figure 1. This chart illustrates that there
are only three components with an amount of waste greater than plastic: paper, yard
trimmings, and food scraps. Plastic, being the fourth most abundant waste compound, is
most common compound contained in landfills that does not degrade within a few years.
Table 1 demonstrates this fact, with the amount of time it takes for various materials to
biodegrade. Paper takes 2 to 5 months, and yard trimmings and food scraps are all
natural products. The plastic components, shown in bold, range from 5 years to never
actually being able to biodegrade.
Glass
6%
Wood
6%
Other
3%
Paper
36%
Rubber, Leather,
and Textiles
7%
Metals
8%
Plastics
11%
Food Scraps
11%
Yard Trimmings
12%
Figure 1: Breakdown of Waste, 2001 [28]
2
Table 1: Breakdown Times for Common Materials to Biodegrade [15]
Products
Time to biodegrade
Cotton rags
1 to 5 months
45 to 60 days
Polylactic acid plastic
Paper
2 to 5 months
Rope
3 to 14 months
Orange peels
6 months
Wool socks
1 to 5 years
Cigarette butts
1 to 12 years
Plastic coated paper milk cartons
5 years
Plastic bags
10 to 20 years
Leather shoes
25 to 40 years
Nylon fabric
30 to 40 years
Tin cans
50 to 100 years
Aluminum cans
80 to 100 years
Plastic 6-pack holder rings
450 years
Glass bottles
1 million years
Plastic bottles
Forever
The majority of plastics produced can be recycled, and most states have made recycling
very easy for their citizens by providing curbside pickup programs and recycling stations.
However, recycling can actually be a tiresome task, as there are seven different categories
of plastic which cannot be mixed together. Plus, the strength of plastics decrease with
each time they are recycled. Despite this fact, recycled plastics can still be useful in some
applications such as outdoor furniture, toys, jacket insulation, and egg cartons. Figure 2
demonstrates that even though plastics can be somewhat useful when recycled, the
recycling efforts do not make much of an impact in how much plastic is produced per
year. The only good thing this chart demonstrates is that more plastic is being recycled,
as the amount of plastic produced increases.
3
30,000
Amount/thousands of tons
25,000
20,000
Amount Recycled
15,000
Generated
10,000
5,000
0
1960
1970
1980
1990
2000
Figure 2: Amount of Plastic Generated and Recycled, 1960 – 2000 [28]
Marketing Aspects
As of the year 2000, overall plastic consumption was at 150 million tons per year. The
projected consumption of plastic products in 2010 is expected to be 258 million tons.
This averages out to be an annual growth rate of five percent. Currently, over one fourth
of the total consumption of plastic takes place in North America in 2000. It can be
expected that North America will continue to consume roughly the same fraction of the
market in 2010.
Figure 3 illustrates the breakdown of plastic products in the United States for the year
2001. The durable goods category, which includes appliances and furniture, makes up
the largest amount of plastic products, at approximately eight million tons. The other
categories imply that they are disposable goods, meaning that they are not meant to last,
and are therefore used only for a short time before the consumer throws them away.
4
Soft drink, milk, and water
containers
Other containers
Amount Recycled
Generated
Other packaging
Bags, sacks, and wraps
Nondurable Goods
Durable Goods
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
Amount/thousands of tons
Figure 3: Breakdown of Plastic Products, 2001 [28]
The physical properties of polylactic acid make it an extremely appealing venture. PLA
is insoluble in water, as well as moisture and grease resistant, which makes it ideal for
applications such as food packaging. It is biodegradable, compostable, and does not need
an enzyme to break down. Its clarity and glossiness make it very much like its other
plastic competitors. By changing its molecular weight, the properties of polylactic acid
can be changed to make it a perfect substitute for various types of plastic.
One of the key markets to be targeted is food packaging, considering this area has the
shortest lifespan in terms of actual use. This type of plastic is normally thin and is not
required to be durable, and therefore is relatively cheap to produce (in comparison to
other grades). Though polylactic acid may not be cheaper than petroleum based plastics,
there are some advantages to this. One main advantage is that polylactic acid is
extremely environmentally friendly, due to the fact that it can biodegrade. There are a
number of other reasons PLA could become very helpful as a large commodity on the
market. For example, in small farms in Japan, mulch is removed after the growing
season and placed into polyethylene bags, which then need to be eliminated and burned.
Labor cost is therefore increased. If biodegradable bags were used instead, which could
initially be more expensive, the farmers may actually save money through reduced labor
requirements. Another area where PLA appears to be very useful is biomedical
applications, such as drug delivery. However, this area is still being researched and
tested, and probably will not be a key market for awhile considering the higher costs of
producing polylactic acid and the need to be approved by the United States Food and
Drug Administration.
Another important factor in the sustainability of the PLA market is that it is made from a
renewable feedstock. Unlike traditional plastics, which are petroleum based, PLA can be
5
manufactured using dextrose from corn as a starting material. Dextrose can be fermented
to lactic acid, which after much further processing is eventually polymerized to PLA. As
the energy costs continue to increase, this factor will become more and more important.
Currently, dextrose can be purchased at roughly $0.21 per pound. The primary target
market will be the disposable plastics market which is traditionally supplied by
polyethylene or polyethylene terephthalate (PET). The current market price for PET
typically ranges between $0.55 and $0.65 per pound, and is continuing to rise. In order to
make polylactic acid competitive, production costs need to be low enough to make a
profit selling polylactic acid at a similar price. If these prices are not met, other methods
would need to be employed to convince consumers that polylactic acid is an alternative to
traditional plastics. The current market price for PLA is $1.50 per pound, which leaves a
large margin for profitable production even if the goal of joining the PET market is not
met. Government stipends to aid in development of environmentally friendly products
may alleviate a portion of this problem.
Process Overview
Figure 5 illustrates the process in a block flow diagram. For a more detailed process
diagram, see Appendix H. A detailed process description can be found under Process
Description. Complete process chemistry is found in the process design section under
process chemistry. The initial feed for the polylactic acid process would be corn;
however, our project will begin with a dextrose feed derived from corn, because this is
the first process that is somewhat unique to the production of polylactic acid. The
dextrose will first be placed into a fermentation reactor. Once it has left the reactor, it
must undergo esterification, and hydrolysis to obtain lactic acid. The lactic acid is then
polymerized to the low molecular weight, unstable polylactic acid. Meanwhile, the
ammonia used for pH control, and the butanol from the esterification are recycled. The
low molecular weight polylactic acid is then converted to the pre-polymer, lactide. The
lactide will then be distilled to remove any impurities. After that, polymerization will
take place. Any unconverted monomer will be recycled to the acidification step while the
polylactic acid will be further processed into pellets.
Key Challenges or Issues
The market for normal plastic resins has been well established over the past century.
However, it is hard for companies to switch to a new, biodegradable plastic for their
products if nothing is wrong with their current product. Biodegradable plastics have
undergone extensive testing of their properties, but since they have only been around for
a few years, not all aspects have been looked at.
Finding a market for polylactic acid can become a problem. Even though polylactic acid
can be substituted into a number of markets, it may be difficult to convince consumers
that this new product is of the same quality as the original product in each market. This
becomes extremely important if the polylactic acid products are more expensive. In order
to be successful in the market, polylactic acid must be sustainable as well as similar in
properties.
6
The future does look promising for polylactic acid as the United States strives to reduce
landfill growth. Yet, two major factors may change polylactic acid’s market. On the
positive side, the United States is currently not producing as much corn as they could,
since the demand for this crop is not very high. In twenty-five years, a total of 33% of
today’s used land could be taken out of production, requiring improved production on a
per acre basis [26]. Therefore, if an increase in demand occurs, possibly due to the
emergence of biodegradable plastics and biodiesel, more land will be needed for corn –
still, there should always be enough farm land. Yet, during the last century, the
population of Earth tripled, and is only expected to continue this trend throughout this
next century. Land is already a concern as it is lost to new developments, such as
housing, roads, and businesses. In addition, water supply will become a growing
concern. In summary, the immediate future of polylactic acid looks promising and will
continue to be as long as farmland is available.
Key Engineering/Design Issues
In general, considering the polylactic acid process has only been around for a few years,
there are limited amounts of information available to the public. Companies want to keep
their process chemistry “top secret”, so that no one else can steal their ideas. As a result
our group has been forced to look at vague lab scale studies and attempt to scale them up.
To add to the difficulty the lab scale experiments are generally so specific that they only
include one or two smaller steps in the whole process. Therefore, the scaled up
experiments also had to be pieced together to form the complete PLA process. This
brings about concerns involving mixing requirements and capabilities for the larger scale
system, as well as continuity of the process.
The fermentation step is the most involved step of the polylactic acid process. The pH
and the temperature must be carefully watched to ensure a high percent conversion of
dextrose to lactic acid
Temperature affects reaction rate and culture growth. It is found that as temperature
increases, lactic acid production increases until cells begin to die, usually at about 50°C.
To achieve adequate lactic acid production rates, yet prevent death of the culture, a
temperature of 30°C has been chosen. Testing specific to the bacteria chosen would need
to be performed to fully optimize this portion of the process. However, the productivity
of bacteria tends to decrease more sharply with temperature fluctuations as temperature
approaches death conditions.
It has also been found that certain growth media give better production rates. However,
many of these materials have high costs which may not make up for the gain in
production seen. Estimations for reactor sizing, bleed rate, and dilution rate were
conducted in Mathcad (Appendix B). The pH control for the fermenters was maintained
by the addition of 8 molar ammonia. Though this appears to be the best option for
neutralization, the separation process that follows may need further tuning.
7
During the fermentation step, the pH within the reactor drops as dextrose is transformed
into lactic acid. If the pH drops too far, the rate of reaction decreases, the reactivity of
the enzyme could decrease, and eventually, the bacteria would die. Yet, as pH increases
above the pKa for lactic acid, lactate salt is preferentially formed. For the given
conditions, pKa of lactic acid is approximately 3.8. To maintain adequate culture growth,
and reasonable amounts of free lactic acid, pH just below neutral is generally used in this
type of fermentation. We will use a pH of 6.0. The optimum pH would need to be
determined from additional testing.
Because the lactate salt is preferentially formed under these conditions, the choice of
neutralizing agent is also important. Ammonia as a neutralizing agent allows for
sufficient pH control while preventing the need to process and dispose of waste salts. As
can be seen in the process description, nearly all of the ammonia can be captured in later
processing steps and recycled to the reactor.
In addition to reactor conditions, there are also many differing types of bioreactors. Each
type of bioreactor has benefits and drawbacks. The main concern in choosing a
bioreactor for this process is that the choice in bioreactor affects downstream processing.
Because a continuous fermentation process is desirable, a cell recycle reactor is chosen in
this study. Not only does the cell recycle reactor allow for continuous lactic acid
production, but it also gives the highest yield of any reactor investigated.
The other major design portion of the process is the polymerization step. There are many
catalysts available for the formation of polylactic acid through ring opening
polymerization. Because the main focus of our process is to sell our product to a food
packaging manufacturer, the catalyst must either be safe for contact with humans, or fully
removable from the end product. The easiest way to ensure the catalyst will not affect
the final consumer is to use an immobilized catalyst. This allows for the use of a packed
bed reactor and prevents the catalyst from incorporating into the polymer. Though
immobilization is possible with many catalysts, reaction times for the majority of these
catalysts are not reasonable. Hence, a catalyst was chosen with reaction time near half an
hour for the desired molecular weight range.
Aside from time and catalyst, temperature, pressure, and stereo-isomer of lactide affect
polymer properties. Temperature should be kept at a reasonable value to reduce utilities
cost for heating and minimize safety concerns. Pressure should also be maintained at a
point that optimizes cost while preventing extreme safety risk to employees. The stereoisomer of lactide used for the food packaging industry is L-lactide with 95% purity. In
order to form L-lactide, L-lactic acid must be fed into the pre-polymerization step. To
best achieve this, a micro-organism capable of preferentially producing L-lactic acid
versus D-lactic acid is necessary. Much research around optically pure lactic acid
production has been performed recently. Because one such experiment performed with
much higher yields than all others, the bacteria from this experiment is to be used.
8
Within the polymerization step, not only are there many catalysts to choose from, there
are a variety of properties and chemical structures that can result based on reactor
conditions. At this point, a preliminary look at catalyst safety and ease of separation has
been performed. A molecular weight between 60,000 and 150,000 is what is typically
required of PLA to possess the properties required for one time use products. There are a
number of finely tuned grades of these one time use plastics. In order to vary the grades
of PLA produced, temperatures, pressures, and residence times within the reactor will
need to be able to be adjusted to give the desired product. In addition to varying reactor
conditions, there are often a number of additives that are used to obtain the needed
properties and appearance. One desirable effect of our catalyst choice is that silica
immobilized zinc β-diiminate complex produces a 94% conversion rate in approximately
a half an hour (51). This is very helpful in reducing the reactor size due to space time
considerations. Unfortunately, as we got further into the project we became aware of the
fact that this catalyst was very expensive. Another complication with this catalyst is the
fact that one can only obtain a molecular weight of approximately 28000 (51). In
addition, the price of the catalyst we used in our process was not available until near the
end of the project development. Once we found out the price of our catalyst it became
apparent that any fouling of our catalyst would destroy profitability.
In the proposed process, an extremely high raw material cost is associated with the nbutanol due to its lack of recovery from the process and discarding it as waste. A liquidliquid extractor utilizing di-ethyl-ether to separate the n-butanol from the waste water
stream was proposed. This process would also require a few additional flashtanks and an
extra distillation colum which would increase capitol costs but would significantly lower
the raw material cost for n-butanol. Further research in this area is required and would be
beneficial in decreasing raw material costs and complying with environmental regulations
for the disposal of n-butanol.
There are solvent-free ring opening polymerization processes that should also be
explored in an attempt to alleviate some safety and environmental concerns brought about
with the use of solvents, as well as elimination of process equipment. The main source of
design information would be patents and journal articles. The hope is that these articles
and patents will give the necessary design equations to size the polymerization reactor
with the aid of Mathcad and Excel for calculations.
Additional research should be performed in the area of lactide formation. It does not
appear as though there are as many people interested in this area of study as some of the
other process steps involved, which has lead to the belief that optimization here will not
be as important as the afore mentioned portions of the process. However, as this is a
major step in the process, it should not be overlooked entirely. Additional work could be
done in the area of catalyst selection, as well as pre-polymer formation optimization.
These would be completed with the assistance of journal articles and Mathcad.
9
Plant Scope
Plant Information
Market demand for degradable plastics was 20 million pounds in 1997 and is expected to
reach 167 million pounds, resulting in revenue of $185 million this year [39]. Total
capacity of this proposed polylactic acid plant is anticipated to produce 500 million
pounds annually. This capacity allows for commodity production levels, while keeping
dextrose demands in line with expected production from an average corn processing
facility. Even though biodegradable plastics demand has not yet reached this level,
demand is rising and it will be assumed that when this plant is built, market demand will
be in accord. Currently, a co-venture facility with Cargill and Dow Chemical resulted in
a 300 million pound polymer plant recently; a second similar plant is under consideration.
Two assumptions of this proposed plant is that it is built next to a currently operating
corn milling facility and that the dextrose production can be increased to provide the
required amount of dextrose.
For the best market stability, the process would be an addition to an existing corn milling
facility. This alleviates the cost of transportation of one raw material: dextrose. It also
allows for production swings to mirror demand on dextrose and polylactic acid. Ideally,
the plant would be located in southwestern Minnesota, Iowa, Nebraska, Oklahoma, or
Wisconsin. These are some of the largest corn producing states as well as being centrally
located to American consumers. The plastics industry had a trade surplus of $7.3 billion
in 2000 [19], so Minnesota may be highly considered because it is in close proximity to
the Great Lakes, a major waterway that could be used for product distribution during the
warmer portion of the year.
Product Description
The only product created in this process is polylactic acid. There are no by-products
formed in our process. Polylactic acid can be used in a wide variety of areas. As
mentioned before, it is primarily used for single-use items such as plates, utensils, cups,
and film wrap. It is especially marketed towards plastic bottling and fast-food
companies, which will be our primary market. Nevertheless, polylactic acid can also be
used for paper coatings, as clothing fibers, and for compost bags. It is also being
investigated for biomedical applications, such as in the aiding of drug delivery (as
microcapsules), as well as in the cosmetics industry as a filler-substance for wrinkles.
Product purity will vary according to the grade demanded by the consumer. Generally,
polylactic acid is relatively pure with the only impurities being those intentionally added
to assist or delay deterioration.
This process would produce polylactic acid as a raw pellet for further processing and
possibly film to be sold to consumers. By producing polylactic acid as a pellet, plastic
companies can transform it into whatever form is needed for their product. Pellets are
also much easier to ship than rolls of plastic sheeting and are less susceptible to damage
during transportation. In addition, a wide variety of polymer grades can be produced
from polylactic acid, depending on the market demand for specific properties such as
10
strength, elasticity, crystallinity, melting temperature, glass transition temperature,
density, and hardness.
Storage Requirements
The main raw material, dextrose, will be acquired from the neighboring corn milling
facility. A raw material storage capacity to accommodate up to one week of production
during process upsets or equipment upgrades in the corn milling facility should be
sufficient. Other minor raw materials should have about the same storage capacity.
Large quantities of n-butanol will be needed to reduce the lactate salt into the salt free
acid form. The n-butanol could be shipped in by railcar. Ammonia will also be needed.
This will be supplied by truck in eight molar solution with water and stored in totes.
Additional water would be obtained from the local utilities company.
Process Utilities
A large amount of energy will be required to support this process. The easiest and
probably most economically feasible option is to purchase power from the local utilities
vendor. Alternatives could include supplemental wind power, solar power, and burning
of waste biomass.
Fairly large quantities of water will be required for this process, placement of the facility
near a hefty source of water would be beneficial. In addition, wastewater treatment will
be a fairly involved task. The fermentation reaction brings about a fairly high quantity of
biochemical oxygen demand in the waste water; in order to meet government regulations
this water will need tertiary wastewater treatment. This will either make the initial
construction cost of the plant increase, or the plant will absorb the significant cost to aid
the local waste treatment facility in adding or maintaining the proper equipment for
handling a waste stream with the biochemical oxygen demand.
The amount of product storage will depend on the number of grades of polylactic acid
polymer that are produced and the change-over time required for each grade. Long grade
change times would result in fewer grade changes and vice versa. Longer grade change
times translate into greater desired final product storage capacity.
Product Distribution
Railcars will provide transport to select regions to be further transported by cargo ships or
trucks. Cargo ships will provide the best means to transport cargo to markets in Europe,
and Asia. Trucks can pick up the product from railcars and then deliver it to specific
processing sites as needed.
Other Considerations
The largest environmental concerns revolve around the fermentation reactor. First, a
biochemical waste stream is created which will be difficult to treat. Also, because of the
necessary pH control and the resulting lactate salt, there are some undesirable process
chemicals present. Furthermore, there is a need for organic solvents in the final
11
polymerization steps. Although the finished product is environmentally friendly, the
challenge now becomes making the process match.
Neutralization and acidification chemicals surrounding the fermentation reactor could
cause concern for operator safety if used in high concentrations. Nevertheless, it will
probably be most beneficial to the process to have a higher concentration of these
chemicals.
12
Process Design
Process Chemistry
The first reaction of the chosen polylactic acid process occurs in the fermentation reactor.
This reaction produces two molecules of lactic acid for every molecule of dextrose that
enters. This reaction occurs in an aqueous phase with the use of bacteria. The
fermentation reaction is as follows:
C6 H12O6  2C3 H 6O3
Given that pH control is necessary in the reactor, the product collected from the
bioreactor usually takes the form of a soluble lactate salt, in this case, it is ammonium
lactate. There are various methods of reducing this salt to lactic acid. The method
chosen is esterification with n-butanol. This frees the ammonia, but leads to the
formation of butyl lactate. The butyl lactate is then hydrolyzed freeing the lactic acid.
Once lactic acid has been isolated, excess water must be removed from the system. With
the removal of water, the lactic acid will undergo polymerization to an unstable, low
molecular weight polymer. With further heating in the presence of a catalyst, the low
molecular weight polymer breaks down and evaporates in the form of a lactide ring. The
lactide stream is then purified, and sent to a polymerization reactor. There, it undergoes
the final polymerization step in the presence of a catalyst. The chemical form of lactic
acid, the lactide, and the basic polylactic acid building block formula are shown in Figure
2.
Figure 4: Lactic Acid, Lactide, and Polylactic Acid Molecules
Block Flow Diagram
Below, in Figure 5, is a block flow diagram of the process with relevant flow rates and
major process steps labeled. A more detailed process flow diagram can be found in
Appendix G.
13
Figure 5: Block Flow Diagram
Process Description
The detailed process flow diagram can be found in Appendix G. A block flow diagram is
found above. The main reactant of the production of polylactic acid is dextrose, a
derivative of corn. Dextrose is sent to the reactor in aqueous solution (approximately 20
grams per liter of water) to allow for easier transport. The bioreactors (R-101 and R-102)
are encased with a cooling jacket, and are kept at 30°C. Within the reactors,
Lactobacillus rhamnosus bacteria is utilized to ferment the dextrose into lactic acid.
Two continuous reactors are used in series to achieve higher product yield and
concentration. After each reactor, the product concentration is augmented by using
membrane filtration (V-101 and V-102). This also serves to separate the cells from the
product stream.
Because the product is acidic and the reactor contains living organisms, a pH control
system must be incorporated. The neutralizing agent, ammonia, is added to maintain pH
at 6.0. Although this is not ideal for formation of free lactic acid, it is found that neutral
pH is desired for culture growth. The addition of the neutralizing agent results in
ammonium lactate formation in equilibrium with free lactic acid, the desired product.
After leaving the bioreactor, excess ammonia is allowed to flash off in V-104, and
recycled to the pH control system. Then, the ammonium lactate must undergo
esterification by n-butanol with the addition of small amounts of sulfuric acid in V-106.
This allows the remaining ammonia and any water to be removed in a three phase
separator (V-107). The ammonia is recycled to the esterification step, and the water is
sent to waste treatment. The resulting product, butyl lactate is now hydrolyzed (V-108)
forming free lactic acid and n-butanol. The n-butanol is separated from the lactic acid
through two liquid phase separation (V-109) and recycled to the esterification step.
14
Once free lactic acid is obtained, it is sent to an evaporator (V-201). After a large portion
of the water has been removed, the lactic acid is sent to R-201 where additional water is
removed causing the lactic acid to polymerize. The polylactic acid formed in this step
does not have the required molecular weight, nor is it stable enough for consumer goods.
Therefore, the polylactic acid is then mixed with a tin octanoate catalyst and sent to a
heater (R-201). As heat is added, the polylactic acid decomposes and lactide escapes as a
vapor. To prevent impurities from entering the lactide vapor stream, a bleed is taken
from the lactide reactor (R-202) and sent back to the evaporator (V-201). The lactide
vapor is then further purified in a distillation column (T-201) where water, lactic acid,
and other impurities are removed as a vapor and returned to the lactide reactor (R-201).
The purified lactide is condensed and continues to polymerization.
To achieve polymerization, the lactide is dissolved in toluene and passed through a silicaimmobilized zinc β-diiminate catalyst in a packed bed reactor (R-203) with a residence
time of 30 minutes. The polymer and solvent are sent on to a methanol quench (V-202).
The methanol quench not only stops the reaction, but also causes the polymer to
precipitate. This precipitated polymer can then be filtered (V-203) and dried. The
methanol/toluene mixture is distilled (T-202), and the methanol and toluene are recycled
to the quench (V-202) and the polymerization reactor (R-203) respectively.
Novelty
Given that there is not an abundance of information regarding the best way to obtain
lactic acid in a free form from the fermentation reactor, this allows for novel
developments. Although lactic acid has been produced for some time as a commodity
chemical, the use of renewable agricultural starting materials has lead to new challenges.
The greatest challenge being integrated fermentation and separation. In addition, the
large scale production of polylactic acid as a biodegradable biopolymer is fairly new.
Discussion of Process Alternatives & Design Choices
There were many choices made when developing this process. Most of the options
revolved around the bioreactor and the final polymerization step. Since the fermentation
reactor influenced the rest of the process, this area received the most attention. However,
there are areas of particular importance toward the end of the process which will also be
addressed.
The first choice made involved the type of bioreactor to be used. There are various
reactor systems that could have been used. Early on, it was determined that either a
reactor with high cell concentration or a reactor that incorporated product separation
would be the best choice. This decision is made based on the fact that higher cell
concentration gives higher yield, as well as the knowledge that lactic acid fermentation is
product inhibited. Two promising systems were found. One system involved
electrodialysis for product separation and neutralization. This system alleviates the
product inhibition, but because the cell concentration within the reactor is fairly low, the
productivity is not as high as it could be. The other technique was a two stage cell
15
recycle reactor system which incorporated membrane separation. This system runs at
high cell concentration, and still allows for a more concentrated product than other
systems due to the membrane separation. The two stage cell recycle system is thought to
be the most promising alternative. This alternative has been scaled up to an appropriate
industrial size and utilized in the process design.
Once the reactor style was chosen, the number of stages was determined based on the
information presented in the research article. Within the research article, the affects of
additional reactors was investigated. A sizable production gain was found in having a
second reactor. However, after the addition of a second reactor, the gains in production
did not appear as though they would outweigh the cost of additional reactors.
With this reactor choice, an appropriate neutralizing agent needed to be found. At
acceptable pH for cell growth, lactic acid takes the form of a soluble lactate salt, most
often sodium lactate or calcium lactate. There are various methods of reducing this salt
to lactic acid. Examples include the introduction of gypsum, ion exchange,
electrodialysis, or use of the CO2 process. All of these methods involve the formation of
a low value by-product salt. An alternative was found in ammonia. Ammonia can easily
be dissociated from lactic acid and recycled; decreasing the raw materials cost and
removes the burden of finding a buyer for the by-product. The separation process
becomes somewhat more complicated because of this choice of neutralizing agent,
though it was found that any separation process for a fermentation system is complex.
Another choice involving the reactor was the bacteria to be used. This affects the
orientation of the lactic acid formed. The isomer of lactic acid used in polymerization
determines the properties of the finished product. It was found that most food packaging
materials utilize the L-lactic acid isomer. Therefore, a bacteria was chosen for the reactor
which would preferentially produce L-lactic acid. Fortunately, this bacteria is the same
one used in the research article for the desired reactor. This means that the bacteria is
capable of forming the desired product at the concentration and rate found in the article
for bioreactor set up.
Following the reactor, the esterification of the ammonium lactate could be performed
with any number of solvents. N-butanol was chosen based on information from a
research article. This article tested various solvents and found that n-butanol gave good
conversion in this step while allowing for fairly easy separation through the formation of
two liquid phases.
The next major decision to be made involved the catalyst used in lactide formation.
There were numerous options here. Information involving lactide formation catalysts
was taken from Cargill-Dow patents and evaluated. In the end, tin octonoate was chosen
because of its high conversion and low residence time requirements. This is also the
catalyst used in the Cargill-Dow process.
16
After forming the lactide, decisions concerning the purification of the lactide became
important. Most processes crystallize the lactide prior to polymerization. Because
Cargill-Dow currently seems to have the best process for PLA production, patents were
searched to find their process for this section. It was found that introduction of the
product stream from the lactide reactor as a vapor into a distillation column gives the
greatest product yield while maintaining adequate purity. When crystallization is used,
the meso isomer is often lost to a waste stream, or recycled. Though this accounts for
only a small portion of the lactide produced, any production that can be conserved in a
single pass should be. This approach also reduces the equipment requirements and
utilities.
Then, the final polymerization catalyst needed to be selected. The catalyst used in the
Cargill-Dow process was researched. It was found that this catalyst can be harmful to
humans and the environment if it makes it to the final product. This prompted a search
for an alternative catalyst. The alternative catalyst, silica-immobilized zinc β-diiminate
catalyst, was chosen based on residence time required, molecular weight of polymer that
can be produced, and immobilization capabilities. Unfortunately, cost was not initially
considered. The chosen catalyst may need to be re-evaluated as the cost associated with
it has since been found to be extreme.
Summary of Key Process Design Assumptions
Many assumptions needed to be made for the design of this process as there is only one
operational full scale plant in existence. The largest assumption is that full scale
equipment will behave in a manner comparable to laboratory scale equipment. This
assumption was used to scale up almost every major piece of equipment in the process.
The realization that additional pilot scale, and possibly even laboratory scale testing
would need to be performed exists. However, given the limitations for this project, such
testing could not be performed. Additional assumptions include compatability of
equipment from differing laboratory studies, and all assumptions detailed in Appendix B,
which shows the equipment design calculations, mainly ideal systems.
17
Material and Energy Balances
Overall Material Balance
The overall picture of the entering and exiting materials can be seen below, in Figure 6.
A more detailed diagram is shown in Figure 5. For more specific flows, see Appendix E.
Figure 6: Overall Material Balance
Component Material Balances
Due to the size of our process, the stream table can be found in Appendix E. A detailed
stream table from HYSYS is also available, though this does not specify every stream as
shown on the process flow diagram. For a general balance of materials around major
process areas, see Figure 5.
Energy Balances (Utility Requirements)
Complete utilities cost with breakdowns for each piece of equipment can be found in
Appendix C. A summary of costs by utility type can be found below in Figure 7. As the
graph clearly shows, the major utility cost is low pressure steam. One possible way to
reduce this requirement could be heat exchanger integration. Given the incredible
amount of steam used, this appears to be a good option to investigate.
18
High-Pressure
Steam,
$1,220,000 , 1%
Cooling Water,
$13,949,754 ,
11%
Refrigeration,
$27,600,000 ,
22%
Electricity,
$230,931 , 0%
Low-Pressure
Steam,
$82,878,000 ,
66%
Figure 7: Utility Costs, by Type
Figure 8 gives a breakdown of utilities cost by equipment type. As expected, heat
exchangers utilize the majority of the utilities. It might be possible to look into ways of
optimizing the system to reduce temperature changes within the process. This
information also supports the idea of attempting heat exchanger integration, mentioned
above to reduce low pressure steam costs.
19
Vessels,
$19,396,000 ,
15%
Exchangers,
$68,221,754 ,
55%
Reactors,
$38,030,000 ,
30%
Pumps,
$230,931 , 0%
Figure 8: Utility Costs, by Equipment
20
Equipment Design
Major Equipment Listing
Fermentation Reactors (R-101, R-102)
Dextrose is fed to R-101 where a portion of it is converted to lactic acid. The product of
R-101 is then passed on to R-102 where production of lactic acid continues. The
expected productivity is 52 gL-1hr-1. R-101 and R-102 were scaled up based on desired
flow rate of product and production of lactic acid. The assumption of a height to
diameter ratio of 2:1 was made, as this is generally used in fermentation design. The
experimental reactor assumed Monod kinetics for cell growth. The bleed ratio is
assumed to be optimized at 0.053 for both reactors. The cell concentrations are 100g/L
and 120g/L for R-101 and R-102 respectively. Specific design calculations can be found
in Appendix B for all process equipment.
Membrane Separators (V-101, V-102)
V-101 and V-102 are the two membrane separators utilized to separate the product from
the reactor slurry coming from R-101 and R-102 respectively. The desired lactic acid
concentration leaving the second membrane is 92g/L. The membrane area for each was
scaled up based on the increased flow rate.
Esterification Promoting Can (V-106)
Esterification of ammonium lactate occurs in V-104. During this step, the ammonium
lactate is converted to n-butyl lactate. Sizing of this piece of equipment was based on
required residence time as well as product flow rate.
Phase Separators (V-104, V-105, V-107)
Phase separators were sized based on Reynold’s number calculations and fluid
properties. For the three phase separator, it is assumed that the residence time needed for
the ammonia to flash is negligible in comparison to residence time for n-butanol/water
separation. Therefore, residence time was based on reasonable settling time for the nbutanol/water system. From the residence time and required flow through the system,
vessel sizing was possible.
 V-104 is used to capture excess ammonia from the fermentation reactor prior to
esterification, and is treated as a flash tank.

V-107 is the three phase separator which removes ammonia and water from the
n-butanol/n-butyl lactate solution after esterification.
 V-105 separates n-butanol and water following hydrolysis to allow for n-butanol
recycle.
Hydrolysis Step (V-108)
After the ammonia and water have been separated from the n-butyl lactate/n-butanol
system, the n-butyl lactate is hydrolyzed in T-102. The residence time and required lactic
acid flow rate leaving the vessel were used to determine size.
21
Prepolymer Reactor (R-201)
The formation of the low molecular weight polymer formed directly from lactic acid
takes place in R-201. This reactor vaporizes water from the lactic acid solution, thereby
causing polymerization. R-201 was sized based on the residence time needed for this to
occur, as well as the desired product flow rate. Specific calculations are found in
Appendix B.
Lactide Reactor (R-202)
R-202 is utilized in forming the lactide ring structure from the low molecular weight
polymer formed in R-201. R-202 was sized by utilizing residence time data, and required
product flow rate.
Polymerization Reactor (R-203)
The final polymerization occurs in R-203. The product from this reactor is the desired
product, polylactic acid. To size this reactor, the desired product flow rate and required
residence time were used.
Precipitation Vessel (V-202)
V-201 is where the polylactic acid formed in R-203 is precipitated from the solvent. It is
assumed that precipitation is instantaneous. The main factor in sizing V-201 was the
flow rate through the vessel. A reasonable residence time was assumed. Detailed
calculations outlining these parameters can be found in Appendix B.
Filters (V-103, V-203)
Filters are used for the bleed stream on R-102 to allow for product recovery as well as
cell recycle. They are also used to collect the formed polymer from the solvent. Filters
were sized based on experimental data. Detailed calculations for individual filters can be
found in Appendix B.
 Filter V-103 separates the cells from the products and reactants in the bleed
leaving R-102 before sending on the liquid stream.
 Filter V-203 separates the polymer from the solvent
Heat Exchangers
Heat Exchangers are found mainly before or after reactors in the process and are used to
maintain desired temperatures for each process area. They are also used in our
distillation columns. Negligible heat loss to surroundings, negligible fouling factor,
negligible tube wall thermal resistance, negligible kinetic and potential energy changes,
and constant fluid properties were assumed. All heat exchangers were treated as counterflow models. Specific calculations for each heat exchanger can be found in Appendix B.
Compressors
Compressors are used to increase pressure on gas streams, particularly the ammonia feed
for neutralization in the reactor, and ammonia recycle streams. Compressors were sized
using Bernoulli’s Equation. Individual compressor calculations can be found in
Appendix B.
22
Distillation Columns (T-201, T-202)
Distillation columns are found in various locations in the process for the separation of
impurities from recycle streams as well as product streams from reactors. HYSYS was
used to determine reflux ratio and number of trays for distillation columns. Heights and
diameters were calculated using the spreadsheet found in Appendix B.
 T-201 purifies the lactide stream prior to polymerization by removing water and
unreacted lactic acid.
 T-202 separates the methanol and toluene streams to allow for recycle of both
solvents.
Pumps
All pumps were sized using Bernoulli’s Equation and placed based on frictional pressure
drops and reasonable pressure drop assumptions across equipment. Efficiency of 75%
was assumed. Detailed calculations for individual pumps can be found in Appendix B.
Tanks
Tanks were sized based on volumes from HYSYS and a two to one height to diameter
ratio.
23
Economic Evaluation
Summary of Key Economic Assumptions
Table 2: Key Economic Assumptions
12% after-tax return is the minimum
Benchmark Interest Rate
acceptable rate
Pricing data from Chemical Market
Product/Raw Material Pricing
Reporter and Economic Research Services
of the USDA
End-2003 CEPC Index for Equipment cost
Cost Index
estimates
2 years to construct, with 40% of fixed
Project Construction Schedule
capital expended in the first year of
construction, and 60% in the second year
Plant Starts at the end of year 2. learning
curve for operating rate projected to be:
Start-up Schedule
-75% of design in first year
-90% of design in second year
-100% of design after second year
Working Capital
15% of Fixed Capital
Location
Addition to existing corn milling facility
Land Costs
0
Project Life
15 years (including construction)
Based on Design product flow, assuming
Design Production rate
8000 hours of operation per year
40% tax rate and MACRIS depreciation (5
Financial Assumptions
year accelerated). Minimum acceptable
DCFROR = 12% after-tax
Equipment Cost Summary
Detailed equipment costing can be found in Appendix C. Below is a graph summarizing
equipment cost by equipment type. As expected, reactors dominate the equipment cost.
Vessels and heat exchangers also make up a large portion of the equipment cost. More
exact studies of necessary storage space could be done to reduce the amount, or size of
vessels within the process. Optimization of reactors could also help to reduce size and
cost. In addition, attempts to maintain more even temperature throughout the process
could reduce heat exchanger costs.
24
Compressors,
$772,324 , 1%
Vessels,
$28,176,940 , 28%
Exchangers,
$13,976,800 , 14%
Pumps, $208,360 ,
0%
Towers, $264,124 ,
0%
Tanks, $798,200 ,
1%
Reactors,
$56,698,000 , 56%
Figure 9: Equipment Costs, by Equipment
Fixed Capital Investment
Details on the calculation of the fixed capital investment can be seen in Appendix D. The
fixed capital investment was calculated to be $265 million. This assumes no land costs
due to the fact that this process will be an addition to an existing facility. However,
because of the size of the process, it was assumed that the land was undeveloped.
Therefore, the Grass Roots pricing method was used.
Manufacturing Cost Estimate
The manufacturing cost estimate was found to be $245 million. For a more detailed
account of manufacturing costs, see Appendix D.
Investment Analysis
This venture is profitable at the current raw selling price for PLA and current raw
materials cost. Below, in Figure 10, is the discounted cash flow rate of return. The IRR
of the process is 101%, and the payback period is 0.7 years from the point of start-up.
25
2000.00
Cash Flow (MM$)
1500.00
1000.00
500.00
0.00
0
2
4
6
8
10
12
14
16
(500.00)
Year
Figure 10:Discounted Cash Flow Rate of Return
Sensitivity Analysis
Figure 11 shows the effect of each raw material on IRR. From this graph, it is easy to see
that n-butanol has the greatest affect on the profitability of the process. This information
helps justify the notion of researching better n-butanol recovery systems. Yet even with a
cost swing of 50% in the cost of n-butanol, the process remains profitable.
26
147%
146%
IRR
145%
Dextrose
Ammonia
1-Butanol
Sulfuric Acid
Toluene
Methanol
Tin Octanoate Catalyst
Zinc Diiminate Catalyst
Waste water
144%
143%
142%
141%
140%
-25% -20% -10%
0%
10%
20%
30%
40%
50%
Percent change in price
Figure 10: Effect of percent change in price of material to IRR
Comparison of Base Case to Alternatives
There is only one industrial scale PLA plant in the world, which is owned and operated
by a joint venture between Cargill and Dow. They do not have all of their process
patented, which has created a major hurdle in acquisition of information about their
facility for a comparison of this design to the Cargill Dow design. Since, they do not yet
own the rights to all of their plant design they have to keep them a secret to ensure their
competitive edge. We do know that they sell their product at a price that is competitive
with PET. We have performed a calculation to see if we could also be profitable at this
selling price. We found that we remain profitable, with an IRR of 28% and a payback
period of about 3.5 years after start up. The details of these calculations can be found in
Appendix D. We have just recently made contact with a research engineer and obtained a
list of major patents that are used in the process. This information would allow for a
reasonable comparison of the two processes. However, sufficient time to adequately
perform such a study was not available.
27
Other Important Considerations
Safety
There are many chemicals which pose potential health hazards if equipment is not
properly maintained in this process. A full hazard analysis has not yet been completed,
but as with any process, storage of dangerous chemicals should be kept to a minimum.
Chemical storage units should also be located as far from potential fire hazards as
possible. Toxic chemicals should be stored in low traffic areas and appropriate
monitoring of these chemicals should be in place, such as pressure gauges on tanks, or
chemical sensors/alarms to detect leaks. Below are some more specific chemical risks
followed by process areas that may pose potential risks.
Dextrose can form explosive mixtures with air when in dry dust form; however, for this
process, aqueous forms will be used and this should not be an issue. If a fire should
occur, toxic fumes can be released, and a self contained breathing apparatus should then
be used. Because of this, dextrose should be stored in an area where flammable products
are not kept.
The main risk associated with ammonia is inhalation. Ammonia concentrations in the air
must remain at or below 25 PPM to prevent severe irritation. Piping and tanks will need
to be inspected on a regular basis. Alarm systems will also need to be placed on these
portions of the process in case of rupture. It may also be advisable to double wall all
ammonia piping. Explosion hazards arise only when in the presence of halogens, which
should not be an issue with the desired process. Also, when ignited, ammonia produces
toxic fumes, so storage should take place away from fire hazards.
Ammonium lactate poses almost no risk, though adequate ventilation systems should be
in place to prevent dangerous levels from being reached. It is stable under normal
conditions and does not pose a problem until it is exposed to temperatures nearing 200°C.
Temperature control measures should be taken to ensure maintenance below dangerous
levels.
N-butanol vapors are hazardous to humans and presence of n-butanol vapor should be
monitored. Also, all storage units should be tightly closed as N-butanol is lighter than air
and could travel to a remote ignition source where it may flash back to a storage unit and
cause an explosion hazard. N-butanol should be kept away from excessive heat and
flames. Excess n-butanol should be disposed of in a responsible manner (with
appropriate permits and at acceptable levels), or combusted in a controlled environment.
N-butyl lactate is also flammable and is capable of forming an explosive mix with air.
When burned, hazardous gas may be released. Vapor levels should be periodically
checked to ensure harmful exposure levels are avoided. As there is no storage of n-butyl
lactate, the esterification reactor (R-103), three phase separator (V-106), and hydrolysis
reactor (R-104) should all be monitored for leaks and temperatures and pressures should
be carefully observed. Secondary containment should also be utilized to prevent flow
into wastewater.
28
Toluene vapor levels should be monitored to ensure no health risk to employees.
Equipment that contains toluene (R-203, R-204, V-202, and T-204) should be inspected
for leaks on a regular basis. Although toluene is stable, it should be stored at ambient
temperature and atmospheric pressure. Secondary containment should be utilized to
prevent flow into wastewater streams.
Methanol causes blindness with repeated exposure and vapor levels should be monitored.
R-204, V-202, and T-204 should be inspected regularly for leaks and structural integrity.
Also, methanol is flammable and its vapor is slightly heavier than air allowing for travel
to ignition sources and flashback. Methanol should be stored away from ignition sources,
and in reasonable quantities. Spills should be cleaned quickly, and secondary
containment should be utilized so as not to allow methanol to enter the sewer.
As there are many potentially hazardous solvents within the process, inhalation and
flammability hazards stand out the most. Smoking should preferentially not occur within
the facility. However, if this is not possible, designated smoking areas should be created
in enclosed, well ventilated areas away from solvent storage to reduce explosion hazards.
Temperatures within the lactide formation step will be somewhat high and caution should
be taken in this area. Also, even though exact pressure for the polymerization step has
not yet been determined, this could be a potentially hazardous area of the process as well.
Environmental
Currently, 8300 lbs/hr of the n-Butanol that is used in the esterification step is lost to
waste treatment. N-Butanol becomes fairly miscible in water at high temperatures and
forms a difficult solution to separate by distillation. It has been discovered that a liquidliquid extractor using diethyl-ether successfully removes the n-butanol from the water.
The problem has now transformed into separating the n-butanol from the diethyl-ether. A
distillation column could work for this separation, but further research would need to be
performed, possibly using HYSYS. The only other waste stream is water containing
sulfuric acid, which can be sent to an off site water treatment plant and have little or no
effect on the environment.
Control Issues
The main control issues are the reactors which can be easily affected by disturbances in
temperature, flow rates, and pressure. These disturbances can have negative effects on
product properties, and reaction conversion rate. The fermentation reactors need pH and
temperature control to ensure high product yield and growth of bacterial cells. Cell purge
will also have to be monitored so as to maintain proper cell concentration. If too many
cells are taken out then lactic acid productivity will be decreased, and if not enough are
taken out than the filters will become obstructed also decreasing productivity.
The polymerization reactor will have to have temperature, pressure, and flow controls
which will vary depending upon the grade of polymer that is to be made. This area will
29
not be covered in great depth due primarily to time constraints and technicality of the
nature of this topic.
30
Discussion of Results
The proposed design shows promise to be competitive with the PET market. Given that
another facility has already been designed to meet these criteria, it is feasible to create
such a facility, even with the current market price of PLA being approximately three
times that of PET. The main factor in the cost of the process revolves around the
fermentation step, because the fermentation affects the separation process used. As the
fermentation step of the Cargill Dow process seems to be the area they have not yet
patented, the information surrounding that portion of the process is currently trade secret
and unavailable for comparison. It is possible that they have a system similar to ours that
utilizes a different neutralizing agent, or possibly a different esterification solvent. Based
on the economic analysis, it seems fair to say that they are getting approximately the
same productivity out of their fermentation reactors.
It is already known that Cargill Dow uses a different catalyst than that outlined in this
proposal. For their process, a tin based catalyst is used. Because this catalyst is harmful
not only to the environment, but also to humans, it is assumed that some method of
catalyst immobilization is used. This idea has already been incorporated into the current
design. Additional information on immobilization capabilities, price, and
regeneration/destruction rates of the tin catalyst would need to be found in order to
determine if that catalyst would be a better option.
In general, the proposed process seems to be an approximate model of the Cargill Dow
process. Though process flow data to accurately support this is lacking, through
communication with an employee of Cargill Dow, re-enforcement of the idea that this is
a comparable process has occurred.
31
Conclusions
The proposed process meets the aforementioned goal of being cost competitive with PET.
There are still many areas of improvement, which means this process can only become
more profitable. The main area of improvement seems to be the n-butanol recovery
system. Although the researchers cited observed the greatest product yield and
solvent/water separation factor with n-butanol of all solvents used, it is also possible that
a different solvent could be used in the esterification step. Still, the process is profitable
as a commodities polymer producer. The future of PLA truly does look bright.
32
Recommendations
There are many areas in which optimizations can be performed for this process. Most of
these optimizations require laboratory studies. Starting from the beginning, fermentation
reactor operating temperature and optimal pH should be tested. It would also be
beneficial to test mixing of the system in a pilot scale operation to ensure a proper microenvironment in all areas of the bioreactor for cell growth and lactic acid production.
Though the system was designed for one set of fermentation reactors in series, it could be
of use to have two or three such sets. This would allow for continued production of PLA
should contamination of a fermentation reactor occur. It may also improve mixing within
each reactor.
Additional work on the neutralization/separation system of the fermentation process
should also be carried out. It has already been determined that the vast majority of the nbutanol currently lost to waste treatment in the process can be captured through the use of
liquid-liquid extraction with di-ethyl ether as the solvent. The method of separation of nbutanol from di-ethyl ether has yet to be determined. However, this option gives hope for
a more environmentally friendly and economically sound process, depending on final
equipment design.
Though the formation of the lactide seems fairly straightforward, additional research
should be performed in this area as well. Because the lactide formation had the fewest
design variables, it received the least attention for process optimization. Scale-up
limitations, optimal temperature ranges, and optimal bleed rates could be determined
through laboratory scale, and pilot scale testing. Unless this portion of the process began
to be seen as a major cost area, it may be best to focus on other areas for optimization
first.
Finally, the polyermerization step requires attention. One major problem in the
polymerization is that the catalyst originally chosen has become infeasible if production
of PLA at PET prices is the goal. The limiting factor is the cost of the catalyst, and its
inability to be regenerated. Work to ensure the tin based catalyst does not make it into
the final product or into a waste stream could be performed. Also, additional work with
the chosen catalyst could be performed on a pilot scale, utilizing a plug-flow reactor
followed by a separate purge vessel, since it was concluded that it is the methanol purge
that destroys the catalyst. Also, it should be noted that the desired end form of the
polymer is pellets. This has not yet been achieved by the process currently laid out. The
cost would increase because of the additional equipment associated with this portion of
the process. It is assumed that because of the sheer volume of equipment prior to this
portion of the process, the venture will remain profitable at least for current market PLA
pricing. It cannot assuredly be claimed that this process would remain cost competitive
with PET after the addition of this equipment, though it appears as though it would be.
Cargill Dow has fully patented the polymerization process, and information to gather cost
estimates for finishing the process should be fairly easy to extrapolate.
33
Upon completion of process design and optimization, additional work should be
performed involving hazard evaluation. Obvious hazards have been pointed out.
However, an in depth study of the safety of the process should be conducted, including
environmental hazards. With this study, further process changes may need to be
implemented, but it is assumed they would be minor changes to the overall process.
34
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