Environ. Sci. Technol. 2004, 38, 2721-2728
Novel Aqueous Foams for Suppressing VOC Emission PANKAJ S. GAUTAM AND KISHORE K. MOHANTY* Chemical Engineering Department, University of Houston, 4800 Calhoun Road, Houston, Texas 77204-4004
Reducing volatile organic compound (VOC) emissions from crude oil/gasoline distribution and storage facilities is important in controlling environmental pollution and enhancing workplace safety. Stable aqueous foam formulations are developed to provide a mass transfer barrier to the emission of VOCs during loading of gasoline. Experiments are carried out in a bench-scale foam cell using liquid hexane as oil. The foam columns of 32 cm in height were able to suppress the plateau concentration of hexane vapors in the effluent by 87% under experimental conditions tested. Vapor suppression increased with foam height but was almost insensitive to liquid viscosity. These experiments are then upscaled from bench-scale to a vessel having an exposed surface area of roughly 2 orders of magnitude higher. Gasoline is used as oil in the upscaled experiments, and the concentrations of volatile hydrocarbons in the effluent are measured during oil loading. A 40-cm-thick foam column is found to reduce the emissions by 96% for foams prepared with deionized water and by 93.8% for foams prepared with 3.5 wt % NaCl brine for 10 h of oil loading.
Introduction At the loading and unloading terminals for gasoline/crude oil, some amount of light hydrocarbons are invariably vented into the atmosphere every time a tanker is loaded with gasoline/crude oil. The vapors released from gasoline storage and loading operations contain a mixture of light hydrocarbons (C4-C7), including some aromatic hydrocarbons. Each loading can take from a few hours to half a day. The vapor compositions for crude oil and gasoline terminals are similar but could be different for loading of other organic chemicals. Volatile organic compound (VOC) emission is undesirable since it is an air pollutant, a fire hazard, and a threat to workplace safety. VOC emissions from gasoline storage and distribution facilities are coming under increased scrutiny in both the United States and Europe. U.S. Environmental Protection Agency Standard 40 CFR Part 63 has established an emission limit of 10 mg of total organic compounds (TOC)/L of gasoline loaded. European Community Stage 1 directive has established a limit of 35 mg of TOC/L of gasoline loaded. The most stringent gasoline emission regulation is set forth by the German TA-Luft standard where emission limit is 0.15 mg of TOC/L of gasoline loaded (1). In view of such concerns, it becomes imperative that the methods to reduce VOC emissions be investigated and evaluated. There are two potential technologies for emission controls vapor recovery and vapor suppression. Most of the existing * Corresponding author phone: (713)743-4331; fax: (713)743-4323; e-mail:
[email protected]. 10.1021/es0349599 CCC: $27.50 Published on Web 03/23/2004
2004 American Chemical Society
technologies for emission control use vapor recovery techniques. Vapor recovery techniques include pressure swing adsorption (2) (Sorbathene method), steam-generated temperature swing adsorption (3), and subsequent thermal incineration. Sorbathene solvent recovery technology involves passing a feed stream containing VOCs through an adsorbent bed. For gasoline vapor recovery, the standard Sorbathene technology needs to be supplemented by a compressor or a mechanical refrigeration unit. The steamgenerated temperature swing adsorption is a useful technique but limited to organics that are not thermally sensitive. Additionally, thermal incineration for many chemicals requires tail gas treatment and ultimately releases carbon dioxide into the atmosphere. Vapor suppression techniques aim at creating a mass transfer barrier through which the hydrocarbon vapors must diffuse before they are released into the atmosphere. Corino et al. (4) have suggested using a gelling agent to create a roof by the upper layer of oil in the tank to provide a floating roof of the same material. This technique may create considerable difficulties in cleaning the tanker, especially if the tanker has any plumbing lines or compartment walls. Polyurethane type foams can also be effective against vapor release but leave behind noncollapsible residue. Canevari et al. (5) were the first to suggest a foamed vapor barrier to suppress the release of volatile hydrocarbons using common aqueous foams. Conventional aqueous foams, however, seldom persist for more than a few hours and often have very poor stability in the presence of oil. When a gasoline (or crude oil) tanker is empty, an aqueous foam column of required height could be deposited onto the bottom of the vessel. The gasoline (or crude oil) can then be injected from the bottom. The liquid would displace the foam column, which in turn would displace the air to fill the tank. The challenges in developing an aqueous foam formulation for application in suppression of VOCs are many-fold. First, the foam formulation should persist about 10 h or more for it to be viable. The evolution of foam height with time is a good criterion of foam stability in this regard. Second, hydrocarbon gases should have low solubility in the foam liquid so that the permeability of foam lamellae to diffusing hydrocarbon gases is low. It is important that the foam has high fluidity and covers the surface of gasoline/crude oil completely. Also, the foam blanket should be flexible enough to retain its height when it is being pushed upward by the liquid from below during loading. The foam should not degrade the quality of chemical it is covering and should not pose any additional environmental issues. Last, but most importantly, the foam should be stable in the presence of gasoline/crude oil. Specifically, it must not rupture at the foam/oil contact or the foamed barrier would rapidly deteriorate. The objective of this work is to develop a foam formulation that meets the aforementioned criteria and to evaluate the suppression of VOC emissions. In the next section, foam formulation and stability in the absence and presence of oil is discussed. This is followed by a description of the experimental methods. The results are discussed in the fourth section, followed by the conclusions.
Foam Formulation and Stability The discussion on the foam stability can be divided into two partssbulk foam stability in the absence of oil and stability in the presence of oil. In the absence of oil, the stability of the bulk foam is typically controlled by the liquid drainage from the plateau borders and foam lamellae in the initial phase, which may last from a few minutes to a couple of VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2721
hours depending upon the properties such as viscosity and bubble size among others (6, 7). The foam column shrinks in height during this period depending primarily upon the initial liquid holdup and becomes thinner. In the intermediate phase, the smaller bubbles coalesce with the larger bubbles due to inter-bubble gas diffusion, thus coarsening the foam (8). The extended liquid surface area of the foam decreases in this phase. Nishioka and Ross (9) developed a method to characterize foam stability based on the total area of the extended liquid surface. Lemlich (10) described a theory to predict the evolution of bubble size distribution due to interbubble gas diffusion. Sarma et al. (11) showed that the presence of water-soluble polymers retards the inter-bubble gas diffusion, thereby enhancing foam stability. Finally, the foam films thin down to a critical film thickness subject to capillary suction and disjoining pressure and rupture. In the presence of oil, foam could be destabilized additionally at the foam/oil contact. The stability of foams in the presence of oil has been studied by several authors (12-14). Schramm and co-workers (15, 16) have discussed in detail the interaction of foam with the oil. The important parameters defining foam-oil interaction are the spreading coefficient S () σf - σof - σo) and the entering coefficient E () σf + σof - σo). Based on these parameters, the foam-oil interaction can be classified into three categories (17). If E is negative, then S must be negative, and oil would neither be drawn into the foam lamellae nor spread at the foam liquid-gas interface; thus, the presence of oil is not expected to destabilize the foam in this case. These are called type A foams. If E is positive and S is negative, oil would be drawn into the foam lamellae but is not expected to spread at the foam liquid-gas interface. The foam may or may not destabilize in this case depending upon whether oil drops reduce the coherence of the foam lamellae and whether the oil remains as a lens or is ejected outside the lamellae. These are called type B foams. If both E and S are positive, then oil is drawn into the lamellae and also spreads as a film at the foam liquid-gas interface. This could seriously destabilize the 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 preferentially be negative (type A). This is possible when σf + σof < σo. Since the interfacial tension between oil and water in the presence of surfactants is typically less than a few millinewtons per meter, the key to stability is decreasing the surface tension of the foam solution comfortably below that of the oil. The solubilization of hydrocarbon vapor in foam films can also destabilize them. As Binks et al. (18) point out, hydrocarbon gases affect foam stability at low surfactant concentrations. The foams studied in their study are in the vicinity of the foaming/nonfoaming boundary. We have studied very stable foams with surfactant concentrations much above their critical micelle concentration. These foams last for more than 1 day (not just a few minutes). These foams are almost insensitive to the solubilized hydrocarbon gases; they are destabilized by the hydrocarbon liquids that enter the foam films, as discussed above. We have chosen surfactants in such a way that both the entering and spreading coefficients are negative and foams remain stable in the presence of gasoline.
Experimental Section Materials. The foam formulation consisted of an aqueous solution of two surfactants, a stabilizer and a viscosifier, similar to those used by Thach et al. (19). The first surfactant was a nonionic surfactant, Tergitol NP-10 (T). Tergitol is a nonylphenol polyethylene glycol ether from Sigma-Aldrich. It contained 97% active surfactant, about 2% polyethylene glycol, and about 1% dinonylphenyl polyoxyethylene. The second surfactant is a fluorinated surfactant, F1127 (F). F1127 2722
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004
FIGURE 1. Schematic of experimental setup for bench-scale experiments. is a polyfluoroalkyl betaine from Atofina. It contained 27% active surfactant, 38% water, and 35% ethanol. The stabilizer is glycerol (G) obtained from Sigma-Aldrich. The last component is an anionic polymer xanthan gum (X) from Sigma-Aldrich. Hexane was used as a gasoline substitute in some experiments. It was supplied by Sigma-Aldrich and was 99.99% saturated isomers. The gasoline was an unleaded gasoline from a local gas station. These chemicals were used as supplied. Bench-Scale Setup. The experimental setup used for studying the bulk stability of foams and hydrocarbon emission in bench-scale is shown in Figure 1. The foam cell consists of a glass column having two sectionssa bottom section having an i.d. of 1.4 cm and a top section having an i.d. of 5 cm. The lengths of the bottom and top sections are 10 and 45 cm, respectively. This kind of design enables us to accurately measure the liquid drained from the foam. The foam cell contains a valve at the bottom to regulate the flow of surfactant solution into the cell. There are two additional valves in the lower portion of the cell placed at calculated heights for charging of nitrogen gas and oil. All the components of the foam were carefully measured, and the aqueous solution was then stirred overnight by a magnetic stirrer. The foam solution was charged into the cell, and the initial height of liquid in the lower narrow portion of the cell was noted. Nitrogen was then bubbled through the liquid at a constant rate. The internal diameter of the nozzle (valve on the nitrogen line) generating the foam was 1 mm. The bubbling rate was kept constant at 315 mL/min for all the cases presented in this study. Foam drains into the narrow lower part of the cell, and change in the level of drained liquid is recorded with time. Commercially available hexane was used as oil in these bench-scale mass-transfer experiments. Preliminary experiments with gasoline as the oil suggested that hexane was one of the main components of the volatile hydrocarbons and that mass transfer was higher for hexane than for the lighter components. Thus, hexane represents the worst case from the mass transfer point of view. The oil was introduced into the foam cell after most of the bulk liquid drained out, which is approximately 50 min after the foam was generated in most cases. The hexane vapors issuing from the top of the foam column were swept by the nitrogen gas and carried to a gas chromatograph for compositional analysis. The flow rate of nitrogen gas was controlled by a flow meter and kept constant at 4.8 ( 0.2 mL/min during these experiments. The concentrations of the hydrocarbon vapors in the effluent in the absence of any foam were also measured under experimental conditions to establish a baseline for estimating the suppression of hexane vapors in the effluent in the presence of foam. Experiments were repeated to check the consistency of data.
FIGURE 2. Schematic of experimental setup for the upscaled experiments. The viscosity of the aqueous solutions was measured by a Brookfield rotational rheometer. The surface tensions of aqueous solutions and oil were measured by a du Nouy ring tensiometer. The interfacial tension between the aqueous solution and the oil was measured by a spinning drop tensiometer. Foam texture/bubble size was observed visually. Upscaled Setup. The experiments at the bench-scale were carried out in a narrow (5 cm diameter) cylindrical vessel. The fluidity of foam and the effective coverage of the exposed oil surface by the foam blanket were not tested in these experiments. Moreover, in the bench-scale experiments, the foam column merely sat on the top of the oil layer and was not pushed from the bottom. In field applications, the foam column would be pushed up as the crude oil/gasoline is pumped from the bottom. If the foam is not sufficiently flexible/mobile, it might get partially destroyed because of the mechanical perturbations during the motion. Hexane was used as oil in bench-scale experiments. Polar organics, which could be present in crude oil/gasoline, are more destabilizing than the nonpolar components (19). A second set of experiments, called here “upscaled”, were conducted to evaluate these effects. A cylindrical container having a diameter of 46 cm and a height of 74 cm was chosen for these experiments. This vessel is referred to as the “emission cell”. The schematic of the experimental set up is shown in the Figure 2. The foam was generated separately in a foam cell and transferred to the emission cell through a hose connecting the two. A foam column 40 cm in height was sprayed onto the bottom of the emission cell. The emission cell was then covered with a lid, and gasoline was pumped from the bottom at a low flow rate. The top of the emission cell was connected by a tube to the gas chromatograph where the composition of the effluent was analyzed. After a 1-in.-thick layer of gasoline was deposited inside the emission cell, water was used to push this layer of gasoline up for the remaining duration of the experiment. Here the water mimics the injection of gasoline because it stays at the bottom of gasoline layer and does not interact with the foam. Use of water reduced the use of oil in this laboratory experiment and the associated fire-safety concerns and disposal issues. Approximately 18 L of the total liquid was pumped in 10 h, the duration of the experiment. The flow rate of the gases flowing through the gas chromatograph was approximately 11 mL/min for the experiments with foam. In the absence of foam, a gas flow rate of about 30 mL/min was recorded at the exit of the gas chromatograph. The results of the foam cell experiments are discussed next followed by the emission cell experiments.
Results and Discussion Foam Cell. Several foam formulations were tried. Foam formulations with only surfactants are not very stable and
FIGURE 3. Bulk drainage of foam. The foam solution viscosity is 18.1 cP, and the initial height is 0.32 m. have a half-life less than 1 h. Foam formulations consisting of a nonionic surfactant (T), a fluorinated surfactant (F), a stabilizer (G), and a viscosifier (X) were found to be remarkably stable with a half-life exceeding 1 day. The nonionic surfactant tends to increase the thickness of foam films; this is concluded from visual observations of foams. The fluorinated surfactant reduced the surface tension of the aqueous solution and stabilized the foam column in the presence of oil. The stabilizer (e.g., glycerol) decreased the vaporization of water from foam film and thus increased stability. The viscosifier, xanthan gum, increased the viscosity of the aqueous solution and slowed the drainage of the solution and the thinning of foam lamellae, thus providing stability to the foam column. The following nomenclature is used to specify the foam composition; 1T0.4F0.4G0.16X implies 1 wt % Tergitol, 0.4 wt % F1127, 6 wt % glycerol, and 0.16 wt % xanthan gum. This foam composition was found to be very stable and has been used in most of the results reported in this paper. The surface tension of this aqueous foam solution was measured to be 20.5 mN/m at room temperature (22 °C). At the same temperature, the surface tension of the oil (99.99% assay of C6-saturated isomers) was found to be 23.3 mN/m. The oil and foam formulation were equilibrated for 48 h in a volumetric ratio of 1:2. The interfacial tension (σof) between the fluids was measured using a spinning drop tensiometer and was found to be 1.5 mN/m. Thus, the entering coefficient E is -1.3 mN/m for this system. Hence, this foam falls into the category of type A foam according to the criteria developed by Ross (17). Experimentally, it was observed that this foam did not rupture at the foam/oil contact and exhibited high degree of stability in the presence oil. These foams persisted for 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 columns having a composition of 1T0.4F6G0.16X and an initial height of 32 cm. The liquid drained is plotted as a fraction of the VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2723
FIGURE 4. Concentration of hexane vapors in the effluent with and without foam.
FIGURE 5. Concentration of hexane in the effluent with time for the foam columns with initial heights of 0.15, 0.23, and 0.32 m. The values 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). amount of liquid foamed originally. With time, the fraction drained tends to a limit slightly lower than one. Most of the liquid is drained within 1 h. The amount not drained is in the foam as lamellae and plateau borders. It is clear that the experimental data for bulk drainage rates are fairly reproducible between the two identical experiments. These foams are stable for more than 1 day in the presence of oil, although the data on concentration of hexane vapors in the effluent was collected only for about 10 h. Visual observation confirms that most of the foam bubbles are of similar size, though some smaller bubbles sandwiched between the larger bubbles are also observed. Figure 4 shows the mole percentage of hexane vapors in the effluent with and without foam under experimental conditions as described above. The initial height of the foam column was 32 cm. The height of the foam column remained practically unchanged throughout the duration of the mass transfer experiments. Without foam, the hexane concentration in the effluent increases with time as it mixes with nitrogen in the cylinder and reaches a plateau value of about 4.5 mol % in about 5 h. In contrast, the hexane concentration in the effluent in the presence of foam rises to only about 0.6 mol % at the end of 10 h. It is clear that the presence of foam significantly suppresses the concentration of hexane vapors in the effluent. If the plateau values in both the cases are considered, the presence of the foam suppresses the concentration of the hexane vapors in the effluent by about 87%. Various parameters that could affect the performance of foam such as initial foam height and liquid viscosity were varied experimentally. The following sections discuss the impact of these parameters. Effect of Foam Height. Figures 5-7 show the effect of initial foam height on the concentration of hexane vapors in the effluent. The foam height during the mass transfer experiments was essentially equal to the initial foam height. 2724
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004
FIGURE 6. Concentration of hexane in the effluent with time for the foam columns with initial heights of 0.15, 0.23, and 0.32 m. The values 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 the foam columns with initial heights of 0.15, 0.23, and 0.32 m. The values 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). Three foam formulations of viscosities 8 × 10-3 Ns/m2 () 8 cP) (Figure 5), 18.1 × 10-3 Ns/m2 (Figure 6), and 65.3 × 10-3 Ns/m2 (Figure 7) were chosen by varying the concentration of viscosifier in the foam formulation and keeping the rest of the foam composition unchanged. The concentration of hexane vapors in the effluent increases with time and reaches a plateau after about 5 h for 15 cm column and for about 8 h for the 32 cm column. As expected, the mole fraction of hexane vapors in the effluent decreases with increasing initial foam height for a given viscosity of the foam solution. The plateau concentration of hexane also decreases with the initial foam height. The aqueous solution drains from the lamellae into the adjoining plateau borders subject to the forces of capillary suction and disjoining pressure. The plateau borders form a well-connected network in the foam column through which the liquid drains down under the action of gravity and an opposing capillary pressure gradient is set up as a consequence of the gravity drainage. The plateau borders become leaner in liquid with height in a foam column with the result that the plateau border radius of curvature gets smaller and the capillary suction higher causing rapid drainage of the connected foam lamellae. Thus, the foam lamellae get thinner progressively with height in a foam column. Since the liquid holdup in the foam films decreases with height, the mass transfer resistance to the diffusing hydrocarbon vapors also decreases. If the liquid holdup in the foam lamellae were uniform (i.e., independent of the height), the diffusion time scale of the foam column would quadruple when the foam column height is doubled assuming it is only the liquid in the foam films that provides a significant diffusive mass transfer barrier. Since the mass transfer resistance decreases with height, we expect the diffusional time to increase by less than 4 times. Figures 5-7 show that the time to reach
FIGURE 8. Concentration of hexane in the effluent with time for the foam columns with initial height of 0.23 m. The values of the parameters 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 having an initial height of 15 cm. the plateau concentration increases by only a factor of about 8:5 (which is considerably less than 4 times) when the height is approximately doubled. Figures 5-7 also show that the plateau concentration of hexane vapors in the effluent is suppressed by about 65% (∼1.4-0.5 mol %) when the height of the foam column is doubled. Effect of Viscosity. Figure 8 shows the variation of mole percentage of hexane vapors in the effluent with time for foam columns having an initial height of 23 cm. Three sets of data for three different viscosities of the aqueous foam solution are presented. It is interesting to note that no significant difference in the concentration of hexane vapors in the effluent was observed with varying viscosities during the transient. The plateau hexane concentration value decreases slightly as the viscosity of the aqueous solution increases. The increase in viscosity slows down the drainage of liquid from the foam lamellae resulting in thicker foam films. Figure 9 shows the bulk drainage of foams having an initial height of 15 cm. The drainage fraction is calculated with respect to the total amount of liquid present in the foam at the end of bubbling. The foams made up of aqueous solutions having higher viscosities drain slower. The foam film thickness was not explicitly measured. Although the bulk drainage data reflects plateau border drainage more accurately, a greater amount of liquid in the plateau borders implies weaker capillary suction from the films, thus producing thicker films. If the solubility of hexane is not affected by the polymer concentration, the thicker foam films would have less permeability to the diffusing hydrocarbon vapors; hence, the concentration of hexane vapors in the effluent is expected to decrease with higher viscosities. Moreover, if one were to assume that Stokes-Einstein relationship for diffusivity in liquid holds for the diffusion of hydrocarbon vapors, the diffusivity would decrease for higher viscosities even for the
FIGURE 10. Cumulative hydrocarbon emission with time per liter of gasoline loaded in the presence of foam having a composition of 1T0.4F6G0.24X and without foam. same thickness of the foam film. Thus, the mass transfer of hexane should decrease dramatically with the increase in viscosity. This does not appear to be the case in this study (i.e., Figure 8). One explanation could be that increasing the polymer concentration not only increases the viscosity of the foam formulation but also possibly increases the solubility of the hydrocarbon gases in the foam liquid, annulling any diffusivity decrease due to higher viscosity or thicker films. Another possibility is that the resistance provided by the adsorbed surfactant monolayers at the film-gas interface is the dominant resistance to the mass transfer, as shown by Quoc et al. (20). In that case, the liquid content of the foam becomes less significant and since the number of monolayers through which the hydrocarbon molecules diffuse through remain the same for a given foam height and bubble size irrespective of the viscosity, the mass transfer resistance of the foam column does not vary significantly. Emission Cell. Scaled-up experiments were conducted with an unleaded gasoline in the emission cell. Figure 10 shows the cumulative hydrocarbon emissions per liter of gasoline loaded in the emission cell for the foam composition 1T0.4F6G0.24X and the initial foam height of 40 cm. About 144 mg of total organic compounds are emitted per liter of gasoline loaded in 10 h without the foam. About 5 mg of TOCs are emitted per liter of gasoline loaded in 10 h with the foam. Thus, the suppression of total emission of hydrocarbons is about 96.5%. This emission, 5 mg of TOC/L of gasoline loaded, meets the U.S. EPA (10 mg of TOC/L) and European Community (35 mg of TOC/L) standards (1) in our emission cell. The total hydrocarbon emission is controlled by several parameters, such as loading time, foam height, foam formulation, and exposed surface to container volume ratio. These parameters can be chosen appropriately to obtain the required emission. Figure 11a-c shows the concentration of the individual hydrocarbons in the effluent for the experiment reported above. All the hydrocarbon gases heavier than hexanes are collectively lumped as C6+, the isomers of pentane as C5, and the isomers of butane as C4. For the light hydrocarbons such as butanes, close to 100% suppression of the vapors was observed in the presence of foam, at least for 10 h for both the foam formulations. For pentanes, the emission is cut down by about 97% in the presence of foam for the foam formulation 1T0.4F6G0.24X. It should be noted that the pentanes collectively contribute highest to the total VOC emissions from the gasoline. For hexanes and other heavier gases (C6+), the reduction in the emission is about 71% if the concentration in the effluent at the end of 10 h is considered for 1T0.4F6G0.24X. Thus, these foam formulations achieve 70% to near 100% suppression in the emission of VOCs depending upon the component of the gasoline under consideration. Note that the emission of C6+ is higher than that of C5, even though the diffusivity of C6+ is lower in the aqueous solution. This implies that the solubility of these organic vapors in the foam lamellae may play an important VOL. 38, NO. 9, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2725
FIGURE 12. Comparison of foam liquid content for the foam formulation 1T0.4F6G0.24X prepared with deionized water and salty water.
FIGURE 13. Cumulative hydrocarbon emission with time per liter of gasoline loaded in the presence of foam having a composition of 1T0.4F6G0.24X prepared in 3.5 wt % NaCl solution and without foam.
FIGURE 11. (a) Concentration of C6+ gases in the effluent with and without foam for the upscaled experiment for identified aqueous foam formulations. (b) Concentration of pentanes in the effluent with and without foam for the upscaled experiment for identified aqueous foam formulations. (c) Concentration of butanes in the effluent with and without foam for the upscaled experiment for identified aqueous foam formulations. role. Higher solubility of C6+ in the surfactant solution than that of C5 may be due to the presence of surfactants and viscosifiers that are organic. Often, the loading and unloading terminals for the crude oil/gasoline are located near the large saline water bodies such as oceans. Therefore, we evaluate the effectiveness of the foams developed with seawater. In the open oceans, the salinity based on total dry solids per kilogram of seawater typically varies between 3.36 and 3.68 wt % (21). An approximate value of seawater salinity could be taken as 3.5 wt %, composed entirely of NaCl, although seawater also contains other salts. In the preliminary experiments with the foam formulations prepared in salty water containing 3.5 wt % of NaCl, it was observed that the foam column approximately retains its initial height for at least 10 h, although it has lower initial liquid holdup and drains faster than the foam formulation prepared with deionized water as shown in Figure 12. The lamellae for the foam prepared in salty water also appear thinner visibly. These observations are consistent with the effect of salinity on foam stability observed by Rojas et al. (22). The presence of electrolytes in the foam liquid causes electrostatic double layer in the foam films to shrink, thereby reducing the repulsive forces that stabilize the foam films. This, in turn, leads to thinner foam films in the presence of salt. Thinner films may not reduce the ability of the foam barrier to suppress VOC emissions substantially 2726
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004
if the limiting mass transfer resistance is provided by the film-gas interface (20). Figure 13 shows the cumulative hydrocarbon emissions per liter of gasoline loaded in the emission cell for the foam composition 1T0.4F6G0.24X prepared in saline solution (3.5 wt % NaCl) and the initial foam height of 40 cm. About 9 mg of TOC is emitted per liter of gasoline loaded after 10 h against 144 mg without foam. Thus, the suppression of total emission of hydrocarbons is about 93.8%. Figure 14a-c shows the concentration of components in the effluent with time. As expected, the effectiveness of the foam column in suppressing hydrocarbon emission goes down when compared with foam formulations prepared in deionized water for the same initial foam height; concentrations of all the components are higher in Figure 14 as compared to those in Figure 11. However, it still suppresses the concentration of C6+ gases in the effluent by 59%, of pentanes by 84%, and of butanes by 69% based on the concentration in the effluent at the end of about 10 h. The effectiveness of the foam column can be increased by increasing the column height. An approximate model has been developed to interpret the mass transfer of vapors during the loading process with and without foam barriers. This model considers onedimensional diffusion of dilute VOC components in a semiinfinite air column above the foam (or gasoline in the absence of foam). The distance (h ) h0 - vt) between the foam-air interface and the exit of the vessel shrinks as gasoline is loaded. The initial height of the air column is h0, v is the velocity of the gasoline front, t is the time, and z is the distance from the foam-air interface. The effluent concentration is evaluated at h, which is a function of time. The foam is modeled with a mass transfer coefficient (k) at the foam-air interface. The solubilization of hydrocarbons in the aqueous solution of the foam is important in slowing down the
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)
where Cs is the concentration of the component at the gasoline-vapor interface dictated by the thermodynamics. If the mass transfer coefficient is large (i.e., in the absence of a foam barrier), the last term in eq 2 is negligible. This model has been fitted to the experimental data of hexane 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 and 5.7 × 10-4 for foam with salt solution. These mass transfer coefficients depend on the foam structure, solubility in the aqueous solution, and diffusivity through foam films. These factors will be studied in detail in a future study.
Acknowledgments This work was partially supported by the funding from Texas Hazardous Waste Research Center.
Nomenclature C
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 0
initial volume fraction of liquid in foam
σf
surface tension of foam solution
FIGURE 14. (a) Concentration of C6+ in the effluent with and without foam for the upscaled experiment. Foam liquid composition is 1T0.4F6G0.24X and contains 3.5 wt % NaCl. (b) Concentration of pentanes in the effluent with and without foam for the upscaled experiment. Foam liquid composition is 1T0.4F6G0.24X and contains 3.5 wt % NaCl. (c) Concentration of butanes in the effluent with and without foam for the upscaled experiment. Foam liquid composition is 1T0.4F6G0.24X and contains 3.5 wt % NaCl.
σo
surface tension of oil
σof
interfacial tension between foam liquid and oil
µ
bulk viscosity of foam solution
F
density of foam solution
breakthrough of the gases but is neglected here. The details of diffusion through the liquid-air interface, foam films, and foam cells are lumped into a single mass transfer coefficient. These factors are important but outside the scope of this paper. The concentration of a hydrocarbon component in the air above the foam barrier can be described by (under the assumptions stated above):
(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; Paper SPE 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.
∂C/∂t ) D ∂2C/∂2z
for 0 < z < ∞
(1)
with the initial and boundary conditions:
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 where D is the diffusivity of the component in air. The solution to the above equation, C(z, t), is given by ref 23:
Literature Cited
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
2728
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 9, 2004
Chemistry; Society of Petroleum Engineers: Richardson, TX, 2001; Paper SPE 64999. (23) Welty, J. R.; Wicks, C. E.; Wilson, R. E. Fundamentals of Momentum, Heat & Mass Transfer; John Wiley & Sons: New York, 1984; p 307.
Received for review September 2, 2003. Revised manuscript received January 5, 2004. Accepted February 17, 2004. ES0349599
