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Separations
CHF3-CHClF2 Binary Competitive Adsorption Equilibria in Graphitic Slit Pores: Monte Carlo Simulations and Breakthrough Curve Experiments Qiang Fu, Hideki Tanaka, Minoru T. Miyahara, Yingjie Qin, Yuanhui Shen, and Donghui Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00141 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018
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Industrial & Engineering Chemistry Research
CHF3-CHClF2 Binary Competitive Adsorption Equilibria in Graphitic Slit Pores: Monte Carlo Simulations and Breakthrough Curve Experiments Qiang Fu,† Hideki Tanaka,*‡ Minoru T. Miyahara,‡ Yingjie Qin,† Yuanhui Shen,† DonghuiZhang*† Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Department of Chemical Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 606-8501, Japan
Abstract The separation and recovery of the greenhouse gases CHF3 and CHClF2 from emissions is essential to avoid adverse environmental effects. Nanoporous carbons are excellent candidates for the capture of CHF3. However, little research has been done on the effect of pore size on the adsorption capacity and selectivity of CHClF2 over CHF3 in nanoporous carbon materials. The effect of the pore size on the competitive adsorption of a binary CHF3–CHClF2 mixture in graphitic slit pores has been investigated using a combination of Monte Carlo simulations and breakthrough curve experiments over a wide range of pressures and temperatures. The entropy changes have a considerable effect on selectivity of CHClF2 over CHF3 as the size of the adsorbate molecule being close to the effective height of the pores, conversely, adsorption energy has a significantly impacts. The selectivity of CHClF2 over CHF3 at 0.05–1.0 MPa increases in the order of 19.47 < 16.62 < 13.20 < 11.77 < 8.30 < 5.17 < 5.67 Å. The mechanisms for adsorption and the selectivity of CHClF2 over CHF3 were determined as well as the effects of pore size and pressure on adsorption energy and adsorption entropy. This combination of methods provides an effective strategy for the design and screening of adsorbent materials for CHF3–CHClF2 separation and recovery applications.
Keywords: Grand Canonical Monte Carlo, CHClF2 and CHF3 capture, adsorption enthalpy and entropy, breakthrough curve experiment * E-mail address:
[email protected] * E-mail address:
[email protected] (D.-H. Zhang). † Tianjin University. ‡ Kyoto University.
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1. Introduction Trifluoromethane is a potent greenhouse gas regulated under the Kyoto Protocol with an atmospheric lifetime of 270 years and a global warming potential of 14,800 over 100 years.1-4 CHF3 is an unavoidable byproduct in the production of chlorodifluoromethane which is used in air conditioning, commercial refrigeration, extruded polystyrene foams, and as a feedstock in pentafluoroethane and fluoropolymer manufacturing.5, 6 The Clean Development Mechanism of the Kyoto Protocol has stipulated that the emission of CHF3 in fluorine chemical plants should be reduced as much as possible.1 Therefore, the separation and recovery of CHF3 and CHClF2 from emissions has become a very important area of research. Adsorption separation technology is an effective and desirable method for CHF3–CHClF2 separation because of its low energy costs and there are no hazardous by-products.7, 8 Adsorbent materials with high adsorption capacities and excellent selectivities are essential for adsorption separation process. A wide diversity of adsorbent materials, such as zeolites, ionic liquids, activated carbons, and metal organic frameworks (MOFs), have been designed and synthesized for potential use in the capture of CHF3. Among these materials, nanoporous carbons have been shown to be competitive candidates by virtue of their high surface areas and other unique properties.9-11 In our previous work, a new clean and energy efficient hybrid system of nanoporous carbons was developed to separate and recover CHF3 and CHClF2.12 However, studies on the effects of the pore size and of temperature and pressure on the adsorption capacity and selectivity of CHClF2 over CHF3 in nanoporous carbons are limited. Several models using activated carbon have been developed to illustrate the adsorption behavior of liquids in carbon. For example, Fomin studied the behavior of benzene in graphite and amorphous slit pores and showed that different pore structures greatly affected the behavior of benzene.13 Kojima et al. investigated the adsorption of cyclohexene on sp2 and sp3 hybridized carbon atoms in graphitic slit pores using Grand Canonical Monte Carlo (GCMC) simulations.14 The cyclohexene molecules preferentially adsorbed on the sp2 carbon atoms. Other researchers have used both molecular simulations and experimental studies to investigate the adsorption behavior of CH4, CO2, Ar, and N2 on graphite and in graphitic slit pores.15-20 These studies used a graphite slit pore model and the simulation results were in good agreement with the experimental data indicating that graphite slit pores are an effective porous carbon model. Consequently this model has been extensively adopted for the study of gas capture and separation. In this paper, the effects of pore size on the competitive adsorption of a binary CHF3–CHClF2 mixture in graphitic slit pores were investigated over a wide range of pressures and temperatures. GCMC simulations and breakthrough curve experiments were carried out to predict the thermodynamic equilibrium properties of both single-component CHF3 and CHClF2 gases and
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their binary mixtures in graphitic slit pores. The mechanisms of adsorption and the selectivity of CHClF2 over CHF3 are discussed as well as the effects of pore size and pressure on adsorption energy and adsorption entropy. 2. Methodology 2.1 Graphitic slit pore model for activated carbon A graphite slit-shaped pore was constructed using the interface between two semi-infinite graphite layers. The carbon molecule wall interactions were approximated using the 12-6 Lennard-Jones (LJ) potential.21, 22 The graphitic slit porous carbons have lateral dimensions of 29.5229.82 Å in the x and y directions and various slit widths (H) in the z direction. Various graphite slit pore sizes (H = 8.52, 9.02, 11.65, 15.12, 16.55, 19.97 or 22.82 Å) were used to investigate the effect of the slit pore size. The slit pore size was calculated as the center-to-center distance between carbon atoms in the innermost graphite layers. The distance between the inner graphite sheets is denoted as Heff (effective pore width). For the graphite slit pores, Heff was calculated using: H H－0.335nm
(1)
Each slit wall consists of a single layer of immobile graphite. In each layer, the carbon atoms are arranged in a honeycomb lattice with a separation of 1.42 Å. To simulate an infinite slit surface, periodic boundary conditions were adopted by replicating the simulation domain along the x and y directions. The carbon LJ parameters for εss/k and σss, are 28 K and 3.4 Å respectively (εss, σss are the potential well depth and collision diameter of carbon, respectively). The 3D periodic pore geometry with an upper and lower single graphite layer was represented as the pore wall as shown in Figure 1.
Figure 1 Molecular simulation unit cell of the pore system for CHF3–CHClF2 adsorption. Color code: gray, C; white, H; green, Cl; blue, F. 2.2 Force field potentials CHClF2 and CHF3 were modeled as rigid tetrahedral molecules with five charged interaction sites. The intermolecular interaction potential energies between two fluid molecules were taken as the sum of a short-range term, using the 12-6 LJ potential, and a long-range Coulombic term. The total adsorbate-adsorbent interaction energies were also assumed to be governed by the 12-6 LJ 3
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model.
ϕ 4ε σ
− (2) σ
where σij and εij are the collision diameter and potential well depth, respectively, and rij is the distance between sites i and j. Coulombic potentials (ϕ ) were calculated using Coulomb’s law: # #
ϕ !πε
"
(3)
where ε0 is the permitivity of free space, qi denotes the atomic charge on site i, and rij is the distance between sites i and j. The LJ potential parameters used for the intermolecular interactions of CHF3 and CHClF2 adsorption22 are shown in Table 1, and the atomic partial charges of the CHF3 and CHClF2 sorbates are provided in Table 2. Cross-interactions with other molecules and the framework were computed using Lorentz−Berthelot mixture rules, i.e., εij = (εii﹒εjj)1/2, σij = (σii+σjj)/2. Table 1 Intermolecular interaction parameters for CHF3 and CHClF2 adsorption
σ(Å)
ε/kB (k)
C…C
3.341
25.86
C…Cl
3.377
55.65
C…F
3.020
13.91
C…H
2.993
26.73
Cl…Cl
3.403
119.80
Cl…H
3.020
57.53
Cl…F
3.296
34.67
F…F
2.691
29.70
F…H
2.664
14.38
H…H
2.637
27.63
Table 2 Estimated partial atomic charges for CHF3 and CHClF2 sorbates23, 24 Charges sorbate C
F
Cl
CHF3
0.719
-0.245
CHClF2
0.4642
-0.205
H 0.016
-0.0412
-0.013
2.3 Simulation techniques The Peng-Robinson equation of state was chosen to calculate the gas–phase densities and experimental fugacities.25, 26 A series of Monte Carlo (MC) simulations in the grand-canonical ensemble (or µVT ensemble) were carried out to estimate the adsorption isotherms and the 4
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selectivity of state point at temperatures of 298, 303 and 323 K for pressures ranging from 0 to 1.0 MPa.27-29 A cutoff radius of 12 Å was applied and tail corrections were used. The simulation box was a 111 rigid unit cell with periodic boundary conditions to ensure that the distance of the two periodic images was at least twice the cutoff radius. At each GCMC step, an attempt was made to displace, regrow, rotate, insert, or remove a randomly chosen fluorocarbon chain. For every state and graphite slit pore size, 1.5×107 GCMC configurations were generated. The first 5×106 configurations were discarded to guarantee equilibrium and then the latter 1.0×107 configurations were averaged to obtain the desired thermodynamic properties. 2.4 Enthalpy and entropy of adsorption During an adsorption process, molecules were removed from the reference state and transported into the carbon pores. It is important to have a clear definition of the reference state. In this work, the gas phase in chemical equilibrium with the adsorbed phase was used as the reference state for the calculations of the enthalpies and entropies of adsorption. The changes in enthalpy, free energy and entropy during adsorption were calculated using:30, 31
∆H ,&'( ∆U ,&'( − RT ∆G ,&'( R - T ln
∆S ,&'(
p0 p
∆H ,&'( − ∆G ,&'( T
(4) (5) (6)
where ∆H ,&'( is the enthalpy of adsorption of component i, ∆G ,&'( is the total Gibbs free
energy of component i, ∆S ,&'( is the entropy of adsorption of component i. ∆U ,&'( is the total internal energy change of the system, due to the adsorption of an additional molecule of component i, R is the ideal gas constant, and p0 is the standard pressure, which was taken as 1.0 atm. The heat of adsorption is also given by eq. (7). The change in the total internal energy of the graphitic split pores and adsorbates upon the adsorption of an additional molecule can be estimated by eq.(8). Using a biased Widom’s test particle method with the Rosenbluth algorithm,
〈U〉45,46 ∙∙∙,849:5;
