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Recovery of surfactant from an aqueous solution using continuous multistage foam fractionation: Influence of design parameters
Chemical Engineering and Processing 52 (2012) 41–46
Contents lists available at SciVerse ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Recovery of surfactant from an aqueous solution using continuous multistage foam fractionation: Influence of design parameters Visarut Rujirawanich a , Nopparat Chuyingsakultip a , Manutchanok Triroj a , Pomthong Malakul a,b,∗ , Sumaeth Chavadej a,b,∗ a b
The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand
a r t i c l e
i n f o
Article history: Received 10 March 2011 Received in revised form 16 September 2011 Accepted 4 December 2011 Available online 13 December 2011 Keywords: Foam fractionation Surfactant recovery Cetylpyridinium chloride Bubble caps
a b s t r a c t A multistage foam fractionation column with bubble cap trays was used to recover a surfactant from water at low concentrations. The effects of design parameters—including the number of bubble caps, foam height, and tray spacing—were first investigated under steady state conditions using cetylpyridinium chloride (CPC) as the model surfactant. An increase in bubble caps per tray significantly increased the separation efficiency, both in terms of the enrichment ratio and recovery of the CPC and of the separation factor (ratio of foamate concentration to effluent concentration). The increase in bubble caps per tray also increased the foam production rate, leading to increasing the adsorptive transport. An increase in tray spacing increased both the enrichment ratio and the residual factor of the CPC, whereas the CPC recovery and liquid entrainment in foam were reduced. An increase in foam height produces drier foams, leading to decreasing bulk liquid transport. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The total amount of surfactants, not including soaps, consumed around the world in 2003 was about 9.2 million tons [1]. Hence, a large number of industrial plants generate wastewater containing significant amounts of surfactants, which can directly impact both existing wastewater treatment plants as well as natural receiving waters. Surfactants can retain their foaming properties in natural waters in concentrations as low as 1 mg/L, leading to some environmental problems [2]. Apart from the pollution point of view, it is worthwhile to recover the surfactants for reuse, which, in turn, would make surfactant-based separation processes—such as micellar-enhanced ultrafiltration (MEUF) and surfactant-enhanced remediation of contaminated soil—become more economically feasible [3,4]. Foam fractionation is a surfactant adsorptive process that can be used to concentrate and usually remove dissolved materials, including surfactants, from homogeneous solutions [5]. The process offers several advantages for the treatment of industrial wastewaters with low solute concentrations compared to other treatment
∗ Corresponding authors at: The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand. Tel.: +662 2184139; fax: +662 2184139. E-mail addresses: [email protected] (P. Malakul), [email protected] (S. Chavadej). 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.12.002
processes, including: low space and energy requirements; simple plant design, operation, and scale-up; and low capital and operating costs [6]. Unlike biological treatment, it also allows for the immediate reuse of both water and surfactants. In a foam fractionation operation, air is introduced into the system to generate air bubbles where the surfactant will adsorb preferentially at the air–liquid interface of the air bubbles. The bubbles proceed upwards to the top of the column to produce foam. This foam layer (froth) can be separated physically from the bulk liquid phase, resulting in the removal of the surfactant and any other soluble solutes that adsorb preferentially at the foam surface. Only a small volume of entrained liquid is generally carried with the foam due to liquid film drainage. After the foam is collapsed, the collapsed foam solution (foamate) contains the surface active solute at a much higher concentration than that found in the feed because surfactant molecules adsorb preferentially at the air–water interface of the produced foam whereas entrained liquid, containing much less surfactant molecules, is drained out from the produced foam. Unlike other separation processes, foam fractionation has proven high separation efficiencies—especially at low surfactant concentrations [6]—because maximum equilibrium adsorption density of typical surfactant can be obtained in millimolar level [7]. For kinetic aspect of foam fractionation, the short residence time of air bubbles in the column does not significantly affect the separation performance of the foam fractionation because both the transport rates by diffusion and the convection of the
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surfactant from the bulk liquid phase to the air–water interface of the produced bubble are relatively fast [8]. Therefore, a wastewater—which typically contains very low surfactant concentrations, around or below the critical micelle concentration (CMC)—can be treated to separate the surfactants physically by using foam fractionation. Foam fractionation may be operated in simple mode (batchwise or continuous), or in a more complex mode with enriching and/or stripping [5,9,10]. Foam fractionation columns can also be classified as either single-stage or multistage. A number of studies have investigated the effects of operational parameters of the foam fractionation on the separation efficiency of proteins [11,12]. Most previous studies used either batch mode in single-stage flotation columns [13–15] or simple continuous mode [16–19], whereas the use of multistage pilot plants with continuous mode operation has rarely been reported [20]. Material balance of CPC in the collapsed foam at the top of column, which is the molar flow rate of foamate (Cf Vf ), is equal to the sum of the mass transfer by the bulk liquid and the adsorptive transports [20]: Cf Vf = Ce Vf + A ,
(1)
where Vf is the volumetric flow rate of collapsed foam (foamate) (dm3 /min), Ce is the CPC molar concentration in the lamellae liquid assumed to be equal to that in effluent, Cf is the CPC molar concentration in the collapsed foam, A is the flow rate of the interfacial area of the generated foam (m2 /min), and is the surface excess concentration or adsorption density (mol/m2 ). The term A (mol/min) indicates the amount of CPC adsorbed on the bubble surface, known as adsorptive transport whereas the Ce Vf (mol/min) is the product of the CPC concentration and the liquid volume in the foam lamella, known as bulk liquid transport. The increasing bulk liquid flow rate not only increases the molar flow rate of CPC in foamate but also causes a dilution of adsorbed component which results in a decrease in the enrichment of CPC (concentration in foamate/concentration in feed). The adsorptive transport is responsible for the reduction in the residual factor of CPC (concentration in effluent/concentration in feed) whereas the bulk liquid transport does not reduce the residual factor of CPC if the concentration of the component in the lamellae liquid is not significantly larger than that in the effluent (i.e. single stage foam fractionation) but it does in multistage foam fractionation due to the fact that material enrichment between stages increase concentration of the component in liquid pool of the upper tray. Multistage foam fractionation with bubble caps is basically analogous to a distillation unit. For the ideal distillation unit, a vapor phase is in equilibrium with an aqueous phase in each tray, while a foam phase is in equilibrium with an aqueous phase for the foam fractionation system, as illustrated in Fig. 1a. In multistage foam fractionation with feed position on the top tray (Fig. 1b), surfactant molecules found in a lower tray derived from draining liquid (containing residual surfactant) via downcomer can be recovered back to the top tray. The rising foam from the lower tray will pass the bubble caps of the upper tray, resulting in increasing surfactant concentration in the liquid pool of the upper tray. Thus, the effluent surfactant concentration becomes very low while most of surfactant molecules will be carried upward by rising foam to the top tray. Internal bubble coalescence within the rising foam before passing through the bubble caps is also likely to occur due to sudden change in flow cross sectional area, resulting in increasing internal reflux and enhanced enrichment ratio [21]. Hence, the more trays (or stages) there are, the higher the surfactant concentration in the top stage and the foamate [22]. Our previous study [22,23] demonstrated that the multistage foam fractionation system could provide enrichment ratio up to 240 which is much higher than that of a previous work [17] in single stage, 21.5 in enrichment ratio. Moreover,
Fig. 1. Schematic of bubble caps tray (a) and a multistage foam fractionation unit (b).
it demonstrated that a multistage foam fractionation column with bubble caps could be operated without the problems of excessive pressure drop and flooding. In this present study, a continuous multistage foam fractionator with bubble caps was tested to remove a cationic surfactant from water. The study focused on an analysis of the effects of design parameters (tray spacing and number of bubble caps) on the overall performance of the multistage foam fractionation. In addition, the effects of air flow rate and feed flow rate were also investigated at two different numbers of bubble caps per trays. 2. Experimental 2.1. Materials and methods The surfactant used in this study was cetylpyridinium chloride (CPC), molecular weight of 339.99 g/mol, obtained from Zealand Chemical with >99% purity. The surfactant was used as received without further purification. Distilled water was used in all experiments. The multistage foam fractionation apparatus used in this study (Fig. 1b) had two multistage foam fractionation columns that were made of an acrylic cylinder with an inner diameter of 18 cm but having two different tray spacings (15 cm and 30 cm), which could be assembled to 5 trays. At the top tray, the foam column, which was
V. Rujirawanich et al. / Chemical Engineering and Processing 52 (2012) 41–46
43
Downcomer Bubble cap Ø = 4 cm
Ø = 4 cm
Ø = 1 cm
Ø = 1 cm
Ø= 4 cm
Ø= 4 cm
I.D. Ø= 18 cm
I.D. Ø= 18 cm
a) 22 bubble caps/tray
b) 8 bubble caps/tray
Fig. 2. Positions and dimensions of bubble caps and downcomer: 8 caps/tray (a) and 22 caps/tray (b).
made of an acrylic cylinder with the same inner diameter, had three foam outlets to obtain different foam heights of 30, 60, and 90 cm. Each tray had either 8 or 22 bubble caps with a weir height of 6 cm and a cap diameter of 2 cm. The clearance between the lower edge of bubble cap and each tray was equal to 1.5 cm. The downcomer, having a diameter of 4 cm with a weir height of 3.0 cm was used to control the liquid level in each tray and to allow the liquid to flow down to a lower tray (Fig. 2 shows the configurations and dimensions of the bubble caps and downcomer). A sample port was located at the base of each tray for taking liquid samples. A surfactant solution was prepared at different CPC concentrations of 0.25, 0.5, and 0.75 times the CMC and was continuously fed into the top of the column at a constant flow rate that was varied from 0.98 to 3.93 kg/m2 /min (25–100 cm3 /min) using a peristaltic pump. The pressurized air from an air compressor was regulated to have any desired flow rate in the range of 1.96–6.55 cm/s (30–100 dm3 /min) using a rotameter and it was introduced below the bottom tray. The generated foam at the top of the column was collected at three different foam heights of 30, 60, and 90 cm. The foam was collected, frozen, thawed, and then measured to get the collapsed foam (foamate) volume at room temperature (25–27 ◦ C). The time needed to establish steady state was found to be approximately 7 h, and the steady state time was justified by all measured parameters invariant with time. After steady state was achieved, samples of the feed solution, the foamate, and the effluent were collected and analyzed for surfactant concentration. The CPC concentrations were measured by UV/visible spectrophotometry (Perkin Elmer, Lambda 10) at a wavelength of 260 nm. For each run under steady state condition, the samples were taken at least a few times and the data were averaged and used to assess the process separation performance. The foam fractionation column was thoroughly cleaned with distilled water before starting the next experiment. Overall material balance calculations were performed to check the validity of the experimental data. The surfactant mass balance showed less than 10% error for all runs. The process separation performance of the foam fractionation column was assessed by using the enrichment ratio of CPC, the residual factor of CPC, the separation factor of CPC, and the recovery of CPC, and the foamate volumetric ratio [24,25], described as follows: Enrichment ratio =
% CPC recovery =
Cf Ci
,
Vi Ci − Ve Ce × 100, Vi Ci
(2)
(3)
Residual factor =
Ce , Ci
(4)
Foamate volumetric ratio = Separation factor =
Cf Ce
Vf Vi
,
,
(5) (6)
where Ce , Cf , and Ci are the CPC concentrations in the effluent solution, the foamate (collapsed foam solution), and the influent (feed), respectively. Ve , Vi , and Vf are the volumetric flow rate of effluent, feed, and foamate, respectively. 2.2. Surface tension measurement Measurement of the surface tension of surfactant solutions at different CPC concentrations was carried out by using a drop shape analysis instrument (Krüss, DSA 10) at room temperature (25–27 ◦ C). 3. Results and discussion 3.1. The CMC of CPC From the plot of surface tension versus the log of the initial CPC concentration, the critical micelle concentration (CMC) and the surface tension at the CMC ( cmc ) can be determined from the reflection point in a –log C curve. The CMC and cmc were found to be 0.9 mM and 42.6 mN/m, respectively. Feed CPC concentrations of 0.25, 0.5, and 0.75 times the CMC, corresponding to 0.225, 0.450, and 0.675 mM, respectively, were used to operate the multistage foam fractionation unit in this study. 3.2. Effects of tray spacing and foam height As mentioned above, both tray spacing and foam height were found to be the main factors affecting the process separation performance of the studied multistage foam fractionation unit. As shown in Fig. 3, either increase in tray spacing or foam height increases the enrichment ratio of CPC but decreases the foamate volumetric ratio. With increasing foam height, the increasing enrichment ratio of CPC results from the longer foam residence time, causing more liquid film drainage, leading to drier foam flowing out from the column, as evidenced experimentally by the reduction of the foamate volumetric ratio. From the tray spacing results, it can be explained by the fact that the decreasing amount of lamellae liquid containing in rising foam passing the bubble caps of an upper tray with
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100
96
10
94
8
92
6
90
4
Tray spacing= 30 cm Tray spacing= 15 cm
88
2
86
0
60 Foam height (cm)
0.12
0.24
b
240
Separation Residual
Foamate volumetric ratio
3.3. Effect of the number of bubble caps per tray, air flow rate, and feed flow rate
90
0.20 0.18 0.16
220 0.10
180
Tray spacing = 15 cm Tray spacing = 30 cm
0.08
Separation
0.06
Residual
0.04
0.14 0.12 0.10
200
0.08
160 140 120 100
Separation factor
30
0.22
Enrichment ratio
12
Residual factor
% CPC recovery
98
From the present results, both increases in tray spacing and foam height can provide more liquid film drainage which is an important mechanism for a multistage foam fractionation system to achieve both high enrichment ratio and separation factor of the surfactant. To operate a multistage foam fractionation column, tray spacing should be matched with the ability of surfactant solution to generate foam with sufficient stability to reach an upper tray. Moreover, a highest possible foam height which the generated foam can pass through the foam exit without foam collapse should be used to produce dry foams, leading to both a high enrichment ratio and a high recovery of surfactant in multistage foam fractionation.
14
a
80 60
0.06
0.02
30
60 Foam height (cm)
40
90
Fig. 3. (a and b) Effect of foam height on different separation performance parameters at two tray spacings (15 and 30 cm) of the multistage foam fractionation unit (conditions: feed flow rate = 50 cm3 /min; air flow rate = 60 dm3 /min; feed CPC concentration = 0.75 CMC; number of bubble caps per tray = 8; and number of trays = 5).
increasing tray spacing results from increasing liquid film drainage caused by an increasing foam residence time. The results are verified by the decrease in foamate volumetric ratio with increasing tray spacing (see Fig. 3b). As shown in Fig. 3, both recovery and residual factor of CPC show insignificant change with increasing foam height at any given tray spacing. The results can be explained by the fact that with increasing foam height, an increase in liquid film drainage leads to decreasing both the volumetric flow rate of the foamate and the molar flow rate of CPC in the foamate. The CPC recovery is expected to decrease with increasing foam height. However, the CPC recovery was found to decrease slightly with increasing foam height instead. The results can be explained in that the draining liquid contained an insignificant of amount of CPC while the maximum adsorption density of CPC at air–water interface was achieved under the studied conditions with very low initial concentration about 0.75 CMC or 0.675 mM [7]. For any given tray spacing, the insignificant increase in residual factor of CPC with increasing foam height also suggests that, under the studied conditions, the system could be operated with high foam stability without a significant fraction of foam collapse and most CPC molecules were carried out by the adsorptive transport. However, with increasing tray spacing from 15 cm to 30 cm, the CPC recovery decreased and the residual factor of CPC increased substantially. The results suggest that the system was operated under the existence of foam collapse. The foam collapse can happen if the generated foam has insufficient stability to experience bubble coalescence within the rising foam before passing through the bubble cap of the upper tray because a longer liquid film drainage time can reduce foam stability, especially at a low CPC concentration in liquid pool of the lower tray.
The effect of the number of bubble caps per tray was investigated at different air and feed flow rates under the conditions: a feed concentration of 25% CMC; tray spacing of 15 cm; foam height of 30 cm; and number of trays of 5. The impact of the number of bubble caps per tray on the process separation performance of the multistage foam fractionation unit is shown in Figs. 4 and 5. It can be clearly seen that the column with 22 bubble caps per tray showed a significantly higher separation efficiency in terms of enrichment ratio, recovery, residual factor, and separation factor of CPC at any given air and feed flow rates. For any given air and feed flow rates, the foamate volumetric ratio for the column with 22 bubble caps per tray was lower than that of the column with 8 bubble caps per tray, suggesting that the increase in the number of bubble caps decreases the liquid hold-up in the foam, resulting in drier foams and thus an increase in the enrichment ratio of CPC. This is, probably, because gas velocity is decreased with increasing the number of caps at given air and liquid flow rates, reducing the amount of water entrained by the bubbles. Both separation factor and recovery of CPC were found to increase whereas the residual factor of CPC was found to decrease with an increasing number of bubble caps per tray. This can be explained by the fact that an increase in the number of bubble caps per tray simply increases the masstransfer surface area of the generated foam in each tray as well as allows more foam to pass through from a lower tray to an upper tray, leading to increasing adsorptive transport. Fig. 4 shows the separation process performance of CPC as a function of air flow rate. With increasing air flow rate, the CPC recovery and the foamate volumetric ratio increased whereas the enrichment ratio and the residual factor of CPC decreased for both numbers of bubble caps per tray. Interestingly, for the studied column with 22 bubble caps per tray, the decrease in the residual factor without a significant increase in the foamate volumetric ratio caused the separation factor to reach a maximum at an air flow rate of 80 dm3 /min (Fig. 4). These results suggest that the studied column with 22 bubble caps per tray has high ability to promote adsorptive transport with increasing air flow rate without significant increase in bulk liquid transport, especially at low air flow rates (lower than 80 dm3 /min). For the column with 8 bubble caps per tray, the separation factor and residual factor of CPC decreased whereas the foamate volumetric ratio increased substantially with increasing air flow rate up to 80 dm3 /min. These results suggest that both adsorptive transport and bulk liquid transport increase with increasing air flow rate but the bulk liquid transport tends to be promoted more substantially. The results presented above suggest that the studied foam fractionation column with higher number of bubble caps per tray shows higher ability to increase adsorptive transport and reduce liquid hold-up in foam with increasing air flow rate. Hence, the column with higher number of bubble caps per tray can provide more increasing adsorptive transport and less increasing bulk liquid transport with increasing air flow rate. The role of the increased number of bubble caps per tray is thus
V. Rujirawanich et al. / Chemical Engineering and Processing 52 (2012) 41–46
Fig. 4. Effect of air flow rate on different separation performance parameters of the multistage foam fractionation unit at two numbers of bubble caps per tray (conditions: feed flow rate = 50 cm3 /min; foam height = 30 cm; tray spacing = 15 cm; feed CPC concentration = 0.25 CMC; and number of trays = 5).
to increase the rate of foam production in each tray with a small foamate volumetric ratio and to allow for the passing of foam from a lower tray to a higher tray. As a result, the increasing rate of foam production simply increases the mass-transfer surface area, thus leading to increased adsorptive transport.
45
Fig. 5. Effect of feed flow rate on different separation performance parameters of the multistage foam fractionation unit at two numbers of bubble caps per tray (conditions: air flow rate = 60 dm3 /min; foam height = 30 cm; tray spacing = 15 cm; feed CPC concentration = 0.25 CMC; and number of trays = 5).
As shown in Fig. 5, with increasing feed flow rate, for the column with 8 bubble caps per tray, CPC recovery decreases; but the residual factor of CPC increases. In the case of the column with 22 caps per tray, both the recovery and the residual factor of CPC remained nearly unchanged in the studied range of feed flow rate. These results suggest that the column with higher number of bubble caps
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per tray can separate the CPC more effectively, especially at a higher increasing CPC input rate. Even at a higher CPC input rate (feed flow rate of 80 cm3 /min), the column with higher number of bubble caps per tray can keep the effluent CPC concentration the same as that at a lower CPC input rate (feed flow rate of 25 cm3 /min), contrasting to the column with lower number of bubble caps per tray. These results indicate that an increase in bubble caps per tray can provide more mass-transfer surface area and increasing adsorptive transport. To obtain a better understanding, our next paper will present the interstate data to indicate the tray efficiency as a function of the design parameters. 4. Conclusions An increase in the number of bubble caps per tray results in a drier foam. Hence, the column with 22 caps per tray provided higher values for all process separation performance parameters (enrichment ratio, recovery, separation factor, and residual factor) than that with 8 bubble caps per tray. A highest possible foam height which the generated foam can pass through the foam exit without foam collapse should be used to produce dry foams with high foam stability, leading to both a high enrichment ratio and a high recovery of surfactant in multistage foam fractionation. Tray spacing should be matched with the ability of surfactant solution to generate foam. Acknowledgments The Thailand Research Fund is acknowledged for providing a Royal Golden Jubilee PhD scholarship (Grant no. PHD/0059/2550) to the first author and an Advanced Research Scholar Grant (BRG5080028) to second corresponding author. The Research Unit of Applied Surfactants for Separation and Pollution Control, under the Ratchadapisak Sompoch Fund, Chulalongkorn University is also acknowledged. References [1] B. Brackmann, C.-D. Hager, The statistical world of raw materials, fatty alcohols and surfactants, in: Proceedings of the 6th World Surfactant Congress CESIO, Berlin, Germany, 2004. [2] C. Yapijakis, L.K. Wang, Treatment of soap and detergent, in: L.K. Wang, Y. Hung, H.H. Lo, C. Yapijakis (Eds.), Waste Treatment in the Process Industries, 1st ed., CRC Press, New York, 2005, pp. 307–362. [3] A. Ahmad, S. Puasa, Reactive dyes decolourization from an aqueous solution by combined coagulation/micellar-enhanced ultrafiltration process, Chem. Eng. J. 132 (2007) 257–265.
[4] C.N. Mulligan, R.N. Yong, B.F. Gibbs, Surfactant-enhanced remediation of contaminated soil: a review, Eng. Geol. 60 (2001) 371–380. [5] M.E. Prudich, Alternative solid/liquid separations, in: R.H. Perry, D.W. Green (Eds.), Perry’s Chemical Engineers’ Handbook, Eighth, McGraw-Hill Professional, New York, 2007. [6] C. Wong, M. Hossain, C. Davies, Performance of a continuous foam separation column as a function of process variables, Bioprocess Biosyst. Eng. 24 (2001) 73–81. [7] M.J. Rosen, Surfactants and Interfacial Phenomena, 3rd ed., Wiley-Interscience, Hoboken, NJ, 2004. [8] L. Du, V. Loha, R. Tanner, Modeling a protein foam fractionation process, Appl. Biochem. Biotechnol. 84–86 (2000) 1087–1099. [9] R. Konduru, Operating a foam fractionating column in simple mode, J. Chem. Eng. Jpn. 25 (1992) 548–554. [10] R. Lemlich, Adsorptive Bubble Separation Techniques, Academic Press Inc., New York, 1972. [11] C. Lockwood, P. Bummer, M. Jay, Purification of proteins using foam fractionation, Pharm. Res. 14 (1997) 1511–1515. [12] D. Linke, H. Zorn, B. Gerken, H. Parlar, R.G. Berger, Laccase isolation by foam fractionation—new prospects of an old process, Enzyme Microbial Technol. 40 (2007) 273–277. [13] K. Yamagiwa, M. Iwabuchi, H. Tsubomizu, M. Yoshida, A. Ohkawa, Effect of external reflux on foam fractionation of poly(vinyl alcohol), J. Chem. Eng. Jpn. 34 (2001) 126–131. [14] C. Brett Neely, J. Eiamwat, L. Du, V. Loha, A. Prokop, R. Tanner, Modeling a batch foam fractionation process, Biologia 56 (2001) 583–589. [15] V. Loha, A. Prokop, L. Du, R. Tanner, Preserving the activity of cellulase in a batch foam fractionation process, Appl. Biochem. Biotechnol. 77–79 (1999) 701–712. [16] F. Uraizee, G. Narsimhan, Effect of coalescence on the performance of a continuous foam fractionation column, Sep. Sci. Technol. 27 (1992) 937–953. [17] N. Tharapiwattananon, J. Scamehorn, S. Osuwan, J. Harwell, K. Haller, Surfactant recovery from water using foam fractionation, Sep. Sci. Technol. 31 (1996) 1233–1258. [18] P. Stevenson, G.J. Jameson, Modelling continuous foam fractionation with reflux, Chem. Eng. Process. 46 (2007) 1286–1291. [19] B.M. Gerken, A. Nicolai, D. Linke, H. Zorn, R.G. Berger, H. Parlar, Effective enrichment and recovery of laccase C using continuous foam fractionation, Sep. Purif. Technol. 49 (2006) 291–294. [20] R.C. Darton, S. Supino, K.J. Sweeting, Development of a multistaged foam fractionation column, Chem. Eng. Process. 43 (2004) 477–482. [21] P. Martin, H. Dutton, J. Winterburn, S. Baker, A. Russell, Foam fractionation with reflux, Chem. Eng. Sci. 65 (2010) 3825–3835. [22] S. Boonyasuwat, S. Chavadej, P. Malakul, J.F. Scamehorn, Surfactant recovery from water using a multistage foam fractionator. Part I. Effects of air flow rate, foam height, feed flow rate and number of stages, Sep. Sci. Technol. 40 (2005) 1835–1853. [23] S. Boonyasuwat, S. Chavadej, P. Malakul, J.F. Scamehorn, Anionic and cationic surfactant recovery from water using a multistage foam fractionator, Chem. Eng. J. 93 (2003) 241–252. [24] H. Tsubomizu, R. Horikoshi, K. Yamagiwa, K. Takahashi, M. Yoshida, A. Ohkawa, Effect of perforated plate on concentration of poly(vinyl alcohol) by foam fractionation with external reflux, J. Chem. Eng. Jpn. 36 (2003) 1107–1110. [25] V. Rujirawanich, S. Chavadej, J.H. O’Haver, R. Rujiravanit, Removal of trace Cd2+ using continuous multistage ion foam fractionation. Part I. The effect of feed SDS/Cd molar ratio, J. Hazard. Mater. 182 (2010) 812–819.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Recovery of surfactant from an aqueous solution using continuous multistage foam fractionation: Influence of design parameters Visarut Rujirawanich a , Nopparat Chuyingsakultip a , Manutchanok Triroj a , Pomthong Malakul a,b,∗ , Sumaeth Chavadej a,b,∗ a b
The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand
a r t i c l e
i n f o
Article history: Received 10 March 2011 Received in revised form 16 September 2011 Accepted 4 December 2011 Available online 13 December 2011 Keywords: Foam fractionation Surfactant recovery Cetylpyridinium chloride Bubble caps
a b s t r a c t A multistage foam fractionation column with bubble cap trays was used to recover a surfactant from water at low concentrations. The effects of design parameters—including the number of bubble caps, foam height, and tray spacing—were first investigated under steady state conditions using cetylpyridinium chloride (CPC) as the model surfactant. An increase in bubble caps per tray significantly increased the separation efficiency, both in terms of the enrichment ratio and recovery of the CPC and of the separation factor (ratio of foamate concentration to effluent concentration). The increase in bubble caps per tray also increased the foam production rate, leading to increasing the adsorptive transport. An increase in tray spacing increased both the enrichment ratio and the residual factor of the CPC, whereas the CPC recovery and liquid entrainment in foam were reduced. An increase in foam height produces drier foams, leading to decreasing bulk liquid transport. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The total amount of surfactants, not including soaps, consumed around the world in 2003 was about 9.2 million tons [1]. Hence, a large number of industrial plants generate wastewater containing significant amounts of surfactants, which can directly impact both existing wastewater treatment plants as well as natural receiving waters. Surfactants can retain their foaming properties in natural waters in concentrations as low as 1 mg/L, leading to some environmental problems [2]. Apart from the pollution point of view, it is worthwhile to recover the surfactants for reuse, which, in turn, would make surfactant-based separation processes—such as micellar-enhanced ultrafiltration (MEUF) and surfactant-enhanced remediation of contaminated soil—become more economically feasible [3,4]. Foam fractionation is a surfactant adsorptive process that can be used to concentrate and usually remove dissolved materials, including surfactants, from homogeneous solutions [5]. The process offers several advantages for the treatment of industrial wastewaters with low solute concentrations compared to other treatment
∗ Corresponding authors at: The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand. Tel.: +662 2184139; fax: +662 2184139. E-mail addresses: [email protected] (P. Malakul), [email protected] (S. Chavadej). 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.12.002
processes, including: low space and energy requirements; simple plant design, operation, and scale-up; and low capital and operating costs [6]. Unlike biological treatment, it also allows for the immediate reuse of both water and surfactants. In a foam fractionation operation, air is introduced into the system to generate air bubbles where the surfactant will adsorb preferentially at the air–liquid interface of the air bubbles. The bubbles proceed upwards to the top of the column to produce foam. This foam layer (froth) can be separated physically from the bulk liquid phase, resulting in the removal of the surfactant and any other soluble solutes that adsorb preferentially at the foam surface. Only a small volume of entrained liquid is generally carried with the foam due to liquid film drainage. After the foam is collapsed, the collapsed foam solution (foamate) contains the surface active solute at a much higher concentration than that found in the feed because surfactant molecules adsorb preferentially at the air–water interface of the produced foam whereas entrained liquid, containing much less surfactant molecules, is drained out from the produced foam. Unlike other separation processes, foam fractionation has proven high separation efficiencies—especially at low surfactant concentrations [6]—because maximum equilibrium adsorption density of typical surfactant can be obtained in millimolar level [7]. For kinetic aspect of foam fractionation, the short residence time of air bubbles in the column does not significantly affect the separation performance of the foam fractionation because both the transport rates by diffusion and the convection of the
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surfactant from the bulk liquid phase to the air–water interface of the produced bubble are relatively fast [8]. Therefore, a wastewater—which typically contains very low surfactant concentrations, around or below the critical micelle concentration (CMC)—can be treated to separate the surfactants physically by using foam fractionation. Foam fractionation may be operated in simple mode (batchwise or continuous), or in a more complex mode with enriching and/or stripping [5,9,10]. Foam fractionation columns can also be classified as either single-stage or multistage. A number of studies have investigated the effects of operational parameters of the foam fractionation on the separation efficiency of proteins [11,12]. Most previous studies used either batch mode in single-stage flotation columns [13–15] or simple continuous mode [16–19], whereas the use of multistage pilot plants with continuous mode operation has rarely been reported [20]. Material balance of CPC in the collapsed foam at the top of column, which is the molar flow rate of foamate (Cf Vf ), is equal to the sum of the mass transfer by the bulk liquid and the adsorptive transports [20]: Cf Vf = Ce Vf + A ,
(1)
where Vf is the volumetric flow rate of collapsed foam (foamate) (dm3 /min), Ce is the CPC molar concentration in the lamellae liquid assumed to be equal to that in effluent, Cf is the CPC molar concentration in the collapsed foam, A is the flow rate of the interfacial area of the generated foam (m2 /min), and is the surface excess concentration or adsorption density (mol/m2 ). The term A (mol/min) indicates the amount of CPC adsorbed on the bubble surface, known as adsorptive transport whereas the Ce Vf (mol/min) is the product of the CPC concentration and the liquid volume in the foam lamella, known as bulk liquid transport. The increasing bulk liquid flow rate not only increases the molar flow rate of CPC in foamate but also causes a dilution of adsorbed component which results in a decrease in the enrichment of CPC (concentration in foamate/concentration in feed). The adsorptive transport is responsible for the reduction in the residual factor of CPC (concentration in effluent/concentration in feed) whereas the bulk liquid transport does not reduce the residual factor of CPC if the concentration of the component in the lamellae liquid is not significantly larger than that in the effluent (i.e. single stage foam fractionation) but it does in multistage foam fractionation due to the fact that material enrichment between stages increase concentration of the component in liquid pool of the upper tray. Multistage foam fractionation with bubble caps is basically analogous to a distillation unit. For the ideal distillation unit, a vapor phase is in equilibrium with an aqueous phase in each tray, while a foam phase is in equilibrium with an aqueous phase for the foam fractionation system, as illustrated in Fig. 1a. In multistage foam fractionation with feed position on the top tray (Fig. 1b), surfactant molecules found in a lower tray derived from draining liquid (containing residual surfactant) via downcomer can be recovered back to the top tray. The rising foam from the lower tray will pass the bubble caps of the upper tray, resulting in increasing surfactant concentration in the liquid pool of the upper tray. Thus, the effluent surfactant concentration becomes very low while most of surfactant molecules will be carried upward by rising foam to the top tray. Internal bubble coalescence within the rising foam before passing through the bubble caps is also likely to occur due to sudden change in flow cross sectional area, resulting in increasing internal reflux and enhanced enrichment ratio [21]. Hence, the more trays (or stages) there are, the higher the surfactant concentration in the top stage and the foamate [22]. Our previous study [22,23] demonstrated that the multistage foam fractionation system could provide enrichment ratio up to 240 which is much higher than that of a previous work [17] in single stage, 21.5 in enrichment ratio. Moreover,
Fig. 1. Schematic of bubble caps tray (a) and a multistage foam fractionation unit (b).
it demonstrated that a multistage foam fractionation column with bubble caps could be operated without the problems of excessive pressure drop and flooding. In this present study, a continuous multistage foam fractionator with bubble caps was tested to remove a cationic surfactant from water. The study focused on an analysis of the effects of design parameters (tray spacing and number of bubble caps) on the overall performance of the multistage foam fractionation. In addition, the effects of air flow rate and feed flow rate were also investigated at two different numbers of bubble caps per trays. 2. Experimental 2.1. Materials and methods The surfactant used in this study was cetylpyridinium chloride (CPC), molecular weight of 339.99 g/mol, obtained from Zealand Chemical with >99% purity. The surfactant was used as received without further purification. Distilled water was used in all experiments. The multistage foam fractionation apparatus used in this study (Fig. 1b) had two multistage foam fractionation columns that were made of an acrylic cylinder with an inner diameter of 18 cm but having two different tray spacings (15 cm and 30 cm), which could be assembled to 5 trays. At the top tray, the foam column, which was
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43
Downcomer Bubble cap Ø = 4 cm
Ø = 4 cm
Ø = 1 cm
Ø = 1 cm
Ø= 4 cm
Ø= 4 cm
I.D. Ø= 18 cm
I.D. Ø= 18 cm
a) 22 bubble caps/tray
b) 8 bubble caps/tray
Fig. 2. Positions and dimensions of bubble caps and downcomer: 8 caps/tray (a) and 22 caps/tray (b).
made of an acrylic cylinder with the same inner diameter, had three foam outlets to obtain different foam heights of 30, 60, and 90 cm. Each tray had either 8 or 22 bubble caps with a weir height of 6 cm and a cap diameter of 2 cm. The clearance between the lower edge of bubble cap and each tray was equal to 1.5 cm. The downcomer, having a diameter of 4 cm with a weir height of 3.0 cm was used to control the liquid level in each tray and to allow the liquid to flow down to a lower tray (Fig. 2 shows the configurations and dimensions of the bubble caps and downcomer). A sample port was located at the base of each tray for taking liquid samples. A surfactant solution was prepared at different CPC concentrations of 0.25, 0.5, and 0.75 times the CMC and was continuously fed into the top of the column at a constant flow rate that was varied from 0.98 to 3.93 kg/m2 /min (25–100 cm3 /min) using a peristaltic pump. The pressurized air from an air compressor was regulated to have any desired flow rate in the range of 1.96–6.55 cm/s (30–100 dm3 /min) using a rotameter and it was introduced below the bottom tray. The generated foam at the top of the column was collected at three different foam heights of 30, 60, and 90 cm. The foam was collected, frozen, thawed, and then measured to get the collapsed foam (foamate) volume at room temperature (25–27 ◦ C). The time needed to establish steady state was found to be approximately 7 h, and the steady state time was justified by all measured parameters invariant with time. After steady state was achieved, samples of the feed solution, the foamate, and the effluent were collected and analyzed for surfactant concentration. The CPC concentrations were measured by UV/visible spectrophotometry (Perkin Elmer, Lambda 10) at a wavelength of 260 nm. For each run under steady state condition, the samples were taken at least a few times and the data were averaged and used to assess the process separation performance. The foam fractionation column was thoroughly cleaned with distilled water before starting the next experiment. Overall material balance calculations were performed to check the validity of the experimental data. The surfactant mass balance showed less than 10% error for all runs. The process separation performance of the foam fractionation column was assessed by using the enrichment ratio of CPC, the residual factor of CPC, the separation factor of CPC, and the recovery of CPC, and the foamate volumetric ratio [24,25], described as follows: Enrichment ratio =
% CPC recovery =
Cf Ci
,
Vi Ci − Ve Ce × 100, Vi Ci
(2)
(3)
Residual factor =
Ce , Ci
(4)
Foamate volumetric ratio = Separation factor =
Cf Ce
Vf Vi
,
,
(5) (6)
where Ce , Cf , and Ci are the CPC concentrations in the effluent solution, the foamate (collapsed foam solution), and the influent (feed), respectively. Ve , Vi , and Vf are the volumetric flow rate of effluent, feed, and foamate, respectively. 2.2. Surface tension measurement Measurement of the surface tension of surfactant solutions at different CPC concentrations was carried out by using a drop shape analysis instrument (Krüss, DSA 10) at room temperature (25–27 ◦ C). 3. Results and discussion 3.1. The CMC of CPC From the plot of surface tension versus the log of the initial CPC concentration, the critical micelle concentration (CMC) and the surface tension at the CMC ( cmc ) can be determined from the reflection point in a –log C curve. The CMC and cmc were found to be 0.9 mM and 42.6 mN/m, respectively. Feed CPC concentrations of 0.25, 0.5, and 0.75 times the CMC, corresponding to 0.225, 0.450, and 0.675 mM, respectively, were used to operate the multistage foam fractionation unit in this study. 3.2. Effects of tray spacing and foam height As mentioned above, both tray spacing and foam height were found to be the main factors affecting the process separation performance of the studied multistage foam fractionation unit. As shown in Fig. 3, either increase in tray spacing or foam height increases the enrichment ratio of CPC but decreases the foamate volumetric ratio. With increasing foam height, the increasing enrichment ratio of CPC results from the longer foam residence time, causing more liquid film drainage, leading to drier foam flowing out from the column, as evidenced experimentally by the reduction of the foamate volumetric ratio. From the tray spacing results, it can be explained by the fact that the decreasing amount of lamellae liquid containing in rising foam passing the bubble caps of an upper tray with
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100
96
10
94
8
92
6
90
4
Tray spacing= 30 cm Tray spacing= 15 cm
88
2
86
0
60 Foam height (cm)
0.12
0.24
b
240
Separation Residual
Foamate volumetric ratio
3.3. Effect of the number of bubble caps per tray, air flow rate, and feed flow rate
90
0.20 0.18 0.16
220 0.10
180
Tray spacing = 15 cm Tray spacing = 30 cm
0.08
Separation
0.06
Residual
0.04
0.14 0.12 0.10
200
0.08
160 140 120 100
Separation factor
30
0.22
Enrichment ratio
12
Residual factor
% CPC recovery
98
From the present results, both increases in tray spacing and foam height can provide more liquid film drainage which is an important mechanism for a multistage foam fractionation system to achieve both high enrichment ratio and separation factor of the surfactant. To operate a multistage foam fractionation column, tray spacing should be matched with the ability of surfactant solution to generate foam with sufficient stability to reach an upper tray. Moreover, a highest possible foam height which the generated foam can pass through the foam exit without foam collapse should be used to produce dry foams, leading to both a high enrichment ratio and a high recovery of surfactant in multistage foam fractionation.
14
a
80 60
0.06
0.02
30
60 Foam height (cm)
40
90
Fig. 3. (a and b) Effect of foam height on different separation performance parameters at two tray spacings (15 and 30 cm) of the multistage foam fractionation unit (conditions: feed flow rate = 50 cm3 /min; air flow rate = 60 dm3 /min; feed CPC concentration = 0.75 CMC; number of bubble caps per tray = 8; and number of trays = 5).
increasing tray spacing results from increasing liquid film drainage caused by an increasing foam residence time. The results are verified by the decrease in foamate volumetric ratio with increasing tray spacing (see Fig. 3b). As shown in Fig. 3, both recovery and residual factor of CPC show insignificant change with increasing foam height at any given tray spacing. The results can be explained by the fact that with increasing foam height, an increase in liquid film drainage leads to decreasing both the volumetric flow rate of the foamate and the molar flow rate of CPC in the foamate. The CPC recovery is expected to decrease with increasing foam height. However, the CPC recovery was found to decrease slightly with increasing foam height instead. The results can be explained in that the draining liquid contained an insignificant of amount of CPC while the maximum adsorption density of CPC at air–water interface was achieved under the studied conditions with very low initial concentration about 0.75 CMC or 0.675 mM [7]. For any given tray spacing, the insignificant increase in residual factor of CPC with increasing foam height also suggests that, under the studied conditions, the system could be operated with high foam stability without a significant fraction of foam collapse and most CPC molecules were carried out by the adsorptive transport. However, with increasing tray spacing from 15 cm to 30 cm, the CPC recovery decreased and the residual factor of CPC increased substantially. The results suggest that the system was operated under the existence of foam collapse. The foam collapse can happen if the generated foam has insufficient stability to experience bubble coalescence within the rising foam before passing through the bubble cap of the upper tray because a longer liquid film drainage time can reduce foam stability, especially at a low CPC concentration in liquid pool of the lower tray.
The effect of the number of bubble caps per tray was investigated at different air and feed flow rates under the conditions: a feed concentration of 25% CMC; tray spacing of 15 cm; foam height of 30 cm; and number of trays of 5. The impact of the number of bubble caps per tray on the process separation performance of the multistage foam fractionation unit is shown in Figs. 4 and 5. It can be clearly seen that the column with 22 bubble caps per tray showed a significantly higher separation efficiency in terms of enrichment ratio, recovery, residual factor, and separation factor of CPC at any given air and feed flow rates. For any given air and feed flow rates, the foamate volumetric ratio for the column with 22 bubble caps per tray was lower than that of the column with 8 bubble caps per tray, suggesting that the increase in the number of bubble caps decreases the liquid hold-up in the foam, resulting in drier foams and thus an increase in the enrichment ratio of CPC. This is, probably, because gas velocity is decreased with increasing the number of caps at given air and liquid flow rates, reducing the amount of water entrained by the bubbles. Both separation factor and recovery of CPC were found to increase whereas the residual factor of CPC was found to decrease with an increasing number of bubble caps per tray. This can be explained by the fact that an increase in the number of bubble caps per tray simply increases the masstransfer surface area of the generated foam in each tray as well as allows more foam to pass through from a lower tray to an upper tray, leading to increasing adsorptive transport. Fig. 4 shows the separation process performance of CPC as a function of air flow rate. With increasing air flow rate, the CPC recovery and the foamate volumetric ratio increased whereas the enrichment ratio and the residual factor of CPC decreased for both numbers of bubble caps per tray. Interestingly, for the studied column with 22 bubble caps per tray, the decrease in the residual factor without a significant increase in the foamate volumetric ratio caused the separation factor to reach a maximum at an air flow rate of 80 dm3 /min (Fig. 4). These results suggest that the studied column with 22 bubble caps per tray has high ability to promote adsorptive transport with increasing air flow rate without significant increase in bulk liquid transport, especially at low air flow rates (lower than 80 dm3 /min). For the column with 8 bubble caps per tray, the separation factor and residual factor of CPC decreased whereas the foamate volumetric ratio increased substantially with increasing air flow rate up to 80 dm3 /min. These results suggest that both adsorptive transport and bulk liquid transport increase with increasing air flow rate but the bulk liquid transport tends to be promoted more substantially. The results presented above suggest that the studied foam fractionation column with higher number of bubble caps per tray shows higher ability to increase adsorptive transport and reduce liquid hold-up in foam with increasing air flow rate. Hence, the column with higher number of bubble caps per tray can provide more increasing adsorptive transport and less increasing bulk liquid transport with increasing air flow rate. The role of the increased number of bubble caps per tray is thus
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Fig. 4. Effect of air flow rate on different separation performance parameters of the multistage foam fractionation unit at two numbers of bubble caps per tray (conditions: feed flow rate = 50 cm3 /min; foam height = 30 cm; tray spacing = 15 cm; feed CPC concentration = 0.25 CMC; and number of trays = 5).
to increase the rate of foam production in each tray with a small foamate volumetric ratio and to allow for the passing of foam from a lower tray to a higher tray. As a result, the increasing rate of foam production simply increases the mass-transfer surface area, thus leading to increased adsorptive transport.
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Fig. 5. Effect of feed flow rate on different separation performance parameters of the multistage foam fractionation unit at two numbers of bubble caps per tray (conditions: air flow rate = 60 dm3 /min; foam height = 30 cm; tray spacing = 15 cm; feed CPC concentration = 0.25 CMC; and number of trays = 5).
As shown in Fig. 5, with increasing feed flow rate, for the column with 8 bubble caps per tray, CPC recovery decreases; but the residual factor of CPC increases. In the case of the column with 22 caps per tray, both the recovery and the residual factor of CPC remained nearly unchanged in the studied range of feed flow rate. These results suggest that the column with higher number of bubble caps
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per tray can separate the CPC more effectively, especially at a higher increasing CPC input rate. Even at a higher CPC input rate (feed flow rate of 80 cm3 /min), the column with higher number of bubble caps per tray can keep the effluent CPC concentration the same as that at a lower CPC input rate (feed flow rate of 25 cm3 /min), contrasting to the column with lower number of bubble caps per tray. These results indicate that an increase in bubble caps per tray can provide more mass-transfer surface area and increasing adsorptive transport. To obtain a better understanding, our next paper will present the interstate data to indicate the tray efficiency as a function of the design parameters. 4. Conclusions An increase in the number of bubble caps per tray results in a drier foam. Hence, the column with 22 caps per tray provided higher values for all process separation performance parameters (enrichment ratio, recovery, separation factor, and residual factor) than that with 8 bubble caps per tray. A highest possible foam height which the generated foam can pass through the foam exit without foam collapse should be used to produce dry foams with high foam stability, leading to both a high enrichment ratio and a high recovery of surfactant in multistage foam fractionation. Tray spacing should be matched with the ability of surfactant solution to generate foam. Acknowledgments The Thailand Research Fund is acknowledged for providing a Royal Golden Jubilee PhD scholarship (Grant no. PHD/0059/2550) to the first author and an Advanced Research Scholar Grant (BRG5080028) to second corresponding author. The Research Unit of Applied Surfactants for Separation and Pollution Control, under the Ratchadapisak Sompoch Fund, Chulalongkorn University is also acknowledged. References [1] B. Brackmann, C.-D. Hager, The statistical world of raw materials, fatty alcohols and surfactants, in: Proceedings of the 6th World Surfactant Congress CESIO, Berlin, Germany, 2004. [2] C. Yapijakis, L.K. Wang, Treatment of soap and detergent, in: L.K. Wang, Y. Hung, H.H. Lo, C. Yapijakis (Eds.), Waste Treatment in the Process Industries, 1st ed., CRC Press, New York, 2005, pp. 307–362. [3] A. Ahmad, S. Puasa, Reactive dyes decolourization from an aqueous solution by combined coagulation/micellar-enhanced ultrafiltration process, Chem. Eng. J. 132 (2007) 257–265.
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