398
Ind. Eng. Chem. Res. 2011, 50, 398–403
Microwave Heating Application To Produce Dehydrated Castor Oil ¨ zlem,† and Erciyes Ahmet Tunc¸er‡ Azcan Nezihe,*,† Demirel Elif,† Yılmaz O Faculty of Engineering and Architecture, Department of Chemical Engineering, Anadolu UniVersity, 26470 Eskisehir, Turkey, and Chemical and Metallurgical Engineering Faculty, Department of Chemical Engineering, Istanbul Technical UniVersity, 34469 Maslak, Istanbul, Turkey
Dehydrated castor oil (DCO) is the best known and most widely used of all the oils commonly termed “synthetic” drying oils. DCO imparts good flexibility, rapid drying, excellent color retention, and water resistance to protective coatings. In this study, suitable reaction parameters (reaction time, reaction temperature, catalyst ratio, and pressure) were determined for obtaining DCO from raw castor oil. Reactions were performed at atmospheric pressure with N2 flow as a sweeping gas and at reduced pressure using two different microwave synthesis unit, Start S and Roto Synth, respectively. Iodine value and hydroxyl value of DCO, obtained at atmospheric and reduced pressure (500 mbar), were determined as 135.8 and 140, and 12.3 and 11.9, respectively, using 4% catalyst (w/w) at 250 °C and 20 min reaction time. Fatty acid composition of DCO was determined by gas chromatography analysis to observe the increase of the content of unsaturated fatty acid. Under the applied conditions, dehydration reaction time was decreased from hours (1-2 h) to 20 min using microwave heating system. Introduction Castor oil has been known for a long time as an industrial oil and also has reputation for its medicinal use. Moreover, castor oil and its chemical derivatives are used as raw materials for different types of products in many chemical industries.1 The broad and versatile use of castor oil comes from its main component, ricinoleic acid (12-hydroxy-9-cis-octadecenoic acid), which represents nearly 90% of the vegetable triglycerides.2 The hydroxyl group, double bonds, and ester linkages in castor oil provide reaction sites for the preparation of many useful industrial derivatives, and the hydroxyl groups can be eliminated by dehydration to increase the unsaturation of the molecule (ricinoleic acid). As the name implies, dehydration involves the removal of water from the fatty acid portion of the molecule. The catalytic dehydration results in the formation of new double bond in the fatty acid chain.3 Depending on the double-bond positions and their configuration (cis or trans), different isomers can be found.4 The dehydration process is carried out at about 250 °C and in the presence of catalysts (e.g., sulphuric acid, phosphoric acid, sodium bisulfate, and activated clays) and under inert atmosphere or vacuum.3,5 In the dehydration using mineral acids such as sulfuric acid, phosphoric acid, or sodium acid sulfate, it is postulated that dehydration proceeds through intermediate hydroxonium and carbonium ions as shown in Figure 1 (only one acyl chain of the triglyceride is shown).6 In the proposed mechanism, the hydroxonium is formed by electrophilic attack of a proton on the unshared electron pairs of the hydroxyl group on carbon-12, followed by loss of water to form a carbonium ion, and ejection of a proton from either carbon-11 or carbon-13. Because dehydrated castor oil commonly has a ratio of nonconjugated to conjugated dienoic acid of 4:1 to 3:1, hydrogen removal from the 13-carbon atom must be preferable to that from the 11-carbon atom.7 * To whom correspondence should be addressed. Tel.: +90 222 3350580/6508. Fax: +90 222 3239501. E-mail:
[email protected]. † Anadolu University. ‡ Istanbul Technical University.
As the hydroxyl group is removed during the course of reaction, the viscosity and hydroxyl value decrease, the iodine value increases, and the refractive index changes, allowing these analyses to be utilized to control the degree of dehydration and polymerization.3 The catalyst sodium bisulfate ionizes into Na+ and HSO4and forms sulphuric acid and sodium hydroxide along with the liberated water molecule. This causes the lowering of effective concentration of the catalyst, sodium bisulfate (Figure 2). DCO is noted for nonyellowing and outstanding color retention characteristics in protective coatings. Varnishes, alkyds, and coating resin systems based upon DCO are noted for high speed drying, flexibility, excellent chemical resistance, adhesion, gloss, and water proofness.3
Figure 1. Chemical mechanism of dehydration of ricinoleic acid.6
Figure 2. Chemical mechanism of catalyst.8
10.1021/ie1013037 2011 American Chemical Society Published on Web 11/23/2010
Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011
399
Figure 3. Schematic diagrams of (a) Roto Synth microwave synthesis unit and (b) Start S microwave synthesis unit.
The commercial dehydration of castor oil is carried out in stainless steel, inconel, monel, or glass lined reactors. The reactors generally are equipped with steam-jet ejectors, or highvacuum sources and efficient agitation.3 Guner studied dehydration kinetics of castor oil and obtained DCO with a hydroxyl value of 43.5 (HV) and an iodine value (IV) of 136.2 at 220 °C and 60 min reaction time.9 Ramamurthi et al. obtained DCO with a HV of 9.0 and an IV of 132.0 at 240 °C under vacuum of 5 mmHg at 3 h reaction time.6 Villeneuve et al. studied production of CLA (conjugated linoleic acid) isomers from DCO and obtained DCO with a minimal content of ricinoleic acid after 24 h reaction time.4 Another study of Villeneuve et al. shows that the reaction was maintained for 5 h at 280 °C to obtain DCO with a desired fatty acid distribution.10 Chowdhury et al. performed dehydration reactions using 1% NaHSO4 + 1% KHSO4 under 24-25 in Hg pressure and 250 °C for 60 min reaction time, and the resulting oil had an iodine value of 148.2 and hydroxyl value of 13.3. The ricinoleic acid content was reduced from 97.5% to 7.1%.11 In general, most organic reactions have been heated using traditional heat transfer equipment such as oil baths, sand baths, and heating jackets. These heating techniques are, however, rather slow, and a temperature gradient can develop within the sample. In addition, local overheating can lead to product, substrate, and reagent decomposition. In contrast, in microwave dielectric heating, the microwave energy is introduced into the chemical reactor remotely, and direct access by the energy source to the reaction vessel is obtained. The microwave radiation passes through the walls of the vessel and heats only the reactants and solvent, not the reaction vessel itself. If the apparatus is properly designed, the temperature increase will be uniform throughout the sample, which can lead to less byproducts and/or decomposition products.12 The changing electrical field that interacts with the molecular dipoles and charged ion causes these molecules or ions to have rapid rotation, and heat is generated due to friction of this motion. The increase in the reaction rate most probably is due to an elevated temperature at the local reaction site: the catalytic surface. This is supposed to accelerate various chemical processes. Microwave treatment brings about greater accessibility of the susceptible bonds and, hence, a much more efficient chemical reaction.13 Despite many studies on dehydration of castor oil using conventional heating, microwave heating technique has not been used so far. This research work was undertaken with a view to
develop a simple process for dehydration of castor oil using microwave heating system and to determine suitable reaction conditions. Materials and Experimental Methods Material. Castor oil that was purchased from a local market was used. Fatty acid composition of the oil was determined (87.2% ricinoleic acid, 5.5% linoleic acid, 3.8% oleic acid, 1.6% stearic acid, 1.4% palmitic acid, and 0.5% linolenic acid). All reagents were obtained from commercial suppliers and used without any further purification. Microwave Heating Systems. Reactions were carried out using two different microwave synthesis systems: one is Start S (Milestone-Italy), which is equipped with a magnetic stirrer, a noncontact infrared continuous feedback temperature system under atmospheric pressure (Figure 3a). The other system is Roto Synth (Milestone-Italy), which is suitable for solid and liquid phase synthesis and is equipped with a fiber optic thermocouple. The system has an automatic vacuum control module and a solvent recovery unit (Figure 3b). Dehydration Reactions. a. Reaction under Atmospheric Pressure. Castor oil and different amounts of heterogeneous catalyst (sodium bisulfate-sodium bisulfite (3:1, w/w)) were weighed in a glass reactor and placed into the Start S. The mixture was heated to the desired reaction temperature (220-250 °C) in a short time (3 min), and the reaction was carried on at that temperature for the desired reaction time (5-30 min). Nitrogen flow of 300 mL/min was purged into the system to limit the presence of oxygen and possible side reactions and to remove the emerging water from the reaction media. The contents of the reactor were immediately cooled as soon as the reaction was complete, and the oil was filtered to separate the catalyst. b. Reaction at Reduced Pressure. For the reactions in the Roto Synth microwave heating system, the mixture of castor oil and catalyst (sodium bisulfate as dehydrating agent-sodium bisulfite as antipolymerizing agent (3:1, w/w)) was introduced into the system in a reactor specially designed for the system, and the reactions were carried out at desired temperature (210-250 °C) and time (5-30 min). The reaction medium was maintained under vacuum to allow the removal of water formed during the process (400-700 mbar). The same procedure was applied as with atmospheric conditions after the reactions were completed. Analytical Methods. After the reactions were completed, iodine values and hydroxyl values of the DCO were determined according to the standard methods to observe the degree of dehydration reactions. All titrations were performed using an automatic titrator (Radiometer-TIM 840).
400
Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011
Figure 4. Change of physical properties of DCO with time (reaction conditions: 220 °C, 2% catalyst by mass).
Figure 5. Effect of varying the percentage of catalyst (reaction conditions: 220 °C, 20 min).
Determination of Iodine Value (IV). The iodine values of oil and DCO were determined by Wijs method. In each run, 0.13 g of sample was weighed into an Erlenmeyer flask, and 20 mL of titration solvent (cyclohexane-acetic acid (1:1, v/v)) was added. After oil was dissolved in the solvent, 20 mL of Wijs solution was added, and the resulting solution was kept in the dark for 2 h until the first step of the reaction was completed. Next, 20 mL of 10% KI solution and 150 mL of deionized water were added. The mixture was then titrated with 0.1 N sodium thiosulphate solution until the color changed. The blank determination was conducted without the sample, and the iodine value was calculated.14 Determination of Hydroxyl Value (HV). 1-5 g mass of DCO sample, the mass depending on the expected hydroxyl value, was weighed into a 25 mL calibrated flask and dissolved in toluene, diluting to volume with this solvent. A 5 mL aliquot of the above solution was pipetted into a 200 mL Erlenmeyer flask, followed by 5.0 mL of the acetylating reagent added. The mixture was shaken and left for 10 min. Sodium hydroxide solution (1.3 M, 25 mL) containing sodium sulfate was pipetted into the flask, the flask was shaken, and 10 mL of tert-butanol was added with shaking. After 1 min, the excess alkali was titrated with 0.5 M hydrochloric acid in the presence of 0.5 mL of phenolphthalein indicator. The blank determination was carried out by adding 5 mL of the acetylating reagent to a 200 mL Erlenmeyer flask by pipet, then adding 25 mL of the 1.3 M sodium hydroxide solution by pipet and 10 mL of tert-butanol with a measuring cylinder, with shaking. After 1 min, 5 mL of the original sample solution in toluene was added, and the resulting solution was shaken and titrated with 0.5 M hydrochloric acid in the presence of phenolphthalein. For samples with a low hydroxyl value (20 or less), 10 mL of the original sample solution in toluene, instead of 5 mL, was used for both sample and blank determination.15 Determination of Water Content. Water content of dehydrated castor oil was determined on the basis of Karl Fischer standard procedures using commercially available standard Karl Fischer reagent integrated with a drying oven. Determination of the Fatty Acid Compositions. Relative fatty acid compositions of the obtained DCO samples were
determined by gas chromatography analysis using Agilent 6890N gas chromatography apparatus equipped with HPinnowax column (60 m length × 0.25 mm ID × 0.25 µm film thickness) after converting fatty acids into methyl ester forms using 14% BF3 in methanol.16 Helium was used as a carrier gas at a flow rate of 1.0 mL/min. The injection temperature was 523 K; the oven temperature was kept at 333 K for 10 min, programmed to 493 K at a rate of 4 K/min, kept at this temperature for 10 min, then increased to 513 K at a rate of 1 K/min, and kept 50 min at this temperature. Results and Discussion Physical properties of castor oil were determined as iodine value (IV), 85.0; hydroxyl value (HV), 161.0. Effect of reaction time, reaction temperature, catalyst ratio, and the microwave heating system used (Start S under atmospheric pressure and Roto Synth under vacuum) were investigated on the dehydration of castor oil. Iodine values and hydroxyl values of the obtained DCO samples were determined because these physical properties show the degree of the dehydration. a. Dehydration Reaction under Atmospheric Pressure. Because of the fact that microwave heating reduces reaction time from hours to minutes,13 experiments were conducted from 5 to 30 min with 5 min increments using microwave system under atmospheric and reduced pressure (Figures 4 and 7). The results obtained at atmospheric pressure are given in Figures 4-6. First, a suitable reaction time was determined as 20 min (Figure 4). A suitable catalyst ratio (by mass of oil) was determined as 4% depending on the IV (131.0) and HV (22.4). It can clearly be seen from Figure 5 that there are no significant changes in IV and HV of the oil using further amounts of catalyst. Suitable reaction temperature was determined as 250 °C according to the results given in Figure 6, because maximum IV (135.8) and minimum HV (12.3) were obtained at this condition. b. Dehydration Reaction under Reduced Pressure. The results obtained with Roto Synth are given in Figures 7-11. A suitable reaction time was determined as 15 min at 210 °C and
Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011
401
Figure 6. Effect of temperature on the physical properties of the obtained oils (reaction conditions: 4% catalyst by mass, 20 min).
Figure 7. Change of physical properties of DCO with time (reaction conditions: 210 °C, 700 mbar, 2% catalyst by mass).
Figure 8. Effect of varying the percentage of catalyst (reaction conditions: 210 °C, 700 mbar, 15 min).
Figure 9. Effect of temperature on the physical properties of the obtained oils (reaction conditions: 700 mbar, 4% catalyst by mass, 15 min).
700 mbar (Figure 7). After determining the reaction time, the catalyst ratio was determined as 4% by mass of the oil according to the IV and HV of the obtained oil (Figure 8). Suitable reaction temperature was determined as 250 °C using 4% catalyst and 15 min reaction time, because the IV increased and HV decreased as the temperature was increased (Figure 9). To achieve the lowest hydroxyl value, reaction times were extended to 25 min, and then a suitable reaction time was found as 20 min (Figure 10) because IV increased from 127.2 to 131.0 and hydroxyl value decreased from 30.6 to 21.2, which are more desirable values. Because the maximum operating temperature of the microwave system is 250 °C, instead of increasing the temperature, further experiments were carried out under reduced pressure to obtain oil with more desirable properties, that is,
higher iodine value and lower hydroxyl value. The best result (DCO with IV of 140.0 and HV of 11.9) was obtained at 500 mbar pressure (Figure 11). According to Figure 11, the iodine value of the DCO increased as long as the pressure decreased and it reached a maximum value at 500 mbar. It then started to decrease because further reduction of pressure leads to polymerization with a consequence drop in iodine value and a step rise in hydroxyl value.17 The moisture content of dehydrated castor oil samples obtained at atmospheric conditions was found as 0.25% using Karl Fischer apparatus, whereas there is no water left in the DCO obtained under vacuum conditions.
402
Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011
Figure 10. Change of physical properties with time (reaction conditions: 250 °C, 700 mbar, 4% catalyst by mass).
Figure 11. Effect of pressure on physical properties (reaction conditions: 250 °C, 4% catalyst by mass, 20 min). Table 1. Fatty Acid Compositions of Raw Castor Oil and DCO fatty acid
castor oil
16:0 18:0 18:1 18:2 NCLAa 18:2 CLA (total)b 18:3 18:1, OH total saturated total unsaturated
1.4 1.6 3.8 5.5 0.5 87.2 3.0 97.0
DCO (under N2)
DCO (under vacuum)
1.6 1.8 4.3 49.2 43.1
1.9 2.0 4.7 53.9 37.5
trace 3.4 96.6
trace 3.9 96.1
a Nonconjugated linoleic acid. b 9-cis,11-trans-Linoleic acid, 10-trans,12cis-linoleic acid, 9-cis,11-cis-linoleic acid, 10-trans,12-trans-linoleic acid, and 9-trans,11-trans-linoleic acid.
Fatty acid compositions of raw castor oil and DCO obtained under N2 flow and under vacuum are summarized in Table 1. As it can clearly be seen from the table, almost all of the ricinoleic acid in castor oil (87.2%) has been converted into conjugated and nonconjugated linoleic acids. Conclusion The main fatty acid of castor oil is ricinoleic acid, which comprises about 90% of the triglyceride molecule. Dehydration reaction occurs in the ricinoleic acid molecule, which splits from the hydroxyl group of the molecule with an adjacent hydrogen atom, and water is formed consequently. The resulting products consist of mainly two fatty acids (9,12 linoleic acid and 9,11 linoleic acid). Analytical features like hydroxyl value, iodine value, viscosity, and refractive index show the degree of dehydration reaction, and as hydroxyl groups are removed during the course of the reaction, iodine value increases, hydroxyl value and viscosity decrease, and as a result refractive index changes. For the reactions under atmospheric pressure and N2 flow, DCO with a HV of 12.3 and IV of 135.8 was obtained at 20 min reaction time, 250 °C temperature, and 4% of catalyst by mass of the oil. For the reactions under vacuum, DCO with a HV of 11.9 and IV of 140.0 was obtained at 20 min reaction
time, 250 °C temperature, 500 mbar pressure, and 4% of catalyst by mass of the oil. To increase IV of dehydrated castor oil, the same reaction was carried out under reduced pressure. DCO was obtained with an IV of 140.0, which is slightly higher than the value of the oil obtained under atmospheric pressure (IV of 135.0). Although there is no significant difference between the values (IV and HV) obtained under atmospheric and reduced pressure, oxidative polymerization reactions and side reactions were avoided under vacuum during removal of water from reaction media. The water content of dehydrated castor oil obtained atmospheric pressure is determined as 0.25%, while there is no moisture left in the dehydrated castor oil obtained under vacuum. Water and volatile decomposition products can effectively be removed from the reaction media when the experiments are carried out under vacuum, and therefore water separation steps can be eliminated. Castor oil dehydrated under reduced pressure is always paler in color than DCO obtained at atmospheric pressure,20 which is more preferable in paint industry. As bounded OH group to the ricinoleic acid breaks off the molecule, the number of the double bonds increases, and fatty acids with different geometry and positions were obtained depending on the reaction conditions. At atmospheric conditions, the total CLA content of the oil is 43.1%, whereas under vacuum, the total CLA content is 37.5%. The results show that atmospheric pressure could be preferred to obtain oil having higher CLA, and vacuum pressure could be preferred to produce oil having higher NCLA. The oxidative stability of CLAs relative to other polyunsaturated fatty acids readily decomposes due to the formation of unstable free-radical intermediates during oxidation.18 Formerly, it was believed that the main product was the conjugated 9,11 linoleic acid, but later work showed that 9-12 linoleic acids are usually present in greater proportions than are conjugated acids.19 That is why obtaining DCO under reduced pressure is more favorable.
Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011
The properties of castor oil are in good agreement with the published data.6,9,11 According to the results, it can be concluded that microwave heating has reduced the reaction time from hours (1, 2, 3 h) to minutes (20 min) as compared to literature data carried out with conventional heating techniques.6,9,10 Acknowledgment Financial support from The Scientific and Technological Research Council of Turkey (TUBITAK Project No. 105M 289) is gratefully acknowledged. Literature Cited (1) Ghosh, M.; Bhattacharyya, D. K. Enzymatic Interesterification of Blends of Castor Oil and Some Oils Rich in Saturated Fatty Acids. Fett/ Lipid 1999, 6, 214–216. (2) Schneider, R. C. S.; Baldissarelli, V. Z.; Trombetta, F.; Martinelli, M.; Caramao, E. B. Optimization of Gas Chromatographic-Mass Spectrometric Analysis for Fatty Acids in Hydrogenated Castor Oil Obtained by Catalytic Transfer Hydrogenation. Anal. Chim. Acta 2004, 505, 223–226. (3) Naughton, F. C. Production, Chemistry, and Commercial Applications of Various Chemicals from Castor Oil. J. Am. Oil Chem. Soc. 1974, 51, 65–71. (4) Villeneuve, P.; Lago, R.; Barouh, N.; Barea, B.; Piombo, G.; Dupre, J. Y.; Guillou, A. L.; Pina, M. Production of Conjugated Linoleic Acid Isomers by Dehydration and Isomerization of Castor Bean Oil. J. Am. Oil Chem. Soc. 2005, 82, 261–269. (5) Ogunniyi, D. S. Castor Oil: A Vital Industrial Raw Material. Bioresour. Technol. 2006, 97, 1086–1091. (6) Ramamurthi, S.; Manohar, V.; Mani, V. V. S. Characterization of Fatty Acid Isomers in Dehydrated Castor Oil by Gas Chromatography and Gas Chromatography-Mass Spectrometry Techniques. J. Am. Oil Chem. Soc. 1998, 75, 1297–1303. (7) Achaya, K. T. Chemical Derivatives of Castor Oil. J. Am. Oil Chem. Soc. 1971, 48, 758–763.
403
(8) Bhowmick, D. N.; Sarma, S. A. N. Dehydration of Castor Oil. Ing. Eng. Chem. 1977, 16, 107. (9) Guner, F. S. Castor Oil Dehydration Kinetics. J. Am. Oil Chem. Soc. 1997, 74, 409–412. (10) Villeneuve, P.; Barouh, N.; Barea, B.; Piombo, G.; FigueraEspinoza, M. C.; Turon, F.; Pina, M.; Lago, R. Chemoenzymatic Synthesis of Structured Triacylglycerols with Conjugated Linoleic Acids (CLA) in Central Position. Food Chem. 2007, 100, 1443–1452. (11) Chowdhury, D. K.; Mukherji, B. K. Studies on Dehydrated Castor Oil-Part I. J. Am. Oil Chem. Soc. 1956, 22, 189–198. (12) Lidstro¨m, P.; Tierney, J.; Watley, B.; Westman, J. MicrowaveAssisted Organic Aynthesis-A Review. Tetrahedron 2001, 57, 9225–9283. (13) Azcan, N.; Demirel, E. Obtaining 2-Octanol, 2-Octanone, and Sebacic Acid from Castor Oil by Microwave-Induced Alkali Fusion. Ind. Eng. Chem. Res. 2008, 47, 1774–1778. (14) EN ISO 3961 (1999) and ISO 3961 (1996), Iodine Value of Animal and Vegetable Fats and Oils. (15) Hartman, L.; Lago, R. C. A.; Azeredo, L. C.; Azeredo, M. A. A. Determination ff Hydroxyl Value in Fats and Oils Using an Acid Catalyst. Analyst 1987, 112, 145–147. (16) Williams, S. Officials Methods of Analysis of the Association of Official Analytical Chemists; AOAC Publications: Arlington, VA, 1984. (17) Thi, M. M.; Hlaing, N. N.; Oo, M. M. Production of Alkyd Resin from Vegetable Oils. GMSARN International Conference on Sustainable DeVelopment: Issues and Prospects for the GMS, 2008. (18) Jie, M. S. F. L. K.; Pasha, M. K. Fatty Acids, Fatty Acid Analogues and Their Derivatives. Nat. Prod. Rep. 1998, 15, 609. (19) Hilditch, T. P. The Chemical Constitution of Natural Fats; John Wiley & Sons: New York, 1954. (20) Dole, K. K.; Keskar, V. R. Dehydration of Castor Oil by Substituted Sulphonic Acids and Their Salts. Proc. Math. Sci. 1953, 38, 135–142.
ReceiVed for reView June 17, 2010 ReVised manuscript receiVed September 24, 2010 Accepted November 8, 2010 IE1013037
