Isobutanol Vapor Adsorption on Activated Carbons: Equilibrium and

Extruded pellets of the other adsorbents are made from peat by Norit Ltd. (The Netherlands). Owing to their favorable adsorption properties (Table 1),...
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Isobutanol Vapor Adsorption on Activated Carbons: Equilibrium and Kinetic Studies Dorota Downarowicz* and Tomasz Aleksandrzak Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Szczecin Aleja Piastów 42, 71-065 Szczecin, Poland S Supporting Information *

ABSTRACT: Activated carbon (AC) is commonly used to remove volatile organic compounds (VOCs) from the air. The purification efficiency depends on adsorbent characteristics (surface area, pore size, surface chemistry), adsorbate properties (size, functional groups, polarity), and process conditions. In the present study, the affinity of the Sorbonorit 4 (S4), Sorbonorit B4 (SB4), and BPL 4 × 6 (BPL) activated carbons toward the isobutanol (i-BuOH) vapor adsorption was investigated. The impact of vapor pressure and temperature on the process efficiency was examined. Adsorption equilibrium and kinetic measurements were conducted at 293.15, 313.15, 348.15, 373.15, and 393.15 K using an intelligent gravimetric analyzer (IGA). Experimental isotherm data were analyzed using the multitemperature Toth, Sips, and hybrid Langmuir−Sips models. Results indicated that the Sips model gave somewhat better fit than the others. The adsorption capacities of ACs at the same vapor pressure followed the order: S4 > BPL > SB4. Adsorption kinetic measurements at a constant vapor pressure of i-BuOH (ca. 5.6 Pa) showed that an increase in temperature affects an increase in the adsorption uptake and a decrease in the adsorbent efficiency. The pseudo-first-order model fitted very well the experimental data (R2 > 0.99) which suggests that physisorption plays a decisive role in the process mechanism. used to fit the equilibrium data.4,9 However, multitemperature isotherms are more useful for prediction of the adsorption capacity at any temperature and concentration. On the other hand, pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion models are mostly widely applied in kinetic predictions. They include the effects of mass transfer in the gas and solid phases, physical adsorption, or a chemical reaction.10,11 In our previous works,4,12,13 it was demonstrated that surface and structural heterogeneities of ACs affect their affinity for the polar organic compounds, such as ethanol, propanol-1-ol, and propanol-2-ol. The information about the adsorption of longer chain alcohols on the heterogeneous ACs is minimally available in literature. These compounds can occur in various isomeric forms that differ in carbon chain structures and an arrangement of the hydroxyl (OH) group.14 There are four structural isomers of butyl alcohol, all of them being commercially important. However, the highest increase in production of n-butanol (butan-1-ol) and isobutanol (2methylpropan-2-ol) has been observed recently.15,16 Both compounds are primary alcohols with straight and branched chains, respectively. They are used as solvents, chemical intermediates, extract agents, and recently as liquid fuels or

1. INTRODUCTION Adsorption on activated carbons is one of the most effective removal methods of volatile organic compounds (VOCs) from dilute waste gases.1,2 Its efficiency depends on a successful selection of adsorbents and process operating parameters. The primary requirements for adsorbents should include: sufficient adsorption capacity and appropriate selectivity as well as physical, chemical, and thermal stability.3,4 The purification process can be performed in temperature swing adsorption (TSA) systems containing two or more adsorbent fixed beds, operating in a cyclic adsorption/ regeneration mode.2,4−6 However, this is usually the energyconsuming process. The process can be optimized using mathematical modeling which is an important tool for design of adsorption plants. The models involve a system of partial differential equations describing the mass and energy balances and can be used to simulate cyclic adsorption processes under various operating condition. To get an accurate solution of the model, reliable experimental adsorption equilibrium and kinetic data over a wide temperature range are necessary.6−8 They are also necessary to determine the isosteric heat of adsorption and to explore a process mechanism. A proper choice of isotherm and kinetic equations may be have a significant impact on the correctness of simulation results. The single-compound isotherm models, such as Langmuir, Freundlich, and Toth or their combinations, are commonly © 2017 American Chemical Society

Received: June 9, 2017 Accepted: August 18, 2017 Published: August 31, 2017 3518

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fuel additives.17 Despite the growing demand of both alcohols, only few studies have focused on their adsorption behavior on various adsorbents to date.18,19 In addition, most of them were dedicated to the n-butanol adsorption.20,21The main objective of this study was to compare the adsorption performance of three heterogeneous activated carbons for i-BuOH vapor removal from air. Three multitemperature isotherm models were found to best fit the experimental equilibrium data. The effect of temperature on the adsorption uptake and adsorbent capacity was also investigated. The pseudo-first kinetic model was chosen for kinetic data validation. The fitted isotherm and kinetic model parameters may be helpful for designing and modeling the adsorption processes. It is one of the criteria for adsorbent performance evaluation.

The structural characteristics of the i-BuOH molecule were determined for the lowest energy conformer using ChemAxon’s geometrical descriptors plug-in (MarvinSketch software version: 16.2.1.0). The projection area indicates the surface occupied by a separate i-BuOH molecule on a flat adsorbent surface. 2.2. Apparatus and Procedure. The adsorption kinetic and isotherm measurements of i-BuOH vapor on ACs were conducted using an intelligent gravimetric analyzer (IGA-002, Hiden Isochema Ltd., UK). It is a fully computer-controlled microbalance system with a resolution of 0.1 μg and uncertainty of ±1 μg. The balance and pressure control system were fully thermostated to 0.2 K to eliminate changes in the external environment. A sample of activated carbon (ca. 100 mg) was placed in the thermostated reactor chamber with accurate temperature control (±0.1 K). Prior to isotherm measurements, the adsorbent samples were outgassed to constant weight at a high vacuum level (10−6 Pa) at 393.15 K for 2 h. The adsorbent weight uptake was measured until the saturation state was reached and then the i-BuOH vapor pressure was increased to the next desired value. Measurements were conducted at 293.15, 313.15, 348.15, 373.15, and 393.15 K and pressures of up to 938 Pa. A comprehensive description of the experimental methodology is presented elsewhere.27,28 The adsorption equilibrium data of i-BuOH vapor on S4, SB4, and BPL at 293.15, 313.15, 333.15, 348.15, 373.15, and 393.15 K are presented in Tables S1, S2, and S3 (see Supporting Information section). Each experiment was performed three times, and then data were averaged.

2. EXPERIMENTAL SECTION 2.1. Materials. Three commercial activated carbons, BPL 4 × 6 (BPL), Sorbonorit 4 (S4), and Sorbonorit B4 (SB4), were selected as the adsorbents. The first one is a granular adsorbent manufactured from bituminous coal by Calgon Carbon Corp. (Pittsburgh, PA). Extruded pellets of the other adsorbents are made from peat by Norit Ltd. (The Netherlands). Owing to their favorable adsorption properties (Table 1), the adsorbents are designed for use in solvent vapor recovery applications. Table 1. Properties of ACs12,13,22−25 property

BPL

S4

SB4

apparent density (kg·m−3) total pore volume (cm3·g−1) particle porosity BET (m2·g−1) chemical compositions (wt %) C H N O S Si

400 0.54 0.595 1100

380 0.78 0.61 1400

400 0.48 0.44 1250

95.58

94.02

4.42 0.45 0.48

5.05

95.2 0.4 0.6 2.9 0.9

3. RESULTS AND DISCUSSION 3.1. Adsorption Isotherms at Ambient Temperature. Figure 1 presents experimental isotherm curves of i-BuOH vapor on BPL, S4, and SB4 at 293.15 K in the p/p0 range from 0.01 to 0.9, where p and p0 are the current and saturated vapor pressures, respectively. They are convex curves with a positive slope in higher p/p0 pressure ranges and are similar to Type Ib in the IUPAC scheme.29 It indicates that mainly micropores are to be found in

The high-purity (99.7%) i-BuOH from Chempur (Poland) was used as an adsorbate. This is a colorless flammable liquid with a mild odor, which has limited miscibility in water, although it is easily soluble in most organic solvents such as alcohols, ethers, and ketones. Its vapor forms an explosive mixture with air within the range of 1.2−10.9% by volume.26 The basic properties of i-BuOH are presented in Table 2. Table 2. Physical-Chemical and Structural Characteristics of i-BuOH26 property molecular formula density of liquid at 293.15 K (kg·m−3) boiling point (K) saturation pressure at 293.15 K (Pa) liquid molar volume (cm3·mol−1) minimum projection area (Å2) maximum projection area (Å2) minimum projection diameter (Å) maximum projection diameter (Å)

(CH3)2CHCH2OH 797 380.8 959 93.029 21.85 88.81 6.26 7.34

Figure 1. Experimental and correlated adsorption isotherms at 293.15 K: □, S4; ○, BPL; ◊, SB4, , DR; − − −, DA model. 3519

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wider than micropores.32 It is typical for carbons with high activation degree, leading to a broader micropore size distribution and a complex pore structure. As can be seen from Figure 1, the D−R model gives a good fit to the experimental data in the pressure range p/p0 < 0.1. The two-term Dubinin−Astahov equation (D−A) can be used to describe the adsorption on structurally heterogeneous adsorbents. It is based on the assumption that an adsorbent possesses micro- and supermicropores with the same shapes but different sizes. The equation is expressed as33,34

ACs and i-BuOH vapor adsorption occurs according to the micropore filling mechanism. As can be seen from Figure 1, the adsorption capacities of ACs at the same vapor pressure followed the order: S4 > BPL > SB4. The Dubinin−Radushkevich (D−R) equation can be used to derive specific adsorption characteristics of ACs and to interpret the adsorption behavior of i-BuOH at 293.15 K.30 ⎛ ⎛ A ⎞⎞ 2 W = W0 exp⎜ −⎜ ⎟⎟ ⎝ ⎝ E ⎠⎠

(1)

⎛ ⎛ A ⎞⎞ n ⎛ ⎛ A ⎞⎞ n W = W01 exp⎜⎜ −⎜ ⎟⎟⎟ + W02 exp⎜⎜ −⎜ ⎟⎟⎟ ⎝ ⎝ E 2 ⎠⎠ ⎝ ⎝ E1 ⎠⎠

where W is the volume of adsorbate in the unit mass of adsorbent (cm3·g−1), W0 is the limiting micropore volume (cm3·g−1), E is the characteristic adsorption energy (J·mol−1); A is the adsorption potential, defined as A = RT ln(p0/p), T is the absolute temperature (K), p is the pressure in the gas phase, p0 is the saturation vapor pressure at temperature T, and R is the universal gas constant (8.314 J·mol−1·K−1). A liquid-like adsorbed phase is assumed. Therefore, the values of adsorption micropore capacity W at relative pressure p/p0 can be calculated as the product of adsorbate liquid molar volume vm (m3·mol−1) at 293.15 K and experimental adsorbent loading q (mol·kg−1): W = νmq (2)

where W is the amount adsorbed at a given relative pressure p/ p0; W01 and E1 correspond to micropores, and W02 and E2 to supermicropores; exponent n is a measure of the heterogeneity of the adsorbent. The total micropore volume is defined as the sum of overall volumes of both pore fractions: W0 = W01 + W02 (5) Table 3 presents the D−A model parameters for the investigated ACs and total pore volumes (Wt) determined Table 3. D−A Model Parameters and Total Micropore Volumes (Wt) for i-BuOH-AC Aystems

To determine the parameters of the D−R equation, its linear form was used:30,31 ⎛ RT ⎞2 2⎛ p0 ⎞ ⎟ ln ⎜ ln W = ln W0 − ⎜ ⎟ ⎝ E ⎠ ⎝ p⎠

(4)

(3)

Figure 2 presents plots of ln(W) versus ln2(p0/p) for i-BuOH vapor adsorption on three ACs. As can be seen the graphs are

parameters

SB4

BPL

S4

Wt (cm3·g−1) n W01 (cm3·g−1) W02 (cm3·g−1) W0 (cm3·g−1) E1 (J·mol−1) E2 (J·mol−1)

0.386 2.40 0.362 0.021 0.383 20273 1400

0.463 1.916 0.432 0.028 0.460 15825 1623

0.591 2.096 0.533 0.090 0.585 16040 1346

experimentally at 293.15 K. The parameter values were determined with the least-squares method using Statistica 12.5 software. As can be seen from Figure 1, the D−A model gives a good agreement with the experimental data in whole pressure range of p/p0. The data presented in Tables 1 and 3 indicate that the Wt values are lower than those determined by nitrogen adsorption method at 77 K. It may be caused by molecular packing restrictions of the large i-BuOH molecules within narrow micropores.35 Different interactions of these molecules with surface functional groups located at micropore entrances may also reduce the adsorption capacity of ACs.36 A comparison of W01 values (Table 3) shows that the micropore volume of S4 is larger by ca. 19 and 40% than that of BPL and SB4, respectively. By contrast, the characteristic adsorption energy E1 of SB4 is higher than the others by ca. 20%, and therefore, its micropores are smaller.12 The W02 and E2 values indicate the presence of wider micropores in S4 carbon.12 The evidence of this is a more rounded shape of the isotherm, as shown in Figure 1. 3.2. Adsorption Isotherm Modeling. The adsorption equilibrium data of i-BuOH vapor on S4, SB4, and BPL at 293.15, 313.15, 333.15, 348.15, 373.15, and 393.15 K are presented in Tables S1, S2, and S3 (see Supporting Information). Each experiment was performed three times, and then data were averaged.

Figure 2. i-BuOH characteristic adsorption curves for ACs.

linear over narrow pressure ranges p/p0. According to Marsh and Rand classification,31 two types of deviation from linearity can be observed. In the relative pressure range of p/p0 < 0.1 (high values of ln2(p0/p)), the curves show a deviation of type A, which corresponds to an adsorbent with narrow micropores. In the high pressure range (p/p0 > 0.5), the upward deviation of type C is observed, which is characteristic for adsorption in pores 3520

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The temperature dependence of qmS (mol·kg−1) and bS (Pa−1) parameters of the equations can be expressed as

The Toth, Sips, and hybrid Langmuir−Sips multitemperature isotherm models were used to correlate the experimental data. They can predict adsorption capacity at any temperature and vapor pressure and are useful for proper design and simulation of cyclic adsorption processes. The model parameters were determined with the Levenberg−Marquardt method using Statistica 12.5 software. The goodness-of-fit of the model to experimental data was evaluated on the basis of the determination coefficient (R2) and the average relative error (ARE):37 qexp, i − qcal, i 100 ∑ N i=1 qexp, i

⎛ ⎛ T ⎞⎞ qmS = q0S exp⎜⎜q1S⎜1 − ⎟⎟⎟ T0 ⎠⎠ ⎝ ⎝

⎛ Q ⎛T ⎞⎞ bS = b0S exp⎜ S ⎜ 0 − 1⎟⎟ ⎠⎠ ⎝ RT0 ⎝ T −1

(8b) −1

−1

where q0S (mol·kg ), q1S, b0S (Pa ), and QS (J·mol ) are the parameters of the equations, and T0 is the reference temperature (293.15 K). The nS parameter in eq 8 characterizes the heterogeneity of adsorbent−adsorbate systems. The model parameter values are given in Table 5.

N

ARE =

(8a)

(6)

where qexp is the experimental adsorption capacity, qcal is the calculated adsorption capacity, and N is the number of experimental points. Toth Isotherm Model. The model is an empirical expression which was derived from the Langmuir equation by considering the energetic heterogeneity of adsorption sites. The model is valid at low- and high-pressure limits, and therefore, the Clausius−Clapeyron equation is satisfied.26 The model is expressed as38 p q = qmT (bT + pnT )1/ nT (7)

Table 5. Sips Isotherm Parameters and Error Coefficients model Sips

parameters and errors −1

q0S (mol·kg ) q1S b0S (Pa−1) QS (kJ·mol−1) T0 (K) nS ARE (%) R2

SB4

BPL

S4

4.092 0.311 0.508 57.604 293.15 0.603 2.31 0.9982

5.143 0.148 0.163 60.843 293.15 0.574 1.68 0.9997

6.430 0.320 0.146 60.121 293.15 0.541 1.76 0.9991

where bT (Pa·nT−1) is the parameter of the equation defined as ⎛ n ΔH ⎞ ⎟ bT = b0T exp⎜ − T ⎝ RT ⎠

Hybrid Langmuir−Sips Isotherm. The model incorporates Langmuir and Sips isotherm equations and was developed for the adsorption of condensable vapors on porous adsorbents. It is given by40

(7a) −1

where q is the adsorption capacity (mol·kg ), qmT is maximum adsorption capacity (mol·kg−1), p is the equilibrium pressure of the adsorbate (Pa), T is temperature (K), ΔH is the heat of adsorption (J·mol−1), R is the universal gas constant (8.314 J· mol−1·K−1), b0T (Pa·nT−1) is the equation coefficient, and nT is the adsorbent heterogeneity degree. The heat of adsorption ΔH is considered to be a loading- and temperatureindependent parameter over the whole pressure range. This is the basic advantage of this modified equation in comparison to its classical form presented elsewhere. The model parameter values are given in Table 4.

⎡ b p b2LSpnLS ⎤ ⎥ q = qmLS⎢ 1LS + 1 + b2LSpnLS ⎦ ⎣ 1 + b1LSp

The temperature dependences of the hybrid isotherm parameters are as follows:

Table 4. Toth Isotherm Parameters and Error Coefficients model

parameters and errors

SB4

BPL

S4

Toth

qm (mol·kg−1) b0T (kPa·nT−1) nT ΔH (kJ·mol−1) ARE (%) R2

4.253 35.734 0.444 61.373 3.42 0.9977

5.477 28.138 0.414 61.913 5.33 0.9991

7.084 8.019 0.362 63.480 4.09 0.9861

⎛q ⎛ 1 1 ⎞⎞ qmLS = q0LS exp⎜⎜ 1LS ⎜ − ⎟⎟⎟ T0 ⎠⎠ ⎝ R ⎝T

(9a)

⎛ Q ⎛1 1 ⎞⎞ b1LS = b01LS⎜⎜ − 1LS ⎜ − ⎟⎟⎟ R ⎝T T0 ⎠⎠ ⎝

(9b)

⎛ Q ⎛1 1 ⎞⎞ b2LS = b02LS⎜⎜ − 2LS ⎜ − ⎟⎟⎟ R ⎝T T0 ⎠⎠ ⎝

(9c)

⎛ n ⎛1 1 ⎞⎞ nLS = n0LS⎜⎜ − 1LS ⎜ − ⎟⎟⎟ T0 ⎠⎠ ⎝ R ⎝T

(9d)

−1

where qmLS (mol·kg ), b1LS (Pa ), b2LS (Pa·nLS−1), nLS, q0LS (mol·kg−1), q1LS (J·mol−1), b01LS (Pa−1), Q1LS (J·mol−1), b02LS (Pa·nLS−1), Q2LS (J·mol−1), n0LS and n1LS (J·mol−1) are the

Sips Isotherm. The model is a combination of the Langmuir and Freundlich equations. At low pressure, the model is reduced to a Freundlich isotherm, and thus, it does not follow Henry’s law. At high concentrations, it predicts a monolayer adsorption capacity characteristic of the Langmuir isotherm. The Sips model can be given by39 q = qmS

(bSp)nS 1 + (bSp)nS

(9)

−1

parameters. The model parameters are given in Table 6. The experimental and simulated adsorption isotherms of iBuOH vapor on S4, SB4, and BPL are shown in Figures 3, 4, and 5, respectively. The scatter plots represent experimental data, whereas different types of line plots represent the fitted isotherms. It can be observed that the adsorption capacity of ACs decreases with temperature increases, which is indicative of physical adsorption. As can be seen, all analyzed models give a

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Table 6. Hybrid Langmuir−Sips Isotherm Parameters and Error Coefficients model Hybrid Langmuir− Sips

parameters and errors

SB4

q0LS (mol·kg−1) q1LS (kJ·mol−1)

2.152 803.6

b01LS (Pa−1) Q1LS (kJ·mol−1) b02LS (Pa·nLS−1) Q2LS (kJ·mol−1) n0LS n1LS (kJ·mol−1) ARE (%) R2

0.618 −57.691 0.356 −9.727 0.454 −6.146 2.20 0.9981

BPL

S4

2.430 1.319

3.625 4.000

0.059 −48.868 1.436 −3.861 0.456 1.599 1.75 0.9991

0.444 −62.260 0.068 0.434 0.524 −4.279 3.61 0.9981

Figure 4. Experimental and correlated isotherms for i-BuOH adsorption onto BPL at various temperatures: ●, 293.15 K; ○, 313.15 K; ⧫, 333.15 K; ◊, 348.15 K; ■, 373.15 K; □, 393.15 K; , Toth model; − − −, Sips model; ---, hybrid.

Figure 3. Experimental and correlated isotherms for i-BuOH adsorption onto S4 at various temperatures: ●, 293.15 K; ○, 313.15 K; ⧫, 333.15 K; ◊, 348.15 K; ■, 373.15 K; □, 393.15 K; , Toth model; − − −, Sips model; ---, hybrid.

good fit to the experimental results. However, in high pressure ranges, the calculated curves show some deviations from experimental isotherms at 293.15 K. This issue was discussed in section 3.1. As indicated by the ARE error values in Tables 4−6, the Sips and hybrid Langmuir−Sips models give a somewhat better fitting of the equilibrium data (ARE < 2.35%) than the hybrid Langmuir−Sips and Toth models (ARE 1.42− 5.33%). However, the first one model is more flexible than the others to fit an identical data set. As can be seen from Figure 3, despite of the higher ARE errors, the Toth model correctly estimates the adsorption capacities of all ACs. The maximum capacity values (qm) estimated from this model increase in the order S4 > BPL > SB4 and are 7.084, 5.477, and 4.253 mol· kg−1, respectively. It indicates that 1-BuOH vapor is the most effectively adsorbed by S4. 3.3. Adsorption Kinetic Modeling. Figure 6 shows experimental and simulated kinetic curves of i-BuOH vapor onto S4, SB4, and BPL at the vapor pressure of 5.6 ± 0.3 Pa and at 293.15 K. The adsorption kinetic profiles of i-BUOH vapor on the ACs at 293.15 K are shown in Figure 6. Results showed that the adsorption rate initially increased quite rapidly, which resulted

Figure 5. Experimental and correlated isotherms for i-BuOH adsorption onto SB4 at various temperatures: ●, 293.15 K; ○, 313.15 K; ⧫, 333.15 K; ◊, 348.15 K; ■, 373.15 K; □, 393.15 K; , Toth model; − − −, Sips model;. ---, hybrid.

in the adsorption of about 50−60% of the total alcohol amount within 2000 s. Then the rate gradually decreased, reaching equilibrium capacities of the subsequent ACs in about 7000− 8500 s. The initial faster process rate may be due to the easy availability of the uncovered adsorbent surfaces for the adsorbate molecules. The experimental data were fitted using the pseudo-first order (PFO) kinetic model, whose integrated form is given as41 qt = qe(1 − e−kt )

(10)

−1

where: qe (mol·kg ) is the saturation adsorption capacity, qt (mol·kg−1) is the adsorption capacity at time t (s), and k (s−1) is the PFO rate constant. 3522

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isotherm models (Toth, Sips, and hybrid Langmuir−Sips) were used to fit experimental data. The modeling results showed that all isotherm equations gave a good fit to the experimental results. However, the Sips model provided slightly lower ARE errors than the others. The Toth model parameter values reveal that the maximum adsorption capacity of S4 is higher than that of BPL by about 23% and SB4 by 40%. Therefore, it can be successfully used for air purification from the i-BuOH vapor in adsorption plants. The adsorption kinetic data at constant vapor pressure (ca. 5.6 Pa) were analyzed using the pseudo-first-order model. The results showed that an increase in temperature affects an increase in the adsorption uptake and a decrease in the adsorbent efficiency. The model gives an excellent agreement with the experimental data (R2 > 0.99) which suggests that physisorption plays a decisive role in the process mechanism. The presented results can be used to design and model cyclic adsorption processes.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure 6. Experimental and correlated kinetic curves at 293.15 K: □, S4; ○, BPL; ◊, SB4; , PFO model.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00528. Experimental isotherm data for i-BuOH vapor on S4, BPL, and SB4 (PDF)

The model constants were determined with the least-squares fitting method using Statistica 12.5 software. The values of the k and qe parameters for i-BuOH vapor adsorption on three ACs at 5.6 ± 0.3 Pa are given in Table 7. It can be observed that the increase in temperature from 293.15 to 323.15 K affects an increase in the adsorption rate constants (k) and a decrease in the adsorption capacities (qe). Moreover, the qe values are very close to the experimental equilibrium capacities at constant pressure of 5.6 ± 0.3 Pa (Tables S1, S2, and S3 in the Supporting Information section). The high goodness of fit of the PFO model (R2 > 0.99) suggests that the adsorption of the i-BuOH vapor pointed toward physisorption as the predominant mechanism of the process.10,41 Although the equilibrium capacity and micropore size of S4 are larger than those of BPL, the rate constant values for the former adsorbent are slightly lower than those for the other. This suggests various adsorption behavior of the i-BuOH molecules on S4 and BPL at low coverage that can be due to differences in the surface chemistry of the adsorbents.36,41−43 However, further studies are needed to fully elucidate this phenomenon.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. ORCID

Dorota Downarowicz: 0000-0002-8835-8706 Tomasz Aleksandrzak: 0000-0003-1510-6687 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Ghoshal, A. K.; Manjare, S. D. Selection of Appropriate Adsorption Technique for Recovery of VOCs: An Analysis. J. Loss Prev. Process Ind. 2002, 15, 413−421. (2) Bathen, D.; Breitbach, M. Adsorptionstechnik; Springer-Verlag: Berlin, 2001. (3) Fletcher, A. J.; Yüzak, Y.; Thomas, K. M. Adsorption and Desorption Kinetics for Hydrophilic and Hydrophobic Vapors on Activated Carbon. Carbon 2006, 44, 989−1004. (4) Gabruś, E.; Downarowicz, D. Anhydrous Ethanol Recovery from Wet Air in TSA Systems - Equilibrium and Column Studies. Chem. Eng. J. 2016, 288, 321−331. (5) Downarowicz, D.; Gabruś, E. Electrothermal temperature swing adsorption. A Chance of Effective VOC Recovery from Flue Gases. Przem. Chem. 2008, 87, 768−774.

4. CONCLUSIONS Adsorption equilibrium and kinetic measurements for isobutanol (i-BuOH) vapor (IUPAC name: 2-methyl-1-propanol) on Sorbonorit 4 (S4), Sorbonorit B4 (SB4), and BPL 4 × 6 (BPL) activated carbons were conducted at five temperatures in the range of 293.15−393.15 K. Three multitemperature

Table 7. PFO Model Constants and Determination Coefficients R2 SB4

S4

BPL

T (K)

qe (mol·kg−1)

k × 103 (s−1)

R2

qe (mol·kg−1)

k × 103 (s−1)

R2

qe (mol·kg−1)

k × 103 (s−1)

R2

293.15 313.15 333.15 348.15 373.15 393.15

2.106 1.558 0.908 0.658 0.345 0.229

0.372 0.431 0.580 0.701 0.952 1.152

0.9977 0.9977 0.9976 0.9959 0.9937 0.9944

3.002 1.740 0.957 0.598 0.285 0.190

0.397 0.565 0.840 1.100 1.495 2.029

0.9995 0.9991 0.9998 0.9961 0.9912 0.9912

2.412 1.397 0.735 0.463 0.235 0.124

0.465 0.616 0.945 1.203̀ 1.550 1.820

0.9994 0.9986 0.9976 0.9938 0.9957 0.9773

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DOI: 10.1021/acs.jced.7b00528 J. Chem. Eng. Data 2017, 62, 3518−3524