Membranlarla gaz ayırma
Gas separation by membranes
- Tez No: 39730
- Danışmanlar: DOÇ.DR. BİRGÜL TANTEKİN ERSOLMAZ
- Tez Türü: Yüksek Lisans
- Konular: Kimya Mühendisliği, Chemical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 104
Özet
ÖZET Membran malzemeleri, membran hazırlama, ve modül dizaynı konularındaki çalışmalar ilerledikçe membranlann ticari olarak kullanımı da yaygınlaşmaktadır. Hızla gelişmekte olan membran proseslerinden biri de gaz ayırmadır. Ancak membran gaz ayırma proseslerinin yaygınlaşmasının karşısındaki en büyük engel mevcut polimerik membranlann ayırma özelliklerinin yeterli olmamasıdır. Yani yeterince seçici geçirgen membranlann olmamasıdır. Bu tezin ana amacı gaz ayırmada kullanılabilecek, seçiciliği ve geçirgenliği yüksek membranlann hazırlanmasıdır. Bunu gerçekleştirmenin bir yolu çok ince ayıncı tabakaya sahip asimetrik membranlar hazırlamaktır. Yüksek seçici geçirgenliğe sahip malzemeler hazırlamanın diğer bir yolu da polimer matrisi içerisine adsorban bir dolgu maddesi katmaktır. Bu tezde izlenecek olan bu her iki yol için de membran karakterizasyonu geçirgenlik ölçümleri ile yapılacaktır. Dolayısıyla bu tezin bir amacı da hazırlanacak membranlann seçiciliğini ve geçirgenliğini ölçmede kullanılacak bir geçirgenlik ölçüm sisteminin tasarlanması, kurulması ve test edilmesidir. Bu çalışmada önce sabit hacim, değişken basınç yöntemine dayanılarak bir geçirgenlik ölçme sistemi tasarlanmış ve kurulmuştur. Kurulan sistemin güvenilirliği, yapılan ölçümlerin aynı prensibe göre çalışan bir başka cihazda ölçülen geçirgenlik değerleri ile karşılaştınlması ile test edilmiştir. Deneylerin tekrarlanabilirlikleri ise, aynı örneklerin, aynı şartlarda fakat farklı zamanlarda ölçülen geçirgenliklerinin kıyaslanması ile gerçekleştirilmiştir. Daha sonra asimetrik membranlann seçicilik ve geçirgenliğini etkileyen önemli hazırlama parametrelerinden evaporasyon süresinin membran ayırma özellikleri üzerindeki etkileri incelenmiştir. Çözünme-diffüzyon membranlannın seçici geçirgenliğini arttırmak için ise polimer matrisine X ve Y tipi zeolitlerin katılmasının geçirgenlik ve seçicilikler üzerindeki etkileri araştırılmıştır. Ticari polietilen ve polipropilen membranlann bu çalışmada tasarlanan geçirgenlik ölçme cihazında ölçülen geçirgenlik değerlerinin TÜBİTAK' ta bulunan ticari GDP cihazında ölçülen değerlerle uyum içinde olduğu saptanmıştır. Her iki cihazda belirlenen geçirgenliklerin de literatürde bildirilen değerler ile uyum içinde olduğu görülmüştür. Polimer matrisi içerisine 13X ve Y zeolitlerinin katılmasının O2, N2, CO2 ve He geçirgenliklerinde düşüşe yol açtığı, seçiciliklerde ise önemli bir değişiklik yaratmadığı gözlenmiştir. Bunun nedeninin oldukça hidrofilik olan NaY ve NaX zeolitlerinin yapılannda bulunan suyun, aktivasyon işlemi ile yeteri kadar uzaklaştınlamamış olduğu sanılmaktadır. Asimetrik gaz ayırma membranlann hazırlanmasında evaporasyon süresinin etkileri incelenmiş ve düşük evaporasyon sürelerinde hazırlanan asimetrik membranlann yüzey kusurları içerdiği saptanmıştır. VI
Özet (Çeviri)
SUMMARY GAS SEPARATION BY MEMBRANES Importance of membrane separation processes and their technological applications have been increasing rapidly in the last 20 years. Membrane processes are comparatively more energy efficient than traditional separation processes since there is usually no phase change and no need to go high temperatures. A membrane can be defined as a thin barrier usually polymeric or occasionally ceramic, metal or liquid interface between fluids or solutions. The first large scale commercial application of membrane separation processes is desalination of sea water by reverse osmosis after the development of asymmetric membranes by Loeb and Sourirajan. Asymmetric membranes are composed of a thin dense skin layer which is permselective, supported by a porous substrate which gives mechanical strength to the skin layer. Membrane technology has developed rapidly since Loeb and Sourirajan' s development of the first asymmetric membrane. Present commercial applications of membranes include separation of dissolved ions or molecules via ultrafiltration, microfiltration and reverse osmosis, separation of organic solvents from water or dehydration of solvents via pervaporation and separation of gas mixtures. Advances in membrane technology for the separation of liquid-liquid and solid-liquid streams gradually led to the development of membranes suitable for industrial gas separations. The major applications of membrane based gas separation includes separation of oxygen and nitrogen from air, hydrogen from supercritical gases such as methane, carbon monoxide and hydrogen, removal of CO2 and H2S from natural gas and removal of volatile organics from air. Membrane based gas separation is a concentration driven process. Given a feed stream of a gas mixture at a given pressure, the more permeable component will pass through the membrane to the low pressure side and exit as the permeate while the other remains on the high pressure side and exits as the retentate. A successful membrane based gas separation processes requires attention to three different areas, namely, membrane material selection, membrane preparation, and module design and operating conditions. In spite of all of its advantages, such as low energy consumption, space efficiency, low capital investment, over conventional separation processes, commercial applications of membrane based gas separation processes for VIImany separation problems has failed or cannot compete with well established non-membrane process such as adsorption and cryogenic distillation. A major reason for this is the lack of membranes which give high selectivity and high flux. The gas separation membranes used today are mostly of the solution- diffusion type. In these membranes, gas molecules are first dissolved in the membrane material, in other words, adsorbed by the membrane. The dissolved or adsorbed molecules then diffuse through the membrane across a concentration gradient and finally desorbe at the other side of the membrane. Therefore, separation in solution-diffusion membranes depend on two factors: a) solubility or adsorption, b) mobility or diffusion. Gas 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 and diffusivity. Diffusivity favors the smaller gas molecule, whereas, solubility favors the most condensable gas. Since the number of the molecules moving in the membrane depends on the number of molecules dissolved, separation becomes difficult when these two factors are compatible. Ideal selectivity of a membrane for a binary mixture is the ratio of the pure gas permeabilities of the two species in the gas mixture. The interest in improving the permeability and selectivity of polymeric membranes is growing every year parallel to the increasing demand for new applications. Membranes must be developed rapidly in response to newly emerging needs. This requires continuous research in developing new membranes as well as the capability to produce them on large scale. These research efforts in improving the economics of membrane gas separation applications are condensed mainly in three areas: 1. preparation of ultrathin membranes 2. development of new or modified membrane materials with high permselectivity 3. optimization of operating conditions There are several ways of preparing ultrathin membranes, however, the research efforts towards this had limited success. In asymmetric membranes the important thing is the thickness of the skin layer and its microscopic structure. Optimum membrane properties are obtained when the skin layer thickness is minimized and at the same time contain no surface defects. A defect free skin layer ensures that the gas transport takes place via solution- diffusion mechanism. Selectivity of an asymmetric membrane would be equal to the intrinsic selectivity of the membrane material if the skin layer does not contain any defects and the substrate does not show any resistance to the transport. Asymmetric gas separation membranes are prepared by phase inversion method. In this method, a polymer/solvent solution is cast into a thin layer and subjected to first a solvent evaporation step and then a precipitation step in which the polymer/solvent film is immersed in a nonsolvent bath. The ultrathin VUlasymmetric gas separation membranes prepared in this way usually contain surface defects which is turn cause great decreases in selectivities. In the early 1980' s, it was shown that membrane defects can be overcome by coating the membrane with a thin layer of another polymer whose resistance to flow is less than the skin. Hennis and Tripodi at Monsanto utilized this concept to seal defects in asymmetric polysulfon hollow fibers with silicon rubber. However, these post treatment processes complicates the membrane preparation procedures. Preparation of ultrathin defect-free asymmetric gas separation membranes requires a complete understanding of the membrane formation mechanism and the effect of preparation parameters on the final separation properties of the membrane. The research efforts on the development of new membrane materials with high permselectivity has been focused in recent years on the synthesis of high performance polymers such as polyimides and polytriazols. Another way to achieve an improvement of the intrinsic separation properties of solution- diffusion type membranes is to incorporate specific adsorbents such as zeolites into the polymeric matrix to enhance the permeability of one component with respect to the other. The ultimate goal of this thesis is to prepare gas separation membranes with high selectivity and high permeability. In order to achieve this goal two separate ways are pursued. First, the effect of asymmetric membrane preparation parameters on the final membrane morphology and separation properties is investigated with the objective to understand how to prepare ultrathin membranes. Second, the effect of zeolite filling in the polymeric matrix of solution-diffusion type gas separation membranes and the role of zeolite during gas transport is investigated with the objective to improve the permeability and selectivity of polymeric membranes. Both of these objectives require measurement of gas permeability values in order to test the separation properties of the membranes prepared. Therefore, another objective of this thesis is to design, build and test a gas permeability apparatus. The gas permeation system designed in this thesis is based on the constant volume/variable pressure method. In this method, transport of a gas into a constant volume is obtained by employing a pressure difference between the two sides of a membrane. By observing the pressure increase in the constant volume at the permeate side of the membrane, gas permeability can be determined. The main component of the gas permeation apparatus is a membrane cell which consists of two sections separated by a membrane. A porous quartz disk supports the membrane at the low pressure side of the cell so that the membrane is not damaged due to the pressure difference employed during the experiments. In order to prevent leaks in the cell, two o-rings are placed in each part of the cell. Pressure on the feed side can be monitored by a pressure indicator. Pressure changes in the permeate side and temperature changes in the feed side are measured with a pressure transducer and a IXtemperature sensor respectively. The pressure transducer output is stored in a computer via a data acquisition unit. The cell and the connecting pipes are immersed in a constant temperature oil bath whose temperature is controlled to ± 0. 1 °C sensitivity with an on-off controller. In order to measure steady-state gas permeability coefficient, the permeate side is kept under vacuum at the beginning of the experiment while the feed side is exposed to a pressure of 4 atm until steady-state is reached. At steady-state, the permeate side is closed to vacuum and the pressure increase in the permeate side in time is then evaluated to determine the steady-state gas permeability coefficient. The gas permeability system built in this thesis has been tested for reproducibility and accuracy. The gas permeability values obtained for polyethylene, polypropylene and cellulose acetate dense films have been compared with the results of a commercial gas permeability apparatus in TÜBÎTAK-MAM and literature data. It has been shown that the system used here can give reproducible data and maximum error between the apparatus in TÜBÎTAK-MAM and this system is 15%. All permeability values agree with the literature date range very well. Integrally skinned asymmetric membranes were prepared from cellulose acetate (CA) by phase inversion technique at ambient pressure and temperature. Casting solutions were prepared by dissolving appropriate amounts of CA in acetone. Then the solution is cast in the form of a thin film of a certain thickness on a glass plate using a casting knife built for this purpose. The cast films were first exposed to a fixed evaporation time and then precipitated in a nonsolvent bath, water in this case. In this study the effect of evaporation time on the structure and gas separation properties of the final membrane is investigated. Zeolite filled dense membranes were prepared from silicon rubber (PDMS). Zeolites used as filler are commercial 13X and Y zeolites in Na form. Zeolite particles were activated at 150°C overnight prior to use in membrane preparation. Zeolite filled silicone rubber membranes are prepared by first mixing the two PDMS components and iso-octane as solvent with the zeolite until a homogenous dispersion was obtained. The air bubbles entrapped during mixing were removed by employing vacuum. PDMS membranes were then cast by using the casting knife. Evaporation of the solvent and the cross- linking was carried out at 60°C for 6 hours. All membranes prepared were characterized by gas permeability measurements for He, O2, N2 and CO2 gases using the apparatus built for this purpose. The 02, N2 permeabilities and O2/N2 selectivity was determined for asymmetric membranes prepared from 15(wt)% CA solution with 250 jim initial cast film thickness with 4, 30 and 60 s. evaporation time. The membrane with 4 s. evaporation time gave extremely high permeabilities and very lowselectivities indicating that it contained surface defects causing Knudsen flow or Poisseville flow instead of solution-diffusion mechanism. As the evaporation time is increased to 30 s and 60 s the defects were eliminated and a O2/N2 selectivity of 6.7-6.9 was obtained. The permeability values were slightly lower for the evaporation time of 60 s. The O2 and N2 permeability measurement conducted on membranes prepared from 20 (wt)% CA solution with 250 \im initial cast film thickness and various evaporation times (6, 11, 16, 21, 31, 41, 60, 92, 00) indicated that membranes prepared at low evaporation times contain surface defects. As the evaporation time increases, defects vanish and permeability values decrease due to the fact that the skin thickness increases. The membranes prepared with 60 s and 92 s evaporation times and the completely evaporated membrane exhibited similar O2, N2 permeabilities and 02/N2 selectivity. It was qualitatively observed that optimum evaporation time was around 30-40 s. The addition of zeolites 13X and Y (in Na form) into the silicon rubber matrix caused considerable decreases in O2, N2, CO2 and He permeabilities as the zeolite content increased. No considerable change was observed in selectivities with zeolite addition and the amount of zeolite content. It was expected that the zeolite addition would increase the permeabilities and selectivities. Surprisingly, 13X and Y zeolites behaved as inert fillers inspite of their large pore size and high adsorption capacity. This may be due to the hydrophilic nature of the zeolites 13X and Y and that the hydration water in the zeolite channels could not be removed by activation at moderate temperatures as employed in this study. Similar behaviour for zeolit A is reported in the literature. Therefore, it is recommended that 13X and Y zeolites are activated at high temperatures (~450°C) before use in preparing zeolite filled membranes. Special attention has to be given to avoid moisture adsorption during membrane preparation. It is also observed that NaY decreased all permeabilities more than 13X. This may be the result of the fact that 13X has a higher number of cations and hence adsorbe gases better. In conclusion, the effect of casting parameters on the separation properties of asymmetric gas separation membranes need to be determined in detail, in particular the effect of evaporation time should be investigated in more detail under well controlled conditions. Zeolite 13X and Y may improve the permeability and selectivity of silicon rubber membranes when they incorporated into silicon rubber membranes. However the activation of the zeolites should be carried out at high temperatures. More work needs to be done in both these areas in order to understand the transport mechanism and achieve the goal of obtaining gas separation membranes with high permeability and high selectivity. XI
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