An important part of American industry's efforts to "go green" in the coming decades will be to invent clever ways to extract raw materials from renewable resources. LBL chemical engineer Judson King and coworkers have created two environmentally friendly methods to recover carboxylic acids from plants instead of from oil.
Important in a variety of industries, carboxylic acids serve as the building blocks of nylon (adipic acid), help put the fizz in antacid tablets (citric acid) and give many soft drinks their tart taste (citric acid). Other forms, such as succinic and lactic acid, are important intermediates in many industrial chemical reactions.
The low-cost techniques, devised by King and his colleagues in the Energy and Environment Division, recover carboxylic acids from two important sources: fermentation broths, where microorganisms break down the carbohydrates from plants such as corn; and industrial waste streams, such as the run-off from sugar cane factories.
The techniques themselves produce very little waste since the acids are extracted with chemicals that can be recycled. Traditional carboxylic acid recovery techniques generate massive amounts of calcium sulfate as a waste product.
"Using waste streams as sources also gives companies essentially a zero-cost raw material," King says. "They can recover carboxylic acids and take care of a waste problem at the same time."
The new methods may also allow companies to make acid-based products that in the past have been too expensive to manufacture. King is currently tailoring the techniques to produce a non-corrosive salt from acetic acid for use in de-icing roads. The methods may also help companies trying to create biodegradable plastics from lactic acid. Cargill, Inc., a Midwestern grain manufacturer, constructed an $8 million plant in Minnesota last year to develop plant-based plastics.
Scientists currently separate carboxylic acids formed in fermentation processes by adding calcium, which forms a solid calcium carboxylate salt that drops out of solution. They then add sulfuric acid to the salt, which regenerates carboxylic acid along with calcium sulfate.
"Places that produce citric and lactic acid create piles and piles of calcium sulfate that they can't use," King says. "The hills you see around some of the factories are actually grass-covered mounds of calcium sulfate."
The first invention, recently patented and for use primarily with fermentation broths, removes carboxylic acids from solution with amine-based extractants. The carboxylic acids are freed from the extractants by transferring them to a second, more strongly binding amine molecule, which is subsequently boiled away.
The second recovery method, for use mainly with waste streams, makes use of an unusual solubility phenomenon found for carboxylic acids. Some carboxylic acids that are generally very insoluble will dissolve much more readily into certain organic solvents if the solvents contain a small amount of water.
The technique removes carboxylic acids from solution by letting them diffuse into such a water-containing solvent. Scientists recover the carboxylic acids by boiling the water away, which causes the acid to drop out of the solvent as a solid.