Enhanced Carbon Dioxide Separation by Amine-Promoted Potassium

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Enhanced Carbon Dioxide Separation by Amine-Promoted Potassium Carbonate Solution in a Hollow Fiber Membrane Contactor Sara Masoumi, Peyman Keshavarz, Shahab Ayatollahi, Morteza Mehdipour, and Zahra Rastgoo Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 12 Aug 2013 Downloaded from http://pubs.acs.org on August 13, 2013

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Enhanced Carbon Dioxide Separation by Amine-Promoted Potassium Carbonate Solution in a Hollow Fiber Membrane Contactor Sara Masoumi, Peyman Keshavarz*, Shahab Ayatollahi, Morteza Mehdipour, Zahra Rastgoo Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz, Iran

ABSTRACT Aqueous solution of potassium carbonate is an appropriate absorbent for cost-effective separation of CO2 from flue gas. Amine-promoted potassium carbonate has the potential to take advantage of both absorbents. In this study, a mathematical model has been developed to simulate the absorption of CO2 into promoted potassium carbonate solutions in a hollow fiber membrane contactor, where monoethanolamine, diethanolamine and methyldiethanolamine have been considered as promoters. A numerical scheme was applied to solve the simultaneous partial differential equations in the liquid, membrane and gas phases and the results were validated with available experimental data in the literature for all promoters. The effects of promoter concentration, temperature, gas and liquid flow rates, flow directions, axial diffusion in the gas phase and possible wetting of membrane were investigated using the model. The promoted solution with monoethanolamine had much higher flux, about four times superior to notpromoted absorbent. Simulation results indicated that the promoted potassium carbonate is only effective in a specific range of operating conditions. The membrane wetting can reduce the flux impressively for all solutions; however, the flux was still much higher than not-promoted solution even at high wetting fractions. 1 ACS Paragon Plus Environment

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1. INTRODUCTION Carbon dioxide is an important greenhouse gas, and it is naturally found in the earth's atmosphere as a part of carbon cycle. Human’s activities disturb this cycle by adding more carbon dioxide to the atmosphere than it can be rejuvenated. Among various techniques for capturing carbon dioxide, membrane processes have been observed widespread use in recent decades due to their advantages to traditional processes. Some of these advantages are: 1) Having high and constant contact area per unit volume of contactor. 2) Easily change the capacity by adding and reducing the number of modules. 3) No need a density difference between two fluids. 4) Gas and liquid phases are separated by membrane; therefore, there is no flooding, loading, weeping, foaming etc. For the first time, Zhang and Cussler1,2 used the hollow fiber membrane contactor (HFMC) for CO2 absorption. They employed a microporous non-wetted polypropylene membrane, where aqueous sodium hydroxide solution was used as an absorbent. Since then, HFMCs have been studied to separate some gases such as CO2 and SO2 by Karoor and Sirkar3, among others. These investigations indicated that the mass transfer flux was much higher than those usually found in packed towers. Chemical absorption is one of the most common methods of CO2 capture. Many solvents are available; however, process selection must be according to economics (Solvent cost and energy requirement for solvent regeneration) and clean-up ability. The most widely used chemical solvents are aqueous alkanolamines such as MEA (monoethanolamine), DEA (diethanolamine), MDEA (methyldiethanolamine), AMP (2-Amino-2-methyl-1-propanol) and DIPA (Diisopropanolamine)4. Hot potassium carbonate solution and amino alcohol solutions such as Sulfinol can also be applied for CO2 separation. 2 ACS Paragon Plus Environment

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Kim and young5 used AMP, MEA and MDEA for separating carbon dioxide from a mixture of CO2/N2 in a PTFE (polytetrafluoroethylene) hollow fiber membrane contactor and noticed that among the considered absorbents, AMP exhibited a higher absorption capacity and moderate absorption rate. Keshavarz et al.6-8 presented a mathematical model to simulate the absorption of carbon dioxide and hydrogen sulfide in hollow fiber membrane contactors. They also checked the effects of membrane wetting on separation performance. Dindore et al.9 explored the absorption of CO2 and H2S using aqueous potassium carbonate as a solvent in cross-flow membrane contactors. Aqueous solution of potassium carbonate was also studied by Mehdipour et al.10 in a HFMC. They showed that there was an optimum concentration of potassium carbonate at each solution temperature. Golkhar et al.11 applied nanofluids of nanosilica and carbon nanotube as absorbents in a gas-liquid membrane contactor for CO2 separation. Mixture of amines was noticed in membrane contactors recently12,13. It has been

shown that the absorption performance of the activated-MDEA was much better than that of the non-activated MDEA. While membrane contactors have several advantages compared to traditional towers for CO2 separation from flue gas, selection of absorbent is still an important challenge14. Amine solutions, especially MEA, have relatively higher rate of reactions with CO2 compared to carbonate solutions. Nevertheless, their performance as solvent is limited due to high heat of reaction, amine loss, amine degradation and corrosion. One way to enhance the overall solvent performance is to blend a fast reactant, such as MEA, with a solvent having a low heat of reaction, such as potassium carbonate. The effects of adding amines into carbonate solutions have previously been studied by some researchers in traditional towers15,16. The study of Nii and Takeuchi17 is the only work on the absorption of CO2 and SO2 in microporous membrane

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modules using amine-promoted carbonate solutions. As a comprehensive experimental study, alkanolamines, Na2SO3, K2CO3, NaOH and amine-promoted K2CO3 were applied as absorbent in the fibers. In this work, a comprehensive simulation study was done to analyze the absorption of CO2 from gas mixtures by MEA/DEA/MDEA promoted potassium carbonate solutions in a HFMC. The governing equations in the liquid, membrane and gas phases were derived by applying the mass balance for each diffusing components. A numerical scheme was applied to solve the simultaneous partial differential equations in the three phases and the model results were validated with the experimental data of Nii and Takeuchi17. Absorption performance of aqueous K2CO3 solutions, aqueous amine solutions and amine-promoted K2CO3 solutions were compared. The effects of promoter concentration, temperature, gas and liquid flow rates, flow directions in module, axial diffusion in the gas phase and possible wetting of membrane were investigated using the model.

2. MODEL DESCRIPTION According to Figure 1, a molecule of gas should firstly diffuse from the bulk of gas phase towards the outer of membrane wall. Then, it should diffuse into the non-wetted membrane pores, where the pores are filled with a stagnant gas film. The next step is the dissolution of CO2 into the absorbent and diffusion with chemical reaction inside the wetted pores. Finally, it will penetrate to the liquid bulk inside the fibers. In fiber with a high degree of hydrophobicity, the pores will remain dry and the diffusion inside the wetted pores will not exist. The gas phase environs the fibers and flows in an opposite direction to the liquid inside the fibers. Steady state


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and isothermal conditions are assumed for all sections. The ideal gas behavior can be assumed for the gas phase due to atmospheric pressure.

2.1. Reaction Scheme In an aqueous solution of potassium carbonate and amine as promoter, K2CO3 is ionized into K+ and CO 32- , and also amine is protonated partially9,18. Then, the carbonate reacts with water to form hydroxide and bicarbonate ions. In a carbonate solution (without promoter), absorbed CO2 will react with water and hydroxide to form bicarbonate ions. Reactions (1) to (5) can be applied in this aqueous solution, and reaction (6) is the overall reaction of K2CO3 with CO2 in the system9,18. Kw H2O ←  →H+ +OH−

(1)

K2 Amine + + OH - ←→ H 2 O+ Amine

(2)

K3 HCO 3- + OH - ←→ H 2 O+ CO32-

(3)

K4 CO 2 + H 2 O ←→ H + + HCO 3-

(4)

K5 CO 2 + OH - ←→ HCO 3-

(5)

CO 2 + CO 32- + H 2 O ← → 2 HCO 3-

(6)

Zwitterion mechanism is the most acceptable mechanism for the reaction of carbon dioxide with MEA/DEA according to reactions (7) and (8)19,20. Based on this mechanism, CO2 reacts with MEA/DEA to generate zwitterions, which is an intermediate component and can 5 ACS Paragon Plus Environment

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react with each base in the solution such as MEA/DEA, water and hydroxide ion21,22. The effect of hydroxide ion without causing a substantial loss of accuracy can be negligible22. '

k CO 2 + R1R 2 NH ← → R 1R 2 NH + COO -

(7)

k R 1R 2 NH + CO O - + B  → R 1R 2 NCOO - + BH +

(8)

Where R1 and R2 represent alkyl groups for primary amines, while R2 is a hydrogen atom for secondary amines. The reaction rate of CO2 in an aqueous solution of potassium carbonate promoted by MEA/DEA was proposed as follows22,23,24:

R CO2 = − k 4 [ CO 2 ] +

K 3 K w k 4  HCO3-  K 4 CO32- 

2

−

k 5 [ CO 2 ] CO32-  K 3  HCO3- 

+

k 5  HCO3-  K5

[ CO2 ][ MEA/DEA ]

−

1 k

' MEA/DEA

+

(9)

1

k M/D H2O [ H 2 O] + k MEA/DEA [ MEA/DEA ]

In reactions (7) and (8), for the consumption of one mole CO2, two moles of amine are required. Therefore, the reaction rate for the consumption of amines can be expressed as22:

R MEA/DEA = −

1 k

' MEA/DEA

2 [ CO 2 ][ MEA/DEA ] 1 + k M/D H2O [ H 2 O] + k MEA/DEA [ MEA/DEA ]

The rate of reaction for carbamate generation is:


(10)

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RR R 1

2 NCOO

-

[ CO2 ][ MEA/DEA ]

=

1 k

' MEA/DEA

+

1

(11)

k M/DH2O [ H 2O ] + k MEA/DEA [ MEA/DEA ]

The reaction of CO2 with tertiary alkanolamines such as MDEA has been proposed by Donaldson and Nguyen25 as follows: k MDEA CO 2 + R 3 N+ H 2 O ← → R 3 NH + + HCO 3-

(12)

For CO2 absorption in a mixture of potassium carbonate and MDEA, the reaction rate for CO2 can be expressed as24,26:

R CO2 = − k 4 [ CO2 ] +

K 3K w k 4  HCO3- 

2

−

K 4 CO32- 

k 5 [ CO2 ] CO32-  K 3  HCO3- 

+

k 5 HCO3-  K5

- k MDEA [ CO2 ][ MDEA ]

(13)

According to reaction (6), the reaction rate for HCO 3- and CO32- are:

R HCO-

3

2  K3K w k 4 HCO3-  k 5 [ CO2 ] CO32-  k 5  HCO3-    = 2  k 4 [ CO2 ] − + − 2  K5 K 4 CO3  K3  HCO3   

R CO2- = − k 4 [ CO 2 ] + 3

K 3 K w k 4  HCO3-  K 4 CO32- 

2

−

k 5 [ CO 2 ] CO32-  K 3  HCO3- 

+

k 5  HCO3-  K5

(14)

(15)

2.2. Mass Transfer in Liquid Phase Based on the mentioned assumptions, a component material balance can be written on the liquid phase for each diffusing component:


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Uz ( r )

 ∂2 C jL 1 ∂ C jL  = D jL  + +Rj  ∂ r2  ∂z r ∂ r  

∂ C jL

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(16)

Where subscript 'j' denotes each component in the absorbent. The velocity profile of absorbent inside the fibers is assumed to follow newtonian laminar flow because of low Reynolds number:

U (r) = 2 U z av L

2     r   1 -  R     i   

(17)

The initial condition for each available component at the entrance of fibers is expressed as: C

jL

=C

jL, in

for z = 0

(18)

Symmetrical condition is assumed at the axis of the cylindrical fibers, therefore equation (19) can be used as second boundary condition: ∂C

jL = 0 ∂r

(19)

for r = 0 and all z

2.3. Mass Transfer in Membrane Phase 2.3.1. Non-Wetted Case: In a hydrophobic fiber, a material balance for CO2 transferring inside the gas-filled membrane pores can be considered as follows: 8 ACS Paragon Plus Environment

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D jm

non − w et

 ∂2 C ∂C  jm 1 jm +  2 r ∂r  ∂r 

  = 0  

(20)

For non-wetted pores, diffusion of gas molecules through a pore may follow ordinary diffusion and/or in series with Knudsen diffusion when pore diameters are very small. In microporous membranes, usually both ordinary and Knudsen diffusion should be considered due to small pore diameter. The diffusion coefficient is modified as27:

D jm non −wet

  ε =  τ  1    D j 

  1    1   +    D   kj  

(21)

Where Dkj is the Knudsen diffusion coefficient, which is estimated from the kinetic theory of gases for a straight, cylindrical pore of diameter dp27: 1

 T 2 Dkj = 0.485d p   M

(22)

At the interface of membrane and absorbent, the mass balance leads to another boundary condition: ∂C D jL

jL = D jm non-wet ∂r

∂C

jm ∂r

for r = R and all z i


(23)

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Some component in the absorbent cannot transfer to the gas phase. For these species, we have: ∂C

D

jL =0 jL ∂ r

(24)

for r = R and all z i

At the interface of membrane and absorbent, Henry’s law can be used because of low solubility of CO2: C CO

2 ,L

= H C CO

(25)

for r = R i and all z

2 ,m

At the outer surface of fibers (R=Ro), the boundary condition is:

D jm non-wet

∂ C jm ∂r

= D jG

∂ C jG ∂r

for R = R o and all Z

(26)

2.3.2. Wetted Case: For a hydrophilic membrane, or for a hydrophobic membrane when liquid penetrates partially to the pores, a part of pores is filled with a stagnant film of absorbent. By considering chemical reaction, the governing equation in this part is:  ∂2 C ∂C  jm 1 jm D + jm wet  ∂ r 2 r ∂ r  

  +Rj = 0  

(27)

The membrane diffusivity of species within the wetted pores is defined as:


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D jm wet =

D jL ε

(28)

τ

The boundary conditions at r = R i , R o are the same as non-wetted case, however one additional boundary condition at r = R w is needed:

D jm non-wet

∂ C jm ∂r

= D jm wet

∂ C jm

for r = Rw and all z

∂r

(29)

For this case, Henry’s law is applied at r = R w where Rw=Ri+δl. Here, δl is the liquidfilled membrane thickness.

2.4. Mass Transfer in Gas Phase The steady state conservation equation in the shell side leads to following equation: Uz ( r )

 ∂ 2 C jG 1 ∂ C jG = D jG  +  ∂ r2 ∂z r ∂r 

∂ C jG

   

(30)

The shell side flow is very complex8. Because of uniformity in the distribution of fibers in the HFMC applied in this study, the Happel's free surface model28 is suggested to describe the velocity profile of the gas phase: 2     R o   U (r) = 2 U 1× β (r) z av   R   G   e   

β (r ) =

(r

(31)

R e ) - ( R o R e ) + 2 ln ( R o r ) 2

2

3 + ( R o R e ) - 4 ( R o R e ) + 4 ln ( R o R e ) 4

2


(32)

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In this model, no fiber-fiber interaction is assumed, and therefore each fiber is surrounded by a fluid envelope (i.e., a free surface) with no mass transfer across this area. Re is defined as: 0.5 1 R =  R e θ  o

(33)

Because of the assumption of no fiber-fiber interaction, the proposed boundary condition at Re is: ∂C

jG =0 ∂r

(34)

for r = R and all z e

The initial condition for the gas phase is expressed as the following equations based on the cocurrent or countercurrent conditions:

C C

jG jG

=C =C

jG , in jG , in

for z = 0

(Cocurrent flow)

(35)

for z = L

(Countercurrent flow)

(36)

The kinetic parameters, equilibrium constants, diffusion and solubility coefficients, which are used in calculations, and their references are listed in Table 1 and 2.


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3. NUMERICAL METHOD OF SOLUTION Concentration profile of each component both in the radial and axial directions were obtained by solving four governing equations (16, 19, 26, 29) simultaneously in the liquid, wetted and non-wetted membrane and gas phases. Some of these partial differential equations (PDEs) are highly nonlinear due to the nonlinear chemical reaction terms in the absorbent. A numerical procedure should be utilized to solve these equations due to complexity of the system. To avoid instability, an implicit finite difference method was applied to solve the set of equations in MATLAB. The concentration of some species such as carbamate, amine, carbonate and bicarbonate have relatively small variation in the axial direction, based on which the reaction terms were linearized6. According to this procedure, a set of simultaneous linearized algebraic equations was reached, which is in form of a tridiagonal matrix in the radial direction. Thomas algorithm was a fast method for solving this matrix as an initial guess. Because of linearization method, this procedure was repeated until convergence was obtained. In some cases, for examples in the case of high partial pressure of CO2, the variation in the axial direction will be enlarged. In these cases, the number of required iterations will increase to correct the initial guess. However, even in these cases, the computer run time is much lower than other methods of linearization. To confirm the accuracy of numerical method a material balance error was evaluated at the end of program.


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4. RESULTS AND DISCUSSION 4.1. Model Validation A number of absorbents including MEA, DEA, MDEA and K2CO3 aqueous solutions as well as mixtures of mentioned amines with K2CO3 have been applied in this study. Therefore, the proposed model should be validated for all of these absorbents. Figures 2 and 3 compare the model results with the experimental data of Nii and Takeuchi17 for amines and promoted K2CO3, respectively. It can be seen from these figures that the model results have a good agreement with the experimental data for CO2 absorption in hollow fiber membrane contactors. Figure 2 presents CO2 absorption flux as a function of CO2 partial pressure in the gas phase. Figure 3 shows the effect of adding 1M alkanolamine to 2M potassium carbonate solution on the CO2 absorption flux. Among these three mixtures, the mixture of potassium carbonate and MEA possesses the highest flux due to its highest reaction rate. The specifications of the membrane module modeled in this study are shown in Table 3.

4.2. Absorption Performance of Promoted K2CO3 and Amine Solutions Figure 4 compares the absorption performance of aqueous K2CO3, promoted K2CO3 with MEA and aqueous MEA solutions. This figure shows that the aqueous solution of K2CO3 even at maximum concentration (30% wt) has much lower CO2 flux than promoted K2CO3 with MEA, which is due to low reaction rate of K2CO3 without promoter. The promoted solutions give much higher fluxes, about 4 times superior to not-promoted K2CO3. While the promoted K2CO3 enhances the CO2 flux significantly compared to notpromoted K2CO3, Figure 4 reveals that single MEA has even better performance than promoted 14 ACS Paragon Plus Environment

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K2CO3 at low partial pressures of CO2. Surprisingly, it means that a promoted solution of 15% wt K2CO3 with 5 % wt MEA can have a lower flux than a solution of 5 % wt MEA. To explain this result, it should be noticed that the Henry’s constant and CO2 diffusivity for a mixture of potassium carbonate and amine is less than single amine. On the other hand, it is obvious that the reaction rate and capacity of promoted mixture is higher than single amine for CO2 absorption. At low partial pressure of CO2, the concentration of reactants is enough to react with absorbed CO2. Therefore, the solubility of CO2 is the predominant factor in this condition. However, at higher partial pressures of CO2, the absorbed CO2 increases and the concentration of reactants may not be enough. As a result, the concentration of reactants is more important in this condition. It can also be seen from Figure 4 that a 15% K2CO3 promoted with 5% MEA has a better performance than a 25% K2CO3 promoted with the same amine at given conditions. It can be explained again using the fact that the solubility and diffusivity of CO2 is reduced in the mixtures with higher concentrations of K2CO3. As an important issue, the result of this figure reveals that the promoted K2CO3 (here with MEA) is only effective in a specific range of operating conditions. The effect of concentration of MEA as promoter is illustrated in Figures 5 (a) and (b), where (a) and (b) refer to 10 and 20 mole percent of CO2 in the feed gas, respectively. It is clear from both figures that the CO2 flux enhances with increasing the amount of MEA in the mixture. Figure 5 (a) shows that the promoted solution has a better performance than single amine in the lower concentrations of MEA, because of lower available reactants in the single amine. Nevertheless, the lower solubility of CO2 in the promoted solution at higher concentrations of MEA causes a reduction in the CO2 flux compared to single MEA. 15 ACS Paragon Plus Environment

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Figure 5 (b) reveals that the promoted solution gives always a higher absorption flux than single MEA. In this condition, the concentration of reactants is more important than solubility, because of higher concentration of CO2 in the feed gas. It is concluded that applying a MEApromoted solution is useful where the concentration of CO2 in the feed gas is high enough. In low concentrations of CO2, a single MEA may gives higher absorption fluxes. It again emphasizes that the promoted K2CO3 is only effective in a specific range of operating conditions. If the composition of CO2 in flue gas is specified, such analysis should be done for each module to find this effective range of operation. Figure 6 compares the absorption performance of promoted K2CO3 with DEA and aqueous DEA solutions as well as promoted K2CO3 with MDEA and aqueous MDEA solutions. The same trend of MEA-promoted K2CO3 can be seen here, which shows that a DEA-promoted solution should be applied where the concentration of CO2 in the feed gas is not low. The figure shows that the cross point of two curves is in the lower partial pressure of CO2 compared to Figure 4 because DEA is a weaker solution compared to MEA for CO2 absorption. It can be seen that the promoted K2CO3 with MDEA always has a higher flux than single MDEA, a different trend from MEA and DEA. Because MDEA has a relatively slow reaction rate with CO2, there is no cross point for these two curves. While MDEA has several advantages compared to other amines especially because of lower cost of regeneration, it is not recommended as promoter due to its much lower reaction rate compared to other amines. The values of flux in Figure 3 as well as Figures 4, 6 confirm this limitation of MDEA as promoter in hollow fiber membrane contactors.


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4.3. Effect of Temperature on CO2 Absorption Flux Figure 7 indicates the effect of higher temperatures on the absorption flux of CO2 for MEA-promoted potassium carbonate. A comparison of Figures 4 and 7 shows that the absorption flux of CO2 enhances up to 50% by increasing the system temperature to about 320 K. This can be explained by the fact that the reaction rate increases considerably by rising the temperature of the solution. In addition, the position of cross point of two curves does not change significantly. Figure 8 compares the influence of increasing the temperature for promoted potassium carbonate with MEA, DEA and MDEA. The flux increases with temperature for all promoted solutions, however, the flux of MEA-promoted solution is significantly higher than others at all temperatures. Since this system has a superior performance at higher temperatures, it can be proposed for CO2 sequestration from hot flue gases for energy saving.

4.4. Effect of Gas Flow Rate, Flow Direction and Axial Diffusion Figure 9 shows the absorption flux of CO2 at various gas velocities for promoted K2CO3 with MEA solution. Generally, the CO2 flux increases as increasing the gas velocity at a constant liquid flow rate. An increase in gas velocity reduces the mass transfer resistance of the gas phase; also higher amount of CO2 will exist in the shell side that causes higher driving force. At higher gas velocities, there is no significant change in the CO2 flux by increasing the gas flow rate, because the liquid phase resistance is predominant in this condition. According to Figure 9, the countercurrent flow gives a higher flux than co-current flow; however, this increase is less than 5%. While there is no significant enhancement in the flux in countercurrent condition, the flux keeps its upward trend even at high gas flow rates. 17 ACS Paragon Plus Environment

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Page 18 of 44

The Reynolds number of the gas phase in the shell side is usually low and therefore the axial diffusion might be important. If the axial diffusion in gas phase is considered, equation (30) in shell side is modified as:

Uz ( r )

 ∂ 2 C jG 1 ∂ C jG ∂ 2 C jG  = D jG  + +   ∂ r2 r ∂r ∂z ∂z 2  

∂ C jG

(37)

As it can be seen from Figure 10, the curves with and without considering the axial diffusion are nearly the same. Therefore, the axial diffusion has no significant influence on the CO2 flux, not only in a relatively high gas flow rate (Re = 1.7, Pe = 7), but also in a lower gas flow rate (Re = 0.4, Pe = 2).

4.5. Effect of Liquid Flow Rate The CO2 absorption flux versus liquid flow rate is depicted in Figure 11 for a promoted K2CO3 with MEA, DEA and MDEA. The liquid velocity, within the calculated range, has no significant influence on the CO2 flux for the promoted K2CO3 with MDEA, because this solution has a low reaction rate. However, for the mixtures of K2CO3 with MEA or DEA, and especially for MEA, the CO2 flux increases incessantly, because of much higher reaction rate of these solutions with CO2.


Page 19 of 44

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Energy & Fuels

4.6. Effect of Possible Wettability of Membrane The wetting of membrane pores is one of the most important challenges in the long-term application of membrane contactors. The effect of possible wettability of membrane on the CO2 flux is presented in Figure 12 for different promoters. The figure reveals that increasing the wetting fraction can reduce the flux impressively for all promoted solutions. However, the flux is still much higher than not promoted K2CO3 even at high wetting fractions. For all three solutions, a very fast reduction of flux at low wetting fractions causes the lower CO2 capture from the gas phase, which will increase the overall driving force of the module. Therefore, the slope of curves slows down at higher wetting fraction. The possible wetting of membrane depends on the surface tension of absorbent. The surface tensions of K2CO3, MEA and MEA–promoted K2CO3 solutions are presented in Table 4. These data show that K2CO3 has lower tendency to penetrate into the membrane pores because of much higher surface tension especially at higher concentrations. The estimated surface tension of the promoted solution is lower than not-promoted K2CO3, however it is still higher than single amine and even higher than pure water (72 mN m-1) at higher concentrations of K2CO3. Therefore, a promoter solution with a limited concentration of amine and enough concentration of K2CO3 (for example 25% K2CO3 + 5% MEA) has an enough surface tension to prevent wetting. It should be noted that the calculated surface tension for the promoted solution is a rough estimation based on the weight percent of K2CO3 and MEA because of the lack of experimental data for this promoted solution.


Energy & Fuels

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5. CONCLUSIONS A simulation study was done to analyze the CO2 capture in a HFMC using promoted solutions of K2CO3 with MEA, DEA and MDEA. The model results were in good agreement with the experimental data in the literature for all promoters. Among three solutions, the promoted potassium carbonate with MEA had the highest flux. It was shown that the promoted solutions are only effective in a range of operating conditions. At high partial pressure of CO2, the mixture of K2CO3 with MEA or DEA had better performance than equal amount of single amine. However, in lower CO2 pressure, the performance was opposite. Increasing the system temperature and gas velocity enhanced the rate of CO2 absorption significantly. The higher fluxes of promoted K2CO3 with MEA or DEA, as well as other advantages of K2CO3 solution such as lower cost and easier regeneration, make them good alternative solutions compared to alkanolamines such as single MEA or DEA for CO2 capture in hollow fiber membrane contactors.

AUTHOR INFORMATION Corresponding Author *

Tel.: +987116133713. Fax: +987116473180. E-mail: [email protected]


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Energy & Fuels

NOMENCLATURES C

concentration (kmol m-3)

dp

pore diameter (cm)

D

diffusivity (m2 s-1)

Djk

Knudsen diffusivity of species j (m2 s-1)

H

Henry’s constant (kmol kmol-1)

J

absorption Flux (kmol m-2 s-1)

k

reaction rate constant

K

reaction equilibrium constant

M

molecular weight (kg kmol-1)

P

total pressure (kPa)

Pe

Peclet number (dimensionless)

r

radial coordinate (m)

Re

Reynolds number (dimensionless)

Re

free surface radius of fiber (m)

Ri

inner radius of fiber (m)

Rj

reaction rate of component j (kmol m-3 s-1)

Ro

outer radius of fiber (m)

Rw

radius of gas–liquid interface (m)

T

temperature (K)

U

velocity (m s-1)

z

axial coordinate (m)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Greek letters δl

liquid-filled membrane thickness (m)

ε

membrane porosity

θ

packing density

τ

tortuosity

σ

o CO 2 _ N 2

Ω

Lennard-jones parameter ( A ) collision integrals

Subscripts av

average

e

effective

G

gas

in

input

j

any diffusing species

L

liquid

m

membrane

p

pore

W

water


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Energy & Fuels

REFERENCES (1)

Zhang, Q.; Cussler, E. L. J. Membr. Sci. 1985, 23, 321–332.

(2)

Zhang, Q.; Cussler, E. L. J. Membr. Sci. 1985, 23, 333–345.

(3)

Karoor, S.; Sirkar, K. K. Ind. Eng. Chem. Res. 1993, 32, 674–684.

(4)

Sema, T.; Naami, A.; Liang, Z.; Shi, H.; Rayer, A. V.; Sumon, K. Z.; Wattanaphan, P.; Henni, A.; Idem, R.; Saiwan, C.; Tontiwachwuthikul, P. Carbon Manage. 2012, 3, 201– 220.

(5)

Kim, Y.; Yang, S. Sep. Purif. Technol. 2000, 21, 101–109.

(6)

Keshavarz, P.; Fathikalajahi, J.; Ayatollahi, S. J. Hazard. Mater. 2008, 152, 1237–1247.

(7)

Keshavarz, P.; Fathikalajahi, J.; Ayatollahi, S. Sep. Purif. Technol. 2008, 63, 145–155.

(8)

Keshavarz, P.; Fathikalajahi, J.; Ayatollahi, S. J. Membr. Sci. 2008, 325, 98–108.

(9)

Dindore, V. Y.; Brilman, D. W. F.; Versteeg, G. F. J. Member. Sci. 2005, 225, 275–289.

(10) Mehdipour, M.; Karami, M. R.; Keshavarz, P.; Ayatollahi, S. Energy Fuels 2013, 27, 2185−2193. (11) Golkhar, A.; Keshavarz, P.; Mowla D. J. Member. Sci. 2013, 433, 17–24. (12) Zhu, D.; Fang, M.; Lv, Z.; Wang, Z.; Luo, Z. Energy Fuels 2012, 26, 147–153. (13) Lu, J.; Zheng, Y.; Cheng, M.; Wang, L. J. Member. Sci. 2007, 289, 138–149. (14) Cui, Z.; deMontigny, D. Carbon Manage. 2013, 4, 69–89. (15) Yih, S.; Sun, C. Chem. Eng. 1987, 34, 65–72. (16) Tseng, P. C.; Ho, W. S.; Savage, D. W. AIChE J. 1988, 34, 922–931. (17) Nii, S.; Takeuchi, H. Gas Sep. Purif. 1994, 8, 107–114. (18) Fernandes, D.; Conway, W.; Burns, R.; Lawrance, G.; Maeder, M.; Puxty, G. J. Chem. Thermodyn. 2012, 54, 183–191. 23 ACS Paragon Plus Environment

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(19) Caplow, M. J. Am. Chem. Soc. 1968, 90, 6705–6803. (20) Danckwerts, P. V. Chem. Eng. Sci. 1979, 34, 443–446. (21) Blauwhoff, P. M. M.; Versteeg, G. F.; van Swaaij, W. P. M. Chem. Eng. Sci. 1984, 39, 207–235. (22) Versteeg, G. F.; van Swaaij, W. P. M. Chem. Eng. Sci. 1988, 43, 573–585. (23) Zhang, H. Y.; Wang, R.; Liang, D. T.; Tay, J. H. J. Membr. Sci. 2006, 297, 301–310. (24) Faiz, R.; Al-Marzouqi, M. Sep. Purif. Technol. 2011, 76, 351–361. (25) Donaldson, T. L.; Nguyen, Y. N. Ind. Eng. Chem. Fundam. 1980, 19, 260–266. (26) Little, R. J.; van Swaaij, W. P. M.; Versteeg, G. F. AIChE J. 1990, 36, 1633–1640. (27) Seader, J. D.; Henley, E. J. Separation Process Principles; Wiley: New York, John Wiley & Sons, 2011; pp 500–567. (28) Happel, J. AIChE J. 1959, 5, 174–177. (29) Tsonopolous, C.; Coulson, D. M.; Inman, L. W. J. Chem. Eng. Data 1976, 21, 190–193. (30) Edwards, T. J.; Maurer, G.; Newman, J.; Prausnitz, J. M. AIChE J. 1978, 24, 966–976. (31) Barth, D.; Tondre, C.; Lappai, G.; Delpuech, J. J. Phys. Chem. 1981, 85, 3660–3667. (32) Austgen, D. M.; Rochelle, G. T.; Chen, C. C. Ind. Eng. Chem. Res. 1991, 30, 543–555. (33) Hikita, H.; Asai, S.; Takatsuka, T. Chem. Eng. J. 1976, 11, 131–141. (34) Danckwerts, P. V.; Sharma, M. M. Chem. Eng. 1966, 44, 244–279. (35) Ashour, S. S.; Rinker, E. B.; Sandall, O. C. AIChE J. 1996, 42, 671–682. (36) Ali, S. H. Int. J. Chem. Kinet. 2005, 37, 391–405. (37) Benamor, A.; Ali, B. S.; Aroua, M. K. J. Chem. Eng. 2008, 25, 451–460. (38) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport phenomena; Wiley: New York,

John Wiley & Sons, 2002; pp 511–542.


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(39) Versteeg, G. F.; van Swaaij, W. P. M. J. Chem. Eng. Data 1988, 33, 29–34. (40) Ko, J.; Tsai, T.; Lin, C.; Wang, H.; Li, M. J. Chem. Eng. Data 2001, 46, 160–165. (41) Ratcliff, G. A.; Holdcroft, J. G. Trans. Inst. Chem. Eng. 1963, 41, 315–319. (42) Snijder, E. D.; te Riele, M. J. M.; Versteeg, G. F.; van Swaaij, W. P. M. J. Chem. Eng. Data 1993, 38, 475–480. (43) Otto, N. C.; Quinn, J. A. Chem. Eng. Sci. 1971, 26, 949–961. (44) Ying, J.; Eimer, D. A. Ind. Eng. Chem. Res. 2012, 51, 16517–16524. (45) Tsai, T.; Ko, J.; Wang, H.; Lin, C.; Li, M. J. Chem. Eng. Data 2000, 45, 347–341. (46) Benamor, A.; Aroua, M. K. Korean J. Chem. Eng. 2007, 24, 16–23. (47) Va´zquez, G.; Alvarez E.; Navaza J. M.; Rendo, R.; Romero E. J. Chem. Eng. Data 1997, 42, 57–59.


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LIST OF FIGURES

Figure 1. Schematic diagram of the liquid and gas flow in a hollow fiber membrane module. Figure 2. CO2 absorption in 10 wt% DEA/MDEA or 1M MEA aqueous solutions (feed conditions: T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s) Figure 3. Experimental and model results for promoted potassium carbonate, amines (1M) in K2CO3 solution (2M) (feed conditions: T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s,) Figure 4. Effect of CO2 partial pressure on absorption flux of MEA-promoted solution (feed conditions: T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s) Figure 5 (a), (b). Effect of MEA concentration on CO2 flux (a): Feed composition: 10/90 CO2/N2, (b): Feed composition: 20/80 CO2/N2, (T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s) Figure 6. Effect of CO2 partial pressure on absorption flux of DEA/MDEA-promoted solution (feed conditions: T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s) Figure 7. Absorption flux of CO2 at higher temperatures (feed conditions: T = 320 K, UL = 0.011 m/s, UG = 0.1 m/s) Figure 8. CO2 flux of promoted solutions at different temperatures (feed conditions: 20/80 CO2/N2, UL = 0.011 m/s, UG = 0.1 m/s) Figure 9. Effect of gas velocity and gas flow direction through the module (feed conditions: 20/80 CO2/N2, T = 294 K; absorbent: 25% K2CO3 + 5% MEA)


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Energy & Fuels

Figure 10. CO2 flux with and without considering axial diffusion in gas phase (feed conditions: 20/80 CO2/N2,T = 294 K; absorbent: 2M K2CO3 + 0.5M DEA) Figure 11. Effect of liquid velocity on absorption flux of promoted solutions (feed conditions: 20/80 CO2/N2, T = 294 K, UG = 0.1 m/s) Figure 12. Effect of membrane wetting on absorption flux of promoted solutions (feed conditions: 20/80 CO2/N2, T = 298.15 K, UL = 0.011 m/s, UG = 0.1 m/s)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Schematic diagram of the liquid and gas flow in a hollow fiber membrane module.


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2

1.5 JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1 Model (MEA) Model (DEA) Model (MDEA) 0.5 Exp. (MEA) [17] Exp. (DEA) [17] Exp. (MDEA) [17] 0 0

5

10

15

20

PCO2 (kPa)

Figure 2. CO2 absorption in 10 wt% DEA/MDEA or 1M MEA aqueous solutions (feed conditions: T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s)


Energy & Fuels

2

Exp. (K2CO3+MEA) [17] 1.6

JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 44

Exp. (K2CO3+DEA) [17] Exp. (K2CO3+MDEA) [17]

1.2

Exp.( K2CO3) [17] Model (K2CO3+MEA)

0.8

Model (K2CO3+DEA) Model (K2CO3+MDEA) Model.( K2CO3)

0.4

0 0

2

4

6

8

10

12

14

16

18

20

PCO2 (kPa)

Figure 3. Experimental and model results for promoted potassium carbonate, amines (1M) in K2CO3 solution (2M) (feed conditions: T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s)


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2

1.5 JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5% MEA 1

15%K2CO3+5%MEA 25% K2CO3+5% MEA 30% K2CO3

0.5

0 0

5

10

15

20

25

30

PCO2 (kPa)

Figure 4. Effect of CO2 partial pressure on absorption flux of MEA-promoted solution (feed conditions: T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s)


Energy & Fuels

2

1.5 JCO2×106 (kmol m-2 s-1)

1

0.5

MEA 25% K2CO3+% MEA

0 0

1

2

3

4

5

6

Added %MEA

(a) 2

1.5 JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 44

1

0.5

25% K2CO3+% MEA MEA

0 0

1

2

3

4

5

6

Added %MEA

(b) Figure 5 (a), (b). Effect of MEA concentration on CO2 flux (a): Feed composition: 10/90 CO2/N2, (b): Feed composition: 20/80 CO2/N2, (T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s) 32 ACS Paragon Plus Environment

Page 33 of 44

1.2 25% K2CO3+5% DEA 1

5% DEA 25% K2CO3+5% MDEA

JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.8 5% MDEA

0.6

0.4

0.2

0 0

5

10

15

20

25

30

PCO2 (kPa)

Figure 6. Effect of CO2 partial pressure on absorption flux of DEA/MDEA-promoted solution (feed conditions: T = 294 K, UL = 0.011 m/s, UG = 0.1 m/s)


Energy & Fuels

3

2.5 JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

1.5

1 5% MEA 0.5 25% K2CO3+5% MEA 0 0

5

10

15

20

25

30

PCO2 (kPa)

Figure 7. Absorption flux of CO2 at higher temperatures (feed conditions: T = 320 K, UL = 0.011 m/s, UG = 0.1 m/s)


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3

2.5

JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2

1.5

1 1M K2CO3+0.5M MEA 1M K2CO3+0.5M DEA

0.5

1M K2CO3+0.5 M MDEA 0 290

300

310

320

330

340

350

360

Tempreture (K)

Figure 8. CO2 flux of promoted solutions at different temperatures (feed conditions: 20/80 CO2/N2, UL = 0.011 m/s, UG = 0.1 m/s)


Energy & Fuels

2.7

JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 44

2.3

Countercurrent (Ul=0.04 m/s) Co-current (Ul=0.04 m/s) Countercurrent (Ul=0.011m/s) Co-current (Ul=0.011m/s)

1.9

1.5 0

0.05

0.1

0.15

0.2

0.25

Ug (m/s)

Figure 9. Effect of gas velocity and gas flow direction through the module (feed conditions: 20/80 CO2/N2, T = 294 K, absorbent: 25% K2CO3 + 5% MEA)


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1 Pe = 7

0.8 JCO2×106 (kmol m-2 s-1)

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Energy & Fuels

0.6 Pe = 2 0.4 with neglecting axial diffusion with out neglecting axial diffusion 0.2 with neglecting axial diffusion with out neglecting axial diffusion 0 0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

Ul (m/s)

Figure 10. CO2 flux with and without considering axial diffusion in gas phase (feed conditions: 20/80 CO2/N2,T = 294 K; absorbent: 2M K2CO3 + 0.5M DEA)


Energy & Fuels

2

1.6 JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.2

0.8

2M K2CO3+0.5M MEA

0.4

2M K2CO3+0.5M DEA 2M K2CO3+0.5M MDEA 0 0

0.01

0.02

0.03

0.04

0.05

Ul (m/s)

Figure 11. Effect of liquid velocity on absorption flux of promoted solutions (feed conditions: 20/80 CO2/N2, T = 294 K, UG = 0.1 m/s)


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1.4 2M K2CO3+0.5M MEA 1.2 2M K2CO3+0.5M DEA 2M K2CO3+0.5M MDEA

1 JCO2×106 (kmol m-2 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2M K2CO3 0.8

0.6

0.4

0.2

0 0

20

40

60

80

100

%Wet

Figure 12. Effect of membrane wetting on absorption flux of promoted solutions (feed conditions: 20/80 CO2/N2, T = 298.15 K, UL = 0.011 m/s, UG = 0.1 m/s)


Energy & Fuels

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Page 40 of 44

Table 1. Equilibrium and Rate Constant Equations Used in Calculations Parameter

Expression/Value

References

K W ( kmol2 m-6 )

log ( K W ) = -5839.5/T-22.4773logT+61.2062

29

K 2MEA

ln ( K 2MEA K W ) = 220.1-12432/T-35.5lnT

30

K 2DEA

log ( K 2DEA K W ) = -4.0302-1830.15/T+0.0043T

31

K2MDEA

ln ( K 2MDEA K W ) = -9.4165-4234.98/T

32

K 3 ( m3 kmol-1 )

log ( K 3 ) = 1568.94/T+0.4134-0.006737T

33

k 4 ( s -1 )

log ( k 4 ) = 329.850-110.541logT-17265.4/T

34

K 4 ( kmol m-3 )

log ( K 4 ) = -3404.7/T+14.843-0.03279T

34

k 5 ( m3 kmol-1 s-1 )

log(k5 ) = 13.635-2895/T

35

K 5 ( m3kmol−1 )

log(K5K W ) = 179.648 + 0.019244T- 67.341logT- 7495.441/T

35

k'MEA ( m3 kmol-1 s-1 )

4279a

36

k'DEA ( m3 kmol-1 s-1 )

4317a

37

k MEA ( m6 kmol-2 s-1 )

275090a

36

k DEA ( m6 kmol-2 s-1 )

375a

37

k MH2O ( m6 kmol-2 s -1 )

75a

36

k DH2O ( m6 kmol-2 s-1 )

3.5a

37

k MDEA ( m3 kmol-1 s-1 )

k MDEA = 1.34 ×10 9 exp ( -5771/T )

26

a

The data is presented for 294 K. 40 ACS Paragon Plus Environment

Page 41 of 44

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Energy & Fuels

Table 2. Transport Properties Used in This Study Parameter

Equation

Reference

 1 1 0.00000018583 T3  +  M CO M N  2 2

D CO 2 -N 2

 101.325  2  P σ CO2 _ N 2 Ω

38

D CO 2 -water

2.35×10-6 exp(-2119 /T)

39

D N 2 O-water

5.07×10-6 exp(-2371/T)

39

D CO 2 -MEA

(

D CO 2 -DEA

(

D CO2 -MDEA

(

5.07 × 10

-6

5.07 × 10

+8.65 × 10

-6

5.07 × 10

-6

-7

+2.17 × 10

+1.31 × 10

[MEA] +2.78 × 10-7 [MEA]

-6

-6

2

[DEA] +2.29 × 10-6 [DEA]

2

) ((

-2371-93.4 MEA

) ((

-2371-292 × DEA

× exp

× exp

[MDEA] +8.73 × 10-8 [MDEA]

2

)

[

[

× exp

]) /T ) ×

D CO -water 2

40

D N O-water 2

]) /T ) ×

D CO -water 2

40

D N O-water 2

DCO -water 2

(( -2371-150 [MDEA]) /T) × D

40

N2O-water

D CO2 -K 2CO3

(1- 0.154 [ K CO ]) × 2.35×10

DMEA-MEA

exp ( -13.275-2198.3/T-7.8142 ×10-5 [ MEA])

42

DDEA-DEA

exp ( -13.268-2287.7/T-19.699 ×10-2 [ DEA])

42

DMDEA-MDEA

exp ( -13.088+2360.7/T-24.727 ×10-5 [ MDEA ])

42

DIon HCO2 -water ( kPa m3kmol-1 ) H N2O-water ( kPa m3kmol-1 )

-6

2

3

D CO2 -solution ×

exp ( -2119 /T )

M CO2 M Ion

2.8249 × 10 6 exp ( -2044/T )

8.5470 × 10 6 exp ( -2284/T )


41

43

39

39

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H CO 2 -MEA

0.86a

H CO 2 -DEA

0.81a

H CO2 -MDEA

H CO2 -K 2 CO3 a

44 45

  H 1/  exp (14.964814 -1977.4 /T+ 0.03989 [ MDEA]) × CO2 -water   H N O-water 2   10(

Page 42 of 44

-5.30+1140/T-0.125[ K 2 CO3 ])

× 0.0820578 × T

The data is presented for 294 K and Camine = 1M.


   × RT   

46

34

Page 43 of 44

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Energy & Fuels

Table 3. Specifications of Hollow Fiber Membrane Contactor Parameter

Value

Length of fiber (cm)

30.5

Number of fiber

14

Fiber outer diameter (mm)

1.8

Fiber inner diameter (mm)

1

Pore size ( µ m)

0.02

Tortuosity

2

Porosity

0.5

Module inner diameter (cm)

1.7

Reference

17


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 44

Table 4. Surface Tension of K2CO3, MEA and MEA–Promoted Solutions at 298 K.

σ (mN m −1 ) wt%

K2CO3

MEA

MEA–Promoted K2CO3

0

72

72

72

10

75.1

65.97

(5% K2CO3 + 5% MEA) 70.53 (15% K2CO3 + 5% MEA) 20

78.6

62.63 74.6 (25% K2CO3 + 5% MEA)

30

83.8

60.41 79.91

Reference

10

47

Estimated


Enhanced Carbon Dioxide Separation by Amine-Promoted Potassium

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