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Trace Adsorption of Ethane, Propane, and n‑Butane on Microporous Activated Carbon and Zeolite 13X at Low Temperatures Florian Birkmann,*,† Christoph Pasel,† Michael Luckas,† and Dieter Bathen†,‡ †

University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany IUTA e.V., Bliersheimer Straße 58-60, 47229 Duisburg, Germany

‡

S Supporting Information *

ABSTRACT: Removing of trace-leveled light hydrocarbons from exhaust air or gas streams becomes an increasingly important issue in the field of process and environmental technology, e.g., storage and transport of liquefied natural gas. Adsorption processes at temperatures below 0 °C have great potential to meet process specifications or environmental regulatory limits. Designing of such adsorption processes requires a profound insight into the thermodynamics of adsorption at low temperatures, which is not available yet. Therefore, this work provides adsorption isotherms of ethane, propane, and n-butane on microporous activated carbon and zeolite 13X in a temperature range from −40 to +60 °C and at partial pressures from 5 to 1000 Pa. The influence of temperature on the adsorbed amount on activated carbon and zeolite 13X is discussed for each adsorptive considering isosteric heats of adsorption and specific interactions between the adsorptive and the adsorbent surface.

1. INTRODUCTION The removal of light hydrocarbons in trace concentrations, such as ethane, propane, and n-butane, from exhaust air or process gas is important in multiple environmental and technical applications, e.g., processing of natural gas or separation of olefin/paraffin mixtures. Current growth fields are the increasing demand of particularly high gas purities, e.g., for synthesis in the chemical industry, and the upswing of LNG technology.1−4 To meet future requirements, there is a demand for improving the removal of light hydrocarbons in trace concentrations from gas/air streams. For that purpose, adsorptive processes would be generally suitable.5 However, experience has shown that the capacity of industrially available adsorbents at ambient conditions is often poor. By lowering the adsorption temperature significantly below ambient temperature, as is possible by energy integration at LNG terminals or low temperature rectification columns, the adsorbent’s capacity could be increased. As yet, there are no data on the adsorption of light hydrocarbons at temperatures significantly below ambient temperature, while a profound database is mandatory for designing adsorption columns as well as for simulating dynamic adsorption processes. In this field, a detailed knowledge of the temperature dependence of the adsorption equilibrium as well as of the adsorption enthalpy is indispensable. In principal, microporous activated carbons and 13X zeolites may be suitable for the adsorption of light hydrocarbons. While activated carbons usually have high capacities and fast kinetics, 13X zeolites are mainly used for selective removal of specific components in multilayer adsorption columns. © XXXX American Chemical Society

According to several publications on the adsorption of light hydrocarbons at moderate and high temperatures on activated carbon6−21 and 13X zeolite,22−31 a strong dependence of the adsorption equilibrium on temperature is to be expected. Moreover, Zhu et al. found a slightly increasing temperature dependence of the adsorption equilibrium on activated carbon with increasing organic chain length of n-alkanes at temperatures between 0 and 100 °C.9,10 On the other hand, the choice of adsorbent does not seem to have a crucial influence on the extent of temperature dependence.9,24,29 Cheripally et al. published adsorption enthalpies of n-alkanes on activated carbon.6 With increasing organic chain length, they found a linear increase of the adsorption enthalpy. These results are in good agreement with other adsorption enthalpies of nalkanes on activated carbon.8−10,12 While the adsorption enthalpy of n-alkanes on activated carbon is usually decreasing with increasing equilibrium loading, Walton et al.8 found temperature dependence to be negligible. An overview of adsorption enthalpies of n-alkanes on zeolite 13X was published by Loughlin et al.29 At low coverages, only slight differences of the adsorption enthalpies on activated carbon and zeolite 13X can be observed. Despite the considerable number of studies investigating the adsorption of light hydrocarbons at moderate and high temperatures, there is no systematic work on trace-level adsorption at low temperatures. Against this background, we Received: December 27, 2016 Accepted: June 7, 2017

A

DOI: 10.1021/acs.jced.6b01068 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Flow sheet of the experimental setup: 1, gas-mixing chamber; 2, cooling thermostat; 3, adsorption column; 4, flame ionization detector FID; mass flow controller MFC.

GmbH, Germany) with an accuracy of ±0.002 bar. Hydrocarbon concentration in the carrier gas is continuously analyzed at the outlet of the adsorption column by a flame ionization detector (FID) (Testa GmbH, Germany). 2.2. Materials. All adsorptives were purchased in pressurized gas cylinders from Air Liquide Deutschland GmbH with purities of ≥99.5%. Table 1 shows selected

examine in this work the adsorption of ethane, propane, and nbutane on a microporous activated carbon and a zeolite 13X. Adsorption isotherms are derived from measurements of the adsorbed amount by dynamic breakthrough experiments in a fixed bed at temperatures down to −40 °C, and load dependent isosteric heats of adsorption are calculated. The temperature dependence of the equilibrium loading is discussed considering the adsorbents’ surface chemistry.

Table 1. Critical Molecular Diameters and Polarizabilities of the Adsorptive Molecules32,33

2. EXPERIMENTAL SECTION 2.1. Apparatus. Figure 1 shows a schematic flow sheet of the experimental setup. In a tempered gas-mixing chamber three mass flow controllers (MFC 1−3) (Brooks Instrument GmbH, Germany) provide the raw gas mixture, while a fourth mass flow controller (MFC 4) can be used to dose a nitrogen stream for precooling of the fixed bed. To avoid condensation inside the tubing, the carrier gas is permanently dehumidified to a dew point of −100 °C in a pressure swing adsorption unit. Cooling of the fixed bed is done by channeling raw gas through a cooling coil situated in the bath of a cooling thermostat (Julabo GmbH, Germany), which can set bath temperatures down to −50 °C. As cooling fluids, ethanol is used for adsorption temperatures below 0 °C while water is used for temperatures above 0 °C. After the gas passes the cooling coil, the gas velocity is reduced in a diffuser. Subsequently the gas mixture flows through the fixed bed reactor from bottom to top. The adsorption column is made of glass with an inner diameter of 30 mm and a height of 150 mm. To reduce thermal losses, the adsorption column is double-walled, with the volume between the walls evacuated by a turbomolecular pump. Process temperatures are monitored by five thermocouples of type T with an accuracy of ±0.5 °C. While the first thermocouple is placed right before the fixed bed, characterizing the adsorption temperature, temperatures inside the fixed bed are measured at adsorption column heights of 0, 50, 100, and 150 mm. Before and after the adsorption column process pressure is monitored by pressure transducers (BD Sensors

crit mol diam/nm polarizability/10−24·cm3

ethane

propane

n-butane

0.4499 4.47

0.5230 6.29

0.5228 8.2

adsorptive properties. As carrier gas, nitrogen of a purity >99.999% supplied by the university’s infrastructure was used. To calibrate the FID a certified test gas with 8190 mol ppm (±2%) propane in nitrogen was used. As adsorbents, the steam-activated carbon Norit RX 1.5 Extra (Cabot Corp., Boston, MA, USA) and the zeolite 13X APG MOLSIV 8×12 (Honeywell UOP, USA) were selected. The activated carbon (AC) is a fractured, cylindrical extrudate with an average diameter of 1.5 mm, while the zeolite 13X (13X) consists of spherical particles with an average diameter of 2 mm. Hereby, the ratio of particle diameter to adsorption column diameter is >10 for each adsorbent, while the ratio of packing height to adsorption column diameter is >3 for all experiments. Hence, current standards of adsorption column design are fulfilled and flow effects can be neglected.34−36 The adsorbents’ main properties were determined using nitrogen adsorption at 77 K. Inner surface areas were calculated by the BET method (DIN ISO 9277), pore volumes according to the Gurvich rule at a relative pressure of p/p0 = 0.98, and micropore volumes according to Dubinin−Radushkevich (DIN 66135-3). Results are presented in Table 2. Pore size distributions of the adsorbents were evaluated by nonlocal B



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getting left-bent. Due to saturation of the adsorbent’s surface at higher coverages, the chemical adsorption potential decreases. Consequently, the slope of the isotherm decreases, which leads to a right-bent isotherm. The coverage and the partial pressure at the inflection point of the sigmoidal isotherm depend on the ability of the adsorptive molecules to form lateral dispersion interactions and on the degree of heterogeneity of the adsorbent’s surface. In the literature, for the adsorption on heterogeneous surfaces the inflection point is observed at very low partial pressures, while it is observed at higher partial pressures for the adsorption on homogeneous surfaces43,44 (see also section 2.3). 2.3. Experimental Procedure. The adsorbents were thermally activated at ambient air. Weighing experiments showed no further mass loss of activated carbon after 2 h at 175 °C and of zeolite 13X after 24 h at 300 °C. The FID is calibrated to zero value using pure nitrogen and to 8190 mol ppm propane with test gas. Depending on the adsorptive and adsorbent, between 20 and 85 g of adsorbent was weighed and filled into the adsorption column. Through MFC 4, carrier gas was channeled through the cooling bath to cool down the fixed bed to process temperature, while by bypassing the adsorption column, the desired adsorptive concentration was adjusted by MFCs 1−3. As soon as the average adsorptive concentration’s standard deviation was less than 2% for a duration of 1200 s and the average fixed bed temperature was within ±1 K of the required temperature having a standard deviation less than ±0.5 K for a duration of 3600 s, the experiment was started by leading the nitrogen stream with defined adsorptive concentration into the adsorption column. Thermodynamic equilibrium was assumed when the average outlet concentration equaled the average inlet concentration and only deviated within 1% for a duration of 1200 s. Subsequently, additional equilibrium steps were measured by increasing the adsorptive concentration before the adsorption column. From the measured breakthrough curves the excess amount adsorbed was determined using a mass balance around the adsorption column. The amount of adsorptive molecules remaining in the interparticle and intraparticle gas phase volume was neglected as its contribution is much smaller than the experimental error. By normalization to the mass of adsorbent ma [kg] the equilibrium loading Xeqi [mol·kg−1] was calculated. To ensure comparability of different adsorbents, normalization on the inner surface ABET [m2·g−1] is also common. Equation 1 shows the mass balance, where ṅin [mol· s−1] is the total mole flow, yin [mol ppm] is the inlet concentration, youtj [mol ppm] is the measured outlet concentration at measuring point j, and Δt [s] is the time span between two measuring points, usually 2 s.

Table 2. Results of Nitrogen Physisorption Isotherms at 77 Ka AC 13X

BET/m2·g−1

TPV/cm3·g−1

MPV/cm3·g−1

1600 602

0.756 0.334

0.605 0.229

a

BET: BET surface area. TPV: total pore volume. MPV: micropore volume.37

density-functional theory (NLDFT) assuming slit pores for the activated carbon and cylindrical pores for the zeolite 13X. Data are included in the Supporting Information. Norit RX 1.5 Extra is a mainly microporous material with a multimodal pore size distribution having maxima at pore widths of 0.45−0.85 nm and 1.1−2.4 nm. As the critical diameter of the adsorptives is found between 0.45 and 0.523 nm, steric hindrance is excluded. Presumably, the activated carbon’s surface consists mainly of disordered, nonpolar graphite-like areas forming slit pores. In addition, oxygen containing functional polar groups can be found at the surface. Therefore, dispersion interactions between the surface and the nonpolar alkanes occur due to instantaneous polarization of the molecules. In narrow pore systems, the energetic value of an adsorption site is often dependent on the pore width. Inside micropores with pore widths in the range of the critical diameter of the adsorptive, adsorption is favored due to interactions of the adsorptive with more than one surface. To sum up, the activated carbon’s surface is heterogeneous with adsorption sites of different energetic value. 13X zeolites consist of aluminum and silicon atoms linked via oxygen atoms forming a regular crystalline structure of cagelike pores. The inner diameter of the cages is about 1.27 nm and is accessible through apertures with a diameter of about 0.74 nm. Thus, steric hindrance is excluded for the adsorptives in this work. To compensate the negative charge of the aluminum atoms, sodium cations are incorporated into the crystal lattice structure so that 13X zeolites have polar properties. A part of the sodium cations is accessible for interactions with adsorptives. Therefore, during adsorption of nonpolar adsorptives, interactions of type cation-induced dipole occur in addition to dispersion interactions between the adsorptive molecules and Si−O respectively Al−O segments. Here, the sodium cation induces an instantaneous dipole in the nonpolar molecule leading to mutual attraction. Interactions of type cation-induced dipole are in general energetically more valuable than dispersion interactions.38,39 Zeolite 13X presents a chemically heterogeneous surface to the adsorptives due to sodium cations incorporated in a silica−alumina lattice. However, because of its crystallinity zeolite 13X exhibits a periodical pattern which may offer many adsorption sites of similar energetic value.40,41 Aside from interactions between adsorptive molecules and surface of the adsorbent, lateral dispersion interactions between adsorbate molecules are to be expected for both adsorbents leading to sigmoidal isotherms.42 At low coverage, distances between adsorbate molecules are large and lateral interactions between adsorbate molecules are negligible. Thus, the adsorption isotherm is a Henry isotherm. With increasing coverage, distances between adsorbate molecules become smaller, so that lateral dispersion interactions can occur, increasing in strength with further increasing coverage. This results in an increase of the chemical adsorption potential and an increase of the slope of the isotherm, which is

Xeq = Xeq i

i−1

ṅ + in ma

ji ,eq

⎛y − y ⎞ in i out j ⎟ Δt 1 − yout ⎟ ⎝ j ⎠

∑ ⎜⎜ ji ,0

(1)

The experimental data yield linear, concave, and sigmoidal isotherms. To select a suitable isotherm equation, the correlation coefficients of the parameter fit as well as the consistency of the isotherm equation within the entire isotherm field were considered. Sufficient accuracy was achieved using Henry (eq 2) and Sips (eq 3) isotherms. Xeq = kHpi ,eq C

(2) DOI: 10.1021/acs.jced.6b01068 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data Xeq = X mon

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(bpi ,eq )n 1 + (bpi ,eq )n

adsorption results from the slope of the isostere, which were fitted using linear regression. (3)

⎛ ∂ ln(p ) ⎞ Δhads i ⎟ = ⎜⎜ 1 ⎟ R0 ⎝ ∂T ⎠

The linear Henry isotherms indicate a constant chemical adsorption potential resulting from low coverage on the adsorbent’s surface with adsorption sites of uniform energetic value.45 A deviation from Henry isotherm behavior can lead to convex, concave, or sigmoidal isotherms. In the case of the Sips isotherm, this is considered by introducing a saturation capacity Xmon [mol·kg−1] and a heterogeneity constant n [−]. The saturation capacity corresponds to the capacity where all adsorption sites are occupied by adsorbed molecules. The heterogeneity constant is a measure of the energetic heterogeneity of the adsorbent’s surface. If n equals 1, all adsorption sites have the same energetic value. If n does not equal 1, the energetic values of the adsorption sites exhibit an energy distribution, where the width of the distribution widens with greater deviation of n from 1. The isotherm parameter b [Pa−1] is called affinity constant and describes the ratio of adsorption rate to desorption rate. Therefore, it is a measure of the adsorptive’s affinity to the surface. In general, Sips isotherms have a sigmoidal shape, where the position of the inflection point significantly depends on the heterogeneity constant. Figure 2 shows exemplary Sips

Xeq = const

(4)

2.4. Experimental Error. The measured quantities are afflicted with systematic and random errors leading to uncertainties of the equilibrium partial pressure and the equilibrium load. The systematic uncertainty arises from the uncertainties of the equipment used, namely, the test gas (2%), the drift of the concentration signal of the FID (1%/24 h), the pressure transducers (0.1%), the MFCs (1%), and the balance (0.025−0.15%), while the random error was estimated by Gaussian error propagation. Additionally to the theoretical estimation of the experimental uncertainty, the reproducibility of the experimental setup was investigated by replicate measurements. Uncertainties of 1% for the equilibrium partial pressure and up to 6% for the equilibrium load were found.

3. RESULTS AND DISCUSSION 3.1. Adsorption Isotherms. Adsorption equilibria of ethane, propane, and n-butane were measured in a concentration range of 5 to 1000 Pa and in a temperature range of −40 to +60 °C. The results are presented as isotherm fields and load-dependent isosteric heats of adsorption. The isotherms are plotted as equilibrium load [mol·kg−1] or [mmol·m−2] versus equilibrium partial pressure [Pa]. The symbols and lines represent the experimental data and fitted isotherms, respectively. Since the partial pressure of the carrier gas nitrogen is much higher compared to the adsorptive’s, all presented isotherm fields show mixture adsorption equilibria of the alkane and nitrogen. The experimental data as well as the estimated uncertainties are included in the Supporting Information. All approximated isotherm parameters, regression coefficients, and standard deviations of regression are presented in Table 3. Load-dependent isosteric heats are presented as isosteric heats of adsorption [kJ·mol−1] versus equilibrium load [mmol·m−2]. It should be noted that there are only few equilibrium data in the literature in a comparable concentration and temperature range.31,46,47 However, a comparison with the equilibrium data presented here seems not adequate due to very different preparation of the adsorbent material (temperature and atmosphere). Figures 3−5 show the adsorption isotherms of ethane, propane, and n-butane on the activated carbon Norit RX 1.5 Extra. All isotherms exhibit a concave shape according to IUPAC type I. The slopes decrease with increasing partial pressure of the adsorptive. In the range of pressures covered no isotherm reaches a plateau. The experimental data were fitted to the Sips equation by nonlinear regression with a regression coefficient R2 > 0.99. The heterogeneity constant n is smaller than 1 for all isotherms and increases with temperature. The affinity constant b is in the range of 10−3 to 10−4 Pa and decreases with temperature. The behavior of the isotherm parameters is characteristic for hydrocarbon adsorption on microporous heterogeneous activated carbons.45 At low partial pressure hydrocarbon adsorption takes place in micropores of the activated carbon due to energetically favorable interactions with the pore walls (see section 2.2). In very narrow pores the molecule can also interact with the surface of the opposite wall.

Figure 2. Exemplary Sips isotherms for varying parameter n.

isotherms for varying parameters n and for constant parameters Xmon and b. For n ≤ 1, the inflection point is close to the origin leading to almost concave isotherms. For n > 1, Sips isotherms are sigmoidal with the extent of the sigmoidal shape increasing with increasing values of n. Modeling of the isotherm parameters was carried out by minimizing the sum of square errors using nonlinear regression following the Levenberg−Marquardt algorithm. The isosteric heat of adsorption Δhads [kJ·mol−1] is calculated using eq 4, where R0 [kJ·(mol·K)−1] is the gas constant and T [K] the adsorption temperature. The isotherm equation was rearranged for the partial pressure. To construct the isostere, the logarithmic partial pressure is plotted versus the inverse temperature for given load. The isosteric heat of D



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Table 3. Isotherm Parameters, Regression Coefficients, and Standard Deviations of Regression Sips Model adsorptive

adsorbent

ethane

AC

propane

AC

n-butane

AC

propane

13X

n-butane

13X

T/°C

Xmon/mol·kg−1

−40 −30 −20 0 20 −40 −20 0 20 −40 −30 −20 0 20 40 60 −40 −20 −10 0 20 −40 −30 −20 −10 0 20 40 60

11.96 11.56 11.12 10.08 8.53 9.92 9.51 9.06 8.57 8.35 8.22 8.09 7.81 7.53 7.22 6.88 1.65 1.62 1.59 1.33 4.23 2.21 1.99 1.72 1.53 1.44 1.34 1.29 1.37

u(Xmon)/mol·kg−1 0.392 1.264 1.343 1.735 3.152 0.181 0.496 0.235 0.165 0.241 0.143 0.144 0.208 0.479 1.176 1.767 0.007 0.010 0.016 0.028 1.929 0.373 0.137 0.044 0.008 0.007 0.010 0.018 0.077

b/Pa−1

8.24 7.15 4.66 3.16 2.29 2.07 6.54 1.99 8.99 3.10 1.84 8.83 2.86 9.77 3.82 1.91 3.04 9.78 4.47 3.00 2.85 1.21 3.75 1.06 1.75 6.09 1.77 6.63 2.34 Henry Model

× × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

u(b)/Pa−1 −5

10 10−5 10−5 10−5 10−5 10−3 10−4 10−4 10−5 10−2 10−2 10−3 10−3 10−4 10−4 10−4 10−2 10−3 10−3 10−3 10−4 101 100 100 10−1 10−2 10−2 10−3 10−3

1.73 2.23 2.18 2.59 3.60 8.39 5.30 3.69 3.17 6.00 3.36 1.23 3.11 1.61 1.36 1.15 6.23 1.64 9.08 1.09 1.54 1.59 9.95 3.43 7.89 1.77 4.83 2.03 2.32

× × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

−6

10 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−4 10−4 10−4 10−5 10−5 10−5 10−5 10−4 10−4 10−5 10−4 10−4 101 10−1 10−1 10−3 10−3 10−4 10−4 10−4

n/− 0.628 0.662 0.699 0.748 0.799 0.519 0.533 0.535 0.552 0.409 0.391 0.408 0.473 0.492 0.494 0.518 1.292 1.491 1.618 1.534 1.213 0.143 0.216 0.407 0.918 1.288 1.440 1.475 1.284

R2

u(n)/− 4.27 6.51 9.37 1.60 3.07 1.99 2.82 4.20 6.38 3.73 3.86 3.90 4.58 5.70 9.16 1.28 3.32 3.79 4.29 5.49 8.34 6.65 5.27 7.34 4.18 4.81 5.12 5.52 7.18

× × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

−3

10 10−3 10−3 10−2 10−2 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

0.9999 0.9998 0.9996 0.9999 0.9999 0.9995 0.9997 0.9996 0.9998 0.9917 0.9987 0.9997 0.9991 0.9994 0.9997 0.9998 0.9944 0.9980 0.9995 0.9996 0.9991 0.9972 0.9951 0.9977 0.9811 0.9852 0.9976 0.9996 0.9998

adsorptive

adsorbent

T/°C

kH/Pa−1

u(kH)/Pa

R2

ethane

13X

−40 −30 −20 0 20

75.27 44.78 20.01 11.47 5.17

0.00387 0.24865 0.05772 0.03052 0.00187

0.99939 0.99929 0.99982 0.99984 0.99969

Figure 3. Adsorption isotherms of ethane from nitrogen on activated carbon Norit RX 1.5 Extra at different temperatures.

Figure 4. Adsorption isotherms of propane from nitrogen on activated carbon Norit RX 1.5 Extra at different temperatures.

E



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Figure 6. Adsorption isotherms of ethane from nitrogen on UOP zeolite 13X at different temperatures.

Figure 5. Adsorption isotherms of n-butane from nitrogen on activated carbon Norit RX 1.5 Extra at different temperatures.

Subsequently, as partial pressure increases, the adsorptive molecules occupy energetically less attractive sites in larger micropores, mesopores, and macropores. No influence of lateral interactions on the shape of the isotherms can be observed in the experimental range of partial pressure. However, according to Ritter et al.48 one can assume that in adsorption of nonpolar alkanes on heterogeneous surfaces lateral dispersion interactions occur. However, the partial pressure of the inflection point resulting from the sigmoidal shape typical for these interactions is below the first measurement point. The addition of dispersion interactions with the surface and with other adsorbed molecules on the surface leads to a steep increase of the adsorption isotherm already at low partial pressures. As temperature increases at given partial pressure, adsorption sites of lower energetic value cannot be occupied anymore, so that the energy distribution of the available sites becomes more homogeneous. This results in an increase of the heterogeneity constant and a decrease of the affinity constant. Figures 6−8 represent the isotherms of ethane, propane, and n-butane on zeolite 13X. In the range of experimental partial pressures ethane adsorption follows a Henry isotherm. Henry’s coefficients were fitted by linear regression, and a correlation coefficient R2 of >99% was found. The value rises by a factor of 14.5 as temperature decreases from +20 to −40 °C. Adsorption of ethane on zeolite 13X probably takes places due to dispersion and induced interactions with the surface. As the adsorption isotherms of ethane on zeolite 13X exhibit Henry behavior, the impact of lateral interactions on the adsorbed amount is assumed to be negligible. The isotherms of propane and n-butane are concave at low temperatures and suggest a slight s-shape at higher temperature (see bottom part of Figures 7 and 8). At low temperatures, both adsorptives reach a capacity plateau. The experimental data were fitted to the Sips equation with a regression coefficient R2 > 0.98. The affinity constants decrease as temperature increases, indicating a receding affinity between adsorptive and surface. The heterogeneity constants of propane are >1 at all temperatures, while in the case of n-butane n > 1 at higher temperature but n < 1 at lower temperature.

Figure 7. Adsorption isotherms of propane from nitrogen on UOP zeolite 13X at different temperatures.

As explained in section 2.2, the sigmoidal isotherm shape with an inflection point resulting from n > 1 is ascribed to the addition of induced and dispersion interactions with the surface and lateral dispersion interactions with other adsorbed molecules. This effect is also discussed by other authors.43,45,47,49 The partial pressure required to form an adsorbate phase with lateral interactions increases with temperature. Therefore, in n-butane adsorption at temperatures ≤−20 °C, a wellestablished adsorbate phase is supposed to appear already at F



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Figure 8. Adsorption isotherms of n-butane from nitrogen on UOP zeolite 13X at different temperatures. Figure 9. Adsorption isotherms of all adsorptives on (a) activated carbon Norit RX 1.5 Extra and (b) UOP zeolite 13X.

very low partial pressures. As a result, an inflection point of the isotherm exists at a lower partial pressure than the first measurement point and n < 1. Higher temperatures require higher partial pressures to form the adsorbate phase. Consequently, the inflection point of the isotherms shifts into the measurement range and n > 1. In propane adsorption, affinity is lower than in n-butane adsorption, which is why sigmoidal isotherms with n > 1 are found over the entire temperature range. 3.2. Comparison of Adsorptives. Figure 9 compares the adsorbed amounts of ethane, propane, and n-butane at −40 and +20 °C on activated carbon (upper part) and zeolite 13X (bottom part). In general, the adsorbed amount increases with increasing alkane chain length from ethane to n-butane because both the number of molecular bonding sites and molecular polarizability (see Table 1) increase. Moreover, a pronounced temperature dependence of adsorption is obvious. For example, as temperature decreases from +20 to −40 °C, at a partial pressure of 1000 Pa the adsorbed amount on the activated carbon increases by a factor of 5 to 2 mol·kg−1 in the case of ethane, by a factor of 3.2 to 6 mol·kg−1 in the case of propane, and by a factor of 1.8 to 6.8 mol·kg−1 in the case of n-butane. A similarly strong increase turns out in adsorption of the alkanes on zeolite 13X. Consistently, temperature dependence decreases as the hydrocarbon chain length grows. 3.3. Comparison of Adsorbents. Figure 10 shows isotherms of ethane, propane, and n-butane on activated carbon and zeolite 13X at +20 °C (upper part) and −40 °C (bottom part). In order to work out the influence of adsorbent

pore structure and chemical surface properties, the adsorbed amounts were normalized to the inner surface area because the activated carbon has a much higher surface than the zeolite (see Table 2). The normalized adsorbed amount of ethane is higher on the activated carbon than on the zeolite. On the activated carbon, micropore adsorption occurs where dispersion interactions in narrow pores to more than one surface dominate. This is obviously more favorable than adsorption in the zeolite cage where induced and dispersion interactions are supposed to prevail. However, in the zeolite cage interactions with more than one surface are weaker due to the large cage diameter. Comparing the adsorbents with respect to adsorption of propane and n-butane reveals several effects. Owing to limited space in the cages, zeolite adsorption may approach a plateau within the measurement range. In the case of propane, a plateau appears at −40 °C, while in the case of the more affine n-butane, the plateau is evident at +20 and −40 °C temperatures. At −40 °C a steeper initial isotherm slope is observed on both adsorbents. Compared to the activated carbon, this effect is even more pronounced on the zeolite. As for the activated carbon, more adsorption sites in larger pores may be occupied. In the case of the zeolite, strongly increasing lateral interactions in the adsorbed phase in the cage may be assumed due to the higher loading at low temperature. Generally, with increasing partial pressure the isotherms flatten. The effect is stronger on the activated carbon because of its energetic heterogeneity. The number of free attractive G



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Figure 11. Adsorption isosteres for n-butane on activated carbon Norit RX 1.5 Extra at different loadings.

Figure 10. Adsorption isotherms of all adsorptives on activated carbon Norit RX 1.5 Extra and UOP zeolite 13X at (a) + 20 °C and (b) −40 °C.

micropore sites decreases with increasing load. The zeolite’s surface is more homogeneous so that the isotherm slope is more constant before approaching the plateau. 3.4. Isosteric Heat of Adsorption. In this work, at least 3 experimental points were used to construct an isostere (see section 2.3). To maintain data consistency, no extrapolation beyond the range of experimental partial pressures was done. For that reason, the isotherms employed as well as the number of equilibrium points available for an isostere may vary. This leads to characteristic but undesired jumps in the calculation of the isosteric heat of adsorption. These jumps have no physical meaning, but they are systematic artifacts contributing to the uncertainty of the method. As an example, Figure 11 portrays adsorption isosteres of n-butane adsorption on activated carbon Norit RX 1.5 Extra. Isosteres were calculated for loads from 0.004 to 2.5 mmol m−2 and temperatures from −40 to +60 °C. Figure 12 displays the load-dependent isosteric heats of adsorption resulting from the isostere slopes. The slopes were calculated by linear regression, and correlation coefficients R2 > 99% were found. The isosteric heats of adsorption of the alkanes range from 25 to 45 kJ·mol−1 on both adsorbents. As the alkane chain length increases, the isosteric heat of adsorption increases by about 7 kJ·mol−1 per carbon atom, which may be mainly attributed to stronger dispersion interactions due to more molecular bonding sites. Quantitatively the results agree well with data from the literature.6,29,47 A slight decrease of adsorption enthalpy is found with all adsorptives on activated carbon because of the depletion of energetically favorable sites as the load increases. This behavior is characteristic for adsorbents with heterogeneous surface.9

Figure 12. Isosteric heats of adsorption of all adsorptives on Norit RX 1.5 Extra and UOP zeolite 13X at temperatures from −40 to +60 °C.

The isosteric heat of adsorption of ethane on zeolite 13X is nearly constant over the entire range of measurements, which is typical for adsorptives in the Henry range. Because of energetic homogeneity and low surface coverage, the zeolite provides a large number of equivalent sites, which results in Henry-type adsorption of ethane. The isosteric heats of adsorption of propane and n-butane on zeolite 13X exhibit a slight increase at low partial pressures followed by a broad region of more constant values. Initially, at very low coverage, mainly induced and dispersion interactions with the surface occur. With increasing load, more and more lateral dispersion interactions to neighbors on the inner cage surface form, which enhance the total energetic value of the adsorption sites. At higher loads the enthalpy stagnates, which may be indicative of multilayer adsorption in the cage where the sum of interactions to the surface and to neighbors remains constant. Finally, where the adsorption plateau is reached, another increase of the adsorption enthalpy seems to become apparent. The isosteric heat of adsorption of propane rises only H



Article

slightly from 35 to 37 kJ·mol−1 while n-butane has a significant rise from 42 to the region above 50 kJ·mol−1. A similar effect was discussed by Bläker et al.47 In the adsorption plateau region the cage is being completely filled. Bläker et al. suppose that the position of the molecules adsorbing now allows forming lateral interactions with other molecules in all directions, which in turn increases the energetic value of these final sites.

Funding

The authors wish to express their thanks to DFG Deutsche Forschungsgemeinschaft e.V. for funding the Research Project BA 2012/8-1. Notes

The authors declare no competing financial interest.

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4. SUMMARY AND CONCLUSION Equilibrium loadings of ethane, propane, and n-butane on activated carbon Norit RX 1.5 Extra and UOP zeolite 13X were measured by dynamic breakthrough experiments at temperatures from −40 to +60 °C and partial pressures from 5 to 1000 Pa. The adsorbed amount rises with increasing alkane chain length on both adsorbents because the number of molecular bonding sites and molecular polarizability increase. Decreasing temperature improves the adsorbed amount of all systems. The shorter the chain, the more pronounced is the effect. This finding may turn out to be important for technical applications where trace concentrations of short chain hydrocarbons have to be removed from gas streams. Adsorption isotherms of n-alkanes on activated carbon exhibit a concave pattern, which fits well to a Sips isotherm equation. The addition of dispersion interactions with the surfaces in narrow micropores and lateral interactions with other adsorbed molecules leads to a steep initial isotherm slope. At higher loads, the slope flattens due to adsorption on less attractive sites in larger pores. On zeolite, ethane shows Henry-type adsorption while the isotherms of propane and n-butane exhibit sigmoidal patterns owing to lateral dispersion interactions in the adsorbate phase which increase with the load. The analysis of the load-dependent isosteric heats of adsorption reflects the effects discussed for the isotherms. The heats of adsorption increase on both adsorbents as the alkane chain length increases. On the activated carbon, a slight decrease of the isosteric heats of adsorption is observed with all adsorptives, which may be attributable to the energetic heterogeneity of the adsorption sites. On zeolite 13X, ethane has a constant isosteric heat of adsorption typical for adsorption in the Henry range. The enthalpies of propane and n-butane on zeolite show a slight increase followed by a plateau and a final increase. This may be explained by increasing lateral interactions between the first adsorbing molecules, a broad range of multilayer adsorption in the cage with constant interactions, and a final enhancement effect by complete cage filling close to the plateau.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b01068. Tables of experimental data (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

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

Florian Birkmann: 0000-0001-8950-2282 I



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