Trace Level Adsorption of Toxic Sulfur Compounds ... - ACS Publications

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Trace Level Adsorption of Toxic Sulfur Compounds, Carbon Dioxide, and Water from Methane Bastian Steuten,* Christoph Pasel, Michael Luckas, and Dieter Bathen University of Duisburg-Essen, Lotharstraße 1, D-47057 Duisburg, Germany S Supporting Information *

ABSTRACT: This paper presents breakthrough curves and isotherms of the adsorption of sulfur compounds, carbon dioxide, and water from a carrier gas (methane) on a fixed solid bed at 298 K and 1.3 bar. For the investigation two industrial adsorbents (silica−alumina gel, zeolite 5A) were used. The adsorptives were prepared in trace level concentrations up to 2000 mol-ppm. Common isotherm equations were fitted to the adsorption capacities which were obtained from breakthrough curves by mass balances. Binary systems (one adsorptive in methane) and ternary systems (two adsorptives in methane) are included. Methane is used to duplicate conditions of industrial scale natural gas treatment as far as possible. Though methane is a very weak adsorptive on oxidic adsorbents the reported adsorptive capacities might be slightly lower than pure component loadings accessible from a volumetric or gravimetric method. The adsorption isotherms of the binary systems show distinctly different capacities depending on the polarity of the adsorptive and the structure of the adsorbent. The investigation of the ternary systems reveals significant coadsorption and displacement as well as kinetic effects due to the presence of competing adsorptives.

1. INTRODUCTION Sulfur compounds need to be removed from natural gas, bio gas, and synthesis gas due to their corrosive and toxic properties. Damage of pipelines and power generation equipment as well as catalyst poisoning is observed even for trace concentrations of these compounds. The combustion of sulfur compounds forms SO2 which is also corrosive and causes acid rain. Rendering these gases exploitable for energy generation, for fuel cells, or as raw material for chemical syntheses requires desulfurization. Adsorption processes using impregnated activated carbons, silica−alumina gels, and zeolites are established in industrial desulfurization.1−3 In some applications also metal oxides are found.4 Literature Review. Numerous publications deal with desulfurization by activated carbons. Adsorption capacities largely depend on surface chemistry. Impregnation with, for instance, potassium iodide and sodium hydroxide is found to increase capacities.5−8 In the field of novel adsorbents the combination of a silica gel with metal oxide nanoparticles is reported.9 As a result of chemisorptive bonding impregnated adsorbents can hardly be regenerated. In contrast, zeolites and silica gels are physisorptive adsorbents which may be completely regenerated. Few capacity and kinetic data exist for these adsorbents which usually were not examined in the presence of a carrier gas. Fails, Chi, and Groninger measured the adsorption of H2S on several zeolites type A. They used pure H2S and mixtures of H2S with carbon dioxide and carbon dioxide/methane.10−12 Maddox presents pure component isotherms of CO2, H2O, and H2S on various molecular sieves at different temperatures.4 © XXXX American Chemical Society

Sakano et al. show pure component adsorption isotherms of methyl mercaptan, carbon dioxide, water, and other compounds together with breakthrough curves of carbon dioxide, water, and some multicomponent mixtures on a zeolite 5A.13 Weber et al. investigated the adsorption of ethyl mercaptan on NaX zeolites. They used the pure component as well as binary mixtures with hydrocarbons.14,15 Sarbak reports that modified NaX zeolites can be used for catalyzed conversion of ethyl mercaptan.16 Ryzhikov et al. studied the adsorption of methyl mercaptan and carbonyl sulfide in trace level concentrations on various zeolites.17 Tanada et al. measured the adsorption of pure H2S on a zeolite 5A.18 Table 1 summarizes some characteristic data for H2S adsorption on zeolites. For silica gels published equilibrium and kinetic data are rare. Zhou Li et al. studied the breakthrough characteristics of H2S in the trace level region on an impregnated silica gel.19 The objective of this work is to systematically describe the fundamentals of sulfur component adsorption in natural gas Table 1. H2S Adsorption Capacities on Zeolites in the Range of 2000 mol-ppm (Recalculated) test gas

temp [°C]

adsorbent load [mol/kg]

ref

pure gas pure gas pure gas pure gas gas mixture (H2S + CH4)

20 20 25 25 25

≈ 1.9 ≈ 0.35 to 1.03 ≈ 1.47 to 2.05 ≈ 1.99 1.64

12 18 10 4 this work

Received: March 28, 2013 Accepted: July 29, 2013

A

dx.doi.org/10.1021/je400298r | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Experimental Procedure. The adsorbents were prepared for 12 h in a drying oven according to manufacturer’s guidelines (silica−alumina gel at 448 K, zeolite 5A at 573 K). The experiments were performed as follows: (i) The column was filled with hot adsorbent from the drying oven. (ii) The column was rinsed and cooled down to 298 K using dry nitrogen (< 1 ppmv H2O). Then pure methane was passed through the column to saturate the bed and make the influence of methane and impurities coadsorption comparable for all experiments. (iii) The column was bypassed by the gas flow and the adsorptive was added to the gas flow. Part of the gas flow was directed to the gas analysis. In case of experiments with water a humidifier was used to add water to the gas flow. Concentrations were adjusted by diluting with methane. (iv) The gas mixture was directed into the column. The outlet gas was analyzed and concentrations were plotted versus time. The experiment was finished when the outlet concentrations became equal to the feed concentrations for all components indicating that the column was completely saturated. All experiments were conducted at 298 K and 1.3 bar. Instrumentation. The gas mixtures were prepared by thermal mass flow controllers from Bronkhorst High Tech. The gas analysis was carried out by a gas chromatograph MicroGC Varian CP 4900. The time interval between two analyses was less than 2 min. Water concentration was measured by a capacitive polymeric dew point sensor Testo 6615. The pressure was monitored at the column inlet and outlet by a pressure indicator Bd Sensors DMP 331i. Temperature was measured at the inlet and outlet of the column as well as at 3 points inside the bed by thermocouples type T. The thermocouples were axially distributed to 1/4, 2/4, and 3/4 of the height of the column. Their measuring points were located in the middle of the column’s cross section. To avoid undesired adsorption of sulfur compounds on metal surfaces the column, all tubing and valves in contact with sulfur containing gases were passivated by a cover of SilcoNert 2000 applied by SilcoTek. 2.4. Data Evaluation and Approximation. The area above the breakthrough curve was integrated to calculate the load X of the adsorbent. For a binary gas mixture containing an adsorbing and a nearly inert component the mass balance of the adsorber yields

cleaning. In order to improve the understanding of adsorption mechanisms we correlate the adsorption behavior with suitable molecular properties. To account for the influence of methane in natural gas all experiments were done with methane as carrier gas and the fixed bed was saturated with methane before the experiments. First, binary systems (one adsorptive + methane) were examined. Then, more experiments were done with ternary mixtures by addition of a second adsorptive (CO2, H2O) to analyze coadsorption and displacement effects due to competition of two adsorptives.

2. EXPERIMENTAL SECTION 2.1. Materials. For the adsorption experiments two industrial adsorbents of different structure were used, a microporous zeolite 5A and a mesoporous silica−alumina gel. The adsorbents were spherical particles obtained from BASF Catalysts Germany GmbH. Hydrogen sulfide (99.5 %) and carbon dioxide (99.95 %) were supplied by Air Liquide Deutschland GmbH, methyl mercaptan (99.5 %) and carbonyl sulfide (97.5 %) were delivered by Sigma-Aldrich Chemie GmbH. Ethyl and propyl mercaptan were purchased from Air Liquide Deutschland GmbH as certified calibration gases. For the experiments all gases were diluted with methane to achieve the desired concentration. For calibration of the gas analysis system certified mixtures were obtained from Air Liquide Deutschland GmbH. In all experiments the adsorption was measured with methane as carrier gas (99.95 %, Air Liquide Deutschland GmbH) in order to duplicate practical conditions of natural gas treatment as far as possible. The measurement of the pure methane capacity requires a volumetric or gravimetric apparatus which was not available. 2.2. Characterization of Adsorbents. The adsorbents were characterized by volumetric nitrogen isotherms at 77 K (Belsorp-Max by Bel Japan, Inc.). Before these experiments all adsorbents were prepared thermally under vacuum (< 10−3 Pa) for 6 h according to manufacturer’s guidelines (silica−alumina gel at 448 K, zeolite 5A at 573 K). Specific surfaces were determined by the BET method pursuant to DIN ISO 9277. The pore volume was extracted from the nitrogen isotherm at relative pressure p/p0 = 0.98 by the Gurvich method.20 The particle size range was given by the supplier and checked. All data can be found in Table 2.

X=

Table 2. Structural Characteristics of the Adsorbents trade name BET surface [m2/g] pore volume [cm3/g] particle size range [mm]

silica−alumina gel

5A zeolite

Sorbead H 737 0.49 2.5 to 3.5

5A Molecular Sieve 568 0.35 1.6 to 2.5

nfl̇ mads

∫t

teq 0

⎛ ⎛ 1 − y ⎞⎞ in ⎟⎟ ⎜y − ⎜y ⎜ in ⎜ out 1 − y ⎟⎟ dt ⎝ ⎝ out ⎠⎠

(2.1)

where ṅfl is the total mole flow, mads is the mass of adsorbent, yin is the mole fraction of the adsorbing component in the gas at the adsorber inlet, and yout is the mole fraction at the adsorber outlet. For the ternary case with two adsorbed components the load of adsorbent for component 1 is calculated by eq 2.2:

2.3. Breakthrough Curve Measurement. Experimental Setup. The experiments were conducted as breakthrough curve measurements in a lab scale fixed bed adsorber (Figure 1). This method simultaneously delivers equilibrium and kinetic data. The plant consists of the gas supply and mixing system equipped with an inert gas purge by nitrogen, thermal mass flow controllers (MFC) and a static mixer, the adsorber column with temperature and pressure measurement and a bypass, and the gas analysis system. The entire setup was regulated to 298 K by a Peltier thermostat.

X1 =

n fl̇ mads

∫t

teq 0

⎛ ⎛ 1 − y1,in − y2,in ⎞⎞ ⎜y − ⎜y ⎟ ⎟ dt ⎟⎟ ⎜ 1,in ⎜ 1,out 1 − y − y ⎝ 1,out 2,out ⎠⎠ ⎝

(2.2)

The load capacities were described by common isotherm equations. On zeolite 5A the Langmuir eq 2.3 delivered a good approximation to the experimental data: X = X mon B

b·y 1 + b·y

(2.3)



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Figure 1. Experimental setup.

For the Langmuir fitting established mathematical methods according to Schulthess and Dey were used.21 Uptakes of silica−alumina gel were fitted to the Henry eq 2.4 or the Freundlich eq 2.5:

X = kH · y

(2.4)

X = kF· y n

(2.5)

Following eq 2.6 a value for the coefficient of determination R2→1 was defined as a target for the fit. n

R2 = 1 −

∑i = 1 (X meas, i − Xcalc, i)2 n

∑i = 1 (X meas, i − X mean, i)2

(2.6)

2.5. Experimental Errors. Experimental errors of the adsorbent load calculated using eqs 2.1 and 2.2 arise from concentration measurement by gas chromatography, from mass flow controllers, and weighing of the adsorbent. The gas chromatographic analysis had the largest contribution with a relative error of só = 3 %. Taking into account Gaussian error propagation an estimated standard deviation of só = 4 % to 10 % was found for the adsorbent load. The wide error margin results from varying adsorbent weights and different concentration ranges of the adsorptives (sulfur compounds, 0 to 2000 mol-ppm; CO2, 0 to 5 mol-%).

Figure 2. Adsorption isotherms of sulfur species, CO2, and H2O on silica−alumina gel (T = 298.15 K, p = 1.3 bar).

0.7 to 0.2 mol/kg. The propyl mercaptan isotherm ends at 450 mol-ppm. Because of its high boiling point propyl mercaptan mixtures with methane in pressure cylinders are commercially only available up to a concentration of 450 mol-ppm. Isotherm extension to higher concentrations requires an expensive preparation of higher concentrated mixtures by evaporation of the liquid mercaptan into a methane flow. This was relinquished in this work. The less polar adsorptives (H2S, COS, CO2) show much lower loads. The adsorption behavior of these systems could be well fitted by the Henry and the Freundlich equation. Parameters and coefficients of determination R2 are reported in Table 3. 3.2. Adsorption Isotherms of Binary Systems on Zeolite 5A. Figure 3 depicts adsorption isotherms on zeolite 5A. Because of its molecular dimension the mass transfer of propyl mercaptan inside the pore system of the 5A mole sieve should be more limited by diffusion mechanisms than in case of the smaller mercaptans. Therefore, we expected extremely long experiment times and prohibitive operation costs to reach

3. RESULTS In the following diagrams adsorbent load (mol/kg) is plotted versus adsorptive equilibrium concentration in the gas phase. Symbols indicate the loads calculated from breakthrough curve integration. Lines represent fitted isotherms. The measurements comprise the adsorption of CO2, COS, H2S, CH3SH, C2H5SH, C3H7SH, and H2O from the carrier gas CH4. 3.1. Adsorption Isotherms of Binary Systems on Silica−Alumina-Gel. Figure 2 shows the isotherms of adsorption on the silica−alumina gel. H2O is the strongest adsorptive. A capacity of approximately 0.85 mol/kg is found at a water concentration of 450 mol-ppm. The mercaptans (propyl, ethyl, and methyl) have lower capacities in the range of C



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H2S Adsorption Isotherms of (H2S + CO2 in CH4). Two experimental series with different CO2 contents were performed. The H2S concentration was varied in the range of 0 to 2000 mol-ppm, while the CO2 concentration was held constant during a series. Figure 4 illustrates the experimental

Table 3. Isotherm Fitting Parameters of Binary Systems on Silica Alumina Gel adsorptive CO2 COS H2S CH3SH C2H5SH C3H7SH H2O

kH/[mol/mg]

n

R2

0.689 0.517 0.541 0.679

0.99 0.99 0.99 0.98 0.99 0.99 0.99

kF/[mol/mg]

−6

2.48·10 7.18·10−6 2.76·10−5 2.78·10−3 0.0172 0.0282 0.0136

Figure 4. Adsorption isotherms of H2S on silica−alumina gel in the presence of different CO2 concentrations (T = 298.15 K, p = 1.3 bar).

capacities of the silica−alumina gel and the fitted Henry isotherms. For comparison the H2S isotherm of the binary system (H2S in CH4) is also plotted. It is evident that the H2S capacity of the silica−alumina gel is lowered in the presence of CO2. At 0.5 mol-% CO2 the capacity drops by approximately 17 % related to the capacity without CO2. At the 10-fold CO2 concentration of 5 mol-% the H2S capacity is lowered by another 17 %. Table 5 contains the parameters of the isotherm fitting.

Figure 3. Adsorption isotherms of sulfur species, CO2, and H2O on zeolite 5A (T = 298.15 K, p = 1.3 bar).

equilibrium. For this reason, we relinquished the series of measurements with propyl mercaptan on the 5A zeolite. H2O reaches a load in the range of 7.4 mol/kg. The maximum load of the mercaptans attained in the experimental concentration range is 2.4 mol/kg to 2.9 mol/kg. For H2S and COS a load of 1.5 mol/kg to 1.65 mol/kg is observed. The capacity of the zeolite for CO2 adsorption was investigated up to 5 mol-%. At the end of the displayed concentration range it is very low (0.4 mol/kg). The data were fitted to the Langmuir equation. Langmuir parameters of the measured isotherms and coefficients of determination R2 are listed in Table 4.

Table 5. Isotherm Fitting Parameters of (H2S + CO2 in CH4) on Silica Alumina Gel adsorptive (matrix) H2S (0.5 mol-% CO2 in CH4) H2S (5 mol-% CO2 in CH4)

Table 4. Isotherm Fitting Parameters of Binary Systems on Zeolite 5A adsorptive CO2 COS H2S CH3SH C2H5SH H2O

Xmon/[mol/kg] 2.72 2.55 2.01 3.45 2.90 7.35

−5

7.749·10 6.62·10−4 2.1·10−3 2.98·10−3 5.91·10−3 0.211

−5

2.27·10 1.807·10−5

R2 0.98 0.99

H2S Adsorption Isotherms of (H2S + H2O in CH4). An analogous procedure was chosen to examine the influence of H2O on the adsorption of H2S. H2S concentration was varied from 0 to 2000 mol-ppm, while the H2O concentration was kept constant at 500 and 1000 mol-ppm H2O, respectively. Figure 5 compares the uptakes found for H2S adsorption. The presence of H2O also significantly reduces the H2S capacity of the silica−alumina gel. For a H2O concentration in the gas mixture of 500 mol-ppm the loss is about 60 % at 2000 mol-ppm H2S relative to the experiments without H2O. For 1000 mol-ppm of H2O the H2S capacity is lowered by approximately 66 %. Table 6 shows isotherm parameters and coefficients of determination for the fit. 3.4. Adsorption Isotherms of Ternary Systems on Zeolite 5A. H2S Adsorption Isotherms of (H2S + CO2 in CH4). In Figure 6 we present the H2S isotherms for binary (H2S in CH4) and ternary (H2S + CO2 in CH4) gas mixtures on zeolite 5A. The experiments were designed analogously to the investigation of the silica−alumina gel described in section 3.3.

R2

b [−]

kH/[mol/mg]

0.99 0.99 0.98 0.99 0.99 0.98

3.3. Adsorption Isotherms of Ternary Systems on Silica−Alumina-Gel. Most gas treatment processes deal with gas mixtures with several adsorbing components. For that reason simultaneous adsorption occurs at desulfurization of, for example, natural gas. In that respect CO2 and H2O are of peculiar interest. To describe the impact of these components on the adsorption process constant, amounts of CO2 and H2O were added to binary mixtures of H2S and methane. For ternary systems with mercaptans we expect similar effects. D



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H2S Adsorption Isotherms of (H2S + H2O in CH4). A comparison between H2S adsorption on zeolite 5A from binary and from ternary systems with H2O (500 and 1000 mol-ppm) is made in Figure 7.

Figure 5. Adsorption isotherms of H2S on silica−alumina gel in presence of different H2O concentrations (T = 298.15 K, p = 1.3 bar).

Table 6. Isotherm Fitting Parameters of (H2S + H2O in CH4) on Silica Alumina Gel −5

H2S (500 mol-ppm H2O in CH4) H2S (1000 mol-ppm H2O in CH4)

Figure 7. Adsorption isotherms of H2S on zeolite 5A in the presence of different H2O concentrations (T = 298.15 K, p = 1.3 bar).

R2

kH/[mol/mg]

adsorptive (matrix)

1.103·10 9.27·10−6

0.99 0.98

It is obvious that water strongly reduces H2S capacity. For 500 mol-ppm of H2O in the gas mixture a capacity loss of 75 % relative to the binary system (from 1.6 to 0.4 mol/kg) is observed at the right edge of the H2S concentration range. Doubling the water content to 1000 mol-ppm causes a capacity drop of approximately 90 % (from 1.6 mol/kg to 0.16 mol/kg). The isotherms can still be fitted to the Langmuir equation. Table 8 summarizes the fit parameters. Table 8. Isotherm Fitting Parameters of (H2S + H2O in CH4) on Zeolite 5A

The isotherm representation makes clear that the H2S capacity of the molecular sieve drops in the presence of CO2. In the case of the lower CO2 content (0.5 mol-%) a slight drop of about 4 % related to the binary system is found at 2000 molppm H2S. At a CO2 content of 5 mol-% a higher drop is clearly visible. At 2000 mol-ppm H2S the capacity is lowered by approximately 32 %. The isotherms can still be well fitted to the Langmuir equation. Table 7 summarizes the fit parameters.

H2S (0.5 mol-% CO2 in CH4) H2S (5 mol-% CO2 in CH4)

2.145 2.278

b [−] −3

1.35·10 4.73·10−4

b [−]

R2

H2S (500 mol-ppm H2O in CH4) H2S (1000 mol-ppm H2Oin CH4)

2.178 0.407

1.136·10−4 2.85·10−4

0.97 0.98

Table 9. Dipole Moments of the Adsorptive Molecules22−24

Table 7. Isotherm Fitting Parameters of (H2S + CO2 in CH4) on Zeolite 5A Xmon/[mol/kg]

Xmon/[mol/ kg]

4. DISCUSSION 4.1. Binary Systems. 4.1.1. Adsorption on Silica− Alumina-Gel. The studied adsorptives can be divided into two groups with respect to the adsorption characteristics on mesoporous silica−alumina gel (see Figure 2). In the CO2 molecule the carbon is positively and the oxygen atoms are negatively polarized, but because of the symmetric structure the overall dipole moment is zero (see Table 9). Thus the CO2 is considered to be nonpolar. For the CO2 and the

Figure 6. Adsorption isotherms of H2S on zeolite 5A in presence of different CO2 concentrations (T = 298.15 K, p = 1.3 bar).

adsorptive (matrix)

adsorptive (matrix)

R2 0.99 0.98 E

adsorptive

dipole moment [Debye]

H2O C3H7SH C2H5SH CH3SH H2S COS CO2

1.84 1.60 1.58 1.52 0.98 0.72 0



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Comparison of Binary Adsorption on Silica−Alumina Gel and Zeolite. Apart from the extraordinary behavior of methyl and ethyl mercaptan in the saturation region of the zeolite the capacity increases with increasing adsorptive polarity in the case of both polar adsorbents. As expected, in the examined trace level concentration range the zeolite capacities were higher than the capacities of the silica−alumina gel. The silanol and siloxan groups on the silica surface are less polar than the ionic sites of the molecular sieve. Silica gels reach higher capacities only for higher adsorptive concentrations28 when the molecular sieves are often saturated. 4.2. Ternary Systems. 4.2.1. Adsorption on Silica− Alumina-Gel. Impact of CO2 on H2S Adsorption. The isotherm representation (see Figure 4) illustrates that the presence of CO2 reduces the H2S capacity. The molecules compete for adsorption sites so, depending on CO 2 concentration, a different capacity loss is measured for H2S. This explanation is supported by the breakthrough curves plotted in Figure 8. All curves were measured at 2000 mol-ppm

weakly polar compounds H2S and COS low adsorption capacities are found. In the studied concentration range isotherms of this group follow the Henry equation. The isotherm of CO2 which was measured up to 5 mol-% to approximate realistic concentrations in natural gases is of a Henry-type even in this high concentration range. In contrast, the group of more polar mercaptans and water may be well described by the Freundlich equation. Distinctly higher adsorption capacities are seen. This behavior correlates with the dipole moments listed in Table 9 which may be regarded as a measure for the affinity of the adsorptive to the polar silica−alumina gel. According to this the affinity increases with increasing polarity in the sequence CO2 > COS > H 2S > CH3SH > C2H5SH > C3H 7SH > H 2O

It should be noted that this consideration does not include interactions owing to higher electrostatic moments like the quadrupole and octupole moment. The findings also coincide with the homologous series H2S− CH3SH−C2H5SH−C3H7SH where the next member is formed by the addition of a methylene group. Capacity and dipole moment in the series increase with increasing chain length. It is evident that the mercaptans exhibit adsorption characteristics which are clearly different from the related alcohols. For the alcohols polarity decreases with increasing chain length in the homologous series because the impact of the electronegative hydroxyl oxygen is forced back. In contrast the polarity of thiols is dominated by the electronegative carbons in the hydrocarbon chain since sulfur has a slightly lower electronegativity than carbon. 4.1.2. Adsorption on Zeolite 5A. The capacities found on zeolite 5 A may be well fitted to the Langmuir equation (see Figure 3). As in the case of the silica−alumina gel the capacities of zeolite 5A increase with increasing polarity of the adsorptive. The isotherms of methyl mercaptan and ethyl mercaptan give rise to more differentiated conclusions. In the range of low concentrations (< 500 mol-ppm) the expected behavior is visible. The more polar ethyl mercaptan reaches higher capacities and the Langmuir parameter b is larger. This parameter is a measure for the adsorptive affinity to the adsorbent and dominates the initial slope of the Langmuir isotherm. However, at higher concentrations the isotherms intersect and the Langmuir plateau of the methyl mercaptan is higher. This may be due to two reasons: Zeolite 5A is synthesized from zeolite 4A by the exchange of Na+ ions with Ca2+ ions. The window aperture of the Ca cages is 5.0 Å.25 According to the manufacturer the exchange is not complete (degree of exchange > 65 %), thus zeolite 5A still has Na cages (window aperture of 4.2 Å25). Assuming a simple size exclusion ethyl mercaptan molecules (critical diameter of 5.1 Å26) are not capable to enter Na cages. The given critical diameters of the methyl mercaptan molecule are in the range of 3.8 Å to 4.5 Å.26,27 Therefore the Na cages are available for methyl mercaptan but not for ethyl mercaptan which results in a lower number of adsorption sites for ethyl mercaptan. Additionally ethyl mercaptan requires more space on the surface because of its larger molecular size. Because of this the molar capacity at high coverage in the saturation region should be lower, taking into account the limited space available in the narrow zeolite cages.

Figure 8. Normalized breakthrough curves of H2S adsorption on silica−alumina gel (yH2S ≈ 2000 mol-ppm). Red symbols: H2S signal for binary system. Green and blue symbols: simultaneously detected H2S (dark color) and CO2 (light color) signals for ternary systems.

H2S. The lines do not represent results of modeling or simulation. They are just linking the measuring points to improve the reader’s understanding of the diagram. The heat tone of the experiments with silica−alumina gel was negligible. The nonpolar and small CO2 has a low affinity to the surface and fast kinetics. Thus the concentration front of the CO2 passes the adsorber quickly (light green and light blue curve). The H2S has a higher affinity and larger size. For that reason it passes the adsorber more slowly (dark green and dark blue curve). As a consequence CO2 adsorption of the fresh bed is not yet affected by H2S and in both ternary experiments the binary system isotherm for the adsorption of pure CO2 from methane (CO2 in CH4) applies before the H2S front arrives. The H2S molecules find some adsorption sites occupied by CO2 so that H2S adsorption is influenced by the presence of CO2 right from the beginning. There are isotherms with different slopes which describe equilibrium as can be seen from Figure 4. According to common adsorption theories (e.g., LDF theory28) one of the quantities determining adsorption kinetics F



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is a Lewis acid which may interact with a free electron pair of the Lewis base H2S. CO2 is a Lewis acid,33 which does not undergo interactions with the aluminum. The observed weak affinity to the silica surface may be caused by dispersion forces and higher electrostatic moments. This supports the idea that the capacity loss of H2S arises from a partial coverage of the silica surface by CO2 while aluminum sites are not available to CO2. As an effect the additional capacity loss in the wake of a 10-fold increase in CO2 concentration would be comparatively low because now H2S preferably adsorbs on free aluminum sites. Impact of H2O on H2S Adsorption. As in the case of CO2, the H2S capacity is also significantly reduced by the addition of water (see Figure 5). The drop is much more pronounced because the affinity of water is much higher than the affinity of CO2. A displacement of H2S by H2O molecules was observed in the breakthrough curves. The H2S front advances the water front because H2S adsorption is weaker. Breakthrough curves for the ternary systems with H2O are not shown here because the observable effects are very similar to the effects discussed for the ternary systems with CO2. 4.2.2. Adsorption on Zeolite 5A. Impact of CO2 on H2S Adsorption. The H2S capacity loss by coadsorption of CO2 (see Figure 6) may be illustrated by a comparison of breakthrough curves measured during adsorption of 2000 mol-ppm H2S as shown in Figure 10. The heat tone in the bed

is the slope of the isotherm at the respective concentration in the gas phase. The slighter the isotherm slope is, the faster is adsorption kinetics. Following this theory it is clear why the two CO2 breakthrough curves are congruent: As mentioned above, for both experiments the same isotherm applies for initial adsorption. This isotherm is a Henry isotherm with a constant slope which renders adsorption kinetics independent of gas phase concentration. In contrast, the two H2S breakthrough curves are not congruent because they are influenced by the presence of CO2. Compared to H2S adsorption without CO2 (red curve) the breakthrough curves are steeper and reach equilibrium earlier. Obviously H2S adsorption kinetics is faster in the presence of CO2. The H2S isotherms are also Henry isotherms but the different slopes result in different kinetics corresponding to the LDF theory. The isotherm at 5 mol-% of CO2 has the flattest slope leading to the fastest kinetics with the steepest breakthrough curve. Owing to the low capacities for both components there were no displacement effects detectable in the breakthrough curves which would manifest by an increase of the concentration beyond the feed level. The formation of carbonyl sulfide reported in literature29,30 H 2S + CO2 ⇌ H 2O + COS

(4.1)

was not observed under the conditions of our experiments. The Henry isotherm of CO2 adsorption measured for the binary system (CO2 in CH4) predicts a 10-fold load when the concentration increases by a factor of 10 as in the case of the step from 0.5 mol-% to 5 mol-% of CO2 in the ternary systems. This large increase in CO2 adsorption should correspond to a high loss of H2S capacity. However, the observed capacity loss is only as large as the loss for the small first step from 0 to 0.5 mol-%. This effect gives rise to the hypothesis that part of the adsorption sites of the silica−alumina gel is only available for H2S and cannot be occupied by CO2. In that respect the aluminum oxide in the silica−alumina gel could play an important role. The content of aluminum given as Al2O3 in the silica−alumina gel is 3 %. H2S Bonding Mechanisms on Silica−Alumina-Gel. Generally two interaction mechanisms are proposed for the adsorption of H2S on the surface of the silica−alumina gel (see Figure 9).

Figure 10. Normalized breakthrough curves of H2S adsorption on zeolite 5A (yH2S ≈ 2000 mol-ppm). Red symbols: H2S signal for binary system. Green and blue symbols: simultaneous detected H2S (dark color) and CO2 (light color) signals for ternary systems.

for trace level investigations (sulfur compounds) was clearly below 5 °C. For CO2, which was investigated up to concentrations of 5 mol-%, the heat tone in the bed for experiments with higher concentrations was in the range of 10 °C which may influence the shape of the breakthrough curve. The CO2 breakthrough curves display a significant exceeding of the feed concentration yin which is typical for displacement effects in fixed beds. H2S molecules adsorbing from the retarded H2S front remove a large portion of the adsorbed CO2 molecules during a short time. Therefore, the concentration at the adsorber outlet can become higher than the feed concentration. Contrary to the adsorption on the silica− alumina gel the CO2 curves are not congruent anymore because

Figure 9. Bonding mechanisms on the silica−alumina gel surface: (a) H2S bonding by a hydrogen bridge; (b) H2S bonding by Lewis acid− base interaction.

In H2S the sulfur is negatively polarized and carries a negative partial charge. This enables an attractive interaction by hydrogen bridge bonds of the sulfur and the hydrogen of the silanol groups on the silica surface. Another mechanism is offered by the aluminum which can be considered as being due to trivalent aluminum ions held in tetrahedral coordination to three oxygen atoms.31,32 Aluminum G



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capacity losses compared to the binary system due to coadsorption of the added compound. Displacement effects and kinetic effects in the ternary mixtures due to the presence of the added compound were demonstrated by an analysis of the breakthrough curve patterns.

the underlying Langmuir isotherm does not have a constant slope. The H2S breakthrough curves are influenced by CO2. Above all in the case of the high CO2 content (dark blue curve) a different pattern is found compared to the binary system (red curve). The initial slope is steeper; close to equilibration the slope is flatter. The underlying adsorption isotherms are Langmuir isotherms which do not have a constant slope (see Figure 6). The isotherm at 5 mol% of CO2 has the flattest initial slope leading to the fastest kinetics and the steepest breakthrough curve. Close to equilibration at 2000 mol-ppm the isotherm at 5 mol-% CO2 is a little steeper than the isotherm of the binary system which is closer to the plateau. As a consequence the breakthrough curve is a little flatter. As seen for the silica−alumina gel no COS formation according to eq 4.1 was detected on the zeolite. Obviously H2S and CO2 do not react under the conditions of our experiments. Impact of H2O on H2S Adsorption. Water also significantly reduces the capacities of H2S on the zeolite. For both experimental series with different water content a displacement of H2S by H2O molecules was observed in the breakthrough curves. Since H2S is the weaker adsorptive the H2S front runs faster than the water front. Comparison of Ternary Adsorption on Silica Alumna Gel and Zeolite. The ternary systems (H2S+ CO2 in CH4, H2S + H2O in CH4) provide a similar picture for both adsorbents. The addition of a second compound (CO2 or H2O) leads to a capacity loss of H 2 S which increases with increasing concentration of the added compound. The role of H2S in systems with different other compounds requires a more detailed analysis. H2S + CO2 in CH4. Because of its polarity H2S is the stronger bonding adsorptive with higher affinity in this system. This was confirmed by experiments with binary systems. CO2 adsorbs first on account of faster kinetics and lower capacity. When H2S adsorbs the surface is already partly covered by CO2 molecules. Although the stronger bonding H2S then removes part of the CO2 molecules the H2S capacity is lower. H2S + H2O in CH4. In this system H2S is the weaker adsorptive with faster kinetics which adsorbs first and is removed later by the strong bonding water. A comparison of the results of the binary system experiments underlines that water is a much stronger adsorptive than CO2 on both adsorbents. For that reason the H2S capacity loss in the ternary mixtures is consistently higher in the case of water addition even though water concentrations were much lower than CO2 concentrations (500 and 1000 mol-ppm H2O ⇔ 5000 an 50000 mol-ppm CO2).

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ASSOCIATED CONTENT

S Supporting Information *

Tables of experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

The authors wish to express their thanks to BASF Catalysts Germany GmbH for funding and support with adsorbent materials. Notes

The authors declare no competing financial interest.

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NOTATION ṅfl = mole flow of the gas [mol/s] mads = weighed mass of adsorbent [kg] X = adsorbent load [mol adsorptive/kg adsorbent] y = mole fraction in the gas phase [mol-ppm] b = Langmuir parameter [−] kH = Henry parameter [mol adsorptive/mg adsorbent] kF = Freundlich parameter prefactor [mol adsorptive/mg adsorbent] n = Freundlich parameter exponent [−] REFERENCES

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5. SUMMARY AND CONCLUSION This paper presents a systematic investigation of the adsorption of organic and inorganic sulfur compounds, water, and carbon dioxide from the carrier gas methane on silica−alumina gel and zeolite 5A. The adsorption capacities were calculated from experimental breakthrough curves using mass balances. The data were fitted to common isotherm equations. The adsorption capacities increase with molecular polarity expressed by the dipole moment. The adsorption of the large molecule ethyl mercaptan on the microporous zeolite 5A was subject to steric effects. To approximate real natural gas compositions CO2 and H2O were added to the binary mixture of H2S and methane. H2S adsorption from the resulting ternary mixtures leads to marked H



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