Novel Aqueous Foams forSuppressing VOC EmissionP A N K A J S . G A U T A M A N DK I S H O R E K . M O H A N T Y *

Chemical Engineering Department, University of Houston,4800 Calhoun Road, Houston, Texas 77204-4004

Reducing volatile organic compound (VOC) emissionsfrom crude oil/gasoline distribution and storage facilitiesis important in controlling environmental pollution andenhancing workplace safety. Stable aqueous foamformulations are developed to provide a mass transferbarrier to the emission of VOCs during loading of gasoline.Experiments are carried out in a bench-scale foam cellusing liquid hexane as oil. The foam columns of 32 cm inheight were able to suppress the plateau concentrationof hexane vapors in the effluent by 87% under experimentalconditions tested. Vapor suppression increased withfoam height but was almost insensitive to liquid viscosity.These experiments are then upscaled from bench-scaleto a vessel having an exposed surface area of roughly 2orders of magnitude higher. Gasoline is used as oil in theupscaled experiments, and the concentrations of volatilehydrocarbons in the effluent are measured during oil loading.A 40-cm-thick foam column is found to reduce theemissions by 96% for foams prepared with deionizedwater and by 93.8% for foams prepared with 3.5 wt %NaCl brine for 10 h of oil loading.

IntroductionAt the loading and unloading terminals for gasoline/crudeoil, some amount of light hydrocarbons are invariably ventedinto the atmosphere every time a tanker is loaded withgasoline/crude oil. The vapors released from gasoline storageand loading operations contain a mixture of light hydro-carbons (C4-C7), including some aromatic hydrocarbons.Each loading can take from a few hours to half a day. Thevapor compositions for crude oil and gasoline terminals aresimilar but could be different for loading of other organicchemicals. Volatile organic compound (VOC) emission isundesirable since it is an air pollutant, a fire hazard, and athreat to workplace safety. VOC emissions from gasolinestorage and distribution facilities are coming under increasedscrutiny in both the United States and Europe. U.S. Envi-ronmental Protection Agency Standard 40 CFR Part 63 hasestablished an emission limit of 10 mg of total organiccompounds (TOC)/L of gasoline loaded. European Com-munity Stage 1 directive has established a limit of 35 mg ofTOC/L of gasoline loaded. The most stringent gasolineemission regulation is set forth by the German TA-Luftstandard where emission limit is 0.15 mg of TOC/L of gasolineloaded (1). In view of such concerns, it becomes imperativethat the methods to reduce VOC emissions be investigatedand evaluated.

There are two potential technologies for emission controlsvapor recovery and vapor suppression. Most of the existing

technologies for emission control use vapor recovery tech-niques. Vapor recovery techniques include pressure swingadsorption (2) (Sorbathene method), steam-generated tem-perature swing adsorption (3), and subsequent thermalincineration. Sorbathene solvent recovery technology in-volves passing a feed stream containing VOCs through anadsorbent bed. For gasoline vapor recovery, the standardSorbathene technology needs to be supplemented by acompressor or a mechanical refrigeration unit. The steam-generated temperature swing adsorption is a useful techniquebut limited to organics that are not thermally sensitive.Additionally, thermal incineration for many chemicalsrequires tail gas treatment and ultimately releases carbondioxide into the atmosphere.

Vapor suppression techniques aim at creating a masstransfer barrier through which the hydrocarbon vapors mustdiffuse before they are released into the atmosphere. Corinoet al. (4) have suggested using a gelling agent to create a roofby the upper layer of oil in the tank to provide a floating roofof the same material. This technique may create considerabledifficulties in cleaning the tanker, especially if the tanker hasany plumbing lines or compartment walls. Polyurethane typefoams can also be effective against vapor release but leavebehind noncollapsible residue. Canevari et al. (5) were thefirst to suggest a foamed vapor barrier to suppress the releaseof volatile hydrocarbons using common aqueous foams.Conventional aqueous foams, however, seldom persist formore than a few hours and often have very poor stability inthe presence of oil. When a gasoline (or crude oil) tanker isempty, an aqueous foam column of required height couldbe deposited onto the bottom of the vessel. The gasoline (orcrude oil) can then be injected from the bottom. The liquidwould displace the foam column, which in turn woulddisplace the air to fill the tank. The challenges in developingan aqueous foam formulation for application in suppressionof VOCs are many-fold. First, the foam formulation shouldpersist about 10 h or more for it to be viable. The evolutionof foam height with time is a good criterion of foam stabilityin this regard. Second, hydrocarbon gases should have lowsolubility in the foam liquid so that the permeability of foamlamellae to diffusing hydrocarbon gases is low. It is importantthat the foam has high fluidity and covers the surface ofgasoline/crude oil completely. Also, the foam blanket shouldbe flexible enough to retain its height when it is being pushedupward by the liquid from below during loading. The foamshould not degrade the quality of chemical it is covering andshould not pose any additional environmental issues. Last,but most importantly, the foam should be stable in thepresence of gasoline/crude oil. Specifically, it must notrupture at the foam/oil contact or the foamed barrier wouldrapidly deteriorate.

The objective of this work is to develop a foam formulationthat meets the aforementioned criteria and to evaluate thesuppression of VOC emissions. In the next section, foamformulation and stability in the absence and presence of oilis discussed. This is followed by a description of theexperimental methods. The results are discussed in the fourthsection, followed by the conclusions.

Foam Formulation and StabilityThe discussion on the foam stability can be divided into twopartssbulk foam stability in the absence of oil and stabilityin the presence of oil. In the absence of oil, the stability ofthe bulk foam is typically controlled by the liquid drainagefrom the plateau borders and foam lamellae in the initialphase, which may last from a few minutes to a couple of

* Corresponding author phone: (713)743-4331; fax: (713)743-4323;e-mail: mohanty@uh.edu.

Environ. Sci. Technol. 2004, 38, 2721-2728

10.1021/es0349599 CCC: $27.50 2004 American Chemical Society VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2721Published on Web 03/23/2004

hours depending upon the properties such as viscosity andbubble size among others (6, 7). The foam column shrinksin height during this period depending primarily upon theinitial liquid holdup and becomes thinner. In the intermediatephase, the smaller bubbles coalesce with the larger bubblesdue to inter-bubble gas diffusion, thus coarsening the foam(8). The extended liquid surface area of the foam decreasesin this phase. Nishioka and Ross (9) developed a method tocharacterize foam stability based on the total area of theextended liquid surface. Lemlich (10) described a theory topredict the evolution of bubble size distribution due to inter-bubble gas diffusion. Sarma et al. (11) showed that thepresence of water-soluble polymers retards the inter-bubblegas diffusion, thereby enhancing foam stability. Finally, thefoam films thin down to a critical film thickness subject tocapillary suction and disjoining pressure and rupture.

In the presence of oil, foam could be destabilizedadditionally at the foam/oil contact. The stability of foamsin the presence of oil has been studied by several authors(12-14). Schramm and co-workers (15, 16) have discussedin detail the interaction of foam with the oil. The importantparameters defining foam-oil interaction are the spreadingcoefficient S () σf - σof - σo) and the entering coefficient E() σf + σof - σo). Based on these parameters, the foam-oilinteraction can be classified into three categories (17). If Eis negative, then S must be negative, and oil would neitherbe drawn into the foam lamellae nor spread at the foamliquid-gas interface; thus, the presence of oil is not expectedto destabilize the foam in this case. These are called type Afoams. If E is positive and S is negative, oil would be drawninto the foam lamellae but is not expected to spread at thefoam liquid-gas interface. The foam may or may notdestabilize in this case depending upon whether oil dropsreduce the coherence of the foam lamellae and whether theoil remains as a lens or is ejected outside the lamellae. Theseare called type B foams. If both E and S are positive, then oilis drawn into the lamellae and also spreads as a film at thefoam liquid-gas interface. This could seriously destabilizethe foam. These foams are called type C foams.

For a foam blanket to be an effective mass transfer barrier,the entering and spreading coefficients should preferentiallybe negative (type A). This is possible when σf + σof < σo. Sincethe interfacial tension between oil and water in the presenceof surfactants is typically less than a few millinewtons permeter, the key to stability is decreasing the surface tensionof the foam solution comfortably below that of the oil.

The solubilization of hydrocarbon vapor in foam filmscan also destabilize them. As Binks et al. (18) point out,hydrocarbon gases affect foam stability at low surfactantconcentrations. The foams studied in their study are in thevicinity of the foaming/nonfoaming boundary. We havestudied very stable foams with surfactant concentrationsmuch above their critical micelle concentration. These foamslast for more than 1 day (not just a few minutes). These foamsare almost insensitive to the solubilized hydrocarbon gases;they are destabilized by the hydrocarbon liquids that enterthe foam films, as discussed above. We have chosensurfactants in such a way that both the entering and spreadingcoefficients are negative and foams remain stable in thepresence of gasoline.

Experimental SectionMaterials. The foam formulation consisted of an aqueoussolution of two surfactants, a stabilizer and a viscosifier,similar to those used by Thach et al. (19). The first surfactantwas a nonionic surfactant, Tergitol NP-10 (T). Tergitol is anonylphenol polyethylene glycol ether from Sigma-Aldrich.It contained 97% active surfactant, about 2% polyethyleneglycol, and about 1% dinonylphenyl polyoxyethylene. Thesecond surfactant is a fluorinated surfactant, F1127 (F). F1127

is a polyfluoroalkyl betaine from Atofina. It contained 27%active surfactant, 38% water, and 35% ethanol. The stabilizeris glycerol (G) obtained from Sigma-Aldrich. The lastcomponent is an anionic polymer xanthan gum (X) fromSigma-Aldrich. Hexane was used as a gasoline substitute insome experiments. It was supplied by Sigma-Aldrich andwas 99.99% saturated isomers. The gasoline was an unleadedgasoline from a local gas station. These chemicals were usedas supplied.

Bench-Scale Setup. The experimental setup used forstudying the bulk stability of foams and hydrocarbonemission in bench-scale is shown in Figure 1. The foam cellconsists of a glass column having two sectionssa bottomsection having an i.d. of 1.4 cm and a top section having ani.d. of 5 cm. The lengths of the bottom and top sections are10 and 45 cm, respectively. This kind of design enables usto accurately measure the liquid drained from the foam. Thefoam cell contains a valve at the bottom to regulate the flowof surfactant solution into the cell. There are two additionalvalves in the lower portion of the cell placed at calculatedheights for charging of nitrogen gas and oil. All the com-ponents of the foam were carefully measured, and theaqueous solution was then stirred overnight by a magneticstirrer. The foam solution was charged into the cell, and theinitial height of liquid in the lower narrow portion of the cellwas noted. Nitrogen was then bubbled through the liquid ata constant rate. The internal diameter of the nozzle (valveon the nitrogen line) generating the foam was 1 mm. Thebubbling rate was kept constant at 315 mL/min for all thecases presented in this study. Foam drains into the narrowlower part of the cell, and change in the level of drainedliquid is recorded with time.

Commercially available hexane was used as oil in thesebench-scale mass-transfer experiments. Preliminary experi-ments with gasoline as the oil suggested that hexane wasone of the main components of the volatile hydrocarbonsand that mass transfer was higher for hexane than for thelighter components. Thus, hexane represents the worst casefrom the mass transfer point of view. The oil was introducedinto the foam cell after most of the bulk liquid drained out,which is approximately 50 min after the foam was generatedin most cases. The hexane vapors issuing from the top of thefoam column were swept by the nitrogen gas and carried toa gas chromatograph for compositional analysis. The flowrate of nitrogen gas was controlled by a flow meter and keptconstant at 4.8 ( 0.2 mL/min during these experiments. Theconcentrations of the hydrocarbon vapors in the effluent inthe absence of any foam were also measured under experi-mental conditions to establish a baseline for estimating thesuppression of hexane vapors in the effluent in the presenceof foam. Experiments were repeated to check the consistencyof data.

FIGURE 1. Schematic of experimental setup for bench-scaleexperiments.

2722 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004

The viscosity of the aqueous solutions was measured bya Brookfield rotational rheometer. The surface tensions ofaqueous solutions and oil were measured by a du Nouy ringtensiometer. The interfacial tension between the aqueoussolution and the oil was measured by a spinning droptensiometer. Foam texture/bubble size was observed visually.

Upscaled Setup. The experiments at the bench-scale werecarried out in a narrow (5 cm diameter) cylindrical vessel.The fluidity of foam and the effective coverage of the exposedoil surface by the foam blanket were not tested in theseexperiments. Moreover, in the bench-scale experiments, thefoam column merely sat on the top of the oil layer and wasnot pushed from the bottom. In field applications, the foamcolumn would be pushed up as the crude oil/gasoline ispumped from the bottom. If the foam is not sufficientlyflexible/mobile, it might get partially destroyed because ofthe mechanical perturbations during the motion. Hexanewas used as oil in bench-scale experiments. Polar organics,which could be present in crude oil/gasoline, are moredestabilizing than the nonpolar components (19). A secondset of experiments, called here “upscaled”, were conductedto evaluate these effects.

A cylindrical container having a diameter of 46 cm anda height of 74 cm was chosen for these experiments. Thisvessel is referred to as the “emission cell”. The schematic ofthe experimental set up is shown in the Figure 2. The foamwas generated separately in a foam cell and transferred tothe emission cell through a hose connecting the two. A foamcolumn 40 cm in height was sprayed onto the bottom of theemission cell. The emission cell was then covered with a lid,and gasoline was pumped from the bottom at a low flowrate. The top of the emission cell was connected by a tubeto the gas chromatograph where the composition of theeffluent was analyzed. After a 1-in.-thick layer of gasolinewas deposited inside the emission cell, water was used topush this layer of gasoline up for the remaining duration ofthe experiment. Here the water mimics the injection ofgasoline because it stays at the bottom of gasoline layer anddoes not interact with the foam. Use of water reduced theuse of oil in this laboratory experiment and the associatedfire-safety concerns and disposal issues. Approximately 18L of the total liquid was pumped in 10 h, the duration of theexperiment. The flow rate of the gases flowing through thegas chromatograph was approximately 11 mL/min for theexperiments with foam. In the absence of foam, a gas flowrate of about 30 mL/min was recorded at the exit of the gaschromatograph. The results of the foam cell experiments arediscussed next followed by the emission cell experiments.

Results and DiscussionFoam Cell. Several foam formulations were tried. Foamformulations with only surfactants are not very stable and

have a half-life less than 1 h. Foam formulations consistingof a nonionic surfactant (T), a fluorinated surfactant (F), astabilizer (G), and a viscosifier (X) were found to beremarkably stable with a half-life exceeding 1 day. Thenonionic surfactant tends to increase the thickness of foamfilms; this is concluded from visual observations of foams.The fluorinated surfactant reduced the surface tension ofthe aqueous solution and stabilized the foam column in thepresence of oil. The stabilizer (e.g., glycerol) decreased thevaporization of water from foam film and thus increasedstability. The viscosifier, xanthan gum, increased the viscosityof the aqueous solution and slowed the drainage of thesolution and the thinning of foam lamellae, thus providingstability to the foam column.

The following nomenclature is used to specify the foamcomposition; 1T0.4F0.4G0.16X implies 1 wt % Tergitol, 0.4wt % F1127, 6 wt % glycerol, and 0.16 wt % xanthan gum.This foam composition was found to be very stable and hasbeen used in most of the results reported in this paper. Thesurface tension of this aqueous foam solution was measuredto be 20.5 mN/m at room temperature (22 °C). At the sametemperature, the surface tension of the oil (99.99% assay ofC6-saturated isomers) was found to be 23.3 mN/m. The oiland foam formulation were equilibrated for 48 h in avolumetric ratio of 1:2. The interfacial tension (σof) betweenthe fluids was measured using a spinning drop tensiometerand was found to be 1.5 mN/m. Thus, the entering coefficientE is -1.3 mN/m for this system. Hence, this foam falls intothe category of type A foam according to the criteria developedby Ross (17). Experimentally, it was observed that this foamdid not rupture at the foam/oil contact and exhibited highdegree of stability in the presence oil. These foams persistedfor several days without practically any change in the height,both in the absence and in the presence of oil.

Figure 3 shows the liquid drainage of two foam columnshaving a composition of 1T0.4F6G0.16X and an initial heightof 32 cm. The liquid drained is plotted as a fraction of the

FIGURE 2. Schematic of experimental setup for the upscaled experiments.

FIGURE 3. Bulk drainage of foam. The foam solution viscosity is18.1 cP, and the initial height is 0.32 m.

VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2723

amount of liquid foamed originally. With time, the fractiondrained tends to a limit slightly lower than one. Most of theliquid is drained within 1 h. The amount not drained is inthe foam as lamellae and plateau borders. It is clear that theexperimental data for bulk drainage rates are fairly repro-ducible between the two identical experiments. These foamsare stable for more than 1 day in the presence of oil, althoughthe data on concentration of hexane vapors in the effluentwas collected only for about 10 h. Visual observation confirmsthat most of the foam bubbles are of similar size, thoughsome smaller bubbles sandwiched between the larger bubblesare also observed.

Figure 4 shows the mole percentage of hexane vapors inthe effluent with and without foam under experimentalconditions as described above. The initial height of the foamcolumn was 32 cm. The height of the foam column remainedpractically unchanged throughout the duration of the masstransfer experiments. Without foam, the hexane concentra-tion in the effluent increases with time as it mixes withnitrogen in the cylinder and reaches a plateau value of about4.5 mol % in about 5 h. In contrast, the hexane concentrationin the effluent in the presence of foam rises to only about0.6 mol % at the end of 10 h. It is clear that the presence offoam significantly suppresses the concentration of hexanevapors in the effluent. If the plateau values in both the casesare considered, the presence of the foam suppresses theconcentration of the hexane vapors in the effluent by about87%. Various parameters that could affect the performanceof foam such as initial foam height and liquid viscosity werevaried experimentally. The following sections discuss theimpact of these parameters.

Effect of Foam Height. Figures 5-7 show the effect ofinitial foam height on the concentration of hexane vapors inthe effluent. The foam height during the mass transferexperiments was essentially equal to the initial foam height.

Three foam formulations of viscosities 8 × 10-3 Ns/m2 () 8cP) (Figure 5), 18.1 × 10-3 Ns/m2 (Figure 6), and 65.3 × 10-3

Ns/m2 (Figure 7) were chosen by varying the concentrationof viscosifier in the foam formulation and keeping the restof the foam composition unchanged. The concentration ofhexane vapors in the effluent increases with time and reachesa plateau after about 5 h for 15 cm column and for about 8h for the 32 cm column. As expected, the mole fraction ofhexane vapors in the effluent decreases with increasing initialfoam height for a given viscosity of the foam solution. Theplateau concentration of hexane also decreases with the initialfoam height.

The aqueous solution drains from the lamellae into theadjoining plateau borders subject to the forces of capillarysuction and disjoining pressure. The plateau borders forma well-connected network in the foam column through whichthe liquid drains down under the action of gravity and anopposing capillary pressure gradient is set up as a conse-quence of the gravity drainage. The plateau borders becomeleaner in liquid with height in a foam column with the resultthat the plateau border radius of curvature gets smaller andthe capillary suction higher causing rapid drainage of theconnected foam lamellae. Thus, the foam lamellae get thinnerprogressively with height in a foam column. Since the liquidholdup in the foam films decreases with height, the masstransfer resistance to the diffusing hydrocarbon vapors alsodecreases. If the liquid holdup in the foam lamellae wereuniform (i.e., independent of the height), the diffusion timescale of the foam column would quadruple when the foamcolumn height is doubled assuming it is only the liquid inthe foam films that provides a significant diffusive masstransfer barrier. Since the mass transfer resistance decreaseswith height, we expect the diffusional time to increase byless than 4 times. Figures 5-7 show that the time to reach

FIGURE 4. Concentration of hexane vapors in the effluent with andwithout foam.

FIGURE 5. Concentration of hexane in the effluent with time for thefoam columns with initial heights of 0.15, 0.23, and 0.32 m. Thevalues of the parameters are µ ) 8 cP, G ) 1.03 × 103 kg/m3, σf )20.5 mN/m, Rb ) 10 mm, E0 ) 0.015 (L0 ) 0.15 m), E0 ) 0.009 (L0 )0.23 m), E0 ) 0.007 (L0 ) 0.32 m).

FIGURE 6. Concentration of hexane in the effluent with time for thefoam columns with initial heights of 0.15, 0.23, and 0.32 m. Thevalues of the parameters are µ ) 18.1 cP, G ) 1.03 × 103 kg/m3,σf ) 20.5 mN/m, Rb ) 10 mm, E0 ) 0.024 (L0 ) 0.15 m), E0 ) 0.02 (L0

) 0.23 m), E0 ) 0.016 (L0 ) 0.32 m).

FIGURE 7. Concentration of hexane in the effluent with time for thefoam columns with initial heights of 0.15, 0.23, and 0.32 m. Thevalues of the parameters are µ ) 65.3 cP, G ) 1.03 × 103 kg/m3,σf ) 20.5 mN/m, Rb ) 10 mm, E0 ) 0.034 (L0 ) 0.15 m), E0 ) 0.024(L0 ) 0.23 m), E0 ) 0.018 (L0 ) 0.32 m).

2724 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004

the plateau concentration increases by only a factor of about8:5 (which is considerably less than 4 times) when the heightis approximately doubled. Figures 5-7 also show that theplateau concentration of hexane vapors in the effluent issuppressed by about 65% (∼1.4-0.5 mol %) when the heightof the foam column is doubled.

Effect of Viscosity. Figure 8 shows the variation of molepercentage of hexane vapors in the effluent with time forfoam columns having an initial height of 23 cm. Three setsof data for three different viscosities of the aqueous foamsolution are presented. It is interesting to note that nosignificant difference in the concentration of hexane vaporsin the effluent was observed with varying viscosities duringthe transient. The plateau hexane concentration valuedecreases slightly as the viscosity of the aqueous solutionincreases.

The increase in viscosity slows down the drainage of liquidfrom the foam lamellae resulting in thicker foam films. Figure9 shows the bulk drainage of foams having an initial heightof 15 cm. The drainage fraction is calculated with respect tothe total amount of liquid present in the foam at the end ofbubbling. The foams made up of aqueous solutions havinghigher viscosities drain slower. The foam film thickness wasnot explicitly measured. Although the bulk drainage datareflects plateau border drainage more accurately, a greateramount of liquid in the plateau borders implies weakercapillary suction from the films, thus producing thicker films.If the solubility of hexane is not affected by the polymerconcentration, the thicker foam films would have lesspermeability to the diffusing hydrocarbon vapors; hence,the concentration of hexane vapors in the effluent is expectedto decrease with higher viscosities. Moreover, if one were toassume that Stokes-Einstein relationship for diffusivity inliquid holds for the diffusion of hydrocarbon vapors, thediffusivity would decrease for higher viscosities even for the

same thickness of the foam film. Thus, the mass transfer ofhexane should decrease dramatically with the increase inviscosity. This does not appear to be the case in this study(i.e., Figure 8). One explanation could be that increasing thepolymer concentration not only increases the viscosity ofthe foam formulation but also possibly increases the solubilityof the hydrocarbon gases in the foam liquid, annulling anydiffusivity decrease due to higher viscosity or thicker films.Another possibility is that the resistance provided by theadsorbed surfactant monolayers at the film-gas interface isthe dominant resistance to the mass transfer, as shown byQuoc et al. (20). In that case, the liquid content of the foambecomes less significant and since the number of monolayersthrough which the hydrocarbon molecules diffuse throughremain the same for a given foam height and bubble sizeirrespective of the viscosity, the mass transfer resistance ofthe foam column does not vary significantly.

Emission Cell. Scaled-up experiments were conductedwith an unleaded gasoline in the emission cell. Figure 10shows the cumulative hydrocarbon emissions per liter ofgasoline loaded in the emission cell for the foam composition1T0.4F6G0.24X and the initial foam height of 40 cm. About144 mg of total organic compounds are emitted per liter ofgasoline loaded in 10 h without the foam. About 5 mg ofTOCs are emitted per liter of gasoline loaded in 10 h with thefoam. Thus, the suppression of total emission of hydrocar-bons is about 96.5%. This emission, 5 mg of TOC/L of gasolineloaded, meets the U.S. EPA (10 mg of TOC/L) and EuropeanCommunity (35 mg of TOC/L) standards (1) in our emissioncell. The total hydrocarbon emission is controlled by severalparameters, such as loading time, foam height, foamformulation, and exposed surface to container volume ratio.These parameters can be chosen appropriately to obtain therequired emission.

Figure 11a-c shows the concentration of the individualhydrocarbons in the effluent for the experiment reportedabove. All the hydrocarbon gases heavier than hexanes arecollectively lumped as C6+, the isomers of pentane as C5,and the isomers of butane as C4. For the light hydrocarbonssuch as butanes, close to 100% suppression of the vaporswas observed in the presence of foam, at least for 10 h forboth the foam formulations. For pentanes, the emission iscut down by about 97% in the presence of foam for the foamformulation 1T0.4F6G0.24X. It should be noted that thepentanes collectively contribute highest to the total VOCemissions from the gasoline. For hexanes and other heaviergases (C6+), the reduction in the emission is about 71% ifthe concentration in the effluent at the end of 10 h isconsidered for 1T0.4F6G0.24X. Thus, these foam formulationsachieve 70% to near 100% suppression in the emission ofVOCs depending upon the component of the gasoline underconsideration. Note that the emission of C6+ is higher thanthat of C5, even though the diffusivity of C6+ is lower in theaqueous solution. This implies that the solubility of theseorganic vapors in the foam lamellae may play an important

FIGURE 8. Concentration of hexane in the effluent with time for thefoam columns with initial height of 0.23 m. The values of theparameters are G ) 1.03 × 103 kg/m3, σf ) 20.5 mN/m, Rb ) 10 mm,and E0 ) 0.009, 0.02, and 0.024 for µ ) 8, 18.1, and65.3 cP, respectively(1 cP ) 0.001 Ns/m2).

FIGURE 9. Bulk drainage of foams of different viscosities havingan initial height of 15 cm.

FIGURE 10. Cumulative hydrocarbon emission with time per literof gasoline loaded in the presence of foam having a compositionof 1T0.4F6G0.24X and without foam.

VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2725

role. Higher solubility of C6+ in the surfactant solution thanthat of C5 may be due to the presence of surfactants andviscosifiers that are organic.

Often, the loading and unloading terminals for the crudeoil/gasoline are located near the large saline water bodiessuch as oceans. Therefore, we evaluate the effectiveness ofthe foams developed with seawater. In the open oceans, thesalinity based on total dry solids per kilogram of seawatertypically varies between 3.36 and 3.68 wt % (21). Anapproximate value of seawater salinity could be taken as 3.5wt %, composed entirely of NaCl, although seawater alsocontains other salts. In the preliminary experiments with thefoam formulations prepared in salty water containing 3.5 wt% of NaCl, it was observed that the foam column ap-proximately retains its initial height for at least 10 h, althoughit has lower initial liquid holdup and drains faster than thefoam formulation prepared with deionized water as shownin Figure 12. The lamellae for the foam prepared in saltywater also appear thinner visibly. These observations areconsistent with the effect of salinity on foam stability observedby Rojas et al. (22). The presence of electrolytes in the foamliquid causes electrostatic double layer in the foam films toshrink, thereby reducing the repulsive forces that stabilizethe foam films. This, in turn, leads to thinner foam films inthe presence of salt. Thinner films may not reduce the abilityof the foam barrier to suppress VOC emissions substantially

if the limiting mass transfer resistance is provided by thefilm-gas interface (20).

Figure 13 shows the cumulative hydrocarbon emissionsper liter of gasoline loaded in the emission cell for the foamcomposition 1T0.4F6G0.24X prepared in saline solution (3.5wt % NaCl) and the initial foam height of 40 cm. About 9 mgof TOC is emitted per liter of gasoline loaded after 10 h against144 mg without foam. Thus, the suppression of total emissionof hydrocarbons is about 93.8%. Figure 14a-c shows theconcentration of components in the effluent with time. Asexpected, the effectiveness of the foam column in suppressinghydrocarbon emission goes down when compared with foamformulations prepared in deionized water for the same initialfoam height; concentrations of all the components are higherin Figure 14 as compared to those in Figure 11. However, itstill suppresses the concentration of C6+ gases in the effluentby 59%, of pentanes by 84%, and of butanes by 69% basedon the concentration in the effluent at the end of about 10h. The effectiveness of the foam column can be increased byincreasing the column height.

An approximate model has been developed to interpretthe mass transfer of vapors during the loading process withand without foam barriers. This model considers one-dimensional diffusion of dilute VOC components in a semi-infinite air column above the foam (or gasoline in the absenceof foam). The distance (h ) h0 - vt) between the foam-airinterface and the exit of the vessel shrinks as gasoline isloaded. The initial height of the air column is h0, v is thevelocity of the gasoline front, t is the time, and z is the distancefrom the foam-air interface. The effluent concentration isevaluated at h, which is a function of time. The foam ismodeled with a mass transfer coefficient (k) at the foam-airinterface. The solubilization of hydrocarbons in the aqueoussolution of the foam is important in slowing down the

FIGURE 11. (a) Concentration of C6+ gases in the effluent with andwithout foam for the upscaled experiment for identified aqueousfoam formulations. (b) Concentration of pentanes in the effluentwith and without foam for the upscaled experiment for identifiedaqueous foam formulations. (c) Concentration of butanes in theeffluent with and without foam for the upscaled experiment foridentified aqueous foam formulations.

FIGURE 12. Comparison of foam liquid content for the foamformulation 1T0.4F6G0.24X prepared with deionized water and saltywater.

FIGURE 13. Cumulative hydrocarbon emission with time per literof gasoline loaded in the presence of foam having a compositionof 1T0.4F6G0.24X prepared in 3.5 wt % NaCl solution and withoutfoam.

2726 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004

breakthrough of the gases but is neglected here. The detailsof diffusion through the liquid-air interface, foam films, andfoam cells are lumped into a single mass transfer coefficient.These factors are important but outside the scope of thispaper.

The concentration of a hydrocarbon component in theair above the foam barrier can be described by (under theassumptions stated above):

with the initial and boundary conditions:

where D is the diffusivity of the component in air. The solutionto the above equation, C(z, t), is given by ref 23:

where Cs is the concentration of the component at thegasoline-vapor interface dictated by the thermodynamics.If the mass transfer coefficient is large (i.e., in the absenceof a foam barrier), the last term in eq 2 is negligible.

This model has been fitted to the experimental data ofhexane in Figures 11a and 14a. The mass transfer coefficient(k) estimated for hexane in the 40 cm foam columns studied(from the fitting) are 3.9 × 10-4 cm/s for foam with water and5.7 × 10-4 for foam with salt solution. These mass transfercoefficients depend on the foam structure, solubility in theaqueous solution, and diffusivity through foam films. Thesefactors will be studied in detail in a future study.

AcknowledgmentsThis work was partially supported by the funding from TexasHazardous Waste Research Center.

NomenclatureC hydrocarbon concentration in air column (mol/L)

D diffusivity (m2/s)

E entering coefficient (mN/m)

h height of the air column above foam (m)

k mass transfer coefficient of foam (m/s)

L0 initial foam height (cm)

Rb bubble radius (cm)

S spreading coefficient (mN/m)

Greek Letters

ε0 initial volume fraction of liquid in foam

σf surface tension of foam solution

σo surface tension of oil

σof interfacial tension between foam liquid and oil

µ bulk viscosity of foam solution

F density of foam solution

Literature Cited(1) Pezolt, D. J.; Collick, S. J.; Johnson, H. A.; Robbins, L. A. Environ.

Prog. 1997, 16, 16-19.(2) Robbins, L. A.; Frank, T. C. U.S. Patent 4,857,084, 1989.(3) Skarstrom, C. W. Recent Dev. Sep. Sci. 1975, 2, 95.(4) Corino; et al. U.S. Patent 3,639,258, 1972.(5) Canevari, et al. U.S. Patent 3,850,206, 1974.(6) Weaire, D.; Hutzler, S. Phys. A 1998, 257, 264-269.(7) Neethling, S. J.; Lee, H. T.; Cillier, J. J. L. J. Phys.: Condens. Matter

2002, 14, 331-336.(8) Magrabi, S. A.; Dlugogorski, B. Z.; Jameson, G. J. Chem. Eng. Sci.

1999, 54, 4007-4022.(9) Nishioka, G.; Ross, S. J. Colloid Interface Sci. 1981, 81, 1-7.

(10) Lemlich, R. Ind. Eng. Chem. Fundam. 1978, 17, 89-93.(11) Sarma, D. S. H. S. R.; Pandit, J.; Khillar, K. C. J. Colloid Interface

Sci. 1988, 124, 339-348.(12) Manlowe, D. J.; Radke, C. J. SPE Res. Eng. 1990, 5, 495-502.(13) Ratterman, K. T. Proceedings of the 64th Annual Technical

Conference of SPE; Society of Petroleum Engineers: Richardson,TX, 1989; Paper SPE 19692.

(14) Nikolov, A. D.; Wasan, D. T.; Huang, D. W.; Edwards, D. A.Proceedings of the 61st Annual Technical Conference of SPE;Society of Petroleum Engineers: Richardson, TX, 1986; PaperSPE 15443.

(15) Schramm, L. L.; Novosad, J. J. Colloids Surf. 1990, 46, 21-43.(16) Mannhardt, K.; Novosad, J. J.; Schramm, L. L. SPE Res. Eng.

Eval. 2002, 3, 23-34.

FIGURE 14. (a) Concentration of C6+ in the effluent with and withoutfoam for the upscaled experiment. Foam liquid composition is1T0.4F6G0.24X and contains 3.5 wt % NaCl. (b) Concentration ofpentanes in the effluent with and without foam for the upscaledexperiment. Foam liquid composition is 1T0.4F6G0.24X and contains3.5 wt % NaCl. (c) Concentration of butanes in the effluent with andwithout foam for the upscaled experiment. Foam liquid compositionis 1T0.4F6G0.24X and contains 3.5 wt % NaCl.

∂C/∂t ) D ∂2C/∂2z for 0 < z < ∞ (1)

C ) 0 for all z at t ) 0

C f 0 as Z f ∞, for t > 0

-D ∂C/∂z ) k(C - Cs) at z ) 0, for t > 0

C/Cs ) 1 - erf(zD-1/2t-1/2/2) - exp((kz/D) +(k2t/D))[1 - erf((kD-1/2t1/2) + (zD-1/2t-1/2/2))] (2)

VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2727

(17) Ross, S. J. Phys. Colloid Chem. 1950, 54, 429-436.(18) Binks, B. P.; Fletcher, P. D. I.; Haynes, M. D. Colloids Surf. A

2003, 216, 1-8.(19) Thach; et al. U.S. Patent 5,296,164, 1994.(20) Quoc, P. N.; Zitha, P. L.; Currie, P. K. J. Colloid Interface Sci.

2002, 248, 467-476.(21) Spiegler, K. S. Salt-Water Purification; Plenum Press: New York,

1977; pp 16-17.(22) Rojas, Y.; Kakadjian, S.; Aponte, A.; Marquez, R.; Sanchez, G.

Proceedings of the 2001 SPE International Symposium on Oilfield

Chemistry; Society of Petroleum Engineers: Richardson, TX,2001; Paper SPE 64999.

(23) Welty, J. R.; Wicks, C. E.; Wilson, R. E. Fundamentals ofMomentum, Heat & Mass Transfer; John Wiley & Sons: NewYork, 1984; p 307.

Received for review September 2, 2003. Revised manuscriptreceived January 5, 2004. Accepted February 17, 2004.

ES0349599

2728 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004