Acta Chim. Slov. 2004, 51, 687-698. 687 Scientiflc Paper BATCH FOAM FRACTIONATION OF SURFACTANTS FROM AQUEOUS SOLUTIONS Andrej Šonc and Viktor Grilc National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia Received 08-09-2004 Abstract Surfactants represent a striking problem in water resources. Foam fractionation enables both defoaming and concentration of surfactants. Foam fractionation process is controled by many process and material parameters that are: airflow rate, foam column geometry, feed concentration and added salt. Optimal conditions were tested on a real sample from industry. A mathematical model that describes the changes in concentration of surfactants along the column was developed. Key words: foam fractionation, surfactants and wastewaters Introduction In the last few decades the human society has been facing with an expanding pollution of surface and ground waters. Sources of pollution are numerous, industry being one of most important. Many industries use surfactants as washing agents that may be harmful or even toxic for aquatic organisms. One of the promising methods for removal of surfactants from wastewaters is foam fractionation1. In foam fractionation surfactants adsorb on the gas liquid interface generated by bubbling air into dilute surfactant solution and then are carried along the column to its top. Because of the liquid drainage in the film of foam, there is a concentration effect of surfactants towards the column top. When the outlet foam phase collapses, a concentrated solution (foamate) remains. This method is not limited only to surfactants, but it can be used also for concentration of proteins and other surface-active compounds. Furthermore surfactants can act as a collector and bind metal cations into chelates that can be easily removed by foam fractionation. Aziz and Beheir2 investigated the removal of Cs-134 and Co-60 from radioactive process wastewater using cetyl pyridinium chloride as a collector. Chiu and Huang3 showed the effectiveness of foam fractionation for nonionic organic pollutants removal from an aqueous solution. A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions 688 Acta Chim. Slov. 2004, 51, 687-698. Numerous studies have been published on transport phenomena in foam fractionation. Most of them are related to proteins4'5 and only a few models were proposed for systems containing surfactants.6 The reason is that surfactants are cheaper and display a more complex behavior when they adsorb on the gas liquid interface. These models4'5 were made only for proteins due to their specific properties that simplified the proposed material balances. Material balances for the investigated systems were solved by numerical calculations and the result did not include the concentration changes in bulk solution. Lemlich7 proposed a simple model for foam fractionation of surfactants. He made several assumptions that are valid only for a dilute surfactant solution. The liquid streams (up flow of entrained liquid in foam and the descending drainage liquid) had constant volumetric flow rates in the column. Complex mechanisms of liquid drainage and coalescence were not included. Roustan and Roques8 proposed an interesting model for the continuous foam fractionation. Their theory of foam fractionation is based on dimensional analysis. The model takes into account the internal reflux due to drainage of liquid. A correlation was presented between specific production of the column and the parameters characterizing the column geometry and operating conditions. Unfortunately the model is valid only for the concentrations near the critical micelle concentration of the surfactant. Recently a mathematical model6 for batch foam fractionation has been developed and tested on an aqueous protein solution. The model described the time-varying total protein concentration profiles along the column in both foam and bulk phases. The aim of the present work was to study and determine the optimal parameters that control the process of batch foam fractionation of surfactants and then apply it to real samples of industrial wastewaters. Experimental Experimental setup is shown in Figure 1. It consists of a Plexiglas cylindrical column of 10 cm outer diameter, 3 mm wall thickness and 125 cm effective height. The model surfactant feed solution in distilled water was charged into the column to the height of 28.5 cm above the bottom. The air (pressure from a compressor was maintained by a regulator) was pre-saturated with water and introduced into the column A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions Acta Chim. Slov. 2004, 51, 687-698. 689 through a sintered glass diffuser No. 2, located at the bottom of the column. The air bubbles produced were of 100-160 µm in diameter. Foam from the column top was collected with a vacuum pump into a receiver in 30 minutes tirne intervals, frozen immediately in liquid nitrogen to prevent loss of water and thawed to obtain the collapsed foamate samples. The experiments were conducted in temperature range from 20-25 °C. i «4 g h d a b c Figure 1. Experimental setup for foam fractionation: a- air compressor; b- needle valve; c- flow meter; d-air suppressor; e- humidifier; f- air sparger; g- column; h- bulk liquid; i- foam withdrawal. For the determination of process parameters and the process modeling the following technical grade surfactants were used: dodecylbenzene sulfonic acid sodium salt (DBSA, an anionic surfactant, supplied by TEOL), Hyamine 1622 (a cationic surfactant, supplied by SIGMA) and TRITON X-100 (a nonionic surfactant, by FLUKA). Surface tension of the various feed solutions, determined by the ring method, is presented in Tablel. e f A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions 690 Acta Chim. Slov. 2004, 51, 687-698. Table 1. Surface tension of different surfactant solutions at 20 °C. Surfactant M (g/mol) C0 (M) Y(mN/m) DBSA Hyamine 1622 TRITON X-100 348.5 448.1 624 8.0xl0"4 8.0xl0"4 8.0xl0"4 37.1 49.5 30.8 Industrial samples of wastewaters, containing surfactants, were obtained from various sources: two from cloth dry cleaning companies and one from a car wash station. Their physical properties are given in Table 2. Table 2. Wastewater parameters for different samples from industry. PARAMETERS Usluga* Labod* AC** COD (mg/L) 422 307 156 TOC (mg/L) 99.7 84.5 8.95 Speč. conductivity (mS/cm) 1.306 0.887 0.735 Dry matter (g/L) 1.2 0.768 0.55 Surface tension (mN/m) 35.77 31.9 62.43 C(an. surf.) (mg/l) 11.69 23.37 < 3 C(nonion. surf.) (mg/ml) 32.16 40.5 1 * textile laundry, **car wash service. Anionic surfactant concentrations in the foamate were analytically determined by two-phase titration method.9 Nonionic surfactants were determined by photometric analysis described in literature.10 When the mixture of both surfactant types was present in the solution, they had to be separated by a batch ion exchange method11 and analyzed with the appropriate method cited above. For the separation of anionic surfactants from the mixture an ion exchange resin DOWEX K-21 was used. Foam samples were collected 60 cm above the gas distributor. Feed concentrations of DBSA, hyamine 1622 and TRITON X-100 were c0=8.0xl0"4 M. Samples of feed solution (bulk liquid) were collected every 30 minutes 5 cm above the gas distributor. Superficial air velocity varied from 0.193 to 0.386 mm/s. Results and Discussion The most practical indicator for the performance of the foam column is the enrichment ratio (e), which is defined as the ratio of foamate concentration vs. bulk liquid concentration of the surfactant (1). A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions Acta Chim. Slov. 2004, 51, 687-698. 691 400 350 ' 300 250 200 150 100 50 0,00 e=cf/cb (1) 200 150 0d ,20. '<°e, '^ 0,25 100 C/a 0,30 0,35 50 ^ :^ 0,40 Figure 2. Typical tirne dependence of enrichment ratio e of 0.8 mM DBSA at different air flow rates at 25 °C. As it is shown in Figure 2, there is an optimal air superficial velociry for each system, for DBSA being 0.241 mm/s (airflow rate 100 ml/min). At higher flow rates there is a decrease of enrichment ratio presumably due to lower residual tirne of bubbles in foam phase, which causes lower drainage of liquid from foam and consequently there is a higher content of water in foamate. 400 350 300 250 200 150 100 50 50 100 150 t/min 200 250 Figure 3. Time dependence of enrichment ratio e at different feed solution concentrations of DBSA. 0 0 A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions 692 Acta Chim. Slov. 2004, 51, 687-698. As it is shown in Figure 3 the enrichment ratio increases as feed liquid surfactant concentration decreases, presumably due to higher foam stability at higher surfactant concentration in the foam. The contribution of adsorbed surfactant to enrichment ratio increases since the amount of surfactant in the bulk liquid is smaller. 1000- —•— e for Hyamine 1622 at air superficial velocity 0.289 mm/s —A— 0.338 mm/s T 0.386 mm/s • 800- 600- ^* ^* 400- ^._____.^-^/^ 200- •- """ ^*^^ 0- f ==Y****"* 30 60 90 120 150 180 210 240 270 t/min Figure 4. Time dependence of enrichment ratio e at different airflow rates for Hyamine 1622. As it is shown in Figure 4 the results of foaming efficiency are similar for both surfactants (anionic and cationic) only there is a slight difference in the optimal airflow rate. Optimal airflow rate for Hyamine 1622 is higher than for DBSA for about 25%. This is probably because of the lower surface tension for DBSA at the same conditions. Lower surface tension reduces the bubble size in the system and this enhances the process of foam fractionation. 500 400 300 200 100 50 100 150 t/min 200 250 300 Figure 5. Time dependence of enrichment ratio e at airflow rate 0.241 mm/s and different concentrations of added salt for DBSA. 0 0 A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions Acta Chim. Slov. 2004, 51, 687-698. 693 In Figure 5 a decrease of enrichment ratio with added salt can be seen. The presence of salt presumably causes higher stability of liquid in foam and lower drainage, so the liquid phase content in foam is higher. The electrostatic repulsion of adsorbed surfactants in thin films between foam bubbles induced by salt stabilizes liquid drainage from foam.12 Foam fractionation of nonionic surfactant solutions gave similar results. The optimal airflow rate for TRITON X-100 was 0.338 mm/s; the highest of ali three types used. After obtaining optimal process parameters a mathematical model was developed for batch foam fractionation based on analogies to distillation. This model describes the total surfactant concentrations of both the foam and the bulk solution. Material balances and equilibrium relationships are used to develop the model. Further details are given elsewhere.6'13 Equilibrium in the bulk or foam phase at a theoretical stage n is defined by: c„ = Kn C„ n =l,2, ...N (2) where cn is the surfactant concentration in the upward foamate leaving the equilibrium stage, Cn is the concentration in draining liquid and Kn is the distribution coefficient. In general, Kn varies with the height of the column and tirne. The liquid drainage flow rates F and foamate flow rates f are described by a mass balance: dVn/dt = f„.i - f„ + Fn+1 - Fn - fout;n (3) where Vn is the volume hold-up of the collapsed foam on stage n. The effluent flow rate of collapsed foam leaving each stage fout,„ is determined from experimental data and equals the amount of withdrawn sample from each stage divided by the elapsed tirne (in our čase 30 min). The effluent values are then multiplied by a flow factor to give upward foam flow rate, fn, leaving a given stage n. The flow factor was determined from the parallel experiment with the same conditions, where we established the total volume of generated foam on each stage n. The boundary condition at the top stage is that the flow rate of the foam leaving the stage is set to zero. The column is capped and there is no flow at the top of the column. If we approximate that the left side of equation (3) is negligible or equal to zero, then the F's can be solved from the above equation (3) that A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions 694 Acta Chim. Slov. 2004, 51, 687-698. turns into a set of linear algebraic equations and can be easily solved with the method of Gauss elimination. The stagewise mass balance for the bulk solution is: dVb/dt = F3 - f2 - fout,b (4) The term dVb/dt cannot be neglected since mass is being removed from the column through the sampling ports. Measured bulk liquid volume changes with tirne are presented in table 3. Boundary conditions for equation (3) is: dVn/dt = 0 for 0 < t < tf (5) f6 = F7 = 0 for n = 6 for N = 6 (6) where tf is the sampling tirne interval, Vb is the volume hold-up of the collapsed foam at the bottom. The total surfactant balance can be written for stage n as: d(cnVn)/dt = fn.lCn.i - f„cn + Fn+1Cn+1 - FnC„ - U„c„ (7) d(cbVb)/dt = F3C3 - f2cb - f0Ut,bCb (8) Boundary conditions for equation (7) is: d(cnVn)/dt = 0 for 0 < t < tf (9) The equilibrium relationship from equation (2) was substituted into equations (7,8) and the values Kn's were obtained. Other variables were obtained either from experimental data or calculated from equations (3,4). We assumed steady state in each of the sampling period so that the left side of equation (7) is close to zero and the method of solving these linear equations is the same as in equation (3). After the Kn's were obtained, they were tirne averaged and using these averaged K's, the surfactant concentrations in the foam and bulk liquid was simulated from equations (7,8). Effluent flow rates for liquid and foam phase and distribution coefficients Kn's are listed in tables 4 and 5. The value of equilibrium constant Kb was determined from the equation (2) in A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions Acta Chim. Slov. 2004, 51, 687-698. 695 the beginning of the experiment when the foam phase was a few cm above the interface liquid/foam. We assumed that Kb was not changing with tirne. Table 3. Measured bulk liquid volume changes with tirne. Time (min) Vb (mL) 0 2406 30 2391 60 2376 90 2361 Table 4. Effluent flow rates of collapsed foam and liquid drainage. Stage, n fn (mL/min) Fn (mL/min) foutn (mL/min) Flow factor 2(foam phase bottom) 3 4 5 6 7 5.670 - 0.366 3.867 5.517 0.097 40 1.206 3.810 0.032 38 0.208 1.193 0.017 12 0 0.205 0.012 14.6 - 0 - - Table 5. Distribution coefficients Kn's in the model. Time(min) Kb K3 K4 K5 K6 30 1.240 1.256 2.243 2.497 2.217 60 1.240 1.536 4.444 2.971 1.820 90 1.240 1.608 9.547 2.767 1.560 Kaver. 1.240 1.199 3.820 2.284 1.606 As we can see on Figure 6 there is a difference between experimental data and simulation results on stage 3 (height of the withdrawal of foamate H=48.7 cm). The explanation for the descending slope at the second stage is probably higher airflow rate than in the original model6 that presumably causes an extension of surfactant fractionation in the bulk solution. Because of the withdrawal of surfactant from the bulk solution, the concentration decreases and only at a certain height of the column this effect is reversed (stage 3) to an increasing surfactant concentration in foamate due to fractionation in the foam phase. From stage 3 on the obtained simulation data reasonably good fit the experimental data. A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions 696 Acta Chim. Slov. 2004, 51, 687-698. Experimental data n H=5.0 cm (stage 1) H=35.2 (stage 2) H=48.7 (stage 3) H=62.2 (stage 4) H=75.7 (stage 5) Simulation data ¦— H-5.0 (stage 1) •— H-35.2 (stage 2) k— H-48.7 (stage 3) T— H-62.2 (stage 4) P— H-75.7 (stage 5) 30 40 50 60 t/min 70 80 90 Figure 6. Simulation results of the surfactant concentration profiles in foam and bulk liquid at airflow rate 0.338 mm/s. The points represent the experimental data and the curves represent the simulation results. Stages 1 and 2 represent the height of bulk solution withdrawal and stages 3 to 5 represent the height of foamate withdrawal. Results of foaming of some industrial wastewaters are presented in Figure 7. 40- —¦—anion —•—nonionic 35- 30- 25- »^^^ 20- . ^""¦"¦'• 15- -^^ ^^^^^^ " 10- "*—-----------—. 20 40 60 80 100 120 140 160 t/min Figure 7. Decrease of surfactant concentration in bulk solution with time for sample from Labod at superficial air velocity 0.338 mm/s. As can be seen from figure 7, 66% of surfactants have been removed from the bulk solution and recovered in the foamate during 250 minutes under given circumstances. After that time it was no more possible to provide samples of foam and also the concentration in the bulk solution did not change significantly with time. 4 2 0 0 A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions Acta Chim. Slov. 2004, 51, 687-698. 697 During the three hours process of foaming the COD values in the laundry wastewater USLUGA reduced from 266 to 186 mg/L and TOC values from 84.5 to 62.6 mg/L. Both values did not drop significantly because foam fractionation is a specific method only for the separation of surface-active agents. Other organic impurities (softeners, grease, waxes) remained in the wastewater after the process and could not have been removed by foaming. The process is primarily applicable for the recovery of surfactants from wastewaters in order to be recycled. Conclusions The experimental results shown enable most important parameters that affect the foam fractionation process to be determined. The optimal parameters for model surfactant solutions were applied in modeling purification of real wastewater samples containing surfactants from various industrial cleaning activities. Since foam fractionation has some similarities with the fractional distillation, a mathematical model was applied that describes the changes of concentration profiles of surfactants in the batch-operated foaming column. The simulation results show good consistency with the experimental results. Nomenclature cf = surfactant concentration in foam, mmol/L cb = surfactant concentration in bulk liquid, mmol/L cn = surfactant concentration in rising foamate leaving stage n, mmol/L Cn = surfactant concentration in draining liquid, mmol/L e = enrichment ratio fn = foamate flow rate, ml/min Fn = liquid drainage flow rate, mL/min foutn = effluent flow rate of collapsed foam leaving stage n, mL/min fout,b = sample withdrawal flow rate of bulk liquid, mL/min Kn = distribution coefficient tf = sampling tirne interval, min Vn = volume hold-up of the collapsed foam on stage n, mL Vb = bulk liquid volume, mL Acknowledgements We are grateful to the Ministry of Science and Technology of the Republic of Slovenia for fmancial support through grant No. S2-104-014/20391/99. A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions 698 Acta Chim. Slov. 2004, 51, 687-698. References 1. S. Chen, M. B. Timmons, J. J. Bisogni Jr., D. J. Aneshansley, Aquac. Eng. 1994, 13, 183-200. 2. M. Aziz, S. G. Beheir, J. Radioanal. Nucl. Chem. 1995, 191, 53-66. 3. H. L. Chiu, S. D. Huang, Sep. Sci. Technol. 1991, 26, 73-83. 4. F. Uraizee, G. Narsimhan, Sep. Sci. Technol. 1995, 30, 847-881. 5. S. Chen, PhD Thesis, Cornell University, Ithaca NY, 1991. 6. C. B. Neely, J. Eiamwat, L. Du, V. Loha, A. Prokop, R. D. Tanner, Biologia 2001, 56, 583-589. 7. R. Lemlich, Ind. Eng. Chem. 1968, 60, 16-29. 8. M. Roustan, H. Roques, Can. J. Chem. Eng. 1973, 51, 168-172. 9. V. W. Reid, G. F. Longman, E. Heinerth, Tenside 1967, 4, 292-298. 10. B. M. Milwidsky, Soap, Cosmetics, Chemical Specialties 1971, 47, 66-70. 11. M. J. Rosen, J. Am. Oil Chem. Soc. 1961, 38, 218-220. 12. P. Wungrattanasopon, J. F. Scamehorn, S. Chavedej and C. Saiwan, J. H. Harwell, Sep. Sci. Technol. 1996, 31, 1523-1540. 13. A. Šonc, MSc Thesis, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Slovenia, 2003. Povzetek Detergenti predstavljajo pereč problem v raznih industrijskih in komunalnih odpadnih vodah. Izpenjevanje z zrakom nam omogoča hkratno zmanjšanje organske obremenitve odpadnih vod in koncentriranje detergentov zaradi možne reciklaže. Glavne procesne in snovne spremenljivke, ki kontrolirajo proces izpenjevanja so: koncentracija in tip detergenta v napajalni raztopini, prisotnost soli, pretok plina in geometrija kolone. Optimalne pogoje izpenjevanja smo testirali na realnih vzorcih odpadnih vod iz industrije. Podan je tudi matematični model, ki opisuje potek koncentracije detergenta vzdolž kolone. Model zadovoljivo opisuje proces frakcioniranega izpenjevanja detergentov iz modelnih raztopin, manj uspešno pa iz realnih vzorcev odpadnih vod. A. Šonc, V. Grilc: Batch Foam Fractionation of Surfactants from Aqueous Solutions