Removal of Organic Impurities from Polymer Pellets

Removal of Organic Impurities from Polymer Pellets

By Zhong Chen, U. Flechtner, Dirk Meier, Jan Pruess, and Hans-Joachim Warnecke*

An investigation into the behavior of removing low molecular organics from polymer pellets in a countercurrent extractioncolumn is presented. A mathematical model is developed, describing it in terms of intraparticle diffusion, adsorption, interphaseequilibrium, convection and dispersion. The corresponding parameters are determined using nonlinear regression. The resultingpartial differential equations are solved using a numerical method. The validity of the model is verified by means of experimentaldata.

1 Introduction

Granular polymers often contain volatile organic impuritieswhich can originate from different sources, e.g. solventresidues, unreacted monomer, catalysts and/or products ofside reactions during product handing. The presence ofimpurities has an adverse effect on the properties of thepolymer material and can also increase its odor or toxicity. Theremoval of impurities from granular polymers, in certainproducts, is therefore important.

Extraction is one of the main separation techniques and iswidely used in the production of fine polymers. Manyprocedures for polymer purification involving extraction haveproven to be very efficient and offer significant advantagesover other alternatives.

Extraction on an industrial scale is usually carried outcontinuously. Extractors present three classical configura-tions, which are characterized by the relative direction of theflow of polymer and solvent, viz. cocurrent, countercurrentand crosscurrent. In general, countercurrent operation giveshigher efficiencies. When the extracting solvent and raffinatesolvent are immiscible, countercurrent operations is the mostcommonly used method of extraction.

There are a number of studies in the literature that describeextraction techniques for removing oligomers from polymers,which are already in use. BASF have reported their work onthe extraction of oligomers from polyamide using hot water[1]. An U.S. Patent has described the removal of cyclicoligomers from polyoxyalkylene using supercritical propylene[2]. Barry [3] discussed the upgrading of a silicone polymer,designated OV-17, which is used in gas chromatography as astationary phase. An interesting study on the extraction ofoligomers from polyamide using methanol has been presentedby Sridhar and Hartig [4]. They employed a simple diffusionmodel to simulate extractor behavior.

In a previous paper [5] we discussed the mass transferprocess in discontinuous and continuous extraction processes

accounting for the effective diffusion of the oligomer in thepolymer pearls, axial dispersion of the pellets as well as liquidphase and distribution equilibrium at the interface. The aim ofthe present work is to model the extraction of low molecularorganic compounds from polymer pellets in a countercurrentcontinuous column.

The analysis is based on equations obtained from theapplication of mass balance to the polymer and liquid phase.Besides diffusion inside the pellet and liquid film, microscopicinterfacial transport processes and the macroscopic hydro-dynamical properties of the reactor, the effect of adsorption isalso taken into account.

2 Experimental

The extraction of impurities from polymers was investi-gated using three different experimental setups, batch, semi-batch and continuous. The polymer employed was a granularmaterial with a mean radius of 0.0009 m. Water was used as thesolvent. The impurities referred to here are low molecularorganic compounds. Their densities were measured to be 1340and 1000 kg/m3 at 293 K, respectively. They can not be namedfor secrecy reason.

The solid-liquid equilibrium measurements were carriedout in a well-mixed laboratory batch reactor. A charge ofapproximately 60 g of polymer pellets and 120 g of water wasused. The contacting time was 24 hours. The concentrations ofthe low molecular organic both in the polymer and in the waterwere then analytically determined.

The experimental characterization of the kinetic behaviorwas done in a semi-batch reactor with an internal volume ofabout 2 L at extraction conditions. Approximately 400 mg ofmoistened polymer pellets were loaded into the reactor. Thevolumetric rate of water was 150 L/h. The extraction time wasat least 8 hours in order to remove most organic impurities asrequired by industry. The remaining content in the polymerwas analyzed during the extraction.

Some continuous extraction and impulse response testswere performed on a pilot plant with diameter d = 0.316 m,height L = 8.5 m and volume VR = 0.66 m3. Fig. 1 shows theoperating scheme for the continuous countercurrent column.The swelled polymer pellets enter at the top and flowdownward in the extractor, contrary to the rising water. Freshwater is sent to the extractor and extracts the organic impurity

Chem. Eng. Technol. 24 (2001) 5, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0930-7516/01/0505-0523 $ 17.50+.50/0 523

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[*] Prof. Dr. Z. Chen, Department of Chemical Engineering, WuhanInstitute of Chemical Technology, 430073 Wuhan, China, E-mail:[email protected]; Dr. D. Meier, U. Flechtner, University ofPaderborn, FB13/NW, 33098 Paderborn, Germany; Prof. Dr. J. Pruess,Martin-Luther-University, FB Mathematik and Informatik, 06099Halle, Germany, e-mail [email protected]; Prof.Dr. H.-J. Warnecke, University of Paderborn, FB13/NW, 33098 Pader-born, Germany, e-mail: [email protected]

0930-7516/01/0505-0523 $ 17.50+.50/0

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from the polymer pellets. Almost impurity-free polymers areobtained at the bottom, while the extract stream is withdrawnat the top. Distributors and valve locks ensure uniform, plug-like flows of pellets and liquid. The system operates atconstant temperature and pressure.

Figure 1. Schematic representation of the experimental set up.

In order to simulate the residence time distribution, a dyetracer was injected into the column entrance, and its impulseresponse was traced at the column exit by means of opticaltransmission measurements. These experiments have beendescribed extensively by Meier [6].

3 Mathematical Model

From a microscopic point of view the removal of volatileorganic compounds from a polymer pellet occurs throughthree consecutive mass transport steps, which are:1. intrapellet diffusion from the interior to the outer surface

of the polymer pellet and adsorption/desorption at theinner surface,

2. mass transfer of the organic through a laminar liquid filmsurrounding the pellet,

3. bulk transport of the organic inside the mobile phase.In addition, the thermodynamic equilibrium of the organic

within the polymer phase to the organic in the fluid phaseplays a crucial role in the extraction process.

3.1 Semi-Batch Extraction

If the polymer particles are assumed to be spherical beads ofidentical radius R, the differential material balance across aninternal pellet surface of radius r, referred to the organiccompound, in spherical coordinates is given by1)

0 < r < R:

@cP;f t;r� �@t

� DP

@2 cP;f t;r� �@r2

� 2r

@cP;f t;r� �@r

!ÿk�G � cP;f �t; r� ÿ cP;a�t; r�� (1)

@cP;a t;r� �@t

� k G � cP;f t; r� � ÿ cP;a t; r� ��

(2)

with the initial and boundary conditions

cP;f 0; r� � � cP;s 0;r� �1�G

(3.a)

cP;a 0; r� � � GcP;s 0;r� �1�G

(3.b)

@cP;f t;0� �@r

� 0 (3.c)

DP

@cP;f t;Rÿ� �@r

� DEM@cEM t;R��

@r(3.d)

where cP,f and cP,a are the free diffusive fraction and theadsorptive fraction of the concentration of low molecularorganic in the pellet respectively, as a function of extractiontime t and radial coordinate r. cEM is the concentration of lowmolecular organic in the film, DP and DEM are the diffusioncoefficient in the polymer phase and the film respectively, k isthe rate constant of the adsorption to the pellet, and G is theadsorption equilibrium constant given by

G � cP;a�0;r�cP;f �0;r�

(4)

At equilibrium, the distribution of the organic impuritybetween the polymer phase and the liquid phase is given by

H cP;f t;Rÿ� �h i

� cEM t;R�� cP;f t;Rÿ� � (5)

In order to determine the distribution coefficient, H, weconsidered a simple expression which correlates the experi-mental results and takes into account that H is not a constant:

H�cP;f � � k1 � k2 � ek3 cP;f (6)

where k1, k2 and k3 are correlation coefficients.Mass transfer through the liquid-side film with thickness, d,

at the interface is according to Ficks law, can be expressed by

R < r < �d:

@cEM t;r� �@t

� DEM@2 cEM t;r� �

@r2� 2

r@cEM t;r� �

@r

� �(7)

524 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0930-7516/01/0505-0524 $ 17.50+.50/0 Chem. Eng. Technol. 24 (2001) 5

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1) List of symbols at the end of the paper.

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with

cEM �t;R� d� � cbEM�t� (8)

where cbEM denotes the concentration of the organic impurity

in the bulk phase.The differential material balance for the bulk phase is

@cbEM

t� �@t

� ÿ 1�EM

cbEM

t� � ÿAEMVEM

DEM@cEM t;R��

@r(9)

with initial condition

cbEM�0� � 0 (10)

where sEM means the residence time of the solvent, AEM andVEM are the interfacial area and volume of the filmrespectively.

Assuming that oligomer diffusion in the film is stationary,which means ¶tcEM(t,r) » 0, and combining Eqs. (5) and (8),Eq. (7) can be solved to the result

@cEM t;r� �@r

� R R�� r2

cbEM

t� � ÿ cP;f t;Rÿ� �H

� �(11)

Substituting Eq. (11) into Eq. (3.d), the transition behaviorat the interface between the polymer phase and water phasecan be simplified to

DP

@cP;f t;Rÿ� �@r

�DEM R�� R �

cbEM

t� � ÿ cP;f t;Rÿ� �H

� �(12)

Then the material balance in the bulk phase becomes

@cbEM

t� �@t

�ÿ 1�EM

cbEM

t� �ÿAEMVEM

DEM R�R

cbEM�t�ÿcP;f �t;R�

H

� �(13)

By introducing

z � rR

, ui t� � � ci t� �cP;a 0;r� ��cP;f 0;r� � �

ci t� �cP;s 0;r� �

� DEMDP

1� R�

� �, � � VP

VEM

, �D;P �R2

DP

� � �EM

�D;P

, Sta � k � �D;P (14)

and putting AEM = n4p(R+d)2 in Eq. (13), the semi-batchkinetics can be expressed in dimensionless form as

0 < z < 1:

�D;P

@uP;f t;z� �@t

� @2 uP;f t;z� �@z2

� 2z

@uP;f t;z� �@z

!ÿSta�G � up;f �t; z� ÿ uP;a�t; z�� (15)

�EM

@ubEM

t� �@t

� ÿubEM

t� �

ÿ3gch�ubEM�t� ÿH�cP;f �t; 1�� uP;f �t; 1�� (16)

with initial and boundary conditions

uP;f 0; r� � � 11�G

, uP;a 0; r� � � G1�G

@uP;f t;0� �@z

� 0

@uP;f t;1� �@z

� ubEM

t� � ÿH cP;f t; 1� �h i

� uP;f t; 1� ��

(17)

3.2 Continuous Countercurrent Extraction

Taking into account the dispersion-convection model andthe semi-batch model described above, the mass balanceaccording to Fig. 1, under steady state conditions, can beexpressed by the following equations.

Mass balance for the polymer phase:

0 � 1BoP

@2 uP;f x;z� �@x2

ÿ @uP;f x;z� �@x

�� @2 uP;f �x;z�@z2

� 2z

@uP;f �x;z�@z

" #

ÿSta�G � uP;f �x; z� ÿ uP;a�x; z�� (18)

0 � 1BoP

@2 uP;a x;z� �@x2

ÿ @uP;a x;z� �@x

�Sta�G � uP;f �x; z� ÿ uP;a�x; z��(19)

Mass balance for the solvent phase:

0 � 1BoEM

@2 ubEM

x� �@x2

� @ubEM

x� �@x

ÿ3gc��H�cP;f �x; 1�� uP;f �x; 1� ÿ ubEM�x�� (20)

The boundary conditions according to Danckwerts [7] canbe written as

1BoP

@uP;f 0� ;z� �@x

ÿ uP;f 0� ; z� � � ÿ 1G�1

(21.a)

1BoP

@uP;a 0� ;z� �@x

ÿ uP;a 0� ; z� � � ÿ GG�1

(21.b)

@uP;f 1;z� �@x

� @uP;a 1;z� �@x

� 0 (21.c)

@uP;f x;0� �@z

� 0 (21.d)


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@uP;f x;1� �@z

� ubEM

x� � ÿH cP;f x; 1� �h i

� uP;f x; 1� ��

(21.e)

@ubEM

0� �@x

� 0 (21.f)

1BoEM

@ubEM

1ÿ� �@x

� ubEM

1ÿ� � � 0 (21.g)

The normalized variables here are defined by

x � yL

, z � rR

, Boi �wi �L

Ei, � � �P

�i;P

(22)

where y is the axial coordinate, L the length of column, w theflow rate, E the dispersion coefficient, Bo the Bodensteinnumber, and r is the normalized residence time in polymerphase.

4 Results

4.1 Distribution Coefficient

The experimental results for the distribution coefficientversus the low molecular organic concentration in the polymerphase, can be correlated by Eq. (6) using nonlinear regression[8]. Due to the condition cP,f >> cP;a, H as the above function ofcP,f determined by cP,s is believed to be valid.

Fig. 2 shows the values of the Nernst coefficient and thecurve fitting results. The optimum parameters were deter-mined as

k1 = 1.17, k2 = -0.83 and k3 = 13.45

Figure 2. Determination of the Nernst distribution coefficient.

4.2 Kinetic Parameters

According to the semi-batch kinetics given by Eqs. (15), (16)and (17) the parameters DP, Sta, G and c were estimated byfitting the extraction experimental data. Optimization of theseparameters was performed numerically applying a nonlinearregression method. The corresponding optimums are asDP = 9.0´10±11 m2/s, Sta = 0.76, G = 5.5´10±4 and c = 1.0´105.

Fig. 3 shows the comparison between calculated andexperimental dimensionless concentrations. A logarithmiccoordinate was used in order to also obtain detailedinformation in the lower concentration range. The influenceof the adsorption term in the model equations on theextraction is illustrated in this figure. When the adsorptionterm was used, there is a fair agreement between experimentaldata and modeled results over all concentration ranges.

Figure 3. Semi-batch extraction model. Results of curve fitting and measure-ments.

4.3 Hydrodynamic Parameters

Experimental impulse responses were evaluated accordingto the axial dispersion model [9] with Danckwerts boundaryconditions in closed form. Curve fitting was performednumerically in the frequency domain by means of the fastFourier transform. The residence time distributions measuredand the results of the curve fitting for the polymer phases areshown in Fig. 4. Tab. 1 lists the evaluated parameters for bothphases.

Figure 4. Impulse response function of the polymer phase. Results of curvefitting and measurements.

Table 1. Parameters for the simulation.

VÇ P 20.9 L/hVÇ EM 80 L/hg 1.43R 9.0´10±4 mBpP 6100BoEM 18sP 1114 minsEM 204 minc 20eP 23


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4.4 Numerical Simulation

The model described above consists of non-linear partialdifferential equations. An analytical solution for such a systemdoes not exist. In this study, numerical simulation using afinite-difference based algorithm has been used to analyze theextraction profiles. In order to compare predictions withexperimental results, an average concentration in the particleswas computed as

�cP;s �3N

PNÿ1

i�0cP;s;i

iN

� �2

� iN� 1

3N2

!(23)

where N denotes the number of discrete points along theparticle radius. The corresponding normalized concentrationcan be written as

uP;s ��cP;s x� �

cP;s x�0� � (24)

Details of the algorithm and its advantages are describedelsewhere [5].

Experimental continuous countercurrent extraction pro-files were compared with extraction profiles predicted by theabove model, using the parameters in Tab. 1 (Fig. 5). Theresults showed a faster extraction rate at low extraction timesand a subsequent progressive decrease to an asymptotic valuethat corresponds to complete depletion of the low molecularorganic in the polymer pellets. A good agreement wasobserved between the pilot plant data and the modelpredictions for the overall extraction processes.

Figure 5. Comparison between predicted and measured depletion.

5 Conclusions

The present article proposes a mathematical model for acontinuous, countercurrent operation widely used in thechemical industry, consisting of the purification of granularpolymers containing low molecular organics by solventextraction. Inthismodel, the interparticle interaction is treatedon the basis of kinetic theory and corresponding parameterswere determined by an adequate fitting. The Nernst distribu-tion coefficient was obtained. A good agreement between thepredictions and experimental results was achieved.

Further refinements of the model are possible in order totake into account particle shape and particle size distribution.

Received: August 4, 2000 [CET 1281]

Symbols used

A [m2] interfacial areaBo [±] Bodenstein numberc [mol/m3] concentrationd [m] diameterD [m2/s ] diffusion coefficiente [s±1] residence time distributionE [m2/s ] dispersion coefficientG [±] adsorption equilibrium coefficientH [±] distribution coefficientk [s±1] rate coefficientki [±] correlation coefficientL [m] length of the columnn [±] total number of pelletsN [±] discrete points numberr [m] pellet radial coordinateR [m] pellet radiusSta [±] adsorption numbert [s] timeu [±] normalized concentrationV [m3] volumeVÇ [m3/s] volumetric flow ratew [m/s] local velocityx [±] normalized axial coordinatey [±] axial coordinatez [m] normalized pellet radius coordinate

Greek symbols

d [m] film thicknesse [±] phase fractiong [±] phase ratioc [±] resistance numberr [±] normalized pellet residence timeh [±] normalized bulk residence times [s] residence time

Subscripts

a adsorbentb bulk phaseD diffusionEM solventf freeP polymer phaseR reactors sum


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References

[1] BASF Company, OS Patent (1968) 1,770,097.[2] Copelin, H. B., Method for Reducing Oligometric Cyclic Ether Content

of a Polymerizate, U.S. Patent (1981) 4,306,058.[3] Barry, E. F.; Ferioli, P.; Hubball, J. A., Purification of OV-17 by

Supercritical Fluid Fractionation for Fused Silica Capllary GasChromatography, J. High Resolut. Chromatogr. Chromatogr. Commun.6 (1983) p. 719.

[4] Sridhar, S.; Hartig, H., Chem. Ing. Tech. 13 (1974) p. 46.

[5] Chen, Z.; Pruess, J., Meier, D., Warnecke, H. J., Chem. Eng. J. 68 (1997)pp. 165±172.

[6] Meier, D., Abreicherung niedermolekularer flüchtiger Substanzen ausPolymerdispersion und -granulaten, Dissertation, Univ. Paderborn,Germany, 1998.

[7] Deckwer, W-D.; Maehlmann, E. A., Chem. Eng. J. 11 (1976) pp. 19±25.[8] Marquardt, D. W., J. Soc. Ind. Appl. Mat. 11 (1963) pp. 431±441.[9] Nauman, E. B.; Buffham, B. A., Mixing in Continuous Flow Systems,

John Wiley & Sons Interscience Publication, New York, Chichester1983.


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Removal of Organic Impurities from Polymer Pellets

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