Absorption of carbon dioxide into piperazine activated aqueous N-methyldiethanolamine

Chemical Engineering Journal 171 (2011) 734–741

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Absorption of carbon dioxide into piperazine activated aqueous N-methyldiethanolamine Arunkumar Samanta a , S.S. Bandyopadhyay b,∗ a b

Department of Chemical Engineering, GCET, Vallabh Vidyanagar, Gujarat 388 120, India Separation Science Laboratory, Cryogenic Engineering Centre, IIT Kharagpur, West Bengal 721 302, India

a r t i c l e

i n f o

Article history: Received 31 May 2010 Received in revised form 21 January 2011 Accepted 5 February 2011 Keywords: CO2 capture Absorption Modelling Kinetics MDEA Piperazine

a b s t r a c t This work presents a theoretical and experimental investigation on the absorption of CO2 into piperazine (PZ) activated aqueous N-methyldiethanolamine (MDEA) solvent. A comprehensive mathematical model which is based on Higbie’s penetration theory has been developed to analyze the experimental data. The model involving coupled mass transfer–reaction kinetics–chemical equilibrium incorporates the important reversible reactions in the liquid phase. The model is validated with the experimental results of steady state absorption measurements of CO2 in a 2.81 × 10−2 m o.d. stainless steel wetted wall contactor. The rates of absorption of CO2 into this solvent have been measured over the CO2 partial pressure range of 2–14 kPa and temperature range of 298–313 K under atmospheric pressure. The absorption experiments are performed over the MDEA concentration range of 1.89–2.41 kmol m−3 along with PZ concentrations of 0.24, 0.60 and 0.95 kmol m−3 . The predicted absorption rates and enhancement factors based on the model have been found to be in good agreement with the experimental results, the average absolute deviation between the model predicted and experimental results being 6.8%. The values of the rate constants, k23 and k25 for the PZ-carbamate and PZ-dicarbamate formation reactions determined in this work have been found to be about 17,500 m6 kmol−2 s−1 and 15,500 m6 kmol−2 s−1 at 298 K, respectively. Good agreement between the model predicted and experimental results validates the mathematical model developed in this work to represent CO2 mass transfer in PZ activated aqueous MDEA. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Carbon dioxide capture from the flue gas streams of thermal power plants is very important today to mitigate the problem of global warming. The United Nations’ Intergovernmental Panel on Climate Change (IPCC) identified CO2 capture and storage as one of several approaches to address the problem of global warming and climate change [1]. Hence, efficient and less energy intensive capture of CO2 from flue gases with subsequent geological storage is increasingly being viewed as one of the options for reducing CO2 emissions. Major challenges for CO2 capture from the flue gas stream of a fossil fuel fired power plant are the large volumetric flow rate of flue gas with large amount of CO2 at low partial pressure. The flue gas from a power plant mainly contains of nitrogen, water vapor and CO2 . For coal based thermal power plants the flue gas may also contain SO2 and NOx which are known to form heat stable salts with the amines used for CO2 capture leading to substantial solvent degradation. Therefore, potential solvents for CO2 capture are only those which have high reaction rates, high

∗ Corresponding author. E-mail address: [email protected] (S.S. Bandyopadhyay). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.02.008

equilibrium capacities for CO2 , a higher degradation resistance, and lower energy requirements for regeneration. In view of this, extensive research activities are in progress globally to develop efficient and economic processes for CO2 capture from the flue gas streams of thermal power plants. Besides CO2 capture from the flue gas streams of thermal power plants, the removal of CO2 from industrial gas streams is an important step in processing sour natural gas, refinery off-gases, and synthesis gas [2–4]. Most frequently, the removal of CO2 from various industrial gas streams is done by the regenerative chemical absorption into aqueous solutions of alkanolamines. Commercially available alkanolamine solvents for these processes are monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), diisopropanolamine (DIPA), N-methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP) [3]. Recently, there is a growing interest in the use of piperazine (PZ) activated aqueous MDEA for gas treating processes [5–12] in order to achieve higher overall reaction rate for CO2 and reduced regeneration energy requirement. PZ is used as an activator in the activated MDEA process of BASF [5]. Being a cyclic diamine, each mole of PZ can theoretically absorb two moles of CO2 , and it may favor rapid formation of the carbamates. The apparent second-order rate constant of PZ has been found to be an order higher than that of the

A. Samanta, S.S. Bandyopadhyay / Chemical Engineering Journal 171 (2011) 734–741

primary amine MEA [9]. While there is extensive published literature on the use of primary, secondary and tertiary alkanolamines and their blends for the removal of CO2 , literature on PZ activated MDEA solvent system is very limited. Xu et al. [7] measured the absorption rate of pure CO2 gas at atmospheric pressure into aqueous blends of MDEA/PZ using a disk column in the temperature range of 303–343 K. The concentration range of PZ was 0.014–0.21 M, which was too low to be used commercially for CO2 removal processes. Besides, absorption measurements were done at high partial pressure of CO2 (pure CO2 ) leading to almost complete depletion of PZ at the gas–liquid interface. Hence, their results on the kinetics of CO2 –PZ appear to be erroneous [13]. Zhang et al. [8] suggested the mechanism of CO2 absorption into an aqueous solution of MDEA blended with PZ using a disk column with the PZ concentration in the range of 0.2–0.6 M. Bishnoi and Rochelle [9] reported CO2 absorption data in 0.6 M PZ in 4.0 M aqueous MDEA solutions at 313 and 343 K using wetted wall contactor. Rate model based on eddy diffusivity theory was used for prediction of the rate of absorption and enhancement factor. Zhang et al. [14] extended their investigation to study the effects of CO2 partial pressure on the absorption of CO2 into aqueous MDEA/PZ in a laboratory packed column. They concluded from their results that the absorption rates at elevated CO2 partial pressures were higher than those predicted from the model. Recently, Derks [12] studied experimentally the rates of absorption of CO2 into aqueous solutions containing a mixture of 4.0 M MDEA and (0.5–1.0) M PZ in stirred cell contactor at various CO2 partial pressures and at 298.15 K. However, the absorption model proposed by Derks [12] was unable to predict the experimental results with reasonable accuracy at high CO2 loadings. Hence, it is evident from the foregoing discussion that in spite of great commercial significance of the PZ activated aqueous MDEA solvents, published literature on the theoretical and experimental investigation on the rates of absorption of CO2 into PZ activated aqueous MDEA solvent is extremely limited. Moreover, except the work reported by Samanta [15], there is no literature on the coupled mass transfer–reaction kinetics–equilibrium comprehensive model to represent the absorption of CO2 into aqueous blends of (MDEA + PZ) with various relative compositions of the amines. This work presents an experimental and theoretical investigation on the absorption of CO2 into PZ activated aqueous MDEA solvent. The rates of absorption of CO2 into aqueous solutions of MDEA–PZ have been measured in a wetted wall contactor over the CO2 partial pressure range of 2–14 kPa and temperature range of 298–313 K under atmospheric pressure. In view of industrial gas treating conditions the absorption experiments are performed over the MDEA concentration range of 1.89–2.41 kmol m−3 (22–28 wt%) along with PZ concentrations of 0.24, 0.60 and 0.95 kmol m−3 (2–8 wt%) keeping the total amine concentration in the solution at 30 wt%. Required physicochemical properties, such as density and viscosity of aqueous amine solvents and Henry’s constant and diffusivity of CO2 in the solvents have been determined in this work by appropriate experimental methods or estimated from available literature. A coupled mass transfer–reaction kinetics–equilibrium model, which is capable of predicting gas absorption rates and enhancement factors for absorption of CO2 into activated amine solvents has been developed based on Higbie’s penetration theory.

2. Reaction scheme and mathematical model For the rational design and simulation of gas-treating units involving blended amine solvents, it is essential to develop mass transfer rate-based models to describe the CO2 mass transfer in these solvents. In this section, the diffusion-reaction processes for CO2 mass transfer in activated MDEA are modeled according to

735

Higbie’s penetration theory with the assumption that all reactions are reversible. Since liquid bulk concentrations of all species are essential for the analysis of mass transfer, a bulk liquid equilibrium model has also been developed to estimate the initial liquid bulk concentrations of all chemical species. The developed model that comprises sets of partial differential equations coupled with nonlinear algebraic equations is capable of predicting the CO2 absorption rates and enhancement factors in aqueous (MDEA + PZ) solutions using the relevant physicochemical properties and wetted wall contactor hydrodynamics. 2.1. Reaction scheme and mechanism When CO2 is absorbed into an aqueous mixed amine solution of MDEA (R1 R2 R3 N, where, R1 = CH3 and R2 = R3 = C2 H4 OH) and PZ several reactions may take place. Based on the studies conducted by Bishnoi and Rochelle [16] and Kamps et al. [10] for absorption of CO2 into aqueous solutions of MDEA and PZ, the following reactions are considered for the model development: K ,k

1 21 CO2 + R1 R2 R3 N + H2 O ←→ R1 R2 R3 NH+ + HCO− 3

K2 ,k22

−

(1)

+

CO2 + PZ + H2 O ←→ PZCOO + H3 O K3 ,k23

(2)

−

+

CO2 + R1 R2 R3 N + PZ ←→ PZCOO + R1 R2 R3 NH K4 ,k24

−

−

(3)

+

CO2 + PZCOO + H2 O ←→ PZ(COO )2 + H3 O − K5 ,k25

−

CO2 + R1 R2 R3 N + PZCOO ←→ PZ(COO )2 + R1 R2 R3 NH − K6 ,k26

CO2 + OH ←→ HCO− 3 K7

2− + HCO− 3 + H2 O←→CO3 + H3 O K8

PZ + H3 O+ ←→PZH+ + H2 O K9

PZCOO− + H3 O+ ←→PZH+ COO− + H2 O K10

R1 R2 R3 N + H3 O+ ←→R1 R2 R3 NH+ + H2 O K11

2H2 O←→H3 O+ + OH−

(4) +

(5) (6) (7) (8) (9) (10) (11)

where Ki is the equilibrium constant for reaction (i) and k2i is the forward rate coefficient for reaction (i). Reactions (1)–(6) are considered to be reversible with finite reaction rates, whereas reactions (7)–(11) are considered to be reversible and instantaneous with respect to mass transfer and at equilibrium, since they involve proton transfer only. As proposed by Donaldson and Nguyen [17], the reaction mechanism for CO2 with tertiary amines (e.g., MDEA) is a base-catalyzed hydration reaction as shown in the reaction (1). The mechanism implies that MDEA does not react directly with CO2 . Versteeg and van Swaaij [18], who studied CO2 absorption into nonaqueous solutions of MDEA, found that only physical absorption occurs in the nonaqueous tertiary amine solvent. This supports the validity of the reaction (1). The proposed mechanism for the reaction between CO2 and PZ involves the formation of a zwitterion followed by the deprotonation of zwitterion by a base to produce PZ-carbamate and protonated base [13,19–21]. The most important contribution to the deprotonation of the zwitterion in an aqueous solution of MDEA and PZ would come from PZ, MDEA and to a lesser extent from OH− and H2 O. The presence of significantly more MDEA appears to catalyze the reaction of CO2 and PZ to form carbamate. In the case of PZ, formation of zwitterion should be the rate determining step, while the deprotonation step involves only a proton transfer and is considered to be very fast. In this work, reaction (2) represents the reaction between CO2 and PZ for producing carbamate [13]. The rate constant, k22 , is considered the global rate coefficient for the formation of zwitterion and zwitterion deprotonation

736

A. Samanta, S.S. Bandyopadhyay / Chemical Engineering Journal 171 (2011) 734–741

(Eq. (2)). Similarly, the rate constant, k24 , is viewed as the global rate coefficient for the formation of PZ-dicarbamate, PZ(COO− )2 , by the reaction (4). Therefore, the proposed reaction scheme does not rule out the possible formation of a zwitterion reaction intermediate. Besides, two rate constants for reactions (3) and (5) have been considered here as suggested by Bishnoi and Rochelle [9] for the absorption of CO2 into (MDEA + PZ + H2 O).

2.3. Diffusion-reaction model

R1 = −k21 u1 u2 +

k21 u3 u4 K1

(24)

2.2. Bulk liquid equilibrium model

R2 = −k22 u1 u8 +

k22 u7 u10 K2

(25)

The liquid bulk concentrations of all chemical species can be estimated from the initial concentration of MDEA and PZ, the initial CO2 loading, ˛1 , of the solution and the assumption that all reactions are at equilibrium. We have the following twelve equations for twelve liquid bulk concentrations: overall MDEA balance: u02

+ u03

R6 = −k26 u1 u5 +

u08 + u09 + u010 + u011 + u012 = [PZ]initial

(13)



u01 + u04 + u06 + u010 + u011 + 2u012 = ˛1 [MDEA]initial + [PZ]initial



(14) electroneutrality balance: u03 + u07 + u09 − u04 − u05 − 2u06 − u010 − 2u012 = 0

K4 =

K6 =

K7 =

K8 =

K9 =

u01 u08 u07 u012 u01 u010 u04 u01 u05 u06 u07 u04 u09 u07 u08 u011 u07 u010

K10 =

u03 u02 u07

K11 = u05 u07

k23 u3 u10 K3

k24 u12 u7 K4 k25 u3 u12 K5

k26 u4 K6

(26)

(27)

(28)

(29)

The mass transfer model describing the diffusion-reaction process consists of following equations: CO2 balance: ∂u1 ∂2 u1  = D1 + Ri ∂t ∂x2 6

(30)

i=1

(15)

Total carbon (from CO2 ) balance: ∂u1 ∂u12 ∂2 u1 ∂2 u4 ∂u4 ∂u6 ∂u10 ∂u11 + D4 + + + + +2 = D1 2 ∂t ∂t ∂t ∂t ∂t ∂t ∂x ∂x2

All reactions are in equilibria: u07 u010

R4 = −k24 u1 u10 +

(12)

overall PZ balance:

K2 =

R3 = −k23 u1 u2 u8 +

R5 = −k25 u1 u2 u10 +

= [MDEA]initial

overall CO2 balance:

All reactions are treated as reversible reactions. Reactions (1)–(6) have finite reaction rates which are given by the following reaction rate expressions:

(16)

+D6

∂2 u6 ∂2 u10 ∂2 u11 ∂2 u12 + D10 + D11 + 2D12 ∂x2 ∂x2 ∂x2 ∂x2

(31)

Total MDEA balance: (17)

(18)

(19)

∂u2 ∂u3 ∂2 u2 ∂2 u3 + = D2 + D 3 ∂t ∂t ∂x2 ∂x2 Total PZ balance: ∂u8 ∂u9 ∂u10 ∂2 u8 ∂2 u9 ∂u11 ∂u12 + + + + = D8 + D9 2 ∂t ∂t ∂t ∂t ∂t ∂x ∂x2 +D10

(20)

(21)

(32)

∂2 u10 ∂2 u11 ∂2 u12 + D11 + D12 ∂x2 ∂x2 ∂x2

(33)

Carbamate balance: ∂u10 ∂2 u10 ∂2 u11 ∂u11 + D11 − R2 − R3 + R4 + R5 + = D10 ∂t ∂t ∂x2 ∂x2

(34)

Dicarbamate balance: (22)

∂u12 ∂2 u12 = D12 − R4 − R5 ∂t ∂x2

(23)

Electroneutrality balance:

where u1 = [CO2 ], u2 = [R1 R2 R3 N], u3 = [R1 R2 R3 NH+ ], u4 = [HCO3 − ], u5 = [OH− ], u6 = [CO3 2− ], u7 = [H3 O+ ], u8 = [PZ], u9 = [PZH+ ], u10 = [PZCOO− ], u11 = [PZH+ COO− ] and u12 = [PZ (COO− )2 ] and superscript ‘0’ denotes initial liquid bulk concentration of species i. The twelve simultaneous nonlinear algebraic equations (Eqs. (12)–(23)) have been solved using IMSL Math/Library in FORTRAN 90 for the twelve unknowns (u01 , . . . . . . , u012 ) of the liquid bulk concentrations.

(35)

∂u3 ∂u7 ∂u9 ∂u4 ∂u5 ∂u10 ∂u6 ∂u12 + + − − −2 − −2 ∂t ∂t ∂t ∂t ∂t ∂t ∂t ∂t = D3

∂2 u3 ∂2 u7 ∂2 u9 ∂2 u4 ∂2 u5 + D7 + D9 − D4 − D5 2 2 2 2 ∂x ∂x ∂x ∂x ∂x2

−2D6

∂2 u6 ∂2 u10 ∂2 u12 − D10 − 2D12 ∂x2 ∂x2 ∂x2

(36)

A. Samanta, S.S. Bandyopadhyay / Chemical Engineering Journal 171 (2011) 734–741

737

Table 1 Physicochemical properties aqueous (MDEA + PZ). Mass % MDEA

Mass % PZ

[MDEA] (kmol m−3 )

[PZ] (kmol m−3 )

T (K)

 (kg m−3 )

 × 103 (kg m−1 s−1 )

HCO2 –amine a (kPa m3 kmol−1 )

DCO2 –amine × 109 a (m2 s−1 )

28 28 28 28 25 25 25 25 22 22 22 22 30

2 2 2 2 5 5 5 5 8 8 8 8 0

2.41 2.41 2.41 2.41 2.15 2.15 2.15 2.15 1.89 1.89 1.89 1.89 0

0.24 0.24 0.24 0.24 0.60 0.60 0.60 0.60 0.95 0.95 0.95 0.95 2.56

298 303 308 313 298 303 308 313 298 303 308 313 313

1025.4 1023.1 1020.8 1018.2 1024.5 1022.3 1019.9 1017.5 1023.2 1021.5 1019.3 1016.9 1019.3

3.183 2.692 2.251 1.925 3.341 2.778 2.317 1.965 3.481 2.891 2.413 2.028 1.843

3441 3838 4267 4661 3303 3660 3926 4203 3259 3651 4090 4454 4254

0.84 1.04 1.16 1.30 0.70 0.88 0.94 1.09 0.62 0.74 0.85 0.95 1.37

a

Samanta et al. [28].

Instantaneous reactions (7)–(11) assumed to be at equilibrium are represented by the equilibrium constants using respective species concentrations as follows: K7 =

u6 u7 u4

(37)

K8 =

u9 u8 u7

(38)

u11 K9 = u10 u7

(39)

u3 = u2 u7

(40)

K11 = u5 u7

(41)

K10

Thus, there are twelve partial differential-algebraic equations which can be solved for the concentration profiles of the twelve chemical species (u1 , . . . . . . , u12 ) present in the liquid phase of CO2 –(MDEA + PZ + H2 O). 2.3.1. Initial and boundary conditions At t = 0 (for all x ≥ 0) and at x = ∞ (for all t ≥ 0), the concentration of all chemical species in the liquid are set equal to their equilibrium bulk concentrations, i.e., ui = u0i ,

i = 1, . . . , 12

(42)

The fluxes of the non-volatile components (i.e., for i = 2, 3,. . .,12) at gas–liquid interface (i.e., at x = 0) are equal to zero, which implies following boundary conditions: ∂ui = 0 at x = 0 and t > 0 dx

(43)

For volatile component, CO2 , the mass transfer rate in the gas phase near the gas–liquid interface is equal to the mass transfer rate in the liquid near the interface: −D1

∂u1 = kg (p1 − H1 u1 (0, t)) at x = 0 and t > 0. dx

(44)

For negligible mass transfer resistance in the gas phase, the boundary condition for CO2 at the gas–liquid interface (x = 0, t > 0) reduces to: p1 . (45) u1 (0, t) = u∗1 = H1 The differential equations are integrated from t = 0 to t = , the contact time, using the above initial and boundary conditions. For wetted wall contactor, the contact time  is given by

   2 1/3 3 d

2 = h 3

c

g

VL

(46)

where VL is the volumetric liquid flow rate,  is the density of the liquid,  is the viscosity of the liquid, and dc and h are the outer diameter and the absorption length, respectively, of the wetted wall contactor. The time-averaged absorption rate per unit interfacial area is obtained as given in Eq. (47). R¯ = −

D1 





0

∂u1 (0, t) dt ∂x

(47)

and the enhancement factor, E, for absorption of CO2 is given by Eq. (48): E=

R¯ kL1 (u∗1

− u01 )

(48)

where kL1 is the liquid-phase mass transfer coefficient defined by Higbie’s penetration model. 2.3.2. Method of solution The method-of-lines (MOL) is used to transform each partial differential equation into a set of ordinary differential equations in t by discretizing the spatial variable x. The resulting system of ordinary differential equations coupled with the nonlinear algebraic equations is solved by using the subroutine DDASSL [22] in FORTRAN. The typical number of nodes used in this work is 450 and corresponding nodal spacings are of the order 10−8 m. 2.4. Physicochemical properties and model parameters Knowledge of the physicochemical and transport properties of the alkanolamines and CO2 , e.g., density and viscosity of the aqueous amine solutions and diffusivity and physical solubility of CO2 in the aqueous amine solutions were necessary for analyzing the results of absorption studies using the numerical model developed in this work. The densities and viscosities of the PZ activated aqueous MDEA solutions have been measured by standard procedures and described by Samanta and Bandyopadhyay [23] and also presented in Table 1. The solubility and diffusivity of CO2 in aqueous amine solutions cannot be found out directly because CO2 undergoes reaction with the amines. Hence, these properties were estimated by the “N2 O-analogy” [24–27]. The solubilities of N2 O in aqueous (MDEA + PZ) solutions have been measured using an equilibrium cell. The diffusivities of N2 O in aqueous amine solutions have been measured using the wetted wall column by the volume uptake method. The details of the estimation of these properties are described in Samanta et al. [28]. The physical solubility and diffusivity of CO2 are estimated from measured N2 O solubility and

738

A. Samanta, S.S. Bandyopadhyay / Chemical Engineering Journal 171 (2011) 734–741

Table 2 Equilibrium and forward rate constant correlations used for model. Equilibrium and forward rate constant

Correlation

K2

ln K2 = −29.31 −

5615 T

K4

ln K4 = −30.78 −

5615 T

K7

log10 K7 = 6.498 − 0.0238 T −

1

Reference Bishnoi [30] Bishnoi [30] 2902.4 T

Danckwerts and Sharma [31]

4351 T

= −11.91 −

K8

ln

K11

ln K11 = 132.899 −

K6

log10 (K6 K11 ) = 179.648 + 0.019244 T − 67.341 log10 T −

K8

Pagano et al. [32]

13445.9 T

− 22.4773 ln T

 ln K1 = −8.21 − 5286 T

9  ln K1 = −9.4165 − 4234.98 T 10

4579  7

K9 K10

k21 = 2.91 × 10

k21

4

k22 = 5.8 × 10

k22

exp −

exp 4



k24

k24 = 5.95 × 10

k26

log10 k26 = 13.635 −

exp

T

4 − 3.5×10 R



HCO2 –amine = HN2 O–amine

 DCO2 –amine = DN2 O–amine

HCO2 –water HN2 O–water DCO2 –water

(49)



2360.7 − 24.727 × 10−5 [MDEA] T

1 T

−

1 T

1 298

−



1 298



Rinker et al. [36] Samanta and Bandyopadhyay [37] Samanta and Bandyopadhyay [37] Pinsent et al. [38]

(50)

where HN2 O–amine and HN2 O–water are the measured solubility of N2 O in the amine solution and in water, respectively. DN2 O–amine and DN2 O–water are the measured diffusivity of N2 O in the amine solution and in water, respectively. HCO2 –water and DCO2 –water are the measured solubility and diffusivity of CO2 in water. The estimated HCO2 –amine and DCO2 –amine values are presented in Table 1. The diffusion coefficients of amine in solution have been estimated using the diffusion coefficient of MDEA, corrected for the molecular weight by multiplying with an appropriate factor, e.g., 1.38 for PZ. The diffusion coefficient of MDEA has been calculated from Eq. (51) as given by Snijder et al. [29]. The diffusion coefficients of the ionic species have been assumed to be equal to that of PZ. ln DMDEA = −13.088 −

Austgen [35]

2895 T



DN2 O–water

Read [34] Bishnoi [30]

4 − 3.55×10 R

diffusivity in the blended amine solutions using N2 O analogy as follows (Eqs. (49) and (50)):



Posey [33] 7495.441 T

(51)

The equilibrium constants are needed to interpret the results of CO2 absorption measurement into alkanolamines. Values of the independent equilibrium constants have been found out using reliable correlations presented in Table 2 [30–35] and the dependent ones have been estimated by appropriate combination of the independent equilibrium constants. The correlations for the reaction rate constants are also given in Table 2 [36–38]. 3. Experimental 3.1. Materials Piperazine (>99% pure) and MDEA (>98% pure) were supplied by E. Merck (Germany) and were used without further purification. Distilled water degassed by boiling and cooled to ambient temperature under vacuum, was used for preparing the amine solutions. Carbon dioxide (>99.9% pure) and nitrogen gas (>99.999% pure) used for the absorption measurement were obtained from Chemtron Science Pvt. Ltd. (India).

3.2. Apparatus and procedure The stainless steel wetted wall contactor (2.81 × 10−2 m o.d.) used for absorption measurements in this study was similar to the one used by Saha et al. [39]. Absorption measurements were done with (30 wt% MDEA), (28 wt% MDEA + 2 wt% PZ), (25 wt% MDEA + 5 wt% PZ), and (22 wt% MDEA + 8 wt% PZ) over the CO2 partial pressure range of 2–14 kPa and temperature range of 298–313 K under atmospheric pressure. Appropriate measures, as described by Danckwerts [40] have been incorporated in the design to minimize errors due to entrance effect, ripples and rigid film formation. The liquid was introduced in the contactor from the overhead storage. Flow rates of CO2 , and N2 were controlled by mass flow controllers (Sierra Instruments, USA). Two circulator temperature controllers (JULABO F 32 and FP 55, FRG) were used to control the liquid film and gas phase temperature within ±0.2 K of the desired level. Besides, the absorption measurements were conducted in an enclosure where the environment temperature could be controlled within about ±2 K to ensure that the temperature controllers of the experimental set-up for the gas phase and liquid phase temperature control could efficiently control the absorption temperature. It has been observed earlier by Samanta and Bandyopadhyay [37] that for the contactor used for this work the mass transfer resistance to CO2 absorption is negligible at a gas flow rate above 160 × 10−6 m3 s−1 under the conditions of CO2 partial pressure range and temperature range as used for this study. However, to ensure elimination of the gas phase resistance a volumetric gas flow rate of 180 × 10−6 m3 s−1 was maintained throughout all runs of gas absorption measurement. CO2 diluted with N2 to make the required gas phase composition at the inlet was passed through the mixing tube and the coil immersed in the controlled temperature bath and finally through the saturators immersed in the same bath. When the desired gas phase temperature was reached, the concentration of CO2 at the inlet was determined using the HORIBA NDIR on-line CO2 analyzer (Model: VA 3000, HORIBA, Japan) connected through its continuous sampling unit (Model: VS 3000, HORIBA, Japan). The gas, saturated with water vapor at the temperature of absorption, was fed to the top of absorption space of the wetted wall contactor. The amine solution thermostated at the temperature of absorption was then fed from the overhead storage to the contactor at the desired flow rate of about 2 × 10−6 m3 s−1 . Since turbulence appears when liquid film Reynolds number is greater than 250 [40], liquid flow rate for all runs of this work were fixed at about 2 × 10−6 m3 s−1 . With this flow rate the Reynolds num-

A. Samanta, S.S. Bandyopadhyay / Chemical Engineering Journal 171 (2011) 734–741

739

Table 3 Experimental and model predicted results for the absorption of CO2 into aqueous (MDEA + PZ). [MDEA] −3

[PZ] −3

T

p1



kmol m

kmol m

K

kPa

s

2.564 2.564 2.564 2.41 2.41 2.41 2.41 2.41 2.41 2.41 2.15 2.15 2.15 2.15 2.15 2.15 1.89 1.89 1.89 1.89 1.89 1.89 1.89

0.0 0.0 0.0 0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.60 0.60 0.60 0.60 0.60 0.60 0.95 0.95 0.95 0.95 0.95 0.95 0.95

313 313 313 298 303 308 313 313 313 313 298 303 313 313 313 313 298 303 308 313 313 313 313

5.04 7.06 14.24 4.8 4.89 4.95 4.84 1.69 6.67 13.83 4.76 4.83 4.66 1.98 6.97 13.27 4.93 4.71 4.75 4.70 1.87 6.69 13.51

0.35 0.39 0.40 0.53 0.49 0.43 0.42 0.43 0.40 0.43 0.50 0.41 0.39 0.40 0.40 0.40 0.45 0.42 0.42 0.39 0.46 0.43 0.45

ber was about 40. When the system reached the steady state with respect to the gas and liquid flow rates, gas phase and liquid film temperatures, and the gas phase concentration of CO2 at the exit of the contactor, three liquid samples were collected from liquid outlet located at the end of the absorption length at an interval of about one minute. The amount of total CO2 in each loaded liquid sample was determined immediately by acidulating a known volume of the sample in a glass equilibrium cell placed in a thermostated bath and measuring the volume of evolved gas following the procedure described by Saha et al. [39]. The corresponding rate of absorption of CO2 in the alkanolamine solution was determined from the liquid phase CO2 concentration. 4. Results and discussion Absorption measurements for CO2 into aqueous (MDEA + PZ) at 298, 303, 308 and 313 K have been interpreted according to the model developed and described in Section 2 to obtain estimates for the rate constants, k23 and k25 . This is achieved by adjusting the values of rate constants in our mathematical model until the predicted rates of absorption agreed with the experimentally measured rates of absorption within 5%. Similar approach was also followed by Rinker [41], Mandal and Bandyopadhyay [42], and Samanta and Bandyopadhyay [43]. By comparing the model calculated results with the experimental results, it has been observed that if reaction (3) is not considered the model under-predicts the rate of absorption by about 28% compared with the experimental rates for CO2 partial pressure above 5 kPa. From this observation it appears that the role of MDEA in proton transfer is important. It is also observed that if reactions (4) and (5) are not considered, the model under predicts the rate of absorption by about 12% for CO2 partial pressure above 5 kPa. Thus the formation of PZ-dicarbamate (reactions (4) and (5)) is also important in the overall kinetics of CO2 –MDEA–PZ. Similar view was also made by Derks [12]. The theoretical model developed by Derks [12] predicts reasonably their experimentally determined absorption data at very low CO2 loadings without incorporating reactions (4) and (5). However, the same model underpredicts substantially their experimentally determined absorption rates at high CO2 loadings. This discrepancy may be due to the fact that the reactivity of the

kL × 105

7.04 6.68 6.57 4.5 5.17 5.85 6.28 6.18 6.39 6.17 4.22 5.22 5.98 5.90 5.90 5.90 4.19 4.74 5.08 5.57 5.31 5.19 5.13

Experimental results

Predicted results

R × 106 kmol m−2 s−1

E

R × 106 kmol m−2 s−1

E

1.32 1.48 2.40 6.05 6.57 7.16 7.75 4.69 9.30 16.5 7.99 8.87 10.90 5.70 13.50 23.20 9.93 10.20 11.20 11.90 5.86 15.30 28.80

15.8 13.4 10.9 96.5 99.7 105.5 118.8 209.4 101.8 90.1 131.5 128.8 164.5 204.9 137.9 124.5 164.4 166.9 189.9 202.4 262.2 196.4 185.1

1.02 1.23 2.17 5.65 6.07 6.83 8.11 4.51 9.11 16.3 7.45 8.71 10.77 5.15 14.02 24.57 8.80 9.73 11.10 12.72 5.17 16.65 30.09

12.2 11.1 9.9 90.1 92.1 100.6 124.3 201.4 99.7 88.9 122.6 126.4 162.5 185.2 143.2 131.8 145.7 159.2 188.2 216.4 232.1 213.8 193.4

second amine group of PZ (i.e., reactions (4) and (5)) has not been considered in his model. Therefore, both reactions (4) and (5) to form PZ-dicarbamate and equilibrium between PZ-carbamate and its protonated species play an important role with increasing CO2 loadings for CO2 –(MDEA + PZ + H2 O) system. The average absolute deviation (AAD) in this work for the whole partial pressure range of CO2 when reactions (3)–(5) are considered is about 6.8%. Table 3 presents measured and model predicted rates and enhancement factors for absorption of CO2 into (MDEA + PZ + H2 O). As shown in Table 3, the addition of small amounts of PZ to an aqueous solution of MDEA results in significant increase in the rate of absorption and enhancement factor. For instance, at T = 313 K and CO2 partial pressure of about 5 kPa, the enhancement factors for absorption in aqueous solutions of (30 wt% MDEA), (28 wt% MDEA + 2 wt% PZ), (25 wt% MDEA + 5 wt% PZ), and (22 wt% MDEA + 8 wt% PZ) are about 15.8, 118.8, 164.5 and 202.4, respectively. So, by replacing 2 wt% MDEA with 2 wt% PZ, the enhancement factor increased by about 651%. Replacing an additional 3 wt% MDEA with PZ increased the enhancement factor by an additional 38%. A further replacement of 3 wt% MDEA with an equal amount of PZ resulted in increasing the enhancement factor by an additional 23.0%. The substantial increase in the enhancement factor with the replacement of small amount (2 wt%) of MDEA with PZ may be due to the increase of PZ-carbamate and PZ-dicarbamate ions formation catalyzed by higher concentration of MDEA in the solvent. However, as the concentration of PZ is increased further, say to 8 wt%, in the blend, the change in the enhancement factor does not follow this trend. This may be due to the reduced diffusivity of CO2 in the solvent with higher concentration of PZ (Table 1). Besides, with higher concentration of PZ (and relatively lower concentration of MDEA) there is negligible increase in the equilibrium CO2 capacity of the solvent, which also puts a limit on the rate of CO2 absorption in the solvent. Similar improvement in the specific rate of absorption and enhancement factor with the addition of small amounts of PZ in aqueous AMP has been observed by Samanta and Bandyopadhyay [43] for absorption of CO2 in aqueous (AMP + PZ). The measured and model predicted rates of absorption of CO2 in (MDEA + PZ + H2 O) are compared in the parity plot shown in Fig. 1. As shown in Fig. 1, the model predicted rates of absorption of CO2 into (MDEA + PZ + H2 O) are in good agreement with the experimen-

740

A. Samanta, S.S. Bandyopadhyay / Chemical Engineering Journal 171 (2011) 734–741

Fig. 1. Parity plot of model predicted rates and experimental rates of absorption of CO2 into aqueous (MDEA + PZ).

tal results, the AAD between the experimental and model results being about 6.8%. The estimated k23 and k25 values obtained for CO2 –(MDEA + PZ + H2 O) in this work have been correlated according to the following Arrhenius equations.



4

k23 = 1.75 × 10

exp

 k25 = 1.55 × 104 exp

8.75 × 104 − R −

8.75 × 104 R

1

1 − T 298

1 T

−

1 298



(52)

 (53)

A typical estimate of k23 from this work is plotted in Fig. 2 and compared with the k23 values reported by Bishnoi and Rochelle [9]. The estimated k23 and k25 values for the reaction of CO2 with (MDEA + PZ + H2 O) have been found to be about 17,500 m6 kmol−2 s−1 and 15,500 m6 kmol−2 s−1 at 298 K, respectively. Bishnoi and Rochelle [9] reported the values of k23 and k25 as 14,600 m6 kmol−2 s−1 and 12,700 m6 kmol−2 s−1 at 298 K, respectively. Thus, the values of the rate constants k23 and k25 are in good agreement with those reported by Bishnoi and Rochelle [9]. A sys-

Fig. 3. Calculated concentration profile of species in the liquid near the gasliquid interface at the end of the absorption length: [MDEA] = 2.15 kmol m−3 , [PZ] = 0.6 kmol m−3 , T = 313 K, p1 = 4.66 kPa and  = 0.39 s.

tematic discrepancy between the k23 values of this study and that of Bishnoi and Rochelle [9] is observed in Fig. 2. This may be due to the fact that some of the CO2 absorption studies were conducted by Bishnoi and Rochelle [9] into loaded solutions and very low partial pressures. Bishnoi [30] reported that a combined error of 5% in reading the partial pressure in and out of their contactor can result in a 30% error in the flux. Typical calculated concentration profiles for absorption of CO2 into (25 wt% MDEA + 5 wt% PZ) are shown in Fig. 3. The concentrations of MDEA and PZ near the gas–liquid interface are lower by about 1.54% and 36%, respectively, than their liquid bulk concentrations after a gas–liquid contact time of about 0.39 s, showing substantial depletion of PZ at the gas–liquid interface. The depletion of more PZ could be due to the fast chemical reaction between highly reactive PZ and CO2 near the gas–liquid interface to form PZ-carbamate and PZ-dicarbamate. Although the change in concentration of MDEA between that in the bulk and at the interface is 1.54% only, this change has also been considered in the model for the sake of accuracy of the results, instead of the simplifying consideration that MDEA concentration remains constant at the bulk value. Besides, it has been checked that the computational time and complexity do not reduce substantially by considering MDEA concentration constant at bulk value. 5. Conclusions

Fig. 2. Arrhenius plot for the reaction rate constant, k23 of CO2 – (MDEA + PZ + H2 O): —, this work; , Bishnoi and Rochelle [9].

In this work absorption of CO2 into PZ activated aqueous MDEA solution has been studied over the temperature range of 298–313 K under atmospheric pressure using a wetted wall model contactor. The CO2 partial pressure was varied in the range 2–14 kPa, keeping the total amine concentration at 30 wt%. The absorption data are interpreted by using a coupled mass transfer–kinetics–equilibrium mathematical model in which all reactions are considered reversible. New kinetic parameters, k23 and k25 for the PZ-monocarbamate and PZ-dicarbamate formation reactions have been obtained using the mathematical model and the measured absorption data of this work. The estimated k23 and k25 values obtained by adjustment in the model for the reaction of CO2 with (MDEA + PZ + H2 O) in this work has been found to be about 17,500 m6 kmol−2 s−1 and 15,500 m6 kmol−2 s−1 at 298 K, respectively. Bishnoi and Rochelle [9] reported the values of k23 and k25 as 14,600 m6 kmol−2 s−1 and 12,700 m6 kmol−2 s−1 at 298 K, respectively. Thus, the rate

A. Samanta, S.S. Bandyopadhyay / Chemical Engineering Journal 171 (2011) 734–741

constants are in good agreement with those reported by Bishnoi and Rochelle [9]. From a comparison of the predicted rates from the model developed in this work with the experimental rates of absorption, it has been found that it is important to consider both the PZ-carbamate and PZ-dicarbamate formation reactions for the accuracy in model predicted results. In this work, it has been found that the addition of small amounts of PZ to an aqueous solution of MDEA significantly enhances the rate of absorption and enhancement factor. A close agreement between the model predicted and experimental results justifies the validity of the mathematical model. References [1] IPCC, Carbon dioxide Capture and Storage, Intergovernmental Panel on the Climate Change. Prepared by Working Group III of the Intergovernmental Panel on the Climate Change, Cambridge University Press, Cambridge, New York, 2005. [2] G. Astarita, D.W. Savage, A. Bisio, Gas Treating with Chemical Solvents, John Wiley and Sons, New York, 1983. [3] A.L. Kohl, R.B. Nielsen, Gas Purification, 5th ed., Gulf Publishing Company, Houston, TX, 1997. [4] S.W. Campbell, R.H. Weiland, Modeling of CO2 removal by amine blends. Paper Presented at the AIChE Spring National Meeting, Houston, TX, 1989. [5] M. Appl, U. Wagner, H.J. Henrici, K. Kuessner, F. Volkamer, N. Ernst, Removal of CO2 and/or H2 S and/or COS from gases containing these constituents, US Patent 4,336,233, 1982. [6] R.E. Meissner, A low energy process for purifying natural gas, in: Proceedings of the 1983 Gas Conditioning Conference, University of Oklahoma, Norman, 1983. [7] G.-W. Xu, C.-F. Zhang, S.-J. Qin, Y.-W. Wang, Kinetics study on absorption of carbon dioxide into solutions of activated methyldiethanolamine, Ind. Eng. Chem. Res. 31 (1992) 921–927. [8] X. Zhang, C.-F. Zhang, S.-J. Qin, Z.-S. Zheng, A kinetics study on the absorption of carbon dioxide into a mixed aqueous solution of methyldiethanolamine and piperazine, Ind. Eng. Chem. Res. 40 (2001) 3785–3791. [9] S. Bishnoi, G.T. Rochelle, Absorption of carbon dioxide in aqueous piperazine/methyldiethanolamine, AIChE J. 48 (2002) 2788–2799. [10] P.-S. Kamps A, J. Xia, G. Maurer, Solubility of CO2 in (H2 O + piperazine) and in (H2 O + MDEA + piperazine), AIChE J. 49 (2003) 2662–2670. [11] B.S. Ali, M.K. Aroua, Effect of piperazine on CO2 loading in aqueous solutions of MDEA at low pressure, Int. J. Thermophys. 25 (2004) 1863–1870. [12] P.W.J. Derks, Carbon dioxide absorption in piperazine activated Nmethyldiethanolamine, PhD thesis, University of Twente, The Netherlands, 2006. [13] S. Bishnoi, G.T. Rochelle, Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility, Chem. Eng. Sci. 55 (2000) 5531–5543. [14] X. Zhang, J. Wang, C.-F. Zhang, Y.-H. Yang, J.-J. Xu, Absorption rate into a MDEA aqueous solution blended with piperazine under a high CO2 partial pressure, Ind. Eng. Chem. Res. 42 (2003) 118–122. [15] A. Samanta, Absorption of carbon dioxide into piperazine activated alkanolamines, PhD thesis, Indian Institute of Technology Kharagpur, India, 2008. Bishnoi, G.T. Rochelle, Thermodynamics of piper[16] S. azine/methyldiethanolamine/water/carbon dioxide, Ind. Eng. Chem. Res. 41 (2002) 604–612. [17] T.L. Donaldson, Y.N. Nguyen, Carbon dioxide reaction kinetics and transport in aqueous amine membranes, Ind. Eng. Chem. Fund. 19 (1980) 260–266. [18] G.F. Versteeg, W.P.M. van Swaaij, On the kinetics between CO2 and alkanolamines both in aqueous and nonaqueous solutions – II. Tertiary amines, Chem. Eng. Sci. 43 (1988) 587–591.

741

[19] M. Caplow, Kinetics of carbamate formation and breakdown, J. Am. Chem. Soc. 90 (1968) 6795–6803. [20] P.V. Danckwerts, The reaction of CO2 with ethanolamines, Chem. Eng. Sci. 34 (1979) 443–446. [21] P.W.J. Derks, C. Kleingeld, C. van Aken, J.A. Hogendoorn, G.F. Versteeg, Kinetics of absorption of carbon dioxide in aqueous piperazine solutions, Chem. Eng. Sci. 61 (2006) 6837–6854. [22] L.R. Petzold, A Description of DASSL: A Differential/Algebraic System Solver in Scientific Computing, North-Holland, Amsterdam, 1983. [23] A. Samanta, S.S. Bandyopadhyay, Density and viscosity of aqueous solutions of piperazine and (2-amino-2-methyl-1-propanol + piperazine) from 298 to 333 K, J. Chem. Eng. Data 51 (2006) 467–470. [24] J.K.A. Clarke, Kinetics of absorption of carbon dioxide in monoethanolamine solutions at short contact times, Ind. Eng. Chem. Fund. 3 (1964) 239–245. [25] S.S. Laddha, J.M. Diaz, P.V. Danckwerts, The N2 O analogy: the solubilities of CO2 and N2 O in aqueous solutions of organic compounds, Chem. Eng. Sci. 36 (1981) 228–229. [26] G.F. Versteeg, W.P.M. van Swaaij, Solubility and diffusivity of acid gases (CO2 , H2 S) in aqueous alkanolamine solutions, J. Chem. Eng. Data 33 (1988) 29–34. [27] A.K. Saha, S.S. Bandyopadhyay, A.K. Biswas, Solubility and diffusivity of N2 O and CO2 in aqueous solutions of 2-amino-2-methyl-1-propanol, J. Chem. Eng. Data 38 (1993) 78–82. [28] A. Samanta, S. Roy, S.S. Bandyopadhyay, Physical solubility and diffusivity of N2 O and CO2 in aqueous solutions of piperazine and (Nmethyldiethanolamine + piperazine), J. Chem. Eng. Data 52 (2007) 1381–1385. [29] D.E. Snijder, M.J.M. te Riele, G.F. Versteeg, W.P.M. van Swaaij, Diffusion coefficients of several aqueous alkanolamine solutions, J. Chem. Eng. Data 38 (1993) 475–480. [30] S. Bishnoi, CO2 absorption and solution equilibrium in piperazine activated methyldiethanolamine, PhD thesis, The University of Texas at Austin, USA, 2000. [31] P.V. Danckwerts, M.M. Sharma, The absorption of carbon dioxide into aqueous solutions of alkalis and amines (with some notes on hydrogen sulphide and carbonyl sulphide), Chem. Eng. J. 10 (1966) CE244–CE280. [32] J.M. Pagano, D.E. Goldberg, W.C. Fernelius, A thermodynamic study of homopiperazine, piperazine, and N-(2-aminoethyl)-piperazine and their complexes with copper(II) ion, J. Phys. Chem. 65 (1961) 1062–1064. [33] M.L. Posey, Thermodynamic model for acid gas loaded aqueous alkanolamine solutions, PhD thesis, The University of Texas at Austin, USA, 1996. [34] A.J. Read, The first ionization constant of carbonic acid from 25 to 250 ◦ C and to 2000 bar, J. Solution Chem. 4 (1975) 53–70. [35] D.M. Austgen, A model of vapor–liquid equilibria for acid gas–alkanolamine–water systems, PhD thesis, The University of Texas at Austin, USA, 1989. [36] E.B. Rinker, S.S. Ashour, O.C. Sandall, Kinetics and modeling of carbon dioxide absorption into aqueous solutions of N-methyldiethanolamine, Chem. Eng. Sci. 50 (1995) 755–768. [37] A. Samanta, S.S. Bandyopadhyay, Kinetics and modeling of carbon dioxide adsorption into aqueous solutions of piperazine, Chem. Eng. Sci. 62 (2007) 7312–7319. [38] B.R.W. Pinsent, L. Pearson, F.J.W. Roughton, The kinetics of combination of carbon dioxide with hydroxide ions, Trans. Faraday Soc. 52 (1956) 1512–1520. [39] A.K. Saha, S.S. Bandyopadhyay, A.K. Biswas, Kinetics of absorption of CO2 into aqueous solutions of 2-amino-2-methyl-1-propanol, Chem. Eng. Sci. 50 (1995) 3587–3598. [40] P.V. Danckwerts, Gas–Liquid Reactions, McGraw-Hill, New York, 1970. [41] E.B. Rinker, Acid gas treating with blended alkanolamines, PhD thesis, University of California, Santa Barbara, USA, 1997. [42] B.P. Mandal, S.S. Bandyopadhyay, Absorption of carbon dioxide into aqueous blends of 2-amino-2-methyl-1-propanol and monoethanolamine, Chem. Eng. Sci. 61 (2006) 5440–5447. [43] A. Samanta, S.S. Bandyopadhyay, Absorption of carbon dioxide into aqueous solutions of piperazine activated 2-amino-2-methyl-1-propanol, Chem. Eng. Sci. 64 (2009) 1185–1194.