Oils, fats, soaps and detergents

Oil extraction, refining and hydrogenation, soap making with glycerol recovery, and LAS-based detergents, with saponification and iodine value calculations.

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Why it matters

India is one of the largest consumers and importers of edible oils, and soaps and detergents are everyday products made at very large scale. The industry links mechanical separation (pressing, solvent extraction), refining by physical and chemical steps, catalytic hydrogenation, and surfactant chemistry. Simple oil analyses (saponification, iodine and acid values) decide how much alkali, hydrogen or refining an oil needs.

Key ideas

Oils and fats are triglycerides: esters of glycerol with three fatty acids. Saturated chains (palmitic C16:0, stearic C18:0) pack well and give solid fats; unsaturated chains (oleic C18:1, linoleic C18:2, linolenic C18:3) have cis double bonds that kink the chain and give liquid oils.

Recovery of oil.

  • Mechanical expression in screw presses (expellers) leaves about 5–8 % oil in the cake.
  • Solvent extraction of flakes or press cake with hexane in continuous percolation extractors leaves under 1 % oil. Solvent is recovered from miscella by distillation and stripping, and from the meal in a desolventiser–toaster. Hexane is flammable, so plants are designed for explosion safety.

Refining.

  1. Degumming: water or phosphoric acid removes phospholipids (gums, which give lecithin).
  2. Neutralisation: caustic soda converts free fatty acids (FFA) to soapstock, separated by centrifuge. Physical refining instead strips FFA by steam distillation.
  3. Bleaching: activated bleaching earth adsorbs pigments and traces of soap and metals.
  4. Deodorisation: high-vacuum steam stripping at about 220–260 °C removes odour compounds and remaining FFA.

Hydrogenation (hardening). H₂ is added to C=C double bonds over a nickel catalyst at about 150–200 °C and a few bar in a stirred slurry reactor. It raises the melting point (vanaspati, shortening, margarine) and oxidative stability. Partial hydrogenation also isomerises some cis bonds to trans; trans fats are a health concern, so modern practice favours full hydrogenation plus interesterification or blending. The reaction is limited by H₂ mass transfer into the oil, so agitation and pressure matter.

Soap. Saponification: triglyceride + 3NaOH → glycerol + 3 RCOONa. It is base-promoted, not catalysed: alkali is consumed.

  • Kettle (full-boiled) process: boil fat with lye, "grain out" soap with salt, separate spent lye containing glycerol.
  • Continuous routes: fat splitting by high-pressure hydrolysis (about 250 °C, 50 bar, counter-current water) gives fatty acids and sweet water (glycerol), then fatty acids are neutralised.
  • Glycerol is recovered from spent lye or sweet water by evaporation and vacuum distillation; it is a valuable by-product. Sodium soaps are hard (bars); potassium soaps are soft (liquid soaps, shaving creams). In hard water, soap forms insoluble calcium and magnesium salts (scum); in acid it reverts to free fatty acids.

Synthetic detergents. The main surfactant is linear alkylbenzene sulphonate (LAS): linear alkylbenzene (LAB) is sulphonated with SO₃ in falling-film reactors and neutralised with NaOH. Other surfactants: fatty alcohol sulphates and ether sulphates (anionic), ethoxylates (non-ionic), quaternary ammonium (cationic, fabric softeners). Calcium salts of sulphonates are soluble, so detergents work in hard water. Powders are made by spray drying a slurry, or by agglomeration; they contain builders (zeolite, sodium carbonate, formerly STPP, which causes eutrophication), anti-redeposition agents (CMC), enzymes, optical brighteners and perfume. Branched alkylbenzene sulphonate was replaced by LAS because it did not biodegrade.

Formulas

(RCOO)₃C₃H₅ + 3NaOH → C₃H₅(OH)₃ + 3RCOONa

Saponification value (SV) = mg KOH to saponify 1 g of fat = V·M·56.11 / m

  • V: volume of KOH (mL), M: molarity (mol/L), m: sample mass (g). Typical SV 185–200.

Average molar mass of triglyceride ≈ 3 × 56 110 / SV (g/mol)

NaOH needed (kg) = SV × m_fat (kg) × 40.00 / 56.11 / 1000

Iodine value (IV) = g I₂ absorbed by 100 g of fat; each mole of I₂ corresponds to one C=C bond, i.e. one mole of H₂.

H₂ (kmol) = ΔIV × m_fat (kg) / (100 × 253.81)

  • ΔIV: drop in iodine value on hydrogenation; 253.81 = molar mass of I₂.

Acid value = mg KOH to neutralise the FFA in 1 g of fat (≈ 2 × % FFA as oleic acid).

Worked examples

Example 1 (standard): alkali and glycerol for soap making. 1000 kg of oil has SV 190 mg KOH/g. Find the NaOH needed, the average molar mass of the oil and the glycerol released.

  1. KOH equivalent = 190 mg/g × 10⁶ g = 190 kg KOH = 190 / 56.11 = 3.386 kmol.
  2. NaOH = 3.386 × 40.00 = 135.4 kg.
  3. Triglyceride moles = 3.386 / 3 = 1.129 kmol, so average molar mass = 1000 / 1.129 = 886 kg/kmol (check: 3 × 56 110 / 190 = 886).
  4. Glycerol = 1.129 × 92.09 = 103.9 kg.

Answer: 135 kg NaOH, M ≈ 886 g/mol, about 104 kg glycerol.

Example 2 (GATE level): hydrogen for hardening. Soybean oil with IV 130 is hydrogenated to IV 70. Find the hydrogen consumed per tonne of oil, in kg and in m³ at 0 °C and 1 atm.

  1. ΔIV = 60 g I₂ per 100 g oil, so I₂ equivalent = 0.60 × 1000 kg = 600 kg I₂.
  2. Moles = 600 / 253.81 = 2.364 kmol of C=C saturated = 2.364 kmol H₂.
  3. Mass = 2.364 × 2.016 = 4.77 kg.
  4. Volume = 2.364 × 22.414 = 53.0 m³.

Answer: about 4.8 kg (53 m³ at STP) of H₂ per tonne of oil.

Common mistakes

  • Calling saponification base-catalysed; the base is consumed.
  • Reporting SV in g KOH/g instead of mg KOH/g.
  • Using 1 mol NaOH per mol triglyceride; it is 3.
  • Forgetting that IV refers to 100 g of fat, not 1 g.
  • Thinking hydrogenation only saturates bonds; partial hydrogenation also creates trans isomers.
  • Saying detergents work in hard water because they have a hydrophilic head and hydrophobic tail. Soaps do too; the difference is that calcium salts of sulphonates are soluble.

For GATE CH

Expect sequence questions on oil extraction and refining, the catalyst and conditions for hydrogenation, kettle vs continuous soap processes and glycerol recovery, LAS manufacture and detergent builders, and why detergents work in hard water. Numericals: SV, IV and acid value calculations, alkali requirement, glycerol yield and hydrogen consumption.

Quick check

  1. Which solvent is used for oilseed extraction?
  2. What does a high iodine value indicate?
  3. Name the catalyst used for hydrogenation of oils.
  4. What by-product is recovered from spent lye?

Answers: 1. hexane; 2. a high degree of unsaturation (more C=C bonds); 3. nickel; 4. glycerol.

Interview questions

All Chemical Technology interview questions

Try answering each one aloud before you open it.

  1. 1.What are oils and fats, and how do they differ chemically?Concept

    Oils and fats are both triglycerides, which are esters derived from glycerol and three fatty acids. The primary difference between them is their physical state at room temperature: oils are liquid, while fats are solid. This difference is due to the degree of saturation of the fatty acids; oils typically contain more unsaturated fatty acids, which have double bonds that prevent tight packing, whereas fats contain more saturated fatty acids, which have no double bonds and can pack closely together.

  2. 2.Explain the process of saponification.Concept

    Saponification is the alkaline hydrolysis of a triglyceride: one mole of fat reacts with three moles of NaOH (or KOH) to give one mole of glycerol and three moles of fatty-acid salts, which are soap. The hydroxide is consumed, so the reaction is base-promoted rather than base-catalysed. Industrially it is done in kettles with lye followed by salting out, or continuously by high-pressure fat splitting and neutralisation, with glycerol recovered from the spent lye.

  3. 3.What is the role of surfactants in detergents?Concept

    Surfactants are active agents in detergents that reduce the surface tension of water, allowing it to spread and wet surfaces more effectively. They have a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail, which enables them to emulsify oils and fats, suspending them in water and allowing them to be washed away. This property is crucial for the cleaning action of detergents.

  4. 4.Why are detergents preferred over soaps in hard water conditions?Application

    Detergents are preferred over soaps in hard water because they do not form insoluble precipitates with calcium and magnesium ions present in hard water. Soaps react with these ions to form soap scum, which reduces their effectiveness. Detergents, on the other hand, are formulated to remain soluble and effective in hard water, maintaining their cleaning power.

  5. 5.What happens if a soap solution is added to an acidic solution?Application

    When a soap solution is added to an acidic solution, the fatty acid salts in the soap can react with the acid to form free fatty acids. This reaction reduces the soap's effectiveness as a cleaning agent because free fatty acids do not have the same surfactant properties as the soap salts. The soap may precipitate out of the solution, reducing its cleaning ability.

  6. 6.Explain why hydrogenation is used in the processing of oils.Application

    Hydrogenation is used in the processing of oils to convert unsaturated fats to saturated fats by adding hydrogen atoms to the double bonds. This process increases the melting point of the oil, turning it into a solid or semi-solid form, which is desirable for certain food products like margarine and shortening. Hydrogenation also improves the shelf life and stability of the oil by reducing its susceptibility to oxidation.

  7. 7.What is the iodine value of an oil, and what does it indicate?Concept

    The iodine value of an oil is a measure of the degree of unsaturation in the fatty acids present in the oil. It is defined as the number of grams of iodine that can react with 100 grams of the oil. A higher iodine value indicates a higher degree of unsaturation, meaning the oil contains more double bonds. This value is important for determining the drying properties of oils and their stability against oxidation.

  8. 8.Calculate the saponification value of a fat if 2 g of the fat require 14.2 mL of 0.5 M KOH for complete saponification.Numerical

    Saponification value is mg KOH per g of fat: SV = V x M x 56.11 / m = 14.2 mL x 0.5 mol/L x 56.11 g/mol / 2 g = 199.2 mg KOH/g. (mL x mol/L gives mmol, and mmol x g/mol gives mg.) This is typical of common oils and corresponds to an average triglyceride molar mass of about 3 x 56 110 / 199.2 = 845 g/mol.

  9. 9.What are the environmental impacts of using synthetic detergents?Application

    Synthetic detergents can have several environmental impacts. They often contain phosphates, which can lead to eutrophication in water bodies, causing excessive growth of algae and depletion of oxygen. Some detergents are not biodegradable, leading to accumulation in the environment and potential harm to aquatic life. Additionally, the production and disposal of detergents can contribute to pollution and resource depletion.

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