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KINETICS OF THE ABSORPTION OF C02 INTO MIXED

AQUEOUS LOADED SOLUTIONS OF MONOETHANOLAMINE AND

METHYLDIETHANOLAMINE

A Thesis

Submitted to the Faculty of Graduate Studies and Research

In Partial Fulfillment of the Requirements

for the Degree of

Master of Applied Science

in Industrial Systems Engineering

University o f Regina

by

Naveen Ramachandran

Regina, Saskatchewan

January 2004

Copyright 2004: N. Ramachandran

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ABSTRACT

Alkanolamines have attained a well-established position in gas treating for the removal of

acid gases like Carbon Dioxide(COi). The widely used alkanolamines are

monoethanolamine (MEA), diethanolamine (DBA) etc. Due to the high increase in the

regeneration costs, the motivation is to develop better processes and efficient gas treating

solvents. Tertiary amines have high equilibrium loading capacity but react slowly with

CO2whereas primary amines have limited equilibrium loading capacity but react much

faster with CO2 . By varying the compositions of these solvents one can optimize the CO2

removal process. Kinetic studies are very important in the design of the acid gases

treating processes. This masters thesis presents the kinetics of absorption of CO2 in

loaded methyldiethanolamine (MDEA) and MEA solutions.

The work can be divided into two parts: (a) experimental work to obtain reliable date for

the mixed alkanolamine system at various acid gases process treating conditions, (b)

modeling work Ito interpret the experimental absorption data with the help of reaction

models. The experiments were conducted over the temperature range of 298-333K, the

MDEA/MEA wt.% of 27/03, 25/05 and 23/07, total amine concentration of 30wt% and

the CO2 loading from 0.005 to 0.15 mol/mol. Experimental data were obtained in a

laminar je t absorber at various contact-times between the gas and the liquid. The

physical properties like density, viscosity, diffusivity and solubility of the system were

calculated from the published data and/or models. The reaction mechanisms namely

Zwitterion and Teraiolecular were used to interpret the Kinetic data. It was found that

Zwitterion mechanism in its original form could not predict the individual Kinetic rate

constants, Termolecular mechanism with water in the apparent reaction rate term as

suggested by Crooks and Dormellan (1989) did not yield any results as well. A modified

Termolecular mechanism, which included the contribution of hydroxide ions, was able to

predict the kinetics of CO2 loaded mixed alkanolamine solution better. Individual

reaction rate constants were predicted based on the modified Termolecular mechanism.

Also, it was observed MDEA did not participate in MEA kinetics.

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ACKNOWLEDGMENTS

The thesis would have been impossible without the following people:

I would like to express my sincere gratitude to Professor Paitoon Tonti wachwuthikul,

thesis supervisor for guidance, encouragement, financial support and the independence to

pursue research. I consider myself fortunate to work with Professor Paitoon whose timely

advice has saved memany times while progressing through the research.

I would like to thank Professor Raphael Mem who is not only serving as a member of the

supervisory committee, but for having read the early draft of the thesis. Professor Mem

have given valuable inputs on each aspect of the project which was very important for the

timely completion.

Dr. Ahmed Abondheir deserves special thanks. Right from initial training as to how to

obtain the most reliable data from the laminar jet apparatus until the analysis of

experimental results, he has taken me step by step to successful completion. Thanks so

much for patiently hearing and answering much number of e-mails, frantic phone calls

that I had made when something went wrong with the apparatus. I wish you had a 1-800

number!

I thank Dr. Arar Henni for guidance. We had very interesting discussions about this

masters project. Thank you David deMontigny for helping me with everything (technical

and non technical). I cherish every moment I was with you in the CO2 lab. Thanks to

Thyagarajan Mathialagan for many rides to my home from P.T.R.C during late nights

and to Asok Kumar Tharanivasan for the great time I had with him at the Petroleum

Technology Research Center and especially for wonderful kichhidi.

Last but not the least, I thank my parents for their never ending love, support, motivation

without which I would have not been what I am today.

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4.1 Experimental apparatus and procedure 55

4.2 Calibration of the laminar Jet apparatus aaffi-aaeas


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LIST OF TABLES

Table 1. 1Targets for CO2 removal in process industry (Astarita et ai, 1983)....,.........,,.. 7

Table 1. 2 Guidelines for process selection of CO2removal process................................. 9

Table 1. 3 Compilation of kinetics o f CO2in mixed amine system................................. 19

Table 3. I Parameters for density correlations.................................................................. 39

Table 3.2 Experimental data for absorption rate of CO2in 1MMEA solution at 298K

and atmospheric pressure.......................................................................................... 49

Table 3. 3 Parameters in the diffusivity correlation (Equations .20) for MEA-HaO solution

(Ko et aL, 2001)........................................................................................................ 51

Table 3.4 Parameters in the diffusivity correlation (Equation 3.21) for MEA-MDEA-

H2O (Li and Lai, 1995).......................................................................,..................,.. 51

Table 4. 1Experimental data for the absorption of CO2in water at 298K and atmospheric

pressure............................................................................ 63

Table 4. 2 Diffusivity of CO2in water at 298K and atmospheric pressure...................... 66

Table 5. 1Fitted values of kinetics constants of CO2- absorption into aqueous MDEA-

MEA solution based on the deprotonation of zwitterion mechanism....................... 92

Table C. 1 Experimental data for Kinetics for CO2absorption into aqueous 23 wt. %

MDEA-7 wt.% MEA solutions.............................................. 131

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LIST OF FIGURES

Figure 1.1 Emissions by sector in 2010 (Climate change plan for Canada, 2003)............. 5

Figure 2.1 Liquid-phase speciation and concentration in aqueous MDEA-MEA (27wt%-

3wt.%) solution with CO2loading (a) from Oto 1.0 at 313K. The thermodynamic

model developed in this work predicted the composition of species..... 26

Figure 2. 2 Single step, termolecular reaction mechanism for the formation of carbamate

(Crooks andDonnellan, 1989).................................................................................. 32

Figure 2.3 Flow diagram for overall and apparent kinetics rate constants calculation..... 35

Figure 3. 1The density of aqueous mixed alkanolamine solution as a function of mixed

amine concentration and CO2loading at 298K......................................................... 40

Figure 3 .2 The density of aqueous 23 wt.%MDE A/07 wt.%MEA alkanolamine solution

as a function of temperature and CO2loading......................................................... 41

Figure 3. 3 Solubility of N2O in MEA-MDEA solutions as function of temperature.

Points are predicted by the model and lines are experimental by Li and Sfaen, 1992.

...................................................................................................................................46

Figure 3.4 Rate of CO2absorption into 1M MEA solution at 298K and atmospheric

pressure............................................... 52

Figure 4. 1 Schematic drawing of laminar jet apparatus from Aboudheirs Ph.D. Thesis,

2002........................................................................................................................... 57



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Figure 4. 2 Volumetric technique for measuring the mass transfer rate in laminar jet

absorber..................................................................................................................... 61

Figure 4. 3 Rate of absorption of CO2into water at 298K and atmospheric pressure in a

laminar je t absorber. Experimental (points), theoretical (line)................................ 67

Figure 4. 4 Absorption rate o f COj into aqueous MEA-MDEA solutions: average

loadings 0.031, T=323K.......................................................................................... 71

Figure 4. 5 Absorption rate of CO2into aqueous 23wt.% MDEA - 7wt.% MEA solutions:

loading= 0.031 ......... 72

Figure 4. 6 Absorption rate of CO2into aqueous 23wt.% MDEA - 7wt.% MEA solutions:

T = 323K... .... 73

Figure 5. 1 Apparent rate constant k^p for the reaction of CO2into aqueous 23wt.%

MDEA - 7wt.% MEA solutions as a function of free MEA concentration.............. 78

Figure 5 .2 Apparent rate constant kaPPfor the reaction of CO2into aqueous 25wt.%

MDEA - 5wt.% MEA solutions as a function of freeMEA concentration.............. 79

Figure 5. 3 Apparent rate constant kapPfor the reaction o f CO2 into aqueous 27wt%

MDEA - 3 wt.% MEA solutions as a function of free MEA concentration.............. 80

Figure 5.4 Apparent rate constant kaPPfor the reaction of CO2into aqueous MDEA -

MEA solutions as a function of free MEA concentration at 298K........................... 81

Figure 5. 5 Apparent rate constant kapp for the reaction of CO2into aqueous MDEA -

MEA solutions as a function of free MEA concentration at 303K.. .... 82

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Figure 5, 6 Apparent rate constant kapp for the reaction of CO2into aqueous MDEA -


Figure 5. 7 Apparent rate constant k^p for the reaction of CO2into aqueous MDEA -

MEA solutions as a function of free MEA concentration at 323K.,............... 84

Figure 5. 8 Apparent rate constant kapp for the reaction of CO2into aqueous MDEA -


Figure 5. 9 Apparent rate constant for the reaction of CO2with 27 wt.% MDEA-3wt.%

MEA solution as a function of free MEA concentration. Solid lines are predicted by

Equation 5.4..................................................................

Figure 5.10 Apparent rate constant for the reaction of CO2with 25 wt.% MDEA-5wt.%


Equation 5.4......................................................................................... 95

Figure 5. II Apparent rate constant for the reaction of CO2with 23 wt.% MDEA-7wt.%


Equation 5.4.............................................................................................................. 96

Figure 5. 12 Apparent rate constant for the reaction of CO2with 27 wt.% MDEA-3wt.%


Equation 5.9.............................................................................................................. 99

Figure 5.13 Apparent rate constant for the reaction of CO2with 25 wt.% MDEA-5wt%


Equation 5.9...........................................................................

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Figure 5. 14. Apparent rate constant for the reaction of CO2with 23 wt.% MDEA-7wt%


Equation 5.9......... ........................................................................................... ...... 101



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1. INTRODUCTION

1.1 General

The removal of carbon dioxide, from gaseous mixtures not only is important in the field

of sour gas removal, but it is also used to curb emissions of greenhouse gases. The

international community has concluded that there is compelling evidence that human

activities like energy use and deforestation are accelerating the concentration of

greenhouse gases in the earth's atmosphere. There is a general agreement that the world

is experiencing a change in the earths surface temperature. The average increase is

predicted to be 1.4 to 5.8C by 2100 with serious implications regarding global food and

water requirements (Climate change plan for Canada, 2003).

In Canada, some of the observed effects of climate change are:

declining water levels in the Great lakes

increasing number and intensity of heat waves

melting of polar ice cap and changes in marine life

hotter summers and higher levels of smog in major urban centers

extreme weather conditions such as recent droughts in the Prairies, ice storms in

Eastern Canada, flooding in Manitoba and Quebec to name a few.

The immediate impact of these frequently changing climate-related activities are related

for itsprofound effect on our economy, as well as our health and quality of life.

One of the important methods outlined by the Government of Canada in the recent

approach to curb greenhouse gases is by

promoting new innovation and technologies

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energy efficient homes and vehicles used in day to day life

The plan involves a three-step approach for achieving Canadas climate change objective

of reducing annual greenhouse gas emissions (GHG) by 240 megatormes. As of today,

there are investments that can address one third of the emission reduction (80MT).

Secondly, the plan enunciates a strategy for a further 100 MT reduction, and finally, it

outlines a number of current and potential actions that should enable Canada to address

the remaining 60 MT reduction. As per the Kyoto Accord of 1997 that Canada ratified

recently, the greenhouse gases including CO2are to be reduced to 6% below the 1990

levels by 2008 and 2012.

To attain the goal, some of the key means suggested by the plan are:

Emissions reduction targets for large industrial emitters through agreements

with regulations that would create an incentive for shifting to lower-emissions

technologies and energy sources while providing flexibility for emitters through

emissions trading.

Strategic infrastructure investments in innovative climate change proposals

such as urban transit projects, etc.

A coordinated innovation strategy that allows Canada to benefit fully from

innovation possibilities of the climate change agenda

Targeted measures including information, incentives, regulations and tax

measures that will aid achieve climate change objectives in specific sectors

(Climate Change plan for Canada, 2003).

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Key areas for action in the plan have identified seven initiatives: transportation, housing

and commercial buildings, large scale industrial emitters, renewable energy and cleaner

fuels, small and medium-sized enterprises and fugitive emissions, agriculture, forestry

and landfills, International emission reductions. Though dealing with each topic in detail

is beyond the scope of this thesis, the plan for large industrial emitters and renewable

energy will be dealt with in brief. They are: establishing emissions targets through

covenants, domestic emissions trading, cost-shared strategic investments in the areas like

renewable energy, clean coal demonstration projects, and a CO2 pipeline. Under

renewable energy comes the need for new innovative technologies. To accomplish these

objectives under innovative technologies, new formulated solvents are important in the

removal of CO2(which is a greenhouse gas). In the industry, chemical solvents such are

alkanolamines are commonly used to aid the absorption process of CO2 . Data available

on the kinetics of absorption of the newly formulated mixed amines are scarce at various

CO2 loadings and for a wide range of temperatures (298K to 333K). This research

involves the study of the Kinetics of CO2 absorption into mixed amine systems at

various process parameters such as temperature, acid gas loadings that will effectively

address CO2absorption issues as a foundation to formulate amine technologies for CO2

capture. Before delving into the absorption issues, it would be worthwhile to examine

some basic concepts about greenhouse gases and sources of CO2emissions.

Greenhouse gases

Greenhouses gases occur in nature are mainly water vapor, carbon dioxide, methane,

nitrous oxide and ozone within limits. The list is not exhaustive. Specific human

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activities that produce these gases can create greenhouse gases that do not occur

naturally. It is Interesting to examine how these gases are released in excess to the

earths atmosphere.

Carbon dioxide: An increasing amount of carbon dioxide is being released by the

combustion of fossil fuels (coal, natural gas and oil) for Industrial purposes,

transportation, heating/cooling of buildings, as well as by deforestation.

Methane: Increasing amounts of methane are being released from landfills,

wastewater treatment, and certain agricultural practices, and from grazing of live

stock.

Nitrous oxide: This isbeing emitted into the earths atmosphere through the use

of chemical fertilizers and burning o f fossil fuels

Apart from these naturally occurring greenhouse gases, man-made gases that are included

in the Kyoto Protocol, hydrochlorofluorocarbons (HFCs), perfluorocarbons (PFCs) and

sulfur hexafluoride (SFg), are generated in variety of industrial processes.

Sources of emissions from large scale industrial establishments

The large Industrial emitters are grouped In three sectors: Power Generation, Oil and Gas,

and Mining and Manufacturing. Industry wise emissions break-up is presented in Figure

1.1. From this figure, it can be observed that approximately 51% of the total COa

emissions are from the sector namely Mining and Manufacturing, Power Generation and

Oil Gas. Cleaner technologies have to developed to target these sectors. Relying on

cleaner technologies and in-plant processing by setting absorption-regeneration systems

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IA gr i cu l t u r e

10% I Oil and gas

ILandfill Gas

4% ,

ITransportation

25%

Buidsigs

M i n in g a n d

Power ~ Mam&cturing

Generation

16%

17%

Figure 1.1 Emissions by sector in 2010 (Climate change plan for Canada, 2003)



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can reduce greenhouse gases from electricity sector. For the oil and gas sector, the

potential is by reducing emission intensity of oil and gas distribution.

In the mining and manufacturing sector which is highly diverse, the emissions generally

fall into two categories: those arising from combustion of fossil fuels for energy and

heating, and secondly from industrial processes in which greenhouse gases are emitted as

direct by-products. There hasbeen some successes in this factor in curbing emissions

based on mitigation technologies and increased energy efficiency.

In addition to the climate change problems associated with the greenhouse gas emissions,

the removal of acid gases like CO2 , H2 S etc. is of prime importance in process industry to

meet specific cleanup targets. Some of the industrial applications that entail CO2removal

are given in Table 1.1. It can be clearly seen from the table that clean up targets for

various process industries are different and are industry specific. For example, CO2

specification may be relatively slack for natural gas (


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(Astarita et aL, 1983). Absorption is the most widely used CO2removal technique. The

main methods of absorption include absorption into a liquid solvent, either physically or

chemically. Common physical solvents include organic liquids such as alcohols, glycols,

propylene carbonate, and pyirolidones. Despite the fact that physical solvents may be

important as the energy costs continue to rise, chemical solvents are most preferred for

low-pressure gas treating (< about 50psi) applications because of the economic viability

of chemical solvent. In the case of physical solvent, the acid gas solubility is small at low

partial pressures and absorption capacity of physical solvent is a function of partial

pressure. A chemical solvent exhibits quite a high loading at low CO2 pressures but

becomes saturated at moderate pressures. Thus, when removing acid gases at low partial

pressure in the feed gas, a chemical solvent is preferred. Adsorption also seems to be a

viable option for carbon dioxide removal when CO2partial pressure is high in the feed

and the concentration of CO2in the product required is very small. In small sized plants,

pressure-swing adsorption (PSA) seems to be an attractive choice compared to

absorption. Membrane permeation is an active technology in the field of gas purification.

In this process, polymeric membranes separate gases by selective permeation of one or

more gaseous components from one side of the membrane barrier to the other side.

Hybrid processes are also prevalent where membranes are used for bulk removal of

carbon dioxide and amine process for final clean up (Kohl and Neilsen, 1997). Suitable

gas purification systems can be grouped into six categories based on the plant size and

partial pressure of carbon dioxide in the feed gas stream that is to be treated. Table 1.2

gives the guidelines for selection of CO2removal process.

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Table 1.2 Guidelines for process selection of CO2removal process

(Kohl andNeilsen, 1997)

Type o f process Plant Size,m3/day Partial Pressure, kPa

Absorption in Alkaline solution >600,000 600,000 >700

Membrane permeation 700

Hybrid(amine-membrane) 700

Adsorption 700

Methanation


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solution that will give the optimum conditions. Despite these facte, sufficient data and

operating experience with several alkanolamines serve as a benchmark for the selection

of treating solutions for a wide range of conditions (KoM and Neilsen, 1997).

The latest trends on gas treating are mixed amine system where the properties of one of

the amine help to compensate for the undesirable effects of the second amine. It has been

found that small addition of primary amine to the tertiary amine enhances the rate of

absorption of CO2 largely without changing the stripping characteristics. MEA as a

primary amine has several disadvantages. As mentioned earlier, corrosion is a major

problem associated with MEA. The effect of corrosion increases at high acid gas

loading. MEA cannot be loaded beyond 0.5 mol of CO2per mole of amine as this is

limited by stoichiometry. The advantage of MDEA is that it has high equilibrium loading

capacity (1.0 mol of CO2per mole of amine). Secondly, MDEA has low corrosion rate at

high loading. MDEA has a low heat of reaction and requires a low heat for regeneration.

MDEA also has a low degradation rate. The vapor pressure of MDEA is very low (0.01

mmffg at 20C) whereas for MEA, it is relatively high (0.36 mmHg at 20C), hence low

solution losses(Kohl and Neilsen, 1997). Some of the disadvantages of using MDEA are

the higher cost, tendency of foaming at high concentrations and slow rate of reaction of

CO2 and high viscosity. When the mixed amine solution reacts with CO2 , the

disadvantages are counteracted. It is found that the higher equilibrium capacity of tertiary

amine combined with the higher reaction rate of primary or secondary amine, results in

improvement in gas absorption of CO2 and greater savings in regeneration costs (Kohl

and Neilsen, 1997). Some of the blends that have been extensively utilized in the industry

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are MEA-MDEA, MDEA-DEA. A detailed discussion of the reaction kinetics and

absorption of 0 2 into these systems is included in the literature survey part of the

thesis. Although the mixed amine system is of wide interest, data on the absorption,

kinetics, andphysico-chemical properties are scarce. Proper understanding of the kinetics

of absorption reaction of mixed amine system is essential for the effective design of

absorption units that use the mixed amine system.

Most of the required kinetic data can be obtained from laboratory scale experiments. The

experiments can be broadly classified into: differential and integral (Astarita et al., 1983).

In a differential experiment, the operating variables (i.e. temperature, pressure,

composition, etc.) are fixed at some unique pre-determined values, the values of some

dependent variables (e.g., absorption rate for laminar jet, wetted wall etc.) that is

presumed to depend on the above operating variables is measured. Some of the physico

chemical properties cannot be measured directly from these experiments, but reliable

absorption rate helps to calculate diffusivity from standard equations for physical

absorption. Also the functional dependency of the dependent variable is determined by

repeating the experiments over a wide range of temperature. An example is the wetted-

wall column where the rate of absorption can be determined at wide range of

temperatures, pressure, and contact time. From the dependency of the absorption rate on

temperature and composition, reaction mechanisms can be elucidated. In an integral

experiment, the operating variables are not constant; their values vary from point to point

in time within a single experiment. A typical example is a pilot plant where the

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composition, temperature, and pressure have various values at different points in the pilot

plant. Hence, these fundamental data cannot be obtained only from them.

The key parameter required for the design and simulation of an absorber is the reaction

rate data. The absorption of acid gases into aqueous alkanolamine solution is considered

to be a heterogeneous non-catalytic reaction. As mentioned earlier, the reaction rate is

dependent on composition, temperature, pressure, mass transfer and heat transfer rate.

The reaction between CO2and amines is exothermic and for the reaction to take place

CO2has to diffuse from the gas phase to the liquid phase. Hence heat and mass transfer

effects play an important in alkanolamine-COa reaction system. In the present work, only

the mass transfer aspects will be considered. The heat effects will not be considered.

1.2 Objectives

It has been found in the literature that there are not enough data to represent the mixed

amine system. Chakravarty et al.( 1985) have suggested blends of primary and tertiary

amines such as MEA+MDEA+H2O for CO2 removal. Critchfield and Rochelle, 1987

studied the absorption of CO2 into aqueous solutions of MDEA-MEA in a stirred-cell

absorber. However, the kinetics of the reaction of CO2into mixed amine solution at low

and moderate loadings have not been reported except for Glasscock et al.(1991). The

experimental data obtained by Critchfield and Rochelle, 1987 suggested that a shuttle

mechanism could be used to describe the absorption of CO2 into MEA-MDEA+H2O

instead of the assumption that the reactions occur In parallel paths. Versteeg et al. (1990)

carried out a study of CO2 absorption into various mixed alkanolamines. The aqueous

mixed alkanolamine systems experimented were MMEA-MDEA, MEA-MDEA, DIPA-

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MDEA and MEA-DEA-MDEA, The experiments were carried out in a stirred vessel over

a wide range of process conditions. An absorption model was developed taking into

account the phenomenon of mass transfer accompanied by multiple complex parallel

reversible reactions.

Glasscock et al. (1991) investigated the absorption/desorption of aqueous MDEA/MEA

and MDEA/DEA solutions over the range from 288 to 313K with a gas-stirred cell

reactor. Combined mass transfer/equilibrium modelbased on the zwitterion mechanism

suggested by Danckwerts (1979); Blauwhoff et al.(1984) effectively represented the mass

transfer rate for the mixture of MEA/MDEA and DEA/MDEA. Rangwala et al. (1992)

measured the absorption of CO2into aqueous blends of MDEA and MEA solutions in a

stirred cell absorber. A modified pseudo-first order model based on film theory was used

to predict the rate of absorption into mixed amine solutions. Bulk liquid concentrations

of various species present were obtained from a simplified thermodynamic model. The

model predicted absorption rates, which were in good agreement with experimental

measurements.

Hagewiesche et al. (1995a) reported the absorption of CO2 into aqueous blends of

MDEA and MEA solutions in a laminar jet apparatus at low contact times. The

experiments were performed at 313K. They proposed a model based on the system of

parallel reversible reactions according to Higbies penetration theory for the predicting

gas absorption rates and enhancement factors.

Rinker et al. (2000) measured the rate of absorption of CO2 into aqueous solution of

MDEA and DEA in a laminar jet absorber and stirred cell absorber. A model based on

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Liao and Li, 2002 measured the absorption rates of 02 into aqueous blends of MDEA

and MEA solutions in a laboratory disk column and the kinetics of absorption were

studied. The experiments were performed at 30, 35, 40C. The overall reaction rate for

the system was determined based on the fast pseudo-first-order reaction regime. A

simplified hybrid kinetic model i.e., a zwitterion mechanism for MEA and a pseudo-first

order reaction model for MDEA was adopted to interpret the results.

Details of experiments conducted on the kinetics of COa in mixed amine systems by

various researchers is summarized in Table 1.3. From this table, there is enough scope to

obtain reliable data for various concentrations of mixed amine systems and acid gas

loadings. Secondly, all experiments conducted were either at one temperature

(Hagewiesche et al., 1995a; Zhang et al., 2002) or two or three temperatures (Glasscock

et al, 1991; Homg and Li, 2002; Liao and Li, 2002; Xiao et al., 2000). Since the kinetic

rate constant is not available for a wide range of temperatures (298K to 333K), there can

be some inconsistency in applying the kinetic rate for all temperatures at which an

industrial absorber is operated.

Third, the experiments were not performed for more than three concentrations of mixed

amine solution systems except in two cases: (Homg and Li, 2002; Liao and Li, 2002).

Finally, in most of the above experiments the effect of CO2 loading is not taken into

consideration with the exception of Glasscock et ai.(1991). Even for single amine system,

COa-MEA, Littel et al. (1992b) questioned the data reported by Savage and Kim (1985)

as they did not consider loading effects on the physical properties and kinetics.

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There are two ways of changing the free amine concentrations: either by varying the

individual amine concentration or the loading of the solution (Aboudheir et al, 2003).

The second method was adopted to change the free amine concentration. For each mixed

amine system five different loaded systems were used to conduct the absorption

experiments. The effect of loading on the physical properties is required to interpret the

kinetics. Based on the above points, there appears a need for obtaining more accurate

experimental data on a laboratory scale to determine the kinetics of the mixed amine

system, by taking the following into consideration:

1. Obtain reliable data on the absorption rate of CO2 into loaded mixed amine

solutions under the condition of no Interfacial turbulence. One of thebest apparatus

that can be used to generate reliable absorption data is the laminar jet absorber

because the interfacial area is known accurately. (Danckwerts, 1970; Astarlta et al,

1983).

2. Conduct the kinetics experiments within the typical range of temperature found in

absorber, which Is from 298 to 333K.

3. Develop a kinetic model for absorption of CO2 into loaded MEA-MDEA by

applying the Zwitterion mechanism, Termoleculat Mechanism, and the Power Law

model.

4. Interpret the experimental data o f the CO2-MEA-MDEA system with the aid o f the

simplified absorption model. The physical, chemical properties of the system as a

function of temperature, concentration, and loading are used to interpret the

absorption data. It should be mentioned that in previous studies of mixed amines,

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Table 1.3 Compilation o f kinetics of CO2in mixed amine system

Mixed aminesolution

Composition

(mole/L)

Temp.

(Q

Experimental

technique

Calculation

methodRef.

MDEA+MEA 1.34+0.65 25 Stirred cell Numericalmodel

Glasscock et

al., 1991MDEA +DEA 0.59-1.85+

0.10-0.57

25,40

MDEA+MEA 2.43+0.252.30+0.50

2.17+0.75

40 Laminar jetabsorber

Numericalmodel

Hagewiescheet al., 1995a

AMP+MEA 1 .5+0.1,0 .2 ,0 .

3 and 0.430,35,40 Wetted wall

column

Simplified

modelXiao et al.,

2 0 0 0

1.7+0.1,0.2,0.

3 and 0.4

TEA+MEA 0.5+0.1,0.2,0.

3,0.4 and 0.5

30,35,40 Wetted wall

column

Simplifiedmodel

Homg and

Li, 2002

1 .0 +0 .1 ,0 .2 ,0 .

3,0.4 and 0.5

MDEA+MEA 1 .0 +0 .1 ,0 .2 ,0 .3 ,0.4 and 0.5

30,35,40 Wetted wallcolumn

Simplifiedmodel

Liao and Li,2 0 0 2

1 .5+0.1,0 .2 ,0 .

3,0.4 and 0.5

MDEA+DEA 2 .8 +0 . 22.4+0.6

60 Disc column Simplified

model

Zhang et al,

2 0 0 2

2.7+0.3 40,50,60,70

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mainly from MDEA and MEA and to a lesser extent from OH', H 2O (Hagewiesche et al,

1995a).

The following reactions (which are similar to that of Rinker et al., 2000; Hagewiesche et

al, 1995a; Versteeg et al, 1996) for the reaction of CO2with primary, secondary' and

tertiary amines) may occur when the CO2 is absorbed into and reacts with aqueous

MDEA-MEA solution. The proposed mechanism for the reaction between CO2 and

tertiary amine represented by reaction 2.5 indicates that they do not react directly with

CO2 . Instead, tertiary amines act as bases, which catalyze the hydration of CO2

(Donaldson and Nguyen, 1980; Versteeg and vanSwaaij, 1988; Littel et al., 1990;

Hagewiesche et al, 1995a).

Ionization of water

H20 < - ^ O H ~ + H+ (2.1)

Dissociation of dissolved carbon dioxide through carbonic acid:

C02+H20 < ECO;+H+ (2.2)

Dissociation of bicarbonate

h c o ; C O*' +H + (2.3)

Formation of bicarbonate

C02+ OH~ < >h c o ; (2.4)

Reaction of CO2with tertiary amine

co2+r rch3n + h2o< -&->r r c h3n h++h c o ; (2.5)

RRCH,N + RRCH.NH* (2.6)

21



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Reaction of CO2 withprimary amine

RNH2+ C 0 2H* +RNHCOO- (2.7)

iW if2+ 77+< >RNH3+ (2.8)

RNHCOO~+ i / 20 I M 2+ C03" (2.9)

where K-, is the equilibrium constant, kj is the forward rate coefficient , and Lj is the

backward rate coefficient for the reaction i.Carbamate to bicarbonate reversion (reaction

2.9) is neglected while calculating the bulk concentrations of the species because of low

reaction rates and short contact times which are the characteristics of the laminar jet

absorber, and it has been found that reaction 2.9 does not have a noticeable effect on the

Vapor Liquid Equilibrium (VLE) model and the overall reaction of CO2 in mixed

alkanolamine system (Hagewiesche et ah, 1995a).

2.2 Liquid bulk concentration of all chemical species

A computer thermodynamic model to estimate the liquid bulk concentration of all

chemical species was developed based on the reactions discussed above. The input data

of the model included initial concentration of the MEA and MDEA solution, [MEAjo,

[MDEAJo, initial CO2loading of the mixed amine solution, a ,the equilibrium constants

of the reactions, and the solubility of CO2into MEA and MDEA solution as a function of

both amine concentration and temperature. AH the chemical reactions are at equilibrium

and the concentration of water is assumed to remain constant because its concentration is

much larger than the concentration of all other chemical species and also due to the short

contact time in the laminar jet absorber. The concentrations for the remaining eight

22



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chemical species shown in the above chemical reactions were calculated by solving the

following mass balance equations and the Henrys law correlation:

MEA balance:

[RNH2] + [RNH3 ]+[RNHCOO-] =[MEA\ (2.10)

MDEA balance:

[RRCH3N ]+[RRCH.NH+] =[MDEA]0 (2.11)

Carbon balance:

[CO2] +[HC03~] +[C032~] +[RNHCOO ~] = {[MDEA\ +[MEA}0)a (2.12)

Charge balance:

{RNH +] + { R RCHN H+] + [H*] = [RNHCOO-] + [H COf] +2[C 032'] +[Off~]

(2.13)

Independent equilibrium constants:

Kj=[OH][H+] (2.14)

K3 =[C032l [H +]/[HC03-] (2.15)

k 5= [ r r c h 3n h +] [h c o ; ] /[ co 2] [ r r c h 3n ] (2 .1 6 )

K6=[RRCH3NH+]/[RRCH3N] [H+3 (2.17)

K7=[H+][RMHC00-]/[RNH2][C02] (2.18)

Kg=[RNH3+]/[RNH2][H+] (2.19)

In addition to these mass balance equations, Henrys law relationship between the

equilibrium partial pressure and the free concentration of CO2is required:

23

roduced with permission of the copyright owner. Furthe r reproduction prohibited withou t permission.


36/148

PMj=He[C02] (2.20)

As mentioned earlier, though K is an independent equilibrium constant, it is not included

while solving the system of equations because of the negligible rate of the carbamate

reversion to bicarbonate reaction at short contact time. To solve these non-linear

algebraic equations for the bulk concentrations, the values of the solubility (in terms of

Henrys law constant, He) and the equilibrium constants are required. The solubility of

CO2into the mixed amine solution was calculated using the N2O analogy. Details of die

calculation are based on the work of Wang et al. (1992) and are presented in chapter 3.

The equilibrium constants Kj, K3 , K5 , K7, Kg developedby various researchers expressed

as a function of temperature have been adopted here. The equilibrium constants were

programmed in subroutines within a module calledEquilibrium-Constants,of Appendix

B. These subroutines are accessible for all the computer programs used in this work.

The eleven nonlinear algebraic equations, Equations 2.10 to 2.20, were solved for the

eleven unknowns of the bulk concentrations and equilibrium partial pressures using a

FORTRAN 90 program called VLE-Modelpresented in Appendix A. The FORTRAN 90

program utilizes a subroutine called DNEQNF documented in the IMSL MATH/Library

for solving a system of nonlinear algebraic equations. DNEQNF was successfully used

to solve the non-linear equations by Aboudheir et al. (2003). This routine isbased on the

MINPACK subroutine HYBRD1, which uses a modification of MJ.D. Powells hybrid

algorithm. The algorithm is a variation of Newtons method, which uses finite difference

approximation to the Jacobian and takes precautions to avoid large step sizes or

increasing residuals ( Visual Numerics, Inc., 1994a). By this technique, it has been found

24



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that the program converged to the solution even when the initial guesses of the bulk

concentrations and partial pressure were not close to the solution. The results for bulk

concentrations obtained by the VLE model for aqueous mixed amine system are shown in

Figure 2.1. This figure shows how the species concentration varies with carbon dioxide

loading from 0 to 1 in a 27wt.%MDEA/3wt.%MEA solution at 313K. The speciation

plot given here has a similar trend to those obtained by Austgen et al. (1991) using the

Electrolyte-NRTL model for vapor-liquid equilibrium in acid gas-mixed alkanolamine-

water system.

As shown in Figure 2.1, the main products with high concentrations are ENBV, HCO 3",

RNHCOO', RRCH3H+ at loading below 0.5. Though the speciation was developed for the

CO2 loading from 0 to 1 .0 , the experiments were only performed from 0.05 to 0.15 CO2

loading due to the operational constraints imposed by the laminar jet absorber and the

nature of mixed solvent ( the nature of the solvent being it is slow reacting when loaded

with CO2

unlike primary amine) used in this study. It can be observed that the

concentration of free carbon dioxide, [CCy, is found to increase only at loading beyond

0.8 (see Figure 2.1, the solid line increases after a loading of 0.8). Also, there is sharp

increase in the bicarbonate, (HCO3") concentration at loading beyond 0.3, due carbamate

reversion to bicarbonate. Since the experiments were conducted for loadings, < 0.15, the

exclusion of Equation 2.9 for solving the system of equations is acceptable (see Figure

2.1). OH', CO32', FT concentrations are located near the X-axis and are smaller in

magnitude when compared to the species shown in the figure.

25



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e

S3


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23 Kinetics f Reaction

The reactions between COa and MEA solution have been described in the literature by

two mechanisms namely zwitterion and termolecular mechanisms and the reaction of

CO2and MDEA solution has been explained based on the base-catalyzed hydration of

COa. The zwitterion mechanism consists of the reaction of CO2with primary amines like

MEA to form a zwitterion intermediate (RNH2+COO-) followed by the deprotonation of

the zwitterion by abase to form carbamate (RNHCOO-), where R is CH2 CH2OH.

( Danckwerts, 1979; Blauwhoff et a l, 1984; Versteeg and van Swaaij, 1988, Liao and Li,

2002). Any base in the reaction will contribute to the deprotonation of the zwitterion.

The contribution of each base to the reaction rate depends on its concentration as well as

its strength in the reaction. The main contributions for the deprotonation of zwitterion in

aqueous solution of primary and tertiary amine are from the amines (MEA and MDEA),

hydroxide (OH") and water (Blauwhoff et al., 1984; Versteeg and van Swaaij, 1988:

Rinker et al, 2000; Liao and Li, 2002).

For MDEA and CO2, the base catalyzed hydration of COa by the tertiary amines is the

most widely accepted rate mechanism (Donaldson and Nguyen, 1980; Haimour et al.,

1987; Versteeg and vanSwaaij, 1988; Littel et al, 1990 a,b; Hagewiesche et al., 1995a).

It is found that the tertiary amine does not directly react with CO 2 . Instead, tertiary amine

acts as a base which catalyses the hydration of CO2 .

The zwitterion mechanism consists of the formation of a complex zwitterions followed

by the deprotonation of the zwitterions by a base (Danckwerts, 1979; Blauwhoff et al.,

27



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1984), which can be represented as (refer to Equation 2.7, which is overall carbamate

formation reaction):

where B corresponds to any species in the solution that can act as a base to abstract the

proton from the zwitterions, and in the case of loaded mixed amine solution, such species

are [RNBh], [BqO], [OH"], [R3N], [HCOa"], [CO32' ] . As mentioned earlier, the reaction

with tertiary alkanolamines is essentially a base catalyzed reactions as proposed by

Donaldson and Nguyen, (1980). Based on this, the overall reaction rate of CO2 into

mixed amine system can be expressed as follows (Liao and Li, 2002):

Tov ~ rC02-MEA+ rC02-MDEA+ rco2-OH~ + TC02-H20 (2.23)

where the reaction rates are CO2with MDEA, MEA, hydroxyl ion and water. Owing to

the insignificant contribution, the reaction of CO2with water, i.e., rcc>2_Hi0 in Equation

2.23, is usually neglected (Blauwhoff et al., 1984).

The reaction of CO2with hydroxyl ion is considered as the bicarbonate formation:

This reaction is fast and can enhance mass transfer even when the concentration of the

hydroxyl ion is low. The forward reaction is described (Pinset et a l, 1956) as

C 02 + RNH2 r n h 2+c o c t (2-21)

RNH2+COO~ + B RNHCOO~+BH+ (2 .22)

C0 2 +0H~ *>HC 03~ (2.24)

(2.25)

logj0(k *OH_ im3kmol~is~l) = 13.63 5 -2 8 9 5 /T(K) (2.26)

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23.1 Reaction rate dependence on MEA

Carbamate formation occurs when COj reacts with primary and secondary amines:

RNH2+ C 02 f f ++ RNHCOO- (2.7)

Zwitterion mechanism, which has been explained above, is generally accepted as reaction

mechanism for Carbamate formation. Zwitterion mechanism has been widely used in

aqueous alkanolamine solutions (Blauwhoff et al., 1984; Barth et al , 1984) as well as in

some organic and viscous solution by Sada et al. (1985) and Versteeg and Van Swaaij

(1988b). As described by reactions 2.21 and 2.22, the steps involve the formation of

zwitterions and subsequent removal of proton by a base B (base catalysis).

Danckwerts (1979) reintroduced the mechanism originally proposed by Captow(1968)

and had derived the reaction rate equation for this reaction as quasi-steady state.

Based on this, the general rate of reaction of CO2with primary amine solution such as

MEA by the zwitterions mechanism can be given (Blauwhoff et al., 1984: Glasscock et

al., 1991; Versteeg et al, 1996) by:

[CO!][RNHJ) - - t l - f R N H C O O - ] 2 f A ^ L l

r --------------------------^ tL J (2.27)1 k , 1 7

--------- 1----------- zJ--------

^2MEA ^2MEA^^b[B]

For the system under consideration, MEA with CO2 since the second term in the

numerator is usually negligible under low loading absorption conditions, the rate of

reaction is given by:

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rC02AMINE~ f[C02][MEAj

(2.28)

^2 ,MEA

1

2,MA [] /

The zwitterion mechanism covers the shifting reactions orders, i.e., the overall reaction

order changes between two and three, for the reaction of CO2with primary and secondary

amines. The zwitterion mechanism can be applied in this study, to the reaction between

CO2with MEA for the CO2absorption into MEA+MDEA+H2 O. Since the bases B in

Equation 2.22 can be MEA, MDEA, OH", or H2O, the forward reaction rate for CO2-

MEA, i.e., Equation 2.28 becomes

23.2 Reaction rate dependence on MDEA

For the reaction of CO2 with tertiary alkanolamines (R3N), Donaldson and Nguyen,

(1980) proposed the mechanism as given by Equation 2.30. As mentioned earlier, the

mechanism implies that tertiary amines cannot directly react with CO 2 . In most of the

literature on CO2kinetics with aqueous tertiary amine solutions, it is assumed that the

reaction of CO2 with MDEA is a pseudo-first order reaction with respect to CO2

(Blauwhoff et al, 1984; Versteeg and van Swaaij, 1988b;Tomcej and Otto, 1989) as

follows:

[ C DJPE Aj

1

"*" 2>EA ce-

(2. 29)

rC02-MDEA 2,MDEA[C02 ][MDEA] (2.30)

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The kinetics study based on the pseudo first reaction assumption means that since MDEA

is in large excess there is no concentration gradient of MDEA in the liquid reactant and

therefore the interface conditions can be represented by bulk conditions.

The reaction rate equation, Equation 2.30, will be used in this study to represent the

reaction kinetics of COa with MDEA in MEA+MDE A+H20 .

23 .3 Overall reaction rate for CO2with MEA+MDEA+H2O

Substituting the zwitterion mechanism for CO2 -MEA, i.e., Equation 2.29 and C02-

MDEA, i.e., Equation 2.30 in Equation 2.23,

fov- kovICOa]

(2.31)% M m EC02)[MDEA]^(1I.[CX^][CIT]

and the apparent rate reaction constant, kappis defined as

kw =kw~k*OT[OHl (232)

k app- [M EA ]/(l/ k2>MEA'+(l"Kk2MEAkH2O/ . l)[ H 20 3+(k2MEAkoH-^c- l) [0 lH ']

'+( 23ffiA^MEA :-l)lMEA]+(k2sMEA ;MDEA -l)Pv ^ ] + :2J(EAP^^1]))(2.33)

The overall reaction rate defined above is based on the zwitterions mechanism. The term-

molecular based kinetics will be incorporated below.

23 .4 Termolecular mechanism

Termolecular mechanism assumes that the reaction takes place in one single-step where

the initial product is not a zwitterion, but a loosely- bound encounter complex as shown

31



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in Figure 2.2. The complex breaks up to form reactant molecules, few react to form with

a second molecule of amine, or a water molecule, to give ionic products.

R

R N:/

O

II

B H

Figure 2. 2 Single step, termolecular reaction mechanism for the formation of carbamate(Crooks and Donndlan. 1989)

Crooks and Donnellan (1989); Versteeg et al. (1996) presented the forward reaction rate

for this mechanism by:

23.5 Methodology to determine kinetics

The rate of absorption of COa into mixed amine system can be obtained from the laminar

jet apparatus. Given the data for physical absorption at short contact time and with an

initial gas-free liquid based on the Higbies penetration theory, specific absorption rate

( N a ) and liquid phase mass transfer for physical absorption of CO2(ki) are given by

*-co2=(kH2o[H 20 ] + kMEA[M EAj)[M EAj[C 02j (2.34)

N A = 2 ( D j / t i t ) l!2 ( p A I H ) (2.35)

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(2.43)

The specific absorption of CO2 (Na) into the MEA+MDEA+H2O can be measured for

various systems at different temperatures and CO2 loadings. The koVis calculated by

Equation 2.43 knowing the diffusivity of CO2 in the solution,Dqqi(refer to section 3.4

for diffusivity), partial pressure of CO2 , Pco2s and the Henrys law constant, Hcq2s in the

solution. Henrys law constant is obtained from Wang et al. (1992) model, which is

discussed in section 3.3. kapp is calculated from koVby Equation 2.32. The values of

experimental kaPPvalues are fitted to the zwitteion ion mechanism rate expression or the

termolecular mechanism rate expression to obtain the individual rate constants,

given by Equation 2.33. The kinetic calculations can be summarized in flow diagram

given below.

MDEA, k2MDEA in the expression of kapp

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Evaluate the Statistics

Evaluate kappPhysical

Properties

Equilibrium

constants

Vapor Liquid

Equilibrium

Regress k^p based on

Zwitterion, Termolecularmechanisms and Power law

Raw data at a given temperature, loading, andcomposition.

( ^ 2,ME A ^H20 / ^ -1 ?^ 2, M E A^ O H -

Obtain rate constants

- l* t%'2,MEA t l MEA 1 n--Hn'2,MEAn'MDEA " - I ' 2,MDEA * 2,MBA

Figure 2.3Flow diagram for overall and apparent kinetics rate constants calculation

35



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49/148

account for the amine+ water and amine + carbon dioxide Interaction In addition to the

use of a molar volume for dissolved CO2,which is unrelated to its pure-component value.

For loaded solutions, Weiland et al. (1998) gave the molar volume for mixed amine

solution as:

^ ~ X M E A ^M A + X M DE A^M DE A + X H , 0 ^ H , 0 + X C O , f '^ C O , MEA + X : 0 , MDEA 1

7 (3.2)# # ## ##

+ X MEAX H20 V M E A + X MDEAX H 20 V MDEA + X MEAX C G2 V M E A + X M DEAX O 0 2 V M D E A

The molar volumes associated with the interaction between water and amine (Vs),

between carbon dioxide and amine (V##), and for dissolved CO2(Vcoi) are given in Table

3.1. The molar volumes of pure MEA (Vmea)> MDEA, (Vmdea), and water, (Vmo), can

be obtained from:

Vj =fWj / pj (3.3)

where fwj are the molar weights in g/mol of the pure component and V is the molar

volume in cm3/mol. The density of pure components can be calculated from the

expressions developed by Hsu and LI (1997) as follows:

Pj &j + a2T + a3T2 (3.4)

where p. are densities of the pure components in g/cm3, T is the temperature in Kelvin,

and a, are parameters given in Table 3.1. The molar volume associated with the

interaction between carbon dioxide and the amine, V##

V## = d + e Xmdea (3.5)

This density expression covered wide range of temperatures, from 288K to 473K, and

predicts the density with an average absolute deviation o f 0.038% from the experimental

data (Hsu and LI, 1997).

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Equation 3,1 can be used to calculate the mixed alkanolamine solution density with CO2

loadings up to 0.5 mol CCVmol MEA and MDEA and to a maximum temperature of

398K, with a standard deviation of the fits of the experimental data of 0.00221 for MEA

and .0074 for MDEA (Wetland et al., 1998). The density correlation was programmed in

a FORTRAN 90 subroutine called Density-MEA-MDEA-Soln, Part B.2 of Appendix B,

where the input data of the subroutine are the weight percent of MDEA and MEA,

temperature, and CO2loading. The outputs of the subroutine are presented in Figures 3.1

and 3.2.

Figure 3.1 shows the density change of MEA+ MDEA solution as a function of

concentration and CO2loading at 298K. The densities of unloaded 23/7 and 27/3 wt. %

MDEA and MEA are 1.0214 and 1.0234 g/cm3 respectively; the amount of increase in

the density is less than 0.19%. If the loading of the solution is 0.3 mol CCVmol

MDEA+MEA, the density is found to decrease by 0.64%, instead of the appreciable

increase (13%) as reported for single amine MEA by Aboudheir et al. (2002).

Figure 3.2 represents the effect of loading and temperature on the density of 23/7

MDEA/MEA solution. At a loading of 0.3 mol CO2/ mol of mixed amine, increasing the

temperature of the 23/7 MDEA/MEA solution from 298 to 333K decreases its density

from 1.0651 to 1.046 g/cm3; the decrease being 1.80%. For the same solution at 313K, a

change in CO2 loading from 0.1 to 0.3 mol of CCVmol of mixed amine, changes the

density from 1.0284 to 1.0575 g/cm3; an increase of 2.75%. Based on this analysis, the

CO2 loading has minimal effect on the solution density of mixed alkanolamines when

compared to a single amine, MEA, (Aboudheir et ah 2002).

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Table 3.1 Parameters for density correlations

Parameters MEA MDEA h 2o Reference

ai 1.19093 1.22864 0.863559 Hsu and Li, 1997a

a2 -4.29990X10-4 5.4454x1O'4 1.21494xl03 Hsu and Li, 1997a

a3 -5.66040x1 O'7 -3.35930xi0'7 -2.5708XI0*6 Hsu and Li, 1997a

d 12.983 Weiland et al.,1998

e 397.2 Welland et al.,1998

Vc02 0.04747 -2.8558 Weiland et al.,1998

V# -1.8218 -6.65 Weiland et al.,1998



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Density,g/cm

1.10

1.09

1.08

1.0723/07

25/05

27/03

1.06

1.05

1.04

1.03

1.02

1.01

0.1 0.2 0.3 0.4 0.5 0.60

Loading, mol CCVCmoIe MEA+MDEA)

Figure 3.1 The density of aqueous mixed aikanofamine solution as a function of mixedamine concentration and COa loading at 298K.

40



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Density,g

/cm

1.07 i

&-

1.06 -

1.05~A

1.04 -

1.03 -

1.02 H

1.01

335320 325 330310 315300 305295

Temperature, K

Figure 3 .2 The density of aqueous 23 wi%MDEA/07wt%MEA alkanolamine solution

as a function of temperature and COa loading.



54/148

3.2 Viscosity

Viscosity like Density is an important parameter in the naass-transfer rate modeling of

absorbers and regenerators as it affects the liquid-film mass transfer coefficient and

solubility in amines. Solution viscosity was affected by the concentration, temperature

and CO2 loading. Weiland et al.(1998) combined their experimental viscosity data for

partially loaded amines with literature data and reported a useful correlation to calculate

the viscosity of mixed amines at various temperatures. The implemented program for

the viscosity calculations is attached in Appendix B. Viscosity of the solution was not

required to determine the absorption or kinetics.

3.3 Solubility

To model and simulate CO2 absorption into amines, it is necessary to use the free-gas

solubility of CO2 in amine solutions at various amine concentrations and temperatures.

The solubility is also required to calculate the diffosivity and the kinetics of reactions

between the absorbed gas and liquid. Due to the chemical reaction o f CO2with amines, a

common practice is to use N2O analogy to determine the free- gas solubility of CO2 in

amine solutions (Clarke, 1964; Laddha and Danckwerts, 1981; Versteeg and van Swaaij,

1988; Al-Ghawas et al., 1989; Liao and Li., 2002; Aboudheir et ah, 2003). The solubility

may be defined in different ways, but the definition that is used in this work is the form

of Henrys law, which relates the equilibrium concentration of the gas in the liquid as a

function of its partial pressure in the gas phase:

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He, - Pjf Cei (3.6)

Here Hetis the Henry's law constant for the gas i, and Ceiisequilibrium concentration of

the absorbed gas iin the liquid, which is calculated from the total moles of gas physically

absorbed ina volume of absorbed liquid, (Al-Ghawas et al , 1989).

The N2O analogy for solubility of CO2 in amine solutions, Heco2, can be presented as

follows:

Heco2=HeN20 (Hecoa/HeNio) water (3.7)

where Hcn2o is solubility of N2O in amine solution. The solubility of NaO and CO 2 in

water can be obtained from the following correlation developed by Versteeg and van

Swaaij (1988), based on published experimental data:

(H ecoaW = 2.82 x 106 exp(-2044/T) (3.8)

(HeN2o)water = 8.55 x 106 exp(-2284/T) (3.9)

Here solubility is in kPa.m3.kmol'1(kPa.dm3.moF1) and temperature is in K. Substituting

the free gas solubility from equations 3.8 and 3.9 into 3.7 gives the free gas solubility of

CO2in amine solution.

The solubility of N2O in amine solutions can be calculated from a semi-empirical model

of the excess Henrys law constant that was proposed by Wang et al. (1992).

Wangs model was programmed in a subroutine called Solubility CO2-MEA-MDEA Soln

(see Appendix B) for the solubility prediction of the CO2 and N2O in mixed amine

solutions. The model equations implemented in the computer program are:

The solubility of N2O in mixed amine solutions is given by:

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In fe M=J?234+ ; # ,J n if e ,y (3.10)> 2

Here the subscripts 1 represents N2O absorbed gas, and 2 stands for pure amine (MEA)

and water and 4 stands for another amine solvent (MDEA), S stands for aqueous amine

solution. The volume fraction is calculated by the following equation using the density

subroutines that are called in the program:

4

= * / , / ! > / . , p -11)2

The excess Henry's quantity for the ternary system is defined as a function of volume

fraction:

i?234 = 2^3^23 + 24&24 ^3^4^34 **"^2*^3 4 234 (3.12)

where the two-body and three-body interaction parameters are considered to contribute

to the excess free energy of the ternary solvent system. The two-body interaction

parameters are assumed to be temperature dependent and the three-body interaction

parameter is independent of temperature and is a constant which is unique for a ternary

system (Wang et a l, 1992). Hence, the effect of temperature on the three-body

interaction parameter was neglected by Wang et al.(1992). The temperature dependent

two-body interaction parameter is given by:

a 23=a ,+b, /TK (3.13)

The parameters au bi for the binary system and the parameters aa, bi, as, bs used in

a 3A a 24 for binary interactions between MEA and MDEA, MDEA and water are also

taken from Wang et al.(1992). The solubility of N2O in pure MEA and MDEA required

44



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by Equation 3.04 can be calculated from the following expressions, with less than 1.6%

average regression deviation from the experimental data (Wang et al., 1992):

(Hei2>MEA~l .2075 x 105exp (-1136.5/TK) (3.14)

(Hei2)MDEA= 1.524X 105exp (-1312.7/TK) (3.15)

where the units of the solubility are in kPa.dm3/moi and temperature is in Kelvin.

The accuracy o f the N20 solubility predictions in amine solutions was found to be

satisfactory based on the model output that was compared with experimental results at

various concentrations of aqueous mixed amine solution. As shown in Figure 3.3, the

model predictions are compared with the published experimental data of Li and Shen

(1992) at various mixed amine (MEA+MDEA) solution systems for three temperatures

and an AAD of 0.62% is reported in the case of aqueous MEA-MDEA solution. The

predicted solubility of N20 in amine solution from Equation 3.10 will be used in

subsequent analyses in order to predict the C02 in amine solution according to the N20

analogy (Equation 3.7).

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He/1000,

kPa.m

.kmof

6.5

6

5.5

5

4.5

4

.

o- ' _

" -A-..

24MDEA/6MEA

A 18MDEA/12MEA

O 12MDEA/18MEA

+ 6MDEA/24MEA

30MDEA/0MEA

-4-

3.53.15 3.2 3.25

1000/TpC1)3.3 3.35

Figure 3 .3 Solubility o f NaO in MEA-MDEA solutions as function of temperature.Points are predicted by the model and lines are experimental by Li and Sfaen, 1992.



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3.4 Diffusivity of COa in mixed amines

The molecular diffusivity of the absorbed COa in mixed amine solution is required for

the modeling and simuatlon of gas absorption process. Due to the chemical reaction of

CO2in amines, the N2O analogy is a common approach used to determine the molecular

diffiislvlty of CO2 In amine solutions (Clarke, 1964; Versteeg and van Swaaij, 1988),

similar to solubility.

Li et al. (1995) performed diffusivity experiments using wetted wall column apparatus.

The diffusivity was calculated by knowing the Henrys law constant, specific gas

absorption rate, the contact time, density and viscosity of the solution.

The N2O analogy for diffusivity of CO2 in amine solutions, Hecoa, can be presented as

follows:

D c 0 2= D n 20 (D c 0 2 / Dn 2o ) water (3 .1 6 )

where Dn2 0 is solubility of N2O in amine solution. The diffusivity of N2O and CO2 in

water can be obtained from the following correlation developed by Versteeg and van

Swaaij (1988), based on published experimental data:

{Dco2 W = 2 .3 5 X iO'2exp{-2119/T) (3.17)

(D n 2o W = 5.07 X 10"2 exPC-2371/T) (3.18)

where diffusivity is in cm /s and temperature is in Kelvin. Substituting equations 3.17

and 3.18 in equation 3.16 gives the diffusivity of CO2in mixed amine solution.

There is a direct method called Protonation Method as cited by Abu-Arab! et al.( 2001) to

measure the diffusivity of CO2in alkanolamine solutions. The diffusivity of CO2 in 1M

MEA solution at 298K was measured in the laminar jet apparatus used in this work (refer

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Table 3.2 Experimental data for absorption rate of CO2in 1M MEA solution at 298Kand atmospheric pressure

L H d t R a

Ccm3/s) (n) (cm) (sec) (mol/s)

0.613 2.335 0.047 0.0268 5.748E-07

0.613 2.120 0.049 0.0261 5.123E-07

0.613 1.663 0.051 0.0219 4.854E-07

0.613 1.345 0.051 0.0177 4.533E-07

0.613 0.981 0.051 0.0129 3.868E-07

0.694 2.448 0.047 0.0248 6.460E-07

0.694 1.802 0.049 0.0196 5.418E-07

0.694 1.297 0.051 0.0151 4.523E-07

0.694 0.921 0.051 0.0107 3.863E-07

0.694 0.579 0.051 0.0067 2.9! IE-07

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In the absence of reaction, the total rate of absorption into the jet can be presented

theoretically by the following expression, derived from penetration theory (Astarita,

1967; Danckwerts, 1970):

Ra =4Ce\ /DLh (3.19)

where Ce* is the interfacial-equilibrium concentration of the gas ( CO2) in the liquid

phase. C"can be calculated as explained in theprevious section of solubility, section 3.3.

A plot of the experimental data shown in Figure 3.4 of RAagainst VEh , at various flow-

rates and jet-lengths, should be straight line through the origin with a slope equal to

4Ce*-s/D . From Figure 3.4, the value of the slope for the experimental data was found to

be 4.812xl0'7. The solubility, 3.07x1 O'5mole/cm3 and the slope were used to determine

the experimental diffusivity. The experimental diffusion coefficient of CO2in 1M MEA

3 ?solution at 298K determined by the protonation method is 1.53 xlG' cm /s. The results

thus obtained for diffusivity were compared with the diffusivity values obtained by

substituting in Equation 3.16. The correlation for prediction of N2O diffusivity in amine

is given by Ko et al. (2001):

P n ;0=(b ,+b,M -|-b;Mi )xexpf-b-A 1 (3.20)V *

The correlation (Equation 3.20) is a function of amine concentration in terms of molarity

(M) and temperature.

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Ra,mol/s

7.0E-07

5.0E-07

4.0E-07

3.0E-07

1.0E-O7

0.0E+00

1 1.2 1.40.4 0.80.60 0.2 , vl/2 2 -1/2(Li) , cm s

Figure 3.4 Rate of CO2absorption into 1M MEA solution at 298K and atmospheric

pressure

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The deviation between the experimental diffusivity values and with those calculated with

the help of the N2O analogy is about 11%. The reason for the deviation can be explained

because the predicted values were obtained based on the N2O analogy where the

diffusivity of CO2in primary amines cannot be measured directly.

For mixed alkanolamine system, Li et al. (1995) observed that for the total amine weight

percent of 30, the diffusivity was highest for MEA system alone and lowest for aqueous

MDEA system. When a small amount of MEA was added to MDEA, the diffusivity

increased dramatically and diffusivity values lied in between pure MEA and MDEA

systems. Hagewiesche et al. (1995b) also observed the same trend for

MDEA+MEA+H2 O system where the total amine concentrations were 30 and 40 wt.%

respectively.

Li et al. (1995) developed a correlation for diffusivity of N2O in (MEA+MDEA+H2O)

and (MEA+AMP+H2O) systems. The experiments were performed at 30, 35, 40 C and

total solution concentration 30 % wt. The correlation for determining the diffusivity of

N2O in mixed amine solution is given by:

Dn2q = (b0 +b!1M1+b!2M12 +b21M2 +b22M22+ b22M22+ c12M,M2)exp (-c/T) (3.21)

where bo, bn, bo , bat, bn, C2 1 , c are constants specific for each amine system, Mi, M2are

individual molarities of the amines, T Is the temperature. The correlation has the

molarities of two reacting amines and is a function of temperature. The parameters in the

Equation 3.21 were given in Table 3.4. Equation 3.21 will be used to estimate the

diffusivity of mixed amine for the determination of the kinetics.

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4, LAMINAR JI T ABSORPTION EXPERIEMNTS

Absorption experiments are performed to obtain reliable data of CO2 absorption into

aqueous solution of partly loaded MEA-MDEA. The experimental data obtained and

discussed in this chapter are under the conditions o f no interfacial turbulence(Aboudfaeir,

2002). The data thus generated were obtained by the dynamic method, in which a jet of

liquid moves continuously though the gas for a known contact time. From the literature,

it is observed that only few experiments were performed in the area of chemical

absorption of CO2into MEA and MDEA. Most of these experiments were restricted only

to aqueous solution mixtures without CO2loading and only two or three temperatures.

In this study, three different mixed amine ratios (27wt.%/ 3wt.%, 25wt.%/5wt.% 23wt.%/

7wt%) were considered. The CO2loading into the solution was varied from 0.0095 to

0.1427 moles of CO2per mol of amine. The absorption rates were utilized to determine

the kinetics of the reaction. The absorption rates were measured with a digital soap-film

meter.

This section is organized in the following order.

The design and method of operation of laminar jet absorber will be discussed. Second,

generating the physical absorption data for CC -water for the calibration of the apparatus

and calculating the molecular diffiisivities and; generating the absorption data of CO2into

loaded mixed MEA-MDEA solution for studying the kinetics o f the reaction are outlined.

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4.1 Experimental apparatus andprocedure

The schematic of the laminar jet apparatus used for the measurements of absorption rates

is presented in the Figure 4.1. The apparatus was designed by Dr. Ahmed Aboudheir

based on the work of Danckwerts, 1970, Astarita et al., 1983, and Al-Ghawas et al, 1989.

The absorbing liquid constant-head system was fed by gravity through a constant-

temperature bath. The constant-head system was used in order to maintain a constant

liquid flow rate without any ripples. A jet of liquid is issued from a circular nozzle,

flowed downward through an atmosphere of the absorbing gas, was collected in a

capillary receiver. The absorbed gas was supplied from a pressure cylinder and was

saturated with water vapor at the experimental temperature before entering the absorber

chamber.

Jet ChamberAssembly:The jet chamber was a 25.0 cm long, 5.0 cm inside diameter,

acrylic cylinder and was enclosed in a constant temperature jacket, with dimension 25.0

cm in length and 15 cm in inside diameter, acrylic cylinder. The two cylinders were held

between aluminum flanges, and the ends were sealed with rubber gaskets. The nozzle

was mounted at the end of 0.64 mm insider diameter acrylic delivery tube, which passed

through a swage-lock nut in the aluminum flange. The delivery tube can glide in a

vertical direction, which facilitates the change of jet-lengtfa and can be locked in position

by the Swage-lock nut. The jet chamber was provided with a gas entry near the bottom,

gas outlet at the top, a manometer port, a receiver tube, 0.64-cm inside diameter, fitted

into a funnel shaped base.

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A hole in the base allows the draining of the any liquid overflow through the overflow

port. To avoid vibration, the je t absorber was placed on an anti-vibration pad and all

lines leading to the jet absorber were made from flexible tubing.

Nozzle.- The most important part of the laminar jet apparatus is the nozzle, which

produces the liquid jet. For details regarding the design of the nozzle, refer to Aboudheir

(2002). The extent of departure of behavior of the jet from an ideal rod-like flow depends

on the shape of the nozzle or the orifice from where the liquid-jet is formed,

(Danckwerts, 1970). The nozzle used for the current absorption studies had a circular-

hole orifice of diameter 0.63mm. The circular hole was made in a thin sheet of thickness

0.07Q.005 mm.

Receiver: The receiver was a capillary-fade drilled in an acrylic rod (Aboudheir, 2002).

The length and diameter of the hole were 2.0 and 0.1cm respectively. The receiver

assembly was fitted onto the end of the receiver tube by two o-rings. The downstream

end of the receiver was connected to a constant-level device, which was mounted on a

vertical slide rod andprovided with a fine-adjustment leveling screw

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Gas purge

Soap film meter

:

Constanthead

system

A

Saturator inside

water jacket

Water lacket

Constant level

deviceM2Balloon

Gas feed

Jet chamber inside

water jacket

Exit Liquid Liquid feed

gas flow, -------- liquid flow

Figure 4 .1 Schematic drawing of laminar jet apparatusfrom Aboudheir5s Ph.D. Thesis, 2002.



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Operation &f the laminar le t apparatus: The absorbing liquid was degassed by spraying

it into a vacuum and kept under a blanket of nitrogen. The liquid was fed by gravity to

the jet chamber from a constant-head system, located about 3.9 m above the absorption

chamber. This was to provide sufficient head for the required rate of flow. The constant

-5

head system is a 200 cm capacity cup provided with a liquid inlet and outlet ports.

During the liquid feed operation, the inlet flow rate was kept higher than the outlet

flowrate and the overflow from the cup was returned to the feed tank. The liquid in both

the constant-head system and the feed tank is kept under the blanket of nitrogen. The

nitrogen supply that replaces the used liquid in the liquid feed tank was stored in a

balloon connected to the feed tank and the constant-head system. The flow rate of the

liquid was controlled by rotameter. The actual flow rate of the liquid was determined by

weighing the liquid discharged in a timed interval during each experiment. A 1000 cmJ

volumetric flask was used to collect the discharged liquid. The constant-liquid level is

used to adjust the liquid level in the receiver. Some of the operational problems due to

high liquid levels and low liquid levels are: not all liquid-jet is collected in the receiver,

some of theje t overflowed and the free jet continued several millimeters down into the

receiver and for low liquid levels the small gas bubbles were entrained in the rapidly

moving liquid inside the receiver. The two aforementioned conditions yielded erroneous

gas absorption rates. The level was adjusted to attain the condition in which there is

neither spillover of the liquid nor entrainment of the gas. A two dimensional traveling

microscope was used to measure the jet-length to a precision of one hundredth of a

millimeter and the jet diameter to a precision of thousandth of a millimeter. The gas from

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a pressure cylinder was saturated with water vapor at the experimental temperature before

entering the jet chamber. For the determination of the gas absorption rate, a volumetric

technique was used which will be presented in the following section. Temperature of the

liquid entering and leaving the jet chamber were controlled within 0.3C. This was

achieved by having two separate heating/cooling circular units, one for controlling the

gas-stream temperature and the other for controlling the liquid-stream temperature.

The choice of the operating conditions of the laminar je t is based on a number of design

constraints (mainly the design o f the nozzle and the receiver). First, the length of the jet

cannot be varied over a range. If the jet is too short, end effect becomes too important; if

it is too long, operation of the receiver becomes difficult. Secondly, the liquid flow rate

can be changed only over a limited range: a lower limit is imposed by the requirement of

avoiding dripping and an upper limit being imposed to avoid turbulence (Aboudheir,

2002). As a result, the former constraint restricts the range of the interfacial area and the

latter limits the range o f mass transfer coefficient (Astarita et al., 1983).

Mass flow rate measurements: The best method for measuring the mass transfer rate is

the volumetric technique, which has been used by other authors (Astarita et a l, 1983; Al-

Ghawas et al., 1989; Sinker et al., 1995). A schematic diagram, Figure 4.2 can illustrate

the technique used in this work. After obtaining a stable je t -flow, a stream consisting

only of the gas to be absorbed was forced to flow through the absorption chamber of the

laminar jet and purged through a digital flow meter (soap-film meter). The flow meter

was designed to measure flows from 0.1 to 50 cm3/min with an accuracy of 3% o f

readings. After sufficient time had elapsed to make sure that the jet chamber, the soap

59

roduce d with permission of the copyright owner. Furthe r reproduction prohibited without permission.


72/148

meter, and the long-purge U-tube had been filled with the gas, the gas feed-vaive is

closed. At this point, the flow rate of the gas that was indicated by the soap film meter

equals the volumetric rate of absorption. Experiments were repeated several times and

average was taken.

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Gas purge

A

%OqaQ

3ooOoos

O o

>oO180

0*

O 0 C

oOoC

00

4o

0OoOlO

0Q00

] O O S Q Q O G Q O O Q D O u %

O^ Q

Soap

film

meter

Liquid Feed

*3 Gas Feed

Absorptionchamber of thelaminar jet

Exit Liquid

Figure 4. 2 Volumetric technique for measuring the mass transfer rate in laminar jet

absorber

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4 2 Calibration of the laminar Jet apparatus

The laminar jet absorber designed in this work was initially tested by absorbing CO2 in

water which is a standard method for testing this type of apparatus as the solubility and

the diffusivity of CO2in water are very well known (Astarita et al., 1983, Al Ghawas et

a l, 1989). When there is no chemical reaction, the total absorption into the jet can be

calculated theoretically from the following equation (Danckwerts, 1970; Astarita et al.,

1983);

Ra =4C/VDLh (4.1)

where RAis the total rate of absorption of gas, Ce *is the equilibrium concentration of gas

at the interface, D is the diffusivity of ga

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