Polyethylene terephthalate (PET) is used in the manufacture of blow-moulded plastic bottles (PET bottles) and in films amongst other things and is one of the main synthetic fibres. In order to produce PET, the monomers terephthalic acid and ethylene glycol extracted by petrochemical means are esterified in the reactor at a moderate pressure (2.7–5.5 bar) and high temperature (220–260°C) using a catalyst. In a second stage, gradual pre-polymerisation is performed, whereby separated water and waste products are continuously removed from the reaction mixture, as otherwise the reaction would be inhibited. In parallel to this, reactive intermediate products are returned to the esterification reactor. The final polymerisation stage takes place at up to 280°C and at very low pressure (< 1 mbar). The PET fibres can then be produced from the extracted molten polyester using the melt-spinning method for example.

PET manufacture must not only fulfil food regulatory limit values and taste requirements, but nowadays the plant must also significantly reduce energy consumption and sustainably lower operating costs too. This involves keeping the process variables of flow rate, pressure and temperature within narrow limits, which requires comprehensive monitoring. KROHNE offers the ideal technical measuring systems for this, such as industrial/resistance thermometers for use in piping and tanks. There are also pressure transmitters to simultaneously measure the process pressure and fill level up to +200 °C as well as radar level meters for liquids, pastes and sludges, which are suitable for continuous, non-contact level measurement in storage and process tanks. And 2-wire level transmitters with guided radar come in a wide range of sensors and materials, which means they can even be used in high pressure and temperature conditions.