High-Pressure Sour Gas and Water Adsorption on Zeolite 13X

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Materials and Interfaces

High-pressure sour gas and water adsorption on zeolite 13X Kyle G Wynnyk, Behnaz Hojjati, and Robert A Marriott Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03317 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Industrial & Engineering Chemistry Research

High-pressure sour gas and water adsorption on zeolite 13X

Kyle G. Wynnyk, Behnaz Hojjati, Robert A. Marriott* Chemistry Department, University of Calgary, 2500 University Drive, N.W., Calgary, Alberta, Canada, T2L 1N4 a

Corresponding author: Dr. Robert A. Marriott; E-mail: [email protected]

ABSTRACT The design and optimization of natural gas conditioning with adsorbents requires self-consistent data up to pressures relevant to gathering and transportation pipelines. Adsorption isotherms for methane, carbon dioxide, carbonyl sulfide, and hydrogen sulfide have been measured by a custom-built high-pressure manometric adsorption instrument. These isotherms are measured up to 100 bar on zeolite 13X (significantly above the inflection of isotherm). Furthermore, water adsorption

isotherms

were

measured

by

an

in-house

modified

continuous-flow

thermogravimetric adsorption system. For this work, the zeolite 13X was synthesized and characterized by SEM/EDX, XRD, DLS, and FT-RAMAN. Adsorption isotherms (absolute and excess) were determined for the full range of the adsorptives studied. The manometric system was operated from T = 0 – 50 °C and the gravimetric system from T = 25 – 150 °C. The experimental data are provided in the Supplementary Information, a modified-Tóth equation was reported and adsorption enthalpies calculated. As expected, zeolite 13X shows a much stronger adsorption for water at all pressures, followed by a the acid gases (H2S and CO2), COS and then CH4. We note that CH4 adsorption continues to increase with increasing pressure and approaches that of CO2 and H2S near p = 100 bar due to the higher compressibility of adsorbed CH4. KEYWORDS Sour gas conditioning; zeolite 13X; adsorption; high-pressure; hydrogen sulfide; carbonyl sulfide; carbon dioxide; methane; water; molecular sieves

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. INTRODUCTION In order to bring fuel sources to market efficiently, processes for removing unwanted components from natural gas production sources are necessary. One area of ongoing research is the production of sources containing raw sour gas, where sour gas is natural gas containing H2S and CO2.1 To reach sales specifications, H2S is removed due to its toxicity and CO2 because of its benign heating value. In the presence of H2O, both of these components can be corrosive towards carbon steel and/or form solid hydrates; therefore, H2O is often removed to levels below its dew point (conditioned).2,3 All produced natural gases undergoes conditioning for flow assurance and to ensure the processed gas meets sales specifications.4 Conditioning typically occurs after removal of CO2 and H2S, where contact by an alkanoamine solution (treatment) is the preferred method of acid gas removal. In certain cases, it is more viable to dehydrate upstream of treatment, these cases include: production in remote locations where use of lower cost carbon-steel gas gathering lines is favorable and/or non-aqueous processes are required downstream for bulk H2S/CO2 removal.4,5 In this work, our focus is on dehydration close to production wellheads where adsorbents such as silica or zeolites are used in closed-cycle thermal regeneration adsorption processes. Close to production there are additional challenges in dehydration because of other impurities in the gas stream, which can lead to unwanted surface reactions that are catalyzed by the adsorbent or occur due to high-temperatures during thermal regeneration. One reaction that can occur during sour gas dehydration is the reaction between CO2 and H2S, producing COS and H2O (reverse hydrolysis).5,6 The production of COS is detrimental to downstream operations due to difficulty in separation and its equilibrium shift back to H2S that can lead to safety concerns during storage. Finally, adsorptive dehydration before processing occurs at high-pressure (p > 50 bar) where competitive adsorption is not fully described or well modelled by commercial simulators. Adsorbents have been used for the dehydration of natural gas for over 50 years, but most experimental data are provided at low pressures; this makes it difficult to optimize high-pressure dehydration due to the requirement of temperature dependent high-pressure adsorption models. In the literature, high-pressure data acquired on the same instruments and for the same material are sparse; to the authors knowledge this is the first study containing experimental data for all -2ACS Paragon Plus Environment

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components (CH4, CO2, COS, H2O and H2S) found in a simplified sour gas stream. A previously detailed small-volume manometric adsorption instrument was used to measure CH4, CO2, COS, and H2S on zeolite 4A (T = -30.00 to 150.00 °C; p = 0.01 to 225 bar; sample size ca. 30 mg),1 and a continuous-flow thermogravimetric analyser for measurement of H2O adsorption is described in this work. As a continuation of our previous work on zeolite 4A, zeolite 13X’s adsorption behaviour has been studied due to its selectivity for H2O and use as a commercial desiccant. Zeolite 13X samples were synthesized from high-purity starting materials. The synthesized zeolite 13X sample was rigorously characterized using XRD, SEM, EDX and surface analysis techniques. In-situ FT-RAMAN also was used to analyze the surface of the adsorbent and to identify any reactions that may occur. The adsorption isotherms of CH4, CO2, COS, and H2S were measured at three different temperatures T = 0, 25, and 50 °C with our manometric adsorption instrument. A continuous-flow thermogravimetric instrument was used for the measurement of H2O adsorption and compared to recent literature sources performing similar experiments (T = 25.0 to 150.0 °C; pH2O ≈ 5 ∙ 10-4 – 5 ∙ 10−3 bar; sample size ca. 75 mg). 711

2. MATERIALS AND METHODS 2.1 Materials. Sodium aluminate (CAS No. 1138-49-1: NaAlO2, 50 – 56% Al), sodium hydroxide (CAS No. 1370-73-2: NaOH, 98%) and sodium metasilicate (CAS No. 6834-92-0: Na2SIO3, 44-47% SiO2) were obtained from Sigma-Aldrich and used as received. H2O was polished to a resistivity of 18 MΩ using an in-house EMD Millipore. CO2 (Laser grade), CH4, COS, and H2S with the corresponding purities of 99.9995%, 99.999%, 99.6%, and 99.6% were purchased from Praxair. Gas purities have been confirmed by gas chromatography (TCD/FID and SCD). Zeolite 13X was prepared by adding an 18 mL solution containing 1.5 g NaOH, 1.39 g NaAl2O3, and a small quantity of zeolite 13X seed crystals (50 mg, 13X molecular sieve powder, Sigma-Aldrich, Lot. MKBL4541V). The resulting solution was added to 6.28 mL of saturated Na2SiO2 solution in a Teflon bottle at room temperature and stirred for t = 42 hr for ageing. The bottle was then heated to T = 90 °C in an oven for t = 24 hr. The bottle was cooled to room



temperature; the product was suspended in water and filtered by vacuum. Finally, the product was dried at T = 100 °C for t = 2 hr. 12 The synthesized zeolite 13X was characterized by powder X-Ray diffraction (PXRD) for phase identification and purity evaluation by a Bruker D8 PXRD (CuKα, 40 kV, 40mA). The crystal morphology of zeolite and Si/Al ratio were determined using a Philips XL 30 scanning electron microscope (SEM) and energy dispersive X-Ray spectroscopy (EDX) microanalysis. The particle size was determined by dynamic light scattering (DLS, NanoTex Analysette 22) in a THF solvent. In-situ Fourier transform (FT) Raman spectroscopy (MultiRAM, spectral range of 3600 – 50 cm-1, max. T = 800 °C, p = 34 bar) was utilized to analyze purity and identify and adsorption events on zeolite 13X. The accessible surface area of the zeolite 13X was determined by a Micromeretics 3-Flex Surface Characterization Analyzer. 2.2 Experimental set-up. In this work, a continuous-flow adsorption system was developed and is shown in Figure 1. To measure mass gain or loss, a SETARAM LABSYS EVO TGA was used. This system can control temperature to within δT = ± 0.05 °C over the temperature range of interest T = 25 – 150 °C. The oven block is actively cooled by a PolyScience 6000 Series Chiller flowing a 20 °C 50:50 ethylene glycol/water mixture. The dynamic mass range used in this work was m = ± 0.200 g, where the stated mass precision is δm = ± 0.01% (resolution of 2 ∙ 10−8 g). Flow meter Mass flow controller (MFC) Water analyzer He(g) feed

Zeolite 3A

MFC 1

Oven Sample crucible

MFC 2

Void volume

MFC 3

Water saturator Thermostated balance

Water bath

Figure 1. As schematic of the continuous-flow thermogravimetric adsorption system.


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As shown in Figure 1, a dry He (g) stream is split and passed to three different Brooks SLA-5850 mass-flow controllers. MFC 1 flows He (g) to the thermostated balance to ensure a dry and stable environment (5 mL min-1). MFC 3 flows He (g) through a water saturator that is submerged in a PolyScience PP07R-40 circulating bath with a stability of δT = ± 0.005 °C using a 50:50 ethylene glycol/water mixture as the circulating fluid. MFC 2 is used to dilute the saturated water stream upstream of the TGA. Varying the flow of MFC 2/MFC 3 and the temperature of the water saturator allowed the water concentration to be varied from approximately 500-6000 ppm (T(saturator) = 0.5 to 25.0 °C). Here the total flow of all MFCs is fixed at 25 mL min-1. Note that the ambient temperature is this bay is T = 30 °C and no condensation beyond the saturator was observed. A Meeco M-i ™ moisture monitor was used to verify the concentration of water being delivered to the TGA at lower concentrations. We note that some moisture is accumulated from the atmosphere via the seals in the balance. For higher water concentrations, the water was trapped using a solid desiccant and analyzed gravimetrically. The high-pressure volumetric adsorption apparatus was described in previous work.1 The apparatus has an operating pressure range up to p = 0.01 to 225 bar and temperature range of T = −30 to 150 °C. 2.3. Experimental conditions and procedure. Prior to adsorption experiments, the mass of the TGA support rod and clean stainless-steel crucible were measured at T = 25 – 150 °C and

≈

5 ∙ 10-4 – 5 ∙ 10−3 bar, where was found that the variance was within the error of the balance. For the gravimetric adsorption experiments, the synthesized zeolite 13X experimental mass was determined by regenerating zeolite 13X at T = 200 °C overnight in a custom-built stainless-steel vessel that could hold ultrahigh vacuum (p = 1 ∙ 10-10 bar). After regeneration, this vessel was cooled to room temperature, sealed, and transferred to a He (g) dry box with a water concentration less than 1 ppm. Inside the helium dry box, the zeolite 13X was transferred to the stainless-steel TGA crucible. This process was used to minimize any possible water adsorption prior to initial mass determination. In all dry gas experiments, the mass did not change for T > 325°C. Note that the crucible is placed in a vial before transfer to the TGA. Any adsorbed water during transfer is desorbed prior to the initial water adsorption measurement. The experimental procedure and methodology for the volumetric system has been previously discussed.1 The regeneration was performed at T = 200 °C until a vacuum of p ≈ 10−10 -5ACS Paragon Plus Environment


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bar was achieved; this typically occurred overnight (ca. t = 12 hrs). Regeneration at T = 200°C was chosen because there was no significant difference observed in isotherms after regeneration at higher temperatures (i.e. T = 250 °C). Zhang et al. previously observed a difference in adsorption capacity for an ion-exchanged NaX powder until regeneration temperatures of T = 400 °C (p = 1.33 ∙ 10−7 bar.).13 The combination of the different type of zeolite, stronger vacuum, and shorter regeneration times used by Zhang et al. can explain why we required lower larger regeneration temperatures. 2.3.1 Data handling. There are multiple procedures for handling gravimetric adsorption data and most methods involve accounting for the displacement of gas phase density by the adsorbentadsorbate volume.14,15 For this work the method of Murata et al. was used with the assumption the adsorbent-adsorbate volume was fixed by the crystal density and mass of the adsorbent:15

M n abs  n ob   bulk   mat   mat

  B      M mat

  , 

(1)

where nabs is the absolute amount adsorbed, nob is the measured mass, ρbulk is the bulk phase density, Mmat is the mass of material, ρmat is the density of the material, and B is the dry mass. This definition defines all intra-crystalline adsorption as absolute, i.e., this is not reported as an excess adsorption. The crystal density of zeolite 13X was calculated using dehydrated zeolite 13X crystallographic data reported by the International Zeolite Association (ρmat = 1.4220 g ∙ cm3 16

).

Data handling for the volumetric system and the calculation of absolute/excess adsorption was detailed in previous work and follows the same definition of intra-crystalline adsorption as absolute.1Our absolute adsorption is defined by the difference between the adsorption chamber volume and the crystalline volume. By this definition, all molecules in the pore volume are considered adsorbed, even if at the same density as the bulk fluid outside the crystallites. This definition relies on well-known adsorbent densities versus helium expansion, which also can be an additional source of experimental error. Alternatively other absolute adsorption definitions can rely on defining pore volume through other isotherms, which again can introduce inconsistency.1,17,18


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The reference volume (dosing volume) was calibrated to Vref = 0.054 ± 0.004 cm3. The adsorption chamber was determined to have a volume of Vads = 1.534 ± 0.004 cm3. Isotherms were reproduced using different mass of adsorbent to verify the stability of the calibrated parameters and quoted uncertainties. All measured data for absolute and excess adsorption, including uncertainties at the 95% confidence interval, have been reported in the Supplementary Information. 2.3.2 Isotherm models. Variations of the modified-Tóth equation were fit to the volumetric (CH4, CO2, COS and H2S) and gravimetric (H2O) data. Evaluation and optimization was through minimization of the total sum squared error for the adsorption and are reported later as the mean sum squared error (MSSE). The best modified-Tóth equation fit to the volumetric experimental data was found to be

bf

n abs  n 

1 t t

K ,

(2)

(1  bf )

where n∞ is the amount adsorbed at a saturation adsorbed amount, b corresponds to the adsorption constant, f if the fugacity of the adsorbate, t is the so-called heterogeneity parameter and K is a second linear term.19 Fugacities have been calculated using reference-quality Helmholtz Energy equations of state.20-23 Eq 2 can be expanded to include temperature dependence:24

 H  b  b   exp     RT 

(3)

t  A  BT

(4)

In eq 3 and 4, b° is the infinite adsorption constant, ΔH is the isosteric heat of adsorption, and A/B are empirical parameters that relate temperature to t. In this model, the second linear term is

K  n  b f .

(5)

This term also helps to account for the compressibility of the adsorbed phase at high-pressures and high-loading. To model gravimetric H2O adsorption the K term was not statistically significant; therefore, it was not used. In this research, the excess adsorption is not a focus, but is



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useful in comparison to literature values. From the absolute adsorption isotherm, excess adsorption isotherms can be calculated as follows:

  gVa nexc  1  abs n 

 abs n , 

(6)

where Va represents apparent excess volume of the adsorbed phase and is constant for each adsorbent.25 During the parameterization of eq 2, no constraints were placed on the parameters as fitting accuracy was deemed to be more important than the parameters being theoretically correct. Regardless, eq 2 does obey Henry’s Law and was found to give the best fit to experimental data over other semi-empirical equations tested.


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3. RESULTS AND DISCUSSION 3.1 Material Characterization. The SEM images of zeolite 13X depicted as micrographs in Figure 2 clearly present that the zeolite 13X sample is an agglomerated crystalline material. EDX at multiple points shows that the Si/Al mole ratio of the product was found to be 1.8, which is indicative of zeolite 13X. The DLS measurement found in Supporting Information (Figure SI) shows the synthesized zeolite 13X particles have an average diameter of 21.41 µ.

Figure 2. SEM images of synthesized zeolite 13X with scales of 2 µ (top) and 0.5 µ (bottom).



The XRD pattern of synthesized zeolite 13X compares well with the dehydrated zeolite 13X data reported by the International Zeolite Association (Supplementary Information, Figure S2).16 The intensity and broadening of the XRD peaks are evidence that the synthesized zeolite 13X sample has considerable crystallinity. The limiting pore volume and accessible surface area of the synthesized zeolite 13X sample was determined by the Dubinin-Radushkevich equation using CO2 (g) at T = 273.15 K.26 The limiting micropore volume was found to be 0.270 cm3 ∙ g-1 and the accessible surface area was 583 m2 ∙ g-1. The calculation method is detailed in Supplementary Information S3-S7. 3.2 CH4, CO2 and H2S adsorption followed by in-situ FT-Raman. The zeolite 13X sample was pretreated in the environmental chamber for a Bruker FT-Raman at T = 200 °C for 48 h under vacuum and p ≤ 2.0 ∙ 10-6 bar to remove any contaminants before exposure to the purecomponents at T = 25.00 °C. Raman spectra are shown in Figure 3, where absorption at 450 and 600 cm-1 are attributed to the symmetric T-O-T optical modes in zeolite 13X. After introducing CO2, two strong peaks at 1388 and 1286 cm-1 are recognized to be the Fermi resonance doublet of CO2 in Figure 3a.27-29 Figure 3b depicts two bands at 2570 and 2611 cm-1 in the absorption spectra of H2S on zeolite 13X, where these vibrations are assigned to S-H stretching. The band at 2913 cm-1 in Figure 3c is associated to C-H stretching vibration of CH4. In all these FT-Raman spectra, no unexpected peaks were observed which could attributed to impurities or non-ideal absorption. No chemisorption was observed, where the peaks would have shifted.


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(a) CO2

1600

1400

1000

1200

(b) H2S

0.2 bar 13X Zeolite

bar 22.0bar

3.6 bar 3.6 bar

bar 77.0bar

10.69 bar 10.6 bar

15.0 bar 15 bar

800

600

0.2 bar

0.5 bar

1.8 bar

2.2 bar

400

3.1 bar

2400 2400

(c) CH4

2930 2930

2900

2900

1900 1900

2920 2920

2400

2400

2910 2910

1900

1400 1400 0.9 bar 0.85 bar

900 900 1.5 bar 1.52 bar

2.8 bar 2.78 bar

4.1 bar 4.10 bar

6.3 bar 6.25 bar

10.02 bar 10.0 bar

13.9 bar 13.92 bar

15.0 bar 15.03 bar

1400

1900

1400

900 900

400 400

400

400

ν (cm−1)

Figure 3. FT-Raman spectra of adsorbed species formed by contact of (a) CO2, (b) H2S, and (c) CH4 gases at T = 25.00°C. 3.3. Gravimetric H2O adsorption isotherms. The gravimetric instrument was used to study H2O adsorption for T = 25 – 150 °C at 25 °C intervals. The parameters for the optimized modified-Tóth equation are reported in Table 1. Again, the parameters were treated as empirical parameters and were not constrained during optimization. The absolute isotherm can be calculated directly from eq 2 and is shown in Figure 4 with the experimental data calculated with eq 1. Figure 5 shows a comparison of experimental H2O data from multiple literature sources at room temperature.7-11 Figure 5 shows good agreement across literature given that there are differences between materials, experimental systems and procedures. We note that several literature sources correspond to zeolite 13X formed with binder and in this work the material is binderless zeolite 13X. Mette et al.10 and Rege et al.7 data in Figure 4 also corresponds to binderless 13X. -11ACS Paragon Plus Environment


nabs (mmol ∙ g-1)

16 14

24.82 C 49.94 C

12

75.15 C

10

100.3 C

8.0

125.4 C 150.3 C

6.0 4.0

2.0 0

0.001 0.002 0.003 0.004 0.005 0.006 p (bar)

Figure 4. Absolute adsorption isotherms of H2O on zeolite 13X.

Kim et al.11 (20 C) Mette et al.10 (25 C) 10 nexc (mmol ∙ g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

This work (24.82 C) Wang et al.9 (25 C) Rege et al.7 (22 C)

Kim et al.8 (20 C)

1.0 0.0001

0.0010 p (bar)

0.1000

Figure 5. H2O adsorption on different samples of zeolite 13X at approximately room temperature.7-11


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Table 1: Modified-Tóth parameters for adsorption of CH4, CO2, COS, H2O and H2S on zeolite 13X. n∞

b° -1

-1

−ΔH

A -1

B

Va (m g )

(mmol2 g-2)

2.79•10−1

−2.14•10−4

4.00•10−4

2.85•10−3

26.68

6.02•10−1

−9.49•10−4

4.00•10−4

1.62•10−2

2.25•10−3

17.58

2.43•10−1

2.88•10−3

4.00•10−4

6.24•10−3

1.19•10−1

37.33

4.66•10−1

−3.83•10−4

4.00•10−4

1.00•10−1

H2S 8.55 6.58•10−4 27.83 MSSE is the mean sum squared error.

6.02•10−1

−7.18•10−4

4.00•10−4

5.44•10−2

(bar )

(kJ mol )

CH4

29.18

3.62•10−4

17.31

CO2

7.32

5.79•10−3

COS

4.44

H2O

17.61

3

MSSE

(K )

(mmol g )

-1

-1

3.4 Volumetric CH4, CO2, COS, H2S adsorption isotherms. The volumetric adsorption system was used to study CH4, CO2, COS and H2S at temperatures of T = 0, 25 and 50 °C. Table 1 contains the parameters for the modified-Tóth equation used to fit the volumetric experimental data. In addition to the modified-Tóth parameters, all experimental data are available in Supplementary Information, Tables S9-S11. 3.4.1 Absolute adsorption isotherms. The absolute adsorption isotherms determined by the experimental volumetric system are shown in Figure 6 to 9. To demonstrate the reproducibility of these, isotherms for CH4 and CO2 were reproduced with different mass of adsorbent, dosing pressures and regeneration (activation) temperatures (T = 200 and 250°C)). The reproduced isotherms in Figure 6 for CH4 show less than 2.5 % difference at the highest comparable pressure (p ≈ 96 bar). In Figure 7 CO2 isotherms were compared at T ≈ 50 C and were found to be within 3 % and at the highest comparable pressure (p ≈ 71 bar). For the isotherms at T ≈ 50 C the adsorptive is supercritical CO2 and this fluid is significantly more dense and less compressible than the other measured fluids. This high-density region typically leads to difficulties in measuring adsorption isotherms, thus a slightly lower reproducibility of these isotherms seems reasonable. For CH4, the comparison to literature isotherms at T ≈ 25 C in Figure 6 show large variations, with the measured isotherms agreeing at low pressures, but not at high-pressures.30 Note that our CH4 isotherm resides between the two high-pressure literature sources and up to 10 bar good agreement across all three isostherms.24,30,31 The experimental data of Moreira et al. at T = 50 °C shows larger amounts adsorbed initially for both CH4 and CO2 but at increasing -13ACS Paragon Plus Environment


pressures the adsorption capacity become similar to our measured experimental data.32 The CO2 isotherm measured at T = 25 °C shown in Figure 7 agrees well with Cavenati et al.24 and the shape of the isotherm compares well with Avijegon et al. but the capacity differs between our work.30 Again, these comparisons demonstrate the need for self-consistent high-pressure data.

9.0

nabs (mmol ∙ g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8.0

−0.016 C

5.0 4.0

50.011 C

25.000 C 24.984 C 7.0 6.0 Cavenati et al.24 25 C

3.0

2.0 1.0 0

Moreira et al.32 50 C Rolniak et al.31 25 C Avijegon et al.30 30 C

10 20 30 40 50 60 70 80 90 100 p (bar)

Figure 6. The absolute adsorption of CH4 on zeolite 13X from T = 0 to 50°C with literature isotherms.24.30-32


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9.0 0.009 C 24.978 C

nabs (mmol ∙ g-1)

8.0

7.0 6.0 5.0 4.0 3.0

50.013 C 50.004 C Moreira et al.32 50 C Cavenati et al.24 25 C Avijegon et al.30 30 C

2.0 1.0 0

10

20

30

40 50 p (bar)

60

70

80

Figure 7. The absolute adsorption of CO2 on zeolite 13X from T = 0 to 50°C with the literature isotherms.24,30,32

4.5 4.5

−0.044 C

25.013 C

4.04

nabs (mmol ∙ g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


49.997 C

3.5 3.5 3.03 2.5 2.5 2.02 1.5 1.5

1.01 0.5 0.5 0

0

0.5

1.0

1.5 p (bar)

2.0

2.5

3.0

Figure 8. The absolute adsorption of COS on zeolite 13X from T = 0 to 50°C.



8.0

−0.055 C

7.0 nabs (mmol ∙ g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

24.983 C

6.0 50.009 C

5.0 4.0 3.0 2.0 1.0

0

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10 p (bar)

Figure 9. The absolute adsorption of H2S on zeolite 13X from T = 0 to 50°C.

3.4.2 Excess adsorption isotherms. The He excess adsorption isotherms for CH4 and CO2 can be directly compared with available literature isotherms reported at p > 1 bar.13,33 Isotherms and experimental data from this work and literature13,33 are shown in Figure 10 and 11 for CH4 and CO2, respectively. The comparison of CH4 in Figure 10 shows a small difference in the amount adsorbed within the Henry’s Law region with better agreement at higher-pressures. The comparison for CO2 is shows a less favorable agreement with Bezerra et al.33, our data have a lower adsorption capacity and smaller slope at low-pressure (Henry’s law region). This comparison of literature highlights the need for consistency in measurements if experimental data are to be used to calibrate models for simulation and optimization. Variations are exasperated in high-pressure experiments where measurements become more difficult and errors are cumulative. As discussed previously, all sources are measuring slightly different materials with different instruments and methodologies, where comparisons of adsorptives should be made using the same material.


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Page 17 of 25

−0.016 C nexc (mmol ∙ g-1)

Bezerra et al.29 25 C

50.011 C 24.984 C 25.000 C

1.0 Zhang et al.13 25 C

0.1 1.0

10 p (bar)

100

Figure 10. The excess adsorption of CH4 on zeolite 13X from T = 0 to 50°C.13,33 The solid lines correspond to a conversion or our absolute isotherm fits (eq 6).

10 −0.016 C 50.011 C nexc (mmol ∙ g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24.984 & 25.000 C

1.0 Bezerra et al.29 25 C

0.1 0.01

0.1

1.0 p (bar)

10

100

Figure 11. The excess adsorption of CO2 on zeolite 13X from T = 0 to 50°C.33 The solid lines correspond to a conversion or our absolute isotherm fits (eq 6).



Page 18 of 25

The experimental data collected for zeolite 13X can be readily compared to zeolite 4A which was previously studied by the same experimental instruments.1 Many of the adsorption trends of zeolite 4A are mirrored for zeolite 13X, with H2S and CO2 showing a large initial adsorption compared to COS and CH4. The relative adsorption capacities are also similar but zeolite 4A has a lower capacity for all adsorptives. A major difference is the adsorption of CH4 on zeolite 13X, where after a low initial uptake of CH4, the adsorption capacity continues to increase with increasing pressure. This is due to (a) the definition of absolute adsorption as used in our study, (b) the compressibility of adsorbed CH4 and (c) a larger pore volume for zeolite 13X. In the previous work, an anomalous COS adsorption on zeolite 4A was observed, where the adsorption capacity of two different temperatures were found to be similar, i.e., like a chemisorption phenomenon. This was attributed to the size of COS in relation to the pore opening of zeolite 4A (ca. 4.6 Å and 4.0 Å respectively) and a potentially reversible chemical reaction (dissociation of COS).1 The adsorption of COS on zeolite 13X did not show this different temperature dependent behavior (the isotherms were evenly spaced like other physical adsorbates). Note that the larger pore openings (7.8 Å) are no longer restricting for the COS molecule, which may explain why zeolite 4A shows anomalous adsorption behavior and zeolite 13X does not.1,34 The COS isotherms do show a lower capacity and lower heat of adsorption when compared to CO2. 3.5 Isosteric heat of adsorption. The isosteric (constant surface coverage) heat of adsorption (ΔaH) can be fit through the equations derived by Titoff and later Hückel.35 In this work the isosteric heat is estimated for a pure-component on a solid surface by calculating equilibrium fugacity for corresponding absolute adsorption at different temperatures;36

a H   RT   ln( f ) / T  .

(8)

In eq 8, f is the fugacity of the adsorptive, and Γ is the surface coverage (in this case Γ is replaced by nabs). The slope is obtained using a least square regression for the fugacity data at T = 0, 25 and 50 °C. The fugacities at constant surface coverage were interpolated using eq 2 and the parameters in Table 1. An alternative procedure was the use a spline fit of experimental data, which is not biased towards the semi-empirical isotherm. In this case there were very little -18ACS Paragon Plus Environment

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differences between the two procedures. Thus eq 8 and 2 have been used to calculate ΔaH at over a lower surface coverage range and reported in Table 2 along with other literature.13,24,37-48 The isosteric heats for CH4, CO2, COS, H2O and H2S are shown in Figure 12.

Table 2. Experimental and literature enthalpies of adsorption. ΔaH (this Study)

nabs

ΔaH (Literature)

(kJ mol-1)

(mmol g-1)

CH4

19.2 ± 0.8

2.60 – 4.70

(kJ mol-1) 13 20.1, 2415.3, 3717.6, 3818.4, 39 18.8, 4017.6, 4119.2

CO2

42.9 ± 1.6

3.35 – 5.00

COS

15.3 ± 1.4

0.30 – 3.80

H2O

47.2 ± 3.6

5.10 – 6.50

H2S

35.9 ± 3.5

2.90 – 5.80

24

37.2, 4149.1, 4247.3, 4350, 44 47.5, 4544.2, 4648.7 47

61.3,

48

61.8

-

ΔaH has been averaged for the range of nabs provided.

H2O ΔaH (kJ ∙ mol-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

H2S

30 CH4

20

0

CO2

COS

10

1.0

2.0

3.0 4.0 5.0 6.0 nabs (mmol ∙ g-1)

7.0

8.0

Figure 12. Isosteric heats for the CH4, CO2, H2S, COS and H2O on zeolite 13X.



Isosteric heats are often compared at a specific surface loading or at the lowest surface coverage measured, because the ΔaH should trend toward zero as the surface is loaded with adsorbate, i.e., adsorbate-adsorbent interactions become less favourable. In our case, our experimental data are not collected at very low surface loading; therefore, we have averaged the low-pressure isosteric heat data and the reproducibility in Table 2 has been obtained using the standard deviation for that range. Our ΔaH(CH4) and ΔaH(CO2) agree with the range shown in the literature and show the reduction in the ΔaH at high surface loading. ΔaH(H2S) and ΔaH(H2O) show a small rise in the ΔaH with increasing surface loading. While we realize this seems incorrect, we attribute it to the limited sensitivity of these measurements. ΔaH(H2O) is lower than expected, but this is because the largest nabs used to calculate the isosteric heat in the literature47,48 is smaller than the range investigated here. Note that the averaged ΔaH is more applicable to industrial adsorption bed design, whereas the very low value is more fundamentally revealing. This is because an industrial bed would never become fully loaded or regenerated to a vacuum type pressure. Despite this, our ΔaH values show a similar trend as those found with zeolite 4A [ΔaH(H2O) > ΔaH(CO2) > ΔaH(H2S) > ΔaH(CH4) > ΔaH(COS)]. 3.6 SUPPORTING INFORMATION Supporting information has been provided containing the particle distribution of the synthesized zeolite 13X (Supporting Information; Figure S2. The XRD pattern of the synthesized material is compared to a commercial zeolite 13X sample in the Supporting Information (Figure S2). Supporting Information S3 – S7 shows the estimations of surface area and pore volume by the Dubinin-Radushkevich transformed isotherm with laser-grade CO2 at T = 0 °C. Finally, all experimental data presented in this article is tabulated in Tables S8 – S12. 4. CONCLUSIONS In order for the future design and optimization of adsorptive sour gas conditioning, accurate isotherm equations and/or experimental isotherm data are required. In this respect, it was important for isotherms to be obtained for all species within a sour gas on zeolite 13X, with consistent instrumentation and up to pipeline pressures. We have reported adsorption isotherms for CH4, CO2, COS, H2O, and H2S on an in-house synthesized zeolite 13X at T = 0, 25 and 50 °C. A manometric instrument was utilized for -20ACS Paragon Plus Environment

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measurements of CH4, CO2, COS, and H2S, whereas a continuous flow gravimetric instrument was used for H2O adsorption. All experimental data have been provided for future tuning of simulation tools. The results show that zeolite 13X adsorption is preferential for water at all pressures, followed by a preference for the acid gases (H2S and CO2) and a lower preference for COS and CH4. We note that CH4 adsorption continues to rise with increasing pressure and approaches that of CO2 and H2S near p = 100 bar. The same observation was made for zeolite 4A, where there appears to be a reversal of preference at some higher pressure. This is due to the adsorbed CH4 phase being more compressible than the other adsorbates studied. All adsorption isotherms were compared to literature where possible and it was found that high-pressure isotherms for CH4 and CO2 showed large variations in the adsorption isotherms. A modifiedTóth isotherm was used to model the manometric instruments, and a modified-Tóth with the removal of KHenry for H2O adsorption models. These models were chosen due to their low MSSE and ability to calculate adsorption at different temperatures while using the same parameters. Calculation and comparison of isosteric heats have been reported and show a similar trend as the low-pressure adsorption capacity. Again, all data have been reported for future investigators studying the optimization of molecular sieve dehydration, where temperatures and regeneration gas compositions can be optimized to reduce unwanted reactions at the surface of the adsorbent. Such optimizations will need to consider the thermal degradation of the zeolite with high-temperature regeneration, which is a future sour gas study.

ACKNOWLEDGEMENTS The authors are grateful for funding through the NSERC ASRL Industrial Research Chair in Applied Sulfur Chemistry and the sponsoring companies of Alberta Sulphur Research Ltd. Equipment and infrastructure has been purchased through a grant from the Canadian Foundation for Innovation.



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Graphical Abstract 100 H2O

10

1 0.1

0.01

CO2 COS

CH4

H2S

0.001 0.0001

0.01 p (bar)

1

100


High-Pressure Sour Gas and Water Adsorption on Zeolite 13X

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