Is Your Salad Take-Out Container a Hazardous Material?

With the new year upon us, we can expect the month of January to be filled with gym membership deals and weight-loss ads. In fact, many organizations regard January as “Weight Loss Awareness Month.” To help meet our expectations for a healthy year, along with reaching above our heads during exercise, many of us will be reaching for the pre-made salads at the grocery store.

According to a survey of almost 63,000 Consumer Reports subscribers last March, “more than half buy meals at the fresh prepared-food counter of the grocery store. In fact, prepared meals have become almost a $29 billion-a-year business, growing twice as fast as overall grocery store sales.”

Most of those clear plastic, clamshell takeout containers you grab from the store for your lunch salad, are made from Polyethylene terephthalate (PET). PET is a strong, lightweight, transparent plastic that is recyclable. However, the processing of PET can be a bit dangerous. In fact, maintaining process safety can be crucial to avoidance of an explosion at the ethylene oxide plant.

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. Thus, the EO process is inherently potentially hazardous, so those plastic container manufacturers must take great care in operating the plant safely.

EO is produced commercially by the vapor-phase reaction of ethylene and oxygen over a silver-based catalyst. There are two possible reactions that can occur:

Main Oxidation Reaction

Ag
C₂H₄ + ½ O₂ -> C₂H₄O

Side Oxidation Reaction

Ag
C₂H₄ + 3O₂ -> 2CO₂ + 2H₂O

Both reactions are exothermic. The side reaction is not only completely inefficient, producing no EO, it is over 10 times more exothermic than the main reaction, so much so that large amounts of heat must be removed from the process to avoid the risk of explosion. The key process control strategy is therefore to maximize the primary partial oxidation reaction while minimizing or eliminating the competing side reaction to complete oxidation. This is defined as the selectivity, the number of moles of EO produced in the reactor per 100 moles of ethylene converted. The key role of the catalyst is to maximize this selectivity.

Various organic chloride inhibitors, designed to slow preferentially the side oxidation reaction, can be added. 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 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 plant operation, methane is used as the balance gas to ensure the process streams are ‘fuel rich’, thereby minimizing the explosion risk. However, for maximum plant safety, methane is replaced by nitrogen during plant start-up and shut-down. The gas analyzer is therefore required to analyze two very different stream compositions.

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 more about how mass spectrometers can help overcome the challenges of the ethylene oxide process.

Overcoming the challenges of weight loss… well that’s a different story.