Purification of glycerol/water solutions from biodiesel synthesis by ion exchange: sodium and chloride removal. Part II

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Research ArticleReceived: 5 November 2008 Revised: 9 December 2008 Accepted: 13 January 2009 Published online in Wiley Interscience: 6 March 2009

(www.interscience.wiley.com) DOI 10.1002/jctb.2144

Purification of glycerol/water solutions frombiodiesel synthesis by ion exchange: sodiumand chloride removal. Part II†

Manuel Carmona, Anna Lech, Antonio de Lucas, Angel Perez andJuan F. Rodriguez∗

Abstract

BACKGROUND: Equilibrium studies were carried out with the aim of finding the basic design parameters for ion exchangeplants using a glycerol phase obtained from biodiesel production. The uptake of sodium and potassium ions on a stronglyacidic ion exchanger, Amberlite IR-120, in the proton form from glycerol/water mixtures has been studied. The effect on theselectivity towards sodium of the percentage of water in glycerine/water mixtures on the macroporous resin Amberlite 252has been analyzed. Finally, chloride removal by a strongly basic anionic-exchange resin Amberlite IRA-420 at three differenttemperatures has been studied.

RESULTS: The strongly acidic ion exchanger Amberlite IR-120 exhibits higher selectivity for potassium versus sodium ions.The ideal mass action law model was able to fit the experimental equilibrium data. The equilibrium data obtained at differentpercentages of water in the glycerine/water mixture indicate that as the water content increased the resin selectivity for sodiumuptake is reduced. The selectivity of the anion exchange resin Amberlite IRA-420 for chloride ions decreases with temperature.The ideal mass action law was accurate enough to fit the equilibrium data of the three systems and allowed the equilibriumthermodynamic properties to be obtained.

CONCLUSIONS: These results confirm that macroporous resin Amberlite 252 could be a good choice to remove sodium ionsfrom glycerol/water solutions with a high salt concentration and also that a strongly basic anionic-exchange resin could be usedfor chloride removal.c© 2009 Society of Chemical Industry

Keywords: glycerol; ion exchange; adsorption; biodiesel

INTRODUCTIONIn recent years the diminishing of fossil fuel supplies, theincrease of petroleum prices and the contribution of fossil fuelcombustion to global warming has created increasing interest inthe development of alternative fuels, mainly biodiesel or ethanol,as renewable transportation fuels. Biodiesel reduces the net carbondioxide emissions, produces less smoke, diesel particulate matter,carbon monoxide and hydrocarbon emissions. Additionally, it isbiodegradable, non-toxic, has a high octane number and providesengine lubrication to low-sulfur diesel fuel.1,2 Recent governmentmeasures like the EU Directive 2003/30/EC on the promotion ofthe use of biofuels or other renewable fuels for transport andthe US Energy Policy Act (EPAct) passed in 1992 to acceleratethe use of alternative fuels in the transportation sector favourbiocomponents playing an increasing role in motor fuel.3,4

Biodiesel is made by the transesterification of vegetable oils andanimal fats with methanol or ethanol homogeneously catalyzedby sulfuric or hydrochloric acid, sodium and potassium hydroxide,or an enzyme such as lipase, or heterogeneously catalyzed bycationic or anionic resins.5,6

Alkaline catalyts are the most widely used because they exhibita higher rate of transesterification under moderate reaction

conditions.6 Glycerol is obtained as a by-product with a productionof close to 10 wt % with respect to the biodiesel.

The glycerol stream, 50 wt % glycerol, leaving the separatorcontains methanol, catalyst and soap. Methanol is recovered byvacuum distillation leaving the salt of the alkaline cation andglycerol with a water content of about 85 wt %.7 – 10 Furtherglycerol refining is required to obtain a product with a purity upto 99.5–99.7% using ion exchange for salt removal followed byvacuum distillation for water elimination.8

Although ion exchange seems to be one of the most suitablemethods for the removal of salts in the case of low saltconcentration in the glycerol solution or as a finishing step, there isa lack of fundamental information about the influence of organic

∗ Correspondence to: Juan F. Rodriguez, Department of Chemical Engineering,University of Castilla-La Mancha, Avda. de Camilo Jose Cela s/n, 13004 CiudadReal, Spain. E-mail: [email protected]

† Part 1: J Chem Tech Biotechnol DOI 10.1002/jctb.2016.

Department of Chemical Engineering, University of Castilla-La Mancha, Avda.de Camilo Jose Cela s/n, 13004 Ciudad Real, Spain

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Purification of glycerol/water solutions from biodiesel synthesis. Part II www.soci.org

solvents on the equilibrium and kinetics of this well known watercation/anion/resin system.

Most of the work previously performed with ion exchangersin non-aqueous media has been done with macroporous ionexchangers.11 A practical example of this is the sorption ofnitrogenated compounds from gasoline onto Amberlite 15, whichrequires in excess of 72 h to reach equilibrium.12 Also Amberlite252, designated by the manufacturer as a macroporous strongacid resin, has been used in previous studies of this group for theremoval of the alkaline catalyst after polyol production.13 – 16 Inalmost all the applications of ion exchangers as catalyst in organicsynthesis a macroporous resin has been employed.17,18 Althoughour research group has made efforts to study the behavior of a gel-form resin in the removal of alkaline metal ions from non-aqueousmixtures, experimental data on the performance of such resins innon-aqueous media are scarce.19 – 21

If the studies and practical applications of ion exchange for thepurification of non-aqueous solution are scant, the knowledge ofthe influence of the proportions of different solvents in a mixtureor the polarity of the solvents on the ion exchange equilibriumor kinetic are almost non-existent.22,23 Equilibrium studies in non-aqueous systems are limited and most cases do not progressfrom the pure solvents.24,25 Some studies have shown that theratios between the solvents in a mixture can change the selectivitytowards the ions involved in a process dramatically.20 It wouldtherefore be of interest to know how the relative proportions ofglycerol and water affect the equilibrium uptake and selectivity ofthe resin towards the ions involved in the process.

Studies have been made of a process in which the residualalkaline hydroxide that remains in the glycerol phase after reactionis neutralized with chloride, and obviously for glycerol purificationpurposes, the chloride ion could also be removed afterwardswith an anion exchange resin. The information about anionexchange in non-aqueous media is even more scarce than forcation exchangers.26 – 28 Thus it is important to carry out a firstapproximation of the possibility of using anion exchangers for thispurpose

The aim of this work is therefore to clarify the previously men-tioned subjects. First, to understand the sodium and potassiumequilibrium data on the strongly acidic gell-form cation-exchangeresin Amberlite IR-120 at 318 K. Second, to compare the perfor-mance of Amberlite IR-120 and 252 and to select the more selectivein glycerol-water media by studing the equilibrium of the binarysystem H+/Na+ on the selected resin in mixtures of glycerol/waterin different proportions. Third, to understand the equilibrium be-haviour of chloride ions on the strongly basic anion-exchange resinAmberlite IRA-420 in OH−-form at different temperatures. In thethree cases the basic equilibrium parameters of the H+/Na+ andOH−/Cl− systems was obtained by applying the mass action law.

EXPERIMENTAL SECTIONChemicalsGlycerine (C3H8O3) with a water content of 2 wt % and puritygreater than 97% was supplied by Sigma-Aldrich (Madrid, Spain).Sodium chloride, potassium chloride, hydrogen chloride (37 wt%), sodium hydroxide and potassium hydroxide PA grade qualitywere supplied by Panreac (Barcelona, Spain). Demineralised waterwith conductivity lower than 5 µS was used to prepare thesynthetic glycerol solutions with 10 wt % of water simulatingthe conventional solutions obtained from biodiesel production.

Ion exchange resinAmberlite IR-120 (Rohm and Hass Co., Barcelona, Spain), is a gel-form sulfonated polystyrene-divinylbenzene resin; Amberlite 252(Rohm and Haas Co.), is a macroreticular sulfonated polystyrene-divinylbenzene resin, and Amberlite IRA-420 (Rohm and HaasCo.), is a gell-type strongly basic quaternaryamine cross-linkedstyrene/divinylbenzene copolymer anion exchange resin. Themain physical properties of the three resins are given in Table 1.

The resins were pre-treated in a column by repeated treatmentwith 1.0 mol L−1 NaOH and 1.0 mol L−1 HCl solutions in the propersequence to convert them to the H+-form or OH−-form ofthe cationic or the anionic resin, respectively. Finally they werethoroughly rinsed with demineralised water.

Analytical methodsThe sodium or potassium content of solutions was determinedby atomic absorption spectrophotometry with a Varian 220 ASspectrophotometer (Madrid, Spain). The chloride content wasmeasured by ion chromatography (Metrohm 861 AdvancedCompact IC with sequential suppression, Madrid, Spain). Thestandard uncertainty and reproducibility of measurements wasfound to be ±0.1%.

Equilibrium experimentsThe experimental set-up consisted of seven 250 mL Pyrex contain-ers hermetically sealed and mechanically agitated, submergedin a temperature-controlled thermostatic bath. The temperaturewas kept constant with a maximum deviation of ±0.1 K. Differentknown masses of resin, in the H+-form or OH−-form, were placedin contact with 100 mL of the synthetic mixture of glycerine/water(90/10 by weight) containing a total concentration of 1 mol L−1.Synthetic mixtures were prepared by mixing different amounts ofa 1 mol L−1 solution of NaCl or KCl with solutions containing HCl1 mol L−1 when the cationic resin was used, or NaOH 1 mol L−1

for the anionic resin. This dilution process was carried out withthe aim of decreasing the initial sodium, potassium or chloridecontent and therefore, the amount of ion exchanger required forthe experiments. The accuracy of resin weighing was ±0.0001 g.

The suspension formed by the resin and solution was vigorouslyagitated at 200 rpm with a multipoint magnetic stirrer. To ensure

Table 1. Properties of Amberlite IR-120 and Amberlite IRA-420

ResinActivegroup

Particle size,D (m)

Resin capacity for, sodium orchloride n∞ (mol kg−1 dry resin)

Apparent density, ρR(kg m−3)

Amberlite IR-120 Sulfonic 2.52 × 10−4 5.00 –

Amberlite 252 Sulfonic 4.94 × 10−4 4.83 1.32

Amberlite IRA-420 Quaternary Amine 4.90 × 10−4 3.80 1.15

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that equilibrium was reached, experiments were left stirringovernight. At the end of this period, the mixtures were filtered toremove the ion exchange resin. Then, the filtrate was analyzed forsodium content as described above. The resin phase concentrationin equilibrium with the liquid phase was obtained by means of thefollowing mass balance equation:

q∗ = (C0 − C∗) · V

W· 103 (1)

where C0 and C∗ are the initial and equilibrium concentrationsof sodium in the liquid phase (mol L−1), respectively; q∗ denotesthe resin phase equilibrium concentration of the entering ion inmmol g−1 of dry resin, V and W are the initial volume of thesynthetic glycerine/water solutions in litres and the weight of dryion exchange resin in grams, respectively.

RESULTS AND DISCUSSIONAs shown in Part I of this paper,29 an accurate interpretation of thephysical reality of the ion exchange process can be achieved usinga mathematical approximation based on the ideal mass actionlaw. Briefly, the following ion exchange process occurs for a binarysystem where the ion exchanger (r) is initially in the A-form and Bis the counterion in the solution:

βAα+r + αBβ+

s ⇔ βAα+s + αBβ+

r (2)

where α and β are the valences of the ionic species A and B,respectively.

As the dimensionless form of ideal LAM equation will be used,the ionic fractions of the ions in both phases has to be defined:

xA = C∗A

N; xB = C∗

B

N(3)

yA = q∗A

q0; yB = q∗

B

q0(4)

Here yB and xB represent the ionic fraction of the ion B in thesolid and liquid phases, respectively. N is the total concentrationof the co-ions in the solution phase (mol L−1); and q0 is the usefulcapacity of the resin in the system studied (mmol g−1 of dry resin).

Considering ideal behaviour of both phases the equilibriumconstant equation is as follows:

KAB(T) = yB

(1 − yB)· (1 − xB)

xB(5)

Ion exchange using the resin Amberlite IR-120Figure 1 shows the equilibrium isotherms in dimensionless form forthe system Na+/H+ and K+/H+ at 318 K in glycerol/water solutionson Amberlite IR-120. The experimental data were fitted perfectlyby assuming ideal behaviour in both phases. This behaviour agreeswith that shown in Part I of this paper by the strongly acidic cation-exchanger Amberlite 252.29 The fitting process gave the samemaximum ion-exchange capacity, q0 = 5.0 mmol protons g−1 dryresin, found by Valverde et al.30 for the ions Cu2+, Cd2+ and Zn2+

but a little lower than the values reported by Carmona et al.31 forremoval of the ions Pb2+, Ni2+ and Cr3+ using this resin in aqueousmedia.

As in Part I,29 it is important to point out that this result agreeswith previous work indicating that the presence of water at any

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

y B+

xB+

Experimental data

Sodium

Potassium

Theoretical curves

Sodium

Potassium

Figure 1. Ion exchange isotherms of sodium and potassium ions onAmberlite IR-120 in mixtures of glycerine/water with 10% water at 318 Kand a total concentration of 1 mol L−1.

concentration in a mixture with an organic solvent favours saltionization and furthers the accessibility of the counter ions to theresin active sites and thus, the capacity of the resin known to befully accessible in pure organic solvents is completely availablein glycerol–water mixtures.16 – 20 The equilibrium constants foundfor sodium and potassium are 1.939 and 3.603, respectively. Thesevalues indicate that this resin possesses higher selectivity for thepotassium than the sodium ions. This agrees with the higher affinitythat this resin exhibited for potassium in potassium–sodiummixtures in aqueous media.20 This higher selectivity that theAmberlite IR-120 exhibits for potassium with respect to sodiumions can be attributed to the difference between the respectivehydrated ionic radii, 1.65 and 2.2 Å. Thus the lower the hydratedionic radii the higher the selectivity of the ion-exchanger whenthe exchangeable ions have the same charge.

On the other hand, as shown in Table 1, both the cationicresins Amberlite IR-120 and Amberlite 252 have the same kindof ion exchange sites with high charge density (−SO−

3 ), whichproduce strong electrostatic fields without ion-pair formation.Results reported in Part I for the macroporous resin Amberlite 252at a water content of 10% indicate that its maximum capacityis a little lower than the value exhibited by Amberlite IR-120.Nevertheless, its equilibrium constant (6.802) is at least threetimes higher than that found for Amberlite IR-120.29 This resultindicates that for sodium removal from glycerine–water mixturesthe macroporous Amberlite 252 would be the best choice. Takinginto account the effect of the hydrated radii on ion exchangeprocess, it would be expected that this resin would also exhibit ahigher preference for potassium than for sodium.

Effect of water concentration on sodium removal using Amberlite 252A glycerine–water mixture containing 10% water presents aproblem of high viscosity when an ion-exchange process isemployed. Two ways can be used to make this solution easierto handle; increasing the temperature of the solution or increasingits water content. The effect of temperature on sodium removalwas studied in Part I.

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0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0y N

a+

xNa+

Water content (%)

Experimental data

10

30

50

100

Theoretical curves

10

30

50

100

Figure 2. Ion exchange isotherms of sodium ions on Amberlite 252 inglycerine/water mixtures with different water contents at 318 K and a totalconcentration of 1 mol L−1.

Table 2. Equilibrium parameters of the system H+/Na+ on Amberlite252 obtained by fitting the experimental data to the ideal mass actionlaw

SystemT

(K)

Watercontent

(%)q0

(mmol g−1) KAB R2

H+/Na+ 318 10.0 4.34 6.80229

30.0 3.848 0.988

50.0 4.34 2.085

100.0 1.560

Figure 2 shows the equilibrium isotherms in dimensionless formfor the system H+/Na+ at 318 K using four percentages of waterin glycerine. The equilibrium data for a water content of 10% werereported in Part I.29 As expected, the experimental data for all fourdifferent water contents were perfectly fitted by assuming idealbehaviour in both phases.

As can be seen in Table 2, the maximum ion-exchange capacityis independent of the water content, confirming the abovecomments in which the presence of any concentration of waterin a mixture with an organic solvent favours accessibility of thecounter ions to the resin active sites and thus, the resin capacityof the resin is fully accessible. On the other hand, the selectivityof the resin for the sodium ions decreases as the water contentincreases.

In a previous paper20 it was stated that the presence of waterin any concentration in the mixture exerts a stronger influencethan that of the organic solvent on the behaviour of the system,because the isotherms were quite similar to the aqueous systememploying mixtures of methanol, ethanol and propanol withwater. The dielectric constant of water, methanol, ethanol and2-propanol at 25 ◦C are 78.4, 32.6, 24.3 and 20.1, respectively. Inthis case water seems to be the main solvating agent of the ion

40 50 60 70 800

2

4

6

8

Equ

ilibr

ium

Con

stan

t (K

AB)

Dielectic Constant

Water content (%)103050100

Figure 3. Relationship between dielectric constants and the equilibriumconstants for the binary system H+/Na+ using the Amberlite 252 fordifferent glycerol/water mixtures at 318 K.

exchange sites. As can be seen in Fig. 3, a linear relationship witha correlation factor R2 = 0.988 is obtained between the dielectricconstant and the equilibrium constant at different glycerol–watermixtures (excluding pure water).32,33 These results indicate thatthe glycerol effectively solvates the ions favouring its interactionwith the active sites of the resin and thus the resin selectivity forsodium increases with glycerine content.

The increase in glycerine content produces an increase of theselectivity of the resin for sodium. It seems clear that the greaterdielectric constant of glycerine makes it a good solvating agent.

Taking into account these results, ion exchange removal ofsodium using the macroporous resin Amberlite 252 should becarried out at low water content and at the crude glycerolproduction temperature (333 K).

Chloride removal by strongly basic anionic-exchanger AmberliteIRA-420Once the sodium ions have been eliminated using Amberlite 252,the chloride ions remain in an acidic mixture of glycerine/waterand should be eliminated. To remove the chloride ions, an initialsolution with a concentration 1 mol L−1 NaCl was used becausethe use of an acidic solution containing 1 mol L−1 of HCl undergoesan irreversible process by neutralization of the protons with thehydroxyl ions forming water (Equation (6)). The use of an acidicsolution permits quantification of the maximum ion-exchangecapacity. Thus, using NaCl as solute allows determination of themaximum capacity and also study of the selectivity of the resin forchloride versus hydroxide ions.

R′OH− + H+Cl− ⇔ RCl− + H2O (6)

where R′ is an solid active centre.Figure 4 shows the equilibrium isotherms in dimensionless form

for the system OH−/Cl− at 303, 318, and 333 K in glycerol/watersolutions on the strongly basic anionic-exchange resin AmberliteIRA-420.

As can be seen, all the points are above the diagonal andthe curve shape allows the assumption that the system OH−/Cl−

exhibits ideal behavior, as was found for the cationic resins. Thus,the use of strongly acidic or basic resins for sodium chloride


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0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0y C

l–

xCl–

Experimental data

T = 303 K

T = 333 K

T = 348 K

Theoretical curves

T = 303 K

T = 333 K

T = 348 K

Figure 4. Ion exchange isotherms of chloride ions on Amberlite IRA-420 inglycerine/water mixtures at 303, 318 and 333 K and a total concentrationof 1 mol L−1.

removal from glycerine/water mixtures can be represented by theideal mass action law model.

As was considered in Part I,29 the standard thermodynamicproperties of the binary system OH−/Cl− can be obtained by thefollowing thermodynamic relation:

KAB(T) = 1

e

(1R

)·[�H◦

ABT

−�S◦AB

] (7)

where R is the ideal gas constant, �G◦, �H◦ and �S◦ are changesin free energy, enthalpy and entropy, respectively.

The equilibrium experimental data at the three temperaturesstudied were simultaneously fitted to the model comprised ofEquations (5) and (7). Thus, the three unknown parameters of themodel, i.e. the maximum capacity (q0) and the thermodynamicproperties �H◦ and �S◦ were calculated.

Thermodynamical properties and equilibrium parameter for thesystem OH−/Cl− at each temperature obtained using the idealmass action law model are given in Table 3.

It is important to point out that the value of the available capacityof the resin obtained is the same as in pure water.34,35 Thus inthis case the capacity of the resin is not affected by the presence

Table 3. Equilibrium and thermodynamic properties of the systemOH−/Cl− on Amberlite IRA-420 obtained by fitting the experimentaldata to the ideal mass action law

SystemT

(K) KAB

q0(mmol

g−1)

�H0

(kJmol−1)

�S0

(Jmol−1 K−1)

�G0

(kJmol−1) R2

303 20.60 −7.79

OH−/Cl− 318 16.88 3.8 −13.62 −19.24 −7.20 0.93

333 9.39 −6.91

of such large amounts of glycerine in solution. Thus, it is possibleto infer that the high dielectric constant of the glycerine/watersystem makes the glycerine also a good solvating agent in thiscase and that the slight shrinking of the anionic resin in the mixedmedia does not produce any diminution of the resin capacity.

According to the equilibrium constants, the uptake of chlorideis favoured over hydroxyl ions, as in aqueous media at anytemperature studied. As can be seen, the equilibrium constantis also affected by the presence of glycerine favouring theuptake of chloride because its value in the glycerine–watermixture containing 10% water is almost ten-fold higher thatthat exhibited in aqueous media.33 This result confirms thatthe equilibrium constant is strongly affected by the presenceof glycerine, favouring chloride uptake. KAB values also indicatethat the selectivity of this resin for chloride ions decreases withtemperature.

On the other hand, the negative value of �G◦ indicates that theion exchange process is feasible and spontaneous; the negativevalue of �H◦ confirms that the ion exchange processes areexothermic, whereas the negative value of �S◦ suggests that therandomness decreases at the liquid/solid interface when hydroxylions are eluted from the solid by the chloride ions initially presentin the liquid phase.

CONCLUSIONSEquilibria for the sodium and potassium uptake from a solutionof glycerine/water 90/10 w/w using Amberlite IR-120 in H+-formwere favourable at 318 K and the equilibrium behaviour can beconsidered as quasi-ideal. As expected, this resin presents higherselectivity for potassium ions due to their lower hydrated ionicradii.

The equilibrium data obtained at different water contents usingAmberlite 252 lead to the conclusion that the presence of glycerinein the glycerine/water mixture favours the selectivity of the resinfor sodium uptake and also that its maximum ion exchangecapacity is fully available. In addition, when glycerine is presentin the glycerine/water mixtures a linear relationship is obtainedbetween the equilibrium constant and the dielectric constant.

The equilibrium for chloride uptake from a 90/10 w/w mixtureof glycerine/water using Amberlite IRA-420 in OH−-form wasfavourable over the temperature range 303–333 K and the idealmass action law model was able to fit the experimental equilibriumdata.

As expected, the maximum capacity of ion-exchange of theAmberlite IRA-420 was fully available for exchange in this non-aqueous mixed media. The affinity of the resin for chloride iondecreased with temperature, and the standard thermodynamicproperties confirmed that this ion exchange process is exothermic,spontaneous and feasible.

These results indicate that the macroporous resin Amberlite 252could be a good choice to remove sodium ions from glycerol/watersolutions with a high salt concentration, and also that a stronglybasic anionic-exchange resin could be used for chloride ionremoval.

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Purification of glycerol/water solutions from biodiesel synthesis by ion exchange: sodium and chloride removal. Part II