journal of industrial and engineering chemistry paper.pdf · 2015. 6. 17. · of phenol extraction...

Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

G ModelJIEC 2091 No. of Pages 7

Phenol removal from wastewater by means of nanoporous membranecontactors

Mehrdad Hemmati b, Negin Nazari a, Alireza Hemmati c,*, Saeed Shirazian d

aDepartment of Educational Science, Payame Noor University, P.O. Box 19395-3697, Tehran, IranbResearch and Development Group, Latif Paper Company, Tehran, IrancDepartment of Chemical Engineering, Faculty of Engineering, South Tehran Branch, Islamic Azad University, Tehran, IrandYoung Researchers and Elite Club, South-Tehran Branch, Islamic Azad University, Tehran, Iran

A R T I C L E I N F O

Article history:Received 23 January 2014Received in revised form 11 May 2014Accepted 10 June 2014Available online xxx

Keywords:Phenol removalMembraneWastewater treatmentSeparationNanoporous

A B S T R A C T

A comprehensive mathematical model that takes into account mass and momentum transports wasdeveloped to simulate removal of phenol from wastewater by means of porous membrane contactors.The model was based on the hydrophobic porous membranes, in which the organic phase fills themembrane pores in a co-current and counter current liquid–liquid contact. The model was then validatedfor various aqueous and organic flow rates with the experimental data obtained for phenol removal fromaqueous phase using polypropylene membrane. The results showed that increasing aqueous phase flowrates reduces the percentage of phenol separation from aqueous solution.ã 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

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Introduction

Wastewater discharged from chemical processes includingcoal gasification, petrochemical, oil refineries, and agrochemicalindustries contain phenol and related compounds. These chemi-cal components are recognized as priority toxic organic pollu-tants via the US Environmental Protection Agency. Consequently,effective removal of these toxic pollutants from industrialwastewater is a problem that has major importance and interest.There are a number of processes for removal of phenol fromhazardous waste disposal such as adsorption, liquid–liquidextraction, chemical oxidation, membrane technology, andbiodegradation methods [1–5].

The conventional equipment for phenol removal exhibitconsiderable shortcomings associated with the dispersion oftwo fluids, large land requirement, and emulsion formation. Forthe latter reasons, recently membrane contactors are used as anovel and efficient process for wastewater treatment. Benefits ofthis approach over traditional techniques are the great interfacialarea, lower solvent losses because of non-dispersion operation,downstream phase separation and easy to scale up. On the other

* Corresponding author. Tel.: +98 912 3108736.E-mail address: [email protected] (A. Hemmati).

http://dx.doi.org/10.1016/j.jiec.2014.06.0151226-086X/ã 2014 Published by Elsevier B.V. on behalf of The Korean Society of Indus

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hand, compared to dispersive extraction systems, the keydisadvantage is the additional membrane resistance whichlowers the overall mass transfer rate of phenol in membranecontactors. However, this can be covered via the higher interfacialmass transfer area found within membrane contactors. Severalexperimental studies have been carried out to investigateextraction and separation of phenol from wastewater streams.In spite of great advantages of membrane contactors forwastewater treatment, few efforts have been done to modeland simulate the related transport phenomena throughout theprocess [6–12].

The modeling and simulation of transport phenomena inmembrane contactors have been studied via a number ofresearches. Furthermore, CFD simulation of gas separation usingmembrane contactors has been conducted by some researchers[13–27]. In modeling and simulation of membrane contactors,resistance-in-series model and solving conservation equations forthe substance in all subdomains of membrane contactor are themost used approaches. In the first approach, overall mass transfercoefficients, including resistances in both liquid and membrane areconsidered in series. Resistance-in-series model is the most widelyapplied approach in investigating solvent extraction in membranecontactors [13–16]. In the second one, conservation equationsincluding continuity, energy and momentum equations for thesubstance such as phenol in all subdomains are derived and solved

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at the same time via computational fluid dynamic (CFD) technique.Recently, some authors [17–27] applied this modeling andsimulation approach to simulate the non-disperse liquid–liquidextraction and separation of greenhouse gases via membranecontactors. Their simulation results demonstrate excellent agree-ments with the experimental data. They also suggest that CFDoffers a powerful method for simulation and prediction of gas/liquid separation in membrane contactors.

In this study, two-dimensional modeling was performed tocalculate the concentration and velocity distribution in hollow-fibermembrane contactors for both co-current and counter-currentoperational mode. Incorporating appropriate momentum and masstransfer equations, the effect of lumen and shell flow rate, velocity,and membrane structural parameters on phenol mass transfer, andconsequently, phenol separation efficiency was investigated.

Theory

Mechanism of phenol extractionThe balance between the phenol (Ph), within an aqueous phase

at the membrane interface, and carrier are defined through Eq. (1)at equilibrium. In the organic phase, the carrier (EX) is dissolved inthe suitable diluents [6]:

Ph þ s EX ! Ph�EXs Kex (1)

Also, the extraction equilibrium constant, Kex can be repre-sented by Eq. (2) [6]:

Kex ¼ ½Ph�EXs�½Ph�½EX�s (2)

m ¼ ½Ph�EXs�½Ph� (3)

where m is extraction distribution coefficient. The distributionratio (m) is defined as the ratio of the concentration in the organicphase to that in the aqueous phase.

Transport equations

The conservation equations for the separation of phenol fromwastewater stream in the membrane contactor were derived andsolved to predict the performance of process. A schematic diagramof the membrane module is shown in Fig. 1. Simulation wereperformed in membrane contactors operated in a co-current statefor the parallel-flow handmade module (Memtec membrane,Fig. 1a) and in a counter-current once through state for the cross-flow liqui-cel extra-flow module (Celgrad LLC, Fig. 1b). In the

Fig. 1. Schematic diagram of membrane contactors. (a) Parallel-flow module with MemtCel module in a counter-current once through operational mode.


considered system, the aqueous solution is pumped through thelumen side (tube side), while the organic solvent flows in the shellside of membrane contactor. Hydrophobic polypropylene mem-branes are considered so that the organic phase spontaneouslywets the pores of nanoporous membrane. Therefore, the consid-ered membrane contactor includes three compartments: lumenside, nanoporous membrane, and shell side.

The transport model is built upon the following assumptions:

1. Steady state and isothermal operational conditions.2. The aqueous and organic solutions pass with constant physical

properties and transport coefficients.3. Fully developed parabolic velocity profile for the liquid in the

lumen side of extractor.4. The membrane is completely organic-phase filled.5. The reaction of complex formation takes place at the aqueous

solution–membrane interface.6. The chemical reaction between phenol and carrier is very fast

and reaches equilibrium.

The continuity equation for steady state transport of phenol inthe three subdomains of membrane contactor is found using Fick’slaw of diffusion for estimation of diffusional mass transfer. It is alsoconsidered that chemical reaction does not happen in the bulk ofmodel domains [28]:

ðr�CiVÞþðr�JiÞ ¼ 0 (4)

where Ci is the concentration of phenol (mol/m3), V is the velocityvector (m/s), and Ji denotes the diffusive flux of phenol in all threesubdomains. The velocity vector can be written analytically oracquired by coupling a momentum balance to the equation system.The Navier–Stokes equations for laminar flow in the shell side ofmembrane can be written as follows [28]:

r@Vz

@t� r � h rVzþ rVzð ÞT

� �þr Vz:rð ÞVzþrp ¼ f

r�Vz ¼ 0(5)

where r denotes density of the solvent (kg/m3), Vz the velocityvector in the axial direction of contactor (m/s), h is the dynamicviscosity (kg/m s), p denotes the pressure (Pa) and f is the externalforce (N).

The boundary conditions used for the outer side of fiber are asfollow:

CPh�shell ¼ CPh�membrane;No slip condition @r¼R2(6)

ec membranes in a co-current once through operational mode, (b) cross flow Liqui-



M. Hemmati et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx 3


@CPh�shell

@r¼ 0ðinsulationÞ ¼ No slip condition @r ¼ R3 (7)

Convective flux; p ¼ Patmðoutlet boundaryÞfor counter � current state and CPh�shell ¼ 0; Vz¼ V0ðinlet boundaryÞ for co � current state @z ¼ 0 (8)

Convective flux; p ¼ Patmðoutlet boundaryÞfor co � current state and CPh�shell ¼ 0; Vz

¼V0ðinlet boundaryÞ for counter�current state @z ¼ L (9)

Boundary conditions for the nanoporous membrane are alsogiven as:

CPh�membrane ¼ m�CPh�lumen @r ¼ R1 (10)

CPh�membrane ¼ CPh�lumen @r ¼ R2 (11)

@CPh�membrane

@r¼ 0ðinsulationÞ @z ¼ 0 (12)

@CPh�membrane

@r¼ 0ðinsulationÞ @z ¼ L (13)

where m is distribution coefficient of phenol between the aqueousand organic solutions.

Velocity distribution in the tube side is considered to followNewtonian laminar flow [28]:

Vz�tube ¼ 2u 1 � rR1

� �2" #

(14)

where u (m/s) is average velocity in the lumen side, and R1 is theinner radius of fibers.

The boundary conditions assumed for the lumen side areexpressed as:

@CPh�tube

@r¼ 0ðsymmetryÞ@r ¼ 0 (15)

CPh�lumen ¼ Cph�membrane

m@r ¼ R1(16)

CPh�lumen ¼ CPh�inlet@z ¼ 0 (17)

Convective flux; @z ¼ 0

Tables 1 and 2 demonstrate the geometrical and physicochem-ical parameters of membrane contactor considered in this work.

Numerical simulation

The mass transfer model equations are coupled and solvedsimultaneously for the model geometries in COMSOL Multiphysics

Table 1Characteristics of the hollow-fiber membranes and membrane modules [6].

Type of contactor Unit Han

Fiber type – MemContactor inside diameter mm 7.5

Number of fibers – 60

Inside radius of fiber mm 0.12Outside radius of fiber mm 0.27Porosity – 0.65Tortuosity – 2.5

Nominal pore diameter nm 200Effective length of fiber mm 200Packing fraction – 0.32Operation mode – Co-


(version 3.5a) using the direct (UMFPACK) solver. The direct solverUMFPACK was employed because it is preferable for 1D and 2Dmodels. It also employs the COLAMD and AMD approximateminimum degree preordering algorithms to permute the columnsso that the fill-in is minimized [17,19–21]. A system with thespecifications of Intel1 CoreTM i5CPU M 460 @ 2.53 GHz and 4 GBRAM was used to solve the equations.

Results and discussion

Model validation

The modeling results for separation of phenol from aqueoussolution by the membrane contactor were compared with theexperimental data reported by Shen et al. [6]. The percentage ofphenol separation from wastewater at different aqueous or organicphases flow rate is chosen to compare the modeling data with theexperimental values. The results are presented in Tables 3 and 4. Asit can be seen, the modeling predictions follow the general trend ofthe experimental data with deviations of smaller than approxi-mately 15% which indicate the conformity between experimentsand model predictions. Furthermore, it should be pointed out thatthe Reynolds numbers for all cases are very low (less than 10)therefore, the assumption of laminar flow for both feed and solventare valid (see Table 4).

Diffusive and convective mass transfer flux distribution

The diffusive and convective mass transfer fluxes of the phenolin the hollow-fiber membrane contactor are illustrated in Figs. 2and 3 by arrows respectively. As it can be seen, contribution ofconvective flux in the tube side in the z-direction is considerablecompared to axial diffusive flux near the center of tube. That is dueto the fact that in the axial direction, the velocity is considerableand results in high convective flux for phenol. This convective fluxtends to expel the aqueous solution containing phenol andtherefore the phenol removal reduces. Consequently, this massflux is not interesting for removal of phenol using membranecontactors. Figs. 2 and 3 also reveal that both fluxes are reducedalong the contactor because of decline of driving force in the z-direction.

Velocity field in the shell side of membrane contactor

The velocity of fluid passing through the shell side of membranecontactor is illustrated in Fig. 4 where the solvent phase (organicsolution) flows. The velocity field in the shell is simulated viasolving the momentum equation (Navier–Stokes) with appropriateboundary conditions. The results are shown in two dimensions, i.e.r and z. From Fig. 4, it is clearly observed that the velocitydistribution of organic phase is almost parabolic with a mean

dmade modules Liqui-cel extra-flow module

tec polypropylene Celguard X50–215 polypropylene55.511100

75 0.11025 0.150 0.4

2.5 40 150

0.39current once through Counter-current once through



Table 2Physical properties of the working solutions at 20.5

�C and the distribution ratio estimated from experiments across a range of carrier concentrations [6].

Aqueous phase Organic phase Density (kg m�3) Viscosity cP Diffusivity of solute or complex (m2/s) Distribution ratio of phenols

Phenol solution – 983.5 0.98 1e-9 –

– 10% TBP/shellsol 810.6 1.75 4.9e-10 35.5– 50% pentanol/xylene 831.5 1.28 7e-10 35.7Industrial wastewater – 983.5 0.88 7.2e-10 –

– 50% pentanol/xylene 831.5 1.28 5.9e-10 6

Table 3Comparisons between experimental data and model predictions in a Liqui-Celmodule, treating industrial phenolic wastewater with a 50% (v/v) pentanol/xylene solvent [6].

Qorg (m3/s) Qaq (m3/s) Recovery, %Experimental

Recovery, %Simulation

Error, %

6e-7 6.5e-7 78.6 66 16.035.7e-7 17.3e-7 44 40 9.095.7e-7 38.3e-7 22.3 21.2 4.9318.7e-7 8.1e-7 72.1 66.16 8.2418e-7 17.3e-7 48.4 41 15.2



velocity which increases along the membrane length due tocontinuous phenol permeation to the shell side. Moreover, Fig. 4confirms that at the inlet zones in the shell side, the velocity is notfully developed, and after a short distance from the inlet of fiber thevelocity profile becomes fully developed. The mathematical modelconsiders the inlet effects on the hydrodynamics of organic solventpassing through the shell side of membrane contactor which inturn enhances the accuracy of model.

Concentration distribution and total flux of phenol in the membranecontactor

Concentration distribution in the feed side and total flux ofphenol in the shell side of membrane contactor at counter-currentmode is shown in Fig. 5. The reason for this behavior is as the

Table 4Experimental and simulation results of solvent recovery under various operational con

System Constant Rea

10% TBP/shellsol and 1 kg/m3 phenol solution Reorg = 1.59 3.5.6.7.

10.10% TBP/shellsol and 1 kg/m3 phenolsolution

Reaq = 7.94 0.

1.2.2.3.

50% Pentanol/xylene and 1 kg/m3 phenolsolution

Reorg = 3.61 2.

4.5.5.5.6.7.8.

50% Pentanol/xylene and industrialphenolic wastewater

Reorg = 3.83 4.

5.5.7.9.


aqueous wastewater solution containing phenol flows in the tubecompartment, phenol moves towards the nanoporous membranebecause of the concentration gradient. At the membrane–feedphase interface, a chemical reaction is carried out and an organiccomplex is formed. The formed complex then diffuses through thenanoporous membrane which is filled by the organic phase andreaches at the shell side of membrane contactor. Finally, in the shellcompartment the formed complex is swept by the moving solventand leaves the membrane module. Fig. 6 also shows theconcentration profile of phenol in the tube side at radial direction.Figs. 5 and 6 confirm that the concentration decreases along thecontactor due to the extraction of the phenol. Fig. 5 also illustratesthe total mass transfer flux of the phenol in the membranecontactor. The total mass flux of phenol constitutes two massfluxes including diffusion and convection. At the inlet of tube, thetotal flux of phenol is the highest because the concentrationgradient is the highest.

Effect of feed flow rate on the concentration distribution in the shellside

The influence of aqueous phase flow rate on the concentrationprofile of phenol in the shell side at co-current state in the middleof membrane contactor is illustrated in Fig. 7. As it is observed,enhancement of feed flow rates increases the concentration ofphenol in the shell side of contactor. This is because withincreasing feed flow rate in the tube side, the mass transfer rateincreases in the shell side of membrane contactor at co-current

ditions in the handmade module [6].

q or Reorg Recovery, %experimental

Recovery, %simulation

Error, %

74 77 68 11.6806 67 59 11.918 59 52 11.8689 55 47 14.8247 45 38 15.5597 53 45 15

52 52 45 13.407 53 46 13.256 55 46.5 15.411 55 47 14.598 88 83.33 7.5

02 78 74.5 4.4808 73 67.1 847 68 64.78 4.796 64 62.05 345 61 59.6 2.2965 55 54.42 1.0581 50 50.47 0.945 18 15.25 15

18 14 13 799 11 10 987 11 9 1858 9.5 8.5 10.5



Fig. 2. Diffusive flux of phenol in the hollow-fiber membrane contactor (co-current once through).



mode. The latter results in enhancement of concentrationdistribution in the shell side of hollow-fiber membrane contactor.

Effect of feed flow rate on the concentration distribution inside theporous membrane

The influence of feed phase flow rate on the concentrationdistribution inside the porous membrane at counter-current statein the middle of membrane contactor is demonstrated in Fig. 8. Asit can be seen, increasing feed flow rate increases the concentrationof phenol inside the pores of membrane. This is because withincreasing feed flow rate, mass transfer flux of phenol to themembrane increases. The latter increases the concentration ofphenol inside the membrane pores.

Effect of porosity-to-tortuosity ratio

The different porosity-to-tortuosity ratio between 0.04 and0.55 has been applied in the modeling and simulation of phenol

Fig. 3. Convective flux of phenol in the hollow-fiber membrane contactor (co-current once through).


removal to study the influence of this parameter on the percentageof separation [28]. Fig. 9 shows the effect of the porosity-to-tortuosity ratio on the separation efficiency of phenol. The removalof phenol has increased approximately 6 times when the latterratio increases from 0.04 to 0.55.

The effective diffusion coefficient of the nanoporous membraneis determined using the porosity and tortuosity of the membrane,which are provided by the membrane manufacturer [8]:

D2 ¼ D3 � et

� �(18)

where t and e are membrane tortuosity and porosity, respectively.According to Eq. (18), as the porosity/tortuosity ratio grows, the

diffusion of phenol in the membrane pores increases. Indeed, whenthe porosity/tortuosity ratio increases, mass transfer resistance ofporous membrane decreases. Therefore, the total resistance to themass transfer of phenol becomes lower.

Fig. 4. Velocity field in the shell side of hollow-fiber membrane contactor (co-current once through).



Fig. 5. Concentration distribution in the tube side and total flux of phenol in theshell side (counter-current once through).

Fig. 6. Dimensionless concentration distribution of phenol in the feed side ofmembrane contactor (co-current once through).

Fig. 7. Concentration distribution of phenol in the solvent side of membranecontactor (co-current once through).

Fig. 8. Concentration distribution of phenol inside the membrane pores(counter-current once through).



Please cite this article in press as: M. Hemmati et al., J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.06.015


Fig. 9. Effect of porosity/tortuosity ratio on the separation efficiency of phenol.



Conclusions

A comprehensive mathematical model was developed todescribe the counter-current and co-current separation of phenolfrom aqueous solutions using different solvents with variouscarriers. The mathematical model considered concentration andmomentum transport in the membrane module. The modelfindings were validated through comparisons with the experi-mental data reported in literature. Simulation results demonstrat-ed increasing the feed flow rate reduces the extraction efficiency ofphenol, while doing the same for the solvent phase flow rate doesnot change the extraction efficiency of phenol. Moreover,enhancement of aqueous phase flow rate increases concentrationof phenol in the shell side of hollow-fiber membrane contactor atco-current mode.

Acknowledgement

The authors are grateful to Latif Paper Company for the financialsupport of this work.


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