Glycerine Recovery From Spent Soap Lye

Project and Seminar Material for Chemical Engineering

Glycerine Recovery From Spent Soap Lye


The recovery of glycerine from spent soap lye has been done using soap lye samples obtained from the hot process of soap production using palm kernel oil (P.K.O). The main advantage of hot process is that exact concentration of the lye solution is not known, but to perform this with adequate success, the lye and fat are boiled together at 80 – 1000C at times above 1000C until saponification occurs.

The overall weight of glycerine recovered per 100g of oil used was 9.0g. The quantity of spent soap lye obtained was 250ml, Acid value was 555.39, percentage free acid value was 5.6%, total fatty acid = 7.131, percentage free caustic acid = 0.14%, percentage free fatty acid obtained was 0.07131%, the specific gravity was 1.059, purity of glycerine was 92%.

These parameters obtained were measured relatively to international standards with negligible error due to the type of equipment used. From the result, it is evident that the amount of glycerine recovered depends largely on the quantity of spent soap lye. Other processes the spend soap lye and the glycerine undergo as may be seen in other chapters includes, salting out, filtration, splitting and others.

Chapter One

1.0 Introduction

1.1 Background of the Study

Glycerine, (otherwise known as propan 1, 2, 3 triol) is found dissolved in the soap lye and also as an impurity in the crude soap during the saponification of fats and oils with caustic soda. Customarily, the process of soap manufacture, from fats and oil yields glycerol to about 10% of the value of the soap formed and due to its varied uses, its recovery is pertinent to the manufacturing cost analysis for any soap making business. Glycerol being an important bye product of soap manufacture, many small scale and medium scale soap manufacturers normally discards the lye as an unwanted product.

Glycerol occurs in nature combined in the form of triglycerides (fats and oil) and is obtained during the saponification of these triglycerides. This process was the only means of producing glycerol commercially, until 1949 when synthetic glycerol was produced as the compound recovered as by-product from the soap manufacture was sufficient for the world consumption. Alternatively, glycerol is synthesized from propene by the alternate chlorination and hydroxylation process. It is also obtained from the fermentation of various sugars. Glycerine is also produced by various routes from propylene. The epichlorohydrin process is the most important; it involves the chlorination of propylene to give ally chloride which is oxidized with a strong base to give epichlorohydrin. This epichlorohydrin is then oxidized to give glycerine.

In biodiesel, glycerine is a waste, as a result, the market for glycerol is depressed and the old epichlorohydrin process for glycerol synthesis is no more economically viable.

Fats and oils are esters of glycerine with long-chain fatty acid such as stearic acid (C17H35COOH) Palmitic acid (C15H31COOH) and oleic acid (C17H33COOH). Since glycerine contains three –OH groups, it can form three series of esters thus;

Mono-ester di-esterdi-ester

Some of the major industrial applications of glycerol include the manufacture of alkyd resins and flexible polyurethane for the plastic industry. It is also an important ingredient in cosmetics and adhesive manufacture. Many pharmaceutical preparations such as glycerol phenol mixture which serves as tranquilizers utilize glycerol.

Glycerol residue has been reported to contain 20.2% glycerol, 6.6% fatty acids (as soap) and 64.3% salt. Thus 91.1% of it is potentially useful. It is obviously advantageous, both environmentally and economically, to recover the glycerol in the waste material from soap industries as a potential alternative for the production of glycerol using chemical methods.

1.2 Problem Statement

The method of analyzing glycerine is greatly varied due to the fact that glycerine contained impurities which acted so much like glycerine as to introduce serious errors in the determination of crude glycerine. This however led to the appointment of committees in the United States and Europe to investigate the method of glycerine analysis. It was concluded in the meeting of the International Committee set for this purpose, that acetin method should control the buying and selling of glycerine, but the more convenient bichromate method in a standardized form might be used in factory control and other technical purposes.

The quantity of recovered residual salt is dependent on the point of recovery and nature of lye treatment. Neutralizes of the acid and alkaline content of the soap lye helps to reduce the amount of salt originally combined with soap by precipitation, coagulation and flocculation. This is done through their pH adjustments.

The whole process becomes entirely cumbersome as absolute care is taken to ensure that glycerine passes through all the stages required to obtain pure glycerine devoid of impurities.

1.3 Scope and Limitation of the Study

The recovery of glycerine from spent soap lye indicates that;

  1. Glycerine can be successfully separated from soap lye irrespective of the source or nature of the fatty acids from which the soap was made.
  2. The weight of glycerine obtained from two soap lye depends more on the percentage brine concentration rather than its quantities.
  3. The salt used for slating was at some point in the process recovered. Salt is insoluble in glycerine and hence

1.4 Purpose / Aims / Objectives of the Study

This research project is aimed at establishing and analyzing the methods of recovery of glycerine from spent soap lye. This is also intended to critically analyze the various constituents of spent soap lye such as salt (6 – 16%), caustic soda (0.3 – 0.8%), total fatty matter (0.4%) and water (39.40%) which is economically important to both the end users and the manufacturers.

The physical and chemical properties as well as the applications of glycerol in today’s process industries will be duly and carefully examined in this research project.

At the end of this research work, one would be able to compare the yield of glycerine used in the course of experiments and that obtained from the literature.

1.5 Method of Research

A bulk of the information used in this write up were gotten from the international journals of Engineering and technology (IJET – IJENS Vol No: 02 references were also made to the chemical engineering plant laboratory reports, materials form the internet were also helpful in this research project as well as other relevant textbooks.

The result of the treatment and quality of spent soap lye affects the overall quality and quantity of glycerine from soap lye. Optimum method should be adopted in the recovery of spent soap lye due to the increasing demand for glycerine by various industries. Spent soap lye was sampled and was used for experiment in which case glycerine is the desired product.

1.6 Significance of the Study

This research project had afforded the participating students with the necessary technologies or methodologies applicable in the recovery of glycerine from spent soap lye.

Since glycerine is widely used today for different purposes, the researchers are therefore provided with the opportunity of producing glycerine for market sales thereby bridging the gap of unemployment. Due to the fact that many soap manufacturers do discard spent soap lye as a bye product, the researchers are therefore provided with a very cheap source of their major raw materials.

Chapter Five

Discussion, Conclusion and Recommendation

5.1 Discussion

The recovery of glycerine from spent soap lye indicates that glycerine can be successfully separated from soap lye irrespective of the source or nature of the fatty acids from which the soap was made. The salt used for salting was at some point in the process (glycerine recovery) a viable option for local soap manufacturers as it would be an added source of income to them aside from improving both the texture and overall quality of the soap produced.

5.2 Conclusion

This study shows that treatment and quality of spent soap lye affects the quality and quantity of glycerine from the soap lye. With the high demand for glycerine in various industries, optimum method should be adopted in the recovery of glycerine. The spent soap lye should pass through all the processes of purification in order to obtain high quality glycerol yield during the recovery process. Glycerine should be removed from the soap lye because if left in the soap, it will limit the washing qualities of the soap. In this way, glycerol is produced for other industrial applications while the properties of soap are enhanced by absence of glycerol. The recovered salt is reused in the soap graining operations. Therefore glycerol recovered from spent soap lye is of economic importance to both the soap manufacturer and consumers as well.

5.3 Recommendation

It is suggested that further work on the recovery of glycerine from spent palm kernel and palm oils soap lye using the hot process of soap manufacture be investigated. This is because local soap manufacturers use either the hot or cold process for soap making. Such investigations will provide the much needed platform for effective comparison of glycerine yields from both processes.

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