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Carbon Dioxide Absorption in a Membrane Contactor with Color Change ^ s Pantalea ~ o, Ana F. Portugal, and Ade lio Mendes* Ine LEPAE, Faculdade de Engenharia da Universidade do Porto, Rua Roberto Frias, 4200-465 Porto, Portugal *
[email protected] Joaquim Gabriel IDMEC, Faculdade de Engenharia da Universidade do Porto, Rua Roberto Frias, 4200-465 Porto, Portugal
A membrane contactor is a device where mass transfer occurs between two phases without dispersion of one phase within another, with the fluids passing on opposite sides of a membrane. Hollow fiber membrane contactors (HFMC) are widely studied for CO2 removal from gas streams (1). Baird and Nirdosh (2) describe an educational experiment concerning the absorption of CO2 from a single bubble held stationary in a down flow of water in a vertical tube. The mass transfer rate was calculated from the measured rate at which the bubble volume decreased with time. This article describes an experimental setup to study CO2 absorption into water using HFMC with a color change. The experiment is performed by upper-level undergraduate students and requires 3 h. Experimental values of CO2 flux, JCO2, as a function of liquid velocity, v, are compared with those calculated using a phenomenological model JCO2 ¼ kL ðCCO2 , i - CCO2 , bulk Þ
ð1Þ
where kL is the average liquid-phase mass transfer coefficient, CCO2,i is the interfacial CO2 concentration as given by the Henry's law, and CCO2,bulk is the CO2 concentration in the bulk liquid.1 Equation 1 is valid for physical absorption in a HFMC, with constant gas-liquid interface conditions and a fully developed laminar flow of liquid through the fibers (3- 6). The data collected by the students is also used to estimate CO2 solubility in water,1 which is achieved at high liquid flow rates, when CCO2,bulk becomes negligible and eq 1 becomes a straight line through the origin with a slope that can be used to obtain CO2 solubility. Preferably, the value should be obtained from the plateau of the plot of 1=3 JCO2 v ¼ f ðY Þ; Y ¼ 1:62 dL Y where v is the liquid velocity and d and L are the internal diameter and length, respectively, of the fibers (5). The plateau corresponds to the range of flow rates for which the slope becomes constant. The use of a color change in pedagogical experiments is not new. Mendes (7) described a colorful ion-exchange experiment where the use of a resin with an adsorbed acid-base indicator allowed the students to follow the progress of the ion-exchange front along a column. Madeira et al. (8) described a series of experiments to characterize a laminar-flow tubular reactor, using colored tracers and the difference in color between reagents and products. Making
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use of sulfur dioxide's interaction with amines, King (9) described a detection method for SO2 based on a color change. The experiment described here is followed visually, by the change in color of an acid-base indicator. The color change allows students to visually follow the CO2 permeation through the membranes, providing a qualitative understanding of the process (7, 8). Experimental Procedure The setup for the CO2 absorption experiment is shown in Figure 1. This experiment uses a hollow fiber module contactor built from 18 hollow fiber membranes. The contactor is operated in continuous mode with liquid flowing inside the fibers and the gas on the shell side, concurrently with the liquid. The absorption solvent is distilled water, which is degassed by applying vacuum. A peristaltic pump is used to pump the liquid into the lumen of the hollow fibers. Liquid pressure should be approximately equal to gas pressure to avoid gas bubbling into the liquid phase. This is obtained using a valve downstream, labeled as LOV in Figure 1. Carbon dioxide absorption acidifies the liquid phase making possible the use of a pH indicator to allow visual perception of the CO2 uptake by the flowing liquid stream. Bromothymol blue was chosen, which is blue for pH > 7.6, yellow for pH < 6, and bluish-green for the neutral solution. To make the color change more pronounced, students may slightly adjust the pH of the feeding liquid to pH = 7.6, using, for example, a 0.1 M NaOH aqueous solution. This way the pH indicator is in its deprotonated form, exhibiting the color blue. The inlet gas is a mixture of 15% CO2 balanced with N2, with a flow rate of 95 mL min-1; this stream is obtained using two mass flow meters (MFM) and needle valves. The inlet pressure is measured by a digital pressure indicator (PI). The feed gas is presaturated with water vapor to ensure that mass transfer is not impeded by evaporation effects (5). Composition of the outlet gas stream is determined by an infrared analyzer. A step-by-step detailed procedure can be found in the supporting information. A spreadsheet is available in the supporting information for download where the collected data can be inputted and graphs observed. The spreadsheet contains hidden cells and can only be used to verify if more data is necessary while still in the lab. Results And Discussion Students determine the liquid flow rate and record atmospheric and liquid temperatures, CO2 and N2 flow rates,
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 12 December 2010 10.1021/ed1003355 Published on Web 10/11/2010
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In the Laboratory
Figure 1. Experimental setup: MFM, mass flow meter; PI, digital pressure indicator; LOv, liquid outlet valve.
Figure 2. Color exhibited by the feeding water at the inlet (A) and at the outlet (B) of the HFMC.
Figure 4. Estimation of CO2 solubility and comparison of reference and experimental values.1
Figure 3. Flux of CO2 as a function of liquid velocity.
gas-phase pressure, and composition of the outlet gas stream, corresponding to about 10 equally spaced values of liquid feed flow rates ranging from 3 to 50 mL min-1. Students can visually follow the CO2 absorption, by the blue-to-yellow color change (Figure 2). Students manipulate the collected data to obtain a graph of CO2 flux, JCO2, as a function of liquid velocity, v (Figure 3). At low liquid velocities, experimental values match those predicted by the model (eq 1). However, at high liquid flow rates, experimental absorption flux is lower than flux predicted by the model, and this deviation increases with liquid velocity. This happens because of the change in composition of the gas phase, from the inlet to the exit of the contactor, as CO2 is transferred from the gas to the liquid. Equation 1 is derived for constant gas-liquid interface composition and only takes into account the change in concentration of CO2 in the bulk liquid, with liquid velocity. Therefore, when gas-liquid interface conditions are not constant, a deviation from the model will be 1378
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observed. With increasing liquid velocity, the quantity of CO2 removed is higher, and explains the increase in the deviation for high liquid velocities (10). To assess the axial gradients in gasphase velocity and composition, a coupled differential model for both gas and liquid phases must be solved, which is beyond the scope of the experiment (6). As an additional task, students use the collected data to estimate the CO2 solubility in water1 from the plot of the CO2 flux, JCO2, versus Y (=1.62v/dL)1/3 (Figure 4). In the case described, solubility of CO2 is estimated using the four data values that correspond to the highest liquid velocities. The value obtained for solubility is 4.2 mol m-3 whereas the literature value (11) is 5.077 mol m-3. The difference is due to the relatively high concentration of CO2 in the gas phase, which is related to the precision of the infrared analyzer. A higher velocity water pump would allow reaching higher Graetz numbers, which is favorable to estimate CO2 solubility by the procedure described.1 Conclusions A pedagogical experiment investigating the physical absorption of CO2 in water is described. The absorption is performed in a HFMC specifically built for this purpose. This experiment was designed to be simple, safe, attractive, and environmentally friendly. The use of a pH indicator allows students to visually follow the absorption as the color changes from the inlet to the outlet of the membrane contactor. The results obtained are in
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In the Laboratory
agreement with those predicted by a model, at low liquid velocities. At high liquid velocities, a deviation to the model is obtained, as expected. The data collected is also used to estimate CO2 solubility in water, which is achieved within 16% deviation compared to the literature value. It is important that students obtain experimental results according to what is expected. This way they become more confident about their own capabilities. In this experiment, students obtain good results and this is done while reinforcing important concepts. The use of a low-precision analyzer requires the use of high CO2 gas input concentrations and results in some of the experimental values deviating from the predicted values. As students are asked to comment on all the results obtained, the deviation can become an opportunity for deeper understanding of the implications of this mass transfer phenomenon and of the model limitations. This setup can also be used to teach further subjects, like absorption enhanced by chemical reaction, in a subsequent lesson. Acknowledgment The authors would like to acknowledge Klaus-Viktor Peinemann for kindly providing the PDMS hollow fibers used in this work. Note 1. Additional information is available in the supporting information on how to obtain kL and CCO2,bulk and how to estimate CO2 solubility.
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Literature Cited 1. Alan, G.; Sun-Tak, H. J. Membr. Sci. 1999, 159, 61–106. 2. Baird, M. H. I.; Nirdosh, I. Chem. Eng. Ed. 2001, 35, 198– 201. 3. Leveque, J.. Les lois de la transmission de chaleur par convection. Ph.D. Thesis, Faculty of Science in Paris, France, 1928. 4. Kumar, P. S.. Development and design of membrane gas absorption processes. Ph.D. Thesis, University of Twente, The Netherlands, 2002. 5. Dindore, V. Y.; Brilman, D. W. F.; Versteeg, G. F. Chem. Eng. Sci. 2005, 60, 467–479. 6. Portugal, A. F.; Magalh~aes, F. D.; Mendes, A. J. Membr. Sci. 2009, 339, 275–286. 7. Mendes, A. J. Chem. Educ. 1999, 76, 1538–1540. 8. Madeira, L. M.; Mendes, A.; Magalh~aes, F. D. Int. J. Eng. Ed. 2006, 22, 188–196. 9. King, A. G. J. Chem. Educ. 2006, 83, 684. 10. Dindore, V. Y.; Versteeg, G. F. Int. J. Heat Mass Transfer 2005, 48, 3352–3362. 11. Versteeg, G. F.; Van Swaaij, W. P. M. J. Chem. Eng. Data 1988, 33, 29–34.
Supporting Information Available A step-by-step detailed procedure to operate the experimental setup; spreadsheet where the collected data can be inputted and graphs observed. This material is available via the Internet at http:// pubs.acs.org.
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