Effect of Absorbent Type and Concentration on CO2 Capture from aGas Stream into a Liquid PhaseGustavo Capannelli,† Antonio Comite,† Camilla Costa,† and Renzo Di Felice*,‡
†Dipartimento di Chimica e Chimica Industriale, Universita ̀ degli Studi di Genova, Via Dodecaneso, 31, 16146 Genova, Italy‡Dipartimento di Ingegneria Civile Chimica ed Ambientale, Universita ̀ 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 thatprovided the interfacial area between the two phases. Three different absorbents (monoethanolamine, piperazine, and potassiumcarbonate) were tested, and the effects of their concentrations on the CO2 absorption rate were investigated. The solutioncontaining piperazine showed the best CO2 uptake, whereas that containing potassium carbonate was the least effective. Thisobservation can be fully explained when the effects of the physicochemical parameters governing the absorption process (CO2solubility, diffusivity, and reactivity in the solution) are taken into proper account. Useful guidelines for the correct design of aCO2 capturing system by a liquid solution were obtained.
■ INTRODUCTION
The increasing release of carbon dioxide (CO2) into theatmosphere has created environmental concerns. As a result,significant efforts have been made to develop efficient methodsfor capturing CO2 from various gas streams, particularly fluegas.The current practice for CO2 removal from flue gases
produced by the combustion of fossil fuels in power plants isbased on separation by reactive absorption. Compared tophysical absorption, absorption with chemical reaction resultsin a higher selectivity and an enhanced rate of mass transfer.The most commonly used liquids are aqueous solutions ofethanolamines and proprietary blends of potassium carbonate.The gas stream passes through a washing column, where theCO2 is selectively transferred into the liquid phase, and thepurified gas can be safely discharged into the atmosphere. Theloaded solvent is then fed into a high-temperature and/or low-pressure regenerator, releasing CO2 of high purity, which canthen be reused.The energy required for solvent regeneration is one of the
main problems with this approach, especially if ethanolaminesare employed, because it can have a significant impact on theefficiency of the overall process. Other drawbacks of the processinclude amine degradation at elevated temperatures and/or inthe presence of oxygen, solvent losses in both the absorptionand desorption phases, and the toxicity of amines. Considerableacademic and industrial work has been done to formulate newliquid absorbents and to design alternative technologies thatcould improve the current performances and reduce energyrequirements.Among the different technological solutions proposed, the
membrane contactor process is one of the most interesting andmost studied because of its particular advantages.Many theoretical and experimental studies have been
conducted to evaluate the effects of important factors, such asmembrane characteristics, module configuration, absorbenttype, and operating conditions, on contactor performance.1−3
Mathematical treatments, mainly based on the resistance-in-
series model, have been developed to describe mass transfer inmembrane contactors and to predict the system behavior.4−7
In addition, many liquid absorbents have been investigatedand proposed, as the absorbent is a central component in anytype of absorbing device. At present, several researchers arefocused 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 wasemployed as the absorbing device, and the role of theabsorbent solution was studied by analyzing, in particular, theeffects of reagent type and concentration. CO2 removal ratesfrom a N2/CO2 gas mixture were determined for varioussolutions of three different reagents: monoethanolamine(MEA), potassium carbonate (K2CO3), and piperazine (PZ).The absorption of CO2 into aqueous monoethanolamine
solutions using polytetrafluoroethylene and polypropylenemembranes has been extensively investigated in recentyears.11−13 On the other hand, few studies have explored theperformances of potassium carbonate14−16 and piperazine inthese systems. In particular, piperazine, which was extensivelyinvestigated at the pilot scale by the research group of Rochelleat the University of Texas at Austin,17,18 is a relatively newalternative and is usually employed in combination with otherabsorbents rather than as a stand-alone reagent.19 Moreover,the behavior of pure piperazine solutions as absorbents in amembrane 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, 2013Revised: July 23, 2013Accepted: July 30, 2013
the required physicochemical parameters were extracted fromliterature data.The main objective was to identify simple guidelines that can
be used to promote practical applications of membranecontactors as an innovative process in the field of CO2 capture.This preliminary information is indispensable for addressingmore complicated real gaseous streams, such as reforming andflue gases. The study of the performance of membranecontactors applied to industrial effluents is a research areathat is still relatively unexplored and will be the subject of aforthcoming work.
■ SYSTEM DESCRIPTIONThe system employed in this study is a particular type ofabsorbing device, the membrane contactor, about whichabundant scientific information is available in the literature.1,2,6
In general, this system is characterized by a microporoushydrophobic membrane that provides the contact between agas and a liquid absorbent without dispersing one phase intothe other. Mass transfer occurs as the solute gas diffusesthrough 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 theother hand, the membrane itself adds an additional resistance tothe mass-transfer process. Moreover, if the membrane pores arefilled with the liquid (wetted), this mass-transfer resistanceincreases to unacceptable levels, so the membranes used in gasabsorption must be extremely hydrophobic.20
Gas Mixture. For this study, the gas mixture selectedcontained CO2 and N2 in a composition simulating that of atypical flue gas from a coal combustion plant.Absorbent Solutions. The effects of both the reagent type
and the reagent concentration were investigated. The accessibleconcentration ranges were different for the three reagentsexamined: Monoethanolamine is miscible with water in allproportions, whereas the solubility of potassium carbonate inwater is 8.10 kmol/m3 at 25 °C. In contrast, the binary systemPZ/H2O exhibits a complex behavior with PZ solubility atambient temperature (1.94 kmol/m3 at 25 °C) being limitedbecause of the formation of piperazine hexahydrate. An abruptincrease in solubility (to 7.48 kmol/m3) occurs upon heatingthe liquid above 45 °C (the melting point of PZ·6H2O).
21,22
The chemical reactions occurring between CO2 and each ofthe three reagents in aqueous solutions have been extensivelystudied and described in literature. The three reactionmechanisms are summarized next.CO2−MEA System. CO2 reacts with monoethanolamine to
form a carbamate as follows23−25
+
→ +− +
CO 2HOC H NH
HOC H NHCOO HOC H NH2 2 4 2
2 4 2 4 3
This overall reaction takes place in two steps, involving theformation of a zwitterion intermediate
+ → + −CO HOC H NH HOC H NH COO2 2 4 2 2 4 2
followed by the removal of a proton by a base B (H2O, OH−, or
MEA itself)
+ → ++ − − +HOC H NH COO B HOC H NHCOO BH2 4 2 2 4
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
− =r k( ) [CO ][HOC H NH ]CO 2 2 4 22
The overall reaction rate constant k can be determinedaccording to the equation proposed by Hikita and co-workers23
= −kT
log 10.992152
10
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
→ ++ −K CO 2K CO2 3 32
+ → +− − −CO H O HCO OH32
2 3
When CO2 is absorbed into this solution, the followingreactions take place16,26
+ →− −CO OH HCO2 3
+ → +− +CO 2H O HCO H O2 2 3 3
+ → +− − −CO H O HCO OH32
2 3
→ +− +2H O OH H O2 3
and the overall reaction of carbon dioxide absorption inaqueous carbonate solution is written as
+ + →− −CO CO H O 2HCO2 32
2 3
The reaction between CO2 and OH− is the rate-controllingstep, whereas the reaction between CO2 and H2O is slow andunimportant in basic solutions.16 The rate-controlling step issecond-order according to the law
− = −r k( ) [CO ][OH ]CO 22
The reaction rate constant k can be evaluated using theequation proposed by Astarita27
= − +kT
Ilog 13.6352895
0.0810
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. WhenCO2 is added to aqueous PZ solutions, the main equilibriainvolved are19,28
+ + → +− +CO PZ B PZCOO BH2
+ + → +− − +CO PZCOO B PZ(COO ) BH2 2
+ → ++ +PZ H O PZH H O3 2
+ + → ++ + − +CO PZH B H PZCOO BH2
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 smallerkinetic rate constant.19 Moreover, the experiments can beperformed under conditions such that the reagent concen-
Industrial & Engineering Chemistry Research Article
tration is not markedly decreased by the reaction with CO2.The piperazine carbamate concentration is then always muchlower than the (constant) PZ concentration, and piperazinedicarbamate formation can be disregarded. In this regime, theoverall reaction rate is determined mainly by piperazinecarbamate formation, which takes place in two steps, followingthe mechanism already described for MEA
+ → + −PZ CO PZH COO2
+ → ++ − − +PZH COO B PZCOO BHThe kinetic equation is again of second order
− =r k( ) [CO ][PZ]CO 22
Values of the reaction rate constant k at different temperaturescan 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 andlow cost, in addition to its high hydrophobicity. Measuredmembrane−liquid contact angles ranged from 120° for purewater to 110° for a reagent concentration of 1 kmol/m3,thereby ensuring that the pores would not be filled with liquidduring normal operation and the the liquid would contact onlythe outside area of the membrane. Table 1 summarizes themain characteristics of this membrane, which was characterizedin a preceding work.29
■ EXPERIMENTAL SETUP AND PROCEDUREThe experimental equipment was very simple and is illustratedin Figure 1. It included a gas supply, the absorption device, anda 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
regulated by a fine metering valve and controlled at the inletand 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) andcontinually mixed by means of a magnetic stirrer; the vesselwas also equipped with a thermometer to monitor the liquidtemperature. Two side openings at the bottom enabled themodule housing.The membrane modules were built in-house. Each module
was composed of four hollow fibers (Accurel S6/2 fromMembrana, Wuppertal, Germany), 200 mm in length, with thecharacteristics described in Table 1. In the assembly process,the fiber ends were inserted into small plexiglass tubes to whichthey were potted using an epoxy resin. This allowed themembranes to be held in place and helped even fiber spacing inthe module to be obtained. When the module was installed inthe vessel, it was sealed by rubber O-rings embedded in the twolateral 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 stirredvessel, and then the gas stream was fed to the module at aselected flow rate. The CO2 concentrations in the inlet andoutlet gas streams of the contactor were sampled with a gaschromatograph (SIGMA 3B HWD) equipped with a packedPorapak Q column and a thermal conductivity detector. Theamount of liquid solution in the vessel ensured that, in everycase, the concentration of the reagent did not changeappreciably, and therefore, steady-state conditions wereassumed to be valid.In a set of experiments, the described procedure was
repeated, at a given liquid concentration, for different gasflow 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 keptconstant to ensure constant hydrodynamic conditions, tohighlight only the differences due to reagent type andconcentration.The gas mixture (supplied by Air Liquide Italia) contained
15% (v/v) carbon dioxide with the balance being nitrogen andwas fed to the module at atmospheric pressure.Aqueous solutions were prepared at the desired concen-
trations by dissolving known amounts of reagent in deionizedwater. The different reagents employed during the tests weremonoethanolamine (MEA, ≥99%), potassium carbonate(K2CO3, ≥99%), and piperazine (PZ, ≥99%), all of whichwere obtained from Aldrich.The solution pH was measured at the start of each run and
periodically during the experiments by withdrawing liquidsamples. This monitoring activity was particularly important inthe case of potassium carbonate solution, to determine theconcentration 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 themembrane module
=−
Qv C C
A
( )CO
CO ,in CO ,out
e2
2 2
(1)
where v is the gas flow rate (m3/s), assumed to be constantthroughout the fiber length; CCO2
is the CO2 concentration(kmol/m3) in the gas phase (at the inlet and outlet of the
Table 1. Main Characteristics of the Membrane Used
Figure 1. Experimental setup for the absorption of CO2 in aqueoussolutions: SP, sampling port; F, flow meter; T, thermometer; HF,hollow fiber; S, magnetic stirrer.
Industrial & Engineering Chemistry Research Article
contactor); and Ae is the interfacial area useful for the masstransfer. The gas concentration was measured at regular intervalof 1 min to ensure that steady-state conditions had beenreached. The effective gas/liquid contact area at the poremouths was used for the definition of the absorption flux. Aewas then determined by multiplying the total outer membranearea (6.53 × 10−3 m2) by the porosity of the external surface incontact with the liquid, which was assumed to be identical tothe overall nominal porosity (0.6), giving Ae = 3.92 × 10−3 m2.
■ RESULTS AND DISCUSSIONFigure 2 illustrates the results of the various experimental testscarried out to study the effects of reagent type and
concentration. In this figure, the CO2 absorption flux is plottedas 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 insolution concentration gradually increased the carbon dioxidetransfer 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, becauseof the previously described solubility limits. It was found thatPZ behaved qualitatively similarly to MEA but gave a higherCO2 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 andshowed a weak decrease in the removal rate of CO2 as thesolution concentration increased. Moreover, it can be seen thatthe measured fluxes were 1 order of magnitude lower thanthose obtained for the MEA and PZ systems.Modeling and Quantification of Observed Phenom-
ena. In the case of a membrane contactor with a component
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 theliquid, govern the overall transfer rate.Resistance due to gas diffusion from the bulk gas to the
membrane internal surface is generally negligible compared tothe other two resistances. This assertion is indirectly supportedby Figure 3, where CO2 flow rates were measured for the three
systems considered at varying velocity, and consequently fluiddynamic conditions, in the gas phase. No substantial differencecan 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
runs. Therefore, it can be concluded that CO2 diffusion andreaction in the liquid phase took place only in the liquid film, asCO2 was completely depleted before it reaches the bulk of theliquid phase.Based on these simplifying assumptions, transport and
reaction phenomena had to be considered only in themembrane and in the liquid film.In accordance with the considerations put forward in the
previous section, for the three reagents under consideration, thefollowing common kinetic law can be written
Table 2. Operating Conditions for CO2 Absorption
temperature 25 °Cpressure ambientinlet CO2 content 15% v/vgas flow rate (1−30) × 10−6 m3/sMEA concentration 0.05−3 kmol/m3
K2CO3 concentration 0.4−3.3 kmol/m3
PZ concentration 0.05−1.5 kmol/m3
Figure 2. Effect of the reagent concentration on the averageabsorption flux.
Figure 3. Experimental CO2 transfer rate as a function of gas velocityin the membrane lumen. Reagent concentration = 1 kmol/m3.
Figure 4. Experimental CO2 transfer rate as a function of liquidvolume. Reagent concentration = 1 kmol/m3.
Industrial & Engineering Chemistry Research Article
where [B] represents the monoethanolamine, piperazine, orOH− (for potassium carbonate) concentration.Unfortunately, component transport characteristics could not
be easily determined because the effects of the mixer action onthe fluid dynamic behavior of the liquid film are difficult toexpress quantitatively. However, a limiting situation thatassumes the concentration of the reacting component B to beconstant throughout the film can be considered. For this case,recalling that gas film resistance is negligible and that thechemical reaction can be expressed with overall second-orderkinetics with each reactant being first-order, a very simplemathematical expression similar to that reported in reactionengineering textbooks30 is available for the CO2 flux
=+
Qp
k
H
kC D
COCO
12
2
M
CO2
B CO2 (2)
the only difference lying in the fact that membrane resistance isintroduced instead of gas film resistance.In eq 2 for the CO2 flux, pCO2
is the carbon dioxide partialpressure in the membrane, kM is the membrane mass-transportcoefficient, CB is the reactant concentration in the aqueoussolution, HCO2
is the Henry's law constant determining the CO2
solubility in the particular aqueous solution, k is the reactionkinetic constant, and DCO2
is the diffusion coefficient of CO2 inthe liquid phase.For MEA and PZ, CB coincides with the amine concentration
in the aqueous solution, a known value, whereas in the case ofK2CO3, the effective reagent is the ion OH−. Its concentrationcan be evaluated experimentally from the pH value of theabsorbing solution or calculated theoretically on the basis of thefollowing simple approximations.The weak base CO3
2− in water originates the equilibrium
+ → +− − −CO H O HCO OH32
2 3
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 anysignificant amount of H2CO3. Therefore, neglecting the self-dissociation reaction of water, from the mass and chargebalances, we obtain
= + =− − − −C [CO ] [OH ] and [OH ] [HCO ]S 32
3
where CS is the concentration of the K2CO3 salt, so that theOH− concentration can be obtained by solving the quadraticexpression
+ − =− −K K C[OH ] [OH ] 02b b S
In summary, to predict the absorption rate, eq 2 requires anestimation of the membrane mass-transfer coefficient, theHenry's law constant, and the diffusion coefficient of CO2, inaddition to information on the reaction kinetics.Estimation of the Membrane Mass-Transfer Coefficient.
For the membrane mass-transfer coefficient, a value determinedin a previous work29 was used, namely, kM = 1.72 × 109 kmol/(m2 s Pa). This value was estimated from an effective diffusioncoefficient in the membrane calculated using the Bonsaquetequation.
Estimation of the Physicochemical Parameters. Thesolubility and diffusivity of CO2 in the liquid phase as functionsof reagent type and concentration were estimated fromliterature data.Because of the reactivity of CO2 in aqueous solutions, these
properties cannot be directly measured, but they are commonlyestimated from the corresponding data on similar nonreactinggases. N2O is often used for this purpose: Based on the so-called “N2O analogy”, it is assumed that the ratio of thesolubilities or diffusivities of N2O and CO2 gases is the same inaqueous solutions of various substances and in pure water (at agiven temperature).31−33
The ratio between the diffusivities in pure water can beobtained from Versteeg and co-workers34 as a function oftemperature (T in kelvin) as
= ⎜ ⎟⎛⎝
⎞⎠T
diffusivity of CO in waterdiffusivity of N O in water
0.46 exp2522
2
and is equal to 1.08 at 25 °C.The same authors also gave the ratio between the
distribution coefficients in pure water
= −⎜ ⎟⎛⎝
⎞⎠T
solubility of CO in watersolubility of N O in water
3.04 exp2402
2
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 C T(10.3 0.035 1140/ )MEA
For potassium carbonate solutions, the Henry's law constantwas determined from the experimental results obtained fromKnuutila 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 byDerks 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 thecase of potassium carbonate, the rapid increase of H withconcentration is striking. K2CO3 in water is ionized, and the“salting-out” effect becomes increasingly important when theionic strength of the solution increases. The CO2 solubility in
Figure 5. Henry's law constants as functions of reagent concentration.
Industrial & Engineering Chemistry Research Article
this type of solution is hence considerably lower than those forthe two amines.The CO2 diffusion coefficient in aqueous solutions of MEA
was determined by taking the values measured for N2O fromSada et al.32 For potassium carbonate, the following equationfrom Ratcliff et al.39 was used
= − +D
D1 0.154[K CO ] 0.0723[KHCO ]
CO ,K CO
CO ,water2 3 3
2 2 3
2
For PZ, as for MEA, D was evaluated based on N2O datareported in ref 38.In Figure 6, the three diffusion coefficients at various solution
concentrations are compared and appear to be of the same
order of magnitude. As expected, D decreases with the reagentconcentration, showing a linear dependence and a moremarked slope in the case of PZ solutions.Comparison between Experimental and Calculated
CO2 Fluxes. Figures 7−9 show the theoretical predictionsobtained using eq 2 compared with the experimental results forthe three reagents.In each system, the calculated trend for the absorption flux as
a function of the reagent concentration reproduces theexperimental one, although the predicted solute mass transferis always significantly higher.
The model correctly predicts that the absorption fluxachievable with K2CO3 is about 1 order of magnitude lowerthan that provided by the two amines and provides the sameranking 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 thephysicochemical parameters, in particular by the Henry's lawconstant. The ratio H/(kD)1/2 is a useful overall indicator forcharacterizing absorbent performance,9 because it contains allof the necessary information on physicochemical properties andreaction kinetics.Figure 10 illustrates the variation of this indicator as a
function of the reagent concentration for the three systemsunder consideration, calculated at 25 °C from the previouslymentioned 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 isstrongly influenced by the salt concentration in carbonatesolutions. In this case, although k increases with the ionicstrength 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 6. CO2 diffusion coefficients as functions of reagentconcentration.
Figure 7. Experimental and calculated CO2 fluxes in MEA solutions.
Figure 8. Experimental and calculated CO2 fluxes in K2CO3 solutions.
Figure 9. Experimental and calculated CO2 fluxes in PZ solutions.
Industrial & Engineering Chemistry Research Article
concentration is very low, about 2 orders of magnitude lowerthan that of the carbonate itself, so that the related removalrates are poor.The discrepancy between the experimentally observed CO2
fluxes and those calculated with the limiting hypothesessummarized by eq 2 is due to the invalidity of one of theassumptions made, specifically that regarding the constancy ofthe reagent concentration in the liquid film, CB.This conclusion is supported by the following evidence:
Equation 2 can be rearranged and linearized as
= +p
QR RCO
M L2
(3)
where RM = 1/kM is the membrane resistance and RL, the liquidresistance, is the product of two contributions
=⎛⎝⎜
⎞⎠⎟⎛⎝⎜⎜
⎞⎠⎟⎟R
HkD C
1L
B (4)
Membrane resistance is constant and unchanged for the threedifferent absorbent solutions.For MEA and PZ, as the overall parameter H/(kD)1/2 is
roughly constant, one can expect that RL increases linearly withCB
−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 themembrane resistance.
Figure 12 shows the predicted and experimentally observedbehavior of the MEA system for such a plot. Both sets of data
yield a straight line, as expected. The y-axis intercept isessentially the same for both theory and experiment, indicatingthat the experimental membrane resistance is indeed very closeto the calculated value [more specifically, 1.49 × 109 comparedto 1.72 × 109 kmol/(m2 s Pa)]. On the other hand, the twostraight lines are quite far apart, with the experimental valuesalways being larger than the calculated ones. The obviousconclusion is that the experimental resistance to CO2 transfer inthe liquid phase is always larger than that calculated through eq4. This effect, unless the physical parameters were wronglyestimated, is therefore attributed to the simplifying assumptionof a constant reagent concentration in the liquid film equal tothe concentration in the liquid bulk. The reagent concentrationin the liquid film must be lower than that measured in theliquid bulk, which is quite understandable, as the reagent thereis consumed by the reaction, with the mixing system obviouslynot capable of replacing the reacted material at the necessaryrate. The experimental liquid resistance will be larger than thatcalculated theoretically through eq 4 by a numerical factor, f,whose magnitude indicates how far the system is from the idealsituation of constant maximum reagent concentration in theliquid film. The true liquid resistance can be inferred by fittingeq 3 to the experimental CO2 flux and assuming the calculatedmembrane resistance to be correct (as indicated by Figure 12).f, the ratio between measured and theoretical liquid resistanceRL, can then be calculated, and it is reported in Figure 13 as afunction of the parameter H/(kD)1/2 for the three systemsstudied in this work. The parameter f approaches 1 only for thelargest value of H/(kD)1/2, namely, when the CO2 transfer rateis relatively low as it is severely impeded by the liquidphysicochemical constraints, whereas it grows exponentially forthe lower values of H/(kD)1/2 when larger CO2 fluxes areexpected. Therefore, Figure 13 clearly indicates that the mixingdevice used is not capable of supplying the necessary amount ofreagent to the liquid film and that this deficiency becomesincreasingly 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 carbonatesolution, the membrane resistance always makes a negligiblecontribution, given that the resistance in the liquid film isrelatively high in this case. For the other two cases, however,the membrane resistance tends to become important, although
Figure 10. H/(kD)1/2 ratios as functions of reagent concentration.
Figure 11. OH− concentration as a function of carbonateconcentration.
Figure 12. Experimental and calculated CO2 fluxes, linearized as in eq3, for the MEA system.
Industrial & Engineering Chemistry Research Article
never prominent, as the concentration in the liquid increases,leading to a decrease of the overall resistance in the liquidphase.
■ CONCLUSIONSThe present work has shown how the physicochemicalparameters of the reacting solution influence the carbondioxide uptake rate from a gaseous stream. As expected, theCO2 solubility, diffusivity, and reactivity all play importantroles; their effects, however, cannot be considered separatelybut need to be incorporated correctly into the overall picture. Itwas also shown that the effects on the overall transfer ratesbrought about by the use of a membrane contactor can becomerelevant for the fastest processes. In this case, attention needs toalso be paid to the proper membrane choice.Needless to say, before moving to industrial application of
the present technology, other aspects must be examined, suchas the effects of the temperature and the membrane chemicalstability, which are presently being investigated by our researchgroup.
NotesThe authors declare no competing financial interest.
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Figure 13. Ratios of experimental to theoretical liquid resistances asfunctions of H/(kD)1/2.
Figure 14. Relative importance of the membrane resistance as afunction of liquid concentration.
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