Kinetics of CO2 Absorption in Aqueous Hexamethylenediamine

Subscriber access provided by READING UNIV

Article

Kinetics of CO2 absorption in aqueous hexamethylenediamine blended N-methyldiethanolamine Bikash Kumar Mondal, Syamalendu Sekhar Bandyopadhyay, and Amar Nath Samanta Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02744 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 26, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

Industrial & Engineering Chemistry Research

Kinetics of CO2 absorption in aqueous hexamethylenediamine blended N-methyldiethanolamine Bikash K. Mondala, Syamalendu S. Bandyopadhyayb, Amar N. Samantaa,* a

Chemical Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur 721302, India b

Cryogenic Engineering Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

*Corresponding author: amar@ che.iitkgp.ernet.in Phone: +91-3222-283948 Address: Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur -721302, India.

Abstract Carbon dioxide (CO2) absorption kinetics in the aqueous blend of hexamethylenediamine (HMDA) and N-methyldiethanolamine (MDEA) is studied at the temperatures varying from 303K to 333K using the pressure decay technique in a reaction calorimeter set-up. For the experimental study, HMDA concentration is varied in the range of 5-15mass% keeping total amine (HMDA+MDEA) concentration 30mass%. Overall rate constant for this reaction system is estimated assuming pseudo-first-order reaction condition. CO2 absorption kinetics enhance significantly due to presence of HMDA as compared to single MDEA solvent. The individual rate contribution of CO2-HMDA and CO2-MDEA reaction systems are combined to represent the overall CO2 absorption rate in this mixed amine solvent. The kinetic models (I and II) developed using the zwitterion and termolecular mechanism for HMDA-CO2 reaction system and base catalysed hydration mechanism for MDEA-CO2 reaction are able to predict the kinetic data with good accuracy. Key Words: Kinetics, CO2, HMDA, MDEA, reaction calorimeter

1 ACS Paragon Plus Environment

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

Page 2 of 29

1. Introduction The major global challenge in the coming years is to reduce the carbon dioxide emission in view of its apparent contribution to the global warming. Fossil fuel based power plants release major amount of CO2 in the atmosphere. 40% of the world’s electricity is generated by burning coal which contribute about 44% of the total CO2 emission worldwide1. To mitigate the atmospheric CO2 emission immediate implementation of the post-combustion capture and clean coal technology is essential. For the rapid capture of CO2 from the large scale industrial gas streams, amine scrubbing is the most developed and widely used technology. Aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA) are the conventional amines used for the CO2 capture 2. But, the use of these amines for CO2 capture from power plants flue gases lead to substantial energy penalty due to high regeneration energy requirement to handle large volumetric flow rate of the flue gas stream having 1015% (v/v) CO2 content 3. So the CO2 capture research mainly aimed at developing novel solvent with higher CO2 absorption rate and capacity and lower enthalpy of absorption. The higher loading potential and low regeneration energy requirement of non-carbamate forming amine N-methyldiethanolamine (MDEA)

4–7

, has made it an attractive solvent for CO2

capture. But the CO2 absorption rate with aqueous MDEA is very slow. So, this amine is mixed with another amine of higher CO2 absorption rate to improve the kinetic characteristics of the solvent. Plenty of literatures are available on the MDEA blended conventional amine solvent such as (MDEA+MEA)

8,9

, (MDEA+DEA)

10

to make the solvent more efficient in

terms of kinetics as well as regeneration energy. It is found from the recent literatures that aqueous diamine has high CO2 absorption rate and capacity compared to the conventional amines. Some of the diamines which are found in the recent literature are piperazine (PZ)11 and its derivatives12,13, 2-((2-aminoethyl)amino)ethanol (AEEA)14, hexamethylenediamine (HMDA)15–17,

1,4-Butanediamine

(BDA)18,

ethylenediamine

(EDA)19,

3-

20

(methylamino)propylamine (MAPA) . These diamines can potentially improve the solvent characteristics to a greater extend compared to the conventional MDEA blends because of their superior kinetic and loading capacity. Literatures are available on the PZ promoted MDEA solvent

21–27

which show enhanced CO2 absorption kinetics and loading capacity of

this activated solvent. CO2 absorption study in the aqueous (MDEA+MAPA)28 and (MDEA+AEEA)29 also indicate potential of the diamine as better solvent activator. In our earlier works16,30 it is shown that HMDA has very high CO2 absorption rate (, : 59190 m3kmol-1s-1 at 313K) and loading capacity (CO2 loading: 1.143 mole /mole at 15kPa CO2


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


partial pressure and 313K temperature). These superior solvent properties of HMDA and dearth of literature data motivated us to study the kinetic characteristics of aqueous (HMDA+MDEA) solvent. High CO2 loading potential

31

and low absorption enthalpy

32

of

this blended solvent is already presented in our previous works. In this work, the kinetics of CO2 absorption in aqueous (HMDA+MDEA) solvent is studied in the temperature range of 303-333K for its potential application as CO2 capture solvent. 2. Experimental 2.1. Materials Reagent grade HMDA (minimum purity 98% by mass) and MDEA (minimum purity 99% by mass) are purchased from Sigma Aldrich India. Nitrogen, nitrous oxide and carbon dioxide with minimum purity of 99.99% (by volume) are supplied by Linde India Limited. Aqueous amine solutions are prepared in mass percentage basis without further purification of the chemicals using a precision balance (CITIZEN, CX-301 model, accuracy: ±0.001g). 2.2. Density Density of aqueous (HMDA+MDEA) is required to estimate molar concentration of the solvent from the mass% concentration. It is measured using standard Gay-Lussac pycnometer (~25×10-6 m3 at 298 K). Before experiment, volume of the pycnometer is standardised using double distilled water. Density of the solvent is measured dividing the mass of the solvent contained in the pycnometer by the standard volume of the pycnometer at constant temperature. To maintain experimental temperature, a thermostated water bath (JULABO F32 HL, FRG) with a precession of ± 0.1 K is used. Each experiment is repeated for at least three times and relative standard uncertainty is calculated to be ±0.1%. 2.3. Measurement of viscosity Viscosity data is required to estimate diffusivity of CO2 in the solvent. It is also an important property for solvent pumping cost estimation. A Cannon-Fenske viscometer (Size: 50, Viscometer Constant (VC): 0.004 cSt.s-1) is used to measure the kinematic viscosity (KV) of the solvent. KV (cSt) is estimated by multiplying the efflux time of a specific volume of the solvent through the capillary of the viscometer with VC (0.004 cSt.s-1) at constant temperature. A temperature controlled water bath (JULABO F32 HL, FRG) with a precession of ± 0.1 K is used to maintain experimental condition. Then KV is multiplied by the density (g.ml-1) of the solvent to obtain dynamic viscosity. Each measurement is quintuplicated and the standard relative uncertainty is calculated to be within ±1.5%.



2.4. N2O solubility and liquid phase mass transfer coefficient measurement Liquid phase mass transfer coefficient and physical solubility data are required to analysed the kinetic behaviour of CO2-amine system. Due to the reactive characteristics of CO2 with the amine solution, physical solubility is measured using N2O which does not react with aqueous amine and also have similar molecular and electronic configuration as CO233,34. First, equilibrium N2O solubility in the aqueous amine solution is measured in the experimental temperature range and then it is converted to physical CO2 solubility using the method of N2O-analogy. 2.4.1. Experimental Procedure Experimental set-up (Figure 1) and procedure for the solubility measurement is presented in our previous work16 and it is described here briefly. N2O solubility is measured in a stirred cell contactor (500 ×10-6 m3) connected to a buffer cell (650 ×10-6 m3) through needle valve. Temperature of the cells are maintained using a thermostated water bath (JULABO F 32 HL, FRG, accuracy: ±0.1K). For each experimental run, 200 ×10-6 m3 amine solution at desired temperature is kept in the stirred cell under vacuum condition for at least 1 hour to record the vapour pressure of the solvent. Then N2O gas from the buffer vessel at desired temperature is transferred to the stirred cell ( 100 ) and stirrers (both gas phase and liquid phase) are started. Pressure decrease in the stirred cell indicated by the pressure transmitter (Model: Rosemount 3051TA; Range: 0-50 PSIA; accuracy: ±0.04% of the range) is recorded continuously. Equilibrium condition is indicated when there is no change of cell pressure for at least 1 hour.


Page 4 of 29

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


[1-Stirred-cell contactor; 2- Buffer cell for N2O storage; 3- Gas phase stirrer; 4- Liquid phase stirrer (Magnetic bar); 5- Magnetic stirrer speed controller; 6- Pressure transducers; 7Circulator temperature controller; 8- N2O cylinder; 9-Temperature sensors; 10- Liquid solution inlet; 11-External temperature sensor of circulator temperature controller; 12Thermo-stated water bath.] Figure 1: N2O solubility measurement setup 2.4.2. Estimation of N2O Solubility N2O solubility in aqueous amine solvent is estimated using the following expression33.

mN 2O , m

(

)

PNi 2O − PNe2O Vg RT = = . H N 2O , m PNe2O Vl

(1)

Here, mN 2O ,m is dimensionless physical solubility defined as the ratio of equilibrium N2O concentration in the liquid to the gas phase. H N 2O ,m is Henry constant in the aqueous mixed solvent , PNi 2O is initial N2O partial pressure in the cell, PNe2O is the equilibrium partial pressure of N2O in the cell, Vg and Vl are gas phase and liquid phase volume in the cell. 2.4.3. Estimation of liquid phase mass transfer coefficient Liquid phase mass transfer coefficient (kL) (in absence of chemical reaction) is estimated using the expression 35 given below. ln PN 2O

t =t

=−

mN 2O Ak L Vg

t + ln PN 2O

(2)

t =0

Where, Vg and A are the gas phase volume and interfacial area of gas-liquid interaction respectively.

PN 2O and mN 2O , m denote N2O partial pressure in the stirred cell and

dimensionless physical solubility parameter in the mixed solvent respectively. From the slope of the “ln versus time” plot, can be calculated. Since the physical absorption process (without chemical reaction) is slow, slope is calculated using data up to 100 s. 2.5. Carbon dioxide absorption rate measurement The rate of CO2 absorption in aqueous (HMDA+MDEA) solvent is measured using “pressure decay method”. Formulation of the pressure decay expression and kinetic measurement technique are as follows. 2.5.1. Derivation of the pressure decay expression Absorption of CO2 in aqueous amine solution is a complex phenomenon involving Phase equilibria and reaction kinetics. During absorption process CO2 is dissolved in the liquid 5 ACS Paragon Plus Environment


Page 6 of 29

phase physically and then reacts with amine components forming different ions. Because of the chemical reaction, rate in the chemical absorption is much faster compared to the purely physical absorption process. This improved absorption rate is characterised by the enhancement factor (E) which is the ratio of the chemical absorption flux to the physical absorption flux under same driving force. Assuming film theory, CO2 absorption flux ( ) in an aqueous amine solution can be presented as follows.

(

NCO2 = K G PCO2 − P

* CO2

)=

(P

CO2

* − PCO 2

)

(3)

1 RT + kG mCO2 E k L

with 1 1 RT = + K G kG mCO2 E k L

(4)

∗ Where, is the CO2 partial pressure in the system, is the equilibrium CO2 partial

pressure corresponding to the CO2 concentration in the bulk liquid, KG is the overall mass transfer coefficient and is the physical solubility of CO2 in aqueous amine solvent. kG and kL are the gas phase and liquid phase mass transfer coefficient respectively. The terms involved in the above equation can be simplified using following assumption.

•

∗ Negligible CO2 concentration in the bulk liquid ( and hence = 0)

•

Negligible gas phase resistant (and hence

= 0).

So the equation (3) simplifies to the following. N CO2 = mCO2 E k L

PCO2

(5)

RT

Again, the flux can also be expressed as N CO2 = −

V dPCO2 1 dnCO2 =− G A dt ART dt

(6)

Here, A, VG, T, and R are the mass transfer area, gas phase volume, temperature of the system and universal gas constant respectively. Then, combining and rearranging equation (5) and equation (6),

dPCO2 dt

=−

mCO2 E k L A VG

PCO2

(7)


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


In this expression, E is a function of infinite enhancement factor ( ) and Hatta number (Ha). is the enhancement factor corresponding to the infinitely fast reaction of CO2 in the liquid film when absorption rate is limited by the diffusion process only. for a single irreversible reaction is given by,

DCO2

Ei =

DAm

DAm [ Am]RT DCO2 ν CO2 PCO2 mCO2

+

(8)

In this equation and are the diffusivities of CO2 and amine in the aqueous solution, [Am] is amine solution concentration, ν CO2 is stoichiometric coefficient of CO2 in the reaction. Ha is the ratio of maximum reactive conversion rate of CO2 in the film to the maximum diffusional transport through the film35. According to the film theory of mass transfer, E and Ha are related as follows.

E=

Ha tanh ( Ha )

(9)

The terms involved in the equation (7) can be converted to measurable quantity by assuming pseudo-first order (PFO) reaction condition. Criteria for the PFO reaction condition is as follows36.

3 < Ha

Kinetics of CO2 Absorption in Aqueous Hexamethylenediamine

Recommend Documents