The task seems simple enough. Make a key ingredient for polyester with no pesky impurities. No costly clean-up steps. Sounds easy, right? No such luck.
For more than half a century, industry has been making the blockbuster chemical terephthalic acid, or TA, by a problematic process. It makes TA tainted with an undesirable impurity. Removing this impurity not only costs money, it wastes energy and emits carbon dioxide into the atmosphere.
Researchers at the Center for Environmentally Beneficial Catalysis, or CEBC, are getting closer to developing a radically new way to make pure or so-called ‘polymer grade’ TA.
“Pure TA is used to make polyester for clothing, carpets, curtains and plastic bottles,” said Bala Subramaniam, Director of CEBC. “Our greener process could eliminate 60% of the waste and significantly conserve energy.”
The CEBC and original processes both start with the same recipe: a liquid mixture containing p-xylene, acetic acid and a catalyst. TA is made when liquid p-xylene combines with oxygen gas. Here’s the problem. How do you mix a liquid and a gas?
The old way does this by bubbling oxygen into the liquid mixture. Rapid stirring and heat help some oxygen combine with the liquid. But not enough dissolves to fully oxidize p-xylene. The result is a mix of TA and a troublesome impurity known as 4-CBA.
The CEBC process flips things around. It showers a fine mist of the liquid mixture into a tall vessel loaded with oxygen. Oxygen penetrates the tiny droplets as they cascade downward. This exposes more p-xylene to oxygen, leading to more TA and very little, if any, impurity.
Proof-of-concept experiments show that TA from the spray process contains just a negligible fraction (<25 parts per million) of the unwanted 4-CBA. TA made by the old process may contain 40 times more 4-CBA. A separate and costly clean-up step is needed to remove this nemesis impurity to make a useful polyester.
To demonstrate continuous polymer-grade TA production, CEBC researchers had to fabricate, assemble and install new equipment. "A major challenge has been in fabricating the the specialized reactor," said Subramaniam. "We are fortunate to have our lab director, Dr. Fenghui Niu, helping in this regard."
After a year of tweaking, they are finally able run the reaction continuously while maintaining nearly uniform temperature and pressure in the reactor.
“We are steadily closing in on our goal of making a one-step process for pure TA,” said Subramaniam. “Our process is not just greener. It could cut capital investment costs in half and save roughly $6 billion per year globally in production costs.”
--Story by Claudia Bode