PREPARATION OF BASIC METAL SOAPS AS INTERMEDIATE IN BIOHYDROCARBON PRODUCTION FROM VEGETABLE OIL/FATS

• Main Author: Pratiwi, Meiti
• Format: Dissertations
• Language: Indonesia
• Subjects: Teknik kimia
• Online Access: https://digilib.itb.ac.id/gdl/view/44292
• Institution: Institut Teknologi Bandung

In Indonesia, current domestic crude oil and petroleum fuels production could not match the rapidly increasing demand. Petroleum fuels import are therefore required to fill the widening gap. To reduce petroleum fuels importation, domestic production of biohydrocarbon fuels equivalent to gasoline and diesel is needed. One of the process technology for biohydrocarbon production being developed at Institut Teknologi Bandung is decarboxylation and/or pyrolysis of divalent metallic basic soaps of divalent metals, M(OH)(OOCR), in which M could be either a single metal like Mg and Ca, or mixture of Zn, Mg and Ca. It is very interesting to further study the technology of this biohydrocarbon production process because it does not consume hydrogen gas and operate at atmospheric pressure, Commonly practiced means for producing of basic metal soap are double decomposition technique and fusion method. The former generates alkaline salt coproduct that lead to difficult disposal problems, whereas the latter uses fatty acid as raw material. The present research investigated saponification methods via direct reaction of metal hydroxides with vegetable oils/fats to produce metallic basic soap. This is an appealing alternative process for it utilizes cheap raw material (vegetable oil) and do not yield alkaline salt coproduct. The effectiveness of three quite attractive and patented methods in producing basic metal soaps from palm stearin were investigated. Each of these three methods add an agent to enhance the saponification reaction. This agent is water in the method of Blachford (1982), glycerol in the method of Rogers dan Opem (1962), and an aprotic solvent with dielectric constant larger than > 15 in the method of Akers et.al. (1984). The yardstick of effectivitness is the ability of the method to yields basic metal soap containing only a minimum amount of free fatty acids and, compared to the original hydroxide, having a degree of saponified hydroxide in the neighbourhood of 50 %. Of the three methods, that of Rogers dan Opem (1962) is considered as the best. This method could be carried out at atmospheric pressure and a minimum temperature of 122 oC, to yield basic metal soap with acceptable quality. The present work has also proved that : a. the true catalyst of the saponification process is not glycerol but glycerolate ion produced from calcium hydroxide and glycerol or from calcium diglyceroxide, if the latter substance is added as catalyst in place of glycerol;. b. the mechanism of ther saponification reaction follows the cyclic glycerolate – enolate formations proposed by Dijkstra (2005, 2008), which has successfully explained the catalysis mechanism of fatty oil interesterification and methanolysis reactions, but the final stage of soap formation depends on the basic strength of metal hydroxide to conquer fatty acid moiety from enolate ion. The Blachford (1982) method could also yield basic metal soap of acceptable quality, but requires higher operating temperatures (at least 185 oC) and, because the presence of liquid water in amount of half the fatty oil, high pressure (10 bar or more). The method claimed by Akers et.al. (1984) is not recommended to practiced. Firstly because their claim that that the saponification reaction will proceed at relative low temperatures (< 100 oC) if carried out at reflux/boiling temperature of an aprotic solvent with a dielectric constant > 15, could not be proven by the present work eventhough, following Dijkstra mechanism, calcium glyceroxide was added to the reaction mixture. The saponification reaction could only proceed to produce basic metal soap of acceptable quality by using dimethyl sulfoxide (DMSO) solvent, a solvent that, in addition to having a dielectric constant > 15, has the ability to increase the basicity of the reaction mixture and the reaction is carried out at about 130 oC with DMSO : stearin volume ratio of about 3 : 1; this is clearly inferior compared to the method of Rogers and Opem (1962) which does not require a solvent (a nfd in a relatively large amount) and could start the reaction at 122 oC). If the basic metal soap was made from hydroxide mixtures ?Ca(OH)2.(1- ?)Mg(OH)2.Zn(OH)2 with 0 < ? < 1, X-ray Diffraction analysis showed that the basic soap formed is ?Ca(OH)2.(1-?)Mg(OH)2.Zn(OOCR)2, eventhough Mg and, particularly, Ca have higher basicity than Zn. Decarboxylation tests at 370°C showed that the liquid product of decarboxylation is mainly diesel range hydrocarbons (C12 – C17) with C15 as the component of highest content. The presence of free fatty acids in the decarboxylated soap will cause the liquid product to contain ketonic compounds. The optimum value of ? is 0,50 and the class of hydrocarbons composing the liquid product of decarboxulation were n-parafin, iparafin dan 1-olefin having a sufficient quality for use as diesel fuel, judged based on the quality parameters of cetane number, pour point, and oxidation stability (reactivity toward atmospheric oxygen).