Prediction of the monocomponent adsorption of H2S and mixtures with CO2 and CH4 on activated carbons

Prediction of the monocomponent adsorption of H2S and mixtures with CO2 and CH4 on activated carbons

Accepted Manuscript Title: Prediction of the monocomponent adsorption of H2 S and mixtures with CO2 and CH4 on activated carbons Authors: Daniel V. Go...

1MB Sizes 0 Downloads 41 Views

Accepted Manuscript Title: Prediction of the monocomponent adsorption of H2 S and mixtures with CO2 and CH4 on activated carbons Authors: Daniel V. Gonc¸alves, Mayara A.G. Paiva, Jos´e C.A. Oliveira, Moises Bastos-Neto, Sebasti˜ao M.P. Lucena PII: DOI: Reference:

S0927-7757(18)31241-X https://doi.org/10.1016/j.colsurfa.2018.09.082 COLSUA 22882

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

22-6-2018 24-9-2018 27-9-2018

Please cite this article as: Gonc¸alves DV, Paiva MAG, Oliveira JCA, Bastos-Neto M, Lucena SMP, Prediction of the monocomponent adsorption of H2 S and mixtures with CO2 and CH4 on activated carbons, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2018), https://doi.org/10.1016/j.colsurfa.2018.09.082 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Prediction of the monocomponent adsorption of H2S and mixtures with CO2 and CH4 on activated carbons Daniel V. Gonçalves, Mayara A. G. Paiva, José C. A. Oliveira, Moises Bastos-Neto, Sebastião M. P. Lucena*

IP T

(*) Corresponding author

TE D

M

A

N

U

TOC/abstract graphic

SC R

Dept. Eng. Química, Grupo de Pesquisa em Separações por Adsorção – GPSA, Universidade Federal do Ceará, Campus do Pici, Bl. 709, 60455-760, Fortaleza, CE, Brazil. [email protected]

Abstract

Monocomponent adsorption of hydrogen sulfide and multicomponent adsorption with CO2

EP

and CH4 mixtures were predicted in two samples of commercial activated carbons. We perform Monte Carlo calculations in the grand canonical ensemble in representative slit-pores

CC

of each carbon material and proposed a new energy parameter for H2S-carbon interaction from experimental adsorption heat in selected activated carbons. Further analysis of adsorption in representative pores demonstrated the importance of ultramicropores in H2S

A

retention and a cooperative effect of CO2 molecules favoring H2S adsorption. To the best of our knowledge this is the first time that a method based on molecular simulation is proposed to predict H2S adsorption on carbon. This study contributes to a better understanding of the role of pore size distribution and adsorption regimes in carbon materials.

1

Keywords: Adsorption; Carbon; Molecular simulation; H2S; CO2; Characterization.

1. Introduction

H2S is an extremely toxic gas. The toxicity threshold of H2S is only 10 ppm and exposure to concentrations around 300 ppm for 30 minutes is sufficient to cause

IP T

unconsciousness [1]. H2S occurs naturally during biogas production, it is also associated with oil and natural gas reservoirs. Residual concentrations of H2S in intermediate streams are

SC R

undesirable in the petrochemical industry. The renewed interest in the use of biogas for

automotive purposes [2,3] should stimulate research in beneficiation processes where H2S must be adequately removed. Activated carbons are used to capture H2S at concentrations of 1000 ppm at ambient temperature [4]. These activated carbons can be impregnated with

U

active species (NaOH, Fe2O3, K2O) [5] or used without impregnation [6].

N

The general use of activated carbon is related to the carbon relative low price and

A

mild operating conditions of the retention process, however due to the difficulty of characterizing the activated carbon, which has amorphous structure, the influence of the

M

pores and the role of each mechanism (adsorption, chemisorption) are not yet fully understood [4,6]. This theoretical lack of knowledge, coupled with H2S toxicity, complicated

TE D

the interpretation and performance of experiments, therefore the selection of activated carbon for this application is made through a laborious semi-empirical process where numerous samples need to be tested in the laboratory. The wide range of concentrations and operating

EP

conditions where H2S is present also make difficult the selection of viable adsorbents. These experimental difficulties involving H2S adsorption research make this system ideal to be studied through molecular simulation techniques.

CC

To predict the adsorbed amount of a gas on activated carbon one need to calculate

a sequence of local isotherms involving a range of pores ranging from 7 to 100 Å [7]. This is

A

precisely the standard method for determining the pore size distribution of activated carbon from experimental isotherms of N2 at 77 K or CO2 at 273 K. We propose [8] that the calculations of adsorbed amounts of C1 to C4 in the range of low gas concentrations (up to 0.4 P/P0) could be made with a smaller number of pores if each selected pore represented the different adsorption patterns of pore filing (one, two and three or more layers of gas) normally encountered when the gas adsorbs in slit-pores [9]. A similar procedure had been previously proposed by Do et al [10] in the reproduction of heats of adsorption of Ar in 2

activated carbons. This same approach also allowed obtaining maximum amounts of activated carbon adsorption of dye molecules with unknown vapor pressure using the NVT ensemble [11]. To the best of our knowledge, there is no method based on molecular simulation to predict the adsorption of H2S in carbonaceous materials. As a contribution to the better understanding of the role of the pores and the H2S retention regime in carbon materials, we used H2S monocomponent experimental isotherm data to validate our representative pore

IP T

methodology to predict H2S adsorption by molecular simulation. We propose a new interaction parameter H2S/activated carbon and apply the methodology in two groups of

SC R

materials with different pore distributions. Multicomponent isotherms with CH4 and CO2 are

also presented where an interesting improvement effect of H2S adsorption by CO2 is also observed.

N

U

2. Models and computational methods

A

2.1 Intermolecular interaction

M

The interaction energy between atoms were calculated using 12-6 Lennard-Jones

TE D

potential plus Coulomb potential (Equation 1).

𝜎𝑖𝑗

12

𝑈𝑖𝑗 = 4𝜀𝑖𝑗 [( ) 𝑟

6

𝑞 𝑖 𝑞𝑗

−( ) ]+ 𝑟 4𝜀 𝑖𝑗

0 𝑟𝑖𝑗

(1)

EP

𝑖𝑗

𝜎𝑖𝑗

Where i and j are interacting atoms and rij is the distance between them. εij and σij are the LJ well depth and diameter, respectively. qi and qj are the partial charges of interacting atoms and

CC

0 is the dieletric constant. Lorentz-Berthelot mixing rules were used to calculate the LJ cross

A

parameters.

2.2 Carbon model Slit-pore model was used to represent carbon. Each pore wall is composed by three sheets of graphene with dimensions of 40 Å x 40 Å and interplanar space of 3.35 Å (Figure 1). LJ parameters used for carbon were: σ = 3,4 Å e ε/k = 28 K. Originally, these

3

parameters were proposed to represent the interaction between graphene and N2 [12]. Recently, they were used to predict methane adsorption in slit-pores [13].

a

SC R

IP T

b

U

Figure 1 – (a) Graphene sheet and (b) slit-pore model.

A

N

2.3 Molecular models

M

H2S molecule was represented by the model with four charged sites proposed by Kristof and Liszi [14] (Figure 2a). Atomic charges and LJ parameters for CO2 were taken from TraPPE force field [15] (Figure 2b). TraPPE-UA force field [16] parameters were used

TE D

for methane, ethane and propane molecules (Figures 2c, 2d and 2e). Helium was represented by the united-atom model proposed by Maitland et al. [17] (Figure 2f). These models are known to accurately describe the vapor-liquid coexistence curves of the molecules. Force

A

CC

EP

field parameters and atomic charges for all molecules are presented in Table 1.

4

a

b

c

e

IP T

d

U

SC R

f

N

Figure 2 – Molecular models: (a) H2S, (b) CO2, (c) CH4, (d) C2H6, (e) C3H8 (f) He. Carbon in

A

gray, oxygen in red, sulfur in yellow, hydrogen in white, dummy atom of H2S model in green

M

and helium in light blue. Table 1 – Lennard-Jones parameters and atomic charges of all adsorbates. σ, Å

ε/k, K

q, e-

H

-

-

0.25

S

3.73

250

0.4

Dummy

-

-

-0,9

CH4

3.73

148

-

CH3

3.75

98

-

CH2

3.95

46

-

C

2.8

27

0.7

O

3.05

79

-0.35

He

2.28

10.2

-

A

CC

EP

TE D

Atom/Pseudoatoms

5

2.4 Simulation details Adsorption properties were calculated using Grand Canonical Monte Carlo method with RASPA 2.0 code [18]. A truncated LJ potential without tail correction was used. Periodic boundary conditions and a cutoff of 16 Å were applied. Ewald sum was used to compute electrostatic interactions. At least 4 x 104 Monte Carlo cycles were used. Each cycle

IP T

contains N Monte Carlo steps, where N is the number of molecules in the system (with N ≥

20). All simulations included random insertion, deletion, rotation and translation moves of

SC R

guest molecules with equal probabilities. During the mixture calculations, the number of cycles was triplicated and an identity move was added. In this move, a molecule A is replaced by a molecule B. Heats of adsorption were estimated using the fluctuation method during the

U

GCMC simulations [19].

N

3. Results and discussion

M

A

3.1 H2S-carbon interaction

To be validated, the proposed Lennard-Jones parameters between slit-pore walls

TE D

and the H2S molecule must reproduce the heat of adsorption on a graphite surface. Unfortunately, there is no experimental data for H2S adsorption on graphite. The adsorption equilibrium data of H2S present in the literature are related to activated carbon [20], which in turn are amorphous and have a pore size distribution (PSD).

EP

Bagreev et al. [20] obtained adsorption heats of H2S at low coverage with good precision in a series with three activated carbon species (W, N and S) by reverse phase gas

CC

chromatography. They used a mixture with 0.5% H2S in N2, which is equivalent to a partial pressure of 500 Pa of H2S, and measured values ranging from 39 to 46 kJ/mol for adsorption

A

heat. Since the experiments were performed at low coverage, the values collected are associated to the filling of the smaller micropores. The smallest pore size raised by the PSDs of N2 on the activated carbon W, N and S was 7Å (measured distance from center to center and that is approximately 4 Å effective). Our hypothesis is that the pore of 7 Å is responsible for almost all of the adsorption heat measured experimentally at low coverage (500 Pa). Thus, we take the 7 Å pore and calculate the adsorption heat values at 298 K at pressures between 10 and 1000 Pa (Figure 3). We established a target value of 40 kJ/mol measured in the W 6

carbon series by its PSD to be similar to the activated carbons normally used in the industry and because it is twice the value measured in carbon black, a low porosity carbon [6]. Using the original εss parameter of Steele in conjunction with the parameter εff proposed by Kristof (εsf = 83.7 K), we estimated the heat of adsorption at 500 Pa as 30 kJ/mol, thus below the target value of 40 kJ/mol. This means that even assuming that the most energetic pore identified in the PSD (7 Å) is responsible for all adsorption heat, it is not possible to reach 40 kJ/mol using the original Steele energy parameter. This indicates that the

IP T

interaction of H2S with the graphene surface is underestimated. Therefore, we test increasing values of the parameter εsf. When we apply a 15% increase to the original value (εsf = 96.2 K)

SC R

we reach the target value of 40 kJ/mol. All the calculations referring to H2S were performed

TE D

M

A

N

U

with this new εsf parameter.

EP

Figure 3 – Heats of adsorption of H2S at 298 K in 7 Å (center to center) micropore. Original H2S-carbon parameter (εsf = 83.7 K) and increased values of εsf were applied. Dashed lines

A

CC

show the target value of 40 kJ/mol at 500 Pa.

7

3.2 Monocomponent Adsorption 3.2.1. Adsorption isotherm The following describes in detail how, from the parameters defined in the previous section, we calculated the H2S in the experimental isotherm of the RB4 activated carbon [21]. As discussed before, the complete pore size distribution (PSD) of an activated

IP T

carbon can be approximated by a limited number of representative pores to allow reproduction of the adsorption isotherms.

SC R

Cruz et al. [21] reported H2S adsorption isotherms at 298 K on various adsorbents, including the activated carbon series RB1, RB3 and RB4. In this study, we

selected the experimental isotherm of H2S in carbon RB4 because it was already carefully characterized (N2 at 77 K and CO2 at 273 K) by our group (Figure 4). From the experimental

U

isotherm of N2 at 77 K, we estimated the pore size distribution using a dedicated Monte Carlo

N

kernel [22] (Figure 5a). As RB4 carbon is a microporous material, we performed an additional characterization with CO2 at 273 K [23] to evaluate the existence of

A

ultramicropores (pore size < 7 Å). The characterization with CO2 did not indicate the

M

presence of ultramicropores (Figure 5a), even so, for greater precision of our analysis, we will use the volume measured by CO2 up to the 8 Å pore. Above this pore, we used the values

A

CC

EP

TE D

determined by the N2 isotherm at 77 K. (Figure 5).

Figure 4 – Experimental adsorption isotherms in carbon RB4: (a) N2 at 77 K e (b) CO2 at 273 K.

8

(a)

-1

0.06 0.04 0.02 0.00

0

10

20

30

40 Pore size / Å

50

-1

0.10 0.08 0.06 0.04 0.02 0.00

60

7Å 8.9Å 18.5Å 27.9Å

(b)

0.12

3

0.08

PSD CO2

PSD / cm .g .Å

-1

0.10

3

PSD / cm .g .Å

-1

0.12

0.14

PSD N2

0

10

20

30

40

50

60

IP T

0.14

Pore size / Å

SC R

Figure 5– (a) PSDs determined by N2 at 77 K and CO2 at 273 K. (b) Representative pores of activated carbon RB4.

U

According to the methodology of the representative pores, the PSD of the activated carbon RB4 will be approximated by three pores (8.9, 18.5 and 27.9 Å) extracted

N

from the volumes measured by the N2 isotherm plus one pore (7 Å) extracted from the

A

volume measured by CO2. Thus, the carbon RB4 will be represented by four pores with

M

respective volumes discriminated in Table 2. The next step is to calculate the isotherms for each individual pore. We simulated adsorption isotherms of H2S at 298 K from 0.1 to 10 kPa

TE D

in the four representative pores (Figure 6).

Table 2 – Pore volumes of carbon RB4 predicted by N2 and CO2 PSDs. PSD

8.9 Å, cm3/g

18.5 Å, cm3/g

27.9 Å, cm3/g

-

0.1662

0.2564

0.1330

0.01787

-

-

-

EP

N2 at 77 K

7 Å, cm3/g

A

CC

CO2 at 273 K

9

IP T SC R

Figure 6– Adsorption isotherms of H2S at 298 K in slit-pores. Inset figure shows Y-axis in log scale.

U

One aspect that draws attention is the large adsorption difference between the

N

pore of 7 Å and the other pores. This large capacity of H2S immobilization of the smaller pores must be directly linked to the efficiency of the carbonaceous materials in the retention

A

of H2S in low concentrations.

M

From the simulated isotherms of the individual pores it is possible to predict the adsorption isotherm on activated carbon RB4. For each pressure, the total adsorbed amount

TE D

(Qtotal) in the carbon RB4 is determined by the sum of the products of the volume of each pore (Vp) by the amount adsorbed in each pore (qp) (Equation 2): (2)

EP

𝑄𝑡𝑜𝑡𝑎𝑙 = ∑𝑚 𝑚=1 𝑉𝑝𝑚 𝑞𝑝𝑚

Figure 7 shows the theoretical isotherms of H2S at 298 K on RB4 using the four

CC

significant pores. For comparison, the estimated isotherm with the original H2S-carbon interaction value (εsf = 83.7 K) proposed by Steele is also presented. While the isotherm with

A

the original H2S-carbon energy parameter considerably underestimates the adsorbed quantities, the calculation with the proposed εsf reproduces the experimental isotherm up to the pressure of 4 kPa, being more accurate in the lower pressures (up to 2.5 kPa), exactly the range where we will find the majority of H2S gaseous mixtures of industrial interest. The discrepancy beyond 4 kPa can be related to the homogeneous slit-pore models used in the representation of these materials, a known source of discrepancies between experimental and

10

simulated isotherms on activated carbons [24]. The successful reproduction of the RB4

U

SC R

IP T

carbon isotherm is an excellent validation test of the new proposed energy parameter.

N

Figure 7 – Experimental and estimated isotherms of H2S at 298 K in carbon RB4. Original

A

H2S-carbon parameter (εsf = 83.7 K) and our proposed value (1.15εsf) were used.

M

Analysis in high-pressure range is also possible to compare with existing data in MOF and carbon nanotube [25]. We calculated the adsorption of pure H2S on each representative

TE D

pore and found that the uptake on carbon RB4 at 1600 kPa and 298 K is 12 mmol/g, of the order of magnitude of the (20, 20) single-walled carbon nanotube (16 mmol/g).

EP

3.2.2. Breakthrough curves

Many of the tests carried out for the selection of adsorbents to capture H2S

CC

consist of breakthrough curves. Next, we apply the same previous methodology in predicting the adsorbed value estimated by this type of experiment.

A

Recently, the H2S retention capacity at 298 K on the ultramicroporous carbon

Desorex K43 was performed by our group using breakthrough curves [26]. Activated carbon K43 was carefully characterized (N2 at 77 K and CO2 at 273 K) and the H2S retention capacity was measured at two concentrations: 100 and 200 ppm. This is an excellent opportunity to apply the methodology because we have the detailed characterization and two values of H2S concentration.

11

Characterization with N2 indicates that, unlike RB4, K43 carbon has a narrow PSD with a mean pore size of around 9 Å associated with a reasonable volume of ultramicropores indicated by the characterization with CO2 (Figure 8).

0.10 PSD CO2

0.08

IP T

0.06 0.04 0.02

0 2 4 6 8 10 12 14 16 18 20 22 Pore size / Å

U

0.00

SC R

3

-1

PSD / cm .g .Å

-1

PSD N2

A

N

Figure 8 – Calculated PSDs of carbon Desorex K43: N2 at 77 K and CO2 at 273 K.

M

Since K43 is strictly microporous, it can be modeled with only two representative pores: 7 and 8.9 Å. The pore volume of 7 Å (0.0207 cm3/g) was taken from the

TE D

ultramicroporous volume of the CO2 PSD at 273 K. The volume calculated by PSD from N2 to 77 K was integrally assigned to the 8.9 Å pore (0.37 cm3/g). Using the same force field parameters, we computed adsorption at concentrations of 100 and 200 ppm of H2S (diluted in He) at 298 K and at 1 bar in the two representative

EP

pores of Desorex K43 (Table 3). Using the values of the pore volumes and the adsorbed quantities in Equation 2 we determined the theoretical adsorbed amounts of H2S in Desorex

CC

K43. The carbon adsorbs 0.20 mg/g and 0.40 mg/g of H2S, for mixtures with 100 and 200 ppm, respectively. These values are similar to those measured experimentally: 0.34 mg/g and 0.51 mg/g. It is also important to note that Menezes et al. [26] also performed tests on the

A

regeneration of the Desorex K43 sample. The authors observed that, from the second regeneration cycle, the amount of H2S retained dropped from 0.34 mg/g to 0.2 mg/g, which corresponds to the theoretical value of the physical adsorption we estimate.

12

Table 3 – Adsorption of H2S in 7 and 8.9 Å pores: 100 and 200 ppm. 100 ppm of H2S

200 ppm of H2S

Property 8.9 Å



8.9 Å

Theoretical adsorbed amount, mmol/cm3

0.1621

0.0068

0.3253

0.0141

qpore, mmol/g

0.0033

0.0025

0.0067

0.2

Experimental, mg/g

0.34

0.052

0.4

SC R

Qtotal, mg/g

IP T



0.51

N

U

3.3 Multicomponent Adsorption

Hydrogen sulphide always occurs naturally in mixtures from various sources such

A

as crude oil, biogas and natural gas. In biogas, H2S is mainly associated with CH4 and CO2.

M

The concentration of methane in the biogas varies between 60% and 70%, the concentration of CO2 varies between 30% and 40% and the concentration of H2S can reach up to 0.4% [27].

TE D

Once the H2S force field parameters have been validated, an interesting application would be to evaluate the impact of the presence of CH4 and CO2 from biogas streams in the H2S adsorption on activated carbon. In this evaluation, we will focus on the final phase of the biogas desulfurization (polishing), considering gas mixtures with 200 ppm H2S.

EP

We simulated the adsorption of two mixtures: one with 70% methane + 200 ppm

H2S and another with 40% CO2 + 200 ppm H2S. The remaining percentage of each mixture

CC

up to 1 bar was complemented with helium gas. We compared these results to adsorption of H2S diluted in He at an equivalent partial pressure (20 Pa) and calculated the impact (increase

A

or decrease) on the adsorbed amount of H2S (Table 4).

13

Table 4 – Adsorption of 200 ppm of H2S at 1 bar and 298 K in slit-pores: 7, 8.9 and 18.5 Å. Pore, Å 7 8.9 18.5

Adsorbed amount of H2S, mmol/cm3 With 40% of CO2 (% Diluted in He With 70% of CH4 difference) 3.27E-01 2.25E-01 1.36E-01 (-58%) 1.36E-02 1.36E-02 1.95E-02 (+43%) 4.67E-04 4.79E-04 4.90E-04

IP T

The presence of CH4 (70%) or CO2 (40%) in the mixture reduced H2S adsorption in the 7Å pore. The adsorbed amount of pure H2S (0.37 mmol/cm3) decreases to 0.22 and

SC R

0.13 in the presence of CH4 and CO2 respectively. The CO2 molecules interfere more with adsorption of H2S, although it is in a lower concentration, because its adsorption heat is higher than that of CH4 (29 versus 23 kJ/mol - Table 05).

U

Table 5 – Theoretical heats of adsorption calculated at 10 Pa in representative pores. Values

N

for heavier components of natural gas (ethane and propane) are also presented. CO2

Methane

Ethane

Propane

7

33.92

29.87

23.29

36.13

47.6

8.9

23.2

18.93

16.42

25.37

33.33

18.5

17.11

14.87

11.21

16.99

21.05

TE D

M

H2S

A

Heat of adsorption, kJ/mol

Pore, Å

In the 8.9 Å pore, both adsorption heats decreased and CH4 no longer interferes

EP

with H2S adsorption. However, for CO2 an unexpected result occurs. Surprisingly, we noticed an increase in H2S adsorption in the presence of CO2. The 8.9 Å pore adsorbed 0.0136 mmol/cm3 of H2S diluted in He (200 ppm) increasing to 0.0195 mmol/cm3 for the

CC

mixture with 40% CO2. This corresponds to a 43% increase in H2S adsorption. This effect has an impact on the commercial activated carbons because pores in the size range of 9 Å are

A

present in a considerable volume in the samples used for H2S retention. This is an interesting phenomenon where adsorbates that normally compete with each other, promotes the adsorption of one of the species. Nguyen et al. [28] have identified similar phenomena in which benzene molecules promote the adsorption of water on activated carbon. Small concentrations of water promote the adsorption of CO2 in the Cu-BTC metal-organic structure [29]. The presence of CO2 in the silicalite zeolite considerably increases the

14

diffusivity of C7+ n-paraffins by improving the separation between the long chain n-paraffins and those of smaller chains (C4 to C6) [30]. An explanation for the cooperative behavior observed in the 8.9 Å pore can be obtained by analyzing the H2S potential within the pore. We calculated the energy distribution of a molecule of H2S within the 8.9 Å pore without the presence of CO2 molecules and compared to the pore distribution containing 24 molecules of CO2. Calculations were performed through the canonical ensemble and the number of molecules

IP T

inserted corresponded to the amount adsorbed in the mixture with 40% CO2 which was 4.9

mmol/cm3. The two calculated distributions are present in Figure 9. The presence of CO2

SC R

causes a widening of the energy range towards much stronger values (up to -7 kcal/mol). This

increase in energy within the porosity comes from the fluid-fluid interaction between H2S and CO2. At concentration of 200 ppm without CO2, the number of H2S molecules is very small and the total adsorption energy is basically the solid-fluid component. At 200 ppm of H2S and

U

40% of CO2 in the flue gas, the extra 24 molecules of CO2 provided a fluid-fluid potential

N

component that added to the solid-fluid potential result in increase of the total adsorption energy of H2S. The radial distribution of the distance between the S of the H2S and the C of

A

the CO2 evidence such interaction between the two molecules inside the pore (Figure 10).

M

The molecule of H2S is positioned at a mean distance of 4 Å of the CO2 molecules. A

A

CC

EP

TE D

characteristic image of the system presents this positioning clearly (Figure 11).

Figure 9 – Energy distribution of a single H2S molecule in 8.9 Å pore.

15

IP T SC R U

N

Figure 10 – Radial distribution of the distance between the S of the H2S and the C of the CO2

A

CC

EP

TE D

M

A

in 8.9 Å pore.

16

b

SC R

IP T

a

EP

TE D

M

A

N

U

c

Figure 11– Typical image of the adsorption of 200 ppm of H2S with 40% of CO2 in 8.9 Å

CC

pore. Helium molecules, that compose the remain fraction of the mixture, were removed only for better viewing. (a) Perspective view (b) Upper view (c) Detail of upper view with distances (in Å) between de H2S and CO2. Hydrogen in white, sulfur in yellow, carbon in

A

grey and oxygen in red. In the larger pore of 18.5 Å, we observed slight increases in H2S adsorption in the presence of CH4 and CO2. This pore adsorbed 4.67 x 10-4 mmol/cm3 in the presence of only H2S and adsorbs 4.79 x 10-4 mmol/cm3 and 4.90 x 10-4 mmol/cm3 in the presence of CH4 and CO2, respectively. These values correspond to small increments of 3% and 5% in H2S

17

adsorption. In the case of the mixture with CH4, the observed increase is very close to the error of the simulations, which is 2.5%. With the reduction of the solid-fluid potential, CO2 no longer contributes with increases in the adsorption of H2S mainly because the adsorbed quantity also decreases, being only 3.24 x 10-1 mmol/cm3 of CO2 (which is equivalent to 4 molecules of CO2 against 24 in the pore of 8.9 Å) and, therefore, insufficient to exert some relevant interaction effect in the H2S molecules. Turning our attention again to the 8.9 Å pore, in order to verify the influence of

IP T

CO2 concentration on the adsorbed amount of H2S, we gradually decrease the CO2 percentage in the mixture (up to 5%) and calculate the adsorption of H2S (Figure 12). For the

SC R

purpose of comparison, we performed the same calculation in the mixture with CH4. The adsorbed amount of the 200 ppm H2S diluted mixture is also shown. We find a linear relationship between the adsorbed amount of H2S and the percentage of CO2 in the mixture. The CO2 concentration in the flue gas needs to be greater than 5% to improve H2S adsorption

U

in the 8.9 Å pore. In the case of the CH4 mixture, for the entire CH4 concentration range

N

evaluated, there is no improvement in H2S adsorption. The fluid-fluid contribution between

A

CC

EP

TE D

M

A

H2S and CH4 is not enough to increase H2S adsorption.

Figure 12 – Variation of the adsorbed amount of H2S with the percentage of CO2 or CH4 in the mixture in 8.9 Å pore. Adsorbed amount of H2S diluted in helium (200 ppm) is also presented - Dashed line.

18

We also verified the simultaneous impact of CO2 and CH4 on the adsorbed amount of H2S in the 8.9 Å pore. We calculated the adsorption of a ternary mixture with 30% CO2 + 60% CH4 + 200 ppm H2S (the remaining percentage was attributed to He) (Table 6). We observed an increase of about 20% in the adsorbed amount of H2S relative to the diluted H2S. However, this increase is lower than that observed in the mixture with 30% CO2 (35%). Although CH4 did not interfere with the adsorption of H2S in the binary mixture calculations, it contributed to the decrease in the adsorbed amount of CO2 (3.5 to 3.2 mmol/cm3) in the

IP T

ternary mixture by a dilution effect of the CH4 molecules.

Diluted in He

With 30% of CO2

SC R

Table 6 – Adsorption of different mixtures with 200 ppm of H2S in 8.9 Å pore. With 30% of CO2 + 60% of CH4

H2S,

CO2,

H2S,

mmol/cm3

mmol/cm3

mmol/cm3

mmol/cm3

1.36 x 10-2

1.84 x 10-2

3.5

1.63 x 10-2

CO2,

CH4,

mmol/cm3

mmol/cm3

3.2

2.17

A

N

U

H2S,

Considering that the balance between solid-fluid and fluid-fluid potential of the

M

CO2 is related to the phenomenon of cooperative adsorption of H2S, we verified if molecules with higher adsorption potential values could also contribute to the effect. This effect is no

TE D

longer observed for adsorption heats in the order of 36 kJ/mol (ethane) or 47 kJ/mol (propane) in the 8.9 Å pore. Since the solid-fluid interaction is stronger, a very large amount of C2 or C3 adsorbs in the 8.9 Å pore occupying all available volume, thus any H2S molecule

EP

is allowed. An increase of H2S can be again observed only for the larger pore of 18.5 Å (Tables 7 and 8) with propane exerting a greater influence in the H2S adsorption through the cooperative effect.

CC

We therefore note that the cooperative effect occurs for specific values of

potentials that favor in each pore an optimal number of molecules (solid-fluid potential

A

dependent) with strong interaction with H2S (fluid-fluid potential dependent) without, however, leading pore obstruction.

19

Table 7 – Adsorption of mixtures with 200 ppm of H2S and different percentages of ethane: 8.9 and 18.5 Å pores. Adsorbed amount of H2S diluted in helium is also shown. 8.9 Å pore Diluted in He

With 10% of C2

With 20% of C2

H2S, mmol/cm3

C2, mmol/cm3

H2S, mmol/cm3

C2, mmol/cm3

1.36 x 10-2

1.12 x 10-2

7.77

7.36 x 10-3

9.34

18.5 Å pore Diluted in He

With 10% of C2

IP T

H2S, mmol/cm3

With 20% of C2

H2S, mmol/cm3

C2, mmol/cm3

H2S, mmol/cm3

4.67 x 10-4

4.94 x 10-4

0.219

4.71 x 10-4

C2, mmol/cm3

SC R

H2S, mmol/cm3

0.443

U

Table 8 – Adsorption of mixtures with 200 ppm of H2S and different percentages of propane:

N

8.9 and 18.5 Å pores. Adsorbed amount of H2S diluted in helium is also shown. With 5% of C3 H2S, mmol/cm3

1.36 x 10-2

2.51 x 10-3

Diluted in He

With 20% of C3

C3, mmol/cm3

H2S, mmol/cm3

C3, mmol/cm3

9.26

1.08 x 10-3

9.95

TE D

H2S, mmol/cm3

M

Diluted in He

A

8.9 Å pore

18.5 Å pore

With 5% of C3

With 20% of C3

H2S, mmol/cm3

C3, mmol/cm3

H2S, mmol/cm3

C3, mmol/cm3

4.67 x 10-4

5.10 x 10-4

0.512

5.58 x 10-4

2.31

A

CC

EP

H2S, mmol/cm3

20

4. Conclusion We present a methodological strategy for the prediction of adsorption of H2S on activated carbons from the use of representative pores. Through molecular simulation, we proposed a value for H2S-carbon interaction in slit-pores from calorimetric adsorption data on activated carbons. We validate this proposed value by estimating H2S adsorption at carbons RB4 and Desorex K43. This is the first theoretical study of this nature present in the

IP T

literature.

We evaluated the impact of the presence of CH4 and CO2 on adsorption of H2S in

SC R

three sizes of activated carbon pores. We found that the presence of CO2 favors H2S adsorption in the 8.9 Å pore. The analysis of the energy distribution and the positioning of the molecules inside the pore evidenced that increase in adsorption. The cooperative effect depends on a specific combination of solid-fluid and fluid-fluid interactions that favor the

U

presence of molecules in the pore (without blocking it) able to attract the CO2.

N

Because there are limitations in the H2S multi-component experiments, the methodology and the proposed H2S-carbon interaction parameter can be used to investigate

A

the different H2S retention regimes and the influence of pore size distribution on this

M

retention. We believe that the presented technique will be useful in the optimization of

TE D

carbonaceous materials suitable for capturing H2S. Acknowledgements

The authors wish to acknowledge financial support for this study from CAPES: PNPD20130251 - 22001018035P0, CNPq: 483911/2011-9 and FUNCAP and the use of the

A

CC

EP

computer cluster at National Laboratory of Scientific Computing (LNCC/MCTI, Brazil).

21

References

[7] [8] [9] [10]

IP T

SC R

CC

[11]

U

[6]

N

[5]

A

[4]

M

[3]

TE D

[2]

A.D. Wiheeb, I.K. Shamsudin, M.A. Ahmad, M.N. Murat, J. Kim, M.R. Othman, Present technologies for hydrogen sulfide removal from gaseous mixtures, Rev. Chem. Eng. 29 (2013) 449–470. doi:10.1515/revce-2013-0017. E.G. Lindfeldt, M. Saxe, M. Magnusson, F. Mohseni, Strategies for a road transport system based on renewable resources - The case of an import-independent Sweden in 2025, Appl. Energy. 87 (2010) 1836–1845. doi:10.1016/j.apenergy.2010.02.011. A. Sanches-Pereira, T. Lönnqvist, M.F. Gómez, S.T. Coelho, L.G. Tudeschini, Is natural gas a backup fuel against shortages of biogas or a threat to the Swedish vision of pursuing a vehicle fleet independent of fossil fuels?, Renew. Energy. 83 (2015) 1187–1199. doi:10.1016/j.renene.2015.06.006. X. Zhang, Y. Tang, S. Qu, J. Da, Z. Hao, H2S-selective catalytic oxidation: Catalysts and processes, ACS Catal. 5 (2015) 1053–1067. doi:10.1021/cs501476p. A. Bagreev, T.J. Bandosz, A role of sodium hydroxide in the process of hydrogen sulfide adsorption/oxidation on caustic-impregnated activated carbons, Ind. Eng. Chem. Res. 41 (2002) 672–679. doi:10.1021/ie010599r. T.J. Bandosz, On the Adsorption/Oxidation of Hydrogen Sulfide on Activated Carbons at Ambient Temperatures, J. Colloid Interface Sci. 246 (2002) 1–20. doi:10.1006/jcis.2001.7952. P.I. Ravikovitch, A. Vishnyakov, R. Russo, A. V. Neimark, Unified Approach to Pore Size Characterization of Microporous Carbonaceous Materials from N2, Ar, and CO2 Adsorption Isotherms, Langmuir. 16 (2000) 2311–2320. doi:10.1021/la991011c. S.M.P. Lucena, V.A. Gomes, D. V. Gonçalves, P.G.M. Mileo, P.F.G. Silvino, Molecular simulation of the accumulation of alkanes from natural gas in carbonaceous materials, Carbon N. Y. 61 (2013) 624–632. doi:10.1016/j.carbon.2013.05.046. V.Y. Gusev, J.A. O’Brien, N.A. Seaton, A Self-Consistent Method for Characterization of Activated Carbons Using Supercritical Adsorption and Grand Canonical Monte Carlo Simulations, Langmuir. 13 (1997) 2815–2821. doi:10.1021/la960421n. D.D. Do, D. Nicholson, H.D. Do, Heat of adsorption and density distribution in slit pores with defective walls: GCMC simulation studies and comparison with experimental data, Appl. Surf. Sci. 253 (2007) 5580–5586. doi:10.1016/j.apsusc.2006.12.057. J.E. Aguiar, J.C.A. de Oliveira, P.F.G. Silvino, J.A. Neto, I.J. Silva, S.M.P. Lucena, Correlation between PSD and adsorption of anionic dyes with different molecular weigths on activated carbon, Colloids Surfaces A Physicochem. Eng. Asp. 496 (2016) 125–131. doi:10.1016/j.colsurfa.2015.09.054. W. Steele, The Interactions of Nitrogen Molecules Adsorbed on Graphite, Le J. Phys. Colloq. 38 (1977) C4-61-C4-68. doi:10.1051/jphyscol:1977410. S.M.P. Lucena, L.F.A. Frutuoso, P.F.G. Silvino, D.C.S. Azevedo, J.P. Toso, G. Zgrablich, C.L. Cavalcante, Molecular simulation of collection of methane isotherms in carbon material using all-atom and united atom models, Colloids Surfaces A Physicochem. Eng. Asp. 357 (2010) 53–60. doi:10.1016/j.colsurfa.2009.12.015. T. Kristóf, J. Liszi, Effective intermolecular potential for fluid hydrogen sulfide, J. Phys. Chem. B. 5647 (1997) 5480–5483. doi:10.1021/jp9707495. J.J. Potoff, J.I. Siepmann, Vapor–liquid equilibria of mixtures containing alkanes, carbon dioxide, and nitrogen, AIChE J. 47 (2001) 1676–1682. doi:10.1002/aic.690470719. M.G. Martin, J.I. Siepmann, Transferable potentials for phase equilibria. 1. United-

EP

[1]

[12]

A

[13]

[14] [15] [16]

22

[24] [25] [26] [27]

IP T

CC

[28]

SC R

[23]

U

[22]

N

[21]

A

[20]

M

[19]

TE D

[18]

EP

[17]

atom description of n-Alkanes, J. Phys. Chem. B. 102 (1998) 2569–2577. doi:10.1021/jp0549125. M.P. Allen, D.J. Tildesley, Computer Simulation of Liquids, Oxford University Press, New York, 1987. doi:10.2307/2938686. D. Dubbeldam, S. Calero, D.E. Ellis, R.Q. Snurr, RASPA: Molecular simulation software for adsorption and diffusion in flexible nanoporous materials, Mol. Simul. 42 (2015) 81–101. doi:10.1080/08927022.2015.1010082. R.Q. Snurr, A.T. Bell, D.N. Theodorou, Prediction of adsorption of aromatic hydrocarbons in silicalite from grand canonical Monte Carlo simulations with biased insertions, J. Phys. Chem. 97 (1993) 13742–13752. doi:10.1021/j100153a051. A. Bagreev, F. Adib, T.J. Bandosz, Initial heats of H2S adsorption on activated carbons: Effect of surface features, J. Colloid Interface Sci. 219 (1999) 327–332. doi:10.1006/jcis.1999.6485. A.J. Cruz, J. Pires, A.P. Carvalho, M.B. De Carvalho, Physical adsorption of H2S related to the conservation of works of art: The role of the pore structure at low relative pressure, Adsorption. 11 (2005) 569–576. doi:10.1007/s10450-005-5614-3. S.M.P. Lucena, C.A.S. Paiva, P.F.G. Silvino, D.C.S. Azevedo, C.L. Cavalcante, The effect of heterogeneity in the randomly etched graphite model for carbon pore size characterization, Carbon N. Y. 48 (2010) 2554–2565. doi:10.1016/j.carbon.2010.03.034. J. Jagiello, M. Thommes, Comparison of DFT characterization methods based on N 2 , Ar , CO 2 , and H 2 adsorption applied to carbons with various pore size distributions, 42 (2004) 1227–1232. doi:10.1016/j.carbon.2004.01.022. S.M.P. Lucena, J.C.A. Oliveira, D. V. Gonçalves, P.F.G. Silvino, Second-generation kernel for characterization of carbonaceous material by adsorption, Carbon N. Y. 119 (2017) 378–385. doi:10.1016/j.carbon.2017.04.061. W. Wang, X. Peng, D. Cao, Capture of trace sulfur gases from binary mixtures by single-walled carbon nanotube arrays: A molecular simulation study, Environ. Sci. Technol. 45 (2011) 4832–4838. doi:10.1021/es1043672. R.L.C.B. Menezes, K.O. Moura, S.M.P. De Lucena, D.C.S. Azevedo, M. Bastos-Neto, Insights on the Mechanisms of H2S Retention at Low Concentration on Impregnated Carbons, Ind. Eng. Chem. Res. 57 (2018) 2248–2257. doi:10.1021/acs.iecr.7b03402. M.S. Shah, M. Tsapatsis, J.I. Siepmann, Hydrogen Sulfide Capture: From Absorption in Polar Liquids to Oxide, Zeolite, and Metal-Organic Framework Adsorbents and Membranes, Chem. Rev. 117 (2017) 9755–9803. doi:10.1021/acs.chemrev.7b00095. P.T.M. Nguyen, D.D. Do, D. Nicholson, Computer simulation of benzene-water mixture adsorption in graphitic slit pores, J. Phys. Chem. C. 116 (2012) 13954–13963. doi:10.1021/jp301729x. A.Ö. Yazaydin, A.I. Benin, S.A. Faheem, P. Jakubczak, J.J. Low, R.W. Richard, R.Q. Snurr, Enhanced CO2 adsorption in metal-organic frameworks via occupation of openmetal sites by coordinated water molecules, Chem. Mater. 21 (2009) 1425–1430. doi:10.1021/cm900049x. A.P. Guimarães, A. Möller, R. Staudt, D.C.S. De Azevedo, S.M.P. Lucena, C.L. Cavalcante, Diffusion of linear paraffins in silicalite studied by the ZLC method in the presence of CO2, Adsorption. 16 (2010) 29–36. doi:10.1007/s10450-010-9205-6.

A

[29]

[30]

23