Extracting soybean oil with a switchable-hydrophilicity solvent

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2010

A carbon dioxide is used in a new process that is more environmentally friendly and cost efficient. 

KEY CONCEPTS
• The current method for extraction of soybean oil requires the use of a volatile solvent (hexane) and an energy-intensive distillation process.
• A switchable-hydrophilicity solvent has been found to extract soybean oil without the need for hexane and distillation.
• Green Center Canada wants to use this concept to extract residual motor oil from used plastic bottles to facilitate recycling and reduce waste disposal costs.

Soybean oil is currently obtained from soybeans through a process involving extraction of the oil from flakes. Hexane is used as a solvent in this process because it is very compatible with soybean oil and easily removed by distillation.

Philip Jessop, professor of chemistry and Canada Research Chair in Green Chemistry at Queen’s University in Kingston, Ontario, Canada, indicates that this is not an environmentally friendly process. He says, “Hexane is a volatile solvent that can cause a significant amount of smog formation. In Canada, it is estimated that 4,400 metric tons per year of hexane are emitted with a third sourced from oilseed processing. One other factor is that the final distillation process is very energy-intensive.”

In a previous TLT article, the concept of a switchable solvent was introduced (1). Jessop has been looking at this approach to use solvents in a more cost-effective and environmentally friendly standpoint.

His original switchable solvents could be converted reversibly from a nonpolar to a polar state. Jessop has been successful in doing this using carbon dioxide as a reagent. Carbon dioxide is utilized because it is very cost-effective, readily available and easily removed. Jessop has now reported a new kind of switchable solvent, which he calls a switchable-hydrophilicity solvent (SHS), meaning it can switch from a hydrophobic state to a hydrophilic state.

The switching process is shown in Figure 2. A hydrophobic organic solvent is typically insoluble in water, as shown on the left. Reaction with carbon dioxide in water (carbonated water) switches the solvent to a hydrophilic state that is soluble, leading to the formation of a homogeneous mixture on the right. Eventually, the SHS can be isolated in its original hydrophobic state through removal of the carbon dioxide.


Figure 2. A switchable-hydrophilicity solvent is used in the hydrophobic state to extract soybean oil and then is converted into a hydrophilic solvent through the introduction of carbonated water. Isolation of soybean oil is then achieved followed by switching the solvent back to its hydrophobic state through removal of carbon dioxide. (Courtesy of Queen’s University)

Using a SHS to process soybean oil without the use of hexane and distillation has potential. The solvent in its hydrophobic state could be used to extract soybean oil and then switched to a hydrophilic state to enable the oil to be isolated. A suitable solvent that can reversibly react with carbon dioxide has now been found.

AMIDINES AND AMINES
Carbonation of water produces a weak acid (carbonic acid) which means that the desirable solvent should be basic. Jessop and his coworkers have known that amidines, amines and guanidines can react with carbon dioxide and water to become hydrophilic. Jessop says, “We looked at a number of candidates by varying the number of alkyl groups attached to these nitrogen-based functionalities. Guanidines were eliminated as possible solvents because they are too basic and do not switch back to a hydrophobic state.”

The parameter used to determine the best possible choice is the octanol-water partition coefficient. Jessop says, “We added a tiny amount of the solvent to a beaker with one-part octanol and one-part water. The percentage of solvent in octanol vs. water is measured. A higher value means that the solvent is more hydrophobic.”

The researchers found that a solvent with a logarithmic octanol-water partition coefficient above seven is too hydrophobic and will not be miscible with carbonated water. One specific amidine solvent that worked well exhibited a value just above six.

This amidine solvent also happens to change polarity when exposed to carbon dioxide. Polarity measurements were obtained by determining the wavelength of maximum absorption of Nile Red dye in the absence and presence of carbon dioxide. Jessop says, “We found that the difference in the wavelengths of maximum absorption for the solvent in the absence and presence of water is far greater than any other solvent we have ever tested and shows a change in true solvent polarity.”

The reason for this major change is that the solvent is very immiscible in water in the absence of carbon dioxide. This effect changes dramatically when carbon dioxide is added.

This amidine solvent was then used to extract soybean oil from the soybean flakes. An extraction experiment was conducted to compare the ability of the amidine to extract soybean oil vs. hexane. After stirring soybean flakes in both solvents overnight and then filtering, an equivalent amount of soybean oil was detected in both solvents.

Carbonated water was then introduced to remove the amidine solvent from the water. The efficacy of this technique was evaluated by using 1H nuclear magnetic resonance (NMR) spectroscopy of deuterium water. After only one water wash, 96% of the amidine solvent had been removed. Jessop anticipates that additional washings will remove the remaining solvent.

The solvent can then be isolated from the water through removal of carbon dioxide by heating the mixture at a temperature of 80 C for one hour. Jessop indicates that further work needs to be done to determine the amount of residual solvent left in the flakes and how to remove it. This factor is important because the flakes are used in other applications.

One area of concern for Jessop is solvent durability. He says, “We are uncertain that amidine solvents can be reused in this process because they can hydrolyze over time. We have recently looked at amines which are chemically more robust. Several promising amine candidates have been identified that will perform better than the amidine solvent we initially evaluated.”

Jessop also indicates that further work will be done to determine how much energy can be saved by not distilling hexane.

RECYCLING MOTOR OIL
Jessop works with Green Center Canada, which is looking to commercialize sustainable technologies developed in the academic community. He says, “One new project that Green Center Canada is working on is to evaluate the ability of switchable solvents to extract residual motor oil from used plastic bottles. If an approach can be developed, then recycling of the plastic bottles will become feasible.”

Such a process will significantly reduce waste disposal costs as plastic bottles with residual motor oil now have to be land-filled in Canada.

Additional information on the use of switchable solvents to extract soybean oil can be found in a recent article (2) and by contacting Jessop at jessop@chem.queensu.ca. 

REFERENCES
1. Canter, N. (2006), “Analyzing Switchable Solvents,” TLT, 62 (2), pp. 15–16.
2. Jessop, P., Phan. L., Carrier, A., Robinson, S., Durr, C. and Harjani, J. (2010), “A Solvent having Switchable Hydrophilicity,” Green Chemistry, 12 (5), pp. 809–814.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.