Zeolit zms-5 katkılı silikon kauçuk membranlarla n-parafin/i-parafin ayırımı

Abstract

Membran esaslı buhar ve gaz ayırma proseslerinin henüz yaygınlaşamamasının temel nedeni yeterince yüksek seçicilik ve geçirgenlik gösteren membranların olmamasıdır. Polimerik çözünme- difüzyon membranlarında yüksek taşınım hızlarına yüksek seçicilikle ulaşmanın bir yolu polimer matrisine seçici bir adsorban katkı maddesi ilave etmektedir. Zeolitler, tekdüze mikrogözenek yapıları ve moleküler elek özelikleri ile adsorban katkı maddesi olarak oldukça uygun nitelikler taşımaktadır. Uygun zeolit-polimer kombinasyonu ile zeolit katkısının, polimerik membranların gaz ayırma özelliklerini arttırabileceği literatürde gösterilmiştir. Ancak, zeolit katkılı polimerik membranlarla organik buharların ayrılması ile ilgili çalışmalar yok denecek kadar azdır. Bu çalışmanın amacı polidimetilsiloksan (PDMS) membran matrisi içine, zeolit ZSM-5 katkısının, i-pentan ve n-pentan buharı geçirgenliklerine olan etkilerinin araştırılmasıdır. Bu amaçla, ZSM-5 katkılı membranlar hazırlanmış ve buhar geçirgenlikleri ölçülmüştür. Katkı maddesi olarak kullanılan ZSM-5' in Si/Al oranının ve katyon cinsinin geçirgenlik ve seçicilik değerleri üzerine olan etkileri incelenmiştir. PDMS matrisine ZSM-5' in katılması, n-pentan buharı geçirgenliklerini katkısız membrana kıyasla düşünmüştür. Bunun zeolit kanalları içindeki n-pentan difüzyonun polimerdeki buhar difüzyonundan daha yavaş olmasından kaynaklandığı düşünülmektedir. Katkısız PDMS membrana kıyasla, %20 zeolit katkılı membranlarda daha yüksek i-pentan geçirgenlikleri elde edilmiştir. Zeolit katkısının, i-pentanın seçici adsorpsiyonunu arttırdığı anlaşılmaktadır. Daha toplu ve dallanmış yapısı sebebiyle î-pentanın zeolit kanallarında ve dış yüzeyinde adsorpsiyonu yüksek olabilir. %20 zeolit katkısı için görülen bu artıştan sonra %30 ve %40 zeolit katkılı membranların geçirgenliklerinde düşüş görülmüştür. Bu durumda zeolit katkısı membran üzerinden difüzyonu yavaşlatmakta ve bu etki adsorpsiyondaki artışın etkisinden daha büyük olduğundan geçirgenlikler düşmektedir. Si/Al oranı yüksek olan serilerde daha yüksek geçirgenlikler elde edilmiş ve katyon cinsinin etkisi de yüksek Si/Al oranı söz konusu olduğunda belirgin bir hale gelmiştir. En yüksek geçirgenlik değerleri Si/AI=140 olan NaZSM-5 katkılı membranlara aittir. Katkılı membranlara ait n-pentan/i-pentan seçicilik değerlerinin tamamı katkısız membranlara ait olan seçicilik değerlerinin altındadır. Polimer n-pentanı tercih ederken, membran matrisi içine katılan ZSM-5 i-pentanı tercih ettiği için ZSM-5 katkılı polidimetilsiloksan membranlarla n-pentan/i-pentan karışımlarının aynlamayacağı sonucuna varılmıştır.

In recent years, separations with synthetic membranes have become increasingly important in the chemical industry. Membrane separation systems not only are comparatively more energy efficient than traditional separation process, but often cost much less and can be easier to operate. Membrane processes can be very different in their application, in the structures used as the separating barrier, and in the driving force used for the transport of different chemical components. Membranes are increasingly being used on a large scale, to produce potable water from sea water, in cleaning industrial effluents and recovering valuable constituents, natural gas treatment, hydrogen separation, oxygen enrichment, S02 removal from smelter gas streams, H2S and water removal from natural gas and air streams, NH3 removal from recycle streams in ammonia synthesis and separation of hydrocarbons. One of the major problems confronting the use of membrane separation process, such as gas separation and vapor permeation, in a wider range of applications, is the lack of membranes yielding high flux and high selectivity. Membranes, currently being used for these processes are generally solution-diffusion type, dense, non-porous polymeric membranes. Transport through these membranes occurs in a series of three steps: 1. selective adsorption of feed components into membrane at the feed side, 2. diffusive transport through the membrane, 3. desorption from the membrane at the product or permeate side. Generally the combination of the first two steps determines membrane properties and desorption is not supposed to be rate limiting. Gas and vapor permeability of a membrane is determined by the combined effect of these two factors, and the permeability coefficient is defined as the product of solubility (adsorption) and diffusivity: vi Permeability (P) = Solubility (S) x Diffusivity (D) Solubility is a thermodynamic parameter and gives a measure of the amount of penetrant sorbed by the membrane under equilibrium conditions. The solubility of gases in elastomeric polymers is very low and can be described by Henry" s law. However, with organic vapors or liquids which, can not be considered as ideal, Henry' s law does not apply. Diffusivity is a kinetic parameter which indicates how fast a penetrant is transported through the membrane. Diffusivity depends on the geometry of the penetrant, i.e. as the molecular size increases, the diffusion coefficient decreases. However, the diffusion coefficient is concentration-dependent with interacting systems and even large organic molecules having the ability to swell the polymer can have large diffusion coefficients. The solubility of gases in polymers is generally quite low (< 0.2% by volume) and it is assumed that the gas diffusion coefficient is constant. Such cases can be considered as ideal systems where Fick' s law is obeyed. On the other hand, the solubility of organic liquids and vapors can be relatively high (depending on the specific interaction) and the diffusion coefficient is now assumed to be concentration-dependent, i.e. the diffusivities increase with increasing concentration. Two separate cases must therefore be considered, ideal systems where both the diffusivity and the solubility are constant, and concentration dependent systems where the solubility and the diffusivity are functions of concentration. For ideal systems the sorption isotherm is linear (Henry1 s law), the concentration inside the polymer is proportional to the applied pressure. This behavior is normally observed with gases in elastomers. In concentration dependent systems, the sorption isotherm is generally curved rather than linear, where as such strong interactions occur between organic vapors or liquids and polymer, the sorption isotherms is highly non-linear, especially at high vapor pressures. Such non-ideal sorption behavior can be described by free-volume theory and Flory-Huggins termodynamics. As can be found in the literature the permeability coefficients of various organic components in PDMS can be 4 or 5 orders of magnitude higher than small molecules such as nitrogen. These large differences in permeability arise from differences in interaction and consequently in solubility. Higher solubility increases segmental motion and hence the free volume is increased. Furthermore, since the solubility is non-ideal, the solubility coefficient is a function of concentration (or activity). Since high solubilities occur in glassy as well as rubbery polymers, the diffusion coefficients are also concentration dependent in such a way that the diffusivities increase with increasing penetrant concentration. For such non- ideal systems the solubility can no longer be described by Henry's law and the diffusion coefficient is not a constant. Information on non-ideal or concentration-dependent solubility coefficient can be obtained from sorption isotherms. VII The separation of various hydrocarbonaceous compounds through the use of selective adsorbents is widely employed in the petroleum, chemical and petrochemical industries. The adsorptive separation of various hydrocarbonaceous compounds is a well-developed and commercially practiced process. Presently there are several commercial processes employing zeolite molecular sieve 5A for the separation and recovery of n- paraffins. These processes differ from one another with respect to desorption step, the desorbent, the operating temperatures and pressure, the cycle time, the purging agent, etc. Some of the most important of these processes are Isosiv, Molex, Ensorb and Elf-N-lself processes. Volatile organic compounds produce a large amount of waste emissions which cause not only a severe environmental pollution problem but also a significant economic loss. The recovery of volatile organic compounds from loading, unloading, and other handling operations is under scrutiny from both environmental and economic points of view. However, most existing techniques to control organic vapor emissions, such as adsorption, absorption and condensation have so far proved to be unsatisfactory in view of safety, performance, operating cost and facility space. The concept of organic vapor separation by membranes is not new, but only recently interest has been increased in this process. However, same as in many other gas separation applications, one of the major problems is the lack of high performance membranes. The solution-diffusion model used to describe gas separation and vapor permeation transport mechanism implies that membrane separation properties can be enhanced by either trying to improve the selective sorption or to lower the diffusion barrier or do both. One way to affect membrane sorption is adding zeolites into the membrane matrix. Since the diffusion coefficient is a function of the concentration of the permeants inside the membrane, a higher solubility also affects the diffusional transport mechanism. Zeolites are perfect additives in view of impressive range of zeolite framework structures, pore size distribution and the possibility of modifications by isomorphous substitution, alumination/dealumination and ion-exchange. Recent studies have shown that the performance of polymeric membranes for pervaporation and gas separation applications can be improved by incorporation of zeolite particles in the membrane when the correct zeolite-polymer combination is utilized. However, these studies have been focused on the separation of simple gases. There are no studies in the literature on the separation of organic vapors using zeolite filled membranes. The goal of this study is to investigate the effect of zeolite filling in the polymeric matrix of solution-diffusion type silicon rubber membranes on their separation properties for the application of n-pentane/i-pentane separation and consequently get a better understanding of the transport mechanism during organic vapor transport through the filled membrane. For viii this purpose, commercial ZSM-5 zeolites used as adsorbent fillers. In order to understand the effect of Si/AI ratio of the zeolite on the vapor permeation properties of the membrane, two different ZSM-5 samples with Si/AI=40 and Si/AI=140 have been used. The commercial ZSM-5 zeolites were in H form and in order to study the effect of cations on vapor permeation properties of the zeolite filled membrane, NaZSM-5 samples were prepared by repeated ion-exchange of HZSM-5 samples. All zeolite samples were activated at 500°C for 4 hour prior to preparation of filled membrane. Zeolite filled polydimethylsiloxane (RTV664 A,B) membranes were prepared by casting-evaporation technique. Iso-octane was used as a solvent to provide homogeneous dispersion. The evaporation of the solvent and the crosslinking reaction were carried out at 50°C for 12 hours. The final thickness of the prepared membranes changes from 420 urn to 880 \im. The characterization of membranes was conducted by vapor permeability measurements of n-pentane and i-pentane vapors utilizing a vapor permeation system. Vapor permeability coefficients of n-pentane and i-pentane were measured by two different experimental methods: constant volume/variable pressure method and volumetric method. The vapor permeabilities obtained from volumetric method are much higher than the vapor permeabilities obtained by constant volume/variable pressure method. These differences in permeabilities arise from the strong interaction between the organic vapor and polymer. In constant volume/variable pressure method, it is assumed that the solubility behavior obeys Henry' s law and the diffusion coefficient of the penetrant in the polymer is constant. However in the case of many organic vapors, polymer-organic vapor interaction is so strong that both the solubility and diffusion behavior deviates from the ideal behavior and polymer-organic vapor interaction must be incorporated into the analysis. Volumetric method utilizes the mass of vapor transported through the membrane in the permeability calculations and hence vapor-polymer interaction is already incorporated. Incorporation of ZSM-5 into the membrane matrix, decreased n-pentane permeability compared to the unfilled membrane. The decrease in n-pentane permeabilities may be due to the fact that the rate of diffusion through the zeolite is smaller than the rate of diffusion in the polymer. The vapor permeabilities of i-pentane increased, compared to the unfilled membrane at 20% zeolite content except in the case of NaZSM-5 which has a Si/AI=40. This indicates that incorporation of zeolites into the membrane matrix, increases the adsorption of i-pentane. Incorporation of zeolite ZSM-5 favored i-pentane. After this increase in permeabilities at 20% zeolite content, a decrease is observed at 30% and 40% zeolite contents. This decrease may be due to the decrease in the rate of diffusion with increasing zeolite content through the membrane. be ZSM-5 filled membranes exhibited increases in both n-pentane and i-pentane permeabilities at 50% zeolite content. This may be the result of agiomeration of zeolite particles at high loading of zeolites, since there may be non-selective voids between the zeolite particles and the polymer. Vapor permeabilities increases with increasing Si/AI ratio of the zeolite. Besides, high Si/AI ratio increases the effect of cation type in the zeolites on vapor permeabilities. The highest permeabilities for both n- pentane and i-pentane were obtained in the case of membranes filled with NaZSM-5 with a Si/AI ratio of 140. The n-pentane/i-pentane selectivities of the zeolite filled membranes are lower than the selectivity of unfilled membrane. Although it was expected that incorporation of zeolite would favor n- pentane, zeolite ZSM-5 favored i-pentane. Since PDMS favors n-pentane, there occurs an incompatibility between the zeolite and the polymer, resulting in decreases in selectivities. Therefore n-pentane/i-pentane mixtures can not be separated by ZSM-5 filled silicon rubber membranes.