Process description – SSP for recycled PET pellets.SSP for the PET recycling process meets the same basic requirements as virgin polymers: increase of molecular weight (IV) and removal of organic by-products. However with recycled material the increase of IV is less (due to IV degradation in previous PET heating processes), and organic by-products are not caused by side reactions during PET synthesis, but are mostly migrated substances from consumer usage.
SSP for recycled PET is optimized for the specific recycling requirements: it is more flexible and has only two steps, starting with crystallized material. Inside the solid-state reactor, the product reaches its final temperature and is upgraded to the desired IV. Homogeneous product quality is guaranteed through even heating and the narrow residence time distribution. Maximum IV flexibility is obtained through a reactor design that allows a wide range of process temperatures. During processing, all degradation products, which partially present as volatile contaminants and are partially bound to the polymer molecular structure, are removed or re-polymerized accordingly.
For quick cool-down, a fluid bed cooler is used. The cooling process also provides a final dedusting of the material. Cooling is performed under air; polycondensation under nitrogen prevents any undesirable oxygen degradation of PET. The nitrogen is constantly cleaned in a gas purification loop where all reaction products and any residual contaminants are removed.
Due to reversing degradation from previous processing, the acetaldehyde content is reduced to values below even virgin PET.