rate of absorption for co absorption systems using a

Available online at www.sciencedirect.com

ScienceDirect

Energy Procedia 00 (2017) 000–000

www.elsevier.com/locate/procedia

1876-6102 © 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the organizing committee of GHGT-13.

13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland

Rate of absorption for CO2 absorption systems using a wetted wall

column

Hanna Karlsson1, Helena Svensson1

1Department of Chemical Engineering, Lund University

Abstract

Knowledge on the absorption rate of CO2 in absorption systems is essential for accurate equipment design. This

work is focused on determining the rate of absorption for novel absorption systems using a wetted wall column. This

is a widely used method for determining the absorption flux of carbon dioxide in amine solutions. Two absorption

systems were investigated, namely aqueous solutions of monoethanolamine (MEA), which is considered a

benchmark amine solution, and solutions of 2-amino-2-methyl-1-propanol (AMP) in the organic solvent N-methyl-

2-pyrrolidone (NMP). Aqueous solutions of MEA have been widely researched for CO2 capture whereas solutions

of AMP in NMP are novel absorption systems that have been reported as promising alternatives for CO2 capture,

based on its ability to be regenerated at lower temperatures. Experiments were performed with a wetted wall column

in order to investigate the CO2 absorption rate of the previously mentioned amine solutions. Different amine

concentrations for the aqueous MEA were investigated, 1.5 and 7 m, as well as for solutions of AMP in NMP, 1.5

and 5 m. From the experiments the overall flux of CO2 could be evaluated and used to calculate the mass transfer

coefficients. The results showed a higher flux and higher liquid side mass transfer of CO2 for AMP in NMP

compared to aqueous MEA.

© 2017 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the organizing committee of GHGT-13.

Keywords: CO2 capture; Absorption; Wetted wall column; AMP; NMP; Mass transfer

1. Introduction

In order to properly design an absorption tower for CO2 capture knowledge on the absorption rate of CO2 is

required. For new amine solutions knowledge of the rate of absorption is an important parameter to determine the

suitability of the system for use in CO2 absorption processes.

http://www.sciencedirect.com/science/journal/22126716

2 Author name / Energy Procedia 00 (2017) 000–000

In this work two different absorbing systems are studied, aqueous monoethanolamine (MEA) and solutions of 2-

amino-2-methyl-1-propanol (AMP) in N-methyl-2-pyrrolidone (NMP). Aqueous solutions of MEA is a benchmark

absorbent for CO2 capture that has been extensively studied and documented. Due to the comprehensive evaluations

with different equipment, concentrations and temperatures, there is extensive and well determined rate data for this

system [1–3]. AMP is a sterically hindered amine which has been under evaluation for some time. Sterically

hindered amines form an unstable carbamate intermediate and when mixing them in organic solvents, the formation

of this carbamate will be favoured, since formation of bicarbonate cannot occur without water present. The

instability of the carbamate, due to the steric hindrance, results in regeneration of the amine at lower temperatures

than for benchmark amine solutions, which may decrease the operational cost for the process [4,5]. The maximum

loading for AMP in NMP is limited to 0.5 mole CO2 per mole amine compared to an equimolar loading capacity in

aqueous solutions of AMP, where bicarbonate is formed [6].

This work presents experimental data on the absorption rate of CO2 in solutions of AMP in NMP, determined

using a wetted wall column (WWC). The wetted wall column has been characterized using aqueous solutions of

MEA. The determined rate of absorption for aqueous MEA has also been used for comparison with the rate obtained

with the AMP in NMP system.

Nomenclature

Awwc Area (m2)

C Concentration (mol/m3)

d Diameter (m)

D Diffusivity (m2/s)

ds Hydraulic diameter (m)

E Enhancement factor

g Gravitational acceleration (m/s2)

H Henry’s constant (Pa·m3/mol)

h Hight of column (m)

Ha Hatta number

KG Overall mass transfer coefficient (mol/m2/sec/Pa)

k2 Second order rate constant (Pa/m3/mol)

k’g Liquid side mass transfer coefficient (mol/m2/sec/Pa)

kg Gas side mass transfer coefficient (mol/m2/sec/Pa)

kl Liquid side mass transfer coefficient (m/s)

kl0 Physical liquid mass transfer coefficient (m/s)

kOH Rate constant for bicarbonate formation (Pa/m3/mol)

LMPD Logarithmic mean pressure difference (Pa)

m Molal (mole/kg solvent)

N Flux (mol/m2/sec)

P Pressure (Pa)

Q Flow rate (m3/sec)

Re Reynolds number

Sc Schmidt number

Sh Sherwood number

T Temperature (K) or (°C)

usurf Surface rate (m/s)

v Mean gas velocity (m/s)

W Perimeter (m)

α Fitting parameter

β Fitting parameter

δ Film thickness (m)

ε Dimensionless parameter

Author name / Energy Procedia 00 (2017) 000–000 3

θ Dimensionless parameter

μ Viscosity (Pa·s)

νam Ratio amine to CO2 from mechanism

ρ Density (kg/m3)

2. Theory

2.1. Reaction mechanism

The system of AMP in NMP have shown promising potential for lower temperature regeneration compared to

conventional amine solutions. Desorption of CO2 can be achieved at temperatures ranging from 70-90 °C, which

opens up the possibility of using low-grade heat for the regeneration step, thus making the absorption process more

cost effective [6]. The reaction mechanism for amine based absorption in non-aqueous solutions can be described by

the reactions below. First the carbon dioxide is dissolved in the liquid medium (1) where a reaction following the

zwitterion mechanism takes place (2) and a carbamate is formed (3). In the AMP in NMP system precipitation of a

crystalline solid occurs as the loading of the solution is increased, according to reaction (4) [7,8].

𝐶𝑂2(𝑔) ↔ 𝐶𝑂2(𝑠𝑜𝑙) (1) 𝐶𝑂2(𝑠𝑜𝑙) + 𝑅𝑁𝐻2(𝑠𝑜𝑙) ↔ 𝑅𝑁𝐻2

+𝐶𝑂𝑂−(𝑠𝑜𝑙) (2)

𝑅𝑁𝐻2+𝐶𝑂𝑂−(𝑠𝑜𝑙) + 𝑅𝑁𝐻2(𝑠𝑜𝑙) ↔ 𝑅𝑁𝐻3

+(𝑠𝑜𝑙) + 𝑅𝑁𝐻𝐶𝑂𝑂−(𝑠𝑜𝑙) (3)

𝑅𝑁𝐻3+(𝑠𝑜𝑙) + 𝑅𝑁𝐻𝐶𝑂𝑂−(𝑠𝑜𝑙) ↔ 𝑅𝑁𝐻3

+𝑅𝑁𝐻𝐶𝑂𝑂−(𝑠) (4)

2.2. Film theory

The overall absorption of carbon dioxide can be divided into two steps, physical and chemical absorption. The

physical absorption is described by the diffusion of CO2 from the gaseous phase through an interface in to the liquid

phase. The chemical absorption is then described by the reaction CO2 undergoes while in the liquid phase [3,9].

The physical mass transfer, can be described by the two-film theory [10] where the mass transfer of CO2 is

explained by different diffusion steps. These steps include the transport of CO2 from the bulk gas phase to the

interface and then the transport of CO2 from the interface to the bulk of the solvent.

Figure 1 shows the mass transfer of CO2 from the gas to the bulk liquid, including fast reaction. The transport

through the gas phase is described by Fick’s first law and the driving force is the gradient of the chemical potential,

which can be translated to the concentration gradient and thus be estimated by the partial pressure of carbon dioxide.

For the liquid side the driving force can be estimated by the concentration difference in the film. If a reaction takes

place in the liquid the two-film theory is not valid. To account for the reaction an enhancement factor can be

introduced. Since the reaction occurs on the liquid side of the system, the enhancement factor is introduced in the

description of the flux through the liquid film. The expressions for the flux through the gas and liquid film are given

in equations (6) and (7).

𝑁𝐶𝑂2,𝑔 = 𝑘𝑔(𝑃𝐶𝑂2,𝑏𝑢𝑙𝑘 − 𝑃𝐶𝑂2,𝑖) (6)

𝑁𝐶𝑂2,𝑙 = 𝐸 · 𝑘𝑙(𝐶𝐶𝑂2,𝑖 − 𝐶𝐶𝑂2,𝑏𝑢𝑙𝑘) = 𝑘𝑔′ · (𝑃𝐶𝑂2,𝑖 − 𝑃𝐶𝑂2,𝑏𝑢𝑙𝑘

∗ ) (7)

Here 𝑁𝐶𝑂2,𝑔 and 𝑁𝐶𝑂2,𝑙 are the fluxes while 𝑘𝑔 and 𝑘𝑙 are the mass transfer coefficients for the gas and liquid

film, respectively. 𝑃𝐶𝑂2,𝑏𝑢𝑙𝑘 and 𝑃𝐶𝑂2,𝑖 are the partial pressures of CO2 in the gas bulk and at the gas side of the

interface. 𝐶𝐶𝑂2,𝑖 and 𝐶𝐶𝑂2,𝑏𝑢𝑙𝑘 are the concentrations of CO2 at the liquid interface and in the liquid bulk. P*CO2,bulk is

the liquid bulk concentration of CO2 expressed as a partial pressure, k’g is the liquid side mass transfer coefficient

and E is the enhancement factor.


Figure 1. Illustration of mass transfer from gas to liquid bulk with fast chemical reaction, inspired by Dugas [11]

When the system is at equilibrium the concentration at the liquid interface can be calculated from the equilibrium

gas pressure at the interface using Henry’s constant. According to the two-film theory and the assumption of fast

reaction where the reaction only takes place in the liquid film, an overall mass transfer coefficient, KG, can be

introduced which includes both the gas side and the liquid side mass transfer coefficient, as described in equation

(8).

1

𝐾𝐺=

1

𝑘𝑔+

𝐻𝐶𝑂2

𝐸·𝑘𝑙0 =

1

𝑘𝑔+

1

𝑘𝑔′ (8)

The overall flux of CO2 from the gas phase to the liquid in the system can be described by KG and the driving force:

𝑁𝐶𝑂2 = 𝐾𝐺 · (𝑃𝐶𝑂2,𝑏𝑢𝑙𝑘 − 𝑃𝐶𝑂2,𝑏𝑢𝑙𝑘∗ ) (9)

where PCO2,bulk is described by the logarithmic mean pressure difference (LMPD) which is defined as follows:

𝐿𝑀𝑃𝐷 = 𝑃𝐶𝑂2,𝑏𝑢𝑙𝑘 =𝑃𝐶𝑂2,𝑖𝑛−𝑃𝐶𝑂2,𝑜𝑢𝑡

ln(𝑃𝐶𝑂2,𝑖𝑛

𝑃𝐶𝑂2,𝑜𝑢𝑡)

(10)

2.3. Pseudo-first order reactions

Absorption of CO2 into aqueous amine solutions normally follows a second order reaction which makes the

determination of the enhancement factor quite complex [12]. If instead the experiments are conducted in the pseudo-

first order regime it allows for a simplified model as the enhancement factor can be set equal to the Hatta number

under these conditions [13]. The pseudo-first order regime is defined by a fast reaction and requires the

concentration of amine to be high enough not to change in the gas-liquid interface during the absorption so that it

can be considered constant [14].

To verify that the reaction follows the pseudo-first order regime, the Hatta number must be larger than 3 and

lower than the infinite enhancement factor, E∞ [13].

3 < 𝐻𝑎 ≪ 𝐸∞ (11)


The Hatta number is defined by Equation (12). It includes the rate constants for both the carbamate and

bicarbonate formation (bicarbonate is formed in aqueous solutions) together with the concentrations of the involved

species, CMEA and COH. DCO2 is the diffusivity of CO2, kOH is the rate constant for the bicarbonate formation and kl0

the physical liquid mass transfer.

𝐻𝑎 =√(𝑘2·𝐶𝑀𝐸𝐴+𝑘𝑂𝐻·𝐶𝑂𝐻)·𝐷𝐶𝑂2

𝑘𝑙0 (12)

The infinite enhancement factor is the enhancement factor obtained for instantaneous reactions where the

absorbent is depleted at the interface. It can be calculated according to equation (13), using the diffusivities of amine

and CO2 in the liquid phase and the concentration of amine and CO2 in the bulk and at the interface, respectively. νam

is the ratio amine to CO2 in the reaction obtained from the reaction mechanism.

𝐸∞ = 1 +𝐷𝐴𝑚,𝐿·𝐶𝑎𝑚

𝑏𝑢𝑙𝑘

𝑣𝑎𝑚·𝐷𝐶𝑂2,𝐿·𝐶𝐶𝑂2𝑖𝑛𝑡 (13)

In the case where the condition described in equation (11) is valid, and the reaction is of pseudo-first order, one

can make the assumption that the enhancement factor is equal to the Hatta number based on the following correlation:

𝐸 =𝐻𝑎

tanh 𝐻𝑎 (14)

For values of x higher than or equal to 3, tanh(x) is approximately equal to 1. Thus the enhancement factor can be

set equal to the Hatta number [13].

3. Experimental method and procedure

3.1. Materials

The chemicals used were CO2, N2, He, MEA, NMP and AMP. CO2 was obtained from AGA and He from Air

Products with purities of >99,99 % and >99.98 %, respectively. NMP was obtained from Sigma Aldrich with a purity

of 99.5%. MEA and AMP were obtained from Merck with purities of >99 % and 93-98 %, respectively. All chemicals

were used without further purification. The aqueous solutions of MEA were prepared by mixing with distilled

deionized water. The solutions were prepared using a scale with an accuracy of 0.01 g for weights up to 1200 g.

Gas mixtures with varying partial pressures of CO2 in N2 were used in the experiments, where the pressure of CO2

ranged between 4.4 and 19.5 kPa. He was used in low concentrations as internal standard. Table 1 gives an overview

of the experiments conducted and the conditions for the flux measurements.

Table 1. Overview of the experimental conditions.

Amine Solvent CAmine (mol/kg solvent) T(°C) PCO2 (kPa)

MEA H2O 1.5 27 4.7 - 19.0

MEA H2O 7 26 4.6 - 18.9

MEA H2O 7 26 4.8 - 19.1

AMP NMP 1.5 27 4.4 - 18.7

AMP NMP 1.5 27 5.1 - 18.7

AMP NMP 5 27 4.8 - 19.4

AMP NMP 5 26 4.6 - 15.5


3.2. Experiments with wetted wall column

In order to measure the absorption flux in a gas-liquid system a wetted wall column (WWC) can be used. This is

a counter-current approach where the gas enters at the bottom of the column and exits at the top. An absorbing

liquid medium is pumped and distributed on to a known surface area from the top, exiting at the bottom of the

column. An illustration of the wetted wall column used in this study is presented in Figure 2. It is based on the

existing apparatus used by Darde [12], which in turn has been based on the column used by Dugas [11] with minor

updates.

The gas is introduced to the reactor by three evenly distributed inlets at the bottom of the reactor. The absorbing

medium is pumped through a stainless steel cylindrical tube and distributed as a thin film on the outer surface of the

tube, before it exits at the bottom. The contact area can thus be obtained from calculations using the tube dimensions

and by adding the dimensions of the liquid film and dome (at the top of the tube) resulting from the liquid

distribution. The experimental temperature is kept constant through an external cooling system where water with a

set temperature is circulated in a cooling jacket around the reactor.

Figure 2 also shows the experimental set-up. The pump circulating the absorbing medium is placed directly after

the column to avoid build-up of liquid in the column. Temperature is measured both at the inlet and outlet of the

reactor to monitor any temperature difference due to the exothermic absorption reaction. The gas is saturated with

solvent before it enters the reactor. The saturator is placed in the water bath to make sure the gas holds

approximately the same temperature as the absorbing medium. On/off-valves enables the user to choose whether to

run the gas through the reactor, or in by-pass mode for calibration. The gas is analyzed with a mass spectrometer

(MS) (Hiden Analytical) which analyses part of the gas stream, the rest is led out in the fume hood, making the

system work at atmospheric pressure. The MS has an accuracy of ±1.00 %. Calibration of the MS was carried out

with an inert standard (He) and individual calibration factors were calculated for each data point.

Gas flows were measured using Bronkhorst mass flow controllers, with an accuracy of ±0.6 %, that were

controlled through LabVIEW. The temperatures were measured using thermocouples with an accuracy of 1.5 °C.

The theoretical loading was calculated by the flux obtained from the experiments and the time the gas stream was

lead through the reactor. The molar content of the amine was calculated from the concentration and volume or mass

used in the experiment. The loading is defined as mole CO2 per mole amine, according to equation (15).

𝑙𝑜𝑎𝑑𝑖𝑛𝑔 =(𝑛𝐶𝑂2)𝑎𝑏𝑠

(𝑛𝑎𝑚𝑖𝑛𝑒)0 (15)

Figure 2. Schematic overview of the wetted wall column (left) and experimental set-up (right).


For some experiments regeneration during the experiment was necessary. This was done by introducing pure

nitrogen to the system between each data point. As pure nitrogen is introduced the concentration difference will cause

the previously absorbed CO2 to desorb from the system leading to a decrease of the system loading. Nitrogen was

introduced for 15 min – 1.5 hours, depending on the amount of CO2 absorbed, to obtain sufficient regeneration.

3.3. Characterization of wetted wall column

In order to characterize the WWC, experiments were conducted with a well investigated amine solvent. The flow

rate of gas was altered as well as the partial pressure of carbon dioxide in the gas. The solvent used for the

characterization experiments was 7m MEA. The experiments were performed at room temperature.

To calculate the Henry’s constant for CO2 in the MEA solution a model proposed by Pacheco was used [15]. The

relation is based on the N2O analogy, data obtained by Browning [16] and the assumption that the properties of

MEA changes with temperature similarly to that of diethanolamine (DEA), according to equation (16).

𝐻𝐶𝑂2(𝑃𝑎 ·𝑚3

𝑚𝑜𝑙) = 7.487 · 105 · 𝑒𝑥𝑝 (−

1614.5

𝑇) (16)

The value of the second order rate constant was obtained from Hikita [17], which was also used and re-written by

Darde [12], as can be seen in equation (17).

𝑘2 (𝑚3

𝑚𝑜𝑙·𝑠) = 9.77 · 107 · 𝑒𝑥𝑝 (−

4955

𝑇) (17)

The diffusion of CO2 in the MEA solution was obtained from N2O analogy:

𝐷𝐶𝑂2−𝑤𝑎𝑡𝑒𝑟

𝐷𝑁2𝑂−𝑤𝑎𝑡𝑒𝑟=

𝐷𝐶𝑂2−𝑀𝐸𝐴𝑠𝑜𝑙

𝐷𝑁2𝑂−𝑀𝐸𝐴𝑠𝑜𝑙 (18)

The diffusivity of CO2 and N2O in water were obtained through the correlations described in equation (19) and (20)

derived by Versteeg et.al. [18].

𝐷𝐶𝑂2−𝑤𝑎𝑡𝑒𝑟(𝑚2/𝑠) = 2.35 · 10−6 · 𝑒𝑥𝑝 (−2119

𝑇) (19)

𝐷𝑁2𝑂−𝑤𝑎𝑡𝑒𝑟(𝑚2/𝑠) = 5.07 · 10−6 · 𝑒𝑥𝑝 (−2371

𝑇) (20)

The diffusivity of N2O in MEA solutions were obtained from Ko et.al. according to equation (21), where the

concentration of MEA is expressed in mol/m3 [19].

𝐷𝑁2𝑂−𝑀𝐸𝐴(𝑚2/𝑠) = (5.07 · 10−6 + 8.65 · 10−10 · 𝐶𝑀𝐸𝐴 + 2.78 · 10−13 · 𝐶𝑀𝐸𝐴2 ) · 𝑒𝑥𝑝 (

−2371−9.34·10−2·𝐶𝑀𝐸𝐴

𝑇)

(21)

3.3.1. Physical liquid mass transfer

The physical liquid mass transfer coefficient, 𝑘𝑙0, was calculated using models proposed by Pigford [20] and

Hobler [21]. It includes information of the film thickness, the velocity at the surface of the liquid and a

dimensionless variable which correlates the change in CO2-concentration during the absorption. First the thickness

of the film was calculated using equation (22) [22].

𝛿 = √3·µ𝐿·𝑄𝐿

𝜌𝐿·𝑔·𝑊

3 (22)


Here QL, µL and ρL is the flow rate, dynamic viscosity and density of the liquid medium. W is the perimeter of the

tube and g is the standard acceleration due to gravity. This was then used to calculate the velocity at the surface of

the liquid, usurf [22].

𝑢𝑠𝑢𝑟𝑓 =𝜌𝐿·𝑔·𝛿2

2·µ𝐿 (23)

A dimensionless number ε was then introduced based on the diffusivity of CO2 in the liquid (DCO2,L), the height of

the column (h), the film thickness and the surface velocity [15,20].

휀 =𝐷𝐶𝑂2,𝐿·ℎ

𝑢𝑠𝑢𝑟𝑓·𝛿2 (24)

ε was then used to calculate a variable θ which correlates to the change in CO2-concentration during the

absorption. If ε is larger than 0.01 equation (25) is used, if it is lower than 0.01 it can be approximated with equation

(26) [15,20].

𝜃 = 0.7857 · exp(−5.121휀) + 0.1001 · exp(−39.21휀) + 0.036 · exp(−105.6휀) + 0.0181 · exp(−204.7휀) (25)

𝜃 = 1 − 3√𝜋 (26)

The physical liquid side mass transfer coefficient can then be obtained using θ according to equation (27) [21].

𝑘𝑙0 =

𝑄𝐿

𝐴𝑤𝑤𝑐· (1 − θ) (27)

Here QL is the liquid flow rate and Awwc is the contact area of the column. In this work, 𝑘𝑙0 was calculated with

equation (27) and used to verify pseudo-first order by calculating the Hatta number according to equation (12).

3.3.2. Gas side mass transfer

The gas side mass transfer coefficient, kg, depends on the velocity and the path of the gas flow. Hobler made a

model for this mass transfer coefficient which is valid in the laminar flow regime that is present in the wetted wall

column [21]. This is a correlation that includes the dimensionless Sherwood (Sh), Schmidt (Sc) and Reynolds (Re)

numbers. Pacheco simplified this model to only obtain two parameters for fitting, α and β [15].

𝑆ℎ = 𝛼 · 𝑅𝑒𝛽 · 𝑆𝑐𝛽 · (𝑑𝑠

ℎ)

𝛽

(28)

where ds is the distance between the inner glass tube and the central tube, h is the height of the column and the

dimensionless numbers are defined as follows:

𝑆ℎ =𝑘𝑔·𝑑𝑠

𝐷𝐶𝑂2,𝑔𝑎𝑠 , 𝑅𝑒 =

𝑑𝑠·𝑣·𝜌

µ , 𝑆𝑐 =

µ

𝜌·𝐷𝐶𝑂2,𝑔𝑎𝑠 (29)

Where DCO2,g and v is the diffusivity of CO2 in the gas phase (m2/s) and the mean gas velocity (m/s), respectively.

When the exponent of Re and Sc are equal, as in this model, it becomes independent of the gas viscosity and gas

density.

3.3.3. Liquid side mass transfer

From the experiments the overall mass transfer coefficient is obtained, according to equation (9) previously

described. By using the model in equation (28), with the fitted parameters obtained from the characterization of the


column, the gas side mass transfer coefficient, kg, can be calculated. The liquid side mass transfer coefficient, k’g,

can then be calculated according to equation (8) previously described.

3.4. Repeatability

The investigated repeatability of each experiment was estimated using standard deviation. Two data series was

made for each system and the standard deviation was calculated. In Table 2 the results are presented in % (standard

deviation divided by the mean value). Repeatability was not calculated on the 1.5m MEA system where only one data

series was obtained. This is also the case for the last data point in the AMP 5m series.

Table 2. Repeatability for each data point in form of standard deviation in %.

Data point MEA 7m (%) AMP 1.5m (%) AMP 5m (%)

1 3.31 3.65 0.422

2 2.58 3.47 1.68

3 1.59 0.700 0.514

4 1.32 1.22 2.59

5 0.164 0.541 N.A.

4. Results and discussion

4.1. Characterization of the wetted wall column using aqueous solutions of MEA

The gas side mass transfer coefficient, kg, was calculated and used to obtain the Sh-number. The Sh-number was

plotted as a function of (ReSc(ds/h)) to obtain the parameters for the characterization. The parameters are presented in

Table 3 and had R2 =0.8635.

Table 3. Fitting parameters for characterization of wetted wall column.

Parameter value

α 2.747

β 0.835

Equation (28) was then used, with the parameters from Table 3, to calculate the gas side mass transfer coefficient

(kg) for the MEA experiments, which in turn was used to obtain the liquid side mass transfer coefficient (k’g). Figure

3 shows a comparison of the experimentally obtained k’g value compared to a theoretically calculated value obtained

from a combination of equations (8), (12) and (14). This gives an indication of how good the model is. The values of

k’g presented in Figure 3 are calculated from the data presented in section 4.3.

The comparison of experimental and theoretical k’g shows satisfying results. The individual data points give

scattered values of k’g but show a good fit of the model obtained from the characterization. For the experimental

evaluation of the absorption rate, an average value based on a linear regression was used. For these k’g the average

deviation between theoretical and experimental evaluation is only 4.2 %. Thus the model is considered reliable and

the characterization of the column is deemed sufficient.


4.2. Theoretical loading

The theoretical loading was calculated for each data point, according to equation (15). The results are presented in

Table 4.

Table 4.The theoretical loading, in mol CO2 per mol amine, calculated for each data point.

Sample Name/ Data Point 1 2 3 4 5

AMP5a 0 N.A N.A N.A N.A

AMP5b 0 N.A N.A N.A N.A

AMP1.5a 0 0.014 0.030 0.054 0.081

AMP1.5b 0 0.013 0.031 0.054 0.080

MEA7a 0 0.004 0.012 0.019 0.028

MEA7b 0 0.004 0.010 0.018 0.029

MEA1.5a 0 0.008 0.029 0.051 0.085

Two experiments to investigate the flux as a function of theoretical loading were also conducted in this study. A

gas stream with approximately the same partial pressure of CO2 in N2 was introduced to the reactor multiple times,

to determine how the flux changed as the loading increased. The experiments were conducted at a partial pressure of

CO2 in the range of 8350-8990 Pa. The results are presented in Figure 4. Here it is possible to see the decrease in

flux as the loading increases. This is why it is important to keep the loading as low as possible to obtain a reliable

result of the flux. The overall mass transfer coefficients, KG, obtained from these experiments (AMP in NMP) only

differ 5.2 % when comparing loadings 0 and 0.05 but around 12% when comparing loading 0 and 0.08. However, to

obtain any conclusive result regarding how the flux of the systems varies with increased loading, more experiments

need to be conducted.

0

0,5

1

1,5

2

2,5

3

0 0,5 1 1,5 2 2,5 3

k' g

∙10

6T

heo

reti

cal

(mo

l/m

2/s

ec/P

a)

k'g∙106 Experimental (mol/m2/sec/Pa)

Figure 3. Comparison of the liquid side mass transfer coefficient obtained from the experiments with that calculated from the expression of the

enhancement factor in the pseudo-first order regime.


4.3. MEA

Figure 5 shows the results obtained for the aqueous MEA systems of concentrations 1.5m and 7m, respectively.

Only one experiment was conducted for 1.5m MEA and thus the repeatability for this experiment cannot be

evaluated. The last data point seems to deviate more from a linear correlation and after evaluating the theoretical

loading of the system it was found that the loading for this point is higher than that of the rest, close to 0.08 mol

CO2/mol amine. The overall mass transfer coefficient obtained from all 5 data points was determined to be 8.87·10-7

mol/m2/sec/Pa. To obtain the overall mass transfer coefficient at zero loading, the increase in loading should be held

as low as possible as discussed above. If the last data point is neglected, the maximum loading in the experiment is

close to 0.05 and the result is more comparable to literature data where maximum loadings of around 0.05 have been

reported [11,15]. The overall mass transfer coefficient for the system, using only the first four data points, is

determined to be 1.06·10-6 mol/m2/sec/Pa (a difference of 16.3 % compared to the result obtained when using all

data points).

Two series of experiments of 7m MEA was conducted. All the data points seem to follow a linear behaviour and

an evaluation of the theoretical loading showed that the maximum loading during the experiments (data point 5) is

approximately 0.02 mol CO2/mol amine. The overall mass transfer coefficients obtained by the two series, 1.68·10-6

and 1.71·10-6 mol/m2/sec/Pa, only differ by 1.8% showing a good repeatability of the experiments.

The overall mass transfer coefficients were compared to those found in literature and the results are presented in

Table 5. The overall mass transfer coefficient includes information on both the liquid and the gas mass transfer

coefficient. Another way to compare the results with those obtained in previous studies is by looking at the liquid

side mass transfer coefficient. Here only the characteristics of the absorbing medium is taken into account and the

gas side resistance is subtracted [12].Table 6 shows a comparison of the liquid side mass transfer coefficients

obtained from this study and those found in literature for the same system. The values of the liquid side mass

transfer coefficient obtained in this work for the aqueous solution of MEA are in good agreement with the literature

findings.

0,00

0,20

0,40

0,60

0,80

1,00

1,20

1,40

1,60

1,80

2,00

0 0,02 0,04 0,06 0,08

Flu

x ∙1

0-2

(m

ol/

m2/s

ec)

Loading (mol CO2/mol amine)

MEA 7m

AMP 5m

Figure 4. Flux as a function of theoretical loading for 5 m AMP in NMP and 7 m MEA.


Table 5. Values of overall mass transfer coefficients (KG) compared with those found in literature.

KG·106 (mol/m2/sec/Pa) T (°C) CMEA (mol/dm3) Reference

1.68 27 4.9 This study


2.07 24 5 Luo et.al [3]

1.90* 24 5 Luo et.al [23]

2.25 33 5 Luo et.al [3]

1.12 40 4.9 Han et.al [24]

2.80 40 5 Puxty et.al [9]

* Read from figure with logarithmic scale.

Table 6. Values of liquid side mass transfer coefficient (k’g) compared with those found in literature.

k’g·106 (mol/m2/sec/Pa) T (°C) CMEA (mol/dm3) Reference



1.99 20 4.9 Darde [12]

2.59 25 5 Luo et.al [3]

2.30* 24 5 Luo et.al [23]

2.05* 25 5 Hartono (from Dugas [11])

2.30* 30 5 Hartono (from Dugas [11])

2.88 40 5 Hartono (from Dugas [11])

2.58 40 4.9 Darde [12]

* Read from figure with logarithmic scale.

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

0 5 10 15 20

Flu

x ∙1

0-2

(m

ol/

m2/s

ec)

LMPD (kPa)

Figure 5. Flux as a function of logarithmic mean pressure for 1.5 m MEA (left) and 7 m MEA (right). The slope of the linear regression is the

overall mass transfer coefficient. Circles correspond to the first data set and x to the second data set.

0,00

0,20

0,40

0,60

0,80

1,00

1,20

1,40

1,60

1,80

2,00

0 5 10 15 20

Flu

x ∙1

0-2

(m

ol/

m2/s

ec)

LMPD (kPa)


4.4. AMP in NMP

Figure 6 shows the results for the AMP in NMP systems of concentrations 1.5m and 5m, respectively. Two sets

of data points were collected for each system in order to investigate the repeatability of the experiments.

As for the 1.5m MEA case it was found that the theoretical loading for the last data point of the 1.5 AMP in

NMP system was high (0.08). Using all five data points gave an overall mass transfer coefficient of 8.92·10-7 and

9.31·10-7 mol/m2/sec/Pa for the two experiments, a difference of 4.2 %. Evaluation of the four first data points, with

maximum loading of 0.05, would give a better linear correlation since the loading up to this point has a smaller

impact on the flux (see section 4.2). The overall mass transfer coefficients determined from the regression using

only the first four data points were 1.05·10-6 and 1.10·10-6 mol/m2/sec/Pa for the two data sets, differing 4.5%. In

either case, this shows a good repeatability for the experiment.

For the 5m case precipitation of the carbamate could be observed during the experiment indicating a high

loading of the system. Therefore, the experimental procedure was changed to include regeneration after each data

point collected. Thus the loading is kept to a minimum and the collected data follows the linear trend that is

expected from the experiment. Because of the regeneration performed after each data point, no theoretical loading

could be calculated for this system. For the repeated experiment only four data points were collected due to

malfunction of the set up (tubing broke). The overall mass transfer coefficients obtained were 2.08·10-6 and

2.23·10-6 mol/m2/sec/Pa for the two experiments. The larger deviation of 6.7 % could be due to the difference in

number of data points. It could also be because of the regeneration applied after each data point. If the regeneration

of the solution between each data point has not been achieved to the same degree during the two experimental series,

this could also lead to altering values of the flux.

Figure 7 shows a comparison of the flux obtained for AMP in NMP with that of aqueous MEA and a summary

of the mass transfer coefficients obtained from these experiments are listed in Table 7. The measured flux in this

study is higher for the AMP in NMP solution than that for aqueous MEA, at similar amine molal concentrations,

which is clearly seen in Figure 7. The higher absorption rate is also evident when comparing the values of the liquid

side mass transfer coefficients for 5m AMP in NMP with aqueous 7m MEA, in Table 7. The values of the liquid

side mass transfer coefficient for the 1.5m systems show similar values.

0,00

0,50

1,00

1,50

2,00

2,50

0 5 10 15 20

Flu

x ∙1

0-2

(m

ol/

m2/s

ec)

LMPD (kPa)

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

4,00

0 5 10 15 20

Flu

x ∙1

0-2

(m

ol/

m2/s

ec)

LMPD (kPa)

Figure 6. Flux as a function of logarithmic mean pressure for 1.5 m AMP (left) and 5 m AMP (right). The slope of the linear regression is the

overall mass transfer coefficient. Circles correspond to the first data set and x to the second data set.


Table 7. The KG and k’g values for AMP in NMP and aqueous MEA, obtained from the experiments.

Sample

Name

KG·106

(mol/m2/sec/Pa)

STD

(%)

k’g·106

(mol/m2/sec/Pa)

STD

(%)

AMP5a 2.08 3.48

2.60 4.44

AMP5b 2.23 2.84

AMP1.5a 1.10 3.33

1.23 2.93

AMP1.5b 1.05 1.17

MEA7a 1.68 0.88

2.01 1.24

MEA7b 1.71 2.05

MEA1.5a 1.06 N.A. 1.18 N.A.

5. Conclusions

The absorption rates of CO2 in solutions of AMP in NMP using a wetted wall column have been determined.

The characterization of the wetted wall column showed satisfying results when comparing liquid side mass transfer

coefficients (k’g) obtained experimentally by using equation (28) and the characterized parameters from Table 3,

with those obtained theoretically from equations (8), (12) and (14). This, in combination with the fact that the

experimental values of k’g are in good agreement with those found in literature for aqueous MEA, makes the

characterization of the WWC and the model obtained (equation (28) with the parameters from Table 3) reliable for

further absorption rate evaluations. To strengthen these results more data points for the characterization should be

collected at varying temperatures.

The results obtained in this work showed that the AMP in NMP solutions over all had a higher flux compared to

aqueous MEA solutions. The liquid side mass transfer coefficients were determined to be 2.05·10-6 and 2.84·10-6

mol/(m2 sec Pa) for aqueous MEA and AMP in NMP, respectively. A higher flux means that less absorbing medium

is needed to capture the same amount of CO2. Less absorbing medium in combination with the lower temperature

needed for the regeneration would lead to a lower operational cost for the absorbing process.

Figure 7. Flux as a function of logarithmic mean pressure difference for all the studied mixtures and their duplicate.

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

4,00

0 5 10 15 20

Flu

x ∙1

0-2

(m

ol/

m2/s

ec)

LMPD (kPa)

AMP 5m

AMP 1.5m

MEA 7m

MEA 1.5m


Acknowledgements

The research foundation of Göteborg Energi is gratefully acknowledged for financial support.

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rate of absorption for co absorption systems using a

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