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.