Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Kinetic Study of CO2 Absorption in Aqueous Benzylamine Solvent Using a Stirred Cell Reaction Calorimeter Satyajit Mukherjee,† Syamalendu S. Bandyopadhyay,‡ and Amar N. Samanta*,† †
Department of Chemical Engineering and ‡Cryogenic Engineering Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India S Supporting Information *
ABSTRACT: In the literature, aqueous amine absorbents are widely used for post-combustion CO2 capture. Recently, benzylamine (BZA) aqueous solutions have been identified as promising solvents for CO2 capture. In this work, the kinetics of CO2 absorption in aqueous BZA, containing a primary amino group, has been studied using a stirred cell reactor with a horizontal gas liquid interface in a reaction calorimeter. Experiments were performed over a temperature range from 303.15 K to 333.15 K and the amine concentration ranging from 5 mass % to 30 mass %. Absorption rate experiments were performed in the pseudo-first-order regime to determine the overall kinetic rate constant using a fall-in-pressure technique. Both the zwitterion and termolecular mechanisms were applied to model the kinetic data and to estimate the individual reaction rate constants from experimental overall pseudo-first-order rate constants, kOV. The experimental kinetic data are better correlated by a termolecular mechanism (AAD 14.7%) compared to a zwitterion mechanism (AAD 38.02%). The density and viscosity of pure and aqueous binary mixtures of BZA are also measured over experimental temperature and concentration ranges. Empirical models are proposed to predict pure component density and viscosity data with AAD of 0.006% and 1.16% respectively. A Redlich−Kister type equation in terms of molar fraction is fitted to experimental density data, and the viscosity data for binary mixtures are correlated with Grunberg−Nissan model with AAD of 0.02% and 5.05% respectively. The reaction activation energy (Ea) calculated from the Arrhenius power law model are 25.82 and 25.98 kJ/mol for zwitterion and termolecular mechanisms respectively, which indicates a lower energy barrier (∼26 kJ/mol) for the BZA−H2O−CO2 reaction system.
1. INTRODUCTION
Recently, benzylamine (BZA) has been identified as a potential absorbent for CO2 removal with a higher cyclic capacity similar to that of a tertiary or sterically hindered amine solution.2 BZA, a primary amine functional group attached to a benzyl group, is water-soluble in all proportions. Aqueous BZA is readily biodegradable, and its volatility is comparable to MEA. The corrosion rate of BZA is also lower than MEA.2 Vapor−liquid equilibrium data and kinetics parameters are the key requirements to design absorption and regeneration column. Density, viscosity, and physical solubility data are needed to find kinetic parameters and thus the determination of the required solvent circulation rate, and the size of the absorption column to achieve a specified CO2 removal rate. Solubility of CO2 in new solvents has been reported in recent years to develop a new solvent system for acid gas removal. Density, viscosity, CO2 solubility ,and kinetic measurement data in aqueous BZA solvent are very rare in the open literature. In our recent work,3 the CO2 solubility in BZA aqueous solution has been investigated in a wide range of temperatures and concentrations accompanied by ENRTL thermodynamic modeling to present BZA as a potential solvent for CO2 removal. Penny et al.4 have proposed a Brønsted relationship between the second order rate constant and acid dissociation constant of benzylamine only at 20 °C using the stopped flow indicator
In March 2017, an atmospheric carbon dioxide (CO2) concentration level of 407 ppmv was recorded by the monitoring system of Mauna Loa Observatory, Hawaii (https://www.co2.earth/). This is expected to rise in the future largely due to the increase in global energy demand, 59% of which is met by fossil fuel based power plants.1 Global warming due to the greenhouse effect is one of the most concerning environmental effects due to accumulation of acid gases in the troposphere. Amine based CO2 absorption is the most mature technology to reduce greenhouse gas emissions by coal fired power plants. Chemical solvents remain the best solution to absorb CO2 because of the low partial pressure of CO2 in post-combustion exhaust gases from industrial operations. Commonly used aqueous alkanolamine absorbents such as monoethanolamine (MEA) and diethanolamine (DEA) are widely used for post-combustion CO2 capture. However, several drawbacks like energy efficiency, high reaction enthalpy, amine volatility, thermal and oxidative degradation, low absorption cyclic capacity, and corrosiveness of the solvent are associated with these aqueous amines for CO2 scrubbing processes. At high concentration, the viscosity of MEA is found to be high, which decreases the liquid phase mass transfer and a high corrosion rate is also related to the concentration of MEA. However, unlike secondary amines (like piperizine), carcinogenic nitrosamines are not formed directly with primary amines2 in the presence of NOx, and therefore primary amines are commercially more acceptable for PCC applications. © XXXX American Chemical Society
Received: November 29, 2017 Revised: January 24, 2018 Published: February 5, 2018 A
DOI: 10.1021/acs.energyfuels.7b03743 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels technique. Recently, Richner et al.2 have investigated rate constants of BZA carbamate formation and decomposition using the stopped flow spectrophotometric technique. However, their work is limited to very low BZA concentration in the range of 1−16 mM, which is well below the average amine concentration used in the CO2 capture process. Moreover, they have not specified any reaction mechanism to describe an aqueous CO2−BZA system. In view of the above, a detail investigation on absorption kinetics of CO2 in aqueous BZA solutions having the appropriate range of concentration suitable for PCC application are carried out in this work to propose the best suited kinetic model for a (BZA + H2O + CO2) system considering both zwitterion and termolecular mechanisms.
log10 K w = − 22.759 + 0.0294T
Kp are estimated from the van’t Hoff plot of the protonation constants of benzylamine in the work of Richner et al.2 using the following relationship. log10 K prot = pK a
CO2 + OH XooooY
HCO−3
k2
RNH 2 + CO2 ↔ RN+H 2COO−
k H2O
Step II: Base catalyzed proton removal reaction kB
RN+H 2COO− + B → RNHCOO− + BH+
The reaction rates for CO2 + H2O described by eqs 1 and 2 are expressed as (3)
rCO2 − H2O = k H2O[H 2O][CO2 ]
(4)
rCO2 − BZA =
5
Thomas suggested in a review work that reactions 1 and 2 are very slow considering 0.8% of CO2 depletion at 298.15 K, which is very less compared to other reactions. Therefore, reactions 1 and 2 can be neglected. On the other hand, Pinset et al.6 investigated that the formation of carbonic acid through the reaction 2 is very slow (kH2O = 0.026 s−1 at 298.15 K) compared to the other reactions. They also estimated the rate constant value of bicarbonate formation of reaction (1) is quite high, which cannot be neglected and is given by the following expression. log10 k OH− (m 3/kmol·s) = 13.635 −
2895 T
=
Kw ⎛ 1 − α ⎞ ⎜ ⎟ Kp ⎝ α ⎠ Kw [Am] Kp
k 2[CO2 ][RNH 2] 1+
k −1 ∑ kB[B]
(11)
The reaction rate expressed in eq 11 unveils a reaction order between 1 and 2 with respect to BZA. When deprotonation of zwitterion is very high and the reaction is instantaneous compared to the reverse reaction of zwitterion formation reaction and the zwitterion formation reaction is rate determining, then eq 11 can be expressed in the following form. rCO2 − BZA = k 2,BZA[CO2 ][RNH 2] when
k −1 ∑ kB[B]
99.9%), and nitrous oxide (>99.9% v/v) were procured from Linde India Limited. Aqueous amine solutions are prepared by using double distilled water after degassing by prolonged boiling and cooled to ambient temperature within airtight condition in desiccators. A precision digital balance (CITIZEN, CX-301 model, accuracy ±0.0001g) was used to weigh BZA and MEA for solvent preparation. 3.2. Experimental Methods. 3.2.1. Density Measurement. The density of the aqueous amine solution is needed in order to calculate molar concentrations from weight percent and is also necessary for measurements of dynamic viscosity, physical solubility of CO2, diffusivities, and reaction kinetics. The densities of aqueous BZA solutions were measured using a 25 × 10−6 m3 (at 298 K) Gay-Lussac pycnometer. For each run, the pycnometer containing the amine solution was put in a constant temperature bath which was controlled within ±0.1 K of the desired temperature level by a circulator temperature controller (F 32 ME, JULABO, FRG). Once the solution reached the desired temperature, it was weighed (Citizen CX301, accuracy of ±0.0001 g). Each density data value is the average of at least three measurements. 3.2.2. Viscosity Measurement. The viscosity of the aqueous amine solution is necessary for the measurement of reaction kinetics and diffusivities, and for correlating diffusivities and mass transfer coefficients. The kinematic viscosities were measured using a routine Cannon−Fenske viscometer according to the operating instructions of the ASTM D 445 standard test method in Annual Book of ASTM Standards.16 The viscometers were procured from Zenith Glasswares
RNH 2 + CO2 + B ↔ [encounter complex] → RNHCOO− + BH+
(17)
The forward reaction rate of this mechanism can be expressed as follows considering [H2O], hydroxyl ion, and amine are the dominating bases. kobs = {k H 2O[H 2O] + k OH−[OH−] + kBZA[BZA]}[BZA] (18)
This mechanism suggested that the reaction is second order with respect to BZA when BZA is the most dominant base and therefore the rate is given as rCO2 ‐ BZA = k 2,BZA[CO2 ][RNH 2]2
(19)
rov = rCO2 ‐ OH− + rCO2 ‐ BZA
(20)
k OV[CO2 ] = k OH−[OH−][CO2 ] + k 2,BZA[BZA]2 [CO2 ] (21)
k OV = k OH−[OH−] + k 2,BZA[BZA]2
(22)
The apparent reaction rate constant (kapp) is defined as kapp = k OV − k OH−[OH−] = k 2,BZA[BZA]2
(23)
Termolecular mechanism suggested that the reaction is second order with respect to BZA and the overall reaction is third order. C
DOI: 10.1021/acs.energyfuels.7b03743 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 2. CO2 absorption rate measurement set up: 1- double jacketed glass reactor (reaction calorimeter model: RC1e); 2 - anchor type impeller; 3 - temperature sensor of reactor; 4 - calibrated heater (25W); 5 - pressure transmitter (Rosemount, 3051TA); 6 - magnetically coupled stirrer; 7 inlet for liquid; 8- a- N2 cylinder, b - CO2 cylinder; 9 - a- N2 gas regulator, b- CO2 gas regulator; 10 - thermostated water bath; 11 - temperature sensor to control water bath temperature; 12 - coil type CO2 feeding arrangement. and Instruments Corporation which were calibrated and verified using certified standard liquids. The measurements are based on fluid flow through a capillary tube, which is directly related to fluid viscosity. The efflux time was measured and recorded manually using a digital stop watch whose precision is ±0.01s. Using the efflux time data, the kinematic viscosity was calculated from the equation υ = ct, where υ is the kinematic viscosity in cSt, c is a constant specific to each viscometer in cSt/s, and t is the efflux time in s. For each measurement the viscometer was placed in a constant temperature water bath whose temperature was controlled within ±0.1 K using a JULABO (F 32, FRG) temperature controller to maintain the desired temperature. The dynamic or absolute viscosities in cP were calculated by multiplying the kinematic viscosity by the corresponding density in g/cm3 of each solvent system of this work at each experimental temperature condition. 3.2.3. Physical Solubility Measurement. For the N2O solubility measurements of this work, a stainless steel equilibrium cell with a stirring arrangement was used. The schematic of the experimental setup used in this study as shown in Figure 1 is similar to the set up used by Mondal et al.17 The physical solubility measurement apparatus consists of a high pressure 494 × 10−6 m3 stainless steel equilibrium cell connected to a high pressure 751 × 10−6 m3 stainless steel gas buffer vessel. A magnetic stirrer is used for liquid phase agitation in the equilibrium cell. Both the cell and the gas buffer are equipped with platinum temperature sensors (Pt 100, JULABO, FRG) and Rosemount 3051 pressure transmitter (Emerson Process Management, US) with an accuracy of ±0.04%. The temperatures of the equilibrium cell and buffer vessel were controlled within ±0.1 K of the desired temperature level by a circulator temperature controller (F 32 ME, JULABO, FRG). A vacuum pump (INDVAC model IV 50, India) was employed to evacuate the equilibrium cell and buffer vessel, and also for degassing the amine solutions. For each run, the equilibrium cell and buffer vessel were allowed to reach the desired temperature for solubility measurements. The cell and the buffer vessel were then purged with N2O and evacuated. The gas buffer was filled with pure N2O from the cylinder up to a pressure of 250 kPa. A known volume (200 × 10−6 m3) of amine solution was introduced into the cell, and the solution was then degassed so that the liquid exists under its own vapor pressure. Once the temperature was
uniform throughout, the pressures of the cell (i.e., solution vapor pressure, Pv) and buffer vessel were recorded. Then pure N2O gas from buffer vessel was introduced into the equilibrium cell until the pressure of equilibrium cell attained 101.3 kPa and the magnetic stirrer was switched on. After feeding N2O gas, both equilibrium cell and buffer vessel were closed, and the pressure inside these two vessels was recorded. The attainment of equilibrium was ensured when there was no change in the pressure of the equilibrium cell for at least 1 h at constant temperature. The equilibrium partial pressure of N2O, pN2O was estimated by taking the difference of total pressure of cell, Pt, and vapor pressure, Pv (pN2O = Pt − Pv). 3.2.4. Carbon Dioxide Absorption Rate Measurement. The rate measurements of CO2 in aqueous BZA solutions were performed in the fully automated computerized heat flow reaction calorimeter (Mettler TOLEDO, model RC1e). The reactor, a cylindrical double walled glass vessel (nominal volume: 1.2 × 10−3 m3; internal diameter: 0.082 m), is attached with an anchor type impeller connected to a magnetically coupled stirrer shaft. Separate inlet lines for solvent, CO2, and N2 with purging and evacuation arrangement are made in the reactor. The reactor is instrumented with a temperature sensor (Pt 100), a calibrated heater (25 W), and a digital pressure transducer (Rosemount 3051TA; accuracy: ±0.04% of full scale pressure range), data acquisition and control system to record and control the reactor process variables. The schematic of this experimental setup is shown in Figure 2, which is similar to that reported by Mondal et al.17 The experimental setup was ensured to be leak proof before commencement of each experimental run. Reactor temperature was controlled accurately within ±0.1 K using heat transfer fluid circulating between reactor jacket and thermostat. Feed gas, before sending to the reactor, was kept in coil immersed in a thermostat to maintain the desired feed temperature within accuracy of ±1 K. For the investigation of the reaction kinetics of CO2 in aqueous BZA solutions, a series of experiments were conducted at a temperature range of 303.15−333.15 K and at different aqueous BZA concentrations ranging from 5 to 30 mass %. In each experiment, the reactor and the feed lines were initially purged with nitrogen and evacuated before filling the reactor with 500 g of freshly prepared aqueous BZA solution. The solution was then degassed and heated to D
DOI: 10.1021/acs.energyfuels.7b03743 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels the desired temperature under stirring conditions and kept under a vacuum for 1 h to equilibrate at its own vapor pressure. A small amount of pure CO2 gas was also kept in the thermostatic coil to attain the desired reactor temperature. Afterward, the solution vapor pressure of reactor was recorded and preheated pure CO2 gas from the thermostatic coil was fed to the reactor in a very short duration of time ( >1
(42)
⎛ μ ⎞−2/3 = ⎜⎜ ⎟⎟ ⎝ μw ⎠
° −w = DAm
(43)
7.4 × 10−8 × (2.6M w )1/2 T 0.6 ηw V Am
(44)
where, Mw and μW are the molecular weight and viscosity of water respectively, VAm is the molar volume of amine at normal boiling point, and T is the experimental temperature in K. The liquid phase mass transfer coefficient without chemical reaction, kL, an important parameter for the estimation of Hatta number (Ha), are estimated and presented in Table 6. The experimental measurement of liquid phase mass transfer coefficient (kL) was performed using the absorption rate measurement setup. A mass balance for the solute in both gas and liquid phase is used to calculate kL as described by the following fall in pressure expression of the physical absorption of CO2 in water.
The instantaneous enhancement factor, Ei can be defined as follows:
DAm
2.7163 3.4205 4.2361 5.1333 2.5152 3.1839 3.9365 4.8325 1.9551 2.4931 3.1583 3.9325 1.7362 2.2416 2.8431 3.5644
where μ and μw are viscosity of aqueous amine solution and water, D0Am−w is the diffusion of amine in aqueous solution at infinite dilution which can be determined using the following Wilke−Chang method.29
(40)
DAm [Am]RT DCO2 υCO2PCO2mCO2
Ei
0.47 0.47 0.47 0.47 0.94 0.94 0.94 0.94 1.87 1.87 1.87 1.87 2.78 2.78 2.78 2.78
0 DAm −w
Diffusivity of CO2 in aqueous BZA solutions along with in pure components values is presented in Table 1. 4.2.2. Estimation of kL: Pseudo-First-Order Reaction Criterion. The initial partial pressure of CO2 was maintained very low without depletion of amine in the gas−liquid interface to satisfy the criterion as eq 40 for pseudo-first-order reaction12 regime.
+
Ha
303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15
DAm − w (39)
DCO2
DAm−H2O 109 m2·s−1
According to eq 41, diffusivity of BZA in aqueous amine solution (DAm‑w) is required to calculate the instantaneous enhancement factor, Ei, which can be determined by the following relation.28
DCO2,water (m 2/s) = 2.35 × 10−6 exp(− 2119/(T /K ))
Ei =
[BZA] mol·Lit−1
Ha =
0.74
2 < Ha <
