Membrane-assisted VOC removal from aqueous acrylic latex

Bridget Ulrich a, Timothy C. Frank a,b, Alon McCormick a, E.L. Cussler a,n

a Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USAb Engineering & Process Science Laboratory, The Dow Chemical Company, Midland, MI 48667, USA

a r t i c l e i n f o

Article history:Received 4 March 2013Received in revised form11 October 2013Accepted 12 October 2013Available online 23 October 2013

Keywords:Latex paintVolatile organic compoundsPorous membranes

a b s t r a c t

Volatile organic compounds (VOCs) in model aqueous solutions and in acrylic latex binder used forformulating latex paint can be stripped without foaming by using a nanoporous hydrophobic polypropylenemembrane. The stripping gas employed was dry, room temperature nitrogen or 50 1C water-saturated air. Forthe dry nitrogen stripping gas, fouling was minimal for the hydrophobic polypropylene over several days, insharp contrast to experiments with hydrophilic membranes. No fouling of the polypropylene membrane wasobserved for experiments with the water-saturated strip gas. The rate of VOC removal in these experimentsdepends on mass transfer in the aqueous latex and in the membrane. These results allow estimates of themembrane area to strip VOCs from commercially relevant quantities of acrylic latex paint.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Latex paint has an odor. When a consumer enters a freshlypainted room, he or she will not only notice the color andsmoothness of the painted surfaces, but also the smell. This smellhas been described as chemical or like vodka. Most consumersdo not like this smell; it causes headaches for some consumers.While the smell is usually ushed from a well-ventilated room in aday or so, it is still undesirable. Odor-free latex paint would beregarded as superior [1,2].

Ironically, the smell in latex paint is due to trace amounts of volatileorganic compounds that are not essential to the paint's function. Fortypical acrylic latex binders like those used inmany paint formulations,the most abundant VOCs include acetone and n-butanol. Thesecompounds, typically present at concentrations between 1 and2500 ppm, do not affect performance properties like ease of applicationand hiding power. They are added to facilitate various steps in thepaint's manufacture, or they are present as impurities. Their removalwould improve the nal paint formulation by removing theunpleasant smell.

Ordinarily, the removal or stripping of trace amounts of lowmolecular weight organics is easily accomplished by contactingthe liquid with air, nitrogen or steam. In a typical batch strippingprocess, gas or vapor is blown through a sparger to create largenumbers of small bubbles. Sometimes the contacting is accom-plished using a trayed or a packed tower. The organics transferfrom the paint to the gas phase due to favorable liquidvaporequilibrium partition ratios or relative volatilities. Bubbles of gasrise due to buoyancy, and quickly collect the volatile organicsbecause their large surface area overcomes the liquid's slow mass

transfer. Because air or steam stripping is so effective, it is a widelyused separation process in environmental engineering, and issupported by a large and vibrant literature [39].

However, air or steam stripping is difcult to carry out on latexbecause it is stabilized by large amounts of surfactants. When the latexis spargedwith air or steam, the small bubbles desired for fast, efcientremoval of VOCs can produce large volumes of relatively stable foam.This foam causes major problems in the processing and packaging ofthe paint. Reducing the detergent concentration or changing the latexpropertiesmaymake air or steam stripping easier, but these alterationsare believed to compromise other properties of the paint.

In this paper we explore the use of membranes as an alter-native means to remove VOCs from latex and to avoid foaming.Such a process has been used in other situations [8,9]. In ourexperiments, a nanoporous, hydrophobic membrane separatesowing latex paint from a stream of water-saturated air. Themembrane should stabilize the gaslatex interface, while poten-tially providing a large interfacial area for rapid VOC removal. Themembrane should also suppress bubble formation, and henceeliminate foaming. At the same time, the membrane must notsignicantly retard mass transfer, because the desire to achievefast VOC removal motivates this work in the rst place.

In the following sections, we rst describe our experimentswith model solutions and with latex. We then report our results,emphasizing the search for the mechanism of VOCs removal. Thismechanism is the key to judging if membrane-assisted stripping oflatex paint has commercial value.

2. Theory

To analyze the removal of organic compounds from this system,we consider a reservoir of liquid containing one typical solute at

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0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.memsci.2013.10.025

n Corresponding author. Tel.: 1 612 625 1596; fax: 1 612 626 7246.E-mail address: cussler@umn.edu (E.L. Cussler).

Journal of Membrane Science 452 (2014) 426432

concentration c1. In our experiments, this liquid will either be anaqueous solution of dissolved organics or an acrylic latex contain-ing dissolved organics, that is, a surfactant-stabilized emulsion ofacrylic polymer and dissolved organics in water. This is the latexbinder used in formulating latex paint. The concentrations ofdissolved organics are dilute, well below their solubility in water.The liquid is pumped at a volumetric ow L from the reservoir,through a membrane module, and returned to the reservoir with achanged solute concentration c1. The change in the reservoirconcentration c1 with time t is given by

Vdc1dt

Lc1 c1 1

where V is the volume of solution in the reservoir. In ourexperiments, V is nearly constant. We nd the concentrationc1 by writing a steady state mass balance for the concentrationchange within the module. Because the liquid and gas in themembrane module are nearly well mixed, their concentrations canbe approximated by single values. For the small module used here,this is a good approximation; for cases where these concentrationsvary within the module, more complex analyses are availableelsewhere [10]. A mass balance over the entire module gives

L c1 c1 Gp1RT

0

2

where G is the volumetric ow of gas, and p1 is the solute's partialpressure. A mass balance on the liquid alone yields

Lc1c1 KLAc1 cn1 3where KL is the overall mass transfer coefcient based on a liquidside driving force, A is the membrane area, and cn1 is thehypothetical liquid concentration if the liquid were in equilibriumwith the gas. To dene cn1 we use the following:

cn1 kHp1 4where kH is a type of Henry's law constant for the dissolved solute.We now combine Eqs. (1)(4) to nd

Vdc1dt

11=KL A 1= L kH RT=G

c1 5

This is subject to the condition that the initial concentration isknown

t 0; c1 c10 6Integrating, we nd

c1c10

exp 1V1=KLA1=LkHRT=G

exp KAt

V

7

where K is an overall apparent rate constant

1K

1KL

AL kHART

G8

The apparent rate constant K reects a combination of threedifferent rate processes. If the liquid ow L and the mass transferproduct KLA are both large, then K is just a measure of gas ow G.If the gas ow G and KLA are both large, then K is a measure of L.If both L and G are large, then K represents only the overall masstransfer coefcient KL.

The overall liquid mass transfer coefcient KL also has impor-tant characteristics. It results from three resistances in series:those in the liquid, across the membrane, and in the gas. The uxj1 out of the liquid is given by

j1 kLc1 c1i 9

where kL is the individual mass transfer coefcient in the liquidand c1i is the concentration in the liquid at the liquidmembrane

interface. The ux in the gas is given by

j1 kGRT

p1ip1 10

where kG is the individual mass transfer coefcient in the gas andp1i is the partial pressure of the solute in the gas at the gasmembrane interface. This much is the standard for many analysesof mass transfer [1113].

The less familiar part is the analysis in the membrane itself.Two types of membrane give different results. The rst type,which is considered in this work, is a nanoporous hydrophobiclm. In this case, the solute diffuses through the gas-lled pores.The ux across the lm is

j1 DGlRT

p1ip1 11

where DG is the diffusion coefcient in the gas, and are thelm's porosity and tortuosity, respectively, and l is the lm'sthickness. On the other hand, the second type of membrane,which is not studied in this paper, is a nonporous lm, whichmay be either rubbery or glassy. In this case, solute dissolves in thenonporous lm and diffuses within the polymer itself. The uxacross this type of lm is

j1 PlRT

p1ip1 12

where P is the membrane permeability, the product of the solute'ssolubility in the lm and its diffusion coefcient in the polymer lm.

We now can calculate the overall mass transfer coefcient KL asa function of variables like kL, kG, and l. We do so by using Eq. (9)(12) to eliminate the unknown interfacial concentrations. For thenanoporous lm, the result is

j1 KLc1 cn1 1

1=kLlkHRT=DGkHRT=kG

c1 kHp1

13For the non-porous lm, the result is

j1 KL c10 cn1 1

1=kLlkHRT=PkHRT=kG

c1 kHp1

14Only Eq. (13) is tested experimentally in this paper.

The analysis, summarized by Eqs. (7) and (13), yields fourpredictions which can be tested experimentally. First, a plot of thelogarithm of the concentration difference should be linear in time,as suggested by Eq. (7). The second prediction is that the reciprocalof the apparent rate constant 1/K should vary linearly with theHenry's law constant kH , as suggested by combining Eqs. (8) and(13) to get

1K

1kL

AL

lDG

1kG

AG

kHRT 15

The intercept on this plot is a measure of both the mass transfer inthe liquid and of the liquid ow. We expect the slope on this plotto include effects of the mass transfer through the membrane, ofmass transfer in the gas, and of the gas ow.

The third and fourth predictions deal with changes in K causedby changes in the gas and liquid ows. Again, from Eqs. (7) and(13) we expect

1K

1kL

AL

lkHRTDG

kHRTkG

kH ARTG

16

The third prediction is that K varies with G. Because kG variesnon-linearly with G, this variation of K will be nonlinear. However,if the mass transfer in the gas is fast, then the term containing1=kG will be relatively small, and the reciprocal of K will vary withthe reciprocal of G. The fourth prediction is similar, but applies to

B. Ulrich et al. / Journal of Membrane Science 452 (2014) 426432 427

the variation of K with liquid ow L. From boundary layer theoryand from many empirical correlations, we expect the individualcoefcient in the liquid kL will vary with the square root of liquidow L. Thus the reciprocal of K should be linear with L0:5. We willtest all four of these predictions with the experiments described inthe following section.

3. Experimental

3.1. Materials

An aqueous solution of VOCs was used to model the latex paintfor some experiments. Model solutions were prepared by weightfrom distilled water and reagent grade organics. Methanol, etha-nol, acetone, and t-butanol were from Sigma Aldrich, (St. Louis,MO); ethyl acetate and n-butanol were from Fischer Scientic(Pittsburgh, PA). Each VOC was present at 1000 ppm in the modelsolutions.

A sample of acrylic latex was obtained from the Dow ChemicalCompany (Midland, MI). The viscosity of the latex was between 20and 50 cP at room temperature. The solids' concentration was50.3%, and the mean particle diameter was between 0.122 and0.125 m. The concentrations of VOCs in the latex are shown inTable 1.

3.2. Analysis

VOC concentrations were measured using a Trace GC withan FID. Helium was the carrier gas in an Agilent HP-1 capillarycolumn with a 5.0 m lm thickness. The sample loop was 2.0 mLwith a 1:10 split. The injector temperature was 150 1C and the FIDtemperature was 250 1C. The oven temperature was rst held at35 1C for 7 min, then ramped to 220 1C at 30 1C/min, and then heldat 220 1C for 1 min. The elution order and retention times arelisted in Table 2.

The response of the detector for the VOCs was calibrated forconcentrations from 10 ppm to 10,000 ppm. Calibration samples wereprepared by charging a 2.0 mL GC vial with 500 L of a standardsolution (containing all six VOCs to be analyzed in deionized water)and 2.0 L isopropyl alcohol. The data in Fig. 1 show that the responsefor each VOC is linear.

Concentrations of the VOCs in the aqueous model solution weremeasured by 2.0 L liquid injections. Samples of model solutionwere prepared by the same method as described for the calibra-tion samples. Concentrations of VOCs in the latex samples weremeasured by analyzing their headspace gas. A 1 g sample of latexfrom the reservoir was placed in a 10 mL vial, and the vials were

crimp-sealed with Teon-lined septa (Chromtech, Apple Valley,MN). After the vials were left to cool and equilibrate at roomtemperature overnight, 200 mL of the headspace gas was drawnfrom the vials and injected into the GC.

3.3. Fouling

Several membranes were screened for their ux, fouling behaviorand solvent resistance. Flat membrane samples were placed in a20 cm2 lter holder (Millipore Corp., Billerica, MA; Catalog no.XX4404700) altered to have two inlets and two outlets. Dry, roomtemperature nitrogen owed through the top compartment of thecell and latex at 50 1C was cycled through the bottom compartment.The total amount of water and organics was condensed in a cold trapin a liquid nitrogen bath. The ux was determined by dividing themass of permeate collected by the time and membrane area.

3.4. VOC stripping

Stripping of both the model solution and the latex was measuredwith the apparatus shown in Fig. 2. A at membrane was placed inthe altered Milipore lter holder. Water-saturated air at 50 1C wascycled through the bottom compartment and latex or modelsolution, also at 50 1C, was cycled through the top. The air wassaturated in a humidication column (24 in. tall, 1 in. diameter,0.16 in. Ace Glass ProPac Packing). Heating tape kept the cell at50 1C and aluminum foil wrapped around the cell reduced heat loss.Experiments were typically conducted over 2 h, and samples werecollected from the reservoir every 2030 min. Samples of the modelsolution were 0.5 mL and samples of the latex were 1.0 mL.Apparent rate constants were calculated from Eq. (7).

Table 2Elution Order and Retention Times of VOCs.Isopropyl alcohol (from Fischer Scientic) was usedas an internal standard.

Component Retention time (min)

Methanol 3.3Ethanol 5.2Acetone 6.5Isoproyl alcohol 7.1t-Butanol 0.2n-Butanol 11.4

Fig. 1. GC-FID calibration data for VOCs in model solution. The response is linearover a range of concentrations from 1 ppm to 10,000 ppm.

Table 1Concentrations of VOCs in Latex Sample. Similar VOCs were used in the modelsolution. The Henry's law constants are at 25 1C.

VOCs in latexsample

Conc.(ppm wt)

Henry's lawconstant(M/atm)

Methanol 127 200Ethanol 123 180n-Butanol 4 100Ethyl acetate 25 6.7Acetone 2500 30t-Butanol 236 Benzaldehyde 6 6.3Ethylpropionate

38 4.6

Butyl acetate 35 3.1Di-butyl ether 44 0.17Butyl acrylate 5 0.46

B. Ulrich et al. / Journal of Membrane Science 452 (2014) 426432428

4. Results

In this research we seek to remove the odor from latex binder usedin formulated latex paint by mass transfer across a membrane. Theodor, caused by low molecular weight organics, is difcult to removeby conventional air stripping because the latex foams easily. However,to be successful:

1. Membrane fouling must be minimal.2. The rate of VOC removal must be fast compared to that for air

stripping.3. The main resistances for mass transfer must be identied.

If we can measure these properties, we can make estimates ofthe membrane areas needed to process commercial quantitiesof latex.

4.1. Fouling

Membranes tested for fouling with dry nitrogen at 25 1C, aswell as the overall ux for each membrane, are reported in Table 3.The cellulose acetate membrane crumbled because it was notresistant to the latex, and the overall uxes from the Dowmembranes were approximately an order of magnitude lowerthan the other membranes. The hydrophobic polypropylene andhydrophilic nylon were chosen for further fouling studies.

The nylon membrane fouled signicantly, showing a 73%decrease in ux after 20 h of continuous operation. The ux wasrestored to 92% of the original value after washing with a cleaningsolution with base (sodium hydroxide) and detergent (sodiumdodecyl sulfate, SDS). It was not necessary to scrub or peel anylatex from the membrane. Thus, while the nylon membrane fouledrapidly, it could be cleaned easily.

As Fig. 3 shows, the polypropylene membrane fouled muchmore slowly than the nylon membrane, with a decrease in ux ofonly 5% observed after 20 h of continuous operation. Though thismembrane was cleaned before it was shut down and left over-night, a lm deposited on the membrane, reducing the ux by 69%in subsequent runs. After peeling the lm of latex from themembrane, the ux was restored to 91% of the original value.Film formation can be prevented by keeping both compartmentsof the module moist when the system is shut down.

That the hydrophobic polypropylene membrane fouled moreslowly than the hydrophilic nylon membrane is the opposite ofmany membrane experiments which ultralter biological macro-molecules. There, hydrophobic membranes have more fouling andthe hydrophilic ones remain almost pristine [14]. Perhaps thesurfactant stabilizes the particles in the latex, causing less foulingon the hydrophobic membrane.

We must stress that these results showing fouling are for VOCand water removal with dry nitrogen. For experiments removingVOCs with water-saturated air at 50 1C, no signicant fouling ofthe polypropylene membrane was observed. While our data are

limited, we expect no major fouling with the hydrophilic mem-brane either if the stripping gas is saturated with wateror steam is used at the appropriate pressure to avoid excessivetemperatures.

4.2. Stripping of VOCs

We turn next to stripping experiments. The natural log of theconcentration of the reservoir ln c1=c10 does change linearly withtime, as exemplied by the data in Fig. 4 for acetone both in themodel solution and in the latex paint. The slopes from plots likethis can be used to calculate apparent rate constants K.

The rate constant for acetone mass transfer from modelsolution is 5.170.7104 cm/s, 16% faster than transfer fromlatex, which is 4.371.1104 cm/s. However, since the error onthe rate constant is over 30%, there is little difference in the speedsof mass transfer under the conditions chosen. The value for masstransfer from the latex is a pleasant surprise. The latex is morethan 20 times more viscous than the model solutions, so we wouldexpect solution diffusion and hence mass transfer to be slower.This is not the case, perhaps because the latex is somehowfacilitating solute transport.

4.3. Mechanisms

To understand the characteristics of VOC removal, we mustexamine the mechanism for solute stripping in more detail. Wecan do this by considering how the rate varies with the Henry'sLaw constant kH , the liquid ow rate L, and the gas ow rate G.Because in our experiments we vary kH more than L and G, itsvariation is more instructive. Before we begin, we consider theHenry's law constant of a latex in more detail, and in particularhow mass transfer is related to kH . The total amount of a volatilesolvent which can dissolve in an aqueous latex suspensiondepends on the partitioning of the solute between the water andthe latex particles. Thus the total amount of a hydrophobic solute

Fig. 2. VOC stripping apparatus. Measurements of the reservoir concentrationversus time were used to explore the mass transfer mechanism.

Table 3Membranes Tested for Fouling with a Dry Nitrogen Strip Gas. The most successfulmembrane was the nanoporous polypropylene membrane.

Material Supplier Pore size

(m)Overall ux(g/cm2 h)

NF90 Dow 0.01NF200 Dow 0.01Nylon GE Osmonics 0.10 0.15Polypropylene (PP) W. L. Gore 0.02 0.10Cellulose acetate (CA) GE Osmonics 0.20

Fig. 3. Fouling of the polypropylene membrane. This membrane fouls more slowly,but is more difcult to clean.

B. Ulrich et al. / Journal of Membrane Science 452 (2014) 426432 429

can be much more in an aqueous latex than in water alone.However, because the latex particles have a large surface areaper volume, we expect that they will quickly reach equilibriumwith water. Thus we will correlate our rates both in an aqueoussolution and in aqueous latex using the water-to-vapor Henry'slaw constant. We thus avoid the more complex effective volatilityused in solubility studies [15,16].

After we measure K from plots like those in Fig. 4, we can plot1/K vs. 1=kH as shown in Fig. 5. These results strongly support thetheory leading to Eq. (15). Moreover, the magnitude of theintercept and the slope on this plot are consistent with ourestimates of the magnitudes of these quantities. More specically,the intercept should equal [(1/kL)(A/L)], or the sum of theresistances from mass transfer in the liquid phase and from theliquid ow rate, respectively. The rst term will limit VOC removalif the liquid diffusion coefcient is small, while the second termwill be limiting if the liquid ow rate is small. The slope reectschanges in volatility, both due to the membrane and to the masstransfer in the gas phase.

We can make estimates of the resistances represented by theintercept in Fig. 5. The liquid mass transfer coefcient kL can befound from the lm theory of mass transfer:

kL DLlL

105 cm2=s0:01 cm

103 cm=s 17

where DL is the diffusion coefcient of a typical solute in the liquid,approximately 105 cm2/s, and lL is the liquid boundary layerthickness, typically 0.01 cm [17]. Next we calculate the value of L/A

for our experiments:

LA 1:7 cm

3=s20 cm2

0:085 cm=s 18

Thus the mass transfer resistance in the liquid is more signicantthan the resistance from the liquid ow rate, consistent with whatnormally happens in gas stripping [1113]. Our estimate of theintercept is

1kL

AL 1

103 cm=s 10:085 cm=s

1100 s=cm 19

which is close to our experimental value of 1300 s=cm:We next turn to the slope in Fig. 5, which is about 1 s/cm.

According to Eq. (15), this slope should contain the terms1=kGl=DGA=G. The rst two terms represent the masstransfer resistances in the water-saturated stripping gas and in thegas-lled nanopores of the membrane, respectively. The third termis the effect of the gas ow removing any evaporated organicsfrom the module.

As for our estimate for intercept, we can use the parameters ofour experiments to estimate these quantities. Neglecting for themoment any correction for different solute volatilities, we canestimate kG with the following equation [17]:

kG DGlG

0:1 cm2=s

0:1 cm 1 cm=s 20

where DG and lG are the diffusion coefcient for a solute in thewater-saturated stripping gas and the thickness of the gas bound-ary layer, respectively. Because the membrane has 30% porosity, is300 mm thick, and has pores with a tortuosity of about 3, theresistance to mass transfer across the membrane is about

DGl

0:1 cm2=s 0:3

0:3 cm 3 0:03 cm=s 21

The third term is

GA

20 cm3=s

82 cm3 0:24 cm=s 22

Adding our results from Eqs. (20)(22) together we get ourestimate for the slope:

1kG

lDG

AG

11 cm=s

10:03 cm=s

10:24 cm=s

40 cm=s

23This rough estimate is the same order of magnitude as ourmeasured value of about 80 s/cm.

The success of Fig. 5 may seem surprising because the data onwhich it is based include different VOCs which have differentdiffusion coefcients in water and in gas. We should expect thatdifferent solutes lead to different values of the mass transfer kL andkG. Further study is needed to better understand the extent towhich differences in hydrophilic/hydrophobic character affectmass transfer rates. For example, highly hydrophobic solutes arelikely to be strongly associated with latex polymer particles, andmay prove somewhat slower to strip from the latex compared tohighly hydrophilic solutes that tend to reside in the continuousphase. Similarly, we could expect for the various solutions differ-ent partition coefcients and different boundary layer thicknessesaround latex particles. The fact that these apparently do notoccur afrms our use of a single airwater partition coefcient,as discussed above.

Thus in our experiments, removal of VOCs from latex is largelycontrolled by two resistances. The rst expected resistance is thatdue to solute diffusion in the liquid, represented by 1/kL. Thesecond, unexpected one is mostly due to water-saturated gas inthe membrane's pores. To further evaluate the mass transfermechanism, we examine the effects of varied gas and liquid ow

Fig. 5. VOC stripping from model solution versus Henry's law constant. This plot isconsistent with Eq. (15).

Fig. 4. Acetone removal from model solution and from latex. In both experimentsthe liquid ow L was 1.7 cm3/s and the gas ow G was 82 cm3/s. The rates are equalwithin experimental error.

B. Ulrich et al. / Journal of Membrane Science 452 (2014) 426432430

rates on VOC removal. As discussed above, we expect plots of 1/Kversus G1 and L1/2 to be linear. To test this, stripping experi-ments were conducted with the model solution over a small rangeof ow rates for the liquid (0.81.7 cm3/s) and the stripping gas(80160 cm3/s). The results from these experiments, shown inFig. 6, are consistent with the dependence of these variations of Kwith G and L. However, due to the limited accuracy of the data andthe relatively small ranges of G and L, this prediction is not proven.

5. Discussion

The results above lead to three important conclusions. First,membrane-assisted stripping avoids latex foaming. This is simplynot a problem.

The second conclusion is that membrane fouling is minor if theVOCs are stripped with water-saturated gas (or by extension, withsteam). This conclusion is more restricted than that on foaming,because we have never continuously run our experiments for morethan a few days. To be successful commercially, we believe that anymembranes must be operated for about 3 years. Achieving thislength of service life may require periodic cleaning, an aspect whichwe have not considered carefully. This point merits further study.

The third conclusion is that the mass transfer is about what isobserved in more conventional separation processes. In particular,a nanoporous hydrophobic membrane offers little additional masstransfer resistance. This conclusion is that expected from a widevariety of experiments with many other membrane contactors,including those suggested for gas absorption, liquidliquid extrac-tion, and differential distillation [1822]. It is important to havethis afrmed experimentally for stripping of VOCs from latex,which is what is done in this paper.

However, this positive conclusion should be tempered byother experience using microporous polypropylene membranes.While these membranes often give successful rates for the rstfew days, the membrane's pores can then wet and the membra-ne's mass transfer rates can decrease. These slower rates ofteninvolve solvents or detergents which facilitate pore wetting. Wesaw no evidence of these effects here, but we did not look at thelonger operating times where they would be expected to occur.If these slower rates do occur, they can sometimes be managedby lling the pores with a hydro-gel like polyvinylalcohol.

We can use the experimental results to estimate the membranearea required for latex processing. To make this more specic,imagine that we wish to remove 90% of the VOC content from tentonnes of latex per day, or a ow of 120 cm3 latex/s. We canimagine carrying out this stripping either across a membraneconnected to a stirred tank or in a hollow ber membrane module.For the stirred tank, the membrane would be at sheets, whichgive a small membrane area per latex volume but can be easily

disassembled for cleaning. From a balance on the VOCs in thestirred tank, we obtain

c1c10

11KA=L 24

c1c10

0:1 1=5 104 cm=sA

120 cm3=s25

A 200 m2 26where c1/c10 is the fraction of VOC content remaining, and K is theoverall rate constant determined in this work. For a hollow bermodule, the area needed is smaller [16]. A mass balance on theVOCs in bers surrounded by a high gas ow gives

c1c10

eKLA=Q 27

c1c10

0:1 ef1010 4 cm=sA=120 cm3=sg 28

A 30 m2 29where c1/c10 is again the fraction of VOC content remaining and KLis the overall mass transfer coefcient inferred at high gas ow fromthis work. This assumes that the latex is owing within the hollowbers. Other studies of different module designs suggest that KL canbe increased by about ve times by having the latex ow outside ofand across a bed of hollow bers [23]. This suggests a strategy fordecreasing the membrane area still more, though the hollow berswill be much more difcult to clean than at membranes. On thisbasis we believe this membrane-assisted removal of trace VOCsfrom latex used for paint has considerable potential.

Acknowledgments

The authors are indebted to W.A. Arnold, M.L. Trippeer, andGrant A. Von Wald for helpful discussions, and to Andrew Wagnerfor help with preliminary experiments. This work was supportedby the Dow Chemical Company.

Nomenclature

A membrane areac1 volatile solute concentration in reservoir, moles

per volumec01 volatile solute concentration in module, moles

per volume

Fig. 6. Mass transfer of acetone versus gas and liquid ow. The data are consistent with the expectation that 1/K varies linearly with G1 and L1/2.

B. Ulrich et al. / Journal of Membrane Science 452 (2014) 426432 431

cn1 hypothetical volatile solute concentration in liquidin equilibrium with gas

DG; DL diffusion coefcients in gas and liquid respectivelyG gas ow, volume per timekG individual gas mass transfer coefcient, length

per timekH a Henry's law coefcient, moles per volume per

pressurekL individual liquid mass transfer coefcient, length

per timeK apparent rate constant, length per timeKL overall mass transfer coefcient, length per timeL liquid ow, volume per timel membrane thicknessP permeabilityp1 partial pressure of volatile soluteR gas constantT temperaturet timeV reservoir volume void fraction in membrane tortuosity in membrane

References

[1] D. Stoye, B. Marwald, W. Plehn, Paints and coatings. 1. Introduction, Ullmann'sEncyclopedia of Industrial Chemistry, Wiley, 2010.

[2] P. Wolkoff, How to measure and evaluate volatile organic compound emis-sions from building products. A perspective, Sci. Total Environ. 227 (1999)197213.

[3] J.L. Fleming, Volatilization Technologies for Removing Organics from Water,Noyen data, Park Ridge, NJ, 1990.

[4] Y.L. Hwang, G.E. Keller, J.D. Olson, Steam stripping for removal of organicpollutants from water. 1. Stripping effectiveness and stripper design, Ind. Eng.Chem. Res. 31 (1992) 17531759.

[5] Y.L. Hwang, J.D. Olson, G.E. Keller, Steam stripping for removal of organicpollutants fromwater. 2. Vapor liquid equilibrium data, Ind. Eng. Chem. Res. 31(1992) 17601768.

[6] R.E. Hinchee, Air Sparging for Site Remediation, CRC Press, Boca Raton,FL, 1994.

[7] J.R. Ortiz-Del Castillo, G. Guerrero-Medina, J. Lopez-Toledo, J.A. Rocha, Designof steam-stripping columns for removal of volatile organic compounds fromwater using random and structured packings, Ind. Eng. Chem. Res. 39 (2000)731739.

[8] J.G. Crespo, K.W. Boddeker, Membrane Processes in Separation and Purica-tion, Springer Verlag, Berlin, 2010.

[9] I. Abou-Nemeh, S. Majumdar, A. Saraf, K.K. Sirkar, L.M. Vane, F.R. Alvarez,L. Hitchens, Demonstration of a pilot-scale pervaporation systems for volatileorganic compound removal from a surfactant enhanced aquifer remediationuid, Environ. Prog. 20 (2001) 6473.

[10] L. Dahuron, E.L. Cussler, Protein extractions with hollow bres, AIChE J. 34(1968) 130136.

[11] C.J. Geankoplis, Transport Processes and Separation Process Principles, 3rd ed.,Prentice Hall, Upper Saddle River, NJ, 2003.

[12] W. McCabe, J.C. Smith, P. Harriot, Unit Operations of Chemical Engienering, 7thed., McGraw-Hill, New York, NY, 2005.

[13] J.D. Seader, E.J. Henley, Separation Process Principles, 3rd ed., Wiley, Hoboken,NJ, 2010.

[14] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, J.B. Marinas,A.M. Mayes, Science and Technology for water purication in the comingdecades, Nature 452 (2008) 301310.

[15] T.C.G. Kibbey, K.D. Pennell, K.F. Hayes, Application of sieve-tray air strippers tothe treatment of surfaction-containing wastewaters, AIChE J. 47 (2001)14611470.

[16] C. Zhang, G. Zheng, C.M. Nichols, Micellar partitioning and its effects onHenry's law constants of chlorinated solvents in anionic and nonionicsurfactant solutions, Environ. Sci. Technol. 40 (2006) 20214.

[17] E.L. Cussler, Diffusion, 3rd ed., Cambridge University Press, New York, NY,2009.

[18] Q. Zhang, E.L. Cussler, Microporous hollow bers for gas absorpion, J. Membr.Sci. 23 (1985) 321345.

[19] R. Prasad, K.K. Sirkar, Dispersion-free solvent extraction with microporoushollow bers modules, AIChE J. 34 (1988) 177188.

[20] D. Yang, R.S. Barbero, D.J. Delvin, E.L. Cussler, C.W. Colling, M.E. Carrera,Hollow bers as structured packing for olen/parafn separation, J. Membr.Sci. 279 (2006) 6169.

[21] D. Yang, J.D. Devlin, R.S. Barbero, R. Wright, E.L. Cussler, Hollow bers asstructured packing for olen/parafn distillation, in: Proceedings of the WorldCongress of Chemical Engineering, Montreal, Canada, August 2009.

[22] E. Bringas, M.F. San Roman, J.A. Irabien, I. Ortiz, Overview of the mathematicalmodeling of liquid membrane separation processes in hollow ber contactors,J. Chem. Technol. Biotechnol. 4 (2009) 1531614.

[23] S.R. Wickramasinghe, J.J. Semmens, E.L. Cussler, Mass transfer in varioushollow ber geometries, J. Membr. Sci. 69 (1992) 235250.

B. Ulrich et al. / Journal of Membrane Science 452 (2014) 426432432

Membrane-assisted VOC removal from aqueous acrylic latexIntroductionTheoryExperimentalMaterialsAnalysisFoulingVOC stripping

ResultsFoulingStripping of VOCsMechanisms

DiscussionAcknowledgmentsReferences