Process optimization studies of p-xylene separation from binary xylene mixture over silicalite-1 membrane using response surface methodology (original) (raw)
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Chemical Engineering Journal, 2010
a b s t r a c t m-Xylene isomerization kinetics has been studied using acid-functionalized silicalite-1 catalytic membrane in the temperature range of 355-450 • C. Two types of catalytic membranes: (1) propylsulfonic acid-functionalized silicalite-1 membrane and (2) arenesulfonic acid-functionalized silicalite-1 membrane were synthesized on ␣-alumina support via one-step in situ hydrothermal crystallization and subsequent post-synthesis modifications. The membranes were characterized by scanning electron microscopy (SEM), ammonia temperature-programmed desorption (NH 3 -TPD) and Fourier transform infrared spectroscopy (FT-IR). Arenesulfonic acid-functionalized silicalite-1 membrane with its higher acidity gave better catalytic activity as compared to propylsulfonic acid-functionalized silicalite-1 membrane. The continuous removal of reaction products over the membrane contributed in the higher p-xylene yield. A triangular reaction scheme based on time on stream (TOS) model was used to analyze the experimental data. The simulated results were in good agreement with the experimental results, within an error less than ±5%. The estimated activation energies indicated that conversion of m-xylene to p-xylene in both acid-functionalized silicalite-1 membranes is affected by the mass transfer rate through the membrane, while conversion of m-xylene to o-xylene is controlled by the reaction rate.
Xylene isomerization in an extractor type catalytic membrane reactor
Catalysis Today, 2005
A zeolite/alumina pore plugging membrane was used to successfully separate xylene isomers. It was then applied, as a selective membrane, in an extractor type catalytic membrane reactor (CMR), used to enhance the xylene isomerization reaction selectivity towards para-xylene. The results of the CMR in different configurations (permeate-only and combined permeate-and-retentate mode) were compared to conventional fixed-bed reactor results. In both cases, the selectivity was significantly enhanced (up to 100% in permeate-only mode). In the combined mode, the CMR also provided a net increase in productivity over the conventional reactor. #
Journal of Membrane Science, 2012
The separation of olefin-paraffin mixtures is one of the most important and expensive processes in petrochemical industries. In this research, the performance of cellulose acetate-silica nanocomposite membranes in the separation of ethylene/ethane and propylene/propane has been studied. Silica nanoparticles were prepared via hydrolysis of tetraethoxysilane (TEOS). Membranes were prepared by the solution-casting method. The prepared membranes were characterized using FT-IR, SEM and TGA analyses. Pure gas permeation experiments were performed by the constant volume/variable pressure method at 2 bar feed absolute pressure and 35 1C. The results showed the permeability of ethylene and propylene increased from 0.052 barrer and 0.046 barrer in pure cellulose acetate to 0.11 and 0.098 barrer in the composite membrane containing 30 wt% silica particles, respectively. The comparison of the selectivities of C 2 H 4 /C 2 H 6 and C 3 H 6 /C 3 H 8 indicates an increase from 2.16 and 2.55 in pure cellulose acetate to 4.07 and 6.12 in composite membrane containing 30 wt% silica particles. The diffusion coefficients of prepared hybrid membranes were determined by the time lag method. The solubility coefficient was calculated indirectly from permeability and diffusivity coefficients. The results showed an increase in the solubility coefficient and a decrease in the diffusion coefficient of gases while increasing the silica mass fraction. To investigate the possible plasticization phenomena, the effect of feed pressure on gas permeability and O 2 /N 2 selectivity, before and after exposure of membranes with propylene, was studied. The results showed no plasticization effects up to 8 bar feed pressure.
Theoritical Study of Various Configurations of Membrane Processes for Olefins Separation
International Journal of Membrane Science and Technology
The major purpose of present study, is investigation of the ZIF-8 membrane performance for various configurations, namely; single stage, series multistage and countercurrent recycle cascade (CRC), during separation of propylene from propane and other gases. To this aim, a balck-box model was developed using Pro II software vesion 9 and its validation was carried out by comparing the simulation results with experimental data. The results of simulation validation illustrated a good agreement between theoritical results and experimental data. After simulation validation, for propylene separation, various configurations of ZIF-8 membrane process (single stage, series multistage and countercurrent recycle cascade (CRC) multistage) were compared and the best results were achieved for CRC configuration. In this work, the influence of the some significant operating parameters, namely pressure gradient, feed molar flow rate and membrane surface area on the performance of ZIF-8 membrane was studied in term of purification efficiency of propylene. In general, the simulation results showed that the ZIF-8 membrane presents acceptable performance to produce high purity propylene. It can be concluded that the propylene composition in the permeate side was decreased by membrane surface area, while the feed flow rate and pressure gradient effects were different. However, increasing of feed flow rate indicated the enhancement of propylene molar fraction in retentate side and consequently this result showed decreasing of the membrane performance.
Fixed-Bed Adsorption Separation Of Xylene Isomers over SiO2/Silicallite-1 Core-Shell Adsorbents
Chemical Engineering Research Bulletin, 2013
SiO 2 /Silicalite-1 core-shell material has been demonstrated as potential shape selective adsorbent in gas phase separation of p-xylene from a mixture of p/o-xylene isomers. The core-shell composite comprised of large silica core and thin polycrystalline silicalite-1 shell which was synthesized via a self-assembly of silicalite-1 nanocrystals on core silica surface followed by a secondary seeded growth method. The core materials, SiO 2 used in this study has mesoporosity with an average pore diameter of 60Å and hence offers no shape selectivity for xylene isomers. However, the shell, silicalite-1 contains rigid pore structures and preferentially adsorbs p-xylene from their isomers mixtures. A series of adsorption fixed bed breakthrough adsorption/desorption experiment was performed to obtain the equilibrium isotherms and adsorption isotherm parameters of xylene isomers. The equilibrium isotherms of xylene isomers follow the Langmuir's model. A chromatographic adsorption model has been used to describe the fixed-bed breakthrough profiles of xylene isomers. The model has successfully predicted the responses of the binary mixtures of p/o-xylene isomers. The SiO 2 /silicalite-1 core-shell adsorbents have shown para-selectivity as high as 15.
Separation and Purification Technology, 2001
The permeation of helium, ethane, propane, n-butane, i-butane through a newly developed silicalite-1 membrane was performed using by a batch method and a Wicke-Kallenbach (WK) method. This membrane exhibits high flux properties and maintains a good separation selectivity. A procedure is outlined to interpret measured fluxes and estimate the various contributions of transport modes. The experimental fluxes of helium, ethane, propane and n-butane in the batch method could be divided into different parallel contributions, such as surface diffusion, activated gaseous diffusion and viscous flow. The permeation of helium was mostly governed by activated gaseous diffusion at 303-573 K. For adsorbing gases such as ethane, propane and n-butane, surface diffusion was dominant at temperatures up to 393 K. Their permeation mechanism shifted to activated gaseous diffusion with increasing temperature. In the WK method, both single component measurements and binary mixture separations using n-butane and i-butane were performed in the temperature range of 303-573 K. The selectivity for n-butane in a 1:1 mixture n-butane/i-butane was about 28 up to 400 K, which was higher than the ideal selectivity calculated from the single component measurements because of the competitive adsorption of the butanes. The selectivity of the membrane for n-butane/i-butane mixtures was highly dependent on feed composition and feed pressure.
Industrial & Engineering Chemistry Research, 2010
MFI-type zeolite membranes have shown good selectivity for separation of p-xylene from its isomers. The major problem with the MFI-type zeolite membrane is that the MFI-type zeolite framework loses its size/ shape selectivity under high loadings of p-xylene because of the significant framework distortion experienced by the pore structure and as a result observed high selectivities are not stable over time. This paper proposes changing the interaction of the xylene isomers with MFI-type framework to address this problem. MFI-type zeolite membranes with aluminum and boron isomorphously substituted into the framework were synthesized and subjected to multicomponent xylene separation via pervaporation. It is found that by performing this substitution, slight changes to both surface chemistry and framework flexibility can be introduced. Essentially, the interaction of the xylene molecules with the MFI structure is modified to limit p-xylene loading, as well as diffusion pathway access to o-xylene. As a result improvement in xylene separation performance over silicalite was observed. The boron-substituted membranes demonstrated the highest selectivities for p-xylene under a wide range of feed compositions; the highest selectivity observed was ∼55 (feed, 5% p-xylene; 95% o-xylene). This is higher than any previously reported xylene separation selectivity for pervaporation through MFI-type zeolite membranes. However, the performance stability of substituted membranes over time was also investigated, and it was found that, over a period of 96 h, a reduction in selectivity of about an order of magnitude was observed.
Controlling the Performance of Silicalite-1 Membranes
Chemistry - A European Journal, 2000
The structural and performance characteristics (for n-and i-butane separation) of self-supported silicalite-1 membranes, were optimised by finetuning their syntheses by screening a total of nine silica sources and many reaction conditions. The mass balances indicate that membrane thickness is a function of both the synthesis volume and the silica source used. The excellent properties of the final membrane are demonstrated by its high permselectivity of 31 for n-butane combined with a nbutane flux of 10 mmol m À2 s À1 , indicating perfect performance. For 50/50 mixtures (of n and i) the selectivity for nbutane was 48 and its flux was 3.8 mmol m À2 s À1. For the given selectivities, in relation to the membrane thickness, the theoretical fluxes are the highest values ever reported, underlining the point that high structural integrity is essential to achieve superior functionality.
Separation and Purification Technology, 2008
Na-ZSM-5 membranes were synthesized by secondary growth on the outer surface of stainless steel porous tubes. The membranes were ionexchanged with Cs + , Ba 2+ and Sr 2+ to investigate their effect upon the separation of p-xylene from m-xylene and o-xylene. The permeation through the membranes was measured between 150 and 400 • C using each xylene isomer separately and a ternary mixture. All the membranes were selective to p-xylene in the temperature range studied. N 2 and xylene permeation measurements together with SEM observations were used to determine whether or not cracks and/or pinholes developed after exposure to the xylene isomers at high temperature (400 • C). Neither pore blockage nor extra-zeolitic pores developed after the ion exchange procedure and subsequent calcination. Furthermore, duplicate synthesized membranes of each cation form had similar separation factors and permeances. The duplicate values differ much less than the measurement error. The pxylene permeation flux decreased in the order: Na-ZSM-5 > Ba-ZSM-5 > Sr-ZSM-5 ∼ = Cs-ZSM-5 while the permeation flux of the m-and o-xylene decreased in the order Na-ZSM-5 > Sr-ZSM-5 > Ba-ZSM-5 > Cs-ZSM-5. The membrane that exhibited the best performance was Ba-ZSM-5, with a maximum p/o separation factor of 8.4 and a p-xylene permeance of 0.54 × 10 −7 mol s −1 m −2 Pa −1 at 400 • C.