Hydrogenation of Edible Oils
By:
Trent M. Hebert
Product Information
Catalytic hydrogenation of edible oils and fats has been performed throughout the world for well over fifty years. Large-scale industrial processes were developed to convert liquid oils to hard or liquid fats and convert soft fats to firmer products. Another beneficial property of the process was that it improved the resistance of fats and oils to deterioration through flavor reversion or oxidation. Catalytic hydrogenation can also produce edible oils from oils such cottonseed, whale, and other fish oils which would have otherwise been unfit for human consumption in large quantities. Early literature describes this process as being the first and by far the most important catalytic hydrogenation process to be adapted to commercial operation.
Hydrogenation of a fat molecule directly adds hydrogen atoms at the double bonds of a fatty acid chain to produce an artificial fatty acid. In a complete catalytic, all unsaturated radicals are transformed to saturated radicals. Partially hydrogenated oils are the result of an incomplete hydrogenation reaction, due mainly to poisoned catalysts. Incomplete hydrogenation gives rise to the formation of trans acids and acids produced by the double bond shifting to either side of its original position. The trans acid formed in the production of partially hydrogenated oils, evident in most of your favorite junk food, is currently being linked with such diseases as obesity and coronary disease.
Process
Although there are many different designs, a hydrogenation process must include a heater; a mixer to create surface area contact between the oil, hydrogen gas, and the catalyst; an autoclave; and a means to cool and filter the oil. A necessity for the hydrogenation of edible oils is to bring mixture of liquid oil and gaseous hydrogen in contact with a solid catalyst at a suitable temperature.
Older methods of mixing include: 1) Constant recirculation of the mixture from the base of the autoclave to the top; 2) An impeller positioned inside autoclave; 3) Bubbling gas dispersed at the bottom of the autoclave through agitated oil present in the autoclave. Since its introduction, the Venturi jet mixer has replaced the older methods in most processes.
Most autoclaves present in industry are tall cylidrical vessels of 5-20 ton capacity designed to operate at pressures of 3 6 atm and temperatures of 100 180C. When designing an autoclave, it is important to remember between 74%/2% and 4%/1% hydrogen air forms an explosive mixture. The surrounding areas must be such that any vessel ventilation is dispersed quickly. Also, all equipment located inside the vessel must be flame proofed and no ignition sources can be present.
After leaving the autoclave, the oil is cooled and filtered. The hydrogenated oils have greater melting points than regular oils so separation is not a problem.
Below is a schematic of the process. 1 and 2 are inlet streams of oil and hydrogen, 3 is a fresh charge of catalysts, and 4 is a vacuum. 5 is the autoclave and is preceeded by 6, the Venturi mixer. 7 and 10 are pumps and 9 is a temporary storage tank.
Catalysts
The hydrogenation of edible oils is usually carried out using nickel catalysts with minor amounts of copper, alumina, etc. Powder catalyst composed of specially prepared metals in a finely divided form on an inert, highly porous, refractory material such as kieselguhr. The catalyst is heterogeneous, which means it is present in a phase separate from the reacting mixture. At higher temperatures and pressures, the solubility of hydrogen in oil is increased, thus increasing the rate of the reaction. Selectivity refers to the hydrogenation of the more unsaturated constituent, linoleic acid to oleic acid then oleic acid to stearic acid. Operating at temperatures of 170 180C increases the selectivity of the process. An increased rate results in lower selectivity because droplets of oil at the catalyst surface are much smaller than in less soluble conditions. As the catalyst becomes poisoned, selectivity is greater, but formation of the trans acid is promoted. The catalysts can become poisoned by impurities present in the system that are held to the catalysts surfaces until they saturate the active atoms.
There are numerous catalysts and catalysts preparations available for this process. One preparation provided by DeVine and Williams is the precipitation of nickel carbonate and hydroxide by the addition of an acqueous solution of nickel sulphate. A reduction of the dried precipitate at 450 550C follows this addition. The conditions of the precipitation are the most important factor determining the activity of the catalysts.
Alternatives are copper, which is less active but more selective; palladium; platinum; and rhodium.
Reference:
Bailey, Alton E. Industrial Oil and Fat Products. Interscience Publishers, Inc. New York, 1945.
Devine, J. and P. N. Williams. The Chemistry and Technology of Edible Oils and Fats. Pergamon Press. New York, 1961.
Hamilton, R. J. and A. Bhati. Recent Advances in Chemistry and Technology of Fats and Oils. Elsevier Applied Science. London, 1987.
Peterson, Robert J. Hydrogenation Catalysts. Noyes Data Corporation. Park Ridge, New Jersey, 1977.