Soaps, detergents and agrochemicals: Fats and oils: the renewable raw material

1.1 A fat is three fatty acids on a glycerine backbone, and the chains decide its use

More than 95% of the weight of most extracted fats is triglyceride. Kent describes it as glycerol, an alcohol with three OH groups, joined by ester links to three fatty acids. An ester link is what forms when an acid and an alcohol combine and give off a molecule of water. Picture a capital E: the upright is the glycerine and each arm is a fatty acid chain.

Two features of each chain matter. The first is length. Lauric acid (12 carbons) and myristic acid (14) dominate coconut and palm kernel oil, the so-called lauric oils. Palmitic (16), stearic (18) and oleic (18 carbons with one double bond) dominate palm oil, tallow and most seed oils. The second is the number of carbon-carbon double bonds, called unsaturation. Each double bond kinks the chain, so the molecules pack less tightly and melt lower. Dryden gives the series: stearic acid, with no double bond, melts at 69 C; oleic, with one, at 14 C; linoleic, with two, at -5 C; linolenic, with three, at -11 C. The same double bonds react with oxygen, which dries linseed paints and turns food rancid.

The trade measures these properties with two numbers. The iodine value (IV) rises with unsaturation. The saponification value (SV), in milligrams of potassium hydroxide per gram, rises as chains get shorter, because a gram of short chains holds more ester links. Kent's table gives coconut fatty acids an SV of 270 and an IV of 10, and tallow 205 and 55.

For a soapmaker this profile is the whole story. Kent explains that C10-C12 chains give quick, voluminous lather and C16-C18 chains a creamier, longer-lasting foam. A standard toilet bar therefore uses 80-90% tallow with 10-20% lauric oil, and a high-lathering bar 60-75% tallow with 25-40% lauric oil.

• Kent (2012), Ch 34 Animal and Vegetable Fats, Oils, and Waxes · pdf pp. 1333-1337
• Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf p. 1433
• Dryden (1997), IIIA Edible and Essential Oils · pdf p. 297
• Shreve (1984), Ch 29 Soap and Detergents · pdf p. 566

1.2 Most oil is eaten, so oleochemicals ride on a food market

Fats and oils are huge commodities. Wittcoff puts world production in 2000 at 110.8 million tonnes, similar to the volume of propylene or benzene. The difference is where it goes: 88% is eaten, 6% of poorer grades goes to animal feed, and only about 6% is used by the oleochemical industry, the branch that turns fats into Fatty acids, Fatty alcohols, Glycerine and their derivatives. Kent's 2010/11 figures show palm oil (47.5 million tonnes) ahead of soybean oil (42.0 million).

Palm has grown fastest for an agronomic reason. Wittcoff gives yields of about 4,000 kg of oil per hectare for oil palm, against 875 for groundnut, 675 for rapeseed, 355 for coconut and 350 for soybean. Palm is also the cheapest edible fat, and at 54.7% of exported oil it is the most traded (Kent).

India is both a large grower and a large importer. Dryden reports oilseeds on 19-20 million hectares, about 11% of cultivated land, and about 20 million tonnes of oilseeds in 1992-93, giving about 6 million tonnes of edible oil. Only about 10% of vegetable oil went to soaps and detergents, against 57% to liquid cooking oil and 19% to vanaspati, the hydrogenated cooking fat. Tallow is not allowed in Indian toilet soap, so soapmakers compete with kitchens for the same oils and fill the gap with imported palm products and distilled fatty acids.

The consequence is that a soap or detergent maker using natural raw materials is a small buyer in a food market. Wittcoff's February 2003 figures show the spread: Malaysian palm oil cost $698 a tonne to produce and deliver to the United States, US soybean oil $489 and European rapeseed oil $750, against a US market price of about $560.

• Wittcoff (2004), Ch 13 Fats and Oils · pdf pp. 438-439
• Kent (2012), Ch 34 Animal and Vegetable Fats, Oils, and Waxes · pdf p. 1347
• Dryden (1997), IIIA Edible and Essential Oils · pdf pp. 295-298, 304-305
• Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf p. 311

1.3 Refining and hydrogenation turn a crude oil into a predictable feedstock

Crude oil from a press or a hexane extractor carries phosphatides (gums), free fatty acids, colour bodies, trace metals and odours. A modern press leaves 3-5% of the oil in the cake and solvent extraction under 1% (Wittcoff).

Refining runs in a fixed order, each step protecting the next. Degumming with water or a little phosphoric or citric acid removes phosphatides; left in, Kent says, they foul bleaching earth and poison hydrogenation catalyst. Neutralisation with caustic soda turns free fatty acids into soap, which is centrifuged off as soapstock, or "foots", and later acidified back to fatty acids. Bleaching stirs the oil with 0.1-2% activated clay, which adsorbs chlorophyll, traces of soap and metals. Deodorisation is steam stripping under deep vacuum: Kent gives 250-260 C, 2-4 mbar, 0.5-2% sparge steam and 20-40 minutes, leaving 0.03-0.05% free fatty acid. Palm oil, low in gums, skips caustic neutralisation and loses its free acids in the deodoriser instead, a route called physical refining.

Hydrogenation adds hydrogen across double bonds over a nickel catalyst. It raises the melting point and slows oxidation. Dryden describes Indian practice for vanaspati: batch reactors of 5-30 tonnes on 1-3 hour cycles, with nickel slurried at 5-15 kg per tonne of oil and mostly filtered out for reuse. Conditions steer the product. Low pressure (1-2 atm gauge) and high temperature (135-180 C) give a butter-like ghee base; higher pressure (2-3.5 atm) and lower temperature (120-160 C) give a firmer, more stable shortening. The reaction releases only mild heat, but sulphur compounds in the hydrogen poison the catalyst and hydrogen's wide explosive range (4.1-74.2%) demands leak-tested plant. Partial hydrogenation also twists some double bonds into the trans form: Kent reports about 50% trans isomers in margarine bases taken down to an IV of about 70.

• Wittcoff (2004), Ch 13 Fats and Oils · pdf pp. 441-442
• Kent (2012), Ch 34 Animal and Vegetable Fats, Oils, and Waxes · pdf pp. 1363-1372, 1377
• Dryden (1997), IIIA Edible and Essential Oils · pdf pp. 299-303
• Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf p. 1444

See it in the table

Check your understanding

Why does a high-lathering toilet bar use more coconut or palm kernel oil than a standard bar?

A refinery sends soybean oil to the hydrogenator without degumming. What is the most likely result?

Using Wittcoff's yields, roughly how many hectares of soybean give the same oil as one hectare of oil palm?