Separation of phenol and formaldehyde from industrial wastes. Modelling of the phenol extraction...

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Research ArticleReceived: 22 December 2009 Revised: 18 March 2010 Accepted: 30 March 2010 Published online in Wiley Interscience: 13 May 2010

(www.interscience.wiley.com) DOI 10.1002/jctb.2419

Separation of phenol and formaldehyde fromindustrial wastes. Modelling of the phenolextraction equilibriumRaquel Gutierrez, Ane Urtiaga∗ and Inmaculada Ortiz

Abstract

BACKGROUND: The manufacture of phenolic resins causes the generation of hazardous wastes composed of high concentrationsof phenol and formaldehyde together with low molecular weight polymers in lower concentrations. The separation of phenol,mainly from synthetic aqueous solutions, has been successfully achieved by means of solvent extraction,8 – 17 but few referencestackle the treatment of industrial wastes because of their complex behaviour. This work aims at the experimental and theoreticalanalysis of the recovery of phenol from industrial aqueous wastes using CYANEX 923 as organic extractant.

RESULTS: Aqueous condensates containing phenol in the concentration range 40–280 g L−1, and formaldehyde in the range30–110 g L−1, were contacted with CYANEX 923 to analyse the influence of feed pH and of concentration of the selectiveextractant on the extraction equilibrium. Concerning the pH of the feed phase, it was observed that for values higher than8.0 a decrease in the distribution ratio of phenol between the organic and the aqueous phases took place. Additionally,caustic conditions promoted formaldehyde degradation reactions in the feed phase. Phenol recovery from the loaded organicextractant was obtained by stripping with NaOH solutions. Best results were obtained working with a CYANEX 923 concentration0.6 mol L−1.

CONCLUSION: Analysis of the experimental data established the optimum conditions of the selective extraction of phenol fromindustrial condensates. A mathematical model based on the extraction reaction of 2 moles of phenol per mole of Cyanex 923described successfully the experimental results. The equilibrium parameter was estimated from the fitting of experimental datato the mathematical model obtaining a value of K = 750.9 (mol L−1)−2.c© 2010 Society of Chemical Industry

Keywords: phenol recovery; phenolic condensates; Cyanex 923; equilibrium analysis

INTRODUCTIONPhenolic resins are a large family of polymers and oligomers,composed of a wide variety of structures based on the reactionproducts of phenols with formaldehyde. Some of their commonuses are as adhesives, for coating moulding, and in laminate orplastics manufacturing. The main process in the manufacturing ofphenolic resins is discontinuous polymerization by condensationbetween phenol and formaldehyde in strong acid or alkalinemedium.1 Once the reaction is finished, two phases are formed:a highly viscous layer (pre-polymer), and an aqueous phase. Thelatter is separated by vacuum distillation obtaining an aqueouswaste called condensate.2 This effluent, which contains oligomers,water and high concentrations of phenol (15–25 wt%) andformaldehyde (10–15 wt%), is usually further distilled in orderto recover the unreacted phenol and formaldehyde. Thus, aconcentrated condensate that can be reused in the manufactureof low grade resins is generated at the bottom of the distillationcolumn. The aqueous waste distillate obtained at the top of thecolumn is less concentrated in phenol and formaldehyde makingmore difficult its further reuse; however, it cannot be directlydischarged due to its high toxicity.

Microbial degradation, thermal combustion, chemical oxidationor resinification reactions are among the treatment possibilities

that have been employed to reduce the toxicity of phenoliceffluents.2,3 Some work has been published on the use of advancedprocesses for phenol recovery such as adsorption methods,liquid–liquid extraction, and pervaporation.3 – 7

Selective liquid membranes have been reported as efficienttechnologies for phenol recovery. Ferreira et al.8 recovered phenolfrom an aqueous wastewater with 2–8 wt % of phenol throughits selective permeation across a dense membrane. Emulsionliquid membranes (ELMs) have been successfully applied to therecovery of phenol showing high efficiency in the treatmentof low concentration solutions allowing at the same time highconcentrations of phenol in the stripping phase.9 – 11 The ELMtechnology has also been applied to the phenolic effluentsgenerated in resin manufacturing.12 Liquid membranes togetherwith hollow fibre contactors have been shown as efficienttechnologies.13 – 17 Finally, a new configuration based on the

∗ Correspondence to: Ane Urtiaga, Department of Chemical Engineering,Universidad de Cantabria, Avda. de los Castros, s/n-39005 Santander, Spain.E-mail: [email protected]

Department of Chemical Engineering, Universidad de Cantabria, Avda. de losCastros, s/n-39005 Santander, Spain

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www.soci.org R Gutierrez, Ane Urtiaga, Inmaculada Ortiz

combination of emulsion liquid membranes and hollow fibrecontactors has been successfully employed for phenol recoveryfrom phenolic effluents.18,19

The use of liquid membranes is based on the fundamentalsof solvent extraction and therefore, the chemical equilibriumbetween aqueous phenol and dissolved phenol in the organicphase plays an important role in mass-transfer and selectivity.

Among the solvents and extractants that have been usedfor phenol recovery, the groups of solvating extractants havebeen extensively studied improving the capacity of the organicphase by their reversible complexation with phenol. For instance,the nitrogen-based extractants20 – 25 and phosphorous-basedextractants15 – 18,21 – 27 are shown to be excellent phenol extractantsdue to their high distribution ratios. These solvating agentsinclude Cyanex 92315 – 18,23 – 26 – 28, a commercial mixture oftrioctylphosphine oxides. This extractant forms relatively strongand reversible hydrogen bonds with phenol, showing the highestcapacity for phenol extraction. Recently, the phenol extraction ofionic liquids has also been characterized.29,30

The theoretical treatment of the equilibrium between phenolfrom highly concentrated wastes and Cyanex 923 is difficultdue to the strong non-ideality of the liquid phases. In orderto interpret equilibrium data, the formation of the solublecomplexes iPhOH•jCyanex923 have been considered by Correiaet al.5 Working with low phenol concentrations and syntheticaqueous solutions the authors found that the predominantspecies at high excess of extractant was the 1: 1 stoichiometricspecies.5,17,23 In practice, the reported model is difficult to applybecause it requires a large number of parameters.

This work aims at the analysis of the extraction equilibrium ofphenol contained in mixed aqueous wastes that also contain highconcentrations of formaldehyde, using Cyanex 923 as extractant.The equilibrium model and parameters that are necessary toolsfor the description and simulation of a phenol recovery processhave been determined.

EXPERIMENTALReagentsThe extraction tests were performed using samples of industrialaqueous effluents generated in the manufacture of phenolicresins. Equilibrium experiments were performed by mixing acertain volume of the aqueous phenol solution with a volumeof the organic extractant phase. Two different aqueous wasteswere employed: i) phenolic condensates characterized by highconcentrations of phenol and formaldehyde; and ii) phenolicdistillates, with lower concentrations of both compounds. Thecharacteristics of the solutions are shown in Table 1.

The organic phase was prepared using a commercial extractantbased on phosphine oxides, Cyanex 92331 (kindly supplied by

Table 1. Characterization of phenolic wastewaters

Condensate Distillate

pH 3.1 2.4

Colour Violet Colourless

Phenol (g L−1) 279.5 47.6

Formaldehyde (g L−1) 105.0 31.8

Water (g L−1) 551.7 –

Non-identified (g L−1) 141.1 –

Cytec Inc., Canada) and kerosene as solvent (kindly supplied byPetronor, Spain). Cyanex 923 is a commercial reagent selectedbecause of its high phenol distribution ratio.

Equilibrium experimentsThe influence of extractant concentration on phenol extractionand back extraction was analysed by contacting the aqueouscondensate with the organic phase, which was prepared witha concentration of Cyanex 923 in the range 0–2.5 mol L−1,diluted in kerosene. The back-extraction tests were carried out bycontacting the previously loaded organic phase with the strippingagent solution, sodium hydroxide (3 mol L−1), a concentrationhigher than the stoichiometric quantity. The contact betweenaqueous and organic phases was carried out in closed tubeswith a maximum capacity of 50 mL, at room temperature. Theexperimental procedure involved measuring and adding to thetubes the proper volume of each phase according to the selectedvolume ratios A/O = 1 : 1 and O/S = 1 : 1. The tubes were shaken inan orbital stirrer at 70 rpm for 4 h. At the end of the contact time,the phases were separated by centrifugation at 3000 rpm. Finalvolumes were measured and the aqueous phase was sampled foranalysis. All experiments were conducted on a replicate basis.

For analysis of the influence of aqueous pH on the extractionequilibrium, two types of aqueous phenolic wastes were used:distillates and condensates (Table 1), although the latter wasdiluted by adding distilled water. The phenol and formaldehydeconcentration of the diluted condensates were 47.6 g L−1 and31.8 g L−1, respectively. The initial pH of the aqueous phase wasmodified by adding sodium hydroxide. Once the selected pH hadbeen attained, the solution was preserved at room temperature indark conditions for 15 h. Then, it was contacted with the organicphase prepared with a Cyanex 923 concentration 0.6 mol L−1,using volumetric ratios aqueous/organic (A/O) in the range 10 : 1to 1 : 2. The experimental conditions are summarized in Table 2.

Analytical methodsPhenol concentration in the aqueous samples was determinedspectrophotometrically after appropriate dilution with a sodiumhydroxide solution (1 mol L−1) to adjust the pH well abovethe pKa of 10. The content of phenolate was determinedat λ = 235 nm with a Genesys 10 UV (Thermo Electro Co.,England) spectrophotometer. Formaldehyde was determinedby potentiometric titration based on the hydroxylammoniumchloride method (ISO 11 402 : 2004). The water concentration wasanalyzed by Karl-Fischer titration using a DL31 titrator (MetterToledo, Switzerland). Phenol and formaldehyde concentrations inthe organic phase were calculated from the mass balances.

RESULTS AND DISCUSSIONLiquid–Liquid equilibrium between residual phenoland Cyanex 923The influence of two variables was analysed experimentally: (i) theconcentration of extractant Cyanex 923 in the organic phase, and(ii) the initial pH of the aqueous phase. In addition, reuse of theregenerated organic phase in consecutive extractions was alsoevaluated.

Initially, the influence of extractant concentration was studiedusing the condensates as aqueous phase and varying theconcentration of Cyanex 923 between 0 and 2.5 mol L−1. Theformer value corresponds to the use of kerosene as solvent.

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Separation of phenol and formaldehyde from industrial wastes www.soci.org

Table 2. Equilibrium experiments

Aqueous phase Organic phase

Variable under study Step[PhOH]initial

[gL−1][CH2O]initial

[gL−1][PhOH]initial

[gL−1][Cyanex923]

[mol.L−1]Volumetricratio A/O

Influence of [Cyanex 923]org. First Extraction 279.5 105.0 0.0 0–2.5 1 : 1

Back-Extraction 0.0 0.0 65.4–266.8 0–2.5

Secondextraction withstrippedorganic phase

279.5 105.0 56.5–160.9 0–2.5

Initial pH of aqueous phase Extraction 47.6 31.8 0.0 0.6 10 : 1–1 : 2

Experiments carried out at room temperature.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.5 1.0 1.5 2.0 2.5

Cyanex 923 (M)

PhO

H (

M)

Figure 1. Influence of Cyanex 923 concentration on the equilibrium phenolconcentration. (�) aqueous condensates; (×) organic phase; and (�)stripping phase.

Figure 1 shows the equilibrium concentrations of phenol in theaqueous condensates and in the organic phase, as a function of theinitial concentration of Cyanex 923. As illustrated, an increase inthe Cyanex 923 concentration improves the extraction of phenolin the concentration range studied. Furthermore, in Fig. 2 a linearrelationship is observed between the extraction percentage ofphenol and the initial Cyanex 923 concentration; the ordinate atthe origin corresponds to the solubility of phenol in kerosene usedas solvent in the formulation of the organic phase. Moreover, atthe end of the extraction tests the mass of formaldehyde remainedconstant in the aqueous phase. Hence, it could be concluded thatliquid–liquid extraction using solutions of Cyanex 923 in keroseneallows the selective separation of phenol from formaldehydecontained in aqueous condensates generated in the course of themanufacture of phenolic resins.

Next, the stripping of phenol from the loaded organic phaseswas performed using sodium hydroxide solutions (3 mol L−1)and data are reported in Fig. 1. After equilibrium, the phenolconcentration in the stripping phase was analysed. The phenolrecovery ratio (R) was calculated as the ratio between the strippedmass of phenol and the phenol mass extracted in the previousstep, which was calculated as the initial phenol mass minus theequilibrium phenol mass in the aqueous waste condensate as

0

10

20

30

40

50

60

70

80

90

100

0 0.5 1 1.5 2 2.5

Cyanex 923 (M)

Ext

ract

ion

ratio

, Rec

over

y ra

tio (

%)

Figure 2. Influence of Cyanex 923 concentration on the extraction andrecovery of phenol and formaldehyde from phenolic condensates.Volumetric ratio A : O = 1 : 1. (�) phenol extraction (R2 = 0.987); (�)formaldehyde extraction; and (×) phenol recovery ratio.

shown in Equation (1). The values of R are plotted in Fig. 2.

R(%) = Vs,f · [PhOHs]f

Va,i · [PhOHa]i − Va,f · [PhOHa]f× 100 (1)

Figure 1 shows the increase of phenol concentration in thestripping phase with the concentration of Cyanex 923 in theorganic phase. However, an almost constant concentration wasattained for Cyanex 923 concentrations higher than 1 mol L−1,probably due to the increasing concentration of free extractantin the organic phase and its negative influence on the back-extraction reaction. On the other hand, phenol concentration inthe organic phase increased as the Cyanex 923 concentrationwas increased, as observed in Fig. 1. Owing to these differenttendencies, phenol recovery ratio exhibits a maximum value in theCyanex 923 concentration range 0.5 to 0.8 mol L−1, thus definingthe optimum composition of the extractant phase.

To evaluate the stability of the organic phase during equilibriumexperiments, its reuse was assessed in the present study. Thus,the previously stripped organic phase was contacted again witha fresh aqueous phenolic condensate. The values of the phenoldistribution ratio, obtained with fresh and reused organic phasesare shown in Fig. 3. Furthermore, the formaldehyde concentrationand pH of the condensate after equilibrium are also plotted againstthe concentration of Cyanex 923.

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(a)

0.0

2.0

4.0

6.0

8.0

10.0

0 0.5 1 1.5 2 2.5

Cyanex 923 (M)

DPh

OH

(b)

0.0

1.0

2.0

3.0

4.0

0 0.5 1 1.5 2 2.5

Cyanex 923 (M)

CH

2O (

M)

0

2

4

6

8

10

12

pH (

-)

Figure 3. Reuse of organic phase: (a) stability of the organic phase, D,phenol distribution ratio (mol/mol) (�) fresh organic phase; (�) re-usedorganic phase; (b) evolution of the pH and CH2O in aqueous condensatephase (�) formaldehyde concentration; and (◦) pH.

The results show that the distribution ratio (D) of phenol issimilar for both fresh and reused extractants. On the other hand, itsvalue is lower than the distribution ratio found in literature wherevalues higher than 103 have been reported.21 This differencecan be attributed to the dependency of the distribution ratioon the initial phenol concentration.24,28 In the present work, theconcentration of phenol (279.5 g L−1) is much higher (more thantwenty times) than the initial phenol concentration employed inother studies, and it is present together with high concentrations offormaldehyde. Even so, it is observed that the distribution ratio ofphenol in Cyanex 923 mixtures is higher than the values reportedwith other organic solvents14 as well as with nitrogen-basedextractants such as Amberlite LA-2 and TOA23,25 (Table 3).

With regard to formaldehyde, a decrease of its concentration inthe aqueous phase was observed at the end of the experimentsperformed with the reused organic phase. As was previouslyshown in Fig. 2, formaldehyde was not extracted when employingan organic phase composed of mixtures of Cyanex 923 dilutedin kerosene. Therefore, the formaldehyde decrease observed

in Fig. 3 could be due to the increase of the equilibrium pHattained in the aqueous phase, which is also represented inFig. 3. In previous work it has been reported that formaldehydedegradation reactions could take place in alkaline solutions suchas the auto-condensation reaction (aldol condensation) that givesa carbohydrate mixture, and the oxidation-reduction reaction(Cannizzaro’s reaction) to give methanol and formic acid.32,33 Bothreactions are shown below:

nHCHOOH−

−−−−−−−−→H(CH − OH)n−1CHO (2)

2HCHO + H2OOH−

,�−−−−−−−−−−→HCOOH + CH3OH (3)

The increase of equilibrium pH observed in the aqueousphase at the end of the second extraction run is assigned tothe solubilisation of sodium hydroxide loaded in the organicphase during the previous back-extraction step.26 Furthermore,extractant impurities can cause an atypical change of the interfacialtension and the formation of emulsions when the aqueous phasecontains sodium hydroxide.24 In this work, during contact ofthe loaded extractant with the stripping solution, the uptakeof water was observed to depend linearly on the extractantconcentration. In addition, the formation of an incipient emulsionwas observed. The droplets of the stripping phase contained inthe organic phase were later desorbed in the aqueous condensatein the next extraction run resulting in the increase of pH ofthe aqueous solution. Figure 3 shows that the increase of thecondensate pH depends on the concentration of Cyanex 923in the organic solution. Nevertheless, this phenomenon couldbe avoided by including a step neutralizing the organic phasebetween consecutive stripping and extraction stages.

Once the effect of the organic phase composition was assessed,the influence of the pH of the aqueous phase on the extractionequilibrium of phenol was studied. The experiments were carriedout with an organic phase composed of 0.6 mol L−1 Cyanex 923diluted in kerosene. This concentration is within the optimalrange shown in Fig. 2. The organic phase was contacted with twodifferent aqueous samples: (i) phenolic distillates; and (ii) dilutedphenolic condensates, to provide a phenol concentration of47.6 g L−1, and a formaldehyde concentration of 31.8 g L−1. Theequilibrium isotherms for phenol partition between the organicphase and the aqueous phases are presented in Fig. 4. The trend issimilar for both types of aqueous samples, although the dispersionof data is less pronounced when working with distillates, probablydue to the absence of unknown organic compounds in this kindof waste.

It is observed that the pH of the aqueous phase has an importanteffect on the equilibrium behaviour. This effect is explained bythe dissociation of phenol. In the alkaline solution, an acid-basereaction with phenol takes place converting the phenol weak

Table 3. Distribution ratio of phenol using different extractans

Extractant Distribution ratio Phenol concentration (g L−1) Formaldehyde concentration (g L−1)

Organic solvents Toluene 1.521 1.0–3.0 –

Kerosene 0.2214, 0.1223 0.5–10.0

Nitrogen-based Amberlite LA-2 2.223,24 0.5–10.0

TOA 1.3523,24 0.5–10.0

This work Cyanex 923 0.6–9 279.5 105.0


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Diluted Condensates

0.0

0.4

0.8

1.2

1.6

2.0

0.0 0.1 0.2 0.3 0.4 0.5

Ca (M)

Co

(M)

Distillates

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.1 0.2 0.3

Ca (M)

Co

(M)

(a)

(b)

Figure 4. Equilibrium isotherms of phenol extraction with 0.6 mol L−1

Cyanex 923 in kerosene: (•) pH = 2.4; (�) pH = 6.0; (�) pH = 8.0; (♦) pH =10.0; and (×) pH = 12.0.

acid into a salt (sodium phenolate), which is insoluble in theorganic phase. Thus, a drop in the distribution ratio of phenolfor pH values higher than 8 is observed in Fig. 4. This findingis in agreement with the work of Cichy et al.24 who reported asimilar drop in the distribution ratio at pH > 8 working in aninitial phenol concentration range from 0.01 to 0.1 mol L−1 and0.4 mol L−1 Cyanex 923 in n-alkanes.

For pH < 8 equilibrium data falls on the same curve. A linearrelationship between log D and log Ca is seen for this pH rangefor the aqueous distillates (Fig. 5). Linear regression of the dataprovided satisfactory correlation coefficients (R2 = 0.99) with aslope value of 0.6. Previous studies on phenol extraction with0.6 mol L−1 Cyanex 923 were carried out using synthetic aqueouseffluents with initial concentrations of phenol between 1 and50 g L−1 and no formaldehyde, similar slopes (0.58–0.64) werereported.28

Samples were analysed for formaldehyde 15 h after pHmodification. The degradation of formaldehyde occurs at pH> 8 (Fig. 6) and is attributed to the degradation reactions inalkaline solutions, previously described. However, degradationof formaldehyde takes place at pH values not relevant for theextraction of phenol. Thus, operating conditions can be fitted toseparate both components from the analysed condensates.

Modelling the extraction equilibrium of phenolFirst, the mathematical model proposed by Correia et al.5, basedon speciation, in which the individual complexes present inthe organic phase and their contributions to equilibrium wereidentified, was applied in order to describe the experimental

-0.7

-0.2

0.3

0.8

1.3

1.8

2.3

-0.85 -0.35 0.15 0.65 1.15 1.65

Log Ca (gL-1)

Log

DPh

OH

Figure 5. Log of the partition coefficient against the log of equilibriumconcentration of phenol in the distillates: (•) pH=2.4; (�) pH=6.0; and (�)pH=8.0.

0.0

0.2

0.4

0.6

0.8

1.0

2.0 4.0 6.0 8.0 10.0 12.0

pH (-)

CH

2O/C

H2O

i (M

/M)

Figure 6. Influence of pH in formaldehyde degradation. (•) dilutedcondensates; and (�) distillates.

behaviour observed in this work. The model described theformation of the soluble complexes iPhOH•jCyanex923 includingthe dimerization of the extractant for mole ratios (Cyanex923TOTAL/PhenolTOTAL) lower than 0.6. The mass balance equationsof the equilibrium model are the following:

iPhOH + jCKij↔PhOHiCj (4)

(PhOH)total = (PhOH)aq + (PhOH · C)org

+ 2(PhOH2 · C)org + 3(PhOH3 · C)org

+ 4(PhOH4 · C)org + (PhOH · 2C)org (5)

(C)total = (C)org + (PhOH · C)org

+ (PhOH2 · C)org

+ (PhOH3 · C)org(PhOH4 · C)org

+ 2(PhOH · 2C)org + 2(C2)org (6)

and the equilibrium constants are defined as follows:

Kij = [iPhOH · jC]

[PhOH]iaq[C]j

with i = 0 to 4 and j = 1 to 2 (7)

In the results reported by Correia et al., the main individualcomplex species present in the organic phase were identified asPhOH•C, PhOH2.C and PhOH•

4C and the values of the conditionalequilibrium constants were: K11 = 911 M−1: K21 = 2.79 × 104

M−2; and K41 = 8.50 × 105 M−4, respectively. Figure 7 comparesthe experimental data obtained in the present work together withthe simulated curve (upper dotted line) obtained with the model


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and constants values reported by Correia et al. All experimentalequilibrium data obtained both from distillates at pH < 8 areincluded in the fitting.

As shown in Fig. 7, the conditional equilibrium constantsproposed by Correia et al. did not describe the experimentalisotherm satisfactorily, showing a high value of standard deviationσ = 0.73, calculated using

σ =

√√√√√∑(

Cexp − Csim

Cexp

)2

N − 1(8)

Next, experimental data were fitted to the mathematical model,Equations (4) to (7), obtaining a new set of values of the estimatedconstants K11 = (1.50±1.60)×102 M−1; K21 = (1.74±1.32)×103

M−2; and K41 = (1.01 ± 1.10) × 104 M−4. The simulated isotherm,plotted in Fig. 7 by a semi-dotted line, described the experimentaldata adequately with a standard deviation of σ = 0.09. On theother hand, when compared with the values of the conditionalconstants proposed by Correia et al., the values of the equilibriumparameters estimated from the experimental data obtained inthis work, were 1 or 2 orders of magnitude lower. A possibleexplanation is the complex composition of the phenolic wastesused in the present work.

Alternatively to reaction (4), the equilibrium extraction can bedescribed by the overall chemical reaction shown in Equation (9).The advantage of this model is its relative simplicity because itinvolves only one parameter, the overall equilibrium constant, Kr :

r · PhOH + C ↔ PhOHr · C (9)

The mass balance equations of phenol and solvating agentresulting from reaction (9) are as follows:

(PhOHT) = (PhOHa) + r · (PhOHr · C) (10)

(CT) = (PhOHr · C) + (Co) (11)

0.00

0.40

0.80

1.20

1.60

0.00 0.05 0.10 0.15 0.20 0.25

Ca (M)

Co

(M)

Figure 7. Simulated isotherms and equilibrium experimental data (◦)Experimental data; (--) predicted curve by Correia et al. model andconstants; ( − ·−) predicted curve by Correia model and the new estimatedconstants obtained in this work; and ( – ) predicted curve by simplifiedmodel considering only the formation of species PhOH

•2 C.

and the equilibrium constant according to the mass action law isdescribed as:

Kr = �PhOHr · C�[PhOHa]r · [C]

(12)

Equations (10) to (12) were solved by means of a least squaresparameter estimation software, obtaining the stoichiometric co-efficient r and equilibrium parameter Kr . This mathematical tech-nique minimizes the weighted absolute squared error betweenthe experimental and predicted values of the measurements.

The fitting of the results led to a stoichiometric coefficientr = 2 and the overall equilibrium constant took a value ofKr = 794.8 ± 405.0 M−2. Figure 7 depicts the experimentaltogether with the predicted data. Solid lines represent thefitting of the experimental data to the proposed model. It isworth noticing that simulated curves describe reasonably well theexperimental behaviour of the extraction equilibrium of phenolalthough the standard deviation took a value of σ = 0.55. Thishigh value is mainly due to the residual errors between themeasured and predicted values at the low phenol concentrationsrange. If aqueous phenol concentrations higher than 0.03 mol L−1

are considered, the standard deviation between experimentaland predicted data is reduced to σ = 0.10, similar to thevalue provided by the previous model with three conditionalequilibrium constants. In conclusion, the model based on an overallchemical reaction was able to describe the equilibrium isothermssatisfactorily in the phenol concentration range in aqueous phasebetween 0.03 and 0.24 mol L−1.

Finally, based on the results obtained in the present study andon previous literature, the separation process for phenol recoveryfrom phenolic condensates could include a first step aimingat phenol separation and recovery followed by degradation offormaldehyde in alkaline solution; Fig. 8 shows a block diagram ofa tentative process.

In liquid–liquid extraction the aqueous condensate is contactedwith an organic phase with 0.6 mol L−1 Cyanex 923. When theequilibrium is reached, the phases are split into an aqueous wasteand a loaded organic phase. Then, the loaded organic phaseis contacted with an alkaline solution for phenol stripping. Thedischarged organic phase is washed with an acidic solution andrecycled.

The aqueous waste obtained in the extraction step is still highlyconcentrated in formaldehyde, although it has a low concentrationof phenol. This effluent is treated by addition of a strong alkalinesolution achieving formaldehyde degradation and thus reducingthe toxicity of the wastewater.

Phenol from the alkaline solution generated in the stripping stepis recovered by means of the addition of a strong acidic solution.18

As a result, in the experimental tests performed, two phaseswere obtained: a highly saline aqueous waste with low phenolconcentration and an organic phase concentrated in phenol.The latter could be used as raw material in the phenolic resinmanufacturing.

CONCLUSIONSThe extraction equilibrium of phenol from industrial aqueouswastes generated in phenolic resin manufacture has been analysedand modelled. Cyanex 923 was used as selective extraction agent.

Study of the influence of the initial concentration of Cyanex923 on phenol extraction and back-extraction reported a linearrelationship between percentage phenol extraction and initial


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Fresh-Organicphase

Recycling-Organicphase

Condensate

Alkalinesolution

Acidsolution

Alkalinesolution

Phenol(raw material)

Acidsolution

Phenolatesolution

Conventionaltreatment

SalineWaste

LIQUID-LIQUID EXTRACTION

FORMALDEHYDE DEGRADATION

PHENOL RECOVERY

Neutralsolution

Extraction

Stripping

Neutralization

Figure 8. Combined process for the treatment of aqueous wastes from phenolic resin manufacturing.

Cyanex 923 concentration. However, the stripping of phenol usingsodium hydroxide reaches a plateau at Cyanex 923 concentrationshigher than 1 mol L−1. As a result of the combined behaviourof extraction and back-extraction steps, a range of optimumcomposition of the extractant Cyanex 923 in the organic phase of0.5–0.8 mol L−1 was determined.

The initial pH of phenolic wastes exerts an important effect onthe chemical equilibrium extraction of phenol, observing that thedistribution ratio of phenol falls to its value at pH values higherthan 8. Extraction of formaldehyde was not observed. However,a decrease in formaldehyde concentration was observed whenworking at a feed pH > 8, behaviour assigned to formaldehydedegradation.

Finally, two mathematical models were proposed to describe theequilibrium isotherms between phenol and Cyanex 923: (i) a modelbased on the speciation reaction considering the main individualcomplex species: PhOH•C, PhOH•

2C and PhOH•4C (Correia et al.5);

and (ii) a model based on overall chemical reaction, obtaining thebest fitting with a stoichiometric coefficient of 2PhOH•C. Takinginto account that both models provide similar values for thedeviation coefficients, it is considered that the overall chemicalreaction model is able to describe reasonably well the extractionequilibrium data for the phenol concentration range in aqueousphase between 0.03 and 0.24 mol L−1. The main advantage ofthis model is its simplicity using only one parameter, the overallequilibrium constant with a value of 794.8 M−2. This simplicitycontributes to its use for design purposes.

Based on the knowledge gained in the present study, a newprocess flowsheet for the treatment of industrial wastes generatedin the manufacture of phenolic resins is proposed. The processinvolves the recovery of phenol by liquid–liquid extraction andacidification and formaldehyde degradation. The re-use of theextractant requires intermediate neutralization of the organicphase, due to the observed uptake of water by the extractant.

ACKNOWLEDGEMENTSFinancial support of projects CTM2006-00317, CTQ 2008-00690(MICINN) and Auxiliar Industrial S.A is gratefully acknowledged.

REFERENCES1 Kopf PW, Kirk-Othmer Encyclopedia of Chemical Technology, 4th edn.

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Separation of phenol and formaldehyde from industrial wastes. Modelling of the phenol extraction...

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