Leticia Ledo Marciniuk; Camila Martins Garcia; Ulf Friedrich Schuchardt. "USE OF ETHANOL IN TRANSESTERIFICATION", p.853-860. In Luis Augusto Barbosa Cortez (Coord.). Sugarcane bioethanol — R&D for Productivity and Sustainability, São Paulo: Editora Edgard Blücher, 2014. http://dx.doi.org/10.5151/BlucherOA-Sugarcane-SUGARCANEBIOETHANOL_71 24 USE OF ETHANOL IN TRANSESTERIFICATION Leticia Ledo Marciniuk, Camila Martins Garcia and Ulf Friedrich Schuchardt Due to the unrestrained energy consumption reduction of its availability over the years, as it is associated with growing environmental concern, presently happening in most developing countries research is being encouraged in many diverse (GARCIA, 2006). areas, to search for alternative resources, which As readily available sources, vegetable oils were neglected until recently, principally re- have been gaining in importance in current energy newable energy resources such as wind, tides programs. They provide a decentralized energy and biomass. generation, bringing benefits to more distant loca- Among these energy resources, biomass has tions and less affluent regions. Its production also received special attention because due to the large means supporting family farming and enhancing number of applications e.g. ethanol and biodie- regional development (RAMOS, 2003). sel, which are the object of great interest in the Several researchers propose the direct use of world economy. vegetable oils as alternative fuels for petroleum- They can be burned directly or used indirectly based such as diesel oil. However, despite being after chemical, biological or thermochemical trans- vegetable oils energetically favorable due to their formations. Among these are the transesterifica- high calorific value, their direct use may prejudice tion of vegetable oils, fatty acids esterification, operations and durability of diesel engines, increas- pyrolysis, gasification, extractions with super- ing their maintenance costs (MACEDO, 2004). critical fluid, anaerobic digestion and fermentation. This is due to the high viscosity of these oils, These processes can use various feedstocks such approximately 11 to 17 times greater than diesel as agricultural wastes, which otherwise would fuel, low volatility and high molecular weight be- cause serious environmental problems if left in cause of the large chain of triacylglycerides. These the fields as it is the case of sugarcane bagasse factors may prevent their complete combustion, (MARCINIUK, 2007). leading to the formation of carbon deposits inside In addition, the use of biomass reduces pol- the engine and obstruction of oil filters and injec- lution, because its main components are formed tion systems. Furthermore, the thermal decom- from carbon dioxide and water, using sunlight as a position of glycerol, present in vegetable oils, can source of energy. This gives biomass a strategic po- lead to the release of acrolein, a highly toxic and sition for the solution of problems related to global carcinogenic substance (TEIXEIRA, 2006). warming. If you consider that during combustion of Several processes have been studied which biomass, carbonic gas emitted to the atmosphere would allow to obtain renewable fuels with physi- is absorbed by photosynthesis during growth, the cochemical properties similar to diesel oil, with- mass balance for CO2 is more favorable than zero. out requiring engine modifications or additional However, for this becoming a reality, a sustain- technology investment. An example is the use of able consumption of biomass is needed, without biodiesel, which can be obtained from vegetable 854 A New Model for Industrial Production and Final Uses of Ethanol oils or other sources of fatty materials, such as consumption of a plant that produces ethylic animal fats or waste oils, by transesterification biodiesel is higher when compared to methylic with short-chain alcohols. The fuel compatibility biodiesel. However, for Brazil, from the economic with conventional diesel has characterized it as the point of view, the use of ethanol is more advanta- most appropriate alternative which can be used in geous, since the country is considered to be the fleet of most diesel vehicles existing on the market second largest producer. (XIE et al., 2006). The transesterification of vegetable oils is a When compared to mineral diesel, biodiesel reversible reaction, whose kinetics is governed by has as major advantages such as the reduction of the principle of Le Chatelier. Thus, the conversion emissions, biodegradability, higher flash point and of the reaction depends on shifting the chemical increased lubrication. equilibrium towards the formation of products The term “transesterification” (or alcoholysis) through the optimization of the variables such as describes an important class of organic reactions in temperature, concentration of catalyst, its acidic which an ester is transformed into another through or basic strength and the amount of reagents. the exchange of their alkoxide groups. In this reac- One of the most important variables that af- tion, the triacylglycerides present in vegetable oils, fect the conversion to esters is the vegetable oil react with an alcohol in the presence of a catalyst to alcohol molar ratio. Using an excess of alcohol, producing a mixture of monoalkyl esters of fatty the equilibrium is shifted towards the product, acids and glycerol, as shown in Figure 1. however, an excessive increase will also favor Various alcohols can be used in such reactions, the solubility of glycerol in biodiesel, making its however, only methanol or ethanol will produce separation difficult (GARCIA, 2006). According biodiesel. Both can be obtained from renewable to the literature, vegetable oil to alcohol molar sources such as dry distillation of wood and fer- ratios are normally in the range of 1:6 to 1:30. In mentation of sugarcane, respectively. the case of ethanol, ratios of 1:6 and 1:12 show According to the technical aspects of trans- satisfactory results. esterification, the use of methanol (methanolysis) ENCINAR et al. (2002) have studied the is advantageous because it allows the spontaneous ethanolysis of Cynara oil by varying the oil to separation of glycerol and as a consequence the re- ethanol molar ratio from 1:3 to 1:15. The best duction of the number of process steps. Moreover, results were obtained for reactions carried out it gives a high conversions using homogeneous with the molar ratio between 1:9 and 1:12. The catalysts in basic or acidic conditions. On the other reactions employing molar ratios below 1:6 were hand, this alcohol has a high toxicity. incomplete and problems in the glycerol separa- The use of ethanol (ethanolysis), even an- tion step were found when using a molar ratio hydrous, creates problems in the separation of of 1:15. The temperature and reaction time also glycerin from the reaction mixture. The power influence these reactions. High temperatures al- FIGURE 1 Transesterification of a triacylglycerides. Use of Ethanol in Transesterification 855 low higher conversions in shorter reaction times. reaction; after this period it is preferable that the However, it is necessary to evaluate whether the reaction is conducted under milder agitation. This energy expenses used for heating do not exceed procedure reduces the dispersion of glycerol drop- the economic gains. lets and thus shortens the glycerin coalescence at Methanol and ethanol are not miscible with the end of the reaction. vegetable oils at room temperature. To increase GARCIA (2006) found that the yield of esters the reagents’ miscibility, transesterification re- in transesterification reactions of soybean oil with actions are largely promoted at temperatures ethanol at 70 ºC in the presence of potassium hy- between 60 ºC and 70 ºC under vigorous stirring droxide was directly proportional to the amount in order to promote the phase transfer. These of catalyst used, however, the formation of a conditions usually cause the formation of an emul- transparent and thermodynamically stable micro- sion which, in the case of methanolysis, is rapidly emulsion was observed, which does not allow the broken with the interruption of agitation, leading coalescence of the glycerol, reducing the amount to the separation of glycerol. In ethanolysis, these of ethyl esters isolated and increasing the difficulty emulsions are much more stable and sometimes to purify the product. On the other hand, the alco- difficult to brake (MEHER et al., 2006). holysis at 25 ºC, catalyzed by sodium methoxide The emulsions formed during transesterifica- or by sodium or potassium hydroxide, resulted in tion are partly due to the presence of diacilglyc- systems that allowed the complete separation of erides (DAG) and monoacilglycerides (MAG), glycerin which was observed immediately after intermediaries in the alcoholysis of triacylglyc- switching off the agitation. For all three reactions erides (TAG), which are good surfactants. In the total conversion was observed. alcoholysis process the base catalyst is dissolved Since methanolic solutions of sodium or potas- in the alcohol and, after its complete dissolution, sium methoxide are now commercially available, is mixed with the triacylglyceride. The reaction the problems of the ethyl esters and glycerin is initially controlled by phase transfer, however, separation are reduced, so that the water con- in the course of the reaction the concentration of tent of ethanol is now a major variable causing intermediaries decreases, reaching a