CO2 Mixture Adsorption on a Characterized Activated Carbon

Mar 8, 2017 - ABSTRACT: The aim of this work is to provide new experimental data of the adsorption equilibria of CH4/CO2 binary mixtures on a micropor...
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CH4/CO2 Mixture Adsorption on a Characterized Activated Carbon David Pino* and David Bessieres TOTAL-UMR 5150- LFC-R-Laboratoire des Fluides Complexes et leurs Réservoirs, University Pau & Pays Adour, CNRS, BP 1155, PAU, F-64013, France ABSTRACT: The aim of this work is to provide new experimental data of the adsorption equilibria of CH4/CO2 binary mixtures on a microporous activated carbon Ecosorb. The mixtures studied were made for two nominal feed-gas compositions of 25 and 75 mol %. Adsorption isotherms were performed using a homemade manometric technique, combined with gas chromatographic analysis of the equilibrium gas-phase composition. Experimental isotherms are provided up to 3 MPa for three temperatures at 303.15 K, 323.15 K, and 353.15 K. The equimolar mixture was previously studied thus allowing a description of the CO2/CH4 mixture adsorption in a full range of pressure, temperature, and composition. As expected, the binary adsorption isotherms mixture reveals a preferential adsorption of CO2 compared to CH4 over the whole pressure and temperature range. Additionally, these adsorption data were used to check the capability of the ideal adsorbed solution (IAS) theory to estimate the mixture adsorption of CH4/CO2. At low pressures (up to 1.5 (MPa)) the IAS model is in good agreement with the experimental data.

I. INTRODUCTION Carbon dioxide and methane are the main gases responsible for global warming. Both gases are founded in gases mixtures such as natural and landfill gas, coalbed methane, or more recently biogas.1 Therefore, the separation of the CO2/CH4 gas mixture, which is still an enduring challenge2−23 with economic and environmental impacts, is the subject of fundamental and applied research. Among the possible procedures to remove CO2 from the natural gas stream, the pressure swing adsorption is known to be one of the most efficient and economic processes. The experimental investigation of mixture adsorption has received less attention with respect to adsorption studies of pure components, and this is certainly due to the complexity of the measurements. A literature review shows that, in the framework of separation processes, gas mixture adsorption has been generally studied in the vicinity of the standard conditions corresponding to low pressure. However, some adsorbents able to sustain under more severe conditions are an attractive option for gas separation with the (PSA) process. Very few studies are reported for gas mixture adsorption under extended pressure and temperature conditions.1,6,10,12,14,19,21,24−27 Additionally, most of the studies target the development of new materials as potential adsorbents. An adsorbent performance indicator has been proposed to highlight porous materials for the PSA separation process.28 Its expression takes into account the working capacities, adsorption energies, and selectivities studied from the equimolar mixture. The purpose of this work is somewhat removed from these studies. The first objective is to provide a new set of data on extended ranges of pressure, temperature, © XXXX American Chemical Society

and concentrations for the binary mixture CO2/CH4. In a previous work,29 the adsorption isotherms of pure carbon dioxide and methane, and their equimolar mixture were explored in a microporous activated carbon (Ecosorb).29−31 In the present study, we report mixture measurements were made at nominal feed-gas compositions of 25 and 75 mol %. Measurements were made through the use of a manometric technique,29 coupled with gas chromatographic analysis of the equilibrium gas-phase composition. Experimental isotherms are provided up to 3 MPa for three temperatures at 303.15, 323.15, and 353.15 K. On the basis of these experimental data, we study the ability and the limitations of the ideal adsorbed solution (IAS) model to predict mixture adsorption. The paper is organized as follows. Section II is devoted to a detailed presentation of both the experimental set up and measurement principle. In section III we report the experimental data for the two binary mixtures. Finally a comparative study is performed with the IAST predictions.

II. EXPERIMENTAL SECTION Description of the Instrument. The instrument designed and built in the present study is a manometric device combined with a gas chromatograph (Agilent Technologies 7890A). A schematic view of this “homemade” apparatus is provided in Figure 1. This experiment and the measurement principle have been previously described.29 The main elements of this set up Received: December 12, 2016 Accepted: February 28, 2017

A

DOI: 10.1021/acs.jced.6b01029 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Article a ntot = n1a + n2a

(2)

g ntot

(3)

=

n1g

+

n2g

Individual Components Adsorbed. To calculate the amounts of each individual gas adsorbed, it is necessary to know the mole fraction y1 and y2 (CH4 and CO2, respectively) of each gas in the bulk gas phase. y1 is defined as

y1 =

n1g n1g + n2g

(4)

This ratio is obtained using the gas chromatograph. In a similar way, one can define a mole fraction x1 on the adsorbed phase: Figure 1. A schematic view of the apparatus.

x1 =

3

are the dosing volume (16.57 cm ) (1), the measuring vessel (18.29 cm3) in which the adsorbent is placed (2), the pressure transducer (3) (MKS Baratron type 121 A, with an uncertainty of 0.01% in the full scale ranging from the vacuum to 3.3 (MPa)), and the recirculation pump (4). The various parts are isolated with spherical valves, thus limiting the “dead space” volume. The whole apparatus is regulated under isothermal conditions through the use of a heater wire controlled by a PID regulator (Eurotherm 3208) and there were installed five thermocouples (type K, accuracy of ±0.1 K) in different parts of the circuit, in order to check the value of the constant temperature and ensure there is not temperature gradient during the isotherm measured. The overall uncertainty of the amount adsorbed (due to helium calibration procedure and pressure accuracy) is determined to be lower than 1% over the entire range investigated in this study. Measurement Principle. The experimental procedure is based on a mass balance principle, with accurate measurements of the pressure, temperature, and volumes, as it has been described in details by several authors (see, for example, ref 32). The adsorbent sample is placed in the cell and its mass is carefully measured (the mass is measured before outgassing) and chosen in order to get a large enough available adsorption area; about 30 m2 is the minimum area required.32 Then, a vacuum (vacuum quality: ultimate pressure ≤1 × 10−2 (Pa)) is created and the volume accessible with the adsorbent inside the cell is determined through successive helium expansions. The experimental protocol thus follows with sample outgassing.32 The gas mixture which is previously analyzed using the gas chromatograph, is then introduced from the dosing volume to the sample via a step-by-step procedure. At each dose, the mixture is circulated around the circuit and across the sample for at least 15 min in order to ensure that a homogeneous final mixture is obtained. Discrete quantities (0.5 cm3) are then extracted from the isolated dosing volume and analyzed through the use of the GC (more information is provided in ref 29). Amount Adsorbed. For each dose, the total amount adsorbed is calculated by a mass balance before and after adsorption equilibrium: a d g ntot = ntot − ntot

natot

n1a n1a + n2a

(5)

where x1 + x 2 = 1

(6)

When the adsorption cell is filled with the first dose, the amount adsorbed for an individual gas adsorbed is a n1a = ntot ·x1

(7)

After each step (i), the dosing volume is evacuated and filled again with the initial mixture. As a consequence, the general expression of the individual adsorbed gas, at each step, is provided by the relation: d g, i − 1 i − 1 g, i i n1a, i = (ntot 0.5 + ntot y1 ) − (ntot y1 )

(8)

The individual amount adsorbed is then obtained by a cumulative process:

n1a =

∑ n1a,i i

(9)

III. RESULTS AND DISCUSSION III.1. Materials. The Ecosorb activated carbon used as adsorbent material in this study was kindly supplied by JACOBI. CH4 and CO2 were provided by Linde Gas with a minimum purity of 99.995%. The mixtures with nominal compositions of 25% and 75% were provided by Linde Gas. The accuracy claimed to be better than 1% and was checked by gas chromatograph analysis. This activated carbon was characterized with a Micromeretics ASAP 2020 system. A nitrogen isotherm at 77 K was used to determine the pore size distribution (5 × 10−7 to 0.99 p/p0 in terms of relative pressure) by applying the Horvath−Kawazoe model.33 The pore size distribution (see Figure 2) shows a peak around 4.6 Å. The value of the specific surface Brunauer−Emmett−Teller is 1290 ± 30 m2·g−1, and the pore volume is 0.6 cm3·g−1. Prior to the experiment, the adsorbents were systematically purified under vacuum at 423.15 K during 24 h. The mass placed in the experimental cell was approximately 1 g. III.2. CO2/CH4 Coadsorption. Adsorption isotherms were measured at 303,15 K, 323,15 K, and 353,15 K for pressures ranging from 0.1 to 2.5 MPa. The data for the mixture 25% CO2/ 75%CH4 are reported in Table 1. In Tables 2−4, we report the data for each component of this mixture at 303.15 K, 323.15 K, and 353.15 K, respectively. Figures 3, 4, and 5 represent the adsorption isotherms for the mol 25% CO2 mixture at the three temperatures investigated. A similar behavior is observed for the whole temperature range

(1)

ndtot ngtot

in which represents the total amount adsorbed, is the total amount introduced in the adsorption cell and is the total amount of gas remaining in the gas phase. Both terms in the gas phase include a contribution from each of the individual gas mixture components (eqs 2 and 3): B

DOI: 10.1021/acs.jced.6b01029 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 3. Adsorption Data for Each Component of the Mixture 25%CO2/75%CH4 at 323.15 K P

nexc/mol·kg−1

MPa

CO2

CH4

y(CH4)

y(CO2)

x(CH4)

x(CO2)

0.55 1.17 1.55 1.99 2.39 2.76 3.07

0.78 1.44 1.87 2.25 2.58 2.86 3.10

1.35 2.22 2.59 3.00 3.28 3.55 3.74

0.780 0.770 0.764 0.759 0.756 0.755 0.754

0.220 0.230 0.236 0.241 0.244 0.245 0.246

0.635 0.606 0.580 0.571 0.560 0.554 0.547

0.365 0.394 0.420 0.429 0.440 0.446 0.453

Table 4. Adsorption Data for Each Component of the Mixture 25%CO2/75%CH4 at 353.15 K P

Figure 2. Pore size distribution of Ecosorb.

Table 1. Isotherms Adsorption Data of the Mixture 25% CO2/75%CH4a 303.15 K

a

323.15 K

nexc/mol·kg−1

MPa

CO2

CH4

y(CH4)

y(CO2)

x(CH4)

x(CO2)

0.58 1.22 1.58 2.01 2.37 2.65 3.10

0.58 1.11 1.42 1.70 1.91 2.07 2.27

0.97 1.63 1.92 2.21 2.40 2.53 2.78

0.773 0.766 0.760 0.757 0.755 0.753 0.753

0.227 0.234 0.240 0.243 0.245 0.247 0.247

0.627 0.595 0.575 0.565 0.556 0.550 0.550

0.373 0.405 0.425 0.435 0.444 0.450 0.450

353.15 K

P

nexc

P

nexc

P

nexc

MPa

mol·kg−1

MPa

mol·kg−1

MPa

mol·kg−1

0.58 1.15 1.57 2.02 2.43 2.86 3.10

2.68 4.18 5.14 5.90 6.57 7.24 7.64

0.55 1.17 1.55 1.99 2.39 2.76 3.07

2.13 3.66 4.47 5.25 5.86 6.41 6.84

0.58 1.22 1.58 2.01 2.37 2.65 3.10

1.55 2.73 3.34 3.91 4.31 4.60 5.05

Uncertainties: ΔT = 0.2 K, ΔP = 0.01 bar, Δn/n = 1%.

Table 2. Adsorption Data for Each Component of the Mixture 25%CO2/75%CH4 at 303.15 K P

Figure 3. Mixture (75%CH4/25%CO2) and individual sorption at 303.15 K: (circle) mixture; (filled diamond) CO2 individual; (filled triangle) CO2-mixture; (filled square) CH4 individual; (times) CH4mixture.

nexc/mol·kg−1

MPa

CO2

CH4

y(CH4)

y(CO2)

x(CH4)

x(CO2)

0.58 1.15 1.57 2.02 2.43 2.86 3.10

0.99 1.73 2.29 2.78 3.18 3.54 3.77

1.69 2.45 2.85 3.12 3.39 3.70 3.88

0.785 0.774 0.767 0.762 0.759 0.756 0.754

0.215 0.226 0.233 0.238 0.241 0.244 0.246

0.630 0.586 0.554 0.529 0.516 0.511 0.507

0.370 0.414 0.446 0.471 0.484 0.489 0.493

covered. The total adsorbed amount is lower than in the case of pure CO2. This is a first indicator of the preferential selectivity of CO2. This tendency is highlighted by the study of the mol 75% CO2 mixture plotted in Figures 6, 7, and 8, for which the data are reported in the Tables 5−8. As expected, CO2 is much more preferentially adsorbed than CH4. When one compares these isotherms with those of the single gas component, it is relevant that the CO2 adsorbed amount is less affected by the presence of CH4 in the gas phase, whereas the CH4 amount significantly decreases. As plotted in Figure 9, the experimental tendency presents a consistent and continuous evolution of the

Figure 4. Mixture (75%CH4/25%CO2) and individual sorption at 323.15 K: (circle) mixture; (filled diamond) CO2 individual; (filled triangle) CO2-mixture; (filled square) CH4 individual; (times) CH4mixture.

C

DOI: 10.1021/acs.jced.6b01029 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 8. Mixture (25%CH4/75%CO2) and individual sorption at 353.15 K: (circle) mixture; (filled diamond) CO2 individual; (filled triangle) CO2-mixture; (filled square) CH4 individual; (times) CH4mixture.

Figure 5. Mixture (75%CH4/25%CO2) and individual sorption at 353.15 K: (circle) mixture; (filled diamond) CO2 individual; (filled triangle) CO2-mixture; (filled square) CH4 individual; (times) CH4mixture.

Table 5. Isotherms Adsorption Data of the Mixture 75% CO2/25%CH4 303.15 K

Figure 6. Mixture (25%CH4/75%CO2) and individual sorption at 303.15 K: (circle) mixture; (filled diamond) CO2 individual; (filled triangle) CO2-mixture; (filled square) CH4 individual; (times) CH4mixture..

323.15 K

353.15 K

P

nexc

P

nexc

P

nexc

MPa

mol·kg−1

MPa

mol·kg−1

MPa

mol·kg−1

0.40 0.68 0.96 1.20 1.55 1.93 2.17

3.47 5.07 6.33 7.37 8.59 9.88 10.79

0.41 0.80 1.19 1.56 1.84 2.06 2.24

2.73 4.20 5.65 6.94 7.85 8.66 9.21

0.48 0.93 1.35 1.85 2.20 2.45

2.28 3.67 4.89 5.99 6.80 7.39

Table 6. Adsorption Data for Each Component of the Mixture 75%CO2/25%CH4 at 303.15 K P

nexc/mol·kg−1

MPa

CO2

CH4

y(CH4)

y(CO2)

x(CH4)

x(CO2)

0.40 0.68 0.96 1.20 1.55 1.93 2.17

2.97 4.30 5.58 6.57 7.71 8.90 9.73

0.51 0.77 0.75 0.80 0.88 0.98 1.06

0.321 0.281 0.285 0.274 0.268 0.264 0.260

0.679 0.719 0.715 0.726 0.732 0.736 0.740

0.146 0.151 0.119 0.109 0.102 0.099 0.098

0.854 0.849 0.881 0.891 0.898 0.901 0.902

Table 7. Adsorption data for each component of the mixture 75%CO2/25%CH4 at 323.15 K P

Figure 7. Mixture (25%CH4/75%CO2) and individual sorption at 323.15 K: (circle) mixture; (filled diamond) CO2 individual; (filled triangle) CO2-mixture; (filled square) CH4 individual; (times) CH4mixture.

adsorbed amount when one plots the CO2 amount adsorbed (xCO2) versus the CO2 mole fraction. III.3. Comparison of the Results with the IAS theory. Introduced in 1964 by Myers and Prausnitz,34 the ideal adsorbed solution theory (IAST) can be used to predict adsorption equilibria of gas mixtures. Here, the present study proposes to check and analyze the consistency and the limitations of this theory on the basis of the experimental database. This model, which is an independent geometric approach, represents the adsorption of mixtures as a thermodynamic

nexc/mol·kg−1

MPa

CO2

CH4

y(CH4)

y(CO2)

x(CH4)

x(CO2)

0.41 0.80 1.19 1.56 1.84 2.06 2.24

2.10 3.55 4.84 5.97 6.80 7.59 8.12

0.63 0.64 0.81 0.97 1.04 1.07 1.09

0.260 0.289 0.273 0.265 0.261 0.261 0.258

0.740 0.711 0.727 0.735 0.739 0.739 0.742

0.232 0.153 0.143 0.140 0.133 0.124 0.118

0.768 0.847 0.857 0.860 0.867 0.876 0.882

equilibrium between the adsorbed phase and the surrounding free gas phase. This equilibrium is described by an equation analogous to Raoult’s law for vapor−liquid equilibria (VLE). To predict mixture adsorption, we consider the fluids and mixtures as ideal; that is, the fugacity is equal to the partial pressure. The IAST is then reduced its most used forms: D

DOI: 10.1021/acs.jced.6b01029 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 8. Adsorption data for each component of the mixture 75%CO2/25%CH4 at 353.15 K

Table 9. AAD (%): Absolute Average Deviations between Experimental and IAST Adsorption Data (Mixture 75% CO2 at 323.15 K)

nexc/mol·kg−1

P

nexc/mol·kg−1

MPa

CO2

CH4

y(CH4)

y(CO2)

x(CH4)

x(CO2)

P

0.48 0.93 1.35 1.85 2.20 2.45

1.97 3.17 4.21 5.17 5.90 6.46

0.31 0.51 0.68 0.82 0.90 0.93

0.302 0.275 0.265 0.261 0.258 0.257

0.698 0.725 0.735 0.739 0.742 0.743

0.136 0.139 0.140 0.137 0.133 0.126

0.864 0.861 0.860 0.863 0.867 0.874

MPa

CO2

CH4

CH4

CO2

0.80 1.19 1.56 1.84 2.06 2.24

3.55 4.84 5.97 6.80 7.59 8.12

0.64 0.81 0.97 1.04 1.07 1.09

1.04 −2.17 −3.26 −7.89 −16.10 −20.98

−0.23 0.31 0.45 1.12 2.25 3.22

IV. CONCLUSIONS In this study, a new set of experimental data of the coadsorption equilibria of CH4/CO2 binary mixtures was obtained on a well characterized microporous activated carbon Ecosorb. The isotherms were performed for two gas compositions of 25 and 75 mol %. As the equimolar mixture was investigated in a previous work, the present work provide a description of the mixture CO2/CH4 adsorption under extended ranges of pressure (3 MPa) and for three temperatures, 303.15 K, 323.15 K, and 353.15 K, respectively. As expected, the binary adsorption isotherms mixture reveals a preferential adsorption of CO2 compared to CH4 over the whole pressure and temperature range. Additionally, the experimental data were compared with IAST predictions obtained from pure absolutes compounds. A good agreement is observed between the two approaches at pressures up to 1.5 (MPA)). At higher pressures, a deterioration is observed on the predictions of the individual adsorbed amounts due to the formulation of the IAST.

Figure 9. Comparison of CO2 amount adsorbed as a function of CO2 mole fraction: (circle) our experimental data; (filled diamond) experimental data.14

Pyi = xiPio

(10)

where Poi is the pressure of the pure component i at the same spreading pressure of the mixture, P is the total adsorptive pressure and yi and xi are the mole fractions of the specie i of the gas mixture (vapor) and in the adsorbed phase, respectively. Here, the superscript o denotes the quantities of pure components. The spreading pressure represents a two-dimensional confinement pressure of the adsorbed phase against the adsorbent surface. The spreading pressure itself cannot be measured, but can be derived using a modified Gibbs−Duhem equation under isothermal conditions: Π=

∫0

Pi0

ni(Pi) dPi Pi

AAD%



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

David Pino: 0000-0002-2810-6195 Notes

The authors declare no competing financial interest.



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(11)

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DOI: 10.1021/acs.jced.6b01029 J. Chem. Eng. Data XXXX, XXX, XXX−XXX