Wednesday, January 6, 2010

Soap Making

Soap comprises the sodium or potassium salts of various fatty acids, but chiefly or oleic, stearic, palmitic, lauric, and myristic acids. For generations its use has increased until its manufacture has become an industry essential t the comfort and health of civilized human beings.

Raw Materials
Tallow is the principal fatty material in soap making; the quantities used represent about three-fourths of the total oils and fats consumed by the soap industry. It contains the mixed glycerides obtained from the solid fat of cattle by stream rendering. This solid fat is digested with steam, the tallow forms a layer above the water, so that it can easily be removed. Tallow is usually mixed with coconut oil in the soap kettle or hydrolyser in order to increase the solubility of the soap. Greases (about 20 percent) are the second most important raw material in soap making. They are obtained from hogs and smaller domestic animals and are an important source of glycerides of fatty acids. They are refined by stream rendering or by solvent extraction and are seldom used without being blended with other fats. In some cases, they are treated so as to free their fatty acids, which are used in soap instead of the grease itself. It contains large proportions of the very desirable glycerides of lauric acid and myristic acids. Free fatty acids are utilized in soap, detergent, cosmetic, paint, textile, and many other industries. The acidification of “foots,” or stock resulting from alkaline refining of oils, also produces fatty acids. The Twitchell process is the oldest, continuous countercurrent process are now most commonly used.

The soapmaker is also a large consumer of chemicals, especially caustic soda, salt, soda ash, and caustic potash, as well as sodium silicate, sodium bicarbonate, and trisodium phosphate. Inorganic chemical added to the soap are the so-called builders. Important work by Harris of Monsanto and his coworkers demonstrated conclusively that, in particular, tetrasodium pyrophosphate and sodium tripolyphosphate were unusually effective synergistic soap builders. Of considerable economic importance was the demonstration that combination of inexpensive builders, such as soda ash, with the more effective (and expensive) tetrasodium pyrophosphate or sodium tripolyphosphate, were sometimes superior to the phosphate used alone. It was further shown that less soap could be used in these mixtures to attain the same or more effective soil removal.

Manufacture
The long established kettle process, however, is mainly used by smaller factories of for special and limited production. As soap technology changed, continuous alkaline saponification was introduced. Computer control allow an automated plant for continuous saponification by NaOH of oils and fats to produce in 2 h the same amount of soap (more than 300 t/day) made in 2 to 5 days by traditional batch methods.

The basic chemical reaction in making of soap is saponification.

3 NaOH + (C17H35COO)3C3H5 ――→ 3 C17H35COONa + C3H5(OH)5
Casutic Soda Glyceryl stearate Sodium Stearate Glycerin

The procedure is to split, or hydrolyze, the fat, and then, after separation from the valuable glycerin, to neutralize the fatty acids with a caustic soda solution:

(C17H35COO)3C3H5 + 3 H2O ――→ 3 C17H35COOH + C3H5(OH)5
Glyceryl Stearate Stearic Acid Glycerin

C17H35COOH + NaOH ――→ C17H35COONa + H2O
Stearic Acid Caustic Soda Sodium Stearate

The usual fats and oils of commerce are composed of the glyceride of any one fatty acid, but of a mixture. However, some individual fatty acids of 90% purity or better are available from special processing. Since the solubility and hardness of sodium salts of various fatty acids differ considerably, the soap maker chooses the raw material according to the properties desired, with due consideration of the market price.

In continuous, countercurrent splitting the fatty oil is deaerated under a vacuum to prevent darkening by oxidation during processing. It is charged at a controlled rate to the bottom of the hydrolyzing tower through a sparge ring, which breaks the fat into droplets. To cook the oil can use of stainless steel tank with about 20 m in high and 60 cm in diameter. The oil in the bottom contacting section rises because of its lower density and extracts the small amount of fatty material dissolved in the aqueous glycerin phase. At the same time deaerated, demineralized water is fed to the top contacting section, where it extracts the glycerin dissolved in the fatty phase. After leaving the contacting sections, the two streams enter the reaction zone. Here they are brought to reaction temperature by the direct injection of high pressure steam, and then the final phase of splitting occur. The fatty acid are discharged from the top of the splitter or hydrolyzer to a decanter, where the entrained water is separated or flashed off. The glycerin-water solution is then discharged from the bottom of an automatic interface controller to a settling tank.

Although the crude mixtures of fatty acids resulting from any of the above methods may be used as such, usually a separation into more useful components is made. The composition of the fatty acids from the splitter depends upon the fat or oil from they were derived. Those most commonly used for fatty acid production include beef tallow and coconut, palm, cottonseed, and soybean oil. Probably the most used of the older processes is panning and pressing.

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