Centrifugal Reactor Separator

Production of Biodiesel at the Kinetic Limit in a Centrifugal Reactor/Separator

Joanna McFarlane,* Costas Tsouris, Joseph F. Birdwell, Jr., Denise L. Schuh, Hal L. Jennings,Amy M. Palmer Boitrago, and Sarah M. Terpstra

Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, Tennessee 37831-6181

The kinetics of the transesterification of soybean oil has been investigated in a centrifugal contactor reactor/separator at temperatures from 45 to 80 °C and pressures up to 2.6 bar. The high shear force and turbulentmixing achieved in the contactor minimized the effect of diffusion on the apparent reaction rate, and henceit could be assumed that the transesterification rate was limited by the reaction kinetics. The yields of productmethyl esters were quantified using gas chromatography flame ionization detection (GC-FID), infrared (IR)spectroscopy, proton nuclear magnetic resonance (H1NMR), and viscosity measurements and typically werefound to achieve 90% of complete conversion within 2 min. However, to meet American Society for Testingand Materials (ASTM) specifications with one pass through the reactor, a minimum 22-min residence timeat 80 °C was needed. Performance was improved by stepwise processing, allowing separation of byproductglycerine and injection of additional small aliquots of methanol at each step. The chemical kinetics wassuccessfully modeled using a three-step mechanism of reversible reactions, and employing activation energiesfrom the literature, with some modification in pre-exponential factors. The mechanism correctly predictedthe exponential decline in reaction rate as increasing methyl ester and glycerine concentrations allow reversereactions to occur at significant rates.

Introduction

A variety of alternatives to petroleum are being developedas vehicular fuels to reduce dependence on gasoline and dieselfuel.1 Biodiesel, a product of the transesterification of fats andoils, is commercially available for blending with standard dieselfuel. Industrial production of biodiesel from oil of low fatty-acid content follows homogeneous base-catalyzed transesteri-fication, a sequential reaction of the parent triglyceride with analcohol, usually methanol, into methyl ester and glycerolproducts. Although the price of diesel fuel has increased,economical production of biodiesel is a challenge because of(1) the increasing price of oil feedstocks and reagent methanol,(2) a distributed supply of feedstocks that reduces the potentialfor economies of scale, (3) processing conditions that includepressures and temperatures above ambient, and (4) multipleprocessing steps to reduce contaminants to ASTM specificationD6751 limits.2,3

Commercially produced biodiesel is made in batch reactorsto achieve high enough yields. Producers and investigators havefocused on the kinetics of transesterification to see if conversionsto methyl ester are limited by mass transfer effects or by slowkinetics.4,5 Much of the cost of biodiesel production is relatedto the conversion of the oil to the methyl ester; hence, theprocess would be improved by the use of a continuous ratherthan batch process, with energy savings generated by combinedreaction and separation, online analysis, and reagent methanoladded by titration as needed to produce ASTM specificationgrade fuel. Hence, Oak Ridge National Laboratory (ORNL) hasbeen investigating the use of a centrifugal reactor/separator toincrease mass transfer and reduce the transesterification process-ing time to that limited by chemical kinetics.

Centrifugal contactors were initially developed as solventextraction devices for use in actinide recovery from spent nuclearreactor fuel in place of mixer-settlers.6,7 Because of its ease ofoperation, rapid attainment of steady state, high mass transfer

and phase separation efficiencies, and compact size, thecentrifugal contactor was chosen for intensification of thebiodiesel production process. In these experiments the im-miscible liquid reagents were introduced into the reactor as twoseparate feed streams: a mixture of methanol and base catalyst,and vegetable oil. Following reaction, the immiscible glyceroland methyl ester products were separated by centrifugal forcein the same vessel. A commercial unit was modified to increasethe residence time from a few seconds to a few minutes byachieving hold-up in the mixing zone.8,9 The advantage of thisdevice over previously described centrifugal reactors (e.g., byPeterson and co-workers10) is that the increased residence timeis integrated into the unit design and additional delay loops andprocessing are not required.

Through understanding of the chemical kinetics of transes-terification in the centrifugal reactor/separator, the objective wasto develop an efficient, high-yield, continuous process for base-catalyzed biodiesel production. The model developed to describethe data can be used to optimize the process for maximumthroughput or minimum residence time and minimal use ofmethanol, key factors in the viability of biodiesel productionfrom triglyceride.

Experimental Section

Centrifugal Contactor. Transesterification of commercialgrade soy oil was carried out in a modified centrifugal contactor.The distinctive feature of the centrifugal contactor apparatus isthe use of a rotor within a stationary cylinder (the housing) toaccomplish the intimate mixing of two immiscible liquids toproduce a dispersion, which is then subjected to a centrifugalforce used to separate the two liquid phases.11 In solventextraction applications, the generation of a finely divideddispersion promotes the transfer of one or more solutes fromone liquid phase into another by maximizing interfacial area.Similarly, in the case of a chemical reaction between immisciblereactants, the creation of a dispersion minimizes the diffusionpathway of reactants to the interfacial boundary. In both cases,

* To whom correspondence should be addressed. Tel.: 865-574-4941.Fax: 865-241-4829. E-mail: [email protected].

Ind. Eng. Chem. Res. 2010, 49, 3160–31693160

10.1021/ie901229x 2010 American Chemical SocietyPublished on Web 03/02/2010

dispersion formation is accomplished by means of Couettemixing, in which shear forces are created in the annulus betweena stationary outer cylinder and, in this case, a 5-cm-diameterrotating inner cylinder, as shown in Figure 1.8

As the dispersion flows downward inside the housing, it entersthe rotor (i.e., the separation zone) through an opening locatedat the bottom end of the rotor at the centerline. Angular inertiacreated by the rotor causes the liquid to be pushed against theinner rotor wall, with fluid slip being prevented by axial vanesthat are attached to the inner wall. The resulting centrifugal forcecoalesces the dispersion into its constituent phases, whichbecome layered axially based on their relative densities. Weirslocated at the upper end of the rotor control both the flowthrough the rotor and the axial location of the interface betweenthe liquid phases; hence the separation of liquid phases passingthrough the rotor is based on density difference.

Although the main goal of the project was to evaluate acontinuous production of methyl ester in the reactor/separator,kinetics studies were also carried out in batch mode to facilitatesampling at frequent intervals, and batch experiments wereundertaken at above ambient pressure in the centrifugal contactorreaction vessel to increase the reaction yield. Hence, transes-terification kinetics was studied in three configurations: (1) ina stirred glass reaction vessel to enable visual monitoring ofthe fluids and in a 200-mL centrifugal reactor/separator run in(2) batch and (3) flow-through mode.

Materials. Reagents were soybean oil, methanol, and 30%methanol/methylate (all from Nu-Energie, LLC, http://www.nu-energie.com) at volumetric phase ratios from 4:1 to 6:1 oil/methanol, or mole ratios with an excess of methanol, from 5.62to 4.71. These phase ratios were used for both the static andflow-through experiments. The benchmark methanol-to-oil moleratio was the same as the one used at the Nu-Energie plant,that is, ∼4.81. All reagents were used without further purifica-tion to simulate conditions at the manufacturing facility. Theacid number of the feedstock oil was determined by titrationwith alcoholic KOH to be 0.06 ( 0.01 mg KOH/g. In the batchexperiments, the oil was preheated to the desired temperaturebefore the addition of the methanol/methylate mixture.

Kinetics Experiments in Stirred Glass Vessel. A magneti-cally stirred (∼100 rpm) glass reaction vessel (500 mL) was

heated to temperatures between 45 and 60 °C using flow froma VWR thermostatted bath through a temperature-controlledwater jacket. Temperatures were measured using a Type Kthermocouple. Reaction mixtures of total volumes between 100and 200 mL, methanol-to-oil molar phase ratios of 4.8 to 5.3,were heated separately, then combined in the reaction vessel atthe initial time of the experiment. Sampling from the reactionvessel was done by pipetting a portion from the mixture intovials containing 1 mL of 1 M HCl to stop the transesterificationprocess. Sampling was performed at several intervals afterinjection of the methanol: 15, 30, 45, 60, 90, 120, 180, 240,300, 420, 600, and 900 s, to within (5 s. Samples were analyzedby gas chromatography (GC).

Flow-Through Experiments in Centrifugal Contactor.Base-catalyzed biodiesel synthesis and simultaneous separationof methyl ester and glycerol products were carried out in amodified centrifugal contactor.9 The reactor was operated incontinuous mode at temperatures from 50 to 60 °C, with resistiveheating controlled with a Thermolyne controller (model 45500).The methanol-to-oil molar phase ratio, nominally 4.87, wascontrolled through the volumetric flow rates pumped (FluidMetering, Inc.) to the reactor: the oil flow rate ranging from50.0 to 150.0 ( 0.5 mL ·min-1, and the methanol with basecatalyst flow rate ranging from 10.0 to 30.0 ( 0.5 mL ·min-1.Samples of product were taken from the lighter-flow andheavier-flow discharges of the reactor/separator. The totalvolumetric flow was set so as to obtain a mean reactor residencetime of 1-3 min for a total volume of 180 mL. A time of 5min was needed to establish stable flows through the contactoras the glycerine phase was only one-tenth of the volume of themethyl ester phase. Samples were taken at intervals between 1and 20 min after flow was initiated. These samples were alsoacidified by contact with 1 M HCl to prevent further reactionand were analyzed using GC.

Observations made during testing using a contactor with atransparent Lucite housing and dyed feed solution indicated thatthe residence time distribution was broad. Although the desiredaverage residence time was set as 1 min, a small amount ofmaterial exited the apparatus less than 10 s after introductionbecause of pulsing in the mixing zone. This effect was reducedin later experiments by lengthening the contactor housing.

Figure 1. Schematic showing fluid flows in a centrifugal contactor. The reagents can be introduced separately through one or more of the solution inlets. Theflow-through apparatus includes additional side ports, allowing recirculation of the mixture through the mixing zone.

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Elevated Pressure Tests in Contactor Done with SingleCharge of Reagents. An important performance goal forbiodiesel production as a fuel is the achievement of an ASTM-defined level of conversion from the triglycerides. Tests wereperformed at 50 and 60 °C as well as temperatures above theboiling point of methanol in a sealed vessel. For tests at ambientpressure, the procedure followed was very similar to thatdescribed for tests in the stirred glass vessel. In testing performedat elevated (i.e., above ambient) pressure, however, the contactorhad to be operated in batch mode because pressurizable feedand product tanks were not available. The oil was heated in thecontactor to the desired temperature, 80 °C, under a blanket ofabout 1.5 bar N2 with the rotor at full speed. Methanol and basewere then injected under pressure, at ∼2 bar, into the housing.During the reaction, the pressure in the vessel increased, usuallyto 2.6 bar. In these tests, the device was configured so as toprevent transfer of fluids from the reaction zone into theseparation zone. Reaction products were removed directly fromthe reaction zone at the completion of each test and analyzedusing infrared spectroscopy (IR), viscosity measurements, andnuclear magnetic resonance spectroscopy (NMR). The lengthsof batch-mode runs were varied to determine the progress ofthe reaction under pressures up to 2.6 bar and temperatures upto 80 °C. In some cases the glycerine was separated from theoil/methyl ester mixture, the latter being again reacted in thecontactor to see if removing the byproduct could improve theyield of methyl ester.

Chemical Composition Analysis. Gas chromatography wasused to determine changes in composition as a function of time;however, because the sample preparation removed the productglycerine and the unreacted triglyceride peaks were not detect-able, these data could not be used for quantification of thefraction of bound glycerine. For these measurements, IR,viscosity, and NMR analyses were used.

For the kinetics data, reactions were halted by contact of thesample with an equal volume of 1 M HCl. The samples werecentrifuged, and the organic layer was separated and rinsed withtwice the volume of deionized water to remove much of theunreacted methanol. Although most of the methanol wasassociated with the glycerine phase, a significant fractionremained in the methyl ester, and had to be removed prior toanalysis.

Reaction products were analyzed by GC FID, with a Hewlett-Packard 5890 II GC. The analysis procedure followed ASTMD6854.12 The precision in the measured peak areas wasestimated to be (10% from the injection of reference standards.Infrared (IR) analysis of the products was performed using botha Thermo-Nicolet Magna-IR 560 Fourier transform infrared(FTIR) spectrometer and a Bruker FTIR spectrometer.The Bruker analyses were conducted off-site by using theCognis model QTA system (http://www.cognis.com/products/Business+Units/AgroSolutions/Grain+Analysis/) through Nu-Energie LLC, and were reported with a precision as low as (0.01 wt % in the triglyceride, diglyceride, and monoglyceridecomponents of the “bound” glycerine fraction. (Bound glycerinerefers to the wt% of the glycerine backbone in the acylglyceridemolecule.) However, the accuracy of the results was estimatedto be (0.1 wt % as a result of uncertainties introduced duringsample preparation. For instance, it was found that the analyticalresults were biased if unreacted methanol was not removed bywashing either with deionized water or with 1 M HCl.

Changes in viscosity, measured with a Brookfield modelDV-E viscometer, were correlated with reaction yield and wereused to monitor progress toward the ASTM standard in bound

glycerine concentration (<0.24 wt %). Although the mainpurpose for conversion of the triglyceride to methyl ester is toreduce viscosity, this type of analysis is not standard formonitoring reaction progress and so is discussed here in somedetail. A calibration curve for viscosity measurements wasdeveloped using the Nu-Energie, LLC, commercial methyl esterproduct mixed with unreacted-soy oil, washed, and heated toremove any residual methanol in the same manner as thesamples from the experiments (Figure 2). All viscosities weremeasured at 25 °C, as established by using a water jacket onthe viscometer fed by a thermostatted water bath. The biodieselcalibration samples were found to degrade over time. Hence,the overall accuracy of the viscosity analysis was estimated tobe (1 wt % unreacted oil.

Samples were analyzed by H1NMR in a Bruker Avance-400NMR after dissolution in CDCl3 (Aldrich, lot no. 00808TH,0.03% v/v tetramethylsilane, TMS). Peak shifts are given inTable 1 and compared to the Sadtler compilation.13 Because ofthe variety of constituent fatty acid chains, the peaks weregenerally quite broad, with the sharpest being the methyl groupon the methyl-ester product, at δ ) 3.7. The peaks that wereuniquely attributed to the acylglyceride were present at a shiftbetween 4.1 and 4.3 ppm. A small sidebar on the 3.7 peak couldbe attributed to hydrogen on the C-O-H of the glycerinebackbone of the partially reacted acylglyceride.

Results

The focus of the study was to demonstrate the continuousproduction of biodiesel in the reactor/separator. Samples fromthe flow-through contactor experiments (phase ratios rangingfrom 4.1 to 4.9 mols methanol-to-oil) were analyzed using gas

Figure 2. Viscosity as a function of bound glycerine in biodiesel at 25 °C.The weight fraction of bound glycerine (9) is not identical to the amountof unreacted oil ([) as the latter does not include the diglycerides andmonoglycerides.

Table 1. NMR Peak Shifts Observed from Samples of Methyl Estersand Soybean Oil

shiftrelativeto TMS splitting assignment sample

0.8 multiplet Terminal CH3 soybean oil, methyl ester1.3 doublet -CH2- soybean oil, methyl ester2.0 multiplet -CH2- beta to CdO soybean oil, methyl ester2.3 triplet -CH2- alpha to CdO soybean oil, methyl ester2.8 triplet �-H to double bond soybean oil, methyl ester3.6 singlet free-glycerol H methyl ester3.7 singlet CH3-O- methyl ester4.1-4.3 multiplet H on glycerine backbone soybean oil5.3 multiplet R-H to double bond soybean oil, methyl ester

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chromatography (Figures 3 and 4) and IR spectroscopy. Figure3 shows a typical gas chromatograph for samples taken from amethanolysis reaction. Gas chromatographic data from flowthrough experiments are shown in Figure 4, where results arepresented in a bar chart showing the analysis of flows from theheavy and light phase ports of the contactor, taken by summingGC peak areas corresponding to C16, C18, C20, C22, and C24methyl esters. Figure 4 shows the relative change in compositionas a function of time after initiating flow through the contactor,hence the units on the ordinate are arbitrary.

The gas chromatograms show significant production of methylesters with little residual acylglyceride. However, results fromthe IR analysis indicate some contamination of the methyl esterwith oil. This was thought to have occurred because some ofthe reagents experienced a residence time much shorter thanthat predicted by the average flow rate through the reactor. Afairly broad distribution in residence time was confirmed byvisualization experiments in the clear-housing contactor, wheresome of the dyed solution passed through the reactor in a fewseconds. Hence, although qualitative proof-of-principle dem-onstration of the reactor/separator was obtained in these experi-ments, they did not give data that could be used to develop orconfirm a kinetic model. After a couple of minutes of operation,steady flows were established through the reactor outlets. Thehigher the methanol-to-oil molar ratio and the greater the rotorspeed was, which was varied between 3600 and 4200 rpm inthe tests, the greater was the reaction yield as demonstrated bysteady flows of products from both outlets of the contactor. Withthe exception of the flow at the mole ratio of 4.9 and rotor speedof 3600 rpm, all of the tests indicated steady flows beingachieved within 5 min.

In Figure 4, it can be seen that it took longer to establish aflow through the heavy side port than the light side port meaningthat at first the flow was mainly methyl ester product. Per unitvolume, glycerine was produced at a much lower rate thanmethyl ester. This fact, coupled with the high viscosity ofglycerine, meant that several minutes were required to establishflows through the reactor. In addition, the weir in the dispersionzone was deliberately set so as to produce as pure a methylester product as possible, leaving residual unreacted oil in theglycerine stream. All of the factors discussed above suggested

that a quantitative kinetic analysis and comparison with a modelbe done using well-stirred batch reactors rather than in the flow-through configuration.

Typical kinetic results from the glass stirred vessel at 45,52, and 60 °C as analyzed by gas chromatography are shownin Figure 5. In these tests a steep rise in the peak area attributedto the C16 (methyl palmitate) and C18 (methyl oleate, methyllinolate, and methyl linoleate) esters was observed in the firstminute of residence time, and then a leveling off, or evendecrease, in the signal occurred. The relative signal intensitywas corrected by a naphthalene-d8 internal standard; hence,these data have arbitrary units. As expected, the initial rise insignal was steeper at 60 °C than at 45 °C because of increasingreaction rates. Because conversion to methyl ester in the reactorappeared to plateau after an initially rapid increase, this behaviorsuggested that the reaction rate was not limited by mass transferto the interface. In fact, a slow degradation of product over timecould be seen in decreased and broadened methyl ester peaks.Discoloration and darkening was also observed. Thermaldegradation is expected to occur through beta scission adjacentto the carbonyl group and the unsaturated bonds in the fattyacid chains.14,15 As the triglycerides that comprise soybean oilare significantly unsaturated, it is possible that this mechanismof decomposition occurred when the oil was heated for severalminutessup to half an hour when including the preheatingperiod before the addition of methanol.

Kinetic results from batch ambient-pressure tests in thereactor/separator done at 50 °C and at a methanol-to-oil moleratio of 5.74 are presented in Figure 6. Because the system wasnot pressurized, sampling was done online through a port atthe base of the vessel. Data were taken from IR measurementsusing the Bruker spectrometer, starting at 3 min after thereagents were first mixed in the reactor. Infrared data at earliertimes were not available because the commercial spectrometerhad been calibrated only for acylglyceride concentrations lessthan 7 wt % triglyceride, 5 wt % diglyceride, and 3 wt %monoglyceride. At a time of 3 min after the reaction wasinitiated the amount of triglyceride was less than 6 wt % (Figure6) and decreasing exponentially (Figure 7), and the ratiosbetween the diglycerides and monoglycerides were fairlyconstant, within the reproducibility of the data, or (0.2 (80 °C).These fairly static concentration ratios between the acylglyc-erides are discussed later in terms of the chemical kinetic modeland used to explain the difficulty in accelerating the productionof biodiesel.

In the batch tests, data were not collected beyond 10 min asthe increase in reaction yield had tapered off. An experimentwas conducted at a slow rotor speed, 3000 rpm at 50 °C, andshowed a reduced rate of conversion to methyl ester. However,even at higher rotor speeds, the overall reaction yield attemperatures up to 60 °C did not go above 90%, indicating thata substantial increase in reaction temperature was needed toreach ASTM grade fuel in a reasonable time. Increasedtemperature required operation at above ambient pressure toprevent vaporization and loss of the methanol reagent.

Results from the pressurized reactor tests from both IRanalysis and viscosity measurements are given in Table 2.Conditions of the reaction are also shown in the table, includingthe maximum pressure and temperature reached. For tests doneunder pressure, the operating condition that worked best wasto slightly pressurize the reactor containing the oil with drynitrogen as it was heated to approximately 80 °C. The pressureincreased rapidly after injection of the methanol, indicative ofthe progress of the transesterification reaction, and dropped again

Figure 3. Typical gas chromatogram of transesterification product fromeither centrifugal reactor or open stirred reaction vessel. This particularanalysis was done on product from an open stirred vessel after 1-min reactiontime, 60 °C, and methanol-to-oil mole ratio of 4.9. Unlabeled peaks includethe solvent (4 min) and free glycerine (8.6 min).


after a few minutes as the system equilibrated between theacylglycerides. Sampling in this case could only be done at theend of the test, as it involved depressurizing the vessel, so eachof the data points came from a separate run. The difference inthe bound glycerine concentrations from the two types ofmeasurement, IR and viscosity analysis, shows that above thedetection limit for viscosity, the two sets of data agreed withinthe reproducibility of the viscosity analysis, or (0.2%, for shortreaction times up to 10 min. Significant discrepancy wasobserved at longer times, which indicated possible degradationof the biodiesel, as observed earlier in the GC analysis.

Results from viscosity measurements are also presentedgraphically in Figure 8, showing the approach to ASTM standardfor bound glycerine (<0.24 wt %). The graph shows a slowincrease in triglyceride conversion from 90% completion at 2min. From these data the minimum time for achievement ofASTM standard in one pass appears to be 22 min at 80 °C.

Reaction yields from IR spectroscopy are presented in Table2, columns 5-8 and 10, and from viscosity measurements,column 9. As can be seen from table for entries correspondingto a total reaction time of 10 min, the reaction yield was high(<95% conversion) for mixtures that were reacted twice at 5min, which can be compared with 96% for one pass at 15 min.In some cases a small amount of methanol, less than 1 mL,was injected into the second reaction stage; however, theresulting increase in methyl ester yield was not significant. Thevariability in the results was indicative of the sensitivity of theanalysis to small errors in sampling, such as contamination ofsampling lines in the contactor with residual oil. Hence, thereported uncertainties in Table 2 were considerably greater thanthe measurement errors discussed in the Experimental Section.

Methanolysis yields at above ambient pressure and temper-ature (temperature of 80 °C at 2.6 bar) suggest that a 2 minresidence time giving a 90% reaction yield would be optimalfor the design of a commercial reactor/separator. Thus, toachieve ASTM specification fuel, that is, <0.24 wt % totalglycerine, two or three cross-current reactors would be neededin series, with the byproduct glycerine being separated fromthe triglyceride/methyl ester mixture in stages. This experiment

Figure 4. Gas chromatographic data showing flows from (A) “light” and (B) “heavy” ports of the centrifugal contactor (flow-through experiment withmethanol-to-oil mole ratio of 4.9, 60 °C, 3600 rpm), at intervals of 1, 3, 6, and 10 min after the flows of reagents were initiated.

Figure 5. Gas chromatographic results for production of C16 and C18methyl esters as a function of reaction time at 45° and 60 °C under ambientpressure in a stirred glass vessel. In both cases, the methanol-to-oil moleratio was 4.9.

Figure 6. Concentration of reagents and intermediates ((0.1 wt %) as afunction of time in batch contactor tests (3600 rpm, 5.73 methanol-to-oilmole ratio, 50 °C): triglyceride (∆), diglyceride (0), monoglyceride (]),total wt % bound glycerine (O), calculated from adding the contributionsfrom the glycerine backbone of each of the acylglycerides. The dashed linerising to the right refers to the extent of the reaction or fraction of boundglycerine converted to methyl esters.

Figure 7. Ratio of triglyceride concentration to initial concentration as afunction of time, taken under batch conditions in the centrifugal contactor,rotor speed of 3600 rpm, 4.87 methanol-to-oil mole ratio. In the case ofthe reaction at 50 °C (upper line), a close-to-linear fit suggests that a pseudo-first-order removal process may be assumed. Deviations from linearity at60 °C ([), suggest that a pseudo-first-order assumption that reagentmethanol is in excess is not valid under these conditions.


was carried out with analysis by NMR, and the results arepresented in Figure 9. In these tests, a small injection ofmethanol into the second and third stages further increased thereaction yield beyond that predicted for a model of a singlestage reactor. The experimental acylglyceride concentration at2 min was lower than predicted, which could have arisen fromsampling errors, but could also be explained by an expectedinitial increase in the amount of bound glycerine as thetriglyceride is converted into di- and monoglycerides. Thedifferent acylglycerides could not be separated in the protonNMR spectra.

Kinetic Analysis and Discussion

The biodiesel production experiments used angular momen-tum to maximize process intensification, a principle espousedby Ramshaw and co-workers,16 and to reduce the effect of masstransfer at the kinetic limit. In these experiments, the quality ofthe biodiesel produced, or the yield of the reaction, reached amaximum at 3600 rpm. At higher rpm (4800) the yielddecreased, as it also did at lower rotational speeds (e.g., 3000rpm). This can be explained from simulation of Taylor-Couette

flow, characteristic of annular centrifugal extractors as done byDeshmukh and co-workers17 using computational fluid dynamics(CFD). At high turbulence, CFD calculations show that intra-vortex transport is fast relative to intervortex transport; thus,increasing rotational speed does not necessarily increase mixingin the reactor. As the contact time between reagents is governedby mixing, higher angular momentum does not necessarilytranslate to higher yields. This effect has also been modeled byYacoub and Maron18 for a reactive systemssequential oxidationof liquids in an annular reactor.

Hence, a kinetic analysis ignoring transport effects wasperformed using a three-step reaction (including back reactions)as proposed by Noureddini and Zhu19 as well as Freedman andco-workers20 for soybean oil and by Karmee and co-workers21

for Pongamia oil and computed using the kinetics codeCHEMKIN.22 In addition to parameters describing the rateequations, thermodynamic data are needed for the computationof changes of state in the reactor, namely, heat capacities,enthalpies, and entropies, over the temperature range of thesimulation. Derivation of functions describing these statevariables is discussed below.

Table 2. Pressurized Transesterification, 3600 rpm

total reaction time((0.08 min)

methanol-to-oilmolar ratio

max temp((1 °C)

max pressure((0.03 bar)

MONOwt %

DIwt %

TRIwt %

total boundglycerine

((0.01 wt %)

bound glycerineby viscosity

((0.2 wt %)afree glycerine

((0.001 wt %)

5 4.8 75.0 2.52 0.375 0.251 0.157 0.783 0.69 0.0015 4.8 82.0 2.52 0.127 0.132 0.073 0.332 NM 0.0005 4.8 82.0 2.52 0.121 0.140 0.078 0.339 NM 0.045

10b 5.0 80.0 1.97 0.255 0.269 0.171 0.696 0.72 0.00110b 5.1 79.5 2.52 0.164 0.159 0.165 0.489 0.51 0.01810b 4.9 82 2.17 0.120 0.082 0.050 0.253 NM 0.00010b 5.9 82 2.17 0.124 0.088 0.050 0.261 NM 0.09810c 4.8 79.5 2.52 0.120 0.107 0.050 0.277 NM 0.00010c 4.8 79.5 2.52 0.119 0.120 0.070 0.309 NM 0.03415 4.8 80 2.48 0.159 0.118 0.070 0.346 1.03 0.00015 4.8 60 1.00 0.115 0.182 0.226 0.523 0.84 0.00815 4.8 80 2.52 0.152 0.120 0.048 0.321 0.43 0.00015 4.8 80 1.93 0.238 0.352 0.194 0.784 2.07 0.00015 4.8 80 2.52 0.179 0.340 0.086 0.604 1.13 0.07830 4.8 80 2.41 0.139 0.070 0.031 0.239 0.07 0.00045 4.8 80 2.48 0.179 0.210 0.117 0.506 1.4 0.04145 4.8 78.9 2.59 0.136 0.105 0.087 0.328 0.66 0.000

a NM ) not measured because the bound glycerine was below the detection limit of the analytical method. b Two 5-min residence periods in thereactor with methanol added to the second batch. c Two 5-min residence periods in the reactor, with no addition of methanol.

Figure 8. Yield of batch transesterification reaction in terms of the weightpercent of triglyceride reacted (2) and remaining total bound glycerine (∆)as a function of reaction time (80 °C, above ambient pressure to 2.6 bar,3600 rpm rotor speed). The arrows indicate the conversion goal of <0.24wt % bound glycerine or 97.8% conversion of acylglyceride.

Figure 9. Bound glycerine (wt %) is plotted as a function of time (min)for the staged production of biodiesel. Both experimental data (80 °C, 3600rpm, pressure up to 2.6 bar, methanol-to-oil mole ratio of 4.81) are presented,along with simulations from a chemical kinetic analysis.


Chemical Kinetics Reaction Scheme.

Assumptions had to be made about the composition of thesoybean oil as the fatty acid composition was not available forthe commercial samples used in this experiment. However, thekinetic mechanism used to model the transesterification processwas not expected to be sensitive to small changes in triglyceridecomposition as reflected in the calculated physical propertiesof the mixture. Hence, the oil composition was assumed to besimilar to that analyzed using high-performance liquid chro-matography by Holcapek and co-workers.23 Their analysisshowed that the soybean oil comprised 52 substituent triglyc-erides, of which the 4 most prevalent were used in our predictionof thermodynamic properties (substituent chains including Ln) linolenic, L ) linoleic, O ) oleic, and P ) palmitic acids).The relative concentrations of the fatty chains in soybean oilconstituent triglycerides are listed in Table 3, along with thepredictions of critical temperature, Tc, and acentric factor, ω,following the procedure of Morad and colleagues.24

Although heat capacities have been measured for selectedtriglycerides,25 the data for this particular mixture for soybeanoil are not available. Hence, these thermodynamic properties,along with ideal gas heat capacities calculated according to themethod of Rihani and Dorasiwamy,26 were used to predict theheat capacity as a function of temperature following the methodgiven by Morad and colleagues24,27 for a mixture of triglycer-ides. The average composition of the triglyceride was estimatedto be C57H92O6. A correction factor was applied to the liquidspecific heat capacity as suggested by Morad et al. Similarpredictions were carried out for the diglyceride and monoglyc-eride intermediates, as well as the methyl esters, using nominalcompositions of C29H64O5, C21H36O4, and C19H32O2, respectively.Over the temperature range of the experiments being simulated,the heat capacities were calculated to vary linearly withtemperature. Previous work28 has indicated that the thermody-namic properties of the various triglycerides do not vary greatlybetween chain lengths of (2, and hence these predictedproperties were considered to be sufficiently accurate for thekinetic model used in this analysis. Thermodynamic data for

glycerine, methanol, and water were taken from the NationalInstitute of Standards and Technology (NIST) compilation.29

The second-order rate constants, k1 through k-3, were assumedto have no temperature dependence in the pre-exponential factor,following the standard Arrhenius expression: k ) ATne-Ea/(RT),where n ) 0. The shunt reaction proposed by Noureddini wasnot incorporated into the model. The activation energies Ea andpre-exponential factors A were taken from Noureddini and Zhu19

for a well-mixed system (Reynolds number ) 12 400), and arereported in Table 4, along with rate constants for the system at80 °C. The rate constants for the reactions involving thediglyceride were reported to be an order of magnitude greaterthan the reactions involving the triglyceride and monoglyceride.Although Noureddini does not explicitly discuss this, anargument could be made that the formation of methyl ester atthe reaction site improves the local miscibility of the reagents,thus increasing the rate of reaction. Mechanisms proposed by adifferent group20 do not show the same discrepancy, but includefitting parameters, such as shunt reactions, to better simulatetheir data.

Results from the kinetic model are presented in Figures 9-11.Overall, the long-time predicted behavior approximated theapproach to limiting methyl ester yield observed in the pres-surized batch contactor reactions, also shown earlier in Figure9. The overall rate of the production of methyl ester can bewritten as

The mechanism above also includes the reaction for thehydrolysis of methyl ester, k4. When the rates of forward and

Table 3. Triglyceride Composition of Soybean Oil and AssociatedThermodynamic Properties

triglyceride

normalizedmole

fraction

Ln(linolenic)

C18:3

L(linoleic)

C18:2

O(oleic)C18:1

P(palmitic)

C16:0

LLLn 0.18 1 2 0 0LLL 0.34 0 3 0 0OLL 0.27 0 2 1 0LLP 0.21 0 2 0 1mole fraction 0.083 0.75 0.083 0.083molecular

weight, g278 278 280 282 256

Tc, °C 545 547 547 546 526ω 1.13 1.07 1.13 1.18 1.11correction factor,

J g-1 K-1-0.2946

C57H92O6 (TRI) + CH4O (methanol) y\zk1

k-1

C39H64O5 (DI) +

C19H32O2 (ME)

C39H64O5 + CH4O y\zk2

k-2

C21H36O4 (MONO) + C19H32O2

C21H36O4 + CH4O y\zk3

k-3

C3H8O3 (glycerine) + C19H32O2

Table 4. Reaction Mechanism Values for CHEMKIN Analysis

rate constants(cm3 ·mole-1 · s-1)

Ea/R(K)

A(cm3 ·mole-1 · s-1)

k (80 °C)(cm3 ·mole-1 · s-1)

k1 6844 1.33 × 109 5.04k-1 4923 7.64 × 106 6.70k2 9445 1.80 × 1013 43.0k-2 5625 7.48 × 108 89.9k3 2607 1.29 × 104 8.01k-3 4969 5.59 × 105 0.431

Figure 10. Experimental mole ratios of diglyceride/triglyceride (+) andmonoglyceride/diglyceride (/), at 80 °C and 3600 rpm in the pressurizedbatch reactor, and simulation results for the diglyceride/triglyceride ratio(solid line) and the monoglyceride/diglyceride ratio (dashed line). Simula-tions shown on the graph use rate constants from Noureddini and Zhu,19

brown lines, and modifications to k1 and k3 (blue lines), as specified in thetext.

d[ME]dt

) k1[TRI][CH3OH] - k-1[DI][ME] +

k2[DI][CH3OH] - k-2[MONO][ME] +k3[MONO][CH3OH] - k-3[ME][C3H8O3] - k4[ME][H2O]


backward reactions become equivalent in each of the steps ofthe methanolysis mechanism, the rate of production of methylester approaches zero, which is what was observed in theseexperiments.

A sensitivity analysis was performed on the rate of productionof methyl ester with respect to the rates of conversion oftriglyceride, diglyceride, and monoglyceride (Figure 11). Firstorder sensitivity coefficients for species concentration andreaction rate on model parameters were simultaneously com-puted along with the solution of the differential equationsdescribing the reaction mechanism.22 At early times or up to2.5 min, the rate of production of methyl ester was mostsensitive to the rate of the decomposition of the triglyceride(the blue line), while at later times, the rate of conversion ofthe monoglyceride to methyl ester (the green line) became moreimportant. These sensitivity results were used to select whichrate constants to vary to achieve better simulation of the data.Although the simulation shown in Figure 11 is for transesteri-fication at 80 °C, similar results were observed at higher andlower temperatures, with the time scale being shortened orlengthened, respectively.

Results of the sensitivity analysis were used in furthermodeling of the transesterification process to optimize the rateconstants and reproduce the experimental data. The brown linesin Figure 10 were calculated using the rate constants given inTable 4. The blue lines show results of simulations in whichthe pre-exponential factor for k1 and k3 had each been increasedby a factor of 2 to better fit the experimentally determined ratioof diglyceride/triglyceride. The reason for the better fit withhigher pre-exponential factors is not understood, but could bedue to differences in soy oil composition or to differences inmass transfer between this experiment and those done byNoureddini and co-workers,19 even though an attempt was madeby us and them to minimize the effect of mixing.

The kinetic calculations performed here included saponifica-tion, or the hydrolysis of methyl ester. Activation energies werederived from Smith and Levenson30 for the saponification ofthe ethyl ester, as a comparable value for the methyl ester wasnot available. A reaction was also included to model thepreferential partitioning of methanol into glycerine and awayfrom the oil and methyl ester. The rate constant was estimatedfrom values for hydrocarbons measured by Castells and col-leagues.31 However, inclusion of these reactions was found to

have little effect on the overall yield of methyl ester, althoughsmall amounts of soap (hydrolysis product) were predicted whenwater was present in the mixture.

Similar steady-state behavior has been observed by Karmeeand co-workers21 in the transesterification of Pongamia oil, butthe latter experiments were conducted for hundreds of minutes.Hence, with the high degree of mixing in the contactor system,the yield of the reaction appears to be driven by the kinetics upto the thermodynamic limit. In particular, the transesterificationprocess is impeded by the build up of products in the reactor,including glycerine that can recombine with methyl ester. Hence,if the reaction mixture enters the rotor after a couple of minutes,and methyl esters are separated from byproduct glycerine, itshould be possible to reduce the rate of back reactions and drivethe process to >98% conversion.

As discussed earlier, the gas chromatographic results (Figure5) indicated a degradation of product with heating time, likelyarising from beta fission of the alkyl chain. Degradationreactions were not included in the kinetics mechanism as specificdegradation products were not identified in the gas chromato-gram, rather, a broadening of the peaks was attributed togeneration of free fatty acids and fragments along with adecreased signal-to-noise ratio.

The kinetic analysis of transesterification in the centrifugalcontactor will be employed to explain and optimize futureexperiments where two or three contactors will be taggedtogether in a cross-flow configuration. For instance, reactionyield in single-stage reactors at longer residence times can becompared with yields in reactors at multiple stages with shortresidence times. Also, yields can be calculated with differingamounts of methanol added between the reactor stages. Finally,the transesterification reactions actually take place at theinterface between the immiscible reagents, thus the system isinhomogeneous. A model that accounts for fluid flow and masstransfer is needed before the reactor is scaled-up to a pilot-scale or commercial sized plant.

Conclusions

Kinetics experiments were carried out to understand theprocess of transesterification in a centrifugal contactor. As theresults describe, the ASTM specification for bound acylglyc-erides was achieved only at long reaction times in a single-stage batch reactor, greater than 22 min, even when conductedat elevated temperature and pressure. The approach to equilib-rium between the acylglycerides limits the overall rate ofproduction of methyl ester. After the reactants were in a well-mixed reactor for a few minutes, the centrifugal contactor forexample, their overall conversion to methyl esters was foundto be only dependent on the temperature.

Hence, in the single-pass configuration, the residence timerequired to achieve ASTM specification gives little throughputadvantage over the current batch reaction process. In addition,achieving a residence time of several minutes is not feasible inthe contactor as it is currently engineered. Aside from temper-ature, the limitation to achieving complete reaction seems tobe the presence of the products, including glycerine, whichhinder complete conversion because of reversible reactions.Significant improvement in quality was indicated after a secondand third pass, where product and unreacted oil from the firstand second stages were collected and separated from theglycerine. The separated mixture was further reacted with aminor addition of methanol. The kinetic analysis suggests thatthe explanation for this improvement is the reduction in the rateof the back reactions that hinder the overall conversion to methylesters.

Figure 11. Relative sensitivity coefficients (arbitrary units) for productionof methyl ester on the rate of conversion of triglyceride, diglyceride,monoglyceride, and hydrolysis of methyl ester over the time of thesimulation (10 min). The simulation was carried out for a reaction at 80°C, 2.6 bar, similar to the kinetic tests in the pressurized batch reactor.


These findings could be used to develop a pilot scale systemfor the continuous production of biodiesel. A second or thirdstage will increase the capital investment required to implementthe process improvement, but the increase should be offset byreduced operating costs.

Acknowledgment

The authors thank Nu-Energie (particularly Brian Hulletteand Joel Day) for providing the reaction materials and help withthe analysis of the product, and Elizabeth Ashby for herassistance with the chemical kinetic modeling. Preliminaryresearch was sponsored by the Laboratory Directed Researchand Development Program of Oak Ridge National Laboratory,managed by UT-Battelle, LLC, for the U.S. Department ofEnergy. Funding for this project was provided in part by theDepartment of Energy’s Office of Energy Efficiency andRenewable Energy’s Technology Commercialization and De-ployment Program’s Technology Commercialization Fund andby Nu-Energie, LLC, under CRADA No. 01377.

Supporting Information Available: Experimental details andchemical kinetics calculations, namely, input files formatted forCHEMKIN III. This material is available free of charge via theInternet at http://pubs.acs.org.

Appendix

AbbreViationsASTM ) American Society for Testing and MaterialsCFD ) computational fluid dynamicsCRADA ) Cooperative Research and Development AgreementDI ) diglycerideFT-IR ) Fourier transform infrared spectroscopyFT-Raman ) Fourier transform Raman spectroscopyGC-FID ) gas chromatography flame ionization detection1H NMR ) proton nuclear magnetic resonance spectroscopyIR ) infrared spectroscopy (includes FTIR, and FT-Raman)L ) linoleic, -O(CdO)C17H29

LLC ) limited liability companyLn ) linolenic, -O(CdO)C17H27

ME ) methyl esterMONO ) monoglycerideNIST ) National Institute of Standards and TechnologyO ) oleic, -O(CdO)C17H31

ORNL ) Oak Ridge National LaboratoryP ) palmitic, -O(CdO)C15H31

TMS ) tetramethylsilaneTRI ) triglycerideUT ) University of TennesseeSymbolsA ) preexponential factorC16, etc. ) the number of carbons in the fatty acid fragment of

the methyl ester.Ea ) activation energyk ) second order rate constantR ) gas constantT, Tc ) temperature, critical temperatureω ) acentric factor

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ReceiVed for reView August 4, 2009ReVised manuscript receiVed January 26, 2010

Accepted February 10, 2010

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