Microwave Heating Application To Produce Dehydrated Castor Oil

Azcan Nezihe,*,† Demirel Elif,† Yılmaz Ozlem,† and Erciyes Ahmet Tuncer‡

Faculty of Engineering and Architecture, Department of Chemical Engineering, Anadolu UniVersity, 26470Eskisehir, 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 waterresistance 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 performedat atmospheric pressure with N2 flow as a sweeping gas and at reduced pressure using two different microwavesynthesis unit, Start S and Roto Synth, respectively. Iodine value and hydroxyl value of DCO, obtained atatmospheric 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 DCOwas determined by gas chromatography analysis to observe the increase of the content of unsaturated fattyacid. Under the applied conditions, dehydration reaction time was decreased from hours (1-2 h) to 20 minusing microwave heating system.

Introduction

Castor oil has been known for a long time as an industrialoil and also has reputation for its medicinal use. Moreover,castor oil and its chemical derivatives are used as raw materialsfor different types of products in many chemical industries.1

The broad and versatile use of castor oil comes from its maincomponent, ricinoleic acid (12-hydroxy-9-cis-octadecenoic acid),which represents nearly 90% of the vegetable triglycerides.2 Thehydroxyl group, double bonds, and ester linkages in castor oilprovide reaction sites for the preparation of many usefulindustrial derivatives, and the hydroxyl groups can be eliminatedby dehydration to increase the unsaturation of the molecule(ricinoleic acid).

As the name implies, dehydration involves the removal ofwater from the fatty acid portion of the molecule. The catalyticdehydration results in the formation of new double bond in thefatty acid chain.3 Depending on the double-bond positions andtheir configuration (cis or trans), different isomers can be found.4

The dehydration process is carried out at about 250 °C and inthe presence of catalysts (e.g., sulphuric acid, phosphoric acid,sodium bisulfate, and activated clays) and under inert atmo-sphere or vacuum.3,5

In the dehydration using mineral acids such as sulfuric acid,phosphoric acid, or sodium acid sulfate, it is postulated thatdehydration proceeds through intermediate hydroxonium andcarbonium ions as shown in Figure 1 (only one acyl chain ofthe triglyceride is shown).6

In the proposed mechanism, the hydroxonium is formed byelectrophilic attack of a proton on the unshared electron pairsof the hydroxyl group on carbon-12, followed by loss of waterto form a carbonium ion, and ejection of a proton from eithercarbon-11 or carbon-13. Because dehydrated castor oil com-monly has a ratio of nonconjugated to conjugated dienoic acidof 4:1 to 3:1, hydrogen removal from the 13-carbon atom mustbe preferable to that from the 11-carbon atom.7

As the hydroxyl group is removed during the course ofreaction, the viscosity and hydroxyl value decrease, the iodinevalue increases, and the refractive index changes, allowing theseanalyses to be utilized to control the degree of dehydration andpolymerization.3

The catalyst sodium bisulfate ionizes into Na+ and HSO4-

and forms sulphuric acid and sodium hydroxide along with theliberated water molecule. This causes the lowering of effectiveconcentration of the catalyst, sodium bisulfate (Figure 2).

DCO is noted for nonyellowing and outstanding colorretention characteristics in protective coatings. Varnishes, alkyds,and coating resin systems based upon DCO are noted for highspeed drying, flexibility, excellent chemical resistance, adhesion,gloss, and water proofness.3

* To whom correspondence should be addressed. Tel.: +90 2223350580/6508. Fax: +90 222 3239501. E-mail: nazcan@anadolu.edu.tr.

† Anadolu University.‡ Istanbul Technical University.

Figure 1. Chemical mechanism of dehydration of ricinoleic acid.6

Figure 2. Chemical mechanism of catalyst.8

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10.1021/ie1013037 2011 American Chemical SocietyPublished on Web 11/23/2010

The commercial dehydration of castor oil is carried out instainless steel, inconel, monel, or glass lined reactors. Thereactors generally are equipped with steam-jet ejectors, or high-vacuum sources and efficient agitation.3 Guner studied dehydra-tion kinetics of castor oil and obtained DCO with a hydroxylvalue of 43.5 (HV) and an iodine value (IV) of 136.2 at 220 °Cand 60 min reaction time.9 Ramamurthi et al. obtained DCOwith a HV of 9.0 and an IV of 132.0 at 240 °C under vacuumof 5 mmHg at 3 h reaction time.6 Villeneuve et al. studiedproduction of CLA (conjugated linoleic acid) isomers from DCOand obtained DCO with a minimal content of ricinoleic acidafter 24 h reaction time.4 Another study of Villeneuve et al.shows that the reaction was maintained for 5 h at 280 °C toobtain DCO with a desired fatty acid distribution.10 Chowdhuryet al. performed dehydration reactions using 1% NaHSO4 +1% KHSO4 under 24-25 in Hg pressure and 250 °C for 60min reaction time, and the resulting oil had an iodine value of148.2 and hydroxyl value of 13.3. The ricinoleic acid contentwas reduced from 97.5% to 7.1%.11

In general, most organic reactions have been heated usingtraditional 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 thesample. In addition, local overheating can lead to product,substrate, and reagent decomposition. In contrast, in microwavedielectric heating, the microwave energy is introduced into thechemical reactor remotely, and direct access by the energysource to the reaction vessel is obtained. The microwaveradiation passes through the walls of the vessel and heats onlythe reactants and solvent, not the reaction vessel itself. If theapparatus is properly designed, the temperature increase willbe uniform throughout the sample, which can lead to lessbyproducts and/or decomposition products.12

The changing electrical field that interacts with the moleculardipoles and charged ion causes these molecules or ions to haverapid rotation, and heat is generated due to friction of thismotion. The increase in the reaction rate most probably is dueto an elevated temperature at the local reaction site: the catalyticsurface. This is supposed to accelerate various chemicalprocesses. Microwave treatment brings about greater acces-sibility of the susceptible bonds and, hence, a much moreefficient chemical reaction.13

Despite many studies on dehydration of castor oil usingconventional heating, microwave heating technique has not beenused so far. This research work was undertaken with a view to

develop a simple process for dehydration of castor oil usingmicrowave heating system and to determine suitable reactionconditions.

Materials and Experimental Methods

Material. Castor oil that was purchased from a local marketwas 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). Allreagents were obtained from commercial suppliers and usedwithout any further purification.

Microwave Heating Systems. Reactions were carried outusing two different microwave synthesis systems: one is StartS (Milestone-Italy), which is equipped with a magnetic stirrer,a noncontact infrared continuous feedback temperature systemunder atmospheric pressure (Figure 3a). The other system isRoto Synth (Milestone-Italy), which is suitable for solid andliquid phase synthesis and is equipped with a fiber opticthermocouple. The system has an automatic vacuum controlmodule and a solvent recovery unit (Figure 3b).

Dehydration Reactions. a. Reaction under AtmosphericPressure. Castor oil and different amounts of heterogeneouscatalyst (sodium bisulfate-sodium bisulfite (3:1, w/w)) wereweighed in a glass reactor and placed into the Start S. Themixture was heated to the desired reaction temperature (220-250°C) in a short time (3 min), and the reaction was carried on atthat temperature for the desired reaction time (5-30 min).Nitrogen flow of 300 mL/min was purged into the system tolimit the presence of oxygen and possible side reactions and toremove the emerging water from the reaction media. Thecontents of the reactor were immediately cooled as soon asthe reaction was complete, and the oil was filtered to separatethe catalyst.

b. Reaction at Reduced Pressure. For the reactions in theRoto Synth microwave heating system, the mixture of castoroil and catalyst (sodium bisulfate as dehydrating agent-sodiumbisulfite as antipolymerizing agent (3:1, w/w)) was introducedinto 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 wasmaintained under vacuum to allow the removal of water formedduring the process (400-700 mbar). The same procedure wasapplied as with atmospheric conditions after the reactions werecompleted.

Analytical Methods. After the reactions were completed,iodine values and hydroxyl values of the DCO were determinedaccording to the standard methods to observe the degree ofdehydration reactions. All titrations were performed using anautomatic titrator (Radiometer-TIM 840).

Figure 3. Schematic diagrams of (a) Roto Synth microwave synthesis unit and (b) Start S microwave synthesis unit.

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Determination of Iodine Value (IV). The iodine values ofoil and DCO were determined by Wijs method. In each run,0.13 g of sample was weighed into an Erlenmeyer flask, and20 mL of titration solvent (cyclohexane-acetic acid (1:1, v/v))was added. After oil was dissolved in the solvent, 20 mL ofWijs solution was added, and the resulting solution was kept inthe 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 waterwere added. The mixture was then titrated with 0.1 N sodiumthiosulphate solution until the color changed. The blank deter-mination was conducted without the sample, and the iodinevalue was calculated.14

Determination of Hydroxyl Value (HV). 1-5 g mass ofDCO sample, the mass depending on the expected hydroxylvalue, was weighed into a 25 mL calibrated flask and dissolvedin toluene, diluting to volume with this solvent. A 5 mL aliquotof the above solution was pipetted into a 200 mL Erlenmeyerflask, followed by 5.0 mL of the acetylating reagent added. Themixture was shaken and left for 10 min. Sodium hydroxidesolution (1.3 M, 25 mL) containing sodium sulfate was pipettedinto the flask, the flask was shaken, and 10 mL of tert-butanolwas added with shaking. After 1 min, the excess alkali wastitrated with 0.5 M hydrochloric acid in the presence of 0.5 mLof phenolphthalein indicator. The blank determination wascarried out by adding 5 mL of the acetylating reagent to a 200mL Erlenmeyer flask by pipet, then adding 25 mL of the 1.3 Msodium hydroxide solution by pipet and 10 mL of tert-butanolwith a measuring cylinder, with shaking. After 1 min, 5 mL ofthe original sample solution in toluene was added, and theresulting solution was shaken and titrated with 0.5 M hydro-chloric acid in the presence of phenolphthalein. For sampleswith a low hydroxyl value (20 or less), 10 mL of the originalsample solution in toluene, instead of 5 mL, was used for bothsample and blank determination.15

Determination of Water Content. Water content of dehy-drated castor oil was determined on the basis of Karl Fischerstandard procedures using commercially available standard KarlFischer reagent integrated with a drying oven.

Determination of the Fatty Acid Compositions. Relativefatty acid compositions of the obtained DCO samples were

determined by gas chromatography analysis using Agilent6890N gas chromatography apparatus equipped with HP-innowax column (60 m length × 0.25 mm ID × 0.25 µm filmthickness) after converting fatty acids into methyl ester formsusing 14% BF3 in methanol.16 Helium was used as a carriergas at a flow rate of 1.0 mL/min. The injection temperaturewas 523 K; the oven temperature was kept at 333 K for 10min, programmed to 493 K at a rate of 4 K/min, kept at thistemperature for 10 min, then increased to 513 K at a rate of 1K/min, and kept 50 min at this temperature.

Results and Discussion

Physical properties of castor oil were determined as iodinevalue (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 atmo-spheric pressure and Roto Synth under vacuum) were investi-gated on the dehydration of castor oil. Iodine values andhydroxyl values of the obtained DCO samples were determinedbecause these physical properties show the degree of thedehydration.

a. Dehydration Reaction under Atmospheric Pressure.Because of the fact that microwave heating reduces reactiontime from hours to minutes,13 experiments were conducted from5 to 30 min with 5 min increments using microwave systemunder atmospheric and reduced pressure (Figures 4 and 7). Theresults obtained at atmospheric pressure are given in Figures4-6. First, a suitable reaction time was determined as 20 min(Figure 4). A suitable catalyst ratio (by mass of oil) wasdetermined as 4% depending on the IV (131.0) and HV (22.4).It can clearly be seen from Figure 5 that there are no significantchanges in IV and HV of the oil using further amounts ofcatalyst. Suitable reaction temperature was determined as 250°C according to the results given in Figure 6, because maximumIV (135.8) and minimum HV (12.3) were obtained at thiscondition.

b. Dehydration Reaction under Reduced Pressure. Theresults obtained with Roto Synth are given in Figures 7-11. Asuitable reaction time was determined as 15 min at 210 °C and

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).

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700 mbar (Figure 7). After determining the reaction time, thecatalyst ratio was determined as 4% by mass of the oil accordingto the IV and HV of the obtained oil (Figure 8). Suitable reactiontemperature was determined as 250 °C using 4% catalyst and15 min reaction time, because the IV increased and HV decreasedas the temperature was increased (Figure 9).

To achieve the lowest hydroxyl value, reaction times wereextended to 25 min, and then a suitable reaction time was foundas 20 min (Figure 10) because IV increased from 127.2 to 131.0and hydroxyl value decreased from 30.6 to 21.2, which are moredesirable values. Because the maximum operating temperatureof the microwave system is 250 °C, instead of increasing thetemperature, further experiments were carried out under reducedpressure 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 500mbar pressure (Figure 11).

According to Figure 11, the iodine value of the DCOincreased as long as the pressure decreased and it reached amaximum value at 500 mbar. It then started to decrease becausefurther reduction of pressure leads to polymerization with aconsequence drop in iodine value and a step rise in hydroxylvalue.17

The moisture content of dehydrated castor oil samplesobtained at atmospheric conditions was found as 0.25% usingKarl Fischer apparatus, whereas there is no water left in theDCO obtained under vacuum conditions.

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).

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Fatty acid compositions of raw castor oil and DCO obtainedunder N2 flow and under vacuum are summarized in Table 1.

As it can clearly be seen from the table, almost all of thericinoleic acid in castor oil (87.2%) has been converted intoconjugated and nonconjugated linoleic acids.

Conclusion

The main fatty acid of castor oil is ricinoleic acid, whichcomprises about 90% of the triglyceride molecule. Dehydrationreaction occurs in the ricinoleic acid molecule, which splits fromthe hydroxyl group of the molecule with an adjacent hydrogenatom, and water is formed consequently. The resulting productsconsist of mainly two fatty acids (9,12 linoleic acid and 9,11linoleic acid). Analytical features like hydroxyl value, iodinevalue, viscosity, and refractive index show the degree ofdehydration reaction, and as hydroxyl groups are removedduring the course of the reaction, iodine value increases,hydroxyl value and viscosity decrease, and as a result refractiveindex 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 20min reaction time, 250 °C temperature, and 4% of catalyst bymass of the oil. For the reactions under vacuum, DCO with aHV of 11.9 and IV of 140.0 was obtained at 20 min reaction

time, 250 °C temperature, 500 mbar pressure, and 4% of catalystby mass of the oil.

To increase IV of dehydrated castor oil, the same reactionwas carried out under reduced pressure. DCO was obtained withan IV of 140.0, which is slightly higher than the value of theoil 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 wereavoided under vacuum during removal of water from reactionmedia.

The water content of dehydrated castor oil obtained atmo-spheric pressure is determined as 0.25%, while there is nomoisture left in the dehydrated castor oil obtained under vacuum.Water and volatile decomposition products can effectively beremoved from the reaction media when the experiments arecarried out under vacuum, and therefore water separation stepscan be eliminated.

Castor oil dehydrated under reduced pressure is always palerin color than DCO obtained at atmospheric pressure,20 whichis more preferable in paint industry.

As bounded OH group to the ricinoleic acid breaks off themolecule, the number of the double bonds increases, and fattyacids with different geometry and positions were obtaineddepending on the reaction conditions. At atmospheric conditions,the total CLA content of the oil is 43.1%, whereas undervacuum, the total CLA content is 37.5%. The results show thatatmospheric pressure could be preferred to obtain oil havinghigher CLA, and vacuum pressure could be preferred to produceoil having higher NCLA. The oxidative stability of CLAsrelative to other polyunsaturated fatty acids readily decomposesdue to the formation of unstable free-radical intermediates duringoxidation.18 Formerly, it was believed that the main productwas the conjugated 9,11 linoleic acid, but later work showedthat 9-12 linoleic acids are usually present in greater propor-tions than are conjugated acids.19 That is why obtaining DCOunder reduced pressure is more favorable.

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 DCO (under N2) DCO (under vacuum)

16:0 1.4 1.6 1.918:0 1.6 1.8 2.018:1 3.8 4.3 4.718:2 NCLAa 5.5 49.2 53.918:2 CLA (total)b 43.1 37.518:3 0.518:1, OH 87.2 trace tracetotal saturated 3.0 3.4 3.9total unsaturated 97.0 96.6 96.1

a Nonconjugated linoleic acid. b 9-cis,11-trans-Linoleic acid, 10-trans,12-cis-linoleic acid, 9-cis,11-cis-linoleic acid, 10-trans,12-trans-linoleic acid, and9-trans,11-trans-linoleic acid.

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The properties of castor oil are in good agreement with thepublished data.6,9,11 According to the results, it can be concludedthat microwave heating has reduced the reaction time from hours(1, 2, 3 h) to minutes (20 min) as compared to literature datacarried out with conventional heating techniques.6,9,10

Acknowledgment

Financial support from The Scientific and TechnologicalResearch Council of Turkey (TUBITAK Project No. 105M 289)is gratefully acknowledged.

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(10) Villeneuve, P.; Barouh, N.; Barea, B.; Piombo, G.; Figuera-Espinoza, M. C.; Turon, F.; Pina, M.; Lago, R. Chemoenzymatic Synthesisof Structured Triacylglycerols with Conjugated Linoleic Acids (CLA) inCentral Position. Food Chem. 2007, 100, 1443–1452.

(11) Chowdhury, D. K.; Mukherji, B. K. Studies on Dehydrated CastorOil-Part I. J. Am. Oil Chem. Soc. 1956, 22, 189–198.

(12) Lidstrom, P.; Tierney, J.; Watley, B.; Westman, J. Microwave-Assisted Organic Aynthesis-A Review. Tetrahedron 2001, 57, 9225–9283.

(13) Azcan, N.; Demirel, E. Obtaining 2-Octanol, 2-Octanone, andSebacic 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 Animaland 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 ofOfficial Analytical Chemists; AOAC Publications: Arlington, VA, 1984.

(17) Thi, M. M.; Hlaing, N. N.; Oo, M. M. Production of Alkyd Resinfrom Vegetable Oils. GMSARN International Conference on SustainableDeVelopment: Issues and Prospects for the GMS, 2008.

(18) Jie, M. S. F. L. K.; Pasha, M. K. Fatty Acids, Fatty Acid Analoguesand Their Derivatives. Nat. Prod. Rep. 1998, 15, 609.

(19) Hilditch, T. P. The Chemical Constitution of Natural Fats; JohnWiley & Sons: New York, 1954.

(20) Dole, K. K.; Keskar, V. R. Dehydration of Castor Oil by SubstitutedSulphonic Acids and Their Salts. Proc. Math. Sci. 1953, 38, 135–142.

ReceiVed for reView June 17, 2010ReVised manuscript receiVed September 24, 2010

Accepted November 8, 2010

IE1013037

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