Adsorption of Pure and Binary CO2, CH4, and N2 Gas Components

Article pubs.acs.org/jced

Adsorption of Pure and Binary CO2, CH4, and N2 Gas Components on Activated Carbon Beads Yi-Jiang Wu,† Ying Yang,† Xiang-Ming Kong,† Ping Li,*,† Jian-Guo Yu,† Ana M. Ribeiro,‡ and Alirio E. Rodrigues‡ †

State Key Laboratory of Chemical Engineering, College of Chemical Engineering, East China University of Science and Technology, Shanghai 20037, China ‡ Laboratory of Separation and Reaction Engineering (LSRE), Associated Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal S Supporting Information *

ABSTRACT: The adsorption equilibriums of CO2, CH4, and N2 pure gases on pitch-based activated carbon beads have been studied using a magnetic suspension microbalance at (293, 303, 333, and 363) K within a pressure range of 0 kPa to 4000 kPa. It is found that experimental adsorption capacities can be successfully described with both the Sips and Multisite Langmuir (MSL) isotherm models. Afterward, binary competitive adsorption breakthrough experiments (CO2/CH4 and CH4/N2) at 303 K and adsorption isotherms of gas mixtures under different conditions have been measured. Theoretical calculations from Sips model-based ideal adsorbed solution theory are found to have better agreement with experimental data of competitive binary adsorption than that from MSL model. Promising adsorption selectivity (5.5) between CH4 and N2 is obtained at 303 K as the pressure of a binary gas mixture is 100 kPa with yCH4 = 0.5 in the feed. Therefore, the activated carbon beads reported in this study can be considered as a promising adsorbent for CH4 enrichment from coalbed methane (CH4/N2) gas mixture.

1. INTRODUCTION Natural gas (mainly CH4) is one of the most important energy sources around the world, and the consumption of natural gas is expected to grow more than 50 % over the next 30 years.1 Meanwhile, CH4 is a powerful greenhouse gas, and the comparative global warming potential of CH4 is over 20 times greater than CO2 in a time horizon of 100 years.2 Landfills, petroleum extraction, and coal mining processes can release considerable amount of CH4 into the atmosphere. Capturing and utilizing CH4 from these resources will provide numerous energy, economic, and environmental benefits.2−4 However, owing to the high N2 and/or CO2 content within coalbed methane, associated petroleum, and landfill gases, CH4 separation and enrichment processes are always required in order to use these unconventional CH4 resources for civil and chemical processing applications (CH4 > 80 vol %).5 In recent years, lots of research efforts and various separation techniques have been devoted to the separation of CH4/N2 and CH4/CO2 gas mixtures. Pressure swing adsorption (PSA) is known as an energy efficient gas separation technology compared with the conventional cryogenic distillation process for CH4 enrichment,6 and the adsorption performance of adsorbent material plays a decisive role in the development of the PSA process. Development of adsorbents that have high capacity, high selectivity, and good regenerability for adsorption/desorption is critical for the success of methane enrichment processes via PSA.7 However, the relatively low © 2015 American Chemical Society

separation efficiency of adsorbent materials for methane enrichment is the major challenge that must be overcome,8 especially the development of a selective adsorbent for CH4/N2 separation has been regarded as one of the future challenges in the adsorption research area.9 Many potential candidates have been reported in the literature where zeolite-type materials have been intensively studied.10−16 The adsorption behaviors of the CH4/N2 gas system on several commercial zeolites, 4A,10 5A,11 and 13X,16 are studied, but relatively low selectivities (≤ 2.5) between CH4 and N2 have been found. Harlick et al.17,18 investigated the competing adsorption behavior of CO2, CH4, and N2 gas mixtures on H-ZSM-5 as the adsorbent, using Extended Langmuir (EL), Ideal Adsorbed Solution Theory (IAST), and the Flory−Huggins form of the vacancy solution theory (FH-VST) for the prediction of binary adsorption results, of which the IAST model is found to have a good prediction for experimental data except the CO2−N2 binary system. Recently, mesoporous silica materials (MCM) and metal organic framework (MOF) adsorbents have received extensive attention due to the high adsorption capacity of CH4, N2, and CO2 on those materials.12,14,19−22 Belmabkhout et al.20,21 studied the separation of the CO2/CH4 mixture by adsorption on mesoporous MCM-41 silica; they found that the Received: April 6, 2015 Accepted: July 28, 2015 Published: August 7, 2015 2684

DOI: 10.1021/acs.jced.5b00321 J. Chem. Eng. Data 2015, 60, 2684−2693

Journal of Chemical & Engineering Data

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material exhibits very high CO2 adsorption capacity even under low CO2 concentration (< 5 %) conditions. But the extremely high unit price of MCM and MOF materials may become the major limitation for large scale industrial applications. Carbonaceous materials have also been found as promising adsorbents for methane enrichment19,23−28 because of the high specific surface area, good CO2 adsorption capacity, etc. Above all, carbonaceous adsorbents are water tolerant,29 since coalbed methane and landfill gases are always associated with very high moisture content which may dramatically affect the adsorption performance of most adsorbent materials. Additionally, carbonaceous materials are generally less expensive than other synthetic adsorbents, which is another major advantage for commercial applications. In the present work, adsorption equilibriums of pure CO2, CH4, and N2 compounds on pitch-based activated carbon beads have been investigated thoroughly with both experiments and theoretical predictions. Afterward, competing adsorption behaviors of binary CO2/CH4 and CH4/N2 gas systems have been determined by comparing experimental data with calculation results from both ideal adsorption solution theory as well as Multisite Langmuir. Finally, the selectivity of activated carbon beads for CO2/CH4 and CH4/N2 gas mixtures have been studied and compared with those on other materials.

qi qm , i

(K ipi )1/ ni 1 + (K ipi )1/ ni

,

⎛ ΔH0, i ⎞ K i = K 0, i exp⎜ − ⎟ ⎝ RT ⎠

⎛ ΔH0, i ⎞ K i = K 0, i exp⎜ − ⎟ ⎝ RT ⎠

(2)

Correlations of Sips and MSL models with experimental data have been performed by MATLAB 2012a software with a Nelder−Mead simplex search method. The objective function, which is also the error function, can be defined as F = ERR =

1 N

qexp − qcal

∑∑ T

qexp

p

(3)

where T and p are the temperature and pressure parameters for each experiment, respectively. qcal is the calculated adsorption capacity and qexp is the amount adsorbed recorded in the experiment. 2.2. Adsorption of Binary Gas Mixtures. The binary gas adsorption amounts can be directly obtained according to the combination of pure gas compound isotherms through the MSL model by using qtotal

⎡ ⎛ = ∑ Miqi = ∑ ⎢Miqm , iK ipyi ⎜⎜1 − ⎢ ⎝ i i ⎣

∑ i

ai qi ⎞ ⎤⎥ ⎟ qm , i ⎟⎠ ⎥⎦

(4)

where qtotal is the weight of total adsorbed amount, Mi is the molecular weight of compound i, and qi it the uptake capacity of compound i. Besides, ideal adsorption solution theory (IAST)36,37 can be employed to predict the competing adsorption equilibrium behaviors of a binary gas mixture systems. The theory is based on the concept of an ideal adsorbed solution in which the adsorbed phase is in equilibrium with the gas phase. The analogy with Raoult’s law for equilibrium between adsorbed phase and gas phase can be expressed as

2. ADSORPTION THEORY 2.1. Adsorption of Pure Gas Component. A variety of isotherm models have been developed to describe the adsorption equilibrium of a pure gas compound. Different approaches on isotherm models can be found over past decades.30,31 Among these isotherm models, the extended Langmuir isotherm models have been wildly used for adsorption data correlation due to their simplicity and promising fitting performance, especially applicable for the small gas molecule adsorption process. Therefore, two extended Langmuir type approaches, Sips and Multisite Langmuir (MSL) models have been employed in this work. The Sips model,32 which can be regarded as a combination of conventional Langmuir and Freundlich equations, is employed to describe the adsorption equilibrium of pure gases. The limitation of adsorbate concentration rising associated with conventional Freundlich isotherm model can be circumvented,31 the Sips model is found able to describe the adsorption capacity over a relatively large pressure range without compromising the degree of accuracy for CH4/N2/ CO2 adsorption over activated carbon.33 The model is expressed as qi = qm , i

ai ⎛ qi ⎞ ⎟ , = K ipi ⎜⎜1 − qm , i ⎟⎠ ⎝

ptotal yi = xipi0 (π )

(5)

where yi and xi is the molar fraction of component i in the gas phase and adsorbed phase, respectively. p0i (π) is the equilibrium gas phase pressure corresponding to the solution temperature and to the solution spreading pressure π for the adsorption of component i. For a pure component i, the integration of Gibbs equation will give:

πi0A = RT

∫0

pi0

qi(p) p

dp

(6)

where π0i is the spreading pressure of component i in adsorbed phase, A is the surface area of adsorbent, qi(p) is the pure gas adsorption equilibrium equation of component i. Since the mixing process is carried out at a constant spreading pressure, eq 6 becomes

(1)

where Ki is the adsorption equilibrium constant of component i, K0,i is the adsorption equilibrium constant of component i at infinite temperature, −ΔH0,i is the heat of adsorption, and ni is the isotherm parameter on nonuniform surfaces. On the other hand, the MSL model34 is another extension of the Langmuir isotherm, which is based on the assumption that the adsorbent material has a fixed number of adsorption sites (qs) and each adsorbate molecule is able to occupy more than one adsorption site (ai). In addition, for each and every component i, qs = qmiai should be imposed according to thermodynamic consistency.35 The model can be expressed as

∫0

p10

q1(p) p

d p=

∫0

p20

q2(p) p

dp

(7)

and combining eq 7 with Lewis relations yields x1 x2 1 , = + 0 qtotal q1(p1 ) q2(p20 ) q2 = qt x 2 2685

x1 + x 2 = 1,

q1 = qt x1 , (8)



Article

The predictions of binary mixture adsorption isotherms through IAST are based on the pure component adsorption isotherms through the Sips model without any mixture parameters. Therefore, the accuracy of predictions depends on the adsorption isotherms of pure components.

mex = mabs − ρg (Vs + Vc + Vads)

where mex is the excess amount adsorbed and mabs is the absolute amount adsorbed. ρg is the density of the gas, which can be obtained from thermophysical properties database. Additionally, Vs is the volume of the solid adsorbent, and VC is the volume of the cell where the adsorbent is located. Vads is the volume of adsorbed phase; the calculation of Vads is based on the assumption that the density of the adsorbed phase can be approximated with the density of the liquid at the boiling point at atmospheric pressure.13 A calibration with helium has been carried out to determine the volume (Vs + Vc) that contribute to the buoyancy effect40 under the assumption that helium is not adsorbed. The data reported in this work all correspond to the absolute amount adsorbed. Gases used in all experiments are supplied by Jiajie Gas Company (Shanghai, China) with purities of CO2 > 99.998 vol %, CH4 > 99.99 vol %, N2 > 99.995 vol % and He > 99.999 vol %. 3.3. Binary Adsorption Equilibrium Measurements. Competitive adsorption performance of CO2/CH4 and CH4/ N2 gas mixtures on the adsorbent material have been measured by the magnetic suspension microbalance within a total pressure range of 0 kPa to 500 kPa at 303 K and 333 K with different feeding gas concentrations, and the desired constituents of CO2/CH4 and CH4/N2 gas mixtures in the feed are obtained by adjusting and fixing the mass flow controllers. The flow of binary gas mixture feed is continued until the equilibrium is reached. The gas composition at equilibrium in the microbalance is measured by gas chromatography with the thermal conductivity detector. The same operating procedures for degassing and regeneration processes of the adsorbent material have been carried out as explained in section 3.2. Additionally, to obtain the competitive adsorption behavior of CO2/CH4 and CH4/N2 gas mixtures and the adsorbed amount of each gas component, binary breakthrough curves have been measured with an already existing apparatus in our lab.41 To remove moisture, adsorbent material has been heated at 423 K under a flow of helium for 10 h as pretreatment. Different feeding gas concentrations are used at 303 K, and the isothermal of the adsorption column is maintained with a water bath. For all the breakthrough tests, the bed was filled with helium under the desired pressure before experiments. Detailed information on the column can be found in Table 2. The molar amount of each compound retained in the bed can be obtained by the integration of breakthrough curves measured in terms of molar flow rate at the outlet as a function of time. An illustration of binary breakthrough curves is shown in Figure 1, where F0,A/B is the inlet molar flow rate of A and B.

3. EXPERIMENT 3.1. Adsorbent Material. Pitch-based activated carbon beads material, which is used as the adsorbent in this work, has been prepared within the East China University of Science and Technology, where no binder material has been used in the production process; detailed preparation and characterization information can be found in a previous publication.38 Mercury porosimetry has been performed using the PoreMaster 60 (Quantachrome Corporation, Boynton Beach, USA) to measure the void fraction and macropore size distribution of adsorbent beads. The specific surface area of the material has been calculated by the Brunauer−Emmett−Teller (BET) method from the nitrogen equilibrium adsorption isotherm at 77 K, using ASAP-2020 M (Micromeritics Instrument, Atlanta, USA), and Density Functional Theory (DFT) has been used to obtain the micropore size distribution. Characteristic parameters of the adsorbent material can be found in Table 1. Table 1. Properties of the Activated Carbon Beads As Adsorbent Material parameter

value

unit

pellet diameter pellet density pellet porosity specific surface area total pore volume micropore volume micropore average size macropore average size

1 to 1.2 984.3 0.51 1457 0.57 0.35 0.60 0.40

mm kg·m−3

(9)

m2·g−1 cm3·g−1 cm3·g−1 nm μm

3.2. Pure Gas Adsorption Equilibrium Measurements. Adsorption equilibrium measurements for pure gas systems (CO2, CH4, and N2) have been performed with a magnetic suspension microbalance (Rubotherm GmbH, Bochum, Germany) operated in closed system. The accuracy of the microbalance used is ± 2·10−8 kg. Two Lucas Schaevitz pressure transducers have been used for pressure control and measurement, one from 0 kPa to 100 kPa with an accuracy of ± 2·10−2 kPa and the other from 0 kPa to 5000 kPa with an accuracy of ± 1 kPa. The temperature of the system is controlled by a constant temperature oil bath with an accuracy of ± 0.3 K. The schematic diagram of the experimental setup used in this work can be found in our previous publication.39 Degassing of the adsorbent material has been performed under vacuum condition (< 1 kPa) at 423 K overnight prior to the experiments. Besides, adsorbent regenerations for different experiments have also been performed under vacuum condition (< 1 kPa) at desired temperatures overnight. Finally, adsorption isotherms of CO2, CH4, and N2 are measured at 293 K, 303 K, 333 K, and 363 K in a pressure range from 0 kPa to 4000 kPa. The experimental data directly obtained from the balance is the excess amount adsorbed, and the effect of buoyancy should be considered especially under higher pressure conditions. Therefore, the absolute adsorption and excess adsorption can be calculated by

Table 2. Packed Column and Operating Conditions Used in the Breakthrough Experiments

2686

parameter

value

column length column diameter bed void adsorbent weight temperature flow rate of gas mixture flow rate of helium total pressure

0.482 0.025 0.32 115.93 303 400 400 200

unit m m g K Ncm3·min−1 Ncm3·min−1 kPa



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beads (SBET = 1457 m2·g−1, 3.7 mol·kg−1 at T = 303 K, pCH4 = 1000 kPa) is similar to the Norit R1 Extra (SBET = 1450 m2·g−1, ∼ 3.8 mol·kg−1 at T = 298 K, pCH4 = 1000 kPa), and higher than the adsorption capacities of carbonaceous materials (BPL, A10, and activated carbon A) with lower specific surface area. While the powdered Maxsorb activated carbon with a specific surface area higher than 3000 m2·g−1 has an even higher adsorption of CH4 (∼6 mol·kg−1) at T = 298 K, pCH4 = 1000 kPa.24,42 The regressed parameters for Sips model and MSL model can be found in Tables 3 and 4, respectively. Both models used Figure 1. Illustration of binary (A and B) breakthrough curves obtained at the outlet of the column during a breakthrough test, F0,A/B is the inlet molar flow rate of A and B.

Table 3. Parameters Calculated for the Sips Model According to Experimental Data of CO2, CH4, and N2 Adsorption on Activated Carbon Beads

The molar amount of A retained in the bed is equal to the area 1 minus the roll-up area 2, and the number of moles for B is equal to area 1 plus area 3. Finally, the equilibrium loadings of each compound are equal to the total amounts retained in the bed minus the number of moles within the gas phase (void fraction of column).

−ΔH0

K0 gas CO2 CH4 N2

kPa

−1

kJ·mol

1.10·10−7 3.66·10−7 1.40·10−6

−1

24.72 20.55 14.63

qm,i mol·kg

ERR −1

10.66 6.79 5.52

n

%

1.30 1.23 1.09

1.72 1.62 3.38

Table 4. Parameters Calculated for the MSL Model According to Experimental Data of CO2, CH4, and N2 Adsorption on Activated Carbon Beads

4. RESULTS AND DISCUSSIONS 4.1. Pure Gas Adsorption Performance. The experimental data of carbon dioxide, methane, and nitrogen adsorption on activated carbon beads in a pressure range from 0 kPa to 100 kPa (left side) and 0 kPa to 4000 kPa (right side) at 293 K, 303 K, 333 K, and 363 K are presented in Figure 2, where the symbols are the experimental data and the solid

K0

−ΔH0

qm,i

gas

kPa−1

kJ·mol−1

mol·kg−1

ai

%

CO2 CH4 N2

1.57·10−7 6.70·10−7 1.01·10−6

24.96 19.61 15.06

11.29 8.64 8.42

1.84 2.40 2.47

2.48 3.17 3.22

ERR

in this work can describe experimental results with small mean relative errors (< 4 %), and the Sips model shows a better correlation than that from the MSL model. Therefore, the heat of adsorption calculated from the Sips model has been used for further discussion and comparison. The heat of adsorption for CO2 in this study is similar as the BPL activated carbon24 (−ΔHCO2 = 25.7 kJ·mol−1), while CH4 and N2 adsorption heats are more close to the previous reports on Norit R1 Extra24 (-ΔHCH4 = 20.6 kJ·mol−1) and activated carbon beads41 with -ΔHN2 = 18.1 kJ·mol−1. It can be found that the heat of adsorption for all the adsorbates are less than 25 kJ·mol−1, which indicates that the adsorption of these gas compounds on activated carbon beads are a typical physisorption process.29 Besides, the values of adsorption enthalpy for CO2, CH4, and N2 are in descending order, which might be related with the physical properties of these gas molecules, as shown in Table 5. The adsorption process occurs when the interaction potential is equal to the energy change of molecular from gas to adsorbed state, and the interaction potential mainly depends on the

Figure 2. Adsorption isotherms of CO2 (a,b), CH4 (c,d) and N2 (e,f) on activated carbon beads at (0 to 100) kPa and (0 to 4000) kPa with different temperature conditions. Symbols, experimental data; solid lines, Sips model; dashed lines, MSL model.

Table 5. Physical Properties of CO2, CH4, and N2 Molecules44

lines represent the calculated results obtained from Sips model while dashed lines are obtained according to the MSL isotherm model. Adsorption capacities for pure components under the same pressure and temperature conditions are CO2 > CH4 > N2. The promising adsorption capacity of CH4 for pitch-based activated carbon beads might be related to the high specific surface area, especially under high pressure conditions. Compared with a previous report from Himeno et al.,24 the adsorption capacity of CH4 for pitch-based activated carbon

diameter

2687

quadrupole moment· 10−40

polarizability· 10−25

gas

nm

dipole moment

cm2

cm3

CO2 CH4 N2

0.33 0.38 0.36

0 0 0

−13.7 0 −4.9

29.1 25.9 17.4



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Figure 3. Isosteric heat for CO2, CH4, and N2 adsorption on activated carbon beads. Symbols, experimental data by van’t Hoff equation; solid lines, Sips model; dashed lines, MSL model.

for MSL model it does not depend on loading. Besides, we can find the heats of adsorption calculated from both the Sips model and MSL model are in the range of values obtained from experimental data calculated by van’t Hoff equation. Therefore, Sips model and MSL model based equilibrium adsorption isotherms and the obtained parameters for eq 1 and 2 will be used in the following section for the prediction of multicomponent adsorption behaviors. Finally, loading ratios can be compared by using qi/qj under the same partial pressure and temperature conditions for a preliminary evaluation. According to Figure 2, we can find that the loading ratios between CH4 and N2 at equilibrium are higher than that between CH4 and CO2 at the same condition, for example, qCH4/qN2 = 2.1 and qCO2/qCH4 = 1.8 at 303 K and 1000 kPa. However, we cannot simply conclude that the activated carbon material is more suitable for the separation of CH4/N2 (coalbed methane) than for the separation of CO2/ CH4 (landfill gases) because of the possible existence of competing adsorption effects, which will be investigated in the following section. 4.2. Binary Adsorption Performance. Binary adsorption performance on the pitch-based activated carbon beads have been measured through binary breakthrough curves. Figure 4 shows the binary adsorption behaviors for CO2/CH4 and CH4/ N2 gas mixtures on activated carbon beads at 303 K with

permanent dipole, Debye, and dispersion forces. Since CO2, CH4, and N2 are nonpolar molecules (Table 5), the interaction of these adsorbates with activated carbon beads mainly depends on the dispersion force, which is determined by the polarizability.29 Besides, Bae and Lee43 have found that the strong affinity of CO2 to carbon surface is directly related with the change of excess surface work during CO2 adsorption over carbonaceous materials. As a result, the carbon dioxide molecule which has the highest polarizability among these adsorbates leads to the highest adsorption enthalpy. Additionally, isosteric heats of adsorption as a function of loading have been calculated with experimental data using the van’t Hoff equation:45 −

⎡ ∂ ln p ⎤ ΔH =⎢ 2 ⎣ ∂T ⎥⎦q RT

(10)

where (−ΔH) is the isosteric heat of adsorption at specific loading (q). Results obtained are presented in Figure 3. It can be observed that the isosteric heats of adsorption for CO2, CH4, and N2 on activated carbon beads vary with the surface loading, indicating that the adsorbent material has a heterogeneous surface in agreement with previous reports.38,41 Although the experimental data of the adsorption heat are dependent on the loading amount, for Sips model, the adsorption heat parameter is the value for q/qm = 0.5, whereas 2688



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Figure 4. Completive adsorption behaviors of CO2/CH4 (a,c) and CH4/N2 (b,d) binary gas mixtures on pitch-based activated carbon beads at 303 K and 100 kPa. Symbols, experimental data; solid lines, Sips-based IAST model; dashed lines, MSL model.

Figure 5. Adsorption amounts of CO2/CH4 gas mixtures on pitch-based activated carbon beads under the pressure range of (0−500) kPa, T = 303 K (a) and 333 K (b), yCH4 = 0.254, 0.699, and 0.794. Symbols, experimental data; solid lines, Sips-based IAST model; dashed lines, MSL model.

of adsorbed amount and molar fraction of methane in the adsorbed phase. The difference of such a prediction performance might be caused by the heterogeneity of adsorbent shown in Figure 3; such behavior is in a good agreement with the previous analysis by Sircar46 that IAST is able to work very well when the adsorbates have similar sizes (Table 5) with weakly heterogeneous adsorbent, while the MSL model is generally applicable to account differences in the adsorbate sizes on homogeneous adsorbents. To investigate the binary adsorption performance in a wider range of operating conditions (temperature and pressure), competitive adsorption performance of CO2/CH4 and CH4/N2 gas mixtures on the adsorbent material have been measured by the magnetic suspension microbalance within a pressure range

different compositions, the total pressure is 200 kPa and the partial pressure for binary gas mixtures has been kept as 100 kPa. Figure 4 panels a and b are the total and partial adsorbed amounts for CO2/CH4 and CH4/N2 binary adsorption on activated carbon beads, respectively. Figure 4 panels c and d are the x−y diagrams for CO 2 /CH 4 and CH 4/N 2 binary adsorptions, respectively. It can be found from Figure 4 that the results calculated according to Sips-based IAST model eqs 8 with adsorption isotherms of pure compounds given in section 2.1 are in a good agreement with the experimental data collected from binary breakthrough curves. With the increase of methane molar fraction in the gas phase, the lines obtained from the MSL model are found with an acceptable accuracy in the prediction 2689



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Figure 6. Adsorption amounts of CH4/N2 gas mixtures on pitch-based activated carbon beads under the pressure range of (0−500) kPa, T = 303 K (a) and 333 K (b), yCH4 = 0.766, 0.551, and 0.302. Symbols, experimental data; solid lines, Sips-based IAST model; dashed lines, MSL model.

Figure 7. Adsorption selectivity of CO2/CH4 (a) and CH4/N2 (b) mixtures over pitch-based activated carbon beads as a function of total pressure at 303 K and 333 K obtained according to Sips-based IAST model with experimental results from binary adsorption amounts.

investigate the adsorption selectivity of activated carbon beads for CO2/CH4 and CH4/N2 binary gas mixtures. 4.3. Selectivity. The adsorption selectivity is usually used for a comparison of adsorption performances with different adsorbents, the selectivity (or sometimes referred as separation coefficient47) can be defined as the ratio of adsorbed amounts of two gases at a specified temperature and pressure condition:45

from 0 kPa to 500 kPa at 303 K and 333 K with different feeding gas concentrations. Figures 5 and 6 show the adsorption amounts for CO2/CH4 and CH4/N2 binary gas mixtures. Methane molar fractions are 25.4 %, 69.9 %, 79.4 % in CO2/CH4 gas mixtures and 24.9 %, 55.1 %, 76.6 % in CH4/N2 gas mixtures, respectively. Sips model-based IAST has been employed to calculate the total adsorbed amounts with the effects of competing adsorption, and the results from MSL model are also included for comparison. Symbols represent the experimental data, solid lines are obtained according to the Sips model-based IAST and dashed lines are calculated according to MSL model. On one hand, for the CO2/CH4 mixture, the total adsorbed amount decreases significantly with the rise of CH4 molar fraction in the feeding under the same total pressure conditions as shown in Figure 5, which is in a good agreement with the results of CO2/CH4 loading ratios obtained in section 4.1. On the other hand, we can observe that all the adsorption capacities at 333 K are much less than those obtained at 303 K with same feed compositions, which indicates that the temperature variation has an apparent effect on the adsorption capacity. In addition, similar behavior can also be found with the results obtained from the CH4/N2 gas mixture systems shown in Figure 6. Similar to Figure 4, dashed lines calculated by MSL isotherm models are unable to follow all the experimental results in Figures 5 and 6, especially at low pressure conditions, while Sips model-based IAST can be used to predicate the adsorption behavior of binary gas mixtures on activated carbon beads with a promising accuracy. As a result, the data calculated from Sips model-based IAST will be used in the following section to

α1/2 =

x1 y2 · x 2 y1

(11)

where xi is the molar fraction of component i in adsorbed phase, yi is the corresponding molar fraction in gas phase. In Figure 7, the selectivities of CO2/CH4 and CH4/N2 mixtures as a function of pressure are calculated using Sips model-based ideal adsorbed solution theory with different temperature and composition conditions according to the experimental data shown in Figure 5 and 6. Figure 7a demonstrates the selectivity of the CO2/CH4 mixture on pitch-based activated carbon beads, and the selectivity over the whole range of pressure (0 kPa to 500 kPa) remains almost constant. Besides, it can be found from Figure 7a that the selectivity has not been affected by the change of feeding composition. Figure 7b shows the selectivity of CH4/N2 mixture on pitch-based activated carbon beads, where the selectivity between CO2 and CH4 is higher in a low pressure region ( Norit R1 Extra48 > monolith activated carbon27 (loading ratio) > Maxsorb activated carbon49 > mesoporous carbon26 > coalderived activated carbon33 (loading ratio) > CMK-5.50 A very high selectivity (5.5) between CH4 and N2 on the adsorbent material can be found in Figure 4d at 303 K with yCH4 = 0.5 in the feed and total pressure is 100 kPa. Besides, such selectivity is also higher than most noncarbonaceous materials such as Basolite A10015 (αCH4/N2 = 4.8) and MOF-17714 (αCH4/N2 = 4.0). Therefore, we may come to the conclusion that activated carbon beads studied in this work constitute a very promising adsorbent material for the separation of the CH4/N2 gas mixture.

values for CO2/CH4 and CH4/N2 selectivity are strongly dependent on the model precision. Although the MSL model and Sips-based IAST model are able to give the good prediction for the competitive adsorption isotherms of CO2/CH4 and CH4/N2, the use of different models may affect the selectivity value significantly, as listed in Table 6. Therefore, in the future more attention should be paid to the deviation of selectivities from different isotherm models for competitive adsorption. Since a better correlation with Sips-based IAST model can be found in Figure 4, the selectivities obtained from the Sips-based IAST model have been used in the following comparison and discussion. A brief survey of the adsorption performance of CO2/CH4 and CH4/N2 gas mixtures on these adsorbent materials can be found in Table 7. Table 7 shows that the selectivity between the CO2 and CH4 gas mixture over monolith activated carbon27 (loading ratio) > activated carbon beads (this work) > mesoporous carbon26 > activated carbon (Shao et al.25) > commercial activated carbon materials24 (Norit R1 Extra, BPL, Maxsorb, A10). It can be found that the activated carbon beads in our work have a good selectivity for the CO2/CH4 gas mixture among the carbonaceous materials reported. However, as compared with the adsorption performance on noncarbonaceous materials such as zeolite 13X13 (αCO2/CH4 = 16) and MOF-514 (αCO2/CH4 = 15.5), activated carbon beads used in this

5. CONCLUSIONS Pitch-based activated carbon beads have been used as the adsorbent material to obtain the adsorption equilibrium of CO2, CH4, and N2 pure and binary gas systems using a magnetic suspension microbalance. The experimental data obtained at (293, 303, 333, and 363) K in a pressure range of 0 kPa to 4000 kPa for pure gas compounds can be predicated by both the Sips adsorption isotherm and Multisite Langmuir (MSL) models with high correlation degrees. The heats of adsorption for CO2, CH4, and N2 calculated from the Sips and MSL models are in a good agreement with the values of 2691



Article

Vadsorbate Vadsorbent Vc α1/2 xi yi π π0i

isosteric heat obtained from experimental data by van’t Hoff equation. In addition, the effects of competitive adsorption have been studied according to the binary gas adsorption capacities obtained from breakthrough experiments, and the Sips modelbased ideal adsorbed solution theory (IAST) model used in this work can predict the binary gas mixture adsorption behavior with a very high accuracy, while the MSL model is able to give the prediction for the competitive adsorption behavior with an acceptable accuracy. Finally, the selectivity of activated carbon beads for CO2/ CH4 and CH4/N2 gas adsorption separation have been calculated and compared with the results reported from various carbonaceous materials available in the literature. Promising adsorption selectivity (5.5) between CH4 and N2 is obtained at 303 K in the binary breakthrough experiments, with a total pressure of 100 kPa and yCH4 = 0.5 in the feed, which makes the adsorbent used in this work a promising candidate for methane enrichment from coalbed methane gas (CH4/N2).

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ρ −ΔHi0

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

All experimental data reported in this work. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00321. (PDF)

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volume of adsorbed phase, m3 volume of the solid adsorbent, m3 volume of the cell where the adsorbent is located, m3 selectivity the molar fraction of component i in adsorbed phase the molar fraction of component i in gas phase spreading pressure in adsorbed phase, kPa·m spreading pressure of component i in adsorbed phase, kPa·m gas density, kg·m−3 isosteric heat of adsorption of component i, kJ·mol−1

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-021-64250981. Fax: +86-021-64250981. Funding

Authors would like to acknowledgement the finance support from MOST of China under the Program of International S&T Cooperation (2015DFG42220). China Postdoctoral Science Foundation funded project (2015M570339). This work is also financed by FCT of Portugal under the Cooperation Project FCT/CHINA 441.00. Notes

The authors declare no competing financial interest.

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NOMENCLATURE A surface area of adsorbent, m2 ERR mean relative error Ki adsorption equilibrium constant of component i, kPa−1 K0,i adsorption equilibrium constant of component i at infinite temperature, kPa−1 mabs absolute amount adsorbed, kg mex excess amount adsorbed, kg Mi molecular weight of compound i, g·mol−1 ni isotherm parameter on nonuniform surfaces N number of experimental points p total pressure, kPa qcal calculated amount adsorbed, mol·kg−1 qexp experimental amount adsorbed, mol·kg−1 qi adsorbed amount of component i, mol·kg−1 qm,i saturation capacity of component i, mol·kg−1 qtotal total adsorbed amount, g·kg−1 R ideal gas constant, J·mol−1·K−1 T experimental temperature, K 2692



Article

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Adsorption of Pure and Binary CO2, CH4, and N2 Gas Components

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