chemical engineering research and design 9 0 ( 2 0 1 2 ) 1407–1415Contents lists available at SciVerse ScienceDirect Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd Adsorptive separation of meta-xylene from C8 aromatics Milad Rasouli a , Nakisa Yaghobi b,∗ , Sahar Chitsazan a , Mohammad Hossein Sayyar c a b c Faculty of Chemical Engineering, Islamic Azad University, South Tehran Branch, P.O. Box 11365/4435, Tehran, Iran Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran Pooshesh Fan Avar Pars Co., P.O. Box 14965/115, Tehran, Iran a b s t r a c t Industrial adsorptive separation process for liquids is most successful when the involved species have very close boiling points, making distillation expensive or are thermally sensitive at convenient distillation temperatures. The adsorption process was studied for separating meta-xylene from a feed mixture containing all C8 aromatics on binder-free X and Y zeolites in the liquid phase. Zeolitic adsorbents with different SiO2 /Al2 O3 were synthesized by the hydrothermal method and ion-exchanged with alkaline metal cations like lithium, sodium and potassium. The adsorption process was carried out in a breakthrough system at temperature of 110–160 ◦ C and pressure of 6–8 atm. The influence of adsorbent moisture content on the separation process was studied. The optimization of adsorption process was also investigated by the changing operation conditions. The isotherms for each isomer of C8 aromatics and the desorbent possess the adsorption characteristics of Langmuir type. The selectivity factor of meta-xylene and the saturation adsorption capacities of adsorbates were determined. It was observed that the selectivity of meta-xylene increased by sodium ion-exchanging of cationic sites in Y zeolite and the selectivity factor of meta-xylene/para-xylene, meta-xylene/ortho-xylene and meta-xylene/ethylbenzene in the optimum conditions was determined to be 2.62, 2.83 and 5.93, respectively. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Adsorptive separation; Na–Y zeolite; Selectivity factor; C8 aromatics 1. Introduction The growth in demand of meta-xylene (MX) is closely linked to the demand growth of para-xylene (PX), as it is used as a co-monomer in the production of polyethylene terephthalate (PET) based packaging resins with very low oxygen permeability. MX is first oxidized to isophthalic acid (IPA) prior to copolymerization with terephthalic acid (PTA) and ethylene glycol to produce the enhanced gas barrier resins. Significant progress is being made in using MX as the primary monomer in other resins with low oxygen permeability such as metaxylenediamine (MXDA) (Harada, 1988; Toft and Postoaca, 2002). Industrial production of MX was first accomplished by Mitsubishi Gas Chemical in the early 1980s using an HF/BF3 ionic liquid (Kulprathipanja, 2010a,b,c,d,e). Other methods like fractionation (Berger et al., 1972), crystallization (Mohameed et al., 2007) and also extractive distillation have been proposed to recover MX from C8 aromatic isomers (MX, PX, ortho-xylene (OX), ethylbenzene (EB)). Fractionation method is used to ∗ separate two or more solutes, by making use of their difference in solubility. Mikitenko and MacPherson (2000) reported a process for separating PX containing at least two crystallization stages at high temperature. The crystallization process consisted of two major events, nucleation and crystal growth. It was concluded that ultimate recovery of pure MX was around 99%. Hotier et al. (2008) reported a process for producing high purity MX, containing simulated moving adsorption and crystallization. In this system adsorption unit with 15 beds of adsorbent coupled with a crystallization zone which produced MX with a purity of 99.7%. Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture. In the process for separation of MX from a mixture containing MX and OX by extractive distillation MX was recovered in a rectification column in the presence of about one part of extractive agent. Final mixture included a vapor composition of 57.6% MX and 42.4% OX (Berg and Yeh, 1987). It has been also suggested Corresponding author. Tel.: +98 2144580000; fax: +98 2144580000. E-mail address:
[email protected] (N. Yaghobi). Received 26 September 2011; Received in revised form 14 November 2011; Accepted 25 November 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.11.016 ethylbenzene (EB).2. 2000. The product was analyzed by X-ray diffraction and found to be 99% zeolites Y. The chemicals of meta-xylene (MX). Adsorbents preparation 2.. However. Chemicals 2. the aluminosilicate structure is a threedimensional open framework of AlO4 and SiO4 tetrahedrals linked to each other by oxygen molecules.1408 chemical engineering research and design 9 0 ( 2 0 1 2 ) 1407–1415 to recover MX from the process streams circulating in xylene isomerization units prior to or subsequent to the recovery of other desired xylene isomers (http://www. The solution was slowly heated up to 80 ◦ C and maintained at this temperature. 1996. Then.5 g aluminum trihydrate was dissolved in the sodium hydroxide solution which was previously heated to 100 ◦ C. 2010a. Operation conditions and mechanism were discussed. then. 1999). purchased from Merck Chemical Reagent Corporation. for the first time effect of monovalent alkaline metal cations on zeolites X and Y (FAU structure) was investigated in the process for adsorptive separation of MX from a feed mixture containing all C8 aromatics. Since 1980 all new MX has been produced via the UOP MX SorbexTM process.5 g of nucleation center (seed) was added to other reactants (solutions A and B). The synthesized zeolite was changed to ammonium form by ion-exchange method using ammonium chloride 10% at 80 ◦ C followed by sodium chloride 10% at 80 ◦ C (Sanders.uop. Minceva and Rodrigues.e). The synthesized zeolite was changed to the ammonium form by ion-exchanged method using ammonium chloride 10% at 80 ◦ C followed by sodium chloride 10% at 80 ◦ C (Hu and Hiimatta. Adsorption process was carried out on a breakthrough system at the desired temperature and pressure. The framework contains channels and interconnected voids occupied by cations and water molecules. respectively. The degree of separation was characterized by the zeolite selectivity factor. About 300. at temperature of 125 ◦ C. 2008. 2. Elliott and McDaniel.2. 100 g of the prepared solution was mixed with 612 g distilled water and 59 g of sodium hydroxide until being dissolved (solution A). Priegnitz et al. and isooctane (IO) were chemical grade and toluene (TB) was of analytical grade.186 (g/g zeolite). The adsorbents of zeolite used in the MX Sorbex process are metal ion-exchanged Beta and Faujasite (FAU) type zeolites (Zinnen et al. which in turn. ammonium chloride. 2. Materials and methods 100 g of sodium hydroxide was mixed with 300 g distilled water until being dissolved. choice of exchanged metal cations. 1979).. Gomes. Zeolites X and Y were hydrothermally synthesized. Seko et al. Zeolite X preparation 20 g of sodium aluminate was dissolved in 30 g de-ionized (DI) water.b. sodium chloride. 2. the content of autoclave was filtered in the Buckner funnel and it was washed with distilled water until the filtrate had the pH of about 9. orthoxylene (OX).. para-xylene (PX). sodium and potassium cations for MX separation. The liquid phase adsorption process is favored for its operational. Weber. 97. The materials were dried at 110 ◦ C overnight. The synthesis gel was poured in the Teflon-lined stainless steel autoclave and slowly heated to 80–100 ◦ C for 48–72 h.com).2. 8 g of tetra-methylammonium chloride was dissolved in 10 g DI water and the solution was slowly added to the other component. the materials were dried at 100 ◦ C. Leflaive and Berthelet (2007) reported the separation of MX from a mixture including MX and OX. were mixed until being dissolved (solution B). 2010. 1996. 2. and the operating conditions such as temperature. 1974. The crystalline material was separated by filtration and washed with distilled water until the pH was neutral (pH < 10). maintenance and environmental advantages over the previous technologies.23.4 A (Song et al.3. SiO2 /Al2 O3 ratio and water content were manipulated in order to influence the acidity of zeolites. OX and EB on sodium-exchanged zeolite Y were 2. lithium chloride and potassium chloride were purchased from Fluka chemical company. Ion-exchange by monovalent cations There is a strong correlation between the total acidity of a zeolite (the sum of both Brönsted and Lewis acids) and the ionic radius of the cations as well as the valence charge of the exchanged cations (Barthomeuf. 1970). 2009). Zeolite Y preparation 2. 78 g of sodium silicate was diluted with 120 g DI water and the solution was mixed with 16 g aluminum sulfate solution (solution B).000 tons/year of MX capacity was constructed in the late 1990s using the MX SorbexTM process. The cations are mobile and can be . The framework for FAU type zeolite can be built by linking sodalite cages through double six-rings.1. sodium silicate. The agitator was turned off and the mixture aged for 48 h at 25 ◦ C. 2004). In the present work. For FAU zeolites. The mixtures of hydrocarbon feed were prepared from the chemicals mentioned above to simulate the stream of the typical C8 aromatic product from reformers. pressure and feed rate were specified. The solution was transferred to a stainless steel autoclave lined with PTFE (Teflon) and kept in a static air oven at 80–100 ◦ C for 48–72 h. aluminum sulfate.18 and 5. sodium aluminate. 2004). 2. Finally. Specific physical properties of zeolites such as framework structure. affects the separation performance. Finally.. Masini and Plee.1. After that. This creates a large cavity in FAU called the “supercage” (which should really be called a supercavity) accessible by a three-dimensional 12-ring pore system (Kulprathipanja. In this system the selectivity factor of MX to PX. The solution of 220 g sodium silicate was slowly added to the solution containing 59 g sodium hydroxide and 612 g distilled water.c. Solution A was slowly added to solution B and mixture was well agitated for 30 min.88.2. 2005). then it was slowly added to the silicate and aluminate mixture and was mixed (seed).2. 1984. In their research the selectivity factor of MX/OX and adsorption capacity on Na–Y adsorbent was obtained to be 1. Also sodium hydroxide.92 and 0.. Kulprathipanja and co-workers reported a process for separating MX from aromatic hydrocarbons by adsorption separation method (Kulprathipanja et al. 10 g sodium aluminate was diluted with 10 g DI water. characterized and ion-exchanged with lithium. aluminum trihydrate. Zeolites X and Y are versatile molecular sieves from the faujasite family of zeo˚ pore size and three-dimensional pore structure lites with 7. respectively. the recovery of MX has not been commercial success and much MX in these sources is simply converted to other materials such as benzene or PX. which utilizes simulated moving bed (SMB) principles and can process a variety of C8 aromatics feeds (Kulprathipanja.d. 55 g sodium silicate was slowly added to the sodium aluminate solution and stirred for 2 h (solution A). The X-ray diffraction pattern showed that it contained 99% zeolite X. 93 0. The products were sampled and analyzed by GC–MS using capillary columns CP-SilPona CB. toluene Table 1 – Elemental composition of various zeolite adsorbents used.27 5.88 0. The feed rate was controlled using a syringe dosing system (Harvard Apparatus.1 1.27 – – 0. a few droplets of the sample suspended in acetone were placed on a carbon-coated copper grid followed by evaporation at ambient conditions.1 1.83 5. 2003).45 2. toluene as a solvent was introduced in to a packed adsorbent column at a desired flow rate. The samples were scanned 4 times for structure conformation.83 5. 2. In the breakthrough method.5 4.31 – – – – – 1. pore size and the pore volume were determined with nitrogen adsorption isomers at 77 K using a Micrometric ASAP 2010 analyzer. Scanning electron microscopy (SEM) was carried out in a Shimadu S-520 electron microscope operated at 80 kV and equipped with a SIS Megaview III CCD camera. Adsorbent Li (wt%) K (wt%) Al (wt%) Na (wt%) SiO2 /Al2 O3 Y (before ion-exchange) LY NY KY NLY NKY X (before ion-exchange) LX NX KX – 1. Typically the solutions of LiCl 10%.26 5. For this visualization.5 1.83 5.1409 chemical engineering research and design 9 0 ( 2 0 1 2 ) 1407–1415 Fig. First. 2. about 4 g of 0.3. Finally. In the present study.1◦ (2) min−1 .5–1 mm diameter adsorbent particles were placed in a stainless steel column with an inner diameter of 1 cm. Second.5 4.83 5. a large amount of high feed concentration was used. In the present study.51 2. Characterization The chemical composition of the adsorbents was determined by ICP-AES. The suspensions were further diluted by adding additional water up to a solid to liquid (10–15 g solid to 1 l liquid).1 .26 5. sodium and potassium.94 0. NaCl 10% and KCl 10% were dissolved in demineralised water and added to the zeolite for exchanging acidic sites with lithium. exchanged with other cations to varying degrees.00044–77 ml/min). a breakthrough system was used for separating MX from C8 aromatics in the liquid phase adsorption. 18% OX and 4% EB. The feed components were controlled using mass flow controllers (Brooks)..5 4.5 4. X-ray diffraction (XRD) patterns were collected at room temperature using a D-MAX/II A X-ray powder diffractometer with Cu Ka radiation in the range of 5◦ < 2 < 50◦ using Cu K␣ radiation with the scanning speed of 0.66 – – 1.83 5.26 1. respectively. synthesized zeolites were ion-exchanged by monovalent metal cations like lithium. 24% PX.1 0. About 100 mg of a zeolite sample was used in a test.21 – 0. The samples were washed and filtered under vacuum and finally dried overnight at 70 ◦ C (Pieterse et al.26 5. the feed mixture was replaced with toluene and separation process was continued until the effluent reached the feed composition.5 4.71 – – – 1.1 1.90 1. Experimental procedure For each experiment. sodium and potassium. 0. FT-IR spectra were run on a Perkin Elmer system 2000 spectroscopy in the scanning range of 2500–4000 cm−1 for a sample disc of zeolite with KBr. pressure and temperature.4. 1 – Schematic of breakthrough set up.44 0. The specific surface area.83 5.18 0.49 4. The ion-exchange processes were carried out at temperature of 80–85 ◦ C for a period of 6–24 h while stirring at 500 rpm. The experiments were carried out at the temperature of 110–160 ◦ C and pressure of 6–8 atm with a feed mixture containing 54% MX. Vanwell et al.21. Guo et al. Adsorption capacity is defined as the quantity of desired component for example C8 aromatic isomers adsorbed from the feed while the adsorbent is exposed to the feed. 8000  B X X 4000 Y Adsorption selectivity and capacity MX xylene isomer = × MX in zeolite MX in liquid phase A Y 0 xylene isomer in liquid phase Y 2000 Selectivity is a relative term and is defined as the adsorbent’s preference for desired component over the undesired feed components. Fig.c. XRD patterns show that zeolites have crystallinity almost identical to the parents NY and NX. 2009).2.d.86 and 15. There are many adsorption mechanisms that zeolite separation processes can account for such as equilibrium-selective adsorption. These variables can carefully modify to selectively adsorb one particular component over others.e). 2 – XRD pattern of (A) zeolite Y and (B) zeolite X. The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent. 2010a.c. It can be seen that the adsorbents possess ideal porous system. pore size and the pore volume of the adsorbents are given in Table 2.b. The XRD profiles of the synthesized X and Y zeolites (Fig. Ci. 2000).1. chemical composition. zeolite particle sizes. 2003.. Ogura et al. The identification peaks for FAU zeolites are at 2 = 6.0 − Ci Cm Zeolite structures are determined by their SiO2 /Al2 O3 molar ratio. The mechanism present in this study for MX separation is equilibriumselective adsorption. Samples with different SiO2 /Al2 O3 ratios were digested using a sodium peroxide fusion before analysis by ICP-AES. xylene isomer in zeolite where Ai is the adsorption capacity of component i (g/g of zeolite).. While all the adsorbates have access to the adsorbent sites. atomic radius and their hydrophilic specifications. Results point to the incorporation of species into micropore space of Separation of MX from C8 aromatics .5. This in turn results in higher adsorbent selectivity for one component than the others (Kulprathipanja.d. One important parameter that affects the equilibrium-selective adsorption mechanism It is certain that the higher the selectivity factor. the extent of cation-exchange. For different adsorbents intense band observed around 3452 cm−1 which assigned to hydroxyl groups of zeolites.1410 chemical engineering research and design 9 0 ( 2 0 1 2 ) 1407–1415 2.0 is the concentration of component i in the feed (g/g).b. 2001)... The adsorbents to be used in this study comprised a specific crystalline aluminosilicate cage structures in which the alumina and silica tetrahedral were intimately connected in an open three dimensional network and were composed of zeolite single crystals as showed in the SEM photo (Fig. The elemental compositions of the various zeolite samples are given in Table 1. Results and discussion 3. infrared spectra of zeolites provide a wealth of information on hydroxyl groups attached to zeolite structures. Ci is the concentration of component i in the equilibrium solution (g/g). Particularly important is the sequence and time when each of the feed components exits the packed adsorbent column because these characteristics describe the specific adsorbate and adsorbent interactions. Average crystal size around 1 m was observed for both zeolite Y and X. counter exchange ion and water content. 11. 1995).e. the FT-IR spectra matched well with the characteristics of zeolites X and Y absorption peaks presented in this work. The selectivity factor for MX over a particular xylene isomer on the adsorbent was determined as: X 6000 Intensity was replaced and introduced until no C8 aromatics in the feed mixture were detected in the effluents. 2010a.e). 4.61 which can be seen in Fig.0 Cm. 2. Cm is the concentration of IO in the equilibrium solution (g/g) (Kulprathipanja. thermal stability. K0 is the IO charge in the feed (g/g of zeolite). the better the effect of the separation between two components (Kulprathipanja et al. FT-IR spectra for different adsorbents are shown in Fig. Cm. Key zeolitic adsorbent characteristics include framework structure. The hydroxyl groups are important for the chemistry of zeolite materials (Fan et al. consistent with previous works (Olson et al. 2010a. rates-selective adsorption. ion-exchange and reactive adsorption (Kulprathipanja.. shape-selective adsorption. the specific adsorbate is selectively adsorbed based on differences in the adsorbate–adsorbent interaction. respectively.0 is the concentration of IO in the feed (g/g).b. 2001).c. 1 indicates the schematic of the breakthrough set up used in our experiments. Increasing the ratio improves the acidity. The surface area. zeolite Y and zeolite X. The amount of the sample used in this case was much more than that used in the experiments of its kind in other laboratories. 3. The textual properties of the adsorbent samples were investigated by nitrogen adsorption.. The adsorption capacity is determined as follows: Ai = K0 C i. 2006. 2) matched well with the standard patterns (Treacy and Higgins. 3). 5 15 25 35 45 55 2-Theta-Scale Fig.d. Adsorbents structural characters The degree of liquid phase adsorption is almost infinite due to the number of ways of modifying the zeolite characteristics. 3. Jentys and Lercher. 1981. In the O–H stretching region. Overall. 33 0. Specific physical properties of zeolites.14 7. 24% PX.3 47.28 0.05 7.27 .6 45.33 0.28 0. choice of exchanged metal cations. Baerlocher et al. 18% OX and 4% EB which was diluted with IO as an inert component. As mentioned above. SiO2 /Al2 O3 ratio and water content can be manipulated to influence the acidity of zeolites.17 7. is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate.29 0.. Adsorbent framework structure According to the critical molecule dimensions of C8 aromatic isomer. The FAU-type zeolites possess straight ´˚ × 7.4 A et al. For each experiment feed mixture included 54% MX.09 7.09 7.5 48. which in turn affects separation performance.or two-dimensional channels for the liquid adsorption separation.30 0. 3.1411 chemical engineering research and design 9 0 ( 2 0 1 2 ) 1407–1415 Fig. such as framework structure. The degree of separation is characterized by the zeolite selectivity and capacity as well as the operating conditions like temperature. 1984.2.36 0.7 43.6 42.4 A ´˚ (Namba channel system with a window size of 7.4 47.1.7 44.27 0.7 48. large pore size zeolites are more suitable for separating MX from other isomers.21 7. Zeolites X and Y from FAU family were used as the adsorbent to investigate the separation of MX from its isomers. Three-dimensional channel zeolites because of expected to have higher adsorption and desorption rates are the preferred mass separating agent of choice compared to one. 2007)..32 7. Adsorbent Surface area (m2 /g) External surface area (m2 /g) ´˚ Pore size (A) Pore volume (cm3 /g) Y LY NY KY NLY NKY X LX NX KX 691 678 665 657 661 656 683 671 659 652 48. pressure and feed flow rate.08 7.22 7.04 0.8 47. Because MX Table 2 – Surface area of zeolite Y and zeolite X.5 7.32 0. 3 – SEM photographs of (A) zeolite Y and (B) zeolite X. 1) and is more suitable for separation of MX which has higher basicity than other xylene isomers. Based on the most basicity of MX than other xylene isomers.5 ml/min are given in Table 3 which shows the selectivity factor of the MX to PX.83 for MX/OX and 5.69 2. Lower acidic zeolite such as NY demonstrated higher MX separation and was more effective to this purpose.3.93 2. methyl red. NY adsorbent with SiO2 /Al2 O3 ratio of 4.21 1. The influence of water content Adsorbed water molecules on a zeolite adsorbent are polarizable due to a strong electrostatic field between the exchanged cations and alumina framework.83 5. The influence of ion-exchange on the adsorption process Exchanged cations with lower ionic radius have higher zeolite acidity.62 for MX/PX.14 1.61 2. and (F) NKY.7.b.52 4.44 1. sodium and potassium on the selectivity of Y and X zeolitic adsorbents was investigated. SiO2 /Al2 O3 Al (wt.2.28 1.93 1.5 5.61 2. Adsorbent Fig.5.06 1. Zeolite acidity increases for monovalent exchanged cations from K+ < Na+ < Li+ . 2010a. water molecules enhance the acidic properties of the zeolite’s Brönsted acids. 3. 1979).07 1.%) Na (wt. In order to determine the effect of the silica to alumina mol ratio in NY zeolite on the above relative selectivities. In this study. Hence.62 2. Acid–base interactions between zeolitic adsorbents and adsorbates do not always correctly predict the trend of adsorbent selectivity (Kulprathipanja.27 2. adsorbent selectivity and adsorbate mass transfer rates are altered due Table 4 – Influence of SiO2 /Al2 O3 on selectivity of MX. Adsorbent Selectivity factor (MX/PX) (MX/OX) (MX/EB) LY NY KY NLY NKY LX NX KX 1.5) has lower acidity than zeolite X (SiO2 /Al2 O3 : 1.83 5. Ion-exchanged Y adsorbents because of the higher amount of SiO2 /Al2 O3 indicated higher selectivity of MX than ion-exchanged X zeolites.96 1.37 2.33 1. temperature of 130 ◦ C and feed rate of 2..12 1. Experimental results at the pressure of 8 atm.07 1.01 1.10 was by far the most basic of the four mixed xylene isomers.33 1. respectively.5 and 5. Zeolite NY as an efficient adsorbent was applied to continue the experiments. It can be seen that MX adsorption from the feed mixture containing all C8 aromatics highly depended on the exchanged cationic sites.1412 chemical engineering research and design 9 0 ( 2 0 1 2 ) 1407–1415 Table 3 – Influence of ion-exchange on selectivity factor of MX to other isomers. OX and EB on ion-exchanged X and Y adsorbents.65 2.c.09 1. (D) KY.93 for MX/EB. 4 – FT-IR spectra of (A) HY. a breakthrough test was repeated using NY zeolite having silica to alumina molar ratio of 2.18 1.39 2.44 1.15 1.09 1. 3. The correlation between zeolite acidity and ionic radius or exchanged cation valence is measured by titration with n-butylamine and a Hamett indicator. The selectivity factor on NY adsorbent was calculated as 2.83 4.d. Zeolite Y with lower SiO2 /Al2 O3 ratio (4. (E) NLY.35 1.03 1. therefore.62 2.43 2.2.4.83 2.10 5 5. This indicated the excellent characteristic of the adsorbent for the separation of MX from its isomers. 2.5 6. The influence of SiO2 /Al2 O3 molar ratio Zeolite acidity increases in strength as the molar ratio of SiO2 /Al2 O3 decreases due to the increase in AlO4 − sites. 3. 3. (B) LY.e).09 . after post-treatment. (C) NY. This is illustrated by the lower adsorptive separation of MX on KY adsorbent despite the lower amount of acidity than NY adsorbent. Adsorbate–adsorbent interactions and.%) Selectivity factor (MX/PX) (MX/OX) (MX/EB) NY 2.65 3. Table 4 shows results including a comparison with the NY zeolite for which the SiO2 /Al2 O3 was 4. the selectivity of the zeolite was tailored for this chemical trait.7 7.5 indicated higher MX selectivity and was found to be more efficient for separation of MX from its isomers.36 1. the acidity of each ion-exchanged zeolite Y can be properly adjusted to selectively adsorb MX.2.32 1. the influence of alkaline metal cations like lithium. Results point to the criticality of the silica to alumina ratio for the zeolite to be effective in MX separation. which strengthens the electrostatic field in the zeolite and increases the number of acid sites (Seko et al.2.18 1.31 1. 2 0. the raffinate solution was analyzed with GC for determining the adsorption capacities.08 0. 5 – Adsorption isotherm of xylene isomers on NX adsorbent. Results are listed in Table 5.14 2.47 2.5.16 0.06 EB 0. The typical Langmuir types of isotherms were observed.4 when the moisture of the adsorbent was in the range of 0. 8 – Selectivity of MX in the pressure of 6 atm and feed rate of 2.06 0.12 Chemical 0 0 0 0 (MX/PX) (MX/OX) (MX/EB) 2. With a respect to the result for NY adsorbent in reference (Kulprathipanja et al.21 5. it was applied for continuing the optimization of 0.3 and 19.02 Selecvity factor Adsorpon Capacity [g/g] 2. 3.93 5.55 2. For instance.83 2. 7 – Adsorption isotherm of toluene on NX and NY adsorbents.1 Adsorpon Capacity [g/g] Physical Selectivity factor () MX 0.. about 1. this part of physically adsorbed water had no influence on the adsorption behavior of the adsorbent. 2012)..5 ml/min.2. The adsorption isotherm of toluene as a desorbent on NX and NY adsorbents is shown in Fig. it is evident that the NY adsorbent with a perfect framework used in 0 0 10 20 30 40 50 60 70 Fig.3.35 5. Adsorption isotherms of C8 aromatic isomers in liquid phase of IO on NX and NY adsorbents are shown in Figs. Zeolite NY was applied for these tests. 6 – Adsorption isotherm of xylene isomers on NY adsorbent.04 MX/EB MX/OX MX PX OX EB 0.02 0 10 0 20 30 40 50 60 Equilibrium concentraon [wt%] Fig. The maximum water content of the dry sample of the NY adsorbent was 0. the selectivity of MX/EB was obtained 5. 0. when the moisture content of the NY adsorbent was increased. 51.02 A series of tests were performed in batch system to determine the adsorption isotherms on ion-exchanged zeolites X and Y. respectively.5.15–2.12 Toluene (NX) 0.15 0.%) 0. NX and NY zeolites were selected. It was observed that the moisture content in the adsorbent had little influence on the selectivity factor of MX.47 5.5 mg/g for OX. 5 and 6.04 0. 0.1 Toluene (NY) 0. The water just remained on the external surface of the zeolite crystallites in the adsorbent. 99.5 1 0 10 20 30 40 Equilibrium concentraon [wt%] 50 Fig.1413 chemical engineering research and design 9 0 ( 2 0 1 2 ) 1407–1415 0. PX and EB. In one experimental run. .39 2. our experiments play a key role in the course of the adsorptive process. For this reason.18 Adsorpon Capacity [g/g] Total water content (wt.5 g of feed solution was contacted with about 1 g of the adsorbent for 1–2 h at desired adsorption condition.15 wt%.5 ± 0. MX.14 0.09 to water polarization (Rasouli et al. Then. 2009). Optimization of adsorption conditions Since NY adsorbent had higher selectivity than other prepared zeolites.5 0 80 Equilibrium concentraon [wt%] 1. 0. Thus. The saturation equilibrium capacities of the adsorption on NY adsorbent was found higher than NX adsorbent and obtained to be 41. 7.12 wt%.6.49 2. 60 MX/PX 2 110 120 130 140 150 160 Temperature(°C) Fig.06 0.12 Table 5 – Influence of water content in NY adsorbent on selectivity of MX. The remaining water was referred to as physically sorbed water.62 2.14 2. Isotherms indicated the typical type of Langmuir adsorption. Adsorption isotherms 0. Series of tests were performed to investigate the effects of adsorbent water content as measured by loss of ignition (LOI) at 500 ◦ C on MX selectivity factor.39 1.08 PX OX 0.04 3. The adsorption capacity of toluene in both zeolites was more than that of MX and this implied that the affinity of these compounds with the adsorbent was stronger than that with MX. in which no chemical adsorbed water was measured with the weight loss at temperature of 500 ◦ C in TGA. 5 2 1. sufficient pressure and feed mixture flow rate must be applied for maintaining the system in the liquid phase during the entire process.5 2 1. the selectivity factor of MX to other isomers was calculated.6 mg/g of MX. Adsorption process yields good separation when the diffusion rates of the feed components through the permeable barrier differ by a wide margin (Kulprathipanja. According to the principle described in the section of experimental procedure.e).5 MX/EB MX/OX Selecvity factor MX/PX 2 1. 11 – Selectivity of MX in the pressure of 7 atm and feed rate of 5 ml/min.5 2 1 110 1. MX had the highest selectivity to other C8 aromatic isomers. The process was carried out at the temperature of 110–160 ◦ C. One of the critical variables in the liquid phase adsorptive separation is the operating temperature.5–5 ml/min.5 ml/min. pressure of 8 atm and flow rate of 2. Conclusion Binder-free X and Y zeolites were hydrothermally synthesized. 8–13. Fig. The results indicated that MX selectivity decreased by decreasing the SiO2 /Al2 O3 ratio from 4.5 ml/min. Toluene as a desorbent stream was used for the desorption section. Also. 19. 12 – Selectivity of MX in the pressure of 8 atm and feed rate of 2. 2.5 MX/EB MX/EB 6 MX/OX 5.3 mg/g of PX.5 1 110 12 0 13 0 14 0 15 0 160 Temperature(°C) Fig.5 ml/min. 10 – Selectivity of MX in the pressure of 7 atm and feed rate of 2.5 12 0 13 0 14 0 150 16 0 1 110 Temperature(°C) 13 0 140 150 16 0 Temperature (°C) Fig. Liquid phase adsorption must be operated at the temperature that optimizes selectivity and mass transfer rates. characterized and ion-exchanged with metal cations such as lithium.5 mg/g of . sodium and potassium. Zeolite Y with a large-pore size possesses a perfect framework which leads to high selectivity.5 4 3.5 1 110 12 0 13 0 14 0 15 0 1 110 16 0 12 0 Temperature (°C) 13 0 14 0 15 0 16 0 Temperature (°C) Fig. 3 3. All isotherms of each isomer of xylene and toluene as a desorbent showed the typical Langmuir type of adsorption.7.5 3 2.5 MX/EB MX/OX 2.5. It was observed that ionexchanged zeolite Y was stronger than zeolite X in the MX adsorption process. they were tested for the separation of MX from its isomers in the liquid phase adsorption. Fig.5 MX/OX MX/PX 4. finally.5 mg/g of OX. 2010a. pressure of 6–8 atm and also feed mixture flow rate of 2.5 MX/PX 5 Selecvity factor Selecvity factor 2 1.39 at 500 ◦ C. 4. It can be seen that at the temperature of 130 ◦ C. 99. The influence of zeolite water content was also investigated. The best SiO2 /Al2 O3 ratio was obtained for MX separation as 4. The results indicated a little difference for the selectivity factor of MX to other isomers and that increasing water content may keep the selectivity factor low. capacity and high rate of adsorption. These experiments showed that sodium was more effective than other cations in separating MX. The selectivity of MX to other isomers was calculated for each adsorbent. The LOI of the used adsorbent was 0. The experimental results are given in Figs. 9 – Selectivity of MX in the pressure of 6 atm and feed rate of 5 ml/min. 13 – Selectivity of MX in the pressure of 8 atm and feed rate of 5 ml/min.5 MX/EB MX/OX 3 MX/PX Selecvity factor Selecvity factor 12 0 MX/PX 2. 51. adsorption conditions. The saturation capacities of the adsorption on NY were 41.1414 chemical engineering research and design 9 0 ( 2 0 1 2 ) 1407–1415 6.b.d.5 to 2.c. 2007.. 38–40. S. Chem. 46. Okubo.M.. New barrier resin for food packaging.. vanden Brink.. Separation of P-xylene from C8 aromatics on binder-free hydrophobic adsorbent of MFI zeolite I.W. Mikitenko.A. Stud. Meier. Van Hoo. Ch. R... Kawazu. Oxford..S. 140 (1–3). Acknowledgment Author is gratefully acknowledged to SPEC Company. 211–216... L. S. W. Sci. US Patent 3. 1974. pp.H. B. US Patent 4. Zeolites in Industrial Separation and Catalysis..J. 2010d.. Frey. Appl. S. Kanai. 85. 150. G.). US Patent 5.. Eng... Catal. 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