# Soaps, detergents and agrochemicals: Making soap and recovering glycerine Source page: https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p2/ [Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 9: Soaps, detergents and agrochemicals](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/) Part 2 of 5 ## Making soap and recovering glycerine Soap making is one reaction, alkali plus fat, run in very different equipment. The old kettle took days and experienced operators; a continuous plant does the same in hours under instrument control. The choice of route also decides how much glycerine, the valuable co-product, is recovered. This part works through the chemistry, the kettle and continuous routes, fat splitting and the glycerine plant. ### 2.1 Saponification: a fat plus caustic soda gives soap and glycerine Saponification is the reaction of a triglyceride with an alkali: triglyceride + 3NaOH → 3RCOONa + glycerine. RCOONa is soap, the sodium salt of a fatty acid. Its long hydrocarbon tail dissolves in grease and its charged head dissolves in water, which is how it lifts dirt. Two practical problems shape every soap plant. The first is that fat and caustic solution do not mix, like oil and vinegar. Kent explains that high-shear mixing, high temperature and enough pressure to stop boiling raise the rate. The reaction also speeds itself up: soap is an emulsifier, so the more soap already present, the finer the droplets and the faster the rest reacts. Continuous reactors exploit this by recirculating 13-25 times as much reacted mass as fresh feed. The second problem is separating the soap from glycerine and excess alkali. Soapmakers do this with salt: the right brine strength splits the mass into a soap layer and a heavier salt solution, the lye, which carries glycerine and impurities. Kent gives the densities as about 0.97 g/cc for neat soap and 1.05 for spent lye. The gap is small, so separators need time and a centrifuge often follows. The caustic dose follows from the saponification value. Kent gives 14.09 g of NaOH per 100 g of beef tallow and 18.32 g for coconut oil, whose shorter chains carry more ester links per kilogram. With 50% caustic, 100 g of tallow needs 28.2 g of solution. Neat soap leaves the process at about 30% water. Vacuum spray dryers take it to 8-18% moisture as pellets of 70-82% total fatty matter. Finishing lines add perfume and colour, mill, extrude under vacuum, cut and stamp the bars. Process flowchart Continuous saponification of neutral fat Inputs **Refined fat blend**1.1 t per t anhydrous soap **Caustic soda 50%**0.3 t **Brine and water**to hold the neat soap phase **Steam**1.5 t Process steps 1 **Saponification reactor** Pressurised loop with high-shear pump; recycle ratio 13-25; fat almost fully saponified 2 **Cooling** Mixer cooler, or flash cooler at about 0.5 atm holding 85-92 C 3 **Static separator** Neat soap (0.97 g/cc) over spent lye (1.05 g/cc); 15-25% lye stays entrained 4 **Glycerine extraction column** Fresh lye falls through rising soap and washes out glycerine; column lye returns to reactor 5 **Centrifuge** Removes entrained lye, lowering electrolyte and moisture 6 **Neutraliser** 80-90 C, at or below 1 bar; trims residual free alkali 7 **Vacuum spray dryer** Soap heated to 135-155 C, sprayed into a chamber at 40-60 mmHg; 30-35% to 8-18% moisture Outputs **Soap pellets**1 t, 70-82% total fatty matter **Spent lye to glycerine plant**over 40% glycerine possible with flash cooling Cooling, separation, extraction and centrifuging exist only to remove by-product glycerine. Source: Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents, pdf pp. 1434-1440; Dryden (1997) pdf p. 311 Worked example #### Check one book's recipe against another's chemistry Dryden's inputs per tonne of anhydrous soap include 1.1 t of fat, 0.3 t of 50% caustic soda and 1.5 t of steam. The caustic contains 0.15 t of NaOH. Kent's 14.09 kg NaOH per 100 kg of tallow, applied to 1.1 t of fat, gives 1.1 x 0.1409 = 0.155 t. Glycerine follows from the same numbers: 155 kg of NaOH is 3.9 kmol, which frees 1.3 kmol of glycerine (molecular weight 92.1), about 119 kg, or roughly 11% of the fat charged. - Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1431-1440 - Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 563-566 - Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf p. 311 ### 2.2 Kettle or continuous: a trade between fat quality, labour and glycerine The kettle is the traditional batch route. Fat is saponified with caustic in a large boiling vessel, and the soap is kept in contact with lye, which carries away glycerine and water-soluble impurities (Kent). A final "fitting" step separates a small quantity of lower-quality soap. Shreve says the whole sequence takes more than a week, and that by the 1980s the kettle survived mainly in smaller factories. Continuous plants won on almost every count. Shreve reports that a computer-controlled continuous saponifier makes in 2 hours what batch methods make in 2 to 5 days, at more than 300 tonnes a day. Kent lists the reasons large plants switched: savings in utilities, simpler operator training, no operator bias in control, and a more consistent soap base. Dryden adds higher glycerine recovery (over 80%), less discoloured product, and less space and manpower. There is a price. In the kettle, fitting drops 1-2% of poorer "slurry soap", which takes colour and odour bodies with it. A continuous plant has no such step, so colour components equal to roughly 5-10% of a kettle charge stay in the product, and Kent warns that fats for continuous saponification must start with better natural colour and odour. In effect the continuous route swaps labour and steam for better feedstock and instruments. Scale varies widely. Dryden gives Indian soap plants of only 2-15 tonnes a day and notes that much Indian soap was made in villages by crude methods, with no glycerine recovered at all. At that scale a recovery plant cannot pay for itself. Data Routes to soap and fatty acids compared | Route | Conditions | Time | Conversion or recovery | Main trade-off | |---|---|---|---|---| | Kettle, full boiled | Batch boiling; soap washed with lye; final fitting | Several days; over a week in all | Fitting drops 1-2% slurry soap with colour bodies | Tolerates poorer fats; heavy on labour and steam | | Continuous saponification | Pressurised loop reactor, recycle 13-25 | About 2 h; plants over 300 t/day | Glycerine recovery over 80% | Needs better fats and frequent analysis | | Twitchell splitting | 100-105 C, sulphonic acid catalyst | 12-48 h (Kent: 8-24 h) | 85-98% split | Cheap equipment; dark acids; batch | | Autoclave splitting | 150-175 C with ZnO, or 240 C without; 2.9-10 MPa | 5-10 h (2-4 h at 240 C) | 85-98% split | Suits small scale; batch | | Continuous countercurrent splitting | About 250 C, 4.1-4.9 MPa (Kent: up to 260 C, 60 bar) | 2-3 h | 97-99% split | High first cost; uniform acids, 10-25% glycerine water | | Fatty acid neutralisation | 85-98 C, high-shear mixer | Almost instantaneous | Single-phase neat soap | Low capital; needs distilled fatty acids | Source: Shreve (1984), Ch 29 Soap and Detergents, pdf pp. 554, 563-566; Kent (2012) Ch 36 pdf pp. 1434-1447 Case #### India's small soapmakers and the central fat splitter Dryden describes a structural problem in Indian soap. Laundry soap manufacture was reserved for small-scale industry, and many units were far too small to recover glycerine, so the co-product was lost. The proposed remedy was central fat-splitting plants: a large plant would hydrolyse fat, recover glycerine at an economic throughput and ship fatty acids to small soapmakers, who would then only need the simple neutralisation step. - Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 563-566 - Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1431-1435, 1440 - Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf pp. 311-313, 318 ### 2.3 Splitting the fat first gives fatty acids, sweet water and a simpler soap plant Splitting is hydrolysis: the fat reacts with water instead of caustic, triglyceride + 3H2O → 3RCOOH + glycerine. It is an equilibrium, so the trick is to keep removing glycerine. In the continuous countercurrent splitter that Shreve and Kent describe, a stainless steel tower about 20 m high and 60 cm across runs at around 250 C and 4-6 MPa. Fat enters at the bottom and rises; hot demineralised water enters at the top and falls. The falling water collects glycerine and leaves as "sweet water" with 10-25% glycerine, while fatty acids leave the top with 97-99% of the fat split. The crude acids are then distilled under high vacuum, about 10 mbar with live steam, because they decompose if boiled at atmospheric pressure. Kent's typical still cut is 1-5% light ends, 85-91% refined product and 7-11% residue, or pitch. Fractional distillation separates chain lengths, giving for example 99.5% pure C12 from coconut. It cannot separate stearic from oleic acid, which boil too close together. Their different melting points are used instead, by pressing, solvent crystallisation or hydrophilisation. Neutralising the distilled acids is close to trivial: RCOOH + NaOH → RCOONa + H2O, essentially instantaneous in a high-shear mixer at 85-98 C. Some plants neutralise 70-85% of the acid with soda ash and finish with caustic; the carbon dioxide released makes the paste very viscous, so this only pays where soda ash is cheaper or purer. The same splitter feeds the wider market for Fatty acids, about 5.7 billion lb (2.6 million tonnes) worldwide in 2000 according to Wittcoff. Fatty acids or their methyl esters are also hydrogenated at 200-300 C and 200-300 bar over copper chromite to make Fatty alcohols, the raw material for many detergent surfactants. Process flowchart Continuous fat splitting, distillation and neutralisation Inputs **Fat blend**with optional zinc oxide catalyst **Demineralised water**fed countercurrent **High-pressure steam**to reach about 250 C **Caustic soda 50%**for neutralisation Process steps 1 **Deaeration** Vacuum removes air that would darken the acids 2 **Hydrolyser tower** About 250 C, 4.1 MPa, 20 m tall; fat rises, water falls; 97-99% split 3 **Flash drying** Water flashed off the fatty acid phase 4 **Vacuum still** About 10 mbar with live steam: 1-5% heads, 85-91% product, 7-11% pitch 5 **Mixer-neutraliser** High shear, 85-98 C; neat soap at about 93 C with about 30% water 6 **Finishing** Extrude, mill or spray dry; cut, stamp and wrap Outputs **Bar soap or soap base**- **Sweet water**10-25% glycerine, to evaporators **Distilled fatty acids**for sale or fatty alcohols **Pitch**7-11% of crude acid Source: Shreve (1984), Ch 29 Soap and Detergents, pdf pp. 555, 564-566; Kent (2012) Ch 36 pdf pp. 1440-1449 In the data #### Every point of conversion is saleable product Kent compares hydrolysis routes: Twitchell batch splitting takes 8-24 hours for 95% conversion, continuous splitting 1-3 hours for 98-99%, and enzyme splitting with lipase 72 hours for 98% at only 35-37 C. Fat left unsplit ends up in the still bottoms. With fat dominating cost, 95% versus 99% conversion is a margin item, not a technical detail. - Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 553-557, 564-565 - Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1440-1450, 1454 - Wittcoff (2004), Ch 13 Fats and Oils · pdf pp. 442, 451 ### 2.4 Glycerine is worth recovering, but its supply follows soap and biodiesel Every route from fat releases glycerine, about one molecule for every three fatty acids. Recovering it means removing water and salt. Sweet water from a splitter is the easy case. Shreve describes the Procter & Gamble scheme: sweet water at about 12% glycerine passes through a triple-effect evaporator, where vapour from one stage boils the next, and reaches 75-80% glycerine, usually with no heat beyond what the hot sweet water already carries. The crude is settled hot for 48 hours to drop fatty impurities, giving about 78% glycerine. It is then distilled under vacuum (8 kPa, about 200 C) with a little caustic, and condensed in three stages; the first gives 99% chemically pure glycerine. Carbon bleaching and filtration or ion exchange finish it. Spent lye from saponification is harder because it is loaded with salt, which comes out in the evaporators and must be removed constantly. Glycerine has many modest uses rather than one big one. Dryden's pattern puts alkyd resins and plastics at 35%, tobacco humidification 15%, cellophane 12%, explosives 10% and food and pharmaceuticals 10%. The supply side is the weak point. Natural glycerine is made because soap and fatty acids are made, not because anyone ordered more glycerine. As detergents replaced soap, supply lagged demand, and from about 1948 Shell's route from propylene via allyl chloride and epichlorohydrin filled the gap; Shreve puts synthetic glycerine at about 40% of the market, and Dryden's table shows the world synthetic share falling to 30% by 1988. Wittcoff shows the opposite risk: Procter & Gamble's fat substitute Olestra, made from methyl esters, could have caused a 50% world oversupply of glycerine had it succeeded, and biodiesel from methanolysis raises the same disposal problem. Process flowchart Glycerine recovery from hydrolyser sweet water Inputs **Sweet water**about 12% glycerine **Steam**mainly for distillation **Caustic soda**small dose to still feed **Activated carbon**for bleaching Process steps 1 **Triple-effect evaporator** 12% to 75-80% glycerine, using heat in the sweet water 2 **Settling tank** 48 h hot; crude at about 78% glycerine, 0.2% fatty acids, 22% water 3 **Vacuum still** About 8 kPa and 200 C 4 **Three-stage condensers** First stage gives 99% CP glycerine; lower grades condense later 5 **Bleaching and polishing** Activated carbon, then filtration or ion exchange Outputs **Refined glycerine**95-99% glycerol (CP and USP grades) **High-gravity and yellow distilled grades**industrial uses **Still residue**- Source: Shreve (1984), Ch 29 Soap and Detergents, pdf pp. 568-569 Common misreading #### "Glycerine is a free bonus" It is valuable but not free. A tonne of tallow soap carries roughly 110-120 kg of glycerine potential (worked example above), but in spent lye it is dilute and mixed with salt. Recovery needs evaporators, a vacuum still, steam and capital. Dryden notes that village soapmakers recovered none, and that US synthetic glycerine plants ran at 33-100 tonnes of glycerine a day, larger than a typical Indian soap plant's entire output. - Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 567-570 - Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf pp. 316-318 - Wittcoff (2004), Ch 13 Fats and Oils · pdf pp. 454, 456-457 ### See it in the table [H₂O Water](https://www.materialinsights.org/commodities/water/)[Cu Copper](https://www.materialinsights.org/commodities/copper/)[St Steel](https://www.materialinsights.org/commodities/steel/)[NaCl Salt](https://www.materialinsights.org/commodities/salt/)[Sda Soda ash](https://www.materialinsights.org/commodities/soda_ash/)[Cnt Coconut oil](https://www.materialinsights.org/commodities/coconut/)[Tb Tobacco](https://www.materialinsights.org/commodities/tobacco/)[Bf Beef](https://www.materialinsights.org/commodities/beef/)[NaOH Caustic soda](https://www.materialinsights.org/chemicals/caustic_soda/)[CO2 Carbon dioxide](https://www.materialinsights.org/chemicals/carbon_dioxide/)[C3H6 Propylene](https://www.materialinsights.org/chemicals/propylene/)[Srf Surfactants](https://www.materialinsights.org/chemicals/surfactants/)[FA Fatty acids](https://www.materialinsights.org/chemicals/fatty_acids/)[FAl Fatty alcohols](https://www.materialinsights.org/chemicals/fatty_alcohols/) ### Check your understanding Why does a continuous saponification reactor recirculate 13-25 times as much reacted mass as fresh feed? Kent calls the reaction autocatalytic in effect: soap emulsifies the immiscible reactants, so a large mass of recycled soap gives fine droplets and rapid saponification. Kent counts seven steps for neutral fat saponification but three for fatty acid neutralisation. What explains most of the difference? Cooling, static separation, glycerine extraction and centrifuging all deal with glycerine. Neutralising pre-split, distilled acids forms a single neat soap phase with no glycerine to remove. Why did synthetic glycerine from propylene become important from about 1948? Shreve explains that glycerine was a co-product of soap, so supply depended on soap; demand growth, and soap's loss of share to detergents, made a dedicated synthetic route worthwhile. [← Part 1: Fats and oils: the renewable raw material](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p1/)[Mark read · Part 3: Synthetic detergents: surfactants, formulation and manufacture →](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p3/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/index.md) - [Fats and oils: the renewable raw material](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p1/index.md) - [Synthetic detergents: surfactants, formulation and manufacture](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p3/index.md) - [Agrochemicals: classes, modes of action and discovery](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p4/index.md) - [Making, formulating and selling pesticides](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/review/index.md)