Article pubs.acs.org/jced
Cite This: J. Chem. Eng. Data 2019, 64, 3156−3163
Sour Gas and Water Adsorption on Common High-Pressure Desiccant Materials: Zeolite 3A, Zeolite 4A, and Silica Gel Kyle G. Wynnyk, Behnaz Hojjati, and Robert A. Marriott* Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
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S Supporting Information *
ABSTRACT: When raw sour gas is gathered for transportation over long distances, adsorptive dehydration is often employed to avoid condensable water prior to purification at a gas processing plant, i.e., adsorptive dehydration is used to avoid excessive corrosion or hydrate plugging within transportation/gathering lines. To optimize and design adsorptive conditioning facilities, simulation requires self-consistent experimental adsorption measurements at pressures relevant to gathering and transportation. To this end, manometric adsorption measurements on methane, carbon dioxide, carbonyl sulfide, and hydrogen sulfide on silica gel were measured at high pressures. This data adds to our previous work with sour gas adsorption on zeolites 4A and 13X. Also, in this study, gravimetric measurements for water adsorption are reported for silica gel, zeolite 3A, and zeolite 4A. Absolute and excess adsorption isotherms were determined for the adsorptives studied with operating temperatures ranging from T = 0 to 50 °C and T = 25 to 150 °C for the manometric and gravimetric measurements, respectively. Across all of the desiccant materials studied, H2O has the largest adsorption affinity, followed by the acid gases (H2S and CO2), COS, and finally CH4. In terms of H2O adsorption, zeolites 3A and 4A show similar affinity (low-pressure Henry’s law slope), whereas zeolite 4A shows a slightly larger capacity. Both zeolites 3A and 4A exceed silica gel in terms of capacity and affinity.
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thermal regeneration at high temperatures.2−5 The unwanted formation of COS negatively affects downstream operations as it is difficult to separate and can follow other valuable liquid products, such as propane.2 Finally, on-site adsorptive dehydration occurs at large pressures (p > 50 bar) where competitive adsorption is not fully described in most commercial simulators due to the paucity of adsorption data. Due to differences among materials, measurements, and experimental systems, an added benefit of our recent work is the measurement with self-consistent adsorption techniques on consistent adsorbents. Recent studies have reported sour gas adsorption on zeolite 4A and sour gas plus H2O adsorption on zeolite 13X.3,4 This work completes our planned purecomponent adsorption studies by reporting similar measurements on zeolite 3A (H2O), zeolite 4A (H2O), and 60 Å silica gel (CH4, CO2, COS, H2S, and H2O). With our previous studies and this work, the components of a simplified sour natural gas stream (CH4, CO2, COS, H2O, and H2S) for zeolite 3A, zeolite 4A,4 zeolite 13X,3 and 60 Å silica gel, alongside H2O adsorption, can be used to parameterize our multicomponent models or of other authors who may be developing competitive models. Within this study, zeolite 3A was prepared by cation exchange with our previously synthesized zeolite 4A4 material
INTRODUCTION A primary concern in transporting raw sour gas (natural gas containing H2S and CO2) from remote production wells to distant processing plants is often the possibility of condensable H2O (liquid or hydrate). If the distance is long enough, a normal carbon steel pipeline is often utilized, where condensable H2O would lead to higher corrosion rates. In these cases, it is preferable to dehydrate the sour gas in the field (close to wellhead) before flowing through the long transportation line. Unlike the water conditioning of treated gas in a processing plant (natural gas after removal of H2S and CO2),1 removing water from the raw sour gas in the field is often completed using adsorption processes that rely on solid desiccants.1,2 Collecting data to optimize these adsorptive dehydration processes has been an ongoing focus of our group.3,4 Optimization can increase the life expectancy of the adsorbent, decrease undesired chemical reactions, and increase energy efficiency. In this area, there are very few studies that (i) look at the adsorption of sour gas species on the same material, (ii) make measurements at production pressures, and/or (iii) have utilized measurements with self-consistent techniques. Typical adsorbents used in closed-cycle thermal regeneration processes are materials such as silica gel or zeolites. An unwanted reaction that can occur during adsorptive dehydration at the production site is the reaction between CO2 and H2S, which produces COS and H2O. This reverse hydrolysis reaction can be catalyzed by the adsorbent or occur during © 2019 American Chemical Society
Received: March 15, 2019 Accepted: May 28, 2019 Published: June 7, 2019 3156
DOI: 10.1021/acs.jced.9b00233 J. Chem. Eng. Data 2019, 64, 3156−3163
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microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX) for qualification of crystal morphology and Si/Al ratios. A Bruker D8 Advance Eco, equipped with a variable temperature MTC-HIGHTEMP sample stage, was used for the acquisition of the powder X-ray diffraction pattern (PXRD/Cu Kα, 40 kV, 25 mA). A NanoTex Analysette 22 was used to determine particle size distribution in THF solvent by dynamic light scattering (DLS). No observable changes in crystal morphology were made upon cation exchange from zeolite 4A to zeolite 3A. Accessible surface area and pore volume for the studied silica gel were determined using a Micromeritics 3-Flex surface characterization analyzer (N2). Experimental Procedures. A previous work outlined the detailed design of a high-pressure volumetric adsorption apparatus4 suitable for measurement of CH4, CO2, COS, and H2S adsorption. Adsorption measurements for H2O were performed using a continuous-flow thermogravimetric analyzer.3 The high-pressure volumetric adsorption apparatus has an operating range of p = 0.01−225 bar and a temperature range of T = −30−150 °C. The uncertainty in reference volume was δVref ≈ 3.50 × 10−5 cm3, and the uncertainty in the adsorption chamber volume was δVads ≈ 3.50 × 10−3 cm3 for the studied silica gel. The continuous-flow thermogravimetric analyzer (SETARAM LABSYSevo TGA) operates from T = 25 to 1600 °C. The dynamic mass range used was m = ±0.200 g, mass precision was δm = ±0.01%, and resolution was rm = 2 × 10−8 g. A wet He (g) stream was generated by three mass-flow controllers: one He (g) stream flows through a temperature-controlled H2O saturator, a second stream dilutes the H2O saturator effluent, and then the combined stream flows directly into the oven of the thermogravimetric analyzer. The third He (g) stream flows through the thermostated balance toward the oven ensuring an inert and stable atmosphere for the balance portion of the TGA. By controlling the flow rates and the saturator temperature, a range of H2O concentrations can be generated (200−8000 ppmv). The H2O concentration was measured by a Honeywell HIH-4010 relative humidity sensor, which was externally calibrated and then verified by a Meeco MI moisture meter (±5% or 0.4 ppmv; whichever was greater). Activation of the adsorbent while loaded in the volumetric system was achieved by a ceramic oven and turbo molecular vacuum pump (Agilent TPS Compact). The silica gel studied in the volumetric system was regenerated at T = 200 °C for t = 4 h or until a vacuum of p = 1 × 10−10 bar was achieved. Activation of the adsorbent in the gravimetric system was achieved using dry He (g) at elevated temperatures. For silica gel and zeolite 3A, an activation temperature of T = 200 °C was used, whereas zeolite 4A required a higher temperature of T = 300 °C. The regeneration temperatures were chosen because a mass difference was not observed when higher temperatures were applied to the adsorbent, i.e., no additional water was released. Data Handling. The method to calculate the amount adsorbed for both systems has been described previously but will be briefly highlighted here.3,4 All densities are calculated using the appropriate reference quality equation of state for each of CH4,6 CO2,7 He,8 COS,9 H2S,9 and H2O10 as implemented by Reference Fluid Thermodynamic and Transport Properties 9.1 (REFPROP NIST).11 Helium and water mixture densities were calculated following the mixing rules of GERG-2008,12 also implemented by REFPROP 9.1. For the
and then characterization by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDX). Water adsorption for zeolite 3A, zeolite 4A, and purchased silica gel was measured using a previously described continuous-flow thermogravimetric instrument for measurement conditions of T = 25−150 °C and p = 8 × 10−3− 3 × 10−4 bar.3 Using a previously described small-volume manometric adsorption instrument, CH4, CO2, COS, and H2S were measured on the purchased 60 Å silica gel at T = 0, 25 and 50 °C.4 Empirical isotherms parameters have been reported in addition to the raw data from other authors. The experimental measurements or adsorption isotherm equations were used to calculate the isosteric heat of adsorption for each adsorbate−adsorbent system where microporous materials (zeolites) show a much larger range of adsorption enthalpies versus mesoporous (60 Å silica gel).
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EXPERIMENTAL SECTION Materials. Table 1 includes all starting materials, sources, and stated purities. 60 Å silica gel grade 9385 (CAS No. Table 1. Reagent Purities and Suppliers chemical formula
source
final purity
NaAlO2 Na2SiO3 NaOH KCl AgNO3 SiO2 H2O CO2 COS CH4 H2S He
Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Fisher Scientific Merck In-house Praxair Praxair Praxair Praxair Praxair
50−56% Al 44−47% SiO2 >98% >99.0% >99.5% >99% 18 MΩ cm−1 >99.9995% >99.6% >99.999% >99.6% >99.999%
112926-00-8), sodium aluminate (CAS No. 1138-49-1), sodium hydroxide (CAS No. 1370-73-2), sodium metasilicate (CAS No. 6834-92-0), silver nitrate (CAS. No. 7761-88-8), and potassium chloride (CAS No. 7447-40-7) were used as received. An in-house EMD Millipore system was used to polish H2O to 18 MΩ cm. CO2, COS, H2S, CH4, and He were purchased from Praxair and used as received. Gas purities of CO 2 , COS, H 2 S, and CH 4 were confirmed by gas chromatography (TCD/FID and SCD). The synthesis of zeolite 3A was performed by cation exchange of K+ with Na+ from the previously synthesized zeolite 4A.4 Approximately 10 g of zeolite 4A was dissolved in 250 mL of 0.5 M KCl solution heated to T = 80 °C. The suspension was stirred for approximately 1 h at T = 80 °C, and thereafter, the solid phases were separated from the solution by filtration. The filtered residue was redispersed in a fresh 250 mL portion of 0.5 M solution of KCl and stirred again for approximately 1 h at T = 80 °C; this procedure was carried out three times. After the final solid−liquid separation, the residue on the filter paper was rinsed with H2O until the reaction of AgNO3 with filtrate yielded a negative result (i.e., all KCl (aq) was washed) and then the residue was dried overnight at T = 105 °C. The majority of the material characterization can be found in the Supporting Information. The cation-exchanged zeolite 3A was characterized using a Philips XL 30 scanning electron 3157
DOI: 10.1021/acs.jced.9b00233 J. Chem. Eng. Data 2019, 64, 3156−3163
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to other empirical and semiempirical isotherms. The modified Tóth equation was based on the work by Cavenati et al.20 with an addition a linear term, K, to the equation3
volumetric system, gas was introduced into the adsorption cell after equilibration in the reference chamber. The pressure transducers had a full-scale accuracy within ±0.007% and were recalibrated against a Pressurements Limited T 3800/4 deadweight tester. The temperature probes were recalibrated to an accuracy of δT = ±0.005 °C. The amount of adsorptive introduced (nint) was n int = (ρref − ρsys ) ·Vref
nabs = n∞·
m=0
(1)
mabs
(4)
b = b°·e(−ΔH / RT )
(5)
t = A + BT
(6)
In eq 5, b° is the infinite adsorption constant, ΔH is the enthalpy of adsorption, and A/B are empirical parameters that relate the heterogeneity parameter to temperature. The linear term is K = n∞b°f
(2)
(7)
The linear term in eq 7 accounts for the compressibility of the adsorbate at large loadings and pressures. Several other semiempirical and empirical isotherms did not fit as well as this modified Tóth isotherm or required extraneous adjustable coefficients (poor statistical degrees of freedom). To represent gravimetric H2O adsorption, the isotherm of Gorbach et al.21 was used because this model was found to have the smallest MSSE. This is a two-site Langmuir isotherm
The void volume (Vvoid) is determined by He (g) expansion at T = 50 °C for excess adsorption,13 or the adsorbent−adsorbate volume is fixed by crystal density and mass of adsorbent for absolute adsorption.14,15 This definition of absolute adsorption assumes that all intracrystalline molecules are adsorbed and (i) has been used in our previous studies and (ii) is used for our developing multicomponent models. Excess H2O adsorption can be determined directly using the gravimetric system as the baseline is measured for dry He (g); therefore, any mass gain during wet flow is in excess to He (g) adsorption. To determine absolute amounts adsorbed, the displacement of gas-phase density by adsorbent−adsorbate volume is accounted for.16,17 The method of Murata et al. was used with the same assumption as our absolute volumetric adsorption, which is that the adsorbent−adsorbate volume was fixed by crystal density and mass of the adsorbent. To determine mass adsorbed (mabs)16 ij m yz ji D zy zz = mob + ρbulk ·jjjj mat zzzz − jjj j mmat zz ρ mat { k { k
+K
where n is the amount adsorbed at saturation and f is the fugacity of the adsorptive. The parameters b and t are related to the adsorption constant and heterogeneity parameter, respectively, and both are expanded to include temperature dependence
m
∑ nmint − ρsys Vvoid
(1 + bf t )1/ t
∞
where ρref is the reference molar density (density of adsorptive within the dosing loop), ρsys is the system molar density (density of the adsorptive within the adsorption chamber), and Vref is the volume of the reference chamber. The amount adsorbed (nads) is determined by the sum of adsorptive introduced (nint) subtracted from the gas accounted for in the head space of the adsorption cell, where m is the number of injections nads =
bf
nabs = b0(T ) ·
b1(T ) ·f + b2(T ) ·f 2 1 + b3(T ) ·f + b4(T ) ·f 2
(8)
The temperature dependence for the parameters of eq 8 were assumed to follow a van’t Hoff relationship. b0(T ) = b0,0 ·eb0,T ·(1 − T / T0)
(9)
bi(T ) = bi ,0 ·e(bi ,T ·T0/ T ) − 1 for i = 1 to 4
(10)
where b0,0, b0,T, bi,0, and bi,T are fitting parameters and the empirical reference temperature (T0) is set to the lowest measured temperature. Although not the focus of our research, excess experimental data were also calculated for ease of comparison to literature values, i.e., many other studies and models utilize helium excess adsorption. The excess adsorption isotherms can be calculated from absolute adsorption isotherm by eq 11
(3)
where mob is the measured mass. The second term represents displaced adsorptive, where the density of the adsorptive is ρbulk, the mass of material is mmat, and the density of the material is ρmat. The final term fixes the baseline by the dry weight (D). Note that the units for mabs and mob are in units of a gram of adsorbate per gram of adsorbent, which is then converted to millimole of adsorbate per gram of adsorbent using the molar mass of the adsorbate. For the previous studies, the crystal density, measured by XRD, was used in the definition of absolute adsorption, which is not possible for amorphous silica gel. To calculate the silica gel solid density, the volume of nonporous silica and pore volume of the studied silica gel were added and the inverse was taken. This was used as the studied silica gel density for absolute adsorption measurements. The pore volume was calculated using a measured N2 Brunauer−Emmett−Teller (BET) isotherm at T = −196 °C.18,19 Adsorption Isotherm Equations. A variation of the modified Tóth equation was fit to the volumetric adsorption data (CH4, CO2, COS, and H2S) on silica gel, which was found to give the lowest mean sum squared error (MSSE) compared
ρg Va yz ij nexc = jjjj1 − abs zzzz·nabs n { k
(11)
where ρg is the density of the adsorptive and Va is the apparent excess volume of the adsorbate, which is a fitted parameter that is constant for each unique adsorbent. Isosteric Heats of Adsorption. Equations derived by Titoff and later Hückel can be used to calculate the isosteric (constant surface coverage) heat of adsorption, (ΔaH), through fitted experimental data.22 The isosteric heat is calculated by the following equation23 Δa H = −R(∂ ln f /∂(1/T ))nabs
(12)
ΔaH is obtained numerically by a linear least-squared regression of ln f versus 1/T for each nabs selected. Two 3158
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Table 2. Parameters for Adsorption on Silica Gel (eqs 4−7) nabs (mmol g−1) CH4 CO2 COS H2S
3.44 × 10 3.28 × 102 18.0 18.0 2
b° (bar−1) 7.99 1.11 4.58 5.80
× × × ×
−6
10 10−6 10−6 10−6
−ΔH (kJ mol−1)
A
B (K−1)
8.23 19.67 22.77 23.72
1 7.91 × 10−2 4.26 × 10−1 −1.33 × 10−1
0 6.17 × 10−4 5.70 × 10−4 2.15 × 10−3
MSSE (mmol−2 g−2) 7.86 1.54 6.99 3.77
× × × ×
10−3 10−2 10−4 10−3
methods are used to determine f at constant nabs: (i) by a spline fit of experimental data when there are measurements at similar nabs at each temperature (interpolation; silica gel), or (ii) by calculation of fugacity at a constant amount adsorbed using the appropriate isotherm equation (extrapolation; zeolites 3A and 4A).
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RESULTS AND DISCUSSION Material Characterization. The SEM micrographs of zeolite 3A are shown in the Supporting Information, S1 and show a well-crystallized adsorbent with a chamfered-edge cube morphology. No change was observed in the morphology after cation exchange. Zeolite 3A was found to have an average K/ Na ratio of approximately 25 from EDX measurements at multiple points, suggesting that the majority of Na+ was replaced by K+. DLS measurements (Supporting Information, S2) show an average diameter of 4.13 μm. The PXRD pattern of zeolite 3A showed a favorable comparison to a commercial zeolite 3A (Supporting Information, S3).24,25 The PXRD had sharp peaks and a small baseline, which are both indications of a high degree of crystallinity. Despite assuming that CO2, CH4, COS, and H2S cannot enter zeolite 3A pores, we verified that our zeolite 3A did not adsorb CO2 (
