Drying and Crystallization Systems for Reclaim Extrusion¶
Jeff Courter
ACS-Walton/Stout
02/21/13Abstract
Many reclaim lines require moisture removal from the
regrind material, and a PET reclaim extrusion line will not
operate properly if the drying and crystallizing system cannot
supply the material into the feed throat at the desired moisture
content, temperature and intrinsic viscosity (I.V.). A proper
drying and crystallizing system can be the difference between
quality product and junk, so it is worthwhile to consider some
features of the new equipment for your system:
- Correct sizing and proper operation
- New filter systems for increased performance
- High-efficiency motors
- New user-friendly PLC-based control systems
- Heat recovery systems
- Gas-fired options
- Integration with extruder control system
The following equipment is crucial to any reclaim extrusion
line:
- Hot air dryer (material dependent)
- Crystallizer (material dependent)
- Dust collection system
- Dryer and hopper sized for the application
- Loading system
Introduction
A properly configured crystallizing and drying system
will help produce your product with higher quality, improved
efficiency and lower cost if they are reviewed and specified in
advance.
Auxiliary equipment sized to match downstream production needs
Many reclaim lines require moisture removal from
regrind and reclaimed materials- with further processing
required in the case of Polyethylene (PET); based on its
chemical characteristics.
A PET reclaim extrusion line will not operate properly if the
drying and crystallizing system cannot supply properly
conditioned material to the feed throat at the desired moisture
content, temperature and intrinsic viscosity (I.V.). A properly
engineered drying and crystallizing system can mean the
difference between having a quality product or an unusable
finished good. It is well worth the time investment to consider
multiple processing features of new equipment for your
system:
Correct sizing of your upstream equipment relative to the
desired throughput rates for recovery, as well as overall
extrusion rates will be the first consideration for successful
processing of reclaimed materials.
For example: many sheet producers – especially thermoformers
producing shapes with round configurations (i.e.: cups
and lids) could easily have a 60% or greater scrap factor to
consider. For PET sheet lines, this percentage of potential
“loss” would quickly convert their business to a “non-profit”
organization.
Considering typical reclaim percentages then becomes a
relatively simple mathematical calculation. If a processor is
running a thermoforming sheet line with a throughput rate of
1000 pounds per hour and has a maximum potential scrap
factor of 60%, the recovery or reclaim rate desired should be
sized to accommodate this potential, and will be equivalent to
the needed rate of recovery at a reclaim rate of 600 pounds per
hour.
Size Reduction
The first consideration for reclaiming materials for
re-use would be for size reduction equipment to obtain reclaim
characteristics that can be processed more similarly to the
original virgin material. Screening of the material to a
consistency that provides greater bulk-densities and good flow
characteristics are always deliberate and essential for further
downstream processes.
Granulators configured with accessories to accept materials
as-processed by the extrusion line are also desirable to avoid
undue intervention by an attendee for this portion of the
process.
Crystallization Methodology
For PET processors, there are further considerations
for the conditioning of reclaimed PET. In its natural state, PET
is an opaque crystalline resin. Clear products can be produced
by rapidly cooling the molten polymer to form an amorphous
solid. In a process called “glass transition”, amorphous PET
forms when its molecules are not given enough time to
arrange themselves in an orderly fashion as the melt is cooled.
At room temperature these molecules are frozen in place, but
if enough heat energy is put back into a controlled process, the
molecular structure begins to migrate, allowing crystals to
nucleate and grow. This procedure is known as solid-state
crystallization.
Crystallization, or re-crystallization, has become an important
process when considering potential losses of valuable petrochemical
“waste”. The massive amounts of post-consumer
waste produced daily from food and beverage containers alone
is staggering.
As mentioned previously, losses of in-house scrap-factors for
large sheet producing facilities would accumulate startling
amounts of waste in a just a matter of weeks – not to mention
a stunning housekeeping problem.
Consider these statics for general reclaim of consumer
products:
• Each year, 29 billion plastic water bottles are
produced for use in the United States, according to
the Earth Policy Institute, an environmental
organization in Washington, D.C.
• Manufacturing them requires the equivalent of 17
million barrels of crude oil, so rising oil and natural
gas prices have only exacerbated the high price of
virgin plastic.
• Plastics News lists the price of PET virgin bottle
resin pellets approaching $1.00 a pound, compared
to 60 to 70 cents a pound for PET recycled pellets.
Scienceline.org
Selection of a properly sized crystallizer vessel is
directly related to the bulk-density of the material, the size and
shape of the particulates and the relative residence time or
dwell time required to orient the molecular structure.
Consistent agitation of the vessel is essential to disturb the
tendency of the glass-transition phase to agglomerate. Thinner
materials such as sheet or film will crystallize much more
rapidly than more dense pellets, thicker sheets or blends of
these materials.
Crystallization requires a heat source of forced-air through the
column of material. A Hot-Air dryer is employed to control
airflow with consistent temperature-controlled heating at the
correct transition temperature to create an environment for
molecular migration.
Special care in controlling the transition near the upper third
of the vessel will have significant impact on successful
crystallization.
Dust Removal
Re-processing of sheet and other regrinds, inherently
produces a small percentage of nuisance dust from size
reduction granulation. There are several methods of
separating dust from the process, including; high-tech dedusting
equipment, cyclonic elutriation, central dust-collection
systems, and/or simple cyclonic separators. All of these
solutions have the ability to remove significant amounts of
dust and fine particles post-granulator, or further downstream
– with price variations to match the amount of dust removal
desired.
Energy Conservation
Recent mandates for the use and distribution of
energy-efficient motors have had an impact on the cost to
operate machinery in the United States and abroad. Since the
cost to operate generally rapidly outweighs the initial
equipment investment, consumers and suppliers are always
looking for ways to reduce, recoup and reuse energy –
especially when heating and cooling are necessary processes
involved in the manufacturing of finished goods. Engineers
are always looking for ways to use otherwise lost energy
sources and to return them to the process or use them in other
localized process.
Off-the-shelf PLC Control Solutions and Controls Integration¶
Customers requiring feedback and historical data
from their equipment have distanced themselves from
simplified “relay-logic” control systems to avoid
troubleshooting issues and to have nearly infinite control of
their processes.
This ability is not possible with a series of block timers and
relays.
In an effort to answer this need, equipment manufacturers
looked to the electronics industry for inexpensive solutions,
which were generally proprietary boards with no standardized
features for generally accepted protocols.
As technologies rapidly advanced in processing speeds, many
of these components quickly became obsolete, rendering
entire lines useless for a lack of viable replacements.
Initially, Programmable Logic Controllers (PLCs) were an
expensive luxury that few could justify.
Now that robust PLCs have become quite affordable, many
equipment producers have realized that the era of proprietary
components has reached an end-of-life cycle, and are
supplying PLCs with greater expansion capabilities, faster
processing, and recoverable data that can be logged and
tracked – also lending itself to nearly fool-proof
troubleshooting of equipment – or entire systems.
PLC manufacturers, as well as governing institutions such as
SPI have realized the need to integrate entire systems with a
common communication standard generally referred to as
“protocol”. These firmly established protocols allow users to
have a common, standardized and regulated transmission of
data between all equipment, computers and peripherals.
Gas-Fired Options
Natural Gas and Propane remain as efficient and
viable options for processes requiring reliable heating.
In some areas of the United States and abroad, natural gas
supplies are prevalent and readily available. High
consumption users have been awarded subsides and deep
reductions from gas producers. For these consumers with
equivalent thermal usages for gas, the use of this natural
resource outweighs electric heating costs.
Most equipment manufacturers have options available for gasfired
heater in lieu of electric heating elements.
Typical Reclaim Extrusion Line - Summary of Equipment Focus on PET
Granulation
Select size-appropriate granulator for rates equivalent
to the highest scrap-recovery rate.
Select screen size for best bulk-density and flow
characteristics
Evacuate granulator chamber with method best suited to dust
separation and recovery of usable materials
Dust Collection
Several methods are generally employed – and best
solved immediately following granulation
Crystallization
Amorphous PET requires crystallization.
Controlled heat and forced-air provide the methodology to
initiate crystallization.
Agitation is essential as the transition phase creates tacky
agglomerates that must be broken apart to achieve optimal
crystallization and to avoid “chunks” of agglomerates.
Blending
Once the amorphous material has been reduced in
size for efficient handling; conveying and flow characteristics,
it can blended with virgin material, colorants or other
additives.
Desiccant Drying
PET is a hydroscopic material and will readily bond
with atmospheric moisture which must be removed prior to
entry to the extruder or molding machine.
The drying vessel is sized for the “worst-case” bulk-density
that will be processed at any time, thus allowing for adequate
dwell-time in the vessel for proper drying.
This residence time is commonly 4-6 hours at ~350degF with
1CFM per pound, per hour airflow at the established
temperature.
Loading Systems
Generally, sheet recovery conveying and handling
(post-granulator with dust removed) can be accomplished with
a common vacuum conveying system – with considerations
for the bulk densities of the reclaim and rates desired.
Where simplified dust-removal systems are employed and
removal is not accomplish 100%, and/or the processor can use
the fine particulates, filtered-receivers can be installed to avoid
undue maintenance of the rudimentary screen filter found with
most standard receivers.
Poly-Lactic Acid (PLA) – “Going Green”
PLA, a plant-based polymer typically derived from
corn nucleotides, has gained a foothold in the market as a
viable replacement for petroleum-based products.
This product group has been touted as the new “renewable
resource” that is also a bio-degradable solution to off-set the
fact that we have become a “throw-away
nation.”
PLA is also a hydroscopic material as is PET, and will become
amorphous requiring crystallization.
Although this material has similar characteristics to PET, it is
significantly more “finicky” to process and prep for extrusion.
The crystallization phase uses much lower (and tightly
controlled) temperatures, becoming tackier and “gummy”
during transition to the crystalline state.
Once stabilized, it has more common “plastic” characteristics.
Desiccant dryer is a bit more conventional, but also requires
lower temperatures.
Newer Processing Technologies
Extruder designs
Several new technologies are being developed to help
plastics processors to increase throughput rates, process lower
quality regrinds and blends, and/or to eliminate conventional
equipment such as dehumidifiers.
At least one manufacturer is working toward
developing a new main screw and barrel design that utilizes a
single main screw with peripheral “satellite” screws rotating in
the opposite direction, driven by a ring gear in the main barrel
This design provides for more of internal exposure of the
polymer and thus allowing for vapor or moisture removal at
the machine.
The drawback of these technologies today is the extreme cost
for the initial investment, and the fact that most existing
machines cannot to retro-fitted with the necessary
components, necessitating the purchase of a new extruder or
main extrusion components.
Infrared Processing
Infrared (IR) technology is being developed as a
well-controlled heat source that generates IR “waves”.
IR waves excite and heat materials from the inside
out to assist in expediting the release of moisture from wet and
hydroscopic materials. Heated materials can also be tumbled
in IR equipment to assist in the crystallization of some
polymers.
IR has become an interesting and promising
technology for some processing requirements.
The limiting factors of IR processing:
These systems do not operate in the necessary closedloop
/ low-dew-point environments needed for many popular
polymers and thus require further heated, and closed-loop, low
dew-point drying.
Lamps and associated hardware can become
overheated and require consistant air movement to keep the
lamps and bases cool.
The lamps cannot tolerate dusty environments and
can become fouled easily when processing dusty materials,
and regrinds.
Complex zone controls and lots of moving parts for
the internal tumbler beds.
Re-Pelletizing and Conditioning of Recovered and Recycled Materials¶
Several companies have worked closely with
polymer chemists to provide innovative machinery to convert
reclaimed plastics from granulated sheets, shapes and
containers back to a uniform pellet that increases the bulkdensity,
provides for much better flow characteristics, and for
controlling the process for a wide variety of polymers to
“condition” these materials to nearly approach virgin specs in
some cases.
Conclusion
A properly sized and configured crystallizing and
drying system can provide the most economical and modular
solution to produce a product with higher quality, improved
efficiency and lower cost when reviewed and specified in
advance.
New innovations by machinery manufactures and
polymer chemists will assist the industry to reclaim and
reprocess otherwise lost resources.
References
1. Walton/Stout, Inc., New Berlin, WI
2. Cumberland Engineering, Inc., New Berlin, WI
3. AEC/HydReclaim, Inc., Schaumburg, IL
4. Scienceline.org a project of the Arthur L. Carter
Journalism Institute at New York University.
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