INTRODUCTION TO PET PRODUCTION

PET may be produced from ethylene glycol and either dimethyl terephthalate (DMT) or terephthalic acid. High purity is required of all raw materials. In either case, the first step of the reaction is the formation of a prepolymer, bis-hydroxyethyl terephthalate (bis-HET). Subsequent polymerization of this material (with the removal of ethylene glycol) forms the polymeric polyethylene terephthalate. The extent of polymerization (apparent from the molecular weight of the polymer) is a function of the polymerization conditions and significantly affects the properties of the resin that is produced.

Until the mid-1960s, DMT had been the preferred feedstock for PET manufacture, partly because the ester could generally be made in purer form than the acid. With the development of high-purity terephthalic acid processes, notably by Amoco, the free acid gained acceptance and is now the preferred feedstock. The use of high purity terephthalic acid (purified terephthalic acid - PTA) eliminates the need to recover or recycle methanol and has the added advantage that esterification to the prepolymer is considerably more rapid than the transesterification reaction, which is the first step when starting from DMT.

The conventional PET process consists of two discrete plant sections; the melt phase reaction to lower intrinsic viscosity product that is suitable for textile applications, but not for bottle grade and other high molecular weight applications. That lower intrinsic viscosity material is then further polymerized to bottle-grade intrinsic viscosity in a solid-state polymerization section. Note that intrinsic viscosity (as well as molecular weight) indicates the degree of polymerization, which in turn is the main factor in setting the properties of PET.

COMMERCIAL ANALYSIS

Fiber producers have for some time been converting older plants to PET bottle grade production by adding SSP capacity, in order to escape the more difficult market conditions in the fibers industry. This is still ongoing in certain areas, although the amount of remaining capacity that could be converted to competitive PET resin capacity is minimal relative to the amount of new capacity expected.

RESOURCE LINKS

Phoenix Equipment sells used PET and PTA plants that are immediately available for purchase and relocation. Buying a used plant can save you significant capital and drastically shorten the time required to build or expand manufacturing operations.