CHEMICAL ENGINEERING TRANSACTIONS VOL. 52, 2016 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Peng-Yen Liew, Jun-Yow Yong, Jiří Jaromír Klemeš, Hon Loong Lam Copyright © 2016, AIDIC Servizi S.r.l., ISBN 978-88-95608-42-6; ISSN 2283-9216 A New CO2 Absorption Data for Aqueous Solutions of N-methyldiethanolamine + Hexylamine Ammar Mehassouel*a, Ratiba Derrichea, Chakib Boualloub aEcole Nationale Polytechnique, Laboratoire de valorisation des énergies fossiles, El Harrach, Algérie bMINES ParisTech PSL-Research University, CES - Centre d'efficacité énergétique des systèmes, Paris, France ammar.mehassouel@g.enp.edu.dz The aim of this work is to develop a new chemical solvent for CO2 capture and compare its performance with a reference based solvent MDEA (MethylDiEthanolAmine). A mixture of MDEA and Hexylamine activator (HA) is designed to improve the performance of the process and allow a substantial gain in terms of energy regeneration and reducing the cost of capture. New CO2 absorption data is obtained using a Lewis cell gas- liquid interface at 298, 313 and 333 K with mass concentrations MDEA 37 wt%+ HA 3 wt%, MDEA 35 wt%+ HA 5 wt% and MDEA 33 wt%+ HA 7 wt%, to assess the CO2 absorption flux and the absorption capacity of the new solvent. Our results show that for the mixture of MDEA 37 wt%+ HA 3 wt%, the kinetics reaction is fast with reduced activation energy compared to that of MDEA 40 wt%. 1. Introduction Carbon dioxide is non-flammable and non-toxic gas, however, its concentration in the atmosphere has increased because of human activities (power generation, oil and gas ...). This increase has caused an imbalance in the ecosystem, so, it is necessary to minimize CO2 emissions and especially those for the most polluting industries with fixed sources of emissions. There are different techniques for capturing CO2 (Kanniche et al., 2010), the chemical absorption method is the most used one for CO2 capture using a chemical solvent usually amine-based. There are two main features to be taken into account. The first is the kinetics of the CO2 absorption reaction: primary amines like MEA (MonoEthanolAmine) are more reactive than secondary amines like DEA (DiEthanolAmine) which are more reactive than tertiary amines like MDEA (MethylDiEthanolamine). The CO2 absorption rate will affect the design of the absorption column and thus the investment cost of the capture process. Secondly, the CO2 solubility in the solvent: a reactive amine with CO2 will have a very good solubility of CO2 but will be less easily generable. Selecting more easily generable amine is tempting but this comes at the expense of the reaction rate and the solubility of CO2. The size of the installation will be greater and thus more costly. The chemical absorption methods are therefore too expensive to be used for CO2 capture. This study aims to develop a new chemical solvent to reduce the amount of energy required for regeneration, while maintaining a sufficient absorption flow to limit the size of the columns. Experimental measurements characterizing the reaction kinetics as well as CO2 solubility measurements were performed in a new solvent consisting of a mixture of MethylDiEthanolAmine (MDEA) activated by Hexylamine (HA) at temperatures of 298, 313 and 333 K and CO2 partial pressures below 1 MPa. Hexylamine is a primary amine, its solubility in water varies depending on the mass fraction and the temperature (Zhang et al., 2012), it has a very fast kinetics with CO2, the more lipophilic nature makes Hexylamine very interesting for CO2 capture (minimizing energy regeneration), this is why we have chosen to activate the MDEA by Hexylamine. DOI: 10.3303/CET1652100 Please cite this article as: Mehassouel A., Derriche R., Bouallou C., 2016, A new co2 absorption data for aqueous solutions of n- methyldiethanolamine + hexylamine, Chemical Engineering Transactions, 52, 595-600 DOI:10.3303/CET1652100 595 2. Experimental Procedure The apparatus used is a Lewis cell (Figure 1) consisting of a jacket Pyrex glass which allows the circulation of a fluid temperature regulator (Molina and Bouallou, 2013). The cell can withstand a maximum pressure of 3 bars and a temperature of 423 K. The inside diameter is 63 × 10-3 m. The effective cell volume is 369.7 × 10-6 m3. The ends of the cell are fitted two metal flanges and are sealed by two seals. The upper flange has a DRUCK pressure (thermostatically controlled at 373 K to avoid condensation problems), that allows monitoring changes in pressure over time in the gas phase and makes the gas injection for kinetic measurements through the valve. The lower flange has a platinum probe to measure the temperature of the liquid phase at any time and a charging valve for the solution. The agitation of the gas phase is accomplished by a four-blade impeller 40 × 10-3 m in diameter. It is driven by a magnetic stirrer located inside the cell and set in motion by a magnetic stirrer located outside the cell. Magnetic stirring for the liquid phase is assured by a Rushton turbine with six blades 42.5 × 10-3 m in diameter. The stirring speed in the liquid phase is measured by a photo tachymeter reflection. Four Teflon baffles are located inside the cell to avoid the formation of vortexes and to hold in place a central ring to stabilize and then to fix the interfacial area. The acquisition allows the pressure to determine the absorption flows. Figure1: Experimental apparatus 3. Result and discussion The absorption measurements were made at three different temperatures 298, 313 and 333 K, for the three solvents (MDEA 37 wt% + HA 3 wt%), (MDEA 35 wt%+ HA 5 wt%) and (MDEA 33 wt% + HA 7 wt%). On the basis of mass balance of the CO2 in the gas phase of the reactor we can to determine the absorption flux. dt Pd RT V dt nd A COgCO iCO )( . )( . 22 2  (1) Ai is the gas liquid interfacial area (m2) and Vg the volume of the gaseous phase (m3) The influence of the absorption kinetics of the chemical reaction between the CO2 and the reagent solutions is expressed by the enhancement factor E according to the following relationship iCOL G CO AECk V RT dt dP 2 2 . (2) kL is the liquid side mass transfer coefficient; R is the ideal gas constant 8.3143 J.K-1.mol-1, T is the absolute temperature, VG is the volume of gas. 596 The gas is considered ideal and the CO2 concentration in the liquid is negligible to its concentration at the interface CCO2i . At the interface the vapor liquid equilibrium is reached, the partial pressure PCO2 is connected to its concentration in the gas phase according to Henry's law: iCOCO CHP 22 . (3) Where H is the Henry’s law constant, PCO2 is obtained by solCO PPP  2 (4) The concentration of CO2 resulting from absorption does not change much the composition of the solution So , kl, H, and E remain constant with time, and integration of Eq(1) yields: bt PP PP Ln sol sol            0 (5) Where P is the total pressure in the apparatus and Psol is the solvent pressure before introducing CO2 and AEk HV RT b L G ... ..  (6) The kL coefficient is calculated using the correlation obtained by (Amararene and Bouallou, 2004), from physical measurements of N2O absorption into aqueous solutions of MDEA. And that involves the Reynolds number   2 Ag dN Re  , Schmidt Sc i    D and Sherwood i cellL D dk Sh  . 434.0618.0 Re352.0 ScSh  (7) This correlation is valid for a Reynolds number ranging from 215 to 5,666, a Schmidt number varying from 46 until 21,710 and a Sherwood number included in the range 378 – 985. μ = dynamic viscosity (Pa s) dcell = inner diameter of the Lewis cell (m) D = diffusivity (m2 s-1) dAg = diameter of the Rushton turbine (m) ρ= solvent density (kg m-3). The enhancement factor E is obtained from b by using Eq(6), the density and viscosity of the solutions are measured for different temperatures. The diffusion coefficient and the Henry constant are obtained by applying the correlation of (Al-Ghawas et al.,1989), N = 100 rpm, is the stirring speed of the liquid phase, it is maintained constant during the experiment. It is possible to determine the reaction system through the test of Hatta, noted Ha. This criterion is used to differentiate the mass transfer phenomena unique to a chemical reaction between the liquid within, the interface and the boundary layers. For an order of 1-1 reaction with rate constant k, the criterion of Hatta is calculated using Eq(8) 0,2 1 reactifco L CkD k Ha  (8) Dimensionless number that allows us to locate the place of the reaction: Ha <0.3: in the liquid phase 0.3 3 in the film diffusion In several literatures, kinetics of CO2 absorption by an aqueous mixture of MDEA is second order, the reaction of CO2 with MDEA is expressed by MDEACOMDEACOMDEACO CCkr 222   (9) kCO2-MDEA : is the second order constant kinetics reaction (m3.s-1.mol-1) 597 Therefore, for the absorption of CO2 into (HA+MDEA+H2O), the overall CO2 reaction kov (s-1), is expressed as follows: MDEACOHACOov rrr   22 (10) MDEACOMDEACOHACOHACOov CCkCCkr 2222   (11) 2COovov Ckr  (12)   2 1 2 1 COOV L DK k E  (13) The instantaneous enhancement factor, noted Ei, is also determined according to Eq (14) 2 * * 1 2 COCO reactifreactif i CD CD E  (14) The calculation of different parameters of CO2 absorption by the different amine mixtures at different temperatures are summarized in Table1. Table 1: Calculation of different absorption parameters Solvant (wt %) T (K) VG.106 (m3) b.103 s-1 Damine109 m2.s-1 DCO2109 m2.s-1 HCO2 Pa.m3.mol-1 kL10-5 m.s-1 E Ei MDEA 40 298.15 196.24 0.4 0.148 0.50 3,663.72 0.772 9.80 19.81 MDEA 37 +HA 3 298.15 313.15 333.15 192.27 184.01 188.69 0.3 0 .6 0.78 0.14 0.24 0.46 0.48 0.80 1.45 3,670.58 4,625.24 6,024.47 0.74 1.12 1.77 7.46 11.38 11.76 14.20 26.20 25.13 MDEA 35 +HA 5 298.15 313.15 333.15 208.41 207.55 203.92 9 9.5 9.2 0.14 0.25 0.46 0.5 0.82 1.45 3,677.41 4,629.39 6,020.08 0.76 1.14 1.78 236.92 200.32 148.87 25.11 32.31 19.87 MDEA 33 +HA 7 298.15 313.15 333.15 185.48 197.85 184.72 5.9 5.42 4.6 0.14 0.24 0.46 0.48 0.8 1.46 3,578.97 4,633.51 5,884.63 0.74 1.25 1.19 138.31 99.56 66.03 22.18 17.52 23.11 According to the results we note that Ha 3 for all examined mixtures, which means that the reaction takes place in the film diffusion, the condition of the reaction of pseudo first order (Ei/2 Ha 3) is satisfied for the mixture MDEA 40 wt% and the mixture MDEA 37 wt% + HA 3 wt% at 298.15, 313.15 and 333.15 K. The calculation of the activation energy for the reaction of CO2 with aqueous mixture MDEA 37 wt% + HA 3 wt% is carried out by applying the Arrhenius law. RT Ea Ak  )ln()ln( (15) The calculated activation energy (Ea) is 22. 25 kJ.mol-1, On the other hand, the overall rate law is ) 5.2678 exp(*10*78.26 2 T k   (16) 598 Figure 2: Arrhenius law for the CO2 absorption in aqueous (MDEA 37 wt% + HA 3 wt%) solution The Comparison of the activation energy and the kinetic constant with the literature is given in the table 2. Table 2: Comparison of the activation energy and the kinetic constant with the literature References Temperature K Solvent Concentration wt % k (m3.mol-1s-1) at 298.15K Ea (kJ.mol-1) Amann and Bouallou , 2009 298 -333 MDEA+TETA (40 + 3) 1.5 20.30 Our result 298 MDEA 40 3.38 × 10-3 Cadours and Bouallou, 1998 296 - 343 MDEA (10 - 50) 5.05 × 10-3 44.30 Pani et al.,1997 296 -343 MDEA (0- 50) 4.55 × 10-3 44.30 Gonzalez-Garza et al., 2009 278 - 303 MEA 30 1.53 37.73 Sema et al., 2013 298 - 318 DEAB (14.5-29) 0.43 62.55 Our result 298 - 333 MDEA+HA (37+3) 3.44 × 10-3 22.25 We note that adding a small amount of Hexylamine activates the absorption reaction, activation energy decreases from 44.30 kJ. mol-1 for MDEA (10- 50) wt% to 22.25 kJ. mol-1 for the studied solvent, we note also that the activation energy of the solvent studied is the same order compared to solvent mixture of MDEA with TriethyleneTetraAmine (TETA) with mass concentration (40 + 3) wt %, latter is more reactive than our solvent, because TETA has four amino group in its structure , we can also confirm that the 4-diethylamino-2-butanol (DEAB), which is a tertiary amine is more reactive than our solvent and the MEA is the most reactive solvent. The absorption rate of the mixture MDEA 37 wt% + HA 3 wt%, is better compared to MDEA 40 wt% and compared to other solvents, MDEA 35 wt% + HA 5 wt% and MDEA 33 wt% + HA 7 wt% at 298.15 K. 599 Figure 3: CO2 loaded versus time for different solvents at 298.15 K Figure 4: CO2 loaded versus time for different temperatures and MDEA 37+HA 3 solvent 4. Conclusions New CO2 absorption data with different solvent are obtained. The kinetic results are in agreement with a pseudo first order regime of absorption according to film theory for the mixture of MDEA 37 wt% + HA 3 wt%. The activation energy decreased from 43.3 kJ.mol-1 for MDEA 40 w% to 22.25 kJ.mol-1 for the new formulated solvent leading to an energy saving during the regeneration step. The addition of small amount of HA leads to a high increase in the CO2 absorption rates and improve the performance of the process allowing a substantial gain in terms of energy regeneration and reducing the cost of CO2 capture. 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Zhang J., Qiao Y., Agar D.W., 2012, Intensification of low temperature thermomorphic biphasic amine solvent regeneration for CO2 capture, Chemical Engineering Research and Design 90, 743–749. 600 http://www.scopus.com/authid/detail.uri?authorId=6505859600&eid=2-s2.0-4444373578 http://www.scopus.com/authid/detail.uri?authorId=6603363939&eid=2-s2.0-4444373578 http://www.scopus.com/source/sourceInfo.uri?sourceId=13057&origin=recordpage http://www.scopus.com/authid/detail.uri?authorId=37058753400&eid=2-s2.0-75749152974 http://www.scopus.com/authid/detail.uri?authorId=56013628300&eid=2-s2.0-75749152974 http://www.scopus.com/authid/detail.uri?authorId=6603363939&eid=2-s2.0-75749152974 http://www.scopus.com/source/sourceInfo.uri?sourceId=19600161818&origin=recordpage http://www.scopus.com/source/sourceInfo.uri?sourceId=19600161818&origin=recordpage http://www.scopus.com/authid/detail.uri?authorId=36809912100&eid=2-s2.0-84886393178 http://www.scopus.com/authid/detail.uri?authorId=6603363939&eid=2-s2.0-84886393178 http://www.scopus.com/source/sourceInfo.uri?sourceId=19600161818&origin=recordpage