Single and Binary Mixture Adsorption Behavior of C6 - C8

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Single and Binary Mixture Adsorption Behaviors of C6−C8 Hydrocarbons on Silica−Alumina Gel Frederik Berg,*,† Karina Gohlke,† Christoph Pasel,† Michael Luckas,† Tobias Eckardt,‡ and Dieter Bathen†,§ †

Thermal Process Engineering, University of Duisburg−Essen, Lotharstraße 1, D-47057 Duisburg, Germany BASF Catalysts Germany GmbH, Große Drakenburger Straße 93-97, D-31582 Nienburg, Germany § Institute of Energy and Environmental Technology, IUTA e. V., Bliersheimer Straße 60, D-47229 Duisburg, Germany Downloaded via UNIV OF NEW ENGLAND on November 20, 2018 at 19:25:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: To ensure the technical use of natural gas, heavy hydrocarbons and other components must be separated. Therefore, in this work the adsorption of C6−C8 hydrocarbons n-hexane, n-heptane, n-octane, benzene, toluene, and cyclohexane on a commercial silica−alumina gel (Sorbead H) was investigated in experiments and dynamic simulations. Single and binary mixture equilibrium isotherms were determined from cumulative breakthrough curve experiments on a fixed bed adsorber at 25−75 °C and 1.3 bar (abs) in the trace range up to approximately 5000 ppmmol. The adsorption behavior is interpreted on the basis of the molecular structure of the adsorptive, the surface chemistry, and the pore structure of the adsorbents. Binary mixture isotherms were calculated using the theory of the ideal adsorbed solution (IAST). The results were in good agreement with experimental data. The adsorption dynamics was investigated by breakthrough curve experiments and simulated dynamically using an isothermal model based on mass balances and a linear driving force (LDF) model for adsorption kinetics. zeolites.8 The highest equilibrium load and the highest adsorption enthalpy were determined for benzene. Diaz et al. found the same result when adsorbing different hydrocarbons on 13X and 5A zeolites.9 Furthermore, an increase in both loading and adsorption enthalpy with increasing chain length was observed by investigating the adsorption behavior of nalkanes. Papaioannou et al. investigated the adsorption of various hydrocarbons in the trace range on a zeolite.10 Capacity and enthalpy data from our own work on the adsorption of C6 hydrocarbons on silica−alumina gels prove that the aromatic benzene has both the highest capacity and the highest enthalpy.11,12 Song et al. have published several studies on kinetics and diffusion processes in the adsorption of cyclic hydrocarbons on zeolites.13,14 The adsorption dynamics of pure heavy hydrocarbons on different zeolites as well as different approaches for numerical simulation of breakthrough curves were also presented in some investigations.15−20 In addition to single-component measurements, some work on the simultaneous adsorption of heavy hydrocarbons has also been published. Steffan and Akgerman have carried out singlecomponent and multicomponent measurements for the

1. INTRODUCTION In addition to the main component methane, natural gas contains other components such as water, sulfur compounds, carbon dioxide, and heavy hydrocarbons. Due to the high dew point, heavy hydrocarbons can cause damage to the entire processing chain, e.g., to heat exchangers or CO2 membranes. Therefore, strict purity requirements apply to these key components for use as pipeline, liquefied natural gas (LNG), or gas-to-liquids (GTL) gas. Furthermore, heavy hydrocarbons are valuable basic chemicals used in the chemical industry. For these reasons a separation from the raw natural gas is necessary.1 Adsorption is a widely used process for dew point reduction and hydrocarbon recovery in natural gas processing. Due to their good regenerability, polar adsorbents such as zeolites, silica gels, and silica−alumina gels are often used.2,3 Silica gels in particular have a high cyclic stability and thus a long service life in the treatment process. The key components to be separated in the process are C6+ hydrocarbons.1 From a scientific point of view, it is expected that the adsorption of C6−C8 hydrocarbons is especially influenced by the different molecular structures of chain-shaped alkanes, cyclic aliphatics, and aromatic molecules. In the literature there are some research papers on singlecomponent adsorption of heavy hydrocarbons on different zeolites.4−7 Inel et al. compared the adsorption of C6 hydrocarbons n-hexane, benzene, and cyclohexane on two © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

September 14, 2018 November 8, 2018 November 9, 2018 November 9, 2018 DOI: 10.1021/acs.iecr.8b04498 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research adsorption of cyclic and chain-shaped hydrocarbons on silica gel.21,22 They found a decrease in load due to the competition of the adsorptives for the adsorption sites. During the recording of breakthrough curves several authors observed the so-called “roll-up” effect, which describes an exceeding of the inlet concentration due to displacement of the component by another adsorptive.23−25 Some studies deal with the measurement of multicomponent isotherms for short-chain hydrocarbons and compare the experimental results with IAST predictions. The ideal adsorbed solution theory (IAST), which was thermodynamically derived by Myers and Prausnitz in 1965,26 assumes an ideal mixture; the loads in multicomponent cases can be calculated iteratively from the pure component isotherms. There are also approaches which allow a direct calculation of the state of equilibrium according to IAST under certain conditions, e.g., the approach of LeVan and Vermeulen in the case that both pure component isotherms can be easily approximated by Freundlich or Langmuir isotherms.27 An overview about the potential, fields of application and further development of IAST is presented by Walton et al. in a review article.28 For all chemically similar adsorbents a good prediction is achieved.29−33 Since IAST requires an iterative solution, Seidel-Morgenstern et al. have proposed a numerically efficient IAST approach which allows faster calculations of the dynamics in fixed-bed adsorbers.34 Yun et al. investigated the adsorption of binary and ternary mixtures of benzene, toluene, and p-xylene in the trace range on activated carbon and found good agreement with IAST predictions.35 Furthermore, using the linear driving force (LDF) approach and the extended Langmuir equation, they present a mathematical model for the dynamic simulation of breakthrough curves in multicomponent adsorption, which shows good agreement with experimental data. Möller et al. also published studies on multicomponent dynamics in 2017.36 The adsorption dynamics of methane and carbon dioxide in helium on a zeolite, activated carbon, and carbon molecular sieve were simulated with the LDF model and fitting of a kinetic parameter. Data on thermodynamics of heavy hydrocarbon adsorption in trace concentrations on silica−alumina gels are barely published. For these systems, a systematic study of adsorption dynamics in single-component and binary mixture adsorption is not available in the literature. For a deeper understanding of the adsorption mechanisms occurring, the Chair of Thermal Process Engineering at the University of Duisburg−Essen is investigating the kinetics and thermodynamics of these systems by measuring cumulative breakthrough curves. From the isotherm data at different temperatures, isosteric adsorption enthalpies are calculated to support the mechanistic interpretation. IAST is used to predict experimental binary mixture isotherms, and the accuracy of predictions is discussed. Based on the LDF approach, a dynamic simulation of singlecomponent and binary mixture adsorption is developed and calculations are compared to measured dynamic breakthrough curves.

Figure 1. Flow diagram of the fixed bed apparatus: a, shell-tempered bubbler; b, static mixer; c, shell-tempered adsorber column; d, air cooler; MFC, mass flow controller; EPC, electronic pressure control; μ-GC, micro gas chromatograph.

bubbler system. The adsorber column with a diameter of 3.8 cm and a height of 17.5 cm meets the technical design rules recommending a ratio of at least 4:1.37 Also, the adsorber diameter (3.8 cm) and the adsorbent particle diameter (0.3 cm) conform to the recommended ratio of at least 10 to reduce wall effects.38 Shell tempering of the vertical column and electrical heating of the pipes allow an adjustment of process temperature between 25 and 300 °C. Thermal process monitoring is performed by type T thermocouples with an accuracy of ±0.5 K, one of which is installed upstream and one is downstream of the adsorber column while another five are distributed at equal distances inside the fixed bed. Two pressure sensors and an electronic pressure control (EPC) guarantee an operating pressure of 1.3 bar. The components of the gas mixture are separated by a Varian micro gas chromatograph (CP-4900) and analyzed using a thermal conductivity detector. Before starting an experiment, the adsorbent material is preconditioned at 300 °C and ambient atmosphere for at least 12 h. Then, the hot material is filled into the adsorber column. Next, the bed is purged with dry nitrogen (99%, SigmaAldrich), the alkanes n-hexane (>99%, Sigma-Aldrich), nheptane (>99%, Alfa Aesar), and n-octane (>98%, Alfa Aesar), and the cyclic hydrocarbon cyclohexane (>99%, SigmaAldrich) were used as adsorptives. Table S244−51 (Supporting Information) lists some relevant structural and thermodynamic properties of the adsorptives. Certified test gases from Air Liquide were used to calibrate the analytics. The carrier gas nitrogen was provided via the university’s infrastructure with a purity of 99.999% and a dew point less than −80 °C. The adsorptives can be divided into three groups. The aromatic hydrocarbons benzene and toluene have a planar ring-shaped structure with delocalized π electrons in the conjugated double bonds. Both aromatics have a similar quadrupole moment and an anisotropic polarizability, which is slightly larger for toluene because of the additional methyl group. For the same reason, in contrast to benzene, toluene also has a dipole moment. The critical molecule diameter for both aromatics is 0.58 nm. The chain-shaped alkanes n-hexane, n-heptane, and n-octane are unbranched, saturated hydrocarbons with a critical molecular diameter of 0.43 nm. In contrast to aromatic compounds, they have neither a quadrupole moment nor a measurable anisotropic polarizability. The isotropic polarizability of the n-alkanes increases with increasing chain length. Cyclohexane belongs to the group of cyclic hydrocarbons and usually has a “chair-shaped” structure. This stable conformation results in the largest critical molecule diameter of 0.62 nm. The molecule has neither conjugated double bonds nor a significant quadrupole moment. As explained above, the IAST calculation is based on the assumption that the interactions with the surface and with adjacent adsorbed molecules are the same for adsorption in the mixture and in the pure component. To estimate the validity of this assumption, the limiting activity coefficients γ∞ i in the investigated binary mixtures are determined using the activity coefficient model UNIFAC. Table 1 lists the limiting activity coefficients of the investigated adsorptive pairs. All limiting activity coefficients are close to 1, which means that all mixtures behave approximately ideally. Therefore, an acceptable prediction by IAST can be expected. The binary mixtures of the alkanes n-hexane/n-heptane come closest to an ideal mixture with limiting activity coefficients of 0.999 each.

cumulative isotherms of up to 14 equilibrium points are measured. We are working on the assumption that the carrier gas (methane or nitrogen) has no influence on the adsorption of C6+ hydrocarbons at mesoporous silica gels. This has been shown for a variety of adsorptives in previous studies at our chair.39−41 (Additional remark: Of course this is not true for microporous zeolites.) Therefore, in the following experiments we use nitrogen as carrier gas. As already shown in refs 11 and 12, the equilibrium load of the adsorbent qeq is determined by mass balances from the measured breakthrough curve and can be described using the general Freundlich isotherm, eq 1. The Freundlich parameters n and kf are fitted using nonlinear regression;11,39 parameters and correlation coefficients R2 can be found in Table S1 (Supporting Information). qeq = k f y n

(1)

The load-dependent isosteric heat of adsorption ΔhAds is determined from the isotherms via load-dependent isosteres analogously to the Clausius−Clapeyron approach, eq 2.23 ∂ ln(pi )

ΔhAds = R

1

∂T

(2)

q

The slope of the adsorption isosteres required for the calculation of ΔhAds is determined by linear regression. To describe and predict the state of equilibrium in binary mixture adsorption, the applicability of the ideal adsorbed solution theory (IAST) according to Myers and Prausnitz is investigated. The binary mixture isotherms are calculated from the Freundlich parameters of the measured single-component isotherms. IAST is based on the assumption that adsorption of a component in the mixture is not influenced by any other component and therefore can be calculated using information from the single-component isotherms only. This assumption is particularly well fulfilled with very low loads, even for heterogeneous surfaces. At higher loads, IAST only provides good results for systems whose components differ only slightly in terms of their intermolecular interactions (lateral and with the surface) and the area occupied by molecules on the surface.28 For an exact derivation and discussion of the applicability of IAST, see refs 26, 28, 29, and 42. LeVan and Vermeulen give a mathematical modification of IAST which was explicitly developed for Freundlich isotherms, according to eq 3.27,43 qeq, i = Ä ÅÅ ÅÅ ÅÅ ÅÅ ÅÅÇ

n̅ k f, i

1/ ni

( ) ni

k f, i

1/ ni

( )

pi

i k f,j y pi + jjj n zzz k j{ ni

1/ nj

+ ΔF ÑÉÑ1 − n̅ ÑÑ pj ÑÑÑ ÑÑ ÑÖ

(3)

In this paper, this approach was integrated in the physical model described in section 3 for the dynamic simulation of breakthrough curves in the binary mixture. The correction term ΔF and the mean exponent n̅ are also only calculated from the parameters of the two single-component isotherms. The complete formulation of the quantities can be found in ref 43. Since the origin of this equation and the fundamental assumptions are identical to general IAST, only the term “IAST modeling” is used in the following. C

DOI: 10.1021/acs.iecr.8b04498 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Limiting Activity Coefficients γ∞ i of the Binary Mixtures Calculated with UNIFAC adsorptive mixture (1)/(2)

γ∞ 1

γ∞ 2

n-hexane/n-heptane benzene/toluene n-hexane/cyclohexane toluene/cyclohexane benzene/n-hexane

0.999 0.987 1.037 1.301 1.466

0.999 0.991 1.052 1.280 1.660

3. PHYSICAL MODEL OF GAS PHASE ADSORPTION For investigation of adsorption kinetics, dynamic simulations of experimental breakthrough curves were done using the solver Aspen Custom Modeler by AspenTech. The simulation relies on a finite-difference method. It uses a physical model based on partial differential equations to describe mass and heat transfer. All mass transfer resistances are placed in the boundary film and combined in an effective mass transfer coefficient keff. Thus, mass transfer is described by the effective mass transfer coefficient and a linear driving force (LDF). To model adsorption dynamics, the fixed-bed adsorber is divided into increments. For each increment the mass balances of the fluid and the solid phase are calculated. Since adsorption is investigated in the trace range, the heat released is so low that energetic balancing can be neglected. Additionally, the following assumptions are made: • There is idealized plug flow in the fixed bed. • The fluid phase is considered an ideal gas. • The carrier gas does not adsorb in any case. • The adsorbent is approximated by spherical particles of uniform diameter. • Any radial gradients of concentration and temperature can be neglected. • There is no pressure drop along the fixed bed. • The mass transfer through the external boundary layer can be estimated by a Sherwood correlation for a film diffusion coefficient βF.43 • The calculation of axial dispersion is according to Wakao’s approach.54 For each component i, the following differential equation results from a mass balance of the fluid phase:

With values of 1.466 and 1.660, benzene/n-hexane shows the highest deviation from ideal behavior. 2.3.1. Nitrogen Isotherms. For the adsorbent Sorbead H a nitrogen isotherm was measured at a temperature of 77 K with a volumetric measuring device, Belsorp-Max from Bel-Japan, Inc. Before the measurement, the adsorbent was conditioned under vacuum (p < 10−3 Pa) for approximately 6 h. The Brunauer−Emmett−Teller (BET) method according to DIN ISO 9277 was used to determine the specific surface area. The pore size distribution shown in Figure 2 was calculated using the nonlocal density functional theory (NLDFT) method. The total pore volume was computed using the Gurvich method. The micropore volume was determined by the Dubinin− Radushkevich method according to DIN 66135. Table S3 (Supporting Information) gives an overview of the structural properties of Sorbead H. 2.3.2. Surface Chemistry. The surface of the silica−alumina gel mainly consists of weakly polar siloxane (Si−O−Si) and polar silanol groups (Si−OH).52 Due to the negative partial charge of oxygen and the positive partial charge of hydrogen, strong dipole−multipole interactions with permanent or induced multipoles can occur. Surface silanol groups occur in different forms (isolated, terminal, and vicinal), which cause differently strong interactions.53 The total number of silanol groups and the proportion of forms depend on conditioning parameters such as temperature and time during drying.52 The aluminum contained in the silica−alumina gel can be incorporated into the silicone network in various ways. The tetrahedral and octahedral coordination of aluminum is described in the literature.41,52

keff, iA sp(1 − εL) ∂ci ∂ 2c V̇ ∂ci = Dax, i 2i − G − (qeq, i − qi) ∂t AεL ∂z εL ∂z (4)

ci denotes the adsorptive concentration of component i in the fluid phase, Dax,i is the axial dispersion coefficient, V̇ G is the volume flow, εL is the bed porosity, Asp is the specific external particle surface, and A is the column cross-sectional area. The change in load of the solid phase for each component i is obtained from a mass balance for the solid phase:

Figure 2. Logarithmic plot of differential pore volume versus pore radius of Sorbead H. D


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Figure 3. Adsorption isotherms at 25 °C (top) and isosteric heats of adsorption (bottom) of C6−C8 hydrocarbons on Sorbead H.

∂qi ∂t

=

keff, iA sp ρs

(qeq, i − qi)

the mass balance of the solid phase (for a detailed derivation of the equation, see ref 57):

(5)

∂qi

The driving force of the process is expressed by the load difference (qeq,i − qi), and the effective mass transfer coefficient keff,i describes the velocity of the adsorption process. Assuming that the effective mass transfer coefficient is composed of film diffusion and diffusion inside the pore system, it is calculated as follows:55,56 keff, i =

ρs 15 Deff, i 1 2 ρp ∂q ε p(15)Deff, i A sp R p 2 εp ∂ci 1 + R p βF, iA sp

∂t

=

15 Deff, i 1 (qeq, i − qi) 2 ρp ∂q ε p(15)Deff, i Rp 1 + 2 εp ∂ci R p βF, iA sp

(7)

In binary mixture adsorption, two mass balances must be solved for both the solid phase and the fluid phase. For a detailed derivation of the balances and differential equations, please refer to refs 55−57. Coupling of mass balances and thermodynamic equilibrium is finally done using the equation of LeVan and Vermeulen to calculate the mixture loads according to IAST, based on the Freundlich single-component isotherms of the two adsorptives. The following initial and boundary conditions are required to solve the equation system numerically:

(6)

The effective diffusion coefficient Deff,i takes into account all diffusion mechanisms within the pore system and describes the influence of kinetics on adsorption dynamics. This results in E


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Figure 4. Experimental breakthrough curves (data points) and simulated breakthrough curves (dashed lines) of adsorption of benzene on Sorbead H at 25 °C.

qi(z , t = 0) = 0

(8)

ci(z , t = 0) = 0

(9)

̇ VĠ (z = 0, t ) = VG,in

(10)

ci(z = 0, t ) = ci,in

(11)

∂ci (z = L , t ) = 0 ∂z

(12)

physical interactions between the adsorptive molecule and adsorbent surface. Compared to alkanes with the same number of carbon atoms, the aromatic hydrocarbons toluene and benzene have a significantly higher equilibrium load and adsorption enthalpy, since the quadrupole moment (see Table S2 and section 2.3) interacts with the permanent dipoles of the silanol groups on the surface. The dipoles of the silanol groups also induce dipole moments in the adsorptive molecules and dipole− induced dipole interactions occur. Furthermore, dispersion interactions occur between surface and adsorbent molecules, which increase with the number of carbon atoms in the molecule, because the number of binding sites and the polarizability increase. The higher load and higher isosteric heat of adsorption of toluene compared to benzene can therefore be attributed to the additional methyl group. Only induced and dispersion interactions are available for the adsorption of the almost nonpolar n-alkanes. Compared to the alkanes, the aromatics have a higher polarizability, which is why these interactions are stronger with the aromatics than with the alkanes. Loading and isosteric heat of adsorption of nalkanes increase due to the above-mentioned increase in interactions in the homologous series (n-octane > n-heptane > n-hexane). The slightly increased load and adsorption enthalpy of nhexane compared to cyclohexane can be attributed to the slightly higher polarizability of n-hexane and a more favorable arrangement on the adsorbent surface. The linear n-alkane can approach the surface better than the cycloalkane in the bulky chair conformation. 4.1.2. Kinetics of Adsorption. Five single-component breakthrough curves were recorded for each of the hydrocarbons investigated at adsorptive concentrations of 200, 500, 1000, 2000, and 3000 ppmmol. The breakthrough curves were simulated with the described transport model, and the effective diffusion coefficients were determined by fitting to the measured data. Figure 4 shows the measured and simulated breakthrough curves using the example of the adsorption of

All required parameters can either be calculated physically or estimated with sufficient accuracy with the exception of the empirical effective diffusion coefficient Deff,i. This parameter is calculated from a fitting of experimental single-component breakthrough curves using a least-squares method. It is assumed that mass transfer in binary adsorption in the trace range is dominated by the same mechanisms as in singlecomponent adsorption. Therefore, the diffusion coefficients determined from pure component measurements were also used in multicomponent adsorption simulations without any further fitting.

4. RESULTS AND DISCUSSION 4.1. Adsorption of a Single Component. 4.1.1. Physical Interactions during Adsorption. Figure 3 represents the isotherms of all systems at 25 °C. The concentration of each adsorptive in ppmmol is plotted on the x-axis, and the corresponding load [mol·kg−1] is shown on the y-axis. The measuring points are displayed as symbols, while the lines show the fitted isotherms. To determine the isosteric heats of adsorption on Sorbead H (see Figure 3), isotherms of 25−75 °C were measured at 10 °C intervals for all hydrocarbons investigated.11,12 As can be seen in Figure 3, the highest values for equilibrium load and isosteric heat of adsorption on the silica−alumina gel Sorbead H are achieved with toluene. The values descend in the sequence toluene > n-octane > benzene > n-heptane > nhexane > cyclohexane. This behavior can be explained by the F


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Figure 5. Binary isotherms for n-hexane/benzene mixtures on Sorbead H at 25 °C.

Figure 6. Binary isotherms for n-hexane/cyclohexane mixtures on Sorbead H at 25 °C.

The effective diffusion coefficient represents all diffusion mechanisms within the pore system and therefore does not allow an in-depth mechanistic discussion of the transport processes. We presume that a superposition of free gas diffusion and Knudsen diffusion takes place in the gas phase and that surface diffusion makes another important contribution to mass transport within the pore system. Due to the low pressure gradients prevailing during the experiments, it can be assumed that viscous flow has no significant influence. Nevertheless, the numerical values allow a comparison of the mass transport velocities of the six adsorptives which can be divided into three groups. The effective diffusion coefficients of the aromatics benzene and toluene are approximately the same and indicate the fastest mass transport within the pore system of Sorbead H. The three n-alkanes n-hexane, n-heptane, and noctane also have similar effective diffusion coefficients which are somewhat smaller than those of the aromatics. The effective diffusion coefficient of cyclohexane has the smallest value due to steric hindrance. Cyclohexane has the largest critical molecule diameter of the adsorptives.

benzene on Sorbead H. The time in seconds is plotted on the x-axis, and the adsorptive concentration is shown on the y-axis. In order to facilitate the comparison of different experiments, the adsorptive concentrations were normalized to the respective inlet concentration. The measured breakthrough curves show a typical S-shape and become steeper with increasing adsorptive concentration. A very good agreement between the simulation and the measured breakthrough curves can be seen for all investigated systems. Only in the simulation of the 200 ppm breakthrough curve a slight deviation between the simulation and the measured data can be detected, which, however, appears to be negligible for the design of adsorption units and mechanistic discussions. The deviation can be attributed to higher experimental uncertainties in the lower concentration range of the adsorption isotherms used for the simulation (Figure 3). The fitted effective diffusion coefficients are almost constant for all adsorbents in the investigated concentration range. Table S4 (Supporting Information) shows the mean values used for the binary mixture simulation presented in section 4.2. G


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Figure 7. Binary isotherms for n-heptane/n-hexane mixtures on Sorbead H at 25 °C.

Figure 8. Binary isotherms for benzene/toluene mixtures on Sorbead H at 25 °C.

4.2. Thermodynamics of Adsorption. In the following, the experimental binary mixture isotherms are compared with the prediction of the multicomponent equilibrium by IAST. Each mixture is described by two measurements. In the first experiment, the concentration of component A is kept constant at 2000 ppm and the concentration of component B is gradually increased. In the second experiment, the concentration of B is exchanged and kept constant while the concentration of A is varied. In the following diagrams the measured loads are shown as symbols, the loads according to the IAST calculation are shown as dashed lines, and the pure substance loads of both adsorptives are shown as solid lines. Figure 5 shows the mixture isotherms of the pair benzene/n-hexane, which has the highest deviations from the ideal solution according to the limiting activity coefficients in the binary mixture (see Table 1). The shape of both mixture isotherms corresponds to the expected competitive behavior in the binary system. Due to the competitive situation and the limited number of adsorption sites, both mixture loads are below the pure component load.

In the case of an increase in concentration of the varied component, a drop in load can also be seen for the component with constant concentration due to this behavior. As benzene adsorbs more strongly, it exhibits a smaller decrease in capacity in the presence of n-hexane than n-hexane does in the opposite case. This effect is also observed with all other mixtures. The IAST prediction describes the experimental results very well, although the largest deviation from the ideal solution was expected for this pair (see Table 1). The load of benzene is slightly overestimated by IAST, while the load of n-hexane is slightly underestimated. As the adsorptive concentration rises, the deviation of the IAST calculation from the measured values increases. It can be assumed that at low surface coverage in single-component as well as in mixture adsorption more adsorbed molecules can occupy energetically equivalent adsorption sites than is possible at high degrees of coverage, and thus mixture adsorption at low coverage is closer to the ideal case. The differences between the IAST prediction and the experimental data are shown in Table S5. H


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Figure 9. Isotherms of mixture of cyclohexane/toluene on Sorbead H at 25 °C.

As an example of a mixture of a cyclic and a linear alkane, the pair n-hexane/cyclohexane was investigated (see Figure 6). Both components show a similar load decrease in the mixture. The similar competitiveness can be explained by the similar nature and strength of interactions with the surface of Sorbead H. Both adsorptives show comparable pure component adsorption capacities as well as adsorption enthalpies (see Figure 3). IAST provides a good approximation of the measured values. Both the shape of the isotherms and the load decrease of the adsorbent with constant concentration are well predicted. Figure 7 shows the results of the homologous neighbors nhexane/n-heptane, which behave almost ideally in the binary liquid mixture. Accordingly, the IAST prediction is also very accurate. In competition with n-hexane, n-heptane shows a significantly lower load loss as n-heptane is more strongly adsorbed (see Figure 3). As an example of a mixture of two aromatics, the adsorption of the benzene/toluene pair on Sorbead H was investigated (see Figure 8). The isotherms show a strong decrease for benzene in the presence of toluene compared to the pure component measurement. The load of toluene decreases considerably less. This is in line with previous observations, as toluene adsorbs more strongly. Also for this mixture the prediction of loads according to IAST provides very good results with a slight overestimation of the stronger adsorbing and slight underestimation of the weaker adsorbing component. Finally, as an example of a mixture of a cycloalkane and an aromatic, the adsorption of the pair cyclohexane/toluene was investigated (see Figure 9). In this system the largest load difference is found. The loading of toluene, which adsorbs much more strongly, decreases only slightly in the presence of cyclohexane compared to the pure component loading. The measured cyclohexane load, on the other hand, is reduced in the mixture by approximately 50% compared to the pure component load. The prediction of loads by IAST reveals once again that the load of the more strongly adsorbing component, toluene, is slightly overestimated and the load of the weakly adsorbing component, cyclohexane, is slightly underestimated. The

deviations between experiment and IAST prediction are significantly larger than in the other systems (see Table S3). For most binary systems investigated in this paper IAST predictions are in very good agreement with experimental results. It can therefore be assumed that the interactions with the surface and the adjacent molecules do not differ greatly in the mixture and in the pure component on Sorbead H. According to Walton et al. deviations can be correlated with the surface coverage, the ideality of the mixture, and the load difference between the adsorptives.28 The correlation with surface coverage is confirmed by our results, as we observe an increasing deviation between experiment and prediction with an increase in adsorptive concentration. The effect of ideality and load difference of the hydrocarbon pairs can also be seen in our data (Table S5, Supporting Information). The accuracy of IAST prediction increases with increasing ideality of binary mixtures. The mixtures of the n-alkanes, the two aromatics, and the two C6 hydrocarbons n-hexane and cyclohexane exhibit an almost ideal mixing behavior with limiting activity coefficients close to 1. The mean deviations are between 2 and 9% for all three mixtures. In contrast, the mean deviations of the least ideal mixture are 10 and 35%. Compared to n-hexane/benzene the binary mixture toluene/ cyclohexane has slightly lower limiting activity coefficients, so a better prediction is expected. However, the measurement of the cyclohexane isotherm in the presence of toluene shows the highest deviation of all investigated systems with an average value of 65%. In this mixture, the load difference between the pure components is significantly larger than in the case of the other pairs. 4.3. Kinetics of Adsorption. The experimental and simulated breakthrough curves of the simultaneous adsorption of all binary systems on Sorbead H at 25 °C and 1.3 bar are shown in Figure 10. The concentrations of the hydrocarbons were kept constant at 2000 ppm. The measured breakthrough curves are represented as dashed lines, and the curves calculated using the simulation model are represented as solid lines. In all mixtures, the weaker adsorbing component, e.g., n-hexane in the mixture n-hexane/benzene, breaks through much faster than the more strongly adsorbing component, such as benzene. Furthermore, the weaker component, except for n-hexane/cyclohexane, shows a steeper increase than that I


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Figure 10. Experimental and simulated breakthrough curves of binary mixtures on Sorbead H at 25 °C.

example n-hexane penetrates higher layers of the bed faster because the lower part of the bed is saturated earlier due to the

of the stronger one and an exceeding of the inlet concentration (“roll-up” effect). When loading the fresh fixed bed, for J


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Industrial & Engineering Chemistry Research lower capacity. When benzene reaches the higher layers saturated with n-hexane, benzene displaces adsorbed n-hexane molecules, which add to the n-hexane molecules in the feed and thus exceed the inlet concentration. Only in the simultaneous adsorption of n-hexane and cyclohexane no roll-up is observed due to the similar pure component loads and enthalpies (see Figure 3). The breakthrough curves simulated with IAST using the pure component diffusion coefficients correspond very well to the experimental data. The roll-up effects and the breakthrough times are reproduced with very good accuracy. Even the dynamics of cyclohexane/toluene are well modeled, although the deviation between predicted and measured load is the highest. The slight deviations between the simulated and the measured breakthrough curves are due to the inaccuracies in the prediction of the experimental binary equilibria (Table S5) by IAST. In all systems IAST overestimates the adsorption of the stronger component which results in a delayed breakthrough predicted by the simulation compared to the experiment. The small deviations observed for the simulation of the weaker components can be explained analogously. Nevertheless, we can conclude that the two-component adsorption of C6−C8 hydrocarbons on the silica−alumina gel Sorbead H can be described by an LDF model coupled with IAST with surprisingly good accuracy.

The dynamic breakthrough curves of the binary hydrocarbon mixtures were predicted by the model with very good accuracy. The behavior of components with different loads, which generate a “roll-up” effect, as well as the breakthrough of components with similar adsorption affinity, whose breakthrough curves both show a characteristic S-shape, was described with good accuracy. The deviations between the simulated and the experimental curves can be attributed to inaccuries in the IAST description of the binary equilibria. As a model relying only on pure component data is widely applicable, no further fitting was done to minimize the deviations.

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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.iecr.8b04498. Tables showing isotherm parameters and regression coefficients, structural and thermodynamic properties of the adsorptives, structural properties of Sorbead H, mean effective diffusion coefficients as well as deviations between experimental mixture isotherms and IAST calculation (PDF)

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AUTHOR INFORMATION

Corresponding Author

5. CONCLUSION For the adsorption of C6−C8 hydrocarbons on the silica− alumina gel Sorbead H, equilibrium isotherms for single components and for binary mixtures at 25 °C were determined. Cumulative breakthrough curves were measured in a fixed-bed adsorber with continuous flow, and the load of the adsorbent was calculated using a mass balance. A physical model based on the LDF approach was used to describe the adsorption dynamics. Effective diffusion coefficients were determined from the fitting to breakthrough curves of singlecomponent adsorption. Using IAST, binary isotherms were predicted and the breakthrough behavior of the binary mixtures was simulated. The results were compared with the measurements. The aromatics benzene and toluene have the highest capacity and adsorption enthalpy compared to n-alkanes and cyclohexane. This is attributed to stronger interactions of the aromatics with the adsorbent surface due to the quadrupole moment and the higher polarizability. A comparison of nalkanes showed a proportional increase in equilibrium capacity and adsorption enthalpy with increasing chain length. A comparison between n-hexane and cyclohexane showed slightly higher capacities and adsorption enthalpies of the less bulky linear alkane. For most measured binary systems IAST predictions are in very good agreement with experimental data. In most cases, the load of the more strongly adsorbing component is slightly overestimated while the load of the less strongly adsorbing component is underestimated. Prediction accuracy is better if the two components in the mixture interact with the surface and with adjacent adsorbed molecules similar to pure component adsorption. This can be correlated well with the deviation of the limiting activity coefficients in the liquid phase, which do not differ strongly from 1 at very similar interactions. In addition, the IAST prediction is better for small degrees of coverage and small load differences of the pure components.

*E-mail: [email protected]. ORCID

Frederik Berg: 0000-0001-6060-586X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Chair of Thermal Process Engineering would like to thank BASF Catalysts Germany GmbH for funding and providing the adsorbents.

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NOMENCLATURE A = cross-sectional area of adsorber column, m2 Asp = specific area of particle, m2·m−3 ci = adsorptive concentration, kg·m−3 Dax = axial dispersion coefficient, m2·s−1 Deff = effective diffusion coefficient, m2·s−1 keff = effective mass transfer coefficient, s−1 kf = Freundlich coefficient, 10−6·mol·kg−1 L = length of fixed bed, m M = molar mass, g·mol−1 n = heterogeneity constant of Freundlich isotherm n̅ = mean heterogeneity constant of Freundlich isotherm p = total pressure, bar pi = partial pressure, Pa qeq = equilibrium load of adsorbent, mol·kg−1, kg·kg−1 qi = average load of solid phase, kg·kg−1 R = gas constant, J·mol−1·K−1 R2 = correlation coefficient Rp = particle radius, m t = time, s T = temperature, K V̇ G = volume flow, m3·s−1 yin = inlet concentration, ppmmol z = axial position, m

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Industrial & Engineering Chemistry Research ΔF = correction term ΔhAds = adsorption enthalpy, J·mol−1 βF = film transport coefficient, m·s−1 εL = bed porosity εp = particle porosity ρp = average density of particle, kg·m−3 ρs = apparent density of particle, kg·m−3

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Single and Binary Mixture Adsorption Behavior of C6 - C8

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