Absorption of CO2 into Aqueous Solutions of Methyldiethanolamine

and Activated Methyldiethanolamine from a Gas Mixture in a ... aqueous solutions of activated methyldiethanolamine (MDEA) and MDEA as absorbents was...
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Ind. Eng. Chem. Res. 2005, 44, 9230-9238

Absorption of CO2 into Aqueous Solutions of Methyldiethanolamine and Activated Methyldiethanolamine from a Gas Mixture in a Hollow Fiber Contactor Jiangang Lu, Lianjun Wang,* Xiuyun Sun, Jiansheng Li, and Xiaodong Liu School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

A membrane gas absorption (MGA) process was evaluated in this work for CO2 capture from CO2/N2 gas-mixed streams at room temperature. The experimental study of a polypropylene (PP) hollow fiber membrane contactor in combination with the technique of CO2 absorption into aqueous solutions of activated methyldiethanolamine (MDEA) and MDEA as absorbents was carried out. A laboratory-scale setup, in which the solution with CO2 loading was able to be hot-regenerated into the solution without CO2 loading and used circularly, was established in this study. The effects of a variety of operation factors, such as gas and liquid flow rates, membrane pore-wetting, and liquid CO2 loading, on the separation performance of the membrane contactor were investigated. The absorption performances were compared between activated MDEA and MDEA. A mathematical model was developed to simulate the mass-transfer behavior of the membrane gas-liquid contactor. The experimental results show that the use of a membrane gas-liquid contactor with improved alkanolamines such as activated MDEA can be completely applied to CO2 capture. Low and steady liquid CO2 loading was able to be controlled by hot-regeneration. The CO2 absorption performance of activated MDEA was remarkably better than that of MDEA. The CO2 removal efficiency could reach more than 99% with activated MDEA. The average overall mass-transfer coefficient with activated MDEA was 2.25 times that with MDEA. The activator piperazine (PZ), even with a small quantity in the activated MDEA, plays a significant role in the improvement of mass transfer in MGA. A comparison of model estimations with experimental results indicates that estimations of the nonwetting mode are divaricated from experimental data. Taking partial-wetting of the membrane into account, the model simulation is validated with experimental data. Partial-wetting can result in significant membrane resistance to mass transfer in MGA. 1. Introduction Over the last 200 years, intensive human activities have caused the concentration of greenhouse gases in the atmosphere to rise significantly. Excessive greenhouse gases have contributed to global warming that resulted in serious environmental problems.1 Signed by 124 countries, the core of the 1997 Kyoto Protocol was to require acceding participants to reduce greenhouse gas emissions from industrial sources. CO2 is the main species of greenhouse gases, and the bulk of the CO2 is emitted from the fossil fuel-based energy infrastructure, such as coal-combustion power generators. These emissions create the need for low energy-consumption, available, efficient technologies for the capture and removal of CO2 from gas mixtures produced by industrial sources. Current capture technologies based on a variety of physical and chemical processes include absorption, adsorption, and cryogenic and membrane techniques. Membrane gas absorption (MGA) is a hybrid process that combines the conventional technique of gas absorption into solvents and a membrane separation module as a mass-transfer contactor (namely, a membrane contactor). MGA has the advantages of both membrane contactor and gas absorption processes. The membrane * To whom correspondence should be addressed. Tel. and Fax: +86-25-8431-5518. E-mail: [email protected].

contactor furnishes a known high specific surface area, independent control of gas and liquid flow rates, and a compact and energy efficient device, while the gas absorption offers a high selectivity and a high driving force for mass transfer.2 MGA can avoid the disadvantages of conventional processes using packed and tray columns, such as flooding, foaming, entraining, channeling, and limiting the opening flow rates. Meanwhile, the operation of MGA differs from that of other membrane separation processes. For example, convective flow through the pores occurs in filtration, whereas only diffusive transport of certain components occurs in MGA. Compared to the conventional processes, the other important advantages of MGA are the following: capably linear scale-up, low corrosion problems, low operation costs and low capital costs, etc.3 MGA has been considered to be a promising and potential largescale application technology for the recovery and removal of CO2.4 In general, the membrane in MGA adopts hydrophobic microporous hollow fibers and the membrane works only as a physical barrier between the gas and liquid phases. For the transmembrane pressure to not be more than the liquid critical entry pressure, a condition which complies with the Young-Laplace equation, hydrophobic membrane materials, such as polytetrafluoroethylene (PTFE) and polypropylene (PP), must have a low surface energy and be able to resist the solution seeping

10.1021/ie058023f CCC: $30.25 © 2005 American Chemical Society Published on Web 10/27/2005

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into membrane pores to ensure the nonwettability of the membrane. The nonwetting operation mode possesses a high mass-transfer coefficient. Various liquid absorbents can be considered to apply in MGA. These absorbents include pure water and aqueous solutions of NaOH, KOH, K2CO3, amines, and amino acid salts. Aqueous solutions of alkanolamines have been the most widely used solvents for removing CO2. Various alkanolamines have been proposed in the literature and in industrial applications, such as monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N-methyldiethanolamine (MDEA), diglycolamine (DGA), di-2-propanolamine (DIPA), and 2-amino-2-methyl-1propanol (AMP).5-7 Among these alkanolamines, MDEA is one of the most widely used alkanolamine solvents because it posseses the characteristics of high selectivity, a large absorption capacity, low regeneration energy, small degradation, and little corrosion.8 In the mid 1980s, in order to further improve the absorption performance of the solvent, an activated MDEA solution was developed by adding an amine activator, piperazine (PZ), into MDEA.9 It was proven that PZ is more effective than other traditional activators, such as monoethanolamine (MEA), diethanolamine (DEA), etc. Activated MDEA solutions have been successfully used in packed tower processes for the removal of CO2.10 Up to now, considerable academic and industrial research about MGA has been carried out. Cussler11,12 initially studied the field of MGA. Currently, a number of articles13-19 on the membrane-based absorption of acid gases can be found. The literature mostly deals with those investigations involving the mass-transfer performance, the effects of operation conditions, and the membrane configuration as well as module structures of absorption flux and mass-transfer coefficients, models for mass transfer, and mass-transfer kinetics. The absorbents used in the investigations were aqueous solutions of NaOH, KOH, MEA, DEA, AMP, the potassium salt of glycine, and taurine. Recently, Mavroudi et al.20 investigated the absorption of CO2 from a CO2/ N2 mixture using a commercial hollow fiber membrane contactor and water or diethanolamine as the absorbing solvent and presented a mathematical model to simulate the process. Their results showed that membrane contactors were significantly more efficient and compact than conventional absorption towers for acid gas removal. Wang et al.21 developed a theoretical simulation to study CO2 capture by absorption in a hollow fiber membrane contactor. Three typical alkanolamine solutions of AMP, DEA, and MDEA were employed as absorbents. The simulation results indicated that AMP and DEA solutions had much higher CO2 absorption fluxes than the MDEA solution, but the concentrations of both AMP and DEA dropped dramatically due to depletion. The reaction kinetics of MDEA with CO2 has been found to be the controlling factor in the process of CO2 capture in the membrane contactor. The theoretical solution also confirms that the nonwetted mode of operation is favored, taking advantage of higher gas diffusivity in order to optimize the CO2 capture performance. Dindore et al.22 evaluated a criterion for the selection of the membrane-solvent combination for CO2 removal in membrane contactors and suggested that a PP membrane with propylene carbonate as an absorbent was a suitable combination for bulk CO2 removal using membrane gas-liquid contactors. In addition, they23 studied CO2 absorption at elevated pressures using a

Figure 1. Molecule structures of MDEA and PZ.

PP hollow fiber membrane contactor and propylene carbonate. Through analyzing all of the above research, the information is obtained that components of the absorbents were aqueous solutions of a single solvent, namely an alkanolamine (e.g., MEA or DEA) dissolved in water to form a solution, and that the solutions were not used circularly in the experimental systems. However, the majority of solutions used in practical applications are composed of blended alkanolamines because blended alkanolamines demonstrate better absorption performance than single alkanolamine solutions. There are few articles involving blended amines such as activated MDEA in MGA. In this work, an experimental setup for the absorption of CO2 into aqueous solutions of MDEA or activated MDEA in a PP microporous hollow fiber membrane contactor was established. The MDEA absorbent was an aqueous solution composed of the single solvent MDEA. The activated MDEA absorbent was a blended aqueous solution composed of both MDEA and PZ; MDEA was the main solvent, and PZ was an activator. The solutions with CO2 loading (i.e., rich solutions) were hot-regenerated in a regenerator and were used continually and circularly in the setup. The mass-transfer performance of MGA for CO2 capture was studied. The absorption performance of the activated MDEA was compared with that of MDEA. A numerical model was presented to simulate CO2 capture in the MGA system. The mass-transfer performance of the membrane contactor combined with activated MDEA or MDEA was evaluated based on both model data and experimental data.

2. Theory 2.1. Activated Mechanism of PZ in Activated MDEA for CO2 Capture. When CO2 in a gas mixture diffuses across the wall of a membrane and enters the liquid in MGA, CO2-MDEA and CO2-PZ chemical reactions occur in the solution. In general, amines are subdivided into primary, secondary, and tertiary amines according to the number of alkyl groups attached to the N atom in the molecular structure of the compound. MDEA and PZ belong, respectively, to the tertiary and secondary amines, and their molecular structures are shown in Figure 1. PZ has a special molecular structure in which a symmetrical diamino cyclic structure exists. The reaction mechanisms of carbon dioxide with alkanolamines have generally been agreed upon. A zwitterionic mechanism and proton transfer can describe the reactions of carbon dioxide with primary, secondary, and tertiary amines.24,25 For activated MDEA mixed with PZ, the reaction mechanism with CO2 can be explained by a homogeneous activation mechanism26

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or a shuttle mechanism.27 The following reactions may occur in the system:

PZ + CO2 h PZ-CO2

(1)

CO2 + H2O h H+ + HCO3-

(2)

PZ + H+ h PZH+

(3)

PZ-CO2 + H2O h PZCOO- + H+

(4)

PZ-CO2 + MDEA h MDEA-CO2 + PZ

(5)

MDEA + H+ h MDEAH+

(6)

MDEA-CO2 + H2O h MDEAH+ + HCO3- (7) PZH+ + H+ h PZH22+

(8)

PZCOO- + H+ h PZH+COO-

(9)

PZCOO- + HCO3- h PZ(COO-)2

(10)

PZCOO- + MDEA + H2O h MDEAH+ + HCO3- + PZ (11) The reaction of CO2 with PZ can be regarded as the rapid pseudo-first-order reaction in parallel with that of CO2 with MDEA.28 The reaction of eq 2 is very weak. Piperazine contains two amino groups. Theoretically, the reaction of eq 10 exists. However, the reaction of eq 10 in the system can be neglected.29 In the course of activation, on one hand, PZ coalesces with CO2 and transfers CO2 to MDEA rapidly (see eqs 1 and 5). On the other hand, PZ reacts with CO2 to form the carbamate product, and then, the carbamate reacts with MDEA to convert its CO2 into bicarbonate (see eqs 4 and 11). PZ itself is reverted in these reactions. Therefore, for activated MDEA, the activator PZ can promote the CO2 absorption rate of an MDEA aqueous solution. 2.2. Mathematical Model. In MGA, the gas and liquid phases flow on opposite sides of the membrane in a countercurrent fashion. CO2 contained in the gas phase crosses the hydrophobic microporous membrane and enters the liquid phase. The gas preferentially fills the membrane pores to meet minimal membrane resistance. Figure 2 describes the mass transfer in MGA. The mass-transfer process consists of three consecutive steps: (1) diffusion of a gaseous component i (i ) CO2) from the bulk gas phase to the outer surface of the membrane; (2) diffusion through membrane pores to the membrane-liquid interface; and (3) dissolution into the absorption liquid and then liquid-phase diffusion/chemical reaction. Thus, the overall mass-transfer process consists of three resistances in series: the gaseous phase boundary layer (1/kg), the membrane (1/kM), and the liquid-phase boundary layer (1/kL). The mass-transfer flux (Ji) of component i is given as eq 12. The overall mass-transfer resistance (1/Kov) can be expressed in a resistance-in-series model

Ji ) kg(Ci,g - Ci,g,mem) ) kM(Ci,g,mem - Ci,g,int) ) mkL(Ci,L,int - Ci,L) ) Kov(Ci,g - Ci,L) (12) 1/Kov ) 1/kg + 1/kM ) 1/(mkL)

(13)

Figure 2. Mass-transfer regions, dominant resistances, and flow configuration in MGA.

where Ci,g, Ci,g,mem, Ci,g,int, Ci,L,int, and Ci,L are the concentration of CO2 in the gas bulk, at the interface of the gas and the membrane, at the interface of the liquid and the membrane, at the interface of the gas and the liquid, and in the liquid bulk, respectively. Kov is the overall mass transfer coefficient; kg, kM, and kL are individual mass transfer coefficients of the gas side, the membrane, and the liquid side, respectively. The variable m represents the distribution coefficient of CO2 between the gas and liquid phases. For chemical absorption, kL ) Ek°L; E is the enhancement factor due to chemical reaction, and k°L is a physical mass transfer coefficient. To develop a mathematical model to describe the absorption of a gas into a liquid flowing through a membrane hollow fiber in MGA, the resistance-in-series concept is used. On the basis of the case of the mass transfer of a gas mixture in the lumen of a hollow fiber membrane and liquid on the shell side, as described schematically in Figure 2, and by combining solution properties, membrane and module geometric characteristics, and process conditions, the following assumptions are presented: (1) steady operation state and isothermal conditions exist; (2) Newtonian fluids with constant physical properties and axis-symmetry are used; (3) ideal gas behavior and obedience of Henry’s law are exhibited; (4) fully developed laminar flow occurs in the lumen; and (5) negligible radial convection and axial diffusion also occur. With these assumptions, the differential equation from a mass balance can describe the CO2 concentration profile in the lumen, and this equation is given as

uz

[ (

)]

∂Ci,g ∂Ci,g 1 ∂ r ) Di,g ∂z r ∂r ∂r

(14)

where Di,g is CO2 diffusion coefficient in the gas phase and uz is the gas velocity inside the lumen. In a laminar flow and fully developed velocity profile, uz can be

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described with the following equation based on Chapman-Enskog theory:30

[ ( )] r rin

uz ) umax 1 -

2

(15)

where umax is the maximum gas velocity in the lumen and rin is the inner radius of the fiber. With the initial conditions for the differential equation eq 14,

Ci,g ) Ci,g,in

for z ) 0 and all values of r (16)

∂Ci,g )0 ∂r

for r ) 0 and all values of z (17)

where Ci,g,in is the concentration of CO2 in the inlet of the gas phase. At the membrane-liquid interface, the flux of CO2 is equal to its flux in the gas phase, based on eq 12; that is, mass is conserved with respect to CO2. Thus, the boundary condition exists

where rmax is the maximal pore size, g(r) represents the pore size distribution function, σ is the surface tension, θ is the contact angle, ∆P is the transmembrane pressure difference, and do and din are the hollow fiber membrane outside and inside diameters, respectively. CO2 diffusion behavior in membrane gas-filled pores depends on the pore size and the molecule average free path. For a binary gas mixture with a smaller pore size at atmospheric pressure, the diffusion behavior is a combination of molecular and Knudsen diffusion.12

1/Di,M ) 1/Di,g + 1/Dk

The molecular diffusion coefficient, Di,g, can be calculated by the Fuller equation (eq 23),34 based on the kinetic theory of gases. The Knudsen diffusion coefficient, Dk, can be determined by eq 24.

10-7T1.75 Di,g )

Di,g(∂Ci,g/∂r) ) Kext(mCi,L - Ci,g) for r ) rin and all values of z (18) where Kext is the external (membrane wall and shell side) mass-transfer coefficient. Kext is a combination of the mass-transfer coefficients of both the liquid phase and the membrane and is derived from eq 13.

1 1 1 1 1 ) + ) + Kext kM mkL kM mEko

(19)

(1 - η)τδ ητδ 1 + ) kM Di,L Di,M

(20)

where Di,L and Di,M are CO2 diffusion coefficients in the liquid and in the membrane pores, respectively, and where η, τ, δ, and  are the pore-wetted ratio, the tortuosity of the membrane pore, the thickness of membrane wall, and the porosity, respectively. The tortuosity33 for eq 20 can be written as τ ) 1/. The value of η is estimated by the following equation:42

η)

4δτ

∫2σcosθ/∆P r g(r) dr rmax

2

i

2

2

L(do - din )

(21)

(

1 1 + MCO2 Mmix 1/3

P(VCO2 Dk )

)

0.5

1/3

+ Vmix )

x8RT πM

4dp 3τ

(23)

(24)

where T is the temperature, P is the pressure of the gas phase, dp is the membrane pore diameter, M is the molecule weight, R is the gas constant, and V is the structural volume of a molecule. The diffusion coefficient, Di,L, can be given below as35

Di,Lµ0.54/T ) 6.109 × 10-8

L

The PP microporous membrane is hydrophobic, and the liquid phase operation pressure is slightly higher than that of the gas phase to prevent dispersion of gas bubbles into the liquid. The transmembrane pressure is controlled to not be more than the liquid critical entry pressure to ensure that the operation mode of the gasfilled membrane pores is correct. Theoretically, the solution does not penetrate the pores and the gas-liquid interface is immobilized at the pore mouth of the membrane on the solution side.31 However, partialwetting of the membrane pores is actually found in MGA because of the use of aqueous solutions of organic compounds as absorbents.20,32 Therefore, partial-wetting should be considered in the model in order to obtain more accurate simulation values. Mavroudi et al. also considered the factor of partial-wetting in their study.20 The membrane mass-transfer coefficient, kM, is calculated by the following equation:

(22)

(25)

where µ is the viscosity of the solution. The calculation of the enhancement factor, E, from eq 19 uses the conventional mass-transfer model. A number of other methods of calculation for E can be found in the literature (ref 36). The reaction of CO2 with an aqueous solution of MDEA or activated MDEA can be treated as a fast pseudo-first-order reaction.37 The calculation of E is given by the following based on the bifilm model:

E)

Ha tanh Ha

(26)

where the Hatta number Ha is given by

Ha )

xkovDi,L koL

(27)

For Ha > 3, tanh Ha is approaching 1; thus, E is approximately equal to Ha. In eq 27, kov is the overall pseudo-first-order reaction rate constant. When MDEA is the absorbent, the kov is expressed as

kov ) kMDEACMDEA

(28)

where CMDEA is the concentration of the solvent and the reaction rate constant, kMDEA, is determined as follows: 38

kMDEA ) 4.01 × 108 exp(-5400/T)

(29)

With activated MDEA as the absorbent, the reaction of CO2 with PZ can be regarded as the rapid pseudo-firstorder reaction that is in parallel with that of CO2 with

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Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 Table 1. Properties of the Hollow Fiber Membrane Module module i.d. (mm) fiber o.d., dout (µm) fiber wall thickness, δ (µm) average pore size, dp (µm)

Figure 3. Schematic diagram of the experimental setup for CO2 absorption with a membrane contactor: (1) mixed-gas cylinder; (2 and 7) flow meters; (3) membrane contactor; (4) stripper; (5 and 11) coolers; (9) gas-liquid separator; (6) pump; (8) solution tank; (10) valve controller; (12) gas chromatography; (S) sample point for the liquid; (P) pressure meter.

MDEA.28 The concentration of free piperazine will not be depleted because CO2 is rapidly transferred to MDEA. The kov is expressed as

kov ) kMDEACMDEA + kpCp

(30)

where CP is the concentration of piperazine in the solution and the reaction rate constant, kP, is determined as follows:39

kp ) 4.49 × 1012 exp(-5712/T)

(31)

The distribution coefficient, m, and the viscosity, µ, are estimated from the literature.40,41 The numerical scheme for the partial differential equations can be solved along with the initial and boundary conditions as well as the reaction kinetics of CO2 with amine solutions. Discretization for the partial differential equation adopts the Euler equation. The initial and boundary conditions are accordingly transferred into an equation corresponding to the discrete function. The calculation values are gained using the Matlab program. 3. Experimental Section 3.1. Experimental Unit and Procedures. Figure 3 is a schematic diagram of the experimental setup for CO2 capture. Prior to the experiments, the gas mixture (i.e. feed gas) was prepared in a gas-prepared system to a given concentration based on the partial-pressure principle, while the aqueous solutions of amines (e.g., MDEA, MDEA + PZ) were prepared in the feed tank with deionized water to a given concentration. By turning the valve of the mixed-gas cylinder to the desired flow rate, the mixed-gas stream, through a gasflow meter, was fed into the fiber lumen of the end of the module at a pressure which was slightly lower than that of the liquid side to prevent dispersion of gas bubbles into the liquid. The differential pressure between the gas and liquid sides was indicated by a pressure-differential meter. The liquid absorbent was introduced by a gear pump from the solution tank, through a liquid-flow meter, to the shell side of the module in counterflow mode in the opposite direction as the gas stream. CO2 in the gas mixture in the fiber lumen diffused through the membrane pores into the

32 500 100 0.045

porosity,  0.60 fiber length, L (mm) 200 number of fibers, n 1500

liquid in the shell side and was absorbed by the solution. The treated gas stream was released from another end of the module. The solution which contained CO2 entered a stripper and was hot-regenerated. CO2 was released from the solution, and the vapor stream containing CO2 was exhausted out of the top of the stripper. The vapor stream was cooled through the water-cooler, and in the gas-liquid separator, CO2 was separated from the condensate water that was returned to the top of the stripper. An electric-heating device heated the solution collected in the bottom of the stripper to form the steam. To ensure that the solution would be regenerated completely, a high power (2500 W) electric heater was used in the system. The stripper size was 57 mm × 3.5 mm × 300 mm (outside diameter × wall thickness × height). The outside of the stripper was wrapped with heat-preserved material. Stainless steel threadlike reticulation of a 200 mm height was packed into the stripper as packing. The regeneration temperature could be controlled by regulating the voltage. The solution which had been regenerated was cooled through the water-cooler and returned to the solution tank. The solution was used circularly in this system. The flow meters were calibrated beforehand. The experiment was continued for about 45 min to allow the system to reach steady state. Steady state conditions were indicated by a constant CO2 concentration from gas chromatography in the outlet gas stream of the module and a constant liquid loading of CO2 of the samples after regeneration. Within the range of the liquid flow rates used in the experiments, CO2 loading of the regenerated solution was not more than 0.001 mol/mol-amine. So, the CO2 and amine concentrations of the solution in the inlet of the module could be thought to be 0 and constant, respectively. Experiments were carried out at atmospheric pressure (0.1 MPa) and at room temperature (19-24 °C). The transmembrane pressure was maintained at 160-280 mmH2O. The solution concentrations were the following: single amine, 2.5 kmol/m3 MDEA; activated MDEA, 2.0 kmol/m3 MDEA + 0.5 kmol/m3 PZ. The properties of the hollow fiber membrane module are summarized in Table 1. 3.2. Materials and Methods of Analysis. In this research, N2 and CO2 were commercial cylinder gases and their purity was more than 99.99% (Nanjing Real Special-gas Co., China). The purity of MDEA was 99.5% (Changzhou Jusheng Chemical Co., Jiangshu China), and the purity of PZ was 99.9% (Shanghai Chemical Co., China). The total amine concentration was determined by titration with a standard 0.1 kmol/m3 sulfuric acid (H2SO4) solution using a methyl orange indicator. The content of CO2 in the gas phase was analyzed by gas chromatography (CP3380, Varian Co., USA). For the determination of the CO2 content in the liquid phase, a known volume of the liquid sample was acidulated with a 1:4 ratio of a H2SO4 aqueous solution and the volume of the evolved gas was measured with a gas buret. After temperature and pressure corrections, the CO2 content of the liquid sample was calculated. The error was (0.02 mL.

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Figure 4. Effect of pore-wetting on the gas outlet concentration.

4. Results and Discussion 4.1. Effect of Pore-Wetting on Gas Outlet Concentration. Usually, water or aqueous solutions of inorganic compounds have difficulty wetting the hydrophobic membranes because of high surface tensions and large contact angles (contact angles are 90-180° for hydrophobic materials). Contrary to that, aqueous solutions of organic compounds easily wet the hydrophobic membranes, possibly because of their lower surface tensions and smaller contact angles. Partial-wetting of membrane pores would result in a significant increase in the resistance of the membrane,42 even though the fraction of wetted pores is small, which would affect operation stability and the long-term running of a membrane contactor. Experiments into the effect of pore-wetting on gas outlet concentrations, expressed in a dimensionless form (Ci,g,out/Ci,g,in) as the increase with the liquid flow rate VL, were carried out, and experimental data were compared with simulation data. The result is given as Figure 4. The following conclusions can be drawn: (a) The CO2 outlet concentration with activated MDEA is obviously lower than that with MDEA. When a small quantity of PZ was added into MDEA and the overall concentration of amines in activated MDEA was the same as that of MDEA, the activation of MDEA by PZ for CO2 absorption was revealed. This performance of PZ partially depends on the special molecular structure of PZ. In addition, both the activated mechanism and reaction kinetics can also account for the performance. For the same operation conditions, the reaction rate constant of CO2 with activated MDEA is significantly larger than that with MDEA. Absorption is more efficiently facilitated by the chemical reaction of PZ. (b) The concentration of CO2 in the gas outlet decreases with an increase of the liquid flow rate. An increase of the liquid flow rate makes the thickness of the liquid boundary layer decrease. The resistance of the liquid side decreases. Hence, this results in more efficient mass transfer. (c) The mathematical model can well describe the mass transfer of MGA. Taking nonwetting (η ) 0) behavior into account, the simulation values diverge from the experimental values. The simulation values of the CO2 outlet concentration are considerably lower than the experimental values. The average deviation is 23.6%. Pore-wetting depends on the membrane configuration (e.g., pore size and distribution), physiochemical properties of the liquid (e.g., surface tension), and operation parameters (e.g., transmembrane pressure difference). According to

Figure 5. Effect of the gas and liquid flow rates on the removal efficiency.

previous work42 and as estimated from eq 21, a proper value for partial-wetting behavior (η ) 8%) was obtained. Taking partial-wetting (η ) 8%) into account, the simulation values have good agreement with the experimental values. This demonstrates that the aqueous solutions of MDEA and activated MDEA have to some extent an effect on the pore-wetting of the PP membrane. Under the operation conditions of this study, the pore-wetted ratio average, η ) 8%, fits the experimental system. 4.2. Effect of Gas and Liquid Flow Rates on Removal Efficiency. Removal efficiency is one of the separation properties of MGA. Removal efficiency, γi, is expressed as the percentage of component i (i ) CO2) in the gas stream that was removed during the absorption operation. The value of γi is calculated by the following equation, based on the material balance:

γi ) 1 -

(

)(

)

yi,out 1 - yi,in 1 - yi,out yi,in

(32)

Figure 5 shows the effect of the gas and liquid flow rates on the removal efficiency with activated MDEA. The removal efficiencies increase with an increase of the liquid flow rates, while the efficiencies decrease with an increase of the gas flow rates. As the gas and liquid flow rates approach high values, the gas and liquid side resistances to mass transfer become negligible and the membrane resistance becomes the dominant factor. The increase of gas flow rates brings on a decrease of gasliquid contact time. Sequentially, this induces a decrease in the removal efficiency. The model prediction of a nonwetting mode of operation gives higher CO2 removal compared with experimental data. The model prediction of a partial-wetting mode (η ) 8%) is in better accordance with the experimental values. The values of the partial-wetting prediction are illustrated in Figure 5. The data from Figure 5 also indicate that, the higher the liquid flow rates, the more pores are wetted. In the meantime, when an appropriate operation condition is adjusted, a removal efficiency of over 99% is easy to obtain with activated MDEA using the process of MGA. 4.3. Comparison of Overall Mass-Transfer Coefficients between Activated MDEA and MDEA. For a given module, the membrane just acts as a barrier between the gas and liquid phases in MGA. The facilitation of mass transfer relies essentially on chemical reaction. Although alteration of hydrodynamics such

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Figure 6. Comparison of the overall mass-transfer coefficients of MDEA and activated MDEA.

Figure 7. Effect of liquid CO2 loading on the separation performance: VL ) 3.5 × 10-3 m/s, Vg ) 0.055 m/s, yCO2 ) 10.88%.

as the flow rate can also promote mass transfer, this promotion is limited. Thereby, it is important to elect a high performance absorbent with a high reaction rate constant. Figure 6 illustrates the effect of the absorbents MDEA and activated MDEA on the overall masstransfer coefficient, Kov. The value of the overall masstransfer coefficient was theoretically calculated from eq 13, and the value was experimentally calculated based on the literature.20 For completely identical operation conditions, the overall mass-transfer coefficient, Kov, with activated MDEA is much higher than that with MDEA. The average value of Kov with activated MDEA is 2.25 times that with MDEA. Comparison of the overall pseudofirst-order reaction rate constant, kov, between eqs 27 and 29, although there is a small quantity of PZ in the activated MDEA, shows that the kov of activated MDEA is much larger than that of MDEA because PZ has a high reaction rate with CO2.28,39 So, activated MDEA significantly enhances chemical reaction with CO2. Meanwhile, the model predictions are in good agreement with the experimental results for a partial-wetting mode (η ) 8%). In addition, a decrease of the CO2 concentration in the feed gas results in an increase of Kov from the information in Figure 6. The CO2 concentration in the outlet largely increases with an increase of the CO2 concentration in the feed gas. The effect of the CO2 concentration in the feed gas on the overall masstransfer coefficient will be further investigated. 4.4. Effect of Liquid CO2 Loading on Separation Performance. The parameter R is the content of CO2 dissolved in the solution, i.e., the quantity of CO2 held by both physical and chemical absorption. It is expressed as moles of CO2 per mole of amine. Different levels of liquid CO2 loading were obtained by controlling the regeneration heat by regulating voltage. Liquid CO2 loading increased with a decrease in the regeneration heat. For a given operation condition, the apparatus continued to run until the system reached a steady state value of the required liquid CO2 loading. All experimental data were determined at the steady state. The experimental results are given as Figure 7. With an increase of the liquid CO2 loading, the overall mass-transfer coefficient decreased and the outlet CO2 concentration of the gas phase increased. Essentially, an increase of the liquid CO2 loading results in a decrease of the free amine concentration.43 The chemical enhancement factor, E, is proportional to (Camine)0.5 in the present case. Hence, E decreases with an increase of the liquid CO2

loading, and Kov decreases with it. In addition, from an analysis of thermodynamics, the solubility of CO2 in the liquid decreases with an increase of the liquid CO2 loading. The driving force decreases on the liquid side. A comparison of the performance between activated MDEA and MDEA shows that the decreasing trend of Kov with activated MDEA is clearly more laggardly than that with MDEA. Within the range 0.05-0.30 mol/(mol of liquid CO2 loading), the Kov with activated MDEA declines from 4.78 × 10-5 to 3.11 × 10-5 m/s, while the Kov with MDEA declines from 2.71 × 10-5 to 0.76 × 10-5 m/s. The rate of decline of the value of Kov with MDEA is about 2 times that with activated MDEA. Owing to the activation of PZ in the course of CO2 absorption, the overall amine concentration in the liquid boundary layer of activated MDEA is less easily depleted than that of MDEA. This demonstrates again that the process of CO2 absorption into activated MDEA has an advantage over that into MDEA in MGA. 5. Conclusions The CO2 capture from CO2/N2 mixtures was investigated using a MGA process with aqueous solutions of activated MDEA and MDEA as the absorbing solvents. A mathematical model was developed to simulate the mass-transfer behavior of the membrane gas-liquid contactor. The experimental results show: (1) A membrane gas-liquid contactor with improved alkanolamines such as activated MDEA can be sucessfully applied to CO2 capture. A low and constant liquid CO2 loading is can be obtained via hot-regeneration. The regenerated solution can be used continuously and circularly in the experimental system. The properties of the experimental system are important for MGA to be applied to practical industry. (2) By means of regulating the operation conditions (e.g., gas or flow rate, liquid CO2 loading, etc.), a low outlet CO2 concentration is very easy to obtain. The membrane contactor displays the characteristic of flexible operation. Notably, achieving a low outlet CO2 concentration is linked to using a high performance absorbent. (3) From estimations of the removal efficiency and the overall masstransfer coefficient, the performance of activated MDEA for the absorption of CO2 remarkably exceeds that of MDEA. The CO2 removal efficiency can reach more than 99% with activated MDEA. The average value of Kov with activated MDEA is 2.25 times that with MDEA. The activator PZ plays a significant role in the improvement of mass transfer in MGA, even with a small

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quantity in the activated MDEA. (4) The model can well describe the mass transfer of the membrane contactor. Comparison of estimations with experimental results indicates that model estimations of nonwetting mode behavior deviate from experimental data. Taking partialwetting of the membrane pores into account, the model simulation is validated with experimental data. This demonstrates that aqueous solutions of alkanolamines can, to some extent, wet hydrophobic PP microporous membranes, especially with a high flow rate, and that partial-wetting can result in significant membrane resistance to mass transfer. Acknowledgment The authors gratefully acknowledge the support of the Doctor Project Foundation of the Ministry of Education, China. Nomenclature C ) concentration (mol/m3) CP ) concentration of piperazine (mol/m3) dout ) fiber outer diameter (µm) dp ) average pore size (m or µm) D ) diffusion coefficient (m2/s) E ) enhancement factor for chemical reaction Ha ) Hatta number J ) mass-transfer flux (mol/(m2 s)) Kov ) overall mass-transfer coefficient (m/s) kov ) overall pseudo-first-order reaction rate constant (1/ s) k ) individual mass-transfer coefficient (m/s) or reaction rate constant (m3/(mol s)) L ) length of a hollow fiber (m) m ) distribution coefficient M ) molecular weight (kg/kmol) P ) pressure (atm) r ) radial coordinate (m) R ) gas constant T ) temperature (K) u ) gas-phase velocity (m/s) V ) linear flow rate (m/s) or structural volume of a molecule y ) mole fraction of i in the gas phase z ) axial coordinate (m) Subscripts ext ) external g ) gas phase i ) component () CO2) in ) inlet or inner int ) interface L ) liquid phase mix ) mixture M ) membrane MDEA ) methyldiethanolamine out ) outer PZ ) piperazine Greek Letters R ) liquid CO2 loading (mol/mol) γ ) removal efficiency δ ) thickness of membrane wall (m or µm)  ) porosity η ) pore-wetted ratio θ ) contact angle µ ) viscosity (Pa s) σ ) surface tension (N/m) τ ) tortuosity of the membrane pore

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Received for review March 7, 2005 Revised manuscript received June 7, 2005 Accepted September 19, 2005 IE058023F