Chemical Engineering May 2021 - 29

Technology Profile
Producing Polylactic Acid from Corn
By Intratec Solutions
P
olylactic acid (PLA, polylactide;
Figure 1) is a thermoplastic
aliphatic polyester
that is derived from a number
of renewable starting materials, such
as starch and sugarcane, via bacterial
fermentation. PLA is semicrystalline
or amorphous, rigid, resilient and
an ultraviolet-light-resistant material,
with similar characteristics to common
petrochemical industry plastics,
such as polyethylene, polypropylene
and polystyrene. PLA is used in plastic
fi lms, bottles and biodegradable
medical devices. It fi gures among
the most highly consumed bio-based
plastics worldwide.
PLA rapidly absorbs moisture from
the atmosphere and is also temperature-sensitive
(storage temperatures
should be below 122ºF). In order to
preserve PLA properties and characteristics,
exposure to ambient (undried)
air should be minimized. PLA is
usually delivered in moisture-resistant
containers, including foil-lined boxes,
which can prevent moisture regain
during shipping and storage. PLA is
mainly used in packaging, textiles,
nonwoven fabrics, electronics, automotive
parts, medical devices and
cutlery. Packaging is the largest application
market for PLA, and will
likely grow as new heat-resistant PLA
blends are developed.
Polylactic acid process
The production of PLA from corn
(Figure 2) involves three major sections:
(1) Starch hydrolysis; (2) Glucose
fermentation; and (3) Lactic
acid polymerization.
Starch hydrolysis. Shelled corn is
fed to grain cleaners, then sent to
steeping vessels, where it is soaked
in a dilute solution of sulfur dioxide,
produced in a sulfur burner. The solid
matter slurry obtained is submitted
to a sequence of unit operations to
recover cornstarch, while the steep
water is concentrated in evaporators
and sent as corn steep liquor (CSL) to
the downstream process stages. The
starch is then mixed with enzymes
and sodium carbonate in the hydrolysis
area to convert it to glucose, which
is sent to the fermentation stage.
Glucose fermentation. The fermentation
is carried out in an agitated
jacketed fermenter, fed with glucose,
CSL and sodium carbonate. The fermentation
broth is sent to an ultrafi
ltration step to remove cell debris,
concentrated in an evaporator, and
then to an extraction column to extract
the lactic acid (LA) in an organic
phase, which is routed to a distillation
column to separate the extractant solvent
from the LA. The LA-rich stream
is then back-extracted with hot water
in an extractor train, generating a LAlean
organic phase, which is recycled
to the fi rst extraction stage, and an
aqueous phase rich in LA that is concentrated
in an evaporator and sent
to the polymerization stage.
Lactic acid polymerization. The
concentrated LA stream is fed to a
prepolymer reactor, where it forms
low-molecular-weight PLA and water,
which is removed and disposed.
The low-molecular-weight PLA is
FIGURE 1. PLA is an aliphatic polyester derived
from renewable starting materials
sent to the depolymerizer and broken
into lactide molecules (the cyclic
lactone di-ester derived from lactic
acid), which are purifi ed in a drying
column and in a meso-lactide
removal column. Lactide is fed to a
polymerization reactor, forming PLA,
which is sent to a devolatilizer to remove
unreacted lactide. The PLA is
routed to an extrusion step and then
packed in bags for storage.
Production pathways
This biodegradable polyester can be
synthesized via two major routes.
The main production route for highmolecular-weight
PLA is based on an
initial polycondensation of lactic acid,
followed by a depolymerization at reduced
pressure and a ring-opening
polymerization step. The other route
is based on a direct polymerization
of lactic acid by polycondensation.
Most companies employ the indirect
route, since the production of highmolecular-weight
polymers is diffi cult
via direct polymerization.
n
Edited by Scott Jenkins
Editor's note: Content for this column was originally developed by
Intratec Solutions LLC (Houston; www.intratec.us) and is edited by
Chemical Engineering. The analyses presented are based on publicly
available and non-confidential information. The content represents
the opinions of Intratec only. More information about the methodology
for preparing the analyses can be found, along with terms of use,
at www.intratec.us/che.
FIGURE 2. The diagram shows the production process for PLA, starting from corn
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2021
1. Sulfur burner
2. Grain cleaners
3. Steeping vessel
4. Steep water evaporator
5. Starch recovery
6. Starch hydrolysis
7. Fermenter
8. Ultrafi ltration
9. Broth evaporator
10. LA extractor
11. Extractive distillation
12 LA back extractor
13. LA evaporator
14. Prepolymer reactor
15. Depolymerizer
16. Lactide purifi cation
17. Polymerizer
18. Devolatilizer
19. Extruder
20. Packaging
21. Cooling tower
22. Steam generation
CW Cooling water
ST Steam
29
http://www.intratec.us http://www.intratec.us/che http://WWW.CHEMENGONLINE.COM
Chemical Engineering May 2021

Table of Contents for the Digital Edition of Chemical Engineering May 2021
