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Methyl mercaptan removal from natural gas using MIL-53(Al)
Journal of Natural Gas Science and Engineering 38 (2017) 272e282
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Methyl mercaptan removal from natural gas using MIL-53(Al) Armin Taheri a, Ensieh Ganji Babakhani a, *, Jafar Towfighi b a b
Gas Refining Technologies Division, Research Institute of Petroleum Industry, Tehran, Iran Chemical Engineering Department, Engineering Faculty, University of Tarbiat Modares, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2016 Received in revised form 29 November 2016 Accepted 20 December 2016 Available online 28 December 2016
MIL-53(Al) was synthesized using hydrothermal method with various synthesis and calcination conditions. The synthesized materials were characterized by Powder X-ray Diffraction (PXRD), Thermal Gravimetric Analysis (TGA) and N2 adsorption-desorption isotherms (BET surface area measurement) methods. The results showed that synthesis time and calcination time can be decreased two days and one day, respectively, with respect to the usual method with no significantly decrement in the BET surface area and the crystallinity of materials. The sorption equilibrium, thermodynamic and kinetic of methyl mercaptan (MeSH) and methane adsorption on the MIL-53(Al) were studied by the volumetric method using a home-made apparatus. The sorption isotherm of MeSH revealed type IV according to IUPAC classification, while a type I was observed in the profile of the methane sorption isotherm. The synthesized MIL-53(Al) displayed high methyl mercaptan adsorption capacity (more than 9 mmol/g) which is about 2e3 times more than 13X, the usual industrial adsorbent for mercaptan removal from natural gas. The various models such as Langmuir, Sips and Thoth were successfully used to fit the adsorption experimental data. The Extended Langmuir (EL) and Ideal Adsorbed Solution Theory (IAST) models along with the pure component isotherm fitted to the Langmuir model were used to predict the removal of MeSH from methane-rich mixture. MIL-53(Al) presented high selectivity (EL selectivity of 263) for MeSH over methane. The isosteric heat of MeSH and CH4 adsorption on the MIL-53(Al) were within the range of 26e50 and about 18 kJ mol1 respectively. Multiple adsorptionedesorption cycles showed that the MeSH adsorption on the MIL-53 was highly reversible with a desorption efficiency of up to 95%. The diffusion time constants of MeSH and CH4 in the MIL-53(Al) at 298 K were estimated to be 1.48 102 and 1.99 102 s1, respectively, leading to relatively equal adsorption kinetics of both molecules. High adsorption capacity, good kinetics, easy regeneration and reversible process of MeSH adsorption, suggested that the MIL-53(Al) can be used as a favorable adsorbent for purification of natural gas or biogas in the Pressure Swing Adsorption (PSA) process. © 2016 Elsevier B.V. All rights reserved.
Keywords: MIL-53(Al) Methyl mercaptan Natural gas Purification Adsorption
1. Introduction Mercaptans (thiols) are highly toxic, colorless, reactive and corrosive volatile organic compounds. Mercaptans comprising SH groups attached to a hydrocarbon chain with a general formula of RSH. They are known to be emitted from gas and petroleum refining processes, wood industry, food industries, sewage treatment, energy-related activities and other sources (Jiun-Horng Tsai et al., 1999; Kim and Yie, 2005; Sakano et al., 1996a,b). The content of mercaptans in the air shall be reduced to very low level
* Corresponding author. Research Institute of Petroleum Industry (RIPI), West Blvd., Azadi Sport Complex, P.O. Box: 14665-137, Tehran, Iran. E-mail address: [email protected] (E.G. Babakhani). http://dx.doi.org/10.1016/j.jngse.2016.12.029 1875-5100/© 2016 Elsevier B.V. All rights reserved.
according to environmental laws and living standards (Lee et al., 2010). The concentration of mercaptans in natural sour gas has to be reduced to a range of 20 ppm according to the development and environmental laws (Weber et al., 2008), although typically 200 ppb mercaptan added to odorless gas for leak detection (Golebiowska et al., 2012). Mercaptans can poison the catalyst of fuel cells so they must be removed before the fuel reforming operation (Bashkova et al., 2005; de Wild et al., 2006; Lim et al., 2008; Seyedeyn-Azad et al., 2009). Methyl Mercaptan (MeSH) is the lightest and the most volatile compound of thiols (Bashkova et al., 2003). It smells like rotten cabbage and has a very low odor threshold (2e7 ppb) and low occupational threshold (0.5 ppm) in air (Bashkova et al., 2003; Kim n et al., 2005; Kim and Yie, 2005; Liu et al., 2008; Moreno-Piraja et al., 2010; Zhao et al., 2015). MeSH is the one of impurities of
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natural gas and biogas which must be removed to low level according to environmental regulations, specific characteristics of transmission lines and consumer specifications (Abatzoglou and Boivin, 2009; Farahzadi et al., 2012). However, the traditional techniques for mercaptan removal have low efficiency for light mercaptans. When the amount of sulfur is small, adsorption technology is common for mercaptan removal (Benoit et al., 2005; Kohl and Nielsen, 1997). Selection of a proper adsorbent plays a key role in this process. Metal oxide (specially iron oxide) (Abatzoglou and Boivin, 2009; Kang et al., 2007; Kim et al., 2007, 2014; Ratnasamy et al., 2012; Turbeville and Yap, 2006) Carbon compounds (specially activated carbon) (Bashkova et al., 2002; Cui et al., 2009; Jiun-Horng Tsai et al., 1999; Kim et al., 2005; Kim and Yie, 2005; Mirzaeian et al., 2014; Park et al., 2010; Salamanka et al., 2010; Tamai et al., 2006; Vega et al., 2013) and zeolite (specially Faujasites) (Bellat et al., 2008; Benoit et al., 2005; Sakano et al., 1996a,b; Satokawa et al., 2005; Shirani et al., 2010; Steuten et al., 2013; Wakita et al., 2001; Weber et al., 2008, 2005) had been utilized for mercaptan removal from gas streams. Iron sponge (iron oxide supported on wood chips) is one of the oldest industrial adsorbents for removal of sulfur components from gas. Iron oxide theoretically can remove a large amount of mercaptan by following eq. (1) but its activity reduced about onethird after each regeneration, so practically they must be replaced by new iron sponge after once or twice regeneration.
Fe2 O3 þ 6 RSH/2 FeðRSÞ3 þ 3 H2 O
(1)
Due to the high operating costs, waste accumulation and hazardous operation, today, the usages of these materials are rather limited to small -scale applications (Abatzoglou and Boivin, 2009). The most effective adsorbents for mercaptan removal are zeolite and activated carbon. Activated carbons are generally used for removal of mercaptans from the air. It is inflammable and sensitive to the regeneration process (Ryzhikov et al., 2011; Tamai et al., 2011; Weber et al., 2008). Thus zeolite is commonly used for mercaptan removal from natural gas. It has been shown that zeolite with effective pore diameter larger than 3 A could adsorbed the MeSH (Sakano et al., 1996a,b), nonetheless 5 A and 13X are generally used for the mercaptan removal from the Persian Gulf natural gas due to the existence of a wide spectrum of mercaptans with the kinetic size larger than MeSH (Kohl and Nielsen, 1997; Kulprathipanja, 2010). However the capacity of 5 A and 13X for MeSH are limited to about 3.5 and 3.8 mmolg1 respectively (Ryzhikov et al., 2011; Sakano et al., 1996a,b). An emerging new class of crystalline porous solids called metaleorganic frameworks (MOFs) has recently been investigated to use as adsorbents. They comprise metal ions (or metal cluster) and organic ligands that link by coordination bonds (Furukawa et al., 2013; Li et al., 2012, 2009; Mueller et al., 2006). Among them, MIL-53(Al) shows easy synthesis, highest thermal stability, cheap and available raw materials. It is also moisture resistant and provides relatively high surface area. Therefore it is a proper MOF candidate for the adsorption process (Alhamami et al., 2014; Burtch et al., 2014; Camacho et al., 2015; Heymans et al., 2012; Loiseau et al., 2004; Mowat et al., 2011; Patil et al., 2011). In recent years, the adsorption of sulfur compounds on some MOFs has been reported (Achmann et al., 2010; Ahmed and Jhung, 2016; Khan et al., 2011). Generally, the studies related to the adsorption of mercaptans, especially adsorption data for light mercaptans, are limited and scarce up to now (Benoit et al., 2005; Lee et al., 2010; Shams et al., 2008; Weber et al., 2005). To the best of our knowledge, there is no work on the adsorption of MeSH on MOFs. However, there are limited numbers of studies related to the other mercaptans
273
adsorption on MOFs (Chen et al., 2015; Li et al., 2015; Wang et al., 2014; Wang et al., 2014). In this study, MIL-53(Al) samples were synthesized by the hydrothermal method which is a conventional and developed method for synthesis of zeolite and a simple and an environmentally friendly technique. The synthesized materials were characterized by PXRD (Powder X-ray diffraction), N2 adsorption-desorption isotherms and TGA (Thermal Gravimetric Analysis) techniques. The adsorption isotherms of pure MeSH and methane on MIL53(Al) were determined at various temperatures using a built inhouse apparatus based on the volumetric method. The adsorption heat of MeSH on MIL-53(Al) was estimated by Clausius-Clapeyron equation. The equilibrium isotherms were fitted by different models. Finally, the selectivity of MeSH over CH4 was predicted based on the IAST (Ideal Adsorbed Solution Theory) and Extended Langmuir models (Myers and Prausnitz, 1965). 2. Material and methods All chemicals were of the reagent-grade quality and were used without further purification. MIL-53(Al) was synthesized from aluminum nitrate (Al (NO3)3), terephthalic acid (TPA or H2BDC, C6H4 (CO2H)2) and deionized water similar to the reported method (Loiseau et al., 2004). The mixture with molar ratio of 1 Al: 0.5 H2BDC: 80 H2O (13gr: 2.88 gr: 50 ml) was stirred for 0.5 h in Teflon lined stainless steel autoclave. The autoclave was sealed and placed in a furnace at 220 C for 72 h. Then the reactor was gradually cooled down to the room temperature. The resulting solid was filtered and washed with deionized water until the filtrated water reached pH~7. Finally, the product was calcined in the air stream at 330 C for 72 h. In addition to the primary sample (S-1) with the preparation conditions described above, three other samples were synthesized according to Table 1 while maintained the other preparation conditions, to investigate the effect of synthesis time and activation (calcination) conditions. PXRD by Philips 1840 diffractometer was used to determine the crystalline structure of the samples. Specific surface area of the samples was determined by BET method analyzing nitrogen adsorption isotherms at liquid nitrogen temperature using a Micrometrics apparatus (2020 ASAP). TGA technique was performed from room temperature to 700 C at a heating rate of 10 C/ min under air atmosphere using a Mettler Toledo 851 TGA analyzer. Adsorption isotherm measurement of MeSH and methane have been performed with a built in-house apparatus by volumetric method. 2.1. Setup and sorption tests description Schematic of experimental apparatus was shown in Fig. 1. Firstly, 1 g of sample was loaded to the adsorption column. The temperature of the adsorption column was recorded and controlled by an electrical tracing equipped with thermocouple and controller (TIC). Before each adsorption test the sample was outgassed at 200 C insitu for 4 h under vacuum pressure imposed by a vacuum pump. The pressure was measured with two pressure transmitters for two ranges (low or vacuum and high pressure). The dead volume was calculated using helium gas measurement by assuming helium not to be adsorbed on adsorbent. The sorption tests were performed using the volumetric method with changing the pressure of holder cell. The trend of pressure changes with time was recorded in a PC using a data acquisition system. After finishing the sorption test, the system was purged with nitrogen gas and exhaust gas pass through an absorption column which filled with NaOH solution and then was conducted to a safe place. The experimental setup was
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Table 1 Synthesis and calcination conditions of trials. sample No.
Synthesis Temperature( C)
Synthesis Time(h.)
Calcination Temperature( C)
Calcination Time(h.)
S-1 S-2 S-3 S-4
220 220 220 220
72 24 24 72
330 330 400 330
72 48 48 48
Fig. 1. Schematic diagram of experimental setup.
implanted in an isolated room with well-ventilated environment. The atmosphere temperature of the room was regulated by a ventilation system.
3. Results and discussion 3.1. Characterization The powder XRD of samples are indicated in Fig. 2. The PXRD patterns of all samples provide a good agreement with the literature (Loiseau et al., 2004) and prove the formation of MIL-53(Al).
The strong and narrow diffraction peaks indicate a high purity and good crystallinity of the synthesized materials. Relative crystallinity of the samples was compared to each other according to the ASTM D3906 method. The results confirmed that the highest relative crystallinity was obtained for S-3 sample. It is obvious that presence of the disordered terephthalic acid, as the main impurity, reduces the relative crystallinity of the final product which is resulted from differences in the crystal structure of MIL-53 before and after BDC removal (Loiseau et al., 2004; Serre et al., 2002). Therefore the S-3 presented highest relative crystallinity, due to higher impurities removal at
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275
Fig. 2. PXRD patterns of the samples.
the highest temperature. Loiseau et al. reported that heating to 275e420 C can remove the unreacted BDC from the framework of MIL-53(Al) and decomposition of MIL-53(Al) occurs at the temperature above 500 C (Loiseau et al., 2004). At high temperature (400 C), which is near the sublimation temperature of BDC (402 C) (Lucchesi and Lewis, 1968), the most of unreacted terephthalic acid could be removed from the structure of the synthesis material without structural collapse of the framework (Sun et al., 2015). The N2 adsorption and desorption isotherms on the samples at 77 K (196 C) are depicted in Fig. 3. The isotherms are reversible and do not show any hysteresis upon desorption. The obtained isotherms of the samples can be classified as type-I according to the IUPAC classification (Sing, 1985; Thommes et al., 2015). The BET (Brunnauer, Emmett and Teller) technique was used in order to estimate the specific surface area and pore volume of the samples from N2 isotherm. The obtained results are tabulated in Table 2. As this table, the highest BET specific surface area (1165.0 m2/g), which is slightly higher than that for the other
Table 2 Results of nitrogen sorption at 77 K up to 1 bar. Sample
ABET(m2/g)
Micropore Area
Micropore Volume(cm3/g)
S-1 S-2 S-3 S-4
1150.6 1074.1 1165.0 1103.9
1126.7 999.9 1136.4 1050.3
0.53 0.46 0.54 0.48
samples, was obtained for the sample S-3 calcined at 400 C. Thermal stability of the selected sample (S-3) was investigated before calcination by TGA method which is depicted in Fig. 4. Two steps were seen in Fig. 4 which the first one (300e400 C) was attributed to removal of the entrapped BDC within the pores and another one (500e600 C) was ascribed to the collapse of MIL-53 structure and BDC removal from the framework (Loiseau et al., 2004). The TGA results indicated that the temperature of 330 C might be not enough for perfect removal of unreacted BDC. This TGA result confirmed the BET results.
Fig. 3. Isotherms of nitrogen adsorption and desorption at 77 K up to 1 bar.
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Fig. 4. Thermal stability of MIL-53(Al) (sample S-3).
3.2. Single component adsorption The MeSH adsorption isotherms of the samples at low-pressure ranges are illustrated in Fig. 5. As it was expected, the S-3 had the best performance because of the highest capacity and steep slope at very low-pressure isotherm. It could be related to the higher available adsorption site particularly the hydroxyl groups. These sites play a major role in the selective adsorption of polar molecules rey, 2008). These sites may partially be occupied by unreacted (Fe disordered BDC which were trapped in the pores of the adsorbents. The methane and MeSH sorption isotherms of MIL-53(Al) (S-3 as the best sample) powder, respectively, for pressure up to 40 bar and 1.6 bar at 298 K, are depicted in Figs. 6 and 7. From Fig. 6, the sorption isotherm of methane is reversible upon desorption without any hysteresis and presents a monotic type-I shape isotherm according to IUPAC. Maximum adsorption capacity of methane on MIL-53(Al) at 298 K (25 C) is about 6 mmol/g at high pressure. As Fig. 7, the MIL-53(Al) showed very high capacity (~9 mmol/gr) which is about 2e3 times higher than adsorption capacity of 13X, the usual industrial adsorbent for mercaptan removal from natural gas (Ryzhikov et al., 2011) and even more than activated carbon, traditional adsorbent for mercaptan removal from air (Bashkova et al., 2002b). The comparison of the capacity of various adsorbents for physical adsorption of light mercaptans (Ethyl
mercaptan (ESH) and MeSH) is tabulated in Table 3. Also, it can be seen in Figs. 6 and 7 that MeSH isotherm is differed from that for methane. It exhibited one step at about 0.25 bar pressure, which is not regular for microporous materials. A type IV isotherm is noticed in the profile of the MeSH adsorption isotherm. The maximum adsorption of MeSH is about 3 mmol/g and 9 mmol/g respectively for the first and the second section of isotherm. This unusual behavior could be attributed to a wellknown phenomenon in some MOFs, called breathing effect. MIL53 can exist in two forms (but the same topology) including narrow-pore with rectangular pores (0.85 0.85 nm), and largepore forms with trapezoidal pores (0.26 1.36 nm). They exhibited reversible structural transformation from large-pore form to narrow-pore form (and vice versa) upon adsorption of polar guest molecules (such as CO2, H2O, H2S) at ambient temperature which is called “breathing effect”(Alhamami et al., 2014). Due to the existence of stepwise behavior in the adsorption isotherm of MeSH, it was divided into two sections for fitting appreciate model for each section. One model for the low-pressure region (LP) and another one for the high-pressure region (HP) was fitted to the experimental data. Using Matlab software (MathWork, Inc. R2012a), adjustable parameters of three well-known isotherms models, Langmuir, Sips and Toth, as follows respectively, were obtained for pure methane
Fig. 5. Adsorption isotherm of MeSH on the samples at 298 K up to 0.2 bar.
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277
Fig. 6. Sorption isotherm of CH4 on MIL-53(Al) at 298 K up to 40 bar.
Fig. 7. Sorption isotherm of MeSH on MIL-53(Al) at 298 K up to 1.6 bar.
Table 3 Light mercaptan adsorption capacity on different adsorbent. Adsorbent
Temperature (K)
Adsorbate
Adsorbent load (mmol/g)
Ref.
Silica-Alumina Gel Silica-Alumina Gel Zeolite -5 A Zeolite -5 A Activated carbon-coconut-based Activated carbon Zeolite-13X (NaX) Zeolite-10X (CaX) Zeolite-5A Zeolite-13X(NaX) MIL-53(Al)
298 298 298 298 293 Amb. 298 298 303 298 298
MeSH ESH MeSH ESH MeSH MeSH MeSH MeSH MeSH ESH MeSH
~0.5 ~0.9 ~2.7 ~3 ~1 ~1.2 3.80 2.92 ~3.5 ~3.9 ~9
(Steuten et al., 2013)
(Lee et al., 2010) (Tamai et al., 2006) (Ryzhikov et al., 2011) (Sakano et al., 1996a,b) (Benoit et al., 2005) This work
and MeSH at 298 K and are tabulated in Table 4.
q ¼ qs
bp 1 þ bp
(2)
1
q ¼ qs
ðbpÞn 1
1 þ ðbpÞn
(3)
bp q ¼ qs 1 1 þ ðbpÞn n
(4)
p: Pressure of gas at equilibrium with adsorbed phase (bar).q: Adsorbed amount per mass of adsorbent (mmol/g).qs: Saturation or maximum capacity of adsorbent (mmol/g).b: Affinity coefficient (bar1).n: Adsorbate-adsorbent interaction (surface heterogeneity) parameter-not applicable for Langmuir model.
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Table 4 Langmuir, Sips and Toth fitting parameter for MeSH and CH4 adsorption on MIL-53(Al) at 298 K. parameter
b (bar
Region
1
)
qs (mmolg
1
)
n SSE R2
LP HP LP HP LP HP LP HP LP HP
CH3SH
CH4
Langmuir
Sips
Toth
Langmuir
Sips
Toth
49.62 1.74 2.9 12.59 e e 0.08762 0.2932 0.992 0.9965
3.29 2.74 3.68 11.5 0.64 1.17 0.02115 0.1944 0.9957 0.9977
2.43 2.29 4.36 11.23 0.43 1.31 0.01623 0.2786 0.9967 0.9966
0.079
0.054
0.076
6.93
8.13
9.51
e
1.14
0.69
0.04163
0.006337
0.003648
0.9992
0.9999
0.9999
Sum of the square of errors (SSE) as the most commonly used error function (Foo and Hameed, 2010) was considered as an objective function for fitting these isotherms to the experimental values. R-square or coefficient of determination denoted as R2, and SSE of each fitting are provided in Table 4. According to the obtained R2 and SSE values for all models, a good agreement between experimental adsorption data and models results was observed. The maximum adsorption capacity of pure gases (qs), which were estimated from the models, were relatively in agreement with each other according to Table 4. These values were observed to be higher than the results were obtained from Fig. 7 which were measured at pressures below MeSH saturation pressure. In all pressure ranges and in all models, the affinity of MIL-53(Al) for MeSH was observed to be so much higher than that for CH4. The MIL-53(Al) affinity for MeSH at high pressure was less than low-pressure. This could be related to the weaker interaction of MeSH molecules within the large-pore than narrow-pore structure as previously observed for some polar molecules (Hamon et al., 2009; Ramsahye et al., 2007). To examine the reversibility of MeSH adsorption on the MIL53(Al), experiments of consecutive adsorption-desorption cycles were performed using the same adsorbent at 298 K. The effect of 5 consecutive adsorptionedesorption cycles are shown in Fig. 8. As this figure, after 5 cycles, more than 95% of initial adsorption capacity was preserved. It suggested that the MIL-53(Al) had excellent reversibility of MeSH adsorption and did not disclose significant loss in capacity. 3.3. Heat of adsorption The isosteric heat of adsorption can provide useful information about the energetic heterogeneity of an adsorbent and its evaluation for potential adsorption processes. The isosteric heat
(enthalpy) of adsorption characterized the interaction between the adsorbate molecules and the adsorbent surface. The isosteric heat of adsorption could be estimated using Clausius-Clapeyron equation by driving the slope of ln(P) versus T1 at a constant loading(q). Therefore, the isotherms of MeSH and CH4 on MIL-53(Al) at three different various (298, 323, 348 K) were obtained and used to calculate the heat of adsorption depicted in Fig. 9. From Fig. 9, the adsorption enthalpy of MIL-53(Al) for MeSH and CH4 at zero coverage (or pressure) were about 50 and 18 kJ/mol respectively. The heat of MeSH adsorption on MIL-53(Al) was in the range of 26e50 kJ/mol while that for CH4 was at around 18 kJ/mol and was nearly constant. The higher isosteric heat of adsorption for MeSH than that of CH4 indicated higher interaction of MeSH than CH4 with the adsorption sites of MIL-53(Al) and it can be an explanation for high selectivity of MIL-53 toward MeSH/CH4. This phenomenon was attributed to the nonpolar nature of CH4 molecules respect to the polar nature of MeSH. The MeSH isosteric heat of adsorption showed a decreasing trend with increasing of loading pressure while the enthalpy of CH4 adsorption is rather constant. At higher pressure, the MeSH heat of adsorption remained constant to a value close to vaporization (or condensation) enthalpy of MeSH. The initial drop in the isosteric heats of MeSH adsorption could be ascribed to the energetic heterogeneity of the adsorbent surface and the strong interaction between MeSH and hydroxyl groups (OH sites) of MIL-53(Al). However the relatively constant isosteric heat with increasing CH4 loading suggested that the surface of MIL53(Al) is homogenous toward CH4 molecules. 3.4. Adsorption kinetics The kinetic selectivity of adsorbent was measured by the ratio of
Fig. 8. Effect of regeneration cycles on the performances of adsorptive removal of MeSH on MIL-53(Al) at 298 K.
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279
Fig. 9. Heat of MeSH and CH4 adsorption on MIL-53(Al).
interacrystalline diffusivities of MeSH to CH4 (Ruthven, 1984). As described in the experimental section, the trend of decreasing pressure with time was recorded while adsorption equilibrium data were recorded. The uptake ratio versus time at 298 K for MeSH and CH4 are plotted in Fig. 10. It can be seen from Fig. 10 that the adsorption kinetic of CH4 and MeSH are relatively fast. It could be related to the larger pore size of adsorbent with respect to the kinetic diameter of molecules. To extract the interacrystalline diffusivity of adsorbates within the adsorbent, classical micropore diffusion model was applied. If the adsorbent regarded as a spherical object and heat transfer is rapid, the diffusion equation is written as (Ruthven, 1984):
vq 1 v vq ¼ 2 r 2 Dc vt r vr vr
(5)
where r is the radius of the adsorbent, Dc is the interacrystalline diffusivity and q(t,r) is the adsorbed amount at time t and radial position r. According to the crystal symmetry and assuming constant adsorbate concentration, initial and boundary conditions are as follows:
qðr; 0Þ ¼ q 0 0 ; qðrc ; tÞ ¼ q0 ;
vq ¼0 vr r¼0
q0 0 : the initial adsorption amount in the adsorbent particle.q0 : the equilibrium uptake in the adsorbent particle.rc: particle diameter. By considering above boundary conditions and assuming constant diffusivity, the solution of Eq. (5) is given by familiar equation:
∞ q q0 0 mt 6 X 1 p2 n2 Dc t ¼ ¼ 1 exp q0 q0 0 m∞ rc 2 p2 n¼1 n2
(6)
mt q : the average adsorption amount in the adsorbent particle.m : the ∞ fractional adsorption. At short times, if the fractional adsorption uptake is less than % 85, Eq. (6) can be reduced to the following equation with less than 1% error (Ruthven and S. Farooq, 1993)
1 mt 6 Dc t 2 3Dc t ¼ pffiffiffi 2 m∞ rc p rc 2 Therefore, by drawing fractional adsorption uptake
Fig. 10. Fractional adsorption uptakes of MeSH and CH4 adsorption on MIL-53(Al) at 298 K.
(7) mt m∞
versus
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time as depicted in Fig. 10, the diffusion time constant be extracted.
3Dc rc 2
could
Dc for MeSH ¼ 0:01483 s1 rc 2 Dc for CH4 ¼ 0:01995 s1 rc 2 Therefore, CH4 diffuse slightly faster than MeSH due to smaller kinetic diameter. The adsorption kinetic measurements suggested that the diffusivity of MeSH on MIL-53(Al) was comparable to that for CH4. So, it can be concluded that the overall selectivity (combining the equilibrium and kinetic effects) was controlled by the equilibrium selectivity. 3.5. Separation selectivity of binary adsorption of MeSH and methane The equilibrium selectivity is one of the most factor in adsorbent selection for pressure swing adsorption (PSA) is defined as (Yang, 2003):
x xj
aij ¼ i.
(8)
yi yj
xi and xj: the mole fractions of the two components on the adsorbent.yi and yj: the corresponding mole fractions in the gas phase. To evaluate the MeSH/CH4 selectivities of MIL-53(Al) under mixture gas conditions, the IAST and EL models were used to predict multicomponent adsorption behaviors. Fitted parameters of low pressure region were applied because of very low concentrations of mercaptans usually found in natural gas and biogas. Using the EL equation in conjunction with eq. (8), revealed that the adsorbent selectivity is the ratio of the product of qs b or Henry's constant (K) of two components. Therefore, the EL selectivity of MIL-53(Al) for MeSH/CH4 could be expressed as:
aij ¼
Ki qsi bi ¼ 263:5 Kj qsj bj
By using Matlab software, IAST model was used to predict selectivity of the binary mixture, based on fitted parameters of Langmuir equation for pure MeSH and CH4 (due to its simplicity). The purification of gas stream with MeSH mole fraction of 0.001, 0.01 and 0.1 were investigated at the pressure up to 10 bar. The obtained selectivities from IAST and EL models for three different compositions in the pressure range of 0e10 bar, are depicted in Fig. 11. It can be seen that, MIL-53(Al) showed high MeSH/CH4 selectivities for all gas compositions and over entire pressures ranges as the EL selectivity was a constant value of 263.5. The selectivity is a key factor for evaluation of a new adsorbent. The minimum acceptable value of selectivity factor for PSA application was reported to be 2 (Tagliabue et al., 2009). Also, as resulted above, the adsorption capacity MIL-53(Al) samples for MeSH was at least twofold of 5 A or 13X zeolite which are mostly used for mercaptan removal. Therefore, it can be concluded that MIL-53(Al) can be a proper adsorbent for MeSH removal from CH4 gas stream with high adsorption capacity and selectivity. From Fig. 11, the IAST selectivity factors were very sensitive to both total pressure and composition. At very low pressure, the selectivity calculated based on EL equation and IAST were almost similar. The IAST model results indicated that by increasing the total pressure from 0 to 10 bar for a particular gas composition, the selectivity decreases. In low-pressure region, the selectivity mostly was controlled with the interaction of the adsorbent with the adsorbate. Hence, MeSH molecules were mostly adsorbed by the adsorbent due to the high affinity of MIL-53 for these molecules. As the pressure was increased, the interaction between molecules was become more significant respect to the interaction with the adsorbent. Therefore, the MeSH molecules needed to compete with CH4 molecules for available surface sites. By increasing MeSH molar composition, the selectivity, resulted from IAST model, decreases. The reasons for this phenomenon could be explained by changing of MeSH adsorption mechanism. At low MeSH contents, MeSH molecules interact strongly with the hydroxyl groups of MIL-53 (two hydroxyl group per unit cell) and selectively were adsorbed while at higher MeSH content, they may be adsorbed on the remaining hydroxyl sites and other available sites (at nonspecific way) which led to the lower selectivity.
Fig. 11. The predicted IAST selectivity toward MeSH/CH4 as a function of pressure and composition at 298 K on the MIL-53(Al).
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4. Conclusion MIL-53(Al) was synthesized in various conditions using hydrothermal method. The obtained materials were characterized by XRD, BET and TGA techniques. Results showed that time of synthesis and calcination of MIL-53(Al) could be reduced with a small decrement in BET surface area and crystallinity of the materials. Also, calcination of MIL-53(Al) at 400 C resulted in higher crystallinity and BET surface area compared to the regular calcination method (at 330 C). Furthermore, MeSH and methane adsorption capacities, thermodynamics and adsorption kinetics of the selected sample were examined by the volumetric method. The isotherm of MeSH adsorption on MIL-53(Al) showed a type IV due to the structural transformation of MIL-53 adsorption. MIL-53(Al) displayed considerably higher adsorption capacity for MeSH than the most used adsorbents for mercaptans removal (zeolite and activated carbon). The adsorption process of MeSH on MIL-53(Al) was highly reversible with a desorption efficiency of up to 95%. The micropore diffusivities of CH4 and MeSH in the MIL-53(Al) were relatively high. Also, the selectivity of MIL-53(Al) for CH4 stream contained MeSH impurity was investigated by EL and IAST methods. Results indicated that MIL-53(Al) had very great selectivity for MeSH/CH4 stream particularly at low pressure and concentration. Finally, The obtained remarkable MeSH adsorption capacity, great selectivity, good kinetic rate, easy regeneration and reversibility of process confirmed that the MIL-53(Al) can be appropriately applied in a PSA process for CH4 purification. Abbreviations Amb Ambient BET Brunnauer, Emmett and Teller EL Extended Langmuir ESH Ethyl mercaptan HP High-Pressure IAST Ideal Adsorbed Solution Theory IUPAC International Union of Pure and Applied Chemistry LP Low-Pressure MeSH Methyl Mercaptan MOF Metal Organic Framework PSA Pressure Swing Adsorption PXRD Powder X-ray Diffraction SSE Sum of the Square of Errors TIC Temperature Indicator Controller TGA Thermal Gravimetric Analysis TPA (H2BDC) Terephthalic Acid References Abatzoglou, N., Boivin, S., 2009. A review of biogas purification processes. Biofuels, Bioprod. Biorefining 3, 42e71. http://dx.doi.org/10.1002/bbb.117. Achmann, S., Hagen, G., H€ ammerle, M., Malkowsky, I., Kiener, C., Moos, R., 2010. Sulfur removal from low-sulfur gasoline and diesel fuel by metal-organic frameworks. Chem. Eng. Technol. 33, 275e280. http://dx.doi.org/10.1002/ ceat.200900426. Ahmed, I., Jhung, S.H., 2016. Adsorptive desulfurization and denitrogenation using metal-organic frameworks. J. Hazard. Mater 301, 259e276. http://dx.doi.org/ 10.1016/j.jhazmat.2015.08.045. Alhamami, M., Doan, H., Cheng, C.-H., 2014. A review on breathing behaviors of metal-organic-frameworks (MOFs) for gas adsorption. Mater. (Basel) 7, 3198e3250. http://dx.doi.org/10.3390/ma7043198. Bashkova, S., Bagreev, A., Bandosz, T.J., 2002. Adsorption of methyl mercaptan on activated carbons. Environ. Sci. Technol. 36, 2777e2782. http://dx.doi.org/ 10.1021/es011416v. Bashkova, Svetlana, Bagreev, Andrey, B., T.J., 2002. Activated carnbon as adsorbent of methyl mercaptan. Abstr. Pap. Am. Chem. Soc. 224, 573e574. Bashkova, S., Bagreev, A., Bandosz, T.J., 2003. Adsorption/oxidation of methyl
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Methyl mercaptan removal from natural gas using MIL-53(Al) Armin Taheri a, Ensieh Ganji Babakhani a, *, Jafar Towfighi b a b
Gas Refining Technologies Division, Research Institute of Petroleum Industry, Tehran, Iran Chemical Engineering Department, Engineering Faculty, University of Tarbiat Modares, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2016 Received in revised form 29 November 2016 Accepted 20 December 2016 Available online 28 December 2016
MIL-53(Al) was synthesized using hydrothermal method with various synthesis and calcination conditions. The synthesized materials were characterized by Powder X-ray Diffraction (PXRD), Thermal Gravimetric Analysis (TGA) and N2 adsorption-desorption isotherms (BET surface area measurement) methods. The results showed that synthesis time and calcination time can be decreased two days and one day, respectively, with respect to the usual method with no significantly decrement in the BET surface area and the crystallinity of materials. The sorption equilibrium, thermodynamic and kinetic of methyl mercaptan (MeSH) and methane adsorption on the MIL-53(Al) were studied by the volumetric method using a home-made apparatus. The sorption isotherm of MeSH revealed type IV according to IUPAC classification, while a type I was observed in the profile of the methane sorption isotherm. The synthesized MIL-53(Al) displayed high methyl mercaptan adsorption capacity (more than 9 mmol/g) which is about 2e3 times more than 13X, the usual industrial adsorbent for mercaptan removal from natural gas. The various models such as Langmuir, Sips and Thoth were successfully used to fit the adsorption experimental data. The Extended Langmuir (EL) and Ideal Adsorbed Solution Theory (IAST) models along with the pure component isotherm fitted to the Langmuir model were used to predict the removal of MeSH from methane-rich mixture. MIL-53(Al) presented high selectivity (EL selectivity of 263) for MeSH over methane. The isosteric heat of MeSH and CH4 adsorption on the MIL-53(Al) were within the range of 26e50 and about 18 kJ mol1 respectively. Multiple adsorptionedesorption cycles showed that the MeSH adsorption on the MIL-53 was highly reversible with a desorption efficiency of up to 95%. The diffusion time constants of MeSH and CH4 in the MIL-53(Al) at 298 K were estimated to be 1.48 102 and 1.99 102 s1, respectively, leading to relatively equal adsorption kinetics of both molecules. High adsorption capacity, good kinetics, easy regeneration and reversible process of MeSH adsorption, suggested that the MIL-53(Al) can be used as a favorable adsorbent for purification of natural gas or biogas in the Pressure Swing Adsorption (PSA) process. © 2016 Elsevier B.V. All rights reserved.
Keywords: MIL-53(Al) Methyl mercaptan Natural gas Purification Adsorption
1. Introduction Mercaptans (thiols) are highly toxic, colorless, reactive and corrosive volatile organic compounds. Mercaptans comprising SH groups attached to a hydrocarbon chain with a general formula of RSH. They are known to be emitted from gas and petroleum refining processes, wood industry, food industries, sewage treatment, energy-related activities and other sources (Jiun-Horng Tsai et al., 1999; Kim and Yie, 2005; Sakano et al., 1996a,b). The content of mercaptans in the air shall be reduced to very low level
* Corresponding author. Research Institute of Petroleum Industry (RIPI), West Blvd., Azadi Sport Complex, P.O. Box: 14665-137, Tehran, Iran. E-mail address: [email protected] (E.G. Babakhani). http://dx.doi.org/10.1016/j.jngse.2016.12.029 1875-5100/© 2016 Elsevier B.V. All rights reserved.
according to environmental laws and living standards (Lee et al., 2010). The concentration of mercaptans in natural sour gas has to be reduced to a range of 20 ppm according to the development and environmental laws (Weber et al., 2008), although typically 200 ppb mercaptan added to odorless gas for leak detection (Golebiowska et al., 2012). Mercaptans can poison the catalyst of fuel cells so they must be removed before the fuel reforming operation (Bashkova et al., 2005; de Wild et al., 2006; Lim et al., 2008; Seyedeyn-Azad et al., 2009). Methyl Mercaptan (MeSH) is the lightest and the most volatile compound of thiols (Bashkova et al., 2003). It smells like rotten cabbage and has a very low odor threshold (2e7 ppb) and low occupational threshold (0.5 ppm) in air (Bashkova et al., 2003; Kim n et al., 2005; Kim and Yie, 2005; Liu et al., 2008; Moreno-Piraja et al., 2010; Zhao et al., 2015). MeSH is the one of impurities of
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natural gas and biogas which must be removed to low level according to environmental regulations, specific characteristics of transmission lines and consumer specifications (Abatzoglou and Boivin, 2009; Farahzadi et al., 2012). However, the traditional techniques for mercaptan removal have low efficiency for light mercaptans. When the amount of sulfur is small, adsorption technology is common for mercaptan removal (Benoit et al., 2005; Kohl and Nielsen, 1997). Selection of a proper adsorbent plays a key role in this process. Metal oxide (specially iron oxide) (Abatzoglou and Boivin, 2009; Kang et al., 2007; Kim et al., 2007, 2014; Ratnasamy et al., 2012; Turbeville and Yap, 2006) Carbon compounds (specially activated carbon) (Bashkova et al., 2002; Cui et al., 2009; Jiun-Horng Tsai et al., 1999; Kim et al., 2005; Kim and Yie, 2005; Mirzaeian et al., 2014; Park et al., 2010; Salamanka et al., 2010; Tamai et al., 2006; Vega et al., 2013) and zeolite (specially Faujasites) (Bellat et al., 2008; Benoit et al., 2005; Sakano et al., 1996a,b; Satokawa et al., 2005; Shirani et al., 2010; Steuten et al., 2013; Wakita et al., 2001; Weber et al., 2008, 2005) had been utilized for mercaptan removal from gas streams. Iron sponge (iron oxide supported on wood chips) is one of the oldest industrial adsorbents for removal of sulfur components from gas. Iron oxide theoretically can remove a large amount of mercaptan by following eq. (1) but its activity reduced about onethird after each regeneration, so practically they must be replaced by new iron sponge after once or twice regeneration.
Fe2 O3 þ 6 RSH/2 FeðRSÞ3 þ 3 H2 O
(1)
Due to the high operating costs, waste accumulation and hazardous operation, today, the usages of these materials are rather limited to small -scale applications (Abatzoglou and Boivin, 2009). The most effective adsorbents for mercaptan removal are zeolite and activated carbon. Activated carbons are generally used for removal of mercaptans from the air. It is inflammable and sensitive to the regeneration process (Ryzhikov et al., 2011; Tamai et al., 2011; Weber et al., 2008). Thus zeolite is commonly used for mercaptan removal from natural gas. It has been shown that zeolite with effective pore diameter larger than 3 A could adsorbed the MeSH (Sakano et al., 1996a,b), nonetheless 5 A and 13X are generally used for the mercaptan removal from the Persian Gulf natural gas due to the existence of a wide spectrum of mercaptans with the kinetic size larger than MeSH (Kohl and Nielsen, 1997; Kulprathipanja, 2010). However the capacity of 5 A and 13X for MeSH are limited to about 3.5 and 3.8 mmolg1 respectively (Ryzhikov et al., 2011; Sakano et al., 1996a,b). An emerging new class of crystalline porous solids called metaleorganic frameworks (MOFs) has recently been investigated to use as adsorbents. They comprise metal ions (or metal cluster) and organic ligands that link by coordination bonds (Furukawa et al., 2013; Li et al., 2012, 2009; Mueller et al., 2006). Among them, MIL-53(Al) shows easy synthesis, highest thermal stability, cheap and available raw materials. It is also moisture resistant and provides relatively high surface area. Therefore it is a proper MOF candidate for the adsorption process (Alhamami et al., 2014; Burtch et al., 2014; Camacho et al., 2015; Heymans et al., 2012; Loiseau et al., 2004; Mowat et al., 2011; Patil et al., 2011). In recent years, the adsorption of sulfur compounds on some MOFs has been reported (Achmann et al., 2010; Ahmed and Jhung, 2016; Khan et al., 2011). Generally, the studies related to the adsorption of mercaptans, especially adsorption data for light mercaptans, are limited and scarce up to now (Benoit et al., 2005; Lee et al., 2010; Shams et al., 2008; Weber et al., 2005). To the best of our knowledge, there is no work on the adsorption of MeSH on MOFs. However, there are limited numbers of studies related to the other mercaptans
273
adsorption on MOFs (Chen et al., 2015; Li et al., 2015; Wang et al., 2014; Wang et al., 2014). In this study, MIL-53(Al) samples were synthesized by the hydrothermal method which is a conventional and developed method for synthesis of zeolite and a simple and an environmentally friendly technique. The synthesized materials were characterized by PXRD (Powder X-ray diffraction), N2 adsorption-desorption isotherms and TGA (Thermal Gravimetric Analysis) techniques. The adsorption isotherms of pure MeSH and methane on MIL53(Al) were determined at various temperatures using a built inhouse apparatus based on the volumetric method. The adsorption heat of MeSH on MIL-53(Al) was estimated by Clausius-Clapeyron equation. The equilibrium isotherms were fitted by different models. Finally, the selectivity of MeSH over CH4 was predicted based on the IAST (Ideal Adsorbed Solution Theory) and Extended Langmuir models (Myers and Prausnitz, 1965). 2. Material and methods All chemicals were of the reagent-grade quality and were used without further purification. MIL-53(Al) was synthesized from aluminum nitrate (Al (NO3)3), terephthalic acid (TPA or H2BDC, C6H4 (CO2H)2) and deionized water similar to the reported method (Loiseau et al., 2004). The mixture with molar ratio of 1 Al: 0.5 H2BDC: 80 H2O (13gr: 2.88 gr: 50 ml) was stirred for 0.5 h in Teflon lined stainless steel autoclave. The autoclave was sealed and placed in a furnace at 220 C for 72 h. Then the reactor was gradually cooled down to the room temperature. The resulting solid was filtered and washed with deionized water until the filtrated water reached pH~7. Finally, the product was calcined in the air stream at 330 C for 72 h. In addition to the primary sample (S-1) with the preparation conditions described above, three other samples were synthesized according to Table 1 while maintained the other preparation conditions, to investigate the effect of synthesis time and activation (calcination) conditions. PXRD by Philips 1840 diffractometer was used to determine the crystalline structure of the samples. Specific surface area of the samples was determined by BET method analyzing nitrogen adsorption isotherms at liquid nitrogen temperature using a Micrometrics apparatus (2020 ASAP). TGA technique was performed from room temperature to 700 C at a heating rate of 10 C/ min under air atmosphere using a Mettler Toledo 851 TGA analyzer. Adsorption isotherm measurement of MeSH and methane have been performed with a built in-house apparatus by volumetric method. 2.1. Setup and sorption tests description Schematic of experimental apparatus was shown in Fig. 1. Firstly, 1 g of sample was loaded to the adsorption column. The temperature of the adsorption column was recorded and controlled by an electrical tracing equipped with thermocouple and controller (TIC). Before each adsorption test the sample was outgassed at 200 C insitu for 4 h under vacuum pressure imposed by a vacuum pump. The pressure was measured with two pressure transmitters for two ranges (low or vacuum and high pressure). The dead volume was calculated using helium gas measurement by assuming helium not to be adsorbed on adsorbent. The sorption tests were performed using the volumetric method with changing the pressure of holder cell. The trend of pressure changes with time was recorded in a PC using a data acquisition system. After finishing the sorption test, the system was purged with nitrogen gas and exhaust gas pass through an absorption column which filled with NaOH solution and then was conducted to a safe place. The experimental setup was
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Table 1 Synthesis and calcination conditions of trials. sample No.
Synthesis Temperature( C)
Synthesis Time(h.)
Calcination Temperature( C)
Calcination Time(h.)
S-1 S-2 S-3 S-4
220 220 220 220
72 24 24 72
330 330 400 330
72 48 48 48
Fig. 1. Schematic diagram of experimental setup.
implanted in an isolated room with well-ventilated environment. The atmosphere temperature of the room was regulated by a ventilation system.
3. Results and discussion 3.1. Characterization The powder XRD of samples are indicated in Fig. 2. The PXRD patterns of all samples provide a good agreement with the literature (Loiseau et al., 2004) and prove the formation of MIL-53(Al).
The strong and narrow diffraction peaks indicate a high purity and good crystallinity of the synthesized materials. Relative crystallinity of the samples was compared to each other according to the ASTM D3906 method. The results confirmed that the highest relative crystallinity was obtained for S-3 sample. It is obvious that presence of the disordered terephthalic acid, as the main impurity, reduces the relative crystallinity of the final product which is resulted from differences in the crystal structure of MIL-53 before and after BDC removal (Loiseau et al., 2004; Serre et al., 2002). Therefore the S-3 presented highest relative crystallinity, due to higher impurities removal at
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275
Fig. 2. PXRD patterns of the samples.
the highest temperature. Loiseau et al. reported that heating to 275e420 C can remove the unreacted BDC from the framework of MIL-53(Al) and decomposition of MIL-53(Al) occurs at the temperature above 500 C (Loiseau et al., 2004). At high temperature (400 C), which is near the sublimation temperature of BDC (402 C) (Lucchesi and Lewis, 1968), the most of unreacted terephthalic acid could be removed from the structure of the synthesis material without structural collapse of the framework (Sun et al., 2015). The N2 adsorption and desorption isotherms on the samples at 77 K (196 C) are depicted in Fig. 3. The isotherms are reversible and do not show any hysteresis upon desorption. The obtained isotherms of the samples can be classified as type-I according to the IUPAC classification (Sing, 1985; Thommes et al., 2015). The BET (Brunnauer, Emmett and Teller) technique was used in order to estimate the specific surface area and pore volume of the samples from N2 isotherm. The obtained results are tabulated in Table 2. As this table, the highest BET specific surface area (1165.0 m2/g), which is slightly higher than that for the other
Table 2 Results of nitrogen sorption at 77 K up to 1 bar. Sample
ABET(m2/g)
Micropore Area
Micropore Volume(cm3/g)
S-1 S-2 S-3 S-4
1150.6 1074.1 1165.0 1103.9
1126.7 999.9 1136.4 1050.3
0.53 0.46 0.54 0.48
samples, was obtained for the sample S-3 calcined at 400 C. Thermal stability of the selected sample (S-3) was investigated before calcination by TGA method which is depicted in Fig. 4. Two steps were seen in Fig. 4 which the first one (300e400 C) was attributed to removal of the entrapped BDC within the pores and another one (500e600 C) was ascribed to the collapse of MIL-53 structure and BDC removal from the framework (Loiseau et al., 2004). The TGA results indicated that the temperature of 330 C might be not enough for perfect removal of unreacted BDC. This TGA result confirmed the BET results.
Fig. 3. Isotherms of nitrogen adsorption and desorption at 77 K up to 1 bar.
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Fig. 4. Thermal stability of MIL-53(Al) (sample S-3).
3.2. Single component adsorption The MeSH adsorption isotherms of the samples at low-pressure ranges are illustrated in Fig. 5. As it was expected, the S-3 had the best performance because of the highest capacity and steep slope at very low-pressure isotherm. It could be related to the higher available adsorption site particularly the hydroxyl groups. These sites play a major role in the selective adsorption of polar molecules rey, 2008). These sites may partially be occupied by unreacted (Fe disordered BDC which were trapped in the pores of the adsorbents. The methane and MeSH sorption isotherms of MIL-53(Al) (S-3 as the best sample) powder, respectively, for pressure up to 40 bar and 1.6 bar at 298 K, are depicted in Figs. 6 and 7. From Fig. 6, the sorption isotherm of methane is reversible upon desorption without any hysteresis and presents a monotic type-I shape isotherm according to IUPAC. Maximum adsorption capacity of methane on MIL-53(Al) at 298 K (25 C) is about 6 mmol/g at high pressure. As Fig. 7, the MIL-53(Al) showed very high capacity (~9 mmol/gr) which is about 2e3 times higher than adsorption capacity of 13X, the usual industrial adsorbent for mercaptan removal from natural gas (Ryzhikov et al., 2011) and even more than activated carbon, traditional adsorbent for mercaptan removal from air (Bashkova et al., 2002b). The comparison of the capacity of various adsorbents for physical adsorption of light mercaptans (Ethyl
mercaptan (ESH) and MeSH) is tabulated in Table 3. Also, it can be seen in Figs. 6 and 7 that MeSH isotherm is differed from that for methane. It exhibited one step at about 0.25 bar pressure, which is not regular for microporous materials. A type IV isotherm is noticed in the profile of the MeSH adsorption isotherm. The maximum adsorption of MeSH is about 3 mmol/g and 9 mmol/g respectively for the first and the second section of isotherm. This unusual behavior could be attributed to a wellknown phenomenon in some MOFs, called breathing effect. MIL53 can exist in two forms (but the same topology) including narrow-pore with rectangular pores (0.85 0.85 nm), and largepore forms with trapezoidal pores (0.26 1.36 nm). They exhibited reversible structural transformation from large-pore form to narrow-pore form (and vice versa) upon adsorption of polar guest molecules (such as CO2, H2O, H2S) at ambient temperature which is called “breathing effect”(Alhamami et al., 2014). Due to the existence of stepwise behavior in the adsorption isotherm of MeSH, it was divided into two sections for fitting appreciate model for each section. One model for the low-pressure region (LP) and another one for the high-pressure region (HP) was fitted to the experimental data. Using Matlab software (MathWork, Inc. R2012a), adjustable parameters of three well-known isotherms models, Langmuir, Sips and Toth, as follows respectively, were obtained for pure methane
Fig. 5. Adsorption isotherm of MeSH on the samples at 298 K up to 0.2 bar.
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Fig. 6. Sorption isotherm of CH4 on MIL-53(Al) at 298 K up to 40 bar.
Fig. 7. Sorption isotherm of MeSH on MIL-53(Al) at 298 K up to 1.6 bar.
Table 3 Light mercaptan adsorption capacity on different adsorbent. Adsorbent
Temperature (K)
Adsorbate
Adsorbent load (mmol/g)
Ref.
Silica-Alumina Gel Silica-Alumina Gel Zeolite -5 A Zeolite -5 A Activated carbon-coconut-based Activated carbon Zeolite-13X (NaX) Zeolite-10X (CaX) Zeolite-5A Zeolite-13X(NaX) MIL-53(Al)
298 298 298 298 293 Amb. 298 298 303 298 298
MeSH ESH MeSH ESH MeSH MeSH MeSH MeSH MeSH ESH MeSH
~0.5 ~0.9 ~2.7 ~3 ~1 ~1.2 3.80 2.92 ~3.5 ~3.9 ~9
(Steuten et al., 2013)
(Lee et al., 2010) (Tamai et al., 2006) (Ryzhikov et al., 2011) (Sakano et al., 1996a,b) (Benoit et al., 2005) This work
and MeSH at 298 K and are tabulated in Table 4.
q ¼ qs
bp 1 þ bp
(2)
1
q ¼ qs
ðbpÞn 1
1 þ ðbpÞn
(3)
bp q ¼ qs 1 1 þ ðbpÞn n
(4)
p: Pressure of gas at equilibrium with adsorbed phase (bar).q: Adsorbed amount per mass of adsorbent (mmol/g).qs: Saturation or maximum capacity of adsorbent (mmol/g).b: Affinity coefficient (bar1).n: Adsorbate-adsorbent interaction (surface heterogeneity) parameter-not applicable for Langmuir model.
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Table 4 Langmuir, Sips and Toth fitting parameter for MeSH and CH4 adsorption on MIL-53(Al) at 298 K. parameter
b (bar
Region
1
)
qs (mmolg
1
)
n SSE R2
LP HP LP HP LP HP LP HP LP HP
CH3SH
CH4
Langmuir
Sips
Toth
Langmuir
Sips
Toth
49.62 1.74 2.9 12.59 e e 0.08762 0.2932 0.992 0.9965
3.29 2.74 3.68 11.5 0.64 1.17 0.02115 0.1944 0.9957 0.9977
2.43 2.29 4.36 11.23 0.43 1.31 0.01623 0.2786 0.9967 0.9966
0.079
0.054
0.076
6.93
8.13
9.51
e
1.14
0.69
0.04163
0.006337
0.003648
0.9992
0.9999
0.9999
Sum of the square of errors (SSE) as the most commonly used error function (Foo and Hameed, 2010) was considered as an objective function for fitting these isotherms to the experimental values. R-square or coefficient of determination denoted as R2, and SSE of each fitting are provided in Table 4. According to the obtained R2 and SSE values for all models, a good agreement between experimental adsorption data and models results was observed. The maximum adsorption capacity of pure gases (qs), which were estimated from the models, were relatively in agreement with each other according to Table 4. These values were observed to be higher than the results were obtained from Fig. 7 which were measured at pressures below MeSH saturation pressure. In all pressure ranges and in all models, the affinity of MIL-53(Al) for MeSH was observed to be so much higher than that for CH4. The MIL-53(Al) affinity for MeSH at high pressure was less than low-pressure. This could be related to the weaker interaction of MeSH molecules within the large-pore than narrow-pore structure as previously observed for some polar molecules (Hamon et al., 2009; Ramsahye et al., 2007). To examine the reversibility of MeSH adsorption on the MIL53(Al), experiments of consecutive adsorption-desorption cycles were performed using the same adsorbent at 298 K. The effect of 5 consecutive adsorptionedesorption cycles are shown in Fig. 8. As this figure, after 5 cycles, more than 95% of initial adsorption capacity was preserved. It suggested that the MIL-53(Al) had excellent reversibility of MeSH adsorption and did not disclose significant loss in capacity. 3.3. Heat of adsorption The isosteric heat of adsorption can provide useful information about the energetic heterogeneity of an adsorbent and its evaluation for potential adsorption processes. The isosteric heat
(enthalpy) of adsorption characterized the interaction between the adsorbate molecules and the adsorbent surface. The isosteric heat of adsorption could be estimated using Clausius-Clapeyron equation by driving the slope of ln(P) versus T1 at a constant loading(q). Therefore, the isotherms of MeSH and CH4 on MIL-53(Al) at three different various (298, 323, 348 K) were obtained and used to calculate the heat of adsorption depicted in Fig. 9. From Fig. 9, the adsorption enthalpy of MIL-53(Al) for MeSH and CH4 at zero coverage (or pressure) were about 50 and 18 kJ/mol respectively. The heat of MeSH adsorption on MIL-53(Al) was in the range of 26e50 kJ/mol while that for CH4 was at around 18 kJ/mol and was nearly constant. The higher isosteric heat of adsorption for MeSH than that of CH4 indicated higher interaction of MeSH than CH4 with the adsorption sites of MIL-53(Al) and it can be an explanation for high selectivity of MIL-53 toward MeSH/CH4. This phenomenon was attributed to the nonpolar nature of CH4 molecules respect to the polar nature of MeSH. The MeSH isosteric heat of adsorption showed a decreasing trend with increasing of loading pressure while the enthalpy of CH4 adsorption is rather constant. At higher pressure, the MeSH heat of adsorption remained constant to a value close to vaporization (or condensation) enthalpy of MeSH. The initial drop in the isosteric heats of MeSH adsorption could be ascribed to the energetic heterogeneity of the adsorbent surface and the strong interaction between MeSH and hydroxyl groups (OH sites) of MIL-53(Al). However the relatively constant isosteric heat with increasing CH4 loading suggested that the surface of MIL53(Al) is homogenous toward CH4 molecules. 3.4. Adsorption kinetics The kinetic selectivity of adsorbent was measured by the ratio of
Fig. 8. Effect of regeneration cycles on the performances of adsorptive removal of MeSH on MIL-53(Al) at 298 K.
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279
Fig. 9. Heat of MeSH and CH4 adsorption on MIL-53(Al).
interacrystalline diffusivities of MeSH to CH4 (Ruthven, 1984). As described in the experimental section, the trend of decreasing pressure with time was recorded while adsorption equilibrium data were recorded. The uptake ratio versus time at 298 K for MeSH and CH4 are plotted in Fig. 10. It can be seen from Fig. 10 that the adsorption kinetic of CH4 and MeSH are relatively fast. It could be related to the larger pore size of adsorbent with respect to the kinetic diameter of molecules. To extract the interacrystalline diffusivity of adsorbates within the adsorbent, classical micropore diffusion model was applied. If the adsorbent regarded as a spherical object and heat transfer is rapid, the diffusion equation is written as (Ruthven, 1984):
vq 1 v vq ¼ 2 r 2 Dc vt r vr vr
(5)
where r is the radius of the adsorbent, Dc is the interacrystalline diffusivity and q(t,r) is the adsorbed amount at time t and radial position r. According to the crystal symmetry and assuming constant adsorbate concentration, initial and boundary conditions are as follows:
qðr; 0Þ ¼ q 0 0 ; qðrc ; tÞ ¼ q0 ;
vq ¼0 vr r¼0
q0 0 : the initial adsorption amount in the adsorbent particle.q0 : the equilibrium uptake in the adsorbent particle.rc: particle diameter. By considering above boundary conditions and assuming constant diffusivity, the solution of Eq. (5) is given by familiar equation:
∞ q q0 0 mt 6 X 1 p2 n2 Dc t ¼ ¼ 1 exp q0 q0 0 m∞ rc 2 p2 n¼1 n2
(6)
mt q : the average adsorption amount in the adsorbent particle.m : the ∞ fractional adsorption. At short times, if the fractional adsorption uptake is less than % 85, Eq. (6) can be reduced to the following equation with less than 1% error (Ruthven and S. Farooq, 1993)
1 mt 6 Dc t 2 3Dc t ¼ pffiffiffi 2 m∞ rc p rc 2 Therefore, by drawing fractional adsorption uptake
Fig. 10. Fractional adsorption uptakes of MeSH and CH4 adsorption on MIL-53(Al) at 298 K.
(7) mt m∞
versus
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time as depicted in Fig. 10, the diffusion time constant be extracted.
3Dc rc 2
could
Dc for MeSH ¼ 0:01483 s1 rc 2 Dc for CH4 ¼ 0:01995 s1 rc 2 Therefore, CH4 diffuse slightly faster than MeSH due to smaller kinetic diameter. The adsorption kinetic measurements suggested that the diffusivity of MeSH on MIL-53(Al) was comparable to that for CH4. So, it can be concluded that the overall selectivity (combining the equilibrium and kinetic effects) was controlled by the equilibrium selectivity. 3.5. Separation selectivity of binary adsorption of MeSH and methane The equilibrium selectivity is one of the most factor in adsorbent selection for pressure swing adsorption (PSA) is defined as (Yang, 2003):
x xj
aij ¼ i.
(8)
yi yj
xi and xj: the mole fractions of the two components on the adsorbent.yi and yj: the corresponding mole fractions in the gas phase. To evaluate the MeSH/CH4 selectivities of MIL-53(Al) under mixture gas conditions, the IAST and EL models were used to predict multicomponent adsorption behaviors. Fitted parameters of low pressure region were applied because of very low concentrations of mercaptans usually found in natural gas and biogas. Using the EL equation in conjunction with eq. (8), revealed that the adsorbent selectivity is the ratio of the product of qs b or Henry's constant (K) of two components. Therefore, the EL selectivity of MIL-53(Al) for MeSH/CH4 could be expressed as:
aij ¼
Ki qsi bi ¼ 263:5 Kj qsj bj
By using Matlab software, IAST model was used to predict selectivity of the binary mixture, based on fitted parameters of Langmuir equation for pure MeSH and CH4 (due to its simplicity). The purification of gas stream with MeSH mole fraction of 0.001, 0.01 and 0.1 were investigated at the pressure up to 10 bar. The obtained selectivities from IAST and EL models for three different compositions in the pressure range of 0e10 bar, are depicted in Fig. 11. It can be seen that, MIL-53(Al) showed high MeSH/CH4 selectivities for all gas compositions and over entire pressures ranges as the EL selectivity was a constant value of 263.5. The selectivity is a key factor for evaluation of a new adsorbent. The minimum acceptable value of selectivity factor for PSA application was reported to be 2 (Tagliabue et al., 2009). Also, as resulted above, the adsorption capacity MIL-53(Al) samples for MeSH was at least twofold of 5 A or 13X zeolite which are mostly used for mercaptan removal. Therefore, it can be concluded that MIL-53(Al) can be a proper adsorbent for MeSH removal from CH4 gas stream with high adsorption capacity and selectivity. From Fig. 11, the IAST selectivity factors were very sensitive to both total pressure and composition. At very low pressure, the selectivity calculated based on EL equation and IAST were almost similar. The IAST model results indicated that by increasing the total pressure from 0 to 10 bar for a particular gas composition, the selectivity decreases. In low-pressure region, the selectivity mostly was controlled with the interaction of the adsorbent with the adsorbate. Hence, MeSH molecules were mostly adsorbed by the adsorbent due to the high affinity of MIL-53 for these molecules. As the pressure was increased, the interaction between molecules was become more significant respect to the interaction with the adsorbent. Therefore, the MeSH molecules needed to compete with CH4 molecules for available surface sites. By increasing MeSH molar composition, the selectivity, resulted from IAST model, decreases. The reasons for this phenomenon could be explained by changing of MeSH adsorption mechanism. At low MeSH contents, MeSH molecules interact strongly with the hydroxyl groups of MIL-53 (two hydroxyl group per unit cell) and selectively were adsorbed while at higher MeSH content, they may be adsorbed on the remaining hydroxyl sites and other available sites (at nonspecific way) which led to the lower selectivity.
Fig. 11. The predicted IAST selectivity toward MeSH/CH4 as a function of pressure and composition at 298 K on the MIL-53(Al).
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4. Conclusion MIL-53(Al) was synthesized in various conditions using hydrothermal method. The obtained materials were characterized by XRD, BET and TGA techniques. Results showed that time of synthesis and calcination of MIL-53(Al) could be reduced with a small decrement in BET surface area and crystallinity of the materials. Also, calcination of MIL-53(Al) at 400 C resulted in higher crystallinity and BET surface area compared to the regular calcination method (at 330 C). Furthermore, MeSH and methane adsorption capacities, thermodynamics and adsorption kinetics of the selected sample were examined by the volumetric method. The isotherm of MeSH adsorption on MIL-53(Al) showed a type IV due to the structural transformation of MIL-53 adsorption. MIL-53(Al) displayed considerably higher adsorption capacity for MeSH than the most used adsorbents for mercaptans removal (zeolite and activated carbon). The adsorption process of MeSH on MIL-53(Al) was highly reversible with a desorption efficiency of up to 95%. The micropore diffusivities of CH4 and MeSH in the MIL-53(Al) were relatively high. Also, the selectivity of MIL-53(Al) for CH4 stream contained MeSH impurity was investigated by EL and IAST methods. Results indicated that MIL-53(Al) had very great selectivity for MeSH/CH4 stream particularly at low pressure and concentration. Finally, The obtained remarkable MeSH adsorption capacity, great selectivity, good kinetic rate, easy regeneration and reversibility of process confirmed that the MIL-53(Al) can be appropriately applied in a PSA process for CH4 purification. Abbreviations Amb Ambient BET Brunnauer, Emmett and Teller EL Extended Langmuir ESH Ethyl mercaptan HP High-Pressure IAST Ideal Adsorbed Solution Theory IUPAC International Union of Pure and Applied Chemistry LP Low-Pressure MeSH Methyl Mercaptan MOF Metal Organic Framework PSA Pressure Swing Adsorption PXRD Powder X-ray Diffraction SSE Sum of the Square of Errors TIC Temperature Indicator Controller TGA Thermal Gravimetric Analysis TPA (H2BDC) Terephthalic Acid References Abatzoglou, N., Boivin, S., 2009. A review of biogas purification processes. Biofuels, Bioprod. Biorefining 3, 42e71. http://dx.doi.org/10.1002/bbb.117. Achmann, S., Hagen, G., H€ ammerle, M., Malkowsky, I., Kiener, C., Moos, R., 2010. Sulfur removal from low-sulfur gasoline and diesel fuel by metal-organic frameworks. Chem. Eng. Technol. 33, 275e280. http://dx.doi.org/10.1002/ ceat.200900426. Ahmed, I., Jhung, S.H., 2016. Adsorptive desulfurization and denitrogenation using metal-organic frameworks. J. Hazard. Mater 301, 259e276. http://dx.doi.org/ 10.1016/j.jhazmat.2015.08.045. Alhamami, M., Doan, H., Cheng, C.-H., 2014. A review on breathing behaviors of metal-organic-frameworks (MOFs) for gas adsorption. Mater. (Basel) 7, 3198e3250. http://dx.doi.org/10.3390/ma7043198. Bashkova, S., Bagreev, A., Bandosz, T.J., 2002. Adsorption of methyl mercaptan on activated carbons. Environ. Sci. Technol. 36, 2777e2782. http://dx.doi.org/ 10.1021/es011416v. Bashkova, Svetlana, Bagreev, Andrey, B., T.J., 2002. Activated carnbon as adsorbent of methyl mercaptan. Abstr. Pap. Am. Chem. Soc. 224, 573e574. Bashkova, S., Bagreev, A., Bandosz, T.J., 2003. Adsorption/oxidation of methyl
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