Polyethylene terephthalate (PET) is a strong, lightweight, transparent plastic. PET is used to make polyester, drink bottles, cosmetic packaging, and food-grade packaging including take-out and microwave safe food containers. Because of its good texture compatibility, PET is also used as basic material for blood vessel implants.
PET is produced using ethylene glycols, which are derived from ethylene oxide (EO). About 75% of EO is converted to ethylene glycol. EO is extremely reactive because its highly strained ring structure can easily be opened, and it is therefore one of the most useful and versatile chemical intermediates. Products derived from EO include:
- Ethylene glycols: used to produce antifreeze, coolants, polyester and PET
- Glycol ethers: used to produce brake fluids, hydraulic fluids, detergents, paints, lacquers and solvents
- Ethanolamines: used to manufacture detergents, and to purify natural gas
- Ethoxylates: used to manufacture detergents, dispersants, surfactants and emulsifiers.
The EO derivatives market is highly competitive, and feedstock prices are volatile, so it is vital that EO process economics are optimized. At the same time, the EO process is inherently potentially hazardous, so great care must be taken to operate the plant safely. EO is produced commercially by the vapor-phase reaction of ethylene and oxygen over a silver-based catalyst. Process control must ensure that the catalyst maintains its activity for as long as possible, since this is costly and time consuming to replace. Selectivity is improved by adding various organic chloride inhibitors, designed to slow preferentially the side oxidation reaction. These must be maintained and monitored at low ppm levels in the reaction mixture.
Gas analysis using a process mass spectrometer plays an important part in optimizing the EO process to improve efficiency while maintaining safety. The EO process is challenging; analysis speed is critical, as is a wide dynamic range. Additionally, the analyzer must be able to cope with two different balance gases — during normal operation methane is used as the bulk gas, but for enhanced safety during plant start-up and shut-down this is replaced by nitrogen.
Mass spectrometers offer fast, complete stream analysis; typical analysis time including stream switching is less than 30 seconds per stream for all components including trace chlorides. By measuring the inlet and outlet gases, important parameters such as Selectivity, Carbon Balance and Oxygen Balance can be derived. Read Improving Ethylene Oxide Process Control with the Thermo Scientific Prima PRO Process Mass Spectrometer to learn about an instrument that has been designed to meet and beat the challenges of the ethylene oxide process.