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Question: Biodiesel Production from Vegetal Oil and Ethanol Via Transesterification in Supercritical Conditions

Answer: Introduction
The main replacement fuel for the petroleum based Diesel is the biodiesel. Over the last years several techniques have been proposed for the production of this fuel which is totally or partially derived from renewable feedstocks.

Because of its relative simplicity of implementation, the transesterification through basic homogeneous catalysis of monoglycerides, diglycerides and triglycerides with methanol in a batch process, is the most common form among the various biodiesel production methods. However, this route raises some problems and imposes some limitations on the process, among which we can highlight:

a) Inhibition of transesterification reactions due to the presence of water and generation of free fatty acids and soap products. Raw materials pre-treatment is required or the use of raw materials free from water (IEA, 2004) in order to avoid that inhibition.

b) The traditional catalyst leads to saponification of free fatty acids and triglycerides by the expenses of catalyst consumption

c) Limitations in reactions with other alcohols such as ethanol because this alcohol requires a mass 44 % higher when compared to that of the methanol, requires raw materials with low levels of free fatty acids, consumes a mass approximately 50 % of catalyst and presents more difficulties in separating glycerin/ethyl esters (IEA, 2004), need to be used in its anhydrous form;

d) Need of multistep separation of products, by-products, reagents and catalysts;

e) Variation of total time of reaction from one to eight hours (IEA, 2004);

f) Formation of two distinct liquid phases alcohol/triglyceride (SAKA and KUSIDIANA, 2001), making contact between the reagents more difficult.

The above-mentioned problems brings technical difficulties to the productive process and increase its cost. Methods capable of overcoming such technical challenges, generating products with quality equal to or greater than that of the products obtained currently, would make the process more attractive in both the economic point of view and from a technical point of view.

According to the literature the supercritical process is capable of generate high purity esters, without being influenced by the low quality of feedstocks and requiring separation processes that are simpler than those applied to the traditional production techniques such as the transesterification using basic homogeneous catalysts.

Objective
The present study aimed to produce biodiesel by supercritical method using soybean oil, oleic acid, and ethanol. The reactions were performed with anhydrous ethanol or hydrated ethanol in order to check how the presence of water in supercritical ethanol influences transesterification reactions and compare with literature data obtained with other experimental apparatus.

Experimental Procedure
Ethanol was used in its anhydrous form, the anhydrous ethyl alcohol fuel 99,7 % and in its hydrous form, the hydrated ethanol fuel 92,8 %, both specified in accordance with current Brazilian legislation. The refined food grade soybean oil and was the oleic acid analytical grades were used as fatty acid sources. Glicerin used in reversibility experiments was analytical grade.

The reactions were conducted in batch experiments in an autoclave Parr model 4578 HT/HP 1,8 L constructed in 316L stainless steel. The temperatures used in reactions were 280, 310 and 340 oC. Samples of experiments were collected at 0, 20, 40, 60 and 120 minutes after the temperature were first registered in the controller of temperature of the reactor.

The samples were analyzed by gas chromatograph using on-column injection, FID in a CP-SimDist 5 m x 0,53 mm x 0,09 µm column.

The water content in the reagents and products was determined by Karl Fischer technique. The material balance was calculated and conversions were determined for each experiment. Modeling was done trying to understand the integral phenomena.

Discussion and Conclusions
The experiments elucidated several critical points regarding transesterification processes and supercritical esterification with ethanol. Conversion values and total time of reaction were evaluated and compared with the literature. For the transesterification reactions with the vegetable oil, the highest value of conversion to ethyl esters was around 90 %. Although present around 90 % conversions, transesterification reactions, under the conditions tested, failed to achieve the conversion levels reported by Warabi et al. (2004) for the transesterification using rapeseed oil with ethanol, which was exceeding 95 % with the reaction to 300 °C for 45 minutes.

The presence of water in the reaction medium was decisive in reactions with vegetable oil. It allowed larger concentrations of ethyl esters were reached at intervals of times smaller.The use of hydrated ethanol could be positive for the productive process since this presents lower cost which eventually would encourage its use in industrial production. These data indicates that this method could be able to operate with lower quality raw materials and that pretreatment steps are not mandatory.

In esterification reactions with oleic acid conversion values reached more than 96 %. The largest conversion of oleic acid confirms the data published by Warabi et al. (2004), which demonstrate the ease of reaction using fatty acids. These experiments showed higher conversions even on lowest temperature, however, the conversion rate was decreased by the presence of water in the reaction medium.

With the implementation of inverse reactions in models was possible to conclude that even using excess ethanol on the reaction medium, transesterification reactions at 340 ° C showed reversibility indicating that equilibrium is not favorable. For the tests performed at 340° C with vegetable oil and hydrated ethanol, apparent signs of reverse reactions might be noticed by the reduction of concentrations of ethyl esters after 60 and 120 minutes of reaction, however, further tests are needed to confirm the final concentrations, the role of reversibility in the reactions and the equilibrium of the reaction.

It was not possible to confirm the reduced time of reaction, one of the advantages of supercritical method presented in the literature for the chosen test conditions. The results indicate temperatures between 310 and 340 °C with hydrated ethanol as being the more efficient conditions for transesterification reactions. With increasing temperature there was a reduction in the time required for the ester concentration get stable around 85 %.
It was detected water formation in the reaction medium, indicating the occurrence of unwanted reactions that could not be identified by the analytical methods employed.

References
IEA - INTERNATIONAL ENERGY AGENCY. Biodiesel in North America: implementation issues. s.l.: (S&T) 2 Consultants Inc. 2004, 142p.

KUSDIANA, D; SAKA, S. Kinetics of transesterification in rapeseed oil to biodiesel fuel as treated in supercritical methanol. Fuel, s.l., v. 80, p 693-698, 2001.

SAKA, S; KUSDIANA, D. Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel, s.l., v. 80, n. 2, p. 225-231, 2001.

WARABI, Y. et al. Reactivity of triglycerides and fatty acids of rapeseed oil in supercritical alcohols. Bioresource Technology, s.l., v. 91, p 283-287, 2004.

(Source: Joao G. R. Poco, 12 Jun 2021)

Source Link: https://www.aiche.org