Membrane-Assisted Volatile Organic Compound Removal from

Jul 15, 2014 - J. Kent Carpenter,. §. William A. ... The Dow Chemical Company, 727 Norristown Road, Spring House, Pennsylvania 19477, United States. ...
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Membrane-Assisted Volatile Organic Compound Removal from Aqueous Acrylic Latex Is Faster Than from Aqueous Solutions Teng Zeng,†,⊥ Andrew J. McCabe,† Timothy C. Frank,‡ J. Kent Carpenter,§ William A. Arnold,*,† and Edward L. Cussler*,∥ †

Department of Civil, Environmental, and Geo- Engineering, University of Minnesota, 500 Pillsbury Drive SE, Minneapolis, Minnesota 55455, United States ‡ Engineering & Process Science Lab, The Dow Chemical Company, 1319 Building, Michigan Operations, Midland, Michigan 48667, United States § The Dow Chemical Company, 727 Norristown Road, Spring House, Pennsylvania 19477, United States ∥ Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States ABSTRACT: Solutes such as butyl ether and acetone, which cause unpleasant smells in latex paint, can be stripped from aqueous solutions using a porous membrane. The different mass transfer coefficients in this stripping are dominated by diffusion in the aqueous solution, and behave as expected from boundary layer theory. These solutes can also be stripped without foaming from latex paints whose viscosities are 20−50 times higher than those of aqueous solutions. For the more hydrophobic solutes such as butyl ether, the mass transfer coefficients in latex are smaller than those in water, as expected from theory. For more hydrophilic solutes such as ethanol, the mass transfer coefficients can be as much as 4 times larger. This unexpected result, inconsistent with conventional theory, may result from latex particles serving as a source and can be described by coupled diffusion and desorption.

1. INTRODUCTION After a room is painted with latex paint, the room smells. This smell comes from volatile organic compounds (VOCs), such as acetone, tert-butyl alcohol, and butyl propionate, which are present in the paint at concentrations of a few parts per million to several tenths of a percent.1 While the smell is usually gone in a few days, consumers do not like it. Latex paints that do not smell would be a superior product. Ironically, the small amounts of VOCs present in paint are not necessary for the paint’s function. They are not required for such properties as spreadability, hiding power, and scrub. These VOCs are added or produced as byproducts during paint manufacture, but their removal will not significantly affect paint properties. Moreover, the removal of VOCs from many dilute solutions is easy: small air or steam bubbles are blown through the solution, the solvents evaporate into the bubbles, and this evaporation is fast because the bubbles’ small size provides a large area per volume. Such “air sparging” is a common and reliable method of removing VOCs and a basic technique of environmental engineering.2−4 Unfortunately, air sparging does not work well for removing the smell from latex paint for two reasons. First, the use of air can destabilize the latex emulsion, producing clots in the paint. This can be avoided by stripping not with dry air but with water-saturated air or low pressure steam. The second reason that sparging can be ineffective for latex emulsions is the production of foam. The foam may not be serious when using large bubbles, but it can be a major problem for small bubbles.5 Such small bubbles are required to effect fast VOC removal from the latex emulsion. © 2014 American Chemical Society

To avoid foaming, we have been investigating VOC removal using a nonselective, porous membrane.6 In these investigations, a latex emulsion containing VOCs was pumped past one side of the membrane and a water-saturated air stream was pumped past the other side. The VOCs are removed from the paint by diffusion across the membrane. Such a membrane geometry, sometimes called a “membrane contactor”, has been widely explored for gas treating7,8 and liquid−liquid extraction.9 Such removal has also been explored with nonporous pervaporation membranes,10 and may find application in shale gas liquids. Our preliminary experiments6 showed that hydrophobic, porous membranes were not fouled by the latex used to formulate paint. These membranes completely avoided foaming. They showed that mass transfer both from model aqueous solutions and from latex was usually controlled by diffusion of the VOCs in the liquid phase. Finally, mass transfer rates were consistent with estimates based on literature correlations. These preliminary experiments suggested that removing VOCs from 10 metric tons of latex per day would take around 30 m2 of membrane, which is encouraging. These preliminary data, however, were fragmentary and not sufficiently complete to let the value of this method be evaluated. Received: Revised: Accepted: Published: 12420

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⎛A 1 A 1 1 ⎞ = + + H⎜ + ⎟ K L kL kG ⎠ ⎝G

In this paper, we report many more results which allow the practical value of these ideas to be more completely judged. We are especially interested in the mass transfer mechanism by which the VOCs are removed from acrylic latex. We begin our report by reviewing the theory for mass transfer out of aqueous acrylic latex. We then describe our experiments, stressing the details of the analysis of dilute solutions of the VOCs. Our results include studies of mass transfer in water, in detergent solutions, and in two commercially sold formulations used in latex paints. Armed with these experimental results, we discuss the mechanisms for VOC removal.

where L is the volumetric flow rate in the liquid and kL is the mass transfer coefficient in the liquid. Similarly, the third limit when latex is present leads to a still more elaborate equation for K: ⎡A ⎛A 1 1 1 1 ⎞⎤ = + H′⎢ + + H⎜ + ⎟⎥ K kp kL k G ⎠⎥⎦ ⎝G ⎣⎢ L

⎛ ⎞ dc1 ⎜ 1 ⎟ c1 =⎜1 1 ⎟ dt H ⎝ G + kGA ⎠

3. EXPERIMENTAL SECTION 3.1. Chemicals and Reagents. Acetone (≥99.9%), ethanol (≥99.5%), tert-butyl alcohol (≥99.5%), n-butyl ether (≥99.6%), n-butyl propionate (≥99.5%), benzoic acid (BA; ≥99.5%), and methylene blue (MB; dye content ≥ 82%) were obtained from Sigma-Aldrich. Sodium dodecyl sulfate (SDS; ≥98.0%) was obtained from Fluka. All chemicals were used as received without further purification. Two commercial latex formulations used to make paints, PRIMAL SF-012 (hereafter abbreviated as “paint SF”) and RHOPLEX AC-261LF (abbreviated as “paint AC”), were supplied by The Dow Chemical Company. Benzoic acid (20 mM) and methylene blue (1 mg/L) solutions were prepared by dissolving appropriate amounts of solids into deionized (DI) water. Sodium dodecyl sulfate solution (19 mM SDS) was prepared with DI water, followed by sonication at room temperature for 90 min. A multisolute aqueous VOC solution containing three hydrophilic VOCs (ethanol, acetone, and tert-butyl alcohol at 1000 ppm (w/w)) and two hydrophobic VOCs (n-butyl ether and n-butyl propionate at 1000 ppm (w/w)) was freshly prepared prior to each experiment by spiking appropriate volumes of liquid VOCs into DI water. An aqueous VOC solution with SDS was prepared by spiking appropriate volumes of liquid VOCs into the SDS solution and was equilibrated for 48 h to ensure homogeneous micelle formation. Diluted latex solutions were prepared by diluting the original latex sample with DI water to achieve a desired latex-to-water ratio (v/v) of 75:25, 50:50, or 25:75 in the final mixture. 3.2. VOC Partitioning Experiment. The VOC partitioning between latex and water as a function of time was measured using a closed-system method. To generate solid latex samples, the latex suspension was evenly spread over a flat glass plate, dried at room temperature for 48 h, and cut into small pieces. Varying amounts (30−50 mg) of the dried latex were weighed into crimp top vials which were subsequently filled with the aqueous VOC solution with no headspace and immediately sealed with PTFE-lined caps. Control vials containing latex-free model aqueous VOC solution or dried latex with DI water were also prepared in a similar manner to account for VOC loss via

(1)

where V is the volume of the aqueous phase, G is the volumetric flow rate of gas, kG is the overall mass transfer coefficient from the gas−liquid interface across the porous hydrophobic membrane into the bulk gas, and A is the total membrane area. The Henry’s law constant H is the ratio at equilibrium of the concentration in the liquid divided by that in the gas.15 This equation is easily integrated subject to the initial condition that

t=0

c1 = c10

The result is c1 = e−(KA / V )t c10

(2)

(3)

where K is an overall apparent rate coefficient given by ⎛A 1 1 ⎞ = H⎜ + ⎟ K kG ⎠ ⎝G

(6)

where kp is an apparent mass transfer coefficient in the latex particles themselves and H′ is another partition coefficient, the ratio at equilibrium of the VOC concentrations in the latex polymer and the liquid. Note that kp includes the effects of latex size and volume fraction. Note also that the product H′H is the ratio at equilibrium of latex polymer to gas concentration. Strictly speaking, eq 6 implies that c1 is now the total VOC amount in the latex polymer plus that in the water per volume of liquid, which is, in our case (H′ ≫ 1), dominated by the VOCs in the latex particles. We will return to this point later in the discussion below, after discussing our experiments.

2. THEORY In this research, we are trying to quickly remove small amounts of organic solutes from a variety of aqueous solutions, ranging from nearly pure water to latex containing 50% acrylic polymer by weight. To explore this removal, we consider transport out of a relatively small reservoir of solution into a much larger volume of gas using a small membrane module. The removal rates are most easily discussed as three limits: that of small gas flow, that of dilute aqueous solution, and that of the latex. Each of the three limits is based on an analysis of resistances in series.11−14 For the case of small gas flow, a solute concentration c1 in the liquid changes with time t as V

(5)

(4)

Thus, we expect that the logarithm of the liquid concentration will decrease linearly with time for the small gas flow case. Two special cases of this limit are obvious. First, if the gas flow G is near zero, then the solute concentration in the gas adjacent to the membrane will be saturated, and the solute removal will depend only on how fast the gas flows. The other special case occurs when the gas flow G is large and the membrane is relatively impermeable. In this case, the removal rate will not be directly affected much by G, but more by changes in kG. Both of these cases show a removal rate which depends on the Henry’s law constant, H. The second, more important limit is that for a dilute aqueous solution when the gas and liquid flows are both small. An analysis which parallels that above for c1 leads again to an expression like eq 3 but with 12421

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Figure 1. Schematic of the system. The VOCs from the reservoir are removed into the moist air across the membrane.

monitored by a Cole-Parmer 150 mm correlated flow meter. The feed liquid, either a model VOC solution or an acrylic latex, was delivered at varying flow rates into the membrane module by a MasterFlex L/S pump with an EASY-LOAD II head from a collapsible Teflon gas sampling bag. Teflon spaghetti tubing of 0.097 cm i.d. and MasterFlex PharMed pump tubing of 0.15 cm i.d. were used for all connecting lines from the Teflon bag to and from the membrane module. The membrane module was wrapped with a silicone heating tape interfaced to a PowerStat variable autotransformer to minimize the heat loss. A long-stem thermometer was attached to monitor the temperature of the membrane module. Model solution or latex samples (∼100 mg) were collected from the outlet of the membrane module into autosampler vials which were immediately sealed with PTFE-lined crimp caps. The exact mass of sample was determined gravimetrically. For all stripping experiments, a single layer of GORE-TEX polypropylene membrane (pore size 0.02 μm; thickness 100 μm) was used. 3.4. VOC Analysis. Concentrations of VOCs were analyzed by a ThermoQuest Trace 2000 gas chromatograph (GC) equipped with a flame ionization detector (FID). Chromatographic separations were performed using a DB-1 fused silica capillary column (100% dimethylpolysiloxane bonded phase; 30 m × 0.32 mm i.d. × 5 μm film thickness; J & W Scientific Inc.). Helium at a constant pressure of 60 kPa was used as carrier gas. The injector (split/splitless) and detector temperatures were set at 150 and 250 °C, respectively. For the analysis of VOCs in the model aqueous solutions, a 1 μL sample was directly injected using a Hamilton Microliter syringe equipped with a Chaney adapter in the split mode (split ratio 33). For VOC analysis from latex samples, the vial containing the collected latex was incubated at 150 °C for 20 min to allow for drying of the latex/evaporation of the VOCs into the gas headspace. To quantify the VOC concentrations,

other processes or any residual VOC release from latex. The exact mass of latex and volume of aqueous solution were determined gravimetrically. One vial was sacrificed at selected times for VOC analysis over a 48-h period. The partition coefficient H′ (in L/kg) for target VOCs at each time point was calculated from q1e =

H′ =

(c10 − c1e)V m

(7)

q1e c1e

(8)

where c10 (mg/L) is the initial aqueous concentration of VOC, c1e (mg/L) is the equilibrium aqueous concentration of VOC at time t, V is the volume of aqueous phase, m is the mass of latex, and q1e (mg/kg) is the amount of VOC in the latex. After 120 h of equilibration, the aqueous VOC content in each vial was analyzed. 3.3. VOC Stripping Experiment. A schematic of membrane-assisted VOC stripping apparatus is shown in Figure 1. The apparatus had three major components: a feed reservoir, a humidification tower, and a membrane module. The membrane module, constructed from stainless steel, consisted of symmetrical upper and lower compartments that were bolted together to form a unit of length 15 cm, width 5 cm, and height 1.5 cm. Both compartments had an inner surface cavity of length 10 cm, width 2 cm, and height 0.5 cm, which sandwiched a flat-sheet membrane having an effective area of 20 cm2. A Viton gasket was used to seal the membrane, and a stainless steel porous mesh was placed underneath the membrane to minimize deflection. Prior to a stripping experiment, heated DI water was circulated through the top of the humidification tower, while a counterflow of dry air was passed through the tower before entering the lower compartment of the membrane module. The air flow rate was 12422

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200 μL of headspace vapor was taken from the vial and injected using a Hamilton gastight syringe in the splitless mode. The oven temperature was programmed as follows to achieve baseline resolution of five target VOCs: initial temperature 35 °C (hold for 1 min), initial rate 10 °C/min to 100 °C (hold for 0 min), and then 20 °C/min to 200 °C (hold for 5 min). Retention times for ethanol, acetone, tert-butyl alcohol, n-butyl ether, and n-butyl propionate were 4.08, 4.75, 5.64, 13.14, and 13.34 min, respectively. The GC was periodically calibrated using VOC standards, and the detector response for target VOCs was linear over a concentration range of 1−10 000 ppm. 3.5. Benzoic Acid and Methylene Blue Supporting Experiments. Benzoic acid was chosen as a model solute for membrane transfer, while the dye methylene blue was used as a tracer to test for dispersion. Both experiments were carried out using the setup described above. For the benzoic acid experiment, however, a custom-made poly(vinyl alcohol) membrane was used, and the air flow was replaced with a counterflow of DI water. For the dye experiment, the feed was switched back and forth between the dye solution and water while other experimental conditions remained the same. The absorbance of the solutes was measured at 193 nm (benzoic acid) and 664 nm (methylene blue) by a Shimadzu UV-1601 PC spectrophotometer using 1 cm path length quartz cuvettes.

into a stream of pure water. We plot the logarithm of c1/c10 vs time and calculate the overall rate constant K from the slope on this plot using eq 3. The actual variation of K with the liquid flow L is shown in the inset of Figure 2. A plot of the reciprocal

Figure 2. Variation of the overall mass transfer coefficient with liquid flow. These data, for stripping of a benzoic acid solution across a poly(vinyl alcohol) membrane into DI water, are consistent with boundary layer theory.

4. RESULTS The focus of this research is the removal of organic solutes from latex using a membrane. Understanding this removal is aided by knowing the partition coefficients of the solutes between the various phases present. Those between water and gas are summarized in the second column of Table 1, and those

of K does vary with the inverse square root of L, as shown in Figure 2. This variation is that expected from boundary layer theory.12,15 The zero intercept shows that there is no significant membrane resistance, consistent with extensive earlier literature.16−18 Thus, the data in Figure 2 show that the membrane module is working as expected for benzoic acid diffusion through a poly(vinyl alcohol) membrane. We next measured acetone mass transfer while varying the flow both of air and of aqueous acetone solution. In these experiments, we found steady water flow only at air flows less than about 40 mL/s. At higher air flows, the water flow became erratic, first squirting and then dribbling. Such pulsing flows cause a maximum in K as the liquid flow increased, as shown in Figure 3. Presoaking the membrane in the acetone−water solution makes no difference. We believe high air flows change the shape of the flow channel, and as a result, we ran all further experiments at air flows of 20 mL/s. We were also concerned that we could have significant dispersion and bypassing in our membrane module. After all, the cell has a cross section of 2 cm × 0.5 cm, but the circular inlet and outlet tubes have a diameter of 0.096 cm. To test the effect of the sudden expansion and contraction in crosssectional area, we injected methylene blue solution into the incoming water, and we measured the absorbance of the dye coming out. If dispersion is significant, the maximum absorbance exiting the cell would come sooner and the spread of the absorbance would be larger when the flow was faster. To check for this, we plot the absorbance of the exiting dye solution vs the product of liquid flow and time, as shown in Figure 4. In Figure 4, both the flow and the time vary by more than a factor of 10, but the concentration vs the product of flow and time varies by about ±20%. While the data in Figure 4 do appear to show a slightly broader peak for higher liquid flow than for lower liquid flow, any effect is small. Because of the results in Figures 2−4, we believe that the apparatus is operating as expected, without significant dispersion.

Table 1. Partition Coefficients for the Five Solutes in Water and Two Latex Formulationsa H′ (latex/water) (L/kg) −3

compound (×10 )

H (water/gas)

AC

SF

ethanol acetone t-butyl alcohol n-butyl ether n-butyl propionate

4.9 0.74 3.2 0.004 0.049

5.2 ± 0.3 8.3 ± 0.1 8.3 ± 0.1 101 ± 8 49 ± 7

5.1 ± 0.5 8.8 ± 0.5 8.6 ± 0.8 105 ± 14 54 ± 6

a

At equilibrium, the solutes are more concentrated in the liquid than in the gas, and in the latex more than the liquid.

measured between the latex polymers and water are shown in the third and fourth columns of Table 1. Note that all latex/ water partition coefficients are much greater than 1. We next turn to the membrane experiments themselves, made using the apparatus in Figure 1. These are conveniently divided into three groups. The first group consists of experiments which test whether the membrane module is behaving as expected. The second group centers on the study of aqueous solutions, showing that these behave as expected from the conventional theory of mass transfer, as reviewed in section 2. The third group of experiments centers on measurements with two latexes. Note that these are the latexes only: they are not paint until they are formulated with pigments and other added ingredients. These three groups are discussed sequentially in sections 4.1, 4.2, and 4.3. 4.1. Tests of Membrane Apparatus. We begin with the experiments testing the membrane apparatus itself. The first test was to extract benzoic acid from a nearly saturated solution 12423

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Figure 3. Mass transfer coefficients at high air flows. The data, for acetone transfer, are apparently a consequence of membrane fluctuations. Experiments reported below used lower air flows without these fluctuations. Error bars represent the standard deviation of duplicate measurements; where absent, bars fall within symbols.

Figure 5. Removal of VOCs from a model aqueous solution. The variation with liquid flow is consistent with eq 5.

membrane and in the gas phase. For n-butyl ether and n-butyl propionate, the plots show a small positive intercept, implying some resistance in the vapor or the membrane. We are uncertain about the detailed causes of these differences. 4.3. Latex Emulsion. The mass transfer coefficients for these same five solutes stripped out of two different latexes are startlingly different, as shown in Figure 7. These differences are emphasized by the results for aqueous acetone, replotted from Figure 5 as the solid line in Figure 7. To begin, the values of K for ethanol, acetone, and tert-butyl alcohol can be bigger for the latex than those in the model solution. This is in spite of the viscosity of the latex, which is 20−50 times higher than that of water. Such an increased viscosity normally should decrease the mass transfer,15 exactly the opposite of the effect which is observed. In contrast, the more hydrophobic solutes, n-butyl ether and n-butyl propionate, have mass transfer coefficients less than 10−5 cm/s, too small for us to measure accurately. These solutes show mass transfer rates over 50 times slower than those in the model aqueous solutions. Finally, the values of K vary differently with liquid flow than the K values for aqueous solutions, as shown in Figure 8. In particular, data on the left of Figure 8 show that 1/K does not vary linearly with the inverse square root of the liquid flow L, as observed for the latex-free aqueous data shown in Figure 6. The data on the right-hand side of Figure 8 show that 1/K is not proportional to 1/L either. To explore these differences, we also measured mass transfer in detergent solutions and in diluted latex. Data for VOCs in detergent solutions show behavior between that in aqueous solutions and that in latex. Like aqueous solutions, mass transfer coefficients of different solutes are different, as shown by the values in Figure 9. These values are slightly larger than those in aqueous solutions. The variation of 1/K with liquid flow seems correlated somewhat less well with 1/L1/2 than with 1/L, as Figure 10 shows. Data for latex diluted with different amounts of water and flowing at 0.04 mL/s show that K appears to vary linearly with the volume fraction of latex, as shown in Figure 11. The values for n-butyl ether and n-butyl propionate are not shown because they were too small to measure. The meaning of these and other results are explored in section 5.

Figure 4. Tracer tests of the membrane module. The results show that dispersion is not major, and is largely independent of flow.

4.2. Aqueous Solutions. We next turn to the removal of the volatile, low molecular weight organic solutes from aqueous solutions. The mass transfer coefficients for five of these solutes are plotted vs solution flow in Figure 5. As expected, different solutes have different K values: butyl ether is the fastest and ethanol is the slowest. As expected, the K values rise quickly with slower flow rates and more slowly with faster flow rates. This variation with flow is consistent with eq 5, but the mechanism is not completely identified. If the solute removal is dominated by liquid convection, then the first term on the right-hand side of eq 5 is key, and 1/K should vary with 1/L. On the other hand, if removal is controlled by mass transfer in the liquid, then 1/K should vary with 1/kL, which, in turn, is proportional to 1/L1/2. When the data in Figure 5 are replotted as reciprocals (as a “Wilson plot”19), they appear to better support the second, mass-transfer-controlled mechanism, as shown in Figure 6. The apparent mass transfer limitation is consistent with other membrane experiments,16−18 including those in Figure 2. These data, however, do not completely behave as expected. For ethanol, acetone, and tert-butyl alcohol, the plots show a zero intercept, consistent with no significant resistance in the

5. DISCUSSION The results above show that diffusion across a membrane is an attractive route for removing hydrophilic VOCs from latex, and 12424

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Figure 6. Double reciprocal plots of mass transfer from water. The linear relationship between 1/K and 1/L1/2 suggests that mass transfer in the liquid controls removal of VOCs from water (cf. eq 5).

Figure 8. Double reciprocal plots for the latexes. The relationship between 1/K and 1/L1/2 or 1/L differs from that in Figure 6.

experiments using a membrane which is more permeable to these solutes would be expected to have little effect. In the latex solutions, the results are more complicated. For the more hydrophobic solutes n-butyl ether and n-butyl propionate, the mass transfer is much slower. This is predicted by eq 6 to be the result of the larger value of the partition coefficient H′, as given in Table 1. Strictly speaking, H′ should be reduced by the volume fraction of polymer particles in the latex, but this will not dramatically affect the estimates. For the moment, this conclusion seems secure. The results for stripping the hydrophilic solutes from the latex are the surprise in this paper. The mass transfer coefficient K of these solutes is about the same size or bigger than those observed in the model aqueous solutions. For example, the values for acetone in aqueous solutions, shown as the solid curve in Figure 7, are about 3 times less than the experimental values in latex, shown by the filled and empty circles. This is in spite of the increased viscosity of the latex. At the same time, the variation of the latex K values with flow, detailed in Figure

Figure 7. Removal of VOCs from two latexes. Compared to the model aqueous solution, the more hydrophilic VOCs have faster removal and the hydrophobic VOCs have slower removal. The solid line, which is the removal of acetone from the model aqueous solution, is shown for comparison. Error bars represent the standard deviation of duplicate measurements; where absent, bars fall within symbols.

hence removing much of the smell from freshly painted rooms. Such smell-free rooms are expected to be preferred by consumers. The membrane-assisted process shows no membrane fouling and no major membrane resistance to VOC removal. The detailed mechanism for the mass transfer out of latex is not always clear. In aqueous solutions, the removal of all solutes seems to be controlled by diffusion in the water itself. There is little or no membrane resistance, so that repeating the 12425

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a series of resistances for diffusion out of the latex, through the water, across the membrane, and into the gas phase. If so, the rates of transfer would go down at least 1000 times. The rates never go down enough. In some cases, the rates even go up. Such larger values of kL probably result from mass transfer steps which include coupled diffusion and desorption from the latex. Most obviously, the submicrometer latex particles may reduce the effective boundary layer thickness, which in aqueous solution is typically about 100 μm. Here, the latex particles measured to be 0.15 μm across,15 and these particles contain at least 100 times more solute than the water itself. Still, the latex particles are over 100 times larger than the solute molecules, so they are relatively immobile. Thus, we have a boundary layer which has a lot of solute held in effectively immobile spheres. Two ways in which such latex spheres could enhance mass transfer are as shunts to diffusion or as sources for desorption. To serve as shunts, the latex would form a second discontinuous phase through which permeation is very rapid. Thus, a solute slowly gets across a thin layer of water, and then zips across a latex sphere, and then slowly struggles through more water. In this case, H′Dlatex for these solutes would be much greater than Dwater. The apparent diffusion coefficient in this composite of suspended spheres Dcomposite is then12

Figure 9. Removal of VOCs from aqueous sodium dodecyl sulfate. The behavior is between that of aqueous solutions (Figure 5) and that of latexes (Figure 7). Error bars represent the standard deviation of duplicate measurements; where absent, bars fall within symbols.

⎛ 1 + 2ϕ ⎞ Dcomposite = Dwater ⎜ ⎟ ⎝1−ϕ ⎠

(9)

where ϕ is the volume fraction of latex. This estimate, originally suggested by Maxwell, says that, for 50% latex, mass transfer will increase by a factor of around 4. Alternatively, the latex could supply a local source of solute which rapidly desorbs into the surrounding water. Now the diffusion in the composite D is given by15

Figure 10. Double reciprocal plots for sodium dodecyl sulfate solutions. The variations lie between the aqueous and latex data.

Dcomposite = Dwater (1 + H′ϕ)0.5

(10)

Again, the mass transfer rate will be increased by the presence of the latex. These results can serve as a guide for removing dissolved solutes from systems other than latex paint. In particular, we wondered about removing VOCs from suspensions of solid materials, including muds. Our data suggest that membranes could effectively aid removal of more hydrophilic solutes such as trichloroethylene (TCE), which has a partition coefficient roughly like that of acetone. Our data imply membranes would be less effective for hydrophobic solutes such as polychlorinated biphenyls (PCBs). In addition to being effective for systems which foam, our designs are parallels to membrane bioreactors.20 However, we can only speculate about the costs of these separations. We conclude that more hydrophilic solutes can easily be stripped from latex across a membrane, but that more hydrophobic solutes may not be. Because the dominant mass transfer resistance is in the liquid and not in the membrane, the same conclusions hold for steam stripping in general: hydrophilic solutes will be much more easily stripped than hydrophobic ones. The membrane-facilitated stripping developed here is not compromised by foaming. Thus, this technique is superior to conventional stripping, but it is not a panacea for removing the smell from freshly painted rooms.

Figure 11. Mass transfer vs latex concentration. The rate for these more hydrophilic solutes increases with the amount of latex.

8, seems different from those in model aqueous solutions, shown in Figure 6. This result is completely inconsistent with the predictions of eq 7, which says that because the partition coefficient between latex and water is much greater than 1, the values of the mass transfer coefficients should decrease by H′. They do not, even for the more hydrophobic solutes. Thus, the simple picture of resistances in series, the standard for the analysis of mass transfer,12−15 is wrong for these latex solutions. We cannot have

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*E-mail: [email protected]. Tel.: 612-625-8582. 12426

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Present Address ⊥

T.Z.: Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, CA 94305, USA. Notes

The authors declare the following competing financial interest(s): coauthors Timothy C. Frank and Edward L. Cussler are authors of a patent assigned to the Regents of the University of Minnesota and The Dow Chemical Company based on this research, “Membrane Stripping Process for Removing Volatile Organic Compounds from a Latex”, US 20130237679 A1. J. Kent Carpenter and Timothy C. Frank are authors of another patent application assigned to The Dow Chemical Company, “Multiple Membranes for Removing Volatile Organic Compounds from Liquids,” US 20140073718 A1.

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ACKNOWLEDGMENTS

Bridget Ulrich and Chaitanya Rachabattuni helped develop the apparatus used. The work was supported by the Dow Chemical Co.

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ABBREVIATIONS A = membrane area ci = concentration of solute i in water or in latex Dwater, Dlatex, Dcomposite = diffusion coefficients of solute i in water, inside a latex particle, and in a latex suspension G = volumetric gas flow rate H, H′ = partition coefficients of solute between water and gas, and between latex and water kG, kL, kp = mass transfer coefficients in gas, liquid, and latex, respectively K = overall mass transfer coefficient L = volumetric flow rate of liquid m = mass of latex qi = concentration of solute i in latex solid t = time V = volume of aqueous phase ϕ = volume fraction of latex

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REFERENCES

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dx.doi.org/10.1021/ie5012239 | Ind. Eng. Chem. Res. 2014, 53, 12420−12427