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 Application
PET Molding
Epoch-Making Double-Level Drying Hopper For PET Resin
1. Lower heat use
(about 20% less than conventional models)
2. The resin temperature in the main drying
hopper is kept at 130 degC, even if
the drying operation is stopped, so that
the physical properties of the PET resin
do not deteriorate.
PET resin drying hopper dries material PET resin chips by exchanging heat
calories with dry heated air. The PET resin chips are fed into the hopper
at the top and move slowly to the bottom of the hopper while dry heated
air flows from the bottom to the top. The moisture diffused from inside
of the chips by heat vaporizes, and is carries away by the air stream,
thus drying the resin chips.
The dry heated air entering the bottom of the hopper is cooled by the chips.
The drop in air temperature due to the air absorbing the moisture varies
with the air flow rate. The higher the air flow rate, the warmer the air
in the upper zone of of the hopper will be. This provides and advantageous
conditions for controlling the temperature increase and drying the chips.
However, a hopper with a larger L/D (Length to Diameter ratio) will increase
the linear velocity of the air flow. A linear velocity exceeding 0.6 m/s
may cause eddies to form in the air. Therefore, the L/D of the hopper is
a key factor in establishing a piston flow of the resin chips through air
flow.
In other words, it is critical to maintain an optimum air flow rate that
will provide enough heat calorie transfer to the material in the hopper
by maintaining the best L/D ratio, without creating eddies.
In order to achieve an optimizes air flow, OSAKA REIKEN adopted a double-level
drying hopper that supplies dry heated air both from below and in the middle
of the hopper. This design allows finer control of the temperature curve,
from the bottom to top of the hopper by introducing piston flow at the
lower and middle positions, where stable piston flow is required. Also,
this system allows a reduced overall hopper height because the hopper diameter
increase just above the mid level air intake.
In addition, our hopper uses an exhaust air recirculation system that provides
energy savings of about 20 %.
Importance of Drying Temperature
The higher drying temperature of PET resin, the higher diffusion rate of
moisture in a chip so that drying rate increases. However, temperature
increase exceeding a certain limit may cause degradation of PET due to
heat, oxygen, and moisture. Reaction rate of this degradation depends upon
temperature level. At higher temperatures, reaction rate increases. In
particular, hydrolysis commences when temperature exceeds 150 degC through
it is extremely gradual at this level. Above 160 degC, hydrolysis advances
rapidly and its advancement rate increase at a high pace as temperature
increases. Drop of melt viscosity and intrinsic viscosity indicates deterioration
of mechanical strength. In addition, Carboxyl end group functions as acid
catalyst and promotes hydrolysis of PET resin so that it may affect stability
of hydrolysis later. For the reasons above, PET resin should be dried below
160 degC.
Purpose
In order to prevent hydrolysis during molding, Hydrolysis -> Drop polymerization
degree, and increase carboxyl end group. One molecule of water cut one
position of polymer chain of PET resin (At the same moisture level in the
higher [n] drop of [n] will be larger). This is because moisture at temperatures
higher than the melting point (253 degC) rapidly advances hydrolysis of
polymer (cut polymer chain). Therefore, it drops [n] and related physical
characteristics. Our experimentation that PET resin of 0.76[n], [n] value
increase 0.01 pitch as moisture retained in the melted PET resin increases
16 ppm. Drying PET resin is the reverse phenomenon of the moisture absorption
process. Absorbed moisture defuses to the center a pellet. In order to
get the reverse effect of this process, a pellet which has absorbed moisture
shall be exposed to extremely dry conditions and heated in order to accelerate
diffusion of moisture outside. However, there is a limit for maximum drying
temperature.
Reason 1.
Hydrolysis depends upon increase of temperature. This phenomenon starts
at 150 degC in a solid PET through it is quite gradual, and hydrolysis
rate increases as temperature rises.
Reason 2.
Excessive drying temperature leads to thermal degradation and oxidative
degradation of polymers. As a result, undesirable final products containing
acetaldehyde may be produced. Degradation reaction during melting acetaldehyde
is larger using germanium catalysis (GeO2) than using antimony oxide (Sb2O3) catalysis. In the mean time, physical change of coloring compound produced
as visually recognizable phenomenon, and yellow coloring occurs.
Chips having constantly stable residual moisture are produced. This system,
needless to say to maintaining stable dew point and sending dried air adopts
highly technical measures to realize equal storage time and stable residual
moisture.
Homogeneity
To realize homogeneous drying, drying air should have stable low dew point
and its flow should maintain the same rate against the surface inside the
hopper. Flow rate of dried air (surface flow volume) in our system is 0.6
m/s at 150 degC so that we can get higher "U" value (overall
heat transfer coefficient). Or, as minimum requirement, drying ratio approximately
0.06 m3/min per hour for 1 kg of polymer resin shall be obtained. In our unique
system, circular air flow occurs between the lower cone section and diamond
shaped section. Polymer resin drops along inside wall of the cone section,
and air flows along the surface of diamond shaped section. Therefore, polymer
is fluidized. As the entire polymer resin rises (bulk density decreases)
and air portions increase, air resistance becomes small so that dried air
can flow at constant velocity against sectional area. With the above flow,
polymer resin is not damaged and can discharged with piston flow (making
fluctuation range of staying time of resin as small as possible to get
constant drying).
Fine Particles and Fine Chips
Fine particles, pull-ups and the like may cause excessive heating, abnormal
high melting point, or (appearance) feed fault of an injection machine.
Difference of heat history for some area may cause inferior luster on formed
products, or opaque area in fine potion. For collection of dust such as
fine particles and tiny chip, contained air in polymer increases and air
resistance decrease (bulk density decreases) so that fine particles and
tiny chip are discharged together with dried air and collected in bug filter.
Molecular Sieve Rotor
The dehumidifier rotor is a honeycomb rotor consisting of walls of molecular
sieve adsorbent. As molecular sieve does not lose moisture adsorption performance
even at low dew point, it can create ultra low dew point air condition.
The honeycomb structure rotor provides larger contact area between adsorbent
and processing air, and effectively absorbs moisture. In addition, it can
offer stable dehumidified air flow. This rotor deteriorates little with
high temperature regeneration, and can offer repeated dehumidification
and regeneration with almost no maintenance work.
Drying Capacity
Enough margin for the required dry air flow rate. As the system has capacity
to generate air below -50 degC dew point, it can be keep the residual moisture
level in chips quite low. The experimental result, residual moisture is
less than 10 ppm with drying temperature at 150 degC.
Thermal Degradation, Oxidative Degradation and Hydrolysis
Concerning thermal degradation, oxidative degradation and hydrolysis of
PET resin, It is known that with the same temperature, oxidative degradation
advances at approximately twice the rate of thermal degradation and hydrolysis
advances at approximately 5,000 times the rate of oxidative degradation.
Degradation rate of PET resin is
"Thermal Degradation" <<
"Oxidative Degradation" << "Hydrolysis"
These three degradations occur at the same time in parallel, so that degradation
process of PET is extremely complicated.
Thermal Degradation
It is said that influence of thermal degradation is relatively small in
drying temperature. However, it is possible when errors occur involving
excessive temperature rise. In this case, carboxyl end group increases
and unfavorable decomposed object having acetaldehyde may be produced.
Oxidative Degradation
Oxidative degradation occurs at 160 degC to 180 degC. Thermal and oxidative
degradation cause drop of malt viscosity and intrinsic viscosity as well
as increase of carboxyl end group and decrease of mass weight.
Hydrolysis
PET resin contains ester bond which may cause hydrolysis in polymer chains.
Hydrolysis reaction involves moisture in resin which cuts molecular chains
at ester bond. As polymer chains shorten, polymerization degree drops so
that melt viscosity and intrinsic viscosity drop. Carboxyl end group also
increases. This reaction rate increase from around 160 degC and up.
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