Soaps are sodium or potassium salts of fatty acid. Common fatty acids used are oleic acid, stearic acid, palmitic acid, lauric acid and myristic acid . Soaps are used for human comfort, cleanliness and for industrial use.
Soap is a surface active agent or surfactant. The soap molecules contain both hydrophilic part and hydrophobic part. The hydrophilic part of soap is carboxylate head group and hydrophobic part is aliphatic chain. The dirt or grease is cleaned by key mechanism.
Most marketed bar soaps contains TiO2 as an opacifier or as a whitener. A variety of dyes are also used to produce colour soaps.
Fat splitting reaction:
Process flow sheet: Illustrated in Figure.
Figure 30.1 Flow sheet of manufacture of soaps, fatty acids and glycerin
30.2 Functional role of various processes (Figure 30.1)
- The fat and catalyst are mixed together and enter the hydrolyser.
- Hot water is added here.
- Intimate mixing is required for the water oil immiscible phase.
- Water has 10 – 15% solubility in oil and fats.
- Temperature is maintained at 230˚C - 250˚C and pressure at 40 – 45 atm for reaction between water and organic compound.
- The fatty acids stream is produced as top product while glycerin stream is produced as bottom product.
Fatty acid stream block
(p) Flash tank:
- Fatty acids from the hydrolyser enters the steam flash tank to remove water & concentrate the fatty acids.
(q) Vacuum still:
- The concentrated fatty acids enter a high vacuum still.
- Jet ejecter is used to generate vacuum in the still.
- Wastes are separated from the bottom.
- Fatty acid is also taken out as a marketable product.
- Fatty acid produced from vacuum still is now pumped to mixer.
- Base such as NaOH or KOH is mixed in appropriate proportions and mixed thoroughly.
- After mixing a viscous mass is produced.
- The viscous mass from the mixer is sent to the blender.
- Ingredients like scents, anti fungal & anti bacterial chemicals etc. is mixed in the blender.
- The blender removes the solid mass which is then sent to:
1. Chipping rolls: to manufacture soap strips.
2. Spray dryer: To get soap powder (used in manufacture of liquid soaps).
3. Bar Press Stock: To press and cut the viscous mass into bars of soap.
Glycerin stream block
(t) Ion Exchange:
- 15 – 20% glycerin along with impurities from the hydrolyser goes for ion exchange.
- Here salt and colour is removed from glycerin.
(u) Triple effect evaporator:
- The product stream from ion exchanger is sent to triple effect evaporator.
- The glycerin solution is concentrated in evaporator.
(v) Vacuum still:
- The glycerin obtained from vacuum still is called yellow glycerin. It is used for industrial use.
- The steam produced from evaporator is used in vacuum still to further concentrate the glycerin.
- For removing color, yellow glycerin is treated with activated carbon in mixer.
- Activated carbon is separated from the solution.
- 99% white glycerin is produced. The yield is 30 – 35kg per ton of soap produced.
Detergents have better surface tension lowering action than soaps. Due to excessive foaming, it is unable to reduce organic content of sewage effluent. Biodegradation of detergent is an important environmental factor. Detergents react with hard water ions. Detergents are of four types - anionic, cationic, nonionic and amphoteric. Most common type is anionic which is generally made up of sodium salts of an organic sulfate or sulfonate.
General method to make synthetic detergents:
Alkylbenzene + oleum→ alkylbenzene sulfonate
Tallow fatty alcohol + oleum →fatty alcohol sulfate
Sulfonate + sulfate + NaOH → sodium salts
Sodium salts + builders → Detergents
30.4 Technical Questions
1. Discuss about raw material of soap.
- Fatty constituents from refined tallow, coconut and palm oil, refined grease, byproducts of vegetable oil refining etc.
- In India, for the source of fatty acids mostly depends on imported tallow and palm oil.
- Coconut oil is the primary vegetable oil used for the manufacture of soaps.
2. Discuss the economy in soap manufacturing process.
- The vacuum still uses the steam from triple effect evaporator thus reducing the make up steam in the still.
- Good amount of glycerin is produced as a byproduct which is sold in market.
- Fatty acid is also taken out as a marketable product. These fatty acids find use in manufacturing cosmetics, beauty products etc.
3. Why is ion exchanger used?
Ans: 15 – 20% glycerin (sweet water) along with impurities from the hydrolyser is put to the successive beds of anion and cation exchange resin.
The glycerin reacts with salts. Ion exchange removes the colour and dissolved salts.
4. What is the difference between soft soaps and hard soaps?
- Sodium salts are called hard soap where as potassium salts are soft soaps.
- A hard soap is moderately soluble in water and does not lather easily whereas soft soap dissolve faster and lather readily.
- Soft soap performs better than hard soaps in cleansing action.
- Tallow, animal fats and coconut oils are the main sources of hard soaps, while linseed oil, castor oil etc produces soft soaps. Mahua and ground nut oil produce intermediate consistency soaps.