Presented at the EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the EuropeanDesalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006.

Desalination 206 (2007) 594–601

Removal of polyether–polyols by means of ultrafiltration

P. Cañizares, Á. Pérez*, R. Camarillo, J. LlanosDepartamento de Ingeniería Química, Facultad de Ciencias Químicas, Universidad de Castilla-La Mancha,

Avenida Camilo J. Cela, 10, 13005 Ciudad Real, SpainTel. +34 (926) 29 53 00, ext. 3413; email: Angel.Perez@uclm.es

Received 19 March 2006; Accepted 30 March 2006

Abstract

The removal of diethyleneglycol (DEG) and several polyether–polyols of different molecular weights and natureby ultrafiltration was studied. These polyether–polyols are polyethyleneglycols (PEG) of different molecular weights(800 and 6,000) and two ethylene oxide–propylene oxide copolymers: Pluronic PE6100 and Alcupol F4811.Rejection coefficients and permeate fluxes were measured for these compounds and for mixtures of PEG-6000+SDS,glycols + Pluronic and glycols + Alcupol. The main purpose of working with these mixtures is to study how theaddition of SDS, Pluronic or Alcupol can enhance rejection coefficients observed for total organic matter present inthe target effluent. In a first stage, 0.1% w/w solutions of DEG, PEG-800 and PEG-6000 were ultrafiltered using aCARBOSEP M5 ceramic membrane (MWCO = 10 kDa). No significant retention was obtained for DEG and PEG-800, but for PEG-6000, rejection coefficients higher than 80% were reached. The effects of temperature, trans-membrane pressure and feed rate on both permeate flux and rejection coefficient were also studied. Secondly, thesame procedure was followed for 0.1% w/w solutions of Alcupol and Pluronic, obtaining rejection coefficients ofalmost 100%. With respect to the above-mentioned mixtures, total organic carbon (TOC) retention was not enhancedby the addition of either SDS or Pluronic. However, a clear increase was obtained for TOC rejection when Alcupolwas added due to the distribution of the PEG-6000 molecules between the aqueous phase and the organic phaseformed by the Alcupol emulsion.

Keywords: Glycols; Polyether–polyols; TOC removal, Ultrafiltration

*Corresponding author.

0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.582

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1. IntroductionThis work confronts the elimination of a prob-

lem that involves the treatment of petrochemicalindustry effluents. Among the different contami-nants produced by the petrochemical industry, wefocus on the treatment of aqueous solutions ofDEG, PEG-800, PEG-6000, Alcupol F4811 andPluronic PE6100. Apart from the effluents gene-rated in their own manufacturing processes, wastecontaining these compounds can be found in awide variety of industries related to their appli-cation. DEG, PEG-800 and PEG-6000 are used inthe petrochemical industry mainly in polyesterresin and polyurethane manufacturing. Moreover,DEG is used as an agent for dehydrating naturalgas and as a functional fluid, PEG-800 is impor-tant in leather and textile processing, cosmeticand pharmaceutical formulations and as lubricant,and PEG-6000 is used as humectant and as aplastics additive. Alcupol F-4811 is used prin-cipally in manufacturing of polyurethane foamsof different hardness and Pluronic PE6100 is anon-ionic sur-factant that can act as an impreg-nating agent, humectant, plasticizer and lubricant.Although these compounds are not highly toxic,they increase the target effluent total organiccarbon (TOC) content and, consequently, theirchemical oxygen demand. The principal aim ofthis work is the reduction of the organic contentof these effluents as a way of decreasing itsenvironmental hazard.

DEG, PEG-800 and PEG-6000 are water-soluble compounds because their ethylene oxidesegments and terminal hydroxyl groups arehighly hydrophilic. Pluronic and Alcupol areethylene oxide–propylene oxide copolymers.They are soluble at low temperatures but thehydrophobic nature of propylene oxide segmentscauses these compounds at higher temperatures toform an emulsion that can be retained by ultra-filtration (UF) and microfiltration membranes.

On one hand, glycols have been widely usedin UF for the development and testing of different

models based on thermodynamics [1], mass trans-fer [2], reversible adsorption [3], reversible pore-plugging [4], filtration theory [5] or non-steady-state models [6]. Nevertheless, their removal hashad far less attention, although membranetechnology has been applied, for instance, for theelimination of PEG using UF with a dynamicmembrane formed by gelatine [7] or for the sepa-ration of ethylene glycol by means of vacuummembrane distillation [8].

On the other hand, the treatment of oil–wateremulsions, similar to those formed by Alcupol orPluronic, has been studied using both UF andmicrofiltration techniques [9–11]. These worksallow confronting the elimination of emulsionsformed by these two copolymers by means of UF.Furthermore, the formation of an organic phasecan lead to the distribution of other organicwater-soluble compounds present in the solutionbetween the aqueous and organic phase. Thisbehaviour allows working with mixtures ofglycols and Alcupol or Pluronic in order toincrease retention coefficients observed and, as aconsequence, reducing the organic content of thetarget effluent.

Some authors have observed an interactionbetween PEG molecules and non-ionic surfac-tants like sodium dodecyl sulphate (SDS) [12,13]. At concentrations lower than its criticalmicellar concentration (CMC), a polymer-induced micellization process can occur [12] and,at higher SDS concentrations, PEG chains arewrapped around free SDS micelles [13]. Theseinteractions allow a synergic effect with respectto TOC retention that may be observed whenPEG–SDS mixtures are ultrafiltered.

Finally, the study and optimisation of hydro-dynamic conditions are interesting to analysetheir influence on design parameters (permeateflux and rejection coefficient) [14–17]. Here, theinfluence of transmembrane pressure, feed rateand temperature have been studied on bothpermeate flux and rejection coefficient.

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2. Experimental

The experimental installation is schematised inFig. 1. It works in total recirculation mode andconsists of a 25 L bath in which the targetsolution is stored, a positive displacement pumpDosapro R856J7H6, a hydropneumatic accumu-lator Hidrocar F015A04T1-Al that reduces thevariation in pressure and flux caused by thepositive displacement pump, the UF module witha CARBOSEP M5 tubular ceramic membrane(MWCO = 10 kDa, inner diameter = 6 mm, mem-brane area = 75 cm2) and a rotameter ComaquinsaR-005 Inox. A positive displacement pump wasselected to minimise fouling problems workingwith oscillatory flow [18], although we con-sidered it advisable to place the hydropneumaticaccumulator to reduce mechanical stress sup-ported by the experimental installation. Tomeasure and control transmembrane pressure, twoBourdon Sedeme manometers (before and afterthe UF module) and a valve were placed. Tem-perature was measured by a Crison TM65thermopar submerged in the bath and controlledby a Selecta Digiterm 100 temperature controller.

Reagents used in this investigation weredi(ethylene glycol) (DEG) from Panreac;poly(ethylene glycol) with a molecular weight of800 Da (PEG-800) from Clariant; poly(ethyleneglycol) with a molecular weight of 6,000 Da fromPanreac (PEG-6000); Pluronic PE6100, a non-

Fig. 1. Schematic UF installation.

ionic surfactant made by BASF (Mw = 2,100);Alcupol F4811, a trifunctional polyol (used inpolyurethane foam manufacturing) made byREPSOL-YPF (Mw = 3,500); and SDS fromPanreac. All the solutions were prepared usingultrapure water produced in a Milli-Q plant fromMillipore.

All reagents concentrations were measured bya Shimadzu TOC-5050A analyser. Molecularweight distribution analysis for PEG-6000 wascarried out with a Shimadzu gel permeationchromatographer (GPC).

3. Results and discussion

3.1. DEG and PEG ultrafiltration

In the first stage, 0.1% w/w solutions of DEG,PEG-800 and PEG-6000 were ultrafiltered. Fig. 2represents the evolution of permeate fluxes (Jp) ofdifferent glycols with transmembrane pressure(ΔP) at 25EC and with a feed rate of 500 L/h(tangential velocity = 4.9 m/s). As can bededuced from this figure, fluxes of these solutionsare very close to those obtained for pure water.The evolution of permeate flux with trans-membrane pressure shows a clear linear trend,independent of PEG molecular weight. The smallvariations from pure water flux can be explainedby an increase in the viscosity of glycol solutions.This evolution is similar for all the followingexperiments with these kinds of compounds. Forthis reason, permeate fluxes have not beenreported in the rest of the experiments with DEGand PEGs.

Fig. 3 represents rejection coefficients of DEGand PEG-800 as a function of transmembranepressure and feed rate at 25EC. As it can beobserved, rejection coefficients were very low(lower than 0.1) as expected from a membranewith a MWCO of 10 kDa.

Fig. 4 corresponds to the effect of trans-membrane pressure, feed rate and temperature on

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Fig. 2. Influence of transmembrane pressure on permeateflux of 0.1% w/w glycols solutions at 25EC and 500 L/h.

Fig. 3. Influence of transmembrane pressure and feed rateon rejection coefficients of 0.1% w/w DEG and PEG-800solutions at 25EC.

Fig. 4. Influence of transmembrane pressure, temperatureand feed rate on rejection coefficients of 0.1% w/w PEG-6000 solutions.

PEG-6000 retention. Rejection coefficients ashigh as 0.87 at 25EC and 0.68 at 50EC wereobtained for this compound. These values arevery high if we compare the membrane MWCOand the polymer molecular weight. One possibleexplanation could be the polymer polydispersity,but it has been ruled out by carrying a GPCanalysis in which PEG-6000 showed a clearmonodispersity with an average molecular weightof approximately 6,100 Da. A more feasibleexplanation is an interaction between PEG ethy-lene oxide segments, acting as basic sites [13],and the ZrO2 sites on the membrane active layer,acting as strong lewis acid sites [17]. Thisinteraction could be a reversible adsorption as itis proposed by Churaev et al. [3].

From Fig. 4 it can be seen how an increase inboth temperature and transmembrane pressureproduces a decrease in the rejection coefficientsobserved. Moreover, an increase in feed rate leadsto higher PEG retention coefficients. The de-crease of retention coefficients with temperatureand transmembrane pressure can be explained bya higher value of convective transport of PEGmolecules. The most significant effect is theincrease in transmembrane pressure, especially atlow feed rates at which rejection coefficients at5 bar decrease below 0.35.

3.2. Alcupol and Pluronic removalSecondly, the behaviour of two different poly-

ether alcohols was studied: Pluronic PE6100 andAlcupol F4811. Fig. 5 depicts the evolution ofpermeate fluxes with transmembrane pressure of0.1% w/w solutions of Pluronic and Alcupol at50EC and 500 L/h. It can be affirmed that theformation of this heterogeneous solution pro-vokes neither concentration polarization orimportant fouling. This behaviour is maintainedat the different temperatures and feed ratesstudied.

Figs. 6 and 7 represent the evolution of rejec-tion coefficients with transmembrane pressure of

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Fig. 5. Evolution of permeate fluxes with transmembranepressure of 0.1% w/w solutions of Pluronic and Alcupolat 50EC and 500 L/h.

Fig. 6. Influence of transmembrane pressure, temperatureand feed rate on rejection coefficients of 0.1% w/wAlcupol solutions.

Fig. 7. Influence of transmembrane pressure, temperatureand feed rate on rejection coefficients of 0.1% w/wPluronic solutions.

solutions of 0.1% w/w of Alcupol and Pluronic,respectively. As can be observed in both figures,for temperatures equal to or higher than 50EC,retention coefficients are very high for bothcompounds. This is because at higher tempera-tures, hydrophobic behaviour of propylene glycolsegments, which represent almost 90% w/w ofboth copolymers, is more important than ethyleneglycol hydrophilic segments. It produces theappearance of an organic emulsion that is retainedby the UF membrane and leads to high retentioncoefficients.

With respect to the influence of feed rate, it isthe same that was previously explained for PEG-6000 retention. On the contrary, transmembranepressure has a lower influence in rejection coeffi-cients and, over 50EC, they are practically con-stant with applied pressure. This can be explainedbecause the majority of the molecules are placedin the organic phase, so the increase in convectivetransport with transmembrane pressure of per-meate molecules clearly does not affect rejectioncoefficients observed.

3.3. Ultrafiltration of mixtures of PEGFrom this point, all experiments were carried

out at 500 L/h because it was demonstrated thatthis is the feed rate at which PEG retention ismaximised.

In order to enhance glycol retention coeffi-cients, two options were proposed. First of all, toincrease rejection coefficients for PEG-6000 wetried to take advantage of the interaction observedby other authors between SDS and PEG ofmolecular weights higher than 1,500 Da [12,13].With this aim, different experiments were set upwith an amount of 0.1% w/w of PEG-6000 andconcentrations of 0.6 CMC, CMC and 2 CMC ofSDS in order to vary the SDS/PEG ratio. Theseconcentrations were chosen with the intention ofworking in the different regions (leading todifferent interactions) proposed by Dai and Tam[13]. Fig. 8 represents the influence of trans-

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Fig. 8. Influence of transmembrane pressure and PEG/SDS ratio on rejection coefficients of total organic matterat 25EC and a feed rate of 500 L/h.

membrane pressure and PEG/SDS ratio onrejection coefficients of total organic carbon at25EC and with a feed rate of 500 L/h. As can bededuced, total retention was not enhanced by theaddition of SDS with respect to retentionobtained for PEG-6000 at this temperature andfeed rate. In theory, a polymer-induced micelli-sation process must occur when a 3 to 4 ratio ofmoles of PEG repeat unit per mole of surfactant(0.6 CMC) are mixed [12]. In this case, if thismicellisation process exists, its strength is nothigh enough to produce an enhancement of theretention obtained for PEG-6000. Moreover,when SDS is added in concentrations equal orhigher to its CMC, there are SDS molecules thatare not forming micelles, which has been studiedin different works in which SDS retention is nothigher than 0.7 with 2 CMC [19]. It produces anincrease in the permeate TOC and, as a conse-quence, a lower retention coefficient for the totalorganic matter.

Finally, mixtures of 0.05% w/w of Polyol orPluronic and 0.05% w/w of glycols (DEG, PEG-800, PEG-6000) were ultrafiltered with the aim ofenhancing total rejection coefficients. Theseexperiments were carried out only at temperatures

Fig. 9. Influence of transmembrane pressure and tem-perature on rejection coefficients of total organic matterfor mixtures of Alcupol+PEG-6000 and Pluronic+PEG-6000 with a feed rate of 500 L/h.

at which rejection coefficients of Alcupol orPluronic were close to 1. When mixtures of DEGor PEG-800 and Alcupol or Pluronic were ultra-filtered, total retention was not enhanced, so thesegraphics are not reported. Fig. 9 depicts the evo-lution of TOC rejection coefficients vs. trans-membrane pressure for experiments carried out atdifferent temperatures for mixtures with Pluronicand Alcupol, respectively. With Pluronic, totalretention is similar to that obtained for PEG-6000alone. However, a clear increase was obtained fortotal retention when Alcupol is added due to thedistribution of PEG molecules between theaqueous phase and the organic phase formed bythe Alcupol emulsion. Total retention is increasedfrom 0.65 for PEG to 0.77 for mixture at 50ECand 4.5 bar and reaches a maximum of 0.97 at2.5 bar and 75EC.

The dissimilar behaviour of different mole-cular weight glycols can be explained because thehydrophobicity of PEG increases with its mole-cular weight. Therefore, the affinity of a moleculeof PEG to migrate from the aqueous to theorganic phase formed by the Alcupol emulsionincreases with PEG molecular weight. The fact

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that the Alcupol addition can enhance TOCretention (while Pluronic does not have the sameeffect) could be explained due to the greatersimilarity between PEG and Alcupol in terms ofmolecular weights (6,100 vs. 3,500) in com-parison with Pluronic (6,100 vs. 2,100). Further-more, retention is enhanced with temperaturebecause the formation of an organic emulsion isfavoured by increasing working temperature, ashas been previously noted. Finally, for bothcompounds, Pluronic and Alcupol, the influenceof transmembrane pressure is lower than forglycols, which allows working with higherpermeate fluxes without significantly affectingrejection coefficients.

4. Conclusions

The removal of different organic compoundsthat cause a TOC increase in industry effluents,mainly in the petrochemical industry, has beenstudied using UF as the separation technique.

The removal of DEG and PEG-800 is noteffective due to the membrane MWCO (10 kDa).Retention of PEG-6000 is clearly higher, reach-ing rejection coefficients close to 0.9 at 25EC,1 bar and 500 L/h. These values of rejectioncoefficients can be explained by an interactionbetween PEG-6000 ethylene oxide segments andZrO2 lewis acid sites in the membrane’s activelayer. The influence of transmembrane pressureis noticeable, causing a clear decrease in rejectioncoefficients when pressure is increased, mainly atlow feed rates (from 0.72 to 0.34 for PEG-6000 at25EC and a feed rate of 250 L/h). An increase intemperature caused a decrease in rejection coeffi-cients, which is explained by a higher convectivetransport of PEG molecules. However, a higherfeed rate (tangential velocity = 4.9 m/s) resultedin higher rejection coefficients.

For Alcupol and Pluronic, very high rejectioncoefficients were reached for temperatures higherthan 50EC. At this temperature, the hydrophobic

propylene oxide segments cause the formation ofan emulsion than can be retained by the UFmembrane. Based on these results, UF seems tobe an effective separation technique for theremoval of these kinds of compounds.

Mixtures of PEG-6000 and SDS were ultra-filtered without obtaining a clear enhancement inTOC retention. Finally, mixtures of glycols andAlcupol or Pluronic were tested in different UFruns. For DEG and PEG-800 the total organicretention was not increased. Nevertheless, formixtures of PEG-6000 and Alcupol retention ofthe total organic matter was increased up to 0.97at 75EC and rejections coefficients became lesssensitive to the effect of transmembrane pressure.This result can be explained by a distribution ofPEG-6000 molecules between the aqueous andorganic phases formed by Alcupol. In this case,UF again proves to be an effective separationtechnique.

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