Effect of Absorbent Type and Concentration on CO - American

Article pubs.acs.org/IECR

Effect of Absorbent Type and Concentration on CO2 Capture from a Gas Stream into a Liquid Phase Gustavo Capannelli,† Antonio Comite,† Camilla Costa,† and Renzo Di Felice*,‡ †

Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso, 31, 16146 Genova, Italy Dipartimento di Ingegneria Civile Chimica ed Ambientale, Università degli Studi di Genova, Via Opera Pia, 15, 16145 Genova, Italy

‡

ABSTRACT: Carbon dioxide from a gas stream was captured in a liquid water solution through a polymeric membrane that provided the interfacial area between the two phases. Three different absorbents (monoethanolamine, piperazine, and potassium carbonate) were tested, and the effects of their concentrations on the CO2 absorption rate were investigated. The solution containing piperazine showed the best CO2 uptake, whereas that containing potassium carbonate was the least effective. This observation can be fully explained when the effects of the physicochemical parameters governing the absorption process (CO2 solubility, diffusivity, and reactivity in the solution) are taken into proper account. Useful guidelines for the correct design of a CO2 capturing system by a liquid solution were obtained.

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INTRODUCTION The increasing release of carbon dioxide (CO2) into the atmosphere has created environmental concerns. As a result, significant efforts have been made to develop efficient methods for capturing CO2 from various gas streams, particularly flue gas. The current practice for CO2 removal from flue gases produced by the combustion of fossil fuels in power plants is based on separation by reactive absorption. Compared to physical absorption, absorption with chemical reaction results in a higher selectivity and an enhanced rate of mass transfer. The most commonly used liquids are aqueous solutions of ethanolamines and proprietary blends of potassium carbonate. The gas stream passes through a washing column, where the CO2 is selectively transferred into the liquid phase, and the purified gas can be safely discharged into the atmosphere. The loaded solvent is then fed into a high-temperature and/or lowpressure regenerator, releasing CO2 of high purity, which can then be reused. The energy required for solvent regeneration is one of the main problems with this approach, especially if ethanolamines are employed, because it can have a significant impact on the efficiency of the overall process. Other drawbacks of the process include amine degradation at elevated temperatures and/or in the presence of oxygen, solvent losses in both the absorption and desorption phases, and the toxicity of amines. Considerable academic and industrial work has been done to formulate new liquid absorbents and to design alternative technologies that could improve the current performances and reduce energy requirements. Among the different technological solutions proposed, the membrane contactor process is one of the most interesting and most studied because of its particular advantages. Many theoretical and experimental studies have been conducted to evaluate the effects of important factors, such as membrane characteristics, module configuration, absorbent type, and operating conditions, on contactor performance.1−3 Mathematical treatments, mainly based on the resistance-in© XXXX American Chemical Society

series model, have been developed to describe mass transfer in membrane contactors and to predict the system behavior.4−7 In addition, many liquid absorbents have been investigated and proposed, as the absorbent is a central component in any type of absorbing device. At present, several researchers are focused on the formulation of blended (also called complex) absorbents, composed of mixtures of various different reagents, in an effort to achieve higher removal efficiencies.8−10 In this study, a hollow-fiber membrane contactor was employed as the absorbing device, and the role of the absorbent solution was studied by analyzing, in particular, the effects of reagent type and concentration. CO2 removal rates from a N2/CO2 gas mixture were determined for various solutions of three different reagents: monoethanolamine (MEA), potassium carbonate (K2CO3), and piperazine (PZ). The absorption of CO2 into aqueous monoethanolamine solutions using polytetrafluoroethylene and polypropylene membranes has been extensively investigated in recent years.11−13 On the other hand, few studies have explored the performances of potassium carbonate14−16 and piperazine in these systems. In particular, piperazine, which was extensively investigated at the pilot scale by the research group of Rochelle at the University of Texas at Austin,17,18 is a relatively new alternative and is usually employed in combination with other absorbents rather than as a stand-alone reagent.19 Moreover, the behavior of pure piperazine solutions as absorbents in a membrane contactor has never been investigated. In this work, the three reagents were separately examined, to understand and compare the relative processes of CO2 absorption. In addition, the experimental results were compared with theoretical predictions from a well-established model, for which Received: April 30, 2013 Revised: July 23, 2013 Accepted: July 30, 2013

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dx.doi.org/10.1021/ie401376w | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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the required physicochemical parameters were extracted from literature data. The main objective was to identify simple guidelines that can be used to promote practical applications of membrane contactors as an innovative process in the field of CO2 capture. This preliminary information is indispensable for addressing more complicated real gaseous streams, such as reforming and flue gases. The study of the performance of membrane contactors applied to industrial effluents is a research area that is still relatively unexplored and will be the subject of a forthcoming work.

The second step is a proton exchange that is instantaneous, whereas the first step is second-order and rate-controlling. Thus, the overall reaction follows the simple kinetic law ( −rCO2) = k[CO2 ][HOC2H4NH 2]

The overall reaction rate constant k can be determined according to the equation proposed by Hikita and co-workers23 2152 log10 k = 10.99 − T where T is in kelvin. At T = 298 K, k = 5868 m3/(kmol s). CO2− K2CO3 System. When potassium carbonate is dissolved in water, it is ionized into K+ and CO32− ions; then, by hydrolysis, carbonate ions form HCO3− and OH− ions

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SYSTEM DESCRIPTION The system employed in this study is a particular type of absorbing device, the membrane contactor, about which abundant scientific information is available in the literature.1,2,6 In general, this system is characterized by a microporous hydrophobic membrane that provides the contact between a gas and a liquid absorbent without dispersing one phase into the other. Mass transfer occurs as the solute gas diffuses through the membrane and absorbs into the liquid solvent (physically or chemically). Compared to conventional absorbers, this device combines important advantages such as independent liquid and gas flows, very high contact area, compactness, and modularity. On the other hand, the membrane itself adds an additional resistance to the mass-transfer process. Moreover, if the membrane pores are filled with the liquid (wetted), this mass-transfer resistance increases to unacceptable levels, so the membranes used in gas absorption must be extremely hydrophobic.20 Gas Mixture. For this study, the gas mixture selected contained CO2 and N2 in a composition simulating that of a typical flue gas from a coal combustion plant. Absorbent Solutions. The effects of both the reagent type and the reagent concentration were investigated. The accessible concentration ranges were different for the three reagents examined: Monoethanolamine is miscible with water in all proportions, whereas the solubility of potassium carbonate in water is 8.10 kmol/m3 at 25 °C. In contrast, the binary system PZ/H2O exhibits a complex behavior with PZ solubility at ambient temperature (1.94 kmol/m3 at 25 °C) being limited because of the formation of piperazine hexahydrate. An abrupt increase in solubility (to 7.48 kmol/m3) occurs upon heating the liquid above 45 °C (the melting point of PZ·6H2O).21,22 The chemical reactions occurring between CO2 and each of the three reagents in aqueous solutions have been extensively studied and described in literature. The three reaction mechanisms are summarized next. CO2−MEA System. CO2 reacts with monoethanolamine to form a carbamate as follows23−25

K 2CO3 → 2K+ + CO32 − CO32 − + H 2O → HCO3− + OH−

When CO2 is absorbed into this solution, the following reactions take place16,26 CO2 + OH− → HCO3− CO2 + 2H 2O → HCO3− + H3O+ CO32 − + H 2O → HCO3− + OH−

2H 2O → OH− + H3O+

and the overall reaction of carbon dioxide absorption in aqueous carbonate solution is written as CO2 + CO32 − + H 2O → 2HCO3−

The reaction between CO2 and OH− is the rate-controlling step, whereas the reaction between CO2 and H2O is slow and unimportant in basic solutions.16 The rate-controlling step is second-order according to the law ( −rCO2) = k[CO2 ][OH−]

The reaction rate constant k can be evaluated using the equation proposed by Astarita27 2895 log10 k = 13.635 − + 0.08I T where I is the solution ionic strength and T is in kelvin. Therefore, the kinetic constant depends on the ionic strength, as well as the temperature. CO2−PZ System. Piperazine is a bifunctional reagent. When CO2 is added to aqueous PZ solutions, the main equilibria involved are19,28 CO2 + PZ + B → PZCOO− + BH+

CO2 + 2HOC2H4NH 2 −

→ HOC2H4NHCOO + HOC2H4NH3

CO2 + PZCOO− + B → PZ(COO−)2 + BH+

+

PZ + H3O+ → PZH+ + H 2O

This overall reaction takes place in two steps, involving the formation of a zwitterion intermediate

CO2 + PZH+ + B → H+PZCOO− + BH+

CO2 + HOC2H4NH 2 → HOC2H4NH 2+COO−

where B is any basic compound present in the system (H2O, OH−, PZ, PZCOO−, PZH+). The reaction of CO2 with protonated PZ can be neglected with respect to the first two reactions, having a much smaller kinetic rate constant.19 Moreover, the experiments can be performed under conditions such that the reagent concen-

followed by the removal of a proton by a base B (H2O, OH−, or MEA itself) HOC2H4NH 2+COO− + B → HOC2H4NHCOO− + BH+ B



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regulated by a fine metering valve and controlled at the inlet and outlet of the contactor using a soap-bubble flow meter. The absorbent solution was contained in a thermally insulated vessel (vertical cylinder of 5 L capacity) and continually mixed by means of a magnetic stirrer; the vessel was also equipped with a thermometer to monitor the liquid temperature. Two side openings at the bottom enabled the module housing. The membrane modules were built in-house. Each module was composed of four hollow fibers (Accurel S6/2 from Membrana, Wuppertal, Germany), 200 mm in length, with the characteristics described in Table 1. In the assembly process, the fiber ends were inserted into small plexiglass tubes to which they were potted using an epoxy resin. This allowed the membranes to be held in place and helped even fiber spacing in the module to be obtained. When the module was installed in the vessel, it was sealed by rubber O-rings embedded in the two lateral openings in the vessel wall, to avoid any leak problems. At the start of a typical individual run, 3 L of liquid solution of a known concentration and pH was charged into the stirred vessel, and then the gas stream was fed to the module at a selected flow rate. The CO2 concentrations in the inlet and outlet gas streams of the contactor were sampled with a gas chromatograph (SIGMA 3B HWD) equipped with a packed Porapak Q column and a thermal conductivity detector. The amount of liquid solution in the vessel ensured that, in every case, the concentration of the reagent did not change appreciably, and therefore, steady-state conditions were assumed to be valid. In a set of experiments, the described procedure was repeated, at a given liquid concentration, for different gas flow rates in the range of (1−30) × 10−6 m3/s. All experiments were carried out at a temperature of 25 °C; both the liquid volume and the stirring speed were kept constant to ensure constant hydrodynamic conditions, to highlight only the differences due to reagent type and concentration. The gas mixture (supplied by Air Liquide Italia) contained 15% (v/v) carbon dioxide with the balance being nitrogen and was fed to the module at atmospheric pressure. Aqueous solutions were prepared at the desired concentrations by dissolving known amounts of reagent in deionized water. The different reagents employed during the tests were monoethanolamine (MEA, ≥99%), potassium carbonate (K2CO3, ≥99%), and piperazine (PZ, ≥99%), all of which were obtained from Aldrich. The solution pH was measured at the start of each run and periodically during the experiments by withdrawing liquid samples. This monitoring activity was particularly important in the case of potassium carbonate solution, to determine the concentration of OH− ions. The operating conditions employed in this work are listed in Table 2. For each individual run, the absorption flux Q [kmol/(m2 s)] was determined by performeing a CO2 mass balance over the membrane module

tration is not markedly decreased by the reaction with CO2. The piperazine carbamate concentration is then always much lower than the (constant) PZ concentration, and piperazine dicarbamate formation can be disregarded. In this regime, the overall reaction rate is determined mainly by piperazine carbamate formation, which takes place in two steps, following the mechanism already described for MEA PZ + CO2 → PZH+COO−

PZH+COO− + B → PZCOO− + BH+

The kinetic equation is again of second order ( −rCO2) = k[CO2 ][PZ]

Values of the reaction rate constant k at different temperatures can be obtained from the literature. Derks and co-workers19 reported k = 71000 m3/(kmol s) at 25 °C. Membrane. A polypropylene hollow fiber was selected for module construction, owing to its commercial availability and low cost, in addition to its high hydrophobicity. Measured membrane−liquid contact angles ranged from 120° for pure water to 110° for a reagent concentration of 1 kmol/m3, thereby ensuring that the pores would not be filled with liquid during normal operation and the the liquid would contact only the outside area of the membrane. Table 1 summarizes the main characteristics of this membrane, which was characterized in a preceding work.29 Table 1. Main Characteristics of the Membrane Used material average pore size porosity inner diameter outer diameter thickness

polypropylene 0.2 μm 60% 1800 μm 2600 μm 400 μm

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EXPERIMENTAL SETUP AND PROCEDURE The experimental equipment was very simple and is illustrated in Figure 1. It included a gas supply, the absorption device, and a gas sampling section. The gas mixture was fed to the membrane module by entering the gas flow in the fiber lumen. The gas flow rate was

Q CO = 2

v(CCO2,in − CCO2,out) Ae

(1)

where v is the gas flow rate (m3/s), assumed to be constant throughout the fiber length; CCO2 is the CO2 concentration (kmol/m3) in the gas phase (at the inlet and outlet of the

Figure 1. Experimental setup for the absorption of CO2 in aqueous solutions: SP, sampling port; F, flow meter; T, thermometer; HF, hollow fiber; S, magnetic stirrer. C



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transferring and reacting from the gas phase to the liquid phase, three resistances in series, namely, transport in the gas film, transport in the membrane, and diffusion and reaction in the liquid, govern the overall transfer rate. Resistance due to gas diffusion from the bulk gas to the membrane internal surface is generally negligible compared to the other two resistances. This assertion is indirectly supported by Figure 3, where CO2 flow rates were measured for the three

Table 2. Operating Conditions for CO2 Absorption temperature pressure inlet CO2 content gas flow rate MEA concentration K2CO3 concentration PZ concentration

25 °C ambient 15% v/v (1−30) × 10−6 m3/s 0.05−3 kmol/m3 0.4−3.3 kmol/m3 0.05−1.5 kmol/m3

contactor); and Ae is the interfacial area useful for the mass transfer. The gas concentration was measured at regular interval of 1 min to ensure that steady-state conditions had been reached. The effective gas/liquid contact area at the pore mouths was used for the definition of the absorption flux. Ae was then determined by multiplying the total outer membrane area (6.53 × 10−3 m2) by the porosity of the external surface in contact with the liquid, which was assumed to be identical to the overall nominal porosity (0.6), giving Ae = 3.92 × 10−3 m2.

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RESULTS AND DISCUSSION Figure 2 illustrates the results of the various experimental tests carried out to study the effects of reagent type and Figure 3. Experimental CO2 transfer rate as a function of gas velocity in the membrane lumen. Reagent concentration = 1 kmol/m3.

systems considered at varying velocity, and consequently fluid dynamic conditions, in the gas phase. No substantial difference can be detected in any of the systems in the range investigated. Moreover, as shown in Figure 4, the CO2 flux was also found to be independent of the liquid volume utilized in experimental

Figure 2. Effect of the reagent concentration on the average absorption flux.

concentration. In this figure, the CO2 absorption flux is plotted as a function of reagent concentration in the aqueous solution. For MEA, the concentration range between 0.05 and 3 kmol/m3 was explored. Figure 2 shows that an increase in solution concentration gradually increased the carbon dioxide transfer rate from the gas stream to the liquid. For PZ, the minimum and maximum concentrations examined were 0.05 and 1.5 kmol/m3, respectively, because of the previously described solubility limits. It was found that PZ behaved qualitatively similarly to MEA but gave a higher CO2 absorption rate (for the same concentration). For K2CO3, a series of solutions with concentrations of 5− 35% (p/p), or 0.4−3.3 kmol/m3, were prepared and analyzed. In this case, the observed behavior was somewhat unexpected, as it seemed to be opposite to those of MEA and PZ and showed a weak decrease in the removal rate of CO2 as the solution concentration increased. Moreover, it can be seen that the measured fluxes were 1 order of magnitude lower than those obtained for the MEA and PZ systems. Modeling and Quantification of Observed Phenomena. In the case of a membrane contactor with a component

Figure 4. Experimental CO2 transfer rate as a function of liquid volume. Reagent concentration = 1 kmol/m3.

runs. Therefore, it can be concluded that CO2 diffusion and reaction in the liquid phase took place only in the liquid film, as CO2 was completely depleted before it reaches the bulk of the liquid phase. Based on these simplifying assumptions, transport and reaction phenomena had to be considered only in the membrane and in the liquid film. In accordance with the considerations put forward in the previous section, for the three reagents under consideration, the following common kinetic law can be written D



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Estimation of the Physicochemical Parameters. The solubility and diffusivity of CO2 in the liquid phase as functions of reagent type and concentration were estimated from literature data. Because of the reactivity of CO2 in aqueous solutions, these properties cannot be directly measured, but they are commonly estimated from the corresponding data on similar nonreacting gases. N2O is often used for this purpose: Based on the socalled “N2O analogy”, it is assumed that the ratio of the solubilities or diffusivities of N2O and CO2 gases is the same in aqueous solutions of various substances and in pure water (at a given temperature).31−33 The ratio between the diffusivities in pure water can be obtained from Versteeg and co-workers34 as a function of temperature (T in kelvin) as

( −rCO2) = k[CO2 ][B]

where [B] represents the monoethanolamine, piperazine, or OH− (for potassium carbonate) concentration. Unfortunately, component transport characteristics could not be easily determined because the effects of the mixer action on the fluid dynamic behavior of the liquid film are difficult to express quantitatively. However, a limiting situation that assumes the concentration of the reacting component B to be constant throughout the film can be considered. For this case, recalling that gas film resistance is negligible and that the chemical reaction can be expressed with overall second-order kinetics with each reactant being first-order, a very simple mathematical expression similar to that reported in reaction engineering textbooks30 is available for the CO2 flux Q CO = 2

pCO

diffusivity of CO2 in water ⎛ 252 ⎞ ⎟ = 0.46 exp⎜ ⎝ T ⎠ diffusivity of N2O in water

2

1 kM

+

HCO2 kC BDCO2

(2)

and is equal to 1.08 at 25 °C. The same authors also gave the ratio between the distribution coefficients in pure water

the only difference lying in the fact that membrane resistance is introduced instead of gas film resistance. In eq 2 for the CO2 flux, pCO2 is the carbon dioxide partial pressure in the membrane, kM is the membrane mass-transport coefficient, CB is the reactant concentration in the aqueous solution, HCO2 is the Henry's law constant determining the CO2 solubility in the particular aqueous solution, k is the reaction kinetic constant, and DCO2 is the diffusion coefficient of CO2 in the liquid phase. For MEA and PZ, CB coincides with the amine concentration in the aqueous solution, a known value, whereas in the case of K2CO3, the effective reagent is the ion OH−. Its concentration can be evaluated experimentally from the pH value of the absorbing solution or calculated theoretically on the basis of the following simple approximations. The weak base CO32− in water originates the equilibrium

solubility of CO2 in water ⎛ 240 ⎞ ⎟ = 3.04 exp⎜ − ⎝ T ⎠ solubility of N2O in water

which amounts to 1.36 at 25 °C. The CO2 Henry's law constant for aqueous solutions of MEA was calculated using an expression proposed by Danckwerts35 and used by Maceiras et al.,36 where H is expressed in Pa m3/ kmol H = 10(10.3 + 0.035CMEA − 1140/ T )

For potassium carbonate solutions, the Henry's law constant was determined from the experimental results obtained from Knuutila and co-workers37 for nitrous oxide. The Henry's law constant for aqueous solutions of PZ was estimated from data on the distribution coefficient reported by Derks et al.38 for N2O. Figure 5 shows the effects of solution concentration on the Henry's law constants for the three reagents examined. In the case of potassium carbonate, the rapid increase of H with concentration is striking. K2CO3 in water is ionized, and the “salting-out” effect becomes increasingly important when the ionic strength of the solution increases. The CO2 solubility in

CO32 − + H 2O → HCO3− + OH−

whose constant at 25 °C is Kb = 10−3.67. In the concentration range examined in this work, the pH is always sufficiently high to preclude the formation of any significant amount of H2CO3. Therefore, neglecting the selfdissociation reaction of water, from the mass and charge balances, we obtain CS = [CO32 −] + [OH−]

and

[OH−] = [HCO3−]

where CS is the concentration of the K2CO3 salt, so that the OH− concentration can be obtained by solving the quadratic expression [OH−]2 + Kb[OH−] − KbCS = 0

In summary, to predict the absorption rate, eq 2 requires an estimation of the membrane mass-transfer coefficient, the Henry's law constant, and the diffusion coefficient of CO2, in addition to information on the reaction kinetics. Estimation of the Membrane Mass-Transfer Coefficient. For the membrane mass-transfer coefficient, a value determined in a previous work29 was used, namely, kM = 1.72 × 109 kmol/ (m2 s Pa). This value was estimated from an effective diffusion coefficient in the membrane calculated using the Bonsaquet equation.

Figure 5. Henry's law constants as functions of reagent concentration. E



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this type of solution is hence considerably lower than those for the two amines. The CO2 diffusion coefficient in aqueous solutions of MEA was determined by taking the values measured for N2O from Sada et al.32 For potassium carbonate, the following equation from Ratcliff et al.39 was used DCO2,K 2CO3 = 1 − 0.154[K 2CO3] + 0.0723[KHCO3] DCO2,water For PZ, as for MEA, D was evaluated based on N2O data reported in ref 38. In Figure 6, the three diffusion coefficients at various solution concentrations are compared and appear to be of the same

Figure 8. Experimental and calculated CO2 fluxes in K2CO3 solutions.

Figure 6. CO2 diffusion coefficients as functions of reagent concentration. Figure 9. Experimental and calculated CO2 fluxes in PZ solutions.

order of magnitude. As expected, D decreases with the reagent concentration, showing a linear dependence and a more marked slope in the case of PZ solutions. Comparison between Experimental and Calculated CO2 Fluxes. Figures 7−9 show the theoretical predictions obtained using eq 2 compared with the experimental results for the three reagents. In each system, the calculated trend for the absorption flux as a function of the reagent concentration reproduces the experimental one, although the predicted solute mass transfer is always significantly higher.

The model correctly predicts that the absorption flux achievable with K2CO3 is about 1 order of magnitude lower than that provided by the two amines and provides the same ranking as found experimentally for the removal efficiency (PZ > MEA > K2CO3). The comparatively poor performance of K2CO3 can be ascribed to two factors: First, an important role is played by the physicochemical parameters, in particular by the Henry's law constant. The ratio H/(kD)1/2 is a useful overall indicator for characterizing absorbent performance,9 because it contains all of the necessary information on physicochemical properties and reaction kinetics. Figure 10 illustrates the variation of this indicator as a function of the reagent concentration for the three systems under consideration, calculated at 25 °C from the previously mentioned literature data. It can be seen that PZ presents the lowest H/(kD)1/2 ratio, which is moderate for MEA solutions as well. More importantly, for MEA and PZ, this overall indicator is almost independent of amine concentration, whereas it is strongly influenced by the salt concentration in carbonate solutions. In this case, although k increases with the ionic strength of the solution,27 the effect of H is dominant. According to eq 2, the higher the H/(kD)1/2 ratio, the lower the CO2 absorption flux. Second, as already mentioned, in a potassium carbonate solution the effective reagent is the ion OH−. It can be seen (Figure 11) that, for all examined solutions, the OH −

Figure 7. Experimental and calculated CO2 fluxes in MEA solutions. F



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Figure 12 shows the predicted and experimentally observed behavior of the MEA system for such a plot. Both sets of data

Figure 10. H/(kD)1/2 ratios as functions of reagent concentration. Figure 12. Experimental and calculated CO2 fluxes, linearized as in eq 3, for the MEA system.

yield a straight line, as expected. The y-axis intercept is essentially the same for both theory and experiment, indicating that the experimental membrane resistance is indeed very close to the calculated value [more specifically, 1.49 × 109 compared to 1.72 × 109 kmol/(m2 s Pa)]. On the other hand, the two straight lines are quite far apart, with the experimental values always being larger than the calculated ones. The obvious conclusion is that the experimental resistance to CO2 transfer in the liquid phase is always larger than that calculated through eq 4. This effect, unless the physical parameters were wrongly estimated, is therefore attributed to the simplifying assumption of a constant reagent concentration in the liquid film equal to the concentration in the liquid bulk. The reagent concentration in the liquid film must be lower than that measured in the liquid bulk, which is quite understandable, as the reagent there is consumed by the reaction, with the mixing system obviously not capable of replacing the reacted material at the necessary rate. The experimental liquid resistance will be larger than that calculated theoretically through eq 4 by a numerical factor, f, whose magnitude indicates how far the system is from the ideal situation of constant maximum reagent concentration in the liquid film. The true liquid resistance can be inferred by fitting eq 3 to the experimental CO2 flux and assuming the calculated membrane resistance to be correct (as indicated by Figure 12). f, the ratio between measured and theoretical liquid resistance RL, can then be calculated, and it is reported in Figure 13 as a function of the parameter H/(kD)1/2 for the three systems studied in this work. The parameter f approaches 1 only for the largest value of H/(kD)1/2, namely, when the CO2 transfer rate is relatively low as it is severely impeded by the liquid physicochemical constraints, whereas it grows exponentially for the lower values of H/(kD)1/2 when larger CO2 fluxes are expected. Therefore, Figure 13 clearly indicates that the mixing device used is not capable of supplying the necessary amount of reagent to the liquid film and that this deficiency becomes increasingly severe as the amount of reagent needed increases. Finally, the relative importance of RL and RM for the different systems investigated is depicted in Figure 14. For the carbonate solution, the membrane resistance always makes a negligible contribution, given that the resistance in the liquid film is relatively high in this case. For the other two cases, however, the membrane resistance tends to become important, although

Figure 11. OH− concentration as a function of carbonate concentration.

concentration is very low, about 2 orders of magnitude lower than that of the carbonate itself, so that the related removal rates are poor. The discrepancy between the experimentally observed CO2 fluxes and those calculated with the limiting hypotheses summarized by eq 2 is due to the invalidity of one of the assumptions made, specifically that regarding the constancy of the reagent concentration in the liquid film, CB. This conclusion is supported by the following evidence: Equation 2 can be rearranged and linearized as pCO 2 = RM + R L Q (3) where RM = 1/kM is the membrane resistance and RL, the liquid resistance, is the product of two contributions ⎛ H ⎞⎛ 1 ⎞ ⎟⎟ RL = ⎜ ⎟⎜⎜ ⎝ kD ⎠⎝ C B ⎠

(4)

Membrane resistance is constant and unchanged for the three different absorbent solutions. For MEA and PZ, as the overall parameter H/(kD)1/2 is roughly constant, one can expect that RL increases linearly with CB−1/2. Therefore, a plot of pCO2/Q as a function of (1/CB)1/2 should be linear with the y-axis intercept corresponding to the membrane resistance. G



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Notes

The authors declare no competing financial interest.

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Figure 13. Ratios of experimental to theoretical liquid resistances as functions of H/(kD)1/2.

Figure 14. Relative importance of the membrane resistance as a function of liquid concentration.

never prominent, as the concentration in the liquid increases, leading to a decrease of the overall resistance in the liquid phase.

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CONCLUSIONS The present work has shown how the physicochemical parameters of the reacting solution influence the carbon dioxide uptake rate from a gaseous stream. As expected, the CO2 solubility, diffusivity, and reactivity all play important roles; their effects, however, cannot be considered separately but need to be incorporated correctly into the overall picture. It was also shown that the effects on the overall transfer rates brought about by the use of a membrane contactor can become relevant for the fastest processes. In this case, attention needs to also be paid to the proper membrane choice. Needless to say, before moving to industrial application of the present technology, other aspects must be examined, such as the effects of the temperature and the membrane chemical stability, which are presently being investigated by our research group.

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