Ind. Eng. Chem. Res. 2008, 47, 1253-1260
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Influence of Surface Tension on Effective Packing Area Robert E. Tsai, Peter Schultheiss, Andreas Kettner, J. Christopher Lewis, A. Frank Seibert, R. Bruce Eldridge,* and Gary T. Rochelle Department of Chemical Engineering, The Separations Research Program, The UniVersity of Texas at Austin, Austin, Texas 78712-1062
Effective packing areas of Sulzer Mellapak 250Y and 500Y structured packings were determined in a 0.46-m OD packed column. A chemical method (absorption of CO2 from air using 0.1 kmol/m3 NaOH) was employed for the measurements. The packing performance was reported as fractional area, defined as effective area normalized by specific packing area. Under high surface tension conditions (∼72 mN/m), the fractional area decreased by approximately a factor of 2 in going from the 250Y to the 500Y packing. When similar tests were conducted at reduced surface tensions (∼35 mN/m, achieved by the addition of surfactant), the fractional area of the 250Y was unchanged, whereas that of the 500Y increased by 50% relative to the base case. The results indicate that, at high surface tension, access to the surface of the 500Y packing was being inhibited. Lowering the surface tension served to maximize the effective area of the packing. 1. Introduction Packed columns are commonly used in distillation, absorption, and stripping processes as a means of promoting efficient contact between gases and liquids. For design and analysis purposes, the development of reliable mass transfer models is clearly necessary. Numerous empirical or semiempirical packing area correlations have been presented in the literature, but none have been shown to be truly predictive over a wide range of conditions. Wang et al.1 provided a comprehensive review of these models. The correlations predict substantially different effects of physical parameters such as viscosity and surface tension on the effective interfacial area (ae), indicating the role of these properties in the “creation” of area is not well understood. A lack of understanding of the fundamental fluid mechanics and mass transfer phenomena occurring in packed columns has been noted as a general weakness associated with the correlations, and the need for experiments over a broader range of conditions has been identified.1 To address these issues, the influence of individual parameters on effective packing area is being determined through direct measurements. In this work, the effect of surface tension on structured packing areas was examined. Surfactants were chosen as a means of simulating low, organic-like surface tension conditions (∼35 mN/m). The surfactant selection rationale included the small concentrations required and the limited interference with the caustic reaction mechanism used for packing area characterization. Packing interfacial areas have been the subject of many publications in the literature. Nevertheless, the number of studies that have focused on the relationship between surface tension and effective area (unconfounded by variation of other physical properties) in packed columns is quite sparse. Sedelies et al.2 employed an air-aqueous sodium sulfite system to characterize effective areas of random packing (polypropylene pall rings). To test the effect of surface tension, surfactant (Tween 20) was added to solutions, resulting in surface tension values of 35 mN/ m. Unfortunately, massive foam formation was encountered, which severely limited the feasibility of the reduced surface
tension studies. The data that the authors managed to collect, however, indicated no statistically confirmable influence of surface tension. Nicolaiewsky et al.3 considered the ratio of wetted area to total geometric area to be directly proportional to rivulet width for structured packing. Rivulet dimensions on a variety of surfaces were measured. Lower surface tension solutions (32.4 mN/m, achieved with 50 wt % acetone) resulted in increased spreading, up to 50% greater than in the case of water. The data were subsequently employed in the development of a correlation for rivulet width. Shi and Mersmann4 conducted similar rivulet spreading experiments but presented a limited amount of raw data. Thus, it is difficult to draw conclusions regarding the sole effects of surface tension on area from their work. Chemical absorption, in which a fast chemical reaction is relied on to make mass transfer measurements, is frequently employed to determine effective areas in packed columns. Henriques de Brito et al.5 and Sharma and Danckwerts6 reviewed the large variety of two-fluid systems that have been used for this purpose. Among these, absorption of CO2 into dilute caustic solution (the method employed in this work) can be considered as one of the more attractive options. The kinetics of the system has been extensively studied and characterized,7-12 and from a practical standpoint, the reagents are inexpensive and nonhazardous. Provided that there is appreciable free hydroxide available, the concentration of bicarbonate ion (HCO3-) is negligible, and the overall reaction may be written as:
CO2 (aq) + 2OH- f CO32- + H2O
(1)
The reaction can be considered as practically irreversible, with a rate expression given by eq 2:
r ) kOH-[OH-][CO2]
(2)
When CO2 partial pressures are low and hydroxide ion is present in relative excess, the reaction can be treated as pseudo-firstorder. Equation 2 consequently simplifies to:
r ) k1[CO2]
(3)
2. Experimental Section * To whom correspondence should be addressed. Tel.: (512) 2321407, (512) 471-7067. Fax: (512) 471-7060. E-mail: rbeldr@ che.utexas.edu.
2.1. Packed Column. The packed column was the same apparatus that Seibert et al.13 used to measure the mass transfer
10.1021/ie070780l CCC: $40.75 © 2008 American Chemical Society Published on Web 01/12/2008
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Figure 1. Process flow diagram for the 0.46-m OD packed tower.
areas of various packings. The column had an outside diameter of 0.46 m, an inside diameter of 0.427 m, and a 3-m packed height. The total height of the tower was 7.62 m. The column, piping, and valves were all constructed from PVC. A 30 kW blower with a variable speed motor drive was used to supply ambient air (CO2 source) to the column. Air was introduced below the packing region with the flow rate monitored by an annubar flow meter. A centrifugal pump capable of discharging 0.57 m3/min was used to transport liquid from a 1.3 m3 storage tank located near the base of the column up to the top of the column. Liquid flow rate was regulated with a variable speed drive on the pump and was measured using a Micromotion coriolis meter. The liquid was distributed using a pressurized fractal distributor with 108 drip points/m2 (F10 distributor). Pressure drop was determined using parallel differential pressure cells (DPCs) calibrated at low and high ranges of 0-7.62 cm H2O and 0-63.5 cm H2O, respectively. The CO2 concentration of either the inlet or outlet air was measured using an infrared gas analyzer (Horiba VIA-510) with a reading range of 0-0.05% CO2 by volume and a reproducibility of (0.5% of full scale. The experimental setup is shown in Figure 1. At the start of an experiment, approximately 0.75-1.15 m3 of 0.1 kmol/m3 NaOH solution, prepared by dissolving NaOH pellets (PHARMCO-AAPER, 98.5%) in water, was charged to the storage tank. The solution was mixed between the column sump and storage tank for 45-60 min. Next, for prewetting purposes, the valve to the top of the column was opened, and the solution was allowed to circulate through the packing for 15-30 min. A sample was collected and titrated with 0.1 kmol/ m3 HCl (Ricca Chemical Company, ACS reagent grade) and phenolphthalein indicator (MCB Manufacturing Chemists, Inc.) to verify the NaOH concentration. For the low surface tension studies, surfactant (Tergitol NP-7) and antifoam (Dow Corning Q2-3183A) were added to the solution at this point, and the solution was mixed between the sump and storage tank for an additional 15 min. The pathway through the packing was closed during this time to limit the possibility of foaming. The Horiba analyzer was calibrated with N2 for a zero point and 470-490 ppm CO2/N2 span gas, and the CO2 concentration of the incoming air was measured. The system was then switched to monitor the outlet CO2 concentration, and the experiment was started. The inlet concentration was periodically checked during the experiment. Air velocity was held constant at 1 or 1.5 m/s, and liquid load was incremented, with a given liquid load being maintained until the outlet CO2 concentration reached steady state. Data point collection was limited to the low end of the loading region, where constant holdup was expected, and was typically halted when the pressure drop approached 8.33 cm
Figure 2. Detailed WWC diagram. T: thermocouple, P: pressure gauge.
Figure 3. Process flow diagram for the WWC experiment.
H2O/m packing. Samples for CO2 analysis were obtained from a valve on the column sump. 2.2. Wetted-Wall Column (WWC). The WWC was a gasliquid contactor with an interfacial area of 38.52 cm2 and was previously used by Mshewa,14 Bishnoi,15 and Cullinane16 to measure the kinetics of various CO2-amine systems (Figure 2). The column was constructed from a stainless steel tube of 1.26-cm OD and had an exposed length of 9.1 cm. Liquid was pumped up through the inside, emerging at the top and overflowing down the outside in a smooth, thin film. The column was enclosed in a glass tube (2.54-cm OD), forming the reaction chamber. This chamber was further enclosed in a second thick-walled glass cylinder (10.16-cm OD) in which paraffin oil was circulated to maintain a constant, uniform temperature in the reaction chamber. CO2 concentrations were measured using an infrared gas analyzer (Horiba PIR-2000). The range of the analyzer was adjustable (0-0.05, 0-0.15, or 0-0.25 vol %), with an expected repeatability of (0.5% of full scale. Auxiliary equipment included a liquid pump (ColeParmer Micropump), calibrated Brooks Instrument Series 5850 mass flow controllers for the gas streams, and two temperature baths (Grant Heating Bath, Fisher Scientific Isotemp 3016H). Figure 3 illustrates the WWC experimental setup. Gas was supplied from both a N2 cylinder (supplied by the Cryogenics Laboratory at the University of Texas at Austin) and a 5000
Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1255
ppm CO2/N2 cylinder (Praxair). The mixed gas stream was first saturated via bubbling through a water-filled cylinder. The gas was then introduced at the bottom of the reaction chamber and flowed upward in a countercurrent contacting process. A needle valve located immediately downstream allowed for pressurization of the chamber. The gas traveled through a condenser system consisting of a 500 cm3 Erlenmeyer flask submerged in an ice bath and a glass tube (2.54-cm OD) packed with anhydrous CaSO4 (W.A. Hammond Drierite Co.) before finally entering the Horiba analyzer. CO2 concentration was displayed on a computer as a voltage reading by means of PicoLog software. Solvent was circulated in a closed loop with a rotameter used to monitor the flow rate. An inline septum allowed for the injection of additional liquid into the system, either to purge air during startup or to maintain liquid inventory and prevent gas from entering the liquid line during operation. Samples for CO2 content analysis could also be withdrawn through the septum. In a typical experiment, the 1000 cm3 solution reservoir was filled with 0.1 kmol/m3 NaOH solution (Fisher Scientific, certified grade). The liquid was circulated at a constant rate (2-4 cm3/s), and its temperature was allowed to stabilize before experiments were started. A gas stream with a known CO2 concentration (based on the outputs of the mass flow controllers) was sent directly to the Horiba analyzer by means of the bypass line shown in Figure 3. Once steady state was reached, indicated by a constant voltage reading, the bypass was closed, forcing the gas to flow through the reaction chamber. On achievement of steady state, the system was switched back into bypass mode, and a new inlet CO2 concentration was run. This alternating process was repeated for five different concentrations in a given experiment. CO2 partial pressures ranged from 130 to 600 Pa and were intentionally kept low to minimize interfacial depletion of hydroxide (calculated to be no more than 3%). The bypass mode data points were used to generate a calibration curve relating CO2 concentration to voltage, which was subsequently utilized to interpret the absorption data. The procedure with low surface tension solutions was identical. Surfactant (Tergitol NP-7) and antifoam (Dow Corning Q2-3183A) were first dissolved in the caustic solution before running the experiment. 2.3. Supplementary Equipment. A goniometer (rame´-hart Inc., model #100-00) setup consisting of an adjustable stage, a computer-linked camera for live image display, and a light source was utilized in conjunction with FTA32 Video 2.0 software (developed by First Ten Angstroms, Inc.) to make surface tension measurements via the pendant drop method. The apparatus was also used to obtain contact angle data via the sessile drop technique. For this purpose, surfaces were thoroughly rinsed with 1% Alconox solution, acetone, and distilled water and were dried in a clean oven. After the surfaces were allowed to cool, sessile drop measurements were conducted immediately to minimize the potential for surface contamination. Drops were 5 µL in size, and contact angles were evaluated quickly after deposition before significant evaporation could occur. DROPimage software was employed for the analysis of the drops. To investigate the foaming behavior of solutions, a foam cell and a 10.16-cm ID packed column were used. The foam cell consisted of a 1000 cm3 graduated cylinder and typically contained 200 cm3 of solution. N2 was flowed at a set rate (regulated by a mass flow controller) through tubing with a diffusing stone attached at the outlet. The stone was placed at
the bottom of the cylinder, and foaming intensity was quantified in terms of the ratio of the expanded foam volume to the volumetric gas rate. The 10.16-cm ID column was constructed from segments (60.96, 30.48, or 15.24 cm) of beaded glass. The total height was approximately 2.7 m, with the packing (15-mm Nutter rings) occupying around 1.4 m. Gas and liquid flow were regulated using Micromotion flow meters. The gas (N2) entered the column via a sparging ring about 30 cm above the base. The liquid was introduced into the column through a single point at the top and was recycled through a 0.15 m3 storage tank. Pressure drop was measured with a Rosemount differential pressure transmitter. 2.4. CO2 Content Analysis. To account for bulk hydroxide depletion, samples from the WWC and packed column experiments were analyzed for CO2 content by acidic evolution. The apparatus consisted of four mounted glass tubessa T-shaped evolution chamber with a septum insert, followed by three drying columns packed with anhydrous Mg(ClO4)2 (Acros)s and a Horiba analyzer (same as that used for the WWC). The evolution chamber was filled with a 30 wt % H3PO4 solution, and N2 carrier gas was allowed to flow through. Samples were injected into the acid, resulting in the liberation of any dissolved CO2. By injecting known quantities of a 1000 ppmv Na2CO3 standard solution (Aqua Solutions), a calibration curve could be generated and subsequently used to determine the quantity of CO2 (or equivalently, CO32-) in a sample. On the basis of the CO2-NaOH reaction stoichiometry (eq 1), the OHconcentrations at various points in an experiment could be backcalculated. Bulk OH- depletion was never greater than 5% for a given WWC trial. By the end of an experiment in the packed tower, however, OH- concentration could be depleted by as much as 15-20%. 3. Results and Discussion 3.1. Preliminary Surfactant Screening and Foaming Tests. It was discovered that the 0.46-m OD column could not be operated with surfactant-containing solutions without using an antifoam agent. However, it was observed that the presence of antifoam tended to have a destabilizing effect on liquid films in the WWC. Screening tests of various solutions were conducted using the WWC, foam cell, and 10.16-cm glass column, and WWC film stability, surfactant potency, and foaming intensity were investigated. Test solutions consisted of mixtures of a commercial surfactant (Dowfax C6L, Tergitol TMN-6, Tergitol TMN-100X, Tergitol NP-7, or Triton X-114), antifoam (Dow Corning Q2-3183A), and 0.1 kmol/m3 NaOH. A formulation consisting of a nonionic surfactant (125 ppmv Tergitol NP-7) and antifoam (50 ppmw/v Dow Corning Q23183A antifoam) was identified that yielded a satisfactorily low surface tension (∼35 mN/m) and allowed for a stable film to be maintained in the WWC. In general, hydraulic tests for both systems (high and low surface tension) performed in the glass column displayed similar pressure drop behavior, and minimal foaming in the packed columns was observed. This formulation was used for all low surface tension experiments. 3.2. Theoretical Analysis of Data. The fundamental equation used to interpret the results is presented in eq 4. The overall mass transfer resistance is expressed as a series relationship of the gas and liquid contributions. KG, kg, and kg′, respectively, represent the overall, gas-side, and liquid-side mass transfer coefficients.
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1 1 1 ) + KG kg kg′
(4)
For the WWC, the overall mass transfer coefficient is calculated from the CO2 flux and the partial pressure driving force.
KG )
NCO2 PCO2LM
)
( )
NCO2
ln
P(yCO2in - yCO2out)
yCO2in
yCO2out
( )( ) uGd2 hDCO2,G
0.85
DCO2,G
(6)
RTd
Gas-side resistance was calculated to account for 10-20% of the overall mass transfer resistance in the WWC experiments that were performed in this work. Equations 4-6 are used to calculate kg′, which has been defined as a liquid-side mass transfer coefficient expressed in terms of a CO2 partial pressure driving force.
NCO2 ) kg′(PCO2i - PCO2*) ) kg′PCO2i
(7)
In eq 7, the equilibrium partial pressure of CO2 is zero because of the irreversibility of the CO2-NaOH reaction. The operating conditions of the WWC were selected to ensure that the reaction could be approximated as pseudo-first-order (eq 3). Under such conditions, surface renewal theory may be used to present the flux as:18
x
NCO2 ) kL°
1+
k1DCO2,L PCO2i (kL°)2 HCO2
) kL°x1 + Ha
2
PCO2i HCO2
(8)
The 1 under the square root in eq 8 can be neglected when the Ha number is relatively large. Ha2 was calculated to be around 50 for the worst case scenario in the WWC, which meant that the 1 term could be ignored with minimal error. With this simplification, eq 8 becomes:
NCO2 )
xk1DCO ,LP 2
HCO2
i CO2
log10 DCO2,w ) -8.1764 + log10
xk1DCO ,L ) xkOH [OH 2
HCO2
-
-
]DCO2,L
HCO2
(10)
For comparison, eq 10 can be evaluated using literature values for the diffusivity of CO2 (DCO2,L), the Henry’s constant of CO2 (HCO2), and the reaction rate constant (kOH-). The correlations of Barrett10 and Danckwerts18 can be applied toward the first two parameters:
) Σ Iihi
(13) (14)
7.8857 × 10-5T2 (15) The values of the contributions in eq 14 suggested by Barrett10 are listed in Table 1. In regard to the rate constant, Pohorecki and Moniuk11 thoroughly investigated the kinetics of CO2 absorption into aqueous alkaline solutions. They performed many laminar-jet absorption experiments with aqueous solutions of KOH, NaOH, and LiOH over a range of concentrations (0.1-4.0 kmol/m3) and temperatures (18-41 °C) in the presence of neutral electrolytes (carbonates, chlorides, bromides, nitrates, and sulfates). Using data and the correlations described in eqs 1115, Pohorecki and Moniuk developed the following expressions for the CO2-NaOH rate constant, dependent solely on temperature and ionic strength:
log10
kOHkOH-∞
) 0.221I - 0.016I2
log10 kOH-∞ ) 11.895 -
2382 T
(16) (17)
Their work was found to agree fairly well with previous related investigations.9,10 The above correlations were used throughout this entire study. Equation 4 is also central to the analysis of data gathered using the 0.46-m OD packed column. The gas-side resistance was intentionally limited by using dilute caustic solution (0.1 kmol/m3) and operating at high air velocities (1 or 1.5 m/s). Even under the worst circumstances, this resistance (estimated using the correlation for kg proposed by Rocha et al.19) was accountable for no more than 1.5% of the overall mass transfer resistance. Therefore, gas-side resistance was ignored in the analysis, and KG was assumed to be equal to kg′. This approximation enabled the effective packing area (ae) to be determined by separating it from the volumetric mass transfer coefficient, KGae.
uG ln
k g′ )
HCO2,w
-log10(HCO2,w ) ) 9.1229 - 5.9044 × 10-2T +
(9)
Thus, from eqs 7 and 9, we have the following expression for kg′:
HCO2
(11)
712.5 2.591 × 105 (12) T T2
hi ) h+ + h- + hg
(5)
A gas-side mass transfer coefficient correlation for the WWC was developed by absorption of SO2 into 0.1 kmol/m3 NaOH, an entirely gas-film controlled process.17 Equation 6 is a rearrangement of this correlation, which involved the Sherwood, Reynolds, and Schmidt numbers and the physical dimensions of the system.
kg ) 1.075
(DCO2,L‚µ)T ) (DCO2,w‚µw)T ) const.
ae )
( ) ( ) yCO2in
yCO2out
ZKGRT
uG ln
≈
yCO2in
yCO2out
Zkg′RT
(18)
It should be noted that the same assumptions as discussed previously (pseudo-first-order conditions, large Ha number) were applied so that eq 10 could be used to calculate kg′. The physical liquid-side mass transfer coefficient in the packed column (estimated using the correlation for kL° proposed by Rocha et al.19) was larger than that in the WWC, however, which weakened the large Ha number approximation. Neglecting the 1 under the square root in eq 8 introduced an error of around 6% in the most extreme case (Ha2 ≈ 7).
Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1257
Figure 4. Normalized kg′ data for base case and low surface tension systems. Table 1. Contributions of Cations, Anions, and Gas in Eq 14 h+
h-
hg (CO2)
Na+ 0.091
OH- 0.066 CO32- 0.021
0.2 °C -0.007 15 °C -0.010 25 °C -0.019 40 °C -0.026 50 °C -0.029 60 °C -0.016
3.3. WWC Results. A series of baseline experiments, consisting of the absorption of CO2 into 0.1 kmol/m3 NaOH, were first performed on the WWC to verify the work of Pohorecki and Moniuk. Results were interpreted in terms of normalized kg′, which has been defined as the experimental kg′ value divided by the predicted value using eq 10. Overall, the average normalized kg′ was found to be 1.10 ( 0.08, implying a reasonable agreement between our data and the correlation of Pohorecki and Moniuk. The use of their kinetic expressions in the interpretation of the packed tower results was therefore concluded to be acceptable. The kinetic results of Pohorecki and Moniuk may be slower than ours because their corrections for hydroxide depletion, which were only applied when dealing with the most dilute solutions tested (0.1 kmol/m3), are questionable. An average bulk/interface hydroxide concentration was used in their analysis, rather than the actual interface concentration. Pohorecki and Moniuk estimated bulk-to-interface depletion to be as high as 25% for dilute solutions, so the specified hydroxide concentration would be expected to impact the interpretation of the results. Interestingly, a plot comparing the model with their own kOHdata shows that the lowest ionic strength points tend to be above the predicted value (in agreement with our findings), whereas the data for more concentrated solutions are fit better. As ionic strength decreases, the data also seem to become more scattered, implying that greater error may have been inherently associated with such experimental conditions. For these reasons, it is suspected that the correlation of Pohorecki and Moniuk may be weakest in the dilute region. Experiments with 125 ppmv Tergitol NP-7 and 50 ppmw/v Dow Corning Q2-3183A antifoam dissolved in 0.1 kmol/m3 NaOH solution were performed to determine the impact of these additives on the CO2-NaOH kinetics. The results from these WWC tests are displayed in Figure 4. For comparison, the baseline data (0.1 kmol/m3 NaOH with no additives) obtained over the same time frame are shown as well. The experiments with NP-7 and antifoam do not show a significant deviation in kg′ from the base case. Thus, it is
Figure 5. Fractional area measurements for 250-series packings (ap ) 250 m2/m3).
appropriate to conclude that the kinetics are unaffected by these additives, and therefore, eqs 10-17 remain valid. The influence of surfactants on mass transfer across gasliquid interfaces has been investigated in numerous studies in the literature. Generally, it is agreed that surfactants tend to reduce mass transfer by means of two mechanisms: the dampening or elimination of interfacial turbulence (i.e., rippling) and the interposition of a physical barrier hindering transfer.20-22 The latter scenario implies that a surfactant-related resistance term, in addition to the generic gas-side and liquid-side resistances, should be incorporated into the overall resistance. It is probable that the magnitude of this mass transfer resistance is a function of not only surfactant concentration but also specific surfactant type, making it difficult to characterize. For the most part, decreases in mass transfer are believed to be dominated by the elimination of ripples, rather than the barrier effect.23,24 Similar (smooth) film behavior has been observed for both the base case and surfactant systems, so the first mechanism proposed above is not a factor. Furthermore, the barrier effect should be insignificant on account of the low concentrations of NP-7 and antifoam. The fact that the additives are nonionic eliminates the possibility of induced charge gradients, which could affect local ion concentrations (and hence, kg′). In conclusion, the comparable kg′ results for the baseline and surfactant systems are justifiable. 3.4. Packing Area Results. 3.4.1. Sulzer Mellapak 250Y Results. The effective area of Mellapak 250Y structured packing was measured in the 0.46-m OD packed column, and the results are displayed in Figure 5. The data are interpreted in terms of fractional area (af), defined as effective area (ae) normalized by specific packing area (ap). Because of the perforations in the packing, the specific area might be anticipated to be slightly less than the vendor supplied value of 250 m2/m3. However, X-ray imaging work performed by Green25 suggests that, for the majority of liquid loads operated at in this study, liquid films would be expected to cover these perforations, thereby resulting in minimal area loss. A specific area of 250 m2/m3 was therefore assumed for Mellapak 250Y. For comparison, data obtained in a prior investigation by the Separations Research Program (SRP) at the University of Texas at Austin with a prototype 250-series packing are also shown in Figure 5. This packing did not have perforations and was assigned a specific area of 250 m2/m3. Tests with this packing employed a SRP 3C liquid distributor (gravity-fed orifice pipe with 430 drip points/m2), in contrast to the F10 distributor used in all of the studies with the Mellapak packing. The two
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cos γ ) 5.211 × 10-16.835σ for σ > 0.055 N/m (19) cos γ ) 0.9 for σ < 0.055 N/m
Figure 6. Comparison of Mellapak 250Y (ap ) 250 m2/m3) baseline data with existing models at air velocity of 1 m/s.
Figure 7. Comparison of Mellapak 250Y (ap ) 250 m2/m3) low surface tension data with existing models at air velocity of 1 m/s.
distributors were anticipated to perform similarly, on the basis of past SRP experience. The four data sets presented in Figure 5 are comparable. For the most part, the data spread at each liquid load was within typical error limits (10-15%). Fractional areas all approached unity, implying that the geometric area of the packing was being well utilized in every case. The baseline Mellapak 250Y results at the two air velocities (1 and 1.5 m/s) coincided, supporting our contention that gas-side resistance should be negligible at either condition. The Mellapak 250Y data also compared favorably with the prototype packing data. This served to quantitatively validate our results with the Mellapak packing and suggested that similar performance (i.e., high fractional areas) might be expected for other analogous 250-series structured packings. Finally, it can be seen that lowering the surface tension did not have a statistically confirmable effect on the fractional area of Mellapak 250Y. Fractional areas were near unity, regardless of surface tension. Figures 6 and 7 compare the experimental data with available models. The correlation proposed by Henriques de Brito et al.5 does not include surface tension as a parameter and for both the baseline and surfactant systems was found to significantly overpredict the data. Rocha et al.19 consistently underpredicted the base case and overpredicted the results at low surface tension. This drastic predictive swing was primarily due to the effect of the contact angle term: (1 - 0.93 cos γ)-1. The Rocha-Bravo-Fair model estimates contact angle for sheet metal packing using eqs 19 and 20.
(20)
The discontinuity at σ ) 55 mN/m indicates that the relation between contact angle and surface tension may not be properly characterized. It has also been suggested by Nicolaiewsky et al.3 that the contact angle term may be weighted too heavily in the correlation, with its exponent of -1. The Olujic model26 matched the data reasonably well, although the fact that it basically only relies on liquid velocity casts some doubt on the significance of this result. Interestingly, the Bravo-Fair27 correlation, despite having been developed for random packing, was in better agreement with the data than the Rocha-BravoFair or Henriques de Brito models. 3.4.2. Sulzer Mellapak 500Y Results. The effective area of Mellapak 500Y structured packing was also measured in the 0.46-m OD packed column with the F10 distributor. The results are displayed in Figure 8. As in the case of the Mellapak 250Y packing, the loss in surface area on account of the perforations was assumed to be negligible. A geometric area of 500 m2/m3 was used for this packing. Results from a previous SRP test with a prototype 500-series packing (perforated, ap specified as 500 m2/m3) are displayed in Figure 8 as well. An F40 liquid distributor (pressurized fractal distributor with 430 drip points/m2) was employed in the experiments with this prototype packing. Prior tests have suggested that comparable performances can be expected from the F10 and F40 distributors. Therefore, it was not surprising that the two 500-series packings yielded approximately the same results. Fractional areas were relatively low in both instances, approaching a value of 0.6 at the higher liquid loads. Consistent data were generated with two different Mellapak 500Y experiments at low surface tension. In contrast to the results with Mellapak 250Y, the fractional area was 50% larger when the surface tension was reduced. 3.4.3. Comparison of Mellapak 250Y and Mellapak 500Y. The difference in baseline fractional areas of Mellapak 250Y and 500Y is striking. The fact that the corresponding prototype packings exhibited similar results indicates that the decline in efficiency from lower to higher surface area packing was not merely an isolated incident and therefore deserves an explanation. It is suspected that capillary phenomena are responsible for this trend. With a high surface tension liquid such as water, “pooling” or “bridging” is likely to occur within the corrugation troughs (in meniscus form) and possibly across adjacent sheets, as seen by Green et al.25,28 via X-ray imaging. This effectively reduces the available mass transfer area. High surface area packings, being structurally denser than their lower surface area counterparts, are more strongly impacted by this effect. Consequently, the advantage of having a higher geometric area is partially offset. The baseline vs surfactant performances of Mellapak 250Y and 500Y are noteworthy, too, and are related to the previous discussion. For structured packings such as these, liquid would be expected to primarily flow in the form of a film over the packing surface (rather than free-falling droplets), with greater spreading typically being associated with lower surface tension. The effective mass transfer area of Mellapak 250Y was equivalent at high and low surface tension, however, which indicates that the wetting of the packing surface was similar in both cases. An analogous argument can be made for Mellapak 500Y, since it shares the same surface features as the 250Y packing and actually has a shorter crimp length. This suggests that the key effect of the reduced surface tension was not
Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1259 Table 2. Comparison of Experimental and Calculated Contact Angles measured σ (mN/m)
measured γ (deg)
calculated γ (deg)
0.1 kmol/m3 NaOH
70.09 ( 0.60
73.3 ( 3.82
0.1 kmol/m3 NaOH/ 125 ppmv Tergitol NP-7/50 ppmw/v Dow Corning Q2-3183A antifoam
35.55 ( 2.43
41.91 ( 1.81
69.86 (from eq 19) 25.84 (from eq 20)
sample
Figure 8. Fractional area measurements for 500-series packings (ap ) 500 m2/m3).
improved wetting of the bulk packing surface, as one might expect, but rather alleviation of wasteful menisci. In comparison to the 250Y packing, the 500Y packing would have a considerably greater portion of its surface rendered inaccessible on account of these capillary effects at high surface tension so it should benefit much more from a decreased surface tension. Indeed, its fractional area significantly improved, whereas the performance of the 250Y packing hardly changed. At low surface tension, the effectiveness of the 500Y packing approached that of the 250Y packing, as evidenced from Figures 5 and 8. If the surface tension reduction was indeed accompanied by a decrease in contact angle (a natural presumption, as asserted by Nicolaiewsky and Fair29), then the data signify that contact angle may not be as critical to the wetting of the packing surface as many models (e.g., Rocha-Bravo-Fair19) contend. However, it is also possible that the observed behavior was due to the contact angles actually being equivalent for the high and low surface tension scenarios. Examining Young’s relation30 (eq 21), one could hypothesize that the presence of surfactant could have altered the solid-liquid interfacial energy (σSL) in addition to the vapor-liquid interfacial energy (“surface tension”) in a manner that ultimately resulted in the contact angle being balanced between the two changes.
cos γ )
σSG - σSL σ
(21)
Alternatively, the “waffled” surface texture of the Mellapak packing could have strongly dictated liquid spreading tendencies, completely dominating any surface tension-related effects. To test these hypotheses, contact angle measurements were conducted via the sessile drop technique on a noncorrugated piece of sheet metal sharing the same characteristics as that of the Mellapak packing. Unfortunately, the surface features made these measurements difficult and unreliable; vastly different angles could be obtained depending on the drop volume and placement (i.e., within a “crest” or “valley” of the textured surface). As a temporary approximation to the problem, a smooth, stainless steel sheet was examined instead. The obtained data were useful in two regards. First, they established the anticipated reproducibility of the experimental technique, which was quite satisfactory; variation in measurements for a given system was confined to a 5° range. Furthermore, they showed that the contact angle was notably different for the base case (∼70°) and surfactant solutions (∼40°), thereby invalidating the
Young’s relation argument discussed above. For comparison, the data are presented together with predicted values from eqs 19 and 20 in Table 2. The contact angles for the high surface tension case match up rather well. The angles for the low surface tension case, on the other hand, do not, which is not too surprising given that eq 20 presents a fairly broad generalization. 4. Conclusions Rates of absorption of CO2 into 0.1 kmol/m3 NaOH were measured. The value of kg′ was found to be 10% greater than predicted by the correlation of Pohorecki and Moniuk. Use of their correlation was nevertheless believed to be acceptable. The addition of 125 ppmv Tergitol NP-7 surfactant and 50 ppmw/v Dow Corning Q2-3183A antifoam did not appreciably affect the kinetics (kg′) of the CO2-NaOH reaction. At a high surface tension of around 72 mN/m (0.1 kmol/m3 NaOH without surfactant), the fractional mass transfer area of Mellapak 250Y structured packing was found to be roughly twice that of Mellapak 500Y. The fractional area of 250Y increased from 0.78 to 1.11 over liquid loads of 12.1 to 73.3 m3/m2-h, whereas that of the 500Y varied from 0.34 to 0.56 as liquid load was incremented from 2.5 to 30.5 m3/m2-h. This decrease in performance from the 250Y to the 500Y packing is attributed to undesirable liquid pooling and bridging, which would be expected to be especially prominent at high surface tensions in higher surface area packings. Reducing the surface tension to 35 mN/m (0.1 kmol/m3 NaOH with surfactant) was found to minimally impact the effective mass transfer area of Mellapak 250Y but yielded substantially higher areas for Mellapak 500Y. The fractional area of 250Y ranged from 0.6 to 1.08 as liquid load was ramped from 2.4 to 85.5 m3/m2-h. The fractional area of 500Y increased from 0.54 to 0.79 as liquid load ranged from 2.4 to 42.8 m3/ m2-h. The high and low surface tension fluids were found to exhibit notably different contact angles (70° and 40°) on a smooth, stainless steel surface. These results, coupled with the Mellapak 250Y mass transfer data, insinuate that for this packing type, contact angle may be relatively unimportant in the context of surface wetting. It appears that the most prominent effect of a reduced surface tension is the inhibition of capillary phenomena. This behavior causes lower capacity (higher surface area) packings to experience a greater improvement in effective mass transfer area than higher capacity (lower surface area) packings under low surface tension conditions. Acknowledgment This work was supported by the UT-SRP Distillation Consortium and the Industrial Associates Program for CO2 Capture. We acknowledge Sulzer Chemtech for supplying the packing materials for this research and the SRP staff members for their help and support.
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Nomenclature ae ) effective area of packing, m2/m3 ap ) specific area of packing, m2/m3 DCO2 ) diffusivity of CO2, m2/s d ) estimated hydraulic diameter of WWC reaction chamber, m HCO2 ) Henry’s constant of CO2, m3‚bar/kmol h ) exposed length of WWC, m h+, h-, hg, hi ) contributions of a cation, anion, gas, and compound “i”, respectively, in eqs 13 and 14, m3/kmol I ) ionic strength of solution, kmol/m3 Ii ) ionic strength contribution of salt “i”, kmol/m3 KG ) overall gas-side mass transfer coefficient, kmol/(m2‚Pa‚ s) k1 ) pseudo-first-order reaction rate constant, s-1 kg ) gas-side mass transfer coefficient, kmol/(m2‚Pa‚s) kg′ ) liquid-side mass transfer coefficient, kmol/(m2‚Pa‚s) kL° ) physical liquid-side mass transfer coefficient, m/s kOH- ) second-order reaction rate constant, m3/(kmol‚s) kOH-∞ ) second-order reaction rate constant at infinite dilution, m3/(kmol‚s) NCO2 ) molar flux of CO2, kmol/(m2‚s) P ) pressure, Pa PCO2* ) equilibrium partial pressure of CO2, Pa PCO2i ) partial pressure of CO2 at gas-liquid interface, Pa R ) ideal gas constant, (m3‚Pa)/(kmol‚K) r ) chemical reaction rate, kmol/(m3‚s) T ) absolute temperature, K u ) velocity, m/s yCO2in/out ) mole fraction of CO2 at inlet/outlet Z ) packed height, m Greek Symbols γ ) contact angle, degrees µ ) viscosity, Pa‚s σ ) surface tension, N/m Subscripts G ) gas phase L ) liquid phase S ) solid phase w ) water Dimensionless Groups af ) fractional area of packing, ae/ap Ha ) Hatta number, xk1DCO2,L/kL° Literature Cited (1) Wang, G. Q.; Yuan, X. G.; Yu, K. T. Review of Mass-Transfer Correlations for Packed Columns. Ind. Eng. Chem. Res. 2005, 44 (23), 8715. (2) Sedelies, R.; Steiff, A.; Weinspach, P.-M. Mass Transfer Area in Different Gas-Liquid Reactors as a Function of Liquid Properties. Chem. Eng. Technol. 1987, 10 (1), 1. (3) Nicolaiewsky, E. M. A.; Tavares, F. W.; Rajagopal, K.; Fair, J. R. Liquid Film Flow and Area Generation in Structured Packed Columns. Powder Technol. 1999, 104 (1), 84. (4) Shi, M. G.; Mersmann, A. Effective Interfacial Area in Packed Columns. Ger. Chem. Eng. 1985, 8, 87. (5) Henriques de Brito, M.; von Stockar, U.; Bangerter, A. M.; Bomio, P.; Laso, M. Effective Mass-Transfer Area in a Pilot Plant Column Equipped with Structured Packings and with Ceramic Rings. Ind. Eng. Chem. Res. 1994, 33 (3), 647.
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ReceiVed for reView June 5, 2007 ReVised manuscript receiVed October 23, 2007 Accepted October 30, 2007 IE070780L
